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

Abstract

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 α-Fe2O3 (hematite) onto a fused silica substrate which was analyzed using second harmonic generation (SHG), a coherent, surface-specific, nonlinear optical technique. Specifically, the χ(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) × 103 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/m2, which corresponds to 1.0 × 1012 ions/cm2 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) × 104 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/m2 for the reversibly bound chromium species, which corresponds to 8.3 × 1011 reversibly bound ions per cm2, again assuming a +3 charge of chromium. The data also allows us to estimate that about 6.7 × 1012 Cr(III) ions are irreversibly bound per cm2 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.

Chemical Insight into the Adsorption of Chromium(III) on Iron Oxide/Mesoporous Silica Nanocomposites

“…The XPS spectra show Cr(III) 2p 1/2 and 2p 3/2 peaks at 587 and 577 eV, respectively. 47 The XPS data were consistent with the presence of an adsorbed chromium(III) hydroxide species. The broadness of the peaks may also indicate the presence of multiple Cr species.…”

Hydrothermal and metamorphic fluid-rock interaction associated with hypogene “hard” iron ore mineralisation in the Quadrilátero Ferrífero, Brazil: Implications from in-situ laser ablation ICP-MS iron oxide chemistry

“…These different enrichment/depletion patterns may be linked to various geological processes: 1) differences in the element patterns may be attributed to variations in the itabirite and country rock lithologies (e.g., elevated Mn concentrations in iron oxides in the vicinity of Mn-rich phyllites); 2) soluble Cr 4+ , for example, may have been transported in an oxidative environment/fluid and eventually reduced by hematite to less mobile Cr 3+ , resulting in the absorption of Cr onto the hematite. This effect is also described by several surface-adsorption experiments in aqueous solutions, e.g., Eggleston (1993) and Troiano et al (2013). The enrichment of other redoxsensitive elements (e.g., As, Co) in an oxidative environment may also be related to these reduction-induced adsorption on hematite; 3) an elevated Ti mobility during microplaty hematite formation is established based on the mass balance calculations (e.g., in Casa de Pedra; Fig.…”

Removal of chromium from industrial effluents using nanotechnology: a review

“…These PANI/silica composites have shown a maximum adsorption capacity of 63.41 mg/g at an equilibrium 9. Polycrystalline haematite Physicochemical Iron displays the ability of physical adsorption of Cr(VI) and its reduction to Cr(III) [64] 10. Biomass Biological Adsorption of Cr(VI) on dead fungal biomass (Aspergillus sydoni) Reduction of Cr(VI) to Cr(III) using algae such as Chlorella miniata, certain cyanobacteria, Microbacterium liquefaciens immobilised in polyvinyl alcohol, Neurospora crassa, and many more species [65][66][67][68][69] concentration of 50 mg/l [77].…”