molecules

Review
Factors Affecting Synthetic Dye Adsorption; Desorption
Studies: A Review of Results from the Last Five Years
(2017–2021)
Eszter Rápó 1,2, * and Szende Tonk 1, *

1 Environmental Science Department, Sapientia Hungarian University of Transylvania, Calea Turzii No. 4,
400193 Cluj-Napoca, Romania
2 Department of Genetics, Microbiology and Biotechnology, Hungarian University of Agriculture and Life
Sciences, Páter Károly No. 1, H-2100 Gödöllő, Hungary
* Correspondence: rapo.eszter@phd.uni-szie.hu (E.R.); tonk.szende@sapientia.ro (S.T.)

Abstract: The primary, most obvious parameter indicating water quality is the color of the water.
Not only can it be aesthetically disturbing, but it can also be an indicator of contamination. Clean,
high-quality water is a valuable, essential asset. Of the available technologies for removing dyes,
adsorption is the most used method due to its ease of use, cost-effectiveness, and high efficiency.
The adsorption process is influenced by several parameters, which are the basis of all laboratories
researching the optimum conditions. The main objective of this review is to provide up-to-date
information on the most studied influencing factors. The effects of initial dye concentration, pH,
adsorbent dosage, particle size and temperature are illustrated through examples from the last five



years (2017–2021) of research. Moreover, general trends are drawn based on these findings. The


removal time ranged from 5 min to 36 h (E = 100% was achieved within 5–60 min). In addition, nearly
Citation: Rápó, E.; Tonk, S. Factors 80% efficiency can be achieved with just 0.05 g of adsorbent. It is important to reduce adsorbent
Affecting Synthetic Dye Adsorption;
particle size (with Φ decrease E = 8–99%). Among the dyes analyzed in this paper, Methylene Blue,
Desorption Studies: A Review of
Congo Red, Malachite Green, Crystal Violet were the most frequently studied. Our conclusions are
Results from the Last Five Years
based on previously published literature.
(2017–2021). Molecules 2021, 26, 5419.
https://doi.org/10.3390/
Keywords: synthetic dyes; historical briefing of dye usage; adsorption influencing parameters;
molecules26175419
desorption eluents
Academic Editor: Sebastian
Schwaminger

Received: 2 August 2021 1. Introduction

Accepted: 2 September 2021 Over the centuries, human ambition and the desire for comfort have brought with
Published: 6 September 2021 them the degradation of the natural environment. This has led to a deterioration in air
quality, over-exploitation of soils and their barrenness through inappropriate management,
Publisher’s Note: MDPI stays neutral and left our natural waters heavily polluted—a problem that needs to be solved [1].
with regard to jurisdictional claims in Between 2000 and 2020, the global population increased from 6.1 billion to 7.8 billion
published maps and institutional affil- people. During this period, 2 billion people gained access to safely managed drinking
iations.
water services, and the number of people lacking safely managed services decreased by
342 million [2]. The rapid population growth is leading to agricultural and industrial
overproduction, with a concomitant decline in water quality and a reduction in quantity as
well. One of the causes of the freshwater crisis, which is slowly unfolding worldwide, is the
Copyright: © 2021 by the authors. presence of various natural or man-made contaminants [3]. As a result of the development
Licensee MDPI, Basel, Switzerland. of human civilization, the pollution caused by the release and/or use of a wide range
This article is an open access article of chemicals has reached serious proportions. Global anthropogenic pollution has led to
distributed under the terms and the accumulation of a wide range of organic xenobiotic compounds that have adverse
conditions of the Creative Commons
effects on human health and intact ecosystems. Xenobiotics are compounds that do not
Attribution (CC BY) license (https://
exist as natural products or may contain structural elements that cannot be synthesized
creativecommons.org/licenses/by/
biochemically [4].
4.0/).

Molecules 2021, 26, 5419. https://doi.org/10.3390/molecules26175419 https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 5419 2 of 31

Pesticides, pharmaceuticals, heavy metals, oils, detergents, industrial chemicals and

dyes can reduce taste quality. The sources of dye contaminants in freshwater can be
the textile, pharmaceutical, food, leather, paint and varnishing industry effluents. Other
sources are households, and moreover the untreated or partially treated effluents from
wastewater treatment plants [3]. According to the literature, five major industries are
known to be responsible for the presence of dye effluents in the environment: the textile
industry (54%), the dyeing industry (21%), paper and pulp industry (10%), tannery and
paint industry (8%), and the dye manufacturing industry (7%) [5,6].
After the dyeing process of textiles, the resulting dye-concentrated wastewater is often
discharged into nature at high pH and temperatures without any treatment. The oxygen
transfer mechanism and the self-purification process of environmental water bodies will
get disturbed by this phenomenon [5,7,8]. Wastewater from the paint industry is a difficult
effluent to treat, not only because of its high biological and chemical oxygen demand, high
suspended solids content and other hazardous substances, but also because of the aesthetic
harm it causes to the visual appearance [9,10]. These substances are often of synthetic
origin and have a complex aromatic molecular structure, which increases their chemical
and microbiological stability, hence their difficult removal from water. The introduction of
dyes into the water system causes a number of health and environmental problems:
• Dyes increases the water turbidity;
• Dyes have a major impact on the photosynthetic activity of the aquatic environment
because they block the penetration of light into the water, thus inhibiting the growth
of algae, which are not only important for oxygen production but are a pillar of the
food chain;
• Most of the dyes are carcinogenic (bladder, kidney, liver), mutagenic and toxic to
living organisms;
• They can cause allergic reactions: skin, eye, mucous membrane irritation, dermatitis,
respiratory problems; and
• They cause harm to aquatic environment, and may be toxic to aquatic organisms due
to their aromatic, heavy metal and chlorine content [3,11,12].
The presence of dyes in natural waters has not received attention in the last 30 years,
and has only recently become part of environmental legislation. As per this law, dye
utilizing industries have to ensure wastewater released from their factories abide by the
International Dye Industry Wastewater Discharge Quality Standards that were adopted
from the Zero Discharge of Hazardous Chemicals Programme (ZDHC) [5,13].
The aim of this review article is to provide up-to-date information on the adsorption
of dyes from aqueous solutions, highlighting the parameter influencing processes. As to
the best of our knowledge, there is a niche in articles that summarize this aspect from the
last five years (2017–2021). The focal aim of this paper is to review the effects of initial
dye concentration, pH, adsorbent dosage, particle size and temperature through examples
from the last five years of research. Moreover, general trends are drawn based on these
findings. In addition, different definitions of dyes are presented at the beginning of the
article, with a brief overview of the historical background and the numerical, statistical
data of their usage and application. The general structure and classification methods are
also described. Finally, the eluents used for adsorbent regeneration and desorption are
listed, and desorption examples are presented.

1.1. Definition of Dyestuff

Dyestuffs are hydro or oil-soluble, colored organic chemical compounds that are
usually dissolved in water and bound to surfaces or fabrics to impart color to textiles. The
majority of dyes are complex organic molecules that are designed to bind strongly to the
polymer molecules that make up the textile fiber, and must be able to withstand a wide
range of external effects [14–16].
In his book “Synthetic dyes”, Gurdeep R. Chatwal [17] defines dyes as colored organic
compounds or mixtures used to color paper, cloth, plastics and leather. The dye substrate
Molecules 2021, 26, 5419 3 of 31

must be resistant to washing and stable to light. It is important to note that not all colored
materials are dyes, as a dye must be fixed to the material to give it a permanent color [17].
According to the internationally accepted convention of Colour Index International,
dyes are defined as intensely colored or fluorescent organic substances that impart color
to a substrate by selective light absorption. These substances dissolve and/or undergo a
process that destroys, if not permanently, the crystal structure by adsorption, mechanical
action, ionic or chemical bonding [18].
Dyes are usually large aromatic molecules, often with many rings linked together. An
aromatic ring structure linked to a side chain in the dye molecule structure is necessary for
resonance and hence for the transfer of color [19]. The resonance structures responsible
for color are those that cause the shifting or appearance of absorption bands in the visible
spectrum of light. In the synthesis of a dye, the correlation of chemical structure and color is
achieved by a chromogen-chromophore-auxochrome combination. Three essential groups
can be found in a dye molecule: the chromophore, auxochrome and matrix [16]. Thus, dyes
are organic colorants that contain at least one unsaturated compound (chromophores) and
one functional group (auxochromes). The chromophore present in the structure may be an
aromatic structure containing benzene, naphthalene, or anthracene rings. The chromophore
group responsible for the color formation is represented by the following radicals: azo
(-N=N-); carbonyl (=C=O); carbon (=C=C=C=); carbon-nitrogen (>C=NH or -CH=N-);
nitroso (-NO or N-OH); nitro (-NO or =NO-OH); and sulfur (>C=S, and other carbon-
sulfur groups). These, in combination with a chromogen, form the basis for the chemical
classification of dyes. Since the chromogen-chromophore structure is often insufficient to
provide adequate solubility and thus the dye cannot adhere to the fiber of the material,
auxochromes are required. Auxochromes enhance the color of the dye. Auxochromes, also
known as binding affinity groups, can be amine (-NHX2 ), hydroxyl (-OH), carboxyl groups
(-COOH), aldehydes (-CHO), sulfonic acid (-SO3 H) or their derivatives [20–23].

