Green Route for Hydrogen Evolution from Real Electroplating Waste

Research Article pubs.acs.org/journal/ascecg

Green Route for Hydrogen Evolution from Real Electroplating Waste Liquids Induced by a Solar Light Responsive Photocatalyst En-Chin Su† †

Department of Environmental Engineering, National Chung Hsing University, 145 Xingda Road, South Dist., Taichung 40227, Taiwan, R.O.C.

Bing-Shun Huang‡ Downloaded via UNIV OF TEXAS MEDICAL BRANCH on June 26, 2018 at 21:04:04 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

Taiwan Research Institute, 29F., No. 27, Sec. 2, Zhongzheng E. Road, Tamsui Dist., New Taipei City 251, Taiwan, R.O.C.

Ming-Yen Wey*,§ §

Department of Environmental Engineering, National Chung Hsing University, 145 Xingda Road, South Dist., Taichung 40227, Taiwan, R.O.C. ABSTRACT: Real electroplating wastewater/liquid containing disodium ethylenediaminetetraacetate (Na2EDTA) and nickel (Ni), a platinum/nitrogen-doped TiO2/strontium titanateTiO2 tube (a solar light-responsive material), and simulated sunlight were used as the photogenerated hole scavenger, photocatalyst, and light source, respectively, in the development and evaluation of a waste-to-energy system. Effects of the wastewater/liquid source, pH, EDTA concentration, EDTA/Ni molar ratio, EDTA/photocatalyst weight ratio, and the degree of Ni removal on the photocatalytic hydrogen evolution efficiency are discussed. Our results indicate that the hydrogen evolution efficiency is affected by these factors in the following order: EDTA/Ni molar ratio > EDTA/photocatalyst weight ratio > pH value. When the initial EDTA/Ni molar ratio and EDTA/photocatalyst weight ratios were 30 and 0.7, respectively, economically optimal hydrogen evolution efficiency (480.0 μmol/h/g) was achieved; when the waste liquid was further treated for Ni removal, improved and stable hydrogen evolution was achieved. These results indicate that our hydrogen evolution system increases the recycling value of the electroplating wastewater/liquids and reduces the pollution caused by their high heavy-metal concentrations and the species that increase their chemical oxygen demand. KEYWORDS: Waste-to-energy, Electroplating wastewater, EDTA, Photocatalytic hydrogen evolution, Pollution reduction

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INTRODUCTION Electroplating is a common process for the deposition of films on objects (e.g., water taps, decoration, and wafer circuits) to increase their resistance to oxidation.1 The electroplating process of the specific factories in Taiwan consists of three steps: degreasing, acid washing, and electroplating; however, the actual process is typically adjusted depending on the individual requirements. In the specific process in Taiwan, heavy metals (e.g., Ni, Zn, Cd, and Cu) are applied as the components of thin films2,3 and the disodium salt of ethylenediaminetetraacetic acid (Na2EDTA), with good water solubility, is used as a buffer for the control of the metal ion concentration. Large amounts of water are consumed for washing the electroplated objects, a necessary process to avoid cross-contamination between the electroplating tanks. Therefore, an enormous amount of wastewater/liquid is produced from electroplating processes, and the wastewater/liquid © 2017 American Chemical Society

usually contains a high concentration of EDTA, oil, suspended solids, unchelated metal ions, and metal−EDTA chelates, resulting in an increase in the chemical oxygen demand (COD) of the solutions, in addition to high heavy metal concentrations. This significantly increases the cost and complexity of wastewater/liquid treatments before discharge, as untreated waste liquids have a negative impact on the environment.4,5 Photocatalysts with reduction potentials below 0 eV (NHE, pH = 0) and oxidation potentials greater than 1.23 eV (NHE, pH = 0) (e.g., TiO2, CdS, and SrTiO3) exhibit excellent photocatalytic hydrogen evolution activities.6−9 As shown in eqs 1 and 2, the transition of a photogenerated electron from the valence band (VB) to the conduction band (CB) of the Received: September 18, 2016 Revised: January 6, 2017 Published: January 26, 2017 2146

DOI: 10.1021/acssuschemeng.6b01999 ACS Sustainable Chem. Eng. 2017, 5, 2146−2153

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic representation of the mechanism for photocatalytic hydrogen evolution from real electroplating wastewater/liquid photocatalytic decomposition over a solar light-responsive photocatalyst.

