RETENTION OF FLUORIDE FROM INDUSTRIAL ACIDIC WASTEWATER AND NaF SOLUTION BY THREE TUNISIAN CLAYEY SOILS

Clayey soil samples, Sebkhat El Malah-El Mkhacherma (MK), Jebel Haidoudi (H), and Zemlet El Beidha (ZB), from three regions in Southeast Tunisia were evaluated for their effectiveness to remove fluoride (F) from acidic industrial waste water with pH 2.75-3.0 and [F] = 2360 mg/L. F removal by these soils from solutions containing 50 to 10,000 mg/NaF/L at PH 3.0, 4.0, 5.5, and 6.5 with a contact time > 24 hr and PH = 3.0 was optimal for treating the acidic wastewater. For the three soils the sorption capacity was found to be in the order of 50-60 mg F/g, and it reached to 85% of the fluoride removal from the wastewater in the case of the MK soil. Fitting the results to the Langmuir isotherm adsorption model gave correlation coefficients (r(2)) ranging from 0.982 to 0.994 and a better fit than with the Freundlich model. The MK soil containing palygorskite clay and calcite was superior as a F sorbent (MK>H>ZB) where the adsorption capacities (Q(0)) calculated from the Langmuir isotherms were, respectively, 71.94, 66.62, and 55.80 mg F/g soil from the NaF solution and 93.45, 84.03, and 69.44 mg F/g soil from the acidic wastewater.