Efficient liquid phase confiscation of nile blue using a novel hybrid nanocomposite synthesized from guar gum-polyacrylamide and erbium oxide

In recent times, biopolymer-metal oxide nanocomposites have gained prominent importance in the attenuation of environmental toxicants from aqueous phase. But lanthanide oxide-based biopolymer nanocomposites have scantly been evaluated for their adsorption potential. A novel guar gum-polyacrylamide/erbium oxide nanocomposite (GG-PAAm/Er2O3 NC) adsorbent was synthesized by copolymerization of guar gum (GG) and acrylamide (AAm) utilizing N-N′-methylenebisacrylamide as a crosslinker and Er2O3 as a reinforcing agent. The adsorptive efficacy of GG-PAAm/Er2O3 nanocomposite was evaluated using nile blue (NB) as a model pollutant dye from aquatic system. The prepared adsorbent was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) analysis, thermogravimetric analysis, scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM–EDX), and high-resolution transmission electron microscopy (HRTEM). The optimal process parameters, which include dosage (0.8 g/L), agitation time (40 min), initial solution pH (6), and initial NB concentration (80 mg/L) were determined by batch methodology. The equilibrium data for NB confiscation was better expressed by Langmuir isotherm model, with maximal adsorption effectiveness (Qm) of 225.88 mg NB/g demonstrating the actively monolayer adsorption onto homogeneous surface of GG-PAAm/Er2O3 NC. The kinetics of NB sorption process onto GG-PAAm/Er2O3 NC was reliable with pseudo-second order model. Thermodynamic parameters such as ΔH° (15–17 kJ/mol) and ΔS° (0.079–0.087 kJ/mol/K), and − ΔG° (8.81–10.55 kJ/mol) for NB validated the endothermic, an increased randomness at the GG-PAAm/Er2O3–NB interface, and spontaneity and feasibility of the process, respectively. The spent nanocomposite was effectively regenerated with NaOH, and could be reused proficiently for five runs demonstrating the high reusability potential of the nanocomposite. The commendable removal efficiency and high reusability of GG-PAAm/Er2O3 NC recommended it to be a highly competent adsorbent for cationic dyes particularly NB diminution from aqueous waste.

The present investigation aims at developing a novel and recyclable nanocomposite adsorbent for organic dyes remediation based on erbium oxide-reinforced guar gum-polyacrylamide biopolymer matrix and to evaluate its adsorption competence towards an organic cationic dye. The synthesis of guar gum-polyacrylamide/erbium oxide nanocomposite (GG-PAAm/Er 2 O 3 NC), its characterization through infrared and nuclear magnetic resonance spectroscopy, X-ray diffraction, N 2 adsorption/desorption, and thermogravimetric analyses, and scanning and transmission electron microscopy, and its liquid phase removal efficacy for nile blue (NB) is depicted in the current study. The process variables (agitation time, initial solution pH, dosage, and initial NB concentration) impacting the NB removal was explored to ascertain the optimal operating conditions, and various facets of the adsorptive uptake phenomena was explicated in terms of isotherm and kinetic parameters deduced by applying non-linear regression analyses of the adsorption data using the corresponding model equations. Thermodynamics parameters were also examined to estimate the energetic changes accompanying the adsorption process. Several adsorption-desorption cycles were performed in order to estimate the reuse potential of the adsorbent.

