Abstract

 
 
 Groundwater is a widely used and affordable source of drinking water in most of the rural areas of South Africa. Several studies have indicated that groundwater in some boreholes in South Africa has a fluoride concentration above the level recommended by the World Health Organization (1.5 mg/L). Fluoride concentrations above the permissible limit (>1.5 mg/L) lead to dental fluorosis, with even higher concentrations leading to skeletal fluorosis. In the present work, we evaluate the application of smectite-rich clay soil from Mukondeni (Limpopo Province, South Africa) in defluoridation of groundwater. The clay soil was characterised by mineralogy using X-ray diffraction, by elemental composition using X-ray fluorescence and by morphology using scanning electron microscopy. Surface area and pore volume was determined by the Brunauer–Emmett–Teller surface analysis method. Cation exchange capacity and pHpzc of the soil were also evaluated using standard laboratory methods. Batch experiments were conducted to evaluate and optimise various operational parameters such as contact time, adsorbent dose, pH and initial adsorbate concentration. It was observed that 0.8 g/100 mL of smectite-rich clay soil removed up to 92% of fluoride from the initial concentration of 3 mg/L at a pH of 2 with a contact time of 30 min. The experimental data fitted well to a Langmuir adsorption isotherm and followed pseudo second order reaction kinetics. Smectite-rich clay soil showed 52% fluoride removal from field groundwater with an initial fluoride concentration of 5.4 mg/L at an initial pH of 2 and 44% removal at a natural pH of 7.8. Therefore smectite-rich clay soil from Mukondeni has potential for application in defluoridation of groundwater. Chemical modification is recommended to improve the defluoridation capacity.
 
 
 
 
 Significance: 
 
 
 
 Physicochemical and mineralogical characterisation of smectite-rich clay soil
 Defluoridation of groundwater using smectite-rich clay soil
 Adsorption modelling using adsorption isotherms and kinetic models
 
 
 
Highlights
Groundwater is the most affordable and widely used natural resource for many of the rural areas in South Africa
We evaluate the application of smectiterich clay soil from Mukondeni (Limpopo Province, South Africa) in defluoridation of groundwater
Fluorosis is an incurable disease caused by drinking water with fluoride concentrations of greater than 1.5 mg/L for extended periods of time and efforts must be made to prevent it by providing drinking water with fluoride concentrations below 1.5 mg/L
Summary
Among all the methods of defluoridation which have been reported, adsorption has been considered an effective method for defluoridation of groundwater in rural areas because it uses materials which are readily available and inexpensive. Different materials such as clay soils[6], activated alumina[7], activated carbon[8] and other low-cost materials have been tested for defluoridation of groundwater. Among the tested adsorbents, activated alumina is widely used because it is inexpensive and it has a high defluoridation capacity.[5] Recently, more studies have focused on the use of clay soils for defluoridation because of their good adsorptive properties such as a large specific surface area, chemical and mechanical stability, layered structure and high cation exchange capacity. Smectite-rich clay soil is available naturally in large abundance at no cost
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