Figure 2. Electroplating process flowchart and sampling locations of the waste liquid and wastewater.

photocatalyst is induced under suitable light irradiation. The photogenerated holes (h+) and electrons (e−) then further migrate to the surface of the photocatalyst, where the photogenerated holes participate in the oxidation of H2O, resulting in the formation of H+ and hydroxyl radicals (·OH) (eq 3).10 The photogenerated electrons participate in the reduction of H+ ions, resulting in the evolution of hydrogen (eq 4).11 In addition, the formation of ·OH radicals facilitates the simultaneous oxidation of organic molecules during the photocatalytic hydrogen evolution process (eq 5).12,13 However, owing to the rapid photogenerated charge recombination rates, charge separation in the photocatalyst is difficult, resulting in inhibition of the photocatalytic hydrogen evolution activity.14−16 It has been reported that the addition of an appropriate amount of an organic compound (e.g., EDTA, methanol, ethanol, or glycerol) as a hole scavenger (electron donor) delays the charge recombination effectively, which is a prerequisite for enhanced hydrogen evolution.17,18 hυ > energy gap (Eg )

photocatalyst → photocatalyst (e− + h+)

(2)

2h+ + 2H 2O → 2·OH + 2H+

(3)

2e− + 2H+ → H 2

(4)

·OH + organic molecule → CO2 + H 2

(5)

On the basis of the electroplating wastewater/liquid composition, the redox ability of the photocatalyst, and the prerequisite for photogenerated charge separation, we envisioned that using EDTA from the electroplating wastewater/ liquid as a hole scavenger in a photocatalytic hydrogen evolution system would present a 3-fold advantage. First, it would satisfy the requirements for photocatalytic hydrogen evolution. Second, EDTA could be oxidized during the photocatalytic reaction, which would reduce the COD of the wastewater/liquid. Third, the metal may be adsorbed on the surface of the photocatalyst, which could reduce the cost and complexity of wastewater/liquid treatments before discharge. However, the feasibility of photocatalytic hydrogen evolution

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between the metal and EDTA and form NiS precipitates. Subsequently, suspended brown NiS particles were obtained after 20 h of stirring. The waste liquid from source 7 was then filtered again through a 0.2 μm membrane to remove the NiS precipitate, and a transparent liquid was obtained. The pH value of the treated waste liquid was finally adjusted from pH 10.5 to 7 with HCl before the photocatalytic reaction. Wastewater Composition Analysis. The total EDTA sodium salt and Ni ion concentrations in wastewater/liquid and treated waste liquid were determined before and after the photocatalytic reaction and before and after Ni ion removal. The ASTM D3113-92 standard method was used to determine the total EDTA sodium salt concentration in the wastewater/liquid. The Ni ion concentration was determined photometrically using a water quality analyzer (Suntex, PhotoLab S6).

from real electroplating wastewater/liquid has not been explored so far. Moreover, the influence of the initial properties of wastewater/liquid on the photocatalytic hydrogen production efficiency has not been investigated. To develop a sustainable alternative energy production technology with practical industrial applications, we designed and built a practical waste-to-hydrogen system using recycled electroplating wastewater/liquid as the hole scavenger, a platinum/nitrogen-doped TiO2/strontium titanate-TiO2 tube (Pt/NT/SrTiO3-TiO2 tube) responsive to the solar spectrum19 (developed by us in a previous study) as the photocatalyst, and simulated sunlight as the light source. As shown in Figure 1, the feasibility of photocatalytic hydrogen evolution from real electroplating wastewater/liquid was evaluated. In addition, an effective metal removal method (established in our previous study19) was applied to remove Ni from the wastewater/liquid, and the influence of the wastewater/liquid source, initial reaction pH, initial EDTA concentration, initial EDTA/Ni molar ratio, initial waste liquid concentration, and the degree of Ni removal on the efficiency and stability of the photocatalytic hydrogen evolution reaction are discussed.

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RESULTS AND DISCUSSION Influence of Wastewater/Liquid Source on the Hydrogen Evolution. The hydrogen evolution efficiencies are shown in Figure 3, and these revealed that the Pt/NT/SrTiO3-TiO2

