Prospects of Silk Sericin as an Adsorbent for ... - ACS Publications

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Prospects of Silk Sericin as an Adsorbent for Removal of Ibuprofen from Aqueous Solution Vishal Kumar Verma and Senthilmurugan Subbiah* Department of Chemical Engineering, Indian Institute of Technology, Guwahati-781039, India S Supporting Information *

ABSTRACT: This Article presents removal of ibuprofen from aqueous solution using commercially available silk sericin as an adsorbent in an integrated adsorbent-membrane process. The adsorption study was performed at different physiological conditions, such as adsorbent concentration (1−10g), ibuprofen concentration (10−70 mg/L), temperatures (20, 30, and 40 °C), and pH (5, 6, 7, and 8). The occurrence of adsorption before membrane separation was confirmed by performing analysis of sericin−ibuprofen interaction and complex formation. Sericin−ibuprofen interaction and complex formation was investigated using FTIR, FESEM, fluorescence spectroscopy, XRD, and ITC. Sericin and ibuprofen interaction is spontaneous and endothermic in nature, while random coil transition of Sericin governed the adsorption system. ITC analysis exhibited a binding affinity (Kb) value of 2.51 × 104 ± 1.4 and one binding site (n ≈ 1) per molecule at 27 °C, revealing moderate binding of ibuprofen to the sericin protein. Complete removal of ibuprofen (at 10 mg/L, pH 8) was achieved using 10 g of sericin (pH 4) and at a temperature of 40 °C in reverse osmosis membrane process. The results established in this work concludes that sericin may be used as an adsorbent for the removal of micropollutants, such as ibuprofen from drinking water.

1. INTRODUCTION Micropollutants are generated from many sources, and they are called contaminants, which are persistent and bioactive nature. These contaminants are dissolved in wastewater streams from multiple sources, such as pharmaceuticals and personal care products (PPCPs), endocrine disrupting chemicals (EDCs), steroid hormones, industrial chemical waste, pesticides, and many other recalcitrant compounds. These micropollutants cannot be removed with conventional wastewater treatment technologies. Therefore, removal of these substances during municipal wastewater treatment has been found to be incomplete.1−3 The continuous discharges of micropollutants in wastewater stream is expected to cause long-term hazards since the contaminants are bioaccumulating nature and even forming new mixtures in our water reuse cycle. The exact impact on ecological system by micropollutants are yet to be fully discovered. As per the recent lab analysis,4 the levels of drug pollutants measured in streams, lakes and well water near pharmaceutical factories in India are 100 000−1 000 000 times higher in comparison to the levels measured in waters that receive sewage effluent in the US or China. Many studies report the adverse effects of pharmaceuticals to the aquatic organisms, chronic long-term exposure to these drugs may have detrimental effects on metabolism of nontarget organisms including microbes fish and other aquatic organisms.5−7 Because of the profound use of diclofenac (a widely used nonsteroidal anti-inflammatory drug, NSAID), an unusual high © 2017 American Chemical Society

death rate among three different species of vulture was reported in India and Pakistan.8 The extent of the impact of pharmaceutical waste on human beings is not as defined and well studied. Still it has been a topic of debate recently and even in low dosages, long-term exposure to pharmaceutical drug remnants may be hazardous to humans.5 Both conventional and advanced water treatment technologies such as coagulation, flocculation, chlorination, advance oxidation process (AOP), adsorption with activated carbon and membrane filtration are widely used for drinking water treatment application.9 Coagulation and flocculation are primary treatment process and cannot be used for removal of pharmaceutical micro pollutants.10 While free chlorine was found to oxidize approximately half of the pharmaceuticals investigated, and chloramine was found to be less efficient.11 AOP, such as ozonation, UV/H2O2, UV/O3, UV/TiO2, and UV/Fenton, are very effective, however, these process may lead to formation of potentially harmful byproducts by reaction with background compounds in water, incomplete degradation products and interference of radical scavengers.12−15 Pressure-driven membrane filtration processes, such as nanofiltration (NF) and reverse osmosis (RO), are widely Received: Revised: Accepted: Published: 10142

May 1, 2017 July 28, 2017 August 18, 2017 August 18, 2017 DOI: 10.1021/acs.iecr.7b01827 Ind. Eng. Chem. Res. 2017, 56, 10142−10154

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Industrial & Engineering Chemistry Research

of micropollutants like heavy metals and textile dyes, there has been no report of any study involving sericin utilization in a membrane process for removal of drug-based micropollutants from drinking water. This work will establish viability of sericin as a potent candidate for membrane modification and deliver a modified membrane-adsorption system to remove ibuprofen from drinking water. Six different methods were used to study the adsorption behavior of ibuprofen in sericin.

