| The rapid development of industrialization, urbanization and transportation and high input of pesticides, herbicides, fertilizers and sewage water have caused severe heavy metal pollution to agro-fields in China. Investigation on the factors affecting the behaviors of heavy metals in polluted soil and water is of great importance in heavy metal pollution control. China produces large amount of crop residues annually which has not been fully used yet. Many studies have proved that soil dissolved organic carbon (DOC) could enhance heavy metal solubility through forming chelated complex, while incorporation of crop residues into soil produces large amount of interdediates which is important source of soil DOC. However, report on the effect of adding crop residues on heavy metal solubility in polluted soil and its relation with soil DOC is in scarce. In addition, there exist aboundent active sites on the surface of crop residue materials which may have the capability to adsorb heavy metals in polluted water. The residues of rapeseed, wheat and rice were used in this study to investigate the effect of adding crop residues on solubility and removal of heavy metals in polluted soil and water, in an attempt to provide valuable information for ecological risk evaluation in the practice of incorporating crop residues to agro-filed and for the possible use of crop redidues in treating heavy metal polluted water. Results were as followings: (1) The solubility of Cd and Cu in polluted soil was significantly enhanced by adding rapeseed straw. The concentration of Cd and Cu in soil solution inceared with the rate of straw added. Soil DOC was also enhanced by straw addition and solubility of Cd and Cu was positively correlated with DOC during incubation period, which primarily indicated DOC might contribute to the enhancement of Cd/Cu solubility after straw addition.(2) EDTA greatly elevated the solubility of Cd and Cu in polluted soil. But leaching of Cd and Cu into deeper soil layer and water may increase ecological risk of using EDTA to enhance the solubility of heavy metals in polluted soil.(3) Cu and Cd solubility increased significantly in both wheat and rice treatments, but the peak concentration of Cd and Cu in soil solution in rice straw treatment appeared 2 days earlier than that in wheat straw treatment.(4) Soil DOC concentration was enhanced significantly by adding wheat or rice straw, but DOC concentration was higher in wheat straw treatment than that in rice straw treatment. The change pattern of DOC with incubation time was much similar to that of Cd and Cu solubility, which indicated that DOC contributed to the solubility of Cd and Cu.(5) The C:N ratio of wheat straw was approximately 2 times that of rice straw, and higher DOC was produced during incubation process in wheat straw treatment, which could be the reason why Cd and Cu solubility was more enhanced by adding wheat straw.(6) The solubility of Cu was more sensitive to the addition of straw materials, compared to that of Cd, possibly because chelating ability of the active sites in DOC to Cu was more efficient than that to Cd.(7) Soil pH and Eh may also affect the solubility of heavy metals in polluted soil. However, the difference of pH and Eh between wheat and rice straw treatments was negligible, which indicated that the DOC difference between the two treatments caused the difference of Cd and Cu solubility.(8) A considerable adsorption effect of rapeseed straw to heavy metals in polluted water was observed in this study. The maximum adsorption capacity of straw material was 39.06mg·g-1 for Cd and 29.67μg·g-1 for Cu, respectively, but the straw material had higher affinity to Cu than to Cd.This result provided valuable information for the removal of heavy metals in polluted water using straw materials. |
PDF Full Text Request