Biogeochemical Control on the Mobilization of Cd in Soil

Abstract

Cadmium (Cd) is a toxic element that can easily enter the human body through the food chain. Rice grain is the main contributor to dietary Cd intake, especially for those populations who consume rice as the staple. Therefore, reducing Cd accumulation in rice is of significance for food safety and human health. In this review, we summarized the major factors underlying the biogeochemical processes controlling Cd availability in soils. Soil pH is one of the most important factors affecting soil Cd availability. An increase in soil pH can dramatically decrease soil Cd solubility; thus, liming is often found to be an effective and economical method in acidic contaminated soils to reduce grain Cd accumulation. The voltaic effect between Cd-sulfide and other metal sulfides formed during soil flooding regulates the remobilization of Cd during soil drainage. Therefore, addition of small amounts of ZnSO4 or MnSO4 to soils can form a protective voltaic cell partner against the oxidative dissolution of Cd sulfides, thereby decreasing Cd remobilization during soil drainage and Cd accumulation in rice grain. The voltaic effect is expected to be more important than their direct competition between Zn2+ and Cd2+. Besides, water management practices markedly affect soil Cd availability by altering the redox status of soils; continuous flooding and delaying drainage of paddy soils during the grain filling period can effectively reduce grain Cd accumulation. These effective biogeochemical strategies can be used singly or combinedly to reduce soil Cd availability and subsequent Cd uptake/accumulation in rice to ensure food safety.