Selective separation and preconcentration studies of chromium(VI ...

Toxicological & Environmental Chemistry, Apr.–June 2006; 88(2): 187–196

Selective separation and preconcentration studies of chromium(VI) with Alamine 336 supported liquid membrane

WAQAR ASHRAF1 & ATIQ MIAN2 1

Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Kingdom of Saudi Arabia and 2Center for Environment and Water, King Fahd University of Petroleum & Minerals, Dhahran 31261, Kingdom of Saudi Arabia (Received 12 September 2005; revised 14 December 2005; in final form 27 February 2006)

Abstract Chromium compounds have received considerable attention because these are used extensively in such industrial applications as electroplating, steelmaking, tanning of leather goods, and corrosion inhibition. The use of supported liquid membranes (SLMs) to remove metals from wastewaters has actively been pursued by the scientific and industrial community. In the present work, the selective separation and preconcentration of Cr(VI) ions has been studied by using a commercial amine as the membrane liquid on the porous polypropylene support. Permeation experiments were conducted on a laboratory scale batch reactor made up of perspex, with a memberane fixed amid the two chambers. The flux of Cr(VI) ions was found to be maximum (3.15 105 mol cm2 s1) around pH 1. Above and below this pH the flux decreases. Distribution studies show that an increase in the amine concentration leads to higher distribution coefficients at fixed pH values. At pH around unity, the distribution of Cr(VI) ions into the organic phase was found to be maximum, of the order of 56.3. The Cr(VI) transport through the membrane increases with rise in temperature. In order to check the long-term efficiency of the flat sheet SLM, an experiment was conducted with higher Cr(VI) concentration (5000 ppm) for 24 h, at optimised parameters. It was observed that in 24 h, about 1/5th of the feed Cr(VI) is left over while the rest is transported. However, minute organic droplets were also seen in the feed and strip compartments, after 1 day. This observation suggested the loss of membrane liquid. The feasibility of preconcentration of Cr(VI) by using the proposed SLM parameters, was also studied by using the hollow fibre (HF) system. Highest enrichment factor (E.F) value was obtained for 50 mg L1 whereby all of the metal was transported to the stripping phase and the resulting Cr concentration was 688 mg L1 (E.F ¼ 13.8). It was observed that while treating more diluted solutions, the enrichment factor decreases. The values of E.F equal to 8.9 and 11.3 were found for initial Cr concentration of 10 and 30 mg L1. Keywords: Supported liquid membranes, selective separation, hollow fibre system, Cr(VI)

Correspondence: Dr Mohammad Waqar Ashraf, Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Kingdom of Saudi Arabia. Tel: þ966 3860 3127. Fax: þ966 3860 4277. E-mail: [email protected]; [email protected] ISSN 0277-2248 print/ISSN 1029-0486 online ß 2006 Taylor & Francis DOI: 10.1080/02772240600668036

