The phytoaccumulation ability of duckweed Spirodela polyrhiza on manganese (Mn) and chromium (Cr) was assessed by exposing the plant to various concentrations of single or dual metals (5-70 mg L-1 Mn, 2-12 mg L-1 Cr(VI)) under laboratory conditions. The results showed that S. polyrhiza can tolerate Mn at high concentrations of up to 70 mg L-1, and its growth rate was barely affected by Mn. The effects of Cr on S. polyrhiza growth were dose-dependent, and the growth was completely inhibited in the presence of 12 mg L-1 Cr. Analysis of metal content in the plant biomass revealed a high accumulation of Mn (up to 15.75 mg per g of duckweed dry weight). The Cr bioaccumulation (from below detection limit to 2.85 mg Cr (11.84 mg Cr2O72-) per g of duckweed dry weight) increased with cultivation time and metal concentration in the medium. Further study with the concurrence of Mn and Cr showed increased toxicity to plant growth and photosynthesis. The metal accumulations in the dual metal treatments were also significantly decreased as compared to the single metal treatments. Nevertheless, the phytoaccumulation of these two metals in S. polyrhiza in the dual metal treatments were still comparable to or higher than in previous reports. Thus, it was concluded that duckweed S. polyrhiza has the potential to be used as a phytoremediator in aquatic environments for Mn and Cr removal.
Keywords: Chromium; Duckweed; Heavy metal; Manganese; Phytoaccumulation; Spirodela polyrhiza.