The Study Of Preferential Adsorption To Phthalic Acid Esters On Y Ellow And Purple Soil

Abstract:Environmental hormone is a kind of chemicals in the environment which can change the endocrine system of both animals and human beings,and may further cause abnormality in reproduction, growth and behaviors. Phthalate Esters (PAEs) is an important environmental hormone and is a key component of plasticizer. With the heavy use of plastic products, PAEs has been constantly brought into the environment. Now, it has become one of the most common organic pollutants. This paper mainly studies the adsorption-desorption by the typical yellow soil and purple soil in Chongqing of three kinds of PAEs--DMP, DEP and DBP. We focus on kinetic features, thermodynamic features and influencing factors and the result goes as follows,(1) In terms of kinetics, the adsorption-desorption process goes at two stages--an initial fast adsorptive stage and then a slow stage. At the former stage, PAEs molecules are quickly adsorbed onto the surface of soil particles and the reaction takes no more than4hours. As the reaction keeps going on, the adsorptive speed begins to slow down and the process reaches a balance in about24hours. The whole adsorptive process can be divided into the initial fast surface adsorption and organic pollutants’ slow spreading inside the soil particles(organic pollutants-dominant).The pseudo-second-order kinetic equation can well explain the kinetic features in the adsorption and it shows that chemical mechanism exists in the adsorption. The intra-particle diffusion models reveal that the adsorption accord with the internal diffusion model, but the intra-particle diffusion is not the only rate controlling step. The relationship of intra-particle diffusion rate constant kp and intercept C in the three kinds of PAEs can be expressed as DBP>DEP>DMP; while the adsorption rate by the yellow soil is higher than that by purple soil. The difference of the adsorbing rates between yellow soil and purple soil is a combined effect of soil structure(specific surface area), organic matter content in soil, module polarity of PAEs and steric hinderance etc.(2)The adsorbing capacity by yellow soil and purple soil in isothermal condition increases with the increase of equilibrium concentration, and the equilibrium adsorbing capacity is arranged in the order of DBP>DEP>DMP. The Langmuir equation fits in the adsorption activity best. According to the equation, the maximal adsorbing capacity by yellow soil is0.2034mg/g,0.3590mg/g and0.5282mg/g respectively and that by purple soil is0.1930mg/g,0.2258mg/g and0.3813mg/g. The desorption capacity increases in direct proposition to the increase of adsorbing capacity. Adsorption of these three kinds of PAEs is irreversible to some extent. As a result, the maximal maintenance of DBP, DEP and DMP by yellow soil are0.177mg/g,0.115mg/g and0.051mg/g which account for25.08%,32%and33.5%of the maximal adsorbing capacity respectively. And those by purple soil are0.157mg/g,0.053mg/g and0.045mg/g which account for23.32%,23.48%and46.42%of the maximal adsorbing capacity.(3)An experiment of competitive adsorption shows that adsorbing amount of several kinds of PAEs is obviously smaller than that of a single PAEs whether by yellow soil or purple soil. The adsorbing amount of the three kinds of PAEs can be arranged in the order of DBP>DEP>DMP and the order also applies to the one in the competitive adsorption.(4)An experiment based on thermodynamic reveals that temperature has obvious influence on the adsorption of the three PAEs. The increase in temperature of both soils leads to the decrease of the adsorbing amount. However, the influence differs in the three PAEs separately of which the most affected is DBP and the least is DMP, with DEP being moderately affected. The kinetic parameters△G,△H and△S in the adsorptive process tells that the whole adsorption reaction is spontaneous and exothermic and is controlled together by surface physical adsorption and chemisorption.(5)In reaction system, pH has the largest influence on the adsorption. The adsorbing amount decreases with the rise of pH. The least adsorption of DBP by yellow soil happens when pH is7.0. With pH increasing within the scope of weak acid, the adsorption amount will decrease. While within the scope of weak base, it will increase. (6) The adsorbing amount of DBP, DEP and DMP by yellow soil and purple soil will increase as ionic strength increases. The influence that the ionic strength has on the adsorption of PAEs by purple soil is stronger.