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In order to understand the influence of feedstock type on biochar adsorption of heavy metals, the adsorption characteristics of nickel (Ni2+), copper (Cu2+) and lead (Pb2+) onto biochars derived from mixed softwood and Miscanthus straw were compared. The biochars were produced from mixed softwood pellets (SWP) and Miscanthus straw pellets (MSP), at both 550 and 700 °C for each material, using a standardised production procedure recommended by the UK Biochar Research Centre. Kinetics analyses show that the adsorption of Ni2+ to all four biochars reached equilibrium within 5 min. The degree of Ni2+ removal for all four biochars remained nearly constant within initial pH values of 3-8, because the equilibrium pH values within this range were similar due to the buffering effect of the biochars. A sharp increase of Ni2+ removal percentage for all biochars at initial solution pH 8-10 was observed as the equilibrium pH also increased. MSP derived biochars generally had higher maximum adsorption capacities (Qmax) for the three tested metals as compared with those from SWP, which was likely due to their higher degree of carbonisation during production. This study shows that feedstock type is a primary factor affecting the adsorption capacities of the tested biochars for heavy metals.

The adsorption mechanisms of lead (Pb2 +) on four biochars (SB produced from British hardwood at 600 °C and three standard biochars produced from wheat straw pellets at 700 °C (WSP700), rice husk at 700 °C (RH700) and soft wood pellets at 550 °C (SWP550)) were characterised qualitatively and quantitatively, using a combination of chemical and micro-structural methods. Sequential extraction test results show that Pb2 + was predominantly adsorbed on SB (85.31%), WSP700 (75.61%) and RH700 (85.76%) as acidic soluble fraction, which was potentially bioavailable if applied in soil. The exchangeable fraction for SB, WSP700 and RH700 was low (1.38–4.29%) and their water soluble fraction was negligible (0–0.14%). Micro-structural analysis further investigated this fraction and confirmed the presence of cerussite (PbCO3) on SB and hydrocerussite (Pb3(CO3)2(OH)2) on WSP700, RH 700 and SWP550, suggesting a mechanism of surface precipitation for Pb2 + adsorption on the biochars. The percentages of Pb2 + in the form of PbCO3 on SB (82.24%) and Pb3(CO3)2(OH)2 on WSP700 (13.00%), RH 700 (19.19%) and SWP550 (29.70%) were quantified using thermogravimetric analysis (TGA). This study suggests that it is feasible to quantify different adsorption mechanisms of Pb2 + on biochars, which is important for the practical application of biochar in water and/or soil treatment.

Biochar is a solid carbon-rich material produced by thermal decomposition of biomass without oxygen (pyrolysis). Among other applications, it can be used to treat soils contaminated with heavy metals or organic pollutants, which has sustainability benefits over traditional remediation technologies, including waste reuse, site greening, carbon storage, and lower life-cycle footprints. This chapter summarizes the development, production, and properties of biochars produced for this purpose. The physicochemical properties of biochar that are most influential on its remediation performance include surface area, pH, cation-exchange capacity , elemental composition, particle size, ash content, aromaticity, and volatile matter content. The specific mechanisms behind biochar–contaminant interactions, and its effectiveness as an in situ stabilization reagent, permeable reactive barrier reactant, or phytoremediation amendment, are discussed in detail. Finally, remaining research gaps are summarized, and implications for green and sustainable remediation are presented.

