Chromate removal as influenced by the structural changes of soil components upon carbonization at different temperatures

Surface fire could induce heat transferring into the soil, creating a carbonized environment, which may alter the chemical compositions of soil organic matters (SOM). In the study, a surface soil was carbonized at up to 600 °C with limited air to simulate soils experiencing a surface fire, and Cr(VI) removal on the carbonized soils was investigated. NMR and FTIR analyses demonstrated a remarkable change of SOM structures at 300–400 °C. TGA-MS spectra indicated that (e.g. C2H4, CH3OH and C3H8) were the major components in the evolved gases from the pyrolyzed soil. A maximum amount of Cr(VI) removal (ca. 4 mg g−1 soil) occurred for the 200 °C-carbonized soils, attributed mainly to a significant increase of Cr(VI) reduction by 0.1 M KCl extractable organic carbon (EOC) with abundant carboxylic groups. Nonetheless, the formation of aromatic C upon carbonization of the soil at >400 °C may be responsible for Cr(VI) reduction.

► A maximum amount of Cr(VI) removal occurred for the 200 °C-carbonized soil.
► Extractable organic carbon (EOC) was increased upon carbonization of soil.
► EOC, enriched with carboxylic groups, enhances Cr(VI) reduction by the soil.
► The formation of aromatic C on a carbonized soil may be responsible for Cr(VI) reduction.
► Reductive product of Cr(III) tends to bond on high-temperature-modified soil.