Fluoride removal from water using Al(OH)3-surface modified diatomite mixed with brick: optimization, isotherm and kinetic studies

Abstract

Excess fluoride in drinking water causes both dental and skeletal fluorosis among other problems. As such there is need to develop affordable and easily accessible techniques for fluoride removal from drinking water. This work assessed surface modified diatomite mixed with brick for f luoride removal. Diatomite samples were modified using aluminium hydroxide and the mixture was optimized for fluoride removal through response surface methodology (RSM) using the Box-Wilson central composite design. Batch experiments showed that, individually, a 28 g/L dose of the surface modified diatomite sufficiently removed fluoride to the acceptable level of 1.5 mg/L from an initial concentration of 10 mg/L fluoride while a 300 g/L dose of brick powder was required to remove an equal amount of fluoride in the same water samples. RSM optimization showed that a mixture of surface modified diatomite and brick in the mass ratio 1.8:17.8 grams per milligram of fluoride in water can be used to remove fluoride in water to an acceptable level. Adsorption of fluoride by surface modified diatomite fit better into the Freundlich adsorption isotherm (R2=0.9753) compared to the Langmuir (R2=0.8954), while adsorption by brick better fit the Langmuir adsorption mechanism (R2=0.9804) in comparison to the Freundlich adsorption (R2 =0.9372). Kinetic studies revealed that chemisorption was the main mechanism for both surface modified diatomite and brick adsorbents. Conclusively, an optimal mixture of surface modified diatomite and brick can be successfully used fo