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Chemosphere 252 (2020) 126539

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Chemosphere

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Review

Biochar technology in wastewater treatment: A critical review

Wei Xiang a, b, Xueyang Zhang a, b, *, Jianjun Chen c, Weixin Zou d, Feng He e, Xin Hu f, Daniel C.W. Tsang g, Yong Sik Ok h, Bin Gao b, **

a School of Environmental Engineering, Jiangsu Key Laboratory of Industrial Pollution Control and Resource Reuse, Xuzhou University of Technology, Xuzhou, 221018, China b Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, 32611, USA c Mid-Florida Research & Education Center, University of Florida, Apopka, FL, 32703, USA d Jiangsu Key Laboratory of Vehicle Emissions Control, Nanjing, 210093, China e College of Environment, Zhejiang University of Technology, Hangzhou, 310014, China f Center of Material Analysis, Nanjing University, Nanjing, 210093, China g Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China h Korea Biochar Research Centre & Division of Environmental Science and Ecological Engineering, Korea University, Seoul, South Korea

h i g h l i g h t s

* Corresponding author. School of Environmental En Xuzhou, 221018, China. ** Corresponding author.

E-mail addresses: [email protected] (X. Zhang), bg

https://doi.org/10.1016/j.chemosphere.2020.126539 0045-6535/© 2020 Elsevier Ltd. All rights reserved.

g r a p h i c a l a b s t r a c t

� Biochar technologies in various wastewater treatment are elucidated.

� Feedstock pre-treatment and post- treatment effect on biochar produc- tion is reviewed.

� Biochar as an innovative adsorbent to remove aqueous contaminants is discussed.

� Future perspectives of biochar tech- nology in wastewater treatment are summarized.

a r t i c l e i n f o

Article history: Received 27 January 2020 Received in revised form 11 March 2020 Accepted 17 March 2020 Available online 18 March 2020

Handling Editor: X. Cao

Keywords: Engineered biochar Wastewater treatment Production technologies Modification methods Carbonaceous adsorbents

a b s t r a c t

Biochar is a promising agent for wastewater treatment, soil remediation, and gas storage and separation. This review summarizes recent research development on biochar production and applications with a focus on the application of biochar technology in wastewater treatment. Different technologies for biochar production, with an emphasis on pre-treatment of feedstock and post treatment, are succinctly summarized. Biochar has been extensively used as an adsorbent to remove toxic metals, organic pol- lutants, and nutrients from wastewater. Compared to pristine biochar, engineered/designer biochar generally has larger surface area, stronger adsorption capacity, or more abundant surface functional groups (SFG), which represents a new type of carbon material with great application prospects in various wastewater treatments. As the first of its kind, this critical review emphasizes the promising prospects of biochar technology in the treatment of various wastewater including industrial wastewater (dye, battery manufacture, and dairy wastewater), municipal wastewater, agricultural wastewater, and stormwater. Future research on engineered/designer biochar production and its field-scale application is discussed. Based on the review, it can be concluded that biochar technology represents a new, cost effective, and environmentally-friendly solution for the treatment of wastewater.

© 2020 Elsevier Ltd. All rights reserved.

gineering, Jiangsu Key Laboratory of Industrial Pollution Control and Resource Reuse, Xuzhou University of Technology,

[email protected] (B. Gao).

W. Xiang et al. / Chemosphere 252 (2020) 1265392

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Production technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2.1. Pre-treatment technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.2. Thermal carbonization technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Post-treatment technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3. Biochar as an adsorbent for aqueous contaminant removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.1. Heavy metal removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3.2. Organic contaminant removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.3. Nitrogen and phosphorus removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

4. Biochar technology in wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Industrial wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.2. Municipal wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.3. Agricultural wastewater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.4. Stormwater treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

5. Conclusions and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Declaration of competing interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

1. Introduction

Biochar is a porous carbonaceous material produced during the thermochemical decomposition of biomass feedstock in the pres- ence of little or no oxygen. Biomass feedstock can be any organic waste materials which include crop and forest residues, wood chip, algae, sewage sludge, manures, and organic municipal solid wastes (Colantoni et al., 2016; Xiong et al., 2019). Methods for thermo- chemical decomposition include pyrolysis, hydrothermal carbon- ization, gasification, torrefaction, and microwave heating, varying in thermochemical temperature and duration (Mohan et al., 2014; Gonz�alez et al., 2017; Fang et al., 2018). Interest in biochar is largely based on its two distinct benefits: First, biochar production per se can offset greenhouse gas emission because it stores carbon in a stable form, preventing the release of greenhouse gases into the atmosphere from biomass degradation (Creamer and Gao, 2016; Yang et al., 2018a). Second, biochar is an effective, low-cost, and environment-friendly adsorbent (Cha et al., 2016; Inyang et al., 2016), which is related to its relatively large surface area and abundant surface functional groups (SFG) (Wang et al., 2017a; Zhang et al., 2017a). Biochar can be used for adsorbing metals/ metalloids and purifying water (Agrafioti et al., 2013; Van Vinh et al., 2015; Palansooriya et al., 2019), applied to soils for improving soil fertility and crop productivity (Yoo et al., 2018), employed for clean energy production to partially replace the fossil fuels (Fang et al., 2018; Cao et al., 2019), and utilized as adsorbent and catalysts to various pollutants and reduce greenhouse gas emission (Xiong et al., 2017). As a result, biochar becomes increasingly important as a solution to some global problems, such as climate change, environmental pollution, and soil degradation (Creamer and Gao, 2016).

It has been well documented that feedstock, thermochemical decomposition methods and their temperature and duration can significantly affect biochar physical and chemical properties (Yu et al., 2019). Several previous review articles have discussed decomposition methods, characterization, and applications of biochar in removal of different contaminants from aqueous solu- tions (Mohan et al., 2014; Cha et al., 2016; Tan et al., 2016). Biochar properties can also be affected by feedstock pre-treatments and biochar post-treatments (Wang et al., 2017a; Yang et al., 2019). As

shown in Fig. 1, pre-treatments vary depending on feedstock and the purposes for biochar use, including physical (dry, crush, sieve, wash, etc.), chemical (treat with chemicals or functional materials, load of precursors and functional agents, etc.), and biological (bacterial treatment, etc.) methods. Post-treatments mainly rely on physical (ball milling, magnetization, etc.) and chemical (corrosive treatment, etc.) methods (Zhang and Gao, 2013; Tan et al., 2016; Usman et al., 2016). Thus far, only few review articles have emphasized pre-treatments in relation to feedstock decomposition methods and resultant biochar properties as well as post-treatment technologies on biochar properties and their effects on wastewater treatment.

