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改革开放以来,中国工业飞速发展,表面处理作为现代工业的一个基础行业更是发展迅猛。表面处理中的基础工艺化学镀和电镀,因其能增强镀件耐磨性、耐腐蚀性、硬度和光泽等在许多领域得到广泛应用[1-3]。镀镍废水是化学镀镍或电镀镍工艺各操作单元汇总的工业废水,其中主要污染物包括重金属镍离子,且因络合剂的存在,镍离子拥有较好的溶解性和稳定性。传统的重金属废水处理方法如化学沉淀法、离子交换法和吸附法去除络合态重金属污染物的效果不佳[4-6]。针对络合态重金属的处理,目前普遍采用的方法是先氧化破络释放出重金属离子,再通过沉淀、絮凝、吸附等传统方法分离去除[7-8].
已有许多学者将高级氧化技术应用于电镀废水的处理中,以实现重金属络合物的氧化破络。这些处理体系主要为UV/H2O2、UV/Chlorine、Fenton及类Fenton反应[8-12]。为避免上述反应过程需外源引入氧化剂、处理成本高等问题,近些年逐渐发展出电化学高级氧化技术[13-14],这些电化学技术主要聚焦于电催化阳极的研发及不同工艺之间的耦合实现污染物的高效去除[15-17],而关于电化学体系中重金属络合物的降解规律、机理解析及重金属是否会参与络合物矿化过程鲜有研究。已有研究报道了Cu-EDTA在UV/Chlorine过程中络合态铜能够介导氯自由基的生成从而形成自催化效果,加快反应速率[18]。在电化学膜过滤体系处理Cu-EDTA过程中也发现了类似的自催化降解现象,络合物态铜能够催化H2O2分解生成羟基自由基从而形成自催化破络效果[19]。也有研究报道Ni(Ⅱ)可以催化活性物种生成,Ni(Ⅱ)/过一硫酸盐体系中生成硫酸根自由基和单线态氧能有效降解柠檬酸镍络合物[20];Ni(Ⅱ)物种如Ni(OH)2作为阳极材料通电后能转为高价Ni物种(NiOOH),后者能有效降解甲醛[21]。Ni-EDTA作为镀镍废水中典型污染物,其在电化学氧化体系中的降解规律还未有详细的报道,镍作为一种变价金属是否会形成自催化效果也未知。
本研究旨在构建以混合金属氧化物(MMO)为阳极的电化学氧化体系处理Ni-EDTA,其核心是当电解质溶液中存在一定浓度的Cl−时,MMO阳极产生的活性氯与小分子配体络合态Ni(Ⅱ)反应生成次生活性物种,促进污染物的降解。以MMO为阳极,不锈钢为阴极建立的电化学体系用于Ni-EDTA破络研究,通过检测Ni-EDTA在不同电解质溶液和电流下的降解规律,探讨相关反应过程与机理,明确Ni2+在Ni-EDTA降解过程中起到的作用,为电化学技术用于镀镍废水处理提供理论借鉴。
Ni-EDTA在电化学氧化体系中的自催化降解
Autocatalytic degradation of Ni-EDTA in electrochemical oxidation systems
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摘要: 氧化破络是去除络合态重金属的关键步骤,本文构建了以商用混合金属氧化物(MMO)为阳极的电化学氧化体系用于降解Ni-EDTA。研究发现,当电流密度为27.8 mA·cm−2时,在溶液有氯和无氯情况下,电化学体系处理180 min后Ni-EDTA的去除率分别达到96.3%和77.8%;动力学拟合发现Ni-EDTA降解符合一级反应动力学,其中在有氯情况下随着反应进行降解速率呈现出加快的现象,降解速率常数在0到90 min阶段为0.0124 min−1,90 min到180 min上升至0.0216 min−1。对含氯体系中出现的固体进行X射线衍射(XRD)、X射线光电子能谱(XPS)和苯基甲基亚砜(PMSO)探针实验,确定反应过程生成了高价Ni物种。通过研究电化学体系中有无Ni2+对Fe(Ⅲ)-EDTA降解的影响,发现高价Ni物种能够促进金属有机络合物的降解,从而提高反应速率。Ni-EDTA在电化学氧化体系中自催化降解现象的揭示,为金属有机络合物去除机制提供了新见解。Abstract: Decomplexation by oxidation is a prerequisite step to remove complexed heavy metals from the aqueous phase. Herein, this study established an electrochemical oxidation system with a commercial mixed metal oxide (MMO) anode for degrading Ni-EDTA. When the current density is 27.8 mA·cm−2, the removal efficiencies of Ni-EDTA were 96.3% and 77.8% after 180-min treatment in the presence and absence of chloride, respectively. The abatement of Ni-EDTA obeyed the first-order kinetic law, and the rate constants increased with the operation time from 0.0124 min−1 to 0.0216 min−1 when chlorine was available