used for drinking water treatment application to improve taste of the water by removing dissolved salts.16 The complete removal of salt from drinking water is also not desirable because that leads to mineral deficit issue. Both RO and NF process highly effective alternative methods for removal of organic micropollutants.17,18 However, selective and complete removal of micropollutants are not possible, and it is limited by membrane separation efficiency.17,19−22 Adsorption process have earned preference as a sustainable solution with respect to complete removal of micro pollutants from drinking water source.23 In addition, adsorption process enables complete capturing and safe disposal without producing toxic byproducts. PAC (powdered activated carbon) and GAC (granular activated carbon) has been reported to have high removal of pharmaceutical compounds, especially hydrophobic compounds.10,24 However, the selective removal and isolation of micro pollutant may be difficult task with PAC and GAC. Therefore, different materials have been investigated for their characteristics and adsorption capability for pharmaceutical drugs, some of the reported adsorbents includes soil minerals, metal oxide, siliceous material, zeolites, agricultural waste, industrial waste, and other materials.25−28 Still the identification of adsorbents with following properties are very important in the field of drinking water treatment: (i) nontoxic, (ii) low-cost, and (iii) selective removal capacity of pharmaceutical micropollutants. Moreover, most of the existing adsorbents are not compatible for surface modification of membrane (via coating, blending, or grafting) which becomes major issue for specific removal of desired micropollutants.29 In this Article, we have evaluated the feasibility of sericin uses as a material for surface modification in UF-membrane for selective removal of drugbased micropollutants. Sericin is a highly hygroscopic globular protein derived from silkworm cocoons with molecular weight in the range of 10− 400 kDa, depending on the method of extraction.30 It comprises of 18 amino acids, majority of which have strong polar side groups, such as hydroxyl, carboxyl, and amino groups.31 Sericin is a byproduct during silk degumming process. Recently, sericin has found its application in biomedical science, regenerative medicine, textile industry, and membrane fabrication.32 Sericin exhibits inherent properties like hydrophilicity, amphoteric, antioxidant, antifouling, antimicrobial and are nontoxic in nature.33,34 These physicochemical properties make it very suitable as a biosorbent for enhancing membrane surface properties (via coating immobilization or blending) for selective removal of drug based micropollutants from aqueous environment. Even though NF and RO are regarded as highly effective in removal of pharmaceuticals with varying degree of rejection (from 45% to 90%),19,20 trace quantities of target pharmaceutical compounds have been found to breach membrane barrier. But, because of the higher molecular weight of sericin molecule, RO membrane is enabling complete removal of sericin, while passing aqueous sericin solution.35 Therefore, the integrated Sericin based adsorption with RO process can be enable dual activity, that is, (i) selective adsorption of micropollutant with sericin molecule and (ii) removal of combined molecule in RO membrane. This novel approach will enable complete removal of micro pollutant from aqueous solution. This project aims harnessing the physical and chemical properties of sericin for synthesis of membrane filtration unit with desired qualities targeting removal of Ibuprofen from drinking water. Though, there has been few recent studies on use of sericin for removal

2. EXPERIMENTAL SECTION 2.1. Materials. Ibuprofen is a prototypical nonsteroidal antiinflammatory, over the counter (OTC) drug. Its wide therapeutic application for pain administration, high consumption rate, reported persistent occurrence in water, ecotoxicity, physicochemical properties, and validated analytical methods have prompted us to select it for current study.36,37 The target pharmaceutical compound Ibuprofen (CAS no. 15687-27-1) was obtained from Sigma-Aldrich Chemicals Pvt. Limited. The stock solutions for ibuprofen were prepared in Millipore water. The stock solutions were used to obtain the calibration curves and for batch adsorption studies. All dilutions were done in Millipore water. The calibration curve for high performance liquid chromatography (HPLC) analysis was prepared (see Supporting Information S1) and was found linear up to 10 mg/L of concentrations with R2 value of 0.99. Acetonitrile and buffer solution used for mobile phase preparation were of HPLC grade. All other chemicals used in the study were of analytical grade and were procured from Merck, India. Commercially available pharmaceutical grade silk sericin was used in this study, and was procured from Swapnaroop Drugs and Pharmaceuticals, India. 2.2. Methods. 2.2.1. Characterization. 2.2.1.1. Sericin Adsorbent. Sericin used in this study had a molecular weight of ∼250 kDa. The structural morphology of the Sericin powder used was visually analyzed using (FE-SEM) field emission scanning electron microscopy (make, Zeiss; model, Sigma) instrument. Powder sample was fixed on top of the stub using carbon tape and layered with gold using an auto fine coating instrument (JEOL JFC-1300) prior to morphology analysis. Surface area and pore size for sericin powder was analyzed using BET method in Quantachrome surface area and pore size analyzer (Model, Autosorb-IQ MP). 2.2.1.2. RO-Membrane. Flat sheet RO-membrane from DOW FILMTEC of dimension 7.007 × 3.293 cm with an effective area of 22.935 cm2 was used for filtration purpose (see Supporting Information S2). FE-SEM of RO-membrane sheet was analyzed before and after filtration process to see the fouling and deposition on to the membrane. Membrane sheet was dried at room temperature and was fixed on top of the stub using carbon tape and double layer coated with gold preceding the morphology analysis. 2.2.2. Methods Used to Study Sericin and Ibuprofen Interaction. 2.2.2.1. Method 1: Integrated Adsorption-CumRO Process. Generally, the MWCO (molecular weight cutoff) value of DOW Filmtec RO membrane lies in the range of ∼200−400 Da. Ibuprofen has a molecular weight of 206 Da, and hence, RO membrane are not supposed to retain it efficiently. On the other hand, the MWCO for sericin is found to be 10−400 kDa; therefore, sericin is expected to be retained completely by RO membrane. Fabiani et al. (1996)38 used UF/ RO filtration method for filtration of wastewater from silk degumming processes and the overall rejection of sericin was found to be more than 97%. Li et al. (2015)35 performed 10143

