Abstract
Biochar is a stable carbon-rich product synthesized from biological materials through different heating methods above the decomposition temperature. The potential uses of biochar in various fields include soil fertility improvement, C sequestration, pollutant removal and waste minimization/reuse. In recent years, large number of research has confirmed that biochar can be used successfully for the removal of heavy metal ions from aqueous solutions. The main aim of this chapter is to summarize and assess the sorption capacity of biochar toward various heavy metal ions. Considering that sorption is a surface phenomenon, the key parameters controlling the formation of biochar including pyrolysis temperature, residence time, and feedstock type will be discussed in detail. In addition, the mechanism associated with remediation of heavy metal ions and the physicochemical factors affecting the sorption potential will be discussed. Mathematical models employed in the sorption studies will be given special importance. The modification procedures used to enhance the sorption capacity of biochar will also be highlighted.
Keywords
- biochar
- adsorption
- heavy metals
- water quality
- pyrolysis
1. Introduction
Water usage has been rising immensely with growing population and industrial activities in both developed and developing countries [1]. This resulted in deterioration of water sources as various contaminants such as dyes [2], toxic heavy metals [3], organic compounds like detergents, phenols, dyes, pesticides in addition to the other persistent organic pollutants [4] are increasingly being dumped into the water bodies [5]. Among these contaminants, heavy metals are of high priority because they persist in soils and do not undergo biodegradation [6]. This might affect significantly the suitability and sustainability of the water resources [7]. These contaminants reach water bodies through various industrial activities, including mining, electrolysis, metallurgy, battery manufacture, metal finishing, electroplating, electro-osmosis, pigment manufacture, tanneries, etc. [8, 9]. Heavy metals are then taken up by the biological systems through food intake and thereby cause major hazardous health impacts [10, 11]. Owing to this, different biological and physico-chemical treatment techniques have been proposed to remediate heavy metal-bearing contaminated waters. These remediation technologies include adsorption, biosorption, ion-exchange, electrocoagulation, membrane technologies and precipitation [12]. Adsorption is one of the widely used remediation approaches, which has its unique advantages including cost-effectiveness, high performance toward metal ion of interest, flexibility and ease in operation. [13]. Some of the widely used adsorbents for heavy metal remediation include fly ash, activated carbon, sorbents prepared from agricultural, industrial and biological waste materials [14]. Recently, char derived from biological materials under oxygen free condition, popularly known as “biochar” has been recently been introduced as an effective sorbent for various toxins [4, 15].
Biochar is a stable carbon-rich product synthesized from biological materials through different heating methods above the decomposition temperature. Biochar is produced through thermal degradation of organic components in absence of O2 or under limited oxygen conditions (pyrolysis) [16]. In recent years, owing to the inherent biochar properties such as surplus surface binding sites (hydroxyl, carboxyl, phenolic hydroxyl and carbony groups), porous surface, high cation exchange capacity and its surface area, this organic amendment be utilized as an efficient and practical sorbent for remediation [17]. Biochars from different feed stock materials were prepared and successfully examined for their adsorption potential toward various metal ions [3] and nutrients [18]. The pyrolysis conditions such as temperature, rate of heating and residence time are all critical factors influenceing the potentiality of metal sorption on biochar. In addition, changes in sorption potential can also be obtained through physico-chemical techniques. Thus through this chapter, we critically explore the state of knowledge on different methods to modify biochar with preferred properties. We focused on various aspects including biochar preparation methods, factors affecting the biochar sorption potential and metal removal mechanisms.
2. Production of biochar and the factors influencing the properties
Biochar has been receiving considerable attention in recent years as an important material for sorption of contaminants from polluted waters [16, 19]. Carbon-rich materials are established adsorbents for inorganic and organic contaminants. For instance, activated carbon (AC) is a well-known adsorbent extensively used for removal of heavy metal ions [20]. It is worth noting that the preparation of AC requires high temperature, costly and an additional activation step (activated carbon, sorbents prepared from agricultural, industrial and biological waste materials) [14]. Hence the cost of remediation is high. Alternatively, biochar is a recommended cost-effective alternative sorbent because of its binding sites, high permeable structure and extensive surface area [21]. Its preparation is comparatively cheaper while considering its fewer requirements for energy [21, 22]. In addition, biochar consist of non-carbonized portion that may react with soil pollutants [19]. Uchimiya et al. [23] also indicated that degree of O-comprising hydroxyl, phenolic and carboxyl binding sites on the biochar surface can play crucial roles to immobilize soil pollutants.
