Three-dimensional (3D) polymer—metal–carbon framework for efficient removal of chemical and biological contaminants

The continuously increased existence of contaminants such as chemical and biological mainly dye, bacteria, and heavy metals ions (HMI) in water bodies has increased environmental concern due to their hostile effects on living things. Therefore, there is necessity to be developed newer materials that skirmishes such environmental menace. The present works focus on the synthesis of a novel three-dimensional (3D) polymer-metal–carbon (3D-PMC) framework for the exclusion of contaminants (chemical and biological) from water bodies. Initially, polyurethane (PU) foam was treated with nitric acid and used as a framework for the development of 3D-PMC materials. The copper nanosheet (Cu-NS) was deposited onto the functionalized PU foam to produce Cu-NS-PU material. The mechanically exfoliated graphene was mixed with chitosan to produce a graphene-chitosan homogenous suspension. The produce homogenous suspension was deposited Cu-NS-PU for the development of the 3D-PMC framework. The prepared 3D-PMC framework was characterized by scanning electron microscopy (SEM), Energy Dispersive X-Ray Analysis (EDX), Fourier-transform infrared spectroscopy (FT-IR), and X-rays diffraction (XRD) analysis. The prepared 3D-PMC framework was subjected to various adsorption parameters to assess the sorption ability of the material. The prepared 3D-PMC framework was effectively used for the removal of chromium (Cr) metal ions and Congo-red (CR) dye from the water system. The synthesis of the 3D-PMC framework is simple, novel, cost-effective, and economically viable. Therefore, the prepared 3D-PMC framework has the potential to be used as a filter assembly in water treatment technologies.


Scientific Reports
| (2021) 11:7708 | https://doi.org/10.1038/s41598-021-86661-w www.nature.com/scientificreports/ 80 °C for 24 h. The developed PU-Cu-NS samples were used for the preparation of PMC based adsorbent or filter materials. The graphene was synthesized from graphite powder using a mechanical exfoliation process 34 . Approximately 0.5-1.5 g of chitosan granules were dissolved in 120 mL of DI water with the help of continuous stirring and heating at 90 °C for 60 min. Next, 0.05% acetic acid was added into the chitosan solution to produce a homogenous solution of chitosan. Approximately, 0.1 to 0.5 g of mechanically exfoliated graphene was added into the homogenous solution of chitosan using continuous stirring at 300 rpm for 2 h at room temperature (~ 30 °C) to produce a chitosan-graphene homogenous solution. The produce chitosan-graphene homogenous solution used as an encapsulating agent for PU-Cu-NS samples by continuously stirring at 200 rpm for 2 h to produce PMC based adsorbents or filter materials. The developed 3D-PMC framework was used as a filter or adsorbent materials for the confiscation of all contaminants from water. Figure 1 shows a schematic illustration of the synthesis of a PMC framework used as the filter or adsorbent materials for environmental remediation application.
Batch adsorption studies. A batch experiment was performed to analyze different influential factors such as time of adsorption, concentration, pH, and temperature over adsorption of Cr(VI) onto PMC framework material. Adsorption kinetics was studied at 150 mg L −1 of Cr(VI) and 10 mg L −1 CR using 10 mg PMC with different time intervals from 0-24 h at a constant speed of 100 rpm at 35 °C. pH study on Cr (VI) and CR using PMC was performed to analyze the better absorbance efficiency (mg L −1 ). Solution pH was adjusted by adding 1 N HCl and 1 N NaOH to maintain the pH range (2 to 9), while the adsorbent dose, temperature, and other parameters were kept constant. A stock solution of 1000 mg L −1 of Cr(VI) and CR was prepared. The test solutions of 25 mL volume having different adsorbate concentrations were prepared in conical flasks from the stock solution. A small amount (~ 0.01 g) of the prepared PMC was transferred to the conical flasks. The flasks containing the test solution and adsorbents were kept in a mechanical shaker (150 rpm) at room temperature (35 °C). The concentration of Cr(VI) in the solution was ascertained by using 1,5-diphenylcarbazide (DPC) method while CR was measured directly using a UV-VIS spectrophotometer. The wavelength of the detector was set at 540 nm and 496 nm, respectively. From the species balance equation, the amount of Cr (VI) and CR adsorbed by the prepared materials was calculated.  where q is the loading (mg g −1 ) of Cr(VI) and CR and Ci and Ce are the initial and final (equilibrium) concentrations (mg L −1 ), respectively, of the solution. V is the volume (L) of the solution and W is the weight (g) of the adsorbent. All tests were done in triplicates to check the reproducibility.

