The influence of pH and BSA on the retention of selected heavy metals in the nanofiltration process using ceramic membrane

The influence of pH and BSA on the retention of selected heavy metals in the nanofiltration process using ceramic membrane

Desalination 369 (2015) 62–67 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal The influence o...

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Desalination 369 (2015) 62–67

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

The influence of pH and BSA on the retention of selected heavy metals in the nanofiltration process using ceramic membrane Arkadiusz Nędzarek ⁎, Arkadiusz Drost, Filip Bronisław Harasimiuk, Agnieszka Tórz Department of Aquatic Sozology, West Pomeranian University of Technology in Szczecin, Kazimierza Królewicza Street 4, 71-550 Szczecin, Poland

H I G H L I G H T S • • • •

The effect of BSA and the pH of the complexing retention of heavy metals Membrane separation Elimination of heavy metals is membrane separation. Recovering metals from wastewater

a r t i c l e

i n f o

Article history: Received 10 November 2014 Received in revised form 16 April 2015 Accepted 18 April 2015 Available online xxxx Keywords: Ceramic membranes Nanofiltration Heavy metals BSA

a b s t r a c t One of the alternative wastewater treatment methods aimed at the elimination of heavy metals is membrane separation supported by the use of metal-complexing polymers. Many sectors of industry generate wastewater containing both heavy metals and protein substances which show complex-forming properties. In this study, a 450 Da molecular weight cut-off nanofiltration ceramic membrane (TAMI Industries, France) was tested to evaluate the retention of Zn, Cd, Pb, Cu and Fe complexed with bovine serum albumin (BSA). Retention levels were tested in water solutions solely for the metals and for the metals with the addition of BSA, depending on the pH of the solutions (from 2.0 to 9.0). It was noted that the retention tended to increase as the pH increased. For solutions composed with only metals, retentions exceeding 80% were noted for pH N 6.9. The addition of a complexing agent resulted in high retentions of Cu, Cd and Zn already at pH = 2.0. Maximum retentions for Zn, Cd, Pb, Cu and Fe were noted for solutions containing the mixture of metals and BSA at pH = 6.0 (97, 96, 90, 95 and 71%, respectively) and at pH = 9.0 (93, 99, 93, 99 and 85%, respectively). The studies confirmed the high metal-complexing capacity of BSA and indicated that heavy metals might be effectively removed from wastewater containing protein in the process of membrane separation using a ceramic membrane. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The most frequently used methods of removing and/or recovering metals from wastewater include: chemical precipitation, extraction, adsorption, ionic exchange, nanofiltration and reverse osmosis. Each type of the abovementioned physicochemical treatment has its own advantages (Table 1). The efficacy of these methods varies [16,27]. Highly efficient is chemical precipitation; according to Brbooti et al. [9], it can be higher than 99%. In the study reported by Chen et al. [11], the efficiency of precipitation of Cu, Zn, Cr and Pb with the help of CaO varied in the range 99.37–99.6. Also removal of heavy metals by adsorption is highly efficient, although its efficiency depends on the adsorbents used. For instance, Moayyeri et al. [24] have proved that adsorption of lead (84.72%) by hydroxy-apatite microparticles is greater than that of cadmium ⁎ Corresponding author. E-mail address: [email protected] (A. Nędzarek).

http://dx.doi.org/10.1016/j.desal.2015.04.019 0011-9164/© 2015 Elsevier B.V. All rights reserved.

(49.89%), zinc (72.90%), iron (74.50%) and nickel (79.25%). Aziz et al. [4] have shown 57.7% removal of Zn with the use of lignite fly ash as an adsorbent. Another highly efficient procedure is based on ionexchange, e.g. Al-Enezi et al., [1] proving 99% removal of heavy metal by extraction from wastewater and precipitates. Especially good effects (removal exceeding 90%) are obtained through the processes combining metal complexing and filtration. Potential complexing agents include natural and synthetic polymers, e.g. chitosan, polyethylene glycol, and polyacrylic acid. As regards the filtration membranes, polymer membranes have been studied most extensively [17,29,31]. Also the cost of each metal elimination process differs. The cheapest methods are those that use the chemical process of precipitation, as the cost of precipitation agent is the lowest. The cost of metal precipitation together with the production of sulphide and hydroxide precipitants was estimated by Peters and Ku (1985) [26] at US$9.27/1000 gal and US$9.45/ 1000 gal, respectively. It is also the price of absorbents that makes the total cost relatively low. For example activated carbon costs, depending

