resources

Article
Activated Carbons from Hydrothermal Carbonization and
Chemical Activation of Olive Stones: Application in
Sulfamethoxazole Adsorption
Elena Diaz 1, * , Ines Sanchis 1 , Charles J. Coronella 2 and Angel F. Mohedano 1

1 Chemical Engineering Department, Faculty of Sciences, Universidad Autonoma de Madrid,

28049 Madrid, Spain; ines.sanchis@uam.es (I.S.); angelf.mohedano@uam.es (A.F.M.)
2 Department of Chemical and Materials Engineering, University of Nevada, Reno, NV 89557, USA;
coronella@unr.edu
* Correspondence: elena.diaz@uam.es

Abstract: This work focuses on the production of activated carbons by hydrothermal carbonization of
olive stones at 220 ◦ C, followed by chemical activation with KOH, FeCl3 and H3 PO4 of the hydrochar
obtained. In addition, N-doped hydrochars were also obtained by performing the hydrothermal
carbonization process with the addition of (NH4 )2 SO4 . All hydrochars, N-doped and non-doped,
showed low BET surface areas (4–18 m2 g−1 ). Activated hydrochars prepared using H3 PO4 or KOH
as activating agents presented BET surface areas of 1115 and 2122 m2 g−1 , respectively, and those
prepared from N-doped hydrochar showed BET surface area values between 1116 and 2048 m2 g−1
with an important contribution of mesoporosity (0.55–1.24 cm3 g−1 ). The preparation procedure
also derived inactivated hydrochars with predominantly acidic or basic groups on their surface.
The resulting materials were tested in the adsorption of sulfamethoxazole in water. The adsorption
capacity depended on both the porous texture and the electrostatic interactions between the adsorbent
Citation: Diaz, E.; Sanchis, I.;
Coronella, C.J.; Mohedano, A.F.
and the adsorbate. The adsorption equilibrium data (20 ◦ C) fitted fairly well to the Langmuir equation,
Activated Carbons from and even better to the Freundlich equation, resulting in the non-doped hydrochar activated with the
Hydrothermal Carbonization and KOH as the best adsorbent.
Chemical Activation of Olive Stones:
Application in Sulfamethoxazole Keywords: activated carbon; adsorption; hydrochar; hydrothermal carbonization; N-doped materials;
Adsorption. Resources 2022, 11, 43. olive stones; sulfamethoxazole
https://doi.org/10.3390/
resources11050043

Academic Editor: Eveliina Repo

1. Introduction
Received: 22 March 2022
Hydrothermal carbonization (HTC) is becoming an increasingly attractive way for
Accepted: 24 April 2022
the valorization of wet biomass wastes, which is carried out in the presence of water, at
Published: 28 April 2022
temperatures between 180 and 250 ◦ C and the corresponding saturation pressure, and
Publisher’s Note: MDPI stays neutral residence times between 5 min to 24 h [1–3]. The main reaction product is a solid known as
with regard to jurisdictional claims in hydrochar (HC), which is more stable and has a higher carbon content than the raw biomass.
published maps and institutional affil- In addition, HTC produces a high organic loading of process water and a minimal gas
iations. fraction consisting mainly of CO2 . Hydrochar is commonly used: (i) for soil amendment [4];
(ii) as a source of energy [5–7]; or (iii) as a precursor of activated carbon materials [8–10].
In this sense, hydrothermal carbonization can be considered as a biomass pre-processing
treatment to alter precursor properties prior to activation. HTC maintains the surface
Copyright: © 2022 by the authors.
chemical functionality in the hydrochar and allows a more complete activation by favoring
Licensee MDPI, Basel, Switzerland.
the development of a high porosity in the activation stage [11,12]. Physical activation
This article is an open access article
distributed under the terms and
consists of a high-temperature treatment (800–1100 ◦ C) in a partially oxidizing atmosphere
conditions of the Creative Commons
using steam, air or CO2 . Chemical activation requires the use of chemical reagents (KOH,
Attribution (CC BY) license (https://
ZnCl2 , K2 CO3 , H3 PO4 ), a subsequent thermal treatment (usually 500–850 ◦ C) for 1–24 h
creativecommons.org/licenses/by/ in an inert atmosphere and a final washing (usually with acid and water) to remove the
4.0/). excess activating agent [12–14]. As a result of the activation process, an increase in the

Resources 2022, 11, 43. https://doi.org/10.3390/resources11050043 https://www.mdpi.com/journal/resources