1.2. Brief History of Dye Usage

The word dye is from Middle English “deie” and from Old English “dag” and “dah”.
The first known use of the word dye was before the 12th century [24].
Human eyes can see more than one million colors, all of which can be found in
our natural habitats. These wonderful and unique colors attract humans’ attention from
the surroundings, and everyday tools were made to mimic these colors. Archeological
excavations prove that the art of dying can be dated back to the appearance of human
civilization. Figure 1 contains a timeline, based on the detailed historical overview of Susan
C. Druding (unfortunately, the literature data has been lost, so its references are missing),
where some important historical milestones regarding dyestuffs are represented [25,26].
Without wishing to be exhaustive, we would like to mention a few interesting facts, as a
detailed list can be found in the literature. According to these data, colored garments of
cloth and traces of madder dye were found in the ruins of the Indus Valley Civilization
dated between 2600 and 1900 BC. Moreover, the first written record about dyestuff usage
was found in China during this period [27]. Another interesting investigation showed that
the cave paintings of “El Castillo” in Spain were painted about 40,000 years ago. Probably
the oldest colored flax fiber dated around 34,000 BC was found in the Republic of Georgia
(in a prehistoric cave) [28]. Several mentions are made between 715 and 55 BC, from the
Roman Empire, where wool dyeing appeared as a craft, and purple has been used for
dyeing their clothing, like robes. After the conquest of Susa in 333 BC (the capital of Persia),
Alexander the Great mentions that he found purple cloths in the royal treasury (dating
from 541 BC) [24–26]. The 5000 talents of purple cloth colored with mucus (yellowish
material from sea snail’s tiny gland near its neck) today is worth about $68 million [29].
Molecules 2021, 26, 5419 4 of 31
Molecules 2021, 26, x FOR PEER REVIEW 4 of 32

Figure 1.
Figure Historical timeline
1. Historical timeline of
of dye
dye usage,
usage, invention
invention and
and interesting
interesting facts.
facts.

Jumping ahead in time, the 12th century saw the establishment of several painters’
Without wishing to be exhaustive, we would like to mention a few interesting facts,
guilds in Europe’s major cities (e.g., London in 1188). In Florence, in the middle of the
as a detailed list can be found in the literature. According to these data, colored garments
century, there were more than 200 registered painters, clothiers and tailors. Several rulers
of cloth and traces of madder dye were found in the ruins of the Indus Valley Civilization
took measures to protect merchants and quality [25].
dated between 2600 and 1900 BC. Moreover, the first written record about dyestuff usage
At the beginning of the 15th century, Cennino Cennini (Padua, Italy) published his
was found in China during this period [27]. Another interesting investigation showed that
treatise, the Method of Painting Cloths by Means of Moulds, in which he described the
the cave paintings of “El Castillo” in Spain were painted about 40,000 years ago. Probably
method of printing cloth. The first European book on painting, Mariegola Dell’Arte de
the oldest colored flax fiber dated around 34,000 BC was found in the Republic of Georgia
Tentori, was published in Italy in 1429. From 1507 onwards, several European countries
(in a prehistoric cave) [28]. Several mentions are made between 715 and 55 BC, from the
(France, the Netherlands and Germany) began to grow dye plants on an industrial scale [25].
Roman Empire, where wool dyeing appeared as a craft, and purple has been used for
Prior to the industrial revolution, to the middle of the 19th century, all dyestuff was
dyeing their natural
made from clothing, like robes.
sources: After
plants, the conquest
animals, of Susa inThe
and minerals. 333small
BC (the capital ofofPer-
quantities the
sia),
mainAlexander
components theofGreat
dyes,mentions
the longthat he found
distances purpleand
involved, cloths
the in the royal
weather treasurywere
conditions (da-
ting from 541 BC)
the economic [24–26]. Theof
disadvantages 5000 talents
using of purple
natural dyes. cloth colored
For this reason,with mucus
there was(yellowish
a need to
material from sea snail’s tiny gland near its neck) today is worth
be able to produce commonly used dyes quickly and easily by synthetic means in about $68 million [29].any
Jumping ahead in time, the 12th century saw the establishment
region, thus making the product cheaper, and transport and trade more reliable. Literatureof several painters’
guilds
recordsinsuggest
Europe’s thatmajor cities (e.g., London
the substitution and thusinproduction
1188). In Florence,
of naturallyin the middle indigo
occurring of the
century,
and maddertheredyes
wereposed
moredifficulties
than 200 registered
for chemistspainters,
of theclothiers
time [29].and tailors. Several rulers
took measures to protect merchants and quality [25].
The root of the Rubia tinctorum plant, most commonly cultivated in Turkey, was used
At thecadherin,
to extract beginning of thecoloring
whose 15th century,
principleCennino Cennini
is alizarin. In a(Padua,
complicatedItaly)process,
published his
it was
treatise, the Method of Painting Cloths by Means of Moulds, in which
mixed with aluminum to form an insoluble red metal complex, bright red in color, with he described the
method
celluloseoffibers.
printing cloth. The first European book on painting, Mariegola Dell’Arte de
Tentori, was published
Indigo, also a plant indye
Italy(Indigofera
in 1429. From 1507 was
tinctoria), onwards, several
the most European
important countries
natural blue
(France, the Netherlands
dye. In ancient times, theand Germany)
flowering began
indigo plantto was
growcutdyeandplants on an in
fermented industrial
woodenscalevats
[25].
underwater for 10–15 h. A yellow solution was obtained, from which the raw indigo was
Prioras
released toblue
the industrial
flakes in therevolution,
air. Thetoleaves
the middle
of the of the 19th
plant century,
are rich all dyestuff
in indoxyl, was
and after
made from natural sources: plants, animals, and minerals. The small quantities of the main
Molecules 2021, 26, 5419 5 of 31

fermentation, free indoxyl is released, which is rapidly oxidized in air to the desired color,
and is insoluble in water [29].
Therefore, the discovery and development of synthetic dyes are closely intertwined with
the development of organic chemistry and the industrial, economic, and social demands of
the 19th century. There were a lot of attempts to produce synthetic dyes; however, these were
not successful due to their poor lightfastness. The discoverer and pioneer of synthetic dyes is
said to be William Henry Perkin. On Easter 1856, while studying the production of artificial
quinine for the treatment of malaria (oxidized dichromate), he isolated a small amount of
purple dye. He named the dye ‘mauve’, which soon became a favorite of the royal family,
and a new industry was launched [30]. Until the beginning of the twentieth century, the dye
industry continued to flourish, with many different types of dyes being produced, making it
essential to classify, record and catalogue them. In 1924, the first edition of the Color Index was
published, listing over 1200 organic and synthetic dyes.
It was reported that in 2014, more than 1.5 million tons of dyes were produced
worldwide, out of which 50% were used by the textile industry [31,32]. According to
an article published in 2016, over 50,000 tons of different synthetic dyes were annually
produced and approximately up to 10% were mixed with water bodies [33].
Up to date statistics show that the global dyes market size was valued at USD 33.2
billion in 2021. The Colour Index™ contains 27,000 individual products under 13,000
generic names and properties [34,35]. It is projected that the revenue generated by the
manufacture of dyes and pigments in Romania will amount to approximately $65.1 million
by 2023 [36].

1.3. Classification of Dyes

As the quantity and variety of dyes has increased throughout history, it has become
essential to classify them. There are several different classifications, based on their structure,
source, color, solubility and application methods. Basically, the most common classification
is based on their chemical structure and application [37]. Figure 2 combines the grouping
by ionic nature (particle charge upon dissolution in aqueous medium) with the application.
Accordingly, we can speak of non-ionic and ionic dyes; the latter being cationic and anionic
in nature. They are classified according to the method of application as reactive, direct and
acid (anionic dyes), basic (cationic dyes), or disperse and vat (non-ionic dyes) [20,38].
Molecules 2021, 26, 5419 6 of 31
Molecules 2021, 26, x FOR PEER REVIEW 6 of 32

Figure
Figure 2.
2. Dye
Dye classification
classification based
based on
on ionic
ionic nature.
nature.

1.3.1. Reactive Dyes

1.3.1.Reactive
Reactivedyes
Dyesmake it possible to obtain a high wet strength compared to the less
Reactive dyes
expensive direct dyes. make it possible
However, theirtouseobtain
is nota always
high wet strength
possible compared
because of thetodifficulty
the less
expensive
in obtainingdirect
gooddyes. However,
unison. Anothertheir use is not always
characteristic is thatpossible because of theisdifficulty
the chlorine-fastness slightly
in obtaining
lower good
than that of unison.
vat dyes,Another
as is its characteristic
light fastness isunder
that the chlorine-fastness
extreme is slightly
conditions [39]. It has
lower than that of vat dyes, as is its light fastness under extreme conditions
been reported that the reactive dyes are the only textile colorants that form a covalent [39]. It has
been reported that the reactive dyes are the only textile colorants that form a covalent
bond with the substrate/textile fiber, usually cotton, during the application process under bond
the influence of alkaline pH and heat [5,40,41]. Reactive dyes contain reactive groups such
Molecules 2021, 26, 5419 7 of 31

with the substrate/textile fiber, usually cotton, during the application process under the
influence of alkaline pH and heat [5,40,41]. Reactive dyes contain reactive groups such as
vinyl-sulfone, chlorotriazine, trichloro pyrimidine, and difluoro-chloro pyrimidine, that
covalently bond with the fiber during the dyeing process [42,43]. Adsorption results show
that since reactive dyes are soluble in aqueous medium and have a greater negative charge
density, the adsorption process was related to electrical attraction between anionic dyes
and positively charged surfaces of adsorbent [43,44]. Initially, these dyes were designed
for cellulose fibers, but nowadays they are used for cotton, wool and poly-amide fabrics;
moreover some fiber-reactive dyes for protein and polyamide fibers are also commercially
available [45]. With about 1150 entries in Color Index and ever rising volumes, the im-
portance of reactive dyes in the global coloration business cannot be overemphasized.
An equally well-known entrenched position is enjoyed by the chlorotriazines and vinyl-
sulphones in the reactive system space, despite the introduction of at least one new reactive
group every year from 1956 until 1971, except 1969 [46,47]. It is estimated that losses of
1–2% occur during the manufacturing process of dyes, while up to 1–10% of dyes are
released back into the environment during use. For reactive dyes, the estimated loss is
around 4%. [32,48]. According to other sources after the colorization process, approxi-
mately 10–50% of the initial dye load remains unused [49–51]. Reactive dyes are said to
be the most problematic among other dyes, as they tend to pass through conventional
treatment systems unaffected, therefore their removal is a difficult task [44,52].

