The effect of anionic surfactant on the surface structure of nanofiltration membranes

Anna Kowalik-Klimczak1, Paweł Religa2, Paweł Gierycz1, Marta Bojarska1

1Warsaw University of Technology, Faculty of Chemical and Process Engineering, Waryńskiego 1, 00-645 Warsaw, Polande-mail: a.kowalik@ichip.pw.edu.pl2 Kazimierz Pulaski University of Technology and Humanities in Radom, Department of Environmental Protection, Chrobrego 27, Radom, Poland

The effect of cleaning bath – sodium dodecyl sulphate solution on the surface structure of the polymer membrane used during nanofiltration of con-centrated salt solutions have been presented in this paper. It was found that the use of the cleaning bath with sodium dodecyl sulphate caused a sig-nificant reduction in the separation and permeability possibilities of tested membrane.

Keywords: nanofiltration membrane, concentrated salt solution, anionic surfactant, scaling

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Introduction

Nanofiltration is a membrane pressure technique, whichhas many applications. Till now, it was successfully appliedfor treatment of both surface and underground waters [1-3] as well as purification of industrial wastewaters [4,5].The nanofiltration process was mostly applied for separa-tion of organic compounds, water softening and bacteriaremoval from water used for housing and industrial pur-poses [1-3] as well as separation and concentration of metalions [4,5] and dyes [6,7]. According to the literature data[8-10] and our own investigations [11-13], one of the mostimportant and interesting research area of nanofiltration isconnected with separation of mono- and multivalent ionspresent in salt solutions. The nanofiltration membrane insuch processes, according to its ion-selectivity, becomesnon-permeable for multivalent ions and permeable formonovalent anions and cations [10,14,15].

The wide application of nanofiltration is limited by re-duction of the process efficiency caused by membrane foul-ing and/or scaling [16,17]. This phenomenon dependstrongly on nanofiltration membrane properties [13,15-17]. Membrane charge plays here main role. It is dependenton functional groups present in the membrane structure[12,13] as well as on pH and kind of solution (mainly con-centration and salt type) [10,14]. The membrane chargechanges dependently on process [18,19] and cleaning bathconditions [20-22]. Our previous investigations [13,18]showed that at low pH positively charged groups appearon nanofiltration membrane surface. They are responsiblefor adsorption of negatively charged ions on the membranesurface. In consequence, it leads to scaling and reducesmembrane selectivity [18]. Change of membrane selectivity

strongly influences and changes retention of feed compo-nents, what can lead to further increasing of membraneconcentration polarization and decreasing its permeability.From the point of view of mono- and multivalent ions (pre-sent in salt solutions) separation between permeate and re-tentate fluxes leaving the membrane module it is useful tokeep high membrane permeability and initial selectivity.That is why removal of adsorbed on the membrane surfaceions layer seems to crucial for the nanofiltration [18]. Bathsbased on mineral acids (H2SO4, HCl), lye (NaOH) andsurfactants are commonly used for the cleaning of polymernanofiltration membranes [17-24]. Besides of type anddensity of membrane charge, structure of membrane sur-face is a main factor for determination of membrane ten-dency for scaling [25-27]. According to Vrijenhoek et al.[16] and Nanda et al. [17] nanofiltration membranes char-acterized by less roughness surface showed less fouling andscaling tendency. It is concerning with the fact that bothorganic and inorganic compounds gathers, in a preferentialway, in any cavities of roughness membrane surface [25,26]. That is why the selection of membrane assuring effec-tive nanofiltration should take into account a structure ofmembrane surface.

Recently, atomic force microscopy (AFM) [13,25,27]and scanning electron microscopy (SEM) [17,18,27] be-came the most used methods for analysis of membrane sur-face structure.

Atomic force microscopy allows for determination ofaverage roughness of the membrane surface and such pa-rameters as: maximal and minimal values of deflection, ave -rage deviation from the sample profile or determination ofa roughness class. The average roughness is determined bythe topographic image of the sample. It is a kind of map

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where a certain heights is attached to every point. The ave -raging of all those data gives an average height of the sam-ple. The difference between the average and local (for eachpoint) height is a roughness measure of the investigatedsurface [25,27]. Results obtained by the use of atomic forcemicroscopy can be displayed on histogram which representsdependence of sum of number of the surface hills on theirheights.

Scanning electron spectroscopy enables visual observa-tion of surface and structural changes of analyzed mem-brane samples. This technique gives the quantitative ana lysison the investigated samples and direct determination of de-tails of the investigated objects [18,27].

The aim of this work was to determined the analysis ofstructural changes of membrane surface used to nanofiltra-tion of concentrated salt solutions at low pH and cleanedby anionic surfactant bath. During the investigations topo-graphic images and histograms of the tested membrane sur-faces as well as their average roughness values has beendetermined by the use of AFM method. Moreover to sup-port the investigations, SEM images of the tested mem-brane surfaces have been taken.

