Defluoridation of groundwater using diatomaceous earth: optimization of adsorption conditions, kinetics and leached metals risk assessment

Abstract

In Sub-Saharan Africa, many rural communities depend on boreholes as the most appropriate source of water supply. Sadly, water from some of the boreholes contains fluoride above the WHO guideline of 1.5 mg/L. Hence, defluoridation is necessary. The aim of this study is to investigate the fluoride uptake capacity of diatomaceous earth (DE), a natural resource at optimized conditions by batch method. X-ray fluorescence analysis showed that the major component is silica (83.1%), while Al₂O₃ is the main minor component. XRD shows it is an amorphous material. For 8 mg/L fluoride spiked water, the highest per cent fluoride removal at optimum adsorption conditions (contact time: 30 min, adsorbent dosage: 8 g/L, pH 2, temperature: 298 K and shaking speed: 200 rpm) was between 23.4 and 25.6%. in tested field water was observed to reduce the fluoride uptake capacity of fluoride. The sorption data fitted better into Freundlich isotherm than Langmuir isotherm. Fluoride sorption process was found to be a second-order kinetic. Inductively coupled plasma-mass spectrometer analysis of treated water revealed that metal and non-metal species were released at trace levels. Modification of DE would be necessary to enhance the fluoride adsorption capacity of DE.

Keywords:

• Defluoridation
• Optimization
• Kinetics
• Leached metals
• Assessment

Acknowledgements

The authors hereby acknowledge the financial support from WRC Project No. K5/2363/3, NRF Project No. CSUR13092849176, Grant No. 90288 and THRIP Project No. TP12082610644 and Research & Innovation Directorate, University of Venda.

The opinions, findings and conclusions expressed in this work are those of the authors and not of WRC and NRF/THRIP funders, hence cannot accept any liability.