Modified Natural Materals For Enhanced Removal Of Fluorine From Aqueous Solution:Characteristics And Mechanisms

Endemic fluorosis caused by high fluoride level in drinking water has always troubled the lives of people in poor and remote areas. Due to the terrain in these areas and other environmental factors relating to this, natural fluoride would easily accumulate in drinking water source. More than that, underdeveloped economic condition in these mountainous areas has limited the application of centralized water distribution network, which makes it difficult to fundamentally solve the local people’s drinking problem. Decentralized drinking water treatment is an effective and feasible solution to this difficulty, among which adsorption is currently the most studied treatment owing to its high removal efficiency, low cost and less dependence on equipment, etc. Considering the regional characteristics of high fluoride-polluted drinking water, we designed an efficient, environmentally friendly and low-cost novel adsorbent which can adapt to the typical water. In this work, chitosan-modified titanium hydrate, aluminum sulfate-modified zeolites and titanium sulfate-modified zeolite were systematically investigated to remove fluoride. The results are shown as follows:(1) The nonlinear fitting methods including Langmuir, Freundlich, Redlich-Peterson adsorption isotherm were used in this study, in good agreement with the adsorption process of fluorine ion on natural mineral rectorite, of which the relative ratio R2 approaching 1;(2) Composite materials such as chitosan-modified titanium hydrate, aluminum sulfate-modified zeolites and titanium sulfate-modified zeolite all showed high efficiency of fluoride removal particularly in alkalescent water, and the effluent quality reached the national standard of drinking water;(3) A cost effective bio-adsorbent, titanium(Ⅳ) hydrate based on chitosan template (Ti-CHI), was synthesized using Ti(SO4)(2) and chitosan for defluoridation from aqueous solutions. The Ti-CHI material was characterized using FTIR, SEM with EDX, XRD, XPS and Zeta potential. The adsorption kinetics of fluoride onto Ti-CHI followed pseudo-second-order and intra-particle diffusion models. The Ti-CHI was effective for defluoridating systems with low fluoride concentrations (4.52 mg/L), exhibiting a 87.50% removal rate and permitting a residual fluoride concentration (0.55 mg/L), and the effluent quality reached the national standard of drinking water (1.0 mg/L). The most stable pH range for fluoride removal was from 4 to 9. After combining the results of pH effects, FTIR and Zeta potential, the-NH2 groups and Ti-OH groups were revealed as the fluoride adsorption sites on Ti-CHI. In addition, the carbonate ions present in the aqueous solutions adversely impact the fluoride adsorption. The Langmuir, Freundlich, and Langmuir-Freundlich models were used to illustrate the isotherms of the adsorption process. The adsorption of fluoride onto Ti-CHI fits the Langmuir and Langmuir-Freundlich isotherm models well. The maximum adsorption capacity for fluoride on the Ti-CHI was 16.12 mg/g. Thermodynamic studies revealed that the adsorption process was spontaneous.(4) Aluminium sulfate modified zeolite (Al-Z) and titanous sulfate modified zeolite (Ti-Z) were synthesized and used as adsorbents for fluorine removal. The primordial zeolite has no effect on defluoridation, while Al-Z and Ti-Z have high defluoridation level. The optimum pH for fluorine adsorption of Al-Z was 6-9 with the maximum adsorption 22.20mg/g, while Ti-Z was at 3 with the maximum adsorption 6.12mg/g。The adsorption capacity has not obviously changed when NO3-presented. However, CO32- and PO43- have negative effect on adsorption process. The effects of the three selected ions on F- adsorptionc were in the following sequence: PO43-> CO32-> NO3-. The adsorption process of Al-Zn and Ti-Z on fluorine removal can be well described by the pseudo-second order kinetic model. The experimental data of isotherm followed the Freundlich isotherm model. Moreover, the thermodynamic parameters calculated from the temperature-dependent isotherms indicated that the adsorption reaction was an spontaneous process.(5)This work developed a new technique for fluoride removal in drinking water on the base of the modified natural composite material, which would give successful access to practical application.