application of nanofiltration in the recovery of chromium (iii) from tannery effluents

Separation and Purification Technology 44 (2005) 45–52

Application of nanofiltration in the recovery of chromium (III) fromtannery effluents

Lina M. Ortegaa, Remi Lebrunb,∗, Isabelle M. Noelc, Robert Hauslera

a Departement de Chimie, Université du Quebeca Montreal, C.P. 8888 Succ. Centre-Ville, Montréal, Que., Canada H3C 3P8b Departement de Génie Chimique,Ecole d’ingenierie, Université du Quebeca Trois-Rivieres, C.P. 500,

Trois-Rivieres Que., Canada G9A 5H7c Departement de Génie Civil, Universite Laval, Ste-Foy Que., Canada G1K 7P4

Received 3 June 2003; received in revised form 10 November 2004; accepted 10 December 2004

Abstract

The aim of this study was to investigate, at laboratory scale, the behaviour of four different nanofiltration membranes (DS 5, CA, DS 5* andBQ 01) and their efficiency for chromium (III) removal. Principally, a solution of chromium basic sulphate Cr(OH)SO4 (CBS) (33% basicity),utilized in tanning processes, was tested with the different membranes in order to recuperate and concentrate the chromium (III) present inaNtt©

K

1

titiwpt

aamc9

1d

queous solutions. Several different transmembrane pressures, flow rates and concentrations were evaluated. In addition, the permeation ofaCl and Cr2(SO4)3·5H2O salts were studied. The results demonstrated acceptable chromium (III) retentions depending on the membrane

ype, the concentrations used (15.2 and 30.3 mol m−3) and the operating conditions. The four membranes exhibited different behaviours dueo their charge during permeation. The results obtained in this study were analyzed according to the dynamic permeability model.

2004 Elsevier B.V. All rights reserved.

eywords:Nanofiltration; Chromium basic sulphate; Water permeability; Dynamic permeability; Tannery

. Introduction

The leather industry is a well-known source of water con-amination. Large quantities of toxic effluents (organic andnorganic pollutants) are discharged, predominantly from theanning processes. Petruzzelli et al. reported that approx-mately 50% of the chemicals in these processes becomeastewater or sludge [1]. The primary consequence of theserocesses is water and soil contamination, potentially leadingo health problems.

Several different methods for tanning leather exist suchs vegetable, oil, aldehyde, synthetic and organic tanningnd also different mineral tanning using zirconium (IV), alu-inium (III), titanium (IV) salts and metal salts (especially

hromium basic sulphate [Cr(OH)SO4] (CBS)) [2]. However,0% of tanneries in the world are using chromium salts to pro-

∗ Corresponding author. Tel.: +1 819 736 5011; fax: +1 819 736 5011.E-mail address:remi [email protected] (R. Lebrun).

duce leather [2] given that it provides better leather flexibility,water resistance and prevents putrefaction, properties that areall important for good leather quality.

Regardless, this procedure creates a significant pollutionproblem due to the large amounts of Cr (III) discharged.Approximately 40% of the chromium, used in tanning pro-cesses, remains in the solid and liquid wastes [3]. For ex-ample, the leather industry in Taiwan discharges about 332metric tons of chromium salts per annum into the environ-ment [4].

In order to reduce contamination, recuperate and con-centrate the chemicals and improve the quality of waterin this industry, the following processes are used: mem-brane processes, chemical precipitation, adsorption, redox-adsorption and ion-exchange [5]. Membrane technology, es-pecially nanofiltration (NF) is considered a viable option forwater treatment because of its efficiency for chromium re-moval. It can operate at low pressures and can provide agood permeation flow rate [6,7]. It can also recover, entirely

383-5866/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
oi:10.1016/j.seppur.2004.12.002

46 L.M. Ortega et al. / Separation and Purification Technology 44 (2005) 45–52

or partially, most of the salts present (mono-, di- and trivalentforms) and has the potential to concentrate them.

The objective of this study was to evaluate the possibilityof obtaining a high separation factor of Cr (III) and other ionson the system using four types of nanofiltration membranes(DS 5, CA, DS 5* and BQ 01) and to investigate the per-formance and behaviour of each membrane operating underseveral conditions.

