Arsenic speciation in titanium dioxide (TiO2) waste produced via drinking water filtration: Potential environmental implications for soils, sediments, and human health: Potential environmental implications for soils, sediments, and human health

Arsenic (As) contamination in soil and water is a global human health and environmental problem due to its common occurrence and high toxicity. At least four million people in Latin America have long-term exposure to As in drinking water. In northwestern Costa Rica, As has been measured in drinking water at concentrations ranging from 20 to over 80 µg As L⁻¹. Arsenic contaminated drinking water can be treated using filter technologies; titanium dioxide (TiO₂)-based drinking water filters decrease As concentration and help mitigate As exposure. However, this process creates an As-enriched TiO₂ solid-phase waste product. An objective of this study is to determine the chemical forms of As in this waste product, which is commonly distributed on soil surfaces and throughout urban and rural areas adjacent to drinking water treatment facilities in the sample area. An additional objective is to determine biogeochemical changes in As speciation in filter waste and affected soils which may be induced through earthworm ingestion, which also occurs in the sample area. Arsenic concentrations in the solid-phase filter waste were measured to be up to 600 mg kg⁻¹. There was a strong correlation between As and Ti (R² > 0.90) indicating As accumulates within or on the surface of the TiO₂ mineral. Earthworm castings enriched in TiO₂ contained up to 530 mg As kg⁻¹, which provided direct evidence soil macroorganisms ingest filter waste. X-ray diffraction (XRD) confirmed Ti is predominantly anatase with minor amounts of quartz and feldspar. These results are novel and innovative because they identify and characterize a previously unexplored problem of anthropogenic soil and sediment As contamination and redistribution in urban and rural environments. Additionally, dry windy seasons in the region increase potential risk of As and TiO₂ exposure through dust inhalation. Earthworm consumption did not decrease As concentrations in earthworm castings. Synchrotron-based X-ray absorption spectroscopy (XAS), specifically X-ray absorption near edge structure (XANES) spectroscopy, indicated the dominant species of As in soil and sediment samples was arsenate [As(V)], which is similar to the chemical form of As in the groundwaters of this region. No evidence of arsenite [As(III)] was identified in any of the solid-phase samples, including earthworm castings.