Adsorption Capacity of Some Metal Ions Using Polyurethane Modified Magnetic Nanoparticles as Adsorbent

INTRODUCTION 
Polyurethane (PU) is one of the most frequently used and known polymers in the world, with annual output estimated to be around 28 million tons in 2020 [1]. The total spread in Europe was on the level of roughly 4 million tons in 2017 and 2018, while the recent total spread outside Europe was to the tune of 2 million tons [2]. The condensed polymerization reaction between polyisocyanates and polyols produces polyurethane [3], and polyurethane is produced when OH alcohol and isocyanate react with NCO. In the present study, polyurethane is made by reacting isocyanates with homo-cyclic azo compounds, which have two groups connected by a homogeneous ring at one end, therefore, the bond varies depending on the type of ring or substituent group that connects the azo group’s sides [4]. Since there are no other binding sites available, the nitrogen atom of the azo bridge is the sole one available for bonding with transitional or comparable metal ions [5]. Rings can be substituted with one or more acidic or basic groups such as (-NH2, -NHR) or (-OH, -SH, -COOH, etc.), the others can contain both categories in a single loop [6]. Isocyanate is a polyurethane component that can be divided into two types of aromatic materials: toluene diisocyanate (TDI) and methylene diisocyanate (MDI). Even though thermoplastic polyurethane is employed in several industries, the majority of Polyurethane – is hard thermoplastic. The polymers resist remelting and reformulation due to thermally cross-linked linkages, and therefore, they are widely used in manufacturing processes. Mechanical processing is not used on thermoplastic polymers like polyethylene or polystyrene.
Methods such as re-milling, bonding, glue pressing, pressure molding, and particle injection limit the use of polyurethane. Fused polyurethane polymer monomer resin or extra isocyanate [7-9] Heavy metals are extremely soluble in aquatic environments, allowing them to be easily absorbed by living organisms. According to previous research, heavy metals were found in the gills, liver tissue, and muscle of many fish species in polluted marine habitats [10]. These metals may wind up accumulating in the human body because of the food chain [11]. The permissible safe limits for heavy metals are connected to when implemented in industries because the majority of heavy metal bands are on a large range of possibilities [12]. Human health risks are lower in food samples [13, 14]. 
Heavy metals are defined as metallic elements that have a relatively high density compared to water, examples of these metals are metals, Cobalt (Co), and Nickel (Ni). Various methods are used to remove heavy metal ions from aqueous solutions including the adsorption, which have two kinds of techniques: physical and chemical. The initial concentration, acid function, temperature, the character of the adsorbing surface, the equilibrium time, and the solvent are factors that influence the kind of adsorption [16-18].  
Magnetic nanoparticles have recently been the focus of several studies because they have promising properties that can be used for stimulation, including nanomaterial-based catalysts, biomedical and tissue-specific targeting, magnetically tunable colloidal photonic crystals, microfluidics, magnetic resonance, magnetic field imaging, data storage, environmental processing, Nano fluids, optical filters, fault sensors, magnetic cooling, and positive sensors [19].  The iron oxide nanoparticles, Fe3O4, have attracted the most attention due to their superior magnetic strength. Fe 3+ evenly across the remaining octahedral and tetrahedral sites. These particles have the characteristic of being highly magnetic, so they can be separated from other solutions using an external magnetic field without using any thermal energy or chemical substances, and therefore can be reused again which reduces the total cost [20]. These nanoparticles are chemically stable [21] and their surface can be easily coated with inorganic, organic, and polymeric materials. There are several methods for preparing magnetite particles (Fe3O4), including the liquid-gel method Sol-gel method [22], thermal decomposition [23], hydrothermal method [24], and co-sedimentation method [25]. The current study aimed to use the Fe3O4-PU nanoparticles as a potential technique to adsorb some heavy metals from the polluted water.

MATERIALS AND METHODS
Chemicals
All chemical reagents used in this work were analytical high-purity and were used without further purification, including 2-Naphthol (%95, OEM), Sodium Hydroxide (%98, Thomas Baker), Hydrogen chloride (%37, Romil-SA), Chloroform (%99, GCC), Sodium Nitrite (99%, B.D.H), P-phenylene Di amine (99%, Fisher Chemical), Ethanol absolute (%99.9, Romil-SA), Diphenyl Methane Diisocyanate (%99.5, Industry Engineering Co,.Ltd), Preparation ions solution using Deionized Water, Nickel Nitrate (%98, CDH), Cobalt Nitrate (%99, Thomas Baker), Ferric Chloride Anhydrous (%97, Thomas Baker), Ammonium ferrous sulfate hexahydrate (%99, CDH), Ammonium hydroxide (%98, Shijiazhuang Xinlongwei Chemical Co., Ltd), Methanol (Absolute) (%99.9, Xilong Scientific), N, N-Dimethyl Form amide(%99, CDH).