DOI: 10.1021/acs.iecr.7b01827 Ind. Eng. Chem. Res. 2017, 56, 10142−10154

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Industrial & Engineering Chemistry Research sericin separation experiment using both silk degumming wastewater and commercial grade purified sericin in NF hollow fiber membrane to study effect of sericin molecular weight. They observed complete removal with commercial grade pure Sericin and partial removal achieved for silk degumming wastewater and similar results are observed by Capar et al. (2012).39 Further, to achieve complete removal of sericin by RO membrane, the purified higher molecular weight sericin has to be used. On the basis of these facts, we have designed an integrated method where sericin specific affinity toward ibuprofen has been coupled with membrane filtration for its removal from aqueous solution. Schematics of size based separation of ibuprofen using sericin adsorbent together with RO-membrane shown in Figure 1. The drug molecules are

Figure 2. Integrated adsorption−membrane filtration setup.

UV-Detector to quantify the concentration of Ibuprofen at λmax = 222 nm. The mobile phase was a mixture of acetonitrile and 10 mM phosphate buffer, pH 2.6 (70:30 v/v) with a flow rate of 1.8 mL/min in isocratic mode. 2.2.2.2. Method 2: ATR-Fourier Transform Infrared Spectroscopy. The absorption of IR-radiation causes vibrational transition in polypeptide units of protein (sericin) and this phenomenon gives nine characteristics bands named as amideA, amide-B, and amide-I−VII. Among these the amide-I (1600 to 1700 cm−1) is most sensitive to composition of protein secondary structure. The variation in corresponding IR vibrational frequencies can provide information related to structural conformation of Sericin due to Sericin and Ibuprofen interaction.40 Hence ATR-FTIR spectra (4000−400 cm−1) were acquired using ATR spectrophotometer (model, Spectrum TWO from PerkinElmer) to confirm structural conformation of Sericin after complex formation with ibuprofen. Experiments were conducted at room temperature with 32 scans per second, at a resolution of 4 cm−1 and obtained data were analyzed using OMNIC software. 2.2.2.3. Method 3: Fluorescence Spectroscopy. The protein folding and unfolding is expected while interacting with ligand. This phenomenon leads to exposure of tyrosine and tryptophan present in the protein from core to solvent phase and similar phenomenon is expected to happen while Sericin and Ibuprofen interaction. The fluorescence intensity of both tyrosine and tryptophan can be measured by fluorescence quenching mechanism.40 Therefore, the drug-mediated sericin conformational changes were monitored by tyrosine intrinsic fluorescence. Drug (ibuprofen)-induced tyrosine intrinsic fluorescence quenching studies were carried out on spectrofluorometer (model, fluoromax-4) at room temperature. One micromolar of sericin protein was incubated with diverse range of ibuprofen drug concentrations from 0.1 mM to 1 mM, while maintaining pH 7.5 using 10 mM of phosphate buffer at room temperature. Samples were excited at 280 nm (2 nm slit width) and collect emissions between 300 and 450 nm and averaging three scans. 2.2.2.4. Method 4: Isothermal Titration Calorimeter (ITC). ITC measures the changes in the power needed to maintain the isothermal conditions between the reference and sample cell, which gives the measures of heat absorbed or generated when molecules interact.41 These heat changes are very low level (submillionths of a degree), but can be detected using high sensitivity thermocouple. The binding of ibuprofen drug with sericin protein is expected to be endothermic/exothermic in nature. The heat of adsorption during complex formation was measured by isothermal titration calorimetry using MicroCal iTC-200 (MicroCal, Northampton, MA, USA). Sericin protein

Figure 1. Schematic of size based separation of ibuprofen using sericin adsorbent and RO-membrane.

supposed to adsorb on sericin and thereafter based on size based steric exclusion mechanism, efficient removal of these drugs can be expected from RO-membrane with almost complete rejection of sericin. In this method, initially batch adsorption studies were carried out on a magnetic stirrer operated at a constant speed of 350 rpm, coupled with digital temperature controller for temperature regulation. In each test, two liter of Ibuprofen solution (of required concentration) was placed in a 5-L beaker and required concentration of Sericin was added from stock solution (1000 ppt). Ibuprofen has a pKa value of ∼4.9 and hence, exist in anionic form when released in to the environment.26 While, Sericin has an isoelectric point (pI) close to pH ≈ 5−6, where it has zero overall charge and the smallest intermolecular repulsive force.39 Since, proteins have net negative charge above their pI values (pH > pI) and net positive charge below their pI values (pH < pI). Sericin used throughout adsorption experiment was kept fixed at pH 4 (to keep it in protonated state), while ibuprofen pH was changed from 5 to 8 to facilitate charged based interaction. The solution was stirred continuously for required time and temperature was monitored using temperature controller. Sericin−ibuprofen solution was passed as feed through a RO-flat sheet membrane at regulated flow rate using bypass valve and dampner (see Figure 2). The setup was run in cross-flow filtration mode and recirculation mode, that is, permeate and reject recirculated back to the feed tank. As per the membrane manufacture’s guideline, a feed flow rate of 100 L/h and transmembrane pressure of 75 psi was maintained throughout the process. Permeate samples were taken at regular intervals of time to measure permeate flux and Ibuprofen concentration. The feed and permeate samples were analyzed using HPLC analyzer (Shimadzu, model UFLC SPD-20A) equipped with a reverse-phase C-18 column (5 μm, 4.6 mm × 250 mm) and 10144


