Membranes and Membrane Processes in Environmental Protection Monographs of the Environmental Engineering Committee Polish Academy of Sciences 2014, vol. 119, 261-269 ISBN 978-83-63714-18-5 MECHANISM OF CHROMIUM(III) IONS CONCENTRATION IN SALT MIXTURE SOLUTION USING POLYMERIC NANOFILTRATION MEMBRANES Anna KOWALIK-KLIMCZAK1*, Paweł RELIGA2, Paweł GIERYCZ1 Abstract: The mechanism of chromium(III) concentration factor in salt mixture solution on polymeric nanofiltration membranes proposed in this paper. Presented results allowed to conclude that chromium(III) concentration factor in salt mixture solution depend on the structure and charge membrane surface. The lower chromium(III) concentration factor observed in the case of the membrane characterized by a loose structure and a low density of negative groups in conditions of studied solutions. It caused by the formation stable and ionic adsorption layer on the surface and inside of the membrane. Therefore, the highest of chromium(III) concentration factor in the tested solution obtained for membranes characterized by a dense structure and negative surface charge in process conditions (pH ≈ 4). Keywords: nanofiltration, polymeric membranes, concentrated salt solution. INTRODUCTION Nanofiltration is a membrane pressure technique, which allows for both high efficiency and high selectivity of a process [1]. That is why it can be successfully applied for treatment of both surface and underground waters, mostly for water softening [2,3] and separation of metal ions from industrial wastewaters [4-6]. According to the literature data [6,7] and our previous investigations [8-10], one of the most important and interesting research area of nanofiltration separation is connected with mono- and multivalent ions separation from concentrated salt solutions characterized by low pH. The nanofiltration membrane in such processes, according to its properties, becomes non-permeable for multi-charged ions and permeable for one-charged anions and cations [10,11]. That is why nanofiltration Warsaw University of Technology, Faculty of Chemical and Process Engineering, Waryńskiego 1, Warsaw, Poland 2 Kazimierz Pulaski University of Technology and Humanities in Radom, Department of Environmental Protection, Chrobrego 27, Radom, Poland * corresponding author: [email protected] 1 262 Kowalik-Klimczak A., Religa P., Gierycz P. seems to be a promising process allowing for effective and efficient separation of mono- and multivalent ions from concentrated salts solutions characterized by low pH. In spite of many applications of the nanofiltration [3,6,8,11] the separation mechanism of the ions salt still entirely isn't clarified. Many different theories associated with attempts to explain of the ions separation mechanism on nanofiltration membranes [12-13]. Peculiarly it’s regarding compound concentrated salt solutions. After analysing literatures data [14-19] stated that both theoretical as well as experimental works about explaining the ions separation mechanism on polymeric nanofiltration membranes aren't still satisfaction and require further actions. The aim of this paper was an identification of the mechanism of chromium(III) ions concentration in salt mixture solution characterized by low pH using polymeric nanofiltration membranes. MATERIALS AND METHODS The nanofiltration of concentrated salt solution was performed in laboratory installation, which was described in details previous work [8]. The concentrated salt solution was pumped from the feed tank toward to nanofiltration membrane, obtaining a retentate that was returned to the feed/retentate tank and permeate that was collected in the permeate tank. All the experimental runs were carried out with a feed volume of 3 dm3 at the beginning of each run. The temperature of feed solution during the process was kept at 25±1°C by a cooling apparatus. Nanofiltration experiments were performed at TMP = 14 bar and QR= 800 dm3/h for 1 h. These operation conditions were fixed based on the previous studies [8-10]. The concentrated salt solution contained 2 gCr3+/dm3, 10 gCl-/dm3 and 0-20 gSO42-/dm3 and characterized by pH ≈ 4. The feed solution has been prepared using the following chemicals: CrCl3·9H2O (Sigma-Aldrich), pure NaCl (Chempur®), pure Na2SO4 (Chempur®) and the deionised water. For initial pH correction the pure HCl (Lachner) and pure NaOH (Chempur®) was used. The pH was measurement by pH-meter (Mettler Toledo SevenEasy). The samples of permeate, feed and retentate have been analyzed using the following methods:  chromium(III) - spectrophotometer NANOCOLOR UV/VIS using 1,5difenylokarbazyde method with wave length λ=540 nm,  chlorides - the Mohr titration method,  sulfates - gravimetric method with the use of BaCl2. Two kinds of commercial nanofiltration flat sheet membranes (under symbol DL and HL) provided by GE Osmonics with an effective surface area of 0.0155 m2 in all cases were used in the experiments. The tested nanofiltration membranes had an active layer made