Elsevier

Powder Technology

Volume 347, 1 April 2019, Pages 93-102
Powder Technology

Removal of methylene blue by Polyaniline/TiO2 hydrate: Adsorption kinetic, isotherm and mechanism studies

https://doi.org/10.1016/j.powtec.2019.02.049Get rights and content

Highlights

  • The adsorption sites on TiO2 were the hydroxyl and carboxyl groups.

  • TiO2 interacted with MB by hydrogen bonding and electrostatic interaction.

  • The adsorption sites on PANI were amino and benzene ring.

  • PANI interacted with MB mainly by π-π stacking and hydrophobic effect.

  • PANI/TiO2 was a potential widely-applied adsorbent and worth further exploring.

Abstract

Polyaniline/TiO2 (PANI/TiO2), as a promising adsorbent, was synthesized and applied in the adsorption of methylene blue (MB) to evaluate the adsorption performance of PANI/TiO2 for cationic dye. The effects of dosage, pH, temperature and ionic strength on the adsorption efficiency were investigated in details. The adsorption isotherm and kinetic studies were conducted and the results revealed that the adsorption of MB on PANI/TiO2 was mainly controlled by chemical adsorption, membrane diffusion and intraparticle diffusion. The maximum adsorption capacity was 458.10 mg/g. HCl solution (0.1 mol/L) was used as stripping agent to study the regeneration stability of PANI/TiO2, and the adsorption efficiency can be maintained over 99% after 10 cycles. The N2 adsorption-desorption and Fourier Transform Infrared Spectroscopy of PANI/TiO2 before and after adsorption were compared and analyzed to obtain the information of adsorption mechanisms. The adsorption of MB by TiO2 mainly occurred on the surface hydroxyl and carboxyl groups, and was mainly heterogeneous multilayer physisorption. However, the adsorption of MB on PANI part occurred mainly on amino groups by hydrogen bonding and electrostatic interaction, and on benzene ring by π-π stacking and hydrophobic interaction, and was assigned to heterogeneous multilayer chemical adsorption by comparing the theoretical monolayer adsorption capacity and the actual adsorption capacity. After composition, PANI and TiO2 cooperated in the adsorption process of MB and exhibited their respective advantages. The PANI/TiO2 was an excellent adsorbent for organic cation dyes and was expected to be a kind of universal adsorbent for multiple pollutants.

Introduction

Nowadays, various kinds of organic dyes are developed and used in leather, electroplating, textile, paper-making, plastics and printing industries [1]. It was reported that 7 × 108 kg of dyestuffs were produced every year all over the world [2]. The complex aromatic structure of dyes make them stable and more difficult to be removed from the effluents [3]. However, the effluents containing dyestuffs without properly treatment can cause severe environmental problems [4]. Methylene blue (MB) is commonly used cationic dye for coloring cotton, wood, and silk. This dye has been studied because of its known strong adsorption affinity onto solid [5]. However, MB can cause a series of harms to humans and animals, such as eye burns, methemoglobinemia, dyspnea, irritation to the skin [6]. Thus, the MB, as a typical cationic dye, has often been chosen as a model compound to investigate the adsorption process from aqueous solutions [7].

Chemical, biological and physical treatment processes aiming at removing pollutants from wastewater have been developed and widely applied [8]. Among these techniques, adsorption has been widely researched and applied due to its low-cost and high-efficient features, as well as operational ease [9,10]. It demonstrates superior removal efficiency to other techniques, especially for refractory wastewater and low concentration wastewater. Currently, the adsorbents including activated carbon [11], resin and gel [12], graphene [13], metal oxide [14], and polymer [[15], [16], [17]] had been well investigated. They all possess their own unique advantages, and limitations as well. There is a lack of widely applicable adsorbents for simultaneous adsorption of various pollutants in complex printing and dye wastewater.

An excellent adsorbent should possess large specific surface area, suitable porous structure, ease of preparation and regeneration, high adsorption capacity and robustness [18,19]. Organic and inorganic composite material as adsorbent can improve the synergistic adsorption and expand the application [20,21]. In our previous studies, the PANI/TiO2 composite had been synthesized and demonstrated as an adsorbent for anion-type pollutants such as phosphate ions [22] and anionic azo dyes (Acid red G, ARG) [23]. The adsorption performance of TiO2 was due to the porous structure and the hydroxyl groups on the surface. However, the regeneration efficiency was very low and the cost for regeneration was high. The conductive polyaniline (PANI) possesses rigid organic skeleton and reversible doping-dedoping property, and the morphology is easily modulated [24], which benefit the adsorption and regeneration. PANI and TiO2 had been individually studied as adsorbent for different pollutants such as Cr(VI) [25], Pb(II) [26], methyl orange [27], Reactive Red 195 [28], phosphate ions [29]. And outstanding adsorption capacity and high stability of PANI/TiO2 for phosphate ions and anionic dyes were observed in our previous studies. It is necessary to further study the adsorption behavior and mechanism of this adsorbent for other typical pollutants.

In this paper, the adsorption behavior and mechanism of PANI/TiO2 for MB cations were detailed to further study the wide applicability of PANI/TiO2. The influence factors on adsorption, like pH, temperature, ionic strength, and initial MB concentration were investigated. The potential adsorption mechanisms were finally analyzed and proposed with the aid of N2 adsorption-desorption and FT-IR characterizations before and after adsorption. In order to have a more accurate and in-depth understanding of the adsorption mechanism of the composite adsorbent, the control studies of PANI and TiO2 were made for all the above items. These studies can not only expand PANI/TiO2 applications but also be helpful to understand the adsorption mechanism of PANI/TiO2 for MB.

Section snippets

Materials

Methylene blue (MB, C16H18N3ClS, Mw 319.85) was purchased from Beijing Chemical Reagents Company, and other reagents were from Sinopharm Chemical Reagent Co., Ltd. Aniline (99.5%) was distilled under reduced pressure, and stored in darkness at 0 °C under N2 atmosphere prior to use. All other chemicals were used as received.

Preparation of PANI/TiO2

PANI/TiO2 was synthesized using a one-pot chemical oxidative polymerization method. TiO2 nanoparticles were hydrolyzed from titanium isopropoxide in 100 mL mixture solution

PANI, TiO2 and PANI/TiO2 characterization

The samples PANI, TiO2 and PANI/TiO2 were characterized by SEM, N2 adsorption-desorption and FTIR, and the results are displayed in Fig. 1 and Figs. S1, S2. The TiO2 particles were uniform in a diameter of <1 μm (Fig. 1a), but the PANI was laminated structure (Fig. 1b) from the SEM patterns. After composition, the particles were more rough than that of TiO2, making the particles appeared to have a layered surface structure (Fig. 1c). This was speculated to the result of the deposition of PANI

Conclusion

PANI/TiO2 synthesized in this study possessed out-standing adsorption performance for MB, such as high adsorption capacity (458.10 mg/g), wide pH applicability (pH 3–11), and good performance of regeneration. By the composition of PANI and TiO2, PANI/TiO2 exhibited more stable and higher regeneration efficiency than PANI and TiO2; after 10 cycles, the adsorption efficiency maintained above 99% using 0.1 mol/L HCl as stripping agent (100 mg/L MB solution). Adsorption equilibrium could be

Acknowledgments

The authors gratefully acknowledge the Shaanxi Key Research and Development Projects, China (Grant No. 2017SF-386), Science and technology project of water resources department of Shaanxi province (2017s1kj-9), the financial supports from the National Natural Science Foundation of China (Grant No. 21307098 and No. 21703165) and the Fundamental Research Funds for the Central Universities of China.

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