Abstract
Fluoride removal from aqueous solution was investigated using the chemical coagulation-flocculation by alum in reason of the efficiency and the simplicity of the process. Preliminary experiments were carried out to study the effect of some operating parameters such as: pH, initial fluoride concentration, coagulant dose, adjuvant coagulation dose, flocculent dose and settling time. Obtained experimental results showed that the treatment of fluoride by alum coagulation causes acidification of the solution. The effect of the initial pH solution on fluoride removal is negligible. Experimental results proved that coagulation process is effective in waters with low or medium levels of fluoride. Defluoridation rate was seen to increase with increasing coagulant and adjuvant coagulation doses. Optimal flocculent dose and optimal settling time were found 4.2 mg L-1 of FABI and 60 min respectively. Subsequently, experimental design methodology using two-level full factorial design was applied to optimize the defluoridation by coagulation-flocculation. Therefore, four operating parameters which are supposed to affect the removal efficiency were chosen: initial fluoride concentration, coagulant dose, coagulation adjuvant dose and settling time. From the statistical analysis, it is inferred that the four studied parameters have an influence on the fluoride removal. In fact, initial fluoride concentration has a positive effect unlike coagulant dose, coagulation adjuvant dose and settling time. Experiment tests were performed to evaluate the efficiency of coagulation-flocculation process for south Tunisian brackish water with high fluoride concentration. The obtained results showed that fluoride removal reached 59%.
Highlights
The fluoride occurs mainly as sellaite MgF2, fluorspar CaF2, cryolite (Na3AlF6), and fluorapatite (3Ca3(PO4)2.Ca(F,Cl)2) [1, 2]
We have investigated the influence of four main parameters: initial fluoride concentration ([F-]0), coagulant dose ([Al3+]), coagulation adjuvant dose ([MgCl2.6H2O]) and settling time on the defluoridation by coagulation-flocculation using the experimental design methodology by means of factorial matrix (2k) [18]
In this study coagulation process with alum was applied in the treatment of water containing fluoride
Summary
The fluoride occurs mainly as sellaite MgF2, fluorspar CaF2, cryolite (Na3AlF6), and fluorapatite (3Ca3(PO4).Ca(F,Cl)2) [1, 2]. Fluorspar is found in sedimentary rocks while cryolite is found in igneous rocks. These fluoride minerals are nearly insoluble in water. Fluorides will be present in groundwater only when conditions favor their dissolution or high fluoride containing effluents are discharged to the water bodies from industries. Fluoride removal receives much attention due to the dental and skeletal fluorosis caused by the excessive intake of fluoride [3]. In the south Tunisian brackish water, the range level of fluoride ions is situated between 1.5 and 2.5 mg L-1. In the region of Gafsa fluoride content greater than 2.5 mg L-1 are encountered [4]
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