Removal of fluoride in aqueous solution by adsorption on acid activated water treatment sludge

Abstract

This paper reports the use of a pellet of adsorbent made from water treatment sludge (S) and acid activated water treatment sludge (SH) for removal of fluoride in the batch equilibration technique. The influence of pH, adsorbent dosage, temperature and effect of other ions were employed to find out the feasibility of acid activated adsorbent to remove fluoride to the permissible concentration of 0.7 mg/L. The results from the adsorption isotherm followed both Langmuir and Freundlich models and the highest fluoride removal was found for adsorbent activated with acetic acid at 2.0 mol/L. The optimum adsorbent dosage was found at 40 g/L, 0.01 mol/L acid activated adsorbent which was able to adsorb fluoride from 10 down to 0.11 mg/L. The adsorption capacity was decreased when the temperature increased. This revealed that the adsorption of fluoride on SH was exothermic. In the presence of nitrate and carbonate ions in the aqueous solution, fluoride removal efficiency of SH decreased from 94.4% to 86.6% and 90.8%, respectively. However, there is no significant effect in the presence of sulfate and chloride ions.

Introduction

Fluoride is an essential micronutrient for human health. However, excessive body intake of fluoride leads to dental fluorosis and crippling skeletal. High groundwater fluoride concentrations associated with igneous and metamorphic rocks which have been reported from India, Pakistan, West Africa, Thailand, China, Sri Lanka, and Southern Africa. In Thailand, groundwater in rural area were used as drinking water. Northern and western parts of Thailand were most likely to have high fluoride levels and it has been estimated that approximately 1% of natural water sources contain greater than 2 mg/L [1]. The maximum permissible level of fluoride in drinking water as regulated by the World Health Organization is 1.5 mg/L [2]. However, fluoride level in drinking water that is suitable for Thai people is lower than that used by the western country. This could be explained by the difference of climate temperature and water consumption behaviour. Thai people generally have lower body weight but consume more water than people in the western country. The Thailand Ministry of Industry suggested that the maximum acceptable concentration for drinking water should be 0.7 mg/L. Excellent technologies for defluoridation are available based on using activated alumina and reverse osmosis. These technologies are not always applicable in rural area due to cost and technology requirements. Other adsorbents derived from natural minerals and by-product from industrial activities such as clay minerals [3], [4] and bleaching powder [5] have been developed for defluoridation. Raw water for the water supply in Thailand normally was withdrawn from the river water which typically has high turbidity. Thus, the coagulation process is needed to remove the suspended solids. Large amounts of sludge from the water supply process are generally disposed by dumping on wetland and land filling. Due to similar mineralogical composition of clay and water treatment sludge, it can be used in brick manufacturing [6] and as an adsorbent after heat treatment for heavy metal removal [7]. Many researchers used adsorbents in a powder form for adsorption study. However, the fine particle is not suitable for the column adsorption. To make a densified adsorbent of high bulk density, good handling and bulk flow properties, the adsorbent should be made as a pellet. Papandreou et al. [8] also reported the use of fly ash in a pellet form was better than a powder form due to easy separation and risk of clogging prevention with time. Moreover, the pH value of effluent was decreased from 11 to 8. The objective of this research is to assess the ability of pellets made from water treatment sludge for the removal of fluoride from aqueous solution with and without acid activation. The adsorption thermodynamics, the effects of acid concentrations for activation, fluoride concentrations, adsorbent dosages, and influence of other ions on the uptake of fluoride are investigated.