Sustainable hydrogen production from electroplating wastewater over a solar light responsive photocatalyst

Abstract

An environmentally friendly and sustainable photocatalytic hydrogen production system was successfully developed using ethylenediaminetetraacetic acid (EDTA) found in the wastewater from electroplating plants as the photo-excited hole scavenger and a solar light responsive multi-junction material as the photocatalyst. Additionally, a facile method of metal removal was established to increase the photocatalytic hydrogen production efficiency. The influences of the metal concentration, pH of the electroplating wastewater, and the photocatalyst concentration on the efficacy of hydrogen production were studied in detail. The results show that before metal removal, the unchelated metal ions and/or the metal–EDTA chelates block the active sites of the photocatalyst, resulting in suppressed adsorptions of H⁺ ions and/or H₂O, which results in negligible hydrogen production efficiency. Through the metal removal process developed in this study, most of the metals could be removed and most of the EDTA could be retained. The removal efficiencies of copper, nickel, and zinc ions were all higher than 90%, facilitating better reaction between the liberated EDTA molecules and the photo-excited holes, improved charge separation, and enhanced hydrogen production efficiency. The system showed economically optimal hydrogen production when the reaction pH was maintained at pH = 6 and the photocatalyst concentration was maintained at 2 g L⁻¹ under simulated sunlight irradiation.

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Introduction

Photocatalysis is viewed as a clean, promising route to produce hydrogen and alternative energy because the route is simple, effective, and non-polluting.^1–3 As shown in eqn (1)–(4), the transition of a photo-excited electron from the valence band to the conduction band of the photocatalyst is induced under suitable light irradiation. The photo-excited hole (h⁺) and electron (e⁻) then migrate to the surface of the photocatalyst, where they participate in the oxidation of H₂O and reduction of H⁺, respectively.^4–7

hν > E_g (1)

Photocatalyst → photocatalyst (e⁻ + h⁺) (2)

2h⁺ + 2H₂O → 2·OH + 2H⁺ (3)

2e⁻ + 2H⁺ → H₂ (4)

Generally, an appropriate amount of methanol needs to be used as a hole scavenger for the suppression of the recombination of an electron and a hole. The addition of the scavenger then results in the improvement of hydrogen production efficiency in the photocatalytic hydrogen production system.^8,9 However, because methanol is also a valuable resource for electricity generation and other applications,^10–12 adding methanol to improve the photocatalytic hydrogen production efficiency is not a green and sustainable solution.

It has been reported that using ethylenediaminetetraacetic acid (EDTA), an organic compound widely applied in chemical industries,^13,14 as the hole scavenger might also retard the charge recombination rate.^15,16 During photocatalytic hydrogen production, the EDTA is degraded by the hydroxyl radical (·OH) produced by the photo-excited hole (h⁺), and the superoxide radical (·O₂⁻) produced by the photo-excited electron (e⁻), simultaneously.^17–19 Depending on the point of attack on the EDTA molecule, EDTA decomposes into intermediates (glyoxylic acid, ED3A, or IMDA),^20,21 which further decompose into NH₃, CO₂, and the H₂.^22,23 The detailed degradation mechanism of EDTA is shown by eqn (5)–(7).

CH₂–COO⁻ → CH₂–COO· + e⁻ → H + CO₂ + CH₂O (5)

CH₂O + H₂O → HCOOH + H₂ (6)

HCOOH → CO₂ + H₂ (7)