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Industrial & Engineering Chemistry Research and drug sample were prepared with 10 mM phosphate buffer (pH 7.5) and they were degassed through thermovac vacuum pump prior to use in ITC. In this experiment, 50 μM of sericin protein was filled in the sample cell and titrated with 4.5 mM of ibuprofen drug at 27 °C. Typically, 25 consecutive injections of 1.5 μL in two min interval were injected into the sample cell with adequate mixing. Heat of mixing due to the buffer solution is measured in separate experiment and were subtracted from total heat of adsorption. 2.2.2.5. Method 5: FE-SEM. FE-SEM images for powder sample of ibuprofen, sericin, and ibuprofen−sericin complex were acquired and analyzed for structural changes. The sericin− ibuprofen complex aqueous solution was prepared by mixing them in distilled water, then using open pan dry method (40 °C) the aqueous solution was dried to remove the water molecules from sericin and ibuprofen−sericin complex. Adsorption of ibuprofen on sericin was visually confirmed using FE-SEM. All the samples were dried at room temperature prior to analysis. 2.2.2.6. Method 6: X-ray Diffractometer. Amorphous and crystalline structure of sericin, ibuprofen, and complex were examined using XRD and analyzed for any change noticed after complex formation. X-ray diffraction pattern for native sericin, ibuprofen, and sericin−ibuprofen complex was obtained at ambient temperature using an X-ray diffractometer (XRD; make, Bruker; model, D8 Advance) using a step size of 0.05 deg/sin the range of 2θ = 5−80° under the acceleration voltage of 40 kV and 40 mA.

Figure 3. Flux behavior for sericin (10 g/L), ibuprofen (10 mg/L), and sericin-ibuprofen complex (1000:1 mg/L) at 517 kPa, pH 6, and 30 °C. (Sericin was removed completely in permeate in all cases.)

ibuprofen complex, which may be attributed to deposition of Sericin/complex on to the membrane surface. 3.2.2. Effect of Sericin Concentration. Effect of sericin concentration was evaluated by adding different concentration of sericin (1−10 g/L) with fixed initial concentration of ibuprofen (at 10 ppm) solution. The solution was stirred for 3 h on a magnetic stirrer. Experiment were conducted at 25 °C and without altering ambient pH. After 3 h the solution was filtered through RO membrane in complete recirculation mode and obtained results were evaluated. Sericin solution gives a pH of 6 with 10g/L of concentration without any alteration (see Supporting Information S4). Figure 4a shows variation observed in permeate concentration of ibuprofen with gradual increase of sericin concentration in feed. It was noticed that removal of ibuprofen increases with increasing amount of sericin and around 10g/L of sericin was needed for complete removal of ibuprofen at 10 ppm. Since, commercial ibuprofen is a racemic mixture and contains equal quantities of R(−)ibuprofen and S(+)-ibuprofen enantiomer, and hence larger amount of sericin is expected to be used for nonspecific binding and complete removal of ibuprofen. In another aspect, protein conformation also plays a vital role and binding is supposed to be site specific, which was later found in FTIR, ITC, and fluorescence studies that single binding site (n ≈ 1) is responsible for sericin−ibuprofen complex formation. More than 99% rejection was noticed with 10g/L of sericin for ibuprofen. The observed rejection of ibuprofen drug demonstrates the competence of sericin as an adsorbent to remove ibuprofen from aqueous solution. 3.2.3. Effect of Ibuprofen Concentration. Ibuprofen solution at different concentration were first filtered through the RO membrane and its inherent removal capacity was evaluated. Since it was found that around 10 g/L of sericin was required for complete removal of ibuprofen from feed solution from 10 ppm. Solution initial concentration of sericin is fixed at 10 g/L and corresponding Ibuprofen rejection efficiency was assessed at different concentration of Ibuprofen (from 10 to 70 ppm). RO experiments were conducted at room temperature and with Ibuprofen solution at pH 8. Figure 4b shows the ibuprofen concentration in RO permeate, while varying RO feed concentration. Permeate ibuprofen concentration is always found to be higher when feed solution is without sericin. On

3. RESULTS AND DISCUSSION 3.1. Sericin Characterization. The results obtained from BET analysis of sericin powder (Table 1) showed surface area Table 1. BET Analysis of Sericin sample

silk sericin

multi-point BET

surface area pore volume pore diameter surface area pore volume pore diameter surface area pore volume pore diameter