MATERIALS AND METHODS

Electroplating Wastewater/Liquid Sampling. The samples of electroplating wastewater/liquid were discharged from an electroplating factory located in Changhua, a city in southern Taiwan. As shown in Figure 2, the electroplating process consists of degreasing, acid washing, electroplating, and an electroless plating step. First, the raw material is immersed in a solution of sodium hydroxide (NaOH), toluene, and EDTA for surface degreasing. Subsequently, the treated material is transferred into a diluted acid solution to remove the impurities and obtain a smooth surface. Next, the material with now a smooth surface is immersed in a tank containing an electroplating solution consisting of nickel sulfate (NiSO4), nickel chloride (NiCl2), ammonium hydroxide (NH4OH), boric acid, and EDTA, and Ni ions are gradually deposited on the surface of the material under the electroplating potential. Finally, the material with a Ni layer is immersed in a tank containing a solution of NiSO4, sodium hypophosphite (NaH2PO2·H2O) and EDTA, resulting in the gradual deposition of Ni ions on the surface of the material to form a second Ni layer during an electroless plating process. Between steps, the material is washed with water several times. The waste liquid (source 1, source 3, source 5, and source 7) and the wastewater (source 2, source 4, source 6, and source 8) were sampled and tested. Experimental Procedure. The solar light-responsive Pt/NT/ SrTiO3-TiO2 tube photocatalyst developed in our previous study and 50 mL of the wastewater/liquid were added to a cylindrical reactor. The light source (a simulated sunlight lamp, 420 W/m2, XHA500) was fixed 10 cm above the reactor. The reaction temperature (20 °C) was controlled by a circulating bath. The suspension was deaerated with nitrogen for 30 min in the dark prior to photocatalytic H2 production. The produced gases were autosampled every 10 min to an online gas chromatograph (GC-TCD, Clarus 500 PerkinElmer, Carboxen 1000 column, N2 carrier) to determine the hydrogen concentration. Ni Removal Process. The Ni ion removal process developed in our previous study,19 based on Ni ion precipitation, was used to reduce the interference of the metal on the hydrogen evolution reaction. The pH value of the waste liquid from source 7 with an initial EDTA concentration of 1404.9 mg/L was adjusted from 4.7 to 10.5 with NaOH to precipitate unchelated Ni ions as Ni(OH)2. After 3 h of stirring, the turbidity of the waste liquid from source 7 increased. This liquid was filtered through a 0.2 μm membrane filter to separate the Ni(OH)2 precipitates, after which a transparent waste liquid was obtained. The number of moles of chelated Ni ions was calculated based on the initial addition of EDTA and the Ni ion valence (2+). An equimolar quantity of sodium sulfide (Na2S, 98%, Acros Organics) was added to the filtered waste liquid from source 7 to break the bonds

Figure 3. Influence of the wastewater/liquid source on the hydrogen evolution.

tube exhibited no photocatalytic hydrogen evolution activity when used with wastewater/liquids from sources 1−4; however, hydrogen evolution was observed with wastewater/ liquids from sources 5−8. These differences in activity are attributed to the negligible EDTA concentration in the degreasing treatment process and the absence of EDTA in the acid washing process. Because of the lack of EDTA in wastewater/liquids from sources 1−4, photogenerated electrons and holes in the Pt/NT/SrTiO3-TiO2 tube cannot separated effectively, resulting in charge recombination and no hydrogen evolution. However, notably, the hydrogen evolution efficiencies gradually decreased after 40 min, possibly because EDTA was gradually oxidized by ·OH and ·O2−, finally decomposing into NH3, CO2, and H2 during the hole-trapping process. This would result in a reduction in the hydrogen evolution efficiency with time.20−23 Influence of the Initial Reaction pH on the Hydrogen Evolution. On the basis of the results from the previous section, we focused on studying the influence of the intrinsic properties of wastewater/liquids from sources 5−8 on the hydrogen evolution efficiency. The hydrogen evolution rate after 30 min was chosen as a representative value to measure the performance of the reaction. As reported in previous 2148


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ACS Sustainable Chemistry & Engineering studies,24,25 the surface charge of the catalyst becomes positive when the reaction pH value is lower than the zeta potential (pHZPC) of the catalyst, which improves the adsorption of anionic molecules. However, because the positive charge density of the catalyst surface increases with the decreasing reaction pH value, anionic molecule adsorption on the catalyst decreases with the decreasing pH due to the competing reactions with the H+ ions at the catalyst surface.26 Thus, the catalyst exhibits optimal adsorption of anionic molecules when the reaction pH is slightly lower than the pHZPC of the catalyst.27,28 As shown in Figure 4, the initial pH values of

