Removal of chloride ions from strongly acidic wastewater using Cu(0

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Removal of chloride ions from strongly acidic wastewater using Cu(0)/Cu(II): Efficiency enhancement by UV irradiation and the mechanism for chloride ions removal Xianjia Peng, Wenyue Dou, Linghao Kong, Xingyun Hu, and Xianliang Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05787 • Publication Date (Web): 07 Dec 2018 Downloaded from http://pubs.acs.org on December 8, 2018

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Environmental Science & Technology

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Removal of chloride ions from strongly acidic wastewater using

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Cu(0)/Cu(II): Efficiency enhancement by UV irradiation and the

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mechanism for chloride ions removal

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Xianjia Peng*1,2,3, Wenyue Dou1,2,3, Linghao Kong1,2, Xingyun Hu1,2, Xianliang

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Wang4

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1. Key Laboratory of Drinking Water Science and Technology, Research Center for

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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2. Beijing Key Laboratory of Industrial Wastewater Treatment and Resource Recovery,

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Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing

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100085, China

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3. University of Chinese Academy of Sciences, Beijing 100049, China

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4. National Institute of Environmental Health, Chinese Center for Disease Control and

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Prevention, Beijing 100021, China

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* Corresponding author:

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Xianjia Peng

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1. Key Laboratory of Drinking Water Science and Technology, Research Center for

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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2. Beijing Key Laboratory of Industrial Wastewater Treatment and Resource Recovery,

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Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing

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100085, China

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3. University of Chinese Academy of Sciences, Beijing 100049, China

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ACS Paragon Plus Environment


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Tel: +86-10-6284 9198

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Fax: +86-10-6284 9198

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E-mail: [email protected]

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ABSTRACT

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Strongly acidic wastewater, which is usually generated from non-ferrous metal smelting

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industries, has the ability to be recycled as sulfuric acid. Before this wastewater is recycled,

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the removal of chloride ions is necessary to improve the quality of the recycled sulfuric acid.

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At present, the widely used method to remove chloride ions from acidic wastewater in the

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form of CuCl precipitate has several disadvantages, including a low removal efficiency, high

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temperature, long treatment time and high dosage of Cu(II). This study proposed an improved

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new method of removing Cl(-I) using Cu(0)/Cu(II) under UV irradiation, and the mechanism

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was investigated. The Cl(-I) concentration was lowered to below 50 mg/L at a Cu(II) dosage

36

of 1200 mg/L. Under UV irradiation, ligand-to-metal charge transfer (LMCT) takes place,

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thereby resulting in the formation of Cl•. Next, CuCl precipitates form through the reaction

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between Cu(0) and Cl• and produce h+/•OH under UV irradiation, which can oxidize Cl(-I) to

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Cl•. Simultaneously, Cl2 gas also forms directly from Cl•. This study offered a theoretical

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foundation for the application of UV irradiation for the enhanced removal of chloride ions

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from strongly acidic wastewater.

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Introduction

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Strongly acidic wastewater containing chloride ions and arsenic is usually generated from

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non-ferrous metal smelting industries. At present, the most common method to treat this type

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of wastewater is neutralization using lime or limestone, which has a major problem of

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generating massive complex CaSO4 waste 1, 2. Therefore, several scholars proposed recycling

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this type of wastewater as diluted sulfuric acid 3-5, which is more environmentally friendly.

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However, high concentrations of chloride ions and arsenic will affect the quality of the

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recycled sulfuric acid 6, 7. In view of this, the removal of chloride ions and arsenic is necessary

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before wastewater recycling.

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The removal of arsenic using sulfide compounds, such as FeS and H2S, is currently an

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effective method and attracts considerable attentions from scholars

8-10.

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remove chloride ions from acidic wastewater in the form of cuprous chloride (CuCl)

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precipitates using Cu(II) and Cu(0) is most widely used in metallurgical industries due to its

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simple operation and high removal efficiency of chloride ions

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disadvantages of this method also exist. Among them, the high dosage of Cu(II) is a major

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one, which is caused by the low reaction constant (K298=5.88×10-7) of eq 1 (Table 1) 12. At

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present, only approximately 70% of Cl(-I) can be removed at a rational Cu(II) dosage in

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industry. Meanwhile, to improve the removal efficiency of Cl(-I), a long treatment time

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longer than 1 h and a high temperature above 60°C should be employed 11. Accordingly,

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further effort is required to improve the traditional method.

