10.1016@j.seppur.2020.117536

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Separation and Purification Technology
journal homepage: www.elsevier.com/locate/seppur
Biomass-derived activated carbons for the removal of pharmaceutical
mircopollutants from wastewater: A review
Jinbo Ouyanga,b,⁎
, Limin Zhoua
, Zhirong Liua
, Jerry Y.Y. Hengb
, Wenqian Chenb
a
State Key Laboratory for Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, PR China
b
Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
A R T I C L E I N F O
Keywords:
Biomass
Biochars
Pharmaceutical
Adsorption
A B S T R A C T
Biomass-derived activated carbons (biochars) have attracted great attention due to their excellent physico-
chemical properties such as high specific area, large pore volume, well-defined microporous structure and
tunable surface chemistry. Although pharmaceuticals are an emerging class of micropollutants in wastewater
through the sewerage disposal by pharmaceutical factories, hospitals and households, only a few recent studies
have reviewed the adsorption and removal of pharmaceuticals from wastewater by biochars and they lack the
systematic insights into total adsorption process from biochars preparation to adsorption mechanism. This paper
aims to provide a comprehensive review on recent publications and to propose future research directions. The
effects of lignocellulosic biomass as well as the pyrolysis, activation and modification conditions on the physi-
cochemical properties of biochars and their adsorption capacities are discussed. The adsorption kinetics and
isotherms of different pharmaceuticals onto various biochars are analyzed based on commonly used models.
Finally, the potential adsorption mechanisms of pharmaceuticals by biochars are summarized.
1. Introduction
Rapid urbanization and industrialization have caused serious en-
vironmental problems, especially water contamination, over the last
few decades [1]. This has resulted in a decrease in water quality, mostly
due to emerging pollutants such as organic micropollutants and in-
organic heavy metal ions [2,3]. It is estimated that about 300 million
tons of micropollutants, including pharmaceuticals, hormones, in-
dustrial chemicals, pesticides, and flame retardants are released into
natural water via wastewater discharges each year [4–6]. Among these
organic micropollutants, pharmaceuticals are of increasing concern
because of their toxicity and non-biodegradability, which can lead to
irreversible long-term side effects to aquatic organisms [7–9]. It is re-
ported that the content of pharmaceuticals is about ng ~ μg L−1
due to
their continuous release into natural and wastewater [10]. Accordingly,
the removal of these pharmaceuticals micropollutants from wastewater
has become one of the most challenging issues, requiring the develop-
ment of a sustainable, efficient, and flexible treatment method.
Numerous methods, such as biodegradation [11], electrochemical
catalysis [12], ozonation [13], coagulation and flocculation [14–16],
and membrane filtration [17], have been extensively used to remove
pharmaceuticals from wastewaters. However, the complex structure of
the pharmaceutical, the formation of toxic by-products, and the high
cost of operation or maintenance are the main disadvantages of these
methods [18]. Compared with the above methods, adsorption is con-
sidered as a promising method for removing various pollutants from
wastewater due to its economical, renewable, and flexible operation.
To improve the adsorption capacity of pharmaceuticals onto ad-
sorbents, various porous materials such as activated carbon [19], resin
[20,21], silica [22], clay [23], multi-walled carbon nanotubes [24],
zeolite [25], graphene oxide [26,27], and chitosan [28,29], have been
explored. The main adsorbents for the removal of pharmaceuticals from
wastewater are shown in Fig. 1. Among them, activated carbon has
attracted more attention due to its better adsorption performance re-
lative to others [30]. Activated carbon refers to a kind of carbonaceous
materials, which has a well-defined hierarchical microporous structure,
high specific area, large pore volume and tunable surface chemicals.
Despite its wide ranging applications such as adsorption, photocatalysis
and electrochemistry, the cost of production prevents its large-scale use
in industries [31–33]. In the early stage of activated carbon develop-
ment, coal was considered as the best precursor to produce activated
carbon due to its high carbon content [34,35]. Due to the limited and
non-renewable nature of coal, the development of activated carbon
from other low-cost renewable resources such as biomass or any solid
waste rich in carbon has become a research focus.
Agricultural and forestry biomass wastes such as corn straw, rice
https://doi.org/10.1016/j.seppur.2020.117536
Received 23 June 2020; Received in revised form 31 July 2020; Accepted 4 August 2020
⁎
Corresponding author at: State Key Laboratory for Nuclear Resources and Environment, East China University of Technology, Nanchang 330013, PR China.
E-mail address: oyjb1001@163.com (J. Ouyang).
Separation and Purification Technology 253 (2020) 117536
Available online 08 August 2020
1383-5866/ © 2020 Elsevier B.V. All rights reserved.
T

husk, coconut shell, pomelo peel, sugarcane bagasse, wheat stem, wood
chips, leaves, etc., are mostly composed of carbon-rich chemical com-
pounds, which make them more suitable for biochars production
[36,37]. These lignocellulosic biomass-derived activated carbons, also
known as biochars, are produced by pyrolysis or carbonization under
limited oxygen atmosphere [38,39]. Furthermore, the conversion of
agricultural and forestry solid residues into valuable biochars not only
removes pollutants from wastewater by using biochars as adsorbent,
but also addresses environmental issues like the accumulation of solid
wastes, which result in air and water contamination during natural
degradation process.
Despite its wide application, the study of biochar as a pharmaceu-
ticals adsorbent is still in the early stage and more research work should
be conducted on biochars preparation, influence factors, adsorption
kinetics and adsorption mechanism. The purpose of this article is to
provide a comprehensive literature review on recent publications and
to propose future research prospects. We summarized characteristics of
the mainly pharmaceuticals in wastewater, discussed the effects of
lignocellulosic biomass, pyrolysis, activation and modification condi-
tions on the physicochemical properties of biochars. Thereafter, the
adsorption kinetics and isotherms models were elaborated to analyze
the adsorption performance of different pharmaceuticals on various
biochars, and the potential adsorption mechanism was also summar-
ized.
2. Pharmaceutical micropollutants
In the past few decades, due to their non-degradability and sus-
tained release, pharmaceuticals have become the most emerging pol-
lutants in natural water and wastewater [40,41]. These pharmaceu-
ticals are released into the water through sewerage discharges of
pharmaceutical factories, hospitals and households [42–44]. Despite
their low concentration (ng L−1
to μg L−1
) in wastewater, pharma-
ceuticals can cause long-term harmful effects on environment [45,46].
Table 1 and Fig. 2 summarize the classification and main characteristics
of some pharmaceuticals, which are commonly studied in recent re-
searches. According to Table 1, antibiotics are the main pharmaceutical
pollutants, followed by non-steroidal anti-inflammatories (NSAIDs),
antidepressant, lipid regulators, β-blockers and hormones. The ex-
istence of these pharmaceuticals in water contributes to water con-
tamination and poses a threat to aquatic life and human health [47].
Fig. 2 also shows the 15 mostly studied pharmaceuticals by biochars,
which are followed by tetracycline, sulfamethoxazole, diclofenac, ibu-
profen, carbamazepine, ciprofloxacin, sulfamethazine, ketoprofen, tri-
closan, estradiol, caffeine, clofibric acid, oxytetracyline and nor-
floxacin.
3. Lignocellulosic biomass and biochar
3.1. Lignocellulosic biomass
Biochar production from lignocellulosic biomass has attracted much
attention due to its low cost, renewable nature, and high availability
[85]. Solid residues of agriculture and forest and other carbon-rich solid
wastes constitute the main resources of lignocellulosic biomass feed-
stocks for biochar production. The overall carbon content and physi-
cochemical properties of biochars are dependent on the type of lig-
nocellulosic biomass, pyrolysis process, and activation/modification
method etc. [86,87]. Fig. 3 shows the main structure and component of
typical lignocellulosic biomass residues, and the three major con-
stituents are lignin (27%), cellulose (43%) and hemicellulose (20%),
respectively.
Cellulose has a molecular structure of linear chain with several
hundreds to thousands of β(1 → 4) linked D-glucose units, and the
linear structure of cellulose is determined by the dehydration of glucose
[89]. Cellulose is the main substance in the plant cell walls and keeps
the plant stiff and upright. Cellulose has crystalline and non-crystalline
phases which are intertwined to form microfibrils [90]. The crystalline
structure is commonly stabilized by the hydrogen bonds formed be-
tween multiple hydroxyl groups on the pyranose ring. Cellulose is in-
soluble in water and most organic solvents, and it is mainly used to
produce paperboard and paper. It can be decomposed into glucose units
by soaking cellulose in mineral acids at high temperature.
