Assessment And Control On Phytoavailablity Of Copper, Cadmium In Contaminated Soils

Heavy metals (HMs) pollution in soils has seriously affected soil quality and led to lots of food safety problems. Since plants have great interrelationship with soil, in the ecological environment research field there are hot discussion about the assessment of the phytoavailability of HMs in soil, remediation of soil contaminated with HMs and its safety utilization. With the purpose of improving the polluted soil management and its safety utilization, this study built and tested the statistical model of Cu, Cd phytoavailablity in contaminated soils, and focused on the effect of contaminated soil on plant growth and the control measures.To built and test the model of Cu, Cd phytoavailability in contaminated soilsThe first part of this study was to set up the appropriate extracting methods about phytoavailability of Cu, Cd in soils. Yellow brown earth-rice(oryza sativa) grain system near one polluted enterprises was studied. Chemical extractants, including 0.1mol·L-1 HC1, 0.5mol·L-1 HC1,2.5% HOAc,5% HOAc, DTPA(pH7.3),0.05 mol·L-1 EDTA,0.01 mol·L CaCl2,0.05ml·L-1 CaCl2,0.5 mol·L-1NH4OAc,1.0 mol·L-1 NH4OAc,0.1 mol·L-1 NaNO3, 1.0 mol·L-1NH4 NO3,0.5 mol·L-1 NH4NO3+0.02 mol·L-1 EDTA,0.1mol·L-1 C6H8O7(citric acid) and 0.1mol·L-1 C2H2O4(oxalic acid), were used to extract Cu and Cd from the soil samples. The correlation of the amount of Cu, Cd extracted from soil and the amount of Cu, Cd in plant was analyzed using the single linear regression models. The results showed that, among the eleven extractants, 0.05mol·L-1 EDTA, DTPA (pH7.3), 0.1mol·L-1 HC1, 0.1mol·L-1 C6H8O7 (citric acid) and 2.5% HoAc could reflect the phytoavailability of Cu, Cd in the test soils. And the 5 extractants were proved valid in the red soil-pakcoi (Brassica chinensis) system adjacent to different polluted enterprises.It was difficult for any extractant to characterize the phytoavailablity of Cu and Cd in different soil-plant system simultaneously, since the phytoavailability was determined by interaction of soil, HMs and plant. Weight ion impulse (ωI=(?)ω(Ci1/n)) was used to correct toxic difference of Cu and Cd, and the weight index of Cu and Cd was 1 and 6 respectively. The extracted content of alkaline soil should be cut half for soil correction. In addition, the water content was used to correct the absorption difference among different plants. After the correction from these three factors, total WI of Cu-Cd extracted by 0.1mol·L-1 HCl could well reflect the phytoavailablity of the test soils. The total WI in the soil has significant correlationship with the total WI in the plant(r=0.901, n=35, P<0.01).Then, the study was to build a statistical model of Cu,Cd phytoavailablity in different kinds of soils. The parameters included a series of physicochemical properties (including pH, soil organic matter(SOM), available phosphorous content(Pavailable), CEC and clay content, total-Cu and total-Cd) of 57 soil samples that covered 12 soil classes, i.e., black soil, dark brown soil, brown soil, yellow brown soil, latosol, latosolic red soil, red soil, zheltozem, purple soil, light sierozem, alluvial soil and paddy soil. The Cu, Cd phytoavailable contents were extracted by 0.lmol·L-1 HC1. The statistical model of Cd phytoavailablity and Cu phytoavailability against soil physicochemical properties were analyzed with SPSS 13.0. Under the experimental condition, there was significant positive correlation between Cd phytoavailabilty and total-Cd, SOM and Pavailable and negative correlation with pH. And there was positive correlation between Cu phytoavailability and total-Cu, total-Cd and Pavailable, and negative correlation with pH. However, the phytoavailability of Cu was affected by total-Cd significantly. The statistical model expressed in terms of total WI could characterize the relationship between the phytoavailability of Cu, Cd and physicochemical properties in different soils. The model could be mathematically described by lgWIpian=0.995 lgWIsoil+0.1531gPavailable+0.841gSOM-0.058pH-0.365(r=0.875, F=42.570, n=57, P<0.001).The