Little is known about the exact mechanism of cadmium (Cd) immobilization by phosphate (P) in arable soil containing low Cd concentration. This study was designed to describe this mechanism in detail. We determined the phosphorus (P) addition rate allowing Cd to precipitate as Cd minerals in Cd-contaminated arable soils and identified the main species of Cd minerals formed by the reaction of Cd and P, using the chemical equilibrium model MINTEQ. To determine the amount of Cd adsorption to soil adsorbed P, 0 –10,000 mgPL−1 of K2HPO4 solution was reacted with soil, then the P adsorbed soil was reacted with a CdCl2 solution (500 mg Cd L−1). Cadmium might not precipitate as Cd minerals such as Cd3(PO4)2 and CdCO3 with a recommended application rate of P fertilizer in field scale. Cadmium might be immobilized by Cd2+ adsorption instead of precipitation under a low P application system. Phosphate adsorption increased the negative charge of soil and Cd adsorption. The contributions of the increase in pH- and P-induced negative charges to the total increase in the soil negative charge were 93.2 and 6.8%, respectively. The increase in Cd adsorption caused by P adsorption was mainly attributed to the increase in pH-induced negative charge.

中文翻译：

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磷酸盐改良耕作土壤中镉的固定机理

关于在低Cd浓度的耕作土壤中通过磷酸盐（P）固定镉（Cd）的确切机理知之甚少。本研究旨在详细描述这种机制。我们使用化学平衡模型MINTEQ，确定了允许Cd作为Cd矿物在受Cd污染的耕地中沉淀的磷（P）添加速率，并确定了由Cd与P反应形成的Cd矿物的主要种类。为了确定Cd对土壤吸附的P的吸附量，先使0–10,000 mgPL-1的K2HPO4溶液与土壤反应，然后使P吸附的土壤与CdCl2溶液（500 mg Cd L-1）反应。在田间规模内，建议的P肥料施用量下，镉可能不会以Cd3（PO4）2和CdCO3等Cd矿物的形式沉淀。镉可能通过Cd2 +吸附而不是在低磷施用系统下沉淀来固定。磷酸盐的吸附增加了土壤的负电荷和Cd的吸附。pH和P引起的负电荷增加对土壤负电荷总量增加的贡献分别为93.2和6.8％。P吸附引起的Cd吸附增加主要归因于pH诱导的负电荷增加。