Ca–alginate-entrapped nanoscale iron: arsenic treatability and mechanism studies

The use of nanoscale zero-valent iron (NZVI, diameter 10–90 nm with an average value of 35 nm) entrapped in calcium (Ca)–alginate beads shows great promise for aqueous arsenic treatment. This research evaluated Ca–alginate-entrapped NZVI as an advanced treatment technique for aqueous arsenic removal. Arsenic is a serious threat to human health and millions of people are affected by arsenic contamination in various parts of the world including the USA. In bench scale batch studies with initial As(V) concentrations of 1–10 mg L⁻¹, ~85–100 % arsenic removal was achieved within 2 h. While the reaction kinetics differ between bare and entrapped NZVI, the overall reductions of arsenic are comparable. Surface area-normalized arsenic reduction reaction rate constants (k _sa) for bare and entrapped NZVI were 3.40–5.96 × 10⁻³ and 3.92–4.43 × 10⁻³ L m⁻² min⁻¹, respectively. The entrapped NZVI removed ~100 μg L⁻¹ As(V) to below detection limit within 2 h and groundwater with 53 μg L⁻¹ As(V) was remediated to below instrument detection limit (10 μg L⁻¹) within 1 h. The presence of Na⁺, Ca²⁺, Cl⁻, and \({\rm HCO}_{3}^{ - }\)did not affect arsenic removal by entrapped NZVI and there was no leaching of iron from the beads. X-ray diffraction and Fourier transform infrared spectroscopic techniques have been used to understand the mechanism of arsenic removal by the entrapped NZVI. Ca–alginate polymer is an excellent choice as an entrapment medium as it is non-toxic and has little solubility in water.