Effects of Changing pH, Incubation Time, and As(V) Competition, on F Retention on Soils, Natural Adsorbents, By-Products, and Waste Materials.

The purpose of this work was to elucidate the repercussion of changing pH, incubation time and As(V) competition on fluoride (F) sorption on forest and vineyard soil samples, pyritic, and granitic materials, as well as on the by-products pine sawdust, oak wood ash, mussel shell ash, fine and coarse mussel shell, and slate processing waste fines. To reach this end, the methodological approach was based on batch-type experiments. The results indicate that, for most materials, F sorption was very high at the start, but was clearly diminished when the pH value increased. However, oak wood ash and shell ash showed high F sorption even at alkaline pH, and pine sawdust showed low F sorption for any pH value. Specifically, F sorption was close to 100% for both ashes at pH < 6, and around 70% at pH 10, while for forest soil it was close to 90% at pH < 2, and around 60% at pH values near 8. Regarding the effect of incubation time on F sorption, it was very low for both soils, pyritic material, granitic material, and both kinds of ashes, as all of them showed very rapid F sorption from the start, with differences being lesser than 10% between sorption at 30 min and 1 month of incubation. However, sawdust and slate fines sorbed 20% of added F in 30 min, remaining constant up to 12 h, and doubling after 30 days. And finally, mussel shell sorbed 20% at 30 min, increasing to close to 60% when incubation time was 30 days. This means that some of the materials showed a first sorption phase characterized by rapid F sorption, and a slower sorption in a second phase. As regards the effect of the presence of As(V) on F sorption, it was almost negligible, indicating the absence of competition for sorption sites. In view of that all, these results could aid to appropriately manage soils and by-products when focusing on F removal, in circumstances where pH value changes, contact time vary from hours to days, and potential competition between F and As(V) could take place.