Phytoremediation of single and combined pollution of Cu and Pb by Medicago sativa, Lolium perenne, and Pennisetum alopecuroides

With rapid socio-economic development, the problem of heavy metal contamination of soils is increasingly threatening food security and human health. So far, phytoremediation has been the main mode of treatment of soils for heavy metal pollution. To develop pastures suitable for remediation of complex Cu and Pb contaminated soils, a pot experiment consisting of 9 treatments was set up. The control (CK) was not treated with heavy metal salts. Four other treatments had low and high pollutions of single metals including Cu (Cu₁:200 mg·kg^-1 Cu²⁺; Cu₂:400 mg·kg^-1 Cu²⁺), Pb (Pb₁:300 mg·kg^-1 Pb²⁺; Pb₂:800 mg·kg^-1 Pb²⁺). The other four treatments consisted of both Cu and Pb pollutions of Cu₁Pb₁, Cu₁Pb₂, Cu₂Pb₁ and Cu₂Pb₂. The effects of pastures (forages) plantation on the Cu and Pb contaminated soil remediation were determined by comparing the adaptability and enrichment characteristics of alfalfa (Medicago sativa), ryegrass (Lolium perenne) and pennisetum (Pennisetum alopecuroides). The results showed that:1) the aboveground and belowground biomasses of M. sativa were largest under Pb₁ treatment, which were significantly larger than those for the other treatments. The aboveground biomass of L. perenne was largest under Cu₁Pb₁ treatment and belowground biomass of L. perenne largest under Pb₁ treatment. The aboveground biomass of P. alopecuroides was largest under Cu₂Pb₂ treatment and belowground biomass of P. alopecuroides largest under Cu₂ treatment. 2) When the soil was treated solely with Cu, the resistance coefficient of P. alopecuroides was largest among the three pasture species. Then when the soil was treated solely with Pb, the resistance coefficient of M. sativa was largest. For Cu and Pb combined treatment, the resistance coefficient of P. alopecuroides was largest. The order of aboveground biomass, belowground biomass and resistance coefficient of the three pasture plants under high Cu concentration treatments was P. alopecuroides > L. perenne > M. sativa. Then the aboveground biomass, belowground biomass and resistance coefficient of P. alopecuroides were significantly higher than those of L. perenne and M. sativa. 3) Soil contents of Cu and Pb decreased after planting the forage plants. At a certain concentration, soil Cu-Pb promoted the absorption of each other by the pasture plants. 4) The enrichment coefficient of Cu for aboveground biomass of M. sativa was highest under Cu₂Pb₂ treatment, which was 1.61. Then the enrichment coefficient of Cu for belowground biomass of L. perenne was highest under Cu₂Pb₂ treatment, which was 3.80. Only the enrichment coefficient of Pb for aboveground and belowground biomass of L. perenne exceeded 1.0, reaching 1.46. 5) The absorption ability of Pb by L. perenne was stronger and accumulated mainly in belowground biomass. M. sativa had the best comprehensive repair effect of complex Cu-Pb pollution. The transport coefficients of Pb in M. sativa and L. perenne were higher than 1.0 in complex Cu-Pb and single Pb polluted soils, respectively, and were 2.72 and 2.06, which reflected their respective potentials for enrichment of Pb in the soil. Thus L. perenne had a stronger tolerance to Pb pollution and was therefore a better remedy for Pb-polluted soils. M. sativa had a stronger tolerance to Cu and Pb and was therefore a better remedy for soils polluted with single Cu or composite Cu-Pb.