of the poly(piperazine-amide) The support of tested Mechanism of chromium(III) ions concentration in salt mixture solution… 263 membranes were made of polysulfone. Tested membranes were marked out based on previous study [9]. They are characterized by a high permeability both in relation to the deionised water as well as also concentrated salt solutions [8-10]. Moreover, the DL membrane had active three-layer - dense membrane structure, while the HL membrane had active two-layer - loose membrane structure [8]. Membrane surface zeta potential was determined by streaming potential using an apparatus and procedure described in the literature [20,21]. KCl (Chempur®) solution (0.001 M) was used as the electrolyte solution to measure the streaming potential of nanofiltration membranes. The pH was set by adding NaOH (Chempur®) and HCl (Chempur®). The zeta potential was calculated from the streaming potential using the Helmholtz-Smoluchowski equation taking into account a dielectric constant, viscosity and electrolytic conductivity of the solution. The effectiveness and efficiency of tested nanofiltration membranes were analyzed using the equations shown in Table 1. Table 1. The equations used for analysis of effectiveness and efficiency of tested nanofiltration membranes. Parameter Equation Retention, R Concentration factor, CF Volume reduction factor, VRF where: CP – concentration of component in permeate [g/dm3], CR – concentration of component in retentate [g/dm3], CF – concentration of component in feed [g/dm3], VF – feed volume [dm3], VR – retentate volume [dm3] RESULTS AND DISCUSSION The mechanism of chromium(III) ions concentration in salt solutions characterized by pH ≈ 4 on nanofiltration polymeric membranes was identification. In the first stage of the study, the electrokinetic characteristic of the tested nanofiltration membrane were determined. The obtained results were presented in Fig. 1. 264 Kowalik-Klimczak A., Religa P., Gierycz P. (a) (b) Fig. 1. Zeta potential of new and “soaked” DL (a) and HL (b) membrane (after 20 hours in salt mixture solution: 2 gCr3+/dm3, 10 gCl-/dm3, 10 gSO42-/dm3, pH ≈ 4) at various pH, (□ – new tested membrane, ● – tested membrane after soaking at 20 hours). In the case of new tested membranes, it was found that the HL membrane was characterized by a less negative zeta potential than the DL membrane. In process conditions (pH ≈ 4) the HL membrane demonstrated zeta potential on the level of -4, whereas, the DL membrane was characterized by a zeta potential equal to -14. According to the examinations conducted by Tang et al. [22] for solution with defined pH, the zeta potential of the membrane depends on the ratio between acidic and basic surface groups. Hence, analysis of the obtained results (Fig. 1) allows conclude that the less negative zeta potential of HL than DL membranes caused by the fact that the HL membrane has a higher density of amine than carboxyl groups on its active layer. The results show also that the concentrated salt solution characterized by low pH has a significant impact on the change in zeta potential of the tested membranes (Fig. 1). Both in the case of DL as well as HL membranes soaked for 20 hours in concentrated salt solutions the change of the zeta potential from negative to positive was observed. The change of the zeta potential of tested membranes caused the formation of the ionic adsorption layer on their surfaces. The created of adsorption layer change the membranes separation properties. Character of these changes was analyzed based on the effect of sulfates concentration in feed on chloride ions retention (Fig. 2a), volume reduction factor (Fig. 2b), chromium(III) ions retention (Fig. 3a) and chromium(III) concentration factor (Fig. 3b). Mechanism of chromium(III) ions concentration in salt mixture solution… (a) 265 (b) Fig. 2. The effect of concentration of sulfate ions in feed on retention of chloride (a) and volume reduction factor (b). The experimental results showed that the increase of the sulfate concentration in the feed caused the height permeation stream of the chloride ions through the membrane to permeate, and hence a fall in the rate of these ions retention was observed (Fig. 2a). Moreover, the increase of the sulfate concentration in the feed caused decline the retention of these ions. The retention of sulfate ions fluctuated on the level 94-96% and 90-95% appropriately for DL and HL membranes. The high retention of sulfate ions caused, that for keeping the electric indifference in the arrangement, chloride ions permeation more quickly and willingly through the membrane, according to the Donnan effect [10,11]. It was also noted that in the examined scope of sulfate ions concentration, they had slight effect on the chromium(III) retention (Fig. 3a). In case of both tested membranes the chromium(III) retention remained at the highest level about the 99% and 98% appropriately for DL and HL membranes. The increase of the sulfate concentration in feed contributed to