11.

The technology to

Nonetheless, several

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In the process of wastewater treatment, ultraviolet (UV) irradiation is a well-established in

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situ treatment technology to remove contaminants 9. Previous studies have found that the

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ligand-to-metal charge transfer (LMCT) of metal complexes is an essential process in the

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aqueous environment under UV light and plays a significant role in metal speciation 13-18. For

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instance, in the LMCT process of Fe(III)/Cl(-I) complexes in seawater, Fe(III) can be reduced

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to Fe(II), and Cl(-I) will be oxidized to Cl•. Similarly, in the solution containing Cu(II)/Cl(-I)

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complexes, LMCT is also likely to take place, and as a result, Cu(II) could be reduced to

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Cu(I). It is reasonable to expect that the traditional chloride-ions-removal method using Cu(0)

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and Cu(II) could be improved under UV irradiation.

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In this study, an improved approach to remove chloride ions from strongly acidic

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wastewater using Cu(0) and Cu(II) under UV irradiation was investigated. First, batch

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experiments were performed to compare the removal efficiency of Cl(-I) and the dosage of

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Cu(II) under UV irradiation with those without UV irradiation, as well as to determine the

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optimum parameters. And next, the immediate radicals and final products were carefully

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identified. Finally, the mechanism was proposed. This study offered a theoretical foundation

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for the application of UV irradiation in the removal of Cl(-I) from strongly acidic wastewater,

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which effectively solved the problems existing in the present method.

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Materials and methods

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Reagents and materials

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The reagents used in the experiments were described in the Supporting Information (SI).

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Deionized (DI) water from a Milli-Q water purification system (Millipore) was used in all

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reagents and samples. The simulated wastewater was prepared by adding H2SO4 and NaCl to

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the DI water. Unless otherwise specified, the concentration of H2SO4 in all simulated

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wastewater was 50 g/L. The real wastewater was sampled from a zinc smelting plant in

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Guangxi Province, China. The major chemical compositions of the real wastewater were

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listed in Table S1. The concentrations of Cl(-I), SO42- and H+ were 428.8 mg/L, 29.9 g/L and

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0.491 M, respectively.

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Experimental system

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To explore the mechanisms, experiments were performed in a submerged UV-light reactor

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(500 mL), which was equipped with a low-pressure Hg lamp (28 w, 25 cm×3 cm2, Shanghai

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Jiguang Light and Electricity Plant, China), at room temperature. The detailed description of

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the apparatus was provided in the SI (Figure S1). The main emission wavelength of the lamp

97

is 253.7 nm (Figure S2). Control experiments without UV irradiation were also performed in

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the above-mentioned reactor. For comparison, Cl(-I) removal experiments at 70°C were

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conducted in a container (500 mL), which was immersed in a thermostatic magnetic stirring

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apparatus (Zhengzhou Hengyan Instrument Co., Ltd., China.).

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Analytical methods

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The concentration of Cl(-I) in the solution was determined by means of titration, before

103

which, the pH of the sample should be adjusted to 6.5-10.5 using NaOH solution. However,

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the absorption of CO2 gas by the NaOH solid probably leads to the existence of CO32- in the

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sample. Considering the possible influences caused by SO42-, CO32- and Cu(II), we analyzed

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the precipitation conditions of Ag2SO4, Ag2CO3 and CuCrO4, and found that CO32- was the

107

only interfering ion (SI). Therefore, we chose the H2SO4 solution (50 g/L) as the blank

108

solution to eliminate the interference of CO32-. The detailed analytical steps are as follows. (1)

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3.00 mL of sample, which was filtered through a 0.22-μm cellulose acetate membrane in

110

advance, 1.00 mL of phenolphthalein and 10 mL of DI water were transferred to a conical

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flask (250 mL); (2) The pH of the solution was adjusted to 6.5-10.5 by adding NaOH solution

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(8 g/L) until the mixed solution turned reddish; (3) 1.00 mL of K2CrO4 solution (50 g/L) was

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added to the mixed solution, and then the solution was diluted with DI water to a final volume

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of 100 mL; (4) The mixed solution was titrated by AgNO3 solution, whose concentration was

115

determined in advance by 0.0141 mol/L NaCl standard solution, until it turned to orange from

116

yellow. The consumed volume of AgNO3 solution was recorded as V1. To obtain the volume

117

of CO32--consumed AgNO3 solution, we also titrated the blank sample using the

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above-mentioned analytical steps, with the consumable volume of AgNO3 solution being

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recorded as V2. The concentration of Cl(-I) can be calculated using equation (1).

