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Energy recovery from tannery sludge wastewaters through photocatalytic hydrogen production
Journal of Environmental Chemical Engineering 4 (2016) 2114–2120
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Energy recovery from tannery sludge wastewaters through photocatalytic hydrogen production Elisson A. Souzaa , Luciana A. Silvaa,b,* a b
Instituto de Química, Universidade Federal da Bahia, Campus de Ondina, 40170-155 Salvador, BA, Brazil Instituto Nacional de Ciência e Tecnologia, INCT, de Energia e Ambiente, Universidade Federal da Bahia, 40170-290 Salvador, BA, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 November 2015 Received in revised form 23 March 2016 Accepted 25 March 2016 Available online 26 March 2016
The presented work focuses on energy recovery through the photocatalytic conversion of sulfide-rich tannery sludge into hydrogen using CdS as a photocatalyst, platinum as a co-catalyst and visible light. Four reaction parameters were evaluated through a multivariate experimental design that included investigations of the mass of the photocatalyst (CdS), sacrificial reagent (tannery sludge) concentration, pH and amount of co-catalyst (Pt). The results demonstrated that the tannery sludge concentration and pH were the most important factors in producing the highest hydrogen levels. The strong interaction between these two factors is associated with the consumption of hydrogensulfide ions during the reaction, which can be replenished in basic medium. In contrast, the Pt content and mass of CdS were less relevant factors. The hydrogen production rate under optimal reaction conditions was only 12% lower than that obtained under simulated conditions after the first 5 h of irradiation. However, this rate decreased with longer reaction times. ã 2016 Published by Elsevier Ltd.
Keywords: Tannery sludge Energy recovery Hydrogen Photocatalysis Experimental design
1. Introduction Leather is a natural product with special features that arise from animal rearing conditions and the industrialization process. Animal hide gives unique characteristics to leather, making it irreplaceable. Brazil is one of the world’s leading leather producers and processes more than 40 million hides annually [1]. According to the Center of the Leather Industries of Brazil (CICB), the leather industry currently employs more than 50,000 workers throughout the country, but part of them are dedicated on actions aimed to mitigate the environmental damage that is currently caused by tanneries. It is estimated that the environmental impact of the leather industry is equivalent to the pollution generated by 1000–4000 Brazilian citizens for each ton of animal hide treated [2]. Thus, the use of new environmentally friendly technologies is necessary to alleviate these environmental effects. During the tanning process to produce wet blue leather, the hides are treated with chemical products such as sodium hydroxide, ammonium hydroxide, nonionic tenso-active
* Corresponding author at: Instituto de Química, Universidade Federal da Bahia, Campus de Ondina, 40170-155, Salvador, BA, Brazil. E-mail addresses: [email protected] (E.A. Souza), [email protected] (L.A. Silva). http://dx.doi.org/10.1016/j.jece.2016.03.040 2213-3437/ ã 2016 Published by Elsevier Ltd.
compounds, bactericides, proteolytic enzymes, hydrated lime, sodium sulfide, ammonium chlorite, ammonium sulfate, sulfuric acid, formic acid, and chromium salts to transform them into unalterable and imputrescible products [3–5]. Of these chemicals, Na2S causes the most discomfort in tanneries due to its characteristic odor. Na2S is used to remove hair from the hides and to destroy the epidermis by breaking the cysteine-disulfide bridges (keratolysis) via reductive division through a step known as liming. This process generates emissions with high chemical oxygen demands (COD), biological oxygen demands (BOD), and total suspended solid (TSS) loads in the resulting industry effluent [6]. Currently there are many physical, chemical and biological methods for treatment of waste streams [7–13]. Among these methods, it is worth mentioning photocatalysis due to its low cost, sustainability and high efficiency in the degradation of environmental hazardous substances [11,12]. In general, this process includes oxidation of organic species and reduction of inorganic. Most studies on the photocatalytic treatment of tannery waste investigate the advanced oxidation process of organics and the removal of hexavalent chromium via photoreduction [4,14–16]. On the other hand, the photocatalytic degradation of organic compounds under non-aerated conditions takes place with the simultaneous production of hydrogen over irradiated
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semiconductor photocatalyst suspensions [17]. Based on this principle, some studies have focused on energy recovery by photocatalytically converting waste into hydrogen [18–21]. It has been reported that photocatalytic activity of hexagonal CdS for hydrogen evolution is higher in the presence of sulfide ions instead of organic compounds [22]. Liming bath sludge is rich in sulfide, and therefore this wastewater can be used for photocatalytic hydrogen generation, since sulfide ions in aqueous solutions act as efficient hole scavengers when metal sulfidebased photocatalysts are employed [6,22,23]. In these systems, sulfide (S2) and hydrogensulfide (HS) are readily oxidized to SO42 and polysulfide ions (Sn2), such as S42 and S52, on hydrous CdS surfaces at pH 14. These polysulfide ions impart a yellow hue to the aqueous suspensions and may act as optical filters and effectively compete in the photoreduction of protons [22]. The formation of yellow polysulfide ions can be suppressed by adding sulfite ions to the reaction media to generate HS and thiosulfate (S2O32) [22]. In general, the presence of sulfide ions in a reaction medium increases the hydrogen production rate due to suppression of the recombination charge effect [22]. Moreover, the sulfide ions in solution stabilize CdS surfaces to eliminate surface defects created by photocorrosion. Several aspects regarding the surface chemistry of hydrous CdS have been studied previously [24–26]. Metal sulfides suspended in aqueous solutions have been shown to behave as diprotic acids, much like metal oxides. In the case of CdS prior to irradiation, hydroxyl and thiol (in protonated and deprotonated forms) groups are developed on photocatalyst surface and involved in the reaction equilibria that are strongly pH dependent (equations 1-4) [22,24]: >CdSH2+ Ð >CdSH + H+
(1)
>CdSH Ð >CdS + H+
(2)
>CdOH2+ Ð >CdOH + H+
(3)
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>CdOH Ð >CdO + H+
(4)
Bastos et al. [24] utilized an adsorption study to investigate which species were most prevalent on CdS surfaces under different pH and salinity conditions prior to irradiation. At a lower pH, the protonated functional groups were predominant (i.e., >CdSH2+at pH 4.1–4.3, more acid groups, and >CdOH2+ at pH 5.8–7.7), while a higher pH generated more of the deprotonated forms (i.e., >CdSat pH 7.3–7.7 and >CdOat pH 8.9–9.9). During irradiation, the groups attached to the surface acted as reversible charge-carrier traps in the primary steps of the photoelectrochemical mechanism. For solutions rich in sulfide ions, it was necessary to maintain a strongly basic medium to prevent the loss of sulfide ions as H2S. In this work, the feasibility of using wastewater from a leather tannery’sliming bath to generate hydrogen was evaluated. In this study, sludge was treated photocatalytically with visible light irradiation, under anaerobic conditions, using CdS as a photocatalyst and Pt as co-catalyst. In general, the addition of a noble metal, mainly Pt, improves the photocatalytic activity, especially for hydrogen-involving reactions [27]. The main purpose is to investigate the influence of the parameters that traditionally affect quantum yields in this reaction. A multivariate experimental design was employed to decrease the total number of experiments required and evaluate the interactions between the parameters. 2. Experimental 2.1. Chemicals All the reagents used in our experiments were of analytical purity and were used without further purification. Cadmium sulfide, CdS, and hexachloroplatinic acid solution, H2PtCl66H2O wt. 8%, were purchased from Aldrich.