The sorption potential of biochar is critically dependent on the pyrolysis conditions (temperature and residence time) as well as chemical composition of the feedstock. Overall, pyrolysis temperature markedly affects the functional groups and surface area of biochar. Jung et al. [24] recorded that the pores of biochar were blocked at pyrolysis temperatures greater than 400°C. Similarly, Gai et al. [25] examined the effects of feedstock (wheat-straw, corn-straw and peanut-shell) and pyrolysis temperature (400–700°C) on the properties and adsorption potential of biochar and found that yield of biochar as well as composition of H2, O2 and N2 decreased as pyrolytic temperature surges from 400 to 700°C. Xiao et al. [26] studied influence of pyrolysis temperature during synthesis of biochar from crayfish shell for adsorption of Pb(II) ions from aqueous solutions. The authors observed that pyrolysis temperature exhibited varied effects on active functional groups, surface area and elemental composition of the produced biochar. Also, the sorption performance of biochar increased with increase in pyrolysis temperature, with biochar produced at 600°C exhibited maximum Pb(II) uptake of 190.7 mg/g.
Biochar are generally synthesized from cheap and copiously available waste biomaterials [27]. To be specific, biochar feedstocks are primarily produced from solid wastes and biomasses of agricultural activities. The agricultural residues are generally available in vast quantities and frequently pose disposal challenges [3]. For instance, preparing biochar from invasive plant can resolve challenges posed during disposal as well as help in waste management. Similarly, marine algae are generally abundant and can clog waterways; therefore other usages such as biochar synthesis can be beneficial to the local people. In recent years, a wide range of biomaterials were proposed as feedstocks for biochar, including animal manure, plant waste, seaweed, municipal solid waste and wood chip [27]. The type of feed stock strongly influences the biochar attributes and subsequently its further applications. Sohi et al. [28] indicated that the size of pores, surface area and functional groups in biochars are strongly influenced by the feedstock type. Hodgson et al. [29] examined the feasibility of different feedstocks (
3. Sorption of heavy metals and associated mechanisms
Biochar has been investigated for adsorption of pesticides, heavy metals, nutrients, and organic compounds. Several researchers explored the adsorption capacity of biochar and confirmed favorable results for heavy metal ions [3, 31], nutrients [15], and organic pollutants [4]. Shakya and Agarwal [32] derived biochar from pineapple peel at different pyrolysis temperatures and investigated its efficiency for Cr(VI) sorption from aqueous solution. The results indicated that biochar synthesized at 350°C exhibited maximum sorption potential of 41.7 mg/g. Liu et al. [33] prepared biochar from corn stalk to test its capability of removing Pb(II) from aqueous solutions. Using X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Scanning electron microscopy with energy dispersive spectrometer analyses, the authors identified combined complexation, mineral precipitation and ion exchange mechanisms contributed to Pb(II) sorption onto corn stalk-derived biochar. The maximum Pb(II) adsorption capacity of biochar was identified to be 49.7 mg/g.
Biochar sorption experimental trials are generally performed in continuous/batch operational modes. In most batch trials, the researchers aim at examining the effects of initial metal concentration, adsorbent dosages, temperatures, and solution pH. On the other hand, continuous trials aimed at understanding the continuous contaminant removal potential of biochar.