Resources 2022, 11, 43 2 of 13

specific surface area of carbonaceous materials is achieved, which is more significant for
chemical activation, and structural modifications and changes in their composition are also
observed, such as the loss of oxygen functional groups or an increase in ash content [15].
Recently, N doping of hydrochar during the hydrothermal process has been used to
modify the physical and chemical properties of hydrochar (e.g., increase in aromatic char-
acter, bulk N content, oxidation resistance and conductivity), without causing significant
changes in the textural properties of hydrochar. Doped hydrochar requires an activation
process to increase pore volume and surface area [16–20]. The incorporation of nitrogen into
carbon structures has been shown to influence the performance of carbon materials used
as energy-storage materials [21,22] and adsorbents [20,23–26]. Roldan et al. [20] prepared
N-doped activated carbon by hydrothermal carbonization with ZnCl2 as an activating agent
(190 ◦ C, 19 h) using glucose as a C precursor and pyrrole carboxaldehyde (C5 H5 NO) as a
doping agent. They observed that the N-doped material showed higher mesoporosity (BET
surface area (ABET ) = 503 m2 g−1 , Vmesopore = 0.44 cm3 g−1 , pore diameter = 12 nm, N = 3.9
wt.%) than the non-modified hydrochar (ABET = 373 m2 g−1 , Vmesopore = 0.14 cm3 g−1 , pore
diameter = 3.5 nm, N = 0 wt.%). This property was responsible for the higher adsorption
capacity (qe ) of N-doped carbon (qe = 159 mg g−1 methylene blue; qe = 105 mg g−1 Rho-
damine B) than the non-modified material (qe = 88 mg g−1 methylene blue; qe = 13 mg g−1
Rhodamine B). The porosity and N content of N-doped porous carbons can be tuned by
adjusting the doping agent and C precursor ratio. In this regard, Huang et al. [26] prepared
an N-doped carbon using NaNH2 as a doping agent and a hydrochar from furfural. They
doped the material with a NaNH2 /hydrochar mass ratio of 2, 3 and 4 at 600 ◦ C. The results
showed an increase in porosity from 1068 to 2436 m2 g−1 with the ratio 2 (N = 0.78 wt.%)
and 4 (N = 4.30 wt.%), achieving a CO2 adsorption capacity (100 kPa and 0 ◦ C) of 4.5 and
5.4 mmol g−1 , respectively.
For the preparation of low-cost adsorbents from biomass waste, both the amount
of waste generated, and its location must be considered. Spain is the world’s leading
source of olives. The olive industry is an area of intensive agricultural activity and there
are by-products that are not widely used and accumulate as waste. In the 2020–2021
campaign, 2.75 million hectares have been dedicated to achieving an olive production of
1.39 million tons [27]. Considering that the weight of the olive stone represents between
10–20 % of the total weight of the olive, the generation in this period of olive stones
could exceed 200,000 tons. Olive stones are being widely used as fuel in southern Europe,
especially in Croatia, Greece, Italy, Portugal, Slovenia, Spain and Turkey [28,29]. Recently,
different alternatives have emerged to give value to this by-product as the production
of catalysts [30], food supplements [31] and adsorbents for CO2 removal [32,33], heavy
metals [34,35], textile dyes [36] and pharmaceuticals [37,38]. Several preparation methods
have been carried out to transform olive stones into a good adsorbent material. Moussa
et al. [32,33] prepared activated carbon from olive stones by a first pyrolysis step (300 ◦ C,
1 h, N2 ) of the raw material, followed by KOH impregnation (85 ◦ C, 3 h, KOH/OS mass
ratio = 7) and a second pyrolysis step (350 ◦ C, 2 h, N2 ). They obtained activated carbons
with an ABET of 1345 m2 g−1 , with an adsorption capacity of 5.7 mmol g−1 for CO2 (0 ◦ C,
1 bar), higher than those of other activated carbons prepared from algae (ABET = 418 m2
g−1 , qe = 2.4 mmol g−1 ) [39] or from empty fruit bunches of oil palms (ABET = 2510 m2 g−1 ,
qe = 5.2 mmol g−1 ) [40]. Bohli et al. [34] impregnated olive stones with H3 PO4 (110 ◦ C, 9 h)
followed by a calcination process (380 ◦ C, 2.5 h, N2 ). They obtained an activated carbon
characterized by an ABET of 1194 m2 g−1 and a pHPZC of 3.4, which was a good adsorbent
to remove Cu (qe = 17.7 mg g−1 ), Cd (qe = 57.1 mg g−1 ) and Pb (qe = 147.5 mg g−1 ). These
results were appreciably better than those obtained by other authors with apricot stones
(ABET = 566 m2 g−1 , qe = 24.1, 33.6 and 22.8 mg g−1 for Cu, Cd and Pb, respectively) [41].
The same preparation process was used by Limousy et al. [38] to prepare activated carbons
for amoxicillin adsorption, obtaining a material (ABET = 1174 m2 g−1 ) able to remove 93%
of the antibiotic (20 ◦ C for 25 mg L−1 initial concentration) with an adsorption capacity of
22.1 mg g−1 .
Resources 2022, 11, 43 3 of 13

