BIOSORPTION: TECHNOLOGY
FOR COLOUR REMOVAL
INTRODUCTION
In most developed and developing countries, stricter environmental regulations, with
regard to contaminants discharged from industrial operations, are being introduced. This
means that industries need to develop on-site or in-plant facilities to their own effluents and
minimize the contaminant concentrations to acceptable limits prior to their discharge. The
discharge of these dye stuffs and metals from industries into rivers and lakes results in a
reduced dissolved oxygen concentration causing anoxic conditions, which subsequently
affect aerobic organisms (1). Apart from the toxicological properties of dyes, their colour is
one of the first signs of contamination recognized in a wastewater Biosorption is a technique
that can be used for the removal of pollutants from water, especially those that are not easily
biodegradable such as metals and dyes. A variety of biomaterials are known to bind these
pollutants, including bacteria, fungi, algae, and industrial and agricultural wastes. There are
three principle advantages of biological technologies for the removal of pollutants; first,
biological processes can be carried out in situ at the contaminated site; Second, bioprocess
technologies are usually environmentally benign (no secondary pollution) and third, they are
cost effective. Of the different biological methods, bioaccumulation and biosorption have
been demonstrated to possess good potential to replace conventional methods for the removal
of dyes. In this review, the biosorption abilities of bacterial biomass towards dyes are
emphasized (9).
Water resources are of critical importance to both natural ecosystem and human
developments. Increasing environmental pollution from industrial wastewater particularly in
developing countries is of major concern. Many industries like dye industries, textile, paper
and plastics use dyes in order to colour their products and also consume substantial volumes
of water. As a result they generate a considerable amount of coloured wastewater. The
presence of small amount of dyes (less than 1 ppm) is highly visible and undesirable. Many
of these dyes are also toxic and even carcinogenic and pose a serious threat to living
organisms. Hence, there is a need to treat the wastewaters containing toxic dyes before they
are discharged into the water bodies (18). Many researches have been investigated in order
to remove dyes molecules before disposal of wastewater such as chemical coagulation and
ozonation. Recent research works had proved that adsorption method is becoming the most
promisingalternative in this domain. The most common adsorbent is the activated carbon
but its use remains quite expensive. This method do not show significant effectiveness and
economic advantage and higher the quality greater the cost. Many low cost biosorbents were
tested for the removal of dyes from aqueous solutions such as wheat bran (Çiçek et al., 2007),
Posidonia oceanica; agricultural waste, pine sawdust orange peel and rice husk(1)
Many physico-chemical methods like coagulation, flocculation, ion exchange, etc are
available for the treatment of heavy metals and dyes. Major drawbacks of these methods are
high sludge production, handling and disposal problems, high cost, technical constraints, etc.
The increasing awareness and concern about the environment motivated research for new
efficient technologies that would be capable of treating inexpensively, waste waters polluted
by dyes. This search brought biosorption to the foreground of scientific interest as a potential
basis for the design of novel wastewater treatment processes. Several adsorbents are currently
used which are by-products from agriculture and industries, which include seaweeds, molds,
yeast, bacteria, crabshells, agricultural products such as wool, rice, straw, coconut husks, peat
moss, exhausted coffee waste tea leaves, walnut skin, coconut fibres, etc.Thus
adsorption/biosorption using low cost adsorbents could be technically feasible and
economically viable sustainable technology for the treatment of wastewater streams (10).
Structures of some majourly used dyes are as follows-
(13)
(18)
(15)
(17)
(14)
(16)
KEYWORDS : Wastewater treatment, conventional methods, decolourization, biosorption,
biosorbent, dye.
TOXICOLOGICAL ASPECTS OF DYES
Dyeing industry effluents are one of the most problematic wastewater to be treated not
only for their high chemical oxygen demand, but also for high biological oxygen demand,
suspended solids, turbidity, toxic constituents but also for colour, which is the first
contaminant discernible by the human eye. Dyes may affect the photosynthetic activity in
aquatic life due to reduced light penetration and may also be toxic to some aquatic life due to
the presence of aromatics, metals, etc. in them. Dyes usually have a synthetic origin and
complex aromatic molecular structure, which make them more stable and more difficult to
biodegrade (19). Water soluble reactive and acid dyes are problematic; as they pass through
the conventional treatment system unaffected, posing problems. Dyes have generated much
concern regarding its use, due to its toxic effects. It has been reported to cause
carcinogenesis, mutagenesis, chromosomal fractures, teratogenecity and respiratory toxicity
(12).
NEED FOR THE REMOVAL OF DYES
India produces 64,000 tonnes of dyes, 2 per cent of which - 7,040 tonnes - are directly
discharged into the environment. There are around 700 varieties of dyes and dye
intermediaries produced in India. In India only a third of the dyestuff producing industries are
in organised sector (17).
Dyes and colour pigments also contain metals such as copper, nickel, chromium,
mercury and cobalt. Metals are difficult to remove from wastewater and may escape the
capacities of the effluent treatment system. Moreover, the unused dyes and colour released in
effluent from dyeing vats, interferes with the transmission of light in the water bodies that
receives the effluent. This in turn inhibits the photosynthesis activity of aquatic biota besides
direct toxic effects on biota. Several textile and food dyes have been linked to
carcinogenicity, such as dye intermediaries like Benz dines. Hence the ubiquitous colour
needs to be regulated. But removing the colour from effluents is extremely difficult. There is
no universally applicable technique for all conditions (18).