Results and discussion
Characterization of GG-PAAm/Er 2 O 3 nanocomposite. The FTIR spectra of GG-PAAm/Er 2 O 3 nanocomposite, prior and post adsorption, is shown in Fig. 1. The diminished intensity of peaks around 3000-3500 cm −1 for GG-PAAm/Er 2 O 3 NC signified the interaction of hydroxyl groups of guar gum with amide group of polyacrylamide 46 . The peaks detected in the spectra around 1200 cm −1 depicted the C-C-O, C-OH and C-O-C stretching modes of polysaccharides 14 . The peaks at 1654 cm −1 and 1604 cm −1 were due to C=O stretching vibrations in acrylamide 47 . The peak observed at 1081 cm −1 was attributed to bending vibration for CH 2 -O-CH 2 48 , whereas the band at 1410 cm −1 due to C-N stretching vibrations 49 . The peak at 3028 cm −1 was ascribed to NH 2 stretching vibrations of the polyacrylamide 50 . Additionally, the band at 659 and 608 cm −1 were attributed to Er-O-Er and Er-O linkage, respectively 51 , which clearly indicated the existence of metal-oxygen bond participating in the biopolymer nanocomposite. After NB adsorption (Fig. 1b), the FTIR spectrum showed slight shift in the peak position assigned to NH 2 from 3028 to 3181 cm −1 , and diminution of intensities together with slight shifting of absorption bands at 1654 and 1081 cm −1 to lower wavelengths indicating that the relevant functional groups were involved in the adsorption procedure either by van der Waals forces or hydrogen bonding 9 .    53 . The existence of characteristic peaks for erbium oxide in the diffraction pattern along with some shifting of the diffraction peaks and the formation of a semi-crystalline network due to the amalgamation of Er 2 O 3 into the amorphous guar gum phase signified the fabrication of GG-PAAm/Er 2 O 3 NC (Fig. 2b). Average crystallite size of GG-PAAm/ Er 2 O 3 NC was 57 nm.
The N 2 adsorption/desorption isotherm was used to determine the specific surface area and porosity of the nanocomposite. Higher surface area is related to greater accessibility of adsorptive sites, hence boosted adsorption aptitude of an adsorbent. The N 2 adsorption/desorption isotherms of GG-PAAm/Er 2 O 3 NC is shown in Fig. 2c,d. The specific surface area, pore volume and pore diameter of nanocomposite were 70 m 2 /g, 0.024 cm 3 /g and 5.796 nm, respectively. The pore diameter in the range of 2-50 nm confirmed the mesoporous nature of the prepared NC.
The thermal stability and rate of decomposition of the nanocomposite was gauged by TGA. This analysis could manifest the change in mass of the sample throughout the heating process. The TGA curves of GG and GG-PAAm/Er 2 O 3 NC are shown in Fig. S1 revealing that the decomposition occurred in two stages. The first decomposition stage of GG started at 68.6 °C and ended at 168.9 °C with a loss of 10.13%, while in GG-PAAm/ Er 2 O 3 NC it started at 86.3 °C and terminated at 199.8 °C with a loss of 2.1%. The logical explanation for these results could be the moisture content loss associated with the gum. The second phase corresponding to the decomposition of sugars in GG started at 255.9 °C and terminated at 340.7 °C with a weight loss of 42%, while in GG-PAAm/Er 2 O 3 this phase began at 264.7 °C and lasted till 342.9 °C with a weight loss of 38%. The results portrayed that the nanocomposite with Er 2 O 3 as filler has acquired higher thermal stability with lesser mass loss than the parent gum.
The SEM micrographs along with related EDX spectra of GG-PAAm/Er 2 O 3 NC and NB-loaded GG-PAAm/ Er 2 O 3 NC are presented in Fig. 3a,b. The morphology of GG-PAAm/Er 2 O 3 NC displayed an irregular, uneven  Figure 3b of GG-PAAm/Er 2 O 3 NC, after confiscation of NB, exhibited almost smooth texture confirming the adsorption of NB onto GG-PAAm/ Er 2 O 3 NC. The EDX spectra for corresponding SEM micrographs are also revealed in Fig. 3a,b. The existence of C, N in the EDX spectra of NB-sorbed GG-PAAm/Er 2 O 3 NC recommended the successful confiscation of NB onto the adsorbent surface. Considerable changes in the surface morphology of GG-PAAm/Er 2 O 3 NC occurred after the sequestration of NB. The pores nearly disappeared, which might possibly be due to the occupation and entrapment of NB molecules in the pore structures.
