Research On Feasibility Of Modified Bentonite As Impermeability Material

Sanitary landfill as the ultimate disposal of municipal solid waste has unmatched economic and technological advantages compared with other treatment options, so it is used in many countries as the primary means. Although the city Sanitary Landfill Site has many advantages, the leachate produced in the landfill stage and during the period of monitoring when landfill is post-closured is a more difficult problem. Landfill leachate contains a large number of pollutions, at the same time, the pollution intensity and the pollution characteristics of the landfill will also timely change constantly. Since the deficiencies in landfill design,construction and operation, it is easy to result in system failure of landfill leachate controlling, and ultimately the leachate will pollute groundwater under the landfill.On the prevention of landfill leachate, study is mainly concentrated in the high-performance protection system at the international level , which is the most common to use of clay and bentonite. Many scholars who research impermeable material have done a lot of research on the modified clay, but bentonite is vulnerable to be influenced by certain pollutions. In this case, the performance of bentonite will decrease. As an ideal cushion security systems, in addition to the low permeability, they need to play a reaction medium of adsorbing organic pollutants, in order to facilitate biodegradation of organic pollutants. Therefore, investigations were conducted into availability of organic clay liner to remove organic pollutants by some experts and scholars.Seeking economic, reasonable and effective methods and techniques to prevent leakage of leachate, as far as possible closure of pollutions in the impervious layer of the system to reduce the pollution of underground environment is the key solution. In this paper, through a lot of static and single and double mineral base liner experiment, exploration and experimental model were conducted to founding the effective liners with low permeability, which could keep long-term closure of pollutions. First of all, according to the methods of modified inorganobentonites, the best modified conditions were determined for: Salt modification, acidification and roasting. At the same time, the four kinds of mature organic modified methods were used to modify organobentonites. After using the seven kinds of modified and non-modified bentonite above as original single mineral base liners, study was done on the removal of leachate pollutants and the permeability in various modified bentonite liners. The comparisons were made to choose best inorganobentonites and organobentonites. Double mineral base liner experiment was designed by using clay as topper "the inactive layer" and respectively using best inorganobentonites and organobentonites as lower "the active layer". Moreover, this article made quantitative analysis of the adsorption of material (clay and modified bentonite) on pollutants in leachate. Investigations were conducted into availability of controlling the permeating of landfill leachate and feasibility of removing the main pollutants in leachate. Ultimately, conclusions were draw that different types of modified bentonite should be chosen as"the active layer"according to different style of landfill pollutants. Specific research and experimental results are as follows:1. Preparation of bentoniteIn the three kinds of inorganobentonites methods, when modifying salt modified bentonite, the best concentration of modifier magnesium chloride was 2 mol/L; when modifying acidification bentonite, the best concentration of modifier sulfate was 0.5 mol/L; when roasting bentonite, the best temperature was at 450℃. According to mature organobentonite preparation process,4 kind of organobentonites: 100 TMAB short carbon chain, 100 HDTMA long carbon chain, 20 TMAB/80HDTMA dual-cation and 100 HDTMA/20SDS anion-cation organobentonites can be made.2. Experiment of single mineral base linerIn the impermeable liner of inorganobentonites, penetration coefficient of salt modified bentonite was the minimum, and the impermeable properties of roating bentonite was most sacrifices, but all kinds of modified bentonite can be reached the requirements of minimum national standards 1.00×10-7cm/s-1; In impermeable liner of organobentonites, the penetration coefficient of 100 TMAB short carbon chain organobentonites was a little higher than the non-modified bentonite, but lower than any other modified bentonite, and the impermeable performance of dual-cation organobentonites shown the worst, close to the national standard in late experiment.In inorganobentonites impermeable layer, the removal rates of COD, NH4+ and TN in leachate were the largest by roasting bentonite; In organobentonites impermeable layer, the dual-cation organobentonites had the largest removal ability of COD, and in the early experiment compared with the long carbon chain organobentonites and anion-cation organobentonites, the dual-cation organobentonites removal more than 50% NH4+. In the middle of the experiment, the value of NH4+ in all of the organobentonites impermeable layers was basically consistent. After penetration, the value of NH4+ had rised. At the same time, the breakthrough time of TN was later than NH4+. In addition, modified bentonites could block various heavy metals strongly.Because of the roasting bentonites and dual-cation organobentonites had better removal ability of pollutions in leachate compared with the other inorganobentonites and organobentonites, these two materials were chosen to use as"the active layer"of double mineral base liner.3. Experiment of double mineral base linerThe adsorption equilibrium time of pollutants in leachate in bentonite,clay,roasting bentonites and dual-cation organobentonites were 24h; The adsorption of COD and NH4+ in landfill leachate in roasting bentonites and dual-cation organobentonites were larger than that of clay, and the adsorption of COD in dual-cation organobentonites was the largest, at the same time the adsorption of NH4+ in roasting bentonites was the largest one; The maximum adsorption of COD was larger than that of NH4+ in clay and dual-cation organobentonites. The experiment of double mineral base liner was conducted by using clay as"the inactive layer"and respectively roasting bentonites and dual-cation organobentonites as"the active layer". The results showed that: The permeability coefficient of the two impermeable layers can both reach the national minimum standard of 1×10-7cm/s. Because the interlayer space of dual-cation organobentonites were obviously enlarged, certain permeability was sacrificed, but the permeability of roasting bentonites indicated better.Though the breakthrough time in the double mineral base liner which use roadting bentonites as"the active layer"was prolonged 20d later than that in the double mineral base liner which use dual-cation organobentonites as"the active layer", the latter had a very strong adsorption capacity of contamination in leachate. On the contrary, the double mineral base liner using roasting bentonites as"the active layer"had a strong adsorption capacity of NH4+. The adsorption of NO3- in the two layers were more or less the same. Totally, the adsorption of TN in the double mineral base liner using roasting bentonites as"the active layer"had also indicated better than that in the double mineral base liner using dual-cation organobentonites as"the active layer". Two columns had blocked Mn, Fe and Cr strongly, but the removal ability of Zn was shown lower. Because copper meshes were used in the two layers, the concentrations of Cu were tested increased.Totally, the removal capacity of inorganic NH4+ in roasting bentonites was better, and the adsorption of organic pollutants in dual-cation organobentonites were better than that of roasting bentonites. The two modified bentonites both can be used as"the active layer", placed under"the inactive layer". At the same time, different modified bentonites should be chosen according to different types of pollutants in landfill.