Biosorption of hexavalent chromium from aqueous solution by Sargassum muticum brown alga. Application of statistical design for process optimization
Highlights
► Biosorption of chromium(VI) by the brown seaweed Sargassum muticum has been investigated. ► Response surface methodology was used for determining conditions that maximized removal of Cr(VI). ► Kinetics study showed that the process was satisfactorily described by pseudo-second order model. ► Experimental equilibrium data have been analyzed using two-parameter isotherm models. ► The most appropriate equation for describing the isotherm profiles was the Langmuir model.
Introduction
Many traditional methods are used for the elimination of the environmental pollution. Specially, the contamination of water by toxic heavy metal ions is a worldwide environmental problem. Biosorption is an alternative method suitable for the treatment of wastewater. It is characterized by low cost and high removal of metals, especially at low concentration of metal in solution. Among different biosorbents, some species of brown marine macroalgae exhibit much higher uptake values than other types of biomass, higher than activated carbon and natural zeolite, and comparable to those of synthetic ion exchange resins [1]. This is the case of the different species of Sargassum genus, which have been studied by several authors [2], [3], [4], especially Volesky and co-workers [5], [6], [7]. In this work, interest has been focused on Sargassum muticum because it is typically found in tropical countries and abundant in South America, and it is easily collected on beaches without any environmental damage due to its occurrence/presence in the waters of Cuba and Europe. Furthermore, S. muticum has been considered an invasive species in European waters, making its obliteration very important.
Chromium as a toxic heavy metal often exists in the waste streams from various industries such as mining, metal cleaning, plating, dyeing and metal processing. According to the World Health Organization (WHO) drinking water guidelines, the maximum allowable limit for total chromium is 0.05 mg/L [8]. Trivalent chromium is the most prevalent form found in natural water. Almost all the contaminant hexavalent chromium comes from human activities. Hexavalent chromium is an acute carcinogen and more mobile and toxic than trivalent chromium. Hence, hexavalent chromium is more important than trivalent chromium in water pollution control [9].
Taking into consideration the necessity to reduce the environmental negative impacts caused by the residuals contaminated with chromium(VI) and their possible repercussion in the health of the population, it becomes necessary to look for feasible economic and environmental alternatives that allow keeping the levels of these polluting agent in the permissible range.
The traditional physico-chemical processes used to eliminate heavy metals from the liquid effluents, such as chemical precipitation, electrochemical oxidation reduction, treatments, are not usually viable from the economic and energy consumption point of view, especially when the concentrations of heavy metals are low. For that reason the feasibility of the use of biosorption process to remove heavy metals from industrial effluents is investigated.
The biosorption can be carried out by physical adsorption, ion exchange, complexation and precipitation mechanisms. In many processes of biosorption, several mechanisms can act in combination and is difficult to define it [10].
According to Fourest and Volesky, the major mechanisms responsible for the accumulation of heavy metals by biomass of bacteria, fungi and algae include ionic interactions and complex formation between metal cations and ligands contained in the structure of the biomaterials [11]. Its effectiveness to remove heavy metals depends on the biosorbent concentration, the pH of the solution, the reaction kinetics and the effluent composition. However it is necessary to investigate on the relationship between biosorption efficiency and the parameters affecting it.
The application of statistical experimental design techniques in biosorption process development can result in improved product yields, reduced process variability, closer confirmation of the output response to nominal and target requirements and reduced development time and overall costs [12], [13].
The main objective of this work was to develop the potential of the marine algae S. muticum to remove of hexavalent chromium ions from aqueous solutions. The effect of pH, temperature, initial metal concentration and sorbent dosage was analyzed using a factorial experimental design.
The factorial experimental design methodology involves changing all variables from one experiment to the next. As the variables can influence each other and the ideal value for one of them can depend on the values of the others, the interaction between the parameters was studied and optimized using response surface methodology.
Section snippets
Biomass
The S. muticum brown alga was collected from the north coasts of Cuba. The biomass was washed with abundant deionized water to remove the impurities and ions such as Ca2+ or Na+ that can influence the biosorption process. After that, it was dried at 60 °C and stored in desiccators. After that, the biomass was grinded and sieved into fractions <1000 μm.
Scanning electron microscopy
The surface morphology of S. muticum was investigated by scanning electron microscopy (SEM), Leo 1430VP, combined with a system of analysis for
Chemical and SEM analysis
The SEM image (Fig. 1a) shows irregular and porous structure of the biosorbent. Chemical analysis of the biomass showed that carbon was the main constituent along with small amount of magnesium, calcium, sulphur, potassium and sodium. For contrasting this data an elemental analysis has been carried out. The contents of total carbon on the surface of S. muticum range from 40% to 42% based on the SEM-EDX and elemental analyses. Calcium is a major metal ion on the surface of S. muticum (may be in
Conclusions
This work demonstrated that the factorial design is a useful tool in determining the operating variables that significantly influence the percentage of Cr(VI) removed by S. muticum biosorbent. At 95% confidence level, the factorial design showed that all three variables tested in this work, temperature, initial Cr(VI) concentration, and sorbent dosage, have significant effects on the percent of hexavalent chromium removed by S. muticum.
The temperature and amount of biosorbent increases the
Acknowledgement
Authors thank European Union for financial support under the ALFA program (Contract AML/190901/06/18414/II-0548-FC-FA).
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