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c (mg/L) 

121

where c’ (mg/L) is the concentration of AgNO3 solution; and V0 is the volume of added

122

sample, which is 3 mL.

(V1  V2 )  c'35.45 1000 V0

(1)

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To verify the reliability of this method, we also titrated the H2SO4/Cl(-I) mixed solutions,

124

which were prepared using chloride ion standard solution. The results show that this

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analytical method has good reliability.

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To identify the generated free radicals, 50 μL of reaction solution was mixed with 10 μL of

127

5,5-dimethyl-1-pyrroline N-oxide (DMPO) in a 1.5 mL centrifuge tube, and hereafter, 40 μL

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of the mixed solution was sucked into a 50 μL quartz-glass capillary tube. The radicals in the

129

reaction system were generated via the irradiation of a xenon lamp (200-800 nm) and

130

subsequently detected by electron spin resonance (ESR) (E500, Bruker EleXsys, Germany)

131

(see the SI).

132

Based on the fact that CuCl can dissolve in the high-concentration hydrochloric acid 8


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solution via the formation of the [CuIClx]1-x complex, the mass of generated CuCl was

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measured as follows. (1) The precipitate, filtered from the solution, was dried in the electric

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blast driving oven (DHG-9070A, Shanghai Yiheng Science Instrument Co., Ltd. China) at

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70°C for 60 minutes, weighed up as m1 and put into a 50 mL centrifuge tube; (2) 40 mL of

137

hydrochloric acid solution (15%) was added to dissolve the CuCl, and after that, the residual

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solid was filtered, dried again and weighed up as m2. The mass of generated CuCl can be

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calculated using equation (2). In addition, the crystal texture and valence of the precipitate

140

were recorded correspondingly by X-ray diffraction (XRD) (X’Pert PRO MPD, PANalytical,

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Holland) and X-ray photoelectron spectra (XPS) (ESCALAB 250Xi, Thermo Fisher, Japan).

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m g   m1  m 2

(2)

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The residual Cu(II) in the solution was quantified using an inductively coupled plasma

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optical emission spectrometer (ICP-OES) (NexION 300X, PerkinElmer, USA). In our

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experiments, the gas product was determined to be Cl2 and the quantity can be calculated

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using equation (3).

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n (mol) 

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where c0 (mg/L) is the concentration of Cl(-I) before treatment, mg/L; c is the concentration

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of the residual Cl(-I) after reaction, mg/L; V is the volume of the reaction solution, 400 mL;

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and m is the mass of generated CuCl, g.

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Results and discussion

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Enhancement of Cl(-I) removal using Cu(0)/Cu(II) under UV Irradiation

153 154

1  (c 0  c)  V 1000m   2  35.45 1000 99.45 

(3)

Figure 1 shows the removal efficiency of Cl(-I) using different amounts of Cu(0) and Cu(II) under UV irradiation at room temperature. In our experiments, the Cl(-I) removal efficiency 9



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was zero by adding individual Cu(0) or Cu(II), which indicates that both Cu(0) and Cu(II)

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were necessary in the treatment. And next, experiments were conducted to obtain the

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appropriate reaction time and dosages of Cu(0) and Cu(II). In the condition that the total

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molar quantity of added Cu(0) and Cu(II) was twice as much as that of Cl(-I), approximately

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60% of Cl(-I) was removed from the solution at a Cu(0)/Cu(II) ratio of 1:1 after 25 min of

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UV irradiation (Figure 1a). On the basis of fixing the Cu(0)/Cu(II) ratio as 1:1, the removal

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efficiency of Cl(-I) was up to 90% at a Cut/Cl(-I) ratio of 4:1 after 25 min of UV irradiation

