Pollution Characteristics Of Heavy Metal In Soil From Lead And Zinc Mine And Its Stabilization Study

In recent years,with the rapid development of China’s economy, heavy metal pollution incidents occurred frequently, and the soil heavy metal pollution has caused widespread concern in society. The ―Action plan of soil pollution prevention‖ expected to be introduced during the year. Therefore, strengthening researches on heavy metal pollution is an important trend, and has very important significance.Effectiveness is an important factor for soil heavy metal pollution. It has become an important issue in environmental sciences field that how accurately and reasonably evaluate the biological effectiveness of heavy metals in contaminated soil. At the same time, heavy metal elements existing in different forms can show the different geochemical behavior and chemical activity. Therefore, it is important that the morphology of the objective elements in environment can be measured quantitatively and qualitatively, and it has gradually developed into an important branch of analytical chemistry. For various comprehensive evaluation methods of heavy metal pollution levels, index method with a certain objectivity and comparability has been widely used in environmental quality assessment, and has been recommended by China Green Food Development Center. The evaluation model comprises single factor pollution index and integrated pollution index. But, it is noteworthy that the limit of heavy metal contamination in soil is gradual and vague in some practical cases. Fuzzy evaluation based on fuzzy mathematics and the principle of fuzzy relation synthesis is a method of comprehensive evaluation that some ill-defined, easily quantified quantitative factors. Leaching effect refers that pollutants with the infiltration water move down along with vertical section of soil. It is also a composite behavior of adsorption, desorption or distribution between contaminants and water-soil particles. How to effectively evaluate the emission of heavy metal elements in soil, undoubtedly, the leaching experiment can simulate furthest. Leaching simulation can effectively simulate the release law and morphological characteristic of heavy metals in soil, as the soil goes through the leaching solution. Relatively speaking, the In situ is more economical, and the operation and maintenance are simple, and the deep contaminated soil can also be repaired. Stabilization remediation technology contained the technologies of In situ refers that with the use of the chemical properties of the added substance, the pollutants are transformed into the patterns with low solubility, low toxicity or low mobility, and reduce the perniciousness of pollutants for environment.In this paper, the pollution of heavy metal of the area of a lead-zinc mine and the surrounding was studied. In the studied area, the sampling points were laid, and the samples were collected. Then, the type and content of heavy metal and the correlation of different kinds of heavy metals were analyzed, and the Pollution sources of heavy metals in mining soil were determined. Using some chemical effective state extraction methods which comprises TCLP, DTPA, PBET and so on, some characteristics of heavy metals, such as the leaching characteristics, validity, and so on, were analyzed. Meanwhile, the existent form of the heavy metals in soil were analyzed by BCR method,and based on the above study, the methods of heavy metal pollution index, potential ecological risk and fuzzy set were further used to evaluate the risk of heavy metal pollution. The pollution characteristics of heavy metals of Zn mining area was disclosed fully. The tread of the p H, conductivity of leaching water in different leaching conditions was studied, also did the release concentration of heavy metals, and the relevance of heavy Metals was determined. The tread of vertical migration of heavy metals on different leaching conditions and the change of existent form of heavy metals between leaching before and after were analyzed. The cumulative release characteristics of heavy metals in the soil under the condition of leaching was confirmed, furthermore, using the first order kinetics equation, modified Elovich equation, double constant rate equation(then fix the formal) and the parabolic equation(equation) for fitting to explore dynamic model of release of heavy metals. The heavy metals in lead and zinc mine soil were stabilized by chitosan, zeolite modified chitosan, hydroxyapatite modified chitosan, phosphorus-bearing matter, phosphorus substances and different combination of biochar, and discussed the stable mechanism.(1) The average concentration of heavy metal in the lead and zinc mine from high to low in the order is Zn >Pb>Ni>Cd>Cu>Cr >As, and average cumulative heavy metal content were higher than China soil element background values; From the analysis of skewness and kurtosis coefficient, content of Ni, Cr, Zn, Pb, As and Cd obey normal distribution; From the aspects of coefficient of variation, coefficient of variation of heavy metals in soil are belong to medium strength variation except Cr; There is great difference among the mode of occurrence of different heavy metals in the soil; Acid extracted state proportion of each heavy metal element in order form large to small is Zn>Pb>Cd>Cu>Ni>As>Cr, and acid extracted state is easy to cause harm to human body or natural for it is easy to migration and transformation; Residue ratio of Ni, As and Cr is more than 50%, and residue ratio of Ni reach up to 76.11% has stable form.