3.1 Solution pH
Solution pH is a vital operating factor influencing the adsorption process and usually plays a critical part in overall success of adsorption. Precisely, the solution pH influences the surface properties of the sorbent, as well as the metal speciation and finally the extent of metal sorption. The pH also decides the extent of adsorbent protonation, thereby affecting the specific charge of functional groups and finally the adsorption capacity of adsorbent [13]. In general, under acidic (low pH) conditions, the uptake of cationic metal ions is low owing to strong competition from H+ ions. As the pH surges, the amount of H+ ions declines and sorption of cationic metal species increases [13]. In order to endorse the effect of pH on the adsorption potential, few researchers investigated the impact of pH on sorption capacity of biochar. Liu et al. [33] witnessed that Pb(II) sorption capacity of corn stalk derived biochar surged as the solution pH increased. The removal performance was improved within the pH ranges of 4–6. The authors suggested that under acidic conditions, the existence of H+ inhibited the sorption of Pb cations. On the other hand, Senthilkumar et al. [34] observed that remediation of As(V) by
3.2 Temperature
Temperature tends to affect the kinetics rate and adsorption capacity of any adsorbent. The increase or decrease of the adsorption capacity upon varying the temperature will be useful to establish the type of the sorption process. On the basis of change in temperature, the process is identified to be endothermic when the adsorption capacity rises with the increase in temperatures; whereas the process is exothermic when the sorptional capacity decreases with temperature [13]. Several research studies have confirmed that temperature plays a critical part during adsorption of heavy metal ions by biochar [33, 35].
3.3 Biochar dosage
In an attempt to determine the optimum adsorbent dose essential to attain maximum adsorption, many researchers have performed adsorbent dosage optimization experiments during metal removal studies [36, 37]. In general, the % metal removal is directly linked with the adsorbent dosage. Precisely, the increase in adsorbent dosage generally increases the % metal removal of the adsorbent. This general trend can be explained as follows: as the sorbent dose increases, the total number of binding groups present on the surface of the adsorbent increases which, in turn, increases the overall binding of metal ions [38]. On the other hand, the sorptional capacity decreases with increasing adsorbent doses [39]. This is due to nature of interaction between sorbent and sorbate. The important factor being at high biochar dose, the metal ions in the solution are less compared to the exchangeable groups on the biochar, typically results in in less metal uptakes [13].
3.4 Initial solute concentration
Initial solute concentration is a critical parameter that influences the adsorption potential of any adsorbent. Past studies have shown that increase in initial metal concentration generally resulted in decline in the % metal removals [33, 34]. However, the sorptional uptake normally improves with the increase in the initial metal concentration. This was because at lower initial metal concentration, the ratio of the initial moles of metals in the solution to the biochar surface area was low and consequently, the adsorption became independent of initial concentration. Nevertheless, at higher metal concentration the accessible binding groups of sorbent become fewer in comparison to the moles of metal ions available in solution and hence, the percentage metal removal would be severely impacted by the initial metal ion concentration. During adsorption of arsenic(V) by
3.5 Parameters influencing column sorption of metals
In comparison to batch sorption research, very little background literature is available about the possibility of utilizing column sorption in the removal of metal ions from aqueous solutions. Packed column sorption refers to feeding contaminated solution into the column packed with sorbent for continuous treatment. Of these little continuous-flow studies, it was identified that column adsorption potential strongly depends on operational parameters such as flow rate, influent metal concentration and bed depth [13]. The batch experimental trials are helpful in elucidating the fundamental information about the characteristics of adsorbent and the factors affecting the adsorption process [38]. Nevertheless, the batch experimental results cannot be utilized for accurate scale-up in real industrial wastewater systems [40]. This is due to the fact that in industrial wastewater systems, continuous adsorption column setup are generally used [13]. For cyclic adsorption/elution processes, packed columns are effective and practical arrangement, as they efficiently utilizes the concentration difference which is known to be the driving force for sorption of heavy metals [41]. Also, the column assembly allows more efficient utilization of the adsorbent capacity and generally results in superior effluent quality. Thus, adsorption using packed columns has important advantages including fast and high yield operations as well as easy scaling up [42]. Additionally, packed columns permit large amount of wastewater to be continuously remediated using a small amount of sorbent loaded inside the column [43]. Regeneration and subsequent reuse of sorbent is also possible using appropriate elutant. After adsorption, metal ions loaded-adsorbent can be eluted using suitable desorbent, or otherwise can be contained/disposed [44].
