Effect of low molecular weight organic acids on adsorption and desorption of fluoride on variable charge soils

The effect of four low molecular weight organic acids on F- adsorption by two variable charge soils was investigated using a batch method. The organic acids reduced F- adsorption through competition by the acids with F- for sorption sites. Oxalic and malonic acids, both of which have simpler chemical structures, were more effective than citric or malic acid. The effect of organic acids on F- adsorption was more prominent at higher pH values and with larger amounts of the organic acids. The desorption study showed that the organic acids enhanced the desorption of F- adsorbed by the soils. In the control and malic acid systems, desorption increased sharply with decreasing pH, while in the oxalic acid system, desorption rose slightly with decreasing pH. Desorption also increased with increasing amount of organic acid added. There are two possible mechanisms for the effect of the organic acids on F- adsorption and desorption: (1) competition of the organic acids with F- for adsorption sites and (2) dissolution of the adsorbents, especially dissolution of soil Al.