Study On The Nanoparticle Stability In Aqueous Solution And The Mechanism For Atrazine Removal Of Nano-TiO₂

Nowdays,with the rapid development of nanotechnology,a large number of nanomaterials have been used in all walks of life.Among these nanomaterials,titanium dioxide nanomaterial(nano-TiO2)is widely used in material additives,cosmetics,environmental governance,etc,because nano-TiO2 has the unique ultraviolet absorption effect,chemical inertness and photocatalytic properties,and so on.Usually,nano-TiO2 is mixed in the liquid phase before use.However,the high specific surface area and large surface energy of nano-TiO2 could make the nanomaterials gathered together in the liquid phase,which influences the colloidal stability of nano-TiO2 in liquid phase.Besides,the other geochemical characteristics of the liquid phase will also influence the colloidal stability of nano-TiO2 particles,and then affect the activity of nanoparticles.In this study,the commercial nano-TiO2 particles(anatase)are employed as the test subjects,the influences of inorganic anions(NO3-,SO42-,and PO43-),cations(Na+,Ca2+,and Mg2+)and natural organic matter(fulvic acid,FA)on the dispersion,aggregation and sedimentation dynamics of nano-TiO2 particles in solution are studied systemically.The aggregating mechanism of nano-TiO2 in the solution is also illuminated.Meanwhile,atrazine as one kind of pesticides is selected as the targeted pollutant,the effects of different inorganic salts and FA on the degradation of atrazine by nano-TiO2 are systematically studied.And the influence mechanisms of the geochemical characteristics on the activity of nano-TiO2 particles are discussed in detail.This study preliminarily illustrates the environmental behavior of nano-TiO2 in aqueous solution and the degradation mechanism of organic pollutants by nano-TiO2.The dispersion and aggregation of nano-TiO2 in aqueous solutions containing various anions(NV3-,SO42-,PO43-)are investigated.The influences of anion concentration and valence on the aggregation size,zeta potential and aggregation kinetics are individually investigated,especially.Results show that the zeta potential decreases from 19.8 to-41.4 mV when PO43-concentration is increased from 0 to 50 mg/L,while the corresponding average size of nano-TiO2 particles decreases from 613.2 to 540.3 nm.Both SO42-and NO3-enhance aggregation of nano-TiO2 in solution.As SO42-concentration is increased from 0 to 500 mg/L,the zeta potential decreases from 19.8 to 1.4 mV,and aggregate sizes increase from 613.2 to 961.3 nm.The trend for NO3-fluctuation is similar to that for SO42-although the range of variation for NO3-is relatively narrow.SO42-and NO3-accelerate the aggregation rapidly,while PO43-does so slowly.The colloid stability of nano-TiO2 in aqueous solutions containing different cations(Na+,Ca2+,and Mg2+)and FA are studied in detail.When the concentration of NaCl is increased from 0 to 100 mmol/L,the zeta potential of nano-TiO2 decreases from 8.67 to 1.42 mV,and correspondingly the aggregate size of nanoparticles increases from 625.6 to 1007.7 nm.The zeta potential and aggregate size of nano-TiO2 show a similar tendency in the presence of CaCl2 and MgCl2.It shows that increasing the concentration of the three cations reduces the absolute value of zeta potential of nano-TiO2,lead to the intergranular gather and increases the size of nanoparticles,thus reducing the colloid stability of nanoparticles in water phase.The strength of the three cations on the aggregation of nano-TiO2 particles is that,the maximum particle size of nanoparticles is appeared in MgCl2 solution,followed by CaCl2,and the smallest particle size in NaCl solution.When there are three cations in the solutions,the absolute value of zeta potential of nano-TiO2 is large at pH=3.0,and the particle size is small due to the electrostatic repulsion between the particles.However,as the pH=9.0,the particle size of nano-TiO2 is largest in the presence of Ca2+ and Mg2+,because the micro-precipitation is formed at this condition.When the coexistence of NaCl or MgCl2 with FA in solution,the particle size is small because of the steric hindrance between nanoparticles by the effect of FA.However,the particle size is large in the presence of CaCl2 and FA,because CaCl2 and FA could form the Ca-NOM bridge which accelerates the aggregation of nanoparticles.The adsorption and photocatalytic degradation of atrazine by nano-TiO2 in solutions are investigated using UV irradiation,and the colloidal stability of nano-TiO2 particles in the solutions is studied.Experiments are conducted at different concentrations of CaCl2 and/or fulvic acids(FA)and at pH=7.0.Results show that the adsorption efficiency of atrazine onto nano-TiO2 decreases with increasing the concentration of CaCl2 and/or FA,which could be due to the aggregation of nanoparticles by compressing the electrical double layer by Ca2+ and the competitive adsorption by FA.In the photocatalytic degradation,the increase of CaCl2 and/or FA concentration accelerate the aggregation of nano-TiO2 and decrease the generation of hydroxyl radical,and also FA could quench the hydroxyl radical.These reduce the degradation rate of atrazine by nano-TiO2.The sedimentation of nano-TiO2 under the conditions of 10 mmol/L CaCl2 and/or 10 mg/L FA decrease the photocatalytic degradation of atrazine because of the decrements in the wavelength of absorption for nano-TiO2 and less surface exposure of TiO2 to the organic pollutant.The results show that the removal efficiency of atrazine by nano-TiO2 could be well explained by the colloidal stability in the presence of CaCl2 and/or FA.Besides,the colloidal stability and photocatalysis of nano-TiO2 particles in aqueous solutions are investigated in the presence of fulvic acids(FA)and three electrolytes(NaNO3,Na2SO4,and Na3PO4)at different pH values.Results show that with the increase in FA concentration,the absolute value of the zeta potential of nano-TiO2 particles increases and particle size decreases at all pH values promoting colloidal stability.When the concentration of electrolytes is equal to or greater than 0.01 mol/L,the particle size of nano-TiO2 is larger with FA and electrolytes than with FA only.The largest particle size occurred in the presence of Na3PO4,followed by Na2SO4 and NaNO3,The particle size of nano-TiO2 decreased with higher pH and stability further increased in the presence of FA and/or electrolytes.Nano-TiO2 particle stabilization is enhanced by increasing the FA concentration and the amount of adsorbed FA on the particle surfaces(at pH 7.0).However,with FA occupying a greater number of active sites and/or consuming more reactive oxygen species,the removal of atrazine by nano-TiO2 decreased.With the addition of electrolytes(0.01 mol/L),atrazine removal efficiency by adsorption and photocatalytic degradation of the nano-TiO2 particles further decreased,albeit slightly.The inhibitory effect of electrolytes on nano-TiO2 removal of atrazine was highest for Na3PO4,followed by Na2SO4,and NaNO3 and were consistent with the results of colloidal stability of nano-TiO2.This study provides the fundamental information about the dispersion,aggregation and sedimentation dynamics of titanium dioxide nanomaterials under different water quality parameters,and also the removal effect and mechanism of atrazine by nano-TiO2 are investigated systemically.The relationship between the colloid stability and reactivity of nano-TiO2 is revealed preliminarily.The results of the study could provide scientific basis on the evaluation of ecological risk and the degradation performance of nano-TiO2 nanoparticles.