ARTICLE pubs.acs.org/est
Nano-TiO2 Enhanced Photofermentative Hydrogen Produced from the Dark Fermentation Liquid of Waste Activated Sludge Yuxiao Zhao and Yinguang Chen* State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China
bS Supporting Information ABSTRACT: After anaerobic dark fermentation of waste activated sludge (WAS) for hydrogen production, there are a large number of organic compounds including protein, polysaccharide, and volatile fatty acids left in the dark fermentation liquid, which can be further bioconverted to hydrogen by photofermentation techniquea. In this study, the enhancement of photofermentative hydrogen produced from WAS dark fermentation liquid by using nano-TiO2 is reported. First, high concentration of NH4+-N in the dark fermentation liquid was observed to inhibit the photofermentative hydrogen production, and its removal was essential. Then the effect of nano-TiO2 on photofermentative hydrogen generation was investigated, and the addition of 100 mg/L nano-TiO2 increased hydrogen by 46.1%. Finally, the mechanisms for nano-TiO2 improving hydrogen production were investigated. It was found that nano-TiO2 improved the decomposition of protein and polysaccharide to smallmolecule organic compounds and promoted the growth of photosynthetic bacteria and the activity of nitrogenase but decreased the H2-uptake hydrogenase activity.
’ INTRODUCTION Waste activated sludge (WAS) is a byproduct of biological municipal wastewater treatment plant. It contains significant amounts of protein and polysaccharide. Recently, much attention has been paid to sustainable hydrogen biological production from WAS, by which human resources, such as fossil fuels, are saved, and sludge is reused.14 The most common method for biological hydrogen production from WAS is anaerobic dark fermentation. Hydrogen production by anaerobic dark fermentation includes several stages, such as solubilization of sludge particulate organic matter, hydrolysis of solubilized sludge organic materials, acidification of hydrolyzed products with hydrogen generation, and hydrogen consumption. The hydrogen production of anaerobic dark fermentation is rather low (0.08 mmol H2/g dried sludge), which has been widely believed to be caused by the low WAS hydrolysis efficiency and the rapid consumption of generated H2 by hydrogen consumers.3 Thus, most of the strategies reported in the literature for improving biohydrogen production are from the aspects of increasing sludge hydrolysis and/or killing hydrogen consumers by pretreating sludge with ultrasonic, acidic, alkaline, heat-shocking, or freezingthawing methods.58 It is well-known that anaerobic dark fermentation of organic matter produces not only hydrogen but volatile fatty acids (VFAs), which results in a low observed hydrogen production.3 Some of the soluble intermediate metabolites left in the anaerobic dark fermentation liquid can be further degraded or r 2011 American Chemical Society
mineralized via photofermentation with photosynthetic bacteria (PSB) to produce hydrogen.9,10 The stoichiometries of photofermentative hydrogen production from acetic, propionic, and butyric acids can be described by the following equations:11 CH3 COOH þ 2H2 O f 4H2 þ 2CO2
ð1Þ
CH3 CH2 COOH þ 2H2 O f 3H2 þ CH3 COOH þ CO2
ð2Þ CH3 CH2 CH2 COOH þ 2H2 O f 2H2 þ 2CH3 COOH
ð3Þ It seems that when biohydrogen is recovered from waste activated sludge, a higher hydrogen production can be achieved through the combination of dark fermentation and photofermentation. Figure 1 illustrates the proposed schematic diagram of two-step fermentation for hydrogen production from WAS (Figure 1). Our previous work showed that by controlling the anaerobic dark fermentation of WAS at constant pH 10 (i.e., step 1 in Figure 1), a remarkably higher hydrogen production of 0.87 mmol H2/g dried sludge was achieved due to the improvement of the anaerobic process (sludge solubilization, Received: May 12, 2011 Accepted: August 18, 2011 Revised: August 10, 2011 Published: August 18, 2011 8589
dx.doi.org/10.1021/es2016186 | Environ. Sci. Technol. 2011, 45, 8589–8595
Environmental Science & Technology
ARTICLE
Figure 1. Two-step fermentation for biohydrogen production from waste activated sludge.
hydrolysis, and acidification) and the inhibition of hydrogen consumption.12 Nevertheless, it is unclear whether the dark fermentation liquid can be used as the substrate of PSB for efficient photofermentative hydrogen production (i.e., step 2 in Figure 1). The purpose of this study was to investigate the feasibility of photofermentative hydrogen production from WAS anaerobic dark fermentation liquid. First, the influence of high concentrations of NH4+-N in the dark fermentation liquid on photofermentative hydrogen production was studied, and its removal was found to be essential for hydrogen generation by PSB. Then the enhancement of photofermentative hydrogen production by the use of nano-TiO2 was reported. As there were no references available on the mechanisms of nano-TiO2 improving photofermentative hydrogen production, the role of nano-TiO2 was finally investigated in this paper.
’ MATERIALS AND METHODS Enrichment of PSB and Preparation of WAS Anaerobic Dark Fermentation Liquid. The photosynthetic bacterium used
in this study was isolated from waste activated sludge and was identified as Rhodopseudomonas palustris after analysis of its 16S rDNA sequence. The isolation method was as follows: 5 g of sludge and 50 mL of deionized water were put in a conical flask, which was then oscillated for 20 min at 120 rpm. After settling for 1 h, 5 mL of the suspension was transferred to a 600 mL serum bottle and mixed with a common phototrophic medium 27 (see Supporting Information). The serum bottle was purged with argon gas for 10 min to remove oxygen and then sealed with rubber stopper. The bacteria were enriched at 30 ( 1 °C by use of halogen tungsten lamps as a light source at a light intensity of 200 W/m2 (60007000 lx, 350820 nm, unless otherwise stated) for 7 days until small quantities of red bacteria were observed on the bottle wall. Then the red bacteria were cultured again for 3 days by the same method, and the same procedure was repeated 3 times. The bacteria were finally purified by plate isolation, and the isolated PSB were cultured at 30 ( 1 °C and a light intensity of 200 W/m2 under sterilized conditions with phototrophic medium 27 for 7 days for the photofermentative hydrogen production experiments. The preparation of WAS anaerobic dark fermentation liquid and its characteristics are shown in Supporting Information All the following experiments were conducted in triplicate, and oneway analysis of variance (ANOVA) at 0.05 level was used to analyze the data. TiO2 Nanoparticles. Nano-TiO2 (
