Fluoride Adsorption from Aqueous Solution by Modified Zeolite-Kinetic and Isotherm Studies

Fluorine is a very common element in the Earth's crust and is present in the air, food, and in natural waters. It never meets in the free state in nature due to its high reactivity, and it comes in the form of fluorides. Depending on the concentration of fluorine absorbed, it may be beneficial or harmful to human health. Similar to any trace element, fluoride ion is beneficial for the human body at low levels, but as soon as its concentration becomes too high, it is toxic, inducing dental and bone fluorosis. The lowering of fluoride concentrations that exceed the recommended standards in drinking water is practiced in various ways around the world. The adsorption process has been classified as one of the most efficient methods for the removal of fluoride from water as it is environmentally friendly, easy to operate, and cost-effective. The present study deals with fluoride ion adsorption on modified zeolite. There are several influential parameters, such as zeolite particle size, stirring rate, solution pH, initial concentration of fluoride, contact time, and solution temperature. The maximum removal efficiency of the modified zeolite adsorbent was 94% at 5 mg/L fluoride initial concentration, pH 6.3, and 0.5 g modified zeolite mass. The adsorption rate increases accordingly with increases in the stirring rate and pH value and decreases when the initial fluoride concentration is increased. The evaluation was enhanced by the study of adsorption isotherms using the Langmuir and Freundlich models. The Langmuir isotherm corresponds with the experimental results of the fluoride ions adsorption with a correlation value of 0.994. The kinetic analysis results of the fluoride ions adsorption on modified zeolite allowed us to demonstrate that the process primarily follows a pseudo-second-order and then, in the next step, follows a pseudo-first-order model. Thermodynamic parameters were calculated, and the ΔG° value is found to be in the range of -0.266 kJ/mol up to 1.613 kJ/mol amidst an increase in temperature from 298.2 to 331.7 K. The negative values of the free enthalpy ΔG° mean that the adsorption of fluoride ions on the modified zeolite is spontaneous, and the positive value of the enthalpy ∆H° shows that the adsorption process is endothermic. The ∆S° values of entropy indicate the fluoride adsorption randomness characteristics at the zeolite-solution interface.

Removal of fluoride ions (F-) from aqueous solutions using modified Turkish zeolite with quaternary ammonium

The natural Turkish zeolite has been modified with hexadecyltrimethylammonium bromide (CTAB) for the elimination of fluoride (F^-) from aqueous solutions. The parent natural zeolite (NZ) and modified zeolite (MZ) have been characterized by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), zeta potentials and Brunauer-Emmett-Teller (BET) method. The effect of pH, adsorbent dose, contact time, initial concentration and temperature on adsorption of fluoride ions onto modified zeolite (MZ) has been determined in batch experiments. Fluoride concentration can be reduced to 1.5 mg/L under the optimum condition (pH = 5, adsorbent dose = 20 mg/L, contact time = 60 min and T = 293 K) when initial fluoride concentration of 10 mg/L is employed. The fluoride adsorption on MZ has been described by the Langmuir isotherm and the maximum fluoride adsorption capacity was found as 2.994 mg/g. Kinetics data were best described by the pseudo-second-order model. The thermodynamic studies proved that the adsorption was exothermic and spontaneous.

[Phosphorus Removal Performance and Mechanism of Modified Zeolite Using Alum Sludge Acidified Extraction Liquid]

To improve the capacity for phosphorus removal and to reduce the cost of treatment, alum sludge from a water supply plant was obtained to prepare modified zeolite coated with layered double hydroxide (LDHs), which was synthesized using an acidified extraction liquid. The surface characteristics and chemical composition of raw zeolites, Al-Zn modified zeolites, and alum sludge modified zeolites were determined, and the isothermal adsorption and adsorption kinetics were analyzed. The performance and mechanism of phosphorus removal by the alum sludge modified zeolites were evaluated. The results showed that the optimum conditions for acidification extraction were 60 min treatment time, 150 r·min^-1, and pH 1.0, which achieved 77 mg aluminum extraction rate per gram of alum sludge. Compared to raw zeolites, the maximum saturated adsorption capacity and the desorption performance of the zeolites were significantly increased by this modification. In addition, the theoretical maximum adsorption capacity of alum sludge modified zeolites was increased from 30.24 mg·kg^-1 to 170.40 mg·kg^-1. The modification changed the main mode of phosphate adsorption by zeolite from physical adsorption to chemical adsorption. Alum sludge modification could improve the efficiency of phosphate adsorption and the regeneration capacity of zeolite, which could reduce the risk of water eutrophication caused by high phosphate concentrations, and could achieve the purpose of waste control by waste.