Methods
Synthesis of iron oxide nanoparticles (Fe4O3)
Magnetic iron nanoparticles were prepared by co-precipitation of ferrous ions Fe +3 and ferrous Fe +2 with ammonium hydroxide solution, according to the equation below [26, 27]:

Fe2+ ​​+ 2Fe3+ + 8OH→ Fe3O4 + 4H2O

In this method, 10 mmol (1.6220 gm) of anhydrous ferric chloride (FeCl3) and 5 mmol (gm 1.9608) of ammoniac ferrous sulfate [(NH4)2 (FeSO4)2.6H2O] were dissolved in 150 ml of deionized water, in a flask with a capacity of 500 ml with continuous stirring of this solution using a magnetic stirrer (hot plate with a magnetic stirrer) with heating to a temperature of 90° C, where the chemical precipitation of magnetic nanoparticles of iron (Fe3O4) was conducted when adding 50 ml of 32% NH4OH ammonium hydroxide solution in the form of drops with continuous stirring until the acidity function reaches 10.5-10.0, and when the Fe3O4 particles are formed, a change in the color of the solution was noticed from brown to dark black and a black precipitate is formed from magnetic Nano-iron particles (Fe3O4NPs). This precipitate was collected using an external magnet. After the solution above it remained transparent, indicating the completion of the sedimentation process as shown in Fig. 1. The increase from the ammonium hydroxide solution, so that the acidity function of the wash water became neutral, then the formed precipitate was filtered using a filter paper with small pores and the resulting precipitate was dried by placing it in an oven at 50°C for 4 hours.
    
Grafting a polyurethane (PU) polymer on the surface of magnetic iron nanoparticles (Fe3O4-PU)
The polyurethane was grafted on the Nano iron oxide (Fe3O4) at a certain weight ratio and mixed with the previously prepared azo (1:2), where both are dissolved in chloroform solvent in an amount of (30ml) during mixing, then the mixture is placed in an ultrasonic device (Bath Sonicator) for an hour, so the mixture became homogeneous by the interaction of azo with Nano iron oxide (Fe3O4), then MDI is added in an equal amount to the amount of azo mixed with The above nanomaterial and the mixture is placed in the ultrasonic device for half an hour to homogenize the mixture of the polymer components formed with (Fe3O4), and to get rid of the solvent, the mixture is placed in a hot plate at a temperature of (50 Cᵒ) for (15 minutes) to boil the solvent and volatilize until The precipitate dried brownish-black and to complete the polymerization process, the precipitate is placed in the oven at a temperature of (50 Cᵒ) for three hours and then raised to (100 Cᵒ) for two hours. The solidification process is completed. It was confirmed by magnets that nanoparticles were mixed with the polymer and the mixture was subtitled to some characterizations such as IR, TEM, FESEM, TEM, and Zeta potential.

Batch adsorption studies
To find the adsorption isotherm by preparing different concentrations of the Adsorbate within the range (of 100-300 mg/L) (50 ml) was added in a series of solutions of heavy metals to (0.025gm) from the prepared (Fe3O4-PU) adsorbents, and these solutions were placed in a shaker at a speed of (300) cycles/ minute for 24 hours and upon reaching the equilibrium time and after then separation and clear solutions were measured by atomic absorption spectroscopy and from knowing the values ​​of absorption at the concentration at equilibrium, then finding the amount of the adsorbed substance from the below equation:

Qe = (Vsol (Co-Ce))/m

Where the initial Concentration is Co، ،(L) Total volume of the adsorbent solution is Vsol, (mg/g) Adsorbent for Final Concentration is Ce، (mg/L) Adsorbent, (mg/g) Adsorbent Quantity is Qe ، (g) Adsorbent Weight is m.

