The reaction of FeII-solids with chromate ions – A general kinetics model involving the progressive formation of blocking surface multilayers of Cr(III)

Abstract

In this study, we investigated the heterogeneous redox reaction between Fe II solids and chromate ions. The reaction kinetics model that we have established earlier for carbonate green rust GRc, has been considered here and we have demonstrated that it could also be applied to the other Fe(II) solids, sulphate green rust GRs and chukanovite Chu. The model implies the solid-state oxidation of Fe II and the concomitant progressive formation of Cr III monolayers, which have different and decreasing k i rate constants. At 25°C synthesis and interaction temperatures, the k 1 values for GRs, GRc and Chu are 3.5, 1.35 and 2 min -1 , the k 2 values for GRc and Chu are 0.036 and 0.009 min -1 and the k 3 values for GRc and Chu are 0.0060 and 0.00025 min -1 , suggesting that the reactivity of our Fe II -solids is in the following way, GRs ≈ GRc > Chu. The chromate removal efficiency is strongly dependent on n 0 , the saturation mole number of Cr III in a monolayer, which is related to the total exposed surface area of the particles. The reaction rate increases with the interaction temperature. The activation energies for the formation of the second and third Cr III monolayers were determined and the obtained values, E a2 = 45,8 kJ mol -1 and E a3 = 97,3 kJ mol -1 , suggest that the rate determining step is the transfer of electrons through the monolayers of Cr III . • The reduction reaction of chromate ions at the surface of different Fe II solids • A general kinetics model suitable for green rusts, ferrous hydroxides, chukanovite • Solid-state oxidation of Fe II and formation of Cr III monolayers with decreasing k i • The rate determining step is the transfer of electrons through the layers of Cr III • The saturation n(Cr III ) in a monolayer is the key-value for a fast chromate removal