An innovative strategy for efficient and economical arsenic removal in hydrometallurgical waste sulfuric acid by co-treatment with Fe–As coprecipitation residue via scorodite formation
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In order to nondestructively characterize chemical forms of ferric hydroxides in weathered rock, charge-coupled device type visible microspectroscopy was applied to brown stains produced in weathered granite surfaces. The combination of visible microspectra and color parameters ( a* and b*) was effective in examining chemical forms of ferric hydroxides in the analytical area. Color parameters in an a*– b* diagram of the brown stains, mostly lying between goethite and ferrihydrite trends, indicated that the brown stains contain ferrihydrite or hematite in addition to goethite. Similarity of the visible microspectra of the brown stains and their first derivatives to those of goethite or ferrihydrite suggests that goethite and/or ferrihydrite are the main weathering products of the granite. Occurrence of ferrihydrite as well as goethite in the brown stains implies that crystallization of ferrihydrite to goethite might be hindered during the granite weathering. This fact suggests the possibility of toxic metal retention in ferrihydrite by its long-term persistence during water–rock interactions at the earth's surface.
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X-ray diffraction and Raman spectroscopy were used in this study to characterize arsenate phases in the arsenate-ferrihydrite sorption system. Evidence has been obtained for surface precipitation of ferric arsenate on synthetic ferrihydrite at acidic pH (3-5) underthe following experimental conditions: sorption density of As/Fe approximately 0.125-0.49 and arsenic equilibrium concentration of <0.02-440 mg/L. Surface precipitation occurred under apparently undersaturated (in the bulk solution phase) conditions, and probably involved initial uptake of arsenate by surface complexation followed by transition to ferric arsenate formation on the surface as indicated by XRD analysis. At basic pH (i.e., pH 8), however, no ferric arsenate was observed in arsenate-ferrihydrite samples at a sorption density of As/Fe approximately 0.125-0.30 and an arsenic equilibrium concentration of 2.0-1100 mg/ L. At pH 8, arsenate is sorbed on ferrihydrite predominantly via surface adsorption, and the XRD patterns resemble basically that of ferrihydrite.
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Despite the number of detailed studies on arsenate adsorption onto synthetic 2-line ferrihydrite carried out during the past few decades, questions remain regarding the fate of adsorbed arsenate during phase transformation of this poorly crystalline iron oxy-hydroxide. We assessed arsenate partitioning during this transformation by aging synthetic 2-line ferrihydrite with adsorbed arsenate (at an As/Fe molar ratio of ~0.017) for 7 days at 75 °C under highly alkaline conditions (pH ~10). X-ray diffraction patterns show that ~55% of the ferrihydrite converted almost entirely to hematite (with traces of goethite) after aging 7 days, accompanied by a ~54% loss of reactive surface area (BET). ICP-MS analyses indicate that despite this conversion and significant loss of surface area, the aqueous arsenate concentration decreased from ~1.48 to ~0.51 mg/L during the course of the experiment. XAS analyses suggest that the concentration of arsenate and its speciation are controlled by its incorporation into the hematite.
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In redox-affected soil environments, electron transfer between aqueous Fe(II) and solid-phase Fe(III) catalyzes mineral transformation and recrystallization processes. While these processes have been studied extensively as independent systems, the coexistence of iron minerals is common in nature. Yet it remains unclear how coexisting goethite influences ferrihydrite transformation. Here, we reacted ferrihydrite and goethite mixtures with Fe(II) for 24 h. Our results demonstrate that with more goethite initially present in the mixture more ferrihydrite turned into goethite. We further used stable Fe isotopes to label different Fe pools and probed ferrihydrite transformation in the presence of goethite using
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Aqueous Fe(II) is known to catalyze the abiotic transformation of ferrihydrite to more stable Fe minerals. However, little is known about the impacts of coprecipitated OM on Fe(II)-catalyzed ferrihydrite transformation and its consequences for C dynamics. Accordingly, we investigated the extent and pathway of Fe(II)-induced transformation of OM-ferrihydrite coprecipitates as a function of C/Fe ratios and aqueous Fe(II) concentrations, and its implications for subsequent C dynamics. The coprecipitated OM resulted in a linear decrease in ferrihydrite transformation with increasing C/Fe ratios. The secondary mineral profiles upon Fe(II) reaction with OM-ferrihydrite coprecipitates depend on Fe(II) concentrations At 0.2 mM Fe(II), OM completely inhibited goethite formation and stimulated lepidocrocite formation. At 2 mM Fe(II), whereas goethite was formed in the presence of OM, OM reduced the amount of goethite and magnetite formation and increased the formation of lepidocrocite. The solid-phase C content remained unchanged after reaction, suggesting that OM remains associated with Fe minerals following ferrihydrite transformation to more stable Fe minerals. However, C desorbability by H2PO4– from the resulting Fe minerals following reaction was enhanced. The study indicates a "lepidocrocite favoring effect" by OM and suggests that Fe(II)-catalyzed transformation of ferrihydrite may decrease OM stability in natural environments under moderately reducing conditions.
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The constant capacitance model and the Elovich equation were combined in the following mathematical expression enabling calculation of the amount of silicic acid adsorbed by iron oxides as a function of the Si concentration, pH, soil:solution ratio, and reaction time: Ka1 is the protolytic surface constant and Ksi is the stability constant for the Fe oxide-silicate surface complex; -logKa1 = 6.40–0.54(8-pH) and logKsi = 3.85 for ferrihydrite and goethite. Good agreement was found between calculated Si adsorption and the amount actually found to be adsorbed by synthetic ferrihydrite and goethite at different pH (3–6), Si concentration (1–600 μM), solid:solution ratio (1:300, 1:1200) and reaction time (1–480h).
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