Grape pomace as a biosorbent for fluoride removal from groundwater

This study presents a new type of biomass material for defluoridation from water; the material was prepared by loading tetravalent zirconium ions onto grape pomace produced from grape juicing and wine factories. Experiments showed that the optimum pH of defluoridation is around 3.0, and the fluorine removal efficiency could reach 96.13% for one-time contact. In batchwise adsorption tests, it was very interesting to find that even at pH values near 10, at which traditional adsorbents usually do not function for defluoridation, the removal efficiency of fluoride was still more than 90% for the Zr(iv)-loaded grape pomace (Zr(iv)-GP) biosorbent; proton release from Zr(iv)-GP was confirmed to cause an automatic decrease of the pH, which can save additional acid consumption in the case of one-time use and render the defluoridation more convenient and efficient. The maximum adsorption capacity of Zr(iv)-GP was 7.54 mg g^-1; as a comparison, the maximum adsorption capacities of zirconium-loaded strongly acidic ion exchange resin D001 and zirconium-loaded weakly acidic ion exchange resin D113 were evaluated to be 4.85 mg g^-1 and 1.14 mg g^-1, respectively. The effects of coexisting anions, such as Cl^-, NO^3-, SO₄ ^2-, CO₃ ^2- and HPO₄ ^2-, on the fluorine removal efficiency were also examined; it was found that CO₃ ^2- and HPO₄ ^2- anions had drastically adverse effects on defluoridation, while Cl^-, NO^3-, and SO₄ ^2- appeared not to interfere. Real groundwater containing 1.8 mg L^-1 fluoride sampled from Guanzhuang Village in Haixing County of Hebei Province was used for defluoridation through a continuous column adsorption process; it was found that pre-adjusting the groundwater pH affected the purification efficiency drastically, i.e., the time of the breakthrough point for the inlet groundwater pH at 3.0 was about 8 times longer than that at the original pH of 8.18. In addition, the Zr(iv)-GP adsorbent retained good adsorption capacity even after 3 cycles of adsorption-desorption-adsorption operations, indicating that the synthesized zirconium-loaded grape pomace is a very promising new fluorine-removing material for groundwater purification.