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International Biodeterioration & Biodegradation 85 (2013) 413e420

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International Biodeterioration & Biodegradation

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Evaluation of the characteristics of polyvinyl alcohol (PVA) as matricesfor the immobilization of Pseudomonas putida

Muftah H. El-Naas a, *, Abdel-Hamid I. Mourad b, Riham Surkatti a

a Chemical and Petroleum Engineering Department, UAE University, P.O. Box 15551, Al-Ain, United Arab Emiratesb Mechanical Engineering Department, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates

a r t i c l e i n f o

Article history:Received 12 August 2013Received in revised form5 September 2013Accepted 6 September 2013Available online 1 October 2013

Keywords:BiodegradationImmobilizationPhenolFreezeethawMechanical properties

* Corresponding author. Tel.: þ971 3 713 5188.E-mail address: Muftah@uaeu.ac.ae (M.H. El-Naas)

0964-8305/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.ibiod.2013.09.006

a b s t r a c t

This study evaluates the characteristics and viability of polyvinyl alcohol (PVA) as a support material forbiomass immobilization. PVA gel pellets were prepared by iterative freezingethawing method usingdifferent PVA compositions. The porous structures of the PVA pellets were examined using a CompoundMicroscope and SEM and revealed that the matrix structure and pore size distribution were affected byPVA composition. Mechanical properties of the PVA gel were characterized to evaluate its integrity asmatrices for immobilizing microbial cells and found to be dependent on the PVA mass composition. Asthe PVA mass % increases the mechanical performance improved. Experiments were also carried out toutilize the prepared gel pellets for the immobilization of Pseudomonas putida and biodegradation ofphenol over a long period of time. The results revealed that the capabilities of the biomass to degradephenol increased with time and depended on the PVA mass content and porous structure.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Wastewater containing phenols and other toxic compoundsrequires careful treatment before discharging into a receivingwaterbody (Tziotzios et al., 2005). Several physical, chemical and bio-logical techniques such as biodegradation, adsorption, ion ex-change and activated carbon have had been proposed in the lastdecades to remove phenolic compounds from wastewater stream(González et al., 2001; El-Naas et al., 2009). Among the variousavailable methods, biological treatment is universally preferredbecause it is economical, environmental friendly and can offer thepossibility of complete mineralization of hydrocarbons (Liu et al.,2009). Many aerobic bacteria are capable of using phenols as thesole source of carbon and energy; these include Acinetobacter sp(Wang et al., 2007; Liu et al., 2009), Peacilomyces variotii (Wanget al., 2010), Panarochaete chrysosporium (Lu et al., 2009), Bur-kholderia vietnamiensis (Cheng et al., 2012), Ralstonia eutropha(Dursun and Tepe, 2005) and Pseudomonas putida (Sarkar et al.,2010).

.

All rights reserved.

P. putida has been studied by many researchers for the degra-dation of different forms of phenols in several kinds of batch andcontinuous bioreactors at different conditions (El-Naas et al.,2010b; Al-Khalid and El-Naas, 2012). These include: degradationof 2,4-dichlorophenol and 4-Cl-m-cresol (Ziagova andLiakopoulou-Kyriakides, 2007); kinetics biodegradation of phenoland catechol (Kumar et al., 2005); degradation of phenol and so-dium salicylate mixture (Tsai and Juang, 2006); biodegradation ofphenolic industrial wastewater in fluidized bed bioreactor(González et al., 2001); and degradation of phenol and TCE (Chenet al., 2007).

Immobilization of microbial cells has received increasing inter-est in the field of wastewater treatment (Kuyukina et al., 2006;Yujian et al., 2006; Al-Zuhair and El-Naas, 2011). Compared withfree cells, immobilized cells have several advantages, such asincreasing the biodegradation rate through a higher cell loading,easily controlled, protecting microorganisms from harsh environ-mental conditions and wash out from the reactors, improving thebiomass performance, increasing mechanical strength, reducingreactor volume and providing stable treatment (Chen et al., 2007;Zhang et al., 2007; Magrí et al., 2012). Synthetic polymers areknown to have good mechanical properties and durabilitycomparing with natural polymers (Masunaga et al., 1986; Lozinskyand Plieva,1998). Among these polymers, polyvinyl alcohol (PVA) isa promising carrier for bacterial immobilization (El-Naas et al.,

Fig. 1. A schematic diagram of the spouted bed bioreactor (SBBR).