To examine the structural morphology of GG-PAAm/Er 2 O 3 NC, TEM investigation was performed. Figure 4a displays the TEM image for GG-PAAm/Er 2 O 3 NC. The average size of GG-PAAm/Er 2 O 3 NC was determined employing Image J software. The particle size distribution curve (Fig. 4b) suggested that the typical particles size ranged from 60 to 70 nm, that was in good agreement with XRD data. Moreover, the TEM image also confirmed the efficacious incorporation of Er 2 O 3 within the biopolymer matrix. The grey portions in Fig. 5a demonstrated the GG and PAAm matrix, while the darker portions were accredited to Er 2 O 3 nanoparticles randomly distributed into the GG-PAAm polymer matrix. In addition, such morphological features offered the GG-PAAm/Er 2 O 3 NC a larger surface area.
Effect of operational parameters on the removal of nile blue. The sorption characteristics of the nanocomposite for NB elimination was investigated in terms of different influencing parameters such as adsorbent dosage, initial NB concentration, agitation time, and initial solution pH at 303 K.
The reliance of aqueous phase removal of NB on variation in dosage was explored in 0.2-1.2 g/L range at pH 6, and the data is presented in Fig. 5a. The %R initially increased from 93.87 to 97.37 with an increment in adsorbent loading up to 0.8 g/L due to greater available active sites onto nanocomposite surface or an overall improved surface area, followed by a slow decrease thereafter. This decline in uptake efficacy on further increment in dosage might be due to higher accessible active sites relative to the number of NB molecules 54 . The removal capacity, however, showed an opposing trend with increase in adsorbent loading, which might be attributed to the overlapping of active sites, and/or decrease in surface area due to aggregation of adsorbent particles at higher dose that limited the removal capacity 54,55 . Therefore, 0.8 g/L of nanocomposite was selected as optimal dose for     56 and CuWO 4 nanoparticles 57 are described in the literature. The agitation time is critical and significant factor in removing perilous pollutants. The effect of shaking time (10-60 min) on the sorbed NB onto the nanocomposite (0.8 g/L) is depicted in Fig. 5b. A rapid adsorption of NB occurred in the first 30 min (94.87-96.95%) suggesting a fast confiscation rate of NB molecules, gradually attaining equilibrium at 40 min with 97.62% removal. So, 40 min was preferred as the optimal equilibrium time for further studies. Initially, the removal process was rapid because of the existence of sufficient active surfacesites for absorbing NB. Subsequently, a decrement in the adsorption rate on increasing the shaking time was associated with the saturation of surface-active sites leading to decrease in the adsorption effectiveness. Similar trend in the NB removal was reported for clay/starch/MnFe 2 O 4 58 and CNT/MgO/CuFe 2 O 4 magnetic composite 59 but with relatively higher equilibrium time of 60 min and 50 min, respectively.
The impact of change in the initial NB concentration (30-80 mg/L) on removal process was examined using optimal GG-PAAm/Er 2 O 3 NC dosage (0.8 g/L) and time (40 min) at pH 6. The obtained results, depicted in Fig. 5c, revealed that the sequestration of NB declined from 97.08 to 95.93% with increasing initial NB concentration from 30 to 80 mg/L, which could be understood in terms of two adversative effects. A fixed mass of GG-PAAm/Er 2 O 3 NC (0.8 g/L) has a definite number of surface-active sites. At low concentrations, the surface binding sites overwhelm the feeble number of NB molecules, which resulted in higher removal efficacy. However, when the solution concentration was increased further, the NB molecules progressively occupied the vacant sites, which reduced the number of available active sites. At high concentrations, fewer dye molecules occupied the remaining surface sites, hence decrease in removal efficiency. The increment in q e (36.40-95.93 mg/g) at higher solution concentration might be as a consequence of higher interactions between dye molecules and GG-PAAm/ Er 2 O 3 NC, or higher concentration gradient and/or increased driving force surpassing the mass transfer 60 . The optimal concentration for NB was 80 mg/L. Similar outcomes for NB adsorption with change in initial NB concentration were noticed for iron oxide nanoparticles 61 and acrylamide-or 2-hydroxyethyl methacrylate-based copolymeric hydrogels 62 .