Electroplating plants typically use EDTA for the fabrication of electroplates.^24,25 EDTA acts as a buffer controlling the metal ion concentration. The electroplating process generally includes a degreasing step, an acid washing step, and finally the electroplating step. Between each step, the electroplates were washed with water. According to the data shown in Table 1, the discharged waste liquid (the liquid from electroplating) and wastewater (the water from washing) generated from the electroplating industry usually contain significant amounts of organic compounds (e.g., EDTA) and heavy metals (e.g., chromium, copper, nickel, and zinc). The waste liquid and wastewater have been among the main environmental pollutants because of the high concentrations of metal ions, metal–EDTA chelates, and suspended solids and a high chemical oxygen demand (COD).^26,27 Although physicochemical treatment is widely used for electroplating wastewater, the problems of a high COD and ineffective metal ion removal in the real wastewater still exist because of the high concentrations of organics and metals in the real wastewater and the degradation resistance of metal–EDTA chelates. Bioaccumulation of the heavy metals discharge from the industrial wastewater may destroy the balance of ecosystem;^28–30 the high COD value renders the waste liquid and wastewater uninhabitable for aerobic organisms and many anaerobic organisms are able to grow and flourish.³¹

Table 1 Characteristics of the electroplating wastewater discharged from the manufactories in southern Taiwan

Characteristics of wastewater	Concentration or value of each manufactory
	A	B	C	D	E
Temperature (°C)	15∼35	20∼40	25∼32	10∼35	15∼40
pH value	2∼8	3∼6	3∼10	7∼12	2∼6
COD (mg-O2 L^−1)	100∼500	100∼1000	100∼400	50∼300	0.01∼400
Suspended solids (mg L^−1)	100∼300	100∼500	30∼120	50∼300	0.01∼200
Nickel (mg L^−1)	100∼300	100∼500	—	—	—
Copper (mg L^−1)	100∼300	—	1∼30	—	—
Zinc (mg L^−1)	—	—	—	—	0.01∼500
Total chromium (mg L^−1)	100∼350	50∼200	—	10∼50	0.01∼700

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Based on the oxidation ability of the photocatalyst, the requirement for photocatalytic hydrogen production, and the characteristics of the electroplating waste liquid and wastewater, we concluded that using EDTA as the hole scavenger in the electroplating waste liquid or wastewater has a two-fold benefit. First, it satisfies the demand for a photocatalytic method to produce hydrogen. Second, it simultaneously reduces the negative economic and environmental impact caused by the pollutants. However, to our knowledge, the conversion of simulated electroplating wastewater, with similar characteristics as the real electroplating wastewater, to hydrogen using a photocatalyst has not been studied. Furthermore, the influence of the metal concentration in the electroplating wastewater on the photocatalytic hydrogen production efficiency has not been adequately evaluated.

For a comprehensive study that can be used as a model reference to evaluate the feasibility of the conversion of electroplating wastewater to hydrogen and its industrial applications, we designed a practical waste-to-hydrogen system. As shown in Fig. 1, a combination of hydrogen production and the recycling of the EDTA in electroplating wastewater was at the core of this study. Simulated electroplating wastewater was prepared according to the characteristics of real electroplating wastewater and a solar light responsive photocatalyst was applied to convert the electroplating wastewater to hydrogen under simulated sunlight irradiation. Additionally, a facile method of metal removal was developed to increase the photocatalytic hydrogen production efficiency in this study. Moreover, the influences of the metal concentration, pH of the electroplating wastewater, and photocatalyst concentration on the efficiency of hydrogen production were studied in detail.

Fig. 1 Schematic representation of the mechanism for hydrogen production from electroplating wastewater over a solar light responsive photocatalyst.

Experimental

Solar light responsive photocatalyst preparation

A multi-junction photocatalyst was synthesized with the similar procedure as the study reported by our group.³²

Preparation of hydrogen titanate (H-titanate) tubes. A sodium hydroxide solution (NaOH, 28%, Union Chemical Work Ltd.), which contained 3 g of TiO₂ particles (Degussa P25, UR) was prepared first. The mixture was heated in a Teflon-lined stainless steel autoclave at 150 °C for 24 h and then cooled to room temperature. The products were neutralized by washing with a 0.1 M hydrochloric acid solution (HCl, 37%, Sigma-Aldrich). The H-titanate tubes were obtained after drying at 80 °C overnight.

