Oxidized g-C3N4/polyaniline nanofiber composite for the selective removal of hexavalent chromium

Nanomaterials with selective adsorption properties are in demand for environmental applications. Herein, acid etching and oxidative decomposition of melon units of graphitic carbon nitride (g-C3N4) was performed to obtain the oxidized graphitic carbon nitride (Ox-g-C3N4) nanosheets. Ox- g-C3N4 nanosheets were further decorated on the polyaniline nanofiber (Ox-g-C3N4/Pani-NF). Ox-g-C3N4/Pani-NF was well characterized and further applied for a selective removal of hexavalent chromium (Cr(VI)) form aqueous solution. The zeta potential analysis indicate that the surface of Ox-g-C3N4/Pani-NF was positively charged which could be beneficial to bind anionic Cr(VI) ions electrostatically. In addition, nitrogen and oxygen containing functional groups exist on the Ox-g-C3N4/Pani-NF were mainly responsible for adsorption of Cr(VI) ions from aqueous solution. Moreover, the adsorption of Cr(VI) ions was also dependent on solution pH, reaction temperature and initial concentration of Cr(VI) ions. The maximum monolayer adsorption capacity of Ox-g-C3N4/Pani-NF for Cr(VI), calculated from Langmuir isotherm was 178.57 mg/g at pH = 2 and 30 °C. The activation energy (Ea = −20.66 kJ/mol) and the enthalpy change (ΔH° = −22.055 kJ/mol) validate the role of physical forces in adsorption of Cr(VI). These results demonstrate that Ox-g-C3N4/Pani-NF can be used as a potential adsorbent for environmental remediation applications.


Results and Discussion
The strong π−π stacking among sp 2 carbon atoms is responsible for poor solubility, hydrophobicity and agglomeration of g-C 3 N 4 nanosheets in various solvents. To overcome this problem, two strategies are applied: (i) introduction of hydrophilic groups on the g-C 3 N 4 nanosheets and (ii) an auxiliary segregation of hydrophilic g-C 3 N 4 nanosheets on polyaniline. Acid etching and oxidation process are applied to fabricate highly dispersible hydrophilic g -C 3 N 4 nanosheets. It is reported in the literature that the oxygen containing functional groups along with defects in the materials can be created effectively using a mixture of strong acid and oxidizing agent at elevated temperature 29,30 . A schematic reaction route for an acid treatment of g-C 3 N 4 nanosheets ( Figure S1) and further synthesis of Ox-g-C 3 N 4 /Pani-NF composite is shown in Fig. 1. In this work, Pani-NF are synthesized through a soft template approach using ascorbic acid. Hydrogen bonding interactions play a vital role in an elongation of Pani nanostructure 31,32 . Ascorbic acid consists four hydroxyl groups that are self-assembled by hydrogen boding interactions and thus, road like structure is formed. The road like self-assembled structure of ascorbic acid helps in the formation of self-assembled Pani-NF 33 . However, ascorbic acid is a reducing agent and hindered the oxidative polymerization Pani-NF. Therefore, polymerization occurs slowly and completed in long time.
TEM analysis was carried out to observe the morphology of pristine g-C 3 N 4 ; Ox-g-C 3 N 4 , Pani-NF and Ox-g-C 3 N 4 /Pani-NF composite. TEM image of bulk g-C 3 N 4 exhibits solid agglomerates with the size of several micropeters ( Figure S2a). It can be visualized from Figure S2b that the interconnected irregular small sheets like particles are obtained after acid-oxidative process. This TEM image confirms a successful reduction in size and alteration of pristine g-C 3 N 4 nanosheets. Furthermore, a self-assembled ribbon like morphology is appeared for pure Pani powder ( Figure S2c). It is also observed that the interconnected small sheets of acid oxidized g-C 3 N 4 decoration on Pani-NF (Fig. 2), These results are revealing a successful synthesis of Ox-g-C 3 N 4 /Pani-NF composite.
