In this study, a comparative evaluation of floating and submerged macrophytes was performed. Azolla filiculoides (free floating) and Hydrilla verticillata (submerged) aquatic macrophytes were utilized for arsenic, copper, chromium and lead removal from the respective metallic ion solutions. Batch experiments were performed initially with optimization of different physical parameters viz., pH, initial heavy metal concentration, biosorbent dosage, contact time, temperature and agitation speed. Submerged (Hydrilla verticillata) had depicted better removal efficiency in comparison to the floating macrophyte (Azolla filiculoides). Field emission scanning electron microscopy equipped with energy dispersive spectroscopy and Fourier transform infrared spectroscopy analysis was performed for the characterization of the metal loaded biosorbents. Biosorption of the respective heavy metal was clearly depicted in the FESEM-EDX spectrum, although not much change in the morphology of the biosorbents were examined. FTIR spectra of the biosorbents obtained after the experiments confirmed the involvement of C-H bend, -CH 2 -(C=O), N-H, -C-O, R 2 -C= bending and -C-C=O on the biomass. Furthermore, the biosorbent regeneration followed by heavy metal biosorption confirmed the reusability of the prepared biosorbent for at least two consecutive cycles without much significant change in the heavy metal biosorption capacity.

This study investigated the biosorption of Pb (II) and Cd (II) on powdered seeds of Adenanthera pavonina from single and binary heavy metal solutions. The adsorption capacity of the metal ions was studied as a function of pH, adsorbent dose, initial metal ion concentration, and contact time. In the single heavy metal system, maximum removal of Pb (II) and Cd (II) were achieved at pH 4 and 6, respectively, while the equilibrium was attained at a contact duration of 35 min at a constant dose of 5 g L −1 was maintained for both Pb (II) and Cd (II). Further, the kinetic study revealed that the biosorption process was best described by the second-order kinetic model. In the competitive biosorption of binary mixtures of Pb (II) and Cd (II) were studied in terms of percentage removal of metal ions, adsorption capacity and equilibrium sorption isotherm. The percentage removal of metal ions in binary solutions was suppressed in presence of the other competitive metal ion. Field emission scanning electron microscopy integrated with energy dispersive X-ray spectroscopy of the biomass before (control) and after (metal loaded biomass) the biosoption process, confirmed the presence of respective heavy metal on to the biomass. Fourier transmission infrared spectroscopy spectra supported the activity of hydroxyl, amines, amides, and carboxyl groups in uptake of both heavy metal ions.

Citric acid is one of the important commercially produced organic acid. The aim of the present work was to study the utilization of pre-treated sugar cane bagasse for citric acid production using Aspergillus niger under solid state fermentation. The maximum value of citric acid was observed in acid treated substrate followed by urea and heat respectively. Fermentation conditions were also optimized and maximum production of citric acid occurred when the pH, Initial moisture content and temperature of the fermentation medium were 5, 65 per cent and 30 0 C, respectively. As a globally required organic acid for various industrial applications, citric acid can be produced at large scale by utilizing pre-treated agro residues such as sugarcane bagasse. Application of agro residues in the production of value added product can be a positive step towards agricultural waste management.

This study comprises the comparative evaluation of floating (Azolla filiculoides) and submerged (Hydrilla verticillata) macrophytes for potential biosorption of Cu(II), Cr(VI), As(III) and Pb(II) from aqueous solution in a multi-component study. Statistically valid Plackett-Burman design of experiments was employed with four heavy metals at two different levels by varying their initial concentration in the range 10-50 mg L −1 for both the macrophytes. The maximum removal efficiency for Pb(II) was obtained for both the biosorbents, i.e., Azolla sp. (81.4%) and Hydrilla sp. (84.3%) within 4 h of the experimental runs, with an initial concentration of 10 mg L −1 of all the heavy metals. Followed by Pb(II) removal, a declining trend for the removal (%) for Cu(II), As(III) and Cr(VI) was obtained, for all the experimental runs. Also, the minimum removal (%) for all the experimental runs was attained at 25 mg L −1 (maximum) for all the heavy-metal concentration level. The removal efficiency (%) trends follow the order: Pb(II) > Cu(II) > As(III) > Cr(VI) for both the biosorbents. Analysis of variance and Student's t test of the metal bioremoval revealed that main (individual) effect due to the metals was highly significant (P value < 0.05) on each other's removal. Student's t test results revealed that both Pb(II) and Cu(II) strongly inhibited both Cu(II) removal (P value < 0.01), while Cr(VI) has only inhibitory effect on Pb(II) removal. Henceforth, all these results simultaneously depicted good potential of the aquatic macrophytes for the biosorption capability of heavy metal and the effect of individual metals on each other's removal in the multi-component system. Keywords Heavy metals • Multi-component system • Aquatic macrophytes • Plackett-Burman design of experiments • Azolla filiculoides • Hydrilla verticillata