Enhanced efficiency of swine wastewater treatment by the composite of modified zeolite and a bioflocculant enriched from biological sludge

A high-ammonia-resistant strain was firstly isolated from activated sludge and applied to harvest a bioflocculant from a swine wastewater. Enhancement of swine wastewater treatment was investigated by a composite of the harvested bioflocculant and a zeolite modified by integrating calcinations with MgO at 400°C. Results have demonstrated that 71.8% of Chemical Oxygen Demand (COD), 54.5% of ammonia, and 81.2% of turbidity can be removed from the swine wastewater by the bioflocculant alone. Results have also demonstrated that 73.4% of ammonia could be removed from the swine wastewater by the modified zeolite alone, while almost no COD was removed. Thus, the bioflocculant and modified zeolite were used simultaneously to enhance swine wastewater treatment, and response surface methodology (RSM) was employed to optimize the treatment process. Under the optimal treatment conditions of bioflocculant of 12 mg/L, modified zeolite of 8 g/L, pH of 7.5, and agitation speed of 200 r/min, obtained by the RSM, 88.6% of COD, 85.8% of ammonia, and 95.5% of turbidity could be removed from swine wastewater, which were significantly improved compared with that by the bioflocculant or modified zeolite alone. The use of the composite exerted advantages of the bioflocculant and modified zeolite, and provided a feasible way to improve pollutants' removal from wastewaters.

[Efficiency and Mechanism of Nitrogen and Phosphorus Removal in Modified Zeolite Wetland]

To study the efficiency and mechanism of nitrogen and phosphorus removal for decentralized rural sewage in modified zeolite wetland, the modified zeolite was applied as substrate into a combined process composed of anaerobic baffled reactor (ABR) and baffled flow constructed wetland (BFCW), providing a new way for rural sewage treatment in Suzhou City. The study was contrasted with zeolite wetland. The results showed that the modified zeolite wetland had high efficiency and stability of nitrogen and phosphorus removal, and the nitrogen and phosphorus removal quantities of modified zeolite wetland were 1.8% and 1 times higher than those of zeolite wetland during the trial. The modified zeolite wetland mainly removed nitrogen and phosphorus by substrate adsorption, and the main fractions of modified zeolite were Ca-P and Al-P. The oxygen-secretion and absorption of plants stabilized the water quality of the effluent. The substrate adsorption was the main nitrification removal pathway in front of the wetland, and nitrification and denitrification were the main nitrification removal pathways at the end of the wetland. The nitrogen and phosphorus adsorption capacities during the pilot test were much higher than those of the static test. The optimization of phosphorus adsorption capacity for modified zeolite was achieved under the synergy of multiple pathways. The effect of configuration and plant root was the main reason for the difference of nitrogen and phosphorus adsorption quantities. Nitrification intensity led to the seasonal fluctuation of nitrogen removal effect and stability in modified zeolite wetland, and the low nitrification intensity in the front of wetland was related to the strong adsorption of NH₄⁺-N by the modified zeolite.

Removal of toluene vapors from the polluted air with modified natural zeolite and titanium dioxide nanoparticles

Toluene is a colorless and flammab1le liquid with the same solubilizing capacity as benzene that is in many cases used as an alternative to benzene, because of the uncertainty of being carcinogens. Workers can be exposed to toluene by breathing the chemical. To avoid inhalation and dermal effects caused by exposure to toluene, solutions such as adsorption, thermal oxidation, membrane separation and photocatalytic processes are applied. In this study, removal of toluene vapors with modified natural zeolite and titanium dioxide nanoparticles was discussed. The natural zeolite was modified using chemical and thermal methods. The samples characterized by Brunauer-Emmet-Teller, Barrett-Joyner-Halenda, X-ray diffraction and scanning electron microscopy tests. After stabilization of TiO₂ nanoparticles, the removal efficiency for the toluene vapors at a concentration of 50, 150 and 300 ppm were evaluated using a dynamic system. The results showed that the zeolite has a very porous surface and after modifying the context its specific surface area increased 2.54 times. The results of the adsorption capacity calculation and photocatalytic process showed that modified zeolite samples-TiO₂ bed has greater efficiency in the adsorption capacity and better photocatalytic activity than a Ze-TiO₂ bed. Ze-TiO₂ bed was able to remove 26% of toluene vapors at the concentration of 50 ppm and inlet flow rate of 1 L/m, which was 1.26 times more than a bed Ze-TiO₂. According to the results of this study, while modifying the natural zeolite increased desirable properties such as specific surface area and Si/Al ratio, but in comparison with similar studies with synthetic bed such as zeolite Y and ZSM-5, could not achieve desired results in a photocatalytic activity for its application in industry. However, because of its abundance in the world and Iran and therefore low cost of preparation and also due to its unique characteristics, it is recommended that more studies to be done about modifying and its application in photocatalytic processes.