Qualitative and quantitative characterisation of adsorption mechanisms of lead on four biochars

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.