Table 1 Experiment matrix and results obtained from the 2k factorial design. Exp.
hex-CdS (mg)
pH
Residue (% v/v)
H2PtCl66H2O (mg L1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 (CP)a 18 (CP) 19 (CP) 20 (CP) 21(CP)
+(120) + + + + + + + (60) 0(90) 0 0 0 0
+(13) + + + (9) + + + + 0(11) 0 0 0 0
+(50) + (10) + + + + + + 0(30) 0 0 0 0
+(20) (0) + + + + + + + 0(10) 0 0 0 0
nH2 (mmol)/ Irradiation time 1h
2h
3h
3.54 3.91 2.20 2.69 4.32 4.42 1.00 0.64 7.41 4.60 1.90 2.81 4.82 4.79 0.77 0.58 3.52 3.74 3.53 3.80 3.48
20.48 21.07 4.44 4.37 7.58 7.34 1.58 1.16 24.93 21.14 3.90 4.61 8.60 9.40 1.52 1.15 6.32 6.42 6.26 6.51 6.35
54.60 52.53 6.24 6.06 11.04 10.62 2.16 1.57 48.04 52.99 5.80 6.05 12.33 12.90 2.10 1.48 8.74 8.80 8.64 8.88 8.70
+: top level, 0: central point, and : lower level of the design. These signs are coded values, and the actual values are shown in brackets. a (CP): Central point.
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Hexagonal-phase CdS was prepared following a well-established procedure by heat-treating commercial-grade CdS (Aldrich) at 700 C under a nitrogen flow for one hour [22,24]. This material, named hex-CdS, was employed as the photocatalyst in all the photocatalytic experiments.
in situ on the photocatalyst surface by the photodecomposition of [PtCl6]2. Platinum nanoparticles act as electron traps and are responsible for minimizing the recombination of photogenerated charge effects, thereby playing a fundamental role in the electron transfer process of the protons adsorbed on the surface. Thus, hydrogen production rates were expected to increase with an increase in the amount of the Pt co-catalyst.
2.3. Photocatalyst characterization
2.6. Photocatalytic tests
Hexagonal CdS powder, before and after reaction, was characterized by X-ray diffraction (Shimadzu XRD6000) using CuKa, Ni-filtered radiation, and a scanning rate of 2 2u min1 in a 2u range of 10–80 at 35 kV and 15 mA.
For each test, the photocatalyst was dispersed in 60 mL of an aqueous solution containing a predetermined concentration of the liming bath sludge, which was pH adjusted by adding NaOH or HCl solutions. However, an appropriate amount of H2PtCl66H2O wt. 8% was added before any pH adjustments. Platinum was deposited on the photocatalyst surface in situ by the photodecomposition of [PtCl6]2 from hexachloroplatinic acid. Subsequent, 1 mL aliquots of the gas phase were injected into the gas chromatography (GC) system in one-hour intervals. In order to ensure the accurate determination and quantification of hydrogen production during the reaction, a 5% (v/v) H2 standard diluted in argon was injected before each experiment. A high-pressure 500 W Hg–Xe arc lamp was used as the light source for the photocatalytic reactions. The collimated light beam was passed through an IR filter, a focus lens and a 418 nm cutoff filter before reaching the photocatalytic cell, which was air cooled to maintain a constant temperature. Before each experiment, the photocatalytic cell was purged with argon for 30 min to eliminate traces of O2. The photocatalytic cell was equipped with a flat window and inlet/outlet tubes, which served to collect and transfer gaseous products to the analytical system. The evolution of hydrogen gas was measured by gas chromatography using a Shimadzu (GC2014) instrument in conjunction with a thermal conductivity detector (TCD). Because He and H2 have similar conductivities, argon was used as a carrier gas. The results were expressed as amount of matter of H2 (nH2) produced over the 3-h irradiation process. The hydrogen production rate was calculated as follows:
2.2. Photocatalyst preparation
2.4. Sacrificial reactant The liming bath sludge, which was used as the sacrificial reactant, was provided by a tannery located in Petrolina-PE, Brazil. According to the analysis report provided by the tannery, the liming bath sludge presents a content of total sulfur of 73 g kg1 (2.2 mol L1) and of total organic carbon of 430 g kg1. The material was packaged in plastic bottles and kept in the refrigerator until their use in the photocatalytic tests. The aqueous tannery waste solutions were prepared by removing the supernatant upon decantation of putrescible materials. Total sulfide, before and after irradiation, was also determined by iodimetric method of analysis using sodium thiosulfate (for details see reference [28]). 2.5. Multivariate experimental design A 2k factorial design was performed using Statistica 10.0 software to evaluate the four reaction parameters that traditionally influence photocatalytic hydrogen production: the mass of the photocatalyst, pH, concentration of the sacrificial reactant, and amount of co-catalyst. The last parameter was determined by the concentration of the platinum precursor solution, H2PtCl66H2O. This resulted in a matrix with sixteen experiments and five central points (total 21 experiments) with the amount of hydrogen (n) produced during reaction as the response (Table 1). The (+) sign represents the top level, (0) represents the central point, and the () sign represents the lower level of the design. These signs are coded values, and the actual values are shown in brackets. The multivariate experimental design evaluated the mass of the photocatalyst in the experimental domain comprising the levels represented by 60 mg () and 120 mg (+). In general, an increase in the mass of the photocatalyst was expected to increase hydrogen production levels in function of increasing absorption centers, which would increase the number of photogenerated charge carriers. Charge carriers are fundamental for redox processes because they are responsible for the formation of hydrogen and the consumption of sulfide ions. The pH was evaluated in an experimental domain comprising levels represented by 9 () and 13 (+). In this case, the best results were expected to be found in the most basic solutions because these conditions minimize H2S formation and consequently the loss of sulfide ions (sacrificial reagent). Tannery sludge wastewater, represented by residue concentrations (% v/v), was evaluated at a range between 10% () and 50% (+). An increase in hydrogen production was expected with increasing concentrations, since the amount of residue is proportional to the amount of sulfide ions (sacrificial reagent). The effects of Pt co-catalyst quantities were evaluated taking into account the concentration of the platinum source, hexachloroplatinic acid solution, in a range between 0 () to 20 mg L1 (+). Metallic platinum was deposited
