Barium-Encapsulated Biodegradable Polycaprolactone for Sulfate Removal

water

Article Barium-Encapsulated Biodegradable Polycaprolactone for Sulfate Removal

Changseok Han 1,* and Mallikarjuna N. Nadagouda 2,* 1 Department of Environmental Engineering, Inha University, Incheon 22212, Korea 2 Center for Nanoscale Multifunctional Materials, Mechanical & Material Engineering, Wright State University, Dayton, OH 45431, USA * Correspondence: [email protected] (C.H.); [email protected] (M.N.N.); Tel.: +82-32-860-7505 (C.H.); +1-513-569-7232 (M.N.N.) Received: 14 September 2018; Accepted: 28 November 2018; Published: 6 December 2018

Abstract: Various compositions of barium carbonate (BaCO3) loaded polycaprolactone (PCL) composites were prepared, including 2.5/97.5, 10/90, 30/70, 50/50 and 90/10 (PCL/BaCO3), via re-precipitation technique. Small-scale column tests were conducted to study the efﬁciency of sulfate removal using the PCL/BaCO3 composites. The composites before and after their use to remove sulfate were extensively characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), and thermogravimetric analysis (TGA). As PCL is a biodegradable polymer, these composites are environmentally friendly and have several advantages over barium sulfate precipitation in overcoming clogging issues in ﬁlters or resins due to collection of natural organic matter (NOM). The media used in this study exhibited high capacity and was able to remove more than 90% sulfate from synthetic sulfate containing waters and NOM samples collected from the Ohio River.

Keywords: barium sulfate; biodegradable polymer; polycaprolactone; environmental remediation; water treatment

1. Introduction Environmental remediation methods often employ the use of synthetic composites due to their enhanced material properties [1–5]. Sulfate is one of commonly found anions in sources of water supplies, due the presence of atmospheric sulfur dioxide and many high water-soluble minerals containing sulfate, such as sodium sulfate, potassium sulfate, and magnesium sulfate in the environment [6]. Recently, the removal of sulfate from drinking water has been targeted due to deleterious effects of sulfate on the environment [7,8]. Sulfates affect not only the environment but also human health. Abnormal changes resulting from sulfate consumption from drinking water may include diarrhea and dehydration. These ill effects are more prominent during infant years and decline with age [6]. Previously, a novel composite made of polyvinyl chloride (PVC) loaded with barium carbonate (BaCO3) was reported for sulfate removal [9]. The implementation of such a material in a small-scale column test (SST) yielded promising results. However, reports indicate that potential contamination and leaching from conventional plastic like PVC can pose a severe threat to human health and the environment [10,11]. Monomethyl-, dimethyl-, monobutyl-, and dibutyl levels have been found at values up to 291 ng Tin (Sn)/L, 49.1 ng Sn/L, 28.5 ng Sn/L, and 52.3 ng Sn/L, respectively, in drinking water distributed through PVC pipes [12,13]. Successful remediation must involve the reduction of hazardous materials without introducing any further complications. Although the previously synthesized PVC/BaCO3 composite [10] showed promising results, the necessity for a benign and environmentally friendly remediation method still exists [14].

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TheWater 2018 hydrolytic, 10, x FOR PEER degradation REVIEW of several common polyesters has provided an attractive2 of 13 new platform for composite synthesis. In particular, polycaprolactone (PCL) has become a focus of various medicalThe hydrolytic device applications,degradation of including several common drug and polyesters bacterial encapsulationhas provided an [15 attractive,16], antimicrobial new film developmentplatform for composite [17], and synthesis. scaffolding In particular, in tissue po engineeringlycaprolactone [18 (PCL)]. PCL has exhibitsbecome a not focus only of various significant medical device applications, including drug and bacterial encapsulation [15,16], antimicrobial film biocompatibility but also undergoes slow degradation to produce harmless byproducts. Additionally, development [17], and scaffolding in tissue engineering [18]. PCL exhibits not only significant the glass transition temperature of −60 ◦C and a low melting temperature near 60 ◦C result in PCL’s biocompatibility but also undergoes slow degradation to produce harmless byproducts. substantialAdditionally, amorphous the glass thermoplastic transition temperature properties. of The−60 °C simple and a ring-opening low melting temperature polymerization near 60 synthesis °C makesresult this polymerin PCL’s matrix substantial attractive amorphous for nanocomposite thermoplastic formation. properties. PCL The shows simple significant ring-opening solubility at roompolymerization temperature in synthesis a gamut makes of solvents, this polymer including matr chloroform,ix attractive dichloromethane,for nanocomposite carbon formation. tetrachloride, PCL benzene,shows toluene, significant cyclohexanone, solubility at androom 2-nitropropane. temperature in Such a gamut strong of propertiessolvents, including of PCL make chloroform, it a suitable alternativedichloromethane, to PVC in thecarbon previously tetrachloride, reported benzene, facile toluene, synthesis cyclohexanone, of polymer/BaCO and 2-nitropropane.3 composites Such [7]. Instrong the presentproperties work, of PCL incorporation make it a suitable of barium alternative into a biodegradableto PVC in the previously and biocompatible reported facile polymer matrixsynthesis for removal of polymer/BaCO of sulfate from3 composites contaminated [7]. waters has been investigated. The porous nature of In the present work, incorporation of barium into a biodegradable and biocompatible polymer the PCL matrix promotes the use of such a material in column packing applications for environmental matrix for removal of sulfate from contaminated waters has been investigated. The porous nature of remediation. Several issues of previous schemes were addressed in the implementation of PCL. the PCL matrix promotes the use of such a material in column packing applications for environmental Primarily,remediation. hazardous Several and issues environmentally of previous damagingschemes were PVC addressed was replaced in thewith implementation PCL as a benign of PCL. matrix. Furthermore,Primarily, thehazardous easily acceleratedand environmentally hydrolytic damaging degradation PVC was elucidates replaced a with new PCL method as a forbenign barium sulfatematrix. (BaSO Furthermore,4) recovery the following easily accelerated its use in hydr theolytic column degradation studies. elucidates The resulting a new PCL method and for BaCO 3 compositebarium (PCL/Ba) sulfate (BaSO showed4) recovery significant following sulfate its use in removal the column capacity studies. similarThe resulting to that PCL observed and BaCO in3 the previouscomposite studies (PCL/Ba) [7]. The showed process significant of precipitation sulfate remo is shownval capacity in Scheme similar1. Concernsto that observed of wall in effects the in columnprevious applications studies [7]. were The addressed, process of andprecipitation the results is shown have beenin Scheme reported 1. Concerns in this work.of wall effects in column applications were addressed, and the results have been reported in this work.

