The removal of Cr(VI)through polymeric supported liquid membrane by using calix[4]arene as a carrier

Canan Onac,Ahmet Kaya,Duygu Ataman,Ne fise Ayhan Gunduz,H.Korkmaz Alpoguz*

Department of Chemistry,Pamukkale University,20070 Campus,Denizli,Turkey

Keywords:Resorcinarene Liquid membrane Cr(VI)Surface characterization

ABSTRACT In this work,the transport and removal of Cr(VI)were achieved through supported liquid membrane(SLM)by using a 5,17,di-tert-butyl-11,23-bis[(1,4-dioxa-8-azaspiro[4,5]decanyl)methyl]-25,26,27,28-tetrahydroxy calix[4]arene carrier,dissolved in 2-nitrophenyl octylether dichloromethane.The studied parameters are the solvent effect in the membrane phase,the effect of carrier concentration,and the acid type in the donor phase.The Celgard 2500 was used as a membrane support.We used the Danesi mass transfer model to calculate the permeability coefficients for each studied parameter.In addition,AFM and SEM techniques were used to characterize the surface morphology of the prepared Celgard 2500 membrane that included the calix[4]arene carrier.

1.Introduction

The earth is degraded through environmentalpollution due to problems caused by technological advances and industrialization,combined with rapidly increasing populations.Environmental pollution emerged forthe firsttime atthe beginning ofurban life,and has increased parallel with industrialdevelopment.Particularly in the second halfofthe twentieth century,increasing environmental pollution due to accelerated population growth caused more pollution of living resources,and as a result,the degradation of the ecosystem balance became increasingly more serious.Irregular urbanization,uncontrolled population growth,and unplanned development of the industry cause problems known as pollutant disposal and pollution.These wastes are defined as solid,liquid,and/or gaseous substances,which have physical,chemical,and/or bacteriological characteristics that change the properties of the entrances by altering their properties[1,2].

Certain wastes also fall in the class of hazardous and harmful wastes.This is waste that pollutes underground and surface waters,which have chronic toxicity and carcinogenic effects,and accumulates in the environment;it enters into living organisms through respiration,digestion,or skin absorption,and it cannot be discharged.The main source of toxicity is the heavy metals in this class.Therefore,environmental pollution from these heavy metals is currently one of the most important problems in the world,and it is necessary to clean the industrial wastes containing these metals before they are left to the environment[3].

For these reasons,separation and purification technologies and processes have become very important.Various techniques are used to remove some metals and compounds that cause environmental pollution(especially water pollution)and that are important for industrial applications.Chemical precipitation,electrodialysis,ion exchange,distillation,reverse osmosis,solvent extraction,ultra filtration,and liquid membranes are some of these techniques[4].In the separation and purification processes in which these techniques are used,it is very important that the toxic metals be selectively separated and removed[5].

Liquid membrane technology is one alternative method of membrane technology that has recently become very popular[6].Liquid membranes are crucial in separation technology and purification processes,as they are effective systems for separating substances from diluted solutions.Membrane processes are primarily used for filtration,concentration,and purification purposes.Liquid membranes have a distinctive feature among these processes and have been studied extensively in recent years,in order to develop new,selective,and stable separation systems with special chemical reactions[7].Liquid membranes normally combine the extraction and stripping processes into a single process.The single-stage liquid membrane process provides maximum driving force for separation of the respective components,and thus provides the best separation and recovery.

In SLMs,the liquid membrane phase is an organic liquid thatimpregnates in the pores of a microporous support material.When the organic liquid comes into contact with the microporous support,it easily impregnates the pores and forms the SLM[8].For the extraction of target material,the organic-based SLM is placed between the donor and acceptor solutions.The SLM acts as a semi-permeable membrane for the transport of the target material from the feed solution to the stripping solution.The organic phase in SLMis immiscible with aqueous feed and stripping solutions,and it generally contains an inert organic solvent or extractant[9].

Fig.1.Structure of calix[4]arene derivative used as carrier(5,17,di-tert-butyl-11,23-bis[(1,4-dioxa-8-azaspiro[4,5]decanyl)methyl]-25,26,27,28-tetra-hydroxy calix[4]arene).