in the solution. The characterizations of X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) and the methyl phenyl sulfoxide (PMSO) probe tests were conducted on the solids obtained from the chloride-contained system, suggesting the formation of high-valence Ni species. By investigating the effect of Ni2+ on the degradation of Fe(Ⅲ)-EDTA in the electrochemical system, it was revealed that high-valence Ni species can promote the destruction of metal organic complexes, and thus increase the reaction rate. The autocatalytic degradation of Ni-EDTA in the electrochemical oxidation system provides a new insight into the removal mechanism of metal-organic complexes.
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Key words:
- electro-oxidation /
- active chlorine /
- Ni-EDTA treatment /
- autocatalytic degradation /
- high-valance metal
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图 5 电化学体系中Fe(Ⅲ)-EDTA(a)降解效果(b)降解动力学拟合;不同Ni2+浓度下Fe(Ⅲ)-EDTA(c)降解效果(d)降解动力学拟合.
Figure 5. Comparisons of time-course (a) Fe-EDTA concentration and (b) kinetic-fitting results in the electrochemical systems. The effects of concentration of added Ni2+ on (c) degradation of Fe(Ⅲ)-EDTA and (d) kinetic-fitting results
表 1 不同电流密度下Ni-EDTA降解一级动力学拟合结果
Table 1. First-order kinetic fitting results of Ni-EDTA degradation under different current density
电流密度/(mA·cm−2)
Current density0—60 min 60—180 min Kobs/min−1 R2 Kobs/min−1 R2 11.1 0.0064 0.9975 0.0064 0.9975 16.7 0.0086 0.9985 0.0139 0.9918 22.2 0.0120 0.9981 0.0189 0.9914 27.8 0.0124 0.9983 0.0216 0.9961 33.3 0.0133 0.9991 0.0244 0.9977 表 2 不同氯离子浓度下Ni-EDTA降解一级动力学拟合结果
Table 2. First-order kinetic fitting results of Ni-EDTA degradation under different Cl− concentration
Cl−/( mmol·L−1) 0—60 min 60—180 min Kobs / min−1 R2 Kobs / min−1 R2 0 0.0077 0.9975 0.0077 0.9975 25 0.0100 0.9965 0.0174 0.9868 50 0.0120 0.9981 0.0193 0.9834 75 0.0119 0.9984 0.0197 0.9811 100 0.0117 0.9965 0.0202 0.9865 表 3 不同镍离子浓度下Fe(Ⅲ)-EDTA降解一级动力学拟合结果
Table 3. First-order kinetic fitting results of Fe(Ⅲ)-EDTA degradation under different Ni2+ concentration
Ni2+/(mmol·L−1) 0—30 min 30—180 min Kobs/min−1 R2 Kobs/min−1 R2 0 0.0028 0.9986 0.0028 0.9986 20 0.0025 0.9859 0.0059 0.9895 40 0.0026 0.9917 0.0069 0.9935 60 0.0032 0.9868 0.0081 0.9949 -
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