Rapid and effective removal of Cr(VI) from aqueous solutions using the FeCl3/NaBH4 system

A new emergency treatment technology for rapid and effective removal of hexavalent chromium [Cr(VI)] from aqueous solutions by direct addition of ferric chloride (FeCl3) and sodium borohydride (NaBH4) in sequence to Cr(VI) solutions under magnetic stirring conditions was proposed in this study. Various reaction conditions, such as solution pH and Fe(III)/Cr(VI) and FeCl3/NaBH4 molar ratios were investigated. The results showed that 97.6–99.9% of 50mg/L Cr(VI) can be removed from aqueous solutions within 3min in the initial pH range of 3.5–6.0 with Fe(III)/Cr(VI) and FeCl3/NaBH4 molar ratios of 1.0:1 and 1:3.0 respectively at 298K, which was significantly better than that of FeCl3 and NaBH4 alone. This indicated that a synergistic effect existed. Jar tests demonstrated that Cr(VI) was reduced to Cr(III) mainly by NaBH4 under a strong acidic condition in which the hydrogen ions were largely provided by the hydrolysis of FeCl3. In addition, co-precipitation of ferric and trivalent chromic ions occurred. The adsorption of ferric compounds also contributed to the loss of a relatively higher concentration of Cr(VI). Experimental results and especially theoretical calculations of redox potentials suggested that the expected nanoscale zero-valent iron (nZVI) or ferroferric oxide (Fe3O4) was formed only with the addition of excess FeCl3/NaBH4. X-ray photoelectron spectroscopy (XPS) indicated that these precipitates were complex Fe–Cr oxides/hydroxides. The results demonstrated the potential use of the FeCl3/NaBH4 system in copping with Cr(VI) pollution accidents.