Sustainable Valorization of Biowaste for Environmental RemediationView this Special Issue
A Comprehensive Insight on Adsorption of Polyaromatic Hydrocarbons, Chemical Oxygen Demand, Pharmaceuticals, and Chemical Dyes in Wastewaters Using Biowaste Carbonaceous Adsorbents
Recent trends in adsorption of hazardous organic pollutants including Polyaromatic Hydrocarbons (PAHs), Chemical Oxygen Demand (COD), Pharmaceuticals, and Chemical Dyes in wastewater using carbonaceous materials such as activated carbon (AC) and biochar (BC) have been discussed in this paper. Utilization of biomass waste in the preparation of AC and BC has gained a lot of attention recently. This review outlines the techniques used for preparation, modification, characterization, and application of the above-mentioned materials in batch studies. The approaches towards understanding the adsorption mechanisms have also been discussed. It is observed that in the majority of the studies, high removal efficiencies were reported using biowaste adsorbents. Regarding the full potential of adsorption, varying values were obtained that are strongly influenced by the adsorbent preparation technique and adsorption method. In addition, most of the studies were concentrated on the kinetic, isotherm equilibrium, and thermodynamic aspects of adsorption, suggesting the dominant isotherm and kinetic models as Langmuir or Freundlich and pseudo-second-order models. Due to development in biosorbents, adsorption has been found to be increasingly economical. However, application of these adsorbents at commercial scale has not been adequately investigated and needs to be studied. Most of the studies have been conducted on synthetic solutions that do not completely represent the discharged effluents. This also needs attention in future studies.
Protection of the environment is of great concern and has gained a lot of attention over the years. In particular, it has been increasingly crucial to meet the demand of clean and safe water. Various types of contaminants are continuously being added into freshwater streams due to increased industrialization and urbanization. Sewage treatment plants, industrial discharges, agricultural discharges, and other anthropogenic activities are major causes of increasing water contamination. Water pollutants are broadly classified into organic and inorganic contaminants . Organic contaminants include pesticides, plasticizers, fertilizers, hydrocarbons, biphenyls, phenols, detergents, oils, greases, pharmaceuticals, and plant and animal tissues [2, 3]. These contaminants are harmful even in small concentrations. Many treatment methods such as physical (i.e., filtration, electrodialysis, flotation, and adsorption), chemical (i.e., precipitation, chemical oxidation, and electrochemical technologies), and biological (i.e., activated sludge, biological aerated filters, microbial capacitive desalination, and microalgae-based treatment) have shown the ability to remove hazardous substances from polluted water [4–7]. Adsorption is regarded as one of the most suitable techniques for wastewater treatment due to its low cost, high efficiency, less harmful secondary products, and ease of operation . However, the efficiency of the method significantly depends on the adsorbent properties, i.e., surface area, pore size, pore diameter, and functional groups, etc. Traditionally, expensive activated carbons were used as adsorbents for removal of pollutants. Currently, the development of numerous novel adsorbents particularly from biomass waste has generated a lot of interest in researchers. Many studies have reported successful removal of water pollutants using economical biowaste adsorbents, e.g., tea waste [9, 10], palm tree leaves (Lee et al., ), corn silk , oil palm fruit , teff straw , rice husk , banana peels , rape straw , and mangosteen peels . These adsorbents have gained attention recently because of the development in modification techniques, which enable the production of a large surface area and prominent physicochemical and biological properties ([19, 20]; L. Zhang et al., ). There is much interest among researchers to explore more materials and methods for the preparation of these economical biowaste adsorbents.
In this review, recent trends in the utilization of carbonaceous biowaste materials, viz., activated carbon (AC) and biochar (BC), their preparation and modification techniques, targeted pollutants, and their removal efficiencies in batch studies, have been discussed in detail for anthropogenic organic pollutant adsorption such as Polyaromatic Hydrocarbons (PAHs), Chemical Oxygen Demand (COD), Pharmaceuticals, and Chemical Dyes. The adsorption mechanism has also been discussed. Peer-reviewed articles published between 2010 and 2020 have been mainly considered. A few reviews have been published in this area concentrating on single pollutant or single adsorbent material. Even so, to date, few reviews outline the adsorption of a variety of pollutants, e.g., organic pollutants. This review will help researchers and planners in understanding the complete adsorption process including biowaste adsorbent preparation, application for various types of priority organic pollutant adsorption, and adsorption modelling.
2. Adsorbent Selection, Preparation, Modification, and Characterization
Adsorption is a well-known and effective technique for wastewater treatment. Choice of the starting material for adsorbent development is based on several factors. The material should be high in carbon and oxygen content; have strong abrasion tolerance, good thermal stability, small pore diameter, and high exposed surface area; and be easily accessible. Low-cost materials used as adsorbents have been derived from high carbon content plants, livestock, and other products, such as fruit wastes, rice husk, woods, seaweed, algae, peat moss, hair, and keratin . Pollutants can be removed effectively by converting organic low-cost materials into carbonaceous adsorbents, e.g., AC and BC. Preparation, modification, and characterization techniques of these materials are discussed further in subsequent sections.
2.1. Preparation of Activated Carbon
Activated carbon (AC) is a widely utilized adsorbent for the treatment of wastewater and drinking water. It is prepared by chemical or steam activation of char that can be made of different materials such as fossils or biomass. Nowadays, due to the demand for affordable adsorbents, attempts are made to replace commercially available ACs by biomass waste-produced ACs, e.g., waste tea [23, 24], rice husk , and oak wood (Liu et al., ). These ACs are not only economical but also proven to be environmentally sustainable.