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(Figure 1b). In addition, a phenomenon was observed in our experiments that the removal

163

efficiency of Cl(-I) began to obviously decrease after 25 min, as a result of the oxidation of

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CuCl by O2 (see the SI). It is concluded that the removal efficiency and residual concentration

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of Cl(-I) were approximately 90% and 30 mg/L respectively, under the conditions that the

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dosages of Cu(0) and Cu(II) were at a Cu(0)/Cu(II)/Cl(-I) ratio of 2:2:1 and the reaction time

167

was 25 min. Therefore, in the following experiments, a Cu(0)/Cu(II)/Cl(-I) ratio of 2:2:1 and

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a reaction time of 25 min were used unless otherwise specified.

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The UV-improved removal of Cl(-I) was compared with that using the traditional heating

170

method. Under the same dosages of Cu(0) and Cu(II), approximately 37.1% of Cl(-I) was

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removed at 70°C after 55 min without UV irradiation. However, approximately 83% of Cl(-I)

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was removed at room temperature after 25 min under UV irradiation (Figure 2a). Moreover,

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to reach the maximum removal efficiency of Cl(-I), the dosage of Cu(II) under the condition

174

of heating was approximately 5000 mg/L at a Cu(II)/Cl(-I) ratio of 8:1. By contrast, the

175

dosage of Cu(II) under UV irradiation was much lower and was approximately 1200 mg/L at

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a Cu(II)/Cl(-I) ratio of 2:1 (Figure 2b). In our experiments under UV irradiation, more black

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CuCl precipitates formed, along with a kind of yellow-green gas product (Figure 2c), which

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could make the wet hydrogen ion test paper turn red and then fade. Therefore, it can be

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confirmed that this gas product was Cl2. In addition, XRD and XPS were performed on the

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precipitate to identify the composition. Figures S3 and S4 show that except for Cl2 gas, CuCl

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formation was also a route to remove Cl(-I) under UV irradiation, with a Cl2/CuCl

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contribution ratio of approximately 1.89:1 (Table S5). The experimental results signify that

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the removal of Cl(-I) in strongly acidic wastewater was significantly improved under UV

184

irradiation.

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To prevent secondary pollution, further treatment of the generated Cl2 and residual Cu(II)

186

in the solution is required. In this study, NaOH solution (1 g/L) was used to absorb Cl2. The

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residual Cu(II) in the solution was precipitated by introducing H2S gas, and after this

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procedure, the residual concentration of Cu(II) was lowered to 0.03 mg/L (Figure S5).

189

Actually, the reactions among Cu(II), Cu(0) and Cl(-I) in acidic solutions without UV

190

irradiation have been identified before 11, 12. At a high temperature (≧60°C), Cu(0) first reacts

191

with Cu(II), leading to the formation of Cu(I) in the solution (eq 1, Table 1). What follows is

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that Cu(I) precipitates fast with Cl(-I) in the form of CuCl (eq 2, Table 1).

193

However, in our experiments, several types of free radicals, including Cl•/Cl2-• and •O2-,

194

might be generated due to LMCT (eqs 3 and 4, Table 1), thereby probably inducing new

195

reactions 13, 19. Moreover, CuCl is a type of semiconductor able to produce h+ and •OH under

196

UV irradiation

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identified before exploring the formation mechanisms of CuCl and Cl2.

198

Identification of the intermediate free radicals

24

.

Therefore, the types and functions of intermediate free radicals were

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As mentioned above, Cl•/Cl2-•, •O2- and •OH radicals can be generated in the freshwater

200

and seawater containing Cl(-I), Cu(II) and CuCl under UV irradiation

13, 15, 17, 23, 25.

To

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distinguish the roles of Cl•/Cl2-•, •O2- and •OH in the Cl(-I) removal process, the experiments

202

were conducted after tert-butyl alcohol (tBuOH), nitrobenzene (NB) and p-benzoquinone

203

(BQ), which are excellent scavengers for Cl•/Cl2-•/•OH, •OH and •O2-, respectively 26, were

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added into the reaction system.