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420414

2010c), that is non-toxic and non-carcinogenetic (Zhang et al.,2007). It has relatively good tensile stress, impact strength, highwater affinity, wear resistance, good biocompatibility, process-ability, minimal cell and protein adhesion, and excellent electricalinsulation (Awad and Abuzaid, 2000; Yujian et al., 2006; Zhanget al., 2007; Ma and Xiong, 2008). The iterative freeze thaw tech-nique has attracted much attention because it gives the capabilityto cross-link an aqueous solution of PVA without leaving chemicalremnant in the gel matrix. In addition, these freeze-thawed gelshave demonstrated enhanced mechanical properties (Hassan andPeppas, 2000a, b; Hernández et al., 2004; Magrí et al., 2012). PVAis also preferred for cell immobilization in wastewater treatmentbecause of the allowance of oxygen and substrate to diffuse insidethe pores to make the biodegradation process possible (Al-Zuhairand El-Naas, 2011; Cheng et al., 2012).

Generally, hydrogels are polymeric material with three-dimensional networks that have the ability to swell, but notdissolve or absorb a significant fraction of water within theirstructure (Jiang et al., 2011). The physically cross-linked PVA gelsare called cryogels that result in a matrix with good elastic prop-erties and strong mechanical strength (Hickey and Peppas, 1997;Hassan and Peppas, 2000a; Doria-Serrano et al., 2001; Hernándezet al., 2004; Bai, 2010). The properties of the cryogels depend on themolecular weight of the polymer, the concentration of the aqueousPVA solution, the temperature, time of freezing and thawing, andthe number of freezingethawing cycles (Hernández et al., 2004;Gonzalez et al., 2012). Many researchers have been focusing onstudying the mechanical properties of PVA gels (cryogels andhydrogels); Gauthier et al. (2004) have studied the degree of cross-linking and mechanical properties of PVA beads cross-linked withepichlorohydrin (PVA-EP) and used for solid-phase organic syn-thesis. They have shown that the shear modulus increases as thecross-linking density increases. Stammen et al. (2001) have re-ported that the critical barrier to Polyvinyl alcohol (PVA) hydrogelsapplication as biomaterials (load-bearing tissue replacements) isthe lack of sufficient mechanical properties.

The aim of this study is to characterize PVA gel prepared by thefreezingethawing technique to be used as support material forbiomass immobilization. The effect of PVA mass % on the porousstructure and mechanical properties was analyzed and comparedfor several PVA compositions. Phenol biodegradation rates for theimmobilized bacteria using different PVA pellets were evaluatedand compared over a period of three months.

2. Materials and methods

2.1. Reagents

Analytical grade phenol was purchased from BDH Chemicals,UK. Synthetic phenol solutions were prepared for the desiredconcentration in mineral nutrient solution before each experi-mental run. All other chemicals and PVA powder were of analyticalgrade and were also obtained from BDH, UK.

2.2. Preparation of bacterial cereal

Bacterial suspension was prepared as reported by El-Naas et al.(2009). A special strain of the bacterium P. putida was obtainedfrom Cleveland Biotech. LTD., UK in a cereal form (AMNITE P300).100 g of the bacterial cereal (powder) was mixed with 1000 ml of0.22% hexametaphosphate solution buffered by Na2CO3 to a pH of8.5. The mixture was homogenized in a blender for about 1 h,decanted and kept in the refrigerator at 4 �C for 24 h. Bacteria slurrywas prepared by four consecutive steps of low speed centrifugationat 6000 rpm for 15 min. The supernatants were collected and

centrifuged again at 10,000 rpm for 20 min. The precipitatedamount from the three centrifugations (which contains the har-vested bacteria cells) were collected, suspended as slurry indistilled water and kept in the refrigerator for subsequentimmobilization.