2. Theory

Nanofiltration is a pressure-driven membrane processcharacterized by the separation of ions in the range of200–2000 Da. The retention by nanofiltration membranes isdue to two factors: a preferential sorption at the membranesurface and a kinetic effect resulting in a differential velocitybetween solute and solvent trough the membrane pore [8].The performance of NF membranes, in terms of pure waterpermeability, dynamic permeability and separation factors,are calculated as shown below.

2.1. Intrinsic pure water permeability (Aipw)

Aipw is calculated based on the viscosity (µ), water perme-ate flux (J ) and transmembrane pressure (�P). A corre-sbs

A

J

wtt

2s

wX

J

�

�

wtsb

When the circulation velocity and/or the turbulence dimin-ished the concentration polarization influence, thenXA2 tendstowards XA1 (molar fraction of the feed and in the reservoir)and consequently �Pe = �Pa. Thus

�Pa = �P − Π(XA1) + Π(XA3) (6)

�Pa = �Pe + Π(XA2) − Π(XA1) (7)

where �Pa is the apparent gradient pressure.Osmotic pressure for diluted solutions is calculated ac-

cording to Van’t Hoff’s equation:

Π =∑

iRTC (8)

where∑i is the number of ions per molecule of solute,R the

universal gas constant, T the absolute temperature and C theconcentration of the inorganic solution (mol m−3).

2.3. Global separation factor (f)

The global separation factor of the solute is defined by thefollowing expression:

f = XA1 − XA3

XA1= 1 − XA3

XA1(9)

2.4. Intrinsic separation factor (f′)

f

Il

3

3

rowQlp

3

Ccb9(p

p ipwponds to the slope of the straight line which origin is passingy (0, 0). Aipw is an intrinsic characteristic of the membranetructure.Aipw is calculated as follows:

ipw = µJp

�P(1)

p = Qp

Sm(2)

here Qp is the permeate flow rate and Sm the surface ofhe membrane. The permeate flux value is affected by theemperature of water in the tank.

.2. Intrinsic dynamic permeability of inorganicolutions (Aid)

The intrinsic dynamic permeability of inorganic solutionsas investigated according to a study reported by Lebrun andu [9]:

p = Aid�Pe

µ(3)

Pe = �P − �Π (4)

Π = Π(XA2) − Π(XA3) (5)

here Aid is the intrinsic dynamic permeability of the solu-ion, �Pe the effective pressure gradient, Π the osmotic pres-ure, XA2 and XA3 are the molar fractions of the concentratedoundary solution and the permeate solution, respectively.

The intrinsic separation factor is calculated by

′ = XA2 − XA3

XA2= 1 − XA3

XA2(10)

n this study, the concentration polarization is considered neg-igible, thus XA2 ≈XA1, subsequently f′ = f.

. Materials and methods

.1. Chemicals

The salts used in these experiments were NaCl (99% of pu-ity, EM Science, Gibbstown, NJ), Cr2(SO4)3·5H2O (21.6%f chromium and 57.9% of sulphate, Aldrich Chemicals, Mil-aukee, WI) and Cr(OH)SO4 (33% of basicity, supplied byuimica Central de Mexico, Mexico, DF). These salts, in so-

ution, displayed a pH between 2.8–3.5. All solutions wererepared with demineralized water, at room temperature.

.2. Quantitative analysis

The chemical analysis of chromium salts,r2(SO4)3·5H2O and Cr(OH)SO4), and sodium, NaCl,oncentrations from the feed and permeate were determinedy flame atomic absorption spectroscopy (AAS) (ARL GBC06 AA, EquiLab, New Castle, DE), conductivity metermodel CDM 81, Radiometer, Copenhagen, Germany),H meter (Fisher Acumet model 915, Pittsburgh, PA),

L.M. Ortega et al. / Separation and Purification Technology 44 (2005) 45–52 47

Fig. 1. Concentration vs. conductivity of the industrial-grade tanning saltCr(OH)SO4 (mol m−3).

UV spectrophotometer (Hewlett-Packard 8452 A, PaloAlto, CA) and inductively coupled plasma atomic emissionspectrometry (ICP-AES), Varian model (Varian Canada,Inc., Mississauga, Ont.). Quality controls were performedwith certified liquid samples (multi-elements standard,catalogue number 900-Q30-002, lot number SC0019251,SCP Science, Lasalle, Quebec) to ensure the conformity ofthe measurement apparatus.

