Development of Highly Water Stable Graphene Oxide-Based

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Materials and Interfaces

Development of highly water stable graphene oxide based composites for the removal of pharmaceuticals and personal care products Krithika Delhiraja, Kowsalya Vellingiri, Danil W. Boukhvalov, and Ligy Philip Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02668 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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Industrial & Engineering Chemistry Research

Development of highly water stable graphene oxide based composites for the removal of pharmaceuticals and personal care products Krithika Delhirajaa**, Kowsalya Vellingiria**, Danil W. Boukhvalovb,c, Ligy Philipa,* a Environmental

and Water Resources Engineering Division, Department of Civil Engineering,

IIT Madras, Chennai, 600 036, India, bCollege of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, P. R. China, c Theoretical Physics and Applied Mathematics Department, Ural Federal University, Mira Street 19, 620002 Yekaterinburg, Russia.

KEYWORDS: Graphene oxide, Activated carbon, Chitosan, Pharmaceuticals, Personal care products, Density functional theory ** Authors one and two contributed equally *Corresponding author: [email protected]

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ABSTRACT In the present study, sorption of pharmaceuticals ((acetaminophen (ACP), carbamazepine (CBZ)) and personal care products (bisphenol-A (BPA), caffeine (CAFF), triclosan (TCS)) using graphene oxide (GO) based composite adsorbent was extensively studied. Structural characterization indicated the incorporation of activated carbon (AC) and chitosan (CS) onto the GO. For both classes of pollutants, the adsorption kinetics followed pseudo-second order kinetic model. On the other hand, Langmuir isotherm showed a better fit compared to Freundlich isotherm.

Maximum

adsorption

capacities

of

ACP,

CBZ,

BPA, CAFF, and TCS were found to be 13.7, 11.2, 13.2, 14.8 and 14.5 mg/g, respectively. Desorption of ACP, BPA, and CAFF was significantly higher in a polar solvent (70-80%), whereas CBZ and TCS were able to desorb (60%) in a non-polar solvent. DFT calculation indicated that the adsorption mechanism is majorly accompanied by size related diffusion and also minor contribution by synergetic combination of hydrophobic/hydrophilic, hydrogen bonding, electrostatic, and π-π interactions. Overall, the prepared composite material showed enhanced structural stability along with higher removal characteristics towards multiple emerging contaminants in water and wastewater.


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1. INTRODUCTION Modern lifestyle and associated economic growth have increased the consumption of wide varieties of emerging contaminants (ECs).1 These ECs are organic chemicals which include medicated products, plasticizers, additives, cosmetics, flame retardants, detergents etc. Among ECs, pharmaceuticals and personal care products (PPCPs) have attracted much attention because of their necessity in everyday life, massive production, and consumption.2 PPCPs include analgesics, lipid regulators, antibiotics, diuretics, non-steroid anti-inflammatory drugs (NSAIDs), stimulant drugs, antiseptics, beta blockers, cosmetics, sunscreen agents, food supplements, plasticizers, fragrances, and their metabolites or transformation products. A study conducted by Ellis in 2008 on priority PPCPs indicated that carbamazepine (CBZ), diclofenac, iopamidol, musks, ibuprofen, clofibric acid, triclosan (TCS), phthalates and bisphenol A (BPA) are the future emerging priority candidates in wastewater systems.3 Among these listed candidates, TCS is used in many products as an anti-bacterial agent, and it was estimated that approximately 350 tons of TCS were being used in European countries.4 Likewise, BPA is one of the phenolic compounds which is widely used as a monomer in the production of plastics. This compound is found to be endocrine disturber, even at extremely low concentrations.5,6 Several studies showed that caffeine (CAFF) and CBZ were dominant in the domestic wastewater and have toxic effects to the environment.7 A prolonged bio-accumulation of these PPCPs on flora and fauna in the environment can cause antibiotic resistance and act as carcinogenic agents. To date, the treatment of these compounds is a challenging task because of their complex physicochemical properties (e.g., non-biodegradability).8 Therefore, there is an emerging need for the complete elimination of these PPCPs from water and wastewater.



A range of treatment technologies such as coagulation-flocculation,8 biodegradation,9 chlorination,10 constructed wetlands,11 adsorption,12 and advanced technologies such as catalytic ozonation,13 photodegradation,14 and advanced oxidation processes (AOPs)15 have been tested for the removal of PPCPs. However, the evaluated efficiencies of these traditional processes such as coagulation and biodegradation were not sufficient. In contrast, AOPs are also not accessible easily in large scale due to high energy consumption and the formation of residual by-products.16 In this context, adsorption using solid adsorbent could be an appropriate method for treating PPCPs in the aqueous environment because of low cost, mild operation conditions, low energy consumption and so on. A wide variety of solid sorbents including activated carbon (AC), clays, graphene based materials, metal nanoparticles, and metalorganic frameworks (MOFs) have been tested and even implemented in the pilot and field scales. The inherent advantages of graphene oxide (GO) such as dispersibility in water, high colloidal stability and biocompatibility,17 superior optical absorptivity,18 enhanced surface area, and lower-cost compared with other nanomaterials (e.g., CNTs and noble metal doped nanoparticles (NPs))19 have opened up a large number of opportunities for GO and its composites in the field of adsorption. From the literature, not many studies have looked into the behaviour of an adsorbent under multi-pollutant conditions, effective solid-liquid separation after the adsorption process and the thermodynamic relationship between the adsorbate and adsorbent. Hence, there is a need for the development of a novel, stable and easily separable composite material for the removal of PPCPs in water and wastewater and understand the mechanism of the process. In the present study, a multifunctional composite material was developed using AC, graphene oxide (GO), and chitosan (CS). The presence of hydrophobic/hydrophilic surfaces along with the large surface area of graphene provided strong adsorption capacity for diverse organic