Vilvanathan and Shanthakumar [45] conducted continuous column adsorption experiments using biochar prepared from
As indicated before, very limited research studies focused on column applications compared to batch applications. Thus, serious efforts ate needed to explore the adsorption capacity of adsorbent in continuous operational mode to elucidate the adsorbent compatibility in real wastewater plants.
3.6 Biochar modification
Although biochar exhibits good sorption properties; however, it can be additionally altered to improve its sorption efficiency. The modification procedures employed include acid/base modification, functional group modification and impregnation with mineral oxides.
Through acid/base modification, alteration of surface acidities and porous nature of biochar can be obtained [15]. After exposure to chemicals including HNO3, H2SO4, HCl, KOH and NaOH, El-Hendawy [46] identified that HNO3 exposure resulted in improved adsorption and pore diffusion of hydrated Pb2+ with O2 groups, and therefore improved the hydrophilic nature of biochar. Li et al. [31] evaluated lead adsorption capacity of two biochar materials (low mesopore char (AC1) and high mesopore char (AC2)) derived from bagasse modified using nitric acid. The results indicated that the adsorption capacities of AC2 and AC1 toward lead ions were recorded by 27 and 15 mg/g, respectively, due to high mesopore volume of AC2. Precisely, the lead removal rate of by AC1 surged from 46 to 99% after treatment with HNO3. Liu et al. [47] investigated the influence of KOH and H2SO4 modifications onto biochar during sorption of tetracycline. The results indicated that the KOH-exposed biochar showed high porosity, larger specific surface area, and high C and O composition than the H2SO4-exposed and virgin biochars. The remediation of inorganic constituents during alkali treatment allowed the biochar to sorb more pollutant.
The biochar hydrophilicity and surface functional sites can be chemically modified for remediation of specific pollutants at a specific rate from solutions [48]. It is well-known that carboxyl, amine, hydroxyl, phosphonate, and phenolic groups are functional groups often responsible for adsorption of different dyes/metals [49]. The biochar material exhibit low pollutant uptake capacities if the amount of these binding sites is low. Nevertheless, several modification techniques are present to improve the number of these functional sites on the surface of biochar. Xue et al. [50] highlighted that modification using H2O2 for peanut hull-derived biochar enhanced the oxygen-comprising functional groups particularly carboxyl groups on surface of biochar, which caused enhanced Pb(II) adsorption potential of over 20 times compared to raw biochar.
Biochar can also be prepared for particular applications through mineral impregnation methods. Yao et al. [51] improved the biochar functionality by distributing clay particles in biochar matrix. The authors mixed the biomaterial (bamboo, bagasse and hickory chips) with clay and consequently pyrolysed at 600°C without O2 for 1 h. The adsorption potential of clay-biochar composite was enhanced five times compared to virgin biochar due to highly porous structure and presence of clay. Magnetic biochar can be prepared through chemical co-precipitation of Fe2+/Fe3+ onto biomass and subsequent pyrolysis [17]. The hybrid nature of magnetic biochar permits enhanced adsorption of various organic and inorganic toxins. Through exposure of peanut hull biochar to FeCl3, Han et al. [52] synthesized magnetic biochar for removal of Cr(VI) ions. The prepared magnetic biochar showed improved adsorption potential toward Cr(VI), around 1–2 times compared to raw biochar. The study also identified the removal mechanism through XPS, XRD and SEM and revealed that Cr(VI) was interacted electrostatically to the protonated -OH onto the surface of γ-Fe2O3.