1.3.2. Direct Dyes

Direct dye is still the most widely applied in the dying and printing processes of
the textile industry [53]. Direct dyes are water-soluble and anionic in nature, and they
contribute 17% share in the textile industry, having wide utility in printing and dyeing
cotton, viscose, silk, wool and leather [54–56]. Although these dyes are water-soluble
anionic dyes, they cannot be classified as acid dyes because the acid groups are not the
means of attachment to the fiber. Since these dyes do not require any kind of fixing, they
are called direct dyes [45]. The major chromophore types are as follows: azo, stilbene,
phthalocyanine, dioxazine, formazan, anthraquinone, quinolone and thiazole. Direct dyes
are known to be easy to use, with a wide range of colors and shades, but have a low
resistance to washing; this is what drives them out of the market compared to reactive
dyes [39,57,58].

1.3.3. Acid Dyes

Acid dyes, as their name implies, contain one or more acidic functions (SO3 H and
COOH) in their molecules [16]. They have excellent chemical and photochemical stability,
which is why their industrial effluents have a complex composition, poor biodegradability
and high tinctorial value [59–61]. This makes them difficult to remove by conventional
methods. Their degradation products or metabolites can be potentially mutagenic or
carcinogenic and can damage aquatic ecosystems. The use of water-soluble acid dyes, in
particular sulphonic acid dyes, is very widespread due to their bright color and high solu-
bility [16,60,62]. Acid dyes account for about 30% to 40% of total dye consumption. They
are used in textiles, printing and dyeing, paper, leather, food, cosmetics, pharmaceutical
and other industries for dyeing, e.g., nylon, wool, silk and modified acrylic [16]. The dye
molecules are structurally very different and often contain some metal complexes. The
defining characteristic of the group is the presence of sulphonated groups, which ensure
water solubility, and azo-chromophore systems (the most important group), anthraquinone,
triphenylmethane or copper phthalocyanine [39,43,45].

1.3.4. Cationic-Basic Dyes

Basic dyes belong to the group of cationic dyes because they form a colored cationic
salt in aqueous solution. Later, these cationic salts react with the anionic surface of the
Molecules 2021, 26, 5419 8 of 31

substrate (acrylic, paper and nylon). The resulting cations are electrostatically attracted to
the negatively charged substrates [63].
The cationic functional groups (-NR3+ or =NR2+ ) are usually acid-soluble amino and
substituted amino compounds. They would bind to the fiber by forming ionic bonds with
its anionic groups [45].
In a literature study, it is recorded that this class of dyes is readily visible even at very
low concentrations. This property contributes to the reduced efficiency of natural biologi-
cal self-cleansing by blocking the penetration of sunlight, thus reducing photosynthetic
activity. Basic dyes are highly resistant to degradation due to the number of aromatic rings
associated with their resonance capacity, and their complex and large structure, which
makes them durable and stable in the environment [64–67].

1.3.5. Disperse Dyes

Disperse dyes are water-insoluble dyes; their structure is small and non-ionic with
attached polar functional groups, such as -NO2 and -CN. They are applied to hydrophobic
fibers from an aqueous dispersion [45]. They are mainly used for the dyeing of polyesters
because they can interact with the polyester chains by forming dispersed particles. Disperse
dyes are employed on cellulose acetate, nylon, acrylic fibers and cellulose fibers. The main
classes are benzodifuranone, nitro, styryl, azo and anthraquinone groups [68]. Disperse
dyes have a low solubility in water, therefore they must be applied with a dispersing aid,
and are mainly used for acetate or polyester fiber [69]. From a chemical point of view, more
than 50% of disperse dyes are simple azo compounds, about 25% are anthraquinones, and
the rest are methine, nitro or naphthoquinone dyes [70]. Disperse dyes are also described
as “sublimation” inks, as the ink molecules “sublimate” or change directly from solid to
gas due to the application of heat, skipping any liquid state entirely [71]. The majority
of disperse dyes are based on azo structures; however, violet and blue colors are often
obtained from anthraquinone derivatives [16,72,73]. Disperse dye particles, due to their
nano size, can keep better stability, especially in high temperature dyeing processes [74].

1.3.6. Vat Dyes

Vat dyes are the main sources of pollution in the wastewater of textile and other
industrial effluents, and they are widely used in dyeing cellulosic cotton fabrics [75]. These
types of dyes are water-insoluble. Their main application is for cellulosic fiber, notably
cotton dying [43]. Vat dyes are characterized by excellent color fastness, washability and
chlorine-bleachable colored fibers [16,76]. The disadvantage of their application is that, as
they are practically insoluble in water and thus have no affinity for cellulosic fibers, they
are difficult to use (reduction and oxidation mechanisms) [77]. In conventional tank dyeing
processes, the dye is reduced in alkaline medium with strong reducing agents, from which
the most important is sodium dithionite (Na2 S2 O4 ) [78,79].
Nirav P. Raval et al. [45] made a detailed classification in their article, where the dyes
are grouped based on:
• source of materials/origin (natural–substantive and adjective–synthetic);
• method of application to the substrate (acid, basic, direct, mordant, reactive, disperse,
solvent, sulfur);
• their chemical structure (azo, nitro, indigoid, cyanine, xanthene, quinione-imine, acridine,
oxazine, anthraquinone, phthalein, triphenylmethane, nitroso, diarylmethane); and
• the electronic origins of color (donor–acceptor chromogens, polyene chromogens,
n→π2 chromogens, cyanine type chromogens) [16,43,45].

2. Dye Removing Methods, Technologies

Dye removal methods have been summarized in review articles by many
authors [5,80–93]. The importance of removing dyes is driven by a number of factors;
they are harmful to health, often mutagenic and carcinogenic, inhibit photosynthetic ac-
tivity in the aqueous medium, and even at very low levels (<1 ppm) are highly visible
Molecules 2021, 26, x FOR PEER REVIEW 9 of 32
Molecules 2021, 26, 5419 9 of 31

aqueous medium, and even at very low levels (<1 ppm) are highly visible and undesirable
in
andwater bodies, in
undesirable with color
water beingwith
bodies, the color
most being
obviousthe parameter
most obviousaffecting wateraffecting
parameter quality
[94,95]. Hessel C. et al. described the percentage of non-fixed dye that may be
water quality [94,95]. Hessel C. et al. described the percentage of non-fixed dye that may be discharged
in the effluent
discharged as aeffluent
in the function asofa dye classes
function fromclasses
of dye EPA and fromOECD legislation
EPA and OECD [5].
legislation [5].
Throughout
Throughout recent years, numerous investigations have been made to
recent years, numerous investigations have been made find the
to find the ideal
ideal
technology
technology forfor dye
dye wastewater
wastewater purification.
purification. Even
Even though
though aa high range of
high range of methods
methods have
have
been studied in the past 30 years, only several are truly being implemented
been studied in the past 30 years, only several are truly being implemented by the concern- by the con-
cerning industries these days due to the limitations they
ing industries these days due to the limitations they possess [5]. possess [5].
As
As it
it appears
appears inin the review articles
the review articles referred
referred toto above,
above, dye
dye remediation
remediation technologies
technologies
can be divided into three main categories: physical, chemical, and
can be divided into three main categories: physical, chemical, and biological biological methods. As
methods.
aAssummary,
a summary,Figure 3 contains
Figure somesome
3 contains of theofused methods,
the used and their
methods, andadvantages and disad-
their advantages and
vantages [89]. [89].
disadvantages

Figure 3.
Figure 3. Dye removing
removing methods
methods and
and their
their advantages/disadvantages
advantages/disadvantages [89].

Reviewarticles
Review articlesexclusively
exclusively analyze
analyze and
and compare
compare paint
paint removal
removal methods.
methods. Often,
Often, pub-
pub-
lished studies
studiesare
areused
usedto to
illustrate the the
illustrate effectiveness of theofmethods
effectiveness presented.
the methods In theseInstudies,
presented. these
several methods
studies, are classified
several methods are into the three
classified intomain
the categories
three main ofcategories
paint removal [5,82,83,87,89].
of paint removal
Physical dye removing
[5,82,83,87,89]. Physicaltechniques
dye removing can be: adsorption,
techniques canmembrane separation,
be: adsorption, reversesepara-
membrane osmo-
sis, ion exchange, ultrasonic mineralization, nano-remediation and photo-Fenton
tion, reverse osmosis, ion exchange, ultrasonic mineralization, nano-remediation and processes.
Chemical methods
photo-Fenton are: catalytic
processes. Chemical reduction,
methods coagulation/flocculation,
are: catalytic reduction,electrochemical reduc-
coagulation/floccula-
tion, photolysis/photochemical reduction, advance oxidation processes, ultraviolet
tion, electrochemical reduction, photolysis/photochemical reduction, advance oxidation irradiation
ozonation, clay
processes, minerals irradiation
ultraviolet and zeolites. ozonation,
Biological methods can be divided
clay minerals to phytoremediation
and zeolites. Biological
 Molecules 2021, 26, x FOR PEER REVIEW 10 of 32
Molecules 2021, 26, 5419 10 of 31

methods can be divided to phytoremediation and microbial remediation (bacterial, algae,

