Novel Three-Dimensional (3D) Carbon-Metal-Polymeric framework: Efficient removal of Chemical and Biological Contaminants

The continuously increasing existence of contaminants such as chemical and biological mainly dye, bacteria, and heavy metals ions in water bodies has increased environmental concern due to their hostile effects on living things. Therefore, it the necessity to be developing newer material 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 scaffold for the development of 3D-PMC. The copper nanosheet (Cu-NS) were deposited on to the functionalized PU foam to produce Cu-NS-PU material. The mechanically exfoliated graphene and 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.


Introduction
The incessant increasing occurrence of biological and chemical pollutants such as heavy metal ions (HMI), dyes, and biological contamination (bacteria and fungi) in the water system increased the environmental menace globally. Contamination of water bodies by toxic HMI such as chromium (Cr), arsenic (As), lead (Pb), cadmium (Cd), and mercury (Hg) is one of the solemn threats globally due to their high toxicity to living being including human health. The existence of pollutants in water bodies has increased environmental worry worldwide due to its adverse effects on animals, plants as well as human health [1][2][3][4][5][6] .
Textile industrial waste mainly contains various pollutants like alkalis, inorganic and organic salts, dye, and HMI. Cr(VI) is extensively used for the leather dyeing procedure.
Moreover, high solubility in water makes it easier to enter the food chains, thereby higher accumulation within the body that leads to severe health issues. The incessant exposure of Cr(VI) to a human might because damaging various organs like the liver, kidney, and improper functioning of the circulatory and nervous system. Congo Red (CR) dye is a recognized pollutant and highly toxic and cancerous. Gram-negative (Escherichia coli (E. coli)), and Gram-positive (Staphylococcus aureus (S. aureus)) bacterial strains some of the most prevalent bacteria in water.
Biological contamination is one of the major issues nowadays that developed various healthrelated problems such as diarrhea, cholera, and bacterial infectious disease [7][8][9][10][11][12] . In this context, treatment or confiscation of such contaminants (both chemical and biological) is necessary for pure drinking water.
Numerous processes mainly physical, chemical, and biological treatment has been used to treat all contaminants from water. However, these processes are efficient for the confiscation of either chemical or biological pollutants from water bodies. Hence, there is a necessity to be developed newer competent, economically viable adsorbent materials or filters that might be eliminated chemical and biological contaminants from the water stream [13][14][15][16] . In this context, polymeric composite contains different surface functional groups that efficiently remove various contaminants from water.
Numerous polymers such as polyvinyl alcohol (PVA), chitosan, poly (methyl methacrylate) (PMA), poly-aniline, poly-pyrrole, poly-urea, poly-urethane have been used to a developed adsorbent or filter materials for the confiscation of a pollutant from water. However, high swelling ability and low affinity of adsorbent or filter materials towards contamination remain a concern 17-21 . In this context, hybrid materials or a combination of more than two or three materials might be overcome such associated issues with high removal efficiency.
Chitosan has considered necessary filtration characteristics. Moreover, Chitosan is a nontoxic, biocompatible, biodegradable polysaccharide, extraordinary adsorbing ability towards anionic dyes like CR-dye, and exceptional antibacterial ability, thereby extensively used in numerous filtration applications (water to air purification) 21-23 . Carbon-based nanomaterials such as CNTs, CNFs, graphene, and graphene oxide exhibit a promising role in the cleansing of pollutants from water bodies. Graphene is considered a futuristic material due to its exceptional characteristics like large specific surface area, and easily tunable properties according to the specific applications 24-26 . Therefore, graphene becomes a suitable candidate for the confiscation of contaminants from water.
Metal-NPs mainly Ag, Au, Zn, and Cu extensively used to treat biological pollutants like bacteria and fungi that inhibit the synthesis of protein, disruption of cells, and damaging DNA, thereby inhibit or kill microorganisms. The nano-sized Cu-NPs have the tremendous quality to clean out chemical as well as biological waste due to high reactivity 12,27,28 . The unique combination of all these materials makes a suitable candidate for the development of PMC based adsorbent or filter materials.
The proposed method of development of PMC framework based filter or adsorbent is simple, novel, and economically viable, and efficiently remove both chemicals as well as biological contaminants from water. The prepared PMC scaffold was used for the confiscation of contaminants from water and proposed as the filter or adsorbent materials. The graphene and chitosan-coated on the PU-foams enhanced the efficiency of the filter or adsorbent materials (3) Enhance the sorption capacity of the Cr(VI) and CR-dye from water. Graphene enhances the sorption ability of the Cr(VI) and CR-dye from water. The unique combination of the polymertransition metal-carbon composite dispersed with Cu-NS and graphene yielded the efficient adsorbent or filter material, used to remove both chemical and biological contaminants from water or provide pure water. The produced PMC materials were highly capable to remove Cr(VI), CRdye, and bacterial contaminants from water. The method of synthesizing the PMC materials or polymer-transition metal-carbon-composite adsorbent or filter material in the present study is novel, facile, and economically viable.

