Harnessing of Newly Tailored Poly (Acrylonitrile)-Starch Nanoparticle Graft Copolymer for Copper Ion Removal via Oximation Reaction

: Our recently tailored and fully characterized poly (AN)-starch nanoparticle graft copolymer having 60.1 G.Y. % was used as a starting substrate for copper ions removal from waste water effluent after chemical modification with hydroxyl amine via oximation reaction. This was done to change the abundant nitrile groups in the above copolymer into amidoxime one and the resultant poly (amidoxime) resin was used as adsorbent for copper ions. The resin was characterized qualitatively via rapid vanadium ion test and instrumentally by FT-IR spectra and SEM morphological analysis to confirm the presence of amidoxime groups. The adsorption capacity of the resin was done using the batch technique, whereas the residual copper ions content in the filtrate before and after adsorption was measured using atomic adsorption spectrometry. It was found that the maximum adsorption capacity of poly (amidoxime) resin was 115.2 mg/g at pH 7, 400ppm copper ions concentration and 0.25 g adsorbent at room temperature. The adsorption, kinetics and isothermal study of the process is scrutinized using different variables, such as pH, contact time, copper ion concentration and adsorbent dosage. Different kinetics models comprising the pseudo-first-order and pseudo-second-order have been applied to the experimental data to envisage the adsorption kinetics. It was found from kinetic study that pseudo-second-order rate equation was better than pseudo-first-order supporting the formation of chemisorption process. While, in case of isothermal study, the examination of calculated correlation coefficient (R2) values showed that the Langmuir model provide the best fit to experimental data than Freundlich one.


1-Introduction
Worldwide growth and rapid expansion of industrialization mainly for pollutant one likes textile, mining, metal plating, leather tanning and pesticides; results a massive release of the sewages embracing toxic substance especially heavy metal ions into the environment [1] . Heavy metal ions, when existing in higher quantities than acceptable limits, are dangerous for human and aquatic life at which various damage and disorders have been observed due to their toxicity [2].
Examples of such toxic heavy metal ions that cause serious challenge to health and are persistent during treatment of waste water include; mercury, cadmium, zinc, lead, chromium, nickel and copper [3] . Copper as in our case is considered as one of the most vital elements in activities of the human body, however, if it was swallowed in more amount than required can lead to serious health problems such as; tremor, vomiting, cramps and finally may be lead to the death [4] . Due to the toxicity and carcinogenic effect of the above effluents; discharge of toxic waste in the environment should be controlled [1] . So, numerous techniques have been developed for the removal and recovery of metal ions from sewage and industrial wastewater such as precipitation, ion exchange, membrane filtration, coagulation, flocculation, flotation, electrochemical treatment, and adsorption [2,3,[5][6][7][8][9][10][11] . The adsorption is consider as one of the low cost, simple, and effective methods for the removal of heavy metal ions in wastewater with small and high concentrations of contamination without leaving any unwanted residue. Besides, the process is convenient to various adsorbents or bio sorbents which are effective and cheap as compared with other processes [12] . On the other hand, poly acrylonitrile as a reactant synthetic resin is widely used for adsorption of heavy metal ions due to its distinctive structures that embrace hardness, chemical resistance, consistency and permeability with other polar materials [13] . Besides, the presence of a large number of nitrile groups along the polymer backbone structure that could be converted into other functional groups via chemical modification with various reagents such as ethylene diamine, hydrazine, thioamide and imidazoline as well as amidoxime to develop new moieties that are vital for the removal of cationic metal ions in wastewater treatment [14][15][16] . So, a number of articles have been published for describing the synthesis of adsorbent using amidoxime functional group via incorporating the nitrile group into a polymer matrix, followed by the conversion of the nitrile group into amidoxime one by treatment with an alkaline solution of hydroxylamine. In this regards, Egawa et al. [17] prepared a chelating resin encompassing amidoxime by reacting acrylonitrile co-divinyl benzene copolymer beads as a synthetic one with hydroxylamine by suspension polymerization. While, Kobuke et al. [18] synthesized a poly acryloamidoxime resin from various copolymers of acrylonitrile and cross-linking agents. On the other hand, Faraj et al. [19] studied the preparation and characterization of poly amidoxime chelating resin from rubber wood fiber-g-poly acrylonitrile. But, one of the major drawbacks on the adsorption capacity of poly acrylonitrile in an aqueous solution exists due to nonbiodegradability and high cost as well as low yield. To solve this problem as one of main target in this manuscript, developing cheaper, biodegradable, low cost and effective adsorbents based on starch nanoparticles using acrylonitrile as a reactive monomer via graft copolymerization to overcome the latter drawbacks of the synthetic nature of poly acrylonitrile as an adsorbent resin was studied. Recently, our research team studied a green and efficient tool for grafting acrylonitrile onto starch nanoparticles using microwave irradiation [20] . Besides, graft copolymerization of acrylonitrile onto starch nanoparticles using peroxymonopersulphate/ mandelic acid redox pair to increase its utilization was also studied in details for maximizing the graft yield % [21] . Therefore, our research team attempt to explore the adsorption behavior of copper ions from aqueous solutions based on our newly tailored poly (AN)-starch nanoparticles graft copolymer with higher graft yield 60.1 % after oximation reaction (a point that has not been reported in the literature). This was done to change the abundant nitrile groups in the above copolymer into amidoxime one and the resultant insoluble poly (amidoxime) resin was used as adsorbent for copper ions from their solutions. Most of the published works using poly (amidoxime) resin is focused on uranium extraction in sea water [17,18] ; while, there are very few articles published on transition metal uptake by poly (amidoxime) resin [19,[22][23][24] . Therefore, different factors affecting the adsorption, such as pH, treatment time, poly (amidoxime) resin dose and copper ions concentration were deliberated in detail. Besides, various adsorption kinetics like pseudo-first-order and pseudo-second-order rate equation as well as isothermal models have been applied to the experimental data especially with respect to Langmuir and Freundlich isotherm.