Unlike cellulose, hemicellulose contains shorter chains with about
500–3000 sugar units [89]. Hemicelluloses are branched hetero-
polysaccharides that exist along with cellulose in the cell walls of most
plants. Although it has random, amorphous structure with little
strength, hemicellulose plays an important role in linking cellulose and
lignin. Due to the structural difference, hemicellulose can be divided
into four groups, including xylans, mannans, mixed linkage β-glucans
and xyloglucans, which are composed of different sugar monomers
[91]. The main application of hemicellulose is to produce ethanol by
hydrolyzing its fermentable sugars with dilute acid or alkali and various
hemicellulose enzymes.
Lignin is a cross-linked phenolic biopolymer with molecular mass
greater than 10000 Da. The three common monolignols that make up
lignin are paracoumaryl alcohol, coniferyl alcohol, and sinapyl alcohol,
which are incorporated into lignin in the form of p-hydroxyphenyl,
guaiacyl, and syringyl, respectively [92]. Lignin fills the cell wall space
between pectin, cellulose and hemicellulose, providing mechanical
strength to support plant structure. For lignocellulosic biomass, the
lignin content mainly depends on the raw material, with 33% and 25%
in softwood and hardwoods respectively [93]. This difference in com-
position leads to the branching structural difference, where hardwood
lignin is less branched than softwood lignin. It has been shown that the
additional steric hindrance caused by the second methoxy group of si-
napyl alcohol in hardwood blocks the formation of branched polymers
[94].
In addition to the three main organic components mentioned above,
lignocellulosic biomass also consists of inorganic components and other
extracts [95]. The former includes potassium, sodium, silicon, calcium,
phosphors and chlorine in the form of ash, while the latter mainly
contains proteins, terpenes, sugars, gums, resins, alkaloids, saponins
and fats, which can be separated from biomass with an extraction sol-
vent. Despite of their low content (2%) in biomass, the effects on pro-
duction via pyrolysis of biomass cannot be neglected.
3.2. Biochar preparation
3.2.1. Pyrolysis
The conversion of biomass into biochars has become a hot topic due
to the wide application of biochars, especially for the adsorption of
pharmaceuticals [96]. The extensively used lignocellulosic biomasses
Fig. 1. Main adsorbents for the removal of pharmaceuticals from wastewater.
J. Ouyang, et al. Separation and Purification Technology 253 (2020) 117536
2

Table 1
The classification and main characteristics of some pharmaceuticals in wastewater.
Pharmaceuticals Compounds Formula Molecular weight (g mol−1
) Solubility (mg L−1
) pK1 pK2 log Kow
a
Ref.
Antibiotics sulfamethoxazole C10H11N3O3S 253.28 610 5.60 0.97 [48]
sulfadiazine C10H10N4O2S 250.28 77 6.28 −0.14 [49]
sulfadimethoxine C12H14N4O4S 310.33 343 5.97 1.76 [50]
sulfamethazine C12H14N4O2S 278.33 1500 7.42 0.05 [51]
tetracycline C22H24N2O8 444.4 231 3.4 7.6/9.7 −1.37 [52]
amoxicillin C16H19N3O5S 365.4 3430 3.2 11.7 0.87 [53]
penicillin V C16H18N2O4S 334.4 210 2.74 1.83 [54]
ciprofloxacin C17H18FN3O3 331.4 150 5.9 8.8 0.28 [55]
oxytetracycline C22H24N2O9 460.4 313 3.27 9.5 −0.9 [56]
enrofloxacin C19H22FN3O3 359.4 612 5.69 6.68 −2.8 [57]
chlortetracycline C22H23ClN2O8 478.9 259 3.3 7.4/9.33 −3.60 [58]
trimethoprim C14H18N4O3 290.32 400 7.12 0.9 [59]
gemifloxacin C18H20FN5O4 389.38 210 5.53 9.53 1.52 [12]
tylosin C46H77NO17 916.1 5000 7.1 1.63 [60]
triclosan C12H7Cl3O2 289.5 10,000 7.9 4.76 [61]
lincomycin C18H34N2O6S 406.53 1693 7.6 0.2 [62]
norfloxacin C16H18FN3O3 319.33 280 6.22 8.51 0.46 [63]
fluconazole C13H12F2N6O 306.27 1390 1.72 0.4 [64]
NSAIDs paracetamol C8H9NO2 151.16 14,000 9.38 0.46 [65]
naproxen C14H14O3 230.26 15.9 4.19 3.18 [66]
ketoprofen C16H14O3 254.28 51 4.45 3.12 [67]
diclofenac C14H11Cl2NO2 296.1 2370 4.15 4.51 [68]
ibuprofen C13H18O2 206.28 21,000 4.91 3.97 [69]
piroxicam C15H13N3O4S 331.37 23 6.3 1.71 [5]
Acetylsalicylic acid C9H8O4 180.16 4600 3.49 1.19 [70]
Antidepressants fluoxetine C17H18F3NO 309.33 1.7 9.8 4.6 [71]
oxazepam C15H11ClN2O2 286.71 20 1.55 10.9 2.8 [72]
venlafaxine C17H27NO2 313.87 267,000 8.91 14.42 2.91 [5]
citalopram C20H21FN2O 324.4 31,090 9.78 1.39 [2]
Hormones 17 α-ethinyl estradiol C20H24O2 296.4 11,300 10.47 3.9 [73]
estriol C18H24O3 288.4 13,250 10.54 2.45 [61]
mestranol C21H26O2 310.4 1132 17.59 4.61 [74]
17-β estradiol C18H24O2 272.4 3900 10.46 4.01 [75]
β-blockers tramadol C16H25NO2 263.37 1151 9.23 13.08 2.4 [76]
atenolol C14H22N2O3 266.34 13,300 9.6 0.16 [77]
metoprolol C15H25NO3 267.36 60 9.7 1.88 [78]
Lipid regulators clofibric acid C10H11ClO3 214.64 583 4 2.84 [79]
gemfibrozil C15H22O3 250.33 11 4.5 4.77 [80]
Anti-diabetics metformin C4H11N5 129.16 1,060,000 12.4 −2.64 [9]
Diuretics furosemide C12H10ClN2O5S 330.74 73.1 3.5 9.0 2.03 [81]
H2 blockers ranitidine hydrochloride C13H23ClN4O3S 350.86 79.5 2.7 8.2 0.99 [82]
Stimultant caffeine C8H10N4O2 194.19 21,600 14 −0.07 [83]
Anti-epileptic carbamazepine C15H12N2O 236.27 18 2.3 13.9 2.25 [84]
a
Kow: defined as the ratio of the concentration of a chemical in n-octanol and water at equilibrium, a relative indicator of the tendency of an organic compound to
adsorb to soil and living organism.
Fig. 2. The most adsorbed 15 pharmaceuticals by biochars in last decades.
3

are agricultural residues (e.g. seed, sugarcane bagasse, root, straw,
stem, husk etc.), forest residues (e.g. wood chips, sawdusts, branches,
leaves, roots etc.), and other solid wastes (eg. municipal waste sludge,
paper mill sludge, animal manure etc.) [97,98]. Table 2 summarizes the
typical biomass residues that are used as precursors to produce biochars
for different pharmaceuticals adsorption. The pyrolysis condition and
biochar properties are also covered here.
Prolysis, carbonization, gasification and torrefaction are the main
technologies for biochar production from biomass [102]. Specifically,
the conversion of biomass to biochar using all of the above technologies
requires thermal treatment to increase the relative carbon content
under oxygen limited atmosphere [103]. Fig. 4 shows the 15 most
commonly used biomass feedstocks, which are paper sludge, corn
straw, tea waste, rice husk, municipal waste, seed shell, palm tree,
bamboo, wheat husks, manure, sawdust, fruit shell, pomelo peel, begass
and cotton stalks. During pyrolysis, the moisture and light volatiles are
first released, followed by aromatic components and hydrogen gas. The
remaining solid residues are biochars with well-defined porous
Fig. 3. Main structure and constituent distribution of typical biomass residues [88].
Table 2
Pyrolysis of different biomass for pharmaceuticals adsorption.
Biomass residues Pharmaceuticals Pyrolysis
condition
Activation/modification Biochar properties (S-Surface area, m2
g−1
;
V-Total pore volume, cm3
g−1
)
Ref.
