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pH-Dependent Transformation of Ag Nanoparticles in Anaerobic Processes Ze-hua Liu,*,†,‡ Yan Zhou,‡ Abdul Majid Maszenan,‡ Wun Jern Ng,‡,§ and Yu Liu‡,§ †
College of Environment and Energy, South China University of Technology, Guangzhou 510006, China Advanced Environmental Biotechnology Center, Nanyang Environment and Water Research Institute, Nanyang Technological University, CleanTech One, Singapore 637141 § School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798 reduction in their toxicity.1 Under oxic conditions, Ag-NPs can be transformed to Ag2S via: 2 4Ag + O2 + 2H2S → 2Ag2S + 2H2O and 4Ag + O2 + 2HS− → 2Ag2S + 2OH−. However, the transformation mechanism of Ag-NPs in anaerobic processes where oxygen is not available has not yet been well understood.3 This viewpoint shows that the transformation of Ag-NP to Ag2S in anaerobic processes is possible, but pHdependent. Moreover, the pH-dependent transformation mechanism of Ag-NPs can likely be extended to other types of metal NPs present in anaerobic process. Therefore, in future evaluation of release and toxicity of metal NPs present in anaerobic processes, the pH effect on their transformations should be carefully taken into account. Hydrogen sulphide (H2S) generated in anaerobic processes undergoes dissociation and the distribution between H2S and HS− is pH-dependent (Figure 1). In a chemical sense, the reaction between Ag-NPs and H2S is possible, whereas Ag-NPs would unlikely react with HS− under anaerobic conditions. In fact, Chen et al has recently confirmed that Ag-NPs could be easily converted to Ag2S with pure H2S gas (i.e., anaerobic condition) through Ag + H2S → Ag2S + H2.5 On the other hand, Liu et al reported no transformation of Ag-NP to Ag2S at ilver nanoparticles (Ag-NPs) with increasing concentration pH 11.1.2 As can be seen in Figure 1, at condition of pH 11.1, have often been found in industrial and municipal HS− possesses over 99.9% of sulphide species, and no form of H2S exists. Thus Liu et al’s investigation confirmed that HS− wastewaters, which may potentially pose a new challenge on could not react with Ag-NP.2 Therefore, it is reasonable to the human health and ecology. It has been reported that conclude that transformation of Ag-NPs to Ag2S in anaerobic transformation of Ag-NPs to Ag2S led to more than 71% process is conditional, and would be pH-dependent, and theoretically such transformation in anaerobic processes would be occur only at pH lower than 9.6, where H2S form of sulphide species exists. These in turn offer a plausible explanation for observed transformation of Ag-NPs to Ag2S in anaerobic bioreactors which were operated at pH below 9.3 ‡
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AUTHOR INFORMATION
Corresponding Author
*(Z-h. Liu) Phone/fax: +86-20-39380507; e-mail: zehualiu@ scut.edu.cn. Notes
The authors declare no competing financial interest. Received: Revised: Accepted: Published:
Figure 1. Distribution of sulphide species at different pH 4. © 2013 American Chemical Society
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October 9, 2013 October 19, 2013 October 28, 2013 November 8, 2013 dx.doi.org/10.1021/es404514g | Environ. Sci. Technol. 2013, 47, 12630−12631
Environmental Science & Technology
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REFERENCES
(1) Choi, O.; Clevenger, T. E.; Deng, B.; Surampalli, R. Y.; Ross, L., Jr.; Hu, Z. Role of sulfide and ligand strength in controlling nanosilver toxicity. Water Res. 2009, 43 (7), 1879−1886. (2) Liu, J.; Pennell, K. G.; Hurt, R. H. Kinetics and mechanisms of nanosilver oxysulfidation. Environ. Sci. Technol. 2011, 45 (17), 7345− 7353. (3) Lombi, E.; Donner, E.; Taheri, S.; Tavakkoli, E.; Jämting, Å.K.; McClure, S.; Naidu, R.; Miller, B. W.; Scheckel, K. G.; Vasilev, K. Transformation of four silver/silver chloride nanoparticles during anaerobic treatment of wastewater and post-processing of sewage sludge. Environ. Pollut. 2013, 176, 193−197. (4) Goueffon, Y.; Arurault, L.; Fontorbes, S.; Mabru, C.; Tonon, C.; Guigue, P. Chemical characteristics, mechanical and thermo-optical properties of black anodic films prepared on 7175 aluminium alloy for space applications. Mat. Chem. Phy. 2010, 120, 636−642. (5) Chen, R.; Morris, H. R.; Whitmore, P. M. Fast detection of hydrogen sulfide gas in the ppmv range with silver nanoparticle films at ambient conditions. Sens. Actuators, B 2013, 186, 431−438.
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dx.doi.org/10.1021/es404514g | Environ. Sci. Technol. 2013, 47, 12630−12631