BJH adsorption

BJH desorption

5.473 m2/g 0.005 cc/g 4.05592 nm 2.939 m2/g 0.006 cc/g 7.111 nm 4.542 m2/g 0.007 cc/g 1.872 nm

of 5.47 m2g−1 with average pore diameter of 4.055 nm and pore volume of 5.550 × 10−3 cc/g, indicating a nonporous morphology. In a study involving removal of dye, Chen et al. (2012)42 reported a surface area of 1.5 m2g−1 for sericin powder obtained from a commercial source. 3.2. Experimental Studies in Integrated Adsorption− RO Process. 3.2.1. Flux through the RO Membrane. Pure water flux (see Supporting Information S3) curve was obtained for the RO-membrane sheet and the estimated membrane permeability is 0.281017 × 10−12 m3/m2·s·Pa. The membrane was preconditioned by operating for one hr at 50 Psig with distilled water before conducting the experiments with Sericinibuprofen solution. Permeate flux with respect to time for ibuprofen (10 mg/L), sericin (10g/L), and sericin−ibuprofen complex (1000:1) is shown in Figure 3. The flux study shows constant decline in water flux with time for Sericin and Sericin10145


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Figure 4. Variation in permeate-ibuprofen concentration (a) with increasing feed sericin concentration and at 10 ppm of initial ibuprofen conc. and (b) with increasing feed ibuprofen concentration, at 10 mg/L of sericin initial conc.

Figure 5. Variation in permeate-ibuprofen concentration (a) with increasing pH of ibuprofen feed solution and (b) at different temperature (20, 30, and 40 °C) with increasing time of contact.

the other hand, the rejection with sericin is always found to be higher than in the case without sericin. 3.2.4. Effect of Ibuprofen Solution pH. Solution pH is supposed to affect the surface charge on solute particles and hence play an important role in adsorption. Effect of pH on ibuprofen removal was assessed from a range of 5−8, to assess its effect on interaction of ibuprofen with sericin particles. Experiments were performed at 10g/L of sericin (at pH 4) and room temperature, while required pH of ibuprofen solution was adjusted using 1 M NaOH and 1 M HCl. Figure 5a shows effect of pH on adsorption of Ibuprofen and it was found that higher pH shows better removal capacity. This was reasonable because ibuprofen has pKa value of 4.9 and hence exists as neutral species below this value, and above this value, ibuprofen attains a negative charge and hence exists in anionic form. Sericin solubility has been found to decrease with increasing acidity of water. Moreover, it has been reported that Sericin solubility in

water is least at low pH, slowly escalates at a pH 5−8, and has high solubility from 8 onward.43 Hence, higher pH must have escalated charge-based interaction and nonspecific binding of negatively charged ibuprofen enantiomers with protonated Sericin particles, resulting in better removal capacity. 3.2.5. Effect of Solution Temperature. The separation efficiency for Ibuprofen with respect to temperature at constant presence of sericin (at 10g/L) is shown in Figure 5b. Since, temperature is one of the important factor that affects heat of mixing and heat of adsorption. The experiments were performed at three different temperatures (20, 30, and 40 °C) to assess the effect on seasonal removal capacity. It was observed that the adsorption of ibuprofen on sericin was a fast process and spontaneously almost 80% of drug was adsorbed within 5 min of contact time. However, after 5 min, the adsorption rate decreases over time until saturation was 10146


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Industrial & Engineering Chemistry Research Table 2. Comparison of Different Adsorbents for Removal of Drug-Based Micropollutants Sl. no. 1

adsorbents

3 4

granular activated carbon powdered activated carbon carbon nanotubes chitosan

5 6 7 8 9

cyclodextrin zeolites clay silica sericin

2

efficiency(%) 80−98 >90−99 >90 >90 >80 99 88−90 >95 complete removal

remarks

refs

not specific toward drugs, quick breakthrough, regeneration issue, not suitable for polymeric membrane modification, prone to fouling not specific toward drugs, high doses, long contact time, not suitable for polymeric membrane modification, prone to fouling not specific toward drugs, not suitable for polymeric membrane modification specific toward some drug, high reusability, suitable for polymeric membrane application, antifouling properties specific toward drug, high reusability, suitable for polymeric membrane application nonspecific, highly pH dependent, not suitable for polymeric membrane modification long contact time, strongly affected by temp. and pH drug specific, long contact time, suitable for membrane grafting highly specific, very spontaneous fast removal, applicable at neutral ph, suitable for membrane application, antifouling and antibacterial properties