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pollutants.20 Unlike carbon nanotubes, graphene requires unique

oxidation processes to

introduce hydrophilic groups on the surface. For instance, the preparation of GO from graphite using modified Hummer’s method introduces many oxygen-containing functional groups such as –COOH, –C=O, and –OH on the surfaces of GO. Further functionalization of GO can be obtained by post-synthetic grafting of suitable functional groups on the surface.21 On the other hand, AC is a low cost and efficient adsorbent for the removal of diverse organic pollutants. Nonetheless the removal efficiencies of AC for hydrophilic compounds were moderate.22 CS is another most abundantly available natural biopolymer and used as a binder in composite preparation. Thus, in the present work, we have utilized the hydrophilic, hydrophobic, and binding properties of GO, AC, and CS for the development of stable and easily separable GOAC-CS composites. The synthesized composite material was characterised using various techniques such as BET (Brunauer, Emmett and Teller), scanning electron microscope (SEM), powder X-ray diffraction (PXRD), Fourier transform infrared (FTIR) spectroscopy, and thermo gravimetric (TGA) analysis. In this regard, three personal care products (TCS, BPA, and CAFF) and two pharmaceuticals (CBZ and ACP) were selected as the pollutants for adsorption study. The selection of these five PPCPs was based on the frequency of occurrence in wastewater and diverse

physical

and

chemical

characteristics

(e.g.,

molecular

mass,

solubility,

hydrophobicity/hydrophilicity, functional groups etc.). The adsorption profiles of prepared composites were assessed through kinetic and equilibrium models. Statistical analysis was also conducted to evaluate the performance of adsorbents against various PPCPs. The influence of operational and environmental parameters such as pH and organic matter on the adsorption capacity of composites was investigated. The results indicated that the synthesized materials were sensitive to pH variation and the presence of organic matter. Especially at lower pH, the



protonation of –NH2 group in CS competes with the PPCPs for adsorption sites. The working pH range of the composites was found to be between 4 and 7. At pH below 4 or higher than 8, the sorption capacity was drastically decreased because of the surface forces. Likewise, the presence of humic acid also influenced the sorption capacity of GO-AC-CS slightly. Therefore, the main demerit of the synthesized GO-AC-CS is the sensitivity towards pH and organic matter. The mechanism responsible for the structural stability was analysed through density functional theory (DFT) calculations, and additionally, adsorption characteristics of GO-AC-CS towards PPCPs were also evaluated. Finally, the desorption experiment was carried out, and suggestions for the scale-up of the treatment process were recommended. Overall, the present study provides a new roadmap for the preparation of multifunctional easily separable adsorbent for the removal of emerging contaminants from water and wastewater.

2. MATERIALS AND METHODS

2.1. Chemicals Materials used for the synthesis of adsorbents such as graphite (100-micron size), concentrated hydrochloric acid (HCl), concentrated sulphuric acid (H2SO4), potassium permanganate (KMNO4), and sodium hydroxide pellets (NaOH) were procured from SD Fine Chem Limited (Mumbai, India), Merck Chemicals (Mumbai, India), and EMPLURA Chemicals (Mumbai, India), respectively. CS flakes, with deacetylation degree of 80%, were supplied by TCI, Japan. Target compounds namely BPA (>97%) was purchased from Alfa Aesar (Massachusetts, USA). ACP (98%), CBZ (99%), TCS (99%), and CAFF (99.9%) were procured from Sigma Aldrich (Missouri, USA). HPLC grade acetonitrile was obtained from Fisher Scientific, India. All other chemicals used in this study were supplied by Rankem, India. The structural and physicochemical properties of selected PPCPs are presented in Table S1 (a, b). All glasswares used were 4 ACS Paragon Plus Environment

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supplied by Borosil, India and were cleaned with distilled water and dried at 110 ºC for 5 h prior to the sorption experiments.

2.2. Synthesis of graphene oxide (GO) Modified Hummer’s method was used for the synthesis of GO.23 In a typical process 1 g of graphite, 0.5 g of sodium nitrate, and 25 mL of concentrated H2SO4 were added into 1 L beaker, and the mixture was stirred for 1 h using magnetic stirrer. Later, 3 g of KMnO4 was added slowly to the above mixture. After the addition of KMnO4, the colour of the slurry turned from grey to dark green, and the content was mixed for 12 h at 40 °C. The brown colour suspension was washed with 1M HCl and centrifuged at 9000 rpm. Then, the precipitate was rewashed with deionised (DI) water and centrifuged. In order to remove possible impurities, this wash cycle was repeated for five times with 1 M HCl and DI water. At the end of the washing cycle, the GO was filtered and dried in a hot air oven at 70 °C for 5 h. Finally, the dried GO was grounded using mortar and pestle and stored in amber bottles at ambient conditions.

2.3. Synthesis of composites as flakes The concept of hydrogel synthesis was adopted from Chen et al.24 with some modifications. Initially, CS stock solution with a concentration of 2.5% was prepared by dissolving 1 g of CS flakes in 100 mL of 2.5% (w/w) aqueous acetic acid. The detailed information about the selection of acid and the concentration of acid is provided in section 1.1 of supporting information (SI). GO and AC suspensions were prepared by dissolving 0.5 g of GO or AC in 0.05 L of DI water, and the content was sonicated for 5 min. Before the preparation of GO and AC suspension, the sorbents were activated at 70 °C for 1 h. In this study, three types of



composites namely GO-CS, AC-CS, and GO-AC-CS were initially prepared. In a typical process, equi-molar mixture (1:1) of GO and CS was mixed in a glass beaker, and the content was sonicated for 5 min. The resulted hydrogel was introduced into the 0.5 M NaOH solution using 1 mL micropipette. Finally, the prepared hydrogels were thoroughly washed with water (10 mL × 5 times) and then dried in an oven at 60 °C for 12 h. Similar procedure was adopted for the preparation of AC-CS and GO-AC-CS hydrogels. Pictures of the prepared composite as hydrogels (GO-AC- CS) as well as flakes are shown in Figure S1 (a, b). The aqueous stability tests of the synthesized adsorbent were also conducted, and the procedure is given in section 1.1 of SI.

