Metal sorption by biochars: A trade-off between phosphate and carbonate concentration as governed by pyrolysis conditions

Three feedstocks, pine wood, grass and cow manure, were pyrolyzed under various conditions and tested on their ability to sorb metals in aquatic systems. The feedstocks were pyrolyzed at 2 different temperatures (350 °C and 550 °C) and 2 different residence times (10 and 60 min) and resulting biochars were assessed on their capability to immobilize Pb, Cu, Cd and Zn. Manure-based chars, and to a lesser extent grass-based chars, featured high concentrations of phosphates and carbonates. These anions play an important role in metal sorption because they form insoluble complexes with the metals. Washing reduced the concentration of these anions, leading to a reduced sorption of metals by the biochar. The carbonate concentration on the biochars’ surface increased at higher reactor temperature and longer residence times. The opposite trend was observed for the phosphate concentration and the cation exchange capacity. Accordingly, the optimal temperature-residence time combination for sorption was a trade-off between these properties. Biochar produced from cow manure and pyrolyzed at 550 °C for 10 min showed the best sorption for all metals considered.