The aim of this work is the valorization of olive stones into activated carbon by
means of hydrothermal carbonization and chemical activation of the resulting hydrochar.
The effect of the N doping of the carbonaceous materials along the hydrothermal car-
bonization process on the characteristics of the adsorbent materials has been also studied.
Hydrochars and activated hydrochars have been characterized by several techniques cover-
ing proximate and ultimate analyses, N2 adsorption-desorption at 77 K, Scanning Electron
Microscope (SEM) and pHslurry . The potential of the activated hydrochars as adsorbents for
the removal of pollutants in water has been tested using as a model compound an emerging
pollutant, the sulfamethoxazole (SMX), a sulfonamide antibiotic used to treat urinary tract
infections, bronchitis and prostatitis, which is effective against gram-negative and positive
bacteria. That compound is frequently detected in wastewater and in drinking water at
concentrations of 100–2500 ng L−1 and 12 ng L−1 , respectively [42]. Table 1 collects some
SMX adsorption results using non-commercial activated carbons from biomass wastes as
adsorbents. The reported materials showed low specific surfaces, except those that follow
preparation procedures based on the hydrothermal carbonization of biomass wastes and
KOH chemical activation or pyrolysis at high temperatures. The results suggest that the
electrostatic interaction between functionalized adsorbents and SMX plays an important
role in pollutant removal.

Table 1. Summary of representative studies on sulfamethoxazole adsorption from water by biomass-

derived adsorbents (the data correspond to the adsorbent that exhibits the best performance in each
study).

Adsorbent Adsorption
Biomass Precursor Adsorbent Preparation Ref.
Characteristics Conditions/Parameters
C = 73.8% SMX0 = 50 mg L−1
Magnetic biochar prepared by O = 21.8% W = 0.2 g L−1
Bagasse FeCl3 impregnation and N = 0.96% T = 25 ◦ C [43]
pyrolysis at 800 ◦ C pHPZC = 2.8 pH = 5
ABET = 606 m2 g−1 qL = 205 mg g−1

C = 34.9% SMX0 = 25–175 mg L−1

Hydrothermal carbonization N = 5.8% W = 0.25 g L−1
Dewatered waste
at 208 ◦ C and KOH activation pHslurry = 5.5 T = 20 ◦ C [44]
activated sludge
at 850 ◦ C pH = 4.6
ABET = 832 m2 g−1
qL = 423 mg g−1
SMX0 = 25–150 mg L−1
Hydrothermal carbonization C = 73.3% W = 0.25 g L−1
Grape Seeds at 220 ◦ C and KOH activation pHslurry = 7.6 T = 20 ◦ C [45]
at 750 ◦ C ABET = 2194 m2 g−1 pH = 4.6
qL = 650 mg g−1
C = 55.8% SMX0 = 0.5–9.0 mg L−1
Magnetic biochar prepared by O = 14.2% W = 20 mg L−1
Pine sawdust FeCl2 , KOH and KNO3 N < 0.3% T = 25 ◦ C [46]
impregnation at 90 ◦ C pHPZC = 9.5 pH = 4.5
ABET = 126 m2 g−1 qL = 19.1 mg g−1
SMX0 = 10 mg L−1
Thermal treatment (N2 ) at 380
◦ C and 2.5–10 psi followed by C = 52.0% W = 100 mg L−1
Bamboo O = 39.5% T = 25 ◦ C [47]
an activation with H3 PO4 at
ABET = 1.12 m2 g−1 pH = 3.3
600 ◦ C
qL = 88.1 mg g−1
W = 0.45–0.68 mg L−1
C = 87.6%
Room temperature
Pine wood Carbonization at 500 ◦ C O = 12.2% [48]
pH = 6
ABET = 328 m2 g−1
KF =131 n = 0.24
ABET : specific surface area by Brunauer–Emmett–Teller (BET) equation; C: carbon content (wt.% d.b.); O: oxygen
content (wt.% d.b.); N: nitrogen content (wt.% d.b.); Ash: ash content (wt.% d.b.); pHPZC : pH at point of
zero charge; SMX0 : initial sulfamethoxazole concentration; W: adsorbent dose; T: adsorption temperature; pH:
adsorption pH; qL ; adsorption capacity from Langmuir model; KF : Freundlich constant; n: adsorption intensity
from Freundlich model.
Resources 2022, 11, 43 4 of 13

2. Materials and Methods

2.1. Preparation of Hydrochars
Olive stones (OS), provided by a company located in Jaén (Spain), with an 18 wt.%
of humidity and with a particle size of 5 mm were used as hydrochar precursors. In the
first stage, the HTC process was carried out in a Teflon-lined stainless steel vessel (100 mL),
using 20 g of dried olive stones and multiple water amounts to obtain an OS concentration
in the reactor between 30–50 wt.% on a dry basis. The reactor was inserted in a muffle
furnace (Hobersal series 8B Mod 12 PR/400) and heated up to 220 ◦ C for 16 h [45]. The
hydrochar was recovered by filtration, washed with distilled water and oven-dried at
105 ◦ C for 24 h (Nabertherm R 60/750/12-C6).
Nitrogen doping of hydrochars was carried out using 20 g of dried OS and 50 wt.%
water spiked with 7.9, 13.2 and 19.8 g of (NH4 )2 SO4 (Panreac, 99%) in an initial C/N mass
ratio of 5, 3 and 2, respectively [19]. HTC process and subsequent hydrochar washing were
carried out under the same conditions abovementioned. Hydrochars were denoted as HC,
followed by NX (HCNX) in cases of N-doped hydrochars, where X represents the initial
C/N mass ratio.