2.4. Characterization of adsorbents Characterization of pristine and synthesized adsorbent was performed using BET, PXRD, FTIR, SEM, TGA, and particle size analyzer. The BET surface area, pore size, and pore volume were measured using ASAP 2020-porosimeter (Micromeritics, USA). Prior to measurements, the samples were dried at 70 ºC in a vacuum oven for 4 h. PXRD patterns of adsorbents were recorded using a D8 Discover diffractometer (Bruker, Billerica, Massachusetts). Before the analysis, the prepared flakes were crushed to a fine powder using mortar and pestle. PXRD data were collected with the angular 2θ range, step size, and scan speed of 5-50 °, 0.01, and 1 sec/step, respectively. The morphology was characterized using a Quanta 200 SEM (FEI, Hillsboro, Oregon). FTIR spectroscopy in the attenuated total reflectance (ATR) mode was carried out using a Perkin Elmer spectrometer (Akron, Ohio, USA). TGA analysis was carried out using SDT Q600 with temperature ranging from 20 ºC to 1200 ºC at N2 gas flow rate of 100


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mL/min. Particle size and Zeta potential measurements were carried out using nano-particles SZ100 (Horiba, Japan).

2.5. Batch kinetic studies Adsorption experiments were performed using Erlenmeyer flasks (100 mL) containing 50 mL of 10 mg/L of each pharmaceutical (ACP and CBZ) and personal care products (BPA, CAFF, and TCS). 25 mg of sorbent material was added to the pollutant solution and kept in a shaker at an isothermal condition (30 °C) until equilibrium was attained. The aqueous samples were taken in a sacrificial mode at regular time intervals, and the residual concentrations of pollutants were analysed. Kinetic studies were performed for both pristine (GO, AC, CS) and composite (GOCS, AC-CS and GO-AC-CS) adsorbents. The adsorption capacity of these composites was calculated using equation1, (1) where C0 and Ct (mg/L) are the residual concentration of pollutants at initial and at any time t, respectively. V is the volume of the pollutant solution (L), and w is the weight of dry adsorbent (g). The rate constant of adsorption kinetics is determined from the pseudo first-order and second order kinetics. The details of equations are provided in Section 1.2 of SI.

2.6. Equilibrium studies For batch equilibrium studies 50 mL of various concentrations of PPCPs (0.1, 0.5, 1, 5, and 10 mg/L) were prepared and equilibrated for 8 h (at 120 rpm and 30 °C) with 25 mg of GO-AC-CS flakes. Here, the maximum initial concentration of two pharmaceuticals (ACP and CBZ) was fixed as 10 mg/L. This is due to the poor water solubility of CBZ (17.7 mg/L). Thus, to make 7 ACS Paragon Plus Environment


comparative data, the maximum initial concentrations of personal care products were also taken as 10 mg/L. Equilibrium data were modelled using Langmuir and Freundlich isotherms. Details of isotherms models are provided in Section 1.3 of SI.

2.7. Desorption studies The practical applicability of this process on a large scale depends on the reusability characteristics. For the chemical desorption experiment, the spent sorbent material was filtered and mildly washed with DI water. Then the material was separately introduced into 50 mL of DI water, tap water, acid (0.5 M HCl), base (0.5M NaOH), polar (methanol), and nonpolar solvent (hexane). The resulted contents were thoroughly mixed in isothermal condition for 8 h, and the liquid part of the samples was drawn at regular intervals for further analysis. The residual concentration of the pollutant was analyzed. Also, regeneration of adsorbent was investigated using thermal treatment. For this purpose, the spent adsorbent material was filtered, and flakes were dried at 100 ºC for 2 h. After thermal desorption, the same flakes were used for the adsorption, and the cycles were repeated for three times.

2.8. Analysis of pollutants The selected compounds were analyzed using UHPLC (Dionex 3000, USA) equipped with a UV detector. The detailed description of analysis is provided in section 1.4 of SI.

2.9. Density functional theory (DFT) calculations Modelling was performed using DFT, implemented using the pseudo potential code SIESTA,25 as per previous work.25-27 All calculations were performed using the generalized gradient


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approximation (GGA-PBE) including spin polarization with taking into account van der Waals correction. Detailed information of selected models and equations are provided in section 1.5 of SI.

2.10. Statistical analysis. To explore the relationship between the physicochemical properties and removal efficiency of selected target PPCPs using different adsorbents,28 Pearson correlation analysis was performed. Detailed description of correlation study is provided in section 1.6 of SI. Further, the two-way ANOVA analysis without replication was conducted to compare the overall removal efficiency of the selected PPCPs against pristine and composite adsorbent. All the above mentioned statistical analysis was performed using IBM SPSS 22.0 software package.

3. RESULTS AND DISCUSSION 3.1. Synthesis and characterization PXRD patterns of synthesized pristine (GO, AC and CS) and composites (GO-CS, AC-CS and GO-AC-CS) are shown in Figure 1a and b. PXRD pattern of GO showed three major peaks located at 10.1°, 26.4°, and 44.4° corresponding to d-space inter-planar distances of GO sheets (Figure 1a). These peaks can be attributed to (0 0 1) reflection of GO, (0 0 2) and (1 0 1) reflection of graphite, respectively.29,30 For pristine AC the PXRD pattern showed one strong peak at 2θ =25.2º which resembles the presence of minor graphitic/turbostratic carbon domains. Pristine CS flakes registered weak and broad peaks at 2θ = 21º which correspond to intermolecular interactions of CS. In the case of AC-CS, a broad peak was obtained in the range of 20-30º corresponding to the merged peaks of AC and CS. On the other hand, the prepared all



three composites were registered a weak and broad peak in the 2θ range of 10° and 20-25° (Figure 1b), which clearly indicates the proper binding between the GO, AC, and CS.

FTIR analyses of pristine and composite adsorbents are shown in Figure 1c and d. The –OH stretching vibrations in the GO was observed in the region of 3500-3000 cm-1. The C-O, C=C, and C=O asymmetric stretching vibrations of GO were noticed at 1220, 1620, and 1725 cm-1, respectively.31 AC registered a broad overlapping band at 3500-3000 cm-1 and 1060 cm-1 which was assigned to the –OH stretching of the hydroxyl groups.32 In the case of CS, a strong band near 1800 and 1500 cm-1 corresponds to symmetric C-O and asymmetric N-H stretching vibrations were noticed. The observed peak further confirmed the presence of primary alcoholic group in CS at 1000 cm-1. All the three composites showed well-distinguished peaks in the regions of 3500-3000, 1700-1600, 1060 cm-1 corresponding to hydroxyl (–OH), carboxylic (COOH), and amine (-NH) functional groups (Figure 1d). Proper grafting of CS and AC on the GO layers was also confirmed by the observed decrease in the transmittance intensity of GO peaks in the carboxyl group stretching region (1600 cm-1). This decrease in intensity of GO based composites was contributed from the new bond formation between the hydroxyl group of GO and a hexatomic ring of CS.