Preparation of SrTiO₃–TiO₂ tubes. A SrTiO₃–TiO₂ tube was prepared in the second hydrothermal process. A solution containing calcium chloride (CaCl₂, 99%, Showa Chemicals, Inc.), potassium hydroxide (KOH, ≥85%, Sigma-Aldrich), and 0.3 M strontium chloride (SrCl₂, 99%, J. T. Baker) was prepared. To obtain the supernatant solution, we centrifuged the solution after 1 h of agitation. The as-prepared H-titanate tubes were then added into the supernatant solution under agitation. The mixture was heated in a Teflon-lined stainless steel autoclave at 150 °C for 24 h and then cooled to room temperature. The white products were neutralized by washing with a 0.1 M hydrochloric acid solution (HCl, 37%, Sigma-Aldrich). The dried white products were calcined under ambient atmosphere for 2 h to obtain the SrTiO₃–TiO₂ tubes (STO–TNTs).

Preparation of the solar light responsive photocatalyst. A sol–gel method was applied to synthesize N-doped TiO₂/STO–TNT. Nitric acid (HNO₃, 69%, Fu Yuan Chemicals Co., Ltd) was dissolved in deionized water following titanium isopropoxide (TTIP, 97%, Aldrich). 5% (w/w) of the as-prepared STO–TNTs and ammonium hydroxide (NH₄OH, 28%, Union Chemical Work Ltd.) were subsequently added to the above solution after 2 h of stirring. The N–Ti–O molecule was gradually grown on the as-prepared STO–TNT under continuous agitation. The products were washed with abundant deionized water until pH = 7 was reached. Then the products were subsequently dried at 80 °C and calcined at 350 °C for 3 h. The as-prepared N-doped TiO₂/STO–TNT tubes (N-TiO₂/STO–TNTs) were obtained after calcination.

The surface of the N-TiO₂/STO–TNT was modified with a small amount of Pt (0.2 wt%) by photodeposition. The N-TiO₂/STO–TNTs were added into a methanol (CH₃OH, ≥98%, Union Chemical Work Ltd.) solution (50% v/v), which contained the appropriate amount of chloroplatinic acid hydrate (H₂PtCl₆·xH₂O, 99.95%, UR). The mixture was then irradiated under UV light (Ultra-Vitalux, 300 W, Osram) and calcined at 250 °C under a reduced atmosphere (5% H₂/95% He) for 2 h. The obtained solar light responsive photocatalyst was named Pt/N-TiO₂/STO–TNT.

Photocatalyst characterization technique. The crystalline structure of the multi-junction photocatalyst was examined by X-ray diffraction (XRD, M18XHF, Mac Science Co.). The morphology of the multi-junction photocatalyst was analyzed by field-emission scanning electron microscopy (FESEM, JSM-6700F, JEOL) and field-emission transmission electron microscopy (FETEM, JEM-2100F, JEOL). Photoluminescence (PL, LS 45, Perkin Elmer) with a Xe lamp (λ = 200 nm) was utilized to evaluate the charge recombination probability of the multi-junction photocatalyst at 25 °C. UV-vis spectrophotometer (Lambda 35, Perkin Elmer) was applied to evaluate the optical reflection of the multi-junction photocatalyst.

Simulated electroplating wastewater preparation

A simulated wastewater was prepared according to the average of each composition shown in Table 1. Copper, nickel, and zinc were chosen as the representative metals and EDTA was used as the source of COD in the electroplating wastewater. The additional amount of EDTA added was calculated by the following equations:

Molecular weight = EDTA–2Na·H₂O = 372.24 g mol⁻¹

0.00184 mol × 372.24 g mol⁻¹ = 685 mg

0.982 g of cupric sulfate pentahydrate (CuSO₄·5H₂O, 100%, J. T. Baker), 1.1425 g of nickel sulfate hexahydrate (NiSO₄·6H₂O, 98%, Alfa Aesar), 0.5315 g of zinc chloride (ZnCl₂, 98%, Alfa Aesar), and 685 mg of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA–2Na·2H₂O, ≧99%, Sigma-Aldrich) were dissolved in deionized water and diluted to 1000 mL with deionized water in a volumetric flask to prepare a simulated electroplating wastewater with [Cu] = 250 mg L⁻¹, [Ni] = 250 mg L⁻¹, [Zn] = 250 mg L⁻¹, and [COD] = 500 mg-O₂ L⁻¹. The solution was mixed with vigorous agitation and was stirred overnight to ensure homogeneity before using.

