Fate and lability of silver in soils: Effect of ageing Lara Settimio a, * , Mike J. McLaughlin a, b , Jason K. Kirby b , Kate A. Langdon b , Enzo Lombi c , Erica Donner c , Kirk G. Scheckel d a Waite Research Institute, School of Agriculture, Food and Wine, University of Adelaide, Waite Rd, SA 5064, Australia b CSIRO Land and Water, Contaminant Chemistryand Ecotoxicology Program, Minerals Down Under Flagship, Waite Campus, Waite Rd, SA 5064, Australia c Centre for Environmental Risk Assessment and Remediation, University of South Australia, Mawson Lakes, SA 5095, Australia d National Risk Management Research Laboratory, US EPA, Cincinnati, US article info Article history: Received 2 February 2014 Received in revised form 15 April 2014 Accepted 23 April 2014 Available online xxx Keywords: Silver Lability Speciation Ageing abstract The fate and lability of added soluble Ag in soils over time was examined by measurement of labile metal (E-value) by isotopic dilution using the 110m Ag radioactive isotope and the solid-phase speciation of Ag by X-ray absorption near edge structure (XANES) spectroscopy. After two weeks of ageing the E-values for Ag decreased by 20e90% with a further decrease of 10e40% after six months. The overall decrease in labile Ag for all soils after the 6 month ageing period was 50e100%. The ageing was more rapid and pronounced in the alkaline soils. XANES results for Ag in soils indicated that for the majority of soils the added Ag þ was reduced to metallic Ag over time, and associations with Fe-oxohydroxides and reduced S groups in organic matter also decreased Ag lability. Strong positive correlations were found between metallic Ag and non-labile Ag and between organic carbon and Ag bonded with S species. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The use of silver (Ag) and silver nanoparticles (AgNPs) in ap- plications such as solar energy, water purification, medicine and nanotechnology is rapidly increasing (Elvers et al., 1993). Commonly exploited for their antibacterial properties, Ag and increasingly AgNPs are being used in a range of consumer products but this may also lead to environmental risks following their po- tential release into the environment (Morones et al., 2005; Benn et al., 2010). Apart from mining processes, the exposure pathways for Ag into the environment from urban areas (e.g. consumer products and industry) will be from wastewater treatment plants (WWTPs), predominantly via land application of biosolids (Johnson et al., 2005; Oliver et al., 2005) and through agriculture by application of Ag containing pesticides such as Agress Ò (Marques et al., 2011; Innovotech Products w Agr). As a result of these exposure path- ways, soil environments may be one of the major sinks for Ag in the environment. Elevated concentrations of Ag may pose a risk to soil environments due to the potential toxicity of Ag to a range of or- ganisms such as plants, invertebrates, microbes and bacteria living in soil (Ratte, 1999; Roh et al., 2009; Blamey et al., 2010; Throback et al., 2007; Murata et al., 2005). When soluble metals are added to soils there are initial fast reactions which remove metals from soil solution (adsorption and precipitation), followed by slower reactions that continue to remove metals from the labile (i.e. exchangeable) pool. The decrease in metal lability that occurs with time after metal addition to soil is thought to occur through the following general mecha- nisms: micropore diffusion, cavity entrapment, occlusion in solid phases by co-precipitation and co-flocculation and surface precipitation (Ma et al., 2006a; Fendorf et al., 2004; Kabata-Pendias and Pendias, 2000). These processes are generally referred to as “aging”. As a consequence the bioavailability and toxicity of the metal to soil organisms may decrease with time as the actual exposure concentration decreases. The ageing of other metals in soils has been studied (e.g. As, Cu, Mo, and Zn) by measuring metal lability after various periods of time up to 27 months (Ma et al., 2006a; Fendorf et al., 2004; Ma et al., 2006b; Crout et al., 2006; Kirby et al., 2012). These studies have shown that the extent of ageing is dependent on the soil type and chemical properties of the metal, as well as environmental conditions such as temperature (Barrow, 1998). To date, little is known of the extent of ageing of soluble Ag in soils and with the predicted increase in Ag concentrations in the environment (due to increased use of Ag- and AgNP-containing products), it is important to examine the fate and potential bioavailability (lability) of soluble Ag in soils over time. * Corresponding author. E-mail addresses: Lara.Settimio@adelaide.edu.au, lara.settimio@adelaide.edu.au (L. Settimio). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2014.04.030 0269-7491/Ó 2014 Elsevier Ltd. All rights reserved. Environmental Pollution 191 (2014) 151e157