2.2. Preparation of Activated Hydrochars

The hydrochar was physically mixed with the different activating agents, such as
potassium hydroxide (KOH, Panreac, 85%), iron chloride (FeCl3 , Panreac, 97%) or phospho-
ric acid (H3 PO4 , Panreac, 85%) in a 3:1 mass ratio at room temperature. They were activated
in a horizontal tube furnace (Nabertherm RHTH 120/300/18/C42) under a continuous
N2 flow (100 NmL min−1 ). Activation with KOH and FeCl3 was performed at 750 ◦ C for
1 h with a heating rate of 10 ◦ C min−1 [13,49,50]. In the case of H3 PO4 , the mixture was
left overnight at 60 ◦ C and then heated at 500 ◦ C (10 ◦ C min−1 heating rate) for 2 h [51].
In addition, chemical activation with KOH, FeCl3 and H3 PO4 of the non-carbonized olive
stones was carried out following the abovementioned procedure. In the cases of N-doped
hydrochar, the activation process was carried out only with KOH under the same operating
conditions. All activated materials were washed with 0.1 M HCl or NaOH and rinsed with
deionized water to neutral pH. They were then recovered by filtration and finally dried at
105 ◦ C overnight. The activated hydrochars were denoted, including the activating agent.
For instance, HCN3-KOH represents the activated carbon obtained with KOH from the
olive stone N-doped hydrochar with an initial mass ratio, C/N, of 3.

2.3. Characterization of Hydrochar and Activated Hydrochars

The proximate analyses were performed according to ASTM methods D3173-11 (mois-
ture), D3174-11 (ash) and D3175-11 (volatile matter (VM)) using a Mettler Toledo apparatus
(TGA/SDTA851e). The elemental analysis (C, N, S and H) was determined in a LECO
Model CHNS-932 elemental analyzer. The analyses were performed in triplicate, with the
standard deviation being less than 5% in all the cases.
N2 adsorption-desorption isotherms at 77 K were performed in a Micromeritics ap-
paratus (Tristar 3020). The samples were previously outgassed at 150 ◦ C and a residual
pressure of 10−3 Torr for 6 h in a Micromeritics VacPrep 061 device. The surface area was
calculated by the BET equation and the micropore volume (Vmicro ) was obtained by the
t-method. The difference between the volume of N2 adsorbed at 0.95 relative pressure (as
liquid) and the micropore volume was taken as the mesopore volume (Vmeso ).
Scanning Electron Microscope (SEM) images of the hydrochar and activated hydrochar
samples were obtained in Hitachi S-3000N apparatus. The samples were metalized with
gold using a Sputter Coater SC502. Images were obtained in the high vacuum mode under
an accelerating voltage of 20 kV, using secondary and back-scattered electrons.
The pHslurry of the activated hydrochars was determined by measuring the pH (pH-
meter, Crison) of an aqueous suspension of the sample (1 g) in deionized water (10 mL)
after being stirred overnight [52].
Resources 2022, 11, 43 5 of 13

2.4. Adsorption Tests

The potential application of activated hydrochar (≈100 µm particle size) as an aqueous
phase adsorbent was assessed using sulfamethoxazole (SMX) as a model compound. SMX
has a solubility in water of 610 mg L−1 at 298 and a pKa of 1.7/5.6 [53]. Adsorption tests
were performed by adding activated hydrochar (12.5 mg) into aqueous SMX solutions
(25 to 300 mg L−1 ) in 50 mL glass bottles. A commercial activated carbon (C: 89.5 wt.%.,
ABET : 800 m2 g−1 ; Vmicro : 0.67 cm3 g−1 ; Vmeso : 0.53 cm3 g−1 ; pHslurry : 7.7), supplied by
Merck, was used as a reference for comparison purposes. Experiments were carried out in
a thermostatized shaker bath (Optic Ivuymen System) at 20 ◦ C, 200 rpm and the natural pH
of the SMX solution (4.6) for 72 h, which was more than enough time to reach equilibrium.
SMX concentration was determined by UV-vis spectrophotometry (Cary 60 UV-Vis Agilent
Technologies) at 265 nm. The reported results are the average values from triplicate runs,
being the standard errors below 5%. Langmuir and Freundlich equations were used to fit
the equilibrium data and parameters were calculated using Origin 9.1 software.