SchemeScheme 1. The 1. The phase phase change change from from barium barium carbonate carbonate to barium barium sulfate sulfate in inthe the polymer polymer composite composite on on passingpassing sulfate sulfate containing containing water. water.

2. Experimental2. Experimental Section Section

2.1. Synthesis2.1. Synthesis of Polymer of Polymer Composites Composites TenTen grams grams of of PCL PCL (Aldrich; (Aldrich; Mn Mn 70–90 70–90 kDa) kDa) was wasdissolved dissolved in tetrahydrofuran in tetrahydrofuran (THF) at 10.0% (THF) at 10.0%weight weight per per volume volume ratio. ratio. The solid The solidBaCO BaCO3 powder3 powder was added was to added the resulting to the resultingsolution. solution.The The heterogeneousheterogeneous mixture mixture was was agitated agitated by byan anIKA IKA Lab Lab Egg Egg compact compact mixer mixer to create to create a milky a milky slurry. slurry. Subsequently,Subsequently, 100 mL100 methanolmL methanol was was slowly slowly added added to to the the slurry. slurry. Finally, Finally, 100 100 mL mL ofof distilleddistilled water was was added to induce precipitation. added to induce precipitation. Following the complete addition of water, solid PCL/BaCO3 precipitates settled from the Following the complete addition of water, solid PCL/BaCO precipitates settled from the solution solution to yield a clear supernatant. The composite material 3was purified by decanting organic to yieldsolvents a clear and supernatant. washing three The times composite with distilled material water. was Various puriﬁed compositions by decanting prepared organic are shown solvents in and washingTable three 1. times with distilled water. Various compositions prepared are shown in Table1.

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Table 1. Various PCL/Ba media compositions.

Sample Composition

PCL/Ba(2.5/97.5) Polycaprolactone 2.5 wt% + BaCO3 97.5 wt% PCL/Ba(10/90) Polycaprolactone 10 wt% + BaCO3 90 wt% PCL/Ba(30/70) Polycaprolactone 30 wt% + BaCO3 70 wt% PCL/Ba(50/50) Polycaprolactone 50 wt% + BaCO3 50 wt% PCL/Ba(90/10) Polycaprolactone 90 wt% + BaCO3 10 wt%

2.2. Characterization of PCL/BaCO3 Composites X-ray diffraction (XRD) patterns were measured using an X'Pert PRO XRD diffractometer (Philips, Almelo, The Netherlands) with Cu K (λ = 1.5406 Å) radiation. To characterize the morphology of PCL/BaCO3 composite samples, an environmental scanning electron microscope (ESEM, Philips XL 30 ESEM-FEG, Eindhoven, The Netherlands) was employed. An energy dispersive spectroscopy (EDS) installed in the ESEM was used to determine Ba content in the samples. A transmission electron microscopy (TEM, Philips CM20, Eindhoven, The Netherlands) and a high resolution-TEM (HR-TEM, JEM-2010F, JEOL, Tokyo, Japan) with a ﬁeld emission gun at 200 kV were used to investigate the crystal structure and crystal size. Thermogravimetric analysis (TGA) curves were obtained using a Perkin–Elmer Thermal Analyzer with a heating rate of 10 ◦C/min under air.

2.3. Investigation of the Capacity of PCL/BaCO3 Composites for Sulfate Removal

Column tests were performed to investigate the adsorption capacity of the developed PCL/BaCO3 composite adsorbents for sulfate removal. Two different types of water were tested, one being deionized (DI) water only, and the other containing natural organic matter (NOM). First, sulfate removal using PCL/Ba samples was investigated using clean water without NOM. Sulfate solution of 1100 mg/L was prepared using DI water with a pH of 7.0. The sulfate solution was continuously injected from the top of a column type of reactor. The flow rate of 1 mL/min was kept using a Thermo ScientificTM FH100M Series Peristaltic pump. To investigate the effect of column diameter on the sulfate removal, two reactors with different diameters (i.e., d = 2.25 cm and d = 1.0 cm) were used. Second, to investigate the sulfate removal in the presence of NOM, raw NOM was collected from the Ohio River, Cincinnati, Ohio, USA, which contains sulfate about 100–500 mg/L. Concentrated NOM with a sulfate concentration of 5000 mg/L was diluted to 328 mg/L using DI water and continuously injected into the column type of reactor with a diameter of 2.25 cm. For all experiments, PCL/Ba samples of 1 g were added in the column reactor. The experimental conditions were decided based on the preliminary tests performed at the beginning of this study (not shown). The concentration of sulfate was analyzed using a HACH spectrophotometer (DR 2700) with the USEPA SlulfaVer 4 method, which is equivalent to USEPA method 375.4. The recently updated SlulfaVer 4 method (10th edition, released in February 2018) reported that Ba interferes with sulfate quantification. The maximum error may occur at a concentration about 20% lower than the actual sulfate concentration when Ba concentration is very high.