CONVENTIONAL METHODS FOR THE TREATMENT OF DYES
Synthetic dyes often receive considerable attention from researchers interested in textile
wastewater effluents treatment processes. As discharge standards are becoming more
stringent, the development of technological systems for minimizing concentration of dyes and
their break down products in wastewater are nowadays necessary. The following are
generally used for the removal of colour from wastewaters (20).
1) Physicochemical methods for dye removal
2) Photo catalytic decolourisation and oxidation of synthetic dyes
3) Membrane processing
4) Microbiological decomposition of synthetic dyes
5) Enzymatic decomposition of synthetic dyes
6) Adsorption
DISADVANTAGES OF USING CONVENTIONAL METHODS FOR DYE REMOVAL
(21).
BIOSORPTION
The bio sorption process involves a solid phase (sorbet or biosorbent; adsorbent;
biological material) and a liquid phase (solvent, normally water) containing a dissolved
species to be sorbed (adsorb ate, dyes). Due to the higher affinity of the adsorbent for the
adsorb ate species, the latter is attracted and bound there by different mechanisms. The
process continues till equilibrium is established between the amount of solid-bound adsorb
ate species and its portion remaining in the solution. The degree of adsorbent affinity for the
adsorb ate determines its distribution between the solid and liquid phases. The pH of the
solution is an important controlling parameter in the sorption process (3). Salt concentration
also has an adverse effect on sorption process (6).
Bio sorption has advantages compared with conventional techniques. Some of these are
listed below:
• Cheap: the cost of the biosorbent is low since they often are made from abundant or waste
Material.
• Dye selective: the dye sorbing performance of different types of biomass can be more or
Less selective on different dyes. This depends on various factors such as type of biomass,
Mixture in the solution, type of biomass preparation and physico-chemical treatment.
• Regenerative: biosorbents can be reused, after the dye is recycled.
• No sludge generation: no secondary problems with sludge occur with bio sorption, as is the
Case with many other techniques, for example, precipitation.
• Competitive performance: bio sorption is capable of a performance comparable to the most
similar technique, ion exchange treatment. Ion exchange is, as mentioned above, rather
costly, making the low cost of bio sorption a major factor.
Bio sorption by seaweed has been demonstrated to be a practical alternative to
conventional systems for the removal of dyes (7).
BIOSORPTION OF DYES BY MICRORGANISMS
A wide variety of micro organisms including bacteria , fungi and yeasts are used for the
bio sorption of a broad range of dyes. Textile dyes vary greatly in their chemistries, and
therefore their interactions with micro organisms depend on the chemical structure of a
particular dye, the specific chemistry of the microbial biomass and characteristics of the dye
solution or wastewater (11). Depending on the dye and the species of micro organism used
different binding capacities have been observed. Magnetically modified yeast cells can thus
be a promising magnetic affinity adsorbent which may be used to the removal of dyes (5).
Decarboxylation is a good option for pre-treatment of the raw biomass to make effective
biosorbents able to remove reactive dyes from dye-containing wastewaters, even at low
concentrations (14).
There are certain inherent disadvantages of using micro organisms for the bio sorption of
heavy dyes and they are as follows: the protein rich algal and fungal biomass projected as dye
biosorbents have limitations as proteinious materials are likely to putrefy under moist
conditions. Further, most dye sorption reported in literature is based on algal and fungal
biomass, which must be cultured, collected from their natural habitats and pre-processed, if
available as discards and transported under special conditions, thus introducing the factor of
additional costs (4).
ALTERNATIVE LOW COST ABSORBENTS
The disadvantages of using micro organisms can be overcome by using low cost
adsorbents. In general, a sorbent can be assumed to be “low cost” if it requires little
processing and is abundant in nature, or is a by product or waste material from another
industry (18), which has lost its economic or further processing values The by-products from
the agriculture and industries could be assumed to be low-cost adsorbents since they are
abundant in nature, inexpensive, require little processing and are effective materials (16).
Palm kernel fibre, a very abundant agricultural by-production Nigeria has been shown to be a
good sorbent for the anionic dye (2). Another agricultural solid waste, tamarind fruit shell is
also a potential sorbent. (13).
Commercially available activated carbons (AC) are usually derived from natural
materials such as wood, coconut shell, lignite or coal, but almost any carbonaceous material
may be used as precursor for the preparation of carbon adsorbents. Hazelnut shell, low cost
material, also has suitable adsorption capacity to the remove dyes from its aqueous solution
(15). Raw agricultural solid wastes and waste materials from forest industries such as sawdust
and bark have been used as adsorbents. These materials are available in large quantities and
may have potential as sorbents due to their physico-chemical characteristics and low-cost.
Amino acid fermentation industry waste, Corynebacterium glutamicum, has been found to
possess excellent bio sorption capacity (8).
Water-pollution control thus is presently one of the major areas of scientific activity as
effluent discharge from textile and dyestuff industries to neighbouring water bodies and
wastewater treatment systems is currently causing significant health concerns to
environmental regulatory agencies. So colour removal, in particular by bio sorption is a
proven technique potentially for the removal of dyes from aqueous solution (12).
SUMMARY:
Biosorption is a technique that can be used for the removal of pollutants from water,
especially those that are not easily biodegradable such as metals and dyes. A variety of
biomaterials are known to bind these pollutants, including bacteria, fungi, algae, and
industrial and agricultural wastes. There are three principle advantages of biological
technologies for the removal of pollutants; first, biological processes can be carried out in situ
at the contaminated site; Second, bioprocess technologies are usually environmentally benign
(no secondary pollution) and third, they are cost effective. Of the different biological
methods, bioaccumulation and biosorption have been demonstrated to possess good potential
to replace conventional methods for the removal of dyes. In this review, the biosorption
abilities of bacterial biomass towards dyes are emphasized.
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