Chromium and fluoride sorption/desorption on un-amended and waste-amended forest and vineyard soils and pyritic material.

Using batch-type experiments, chromium (Cr(VI)) and fluoride (F^–) sorption/desorption were studied in forest and vineyard soil samples, pyritic material, pine bark, oak ash, hemp waste and mussel shell, as well as on samples of forest and vineyard soil, and of pyritic material, individually treated with 48 t ha^-1 of pine bark, oak ash, and mussel shell. Pine bark showed the highest Cr(VI) sorption (always > 97% of the concentration added) and low desorption (<1.5%). Pyritic material sorbed between 55 and 98%, and desorbed between 0.6 and 9%. Forest and vineyard soils, oak ash, mussel shell and hemp waste showed Cr(VI) sorption always < 32%, and desorption between 22 and 100%. Pine bark also showed the highest F^– retention (sorption between 62 and 73%, desorption between 10 and 15%), followed by oak ash (sorption 60-69%, desorption 11-14%), forest soil (sorption 60-73%, desorption 19-36%), and pyritic material (sorption 60-67%, desorption 13-15%), whereas in vineyard sorption was 49-64%, and desorption 24-27%, and in hemp waste sorption was 26-36%, and desorption 41-59%. Sorption data showed better fitting to the Freundlich than to the Langmuir model, especially in the case of Cr(VI), indicating that multilayer sorption dominated. The addition of by-products to the forest and vineyard soils, and to the pyritic material, caused an overall increase in F^– sorption, and decreased desorption. Furthermore, the pine bark amendment resulted in increases in Cr(VI) retention by both soils and the pyritic material. These results could be useful to favor the recycling of the by-products studied, aiding in the management of soils and degraded areas affected by Cr(VI) and F^– pollution, and in the removal of both anions from polluted waters.