3. Results and Discussion

XRD analysis was performed to conﬁrm the phase changes from BaCO3 to BaSO4 after sulfate removal. The obtained XRD patterns for PLC/Ba samples before and after sulfate adsorption are shown in Figure1. All the diffraction peaks were indexed with reference to the unit cell of the barite structure (JCPDS card: 24-1035) or witherite structure (JCPDS card: 00-044-1487). The diffraction peaks of (101), (111), (021), (121), (002), and (212) were characteristic peaks of orthorhombic BaSO4 crystal, demonstrating that the product BaSO4 formed. These results indicate that PCL/Ba samples effectively removed sulfate through the formation of BaSO4. Additionally, these results are in good agreement with a previous report [9], which showed the formation of BaSO4 after sulfate adsorption using BaCO3/PVC composites. However, in the case of the PCL/Ba (50/50) sample, the intensity of Water 2018, 10, 1789 4 of 13

Water 2018, 10, x FOR PEER REVIEW 4 of 13 the peak corresponding to barium sulfate is weak due to the presence of increased polymer content peak corresponding to barium sulfate is weak due to the presence of increased polymer content that that restricts the diffusion of sulfate molecules towards buried barium ions. restricts the diffusion of sulfate molecules towards buried barium ions.

FigureFigure 1. XRD 1. XRD pattern patternof of beforebefore sulfate sulfate adsorption adsorption (a) PCL/Ba (a) PCL/Ba (50/50), (50/50), (b) PCL/Ba (b) (2.5/97.5), PCL/Ba and (2.5/97.5), (c)

and (PCL/Bac) PCL/Ba (10/90), (10/90), and after and sulfate after adsorption sulfate adsorption (d) PCL/Ba (50/50), (d) PCL/Ba (e) PCL/Ba (50/50), (2.5/97.5), (e) PCL/Ba and (f) PCL/Ba (2.5/97.5), and (f(10/90).) PCL/Ba (10/90).

FigureFigure2a–h shows2a–h shows SEM images SEM ofimages control of BaCOcontrol3 and BaCO PCL/Ba3 and samplesPCL/Ba withsamples different withmagnifications. different Figuremagnifications.2a,b represents Figure control 2a,b BaCO represents3 with control low andBaCO high3 with magnification, low and high respectively.magnification,The respectively. particle sizes rangedThe from particle a few sizes hundred ranged from nanometers a few hundred to microns nanometers in length. to microns Elongated in length. rod-shaped Elongated rod-shaped structures with hexagonalstructures phases with were hexagonal observed. phases Figure were observed.2c–h show Figure SEM 2c–h images show of SEM PCL/Ba images samples of PCL/Ba before samples barium before barium sulfate precipitation. BaCO3 particles were dispersed without surface morphology sulfate precipitation. BaCO3 particles were dispersed without surface morphology changes in the changes in the polymer matrix. BaCO3 particles aggregated with an increase in the amount of polymer polymer matrix. BaCO particles aggregated with an increase in the amount of polymer in the PCL/Ba in the PCL/Ba composite3 (Figure 2c), indicating less polymer content in the composites is better to composite (Figure2c), indicating less polymer content in the composites is better to effectively disperse effectively disperse BaCO3 in the composites (Figure 2e,g). The control BaCO3 crystals dispersed in BaCOPCL3 in had the elongated composites structures (Figure with2e,g). hexago Thenal control faces BaCOas seen3 incrystals Figure 2d,f,h. dispersed in PCL had elongated structuresFigure with hexagonal3a–f shows facesSEM images as seen of in PCL/Ba Figure (50/50),2d,f,h. (10/90), and (2.5/97.5) with lower and higher Figuremagnification3a–f shows after SEM sulfate images adsorption, of PCL/Ba respectively. (50/50), (10/90),Well-defined and (2.5/97.5) barium sulfate with lower cubes and were higher magnificationobserved in after all sulfatePCL/Ba adsorption,composites. respectively.In a previous Well-definedstudy [9], the bariumformation sulfate of BaSO cubes4 with were a cubic observed in allstructure PCL/Ba was composites. reported after In a previoussulfate adsorption study [9 using], the BaCO formation3. In addition of BaSO to4 withthe formation a cubic structureof BaSO4 was cubes, the bulk morphology of PCL in PCL/Ba (50/50) sample was observed due to the high content reported after sulfate adsorption using BaCO3. In addition to the formation of BaSO4 cubes, the bulk morphologyof PCL in of the PCL sample. in PCL/Ba Interestingly, (50/50) random sample wassized observed BaSO4 cubes due were to the observed high content in theof samples PCL in the containing less BaCO3. Based on these results, it could be hypothesized that more controlled and well- sample. Interestingly, random sized BaSO4 cubes were observed in the samples containing less BaCO3. defined BaSO4 crystals can be precipitated when the barium content of the PCL/Ba composite is Basedlower. on these The results,reasoning it couldbehind be such hypothesized a trend has that been more explained controlled by Kucher and well-defined et al., who studied BaSO4 thecrystals can beinfluence precipitated of supersaturation when the barium and free content lattice of ion the ra PCL/Batio on barium composite sulfate is lower.particle Theformation. reasoning While behind suchheterogeneous a trend has been nucleation explained dominates by Kucher crystal et al., formation who studied at lower the influencesupersaturation of supersaturation levels, it is and free latticesubstituted ion ratio by onhomogeneous barium sulfate nucleation particle at formation. higher levels While of heterogeneous supersaturation. nucleation Homogeneous dominates crystalnucleation formation promotes at lower the formation supersaturation of smaller levels, crystals it up is substitutedto about 100 nm by in homogeneous size as opposed nucleation to larger at higherwell-defined levels of supersaturation. crystals at lower Homogeneous levels. Similarly, nucleation in the present promotes study, the well-defined formation ofcrystals smaller were crystals up to about 100 nm in size as opposed to larger well-defined crystals at lower levels. Similarly, in the present study, well-defined crystals were observed in composites with lower barium content as compared to random, smaller crystals formed with higher levels of barium-containing composites [19]. Water 2018, 10, x FOR PEER REVIEW 5 of 13

