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

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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.

Introduction

The presence of heavy metals in industrial wastewater represents a serious environmental problem. The most important sources of chromium in the environment are from electroplating, tanning, water-cooling, pulp production as well as ore and petroleum refining processes [1]. While the effluents from these processes contain both Cr(VI) and Cr(III) ranging from tens to hundreds of mg L−1, Cr(VI) is more hazardous to public health due to its greater mobility and mutagenic and carcinogenic properties. Thus the discharge of Cr(VI) into surface water is regulated to below 0.05 mg L−1 while total Cr, including Cr(III), Cr(VI), and its other forms, is regulated to below 2 mg L−1 [2]. Because of its toxicity, it is therefore increasingly important to identify methods for detoxification of Cr(VI) thus facilitating environmental cleanup.

Conventional chromium removal processes, including ion-exchange, activated carbon adsorption, reverse osmosis, and membrane filtration can be expensive or ineffective at low concentrations and may also lead to secondary environmental problems from waste disposal [3]. Thus, much research has been focussed on identifying biological materials that can efficiently remove heavy metals from aqueous environments. These materials are known as biosorbents and the passive binding of metals by living or dead biomass is referred to as biosorption [4].

Seaweeds are extremely efficient biosorbents with the ability to bind various metals from aqueous effluents [5], [6]. Numerous chemical groups including carboxyl, sulphonate, hydroxyl and amino [7] may be responsible for metal biosorption by seaweeds. These functionalities act as binding sites for metals with their relative importance depending on factors such as the quantity of sites, their accessibility and the affinity between site and metal. The main metal binding mechanisms include physical adsorption, ion-exchange and complex formation [5] but these may differ according to biomass type, origin and the processing to which it has been subjected.

Biosorption processes are now becoming an extremely viable option due to the ready availability of non-living biomasses, lower operating costs and high efficiency metal removal. The current worldwide market for ion-exchange resins for heavy metal applications is in the order of 5 billion US dollars [8]. Biosorbents can be marketed for a fraction of ion-exchange costs thus making them extremely competitive products capable of opening whole new markets unavailable to high priced conventional technologies.

The present study investigates the potential of Fucus vesiculosus (brown), Palmaria palmata (red) and Ulva spp. (green) for Cr(VI) removal and detoxification in an industrial context. These seaweeds have not previously been studied for their Cr(VI) detoxification capacity thus illustrating the novelty of approach in this work. This study aims to quantify the removal and reduction of Cr(VI) from concentrated metal solutions as well as evaluating the characteristics of the biomaterial during chromium biosorption. Characterisation is achieved using Scanning Electron Microscopy/Energy dispersive X-ray analysis (SEM/EDX) and X-ray photoelectron spectroscopy (XPS) which identifies key functionalities responsible for metal binding and allows the elucidation of chromium binding mechanisms to these seaweeds.

Section snippets

Preparation of the biomass

The brown seaweed F. vesiculosus, the green seaweed Ulva spp. and the red seaweed P. palmata were harvested from Fethard-on-Sea, Co. Wexford, Ireland (52°11′53.68″N, 64°9′34.36″W). The samples were rinsed thoroughly with distilled water in order to remove any adhering debris, cut into pieces between 3 and 5 mm, then subsequently oven-dried at 60 °C for 24 h. Samples were stored in air-tight containers until required.

Cr(VI) biosorption experiments

Cr(III) and Cr(VI) solutions (2000 mg L−1) were freshly prepared using analytical

Reduction of Cr(VI) to Cr(III)

Seaweed functionalities such as sulphonate (–OSO3) and carboxyl (–COOH) display acidic characteristics and therefore, the pH at which maximum metal uptake occurs is related to the pKa of these groups. The point of zero charge (PZC) of these seaweeds was previously determined by Murphy et al. [11] and on average, a value of approximately of 6.18 was observed. Thus, below the PZC the biomass still has a net positive charge despite the presence of dissociated negatively charged functionalities. As

Conclusions

This study revealed that these three seaweeds effectively removed between 14 and 18% of the total chromium from a concentrated Cr(VI) solution indicating their suitability as biosorbents for detoxification of Cr(VI) over a short time period. Cr(III) was bound to these seaweeds by an ion-exchange mechanism thus altering surface morphology by altering the polymer structures of the seaweed surface. The mechanism of Cr(VI) binding was adsorption coupled with reduction to Cr(III). Conversion of

Acknowledgements

The authors gratefully acknowledge the support of: The Irish Research Council for Science, Engineering and Technology under the Embark Initiative. Technology Sector Research Strand III – Estuarine Research Group (2003). European Regional Development Fund (ERDF) INTERREG IIIA Ireland/Wales Programme 2000–2006, under the SWINGS (Separations, Wales & Ireland – Novel Generation Science) project. Balazs Azalos (Materials & Surface Science Institute, University of Limerick). The Tyndall National

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