Evaluation of the Disinfectant Performance of Silver Nanoparticles in Different Water Chemistry Conditions Hongyin Zhang 1 and Vinka Oyanedel-Craver, A.M.ASCE 2 Abstract: This study aimed to determine the effect of different water chemistry conditions on the bactericidal properties of silver nano- particles (AgNPs). Reduced disinfection performance of AgNPs was obtained in divalent cationic solutions in comparison with monovalent solutions with the same concentration. Average particle size of AgNPs increased with increasing electrolyte concentration as divalent cations (Ca 2þ and Mg 2þ ) produced larger AgNPs aggregates than those formed with monovalent solutions. ξ-potential measurements showed that AgNPs in divalent cationic solutions had low absolute ξ-potential values (À9:8 to À23:2 mV), whereas the values obtained in monovalent solutions were considerably more. The measurements of the concentration of ionic silver released indicated that the fraction of dissolved Ag þ (5:9–18:8 μg=L) was around 0.1% of the total mass of Ag 0 added. The contribution of Ag þ to the overall disinfection performance was negligible at the conditions tested. In this study, different physicochemical properties of silver nanoparticles and the survival rate of Escherichia coli (E. coli) in different AgNPs solutions were analyzed. The data collected lead to a correlation between survival rate of E. coli and average size of AgNPs. The results show a strong correlation between these two parameters that can be fitted to a saturation type curve, reaching a survival plateau around 20% survival at an average particle size of 200 nm for all the water chemistry conditions tested. DOI: 10 .1061/(ASCE)EE.1943-7870.0000460. © 2012 American Society of Civil Engineers. CE Database subject headings: Nanotechnology; Particle size; Water content. Author keywords: Silver nanoparticle; Escherichia coli; Particle size; Saturation type curve. Introduction AgNPs are commonly used in a wide range of applications, includ- ing solar energy absorption, chemical catalysis, and disinfection (Choi et al. 2008; Tiwari et al. 2008; Carlson et al. 2008; Gurunathan et al. 2009; Pradeep and Anshup 2009). Consumer products containing AgNPs accounted for more than 25% of the 1,015 nanotechnology-based consumer products available on the market in 2009 (Liu and Hurt 2010). AgNPs have large surface areas per volume ratio and high reactivity compared with the bulk solid. This feature gives AgNPs antimicrobial properties. Three possible antimicrobial mechanisms of AgNPs have been raised: (1) AgNPs can damage cell membrane and intracellular components (Feng et al. 2000; Sondi and Salopek-Sondi 2004), (2) silver ions released from AgNPs can be sorbed into the cell wall and cause lysis and death (Navarro et al. 2008; Cumberland and Lead 2009; Feng et al. 2000), and (3) reactive oxygen species (ROS) can be formed in AgNPs solution (Hamasaki et al. 2008; Lok et al. 2007; Watts et al. 2003; Park et al. 2009). Although the antibacterial properties of AgNPs have been extensively dem- onstrated (Ju-Nam and Lead 2008; Panacek et al. 2006; Nowack and Bucheli 2007; Tolaymat et al. 2010), their performance at different water chemistries have not been fully understood yet. Some evidence shows that the disinfection effectiveness of AgNPs is size-dependent (Rai et al. 2009), and that the process of aggre- gation reduces their surface area, reducing the cell-particle inter- action, membrane penetration, and the rate of silver ion release (Liu and Hurt 2010). Studies measuring the rate of silver ions release at different dilutions of seawater showed that the salt concentration did not affect the AgNPs’ oxidation kinetics; however, high ionic strength increased the size of the particles from 1.9 to 200 nm after 24 h (Liu and Hurt 2010). Gao et al. (2009) also found that fresh water sam- ples with higher ionic strength produced large AgNPs. Jin et al. (2010) studied the effect of different water matrices on the AgNPs size, silver ions release, and antimicrobial activity using a fixed concentration of Ca 2þ and Mg 2þ . The study revealed that Ca 2þ and Mg 2þ increased the AgNPs aggregation in different electrolyte solutions with the same ionic strength in comparison with mono- valent ions. The antimicrobial test showed that Gram-negative bac- teria Pseudomonas putida was more resistant to AgNPs compared to Gram-positive bacteria Bacillus subtilis. In this work, seven different electrolyte solutions were used to study systematically the influence of different cations and anions on the physicochemical characteristics of the particles and disinfec- tion performance. One objective of this study was to establish a correlation between survival rate of bacteria and particle character- istics that could easily predict the disinfection performance of AgNPs at different water chemistry conditions. Materials and Methods Synthetic Water Solutions Synthetic water solutions were prepared using eight different solu- tions, four monovalent and three divalent salts in addition to one solution containing humic acids (HA). These different solutions 1 Ph.D. Candidate, Bliss Hall 213, Dept. of Civil and Environmental Engineering, Univ. of Rhode Island, Kingston, RI 02881. 2 Assistant Professor, Bliss Hall 213, Dept. of Civil and Environmental Engineering, Univ. of Rhode Island, Kingston, RI 02881 (corresponding author). E-mail: craver@mail.uri.edu Note. This manuscript was submitted on October 22, 2010; approved on July 14, 2011; published online on July 16, 2011. Discussion period open until June 1, 2012; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, Vol. 138, No. 1, January 1, 2012. ©ASCE, ISSN 0733-9372/2012/1-58– 66/$25.00. 58 / JOURNAL OF ENVIRONMENTAL ENGINEERING © ASCE / JANUARY 2012 Downloaded 21 Mar 2012 to 131.128.70.27. Redistribution subject to ASCE license or copyright. Visit http://www.ascelibrary.org