Role of Zn And Polyaniline In Magnetic Nanocomposites And Enhanced Arsenic Adsorption Capacity In Wastewater

Polyaniline/Fe 0.90 Zn 0.10 Fe 2 O 4 (PANI/Zn 0.10 Fe 2.90 O 4 ) nanocomposites were synthesized by a chemical method and an onsite polymerization method. XRD patterns showed that the Zn 0.10 Fe 2.90 O 4 grain size about 12 nm, while TEM image showed grain size from 10 to 20 nm. The results of Raman spectra and DTA analyses showed that PANI participated in part of the PANI/Zn 0.10 Fe 2.90 O 4 nanocomposite samples. The grain size of PANI/Zn 0.10 Fe 2.90 O 4 samples measured by SEM was about 35–50 nm. These results demonstrated the shell–core structures of the nanocomposite material. The magnetization measurements at room temperature showed that in 1250 Oe magnetic eld, the saturation magnetic moment of PANI/Zn 0.10 Fe 2.90 O 4 samples decreased from 71.2 to 42.3 emu/g when the PANI concentration increased from 0 % to 15 %. The surface area and porous structure of nanoparticles were investigated by the BET method at 77 K and a relative pressure P/P 0 of about 1. The arsenic adsorption capacity of the PANI/Zn 0.10 Fe 2.90 O 4 sample with the PANI concentration of 5 % was better than that of Fe 3 O 4 and Zn 0.10 Fe 2.90 O 4 in a solution of pH 7. In the solution with pH P14, the arsenic adsorption of magnetic nanoparticles was insignicant. Due to substitution of Fe ions by Zn transition metal and coating polyaniline, these materials could be reabsorbed and reused. core-shell specic its high capability of (III) aqueous 7 20-minute effective S 5% mass ratio) max = 43.48 mg/g) is S 0, S 2 , S 3 and Fe 3 O 4 nanoparticles. to the of saturation stability of suggests the for heavy metal the in alkaline

