Acrylamide Release Resulting from Sunlight Irradiation of Aqueous Polyacrylamide/Iron Mixtures JAMES E. WOODROW,* ,† JAMES N. SEIBER, § AND GLENN C. MILLER † Natural Resources and Environmental Science, University of Nevada, Reno, Nevada 89557, and Agricultural Research Service, U.S. Department of Agriculture, Albany, California 94710 Linear anionic polyacrylamide (PAM) has been used in irrigation practices as a flocculating agent to minimize water losses through seepage in earthen canals. The stability of PAM is of concern because of the possibility of acrylamide (AMD) monomer release during environmental weathering. Aqueous solutions of commercial PAM mixed with ferric sulfate, subjected to simulated and natural sunlight irradiation, showed polymer chain scission and release of the AMD monomer. At acid/neutral pH, the amount of AMD released was directly related to the concentration of ferric ion and the irradiation time. At alkaline pH (∼8), PAM/Fe 3+ mixtures were stable under irradiation. PAM chain scission involved the hydroxyl radical, but specific AMD release appeared to require PAM-bound iron. Low iron concentrations and alkaline pH of irrigation water would limit AMD release. Residual monomer in PAM can contribute AMD to irrigation water, but concentrations would remain below the U.S. EPA drinking water standard of 0.5 ppb. KEYWORDS: Polyacrylamide; PAM; acrylamide; iron; sunlight INTRODUCTION Commercial crystalline linear anionic polyacrylamide (PAM) is formed by the free radical induced polymerization of a mixture of acrylamide and acrylic acid/acrylate (1). The final dry product is a polymer that is capable of absorbing many times its mass of water by incorporating the water into the polymer structure through hydrogen bonding. When added to an excess of water, the polymer usually requires less than an hour to completely hydrate. In some irrigation practices, PAM is added directly to earthen canals as a flocculating agent to remove suspended sediment and, thereby, seal the canal and minimize seepage. An important concern with the use of PAM is its potential contribution of acrylamide (AMD) monomer to aquifersssome seepage will still occursand to receiving waters (e.g., wetlands, recreational streams and lakes). Commercial PAM already contains a residual amount of AMD remaining from its productionsusually <0.05% w/w. It has been known for some time that AMD is a suspect carcinogen and a cumulative neurotoxin (2). However, its health effects recently garnered worldwide attention and concern through a paper that described the formation of the monomer in cooked high- carbohydrate foods (3). Because of this paper, and others that followed, attention was also focused on other possible sources of AMD (e.g., irrigation practices) that could potentially affect human health. There is little evidence in the literature that suggests AMD can be released when PAM is exposed to certain environmental conditions. One study (4) claimed to have observed the slow release of AMD over several weeks from aqueous solutions of a polyacrylamide thickening agent (PAM mixed with an organic solvent) exposed to environmental sunlight conditions. Depend- ing on the water source (natural surface and well water), AMD content increased by factors of 2–47 over a period of 2–5 weeks and then declined. Other investigators have questioned these results, their interpretation, and overall experimental design (5–7). Also, the results could not be replicated (6). Under the more extreme conditions of 254 nm UV light, aqueous solutions of PAM can release a small fraction of the AMD units (8). Another study observed a significant release of AMD from heated cross- linked polyacrylamide gels (9). However, no explanation for this was offered. The majority of studies to date in the literature have found that under environmental conditions of natural sunlight and/or ambient temperatures, oxidative conditions will commonly lead to chain scission, resulting in smaller molecular weight fragments of PAM but with no AMD release (7, 10–12). In response to the concern over AMD in the environment, the U.S. Bureau of Reclamation (BOR) funded a two-year field and laboratory project to study the efficacy and stability of commercial PAM as a flocculating agent when applied to earthen irrigation canals and ditches. Our study was initiated, as part of the larger BOR project, to find and characterize the environmental conditions that might lead to AMD release and to relate the results to field use conditions. Many of the earlier studies concerned with PAM degradation had not considered how photocatalytic, complex-forming iron might affect the stability of the polymer. One exception was a study that examined the effect on PAM stability of added Fe 2+ in the absence of light, but monomer release was not observed, * Author to whom correspondence should be addressed [fax (775) 784-1142; e-mail jwoodrow@unr.nevada.edu]. † University of Nevada. § U.S. Department of Agriculture. J. Agric. Food Chem. 2008, 56, 2773–2779 2773 10.1021/jf703677v CCC: $40.75  2008 American Chemical Society Published on Web 03/20/2008