H2 ðmmolh
1 1
g
Þ¼
nH2 ðmmolÞ tðhÞ mCdS ðgÞ
ð5Þ
2.7. Stability tests After optimizing the reaction parameters, the photostability of CdS in the reaction medium was evaluated over the 30-h irradiation period. For this photostability test, CO and CO2 gases were monitored simultaneously with hydrogen, which was possible since the GC was also equipped with a flame ionization detector (FID) with a methanator. For this test, a 2% (v/v) CO2 and 500 ppm CO standard diluted in argon was injected in the GC. We also evaluated the stability of the liming baths by exposing them to room-temperature atmospheric conditions. 2.8. Surface chemistry studies The study of the CdS surface chemistry in an aqueous solution containing tannery sludge was conducted using an adsorption method [24,29,30]. A series of seven flasks containing 10 mL of the tannery sludge solution (50%) had their pH levels adjusted to a range of 8–13.3 by adding HCl or NaOH solutions. After adjusting the initial pH of the solutions, 0.040 g of hex-CdS was added to each flask. The suspensions were then manually shaken and allowed to equilibrate for 24 h. Afterwards, the pH values of the supernatants were measured. The differences between the initial and final pH values (DpH) were plotted against the initial pH levels. The point of
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Fig. 1. XRD patterns of the obtained material and compares it with ICSD patterns: (A) ICSD patterns for the hexagonal CdS phase (pdf # 01-075-1545-41-1049); (B) commercial CdS calcinated at 700 C and (*) CdO (pdf # 01-073-2245-5-640).
intersection of the resulting null pH corresponds to the zero point of charge, pHZPC. 3. Results and discussion After the heat-treatment of commercial CdS, the resulting material was submitted to X-ray diffraction analysis, which confirmed the hexagonal phase formation. Fig. 1 shows the XRD patterns of the obtained material and compares it with ICSD patterns. It was possible to confirm the predominant presence of hex-CdS wurtzite structure (pdf # 01-075-1545-41-1049). The low intensity peaks were associated with CdO (pdf # 01-073-2245-5640) as the contaminant phase. This material was used as the photocatalyst in all the photocatalytic tests. Table 1 shows the matrix resulted from the factorial design with the amount of hydrogen produced in the reaction as the response. The analysis presented in Table 1 demonstrates that experiment 10 had the best conditions of those evaluated, and had a maximum hydrogen production rate of 294.38 mmol g1cat h1. It is worth noting that the hydrogen production rate found in our previous studies under controlled laboratory conditions was 332 mmol g1cat h1 [22], in which hex-CdS was used as the photocatalyst and a sulfide/sulfite system, under the basic conditions, was the sacrificial reagent. Therefore, the reaction conditions of experiment 10 generated a hydrogen production rate that was only 12% lower than that obtained in laboratory-simulated conditions. This result opens the possibility of investing in this process at a large scale. In order to identify significant factors and their interactions during the reaction, Pareto charts were generated for each of the time points with 95% confidence (p < 0.05) (Fig. 2). The analysis of the effects and interactions by Pareto charts revealed that hydrogen generation levels were closely related to increases in tannery residue concentrations, which acted as the sacrificial reagent; increases in pH further enhanced the production of hydrogen. As observed from initial stages of the reaction, the significance of pH levels increased throughout the course of the reaction. This is associated with the strong dependence of the equilibria on S(-II) species in aqueous solution and pH levels (Eqs. (6)–(9)) [31,32].
Fig. 2. Pareto charts for each of the time points during the reaction to identify significant factors and their interactions, with 95% confidence (p < 0.05).
H2S(g) Ð H2S(aq) pK = 0.99
(6)
H2S(aq) Ð HS(aq) + H+(aq) pKa1 = 6.88
(7)
HS(aq) Ð S2(aq) + H+(aq) pKa2 = 14.15
(8)
S2(aq) + H2O(l) Ð HS(aq) + OH(aq) pK = 0 (NaOH 8 mol L1)(9)
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[PtCl6]
2
(aq) + 2e (BC) ! [PtCl4]
2
(aq) + 2Cl (aq)
[PtCl4]2(aq) + S2(aq) ! PtS(s) + 4Cl(aq) (basic pH)
(10)
(11)
Platinum(II) sulfide formation was confirmed by XRD analysis of the photocatalyst after a reaction conducted in conditions described in experiment 7 (Fig. 3). In such experiment, the highest concentration of platinum source was used and a low hydrogen production was observed, since platinum(II) sulfide is inactive as a co-catalyst and as an electron trap. However, it should be noted that the absence of platinum does not suppress the photocatalytic generation of hydrogen, as can be seen in experiment 10; rather, its removal reduces process costs. The XRD analysis also reveals that the diffraction peaks associated with the CdO phase in the fresh photocatalyst disappear after the reaction due to the replacement of oxide per sulfide ions present in residue solution, eliminating surface defects.
Fig. 3. XRD patterns of the fresh photocatalyst and after reaction (Experiment 7) and comparison with ICSD patterns.