205

Figure 3 shows the Cl(-I) removal efficiency in the presence of different scavengers. The

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removal efficiency of Cl(-I) within 25 min was affected slightly in the presence of 1.5 mM

207

BQ, indicating O2-• was not the main free radical to participate in the reactions. In contrast,

208

the removal efficiency of Cl(-I) at 25 min decreased to 47.1% with the addition of 1.5 mM

209

NB, and 12.1% with the addition of 1.5 mM tBuOH, which indicates that the removal of

210

Cl(-I), namely, the generation of Cl2 and CuCl, was mainly initiated by Cl•/Cl2-• and •OH

211

radicals.

212

To make further identifications of the generated free radicals, ESR spectroscopy was

213

employed in our experiments. Figure 4a shows the spectrum of our reaction system

214

containing Cu(0), Cu(II), Cl(-I) and H2SO4. To further determine the detected radicals, we

215

compared the spectrum of our reaction system with that of the CHCl3/DMPO solution, which

216

is able to produce Cl• under UV irradiation 27, 28. The spectrum of Cl• is shown in Figure 4b. It

217

can be inferred that Cl• was indeed generated in our reaction system via comparing Figure 4a

218

with Figure 4b. In addition to Cl•, other free radicals were also detected as shown in Figure 4a,

219

which were speculated to be •OH and •O2-, according to the above results for masking

220

experiments. With the above analysis, it can be determined that Cl•, •O2- and •OH radicals

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were produced under UV irradiation and these radicals played important roles in the removal

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of Cl(-I).

223

In the Cu(II)/Cl(-I) mixed system, a series of reactions may occur with the formation of

224

several kinds of free radicals. Cl• can be produced by the LMCT of [CuIIClx]2-x (eq 3), which

225

is consistent with the ESR results for the Cu(II)/Cl(-I) solution (Figure S6). The oxidation of

226

Cl(-I) in aqueous solutions is known to produce Cl2-• as an intermediate 14, 25, 29. However, no

227

Cl2-• was detected in our experiments, probably resulting from the low concentration of Cl(-I).

228

In the solution, the molar quantities of Cu(0), SO42- and Cu(II) were all much higher than that

229

of Cl(-I), thus preventing the combination of Cl(-I) and Cl•.

230

Along with the LMCT of [CuIIClx]2-x, the re-oxidation of [CuIClx-1]2-x by O2 occurs

231

simultaneously, thereby resulting in the formation of O2-•, which reacts fast to produce H2O2

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in the acidic solution (eq 5) 13, 15, 17, 18. H2O2 will induce the transformation of Cu(I) to Cu(II)

233

(eqs 6 and 7), which affects the formation of Cl• from LMCT of [CuIIClx]2-x 15, 16. On account

234

of most [CuIClx-1]2-x being oxidized by O2, the transformation from Cu(I) to Cu(II) initiated by

235

H2O2 was not the main process, which is consistent with the results shown in Figure 3 that the

236

addition of the BQ quencher lowered the removal efficiency of Cl(-I) very slightly.

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It has been mentioned above that CuCl is an excellent semiconductor material, able to

238

produce the electron-hole pair 23, 30. Therefore, in our experiments, a small quantity of •OH

239

can be generated in the H2O2-initiated circulation of Cu(II) and Cu(I) (eq 6), and the most in

240

the photoreaction of CuCl under UV irradiation (eqs 8 and 9). Moreover, •OH and h+ are both

241

able to oxidize Cl(-I) to Cl• (eqs 10 and 11)

242

[CuIIClx]2-x, as well as in the photoreactions of CuCl.

31-33.

Thus, Cl• can form in the LMCT of

13



243

To assess the roles that Cl• and •OH play in producing Cl2 and CuCl, experiments were

244

conducted to determine the yields of Cl2 and CuCl after adding different scavengers into the

245

reaction system. The yield of CuCl was not clearly affected by the addition of NB, but

246

declined fast in the presence of tBuOH (Figure 5a), which indicates that Cl• was the main

247

radical to initiate the formation of CuCl. However, due to the comproportionation between

248

Cu(0) and Cu(II), the formation of CuCl did not totally disappear after NB was added to the

249

solution. Figure 5b shows that the yield of Cl2 after 25 min of reaction declined from 2.3

250

mmol to 0.66 mmol in the presence of NB, and nearly 0 mmol in the presence of tBuOH,

251

indicating that the formation of Cl2 was related to both •OH and Cl•. It has been mentioned

252

above that the function of •OH in our reaction system was oxidizing Cl(-I) to Cl•. Therefore,

253

it could be inferred that the formation of Cl2 was directly related to Cl•.

