Fluoride toxicity affects millions worldwide and is mainly caused by drinking water with elevated fluoride concentrations. It is particularly prevalent in low socio-economic groups, rural communities and volcanic areas. While defluoridation, with adsorbents such as calcium carbonate, is the most viable solution, the majority of technologies are unsuitable for resource-constrained areas. Coral sand is widely available in the Pacific but yet to be thoroughly investigated as a potential adsorbent. Acid enhances fluoride removal in calcium-systems and citric acid is an accessible, palatable and safe choice.

Using adsorbent doses of 1 g/6 mL, the adsorption capacity of coral sand for fluoride was very low at <0.12 and <0.04 mg/g, with and without the addition of 0.025 M citric acid, respectively. Columns with a sand-fluid ratio of 5:1, 0.025 M acid and a 4-hour residence time, could be used seven times to reduce fluoride from 10 mg/L to below the NZ safe limit of 1.5 mg/L. Coarser Vanuatu sand columns could be used 14 times. Increasing citric acid to 0.05 M reduced removal efficiency, likely due to release of pre- existing fluoride in sands or an increase in volume which drained from columns. Scaled-up prototype treatment devices, developed using 1 L HDPE jerrycans and 1 kg of sand, could only be used successfully three times with 0.025 M citric acid. An increase in extractable volume in the prototypes, likely due to fluid mechanics or physical properties of the larger vessel, is a possible cause. Careful control over grain size, sand-fluid ratio, contact time, and other variables will be required to successfully scale-up the treatment system.

Coral sands contained traces of fluoride and contaminants of concern, including: As (0.244 to 1.45 mg/kg); B (46 to 81 mg/kg); Cr (1.9 to 10.4 mg/kg); Mn (7.7 to 45.4 mg/kg); and Ni (0.275 to 2.55 mg/kg). Use of acid and high sand-fluid ratios to remove fluoride from solution concurrently released contaminants from sand, rendering all extracts analysed unsafe to drink. Chromium was present above NZ drinking water maximum allowable value (MAV) in all sand extracts (Kiribati, Vanuatu, and store- bought) until the end of the experimental trials. Arsenic remained above MAV in fine store-bought sand extracts for 2-3 days, and until the end of the trials in Pacific sand extracts. Kiribati and Vanuatu sand column extracts also contained B, Na, and Ni above the MAV in newer columns. Though sand treatment did neutralise acidic waters, treated samples were unpalatable based on total dissolved solids, hardness and high concentrations of Al, Fe, and Mn. A white precipitate also developed in extracts from newer columns some hours after treatment. A further treatment step with activated charcoal reduced fluoride concentrations but not trace elements of concern. Investigations into methods able to reduce contaminants before or after sand treatment, or limit contaminant leaching, will be essential if coral sand is to be used in fluoride remediation of drinking water.