Preparation PMC adsorbent materials
The synthesis of the Cu-NS deposited Pu-foam-graphene-chitosan based scaffold or PMC filter or adsorbent material was started with chemical treatment of PU-foams using 1M HNO3 to produce chemically-treated PU-foams. The produce chemically-treated PU-foams washed several times using DI water until the surface becomes neutral (pH~7). Next, Cu-NS was synthesized by CBD process, for this 0. The graphene was synthesized from graphite powder using a mechanical exfoliation process. 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 ºC) to produce a chitosangraphene 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 mgL -1 of Cr(VI) and 10 mgL -1 CR using 10 mg PMC with different time intervals from 0-24 hrs at a constant speed of 100 rpm at 35 ⁰C. pH study on Cr (VI) and CR using PMC was performed to analyze the better absorbance efficiency (mg/L). Solution pH was adjusted by adding 1N HCl and 1N 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 mgL -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 1,5diphenylcarbazide (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. (1) 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), 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 reproducibility

Antibacterial analysis
The antibacterial test analysis of the PMC based adsorbent materials was determined against both

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 ºC 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). Figure. 2(a) 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. 2(b) shows the nanosheet like the structure of mechanically exfoliated graphene. Multi-layer graphene can be seen from the images with sharp edges. Fig. 2(c-c')

XRD analysis
The XRD diffraction pattern of Cu-NS-PU was shown in Figure 4. 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) [31][32][33] . 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. sample are assigned to be P=O stretching of a phosphate group, NH2, amide band I, and NH2 and OH group, respectively that belongs to chitosan 36 . 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.

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. 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- (2) 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.

Equilibrium study and adsorption isotherms
Where qe is the solute loading (mg/g) at equilibrium, qmax is the maximum adsorption loading

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. 8 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 7 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 from the slope of the line (Eq. (7)) for Cr(VI) and CR dye, confirming the adsorption of Cr(VI) was exothermic respectively. Figure. 9 shows the effect of pH on adsorption of Cr(VI) and CR dye on PMC based adsorbent material. Figure. 9(a) 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) was observed at pH ~2.

Effect of pH on Cr(VI) and CR dye adsorption
The reason for this adsorption capacity is thought to be that the adsorption sites occupy the anionic species, such as HCrO4 − , CrO4 −2 , and Cr2O7 −2 , as the surface of PMC was supposed to be a positive surface charge due to the presence of Cu ions. In the acid medium with pH ranging from 1.0 to 4.0, HCrO4 − is the major species of Cr(VI), and the surface of PMC was surrounded by 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 (HCrO4 − ) attributed to the electrostatic interaction 37 . 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.
Similarly, the effect of pH on adsorption of CR dye was shown in Figure. 9(b). The maximum loading was observed at pH 7 (17.84 mg/g) in 10 mgL -1 of CR concentration. The reason behind that as it is well known that CR is an acidic dye and in the neutral condition. It exists in the anionic form in solution. Additionally, in PMC the surface of aminated graphene (chitosan encapsulated graphene) is positive. It can be expected that the total surface charge over the PMC probably positive during this stage, 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 38 .
Therefore, the prepared PMC based adsorbent materials effectively remove both contaminants (Cr(VI) and CR-dye) from water.

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 and 10b 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 shows 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 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 and 714 mg/g for Cr(VI) and CR-dye, respectively. Pseudosecond-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.