Uranium biosorption by hydroxyapatite and bone meal: evaluation of process variables through experimental design

Biosorption has been examined for the treatment of aqueous solutions containing uranium, a radiotoxic pollutant. Nevertheless, the evaluation of the role of process variables by experimental design on the use of hydroxyapatite and bone meal as biosorbents for uranium has not yet been previously addressed. In this study, the effects of adsorbent dosage (M), initial uranium concentrations ([U]), and solution pH were investigated, using a two-level factorial design and response surface analysis. The experiments were performed in batch, with [U] of 100 and 500 mg L, pH 3 and 5, and adsorbent/uranium solution ratios of 5 and 15 g L. Contact time was fixed at 24 h. Removal rates were higher than 88%, with a maximum of 99% in optimized conditions. [U] and M were found to be the most influential variables in U removal in terms of adsorption capacity (q). The experiments revealed that bone meal holds higher adsorption capacity (49.87 mg g) and achieved the highest uranium removal (~ 100%) when compared to hydroxyapatite (q = 49.20 mg g, removal = 98.5%). The highest value of q for both biomaterials was obtained for [U] = 500 mg L, pH 3, and M = 5 g L. Concerning the removal percentage, bone meal achieved the best performance for [U] = 500 mg L, pH 3, and M = 15 g L. Further experiments were made with real radioactive waste, resulting in a high uranium adsorption capacity for both materials, with 22.11 mg g for hydroxyapatite and 22.08 mg g for bone meal, achieving uranium removal efficiencies higher than 99%.