Optimization of chromium removal by water Fern Azolla

Abstract

Although a large number of plants is known to accumulate heavy metals, but use of small aquatic plants like Lemna, Azolla, Hydrilla, etc. is more feasible for aquatic ecosystems. High multiplication rate, ability to grow in varied environments from normal to polluted waters, high pectin content, moisture content and worldwide distribution of the N2-fixing free floating aquatic fern-Azolla was highly tempting for its selection in the present investigation which has been carried out to test the potential of three different species of Azolla, viz. A. microphylla, A. filiculoides and A. pinnata with a view to select a species having maximum tolerance of chromium (VI) and having maximum growth and chromium removal efficiency. Further optimization of various process parameters has been made so that a suitable application of selected Azolla species for phytoremediation of chromium from polluted aquatic bodies with chromium rich effluents can be carried out on a large-scale. Minimum Inhibitory Concentration (MIC) of Cr (VI) to Azolla spp. was determined as 7.5 ppm by growing them in E and W medium supplemented with 1 to 15 ppm chromium concentrations. A. microphylla excelled in various growth parameters in controls as well as in presence of chromium, and in chromium removal efficiency and was thus selected for further studies. Of the three culture media studied, viz. IRRI medium, Warne medium and E and W medium, A. microphylla excelled in growth performance and chromium removal efficiency in Warne medium supplemented with 5 and 7.5 ppm chromium concentration. Also, it showed better growth performance and chromium removal efficiency in optimized Warne medium (OWM) with combination P2Mg2Ca2Nasc (16.5 ppm 42 ppm + 18 ppm + 4.5 ppm) as compared in standard Warne medium (SWM). Maximum chromium removal was observed in 1L of OWM at pH 6 with 4 g inoculums size of A.microphylla after 6 hour incubation period. This fern showed better growth performance and chromium removal efficiency in effluents diluted with OWM..