ACs are prepared by utilizing physical and chemical methods of activation followed by carbonization at high temperatures . Common methods reported for preparation and activation of ACs have been presented in Table 1. Usual steps followed in the process are as follows: (i) pretreatment including washing, drying (air/oven), and cutting or grinding (powder/small particle size); (ii) carbonization (in furnace/reactor with or without N2 gas); (iii) activation/modification before or after carbonization (steam/acid/oxidizing agent/CO2 gas); and (iv) posttreatment, e.g., washing (until pH is neutral) and drying (air/oven). Various acidic, basic, and oxidizing modification techniques have been reported in the literature for AC preparation. Activating agents produce ACs with improved properties including large porosity, high surface area, variety of functional groups, increased adsorption potential, and regeneration capability. Table 1 shows that surface area of AC may increase or decrease after modification depending on the feedstock properties and activating agent as well. Baghdadi et al.  stated that treatment of AC with nitric acid resulted in degradation of porous framework and modification of the AC exterior framework. On the other hand, carboxylic functional groups were introduced because of the treatment by nitric acid in the AC system. It enhanced the hydrophilicity of AC and consequently reduced the attraction for hydrophobic pollutants towards the adsorbent. Phosphoric acid and steam modifications have shown larger impact on surface area and pore size enhancement of ACs in most of the studies. Furthermore, ACs have also been reported to be modified by metal impregnation, e.g., silver, chromium, or copper , and thermal tension treatment to increase the porosity/surface area .
2.2. Preparation of Biochar
Biochar (BC) is a rich carbon compound developed through combustion at mild temperatures (300–700°C) under minimal to no oxygen condition  or as a byproduct of biocrude oil production . Land wastes, crop residues, and the renewable components of industrial solid wastes have become major source materials for BC preparation. BC has been used to increase soil consistency, promote carbon sequestration, and immobilize pollutants . Its properties rely on the type of raw material, combustion temperature, and residence time. Usually, BCs formed at high temperatures have a higher surface area and carbon content, primarily due to the rise in the number of micropores induced by the removal of volatile organic compounds at high temperatures. However, as the temperature increases, the biochar yield decreases. Thus, in terms of biochar yield and adsorption capacity, an optimal strategy is needed. High temperature BCs are stable in both abiotic and biotic incubations, whereas BC from woody materials have more stable carbons than those generated from rice residues . Amarasinghe et al.  stated that temperature range of 450°C to 500°C and residence time between 45 and 60 minutes had the fastest recovery in BC mass of tea waste. Increased pyrolysis temperature causes reduction in hydrogen and oxygen content while carbon content increases resulting in reduced H/C and O/C ratios at higher temperatures . Yin et al.  observed that when the temperature of pyrolysis rose from 300 to 700°C, BC yield decreased, while pH value, surface area, total pore volume, and ash amount increased. The drop in yield at high pyrolysis temperatures can be attributed to the decrease of volatile components, and the BC’s high ash content suggested the aggregation of inorganic minerals that could elevate the pH and cation exchange capacity (CEC).
BC has a large percentage of organic carbon composition with rich oxygen-containing groups and can provide different sites of adsorption for heavy metal ions and aromatic pollutants. However, BC typically have smaller surface area compared to AC. Through utilizing chemical agents such as acids, bases and oxidizing agents and physical activation methods, the surface area and adsorptivity of the BCs can significantly improve. The modification methods enhance the porosity and add various functional groups at the surface of BC. Table 1 shows that usually acidic modification, e.g., HCl, K2CO3, and H2SO4, introduces O-H or carbon-based functional groups at the surface of the BC which may increase or decrease the surface area, whereas oxidizing agents such as H2O2, H3PO4, and HNO3 introduce hydroxyl and carboxyl groups at the surface of BC increasing the negative charge at the surface. Wang and Liu  reported 63.4% and 101% rise in oxygen and carboxyl group content, respectively, in manure BC after modification by H2O2. Jin et al.  used HNO3 to modify BC and found a very high adsorption of uranium due to increased content of carboxyl groups at the surface after modification. Many lamellar structures appeared on the surface of the dolomite-modified BC as compared with the raw BC, which provided binding sites for phosphate ions in solution . Dolomite was decomposed into calcium and magnesium oxides probably due to the creation of the lamellar system. Luo et al.  used a unique method of BC modification through lignin modification by introducing acrylonitrile into the biochar prepared at various temperatures. It was observed that the BC prepared at 350°C was successfully modified and further utilized for metal adsorption in wastewater. However, at higher temperatures, lignin was not able to be modified. Common methods for preparation and activation of BC have been stated in Table 1. Usual steps involved in preparation include the following: (i) pretreatment, e.g., washing (repeatedly), drying (air/oven), and cutting or grinding (powder/small particle size); (ii) pyrolysis (in furnace/reactor with or without N2) at various temperatures; (iii) modification before or after pyrolysis (steam/acid/bases/oxidizing agent); and (iv) posttreatment, e.g., washing (until pH is neutral) and drying (air/oven). Innovative methods such as ammoniation hydrothermal method have also been introduced recently for BC preparation . Table 1 shows that generally, ACs have much higher surface areas as compared to BCs, but the preparation method of BC is simpler and more economical compared to AC.
2.3. Characterization Techniques
Characterization methods help in identifying the surface properties of the materials and impact of modification on material properties. Figure 1 presents commonly used methods for characterization of adsorbents. Scanning electron microscopy (SEM), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM–EDS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), Brunauer−Emmett−Teller (BET) analysis, field-emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR), surface area analyzer and porosimetry system (SAAPS), gas chromatography mass spectrometry (GCMS), and X-ray diffraction (XRD) are widely used characterization methods for adsorbents. 129Xe NMR has been used as a modern technique for analysis of adsorption mechanism using a porous structure and channel measurement in adsorbent materials . Based on Soxhlet extraction, sodium hydroxide (NaOH) procedure, and two-step acid hydrolysis cycle, respectively, the extractives, hemicellulose, and lignin content of the raw biomass can be evaluated (Lee et al., ).
