Elsevier

Chemosphere

Volume 178, July 2017, Pages 466-478
Chemosphere

Review
Mechanisms of metal sorption by biochars: Biochar characteristics and modifications

https://doi.org/10.1016/j.chemosphere.2017.03.072Get rights and content

Highlights

  • Biochar properties varied with increasing pyrolysis temperature.

  • Complexation and electrostatic interaction are important mechanisms for As sorption.

  • Complexation and reduction are important mechanisms for Cr and Hg sorption.

  • Cation exchange and precipitation are important mechanisms for Cd and Pb sorption.

  • Biochar have been modified to enhance its metal sorption capacity.

Abstract

Biochar produced by thermal decomposition of biomass under oxygen-limited conditions has received increasing attention as a cost-effective sorbent to treat metal-contaminated waters. However, there is a lack of information on the roles of different sorption mechanisms for different metals and recent development of biochar modification to enhance metal sorption capacity, which is critical for biochar field application. This review summarizes the characteristics of biochar (e.g., surface area, porosity, pH, surface charge, functional groups, and mineral components) and main mechanisms governing sorption of As, Cr, Cd, Pb, and Hg by biochar. Biochar properties vary considerably with feedstock material and pyrolysis temperature, with high temperature producing biochars with higher surface area, porosity, pH, and mineral contents, but less functional groups. Different mechanisms dominate sorption of As (complexation and electrostatic interactions), Cr (electrostatic interactions, reduction, and complexation), Cd and Pb (complexation, cation exchange, and precipitation), and Hg (complexation and reduction). Besides sorption mechanisms, recent advance in modifying biochar by loading with minerals, reductants, organic functional groups, and nanoparticles, and activation with alkali solution to enhance metal sorption capacity is discussed. Future research needs for field application of biochar include competitive sorption mechanisms of co-existing metals, biochar reuse, and cost reduction of biochar production.

Introduction

Heavy metals are ubiquitous in the environment, adversely impacting human health (Järup, 2003). However, various anthropogenic activities including mining, smelting, fertilizer and pesticide application, and electronic manufacturing discharge have increased the amount of metal-containing wastewater into the aquatic environment, leading to water contamination with metals. To deal with metal-contaminated water, different methods have been suggested to remove metals from aqueous solution including chemical precipitation, ion exchange, electrochemical treatment, and membrane technologies (Demirbas, 2008). Among the methods, biosorption technique is the most common and cost-effective. This is because biosorbents are environmentally friendly and readily available in large quantities, and one of the most popular biosorbents is biochar.

Biochar is a carbon-rich, fine-grained, and porous material. It is usually produced by thermal decomposition of biomass under oxygen-limited conditions at temperature <900 °C (Lehmann et al., 2006). It has received increasing attention due to its ability to store large amount of carbon, increase crop yield, reduce soil emission of greenhouse gases, improve soil quality, decrease nutrient leaching, and reduce irrigation and fertilizer requirements (Lehmann, 2007, Bird et al., 2008, Kimetu et al., 2008, Nguyen et al., 2009). More importantly, due to the presence of highly-porous structure and various functional groups (e.g., carboxyl, hydroxyl, and phenolic groups), biochar shows a great affinity for heavy metals (Mohan et al., 2007, Cao et al., 2009, Park et al., 2011). Much research has explored its ability for heavy metal removal from water (Ahmad et al., 2014, Mohan et al., 2014). Biochars are produced from various feedstocks (wood bark, dairy manure, sugar beet tailing, pinewood, and rice husk) at different pyrolysis conditions (temperature, heating transfer rate, and residence time) to sorb metals from water, including arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), and lead (Pb) (Qian et al., 2015, Xie et al., 2015, Inyanga et al., 2016). For simplicity, metalloid As is grouped with metals in this review.

Based on literatures, five mechanisms governing metal sorption from water by biochar have been proposed (Ahmad et al., 2014, Mohan et al., 2014, Nartey and Zhao, 2014, Qian et al., 2015, Tan et al., 2015, Xie et al., 2015, Inyanga et al., 2016). They include: (1) electrostatic interactions between metals and biochar surface; (2) cation exchange between metals and protons or alkaline metals on biochar surface; (3) metal complexation with functional groups and π electron rich domain on the aromatic structure of biochar; (4) metal precipitation to form insoluble compounds; and (5) reduction of metal species and subsequent sorption of the reduced metal species. The sorption mechanisms and capacity vary considerably with biochar properties and target metals. Recently, researchers reviewed biochar production technologies and metal removal performance (thermodynamics, kinetics, isotherms, capacity, and mechanisms) from water using biochar (Ahmad et al., 2014, Mohan et al., 2014, Nartey and Zhao, 2014, Qian et al., 2015, Tan et al., 2015, Xie et al., 2015, Inyanga et al., 2016). However, most reviews provided sorption mechanisms for metals as a group, lacking a comparison of the main mechanisms for removal of different metals. Since different metals show different species or valence states at different solution pH conditions, the main mechanisms for their sorption are different.

