Abstract:
High concentration of fluoride in groundwater is a longstanding health problem. Fluoride adsorption capacity of termite mound (TM), containing
mainly silicon, aluminum, iron, and titanium oxides,
was investigated under a batch adsorption system. The
influence of parameters such as contact time, solution
pH, adsorbent dose, initial fluoride concentration, and
the presence of competing anions was investigated.
Equilibrium was achieved within 10 min of agitation
time. A high percentage (~90 %) of fluoride removal
was obtained in a wide pH range 3–8, which is important in the practical application. Kinetics data
followed the pseudo-second-order model (R2 > 0.99).
The Dubinin–Radushkevich isotherm described most
satisfactorily (R2 = 0.968, χ2 = 0.09) the equilibrium
adsorption, giving a sorption capacity of 2.70 mg/g.
The obtained mean free energy (EDR = 11.62 kJ/mol)
suggested that chemisorption should be mainly responsible for fluoride adsorption. Fluoride removal was significantly decreased in the presence of carbonate and
phosphate ions, whereas slightly increased in the presence of chloride, nitrate, and sulfate. The adsorbent
reduced 7.56 mg/L fluoride content of groundwater to
below 1.5 mg/L. The fluoride-loaded TM was successfully regenerated using calcined eggshell or NaOH solution with insignificant loss of metals. The adsorption
efficiency of the regenerated TM was comparable to the
fresh TM. The results obtained from this study could
provide important information for evaluating the application of TM for defluoridation.