3. Application of AC and BC for Adsorption of Organic Pollutants
Application of adsorbents is studied under various controlling parameters such as pH, contact time, initial concentration of adsorbate, temperature, and dosage of adsorbent. The effect of these parameters is commonly studied in batch experiments. The pH at which the charge on the adsorbent surface is zero is called the pH at zero-point charge (pHZPC). It is determined by plotting pHinitial against pHfinal, and the point where is called pHZPC . Below this pH, the material is supposed to be positively charged and favorable for anion adsorption, while above this pH, cations can be removed favorably. The pH and pHZPC of AC or BC can change with change in pyrolysis temperature . Other parameters are highly adsorbent and pollutant dependent and change accordingly. Table 2 shows the summary of various input and optimized parameters studied in batch experiments and resulting maximum removal efficiencies for organic materials using AC and BC. The uptake capacity or removal efficiency of pollutants can be calculated by the following equations :
where is the potential for adsorption at equilibrium in mg/g, represents the initial concentration of pollutants in mg/L, represents the final amount of adsorbate at equilibrium in mg/L, is the volume of solution in L, and represents the mass of adsorbent in g.
3.1. Activated Carbon
Organic pollutants, e.g., pharmaceuticals, dyes, and hydrocarbons, have been widely removed using biomass waste-produced ACs. Increased adsorption of organic compounds on AC is due to its large specific surface area and attractive physicochemical properties such as high mechanical strength, chemical stability in different media, and large pore size . Different organic pollutant adsorption by AC and its composites are discussed here. Organic pollutants such as pharmaceuticals, dyes, and hydrocarbons have been effectively removed from aqueous solutions using biomass waste-produced ACs. Amstaetter et al.  found that biomass-based AC showed stronger adsorption of organic compounds, e.g., PAHs and polychlorinated biphenyls (PCBs) as compared to anthracite-based AC. Increased adsorption of organic compounds on AC is attributed to its large specific surface area and attractive physicochemical properties such as high mechanical strength, chemical stability in different media, and large pore size .
3.1.1. Chemical Dyes
Organic chemical dyes such as methylene blue (MB), crystal violet, reactive blue 19, and thionine are used in various industrial processes. Textile, paper, plastics, leather, and printing industries release a large amount of the dyes in wastewater. These dyes are toxic, cause water pollution, and need to be treated on a priority basis. In excess amounts, they may block the necessary oxygen and sunlight penetration into water bodies causing death of aquatic life. The dyes usually have a complicated aromatic structure and are resistant to degradation by chemical, physical, and biological treatments. Being economical and efficient, adsorption is a potential treatment strategy for the removal of dyes from waste effluents .
Methylene blue is a commonly used cationic colouring dye. If swallowed or inhaled, it can cause gastrointestinal irritation, nausea, vomiting, diarrhea, cyanosis, and dyspnea. It also causes eye irritation and burning . Borah et al.  studied adsorption of methylene blue (MB) and eosine yellow (EY) on tea waste-produced porous carbon. The tea waste was activated by phosphoric acid and adsorption of up to 99% was achieved for both dyes. The maximum monolayer adsorption was 402.25 mg/g and 400 mg/g for MB and EY, respectively, which is much higher than many adsorbents reported in literature, e.g., coconut shell (277.90 mg/g), Rattan-AC (294.12 mg/g), walnut shell-AC (3.53 mg/g), groundnut shell (164.90 mg/g), and granular AC (101.626 mg/g). It was found that AC loaded with nickel sulfide nanoparticles showed 99.9% removal of MB and safranin-O (SO), i.e., 46 and 52 mg/g at optimum condition of pH (8.1), adsorbent dosage (0.022 g), initial concentration of dyes (17.8 mg/L and 5 mg/L), and contact time (5.46 min) . Another study reported development of a high surface area (854.30 m2/g) waste tea AC by chemical activation with potassium acetate for adsorption of MB and Acid Blue 29 (AB29) dyes . The AC was developed at optimized values of impregnation ratio, activation temperature, and activation time that were 1 : 4, 800°C, and 120 min, respectively. The adsorption capacities for MB and AB29 were 554.30 mg/g and 453.12 mg/g, respectively, which were higher than KOH-modified oil palm shell AC, i.e., 243.90 mg/g, and raw fruit AC, i.e., 146 mg/g, for MB . Another study reported the optimized conditions for the synthesis of ZnCl2-activated AC as follows: 1 : 4 impregnation ratio, calcination temperature of 600°C, and calcination time of 30 min . The precursor material of AC synthesis was mangosteen peels, and a very high surface area of 1621.8 m2/g was reported. For methylene blue, the adsorption potential of AC prepared under controlled conditions was 1193 mg/g. Nasrullah et al.  reported the synthesis of a high surface area (890 m2/g) AC-alginate by inserting AC powder extracted from mangosteen fruit peel into calcium-alginate beads. The AC-alginate was used for the removal of MB from aqueous solution. The findings showed that for 100 mg/L initial concentration of MB, , and temperature of 25°C, the optimum adsorption potential of AC-alginate was achieved as 230 mg/g. Maazinejad et al.  found that the initial concentration of MB dye was the most significant factor in adsorption of the dye by single-walled carbon nanotubes functionalized with an amine group (SWCNT-NH2).
3.1.2. Chemical Oxygen Demand (COD)
Chemical Oxygen Demand (COD) is an estimate of organic pollutants in wastewater relevant to the design and assessment of processes for biological treatment (Khairalla and Lu-Xiwu, ; ). It refers to the amount of oxygen required to oxidize organic compounds to carbon dioxide, ammonia, and water and is used as one of the main parameters of water quality in wastewater treatment plants . COD is not only produced by organic compounds but also by some inorganic compounds . Primary treatment of wastewater decreases about 27% of the influent COD, and downstream units are expected to further reduce COD concentration to satisfy indirect or direct discharge limits.
