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

The adsorption mechanisms of lead (Pb) 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 Pb 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 (PbCO) on SB and hydrocerussite (Pb(CO)(OH)) on WSP700, RH 700 and SWP550, suggesting a mechanism of surface precipitation for Pb adsorption on the biochars. The percentages of Pb in the form of PbCO on SB (82.24%) and Pb(CO)(OH) 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 Pb on biochars, which is important for the practical application of biochar in water and/or soil treatment.