RESULTS AND DISCUSSION
IR Spectrum of Azo compound, polyurethane PU, Iron oxide nanoparticles Fe3O4 and Iron oxide nanoparticles –Polyurethane Fe3O4-PU Characterization
The FTIR spectrum of the prepared azo compound showed the effective groups. A strong broadband appears at a frequency of (3452 cm-1) stretching indicating the presence of an (OH group) of the compound (2- naphthole) included in the composition of the resulting compound [28]. The spectrum also showed a weak absorption band at a frequency of (2400 cm-1) belonging to the triple active group (RNC), while the appearance of a peak at frequency (1643) cm-1 belongs to the group (N = N) belonging to the azo group [29] as well as two absorption bands at (1539 and 1512 cm-1) overlapping due to the overlapping of two groups (C=N) Fig. 2.
The FTIR spectrum of Polyurethan1 prepared previously paragraph showed the effective aggregates. The spectrum of the compound showed a band at a frequency of (3375 cm-1) stretching belonging to the (N-H) group of polyurethane NHCOO, with the disappearance of the (OH) band at a frequency of (3400 cm-1) belonging to the azo compound involved in the reaction. The frequency (3032 cm-1) belongs to the aromatic C-H group. This indicated that the compound is aromatic. A strong bundle of symmetric stretching appeared at a frequency of (2276 cm-1) indicating the presence of an isocyanate group [30] belonging to polyurethane, as well as the presence of an absorption band at (1708 cm-1) indicating a group (C= O), as well as the appearance of an absorption band and the appearance of a band at (1500 cm-1) indicating a group (C =C) interferes with the beam at a frequency of (1411 cm-1) due to the azo group (N=N) and the appearance of a beam at a frequency of (1200 cm-1) belongs to the group (C-N) and the appearance of a beam at a frequency of (1006 cm-1) on the group (C-O-O) [31] indicates that the compound formed is a polyurethane Fig. 2.
The FTIR spectrum of the prepared iron oxide nanocomposites (Fe3O4 NPs) showed bands at (3402-3618cm-1) due to the amplitude expansion of the O-H bond caused by the ionic water molecules absorbed by the model when washed with it [32]. Also, found a beam at (574,455sh cm-1) that goes back to the (Fe-O) bond, which indicates an expansive expansion for the metal with the oxygen atom in the tetrahedral position and the octahedral position, they represent stretching and bending (and this applies to previous studies [33-34]. The formation of iron oxide nanocrystals is shown in Fig. 2. The FTIR spectrum of the iron oxide nanocomposite (Polyurethane + Fe3O4 NPs) revealed a group of (O=C=N) at the site (2260 cm-1) because of the Nano-combination with the polymer. In addition, some bands appeared at (497 and 559 cm-1) due to the (Fe-O) bond at the Tetrahedral and Octahedral sites [35], which indicated that the resulting compound is a Nano polymer (Polyurethane + Fe3O4 NPs) as shown in Fig. 2.

XRD Spectrum of Iron oxide nanoparticles and Iron oxide nanoparticles –Polyurethane Characterization
The X-ray diffraction device (Dx-2700B) Haoyuan (Dandong was used, and a Cu-Kα radiation beam with a wavelength (0.15406 nm) and an energy of (30KV) with a current of (20mA, and an angular range of (80-2θ=10). It is shown in Fig. 3a and Fig. 3b. The diffraction pattern in Fig. 3b shows the complete purity and polycrystallization of the Fe3O4 NPs and the crystal structure of the nanoparticles is Cubic [36], where the above information corresponds to the standard card (JCPDS File card) No.01-075-1610, It was also noted that the emergence of new peaks different from the spectrum of the nanocomposite and this is due to the bonding of the polymer to the Nano, forming new mixtures with more peaks as noted in the Fig. 3c.

Transmission Electron Microscopy (TEM)
A TEM study was conducted to find out the exact size of the compound, which is the nanostructure property of magnetic nanoparticles MNPs. Such as particle size distribution, size and morphology, the nanoparticles were observed to have typical spherical Fe3O4 molecular structure in Nano size Fig. 4a. Magnetic nanoparticles agglomerate due to their high magnetic properties. These results follow the previous studies [37, 38].
Iron oxide NPs in the Nano-polyurethane mixture have the characteristic of a large surface-to-volume ratio, and therefore have high surface energies. As a result, they tend to agglomerate to reduce their surface energies [39] Moreover, NPs alone have high chemical activity and are easy to oxidize in the presence of oxygen (especially magnetite) leading to a loss of magnetism or dispersion, but are bound to the polymer it leads to activity Chemically balanced on the surface of the neutral polymer, resulting in increased magnetism and dispersion on the polymer surface. It is necessary to develop an active protection strategy to maintain the stability of these NPs through appropriate surfactant interaction as in the surface of the polymer used in this study [40]. As shown in Fig. 4b.