These results indicate that EDTA is oxidized during the holetrapping process, which supports the premises described in the previous section. Moreover, the initial EDTA concentration was found to follow the order: source 7 > source 5 > source 6 > source 8, suggesting that the photocatalytic hydrogen evolution activity of the Pt/NT/SrTiO3-TiO2 tube should increase in that order. However, the trend for the initial EDTA concentration does not reflect that of the hydrogen evolution efficiency. Consequently, the EDTA concentration is not the critical factor affecting this hydrogen evolution system, and it is possible that the photocatalytic activity of the Pt/NT/SrTiO3-TiO2 tube suffers from interference by the Ni ions. To identify the source of hydrogen in this photocatalytic reaction, we calculated the variation in EDTA concentration before and after the reaction and the theoretical number of moles of hydrogen produced by EDTA oxidation. Taking the initial and residual EDTA concentration of the waste liquid from source 7 as an example, the EDTA concentration difference was 64.4 mg/L, meaning that 9.52 × 10−6 mol Na2EDTA were consumed in the 50 mL reaction volume during the photocatalytic reaction. Theoretically, seven moles of hydrogen can be generated through the oxidation of one mole of Na2EDTA (Na2C10H14O8N2). Thus, 66.6 μmol of hydrogen can be generated through the oxidation of 9.52 × 10−6 mol Na2EDTA. The hydrogen provided by Na2EDTA oxidation is far below the hydrogen evolution rate of 394.0 μmol/h/g. This result indicates that the EDTA in the wastewater/liquid acts as a hole scavenger, and the majority of hydrogen is generated by photocatalytic water splitting. Influence of the Initial EDTA/Ni Molar Ratio on the Hydrogen Evolution. In the photocatalytic reaction system, metal ions might interfere in the adsorption reaction between the photocatalyst and the reactant by occupying the active sites of the photocatalyst surface, resulting in poor photocatalytic reduction or oxidation efficiency. To clarify the critical factor(s) for the photocatalytic activity, we measured the Ni concentration before and after the reaction and calculated the initial EDTA/Ni molar ratio before the reaction. First, it was observed that the Ni concentration was lower after the photocatalytic reaction in all cases, as shown in Table 2. This result indicates

Figure 4. Influence of the initial reaction pH value on the hydrogen evolution.

wastewater/liquids from sources 5−8 were 0.58, 2.68, 4.35, and 6.18, respectively. According to the pHZPC of the Pt/NT/ SrTiO3-TiO2 tube (pHZPC = 6.4)19 and the arguments mentioned above, the theoretical photocatalytic hydrogen evolution activity of the Pt/NT/SrTiO3-TiO2 tube should increase in the following order: source 5 < source 6 < source 7 < source 8. However, Figure 4 shows how the photocatalytic hydrogen evolution activity of the Pt/NT/SrTiO3-TiO2 tube increases in the order: source 5 < source 6 < source 8 < source 7, which does not correspond to the predicted order. Consequently, there is a critical factor influencing the reaction efficiency other than the pH value of the reaction medium. Influence of the Initial EDTA Concentration on the Hydrogen Evolution. In the right concentration range, the charge separation efficiency of the photocatalyst is proportional to the electron donor concentration,29,30 and so the electron donor concentration is one of the key indexes that can be used to evaluate the efficiency of charge separation and hydrogen evolution of a catalyst. The EDTA concentration before and after the reaction were measured by the ASTM D3113-92 standard method. As shown in Table 1, the EDTA concentration decreased after the photocatalytic reaction.

Table 2. Initial EDTA/Ni Molar Ratio of the Wastewater/ Liquid from Sources 5−8 Ni concentration (mg/L) wastewater/ liquid source source source source source

EDTA concentration (mg/L) source source source source

5 6 7 8

before reaction

after reaction

705.2 44.7 22,478.8 31.5

318.6 4.9 22,414.4 1.0

after reaction

weight of adsorbed Ni in 50 mL reaction volume (g)

initial EDTA/Ni molar ratio (before reaction)

5746.7 62.3 129.1 5.5

3056.7 32.8 93.7 0.8

0.1345 0.0015 0.0018 0.0002

2.1 × 10−2 1.3 × 10−1 30.0 1.0

that the photocatalyst acts as a metal ion or metal−EDTA chelate adsorbent during the photocatalytic reaction.31,32 In addition, the data in Table 2 reveal that the adsorbed Ni content in the 50 mL reaction volume after the reaction increased with the increasing initial Ni concentration. This arises because the collision probability between Ni ions or Ni− EDTA chelates and the Pt/NT/SrTiO3-TiO2 tube increases with the increasing initial Ni concentration, resulting in increased adsorption. Moreover, as shown in Figure 5, the initial EDTA/Ni molar ratio of the wastewater/liquid from