The solution pH is a crucial factor that plays a significant role in the removal process of contaminants. The extent of sorption is controlled by surface charge of the nanocomposite and the ionization of sorbate species that are governed by the solution pH 63 . Thus, the impact of the solution pH on NB confiscation was scrutinized in 2-10 pH range at optimum operating conditions (Fig. 5d). The pH of the solution reflects the nature of the physicochemical interactions of the NB molecules and the active sites of the GG-PAAm/Er 2 O 3 NC. The pK a value of NB is 9.27 revealing its existence in the cationic form in the studied pH range. The pH zpc (= 5.6) of the nanocomposite meant that the surface was positively charged at pH < pH zpc implying an electrostatic repulsion between positive GG-PAAm/Er 2 O 3 NC surface and cationic NB, which might have declined the removal rate in the pH range of 2-6. However, as the %uptake was significantly affected, it implied that other type of interactions like hydrogen bonding and π-π interactions might be responsible for higher removal rate (82.43-90.6%) in the 2-6 pH range. Further increase in the pH changed the surface of nanocomposite to negative, which made electrostatic attractions to be a part of removal mechanism and the %uptake augmented to 95.66% till pH 9. After that, the dye became negative and electrostatic repulsion declined the %removal. Similar trend was depicted for NB adsorption onto MoO 3 /Ppy nanocomposite 6 .
Adsorption equilibrium isotherms. The analyses of isothermal equilibrium data at constant temperature by applying different isotherm models provide efficacious perspective with regard to maximum sorption capacity, homogeneity or heterogeneity of the adsorbent surface, affinity of the adsorbent towards adsorbate, coverage type, energy of adsorption, and the mechanism of adsorption. The corresponding isotherm parameters were determined using C e versus q e plots employing Langmuir (Eq. 1), Freundlich (Eq. 2), Temkin (Eq. 3) and D-R (Eq. 4) models.
Langmuir isotherm asserts the sorption of NB onto the nanocomposite surface with finite number of energetically equivalent surface-active sites 64 having equal affinity for NB molecules leading to monolayer formation.
Here, C e (mg/L) and q e (mg/g) is the residual equilibrium NB concentration in fluid phase and the amount of NB sorbed onto the solid phase, respectively, Q m (mg/g) signifies maximum adsorption efficiency of the nanocomposite required to form monolayer of sorbate on its surface, b (L/mol) is a Langmuir constant, K f (mg/g)(L/ mg) 1/n f is an indicator of sorption efficiency, n f indicates heterogeneity of nanocomposite surface and mutual interaction between sorbed species, 1/nf signifies functional strength of sorption, K t (L/g) is binding constant associated with maximal binding energy, β t (= RT/b t ) is a constant related to heat of adsorption, and q D (mg/g) is D-R sorption effectiveness.
The dimensionless factor, R L = 1 1+bC e is used to evaluate the feasibility and favorability of adsorption procedure.
(1) www.nature.com/scientificreports/ The corresponding isotherm model parameters along with correlation coefficients (R 2 ) and standard error of estimate (SEE) at the studied temperatures were estimated from q e versus C e curves for Langmuir (Fig. 6a), Freundlich (Fig. 6b), Temkin (Fig. 6c) and D-R (Fig. 6d) isotherms, and are tabulated in Table 1. An increment in the computed Q m values from 195.16 to 225.88 mg/g with an increase in operating solution temperature (303-313 K) ( Table 1) indicated an improvement in the adsorption aptitude of the nanocomposite at higher temperature probably resulting due to enhanced physical attachment between active binding sites and NB molecules, which designated the removal process as endothermic. The b parameter (0.061-0.069 L/g) varied in the order: 303 K < 308 K < 313 K, which accounted for the best NB-nanocomposite binding at higher temperature. The R L parameter (0.375-0.193) lying between zero to unity validated the energetically favorable sorption, and contemplated a strong NB-nanocomposite interaction 14 probably accounting for high percentage elimination of NB. The parameter, Q m is used to evaluate the sorption potential of a given adsorbent. A considerably higher adsorption efficacy (Q m ) of 225.88 mg NB/g at 313 K relative to most described adsorbents in the literature for NB removal (Table 2) and various tree gum-based nanocomposites for other dyes confiscation (Table 3) validated the admirable sorption effectiveness of GG-PAAm/Er 2 O 3 NC.