Field Emission Scanning Electron Microscopy (FESEM)
Scanning electron microscopy (SEM) is one of the most common and widely used methods. Diagnosing nanostructures and nanomaterials, Figs. 5a illustrate microscopic images of the scanning electron microscope (FESEM) of the nanofabricated iron oxide Nano powder. The crystals range in size (from 15-22) nm and the field of view is about (8.6 mm). These results are consistent with XRD measurement and with previous studies [41]. Fig. 5b showed a large-scale gathering of polymer particles around the nanocomposite for Fe3O4-PU, and this led to the magnetic dispersal of the nanocomposite due to the presence of the polymer on the outer surface of the iron particles [42], and the resulting surface in both mixtures has a rough and shiny characteristic, the size of the crystal ranges from (15-22) nm and the field of view is about (5.46 mm) in the image.
Fig. 5c shows the spectrum of the energy-dispersive X-ray detector (EDX (attached to FESEM), which is usually used to indicate the composition or chemical analysis of the components of (Nano Iron Oxide), where the spectrum shows distinct peaks at (0.6 keV, 6.5, 7) belong to the iron atom, as well as the appearance of a peak at (0.5 keV) belonging to the oxygen atom, which leads to the formation of iron oxide nanoparticles, and this is consistent with the results of X-ray diffraction (XRD) and previous studies [43].

Zeta potential analysis
Zeta potentials study the surface charge of the particle which affects the agglomeration of nanomaterials and the absorption of ions on the Nano surface [44] Particles with a positive zeta potential of +30 mV or more negative than -30 mV are considered naturally stable. The results showed the presence of one beam for Fe3O4 with a value of (mv36.4), with a value of Zeta potential (mv36.4+), and an electrical potential, (cm2/V.S0.000282) as shown in Fig. 6a  measured by the scattered light method (ELS), the large positive charge is centered on the surface of the nanocomposite and indicates a high stability and the small iron oxide particles gather on the large charge to form the basic interface of the compound in a stable manner that represents a uniform distribution of nanoparticles as it matches the XRD calculations and previous studies [45].
Figs. 6b show the zeta potential value of (-10.4mv) for (Fe3O4-PU). Fe3O4 has a positive charge on the surface, which indicates lower affinity stability. To agglomerate over time in groups to form the basic structure, and produce electrostatic forces of attraction to deposit polyurethane on its surface due to the opposite charge of the polyurethane solution, while the produced Fe3O4-PU mixture is characterized by high stability and homogeneity due to the repulsive force between the particles indicated by the huge negative surface charge of the particles by increasing the stability of Fe3O4-PU in plankton, the adsorption and removal of various pollutant particles on the surface of polyurethane will be improved [46,47].

Adsorption of heavy metals ions
The adsorption of heavy metal ions, including Cobalt, and Nickel ions, was studied on the surfaces prepared in this study, including nanomonium oxide grafted with polyurethane at the beast ratio (2:1).

Effect of metal ions Concentration 
Different Co (II) and Ni (II) ions solutions were prepared; the initial concentrations were as follows 100,150,200,250,300 for Cobalt ion and Nickel ion in order to study the initial concentration effect at room temperature and optimum conditions obtained previously to apply the Langmuir and Freundlich adsorption isotherms. It has been noted that when the initial concentration of the adsorbent material increased, a decrease in the removal ratio happened, and an increase in the adsorption capacity may be attributed to the increase in the rate of diffusion and mass transfer on the surface, and then the time required to reach the equilibrium state will be longer. A certain amount of the adsorbent increases with the increase in the initial concentration of the adsorbent at a constant temperature [48].

Equilibrium time
The adsorption process was conducted to calculate the adsorption time of the two mixtures (Fe3O4-PU) with heavy metal ions (Co and Ni) and the results showed that the equilibrium time for Cobalt was (120 min), and the equilibrium time for Nickel (60 min), as shown in Figs. 7a.
While Co + 2 tends to balance at 120 minutes, and Ni + 2 tends to the equilibrium state at about 60 minutes then the adsorption becomes constant for each of the ions used in this study, the rapid increase in the percentage removal at the beginning of the adsorption process is due to a large number of vacant adsorbent surfaces on the nanocomposite material Nano iron oxide nanoparticles and polyurethane. This is a faster diffusion of metal ions in the adsorption material [49].

