Fabrication of polyamide-12/cement nanocomposite and its testing for different dyes removal from aqueous solution: characterization, adsorption, and regeneration studies

Polyamide-12/Portland cement nanocomposite was prepared by using the exfoliated adsorption method. The fabricated nanocomposite was applied first time to remove Congo red (CR), brilliant green (BG), methylene blue (MB), and methyl red (MR) from the synthetic wastewater. The polymer nanocomposite was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, elemental mapping, Brunauer–Emmett–Teller surface area analysis, and X-ray diffraction. The adsorption was rapid and all the studied dyes were absorbed on the surface of the polymer nanocomposite in 90 min. The point of zero charge was found at pH 5 and the factors such as pH, time, and temperature were found to affect the adsorption efficiency. Freundlich isotherm and pseudo-second-order models well-fitted the adsorption isotherm and kinetics data, respectively. The calculated maximum adsorption capacity was 161.63, 148.54, 200.40, and 146.41 mg/g for CR, BG, MB, and MR, respectively. The mode of the adsorption process was endothermic, spontaneous, and physical involving electrostatic attraction. On an industrial scale, the high percentage of desorption and slow decrease in the percentage of adsorption after every five regeneration cycles confirm the potential, practicality, and durability of the nanocomposite as a promising and advanced adsorbent for decolorization of colored wastewater.

www.nature.com/scientificreports/ easy regeneration and separation properties. In another study, electrospun fibers with polyamide-6 and chitosan shell were synthesized and used for the removal of an antibiotic, i.e., tetracycline 17 . For the desalination purpose, a polyamide nanofiltration membrane was prepared through interfacial polymerization and electrostatic assembly by assembling a nontoxic organic compound, i.e., phytic acid on it and their removal efficiency was checked 18 . Recent reviews have highlighted the properties of polyamide desalination membranes in large-scale desalination and membrane separation of toxic organic compounds 19,20 . Saleh et al. have used titania-incorporated polyamide nanocomposite 21 , silica/polyamide nanocomposite 22 , polyamide embedded magnetic palygorskite 23 , polyamidegraphene composite 24 , polyamide grafted carbon microspheres 25 , and clay-based polyamide nanocomposites 26 for the adsorption of dyes and toxic metals. Textile industries use dye products for coloration. The discharged dyes from the cloth industries in water streams are huge toxin chemicals that contain poisonous ingredients such as corrosive alkali and reducing agents 27 . Many dyes don't degrade in water. Some are carcinogenic, and are banned in several European countries. Congo red (CR) is an anionic, organic azo dye. It appears as a red-brown color which turns red in the alkali solution and blue in the acidic solution. CR produces harmful effects even at very low concentrations and thus is highly toxic. Brilliant green (BG) dye is a cationic, organic dye of the malachite-green dye series. BG is used as an antiseptic drug and antibacterial agent in the diluted solution. BG has toxic and mutagenic effects on the gastrointestinal tract which may lead to organ damage with long exposure 28 . Methylene blue (MB) is a thiazine cationic dye. MB has antimicrobial applications. In the form of medicine, MB is used to treat melanoma and methemoglobin levels 29 . MB is known to cause severe central nervous system problems. At high doses (< 2 mg/ kg), MB can cause cardiac and pulmonary vascular issues, and renal and mesenteric blood flow 30 . At doses (> 5 mg/kg), MB may precipitate fatal serotonin in the body 31 . Methyl red (MR) is a benzoic azo dye. It is not easily decomposed in aqueous streams and leads to serious environmental problems. It was found that the toxicity of MR was almost three to fivefold higher near pH 6 32 .
For many decades, adsorption process has been the most famous and most used wastewater treatment technique for removing aqueous pollutants [33][34][35] . Recently, the removal of CR 36,37 , BG 38,39 , MB 40,41 , and MR 42,43 from colored aqueous solution was conducted successfully. PA-12 is new in the area of environmental science. Portland cement (PC) is a hydraulic material that consists primarily of different compounds of calcium silicates (2CaO·SiO 2 , 3CaO·SiO 2 , 3CaO·Al 2 O 3 , 4CaO·Al 2 O 3 Fe 2 O 3 ) and clay minerals (SiO 2 , Al 2 O 3, and Fe 2 O 3 ). It has been used as a low-cost adsorbent [44][45][46][47] . PA incorporated with PC has improved mechanical properties and cost benefits. Gadelmoula and Aldahash evaluated the tensile, compressive, and flexural strength of selective laser sintering (SLS) manufactured PA-12/white cement parts 48 . It was found that a 10% concentration of PC in the mixture was sufficient to improve mechanical properties of PA-12 sintered specimens 49 . Another study showed that the addition of PA enhanced PC's mechanical properties by significantly reducing mechanical cement fatigue by 93.3% 50 . Yuan et al. found that the addition of hot-melt polyamide to hydraulic cement led to excellent crack healing properties and improvement in flexural strength 51 . A further study also showed that adding PA fiber increased the tensile and flexural strength of aggregate concrete 52 .
