Tunning the Physical Properties of PVDF/PVC/ Zinc Ferrite Nanocomposites Films for More E�cient Adsorption of Cd (II)

This work studies the enhancement of the physical properties PVDF/PVC blend by adding ZnFe 2 O 4 with different weight percent (from 0% up to 10%) as a nanofiller. The effect of ZnFe 2 O 4 on behavior of PVDF/PVC was studied through XRD, FTIR, FESEM and UV-Visible spectroscopy. The PVDF/PVC/ 10% ZnFe 2 O 4 nanocomposite film was represented the optimum sample. As it shows maximum crystallinity, roughness average (414 nm) and root mean square roughness (113nm) on another hand this sample has also the lowest value of energy band gap for direct and indirect transition. The removal efficiency of Cd(II) by using PVDF/PVC/ 10% ZnFe 2 O 4 reached to about 50 % at pH 6 after 60min. the absorption mechanism as well as kinetics isotherm have been studied.


Introduction
The destruction of water supplies seems to be one of the most urgent environmental issues in the world.Water resources pollution is caused by heavy metals incorporation, such as, cadmium, mercury, lead and chromium [1].In last decades, whole world becomes very concerned with pollution of water by heavy metals.Because of its highly toxicity and nondegradability dissimilar organic pollutants as they are biodegradable [2].One of the most harmful heavy metal ions is Cd(II), which causes kidney failure and bone brittleness.In 1968, it was considered as the first disease occurred by pollution of the environment [3].Recently, Numerous techniques were used for treatment and reuse of contaminated water by heavy metal ions involving, ion exchange [4], reverse osmosis [5], membrane filtration [6], chemical precipitation [7] and adsorption [8].Among all previous techniques, Adsorption is the most desirable technique due to its low cost, reproducibility, and effectiveness [9].Adsorption of heavy metals are related to Adsorbent as well as adsorbate.For adsorbent, the uptake of heavy metals is affected by its concentration, Ph, and contact time.Also, the adsorbate plays an important role in heavy metal removal.The removal of heavy metals can be done by the lattices which existed in spinel ferrite's structure [10].
ZnFe2O4 is attracted attention due to its magnetic behavior, chemically stability and larger surface area [11].The chemical stability and removal efficiency of ZnFe2O4 can be enhanced by adding different types of polymers [12].As, the polymer have chemical stability, good processability and flexibility [13].Though, there are various types of polymers, poly (vinylidene fluoride) is considered one of the most desirable polymers because of it has high flexibility, electroactive nature, and easily to prepare in films form with different size and shape [14].While PVC (poly vinyl chloride) is used because of its good mechanical, commercially available and easy processibility [15].
Herein, ZnFe2O4 nanosample was synthesized by flash method and combined with different concentrations to PVDF and PVC to form PVDF/PVC/ x ZnFe2O4 (x= 3, 7, 10 wt.%) nanocomposites films to be used for heavy metal Cd(II) removal from wastewater.The batch experiment was studied under various parameters such as pH, contact time and weight ratio of adsorbate, to determine the optimum conditions for maximum removal of Cd(II).

Preparation of Zn-ferrite
Nanosample ZnFe2O4 was synthesized by using flash method.Proper amounts (0.1M) of iron and Zinc nitrates mixed well in powder phase with (0.7) urea.After that, the temperature of mixture was raised to 250 o C till all fumes were ended.The obtained powder was calcinated at 800 o C for 2hrs.

Preparation of PVDF/PVC/ Zinc ferrite nanocomposites films
PVDF and PVC powders were dried at 60 °C in a vacuum oven for around 2 hours to remove any moisture content.Equal quantity of PVDF and PVC was dissolved in Tetrahydrofuran solution (THF) separately until complete solubility, and then polymers added to each other and stirred continuously until homogenous solution was formed.Zinc ferrite NPs was added to blend solution with weight percentage ( 3, 7, 10 wt.%).The nanocomposite solution was sonicated using dip sonicator to prevent aggolomeration of nanoparticles and the solution was transferred into Petri dish and left in drier at 40 °C around 6 hours.