(2) It can be found from the analysis of the single factor pollution index of heavy metals that each element pollution index from big to small is in the order of Cd >Pb >Zn >Ni >Cu >As>Cr, all elements are in severe soil pollution levels except Cr belongs to mild contamination; The nemerow pollution index of heavy metal in the soil is 1472.98, and it is 490.99 times of the upper limit value of serious comprehensive contaminated of soil, which indicates that that the soil in the regional was comprehensive polluted by heavy metals seriously; Form the Eri and RI of heavy metals, the potential environmental risk of heavy metal in soil from big to small is in the order of Cd>Pb>Ni>As>Cu>Zn>Cr, and heavy metal element Cd has the highest environmental risk factor that is up to 24997.33 belongs to the extremely high ecological risk, which is more than 312.47 times high than risk threshold; The comprehensive pollution index RI is up to 25380.63, which is more than 211.51 times than high ecological risk threshold.(3) Referring to HJ557-2009 standard ―Solid Waste-Extraction Procedure for Leaching Toxicity-Horizontal Vibration Method‖, leaching experiments are conducted on soil. The leaching concentrations of Cr, Ni, Cu, Zn, As, Cd, Pb in soil are 8.00E-04 mg/L, 4.83E-03 mg/L, 1.01E-02 mg/L, 1.96E-01 mg/L, 5.22E-03 mg/L, 2.77E-01 mg/L, 1.28E-03 mg/L respectively. The leaching concentrations of the same heavy metal with different leaching methods are different, as well as, the leaching concentrations of the different heavy metals with the same leaching method. The element with highest concentration is Zn in all extract, and the lowest is Cr. Soil is evaluated by soil pollution level and leachate water environment standard level fuzzy comprehensive evaluation method. Each fuzzy comprehensive evaluation vector is calculated and normalized respectively, and it obtains that soil and leachate fuzzy’s evaluation results are B = A ? R = {9.22E- 03,5.74E- 04,0.00,0.00,9.93E-01 }, B = A ? R = {8.04E- 4,1.57E- 03,1.96E- 4,0.00,1.00}. It is analyzed that soil environmental quality level is heavy pollution, and leachate water quality standard level is 5, belonging to ClassⅤ.(4) The p H changes in the leaching process are overall divided into three stages, which are rapid increasing, slow increasing and tending to balance. It is mainly determined by the rate of neutralizing exchange reaction between easily exchangeable base ions and exogenous input H+. There is no significant difference in the p H values of the last three leachate, which indicate that soil has a strong buffering capacity. Electrical conductivity changes are divided into two stages, which are decline and slow decline-leveled off. It is relevant to the quickly releasing ability into the aqueous solution of various base ions on the surface, some soluble oxide ore particles and soluble salt adsorbed on the soil’s surface. The leaching concentrations of heavy metals correspond to the trend of increasing at first and then slowly decreasing basically. The leaching concentrations have a certain negative correlation with leaching volume and p H, as have a certain positive correlation with electrical conductivity. The residual ratios of heavy metals in soil improve significantly, as the acetic acid extractable fraction proportions decrease. This is maily because that heavy metals in the form of more lively in soil are leached into the leachate by leaching. Cumulative release amounts of heavy metals increase slowly. It is found by kinetic equation fitting that amended Elovich equation, double constant rate equation(Freundlich correction formula) and parabolic diffusion equation’s fitting result on the leaching process of heavy metals in soil are all good in addition to first-order kinetics equation. It indicates heavy metals’ release processes in soil under simulated leaching condition are not a simple first-order reaction, but a multifactorial integrated controling process.(5) When a single chitosan is used in the heavy metals stabilization in soil, individule heavy metals concentration may increase in some leaching ways. Zeolite and hydroxyapatite significantly improve the stabilizing effect of heavy metals in soil after modification. The effect of hydroxyapatite modified is generally superior to zeolite modified. The reduction extent order of heavy metals in different leaching ways in the sequence of decending is DTPA effective state, TCLP, PBET effective state. Single chitosan, zeolite modified chitosan and hydroxyapatite modified chitosan are capable of changing occurrence mode of heavy metals in soil. Zeolite modified chitosan and hydroxyapatite modified chitosan can increase the residual fraction proportion of heavy metals in soil better than single chitosan. It is noteworthy that when single chitosan is used, the residual fraction proportions of Ni and Cd appear to reduce.(6) The stable efficiencies of phosphorus-containing substances and biological carbon compound stabilizers on heavy metals have significant differences. For individual heavy metals, such as Cu and Cr, there are risks of activation. The stabilizer’s stabilizing capacities on the heavy metals of different leaching forms are different. Its functioning order in the sequence of decending is DTPA effective state, TCLP, PBET effective state. Under the effect of stabilizer, heavy metals of non-residual fraction have been reduced in different degree. On the basis of the phosphorus-containing substance, adding biological carbon can improve the stability results of heavy metals. It is found by SEM-EDS that there a large number of scattered prismatic and cluster-type products appears on the surface of soil samples, treated by adding stabilizer. It is found that Zn3(PO)4H2O,Pb5(PO4)Cl and Pb10(PO4)6(OH)2 may exist in sample, stabilized by stabilizer, by XRD. But it is noteworthy that soil is a complex system. XRD analysis technology is still mostly used in pure mineral identification, but it can not fully determine the presence(or generate) of some kind of mineral in soil.