Chromate removal by an iron sorbent: mechanism and modeling

A solution containing chromate was treated using waste shot-blast fines recovered from surface finishing operations in a cast-iron foundry as a sorbent in batch and fixed-bed modes. Equilibrium experiments for initial chromate concentrations of 5 to 10 ppm produced a pH-adsorption edge that exhibits removal of chromium (Cr) over a broad pH range, with adsorption capacities that compare favorably to those reported for other adsorbents such as activated carbon and commercial iron oxides. Surface complexation modeling of adsorption equilibria suggests the formation of monodentate, inner-sphere complexes with chromate (CrO4(2-)) and bichromate (HCrO4(-)). Adsorption of Cr(VI) at iron oxy-hydroxide sites appears to be the primary mechanism of chromium removal at neutral pH. At lower pH values (for example, pH 4), reduction to Cr(III) is assumed to contribute to the increasing removal as a function of decrease in pH. There is also evidence to support the formation of Cr(III)-iron (Fe)(III) coprecipitate following Cr(VI) reduction by dissolved Fe(II). Using equilibrium constants for the two surface complexation reactions evaluated from a triple-layer model description of the oxide-water interface, chromate removal in a short fixed bed of fines was simulated using a dual mass-transfer kinetic model. Rate coefficients determined from model calibration of the short column were used to predict experimental breakthrough curves in columns with empty bed contact times (EBCTs) up to four times the short column. For an influent chromium concentration and pH of 5 ppm and 7.0, respectively, a solid-phase loading capacity of 9.5 +/- 0.3 mg/g was achieved at exhaustion. Predictive model runs indicate that, for this case, an EBCT of 2.0 to 2.5 minutes is optimum for achieving a target effluent concentration of less than or equal to 0.05 mg/L chromium as Cr(VI).