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Figure 1. PXRD (a and b) and FTIR (c and d) spectrum of pristine and composite adsorbent.



The pore structures of pristine GO and the newly synthesized composite (GO-AC-CS) were determined by BET isotherm studies. Pristine GO which is synthesized using modified Hummer’s method has a surface area of 132 m2/g which is comparable to earlier reported values.33 The surface area of graphene based composite (GO-AC-CS) was found to be 214 m2/g, and it is high when compared to pristine GO. Surface morphology profiles of pristine adsorbents such as GO, AC, CS, and composites (GO-CS, AC-CS and GO-AC-CS) are presented in Figure 2 (a-g). SEM results of pristine GO, AC, and CS showed clusters, flakes, and tiny ball-like structures, respectively. In contrast, the GO-CS composite possessed smooth clusters which indicate the proper binding between the GO and CS. In the case of AC-CS, CS was intercalated between the crystals like structures of AC. On the other hand, GO-AC-CS showed the porous structure in which homogeneous grafting of AC on the GO layer was observed. Also, the presence of CS on the GO-AC layers was visible (Figure 2 g) which supports the diffusion of guest molecules on the surface of the GO-AC-CS layers.


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Figure 2. SEM images of the synthesized pristine and composite adsorbents: a) GO, b) AC, c) CS, d) GO-CS, e) AC-CS, f) GO-ACCS, g) cross-sectional SEM image of GO-AC-CS.



Finally, the thermal stability of the material was investigated with the help of TGA analysis (Figure S2a and b). The synthesized GO showed a weight loss of about 20%, only in the temperature range of 600-700 ºC. These results support the high thermal stability of GO. AC showed an initial weight loss of 10% at 100 ºC which corresponds to the elimination of water molecules. The second steep weight loss of 20% was observed at 300 ºC which slowly continued until 600 ºC. This slow and steady weight loss of AC indicates the carbonization process. In the case of CS, two weight loss degradation patterns were observed. The first degradation started from 100 oC and continued beyond 200 oC due to the dehydration. The second weight loss started at about 200 oC (70% weight loss) due to decomposition of CS main chains. The total weight loss of the sample at ~500 oC was about 65% and remaining residue was mostly due to the formation of inorganic elements such as C, N, and O.34 The weight loss profiles of composites AC-CS and GO-AC-CS were comparable with the TGA profiles of AC whereas the temperature profile of GO-CS was similar to that of CS. In all three cases, an initial weight loss of about 30% was observed at 200 ºC, and this result was comparable with the result of Chen et al.24, and Song et al.35 Zeta potential values of pristine and composite GO-AC-CS aqueous dispersions at different pH values (4-12) are shown in Figure S3. The value of zeta potential is necessary for characterizing the stability of colloidal dispersions. It measures the effective surface charges around the interface of colloidal particles based on the ionization phenomenon of different functional groups. The pristine GO formed a stable dispersion, and it was found to be negative (-0.6 mV to -42.3 mV) for a wide range of pH (2-12).32 The zeta potential of AC varies from positive to negative for a wide range of pH (2-12). In the case of CS, the zeta potential value varied from +30.8 mV to +10.5 mV in acidic pH range (2-6) which indicates the cationic nature of CS at low pH due to protonation of amine groups.12 The point of zero charge (Pzpc) of


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newly synthesized composite adsorbent GO-AC-CS was found to be 6.5 (Figure S3). The obtained positive surface charge of composite adsorbent at low pH was mainly due to the incorporation AC and CS. Overall results indicated that the synthesized composite adsorbent could effectively absorb anionic pollutants at low pH and cationic pollutants at high pH. The particle sizes of pure GO and composite GO-AC-CS were found to be 237 nm and 336 nm, respectively. The GO particle size obtained in the present study was comparable with the study of Nam et al.30 where they reported the particle size of GO as 250 nm. The particle size of AC, GO-CS and AC-CS were estimated as 200, 317, 380 nm, respectively.

3.2. Stability profiles of synthesized composites 3.2.1 Experimental results The degree of stability of prepared composite in the aqueous phase is an important aspect because it decides the applicability of this material in real-world applications. In this context, during the stability test, no dispersion or leaching of any trace particles from synthesized GOAC-CS was observed (Figure S1 c, d). This result indicates that GO-AC-CS composite showed better stability than GO-CS and AC-CS. For instance, the GO-CS and AC-CS composites started leaching CS after 12 h of shaking time. Later, after 96 h a clear deposition of CS layers on the walls of Erlenmeyer flasks containing GO-CS and AC-CS composites were observed. From this observation, it can be concluded that GO-AC-CS is more stable than GO-CS and AC-CS. The high water stability of GO-AC-CS composite was due to the formation of hydrogel through faceto-face stacking mode which limits the leaching of bonded material in the aqueous system.36 Also, the negatively charged –NH2 groups in CS facilitated stronger electrostatic attraction



towards GO. Beside this interaction, AC provides additional multiple hydrogen bonds between GO-CS layers which enhances aqueous stability of this multifunctional composite material.

3.2.2. DFT calculations 3.2.2.1. Selection of thermodynamic parameters for determining the aqueous stability of composites DFT calculation was carried out to find out the reason for the high stability of GO-AC-CS composite. In this regard, for modelling of CS, three repeating units of CS chains were considered. Likewise, for modelling of GO and AC, previously developed model was considered (Figure S4 a).27 In the case of GO-AC-CS composite formation, modifications such as the use of nano graphene with the edges passivated by hydrogen atoms were considered (Figure S4 b). Here, the composites were considered as planar (two dimensional) structural model. In this twodimensional structure, GO and AC were arranged as a sheet with different sizes and widths, while CS was intercalated in between the GO and AC as a ribbons. The stability of composites in water was calculated in terms of enthalpy and the detailed discussion related to the contribution from entropy, and Gibbs free energy is presented in section 1.5 of SI. The reason for selecting the enthalpy as a tool to determine the stability is based on the properties of selected composites. For instance, simple solids such as LiCl or NaCl undergo a phase transition from solid to liquid with the breaking of inter-atomic bonds. This phase transition provides a significant contribution in the free energy through entropy change. In such cases, it is not possible to discuss about the exothermic and endothermic molar enthalpies as a tool to measure the solubility.