A field remediation treatment was carried out to examine the long-term effect of biochar on the immobilisation of metals and the revegetation of a contaminated site in Castleford, UK. The extracted concentrations of nickel (Ni) (II) and zinc (Zn) (II) in the carbonic acid leaching tests were reduced by 83-98% over three years. The extracted Ni (II) and Zn (II) concentrations three years after the treatment were comparable to a cement-based treatment study carried out in a parallel manner on the same site. The sequential extraction results indicated that biochar addition (0.5-2%) increased the residue fractions of Ni (II) (from 51% to 61-66%) and Zn (II) (from 7% to 27-35%) in the soils through competitive sorption, which may have resulted in the reduction of leachabilities of Ni (II) (from 0.35% to 0.12-0.15%) and Zn (II) (from 0.12% to 0.01%) in the plots with biochar compared with that without biochar three years after the treatment. The germination of grass in the plots on site failed. Further laboratory pot study suggested that larger amounts of biochar (5% or more) and compost (5% or more) were needed for the success of revegetation on this site. This study suggests the effectiveness and potential of biochar application in immobilising heavy metals in contaminated site in the long term.

Salisbury biochar (produced from British broadleaf hardwood) with two different particle sizes (≤2mm and ≤0.15mm) was applied on a kaolin with three different lead (Pb(2+)) contamination levels (50mg/kg, 300mg/kg and 1000mg/kg) at the dosage of 1% in w/w. The short-term impact of biochar on the mobility and speciation of Pb(2+) in the kaolin was investigated using attenuation periods of 1, 7 and 28 days. The leachability and extractability of Pb(2+) in carbonic acid leaching and EDTA extraction tests as well as the speciation of Pb(2+) in soils were not significantly affected by biochar treatment during all periods. The insignificant effects of biochar on Pb(2+) immobilisation were most likely attributed to the high adsorption capacity of Pb(2+) on the kaolin and biochar failed to competitively adsorb Pb(2+) against kaolin. The kaolin immobilised Pb(2+) primarily through cation exchange, which represents the readily bioavailable fractions of Pb(2+) in soils and may still pose environmental risks. This paper suggests the inefficiency of biochar treament on heavy-metal contaminated clay-rich soils. Therefore a laboratory treatablity study with respect to the soil type may be crucial when large-scale biochar applications in heavy-metal associated soil remediation are evaluated.

In this study, the physicochemical properties of Salisbury biochar produced from British broadleaf hardwood and its adsorption characteristics towards lead were investigated. The biochar particle size has a significant effect on its BET surface area, cation exchange capacity and sorption of lead. The kinetics data were well fitted by the Pseudo second order model. The increase of biochar dosage increased the percentage of lead removal in solutions. The increase of initial solution pH increased the percentage of lead removal across the pH range of 2–10. The calculated maximum adsorption capacities of lead by Langmuir model were 47.66 and 30.04 mg/g for 0.15 mm and 2 mm samples. The adsorption capacities of different metals decreased in the order of lead > nickel > copper > zinc calculated in mmol/g. This study suggests a great potential of biochars derived from British broadleaf hardwood to be applied in soil remediation.

The adsorption characteristics and mechanisms of Ni2+ on four-standard biochars produced from wheat straw pellets (WSP550, WSP700) and rice husk (RH550, RH700) at 550 and 700 °C, respectively, were investigated. The kinetic results show that the adsorption of Ni2+ on the biochars reached an equilibrium within 5 min. The increase of the solid to liquid ratio resulted in an increase of Ni2+ removal percentage but a decrease of the adsorbed amount of Ni2+ per weight unit of biochar. The Ni2+ removal percentage increased with the increasing of initial solution pH values at the range of 2–4, was relatively constant at the pH range of 4–8, and significantly increased to ≥98% at pH 9 and stayed constantly at the pH range of 9–10. The calculated maximum adsorption capacities of Ni2+ for the biochars follow the order of WSP700 > WSP550 > RH700 > RH550. Both cation exchange capacity and pH of biochar can be a good indicator of the maximum adsorption capacity for Ni2+ showing a positively linear and exponential relationship, respectively. This study also suggests that a carefully controlled standardised production procedure can make it reliable to compare the adsorption capacities between different biochars and investigate the mechanisms involved. Electronic supplementary material The online version of this article (doi:10.1007/s11356-017-8847-2) contains supplementary material, which is available to authorized users.