Photocatalytic treatment of tannery wastewater using reduced graphene oxide and CdS/ZnO to produce hydrogen with simultaneous sulfide abatement
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This paper addresses the photocatalytic treatment of sulfide-rich liming bath wastewater from tanneries in an oxygen-free atmosphere. The photoactivity assessment of composites obtained from cadmium sulfide (CdS) and zinc oxide (ZnO) hybridization as well the influence of reduced graphene oxide (RGO) on the photocatalytic activity to generate hydrogen and mitigate sulfide was performed under visible-light irradiation. The results of the X-ray diffraction analysis allowed us to identify the crystalline phases in each synthesized sample. The photocatalysts RGO/(CdS)1.0/(ZnO)0.4 and RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57 showed the best performances for hydrogen production of 1.6 mmol g cat −1 h−1 and 1.4 mmol g cat −1 h−1, respectively, with mitigation of 80–90% of the sulfide content in the tannery wastewater. Long-term reaction tests indicated that the photocatalysts are active for a period of 24 h, and they retain the photoactivity after three photocatalytic cycles. The materials have demonstrated great potential for application in energy recovery from tannery wastewater with simultaneous sulfide abatement, especially the photocatalyst RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57, which is more eco-friendly because of its lower cadmium content and minimal leaching of this toxic metal ion during photocatalytic treatment.
KeywordsHydrogen Photocatalysis Energy recovery Tannery sludge Reduced graphene oxide
Mathematics Subject Classification80A50
Hazardous tannery effluents are responsible for many negative environmental impacts. They can be divided into three types based on the operations carried out in the different sections . One is the liming bath wastewater that accounts for 45% of the effluent volume and contributes to 30% of the overall biological oxygen demand (BOD) and chemical oxygen demand (COD). The other two types are tanning wastewater with high salinity and high chrome levels and retanning, dyeing, and fat liquoring wastewater that accounts for ~ 20% of the total COD .
The liming bath treatment eliminates components from skins that are not transformed into leather [1, 2]. This operation involves treatment using lime and sodium sulfide as the primary chemicals to dissolve hair and destroy the epidermis by breaking the cysteine-disulfide bridges (keratolysis) via reductive division . This type of wastewater is highly alkaline (pH 10–12), with a high sodium sulfide content that is responsible for H2S evolution , which is fatal even in concentrations as low as 200 ppm [5, 6].
The removal of pollutants from liquid effluents is currently considered a major challenge. Several strategies are employed to remove pollutants from wastewater depending on their nature. Adsorption processes using porous materials with various functional groups for target ion separation have demonstrated high efficiency for toxic ion removal from water solutions [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19], whereas degradation processes are mostly employed in remediation of a large number of organic and inorganic pollutants [20, 21, 22, 23, 24, 25, 26]. Thus, the use of new environmentally friendly technologies is necessary to mitigate the effects caused by hazardous tannery effluents. Chemical coagulation, electrocoagulation, flocculation, membrane filtration (MF) and advanced oxidation processes (AOPs) are the main solutions proposed and used in the treatment of tannery wastewater [2, 26, 27, 28].
Photocatalysis is one type of AOP considered a green technique that offers the potential for complete elimination of toxic chemicals in the environment through its efficiency and broad applicability, and sunlight can be used as an energy source. On the other hand, this process has been applied for solar energy photoconversion to hydrogen and hydrocarbons . Many works [30, 31, 32, 33, 34] have shown that the photocatalytic treatment of organic and inorganic species, such as alcohols and sulfide ions, in an oxygen-free atmosphere occurs with simultaneous hydrogen evolution over irradiated semiconductor photocatalyst suspensions. In this context, we recently demonstrated the feasibility of hydrogen generation through photocatalytic treatment of tannery sludge, rich in sulfide ions, using solar power . The process is based on light-driven water splitting in the presence of a sacrificial reactant. Sulfide ions in aqueous solutions act as efficient hole scavengers when metal sulfide-based photocatalysts are employed at high pH . In our previous work , the photocatalytic treatment of liming bath wastewater using CdS as a photocatalyst reached a maximum hydrogen production of 294.38 µmol g−1 cat h−1 at pH 13. Despite the high photocatalytic activity under visible light, which accounts for 43% of sunlight, pure CdS, a narrow bandgap semiconductor (Eg = 2.41 eV), can leach during the photoreaction, increasing the cadmium content in the treated effluent , which is a major drawback.