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On the other hand, the materials such as GO does not change its structure drastically because all covalent bonds remain unchanged and contribution from entropy is related only with the possible change of rotational and translational degrees of freedom. Based on this reason, it is proposed that the contribution from entropy is much smaller. In order to further support this approach; authors have suggested several examples. It is wellknown that the graphene is hydrophobic, GO is hydrophilic and soluble in water, and various biomolecules form stable composites with GO. Theory reveals that the enthalpy of the interaction of water with graphene is in the order 0.01~0.02 eV.37 Likewise, the reported enthalpy values for the interaction of hydroxyl portions of graphene with water was in the range of 0.4~0.5 eV.

38

Also, it has been reported that the enthalpy of formation of various bio-

molecules on GO was in the order of 1~2 eV.39 This result indicates the high solubility of GO in water. From the obtained values, it is known that GO possess stable atomic structure and the behaviour of this molecule in the presence of the water environment is similar. Additionally, the graphene is hydrophobic and possess less enthalpy value (0.01~0.02 eV) whereas GO is hydrophilic and dispersible in water which registered higher enthalpy value (1-2 eV).

Therefore, one can rely on the enthalpy of formation instead of entropy change for

determining the aqueous stability of the graphene/GO based composites based on these reasons authors have omitted the calculation related to entropy in the modelling. Also, Djikaev and Ruckenstein40 reported that in molecular dynamics of a single point charge (SPC/E) water model, the hydration thermodynamics was changed from “entropy dominated” to “enthalpy dominated”. This result also indicates that in complex systems the measurement of enthalpy itself provides sufficient information about the hydration behaviour of composites.



3.2.2.2. Limitations of the proposed model During the formation of the composite, two kinds of interaction namely: a) interaction between the GO, AC, CS and b) interaction between these components with water were noticed. It was found that enthalpy value for the formation of composites was dependent on one side of the composite, i.e., where the portions of the composite interacts with water (i.e., probably the hydrophilic ends of the composites). At another end, the composites were interacting among themselves and these portions of the composite were responsible for the adsorption process. Therefore, to describe the water stability of the GO-AC-CS, only the portions where the watercomposites interactions taking place were considered (hydrophilic hydration) and remaining interactions (hydrophobic hydration) were eliminated (Figure S4b). It can be noted that the building of an exact model for determining the energies of interaction of composites with water was difficult and expensive, in contrast to the case of solids. This is mainly due to multiple configurations of water and less stable atomic sturucture. . In addition, unlike gaseous phase, in the aqueous system, the interactions with other interfering molecules cannot be eliminated. In present study, the authors have considered only the bulk part of the composites for the estimation of enthalpies in modelling. Nonetheless, contribution to the surface stability of the repeating unit of the composites from water were not taken into account due to large volume to surface ratio of the selected composites. For instance, the increase in the chain length of the composites increases the interaction between the water molecule and active sites of the composite. While assessing the aqueous stability of the composites in a water environment, two kinds of effect, i.e., cooperative and anti-cooperative effects play a significant role.41 Here, cooperative effect indicates, the interaction between the donor portion of the composites to the donor portion of the water molecule or acceptor portion of the composites bonded to the acceptor portion of the water


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molecule. In case of anti-cooperative effect, the interaction between the donor portion of the composites bonds to the acceptor portion of water molecules and vice versa are considered. In this line, Huš and Urbic41 found that anti-cooperative effect was responsible for determining the solvation phenomenon in complex systems. Also, they have also concluded that quantification of this kind of solvation energies was difficult and mostly avoided in ab initio electronic structure studies. In this regard, the authors have omitted the calculation related to the repeating units of the composites. Thus, the bulk structures of the composites (GO-CS, AC-CS, and GO-AC-CS) only were considered for the calculation of aqueous stability. The calculated value of GO and CS super cell were 1.8 and 1.6 eV/molecule, respectively. In contrast, for hydrophobic AC the energy of interaction of AC with water was found to be 0.02 eV/molecule for each hydrophobic centre. Finally, the total energy of interaction of AC with water was estimated to be 0.92 eV/molecule. Here it is noteworthy to mention that hydrophobicity does not mean that molecules do not interact with water (i.e.) the interaction of molecules with water was less energetically favourable. Similarly, in the case of hydrophilicity, the interaction of molecules with water is more energetically favourable. To this end, the enthalpy of formation (Hform) was estimated by summarizing the interaction enthalpies of hydrophilic GO and CS groups with water (about 0.1 eV/molecule). Similarly, the Hform values for other composites were also calculated.

3.2.2.3. Results of the theoretical study The results of water stability of GO-CS, AC-CS, and GO-AC-CS are summarized in Table 1. From the results, it is clear that both GO-CS (Figure S4 a) and AC-CS composites were stable in water. For instance, Hform for GO-CS and AC-CS was -0.25 and -0.11 eV/molecule, respectively.



The formation of composites AC-CS and GO-CS were energetically less favourable as shown in Table 1. The formation of π-π, hydrogen, and dispersive bonds between GO and CS or AC and CS was minimal. This is because the presence of hydroxyl and epoxy groups present on GO prevent the formation of π-π bonds with CS. The absence of specific functional groups in AC restricts the bond formation with CS (Table 1). Contrastingly, the formation of three-component composite, i.e., GO-AC-CS was highly favourable because the estimated Hform of this composite was -1.17 eV/molecule. Also, GO-AC-CS consisted of a periodic porous structure containing multiple bonds. These multiple bonds provided highly diffusive layers which makes this composite more stable than other two composites. The results presented in these sections confirm a successful formation of the stable multifunctional composite material. For modelling, authors did not include the formation of ice-like layers on the surface of the composites. The structure of the cluster (i.e., composites-water system) resembles ice-like arrangement during the formation of hydrogen bonds. For estimation of this kind of ice-like layers, apart from hydrophilic hydration, the thermodynamic factors related to hydrophobic hydration were included. This hydrophobic hydration helps in studying the effect of the hydrogen bonding ability of water molecules towards the hydrophobic surface.40 In composites, the hydrophobic hydration also plays a dominant role because it helps to form highly ordered (ice-like) structure by forming the water cages around the hydrophobic layers of adsorbent. In this way, the interactions between both hydrophilic and hydrophobic ends of the composites with water were obtained. Through these results, one can easily define the reason for the additional stabilization of GO-AC-CS in comparison with GO-CS and AC-CS. In contrast, for calculating this kind of hydrophobic hydration, along with DFT calculations, one requires probabilistic approach.40 This approach was not yet developed for composites which contain complex


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molecules such as GO, AC, and CS. The current study did not include the values related to hydrophobic hydration, and this is a limitation of the present model.

