Defluoridation using modified mucilage from indigenous plant material in Vhembe District, Limpopo Province, South Africa

Abstract

Access to clean and safe water is one of the Sustainable Development Goals. Unfortunately, it has been reported that groundwater, as another source of water has elevated fluoride levels exceeding the World Health Organisation permissible limit of 1.5 mg/L in some regions around the world. This study aims to apply aluminium (AlDE) and magnesium modified (MgDE) Dicerocaryum eriocarpum (DE) leaves mucilage as adsorbents for fluoride sequestration from aqueous solution. AlDE and MgDE are characterized by Fourier transform infrared spectrometer (FTIR), scanning electron microscopy and energy-dispersive X-ray (SEM-EDX), X-ray fluorescence (XRF), X-ray diffraction (XRD), Thermo-gravimetric analysis (TGA), elemental composition and proximate analysis. Batch adsorption studies as the effects of dosage, pH, time, temperature and change in water chemistry were investigated. The modification process introduced alkanes in AlDE and MgDE while other functional groups such as hydroxyl and carboxyl were improved. Enhancement of sorbent surface was observed in AlDE and MgDE SEM graphs when compared to unmodified DE. Functionalization also increased the chemical composition and elemental composition in both sorbents which were confirmed by EDX data. AlDE and MgDE also displayed good thermal stability. The adsorption studies recorded 84.23% removal by applying 0.25 g dosage of AlDE for 2 hours to 100 mL aqueous solution with 10 mgF-/L at 200 rpm. Change in water chemistry from deionized fluoride water to groundwater recorded an increase of fluoride sorption by the AlDE and MgDE. The increase in temperature led to a decrease in the adsorption capacity when applying both sorbents. Langmuir isotherm best describes the equilibrium sorption based on regression coefficient (R2) ranging from 0.89 to 0.91 (AlDE) and 0.96 to 0.98 (MgDE). A higher fluoride adsorption capacity was recorded at 69.65 mg/g using AlDE and 41.84 mg/g using MgDE. Kinetics studies favoured pseudo-second-order model based on 0.98 - 0.99 regression coefficient (R2) from linear plots of AlDE and MgDE. The sorption process was feasible and exothermic. The adsorbents can be best regenerated using deionized water as a desorbing agent.