2.3. Preparation of PVA gel

Homogenous PVA solutions were prepared by dissolving 50,100, 150 and 200 g of PVA powder in warm distilled water (70e80 �C), to prepare 1 L of solution having PVA contents of 5, 10, 15and 20 mass %, respectively. Heating and mixing by a glass rodcontinued during the entire dissolving process to formwell-mixed,homogenous solutions. The prepared PVA solutions were thenallowed to cool at room temperature before adding 10 ml of thepreviously prepared bacterial suspension. The PVA-biomass solu-tions were then well stirred for 10e15 min to insure homogeneityand then poured into molds and kept in a freezer at about �4 �C.PVA matrices were cross-linked by a repetitive freezingethawingmethod. Freezing was carried out at �4 �C for 24 h followed bythawing at room temperature for 3 h .The freezingethawing pro-cess was repeated for 5 cycles, and the prepared PVA gel was cutinto small cubes (1 cm3) and then rinsed with distilled water toremove any uncross-linked particles.

2.4. Biomass acclimatization

PVA gel pellets with immobilized bacteria were placed in bubblecolumn reactor with 200 mg l�1 glucose (as an easy biodegradationsource of carbon) in addition to other essential mineral nutrientswith continuous aeration. Nutrient mineral medium solution wasprepared by dissolving in 1 L of distilled water 825 mg of mineralsalt mixture consisting of: 299.58 mg MgSO4$7H2O, 249.65 mgK2HPO4, 149.80 mg CaCl2$2H2O, 119.83 mg (NH4)2CO3, 3.50 mgFeSO4$7H2O, 1.30 mg ZnSO4$7H2O, 1.30 mg MnCl2$4H2O, 0.018 mgCuSO4$5H2O, 0.015 mg CoCl2$6H2O and 0.013 mg a 2MoO4$2H2O.The bacteria were acclimatized to phenol by gradually increasingthe concentration of phenol and decreasing the concentration ofglucose. Once the maximum concentration of phenol (200 mg l�1)

Fig. 2. Compound Microscope images of PVA; (a) 5 mass %; (b) 10 mass %; (c) 15 mass %; (d) 20 mass %; (freezingethawing; 24 he3 h).

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420 415

is reached and the concentration of glucose dropped to zero, theimmobilized bacteria are considered fully acclimatized to phenolconcentration up to 200 mg l�1 (El-Naas et al., 2009).

2.5. Spouted bed bioreactor (SBBR)

The spouted bed bioreactor (SBBR) was made of Plexiglas with atotal volume of 1 L. Air was continuously introduced at flow rate of3 l min�1 into the reactor to enhance mixing and at the same timeprovide the aerobic biodegradation process with the excess oxygen.The reactor temperaturewas controlled at 30 �C by the surroundingjacket and the initial pH solution was kept at pH of 7.5; theseoperating parameters were considered to be optimum for phenolbiodegradation as reported by El-Naas et al. (2010a). The SBBR ischaracterized by a systematic intense mixing due to the cyclicmotion of particles within the bed that is generated by a single airjet injected through an orifice in the bottom of the reactor (Al-Zuhair and El-Naas, 2011). A schematic diagram of the SpoutedBed Bioreactor (SBBR) is shown in Fig. 1.

2.6. Specimen preparation for tensile testing

Unlike the majority of polymers, swollen hydrogels areextremely weak materials, which can exhibit poor mechanicalstrength. This weakness and the requirement that the sample doesnot reduce in size during testing need special care. In the currentwork, a specific experimental procedure for testing PVA has beenfollowed. After preparing the viscous PVA solution, as described in

section 2.3, the solution was poured (casted) in a plastic mold withsix rectangular spaces. Each space has nominal dimensions of5.5 mm width, 12 mm depth and 100 mm length. The PVA moldswere kept in the freezer at about -4 �C and PVA matrices werecross-linked by freezingethawing method. After completing thefreezingethawing process with 5 cycles, the samples were allowedto thaw at room temperature for z30 min before performing thetensile tests.

In most of uniaxial tensile testing standard, dumbbell-shapedand dog bone samples are usually used. Dumbbell-shaped speci-mens were used by several investigators (Fouad et al., 2005;Mourad et al., 2009a,b; Mourad, 2010; Dehbi et al., 2012). Dogbone shaped specimenwas used by Mourad et al. (2005) to test diedrawn polymeric samples and by Anseth et al. (1996) to test PVA/Gelatin samples. In these studies an abrasive paper was placedbetween the hydrogel sample and the grip surface to preventslippage during loading. Some difficulties have been encounteredin preparing and testing the standard dumbbell-shaped and dogbone-shaped samples of PVA gel. Therefore, specimens with rect-angular cross section have been used. The specimens have beenprepared by molding technique with nominal dimensions of5.5 mm � 12 mm �100 mm. Specimens were prepared withrelatively long length to ensure good gripping. Special fixture hasbeen prepared to install the specimen in the gripping system of thetensile testing machine. The fixture consists mainly of special typeof soft tissues and rubber bands. The tissue papers and rubberbands were carefully wound around the two ends of the specimen.Then the two ends were placed between two rubber pieces before

Fig. 3. SEM images of PVA. (a) 5 mass %; (b) 10 mass %; (c) 15 mass %; (d) 20 mass %; (freezingethawing; 24 he3 h).

