Removal of fluoride from spent potliner contaminated groundwater using reactive geomaterials

Contamination of groundwater with fluoride has received worldwide attention due to its associated health problems with the prolonged intake of elevated fluoride concentration above 1.5 mg/L. This thesis dealt with the remediation of highly contaminated groundwater with the spent potliner leachate (SPLL) which is characterized by high concentration of fluoride among a cocktail of other chemicals (anions, cations, heavy metals and organics). The viability of natural calcite (limestone) and natural zeolite (clinoptilolite) for fluoride removal associated with SPLL was tested. The purpose was to find the optimal mixture of calcite and zeolite that will maximise fluoride removal and stop/or minimise the inhibitory effect of SPLL chemicals on calcite capacity to remove fluoride. Four calcite-zeolite mixtures were used, that is calcite, zeolite, calcite-zeolite (50:50) and calcite-zeolite (70:30). The removal capacity of these mixtures in batch and column tests was not the same. For a long residence (contact) time (as in batch tests), more fluoride was removed by calcite-zeolite 50:50 and 70:30 mixtures than calcite or zeolite alone. That was true for both synthetic and SPLL solutions. In continuous column tests (where the contact between fluoride and sorbents is spatial and temporal), SPLL tests showed a prevalence of zeolite over calcite or its mixtures (calcite-zeolite 50:50 and 70:30). For synthetic fluoride solution, all columns containing zeolite showed nearly similar fluoride removal which was higher than that obtained using column containing calcite. Overall, in batch tests calcite showed better fluoride removal than zeolite while the inverse was true in column tests. That was attributed to the fact that zeolite has more surface area than calcite and moreover the removal mechanism of fluoride by calcite depends on the slow hydrolysis process. Hence calcite is more effective in batch tests setup where enough time is allowed for calcite dissolution. Fluoride removal by calcite and zeolite was depleted in SPLL by ~40-67% in 1.18 mm and ~12-46% in 150 µm fraction sizes as compared to synthetic fluoride solution. Heavy metals (lead, magnesium and manganese) and organics (3-methylphenol and 4-methylphenol) associated with fluoride in SPLL showed an interfering effect for fluoride removed by calcite. Other major heavy metals (vanadium, copper, barium and cobalt) and organics (aniline, phenol, 2-methyl phenol, 2-methylnaphthalene, acenaphthene, fluorene and carbazole) in SPLL solution have not shown any effect on calcite capacity to remove fluoride.