The Adsorption Performance of Polyaniline/ZnO Synthesized through a Two-Step Method

Abstract

Polyaniline/Zinc oxide (PANI/ZnO) were prepared using a two-step method, and the morphology and the structure of PANI/ZnO composites were characterized through a scanning electron microscope (SEM) and X-ray diffraction (XRD). Factors such as the content of ZnO, the adsorption time and the mass of the adsorbent, and the kinetic equation of PANI/ZnO as adsorbents for the adsorption of methyl orange solution were studied. The results showed that the adsorption efficiency of methyl orange by polyaniline with the increase of adsorbent mass firstly increased and then decreased. Among the composites with the same quality, PANI composites with 8% ZnO have a better adsorption effect for methyl orange, and the maximum adsorption ratio can reach 69% with the increase of adsorption time at 0.033 g; With the increase of adsorbent mass, the adsorption efficiency of PANI composites with 8% ZnO increased continuously. When the mass increased from 0.033 g to 0.132 g, the adsorption rate increased from 69% to 93%, and the adsorption of the methyl orange solution by PANI/ZnO composites was more in line with the quasi-second-order kinetic equation.

1. Introduction

Polyaniline (PANI), as a conducting polymer, has attracted significant attention for the removal of the pollutants such as heavy metal and dyes, owing to excellent environmental stability, easy preparation, abundant morphologies and special doping-de-doping properties [1,2,3,4]. However, it is limited in commercial applications as a result of the weak mechanical processing, which can help to improve the mechanical strength of PANI through forming PANI composites with other materials. Therefore, the adsorption performance of PANI composites as potential adsorbents have been studied [5,6,7], including PANI based biocomposites [8,9], PANI/metal oxides. There are many studies on the adsorption of organic dyes by PANI/TiO₂ [10], PANI/Fe₂O₃ [11,12], PANI/Fe₃O₄ [13,14,15,16] and PANI/ZrO₂ [17,18]. However, the removal of organic dyes on PANI/ZnO was investigated regarding the photocatalytic degradation of pollutants in wastewater by PANI/ZnO nanocomposites as photocatalysts in many reports [19,20,21,22,23], and has not been deeply explored as an adsorbent to treat pollutants in wastewater. In recent years, research on the treatment of dyes in wastewater by PANI/ZnO has increased. Kannusamy and his college [24] synthesized chitosan-polyaniline/ZnO hybrids through the polymerization of aniline hydrochloride in the presence of ZnCl₂ and chitosan, finding that the introduction of ZnCl₂ enhanced the adsorption of reactive dyes and photocatalytic degradation. Nerkar et al. [25] prepared PANI/ZnO composites with a shell structure through a “two-step” and found that the adsorption capacity of the PANI/ZnO composite was much higher than that of pure ZnO and pure PANI, which suggested that PANI/ZnO had a certain potential in wastewater treatment. Akash’s research group [26] prepared microspherical PANI/ZnO composites by the chemical oxidative polymerization method and studied the adsorptive performance under the application of ultrasound, which found that the adsorption capacity of PANI/ZnO can be as high as 240.84 mg/g. These results display that PANI/ZnO has aunique adsorption ability compared to pure PANI. However, the influence of factors such as the morphology of PANI/ZnO composites and the amount of ZnO on their adsorption properties, and the adsorption mechanism of dyes are still unclear; the adsorption behaviour of PANI/ZnO with 3D morphology has been investigated in the present study. In addition, PANI/ZnO composites have the advantages of low cost, simple preparation, non-toxic and environmental protection and strong adsorption capacity for dyes, which makes it a potential adsorbent with low cost and large-scale practical application.

Hence, PANI/ZnO was synthesized through a “two-step” approach in this paper. Factors such as the amount of ZnO, the mass of PANI/ZnO and the contact time on the adsorptive property of PANI/ZnO were investigated through Uv-vis spectra, and the adsorption kinetics of the adsorbent were also studied.

2. Experimental Section

2.1. Chemical Materials

Zinc acetate dehydrate (Zn(CH₃COO)₂-2H₂O), sodium hydroxide (NaOH) and ammonium persulfate ((NH₄)₂S₂O₈, APS)were purchased from Xilong Science Co., Ltd (Guangzhou, China). Aniline monomers were bought from Shanghai MclinBiochemical Technology Co., Ltd. (Shanghai, China); Methyl orange (MO) and ethanol were obtained from Shanghai Shanpu Chemical Co., Ltd. (Shanghai, China) and Shanghai Wokai Biotechnology Co., Ltd. (Shanghai, China), respectively; Salicylic acid (SA) was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), distilled water was obtained from our laboratory. All the reagents used in the experiment were analytical grade (AR).

