Coprecipitation of Arsenate with Metal Oxides. 2. Nature, Mineralogy, and Reactivity of Iron(III) Precipitates

Abstract

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.

Formation, properties and reactivity of coprecipitates and organomineral complexes in soil environments

“…Many studies have been carried out on the formation, surface properties, mineralogy and reactivity of coprecipitates formed by the interaction of low molecular mass organic ligands (LMMOLs; e.g., oxalate, tartrate, citrate and tannate) with Al and/ or Fe hydrolytic products (Violante et al, 2003;2007;Violante, 2013 Acetate < glutarate < succinate = phthalate < glycine < tricarballilate < malonate < acetylacetone < glutamate < aspartate < oxalate < salicylate = malate < tannate < citrate < tartrate. Table 2).…”

“…Violante et al (2006Violante et al ( , 2007Violante et al ( , 2009b evidenced that the mineralogy, the surface properties and chemical composition of the AlAs(V) or Fe-As(V) coprecipitates were affected by the initial pH, initial As/Al (Fe) molar ratio (R) and aging. These authors demonstrated that less As(V) was replaced by P from As(V) iron and/or aluminum coprecipitates than from previously formed iron and/ or aluminum oxides on which As(V) was sorbed.…”

Chemical Processes Affecting the Mobility of Heavy Metals and Metalloids in Soil Environments

“…Some studies were carried out by Violante et al [54][55][56] on samples formed by coprecipitating arsenate with aluminum and/or iron. These authors demonstrated that less ) of arsenite and arsenate in absence and presence of competing anions (initial anion/arsenite or arsenate molar ratio of 1.0), at pH 6.0 and 20°C, and efficiency (%) (i.e., efficiency of the anions in inhibiting arsenite or arsenate sorption on sorbent surfaces) on ferrihydrite (A) [52], noncrystalline Al oxides (B) [53], and MgFe-LDH (C) [26,27] Arsenite Arsenate Anion Sorption (mmol kg arsenate was replaced by phosphate from arsenate iron and/or aluminum coprecipitates than from previously formed iron and/or aluminum oxides on which arsenate was sorbed [57].…”

Dynamic Microbial Community Associated with Iron–Arsenic Co-Precipitation Products from a Groundwater Storage System in Bangladesh

“…While arsenate mobility is limited by chemisorption to the surface of several common minerals, such as iron (Fe) and aluminium oxy(hydr)oxides, arsenite is effectively adsorbed by a fewer minerals and less strongly retained on the surfaces [59]. The strong affinity of Fe oxy(hydr)oxides towards As and the formation of Fe-As co-precipitation products have been widely studied, and the co-precipitation of Fe oxy(hydr) oxides including As and/or other contaminants is a method commonly exploited in As removal plants [26,67].…”