Coral sands as potential low cost treatment for removing fluoride from drinking water

Abstract

Fluoride (F-) toxicity from contaminated drinking water affects over 200 million people worldwide. While defluoridation with adsorbents such as calcium carbonate, is commonly used, most technologies are unsuitable for resource-constrained areas. Coral sand is readily available in tropical regions but is yet to be investigated as a potential F- adsorbent. The effectiveness of coral sand at removing F- from drinking water was assessed using acid-enhanced lime defluoridation with two beach (Kiribati and Vanuatu) and one commercially available coral sands. Citric acid was selected as it is readily accessible, palatable and safe for human consumption. Mini columns with a 5:1 sand-to-fluid ratio, 0.025 M citric acid and a 4-hour residence time, could be used seven times to reduce F- from 10 mg/L to below the World Health Organization (WHO) guideline value of 1.5 mg/L. Mini columns packed with coarser grained (1.3 mm) Vanuatu sand could be used at least 10 times. Increasing citric acid to 0.050 M reduced removal efficiency. Prototype treatment devices, developed using 1 L polyethylene jerrycans and 1 kg of commercial sand, could only be used a maximum of three times with 0.025 M citric acid. All coral sands contained F- as well as other trace elements of human health concern, including As, B, Cr, Mn, and Ni that were released into the treated water, resulting in exceedances of drinking water standards. Total Cr concentrations in treated water always exceeded the 0.05 mg/L WHO health guideline whereas As exceeded the 0.01 mg/L value in the majority of treated water samples. In addition, B, Na, and Ni concentrations exceeded the WHO guidelines in many water samples treated with Kiribati and Vanuatu sands. Treated water samples were also unpalatable as potable water based on hardness, total dissolved solids and elevated concentrations of Al, Fe, and Mn. It is recommended that drinking water treated with coral sand be analysed for the presence of contaminants of health concern.