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Autocatalytic Decomplexation of Cu(II)-EDTA and Simultaneous Removal of Aqueous Cu(II) by UV/Chlorine Xianfeng Huang, Yi Wang, Xuchun Li, Dongxing Guan, Yubao Li, Xiangyong Zheng, Min Zhao, Chao Shan, and Bing-Cai Pan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05346 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019
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Autocatalytic Decomplexation of Cu(II)-EDTA and
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Simultaneous Removal of Aqueous Cu(II) by UV/Chlorine
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Xianfeng Huang1, Yi Wang1, Xuchun Li2*, Dongxing Guan4, Yubao Li1, Xiangyong
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Zheng1, Min Zhao1, Chao Shan3*, Bingcai Pan3
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1. School of Life and Environmental Science, Wenzhou University, Wenzhou,
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325035, China
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2. School of Environmental Science and Engineering, Zhejiang Gongshang University,
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Hangzhou, 310018, China
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3. State Key Laboratory of Pollution Control and Resource Reuse, School of the
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Environment, Nanjing University, Nanjing 210023, China
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4. Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth
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Sciences and Engineering, Nanjing University, Nanjing 210023, China
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* Corresponding authors.
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E-mails:
[email protected] (X. Li),
[email protected] (C. Shan).
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ABSTRACT
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Traditional processes usually cannot enable efficient water decontamination from
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toxic heavy metals complexed with organic ligands. Herein, we first reported the
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removal of Cu(II)-EDTA by UV/chlorine process, where the Cu(II)-EDTA
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degradation obeyed autocatalytic two-stage kinetics, and Cu(II) was simultaneously
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removed as CuO precipitate. The scavenging experiments and EPR analysis indicated
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that Cl• accounted for the Cu(II)-EDTA degradation at diffusion-controlled rate
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(~1010 M-1 s-1). Mechanism study with mass spectrometry evidence of eleven key
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intermediates revealed that the Cu(II)-EDTA degradation by UV/chlorine was an
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autocatalytic successive decarboxylation process mediated by the Cu(II)/Cu(I) redox
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cycle. Under UV irradiation, Cu(I) was generated during the photolysis of the
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Cl•-attacked complexed Cu(II) via ligand-to-metal charge transfer (LMCT). Both free
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and organic ligand-complexed Cu(I) could form binary/ternary complexes with ClO-,
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which were oxidized back to Cu(II) via metal-to-ligand charge transfer (MLCT) with
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simultaneous production of Cl•, resulting in the autocatalytic effect on Cu(II)-EDTA
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removal. Effects of chlorine dosage and pH were examined, and the technological
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practicability was validated with authentic electroplating wastewater and other
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Cu(II)-organic complexes. This study shed light on a new mechanism of
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decomplexation by Cl•, and broadened the applicability of the promising UV/chlorine
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process in water treatment.
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INTRODUCTION
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Toxic heavy metals from anthropogenic wastewater discharge pose great threat to
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environment. Considerable amount of toxic metals are usually stably complexed with
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agents (e.g., ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and
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citric acid) in industrial effluents due to their strong affinity towards carboxyl and
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amino functional groups.1,2 Comparatively, it is more challenging to remove
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complexed heavy metals than the counterpart free ions by traditional methods, such as
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chemical precipitation and adsorption.3,4 Thus, new strategies are highly desired to
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meet the more stringent discharge requirements of toxic metals for industrial
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wastewaters.
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Decomplexation of metal complexes is the critical step for efficient removal of
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heavy metals. Advanced oxidation processes (AOPs) such as Fenton oxidation,4
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ozonation,5,6
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photoelectrocatalytic oxidation,11,12 have been widely employed for decomplexation
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of metal-complexes due to the production of strong oxidizing species (mainly HO•
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and SO4•-). However, most AOPs suffered from greatly suppressed oxidation
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efficiency due to the significant scavenging of poorly selective HO• and SO4•- by
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dissolved organic matter (DOM), carbonate, etc.8,13 Moreover, the conversion of
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SO4•- to HO• and their dramatically declining reaction rates with the deprotonated
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metal-complexes under alkaline conditions also resulted in dramatic inhibition on the
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decomplexation efficiency of AOPs, such as UV/H2O2 and UV/persulfate.8,11,12
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photochemical
oxidation,7-9
electrochemical
oxidation,10
and
The chlorine radical (Cl•) is noted for high reduction potential (~2.47 V)14 and
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could react rapidly with many compounds containing amino and/or carboxylic group
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(e.g. aniline and benzoic acid) at nearly diffusion-controlled rates (~1010 M-1 s-1).14-17
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More importantly, it is less affected by solution chemistry compared with HO• and
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SO4•-.13,14 Zeng et al. found that Cl• significantly enhanced the decomplexation of
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Cu(II)-EDTA in the photo-assisted electrochemical process.18 Similar enhancement
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was also observed in the removal of ammonia.19 Considering the common presence of
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amine and carboxyl groups in the organic complexes such as EDTA and citrate, the
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selective Cl• radical may be promising for efficient destruction of metal-organic
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complexes. UV/chlorine is a classic process to produce Cl• as the dominant reactive
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species under alkaline conditions.16,17,20 Furthermore, the liberated metals (e.g. Cu(II))
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could be naturally precipitated at alkaline pHs free of chemical addition. Nevertheless,
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the performance and mechanism of UV/chlorine-induced decomplexation of
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metal-organic complexes has never been reported.
