Wheat Straw as a Bio-Sorbent for Arsenate, Chromate, Fluoride, and Nickel

Batch-type experiments were used to study As(V), Cr(VI), F−, and Ni2+ sorption/desorption on wheat straw. For the lowest concentration added (0.5 mmol·L−1), the sorption sequence was F− > Ni2+ > Cr(VI) >> As(V) (93%, 61%, 29%, 0.3%), but changed to Ni2+ > F− > Cr(VI) >> As(V) when 3.0 and 6.0 mmol·L−1 were added (with 65%, 54%, 25%, 0%, and 68%, 52%, 27%, 0% sorption, respectively). Overall, As(V) showed the lowest sorption, whereas it was 25–37% for Cr(VI), 61–68% for Ni2+, and 52–93% for F−. For As(V), pH in the equilibrium solution was always above the pH of the point of zero charge (pHPZC) for wheat straw, decreasing sorption efficiency. For Cr(VI), pH was below pHPZC, but not enough to reach high sorption. For F−, pH in the equilibrium was above pHPZC, which could reduce sorption. For Ni2+, pH in the equilibrium was always below pHPZC, which made sorption difficult. The satisfactory fitting of Cr(VI), F−, and Ni2+ data to the Freundlich model suggests multilayer-type adsorption. Desorption was high for F−, whereas Ni2+ showed the lowest desorption. This research could be especially relevant when focusing on the use of wheat straw as a bio-sorbent, and in cases where straw mulching is used.