Materials:
Poly (AN) -starch nanoparticles graft copolymer as a starting substrate was prepared and fully characterized according to our previous works [21] . Details of the conditions used and their main characteristics are given in Table I. Hydroxylamine hydrochloride (Acros organics, USA) was used for chemical modification of the copolymer. Sodium hydroxide (analytical grade) was supplied by R & M (U.K). Copper sulphate pentahydrate (CuSO4.5H2O) (99.8%, Merck) of analytical grade was used as heavy metal ion contaminant in this work. Ethyl alcohol and hydrochloric acid and other chemicals used were of analytical reagent grade. Table I here.

Graft copolymerization technique:
Unless otherwise designated; the graft polymerization reaction was carried out in 100 ml

Modification of poly (AN)-starch nanoparticles graft copolymer via oximation reaction):
Poly (AN) -starch nanoparticles graft copolymer (0.25 g), ethyl alcohol (25 ml) and hydroxylamine hydrochloride (3 g) were added into a 250 ml three-neck round-bottom flask with a reflux condenser. The mixture was stirred at room temperature for 3.0 hour. Then sodium hydroxide solution 6 ml of (1 M) was added to the reaction mixture to neutralize the hydrochloric acid. The pH of the reaction mixture was attuned to pH 7. The reaction was permitted to continue for 6 h at 70 °C under continuous stirring. At the end of the reaction, the resultant amidoxime-modified poly (AN)-starch nanoparticles graft copolymer was then filtered and washed carefully with 50 ml of ethanol and 100 ml of double distilled water. The chelating copolymer was allowed to dry in a vacuum oven at 50 °C till a constant weight.

Adsorption in batch test:
An aqueous working standard stock solution of copper ions (406 ppm) was prepared by dissolving cupric sulphate (CuSO4. 5H2O, 1.6 g) in 1000 ml double distilled water. The amidoxime -modified poly (AN)-starch nanoparticles graft copolymer (0.1-2.0 g) was then added to the copper solution (100 ml), and the dispersion was stirred for 15 min at room temperature (26 0 C ± 0.5) using our calibrated data logger apparatus on the magnetic starrier to form a complex with the copper ions. The amidoxime -modified poly (AN) -starch nanoparticles graft copolymer-copper ions complex was then removed by filtration and the filtrate was used for the residual metal analysis using Atomic Absorption type S Series (U.S.A).
The amount of adsorbed Cu (+2) at equilibrium, qe (mg/g) was calculated using the following equation: Where Co and Ce (mg/L) are the initial copper concentration and copper concentration at equilibrium, respectively; W (g) is the weight of adsorbent used, and V is the volume of Cu (+2) solution (0.1 L).