Water 2018, 10, 1789 5 of 13 observed in composites with lower barium content as compared to random, smaller crystals formed with higher levels of barium-containing composites [19].

FigureFigure 2. SEM 2. SEM images images of of control control BaCO 33 ((aa)) andand ( b(),b), and and PCL/Ba PCL/Ba samples; samples; (50/50) (50/50) (c) and (c) (andd), (10/90) (d), (10/90) (e) (e) andand ( (ff),), and (2.5/97.5)(2.5/97.5) ((gg)) andand ( h(h)) at at low low and and high high resolution. resolution.

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(a) (b)

20 μm 2 μm

(e) (f)

20 μm 2 μm

FigureFigure 3. SEM 3. SEM Images Images after after BaSO BaSO4 precipitation;4 precipitation; (a) (anda) and (b) (PCL/Bab) PCL/Ba (50/50); (50/50); (c) and (c) and(d) PCL/Ba (d) PCL/Ba (10/90), and(10/90), (e) and and (f) PCL/Ba (e) and ( f(2.5/97.5)) PCL/Ba composites. (2.5/97.5) composites. Figure4a–f shows TEM images of PCL/Ba composites before and after sulfate adsorption. Before Figure 4a–f shows TEM images of PCL/Ba composites before and after sulfate adsorption. Before sulfate adsorption, elongated BaCO3 was observed in all samples, as seen in Figure2a–h. After sulfate sulfate adsorption, elongated BaCO3 was observed in all samples, as seen in Figure 2a–h. After sulfate removal, the transformation from a rod-shaped structure to the cubic structure was observed, which is removal, the transformation from a rod-shaped structure to the cubic structure was observed, which in good agreement with the SEM images (Figure3b,d,f). These results indicate the formation of BaSO 4 is in good agreement with the SEM images (Figure 3b,d,f). These results indicate the formation of cubes after sulfate adsorption. However, the formation of the BaSO4 cube with a uniform size was 4 4 BaSOnot cubes observed after in sulfate the PCL/Ba adsorption. (50/50) Ho sample.wever, Due the to formation the high contentof the BaSO of PCL cube in the with sample, a uniform BaCO 3size wasparticles not observed were capped in the withPCL/Ba PCL (50/50) as seen sample. in Figure Due4b. Toto the conﬁrm highthe content formation of PCL of BaSOin the4 sample,after sulfate BaCO3 particlesremoval, were HR-TEM capped analysis with PCL was as performed seen in Figure in PCL/Ba 4b. To (10/90) confirm sample. the Figureformation5a,b showsof BaSO the4 HR-TEMafter sulfate removal,images HR-TEM of the PCL/Ba analysis (10/90) was sampleperformed after in sulfate PCL/Ba removal. (10/90) sample. Figure 5a,b shows the HR-TEM images of the PCL/Ba (10/90) sample after sulfate removal.

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(a) (b)

2 μm 0.5 μm (c) (d)

2 μm 0.5 μm (e) (f)

2 μm 0.5 μm

FigureFigure 4. TEM 4. TEM Images Images before before (a,c (,ea),c and,e) and after after (b,d (b,f,)d BaSO,f) BaSO4 precipitation;4 precipitation; (a) (anda) and (b) (PCL/Bab) PCL/Ba (50/50), (50/50), (c) and(c) and (d) PCL/Ba (d) PCL/Ba (10/90), (10/90), and ( ande) and (e) ( andf) PCL/Ba (f) PCL/Ba (2.5/97.5). (2.5/97.5).