The overarching objective of this work is to present the first comprehensive review on the applications of biochar technology in wastewater treatment. After summarizing new technologies on pre-treatment of feedstock, thermal carbonization process, and post-treatment of biochar (Section 2), this review digests current knowledge of biochar as an innovative adsorbent for aqueous contaminants (Section 3). Most importantly, recent advances of biochar applications in wastewater treatments, including industrial wastewater, municipal wastewater, agricultural wastewater and stormwater are perspicuously and detailly elucidated (Section 4). This critical review also discusses the perspectives and future research directions of the biochar technology in wastewater treatment (Section 5).

2. Production technologies

2.1. Pre-treatment technologies

Pre-treatment is the first step for biochar production from different raw materials. In general, these methods can be classified physical, chemical, and biological pre-treatment technologies (Fig. 1).

Physical pre-treatment technology generally includes drying, crushing, sieving, and washing of biomass feedstock. The feedstock riches in lignocellulosic/plant is usually dried to constant weight at 105 �C or other temperature, ground into smaller particles using a hammer mill, and then cut into different pieces (Wang et al., 2016a; Essandoh et al., 2017; Zhang et al., 2017a). Occasionally, separate

Fig. 1. Biochar production technologies: (a) Biomass. (b) Pre-treatment technologies. (c) Thermal processes. (d) Post-treatment technologies.

W. Xiang et al. / Chemosphere 252 (2020) 126539 3

drying step may be needed for some plant feedstock, because the plant raw materials vary greatly in moisture contents. Physical pre- treatment method for biomass feedstock is related to its own properties. For example, dewatered sludge is often dried in an oven overnight, crushed, sieved, and stored in sealed containers prior to use (Agrafioti et al., 2013). Newspapers and cardboard are commonly shredded and blended into pulp as the feedstock (Randolph et al., 2017). Paper mill sludge is acid-washed, rinsed with deionized distilled water to obtain mineral-free sludge (Cho et al., 2017). Algae is alkaline, it is usually rinsed with fresh water and then dried, granulated or flaked before pyrolysis (Roberts and de Nys, 2016).

Chemical pre-treatment technology often relies on chemical reactions to change the properties or compositions of feedstock materials. One type of most commonly used chemical pre- treatment technique is to treat feedstock biomass materials with chemicals or functional materials to load chemical precursors or functional agents into the feedstock. During the treatment, the biomass feedstock is immersed into a chemical solution or a colloidal suspension, and then dried prior to biochar production (Tan et al., 2016). After pretreated with metal ion solutions such as FeCl3, AlCl3, and MgCl2, biomass feedstock can be successfully converted into biochar-based nanocomposites with metal oxy- hydroxide (e.g. Fe3O4, AlOOH, and MgO) nanoparticles stabilized on carbon surface with the pores of the engineered biochar (Zhang et al., 2012a, 2013; Zhang and Gao, 2013; Son et al., 2018). On the other hand, biomass can be pretreated with engineered nano- particles and natural colloids including carbon nanotubes, gra- phene and clay, which also leads to the successful production of biochar-based nanocomposites (Zhang et al., 2012b; Yao et al., 2014; Inyang et al., 2015). Corrosive chemicals including acid, al- kali, and oxidant have also been applied to pretreat biomass for the

production of engineered biochar with enlarged surface area, unique pore structure, enriched SFG, etc. (Zhou et al., 2017a; Zhao et al., 2018).

Biological pre-treatment technology is a relatively new concept that utilizing biological processes to improve biomass feedstock for engineered biochar production (Wang et al., 2017a). Bacterial treatment, particularly anaerobic digestion or biofuel processes, of biomass feedstock has been proven to be an effective and product ‘biologically activated’ biochar with enhanced properties (Inyang et al., 2010; Yao et al., 2015). In the literature, several biomass materials including sugar beet tailings, bagasse, sludge, and animal waste were subjected to the anaerobic digestion process first and then the residues were converted into biochar through slow py- rolysis (Inyang et al., 2010; Yao et al., 2011a; Tang et al., 2019). The anaerobic digestion pre-treatment would make the obtained bio- char have a larger specific surface area (SSA) and better adsorption performance (Inyang et al., 2010; Yao et al., 2011a). It is recognized that utilizing the biological pre-treatment residue materials to produce biochar can introduce additional benefits such as reducing waste disposal costs, and making bioenergy more eco-friendly (Inyang et al., 2010; Yao et al., 2015). Another biological pre- treatment method uses biomass enriched with high concentra- tions of minerals including heavy metals through bioaccumulation for biochar production (Yao et al., 2013b; Wang et al., 2017c). Wang et al. (2017c) converted a heavy metal hyperaccumulating plant into biochar and suggested that this technology not only provides a safe solution for hyperaccumulator disposal but also produces value-added biochar nanocomposites.

2.2. Thermal carbonization technologies

Thermal processes for biomass conversion into biochar mainly

Fig. 2. Percent yields of biochar from different feedstock at different pyrolysis tem- perature (data are from reference (Yuan et al., 2011; Bian et al., 2016; Colantoni et al., 2016; Irfan et al., 2016; Lin et al., 2016; Wang et al., 2016a; Gonz�alez et al., 2017)).

Fig. 3. Percent of carbon and ash in biochar from different feedstock at different py- rolysis temperature (data are from reference (Hossain et al., 2011; Al-Wabel et al., 2013; Ma�sek et al., 2013)).

W. Xiang et al. / Chemosphere 252 (2020) 1265394

include pyrolysis, microwave-assisted pyrolysis, hydrothermal carbonization and gasification (Mohan et al., 2014; Wang et al., 2017a; Fang et al., 2018). Table 1 summaries and compares these carbonization technologies.

Pyrolysis is a thermochemical process for decomposing biomass in an anoxic or hypoxic environment (Cha et al., 2016). Pyrolysis processes depend on the operating temperature, heating rate, and residence time used, which can affect the compositions and phys- icochemical properties of products. The yields of biochar decrease with increasing pyrolysis temperature (Fig. 2), whereas ash and carbon content increase (Fig. 3). It is mainly related to cellulose, hemicellulose and lignin decomposition, saline-alkali separation, carbonization and other factors in biomass (Hossain et al., 2011; Al- Wabel et al., 2013; Ma�sek et al., 2013; Irfan et al., 2016). The heating rate determines the pyrolysis speed, and it influences the charac- teristics of biochar and the yield of bio-oil and bio-gas (Inyang et al., 2010; Cho et al., 2017). Prolonged residence time provides more complete biomass decomposition while decrease the biochar pro- duction yield (Mohamed et al., 2016).