Pseudo-first order model:
It can be generally expressed by the linear form as in equation 1 below: Where, qe is the equilibrium adsorption capacity of the adsorbent (mg/g), qt represents the adsorption capacity (mg/g) at time t, while K1 (min −1 ) is the equilibrium rate constant of pseudo first-order adsorption model.

Pseudo-second order model:
It can be generally expressed by equation 2.
Where, qe is the equilibrium adsorption capacity of the adsorbent (mg/g), K2 (g mg −1 min −1 ) is the equilibrium rate constants of pseudo second-order adsorption model.
N. B. The values of linear correlation coefficient regression (R2) are used to predict the most suited isotherm and kinetic model for the adsorption process.

Longmuir isotherm:
It can be expressed as shown below by equation 3: Where, Ce is the adsorption capacity at equilibrium concentration (mg/L), qmax represents the maximum adsorption capacity (mg/g) and KL (L/mg) is the Langmuir's constant isotherm.

Freundlich isotherm:
It can be expressed as shown by equation 4.
Where, qe is the equilibrium adsorption capacity of the adsorbent (mg/g), Kf is the Freundlich constant and used to measure the adsorption capacity, 1/n is the adsorption intensity and Ce is the adsorption capacity at equilibrium concentration (mg/l).

Qualitative vanadium ion test:
About 0.1 g of wet resin was shaken with vanadium (V) ion in dilute hydrochloric acid solution and a purple colored complex on the resin beads was detected.

Fourier Transform Infrared Spectroscopy (FTIR):
Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet 380 spectrophotometer (Thermo Scientific) and the IR spectra were scanned 32 times over the range of wave number 4000-400 cm −1 . The sample (0.002 g) was mixed with KBr to reach (0.2 g) to form a round disk appropriate for measurements.

Scanning Electron Microscopy (SEM):
SEM images for surface morphology of the samples were taken using (Joel GM4200, Quanta 200, Holland). The surfaces of all the samples were coated with a gold thin layer under vacuum before SEM studies at an accelerating voltage of 20 kV.

Statistical analysis and metrological precision:
All of the tests were conducted in triplicate. The data were analyzed and expressed as mean values ± standard deviations as well as error bars. This was done to insure about the high precision of metrological measurements all over the work when using our calibrated instruments in our institute either by primary standard apparatus or certified reference materials used especially for this purpose.

Formation of poly (AN)-starch nanoparticles graft copolymer:
Our previously tailored and fully characterized poly (AN)-starch nanoparticles graft copolymer (60.1 % graft yield) was obtained via grafting of acrylonitrile onto starch nanoparticles using free-radical initiating process.

Suggested adsorption mechanism:
Adsorption of copper ions (Cu 2+ ) on the active functional groups of poly (amidoxime) chelating resin occurred between the amidoxime groups and copper cations, at which a certain possible chelation mechanism for the complexation takes place as shown below in scheme 2. It is well known that, several approaches have been applied for confirmation of the formation of amidoxime groups in the resin. Several metal ions bind with amidoxime to yield a visual color in the resin bead. Consequently, the manifestation of amidoxime groups in the resin was confirmed by rapid vanadium ion test through the formation of a purple color complex [25] . given in the text. It was seen from the figure 5 that, both the residual copper ions content in the filtrate and adsorption capacity were decreased from 300 ppm to 113 ppm and from 106 mg/g to 73.25 mg/g respectively as the amount of amidoxime-modified poly (AN) -starch nanoparticles graft copolymer was increased from 0.1 g to 0.5 g, respectively. This was most probably, due to higher concentration of sorbent leads to the accumulation of sorbent particles that decreased the surface area and availability of the active sites to form ligand or complex with copper ion cations [23] .

Effect of initial copper concentration:
The effect of copper ions concentration was studied by varying the initial concentration of copper ions from 50-400 ppm. Figure 6 shows the effect of varying initial concentration with respect to the residual copper ion content in the filtrate and adsorption capacity on the amidoxime-modified poly (AN) -starch nanoparticles graft copolymer. It reveals from figure 6 that, the residual copper ions content in the filtrate and adsorption capacity of amidoximemodified poly (AN) -starch nanoparticles graft copolymer increased by increasing the initial concentration from 50 ppm to 400 ppm. This implies that the adsorption of copper ions is dependent on the initial concentration of heavy metal ions. For more details, the adsorption capacity increased as the number of possible binding sites increased and this is mainly due to the presence of more available copper ions and electrostatic interactions between them and the adsorbent active sites [23,27] .
Pease insert figure 6 here.