As Asseen seen in Figure in Figure 5a,5 thea, the lattice lattice spacing spacing of 0.210 of 0.210 and and 0.312 0.312 nm nm corresponding corresponding to (212) to (212) and and (121) (121) planeplane of Barite, of Barite, respectively, respectively, was was measured, measured, which which indicates indicates the theformation formation of BaSO of BaSO4 after4 after sulfate sulfate adsorption.adsorption. In addition, In addition, the themeasured measured lattice lattice spacin spacingg in the in thesample sample after after NOM NOM removal removal as sulfate as sulfate was was 0.2110.211 nm nmcorresponding corresponding to (212) to (212) plane plane of Barite, of Barite, indicating indicating the the PCL/Ba PCL/Ba sample sample effectively effectively removes removes sulfatesulfate in water. in water. These These results results are arein good in good agreemen agreementt with with the theresults results of XRD of XRD analysis, analysis, confirming conﬁrming the the formationformation of BaSO of BaSO4 after4 after sulfate sulfate adsorption. adsorption. TableTable 2 summarizes2 summarizes theoretical theoretical an andd experimental experimental sulfate sulfate adsorption adsorption capacity capacity of ofPCL/Ba PCL/Ba samples samples obtainedobtained from from various various SST, SST, conducted conducted using using different different size size columns columns and and various various PCL/Ba PCL/Ba composites. composites. TheThe theoretical theoretical capacity capacity of the of themedia media was was determined determined by taking by taking into into account account that, that, according according to the to the chemicalchemical reaction, reaction, 1 mole 1 mole of barium of barium reacts reacts with with 1 mole 1 mole of sulfate of sulfate to form to form barium barium sulfate. sulfate. Therefore, Therefore, the thebarium barium content content in the in themedia media determined determined using using EDS EDS was was used used to calculate to calculate the thetheoretical theoretical sulfate sulfate removalremoval capacity capacity of the of themedia media in mg/g. in mg/g. The results The results shown shown in Table in Table 2 indicate2 indicate that a that lesser a lesser amount amount of polymerof polymer with a with higher a higher barium barium content content in the in comp the compositesosites helps helps improve improve the capacity the capacity of the of themedia. media. TheThe composite composite with with lowest lowest PCL PCL (2.5%) (2.5%) and and highest highest barium barium content content (97.5%) (97.5%) was was capable capable of achieving of achieving 72.4%72.4% of the of thetheoretical theoretical capacity, capacity, as opposed as opposed to the to the PCL/Ba PCL/Ba (50/50) (50/50) which which performed performed poorly. poorly.

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(a) (b)

2 nm 2 nm

FigureFigure 5. 5.HR-TEM HR-TEM images imagesof ofthe thePCL/Ba PCL/Ba (10/90)(10/90) sample sample after after sulfate sulfate removal: removal: (a ()a )dissolved dissolved sulfate sulfate andand ( b(b)) natural natural organic organic matter matter (NOM)(NOM) asas sulfate.sulfate. Table 2. Theoretical and experimental sulfate adsorption capacity of PCL/Ba samples obtained from aTable small-scale 2. Theoretical column testand (SST).experimental sulfate adsorption capacity of PCL/Ba samples obtained from a small-scale column test (SST). Material Theoretical (mg/g) Experimental (mg/g) Percent (%) Material Theoretical (mg/g) Experimental (mg/g) Percent (%) PCL/Ba (50/50); PCL/Ba (50/50); 243.4 2.8 1.2 D = 1 cm 243.4 2.8 1.2 PCL/BaD = 1 cm (10/90); 438.1 262.1 59.8 PCL/BaD (10/90); = 1 cm PCL/Ba (10/90); 438.1 262.1 59.8 D = 1 cm 438.1 223.8 51.1 D = 2.25 cm PCL/Ba (10/90); PCL/Ba (2.5/97.5); 438.1 223.8 51.1 474.6 343.8 72.4 D =D 2.25 = 2.25 cm cm PCL/Ba (2.5/97.5); 474.6 343.8 72.4 D = 2.25 cm Thermogravimetric analysis was done on various BaCO3 loaded polycaprolactone composites and theseThermogravimetric thermograms areanalysis shown was in done Figures on 6various and7. BaCO The control3 loaded polycaprolactone polycaprolactone has composites a broad ◦ decompositionand these thermograms temperature are around shown 380 in toFigures 415 C. 6 Theand addition7. The control of BaCO polycaprolactone3 decreased thedecomposition has a broad temperaturesdecomposition (see temperature Figure6). In addition,around 38 the0 decompositionto 415 °C. The temperatures addition areof veryBaCO sharp3 decreased compared the to puredecomposition polycaprolactone. temperatures The PCL/Ba (see Figure (90/10) 6). has In aad sharpdition, decomposition the decomposition temperature temperatures at around are 335very◦C, ◦ whereassharpWater 2018 compared PCL/Ba, 10, x FOR (50/50) PEERto pure REVIEW has polycaprolactone. a decomposition temperatureThe PCL/Ba of (90/10) 320 C. has a sharp decomposition9 of 13 temperature at around 335 °C, whereas PCL/Ba (50/50) has a decomposition temperature of 320 °C. The increase in the quantity of BaCO3 loaded in the polymer composite in turn accelerated the early decomposition temperatures. For example, PCL/Ba (30/70) has a broad decomposition temperature from 250 to 300 °C and PCL/Ba (10/90) has a broad decomposition temperature of 200 to 250 °C, which is much lower compared to PCL/Ba (90/10).

Figure 6. Thermogram of pure polycaprolactone polymer. Figure 6. Thermogram of pure polycaprolactone polymer.

The drastic decrease in decomposition temperatures resulting from increasing BaCO3 content is unclear and requires detailed mechanistic studies. Similar observations were reported by Liu et al., wherein higher CaSO4 loading in polycaprolactone composites decreased decomposition temperatures [20]. Possible reasons for the abnormal decrease in temperature may have been due to poor recrystallization and weak bonding of polycaprolactone in the presence of BaCO3. However, a different trend was observed by Lee et al. in the case of carbon nanotubes (CNT) dispersed polycaprolactone composites. Thermal stability increased even in the presence of a small amount of multi-walled CNT-Cl, which suggests the homogeneous dispersion in the composites CNT [21]. Trapped solvent decomposition may have also played a role in causing the abnormal decrease in temperature.