The supported liquid membrane technique has become one of the treatment technologies used to remove specific substances from waste water and for recovering valuable substances,and it has considerable advantages over other liquid membrane techniques.The advantages of SLMs are:its use of organic solvent and carrier in small quantities,its mass transfer in one step,its achievementof a high separation factor,its efficient separation of substances,and its low separation cost[10].

Chromium is the most toxic and mutagenic metal ion found in biological systems.Although chromium can be found in many different forms,the most common and stable forms are trivalent chromium(III)and hexavalent chromium(VI)species.Cr(III)is slightly less soluble at neutral and alkaline pH,and less toxic than Cr(VI).Unlike other heavy metals,Cr(VI)oxides or chromates are readily soluble in the liquid phase at almost all pH ranges,and therefore Cr(VI)is one of the most toxic materials that must be urgently controlled.

In this paper,the separation of Cr(VI)metal cation was achieved by using the supported liquid membrane technique,which is an effective separation method.The transport properties of the Cr(VI)metal cation have been investigated,and the removal of Cr(VI)has been effectively carried out by using a calixarene derivative as a carrier.Calixarenes have the ability to complex with cations,anions,and neutral compounds–both in the solid phase and in the solution–as they have cavities in which different guest molecules can be placed.For this reason,they are widely used as a molecule and ion carrier.In this study,the aim was to compare the transport efficiencies of different parameters(carrier ligand concentration,donor phase acid type and concentration,solvent,surface characterization)and then compare them with each other[11–14].

5,17di-tert–butyl-11,23-bis[(1,4-dioxa-8-azaspiro[4,5]decanyl)methyl]-25,26,27,28-tetra-hydroxy calix[4]arene is the carrier ligand(Fig.1),which was synthesized according to the literature method[12,15].Celgard 2500 membrane was used as the solid support.

2.Experimental

2.1.Reagents and chemicals

Organic compounds were purchased from Fluka such as 2-NPOE(2-nitrophenyl octyl ether),and dichloromethane(CH2Cl2)and used without further purification.Inorganic compounds were purchased from Merck such as potassium dichromate(K2Cr2O7),hydrochloric acid(HCl),sulfuric acid(H2SO4),nitric acid(HNO3),ammonium acetate(CH3COONH4),sodium acetate(CH3COONa),and acetic acid(CH3COOH)and they were used without further purification too.The polymeric film Celgard 2500(thickness:25 μm,porosity 55%)was obtained from Celgard Inc.(Germany).

Fig.2.Schematic illustration of mechanism between Cr(VI)and calix[4]arene.

2.2.Experimental procedure

Experimental conditions were prepared the same as in our previous studies[16,17].Transportexperiments were carried outin a permeation cell that consisted of two identical cylindrical compartments.The supported liquid membrane consisted of a thin,microporous polypropylene film that immobilized the solution of calix[4]arene carrier in organic solvents.An aqueous solution of 0.1 mmol·L-1K2Cr2O7in 0.1 mol·L-1HCl was used as a feed phase,and 0.1 mol·L-1sodium acetate solution was used as the stripping phase.The soaking time of the Celgard 2500 membrane was 24 h,and the stirring speed was chosen as 500 r·min-1in all transport experiments.The samples taken were analyzed with a UV spectrometer to determine Cr(VI),as in our previous studies.All kinetic results were calculated according to the mass transfer model described by Danesi[15,18].

The permeability coefficient(P)is given in Eq.(1).

By integrating Eq.(1),the following Eq.(2)was obtained:

where P is the permeability coefficient(cm·s-1),Ciis the initialconcentration ofCr(VI)in the feed phase,and C is the concentration ofCr(VI)in the feed phase at time t(s).ε is the porosity of the supported liquid membrane(55%),A is the effective membrane area(9.08 cm2),and V is the volume of the feed phase(ml).

The knowledge of P values allows an establishment of the initial membrane flux(Ji)according to Eq.(3).

To describe the efficiency of Cr(VI)removal from the feedphase,we calculated the recovery factor(RF)with Eq.(4).

