Arsenite and arsenate sorption was studied on two silt-sized phyllosilicates, namely muscovite and biotite, as a function of solution pH (pH 3-8 for muscovite, and 3-11 for biotite) at an initial As concentration of 13 mu M. The amount of arsenic adsorbed increases with increasing pH, exhibiting a maximum value, before decreasing at higher pH values. Maxima correspond to 3.22 +/- 0.06 mmol kg(-1) As(V) at pH 4.6-5.6 and 2.86 +/- 0.05 mmol kg(-1) As(III) at pH 4.1-6.2 for biotite, and 3.08 +/- 0.06 mmol kg(-1) As(III) and 3.13 +/- 0.05 mmol kg(-1) As(V) at pH 4.2-5.5 for muscovite. The constant capacitance surface complexation model was used to explain the adsorption behavior. Biotite provides greater reactivity than muscovite toward arsenic adsorption. Isotherm data obeyed the Freundlich or Langmuir equation for the arsenic concentration range 10(-7)-10(-4) M. Released total Fe, Si, K, Al, and Mg in solution were analyzed. Calculation of saturation indices by PHREEQC indicated that the solution was undersaturated with respect to aluminum arsenate (AlAsO4 center dot 2H(2)O), scorodite (FeAsO4 center dot 2H(2)O), and claudetite/arsenolite (As4O6). (C) 2007 Elsevier Inc. All rights reserved.