Interaction of Cr(III) and Cr(VI) with Hematite Studied by Second Harmonic Generation

The fate of chromium in the environment relies heavily on its redox chemistry and interaction with iron oxide surfaces. Atomic layer deposition was used to deposit a 10 nm film of polycrystalline alpha-Fe2O3 (hematite) onto a fused silica substrate which was analyzed using second harmonic generation (SHG), a coherent, surface-specific, nonlinear optical technique. Specifically, the chi((3)) technique was used to investigate the adsorption of Cr(III) and Cr(VI) to the hematite/water interface under flow conditions at pH 4 with 10 mM NaCl. We observed partially irreversible adsorption of Cr(III), the extent of which was found to be dependent on the concentration of Cr(III) ions in solution. This result was confirmed using X-ray photoelectron spectroscopy. The interaction of Cr(III) with hematite is compared with the adsorption of Cr(III) to the silica/water interface, which is the substrate for the ALD-prepared hematite films, and found to be fully reversible under the same experimental conditions. The observed binding constant for Cr(III) interacting with the silica surface was found to be 4.0(6) X 10(3) M-1, which corresponds to an adsorption free energy of -30.5(4) kJ/mol when referenced to 55.5 M water. The surface charge density at maximum metal ion surface coverage was found to be 0.005(1) C/m(2), which corresponds to 1.0 X 10(12) ions/cm(2) assuming a +3 charge for chromium. In contrast, the observed binding constant for Cr(III) interacting reversibly with the hematite surface was calculated to be 2(2) X 10(4) M-1, corresponding to an adsorption free energy of -35(2) kJ/mol when referenced to 55.5 M water. The surface charge density at maximum metal ion surface coverage was found to be 0.004(5) C/m(2) for the reversibly bound chromium species, which corresponds to 8.3 X 10(11) reversibly bound ions per cm(2), again assuming a +3 charge of chromium. The data also allows us to estimate that about 6.7 X 10(12) Cr(III) ions are irreversibly bound per cm(2) hematite at saturation coverage. The results of this investigation suggest that the use of hematite in permeable reactive barriers, for cost-effective chromium remediation, allows for Cr(III) remediation at very low concentrations through adsorptive and redox processes but quickly renders the barriers ineffective at high chromium concentrations due to surface saturation.