Leaching Characteristics Of Pb And Cd In The Contaminated Soils By Organic Chelating Agents And Biosurfactant

The national investigation bulletin of soil pollution in 2014 showed that the over limit rate of the cultivated soil was 19.4%. Among them, the slight, mild, moderate and severe pollution soils accounted for 13.7%,2.8%,1.8% and 1.1%, respectively. The soils were mainly polluted by heavy metals, such as Cd, As, Hg, Pb, and Cr, and especially in the southwest and south-central regions. The contents of Cd, Hg, As and Pb appeared a rising trend from the northwest to southwest and from northeast to southwest, which was caused by mining, industry, agriculture and transportation. Pb and Cd in soil can not be degraded by microorganisms, and may not only make soil fertility, crop yield and quality decrease, but also enrich and accumulate into plant and animal, eventually enter into the human body through the food chain. In recent years, Pb, Cd pollution incidents have successively occurred in China, such as "blood lead incident" in Fengxiang of Shaanxi, and "Cd pollution incident" in Xianghe Chemical Plant, " Cd rice contamination incident" in Guangdong and Hunan. These incidents attracted more attention of government, public and academia. Therefore, the development of remediation technology for Pb, Cd contaminated soil becomes a hot issue of environmental science.Until now, the remediation techniques of Pb, Cd contaminated soils mainly include replace the polluted soil, heat treatment, electrokinetic remediation, phytoremediation, chemical fixation and leaching methods. However, these methods have disadvantages of long remediation time, high cost, low efficiency or secondary pollution. The joint remedation of multiple techniques gradually draws more attention by academia. Chemical leaching can improve the bioavailability of Pb, Cd in soil, and it is promising to remedy and control Pb, Cd pollution combined with phytoremediation. In this study, we prepared the single pollution and the combined pollution soils of Pb, Cd, and setted up three pollution levels (mild, moderate and severe pollution), the leaching efficiencies of Pb, Cd in the stimulated pollution soils were examined by organic chelating agents (EDTA, EDDS and citric acid) and biosurfactant (rhamnolipid) by oscillation leaching experiment, and screened the optimal chelators to leach Pb, Cd, and then studied the kinetic characteristics of Pb, Cd by organic chelating agents and biosurfactant. The ratio of EDTA and rhamnolipid was optimized by soil column leaching experiment, and the kinetic characteristics of Pb, Cd in combined pollution soil were obtained by EDTA and rhamnolipid. Compared with the fraction changes of Pb, Cd in soil before and after soil column leaching, and the leaching mechanism of Pb and Cd by EDTA and rhamnolipid was revealed, and hope to provide the basis on phytoremediation of Pb, Cd contaminated soil. The main results were obtained as follows:1. The effects of the concentration and initial pH of EDTA, EDDS and citric acid (CIT) on the leaching efficiacy of Pb, Cd in the contaminated soils were investigated, and then the optimum leaching conditions for Pb, Cd with three pollution levels were obtained, i.e., the eluent concentration was 0.025 mol/L, and the initial pH was 7. The leaching percents of Pb, Cd in combined pollution soils showed the similar variation trend with single pollution soils, and increased with the pollution level increasing. Moreover, the leaching percents of Pb, Cd in single pollution soils were higher than those in combined pollution soils by the three organic chelating agents, and the leaching percents of Cd were higher than those of Pb.Pesudo second-order equation could well describe the leaching process of Pb, Cd in the contaminated soils by the three oganic chelating agents. The leaching processes of Pb, Cd in single pollution soils by EDTA and EDDS were slow ones, and reached leaching equilibrium at 120 and 180 min, respectively. The leaching processes of Pb, Cd by CIT were rapid ones, and reached leaching equilibrium at 45 and 60 min, respectively. The maximal leaching percents of Pb in mild, moderate and severe pollution soils by EDTA were 81.89%,82.91% and 84.4%, and those of Cd were 93.16%,93.62% and 94.09%, respectively. Moreover, the maximal leaching percents of Pb, Cd by EDTA were higher than those by EDDS and CIT.For single and combined pollution soils, the leaching rate constants of Pb, Cd by the three chelating agents showed a decreasing order of slight pollution soil, moderate pollution soil and heavy pollution soil, and the leaching ability showed a decreasing order of CIT, EDDS and EDTA, the leaching rate constant of Pb was greater than that of Cd. On the whole, the leaching rate constants of Pb, Cd in single pollution soils were higher than those of the corresponding combined pollution soils. However, the leaching rate constant of Pb in severe combined pollution soil by EDTA and EDDS were higher than that of the corresponding single pollution soil.2. The effects of the concentration and initial pH of rhamnolipid on the leaching efficiacy of Pb, Cd in the contaminated soils were also investigated, and then the optimum leaching conditions for Pb, Cd with three pollution levels were obtained, i.e., the rhamnolipid concentration was 0.025 mol/L, and the initial pH was 7. The leaching percents of Pb, Cd in combined pollution soils showed the similar variation trend with single pollution soils, and increased with the pollution level increasing. Moreover, the leaching percents of Pb in single pollution soils were lower than those in combined pollution soils by rhamnolipid, but the leaching percents of Cd were just the opposite.The leaching processes of Pb, Cd by rhamnolipid were rapid ones, and reached leaching equilibrium at 30 and 25 min, respectively. The leaching rate constants of Cd were greater than those of Pb, and rhamnolipid leaching was significantly faster than the three organic chelating agents. For single and combined pollution soils, leaching rate constants of Pb, Cd showed a decreasing order of slight pollution soil, moderate pollution soil and severe pollution soil, and leaching rate constants in combined pollution soils were greater than those in the corresponding single pollution soils.3. The leaching characteristics of Pb, Cd in combined pollution soil were investigated with EDTA and rhamnolipid by soil column leaching experiment. The leaching percents of the two heavy metals increased, and reached the equilibrium with the volume of mixed eluents increasing, and showed a trend of increase, reached the maximums and decrease with the ratio of EDTA and rhamnolipid increasing. When the ratio of EDTA and rhamnolipid was 1.5:1, the leaching percents of Pb, Cd reached the maximums, those of Pb in mild, moderate and severe pollution soils were 82.97%, 87.61% and 91.45%, while those of Cd were 85.45%,89.25% and 93.88%, respectively. Morover, the leaching efficiacy was the best in severe pollution soil.Pesudo second-order equation could well describe leaching process of Pb, Cd in combined pollution soil by EDTA and rhamnolipid, and the leaching rate constants of Pb in mild, moderate and severe pollution soils were 0.00092,0.00080 and 0.00072 kg·mg-1·min-1, while those of Cd were 0.0004,0.00025 and 0.00021 kg·mg-1·min-1, respectively. The leaching rate constants were lower than those alone by EDTA and rhamnolipid.4. The fraction changes of Pb, Cd in combined pollution soils were studied before and after joint leaching. The results indicated that joint leaching could effectively remove the exchangeable and carbonate bound forms, followed by organic bound form, and difficult to remove Fe-Mn oxide bound and residual forms in the polluted soil. The joint leaching mechanisms of Pb, Cd in the contaminated soil mainly included the coordination and chelation of EDTA and the micellar solubilization of rhamnolipid.