Characterisation of smectite-rich clay soil: Implication for groundwater defluoridation

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.

Mechanochemically Activated Aluminosilicate Clay Soils and their Application for Defluoridation and Pathogen Removal from Groundwater

“…The results correlate with that reported by Mudzielwana et al [23] and Gitari et al [10] who concluded that CEC is independent of pH. Furthermore, it can also be concluded that the CEC of the clay soil is generally moderate compared to the smectite [23] and montromonilite [22,23].…”

“…pH pzc of the activated clay soil evaluated was found to be 6.1 (adsorbent dosage: 1.0 g/50 mL, contact time: 24 h, agitation speed: 250 rpm and 298 K). This value is close to that of smectite rich (pHpzc 6.0) clay reported in [23] and lower than that of a bentonite clay soil (pHpzc 8.2) reported in [10]. pHpzc refers to the pH at which the clay has zero net charges on the surface.…”

Mineralogical and Chemical Characteristics of Raw and Modified Clays and Their Application in Arsenic and Fluoride Removal: Review

“…Several authors have indicated that total specific area of the clay mineral can be increased through modification to increase their functionality in different areas of application. Hua [27] Fe3+ modified bentonite 188.9 [20] Na-montmorillonite 78 [21] Fe(OH)-montmorillonite 12 [21] Al(OH)-montmorillonite 14 [21] Al3+ modified bentonite 183.3 [22] Mukondeni smectite rich clay soils 79.93 [23] Mixed mukondeni clay soils 137.7 [24] Montmorillonite 91.61 [25] area could be attributed to swelling of bentonite clay during modification. Mishra and Paride [28] also reported increased specific surface area for bentonite pillared with manganese oxides at temperature of 500°C.…”

“…Mudzielwana et al [23] reported the efficiency of Mukondeni smectite rich clay in fluoride removal. They observed that percentage fluoride removal decreases with increasing pH of the solution with about 92% fluoride removal noted at acidic pH of 2.…”

Performance of Mn2+ modified Bentonite Clay for the Removal of Fluoride from Aqueous Solution

“…6,7 Furthermore, adsorption has proved to be the sustainable method since it uses materials that can be regenerated. Some authors have also identified number of promising adsorbent including acid-activated kaolinite clay soils, 8 La 3+ -modified bentonite, 9 organosmectite, 10 smectite-rich clay soils, 11 Al/Fe oxide-coated diatomaceous earth, 12 mixed Mukondeni clay soils, 13 and MnO 2 -coated bentonite clay. 14 Clay minerals are promising materials for defluoridation of groundwater due to their abundance and cost-effectiveness.…”