Material characterization
The surface structure of the prepared PMC based adsorbent materials was characterized by using several characterization techniques such as field emission-scanning electron microscopy (FE-SEM), Energy dispersive X-rays (EDX), X-ray diffraction (XRD), and Fourier transform-infrared (FT-IR) spectroscopy. The surface texture of the prepared PMC based adsorbent materials was characterized by using FE-SEM (MIRA3-, TESCAN, A.S., Brno, Czech Republic). The presence of Cu-NS within the PMC based adsorbent materials was observed by using EDX analysis (Oxford, Inc., Germany). The crystalline pattern of the PMC based adsorbent materials was determined using XRD analysis with Cu Ka radiation (k = 1.54178 A°) at a scan rate of 5 °C per min. The surface functional group of the prepared PMC based adsorbent materials was ascertained by FTIR spectra with a wavelength range (400-4000 cm −1 ) (Brucker, Germany).

Result and discussion
SEM and EDX analysis. Figure 2a shows the SEM images of PU foam. PU foam consists of several macropores. As observed from the SEM image, the size of pores varies from 150 to 500 µm that uniformly distributed over the entire foam. The interconnecting pores through a thin boundary create a framework. Figure 2b shows the nanosheet-like structure of mechanically exfoliated graphene. Multi-layer graphene can be seen from the images with sharp edges. Figure 2c-c' shows images of Cu-NS-PU. As observed from the figure, Cu-NS uniformly deposited over the entire surface of PU foam. The magnified SEM image shows Cu-NS deposited onto the surface of PU foam. Figure 2d-d' shows the SEM images of PMC samples, chitosan encapsulated on Cu-NS-PU can be observed from the images. The encapsulation of chitosan might decrease the Cu leach out from the PU surface. Also, it enhances the sorption ability of dyes and heavy metal ions as well as antibacterial activity 35,36 . Moreover, interconnecting pores might be beneficial for higher adsorption loading of contaminants, discuss later in the manuscript. Figure 3a,b shows the elemental analysis and elemental mapping of the Cr(VI) adsorbed PMC samples. The presence of Cr content in the PMC samples attributed to the adsorption of Cr(VI) on the PMC sample. Moreover, approximately 55% of Cr (VI) indicated a higher sorption ability over the PMC based material. XRD analysis. The XRD diffraction pattern of Cu-NS-PU was shown in Fig. 4a. All the diffraction peaks were well matched with the solo PU, Chitosan, graphene, and Cu of Cu-NS-PU. As shown in Figure, Chitosan had a typical sharp peak at 10°. A broad peak appeared in the pattern at 19.25° and 23.28° was observed for the PU sponge. A characteristic peak of graphene was observed at 26°. The Cu diffraction pattern was observed at 2θ 45°, 50.6° and 74.9° which correspond to the crystallographic indices of the (111), (200) and (220), respectively, which proves that sample contained Cu in its pure metallic FCC phase (JCPDS No. 4-836) [37][38][39] . These results indicate that one-step loading of Cu-NS over PU was successfully done to develop Cu-NS-PU, which allows the function of graphene to be well dispersed and attached to the reticulated matrix of PU sponge to prepared PMC based adsorbent material. Figure 40 . All characteristic peaks were observed in the Cu-NS-PU sample except 1241, 2978 cm −1 . Moreover, the intensity of all characteristic peaks decreased in the Cu-NS-PU sample compare with that of the PU foam sample due to the Cu-NS deposition on the surface of PU foam 41 . All characteristics peaks were observed in PMC materials. Some new characteristics peaks at 1050, 1550, 1640, 1657, and 3490 cm −1 in the PMC sample are assigned to be P=O stretching of a phosphate group, NH 2 , amide band I, and NH 2 and OH group, respectively that belongs to chitosan 42 . The FT-IR spectra confirm that the PMC material was successfully synthesized. These functional groups aided advantageous to adsorb chemical contaminants as well as remove biological contaminants.

