Reduction of chromate by granular iron in the presence of dissolved CaCO3

Hexavalent Chromium Reduction with Zero-Valent Iron (ZVI) in Aquatic Systems

“…The addition of dissolved CaCO 3 also enhanced Cr(VI) reduction significantly by the pH buffering effect, as reported by Yang (2006); although ZVI was passivated by the formation of oxides and secondary carbonate precipitates, the enhancement effect was much greater than the passivation (Yang 2006). Cr(VI) reduction experiments carried out by Gui et al (2009) in the absence and presence (300 mg/L) of dissolved CaCO 3 confirmed the findings of Yang (2006); even though the initial rate constant was greater for the solutions without CaCO 3 , Cr(VI) removal rate was higher in the solution with CaCO 3 because in the presence of CaCO 3 , the increase in pH due to ZVI corrosion and Cr(VI) reduction was buffered (Gui et al 2009). According to Jeen et al (2008), although the presence of a high concentration of dissolved CaCO 3 may enhance Cr(VI) removal, it still may negatively affect the permeability of the PRB; this study further noted that the reactivity of ZVI towards Cr(VI) reduction in the presence of CaCO 3 was affected primarily by a buildup of Fe(III)-Cr(III) (oxy)hydroxides rather than by carbonate minerals (Jeen et al 2008).…”

“…When the pH reaches neutral values, Cr(VI) reduction becomes very slow, almost negligible (Dos Santos Coelho et al 2008;Dutta et al 2010;Gheju and Iovi 2006;Wang et al 2010), whilst under alkaline conditions, Cr(VI) reduction even ceased after a short time period (Alowitz and Scherer 2002). In unbuffered systems, the pH increased during the reaction due to ZVI corrosion and Cr(VI) reduction Gheju and Iovi 2006;Guha and Bhargava 2005;Gui et al 2009;Junyapoon and Weerapong 2006). Although reduction of Cr(VI) occurs very rapidly under strong acidic conditions, decreasing the pH of Cr(VI)-polluted water will lead to a decrease in the pH of the treated effluent, which subsequently must be neutralized before its discharge to the surface water bodies.…”

“…The results reported by Manning et al (2007) suggest that nZVI reduces Cr(VI) and corrodes to form lepidocrocite, which then acts as a substrate for precipitation of Cr(OH) 3 and/or Cr x Fe 1−x (OH) 3 . The analysis of ZVI surface composition used in Cr(VI) column reduction experiments carried out by Gui et al (2009) showed that in the absence of CaCO 3 , Cr(VI) was removed primarily by precipitation of chromite (Fe n Cr m O 4 spinel) and Fe(III)-Cr(III) mixed (oxy)hydroxides; other mineral phases detected included goethite, hematite, magnetite, aragonite, Cr 2 O 3 and CrOOH (Gui et al 2009). The XRD patterns of the ZVI powder, after its reaction with Cr(VI), were assigned by Hou et al (2008) to magnetite, goethite, akaganeite and lepidocrocite.…”

“…Assuming a 0.5 reaction order with respect to Cr(VI) and H + concentration, and using the integrated form of Eq. 52, Gui et al (2009) determined the initial rate constants for Cr(VI) reduction in the presence and absence of CaCO 3 . Semi-batch studies carried out by Franco et al (2009a) using CMC-stabilised nZVI show that the overall removal rate of Cr(VI) follows a pseudo-first-order kinetic model.…”

Mitigation of Chromium Contamination by Copper-ZVI Bimetallic Particles

“…Chromium generally occurs in the Cr 6þ and Cr 3þ oxidation states (Palmer and Puls 1994;Proctor et al 1997;Gui et al 2009). Waterinsoluble chromium(III) compounds are not considered health hazards, whereas chromium(VI) ions are toxic in nature (Barceloux and Barceloux 1999;Jacobs and Testa 2005).…”