| It is important to find suitable plants with metal-accumulative characteristics for the construction of constructed wetlands. However, most of the reported hyperaccumulators up to now are terrestrial plants, which are difficult to survive in water. Therefore, it is a key step to find aquatic plants for the phytoremediation of metal-polluted water bodies. Accuumulation of As, Cu, Zn, Au, B, Cd, Ni, etc. by duckweed (Lemna minor L.) has been well documented and this plant has also been widely used in sewage treatment because of its fast growth, low nutrition requirement and easy harvest. In addition, duckweed can recyle resources in the process of sewage treatment.Because of few harmful by-products in the treatment process, duckweed is an ideal plant and has a great potential for the "green"remediation of polluted water.In present study, firstly, the accumulation of six heavy metals (As, Cu, Zn, Cd, Pb and Hg) by duckweed was investigated under single-metal culture conditions. Secondly, accumulation of Cu, Zn, Cd, As and their effects on physiological and biochemical changes (photosynthetic pigment content, soluble sugar content, free proline content, MDA content, antioxidant enzyme activity) of duckweed were examined under metal-combined conditions by orthogonal design.The changes of these indexes mentioned above were determined with culture time extention (1d,2d,4d,6d,8d and 10d). Thirdly, the factors influencing metal accumulation of duckweed such as pH adjustment, nitrogen (nitrate and ammonium), phosphorus (orthophosphate, metaphosphate, coke phosphate) and iron (Fe2+ and Fe3+) were examined. The results indicated as follows:(1) The bioconcentration factors of duckweed for As, Zn, Pb, Cd, Cu and Hg were over 1, namely, six heavy metals were all enriched by the plant. The bioconcentration factors of different heavy metals and different concentrations of a metal were different. Accumulation of As, Zn, Pb, Cd, Cu and Hg in L.minor were similar. Compared with the control, the concentrations of heavy metals in duckweed significantly increased in high metal concentration treatments. (2) Under the combined pollution of Cu, Cd, Zn and As there is no interaction among these metals was observed. The concentrations of Cu in L.minor were significantly affected by concentrations of Cu in culture solution, but were not by different concentrations of Cd, Zn, As. The maximum concentration of Cu in plant was observed when ingredient 15 was applied [high concentration of Cu (4 mg·L-1), middle concentrations of Cd (0.01 mg·L-1), low concentrations of Zn (0.01 mg·L-1), high concentrations of As (1 mg·L-1)].The concentrations of Zn in L.minor were significantly affected by concentrations of Zn in culture solution, but were not by different the concentrations of Cu, Cd and As. The maximum concentration of Cu in plant was observed when ingredient 4,7,10 and 13 were applied.The maximum bioconcentration factors of plant for Zn, Cu and Cd were observed in ingredient 2 [control Cu (not added), low concentrations of Cd (0.001 mg·L-1), low concentrations of Zn (0.01 mg·L-1), low concentrations of As (0.05 mg·L-1)], and the minimum was ingredient 13 [(high concentration of Cu (4 mg·L-1), were Cd (not added), high concentrations of Zn (50 mg·L-1), low concentrations of As (0.05 mg·L-1)].The concentrtions of heavy metals in L.minor treated by different ingredients were diverse over time. Similar results were also obtained by the same ingredient treatment.(3) Contents of Chlorophyll a, b, total chlorophyll, carotenoids, free proline, soluble sugar, MDA and the CAT, POD, SOD activities in duckweed were not significantly affected by different Cu, Cd, Zn and As concentrations. The physiological and biochemical indexes of L.minor treated by different ingredients were diverse over time. Similar results were also obtained by the same ingredient treatment.(4) The accumulation of copper by duckweed were significantly promoted in acidic conditions (pH=5), not adding nitrogen, different concentrations of phosphate and suitable concentration of iron (1 mg·L-1).The accumulation of copper by duckweed decreased with increasing concentration of nitrogen. Acidic condition (pH=5), low concentration of nitrogen [ammonium nitrogen (25 mg·L-1) and nitrate (500 mg·L-1)],different concentrations of partial phosphate and different forms of iron salt (Fe2+ and Fe3+) significantly promoted zinc accumulation by duckweed. Different concentrations of phosphate and focal phosphate significantly reduced the uptake of zinc by duckweeds. Neutral conditions (pH=7), low concentration for ammonium nitrogen (25 mg·L-1), low concentration of phosphorous (50 mg·L-1) significantly increased the accumulaction of arsenic. High concentration of focal phosphate (150 mg·L-1) and Fe3+(1.5 mg·L-) significantly reduced the absorption of arsenic by duckweed. |
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