Removal of Cr(VI) On a Peat Soil as Influenced by a Carbonization Process.

Removal of Cr(VI) on a peat soil as influenced by a carbonization process

Fire is a natural process that occurs in the terrestrial areas; however, it can be also initiated anthropogenically in many Asian countries. For instance, open-burning of agricultural wastes, such as rice residues, have been a common activity on the farmlands for decades in Taiwan. These natural or human-induced fires influence not only the terrestrial C cycling but also the organic and inorganic compositions of soils beneath due to heat transfer. The heat transfer into the soils is similar to a carbonized or pyrolyzed process due to the limited O₂ content in soil pores. As a result, the adsorptive capacities of the fire-impacted soils for pollutants may be accordingly affected. In this study, a Taiwan peat soil was carbonized in an oven up to 600 ^oC under a limited air to simulate the soils heated under a surface fire. Then, a batch experiment was conducted by adding 10 mg/L Cr(VI) into the carbonized soils (2g/L). Results showed that 200 ^oC-treated peat soil exhibited the highest Cr(VI) removal (3 mg/g soil); however, Cr(VI) removal decreased with an increase in carbonized temperatures. Since the concentration of dissolved organic carbon (DOC) also reached the highest at 200 ^oC (15.9 mg/L), we expected that a portion of large organic molecules may be decomposed into small and loosed bound molecules upon 200^oC carbonization. Thus, high DOC concentration associated with the exposure of some new sorptive sites may contribute to the high Cr(VI) removal on a 200 ^oC carbonized soil. With an increase of carbonized temperature to 600 ^oC, a slight decrease of Cr(VI) removal was observed even if the DOC significantly decreased. The formation of aromatic C upon the carbonization of the peat soil may become a dominant factor controlling Cr(VI) removal.