Activated carbon is used in both powder (PAC) and granular (GAC) forms with typical particle sizes between 15-25 μm and 0.2–5 mm in landfill leachate treatment schemes . Pecan shell-based AC was used for the treatment of COD in municipal wastewater . Various activation methods were employed, e.g., steam, acid, and carbon dioxide activation methods. Among all the carbons, steam and acid-activated carbons produced the largest surface areas of about 917 and 904 m2/g, respectively. Adsorption was found highly dependent on the surface area of the carbons. Halim et al.  conducted an adsorption study for removal of COD in landfill leachate with an initial concentration of 2580 mg/L. Adsorption of 93.7% was obtained after generation of zeolite-activated carbon (Z-C) composite adsorbent. Combination of AC and zeolite in composite media acted as a natural ion exchanger and provided both hydrophobic and hydrophilic surfaces for the removal of organic (COD) and inorganic (ammonia) pollutants . Devi et al.  studied the removal of COD concentration with avocado peel-activated carbon (APC) and compared with commercial activated carbon (CAC). The optimal operating conditions for maximum reduction of COD were also determined in the study. It was found that under ideal operating conditions using APC, the maximum percentage reduction in COD concentration was 98.20% and this reduction was 99.02% with CAC. Mohan et al.  investigated the adsorption efficiency of various ACs prepared from coconut shell fiber (CSF), coconut shell (CS), and rice husk (RH) for removal of COD from industrial wastewater. The removal efficiency obtained was 47-72% by CSFAC, 50-74% by CSAC, and 45-73% by RHAC. El-Naas et al.  reported that COD adsorption efficiency of date pit AC was comparable to commercially available AC when applied to petroleum refinery wastewater. The optimum values of AC dosage, contact time, and temperature were reported as 20 g/L, 30 min, and 60°C, respectively, whereas it was found out that the initial pH of wastewater had no significant impact on the removal efficiency of COD. The COD removal efficiency obtained at optimum conditions was 241.45 mg/g using the date pit AC.
3.1.3. Polycyclic Aromatic Hydrocarbons (PAHs)
Polycyclic aromatic hydrocarbons (PAHs) are organic pollutants containing rings of carbon and hydrogen atoms such as naphthalene, phenanthrene, fluorene, anthracene, and pyrene . They are identified as priority pollutants and are dispersed into the water by incomplete burning of fossil fuels . PAHs are receiving attention because of their carcinogenicity, teratogenicity, and mutagenicity. They are highly lipophilic and carry toxicological effects on both aquatic organisms and humans via food chains . Based on their molecular weight, PAHs are classified into two classes: low molecular weight PAHs emitted to the atmosphere and high molecular weight PAHs that stay in the water or settle to the bottom of water bodies . PAHs have been found to be hydrophobic in nature; nonpolar and hydrophobic adsorbents show high adsorption efficiency for PAHs .
Yakout et al.  correlated hydrophobicity coefficient (lnKow) with adsorption rate of hydrophobic PAHs (naphthalene, phenanthrene, and pyrene) onto rice husk-produced AC. A good correlation was found and increase in the coefficient and adsorption rate K (mg/g/h1/2) were parallel using Weber-Morris equation. Zhang et al.  found that the rise in salinity resulted in an improvement in the rate of naphthalene elimination but had scarce effect on phenanthrene and pyrene removal. The findings revealed that the adsorptive equilibrium capacities of naphthalene, phenanthrene and pyrene on the U detritus were 1.27, 1.97, and 2.49 mg/kg, respectively, at the initial concentration of 10 μg/L. Kumar et al.  investigated the efficiency of PAH removal using palm shell AC. The AC was further modified by 5% KOH and thermal treatment. It was found that thermally modified KOH-soaked AC had superior adsorption capability of 131.1 mg/g for PAHs, whereas initial concentration of the pollutant was an important factor in controlling the process efficiency. Another study reported adsorptive removal of six PAHs, viz., naphthalene, acenaphthene, fluorene, anthracene, pyrene, and fluoranthene, as 145, 111, 145, 232, 109, and 93 g/kg using granular AC . Pore diffusion was found as a prominent mechanism of adsorption onto AC. The reported values were higher than other reported values in literature. When heavy metals and organic compounds exist side by side, they are expected to interfere and therefore have a synergetic impact on human health and other species . Huang et al.  synthesized magnetic nanoparticle adsorbents to simultaneously extract polycyclic aromatic hydrocarbons (acenaphthene) and metal pollutants (Cd2+). The adsorption capacity was up to 1060 mg/kg and 2250 mg/kg for acenaphthene and Cd2+, respectively. It was observed that the sorption capability decreased compared with the individual sorption, suggesting competitive sorption between both adsorbates, but the efficiency of simultaneous sorption was stable over a broad pH spectrum as well as in the presence of competing metal ions or natural organic matter.
The presence of chemical drugs in aquatic ecosystems has been regarded as one of the major environmental concerns over the past decades. Effluents from municipal wastewater treatment plants have been shown to be the primary cause of pharmaceuticals in aquatic ecosystems. These pollutants may be degraded utilizing different specialized methods of oxidation, such as Fenton, Photo-Fenton, Fenton-like, and electrochemical oxidation. Such approaches have a high effectiveness of elimination for contaminants, but high energy and chemical demands are the main economic restrictions for advanced oxidation processes. Adsorption has been effectively used for the removal of drugs in literature. An antipsychotic drug, carbamazepine (CBZ), is one of the most recorded micropollutants in surface waters. Magnetite AC was used for the removal of CBZ from municipal wastewater, and 93% (182.9 mg/g) removal was achieved using 1 : 8 magnetite content and AC . L-Phenylalanine, identical to other amino acids, is important to animals and to the human body. This is commonly used in food or feed additives, in infusion products and in nutraceutical and medicinal applications. In a study by Belhamdi et al. , porous AC was effectively synthesized from date stones, using the chemical activation process. The findings revealed that optimum factors, affecting the adsorption of L-phenylalanine, were temperature range of 20 to 40°C and pH of 2–9.4, with maximum adsorption capacity of 188.3 mg/g by KOH-modified AC.