The crystal and chemical structure of pristine g-C 3 N 4 , Ox-g-C 3 N 4 , Pani-NF and Ox-g-C 3 N 4 /Pani-NF composite are studied in detail using XRD, XPS and FTIR techniques. The XRD pattern of Pani-NF is presented in Fig. 3. A wide peak ~25° validates an amorphous nature of Pani-NF. The XRD peaks for pristine g-C 3 N 4 around 12.7° and 27.4°, corresponding to d spacing 0.693 and 0.324 nm are originated due to the interplanar structure packing of motif and carbon nitride interlayer stacking reflections 24 . A slight variation in the XRD pattern of the acid-oxidized g-C 3 N 4 (Ox-g-C 3 N 4 ) nanosheets is observed. The intensity of the peak decreases and its position shifts from 27.42° to 28.2° due to the reduction in the gallery distance between the layers 21,24 . Due to the chemical oxidation and etching, the oxidized g-C 3 N 4 layers can be planarized by the π−π stacking and H-bonding interactions. These interactions lead to the denser packing and reduction the gallery distance between the layers. The intensity of the XRD peak for Ox-g-C 3 N 4 /Pani-NF composite in compare with pristine g-C 3 N 4 and Ox-g-C 3 N 4 , is further reduced. This could be because of an interactions between Ox-g-C 3 N 4 and Pani-NF. In addition, the characteristic peak for Pani is less pronounced in XRD pattern of Ox-g-C 3 N 4 /Pani-NF composite because Ox-g-C 3 N 4 covered Pani-NF.
The introduction of oxygen containing groups in Ox-g-C 3 N 4 nanosheets after chemical modification is confirmed and analyzed through XPS study and the obtained results are presented in Fig. 4. The atomic percentage  obtained from XPS analysis in Ox-g-C 3 N 4 for O 1 s at 532.06 eV, N1s at 398.86 eV and C1s at 285.01 eV is 21.018, 23.955 and 55.028%, respectively. As shown in Fig. 4a, a strong peak at 532.06 eV for O 1s core level indicates the presence of oxygen containing groups in Ox-g-C 3 N 4 nanosheets. Three peaks at 531.45, 532.16 and 533.47 eV after deconvolution of O 1 s core level, are detected, which confirm the presence of carboxylic and hydroxyl groups 24,29 . These peaks suggest that the oxygen containing groups are introduced on the surface after chemical treatment of g-C 3 N 4 nanosheets. A slight variation in peaks position of oxygen species (O 1 s) is appeared at 530.92, 531.93 and 533.31 eV for Ox-g-C 3 N 4 /Pani-NF composite (Fig. 4b). The C1s core level at 285.01 eV is deconvoluted into three main peaks centered at 285, 287.09 and 288.59 eV (Fig. 4c). These are attributed to graphitic sp 2 C=C bond, C-O bond and sp 2 hybridized C bonded to N in C-N-C coordination 24,34 . Meanwhile, the peak position of C-C, C-O and C-N-C groups appeared at 285, 286.95 and 288.66 eV does not shows major shift in binding energy of C 1 s core level for Ox-g-C 3 N 4 /Pani-NF composite (Fig. 4d). The N1s spectra (398.86 eV) of Ox-g-C 3 N 4 also show three different peaks at around 398.68, 400.1 and 405.4 eV after deconvolution (Fig. 4e). These peaks are typically assigned to sp 2 bonded N atom in C-N=C triazine rings, N-C 3 bridge atoms and π excitation in C=N or uncondensed terminal amine groups 24,34,35 . Three N 1 s peaks are also obtained in XPS spectrum of Ox-g-C 3 N 4 /Pani-NF composite ( Fig. 4f) with slight changes in the peak position at 398.23, 399.89 and 404.85 eV. These results suggest that the chemical states of C, N and O in Ox-g-C 3 N 4 /Pani-NF are similar to Ox-g-C 3 N 4 .
FTIR spectra of pristine g-C 3 N 4 , Ox-g-C 3 N 4 , Pani-NF and Ox-g-C 3 N 4 /Pani-NF composite are shown in Fig. 5. A broad peak ~3000-3400 cm −1 for pristine g-C 3 N 4 nanosheets, is ascribed to the starching vibrations of primary and secondary amine groups. Moreover, broader and sharp peaks are observed for chemically oxidized g-C 3 N 4 nanosheets. This is due to the introduction of oxygeneous functional groups in modified g-C 3 N 4 (Ox-g-C 3 N 4 ) nanosheets. The adsorption bands at 807 and 880 cm −1 are the characteristic peaks for tri-s-triazine units 36 . The peaks at 1220-1450 cm −1 are originated due to C-N stretching of aromatic rings and the peak at 1633 cm −1 is attributed to the stretching vibrations of C=N 37,38 . After chemical etching, the peaks become more intense and sharp in Ox-g-C 3 N 4 , possibly due to the better-ordered packing of H-bond cohered long stand of polymeric melon units that left after chemical treatment 21 . The peaks at 1063, 1452 and 1596 cm −1 in FTIR spectrum of Ox-g-C 3 N 4 appear due to the presence of C-O, O-H and N-O groups, respectively. However, Larkin et al. 39 reported that skeletal stretching vibrations of C-N and C-O appear in almost same IR regions because of their force constant values. The characteristic absorption bands for Ox-g-C 3 N 4 /Pani-NF composite are similar to Ox-g-C 3 N 4 and pure Pani-NF with a slight shift in their peak positions and intensities. In Pani-NF spectrum, the characteristic peaks of benzenoid and quinonoid rings occur at 1479 and 1550 cm −1 . The absorption bands at 1280 cm −1 is ascribed to the C-N stretching vibrations 37 . However, these absorption bands are shifted to 1286 cm −1 in FTIR spectrum for Ox-g-C 3 N 4 /Pani-NF composite. The characteristic band at 790 cm −1 is related to C-H vibration of aromatic ring plane and a slight variation in absorption band from 790 to 794 cm −1 for aromatic C-H ring out plane is observed in FTIR spectrum of Ox-g-C 3 N 4 /Pani-NF composite. The significant shift in the characteristic bands of Ox-g-C 3 N 4 and Pani-NF for Ox-g-C 3 N 4 /Pani-NF composite validate the interfacial interactions between Ox-g-C 3 N 4 and Pani-NF.