Effect of PH
A study was conducted to study the effect of the acidity function of the adsorption process of heavy metal ions Co and Ni on the prepared surfaces, where we notice that the removal percentage increases with the increase in the amount of the acid function, for example for Cobalt, the maximum adsorption at Co equals 5, While Nickel equals 7, since one end of the polymer has a negative charge, so most of the ions are located in some reaction sites in the adsorbent material due to the presence of competing for positive ions in the solution being acidic and did not exceed the base pH values. This leads to a total reduction of the values ​​of the adsorption sites affinity on the metal ion nanocomposite materials. This is due to the increase in the obstacles to the diffusion of metal ions caused by the positive repulsive forces of the increasing cation at moderate pH values ​​[50] The adsorbent material is relatively negatively charged more than the ionic solution, and this contributes to an increase in Affinity for metal ions at pH values ​​5 [51], a higher pH (pH > 7) was not used in the current study to avoid removal of metal ions by precipitation as shown in Fig. 7b.

Effect of the Temperature
The degree of adsorption depends on the temperature of the liquid-solid interface. Adsorption studies were performed at different temperatures 300, 303, 310, 318, 325 and 333. The percentage of metal ion removal decreases with increasing temperature. It was found that a lower temperature is possible for the adsorption process. Thus, the process was concluded to be exothermic and automatic. The decrease in the adsorption capacity of nanocomposite materials with increasing temperature is due to the weak bonds carrying metal ions on the adsorbent surfaces of the nanocomposite. The increase in temperature also degrades the regular structure of the nanocomposites materials, which leads to the loss of adsorption sites, thus reducing the removal efficiency of Co (II), and Ni (II) ions. The decrease in the adsorption efficiency with increasing temperature indicates weak bonding interactions between the adsorbent surfaces and the cations; this is an indication that the process can be exothermic and thus leads to the conclusion that the adsorption process was appropriate at lower temperatures as shown in Fig. 7c.

Adsorption Isotherms
Equilibrium uptake of Co+2 and Ni+2 was investigated with sorption 0.025 gm of (Fe3O4-PU) in contact with 50 ml of solution at pH= 5,7 for Co+2, Ni+2 and time contact of 120, 60 for Co+2, Ni+2 at 25Cᵒ temperature. The graphic between Ce versus Ce/ Qe for the compatibility is greater for the Lankemuir model of the Cobalt ion and Nickel ion, which is in agreement with the Lankemuir model (Fig. 7a and 7c), graphic between Ln Ce versus Ln qe (Fig. 7b and 8b) for the Freundlich model. Freundlich equation is used to describe the adsorption characteristics of heterogeneous surfaces, as it considers the presence of effective sites and a heterogeneous adsorption surface and different energies and adsorption in several layers.
RL is indicative of the isotherm shape and predicts whether a sorption system is either favorable (0<R2<1), unfavorable (R2>1) or irreversible (R2=0). Figs. 8a, b and Fig. 9a, 8b give the plots of the Langmuir and Freundlich isotherms of Co+2, Ni+2 ions adsorbed onto (Fe3O4-PU) respectively, and Table 1. displays qmax, KL, KF, and the correlation coefficient R2 results for the Langmuir and Freundlich isotherms for the adsorption of surface (Fe3O4-PU) by the Prepared adsorbents, The results showed that the adsorption follows the Lankemere and Freundlich equations. It was noticed that the Co (II) follow the Lankemuir isotherms, with great value R in the case of Lankemuir isotherm, which indicates that it is chemical adsorption. The Nickel ion Ni (II) also follow the Lankemuir isotherms, with great value R in the case of Langmuir isotherm, which indicates that it is chemical adsorption. Maximum adsorption of Co (II)> Ni (II).

CONCLUSION 
The adsorption of heavy metals (Co+2, Ni+2) by (Fe3O4-PU) surface has been determined. This study indicates that (Fe3O4-PU) have a good capacity for the adsorption of heavy metal ions. The percentage removal depended on pH, Adsorbent concentration and adsorbent dosage. The maximum adsorption capacity was found at pH=5 for Co+2, pH=7 for Ni+2, adsorbent dosage 0.025 gm and initial concentration 150 mg/L for Co+2, initial concentration 150 mg/L for Ni+2, The best contact time within 120, 60min for Co+2, Ni+2. The adsorption process supplied Freundlich and Langmuir, with isotherm models. This material can be successfully used as an environment-friendly product for the removal of heavy metals from aqueous solutions.  

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.