4. Mathematical modeling
Adsorption isotherm is the mathematical representation of adsorption capacity (
The Langmuir model [53] was fundamentally derived to define the sorption (gas-solid phase) of activated carbon. However, in later years, it was employed to assess and calculate the adsorption behavior of various adsorbents. In its formulation, binding to the surface was primarily by physical forces and implicit in its derivation was the assumption that all sites possess equal affinity for the sorbate. Its use was extended to empirically describe equilibrium relationships between a bulk liquid phase and a solid phase [53]. The model can be expressed as
where
The Freundlich model [54] was empirically derived equation; however it can be applied to adsorption onto diverse surfaces or surfaces with sites of varied affinities. It is assumed that the stronger binding sites are occupied first and that the binding strength decreases with increasing degree of site occupation. It can expressed as,
where
The Sips model [55] is based on the assumption that binding sites on the adsorbent have varied strengths and each active binding site interact with one sorbate molecule. The constant
where
The Toth model [57] is the other three parameter model frequently employed to describe metal-adsorent isotherms. The model assumes quasi-Gaussian energy distribution and is derived from the potential theory. The Toth model can be expressed as
Toth model:
where
For any practical applications, the process design, operation control and sorption kinetics are very important [13]. The sorption kinetics can be described using several models.
The most commonly used method to identify the contribution of intraparticle diffusion during adsorption is through fitting the kinetic data to an intraparticle diffusion plot, as presented by Weber and Morris [58] as below:
where
The pseudo-first-order model assumes that the rate of change of solute uptake with time is directly proportional to the difference in saturation concentration and the amount of solid uptake with time. The model can be expressed as,
where
The pseudo-second-order kinetics is framed to predict adsorption capcity over entire experimental conditions (ranges) as the model based on the adsorption capacity of the solid phase.
where
5. Conclusions
Biochar represents an effective class of sorbent for remediation of heavy metals from solutions. Several studies recognized superior adsorption potential of biochar compared to other established sorbents. The pyrolysis temperature and feedstock type strongly influences the sorption capacity of biochar. In addition, the process operating parameters such as pH, temperature, initial solute concentration and biochar dosage strongly influences the extent of metal sorption by biochar. Despite the application of biochar as sorbents is increasing as indicated through published literatures, more knowledge needed especially in the area of column sorption and real effluent clean-up.
References
- 1.
Arfanuzzaman M, Atiq Rahman A. Sustainable water demand management in the face of rapid urbanization and ground water depletion for social–ecological resilience building. Global Ecology and Conservation. 2017; 10 :9-22 - 2.
Rangabhashiyam S, Balasubramanian P. The potential of lignocellulosic biomass precursors for biochar production: Performance, mechanism and wastewater application—A review. Industrial Crops and Products. 2019; 128 :405-423 - 3.
Inyang MI, Gao B, Yao Y, Xue Y, Zimmerman A, Mosa A, et al. A review of biochar as a low-cost adsorbent for aqueous heavy metal removal. Critical Reviews in Environmental Science and Technology. 2016; 46 (4):406-433 - 4.
Dai Y, Zhang N, Xing C, Cui Q , Sun Q. The adsorption, regeneration and engineering applications of biochar for removal organic pollutants: A review. Chemosphere. 2019; 223 :12-27 - 5.
Abdolali A, Guo WS, Ngo HH, Chen SS, Nguyen NC, Tung KL. Typical lignocellulosic wastes and by-products for biosorption process in water and wastewater treatment: A critical review. Bioresource Technology. 2014; 160 :57-66 - 6.
Anyanwu BO, Ezejiofor AN, Igweze ZN, Orisakwe OE. Heavy metal mixture exposure and effects in developing nations: An update. Toxics. 2018; 6 (4):65 - 7.
Tchounwou PB, Yedjou CG, Patlolla AK, Sutton DJ. Heavy metal toxicity and the environment. Experientia Supplementum. 2012; 101 :133-164 - 8.
Vareda JP, Valente AJM, Durães L. Assessment of heavy metal pollution from anthropogenic activities and remediation strategies: A review. Journal of Environmental Management. 2019; 246 :101-118 - 9.
Vijayaraghavan K, Ashokkumar T. Plant-mediated biosynthesis of metallic nanoparticles: A review of literature, factors affecting synthesis, characterization techniques and applications. Journal of Environmental Chemical Engineering. 2017; 5 (5):4866-4883 - 10.