fungi,
and mycoremediation,
microbial enzyme degradation
remediation (bacterial, and phycoremediation)
algae, fungi, mycoremediation, enzyme[5,37,96].
degradation and
phycoremediation) [5,37,96].
3. General Aspects of Adsorption Process
3. General
The termAspects of Adsorption
adsorption was firstProcess
used in 1881 by the German physicist Heinrich Kayser
[97]. The
Thetermpast adsorption
decade haswas seenfirst
a boom
used inin 1881
environmental
by the German adsorption
physiciststudies
HeinrichonKayser
the adsorp-
[97].
tive removal of pollutants from the aqueous phase. It is
The past decade has seen a boom in environmental adsorption studies on the adsorptive preferred over other methods
becauseof
removal ofpollutants
its relatively
from simple design,phase.
the aqueous operation, cost effectiveness,
It is preferred over otherand energybecause
methods efficiency
of
[98].
its relatively simple design, operation, cost effectiveness, and energy efficiency [98].
ItItisisaamass
masstransfer
transferprocess
processininwhich
whichaasubstance
substance(adsorbate)
(adsorbate)moves movesfromfromaagas gasoror
liquidphase
liquid phasetotoformformaasurface
surfacemonomolecular
monomolecularlayer layeron onaasolid
solidororliquid
liquidcondensed
condensedphase phase
(substrate,the
(substrate, theadsorbent).
adsorbent).ItItusually
usuallyinvolves
involvesthe themolecules,
molecules,atoms atomsor oreven
evenions
ionsof ofaagas,
gas,
liquidororsolid
liquid solidinina adissolved
dissolvedstate
state that
that areare attached
attached to to
thethe surface.
surface. In practice,
In practice, adsorption
adsorption is
is performed
performed as an asoperation,
an operation, eithereither in batch
in batch or continuous
or continuous mode,mode, in a column
in a column packedpacked
with
porous sorbents
with porous [99]. [99].
sorbents
Adsorption
Adsorption is is often confused
confused by bythe
theterm
termabsorption.
absorption. The The difference
difference between
between ab-
absorp-
sorption
tion andand adsorption
adsorption is that
is that in absorption
in absorption the the molecules
molecules penetrate
penetrate a three-dimensional
a three-dimensional ma-
matrix,
trix, whilewhile in adsorption
in adsorption thethe molecules
molecules attach
attach to atotwo-dimensional
a two-dimensional matrix
matrix [100–102].
[100–102]. The
The process is usually reversible (the reverse process is called desorption),
process is usually reversible (the reverse process is called desorption), so that sorption is so that sorption
isresponsible
responsiblenot notonly
onlyforforthe
theextraction
extractionofofsubstances
substancesbut butalsoalsofor
fortheir
theirrelease.
release.
Adsorption
Adsorptioncan canoccur
occurdue duetoto
physical
physical forces
forces or or
chemical
chemical bonds,
bonds,primarily
primarilyas aasresult of
a result
surface
of surfaceenergy.
energy.In general, partially
In general, exposed
partially surface
exposed particles
surface tend tend
particles to attract otherother
to attract particles
par-
into position.
ticles There are
into position. Thereseveral ways ofways
are several classifying adsorption,
of classifying and Figure
adsorption, and 4Figure
provides a
4 pro-
classification based on the nature of the bond (physical or chemical
vides a classification based on the nature of the bond (physical or chemical bonds) formed bonds) formed between
the adsorbent
between and the pollutant,
the adsorbent describingdescribing
and the pollutant, its characteristics [103–105]. [103–105].
its characteristics

Figure4.4.Types
Figure Typesofofadsorption
adsorptionbonds
bondsand
andnature
natureofofadsorption.
adsorption.

Since
Sinceadsorption
adsorptionphenomena
phenomenaoccur occur inin
many
many natural, biological,
natural, biological,physical
physicalandandchemical
chemi-
systems, people
cal systems, tend tend
people to apply it in industrial
to apply processes
it in industrial and take
processes andadvantage of its benefits.
take advantage It
of its ben-
isefits.
increasingly used for purification
It is increasingly or separation
used for purification purposes; purposes;
or separation it is also a wastewater treatment
it is also a wastewater
technique
treatmentfor the removal
technique forofthe
a wide rangeof
removal of acompounds
wide range fromof industrial
compounds wastewater due its
from industrial
low cost and easy operation [102,106]. Adsorption is most commonly performed to remove
low concentrations of non-degradable organic compounds from groundwater, drinking
Molecules 2021, 26, 5419 11 of 31

water production, process water, or as tertiary treatment, for example after biological water
purification [107].
In summary, adsorption, surface enrichment, refers to the binding of atoms, ions and
molecules on the active centers of a solid surface (surface binding).
In most cases, the method does not require unnecessary energy input; the removal rate
often depends on the kinetic equilibrium and is determined by the surface characteristics
and composition of the adsorbent. The progress of adsorption depends largely on the
affinity of the adsorbent, its ability to react with the pollutant and the adsorption mechanism
between the sorbent and the functional groups of the pollutant [108–110]. The end point of
the adsorption process is considered to be the concentration value at which equilibrium
stability between the solid and liquid phase volumes is reached [110].

Possible Adsorbents
A wide range of review articles [93,111–122] discuss the use, classification, effective-
ness and properties of different adsorbents as they are some of the key influencing factors of
the process. The characteristics through the advantages and limitations of most adsorbents
are also reviewed. This is due to the fact that in recent years, researchers have focused
their attention on the use of new, alternative, cost-effective, environmentally friendly, green
adsorbents to replace the commonly used activated carbon [86]. Since adsorption processes
are required to have high removal efficiency even at trace levels, it is crucial to investigate
and develop new adsorbents with better properties, i.e., low cost and easily accessible. The
adsorbents may be collected from agricultural or animal waste, or industrial by-products.
All adsorbents, by their intrinsic nature, have functional groups that play the key role
in adsorption; therefore, the type of the adsorbent is a key factor in the waste removal
process [123].
Each adsorbent has its own characteristics, such as porosity, pore structure, adsor-
bent surface area, and structural specificity [124]. A high range of adsorbents have been
studied to remediate dye contaminated waters: clays [125–127], chitosan [128,129], cy-
clodextrin [130–132], eggshell [51,133–135], orange peel [136], fluorene-based covalent
triazine framework [137], cellulose [138], wool [139], shrimp [140], rice bran hydrogel
beads [141], coccine [142], seeds [143,144].
With the increase in the number of adsorbents used, their classification and sorting
has become indispensable. The different types of adsorbents can be classified in several
ways; however, the most common ones are listed below [145]:
• natural materials: sawdust, wood, fuller’s earth or bauxite;
• natural materials treated to develop their structures and properties: activated carbons,
activated alumina or silica gel;
• manufactured materials: polymeric resins, zeolites or alumino-silicates;
• agricultural solid wastes and industrial by-products: date pits, fly ash or red mud;
• biosorbents: chitosan, fungi or bacterial biomass.
Another classification is based on their origin:
• Natural adsorbents include carbon, clays, clays minerals, zeolites and ores. These natural
materials are often relatively inexpensive, abundant, plentiful and readily available;
• Synthetic adsorbents are adsorbents produced from agricultural products and wastes,
household wastes, industrial wastes, sewage sludges and polymer adsorbents.
We can distinguish five main categories of novel adsorbents [86]: (i) clay/zeolites and
composites; (ii) biosorbents; (iii) agricultural solid wastes; (iv) industrial by-products and
their composites; (v) miscellaneous adsorbents. Biosorbents further include chitosan, cy-
clodextrin, biomass and their composites. Agricultural solid wastes, as adsorbents, include
sawdust, bark and other materials like cotton fiber, coffee/tea residues, rice husk, different
vegetable and fruit peels and their composites. The industrial by-products include metal
hydroxide sludge, fly ash and red mud. Nanomaterials and metal organic frameworks are
examples of miscellaneous adsorbents.
Molecules 2021, 26, x FOR PEER REVIEW 12 of 32

Molecules 2021, 26, 5419 different vegetable and fruit peels and their composites. The industrial by-products
12 ofin-
31
clude metal hydroxide sludge, fly ash and red mud. Nanomaterials and metal organic
frameworks are examples of miscellaneous adsorbents.
Requirements for sorbents [112]:
Requirements for sorbents [112]:
• Ability to work under several wastewater parameters;
• Ability to work under several wastewater parameters;
• Cost effectiveness;
• Cost effectiveness;
• Removal capability of diverse contaminants;
• Removal capability of diverse contaminants;
•• High adsorption capacity;
High adsorption capacity;
•• High selectivity for various concentrations;
High selectivity for various concentrations;
•• High porosity and specific surface
High porosity and specific surface area;area;
•• High
High durability;
durability;
•• Reusability of adsorbent,
Reusability of adsorbent, ease
ease of
of regeneration;
regeneration;
•• Fast kinetics; and
Fast kinetics; and
•• Being
Being present
present in
in large
large quantities.
quantities.

4. Factors
4. Factors Affecting
Affecting Adsorption
Adsorption ProcessProcess
The efficiency
The efficiency of of liquid
liquid phase
phase adsorption,
adsorption, and
and therefore
therefore the
the optimal
optimal operation
operation of
of the
the
water treatment
water treatment process,
process, depends
depends on on several
several parameters.
parameters. The
The sorption
sorption performance,
performance, as as
illustrated in Figure
illustrated Figure 5, 5,isisinfluenced
influencedby byphysico-chemical
physico-chemicalfactors,
factors,thethe
type
typeof of
pollutant
pollutant(in
thisthis
(in study, the the
study, dyes) andand
dyes) its chemical
its chemicalstructure, and the
structure, andproperties of theofadsorbent
the properties used.
the adsorbent
Such physicochemical
used. Such physicochemical parameters are theare
parameters adsorbent/adsorptive
the adsorbent/adsorptiveinteraction, the surface
interaction, the
chemistry and pore structure of the adsorbent, particle size, nature of the adsorbent,
surface chemistry and pore structure of the adsorbent, particle size, nature of the adsorbent, pres-
ence of other
presence ionsions
of other in the aqueous
in the aqueous solution, pH,
solution, pH,temperature,
temperature,pressure,
pressure,and
and contact
contact time.
The properties
The properties of of the
the adsorbate,
adsorbate, its its molecular
molecular weight,
weight, molecular
molecular structure,
structure, molecular
molecular size
size
and polarity
and polarity should
should also
also bebe taken
taken into
into account
account [38,146].
[38,146].

Figure 5. Factors affecting adsorption process.