Table 1. Calculated enthalpies of formation (in eV/molecule) and adsorption for diverse adsorbents Host

Hform

Hads

(composite)

ACP

CBZ

BPA

CAFF

TCS

GO

——

-0.13

-0.40

-1.17

-1.40

-1.22

AC

——

-0.53

-0.90

-1.11

-0.58

-0.43

CS

——

-0.92

-1.14

-0.58

-0.54

-0.42

GO-CS

-0.25

-0.36

-0.50

-0.70

-1.19

-0.78

AC-CS

-0.11

-0.12

-0.40

-0.38

-0.43

-0.40

GO-AC

+0.30

-0.65

-1.02

-0.20

-1.01

-0.84

GO-AC-CS

-1.17

-0.59

-0.71

-0.74

-0.86

-0.59

Note: Hform and Hads values were calculated by considering the aqueous environment using the

equations Hform = Ecomposites – (Epart1 + Epart2 + ··· Eparti) – (Hwater1 + Hwater2 +··· Hwateri)/n and

Hads = Ehost+guest – (Ehost + Eguest) – Hwater(guest)

3.3. Batch kinetic studies The adsorption kinetics study was carried out for two pharmaceutical compounds (ACP, CBZ) and three personal care products (BPA, CAFF and TCS) with the individual (GO, AC, and CS) and composite (GO-CS, AC-CS and GO-AC-CS) adsorbents. The results of kinetic studies are shown in Figure 3 a–e. The results clearly demonstrated that adsorption was a rapid and spontaneous process. This fast sorption was accompanied by the presence of large number of 21 ACS Paragon Plus Environment


available vacant active surface sites of adsorbents. As time increased, the number of available active sites decreased, and the adsorption sites became saturated.42 The maximum adsorption capacities of GO towards ACP, CBZ, BPA, CAFF and TCS were found to be 14.3±1.81, 11.9±2.07, 13.1±0.36, 17.3±0.41 and 14.9±0.37 mg/g, respectively. Similarly, the adsorption capacity of AC towards PPCPs were found to be 15.8±0.05, 17.1±1.83, 17.2±0.15, 17.8±0.03 and 15±0.16 mg/g, respectively. Among the individual adsorbents, AC performed better than GO and CS this may be due to the stronger interaction between the amine (-NH2) or oxygen containing groups of PPCPs with AC.43 Further the developed composites were tested against the removal of selected PPCPs. As expected, the removal efficiency of composites was better than the removal efficiencies of individual sorbent materials (Figure 4). Among the three composites studied, the composite of GO-AC-CS showed higher adsorption capacity than other two composites. For instance, the estimated sorption capacity (mg/g) of GO-AC-CS towards ACP, CBZ, BPA, CAFF, and TCS was 18.6±2.79, 17.9±1.75, 18.4±0.54, 19.8±0.11 and 19.5±0.95, respectively.


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Figure 3. Kinetic plots for the adsorption of pharmaceuticals and personal care products over diverse pristine and composite adsorbents.



When comparing the difference in the removal of selected PPCPs, the composite adsorbent (GOAC-CS) performed much better than individual adsorbent (AC). The percentage difference in the removal of ACP, CBZ, BPA, CAFF, and TCS between AC and composite adsorbent were found to be 10.2, 14.1, 8.42, 5.25, and 17.5% respectively. From the results, it is evident that the newly developed composite adsorbent GO-AC-CS performed well for the removal of both hydrophobic and hydrophilic compounds. The composite adsorbent showed high removal in the case of CAFF (Figure 3 d). The high removal of CAFF (98.9%) could be attributed to the higher hydrophilic nature of compound with lower log Kow (-0.07).44 Followed by CAFF, the composite showed good removal for TCS (98.2%) (Figure 3) due to its possible strong π-π interaction between benzene rings of adsorbate and adsorbent.45 In the case of ACP (Figure 3), the mechanism originated from electrostatic interactions between protonated amino groups (+NH2) of ACP and -COOH/-OH group of the composite.46 The lower adsorption capacities for CBZ (log Kow (2.45)) and BPA (log Kow (3.32)) (Figure 3), when compared to those for other pollutants, may be attributed to their hydrophobicity (lower log Kow value).47 Earlier studies reported that the adsorption of PPCPs using functionalized GO composites was mainly driven by the combination of mechanisms such as hydrophobic/hydrophilic, hydrogen bonding, electrostatic, and π-π interactions. Some earlier developed carbon based adsorbents 48,49 showed better adsorption capacity than newly developed composites for removing single pollutant. However, not many studies have been conducted on the behaviour of functionalized adsorbents in the presence of multi-pollutant conditions. Twoway ANOVA analysis (F value) indicates that the performance of different adsorbents in the removal of selected PPCPs are significantly different (p 0.990) with pseudo-second order kinetic model. Also, the experimental and calculated qmax value of pseudo second order kinetic model was almost equal which indicates that the sorption was predominantly accompanied by chemisorption. This mechanism could be involved by electrostatic forces through sharing or exchange of electrons between composite and ionized species of adsorbate.44,45 For instance, the calculated qmax values of GO-AC-CS for BPA and CAFF from the experiment were 18.4±0.55 and 19.8±0.11 mg/g while the calculated qe values through pseudo second order kinetic model were 18.3 and 19.7 mg/g, respectively. The second order rate constant K2 of GO-AC-CS was found to be 0.088, 0.133, 0.040, 0.487, and 0.650 g/mg/h for ACP, CBZ, BPA, CAFF and TCS, respectively. AlKhateeb et al.44 showed that the experimental data well fitted with pseudo-second order kinetic model for the adsorption of pharmaceuticals onto GO. The obtained rate constants (0.188 and 0.777 g/mg/h for ACP and CAFF) in their study are comparable with the present study (0.088 and 0.650 g/mg/h). It was evident from the batch kinetic studies that the adsorption capacity of GO-AC-CS was higher than that of other pristine and composite adsorbents (GO-CS and AC-CS). Therefore, the composite of GO-AC-CS was selected as an adsorbent for further equilibrium and desorption studies.

