Adsorption of methylene blue onto chitosan–montmorillonite/polyaniline nanocomposite

Highlights

• Chitosan and polyaniline were intercalated into montmorillonite interlayers.

• Adsorption of methylene blue onto chitosan–montmorillonite/polyaniline nanocomposite.

• Eco-friendly candidate for the remediation of organic dyes in industrial effluents.

Abstract

Eco-friendly adsorbent of chitosan–montmorillonite/polyaniline (CH–Mt/PANI) nanocomposite was synthesized by the intercalation of CH into Mt through ion-exchange process followed by the impregnation of aniline. Then in-situ polymerization of aniline using ammonium peroxydisulfate as an oxidant yielded a conducting polymer, polyaniline, as an additional component. The composite with polyaniline salt was also converted to corresponding CH–Mt/PANI base nanocomposite. The composites were characterized by Fourier transform infrared spectroscopy, X-ray diffractometry, thermal gravimetric analysis and electron microscopy and subsequently used for the removal of methylene blue from aqueous solutions. The composite with polyaniline base proved to be better dye adsorbent than that with polyaniline salt and was therefore investigated in detail. The adsorption of the dye onto CH–Mt/PANI base nanocomposite follows the pseudo second-order kinetics and it is well described by Temkin isotherm model. Moreover, the intraparticle diffusion was found to play a significant role in the adsorption mechanism. The maximum adsorption capacity was estimated to be 111 mg g⁻¹, exceeded the adsorption capacities of the individual precursors.

Introduction

Organic dyes are commonly used in various applications e.g. paper and pulp industries, textiles colouring, paints, cosmetics and food production (Chen and Zhao 2009; Gupta 2009). Residual dyes effluents are often disposed into the environment, which cause a destruction of the aquatic environment (Özcan and Özcan 2004) and other harmful effects. Dyes have stable chemical structures due to their complex aromatic moleculararchitectures. Furthermore, most of them affect human and animal health (Srinivasan and Viraraghavan 2010). Many conventional methods are used for the remediation of dye containing wastewater such as coagulation, flocculation, oxidation, photochemical destruction, ion exchange and membrane filtration (Önal et al. 2006), however, those methods are costly and require additional chemicals. The adsorption process has been widely used effectively for the removal of colorants from wastewater due to the low-cost, regeneration and reusability of the adsorbents (Ayad et al. 2018; Minisy et al. 2020). Adsorbents, such as activated carbon (Önal et al. 2006), natural zeolites (Wang et al. 2006) sawdust (Garg et al. 2004) and biomass (Rafatullah et al. 2010) have been commonly used for dye removal.

Chitosan (CH), a low-cost biopolymer, contains large amounts of amino and hydroxyl functional groups which can complex with pollutants (Annadurai et al. 2008; Kyzas and Lazaridis 2009; Dotto and Pinto 2011). In addition, CH has welcome antifungal and antimicrobial activities (Sahariah and Masson 2017). Hence, CH is widely used in food preservation, cosmetics, drug-delivery system and remediation of wastewater (Vakili et al. 2014). However,neat chitosan's hydrophilic character and associated poor mechanical properties limit its application.

Montmorillonite (Mt) is a clay containing dangling hydroxyl end groups on the surface. Mt layered morphology, cation exchangeability, large specific surface area and weak interaction forces between layers allow the intercalation of organic cations and polymers. Many published studies (Zhu et al. 2016; Adeyemo et al. 2017) showed that Mt can be used as a low-cost adsorbent for cationic dyes due to its large cation-exchange capacity.

Clay/polymer nanocomposites have outstanding thermal and mechanical properties when compared to the polymer itself (Unnikrishnan et al. 2011). Montmorillonite mineral has been widely employed in combination with various polymers, including poly(methyl methacrylate) (Salahuddin 2009), epoxy (Salahuddin et al. 2002; Salahuddin 2004) and polyurethane (Salahuddin et al. 2010), thanks to its swelling behavior, ubiquity and economic cost (Pande et al. 2012). Mt was modified by replacing sodium ions by various organic cations to increase its compatibility with polymers (Ayad et al. 2009; Azhar et al. 2014). CH has an extended structure that facilitate the intercalation into Mt layers under acidic conditions to form CH–Mt nanocomposites through cation-exchange process (Darder et al. 2003; Darder et al. 2005; Wang et al. 2005). Qiu et al. (2005) have prepared poly(acrylic acid)/CH-intercalated Mt nanocomposite by two-step method. Moreover, CH-graft-poly(acrylic acid)/Mt nanocomposite was prepared by in-situ intercalative polymerization (Zhang et al. 2007), and was used to remove methylene blue (MB) from aqueous solutions (Wang et al. 2008) with the ability to regenerate and reusethe sorbent.

Conducting polymers have recently come to the forefront as efficient adsorbents of organic dyes (Stejskal 2020). Among them, polyaniline (PANI), is probably the most technologically important. It mainly interacts with organic dyes based on π–π interactions between the aromatic moieties in addition to electrostatic interactions and hydrogen bonding. PANI has good environmental stability, low-cost of synthesis, and relatively high electrical conductivity with units S cm⁻¹ (Stejskal and Gilbert 2002). Even though the last parameter is not of direct importance for the adsorption itself, the future development would probably use it in the control of adsorbent properties by applying electrical potential. It has recently been used for the removal of water pollutants (Stejskal 2020; Ayad and El-Nasr 2010).

In the present investigation, CH, a natural biodegradable polymer, Mt, a clay mineral with high cation-exchange capacity, and conducting PANI with high chemical and environmental stability, are good candidates for the easy preparation of a cost-efficient and eco-friendly composite as adsorbent (Scheme 1). CH–Mt/PANI nanocomposite was synthesized by two-step method, the intercalation of CH into the Mt layers was followed by the impregnation of aniline into CH–Mt layers and finally the in-situ polymerization of aniline. The nanocomposite was then usedfor the adsorption of methylene blue (MB; cationic dye used as a model organic pollutant)from aqueous solutions. It was used indoped and dedoped forms at room temperature for the efficient and fast removal of the dye.