Figure 5. Factors affecting adsorption process.
In a batch process, the mixing speed of the aqueous suspension may affect the time
In a to
required batch process,
remove the the mixing speed
contaminant. Whenof the aqueous
a solid suspension
sample of known may affect
mass the time
is exposed
required to remove the contaminant. When a solid sample of known mass
to a liquid phase of known composition, the concentration varies continuously until is exposed to a
liquid phaseisofreached
equilibrium knownas composition, themultiplication.
a result of the concentration varies continuously
The time required foruntil
this,equilib-
which
riumbeiseffectively
can reached asreduced
a resultbyof shaking
the multiplication.
or stirring,The time required
is determined fromforpreliminary
this, whichkinetic
can be
effectively reduced
measurements. The by shaking
amount or stirring,
adsorbed is determined
can be fromthe
calculated from preliminary
initial andkinetic meas-
equilibrium
urements. The
composition amount
and adsorbed
the amount canmaterials
of the be calculated
(solidfrom
massthe initial
and andvolume).
liquid equilibrium
Thecom-
rate
is also experiment-dependent
position and the amount of the(adsorbent, contaminant,
materials (solid mass andadsorption method).
liquid volume). The In general,
rate is also
increasing the rate will increase the biosorption removal rate of adsorbed impurities
by minimizing mass transfer resistance, but may damage the physical structure of the
biosorbent [147–152].
Molecules 2021, 26, 5419 13 of 31

In contrast to most laboratory experiments, the effluent of industrial water treatment is

not only a single component. Industrial wastewater contaminated with dyes can contain a
number of hazardous chemicals: acetic acid, ammonium sulphate, caustic soda, dispersing
agent, formic acid, hydrochloric acid, hydrogen peroxide, hydrosulphates, leveling agent,
organic resign, organic solvent, oxalic acid, polyethylene emulsion, PV acetate, soap,
softener, sulfuric acid, and wetting agent [5]. A wide range of contaminants occur in
wastewater, such as heavy metals, pesticides, pharmaceutical residues, dyes and colloidal
particles. These can all affect adsorption removal through competition for binding sites or
other interferences. Increasing concentrations of competing contaminants tend to reduce
biosorption removal of the target contaminant [153].
The effects of all these parameters should be taken into account when designing an
adsorption process. Optimization of such conditions will greatly aid the development of
industrial-scale dye removal technology. The most studied influencing factors (initial dye
concentration, aqueous solution pH, adsorbent volume and particle size, and temperature)
are illustrated with the results of research over the last five years. General trends are
formulated based on the results obtained, considering the effects of the factors.

4.1. The Effect of Initial Dye Concentration

The initial dye concentration is perhaps one of the most important factors influencing
the adsorption process, as it indirectly affects the efficiency of dye removal by reducing
or increasing the availability of binding sites on the adsorbent surface. In such water
treatment systems, the efficiency of dye removal (E) and the maximum amount of dye
bound in equilibrium (q) are directly related to the initial dye concentration [38,154].

Ci − Cf
E(%) = ·100 (1)
Ci

(Ci − Cf )·V
q= (2)
m
where: E (%)—efficiency; q (mg/g)—amount of dye bound in equilibrium; Ci
(mg/L)—initial dye concentration; Cf (mg/L)—final dye concentration; m (g)—amount of
adsorbent; and V (L)—volume of aqueous solution.
By examining the effect of initial dye concentration, three trends can be observed
(exemplified in Table 1):
• the removal efficiency decreases as the initial concentration increases;
• removal efficiency increases as the initial concentration increases; and
• no significant change in removal efficiency.

Table 1. Results of various research regarding the effect of initial dye concentration.

Concentration Reaction Quantity in Equilibrium

Dyestuff Adsorbent Efficiency Range (%) Reference
(mg/L) Time (min) Range (qe mg/g)
Algerian
Methylene Blue 3–30 5 up to 97% 2.5–10 [155]
palygorskite
increased but no
Methylene Blue clinoptilolite 50–100 60 significant difference - [156]
> 95%
activated carbon
Brilliant Green derived from 110–200 60 - 100–180 [157]
medlar nucleus
fluctuating values,
Methylene Blue green olive stone 50–1000 24 h - [158]
highest 65.9 at 50 ppm
fluctuating values,
Methylene Blue black olive stone 50–1000 24 h highest 93.5 at 400 - [158]
ppm
Haloxylon
Acid Brown 10–60 180 - 2.846–10.011 [159]
recurvum plant
Molecules 2021, 26, 5419 14 of 31

Table 1. Cont.

Concentration Reaction Quantity in Equilibrium

Dyestuff Adsorbent Efficiency Range (%) Reference
(mg/L) Time (min) Range (qe mg/g)
Congo Red cocoa bean shells 40–120 4–36 h negative linear effect [160]
fava bean peels,
utilizing
Methylene Blue ultrasonic- 3.6–100 70 70–90 - [161]
assisted (US)
shaking
fava bean peels,
Methylene Blue conventional (CV) 3.6–100 70 80–95 - [161]
shaking
Reactive Blue 19 corn silk 10–500 60 - 2.0–71.6 [162]
Reactive Red 218 corn silk 10–500 60 - 2.0–63.3 [162]
Reactive Black 5 pent tea leaves 50–100 5–200 98.7–43.5 24.8 –6.7 [163]
Methyl Orange pent tea leaves 50–100 5–200 88.7–32.7 22.2 –1.6 [163]
Methylene Blue Citrus limetta peel 5–25 10–60 ~100–97 0.06–1.62 [164]
Malachite Green Citrus limetta peel 5–25 10–60 ~97–95 0.17–4.70 [164]
Congo Red Citrus limetta peel 5–25 10–60 ~90–75 0.17–3.77 [164]
mango stone
Crystal Violet 20–50 60 - ~25–352.79 [165]
biocomposite
Congo Red chitosan 50–2000 30 - increased to 0.2 [166]
Methylene Blue chitosan 25–100 30 ~100–50 increased to 1457.1 [166]
Rhodamine B chitosan 25–100 30 ~55–35 increased to 990 [166]
Moringa oleifera
Reactive Red 120 10–100 30 - 18.54–173.99 [167]
seed
olive leaves
Crystal Violet 10–100 5–70 - ~5–45 [168]
powder

Most often, the percentage of dye removal decreases with increasing initial paint
concentration. This phenomenon can be explained by the saturation of adsorption sites
on the adsorbent surface. In this case, as the initial concentration increases, so does the
capacity of the adsorbent, which is due to the high mass transfer driving force at high
initial dye concentrations. The initial concentration of solute acts as a driving force for the
adsorption process, favoring diffusion and mass transfer processes from the solution (with
a higher amount of dye) to the free surface of the adsorbent [158,169].
If the concentration of the solution increases, and with it, the amount of bound material
shows a similar trend, then at low initial solution concentration the surface area of the
adsorbent and thus the number of adsorption binding sites is high, so the contaminant ions
or molecules (in our case dye molecules) can easily bind to the adsorbent surface. At higher
initial solution concentrations, the total available adsorption sites are limited, which may
result in a reduction in the percentage removal of contaminants. The increase at higher
initial concentrations may be attributed to increased driving forces [170,171].
At low concentrations, the ratio of active sites to dye molecules can be high, allow-
ing all molecules to interact with the adsorbent and be removed from solution almost
instantaneously [172].
Arellano G. Rodríguez et al. [160] reported that a negative linear effect between
removal efficiency, amount of bound material and initial concentration occurred when
removing Congo red with cocoa bean shells [160]. Accordingly, as the initial dye concen-
tration increased, the adsorption capacity of the biosorbent decreased. Referring to other
similar studies with Congo red, it was explained that the equilibrium adsorption capacity
increases with increasing initial dye concentration, a process controlled by the mechanism
of resistance to removal of Congo red [160].
Molecules 2021, 26, 5419 15 of 31

Even though it is a driving force, a clear, generalizable influence of the initial concentra-
tion as a parameter is not possible since several experimental conditions act in combination
on the specific contaminant and the adsorbent under study.

4.2. The Effect of Solution pH

According to several papers, the key parameter in almost all adsorption processes
is the pH of the dye solution. This factor affects the capacity of the adsorbent and the
efficiency of the process.
The pH affects the solution chemistry of contaminants, the activity of functional groups
in the adsorbent, the competition with coexisting ions in the solution, and the surface charge
of the adsorbent. The pH of the aqueous medium can also influence the properties of the
adsorbent, the adsorption mechanism, and the dissociation of dye molecules. Not only
the adsorbent but also the chemical structure of the dye can be altered by the pH of the
solution. The pH changes the surface charge and the degree of ionization of the adsorbed
ion [133,173–177].
Practical applications (Table 2) demonstrate that anionic dyes bind more effectively to
the adsorbent surface in acidic media, whereas cationic dyes bind more effectively in basic
media. Usually, the pH of the aqueous dye solution is adjusted with HCl and NaOH.
• When HCl is added to the solution, the surface of the adsorbent in the solution is
protonated, allowing the anionic dye to bind more efficiently on its surface, due to the
electrostatic attraction.
• Conversely, in basic medium, the addition of NaOH deprotonates the biomass surface,
resulting in a repulsive force between the anionic dye and the biomass. Thus, the
reverse phenomenon is observed for cationic dyes.

Table 2. Results of various research regarding the effect of initial solution pH, where E is the efficiency of the adsorption
process and Emax is the highest efficiency calculated in the specific article at a given condition.