3. Results
3.1. Characterization of the Hydrochars and Activated Hydrochars
Table 2 shows the proximate and ultimate analyses of the feedstock and hydrochars.
Elemental analysis indicates that the raw material has the typical mass composition of an
olive stone: carbon (≈43–50 wt.%), oxygen (≈43–49 wt.%), hydrogen (≈6–7 wt.%) and low
nitrogen and sulfur content (≤0.4–0.1 wt.%) [54–58]. In the case of non-doped hydrochar,
the carbonization process led to an increase in C content (≈34 wt.%) and a decrease in
oxygen (≈36–42 wt.%) and hydrogen (≈14–17 wt.%) content, because of the dehydration
and decarboxylation reactions [59,60]. A reduced ash content (less than 0.8 wt.%) was
observed in all cases. Regarding hydrochar yields, calculated as the mass of the hydrochar
per unit mass of olive stone on a dry basis, values around 48–49 wt.% were obtained,
regardless of the olive stone/water ratio (30–50 wt.% of OS on a dry basis) used in the HTC
runs.

Table 2. Main characteristics of feedstock and hydrochars.

Proximate Analysis Ultimate Analysis

Biomass/ (wt.% Dry Basis) (wt.% Dry Basis) ABET
Sample Water
Volatile Fixed (m2 g−1 )
(wt.%) Ash C H N S O*
Matter Carbon
OS - 77.2 22.4 0.4 49.9 5.8 0.1 0.1 43.7 -
30 52.5 47.1 0.4 66.7 4.8 0.2 0.0 27.9 18
HC 40 57.0 42.2 0.8 67.8 5.0 0.2 0.1 26.1 17
50 56.1 43.3 0.6 67.8 4.9 0.2 0.0 26.5 15
HCN2 64.7 35.0 0.3 54.0 4.8 7.7 6.4 26.8 5
HCN3 50 59.9 39.5 0.6 62.2 4.7 5.6 3.2 23.7 10
HCN5 60.5 39.1 0.4 63.9 4.9 4.9 1.4 24.5 4
* Calculated by difference O = 100 − (C + H + N + S + Ash).

N-doped hydrochars (HCN2, HCN3, HCN5) were prepared with an olive stones/water
ratio of 50 wt.%, since it allows one to obtain a higher amount of hydrochar (Table 2). The
C/N ratio in the initial solution (2, 3 and 5) was inferior to that of the hydrochar (7, 11 and
13) because of a less significant increase in carbon content than in nitrogen associated with
the massive incorporation of nitrogen into the hydrochar structure, while no significant
variations in the ash content were found. This fact has been observed previously in the
literature using different nitrogen precursors ((NH4 )2 SO4 , C5 H5 NO, C2 H5 NO2 ) [16,18,20].
Moreover, the use of (NH4 )2 SO4 as a doping agent also provoked an increase in sulfur
content in these hydrochars.
Resources 2022, 11, 43 6 of 13

The N2 adsorption-desorption isotherms at 77 K of the hydrochars (Figure 1) are

associated with solids with low porosity. They showed very low adsorption at low relative
pressures revealing a limited porosity in all cases with specific surface areas values in the
range of 4–18 m2 g−1 . The N-doped materials showed the lowest surface area values, below
10 m2 g−1 , but with a significant contribution of mesoporosity. Table 3 summarizes the
porous structure of the activated hydrochars. Representative N2 adsorption-desorption
isotherms are depicted in Figure 1. All of them correspond to essentially microporous solids
with a higher contribution of mesoporosity for the HC-H3 PO4 and N-doped materials.
For comparison, the olive stones were also chemically activated, without hydrothermal
carbonization treatment (Figure 1). The porous structure developed by these materials was
much lower than that the obtained from the hydrochars, achieving BET surface area values
of 211, 504 and 254 m2 g−1 for olives stones activated with KOH, H3 PO4 and FeCl3 , respec-
tively, showing the crucial role of the HTC treatment in the development of the textural
characteristics of activated hydrochars [44]. In the case of hydrochars, activation with FeCl3
gave rise to a material with a relatively low surface area, slightly superior to the obtained
by direct activation, while activation with H3 PO4 resulted in a microporous material with
a much higher relative contribution of mesoporosity, reaching BET surface area values of
1100 m2 g−1 . Similar BET surface area values were obtained by direct activation of olive
stones with steam, operating at 900 ◦ C, a temperature significantly higher than those used
in this work [61]. Activation with KOH allowed a high porosity development, essentially
microporous, with BET surface area values up to around 2000 m2 g−1 and a significant
ash content (17.1 wt.%). The activation treatments caused some important changes in the
surface characteristics of the materials, specifically in the pHslurry value. As described by
other authors, activation with H3 PO4 resulted in a material with an acidic surface, while
activation with KOH led to a material with a more basic surface [13,62]. Finally, activation
with FeCl3 gave rise to an activated hydrochar with a slightly acidic surface.
Table 3. Main characteristics of feedstock and hydrochars.