Figure 7. Thermograms of PCL/Ba composites: (a) 90/10, (b) 50/50, (c) 30/70, and (d) 10/90.

Figure 8 shows the sulfate removal of PCL/Ba composites with column tests. Symbols of white circles depict the influent sulfate concentration, which was kept constant at 1100 mg/L. There are two significant results shown in Figure 8. Firstly, the sulfate removal depends on the amount of polymer

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The increase in the quantity of BaCO3 loaded in the polymer composite in turn accelerated the early decomposition temperatures. For example, PCL/Ba (30/70) has a broad decomposition temperature from 250 toFigure 300 ◦C 6.and Thermogram PCL/Ba (10/90)of pure polycaprolactone has a broad decomposition polymer. temperature of 200 to 250 ◦C, which is much lower compared to PCL/Ba (90/10). The drastic drastic decrease decrease in in decomposition decomposition temperatures temperatures resulting resulting from from increasing increasing BaCO BaCO3 content3 content is unclearis unclear and and requires requires detailed detailed mechanistic mechanistic studies. studies. Similar observations Similar observations were reported were by reported Liu et al., by 4 whereinLiu et al., whereinhigher CaSO higher CaSOloading4 loading in polycaprolactone in polycaprolactone composites composites decreased decreased decomposition decomposition temperatures [20]. [20]. Possible Possible reason reasonss for for the the abnormal abnormal decrease decrease in in temperature temperature may may have have been been due due to to poor recrystallization and weak bonding of polycaprolactone in the presence of BaCO3. However, a poor recrystallization and weak bonding of polycaprolactone in the presence of BaCO3. However, differenta different trend trend was was observed observed by by Lee Lee et et al. al. in inthe the case case of of carbon carbon nanotubes nanotubes (CNT) (CNT) dispersed dispersed polycaprolactone composites. Thermal Thermal stability stability increa increasedsed even even in in the the presence presence of of aa small small amount amount of of multi-walled CNT-Cl, CNT-Cl, which which suggests suggests the the homogeneous homogeneous dispersion dispersion in in the the composites composites CNT CNT [21]. [21 ]. Trapped solvent decomposition decomposition may may have have also also played played a role a role in incausing causing the theabnormal abnormal decrease decrease in temperature.in temperature.

FigureFigure 7. 7.Thermograms Thermograms of of PCL/Ba PCL/Ba composites:composites: (a)) 90/10,90/10, ( (bb)) 50/50, 50/50, (c ()c )30/70, 30/70, and and (d ()d 10/90.) 10/90.

Figure 88 showsshows thethe sulfatesulfate removalremoval ofof PCL/BaPCL/Ba co compositesmposites with with column column te tests.sts. Symbols Symbols of of white white circles depict the influent influent sulfate concentration, which was was kept kept constant constant at at 1100 1100 mg/L. mg/L. There There are are two two significantsignificant results shown shown in in Figure Figure 88.. Firstly, thethe sulfate removal depends on the the amount amount of of polymer polymer and barium in the composites. The sample, containing a small amount of polymer (and consequently a large amount of barium), demonstrated higher sulfate removal compared to other samples. In the case of the PCL/Ba (50/50) sample, the column is saturated earlier compared to other samples due to a lower barium content, while a similar sulfate removal pattern is observed for PCL/Ba (2.5/97.5) and (10/90) composites. The existence of such a direct proportionality between the efficiency of sulfate removal and the cation content of the media has been studied by other researchers [22]. Sulfate in the effluent was not detected for the first 10 minutes, while after this point it spiked to about 600 mg/L and maintained this level until about 4000 and 7000 min for PCL/Ba (2.5/97.5) and (10/90), respectively. Such a pattern in sulfate removal using PCL/Ba media can be explained by mass transfer principles. In the initial phase, barium available on the exterior of the packed column comes in contact with sulfate and quickly becomes saturated. After that, it takes a longer time for the sulfate molecules to access the barium molecules buried inside of the polymer matrix. Once all the barium molecules are Water 2018, 10, x FOR PEER REVIEW 10 of 13