The samples were analyzed in duplicate. Fig. 1 illus-trates the relationship between conductivity and concentra-tion for Cr(OH)SO4 solution (industrial-grade tanning salt).The Cr(OH)SO4 concentration was measured indirectly us-ing conductivity measurements. The analysis, for a 0.2 g/l(1.21 mol m−3) solution of Cr(OH)SO4, is summarized inTable 1.

3.3. Membranes

The nanofiltration membranes, used during the experi-ments, were thin-film composite polymeric membranes. Thefour membrane coupons were: DS 5 (polypiperazine amideon sulphonated polysulphone) with an isoelectric point of 4[10], positively charged membrane according to the pH ofthe experiments, DS 5* (DS 5 with a surface modificationby sulphuric acid) a slightly negatively charged membrane,

TDt

I

ABBCCCFMNPSS

Fig. 2. Schematic diagram of the experimental set up. 1, feed tank; 2, pump;3, manometer; 4, membrane cell; 5, permeate outlet; 6, three-way valve; 7,pressure valve; 8, flowmeter.

BQ 01 (sulphonated polyphenylene oxide (SPPO) on poly-sulphone) negatively charged membrane [11], all suppliedby Osmonic’s-Desal Corporation (Minnetonka, MN) and CA(cellulose acetate) a slightly negatively charged membranemanufactured in-house [8].

3.4. Equipment

The experiments were carried out at laboratory scale, in abatch mode with recirculation of solutions, as shown in Fig. 2.The experimental set-up contained four cells in series [6–9],where each cell used a different type of coupon. Therefore,the four membranes (DS 5, CA, DS 5* and BQ 01) weretested and characterized at the same time and conditions. Inthe set-up, the solutions were pumped from the feed tanktowards the membrane cells, obtaining a retentate that wasreturned to the feed tank and a permeate that was collectedin four different beakers for analysis. This permeate was notrecirculated into the system, therefore the feed concentrationincreased with time. The effective working area of the mem-brane was 1.26 × 10−3 m2. The maximum feed volume in thetank was 10 l for each experiment.

The membranes were characterized calculating the waterpermeation rate and the solute separation. This was carriedout according to the mass, permeation time and salt con-cec

3

(oC

w2T

able 1ata for 0.2 g/l (1.21 mol m−3) solution of Cr(OH)SO4 (industrial-grade

anning salt)

ons Concentration (g/l)

s 4.74 × 10−6

6.14 × 10−5

a 1.74 × 10−6

a 9.52 × 10−5

r 3.52 × 10−2

u 4.32 × 10−6

e 1.62 × 10−4

g 1.67 × 10−5

a 1.79 × 10−2

b 7.88 × 10−5

3.60 × 10−2

e 2.74 × 10−3

entrations under several operating conditions. At the end ofach experiment, the system was cleaned with pure water toomplete the characterization of each membrane.

.5. Methodology

The experiments were performed using deionized waterpH 5.2 and conductivity 1.23 �S/cm) and three types of in-rganic electrolytes: (1) Cr2(SO4)3·5H2O, (2) NaCl and (3)r(OH)SO4.

The applied pressure used for all experimentsere 4.14 × 105, 6.89 × 105, 1.10 × 106, 1.79 × 106,.76 × 106 Pa. The other operating conditions are shown inable 2.


Table 2Values of concentration (C) and feed flow rate (Qc)

Experiment C (mol m−3) Qc (m3 s−1)

1. Pure water 1.01 × 10−14

2. Cr2(SO4)3·5H2O 0.42 0.76 × 10−14

3. Pure water 1.01 × 10−14

4. NaCl 5.98 1.01 × 10−14

5. Pure water 1.01 × 10−14

6. Cr(OH)SO4 15.2 1.01 × 10−14

7. Pure water 1.01 × 10−14

8. Cr(OH)SO4 30.3 1.01 × 10−14

9. Pure water 1.01 × 10−14

Before starting the experiments, the compaction of themembranes was carried out (Fig. 3) in order to stabilizethe pores and the structure of the membranes [8]. Thisconsisted of pumping deionized water (at room tempera-ture), through the membrane coupons, using high pressure(2.76 × 106 Pa), until the pure water permeability (Aipw) wasconstant. The duration of the compaction was 3 h and mon-itored every 10 min. The feed flow rate was kept constant atQe = 1.01 × 10−4 m3 s−1.

