Abstract

HERNANDEZ-BARCENAS, Gerardo; CASTILLO, Francisco; AVALOS-BORJA, Miguel  and  MARTINEZ-VILLEGAS, Nadia Valentina. SYNTHESIS OF CALCIUM ARSENATES (GUERINITE, HAIDINGERITE AND PHARMACOLITE) MORPHOLOGICALLY SIMILAR TO THOSE FOUND IN CONTAMINATED SOILS. Rev. Int. Contam. Ambient [online]. 2017, vol.33, n.1, pp.153-163. ISSN 0188-4999.  https://doi.org/10.20937/rica.2017.33.01.14.

Total dissolved arsenic concentrations up to 158 mg/L have been reported in the Matehuala and Cerrito Blanco aquifer in San Luis Potosi, Mexico from the dissolution of Ca5H2(AsO4)4·n H2O, as determined by hydrogeochemical modelling. In order to test the precipitation of this calcium arsenate, in this study, calcium arsenates were synthesized at similar pH and Ca/As aquifer conditions. Precipitation experiments were carried out in two synthesis. For synthesis 1, calcium arsenates were prepared by mixing 85.9 mM Na2HAsO4·7H2O solutions with slurries of 0.93 % (w/v) Ca(OH)2. For synthesis 2, calcium arsenates were prepared by mixing 114.4 mM Na2HAsO4·7H2O solutions with slurries of 0.97 % (w/v) Ca(OH)2. A total of six reactors were prepared, they were stored at room temperature for 500 h. Temperature, arsenic, calcium concentrations and pH were measured over time while the precipitates were identified by X-ray diffraction analyses and observed by scanning electron microscopy. According to X-ray diffraction results and the experimental conditions, a calcium arsenate, namely guerinite [Ca5H2(AsO4)4·9H2O] with the predicted stoichiometry, precipitated. Additionally, other two calcium arsenate mineral phases, namely haidingerite (CaHAsO4·H2O) and pharmacolite (CaHAsO4·2H2O) precipitated. The observation of the calcium arsenates precipitates allowed the identification of laminar, platty, and acicular habits associated to haidingerite, guerinite and pharmacolite, respectively. The morphology of the calcium arsenates precipitates in this study agrees with the morphology observed in calcium arsenates found in contaminated soils in Matehuala. Results from this study contribute to the morphological identification of calcium arsenates in the environment.

Keywords : arsenic; synthetic arsenates; crystal habit; X-ray diffraction; scanning electron microscopy; precipitation.

        · abstract in Spanish     · text in Spanish     · Spanish ( pdf )