Material and method

The experiments were carried out at laboratory scale incross flow system equipped with SEPA CF (GE Osmonics)which scheme and detailed description can be found in ourprevious paper [13]. All experiments were performed atTMP = 14 bar and QR = 800 dm3/h. The temperature offeed solution during the process was constant and equal to25±1°C. The feed constituted model saline solution con-tained 2 gCr3+/dm3, 10 gCl-/dm3 and 10 gSO4

2-/dm3 andcharacterized by pH ≈ 4. The composition of model solu-tion was characteristic of chromium industrial wastewater[1,11]. All experiments were performed in batch concen-tration mode, i.e. the permeate stream was collected in thepermeate tank, whilst retentate stream was recycled to thefeed/retentate tank. The nanofiltration flat sheet mem-branes (thin film HL) provided by GE Osmonics were usedin experiments (Table 1). Tested membrane has an effectivearea of 0.0155 m2. Membranes used in experiments alsowere characterized by an isoelectric point (IP) which was3.3, therefore it had slightly negative surface charge at pH≈ 4 [12,13]. It is also characterized by a loose structure ofthe active layer [13,18].

After nanofiltration the tested membranes were cleanedwith anionic surfactants bath - dodecyl sodium sulphate(SDS) solution about concentration 3.5∙10-3 mol/dm3 ac-cording with procedure:• Rinsing in deionized water, pH = 7 (10 minutes),• Detergent cleaning in sodium dodecyl sulphate bath

(SDS), pH = 8 (10 minutes),• Rinsing in deionized water, pH = 7 (20 minutes).

After the end of the experiment, samples of permeateand retentate have been collected for determination of the

chromium(III) and chloride concentration. The samples ofpermeate, feed and retentate have been analyzed using thefollowing methods:• chromium(III) – spectrophotometer NANOCOLOR

UV/VIS using 1,5-difenylokarbazyde method withwave length λ=540 nm,

• chlorides – the Mohr titration method.The feed solution has been prepared using the following

chemicals: CrCl3·6H2O (Sigma-Aldrich), pure NaCl(Chem pur®), pure Na2SO4 (Chempur®) and the deionizedwater (after RO system γ = 46.1 μS/cm, pH = 7.5). Thefeed solution was characterized by pH ≈ 4. For initial pHcorrection the pure HCl (Lachner) was used. The pH wasmea sured by pH-meter (Mettler Toledo SevenEasy).

The tested membrane surfaces were examined usingatomic force microscopy (AFM) and scanning electron mi-croscopy (SEM).

The AFM images were performer with SollverBio In-struments NT-MDT. The cantilever was made out of Siwith a spring of a 4.4 N/m and a nominal tip apex radiusof 10 nm. The membrane surfaces were analyzed in a scansize of 10 μm x 10 μm. The average surfaces roughness oftested membranes were calculated from AFM images usingNova SPM software.

The SEM images were determined by scanning electronmicroscope PHENOM G2 (FEI). For SEM analysis, thedeposition of a gold layer of about 2 nm in thickness wasdone using K550x Sputter Coater (Technologies Quorum).

Results and discussion

The effect of concentrated salt solutions and cleaning bath– sodium dodecyl sulphate solution on the surface structureof the polymer nanofiltration membrane was studied. Forthis purpose, using an atomic force microscope (AFM)were made topographical images and histograms the sur-face of tested membrane: new, after 6 and 20 hours in con-centrated salt solutions at low pH and cleaning by thesodium dodecyl sulphate solution (Fig. 1). Then, using thesoftware Nova SPM determined average roughness valueswhich summarized in Table 2. The obtained resultsshowed a significant effect of sodium dodecyl sulphate onstructure of nanofiltration membranes repeatedly used for

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Table 1. Characteristic of nanofiltration membrane usedin the experiments

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The effect of anionic surfactant on the surface structure of nanofiltration membranes

separation of chromium(III) and chloride ions from theconcentrated salt solutions at low pH. The analysis of theAFM images membranes: new and after 6 and 20 hours inconcentrated salt solutions and cleaned sodium dodecylsulfate allowed to conclude the increase corrugations of thesurface (Fig. 1), and thus there was a significant increase inthe average roughness of the membrane after repeated used

and cleaned by SDS solution (Table 2). The average rough-ness determined from AFM images were 24.0, 25.6 and39.0 nm for the membrane: new, after 6 and 20 hours ofworking in concetrated salt solution and cleaning solutionof sodium dodecyl sulfate, respectively. According toArnold et al. [24] and Petkova et al. [25] concluded thatthe observed structural changes of the tested membrane,

