Biosorption of cadmium by brown, green, and red seaweeds

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Abstract

Seven species of brown, green, and red seaweeds were examined for their abilities to sequester cadmium ions from aqueous solution. Although all the seaweed types investigated were capable of binding appreciable amounts of cadmium, considerable variability in their biosorption performance was observed. Maximum cadmium uptake capacities at pH 5 ranged from the highest value of 0.74 mmol/g for the brown seaweed Sargassum baccularia to the lowest value of 0.16 mmol/g for the red seaweed Gracilaria salicornia, representing a 363% difference. In general, brown seaweeds were found to exhibit the best overall cadmium ion removal. Additional experiments were conducted to evaluate the biosorption characteristics of the brown seaweed S. baccularia. The equilibrium uptakes of cadmium were similar within the pH 3–5 range but decreased significantly when the solution pH was reduced to pH 2. The presence of background cations such as sodium, potassium, and magnesium and anions such as chloride, nitrate, sulphate, and acetate up to a concentration of 3.24 mmol/l was found to have no significant effect on the equilibrium uptake of cadmium. However, the biosorbent uptake of cadmium was markedly inhibited in the presence of calcium ions at 3.24 mmol/l. Kinetic studies revealed that cadmium uptake was fast with 90% or more of the uptake occurring within 30–40 min of contact time.

Introduction

The presence of heavy metals in the environment is of major concern because of their toxicity and threat to plant and animal life. Moreover, recovery of heavy metals from industrial waste streams is becoming increasingly important as society realises the necessity for recycling and conservation of essential metals. Anthropogenic sources of heavy metals include process waste streams from metal plating, mining operations, and semi-conductor manufacturing operations. Increasingly strict discharge limits on heavy metals have accelerated the search for highly efficient yet economically attractive treatment methods for their removal. Biosorption is one such emerging technology that has attracted increased attention in recent years. Biosorption exploits the ability of microbial and plant biomass to sequester heavy metal ions from aqueous solution by physicochemical mechanisms. Charged groups such as carboxylate and hydroxyl present in the biopolymers of biomass cell walls are believed to be responsible for the sequestration of metal ions. Different biomass types such as bacteria, fungi, and algae have been screened and studied extensively in the last decade with the aim of identifying highly efficient metal removal biosorbents.

Recent investigations by various groups have shown that selected species of seaweeds possess impressive sorption capacities for a range of heavy metal ions. Seaweeds are a widely available source of biomass as over two million tonnes are either harvested from the oceans or cultured annually for food or phycocolloid production, especially in the Asia-Pacific region [1]. Seaweeds are found throughout the world’s oceans and come in three basic colours: brown (Phaeophyta), red (Rhodophyta), and green (Chlorophyta). Brown and red seaweeds are almost exclusively marine, but the vast majority of green-coloured seaweeds are freshwater and terrestrial. The brown colour of the Phaeophyta results from the dominance of the xanthophyll pigment fucoxanthin which masks the other pigments while the red colour of the Rhodophyta is due to the presence of the pigment phycoerythrin which reflects red light and absorbs blue light. The green colour of the Chlorophyta comes from chlorophyll a and b. Many of the studies to date on metal biosorption by seaweeds have largely been restricted to various species of brown seaweeds (see Table 1). On the other hand, green and red seaweed species have not been evaluated to any great extent. Lee et al. [2] screened 48 species of brown, green, and red seaweeds for their uptake capacities of hexavalent chromium while Jalali et al. [3] reported the biosorption of lead by eight species of brown, green, and red seaweeds. This study investigated the biosorption properties of seven different species of brown, red, and green seaweeds harvested from tropical coastal areas using cadmium as a model metal ion. Potential variability in the biosorption behaviour of the seaweed species was evaluated on the basis of their equilibrium cadmium uptake. Based on the results of this initial screening, the best-performing species was selected for further studies.

Section snippets

Biomass preparation and chemicals

The seven seaweed species collected from the west coast of Peninsular Malaysia are listed in Table 2. Fresh samples of the seaweeds were washed thoroughly with distilled water, dried in an oven to constant weight, and ground to a size range of 500–710 μm. The resulting biosorbent particles were then stored in sealed containers in a desiccator. All chemicals obtained from Fluka (Switzerland) including the cadmium nitrate salt were of analytical grade.

Biosorption equilibrium

Batch equilibrium experiments were carried out

Biosorption equilibrium

Fig. 1 shows the experimental cadmium uptake isotherms (symbols) for the seven seaweed species at pH 5 and 25 °C (see below for pH effect). All of the equilibrium isotherms exhibited favourable isotherm behaviour, with a maximum capacity that depended on the biomass type. The three brown seaweeds exhibited the largest adsorption capacity, followed by the green seaweed and then the three red seaweeds. Jalali et al. [3] also reported that brown seaweeds outperformed green and red seaweeds in the

Conclusions

Comparison of the biosorption behaviour of seven different species of brown, green, and red seaweeds indicated that the brown seaweeds, especially the Sargassum species, showed excellent cadmium sequestering capabilities. The following ascending order of their Langmuir maximum adsorption capacity (qm) at pH 5 was observed: red < green < brown. However, it was demonstrated that the performance of a biosorbent is dependent upon the two Langmuir equilibrium parameters qm and b. A biosorbent with a

Acknowledgements

Experimental work of V.B. Samuel is gratefully acknowledged.

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