2.2. The Synthesis of ZnO

In total, 25 mL 2.0 M NaOH solution was added into 25 mL 0.09 M Zinc acetate solution, and then the above mixture was stirred 10 min with a magnetic stirrer. Next, the transparent solution was transferred to the stainless steel reactor and heated at 140 °C for 12 h. The precipitates were washed and centrifuged with ethanol and water several times, respectively, and the sample was collected after drying at 60 °C for 12 h.

2.3. The Synthesis of PANI/ZnO

PANI/ZnO composites with a different content of ZnO were fabricated by a chemical approach. Firstly, a series of the solutions with the volume of 20 mL containing 0.20 mol/L aniline monomer and 0.01 mol/L SA were prepared; the pH of the solutions was approximately6.Then the different content of ZnO (0%, 8%, and 16%) was dispersed into the above solution, respectively; 10 mL APS oxidant solution with the concentration of 0.40 mol/L was then quickly added into the above mixture to initiate the polymerization at room temperature. After 24 h, the precipitates were washed several times through water and ethanol, and the samples were collected after drying for 24 h at 70 °C.

2.4. Characterization

The structure and the morphology of the ZnO and PANI/ZnO composites were characterized through X-ray diffraction (XRD, Pert Pro, PANalytical, Almelo, The Netherlands) and a scanning electron microscope (SEM, FEI Quanta250, Thermo Fisher Scientific, Waltham, MA, USA), respectively. The UV-vis spectrophotometer (UV-1800, Shimadzu, Kyoto, Japan) was carried out to measure the absorbance of the MO solution.

2.5. The Adsorption Performance of PANI/ZnO

PANI/ZnO and MO solution were acted as the adsorbent and the adsorbate, respectively. The adsorption experiments were executed in the darkness, and 0.033 g PANI/ZnO adsorbents with a content of 8% ZnO was added into 120 mL MO solution with the concentration of 25 mg/L. The sample with a volume of 5 mL was stirred at regular intervals for 15 min, and the upper solution was collected through centrifugation for UV-vis determination. The adsorption process was investigated through varying and controlling the factors, such as the amount of ZnO, the mass of PANI/ZnO adsorbents and the adsorption time.

To determine exactly the relation of the concentration of MO and the absorbance, a series of the different concentrations of MO solution (5, 10, 12.5, 15, 20 and 25 mg/L) were prepared, and the absorbance values of the as-prepared MO solution were collected using a UV-vis spectrophotometer in the range of 200–800 nm. The relation between the concentration of MO and the absorbance was obtained through linear fitting, according to the peak located at about 464 nm.

3. Results and Discussions

3.1. The Morphology and Structure of PANI/ZnO

Figure 1 display the morphology of the as-synthesized ZnO and PANI composites with different amounts of ZnO. As shown in Figure 1, the morphology of ZnO was rod-like with a size of 13 μm; the morphology of the as-synthesized PANI was sheets, which was aggregated into clusters; when the amount of ZnO was equal to 8%, the flower-like clusters were composed by sheets; with the amount of ZnO increasing to 16%, the size of PANI sheets increased, accompanying the size of the clusters growing, which suggested that the rod-like ZnO was coated by polyaniline nanosheets.

Figure 2 display the XRD pattern of ZnO and PANI composites with different amounts of ZnO. The characteristic peaks of ZnO were observed at 2θ = 31.77°, 34.42°, 36.25°, 47.53°, 56.60°, 62.86°, 66.38°, 67.96° and 69.10°, which corresponded to the planes such as (100), (002), (101), (102), (110), (103), (200), (112) and (201), respectively, and no other peak was determined. The sharp characteristic peaks, in agreement with the hexagonal wurtzite crystal structure of ZnO (JCPDS:36-1451) [27], revealed the good crystallinity of ZnO. The two broad peaks of PANI and PANI composites was observed at 2θ = 20° and 25°, which were attributed to the periodicity in the direction perpendicular and parallel to the polymer chain, respectively. With the increase of the amount of ZnO, the main peaks of ZnO were not clear in Figure 2c,d, which was probably caused by PANI encapsulated rod-like ZnO. Therefore, the FTIR spectra of ZnO, PANI and PANI/ZnO composites was collected and shown in Figure S1 (in Supplementary Materials), which indicated ZnO existed in the PANI/ZnO composites through comparative analysis.

3.2. The Adsorption Performance of PANI/ZnO

3.2.1. The Influence of the Amount of ZnO

The adsorption ratio of PANI composites (0.033 g) containing the different amounts of ZnO as a function of the adsorption time is shown in Figure 3. When PANI acted as the absorbent, the adsorption ratio of MO was equal to 75% in the first 175 min and then slowly increased to 78% at 300 min. For PANI composites, when the amount of ZnO increased from 8% to 15%, the adsorption ratio of MO ZnO decreased from 69% to 54% at 300 min, respectively. The adsorption ratio of the former was lower than that of the latter in the first 75 min, but the former increased markedly with time prolonging. Hence, the adsorption performance of MO adsorbed by PANI was better than that of PANI/ZnO adsorbents, and with the amount of ZnO increasing, the adsorption ratio decreased.