Pine sawdust sulfamethoxazole 650 °C FeCl2 + KOH + KNO3 S = 125.8, V = 0.14 [48]
Cotton stalks sulfadiazine 350 °C None S = 68.4, V = 0.074 [49]
Tea waste sulfamethazine 700 °C Steam S = 576.1, V = 0.109 [51]
Municipal solid waste tetracycline 500 °C Clay S = 8.72 [52]
Giant reed amoxicillin 540–700 W Microwave S = 1372, V = 0.760 [53]
municipal solid Waste ciprofloxacin 450 °C Bentonite None [55]
Cassava waste oxytetracycline 500 °C KOH S = 128.4, V = 0.01 [56]
Paper sludge enrofloxacin 700 °C H3PO4 S = 6493, V = 17.75 [57]
Cauliflowers roots chlortetracycline 500 °C None S = 232.15, V = 0.15 [58]
Garden waste trimethoprim 500 °C None S = 8.89, V = 0.018 [99]
Wheat husks Gemifloxacin 500 °C H2SO4 + HNO3 S = 95.76, V = 0.11 [12]
Palm tree tylosin 600 °C Fe(NO3)3·9H2O + KOH S = 120, V = 2.43 [100]
Manure lincomycin 600 °C None S = 237 [62]
Wheat straw norfloxacin 400 °C montmorillonite S = 112.6, V = 0.604 [63]
Horse manure, tomato waste, olive
waste, rice husks
fluconazole 220 °C none S1 = 4.62, S2 = 0.74, S3 = 0.65,
S4 = 16.92
[64]
Olive stones paracetamol 500 °C H3PO4 S = 990, V = 0.91 [65]
Seed shell naproxen 500 °C H3PO4 S = 1328 [66]
Wheat straw ketoprofen 700 °C HCl S = 605, V = 0.421 [101]
Moringa seed diclofenac 450 °C H3PO4 None [68]
Fruit shell ibuprofen 650 °C steam S = 308, V = 0.384 [69]
Paper sludge piroxicam 800 °C None S = 848 [5]
Hollow tree fluoxetine 500 °C None S = 207, V = 0.1 [71]
Paper sludge oxazepam 800 °C HCl S = 414, V = 0.095 [72]
Paper sludge venlafaxine 800 °C None S = 848 [5]
Bamboo, coconut, yellow pine,
hardwood
citalopram 300–500 °C None S1 = 68.7, S2 = 13, S3 = 313, S4 = 102
V1 = 0.017, V2 = 0.021, V3 = 0.028,
V4 = 0.012
[2]
Micro alga tramadol 80 °C NaOH None [76]
Corncob atenolol 400 °C montmorillonite S = 53.6, V = 0.027 [77]
Woodchips metoprolol 725 °C None S = 3.72, V = 2.83 [78]
Cotton cloth waste clofibric acid 600 °C H3PO4 S = 1175, V = 0.62 [79]
Alligator weed metformin 450 °C H2O2 S = 78.4, V = 0.132 [9]
Vineyard wood furosemide 650 °C none S = 387, V = 0.068 [81]
Hysterophorus weed ranitidine hydrochloride 500 °C NaOH S = 308, V = 0.384 [82]
Bovine bone caffeine 650 °C AlCl3·6H2O + MgCl2·6H2O + NaOH S = 46.3, V = 0.12 [83]
Pomelo peel carbamazepine 600–900 °C KOH S = 904.1, V = 0.506 [84]
4

structure and rich carbon content. Biochar is mainly composed of aro-
matic compounds with abundant tunable surface chemical groups such
as eCOOH, eNH2, eC]O, eOH etc. [104]. Previous studies have
shown that the physicochemical properties of biochars such as specific
surface area, porosity, total pore volume, and surface chemicals are
dependent on the pyrolysis conditions, including heating rate, pyrolysis
temperature, and residence time [91,105]. In most cases of biochar
production, the higher heating rate and temperature, the less residence
time contribute to the lower biochar yield, higher surface area and
larger pore volume, etc. [106,107]. Higher temperatures also increase
the ash and fixed carbon content, reducing the amount of volatile
matter.
3.2.2. Hydrothermal carbonization
Hydrothermal carbonization is a method for producing biochar from
an aqueous solution, which is conducted in an autoclave at high tem-
perature (180–350 °C) for several hours. Compared with pyrolysis,
hydrothermal carbonization is relatively mild and combines with steam
to produce biochar with many chemical functional groups [108]. Pre-
vious studies showed that parameters such as hydrothermal tempera-
ture, residence time, steam pressure and biomass-water ratio have great
influence on the physicochemical properties of the biochars [109,110].
The high temperature reduces the biochar yield, but increases the
specific surface area and total pore volume, making the surface rich in
chemicals and showing good adsorption capacity for pharmaceutical
pollutants [111]. The formation mechanism of the biochar includes
hydrolysis, aromatization, dehydration, decarboxylation and con-
densation. In terms of composition, the main components of biochar are
aliphatic rather than aromatic compounds [112,113]. Although in-
dustrial activated carbon has rich chemical groups, it has a lower spe-
cific surface area, a lower porosity and poorer stability than biochar
[114,115]. Since hydrothermal carbonization takes a long time, it
consumes more energy than pyrolysis, where the energy consumption is
mainly influenced by moisture content of biomass and biomass-water
ratio [116]. Therefore, researchers are more inclined to convert lig-
nocellulosic biomass into biochar through pyrolysis rather than hy-
drothermal carbonization [117].
3.2.3. Microwave-assisted carbonization
Compared with the conventional pyrolysis method, the biochar
produced by microwave assisted carbonization has better quality. Fig. 5
shows the difference between conventional and microwave heating
methods. In a microwave assisted carbonization process, the heating
rate and radiation power are the main factors affecting the performance
and yield of biochar. Lower temperature, higher homogenity and op-
timized heating conditions are conducive to the formation of biochar
with high quality. Many previous studies have reported the preparation
of biochar with higher surface area and larger pore size by microwave-
assisted carbonization [118–120]. The advantage of microwave-as-
sisted carbonization is that it heats the biomass more evenly with the
internal and volumetric effects of microwave, promoting the internal
reactions of biomass and saves more energy. In a microwave assisted
carbonization, electromagnetic radiation induces the polar components
of the biomass to rotate and generate thermal energy, which is known
as electric heating. This increases the local temperature and produces
gases, thereby enhancing the formation of pores in biochar [121].
Some acids, alkalis and other chemical agents were studied for their
contributions in the enhancement of the biochar properties during
microwave assisted carbonization [122]. The results showed that mi-
crowave heating and chemical additives have the synergistic effects for
improving the quality of biochar. Microwave assisted carbonization of
corn stem was used to prepare the biochar, and the pyrolysis tem-
peratures and residence time were considered as the key parameters
affecting the quality of biochar [123]. The surface carbonyl content of
the obtained biochar ranges from 0.27 to 1.70 mmol g−1
, which was
significantly affected by pyrolysis temperature and residence time. The
microwave-assisted biochar has a surface area of 45 m2
g−1
, which is
larger than the biochar prepared by conventional pyrolysis [124].
Noraini et al. obtained high magnetic biochar with a yield of 69% by
optimizing key parameters such as microwave power, heating time,
ratio of iron oxide to bagasse [125].
3.3. Activation
3.3.1. Physical activation
Physical activation is essential for enhancing the physicochemical
properties and adsorption capacity. For the physical activation of lig-
nocellulosic biomass, carbonization is first carried out in a limited
Fig. 4. The 15 most commonly used biomass feedstocks for biochar preparation by pyrolysis.
Fig. 5. Difference between conventional and microwave heating methods.
5

oxygen atmosphere, and then the remaining carbon substances are
pyrolyzed again under a controlled atmosphere, such as carbon dioxide
and steam, at high temperature. The flow diagram for biochar pre-
paration, including pyrolysis, activation and modification is shown in
Fig. 6. During pyrolysis, it is a common practice to use a controlled
atmosphere such as air, steam, or carbon dioxide to better develop the
internal pores of the material. The effect of physical activation on the
specific surface area, total pore volume, average pore size and surface
groups of biochar depends on the activation process parameters, in-
cluding activation temperature, time and activation atmosphere [126].
