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.

As(V) Sorption/Desorption on Different Waste Materials and Soil Samples.

Aiming to investigate the efficacy of different materials as bio-sorbents for the purification of As-polluted waters, batch-type experiments were employed to study As(V) sorption and desorption on oak ash, pine bark, hemp waste, mussel shell, pyritic material, and soil samples, as a function of the As(V) concentration added. Pyritic material and oak ash showed high sorption (90% and >87%) and low desorption (<2% and <7%). Alternatively, hemp waste showed low retention (16% sorption and 100% desorption of the amount previously sorbed), fine shell and pine bark sorbed <3% and desorbed 100%, the vineyard soil sample sorbed 8% and released 85%, and the forest soil sample sorbed 32% and desorbed 38%. Sorption data fitted well to the Langmuir and Freundlich models in the case of both soil samples and the pyritic material, but only to the Freundlich equation in the case of the various by-products. These results indicate that the pyritic material and oak ash can be considered efficient As(V) sorbents (thus, useful in remediation of contaminated sites and removal of that pollutant), even when As(V) concentrations up to 6 mmol L-1 are added, while the other materials that were tested cannot retain or remove As(V) from polluted media.

As(V)/Cr(VI) pollution control in soils, hemp waste, and other by-products: competitive sorption trials.

We study As(V)/Cr(VI) competitive sorption on a forest soil, a vineyard soil, pyritic material, mussel shell, pine bark, oak ash, and hemp waste, adding variable As(V) and Cr(VI) concentrations or displacing each pollutant with the same concentration of the other. When using variable concentrations, As(V) showed more affinity than Cr(VI) for sorption sites on most materials (sorption up to >84 % on oak ash and pyritic material). The only exception was pine bark, with clearly higher Cr(VI) sorption (>90 %) for any Cr(VI)/As(V) concentration added. Regarding the displacement experiments, when As(V) was added and reached sorption equilibrium, the subsequent addition of equal Cr(VI) concentration did not cause relevant As displacement from oak ash and pyritic material, indicating strong As bindings, and/or low competitive effects. When Cr(VI) was added and reached sorption equilibrium, the subsequent addition of equal As(V) concentration caused Cr(VI) displacement from all materials except pine bark, indicating weak Cr bindings. In view of these results, oak ash and the pyritic material could be used to remove As(V) in concentrations as high as 6 mmol L(-1), even in the presence of a wide range of Cr(VI) concentrations, whereas pine bark could be used to remove Cr(VI) concentrations as high as 6 mmol L(-1). The other materials assayed (including hemp waste, studied for the first time as As(V) and Cr(VI) bio-sorbent) cannot be considered appropriate to remove As(V) and/or Cr(VI) from polluted media.

F sorption/desorption on two soils and on different by-products and waste materials.

We used batch-type experiments to study F sorption/desorption on a forest soil, a vineyard soil, pyritic material, granitic material, finely and coarsely ground mussel shell, mussel shell calcination ash, oak wood ash, pine-sawdust, slate processing fines, and three different mixtures that included three components: sewage sludge, mussel shell ash, and calcined mussel shell or pine wood ash. The three waste mixtures, forest soil, pyritic material, and shell ash showed high sorption capacity (73-91 % of added F) and low desorption, even when 100 mg F L(-1) was added. All these materials (and to a lower extent wood ash) could be useful to remove F from polluted media (as certain soils, dumping sites, and contaminated waters). The vineyard soil, the granitic material, mussel shell, slate fines, and pine-sawdust were less effective in F removal. In most cases, sorption data fitted better to the Freundlich than to the Langmuir equation. These results can be useful to program the correct management of the soils, by-products, and waste materials assayed, mostly in situations where F concentrations are excessive and F removal should be promoted.