The slow reactions between soil and anions: 6. Effect of time and temperature of contact on fluoride

Sodium fluoride was incubated with soils for periods of up to 100 days and at temperatures that ranged from 3° to 80°C. With increasing period and temperature of incubation there were decreases in both the concentration of fluoride in the soil solution and in the fluoride that could be desorbed in a given period. It was concluded that this was due to an increase in the firmness of the link between fluoride and the surface of the soil particles. This increase also changed the adsorption characteristics of the soil, making it less able to adsorb further fluoride and less able to adsorb phosphate. The results were analogous to those obtained on the same soils with phosphate and molybdate. This indicates that the increase with time in the strength of the bond between phosphate or molybdate and the soil cannot be entirely explained by postulating that it is due to forming a second link to the surface through a second oxygen atom.