Comparison of nickel adsorption on biochars produced from mixed softwood and Miscanthus straw

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.