Cr Removal by a Soil Carbonized At Different Temperatures.

Surface fires, derived from natural or anthropogenic activities, may lead to heat transferring into the soils beneath, influencing the chemical/physical properties of soil components and subsequently pollutant mobilities. In this study, an organic-rich Taiwan soil was collected from Changhua County. The soil and humic acids extracted from the Changhua soil were carbonized in an oven up to 600^oC under a limited air to simulate the soils heated under a surface fire. The changes in chemical structures of soil organic matter (SOM) upon the carbonization treatments and their influences on Cr transformations were evaluated. Bohem titration and FTIR analyses showed that acidic functional groups (e.g., carboxyl, phenolic and lactonic groups) decreased with an increase in carbonized temperatures. Accompanied with the decrease of acidic groups, the proportion of the basic functional groups increased, particularly, at a carbonized temperature higher than 300 ^oC. Extractable organic C (EOC) reached the maximum at 200~250^oC; however, EOC gradually disappeared behind the specific temperature. 200 and 250^oC-treated soils exhibited the highest Cr removal, attributing mainly to a significant increase in Cr(VI) reduction by either decomposed organic molecules or EOC. Although the Changhua soil showed low adsorptive ability of Cr(VI), it revealed a strong affinity to Cr(III) at higher carbonized temperatures. We expected that exposure of new sorptive sites on clay minerals with an increase of carbonized temperatures may contribute to the increase of Cr(III) sorption in the soil system. Temperature-dependent Cr removal on humic acids exhibited a similar behavior as those in the soil system. At carbonized temperatures higher than 500^oC, the formation of aromatic C upon the carbonization of the soil and humic acids may become a dominant factor controlling Cr(VI) reduction.