10,44−46 10,47,48 49 50 51 52 53, 54 55 current study

interconnected, sponge-like structure with mesoporous voids in between the structure. While, the FE-SEM images of RO-sheet after filtration (see Figure 6c, d) clearly shows deposition of sericin−ibuprofen complex on to the membrane. 3.3. Sericin and Ibuprofen Interaction Study. 3.3.1. ATR-FTIR. The ATR-FTIR spectra of sericin, ibuprofen, and sericin−ibuprofen complex was analyzed for binding confirmation and to study molecular interaction between ibuprofen and sericin (see Figure 7a−c). Ibuprofen spectra shows characteristic peaks at 1700 cm−1 for CO (carbonyl stretching), 3345 cm−1 for O−H stretching of the COOH group, 2800−3100 cm−1 for alkyl stretching (3052 and 3026 cm−1 for C−H symmetric and antisymmetric stretching; 2960 cm−1 for C−H stretching; 2950 cm−1 for CH2 antisymmetric stretching; 2926 cm−1 for CH3 in-phase symmetric stretching; 2905 and 2842 cm−1 for CH stretching, 1500−1600 cm−1 for C−C stretching, 1400−1500 for CH2 deformation, 1300−1400 cm−1 for C−H bend, 1100−1200 cm−1 for C−H and C−O−H in plane bending and 1000−400 cm−1 for aromatic and alkene bends. Ibuprofen characteristic peak at 1700 for CO (carbonyl stretch) and at 3345 cm−1 for OH-stretch of carboxyl group was found to be missing from the sericin−ibuprofen complex spectra, which validates interaction between carboxyl group of ibuprofen and amide groups present in the sericin. The OH-stretch peak of Sericin at 3260 cm−1 shifted to 3280 cm−1, while ibuprofen peaks at 3052, 2950, 2926, and 2868 cm−1 reappeared together with peaks for aromatic and alkene bends (500−1000 cm−1) in sericin−ibuprofen complex and hence confirms ibuprofen adsorption on sericin. Further to calculate the variation in corresponding IR vibrational frequencies in Amide-I region the following analysis is presented. Amide-I bands has been widely used to identify the changes in protein secondary structure.56−58 Considering the complexity involved in analysis of other types of amide bands, they are generally not used for protein structure conformational analysis. Native protein generally consists of more than one secondary structure and hence absorbance usually overlap for amide I region and provide rise to merged featureless band.59 Hence, quantitative information about secondary structure modification was obtained by linear curve-fitting analysis of the component bands. Fourier selfdeconvulation and second derivative procedure was used to characterize individual overlapping absorption59,60 for native sericin, as well as sericin−ibuprofen complex. The broad amideI curve was resolved in to five distinct band using second derivative spectrum peak position which are attributable to

achieved in around 25 min (see Supporting Information S5a− c). The filtration process exhibited better removal capacity with increase in temperature. Since Sericin solubility also increases with temperature because of destabilization of the protein conformation, random coil structure become more dominant with temperature, which may have contributed to better binding capability. Moreover, since the binding of ibuprofen with sericin was found to be endothermic in nature (as found in ITC), increase in temperature must have assisted in improved binding capacity. The adsorption process at 40 °C shows the best performance with almost complete removal of ibuprofen from solution after 25 min of contact time for a period of 15 min of filtration. For 20 and 30 °C, a maximum of 91% and 96% ibuprofen separation efficiency was achieved, respectively. Based on results obtained from current study applicability of sericin for membrane modification and specific removal of micropollutants has been compared with existing adsorbents in Table 2. Sericin shows very spontaneous binding and fast removal compare to other adsorbents, highly specific, suitable for membrane modification because of its functional groups, antifouling properties, and antibacterial properties. 3.2.6. RO-Membrane Sheet Characterization. FE-SEM images were obtained to study the morphological structure of the top surface of the RO sheet (Figure 6). The FE-SEM images before filtration (see Figure 6a) shows densely packed,

Figure 6. FE-SEM images for RO membrane before filtration at (a) 25KX and (b) 50KX and after filtration (c) at 25KX and (d) 50 KX. 10147


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Figure 7. ATR-FTIR spectra of sericin−ibuprofen complex, sericin, and ibuprofen.

different secondary structure folding. To determine the proportion of resolved components Gaussian curves were iteratively fitted to deconvolved spectra. Prior to curve fitting, baseline correction and seven-point savaitzky-golay smoothing was used for the amide I region curve. Excellent agreement was noticed for band positions of the fitted curves with those obtained by second derivative spectrum. Secondary structure conformation to overlapped five

single bands were assigned as follows and according to the previous studies:56,58 band at 1629 cm−1 as β-sheet, 1648 cm−1 as random coil, 1659 cm−1 as α-helices, 1668 and 1680 cm−1 as β-turns, and 1697 cm−1 as antiparallel β-sheet. The percent content of individual secondary structure components in sericin and sericin−ibuprofen complex were calculated (see Table 2 and Table 3) and their distribution were shown in Figure 8a and b. It was found that in native state Sericin has higher 10148


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where F0 and F represents the fluorescence intensity in absence and presence of quencher (ibuprofen), respectively, Ksv is the Stern−Volmer quenching constant, [Q] is the concentration of quencher. The Stern−Volmer plot (see Figure 9b) of the quenching of sericin fluorescence with ibuprofen show a good linear relationship between Q and (F/F0) (R2= 0.9937) and estimated slope (Ksv) is 9.196 × 104 ± 0.02581 L mol−1. The observed fluorescence quenching intensity data can also be used to obtain the binding constant (Ka) and the number of binding sites (n). When drug molecules bind independently to a set of equivalent sites on a protein macromolecule, the equilibrium between free and bound molecules is given by the relationship65,66

Table 3. Assignment of Individual Amide-I Components (Sericin Native Protein) assignment

frequency (cm−1)

peak area (%)

