A new adsorbent for fluoride removal: The utilization of sludge waste from electrocoagulation as adsorbent

This work investigated the potential of calcined electrocoagulation sludge (CES) within metals hydroxide generated during removal of boron using Al electrode for adsorption of fluoride from aqueous solution. The effects of contact time, pH of the solution (2-10), stirrer speed (50-450 rpm), initial concentration (5-100
mg l^-1), adsorbent dose (1-4 mg l^-1), solution temperature (293-333 K) and particle size (0.125-1000 µm) on fluoride removal were investigated. All the experiments were carried out by batch mode. It was found that the maximum adsorption takes place within 2 h at pH 6.0. The adsorption removal increased with increase in the adsorbent dose, but decreased with increase in fluoride concentration. It was found that the adsorption removal decreases with increase in temperature, which showed that the adsorption process was exothermic in nature. The decrease in particle size increased fluoride removal efficiency. The maximum adsorption capacity (q_m) increased from 45.5 to 124.6 mg g^-1 when the adsorbent dosage was adjusted to 1 instead of 4 g l^-1. The Freundlich isotherm and Langmuir isotherm were used to fit the data of equilibrium experiments. The adsorption data fitted well into the linearly transformed Langmuir equation. The efficiency of CES to remove fluoride was found to be 99.99% at pH 6, contact time for 2 h, dose of 4 g l^-1, when 25 mg l^-1 of fluoride was present in 100 ml of water. Comparison with literature reported values of q_m, it was found that CES was an attractive adsorbent.