254

Generation mechanisms of CuCl and Cl2 under UV irradiation

255

The above results from ESR spectroscopy identification experiments and masking

256

experiments show that the formation of CuCl was related to Cl•. We speculated that Cu(0)

257

can react with Cl•, thereby producing CuCl precipitates under UV irradiation. Previous

258

studies have shown that CHCl3 solvent can produce Cl• under UV irradiation 27, 28. To verify

259

our speculation, copper powder was added to CHCl3 solvent and then the mixture was stirred

260

under UV irradiation. The precipitates after reactions were found to consist of Cu(0) and

261

CuCl as tested by means of XRD (Figure S7), which signifies that CuCl can be generated

262

directly from Cu(0) and Cl• (eq 12). In our reaction system, due to the high dosage of Cu(0)

263

as compared to the low concentration of Cl(-I), the reaction between Cl• and Cu(0) first took

264

place, other than the reaction between Cl• and Cl(-I).

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It has been reported before that Cl2 can form directly from Cl(-I) through oxidation by h+ 26,

266

31, 33.

However, it was determined in our experiments that the formation of Cl2 was related to

267

Cl• (Figure 5b). Therefore, considering the large quantity of Cl• in the solution, we draw a

268

conclusion that Cl2 formed directly from Cl• in our experiments (eq 13) 34, 35. Accompanied

269

with the oxidation of Cl(-I) by h+/•OH, electrons reacted with Cu(II) 13, 17, and as a result, Cu(I)

270

was generated and quickly disappeared due to the disproportionation (eqs 14 and 15) 16, 31, 36, 37.

271

Therefore, the final result was that Cl(-I) was oxidized to Cl2 by •OH and h+, and Cu(II) was

272

reduced to Cu(0) by e-.

273

Proposed mechanisms

274

The mechanisms of the UV-improved removal of Cl(-I) by Cu(0) and Cu(II) are proposed

275

as follows (Figure 6). First, the LMCT of [CuIIClx]2-x occurs under UV irradiation near 254

276

nm to generate H2O2 and Cl•. H2O2 in the solution initiates the circulation of Cu(II) and Cu(I),

277

and thereby constantly supplies [CuIIClx]2-x in the reaction system. Then, Cl• radicals

278

efficiently react with Cu(0), thus leading to the formation of CuCl(s), which is one pathway to

279

remove Cl(-I) in the solution. Subsequently, Cl(-I) reacts with •OH or h+ from the photolytic

280

process of CuCl to generate the Cl• radicals. Then, the Cl• radicals, generated from the LMCT

281

of [CuIIClx]2-x and the photoreaction of CuCl, unite fast to produce Cl2 gas, which is another

282

way to remove Cl(-I) in the solution. Along with the oxidation of Cl(-I) by h+/•OH, Cu(II)

283

reacts with the e- released by CuCl to produce Cu(I), which is not stable and decomposes

284

rapidly to Cu(0) and Cu(II).

285

Environmental implications

286

The precipitation of CuCl is a critical process to remove Cl(-I) using Cu(0) and Cu(II) in

15



287

acidic wastewater. And the participation of UV irradiation (254 nm) could obviously improve

288

the removal efficiency of Cl(-I), as well as reducing the dosage of Cu(II) by forming CuCl

289

and Cl2. To verify the practical application of the UV-improved method, experiments were

290

conducted in the real wastewater. After 40 min of irradiation, 87.9% of Cl(-I) was removed,

291

from 428.8 mg/L to 51.7 mg/L (Figure S8). In the process of removing Cl-, Cl2 was absorbed

292

by NaOH solution (1 g/L), and the residual Cu2+ was lowered to 0.03 mg/L by introducing

293

H2S gas. The generated NaCl/NaOCl bleaching agent and CuS solid were both byproducts of

294

high purity. Considering the high efficiency, low dosage of Cu(II) and short reaction time of

295

this method, it could become a potential treatment technology for the removal of Cl(-I) from

296

strongly acidic wastewater. The mechanisms proposed in this study offered a theoretical

297

foundation for the development and application of UV-improved Cl(-I) removal technology.