Although TiO2 is the most commonly used photocatalyst for the degradation of tannery wastewaters with UV irradiation [21, 22, 23], in recent years, ZnO has gained more attention due to its ease of crystallization, nontoxicity, high charge mobility and simplicity in morphology adaptation , in addition to its high performance in the photocatalytic removal of environmental pollutants [24, 25]. ZnO is a wide bandgap semiconductor (Eg = 3.37 eV), which limits its application as an efficient photocatalyst under sunlight irradiation. Additionally, photogenerated charge carriers have a short lifetime. However, its hybridization with CdS in a CdS/ZnO heterostructure can extend the sensitivity of the composite toward the visible-light region and promote effective charge separation through a type II heterojunction [37, 38]. Due to these properties, the CdS/ZnO heterostructure has been successfully used in photoconversion applications [36, 39, 40, 41, 42]. In the present work, we prepared (CdS)x/(ZnO)y composites for hydrogen production from liming bath wastewater rich in sulfide ions that act as sacrificial reactants to improve the photocatalytic activity with solar power.
In general, platinum is used as a cocatalyst for hydrogen reduction. When attached on a photocatalyst, platinum nanoparticles can form a Schottky junction, building a barrier that prevents charge recombination [43, 44, 45]. In our previous work, an experimental design was performed to evaluate the influence of the platinum precursor (H2[PtCl6]·6H2O) concentration on the photocatalytic hydrogen production from liming wastewater. We concluded that the precursor concentration was not a significant factor due to the deposition of PtIIS on the CdS surface instead of Pt(0) during irradiation, which was confirmed by X-ray analysis . Recently, we evaluated the role of graphene oxide (GO) in a hybrid photocatalyst with CdS for hydrogen production using sulfide ions as sacrificial reactants . Our findings demonstrated that reduced graphene oxide (RGO), which is produced during the photoreaction, acts as an efficient electron trap. The trapped electrons can be transferred to adsorbed water molecules on the surface that promote the reduction in water to H2, and the photogenerated holes can oxidize sulfide ions to polysulfides and sulfate on the CdS surface. In the present work, we evaluated the photoactivity of RGO/(CdS)x/(ZnO)y as bifunctional photocatalysts in the treatment of liming bath wastewater to mitigate pollutants with simultaneous hydrogen production.
2 Experimental details
2.1 Materials and chemicals
The chemicals used in the synthesis, photocatalytic tests and wastewater analyses were of analytical grade, and all of them were used as purchased. Graphite, potassium permanganate, sodium sulfide nonahydrate, ethylene glycol and 30% hydrogen peroxide solution were purchased from Sigma-Aldrich. Sulfuric acid was purchased from Qhemis, while citric acid, cadmium chloride monohydrate and zinc nitrate hexahydrate were purchased from Merck.
2.2 Photocatalyst preparation
The binary composites (CdS)x/(ZnO)y were prepared by the Pechini method, followed by sulfurization via ion exchange in a sodium sulfide solution. The precursors used here were CdCl2·H2O and Zn(NO3)2·6H2O with molar ratios y/x = 0, 0.2, 0.4, 0.6, 1.5 and 4.0. Briefly, the procedure consisted of dissolution of an appropriate amount of the precursors in 50 mL of water. This solution was added to an EG/CA solution prepared with 10 mL of ethylene glycol and 7.0 g of citric acid in 50 mL of water. The mixture was kept at 80 °C to obtain the metal-citrate complex; then, the temperature was raised to 120 °C to form a gel, which was polymerized at 300 °C for 4 h and calcined at 500 °C for 4 h. The solids were grounded and suspended in a sodium sulfide solution (0.5 mol L−1) for 12 h to promote ion exchange (IE) and then filtered and dried at 60 °C for 12 h. The samples were labeled Zn/Cd0.0, Zn/Cd0.2, Zn/Cd0.4, Zn/Cd0.6, Zn/Cd1.5 and Zn/Cd4.0 before ion exchange and Zn/Cd0.0-IE, Zn/Cd0.2-IE, Zn/Cd0.4-IE, Zn/Cd0.6-IE, Zn/Cd1.5I-E and Zn/Cd4.0-IE after ion exchange.
