A combined experimental study of vivianite and As (V) reactivity in the pH range 2-11

Four different sets of experiments were completed in order to constrain vivianite [Fe3(PO4)2 · 8H2O] reactivity under conditions pertinent to As(V)-bearing groundwater systems. Firstly, titration experiments were undertaken in the pH range 4-9 to determine the zero point of charge (ZPC) of vivianite; showing that the ZPC lies at a pH of approximately 5.3. Secondly, the steady state dissolution rates of vivianite far from equilibrium were measured in aqueous solutions in the pH range 2-10 at 18.5 °C (±3 °C) using a fluidized bed reactor. The rate of vivianite dissolution, R, is given by. (1)R (moles s- 1 cm- 2) = 1.18 × 10- 10 aH +0.77 + 1 × 10- 15 + 6.92 × 10- 24 aH +- 1. The dissolution rate exhibits an exponential increase with increase in the activity of the H+ ion (aH+) in solution at 2 <pH <5, is apparently pH independent at 5 <pH <8, and increases with increasing pH at pH > 8. Thirdly, the sorption of arsenate [As(V)] onto natural well-crystallized vivianite in the pH range 3-11 under static flow conditions was determined. 25-40% of As(V) from a starting concentration (C0) of 100 μM was adsorbed onto vivianite. Static adsorption experiments were also completed at two lower As(V) concentrations (C0 = 10 and 1 μM). Sorption was determined to be only weakly dependent on pH. Fourthly, the final part of this study investigated the sorption of As(V) onto vivianite at pH 9 under dynamic flow conditions. An input solution of 4 mM As(V) was applied to water saturated columns, followed by leaching with deionised water (DIW). Breakthrough curves show that the retention and exchangeability of As within the column is enhanced with vivianite present, consistent with solid phase analysis of unreacted and reacted solid materials. A simple calculation based on a model shallow Bengal sediment having about 0.2 wt% of vivianite and total initial dissolved As concentrations of 100 μM showed that under such conditions 88% of dissolved As(V) could potentially be adsorbed onto vivianite. These results will help to better understand As mobility in the presence of the Fe(II) mineral phase vivianite, thus providing better prediction of As mobility in partially reduced environments. © 2008 Elsevier Ltd. All rights reserved.