Jan AT, Azam M, Siddiqui K, Ali A, Choi I, Haq QM. Heavy metals and human health: Mechanistic insight into toxicity and counter defense system of antioxidants. International Journal of Molecular Sciences. 2015; 16 (12):29592-29630 - 11.
Vijayaraghavan K, Raja FD. Design and development of green roof substrate to improve runoff water quality: Plant growth experiments and adsorption. Water Research. 2014; 63 :94-101 - 12.
Senthilkumar R, Reddy Prasad DM, Lakshmanarao G, Krishnan S, Naveen Prasad BS. Ocean-based sorbents for decontamination of metal-bearing wastewaters: A review. Environmental Technology Reviews. 2018; 7 (1):139-155 - 13.
Vijayaraghavan K, Yun Y-S. Bacterial biosorbents and biosorption. Biotechnology Advances. 2008; 26 :266-291 - 14.
Aeisyah A, Ismail MHS, Lias K, Izhar S. Adsorption process of heavy metals by low-cost adsorbent: A review. Research Journal of Chemistry and Environment. 2014; 18 (4):91-102 - 15.
Rajapaksha AU, Chen SS, Tsang DCW, Zhang M, Vithanage M, Mandal S, et al. Engineered/designer biochar for contaminant removal/immobilization from soil and water: Potential and implication of biochar modification. Chemosphere. 2016; 148 :276-291 - 16.
Wang J, Wang S. Preparation, modification and environmental application of biochar: A review. Journal of Cleaner Production. 2019; 227 :1002-1022 - 17.
Li L, Zou D, Xiao Z, Zeng X, Zhang L, Jiang L, et al. Biochar as a sorbent for emerging contaminants enables improvements in waste management and sustainable resource use. Journal of Cleaner Production. 2019; 210 :1324-1342 - 18.
Yao Y, Gao B, Chen J, Yang L. Engineered biochar reclaiming phosphate from aqueous solutions: Mechanisms and potential application as a slow-release fertilizer. Environmental Science and Technology. 2013; 47 :8700-8708 - 19.
Beesley L, Moreno-Jiménez E, Gomez-Eyles JL, Harris E, Robinson B, Sizmur T. A review of biochars’ potential role in the remediation, revegetation and restoration of contaminated soils. Environmental Pollution. 2011; 159 (12):3269-3282 - 20.
Saleem J, Shahid UB, Hijab M, Mackey H, McKay G. Production and applications of activated carbons as adsorbents from olive stones. Biomass Conversion and Biorefinery. 2019; 9 (4):775-802 - 21.
Tan X, Liu Y, Zeng G, Wang X, Hu X, Gu Y, et al. Application of biochar for the removal of pollutants from aqueous solutions. Chemosphere. 2015; 125 :70-85 - 22.
Cao X, Ma L, Gao B, Harris W. Dairy-manure derived biochar effectively sorbs lead and atrazine. Environmental Science and Technology. 2009; 43 (9):3285-3291 - 23.
Uchimiya M, Wartelle LH, Klasson KT, Fortier CA, Lima IM. Influence of pyrolysis temperature on biochar property and function as a heavy metal sorbent in soil. Journal of Agricultural and Food Chemistry. 2011; 59 (6):2501-2510 - 24.
Jung KW, Kim K, Jeong TU, Ahn KH. Influence of pyrolysis temperature on characteristics and phosphate adsorption capability of biochar derived from waste-marine macroalgae ( Undaria pinnatifida roots). Bioresource Technology. 2016;200 :1024-1028 - 25.
Gai X, Wang H, Liu J, Zhai L, Liu S, Ren T, et al. Effects of feedstock and pyrolysis temperature on biochar adsorption of ammonium and nitrate. PLOS One. 2014; 9 (12):e113888 - 26.
Xiao Y, Xue Y, Gao F, Mosa A. Sorption of heavy metal ions onto crayfish shell biochar: Effect of pyrolysis temperature, pH and ionic strength. Journal of the Taiwan Institute of Chemical Engineers. 2017; 80 :114-121 - 27.