The surface charge properties of pristine g-C 3 N 4 , Ox-g-C 3 N 4 , and Ox-g-C 3 N 4 /Pani-NF composite were evaluated using a zeta potential analyzer (Malvern, US). The obtained results are shown in Fig. 6a. The zeta potential and surface charge characteristics are increased with alteration in functionality of g-C 3 N 4 (Fig. 6a). It is reported in the literature that the zeta potential of the stable nanomaterial is close to 30 mV. The zeta potential for Ox-g-C 3 N 4 /Pani-NF composite is found to be +21 mV, which validate its good dispersion and stability in compare with oxidized Ox-g-C 3 N 4 (+19.2 mV) and pristine g-C 3 N 4 (11.5 mV) 40 . The positive zeta potential values are attributed to the used of acidic condition for the modification and synthesis of g-C 3 N 4 and Ox-g-C 3 N 4 /Pani-NF composites, respectively. The carboxyl and hydroxyl groups were created when a strong etching and oxidation of pristine g-C 3 N4 were simultaneously carried out using a ternary mixture of H 2 SO 4 , HNO 3 and H 2 O 2 . Hence, the net positive charge on the surface of Ox-g-C 3 N 4 is generated 41 . Similar protocol was used to synthesize Pani-NF and the decoration of Ox-g-C 3 N 4 nanosheets onto Pani-NF in HCl solution. Amine and imine groups available in the Pani-NF backbone are prone to adsorb H + from aqueous solution. Thus, a highly positively charged Ox-g-C 3 N 4 /Pani-NF composite is obtained. Overall, the synthesized Ox-g-C 3 N 4 /Pani-NF composite has ability to selective binding with the anionic Cr(VI) and a poor binding ability with positively charged Cu(II) owing to its net positive surface charge behavior, ( Figure S3). Based on the primary metal adsorption study, Cr(VI) was chosen as a model pollutant to explore adsorption capacity of the synthesized materials.
The effect of adsorbent surface charge and Cr(VI) solution pH on the adsorption process are studied at the varied solution pH in the range from 2 to 9. The results are depicted in Fig. 6b, it can be seen that adsorption of Cr(VI) onto pristine g-C 3 N 4 , Ox-g-C 3 N 4 , and Ox-g-C 3 N 4 /Pani-NF composite increases sharply with decrease in solution pH. The optimum adsorption capacity is attained at pH 2. The solution pH not only influences the surface charge of the adsorbent, but also responsible for the speciation of Cr(VI) in aqueous solution. Cr(VI) exists in various stable forms like H 2 CrO 4 0 HCrO 4 − , CrO 4 2 and Cr 2 O 7 2 , which is highly dependent on solution pH. HCrO 4 − is the main species of Cr(VI) at low pH, which can easily bind with the positively charged adsorbent  . The probability of H 2 CrO 4 0 adsorption onto positively charged adsorbent surface is low compared to ionic HCrO 4 − due to surface charge. Thus higher Cr(VI) adsorption is expected at pH 2. As the solution pH increases, positive charge on Ox-g-C 3 N 4 /Pani-NF composite surface reduces and the adsorption of Cr(VI) decreases with the increase in solution pH. A net negatively charged surface is developed on the adsorbent which shows an electrostatic repulsion with negatively charged Cr(VI) ions 41 . The adsorption of Cr(VI) on the Ox-g-C 3 N 4 /Pani-NF composite is found to be much higher than the Ox-g-C 3 N 4 and pristine g-C 3 N 4 at all the studied pH. This can be attributed to the high positive zeta potential and the large number of surface functional groups (oxygeneous and nitrogenous) present on Ox-g-C 3 N 4 /Pani-NF composite. Because of this reason, Ox-g-C 3 N 4 /Pani-NF composite is further explored for Cr(VI) adsorption at pH = 2. Figure 7 shows the kinetics of Cr(VI) adsorption on the Ox-g-C 3 N 4 /Pani-NF composite at varied temperature. The adsorption of Cr(VI) increases with the increase in reaction time and equilibrium was established within 150 min. Moreover, reaction temperature also plays a positive impact to alleviate Cr(VI) by Ox-g-C 3 N 4 /Pani-NF composite. The adsorption capacity of Ox-g-C 3 N 4 /Pani-NF composite increases from 174.43 to 205.25 mg/g with increase in solution temperature from 30 to 50 °C, suggesting that adsorption process is endothermic in nature 40 . To confirm the nature of Cr(VI) adsorption onto Ox-g-C 3 N 4 /Pani-NF composite, the data is fitted to the Gibbs and Van't Hoff equations.  