Removal and recycling of hexavalent chromium from alkaline wastewater via a new ferrite process to produce the valuable chromium ferrite.

[Display omitted] • CrO 4 2− was almost completely removed from alkaline Cr(VI) contaminated wastewater. • CrO 4 2− was transformed into chromium ferrite (Cr 0.5-x Fe 2.5+x O 4) by one-step process. • Chromium ferrite was formed by reaction of amorphous Cr 0.25 Fe 0.75 (OH) 3 with Fe2+. • Increasing temperature and pH could promote the formation of chromium ferrite. Current technologies for removal of Cr(VI) are generally fit for acidic wastewater. In this study, a new ferrite process for removal and recycling of Cr(VI) from alkaline wastewater to produce the valuable chromium ferrite has been developed. The results show that this new ferrite method is a one-step process which can be divided into two successive reactions including Cr(VI) reduction to form coprecipitation (Cr 0.25 Fe 0.75 (OH) 3) and subsequently magnetic conversion of Cr 0.25 Fe 0.75 (OH) 3 induced by Fe2+ under the same alkaline condition. The total Fe/Cr mole ratio of 5:1 is at least required for the chromium ferrite transformation. Increasing temperature and pH can enhance the interaction of Fe2+ with Cr 0.25 Fe 0.75 (OH) 3 and further promote the formation of chromium ferrite, while suppressing the generation of nonmagnetic by-product goethite. Almost pure chromium ferrite is formed under proposed optimum conditions (Fe/Cr = 7:1, 65 °C and pH of 9) with Cr(VI) removal ratio around 100%. The Cr(VI) remained in the filtrate can be reduced to 0.01 mg/L which is much lower than the limits concentration for surface water (≤0.05 mg/L). The chromium ferrite product whose molecular formula can be expressed as Cr 0.5-x Fe 2.5+x O 4 (where 0 ≤ x < 0.5) presents good magnetic properties and has the potential to be recycled as a useful material. [ABSTRACT FROM AUTHOR]