Experimental procedure

The multi-junction photocatalysts and 50 mL of the simulated electroplating wastewater were added into a reactor. A simulated sunlight lamp (420 W m⁻², XHA500) was fixed 10 cm above the reactor, and the reaction temperature (20 °C) was controlled by a water cooling system. Prior to the photocatalytic H₂ production, the suspension was deaerated by purging with N₂ in the dark. The produced gases were sampled every ten minutes and analyzed using an online gas chromatograph (GC-TCD, Clarus 500 Perkin Elmer, Carboxen 1000 column, N₂ carrier) to determine the H₂ concentration.

Metal removal process

According to the intrinsic properties of each metal, a metal removal process was designed to reduce the metal interference on hydrogen production. To remove the unchelated Cu, Ni, and Zn ions, we adjusted the pH value of the simulated electroplating wastewater from 2.77 to 10.5 with NaOH to precipitate the metal oxides (Cu(OH)₂, Ni(OH)₂, and Zn(OH)₂). After 3 h of agitation, a simulated electroplating wastewater with increased turbidity was obtained. The wastewater was filtered through a 0.2 µm membrane filter for the separation of the metal oxide precipitates from the wastewater, and a transparent pale-blue wastewater was obtained. The numbers of moles of chelated metal ions were deduced according to the initial addition of EDTA and the valency of the Cu, Ni and Zn ions (2+). An equal number of moles of sodium sulfate (Na₂S, 98%, Acros Organics) was added into the filtered wastewater to break the bond between the metal and EDTA, thus forming the metal sulfate precipitates. After 20 h of agitation, a simulated electroplating wastewater with black suspended particles was obtained. The wastewater was filtered through a 0.2 µm membrane filter for the separation of metal sulfate precipitates from the wastewater, and a transparent wastewater was obtained. The pH value of the treated wastewater was finally adjusted from 10.5 to pH = 7 with HCl before the photocatalytic reaction.

Wastewater composition analysis

The total EDTA sodium salt and metal ion concentrations in wastewater were determined before and after removing the metal, and before the photocatalytic reaction. The total EDTA sodium salt concentration in the wastewater was determined using the ASTM D3113-92 standard method, and the metal ion concentrations in wastewater were determined photometrically using a water quality analyzer (Suntex, PhotoLab S6).

Photocatalyst characterization technique

Nano ZS (MALVERN) was applied to analyze the zeta potential of Pt/N-TiO₂/STO–TNT in the aqueous phase.

Results and discussion

Influence of metal concentration on hydrogen production

Fig. 2 shows the XRD patterns of STO–TNT, N-TiO₂, and N-TiO₂/STO–TNT. The crystalline peaks of STO–TNT are identified as the perovskite SrTiO₃ phase (JCPDS card no. 840444) and the anatase TiO₂ phase (JCPDS card no. 211272), and the crystalline peaks of N-TiO₂ are identified as the anatase TiO₂ phase (JCPDS card no. 211272). Besides, the results indicate that the N-TiO₂/STO–TNT was mainly composed of the anatase TiO₂ phase (JCPDS card no. 211272) and partially composed of the perovskite SrTiO₃ phase (JCPDS card no. 840444), and the crystalline ratio of anatase TiO₂/perovskite SrTiO₃ of N-TiO₂/STO–TNT reflected to the STO–TNT addition amount (5 wt%). Moreover, the UV-vis/DRS analysis result shown in Fig. 3(a) indicates that the N-TiO₂/STO–TNT exhibited red-shifts in UV-vis light absorbance, indicating that the N-TiO₂/STO–TNT with well UV-vis light absorbance was successfully synthesized in this study. The FETEM image shown in Fig. 3(c) indicates that the STO–TNT was formed with dispersed SrTiO₃ particles on the anatase TiO₂ tube surface, which was good for photo-exited charge separation and electronic transport. Additionally, from the Fig. 3(d), it was found that the N-TiO₂/STO–TNT was formed with tubes and particles, and the particles were combined with tubes tightly. It means that the N-TiO₂/STO–TNT was synthesized successfully, and the excellent contact between the N-TiO₂ and the STO–TNT was good for charge migration and separation.