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420416

installing the specimen into the gripping system of the machine.These procedures of fixation eliminate both slippage and specimendamage during loading. The tensile tests were conducted on auniversal material testing machine MTS of 100 kN load cell. Allsamples were tested at room temperature and at a constant over-head speed of 25mm/min until the sample reaches ultimate failure.

When testing PVA gel samples, it is expected that the sampleswould be subjected to water loss which can essentially influencethe mechanical performance. In particular, as the temperature isincreased, water loss can become more prominent and lead to in-crease in moduli with temperature which are in reality the result ofwater loss and subsequent changes in the PVA gel structure.Therefore, all tests have been conducted at a relatively high over-head speed of 25 mm/min to minimize the time of the test and inturn water loss. This also helps to avoid coating the hydrogelsamples with any material such as petroleum gel or silicone vac-uum grease that may affect the measured properties.

2.7. Analytical methods

Phenol concentration was determined quantitatively usingChrompack Gas Chromatograph Model CP9001 as reported by ElNaas et al. (2010a). The GC was equipped with capillary column(Stabilwax, 30 m, 0.25 mm ID) and a flame ionization detectorwhich was set at 250 �C. A sample of 1 ml was filtered throughsyringe filter with pore size of 0.45 mmand injected into the GC. Thetemperature programwas started at 100 �C and increased to 180 �Cat a rate of 20 �C min�1. The measurements of each phenol samplewere conducted in duplicates, and a standard solution was used torecheck the accuracy of the GC. PVA pellets were cut into thin

sections in a microtome using stainless steel microtome blade, andthe porous structure was analyzed using Lieca Compound Micro-scope (DM6000) by Bright Field method. The microscopic analysisof PVA gel samples were also carried out for the freeze driedsamples using Scanning Electric Microscope (SEM) (JEOL JSM-5600,Japan). Then tensile tests were conducted to evaluate the me-chanical properties using Universal Material Testing Machine.

3. Results and discussions

3.1. Effect of PVA concentration on porosity

The porous structure of the biomass carrier plays an importantrole in the biodegradation process, since it allows substrate andoxygen diffusion into the internal surface and hence improves thegrowth rate of the biomass within the pellets. In order to investi-gate the porous structure of the PVA gel pellets at different PVAmass contents ranging from 5 to 20%, the gel pellets were cut intothin sections in a microtome sectioning machine and then exam-ined using a Compound Microscope (Leica DM 6000) by the (BrightField) contrast method. Microscopic images for different PVA masscontent (5, 10, 15 and 20%) are presented in Fig. 2 (a)e(d) using 63�magnification. The images clearly show highly porous structure forall PVA mass contents except 15% and the pores seem to bedistributed with varying sizes across the PVA pellet, which in-dicates that PVA matrices have suitable structure for biomassimmobilization. The matrices with 10 mass % PVA had the highestporosity comparing with other pellets and showed a stablemicrostructure with evenly distributed pores as presented inFig. 2(b); the pore size is smaller than that in both 5 and 20 mass %,

(a) Directly after acclimatization.

(b) After three months.

Time (min)

0 50 100 150 200 250

C/C

o

0.0

0.2

0.4

0.6

0.8

1.0

1.2PVA 5 mass%PVA 10 mass%PVA 15 mass%PVA 20 mass%

Time (min)

0 20 40 60 80 100 120 140

C/C

o

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2PVA 5 mass%PVA 10 mass%PVA 15 mass%PVA 20 mass%

Fig. 4. Variation of phenol concentration for several PVA pellets for the biodegradationprocess after different periods; initial phenol concentration ¼ 200 mg l�1; T ¼ 30 �C;pH ¼ 7.5.