1 0.8 0.6 0.4 ΔpH
According to the equilibrium equations presented above, the aqueous solutions predominantly contain HS ions due to the low second dissociation constant of H2S. The alkaline medium contributes to the stabilization of these species in solution and prevents the loss of H2S. The strong correlation between the tannery residue concentration and pH over the three-hour irradiation process presented in the Pareto charts was also associated with the consumption of hydrogensulfide ions during the reaction, which could be replenished in the basic medium. The Pareto charts in Fig. 2 also demonstrate that the concentration of H2PtCl66H2O was not important during the first hour of the reaction, but had significant interactions with the parameters residue concentration and mass of CdS. The insignificance of this factor during the first hour of the reaction may be associated with characteristic induction times for similar systems, which are approximately 1–2 h [33]. During this period, it is known that Pt(IV) is photoreduced to Pt(0) with the assistance of the irradiated semiconductor and a sacrificial reagent [22]. In this study, the concentration of H2PtCl66H2O was significant only in the second hour of the reaction, suggesting that the photoreduction of Pt(IV) took place during this period. In the third hour of the reaction this factor was no longer significant, since during the second hour of the reaction PtIIS was deposited on CdS surface instead of Pt(0) [22,26], as suggested by following the equations:
0.2 0
8
9
10
11
12
13
-0.2 -0.4 -0.6 -0.8
pHinial
Fig. 4. Adsorption equilibrium of CdS-liming sludge solution as a function of initial pH.
A study of the surface chemistry of CdS suspended in the tannery residue solution (50% residue) was conducted using the adsorption method to identify species that acted as charge traps on hydrous CdS surface under reaction conditions and to investigate their pH dependence (Fig. 4). In this system, thiol groups were more likely to prevail on the CdS surface than hydroxyl groups, as the tannery sludge solution was rich in sulfide ions. With respect to this observation, the equilibria described in equations (1) and (2) are the most important. For solutions with pH levels of 8–13, the zero charge pH was identified as 9.3, where the neutral species > CdSH prevailed. In a pH range 8–9, the curve profile suggested the existence of positive surface charges, indicating the presence of the protonated species >CdSH2+, though it was not the predominant species. At pH 10, the maximum concentration of negative surface charges could be identified; the surface was deprotonated and >CdS was the predominant species in the equilibrium. Above pH 10, the surface became electrically neutral again, suggesting that the negative species, >CdS, attracted cations (M+) in the tannery residue solution to form neutral species such as >CdSM. The zero charge on the surface allowed the interaction of negative species such as sulfide and hydrogensulfide ions, which acted as hole scavengers.
Fig. 5. Hydrogen production profile in a long-term reaction under the following conditions: 60 mg of hex-CdS, 50% residue, pH 13 without the addition of H2PtCl66H2O.
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tanning residue using solar power to produce hydrogen. From the obtained results, it can be concluded that the tannery waste concentration and pH are the main factors responsible for maximum hydrogen production rates. In contrast, the addition of platinum source decreases the hydrogen production because it is deposited on CdS surface as platinum sulfide, which is inactive as a co-catalyst. Stability tests also indicated that CdS was deactivated after 27 h of irradiation and the residue lost its efficiency as a sacrificial reagent when exposed to air for long durations at room temperature. In summary, our results open up perspective to recover an appreciable amount of energy from tannery sludge wastewaters through solar photocatalytic production of hydrogen. Acknowledgements
Fig. 6. Comparative hydrogen production profile: residue sample in contact with air for a month (a) and refrigerated residue sample (b).
The authors wish to thank the Brazilian research funding agencies(CNPq, CAPES and FAPESB) for financial support and Professor Sérgio L. C. Ferreira for helpful discussions. References
The results of the multivariate experimental design suggest that experiment 10 generated optimal parameters (60 mg CdS, 50% residue, pH 13 without H2PtCl66H2O addition). Upon establishing the best reaction condition, a test was performed to evaluate the stability of the photocatalyst over the 30-h irradiation period (Fig. 5). A loss of photocatalytic activity of CdS occurred after 27 h of irradiation, when the hydrogen production ceased at 184 mmol, which is equivalent to 3.07 mmol g1cat. In this long-term experiment, no CO or CO2 signals were detected, despite the high content of total organic carbon in tannery sludge, noting that organic species were not photooxidized in appreciable amounts. On the other hand, the sulfide content in the residue determined by iodimetric method decreases considerably, from 2.0 mol L1 to 0.48 mol L1, after 30-h irradiation. These results reinforce the idea that the sulfite ions act as hole scavengers in hydrogen production when tannery residue is photocatalytically treated in an oxygen free environment. According tannery data, 1 kg of animal hide processed in a liming bath generates 10 L of waste residue. If photocatalytic treatment were to be applied to this volume of waste, 1.5 L of hydrogen gas (STP) would be generated under the optimal reaction conditions identified in this study. The amount of energy recovered by this process would be substantial, as the energy supply necessary for conversion could be obtained from solar radiation and more than 40 million hides per year could be treated in Brazil alone [1]. In order to evaluate the long-term stability of the liming sludge, a residue sample was removed from the refrigerator and exposed to the atmosphere for a month at room temperature. The photocatalytic test to evaluate the stability of the residue was carried out under the same conditions as previous tests and the results are shown in Fig. 6. There was a decrease in photocatalytic activity of approximately 45% after residue exposure to atmospheric air at room temperature. These results indicate that most of the species that acted as sacrificial reagents were lost by airinduced oxidation. Thus, for use in the photocatalytic generation of hydrogen, the immediate use of the residue after its generation is recommended; otherwise, samples should be refrigerated until use in the reactions. 4. Conclusion Our findings demonstrate the feasibility of energy recovery from a pollutant through the photocatalytic treatment of leather
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[21] E.C. Su, B.S. Huang, C.C. Liu, M.Y. Wey, Photocatalytic conversion of simulated EDTA wastewater to hydrogen by pH-resistant Pt/TiO2-activated carbon photocatalysts, Renewable Energy 75 (2015) 266–271. [22] L.A. Silva, S.Y. Ryu, J. Choi, W. Choi, M.R. Hoffmann, Photocatalytic hydrogen production with visible light over Pt-interlinked hybrid composites of cubicphase and hexagonal-phase CdS, J. Phys. Chem. C 112 (2008) 12069–12073. [23] I. Tsuji, H. Kato, H. Kobayashi, A. Kudo, Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (AgIn)xZn2(1x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures, J. Am. Chem. Soc. 126 (2004) 13406–13413. [24] S.A.L. Bastos, F.N. Santos, P.A.L. Lopes, L.A. Silva, Experimental design as a tool to study the reaction parameters in hydrogen production from photoinduced reforming of glycerol over CdS photocatalyst, Int. J. Hydrogen Energy 39 (2014) 14588–14595. [25] S.W. Park, C.P. Huang, The surface acidity of hydrous CdS(s), J. Colloid Interface Sci. 117 (1987) 431–441. [26] Z.S. Jin, Q.L. Li, X.H. Zheng, C.J. Xi, C.P. Wang, H.Q. Zhang, L.B. Feng, H.Q. Wang, Z. S. Chen, Z.C. Jiang, Surface properties of Pt–CdS and mechanism of photocatalytic dehydrogenation of aqueous alcohol, J. Photochem. Photobiol. A 71 (1993) 85–96.