The first step, after compaction, was the characterizationof each membrane that consisted in calculating the intrinsicpure water permeability (Aipw). The membranes were char-acterized at five different pressures, as previously mentioned.

Following this characterization was the measurementof the dynamic permeability of each coupon withCr2(SO4)3·5H2O salt solution. This solution served as a ref-erence and a basis for the comparison with Cr(OH)SO4 so-lutions (industrial powder from Mexico). Pure water perme-ability was measured before and after the measurement of thedynamic permeability of each inorganic solution to evaluatemembrane integrity in terms of permeability.

F1

4. Results and discussion

Table 3 presents the values of pure water permeability anddynamic permeability for each experiment and membranetype. It is important to note that the permeability data pre-sented in this table corresponds to the slopes from the graphof Jp versus �Pa (Figs. 3 and 4). Note that �Pa = �P forpure water permeation.

4.1. Pressure treatment

As shown in Fig. 3, the permeate flux decreased with timedue to mechanical stabilization of the membrane. The pres-sure treatment was strongly dependent on the nature of themembrane; therefore the duration of the densification wasdifferent for each membrane. However, for all membranes,the stabilization in permeability was achieved approximatelyafter 2 h of treatment. The highest permeability was obtainedfor BQ 01 and the lowest permeability for CA. The com-paction effect was more pronounced for BQ 01 (40% lost inpermeability), followed by DS 5* and DS 5 (approximately30% decrease in permeability), while only 15% was observedfor CA membrane.

4.2. Intrinsic pure water permeability

mpdmBvp

mf8apir

4

sta

atastm

ig. 3. Permeate flux vs. time (s) during pre-compaction �Pa = 2.76 ×06 Pa.

Fig. 4 illustrates the permeate flux versus apparent trans-embrane pressure with pure water. For all membranes, the

ermeate flux increased linearly with applied pressure, un-er the same operating conditions. The permeability of theembranes decreased according to the following sequence:Q 01 > DS 5* > DS 5 > CA (Table 3, Experiment 1). Theariations in water permeability for all experiments are re-orted in Table 3.

Comparing initial and final water permeability (Experi-ents 1 and 9), the loss in permeability was more pronounced

or BQ 01, which resulted in a reduction of approximately4%, followed by DS 5* with 60%. This decrease in perme-bility suggests that the membranes underwent an irreversiblehenomenon of fouling. On the other hand, the permeabil-ty’s for DS 5 and CA were approximately of 18 and 9%,espectively.

.3. Intrinsic dynamic permeability of Cr2(SO4)3·5H2O

Cr2(SO4)3·5H2O solutions were used to obtain the intrin-ic dynamic permeability (Aid) and global salt separation fac-or (f) according to the conductivity of the solution in the tanknd in the permeate.

Fig. 5 illustrates a linear increase of the permeate fluxccording to the pressure. This linear change, observed inhis experiment, is due to the high recirculation flow ratepplied. In this particular case, it is assumed that the intrin-ic separation factor was equal to the global separation fac-or (f′ = f). The intrinsic dynamic permeability (Aid) of the

embranes (calculated using Eq. (2)) decreased in the fol-


Table 3Evolution of the membrane’s permeability after the compaction with water and inorganic solutions

Experiment DS 5 CA DS 5* BQ 01

Aipw (m) Aid (m) Aipw (m) Aid (m) Aipw (m) Aid (m) Aipw (m) Aid (m)

1. Pure water 2.12 0.88 3.41 3.822. Cr2(SO4)3·5H2O 2.24 1.01 2.60 1.633. Pure water 2.14 0.84 2.40 1.214. NaCl 1.74 0.77 1.31 0.675. Pure water 1.73 0.77 1.43 0.656. Cr(OH)SO4 1.74 0.86 1.30 0.607. Pure water 1.61 0.72 1.23 0.568. Cr(OH)SO4 1.57 0.80 1.16 0.559. Pure water 1.74 0.80 1.34 0.61

The values of the intrinsic water permeability (Aipw) and dynamic permeability (Aid) are given by a factor of 10−14.

lowing order: DS 5*, DS 5, BQ 01 and CA (Experiment 2,Table 3).