Dyes Ionic Observations: with the

Dyestuff Adsorbent pH Reference
Nature Increase (↑) of pH
Direct Red 5B anionic 2 to 10 E% ↓; Emax_pH=2 = 95% [178]
Direct Black 22 anionic 2 to 10 E% ↓; Emax_pH=2 = 98% [178]
spent mushroom waste
Direct Black 71 anionic 2 to 10 E% ↓; Emax_pH=2 = 95% [178]
Reactive Black 5 anionic 2 to 10 E% ↓; Emax_pH=2 = 96% [178]
powdered activated carbon:
4 to 11 E% ↓ [179]
Congo Red rubber seed anionic
powdered activated carbon:
4 to 11 E% ↓ [179]
rubber seed shells
powdered activated carbon:
4 to 11 E% ↑ [179]
Methylene Blue rubber seed cationic
powdered activated carbon:
4 to 11 E% ↑ [179]
rubber seed shells
powdered vegetables wastes 2 to 10 E% ↓; 50.65 to 4.01% [180]
Eriochrome Black T anionic
calcined vegetables wastes 2 to 10 E% ↓; 68.87 to 31.23% [180]
q (mg/g) ↓; 26.4 to 3.3
natural olive stone 2 to 12 [158]
Methyl Orange anionic mg/g
q (mg/g) ↓; 120 to 15
olive stone activated carbons 2 to 12 [158]
mg/g
q (mg/g) ↑; 18 to 120
natural olive stone 2 to 12 [158]
Methylene Blue cationic mg/g
olive stone activated carbons 2 to 12 q (mg/g) ↑ [158]
carbon from Phyllanthus
Reactive Orange 16 anionic 2 to 11 q (mg/g) ↓ [181]
reticulatus
Cationic Red X-5GN cationic 2 to 10 E% ↑ [182]
ceramic
Cationic Blue
cationic 2 to 10 E% ↑ [182]
X-GRRL
activated carbon/cellulose q (mg/g) ↑; 50.54 to 60.48
Methylene Blue cationic 3 to 11 [183]
biocomposite films mg/g
Molecules 2021, 26, 5419 16 of 31

Table 2. Cont.

Dyes Ionic Observations: with the

Dyestuff Adsorbent pH Reference
Nature Increase (↑) of pH
Eriochrome Black T anionic 2 to 11 q (mg/g) ↓ [184]
Malachite Green almond shell cationic 2 to 11 q (mg/g) ↑ [184]
bast fibers: ramie cationic 2 to 12 E% ↑; Emax_pH=12 = 91% [185]
Basic Yellow 37 bast fibers: flax cationic 2 to 12 E% ↑; Emax_pH=12 = 88% [185]
bast fibers: kenaf cationic 2 to 12 E% ↑; Emax_pH=12 = 78% [185]
Remazol Brilliant
Trichoderma viride anionic 4 to 9 E% ↓; 79.05 to 50.25% [186]
Violet
Congo Red anionic 2 to 10 E% ↓; 98.71 to 93.17% [133]
Bromphenol Blue anionic 2 to 10 E% ↓; 67.61 to 1.2% [133]
eggshell powder
Methylene Blue cationic 2 to 10 E% ↑; 14.8 to 75.1% [133]
Malachite Green cationic 2 to 10 E% ↑; 89.95 to 97.92% [133]

4.3. The Effect of Adsorbent Dosage

The amount of adsorbent is an important parameter that influences the adsorption
process, through the quantitative ratio of adsorbent to adsorbent. Since the adsorbent deter-
mines the adsorbent capacity for a given initial concentration, the dosage of the adsorbent
is an important parameter [187]. According to Kroeker’s rule, the specific adsorbed volume,
for a constant initial concentration, decreases with increasing adsorbent mass [188]. Thus,
increasing the adsorbent dose is positively correlated with the efficiency and performance
of dye removal. With increasing adsorbent dosage, at fixed contaminant concentrations,
more active surface area is available for adsorption and more active adsorption sites are
available [189].
As the concentration of biomass (the amount of adsorbent) increases, the efficiency
of pollutant removal (E%) increases, but there is no direct proportionality between the
amount of biomass and the amount of pollutant removed (qe ).
In contrast, as the concentration of biosorbent increases, the amount adsorbed per
species (qe ) decreases. This can be attributed to the fact that the shape of the sorption
isotherm changes with increasing biosorbent concentration. The decrease in the specific
adsorbed amount is probably due to the fact that some of the surface or surface groups
may not be saturated in the more concentrated suspensions [190–193].
During the dye removal process, the capacity may decrease for two reasons [194]:
• adsorption sites remain unsaturated while the number of sites available for adsorption
increases; or
• aggregation or agglomeration of adsorbent particles may occur, reducing the available
surface area and increasing the diffusion path length.
Scientific studies in recent years have investigated the removal of different dyes with
different amounts of broad-spectrum adsorbent. Some examples of these are listed in
Table 3 to support the detected relationships between mass and adsorption. It is observed
that at fixed pollutant concentrations, as the mass of the adsorbent increases, the efficiency
increases, and the maximum amount of material bound decreases.

Table 3. Results of various research regarding the effect of initial adsorbent dosage.

Efficiency Range Quantity in Equilibrium

Adsorbent Dyestuff Adsorbent Dosage Reference
(%) Range (qe mg/g)
walnut shell Methylene Blue 0.5–2 g/L - 178.93–47.51 [195]
magnetic alginate/rice husk
Methylene Blue 0.1–1 g 15–89 338–145 [196]
bio-composite
Tunisian smectite clay Cristal Violet 0.05–0.3 g/L 10–100 - [197]
modified activated carbon
Malachite Green 10–50 mg 31.3–86.6 11.67–6.5 [198]
(PABA@AC)
commercial natural activated
Methylene Blue 0.5–1.5 g 46–78 - [190]
plant-based carbon (CNAC)
commercial natural activated
Eosin Yellow 0.5–1.5 g 51–70 - [190]
plant-based carbon (CNAC)
commercial natural activated
Rhodamine B 0.5–1.5 g 52–60 - [190]
plant-based carbon (CNAC)
Molecules 2021, 26, 5419 17 of 31

Table 3. Cont.

Efficiency Range Quantity in Equilibrium

Adsorbent Dyestuff Adsorbent Dosage Reference
(%) Range (qe mg/g)
Remazol Brilliant
calcined eggshell 0.5–2 g 89.83–96.3 3.59–0.96 [134]
Violet-5R
calcined eggshell Remazol Red F3B 0.5–2 g 92–93.67 3.68–0.94 [135]
calcined eggshell Remazol Blue RR 0.5–2 g 92–93.33 3.68–0.94 [135]
Remazol Brilliant
eggshell 0.5–2.5 g 74.67–93.85 2.96–0.75 [51]
Violet-5R
activated carbon from lotus leaves Methylene Blue 0.5–10 g/L 82.84–98.032 16.57–0.98 [192]
municipial solid waste compost ash Reactive Red 198 0.5–2 g/L 79.25–92.92 - [193]
natural clayey composite Basic Navy Blue 2RN 0.2–1.2 g/50 mL 78–97 - [199]
natural clayey composite Drimaren Yellow CL-2R 0.2–1.2 g/50 mL 87–97 - [199]
geopolymer Methylene Blue (10−5 M) 0.05–0.1 g 79.8–85.6 - [200]
mucilage of Salvia seeds Cationic Blue 41 0.5–4 g/L 34.2–53.9 34.2–6.74 [201]
raw petroleum coke Congo Red 4–24 g/L ~10–60 - [202]
activated petroleum coke Congo Red 4–24 g/L ~15–70 - [202]

Several studies also report that this increase in efficiency lasts until a saturation state
is reached and then steadily decreases, sometimes slightly. This can be explained by the
fact that after a certain adsorbent dose, maximum adsorption is reached and the amount of
ions bound to the adsorbent and the amount of free ions remains constant, even with the
further addition of adsorbent [51,134,135,187,192,197].

4.4. The Effect of Adsorbent Particle Size

Although not regularly investigated in biosorption studies, particle size can be an
important factor in heterogeneous chemical reactions and adsorption [203]. The small
particle sizes result in a higher specific surface area. Specific surface area (SSA), defined
as the total surface area of a solid material per unit of mass, is an important feature for
sorption processes. SSA is dependent on the size of the particles, as well as on the structure
and porosity of the material [204]. The most common unit of measurement is m2 /g.
The relationship of adsorption capacity to particle size depends on two criteria [205,206]:
• the chemical structure of the dye molecule (its ionic charge) and its chemistry (its
ability to form hydrolyzed species); and
• the intrinsic characteristic of the adsorbent (its crystallinity, porosity and rigidity of
the polymeric chains).
In adsorption by static batch methods, smaller particle sizes result in higher adsorption
capacity and efficiency, since there are more active sites for binding [207]. Table 4 represents
results of studies where the effect of particle size was investigated, and a similar trend was
observed. With the decrease of the particle size, the BET surface of the material increases.

Table 4. Results of various research regarding the effect of adsorbent particle size.

Quantity in Equilibrium
Dyestuff Adsorbent Particle Size (µm) Efficiency Range (%) Reference
Range (qe mg/g)
Congo Red cabbage waste powder 150–300 to 360–4750 75.95–8.03 - [208]
Reactive Black 5 macadamia seed husks 150–300 to 2360–4750 98.9–33.2 - [209]
Maxilon Blue GRL coconut shell activated carbon 50, 75, and 106 - ~27.5–22.5–17.5 [205]
Direct Yellow DY 12 coconut shell activated carbon 50, 75, and 107 - ~5.5–4.5–3.5 [205]
80–150, 150–200, and
Methylene Blue Cucumis sativus peel waste 80.25–84.15–85.23 - [210]
>200 BSS mesh
0.15–0.3 to 2.36–4.75
Crystal Violet coffee husks 96.082–89.854 - [211]
mm
Methylene Blue clay3 177–250 to 400–840 99–86.4 - [212]

Figure 6, from the study of Shahul K. Hameed et al. [213], represents the effect of
particle size on adsorption efficiency, where chromotrope dye was adsorbed on the surface
of activated carbons obtained from the seeds of various plants.
Direct Yellow DY 12 coconut shell activated carbon 50, 75, and 107 - ~5.5–4.5–3.5 [205]
Methylene Blue Cucumis sativus peel waste 80–150, 150–200, and >200 BSS mesh 80.25–84.15–85.23 - [210]
Crystal Violet coffee husks 0.15–0.3 to 2.36–4.75 mm 96.082–89.854 - [211]
Methylene Blue clay3 177–250 to 400–840 99–86.4 - [212]

Molecules 2021, 26, 5419 Figure 6, from the study of Shahul K. Hameed et al. [213], represents the effect of
18 of 31
particle size on adsorption efficiency, where chromotrope dye was adsorbed on the sur-
face of activated carbons obtained from the seeds of various plants.

