Addressing technical barriers for reliable, safe removal of fluoride from drinking water using minimally processed bauxite ores

Abstract

Throughout the developing world, over 200 million people drink groundwater containing fluoride concentrations surpassing the World Health Organization's maximum recommended contaminant level (WHO-MCL) of 1.5 mg F^-/L, resulting in adverse health effects ranging from mottled tooth enamel to debilitating skeletal fluorosis. Existing technologies to remove fluoride from water, such as reverse osmosis and filtration with activated alumina, are expensive and are not accessible for low-income communities. Our group and others have demonstrated that minimally-processed bauxite ores can remove fluoride to safe levels at a fraction of the cost of activated alumina. We report results from testing for some technical challenges that may arise in field deployment of this technology at large scale, particularly in a sufficiently robust manner for application in development contexts. Anticipating possible modes of failure and addressing these challenges in advance in the laboratory is particularly important for technologies for vulnerable communities where the opportunity to re-launch pilot projects is limited and small failures can keep solutions from the people that need them most. This work addresses three potential technical barriers to reliable removal of fluoride from drinking water with bauxite ore from Visakhapatnam, Andhra Pradesh, India. We evaluate competition from co-occurring ions, adsorption reversibility, and potability of the product water with regards to leaching of undesirable ions during treatment with various adsorbent materials including raw and thermally activated bauxite, and synthetic gibbsite (a simple model system). Under the conditions tested, the presence of phosphate significantly impacts fluoride adsorption capacity on all adsorbents. Sulfate impacts fluoride adsorption on gibbsite, but not on either bauxite adsorbent. Nitrate and silicate (as silicic acid), tested only with gibbsite, do not affect fluoride adsorption capacity. Both thermally activated bauxite and gibbsite show non-reversible adsorption of fluoride at a pH of 6. Raw bauxite leached arsenic and manganese in a TCLP leaching test at levels indicating the need for ongoing monitoring of treated water, but not precluding safe deployment of bauxite as a fluoride remediation technology. Understanding these phenomena is crucial to ensure field deployment over large diverse geographical areas with aquifers varying in groundwater composition, and for ensuring that the appropriate engineering processes are designed for field implementation of this innovation.

Fig. 1

Adsorption isotherms for fluoride on gibbsite at pH = 6.0 ± 0.2 and ionic strength of 100 mM, for a) phosphate (1, 5, and 25 mM), b) sulfate (1, 5, and 25 mM), c) nitrate (25 mM), and d) silicate (as silicic acid) (1 mM). Data are shown for adsorption in the absence of competing ions (orange) and in the presence of competing ions (yellow = 1 mM, blue = 5 mM, grey = 25 mM). Error bars on all data points represent one standard error above and below the mean for three or more trials. Dashed lines indicate the best fit isotherm model determined by Isofit – isotherm parameters are found in the Supporting Information.

Fig. 1

Adsorption isotherms for fluoride on gibbsite at pH = 6.0 ± 0.2 and ionic strength of 100 mM, for a) phosphate (1, 5, and 25 mM), b) sulfate (1, 5, and 25 mM), c) nitrate (25 mM), and d) silicate (as silicic acid) (1 mM). Data are shown for adsorption in the absence of competing ions (orange) and in the presence of competing ions (yellow = 1 mM, blue = 5 mM, grey = 25 mM). Error bars on all data points represent one standard error above and below the mean for three or more trials. Dashed lines indicate the best fit isotherm model determined by Isofit – isotherm parameters are found in the Supporting Information.

Fig. 2

Adsorption isotherms for fluoride on raw bauxite at pH = 6.0 ± 0.2 and ionic strength of 100 mM, in the absence of competing ions (orange) and in the presence of a) phosphate (5 mM) and b) sulfate (5 mM) (both blue). Error bars on all data points represent one standard error above and below the mean for three trials. Dashed lines indicate the best fit isotherm model determined by Isofit – isotherm parameters are found in the Supporting Information.

Fig. 3

Adsorption isotherms for fluoride on bauxite thermally activated at 300 °C, at pH = 6.0 ± 0.2 and ionic strength of 100 mM, in the absence of competing ions (orange) and in the presence of a) phosphate (5 mM) and b) sulfate (5 mM) (both blue). Error bars on all data points represent one standard error above and below the mean for three trials. Dashed lines indicate the best fit isotherm model determined by Isofit – isotherm parameters are found in the Supporting Information.

Fig. 4

Adsorption and desorption isotherms for fluoride on gibbsite at pH = 6.0 ± 0.2 and ionic strength of 100 mM. Error bars represent one standard error above and below the mean for three or more trials. Dashed line indicates the best fit isotherm model determined by Isofit for forward adsorption – isotherm parameters are found in the Supporting Information.

Fig. 5

Adsorption and desorption isotherms for fluoride on activated bauxite at pH = 6.0 ± 0.2 and ionic strength of 100 mM. Error bars represent one standard error above and below the mean for three or more trials. Dashed line indicates the best fit isotherm model determined by Isofit for forward adsorption – isotherm parameters are found in the Supporting Information.

Fig. 6

Adsorption and desorption isotherms for fluoride on raw bauxite at pH = 6.0 ± 0.2 and ionic strength of 100 mM. Error bars represent one standard error above and below the mean for three or more trials. Dashed line indicates the best fit isotherm model determined by Isofit for forward adsorption – isotherm parameters are found in the Supporting Information.