Comparing the values of the initial pure water perme-ability with the dynamic permeability (Experiments 1 and2, Table 3), both CA and DS 5 membranes had a slight in-crease. This increase was reversible due to the pure waterpermeability measured after this experiment was recovered(Experiment 3 in Table 3). On the other hand, DS 5* andBQ 01 presented a decrease in pure water permeability of 29and 68%, respectively (Experiments 1 and 3, Table 3). Forboth membranes, the decrease observed in dynamic and purewater permeabilities were irreversible (Experiments 2 and3, Table 3). It is assumed that these membranes underwentpreferential sorption of chromium that is irreversible, in otherwords, these membranes are “chromated”. This decrease inthe Aipw value (Experiments 1 and 3, Table 3) presented bythe DS 5* and BQ 01 membranes affected not only the poresstructure (reduction of pore size) but also the nature of thepores surface.

Ft2

4.4. Experiments with reference solute: NaCl

Experiments with NaCl solutions were carried out to char-acterize the “new” membranes after the use of the chromiumsolution (Cr2(SO4)3·5H2O).

The variations in dynamic permeability with NaCl solu-tions are summarized in Table 3 (Experiment 4). The perme-ability ranged from 1.74 × 10−14 for DS 5 membrane, whichhad the highest dynamic permeability, to 0.67 × 10−14 forBQ 01 (Table 3). The separation factors for the sodium chlo-ride solution (tank and permeate) were obtained as a functionof their conductivity.

As illustrated in Fig. 6, for all membranes, the global sep-aration factors increased with the augmentation of the per-meate flux (Jp) through the membranes. In particular, themembrane DS 5 presented the greatest flux, the highest sep-aration factor and the highest permeability. On the other hand,the CA membrane exhibited one of the lowest flux and sep-aration factors. The separation of ions for this membrane

FCQ

ig. 4. Permeate flux vs. apparent transmembrane pressure for pure wa-er (initial pure water permeability) Qc = 1.01 × 10−4 m3 s−1 (1.6 GPM);4 ◦C <T< 27 ◦C, pH 5.2; conductivity = 1.021 �S/cm.

ig. 5. Permeate flux vs. apparent transmembrane pressure forr2(SO4)3·5H2O, initial concentration = 4.2 × 10−1 mol m−3 (0.2 g/l);

c = 7.6 × 10−5 m3 s−1 (1.6 GPM); 24 ◦C <T< 29 ◦C.


Fig. 6. Global separation factor of NaCl vs. flow rate though the sur-face of the membrane. Qc = 1.01 × 10−4 m3 s−1 (1.6 GPM); initial concen-tration of NaCl = 5.98 mol m−3; �Pa = 4.14 × 105, 6.89 × 105, 1.10 × 106,1.79 × 106, 2.76 × 106 Pa; 24 ◦C <T< 29 ◦C.

was assumed to be mainly depended on the sieve mecha-nism. The relative low retention presented by CA membranecan be attributed to the ratio between the pore and solutesize.

In the case of the close retentions presented by DS 5* andBQ 01 (Fig. 6), this behaviour is attributed to the pore sizeand the properties of the membrane surface. Those two mem-branes were negatively charged before the use of Cr(OH)SO4salt solution (industrial grade tanning salt). After the use ofthis salt (Cr(OH)SO4), the two membranes were chromated.The pore size diminished and the links between Cr com-plex and the polymer charge screened the membrane surfacecharge. These effects of the chromation can explain the re-sults with NaCl solution.

4.5. Effect of the concentration of Cr(OH)SO4 salt onthe dynamic permeability

Dynamic permeability values for the membranes are citedin Table 3 (Experiments 6 and 8). In all cases, a decreasein dynamic permeability was obtained as a result of an in-crease in concentration, by a factor of 2. The decrease of Aidranged from 10%, for the DS 5 membrane, to 7%, for theCA membrane. This behaviour was expected since the dy-namic permeability value of a membrane decreases when thec

macs4o

Fig. 7. Permeate flux vs. separation factor of Cr3+.Qc = 1.01 × 10−4 m3 s−1

(1.6 GPM), initial concentration of Cr(OH)SO4 = 30.30 mol m−3 (5 g/l);�Pa = 4.14 × 105, 6.89 × 105, 1.10 × 106, 1.79 × 106, 2.76 × 106 Pa;24 ◦C <T< 29 ◦C; pH (tank) = 3.