The suggested schematic illustration of the mechanism between Cr(VI)and calix[4]arene carrier is presented in Fig.2.

2.3.Surface characterization

Atomic Force Microscopy(AFM,Veeco di Caliber)and Scanning Electron Microscopy(SEM,Zeiss LS-10)measurements were applied to illuminate both the structure of the calix[4]arene carrier and how the carrier was connected to the Celgard 2500 support layer in the supported liquid membrane transport studies.In surface characterization studies,AFM is the most-used method that obtains topographic images of the membrane surface.For this reason,a surface photograph of the Celgard 2500 membrane with and without carrier was taken under optimal conditions.AFM and SEM images of the carrier-free Celgard 2500 membrane are shown in Figs.3(a,b)and 4(a,b).The differences are clear between the Celgard 2500 membrane(carrierfree)and the Celgard 2500 membrane with calix[4]arene derivative.

As can be seen in the AFM images of the Celgard 2500 membrane without carrier,the shaded areas show the highest points,and the darker areas show the pores in the vertical profile.The difference between the membranes in Fig.3(a)and(b)makes the addition of the calix[4]arene derivative carrier quite apparent.In Fig.3(b),pores,light colored areas,and roughness of the AFM image were increased by adding the calix[4]arene derivative carrier to the Celgard 2500 membrane.

Fig.3.(a)AFMimage ofthe originalCelgard 2500 supportlayer,(b)AFMimage ofCalix[4]arene/Celgard 2500/2-NPOE and CH2Cl2 supported liquid membrane.

When SEM images(a)and(b)of the Fig.4 are compared,the difference between the Celgard 2500 membranes with and without carrier is clearly visible.In the SEMimage ofthe Celgard 2500 membrane without carrier(Fig.4(a)),gaps can be seen in the pores of support material and the color is light,while the SEM image of the Celgard 2500 with carrier has darker areas and the size of the pores is decreased.This suggests that our carrier is well-connected to the Celgard 2500 membrane.

3.Results and Discussion

3.1.Effect of carrier concentration

In supported liquid membrane transport studies,the effect of transport concentration on the membrane phase is very important.Therefore,the experiments were performed at four different carrier concentrations(0.75×10-3,1.00×10-3,1.50×10-3and 2.00×10-3mol·L-1),in order to determine the optimal carrier concentration in membrane composition.While working at different carrier concentrations,other conditions were kept constant.The experimental conditions were 1× 10-4mol·L-1K2Cr2O7solution in 0.1 mol·L-1HCl in the donor phase,with 0.75×10-3,1×10-3,1.5×10-3,and 2×10-3mol·L-1calix[4]arene carriers in the membrane composition,which were dissolved in dichloromethane at 5%of 2-NPOE and 0.1 mol·L-1of sodium acetate solution in the acceptor phase.

Fig.4.(a)SEM image of original Celgard 2500 support layer,(b)SEM image of Calix[4]arene/Celgard 2500/2-NPOE and CH2Cl2 supported liquid membrane.

In Fig.5,an In(C/Ci)versus time graph was drawn for the different concentrations of calix[4]arene carrier in supported liquid membrane transport experiments.The values of permeability(P), flow rate(J),and recovery factor(RF)were calculated from the values of the rate constant(k).The kinetic results were given in Table 1.

Fig.6 presents recovery factor percentage(RF)values for each carrier concentration.The transport efficiency increased up to the 1× 10-3mol·L-1carrier concentration,and sharply declined after this concentration.Thus,this concentration was kept as an optimum.The recovery factor is a determinative factor for the transport experiments.As seen in Fig.6,the recovery factors of the supported liquid membrane transportstudies performed at1×10-3and 1.5×10-3mol·L-1carrier concentrations are higher than the results of other carrier concentrations(0.75 × 10-3and 2 × 10-3mol·L-1).There was a regular decrease when the concentration was above 1 × 10-3mol·L-1.

This phenomenon was attributed to the increased carrier concentration,which increased the membrane viscosity.The increased carrier concentration increases the viscosity and so reduces the diffusion constant.Increased membrane viscosity results in a decrease in the transport efficiency.When a certain limit value was exceeded,the kinetic data began to decrease,because the Cr(VI)–carrier complex that occurs at the donor/membrane phase interface was limited in its diffusion.

