Experimental investigation of Al-F species formation and transformation during coagulation for fluoride removal using alum and PACl

Aluminium salts are widely investigated for the coagulation process for removal of fluoride, but the residual Al above the acceptable limit is the main concern which is mainly due to the formation of soluble and colloidal Al-F complexes of aluminium and fluoride. Hence, it is necessary to investigate the nature and mechanism of formation of colloidal, dissolved and precipitated Al-F complexes and other Al associated species, that govern the concentration of residual Al in the defluoridated water. In the present study, experiments were performed to find the optimized dosage of aluminium sulfate (alum) and polyaluminium (PACl) for removal of fluoride from drinking water. Residual fluoride, turbidity and TDS were found to be 1.3 ppm, 7 NTU & 280 ppm respectively, for alum and 0.48 ppm, 4 NTU & 200 ppm respectively for PACT for initial fluoride concentration of 4 ppm. Residual Al was found to be above the acceptable limit (0.2 ppm), but it was less for PACl as compared to alum, however, after the subsequent microfiltration, residual Al was 0.268 & 0.13 for alum & PACl respectively that almost met the acceptable limit. Characterization of the treated water before and after filtration by using microfiltration membrane was done through electrospray ionization mass spectroscopy (ESI-MS) to identify the colloidal, dissolved Al-F species. Aluminium species in the form AlF52-,AlF2+, AlF4-, Al-2(OH)(2+)(2), Al(OH)(+)(2), Al (OH)(2)F-2(-), AlOHF+ etc. as Al-F were recognised. From the ESI-MS results, it was observed that only a few Al-F complexes with lesser concentrations were formed in case of PACI, which confirmed the fact that PACI worked better for the fluoride removal with low residual Al. The settling of the Al-F complexes for PACI was better than alum due to the difference in flocculation mechanism bridging in case of PACI and sweep flocculation for alum. XRD, SEM & FT-IR analysis of alum and PACI sludge was done to identify the precipitated species. (C) 2018 Elsevier B.V. All rights reserved.