Iron reduction coupled to reductive dechlorination in red soil: A review

Iron reduction plays an important role in the reductive transformation of organochlorine pesticides in red paddy soils. This interaction between iron reduction and organochlorine pesticides (OCPs) in red soils in south China is particularly important because of the high abundance and reactivity of iron within unique man-made paddy ecosystems. However, the relationships between iron reduction and reductive dechlorination and the geochemical constraints of these relationships are not fully understood. In this comprehensive review, we summarized current understanding of iron reduction, reductive dechlorination, the relationships between them, and their interactions with the reduction of nitrate, sulfate, Cu(II), and humic substances in red soils. Recent studies showed that iron reduction and dechlorination occur simultaneously in soils and that iron reduction could either stimulate or inhibit dechlorination.Meanwhile, sulfate and Cu(II) reduction can stimulate or inhibit iron reduction and dechlorination. Nitrate reduction can be coupled to iron reduction, but it inhibits dechlorination. Increasing evidence showed that humic substances can enhance the rates of both iron reduction and dechlorination by accelerating electron transfer. However, there is insufficient information in the literature for delineating the effects of several rising environmental problems (e.g., heavy metal pollution, deficiencies in phosphorus and aluminum) on iron reduction, OCP transformation, and the related microbial activities. Future studies are necessary because such information may be key for sustainable development of agriculture and pollution control in red soils.