ABET Vmicro Vmeso

Sample C * (%) N * (%) Ash * (%) pHslurry
(m2 g−1 ) (cm3 g−1 ) (cm3 g−1 )
HC-FeCl3 47.0 0.3 10.0 383 0.18 0.07 6.5
HC-H3 PO4 70.2 0.2 9.1 1155 0.50 0.20 1.8
HC-KOH 74.4 0.1 17.2 2122 0.96 0.14 8.0
HCN2-KOH 49.6 0.99 31.7 1247 0.13 0.60 2.3
HCN3-KOH 40.2 0.63 29.1 1116 0.21 0.55 2.8
HCN5-KOH 59.2 1.52 25.1 2048 0.86 1.24 4.2
* C, N and Ash are represented as wt.% dry basis.

The potential of KOH as an activating agent was also tested in the activation of N-
doped hydrochar, giving rise to materials with high surface area (1100–2000 m2 g−1 ), but
significantly lower than that shown by the HC-KOH activated hydrochar, especially in
the cases where the initial C/N mass ratio was lower (HCN2 and HCN3). This fact is
related to the high ash content of these materials since the BET surface area represents up
to about 1574–2734 m2 g−1 on an ash-free basis. Regarding pore size, the high N content
in the precursor material favored the synthesis of mesoporous activated carbons [20].
The C/N ratio increased significantly after the activation treatment, indicating that the
high temperature of the activation process caused the transformation of N into volatile N
compounds leading to the loss of surface functional groups and resulting in the production
of more stable N-containing species in the carbonaceous structure [63].
As previously discussed, the activation treatment causes some changes in the pHslurry
of the activated hydrochars (Table 3). In the case of the N-doped hydrochars, the low values
of pHslurry (2.3–4.8), despite being activated with KOH, could be attributed to the N-doping
Resources 2022, 11, 43 7 of 13

Resources 2022, 11, x FOR PEER REVIEW 7 of 14

agent ((NH4 )2 SO4 ) causing a modification of oxygen groups leading to an increase in
carboxylic acids [18].

Figure 1.
Figure 1. N
N22adsorption-desorption
adsorption-desorptionisotherms
isothermsat at
7777
K of
K of(a) (a)
hydrochar obtained
hydrochar at different
obtained bio-
at different
mass/water ratio
biomass/water (30–50
ratio wt.%),
(30–50 (b) N-doped
wt.%), hydrochar,
(b) N-doped (c) materials
hydrochar, obtained
(c) materials by direct
obtained activation
by direct acti-
of olive
vation ofstones, (d) activated
olive stones, hydrochar
(d) activated and (e)and
hydrochar activated N-doped
(e) activated hydrochar.
N-doped hydrochar.

Table 3. Main
Figure 2 characteristics of feedstock
shows the scanning and hydrochars.
electron micrographs (SEM) of the surface of the olive
stones, hydrochars and activated hydrochars. The ABET
raw olive stones presented
Vmicro Vmeso
a well-
Sample
defined C *structure,
and regular (%) N * (%) while Ash * (%)
the hydrochars showed some partial pH
degradation of
slurry
(m2 g−1) (cm3 g−1) (cm3 g−1)
the surface,
HC-FeCl3 maintaining
47.0 their original
0.3 morphology,
10.0 indicating
383 the
0.18 good thermo-mechanical
0.07 6.5
stability of
HC-H3PO4
the raw material.
70.2
The
0.2
N-doped9.1
hydrochars
1155
showed multiple
0.50
spheres
0.20
of 1–5
1.8
µm
diameter over the surface, originating in the doping process with (NH4 )2 SO4 . From the
HC-KOH 74.4 0.1 17.2 2122 0.96 0.14 8.0
hydrochar images, non-porous materials were observed. However, activated hydrochars
HCN2-KOH 49.6 0.99 31.7 1247 0.13 0.60 2.3
showed high porosity, with different structures depending on the activating agent. The
HCN3-KOH 40.2 0.63 29.1 1116 0.21 0.55 2.8
hydrochars activated with KOH (HC-KOH and HCN5-KOH) showed cavities and cracks
HCN5-KOH 59.2 1.52 25.1 2048 0.86 1.24 4.2
on their external surface, more regular in the case of the non-doped material. In the case of
* C, N and Ash are represented as wt.% dry basis.

The potential of KOH as an activating agent was also tested in the activation of N-
doped hydrochar, giving rise to materials with high surface area (1100–2000 m2 g−1), but
 fined and regular structure, while the hydrochars showed some partial degradation of the
surface, maintaining their original morphology, indicating the good thermo-mechanical
stability of the raw material. The N-doped hydrochars showed multiple spheres of 1–5
μm diameter over the surface, originating in the doping process with (NH4)2SO4. From the
Resources 2022, 11, 43
hydrochar images, non-porous materials were observed. However, activated hydrochars 8 of 13
showed high porosity, with different structures depending on the activating agent. The
hydrochars activated with KOH (HC-KOH and HCN5-KOH) showed cavities and cracks
on their external surface, more regular in the case of the non-doped material. In the case
FeCl3 -activation, the formation of microspheres was observed, while after activation with
of FeCl3-activation, the formation of microspheres was observed, while after activation
H3 PO4 , prismatic particles can also be seen.
with H3PO4, prismatic particles can also be seen.

