The effect of sulfate and fluoride on the mobility of aluminum in soil

The effect of the coordinating anions sulfate and fluoride on the soil pH and A1 concentration, and on the transport of Al, was studied during unsaturated flow in columns of an Oxisol. The soil was infiltrated with 0-1 m NaCl, 1 or 10 mM SO24, 1 mM F, 1 mM Al, 1 mM Al with 1 or 10 mM SO24, and 1 mM Al with 1 or 10 mM F in a background of 0-1 M NaCl. The pH of the infiltrating solution was adjusted to 3-0.The inflowing solutions all caused a decrease in pH (relative to the initial value) near the column inlet and an increase further away from the inlet. The distance over which the change near the inlet occurred was greatly increased when Al was included in the inflow solutions. The magnitude of the pH change was affected by the accompanying anion; both SO24 and F resulted in a higher pH, with 10 mM F causing the greatest effect. Total solution Al concentration and the level of exchangeable (KCl-extractable) Al showed similar, but inverted, patterns compared with pH. Again, F caused the greatest effect and resulted in large decreases of Al in both the solution and exchange phases. A consistent relationship between pH and pAl was observed which was in part related to that determined in earlier batch experiments, but it was apparently not at equilibrium. The relationship between solution and exchangeable Al measured in the columns agreed closely with the Al adsorption isotherm determined previously for this soil.The effect of SO4- on pH, concentrations of Al species, and Al transport was as predicted from earlier batch studies, the addition of SO4- resulting in enhanced retardation of Al transport caused by a steepening of the Al adsorption isotherm. The effect of F was not totally in accord with earlier findings, which predicted an increase in the proportion of total Al in solution upon addition of F and an enhancement of transport; in fact the opposite occurred. It is argued that the increased importance of the formation of Al-F minerals at the low solution to soil ratios in the column experiments caused this shift in the distribution of Al between solution, KCl-extractable, and solid phases. It was therefore concluded that batch measurements have limited application to the prediction of Al chemistry at realistic soil water contents. It was further concluded that, while F formed very strong complexes with Al, neither SO4- nor F enhanced the transport of Al through soil. © 1995 CSIRO. All Rights Reserved.