Measurement Techniques
XRD analysis of the produced samples was proceeded via PANalyticalX'Pert Pro target Cu-Kα with secondary monochromator Holland radiation with tube running at 45 kV and wavelength= 0.1540 nm.High-resolution transmission electron microscope (HRTEM) was applied by JEM-2100F electron microscope with 200 kV accelerating voltage.ATR-FTIR spectral was used in the range 4000-400 cm -1 via spectrometer VERTEX 80 (Bruker Corporation, Germany).UV-Vis absorption spectra were performed from 200-1000 nm wavelength using Jasco UV-Vis (V-630) made in Japan.FESEM was tested using Quanta 250 FEG worked at 20-30 kV.3D micrographs as well as roughness parameters were obtained from FESEM images for each sample by using Gwyddion soft ware.

Batch experiment of heavy metal (Cd(II)) removal
The experiment was carried out in 250 mL flasks with (0.1g) of produced films in 2ppm of metals nitrate to determine the appropriate pH values for heavy metal (Cd 2+ ) removal.The pH of the solution was changed from 2 to 8. The investigated solutions were thoroughly mixed for 1 hour at room temperature using an electric shaker (ORBITAL SHAKER SO1) at 200 rpm.After that, the solutions were collected using a 0.2m syringe filter.At 25C, atomic absorption spectroscopy (Zeenite 700P, Analytical Jena) was used to determine the heavy metal concentration.Every experiment was repeated three times, with the average results reported.
The optimum contact time was determined by repeating the previous process while keeping the pH at its optimal level and measuring atomic absorption after various contact times (1 to 24h).The film removal efficiency is computed using the following formula [16]. where: C 0 : heavy metal soln at initial concentration (ppm) C f : heavy metal soln at final concentration (ppm)  511) and (440) lattice plane, respectively [20].

Structural study
The Debye-Scherrer equation was used to calculate the average crystallite size (S) of ZnFe2O4 across the (311) crystal plane [21].
Where the X-ray wavelength is λ (0.154 nm), β is the full-width at half-maximum and  is the diffraction angle.It is seen that ZnFe2O4 has average crystal size equal 30 nm.
For PVDF/PVC film displays abroad and weak diffraction peak, relating to the amorphous feature of polymers.By increasing weight ratio of ZnFe2O4 from 3% to 10%, the crystallinity gets better and diffractions peaks become clearer comparing to 0% weight ratio of ZnFe2O4.So, the addition of ZnFe2O4 NPs at different weight percentage has a significant role in crystallinity enhancement of polymers.and 10% films.PVDF has characteristic band at 1400 cm -1 in the FTIR spectra of pure PVDF (Figure 2a) belongs to the CH2 wagging mode associated PVDF chain [22].The asymmetric and symmetric stretching vibrations of CF2 were ascribed to the bands that occurred at 1179 cm -1 and 1061 cm -1 , respectively.The out of plane C-H bending is shown by the band at 973 cm -1 [23].
Asymmetrical stretching vibrations of C-C-C and C-F stretching vibrations were attributed to the bands at 870 cm -1 and 842 cm -1 , respectively [24].CF2 bending vibration was ascribed to the bands that emerged at 614 cm -1 and 489 cm -1 [25].For PVC, the asymmetric stretching vibration of the C-H band is seen at 2913 cm -1 .At 1422 cm -1 , the stretching vibration of the C-H band can be noticed.CH2 deformation and the rocking mode of the C-H bond near Cl are attributed to bands at 1334 cm -1 and 1245 cm -1 , respectively.At 1098 cm -1 , the C-C stretching vibration is detected.Trans -CH and cis -CH wagging modes may be seen at 958 cm -1 and 614 cm -1 , respectively.C-Cl has a stretching vibration of 827 cm -1 [27][28][29].
For PVDF/PVC blend spectra, it combined the characteristic feature of both polymers with changing of intensity of them which affirmed that there is reaction between PVDF and PVC.For PVDF/PVC films doped with different weight ratios of ZnFe2O4 NPs (3%, 7% and 10%).There are new bands 554 cm -1 and 421 cm -1 which are corresponded to Zn-O bonds intrinsic stretching vibration of tetrahedral sites and Fe-O bonds in octahedral sites, respectively [30].Also there is new band appeared with increasing ZnFe2O4 NPs ratio at 603 cm -1 and the intensities of bands decreased by increasing ZnFe2O4 content.The IR intensity variation can be indication on interaction between parts in prepared films.parameters increased with increasing filler content.For example, roughness average has been increased from 319 nm to 414 nm while maximum height increased from 260 nm to 473 nm for PVDF/PVC and PVDF/PVC/10% ZnFe2O4, respectively.Physical adhesion for prepared films is strongly affected with these parameters (roughness average and height).As, roughness height act as hooks and facilitate the connection between films and surrounding [31].The change in roughness parameter may be related to filler in polymeric matrices.Consequently, we can control the roughness parameters through additives in the blend according to the desired applications.

