| The rapid development of industrialization will produce a large amount of industrial waste or result in some industrial accidents, which may cause severe pollution in local areas with the appearance of small range of severe contaminated sites. Both at home and abroad the research on the control and restoration of this kind of contaminated sites has been developed actively and lots of achievements have been obtained. Considering the high cost and long period of remediation technology, blocking technology can limit pollutants migration and reduce the risk of contaminated sites in a relatively short period of time. For contaminated sites with various pollutants the use of remediation technology is also very complicated, and it is very difficult to completely remove all the pollutants, however, blocking technology can hinder various pollutants simultaneously. For sites in need of remediation blocking technology may be used as a part of engineering control technology in order to control pollutants migration and diffusion,avoid further expansion of the pollution risk, and remediate the contaminated sites on the basis of this. As a technical means of the engineering control technology, Soil-Bentonite(SB) slurry wall blocking technology has been widely used in the engineering control technology because it is easy to get and relatively cheap.Abroad in the respect of SB materials used as blocking materials, the research was mainly concentrated on decreasing hydraulic conductivity coefficient of wall materials and increasing its adsorption performance for pollutants by its modification; in the research neither different pollutants were systematically classified, nor was the interaction between different pollutants and wall materials during the transmission process in the wall considered; results of the research were only for some specific contaminated sites or specific pollutants without generality. At home research and application of SB used as blocking materials for contaminated sites has just started and there are few reports on it.With SB slurry wall as the research object, and based on interactions between pollutants(metal ions, soluble organic pollutants, NAPLs) and wall materials as well as pollutants migration process in the wall, this paper studies the blocking performance and mechanisms for different pollutants by SB slurry wall.First, with the reference of American EPA rules and requirements the appropriate ratio of wall materials is selected meeting construction condition requirements and seepage conditions.On the basis of selected wall materials and from the angle of ion valence, the adsorption capacity and blocking performance for metal ions with different valence by SB slurry wall are studied.The experimental results suggest that SB materials’adsorption process for metal ions with different valence(K+、Ca2+、Fe3+)is fast,and it achieves stability in 30 minutes.The higher the valence is,the greater the adsorption capacity is;and the fitting of adsorption process conforms to pseudo-second-order adsorption kinetics and Langmuir-Freundlich adsorption equation.Effect of metal ions with different valence on the penetrating quality of wall materials varies,and with the time increasing permeability coefficient of K+,Ca2+and Fe3+in SB has a maximum respectively(that is,Kmax,Fe=5.303×10-7 cm/s;Kmax,Ca=7.5×10-7cm/s;Kmax,k=1.41363×10-7cm/s),and the maximum time is related to the valence of ions,the higher the valence is,the sooner the maximum appears(that is,tmax,Fe=55h,tmax,Ca=344h,t max,k=420h,),the more obvious the effect on SB blocking performance is.Breakthrough experiment of metal ions in the wall materials shows that the high the valence is,the sooner the ion breaks through the barrier wall completely(t穿透,Fe=246h,t穿透,Ca=400h,t穿透,K=440h),that is,the wall’s blocking performance for higher valent ions may decline.The transmission speed of different ions in the wall materials is different,Fe3+and Cl-are the fastest,and the ions mainly exist in the form of Fe Cl3.The transmission speed of ions in the wall materials is related to the concentration of ions,namely,the greater the concentration is,the faster the permeability speed is;but considering from the aspect of blocking rate,the greater the ion concentration is,the more obvious the blocking effect of wall materials for ions is.Second, with organic pollutants being classified by soluble organic pollutants and NAPLs,their adsorption capacity and blocking performance by the wall materials, as well as their transmission patterns in the wall materials are studied. Phenol and nitrobenzene are taken as target pollutants which represent soluble organic pollutants and NAPL respectively. The experimental results show that adsorption effect for phenol and nitrobenzene is not obvious, andadsorption rate is relatively low( phenol maximum adsorption rate of only 0.75% and nitrobenzene maximum adsorption amount of only 0.0647mg/g). According to infrared spectum curves of before and after adsorption of phenol and nitrobenzene, SB materials’ adsorption of phenol and nitrobenzene belongs to surface adhesion, inner space structure of wall materials does not change after adsorption, and their permeability coefficients in the wall materials remain unchanged and are the same as that of water which is 10-9cm/s. When nitrobenzene that does not dissolve in water migrates in the wall materials, its permeability coefficient is lower than that of water by an order of