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Fig. 1. AFM images of tested membrane surfaces: new (a), after 6 (b) and 20 (c) hours working in concentrated saltmixture solution and cleaning with sodium dodecyl sulphate (SDS)

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and hence increase in the average roughness of the surfacecaused by the adsorption of sodium dodecyl sulphate mol-ecules on the surface. A characteristic feature of the changesin the surface structure of a loose NF membrane was un-even spatial distribution changes. The analysis of his-tograms obtained by AFM (Fig. 1) leads to the conclusionthat the observed increase in the hills formed on the surfacewith working time and subsequent cleanings using sodiumdodecyl sulphate. Probably it was caused by the subsequentadsorption of sodium dodecyl sulphate particles on the sur-face of the tested membrane. At the same time reduced thenumber which indicates the connection of single bulge inthe tight structure – folds. The interactions surfactant-poly-mer can significantly contribute to changes in the surfaceproperties of polymer membranes, which have a direct ef-fect on the permeability and selectivity of the membranes[28,29].

Additionally of AFM analysis were surface images oftested membranes made by using scanning electron micro-scope (Fig. 2). SEM images analysis confirmed the forma-tion on the tested membrane surface the numerous changesunevenly distributed in the shape of folds. Similar obser-

vations were noticed after the addition of surfactant to themembrane polymer by other research group [30-32].

The observed changes in the surfaces of tested mem-brane repeatedly used during the nanofiltration of concen-trated salt solutions and cleaned with sodium dodecylsulphate solution was accompanied by adverse changes inits selectivity (Table 3). The particles of the surfactant ad-sorbed on the membrane surface during the cleaningformed unstable layer, which established the process con-ditions (pH ≈ 4) contributes to the relaxation of the struc-ture [30-32]. Negative phenomenon that accompanied thechanges described was a decrease in retention of ions pres-ent in concentrated salt solutions (Table 3). Therefore, a decrease distribution of chloride and chromium(III) ionsbetween permeate and retentate streams leaving the testednanofiltration module was observed.

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Table 2. Average roughness values for tested membranes

Table 3. The stability of tested membranes new and frequently used during the nanofiltration of concentrated salt so-lutions at low pH and cleaning with sodium dodecyl sulphate (SDS)

Fig. 2. SEM surfaces images of tested membranes after 6 (a) and 20 (b) hours working in concentrated salt solutionand cleaning with sodium dodecyl sulphate (SDS)

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The effect of anionic surfactant on the surface structure of nanofiltration membranes

Application of the cleaning bath constituting an anionicsurfactant (sodium dodecyl sulphate) has caused the de-crease in tested membrane permeability for demineralizedwater (Fig. 3). Probably, the surfactant molecules adsorbedon the surface as a result of interaction with the membranematerial caused relaxation of its structure which provideda constant of permeate flux despite appearing the appear-ance of sediment on the mineral layer deposition. Unfor-tunately, the continue using a further use of the membraneand cleaned it in a bath prepared on the basis of with SDScaused a decrease in the permeability coefficient. Probably,relaxation of the membrane structure facilitated the pene-tration of the ions present in solution into the membranestructure. As a result of interaction of ions with the chargeof the membrane, occurred their adsorption ions into oc-curred in the membrane structure, and thus its scaling intothe internal structure (Fig. 4). This irreversible process ofinternal membrane pores blocking caused probably a sig-nificant and rapid loss of tested membrane permeabilitycoefficient.

Conclusions

The changes of nanofiltration membrane structure, char-acterized by a loose external layers used to during nanofil-tration of concentrated salt and cleaned by sodium dodecylsulphate solution have been analyzed. The tested mem-brane surface intereacted with the surfactant used what re-sulted in further destruction (more loose) of the externallayers structure. It caused the strong decrease of both chlo-ride and chromium(III) ions retention. Easy permeation ofions present in the investigated systems into the membranestructure caused their inside adsorption in the membraneand led to the membrane scaling. Consequently, a decreaseof permeability coefficient for demineralized water was ob-served. The obtained results led to conclusion that the useof cleaning bath formed from anionic surfactant caused asignificant reduction in the separation possibility of testednanofiltration membranes. That is why a further investiga-tions concerning other cleaning bathes enabling for propercleaning of the membranes used for nanofiltration processesof concentrated salt solutions at low pH is absolutely needed.

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Fig. 3. Correlation between the permeability coefficient of tested membrane and time of working in concentration saltmixture solution at low pH and cleaning with sodium dodecyl sulphate (SDS)

Fig. 4. Schema representation of scaling mechanism for the tested nanofiltration membranes

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Acknowledgements

This work was financial supported by the Project No504M/1070/0112/000.

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