Ricinus communis L. is a hyperaccumulation plant newly discovered in an abandoned land of Cu mine in China. A hydroponic experiment was then carried out to determine the root exudates in the Cu-tolerant castor (Ricinus communis L.). Plants were grown in nutrient solution with increasing level of Cu doses (0, 100, 250, 500, and 750 μmol/L Cu) in the form of CuSO4. Cu accumulation in the roots and shoots of castor, and root exudates collected from the castor were measured. The results indicated that the castor had a high Cu accumulation capacity and the Cu concentrations in the shoots and roots of the castor treated with 750 μmol/L Cu were 177.1, 14586.7 mg/kg, respectively. Tartaric was the largest in the root exudates in terms of concentrations, which reached up to 329.13 μmol/g (dry plant) in the level of 750 μmol/L Cu. There was a significantly positive linear relationship between the Cu concentration in root and the concentration of succinic (R = 0.92, P < 0.05), tartaric (R = 0.96, P < 0.01), and citric (R = 0.89, P < 0.05). These results indicated that the difference in root exudation from castor could affect their Cu tolerance. What is more, significant is that the high tartaric and citric, the low oxalic and cysteine in the root exudation of castor contributed to toleration of high Cu concentrations.

Application of fertilizers to supply appropriate nutrients has become an essential agricultural strategy for enhancing the efficiency of phytoremediation in heavy metal contaminated soils. The present study was conducted to investigate the beneficial effects of three types of phosphate fertilizers (i.e., oxalic acid-activated phosphate rock (APR), Ca(HPO), and NaHPO) in the range of 0-600 mg P kg soil, on castor bean growth, antioxidants [antioxidative enzymes and glutathione (GSH)], and Cu uptake. Results showed that with the addition of phosphorus fertilizers, the dry weight of castor bean and the Cu concentration in roots increased significantly, resulting in increased Cu extraction. The phosphorus concentration in both shoots and roots was increased as compared with the control, and the Ca(HPO) treatment had the greatest effect. Application of APR, NaHPO, and Ca(HPO) reduced the malondialdehyde (MDA) content, and the activity of the two antioxidant enzymes superoxide dismustase (SOD, EC 1.15.1.1) and catalase (CAT, EC 1.11.1.6) in the leaves of castor bean. GSH concentration in leaves increased with the increasing levels of phosphorus applied to soil as well as the accumulation of phosphorus in shoots, compared to the control. These results demonstrated that the addition of phosphorus fertilizers can enhance the resistance of castor bean to Cu and increase the Cu extraction efficiency of the plant from contaminated soils.

The efficiency of five different single leaching reagents (tartaric acid (TA), citric acid (CA), CaCl, FeCl, EDTA) and two different composite leaching reagents (CA + FeCl, CA + EDTA) on removing Cu, Pb, Zn, and Cd from contaminated paddy soil in Hunan Province (in China) was studied. The results indicated that the efficiencies of CA, FeCl, and EDTA on extracting Cu, Pb, Cd, and Zn from soil were greater than that of TA and CaCl, and their extraction efficiencies were EDTA ≥ FeCl > CA. The efficiencies of CA + FeCl on extracting Cu, Pb, Cd, and Zn were higher than that of single CA or FeCl. The 25 mmol L CA + 20 mmol L FeCl was a promising composite leaching reagent for paddy soil, and it could remove Cu (57.6 %), Pb (59.3 %), Cd (84.8 %), and Zn (28.0 %), respectively. With the same amount of leaching reagent, the efficiency of continuous leaching by several times was higher than that by once. In addition, the easily reducible and oxidizable fractions of heavy metals showed significant decrease during the process of leaching.

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A dual color fluorescence quantitative detection method for Hg 2+ in soil has been developed by synchronous fluorescence analysis based on graphene oxide (GO) and dye-labeled nucleic acids. In this strategy, two complementary dye-labeled single-stranded nucleic acids (ssDNAs) with thymine-thymine (T-T) mismatches and GO were employed. Two dyes 6-carboxyfluorescein (FAM) and 6-carboxy-xrhodamine (ROX) were labeled on 3 0 ends of two different ssDNAs, respectively. In the absence of Hg 2+ , dye-labeled ssDNAs were adsorbed on GO, then the fluorescence of dyes was quenched by GO and the fluorescence signals were very weak. But in the presence of Hg 2+ , two complementary ssDNAs with T-T mismatches formed double-stranded DNA (dsDNA) via the T-Hg 2+ -T coordination structure. The formation of dsDNA resulted in the release of dye-labeled ssDNA from GO, and the fluorescence of dyes was restored. The wavelength intervals between the maximum excitation and emission of FAM and ROX are very close, so the fluorescence signals of two dyes can be obtained simultaneously using synchronous fluorescence analysis. Thus, dual color fluorescence quantitative detection of Hg 2+ can be realized by simultaneously measuring the fluorescence signals of FAM and ROX, respectively. Under optimum conditions, the total fluorescence intensity of FAM and ROX exhibited a good linear dependence on the Hg 2+ concentration in the range from 8 Â 10 À10 to 8 Â 10 À8 mol L À1 . The fitted regression equation was obtained with a correlation coefficient (R 2 ) of 0.9952 and the detection limit (3s) of Hg 2+ was estimated to be 5 Â 10 À10 mol L À1 . This proposed method can use the total fluorescence intensity of FAM and ROX to realize the quantitative detection of Hg 2+ , so the detection sensitivity of the analytical method can be greatly improved.

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