The Voltaic Effect as a Novel Mechanism Controlling the Remobilization of Cadmium in Paddy Soils during Drainage.

Affiliation

Huang H(1), Chen HP(1), Kopittke PM(2), Kretzschmar R(3), Zhao FJ(1)(4)(5), Wang P(1)(5).
Author information:
(1)College of Resources and Environmental Sciences, Nanjing Agricultural University, Nanjing 210095, China.
(2)School of Agriculture and Food Sciences, The University of Queensland, St Lucia, Queensland 4072, Australia.
(3)Department of Environmental Systems Science, Institute of Biogeochemistry and Pollutant Dynamics CHN, ETH Zurich, Zurich 8092, Switzerland.
(4)State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing 210095, China.
(5)Jiangsu Key Laboratory for Organic Waste Utilization and Jiangsu Collaborative Innovation Center for Solid Organic Waste Resource Utilization, Nanjing Agricultural University, Nanjing 210095, China.

Abstract

Excessive cadmium (Cd) accumulation in rice grain is a global issue that affects human health. The drainage of paddy soils during the grain filling period leads to the remobilization of Cd in soils, resulting in most of the Cd accumulated in rice grain. The rate of Cd remobilization during drainage differs markedly among soils, but the mechanisms underlying these differences remain largely unknown. Using microcosm soil incubation, electrochemical experiments, isotope labeling, and microscopic and spectroscopic analyses, here, we discover the voltaic effect as a novel mechanism controlling the remobilization of Cd during soil drainage. During soil flooding, microbial sulfate reduction results in the formation of various metal sulfides. When the soils are subsequently drained, the various metal sulfides can form within sulfide voltaic cells. The metal sulfides with a lower electrochemical potential act as anodes and are prone to oxidative dissolution, whereas the metal sulfides with a higher potential act as cathodes and are protected from oxidation. This voltaic effect explains why the presence of ZnS (with a low potential) suppresses the oxidative dissolution of Cd sulfides, whereas the presence of CuS (with a high potential) promotes the oxidative dissolution of Cd sulfides. The voltaic effect is applicable to all chalcophile trace metals coupled with the sulfur redox cycle in periodically anoxic-oxic environments, thus playing an important role in the biogeochemistry of trace metals.