| In order to explore the feasibility and mechanism of the phytoremediaton enhancement of Cd-contaminated soil by Euphorbia thymifolia L. with the addition of organic acids, pot experiments were conducted to investigate the effects of exogenous citric acid,oxalic acid and malic acid on Cd accumulation in Euphorbia thymifolia L., as well as on p H and Cd speciation of soils, and on the physiological and biochemical characteristics, chemical forms of Cd of the plants. Cd-contaminated soil was sandy soil from a Cd-contaminated site and Cd concentration of soil is 247 mg/kg.(1) The effects of exogenous natural organic acids on Cd accumulation in Euphorbia thymifolia L.When Cd concentration of soil was 247 mg/kg and implantation time was 15 d, application of organic acids enhanced Cd uptake and accumulation in plant roots and aboveground parts to various degrees. The orders of strengthening effect are: oxalic acid > citric acid > malic acid. Application of 10 mmol·kg-1oxalic acid made the Cd content of the aboveground part reach maximum(77.21mg/kg), increased by 87.72% compared with the control.(2) The effects of exogenous natural organic acids on p H and Cd speciation of soilsThree kind of organic acid all resulted in the drop of the p H, the orders of impact effect are: oxalic acid > citric acid > malic acid, they decrased the p H of soil form 7.65 to 6.01~7.00, 6.62~7.30 and 6.72~7.45, respectively. Different concentration(5, 10, 20 mmol·kg-1)of organic acids also resulted in the reduction of residual Cd and the increase of acid-soluble Cd of soil(form 40.94 mg/kg to51.57~83.01 mg/kg). Compared with the critic acid and oxalic acid treatment groups, the malic acid treatment groups has less change. By lowering soil p H and increasing the content of acid-soluble Cd in soil, Organic acids could improve the bioavailability of Cd, enhanced Cd uptake of Euphorbia thymifolia L..(3) The effects of exogenous natural organic acids on biochemical characteristics of Euphorbia thymifolia L.Compared with the control group, application of 5, 10, 20 mmol·kg-1 of citric acid, oxalic acid and 5, 10 mmol·kg-1 malic acid enhanced root activity and ATPase activity of roots, reduced cell membrane permeability of roots. The oxalic acid treatment group has more obvious effect. However, 20 mmol·kg-1 malic acid decreased root activity and ATPase activity of roots, kept the cell membrane permeability. SEM results of of various organs of Euphorbia thymifolia L. also showed that, organic acids could improve the structure of the roots, stems and leaves of Euphorbia thymifolia L..(4) The effects of exogenous natural organic acids on chemical forms of Cd in Euphorbia thymifolia L.XRD spectras of various organs of Euphorbia thymifolia L. showed that, C4H4 Cd O5·3H2O may be exist in various organs of all the groups, and the malic acid treatment group has stronger peak intensity than the others. C4H4 Cd O4·5H2O may be exist in leaves of all the groups. C2 Cd O4·H2O and C2 Cd O4·3H2O may be exist in various organs of the oxalic acid treatment group. This indicated that C4H4 Cd O5·3H2O and C4H4 Cd O4·5H2O themselves were metabolites of Euphorbia thymifolia L. in the remediation processes, and the exogenous oxalic acid and malic acid may participate in the Cd uptake and accumulation process of Euphorbia thymifolia L.. |
PDF Full Text Request