Removal of Cu2+ using micellar-enhanced ultrafiltration
Removal of Cu2+ using micellar-enhanced ultrafiltration
Received for publication, July15, 2006
Accepted, September15, 2006
MILITINA BOURCEANU*, MARIANA BEZDADEA, DANIELA ZAVASTIN
Technical University «Gh. Asachi» of Iasi, Faculty of Chemical Engineering,
Dept. Of Biochemical Engineering,71D. Mangeron Avenue, 700050, Iasi, Romania
Email:
*the corresponding author
Abstract
Experimental results obtained through the ultra-filtration of the micellesof sodium lauryl sulphate, which contain ions with Cu2+, micellar included. Membrane process has been operated in both cross flow mode, dead-end flow and it was used a polyurethane membrane. The influence of the metallic ion concentration over the superficial tension and implicitly over the CMC and SLS as well as over the conductibility was studied. It was noticed the capacity of SLS to adsorb the copper ions and a high rejection degree of the copper ions was noticed when in small concentrations.
Keywords: havy metal, micellar-enhanced ultrafiltration, membrane
Introduction
In this study we focused on the copper traces in water.The copper present in water comes from industrial, agrarian, urban effluents and from the corrosion of copper pipes, from the fermentation water. The aquatic environments due to their role of drain water and waste water receptacles are in particular exposed to the pollution with copper. The admitted limit provided by the European regulations is of 1,5μg/L, the limit outrun in most of the cases. Copper’s toxicity in comparison to other organisms, as in the case of all heavy metals, varies depending on the shape it is found or in which it can be transformed in mixture with other products existing in waste waters, as well as on the composition of water.
The separation process based on micellar solubilisation followed by ultra-filtration, a process also known as MEUF (micellar-enhanced ultra-filtration), is a separation technique with many applications in the field of used waters. Using surfactants, at a concentration at which their molecules form the association micelles (CMC), this technique allows the removal of metal ion and organic solutes from waste water[2,4].
If to the water containing metallic ions we add an anionic surfactant, it will form aggregates (micelles). The surfactant molecules associate, coming together through the hydrophobic, hydrocarbonated part, oriented towards the interior of the micelle and the polar, anionic groups oriented towards the exterior of the micelle realize a high charge density. The metallic cations will be electrostatic absorbed onto or near the micellar surface. If the water which contains metallic ions micellar solubilised is consequently ultrafiltrated through a membrane whose pores are sufficiently small to allow the passage of the loaded micelles, in the permeate there will be a small concentration of metallic ions and of surfactant(monomer), and in the retentate a very high concentration of both metal ions and surfactant (Fig.1)[2]
Figure 1. Schematic of micellar-enhanced ultrfiltration to remove multivalent metal cations from water [2].
The separation efficiency in MEUF process depends on the micelle formation. That is why we studied the influence of the copper ion over the MCM. The usage of the surfactant at a smaller concentration ensures against a supplementary pollution of the environment.
The membrane process has been operated in both cross flow, dead-end flow. In dead-end filtration the feed-flow is perpendicular to the membrane surface which leads to the formation of a deposit (cake-layer) which grows at the same time with the volume of filtrate. This deposit involves a rapid diminution of the permeate flux, which may reach the value zero, which implies the fact that this is a discontinuous operation. (fig.2a)[3]
In cross flow ultrafiltration, the input flux is introduced tangentially on the membrane. This tangential flux, engages through shearing with the retentate flux, the particles deposited on the filter which prevents the formation of an important deposit. After a rapid decrease of the debit, the thickness of the deposit and the permeate debit are stabilised.(Fig.2b)[3]
Figure 2. a) dead-end ultrafiltration; b) cross-flow ultrafiltration
Materials and method
Thepolyurethane membrane has been prepared by casting and the development of membrane structure during the polymer processing has been achieved.[1] The dimension of the pores of the membrane was determined through the Bubble Point method.
The perm-selectivity of the membrane was tested on a UF-MF 60mm Hg lab installation on 0,003m2 membrane surfaces.
The PU membrane used has a 70% porosity and the dimension of the pore is of 1,00-0,5 micrometer.
The surfactant used was sodium lauryl sulfate (SLS) and CuSO4.5H2O, the source of cupric ions.
Surface tension
For the measures of superficial tension the method of gas bubble was used. The measures of the superficial tensions allowed the determination of the critical concentration of SLS micellisation, as well as the influence of the concentration of the copper ion over this value.
Conductivity
It was determined with a Radelkis conduct-meter.
Experimental set-up
The ultrafiltration experiments were carried out in both cross flow mode, dead-end flow. Probes containing copper ions were ultra-filtrated in concentration of 10, 25, respectively 50 mg/L in solutions of SLS at CMC corresponding to the respective concentration of cooper ions
Metal ion concentration
The metal concentration was measured by Perkin Elmer flame atomic absorption spectrophotometer.
Results and discussion
Crtitical micelle concentration (cmc)
The critical micelle concentration was determined by measuring the superficial tension of the SLS solutions in various concentrations. Figure 3 shows the variation of the superficial tension of the SLS, depending on its concentration.
Figure 3. Determination of CMCof SLS.
The concentration at which the curve σ = f (cSLS) shows an inflexion point represents the CMC value, namely the concentration at which the surfactant molecules begin to form micelles. This value is of 1,8 g/l.
Figure 4 shows the effect of the metal ion concentration on the superficial tension value and implicitly of the CMC. It is noticed that an increase of the Cu ion concentration results in a decrease of the surfactant’s CMC.
Figure 4. Effect of [Cu2+]ion on CMC of SLS.
This decrease is due to the Cu ion binding to the SLS which determines the reduction of the repulsive forces between the surfactant molecules [4] and therefore the possibility of their aggregation at lower concentrations.
Conductivity
Figure 5 shows the variation of the SLS solutions conductibility depending on their concentration.
Figure 5. Determination of CMC of SLS by conductivity
This diagram indicates an inflection point around the same concentration value of 1,8 g/l.
Figure 6 shows the role of the copper ion concentration effect on the SLS conductivity SLS, at various concentrations of the same and at various concentrations of the metal ion.
Figure 6. Effect of concentration ofCu2+ on conductivity.
It is noticed that the increase of the metal ion concentration determines the conductibility increase, due to the higher mobility of the metal ion compared to the surfactant’s molecules.
The cross-flow ultrafiltration of the micelle-incorporated copper ions
The probes containing 10, 25 and respectively 50 mg Cu2+/L were ultrafiltrated in solutions of SLS at CMC corresponding concentrations in the presence of the copper ion.
Figure 7 shows the CMC value for each cooper ion concentration; the increase of its concentration results in a decrease of the CMC value.
Figure 7. Effect ofconcentration of Cu2+ on CMC of SLS.
The results achieved by the analysis of the permeate and retentate probes for cross flow and dead end ultrafiltration are showed in fig 8.
The efficiency of the ultra-filtration is defined by the rejection factor R:
R = 100[1 – permeatCu2+/retentatCu2+
Figure 8. Effect of concentration of Cu2+ in the rejection.
A high rejection degree at low copper concentrations is noticed.
In case of frontal ultrafiltration, the copper rejection at low concentrations was higher, but at higher concentrations, it was lower compared to cross flow ultra-filtration.
Initially high concentrations generally lead to high silting degrees and determine the diminishing of the permeate flow.
Conclusions
The results achieved show an optimum removal of the copper traces by the method of micelle ultrafiltration, using a PU membrane. This method is an alternative technology for removing the heavy metals traces.
References
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Roum. Biotechnol. Lett., Vol. 11, No. 5, 2923-2929 (2006)