Abstract:

Soil contamination by cadmium (Cd) is one of the major environmental hazards recognized in the world, and its migration and enrichment severely threaten national food security and harm human health. A novel sulfhydryl/iron composite functionalized montmorillonite (SH-Fe-Mont) prepared by rational integration of organic/inorganic modification methods and used for efficient removal and passivation of Cd. FTIR, XRD, BET, SEM and XPS were used to confirm that the passivators were successfully prepared and possessed a rich porosity and larger specific surface area without changing the basic layered structure of montmorillonite (Mont). The removal rate and passivation effect of passivators on Cd were examined through a series of aqueous solution adsorption experiments and morphological changes of Cd in soil. The isothermal adsorption experiments were carried out to demonstrate that the adsorption behavior of the passivator was consistent with the Langmuir model over the temperature range of 293-323 K. The theoretical maximum saturated adsorption capacity of Cd onto SH-Fe-Mont and single-modified iron-based functionalized montmorillonite (Fe-Mont) were 55.3 and 42.7 mg·g^-1, which were 46.9 and 36.2 times higher than that of Mont, respectively. The results of the adsorption kinetics experiments showed that the adsorption of Cd(II) by the passivators at the initial concentrations of 20, 50, and 100 mg·L^-1 were in accordance with the pseudo-secondary model, and the faster adsorption kinetics led to the adsorption equilibrium of SH-Fe-Mont within 40 min (Mont: 80 min; Fe-Mont: 50 min). More importantly, the removal rate for Cd could exceed 99.0% (initial Cd concentration 20 mg·L^-1). Acid rain simulation experiments demonstrated only 0.31% Cd desorption and 0.22 mg·kg^-1 Cd leaching after Cd adsorption by SH-Fe-Mont, which was much lower than that of pristine Mont (41.2%, 11.7 mg·kg^-1) and Fe-Mont (4.58%, 1.67 mg·kg^-1). The adsorption mechanism of SH-Fe-Mont on Cd was investigated by XPS, and the results showed that Cd could be immobilized by SH-Fe-Mont in a complex state of -S-Cd and FeOOCd⁺. After the application of 2% SH-Fe-Mont, the available Cd content was reduced by 71.2%, and the sum of the highly active exchangeable and carbonate-bound states was reduced from 83.8% to 47.4%, while the sum of the more stable Fe-Mn oxidized-bound, organically-bound, and residue-bound states was increased from 16.2% to 52.6%, which effectively reduced the Cd bioactivity, and thus reduced the risk of Cd migration and enrichment in soil. The results of this study can provide a theoretical basis for the passivation remediation of Cd-contaminated soils and water.

Key words: montmorillonite, functionalization, compound passivator, cadmium, passivation repair

摘要：

土壤镉（Cd）的迁移和富集严重威胁国家粮食安全和损害人体健康。通过整合有机/无机改性方法制备了新型的巯基/铁基复合功能化蒙脱土（SH-Fe-Mont），用于Cd的高效去除和钝化。利用FTIR、XRD、BET、SEM及XPS等表征手段证实了钝化剂被成功制备，且在没有改变蒙脱土（Mont）基本层状结构的基础上拥有丰富的孔隙结构及更大的比表面积。通过系列水溶液吸附实验和土壤有效态Cd及赋存形态变化考察钝化剂对重金属Cd的去除率和钝化效果。等温吸附实验结果证明在293-323 K温度范围内，钝化剂吸附行为均符合Langmuir模型。在303 K时，SH-Fe-Mont和单改性的铁基功能化蒙脱土（Fe-Mont）的理论最大饱和吸附量分别为55.3、42.7 mg·g^-1，分别是Mont的46.9、36.2倍。吸附动力学实验的结果显示，初始质量浓度分别为20、50、100 mg·L^-1时，钝化剂对Cd(Ⅱ)的吸附过程均符合伪二级模型，更快的吸附动力学使SH-Fe-Mont在40 min内达到吸附平衡（Mont：80 min；Fe-Mont：50 min）。更重要的是，SH-Fe-Mont对Cd的去除率超过99.0%（Cd的初始质量浓度20 mg·L^-1）。酸雨模拟实验证明了SH-Fe-Mont对镉吸附后的高稳定性，仅有0.31%的镉解吸和0.22 mg·kg^-1的镉浸出，远低于原始Mont（41.2%，11.7 mg·kg^-1）和Fe-Mont（4.58%，1.67 mg·kg^-1）。利用XPS研究了SH-Fe-Mont对镉的吸附机制，结果表明，Cd能被SH-Fe-Mont以一种复杂的络合状态-S-Cd和FeOOCd⁺固化。与对照组相比，施加2% SH-Fe-Mont后有效态镉含量降低71.2%，且活性高的可交换态和碳酸盐结合态之和由83.8%降低至47.4%，而稳定性较强的铁锰氧化结合态、有机结合态和残渣态之和由16.2%增加至52.6%，从而有效降低了镉的生物活性，进而减弱土壤中镉迁移的风险。该研究结果可为土壤和水中镉污染的钝化修复提供理论依据。

关键词: 蒙脱土, 功能化, 复合钝化剂, 镉, 钝化修复