Compared to activated carbon, biochar is a promising adsorbent with lower cost for metal removal from water. Metal sorption capacities of biochar are 2.4–147, 19.2–33.4, 0.3–39.1, 3.0–123 mg g−1 for Pb, Ni, Cd, and Cr, respectively (Inyanga et al., 2016). However, they are generally lower than that of activated biochar, which are 255 and 91.4 mg g−1 for Pb and Cd (Wilson et al., 2006). Therefore, biochars have been modified to enhance their metal sorption capacity by loading biochar with minerals, organic functional groups, reductants, and nanoparticles, and by activating biochar with alkali solution (Mohan et al., 2014). However, so far, there is no review on recent progress on biochar modification except Mohan et al. (2014) who briefly discussed biochar modification by incorporating nanoparticles including magnetic particles and carbon nanotubes.

In this review, we aimed to: 1) review the characteristics of biochar to better understand its efficiency in metal sorption, 2) discuss the dominant sorption mechanisms of individual metals by biochar, 3) describe biochar modification to enhance its metal removal from aqueous solutions, and 4) identify research needs and suggest directions for future research. The novelty of the paper is to compare the main mechanisms for removal of different metals by biochar and to review recent progress on biochar modification to enhance metal sorption capacity. This review provides insight into biochar materials and their capability to sorb different heavy metals, which is useful for future research and biochar field application.

Section snippets

Characteristics of biochar

Physicochemical properties of biochar significantly influence its ability to sorb metals. Prior to exploring the mechanisms governing metal removal by biochar, its properties need to be well characterized, including surface area, porosity, pH, surface charge, functional groups, and mineral contents.

Mechanisms of metal sorption by biochar

Table 2 summarizes the sorption capacity and optimum solution pH for metal sorption by biochar. The metal sorption capacity of biochar varies by 1–3 orders of magnitude, ranging from 1 to 200 mg g−1 (Table 2). The pH for maximum metal sorption varies with metals, as solution pH significantly influences both metal speciation and surface charge of biochar. Change in solution pH impacts the complexation behavior of functional groups such as carboxyl, hydroxyl, and amino. For example, the

Modification of biochar to enhance metal sorption

Though biochar has ability to sorb metals from water, its capacity is usually lower compared to other common biosorbents such as activated C. Therefore, recent studies have modified biochar to enhance its metal sorption capacity. For example, efforts have been made to increase its surface area, porosity, pHPZC, and/or functional groups. Approaches to modify biochars include loading with minerals, reductants, organic functional groups, and nano-particles and activation with alkali solution.

Future research directions

Biochar has potential for metal sorption and has received increasing attention during the past decade. However, studies are mostly at a lab scale, focusing on sorption of single metal from spiked solution. In natural waters, different heavy metals may coexist with other pollutants, thereby there is competition for sorption sites on biochar surface between metals and other ions or organic pollutants. However, by far, few studies have assessed the competitive sorption of metals by biochar. Park

References (103)

  • X.C. Chen et al.

    Sorption of copper and zinc by biochars produced from pyrolysis of hardwood and corn straw in aqueous solution

    Bioresour. Technol.

    (2011)
  • A. Demirbas

    Heavy metal sorption onto agro-based waste materials: a review

    J. Hazard. Mater

    (2008)
  • W. Ding et al.

    Pyrolytic temperatures impact lead sorption mechanisms by bagasse biochars

    Chemosphere

    (2014)
  • Z.H. Ding et al.

    Removal of lead, copper, cadmium, zinc, and nickel from aqueous solutions by alkali-modified biochar: batch and column tests

    J. Ind. Eng. Chem.

    (2016)
  • X.L. Dong et al.

    Characteristics and mechanisms of hexavalent chromium removal by biochar from sugar beet tailing

    J. Hazard. Mater

    (2011)
  • X.L. Dong et al.

    Enhanced Cr(VI) reduction and As(III) oxidation in ice phase: important role of dissolved organic matter from biochar

    J. Hazard. Mater

    (2014)
  • Y.T. Han et al.

    Sorption 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)
  • M. Hossain et al.

    Influence of pyrolysis temperature on production and nutrient properties of wastewater sludge biochar

    J. Environ. Manag.

    (2011)
  • H.M. Jin et al.

    Biochar pyrolytically produced from municipal solid wastes for aqueous As(V) removal: sorption property and its improvement with KOH activation

    Bioresour. Technol.

    (2014)
  • J.W. Jin et al.

    Influence of pyrolysis temperature on properties and environmentalsafety of heavy metals in biochars derived from municipal sewage sludge

    J. Hazard. Mater

    (2016)
  • M. Kılıç et al.