Fig.5.In(C/C i)-t graph at different carrier concentrations.

The recovery value was 88.1%in the transport experimentwhen the 1 × 10-3mol·L-1carrier concentration was used,whereas itdecreased to 66.23%forthe 2 × 10-3mol·L-1carrierconcentration.As understood from these kinetic data,the transport of Cr(VI)metal had reached the maximum level when supported liquid membrane experiments were carried out with the 1 × 10-3mol·L-1calix[4]arene carrier concentration.We used this concentration of 1 × 10-3mol·L-1– the maximum carrier concentration–as the optimum carrier concentration in our future supported liquid membrane transport studies.

Table 1 Kinetic results at different carrier concentrations

Fig.6.RF plotted against time for different carrier concentrations.

3.2.Effect of solvents

The choice of solvent is an important parameter in the transport studies of Cr(VI)metal cations carried out via supported liquid membranes.For this purpose,we conducted the experiments with di-octyl terephthalate,tritolyl phosphate,tris(2-butoxy ethyl)phosphate,tris(2-ethylhexyl)phosphate,and 2-NPOE/CH2Cl2.The kinetic data obtained from this experiment are given in Table 2.

Table 2 Kinetic results in different solvents

The maximum kinetic data was reached when 2-NPOE/CH2Cl2was used as a solventin supported liquid membrane transport experiments,as seen in Table 2.The 2-NPOE solvent is an alternative solvent in SLM work.The extremely low water solubility of nitro phenyl alkyl ethers(NPHE-hexyl,NPOE-octyl)allows the membranes to have a very stable structure.When nitro phenyl alkyl ether solvents are used,the organic solvent–carrier interaction is not likely to diffuse into aqueous phases.For this reason,the membrane is extremely stable.The only disadvantage of using this solvent is its price,as it is more expensive than other organic solvents.

It is apparent from the results that the permeability values are remarkably different in different solvents,as found in the order of 2-NPOE/CH2Cl2＞tris(2-ethylhexyl)phosphate＞tris(2-butoxy ethyl)phosphate＞triethyl phosphate＞di-octyl terephthalate.In the case of tris(2-ethylhexyl)phosphate,tris(2-butoxy ethyl)phosphate,triethyl phosphate,and di-octyl terephthalate,the values of the solvents'permeabilities are smaller than 2-NPOE/CH2Cl2.Therefore,we used 2-NPOE/CH2Cl2as a membrane solvent in our SLM studies.In addition,the polymeric support materials used are very important to the stability of the membrane[19,20].A good polymeric support should be have some features such as high porosity,small pore size,good mechanical strength,chemical resistance,thinness,hydrophobicity,and low cost.

Two important features come to the forefront when determining the type of solvent.These are the dielectric constant of the solvent and the viscosity.Table 3 shows the dielectric constants and viscosities of the solvents that we used in our supported membrane transport studies.

As seen in Table 3,the highest transport of Cr(VI)was obtained with 2-NPOE,which has the highest dielectric constant and the lowest viscosity.Ion pairs are more easily dissociated with a high dielectric constant.Each ion has a higher diffusion coefficient than their neutral form;the target solutes and carriers complex easily with these ion pairs.Thus,ions are located more easily between the active sites of neighboring groups of solvents and the carrier[21].As can be understood from Table 3,kinetic data were significantly reduced when tritolyl phosphate was used,as it has the lowest dielectric constant as well as di-octyl terephthalate,which has the highest viscosity.This is an expected result according to the Stokes–Einstein equation(Eq.(5)).

Table 3 Dielectric constants and viscosities of solvents used in supported liquid membrane transport studies

In Eq.(5),D is the diffusion constant,kBis the Boltzmann constant,η is the viscosity ofthe solvent,r is the radius ofthe spherical particle,and T is the temperature in Kelvin.The diffusion coefficient of viscosity and the Stokes–Einstein equation vary with inverse proportions[19]:an increase in viscosity results in a reduction in diffusion,which leads to a reduction permeability of the membrane.Fig.7 presents the permeability, flux,and rate constant values for each solvent(1:di-octyl terephthalate,2:tritolyl phosphate,3:tris(2-butoxy ethyl)phosphate,4:tris(2-ethylhexyl)phosphate and 5:a solution of 5%of 2-NPOE in dichloromethane).