Sodium diclofenac (SD) is used for the therapy of arthritis and is a commonly found pharmaceutical in aquatic environments. It is a sodium salt form of a derivative of benzene acetic acid and is persistent in nature. Malhotra et al.  reported that ZnCl2-modified AC had the highest adsorption potential (62 mg/g) for SD in comparison to KOH- (49.5 mg/g) and K2CO3- (52.4 mg/g) modified AC. Phenol is a very toxic chemical produced by coal purification, paper, pulp, fertilizer, paint, and organic synthesis industries. They can also be transmitted by plants as natural resources. Phenolic compounds are strong irritants for skin and eyes. In most cases, they can cause death of living cells. Phenols can be removed effectively using adsorption method. Particularly, AC as an adsorbent has shown high removal efficiency for phenols due to its large and highly active surface area . Magnetic carbonaceous materials (AC and BC) effectively removed phenol from water samples . Magnetic alteration improved the AC’s surface and pore capacity and retained biochar’s structural properties. It was observed that magnetic AC had lower adsorption rate (10.641 g/mg/min) than virgin AC (20.575 g/mg/min), whereas magnetic BC had a higher adsorption rate (0.618 g/mg/min) compared to virgin BC (0.040 g/mg/min). Yang et al.  developed stable supramolecular gel adsorbent to solve the problem of recyclability. The absorbents in the gel were successful in extracting bicyclic phenols from aqueous solution. Lim et al.  reported the synthesis of novel polyvinyl alcohol cryogel beads with an exterior surface protected by powdered AC. The purpose was to shield the trapped activated sludge biomass from the inhibitory impact of the recalcitrant pollutant 4-chlorophenol. The powder AC dispersed on the outer surface of cryogel beads was shown to have a higher 4-chlorophenol adsorption potential than the homogenized powder AC beads. Overall, the elimination of 4-chlorophenol was achieved via the combined adsorption and biodegradation processes. Maximum 132 mg/g adsorption capacity was obtained using the material for 4-chlorophenol.
Biochar has been used widely to remedy the deterioration of wastewater from organic contaminants. Many considerations, such as the types of feedstock, the dosage applied, the desired contaminants, and their concentration, have influenced the elimination of organic pollutants from wastewater by BC. In general, there is a higher affinity for adsorption of polar and ionic organic compounds on the polar BCs and a lower affinity for hydrophobic compounds. Application of BC for adsorption of various organic pollutants is discussed further.
3.2.1. Chemical Dyes
Fan et al.  developed biochar at 300°C by copyrolysis of waste from municipal sludge and tea waste with 53.21% yield. Up to 100% of MB removal was achieved by application of the BC for more than 24 h contact time and 100 mg/L initial concentration of adsorbent. The mechanisms of MB removal included electrostatic activity, exchanging of ions, complexation of the surface, and physical process. Huff and Lee  revealed that a higher percentage of H2O2 treatment with pinewood biomass BC contributed to a higher cation exchange capacity (CEC) due to the introduction of acidic oxygen functional groups on the BC surface. It also led to the subsequent reduction of the BC’s pH while MB adsorption decreased with higher concentration of H2O2 treatments due to lowering of π-π dispersive forces. Methylene blue (MB), orange G (OG), and Congo red (CR) dye removal was studied using biochar prepared at very high temperature (900°C) . Adsorption of 196.1 mg/g, 38.2 mg/g, and 22.6 mg/g was achieved for the three dyes, respectively. The adsorption was attributed to intraparticle diffusion, high surface area, and electrostatic interaction.
3.2.2. Chemical Oxygen Demand (COD)
Adsorptive removal of total and dissolved COD was investigated by Huggins et al.  in brewery wastewater using granular biochar. Results revealed that BC had higher adsorption capacity for total COD as compared to dissolved COD. High adsorptive removal of total COD was attributed to removal of higher content of total suspended solids (TSS) due to macrostructure of BC, whereas in the case of dissolved COD, the TSS were filtered out and adsorptive removal was reduced. The results of COD removal were compared with granular AC (GAC). It was observed that adsorptive removal of total COD at 1200 mg/L initial concentration using BC (70 mg/g) was 30% higher compared to GAC (49.3 mg/g), whereas at lower initial concentrations and for dissolved COD, the adsorptive removal using BC was lower and almost like GAC. Manyuchi et al.  reported a reduction of 90% COD when the sewage wastewater was subjected to the urban sewage sludge BC. The sewage pH also shifted from alkaline to acidic after treatment. The findings suggested that BC sewage sludge effectively treated the sewage wastewater. Khalil et al.  investigated the effect of pH, COD concentration, temperature, contact time, and the amount of adsorbent dosage on the removal of COD from aqueous solution by biochar obtained from rice straw. It was observed that the removal efficiency of COD increased with increasing initial concentration of COD, which was attributed to higher physical adsorption onto BC. The removal efficiency also improved with increasing pH from 2 to 8.5. A direct correlation was found between the BC dosage, temperature, and adsorption efficiency. The results demonstrated the maximum removal efficiency of COD of 90.44% from aqueous solutions at optimum conditions.
3.2.3. Polycyclic Aromatic Hydrocarbons (PAHs)
Biochar have a smaller surface area than AC, but they can lessen the bioavailability of PAHs, pesticides, and heavy metals efficiently . Yang et al.  investigated the adsorption of aromatic organic compounds onto various biochar prepared at high temperature (700°C). It was found that basic mechanisms involved in the adsorption of aromatic compounds were π-π interaction, hydrogen bonding, and hydrophobic effect. Pyrolysis is unable to fully carbonize the BC and thus forms both carbonized and amorphous organic matter. Higher pyrolysis temperature may favor PAH adsorption because of higher content of carbonized organic matter. Carbonized organic matter has a more compact, aromatic, and nonpolar composition, including associations with planar and aromatic PAHs which are more beneficial in adsorption . Godlewska et al.  reported that during pyrolysis switching from N2 to CO2 gas favored the adsorption of phenanthrene and pyrene onto BC. Increased adsorption was connected to the higher aromatic nature of BC prepared by the former gas. Biochar prepared from municipal waste was found to be a reliable and recyclable adsorbent for PAHs, phenols, and petroleum-based materials in aqueous media . Macroaglae (Enteromorpha prolifera and Ulva lactuca) BC and modified BC (by ferric and zinc chloride) was reported by Cheng et al.  for adsorption of PAHs particularly naphthalene (NAP), acenaphthene (ACE), and phenanthrene (PHE). Adsorption capacity of about 90 mg/g was achieved by pore filling and π-π interaction following Freundlich adsorption model and showing heterogeneous adsorption for all pollutants on the BC. This value was higher than other reported values of adsorption, e.g., mangrove litter BC, 47.27 mg/g for 3-ring PAHs, 66.01 mg/g for 4-ring PAHs, and Phragmites australis BC (1.97 mg/g) for PHE .