β-sheet random coil α-helix β-turn antiparallel β-sheet

1629 1648 1659 1670, 1682 1697

34.35506 39.17869 5.43163 8.478, 11.43774 1.11887

content of random coil (39.17%) and β-sheet (34.35%), similar trend was reported in earlier literatures.61−63 In the sericin− ibuprofen complex random coil conformation (55.24%) increased in content while β-sheet (6.20%) was noticed to decrease subsequently. These indicate that ibuprofen can bind to the exposed unfolded polypeptide chain of sericin. Hence, disordered random coil structure became dominant characteristic of complex after the adsorption of ibuprofen on to sericin. 3.3.2. Steady State Fluorescence Spectroscopy. Although individual content of the tyrosine and phenylalanine vary somewhat across reported literatures,40,62 their presence in sericin can be identified by measuring fluorescence level. Quenching studies employing tyrosine intrinsic fluorescence of sericin is an indispensable approach to analyze the interior location of fluorophore and subsequent structural alteration after ibuprofen binding. Sericin exhibited intrinsic fluorescence when excited at 280 nm to show emission spectra in the 290− 400 nm range with highest intensity peak at 308 ± 1 nm (see Figure 9a). As shown in Figure 9a, the fluorescence intensity is found to be decreasing, while adding ibuprofen. This phenomenon confirms that, fluorophores (i.e., tyrosine) buried inside protein structure is coming out in solvent phase and that leads to decrease in fluorescence intensity.64 Fluorescence quenching can be dynamic, which results from collision of fluorophore and quencher, or static, when ground state complex forms between the fluorophore and quencher. Fluorescence quenching is explained by most studied Stern− Volmer equation64

(Fo − F ) = log K a + n log[Q ] F The value of Ka and n were obtained from intercept and slope of double-logarithm curve (log [(Fo − F)/F] versus log [ibu]), as shown in Figure 9c. The calculates number of binding sites (n) ≈ 0.98, indicating that there was one sericin binding site for ibuprofen and Ka was found to be 9.09 × 104 ± 0.01654 L mol−1. Protein can be unfolded by disturbing the weak interactions that maintains the folded structure (i.e., hydrogen bonding, electrostatic interactions, and hydrophobic interactions). Ibuprofen binding must have modified the polypeptide conformation in an unfolded structure and the same was confirmed in FTIR, using amide I spectrum, where β-sheet conformation decreased and random coil conformation was found to be increased after sericin−ibuprofen complex formation. The binding thermodynamics of ibuprofen−sericin was further investigated through ITC. 3.3.3. Isothermal Titration Calorimetry. ITC is an efficient method to study the binding affinity between protein and drug molecules and provide binding constant (Kb), binding stoichiometry (n), total enthalpy change (ΔH), as well as total entropy change (ΔS), involved during the binding reaction.41 The ITC thermogram of Ibuprofen titration with sericin has shown best fitting in one set of binding model after subtraction of buffer and highlighted the presence of single binding site in Sericin for Ibuprofen with endothermic pattern in thermogram (Figure 10). ITC analysis also revealed the binding affinity (Kb) value 2.51 × 104 ± 1.4 at 27 °C that log

F = 1 + K sv[Q ] Fo

Figure 8. Curve-fitted spectrum of the protein amide-I region: (a) pure sericin and (b) sericin−ibuprofen complex. Five Gaussian curves were fitted iteratively to the deconvolved curve using the peak position obtained from the second derivative spectrum as initial parameters. 10149


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Figure 10. ITC thermogram for the binding of ibuprofen to sericin at 27 °C in 10 mM phosphate buffer: (a) primary raw data and (b) binding curve derived from the raw data.

Table 4. Assignment of Individual Amide-I Components (Sericin−Ibuprofen Complex after Adsorption)

Figure 9. (a) Quenching effect of ibuprofen on sericin fluorescence intensity, λmax = 308 ± 2, (b) Stern−volmer plot, and (c) double logarithmic plot for quenching of sericin by ibuprofen.

conclude moderate binding of ibuprofen to the sericin protein. Diverse potential interaction forces such as hydrophobic, electrostatic, hydrogen bonds and van der Waal’s are usually involved in such interaction. Therefore, ITC analysis also can reveal the driving force of corresponding interaction based on binding thermodynamics. If ΔH < 0 and ΔS < 0, driving force is van der Waal’s and hydrogen; if ΔH > 0 and ΔS > 0, hydrophobic interaction is dominant, while ΔH < 0 and ΔS > 0 signify the dominancy of electrostatic force.41 Therefore, positive values of ΔH and ΔS also has revealed that hydrophobic interactions are the predominant driven force in present binding. Thermodynamic parameters of drug binding with Sericin protein has been listed in Table 4. The values of −TΔS were determined by ΔGapp = ΔH − TΔS, while ΔH and ΔS values were obtained directly from multi-injection mode ITC experiment. The binding of ibuprofen to sericin protein was found to be overall endothermic in nature, the upward peaks indicates heat intake, while adding ibuprofen into sericin solution. ΔH

assignment

frequency (cm−1)

peak area (%)

