A novel study of hexavalent chromium detoxification by selected seaweed species using SEM-EDX and XPS analysis

Abstract

Abstract The toxicity and mobility of Cr(VI) means that it is crucial that the amount of the heavy metal discharged in industrial waste streams is significantly decreased. Seaweed biomass represents a benign, cost-effective, sustainable and efficient solution for the detoxification of anionic Cr(VI) to the less hazardous cationic Cr(III). Chromium binding and reduction behaviour of three seaweeds, Fucus vesiculosus (brown), Ulva spp. (green) and Palmaria palmata (red), was investigated using a colorimetric method while the surface characteristics of the biomasses were examined using SEM/EDX and XPS techniques. This is the first study of red seaweeds for the detoxification of Cr(VI). Results indicated that F. vesiculosus and P. palmata had comparable total Cr removal efficiencies of approximately 18%, while Ulva spp. removed 14% Cr from a 2000 mg L −1 metal solution. Conversion of Cr(VI) to Cr(III) in solution ranged from 51% for F. vesiculosus to 34% for Ulva spp. over a 6-h period. SEM revealed considerable morphological differences between Cr(III) and Cr(VI)-loaded seaweeds while EDX results confirmed an ion-exchange mechanism for Cr(III) binding. XPS results indicated that significant quantities of Cr(VI) solution were reduced when placed in contact with the seaweed surface over a 6-h period, resulting in a 64–75% conversion to Cr(III) bound to the seaweed. Thus, the potential of these seaweeds to bio-reduce and detoxify elevated Cr(VI) concentrations over relatively short time periods has been demonstrated. Cr(VI) binding also altered the relative quantities of carboxyl and alcohol groups in biomass polysaccharides, thus indicating the importance of these functionalities in binding and reduction of Cr(VI) to Cr(III). This work, coupled with existing capacity data points towards the viability of these environmentally friendly biosorbents for use in packed columns in a number of industries including electroplating and tanning.