Ghouma et al. prepared activated carbons from olive stones by the
physical activation with steam at 750 °C, and the activated carbon
shows a high and heterogeneous macroporosity on the surface [127]. In
addition to increasing surface area and total volume, physical activa-
tion with steam also promotes the formation of oxygen-containing
groups on the surface of biochar [128]. Zheng et al. used crofton weed
as a feedstock to prepare activated carbon via microwave carbonization
with CO2 activation [129], and found that a high-quality biochar can be
obtained by modifying the CO2 flow rate, activation temperature and
residence time. The BET surface area, total pore volume and average
pore diameter can reach 1036 m2
g−1
, 0.71 mL g−1
and 2.75 nm re-
spectively. Steam activation of coffee-derived activated carbon makes
surface area and total pore volume much larger than biochar prepared
by conventional pyrolysis [130]. Silvestre et al. reported the activation
of biochars prepared from peach stones with CO2, and the synthesized
activated carbons exhibited a well-defined hierarchical porous structure
with specific surface area about 1500 m2
g−1
and total volume of
0.63 cm3
g−1
[131]. Rey et al. prepared glucose-derived carbon by
hydrothermal carbonization with CO2 activation, and found that the
activation with CO2 of the carbons produced a fivefold increase of the
Fig. 6. Flow diagram for biochar preparation, activation and modification.
6

surface area [132].
It can be seen that physical activation has a greater impact on
surface area, providing more active sites for pollutant adsorption.
Although the mechanism of physical activation is unclear, Foo et al.
proposed the overall reaction that may occur under steam physical
activation [133] as follows:
+ → +C H O CO H2 2 (1)
One possible mechanism for above reaction is shown below:
+ ⟷ +H O C C(O) H2 2 (2)
→ +C(O) CO C (3)
Another underlying mechanism is hydrogen inhibition model:
+ → +C H O H C(O)2 2 (4)
→ +C(O) CO C (5)
+ ⟷H C C(H )2 2 (6)
where C is active site of heteroatoms-free carbon, C(O) and C(H2) re-
present carbon active sites adsorbed by oxygen-containing groups and
hydrogen respectively.
In both mechanisms, the adsorption and desorption of moistures
occurs spontaneously on the active sites of biochar surface under steam
activation to produce CO and H2 based on the above two mechanisms
[134]. Therefore, physical activation can enrich the surface chemicals
and improve surface area, total pore volume and microporosity by
forming oxygen and hydrogen-containing groups.
3.3.2. Chemical activation
Chemical activation refers to the treatment of lignocellulosic bio-
mass with chemical agents such as acids, alkalis and metal compounds.
The most commonly used acids and alkalis are H3PO4, HCl, HNO3,
H2SO4, KOH and NaOH. Other activators such as ZnCl2, H2O2
[135,136], K2CO3 [137], ZnCl2 [138], CaCl2 [139], and others salts
[140] have been also used. In terms of chemical activation, the ratio of
activator to lignocellulosic biomass, soaking temperature, treatment
time and homogeneous mixing should be strictly controlled before
pyrolysis, and the resultant biochars have excellent performance. After
chemical treatment of lignocellulosic biomass, the activator and bio-
mass mixtures are pyrolysed at temperatures ranging from 400 to
1000 °C under a controlled atmosphere, such as limited oxygen, ni-
trogen, argon, carbon dioxide, and steam. The final biochar product
with chemical activation is obtained by washing and removing excess
chemicals.
In a chemical activation, activators can increase the porosity and
enrich its surface chemical groups due to the dehydration and de-
gradation of the lignocellulosic biomass, especially when highly alka-
line activators are used. It is worth noting that the ratio of activator to
lignocellulosic biomass has great influence on pore distribution, surface
chemistry and specific surface area [141,142]. KOH is one of the most
studied activators, and the mechanism of KOH activation to improve
porosity is proposed as follows [143,144]:
→ +2KOH K O H O2 2 (7)
+ → +C H O H CO2 2 (8)
+ → +CO H O H CO2 2 2 (9)
+ →K O CO K CO2 2 2 3 (10)
+ → +K O H 2K H O2 2 2 (11)
+ → +K O C 2K CO2 (12)
The overall mechanism consists of six reactions that produce high
porosity by producing gases at different active sites. A previous study
used KOH to prepare high-porosity activated carbon from lig-
nocellulosic biomass, such as papermaking black liquor [145]. The
study showed that chemical activation after pyrolysis or carbonization
is essential for obtaining a high surface area (2943 m2
g−1
).
Other activators that have been used in biomass-derived carbon
production are H2SO4, H3PO4 and ZnCl2. Activated carbons were pre-
pared from organic sewage sludge with chemical activators, including
H2SO4, H3PO4 and ZnCl2, and results showed that the surface area of
final biochars increased from 137 to 408, 289, 555 m2
g−1
respectively
[146]. With the increase of activation temperature and increase of ac-
tivator/biomass ratio, the porosity in carbon structure increases,
leading to a higher mesoporosity. The biomass of walnut shell was
chemically activated by ZnCl2 and transformed into porous carbon
material [147]. It was found that the ratio of activator to biomass had
significant effects on the biochar properties, including surface area,
pore volume and average pore size. When the mass ratio of activator to
lignin was 1:1, the carbon with the largest specific surface area of
803 m2
g−1
and pore volume of 0.8 cm3
g−1
was obtained. Activation
of sawdust biochar with ZnCl2 can increase the yield of activated bio-
char, since the activator inside biochars can dehydrate the already
converted components, such as cellulose, hemicellulose, and lignin,
during thermal treatment [148]. The activation time also influences the
surface area of activated carbons, and the optimum time for oak cups
pulp activation with H3PO4 and ZnCl2 was found to be 4 h [149]. The
growth and enlargement of pores can be promoted by the increase of
relative amount of activator and activation temperature, which results
in formation of microporous biochars, suggesting an application for
adsorption [150]. In order to absorb larger organic molecules, more
mesopores need to be introduced into the material. The ZnCl2 activator
has demonstrated its potential on several occasions to provide activated
carbons with high proportion of mesopores from agricultural biomasses
including sugarcane bagasse, sunflower seed shells, artichoke stems and
herb residues [151]. Therefore, it is beneficial to use ZnCl2 to prepare
activated carbon with wider pore size from biomass, which can absorb a
wider range of molecules.
3.4. Modification
3.4.1. Atoms-doped modification
In addition to activation, chemical modification is also necessary to
improve the adsorption capacity of biochars and the most commonly
used modification method is atoms doping, which introduces chemical
functional groups that contain oxygen, nitrogen or sulfur atoms on the
surface of biochar. In a chemical modification, the doping agents react
with the lignocellulosics biomass, especially with aromatic moieties
[152]. It has been reported that the surface functionalization of hybrid
atoms such as nitrogen, sulfur, boron and oxygen can greatly affect the
surface chemistry of biochars, thus improving the adsorption perfor-
mance for some pharmaceutical pollutants [153,154].
Nitrogenous groups such as amines and triazoles have high affinity
for metal ions and pharmaceuticals, providing hydrogen bonds sites
that interact with contaminants. The commonly used organic and in-
organic nitrogenous substances are urea, melamine, aniline and am-
monium hydroxide respectively. Ma et al prepared nitrogen-doped
porous carbon from potato waste with melamine, which exhibited ex-
traordinary porous structure and excellent electrochemical capacitance
[154]. The nitrogen content of biochar increased from 3.9% to 17.4%
by impregnating coffee grounds with aniline and melamine [155],
while impregnation of corn with methyl diethanolamine increased the
nitrogen content of biochar from 1.46% to 7.20% with NeH,
eCOeNHe, CeN being introduced [156]. Moreover, N-doped biochar
can also be prepared by using ammonium hydroxide in the ball mill
treatment of biochar. With pyridinic-N and pyrrolic-N being success-
fully introduced into biochar, the nitrogen content of biochar increased
from 0.22% to 1.68% [157]. Microwave assisted carbonization can also
be used to introduce pyrrolic-N and pyridinic-N into biochar with am-
monium chloride and ammonium acetate as doping agents [124]. Wang
et al found that the activated biochars prepared by microwave assisted
7

carbonization had more defective carbon and oxygen atoms, which
offered more active sites for the introduction of N-containing chemical
groups [158].
Similar to the nitrogenous groups, oxygen-containing groups such as
hydroxyl and ethers can form hydrogen bonds with pharmaceutical
contaminants by donating an electron lone pair. Therefore, the ad-
sorption capacity of pharmaceuticals onto biochars was highly en-
hanced by the modification of biochars with oxygen-containing groups
[159]. In the work of Quintana et al. [160], different biomass residues
were oxidized by the acid treatment and their composition and func-
tional group, such as total acidity, hydroxyl, and carbonyl were char-
acterized. The carbonyl content of bagasse was the highest among the
tested materials, but the authors pointed out that the content of func-
tional group could not be determined accurately due to the great in-
terference of ash and carbohydrate. Liang et al. [161] prepared bio-
mass-based ion exchange resin for heavy metal adsorption by the
condensation of sodium lignosulfate and glucose in dilute H2SO4.