Figure 6. Representation
Representation of
of particle
particle size
sizetrends,
trends,where
wherethe
theused
usedadsorbents
adsorbentsare
areASC—aamla
ASC—aamlaseed
seed
carbon, JSC—jambul seed carbon, TSC—tamarind seed carbon, and SNC—soapnut
carbon, JSC—jambul seed carbon, TSC—tamarind seed carbon, and SNC—soapnut carbon carbon[213].
[213].

too small,
If the particle size is too small, the
the adsorption
adsorption capacity
capacity may
may bebe lower,
lower, depending
depending onon
the type
the typeofofadsorbent,
adsorbent,asasthethe lighter
lighter particles
particles float
float andand
thusthus cannot
cannot contact
contact the solution.
the solution. The
The separation
separation of these
of these small small particles
particles from water
from water after biosorption
after biosorption can be challenging
can be challenging [203].
[203].
4.5. The Effect of Solution Temperature
4.5. The
TheEffect
effectofofSolution Temperature
temperature is also a significant physico-chemical factor as it affects the
treatment process by shifting the
The effect of temperature is also nature of the reaction
a significant from endothermic
physico-chemical to it
factor as exothermic,
affects the
or vice versa [9]. Moreover, it has a strong effect on the adsorption as it
treatment process by shifting the nature of the reaction from endothermic to exothermic, can increase or
decrease the amount
or vice versa of adsorption
[9]. Moreover, it has [214].
a strong effect on the adsorption as it can increase or
decrease the amount of adsorption efficiency
The temperature can affect the [214]. of the sorption differently depending on the
adsorbent
The temperature can affect the efficiency of thebiosorption
and the pollutant. In general, it enhances of adsorption
sorption differently impurities
depending by
on the
increasing the surface activity and kinetic energy of the adsorbate, but it can
adsorbent and the pollutant. In general, it enhances biosorption of adsorption impurities also damage
the physical structure
by increasing the surfaceof the biosorbent.
activity and kinetic energy of the adsorbate, but it can also dam-
•age the
As physical structure
the temperature of the
rises, thebiosorbent.
rate of chemical reaction also increases, so if the sorption
• process is chemisorption
As the temperature (∆Hrate
rises, the = −reaction
of chemical
chemisorption 200 kJ/mol), then higher
also increases, so ifsorption effi-
the sorption
ciency will be seen at higher temperatures (this would eventually reach equilibrium).
process is chemisorption (∆Hchemisorption = −200 kJ/mol), then higher sorption efficiency
• On the
will be other
seen athand, if the
higher process is a physical
temperatures adsorption
(this would (∆Hphysisorption
eventually ≈ −20 kJ/mol),
reach equilibrium).
• then the higher temperature will negatively affect the adsorption. Temperature
On the other hand, if the process is a physical adsorption (∆Hphysisorption ≈ −20 kJ/mol), can
chemically alter the adsorbent, its adsorption sites and activity [110].
then the higher temperature will negatively affect the adsorption. Temperature can
chemically
We alter the adsorbent,
can differentiate two types of itsprocesses:
adsorption sites and activity
endothermic [110].
and exothermic (Table 5).
Exotherm: with the increase
We can differentiate two typesof of
temperature, the adsorption
processes: endothermic andprocess (efficiency)
exothermic de-
(Table 5).
creases. It can be explained with the fact that the adsorptive powers among
Exotherm: with the increase of temperature, the adsorption process (efficiency) de- adsorbate
and the active
creases. sites
It can be of the adsorbent
explained become
with the fact weak
that the with the powers
adsorptive increaseamong
in temperature,
adsorbateandand
dye removal efficiency decreased [215]. Exothermic adsorption is usually used to control
the diffusion process, as the mobility of the dye ions increases when heat is added to the
system [216].
Endotherm: with the increase of temperature, the adsorption process (efficiency) increases,
due to more availability of active sites as a result of the activation of the adsorbent surface
at higher temperatures [217]. Increasing the values of adsorption capacity by increasing the
temperature may be attributed to an increase in the mobility of the large dye ions [218].
All in all, better adsorption at higher temperatures may indicate the endothermic
nature of the process, while being exothermic at lower temperatures.
Molecules 2021, 26, 5419 19 of 31

Table 5. Results of various research regarding the effect of temperature.

Quantity in
Efficiency Range
Dyestuff Adsorbent Temperature (K) Type of the Process Equilibrium Range Reference
(%)
(qe mg/g)
Basic Orange 2 alkaline-modified nanoclay 288–308 80–100 endothermic - [219]
Congo Red cross-linked TTU-chitosan 298, 308 and 328 - endothermic increased [218]
Congo Red modified Zeolite A 297–309 - exothermic decreased [216]
ZnO Beyond 313 K, the adsorption capacity was decreased, which is an highest: 40.94 [220]
Direct Sky Blue MgO indication of being endothermic up to 313 K, and exothermic beyond highest: 46.25 [220]
FeO this temperature highest: 42.86 [220]
cationic polymer 293, 303, 328 and
Methyl Orange - endothermic increased [221]
(Amberlite IRA 402) 348
Remazol Red chitosan Schiff base 293, 303, and 313 - endothermic increased [222]
The adsorption of RR-120 on activated carbon is of the physisorption type, as confirmed by
Reactive Red 120 activated carbon [223]
the adsorbed energy values, and it is exothermic as verified by the internal energy
hydroxyapatite/gold 290–305 - endothermic increased [217]
Methylene Blue
nanocomposite 305–330 - exothermic decreased [217]
multiwalled carbon 298, 308, 318 and
63.33–9.07 exothermic - [215]
Reactive Red 35 nanotubes 328
poly (acrylonitrile-styrene)
298, 308, 318 and
impregnated with 67.55–97.61 endothermic - [215]
328
activated carbon
Citrullus colocynthis seed 293–333 93.58–98.00 endothermic - [143]
Methylene Blue
Citrullus colocynthis peel 294–333 91.43–82.52 exothermic - [143]
magnetic carboxyl
Methylene Blue functional nanoporous 298, 308 and 318 - endothermic 52.16–52.58–53.75 [224]
polymer

After studying the effect of initial temperature on adsorption, thermodynamic param-

eters are calculated. It is well known that the adsorption processes are strongly dependent
on the working temperature, which is controlled by thermodynamic parameters including
the standard enthalpy change (∆H0 , J/mol), the standard entropy change (∆S0 , J/mol) and
the standard free Gibbs energy change (∆G0 , J/mol) of the adsorption processes. These
parameters are computed from the Gibbs–Helmholtz equation: ∆G = ∆H − T∆S [225].
Gibbs free energy, enthalpy and entropy are state functions, so ∆G, ∆H and ∆S depend on
the final state and the initial state of the adsorption system. Gibbs free energy, enthalpy
and entropy have extensive property, so attention must be paid to the amount of substance
that these thermodynamic parameters correspond to [226].
During the adsorption of dye molecules, with the increase of temperature, the value
of entropy (∆S) and enthalpy (∆H) can be increased or decreased.
Molecules before adsorption can move in three dimensions, but as they get adsorbed
on the surface, the motion of them is restricted towards the surface, and their disorder
decreases, resulting in the decline in entropy indicating an exothermic process. This may
also be explained on the basis that the solubility of dyes increased at higher temperatures
while adsorbate–adsorbent interactions decreased, resulting in decreased adsorption [227].
The increase in entropy and enthalpy indicates an endothermic process [225,228–231].

4.6. Activation of Solid Sorbent, Surface Modification

In order to increase the adsorption capacity and efficiency, different types of physical
and chemical surface modification methods can be used. The most common physical
modification methods are freezing, crushing, boiling/heating and drying. These types
of surface modification techniques usually destroy the cell membrane of the biomass,
releasing cellular content that may be responsible for contaminant uptake.
Physical modification methods are generally cheap and simple, but not as effective as
chemical methods. Among the chemical modification methods, polymerization, modifica-
tion of the binding site, and washing (or pretreatment) are being experimented with. Of the
chemical methods, washing is preferred for its simplicity and efficiency. The most common
chemical pretreatments include washing of biomass with acid, alkali and detergent, or
crosslinking with organic solvents. Some types of adsorbents produce stable biosorbent
particles after some simple processes such as cutting or grinding. In other cases, the ad-
Molecules 2021, 26, 5419 20 of 31

sorbent must be fixed in a synthetic polymer matrix and/or grafted onto an inorganic
carrier material such as silica in order to obtain particles with the required mechanical
properties. Different ways of manipulating biomass adsorbents to improve various aspects
of biosorption have been described by several authors [232–237].