Fig. 2 XRD patterns of STO–TNT, N-TiO₂, and N-TiO₂/STO–TNT.

Fig. 3 UV-vis/DRS spectra of (a) STO–TNT and N-TiO₂/STO–TNT; PL spectra of (b) STO–TNT, N-TiO₂/STO–TNT, and Pt/N-TiO₂/STO–TNT; FETEM images of (c) STO–TNT; FESEM images of (d) N-TiO₂/STO–TNT.

As shown in Table 2, the initial pH was 2.8 when each metal concentration and EDTA concentration in the electroplating wastewater were 250 mg L⁻¹ and 685 mg L⁻¹, respectively. Fig. 4 shows that hydrogen was not effectively produced from the original electroplating wastewater (before metal removal, pH = 2.8), and it might be attributed to the influence of low pH or the excess amount of metal ions. It has been indicated that the photocatalyst would aggregate together in low pH, resulting in decreased active sites for EDTA and H₂O adsorption and inhibited photocatalytic hydrogen production activity.³³ Besides, the surface of the photocatalyst would be covered with positive charge while the pH was too low,³⁴ which would obstruct the transfer of photo-excited hole to the photocatalyst surface and cause to limited electron and hole separation. To clarify the reason for the inhibited photocatalytic activity, we carried out an additional experiment in where N-TiO₂ catalyst was irradiated in the electroplating wastewater which pH value was adjusted to pH = 7 by diluted NaOH solution. However, it was observed that the N-TiO₂ catalyst also exhibited negligible hydrogen production activity in the neutral electroplating wastewater (before metal removal, pH = 7). The results evidenced that the negligible hydrogen production efficiency were caused by the excess amount of metal ions. According to the calculation, it was found that in 1 L of the electroplating wastewater, 1.2 × 10⁻² mol of metal ions was present, which was significantly higher than the amount of EDTA (1.84 × 10⁻³ mol). This means that the EDTA molecules in the electroplating wastewater were completely chelated with the metal ions to form Cu–EDTA, Zn–EDTA, and Ni–EDTA. The absence of unchelated EDTA molecules results in the inability to trap the photo-excited holes of the N-TiO₂ catalyst directly. Therefore, the photo-excited electron and hole of the photocatalyst would recombine rapidly, resulting in negligible hydrogen production for the N-TiO₂ catalyst. Furthermore, the unchelated metal ions and/or the metal–EDTA chelates in the electroplating wastewater might adsorb on the surface of the N-TiO₂ catalyst and block the active sites, resulting in suppressed adsorption of H⁺ ions and/or H₂O and further decreasing the efficiency of hydrogen production.

Table 2 The electroplating wastewater composition analysis

Electroplating wastewater	Concentration (mg L^−1)				pH value
	EDTA	Cu	Ni	Zn
Before treatment	685.0	250.0	250.0	250.0	2.8
After treatment	529.0	0.1	24.0	8.4	7

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Fig. 4 Influence of metal concentration on hydrogen production. [Catalyst] = 2 g L⁻¹.