Optical Properties
The UV-visible spectrum is an essential measurment to calculate energy gap of amorphous and crystalline materials.The basic absorption, is related to the excitation of electron from the top of valance band to the bottom of conduction band, it also can be used for determining the value and nature of the band gap.The absorption edge defines the excitation of an electron by an incident photon from a lower energy to a higher energy state, which may be calculated by extrapolating the linear part of the absorption coefficient () with photon energy (h).The absorption coefficient is calculated using the following equation, where A and d are the absorbance and thickness of the prepared sample [34].
As observed in Figure (7), when compared to the PVDF/PVC blend sample, the values of the absorption edge are reduced as seen in Table 2.This is due to the formation of localized states within the band gap, as well as the changing number of final states in the system.

Figure (7): Absorption coefficient-photon energy relation of the prepared samples.
The optical absorption spectrum is a useful tool for calculating the optical band-gap energy of any material.The optical band gap's nature and value are checked using critical absorption, which corresponds to the electron's excitation from the valence band to the conduction band.The strong-absorption area can be studied using Tauc's equation, which is provided below [35].

Batch Experiment
The Cd (II) up taking behavior by using PVDF/PVC/x% ZnFe2O4 nanocomposite films was studied at various pH and contact time were used as experimental factors to find the best conditions for heavy metal removal.Cd 2+ over the adsorbent's active sites [36].By increasing value up to 6, consequently, the number of H + decreasing and for Cd (II) adsorption, additional active sites become supplied.[37] the removal efficiency % reached to about 30% for PVDF/PVC/10% ZnFe2O4.OH-ions are present in solution at a basic pH (i.e.8) causing the formation of Cd (OH)2 [38].So, the up taking of Cd(II) ions were due to adsorption and precipitation.The maximum adsorption was observed for PVDF/PVC/10% ZnFe2O4as between all prepared samples this one has maximum roughness as well as maximum roughness height as mentioned above.Finally, we may infer that pH 6 is the best choice for removal of Cd(II).The contact time is another factor that has a significant impact on heavy metal ion adsorption.It is clear, As contact time is increased, the adsorption of Cd(II) ion on surface of prepared samples also is increased.At the beginning, there were many active sites available as a result the adsorption increased by increasing contact time till the equilibrium state is reached.
PVDF/PVC/10% ZnFe2O4 has the highest removal efficiency, reaching nearly 50% after 60 minutes, which could be due to the amount of active sites on its surface [39].Finally, employing a PVDF/PVC/10 % ZnFe2O4 nanocomposite film, the best conditions for Cd(II) adsorption are pH 6 for 60 minutes.The only way for knowing the adsorption mechanisms which were occurred in adsorption process is done through studying adsorption isotherm and kinetics [40].Langmuir and Freundlish isotherms are the two isotherms that are used to describe adsorption.Equations ( 6) and ( 7) were used to represent the Langmuir and Freundlish isotherms, respectively.
ln   = ln   + 1  ln   (7) where qe and qm (mg g -1 ) denote the equilibrium and maximum adsorption capacities, respectively, and Kl (L mg -1 ) denotes the affinity binding constant, and Kf and n (physical constants) denote the adsorption capacity and intensity of adsorption, respectively.
The Langmuir isotherm describes monolayer adsorption at the sites which are equivalent and homogeneous with similar adsorption energies, while the Freundlish isotherm describes heterogeneous surfaces [41].We discovered that all samples matched the Langumir model   Heavy metal adsorption kinetics were primarily explored using pseudo first order or pseudo second order kinetics.The physisorption is explained by pseudo first order kinetics.
Physisorption is a weak form of adhesion that relies on vanderwaals forces rather than chemical bonds.As a result, this type of adsorption can be reversed.However, pseudo-second order kinetics is associated with chemisorption adsorption, which occurs as a result of two processes.It is the first reaction, and it soon extends equilibrium.The second, on the other hand, leaks slowly and takes a long time to attain equilibrium [39].By employing electron sharing, bonds are formed between adsorbents and adsorbates in chemisorption.As a result, it is more difficult than physisorption.The intra particle diffusion kinetic model is also one of kinetic models.This model was studying by Weber and Morris model is studying.They discovered that the intra particle diffusion model is the single rate-determining stage, and adsorbate removal is carried out in a quick process [42].
Three models were used to investigate the kinetics of the adsorption mechanism: Pseudo first-order model: Pseudo second-order model: Inter particle diffusion model Where k1, k2 and k3 are the pseudo first, second order and inter particle diffusion rate constants in (min -1 ) and (g mg -1 min -1 ), respectively.
Further, the majority of heavy metal ions adsorption on the surface of produced samples nanocomposites films fit well with the pseudo-second order kinetic model, refereeing to the