[27] J.M. Herrmann, Fundamentals and misconceptions in photocatalysis, J. Phptochem. Photobio. A 216 (2010) 85–93. [28] J. Mendham, R.C. Denney, J.D. Barnes, M.J.K. Thomas, Vogel’s Textbook of Quantitative Chemical Analysis, 6th edition, Prentice Hall, 2000. [29] D.L.S. Maia, I. Pepe, A. Ferreira da Silva, L.A. Silva, Visible-light-driven photocatalytic hydrogen production over dye-sensitized b-BiTaO4, J. Photochem. Photobiol. A 243 (2012) 61–64. [30] A.P. Vieira, S.A.A. Santana, C.W.B. Bezerra, H.A.S. Silva, J.A.P. Chaves, J.C.P. de Melo, E.C. da Silva Filho, C. Airoldi, Kinetics and thermodynamics of textile dye adsorption from aqueous solutions using babassu coconut mesocarp, J. Hazard. Mater. 166 (2009) 1272–1278. [31] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, 6th edition, John Wiley & Sons Inc., 1999. [32] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd edition, Heinemann, Butterworth, 1997. [33] M.O. Melo, L.A. Silva, Visible light-induced hydrogen production from glycerol aqueous solution on hybrid Pt–CdS–TiO2 photocatalysts, J. Photochem. Photobiol. A 226 (2011) 36–41.
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Energy recovery from tannery sludge wastewaters through photocatalytic hydrogen production Elisson A. Souzaa , Luciana A. Silvaa,b,* a b
Instituto de Química, Universidade Federal da Bahia, Campus de Ondina, 40170-155 Salvador, BA, Brazil Instituto Nacional de Ciência e Tecnologia, INCT, de Energia e Ambiente, Universidade Federal da Bahia, 40170-290 Salvador, BA, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 16 November 2015 Received in revised form 23 March 2016 Accepted 25 March 2016 Available online 26 March 2016
The presented work focuses on energy recovery through the photocatalytic conversion of sulfide-rich tannery sludge into hydrogen using CdS as a photocatalyst, platinum as a co-catalyst and visible light. Four reaction parameters were evaluated through a multivariate experimental design that included investigations of the mass of the photocatalyst (CdS), sacrificial reagent (tannery sludge) concentration, pH and amount of co-catalyst (Pt). The results demonstrated that the tannery sludge concentration and pH were the most important factors in producing the highest hydrogen levels. The strong interaction between these two factors is associated with the consumption of hydrogensulfide ions during the reaction, which can be replenished in basic medium. In contrast, the Pt content and mass of CdS were less relevant factors. The hydrogen production rate under optimal reaction conditions was only 12% lower than that obtained under simulated conditions after the first 5 h of irradiation. However, this rate decreased with longer reaction times. ã 2016 Published by Elsevier Ltd.
Keywords: Tannery sludge Energy recovery Hydrogen Photocatalysis Experimental design
1. Introduction Leather is a natural product with special features that arise from animal rearing conditions and the industrialization process. Animal hide gives unique characteristics to leather, making it irreplaceable. Brazil is one of the world’s leading leather producers and processes more than 40 million hides annually [1]. According to the Center of the Leather Industries of Brazil (CICB), the leather industry currently employs more than 50,000 workers throughout the country, but part of them are dedicated on actions aimed to mitigate the environmental damage that is currently caused by tanneries. It is estimated that the environmental impact of the leather industry is equivalent to the pollution generated by 1000–4000 Brazilian citizens for each ton of animal hide treated [2]. Thus, the use of new environmentally friendly technologies is necessary to alleviate these environmental effects. During the tanning process to produce wet blue leather, the hides are treated with chemical products such as sodium hydroxide, ammonium hydroxide, nonionic tenso-active
* Corresponding author at: Instituto de Química, Universidade Federal da Bahia, Campus de Ondina, 40170-155, Salvador, BA, Brazil. E-mail addresses: [email protected] (E.A. Souza), [email protected] (L.A. Silva). http://dx.doi.org/10.1016/j.jece.2016.03.040 2213-3437/ ã 2016 Published by Elsevier Ltd.
compounds, bactericides, proteolytic enzymes, hydrated lime, sodium sulfide, ammonium chlorite, ammonium sulfate, sulfuric acid, formic acid, and chromium salts to transform them into unalterable and imputrescible products [3–5]. Of these chemicals, Na2S causes the most discomfort in tanneries due to its characteristic odor. Na2S is used to remove hair from the hides and to destroy the epidermis by breaking the cysteine-disulfide bridges (keratolysis) via reductive division through a step known as liming. This process generates emissions with high chemical oxygen demands (COD), biological oxygen demands (BOD), and total suspended solid (TSS) loads in the resulting industry effluent [6]. Currently there are many physical, chemical and biological methods for treatment of waste streams [7–13]. Among these methods, it is worth mentioning photocatalysis due to its low cost, sustainability and high efficiency in the degradation of environmental hazardous substances [11,12]. In general, this process includes oxidation of organic species and reduction of inorganic. Most studies on the photocatalytic treatment of tannery waste investigate the advanced oxidation process of organics and the removal of hexavalent chromium via photoreduction [4,14–16]. On the other hand, the photocatalytic degradation of organic compounds under non-aerated conditions takes place with the simultaneous production of hydrogen over irradiated
