Speciation and bioaccessibility of arsenic in drinking water filtration waste: Environmental implications for soils and human health
Zimmerman, Amanda Jo
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Arsenic contaminated drinking water is a critical problem in many parts of the world, including at least four million people in Latin America. In northwestern Costa Rica, arsenic ranges from 20 to over 80 µg arsenic L-1; well above the World Health Organization (WHO) recommended limit of 10 µg arsenic L-1. This problem is addressed using filtration technologies and large-scale facilities, creating an arsenic-enriched titanium dioxide (TiO2) solid waste product. Improper management of this waste product presents additional risks to humans and the environment. This work systematically describes the solid-phase physicochemical properties of the arsenic-enriched TiO2 by various analytical techniques including portable X-ray fluorescence (PXRF) and inductively coupled – optical emission spectroscopy (ICP – OES), and X-ray diffraction (XRD). The speciation of arsenic in the TiO2 system was determined via X-ray absorption spectroscopy (XAS). Arsenic mobility and potential bioavailability of the waste was determined by employing a suite of wet chemical experiments and spectroscopic measurements. Arsenic concentrations in the TiO2 filter waste were measured up to 600 mg kg−1 with a strong correlation between arsenic and titanium (R2 > 0.90), and arsenic accumulated within or on the surface of the TiO2 mineral. The TiO2 structure was anatase, as identified by X-ray absorption near edge spectroscopy (XANES) and XRD. Digestion by EPA method 3051a showed < 3% total arsenic in the solid phase was released during microwave digestion, and digestion by EPA method 1340 for bioaccessibility was below detection limits. However, arsenic is highly mobile when exposed to simple phosphate solutions with 55% of arsenic extracted from all samples during a 50-day replenishment study. Macroscopic desorption experiments indicated arsenic likely formed inner-sphere bonds with the TiO2 particles in the sediment, which was confirmed by XAS. This also confirmed arsenic was bound to TiO2 as a bidentate binuclear inner-sphere complex, based on the interatomic distance of 3.32 Å and a coordination number (CN) of 1.79 for titanium atoms. Anaerobic incubation actively reduced arsenic(V) to arsenic(III); the amount of arsenic(III) in solution varied from 8 to 38% of total dissolved arsenic.