In this study, a polymer nanocomposite of PA-12 and PC was prepared and tested for the adsorption of four different dyes' , i.e., CR, MG, MB, and MR. According to the literature survey we conducted, the as-prepared nanocomposite was utilized for the first time for the adsorption of targeted dyes. As PC is cheaper than PA-12, the prepared nanocomposite is less expensive than pure PA-12. PC has a porous surface area and is hence used in this study to reduce the cost of synthesized nanocomposite by decreasing the quantity of PA-12. The aim was to synthesize a nanocomposite that has properties such as low-cost and easy preparation that required low mechanical resources. The as-prepared nanocomposite (PA-12/PC nanocomposite) has many advantages such as high surface area, remarkable adsorption properties, ease to use, and cost-effectiveness. The influence of some important factors affecting the adsorption of the dyes was monitored and the isotherm and kinetic modeling and thermodynamic studies of the adsorption process were studied under optimum conditions. The adsorption results were found to be excellent for CR, BG, MB, and MR. Considering its desorption and regeneration utility, the prepared novel PA-12/PC nanocomposite is promising for industrial wastewater treatment.

Results and discussion
Characterization. The Brunauer-Emmett-Teller (BET) method has been used in nanoscience for several decades now. This method is considered best in the determination of the surface area of the material. A H4 hysteresis loop of type-II isotherm with nearly horizontal and parallel lines over a wide p/p° range has been observed ( Fig. 1a), which indicates that the pore shape of PA-12/PC nanocomposite is of microporous characteristics. Type H4 loops are usually shown by many nanoporous adsorbents 53 . Type-II isotherm belongs to successive multilayer formation processes during the adsorption process 54 . A linear BET multipoint plot of 1/[W(P/ P 0 ) − 1] vs P/P 0 (Fig. 1b) was obtained (R 2 = 0.999) and their slope was used to calculate the specific surface area. The BET surface area, total pore volume, and average pore diameter were 2.635 m 2 /g, 0.003 cm 3 /g, and 2.665 nm, respectively. The values suggest the structure characteristics of PA-12/PC nanocomposite as a microporous solid with a relatively small external surface. The small BET surface area and the pore volume of PA-12/PC nanocomposite may be due to the occlusion of internal micropores by incorporating PA-12 in PC. Gedam and Dongre also reported the decrease in porosity and specific surface area of the prepared composite due to the blockage of internal pores by iodate upon doping with chitosan 55 . The mean pore diameter of PA-12/PC nanocomposite was small, leading to a high adsorption performance towards CR, BG, MB, and MR molecules. Recently, Liu et al. also analyzed that the smaller pore diameter of polymeric adsorbent (primary aminated resin) caused higher equilibrium adsorption capacity for phenol and anionic surfactant 56 .   Fig. 2d-f). It can be seen that PC particles cover the round PA-12 particles and some of the PA-12 particles are visible while most of the PA-12 particles are encapsulated in PC confirming the formation of a nanocomposite ( Fig. 2d-f). After dyes' adsorption, the surface of PA-12/PC became brighter. CR was seen arranged onto the adsorbents' surface in the grain shape (Fig. 2g,h). BG was adsorbed in the cluster form (Fig. 2i,j), and MB adsorption created new links between strands of PA-12/ PC nanocomposite (Fig. 2k) and Fig. 2l proves the MR deposition onto PA-12/PC nanocomposite. The element analysis of the PA-12/PC nanocomposite by energy dispersive spectroscopy (EDS) is