During the synthesis process, the desulfonated lignosulfonate was
functionalized by 5-hydroxymethylfurfural and levulinic acid, resulting
in the formation of three types of functional sites, such as carboxyl,
lactones, and phenolic groups. Parajuli et al. [159] immobilized phenol,
catechol, and pyrogallol onto wood biomass to form lignophenol, lig-
nocatechol, and lignopyrogallol gel adsorbents, respectively.
Sulfur-containing functional groups, such as thiols, dithiocarbamate
and xanthate, are also frequently modified with metal adsorbents due to
their strong affinity for heavy metals (e.g., Cu (II), Cd (II), Pb (II), Hg
(II) and others) and weakly binding with light metals (e.g., K (I), Na (I),
Mg (II)) [162,163]. However, one disadvantage of sulphur-containing
biomass is that it produces harmful sulphur at the end of its useful life.
Ge et al. [164] modified an biomass with dithiocarbamate to adsorb
various divalent metal ions. The results showed that dithiocarbamate-
modified biomass contained 17.18% nitrogen and 20.90% sulphur. To
improve adsorption capacity of Hg (II), Ge et al. [165] modified the
organosolv biomass derived carbons with dithiocarbamate, which
possessed 12.9% nitrogen and 16.1% sulphur.
3.4.2. Biochar composites
In the direction of improving adsorption and separation perfor-
mance of biochars, the loading of them onto magnetic, graphene or
other metal substrates to form biochar composites has become an at-
tractive method. Zhou et al. reported the synthesis of magnetic mod-
ified biochar, which is pyrolyzed from nut shell, and found the resultant
biochar exhibited high adsorption capacity and could be easily sepa-
rated from the wastewater [166]. Chitosan is commonly used to com-
bine biochar and Fe2O3, and the composites show excellent perfor-
mance of metal ions due to abundant functional groups of chitosan
[167]. Sometimes the magnetic modified biochar not only has stronger
magnetic property, but also exhibits much greater ability to remove
metal from aqueous solution due to electrostatic interactions between
γ-Fe2O3 and pollutants [168]. A novel biochar-supported magnetic
CuZnFe2O4 composites were synthesized by a facile one-pot hydro-
thermal process, and found that this novel material had fast kinetics,
high adsorption properties, easy magnetic separation, which demon-
strated that it has potential for the removal of bisphenol A and sulfa-
methoxazole from wastewater [169]. Therefore, magnetic activated
biochar composites can be tailored to different pollution problems by
choosing the most appropriate carbonaceous matrix whose properties
are largely preserved during magnetization [170].
Compared to the original biochar, the graphene coated biochar is
more thermally stable and showed enhanced adsorption ability for or-
ganic pollutants due to the formation of π–π interactions between
pollutants and graphene sheets [171]. Du et al. have prepared a novel
Fe3O4-graphene-biochar composite, and structural and morphological
analysis exhibited that a larger surface area, greater thermal stability,
and more functional groups were present after the coating of Fe3O4
nanoparticles, which showed high adsorption capacity of organic
pollutants [172]. Inyang et al. reported the synthesis of multi-walled
carbon nanotube coated biochars, and found that the addition of carbon
nanotubes significantly enhanced the physiochemical properties of the
biochars such as thermal stability, surface areas and pore volumes,
which promoted the adsorption of methylene blue due to enhanced
electrostatic attraction [173]. Xie et al. studied the effects of graphene
amount on the structure, specific surface area, and adsorption capacity
of sulfamthiaze onto graphene coated biochars, and the optimum
amount for graphene loading was 1% with the highest adsorption
amount of 820.27 mg g−1
due to porous and sheet structure [174].
Moreover, some biomass feedstocks are soaked in montmorillonite
or kaolinite suspension before pyrolysis, and the resultant novel biochar
with clay particles distributed on carbon surface makes them suitable
low cost adsorbent with high adsorption ability for pharmaceuticals
[175]. Biochar-Mg composites were prepared by Yao et al. [176], and
the results show that both MgO and Mg(OH)2 were contained within
the matrix, which could be used as a highly efficient adsorbent to re-
move phosphorous contaminants from aqueous solutions. Magnetic
activated biochar nanocomposites derived from wakame were prepared
with nickel via one-step pyrolysis, and it was found that the composites
had a high adsorption capacity for methylene blue [177]. Other metals
like Mn can also be loaded onto biocar surface to improve adsorption
capacity for Pb and enhance adsorption kinetics [178]. Song et al.
proposed a novel engineered adsorbent-MnOx-loaded biochar whose
unique nanostructure provided much stronger adsorption capacity for
Cu2+
than the original biochar due to the formation of surface com-
plexes with MnOx and O-containing groups [179]. Tan et al. reported
the one-pot synthesis of carbon supported calcined-Mg/Al layered
double hydroxides for antibiotics removal by slow pyrolysis of biomass
waste, and it was found that the novel biochar composites exhibited
more than 2 times higher adsorption capacity than that of pristine
biochar due to the interactions such as π–π interaction and hydrogen
bond [180]. Apart from antibiotics, loading Mg/Al layered double hy-
droxides onto biochar can also enhance adsorption ability for diclo-
fenac sodium and caffeine, which is a multi-molecular process, occur-
ring by an angled position with respect to the adsorbent surface
[83,181].
4. Adsorption kinetics and isotherms
Adsorption kinetics is the measure of the adsorption uptake (qt)
with respect to time (t) at a constant initial concentration (C0) and is
employed to measure the diffusion of adsorbate into the pores.
Adsorption kinetics depends on temperature, concentration, interaction
energy between adsorbent and adsorbate as well as the adsorbent
properties such as pore size, surface area and surface chemistry.
Adsorption isotherm is a curve relating the equilibrium amount (qe)
adsorbed onto the adsorbent and the equilibrium concentration (Ce) of
the adsorbate in the solution at a given temperature. The relationship
between qt and t, or qe and Ce can normally be correlated to different
kinetics and isothermal models respectively. Table 3 gives the com-
monly used adsorption kinetics and isothermal models, which are de-
scribed as follows.
4.1. Adsorption kinetics models
The evaluation of adsorption kinetics of pharmaceuticals micro-
pollutants onto different biochars is essential for selecting the desirable
adsorbents. Through model fitting, the adsorption rate and saturation
time can be calculated, revealing the type of adsorption process and
controlling steps [54,194]. The followings are commonly used ad-
sorption kinetics models, including pseudo-first-order, pseudo-second-
order, intra-particle diffusion, Boyd's film-diffusion and Bangham
channel diffusion models.
8

4.1.1. Pesudo-first-order model
Lagergren introduced the pseudo-first-order model (PFO) in 1898 to
describe the adsorption kinetics of organic compounds onto biochar
[182]. In this model, it is assumed that the adsorption rate is dependent
on the deviation between instant adsorption amount and saturated
adsorption amount [195]. Its mathematical expression is given below:
− = −q q q k tln( ) lne t e 1 (13)
where qt (mg g−1
) and qe (mg g−1
) refer to instant adsorption amount
and saturated adsorption amount, respectively. k1 (min−1
) is rate
constant of PFO model. t (min) represents time.
4.1.2. Pseudo-second-order model
Ho and McKay proposed the pseudo-second-order model (PSO) in
1998 to correlate the adsorption data of metal ions onto peat vs time
[184]. It supposes that chemisorption due to covalent bonds and ion
exchange readily take place during adsorption process [196]. The ex-
pression can be written as follows:
= +t q k q t q/ 1/( ) /t 2 e
2
e (14)
in which qt (mg g−1
) and qe (mg g−1
) refer to instant adsorption
amount and saturated adsorption amount, respectively, k2 (g mg−1
min−1
) is the rate constant of PSO model and t (min) represents time.
4.1.3. Intraparticle diffusion model
Weber and Morris [186] introduced intraparticle diffusion me-
chanism to characterize the adsorption process and proposed the in-
traparticle diffusion model (IPD) to analyze kinetic data [197], which is
written as follows:
= +q k t ct i
1/2
(15)
where qt (mg g−1
) refers to instant adsorption amount, ki (mg g−1
min−1/2
) is the rate constant of IPD model and c (mg g−1
) is degree of
adsorption.