    Sorption of heavy metal ions from aqueous solutions by bio-char, a by-product of pyrolysis

    Appl. Surf. Sci.

    (2013)
  • V. Lenoble et al.

    Arsenic removal by sorption on iron(III) phosphate

    J. Hazard. Mater

    (2005)
  • P. Liu et al.

    Mechanisms of mercury removal by biochars produced from different feedstocks determined using X-ray absorption spectroscopy

    J. Hazard. Mater

    (2016)
  • P.J. Lloyd-Jones et al.

    Mercury sorption from aqueous Solution by chelating ion exchange resins, activated carbon and a biosorbent

    Process Saf. Environ. Prot.

    (2004)
  • H. Lu et al.

    Relative distribution of Pb2+ sorption mechanisms by sludge-derived biochar

    Water Res.

    (2012)
  • Y. Ma et al.

    Polyethylenimine modified biochar adsorbent for hexavalent chromium removal from the aqueous solution

    Bioresour. Technol.

    (2014)
  • D. Mohan et al.

    Sorption of arsenic, cadmium, and lead by chars produced from fast pyrolysis of wood and bark during bio-oil production

    J. Colloid Interface Sci.

    (2007)
  • D. Mohan et al.

    Modeling and evaluation of chromium remediation from water using low cost bio-char, a green adsorbent

    J. Hazard. Mater

    (2011)
  • D. Mohan et al.

    Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent – a critical review

    Bioresour. Technol.

    (2014)
  • J.J. Pan et al.

    Sorption of Cr(III) from acidic solutions by crop straw derived biochars

    J. Environ. Sci.

    (2013)
  • J.J. Pan et al.

    Removal of Cr(VI) from aqueous solutions by Na2SO3/FeSO4 combined with peanut straw biochar

    Chemosphere

    (2014)
  • J.H. Park et al.

    Competitive adsorption of heavy metals onto sesame straw biochar in aqueous solutions

    Chemosphere

    (2016)
  • L.B. Qian et al.

    Effective removal of heavy metal by biochar colloids under different pyrolysis temperatures

    Bioresour. Technol.

    (2016)
  • K.Z. Qian et al.

    Recent advances in utilization of biochar

    Renewa. Sust. Ener. Rev.

    (2015)
  • D. Rai et al.

    Environmental chemistry of chromium

    Sci. Total Environ.

    (1989)
  • F.C. Richard et al.

    Aqueous geochemistry of chromium: a review

    Water Res.

    (1991)
  • A.W. Samsuri et al.

    Sorption of As(III) and As(V) by Fe coated biochars and biochars produced from empty fruit bunch and rice husk

    J. Environ. Chem. Eng.

    (2013)
  • Y.S. Shen et al.

    Removal of hexavalent Cr by coconut coir and derived chars – the effect of surface functionality

    Bioresour. Technol.

    (2012)
  • P.L. Smedley et al.

    A review of the source, behaviour and distribution of arsenic in natural waters

    Appl. Geochem

    (2002)
  • R. Subedi et al.

    Greenhouse gas emissions and soil properties following amendment with manure-derived biochars: influence of pyrolysis temperature and feedstock type

    J. Environ. Manag.

    (2016)
  • G.C. Tan et al.

    Sorption of mercury (II) and atrazine by biochar, modified biochars and biochar based activated carbon in aqueous solution

    Bioresour. Technol.

    (2016)
  • X.F. Tan et al.

    Application of biochar for the removal of pollutants from aqueous solutions

    Chemosphere

    (2015)
  • J.C. Tang et al.

    Preparation and characterization of a novel graphene/biochar composite for aqueous phenanthrene and mercury removal

    Bioresour. Technol.

    (2015)
  • L. Trakal et al.

    Geochemical and spectroscopic investigations of Cd and Pb sorption mechanisms on contrasting biochars: engineering implications

    Bioresour. Technol.

    (2014)
  • H. Ucun et al.

    Biosorption of lead (II) from aqueous solution by cone biomass of Pinus sylvestris

    Desalination

    (2003)
  • H.Y. Wang et al.

    Removal of Pb(II), Cu(II), and Cd(II) from aqueous solutions by biochar derived from KMnO4 treated hickory wood

    Bioresour. Technol.

    (2015)
  • S.S. Wang et al.

    Manganese oxide-modified biochars: preparation, characterization, and sorption of arsenate and lead

    Bioresour. Technol.

    (2015)
  • S.S. Wang et al.

    Physicochemical and sorptive properties of biochars derived from woody and herbaceous biomass

    Chemosphere

    (2015)
  • S.S. Wang et al.

    Removal of arsenic by magnetic biochar prepared from pinewood and natural hematite

    Bioresour. Technol.

    (2015)
  • S.Y. Wang et al.

    Regeneration of magnetic biochar derived from eucalyptus leaf residue for lead(II) removal

    Bioresour. Technol.

    (2015)
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