shown in Fig. 3a-e. As carbon (C) element is the main constituent of both PA-12 and PC, it was detected with a sharp peak in the PA-12/PC nanocomposite (Fig. 3a). It can also be seen in Fig. 3a, that PA-12/PC nanocomposite is composed of carbon (74.33 weight percent), oxygen (16.14 weight percent), and nitrogen (9.53 weight percent) which are uniformly distributed on the surface of PA-12/PC nanocomposite. In CR adsorbed PA-12/PC nanocomposite, chlorine (22.41 weight percent) is determined (Fig. 3b), sulfur (23.05 weight percent) in BG adsorbed PA-12/PC nanocomposite ( Fourier transform infrared spectroscopy (FTIR) spectra of PA-12/PC nanocomposite (Fig. 5) show a broad peak at 3285.56 cm −1 , which is attributed to hydrogen-bonded N-H stretching 57 . The peaks recorded at 2915.66 cm −1 and 2848.30 cm −1 belong to symmetrical and asymmetrical stretching of C-H 21 . Peak observed at 1631.50 was related to C=O stretching vibration of the amide I 58 while narrow peaks at 1553.01 and 1461.58 cm −1 were ascribed to N-H bending and C-N stretching vibrations of the amide II 59 , respectively. The sharp peak at 1156.30 cm −1 carrying a small peak at 938.90 cm −1 might be correlated to Si-O-C stretching. The peak at 715.21 cm −1 could be attributed to the substituted aromatic rings. The sharp peak at 530.36 cm −1 commonly corresponds to O-Si-O bending 60 while the peak at 421.44 cm −1 belongs to the Zn-O-Si group 61 . The FTIR spectra of raw PA-12 (Fig. S1a) and PC (Fig. S1b) were also examined to confirm the functional groups existing in the prepared PA-12/PC nanocomposite and it can be seen that many characteristic absorption peaks for PA-12 and PC particles were observed in the produced PA-12/PC nanocomposite. As a result of dyes' adsorption, new peaks at 2359, 1366, 1270, 1191, 824, and 872 cm −1 were detected which indicates that dyes' molecules are involved on the surface of PA-12/PC nanocomposite. Additionally, most of the adsorption peaks remained unchanged, suggesting that the interaction of dyes' molecules with PA-12/PC nanocomposite did not alter the functional groups of the adsorbent after adsorption, since they were physically adsorbed by the PA-12/PC nanocomposite.
The X-ray diffraction pattern (XRD) of PA-12/PC nanocomposite is present in Fig. 6a. According to general observation, the composite seems to possess a slight degree of crystallinity, exhibiting many sharp PC and PA-12 peaks. For PA-12, both α and γ phases were observed in the PA-12/PC nanocomposite. The slight peak at ~ 11° and a distinct peak at 21.2° ~ 2θ correspond to the γ-phase, while the peak at 19.8° ~ 2θ corresponds to the α-phase. The pronounced peak with a peak area from 20 to 25° ~ 2θ suggests the formation of the α-phase during precipitation 62,63 . Apart from these, the peaks of calcium silicates are indicative of the presence of PC at higher ~ 2θ as observed by Ribeiro et al. 64 and Jaya et al. 65 . The crystalline structures of PA-12 powder (Fig. 6b) and PC (Fig. 6c) were also analyzed by using XRD pattern. PA-12 powder is a semi crystalline polymer exhibiting two distinct peaks at 20.9° ~ 2θ and 22.0° ~ 2θ which correspond to the unstable and intermediate structures of α and γ phase 66 (Fig. 6b). The XRD pattern of the PC provides some insight into its elemental composition. From the high resolution transmission electron microscopy (HRTEM) images, it can be concluded that PA-12 particle are spherical and wrapped on the surface of PA-12/PC nanocomposite in the form of globular agglomeration (Fig. 7a). The distribution of the PA-12 particles in the matrix of PA-12/PC nanocomposite is reasonably good and the inter-particle distance of PA-12 is fairly uniform (Fig. 7b).
