| River filtration system is a low-cost and efficient alternative water treatment for drinking-water applications. It is a natural purification, which means during the river water passing through the soil and the aquifer material, potential contaminants present in the river water are filtered and attenuated by a series of physical, chemical and biological action. River filtration system has been used in Europe for more than 100 years and much longer in our country, but studies on purification mechanism of given pollutants has been not reported in quantity currently, while most of our knowledge is restricted to empiricism. Multiple analogue experiments were applied in this study in order to reveal the environmental behaviors and removal mechanism of BTEX in the river filtration system particularly from the standpoint of mass migration and geochemical reaction kinetics. A series of analogue experiments were carried out on account of different transportation and change mechanism of BTEX including volatilization, adsorption and microbiological degradation. Based on volatilization experiments, volatilization dynamic curves of each component of BTEX and mixture on different underlying materials were obtained.(1) Volatilization dynamic of each BTEX component on still water surface accorded with linear model, namely linear positive correlation was found between volatile matter and time. While the volatilization dynamic of the BTEX mixture conformed to exponential form.(2) The results indicated that under the same condition, the average volatility speed of BTEX compounds and mixture on the still water surface was in the order of Benzene > Toluene > BTEX > Ethyl-benzene > M-xylene, on the silt was Benzene > Toluene > BTEX> Ethyl-benzene > M-xylene, and on the sand was Benzene > Toluene > BTEX> M-xylene > Ethyl-benzene.(3) Among three different underlying surfaces, the average volatility speed of BTEX compounds and mixture on the still water surface was the fastest, next was on the sand, and on the silt soil was the slowest.(4) The average volatility speed varied with vapor pressure of each component of BTEX, which submitted linear relationship. So vapor pressure was the key influence factor of volatility speed. Under the same condition, on the soil underlying surfaces, the main influence factors of volatilization dynamic were including particle size and organic matter content of the soil samples. Based on static adsorption experiment, adsorption kinetic curves and adsorption isotherm of each component of BTEX in three different soil samples were plotted.(1) The attenuation processes of the BTEX components in the silt soil and the fine sand accorded with logarithm model, which meant adsorption amount decreased as time went by. The adsorption reaction balance was achieved within 48 hours.(2) Adsorption isotherms and adsorption isotherm equations of four components of BTEX in three soil samples were set up, based on adsorption balance experiment. Adsorption processes in the silt soil and the fine sand were corresponding with Henry model, namely adsorption capacity was directly proportional to equilibrium concentration of the solution. While adsorption process in the coarse sand accorded with Langmuir model.(3) By comparison of the adsorption isotherm and the soil-water partition coefficient Kd of each BTEX component in different soil samples, it was found that the silt soil and the fine sand possessed relatively higher adsorption capacity than that of the coarse sand. The results also showed that the silt soil and fine sand samples displayed strongest adsorption capacity to Benzene, to Toluene the second, and to Ethyl-benzene and M-xylene much weaker.(4) Four influence factors were chosen in this study. Organic matter content had the greatest effect on adsorption among these factors. The influences of pH and salt content were not unimportant standing. The results of temperature effect experiments indicated that adsorption amount increased with temperature rising, which was opposite to the previous understanding.Simulation experiment study of environmental behaviors of BTEX in the river filtration system was carried out by soil column test, and some conclusions were obtained as follow:(1) Environmental behaviors of BTEX in the river filtration system were including adsorption and microbial degradation. The purification effect of adsorption was limited. On the other hand, BTEX contamination could be well removed by degradation.(2) The results indicated that the average removal ratios of BTEX components in the river filtration system ranged from 68.5% to 85.5% in the system of taking nitrate as electron acceptor, and ranged from63.5% to 82.4% in the system of taking sulfate as electron acceptor. Among the removal ratios of the four components of BTEX, M-xylene was the highest, the second was Toluene and Ethyl-benzene, and that of Benzene was the lowest. The stability of the structure of Benzene determined its relatively lower degradation efficiency, and the low removal ratio of Toluene was due to the effect of adsorption on degradation. The soil-water partition coefficient Kd of each component of BTEX was higher, the degradation ratio was lower.(3) As a natural purification river filtration could efficiently remove the BTEX mixed pollution within a quite high concentration range and protect the ground water from being contaminated,but its purification effect was limited within a certain range of time and concentration. When the adsorption saturation was achieved and microbial activity was restrained, BTEX contamination would penetrate through the unsaturated zone and threaten the security of the ground water.
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