Microwave-assisted pyrolysis (MAP) is considered as a sustain- able method to produce bio-energy products, including biochar, bio-oil, and bio-gas (Dai et al., 2017; Mutsengerere et al., 2019). In comparison to the conventional methods, MAP technique offers shorter processing time, lower energy requirement, more effective heat transfer, and better selective heating (Zhang et al., 2017b; Dur�an-Jim�enez et al., 2018). The MAP process is mainly controlled by the microwave power, irradiation time, etc. (Lam et al., 2017; Dur�an-Jim�enez et al., 2018; Nhuchhen et al., 2018; Kadlimatti et al., 2019). The yield of biochar often decreases as the microwave power increases, which can be attributed to the high heating rates at high microwave power levels (Jimenez et al., 2017; Nhuchhen et al., 2018). Biochar with high SSA was obtained in a microwave sys- tem operated at the microwave power of 500 W, irradiation time of 3 min, and frequency of 2450 ± 25 MHz. (Dur�an-Jim�enez et al., 2018). Further microwave treatment, however, resulted in a loss of SSA, which can be attributed to the degradation of micropore structure of the biochar after the microwave overheating (Jimenez et al., 2017).

Hydrothermal carbonization (HTC) is the conversion of wet feedstock at a temperature range of 120e260 �C into biochar without pre-drying (Mohan et al., 2014; Cha et al., 2016; Fang et al., 2018). The wet biomass is heated and pressurized (2e10 MPa) for 5e240 min in a confined system (Kambo and Dutta, 2015; Fang et al., 2018; Zhang et al., 2019a). The biochar produced by HTC is

Table 1 Summary of common thermal carbonization technologies (Cha et al., 2016; You et al., 2017; Mutsengerere et al., 2019; Zhang et al., 2019b).

Thermal carbonization technologies

Key parameters Temperature/ power range

Residence time

Desired product

Advantages

Pyrolysis temperature; heating rate; residence time

300e850 �C 1e3 h Biochar Simple, robust, and cost-effective; applicable to small scale and farm-based biochar production

Microwave-assisted pyrolysis

microwave power; microwave irradiation time

400e500 W 1e10 min Biochar and biofuel

volumetric, fast, selective, and efficient heating

Hydrothermal carbonization

temperature; residence time; pressure; water-to-biomass ratio

120e260 �C 1e16 h Hydrochar More suitable for feedstock with high moisture content

Gasification temperature; particle size; residence time; pressure; gasification agent/ biomass ratio

＞800 �C 10e20 s Syngas Biochar yield of gasification is less than pyrolysis, but the biochar contains a high level of alkali salts (Ca, K, Si, Mg, etc.).

W. Xiang et al. / Chemosphere 252 (2020) 126539 5

usually called hydrochar. Reaction temperature is identified as the governing parameter during the HTC (Kambo and Dutta, 2015). With the increase of temperature, hydrochar contains abundant acidic functional groups on its surface, which can benefit the contaminant adsorption capability (Zhou et al., 2017a; Saha et al., 2019). Increasing holding temperature and holding time can in- crease the porous structure of the hydrochar, which increases the possibility of the application of hydrochar as an adsorbent (Shao et al., 2019).

Gasification is the process converting the biomass to gas fuel using gasification agents. Gasification temperature is generally higher than 800 �C (You et al., 2017). The biochar produced during gasification usually contains high levels of alkali salts and alkaline earth mineral (Kambo and Dutta, 2015; Zhang et al., 2019b), which can precipitate many heavy metal contaminants and thus be used directly as a remediation agent in problem soils (Yang et al., 2018b; Yu et al., 2019). Deal et al. (2012) reported that problem soils amended with gasifier-produced biochar had higher maize yields, and the soluble ash content of the biochar had the greatest influ- ence on soil productivity.

2.3. Post-treatment technologies

Biochar are often post-treated by either physical or chemical modification methods to increase its SSA, pore volume, surface chemistry, and functional agents including SFG and composited nanoparticles (Van Vinh et al., 2015; Tan et al., 2016; Dai et al., 2017). In the literature, there are several good reviews that have provided comprehensive summaries of various post-treatment technologies for biochar modifications (Tan et al., 2016; Wang et al., 2017a). This review thus only slightly discusses three post- treatment technologies including magnetic, ball milling, and cor- rosive (i.e., acid, alkali, or oxidation) treatment (Mohamed et al., 2016; Usman et al., 2016; Wang et al., 2017a), which either are current research hotspots or have not reviewed intensively in the literature.

Magnetization is the method that converts biochar into a magnetic material where magnetic iron oxides including Fe3O4, g- Fe2O3, or CoFe2O4 particles are loaded into biochar (Zhang et al., 2013; Wang et al., 2015b; Tan et al., 2016; Shengsen Wang et al., 2019). Thus, magnetic modified biochar can easily be recovered from the aqueous solution (Zhang et al., 2013; Mohan et al., 2014; Wang et al., 2015b; Son et al., 2018). Magnetic zero-valent iron biochar derived from peanut hull at 800 �C has a higher removal rate for Cr6þ, which is mainly due to its high SSA, pore volume, and loaded reductive iron (Liu et al., 2019b). Another method for pre- paring magnetic biochar composites is directly chemical co- precipitate Fe3þ/Fe2þ on biochar surface (Tan et al., 2016). Mag- netic switchgrass biochar prepared by the precipitation of iron oxide using an aqueous Fe3þ/Fe2þ solution has the highest adsorption capacity for metribuzin (205 mg/g, pH ¼ 2) (Essandoh et al., 2017).

Ball milling is a simple and efficient method which uses the kinetic energy by moving balls to break chemical bonding, chang- ing the particle shape and producing nanoscale particles (Lyu et al., 2017). After ball milling, the characteristics of biochar were enhanced including SSA, pore volume, negative zeta potential, oxygen-containing functional groups, and the adsorption capacity (Wang et al., 2017a; Lyu et al., 2018a, 2018b; Xiang et al., 2020). Ball- milled bagasse biochar has higher Ni2þ removal efficiency than pristine biochar, and the adsorption capacity of Ni2þ and aqueous methylene both increased (Lyu et al., 2018b). This is mainly due to the fact that ball milling can increase the external and internal surface areas of the biochar and expose its graphitic structure and oxygen-containing functional groups (Lyu et al., 2018a). Nitrogen-

doped biochar has been successfully synthesized by simply ball milling pristine biochar with ammonium hydroxide, these N groups improve the adsorption performances of the biochar on acidic carbon dioxide and anionic reactive red (Xu et al., 2019). Ball- milling technology is thus an effective engineering method to produce novel engineered biochar. The ball-milled biochar shows enhanced physicochemical and adsorptive properties, and can be used in various environmental applications.