statistical model was tested and applied by the red soil-vegetable system in the adjacent area of Nanchang. Soil and vegetable samples were taken from the sites around six enterprises including iron and steel, chemical industry, electroplating and plastic manufactory in this area. Predicted phytoavailability was significant correlation with the actual content, while the main related factors were total Cu-Cd, SOM, pH and Pavailable (r=0.88,n=75, P<0.001).Effects of Cu, Cd contamination on plant growth and physiological indexThe purpose of predicting phytoavailability of Cu and Cd in soils is to provide the basis for utilization and management of HMs contaminated soil and to ensure the sustainable development. The second part of the study discussed the germination, seedling growth and physiological quality of ryegrass(Lolium perenne L.) and Elsholtzia Splendens under Cu-Cd combined contamination.The germination test results showed the influence of Cu and Cd on plant germination was little, because plant germination mainly depended on self-nutrition. The height of the seedling could reflect the stress of Cu and Cd. The seedling height of ryegrass treated with Cu or Cd at 10mg·L-1 or 30 mg·L-1 respectively was significantly restrained. The seedling height of Elsholtzia Splendens was significantly restrained when Cu or Cd at 30mg·L-1 or 5mg·L-1 respectively. The height of ryegrass was significant enlonged when the concentration of Cd was low. Therefore it could be concluded that ryegrass was tolerant species of Cd and the same for Elsholtzia Splendens to Cu.Ryegrass and Elsholtzia’s pot experiment results showed that absorption coefficient of Cu and Cd in root was greater than that in shoot. Ryegrass stopped growing when soil pH was lower than 5.0. When total Cu in soil was less than 80 mg·kg-1 with soil pH about 6.0, the biomass, chlorophyll content, antioxidant enzymes, root vigor and MDA content of ryegrass had little difference with the CK treatment. When total Cu was more than 230mg·kg-1, physiological index of the plant had been stressed significantly, and the absorption coefficient of Cu and Cd increased significantly. The physiological index of ryegrass, being a Cd tolerant plant, had little difference with the other treatment under the same copper concentration. In order to ensure the forage safety, ryegrass could not be planted in heavy polluted soil. When total Cu in soil was less than 80 mg·kg-1, Elsholtzia biomass was higher than CK treatment. The biomass was stressed significantly while total Cu was more than 230 mg·kg-1.All of the physiological index in this study, including chlorophyll content, antioxidant enzymes, root vigor and MDA content of the plant, had been stressed significantly by Cu and Cd. Soil proteinase, urease, peroxidase and phosphatase could indicate Cu, Cd contamination. Interaction between Cu and Cd was synergic under Cu, Cd combined contamination, and the higher Cu concentration, synergy stronger. The results were also demonstrated in the solution culture experiments.Control on phytoavailability of Cu-Cd in contaminated soilFinally, the control on phytoavailability of Cu-Cd in contaminated soil were studied. Chitosan could improve phytoavailability of Cu-Cd in red soil whereas TiO2 had the counter-productive reduction ability of heavy metal in that TiO2 can absorb Cu and Cd by hydroxyl group. So the combination control measures, such as "Elsholtzia and chitosan" and "Elsholtzia and TiO2" were adapted to comparing control effects with sole phytoremediation. The results showed that physiological index, such as chlorophyll content, antioxidant enzyme activities, root vigor and MDA content, could reflect the stress of Cu-Cd combined contamination on the growth of plant, no matter what chemical amendment was used. The total content of Cu-Cd in the plant with the combination measure was greater than that with sole phytoremediation, so the effect of former was better than that of the latter. Furthermore, "Elsholtzia and chitosan" measure was suited for lighter contaminated soil, while the other combination measure was suited for heavy contaminated soil.