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

Abstract

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.

Chromium VI and Fluoride Competitive Adsorption on Different Soils and By-Products

“…In Galicia (NW Spain), some environmental problems related to F − and Cr(VI) pollution have been previously pointed out [10][11][12][13]. In fact, the soils and sorbent materials used in this work to study F − and Cr(VI) competitive adsorption were the same previously described when performing individual adsorption tests for these two anions [13], in addition to pine sawdust, also previously described [12]. Specifically, in the current work we used samples of forest and vineyard soils, pyritic material, fine mussel shell, pine bark, oak ash, hemp waste, and pine sawdust.…”

“…Although different methodologies have been developed to remove F − and Cr(VI) from waters, such as precipitation, electrocoagulation, ion exchange, or electro-dialysis, the use of adsorbent materials has been considered as the most economical and sustainable alternative [2]. Previous works have dealt with individual adsorption of F − and Cr(VI), separately, both in soils and in different waste materials [3,[7][8][9][10][11][12][13]. In addition, some studies focused on simultaneous retention of F − and Cr(VI), using adsorbents such as a chitosan-alginate aluminum complex (CSAlg-Al) [4], or synthetic mesoporous alumina [2].…”

“…Galicia (NW Spain) is one of the geographical areas affected by activities which cause environmental pollution by F − and Cr(VI) (mining, aluminum factories, glass, dyes, leather tanning), and where it is also easy to obtain locally different by-products that could be used as low-cost sorbents for both pollutants [10][11][12][13]. Taking into account this previous background, the objectives of this work are: (1) to determine the capacity of forest and vineyard soils to adsorb F − and Cr(VI) simultaneously; (2) to determine the adsorption capacity for both anions of byproducts from forestry (oak ash, pine bark, and pine sawdust), from agriculture (hemp waste), from mining (pyritic material), and from the food industry (mussel shell).…”