The ternary composites RGO/(CdS)x/(ZnO)y were prepared by mechanical mixing of binary composites (CdS)x/(ZnO)y with graphite oxide (2% w/w) obtained by the Hummers’ method [47, 48]. The mixture was placed in a photoreactor with an appropriate amount of tannery wastewater and exposed to high-intensity ultrasound irradiation using an ultrasonic wave source (R2D091109, Unique) under ambient air for 15 min with the power adjusted at 80 W to promote the exfoliation of graphite oxide to produced graphene oxide (GO). The photoreduction of GO to RGO occurred in situ during hydrogen generation.
2.3 Photocatalyst characterization
The synthesized materials were analyzed as powders by XRD (Shimadzu XRD6000) using CuKα, Ni-filtered radiation, and a scanning rate of 2° 2θ min−1 in a 2θ range of 5°–80°, at 35 kV and 15 mA. The chemical composition of the samples was determined by energy-dispersive X-ray fluorescence (Shimadzu EDX720). UV–Vis diffuse reflectance spectra were recorded on a Thermo Scientific Evolution 600 UV–Vis spectrometer using a praying mantis accessory. Selected samples were analyzed before and after irradiation by Raman spectroscopy using a dispersive Raman spectrometer, Jasco NRS-5100, with 0.4 cm−1 resolution. SEM images were taken by a JSM-6610LV scanning electron microscope (JEOL) operated at 20 kV after gold metallization.
2.4 Sacrificial reactant
The target effluent used as a sacrificial reactant was collected from a tannery located in Petrolina-PE, Brazil. All details about the effluent are described elsewhere . To prepare the aqueous solutions used in the photocatalytic tests, first, the wastewater as collected was centrifuged to remove the solid materials and then diluted to 50% V/V with distilled water. Total sulfide, before and after irradiation, was also determined by the iodometric method of analysis using sodium thiosulfate, following the procedure described in . Sulfate was quantified by a standard gravimetric method with precipitation as BaSO4 in the presence of HCl by the addition of BaCl2. Biological demand of oxygen (BDO) and chemical demand of oxygen (CDO) were determined by Standard Methods for the Examination of Water and Wastewater 5210 B and 5220 A/C, respectively. Cadmium was quantified using a FAAS Agilent Technologies 200 Series AA—Model 240FS AA and an analytical curve in the concentration range of 0.5–2.5 mg L−1.
2.5 Photocatalytic reactions
For each photocatalytic test, 60 mg of a given binary composite was mechanically mixed with 1.2 mg of graphite oxide, weighed in a Mettler Toledo microbalance, model MX5, d = 1 μg, and dispersed in 60 mL of diluted tannery sludge (50% V/V) with the pH adjusted to 13 by the addition of a NaOH solution. The mixture was sonicated for 15 min to promote graphite oxide exfoliation. Then, the photocatalytic cell was purged with argon for 30 min to eliminate oxygen gas. The collimated light beam from a high-pressure 500 W Hg–Xe arc lamp was passed through an IR filter, and a 420-nm cutoff filter before reaching the flat window of the photocatalytic cell that was maintained at a constant temperature by air cooling. The photocatalytic cell was equipped with argon gas inlet/outlet tubes to collect and transfer gaseous products to the analytical system. Hydrogen gas evolution was measured by gas chromatography (SHIMADZU, GC2014) with a thermal conductivity detector (TCD) and argon as the carrier gas. Aliquots of 1 mL of the gas phase were injected in the GC system in intervals of 1 h. To quantify the hydrogen produced during the reaction, a 5% H2 standard diluted in argon was injected before each experiment. For comparison, all tests were also performed without GO as well as with CdS obtained by the thermal method . The experiments were performed in triplicate.
