Removal of chromium(VI) from saline wastewaters by Dunaliella species

Abstract

Some industrial wastewaters contain higher quantities of salts besides chromium(VI) ions so the effect of these salts on the biosorption of chromium(VI) should be investigated. The biosorption of chromium (VI) from saline solutions on two strains of living Dunaliella algae were tested under laboratory conditions as a function of pH, initial metal ion and salt (NaCl) concentrations in a batch system. The biosorption capacity of both Dunaliella strains strongly depends on solution pH and maximum. Chromium(VI) sorption capacities of both sorbents were obtained at pH 2.0 in the absence and in the presence of increasing concentrations of salt. Chromium(VI)-salt biosorption studies were also performed at this pH value. Equilibrium uptakes of chromium(VI) increased with increasing chromium(VI) concentration up to 250–300 mg l⁻¹ and decreased sharply by the presence of increasing concentrations of salt for both the sorbents. Dunaliella 1 and Dunaliella 2 biosorbed 58.3 and 45.5 mg g⁻¹ of chromium(VI), respectively, in 72 h at 100 mg l⁻¹ initial chromium(VI) concentration without salt medium. When salt concentration arised to 20% (w/v), these values dropped to 20.7 and 12.2 mg g⁻¹ of chromium(VI) at the same conditions. Both the Freundlich and Langmuir adsorption models were suitable for describing the biosorption of chromium(VI) individually and in salt containing medium by both algal species. The pseudo second-order kinetic model was successfully applied to single chromium(VI) and chromium(VI)-salt mixtures biosorption data.

Introduction

Chromium(VI) is a common pollutant introduced into natural waters from a variety of industrial wastewaters including those from the textile dyeing, leather tanning, electroplating and metal finishing industries [1], [2]. Wastewaters generated by these industries often contain significant quantities of salts. One of the major pollutants from tannery wastewaters has been identified as sodium chloride [1], [2]. Removal of chromium(VI) from waters and wastewaters is obligatory in order to avoid water pollution. Using various microorganisms as biosorbents for chromium(VI) removal offers a potential alternative to existing methods for detoxification and recovery of this component from industrial wastewaters and is a subject of extensive studies [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [23]. Also it is necessary to check the effect of salts on chromium(VI) biosorption. Biosorption has distinct advantages over the conventional methods: the process does not produce chemical sludges (i.e. non-polluting), it could be highly selective, more efficient, easy to operate and hence cost effective for the treatment of large volumes of wastewaters containing low pollutant concentrations. Industrial applications of biosorption often make use of dead or non-growing, living biomass, which does not require nutrients and can be exposed to environments of high toxicity.

Microalgal biomass has been successfully used as sorbing agent, because microalgae use light as an energy source, facilitating the maintenance of metabolism in the absence of organic carbon sources, and electron acceptor required by bacteria or fungi. Thus, the use of metabolically active microalgal systems may be more readily achieved. Also, microalgae cultures can be cultivated in open ponds or in large-scale laboratory culture, providing a reliable and consistent supply of biomass for such studies and eventual scale-up work. Microalgae can sequester heavy metal ions by the same adsorption and absorption mechanisms as other microbial biomass. The mechanism of binding metal ions by inactivated algal biomass may depend on the species and ionic charges of metal ion, the algal organism and the chemical composition of the metal ion solution [4], [5], [12], [13], [14], [15], [16], [17], [18].

During biosorption, a rapid equilibrium is established between adsorbed chromium(VI) ions on the algal cells (q_eq) and unadsorbed chromium(VI) ions in solution (C_eq). This equilibrium can be represented by the Langmuir or Freundlich adsorption isotherms, which are widely used to analyze data for water and wastewater treatment applications. The most widely used isotherm equation for modeling equilibrium is the Langmuir equation, based on the assumption that there is a finite number of binding sites which are homogeneously distributed over the adsorbent surface, these binding sites have the same affinity for adsorption of a single molecular layer and there is no interaction between adsorbed molecules. The mathematical description of this model for a single component adsorption is

$$\text{q}{}_{\mathrm{<mtext>eq</mtext>}}\text{=}\text{Q}{}^0\text{bC}{}_{\mathrm{<mtext>eq</mtext>}}\text{1+bC}{}_{\mathrm{<mtext>eq</mtext>}}$$

where, Q⁰ represents a practical limiting adsorption capacity when the surface is fully covered with metal ions and assists in the comparison of adsorption performance, particularly in cases where the sorbent did not reach its full saturation in experiments, and b is a constant related to the affinity of the binding sites. Q⁰ and b can be determined from the linear plot of 1/q_eq versus 1/C_eq.

The empirical Freundlich model also considers a monomolecular layer coverage of solute by the sorbent. However, it assumes that the sorbent has a heterogeneous surface suggesting (as expected) that binding sites are not equivalent and/or independent. This model takes the following form for a single component adsorption:

$$\text{q}{}_{\mathrm{<mtext>eq</mtext>}}\text{=K}{}_{\mathrm{<mtext>f</mtext>}}\text{C}{}_{\mathrm{<mtext>eq</mtext>}}{}^{\mathrm{1/n}}$$

where K_f and n are the Freundlich constants related to the adsorption capacity and adsorption intensity of the sorbent, respectively. The Freundlich model is more widely used but provides no information on the monolayer adsorption capacity, in contrast to the Langmuir model Eq. (2) can be linearized in logarithmic form and Freundlich constants can be determined.

Kinetic models provide necessary knowledge when the biomass is employed as a free cell suspansion in a well-agitated batch system, all the cell wall binding sites are made readily available for uptake so the effect of external film diffusion on biosorption rate can be assumed not significant and ignored in any engineering analysis. The pseudo second-order kinetic model based on the sorption capacity of the solid phase can be used in this case assuming that measured concentrations are equal to cell surface concentrations. If the rate of sorption is a second-order mechanism, the pseudo second-order chemisorption kinetic rate equation for chromium(VI) both singly and in the mixture is expressed as:

$$\text{d}\text{q}\text{d}\text{t}\text{=k}{}_2\text{(q}{}_{\mathrm{<mtext>eq</mtext>}}\text{−q)}{}^2$$

where, k₂ is the second-order biosorption rate constant of chromium(VI) ions. For the boundary conditions t=0 to t=t and q=0 to q=q_eq the integrated and linear form of Eq. (3) becomes

$$\text{t}\text{q}\text{=}\text{1}\text{k}{}_2\text{q}{}_{\mathrm{<mtext>eq</mtext>}}{}^2\text{+}\text{1}\text{q}{}_{\mathrm{<mtext>eq</mtext>}}\text{t}$$

If second-order kinetics are applicable, the plot of t/q against t of Eq. (4) should give a linear relationship, from which q_eq and k₂ can be determined from the slope and intercept of the plot and there is no need to know any parameter beforehand [4], [16], [19].

Dunaliella, unicellular biflagellate halophilic green algae are the most abundant phytoplanktonic algae in hypersaline habitats [20], [21]. Dunaliella species are also used for the industrial production of fine chemicals, such as glycerol and β-carotene so the biomass of Dunaliella is a biological resource which is available in large quantities and can be used as a potential biosorbent. However, only a limited number of studies have so far been focused on the use of Dunaliella species for heavy metal removal from wastewater. Moreover, little attention has been paid to investigate the role of salts on metal biosorption. The comparative biosorption of chromium(VI), individually and in salt containing environments on two strains of Dunaliella algae isolated from a natural environment was examined in this study. These species were chosen as biosorbents because of the relative lack of information about their sorption abilities.