Research on Copper Ions Removal from Wastewater Using Fe3O4 and Fe3O4- PVP Hybrid Nanomaterials

Wastewater containing Cu (II) ions has become a huge environmental problem worldwide because ingestion of copper ions affects the human body leading to epigastric pain, nausea, vomiting, diarrhea and even death. The researches for new methods of treatment of wastewater polluted with heavy metals more efficient than the conventional ones led us to study Fe3O4 and his corresponding hybrid Fe3O4 -PVP as potential adsorbent nanomaterials for the removal of copper ions from industrial wastewater. First, the two nanomaterials were synthesized by precipitation method and characterized by XRD, FTIR and TEM analyses. Then, the two obtained nanomaterials were tested within the process of copper ions removal from wastewater and factors influencing the process including pH and contact time were investigated. The maximum wastewater treatment capacity reached up to 97 % under the optimal condition using Fe3O4 and 70% for Fe3O4 PVP.


Introduction
It is known that without water no human or animal can live on earth, air and soil having the same importance like water, these being abundant resources for living beings. If these resources are polluted, the human health is in danger. Protection of water, air and soil is the most important concern in the whole world, especially in developed countries. Depollution is a challenge when we take into consideration the legislation, efficiency, recyclability, environmental impact and cost. The most common pollutants found in water are mainly inorganic pollutants like heavy metals, organic compounds such as pesticide, detergents or pharmaceuticals. For example, some of the heavy metal ions found in water are Pb 2+ , Hg 2+ , Cr 6+ , Cd 2+ , As 3+ , As 5+ , Co 2+ , Cu 2+ , Ni 2+ . The pollution of water with heavy metals which are nonbiodegradable in our bodies can lead to huge health problems, such as diarrhea, vomiting, stomach cramp, gastrointestinal distress, coma, jaundice and nausea [1,22].
The conventional methods ( Figure 1) for the removal or decreasing of heavy metal ions from water are: adsorption [2,3], chemical precipitation [4], membrane filtration [5], ion exchange [6], coagulation and flocculation [7], reverse osmosis [8], irradiation [9], flotation and extraction [10], advanced oxidation processes, electrochemical treatment techniques [11] and biosorption processes [12]. Adsorption is the most utilized technique for removing heavy metals from wastewater. Also activated carbons [13], clay minerals [14] or natural zeolites [15] are often used to remove heavy metals from wastewater. They have disadvantages, such as low adsorption over time, unequal pore structure or regeneration issues [16]. The conventional adsorbents already used in the depollution process are hard to remove and those can affect seriously our health [17].
Chemical precipitation used in wastewater depollution is applied for removing ionic compounds from wastewater adding counter-ions to decrease the solubility. It is used for the metallic cations removal, but also for anions and organic molecules elimination form wastewater [4].
The adsorption is the method that makes the pollutant to pass from the industrial wastewater to the adsorbent material surface and links to it through the chemical and physical interactions [24]. Ion exchange method using membranes can enable the passage of counter-ions (oppositely charged ions), while block co-ions (similarly charged ions) [30].
Filtration is a method that refers to solid-liquid separation, being a common method in wastewater depollution. The purpose of solid-liquid separation is to growth the solids concentration by using a driving strength for consolidation of the suspended solid phase [31].
Coagulation-flocculation is a common method in wastewater depollution for precipitating dissolved impurities and destabilizing colloids to make large aggregates, which can be eliminated from wastewater in further processes. The most used coagulants are iron(III) and aluminum(III) compounds [7].
Bio-sorption is a process that use biomass such as yeast, algae, bacteria, fungi for wastewater depollution, being a low cost method and removing heavy metal ions [32].
