Research Article - (2016) Volume 6, Issue 3

Effect of Foulant Layers on the Rejection of Arsenite [As (III)] by Nanofiltration Membranes

Mustapha Chabane1,2* and Benamar Dahmani1
1Department of Chemistry, Laboratory of Spectrochemistry and Structural Pharmacology, 13000, Algeria
2Departement of Science and Technology, Central University Naama, P.O.B 66, 45000, Algeria
*Corresponding Author: Mustapha Chabane, Science Faculty, Department of Chemistry, Laboratory of Spectrochemistry and Structural pharmacology, University of Tlemcen, 13000, Algeria, Tel: 21343242424 Email:

Abstract

The effect of membrane fouling on the rejection efficiency of Arsenite species by nanofiltration membranes were investigated in this research. Two nanofiltration membranes (DESAL DL and N30F) were fouled by various compounds including humic acid; sodium alginate, colloidal silica and CaSO4 and we will evaluate the effect of flux decline and membranes rejection efficiency of As (III) from pH range between 6 and 11. The effect of the interaction mode between foulant layers; membrane and As (III) is related to the difference of membranes characteristics such as MWCO, roughness, hydrophobic/hydrophilic character and the variation of surface charge at different pH values. The monitoring of the rejection of As (III) in the feed and permeate solution will present the decline on permeate flux and an increase of arsenic rejection depending to each type of foulants. The properties of foulants were found to be an important factor for determination the transport of arsenite through the fouling layers. The results show the high rejection values of arsenite ions for membrane DESAL DL compared to the N30F membrane. For pH range situated between 6 to 11, the application of humic acid as foulant substance gives high removal efficiency of As (III) compared to other compounds, with very high values of rejection at pH=11 with 90% for membrane DESAL DL and 37% for N30F membrane. It has been found that the mechanism of size exclusion of arsenite As (III) is predominant for the membrane DESAL DL.

Keywords: Arsenite; Fouling; Nanofiltration; Membranes; Rejection; Toxicity; Water; Exclusion

Introduction

The presence of arsenic in drinking water has an emerging public health causing cancer, the limit of concentration fixed by WHO is 10 ppb [1-3].

The removal of arsenic by nanofiltration (NF) membranes has been extensively investigated in recent years due to a growing interest in brackish ground water desalination to supplement potable water supply and low energy consumption compared to Reverse osmosis [4,5]. In the majority of researches founded in literature [2-8], it has been concluded that NF membranes are able to remove arsenic preferably in pentavalent forms; which is explained by the chemical species forms of arsenic at typical pH range; in natural water (pH 5-8), specially at neutral pH ,the predominant species for As(V) exists as an anion at where in this range of pH; As(III) is mainly present as uncharged species. H3AsO3 and, therefore, is less efficiently rejected.

According to various researches studied, it has been founded that usually the nanofiltration membranes are quite permeable to arsenite As (III) compared to arsenate As (V). This was demonstrated in the absence of membrane fouling which is not connected to the actual operating conditions of the membrane desalination plants, however referring to the presence mixture of organic and mineral compounds in raw water such as sea or brackish water which are deposited on the membrane and reduce the permeate flux, therefore, it would be interesting to study the rejected arsenic species in the presence of organic and inorganic foulants.

The main difference between these two kinds of this foulants is basically related to the interaction. Organic fouling typically exhibits interactions between chemical functional groups of organic foulants and those of the polymeric membrane skin layer [9,10]. A study by Nghiem and Hawkes [11] revealed that permeate flux decline due to membrane fouling would be more severe with membranes having larger pore size. The authors also argued that pore blocking was the predominant fouling mechanism at the first stage of fouling, and the latter stage is governed by cake-enhanced mechanism. Several studies found that higher negative zeta potential and hydrophilicity of the membrane surfaces hould lead to less fouling by organic macromolecules due to higher electrostatic repulsion and lower hydrophobic interactions between the foulant and membrane surfaceit was reported that solution conditions such as pH and ionic strength. Pervov [10] described the inorganic fouling process as the crystal formation took place in the bulk solution due to strong oversaturation in the deadlocks, and then the crystals approached and precipitated on the membrane surface. The impact of membrane scaling on salt rejection has not been extensively investigated. Scaling of divalent cations such as Ca2+ may induce more positive charge to the membrane surface, and consequently reduce the rejection of charge solutes [12].

