As(V) removal by Fe(III) or Al salts and a novel rapid solid-liquid separation in wastewater containing dilute arsenic

The removal of As(V) from wastewater containing low concentration of As(V) was investigated by addition of Fe(III) or Al(III) salts. Results showed that both salts could remove As(V) in the waste-water to the level below the regulated concentration of Japanese effluent standard (0.1 mg/dm³); for example, As(V) could be removed from wastewater of 10 mg/dm³ As(V) after the treatment in 30 mg/dm³ Fe(III) solution at pH 5, 50 mg/dm³ Fe(III) at pH 7, 14 mg/dm³ Al(III) at pH 5, and 20 mg/dm³ Al(III) at pH 7. Chemical equilibrium calculations, based only on the formation of arsenic salts with Fe(III) or Al(III), hardly explained the experimental results, but the diffused layer model (DLM), one of the surface complexation models (SCMs), considering the adsorption of As(V) on the surface of ferric or aluminum hydroxides, could qualitatively explain them. Solid/liquid separation from these As(V) removal experiments was also investigated using a column bed method packed with fibrous ferronickel slag (FS). Generally, solid/liquid separation of ferric or aluminum hydroxide particles, adsorbing dilute As(V) after the treatment of dilute As(V) wastewa-ter by Fe(III) or Al(III) salts, is difficult because of their size of several hundred nm and thinly dispersion, but this method could achieve a favorably rapid separation (2.5 mm/s). This could be explained by the electrostatic attracting force between FS fibers and fine particles of ferric or aluminum hydroxide particles.