Corrosive treatments such as acid, alkali, and oxidation treat- ments are commonly used chemical modification techniques, which alter the surface chemistry of the biochar. The corrosive chemicals, such as HCl, HNO3, KOH, NaOH, KMnO4, and H2O2 have been applied to modify biochar for different purposes (Wang et al., 2015a, 2017a; Cha et al., 2016; Zheng et al., 2019). The chemical modified biochar has higher SSA, more microporous, more func- tional groups, and enhanced sorption capacity (Yang et al., 2019). Alkali (NaOH)-acid (HNO3) combined modification shows an obvious increased BET surface area, porosity and oxygen- containing functional groups of municipal sewage sludge biochar, which enhances tetracycline adsorption, up to 286.9 mg/g (Tang et al., 2018). KMnO4 and KOH treatment increase the SSA of bio- char derived from waste peanut shell, resulting in increased adsorption sites for Ni2þ (An et al., 2019). H2O2 is another strong oxidant for modifying biochar (Xue et al., 2012). H2O2-modified manure biochar can eliminate heavy metals efficiently, due to the increased oxygen and carboxyl group content (Wang and Liu, 2018).

Post-treatment of biochars represent a new area of research. It modifies existing biochars by increasing biochars’ SSA, pore vol- ume, negative zeta potential, oxygen-containing functional groups, and the adsorption capacity. Such modified biochars can be cost- effective and environmentally-friendly carbon materials with great application potential in many fields.

3. Biochar as an adsorbent for aqueous contaminant removal

Biochar can be used as an adsorbent to remove different pol- lutants in water and wastewater. Here, we mainly discuss its use for removal of heavy metals, organic contaminants, nitrogen and phosphorus.

3.1. Heavy metal removal

Heavy metals in wastewater can adversely affect human beings, animals, and plants. Long term exposure to heavy metals in the aqueous phase can cause serious health threats even at low con- centration (Ahmed et al., 2016). Increased evidence suggests that biochar obtained from plants and animal residues can effectively adsorb heavy metals in water and wastewater (Higashikawa et al., 2016; Inyang et al., 2016; Tan et al., 2016; Dai et al., 2017; Zhou et al., 2017a). Table 2 summarizes biochar adsorption of heavy metals in aqueous phase.

Arsenic is an extremely toxic metal and occurs in wastewater as well as drinking water. The adsorption capacity of As3þ is enhanced from 5.7 mg/g to 7.0 mg/g through the surface modification of bio- char by Zn(NO3)2 impregnation (Van Vinh et al., 2015). Biochar produced from paper mill sludge was applied to adsorb As5þ and the maximum adsorptive capacity was 34.1 mg/g (Cho et al., 2017). Biochars produced separately from sugarcane straw, rice husk, sawdust, and chicken manure were mixed with sawdust and used to remove Cd2þ in water. Results show that increased pyrolysis temperature from 350 �C to 650 �C triggers the increasing tread in percentage removal of Cd2þ (Higashikawa et al., 2016). Biochars are also effective in removal of Pb2þ. The removal efficiencies of Pb2þ by biochars produced from fresh and dehydrated banana peels are 359 mg/g and 193 mg/g, respectively (Zhou et al., 2017a). Table 2

Table 2 Biochar adsorption of heavy metals in aqueous solutions.

Biochar feedstock

Pre-treatment Thermal process

Post treatment Pyrolysis temperature (�C)

Biochar dose (g/ L)

Adsorption pH

Heavy metals

Initial concentration (mg/L)

Adsorption capacity (mg/g)

Removal mechanism Ref.

Bamboo wood

Oven dried Pyrolysis HNO3þ nZVI treated

600 2 e Agþ 200 584 Innersphere complexation and electrostatic attraction by outer-layer Fe oxides under oxic conditions

Wang et al. (2017b)

Bamboo wood

Oven dried Pyrolysis H2O2þ nZVI treated

600 2 e Agþ 200 1217 Innersphere complexation and electrostatic attraction by outer-layer Fe oxides under oxic conditions

Wang et al. (2017b)

Pomelo peel

Dried þ H3PO4 impregnated

Pyrolysis Pristine 250 2 6 Agþ 50 137.4 Chemical adsorption with oxygenic functional groups

Zhao et al., (2018)

Pine wood Oven dried and milled

Pyrolysis Ni/Fe-LDH modified

600 2.5 7.5 As3þ 20 4.38 Electrostatic attraction and surface complexation with hydroxyl groups

Wang et al. (2016b)

Pine wood Ni/Fe-LDH modified

Pyrolysis Pristine 600 2.5 7.5 As3þ 20 1.56 Electrostatic attraction and surface complexation with hydroxyl groups

Wang et al. (2016b)

Paper mill sludge

Oven dried and acid washed

Pyrolysis Pristine 720 1 2.7e10.4 As5þ 26.7 34.1 Chemisorption or chemical reaction process between available adsorption sites and adsorbate

Cho et al., (2017)

Sewage sludge

Stirred and heated

Pyrolysis Pristine 300 4 e As5þ 0.05 e Chemical sorption Agrafioti et al., (2013)

Sewage sludge

Stirred and heated

Pyrolysis Pristine 300 4 e Cr3þ 0.2 e Chemical sorption Agrafioti et al., (2013)

Rice husk Washed Pyrolysis Polyethylenimine modified

450e500 1 e Cr6þ 100 435.7 Introduction of amino group facilitate chemical reduction of Cr6þ and increase sorption capacity

Rajapaksha et al., (2016)

Green waste

Dried Pyrolysis HCl modified 600 2 3e8 Cd2þ 5.6 6.72 Chemisorption Zhang et al., (2018)

Peanut shell Washed, dried and milled

Pyrolysis Hydrated manganese oxide treated

400 0.2 6.5 Cd2þ 10 10 Nonspecific outer- sphere surface complexation provided by oxygen-containing groups, specific innersphere complexation offered by the impregnated HMO

Wan et al., (2018)

Marine macro- algal

FeCl3 immersed

Pyrolysis Pristine 500 16.7 e Cu2þ e 69.37 Oxygen-containing functional groups as potential adsorption sites

Son et al., (2018)

Banana peels

Oven dried Pyrolysis Pristine 600 2.5 e Cu2þ 200 75.99 Electrostatic attraction, partial of physisorption, ion exchange and precipitation

Ahmad et al., (2018)