4.5.1. Separation factor of Cr (III)High separation factors for Cr (III) measured by ICP-AES

were obtained for the DS 5 membrane varying from 74.78to 93.57% (Experiment 8) and 61.56 to 91.52% (Experiment9, Fig. 7). Fig. 7 illustrates the Cr3+ retentions for the fourmembranes, using the highest concentration of Cr (III) saltsand four different pressures. In general, the Cr (III) reten-tion increased with an increase in pressure and in flux in themembranes.

As mentioned before, DS 5 displayed the highest sepa-ration factor combined with a high dynamic permeability(1.57 × 10−14) (Experiment 8, Table 3), even at the high con-centration used (30.3 mol m−3).

The high retention of DS 5 can be explained by the chargeof this membrane. In this case, the cation (Cr3+) is rejectedby the membrane charge.

Another factor that contributed to the positive chargeof the membrane was the pH of the solution. In thiscase when the pH > Ip, the membrane was negativelycharged, instead when pH < Ip the membrane was positivelycharged.

The lowest retention was obtained for the CA membrane,characterized as a slightly negative charge [11]. Similar reten-tions of Cr (III) were observed for BQ 01 and DS 5*. Theseretentions were influenced, not only by the charge effect ofthe membrane, but also by the sieving mechanism. Clearly,tt

4

cb(

oncentration increases [9].This change was reversible as pure water permeability was

aintained in both concentrations (Table 3, Experiments 7nd 9). In addition, it is important to note that after the oc-urrence of the first “chromation” with the Cr2(SO4)3·5H2Oolution (Experiment 2) and the NaCl solution (Experiment), no significant variations in pure water permeability wasbserved.

he membrane retention varies according to the membraneype.

.5.2. Effect of pH in the Cr (III) separation factorThe differences in separation factors, among the coupons,

an be attributed to differences in the charge of the mem-rane surfaces, due to the pH of the solution. Generally, CrIII) solutions exhibit a pH range from 2 to 5 [2]. The sepa-


ration factors of the membranes are affected at low pH. Forall experiments, pH of 2.8–3.0 were obtained for the con-centrates while pH of 3.2–3.5 were observed for the perme-ate.

As aforementioned, the pH of the concentrate affected thecharge of the membranes. Fig. 7 illustrates that the highestseparation factor of Cr3+ was obtained for DS 5. This mem-brane is characterized by an isoelectric point (Ip) of 4 [10].This signifies that the membrane will be positively chargedat pH under this value. When this membrane is positivelycharged, the separation factor increases, leading to a repulsionof Cr3+ (co-ion) induced by charge effects. The behaviour ofthe other membranes can be explained using the same princi-ples. The CA, DS 5* and BQ 01 membranes are character-ized to be neutral and/or slightly negatively charged, causingthe effect of the charge repulsions to be less predominant thanthe DS 5 membrane (positively charged). Other factors caninfluence the retention of the membranes such as the poresize distribution, however, further investigation is required.

The results from this research demonstrate that the mem-branes are efficient at separating chromium from effluents,especially tannery effluents [12,13].

4.6. Separation of B3+, Na+ , SO42− and Se2+ ions fromCr(OH)SO4 solution

ssdl

TS

M

DCDB

DCDB

DCDB

DCDB

DCDB

it was observed that the separation factor increased with ap-plied pressure [14].

The highest separation factor was obtained for the DS 5membrane. The CA and DS 5* membranes exhibited thesame behaviour as DS 5. The BQ 01 presented the lowestseparation factor, especially for Na+ ions.

The separation factors for all membranes are mainly af-fected by the size of the ions and less affected by thecharge of the membrane. Monovalent ions, such as Na+,permeate through membrane while divalent ions are re-tained.

The rejection mechanism depends on the type of valenceof the electrolytes. In this case, higher retentions are ob-tained for multivalent ions (B3+, Se2+ and SO4

2− ions) and amoderate retention for monovalent ions (Na+). Exceptionallythe membrane DS 5, followed the ion retention sequence ofSe2+ > SO4

2− > Na+ > B3+.

