Fluoride removal from water by lime-sludge waste

Abstract

The potential application of lime-sludge waste (LSW) from paper mills in removal of excess fluoride from water has been studied. Strings of batch experiments were carried out to assess the fluoride removal ability of dried micrometer-sized LSW in presence of phosphoric acid (PA) by varying operational parameters, viz., initial fluoride concentration, initial PA concentrations, contact time and adsorbent dose. The adsorption data fitted well to Freundlich model and pseudo-second-order kinetics. The mechanism of removal has been suggested as a combination of precipitation of fluorite and fluorapatite (FAP), and adsorption by hydroxyapatite (HAP). Fluoride was found to be reduced from initial 10 mg/L to less than 1 mg/L in 30 min. The maximum fluoride adsorption capacity of LSW was found to be 0.943 mg g–1. The quality parameters of the treated water remain within WHO guidelines and the exhausted used LSW very well passes the toxicity characteristic leaching procedure test of the US-EPA. The study shows LSW, a waste material, as a potential sorbent for remediation of fluoride contaminated water.