Due to the rapid oxidation of Fe 3 O 4 into γ-Fe 2 O 3 in air as well as its decreased magnetization, large numbers of studies have focused on improving the saturation magnetization and chemical stability of the magnetic nanomaterials for effective heavy metal ion adsorption [10,19,[24][25][26][27][28][29]. Two research directions are being considered: (i) partial replacement of divalent metal ions for Fe 2+ ions, and (ii) coating of magnetic nanoparticles by polymers.
Magnetic nanoparticles are studied to treat heavy metals, weak-acid oxyanion contaminants or dyes in the environment [32][33][34][35]. In particular, the coating of magnetic nanoparticles by polymers are also studied to protect the chemical physical properties of magnetic nanoparticles simultaneously ensuring the recovery and reuse of the adsorbent materials [35,36].
Polyaniline (PANI) is well-known as a conductive polymer whose high electrical conductivity has been discovered and is expected to play an increasingly signi cant role in many elds due to its easy synthesis, physicochemical stability, and simple doping chemistry. In recent, the substitution of Mn, Cu, Zn elements for Fe ions and the coating of magnetic nanoparticles by poly(1-naphthylamine), polyvinyl pyrrolidone have been studied [30,31,36] in order to stabilize the physicochemical properties and enhance their applicability. In this work, we discuss the synthesis, magnetic properties, microstructure of PANI/Fe 0.90 Zn 10 Fe 2 O 4 nanocomposites with different mass ratios of PANI/Fe 0.90 Zn 0.10 Fe 2 O 4 and the Zn 2+ substitution for Fe 2+ . Besides, the effect of PANI content on the arsenic adsorption ability in aqueous at different pH values as well as the desorption/re-adsorption capacities of the magnetic nanomaterials are also discussed.
Labconco Freeze Concentrator (USA) was used for material drying in vacuum. The structure and morphology of the samples were investigated by XRD patterns (D5005, Bruker), TEM (JEOL5410), SEM (S4800), IR Prestige -21 and thermal gravimetric analysis by DTG-60H. Meanwhile the magnetization was measured by vibrating sample magnetometer (VSM 8600 S) and the mesopores structure of samples was observed by TriStar 3000 V6.07A with TriStar 3000 V6.08 software. The Flame-Atomic Absorption Spectrophotometer (F-AAS 6300 Shimadzu) was used to determine the arsenic content in the solutions before and after using adsorbent magnetic nanomaterials.
2.2. Synthesis of Fe 0.90 Zn 0.10 Fe 2 O 4 by chemical method A detailed synthesis procedure was described in our previous paper [30,31]. The chemical reactions occur during the synthesized process of Fe 3 O 4 : 2FeCl 3 + Na 2 CO 3 + H 2 O→2FeCl 2 + Na 2 SO 4 + 2HCl To get Fe 2.9 Zn 0.10 O 4 (signed Fe 0.90 Zn 0.10 Fe 2 O 4 ) nanoparticles, the FeCl 3 ·6H 2 O, Zn(CH 3 COO) 2 ·4H 2 O solutions containing Fe 3+ and Zn 2+ with nominal Zn atom content of 0.10 were mixed with a Na 2 SO 3 solution. These mixed solutions were stirred until they turned to yellow in color. Then, the NH 3 solution was added dropwise until the pH was 10. The solution was kept stirring for 30 minutes until it turned to black. These magnetic grains were separated from the mixture solution using external magnets, and then ltered, washed with distilled water. Finally, these materials were desiccated at 50 0 C for 48 hours and nely grinded to obtain Fe 0.90 Zn 0.10 Fe 2 O 4 nanoparticles. Step 1: a calculated amount of Fe 0.90 Zn 0.10 Fe 2 O 4 was added in 60 mL of distilled water, followed by 40 mL of IPA, aniline and well stirred for 60 minutes (mixture A).
Step 2: a stoichiometric amount of (NH 4 ) 2 S 2 O 8 solution, with the monomer/oxidizing agent molar ratio of 1:1.5, was added dropwise into mixture A to obtain a black blue mixture (mixture B) which was allowed to stir for 2 hours with an exothermal reaction.
Step 3 : Filter the mixture using external magnet, then the obtained solid was dried by Labconco Freeze concentrator for 5 hours at 1 mPa and temperature of 40 o C. The nanocomposites with 0%, 5%, 10% and 15% mass ratios of PANI/Fe 0.90 Zn 0.10 Fe 2 O 4 were coded by S 0 , S 1 , S 2 and S 3 as represented in Table 1.

The evaluation of arsenic adsorption
The evaluation of arsenic adsorption capacity of the samples was performed at room temperature. The experiment was conducted by adding 0.01 g of samples (S 0 , S 1 , S 2 and S 3 ) into As(III) solutions of initial content of 106 ppb. Each mixture was then allowed to stir in 20 minutes for a complete adsorption. The arsenic contents before and after adsorption were analyzed by F-AAS.

Results And Discussion
3.1. Structure, morphology and physical properties In Fig. 1, the diffraction peaks of (220), (311), (400), (442), (511), (440) of the Fe 0.90 Zn 0.10 Fe 2 O 4 (S 0 ) sample were completely tted with the standard diffraction pattern of Fe 3 O 4 as in our previous publication [31], and demonstrated the centered face cubic structure. Similarly, the XRD patterns of S 2 and S 3 samples showed the same diffraction peaks as that of Fe 3 O 4 sample, which proved that the Zn 2+ doping as well as the addition of polymer did not affect the crystal structure of materials. Thus, Fig. 1  thus the calculated particle size from XRD patterns is also the particle size for S 1 , S 2 and S 3 samples.
Thus, the average crystal particle sizes of  The infrared spectrum of S 2 (Fig. 3c)