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Table 1 The main advantages and disadvantages of different physicochemical methods for treatment of heavy metal in wastewater (after Barakat [5]). Treatment method

Advantages

Disadvantages

Chemical precipitation Adsorption with new adsorbents

Low capital cost, simple operation Low-cost, easy operating conditions, having wide pH range, high metal-binding capacities Small space requirement, low pressure, high separation selectivity High separation selectivity

Sludge generation, extra operational cost for sludge disposal Low selectivity, production of waste products

Membrane filtration Electrodialysis Photocatalysis

Removal of metals and organic pollutant simultaneously, less harmful by-products

on its quality, costs US$9/kg, zeolite is sold at US$0.03–0.12/kg and montmorillonite is sold at US$0.04–0.12/kg (Kurniawan et al. 2006) [20]. When it comes to membranes, however, even though its price is high, the usage turns to be economically justified. For example Drouiche at al. (2001) [14] proved that purifying the surface water with the usage of ceramic membranes in ultrafiltration process can cost US$ 0.235/m3, while Gluckstern and Priel (1998) [18] estimated the cost of desalting with the usage of ultrafiltration for US$0.50/m3. Many branches of industry, especially food industry, generate wastewater that apart from metals contains also protein substances showing complexing capacities in a wide range of pH values [21,23]. In light of the above, the presented studies evaluated the degrees of retention of Zn, Cu, Pb, Cd and Fe using a ceramic membrane with a 450 Da cut-off. As a protein complexing agent the bovine serum albumin (BSA) was used. This protein has the tendency to aggregate in the macromolecular assemblies and demonstrates high affinity with, inter alia, metals which it forms covalent adducts with. In order to compare the effects of complexing those metals with protein, nanofiltration was conducted for model water solutions of the metals and water solutions of the metals with the BSA. 2. Material and methods Nanofiltration tests were conducted using a quarter-technical scale filtration system (Fig. 1). The experiment was conducted using 19channel ceramic nanofiltration membranes (Inopor, Germany), cut-off

High operational cost due to membrane fouling High operational cost due to membrane fouling and energy consumption Long duration time, limited applications

450 Da, made of Al2O3/TiO2, 1178 mm long, with the external diameter of 25 mm, channel diameter of 3.5 mm, and the filtration area of 0.25 m2. Point of Zero Charge (PZC) of the membrane used in the NF process was 6.9, while the permeate recovery was close to 80%. Transmembrane pressure (TMP) was 0.4 MPa, and the crossflow velocity (CFV) was 4 m/s. The process temperature was fixed and amounted to 293 ± 1.0 K. The system was operated in a continuous mode. Thus, both permeate and retentate were driven to the feed reservoir to keep constant the concentration along the experiments and simulate a continuous filtration process. Four variants of nanofiltration experiments were conducted (Table 2). Working solutions to be subjected to nanofiltration were prepared from standard components, purchased from Merck (Germany), and used in the concentration of 1 g/dm3. The agent complexing metal ion was bovine serum albumin (BSA) with a molecular mass of 66 kDa, was purchased from Sigma-Aldrich. Retention levels of the metals were measured for the following pH values: 2.0, 4.6, 6.0, 6.9, and 9.0. The pH values were controlled using 0.1 M NaOH and 0.1 M HCl and measured with an AD12 pH tester (ADWA Instruments, Hungary). In the feeds and permeates obtained at the respective pH values, heavy metals were traced by a cathodic stripping voltammetry method (CSV), using the 797 VA Computrace system (Metrohm, Switzerland). Samples for volumetric measurements were prepared according to the methodology recommended by APHA [3]. The heavy metal retention R was calculated as usual: R¼

  C 1− P ⋅100½%: CF

ð1Þ

CP is the heavy metal concentration in the permeate stream, while CF is a heavy metal concentration in the feed solution. All experiments were carried out in triplicates and results were reported in the form of a mean ± standard deviation. The significance of differences between the values was determined at p b 0.05 using the analysis of variance (ANOVA) followed by Tukey's multiple range test.

Table 2 Characterisation of water solutions used for the nanofiltration process i. Variant Solution composition V1

V2

V3

Individual aqueous solutions of each metal (Cu, Zn, Cd, Pb, Fe). The initial concentration of each metal in the solution before filtration was 500 μg/dm3. Mixture of the studied metals (Cu + Zn + Cd + Pb + Fe). The initial concentration of each metal in the solution before filtration was 500 μg/dm3. Mixtures of a particular metal and bovine serum albumin (BSA) (Cu + BSA; Zn + BSA; Cd + BSA; Pb + BSA; Fe + BSA). The initial concentration of each metal in the solution before filtration was 500

V4 Fig. 1. Small-scale experimental rig: 1 — feed tank; 2 — pump; 3 — membrane module; 4 — radiator; F — the feed; R — retentate; P — permeate; M —manometer.

