水羟锰矿-高岭石复合物对镉的吸附行为研究

Study on the adsorption of cadmium by vernadite-kaolinite composites

摘要

【目的】重金属镉（Cd）是水体环境中危害极大的有毒污染物，本研究旨在明确水羟锰矿负载量、溶液 pH 和胡敏酸（HA）浓度对 Cd(II)吸附行为的影响。【方法】采用氧化还原共沉淀法合成了 3 种水羟锰矿-高岭石复合物，并以水羟锰矿（V）和高岭石（K）为对照，通过 XRD（X-ray diffraction，X 射线衍射）、FTIR（Fourier-transform infrared spectroscopy，傅里叶变换红外光谱）、TEM（transmission electron microscopy，透射电子显微镜）、比表面积和零电荷点分析表征其结构与表面性质，开展等温吸附、pH 和 HA 影响试验，并采用 Langmuir 及 Freundlich 模型拟合吸附数据。【结果】相较于单一高岭石，复合物晶体结构保持稳定，表面官能团数量增加，比表面积显著提升，提供了更多活性吸附位点；复合物对 Cd 的吸附量较单一高岭石提高 1.5～2 倍，吸附过程符合 Langmuir 等温模型，表明属于单分子层化学吸附。溶液 pH 显著影响吸附性能，中性至碱性条件有利于复合物对 Cd 的吸附，原因在于该条件下复合物表面负电荷增加，有效减弱了与 Cd 之间的静电斥力。胡敏酸（HA）对吸附过程具有调控作用：复合物对 Cd 的吸附量约为无 HA 对照的 2 倍，且随 HA 浓度增加吸附量有所下降；添加 HA 后，低 Cd 浓度时矿物对 Cd 吸附量小幅增加，高 Cd 浓度时吸附性能显著提升，其中高岭石对 Cd 吸附量提升最多。HA 主要通过疏水作用增强吸附效果。进一步研究发现，HA 在酸性条件下对吸附起促进作用，而在中碱性条件下则表现为抑制作用。【结论】水羟锰矿-高岭石复合物能够提高对 Cd 的吸附能力，其吸附性能受水羟锰矿负载比例、溶液 pH 和 HA 浓度共同调控。本研究可为含 Cd 废水吸附处理材料的设计提供实验依据。

Abstract

[Objective] Cadmium (Cd) is a highly toxic heavy-metal pollutant that poses serious risks to aquatic environments. This study aimed to elucidate the effects of vernadite loading, solution pH, and humic acid (HA) concentration on Cd(II) adsorption behavior. [Method] Three vernadite–kaolinite composites with different vernadite loadings were synthesized via redox coprecipitation, using pure vernadite (V) and kaolinite (K) as controls. Their structural and surface properties were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), specific surface area analysis, and point-of-zero-charge determination. Adsorption isotherm experiments and experiments evaluating the effects of pH and HA were conducted, and the adsorption data were fitted using the Langmuir and Freundlich models. [Result] The composites maintained a stable internal structure and exhibited a substantial increase in surface functional groups and specific surface area, thereby providing abundant active adsorption sites. The adsorption capacity of the composites was 1.5–2 times higher than that of kaolinite. The Cd adsorption process conformed to the Langmuir isotherm model, indicating monolayer chemisorption. Solution pH significantly influenced adsorption behavior, with neutral to alkaline conditions favoring Cd removal. This enhancement is attributed to increased surface negative charge under neutral and alkaline conditions, which effectively reduces electrostatic repulsion toward Cd(II). Humic acid (HA) exerted a regulatory effect on adsorption: the Cd adsorption capacity of the test composites was approximately twofold that of the HA-free control; However, adsorption decreased with increasing HA concentration. Upon HA addition, mineral adsorption of Cd exhibited a slight enhancement at low Cd concentrations, whereas at elevated Cd concentrations, adsorption performance improved markedly, with kaolinite demonstrating the most pronounced increase. This enhancement is primarily attributed to strengthened hydrophobic interactions mediated by HA. Further investigation demonstrated that HA promoted adsorption under acidic conditions but inhibited it under neutral to alkaline conditions. [Conclusion] Vernadite–kaolinite composites enhanced Cd adsorption, and their adsorption performance was jointly regulated by the vernadite loading ratio, solution pH, and humic acid concentration. These findings provide an experimental basis for the design of adsorption materials for the treatment of Cd-contaminated wastewater.

关键词

镉 / 水羟锰矿 / 高岭石 / 胡敏酸 / 吸附

Key words

cadmium / vernadite / kaolinite / humic acid / adsorption

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基金资助

安徽省教育厅自然科学重点项目(2024AH050483)

安徽省耕地保护创新示范中心开放基金(ahgbzx202301)