Freundlich isotherm contemplates adsorption onto the surface of adsorbent with heterogeneous distribution of binding sites. It also illustrates the sorption as non-ideal and reversible phenomena where sorbent has nonuniform affinity leading to the multilayer sorption 80 . The magnitude of the parameter, n f is an indicator of the sorbent surface heterogeneity, and its value close to unity expresses a higher surface heterogeneity. The value of 1/n f is a measure of favorable, unfavorable or irreversible sorption process. The 1/n f value < 0.5 specifies the facile sorption, 1/n f > 1.0 denotes cooperative adsorption, while 1/nf > 2 depicts that NB is hardly sorbed 54 . The values 1/n f below 0.5 (0.445-0.475) together with relatively higher K f (57.88-61.44 (mg/g)(L/mg) 1/nf ) ( Table 1) supported the positive and favorable sorption of NB. The increasing trend in K f with rise in temperature confirmed the endothermic trait of sorption.
Temkin isotherm model is employed to investigate the interaction between sorbate and sorbent 81 . It takes into account that heat of adsorption for pollutants diminishes linearly instead of logarithmically with an increase in coverage of the nanocomposite surface. The equilibrium binding constant, K t (L/g) values (4.84-5.46) displayed an incremental change with rise in temperature (303-313 K) pointing out a relatively enhanced electrostatic interaction between nanocomposite surface-sites and NB molecules at high temperature. An increment in the b T values, which is related to the heat of sorption from 0.024 to 0.041 kJ/mol (Table 1) testified to slightly higher   www.nature.com/scientificreports/ bonding probability of NB at elevated temperature (313 K). Further, the endothermic physisorption of NB was evidenced by the positive b T values below 8 kJ/mol, which is confirmed by the pertinent thermodynamic parameter (∆H°) 82 . Dubinin-Radushkevich (D-R) isotherm model adopts a pore filling sorption mechanism with multilayer character involving van der Waals interaction and is usually used to recognize the mode of adsorption, that is, physical or chemical 83 . It also provides reasonable evidence about the adsorption mechanism with possible distribution of energy onto non-homogenous surface of adsorbent. The mean free energy of adsorption (E) was deduced using the equation, E = 1 (2β) 1 2 from the value of β (mol 2 /kJ 2 ), estimated from Eq. (4). If the mean free energy, E is 1-8 kJ/mol then physical interaction governs the adsorption mechanism, while that between 8 and 16 kJ/mol indicates ion-exchange phenomenon. However, E > 16 kJ/mol specifies chemical interaction. The values of E equal to 0.081-0.091 kJ/mol advocated physisorption. The calculated values of q D (mg/g) were 139.74, 143.85, and 149.77 at 303, 308 and 313 K, which are in agreement with the similar trend in Q m values obtained using the Langmuir isotherm plot.
The estimation of excellent fit model is described on the basis of lower SEE and R 2 values close to unity. It was concluded from Table 1  Adsorption kinetics. The adsorption kinetic models provide significant information on the rate of contaminant adsorption. These models are utilized to illustrate the experimental data to conclude the mechanism for the adsorption of contaminants from aquatic system at adsorbent-adsorbate interface. To interpret the adsorption procedure, the kinetic data was examined by pseudo-first order 84 and pseudo-second order 85 models by employing Eqs. (5) and (6), respectively: where k 1 (1/min) and k 2 (g/mg/min) are adsorption rate constants for pseudo-first order and pseudo-second order kinetic model, respectively. The q e and q t signify adsorption capability of NB at equilibrium and time t, respectively. The values of k 1 , k 2 and q e were determined for different initial NB concentration from the slope and intercept of the plot of q t versus t (Fig. 7a,b), and are presented in Table 4 along with R 2 and SEE. The pseudosecond order model approved the best depiction of sorption data based on higher R 2 (0.925-0.978) and lower SEE values (0.077-0.136), which indicated that the removal of NB by nanocomposite was affected by the number of active binding sites instead of the initial NB concentration. The appropriateness of the pseudo-second order kinetic model in describing the experimental data recommended that the rate limiting step for NB confiscation by GG-PAAm/Er 2 O 3 NC probably involved chemisorption mechanism. The decrease in the magnitude of k 2 values with increment in initial NB concentration (0.069-0.059 g/mg/min) ( Table 4) signified rapid adsorption at lower concentration, which could be ascribed to the lesser competition faced by NB molecules for surface active-sites suggestive of physisorption 54 .
