Reduction of aqueous chromate by Fe(11)/Fe(111) carbonate green rust: Kinetic and mechanistic studies

This work describes the heterogeneous reaction between Fell in carbonate green rust and aqueous chromate., in NaHCO3 solutions at 25 degreesC, and at pH values of 9.3-9.6. Evidence for reduction of Cr-VI to Cr-III and concomitant solid-state oxidation of lattice Fe-II to Fe-III was found from Fe-II titration and from structural analysis of the solids using FTIR, XRD, SEM, and XPS methods. Results indicate the formation of ferric oxyhydroxycarbonate and the concomitant precipitation of Cr-III monolayers at the surface of the iron compound that induce passivation effects and progressive rate limitations. The number of Cr-III monolayers formed at the completion of the reaction depends on {Fe-II}(t=0), the molar concentration of Fe-solid(II) at t = 0; on {n(o)}(t=0), the molar concentration of reaction sites present at the surface of the solid phase at t = 0; and on [Cr-VI](t=0), the molar concentration of Cr-VI at t = 0. Kinetic data were modeled using a model based on the formation of successive Cr-III monolayers, -(d[Cr-VI]/dt) = Sigma(1)(j)k(i)({S})[Cr-VI]({n(i-1) - {n(i)}) with k(i)({S}) (in s(-1) L mol(-1)), the rate coefficient of formation of Cr-III monolayer i, and {n(i)} and {n(i-1)}, the molar concentration of Cr-III precipitated in monolayer i and monolayer i - 1, respectively. Good matching curves were obtained with kinetic coefficients: k(1)({S}) = 5-8 x 10(-4), k(2)({S}) = 0.5-3 x 10(-5), and k(3)({S}) about 1.7 x 10(-6) s(-1) m(-2) L. The Cr-VI removal efficiency progressively decreases along with the accumulation of Cr-III monolayers at the surface of carbonate green rust particles. In the case of thick green rust particles resulting from the corrosion of iron in permeable reactive barriers, the quantity of Fe-II readily accessible for efficient Cr-VI removal should be rather low.