44 . The values of ΔG° ranges from −20 to 0 kJ/mol and −80 to −400 kJ/mol are often for physisorption and chemisorption, respectively 45 . In this study, the obtained ΔG° values indicate that the adsorption of Cr(VI) onto Ox-g-C 3 N 4 / Pani-NF composite is physisorption. The positive value of ΔS (80.988 J/mol k) reflects an increase in randomness at the solid-solution interface via adsorption 46 . Furthermore, the magnitude of ΔH° also reflects an interaction between adsorbent (Ox-g-C 3 N 4 /Pani-NF) and adsorbate (Cr(VI)). The ΔH° for chemisorption is usually between 40 and 120 kJ/mol, while the obtained ΔH° value for Cr(VI) adsorption is 22.055 kJ/mol. Thus, the adsorptive removal of Cr(VI) by Ox-g-C 3 N 4 /Pani-NF composite is due to physisorption 45,47 .
The experimental data presented in Fig. 7 is also fitted to the kinetic models to investigate the mechanism and rate controlling step occurs in Cr(VI) adsorption on the Ox-g-C 3 N 4 /Pani-NF. Pseudo-first order 48 and pseudo-second order 49  where q e and q t are the adsorbed amount of Cr(VI) (mg/g) at equilibrium and time t (min). k 1 and k 2 are the pseudo-first order (L/min) and pseudo-second order (g/mg min) rate constants. The plots for the pseudo-first order and pseudo-second order kinetic models are presented in Figure S4a,b and the rate constant values and the calculated equilibrium adsorption capacities, q e cal (mg/g), for the pseudo-first order and pseudo-second order kinetic models are tabulated in Table 1. Pseudo-second order model is fitted well to the experimental data than the pseudo-first order kinetic data at all the temperatures studied because of high R 2 values. The calculated adsorption capacities of Ox-g-C 3 N 4 /Pani-NF composite for Cr(VI) adsorption as predicted from pseudo-second order kinetic model are much closer to the experimental adsorption capacity. This is confirming better fitting of the pseudo-second order kinetic model for adsorption process 50 . Moreover, to find the activation energy (Ea) and type of adsorption forces, a linear relationship between the pseudo-second order rate constant (k 2 ) and temperature (T) is established using Arrhenius equation (5).
The magnitude of Ea clarifies the forces involved in adsorption. The Ea for physisorption varies between 5 to 40 kJ/mol and for chemisorption Ea range from 40 to 800 kJ/mol. The Ea for Cr(VI) adsorption on the Ox-g-C 3 N 4 /Pani-NF composite is 20.660 kJ/mol, indicating the involvement of physical forces in adsorption process 45 .
The impact of initial concentrations of Cr(VI) on the adsorption process is studied to find the maximum adsorption capacity and adsorption mechanism for Cr(VI) removal using Ox-g-C 3 N 4 /Pani-NF composite. As depicted in Fig. 8, adsorption capacity increases with initial concentration of Cr(VI) up to 200 mg/L, and thereafter adsorption reached to the plateau due to the saturation of available adsorption sites. The higher possibility of interaction between Cr(VI) and Ox-g-C 3 N 4 /Pani-NF composite at high initial concentration of Cr(VI) is that increase in the mass transfer driving forces 51 . The equilibrium adsorption data presented in Fig. 8 is analyzed using Langmuir and Freundlich isotherm models. The Langmuir isotherm model is based on the monolayer coverage while the Freundlich isotherm model postulates an equilibrium on the heterogeneous adsorbent surface. The Langmuir equation can be represent as: e e e m m where, q m is the maximum monolayer adsorption capacity (mg/g) and C e is the Cr(VI) concentration at equilibrium (mg/L) and b is a constant related to the energy of adsorption (L/mg). q m and b are calculated from the slope and intercept of a linear plot of C e /q e vs. C e ( Figure S5a). The Freundlich isotherm model can be represented as: where, q e is the adsorption capacity at equilibrium (mg/g), K F and n are constants that stands for the capacity and intensity, respectively. The parameters for Freundlich isotherm model are calculated from a plot of ln q e vs. ln C e ( Figure S5b).