3 Results and discussion
After ion exchange (Fig. 1b), the CdCl2·H2O in sample Zn/Cd0.0 was virtually converted into CdS, with a cadmium residue as Cd3Cl2O2. For samples Zn/Cd0.2-IE, Zn/Cd0.4-IE and Zn/Cd0.6-IE, the ZnO phase was preserved and CdCl2·H2O disappeared to give rise to the CdS cubic phase with residues of the CdOHCl phase and a lamellar compound not identified in samples Zn/Cd0.2-IE and Zn/Cd0.6-IE. For samples Zn/Cd1.5-IE and Zn/Cd4.0-IE, the ZnS cubic phase was identified.
EDX elemental analysis (% wt/wt), y/x ratio for the general formula (CdS)x/(ZnO)y and nCd/nS ratio (mol/mol) obtained from EDX results
Elemental analysis (% wt/wt)
The optical bandgap energies of the samples determined by a geometrical method
The bandgap energy values are quite similar for each group. For group i, samples Zn/Cd0.0-IE and Zn/Cd0.4-IE, the bandgap energies are 2.24 eV and 2.28 eV, respectively. For group ii, samples Zn/Cd0.2-IE and Zn/Cd0.6-IE, the bandgap energies are 2.38 eV and 2.39 eV, respectively. However, for these two groups, the experimental bandgap energies are less than those reported in the literature for CdS (Eg = 2.41 eV). Finally, for group iii, samples Zn/Cd1.5-IE and Zn/Cd4.0-IE, the bandgap energies are 2.50 eV and 2.46 eV, respectively, which are greater than that of CdS due to the presence of ZnS (Eg = 3.7 eV) in these composites.
The amount of hydrogen produced using composites as photocatalysts in both cases is much higher than that obtained from the mechanical mixtures of the individual components, reflecting the heterojunction effect on the composites RGO/(CdS)1.0/(ZnO)0.4 (labeled RGO/(Zn/Cd0.4-IE)) and RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57 (labeled RGO/(Zn/Cd1.5-IE)) obtained by the Pechini method followed by sulfurization.
According to the XRD results, there is no significant difference between samples before (Fig. 6a) and after (Fig. 6b) irradiation. However, for sample RGO/(Zn/Cd1.5-IE), additional peaks appeared. Due to the small amount of RGO component (2%) in the composites and mechanical mixtures, we could identify only a small shoulder around 20° (2θ) in the samples RGO/(Zn/Cd0.4-IE) and RGO/(Zn/Cd1.5-IE), indicating a small stacking degree of RGO in the photocatalysts with the best performance.
The behavior of the two well-known prominent peaks related to graphene, the D band and G band, in the Raman measurements (Fig. 7c) confirms the reduction in GO to RGO . In addition to the increasing ratio ID/IG compared to the ratio for GO , a sp3/sp2 ratio reflex, it is possible to observe a small redshift in the G band, from 1599 to 1582 cm−1, especially for the highest performance photocatalysts, RGO/(Zn/Cd0.4-IE) and RGO/(Zn/Cd1.5-IE). This result is indicative of the restoration of the C=C sp2 bond [59, 60], as found in the graphene hexagonal lattice.
Reduced oxide graphene (RGO) plays an important role in the charge transfer process in photocatalytic reactions since it promotes spatial separation of charges and prevents recombination processes [61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73]. In addition, RGO can transfer electrons to adsorbed water molecules on its surface to produce hydrogen. Thus, a possible mechanism for this process is based on the oxidation of sulfide ions from tannery sludge by photogenerated holes on the ZnO surface, while molecular hydrogen is formed from the reduction in water on the RGO surface. Cadmium sulfide is the component responsible for generating charge carriers upon visible-light absorption. The scheme in Fig. 8 illustrates the proposed generation and charge transfer through the RGO/CdS/ZnO heterojunction.