4.1.4. Pore diffusion model
Bangham et al. [188] proposed the pore diffusion model (PD) to
analyze kinetics data based on the assumption that pore diffusion is the
only rate controlling step during adsorption. It can be written as fol-
lows:
=q k tlog logt (16)
where qt (mg g−1
) refers to instant adsorption amount. k (mg g−1
min−1/2
) is rate constant of PD model.
4.1.5. Elovich model
Roginsky and Zeldovich [190] proposed the Elovich model to
characterize adsorption kinetics data of carbon monoxide onto MnO2. It
is more suitable for describing chemisorption on adsorbents with
complicated structures [198]. It is written as follows:
= +q
β
αβt
1
ln(1 )t
(17)
where qt (mg g−1
) refers to instant adsorption amount. α (mg g−1
min−1
) is the primary rate of adsorption. β represents desorption
parameter, and it is used to characterize activation energy and degree
of chemisorption.
4.1.6. Boyd's film-diffusion model
Boyd [192] suggested that the boundary layer of adsorbent is the
main factor affecting the adsorption process and proposed film-diffu-
sion model (FD) to describe adsorption kinetic data. The following gives
the expression of FD model.
= ⎧
⎨
⎩
− − − < <
− − − < <
Bt
π π F t π πF t F t
F t F t
2 ( )/3 2 (1 ( )/3) 0 ( ) 0.85
0.4977 ln(1 ( ))0.86 ( ) 1
2 1/2
(18)
where B is a parameter about adsorbent characteristics. F(t) is defined
as qt/qe. t is contact time.
4.2. Adsorption isotherm models
Adsorption isotherm models describe the relationship between the
equilibrium adsorption amount and adsorption mechanism [199]. The
adsorption isotherms and their change trends suggest the type of in-
teraction between adsorbent and adsorbate [200]. The information
about pore structure as well as specific surface area can also be ob-
tained from the adsorption isotherms. As a consequence, investigation
into adsorption isotherms and building models are very important for
understanding the adsorption process. The most applied models are
Langmuir, Freundlich, Redlich-Peterson, Temkin and Dubinin-Ra-
dushkevich models, which are discussed below.
4.2.1. Langmuir model
The Langmuir model is widely used to describe monolayer adsorp-
tion and it assumes that adsorbent surface is homogeneous, and each
active site has identical binding ability to adsorbate [183]. It also as-
sumes no interaction between adsorbate molecules on adjacent sites,
and each site can hold at most one molecule of adsorbate to form
monolayer structure. Eq (19) shows the mathematical expression of
Langmuir model.
= +C q K q C q/ 1/( ) /e e L L e L (19)
where qe (mg g−1
) and qL (mg g−1
) represent adsorption amount at
equilibrium and predicted adsorbed amount by Langmuir model, re-
spectively, Ce (mg L−1
) refers to the adsorbate concentration at equi-
librium and KL (L mg−1
) is the Langmuir affinity parameter.
4.2.2. Freundlich model
The Freundlich model [185] is one empirical model that is applic-
able for the characterization of non-ideal and multilayer adsorption on
heterogeneous surface of adsorbent. It supposes that abundant active
sites are initially utilized and the activity is reduced with more site
Table 3
Commonly used kinetics and isotherm models for pharmaceutical adsorption.
Adsorption kinetics models Equations Adsorption isotherm models Equations Ref.
Pseudo-first-order model − = −q q q k tln( ) lne t e 1 Langmuir model = +C q K q C q/ 1/( ) /e e L L e L [182,183]
Pseudo-second-order model = +t q k q t q/ 1/( ) /t 2 e
2
e
Freundlich model = +q K n Cln ln (1/ ) lne F e [184,185]
Intraparticle diffusion model = +q k t ct i
1/2 Dubinin-Radushkevich model = −q q Bεln lne D
2 [186,187]
Pore diffusion model =q k tlog logt Redlich-Peterson model − = +K a β Cln( 1) ln( ) ln( )
C
qR
e
e
R e
[188,189]
Elovich model = +q αβtln(1 )
βt
1 Temkin model = +q K Cln ln
RT
b
RT
be T e
[190,191]
Boyd's film-diffusion model
= ⎧
⎨⎩
− − − < <
− − − < <
Bt
π π F t π πF t F t
F t F t
2 ( )/3 2 (1 ( )/3) 0 ( ) 0.85
0.4977 ln(1 ( )) 0.86 ( ) 1
2 1/2 Sips model
=
+
qe
qsKsCe
m
KSCe
m
1/
1 1/
[192,193]
9

utilization [201]. Eq (20) represents the expression of Freundlich
model.
= +q K n Cln ln (1/ ) lne F e (20)
where qe (mg g−1
) is adsorption amount at equilibrium, Ce (mg L−1
)
refers to adsorbate concentration at equilibrium, KF ((mg g−1
) (L
mg−1
)1/n
) and n are the Freundlich parameters relating the adsorbed
quantity and the adsorption strength, respectively.
4.2.3. Dubinin-Radushkevich model
Dubinin-Radushkevich model is constructed based on the Polayi
theory. It assumes that the adsorption mechanism in micropores is pore-
filling rather than surface coverage with monolayer or multilayer
[187]. It generally applies well to adsorption systems involving only
van der Waals forces. Eq (21) gives the expression as follows:
= −q q Bεln lne D
2
(21)
in which qe (mg g−1
) and qD (mg g−1
) represent adsorption amount at
equilibrium and predicted adsorbed amount by Dubinin-Radushkevich
model, respectively, B (mol2
kJ−2
) is a parameter about mean free
energy of adsorption, ε refers to Polayi potential (ε = RT(1 + 1/Ce))
and Ce (mg L−1
) refers to adsorbate concentration at equilibrium.
4.2.4. Redlich-Peterson model
The Redlich-Peterson model combines the Langmuir and Freundlich
model and can be used to describe more complicated adsorption process
that involves both homogeneous and heterogeneous adsorption [189].
It is written as follows:
− = +K
C
q
a β Cln( 1) ln( ) ln( )R
e
e
R e
(22)
in which qe (mg g−1
) is adsorption amount at equilibrium, Ce (mg L−1
)
refers to adsorbate concentration at equilibrium, KR (L g−1
) and aR (L
mg−1
)β
are the R-P parameters and β has the value from 0 to 1. Plotting
the left-hand side of Eq (22) against ln Ce for obtaining the isotherm
constants is not applicable because of the three unknowns, aR, KR and β.
Therefore, a maximization procedure of the coefficient of correlation
was adopted for solving Eq (22) by minimizing the distance between
experimental data points and theoretical model predictions.
4.2.5. Temkin model
The Temkin isotherm model assumes that the adsorption heat of all
molecules decreases linearly with the increase in coverage of the ad-
sorbent surface, and that adsorption is characterized by a uniform
distribution of binding energies, up to a maximum binding energy
[191]. The Temkin isotherm can be described by Equation (23).
= +q
RT
b
K
RT
b
Cln lne T e
(23)
where KT (L mol−1
) is the equilibrium binding constant corresponding
to the maximum binding energy, b refers to the adsorption heat. R
(8.314 J K−1
mol−1
) is the universal gas constant and T (K) is the
temperature. For Eq (23), plotting qe vs ln Ce results in a straight line of
slope RT/b and intercept (RT/b) ln KT.
4.2.6. Sips model
The Sips model is a combination of the Langmuir and Freundlich
isotherms and it is given the following general expression [193]:
=
+
q
q K C
K C1
e
s s e
m
S e
m
1/
1/
(24)
where qS (mg g−1
) is the Sips maximum adsorption capacity, KS(l/
mg)1/m
is Sips constant related to energy of adsorption and parameter
m could be regarded as the Sips parameter characterizing the system
heterogeneity.
4.3. Model application in pharmaceuticals adsorption
Table 4 summarizes the previous studies that reported the adsorp-
tion kinetics and isotherms of pharmaceuticals onto different biochars.
The best fitting correlation model for pharmaceutical adsorption is
dependent on both the type of biomass feedstock and pharmaceutical.
Using municipal solid waste as an example, the biochar prepared from
municipal solid waste is used to adsorb tetracycline and ciprofloxacin,
and the results showed that the Freundlich model and Sips model re-
present the best correlation isotherm models for tetracycline and ci-
profloxacin, respectively, while the pseudo second order model is the
best correlation model for describing the adsorption kinetics of both
compounds [52,55]. Table 4 shows the most suitable models for cor-
relating adsorption isotherm and kinetics are the Freundlich model and
pseudo second order model respectively. It means that the adsorption
process of pharmaceuticals is mostly characterized by multilayered
adsorption on the heterogeneous surfaces of the biochars.