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semiconductor photocatalyst suspensions [17]. Based on this principle, some studies have focused on energy recovery by photocatalytically converting waste into hydrogen [18–21]. It has been reported that photocatalytic activity of hexagonal CdS for hydrogen evolution is higher in the presence of sulfide ions instead of organic compounds [22]. Liming bath sludge is rich in sulfide, and therefore this wastewater can be used for photocatalytic hydrogen generation, since sulfide ions in aqueous solutions act as efficient hole scavengers when metal sulfidebased photocatalysts are employed [6,22,23]. In these systems, sulfide (S2) and hydrogensulfide (HS) are readily oxidized to SO42 and polysulfide ions (Sn2), such as S42 and S52, on hydrous CdS surfaces at pH 14. These polysulfide ions impart a yellow hue to the aqueous suspensions and may act as optical filters and effectively compete in the photoreduction of protons [22]. The formation of yellow polysulfide ions can be suppressed by adding sulfite ions to the reaction media to generate HS and thiosulfate (S2O32) [22]. In general, the presence of sulfide ions in a reaction medium increases the hydrogen production rate due to suppression of the recombination charge effect [22]. Moreover, the sulfide ions in solution stabilize CdS surfaces to eliminate surface defects created by photocorrosion. Several aspects regarding the surface chemistry of hydrous CdS have been studied previously [24–26]. Metal sulfides suspended in aqueous solutions have been shown to behave as diprotic acids, much like metal oxides. In the case of CdS prior to irradiation, hydroxyl and thiol (in protonated and deprotonated forms) groups are developed on photocatalyst surface and involved in the reaction equilibria that are strongly pH dependent (equations 1-4) [22,24]: >CdSH2+ Ð >CdSH + H+
(1)
>CdSH Ð >CdS + H+
(2)
>CdOH2+ Ð >CdOH + H+
(3)
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>CdOH Ð >CdO + H+
(4)
Bastos et al. [24] utilized an adsorption study to investigate which species were most prevalent on CdS surfaces under different pH and salinity conditions prior to irradiation. At a lower pH, the protonated functional groups were predominant (i.e., >CdSH2+at pH 4.1–4.3, more acid groups, and >CdOH2+ at pH 5.8–7.7), while a higher pH generated more of the deprotonated forms (i.e., >CdSat pH 7.3–7.7 and >CdOat pH 8.9–9.9). During irradiation, the groups attached to the surface acted as reversible charge-carrier traps in the primary steps of the photoelectrochemical mechanism. For solutions rich in sulfide ions, it was necessary to maintain a strongly basic medium to prevent the loss of sulfide ions as H2S. In this work, the feasibility of using wastewater from a leather tannery’sliming bath to generate hydrogen was evaluated. In this study, sludge was treated photocatalytically with visible light irradiation, under anaerobic conditions, using CdS as a photocatalyst and Pt as co-catalyst. In general, the addition of a noble metal, mainly Pt, improves the photocatalytic activity, especially for hydrogen-involving reactions [27]. The main purpose is to investigate the influence of the parameters that traditionally affect quantum yields in this reaction. A multivariate experimental design was employed to decrease the total number of experiments required and evaluate the interactions between the parameters. 2. Experimental 2.1. Chemicals All the reagents used in our experiments were of analytical purity and were used without further purification. Cadmium sulfide, CdS, and hexachloroplatinic acid solution, H2PtCl66H2O wt. 8%, were purchased from Aldrich.
Table 1 Experiment matrix and results obtained from the 2k factorial design. Exp.
hex-CdS (mg)
pH
Residue (% v/v)
H2PtCl66H2O (mg L1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 (CP)a 18 (CP) 19 (CP) 20 (CP) 21(CP)
+(120) + + + + + + + (60) 0(90) 0 0 0 0
+(13) + + + (9) + + + + 0(11) 0 0 0 0
+(50) + (10) + + + + + + 0(30) 0 0 0 0
+(20) (0) + + + + + + + 0(10) 0 0 0 0
nH2 (mmol)/ Irradiation time 1h
2h
3h
3.54 3.91 2.20 2.69 4.32 4.42 1.00 0.64 7.41 4.60 1.90 2.81 4.82 4.79 0.77 0.58 3.52 3.74 3.53 3.80 3.48
20.48 21.07 4.44 4.37 7.58 7.34 1.58 1.16 24.93 21.14 3.90 4.61 8.60 9.40 1.52 1.15 6.32 6.42 6.26 6.51 6.35
54.60 52.53 6.24 6.06 11.04 10.62 2.16 1.57 48.04 52.99 5.80 6.05 12.33 12.90 2.10 1.48 8.74 8.80 8.64 8.88 8.70
+: top level, 0: central point, and : lower level of the design. These signs are coded values, and the actual values are shown in brackets. a (CP): Central point.
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Hexagonal-phase CdS was prepared following a well-established procedure by heat-treating commercial-grade CdS (Aldrich) at 700 C under a nitrogen flow for one hour [22,24]. This material, named hex-CdS, was employed as the photocatalyst in all the photocatalytic experiments.
in situ on the photocatalyst surface by the photodecomposition of [PtCl6]2. Platinum nanoparticles act as electron traps and are responsible for minimizing the recombination of photogenerated charge effects, thereby playing a fundamental role in the electron transfer process of the protons adsorbed on the surface. Thus, hydrogen production rates were expected to increase with an increase in the amount of the Pt co-catalyst.
2.3. Photocatalyst characterization
2.6. Photocatalytic tests
Hexagonal CdS powder, before and after reaction, was characterized by X-ray diffraction (Shimadzu XRD6000) using CuKa, Ni-filtered radiation, and a scanning rate of 2 2u min1 in a 2u range of 10–80 at 35 kV and 15 mA.