The main objective of the current study was to investigate the effects of membrane fouling on the rejection of neutral species of As (III) by NF membranes. Two typical membrane fouling conditions were simulated under controlled chemical and physical conditions in a laboratory membrane system.

The membranes selected for this research has differents characteristics with low salt rejection between 3 to 6% for the N30F membrane [13] in comparison to the DESAL DL membrane [14]. In this study, we will investigate in particular the effect of pH on the flux decline and rejection of arsenites by both membranes.

Arsenic species is in several forms (H3AsO3, H2AsO3–, HAsO32–) depending to the pH of the solution [2]. This is clearly shown in the distribution curve of arsenite species in function of pH at pH less than 9.2 (Figure 1), the uncharged arsenite species H3AsO3 predominates [3-5].

membrane-science-technology-Distribution-species

Figure 1: Distribution species of As (III) versus pH [3,15,16].

Material and methods

Chemicals and reagents

Stock solutions of As(III) using sodium arsenite, NaAsO2 (reagent grade) The preparation, dilution and analytical purposes of solutions. 1000 mg/L of As(III) stock solution is prepared by dissolving 1.320 g of arsenite (As2O3; Merck Chemicals, Germany). The effect of ionic strength will be considered by the addition of the mixture of inorganic salts( NaCl, CaCl2 and NaHCO3 with concentration of 10 mM, 1 mM, and 1 mM, respectively, NaHCO3 was used as a buffer reagent for the each test. The adding NaOH basic solution (1M) and HCl acid solution (1M) will produce the desired pH values.

The use of multiple plugging agents of organic and inorganic nature such as humic acid, sodium alginate, colloidal silica and calcium sulfate, The Use of multiple plugging agents of organic and inorganic nature such as humic acid, sodium alginate, colloidal silica and calcium sulfate. These elements are often present in natural waters Two thin film composite NF membranes noted N30F (Nadir) and Desal DL (GE osmonics) were employed in the present investigation.

Membranes characteristics

The nanofiltration membranes assessed in the present study displayed quite distinct characteristics (Table 1).

It has been proved also in several researches the low rejection of salt by N30F membrane [13,17,18].

Contact angles measurements

Contact angles were measured by the method of water drops, based on the measurement of contact angle between the water drop and the surface of the membrane. The membranes were dried in a desiccator before the measurement. Membrane samples were cut into small pieces and mounted on a support. A drop of approximately 2.0 ml of pure water is placed on the membrane and thanks to a goniometer we want to measure the contact angle.

The contact angle indicates the hydrophobicity of the membrane surface. In the current study, N30F had a larger contact angle and possessed a more hydrophobic surface compared to the Desal DL membrane. An increase in the hydrophobicity was clearly observed by a considerable increase in the contact angle of both membranes (Figures 2-4).

membrane-science-technology-Contact-angles

Figure 2: Contact angles of clean and fouled membranes.

membrane-science-technology-Zeta-potential

Figure 3: Zeta potential of NF membranes at [10 mM of NaCl, 1 mM of CaCl2 and 1 mM of NaHCO3, pH was adjusted with HCl or KOH.

membrane-science-technology-Nanofiltration-membrane

Figure 4: Nanofiltration membrane unit

Preliminary Test

A laboratory pilot dead end membrane test unit type amicon was used in the current study. The cell was pressurized by a high pressure nitrogen cylinder with the pressure determined by a gas pressure regulator. The stirring speed was controlled by a digital magnetic stirrer plate. The permeate samples were collected using a graduated cylinder and the permeate weights were measured by a digital balance (Scout Pro, Ohaus). Prior to each filtration experiment, the pure water hydraulic permeability was measured by dionised water. The Average values of permeability for Desal Dl and N30F membranes are 8.2 L/ m2.h.bar and 4.11 L.h-1.m-2.bar-1, respectively. The permeate samples were collected using a graduated cylinder and the permeate weights were measured by a digital balance (Scout Pro, Ohaus). Prior to each filtration experiment, the pure water hydraulic permeability was measured by dionised water. The Average values of permeability for Desal Dl and N30F membranes are (8.2 L/m2.h.bar. and 4.11 L.h-1.m- 2.bar-1) respectively.