magnitude.By sand-box experiment of organic pollutants’ migration in the wall materials, organic pollutants’ migration speed is related to its original concentration, the greater the original concentration is, the faster the speed is. SB’s blocking performance for phenol and nitrobenzene are rather strong, and the blocking performance is closely related to wall thickness, that is, the thicker the wall is, the greater the performance is. In the experiment after 49 days’ transmission,the blocking performances for phenol and nitrobenzene reach 99.80% and 97.87% at the location of 9cm, respectively.Third, research found that ionic strength might alter the pore structure of the wall materials which in turn might change the permeability coefficients of organic pollutions in the wall materials. Ionic strength can speed up phenol transmission in the wall. However, the influence on phenol transmission speed in the wall by the increase of ionic strength is not unlimited, there is a maximum, and the influence is the most obvious when the ionic strength is 10 ml. This influence is gradually weakened with the increase of the thickness of the wall. The influence of the addition of metal ions on nitrobenzene transmission speed in the wall is also related to ionic strength. When ionic strength is low, it will inhibit nitrobenzene transmission; when ionic strength exceeds a certain value, it will increase nitrobenzene transmission speed.Finally, blocking mechanisms for pollutants by the wall materials is discussed and mathematical model is used to describe transmission process of pollutants in the wall. Pollutants migration in the wall materials mainly includes advection and diffusion, as well as physical,chemical and other reactions between pollutants and media. Therefore blocking mechanisms of pollutants migration by SB slurry wall include controlling advection transmission, lowering permeability coefficient, controlling diffusion transmission, pollutants migration caused by interactions between pollutants and wall materials.The breakthrough and transmission behaviors of pollutants in the wall materials can be described by mathematical models. When the wall thickness is small, it can be described by advection transmission model; when the thickness of the wall materials is great,one-dimensional advection diffusion model should be adopted. Research found that metal ions could influence advection transmission coefficient of the wall materials. Therefore, when the blocking wall for different contaminated sites is designed, groundwater pollutant kinds and levels should be given full consideration. If there are higher valent metal ions which have greater influence on the wall materials, according to one-dimensional advection diffusion model the designer should increase the thickness or the design life of the wall in order to avoid pollutant dispersions caused by the pollutants’ breakthrough of the wall.By studying the literature at home and abroad we found that only a single pollutant was studied in the previous research on the blocking behavior of the wall for pollutants, which is not systematic and representative, so in this paper pollutants are classified by metal ions, soluble organic pollutants and NAPLs, and the blocking rule of the wall is studied systematically.The innovative identity is primarily reflected in the following three aspects:(1) For metal pollutants it is the first time to study from the perspective of ion valence to research on the adsorption and blocking performance for metal ions with different valence by SB. Research found that adsorption performance for metal ions with higher valence by SB is stronger, while blocking performance for metal ions with higher valence by SB is lower, which provides a theoretical basis for the design of blocking wall for contaminated sites by metal ions.Meanwhile, it also found that during the migration of metal ions in the wall, the higher the valence is, the faster the migration is, and the greater the concentration is, the faster the migration is. This conclusion is of crucial importance for monitoring contaminated sites,controlling the wall, and evaluating the distribution of metal ions in the wall as well as their breakthrough behaviors.(2) To research on the blocking effects of wall materials for different pollutants and the distribution of pollutants in the wall, to discuss wall materials’ blocking performance for various pollutants, as well as to describe it by mathematical models. Research found that pollutants migration in the wall materials mainly includes advection and diffusion, as well as physical,chemical and other reactions between pollutants and media. The transmission behaviors of pollutants in the wall materials can be described by mathematical models. When the wallthickness is small, pollutants can quickly break through the wall materials, which can be described by advection transmission model. When the thickness of the wall materials is great,there is no obvious difference to evaluate pollutant concentration distribution in the wall materials in a short period of time by comparing the one-dimensional advection diffusion model and the unsteady molecular diffusion model. In view of the actual blocking wall’s design life is usually longer, one-dimensional advection diffusion model should be adopted, which may provide theoretical support for monitoring and control of various contaminated sites. |
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