Carbofuran (a pesticide) was removed from the aqueous solution by rice husk and tea waste BC produced at 700°C . The equilibrium adsorption potential calculated by the pseudo-second-order kinetic model was 25.2 and 10.2 mg/g for rice husk and tea waste BC, respectively. It showed that carbofuran adsorption on rice husk biochar was 2.5 times higher than that on tea waste. 95% of carbamazepine (CBZ), a famous pharmaceutical, was removed using nano-BC (60 nm particle size) prepared from pinewood residue material . The prepared BC showed a good removal efficiency (up to 57%) for CBZ in the presence of surfactant (Tween 80) as well.
Phosphorus is an important resource in modern agricultural development and pharmaceutical processing. Large amounts of nitrates and phosphates in water threaten the biological equilibrium of the system and exacerbate the water quality. Excessive phosphorous release promotes the uncontrolled development of bacteria and algae (eutrophication) that compete in the water with fish and aquatic insects for dissolved oxygen and deteriorate the aquatic environment . Qiao et al.  prepared a low-cost and highly efficient biosorbent prepared by tea waste and modified by amine cross-linking and tested for its ability to remove phosphate and nitrate ions from aqueous solutions. It was observed that the material had limited nitrate and phosphate adsorption capability before modification. Amine-crosslinked biosorbent had 136.43 mg/g and 98.72 mg/g nitrate and phosphate adsorption capability, respectively, over a wide pH range.
Wan et al.  focused on the optimal conditions for preparation of the eucalyptus sawdust biochar as a possible biosorbent of nitroimidazoles from aqueous solutions. The activation temperature and H3PO4-to-sawdust impregnation ratio were significant factors in maximizing metronidazole’s adsorption capacity. The optimally formulated biochar removal efficiencies were 97.1% and 96.4% for metronidazole and dimetridazole at 20 mg/L concentration and 1.0 g/L dosage within 2 h, respectively, while the thermodynamic analysis showed spontaneity and exothermicity in the adsorption process. Tetracyclines (TCs) are rated as second antibiotics used worldwide and are mostly released in unchanged form in the environment due to poor metabolization. Chen et al.  studied adsorption of TCs onto H3PO4-modified animal manure and rice husk BCs. According to isothermal study, the latter showed higher removal efficiency as compared to the animal manure BC with capacity of 552 mg/g and 365.4 mg/g, respectively. However, the maximum adsorption value was higher than other reported values in literature, i.e., alkali BC (58.8 mg/g) , NaOH-modified BC (455.33 mg/g) , Pinus taeda-activated BC (274.8 mg/g) , wood BC (96.1 mg/g) , and cobalt-impregnated BC (370.37 mg/g) . Simultaneous removal of norfloxacin, sulfamerazine, and oxytetracycline was studied using KOH-modified BC (Luo et al., ). It was observed that in ternary-solute, the sorption kinetics of the three antibiotics were faster than that of the single-solute unit and the process was controlled by BC electrostatic interactions, π-π interaction, and H-bonding. The new BC-supported magnetic CuZnFe2O4 composite (CZF–biochar) was formulated through a simple hydrothermal method by Heo et al.  to extract bisphenol A (BPA) and sulfamethoxazole (SMX) from the aqueous media. The key pathways for adsorption were H-bonding, hydrophobic, and π–π interactions with maximum monolayer adsorption capacity of about 263.2 mg/g and 212.8 mg/g for BPA and SMX, respectively.
4. Adsorption Models
Various models are used to understand and describe the adsorption mechanism. The models are discussed as follows.
4.1. Adsorption Isotherm Models
Isotherm experiments are conducted to obtain the relationship between adsorbate concentration, the amount of adsorbate adsorbed, and the amount of adsorbent at equilibrium. Isotherms play a major function when considering the statistical models for the design and study of adsorption processes. The experimental results obtained in a specific study under one collection of conditions may fail under another. To evaluate the correct fit model for the adsorption process, error analysis involves the use of root mean square error, chi-square, and average relative error. If the value of error measurements is small, the experimental and measured results are more comparable by isothermal and kinetic methods; if they are large, the value would be higher . Table 3 shows that mostly adsorption by carbonaceous materials can be expressed by Langmuir and Freundlich models, and various parameters help in understanding the mechanism. Some common isotherm models are discussed as follows.
The Langmuir model is based on the hypothesis that adsorption happens inside the adsorbent at different homogeneous locations, and there is no substantial interference between the adsorbates . The adsorbate is saturated at the adsorbent surface after one layer (mono layer) of adsorbed molecules is formed . The linearized version of the Langmuir equation is as follows :
where is the mass of adsorbate on the surface of the adsorbent (mg/g) at equilibrium, is the equilibrium adsorbate concentration in solution (mg/L), is the maximum adsorption power (mg/g), and is the Langmuir adsorption constant (L/mg). Linearized forms of isotherm models are commonly reported in most studies. The dimensionless constant , originating from the Langmuir model, is named as the separation factor or equilibrium parameter .
values signify isotherm forms which are either unfavorable (), linear (), desirable (), or irreversible () .