Figure 2. SEM images of (a) OS, (b) HC-50, (c) HCN5, (d) HC-KOH, (e) HC-FeCl3 , (f) HC-H3 PO4
and (g) HCN5-KOH.

3.2. Adsorption of Sulfamethoxazole

The activated hydrochars were used in the SMX adsorption experiments, together with
a commercial activated carbon, whose main characteristics were included in Section 2.4. for
comparison purposes. Figure 3 shows the adsorption isotherms at 20 ◦ C. The experimental
data were fitted to the Langmuir (1) and Freundlich (2) equations:

q L ·K L ·Ce
qe = (1)
1 + K L ·Ce
1
qe = K F ·Cen (2)
where qe is the equilibrium adsorbate loading onto the adsorbent (mg g−1 ); Ce , the equi-
librium liquid phase concentration of adsorbate (mg L−1 ); qL , the monolayer adsorption
capacity of the material (mg g−1 ); KL , the Langmuir constant (L mg−1 ); KF , the Freundlich
constant ((mg g−1 )·(L mg−1 )(1/n) ) and n is the adsorption intensity. The values of the fitting
parameters and correlation coefficients are given in Table 4. As can be seen, the results
fitted the Freundlich equation better than the Langmuir equation, especially in the case of
the adsorbents from HC. The HC-KOH material showed the highest adsorption capacity
of the Langmuir monolayer, reaching almost the value of 760 mg g−1 , followed by the
materials obtained by KOH activation of N-doped hydrochars (qL : 429–695 mg g−1 ). These
adsorption capacity values are comparable to or even superior to those obtained with
 stant ((mg g−1)·(L mg−1)(1/n)) and n is the adsorption intensity. The values of the fitting pa-
rameters and correlation coefficients are given in Table 4. As can be seen, the results fitted
the Freundlich equation better than the Langmuir equation, especially in the case of the
adsorbents from HC. The HC-KOH material showed the highest adsorption capacity of
the Langmuir monolayer, reaching almost the value of 760 mg g−1, followed by the mate-
Resources 2022, 11, 43 9 of 13
rials obtained by KOH activation of N-doped hydrochars (qL: 429–695 mg g−1). These ad-
sorption capacity values are comparable to or even superior to those obtained with adsor-
bents prepared from other biomass wastes (Table 1). Regarding Freundlich isotherms, the
adsorbents
value of the prepared
“n” wasfrom other
lower thanbiomass wastesassociated
1 (0.17–0.33) (Table 1). with
Regarding Freundlich
favorable isotherms,
isotherms, with a
the value of the “n” was lower than 1 (0.17–0.33) associated with favorable
high amount of adsorbate adsorbed at low concentrations [64]. Except in the case of isotherms, with
the
aHC-H
high 3amount
PO4 material, values of “n” were between 0.17 and 0.22, so an approximation ofthe
of adsorbate adsorbed at low concentrations [64]. Except in the case of the
HC-H 3 PO4 material,
adsorption capacity ofvalues of “n” were
the materials couldbetween 0.17 andfrom
be established 0.22, “KsoF”anvalues
approximation
with the fol-of
the adsorption capacity of the materials could
lowing sequence: HC-KOH > HCN5-KOH > HCN2-KOH > HCN3-KOHbe established from “K F ” values with
>> commercialthe
following sequence:
AC > HC-FeCl HC-KOH > HCN5-KOH > HCN2-KOH > HCN3-KOH >> commercial
3. Attending to the value of “qL”, calculated from the Langmuir equation,
AC
the >resultant
HC-FeClsequence
3 . Attending
wouldto the value
be the of “q
same, L ”, calculated
placing HC-H3PO from the Langmuir equation,
4 between HCN3-KOH and
the resultant
commercial AC. sequence would be the same, placing HC-H 3 PO 4 between HCN3-KOH and
commercial AC.

HC-KOH HC-FeCl3 HC-H3PO4 Commercial AC HCN2-KOH HCN3-KOH HCN5-KOH

800 800

600 600
qe (mg·g )

qe (mg·g )
-1

-1
400 400

200 200

0 0
0 20 40 60 80 100 120 140 0 20 40 60 80 100 120 140 160 180

Ce (mg·L-1) Ce (mg·L-1)

Figure3.3.Adsorption
Figure Adsorptionisotherms
isothermsofofSMX
SMXon
onHC-KOH,
HC-KOH,HC-FeCl
HC-FeCl 3 ,3,HC-H
HC-H3 PO , HCN2-KOH, HCN3-
3PO44, HCN2-KOH, HCN3-
KOH and HCN5-KOH and commercial activated carbon at 20 ◦ C (symbols: experimental values;
KOH and HCN5-KOH and commercial activated carbon at 20 °C (symbols: experimental values;
short
shortdot
dotlines:
lines:fitting
fittingto
tothe
theLangmuir
Langmuirequation;
equation;solid
solidlines:
lines:fitting
fittingto tothe
theFreundlich
Freundlichequation).
equation).

