Arsenate Incorporation in Gypsum Probed by Neutron, X-ray Scattering and Density Functional Theory Modeling

Abstract

2The ability of gypsum, a common sulfate mineral, to host arsenic atoms in its crystalline structure, is 1 demonstrated through experimental structural studies of the solid solutions formed upon synthetic co-2 precipitation of gypsum (CaSO 4 ·2H 2 O) and arsenic. Neutron and X-ray diffraction methods show an 3 enlargement of the gypsum unit cell proportional to the concentration of arsenic in the solids and to the 4 pH solution value. The substitution of sulfate ions (SO 4 2-) by arsenate ions is shown to be more likely 5 under alkaline conditions, where the HAsO 4 2-species predominates. A theoretical Density Functional 6Theory model of the arsenic-doped gypsum structure reproduces the experimental volume expansion. 7Extended X-ray Absorption Fine Structure (EXAFS) measurements of the local structure around the 8 arsenic atom in the co-precipitated solids confirm solid state substitution and allow some refinement of 9 the local structure, corroborating the theoretical structure found in the simulations. The charge re-10 distribution within the structure upon substitutions of either the protonated or the unprotonated arsenate 11 species studied by means of Mulliken Population Analyses demonstrates an increase in the covalency in 12 the interaction between Ca 2+ and AsO Arsenic is a metalloid widely distributed in the biosphere and highly toxic [1, 2]. It is present in many 2 industrial sites where mineral ores of lead, copper, zinc, tin, cobalt, gold or silver have been smelted [3]. 3 Some As-bearing minerals (like Arsenopyrite FeAsS) are used as raw materials for some of these 4 processes, causing the release of high quantities of arsenic to the environment in the form of arsenite 5 (As 3+ ) or arsenate (As 5+ ) [4, 5]. The toxicity of arsenic depends on its physico-chemical forms, the 6 arsenite species being more mobile and toxic than arsenate. Redox transformations play as well an 7 important role in the arsenic availability to the environment. Changes in the redox state can give rise to 8 precipitation processes of solid phases, thus decreasing the concentration of arsenic in groundwaters [5-9 7]. The solubility of these solid phases controls the concentration of arsenic aqueous species that are 10 available to the environment. Co-precipitation of As-free minerals like gypsum in the presence of 11 arsenic may lead to long term immobilization of the contaminant, until the host phase is dissolved. For 12 this reason a good understanding of the interactions between the solid and the contaminant and the 13 underlying substitution process is required. 14 The study of ion substitution in minerals has a big impact in the study of the long-term retention of 15 contaminants (as As [6,[8][9][10] Laser Fluorescence Spectroscopy methods [13, 14,[17][18][19][20][21][22]. 22Gypsum is a common industrial by-product from a number of processes involving neutralization of 23 sulfuric acid and SO 2 -rich fumes [23, 24]. Some industrial activities where mineral ores are smelted 24 generate As-rich gypsum sludges, produced up...

Incorporation of arsenic into gypsum: Relevant to arsenic removal and immobilization process in hydrometallurgical industry

“…Gypsum (CaSO 4 ·2H 2 O) is generated during Fe(III)-As(V) coprecipitation process because of the use of lime as base for neutralization [15]. Arsenic can be incorporated into gypsum lattice structure via isomorphic substitution for sulfate as unambiguously evidenced by neutron diffraction [16,17]. This finding is confirmed by a latest study using singlecrystal electron paramagnetic resonance spectroscopy (EPR) and pulsed electron nuclear double resonance spectroscopy (ENDOR) [18].…”

“…This is similar to the incorporation of arsenate ions into jarosite structure by substitution of sulfate [19] and vice versa incorporation of sulfate ions into scorodite structure by substitution of arsenate [20,21]. Fernández-Martínez et al [16,17] estimated the concentration of arsenic incorporated into the bulk of gypsum by interpolating the experimental and theoretical volume expansion data. The results indicated that more arsenic could be incorporated into gypsum at higher pH.…”

“…The Rietveld analysis showed that the incorporation of arsenate into the structure of gypsum induced small volume expansion of its unit cell (Table 2). Previous study also observed the expansion of unit cell volume of the gypsum after incorporation of arsenate by using neutron diffraction [17].…”

Solidification/Stabilization of Arsenic in Red Mud upon Addition of Fe (III) or Fe (III) and Al (III) Dissolved in H₂SO₄

“…The main mechanisms of these include the adsorption of arsenate via surface complex on iron oxides (hematite goethite) and ferrihydrite mainly as an inner-sphere bidentate complex [16][17][18][19] and the formation of insoluble Fe-As compounds, ferric arsenate, scorodite and poorly crystalline ferric arsenate [20][21][22]. Additionally, Al ions, hydroxide and oxide of aluminum also showed good behaviors in adsorbing arsenic as well as Fe [23,24].…”

Hemimorphite as a natural sink for arsenic in zinc deposits and related mine tailings: Evidence from single-crystal EPR spectroscopy and hydrothermal synthesis

“…Numerous studies have demonstrated that the mobility and bioavailability of As are usually closely related to its host phases (i.e., minerals in many cases; Foster et al, 1998;Courtin-Nomade et al, 2009;Walker et al, 2009). Therefore, significant efforts have been devoted to the identification and development of minerals (or their synthetic equivalents) that are capable of fixing or minimizing the mobility and bioavailability of As in the environment (e.g., Bothe and Brown, 1999;Di Benedetto et al, 2006;Blanchard et al, 2007;Fernández-Martínez et al, 2008;Lee et al, 2009;Drahota and Filipi, 2009).…”