For each test, the photocatalyst was dispersed in 60 mL of an aqueous solution containing a predetermined concentration of the liming bath sludge, which was pH adjusted by adding NaOH or HCl solutions. However, an appropriate amount of H2PtCl66H2O wt. 8% was added before any pH adjustments. Platinum was deposited on the photocatalyst surface in situ by the photodecomposition of [PtCl6]2 from hexachloroplatinic acid. Subsequent, 1 mL aliquots of the gas phase were injected into the gas chromatography (GC) system in one-hour intervals. In order to ensure the accurate determination and quantification of hydrogen production during the reaction, a 5% (v/v) H2 standard diluted in argon was injected before each experiment. A high-pressure 500 W Hg–Xe arc lamp was used as the light source for the photocatalytic reactions. The collimated light beam was passed through an IR filter, a focus lens and a 418 nm cutoff filter before reaching the photocatalytic cell, which was air cooled to maintain a constant temperature. Before each experiment, the photocatalytic cell was purged with argon for 30 min to eliminate traces of O2. The photocatalytic cell was equipped with a flat window and inlet/outlet tubes, which served to collect and transfer gaseous products to the analytical system. The evolution of hydrogen gas was measured by gas chromatography using a Shimadzu (GC2014) instrument in conjunction with a thermal conductivity detector (TCD). Because He and H2 have similar conductivities, argon was used as a carrier gas. The results were expressed as amount of matter of H2 (nH2) produced over the 3-h irradiation process. The hydrogen production rate was calculated as follows:
2.2. Photocatalyst preparation
2.4. Sacrificial reactant The liming bath sludge, which was used as the sacrificial reactant, was provided by a tannery located in Petrolina-PE, Brazil. According to the analysis report provided by the tannery, the liming bath sludge presents a content of total sulfur of 73 g kg1 (2.2 mol L1) and of total organic carbon of 430 g kg1. The material was packaged in plastic bottles and kept in the refrigerator until their use in the photocatalytic tests. The aqueous tannery waste solutions were prepared by removing the supernatant upon decantation of putrescible materials. Total sulfide, before and after irradiation, was also determined by iodimetric method of analysis using sodium thiosulfate (for details see reference [28]). 2.5. Multivariate experimental design A 2k factorial design was performed using Statistica 10.0 software to evaluate the four reaction parameters that traditionally influence photocatalytic hydrogen production: the mass of the photocatalyst, pH, concentration of the sacrificial reactant, and amount of co-catalyst. The last parameter was determined by the concentration of the platinum precursor solution, H2PtCl66H2O. This resulted in a matrix with sixteen experiments and five central points (total 21 experiments) with the amount of hydrogen (n) produced during reaction as the response (Table 1). The (+) sign represents the top level, (0) represents the central point, and the () sign represents the lower level of the design. These signs are coded values, and the actual values are shown in brackets. The multivariate experimental design evaluated the mass of the photocatalyst in the experimental domain comprising the levels represented by 60 mg () and 120 mg (+). In general, an increase in the mass of the photocatalyst was expected to increase hydrogen production levels in function of increasing absorption centers, which would increase the number of photogenerated charge carriers. Charge carriers are fundamental for redox processes because they are responsible for the formation of hydrogen and the consumption of sulfide ions. The pH was evaluated in an experimental domain comprising levels represented by 9 () and 13 (+). In this case, the best results were expected to be found in the most basic solutions because these conditions minimize H2S formation and consequently the loss of sulfide ions (sacrificial reagent). Tannery sludge wastewater, represented by residue concentrations (% v/v), was evaluated at a range between 10% () and 50% (+). An increase in hydrogen production was expected with increasing concentrations, since the amount of residue is proportional to the amount of sulfide ions (sacrificial reagent). The effects of Pt co-catalyst quantities were evaluated taking into account the concentration of the platinum source, hexachloroplatinic acid solution, in a range between 0 () to 20 mg L1 (+). Metallic platinum was deposited
H2 ðmmolh
1 1
g
Þ¼
nH2 ðmmolÞ tðhÞ mCdS ðgÞ
ð5Þ
2.7. Stability tests After optimizing the reaction parameters, the photostability of CdS in the reaction medium was evaluated over the 30-h irradiation period. For this photostability test, CO and CO2 gases were monitored simultaneously with hydrogen, which was possible since the GC was also equipped with a flame ionization detector (FID) with a methanator. For this test, a 2% (v/v) CO2 and 500 ppm CO standard diluted in argon was injected in the GC. We also evaluated the stability of the liming baths by exposing them to room-temperature atmospheric conditions. 2.8. Surface chemistry studies The study of the CdS surface chemistry in an aqueous solution containing tannery sludge was conducted using an adsorption method [24,29,30]. A series of seven flasks containing 10 mL of the tannery sludge solution (50%) had their pH levels adjusted to a range of 8–13.3 by adding HCl or NaOH solutions. After adjusting the initial pH of the solutions, 0.040 g of hex-CdS was added to each flask. The suspensions were then manually shaken and allowed to equilibrate for 24 h. Afterwards, the pH values of the supernatants were measured. The differences between the initial and final pH values (DpH) were plotted against the initial pH levels. The point of
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Fig. 1. XRD patterns of the obtained material and compares it with ICSD patterns: (A) ICSD patterns for the hexagonal CdS phase (pdf # 01-075-1545-41-1049); (B) commercial CdS calcinated at 700 C and (*) CdO (pdf # 01-073-2245-5-640).
intersection of the resulting null pH corresponds to the zero point of charge, pHZPC. 3. Results and discussion After the heat-treatment of commercial CdS, the resulting material was submitted to X-ray diffraction analysis, which confirmed the hexagonal phase formation. Fig. 1 shows the XRD patterns of the obtained material and compares it with ICSD patterns. It was possible to confirm the predominant presence of hex-CdS wurtzite structure (pdf # 01-075-1545-41-1049). The low intensity peaks were associated with CdO (pdf # 01-073-2245-5640) as the contaminant phase. This material was used as the photocatalyst in all the photocatalytic tests. Table 1 shows the matrix resulted from the factorial design with the amount of hydrogen produced in the reaction as the response. The analysis presented in Table 1 demonstrates that experiment 10 had the best conditions of those evaluated, and had a maximum hydrogen production rate of 294.38 mmol g1cat h1. It is worth noting that the hydrogen production rate found in our previous studies under controlled laboratory conditions was 332 mmol g1cat h1 [22], in which hex-CdS was used as the photocatalyst and a sulfide/sulfite system, under the basic conditions, was the sacrificial reagent. Therefore, the reaction conditions of experiment 10 generated a hydrogen production rate that was only 12% lower than that obtained in laboratory-simulated conditions. This result opens the possibility of investing in this process at a large scale. In order to identify significant factors and their interactions during the reaction, Pareto charts were generated for each of the time points with 95% confidence (p < 0.05) (Fig. 2). The analysis of the effects and interactions by Pareto charts revealed that hydrogen generation levels were closely related to increases in tannery residue concentrations, which acted as the sacrificial reagent; increases in pH further enhanced the production of hydrogen. As observed from initial stages of the reaction, the significance of pH levels increased throughout the course of the reaction. This is associated with the strong dependence of the equilibria on S(-II) species in aqueous solution and pH levels (Eqs. (6)–(9)) [31,32].
Fig. 2. Pareto charts for each of the time points during the reaction to identify significant factors and their interactions, with 95% confidence (p < 0.05).
H2S(g) Ð H2S(aq) pK = 0.99
(6)
H2S(aq) Ð HS(aq) + H+(aq) pKa1 = 6.88
(7)
HS(aq) Ð S2(aq) + H+(aq) pKa2 = 14.15
(8)
S2(aq) + H2O(l) Ð HS(aq) + OH(aq) pK = 0 (NaOH 8 mol L1)(9)
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[PtCl6]
2
(aq) + 2e (BC) ! [PtCl4]
2
(aq) + 2Cl (aq)
[PtCl4]2(aq) + S2(aq) ! PtS(s) + 4Cl(aq) (basic pH)
(10)
(11)
Platinum(II) sulfide formation was confirmed by XRD analysis of the photocatalyst after a reaction conducted in conditions described in experiment 7 (Fig. 3). In such experiment, the highest concentration of platinum source was used and a low hydrogen production was observed, since platinum(II) sulfide is inactive as a co-catalyst and as an electron trap. However, it should be noted that the absence of platinum does not suppress the photocatalytic generation of hydrogen, as can be seen in experiment 10; rather, its removal reduces process costs. The XRD analysis also reveals that the diffraction peaks associated with the CdO phase in the fresh photocatalyst disappear after the reaction due to the replacement of oxide per sulfide ions present in residue solution, eliminating surface defects.