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Introduction Hexavalent chromium Cr(VI) materials are used in many jobs which involve frequent and/or heavy chromium exposure. These jobs include spraying anticorrosion coatings, dying in textile, welding and cutting stainless steel, tanning, and chrome plating. It is also used as a pigment in paints, inks, and plastics. Cr(VI) has received considerable attention owing to its extensive industrial applications and has long been recognised as a toxic substance due to its strong oxidising potential and the ease with which it can cross the biological membranes [1–4]. The removal of Cr(VI) from industrial wastewater is of great significance due to the high toxicity of Cr(VI) compounds. During recent years, the application of different solvent extraction technologies to the Cr removal and concentration has been widely studied; conventional liquid–liquid (L–L) extraction emulsion liquid membrane technique, and nondispersive extraction are some of the alternatives that have been reported in the literature [5–10]. The tanning industry has been identified as one of the most serious sources of Cr pollution, consuming 30–40 L of water/kg of raw hide to convert into finished leather. The dissolved and suspended impurities are either left to flow onto the open lands or to enter the water channels and rivers. Cr is one of the major toxic elements present in the tannery wastes as a result of chrome tanning process and cannot be eliminated by ordinary treatment processes. A concentration of 2% Cr is applied for the purpose and out of which 60% is absorbed in the tanning process and about 30–40% is discharged in the waste liquors [2]. Analyses of the trace metals in the environmental samples is still a challenging task due to the low concentration of metals and the complexity of matrices. Therefore, the development of methods based on the preconcentration of the target metals with minimum sample perturbation and handling, and which can be coupled to analytical instruments, is of great importance, and the supported liquid membrane (SLM) is one of the tools which can be used for this purpose [11,12]. The SLM technology offers an attractive alternative to the conventional L–L extraction by combining the extraction and stripping in a single step operation. An SLM usually consists of an organic solution immobilised in the pores of a hydrophobic macroporous membrane that contains a complexing agent (carrier) that selectively binds one of the components from the feed solution. The SLM separates, by means of two interfaces, the aqueous solution containing the species that diffuse (feed) and the solution into which the species will diffuse (strip). The species are accumulated in the strip at a concentration generally greater than that in the feed. The permeation of the species is due to a chemical potential gradient (the driving force of the process) that exists between the opposite sides of the SLM. High enrichment factors (E.Fs) can be achieved when using SLM in hollow fibre (HF) configuration which has several additional advantages like the high feed to strip volume ratio leading to higher enrichment of the analyte and easy coupling to the sensitive analytical techniques. Alamine 336 is a water insoluble, tri-octyl/decyl amine which is capable of forming oilsoluble salts of anionic species at low pH. Since Alamine 336 contains a basic nitrogen atom, it typically can react with a variety of inorganic and organic acids to form amine salts, which are capable of undergoing ion exchange reactions with a lot of other anions. The general reactions which are shown below in two steps, protonation and exchange, describe this behavior. Protonation: þ ½R 3 Norg þ ½HAaq ) * ½R 3 NH A org

Selective separation and preconcentration studies of Cr(VI) ions

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Exchange: þ ½R3 NHþ A org þ ½B aq ) * ½R 3 NH B org þ ½A aq :

The extent to which B will exchange for A is a function of the relative affinity of the two anions for the organic cation and the relative salvation energy of the anions by the aqueous phase. Keeping above reactions in view, Cr(VI) in feed solution should be kept acidic, whereas the stripping should be carried out in a basic medium. The chemical reactions occurring on the feed and stripping solution side of membrane are given below: Feed side reaction: þ Cr2 O 7aq þ 2Haq þ 2R 3 Norg () ½ðR 3 NHÞ2 Cr2 O7 org

If the complex formed as above diffuses through the membrane toward the strip side, then in the presence of NaOH there, the following reaction is expected to take place at the membrane face on the stripping or the product solution side. Strip side reaction: ½ðR3 NHÞ2 Cr2 O7 org þ 4NaOHaq () 2R 3 Norg þ 2Na2 CrO4 þ 3H2 O If these two reactions are quite fast then the rate of these chemical reactions taking place on membrane interfaces will not affect the rate of transfer of Cr(VI) from the feed to strip solution. The relationship which correlates the membrane flux ( J) to concentration (C), to the aqueous feed volume V, and to membrane area Q is given below: J¼