Cauliflower leaves

Oven dried Pyrolysis Pristine 600 2.5 e Cu2þ 150 53.96 Electrostatic attraction, partial of physisorption, ion exchange and precipitation

Ahmad et al., (2018)

Pomelo peel

Dried þ H3PO4 impregnated

Pyrolysis Pristine 250 2 6 Pb2þ 50 88.7 Precipitated by phosphorous functional groups

Zhao et al., (2018)

Peanut shell Washed, dried and milled

Pyrolysis Hydrated manganese oxide treated

400 0.2 6.5 Pb2þ 20 36 Nonspecific outer- sphere surface complexation provided by oxygen-containing groups, specific innersphere complexation offered by the impregnated HMO

Wan et al., (2018)

Banana peels

Oven dried Pyrolysis Pristine 600 2.5 e Pb2þ 600 247.1 Electrostatic attraction, partial of physisorption,

W. Xiang et al. / Chemosphere 252 (2020) 1265396

Table 2 (continued )

Biochar feedstock

Pre-treatment Thermal process

Post treatment Pyrolysis temperature (�C)

Biochar dose (g/ L)

Adsorption pH

Heavy metals

Initial concentration (mg/L)

Adsorption capacity (mg/g)

Removal mechanism Ref.

ion exchange and precipitation

Ahmad et al., (2018)

Cauliflower leaves

Oven dried Pyrolysis Pristine 600 2.5 e Pb2þ 200 177.8 Electrostatic attraction, partial of physisorption, ion exchange and precipitation

Ahmad et al., (2018)

Maple wood

Dried Pyrolysis H2O2 modified 500 5 7 Pb 2þ 50 43.3 Complexation by

oxygen functional groups

Wang et al., (2018)

Pecan nutshell

Dried and milled

MAP Pristine e 2 3 Pb2þ 500 80.3 Ion-exchange by calcium ions on the material surface

Jimenez et al., (2017)

Banana peels

Dehydrated and grinded

HTC Pristine 230 0.25 7 Pb2þ 200 359 Ions exchange and surface complexation.

Zhou et al. (2017a)

Banana peels

H3PO4 soaked HTC Pristine 230 0.25 7 Pb 2þ 200 193 Ions exchange and

surface complexation. Zhou et al. (2017a)

Peanut hull Dried HTC Pristine 300 2 e Pb2þ 50 0.88 Complexation with carboxyl surface functional groups

Xue et al., (2012)

Peanut hull Dried HTC H2O2 modified 300 2 e Pb 2þ 50 22.82 Complexation with

carboxyl surface functional groups

Xue et al., (2012)

W. Xiang et al. / Chemosphere 252 (2020) 126539 7

also shows biochar adsorption of Cr3þ, Ni2þ and Cu2þ. Biochar prepared from sewage sludge adsorbed approximately 70% of Cr3þ

from the aqueous solution (Agrafioti et al., 2013). The maximum adsorption capacity of Ni2þ from water by chicken manure mixed with sawdust-derived biochars was 11 mg/g at 650 �C (Higashikawa et al., 2016). Marine macro-algae magnetic biochars are rich in oxygen-functional groups, which attributes to their high selectivity and adsorption capacity to Cu2þ (69.37 mg/g for kelp magnetic biochar and 63.52 mg/g for hijikia magnetic biochar) (Son et al., 2018).

3.2. Organic contaminant removal

Organic contaminants are another major type of pollutants in aquatic environment, which include pesticides, herbicides, and antibiotics etc.. Table 3 summarizes biochar adsorption of some organic contaminants in aqueous phase. Organic pollutants are toxic and can reduce dissolved oxygen in water and cause harm to the aquatic ecosystem and human health (Ahmed et al., 2016).

Switchgrass biochar (SGB) and magnetic switchgrass biochar (MSGB) were employed to remove metribuzin herbicide from aqueous solutions. The low solution pH value is beneficial to bio- char for the metribuzin adsorption compared to the high solution pH value. Metribuzin adsorption onto both SGB and MSGB is un- affected by temperature increase (Essandoh et al., 2017). Biochars can also remove antibiotics, such as sulfonamides and tetracyclines (Yao et al., 2012a; Sun et al., 2018). The mechanism underlying the removal of sulfonamides and tetracyclines is probably due to the electron donor-acceptor interactions and associated with the attracting groups on surface area rings (Peiris et al., 2017). Sulfa- methoxazole (SMX) is one of the typical sulfonamide antibiotics widely used for both human and animals. SMX adsorption onto the digested bagasse biochars is mainly controlled by p-p interaction and effected by the solution pH value (Yao et al., 2018). Iron and zinc doped sawdust biochar shows high simultaneous removal of tetracycline from aqueous solution. The predominant adsorption mechanisms include site recognition, bridge enhancement, and site competition (Zhou et al., 2017b).

In addition, several studies have also suggested biochar’s ap- plications for adsorption of organic matter for water treatment, and

the effectiveness is closely related to the aromaticity index, polarity index, SSA, and the quantity of oxygen functional groups (Mohan et al., 2014; Cha et al., 2016; Braghiroli et al., 2018).

3.3. Nitrogen and phosphorus removal

Biochar can also absorb nutrients, such as nitrogen and phos- phorus in aqueous phase (Zhang et al., 2012a, 2014; Yao et al., 2013b; Zhang and Gao, 2013; Xue et al., 2016). Ammonium, ni- trate and phosphate are the common forms of reactive nitrogen and phosphorus in wastewater, and can lead to eutrophication (Yao et al., 2012b; Yang et al., 2017; Xu et al., 2018). Table 4 lists the adsorptions of nitrogen and phosphorus on various biochars in aqueous phase. The adsorption capacity of modified biochars for nitrogen and phosphorus is significantly higher than pristine bio- chars, because the modified biochars have higher SSA, more reac- tion activity and SFG.

Post-treatment of biochars have significant effects on ammo- nium adsorption. Oxidized maple wood biochar has higher ammonium adsorption capacity than maple wood biochar (Wang et al., 2016a). Additionally, pyrolysis temperatures affect ammo- nium adsorption. Biochars produced from pine sawdust at 300 �C shows the highest NH4

þ adsorption capacity based on the higher H/ C and O/C ratios and presence of more functional groups on the surface of it (Yang et al., 2017). This study demonstrates that chemical bonding and polar interaction between NH4

þ and SFG are likely mechanisms for enhanced NH4

þ adsorption. Pre-treatment of feedstock show pronounced effects on

adsorption of phosphorus. The digested sugar beet tailing biochar shows the highest phosphate removal ability with a removal rate around 73% (Yao et al., 2011a). This is probably because the large amount of colloidal and nano-sized periclase on its surface, which has a strong ability to bind phosphate in aqueous solution. Pre- treatment can be performed during plant growth. For example, the biochar derived from tomato plants that enriched with Mg during their growth, which shows increased adsorption of phos- phate in aqueous solution, reaching more than 100 mg/g (Yao et al., 2013b). Additionally, biochars produced from wood waste pre- treated with magnesium oxides (Mg-biochar) was used to recover ammonium and phosphate (Xu et al., 2018). The struvite

Table 3 Biochar adsorption of organic contaminants in aqueous solutions.