5-Adsorption kinetics:
It is well known that, the kinetic adsorption isotherm models have been extensively explored to study the rate determining step of the adsorption process [29] . The kinetic experiments design of Cu +2 ions adsorption was achieved at pH 7 and 400 mg/g Cu +2 ions as an optimum pH and copper ions concentration respectively at room temperature. In current study, the adsorption kinetic was compared using pseudo-first order and pseudo-second order kinetic models as the commonly used models as described below: Unless otherwise indicated, figure 7 (a, b) display the pseudo-first order and pseudo-second order plot of ln (qe -qt) against (t) and (1/qt) against (t) respectively of Cu (+2) ions solution and their calculated data. The rate constant k1 and k2 were calculated from the slope while the theoretical value of qe (mg/g) was calculated from the intercept. As shown in Table II, the correlation coefficient (R2) of the pseudo-second order kinetic (0.99631) is higher than that obtained from the pseudo-first order kinetic (0.64291). Hence, the pseudo-second order kinetic model is better to signify the experimental data than that of the pseudo-first order kinetic model. Therefore, the pseudo-second order kinetic model is preferred to reflect the chemical process during adsorption of Cu (+2) ions towards amidoxime-modified poly (AN)-starch nanoparticles graft copolymer. Besides, other similar published results have been confirmed the bio sorption of Cu (+2) ions into sugar beet pulp [30] and H3PO4-activated rubber wood sawdust [31] , that obey the second order kinetic equation also.
Please insert figure 7 (a, b) and table II here.

Adsorption isotherm:
Usually, the importance of adsorption isotherms is to deliver evidence on how the adsorbate molecules dispersed between solution and the adsorbent molecule at a given equilibrium conditions [32] .
Where qe is the adsorption capacity (mg/g), Ce is the equilibrium concentration of the adsorbate (mg/ L), qmax represents the maximum adsorption capacity of adsorbents (mg/ g), and KL is the Langmuir adsorption constant (L/mg). The regression of 1/qe against 1/Ce was plotted according to the Langmuir sorption isotherm model (linear regression), as in Fig. 8 (a).

Please insert figure 8 (a) here.
The values of qmax and KL were calculated from the slope (1.23651) and the intercept (0.0026545) of the linear plot of 1/qe and 1/Ce. The calculated data for the maximum sorption capacity (qmax) and the sorption coefficient (KL) are presented in Table III.

Please insert table III here.
While, the in case of Freundlich isotherm, which is described by Eq. 4, Where, Kf is the Freundlich constant and used to measure the adsorption capacity and 1/n is the adsorption intensity. Figure 10 shows a corresponding plot of the data. The values of Kf (mg/g) and 1/n were calculated from the slope (1.05604) and intercept (0.1691) of the linear plot of log qe versus log Ce ( Fig. 8 (b). The calculated data for the Kf and 1/n were shown in (Table III).
As shown above in tables III, the Freundlich isotherm graph provided lower correlation coefficient (R2=0.99526) values as compared to that of Longmire isotherm (R2=0.99956). This indicates that the sorption of Cu (+2) onto amidoxime-modified poly (AN)-starch nanoparticles graft copolymer is tailored well with the Langmuir isotherm model which suggest a higher manifestation of a multilayer adsorption process for the studied copper ions.

Conclusion:
Amidoxime modified poly (AN)-starch nanoparticles graft copolymer as adsorbent resin was by increasing resin dosage up to 1.0 g then leveled off after that. On the other hand, kinetic study was found that pseudo-second-order rate equation was better than pseudo-first-order supporting the formation of chemisorption process. While, in isothermal kinetic study, the examination of R2 values showed that the Langmuir model afford the best fit to experimental data than Freundlich one.

Reaction conditions:
Details of the conditions used are given in the text.

Reaction conditions:
Details of the conditions used are given in the text.

Reaction conditions:
Details of the conditions used are given in the text.

Reaction conditions:
Details of the conditions used are given in the text.

Reaction conditions:
Details of the conditions used are given in the text.