Research On Chemical Washing Of Cadmium-contaminated Soil

This paper, through the analysis of changes of various forms of cadmium concentration before and after chemical soil washing, aims at study main factors of the effects on remediation of cadmium contaminated soil and its washing kinetics characters. The research firstly compares washing efficiency of different eluent at different concentrations and solid-liquid ratio conditions.Then it operates a washing kinetics experiment on the optimum washing conditions to determine the optimum washing time. The main conclusions are as follows:(1) the total cadmium contained in tested soil is 1068 mg/kg, in which the content of exchangeable form is 40.72 percent, carbonate bound form is 31.21 percent, Fe-Mn oxide form is15.86 percent, 12.16 percent organic bound form, 0.05 percent residual form. Its active state(except residual external form) content is at higher proportion.(2) We first selected six kinds of the eluent, which contains four kinds of low molecular weight organic acids(citric, oxalic acid, tartaric acid, acetic acid) and two common inorganic salt(NaCl, CaCl2). These eluent are at three levels of concentration include 0.1 mol/L, 0.5 mol/L and 1.0 mol/L, their solid-liquid ratios are 1:1,1:4 and 1:8. Then we kept washing the contaminated soil at 180r/min in ambient temperature for 160 min. Experimental shows that the total chromium removal in descending order of tartaric acid>citric acid>calcium chloride>oxalic acid>acetic acid>sodium chloride. Tartaric acid is one of the best conditions for solid-liquid ratio of 1:4, the concentration of 1.0mol/L, the optimal removal of 69.98 percent; citric acid in the preferred liquid ratio of 1:4, the optimum conditions for concentration 0.5mol/L, the removal efficiency was 69.38 percent; calcium chloride in the preferred liquid ratio of 1:4, removal under optimal conditions of concentration 1.0mol/L was 65.13 percent; oxalic acid in the preferred liquid ratio of 1:4, the best concentration of 1.0 mol/L of removal was 64.79percent; the best solid-liquid ratio of acetic acid is 1:4, the optimum concentration of 1.0 mol/L, its removal efficiency was 58.71 percent; the best solid-liquid ratio of sodium chloride 1: 4, the optimum concentration of 1.0 mol/L, its removal efficiency was 57.02 percent.(3)Based on different Cadmium removal efficiency of six eluent at each time quantum,we carried on kinetic experiments at optimum washing conditions of each eluent. Experiments show that in 1:4 solid-liquid ratio,citric acid of 0.5mol/L reached relatively stable after 120 min washing, so that is the best washing time; The optimum washing time of oxalic acid of 1.0mol/L is 80min; The best washing time of 1.0mol/L tartaric acid is 80min; The optimum washing time of 1.0mol/L acetic acid is 100min; The optimum washing time of 1.0mol/L NaCl is 140min; The optimum washing time of 0.5mol/L CaCl2 is 80 min.(4) Using Tessier five-step extraction method for soil samples after washing were analyzed and compared to the previous washing soil samples showed chemical eluent has great impact on exchangeable cadmium and carbonate cadmium.Citric acid, oxalic acid, tartaric acid, acetic acid, sodium chloride and calcium chloride for the removal of these two forms are 88.97 percent and 71.47 percent, 94.02 percent and 53.15 percent, 89.89 percent and 53.75 percent, 92.41 percent and 41.14 percent, 89.89 percent and 47.15 percent, 96.32 percent and 47.15 percent. Removal efficiency of residual cadmium is not obvious, it depends on nature.The study shows that these six eluent for removal of the heavy metal cadmium in soil samples were obvious. The concentration, solid-liquid ratio and oscillation washing time of eluent have decisive influence on the repairing effect, which also related to cadmium distribution in the soil.