Sulphate sorption at high equilibrium concentration in Andosols

The main process by which inorganic SO42− inputs are retained in soils is through adsorption on soil colloids. However, in soils subject to intense SO42− addition, precipitation of Alx(SO4)y(OH)z minerals also may contribute to SO42− sorption. While direct observation of Alx(SO4)y(OH)z precipitate in soil has been reported only recently, SO42− sorption isotherms experiments have been rarely performed to distinguish between adsorption and precipitation processes.Traditionally, SO42− sorption experiments have involved relatively low SO42− concentrations in the range 0–5 mM. However, for SO42− precipitation to occur, higher concentrations may be needed. Here, we evaluated SO42− sorption in four contrasting Andosols samples by measuring sorption isotherms with SO42− concentrations ranging from 0.1 to 15 mM and under low pH conditions (pH 4). A 1 h sorption equilibration time was allowed. All sorption isotherms showed a distinct biphasic shape. In the lowest SO42− concentration range (< 5 mM), SO42− sorption obeyed a Langumir isotherm, whereas at higher SO42− concentrations (> 6 mM), SO42− sorption could be described by a linear isotherm. The transition between a Langmuir-type and a linear-type isotherm also corresponded to the transition point where the solution was oversaturated with respect to Alx(OH)y(SO4)z minerals, according to thermodynamic calculations. We interpreted these results in terms of initial SO42− adsorption onto the surface of soil constituents at lower dissolved SO42− concentrations followed by SO42− precipitation at high dissolved SO42− concentrations.