Effect of pH and Weathering Indices on the Reductive Transformation of 2-Nitrophenol in South China

Iron is the first abundant transition metal in the Earth's crust; soils in subtropical and tropical zones contain a large amount of free Fe oxide and have thus been paid increasing attention. In this study, 22 soil samples were collected from the A (0-20-cm) horizon in South China and divided into three groups (i.e., vegetable soils in Pearl River Delta, orchard soils in Pearl River Delta, and forest soils in tropical zone). Two types of experiments, one without buffer 2-(N-morpholino) ethanesulfonic acid (MES) and another with MES, were conducted to investigate the effect of pH and weathering indices on the reductive transformation of 2-nitrophenol (2-NP) on soils. Kinetic measurements showed an increase in pH resulted in an enhanced reaction rate (k) of 2-NP reduction. From cyclic voltammogram tests, the enhanced activity of Fe(II) species was attributed to the negative shift of peak oxidation potential of the Fe(III)-Fe(II) couple. The results were subjected to statistical analysis, including the evaluation of variance and correlation, and the application of stepwise regression. Significant differences in k values were obtained in the different soil types and parent materials. The reaction pH proved to be an essential factor in 2-NP reductive transformation, and the weathering indices of soils had a critical effect on the 2-NP reductive transformation processes in soils. With an increase in the level of soil desilicification and allitization from Groups I to III, the k value increased consistently in the order of the decrease in the weathering indices. These findings improve our general understanding of the effect of pH and weathering indices on soil Fe redox chemistry and provide valuable information on the implications for research on soil pollution control.