Conclusion
ZnFe2O4 nano filler with different weight percentage were synthesized successfully by using flash method.These nanofillers were added to PVDF/PVC blend to form nanocomposite films of formula PVDF/PVC/x% ZnFe2O4 ; x= 0, 3, 7 and 10.The structure of obtained samples was studied through XRD and FTIR, and confirmed the changing in structure and complexation between blend sample and ZnFe2O4 NPs.The morphology and surface of samples were observed by using FESEM, it is observed that by increasing the weight percentage the roughness and porosity have been increased.The absorption edge and band gap energy are decreased compared to blend sample.In addition to, the nanofiller enhanced the removal efficiency for Cd (II) from the wastewater which was reached to 50% at pH 6 for 60 min by using PVDF/PVC/ 10% ZnFe2O4.The adsorption process was occurred through Langmuir mechanism and fellow pseudo-second order isotherm.Finally, we concluded that the sample with composition PVDF/PVC/ 10% ZnFe2O4 has the optimum physical properties and highest removal efficiency for Cd (II).

Figure ( 2 )Figure ( 3 )
Figure (2) represent HRTEM of ZnFe2O4 NPs, it has irregular shapes between semi spherical and deformed cubic with average particle size ranged from 20-55 nm

Figure ( 4 )
Figure (4) were representative of FESEM images of prepared films.The image of surface showed that ZnFe2O4 nanofiller were spread homogeneously in PVDF/PVC matrices which has utilities in different applications.By increasing weigh% content of ZnFe2O4 nano powder, the surfaces of the blend seem to be more roughness.Pure PVDF/PVC image shows smooth surface with high porosity.

Figure ( 5 )
Figure (5) displays the surface roughness of PVDF/PVC films which are doping with different weight percentage of ZnFe2O4 nano powder.As it obtained from Table1 all roughness

Figure ( 6 )
which shows UV-Visible absorbance spectra of PVDF/PVC/ xZnFe2O4; x= 0%, 3%, 7% and 10% nanocomposite films.For PVDF/PVC blend, there are twp peaks at 205 nm and 279 nm which corresponded to n→* and →* transition which corresponded to chromophoric group of PVDF and PVC polymer blend[32,33].As seen in for PVDF/PVC doped with different weight percentage of ZnFe2O4 NPs, the peaks at 205 nm shifts (a) (b )toward higher wavelength (red shift).This means that there are complexation between PVDF/PVC and ZnFe2O4 NPs.

Figure ( 8 ).
photon energy axis is depicted in Figure(8).Fitting the linear part of the curve and determining the intersection of the straight line with the h axis yields the energy gap.As seen in Table2, comparing to PVDF/PVC, the direct and indirect band gap energy values of PVDF/PVC/ xZnFe2O4 decrease.This result revealed that the structure of blend sample had changed, resulting in the formation of a localized state.

Figure ( 9 )
displays the pH solution effect on adsorption performance.It is shown that all prepared films have the same trend which is increasing pH is accompanied by the increasing value of the adsorption of Cd (II) ion.At low pH, adsorption of Cd (II) has low values(from 3 to 10) %maybe it is due to the large amount of H + which compete

behavior by fitting Figure ( 11 ).
On the other hand, fitting Figure(12) revealed that none of the samples followed the Freundlish isotherm.As a result, Cd (II) adsorption on the produced sample's surface occurred via monolayer adsorption.

Figure ( 14 )
Figure (14) compared to that for pseudo-first order Figure (13) and inter particle diffusion model Figure (15).So, the adsorption of Cd(II) by using PVDF/PVC/ x% ZnFe2O4; x=0, 3, 7 and 10 nanocomposites films was done through Langmuir isotherm and the adsorption mechanism kinetics is pseudo second-order model.