Biochar feedstock

Treatment/ Modification

Pyrolysis temperature (�C)

Biochar dose (g/ L)

Organic contaminants

Initial concentration (mg/L)

Adsorption capacity (mg/ g)

Removal mechanism Ref.

Switchgrass Magnetization 425 1 Metribuzin herbicide

100 39.6 Electrostatic attraction and hydrogen bonds Essandoh et al., (2017)

Switchgras Pristine 425 1 Metribuzin herbicide

100 38.2 Electrostatic attraction and hydrogen bonds Essandoh et al., (2017)

Bagasse Anaerobically digested

600 2 Sulfamethoxazole 10 1.6 p-p EDA interaction Yao et al., (2017)

Bagasse Anaerobically digested

600 2 Sulfapyridine 10 3.2 p-p EDA interaction Yao et al., (2017)

Bamboo sawdust

Graphene oxide-coated

600 1 Sulfamethazine 10 6.5 p-p EDA interaction, pore-filling, cation exchange, hydrogen bonding interaction and electrostatic interaction

Huang et al., (2017)

Bamboo sawdust

Pristine 600 1 Sulfamethazine 10 3.1 p-p EDA interaction, pore-filling, cation exchange, hydrogen bonding interaction and electrostatic interaction

Huang et al., (2017)

Sawdust Iron and zinc doped

600 / Tetracycline 150 86 Site recognition, bridge enhancement, and site competition

Zhou et al. (2017b)

Sawdust Iron and zinc doped

600 / Tetracycline 100 53.8 Site recognition, bridge enhancement, and site competition

Zhou et al. (2017b)

Peanut shell

Magnetization 800 2 Trichloroethylene 9.2 4.6 Hydrophobic partitioning, pore-filling and reductive degradation.

Liu et al. (2019b)

Reed Magnetization 600 0.5 Florfenicol 20 5.3 Hydrogen bonding, pore-filling effect and p-p EDA interaction

Zhao and Lang, (2018)

Reed Pristine 600 0.5 Florfenicol 20 2.6 Pore-filling effect and p-p EDA interaction Zhao and Lang, (2018)

Crab shell calcium-rich biomass

800 1 Chlortetracycline hydrochloride

100 70 Cation bridging, electrostatic interaction, hydrogen bonding and p-p interaction

Xu et al., (2020)

Crab shell calcium-rich biomass

800 1 Chlortetracycline hydrochloride

2000 1975 Adsorption and flocculation Xu et al., (2020)

W. Xiang et al. / Chemosphere 252 (2020) 1265398

precipitation on the surface of biochar is the dominant mechanism for the removing ammonium and phosphate. Other reports have also shown modified biochars for removing the nitrate (NO3

�), total Kjeldahl nitrogen (TKN), total nitrogen (TN), total phosphates (TP), and phosphate (PO4

3�) from aqueous solutions (Mohan et al., 2014; Usman et al., 2016; Sun et al., 2017; Vikrant et al., 2017). A general conclusion is that the modifications change biochar surface chemistry, thus resulting in enhanced nutrients sorption capacity compared with pristine biochars.

4. Biochar technology in wastewater treatment

As discussed above, biochars are effective adsorbents for removal of various contaminants due to its special properties, such as large SSA and abundant SFG. Thus, biochars have become increasingly important as a solution to remediate pollutants in the industrial and agricultural sectors for improving environmental quality (Wang et al., 2017a). Wastewater has been a global issue, which is a byproduct of domestic, industrial, commercial or agri- cultural activities. Biochars have great potential to be used for wastewater treatment. This section mainly focuses on discussing biochar’s applications in treatment of industrial wastewater, municipal wastewater, agricultural wastewater and stormwater (Fig. 4).

4.1. Industrial wastewater treatment

The industrial wastewater comes from various sources including mining, smelting, battery manufacturing, chemical industry, leather manufacturing, dyes, and others. And the pollutants are mainly heavy metals and organic pollutants in industrial

wastewater. Biochars have been applied in the treatment of in- dustrial wastewater.

A biochar mixed with chitosan after cross linking can be casted into membranes, beads, and solutions. It can be effectively utilized as an adsorbent for heavy metals adsorption in industrial waste- water. The ratio of biochar and chitosan would affect the adsorption of copper, lead, arsenic, cadmium and other heavy metals in in- dustrial wastewater (Hussain et al., 2017). Gliricidia biochar is a promising material for crystal violet (CV) removal from an aqueous environment in dye-based industries. The CV sorption process is governed by the pH value, surface area and pore volume of biochar (Wathukarage et al., 2017). Bagasse biochar was used to adsorb lead from the battery manufacturing industry effluent. The maximum adsorption capacity can reach 12.7 mg/g and the adsorptive process is related to medium pH value, contact time and dosage (Poonam and Kumar, 2018). Biochar was also used to recapture nutrients from ammonium and phosphate-based dairy wastewater. Biochar can adsorb 20e43% of ammonium and 19e65% of phosphate in flushed dairy manure within 24 h (Ghezzehei et al., 2014). Thus far, most of the experiments on biochar application in removal of contaminants from industrial wastewater were conducted in lab- oratory setting, further research and implementation in real-world conditions is needed.

4.2. Municipal wastewater treatment

Biochar can be directly used or combined with biofilter and other technologies for municipal wastewater treatment, which result in recovery of labile nitrogen and phosphorus (Cole et al., 2017). Engineered biochar loaded with aluminum oxyhydroxides (AlOOH) was applied to recycle and reuse phosphorus from

Table 4 Biochar adsorption of nitrogen and phosphorus in aqueous solutions.

Biochar feedstock Treatment/ Modification

Pyrolysis temperature (�C)

Biochar dose (g/ L)

Nutrient Initial concentration (mg/L)

Adsorption capacity (mg/ g)

Removal mechanism Ref.

Pine sawdust Pristine 300 3 NH4 þ 100 5.38 Chemical bonding and electrostatic

interaction of NH4 þ with the surface functional

groups.

Yang et al., (2017)

Wheat straw Pristine 550 3 NH4 þ 100 2.08 Chemical bonding and electrostatic

interaction of NH4 þ with the surface functional

groups.