Vijayaraghavan K. Recent advancements in biochar preparation, feedstocks, modification, characterization and future applications. Environmental Technology Reviews. 2019; 8 (1):47-64 - 28.
Sohi SP, Krull E, Lopez-Capel E, Bol R. A review of biochar and its use and function in soil. Advances in Agronomy. 2010; 105 :47-82 - 29.
Hodgson E, Lewys-James A, Rao Ravella S, Thomas-Jones S, Perkins W, Gallagher J. Optimisation of slow-pyrolysis process conditions to maximise char yield and heavy metal adsorption of biochar produced from different feedstocks. Bioresource Technology. 2016; 214 :574-581 - 30.
Zhao L, Cao X, Wang Q , Yang F, Xu S. Mineral constituents profile of biochar derived from diversified waste biomasses: Implications for agricultural applications. Journal of Environmental Quality. 2013; 42 (2):545-552 - 31.
Li K, Jiang Y, Wang X, Bai D, Li H, Zheng Z. Effect of nitric acid modification on the lead(II) adsorption of mesoporous biochars with different mesopore size distributions. Clean Technologies and Environmental Policy. 2016; 18 (3):797-805 - 32.
Shakya A, Agarwal T. Removal of Cr(VI) from water using pineapple peel derived biochars: Adsorption potential and re-usability assessment. Journal of Molecular Liquids. 2019; 293 :111497 - 33.
Liu L, Huang Y, Zhang S, Gong Y, Su Y, Cao J, et al. Adsorption characteristics and mechanism of Pb(II) by agricultural waste-derived biochars produced from a pilot-scale pyrolysis system. Waste Management. 2019; 100 :287-295 - 34.
Senthilkumar R, Reddy Prasad DM, Govindarajan L, Saravanakumar K, Naveen Prasad BS. Synthesis of green marine algal-based biochar for remediation of arsenic(V) from contaminated waters in batch and column mode of operation. International Journal of Phytoremediation. 2020; 22 :279-286. DOI: 10.1080/15226514.2019.1658710 - 35.
Ullah A, Kaewsichan L, Tohdee K. Adsorption of hexavalent chromium onto alkali-modified biochar derived from Lepironia articulata : A kinetic, equilibrium, and thermodynamic study. Water Environment Research. 2019;91 (11):1433-1446 - 36.
Cobbina SJ, Duwiejuah AB, Quainoo AK. Single and simultaneous adsorption of heavy metals onto groundnut shell biochar produced under fast and slow pyrolysis. International Journal of Environmental Science and Technology. 2019; 16 (7):3081-3090 - 37.
Peng C, Xiao T, Li Z. Effects of pyrolysis temperature on structural properties of sludge-based biochar and its adsorption for heavy metals. Research of Environmental Sciences. 2017; 30 (10):1637-1644 - 38.
Padmesh TVN, Vijayaraghavan K, Sekaran G, Velan M. Batch and column studies on biosorption of acid dyes on fresh water macro alga Azolla filiculoides . Journal of Hazardous Materials. 2005;125 (1-3):121-129 - 39.
Vijayaraghavan K, Palanivelu K, Velan M. Biosorption of copper(II) and cobalt(II) from aqueous solutions by crab shell particles. Bioresource Technology. 2006; 97 (12):1411-1419 - 40.
Vijayaraghavan K, Thilakavathi M, Palanivelu K, Velan M. Continuous sorption of copper and cobalt by crab shell particles in a packed column. Environmental Technology. 2005; 26 (3):267-276 - 41.
Senthilkumar R, Vijayaraghavan K, Jegan J, Velan M. Batch and column removal of total chromium from aqueous solution using Sargassum polycystum . Environmental Progress & Sustainable Energy. 2010;29 (3):334-341 - 42.
Senthilkumar R, Vijayaraghavan K, Thilakavathi M, Iyer PVR, Velan M. Application of seaweeds for the removal of lead from aqueous solution. Biochemical Engineering Journal. 2007; 33 (3):211-216 - 43.