d½CrðVIÞ V : dt Q

ð1Þ

The integrated form of the flux equation is ln

½CrðVIÞf ,t Q ¼ Pt, V ½CrðVIÞf ,0

ð2Þ

where ½CrðVIÞf ,0 is the initial chromium concentration in feed; ½CrðVIÞf ,t is the total concentration of Cr(VI) at time t, and P is the permeability. A linear dependence of the feed solution with time is obtained, and the permeability can be calculated from the slope of the straight line that fits the experimental data [13]. Winston and Poddar [10] developed a strip dispersion SLM technique for the removal and recovery of chromium from wastewaters. The technology not only removes Cr(VI) to less than 0.05 ppm in the treated effluent allowable for discharge or recycle, but also recovers the product at a high concentration of about 20%. Teramoto et al. [14] developed the extraction model for Cr(VI) transport with Aliquat, by using the measured flux of Cr(VI) and the feed side mass transfer coefficient. Alanso and Pantelides [15] studied the kinetics of extraction of Cr(VI) with Aliquat 336, whereas Park et al. [16] studied the extraction of Cr(VI) at pH 3 through an SLM with the same carrier dissolved in toluene at 25 C. Kilambi et al. [17] used an Amberlite L-A2 carrier in dodecane for the separation of Cr(VI) and afforded a flux of less than 1 gm2h1. Recently, some phenolic compounds have been used as hydrogen bonding donors in enhancing the extraction of hexavalent

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chromium with Alamine 336 [18]. Kozlowski and Walkowiak [19] studied the Cr(VI) transport through SLMs and polymer inclusion membranes (PIMs) from chloride acidic solutions using n-octanol/water system. The purpose of the present work is: to improve the extraction of the chromium ions with Alamine 336 and to find out the optimum conditions for the SLM process to be used later for the pre-treatment of wastewater streams before their disposal to avoid pollution, and also to recover chromium for its further use. The effects of various parameters like the feed pH, the temperature, and the membrane lifetime have been studied. Using the established optimum conditions, the preconcentration studies of Cr(VI) have been conducted by using the hollow fibre system.

Experimental Reagents (a) Chromium (VI) solution: (i) The standard solution of potassium chromate was prepared by dissolving a known amount of this chemical in deionised water, mostly 200 ppm. This concentration was chosen since it is typical of some industrial effluents such as electroplating rinse water. (ii) For L–L extraction experiments, the stock solutions were diluted to the required extent. (b) Alamine 336 (Henkel Co. Hertfordshire, UK) solution for solvent extraction or SLM experiments were prepared by diluting the known volume of this chemical to a given extent with toluene. Supported liquid membrane experiments The permeation cell used for SLM experiments consisted of two compartments separated by a membrane. Each compartment, feed and strip, had a maximum volume of 140 mL. A membrane of effective surface area 14.2 cm2 was fixed amid the two chambers. The agitation of the solutions was carried out by two synchronised motors that relied on the variable power supply with a stirring rate of 1000 rpm. The stirring rate was high enough to minimise the boundary layer resistances [20,21]. The membrane was made by soaking the hydrophobic polypropylene film (Accural PP2E, Akzo Wuppertal, GmbH, Germany), having a thickness of 150 mm, a pore size of 0.2 mm, and a porosity of 75%. The film was kept in this organic solution for at least 24 h before use. Hollow fibre supported liquid membrane experiments Experiments for the simultaneous transport and enrichment of Cr(VI) were conducted by using a hollow fibre module. Hydrophobic polypropylene hollow fibres from Celgard (Charlotte, NC) were used as solid supports for the liquid membrane, with the following characteristics: inner diameter ¼ 240 mm, outer diameter ¼ 300 mm, pore size ¼ 0.04 mm, porosity ¼ 40%, and length ¼ 5.5 in. The HF SLM was prepared by impregnation of the tubular microporous fibre, passing a 5% solution of Alamine 336 in toluene. Experimental set-up is shown in Figure 1. The flow rate of both solutions was fixed at 0.5 mL min1. Each experiment was conducted for 18 h.

Selective separation and preconcentration studies of Cr(VI) ions Out

In

In

191

Out

Strip

Strip pump

Strip reservior

Feed reservior

Feed pump

Figure 1. Single hollow fibre module used in a recycling mode.