FT-IR analysis.
Adsorption study. Figure 5 shows the different PU-based composite materials for the adsorption of Cr(VI) and CR-dye. Figure 5a shows the adsorption of PU-based composite materials against Cr(VI) ions. As observed from the figure, PU has minimum sorption ability. The deposition of Cu-NS onto the PU surface increases the sorption ability of the Cr (VI) ions. Pu-chitosan-based composite has a higher sorption ability compare with Scientific Reports | (2021) 11:7708 | https://doi.org/10.1038/s41598-021-86661-w www.nature.com/scientificreports/ that of PU, and PU-Cu-NS due to positively charged surface and its surface functional group. Interestingly, the incorporation of Cu-NS, chitosan, graphene within the PU surface (PMC) materials shows the superior sorption ability. Figure 5b shows the adsorption of PU-based composite materials against CR-dye molecules. A similar trend like adsorption of Cr (VI) ions was also observed in CR-dye. The data suggested that PMC frameworks show higher sorption ability against both contaminants. Therefore, PMC frameworks might be used for the further analysis.
Kinetics study. Kinetics experiment was performed to study the rate of adsorption of Cr (VI) and CR-dye, at 150 and 10 ppm, respectively, in the time range of 10-1440 min. Figure 6 shows the adsorption kinetics of Cr(VI) and CR-dye on the prepared PMC based material. As observed from the Figure, the adsorption of Cr (VI) and CR-dye on PMC material was rapid during the initial 8 h, after approaching the equilibrium concentration (24 h), the rate slowed down. Pseudo-second-order kinetic models were used to explain the uptake of Cr(VI) and CR dye with time. The model equation was explained as follows: where qe is the equilibrium adsorption loading, and qt is the adsorption capacity (loading) at time t and k is the rate constant. Figure 6, and Table 1, describes the kinetic model and respective parameters as per Eq. (2) fitted to the data. The numerical value of the rate constant k calculated from the intercept of the line, respectively, was 0.0076 and 0.087 g/mg h −1 for Cr(VI) and CR dye, respectively. The relatively higher R 2 value (0.9942 and 0.9889) validates the applicability of the pseudo-second-order rate kinetics to the adsorption of Cr (VI) and CR dye on the PMC materials. (2)

Effect of temperature and thermodynamics of Cr(VI) and CR adsorption. Effect of temperature on Cr(VI)
and CR-dye adsorption on PMC material was studied at three different temperatures viz., 298, 308, and 318 K with keeping rest parameters constant. Figure 8a,b describes the effect of temperature on adsorption. It can be observed from the figure that the maximum adsorption capacity decreased with increasing adsorption temperature, indicating the adsorption to be exothermic. Table 3 shows the calculated values of different parameters from the Langmuir isotherm equation at 298, 308, and 318 K.
Calculations were performed to extract the thermodynamic parameters from the batch data. It was also necessary to conclude whether the adsorption process was exothermic and spontaneous. The thermodynamic parameters, namely Gibb's free energy change (ΔG°), enthalpy change (ΔH°), and entropy changes (ΔS°), were calculated using the following equations: where R is the gas constant, and T is the temperature (K). ΔG° was calculated from the K obtained from the Langmuir equation. Table 3 lists the numerical values of ΔG° obtained for different temperatures for Cr(VI) and CR.
The negative value of ΔG° describes the spontaneous nature of adsorption for both Cr(VI) and CR dye. On increasing the temperature, ΔG° decreased, indicating that a relatively lower temperature was unfavorable for adsorption. Figure 8a,b shows the plot of lnK vs. 1/T and is found to be linear. ΔH° was calculated to be − 37.56 and − 35.4 kJ mole −1 from the slope of the line (Eq. (7)) for Cr(VI) and CR dye, confirming the adsorption of Cr(VI) was exothermic respectively. Figure 8c,d shows the effect of pH on adsorption of Cr(VI) and CR dye on PMC based adsorbent material. Figure 8c it is visible that on changing the pH value from 2 to 9, adsorption of Cr(VI) was significantly affected and the highest adsorption loading (87.5 mg g −1 ) was observed at pH ~ 2. The reason for this adsorption capacity is thought to be that the adsorption sites occupy the anionic species, such as , these anionic form of Cr(VI) mainly found in acidic pH. Next, PMC consist of several amine group, at acidic pH these react with proton to form -NH 3 + , groups which promotes Cr(VI) anion to move towards adsorbent and create electrostatic attraction.