The Freundlich isotherm is a variational sorption model. It suggests monolayer sorption with a heterogeneous energy distribution of active sites, followed by encounters between adsorbed molecules . Linear form of the model is as follows :
where is the mass of adsorbate, at equilibrium, on the surface of the adsorbent (mg/g); is the equilibrium adsorbate concentration in solution (mg/L); is the Freundlich adsorption constant, which implies the adsorption efficiency; and is the nonlinearity measurement. The value of less than 1 shows the physical adsorption phenomena and favors the adsorption process (Liu et al., ).
The Temkin isotherm model is being studied because of the potential adsorption heat interaction. This model suggests that bond energies are defined by a standardized distribution of the molecular adsorption energy. This would mean that the adsorption heat of the molecules present in the surface layer would be reduced linearly with the coverage . The linear form of the Temkin model is given as follows:
where is the ideal constant for gases, 8.314 J/mole K; is the adsorption temperature, K; is the adsorption heat constant, J/mol; and is the Temkin isotherm constant, L/g.
The Dubinin–Radushkevich (D-R) isotherm model is generally used to determine the adsorption mechanism on a heterogeneous surface with a Gaussian energy scattering. The model is well adapted to strongly active solutes and low concentrations. While the D–R isotherm model is also regarded as parallel to the Langmuir isotherm model, it does not consider the possibility for homogeneous surface or persistent adsorption . The linear form of the D-R model is given as follows:
where (mol/L) and (mol/g) are the sum of the adsorption per unit weight and the peak adsorption potential, respectively. The parameter β (mol2·J-2) is the coefficient of activity depending on the mean adsorption free energy and ε () is the Polanyi potential.
4.1.1. Isotherm Types
According to the international union of pure and applied chemistry (IUPAC), the equilibrium of physical adsorption process has different adsorption isotherms that may be categorized as type I to type VI . These isotherms help to understand the multiple adsorption processes including monomolecular, multimolecular adsorption, and condensation in pores or capillaries . Figure 2 shows the classification of the isotherms and associated adsorption characteristics over a particular range of relative pressure () . These isotherms have different patterns due to pore shape, adsorption process, and adsorbent/adsorbate interactions.
Type I isotherms have a very quick rise in adsorbed amount with increasing concentration (or pressure) up to saturation; i.e., they have a partially or completely horizontal plateau . Microporous materials having a small portion of the outer surface are classified as this type. Such isotherms resemble Langmuir isotherms showing monolayer adsorption . Polymolecular adsorption in nonporous or macroporous adsorbents is classified as Type II. It does not represent a saturation limit. Type III sorbents are nonporous and have a low adsorbent-adsorbate interaction energy. Types IV and V are porous adsorbents, which are comparable to types II and III. This shows that a monolayer form first followed by a multilayer . Nonporous adsorbents with a homogenous surface exhibit type VI isotherms. This isotherm is known as stepwise multilayer adsorption, and it only arises when the sample surface comprises many types of adsorption sites with energetically distinct properties .
4.2. Adsorption Kinetic Models
The impact of contact time is investigated to determine the possible application and to gain insight into adsorption kinetics. Generally, the adsorption process initially shows accelerated patterns, which can be due to surface complexation or instant electron transfer. Afterwards, the adsorption seems constant showing the gradual saturation of adsorption sites on the outside surface or reduced efficiency of redox processes . The kinetic adsorption models are typically complex. The rate of adsorption is highly affected by many variables relevant to the solid state and the physical and chemical parameters by which the adsorption takes place . To fully understand the kinetics of adsorption, four widely used kinetic models are pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (ID) models . Table 3 shows that most carbonaceous materials fit either the PFO or the PSO model for describing their adsorption mechanism. However, diffusional results can be very significant for porous sorbent materials, and therefore, the rate constants assessed by diffusion models must be calculated in order to obtain insight into the transfer mechanism.
The best fitting of experimental results to the pseudo-first-order (PFO) model suggests that adsorption to be more inclined towards physisorption interactions and that the mechanism of adsorption depends on the initial adsorbate concentration . The model equation is given as follows:
where (mg/g) and (mg/g) are the amounts of adsorbed ions per unit mass of the adsorbent at equilibrium and at any time (), respectively, and is the PFO model rate constant.
The pseudo-second-order (PSO) model relies on the hypothesis that chemical sorption such as bonding forces by sharing or the exchange of ions/electrons between adsorbate and adsorbent governs the phase reaction rate . The model can be represented through the following equation:
where (mg/g) and (mg/g) are the amounts of adsorbed ions per unit mass of the adsorbent at equilibrium and at any time (), respectively, is the PSO model rate constant. Adsorption rate constants and adsorption percentage may be different for various pollutants because the rate determines the diffusion of pollutants from water sample to solid adsorbent surface . Khan et al.  found the inverse relation between and initial concentration of Cu(II) while was directly related to the initial concentration.
The Elovich equation is often widely used to explain the kinetic second order given that the real solid surfaces are vigorously heterogeneous, but the method does not suggest any specific adsorbate-adsorbent mechanism . It has been widely agreed that this semi-empirical equation can explain the chemisorption cycle. One may measure the Elovich coefficient from the plot, versus . The early adsorption rate, , and the desorption coefficient can be calculated from the interception and slope of the . The linear form of the equation is given as follows:
where is the initial adsorption rate (mg/g/min) and is the desorption constant (g/mg), which is related to the extent of surface coverage and activation energy for chemisorption.
Adsorption mechanism for understanding the diffusion process may be analyzed in terms of the intraparticle diffusion model, interparticle diffusion model, or Boyd’s film diffusion model. According to the Weber and Morris model intraparticle diffusion model, if a linear curve is obtained and it passes through the origin, then the predominant mechanism for adsorption is diffusion. According to this model, an internal diffusion cycle regulates the adsorption capacity .
where (mg/g) is the amount of adsorbed ions per unit mass of the adsorbent at any time (), is the rate of intraparticle diffusion, and is a constant. The values of and are obtained from a plot of versus time . The multilinearity of the plot would indicate that more than one form of adsorption existed while adsorbing the adsorbents .
