Application of Surface Modified Diatomite and bricks as adsorbents for removal of Fluoride contaminants from water samples collected from Lake Baringo

Abstract

High fluoride in drinking water is detrimental to health. Cases of dental and crippling fluorosis have been reported in Lake Baringo in Kenya due to the high fluoride in the lake, which serves as the source of drinking water. The high cost of fluoride removal is an impediment to the efforts for fluoride removal in drinking water. Diatomite and brick are cheap and locally availabe materials in Kenya with potential to remove fluoride but have not been adequately assesed for fluoride adsorption. The main aim of this study was to test the application of surface modified diatomite and bricks as adsorbents for removal of fluoride contaminants from water. The specific objectives of the research were to: develop and optimize aluminum hydroxide surface modified diatomite and brick adsorbent mixture; characterize the diatomite and brick adsorbents in terms of physical features and chemical composition; test the performance of optimized mixture of surface modified diatomite and bricks on fluoride removal from water; and determine the adsorption capacity of the optimized mixture of surface modified diatomite and bricks in the removal of fluoride from water samples collected from Lake Baringo in adsorption columns. Diatomite and brick adsorbents were characterized using scanning electron microscopes equipped with energy dispersive spectroscopy (SEM-EDS). Surface modified diatomite mixed with brick was tested for fluoride removal through batch adsorption and column experiments. Diatomite samples were modified using aluminium hydroxide. Mixture of diatomite and brick samples were optimized for fluoride removal through response 1surface methodology (RSM) using Box-Wilson central composite design (CCD). Fluoride removal studies by batch experiments showed that 28g/L dose of the surface modified diatomite sufficiently removed fluoride to the World Health Organization (WHO) acceptable limit of 1.5mg/L from initial concentration of 10mg/L from fluoride contaminated water simulated in the laboratory. Similarly, 300g/L dose of brick powder was required to achieve the same recommended level. RSM optimization showed that a mixture of 18.4 g of surface modified diatomite and 177.8g of brick could be successfully used to reduce the fluoride levels in water from 10mg/L to 1.5mg/L in 30minutes. Isotherm studies showed that the adsorption of fluoride by surface modified diatomite followed Freundlich adsorption isotherm, with a regression coefficient (R2 ) of 0.9753 compared to Langmuir (R2 =0.8954). The adsorption of fluoride by brick followed the Langmuir adsorption mechanism (R2 =0.9804) as compared to Freundlich adsorption (R2 =0.9372). Kinetic studies revealed that the removal of fluoride by both surface modified diatomite and brick conformed to the pseudo-second order kinetic model. Similarly, the optimized mixture of surface modified diatomite and brick demonstrated high conformity to the pseudo second order kinetic model (R2 =0.9908). Column studies found the breakthrough point to be reached in 1 hour with 5 bed volumes and fluoride uptake capacity of 0.01mg/g of the mixed adsorbent material. The data fit well to the linearized Thomas kinetic model with the regression coefficient of 0.9695. A maximum fluoride uptake capacity of 0.02465mg/g was obtained with a Thomas rate constant of 3.9606mlmg-1min1 . In conclusion, the results indicated that the optimized mixture of surface modified diatomite and brick has high potential for fluoride removal. It is recommended to be used as adsorbents for fluoride removal in areas affected by high fluoride levels in Kenya. Further studies on improving the fluoride uptake capacity of brick are suggested to improve its effectiveness when combined with surface modified diatomite