Removal of Cadmium(II) by hydrated manganese dioxide: behaviour and mechanism at different pH

吸附 化学 Zeta电位 吸热过程 弗伦德利希方程 朗缪尔 无机化学 离子交换 降水 离子 化学工程 物理化学 有机化学 物理 气象学 纳米颗粒 工程类
作者
Yao Wang,Wanzhen Xie,Fencun Xie
出处
期刊:Environmental Technology [Informa]
卷期号:44 (23): 3544-3562 被引量:3
标识
DOI:10.1080/09593330.2022.2064240
摘要

Homogeneous precipitation was proposed to prepare hydrated manganese dioxide (HMO) with KMnO4 as oxidant, NaCl as reductant and HNO3 as reaction auxiliary. HMO was applied to remove Cd(II) and the effect of contact time, initial concentration, adsorbent dose and pH value on adsorption efficiency were investigated. The removal mechanisms at various pH values were analysed in detail. Adsorption thermodynamics parameters were calculated as ΔG < 0, ΔH > 0 and ΔS > 0, which meant that the adsorption process was endothermic. The result of adsorption kinetics indicated the adsorption process conformed to pseudo-second-order kinetics. When adsorbing Cd(II) with initial concentration equaling 100 mg·L−1, the activation energy (Ea) was 62.740 kJ·mol−1. The Langmuir model could describe adsorption behaviour on HMO better than the Freundlich model, indicating that the adsorption sites of HMO were homogeneous and that single-layer adsorption was a dominant way in this process. The maximum adsorption capacity of Cd(II) on MnO2 calculated by the Langmuir model was 267 mg·g−1. The adsorbent HMO could be recycled and reused for several times with a high efficiency above 70% by adding HCl. SEM, EDS, FTIR and XPS were used to analyse the mechanisms of removal of Cd(II) at pH = 3,7 and 10. The mechanisms included electrostatic attraction, ion exchange and chemical precipitation. With pH increasing, the zeta potential decreased and the surface negative charge increased, promoting Cd(II) removal through enhanced electrostatic attraction. Meanwhile, ion exchange mechanisms including inner-sphere complexation and outer-sphere complexation occurred during adsorption process at different pH.
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