298

Acknowledgments

299

This work was supported by the National Natural Science Foundation of China (41473113,

300

21707153) and the Key Research Program of the Chinese Academy of Sciences

301

(ZDRW-ZS-2016-5). We are grateful to the editors and anonymous reviewers for their

302

valuable comments and suggestions for our paper.

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ASSOCIATED CONTENT

304

Supporting Information

305

The major chemical compositions of the real wastewater, the detailed description of the

306

reactor, the spectral distribution of the mercury lamp, the XPS and XRD analysis of the

307

precipitates, the ESR analysis of Cl• radical, and other additional data or descriptions are

308

presented in the Supporting Information.

16


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310 311 312


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Page 22 of 31

403

List of Figure Captions

404

Figure 1. − Treatment using Cu(0) and Cu(II) under UV irradiation at room temperature in

405

simulated acidic wastewater. (a) The removal efficiency of Cl(-I) as a function of

406

time under different ratios of Cu(0)/Cu(II). (b) The removal efficiency of Cl(-I) as a

407

function of time under different ratios of Cut/Cl(-I). Conditions: (a) V=400 mL,

408

[Cl-]=354.7 mg/L, [H2SO4]=50 g/L, n(Cut)/n(Cl-)=2:1, and n(Cu0)/n(Cu2+)=1-5:1; (b)

409

V=400 mL, [Cl-]=354.7 mg/L, [H2SO4]=50 g/L, n(Cu0)/n(Cu2+)=1:1, and

410

n(Cut)/n(Cl-)=1-4:1. The error bars show the standard deviation (n=4).

411

Figure 2. − Treatment using Cu(0) and Cu(II) under the condition of heating and UV

412

irradiation in simulated acidic wastewater. (a) The removal efficiency of Cl(-I) as a

413

function of time. (b) The removal efficiency of Cl(-I) as a function of the

414

Cu(II)/Cl(-I) ratio after 25 min of reaction under UV irradiation at room

415

temperature or 60 min of reaction without UV irradiation at 70°C. (c) Pictures of

416

the generated precipitates and the solutions after 25 min of reaction under UV

417

irradiation at room temperature or 60 min of reaction without UV irradiation at

418

70°C. Conditions: (a, c) V=400 mL, n(Cu0)/n(Cu2+)/n(Cl-)=2:2:1, [Cl-]=342.2 mg/L,

419

and [H2SO4]=50 g/L; (b) V=400 mL, n(Cu0)/n(Cl-)=2:1, n(Cu2+)/n(Cl-)=1-10:1,

420

[Cl-]=342.2 mg/L, and [H2SO4]=50 g/L. The error bars show the standard deviation

421

(n=3).

422

Figure 3. − Removal efficiency of Cl(-I) as a function of time under UV irradiation in the

423

presence

of

different

scavengers.

Conditions:

424

[Cl-]=343.7 mg/L, [H2SO4]=50 g/L, [NB]=1.5 mM, [BQ]=1.5 mM, [tBuOH]=1.5

22


n(Cu0)/n(Cu2+)/n(Cl-)=2:2:1,

Page 23 of 31


425

mM, V=400 mL, and room temp. The error bars show the standard deviation (n=3).

426

Figure 4. − ESR spectra of the reaction solution containing Cu(0), Cu(II), Cl(-I) and H2SO4.

427

(a) The ESR spectrum of the reaction system obtained by DMPO under UV

428

irradiation. (b) The ESR spectrum of the Cl• obtained by DMPO in CHCl3 under UV

429

irradiation.

430

[H2SO4]=50 g/L, [DMPO]=0.5 mM, and t=2 min. (b) [DMPO]=0.5 mM, and t=2

431

min.

