مجله آب و خاک (Feb 2016)
Effect of Stabilized Zero-Valent Iron Nanoparticles on Nitrate Removal from Sandy Soil
Abstract
Introduction: During the recent decades, the use of N fertilizers has undeniable development regardless of their effects on the soil and environment. Increasing nitrate ion concentration in soil solution and then, leaching it into groundwater causes increase nitrate concentration in the water and raise the risk suffering from the people to some diseases. World health organization recommended maximum concentration level for nitrate and nitrite in the drinking water 50 and 3 mg/l, respectively. There are different technologies for the removal of nitrate ions from aqueous solution. The conventional methods are ion exchange, biological denitrification, reverse osmosis and chemical reduction. Using nanoscale Fe0 particles compared to other methods of nitrate omission was preferred because of; its high surface area, more reactive, lower cost and higher efficiency. More studies on the reduction of nitrate by zero-valent iron nanoparticles have been in aqueous solutions or in the soil in batch scale. Nanoparticles surface modified with poly-electrolytes, surfactants and polymers cause colloidal stability of the particles against the forces of attraction between particles and increases nanoparticle transport in porous media. The objectives of this study were to synthesize carboxymethyl cellulose stabilized zero-valent iron nanoparticles and consideration of their application for nitrate removal from sandy soil. Materials and Methods: The nanoparticles were synthesized in a lab using borohydride reduction method and their morphological characteristics were examined via scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier Transmission Infrared Spectroscopy (FTIR). Experiments were conducted on packed sand column (40 cm length and 2.5 cm inner diameter) under conditions of different nanoparticle concentration (1, 2, and 3 g1-1)and high initial NO3- concentration (150, 250, and 350 mgl-1). Homogeneous soil column was filled with the wet packed method. CMC-NZVI suspensions of nanoparticle in aqueous solution (0.01 M CaCl2 and 0.001MKCl) were pumped into the sand column during the injection of nitrate solution. During transport experiment, the flask containing CMC-ZVIN suspension was sonicated using a 50 KH ultrasonicator (DSA100-SK2) to prevent particle agglomeration and ensure homogeneity of the suspensions. In these experiments pore water velocity was 0.16 mms-1. Nitrate and Nitrite concentrations in the samples were measured using UV-VIS.HACH DR 5000 spectrophotometer at wavelengths 220 and 530nm, respectively, and ammonium concentration was measured by Kjeldahl method. All chemicals used in this research were of chemical grades and all solutions were prepared using deionized water (DI). Results and Discussion: Effect of nanoparticles and nitrate concentration on nitrate reduction by stabilized nanoparticle in sand column was investigated. The Results of study indicating at the first of reaction in both cases rate and amount of nitrate reduction was increased gradually. But over time, due to saturation capacity of nanoparticles at higher concentrations of nitrate, reduction speed and amount of reduction was constant approximately. The result showed that increasing dosage of nanoparticles and decreasing the influent nitrate concentration would increase percentage of nitrate reduction. Maximum percentage of reduction (82.56%) were observed at nanoparticles concentration=3 gl-1 and high initial nitrate concentration=150 mgl-1 and minimum percentage of reduction (63.94%) were observed at nanoparticles concentration=1 gl-1 and high initial nitrate concentration=150 mgl-1. After the end of experiment time, amount of observed ammonium and nitrite was a few in the drainage water of sand column. During the reaction nitrate reduction by nano-particles, H + was used and OH- was produced therefore through reaction, environment pH increased continuously. In conditions of alkaline, ammonium release in the form of N2. Therefore reduction of the amount of ammonium may due to high pH of environment reaction or fixation of ammonium in the surface colloidal of particles in porous medium. Nitrite is an intermediate product and due to the reaction conditions can be converted to ammonia or nitrogen gas. The final product of reduction would be nitrogen gas, and produced nitrite and ammonium was less than 2%. Conclusion: The results indicate that, in all experiments (effect of nanoparticle and nitrate concentration on nitrate reduction), amount of observed ammonium and nitrite was a few in the drainage water of sand column and most of the nitrate converted to nitrogen gas. Since maximum concentration level of ammonium in drinking water is 50 times less than nitrate concentration, nitrogen gas is an ideal product in water treatment process. Carboxymethyl cellulose prevents agglomeration ZVI nanoparticles and enhanced the reactivity and transport of nanoparticle in the porous media. The findings of this research demonstrated that carboxymethyl cellulose-stabilized zero-valent iron nanoparticles have a high potential for reduction of nitrate in aqueous solutions and porous media. Therefore, it can be used as an effective method for removing nitrate from water.
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