Adsorption studies. Effect of pH and adsorption mechanism. pH is a significant factor to be accounted for in the adsorption study as it influences the surface binding sites and adsorption capacity of the adsorbent.  www.nature.com/scientificreports/ whereas, above this point, the adsorbent has a negatively charged surface. The PZC value of PA-12/PC nanocomposite came out at pH 5 (Fig. 8b), and therefore below and above this pH, PA-12/PC nanocomposite has positive and negative charged surfaces, respectively. With regard to CR, equilibrium adsorption capacity (qe) increases slightly with pH increasing from 2 to 4, reaches a maximum at pH 4, and then decreases with pH increasing from 5 to 10 (Fig. 8a). CR, an anionic dye, holds a negative sulfonate (SO 3 − ) group in its chemical structure. Therefore, electrostatic attraction between the positively charged surface of PA-12/PC nanocomposite (pH < 5) and negatively charged CR results in adsorption, while from pH 5 to 10, there is electrostatic repulsion between anionic charges of both PA-12/PC nanocomposite and CR, resulting in decreased adsorption. Recent studies have also reported pH 4 as an optimum pH for CR adsorption 69,70 . For other cationic dyes, i.e., BG, MB, and MR, qe increases with increasing pH from 2 to 6 and then decreases slightly with increasing pH from 8 to 10. The reason for low qe at pH < PZC is the competitiveness of dissociated H + ions with BG, MB, and MR adsorption at positive surface sites of PA-12/PC nanocomposite. With increasing pH, negative adsorption sites become available and adsorption reached the maximum at pH 6. At pH 8-10, adsorption again decreases because of the presence of OH − ions in the solution which leads to precipitation of dye molecules and disrupts the bonding between PA-12/ PC nanocomposite and BG, MB, and MR. Several studies have also reported that pH 6 is the optimum pH for   Figure 9 shows the adsorption mechanism of CR, BG, MB, and MR adsorption on PA-12/PC nanocomposite. Isotherm modeling. The experimental data of dyes adsorption onto PA-12/PC nanocomposite were fitted into non-linear plots of the various isotherm models (Fig. 11) and the respective parameters were calculated by using the generated isotherm equations (Table S1). As shown in Table 1, the Freundlich model was found to be the most applicable method for describing the adsorption of dyes onto the PA-12/PC nanocomposite. This outcome indicates that the functional groups present in the CR, BG, MB, and MR molecules were adsorbed in the form isotherm parameters were calculated to confirm the nature of the adsorption process. If the value of E comes out to be < 8 kJ/mol, then the adsorption process is considered in physical mode and if it's equal to or > 8 kJ/mol, then the adsorption process is considered in chemical mode. The calculated E values for all the studied dyes were found to be < 8 kJ/mol which supports the earlier proposed mechanism of multilayered physical adsorption of CR, BG, MB, and MR onto PA-12/PC nanocomposite by electrostatic attraction (Fig. 9). The obtained qm values for the PA-12/PC nanocomposite are in comparison higher than previously reported qm values of various natural and synthetic materials ( Table 2)  Kinetics. To design the dyes adsorption system onto PA-12/PC nanocomposite, the kinetic data of pseudofirst-order (PFO), pseudo-second-order (PSO), Elovich equation, and intraparticle diffusion (IPD) models using equations obtained from linear plots (Table S2) and calculated values of each kinetic parameter (Table 3). It was found that PFO failed to fit the data of dyes' adsorption and PSO was found to be superior as per higher R 2 (Fig. S2) and closer (qecal) to (qeexp) values. Usually, PSO is found suitable for the data as its equation has less chance to take the effect of the experimental error 97 . The constant values of PFO rate constant (K 1 ) and slow decrease in the values of PSO rate constant (K 2 ) with an increase in the dye concentrations indicates that equilibrium was achieved at an increasing concentration of dyes' . The intraparticle diffusion plots were used to assess molecular diffusion. The plots were non-linear with a plateau profile and do not pass through the origin; moreover, the unsatisfactory relationship between qt and t1/2 with low R 2 values suggests that the model was not reasonably fitted ( Table 3). The intercept (C) value increased with dye concentration, which indicates an increase in boundary layer thickness. It means that more than one process influenced dyes' adsorption onto PA-12/PC nanocomposite. Several previous studies have reported that intraparticle diffusion was not the only rate-controlling step in the adsorption process of dye molecules 98,99 . According to Elovich's model linear plots, R 2 values are mostly fall < 0.96 for all the dyes' , and therefore the model does not reasonably fit with experimental data in comparison to PSO's model. Table 3 shows that the values of A (adsorption constant) are higher than the values of B (desorption constant) indicating a higher adsorption rate than desorption 100 , which displays the feasibility of the adsorption process. The increasing value of A with increase in concentration supports enriched adsorption of all the studied dyes' onto PA-12/PC nanocomposite.

Effect of contact time and initial concentration.