μg/dm3, concentration of BSA was 500 mg/dm3. Solution containing BSA and all studied metals (BSA + Cu + Zn + Cd + Pb + Fe). The initial concentration of each metal in the solution before filtration was 500 μg/dm3, concentration of BSA was 500 mg/dm3.

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3. Results

F(4; 195)=25,0983; p=0.0000 90

The processes of metal retention using a ceramic membrane were conducted for specific Zn, Cu, Cd, Pb and Fe concentrations in water solutions (variants V1 and V2). In order to evaluate the retention of metal complexes, the experiments were conducted for water solutions of the studied metals supplemented with the BSA protein (variants V3 and V4). In order to evaluate the effect of pH on the filtration process, each of the studied solutions was examined at several pH values. On the basis of the metal retention, the transfer of mass of the studied metals in particular variant was calculated (Fig. 3). Averaged retention results of all variants at the respective pH values led to a conclusion that the metal retention was low at a pH range of 2.0–4.6 (on average, ca. 57 and 55% respectively) and then increased with increasing pH values, reaching a peak at pH 9.0 (ca. 87% on average). The differences were statistically significant (p b 0.05) (Fig. 2). However, in each of the studied variants it was noted that the tendencies regarding metal retention varied depending on the pH. This variability is illustrated in Fig. 3. Variant V1: the lowest retentions were noted for Zn, from 36% (pH 2.0) to 58% (pH 9.0). For Cd, Fe and Cu the lowest retentions were noted at pH 4.6. At pH ≥ 6.9, the retention increased considerably, being the highest for Cu (98%, pH 9.0). Variant V2: the lowest retentions were noted for Fe, from 29% (pH 2.0) to 73% (pH 9.0). At pH 4.6, a drop in the retention level was noted only for Cd. For pH exceeding 6.9, ca. 90% retention levels were noted for Zn,

85

Retention %

80 75 70 65 60 55 Mean Mean+/-SD

50 45 2.0

4.6

6.0

6.9

9.0

pH

Fig. 2. General correlation between metal retention and the pH of a solution (calculated for all retentions at a given pH; statistically significant differences at p b 0.05).

Linear dependence of metal retention from pH was calculated using Pearson's linear correlation coefficient. Statistical analysis was conducted using STATISTICA 10.0 software [30].

100

100

V1

V2

90

80

80

70

70

Retention %

Retention %

90

60 50

60 50 40

40

Mean+/-SD

Mean+/-SD 30

30

Zn Cd Pb Cu Fe

20

Zn Cd Pb Cu Fe

20

10

10 2,0

4,6

6,0

6,9

9,0

2,0

4,6

pH 100

6,9

9,0

100

V3 90

90

80

80

70

70

Retention %

Retention %

6,0 pH

60 50 40

V4

60 50 40

Mean+/-SD 30

Zn Cd Pb Cu Fe

20

Mean+/-SD 30

Zn Cd Pb Cu Fe

20 10

10 2,0

4,6

6,0 pH

6,9

9,0

2,0

4,6

6,0

6,9

pH

Fig. 3. Content of metal in permeates after the process of nanofiltration with the initial concentration in feed solution at the level of 500 μg/dm3.

9,0

A. Nędzarek et al. / Desalination 369 (2015) 62–67

Cd, Pb and Cu. Variant V3: the lowest retentions were noted for Pb + BSA and Fe + BSA solutions (pH 2.0) and for the Cu + BSA (pH 4.6): 18%, 35% and 40%, respectively. A drop in the retention was noted at pH 4.6 for Cu, Cd, and Zn. For Cd and Zn the lowest retention levels were noted at the same pH value, amounting to 51% and 62%, respectively. Maximum retentions of the studied metals were noted at pH 9.0, ranging from 74% (Cu) to 95% (Cd). Variant V4: the lowest retentions were noted for Pb (18%, pH 2.0). At pH 4.6 retention levels dropped for Zn, Cd and Fe, and were the lowest retentions noted in this variant (74%, 67% and 47%, respectively). At the remaining pH values, the noted trend of retention variability was the same for all studied metals. The retention levels increased at pH 6.0 and slightly dropped at pH 6.9 to increase again at pH 9.0, with maximum retentions noted for Cu and Cd (ca. 99%). General trends regarding changes in metal retention depending on the pH were analysed using Pearson's linear correlation coefficient (Fig. 4). The analysis showed that for the solutions containing the respective metals (V1), across the whole pH range, the lowest retention was noted for Zn and the highest for Cu and Pb. In V2, metal retention levels across the whole pH range were lower than in the remaining variants. In V3, it was noted that for Cu + BSA and Zn + BSA solutions, the retention levels of copper and zinc slightly increased as the pH values increased. For Fe + BSA and Pb + BSA solutions the increase in the retention levels related to the increasing pH values was the most distinct: the values of the linear correlation coefficient (r) were statistically significant for these metals and equalled 0.80 and 0.96, respectively. In