As regarding copper ions, once it reaches our bodies they are transferred in the intestine and then carried through blood to the liver and it links to albumin [23]. The catalytic role of copper can lead to osteoporosis, bone abnormalities, growth disorders, anemia, neutropenia, hypopigmentation or immunodeficiency. The toxic quantity of copper salt for animals is 30 mg/kg. Copper poisoning is not very often for humans because of the transport system in our bodies that speeds elimination of excess copper. These systems can be disabled by the proteins that carry the copper so it will lead to cirrhosis. [24].
Copper containing proteins have another purpose in our bodies for the carriage of electrons and oxygen, for procedures based on the fast transition from Cu + to Cu 2+ . Oxygen is carried by the protein hemocyanin. The level of copper in our bodies is between 1.4 and 2.1 mg/kg of body weight. For the normal behavior of the body, the minimum daily dose of copper is 0.97 mg/day [23].
Because nanomaterials could have a great potential for retain heavy metals from wastewater, are currently studied. Because of their small size, nanomaterials have a high surface area, therefor improving adsorption capacities for the removal of heavy metals [25].
The advantages of using nanosized magnetic particles are that they could have high capacity of adsorption of cooper ions from wastewater compared with other conventional copper adsorbents, have high surface area, reduced cost and can be easily separate from wastewater using magnetic field at the end of the wastewater treatment process [18]. These nanosized magnetic particles have attracted a lot of consideration in the removal process of heavy metals from polluted water [19], such as As 3+ , Pb 2+ , Hg 2+ , Cr 6+ , Cd 2+ which are recognized to cause great damage to living beings and to the environment [17,20].
Fe3O4 nanoparticles have been used for the elimination of some pollutants from wastewater. In the recent years, magnetite (Fe3O4) has been studied in the environmental depollution because of their strong adsorption capacities, magnetic properties and easily separation and collection [21,[26][27][28][29].
Some researchers discovered that the copper adsorption from a copper-ammonia water solution depends on pH, copper removal increases with increasing pH (from pH 2 to pH 9). At pH 8 the removal percentage was 100%. Also, the efficiency of NH3-N removal was about 26% at pH 8. The adsorbent dosage also had an important role. The study consisted in testing of differ&ent magnetic ferrite dosages (0.2, 0.4, 0.6, 0.8 and 1g) into 200mL of Cu(II) ions wastewater [33]. Other researchers observed that initial solution pH influence the Cu(II) ion adsorption on a Fe3O4 composites. They investigated pH range between 3.0 -11.0 and discovered that once increasing solution pH from 3.0 to 7.0 the removal rate increase as well [34].
Poly(vinylpyrrolidone) has various applications because of its unique properties such as biological compatibility, environment friendly, complexing ability and film forming ability. 0.2 6.5 MCGONmagnetic chitosan/graphene oxide-based nanoparticles CNTcarbon nanotubes SDS-Fe3O4 NPssodium dodecyl sulphate-coated Fe3O4 nanoparticles M -Fe3O4 MIA -Fe3O4 nanoparticles incorporated with hydroxyapatite nanorods PFMmagnetic composite of polyacrylamide-modified Fe3O4/MnO2 GFMNPECABsglycine functionalized magnetic nanoparticle entrapped calcium alginate beads FGNsfunctionalized graphene nanosheets Gly@MNPglycine-functionalized maghemite nanoparticles The limit of copper loading of industrial and municipal wastewater discharged into natural receptors is 0.1 mg/dm 3 according to NTPA-001/2005 and the limit value of copper in wastewater discharged into the sewerage of localities is 0.2 mg/dm 3 according to NTPA 002/2005.
The purpose of this study is to use Fe3O4 and his corresponding hybrid Fe3O4 -PVP as adsorbent nanomaterials for copper ions removal from wastewater. Some influential parameters for the wastewater treatment process of copper ions such as contact time, nanomaterials dose and pH value were investigated.