Interpretation of kinetic data is crucial to conclude the adsorption procedure governing the rate controlling steps. Usually, the liquid film diffusion implicating external mass transfer of NB molecules from bulk solution to GG-PAAm/Er 2 O 3 NC surface, intraparticle diffusion and interior pore diffusion are encompassed in the adsorptive scavenging of dyes. The Boyd liquid film and Weber-Morris intraparticle diffusion models 86,87 are respectively expressed as Eqs. (8) and (7) where k i (mg/g/min 0.5 ) and k D (1/min) are rate constants for intraparticle and liquid film diffusion, respectively and C i is the intercept expressing the boundary layer thickness.
The straight-line curves of either q t versus t 0.5 (Fig. 7c) or -ln(1-F) versus t (Fig. 7d) (F = q e /q t ) at 40 mg/L and 50 mg/L initial NB concentration with C i = 0 delineate that the dynamics of the confiscation process of NB is controlled either by intraparticle or liquid film diffusion as the rate-limiting step. The intraparticle diffusion graphs, however, deviated from the linearity with high boundary layer contribution to the rate-controlling step (C i = 45.65 and 58.48) indicating that it did not solely control the adsorption rate. Similarly, the liquid film diffusion plots also were not linear and did not pass through the origin, which precluded the liquid film diffusion as the only rate governing step. The values of k i (mg/g min 0.5 ) and k D (1/min) were 0.306-0.302 and 0.047-0.066, respectively at the studied concentrations. It could, therefore, be inferred that the adsorption process of NB was controlled by both the diffusion mechanisms. However, based on R 2 and SEE values for intraparticle diffusion (0.985-0.987; 0.016-0.020) and liquid film diffusion (0.989-0.991; 0.003-0.007), it could be concluded that the liquid film diffusion has a predominant role.  Fig. 8a, gave the precise value of E a ,, and is listed in Table 5.   www.nature.com/scientificreports/ Thermodynamic studies. The enhancement in NB removal on increasing the temperature from 298 to 313 K represented an endothermic adsorption process. The thermodynamic parameters such as changes in free energy (ΔG°), entropy (∆S°), and enthalpy (∆H°) are used to determine the feasibility, spontaneity, and nature of the sorption procedure. The parameters, ΔG°, ∆H°, and ∆S° were evaluated by employing Eqs. (9) and (10): where k c = q e /C e , R = universal gas constant (8.314 J/mol K), and T = absolute temperature (K). The slope and intercept of the linear curves of logk c versus 1/T (Fig. 8b) provided the ΔH° and ΔS° values, respectively (Table 6). It has been reported that ∆H° values of 2-10 kJ/mol designates physisorption mechanism involving van der Waals interactions, between 2 and 40 kJ/mol denotes hydrogen bonding, while that above 60 kJ/mol deduces chemisorption 90 . The positive ∆H° (15.35-17.87 kJ/mol) directed that the sorption process was endothermic and involved physisorption. Moreover, positive ∆S° (0.079-0.087 kJ/mol/K) ( Table 6) suggested an elevated randomness indicating an escalated degree of freedom at solid-liquid interface. Similarly,  Desorption and regeneration studies. The intention of regeneration is not only to recover the removal efficacy of spent adsorbent, but also to recycle and reuse the valuable adsorbent for several series of sorptiondesorption without loss of efficiency and stability, which might be helpful in sustainable management of the waste adsorbents, and would cut the overall treatment cost. Since operating solution pH had significant impact on NB confiscation by GG-PAAm/Er 2 O 3 NC, it was essential to control pH during desorption. For regeneration investigation, 2.0 g/L of GG-PAAm/Er 2 O 3 NC was agitated with NB solution (50 mg/L) for 1 h, then 0.1 mol/L NaOH was utilized as an eluent. Figure 8c demonstrates  Biodegradation exploration. Viscometry method was used to appraise the biodegradability progress of GG-PAAm and GG-PAAm/Er 2 O 3 NC. The advances in biodegradation was examined by assessing the intrinsic viscosity after every five days. From Fig. 9, it was apparent that both GG-PAAm and GG-PAAm/Er 2 O 3 NC were susceptible to biodegradation. The solution depicted degradation in 5-50 days as the solution disclosed a significant loss in the viscosity, which suggested the biodegradable nature of the biopolymers. However, GG-PAAm/ Er 2 O 3 NC portrayed lesser degradability relative to GG-PAAm matrix, which might be due to an increased mechanical strength owing to erbium oxide doping.
