Phytoremediation Of Heavy Metals By Poplar From Agricultural Soils In Arid Regions

Populus alba L. var. pyramidalis Bunge, Nitraria praevisa Bobr., and Peganum harmala L. are pretty widespread native plants grown on calcareous soils in arid regions of northwest China. In order to examine their uptake and accumulating characteristics for heavy metals such as cadmium (Cd), copper (Cu), lead (Pb), and zinc (Zn), a field investigation experiment was performed on a calcareous agricultural land contaminated by heavy metals in the vicinity of smelters in Baiyin of Gansu Province. On the basis of the field investigation experiment, a pot culture experiment and a field sampling experiment were conducted to examine the Cd phytoextraction potential of P. pyramidalis on single and combined metals-contaminated calcareous soils. In particular, chelating agents were used to enhance the uptake of Cd by P. pyramidalis, the results as below:(1) Significant plant differences in levels of accumulation were found for the four metals. More specifically, P. pyramidalis has an elevated accumulating capability for Cd, while elevated Cu and Pb accumulating capabilities were noted on P. harmala, the Zn accumulating capability, however, was not significant among the three plants. In addition, the sort of metals rather than plant species determined the distribution pattern of metals in plant organs. Cadmium was accumulated mainly in aboveground parts of P. pyramidalis in comparison with the accumulation of Cu, Pb, and Zn in the root.(2) The shoot biomass production of P. pyramidalis grown on single metal contaminated calcareous soils was not significantly reduced with soil Cd concentrations up to or slightly over50mg kg-1. The bioconcentration factor decreased with increasing soil Cd concentration and was greater than1with soil Cd concentrations equal to or less than10mg kg-1. P. pyramidalis showed strong ability to translocate Cd from roots to shoots with the translocation factors for all treatments greater than1. The Cd concentrations in P. pyramidalis organs decreased in the order of leaves> stems> roots, with the highest concentration (35mg kg-1) noted in leaves with100mg kg-1added Cd. Based on the tolerance capability, enrichment capability, translocation capability, and accumulation capability, the results demonstrated that P. pyramidalis is not regarded as a Cd hyperaccumulator. The results suggest that P. pyramidalis is more suitable for remediating slightly Cd contaminated calcareous soils.(3) The root biomass of P. pyramidalis grown on combined metals contaminated calcareous soil was significantly reduced with soil Cd concentrations up to or more than11mg kg"1(P<0.05), showing that the tolerance capability of P. pyramidalis to Cd was higher on single metal contaminated soil compared with the combined metals contaminated soil. The regression models of the Cd concentration between the poplar root and soil changed from linear model on single metal contaminated soil to exponential model on combined metals contaminated soil. Cadmium and Zn accumulated largely in poplar leaves, while Cu and Pb were retained mainly in poplar roots. Although Cd uptake increased on the combined metals contaminated soil, the Cd phytoextraction potential was limited as a result of the reduced shoot biomass production. The results imply that soil additives are needed to enhance the Cd phytoextraction potential.(4) The stem bark of P. pyramidalis always accumulated much higher concentrations of Cd than the stem wood, regardless of the stand age in the field. The concentrations of Cd and Zn in poplar parts decreased in the order of leaves, stem bark> branches> roots> stem wood, while the concentrations of Cu and Pb decreased in the order of roots, stem bark> branches> stem wood> leaves. In general, metal concentrations in poplar parts decreased with an increase in stand age. However, the highest annual amount of Cd taken up by shoots occurred in the5yr old stand rather than the3yr and7yr old stands. The field experiment results suggest that five years may be the optimal rotation period for P. pyramidalis.(5) Applications of chelating agents decreased the stem biomass of P. pyramidalis, but to some extent, stimulated the foliar biomass production. Synthetic chelants ethylenediamine tetraacetic acid (EDTA) and ethylene glycol-bis (2-aminoethylether)-N,N,N’,N’-tetraacetic acid (EGTA) were more efficient both in mobilizing Cd in the soil and in enhancing Cd translocation from poplar roots to shoots than citric acid. The highest Cd concentration (101.88mg kg-1) was noted in leaves of P. pyramidalis after the addition of EGTA at the highest dose of9mmol kg"1, this value was3.54times higher than the control. The foliar Cd amounts on soils treated with chelants increased by more than28%in comparison with the control, thus collection of the litter fall is necessary considering the increased leaf Cd amount as a result of the application of chelating agents. Considering the impacts of applications of chelants on plant biomass, soil Cd mobilization, and Cd translocation and accumulation in poplar organs, additions of the lowest dose (1mmol kg-1) of EDTA and EGTA could assist P. pyramidalis to reach its maximum remediation factor for Cd and avoid the secondary pollution induced by applications of chelants at high doses.In general, P. pyramidalis showed high tolerance, uptake, and accumulating potential to Cd on calcareous soils in arid regions. In addition, synthetic chelants are more efficient in mobilizing Cd in the soil and stimulating Cd translocation from poplar roots to shoots. Therefore, P. pyramidalis is most suitable for remediation of slightly Cd contaminated calcareous soils.