Table 1. Initial EDTA Concentration of the Wastewater/ Liquid from Sources 5−8

wastewater/liquid source

5 6 7 8

before reaction

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Table 3. Conditions of Waste Liquid from Source 7 at Different Dilutions initial EDTA concentration (mg/L)

initial Ni concentration (mg/L)

pH value

22,478.8 11,239.4 5619.7 2809.9 1404.9

129.1 64.5 32.3 16.13 8.1

4.4 4.4 4.5 4.6 4.7

Figure 5. Influence of the initial EDTA/Ni molar ratio on the hydrogen evolution.

sources 5−8 was 2.1 × 10−2, 1.3 × 10−1, 30.0, and 1.0, respectively, and these ratios are proportional to the trend of the hydrogen evolution efficiency. At higher initial EDTA/Ni molar ratios, the Ni interference is correspondingly lower; consequently, there are more unchelated EDTA molecules to trap directly the photoexcited holes of the Pt/NT/SrTiO3-TiO2 tube, resulting in improved H+ ion and H2O molecule adsorption on the active sites of the Pt/NT/SrTiO3-TiO2 tube, enhanced charge separation, and hydrogen evolution. Based on the hydrogen evolution rate, the initial EDTA/Ni molar ratio, and the residual Ni concentration, we deduce that, owing to occupation of the active sites of the Pt/NT/SrTiO3TiO2 tube by Ni ions or Ni−EDTA chelates, the probability of H+ ion and H2O molecule adsorption and hydrogen evolution is reduced. In addition, not only the initial Ni concentration but also the initial EDTA/Ni molar ratio must be considered when evaluating the recycling value of the electroplating wastewater/ liquid. Influence of the Initial Concentration of the Waste Liquid from Source 7 on the Hydrogen Evolution. On the basis of the results of the previous section, we found that the waste liquid from source 7 contained the most appropriate initial Ni concentration and initial EDTA/Ni molar ratio for photocatalytic hydrogen evolution. However, it has been reported that an excess of hole scavengers can adsorb excessively on the surface of the photocatalyst,33 resulting in hindered H+ ion and/or H2O adsorption and photocatalytic activity inhibition.34 Thus, the influence of the initial concentration of the waste liquid from source 7 on the hydrogen evolution efficiency was evaluated. The initial EDTA and Ni concentration of the waste liquid from source 7 was proportionally adjusted by diluting with deionized water. The initial EDTA and Ni concentrations and the pH values are shown in Table 3. The initial pH value was in the range 4.4− 4.7. The effect of the reaction pH was not considered when discussing the hydrogen evolution efficiency due to the narrow pH range. Figure 6 shows that the Pt/NT/SrTiO3-TiO2 tube exhibited better hydrogen evolution activity when the initial EDTA and Ni concentrations were below 22 478.8 and 129.1 mg/L, respectively. The optimal hydrogen evolution efficiency was obtained when the initial EDTA and Ni concentrations were 11 239.4 and 64.5 mg/L, respectively, and the hydrogen evolution rate after 1 h of reaction (557.0 μmol/h/g) was about 1.4 times higher than that with the initial EDTA and Ni

Figure 6. Influence of the initial EDTA concentration of the waste liquid from source 7 on the hydrogen evolution.

concentrations. When the initial EDTA and Ni concentration were reduced, the H+ ion and H2O molecule adsorption increased, resulting in increased hydrogen evolution efficiency. Moreover, the turbidity and chromaticity of the waste liquid from source 7 also decreased when the initial EDTA and Ni concentrations were suitably reduced, leading to decreased screening effects and increased light penetration and generation of photoexcited electron−hole pairs.35,36 However, it was observed that the photocatalytic activity of the Pt/NT/SrTiO3TiO2 tube was slightly reduced with the reduction in the initial EDTA and Ni concentration at values below 11 239.4 and 64.5 mg/L, respectively. This was ascribed to reduced charge separation. Because of the decreased initial EDTA concentration, the hole-trapping probability decreased, resulting in charge recombination and the inhibition of hydrogen evolution. However, when the initial EDTA and Ni concentrations were 1404.9 and 8.1 mg/L, respectively, the Pt/NT/SrTiO3-TiO2 tube still exhibited 86% (480.0 μmol/h/g) of the hydrogen evolution efficiency with initial EDTA and Ni concentrations of 11 239.4 and 64.5 mg/L, respectively. Influence of Ni Ion Removal on the Hydrogen Evolution. Based on the results discussed above, Ni ions or Ni−EDTA chelates were found to interfere with the hydrogen evolution reaction. To reduce the Ni ion or Ni−EDTA interference on the photocatalytic activity of the Pt/NT/ SrTiO3-TiO2 tube, we attempted to remove unchelated Ni ions and break the bonds between Ni and EDTA using the method mentioned in the “Ni removal process” section. The waste liquid from source 7 with initial EDTA and Ni concentrations of 1409.9 and 8.1 mg/L, respectively, was chosen for the test. Figure 7a shows that the untreated waste liquid was originally transparent and pale green; the waste liquid treated with NaOH 2150