Jang et al. [6] studied the adsorption kinetics and isotherm of
Table 4
The kinetics and isotherm models with the best correlation for pharmaceuticals adsorption.
Biomass residues Pharmaceuticals Isotherm model Kinetics model Ref.
Municipal solid waste tetracycline Freundlich pseudo second Order [52]
Giant reed amoxicillin Sips model pseudo-first order model [53]
Municipal solid waste ciprofloxacin Sips model pseudo-second-order [55]
Cassava waste oxytetracycline Langmuir pseudo-second-order [56]
Paper sludge enrofloxacin Langmuir Pseudo-second order [57]
Cauliflowers roots chlortetracycline Langmuir pseudo-second-order [58]
Garden waste trimethoprim Langmuir parameters pseudo-second-order [99]
Palm tylosin Freundlich model pseudo-second-order [100]
Manure lincomycin Langmuir intraparticle diffusion model [62]
Seed shell naproxen Freundlich pseudo-second-order [66]
Wheat straw ketoprofen Freundlich model pseudo-second-order [101]
Moringa seed diclofenac Sips model pseudo-second-order [68]
Fruit shell ibuprofen Langmuir isotherm pseudo-second-order [69]
Paper sludge piroxicam Freundlich model pseudo-second-order [5]
Paper sludge oxazepam Langmuir isotherm pseudo-second-order [72]
Micro alga tramadol Intra-particle diffusion model pseudo-second-order [76]
Corncob atenolol Freundlich model pseudo-second-order [77]
Cotton cloth waste clofibric acid Freundlich model pseudo-second-order [79]
Alligator weed metformin Freundlich model pseudo-second-order [9]
Hysterophorus weed ranitidine hydrochloride Langmuir isotherm pseudo-second-order [82]
Bovine bone caffeine Redlich–Peterson pseudo-second-order [83]
Pomelo peel carbamazepine Langmuir isotherm pseudo-second-order [84]
10

tetracycline on pinus taeda-derived biochar, and found that the best
model to describe the kinetic data was the Elovich model. The ad-
sorption process is depicted in Fig. 7, and kinetics typically includes
four steps: (1) bulk transport, (2) film transport, (3) intra-particle
transport and (4) surface reaction, where step (3) is the major limitation
for adsorption of tetracycline. For isotherm, it is the Freundlich model
rather than the Langmuir model that showed the best correlation result,
and this is different from other cases of tetracycline adsorption
[202,203]. Adsorption kinetics is often controlled simultaneously by
film and intra-particle diffusion. The well-fit pseudo second-order
model implies that the rate limiting step is chemical adsorption invol-
ving electronic forces [204]. The Langmuir model shows relatively
higher regression coefficient (R2
) for sulfonamide antibiotics adsorption
onto biochar, indicating that the overall adsorption process followed
the reversible monolayer sorption mechanism [49]. Intra-particle dif-
fusion model is widely used to predict the rate controlling step, which is
mainly dependent on either surface or pore diffusion. The fitting curves
generally include two parts: the first sharper region is the immediate
adsorption or external surface adsorption and the second region is the
gradual adsorption stage where the intraparticle diffusion is the rate
limiting. In some cases, the third region exists, which is the final bal-
ance stage where intra-particle diffusion starts to slow down due to
extremely low adsorbate concentrations in the solutions [76]. The Sips
model correlates the equilibrium data of amoxicillin adsorption onto
activated biochars with high coefficient R2
of 0.999, suggesting the
heterogeneous surface adsorption [53].
5. Adsorption mechanism
The analysis of adsorption mechanism gives insights into the ad-
sorption performance of different adsorbents to various adsorbates.
Herein, the effects of different interactions between pharmaceutical
micropollutants and biochars during adsorption are described in detail.
It is well known that the adsorption of pharmaceutical micro-con-
taminants on the surface of biochar is because of surface energy. The
atoms or chemical groups on the surface of the biochar can attract the
adsorbate to reduce its surface energy. The driving force of adsorption
is the sum of many interactions, which contributes to the total free
energy of the adsorption process [205]. There are hydrogen bond,
electrostatic attraction, π-π interaction and dipole–dipole interaction
between adsorbent and adsorbate [206,207]. Sometimes van der Waals
forces as well as hydrophobic interactions are also used to explain the
adsorption mechanism of organic molecules onto biochar materials.
Van der Waals forces refer to attraction of intermolecular forces be-
tween molecules. There are two kinds of Van der Waals forces: weak
London Dispersion Forces and stronger dipole–dipole forces. Hydro-
phobic interactions between non-polar groups are also considered as a
binding mechanism of pharmaceutical contaminants with biochars
[208]. However, hydrophobic interaction is a non-specific interaction
that is associated with decreasing entropy related to chemicals leaving
water as opposed to being attracted to adsorbents rather than an in-
termolecular force. Fig. 8 summarizes the potential adsorption me-
chanisms of aqueous pharmaceuticals onto biochar.
Fig. 7. Diagram for adsorption process of tetracycline onto biochar [6].
Fig. 8. Diagram for adsorption mechanism of pharmaceuticals onto biochar.
11

5.1. Hydrogen bond
Hydrogen bond is the intermolecular interaction between a hy-
drogen donor and acceptor. The hydrogen donor is usually bonded to
hydrogen acceptor atoms, which consists mainly of nitrogen (N),
oxygen (O) and fluorine (F) within functional groups such as eCOOH,
eNH2, eOH and electron-rich π-systems [209,210]. In the adsorption
of pharmaceuticals onto biochars, the hydrogen bonds are formed due
to the presence of hydrogen donor/acceptor chemical groups on both
pharmaceuticals and biochars. A previous study showed that the high
content of hydroxyl groups in sludge-derived biochar contributing to a
high adsorption capacity of atrazine, which is rich in amino groups
[211]. Charge-assisted hydrogen bonds include positively charged,
negatively charged and doubly-charged hydrogen bonds, which are
stronger than conventional hydrogen bonds [212]. Among them, ne-
gatively charge-assisted hydrogen bond frequently occurs in the ad-
sorption of ionizable pharmaceuticals, whose ionization degree is
highly dependent on the pH value of the solution and its pKa value
[213]. Gilli et al. [212] found that the negative charge assisted hy-
drogen bonds are easily formed between one hydrogen donor (alkali)/
acceptor (acid) pairs such as a carboxyl group and its conjugate acid
([R-COO⋯H⋯OOC-R]). The closer the pKa values of the acid/alkali
groups in either a pharmaceutical or biochar, the stronger the negative
charge assisted hydrogen bonds [214]. Therefore, the strongest nega-
tive charge assisted hydrogen bonds are generally observed between an
acid and its conjugated alkali. Teixido et al. [215] studied the adsorp-
tion mechanism of sulfamethoxazole onto biochars, and hypothesized
the formation of negative charge assisted hydrogen bond between sul-
famethoxazole and biochar in the pH range of 3–7 due to the similar
pKa values of sulfamethoxazole and biochar [216]. In some cases such
as the adsorption of pharmaceuticals onto oxygen-rich biochar, charge
assisted hydrogen bonds play a more important role than π-π interac-
tion and hydrophobic interaction. Xiao and Pignatello [214] prepared
the desirable adsorbent with acid modification to adsorb several or-
ganic acids such as 2,4-dichlorophenoxyacetic acid and 4-toluic acid.
The results showed that the biochar adsorbed 4-toluic acid five times
more than 2,4-dichlorophenoxyacetic acid by forming charge assisted
hydrogen bond. Ni et al. [206] studied the adsorption process of alle-
lopathic aromatic acid moieties onto biochar, and found that the charge
assisted hydrogen bond was formed between carboxyl acid groups of
biochars and the allelopathic aromatic acid moieties due to the similar
pKa values.
5.2. Electrostatic interaction
Electrostatic interaction includes attraction and repulsion forces
between adsorbate and adsorbent that have electric charges.
