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 Al concentration, and on the transport of Al, was studied duri ng unsaturated how in columns of an Oxisol. The soil was infiltrated w ith 0.1M NaCl, 1 or 10 mM SO42-, 1 mM F-, 1 mM Al, 1 mM Al with 1 or 1 0 mM SO42-, and 1 mM Al with 1 or 10 mM F- in a background of 0.1 M Na Cl. The pH of the infiltrating solution was adjusted to 3.0. The inflo wing solutions all caused a decrease in pH (relative to the initial va lue) near the column inlet and an increase further away from the inlet . The distance over which the change near the inlet occurred was great ly increased when Al was included in the inflow solutions. The magnitu de of the pH change was affected by the accompanying anion; both SO42- and F- resulted in a higher pH, with 10 mM F- causing the greatest ef fect. Total solution Al concentration and the level of exchangeable (K Cl-extractable) Al showed similar, but inverted, patterns compared wit h pH. Again, F- caused the greatest effect and resulted in large decre ases of Al in both the solution and exchange phases. A consistent rela tionship between pH and pAl was observed which was in part related to that determined in earlier batch experiments, but it was apparently no t at equilibrium. The relationship between solution and exchangeable A l measured in the columns agreed closely with the Al adsorption isothe rm determined previously for this soil. The effect of SO42- on pH, con centrations of Al species, and Al transport was as predicted from earl ier batch studies, the addition of SO42- resulting in enhanced retarda tion of Al transport caused by a steepening of the Al adsorption isoth erm. 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 oppo site occurred. It is argued that the increased importance of the forma tion of Al-F minerals at the low solution to soil ratios in the column experiments caused this shift, in the distribution of Al between solu tion, KCl-extractable, and solid phases. It was therefore concluded th at 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 SO42- nor F- enhanced the transport of Al through soil.