MICRO-AND MACROSCALE INVESTIGATIONS OF CHROMATE SORPTION ON BARITE: IMPLICATIONS FOR ALTERNATIVE STRATEGIES TOWARD CHROMIUM IMMOBILIZATION

As an environmental pollutant, Cr (VI) necessitates remediation if detected at sufficiently high concentrations. Cr (VI) cleanup often involves reduction to Cr (III); however, alternative immobilization methods such as sorption of chromate on barite may be more effective in some circumstances. Understanding the Cr (VI) immobilization strategy includes micro and macro scale investigations through laboratory experiments by atomic force microscopy (AFM), mixed flow reactor (MFR) and plug flow reactor (PFR) systems. As an initial investigation into the processes, AFM experiments were carried out on freshly cleaved barite (001) surfaces exposed to aqueous solutions at pH = 8.4 and 70°C under constant flow through conditions. The presence of CrO₄^2- in solutions inhibited dissolution at step edges and subsequently changed the etch pit morphology. A significant effect on the barite dissolution rate was noted at CrO₄^2- concentration as low as 0.001M. The substrate dissolution rates were similar when barite surfaces were exposed to solutions of identical saturation state with respect to barite and hashemite (BaCrO₄) phases, further suggesting an effect of aqueous CrO₄^2- on barite dissolution. To extend the microscopic approach to larger scales, bulk powder suspensions of barite were studied in MFRs under similar conditions at 25°C. Experiments at different [CrO₄^2-] revealed the inhibitory action of CrO₄^2- on barite dissolution. Observations from these experiments suggest CrO₄^2- sorption at step edges, further implying that barite could serve as a possible sorbent in contaminated ground waters. The feasibility of this technique will be discussed in light of PFR experiment results.