Materials and methods
Ferric chloride (FeCl3 . 6H2O), copper chloride (FeCl2 . 6H2O), sodium alginate (ANa) and polyvinylpyrrolidone (PVP), sodium hydroxide (NaOH) were supplied from Sigma Aldrich and were used as received. The preparation of magnetite nanomaterial tested in the wastewater process was done by precipitation method. The precipitation was accomplished using 0.7 mol L -1 aqueous solution of sodium hydroxide and the reaction pH was 12. The molar ratio Fe 2+ : Fe 3+ : ANa: PVP unit was 1:2:5:5. The precipitate was separated by centrifugation and washed several times with water. Finally, Fe3O4 powder was obtained by precursor calcination at 400 o C for 2 h.
The hybrid (Fe3O4-PVP) was obtained by dispersing an amount of Fe3O4 powder in 40% solution of PVP. The Fe3O4-PVP powder was separated by centrifugation and dried at room temperature.
The X-ray diffraction (XRD) analysis was obtained using a X'PERT PRO MPD with Cu-Kα radiation (λ = 0.15418 nm), the transmission electron microscopy (TEM) investigation was done on FEI Tecnai TM G 2 F30 S-TWIN with EDAX energy dispersive X-ray spectrometer and the results of the wastewater treatment process were determined with the PhotoLab S12 photometer.
Adsorption experiments were done using 100 mL of the synthetic wastewater having different concentrations of copper ions, at different pH values and using the two magnetic nanomaterials (Fe3O4 and a Fe3O4-PVP). For experiments were used three concentrations of Cu 2+ ions in water of 0.7, 1 and 1.2 mg/L concentration and 0.2g of adsorbent as in Figure 1. The pH was settled at 4.8 and 11.5.
The system containing copper wastewater solutions and nanomaterials used as adsorbent were homogenized using a ultrasonic bath. All the experiments were done at room temperature. All concentrations were measured at PhotoLab S12. Yields were calculated with the formula (1).
The experimental results were determined in accordance with ISO 8466-1 and DIN 38402 A51 (10mm cell) with a measuring range of 0.10-6.00 mg/L Cu 2+ .

X-ray diffraction
The nanopowder and corresponding hybrid obtained from synthesis were analyzed by XRD. The XRD pattern of oxide powder has proved the formation of single phase compound with inverse spinel structure and cubic symmetry for Fe3O4 (Figure 2a). The average crystallite size was 14 nm.
The XRD pattern of the hybrid material sample showed that the crystalline structure of the oxide nanoparticles is preserved after the coating procedure (Figure 2b).

FT-IR spectroscopy
The FT-IR spectrum of the Fe3O4 -PVP hybrid and PVP polymer are displayed in Figure 3. By comparing the FT-IR spectra of Fe3O4 -PVP ( Figure 3a) and PVP polymer (Figure 3b) were found the following: a) the peaks found at 2958 cm -1 and 2945 cm -1 are due to the asymmetric stretching vibration of CH2 group of PVP from the pyrrolidone group; b) the symmetric CH2 stretching vibration is placed at 2894 cm -1 and 2885 cm −1 ; c) the peaks placed at 1659 cm -1 and 1652 cm -1 correspond to stretching vibration of C=O in the pyrrolidone group; d) the bands placed at 1429 cm -1 in both spectra correspond to the CH deformation modes from the CH2 group [43,44]. In Figure 3a the band at around 1288 cm −1 is owing to C-N bending vibration from the pyrrolidone structure of Fe3O4-PVP while in PVP spectrum is placed at 1285 cm -1 [43].
The FTIR spectrum of Fe3O4-PVP show two main iron-oxygen bands, the highest vibration one, in 600-550 cm -1 range, which is assigned to intrinsic stretching vibrations of the iron from the tetrahedral sites and the lowest vibration band placed in 450-400 cm -1 range corresponding to stretching vibrations of the iron from the octahedral sites (Figure 3a) [45].
As suggested by previously published data a shift of the C=O group in hybrid spectrum in comparison with that of PVP spectrum can be attributed to the change of p-п conjugation associated with the amide group arising from the dissociation of PVP chains due to the interaction with the metals ions from the ferrite nanomaterial (Figure 3a) [43,45].

TEM analysis
The morphology of the obtained hybrid material was investigated by TEM. The Fe3O4-PVP hybrid material exhibit a narrow size distribution, the mean particles size being 12 nm, respectively (Figure 4).