Solvent extraction Equal volumes (20.0 mL) of aqueous Cr(VI) solutions and Alamine solution in toluene were shaken for 15 min at a given temperature and allowed to stand for half an hour to separate to organic and aqueous layers. The aqueous phase was analysed for Cr by atomic absorption spectrophotometry. Flux measurement The membrane was fixed in the cell. The feed and stripping solutions were held in the two half cells. The two solutions were kept stirred at a rate greater than 1500 rpm to avoid concentration polarisation at the membrane with respect to bulk solutions. All the experiments were performed at 25 2 C and 1.0 cm3 samples were drawn after definite time intervals and were analysed spectrophotometrically.

Results and discussion Liquid–liquid extraction Data pertaining to extraction studies are contained in Tables I and II along with the experimental conditions. The results are quoted in terms of extraction efficiency in percentage (E%). Eð%Þ ¼

½Crorg 100 ½Crorg þ ½Craq

Table I shows the variation in extraction efficiency versus Alamine concentration. It is observed that extraction reaches its maximum (89.0%) up to about 8% Alamaine in toluene. It is interesting to note that even at 1% Alamine, 73.6 E% is obtained, thus showing the specific nature and high efficiency of Alamine toward Cr(VI) at the

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W. Ashraf & A. Mian Table I. Extraction (%) of Cr(VI) ions as a function of Alamine concentration. Membrane: Alamine in tolune; Feed phase ¼ 50.0 mg cm3 Cr(VI) at pH 3.0. Alamine conc. (%)

C?aq

E(%)

1.0 2.0 5.0 8.0 10.0

36.8 38.9 44.3 44.5 44.4

73.6 77.8 88.7 89.0 88.8

* Averages of triplicate runs with SD < 0.005.

Table II. Extraction (%) of Cr(VI) ions as a function of pH. Membrane: Alamine in tolune; Feed phase ¼ 50.0 mg cm3 Cr(VI) at various pH values. pH

? Craq

E(%)

0.5 1.0 2.0 3.0

26.3 43.3 31.2 6.2

52.6 86.7 62.3 12.4

* Averages of triplicate runs with SD < 0.005.

prevailing pH. Table II shows how the E% varies with respect to pH of Cr(VI) aqueous solution. The results indicate that extraction is highly pH sensitive and decreases drastically when pH changes from 2 to 3. So, a strict control of pH at the value of 1 0.1 is essential to get maximum extraction. Park et al. [16] have studied extraction of Cr(VI) with Aliquat at pH 3. They have shown the concentration of various ions of Cr(VI) 2 (20 ppm) in acid solution (pH 3); HCrO 4 (98.12%), H2 CrO4 (0.62%), CrO4 (0.03%), 2 Cr2 O7 (1.23%).

Transport against concentration gradient It can be seen in Figure 2 that Cr(VI) ions were transported against their concentration gradient. When concentration of Cr(VI) ions in the feed solution is reduced nearly to zero, then its concentration in the product solution approaches to its initial concentration in the feed solution. It means that chromium in its VI oxidation state can be transported uphill through Alamine–toluene based membrane, with feed pH at about 1. Chaudry et al. [1] also showed that transport of Cr(VI) ions increases with the increase in protons concentration up to a limit. This observation is in accordance with the theory, according to which the flux is directly proportional to the square of hydrogen ion concentration [22]. log J ¼ A þ log T log þ log ½Hþ 2 þ 2 log½R 3 N2 þ log Co ,

ð3Þ

where J is flux, A is a constant, and T are viscosity, and absolute temperature at which the transport takes place, and Co is concentration of Cr in the feed solution.


193

300

Conc. (ppm)

250 200 Feed

150

Strip

100 50 0 0

100

200

300

Time (min)

Figure 2. Curve showing Cr(VI) ions concentration in feed and strip solutions as function of time.

300

Conc. (ppm)

250 200

pH 0.6

150

pH 1.1

100

pH 3.0

50 0 0

100

200

300

Time (min)

Figure 3. Curve showing Cr(VI) ions concentration vs. time at various pH values.