Yang et al., (2017)

Wood waste MgO modified 600 2 NH4 þ 8203 47.5 Struvite precipitation Xu et al.,

(2018) Sugarcane harvest

residue MgO particle- impregnated

550 1.25 NH4 þ 200 22 Struvite crystallization, electrostatic

attraction, and p-p interactions Li et al., (2017)

Wheat straw MgeFe layered double hydroxides (LDH)

600 2 NO3 � 45 24.8 Surface adsorption and interlayer anion

exchange Xue et al., (2016)

Peanut shells MgCl2 solution immersed

600 2 NO3 � 20 94 Surface adsorption Zhang et al.

(2012a) Hickory wood chips Aluminum salt

treated 600 2.5 Phosphorus 6.4 8.346 Electrostatic attraction Zheng et al.

(2019a) Wheat straw Acid wash and

water wash 500e560 12.5 Phosphorus 25 1.06 Adsorption and surface precipitation Dugdug

et al., (2018)

Hardwood Acid wash and water wash

500e550 12.5 Phosphorus 25 1.2 Adsorption and surface precipitation Dugdug et al., (2018)

Willow wood Acid wash and water wash

500e550 12.5 Phosphorus 25 1.93 Adsorption and surface precipitation Dugdug et al., (2018)

Wood waste MgO modified 600 2 PO4 3- 318.5 116.4 Struvite precipitation, surface adsorption Xu et al.,

(2018) Bamboo MgeAl layered

double hydroxides (LDH)

600 2 PO4 3- 50 13.11 Interlayer anion exchange and surface

adsorption Wan et al., (2017)

Anaerobically digested sugar beet tailings

Pristine 600 2 PO4 3- 61.5 25 Surface adsorption by colloidal and nano-

sized MgO particles Yao et al. (2011b)

Cottonwood AlCl3 solution immersed

600 2 PO4 3- 1600 135 Adsorption by unique nanostructure Zhang and

Gao, (2013)

Sugar beet tailings MgCl2 solution immersed

600 2 PO4 3- 1600 835 Surface adsorption Zhang et al.

(2012a) Tomato leaves Mg enriched 600 2 PO4

3- 588.1 100 Precipitation, surface deposition Yao et al. (2013a)

Cottonwood HTC þ LDH 180 2 PO43- 2000 386 Surface adsorption Zhang et al., (2014)

W. Xiang et al. / Chemosphere 252 (2020) 126539 9

secondary treated wastewater (Zheng et al., 2019a). The adsorption mechanism of phosphorus is mainly through electrostatic attrac- tion. Phosphorus adsorbed on engineered biochar can be utilized as a slow-release fertilizer for crop production.

Biochar produced from digested sludge was used as an adsor- bent for ammonium removal from municipal wastewater. Biochar derived at 450 �C has the highest ammonium removal capacity attribute to its higher surface area and functional group density, and the process is controlled by chemisorption (Tang et al., 2019). Biochar derived from waste sludge was used as catalysts to ozonate refinery wastewater and shows high removal rate of the total organic carbon. Because the biochar contains functional carbon groups, Si/O structures, and metallic oxides, it can promote oxida- tion through the formation of hydroxyl radicals and mineralized petroleum contaminants (Chen et al., 2019).

Municipal wastewater can be treated with biochar, produced from municipal biowaste, at the biofiltration stage. Biochar has a high porous surface area that allows it to act as a biofilter in municipal wastewater treatment. The COD, TSS, TKN and TP of wastewater reduce 90%, 89%, 64%, and 78%, respectively, after being passed through the biochar biofilter (Manyuchi et al., 2018).

Wastewater from residential units not connected to any municipal sewage treatment plant was treated with biochar in on-site sewage treatment facility (OSSFs) (Blum et al., 2018). The addition of bio- char obviously increases the removal rate of some polar and hy- drophilic compounds. OSSFs thus can be upgraded with low-cost biochar adsorbents.

4.3. Agricultural wastewater treatment

Agricultural contamination is becoming increasingly serious due to the rapid development of agricultural industry, more and more pesticides or toxic heavy metals are discharged into farm- lands (Wei et al., 2018). Many researchers have applied biochar and its modified forms to treatment of agricultural wastewater contamination.

Pentachlorophenol and atrazine are two most common pesti- cides in agriculture. Rice straw biochar and phosphoric acid modified rice straw biochars show significantly high adsorption for imidacloprid and atrazine from agricultural wastewater (Mandal and Singh, 2017). Soybean and corn straw biochar both show high atrazine removals and the adsorption capacities are mainly

Fig. 4. Biochar application in wastewater treatment.

W. Xiang et al. / Chemosphere 252 (2020) 12653910

due to the pore volume and pH value of biochar (Zhao et al., 2013; Liu et al., 2015). Steam-activated biochar can effectively remove sulfamethazine and the removal rate is pH value dependent (Rajapaksha et al., 2015). Zero valent iron magnetic paper mill sludge biochar (ZVI-MBC) was used for removal of pentachloro- phenol (PCP) from the effluent (Devi and Saroha, 2014). The ZVI- MBC can simultaneously adsorb and dechlorinate the PCP in the effluent and achieve the complete removal of PCP. The removal of glyphosate, diuron and carbaryl from agricultural wastewater by biochar have been also investigated. The adsorption capacity of biochar to pesticides are related to biochar feedstock, functional materials, and target contaminants (Wei et al., 2018).

The toxic heavy metals in agricultural wastewater is another pervasive problem. The common concerned toxic metals include As, Cr, Cu and Pb (Table 2). The adsorption capacity of Cu2þ and As5þ

in agricultural wastewater by biochar can reach 69.4 mg/g and 34.1 mg/g, respectively; and the adsorption quantity of Cd2þ and Pb2þ are ranged from 0.4 mg/g to 12.3 mg/g, and 36 mg/g to 35 mg/ g, respectively (Higashikawa et al., 2016; Cho et al., 2017; Zhou et al., 2017a; Son et al., 2018). For the heavy metals in agricultural wastewater, the possible adsorption mechanisms usually involve electrostatic interactions, surface complexation, ion exchange, intermolecular interaction, cation-p bonding, and p-p interactions (Wei et al., 2018).

The adsorption behavior of biochars for various agricultural contaminants differs widely (Wei et al., 2018). In general, the

adsorption capacities are closely correlated with nano-material content, SSA, SFG, and porous structures (Cha et al., 2016; Braghiroli et al., 2018; Son et al., 2018; Wan et al., 2018; Yao et al., 2018). In addition, the adsorption mechanism by biochars are affected by inner-sphere complexes, p-p interaction, hydrophobic effect, precipitation, ion exchange, and so on (Yuan et al., 2011; Cha et al., 2016; Lef�evre et al., 2018; Wei et al., 2018; Yao et al., 2018).

