The Effects of Structural Characteristics of Repetitively Extracted Humic Acids On the Reduction of Cr(VI).

Humic substances are one of the major components in soils which influence greatly the behavior and fate of pollutants in the environment. Humic substances or humic acids (HA) exhibit diverse physical structures and chemical compositions, depending on their origin, the sampling depth, and the age of the source material. Due to the high chemical and structure heterogeneities of HA, of which domains in humic substances involved in pollutant such as Cr(VI) removal are barely understood. Therefore, in the study, a repetitive extract technique was used to fractionate HA into three fractions containing various proportions of aliphatic and aromatic carbons. The chemical characteristics of the three HA fractions were first examined by EA, FTIR, and ¹³C-NMR spectroscopy. The reactions of target pollutant, i.e., Cr(VI) with different HA fractions were then evaluated. Spectroscopic analysis indicated that an increase in the content of aliphatic C, but a decrease in that of O and aromatic C, in HA was observed with an increase in extraction. Kinetic studies showed that Cr(VI) reduction by HA was more rapid with an increase in extraction, which suggested that the aromatic C of HA may play an important role in Cr(VI) reduction.