and barium in the composites. The sample, containing a small amount of polymer (and consequently a large amount of barium), demonstrated higher sulfate removal compared to other samples. In the case of the PCL/Ba (50/50) sample, the column is saturated earlier compared to other samples due to a lower barium content, while a similar sulfate removal pattern is observed for PCL/Ba (2.5/97.5) and (10/90) composites. The existence of such a direct proportionality between the efficiency of sulfate removal and the cation content of the media has been studied by other researchers [22]. Sulfate in the effluent was not detected for the first 10 minutes, while after this point it spiked to about 600 mg/L and maintained this level until about 4000 and 7000 min for PCL/Ba (2.5/97.5) and (10/90), Water 2018respectively., 10, 1789 Such a pattern in sulfate removal using PCL/Ba media can be explained by mass transfer 10 of 13 principles. In the initial phase, barium available on the exterior of the packed column comes in contact with sulfate and quickly becomes saturated. After that, it takes a longer time for the sulfate molecules saturatedto access with adsorbedthe barium molecules sulfate, the buried media inside reaches of the polymer complete matrix. saturation Once all andthe barium is not molecules capable of any are saturated with adsorbed sulfate, the media reaches complete saturation and is not capable of any further sulfate removal. further sulfate removal. The wallThe effect wall effect was was studied studied in in the the process process byby conducting experiments experiments in smaller in smaller and larger and larger diameterdiameter columns columns loaded loaded with with the the same same amount amount ofof PCL/BaPCL/Ba media. media. As Asseen seen in Figure in Figure 8, the 8friction, the friction was reducedwas reduced signiﬁcantly significantly due due to the to the wall wall surface surface whenwhen larger larger diameter diameter columns columns were wereused. used.Although Although saturationsaturation in the in larger the larger diameter diameter column column (d (d = = 2.25 2.25 cm) was was achieved achieved much much quicker quicker compared compared to the to the smallersmaller diameter diameter (d = 1.0(d = cm)1.0 cm) column, column, the the percentage percentage of of sulfate sulfate removal removal was washigher higher in the insmaller. the smaller. Higher sulfate removal in the smaller column was the result of a longer retention time and hence Higher sulfate removal in the smaller column was the result of a longer retention time and hence better better diffusion of sulfate molecules through the polymer matrix. diffusion of sulfate molecules through the polymer matrix.

Figure 8. Comparison of remediation media for sulfate removal. Figure 8. Comparison of remediation media for sulfate removal. Figure9 shows NOM removal as sulfate using PCL/Ba media. After passing sulfate containing Figure 9 shows NOM removal as sulfate using PCL/Ba media. After passing sulfate containing NOM throughNOM through the column the column for aboutfor about 500 500 min, min, the the PCL/BaPCL/Ba (10/90) (10/90) media media was wasable to able remove to remove more more than 90%than of 90% the of influent the influent sulfate. sulfate. A previousA previous study study on sulfate sulfate removal removal from from wastes wastes by precipitation by precipitation by Beattiby et Beatti al. [23et al.] reported [23] reported that that only only 61.4% 61.4% of of thethe influentinfluent sulfate sulfate was was removed removed using using the barium the barium precipitationprecipitation technique technique under under a sulfate a sulfate concentration concentration of of 80 80 g/L g/L in the in theinfluent. influent. In the In present the present study, study, the effluentthe effluent sulfate sulfate concentrations concentrations remained remained well be belowlow 30–50 30–50 mg/L mg/L for the for first the two first hours two except hours for except an outlier seen at 1 hour, removing more than 90% of influent sulfate because lower sulfate for an outlier seen at 1 hour, removing more than 90% of influent sulfate because lower sulfate concentration (328 mg/L) was fed into the column. Such promising results show that the media used concentrationin this study (328 has mg/L) a very was high fed capacity into the to remove column. low Such concentrations promising of sulfate, results and show further that studies the media are used in this studyrequired has in a pilot very scale high to capacityevaluate a to long-term remove activity low concentrations of these media. of sulfate, and further studies are required in pilotWater scale 2018, 10 to, x FOR evaluate PEER REVIEW a long-term activity of these media. 11 of 13

350

300 effluent Influent 250

200

150

concentration (mg/L) concentration 100 2- 4 50 SO

0 0 5 10 15 20 30 45 60 120 180 210 240 330 390 450

Time (min) Figure 9. SulfateFigure concentration 9. Sulfate concentration in NOM in NOM samples samples in in inﬂuentinfluent and and effluent efﬂuent of PCL/Ba of (10/90) PCL/Ba packed (10/90) packed columns (Diameter: 2.25 cm). columns (Diameter: 2.25 cm). Table 3 shows the cumulative capacity of the PCL/Ba (10/90) composite used to remove sulfate from NOM containing Ohio River water. After passing sulfate containing water through the column for 450 min, the polymer composite was saturated, indicating that all the accessible barium molecules were bound with sulfate molecules to form barium sulfate. From Table 3, it can be seen that PCL/Ba (10/90) exhibited a high capacity and removed more than 90% of the influent sulfate, and the cumulative capacity was calculated to be 175.8 mg/g.

Table 3. Cumulative capacity of PCL/Ba (10/90) composite in the presence of SO4 and NOM (Influent Concentration of sulfate: 328 mg/L and Flow rate: 2.0 mL/min).

Delta Effluent Cumulative Bed Cumulative Elapsed Capacity of Conc Feed Volume Volumes Capacity of Time (min) Media (mg/L) (mL) Fed(mL) Media (mg/g) (mg/g) 0 58 0 0 0.0 0.0 5 32 10 4 2.8 2.8 10 23 20 8 3.0 5.8 15 14 30 13 3.1 8.9 20 19 40 17 3.1 12.0 30 18 60 25 6.2 18.2 45 23 90 38 9.2 27.5 60 103 120 51 8.0 35.4 120 14 240 102 32.3 67.8 180 6 360 153 38.2 105.9 210 5 420 178 19.4 125.3 240 5 480 204 19.4 144.6 330 317 660 280 30.1 174.7 390 324 780 331 0.9 175.6 450 328 900 382 0.2 175.8

Water 2018, 10, 1789 11 of 13

Table3 shows the cumulative capacity of the PCL/Ba (10/90) composite used to remove sulfate from NOM containing Ohio River water. After passing sulfate containing water through the column for 450 min, the polymer composite was saturated, indicating that all the accessible barium molecules were bound with sulfate molecules to form barium sulfate. From Table3, it can be seen that PCL/Ba (10/90) exhibited a high capacity and removed more than 90% of the inﬂuent sulfate, and the cumulative capacity was calculated to be 175.8 mg/g.