4.3. Adsorption Thermodynamics
The adsorption process’s dependency on temperature can be clarified by thermodynamic analysis of the equilibrium results. In adsorption thermodynamics, the adsorption’s spontaneity, thermochemical complexity, and randomness trend with respect to the temperature is correlated by the measurement of certain thermodynamic parameters, like Gibbs free energy change (ΔG°), enthalpy (ΔH°), and entropy (ΔS°). Among these parameters ΔG° is the key indicator used in confirming spontaneity of the adsorption process, and a negative ΔG° value implies that the adsorption process is spontaneous and feasible. The positive value of indicates that the process is endothermic. Positive value of shows affinity of the adsorbent for the adsorbate. These parameters can be calculated as follows :
where , represents adsorbate per unit adsorbent mass (mg/g), is the equilibrium adsorbent concentration in aqueous liquid (mg/L), and is the equilibrium constant for adsorption; is the uniform gas constant (8.314 J/mol/K) and is the absolute temperature (K).
5. Adsorption Mechanism
In general, an adsorbent’s adsorption capability depends basically on its physicochemical properties . Adsorption may take place due to physical, chemical, or both physical and chemical reactions on the surface of the adsorbents. Physical adsorption may be by porous structure (i.e., AC and BC) and chemical adsorption by surface functional groups (i.e., -OH, -COOH, C-O) or others depending on the material properties, surface treatment methods, and reduction sites . Table 3 states the mechanisms of adsorption reported in various studies using AC and BC. Studies showed that organic compounds are usually sorbed onto the carbonaceous materials due to a surface interaction reaction. Other mechanisms, such as π-π bond activity and pore filling, have also been demonstrated to lead to increased sorption and decreased desorption of organic pollutants onto the ACs and BCs (Luo et al., 2018; ). Schematic diagram of adsorption mechanism for organic pollutants on AC and BC is shown in Figure 3.
Characterization and modelling techniques help in identifying the adsorption mechanism. The mechanism of adsorption can be better defined from an FTIR study. Prior to and after adsorption the adsorbent samples are prepared for infrared (IR) study. The dynamic nature of the adsorbent and presence of various functional groups are seen by the amount of adsorption peaks in the spectra . Liu et al.  found the inverse relationship between pyrolysis temperature (PT) and FTIR peaks assigned to -OH, C=C, and C=O groups and direct relationship between PT and peaks assigned to aromatic γ-CH. After adsorption, the peaks were reduced further, and complexation of functional groups was found as adsorption mechanism.
BET surface area analyzer helps in identifying the surface area and pore size variation before and after adsorption. The pore sizes may be micro (<2 nm), meso (2-50 nm), and macro (>50 nm) .
Type IV isotherm specifically shows that the substance being processed is of a mesoporous kind. In the adsorption isotherm, the relative pressure area shows the coexistence of micropores according to Henry’s theorem . There may be increase or decrease in surface area and pore size depending on the type of material. Decrease in pore size or specific surface area indicates the attachment or blockage of activating agent/pollutant at the surface of the adsorbent . Increase in surface area indicates that surface pores opened up after modification/chemical activation .
XRD analysis shows the crystalline properties of adsorbents. It correlates the characteristic peaks with crystal planes in 2D representation (Zhang et al., ). Lee et al.  calculated the size of iron crystal in Fe2O3-carbon foam adsorbent as 38.32 nm using XRD analysis peak value. SEM, TEM, SEM-EDS, and TEM-EDS images of adsorbent materials help to know the surface morphology and elemental composition modification after adsorption . Baghdadi et al.  observed large micropores and rough surface of nitric acid-treated AC after SEM analysis.
It has been observed that higher adsorption capacity, ease of generation, and relatively cheaper costs of carbon materials, e.g., ACs and BCs, have made them an attractive option for the removal of organic pollutants from wastewater. The preparation, modification, characterization, and application methods used for the adsorption of chemical dyes, COD, PAHs, and pharmaceuticals using AC and BC have been discussed in detail in this review. It was observed that in the majority of the studies, high pollutant removal efficiencies were reported using the adsorbents. Surface properties of the materials, e.g., pore size, surface area, pore diameter, and functional groups were found to be important factors in the adsorption process. The adsorption capacity also depends largely on the pyrolysis temperature, modification technique, initial pH of wastewater, dosage of material, contact time, and initial concentration of the pollutants. Parameters of the adsorption process and models used to understand the adsorption mechanisms have also been stated in the review. Characterization and simulation methods aid in the identification of the adsorption mechanism. Most of the studies concentrated on the kinetic, equilibrium, and thermodynamic aspects of adsorption, suggesting the dominant isotherm and kinetic models as Langmuir or Freundlich and pseudo-second-order models. Organic compounds are typically adsorbed on carbonaceous materials due to surface contact reactions such as bonding and pore filling. This review will help readers understand the current state of research in the adsorption of organic pollutants by agricultural waste-based ACs and BCs. However, application of these adsorbents at the commercial scale has not been adequately investigated in research works and needs to be studied. Most of the studies have been conducted on synthetic solutions that do not completely represent the discharged effluents. This also needs attention in future studies.
7. Future Directions and Recommendations
The following are recommendations for future solid–liquid adsorption studies based on the review: (i)There is a strong need for pilot and commercial scale application of adsorbents for removal of organic contaminants.(ii)Research works need to be extended: on competitive adsorption studies of organic pollutants with other pollutant forms such as metals and other organics; also, there is need to demonstrate the molecular level analysis of the adsorbents to determine their main characteristics responsible for adsorption.(iii)In addition, actual wastewaters should be investigated, rather than simulated wastewaters.(iv)To minimize the expenses of the water treatment process, the combination of adsorption and other water management approaches should be explored.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors would like to acknowledge the financial support provided for this study under the YUTP grant with cost center 015LC0-190.
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