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Therefore, this study was focused on the efficiency and mechanism of
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Cu(II)-EDTA degradation at alkaline pHs by UV/chlorine. Kinetics of Cu(II)-EDTA
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degradation and simultaneous removal of Cu from solution were investigated, with
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particular concern on the effects of chlorine dosage and pH. The underlying
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mechanism was explored from the pathways of radical transformation and
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Cu(II)-EDTA
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Cu(II)-organic complexes were employed to validate the technological practicability
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of the UV/chlorine process.
degradation.
Authentic
electroplating
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wastewater
and
other
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MATERIALS AND METHODS
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Materials. The main reagents employed in this study were all of ACS grade from
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Sigma-Aldrich, and are listed in the Supporting Information (Text S1). All the other
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reagents used were at least of analytical grade. All the solutions were prepared with
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Milli-Q water (Millipore, 18.2 MΩ•cm).
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Experimental Procedures. The photochemical experiments were carried out in a
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magnetically-stirred cylindrical borosilicate glass reactor (volume of 500 mL and 60
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mm in diameter). A quartz tube was vertically centered in the reactor and a
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low-pressure mercury UV lamp (Shanghai Jiguang Special Lighting Appliance
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Factory, 28 W, 254 nm) was used. The photon flux (I0, 253.7 nm) entering the
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solution was measured to be 1.32 μEinstein s-1 by the KI/KIO3 method,21 and the
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average fluence rate (Is) was estimated to be about 0.53 × 10-8 einstein s-1 cm-2, or
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2.45 mW cm-2. The effective path light (L) was determined to be 1.97 cm by the
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photolysis kinetics of dilute H2O2.22
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The solution temperature was kept at 25 ± 0.2 oC using a thermostat system. The
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solution pH was adjusted using 1.0 M NaOH or 1.0 M HCl. Various dosage of NaClO
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solution was spiked into 500 ml solution containing 0.30 mM Cu-complexes or
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authentic wastewater to obtain different molar ratio of NaClO and Cu (Rm =
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[NaClO]0/[Cu]0), wherein the solution pH spontaneously increased to 10.5-11.5.
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Samples were withdrawn at predetermined time intervals and were rapidly quenched
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with Na2SO3 solution (1.0 M). Then the samples were filtered through membrane
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(0.45 μm), and were stored at 4 oC in dark before analysis. An authentic industrial
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wastewater effluent was collected from an electroplating plant locating in Wenzhou
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(Zhejiang Province, China), and the water quality is shown in SI Table S1. All the
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experiments were conducted in triplicate.
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Analytical Methods. Solution pH was measured using a pH meter (PB-10,
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Sartorious). Chlorine concentration was measured by DPD/FAS titration method.23
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The concentrations of Cu(II)-EDTA and the decarboxyalted products were
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determined using high performance liquid chromatography (HPLC, E2695, Waters,
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USA) equipped with a PDA detector. The concentrations of BA and NB were also
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analyzed on HPLC. The detailed conditions of HPLC are available in our previous
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studies.5,24 Cu(II)-ethylenediaminetriacetic acid (Cu(II)-ED3A) was identified as
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described in our previous study.5 The total organic carbon (TOC) was measured by a
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TOC-V CPN analyzer (Shimadzu, Japan). Cu ions were quantified by atomic
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absorption spectrophotometer (AAnalytst 800, PE, USA). The concentration of Cu(I)
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was quantified spectrophotometrically using bathocuproine (BC) method.7 UV-Vis
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absorption spectrum of Cu(I) complexes with ClO- was scanned by a UV-Vis
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spectrophotometer (TU-1901, Pgeneral, China) and the detailed procedure was
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provided in Texts S3. The crystal structure of the precipitation dried by vacuum freeze
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drier was recorded by X-ray diffraction analysis (XRD, XTRA, Switzerland) with Cu
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K radiation (40 kV, 25 mA). The reactive species generated in-situ during
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UV/chlorine were detected by electron paramagnetic resonance (EPR) spectroscope
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(Bruker DRX500, Germany) with a 180 W medium pressure mercury lamp using
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DMPO as the trapping agent (Texts S4). The degradation products of Cu(II)-EDTA
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were identified using high resolution mass spectrometry (HRMS, Orbitrap Q
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ExactiveTM Focus, Thermo Scientific) (Texts S5).
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RESULTS AND DISCUSSION
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Cu(II)-EDTA Degradation and Simultaneous Removal of Cu by UV/chlorine.
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The efficiency of Cu-EDTA degradation by UV/chlorine process at pH 11.0 was
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shown in Figure 1a. Compared with negligible degradation of Cu(II)-EDTA (
