Coprecipitation of arsenic with iron or aluminum occurs in natural environments and is a remediation technology used to remove this toxic metalloid from drinking water and hydrometallurgical solutions. In this work, we studied the nature, mineralogy, and reactivity toward phosphate of iron-arsenate coprecipitates formed at As(V)/Fe(III) molar ratios (R) of 0, 0.01, or 0.1 and at pH 4.0, 7.0, and 10.0 aged for 30 or 210 days at 50 degrees C and studied the desorption of arsenate. At R = 0, goethite and hematite (with ferrihydrite at pH 4.0 and 7.0) crystallized, whereas at R = 0.01, the formation of ferrihydrite increased and hematite crystallization was favored over goethite. In some samples, the morphology of hematite changed from rounded platy crystals to ellipsoids. At R = 0.1, ferrihydrite formed in all the coprecipitates and remained unchanged even after 210 days of aging. The surface area and chemical composition of the precipitates were affected by pH, R, and aging. Chemical dissolution of the samples showed that arsenate was present mainly in ferrihydrite, but at R = 0.01, it was partially incorporated into the structures of crystalline Fe oxides. The sorption of phosphate on to the coprecipitates was affected not only by the mineralogy and surface area of the samples but also by the amounts of arsenate present in the oxides. The samples formed at pH 4.0 and 7.0 and at R = 0.1 sorbed lower amounts of phosphate than the precipitates obtained at R = 0 or 0.01, despite the former having a larger surface area and showing only a presence of short-range ordered materials. This is mainly due to the fact that in the coprecipitates at R = 0.1 arsenate occupied many sorption sites, thus preventing phosphate sorption. Less than 20% of the arsenate present in the coprecipitates formed at R = 0.1 was removed by phosphate and more from the samples synthesized at pH 7.0 or 10.0 than at pH 4.0. Moreover, we found that more arsenate was desorbed by phosphate from a ferrihydrite on which arsenate was added than from an iron-arsenate coprecipitate, attributed to the partial occlusion of some arsenate anions into the framework of the coprecipitate. XPS analyses confirmed these findings.
Arsenic mobilization in soils is mainly controlled by sorption/desorption processes, but arsenic also may be coprecipitated with aluminum and/or iron in natural environments. Although coprecipitation of arsenic with aluminum and iron oxides is an effective treatment process for arsenic removal from drinking water, the nature and reactivity of aluminum- or iron-arsenic coprecipitates has received little attention. We studied the mineralogy, chemical composition, and surface properties of aluminum-arsenate coprecipitates, as well as the sorption of phosphate on and the loss of arsenate from these precipitates. Aluminum-arsenate coprecipitates were synthesized at pH 4.0, 7.0, or 10.0 and As/Al molar ratio (R) of 0, 0.01, or 0.1 and were aged 30 or 210 d at 50 degrees C. In the absence of arsenate, gibbsite (pH 4.0 or 7.0) and bayerite (pH 10.0) formed, whereas in the presence of arsenate, very poorly crystalline precipitates formed. Short-range ordered materials (mainly poorly crystalline boehmite) formed at pH 4.0 (R = 0.01 and 0.1), 7.0, and 10.0 (R= 0.1) and did not transform into Al(OH)3 polymorphs even after prolonged aging. The surface properties and chemical composition of the aluminum precipitates were affected by the initial pH, R, and aging. Chemical dissolution of the samples by 6 mol L(-1) HCl and 0.2 mol L(-1) oxalic acid/ oxalate solution indicated that arsenate was present mainly in the short-range ordered precipitates. The sorption of phosphate onto the precipitates was influenced by the nature of the samples and the amounts of arsenate present in the precipitates. Large amounts of phosphate partially replaced arsenate only from the samples formed at R = 0.1. The quantities of arsenate desorbed from these coprecipitates by phosphate increased with increasing phosphate concentration, reaction time, and precipitate age butwere always lessthan 30% of the amounts of arsenate present in the materials and were particularly low (<4%) from the sample prepared at pH 4.0. Arsenate appeared to be occluded within the network of short-range ordered materials and/or sorbed onto the external surfaces of the precipitates, but sorption on the external surfaces seemed to increase by increasing pH of sample preparation and aging. Furthermore, at pH 4.0 more than in neutral or alkaline systems the formation of aluminum arsenate precipitates seemed to be favored. Finally, we have observed that greater amounts of phosphate were sorbed on an aluminum-arsenate coprecipitate than on a preformed aluminum oxide equilibrated with arsenate under the same conditions (R = 0.1, pH 7.0). In contrast, the opposite occurred for arsenate desorption, which was attributed to the larger amounts of arsenate occluded in the coprecipitate.
The aim of this work was to study the influence of tannate concentration (tannic acid/Al molar ratio [R] of 0.01–0.1), pH (5.0 and 8.0), order of aluminum, hydroxyl, and tannate addition, and prolonged ageing at different temperatures (10 months at 50°C and 5 y at room temperature and further ageing for 3–15 days at 140°C) on the nature and crystallinity of synthetic boehmite. Tannate facilitated the formation of boehmite relative to Al(OH)3 polymorphs when coprecipitated with Al and also when added to Al(OH)x phases already formed at pH 4.5 or 8.0. However, the organic ligand was more effective in preventing or perturbing the growth of the crystals of boehmite when coprecipitated with Al than when added to soluble Al(OH)x species or preformed Al precipitates. Boehmite aged at temperatures ⩽50°C typically showed a fibrous morphology. Crystals with a rhombic habit were observed in some treatments after ageing at 140°C. Significant amounts (15–30% C) of tannate were present in the precipitates after prolonged ageing. This tannate was only partly (<50%) oxidized by repeated treatments with H2O2. With further ageing at 140°C, the crystallite size of selected samples increased only slightly, suggesting that much of the remaining tannate was present in the internal structure of these samples and not simply adsorbed to their surfaces. Atomic force microscopy observation showed the presence of globular nanoparticles (probably Al tannate precipitates) attached to the elongated crystals of boehmite. Taken together, our results demonstrate that the crystalline phases that formed under mild conditions in the presence of low concentrations (R = 0.01) of foreign ligands have the same structure as boehmite, but with a poorly ordered and defective ion arrangement. At higher ligand concentrations (R ⩾0.05), mixtures of materials are formed having varying degrees of order, particle size and morphology.
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