A study on the Phytoremediation Potential of Azolla pinnata under laboratory conditions

Abstract

Heavy metal contamination in aquatic environments has become one of the major environmental problems all over the world. Phytoremediation is a plant based technology that utilises special plants known as hyperaccumulators to purify heavy metal contaminated sites. Hyperaccumulators are capable of absorbing heavy metals in greater concentrations. Azolla pinnata is an aquatic macrophyte that has been earmarked for its hyperaccumulation ability. This study was conducted under laboratory conditions to assess the ability of A. pinnata for the removal of Cr, Ni, Cd and Pb through rhizofiltration. Under three main experiments, phytoremediation ability of this species was investigated. In the first experiment, A. pinnata was exposed to prepared solutions of Cr, Ni and Pb of 2 ppm, 4 ppm, 6 ppm, 8 ppm and 10 ppm and of Cd solutions of 0.5 ppm, 1.0 ppm, 1.5 ppm, 2.0 ppm, 2.5 ppm and 3.0 ppm respectively. Experiments were carried out separately for Cr, Ni, Cd and Pb concentrations for 7 days. Presence of Cr, Ni, Cd and Pb caused a maximum inhibition of A. pinnata growth by 47%, 54%, 52% and 45% respectively while the highest removal percentages of Cr-98%, Ni-57%, Cd-88% and Pb-86% were recorded in 2 ppm, 2 ppm, 0.5 ppm and 8 ppm treatments respectively. The highest Bio-concentration Factor (BCF) for Cr was 1,376.67 when treated with 6 ppm, 684.95 at 4 ppm for Ni, 1,120.06 at 0.5 ppm for Cd and 1,332.53 at 8 ppm for Pb respectively. At the end of the experiments toxic symptoms were observed in plants exposed to Cd and Ni. The findings of this experiment revealed that A. pinnata is an excellent candidate for the removal of Pb and Cr even at higher concentrations and for Cd at lower concentrations while it is only partially efficient for Ni removal. The ability of A. pinnata to remove Cr, Ni, Cd and Pb from open dump site leachate was investigated in the experiment three. A. pinnata was exposed to a leachate dilution series of 5%, 15%, 25%, 50%, 75% and 100%. For all four metals, the highest removal percentages as well as the highest BCFs were given by the plants exposed to 5% leachate concentration. Influence of interactive effects of Cr, Cd, Ni and Pb on their removal capacities of A. pinnata and the metal selectivities were determined by the experiment two. The metal selectivity of A. pinnata was Pb> Cd> Cr> Ni. A. pinnata can be designated as a good phytoremediation tool for the mitigation of heavy metal pollution due to its high BCFs (over 1,000) in relation to Pb (2-10 ppm), Cr (2-10 ppm) and Cd (0.5-1 ppm).