TGA and DTG analyses
TGA curve in Fig. 4a showed that at temperature range below 80 0 C, the volume reduction of PANI was due to the evaporation of water, corresponding to 13% with a sharp endothermic peak at 45 0 C in the DTG curve. From 80 -210 0 C, the sample volume stays almost unchanged. However, from 210 -320 0 C, the reduction of sample volume in TGA curve happened due to the decomposition of PANI to form monomers, oligomers, dimers and trimers corresponding to a sharp endothermic peak at 292 0 C in the DTG curve. At temperature higher than 320 0 C, the weight loss caused by the thermal decomposition of the oligomers, dimers, trimers. This led to a complete decomposition of PANI at above 600 0 C.
TGA curve of typical S 2 sample (for S 1 and S 3 ) in Fig. 4b also showed that at temperature range below 80 0 C, the 8.6% reduction of sample volume due to water evaporation corresponding to a sharp endothermic peak at 45.2 0 C in the DTG curve. However, from 100 -300 0 C in the TGA curve, the 3.3% reduction of sample volume may be happened due to the decomposition of the residual monomers and oligomers in the sample. From 300 -600 0 C, the thermal decomposition of the oligomers, dimers, trimers caused a small and broad endothermic peak at 395 0 C in the DTG curve. Thus, the sample mass remained only 30% at 600 0 C. Because PANI is quite stable in the temperature range below 150 o C, it can be seen from above results that the S 1 , S 2 and S 3 nanocomposites have been successfully synthesized and the presence of PANI in these samples has improved the thermal stability of PANI-coated Fe 0.90 Zn 0.10 Fe 2 O 4 in temperature range below 150 0 C. The magnetic moment depends on the content of x that is considered due to the decision of spin direction of Fe x 3+ in B-site, but do not depends on the presence of non-magnetic Zn ions [19,31]. This has been clearly explained in our recent publication [31]. Therefore, the saturation magnetization of Fe 0.90 Zn 0.10 Fe 2 O 4 is higher than Fe 3 O 4 [30,31].

Magnetization and chemical instability
On the other hand, the saturation magnetizations of S 1 , S 2 and S 3 samples decreased from 65 emu/g to 43 emu/g when the non-magnetic PANI content increased from 5-15% (Table 2). However, due to the PANI coating, the magnetization of nanocomposite materials is more stable over time.

Adsorption kinetic, porous properties and arsenic adsorption ability
The adsorption kinetic of S 0 , S 1 , S 2 and S 3 nanocomposites can be explained [30,36] by the relation of adsorption ability based on the inelastic exchange interaction between speci c surface area of nanoparticles and adsorbed materials. The surface and structure of nanoparticle mesopores were studied by the nitrogen adsorption-desorption isotherms of 0.54 g for S 0 , S 1 , S 2 and S 3 samples at 77 K [30,36].
Collision of N 2 gas molecules with nanoparticles is considered to be inelastic, so that the N 2 gas molecules remain in contact with the nanoparticles for a time before returning to the gas phase. This time delay is taken as responsible for the phenomenon of adsorption that demonstrated by equation: P/V a (P 0 − P) = (1/V m )(P/P 0 ) [38]. Here, V a is the quantity of N 2 gas adsorbed at pressure P and V m is the quantity of gas adsorbed when the entire surface is covered with a mono-molecular layer. The N 2 adsorption-desorption isotherm curves of S 0 and S 1 samples at 77 K were presented in Fig. 6. It can be clearly seen that the adsorption and desorption ability of S 1 sample is higher than that of S 0 . By the BET (Brunauer, Emmett, and Taller) theory [38], the pore size distribution at relative pressure P/P 0 ≈ 1 and the speci c surface area at low P/P 0 were calculated as shown in Table 3.

Arsenic adsorption ability
In order to study As adsorption ability of the nanoparticles, the effects of pH in environment and As maximum adsorption capacity also investigate at room temperature.

