Mobility and bioaccessibility of arsenic (As) bound to titanium dioxide (TiO2) water treatment residuals (WTRs)

This work systematically describes arsenic mobility and potential bioaccessibility of arsenic-enriched titanium dioxide water treatment residuals (TiO2 WTRs) by employing a suite of wet chemical experiments and spec-troscopic measurements. Specifically, Environmental Protection Agency (EPA) digestion method 3051a indicated <3% of total arsenic in the solid phase was released, and arsenic assessed by EPA method 1340 for bio-accessibility was below detection limits. A novel finding is while the arsenic appeared to be stable under highly acidic digestion conditions, it is in fact highly mobile when exposed to simple phosphate solutions. On average, 55% of arsenic was extracted from all samples during a 50-day replenishment study. This was equivalent to 169 mg kg-1 arsenic released from the solid phase. Macroscopic desorption experiments indicated arsenic likely formed inner-sphere bonds with the TiO2 particles present in the samples. This was confirmed with X-ray ab-sorption spectroscopy (XAS), where an interatomic distance of 3.32 angstrom and a coordination number (CN) of 1.79 titanium atoms were determined. This translates to a configuration of arsenic on TiO2 surfaces as a bidentate binuclear inner-sphere complex. Thus, both macroscopic and spectroscopic data are in agreement. During in-cubation experiments, arsenic(V) was actively reduced to arsenic(III); the amount of arsenic(III) in solution varied from 8 to 38% of total dissolved arsenic. Lastly, elevated concentrations and mobility of vanadium in these systems merit further investigation. The high mobility of arsenic and its potential for reduction when reintro-duced into the environment, particularly in agriculturally important areas, presents an important risk when waste products are not properly managed.

Automated summary

This study systematically investigates the mobility of arsenic-enriched titanium dioxide water treatment residuals (TiO2 WTRs), and finds that a significant portion of arsenic can be extracted when exposed to phosphate solutions. Macroscopic desorption experiments and X-ray absorption spectroscopy (XAS) confirm the formation of inner-sphere bonds between arsenic and TiO2 particles. Additionally, incubation experiments reveal the reduction of arsenic(V) to arsenic(III) in solution. The high mobility of arsenic and the potential for reduction present important risks, particularly in agriculturally important areas where waste products are not properly managed.