Influence of culture density,pH, organic acids and divalent cations on the removal of nutrients and metals by immobilized Anabaena doliolum and Chlorella vulgaris

The potential of alginate-immobilized Anabaena doliolum and Chlorella vulgaris was assessed for removal of nutrients (NO _inf3 ^sup- and NH _inf4 ^sup+ ) and metals (Cr₂O _inf7 ^sup2- and Ni²⁺) at different biomass concentrations (0.05, 0.1, 0.25, 0.49 and 1.22 g dry wt l^-1) and pH values (4 to 10). Though uptake of all these substances was higher in concentrated algal beads (0.25, 0.49 and 1.22 g dry wt l^-1), their rate of uptake was significantly (P<0.001) lower than that of low (0.05 g dry wt l^-1) cell density beads. For A. doliolum, there was no significant difference in uptake rates for beads having densities of 0.05 and 0.1 g dry wt l^-1. Chlorella vulgaris, however, showed maximum efficiency at 0.1 g dry wt l^-1. Uptake of both the nutrients and the metals was maximal at pH 7 followed by pH 8, 6, 9, 10, 5 and 4. Of the different substances (organic acids and divalent cations) used, humic acid was most efficient in decreasing metal uptake. Mg²⁺ was, however, more efficient than Ca²⁺ in decreasing Ni²⁺ uptake. Immobilized algae with a cell density of 0.1 g dry wt l^-1 were the most efficient for nutrient and metal removal at pH 6 to 8.