Thermodynamics. With the vant's Hoff plot, the thermodynamic parameters affecting the adsorption of CR, BG, MB, and MR were determined for the PA-12/PC nanocomposite at various temperatures (Table 4 and Fig. S3). Based on the results, the values of − ΔG values increased with temperature, indicating that adsorption of the dyes' was increased with increasing temperature due to the endothermic process and spontaneous nature. Additionally, the adsorption process was due to the physical adsorption as ΔG values ranged below − 20 kJ/ mol 101 . The positive value of ΔH also suggests involvement of the endothermic process and physical adsorption. Desorption and regeneration. The chances of dyes adsorbed PA-12/PC nanocomposite retrieval were assessed by desorption experiments using three common desorbing agents (Fig. 12). It was found that a strongly basic solution of sodium hydroxide (NaOH) worked best for CR desorption (88.07%), while hydrochloric acid (HCl) and ethanol (C 2 H 5 OH) solutions desorbed a lesser amount. This outcome supports the result as observed in the pHPZC study that anionic CR was adsorbed onto cationic PA-12/PC nanocomposite. Generally, NaOH has a higher cation exchange capacity and loses its solvation easily during the ion-exchange process. At equilibrium, the quantity of adsorbent is higher than adsorbate 102 , hence when the cations present in the exhausted PA-12/PC  H + promotes the desorption of the cations present in the dye molecules structure i.e., N 2 + in BG, S + in MB, and N + in MR that were previously adsorbed to a negatively charged surface (> pH 5) of PA-12/PC nanocomposite. It can be concluded from desorption results that ion exchange was the main desorption mechanism and physical adsorption was involved in CR, BG, MB, and MR removal by PA-12/PC nanocomposite. Moreover, these results suggest that further research can be conducted to make sure that the four dyes are completely desorbable (> 95%), especially, BG and MB, by increasing the concentration of desorbing agents and changing the parameters such as pH and temperature. Figure 13a-d shows the percentage desorption efficiency of dyes-loaded PA-12/PC nanocomposite at each regeneration cycle (up to 5). It can be seen that the decrease in percentage removal of CR, BG, MB, and MR is constantly high, which is appropriate for the recycling, practicality, and durability of the PA-12/PC nanocomposite. Also, all the dyes were desorbed easily from the PA-12/PC nanocomposite which attributed to the physical adsorption of CR, BG, MB, and MR removal onto the PA-12/PC nanocomposite. This study revealed that PA-12/ PC nanocomposite had 73.6, 68.8, 63.4, and 70.7% stability for CR, BG, MB, and MR adsorption, respectively,  www.nature.com/scientificreports/ after five cycles (Fig. 13a-d). In light of this outcome, it can be concluded that PA-12/PC nanocomposite is economically feasible for the decolorization of industrial wastewater. Characterization. FTIR spectroscopy was used to analyze the structure and functionalization of PA-12/PC nanocomposite were analyzed by FTIR spectroscopy in 400-4000 cm −1 range by using Perkin Elmer Spectrum IR Version 10.6.1. SEM and EDS along with elemental mapping were used to get the elemental composition and   where Ci and Cf are initial and final dye concentrations, respectively, qe is the adsorption capacity of PA-12/PC nanocomposite, V is the volume in L and m is mass in gm.

Desorption and regeneration.
During the desorption experiments, 20 mL of each of the common desorbing agents, including NaOH, HCl, and C 2 H 5 OH were used. Using the same adsorption-desorption method, the used PA-12/PC nanocomposite was washed with distilled water, dried at 80°C for 2 hours, and reused five times for dyes' adsorption.

Conclusion
Easy and facile preparation of PA-12/PC nanocomposite was established throughout this study. SEM images show that PA-12 particles are round in shape while PC particles are angular. A H4 hysteresis loop of type-II isotherm with nearly horizontal and parallel lines over a wide p/p° range has been observed by BET analysis. The BET surface area, total pore volume, and average pore diameter were 2.635 m 2 /g, 0.003 cm 3 /g, and 2.665 nm, respectively. The nanomixture of PA-12 and cement not only had a positive impact on the mechanical properties but also became an excellent adsorbent for removing various dyes and provided novel perspectives for the purification and treatment of drinking water. For the removal of CR, the optimal pH was 4, while it was 6 for the removal of BG, MB, and MR. All dyes were removed after 90 minutes of equilibrium time. The calculated

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