Mechanism of NB adsorption onto GG-PAAm/Er 2 O 3 nanocomposite. The functional groups on the surface of GG-PAAm/Er 2 O 3 NC, and the initial pH of the dye solution play significant roles in the adsorption of NB. The adsorption of pollutants onto different adsorbents generally occurs through various interactions such as electrostatic, hydrogen bonding, dipole-dipole, van der Waals forces and π-π. To explicate the mechanism accountable for the confiscation of NB by GG-PAAm/Er 2 O 3 NC, pH and FTIR studies were utilized. The percentage removal of NB with pH suggested electrostatic interaction as one of the mechanisms responsible for NB uptake. The slight change in the position of IR spectral peak from 3028 to 3181 cm −1 and change in intensity of the vibrational bands at 1654 and 1081 cm −1 specified that the relevant functional groups were involved in the  www.nature.com/scientificreports/ adsorption procedure through hydrogen bonding. However, the change in the peak intensities of O-H, C-O, and C-N groups confirmed the interaction of NB with existing functional groups. It might, therefore, be inferred that electrostatic interaction, hydrogen bonding and π-π interactions were mainly involved in the adsorption of NB onto GG-PAAm/Er 2 O 3 NC surface. The plausible mechanism of NB sequestration by the GG-PAAm/Er 2 O 3 NC is schematically illustrated in Scheme 1.

Conclusions
A novel GG-PAAm/Er 2 O 3 nanocomposite with admirable adsorption capacity (225.88 mg NB/g) was efficaciously fabricated by an efficient, inexpensive, environmentally benign, and easy-to-use ultrasonic-assisted polymerization process. Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray, transmission electron spectroscopy, thermogravimetric analysis, specific surface area (S BET ) and pH zpc measurements were used to characterize the synthesized nanocomposite, and was successfully employed for adsorptive elimination of nile blue from liquid phase. The surface area, pore volume and pore diameter of GG-PAAm/Er 2 O 3 NC were 70 m 2 /g, 0.024 cm 3 /g and 5.796 nm, respectively. The adsorption parameters such as dose (0.8 g/L), concentration (80 mg/L), time (40 min) and pH (6) were optimized. The equilibrium data best fitted to the Langmuir isotherm model signifying homogenous sorption of NB onto the surface of GG-PAAm/ Er 2 O 3 NC. The high Q m value (225.88 mg NB/g) at 313 K validated better sorption competence of the GG-PAAm/ Er 2 O 3 NC for NB confiscation. The rate of NB sorption onto the sorbent surface was governed by pseudo-second order kinetic model with intraparticle and liquid film diffusion controlling the overall rate. The positive ΔH° (15.35-17.86 kJ/mol) suggested endothermic physisorption, whereas ΔS° (0.079-0.087 kJ/mol/K) indicated an increased randomness at the sorbent-NB solution interface. The negative ΔG° (− 8.41 to − 9.67 kJ/mol) governed the spontaneity and feasibility of the process. The regenerated adsorbent demonstrated good performance up to fifth cycles without much loss in efficiency, which implied that GG-PAAm/Er 2 O 3 NC could be employed as an efficacious and potent adsorbent for cationic dyes including NB sequestration from waste water.

Data availability
The datasets used and/or analysed during the current study available from corresponding author on reasonable request.