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...