Chemical Washing Remediation Of Pb?Cd?Cu Contaminated Soil By Extraction With Na₂EDTA And STPP

Soil heavy metal pollution will lead to food and ecological security issues, thus carrying out the rehabilitation and treatment of contaminated soil has become essential. In this paper, batch extraction is applied to study the extraction performance of Sodium phosphate?STPP? and Ethylenediamine sodium oxalate?Na₂EDTA? on Pb, Cd and Cu in the national soil samples in Shuikoushan Pb/Zn mining and smelting area and surrounding areas in Hengyang, Hunan. The emphasis is on the influence of mixed liquid concentration, oscillating time and pH to extraction efficiency of heavy metals in the soil, by comparing the metal fractions of heavy metals before and after the extraction, the extraction behavior of heavy metals in the soil is revealed. Experimental datas were fitted through the first-order kinetic equation, the two order kinetics equation, double constant equation and parabolic diffusion equation to found the optimal fitted equation. At the same time, through the soil column leaching method,the desorption effects in simulative conditions were concluded and were studied to select the optimal restorative materials and provide theoretical basis for practical application. The results show that:?1? Under different conditions, the leaching remediation balance effect of STPP, Na2 EDTA and its composite eluent toward Pb, Cd and Cu in contaminated soil indicated that: when the soil is separately treated with STPP, the heavy metals removal ability in soil is smaller. When the soil is treated with composite eluent, under the condition of lower concentration, STPP can promote the leaching effect of Na2 EDTA toward heavy metals. Along with the increase of concentration, the leaching effect of Na2 EDTA toward heavy metals is inhibited by STPP. The removal rate of the two kinds chelating agents toward heavy metal increases along with the increase of mixture concentration, time, and soil water ratio; on the other hand, it decreases along with the increase of pH value.?2? Studies of two kinds chelating agents toward Pb, Cd and Cu leaching dynamics showed that: the correlation coefficients of pseudo-first order kinetics, intra-particle diffusion model and two constant equation are significantly lower than that of the quasi-second-order kinetics; in addition, the correlation coefficient of quasi-second-order kinetics is the best, and the theoretical equilibrium leaching is very close to the actual balance leaching amount. That indicate that the pseudo second kinetic model is more suitable to describe the leaching process of mixed eluent to lead, cadmium and copper ion in the soil.?3? The heavy metals morphological changes of the soil leached with eluent combined STPP and Na2 EDTA indicate that the combined effects of two kinds chelating agents can effectively leach acid exchangeable, reducible and oxidation state of heavy metal, thereby reducing the activity, direct toxicity and bioavailability of heavy metals.?4? The experiment results of soil column leaching on heavy metals removal in soil shown that in the initial stage of leaching, the leached content of lead, cadmium and copper is relatively large, but over time, the concentration of heavy metal ions in eluent is gradually reduced.