Experimental protocol

The membranes fouled and subsequent rejection experimental protocol were conducted in three steps, including compaction, fouling development, and rejection measurement. First, the membrane was compacted using Milli-Q water at 5 bars for at least 2 h until a stable baseline flux was obtained.

Analysis: The arsenic will be analyzed using inductively coupled plasma-optical emission spectroscopy ICP OES method Argon, air and nitrogen were the used gases. The blank for the analysis was prepared by adding nitric acid to distilled water up to a HNO3 concentration of 2% v/v similarly before measurements, samples and standard.

The solutions were acidified with nitric acid in order to obtain a final solution containing HNO3 at 2% v/v. The wavelength for arsenic was 193.696 nm. The system was equipped with an auto sampler which automatically sent to the torch chamber the solution to be analysed. The deviation of each measurement was of 2% from the average value.

Results And Discussion

Influence of membrane fouling on permeate flux

The results of normalized flux of NF membranes as a function of time will be presented in the (Figure 5). The relative flux (Jv) is the flux at any time (t) during the fouling text divided by the initial flux (Jvo) equation:

image

Jv0: Volumetric flux of distiled water (l/m2h)

Jv: Volumetric flux of permeate (l/m2h)

membrane-science-technology-Relative-permeate

Figure 5: Relative permeate flux as a function of (a) time at accumulated of foulant on the membrane surface. Feed solution: 10 mM NaCl, 1 mM CaCl2, 1 mM NaHCO3, and 30 mgL-1 of each foulant, except CaSO4 was 1 gL-1.,Temperature=250°C.

It has been observed a decrease of the relative permeate flux versus time represented in Figure 5 for each foulant compounds for Two NF membranes, Specially the intensity of decrease for Desal DL is less sever compared to the N30F membrane ,it is explained by the difference membrane pore size and MWCO reported in literature (Table 1) [11]. The other reason is the difference of hydraulic permeability with higher values (8,2 Lm-2h-1bar) for Desal DL compared to N30F membrane with low permeability 4.1 l/m2h bar, it can be explained also that higher permeate flux introduce more foulant compounds which decrease permeate fluxes.

Membranes DESAL DL N30F
Manufacturers GE Nadir
MWCO 150-300 400
Permeability(L/m2hbar) 8.2 4.11
9 40-70 at 40 bar
Roughness (nm) 10 3
9 9
Max Temperature 90 95
Contact angle 44 8
46 82a
Radius of pore(nm) 0.52 0.74
pH range 1-11 0-14
Maximal temperature 90 95
Composition of top layer Polyamide Polyethersulfone

Table 1: Properties of the membranes used in this study

It was observed the effect of humic acid and sodium alginate higher decrease of relative permeates fluxes for two membranes.

The extent of diminution of relative permeates flux for DESAL DL is higher compared to the N30F, this due to the effect of roughness 10.9 nm for DESAL DL and 3.9 nm for the membrane N30F.

Rejection of As(III) by NF membranes

The rejection efficiency of arsenic species was determined using:

image

Where R is the rejection, CO(mole/L) and Cp (mole/L) are the solute concentrations in feed and permeate, respectively. The relation between the feed and permeate concentration was converted into the rejection efficiency.