5. Desorption Studies
Desorption studies help to explain adsorbate and adsorbent recovery, and the ad-
sorption mechanism. Since the regeneration of the adsorbent makes the treatment process
economical, desorption studies were performed to regenerate the spent adsorbent [187]. As
batch adsorption is not a destructive technique and the adsorbents used undergo a phase
transformation, large amounts of often hazardous by-products and waste are generated.
These solids can be regenerated due to their properties, leaving room for the recovery
of the adsorbent and often the contaminant [94]. The process of adsorbent regeneration
is a complex task, as the desorption depends on the adsorbent, the adsorbate (different
types of dyes ionic nature), and the adsorption process. In adsorption–desorption studies,
it is essential to examine the reusability of the adsorbent. Between dye removals, the
adsorbent should be cleaned and regenerated to ensure that it can continue to be used and
the water treatment can be reproduced. The adsorbent lifetime expresses the number of
adsorption–desorption cycles, after which the adsorbent can be used effectively to remove
dye substances. Therefore, the task of scientists who study the desorption process is to
provide information about the reproduction cycles. There are different desorption methods,
and a high range of eluents are employed to regenerate the used adsorbents, out of which
a few examples will be listed below.
The reuse of adsorbent could be considered as one of the most important economic
parameters. Siroos S. et al. studied the recyclability of NaX nanozeolites after malachite
green (MG) and auramine-O (AO) dye adsorption. The NaX nanozeolites used were
washed with a small amount of methanol and then dried for reuse in a vacuum-oven. The
results showed that after up to five cycles, the adsorption efficiency decreases slightly. In
general, this reduction can be due to adsorption degradation during adsorption–desorption
cycles [238].
Feng J. et al. examined the desorption of cationic malachite green (MG) dye on
cellulose nanofibril aerogels. For this purpose, the used aerogels in the first round were put
in deionized water, after the treatment 16% of MG was regenerated. Another desorption
method consisted of putting the material in 50 mL of 0, 50 and 200 mM sodium chloride
solutions. As a result, after 1 h, 65 (50 mM) and 85% (200 mM) recovery was observed [239].
Haq N.Bhatti et al. made a detailed research about the adsorption–desorption behavior
of Direct Orange-26 (DO-26), Direct Red-31 (DR-31), Direct Blue-67 (DB-67) and Ever direct
Orange-3GL (EDO-3) dyes onto native, modified rice husk. The dyes desorption was
investigated using distilled H2 O (pH 8, 10, 12), NaOH and Na2 CO3 (0.1 M) after drying
of the biosorbent at 60 ◦ C. It was observed that the EDO-3, DR-31, DO-26 and DB-67 dye
can be desorbed from rice husk biomass under basic conditions and 75.32, 80.59, 62.88 and
53.97 (mg/g) respectively. The adsorption capacity of rice husk biomass has lost 17% at the
end of ten sorption/desorption cycles [240].
The adsorption–desorption of Acid Violet 17 was examined by İlknur Şentürk and
Mazen Alzein regenerating acid-activated pistachio shell [187]. As a protocol, 1 g of the
dye-loaded adsorbent obtained (0.1, 0.2, 0.4, 0.8 M) was mixed separately with 100 mL of
HCl, NaCl, CH3 COOH, NaOH desorption agents prepared at different concentrations (0.1,
0.2, 0.4, 0.8 M) and solvents (ethanol and distilled water) in the orbital mixer operating at
125 rpm for 24 and 48 h. The desorption efficiency was very low in desorption processes
performed separately with water and ethanol. The AV 17 dye adsorption efficiency after
three cycles of desorption decreased from 94.76 to 75.84% [187].
Mohammad A. Al-Ghouti and Rana S. Al-Absi made desorption studies where spent
black and green olive stones loaded with 600 mg/L methylene blue were added to 50 mL
of acidic mixtures of acetic acid and ethanol (%vol) (10:1, 5:1, and 1:1). The mixture was
 HCl, NaCl, CH3COOH, NaOH desorption agents prepared at different concentrations
(0.1, 0.2, 0.4, 0.8 M) and solvents (ethanol and distilled water) in the orbital mixer operat-
ing at 125 rpm for 24 and 48 h. The desorption efficiency was very low in desorption pro-
cesses performed separately with water and ethanol. The AV 17 dye adsorption efficiency
Molecules 2021, 26, 5419 after three cycles of desorption decreased from 94.76 to 75.84% [187]. 21 of 31
Mohammad A. Al-Ghouti and Rana S. Al-Absi made desorption studies where spent
black and green olive stones loaded with 600 mg/L methylene blue were added to 50 mL
of acidic mixtures of acetic acid and ethanol (%vol) (10:1, 5:1, and 1:1). The mixture was
thenshaken
then 25◦ Cand
shakenatat25°C and 150
150 rpm
rpm forfor
24 24 h. The
h. The totaltotal desorption
desorption removal
removal capacities
capacities of theofMB-
the
MB-loaded black and green olive stones were found to be 92.5 and 88.1%,
loaded black and green olive stones were found to be 92.5 and 88.1%, respectively [158]. respectively [158].
AAchemical
chemicalregeneration
regenerationexperiment
experimentwas wasconducted
conductedby byMomina
Momina et et al.
al. on
on the
the surface
surface of
of
bentonite after methylene blue dye adsorption. The used solvents were:
bentonite after methylene blue dye adsorption. The used solvents were: hydrochloric acid hydrochloric acid
(HCl),nitric
(HCl), nitricacid
acid(HNO
(HNO3),3 ),ethanol
ethanol(C (C
2H2H 5 OH),
5OH), propanol
propanol (C(C H7 OH),
3H37OH), acetone
acetone ((CH((CH 3 )2 CO),
3)2CO), so-
sodium
dium chloride
chloride (NaCl),
(NaCl), sodium
sodium hydroxide
hydroxide (NaOH)(NaOH) and distilled
and distilled water water
(H2O) (H 2 O)Significant
[241]. [241]. Sig-
nificant desorption
desorption of MB (70%) of MB
was(70%) wasusing
achieved achieved using
aqueous aqueous
HCl HCl solution.
solution.
Direct Blue 78 adsorption–desorption on eggshell
Direct Blue 78 adsorption–desorption on eggshell surface was surface was analyzed
analyzed usingusing NaOH
NaOH
solvent by Ainoa Murcia-Salvador et al., where results showed that the
solvent by Ainoa Murcia-Salvador et al., where results showed that the adsorption abili- adsorption abilities
of the
ties eggshell
of the decreased
eggshell withwith
decreased the the
increasing
increasingnumber
numberof cycles [242].
of cycles [242].
Figure 7 contains possible eluents used to desorb contaminantsfrom
Figure 7 contains possible eluents used to desorb contaminants fromadsorbent
adsorbentmate-ma-
rials; therefore, to regenerate them.
terials; therefore, to regenerate them.

Figure
Figure7.7.Possible
Possibleeluents
eluentsused
usedto
todesorb
desorbcontaminants
contaminantsfrom
fromadsorbent
adsorbent materials.
materials.

In the Journal of Saudi Chemical Society, Himanshu Patel wrote a review article
about the comparison, advantages, and disadvantages of different adsorbent regeneration
processes. Moreover, it lists a high range of eluents used by other researchers [243]. As
he writes in the abstract of the article, hazardous solid waste is one of the most serious
problem faced all over the World, which comprises spent solid adsorbents.

6. Conclusions
In the first chapter of the study, we discussed that since ancient times, people have
used dyes to paint their everyday objects. As a result of population growth and a large
increase in industrial production, increasing quantities of dyes were needed. With the
development of science and the chemical industry, researchers have found a solution to this
problem; they have developed various synthetic dyes, the large quantities of which required
classification and catalogization, but have also created another issue that is harmful to
the environment and health. We must therefore tackle the challenge of treating industrial
Molecules 2021, 26, 5419 22 of 31

wastewater (mainly dyes and textiles) and develop appropriate and sustainable water
treatment technologies.
Several possible methods for water treatment have recently become available, but
adsorption is perhaps the most common commercial treatment. The remediation process is
influenced by several external parameters, the optimization of which is essential to ensure
that the system can be applied with low costs, few by-products and high efficiency on a
daily basis, even at low pollutant concentrations.
Looking at the effect of the initial dye concentration, it is observed that a wide range
of adsorbents can be used, with efficiencies of more than 90% even at high concentration
values. In most cases, the increase of the dye concentration negatively influenced the
removal efficiency. The investigated studies covered a concentration range from 3 to
1000 mg/L. In the studies, the removal time ranged from 5 min to 36 h. However, 100%
efficiency was achieved in intervals of up to 5–60 min.
The removal of 16 anionic and cationic dyes was demonstrated. Among the anionic
dyes, direct dyes are the most frequently tested, while Methylene Blue is the model dye for
cationic dyes. Most of the studies have investigated the removal of dyes between pH 2 and
10. Having examined the chemistry of the solution, it can be concluded that anionic and
cationic dyes behave differently in acidic and basic media. When designing the adsorption
process, it is important to keep in mind the ionic nature of the dye, thus reducing the time
required for the optimization study.
Through numerous examples of adsorbents, it has been observed that small amounts
(as small as 0.05 g) have been found to remove dye with efficiencies greater than 85%. The
conclusion of 14 scientific papers (shown in Table 3) is that as the amount of adsorbent
increases, the removal efficiency of dyes increases and the maximum amount of bound
substances decreases. Bearing in mind that the efficiency varied from 8 to up to 99% in
the articles studied by reducing the particle size, it can be said that particle size is a highly
influential factor. Therefore, in future research, if possible and feasible, it is important to
increase surface area and porosity by reducing particle size. The effect of aqueous solution
temperature (Table 5) was investigated between 288 and 348 K. Both endothermic and
exothermic adsorption processes were observed. From a green chemistry point of view, the
exothermic process is preferable, since no excess energy input is required by heating the
system for optimal adsorption. It is observed that the dye does not affect the endothermic
or exothermic nature of the process. Methylene Blue and Congo Red, with different
adsorbents, showed both endothermic and exothermic characteristics. Temperature, in
addition to adsorption efficiency, affects the nature and mechanism of adsorption.
Using the eluents shown in the last figure, it can be seen through examples that many
adsorbents can be recycled over several cycles.

Author Contributions: Both authors are writers of the article. All authors have read and agreed to
the published version of the manuscript.
Funding: This research received no external funding.
Acknowledgments: E. Rápó is thankful for the research fellowship/scholarship from the Sapientia
Hungariae Foundations’ Collegium Talentum scholarship program and for the Forerunner Federation
Székely előfutár scholarship program. We would like to thank Viktor Szentpéteri for his work in the
linguistic proofreading of this article.
Conflicts of Interest: The authors declare no conflict of interest.

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