To reduce the metal interference on the photocatalytic activity of the N-TiO₂ catalyst, we attempted to remove the unchelated metal ions and break the bond between with the metal–EDTA chelates using the method mentioned in “Metal removal process” section. The total EDTA sodium salt and metal ion concentrations in wastewater were monitored before and after removing the metal, and the data are shown in Table 2. The residual Cu, Ni, and Zn ion concentrations were found to have decreased to 0.1 mg L⁻¹, 24.0 mg L⁻¹, and 8.4 mg L⁻¹, respectively. Moreover, the residual EDTA concentration was found to be 529.0 mg L⁻¹, indicating that throughout the metal removal process, most of the metals could be effectively removed and most of the EDTA could be retained. From Fig. 4, we concluded that the N-TiO₂ catalyst showed better hydrogen production activity in the treated wastewater (the hydrogen production rate was increased from 0 to 0.7 µmol h⁻¹ g⁻¹ after 30 min reaction). Because of the removal of unchelated metal ions, the probability of the active sites being occupied by the unchelated metal ions was dramatically decreased. Furthermore, due to the bond breakage within the metal–EDTA chelates, the liberated EDTA molecules were free to react with the photo-excited holes of the N-TiO₂ catalyst directly, resulting in retarded electron and hole recombination rates and an overall improved photocatalytic hydrogen production efficiency. The results confirmed that the role of the unchelated EDTA molecule during photocatalytic hydrogen production is essential. Moreover, it was found that the N-TiO₂/STO–TNT revealed higher hydrogen production efficiency than N-TiO₂ (the hydrogen production activity of N-TiO₂/STO–TNT was 1.8 µmol h⁻¹ g⁻¹ after 30 min reaction). As the photocatalytic mechanism discussed in our previous study,³² both the redox potential differences among N-TiO₂, SrTiO₃, and TiO₂ and the electronic transport of TiO₂ tube could facilitate the photo-exited electron migration from the bulk to the surface, resulting in better charge separation efficiency, lower PL intensity (as shown in Fig. 3(b)), and improved hydrogen production activity. Furthermore, the result shows that the Pt/N-TiO₂/STO–TNT exhibited the optimal hydrogen production activity in the treated wastewater, and the hydrogen production rate was 87 µmol h⁻¹ g⁻¹ after 30 min reaction. It is known that based on certain UV-vis absorption, the charge separation efficiency is the key point for high hydrogen production efficiency. Due to the higher electron acceptance of Pt,²² the charge separation efficiency would be increased when the surface of N-TiO₂/STO–TNT was modified with appropriate amount of Pt, leading to lowest PL intensity (as shown in Fig. 3(b)) and significantly improved hydrogen production activity. Besides, according to the negligible deviation for hydrogen production, it is evident that this study provided a good method to prepare the Pt/N-TiO₂/STO–TNT with excellent reproducibility for hydrogen production.

However, it is important to note that the hydrogen production is accompanied by the consumption of the EDTA. Therefore, the hydrogen production efficiency gradually decreases with time, as the EDTA concentration decreases. Because of the low EDTA concentration in the electroplating wastewater (529.0 mg L⁻¹), the photocatalytic hydrogen production efficiency was greatly diminished around 40 min of reaction time.