Effect of pH on Cr(VI) and CR dye adsorption.
Surface charges of PU, chitosan, and graphene have been reported in several literatures. According to them, the surface charge of PU, and Chitosan is positive 43,44 . However the graphene surface is negative 45 . Next, while synthesizing composite the overall surface charge is positive due to presence of several positive moieties. The reason behind the positive surface of PMC at the acidic pH ranging from 1.0 to 4.0, is availability of adequate H + . Therefore, the amine groups from the chitosan moiety of the adsorbent were easily protonated and positively charged, which promoted the approach of negatively charged Cr(VI) species (HCrO 4 − ) attributed to the electrostatic interaction 46 . However, basic pH exhibits the opposite mechanism as repulsion was the predominant force to expel Cr(VI) far away from the adsorbent surface subsequently low adsorption capacity 47 . Similarly, the effect of pH on adsorption of CR dye was shown in Fig. 8d. The maximum loading was observed at pH 7 (17.84 mg g −1 ) in 10 mg L −1 of CR concentration. Adsorption efficiency changes with changing pH value. As at acidic pH, a large H + concentration results in protonation of the several group such as amine with PMC framework, give rise to electrostatic interactions 48 . However at neutral pH, protonation of amine groups was nominal due to the negligible concentration of free H + . Next, CR is an acidic dye in neutral condition. It exists in the anionic form in solution. Additionally, in PMC the surface of aminated graphene (chitosan encapsulated www.nature.com/scientificreports/ graphene) is positive. It can be expected that the total surface charge over the PMC probably positive during this stage, (as mentioned above) which gives rise to electrostatic attraction. Therefore, adsorption ability is higher at this pH. However, the acidic pH of dye solution gives rise to repulsion between adsorbent and adsorbate surface due to similar polarity over CR molecule and surface of PMC. On further increasing the pH above 7.0, excessive OH ions compete with the sulfonate groups of CR and the active site for dye adsorption is reduced compared to neutral pH which decreases the overall adsorption of CR dye onto PMC based adsorbent material 49 . Therefore, the prepared PMC based adsorbent materials effectively remove both contaminants (Cr(VI) and CR-dye) from water.   Recycling of the materials is an important factor to determine the efficacy of the designed material. Usually, ideal adsorbent materials should be stable after various adsorbent runs. The recyclability test assay of the PMC framework was performed against Cr(VI) and CR-dye using a simple washing procedure. After each adsorption cycle, the PMC framework was washed several times using ethanol: water (1:4) ratio to remove metal ions and dye molecules from the surface of PMC and then dry at 60 °C for 24 h. Figure 9 shows the recyclability test assay of the PMC framework against Cr(VI) and CR-dye contaminants. As observed from the figure, insignificant changes were observed up to five consecutive cycles. The re-usability ensures the high stability during adsorption. Therefore, the prepared PMC frameworks remain stable throughout the adsorption experiments and show extra-ordinary re-usability, which is a chief factor for end applications.
Antibacterial activity. The antimicrobial behavior of different weights (0.01 to 0.20 g) of PMC materials was tested against both Gram-negative (E. Coli) and positive (S.aureus) bacterial strains with different exposure times (3-24 h). Figure 10a,b show the antibacterial analysis of PMC against both E. Coli and S.aureus, respectively. There were numerous significant observations. (1) PMC materials had suppression or inhibitory effects against both E. Coli and S.aureusbacterial strains. (2) PMC materials have comparatively higher performance against S.aureus bacterial strain. (3) The concentration at or below 0.10 g of the PMC sample, the E. Coli and S.aureus bacterial colonies were suppressed or inhibited for lesser exposure time (3 h), and 12 h, respectively, and (4) the higher concentration at 0.20 g was sufficient to kill or inhibit both bacterial strains at 3 h. Table 4 show the comparative data of the different hybrid materials used for the removal of contaminants from water. The data suggested that the prepared materials have superior adsorption loading among all of them. Moreover, most of the materials can remove either chemical or biological contaminants from water. In this  www.nature.com/scientificreports/ context, the prepared PMC based adsorbent materials or filter has the potential ability to remove both chemical and biological contaminants from water, thereby PMC based adsorbent materials or filter efficiently used for the treatment of water.

Conclusion
The PMC framework has synthesized using the CBD process and applied as complete decontamination including chemical and biological contaminants from wastewater. The synthesized framework exhibits excellent adsorption towards Cr(VI) and CR-dye as chemical contaminants and E. coli and S. aureus as biological contaminates. The maximum adsorption capacity was found to be 76.5 mg g −1 and 714 mg g −1 for Cr(VI) and CR-dye, respectively. Pseudo-second-order kinetic model was the best-demonstrated model on the adsorption process. Langmuir and Freundlich's isotherms were the best-fitted models for Cr(VI) and CR adsorption on the PMC framework. pH plays a significant role in the adsorption process as acidic pH was best for Cr(VI) adsorption while neutral pH was the best pH for CR adsorption. Surface charge plays important role in the adsorption process as postulated mechanism proves the electrostatic attraction due to adsorbent charges. The as-prepared hierarchical PMC framework is promising adsorbents for the removal of both biological and chemical contaminants from wastewater because of high surface charge, simple synthesis process, and excellent efficiency towards contaminants.