Soil pollution can be divided into an artificial pollutant and a natural one. Heavy-metal soil pollution caused by the latter indicates the emission of heavy metals, resulting from weathering of a matrix, high in heavy metals so that a contamination l...
Soil pollution can be divided into an artificial pollutant and a natural one. Heavy-metal soil pollution caused by the latter indicates the emission of heavy metals, resulting from weathering of a matrix, high in heavy metals so that a contamination level differs according to the content of materials that absorb them such as clay, hydroxide and organic matter. The former includes mining activities(mining and mineral dressing), agricultural activities(the use of fertilizer and insecticide), combustion of fossil fuel, landfill and all sorts of industrial activities. Among them, tailing, mine waste, acid mine drainage, etc. led by mining activities are a major source of heavy-metal pollution that draws attention in the field of soil pollution nowadays.
There are a variety of ways to restore the soil contaminated by heavy metals. However, when choosing a technology for restoration, we need to make a choice of the optimum way, considering soil characteristics, existence form of heavy metals or economic feasibility and so on. This technology can largely categorized as immobilization(solidification/stabilization) that minimize a possibility of heavy-metal contaminant transports and soil washing/flushing. Even though the latter contains a variety of heavy metals and disposal methods, such a technology should be chosen in consideration of economic feasibility such as utility value of treated soil, reutilization or disposal of heavy metals after soil flushing. The former, a technology to curb the characteristics of contaminant transports by changing physical and chemical characteristics of contaminated soil after injection of additives into the soil, has a problem of remaining as a potential soil pollutant because heavy metals are not destroyed nor removed by a chemical reaction. In addition, these technologies(soil washing, solidification/stabilization, phytoremediation) have a problem of high cost, inflexible reutilization of contaminated sites or a long term. Nevertheless, a way of using phosphate, which has recently been discussed, is a treatment technology for the soil polluted by heavy metals that can solve such a problem.
Accordingly, this research aims at eliminating zinc by immobilizing the artificially zinc-contaminated soil in the form of stable metal phosphate by means of arsenic coagulation and liquid phosphate.
As a result of conducting an experiment by selectively creating reaction of phosphate formulation and alkali one each to a solution contaminated with a zinc density of 2,000 mg/L, the injection of phosphate formulation and alkali one in turn, followed by their mixture, led to 98.7~99.5%, a high immobilizing rate, and the injection by a reaction molar ratio of 1 to 3 resulted in 99.5% of zinc removal rate.
In order to find the optimum immobilizing time of zinc in the artificially contaminated soil in which zinc density measures 400 mg/kg by the standards of soil washing, we carried out an experiment at different reaction hours after immobilizing a proper zinc density confirmed in a liquid reaction, and molar rate(1:3), temperature(15℃) and mixing intensity(100rpm) of immobilizing agents. As a result, more than 86.5 percent of zinc was immobilized within reaction 1hr, showing no big difference in a change to an immobilizing rate with a total of 93.8 percent to 94.9. In this regard, an immobilizing reaction is thought to be mostly completed within 24hr.
We also conducted an immobilizing experiment by injecting immobilizing agents, which had melted into an acid solution, into the artificially contaminated soil and creating further reaction of additional alkali formulation. When 1 mole of zinc and 3 moles of phosphate and alkali formulation, namely the molar ratio of 1 to 3, met zinc contained in the soil, an immobilizing rate turned out to be 93.8 percent, while a constant increase in the molar ratio of immobilizing agents showed only a slight difference, which can be regarded as no increase in an immobilizing rate.
We also changed a reation temperature into 5~25℃ within range of Korea's toil temperature and immobilized a zinc density and a molar ratio of immobilizing agents(1:3), mixing intensity(100rpm), and then caused a reaction for a day. In consequence, soil temperature was found to have little effect on an immobilizing rate as an increase in temperature had been seen to make little change in zinc immobilization efficiency. Contrary to this result, dependence on temperature turned out low within a reaction temperature of zinc immobilization and it is not likely to be a problem to apply an immobilizing reaction, using phosphate formulation, to the spot under the soil temperature(5~25℃) of South Korea with distinct four seasons.
As an experimental result of changing a reaction intensity(the number of times being mixed) into 50~250 rpm in order to enhance zinc immobilizing rate, even such a change turned out to make little difference in removal efficiency since it was still 90.1~95.9 percent. Therefore, the number of times being mixed seems to have no influence on zinc immobilizing reaction as experimental results did not show a big difference in the mixing intensity of 50~250 rpm
Then, we looked into a long-term experiment on soil washing and the characteristics of contaminant transports so as to evaluate the stability of zinc phosphate, created by an interaction of immobilizing agents and zinc, targeting the immobilized soil. In consequence, the soil with a reaction completed showed 93.8 percent of immobilization efficiency and the density of phosphorus residues was under 0.3 mg/L. A long-term experiment on soil washing for 60 days found that zinc has hardly been detected in the immobilized soil with the passage of time, while, in the control group, it flowed out in the contaminated soil due to the injection of a simulated solution of simulated acid rain at the beginning of a reaction time, from 1th to 13th day, which can cause the pollution of ground-water syst