Reductions of Fe(III) and pentachlorophenol linked with geochemical properties of soils from Pearl River Delta

Abstract Soils in the Pearl River Delta (PRD) of South China contain iron with a higher abundance and reactivity formed under a subtropical monsoon climate with a unique biogeochemistry. Iron cycle plays a vital role in transformation of contaminants. However, the linkage between iron cycle and contaminants transformation vs. geochemical properties of soils remains unclear. In this study, a set of experiments for reductions of Fe(III) and pentachlorophenol (PCP) were conducted to fill up the gap between Fe(III) reducibility and PCP transformation on the view of geochemistry. Fourteen soil samples were collected from the A (0 to 20 cm) horizon in the PRD and were divided into three groups based on their land use types (LUTs, i.e., vegetable fields, paddy soils and mangroves). The experiments were designed and subsequently conducted at pH 7.0 ± 0.2 (excluding pH interference) in three different conditions (i.e., soil-sterile, soil, and soil + lactate). Kinetic measurements showed that the reduction rates (μmax) of Fe(III) and PCP could be calculated using a logistic model. The stepwise regression analyses showed that oxalate-extractable iron (Feo) was likely one of the most active iron sources for soil Fe(III) reduction. Feo and dithionite-extractable iron (Fed) had close correlations with the rate of PCP reductive transformation. Moreover, parallel correlations exist between μmax-PCP and μmax-Fe(II)sorbed, illustrating the crucial effect of sorbed Fe(II) on PCP reduction in soils. The variance analysis results showed significant differences in the average μmax-PCP and μmax-Fe(II)sorbed value under different LUTs (vegetable field < paddy soil ≤ mangroves) and soil types (Chinese soil taxonomy (CST), Typic Gleyi-Stagnic Anthrosols < Typic Fe-Leachi-Stagnic Anthrosols ≤ Sulfic Aqui-Orthic Halosols). Moreover, terminal restriction fragment length polymorphism results showed that the differences in reduction of Fe(III) and PCP should be attributable to microbial communities compositions in different soil type (i.e., LUTs or CST). Substrate amendments would obviously impact the bacterial community structure, which could further affect Fe(III) reducibility and PCP transformation. These findings could improve our general understanding of the vital role of iron redox chemistry in PCP transformation vs. geochemical properties of soils. The results also indicated that the linkage of Fe(III) reducibility and PCP transformation on the view of soil microbiology should be gained attention in future studies.