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SOIL CHEMISTRY

Solubility of Symplesite (Ferrous Arsenate): Implications for Reduced Groundwaters and Other Geochemical Environments

United Nations Children's Fund (UNICEF), Water & Environmental Sanitation Section Dhaka, Bangladesh

Philip C. Singer

Dep. of Environ. Sciences and Eng. Univ. of North Carolina Chapel Hill, NC 27599

^* Corresponding author (rjohnston{at}unicef.org).

Arsenic in drinking water represents a major public health concern globally, with tens of millions of people thought to be consuming unsafe amounts of As daily. Groundwater is particularly vulnerable to As contamination, and one geochemical environment conducive to natural mobilization of As is found in shallow alluvial Holocene aquifers. Bacterial consumption of organic matter in these aquifers creates anoxic conditions, leading to dissimulatory reduction of Fe oxides. In these environments, dissolved As and Fe often co-occur. While As(III) is thermodynamically favored under these conditions, As(V) often is found as well. Concentrations of dissolved As(V) and Fe(II) can be constrained by the precipitation of ferrous arsenate solid phases such as symplesite [Fe(II)₃(As(V)O₄)₂·8H₂O]. We present a new solubility product for symplesite (pK_so = 33.25) based on controlled laboratory precipitation experiments. Using this solubility product, we conducted equilibrium geochemical modeling using one of the most extensive groundwater data sets from Bangladesh to show that a number of water samples are oversaturated with a variety of solid phases, including siderite, calcite, rhodochrosite, vivianite, and symplesite. Symplesite or other ferrous arsenate solid phases could be a significant sink for As(V) and Fe(II) in Bangladesh and other countries with similar geochemical environments. Symplesite could also control As(V) solubility in extreme environments such as alkaline lakes or in brines used for regeneration of anion exchange resins.