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砷来源广泛,包括火山喷发、岩石风化等自然来源以及采矿、冶金等人为来源[1-2]。在全球范围内,土壤中砷的平均质量分数为1.8 mg·kg−1,而我国土壤中砷平均质量分数达到9.2 mg·kg−1,超过世界水平的5倍[3]。我国云南、贵州、四川等西南地区的土壤中砷背景值远超全国土壤背景值[4]。土壤中的砷通过食物链进入人体后,可引发色素沉着、慢性肺病、心血管疾病和神经系统紊乱等健康问题[5]。因此,对砷污染土壤的修复十分迫切。
电动修复是常用的一种砷污染土壤修复方法,其利用电渗析、电迁移等电动效应使砷酸根和亚砷酸根定向迁移,从而降低土壤中砷的总量[6-7]。但常规电动修复技术对砷的修复效果有限,KARACA等[8]对沉积物中的砷进行电动修复时发现,运行18 d后砷几乎没有被去除。电极逼近法为电动修复的一种,其在电动过程中每隔一段时间将电极向某一方向移动一定距离,以此来影响土壤pH、氧化还原电位 (Eh) 等环境因子,而砷的溶解性和迁移性与环境因子密切相关。YAO等[9]发现,相比于固定电极法 (FE-EK) ,阴极逼近法 (AC-EK) 通过提高阴极区域pH可将砷的去除率提高4倍。付博等[10]发现,当pH<4时,随着pH的降低,粗砂和细砂中砷的溶出量不断增加。周一敏等[11]发现,当Eh较低时,五价砷[As(V)]会转化为移动性更高的三价砷[As(III)],另外还能驱动土壤中砷的释放。由此可见,电动逼近技术对提高砷污染土壤修复效果具有很大潜力。
目前,常采用向土壤中加入化学药剂[2,7]、增设渗透反应墙[6,12]等方式提高砷去除率,但基于电极逼近技术对砷污染土壤进行修复的研究尚很缺乏。基于此,本研究采用不同的电极逼近方式对砷污染土壤进行修复。研究不同逼近方式对总砷[As(T)]的分布以及As(III)、As(V)迁移转化的影响,探究捕集室土壤中砷赋存形态的转化,以期为砷污染场地修复提供技术参考和理论依据。
电动修复过程中电极逼近对土壤砷迁移与形态转化的影响
Effect of the approaching electrode on the soil arsenic migration and speciation transformation during electrokinetic remediation
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摘要: 以高浓度砷污染土壤为修复对象,探究电极逼近法耦合捕集室对砷污染土壤的修复效果。实验设置4个处理,分别为固定电极 (FE-EK) 、阴极逼近 (AC-EK) 、阳极逼近 (AA-EK) 和两极逼近 (AAC-EK) 。结果表明,AC-EK、AA-EK以及AAC-EK对总砷[As(T)]的迁移具有促进作用,表现为捕集室中As(T)质量分数与初始值相比显著升高 (p<0.05) ,而FE-EK捕集室中As(T)质量分数与初始值相比无显著性差异 (p>0.05) ,As(T)整体迁移率以AAC-EK最高 (31.50%) ,FE-EK最低 (15.38%) 。由于土壤整体氧化还原电位升高,使得FE-EK、AC-EK、AA-EK、AAC-EK处理组三价砷平均质量分数较初始值分别降低9.78%、7.81%、13.65%、4.09%。砷的生物有效性在Fe2O3和电动效应的联合作用下不断降低,表现为可交换态砷、铝结合态砷、钙结合态砷向铁结合态砷、残渣态砷转化。本研究结果表明,AAC-EK促进As(T)迁移的效果最好,可交换态砷占比最低,且单位修复能耗最低,具有良好的砷污染土壤修复潜力。Abstract: The remediation effect of the approaching electrode technique coupled with a capture chamber was evaluated using soil contaminated with a high concentration of arsenic. Four treatments were set up in the experiment: fixed-electrode electrokinetic (FE-EK), approaching cathode electrokinetic (AC-EK), approaching anode electrokinetic (AA-EK), and approaching anode and cathode electrokinetic (AAC-EK) techniques. The results demonstrated that AC-EK, AA-EK, and AAC-EK techniques promoted the migration of total arsenic [As(T)]: the mass fraction of As(T) in their capture chambers was significantly higher than the initial value(p<0.05), while that in the capture chamber of FE-EK did not differ significantly from the initial value(p>0.05). The overall migration rates of As(T) in AAC-EK was the highest (31.50%), and that in FE-EK was the lowest (15.38%). The average mass fraction of trivalent arsenic in FE-EK, AC-EK, AA-EK, and AAC-EK techniques decreased by 9.78%, 7.81%, 13.65%, and 4.09%, respectively, compared with the initial value owing to an increase in the overall soil redox potential. Under the combined influence of Fe2O3 and electric-influence, the bioavailability of arsenic was continuously reduced, as evidenced by the conversion of exchangeable arsenic, aluminum-bound arsenic, and calcium-bound arsenic to iron-bound arsenic and residual arsenic. This study showed that AAC-EK was associated with the highest As(T) migration, lowest proportion of exchangeable arsenic, and lowest energy consumption per unit of remediation. Thus, the AAC-EK technique had good potential for remediating arsenic-contaminated soil.
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