Hydrothermal synthesis of clay-based absorbents and their application to fluoride and pathogen removal from groundwater

Abstract

Groundwater is the main source of drinking water for majority of rural communities worldwide. This essential resource is often contaminated leading to various medical complications in man. Some groundwater have been found to have high fluoride levels beyond the recommended World Health Organization (WHO) limits of 1.5 mg/L and pathogens leading to fluorosis and waterborne diseases. Over 200 million people are at risk of contracting fluorosis in about 26 countries while waterborne diseases accounts for over 2.2 million deaths annually with most of these occurring in South America, Asia and Africa. Water remediation technologies using different materials have been developed yielding various degree of successes. The quest for fluoride and pathogen free groundwater prompted this work. First section of this work focused on mechanochemical activation of aluminosilicate clay soils and evaluation in fluoride and pathogen removal. The clays’ characterisation was conducted using cation exchange capacity (CEC), Brunauer-Emmett-Teller (BET), scanning electron microscopy-electron dispersion spectroscopy (SEM-EDS), X-ray diffraction (XRD), Fourier transform infra-red (FTIR) and X-ray fluorescence (XRF) techniques. Batch fluoride adsorption studies were conducted, and the antibacterial studies were done using the well diffusion assay method. Mechanochemical activation of the clay samples was conducted. The optimum milling time was obtained by evaluation of the activated clays for fluoride removal at a specific pH using batch mode. The clay soil was rich in aluminosilicate, moderate in cation exchange capacity and mesoporous. The sample with highest percent fluoride removal indicated the optimum contact time for the treatment. The surface area of clay increased with increase in activation time. The mechanochemically activated clay (MAC) showed a maximum adsorption capacity of 1.87 mg/g and 32% fluoride removal, from an initial fluoride concentration of 10 mg/L using 2 g/100 mL dosage, 60 min contact time at, 250 rpm shaking speed and 298 K. Adsorption data fitted well to Freundlich isotherms, hence confirming heterogeneous multilayer adsorption when linearised model was used but fitted well to both Langmuir and Freundlich when non-linear model were employed, thereby confirming both monolayer and multilayer adsorption. Kinetics studies revealed fluoride adsorption fitted well to pseudo-second-order model, indicating the dominance of chemisorption mechanism when linear modelling was used but fitted well to both pseudo-first-order and pseudo-second-order model when non-linear modelling were used, thus indicating vi adsorption mechanisms to be both physiosorption and chemosorption respectively. The sorption of fluoride ions onto the clays’ surface followed intra-particle diffusion mode. The clay adsorbent was successfully regenerated for up to five regeneration-reuse cycles. The antibacterial studies revealed no zone of inhibition for all the activated clays, hence, indicating that they are not active against the bacterial strains of Escherichia coli (E. coli). Therefore, MAC showed promising adsorptive capabilities but no antibacterial properties, hence, the material cannot be used to remove pathogens from water. The second section of this work focused on optimisation of synthesis conditions for hydrothermal treatment of the MAC and physicochemical charcterisations. Optimisation of synthesis conditions for hydrothermal treatment of MAC was conducted by varying NaOH concentration and contact time for clay dissolution and followed by concurrent variation of hydrothermal temperature and contact time for the clays’ modification. The solid products obtained were characterised using FTIR, BET, SEM-EDS, and XRD. The optimum conditions for Si and Al dissolution occurred at 1.5 M NaOH and 2 h at 298 K while optimum conditions for synthesis were found to be 2 h ageing time, 140 °C hydrothermal temperature, 48 h treatment time and 9 mL of water. The SEM analysis showed changes from external micro rough irregular morphology in MAC to smooth irregular aggregates of gel-like microsphere particles in the HTAC products as conditions were varied. XRD results revealed appearance of several new mineral phases. The optimally synthesised product showed fine crystalline microspheres’ gel-like particles having microspheric and spheroidal “cotton ball” morphology with sharp peaks around 2θ = 15, 25, 32 and 35 with new mineral phases corresponding to hydroxy sodalite as confirmed by SEM and XRD respectively. XRD pattern which represents quartz, montmorinollite and mullite in MAC gave way to sharp peaks corresponding to new mineral phases corresponding to hydroxy sodalite in the HTAC products. BET surface area also increases from 17 m2/g in MAC to 33.56 m2/g in the products as hydrothermal conditions were varied. However, optimised product with surface area value of 33.56 m2/g was observed to have the highest percent fluoride removal. The pore distribution of all the products was still within mesoporous range while no significant change was observed in pore volumes and sizes in the products. Five best modified products identified from defluoridation and characterised were HTAC-5, HTAC-6, HTAC-7, HTAC-8 and HTAC-12. Preliminary