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化学镀铜工艺是指在无外加电流的条件下,利用合适的还原剂(常用次磷酸盐),使溶液中的铜离子在具有催化活性的基体表面还原沉积出金属铜,形成铜镀层的一种工艺[1-3]。近年来,化学镀铜工艺在表面处理行业中所占的地位在不断上升,在机械工业、航空航天、电子工业等各行各业都有着越来越广泛的应用[4]。在化学镀铜工艺中,会产生大量的化学镀铜废水,将这些废水进行处理和回收,对保护生态环境,变废为宝,提高经济效益,尤为重要[5]。
化学镀铜废水主要来源于清洗零部件时所产生的清洗废水,因此,也可以称为化学镀铜清洗废水,其中主要含有铜离子和次磷酸盐等污染物[6]。过量的铜会刺激人类的消化系统,引起腹痛、呕吐等,严重时可造成中毒。而含铜废水进入水体后,成为持久性污染物,危机植物生长,影响水产养殖。当进入土壤时,会在土壤和作物中富集,经过一系列的环境迁移转化最终进入食物链,对人类健康产生威胁[7]。与正磷酸盐比,次磷酸盐由于其溶解度大且难与沉淀剂反应形成沉淀,导致水体富营养化严重的同时亦造成磷资源的流失[8]。故次磷的去除通常需氧化成正磷,再加入沉淀剂将正磷彻底去除或回收[9]。因此,对化学镀铜清洗废水的处理并回收磷和铜成为当前研究热点之一。
目前,含铜废水处理方法有很多,例如物化沉淀法、膜分离法、吸附法、混凝法和电解法等[10-11]。其中电解法可以使铜离子以金属铜的形式沉积在阴极上,实现了金属铜的回收[12]。除电解法外,其他方法只改变了铜离子存在形态,使铜离子发生迁移,但污染并没有彻底消除。而采用电解法时,当溶液为偏碱性条件下,铜离子很容易水解生成铜的氧化物,累积在阳极或生成沉淀物,使其难以在阴极进行电化学沉积回收[13]。因此,在电解法沉积铜离子实现阴极回收金属铜时,控制溶液的pH较为重要。
光电催化法是一种将光化学和电化学法相结合的方法,通过对半导体光催化剂施加外加偏压作用实现光生电子和光生空穴的有效分离,有效促进自由基的生成,提升污染物的降解效果[14]。光电催化作为高级氧化技术研究热点之一,是一种不仅能产生强氧化性活性物种实现污染物氧化降解,同时也能利用光生电子的还原能力实现阴极还原回收重金属的有效方法[15]。具有运行成本较低、温度和压力适应范围广、可实现有机物矿化且无二次污染等优点,在环境保护水处理领域越来受到关注[16]。二氧化钛(TiO2)纳米管光电极具有高度有序、比表面积大、电池容量高及量子化学效应强等优点,被广泛应用于纳米微电子、光伏器件、水分解产氢、环境污染物降解等领域[17-19]。有研究[20]表明,利用TiO2纳米管电极作为光阳极可有效实现铜氰络合物的氧化破络合同时电还原回收金属铜。
本研究采用电化学阳极氧化法制得的TiO2纳米管电极为光阳极和钛片(Ti)为阴极,在模拟太阳光(AM 1.5)照射进行光电催化处理次磷酸根离子(H2PO2−)和重金属铜离子(Cu2+)同时回收金属铜(Cu)。对TiO2纳米管电极进行了表征分析;对比分析了光电催化(PEC)、电催化(EC)和光催化(PC)体系对次磷氧化和Cu回收效果;考察了电压、初始PH、电解质种类对PEC体系下次磷氧化和Cu回收效率的影响,并进一步探讨该体系的反应机理。本研究结果可为含次磷和重金属铜的工业废水资源化处理提供参考。
TiO2纳米管阳极光电催化氧化次磷酸盐同时阴极回收金属铜
Photoelectrocatalytic oxidation of hypophosphite with TiO2 nanotube arrays anode and simultaneous recovery of copper on the cathode
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摘要: 采用电化学阳极氧化法制备了高度有序阵列的二氧化钛(TiO2)纳米管电极,采用此电极作为光阳极在模拟太阳光(AM 1.5)照射下进行了光电催化氧化次磷酸盐且同时回收金属铜(Cu)的研究,比较了光电催化、电催化和光催化体系对次磷氧化和Cu回收效率的差异,详细探究了电压、初始pH、电解质种类对光电催化体系下次磷氧化和Cu回收效率的影响。结果表明,当电压为2.0 V,初始pH为4.9,电解质为10 mmol·L−1 NaCl,在180 min内,1 mmol·L−1次磷全部被氧化为正磷,同时0.5 mmol·L−1的Cu 100%以金属形式在阴极沉积回收。在该光电催化体系中,在一定范围内,外加偏压的增加、溶液初始pH的升高和NaCl浓度的提高有利于次磷氧化和Cu回收。自由基淬灭实验和电子自旋共振实验结果证明,该体系中羟基自由基为氧化次磷的主要活性物种。以上研究结果可为含次磷和重金属铜废水的资源化处理提供参考。Abstract: Highly ordered TiO2 nanotube arrays electrode fabricated using Ti plate by electrochemical anodization method was used as a photoanode under simulated sunlight (AM 1.5G, 100 mW cm−2) irradiation for hypophosphite (H2PO2−) oxidation and Cu recovery. The performance for H2PO2− oxidation and Cu recovery in the photoelectrocatalytic (PEC) process was evaluated compared with the individual electrocatalytic (EC) or photocatalytic (PC) process. The effects of applied bias, initial solution pH and electrolyte type were investigated in detail. The results show that for 1 mmol·L−1 H2PO2− and 0.5 mmol·L−1 Cu2+, 100 % H2PO2− was oxidized to PO43−, 100 % Cu2+ was recovered as metallic Cu within 180 min at an applied bias of 2.0 V, initial solution pH of 4.9 and electrolyte of 10 mmol·L−1 NaCl. In this constructed PEC system, the increase of applied bias, initial solution pH or NaCl concentration within a certain range was proved to be favorable in the efficiency of H2PO2− oxidation and Cu recovery. The radical quenching experiments and electron spin resonance results indicated that hydroxyl (·OH) radicals were the major active species for H2PO− oxidation. This study can provide a reference for the resource treatment of wastewater containing H2PO2− and heavy metal Cu2+ ions.
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