5. Membranes and the leather industry

The use of membrane technologies applied to the leatherindustry represents an economical benefit, especially inthe recuperation of chromium from leather tanning efflu-ents. Similar to conventional methods, the precipitation ofchromium salts with NaOH have demonstrated that the qual-ioOhnadsdpdpcTnml

6

tuawfnti

The influence of other ions, in the chromium basic sulphateolution from Mexico (Cr(OH)SO4) was studied. Table 4hows the retention of B3+, Na+, SO4

2− and Se2+ ions atifferent pressures. The concentration utilized for the salt so-ution was 30.30 mol m−3 (5 g/l). As expected, in most cases,

able 4eparation factors (%) of B3+, Na+, SO4

2− and Se2+ ions

embranes �Pa (Pa) Separation factor (%)

B3+ Na+ SO4−2 Se2+

S 5 4.14 × 105 78.1 88.4 93.5 97.9A 76.0 64.0 79.9 91.5S 5* 78.9 62.9 81.2 93.6Q 01 70.4 53.2 62.2 68.7

S 5 6.89 × 105 81.5 92.1 95.1 98.3A 77.9 63.9 78.4 88.5S 5* 78.5 60.7 75.9 86.6Q 01 71.6 56.3 64.6 71.1

S 5 1.10 × 106 91.3 94.8 97.1 99.3A 89.5 72.8 83.3 91.2S 5* 91.1 72.4 85.0 93.4Q 01 87.9 65.9 73.9 78.7

S 5 1.79 × 106 94.2 95.0 97.3 99.4A 93.2 77.3 87.8 94.1S 5* 93.2 74.0 86.5 94.5Q 01 90.5 67.8 76.6 81.5

S 5 2.76 × 106 94.7 96.0 97.9 99.5A 92.9 79.9 88.8 93.7S 5* 94.5 78.9 89.1 95.2Q 01 91.5 72.2 80.2 84.0

ty of the recovered waters is not optimal for the presencef metals, lipidic substances and different impurities [15].n the other hand, membrane technologies have achievedigh recovery yields of chromium and low concentrations ofitrogen, oils and suspended solids [3,12]. Membrane oper-tions, integrated into the tanning process, are capable of re-ucing metals in the wastewater, as well as organic species,uspended particles and oils [3]. The experimental resultsemonstrated that it is feasible to recuperate Cr (III), de-ending on the membrane type, as well as reusing the wateruring the process. Laboratory scale and industrial pilot ex-eriments have demonstrated the economic advantages of re-overing chromium using membrane processes [13,15–17].hese technologies require the use of adequate membranes,ot only to separate chromium, but also to separate otheraterials present in the transformation of animal hides into

eather.

. Conclusions

Nanofiltration membranes have been characterized inerms of permeability and retention of several ions, partic-larly Cr (III). The results illustrate variations in dynamicnd pure water permeability. A “chromation” of the couponsas observed mainly for the BQ 01 and DS 5* membranes,

ollowed by a loss in permeability. This irreversible phe-omenon created “new” membranes with different charac-eristics. After this chromation, the membrane permeabil-ty appeared to be almost stable. However, in all cases, the


salt separation increased with an increase in pressure andinflux.

The membranes retained multivalent ions present in solu-tions such as Cr(OH)(SO4) and Cr2(SO4)3·5H2O rather thanunivalent ions such as NaCl, depending on the type of themembrane.

A great removal of trivalent ions, such as chromium (III),was observed for the membrane DS 5, depending on the con-centration used. In addition, the pH of the solution played animportant role in the retention of Cr (III), according to theisoelectric point (Ip) and the charge of each membrane. Themost efficient membrane tested with chromium solution (pHbetween 2.8 and 3.5) was DS 5, which displayed the highestseparation factor (99%) combined with a high permeability.The lowest retention obtained, especially for Cr (III), waswith the CA membrane.

On the other hand, the DS 5* and BQ 01 membranes pre-sented a loss in permeability that was followed by the foulingof the membrane.

Despite the high chromium basic sulphate concentration(30.30 mol m−3), the “new” membranes offered a good per-meability that were preserved for some membranes (DS 5and CA). This study demonstrates the feasibility of usingnanofiltration processes for the removal of chromium presentin tannery effluents.

LAACf

JMPQQSTX

Greek letters� gradientµ fluid viscosity (Pa s)Π osmotic pressure (Pa)∑i number of ions per molecule of solute

Acknowledgements

The authors would like to thank STEPPE-UQAM (Sta-tion experimentale des proc´edes pilotes en environnement),INRS-Ete (Institut National de la recherche Scientifique) fortheir financial support and the HAYKA Industry for the ex-perimental set-up. In addition, we would like to thank to Mrs.Myriam Chartier (INRS) for her technical assistance.

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application of nanofiltration in the recovery of chromium (iii) from tannery effluents

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