β-sheet random coil α-helix β-turn antiparallel β-sheet

1629 1648 1659 1670, 1682 1697

6.20341 55.24681 7.73929 12.25749, 16.92279 1.63021

provide information about total energy of process and include contribution from solute as well as solvent and it is barely plausible to form favorable interaction without affecting each other. Indeed, the ΔH of sericin−ibuprofen binding involves formation and disruption of individual interactions, which includes (i) the loss of hydrogen bonds and van der Waals interaction (between sericin and solvent and between ibuprofen and solvent), (ii) the formation of noncovalent interaction (between sericin and ibuprofen) and (iii) solvent restructuring near the complex surface.67 These phenomenon lead to net enthalpy change. Similarly, binding entropy ΔS is contributed by following three phenomenon such as, (i) solvent entropy change (solvent release upon binding), (ii) conformational entropy change (conformational freedom of both sericin and ibuprofen upon binding), and (iii) change in rotational and translational entropy (reduction in degrees of freedom upon complex formation).67 These three entropic terms represent the net entropy change, with negative or positive contribution 10150


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Table 5. Thermodynamic Parameters of the Sericin and Ibuprofen Interaction Obtained by ITC at 27°C Temperature sericin−ibuprofen interaction

stoichiometry (n)

Kb (mol−1)

ΔH (kcal/mol)

ΔS (kcal/mol·K)

ΔGapp (kcal/mol)

1.15 ± 0.021

2.51 × 104 ± 1.4

4.62 ± 0.36

36.18

−31.9

Figure 11. FE-SEM of sericin, ibuprofen, and sericin−ibuprofen complex at 25 KX (a−c) and at 50 KX (e−g), respectively.

Figure 12. XRD profile for ibuprofen, sericin, and sericin−ibuprofen complex.

play main role to the binding interaction and presence of polar group may be related to the positive values of ΔH° and ΔS°. Hence it was assumed that sericin protein unfolding to random coil conformation was dominant factor for sericin−ibuprofen complex formation, that is, adsorption process. 3.3.4. FE-SEM. FE-SEM images of sericin, ibuprofen, and sericin−ibuprofen complex revealed the adsorption of ibuprofen on sericin protein (see Figure 11). Surface texture and morphology of Ibuprofen crystals are visualized (in Figure

to binding free energy. A negative binding free energy (ΔGapp = −31.9 kcal/mol) shows spontaneous binding mediated by enthalpy−entropy compensation for sericin−ibuprofen complex formation. Ibuprofen adsorption on sericin can be considered to be an analog of protein unfolding, since ibuprofen adsorption and protein denaturation are coalesced with each other. As revealed in FTIR the protein unfolds from β sheet to unordered random coil structure, which may be related to the endothermic effect. Hydrophobic interactions 10151


Article

Industrial & Engineering Chemistry Research Notes

11b and e), which can be seen very clearly to be deposited on the protein surface (in Figure 11c and f). Pure sericin without ibuprofen, as a control, is shown in Figure 11a and d. The complex particles were found to have dotted chiselled ibuprofen structure adsorbed on the native sericin protein surface. 3.3.5. XRD. The XRD results for Sericin exhibited a large diffraction peak at around 2θ = 21.15°, the broad curve indicates amorphous nature (see Figure 12). Similar XRD profile with peak at 2θ = 19.2° has been reported.68 This peak is characteristic of the β-sheet structure due to intermolecular hydrogen bonding between the hydroxyl groups of the amino acid present in Sericin. Similar results have been reported in earlier studies with Sericin obtained from different sources.30,63 Ibuprofen XRD peaks shows its characteristic crystallinity and was found to be in good agreement with previous reported literatures.69 The sericin−ibuprofen complex XRD plot was crystalline in nature and retained most of the ibuprofen characteristic peaks hence depicting adsorption of ibuprofen on sericin.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to Ajeet Singh, Department of Bio Sciences & Bioengineering, IIT-Guwahati, India, for his kind help in fluorescence spectroscopy and ITC analysis and for helpful discussions, and to the Central Instrument Facility, IITGuwahati, India. This research was financially supported by Indian Institute of Technology, Guwahati, India.

■

4. CONCLUSION This work deals with evaluation of silk sericin as an adsorbent for removal of ibuprofen from aqueous solution using an integrated adsorption−membrane filtration setup. The preliminary studies on sericin characterization was done to assess its physiochemical properties relative for adsorption. The nonporous sericin was found to have low surface area and conformational change were noticed with exposure to increasing temperature. Its adsorption ability was analyzed at different operating conditions (concentration, temperature, and pH). High pH and increasing temperature was found to assist in better adsorption of ibuprofen. Complete removal for ibuprofen was achieved at pH 8, 40 °C, and using 10g/L of sericin concentration. Ibuprofen interaction with sericin was investigated and endothermic peaks in ITC together with FTIR study provided information about protein unfolding playing dominant role in adsorption. Ibuprofen binding was related to sericin transition to random coil structure. Sericin−ibuprofen binding and complex formation was established using FTIR, ITC, fluorescence spectroscopy, and XRD analysis. Use of sericin with membrane process seems to enhance and achieve expected removal capacity. We hope this study will help in modification of existing filtration processes and provide a better solution for removal of micropollutants from aqueous solution.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b01827. Standard HPLC calibration curve for ibuprofen, flat sheet RO-membrane dimension and effective area, pure water flux (Jw) for RO-sheet membrane, variation in pH with sericin concentration, and ibuprofen removal at different temperature with time of contact (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Contact No: +919945213864, +913612583527. ORCID

Senthilmurugan Subbiah: 0000-0001-8439-3515 10152


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