Mechanisms for the dissolved biochar promoted iron dissolution and consequential chromium release.

Biochar is widely applied in soil for agricultural and environmental purposes. Soluble fraction of biochar may be released from bulk biochar as dissolved biochar (DBC) after irrigation or rainfall. DBC had been reported to possess high chemical activity in aqueous system, while less attention was paid to the impact of DBC on the soil environmental processes. In this work, the impact of DBC on ferric (hydro) oxides was systematically examined. Our study showed that DBC prepared from rice straw could significantly promote the dissolution of ferric oxides with unstable and metastable crystalline structures, e.g., ferrihydrite under relatively acidic condition. Organic ligand-promoted dissolution was the main mechanism for iron release from ferrihydrite, and the low-molecular-weight DBC component (less than 1000 Da) was the major contributor for this process. Furthermore, the organic carbon content normalized ligand-promoted dissolution capacity for DBC was much higher than common dissolved organic matters. More importantly, DBC could promote the release of Cr from dichromate-adsorbed ferric mineral. Our results suggest that in soils with relatively low pH and high contents of ferric hydroxides, e.g., red soil in southern China, DBC derived from applied biochar could enhance the mobility and bioavailability of iron and other heavy metals. The dissolved metals would play active roles in soil redox cycle and biotic processes. Therefore, it's necessary to evaluate the long-term impact of biochar application on acidic field soils with high iron content.