fluoride removal experiments showed that sample HTAC-8 prepared under optimum vii conditions of 48 h ageing time, 140 °C had highest fluoride adsorption capacity and hence, selected for application in subsequent experiments in the preceeding chapter five. The third section of this work focused on the application of the synthesised hydrothermally-treated aluminosilicate clay (HTAC) obtained in chapter four for application in fluoride and pathogen removal. Characterisation was carried out using FTIR, BET, XRD SEM-EDS and XRF. Batch adsorption studies were conducted. Antibacterial studies employed the well diffusion assay method. The BET surface area of HTAC was 33.56 mg2/g while no significant change was observed in pore sizes and volumes. Batch studies showed maximum adsorption capacity of 1.75 mg/g with 53% fluoride removal from initial fluoride concentration of 6.0 mg/L using 0.8 g/40 mL dosage at initial pH 5.8, 5 min contact time and 298 K. The adsorption kinetics data fitted best to pseudo-second order model, while the adsorption data were best described by Freundlich adsorption model. The regeneration and recyclability potential studies with 0.1 M KCl as regenerant showed it can be used for up to six times. Antibacterial activities result of the optimised HTAC towards Escherichia coli (E. coli) strains indicated some potency. The outcome of this study showed that the synthesised HTAC has strong potential for application in groundwater defluoridation and pathogen removal. The fourth section of this work focused on modification of the surface properties of the HTAC in section three by hydrothermal treatment of MAC in the presence of a pore forming agent (NaClO3) (product designated as PHTAC). Operational parameters were optimised for synthesis of porous-hydrothermally-treated aluminosilicate clay (PHTAC) from MAC. They were all characterised using BET, FT-IR, SEM-EDS and XRD. Preliminary adsorption experiments were conducted on all the products with a view to obtain the six best modified PHTAC materials. The results showed operational parameters such as pore former (NaClO3), contact time and temperature were critical in the synthesis and impacted positively on the PHTACs performance. The six best optimised products with highest percent fluoride removal were selected. XRD and SEM analysis showed the optimised product to have new mineral phases corresponding to hydroxyl sodalite with sharp peak intensities around at 2-degree θ = 15, 25, 32 and 35 with fine crystalline piston rod-like particles interlaced with pores. BET results showed increasing surface area for the products from 33 m2/g to 52.56 m2/g in optimised PHTAC product. The pore-forming agent (NaClO3) introduced during the hydrothermal treatment increased the surface area, which impacted positively on PHTACs’ performance. The best optimised PHTAC-18 was obtained at 6 h treatment viii time with 1.0 g/100 mL adsorbent dosage and 0.20 g/100 mL pore formers in 9 mL water at 300 °C hydrothermal temperature and had the highest fluoride removal of 63% and adsorption capacity of 3.45 mg/g, and was therefore selected for use in batch defluoridation experiments and pathogen removal from groundwater, presented in chapter seven. The fifth section of this work focused on the application of the optimally synthesised porous-hydrothermally-treated aluminosilicate clay (PHTAC) obtained in section four for fluoride and pathogen removal. The optimised PHTAC was characterised by BET, FT-IR, SEM-EDS, XRD and XRF. Batch adsorption experiments were conducted on the optimised PHTAC and regeneration potential was investigated. Antibacterial studies were conducted using well assay diffusion method and modified liquid culture technique & UV-visible spectrophotometry. Batch defluoridation studies of optimised PHTAC showed maximum adsorption capacity of 3.45 mg/g with 68% fluoride removal at 10 min contact time, 1.0 g dosage/100 mL, 4.0 mg/L initial fluoride concentration, pH ≈ 4.0, 250 rpm at 298 K. The adsorption capacity increased from 1.87 mg/g in MAC to 3.45 mg/g in PHTAC. The fluoride sorption fitted well to both Langmuir and Freundlich models, hence confirming both monogeneous unilayer and heterogeneous multilayer adsorption. The adsorption kinetics data also showed a good fit to both pseudo-first-order and pseudo-second-order models suggesting fluoride sorption proceeded via physiosorption and chemisorption pathways respectively. The synthesised PHTAC could be regenerated for up to eight times when compared to MAC and HTAC. The minimum inhibition zone observed for the bacterial strains was about 15 mm. The percent growth inhibition of the bacterial cell was 89 which is very close to 90 - 95% obtained in most government water treatment plants. The efficiency of PHTAC when further tested in field groundwater containing 2.84 mg/L initial fluoride concentration using 0.9 g/ 100 L adsorbent dosage at 10 min contact time showed its capability to reduce fluoride from initial concentration of 2.84 mg/L to 1.35 mg/L, which is within the permissible WHO limits (1.5 mg/L). In conclusion, the adsorption capacity and antibacterial potency of the PHTAC for fluoride and pathogen removal from groundwater improved. It is recommended that its surface be further modified sonochemically and loaded with suitable antibacterial metal oxides to further enhance its defluoridation and antibacterial potency.