Synthesis of Arsenic-Bearing Magnesium Carbonate and Implications for Arsenic in Groundwater, Poster 7

The occurrence of arsenic (As) at harmful concentrations affects groundwater in many regions globally, and its source is usually naturally-occurring minerals. Iron sulfides and hydroxides are the most common As sources, but... [ view full abstract ]

The occurrence of arsenic (As) at harmful concentrations affects groundwater in many regions globally, and its source is usually naturally-occurring minerals. Iron sulfides and hydroxides are the most common As sources, but recent research indicates that carbonate minerals are another potential source. Calcite in veins and fault gouge in New Zealand contains up to 360 mg/kg (crustal average is 2 mg/kg), and magnesite veins in Vermont serpentinites contain up to 1000 mg/kg. This study presents results of synthesis of As-bearing magnesites to assess (1) As uptake capacity of magnesite, and (2) the effect of As on structure of magnesite. Synthesis reactants were milli-Q water and salts (urea, hydrated MgCl2 and Na­HAsO4) in 50 cm3 Teflon-lined reactors at 150 °C for 28 hours. Reaction products were examined by XRD, infrared, ICP-MS, SEM-EDS and grain size analysis. Synthesized minerals include < 60 mm rhombohedral magnesite and bladed hydromagnesite. Increasing As causes decreasing grain size and increasing ratio of hydromagnesite:magnesite. XRD and infrared imply shortening of Mg-O bond length with increasing As (apparent effect of AsO4-3 ↔ CO3-2). SEM-EDS mapping of polished grains shows zones of elevated As in outer 5 mm of crystal rims and in unevenly distributed blotchy zones within crystals.