Fig.7.The effectof solvent type(1:di-octylterephthalate,2:triethylphosphate,3:tris(2-butoxy ethyl)phosphate,4:tris(2-ethylhexyl)phosphate,5:2-NPOE/CH2Cl2).

3.3.Effect of acid type and concentration in donor phase

It is crucial to determine the acid type effect in the donor phase,to provide the Cr(VI)cations in the form of dichromate ions.For this reason,three differentkinds of acid species were used to ensure the acidity of the aqueous solution in the donor phase.The concentration of HCl,HNO3,and H2SO4acids in the donor phase was arranged at 0.1 mol·L-1;the other experimental conditions were kept same.The best kinetic data was obtained when HCl was used in the donor phase in supported liquid membrane transport experiments.The calculated kinetic data were very low when the other two acid types(nitric acid and sulfuric acid)were used.The obtained kinetic data(k,P,J and RF)are given in Table 4.

The most important factor in the selection of donor phase acid type is the complexation reaction between chromate anions and acid type.No complex between Cr(VI)and nitric acid occurred like the complex according to Eqs.(6)and(7).Parallel to this situation,the transport efficiency is much lower when nitric acid is used as an acid type in the donor phase.The complex between sulfate anions and chromate cations limits the diffusion and transport of the donor/membrane interfaces,because the sulfuric acid diameter is larger than hydrochloric acid.As a result,it is expected that the highest transport efficiency is achieved in supported liquid membrane experiments in which the donor phase acid type is selected as HCl.

Table 4 Kinetic results for different acid species in the donor phase

Another important parameter in the supported liquid membrane transport experiments is the donor phase acid concentration effect.For this purpose,we carried out liquid membrane transport studies with different donor phase concentrations(0.025,0.050,0.100,0.250 and 0.500 mol·L-1),to determine the optimal donor phase acid concentration.

Table 5 presents k,P,J,and RF kinetic values.The flux and other kinetic results increased by increasing the acid concentration of the donor phase from 0.025 mol·L-1to 0.1 mol·L-1.The highest result was achieved at 0.1 mol·L-1,and results began to decrease after this concentration value.

Table 5 Kinetic results at different donor phase acid concentrations

The pH difference between the donor and acceptor phases is the most important driving force in supported liquid membrane studies.Dichromate anions bind to the calix[4]arene derivative carrier at low pH values and are removed at higher pH values.Therefore,the pH value of the donor phase should be kept lower than the pH value of the acceptor phase,in order to achieve efficient and effective transport of dichromate ions.

Fig.8 presents the RF values versus the different acid concentration values.These results indicate that the maximum transport efficiency was achieved when 0.1 mol·L-1hydrochloric acid concentration was used in the donor phase.For this reason,0.1 mol·L-1was used as an optimal donor phase acid concentration in supported liquid membrane transport studies.

Fig.8.RF graph against different donor phase acid concentrations.

4.Conclusions

In this study,the separation of Cr(VI)metal cations was achieved by using the supported liquid membrane technique,which is an effective separation method with 88.1%recovery factor.The experiments were performed at different parameters,and the concentration values of the samples were taken at specific time intervals and determined in the UV spectrophotometer.The kinetic values of rate constant(k), flow(J),permeability coefficient(P),and recovery factor(RF)were calculated.As a result of the experiments,it was seen that the flow rate changed linearly with the studied parameters for the transport of the Cr(VI)metal cation.In addition,all other parameters(carrier ligand concentration,donor phase acid species and concentration,solvent effect)were found to be important factors for the transport of the Cr(VI)metal cation.

AFM and SEM pictures demonstrated that the carrier had a good connection with membrane.The experimental results showed that the calix[4]arene had a great selectivity to Cr(VI)metal cation.

Acknowledgments

This study was supported financially by the Scientific Research Projects(BAP)ofPamukkale University,Denizli-Turkey(2013 FBE 045).