Fig. 3. XRD patterns of the fresh photocatalyst and after reaction (Experiment 7) and comparison with ICSD patterns.
1 0.8 0.6 0.4 ΔpH
According to the equilibrium equations presented above, the aqueous solutions predominantly contain HS ions due to the low second dissociation constant of H2S. The alkaline medium contributes to the stabilization of these species in solution and prevents the loss of H2S. The strong correlation between the tannery residue concentration and pH over the three-hour irradiation process presented in the Pareto charts was also associated with the consumption of hydrogensulfide ions during the reaction, which could be replenished in the basic medium. The Pareto charts in Fig. 2 also demonstrate that the concentration of H2PtCl66H2O was not important during the first hour of the reaction, but had significant interactions with the parameters residue concentration and mass of CdS. The insignificance of this factor during the first hour of the reaction may be associated with characteristic induction times for similar systems, which are approximately 1–2 h [33]. During this period, it is known that Pt(IV) is photoreduced to Pt(0) with the assistance of the irradiated semiconductor and a sacrificial reagent [22]. In this study, the concentration of H2PtCl66H2O was significant only in the second hour of the reaction, suggesting that the photoreduction of Pt(IV) took place during this period. In the third hour of the reaction this factor was no longer significant, since during the second hour of the reaction PtIIS was deposited on CdS surface instead of Pt(0) [22,26], as suggested by following the equations:
0.2 0
8
9
10
11
12
13
-0.2 -0.4 -0.6 -0.8
pHinial
Fig. 4. Adsorption equilibrium of CdS-liming sludge solution as a function of initial pH.
A study of the surface chemistry of CdS suspended in the tannery residue solution (50% residue) was conducted using the adsorption method to identify species that acted as charge traps on hydrous CdS surface under reaction conditions and to investigate their pH dependence (Fig. 4). In this system, thiol groups were more likely to prevail on the CdS surface than hydroxyl groups, as the tannery sludge solution was rich in sulfide ions. With respect to this observation, the equilibria described in equations (1) and (2) are the most important. For solutions with pH levels of 8–13, the zero charge pH was identified as 9.3, where the neutral species > CdSH prevailed. In a pH range 8–9, the curve profile suggested the existence of positive surface charges, indicating the presence of the protonated species >CdSH2+, though it was not the predominant species. At pH 10, the maximum concentration of negative surface charges could be identified; the surface was deprotonated and >CdS was the predominant species in the equilibrium. Above pH 10, the surface became electrically neutral again, suggesting that the negative species, >CdS, attracted cations (M+) in the tannery residue solution to form neutral species such as >CdSM. The zero charge on the surface allowed the interaction of negative species such as sulfide and hydrogensulfide ions, which acted as hole scavengers.
Fig. 5. Hydrogen production profile in a long-term reaction under the following conditions: 60 mg of hex-CdS, 50% residue, pH 13 without the addition of H2PtCl66H2O.
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tanning residue using solar power to produce hydrogen. From the obtained results, it can be concluded that the tannery waste concentration and pH are the main factors responsible for maximum hydrogen production rates. In contrast, the addition of platinum source decreases the hydrogen production because it is deposited on CdS surface as platinum sulfide, which is inactive as a co-catalyst. Stability tests also indicated that CdS was deactivated after 27 h of irradiation and the residue lost its efficiency as a sacrificial reagent when exposed to air for long durations at room temperature. In summary, our results open up perspective to recover an appreciable amount of energy from tannery sludge wastewaters through solar photocatalytic production of hydrogen. Acknowledgements
Fig. 6. Comparative hydrogen production profile: residue sample in contact with air for a month (a) and refrigerated residue sample (b).
The authors wish to thank the Brazilian research funding agencies(CNPq, CAPES and FAPESB) for financial support and Professor Sérgio L. C. Ferreira for helpful discussions. References
The results of the multivariate experimental design suggest that experiment 10 generated optimal parameters (60 mg CdS, 50% residue, pH 13 without H2PtCl66H2O addition). Upon establishing the best reaction condition, a test was performed to evaluate the stability of the photocatalyst over the 30-h irradiation period (Fig. 5). A loss of photocatalytic activity of CdS occurred after 27 h of irradiation, when the hydrogen production ceased at 184 mmol, which is equivalent to 3.07 mmol g1cat. In this long-term experiment, no CO or CO2 signals were detected, despite the high content of total organic carbon in tannery sludge, noting that organic species were not photooxidized in appreciable amounts. On the other hand, the sulfide content in the residue determined by iodimetric method decreases considerably, from 2.0 mol L1 to 0.48 mol L1, after 30-h irradiation. These results reinforce the idea that the sulfite ions act as hole scavengers in hydrogen production when tannery residue is photocatalytically treated in an oxygen free environment. According tannery data, 1 kg of animal hide processed in a liming bath generates 10 L of waste residue. If photocatalytic treatment were to be applied to this volume of waste, 1.5 L of hydrogen gas (STP) would be generated under the optimal reaction conditions identified in this study. The amount of energy recovered by this process would be substantial, as the energy supply necessary for conversion could be obtained from solar radiation and more than 40 million hides per year could be treated in Brazil alone [1]. In order to evaluate the long-term stability of the liming sludge, a residue sample was removed from the refrigerator and exposed to the atmosphere for a month at room temperature. The photocatalytic test to evaluate the stability of the residue was carried out under the same conditions as previous tests and the results are shown in Fig. 6. There was a decrease in photocatalytic activity of approximately 45% after residue exposure to atmospheric air at room temperature. These results indicate that most of the species that acted as sacrificial reagents were lost by airinduced oxidation. Thus, for use in the photocatalytic generation of hydrogen, the immediate use of the residue after its generation is recommended; otherwise, samples should be refrigerated until use in the reactions. 4. Conclusion Our findings demonstrate the feasibility of energy recovery from a pollutant through the photocatalytic treatment of leather
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