Electrostatic interaction between ionizable functional groups of phar-
maceuticals and chemical functional groups on the surface of biochar is
frequently observed in the special case of pharmaceutical adsorption in
wastewater [29]. Solution conditions such as pH and ionic strength are
important factors affecting the degree of dissociation of these functional
groups on pharmaceuticals [217]. According to pKa value of one
pharmaceutical and isoelectric point of biochar, the surface charge of
pharmaceutical and biochar can be easily controlled by adjusting the
pH of the solution. For example, methyl violet dye (pKa = 8.64) carries
a positive charge when the pH is below 9, while biochar carries a
surface negative charge in a weak alkaline solution. The oxygen-con-
taining groups on the surface of straw-derived biochar and dissociation
of methyl violet molecules contribute to electrostatic attraction be-
tween them at pH ranging from 7 to 8 [218].
5.3. π-π interaction
π-π interaction is one form of dipole interaction that involves π-
systems and it is weaker than hydrogen bond. The electron-rich π-
system can interact with metal ions, neutral organic molecule or an-
other π-system to form π-π interaction. π-π interactions are polar and
the most common types of aromatic-aromatic interactions, and both of
them involve interactions between aromatic groups and other mole-
cules.
Previous studies showed that due to the destruction of aliphatic
structure, the aromaticity of biochar usually increases with the pyr-
olysis temperature [219,220]. Zhou et al. [29] studied the adsorption
mechanism of pharmaceuticals onto core-brush shaped aromatic rings-
functionalized chitosan magnetic composite particles, and found that
the π-π interaction between aromatic rings contribute to the enhanced
adsorption performance of norfloxacin and diclofenac sodium. Wu et al
[101] used biochars to adsorb ionic and neutral specifics of pharma-
ceuticals (ketoprofen, atenolol, carbamazepine), and π-π interaction
was assumed to be the dominant adsorption mechanism considering the
aromatic-likely structure in biochars and pharmaceuticals. Tan et al.
[180] demonstrated the adsorption mechanism of tetracycline onto
bagasse biomass-derived biochar was π-π interaction by the peak mi-
gration of C]C stretching vibration. The presence of other groups such
as chlorine atoms and carboxylic acid group in pharmaceutical micro-
pollutants like diclofenac could help reduce the π-electron density on its
phenyl ring and facilitate π–π interactions with biochar surface [221].
Mahmudov et al. [222] found that a graphite-like activated carbon
with an aromatic sheet structure favored perchlorate adsorption due to
π-system facilitation. Cation attraction is also favorable due to the
highly electronegative nature of π-systems. π-π interactions occur be-
tween oppositely polarized quadrapoles of arene systems oriented in a
parallel-planar fashion [215]. For example, when the p-aminosulfona-
mide rings of sulfamethazine act as the π-electron acceptors, they can
be adsorbed to the aromatic sheet structures (π-electron donor) of
hardwood waste-derived biochar [215]. Opposite polarization of aro-
matic systems leads to their opposite preference for electrons. There-
fore, the most widely used conceptual model to describe π-π interaction
is the π-electron donor–acceptor model [210]. The conjugated ring
system of the grapheme subunit on the surface of biochar makes it an
electron-rich π-electron donor, which can pair electron-adsorbing sub-
stituents with organics with electron-poor ring systems such as het-
eroaromatic rings or benzene rings. Sander et al. [223] found that the
graphene units of wood biochar had different adsorption affinities for
benzene compounds, which were nitrobenzene, toluene and benzene in
descending order. This trend is due to the strongest electron-with-
drawing effect of nitro group, making nitrobenzene the most electron-
deficient of the three tested compounds. Organic compounds with high-
energy electron donor ability can also be attracted to electron-deficient
areas on biochar, such as the center of graphite units [224]. π-π in-
teractions can be characterized using charge-transfer absorbance in the
UV–visible absorption spectrum [225]. Other types of π interactions
can also be characterized by Raman, nuclear magnetic resonance
(NMR) spectroscopy and fluorescence techniques [209].
5.4. Pore filling
Pore filling is another mechanism for pollutants adsorption onto
biochars. Taking adsorption of phenol onto corn grain-derived biohcar
for example, a non-linear relationship was observed between the sur-
face area and adsorption capacity, and the adsorption capacity in-
creased with the micropore fraction, which suggests the micropore
filling of phenol during adsorption process [226]. Zeng et al. proposed
one g-MoS2 decorated biochar nanocomposites for removing tetra-
cycline hydrochloride from antibiotic-polluted aqueous solution, and
assumed that pore filling was involved in the multiple mechanisms,
since the mesopores in the range of 2–20 nm was found to be sig-
nificantly reduced after the adsorption. It is well known that the su-
perior porosity of biochars could offer more adsorption sites for phar-
maceuticals molecules, and the micropore could decrease the steric
hindrance effect, suggesting pore filling mechanism for
12

pharmaceuticals adsorption [227]. In some cases, the pore filling can
reveal the competitive effects of two or more pharmaceuticals adsorp-
tion, for example, the smaller molecular size of dimetridazole than
metronidazole renders the former to easily and quickly diffuse into the
biochar pores [228].
5.5. Other interactions
Other dipole interactions include permanent dipole interaction, di-
pole-assisted dipole interaction, and fluctuating dipole interaction.
These intermolecular forces contribute to high adsorption for ad-
sorbates containing polar functional groups such as alkyl halide, ether
and nitrile etc. [209,210]. With the increase of thermal treatment
temperature, the development of aromaticity and non-polarity on bio-
char caused by ring fusion reduces dipolar forces. Lattao et al. [229]
observed consistency in the free energy contribution of combined π-π
and dipole interaction to 1,4-dinitrobenzene adsorption on wood bio-
char over a range of fused ring sizes.
Hydrophobic interaction is a type of non-specific interaction that is
primarily driven by entropy. While its underlying basis is not fully
understood, hydrophobic interactions are widely believed to occur due
to the tendency of non-polar groups to aggregate in water to minimize
their contact with water molecules. Since this non-specific interaction is
not driven by intermolecular forces with adsorbents, it is inappropriate
to use the term “bond” for hydrophobic interactions [225,230]. The
octanol–water distribution coefficient (Kow) is an indicator of hydro-
phobicity of organic chemicals. If hydrophobic interaction is the
dominant mechanism, adsorption of nonpolar chemicals on porous
materials would be proportional to Kow values [210]. In the case of
phenanthrene adsorbing on a plant residual-derived biochar, this cor-
relation is generally not observed, implying that hydrophobic interac-
tions are not the dominant mechanism of attraction [231].
6. Conclusions and future perspectives
Over the past few decades, pharmaceuticals have become the most
concerning micropollutants in wastewater due to its long-term release
and non-degradable nature. It is very urgent to develop new cost-ef-
fective adsorbents for the removal of pharmaceuticals from wastewater.
Biochars that are generated from renewable biomass feedstocks have
been widely used as the green adsorbents for pharmaceuticals due to
their high surface area, large pore volume, well defined pore structure
and tunable surface chemicals. Biochars can be prepared from lig-
nocellulosic biomass residues under thermal treatment such as pyr-
olysis, hydrothermal carbonization and microwave assisted carboniza-
tion in an oxygen-limited atmosphere. The biochar adsorption capacity
can be enhanced through physical/chemical activation and modifica-
tion before and after the thermal treatment. Adsorption kinetics and
isotherms of pharmaceuticals onto biochars are dependent on the bio-
char properties, pharmaceuticals characteristics and solution condition.
Biochars can efficiently capture pharmaceuticals through hydrogen
bonds, π–π interactions, electrostatic interactions or pore filling me-
chanism. For the pharmaceuticals removal/recovery application, it is
necessary to create novel biomass-based adsorbents with the aim of
potential commercialization and to explore the adsorption mechanisms
in depth. Based on the comprehensive review of the published studies,
additional research on the activation and modification of biochars into
advanced pharmaceuticals adsorbents is still highly necessary in the
near future. Specific emphasis should be placed on the explanation of
adsorption mechanism, especially providing qualitative and quantita-
tive contributions of each group on the biochar surface to the adsorp-
tion mechanism.
CRediT authorship contribution statement
Jinbo Ouyang: Conceptualization, Supervision, Writing - original
draft, Funding acquisition, Investigation, Methodology. Limin Zhou:
Conceptualization, Methodology. Zhirong Liu: Project administration,
Resources. Jerry Y.Y. Heng: Writing - original draft. Wenqian Chen:
Formal analysis, Software, Data curation.
Declaration of Competing Interest
The authors declared that there is no conflict of interest.
Acknowledgements
The authors are grateful for the financial support from National
Natural Science Foundation of China (No. 21706028), and the scho-
larship from China Scholar Council (CSC) (No. 201908360189).
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