432

Conditions:

(a)

n(Cu0)/n(Cu2+)/n(Cl-)=2:2:1,

[Cl-]=343.7

mg/L,

Figure 5. − Yields of products in the presence of different scavengers under UV irradiation. (a)

433

The yields of CuCl product in the presence of different scavengers under UV

434

irradiation. (b) The yields of Cl2 product in the presence of different scavengers

435

under UV irradiation. Conditions: n(Cu0)/n(Cu2+)/n(Cl-)=2:2:1, [Cl-]=343.7 mg/L,

436

[H2SO4]=50 g/L, [NB]=1.5 mM, [tBuOH]=1.5 mM, V=400 mL, and room temp.

437

The error bars show the standard deviation (n=3).

438 439

Figure 6. − Possible mechanisms of the UV-improved removal of Cl(-I) using Cu(0) and Cu(II) in strongly acidic wastewater.

440 441

List of Table Captions

442

Table 1. − Reactions in the Cu(0)-Cu(II)-Cl(-I) Solution in the Dark and under UV Irradiation

23



Figure 1. Treatment using Cu(0) and Cu(II) under UV irradiation at room temperature in

simulated acidic wastewater. (a) The removal efficiency of Cl(-I) as a function of time under different ratios of Cu(0)/Cu(II). (b) The removal efficiency of Cl(-I) as a function of time under different ratios of Cut/Cl(-I).

24


Page 24 of 31

Page 25 of 31


Figure 2. Treatment using Cu(0) and Cu(II) under the condition of heating and UV irradiation in simulated acidic wastewater. (a) The removal efficiency of Cl(-I) as a function of time. (b) The removal efficiency of Cl(-I) as a function of the Cu(II)/Cl(-I) ratio after 25 min of reaction under UV irradiation at room temperature or 60 min of reaction without UV irradiation at 70°C. (c) Pictures of the generated precipitates and the solutions after 25 min of reaction under UV irradiation at room temperature or 60 min of reaction without UV irradiation at 70°C.

25



Figure 3. Removal efficiency of Cl(-I) as a function of time under UV irradiation in the presence of different scavengers.

26


Page 26 of 31

Page 27 of 31


Figure 4. ESR spectra of the reaction solution containing Cu(0), Cu(II), Cl(-I) and H2SO4. (a) The ESR spectrum of the reaction system obtained by DMPO under UV irradiation. (b) The ESR spectrum of the Cl• obtained by DMPO in CHCl3 under UV irradiation.

27



Figure 5. Yields of products in the presence of different scavengers under UV irradiation. (a) The yields of CuCl product in the presence of different scavengers under UV irradiation. (b) The yields of Cl2 product in the presence of different scavengers under UV irradiation.

28


Page 28 of 31

Page 29 of 31


Figure 6. Possible mechanisms of the UV-improved removal of Cl(-I) using Cu(0) and Cu(II) in strongly acidic wastewater.

29



Page 30 of 31

Table 1. Reactions in the Cu(0)-Cu(II)-Cl(-I) Solution in the Dark and under UV Irradiation Eqs

Reaction

Reference

Formation of CuCl without UV irradiation 1

Cu  Cu 2   2Cu 

11, 12

2

Cu   Cl   CuCl 

11, 12

LMCT of [Cu II Cl x] 2-x under UV irradiation 3

[Cu IICl x ]2 - x  [Cu ICl x -1 ]2 - x + Cl 

4

Cl  + Cl-  Cl 2 

13-15

5

2Cl- + 2H + + 2[Cu ICl x -1 ]2 - x + O 2  2[Cu IICl x ]2 - x + H 2O 2

15, 17

13, 15

-

Circulation of copper under UV irradiation 6

Cu + + H 2O 2  Cu 2 + + OH - + OH

13, 15

7

Cu   OH  Cu 2   OH 

13, 15

Photoreaction of CuCl under UV irradiation 8

hv CuCl  h  e 

19

9

h   H 2O(ads)  H   OH

17, 31

Oxidation of Cl10

h   Cl   Cl 

26, 32

11

 OH  Cl   Cl 

32

Formation of CuCl under UV irradiation 12

Cu  Cl  CuCl

This study

Formation of Cl2 under UV irradiation 13

Cl   Cl  Cl 2

36, 37

30


Page 31 of 31


Reduction of Cu(II) under UV irradiation 14

Cu 2   e   Cu 

13, 17

15

2Cu   Cu 2   Cu 

16

31


Removal of chloride ions from strongly acidic wastewater using Cu(0

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