Competitive adsorption between fluoride and natural organic matter onto activated alumina

Natural organic matter (NOM) is a major water constituent that affects the performance of drinking water treatment processes and the quality of drinking water. Besides, several studies have shown that NOM can be adsorbed on the surface of oxides being thereby able to compete with other ions. The overall goal of this study was essentially to investigate the competitive adsorption between fluoride and NOM on activated alumina (AA). For this purpose, a commercial salt of humic acids (HA) was used as a model compound for NOM. The interaction of NOM with fluoride, the simultaneous competitive adsorption, and the effect of preloading AA with NOM were investigated. The UV/Vis absorbance of HA in the range 200 800 nm was measured and the specific absorbance SUVA254 was determined. High pressure size-exclusion chromatography measurements confirmed the adsorption of aromatic fractions of NOM onto AA and the absence of leaching of the released fractions before equilibrium time. Experimental results indicated that the presence of HA in the system inhibited the adsorption of fluoride onto AA. It was also observed that percentage of removal using fresh AA decreased from 70.4 % to 51.0 % in the presence of HA and this decrease was more pronounced using preloaded AA with HA, reaching 37.7 %. Furthermore, competitive adsorption studies were conducted to evaluate the interference of coexisting ions and their effect on fluoride removal capacity, showing a severe impact of the presence of phosphate on the removal capacity unlike nitrates and sulphates, which slightly improved the fluoride sorption.