The rejection efficiency of Arsenite was affected by pH (Figure 6). The values of arsenite rejection efficiency by N30F membranes are lower than those obtained with the membrane DESAL DL. The high rejection efficiency of As(III)will be obtained in the presence of humic acid for the two membranes. Exceptionally when the membrane N30F is fouled by CaSO4 in the range of pH=8 to pH=9, the arsenite must be rejected with higher values of 18% and 22% respectively with less difference of 0.5% compared to results obtained by using humic acid.

membrane-science-technology-rejection-by-clean

Figure 6: As (III) rejection by clean and fouled N30F and DESAL DL membranes as a function of solution pH. Feed solution: 10 mM NaCl, 1 mM CaCl2, 1 mM NaHCO3,30 mgL-1 of each foulant, except CaSO4 was1 gL1.,Temperature=250°6;C.

When the pH change from 6 to 11, the rejection efficiency of As (III) by the virgin DESAL DL membrane increased by 15%, where as it was approximately increase with 8% for the membranes fouled by sodium alginate and by colloidal silica (Figure 6). The difference of arsenite rejection efficiency by virgin and fouled membranes was apparent. The As(III) rejection efficiency of virgin membranes was considerably higher than that of the CaSO4, alginate and colloidal silica as foulant layers.

The lower arsenite rejection efficiency will be obtained in the presence of colloidal silica and CaSO4 as foulant layers. These compounds neutralize the charge of the membrane by their positive charges and increase the migration of arsenite ions with limitation of the electrostatic repulsion effect.

The Figure 7 shows the high conductivity rejection by virgin and the fouled DESAL DL membranes compared to the results obtained by N30F and consequently the size exclusion mechanism is predominant for the virgin and fouled DESAL DL membranes.

membrane-science-technology-Conductivity-rejection

Figure 7: Conductivity rejection for Virgin and fouled DESAL DL and N30F membranes.

One possible explanation for the lower As(III) rejection efficiency observed with the membranes fouled by sodium alginate, colloidal silica and CaSO4 under high pH conditions.

The effect of concentration polarization layer caused by fouling layers by cake formation has been extensively reported as a major cause of decrease in solute rejection efficiency by NF membranes in several researches [15,19-22].

Effectively, the considerable increase in As (III) concentration at the membrane surface coupled with the decline in permeate flux resulted in a significant decrease in As (III) rejectiosn efficiency by the fouled membranes as observed in Figure 6, this is straight relation between to the cake and concentration polarisation which reduce the rejection of arsenite under either low or high pH conditions.

The Steric exclusion mechanism is favored by the roughness of the membrane surface. In addition, the greatly negative charge of the humic layer resulted in a significant increase in As(III) rejection by charge repulsion mechanism under high pH conditions. Concentration polarization might occur and decrease As(III) rejection by the humic acid fouled DESALDL and N30F membranes. However, the decrease in As(III) rejection caused by this effect was probably compensated by the significant increase in As(III) rejection caused by the fouling and membrane surface charge increase It will be observed in the Figure 6 also that the arsenic removal by humic acid and CaSO4 will decrease from range pH situated between 6 to 7 and increase in the range situated between 7 to 11 ,this is probably due to the reactivity of arsenic species with humic acid and CaSO4 [11,12,21-36]. And interactions between the membrane and organic humic acid due to the some factors: hindrance exclusion and, blocking effect of the valleys of roughness surface and the reactivity of arsenic with humic acid. The effect of concentration polarisation on As (III) on rejection efficiency could also be observed in particularly of colloidal silica and CaSO4 scaling layer.

Conclusion

This research present an important contribution in the understanding of the rejection of neutral form of Arsenic species by virgin and fouled nanofiltration membrane ,it has been found that the efficiency of the remove of As(III) by the NF membrane is generally governed by size exclusion and adsorption to the membrane surface and pore structure. Under both fouled and clean membrane conditions, higher rejection rates of Arsenic were achieved with the tight nanofiltration DESAL DL membrane.

Although, it will be concluded that the fouling had a negligible effect on the rejection of Arsenite with the N30F membrane which has a larger pore size with smooth surface. The enhancement in the rejection of As(III) by fouled DESAL DL membrane was attributed to pore blocking.

References

Citation: Chabane M, Dahmani B (2016) Effect of Foulant Layers on the Rejection of Arsenite [As (III)] by Nanofiltration Membranes. J Membra Sci Technol 6:155.

Copyright: © 2016 Chabane M et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.