Enhanced Cu and Cd sorption after soil aging of woodchip-derived biochar: What were the driving factors?

Biochar (BC) is increasingly tested as a soil amendment for immobilization of heavy metals (HMs) and other pollutants. In our study, an acidic soil amended with wood chip-derived BC showed strongly enhanced Cu and Cd sorption after 15 months of aging under greenhouse conditions. X-ray absorption near edge structure suggested formation of Cu(OH)2 and CuCO3 and upon aging increasingly Cu sorption to the BC organic phase (from 9.2% to 40.7%) as main binding mechanisms of Cu on the BCs. In contrast, Cd was predominantly bound as CdCO3 on the BCs even after 15 months (82.7%). We found indications by mid-infrared spectroscopy that the formation of organic functional groups plays a role for increased HM sorption on aged BCs. Yet, our data suggest that the accessibility of BC's pore network and reactive surfaces is likely to be the overriding factor responsible for aging-related changes in HM sorption capacity, rather than direct interactions of HMs with oxidized functional groups. We observed highly weathered BC surface structures with scanning electron microscopy along with strongly increased wettability of the BCs after 15 months of soil aging as indicated by a decrease of water contact angles (from 62.4° to 4.2°).