the fall in the volume reduction factor (Fig. 2b) and chromium(III) concentration factor (Fig. 3b) caused by reduce of the permeate flux. 266 Kowalik-Klimczak A., Religa P., Gierycz P. (a) (b) Fig. 3. The effect of concentration of sulfate ions in feed on chromium(III) retention (a) and chromium(III) concentration factor (b). The obtained results were presented strong interaction between ions in solution and charge membrane surface. In consequence of these phenomenon, the adsorption ions and created ionic layer was observed. In case of the DL membrane characterized by higher initial density of negative groups than HL membrane, the progressing and distinct fall in the volume reduction factor with the increase of sulfate ion concentration in feed was observed (Fig. 2b). Such results indicated on the formation of the deposit stable layer on membrane surface (Fig. 4a). This layer constituted the additional resistance of the filtration. In case of the HL membrane, in spite of the much smaller initial density of negative groups, the lower volume reduction factor, than for DL membrane, was observed (Fig. 2b). It probably caused by the formation of adsorption layer also inside the nanofiltration membrane (Fig. 4b). In consequence scaling inside of the HL membrane, the decresed of permeate flux was distinct and irreparable. It confirmed by the our previous results [8,10]. The blocking of the HL membrane inside was caused by loose structure. In consequence the formation of mineral deposit inside HL membranes, also considerable loss of selective properties of this membrane was stated. As a result higher retention of chloride ions on this type of membrane was observed. The obtained results shown that for high chromium(III) concentration factor in the tested solution at low pH favourably to used membranes characterized by a dense structure and negative surface charge in process conditions (pH ≈ 4). Mechanism of chromium(III) ions concentration in salt mixture solution… 267 (a) adsorbed multivalent ions dense membrane structure (b) loose membrane structure adsorbed multivalent ions Fig. 4. Schematic presentation of multivalent ions adsorption in active layers of polymeric nanofiltration membranes: dense (a) and loose (b) membrane structure. CONCLUSIONS The aim of this paper was an identification of the mechanism of chromium(III) ions concentration in salt mixture solution characterized by low pH on polymeric nanofiltration membranes. On the basis examinations, it found that the mechanism of chromium(III) ions concentration depend on inside structure of membrane and density of surface charge. The highest of chromium(III) concentration factor in the tested solution obtained for membranes characterized by a dense structure and negative surface charge in process conditions (pH ≈ 4). It was also noted that the sulfate ions caused simultaneously decreases chlorides retention and hampered permeate flux. In consequence, a fall of chromium(III) concentration factor with the increase of sulfate concentration in feed was observed. ACKNOWLEDGEMENTS This publication has been co-financed with the European Union funds by the European Social Fund. REFERENCES 1. Kołtuniewicz A.B., Drioli E., Membrane in clean technology. Theory and practice, Vol.1-2, Wiley-VCH Verlag GmbH & Co. KGaA, 2008. 2. Bellona C., Drewes J.E., Viability of a low-pressure nanofilter in treating recycled water for water reuse applications: A pilot-scale study, Water Research, 2007, 41, 3948-3958. 268 Kowalik-Klimczak A., Religa P., Gierycz P. 3. 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Tang C.Y., Kwon Y-N., Leckie J.O., Effect of membrane chemistry and coating layer on physicochemical properties of thin film composite polyamide RO and NF membranes. I. FTIR and XPS characterization of polyamide and coating layer chemistry, Desalination, 2009, 242, 149-167. MECHANIZM ZATĘŻANIA JONÓW CHROMU(III) W ROZTWORACH SOLI NA POLIMEROWYCH MEMBRANACH NANOFILTRACYJNYCH Anna KOWALIK-KLIMCZAK, Paweł RELIGA, Paweł GIERYCZ Streszczenie: W pracy przedstawiono propozycję mechanizmu zatężania jonów chromu(III) w stężonych roztworach soli na polimerowych membranach nanofiltracyjnych. Na podstawie wyników przeprowadzonych badań stwierdzono, że stopień zatężenia jonów chromu(III) w stężonych roztworach soli uzależniony jest od struktury oraz rodzaju ładunku powierzchniowego polimerowych membran nanofiltracyjnych. Niższy stopień zatężenia jonów chromu(III) zaobserwowano w przypadku membrany charakteryzującej się luźną strukturą oraz niższą gęstością grup ujemnych w warunkach panujących w badanych układach. Jest to spowodowane tworzeniem się stabilnej, jonowej warstwy adsorpcyjnej na powierzchni i wewnątrz membrany. Zatem dla uzyskania jak najwyższego stopnia zatężenia jonów chromu(III) w badanych roztworach korzystne jest zastosowania zwartych membran charakteryzującej się ujemnym ładunkiem powierzchniowym w warunkach procesowych (pH ≈ 4). Słowa kluczowe: nanofiltracja, membrany polimerowe, stężone roztwory soli.