Table 3. Cumulative capacity of PCL/Ba (10/90) composite in the presence of SO4 and NOM (Inﬂuent Concentration of sulfate: 328 mg/L and Flow rate: 2.0 mL/min).

Elapsed Time Efﬂuent Cumulative Feed Bed Volumes Delta Capacity of Cumulative Capacity (min) Conc (mg/L) Volume (mL) Fed (mL) Media (mg/g) of Media (mg/g) 0 58 0 0 0.0 0.0 5 32 10 4 2.8 2.8 10 23 20 8 3.0 5.8 15 14 30 13 3.1 8.9 20 19 40 17 3.1 12.0 30 18 60 25 6.2 18.2 45 23 90 38 9.2 27.5 60 103 120 51 8.0 35.4 120 14 240 102 32.3 67.8 180 6 360 153 38.2 105.9 210 5 420 178 19.4 125.3 240 5 480 204 19.4 144.6 330 317 660 280 30.1 174.7 390 324 780 331 0.9 175.6 Water 2018, 10, x 450FOR PEER REVIEW 328 900 382 0.2 175.8 12 of 13

Figure 10Figure shows 10 shows an SEM an SEM image image of of the the PCL/Ba PCL/Ba (10/90)(10/90) sample sample after after sulfate sulfate removal removal in the Ohio in the Ohio River waterRiver containing water containing NOM. NOM. The The SEM SEM image image demonstrates demonstrates irregular irregular shaped shaped barium barium sulfate sulfate crystals crystals with differentwith different sizes ranging sizes ranging from from200 nm 200 to nm 1 toμm 1 µ werem were formed formed after after sulfate sulfate removal in in the the Ohio Ohio River water, whichRiver water,is in whichcontrast is in contrastto the towell-defined the well-deﬁned cu cubesbes formed formed inin the the presence presence of synthetic of synthetic sulfate sulfate containing solutions as shown in Figures3 and4. containing solutions as shown in Figures 3 and 4.

Figure 10. SEM image of BaSO4 formed in the presence of sulfate and NOM from Ohio River water. Figure 10. SEM image of BaSO4 formed in the presence of sulfate and NOM from Ohio River water.

4. Conclusions Use of barium encapsulated biodegradable polycaprolactone for environmental remediation of sulfate containing water was investigated. Various compositions of PCL/Ba composites were synthesized and tested by conducting SSTs to study the efficiency of sulfate removal. XRD, SEM, TEM, and HR-TEM analyses showed the formation of BaSO4 after sulfate removal, indicating sulfate was effectively removed using PCL/Ba composites. The sulfate removal capacity of PCL/Ba media was dependent on the amount of barium in the composites and was proportional to the amount of barium in the media. It was observed that PCL/Ba (10/90) had the highest capacity and was effective for sulfate removal from NOM samples under these experimental conditions. The sulfate removal was higher in the small column due to a longer retention time for mass transfer compared to the large column. Moreover, the use of a biodegradable polymer for material synthesis helps to overcome the concern over plastic leaching into water supplies.

Funding: INHA University Research Grant (INHA-57849-1).

Acknowledgment: This work was supported by INHA University Research Grant (INHA-57849-1).

Conflicts of Interest: The authors declare no conflict of interest.

References

1. Choi, H.; Stathatos, E.; Dionysiou, D.D. Sol–gel preparation of mesoporous photocatalytic TiO2 films and

TiO2/Al2O3 composite membranes for environmental applications. Appl. Catal. B 2006, 63, 60–67, doi:10.1016/j.apcatb.2005.09.012. 2. Obare, S.O.; Meyer, G.J. Nanostructured Materials for Environmental Remediation of Organic Contaminants in Water. J. Environ. Sci. Health A Tox. Hazard Subst. Environ. Eng. 2004, 39, 2549–2582, doi:10.1081/ESE-200027010. 3. Xu, J.; Bhattacharyya, D. Membrane-based bimetallic nanoparticles for environmental remediation: synthesis and reactive properties. Environ. Prog. 2005, 24, 358–366, doi:10.1002/ep.10106 .

Water 2018, 10, 1789 12 of 13

4. Conclusions Use of barium encapsulated biodegradable polycaprolactone for environmental remediation of sulfate containing water was investigated. Various compositions of PCL/Ba composites were synthesized and tested by conducting SSTs to study the efﬁciency of sulfate removal. XRD, SEM, TEM, and HR-TEM analyses showed the formation of BaSO4 after sulfate removal, indicating sulfate was effectively removed using PCL/Ba composites. The sulfate removal capacity of PCL/Ba media was dependent on the amount of barium in the composites and was proportional to the amount of barium in the media. It was observed that PCL/Ba (10/90) had the highest capacity and was effective for sulfate removal from NOM samples under these experimental conditions. The sulfate removal was higher in the small column due to a longer retention time for mass transfer compared to the large column. Moreover, the use of a biodegradable polymer for material synthesis helps to overcome the concern over plastic leaching into water supplies.

Author Contributions: Conceptualization, Methodology, Validation, Formal Analysis, Investigation, Writing-Original Draft Preparation, Writing-Review & Editing, and Visualization, C.H. and M.N.N.; Project Administration, M.N.N. Funding: INHA University Research Grant (INHA-57849-1). Acknowledgments: This work was supported by INHA University Research Grant (INHA-57849-1). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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