| Cr(VI) reduction and immobilization by Hanford sediment minerals (biotite, magnetite etc.) were investigated under high pH and high ionic strengths conditions similar to the tank waste fluids at the Hanford site.; In the homogeneous system, Cr(VI) reduction by aqueous Fe(II) at high pH proceed very quickly, Cr(VI) removal and reduction increase with Fe(II):Cr(VI) ratio. Reduced Cr(III) precipitate out while all the remaining Cr in the solution phase is Cr(VI). Cr(VI) reduction was non-stoichiometric, either due to the trace amount of O2 oxidizing Fe(II) to Fe(III) or due to the passivation effect of Fe-Cr precipitates on the Fe(OH)2 formed in the alkaline pH conditions. The Fe-Cr precipitates may have spinel structure similar as chromite, with Cr-O distance 1.98 Å, Cr-Cr distance 3.01 Å.; Under alkaline conditions, biotite dissolution increases with NaOH concentration. Cr(VI) reduction by Fe(II) containing silicate minerals such as biotite is closely related to mineral dissolution in different NaOH concentrations. Ionic strength will increase both the biotite dissolution and Cr(VI) reduction. Secondary precipitates formation will have important effect on both biotite dissolution and Cr(VI) reduction. Coprecipitation is an important mechanism of Cr(VI) immobilization at the Hanford site.; Under alkaline pH conditions, in addition to maghemite, goethite formation from magnetite was more prominent, and goethite formation increases with NaOH concentration. Compared to acid and neutral pH conditions, Cr(VI) reduction was much less significant under alkaline pH conditions, and it seems that Cr(VI) reduction decreases with amount of NaOH added. Maghemite and goethite formation might passivate the magnetite surface, inhibit and stop the Cr(VI) reduction by magnetite.; Cr(VI) reduction by magnetic fraction, clay fraction and the Hanford sediment is closely related to dissolution of silicate minerals releasing Fe2+ into solution. Clay fraction dissolution under alkaline pH conditions are much more significant than the Hanford sediment, and in both clay fraction and Hanford sediment systems, dissolution increases with ionic strength. |