Effect of pH The effect of pH of the feed solution fixed with 0.1 M HCl on the transport of Cr(VI) ions through Alamine–toluene based membrane is represented in Figure 3. The effect of pH on flux of chromium (VI) ions is shown in Figure 4. It can be inferred from these figures that the flux of Cr(VI) ions is maximum around pH 1. Above and below this pH the flux decreases. Actually the transport of Cr(VI) ions increases with the increase in proton concentration up to a limit, i.e. pH 1. After reaching a maximum value the flux decreases with increasing acid concentration. This can be explained as follows. þ Cr2 O 7 þ 2H () H2 Cr2O7 :

The increase in proton concentration in the feed solution will form a species like H2Cr2O7 or HCr2O7 which may not ionise completely at a higher acid concentration to form a complex with R3NHþ cation. Hence the extraction of Cr ions will decrease, and thereby the flux and the permeability decrease with the increase in acid concentration. The decrease in the flux with the decrease in proton concentration can be explained by the fact that the concentration of cationic species like R3NHþ decreases due to the less availability of proton as shown below: R3 N þ Hþ ! R 3 NHþ R3NHþ is not formed and hence, the flux will decrease.

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Flux (J )

3 2.5 2 1.5 1 0.5 0 01

1

2 pH

3

4

Figure 4. Curve showing flux, J (mol cm2 s1) of Cr(VI) ions vs. pH of feed solution.

300

Conc. (ppm)

250 200

50°C

150

35°C 25°C

100 50 0 0

50

100 150 Time (min)

200

250

Figure 5. Curve showing Cr(VI) ions concentration vs. time at different temperatures.

Therefore, the optimum composition of the liquid membrane was fixed at 8% Alamine in toluene whereas the strip solution comprised of 2.0 M NaOH. Effect of temperature Effect of temperature on the Cr(VI) ions transport through Alamine–toluene based SLM is shown in Figure 5. From these data, it is clear that Cr transport increases with the increase in temperature. According to kinetic theory, molecules move faster with the increase in temperature and so the diffusion of Cr(VI) ions through SLM increases. This is in accordance with Equation 3, which shows that the flux increases with increase in the temperature. Membrane lifetime In order to check the long-term efficiency of the developed SLM, an experiment was conducted with higher Cr(VI) concentration (5000 ppm) for 24 h, at optimised parameters. The results are depicted in Figure 6. It was observed that in 24 h about 1/5th of the feed


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6000

Cr (ppm)

5000 4000 3000 2000 1000 0 0

500

1000 Time (min)

1500

2000

Figure 6. Cr(VI) ions transport for 1 day at high concentrations. Membrane: Alamine in tolune; Feed phase ¼ 5000 mg cm3 Cr(VI) at pH 1.1.

Cr(VI) is left and the rest is transported. However, minute organic droplets were also seen after one day. This observation suggested the loss of membrane liquid. Preconcentration of Cr(VI) with a hollow fibre system The feasibility of preconcentration of Cr(VI) by using the proposed SLM parameters, was also studied by using hollow fibre system. The E.Fs were calculated by using different initial concentrations in the feed solution. The E.F is defined as ratio of metal concentration in the stripping phase and the initial metal concentration in the feed phase [23]. Highest E.F value was obtained for 50 mg L1 whereby all of the metal was transported to the stripping phase and the resulting Cr concentration was 688 mg L1 (E.F ¼ 13.8). It was observed that while treating more diluted solutions, the E.F decreases. Values of E.F equal to 8.9 and 11.3 were found for initial Cr concentration of 10 and 30 mg L1. The results can be improved by two ways; by using a hollow fibre module with more fibres, and, by increasing the length of experimental time.

Acknowledgment This study has been financed by KACST under project # AR-23-18. Shemsi and Shakeel are acknowledged for their help in laboratory work.

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