厌氧环境下黏土矿物结构Fe（Ⅲ）还原对Cd（Ⅱ）固持特性的影响

黏土矿物结构Fe（Ⅲ）是土壤铁元素主要赋存形态之一，稻田淹水环境下矿物结构Fe（Ⅲ）易发生还原反应，改变矿物晶体微结构和表面化学性质，为研究这种变化对矿物固持Cd（Ⅱ）特性的影响，以合成含铁黏土矿物绿脱石为研究对象，通过化学还原，探讨了厌氧条件下绿脱石结构态Fe（Ⅲ）还原对Cd（Ⅱ）吸附性能的影响及作用机制。结果表明：合成绿脱石中总铁含量为31.3%，结构态Fe（Ⅱ）和Fe（Ⅲ）的含量占总铁的98.6%。还原处理后，矿物结构中Fe（Ⅱ）含量由还原前的3.9%显著提高至67.4%。XRD、SEM、BET和XPS分析表明，还原后绿脱石矿物层间距增大，分散程度更高，比表面积由151.6 m²·g^-1增加至184.0 m²·g^-1，矿物表面的羟基基团数量增加。结构Fe（Ⅲ）还原降低了矿物对Cd（Ⅱ）的吸附和固持性能，Langmuir模型拟合还原后矿物对Cd（Ⅱ）的最大吸附量为23.5 mg·g^-1，明显低于还原前矿物对Cd（Ⅱ）的吸附量（33.4 mg·g^-1）。还原前后矿物对Cd（Ⅱ）的吸附性能均随着离子强度增大而降低，但还原矿物吸附Cd（Ⅱ）的降幅明显低于未还原矿物。黏土矿物结构Fe（Ⅲ）还原后吸附Cd（Ⅱ）的性能降低主要是由于还原过程中结构Fe（Ⅱ）迁移进入层间，抑制了吸附过程中Cd（Ⅱ）进入矿物层间。研究表明，厌氧环境中黏土矿物结构Fe（Ⅲ）显著降低了矿物对Cd（Ⅱ）的固持特性。

Influence of reduction of structural Fe（Ⅲ） in clay on Cd（Ⅱ） capture under anoxic conditions

The structural Fe（Ⅲ） in clay minerals is one of the primary forms of iron in soils. During flooding conditions, the Fe（Ⅲ） in clay minerals'' structure can be reduced to Fe（Ⅱ）, resulting in changes in the microstructure and surface chemical properties of the minerals. However, the effects of structural Fe（Ⅲ） reduction on Cd（Ⅱ） capture by these minerals are still unclear. In this study, iron-containing nontronites prepared through a chemical method were used to investigate the effects of structural Fe（Ⅲ） reduction on Cd（Ⅱ） adsorption performance under anoxic conditions. The results showed that the total iron content in the synthetic nontronite was 31.3%, with structural iron accounting for 98.6% of the total iron in the minerals. After chemical reduction, the Fe（Ⅱ）/Fe ratio in the reduced nontronite significantly increased from 3.9% to 67.4%. XRD, SEM, BET, and XPS analyses revealed an increase in the interlayer spacing and the amounts of surface hydroxyl groups with structural Fe（Ⅲ） reduction. Moreover, the specific surface area of the minerals increased from 151.6 m²·g^-1 to 184.0 m²·g^-1. The adsorption capacities of the reduced minerals for Cd（Ⅱ） were lower than those of the unreduced minerals, whereas the desorption rates of the reduced minerals were slightly higher than those of the unreduced minerals. The Langmuir model best fit the adsorption isotherm data of Cd（Ⅱ） on minerals before and after reduction. The maximum adsorption capacities of Cd（Ⅱ） on reduced and unreduced minerals were 23.5 mg·g^-1 and 33.4 mg·g^-1, respectively. The adsorption capacities of minerals for Cd（Ⅱ） decreased before and after reduction with an increase in ionic strength, and the reduction in Cd（Ⅱ） adsorption on reduced minerals was significantly lower than that on unreduced minerals. The reduction of Cd（Ⅱ） adsorption performance on reduced clay minerals is mainly attributed to the migration of structural Fe（Ⅱ） into the interlayer during the reduction process, inhibiting Cd（Ⅱ） from entering the interlayer of minerals during the adsorption process.