The photocatalyst RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57 exhibits slightly lower photocatalytic activity than the photocatalyst RGO/(CdS)1.0/(ZnO)0.4 for hydrogen production (Figs. 4 and 5), which implies that the presence of a ZnS segregated phase, inactive under visible-light irradiation, decreases the number of visible-light-absorption centers that contribute to the photocatalytic activity.
Characterization of the liming bath wastewater before and after photocatalytic treatment with RGO/(CdS)1.0/(ZnO)0.4 and RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57
The sulfide content in the liming bath wastewater decreased sharply after photocatalytic treatment from 55.0 g L−1 in the untreated wastewater to 5.4 g L−1 when treated with RGO/(CdS)1.0/(ZnO)0.4 and 8.9 g L−1 when treated with RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57. The sulfide is partially converted into sulfate, and its content increased from 3.8 g L−1 in the untreated wastewater to 7.0 g L−1 when the photocatalyst used in the treatment was RGO/(CdS)1.0/(ZnO)0.4 and 6.4 g L−1 when the photocatalyst was RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57. These photocatalysts play a bifunctional role in the photocatalytic treatment of tannery wastewater in an oxygen-free atmosphere, mitigating 80–90% of the sulfide content with simultaneous hydrogen production. Because the photocatalytic treatment of the tannery sludge was conducted in an oxygen-free atmosphere to recover the energy contained in the waste, much of the organic matter remains in the liquid. Although a decrease in both BOD and COD has been observed, this treatment is not sufficient for total abatement of the organic matter present in the residue.
The characterization of the liming bath wastewater before and after photocatalytic treatment allowed the evaluation of the photocatalyst stability with respect to cadmium leaching (Tabela 3). Despite the slightly higher photocatalytic activity of the photocatalyst RGO/(CdS)1.0/(ZnO)0.4, the photocatalyst RGO/(CdS)1.0/(ZnO)0.73/(ZnS)0.57 is more eco-friendly because its cadmium content is lower and there is virtually no loss of this metal by leaching during photocatalytic treatment.
It is possible to conclude that the photocatalysts do not lose activity for at least three photocatalytic cycles. In this way, the halt in hydrogen production after 24 h of irradiation in the long-term reaction (Fig. 9) must be associated with the consumption of the sacrificial reagent (sulfide) instead of photocatalyst deactivation.
The results revealed that this synthesis procedure is capable of obtaining (CdS)x/(ZnO)y composites and that GO reduction takes place in situ during the photoreaction to produce composites such as RGO/(CdS)x/(ZnO)y. Among all photocatalysts tested in this work, the composite named Zn/Cd0.4-IE, with the empirical formula (CdS)1.0/(ZnO)0.4, in the presence of RGO was the most active in the treatment of liming bath wastewater from tanneries to produce hydrogen with simultaneous sulfide abatement. The best activity can be associated with the absence of segregated phases and the narrower bandgap energy, covering a large portion of the visible-light range. The presence of ZnS as a segregated phase in sample Zn/Cd1.5-IE, with the empirical formula (CdS)1.0/(ZnO)0.73/(ZnS)0.57, resulted in a slight decrease in the photocatalytic activity relative to (CdS)1.0/(ZnO)0.4, but this photocatalyst has the advantage of a lower cadmium content and does not suffer from leaching during photocatalytic treatment. In both cases, there is no loss in the photocatalytic activity over three photocatalytic cycles, demonstrating great potential for application in energy recovery from tanning waste with simultaneous abatement of sulfide, one of the main pollutants from leather processing.
The authors acknowledge the Brazilian research funding agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Grant Number 442840/2014-4), Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB, Grant Number APP0050/2016) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Finance Code 001) for financial support. The authors are also thankful to Laboratório Multi-Usuário de Microscopia Eletrônica da UFBA (LAMUME) for Raman and SEM analyses.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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