3.4. Equilibrium studies Adsorption equilibrium studies were carried out for selected PPCPs using GO-AC-CS as adsorbent at ambient temperature and an equilibrium time of 8 h (Figure 5 a). The Langmuir model showed the best fit for interpreting the data with high correlation coefficient value 26 ACS Paragon Plus Environment

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(R2>0.97) (Table 2 and SI Figure S5 a-e). The adsorbed amounts on GO-AC-CS at equilibrium was in the order of CAFF>TCS>ACP>BPA>CBZ. The spontaneous nature of the adsorption process (RL) is calculated (refer to section 1.2 of SI), and the obtained value was between 0 to 1 which clearly indicates the adsorption is highly favourable or spontaneous. Though Freundlich isotherm not showed the best fit, the estimated Freundlich constant (1/n) was found to be less than 1 in all the cases. A lower n value indicates higher bonding between adsorbent and adsorbate.

Table 2. Langmuir and Freundlich isotherm parameters for the adsorption of selected PPCPs over composite adsorbent (GO-AC-CS) Langmuir model

Freundlich model

qmax (mg/g)

KL (mg/L)

R2

RL

KF (mg/g)

1/n

R2

ACP

13.7

10.4

0.995

0.015

10.5

0.516

0.95

CBZ

11.2

22.3

0.999

0.004

14

a) Pharmaceuticals

0.500 0.928

b) Personal care products BPA

13.2

4.00

0.988

0.024

7.7

0.502

0.983

CAFF

14.8

3.39

0.970

0.029

10

0.579

0.964

TCS

14.5

6.90

0.992

0.014

11

0.546

0.949

3.5. Desorption profiles of GO composites Desorption studies were carried out to evaluate the regeneration potential of GO-AC-CS against selected PPCPs. In this study, six eluents such as tap water, DI water, 0.5 M NaOH (base), 0.5 M HCl (acid), polar (methanol) and nonpolar (Hexane) solvents were used (Figure 5b). The higher amount of desorption in DI water indicated the effect of physisorption whereas if desorption occurs in polar and non-polar solvents it implies hydrophobic and 27 ACS Paragon Plus Environment


hydrophilic interactions, respectively.50 Desorption in acid and alkaline solution would indicate the electrostatic interactions occurring between adsorbent and adsorbate. Desorption of ACP was in the order of methanol (79%) > 0.5M NaOH (41.6%) > tap water (13.8%) > hexane (10.5%) > DI water (6.8%), and 0.5 M HCl (4.6%). Desorption of highly hydrophilic ACP in the polar solvent was mainly due to charge transfer interactions. In the case of CBZ, maximum desorption was found in a non-polar solvent (60.6%) due to its hydrophobic nature. In the case of personal care products, BPA was desorbed more than 70% using methanol followed by NaOH (49.8%), and tap water (18.4%). CAFF got desorbed in methanol (70.8%) due to its hydrophilicity. Similar to CBZ, TCS was desorbed more using a non-polar solvent (60.5%). Kwon et al.51 showed the high amount of desorption (>80%) of CAFF, BPA and ACA using methanol as an organic solvent. Negligible desorption in DI water indicates that strong electrostatic and hydrophobic interactions played a major role in the adsorption of target pollutants. Insignificant desorption of PPCPs in tap water and distilled water indicated the strong affinity of selected compounds on the adsorbent surface. In general, it was observed that acid was not a potential desorbing agent for all the types of selected pollutants. Also, leaching of CS and GO in the test solution was observed when HCl was used as a desorbing medium. In addition to chemical regeneration, the thermal regeneration was also explored. Here spent adsorbent was heated at 100 °C and the resulted regenerated sorbent was utilized for further adsorption. The regeneration of selected pollutants using heat treatment is shown in Figure 5c. The adsorption efficiency of adsorbents gradually decreased after each cycle of heat treatment. The adsorption efficiency of ACP, CBZ, BPA, CAFF, and TCS during the first cycle of operation was found to be 68.5, 78.9, 81, 85.8 and 88.2%, respectively. After the third regeneration cycle, the adsorption capacity decreased (47.7, 57.4, 53.6, 60.5, and 58.2%) considerably. From this result, it is clear that using thermal regeneration the spent


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adsorbent can be regenerated and reused maximum up to three cycles and below this the adsorption capacity decreased gradually. Omorogie et al.52 investigated the regeneration of spent adsorbent using different methods such as solvent washing, oxidative, and reductive regeneration and found that solvent washing was effective in desorbing the adsorbed pollutants from the adsorbent surface. In the present study, it was also found that solvent treatment is one of the effective regeneration techniques for PPCPs.

3.6. Performance evaluation of PPCPs sorption on different adsorbents The adsorption efficiency of GO-AC-CS towards selected PPCPs was compared with the adsorption capacities of diverse sorbents reported in previous studies (Table S3). From the literature survey, it was observed that numerous studies were carried out using diverse class of solid sorbents for the removal of ACP, CBZ, BPA, TCS, and CAFF. However, most of the sorbents were used as a powder (e.g., AC, multiwalled carbon nanotubes (MWCNT), reduced graphene oxide (rGO), graphene (Gr)) and removal of this spent sorbent additionally required sophisticated technology (e.g., ultrafiltration or microfiltration). This is the main limitation in the application of powdered sorbent. For instance, the sorption capacity of GO-AC-CS towards ACP (13.7 mg/g) was comparable with the sorption capacity of graphene nanoplatelets (GNPs: 13.0 mg/g).44. In this regard, the achieved adsorption capacity of GO-AC-CS was higher than spent tea leaves derived AC treated with H3PO4 (4.4 mg/g)53 and it was lower than thermally treated AC (53.1 mg/g)54 and NH4Cl treated AC (233 mg/g).55 This might be due to the enhanced surface area of carbon based materials in comparison with GO-AC-CS. In the case of CBZ, the observed sorption capacity of GO-AC-CS was lower than the sorbents listed in Table S3. This might be due to the enhanced hydrophobicity of the CBZ which limits the interaction of GO-AC-CS with CBZ. In the case of BPA, the obtained adsorption capacity (13.2 mg/g) was higher than



