Defluoridation By A Rare-earth Triple-metal Hydrous Oxide:Synthesis, Sorption,Characterization And Emphasis On The Neutral PH Of Treated Water

Access to safe drinking water is not only a basic human need but also a fundamental right, yet millions of world’s populations do not have access to such basic need. Each year, approximately more than5000people die as a result of water borne preventable diseases, thus proved to be a major concern for human health especially in the countries with meager resources. In crux, water of adequate quality and quantity is central to the integrity of the environment&societies, thus incremental efforts are required to control water borne diseases and improve health of human being. This dissertation is a compilation of two studies, which were carried out with the idea to improve the water quality by scavenging fluoride through adsorption onto tri-metal hydrous oxides composites of rare earth metals, mixed with other low priced metals.A disease caused by excessive fluoride, fluorosis, have no medical treatment and only prevention is the best way to avoid it. Water being the vital need of living organisms, is thought to be a major carrier of fluoride to human beings. Fluorosis can be the ultimate result if fluoride exceeds the quantitative limit of1.5mg/L, specified by World Health Organization (WHO) for drinking water. In addition to fluorosis, its usage for a longer period can easily be ended in other complex ailments and disorders. In nutshell, there is a dire need to keep fluoride within the permissible limits in drinking water. Among the different techniques which are used to minimize fluoride, adsorption is one of the tools which can be used as method of choice. Process of adsorption is simple, cost effective and environment friendly. The diversified and unique surfaces of17f block elements of the periodic table, the Rare Earth Metals (REMs), are gaining enormous familiarity and attentions in this extant. In this era of cutting edge technologies where the application of REM are very high, methodologies embedded with conventional technologies are equally applicable and the technique of adsorption is among one of them. Composites containing one or two rare earth metals in combinations with other materials are gaining attention for enhanced removal of anions and fluoride is one of them. Hence, this research.work is also accounts for the use of rare earth metals in one of the conventional technique of adsorption, especially for the fluoride adsorption onto double hexagonal close-packed (dhcp) Lanthanum (La) and face centered cubic (fcc) Cerium [1] in the form of composites with other metals specially with magnesium and/or aluminum. This work approaches with the focal point of not only to remove fluoride at faster rate of adsorption but also to keep the pH of treated water within drinking water pH range (5.5to8.5) or near neutral. For the sake of cost reduction and further enhancement in adsorption potential, rare earth metals are mixed with other spontaneous available low priced metals. Beside other benefits, mixed composites not only reduces the cost of operation and enhances the potential of adsorbents for fluoride uptakes, rather it also makes the process efficient many folds. Batch adsorption approaches along with column studies were performed on composites of La(Ⅲ) with Al(Ⅲ)&Mg (Ⅱ) to have a clear picture of its behavior at various conditions and specially in real time applications. While the composite of La (Ⅲ)&Ce (Ⅲ) with Mg(II), was only studied in batch adsorption mode. The different anions present in water may affect fluoride adsorption differently, hence the effect of those anions were studied at different conditions in both research works. Thermodynamic studies showed the effect of temperature on adsorbents. Pseudo first or second order kinetics along with different isothermal models such as Langmuir and Freundlich predicted system suitability. Column studies gave us a picture of applicability of adsorbent in real time defluoridation which was perform on one of our composite. Finally regenerations carried out on both composites on the idea of the real time cost effectiveness of the adsorbents.In this regard, a mixed composite synthesized by co-precipitation approach of Mg(II), AI(III) and La(III) salts (MAL) in2:1:2ratios was used in first study. The prepared material illustrated relatively stable fluoride adsorption potential and amorphous structure at temperature lower than600℃. The defluoridation capacity of2g L-1adsorbent dose at room temperature was observed between24.08to98.28%for10and20.66mg/L fluoride solution at broad range pH (2to12), while for45.45mg/L solution, it varied between26.49to80.42%. The adsorption process on MAL was endothermic in nature (-ΔG), follows pseudo second order kinetics (R2=0.999) and best fitted with Langmuir sorption isotherm model (R2=0.9985). The prepared material was characterized by XRD, SEM, BET, EDX, FTIR and XPS, whereas contact times, sorption kinetics, dosage and effect of major co-existing anions were carried out for the optimization of adsorbent. In addition to amphoteric behavior of Al (Ⅲ) in composite material, the effect of fluoride concentration and sorbent quantity in batch process appeared to be important factors in maintaining the near neutral pH of treated water. Similarly, one other factor, flow rate was found to affect the pH in column adsorption process. Regeneration up to95.71%of fluoride loaded adsorbent was achieved with methanol and HC1mixture. Results showed that Al (Ⅲ) acted as amphoteric reagent and keep pH within drinking water pH range or near neutral.Similarly, in the second study of fluoride adsorption by tri-metal sorbent, a composite of two rare earth metals, La(Ⅲ)&Ce(Ⅲ) with one alkaline earth metal Mg(Ⅱ), synthesized by same co-precipitation method, somehow produced similar kind of results but the optimized pH was3, which is quite different from the first study, but the dosages of adsorbent and adsorbate were adjusted optimized to get near neutral pH of already processed water. The effect of existing anions was nil at three different concentrations. In both studies, the composites showed a very high affinity towards fluoride uptake.