Influence of pH value on hydrogen production

The protonation of the photocatalyst surface is critical to the production of hydrogen. It facilitates the adsorption of the anionic EDTA molecule,^35,36 which then promotes the photo-excited electron and hole separation. Without the recombination of the hole and the electron, the photocatalytic reduction of the H⁺ ion may proceed via the photo-excited electron, and the photocatalytic oxidation of the adsorbed EDTA may proceed via the photo-excited hole. The pH value of the electroplating wastewater was adjusted with diluted HCl from pH = 7 to pH = 6 and pH = 5 and the effect of the pH value on the activity of the Pt/N-TiO₂/STO–TNT catalyst was examined. Fig. 5(a) shows the hydrogen production rates with time. It was found that the Pt/N-TiO₂/STO–TNT catalyst exhibited the optimal activity when the pH value of the electroplating wastewater was controlled at 6. At pH = 6, the hydrogen production efficiency was 180.6 µmol g⁻¹ h⁻¹, which was 2 times higher than that when the pH value of the wastewater was maintained at 7. Furthermore, the Pt/N-TiO₂/STO–TNT catalyst exhibited the worst hydrogen production activity when the pH value was maintained at 5. As shown in Fig. 6, the pH_ZPC (zeta potential) of the Pt/N-TiO₂/STO–TNT catalyst was 6.4. This indicates that when the pH value of the electroplating wastewater was lower than 6.4, the surface charge of the Pt/N-TiO₂/STO–TNT catalyst is positive, and the density of the positively charged Pt/N-TiO₂/STO–TNT surface increased with the decrease in pH value. When the pH value of the wastewater was maintained at 6, the positive charge density facilitated the adequate adsorption between the unchelated EDTA and the Pt/N-TiO₂/STO–TNT catalyst, resulting in a balance between unchelated EDTA, H⁺ ions, and H₂O adsorption. Therefore, at pH = 5, the higher positive charge density facilitated a higher adsorption of unchelated EDTA, thus decreasing the adsorption of the H⁺ ions and/or the water and diminishing the hydrogen production efficiency. Thus, the pH investigation of the Pt/N-TiO₂/STO–TNT catalyst revealed that the worst hydrogen production activity occurred at pH = 5. To prove the above point, we measured the residual EDTA concentration in the wastewater after reaction. The hydrogen production rate after 30 min reaction time was chosen as the representative rate, and the relationship between hydrogen production rate and the residual EDTA concentration is shown in Fig. 5(b). Fig. 5(b) shows that the residual EDTA concentration decreased when the pH value decreased from 7 to 6, which support the premise that the increased hydrogen production efficiency was ascribed to the increased adsorption probability of EDTA. Furthermore, according to the residual EDTA concentration and the hydrogen production efficiency at pH = 5, it was confirmed that the excess adsorption of EDTA would prevent the H⁺ ion and/or H₂O adsorption, resulting in decreased hydrogen production efficiency. Based on the results, it was found that the Pt/N-TiO₂/STO–TNT catalyst played a significant role in adsorbing EDTA molecules when the pH value of the wastewater was 5.

Fig. 5 Influence of the reaction pH value on hydrogen production. (a) The H₂ production rate varying with reaction time, (b) the H₂ production rate after 30 min reaction. [Catalyst] = 2 g L⁻¹.

Fig. 6 The zeta potential of the Pt/N-TiO₂/STO–TNT catalyst against the pH value.

Influence of the photocatalyst concentration on hydrogen production

Based on the results in “Influence of pH value on hydrogen production” section, the most suitable reaction pH value was found. It this section, the pH value of wastewater was controlled at 6 and the influence of the photocatalyst concentration on hydrogen production was studied.

Theoretically, a higher photocatalyst concentration would provide more active sites for EDTA, H⁺ ions, and H₂O adsorption, resulting in better charge separation and higher hydrogen production probability. Fig. 7(a) shows that when the photocatalyst concentration was in a range of 1–6 g L⁻¹, the activity of the Pt/N-TiO₂/STO–TNT catalyst increased with the increase in photocatalyst concentration. Furthermore, Fig. 7(b) shows that the residual EDTA concentration in the wastewater was inversely proportional to the hydrogen production rate when the photocatalyst concentration was in the range of 1–6 g L⁻¹. This suggests that the increased active sites help the adsorption of EDTA, H⁺ ions, and H₂O, and improve hydrogen production. When the photocatalyst concentration was 12 g L⁻¹, however, the hydrogen production efficiency dramatically declined, even though it provided the highest number of active sites for EDTA adsorption. This phenomenon was attributed to the screening effect caused by the excess amount of the Pt/N-TiO₂/STO–TNT catalyst. The turbidity of the solution increased with the increasing photocatalyst concentration, and the screening effect caused by the turbidity diminished the light penetration,^37–39 resulting in a decreased amount of activated Pt/N-TiO₂/STO–TNT catalyst. Moreover, Neppolian's group⁴⁰ indicated that the ground-state photocatalyst would react with the activated photocatalyst in the reaction system, leading to its de-activation. Therefore, the hydrogen production efficiency declined as a result of both decreased amount of activated Pt/N-TiO₂/STO–TNT and the deactivation of the activated Pt/N-TiO₂/STO–TNT with ground state Pt/N-TiO₂/STO–TNT. It was found that the main role of the Pt/N-TiO₂/STO–TNT catalyst was that of an adsorbent when the photocatalyst concentration was 12 g L⁻¹. Additionally, after assessing the hydrogen production value and the photocatalyst dosage, it was found that the most economically efficient hydrogen production system was established when the photocatalyst concentration was controlled at 2 g L⁻¹.