The calculated values of the Langmuir isotherm parameters q m and b are 178.57 mg/g and 0.370 L/mg. On the other hand, the values of the Freundlich isotherm parameters n and K F , are 5.238 and 72.893 L/mg. It is noted that the correlation coefficient (R 2 ) value for the Freundlich isotherm is lower (0.7247) than that for the Langmuir isotherm (R 2 -0.9986). This indicate that the Freundlich isotherm model is not suitable to describe Cr(VI) removal using Ox-g-C 3 N 4 /Pani-NF composite. The Langmuir isotherm model is much fitted well to the adsorption of Cr(VI) by Ox-g-C 3 N 4 /Pani-NF composite. Thus, adsorption behavior of Cr(VI) on the Ox-g-C 3 N 4 /Pani-NF composite seems to be monolayer and the possibility for interactions between adjacent Cr(VI) ions is negligible 51,52 . In addition, an essential feature of the Langmuir isotherm model is in term of dimensionless separation factor (R L ). For the favorable adsorption of Cr(VI) on the Ox-g-C 3 N 4 /Pani-NF composite, the R L values must be in between 0 and 1. R L > 1 and R L = 0 indicate the unfavorable and irreversible adsorption process, respectively 53 . The R L can be defined as: where, C 0 is initial concentration of Cr(VI) (mg/L) and b is the Langmuir constant (L/mg). The R L values obtained for Cr(VI) adsorption by Ox-g-C 3 N 4 /Pani-NF composite are in the range 0.097 and 0.010, which indicate the favorable adsorption process for Cr(VI), and the suitability of the Langmuir isotherm model for the adsorption equilibrium data.
To find the effectiveness of the synthesized material, the adsorption capacity of Ox-g-C 3 N 4 /Pani-NF composite has been compared with the previously reported adsorbents used for the removal of Cr(VI). The maximum monolayer adsorption capacities of various adsorbents and applied experimental conditions have been reported in Table 2. The results in Table 2 revealed that adsorption capacity of the adsorbents is highly dependent on the experimental conditions and used adsorbent. The adsorption capacity of Ox-g-C 3 N 4 /Pani-NF composite is comparatively higher than the previously reported adsorbents.

Conclusion
A novel anion selective positively charged Ox-g-C 3 N 4 /Pani-NF composite has been synthesized and characterized using various instrumental techniques. The results are showing a capability of H 2 SO 4 -HNO 3 -H 2 O 2 to exfoliate, cut and oxidized the bulk g-C 3 N 4 into oxidized g-C 3 N 4 nanosheets. TEM image clearly shows an alteration in bulk g-C 3 N 4 nanosheets. XPS analysis is confirmed the oxidation of bulk g-C 3 N 4 after chemical modification. The characterization results demonstrate a successful synthesis of multifunctional Ox-g-C 3 N 4 /Pani-NF and its selectivity for adsorption of Cr(VI) from aqueous solution. The adsorption of Cr(VI) significantly increases as the functionality of g-C 3 N 4 changes as g-C 3 N 4 < Ox-g-C 3 N 4 < Ox-g-C 3 N 4 /Pani-NF composite. The optimum adsorption for Cr(VI) using Ox-g-C 3 N 4 /Pani-NF was attained at pH 2 within 180 min. The adsorption capacity of the Ox-g-C 3 N 4 /Pani-NF composite increases with temperature from 30 to 50 °C, revealing the endothermic nature of adsorption process. The Cr(VI) mass transfer rate is well described by pseudo-second order kinetic model. The equilibrium data are well fitted with the Langmuir isotherm model and the obtained values suggest a monolayer adsorption of Cr(VI) on the Ox-g-C 3 N 4 /Pani-NF composite. Based on these observations, Ox-g-C 3 N 4 /Pani-NF composite can be considered as anion selective adsorbent for the separation and removal anionic pollutants present in wastewater.