Effects of pH to arsenic adsorption
To study the pH effect on the adsorption ability of the nanocomposites, the As(III) solutions with different pH in range of 1-14 were prepared. The As(III) adsorption results were presented in Fig. 7. It can be seen from Fig. 7 that the remaining arsenic content was a function of the pH. For all nanocomposite samples, the arsenic adsorption capacity increased as the pH was increased from 1 to 7. Then, the adsorption capacity decreased as the pH was increased above 7. The highest adsorption capacities were obtained in the range of pH 5-9. In both strong acidic and basic solution, the adsorption capacity decreased.
This trend can be explained by the speciation of arsenic (III) at different pH media and the surface charge status of the nanocomposites [39] in strong acidic environment. The surface of nanocomposites will be negatively charged at pH higher than pH pzc (~7). Meanwhile, in neutral media, the un-charge in surface of nanocomposites and As(III) occurs mostly in neutral state (H 3 AsO 3 at pH below 9.2) [40], so at pH 7, the highest As(III) adsorption occurred, due to the electrostatic interaction is not feasible under this condition. Therefore, the As(III) adsorption was controlled by the surface complexation rather than the electrostatic interactions. Similar results were observed with other magnetic nanocomposites as reported earlier [39,40].
When the pH elevates, As(III) exists mainly in form of H 2 AsO 3 − , HAsO 3 2− and AsO 3 3− anions while the surface charge of the nanoparticles is negative. Thus, the sharp decrease in the arsenic adsorption capacity at pH range of 11-14 is likely due to the electrostatic repulsion between the negatively charged surface of the nanocomposites and the deprotonated anionic arsenic. At extreme high pH 14, due to strong electrostatic repulsive force, the material has depicted no arsenic adsorption ability.
Moreover, at very low pH levels (1-2), a decomposition of the Fe 0.90 Zn 0.10 Fe 2 O 4 nanocomposites was observed proving by the presence of iron and zinc in the solution. In neutral and alkaline media, the nanocomposites were stable with no iron and zinc ions detected in the solution. Thus, it's suggested that the de-adsorption process of S 0 , S 1 , S 2 and S 3 should be conducted in solution at pH 14.
As above analyzed results, the best As adsorption occurs in a neutral environment (pH7), where the inelastic exchange interaction takes place, which has the source of the Van der Waal interaction between the magnetic nanoparticles and adsorbent. Thus, maximum arsenic adsorption capacity was investigated by the Langmuir isotherm mode at 300 K in environment with pH 7.
Maximum arsenic adsorption capacity Equilibrium time of As(III) adsorption was analyzed by measuring the remaining arsenic content in solution pH 7. As shown in Fig. 8, the arsenic contents remain in the equilibrium state when the adsorption time is 20 minutes at room temperature. The maximum arsenic adsorption capacity q max of unit volume of adsorbent (mg/g) is calculated by the Langmuir isotherm equation at pH 7 and 300 K [6, 10,17] with the rst order linear relation: where C f : the remaining arsenic content (mg/L) at equilibrium; q: the arsenic adsorption ability at equilibrium for a unit volume of adsorbent (mg/g); b: constants attributed to the interaction of the adsorbent and adsorbed compounds.
The calculated values of q max for S 0 , S 1, S 2 and S 3 are presented in Table 3. Table 3, the arsenic adsorption capability of the materials, q max of S 1 was better than that of S 0 (Fe 0.90 Zn 0.10 Fe 2 O 4 ) sample and Fe 3 O 4 , that reported in [36] with the same condition. The results in Table   2 also are in good agreement with those discussed in Fig. 8.

Conclusion
The PANI/Fe 0.90 Zn 0.10 Fe 2 O 4 nanocomposite materials were successfully synthesized with different mass ratios. These nanocomposite materials with core-shell structure have high saturated magnetic moments and high speci c surface area that contribute to its high capability of adsorbing arsenic (III) in aqueous solutions. The pH 7 and 20-minute adsorption time are suitable factors for an effective adsorption process. The sample S 1 (PANI 5% mass ratio) with As adsorption capacity (q max = 43.48 mg/g) is highest than that of S 0, S 2 , S 3 samples and Fe 3 O 4 nanoparticles. Due to the improving of saturation magnetization and chemical stability of nanomaterials, this work suggests the ability for heavy metal ion adsorption and the desorption in strong alkaline solution, then the materials could reabsorb for further trials.   Remaining arsenic content as a function of pH The C f /q dependence on C f using adsorbents with different PANI ratios in PANI/Fe 0.90 Zn 0.10 Fe 2 O 4