Investigation of Plausible Mechanism of the Synthesized Inorganic Polymeric Coagulant and Its Application toward Fluoride Removal from Drinking Water

Fluoride, an anionic pollutant, existing in concentrations exceeding the allowed limit of 1.5 mg/L in drinking water, has been reported to cause detrimental impact on human health. The traditionally employed methods for water defluoridation mostly involve Al-based coagulants, which however face some limitations, such as requirement of relatively high dosage and production of excessive amounts of chemical sludge posing a problem of its safe disposal. In this study, two inorganic polymeric coagulants of medium (IPC-M) and ultrahigh basicity (IPC-UH) were synthesized using polymerization of aluminum trihydrate (Al2O3 center dot 3H(2)O) with an aqueous solution of 32% hydrochloric acid. The basicity of coagulants was increased by manipulating the redox reaction of the product with the aluminum metal. The synthetic coagulants were analyzed using various characterization techniques, viz., Fourier transform infrared spectroscopy, electrospray ionization-mass spectrometry, and field emission scanning electron microscopy with electron-dispersive X-ray spectroscopy, and the main physicochemical properties such as % Al2O3, relative basicity, and % chloride. The aluminum species distribution was assessed by the ferron assay, and their electrochemical properties such as dissolved charge, conductivity, acidity, and pH were also measured. The application of IPCs was explored for their fluoride removal efficacy using jar tests. The outcome showed that IPC-M was the most efficient when applied in a pH range relevant to fluoride-containing water as it was the only coagulant that showed increasing efficiency at pH values > 7. The uptake capacity of coagulants for using synthetic samples prepared in Milli-Q water containing 9 mg/L of raw fluoride concentrations to achieve residual concentration of less than 1.5 mg F/L at the pH value 6.5 +/- 0.1 was calculated as 87.68 and 68.48 mg F/g Al2O3 for IPC-M and IPC-UH, respectively, which were higher than the reported values of 37.42 and 37.75 mg F/g Al2O3 for alum and polyaluminum chloride in an earlier published paper. The residual aluminum concentration in these experiments ranged at 30 +/- 5 and 20 +/- 5 mu g Al/L, respectively, for IPL-M and IPL-UH, which were well within the WHO norm for drinking water (<200 mu g/L), indicating their immense application in the field.