An Investigation into the Mechanisms of Heavy Metal Binding by Selected Seaweed Species

The Irish Marine Institute has identified the potential of marine natural resources to be exploited as high value-added products. Marine biotechnology is at an early stage of development, and therefore, more of the potential global market is open for development by Ireland than is the case for other sectors. A key area of growth, both in Ireland and Europe is the use of seaweeds for various applications including bioremediation of heavy metal contaminated waters.

This thesis has demonstrated a method for identifying the most promising seaweeds for metal biosorption through the use of multiple analytical techniques. A comprehensive study of dead biomass of six locally derived seaweeds (Fucus vesiculosus, Fucus spiralis, Ulva lactuca, Ulva spp., Palmaria palmata and Polysiphonia lanosa) and three regionally significant metals (Cu (II), Cr (III) and Cr (VI)) was carried out. Fundamental investigations into metal binding were undertaken in order to determine the potential binding capacity of the seaweeds, the factors influencing binding and their potential mechanisms of binding. This work has adapted a number of analytical techniques previously used for seaweed analysis and modified them so that binding information supplementary to that found in the literature could be obtained. Studies indicated that seaweed is polyfunctional in nature with groups of varying affinities for metal ions. The quantity and distribution of these groups varied between species. Variations in experimental parameters were shown to influence the quantity of metal bound to the seaweeds, with optimum conditions dependent on the metal under investigation. Isotherm modelling revealed that Fucus vesiculosus and Polysiphonia lanosa were most effective in removing cations and anions respectively from solutions containing high residual metal concentrations while Palmaria palmata was superior for both cation and anion removal at low residual concentrations. Therefore, this implied that the most suitable seaweed biosorbent was ultimately dependent upon its final application. Changes in seaweed functional groups after metal binding were monitored using FTIR analysis with novel information on the timescale of Cu (II) binding presented.

Ion-exchange and complexation mechanisms were shown to occur for cation binding while a surface reduction mechanism was also apparent during anion binding. The use of multiple chemical modification techniques confirmed binding mechanisms and identified a methodology for capacity enhancement of the seaweeds. Important changes in surface morphology and binding mechanism were established using surface analysis techniques such as SEM/EDX and XPS while a novel methodology for seaweed surface analysis using SFM was also demonstrated.