3.2.2. The Influence of the Mass of the Absorbent

Figure 4 display the mass of the absorbents and the relation of the equilibrium adsorption ratio calculated through the long adsorption. When the mass of PANI rose from 0.033 g to 0.066 g, the final adsorption ratio of MO significantly increased from 78% to 90%.When the mass of PANI increased from 0.099 g to 0.132 g, the adsorption ratio decreased from 93% to 90%. For PANI composites with 8% ZnO, whenthe mass of the adsorbent increased, the adsorption ratio remarkably improved from 69% to 93%.

The adsorption ratio change of MO adsorbed by PANI and PANI/ZnO in the first 150 min is shown in

Table 1. The adsorption ratio changes of MO adsorbed by PANI and PANI/ZnO (8%).

Time/min	PANI				PANI/ZnO
	0.033 g	0.066 g	0.099 g	0.132 g	0.033 g	0.066 g	0.099 g	0.132 g
15	41%	58%	72%	73%	18%	65%	75%	77%
30	51%	70%	82%	83%	28%	77%	83%	85%
45	57%	76%	86%	87%	36%	82%	87%	89%
60	62%	77%	87%	88%	43%	84%	88%	90%

. When the weight of the PANI/ZnOincreased, the adsorption rate of MO absorbed by PANI/ZnO was better than that on PANI, especially in the first 15 min. In the first 60 min, the adsorption ratio of MO absorbed by PANI/ZnO with 0.033 g was equal to 43%, which was lower than that of PANI with the same weight.When the mass of PANI/ZnO increased to 0.066 g, the adsorption ratio increased quickly up to 84%, then, with the mass of the adsorbent increasing to 0.132 g, the adsorption ratio increased to a certain degree.The adsorption ratio of the MO adsorbed by PANI was 77%, 87% and 88%, which was lower than that by PANI/ZnO with increasing themass of PANI from 0.066 g to 0.132 g.

3.2.3. The Adsorption Kinetics of Process of PANI/ZnO

A series of MO solutions with different concentrations were prepared, and the absorbance of the peak at 464 nm was collected through a Uv-visspectrophotometer. The linear fitting between the concentration of MO and the absorbance is shown in Figure 5, and the slope and the correlation coefficient R² were 0.07178 and 0.99936, respectively.

In combination with the linear fitting obtained in Figure 5, the adsorption equilibrium capacity (q_e) and adsorption capacity (q_t) of PANI/ZnO was calculated, according to the below Equations (1) and (2). Then the quasi-first-order kinetic equation and quasi-second-order kinetic equation of the adsorption process were calculatedaccording to the below Equations (3) and (4), and the obtained plot is displayed in Figure 6. The correlation coefficients obtained through linear fitting were 0.98294 and 0.99585, respectively, which indicated the adsorption process of MO adsorbed by PANI/ZnO adsorbent agreed with the quasi-second-order kinetic equation.

q_e = (C₀ − C_e)V/W

(1)

q_t = (C₀ − C_t)V/W

(2)

Ln(q_e − q_t) =lnq_e − k₁t

(3)

t/q_t = 1/(k₂q_e²) + t/q_e

(4)

where q_e and q_t represent the adsorption equilibrium capacity and the adsorption capacity at t min, respectively; C₀and C_e are the concentration of MO solution in the initial solution and at the equilibrium state, C_t is the concentration of MO adsorbed by PAN/ZnO at t min; and k₁ and k₂ are the pseudo-first-order kinetics constant and the pseudo-second-order kinetics constant.

4. Conclusions

The rod-like ZnO was synthesized through a hydrothermal approach, and the flower-like PANI composites with different amounts of ZnO were prepared using a chemical method. The adsorption performance of MO adsorbed by PANI and PANI/ZnO composites was investigated through adjusting and controlling the parameters such as the amount of ZnO, the adsorption time and the weight of the adsorbent. When the amount of ZnO increased, the adsorption effect of MO adsorbed by the PANI/ZnO composites reduced; as the mass of the adsorbents increased, the adsorption effect of the MO adsorbed by PANI composites with 8% ZnO improved, while the adsorption ratio of the MO adsorbed by PANI firstly increased and then decreased. The following results were obtained: the adsorption effect of MO adsorbed by 0.033 g PANI composites with 8% ZnO was equal to 69%, and the adsorption efficiency of MO adsorbed by PANI/ZnO (8%) increased up to 93% when the weight of the absorbent was 0.132 g. In addition, the quasi-second-order kinetic equation was inconsistent with the adsorption process of MO adsorbed by PANI/ZnO.