4.4. Stormwater treatment

With the development of urbanization, urban stormwater runoff has been widely concerned due to its influence on water quality. Stormwater runoff can significantly contribute to the degradation of natural water quality and requires treatment before discharge, which is mainly due to increased concentrations of metals, organic matter and biological pollutants (Mohanty et al., 2014; Gray, 2016; Tian et al., 2016; Ulrich et al., 2017; Ashoori et al., 2019).

Bioretention and biofiltration are commonly used for storm- water treatments, but the purification of stormwater contaminants by these two systems is not ideal (Gray, 2016; Lau et al., 2016; Ulrich et al., 2017). Biochar and its modified forms, as the effective media, have been applied to stormwater treatment systems (Fig. 5). A recent study shows that an aluminum-impregnated biochar can effectively remove As5þ and other runoff pollutants, such as Pb2þ, Zn2þ, Cu2þ, and PO4

3�, in a polluted urban water runoff (Liu et al.,

Fig. 5. Biochar application in stormwater treatment: (a) Potential functions of biochar at different region of bioinfiltration system (Mohanty et al., 2018). (b) Schematic diagram of the enhanced stormwater contaminants removal by biochar-amended biofilters (Lu and Chen, 2018).

W. Xiang et al. / Chemosphere 252 (2020) 126539 11

2019a). A biochar-based filtration medium has been effectively deployed to remove copper and zinc in stormwater runoff, and the remove rate reached more than 85% and 95%, respectively. But the biochar filtration media need to be carefully tested and designed to meet the requirements of stormwater treatment (Gray, 2016).

Biochars have been integrated with biofilters for removing bisphenol A (BPA) from stormwater. Wood dust biochar shows a high adsorption efficiency and increased capacity of BPA attribute to its high SSA and pore volume, which also promotes phragmites australis growth, increases E. coli, TOC, TSS, nitrogen and phos- phorus removal rates (Ashoori et al., 2019). Biochar amendment has improved the removal of contaminant in stormwater biofilters, particularly the toxic trace organic contaminants (TOrCs) that have been poorly removed in conventional systems. Biochar-amended biofilter columns can maintain more than 99% TOrC removal rate compared to the unamended biofilter columns. Meanwhile, biochar-amended biofilter can increase the removal of TOC, TN, and TP greater than 60% (Ulrich et al., 2017).

Poultry litter biochars (PLB) pyrolyzed at 500 �C were applied to adsorb ammonium in stormwater treatment systems. There is a significant positive correlation between NH4

þ sorption and biochar CEC. The ion competition in stormwater adsorption experiments suggests that NH4

þ adsorption is dominated by cation exchange (Tian et al., 2016). Zn-activated sewage sludge-based activated carbon can remove PO4eP and NO3eN effectively from leachate made from stormwater. And the removal rates of PO4eP and NO3eN decrease with increasing pH value (Yue et al., 2018). Biochar and zero valent iron (ZVI) amending bioretention cells can increase the NO3

- removal performance in stormwater system, which pro- vides an important prospect for increasing nitrate removal effi- ciency in bioretention systems (Tian et al., 2019).

Biofilters/bioretention system with biochar can also effectively remove microorganisms from stormwater (Mohanty et al., 2014; Lau et al., 2016). Biofilters amended with 5% biochar can retain up to 3 orders of magnitude more E. coli, and prevent their mobiliza- tion during successive intermittent flows. This indicates that amending biofilters with biochar can improved the removal of bacteria from stormwater (Mohanty et al., 2014). H2SO4-modified wood biochar can be an effective bioretention filter medium for E. coli removal from stormwater. It improves E. coli retention and reduces remobilization. The results indicate that the transport of E. coli is governed by the morphology structures and hydropho- bicity of the biochars (Lau et al., 2016).

In general, biochar has been used as filter media in stormwater treatment. Various removal capacities of contaminants in storm- water depend on biochar properties, pollutant characteristics, and aqueous chemistry (Mohanty et al., 2018). Biochar is more feasible and promising than other materials used in stormwater treatment, because it is inexpensive and readily available and has many beneficial functions in stormwater treatment systems.

5. Conclusions and future perspectives

Biochar is an efficient and low-cost adsorbent, which can be produced from a variety of biomass materials including agricultural crop residues, forestry residues, sewage sludge, manures, solid organic municipal wastes, and thus has been used in wastewater treatment. This article reviews the current technologies for biochar production with an emphasis on feedstock pre-treatment, thermal conversion, and post treatment technologies. It summarizes the biochar application in wastewater treatment including industrial wastewater, municipal wastewater, agricultural wastewater and stormwater. Mechanisms underlying the biochar adsorption of contaminants are discussed.

The main conclusions of this review are as follows: (1) Biochar properties are related to the type of feedstock, feedstock pre- treatment technology, thermal process, and post-treatment of biochars. The modifications of biochars by increasing the SSA, re- action activity or by forming functional groups, become increas- ingly important as a new and exciting area of engineered biochar research and its application for improving environmental quality. (2) Largely due to the modifications, engineered biochar as an adsorbent to remove aqueous contaminant, such as heavy metals, organic contaminants, nitrogen and phosphorus is controlled by various mechanisms, mainly including ion exchange, adsorption, surface precipitation, surface complexation etc. (3) The potential of biochar for removal of pollutants from industrial wastewater, municipal sewage, agricultural sewage, and stormwater has been well demonstrated in laboratory. Its application for onsite appli- cation requires further investigation. Although number of re- searches have been done on production and application of biochar in wastewater treatment, there are still knowledge gaps that need to be filled.

Additional studies are still need to: (1) develop the new low-cost and high-efficiency modification technology of biochar, (2) increase the practical application of biochar in wastewater treatment,

W. Xiang et al. / Chemosphere 252 (2020) 12653912

especially in industrial wastewater and municipal wastewater treatment, and (3) further improve the adsorption capacity of biochar on heavy metals, organic contaminants, nitrogen and phosphorus.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

W.X and X.Z. would like to acknowledge the support of the Natural Science Foundation of the Jiangsu Higher Education In- stitutions of China (Grant No. 18KJA610003), Key R & D Projects of Xuzhou (Grant No. KC18150, KC16SS091), Xuzhou University of Technology (Grant No. XKY2018136), and the Project of Ministry of Housing and Urban-Rural Development (Grant No. 2013-K4-27).

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