PVA mass%

0 5 10 15 20 25

Bio

degr

adat

ion

Rat

e (m

g l-1

h-1)

0

50

100

150

200After acclimatizationAfter 1 monthAfter 2 monthsAfter 3 months

Fig. 5. Comparison of phenol biodegradation rate for different PVA pellets after severalperiods of time.

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420 417

which seemed to be varying in size and unevenly distributed overthe displayed surface. Fig. 2(d) clearly indicates that the porousstructure of the prepared polymer is not density dependent andmay depend more on the crystal formation (Ricciardi et al.,2005a,b) which is related to the concentration of the PVA in thesolution, freezing time and thawing time (Hassan and Pappas,2000a, b). Pellets with 10 mass % PVA seemed to have reachedhigh crystallinity in the 24 h freezing and 3 h thawing as observedin the network structure. The PVA matrices with 15 mass % (Fig. 2c)appears to have diminished porosity due to the inappropriatefreezing thawing time for crystal formation, where the freezingethawing process could not achieve the optimum conditions forcrystallizationwhich consequently affected the pore formation andporosity.

The porous structures for pellets with different PVA mass con-tents were also examined using SEM for freeze-dried samples andpresented in Fig. 3. The SEM images show three-dimensional net-works for the cross-linked polymers, with pore size and pore dis-tribution similar to those observed by the Compound microscopeand thus confirming the above mentioned conclusion.

3.2. Biodegradation rate

The biodegradation capabilities of P. putida immobilized in PVApellets with different mass contents were assessed over long pe-riods of time ranging from one to three months. Batch experi-ments were carried out for the biodegradation of phenol in fourSBBR systems using bacteria immobilized in 5, 10, 15 and 20 mass% PVA for a period of one, two and three months after acclimati-zation. All experiments were duplicated and the repeatability wasachieved; the results presented in this section are the average fortwo to three runs. Plots for the dimensionless concentration as afunction of time for the biodegradation of 200 mg l�1 of phenolare shown in Fig. 4a and b, for experiments carried out immedi-ately after acclimatization, and then after three months. Thebiodegradation rates were calculated from the slopes of the bestfitted straight line for concentration versus time plots and sum-marized in Fig. 5

Directly after acclimatization (Fig. 4a), the biodegradation rateseemed to be similar for the different PVA contents with the 15%showing slightly better rate compared to the other three. Thiscould be attributed to the possibility that for this PVAmatrix, morebiomass was available at the outer surface due the poor cross-linking and diminished porosity as mentioned in the previoussection. However, the biodegradation rate seemed to drasticallyimprove with time and the rate of improvement varied anddepended on the PVA content as shown in Figs. 4b and 5. Thebiodegradation process is expected to improve with time as thenumber of biomass cells within the PVA matrices increasessignificantly and can only be limited by the porous structure of thePVA pellet. This is clearly reflected in the results presented in Fig. 5,as porous pellets (5 and 10%) showed better biodegradation rateafter three months than those that are less porous. The leastporous matrix (15%) exhibited the least biodegradation rate after3 months as shown in the figure. Although the SEM and Compoundmicroscope analysis revealed that the 10% PVA pellets had themost porous structure, it showed slightly lower biodegradationrate than that of the 5% which can only be attributed to the bettermixing and hence better mass transfer for the lighter pellets. Thiswas observed experimentally as the 5% pellets showed bettermovement and better mixing inside the reactor as compared tothose of the 10% that tend to be heavier with more hinderedmovement.

Fig. 6. PVA specimen. (a) before fracture; (b) After fractured.

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420418

3.3. Mechanical behavior

Mechanical tests were carried out to evaluate the tensile prop-erties of PVA gel (fracture strength, percent elongation andModulus of elasticity). Although thematerial exhibits large strain atfracture and rubbery behavior, it is not a ductile material becausethe strain is not permanent and almost the total elongation isrecovered after fracture in away similar to rubber behavior. It is anelastic material and elasticity of the samples is ascertained from thestress-strain profiles. No local deformation or necking/yielding wasobserved while testing as shown in Fig. 6a and b. The fracturesurface is smooth flat and perpendicular to load line in almost allspecimens as the cleavage behavior in brittle material.