Zn=25.82+3.27pH Cd=49.41+3.12pH Pb=52.27+4.33pH Fe=55.26+2.30pH Cu=55.86+4.41pH

V1

r=0.84*, r=0.34, r=0.86*, r=0.48, r=0.79*,

p=0.0023 p=0.3306 p=0.0014 p=0.1616 p=0.0067

V4, in comparison to the remaining variants, the retention levels were the highest for all metals across the whole pH range. In this variant, the lowest retention was noted for Fe: max. 80%, at pH 9.0. The highest increase in retention related to the increasing pH values was noted for Pb, ranging from ca. 20% at pH 2.0 to ca. 94% at pH 9.0. For the remaining metals, the retention levels were the highest across the whole pH range and ranged from 80–86% at pH 2.0 to 95–98% at pH 9.0 (Fig. 4). A comparison of the respective variants showed that V2 was generally characterised by the lowest retention (ca. 61% on average) while V4 was characterised by the highest retention (ca. 80% on average). The differences noted were statistically significant with p b 0.05 (Fig. 5). 4. Discussion The retention levels obtained in our study were similar to those reported in literature. For instance, Almutairi et al. [2] have reported the maximum retentions of Cu, Ni, Zn, Co, Cr and Cd exceeding 90%. Bougen et al. [8] have obtained the retentions of Cu and Zn of 80–90%, while Juang and Shiau [19] of 100% and 95%, respectively. A retention of Cd of about 82% has been reported by Murthy and Chaudhari [25]. The degree of heavy metal retention depends on the pH of the environment and on the presence of metal/ polymer complexes [2]. In general, lower retentions of heavy metals (similarly as in our study) have been obtained at a low pH and for water solutions of the metals, while they increased to pH N 6 and for the solutions in which the metals were complexed, as shown by e.g. Bougen

Zn Cd Pb Fe Cu

Zn=3.39+8.94pH r=0.91*, p=0.0002 Cd=26.41+6.30pH r=0.77*, p=0.0098 Pb=37.90+5.79pH r=0.90*, p=0.0004 Fe=11.96+7.53pH r=0.95*, p=0.00003 Cu=9.61+9.81pH r=0.94*, p=0.00005

V2

Zn Cd Pb Fe Cu

100

100

80

Retention %

80

Retention %

65

60

40

20

60

40

20

0 1

2

3

4

5

6

7

8

9

10

0 1

pH

p=0.6214 p=0.2854 p=0.91 p=0.00001 r=0.0054

Zn Cd Cu Pb Fe

4

5

Zn=81.98+1.17pH Cd=72.87+2.65pH Pb=9.422+10.29pH Fe=54.77+2.45pH Cu=73.28+2.78pH

V4

100

100

80

80

Retention %

Retention %

3

6

7

8

9

10

pH

Zn=77.65+0.79pH r=0.18, Cd=57.89+2.72pH r=0.36, Cu=53.58+0.29pH r=0.05, Pb=4.05+10.26pH r=0.96*, Fe=34.72+6.56pH r=0.80*,

V3

2

60

40

20

r=0.33, r=0.53, r=0.89*, r=0.47, r=0.85*,

Zn Cd Pb Fe Cu

p=0.3461 p=0.1120 p=0.0005 p=0.1720 p=0.0018

60

40

20

0

0 1

2

3

4

5

6 pH

7

8

9

10

1

2

3

4

5

6

7

8

9

pH

Fig. 4. The influence of pH on the % of heavy metal retention in the respective variants (* statistically significant Pearson's linear correlation coefficients (r)).