Vijayaraghavan K, Jegan J, Palanivelu K, Velan M. Removal of nickel(II) ions from aqueous solution using crab shell particles in a packed bed up-flow column. Journal of Hazardous Materials. 2004; 113 (1-3):223-230 - 44.
Vijayaraghavan K, Yun Y-S. Chemical modification and immobilization of Corynebacterium glutamicum for biosorption of reactive black 5 from aqueous solution. Industrial and Engineering Chemistry Research. 2007;46 (2):608-617 - 45.
Vilvanathan S, Shanthakumar S. Column adsorption studies on nickel and cobalt removal from aqueous solution using native and biochar form of Tectona grandis . Environmental Progress & Sustainable Energy. 2017;36 :1030-1038 - 46.
El-Hendawy ANA. Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon. Carbon. 2003; 41 :713-722 - 47.
Liu P, Liu W-J, Jiang H, Chen J-J, Li W-W, Yu H-Q. Modification of bio-char derived from fast pyrolysis of biomass and its application in removal of tetracycline from aqueous solution. Bioresource Technology. 2012; 121 :235-240 - 48.
Yakout SM, Daifullah AEHM, El-Reefy SA. Pore structure characterization of chemically modified biochar derived from rice straw. Environmental Engineering and Management Journal. 2015; 14 :473e480 - 49.
Vijayaraghavan K, Balasubramanian R. Is biosorption suitable for decontamination of metal-bearing wastewaters? A critical review on the state-of-the-art of biosorption processes and future directions. Journal of Environmental Management. 2015; 160 :283-296 - 50.
Xue Y, Gao B, Yao Y, Inyang M, Zhang M, Zimmerman AR, et al. Hydrogen peroxide modification enhances the ability of biochar (hydrochar) produced from hydrothermal carbonization of peanut hull to remove aqueous heavy metals: Batch and column tests. Chemical Engineering Journal. 2012; 200-202 :673-680 - 51.
Yao Y, Gao B, Fang J, Zhang M, Chen H, Zhou Y, et al. Characterization and environmental applications of clay-biochar composites. Chemical Engineering Journal. 2014; 242 :136-143 - 52.
Han Y, Cao X, Ouyang X, Sohi SP, Chen J. Adsorption kinetics of magnetic biochar derived from peanut hull on removal of Cr (VI) from aqueous solution: Effects of production conditions and particle size. Chemosphere. 2016; 145 :336-341 - 53.
Langmuir I. The constitution and fundamental properties of solids and liquids. Journal of the American Chemical Society. 1916; 38 :2221-2295 - 54.
Freundlich HMF. About the adsorption in solution. Zeitschrift für Physikalische Chemie. 1906; 57 :385-471 - 55.
Sips R. On the structure of a catalyst surface. The Journal of Chemical Physics. 1948; 16 :490-495 - 56.
Sathishkumar M, Binupriya AR, Vijayaraghavan K, Yun S-I. Two and three-parameter isothermal modeling for liquid-phase sorption of Procion blue H-B by inactive mycelial biomass of Panus fulvus . Journal of Chemical Technology & Biotechnology. 2007;82 (4):389-398 - 57.
Toth J. State equations of the solid gas interface layer. Acta Chimica Academiae Scientiarum Hungaricae. 1971; 69 :311-317 - 58.
Weber WJ, Morris JC. Kinetics of adsorption on carbon solution. Journal of the Sanitary Engineering Division: American Society of Civil Engineers. 1963; 89 :31 - 59.
Fan J, Cai C, Chi H, Reid BJ, Coulon F, Zhang Y, et al. Remediation of cadmium and lead polluted soil using thiol-modified biochar. Journal of Hazardous Materials. 2020; 388 :122037 - 60.
Xiao J, Hu R, Chen G. Micro-nano-engineered nitrogenous bone biochar developed with a ball-milling technique for high-efficiency removal of aquatic Cd(II), Cu(II) and Pb(II). Journal of Hazardous Materials. 2020; 387 :121980