Cd Contaminated Soil In Situ Remediation Agent And Its Mechanism

With the increase of agricultural pesticide , chemical fertilizer consumption and industrial and mining waste, contaminations containing heavy metals have entered into the soil through various sources which cause serious pollution, decreased agricultural product quality and yield, and produce adverse effects on human health through the biological chain, Over 1.3×104 hm2 farmlands in China have been contaminated with Cd, involving 25 regions of 11 provinces, so treatment of the pollution is very important and urgent. Some sulphydryl (-SH) containing plant offal, modification of clays,mineral slag,industry residue(nanoparticles),organic material(sulfhydrylation)as amendments were applied in this research, the effects and influencing factors of heavy metal amendments were studied by laboratory incubation and pot culture experiment. According to the antagonism between different metal ions in absorption by plant, the effect of seed soaking and foliage spray were studied in pot experiments. The adsorption and desorption of solution Cd to different amendments was studied by the experiment of thermodynamic and kinetic aproaches. The results were given as follows:The hydroxyapatite (HA), phosphate rock powder (PRH and PRX), zeolite, red mud, plant offal, and their modified products were applied to the Cd-contaminated soils to assess the efficiency of these materials in immobilization of soil Cd. After incubation for different times, the concentration of exchangeable Cd was determined. The results showed that the concentration of exchangeable Cd in soils was decreased using red mud nanoparticles and HA by 35%~55%, using red mud, acid-treated red mud, and zeolite by 15%~25%, and using the sulphydryl (-SH) containing materials including garlic leaf, scallion, rape offal by 20%~25%. The immobilization by PRX, PRH, scallion powder, maize straw and sulfhydrylated maize straw were less effective compared with the above materials. The immobilizing effects of HA, ammonium saturated zeolite, red mud nanoparticles, and acid-treated red mud were stable at different incubation times, while the immobilizing effects of red mud, scallion offal, maize straw and its sulfhydrylation dispose increased along with the incubation times, especially for maize straw and its sulfhydrylated materials after eight weeks or longer incubation time. The results in the present study might be of practical importance in screening of amendments for remediation of soils contaminated with cadmium.The biomasses of cucumber aboveground increased by adding red mud into soils, but it decreased gradually along with the increase of the amount of application, it increased effectively with concentration of zeolite and maize straw increasing, the application of sulfhydrylated maize straw showed little effect on biomasses of cucumber. The application of garlic leaf offal also increased biomasses but no distinct change was appeared as the concentration of amendment increasing. The application of red mud decreased the concentration of Cd in cucumber plant by 20.4%～60.2%, and red mud nanoparticles increased the immobilizing effects significantly, saturated zeolite by 11.7%～48.0%, garlic leaf offal by 20.1%～59.4%, maize straw by 20.3%～36.0%, and sulfhydrylated maize straw by about 20%, but the concentration of Cd can be decreased by nearly 50% in 5 mg kg-1 Cd contaminated soils. With the application of different amendments, the enrichment factor of Cd in cucumber appears to decreaseThe pH values of soil increased by the application of red mud, while decreased by maize straw, sulfhydrylated maize straw and garlic leaf offal. The correlation between Cd concentration in cucumber and pH values was not significant. With the application of red mud, the cation exchange capacity decreased, while the cation exchange capacity increased with the zeolite, maize straw and garlic leaf offal applied. The correlation between Cd concentration in cucumber and CEC values was not significant. The application of maize straw, sulfhydrylated maize straw and garlic leaf offal increased organic matter in soils. The negative correlation between organic matter in soil and Cd concentration in plant was significant. The concentration of EDTA and EXC-Cd was the highest, then were FeOx-Cd, and ERMn, CA and OM-Cd were much lower, WS-Cd was the lowest. The EXC and OM-Cd decreased, but ERMn, EDTA, CA-Cd increased under the application of red mud. With zeolite application, the content of EDTA, CA and RES-Cd increased, while WS, EXC, ERMn and OM-Cd decreased. The concentration of EXC, ERMn, CA and FeOx-Cd in soils decreased continuously, while EDTA, WS, OM and RES-Cd increased with the application of maize straw, sulfhydrylated maize straw and garlic leaf offal. The variation of different forms of Cd, pH values, OM and CEC composed the immobilization mechanism of different amendments。During the second crop experiment,the biomasses of cucumber was higher than that in first pot culture experiment. With the application of amendments,Cd concentration in plant was much lower than CK. The effects of immobilizing on red mud in the second crop experiment were better than in the first time. The immobilization effect of maize straw was different in two pot experiments, while zeolite was stable. The garlic leaf offal was better than the first time, and showed no distinct change on effects of different concentration of amendment. The Cd concentration in cucumber plants was significantly decreased under antagonistic effect of foliar application of 300 mg kg-1 ZnSO4. In the complex immobilization experiment, foliage can also decreased uptake of Cd by the plants, but was not so effective as application of amendments. The uptake of Zn by the plants increased significantly under antagonistic effect of foliar spray application, Zn content in plants was not increased obviously under seeds marinated by ZnSO4. The immobilizing effects of red mud by bayer process is much better than by sintering process under different concentration of application, because of the smaller particle size, larger surface area, higher content of Fe2O3 and Al2O3。The adsorption capacity of red mud and zeolite to Cd was very large. The proportion of water soluble or exchangeable Cd was less than 25% in absorbed by red mud, and most absorbed Cd (more than 50%) was stable and nonbioavailable. Red mud nanoparticles and roasting zeolite increased adsorption capacity. The maize straw, sulfhydrylated maize straw powder and garlic leaf offal showed high adsorption capacity of Cd2+ higher than soils, garlic leaf offal appeared more efficient than the sulfhydrylated and maize straw, but the combination stability was lower than soils. In contrast with soil, the proportion of water soluble and exchangeable Cd in absorbed by zeolite, roasting zeolite and garlic leaf offal decreased obviously, but was less effective than red mud. The Cd2+ adsorbing capacity of red mud, zeolite, maize straw, and garlic leaf offal accord with Freundlich and Langmuir adsorption equation model, the parameter of the result showed adsorption character of different amendments. The adsorption isotherms of red mud were affected by the concentration of background ions obviously, and there may be two styles: external layer and special adsorption.The processes of Cd2+ adsorption can be divided into two stages, fast and slow reactions. Adsorptive rate of red mud was higher and the equilibrium time was shorter, red mud nanoparticles increased adsorptive rate obviously and shorten the equilibrium time. However the equilibrium time of zeolite was longer, it may be associated with inner tunnel structure。The pH value of solution under the application of red mud was much higher than zeolite. The adsorptions of red mud were well described by pseudo-second-order kinetics and model, zeolite, fluvo-aquic soil can be simulated better by pseudo-second-order kinetics model, indexⅡand Elovich equation. Fitting parameters of pseudo-second-order kinetics model and indexⅡmodel indicated kinetic different characteristics of amendments, the curve of parabola model indicated that the absorption may have two diffusion processes. Future research should focus on the immobilizing mechanisms at molecular level by advanced analysis technique, such as X-ray absorbance fine structure spectroscopy and Fourier transformed infrared spectroscopy.