10

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A. Nędzarek et al. / Desalination 369 (2015) 62–67 85 F(3, 100)=452.86, p=0.0000 80

Retention %

75

70

65 Mean Mean+/-SD 60

V1

V2

V3

V4

Variant

Fig. 5. Varying retentions of the studied metals in the respective variants (average values calculated for all retentions in the given variants; statistically significant differences at p b 0.05).

et al. [8], Almutairi et al. [2] or Juang and Shiau [19], who tested metal complexation by ethylenediamine, polyethyleneimine and chitosan. Elimination of complexed metals was even up to 10 times greater than that from the solutions without complexing substances. In the acidic environment metals occur in the form of free ions; according to Bernat et al. [7]; the absence of soluble charged metal hydroxides renders the formation of an active layer on the membrane surface impossible. This results in a considerable permeability of membranes and low retention of metal ions (noted also in our studies). As the pH of the environment increases, so does the amount of soluble metal hydroxides. In addition, above the pH values of the isoelectric point (pH N 6.9), the studied membrane has a negative charge. Due to electrostatic effect of the system (metal hydroxides — the membrane), an active filtration layer forms on the membrane surface and the retention level increases [22]. Creation of this layer results in an increase of density of positive charge in membrane, what causes the cation retention to grow. For example this relationship was analysed on copper by using enhanced Nernst–Planck model [10] and the outcomes of modelling were presented in Fig. 6. Variation and value of the Cu retention in pH function are concurrent with the ones acquired through the experiment. It should be noted that the charge density of the membrane determined on the basis of enhanced Nernst–Planck model takes into consideration all the interactions of non-ideal character between ions and ions, and ions and membrane [28]. Metal complexing is a factor increasing metal retention in the process of membrane filtration [23]. For BSA used in the present study, the increase in metal retention was noted in both variants (V3 and

V4). The increase was conditioned by complexing the metals with proteins and by the formation of fouling resulting from the interaction between BSA and the membrane. Negatively charged carboxylic groups and primary amino groups in BSA were the active spots at which the charged metal ions were coordinationally bound [21,23]. However, complexation of different metals is coordinated by different protein fragments. For instance, Zn is usually complexed by cysteine [15]. This variation may explain why the noted retention levels of the respective metals varied. However, we were unable to conduct more detailed studies that would explain how the particular amino acids of which BSA was composed complexed the studied metals. Retention in the solutions in V3 and V4 variants depended also on the pH of the environment. High metal retentions were noted at pH 2.0, and maximum retentions at pH N 6.9. Above this pH value the interaction between the metal ions and BSA led to the formation of stable complexes, analogically to e.g. Barakat [6] observations, who complexed metals with carboxymethyl cellulose and noted maximum retentions at pH ≥ 7.0. At pH = 4.6 and pH = 6.9, a drop in metal retention was noted. At these pH values (the isoelectric points of BSA and the membrane, respectively) the ionic strength was the lowest, meaning that the metal ions–BSA–membrane interactions were the weakest. Especially strong interactions of this kind were noted for the pH range of 4.6 b pH N 6.9, where the ionic strength of interactions between BSA and the membrane was the highest, being conducive to fouling formation and the increase in metal retention [12,13]. 5. Conclusions The retention of Zn, Cu, Cd, Pb and Fe from water solutions and solutions supplemented with a complexing agent: BSA, with the use of ceramic membranes was evaluated. The experiments were conducted within the pH range of 2.0–9.0. The retention tended to increase together with the pH values of the solution in all the tested variants. However, in the case of the solutions composed solely of the metals, retentions exceeding 80% were noted for pH N 6.9. The addition of the complexing agent resulted in high Cu, Cd and Zn retentions already at pH = 2.0. Maximum retentions of all metals were noted for the solutions containing a mixture of the metals and BSA at pH = 6.0 and 9.0. The retentions of Zn, Cd, Pb, Cu and Fe at pH = 6.0 amounted to: 97, 96, 90, 95 and 71%, respectively, whereas at pH = 9.0 they were: 93, 99, 93, 99 and 85%, respectively. The studies have confirmed that BSA displays a high capacity for metal complexation, thus indicating that the effective removal of heavy metals from wastewater containing protein is possible through the process of membrane separation using ceramic membranes. Acknowledgements We would like to express our gratitude to PhD Piotr Mitkowski and MA Agata Marecka for their help in modelling the process of retention carried out on the basis of Nernst–Planck model, as well as to the Reviewers for their valuable comments. References

Fig. 6. The relationship of Cu retention and the effective volume charge density of the membrane depending on pH, on the basis of enhanced Nernst–Planck model.

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