Defluoridation of geothermal water by adsorption onto activated sludge: kinetics, isotherms and thermodynamics studies

In this work, inexpensive, easily available and typical waste activated sludge (generated by anaer-obic/anoxic/oxic (A2/O) and cyclic activated sludge system (CASS) processes) was investigated as an adsorbent for the defluoridation of geothermal water using both batch and column adsorption systems. Initially, the defluoridation efficiency was optimized in batch experiments according to sludge type, contact time, initial F- concentration and adsorbent dose. Fluoride removal efficiency of A2/O was superior to that of CASS. An optimum 86% fluoride removal efficiency occurred at an adsorbent dose of 60 g/L within 180 min. The adsorption behaviors were fitted well by the pseu-do-first-order model and Freundlich model. Moreover, the thermodynamic parameters illustrated that the adsorption process of fluoride on activated sludge was spontaneous and endothermic. The mechanism of fluoride adsorption was exchange with hydroxyls on the activated sludge. The release of OH- due to fluoride adsorption was confirmed by the observed pH values. Finally, a homemade column was used to systematically evaluate the defluoridation performance for real F-containing geothermal water. The dynamic adsorption capacity was 0.55 mg/g. The results demon-strated that this simple, economic and efficient sludge adsorbent has potential for application and can be developed into a feasible defluoridation technology for F- removal from geothermal water.