Copper ions removal using Fe3O4 and Fe3O4 -PVP nanomaterials
The results obtained for testing magnetite material for copper ions removal from the synthetic wastewater are presented in Table 2 and 3. The highest treatment efficiency was obtained at pH of 8 and 11.5 of wastewater for all three concentrations of copper ions. The pH value of wastewater is very important for pollutant-adsorbent system. We made tests on different pH values (4, 8 and 11.5). We observed that at the pH 4, the pollutant is adsorbed, then desorbed (Table 2) and the yield does not increase more than 35% (Table 3). For the other two cases (pH 8 and 11.5) the yield of adsorption got up to 97.14% in less than 24h ( Figure 5, 6).  The removal of copper ions present in wastewater in different concentrations (0.70; 1.00 and 1.20 mg/L respectively) is represented in Figures 5 and 6. Using Fe3O4 as an adsorbent nanomaterial for the removal of copper ions from wastewater with pH 8 ( Figure 5) it was observed that the treatment efficiencies reached 97.14; 97.00 and 96.67% respectively in case of the three studied Cu 2+ concentrations removal. In case of wastewater with pH 11.5 (Figure 6), the same values of treatment efficiencies were obtained as in the case of pH 8 in order to remove 0.70; 1.00 and 1.20 mg/L Cu 2+ , respectively, except that, in the case of the initial concentration of copper ions 0.70 mg/L, it is observed that the process was faster at pH 11.5.
The pH value also had an important role for the copper ions removal using Fe3O4-PVP hybrid and it is observed in Table 4. The acidic environment (pH 4) has a negative impact on the adsorption process. The yield is only 1.45% after 1200 min. When the pH values were 8 and 11.5, we noticed changes of the wastewater treatment yield to 56.92% for pH 8 (Figure 7) and 70.00% for pH 11.5 after 48 h ( Figure  8). where: qe represent the quantity of metal adsorbed/gram of adsorbent at equilibrium (mg/g); Q 0 represent maximum monolayer coverage capacity (mg/g); KL represent the Langmuir isotherm constant (L/mg); C0 represent initial concentration The adsorption process was also characterized by Freundlich isotherm model. Freundlich adsorption isotherm is presented as Eq. (4) qe represent the quantity of metal adsorbed/gram of adsorbent at equilibrium (mg/g); KF represent the Freundlich isotherm constant (mg/g); Ce represent the equilibrium concentration of adsorbate (mg/L); n represent the adsorption intensity; The scientific literature describe the characteristics of adsorption being favorable if RL is greater than zero but less than one [46,47]. Table 5 shows the adsorption isotherms constants for Fe3O4 and Fe3O4 -PVP used as adsorbents. Constants were calculated using the plots below (Figures 9-14).   Two widely used models, represented by Langmuir and Freundlich, were used to model the adsorption at equilibrium. Adsorption isotherms were chosen to show whether the adsorption process takes place in a single layer or in a multilayer. The most appropriate model was chosen based on the correlation coefficient (R 2 ) determined from the equations of the linearized forms of the two models made after using the two adsorbent materials in the treatment process, Fe3O4 and Fe3O4 -PVP hybrid (Figures 9, 10).
In Table 5 it is observed that the RL values are between 0 and 1 both in the case of using the Fe3O4 nanomaterial and in the case of the hybrid material Fe3O4 -PVP, this means that the adsorption went favorably in the studied experimental conditions. The Langmuir isotherms for the two materials used are shown in Figures 11 and 12. From these plots it is observed that the correlation coefficients R 2 are 0.9974 and 0.9291, respectively. Comparing with the values of the correlation coefficients R 2 resulting from the plots presented in Figures 13 and 14 which represent the Freundlich isotherms for each material (R 2 : 0.9938 respectively 0.9125 for the use of Fe3O4 and Fe3O4 -PVP as adsorbent materials) we can see that the R 2 values in the case of Langmuir isotherms are higher than in the case of Freundlich isotherms, which means that the adsorption process of copper ions from wastewater adapts better to the Langmuir model. This model considers that the surface of the adsorbent is homogeneous and the adsorption is monolayer.

Conclusions
The need to find more effective ways of water depollution compare with conventional methods justifies the intense researches that are made in this domain.
In this regard, the study that was made was based on the use of Fe3O4 and Fe3O4 -PVP nanomaterials in order to remove copper ions from wastewater.
Magnetite nanomaterial tested in this research demonstrated high efficiency on removing copper ions from wastewater at pH 8 and 11.5. The wastewater treatment yields obtained at 8 and 11.5 pH values using Fe3O4 as adsorbent were 97% and the maximum percent at pH 4 was 35.71%, the nanomaterial being instable in wastewater. In case of Fe3O4-PVP adsorption at pH 4 the obtained wastewater treatment yield was of only 1.45%, reaching at higher values of 70% at 11.5 pH of wastewater.