Effect of pH and prior phosphate application on the reaction of fluoride with soils from northern India

Abstract

We investigated whether long‐term application of phosphate fertilizers, often as phosphate rock, had led to the accumulation of soluble fluoride in soil from tea estates in northern India. We also compared fluoride sorption and desorption with that of sulphate and of phosphate. We found no evidence of build‐up of soluble fluoride and ascribed this to the very heavy rainfall of about 3 m, nearly all of which occurs during a 4‐month monsoon period. Fluoride sorption was more sensitive than phosphate or sulphate sorption to the build‐up of a negative charge caused by long‐term application of phosphate fertilizers. We think that this occurred because the fluoride ion is smaller and the mean centre of charge is, therefore, located closer to the surface. When the negative charge is increased, it, therefore, experiences a greater decrease in potential than ions for which the mean centre of charge is further from the surface. When the phosphate status of the soil was low, fluoride showed sorption–desorption hysteresis. That is the desorption curve differed from the sorption curve. When the phosphate status of the soil was higher, this was not the case. Similar effects have been observed previously for both phosphate and sulphate. We had speculated that the fluoride ion, being smaller, might have been able to penetrate pores ‘blocked’ by a previous reaction with phosphate, but this did not happen. We conclude that characterizing the charge on sorbed ions, its location and especially the accumulation of a negative charge following a long‐term reaction with phosphate is important for understanding specific sorption on ions.Highlights We investigated the effects of long‐term application of phosphate rock on tea estates. We ascribed the lack of build‐up of soluble fluoride to monsoon rainfall of about 3 m. Fluoride sorption was more sensitive than phosphate or sulphate sorption to the build‐up of a negative charge caused by long‐term application of phosphate fertilizers. Characterizing the charge on sorbed ions, its location and the accumulation of a negative charge is important.