Table 4. Langmuir and Freundlich parameters for SMX adsorption on the activated carbons of
Figure 3.

Langmuir Freundlich
Sample qL KL KF
R2 n R2
(mg g−1 ) (L mg−1 ) (mg g−1 )·(L mg−1 )(1/n)
Commercial AC 221.8 ± 9.2 0.296 ± 0.071 0.951 83.8 ± 12.1 0.220 ± 0.036 0.938
HC-FeCl3 151.2 ± 7.6 0.419 ± 0.187 0.673 69.9 ± 5.7 0.174 ± 0.019 0.945
HC-H3PO4 324.4 ± 7.4 0.078 ± 0.007 0.992 66.3 ± 10.4 0.337 ± 0.039 0.947
HC-KOH 758.7 ± 55.9 1.236 ± 0.630 0.855 338.3 ± 18.3 0.209 ± 0.014 0.987
HCN2-KOH 524.6 ± 34.2 0.579 ± 0.290 0.830 226.2 ± 16.8 0.186 ± 0.017 0.975
HCN3-KOH 429.4 ± 29.3 0.862 ± 0.464 0.829 184.8 ± 11.5 0.187 ± 0.014 0.982
HCN5-KOH 695.4 ± 48.9 0.397 ± 0.185 0.878 278.9 ± 14.6 0.209 ± 0.013 0.989

The adsorption capacity of the activated hydrochars was mainly influenced by the
BET surface area of the carbons followed by the existence of acid or basic surface functional
groups. This fact was also observed in the adsorption of methylene blue using KOH-
modified hydrochar from sewage sludge, where the adsorption process was the result of
several phenomena such as physi- and chemisorption, acid-base and redox equilibria [65].
In this study, consistently with their much larger surface area, HC-KOH and HCN5-KOH
yielded the highest adsorption capacity. Comparing these two materials, electrostatic
interactions could explain the best performance of the HC-KOH material. At solution pH
(4.6), SMX should be mostly a neutral and anionic species. Since the pHslurry of HCN5-
KOH is 4.2, its surface will be negatively charged, while the surface of the HC-KOH
Resources 2022, 11, 43 10 of 13

material will be positively charged, which will favor the interaction of the latter with the
adsorbent [45,66,67]. Then, the effect of N doping did not have a positive effect on the
adsorption of the SMX to provide positively charged functional groups on the surface
of the adsorbent. A comparison of the adsorption capacity of HC-H3 PO4 , HCN2-KOH,
HCN3-KOH materials, which are characterized by similar values of specific surface area
(ABET : 1115–1247 m2 g−1 ) and pHslurry lower than that of the solution, allows the evaluation
of the effect of acidic or basic functional groups on the surface of the activated hydrochars
on the adsorption capacity of SMX. The better performance of the N-doped materials, with
similar properties to the HC-H3 PO4 activated hydrochar could be attributed to the higher
negative charge of the latter, whose pHslurry is like pKa 1 of SMX. Finally, the substantial
differences in adsorption capacity between the HC-FeCl3 and HC-H3 PO4 materials, and
the commercial AC, used as reference material, could be due to the lower surface area of
the former since all of them present similar electrostatic interactions with the adsorbate.

4. Conclusions
Hydrothermal carbonization followed by chemical activation with KOH of olive stones
can be a promising way of valorization of such biomass to produce activated hydrochars.
Activation with FeCl3 , H3 PO4 and KOH of hydrochar resulted in high BET surface area
carbons with predominantly microporous structure and a neutral, acidic and basic surface,
while KOH activation of N-doped hydrochar also provided activated carbons with a high
BET surface area but with a significant contribution of mesoporosity and an acidic surface.
These activated hydrochars can be used as adsorbents for pollutants in water, as can be
deduced from the high adsorption capacity shown for sulfamethoxazole, based on the high
BET area value and the electrostatic interaction between the adsorbent and the adsorbate.

Author Contributions: Conceptualization, E.D., C.J.C., A.F.M.; Methodology, E.D., I.S.; Investigation,
E.D., I.S.; Writing—Original Draft Preparation, E.D., I.S., A.F.M.; Writing—Review and Editing, E.D.,
C.J.C., A.F.M.; Supervision, E.D., C.J.C., A.F.M.; Funding Acquisition, E.D., C.J.C., A.F.M. All authors
have read and agreed to the published version of the manuscript.
Funding: The authors greatly appreciate the financial support from the Spanish MICIIN (PID 2019-
108445RB-100), Comunidad de Madrid (S2018/EMT-4344), US National Science Foundation (NSF
#CBET-1856009), and UAM-Santander (2017/EEUU/07). I. Sanchis wishes to thank the Comunidad
de Madrid for PEJD-2017-PRE/AMB-4616 contract.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments: The authors thank F.J. Manzano for his valuable help.
Conflicts of Interest: The authors declare no conflict of interest.

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