Tensile tests were conducted on samples with different PVAcontents (5%, 10%,15%, 20% and 25%). Three samples were tested foreach PVA % and the curves demonstrated a good level of repeat-ability. The average values of the properties have been reported.Representative stress-strain curves for the different PVA contentsare presented in Fig. 7. The stress varies with the strain followingalmost a linear relationship. The behavior shows that the PVA gelsrespond to stresses with nearly instantaneous and fully reversibledeformation. This may be attributed to the fact that the gels arecross-linked networks (as rubber) with a large free volume thatallows them to respond to external loads with a rapid rearrange-ment of the polymer segments. When a gel is in the region ofrubber-like behavior, its mechanical behavior is dependent pri-marily on the structural design of the polymer network. At lowenough temperature, these gels may lose their rubber elasticproperties and demonstrate viscoelastic behavior. Similar obser-vation has been reported by Anseth et al. (1996).

Fig. 8 demonstrates the variation of the maximum fracturestrength (fracture load) with PVA contents. The material eventuallyoffers increasing resistance to the load, and the curve turns mark-edly upward as PVA concentration increases. The variation follows

Strain %

0 50 100 150 200 250 300

Stre

ss (

MP

a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6 5% PVA10% PVA15% PVA20% PVA25% PVA

Fig. 7. Variation of stress with strain % for different PVA mass %.

almost an exponential function. This improvement in the me-chanical properties (fracture strength, percent elongation, andstiffness) with increasing PVA concentration is due to the combinedeffect of density and/or porosity and cross-linking. It has been re-ported by Ma and Xiong (2008) that the increase in PVA mass %may produce remarkable increase in the cross-linking. This in turnsleads to observed improvement in the properties. In addition,porosity plays an important role in the variation in these properties.PVA matrices that have high porous structure and low density suchas PVA 5% and 10% showed less mechanical properties than others.As has been mentioned before in Section 3.1, the porosity of theproduced PVA gel depends on the process parameters such as PVAmass %, freezing and the thawing cycles (Gupta et al., 2012). PVAmass % of 15% produced a gel with less porosity as presented inFigs. 2 and 3, this may explain the remarkable increase in thefracture stress (see Fig. 8), when PVA mass % increased from 10% to15. The slope of the curves increases with PVA concentrationwhichis an indicator for the apparent improvement in the stiffness(modulus of elasticity) of the material. This behavior is shown inFig. 9. Such improvement is also dependent on the combined effectof the process parameters. These observations have also been re-ported by Gupta et al. (2012).

4. Conclusions

The mechanical properties for PVA gel pellets with differentmass content were evaluated to characterize its suitability as sup-port materials for biomass immobilization. The PVA gels wereprepared through a freezingethawing process for PVA mass con-tent ranging from 5 to 25%. The porous structures were examinedusing compoundmicroscope and SEM and revealed dependency onthe PVA mass content. Tensile testing indicated that the PVA gelspossess a rubbery, elastic nature, fibril network structure. Both

y = 9.4 e 0.104 x

PVA mass%

0 5 10 15 20 25 30

Loa

d (N

)

0

20

40

60

80

100

120

140

160

Fig. 8. Effect of PVA mass % on the fracture load.

PVA mass %

0 5 10 15 20 25 30

Mod

ulus

of

Ela

stic

ity

MP

a

0.0

0.2

0.4

0.6

0.8

Fig. 9. Variation of modulus of elasticity with PVA mass %.

M.H. El-Naas et al. / International Biodeterioration & Biodegradation 85 (2013) 413e420 419

fracture stress, percent elongation and modulus of elasticity in-crease with PVA content. The maximum achieved fracture stressand percent strain were 0.1 MPa and 160% respectively, for 5% PVAand 1.5 MPa and 275% for 25% PVA. The phenol degradability ofP. putida immobilized in the PVA pellets was found to drasticallyimprove with time and depend on the porous structure of thepellets. PVA gels with 20 mass % seemed to have relatively goodporous matrix as well as high mechanical strength and durabilitywith good biodegradation rates, which makes it the most suitablematrix for long term industrial application.

Acknowledgment

The authors would like to acknowledge the financial supportprovided by the Japan Cooperation Center, Petroleum (JCCP) andthe technical support by JX Nippon Research Institute Co., Ltd (JX-NRI). They would also like to thank Manal Abu Alhaija and Abd El-Satar Nour El-Deen for their help with the experimental work.

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