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Figure 7. Illustration of Ni removal from the waste liquid from source 7 with an initial Ni concentration of 8.1 mg/L. (a) Untreated waste liquid, (b) waste liquid treated with NaOH after the first filtration, and (c) waste liquid treated with Na2S after the second filtration.

after the first filtration was colorless (as shown in Figure 7b), and the apple green precipitate was attributed to Ni(OH)2. As shown in Figure 7c, pale brown NiS precipitates were formed, and the waste liquid remained transparent after further treatment with Na2S and a second filtration. As shown in Table 4, about 81.5% of the Ni was removed, indicating that Ni

This result indicates that the lower probability of active sites being occupied by Ni ions or Ni−EDTA chelates prolongs the activity of the Pt/NT/SrTiO3-TiO2 tube effectively, thus resulting in an overall improved photocatalytic hydrogen evolution stability.

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CONCLUSIONS A novel green and sustainable route for alternative energy production was established in this study. We found that EDTA from the specific electroplating wastewater/liquid can be recycled as a photogenerated hole scavenger facilitating the photocatalytic water splitting reaction and leading to excellent hydrogen evolution efficiency. According to the residual EDTA and Ni concentrations, our design of the hydrogen evolution system was found not only to increase the recycling value of the electroplating wastewater/ liquid but also decrease the COD and high levels of heavy metals, resulting in an excellent method to maintain the balance of the ecosystem and decrease the complexity of wastewater/ liquid treatment processes before water discharge. Our results indicate that the initial pH value, initial EDTA/ Ni molar ratio, and initial EDTA/photocatalyst weight ratio are all important parameters to assess the recycling value of electroplating wastewater/liquids, and the hydrogen evolution efficiency is affected by these factors in the following order: the initial EDTA/Ni molar ratio > the initial EDTA/photocatalyst weight ratio > pH value. When the initial EDTA/Ni molar ratio and initial EDTA/photocatalyst weight ratio were 30 and 0.7, respectively, an economically optimal hydrogen evolution efficiency of 480.0 μmol/h/g after 60 min was obtained. When the waste liquid with initial EDTA and Ni concentrations of 1409.9 and 8.1 mg/L, respectively, was further treated for Ni removal, improved and more stable hydrogen evolution performance was achieved.

Table 4. Ni Concentration before and after Ni Removal before removal (mg/L)

after removal (mg/L)

8.1

1.5

could be effectively removed by this process. As shown in Figure 8, in the first hour of the reaction, the Pt/NT/SrTiO3-

Figure 8. Influence of Ni removal from the waste liquid from source 7 on the hydrogen evolution. Initial EDTA and Ni concentrations before Ni removal: 1404.9 and 8.1 mg/L, respectively.

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TiO2 tube displayed a slightly worse hydrogen evolution performance in the treated waste liquid. According to our previous studies,19 some EDTA is lost during the metal removal process. The poorer hydrogen evolution performance in the first hour of the reaction was ascribed to the relatively lower collision probability between EDTA and the Pt/NT/SrTiO3TiO2 tube. However, it was found that the Pt/NT/SrTiO3TiO2 tube exhibited a better and more stable hydrogen evolution performance with the treated waste liquid after 1 h.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.-Y. Wey). Tel.: +886-4-22840441, ext. 533. Fax: +886-4-22862587. ORCID

Ming-Yen Wey: 0000-0003-3035-2683 Notes

The authors declare no competing financial interest. 2151


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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST), Taiwan, R.O.C., for providing financial support under Grant No. NSC 101-2221-E-005-043-MY3.

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ACS Sustainable Chemistry & Engineering (36) Gupta, V. K.; Jain, R.; Mittal, A.; Saleh, T. A.; Nayak, A.; Agarwal, S.; Sikarwar, S. Photo-catalytic degradation of toxic dye amaranth on TiO2/UV in aqueous suspensions. Mater. Sci. Eng., C 2012, 32 (1), 12−17.

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Green Route for Hydrogen Evolution from Real Electroplating Waste

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