Fig. 7 Influence of the photocatalyst concentration value on hydrogen production. (a) The H₂ production rate varying with reaction time, (b) the H₂ production rate after 30 min reaction. Reaction pH value = 6.

Photocatalyst stability

To evaluate the stability of Pt/N-TiO₂/STO–TNT, the reusability of Pt/N-TiO₂/STO–TNT for hydrogen evolution was investigated. The catalyst concentration was controlled at 2 g L⁻¹ based on the results in “Influence of the photocatalyst concentration on hydrogen production” section, and the used Pt/N-TiO₂/STO–TNT was collected, washed with distilled water, and dried at 80 °C overnight before the next reaction. The photocatalytic activity performance shown in Fig. 8 indicates that the recycling test was successfully performed five times without obvious activity drop, and the Pt/N-TiO₂/STO–TNT still exhibited 88% hydrogen evolution efficiency (≒16 µmol h⁻¹) at the fifth cycle, indicating that the Pt/N-TiO₂/STO–TNT possessed excellent photocatalyst stability for hydrogen production. The slightly decreased hydrogen production performance was attributed to the absent active sites of the Pt/N-TiO₂/STO–TNT surface caused by occupation of intermediate species formed during the reaction. According to the result, it was evidenced that the Pt/N-TiO₂/STO–TNT was a promising material for environmental applications.

Fig. 8 Reusability experiment for hydrogen evolution by Pt/N-TiO₂/STO–TNT (the H₂ production rate after 30 min reaction). Reaction pH value = 6; [catalyst] = 2 g L⁻¹.

Conclusions

Currently, efforts are underway to utilize solar light responsive materials in the application of photocatalytic oxidation or reduction, and our preliminary investigation shows further usefulness of the solar light responsive materials in the conversion of organics in wastewater to hydrogen. A photocatalytic hydrogen production system using EDTA in the electroplating wastewater as the photo-excited hole scavenger, Pt/N-TiO₂/STO–TNT as the photocatalyst, and simulated sunlight as the light source was successfully developed.

The results evidenced that the reusability of the electroplating wastewater can be significantly increased through the metal removal process. Additionally, the important role of unchelated EDTA molecules in improving photocatalytic hydrogen production was signified. The positive charge density of the Pt/N-TiO₂/STO–TNT surface increased with the decreasing reaction pH value, and the adsorption probability of unchelated EDTA was proportional to the positive charge density of the Pt/N-TiO₂/STO–TNT surface. However, because of the adsorption competition among the unchelated EDTA, H⁺ ion, and H₂O for the active sites of the Pt/N-TiO₂/STO–TNT, the hydrogen production efficiency was inhibited when the adsorption probability of unchelated EDTA was too high. The system exhibited an economically optimal hydrogen production when the reaction pH value and the photocatalyst concentration were maintained at pH = 6 and 2 g L⁻¹, respectively.

According to the residual EDTA concentration and the result of recycling test of Pt/N-TiO₂/STO–TNT, it is evident that the design of waste-to-hydrogen was a promising route to simultaneously create the recycling value of electroplating wastewater, decrease the concentrations of metal and COD in electroplating wastewater, and reduce the complexity of wastewater treatment before discharge. Additionally, it was found that the initial metal and EDTA concentrations were the important indexes to evaluate the recycling value of the electroplating wastewater. The feasibility of the conversion of EDTA in real wastewater to hydrogen will be our subsequent study in the future.

Acknowledgements

The authors would like to 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.

Notes and references

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