that of graphene (2 mg/g)45 and AC (1.9 mg/g).45 On the other hand, the rGO,48 SWCNTs,48 and MWCNTs48 showed superior sorption capacity than GO-AC-CS (Table S3). In the case of TCS adsorption, the synthesized composite surpassed the sorption capacity of commercial AC (3.5 mg/g)48, and AC derived from coconut pulp waste (2.04 mg/g)56 and the achieved capacity was similar to MWCNTs (19.7 mg/g).57 For the treatment of CAFF, pineapple plant leaves derived AC (77.4 mg/g)58 was found to be the best material followed by the xerogel (38.7 mg/g),59 GNPs (19.7 mg/g),44 and GO-AC-CS (14.5 mg/g) (Table S3). Overall, the performance ability of synthesized GO-AC-CS was better than some of the commonly used adsorbent such as graphene and AC. Nevertheless, steps need to be taken for fabricating graphene and AC in an easily separable form (e.g., hydrogel, beads, and flakes). The synthesized material possesses numerous advantages such as high aqueous stability, easily separable, and reusable.


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3.6. Effect of pH and organic matter on adsorption The role of pH is important in adsorption as it determines the speciation of ionisable pollutants. The effect of a wide range of pH (4-12) on adsorption of the selected PPCPs was shown in Figure 5d. For a given adsorbent, the adsorption capacities for the selected PPCPs varied with respect to pH suggesting that pH effect were adsorbate-specific. The impacts of pH on adsorption can be attributed by three reasons: i) increasing pH might promote the dissociation of the hydrophobic adsorbate molecules (negatively charged species), thus reducing the hydrophobic interaction; ii) electrostatic repulsion would increase with increasing solution pH; and iii) higher pH could increase π donor ability of adsorbate, thus enhancing π–π interaction.60 In the case of ACP, removal efficiency was found to be higher in neutral pH (7-8). At lower pH (pHpKa), deprotonation of the amino group occurred thus result in electrostatic repulsion and decreased the removal efficiency of ACP. In the case of CBZ, removal efficiency decreases as pH increases (pH>pKa) due to the change in frequent bond delocalization of CBZ. A similar trend was also observed in earlier studies.61

BPA adsorption onto the composite adsorbent (GO-AC-CS) increased at lower pH (4-6). However, the sorption levels decreased when the pH increased from 7.0 to 12.0. The composite



adsorbent (Fig S3) carries a negative charge on the surface at higher pH, due to the surface deprotonation of functional groups. Additionally, BPA exists in its neutral form under pH less than 8, but when pH increases above 8 it begins to deprotonate to an anionic form.45 This phenomenon leads to electrostatic repulsion between anionic BPA and negatively charged surface of composite adsorbent (GO-AC-CS) which in turn leads to decreased removal efficiency at higher pH. The removal efficiency of CAFF showed increased efficiency from low to neutral pH, and then efficiency decreased at high pH because of the hydrophilic nature of the compound.44 Similar to BPA, the TCS adsorption efficiency was found to be increasing at lower pH (Fig 5 d). The main driving forces for TCS sorption onto the composite adsorbent were π-π interactions, hydrogen bonding, and hydrophobic interaction.45 The decrease in TCS adsorption with respect to the increase in pH can be attributed to the electrostatic repulsion between TCS anions and negatively charged surface of the composites. Earlier studies have also reported higher removal of TCS at low pH values.45,49 From this study, it is evident that the removal of pollutants was pH dependent and needs to be optimized prior to use.

Interference of organic matter with the adsorption of PPCPs was studied by conducting experiments on adsorption of selected PPCPs in the presence of 20 mg/L of humic acid. The percentage removal of ACP, CBZ, BPA, CAFF and TCS in the presence of humic acid was found to be 42.8%, 54.8%, 62.7%, 72.6% and 70.2%, respectively. From the results, it is clear that the external organic matter competes with PPCPs which in turn decreased the available active sites for PPCPs. Similar observation was reported by Apul et al.62 for the adsorption of phenanthrene (PNT) and biphenyl (BP) onto GO and two types of graphene nanosheets (GNS) (e.g.GNS-A and GNS-B). It has been found that GO showed the least reduction in adsorption


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capacity (60- 74%) than GNS (71-83%) in the presence of natural organic matter (NOM). This higher stability of GO is due to the presence of higher total pore volume. Also, the presence of multifunctional active sites gradually decreased the rigidity of the structure which in turn occupies both organic matter and target pollutants. From this study, it is clear that the synthesized GO based composite can be used as a potential adsorbent for PPCPs at wider pH range as well even in the presence of organic matter.


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Figure 5. a) Sorption of pharmaceuticals and personal care products at different initial concentrations, b) Desorption of pollutants in different solutions, c) Regeneration of pollutants using heat treatment, and d) Effect of pH on adsorption of PPCPs. 34 ACS Paragon Plus Environment

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3.7. Relationship between removal efficiency and physicochemical properties of selected PPCPs. To understand the relationship between removal efficiency of selected adsorbents and PPCPs, various physicochemical properties such as molecular weight (MW), pKa, Log Kow (octanolwater partition coefficient), Log D (distribution coefficient), surface area (SA), density (DS), vapour pressure (VP), complexity (C), molar refractivity (MR), polarizability (PL) etc. of various PPCPs (Table S1b) were correlated using Pearson’s correlation method at 95% confidence interval. The correlation (r-value) between removal efficiencies of selected PPCPs and their properties is shown in Table 3. Also, while monitoring the influence of different variables on the removal efficiencies of selected PPCPs, it is necessary to consider the correlation among different physicochemical properties (Table S4). The scatter plot showing the correlation between physicochemical properties and removal efficiencies of various PPCPs using pristine and composite adsorbent were shown in Figure S6a and b. From the results, it was observed that most of the properties showed a negative correlation with the removal efficiencies of GO against various PPCPs (p

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