Mechanistic aspects for the enhanced adsorption of bromophenol blue and atrazine over cyclodextrin modified polyacrylonitrile nanofiber membranes

https://doi.org/10.1016/j.cherd.2021.02.010Get rights and content

Highlights

  • Natural cyclodextrin polymers enhance the adsorption capacity of PAN nanofiber membranes.

  • Improved adsorption capacity of undesirable bromophenol blue and atrazine in aqueous solutions achieved.

  • Successful incorporation of β-CD monomers into the polymer matrix via a desirable crosslinking process.

  • β-CD gave rise to reduced average fiber sizes translating to high-surface-area-to-volume ratio.

  • Inclusion complexation properties and electrostatic interactions are responsible for enhanced adsorption.

Abstract

The present work describes a unique modification process for electrospun polyacrylonitrile (PAN) nanofiber membranes modified with β-cyclodextrin (β-CD) monomers and their application in the adsorption of bromophenol blue and atrazine from aqueous systems. PAN and β-CD were successfully crosslinked using citric acid followed by electrospinning of PAN-CD nanofiber membranes. XRD and FTIR spectroscopy demonstrated successful crosslinking of PAN, β-CD, and citric acid to form PAN-CD nanofiber membranes. The microstructure and surface morphology of nanofibers were investigated using SEM and AFM and our data show that the nanofibers are uniform, have an average diameter range of 497–557 nm and possess rougher surfaces. UV-Vis spectroscopic technique was used to determine the nanofiber membranes’ adsorption efficiency and to evaluate their adsorption capacity. Batch adsorption studies at optimized conditions revealed that the removal of 66% and 89% was achieved for bromophenol blue using PAN and PAN-CD nanofibers with adsorption capacity of 0.886 and 1.197 mg/g, respectively. PAN and PAN-CD nanofibers removed 67% and 91% atrazine with adsorption capacity of 0.603 and 0.817 mg/g, respectively. The adsorptive removal of these pollutants followed the pseudo-second order kinetics and best fitted the Freundlich isotherm model. The increased removal is therefore credited to the increased surface area per volume ratio obtained from reduced diameters, intermolecular interactions, and inclusion complexation after the incorporation of β-CD monomers. Thus, it is shown that the addition of CDs improved the adsorption capacity of the hybrid materials via distinctive adsorption mechanisms. Thus, the uniqueness of this work lies on the mechanistic adsorption of bromophenol blue and atrazine through modified PAN-CD nanofiber matrix induced by the grafting of citric acid crosslinked β-CD.

Introduction

Organic pollutants such as dyes and emerging pollutants such as micropollutants affect water quality by changing its color, taste and/or odor. These undesirable pollutants are also present in water in acute quantities and are often difficult to remove completely using traditional treatment methods which subsequently poses a serious risk to water users and aquatic flora and fauna (Venkatesha et al., 2012, Mohan et al., 2014). Dyes are mostly used and released to the environment by paper, cosmetic, textile, leather and plastic industries while micropollutants often originate from pharmaceuticals, household detergents, personal care products and pesticides (Patel and Hota, 2016, Shi et al., 2014, Álvarez et al., 2015, Fu et al., 2015). Methods such as photocatalysis (Zikalala et al., 2018), photo-electrochemical treatment (Mafa et al., 2019), ion exchange (Kalaruban et al., 2016), membrane separation (Gumbi et al., 2018), precipitation (Kazadi Mbamba et al., 2015), electroplating (Kim et al., 2017) and adsorption have been used for water treatment targeting various kind of dye compounds and micropollutants. Adsorption technology is one of the methods regarded as effective and economical for water treatment because it is cost effective, has positive environmental impact and due to its ease of operation (Yang et al., 2019, Wu et al., 2005, Wang et al., 2014). In this case, materials such as activated carbon, polymers, chitosan, natural waste, zeolites and bio-sorbents have been studied as adsorbents for the removal of metal ions, micropollutants and organic dyes in aqueous solutions (Venkatesha et al., 2012, Altenor et al., 2009, Ahmad and Kumar, 2010, Sharififard et al., 2018).

Adsorbents are often used in their nanoscale range because they offer larger surface area to volume ratio and pores with uniform sizes which translates to larger active sites and enhanced performance compared to their larger counterparts (Fu et al., 2015, Wu, 2007, Maleki et al., 2016). Nanoadsorbents come in the forms of nanocomposites, nanofibrous membranes, nanobeads, nanospheres and magnetic nanoadsorbents, among others (Kampalanonwat and Supaphol, 2010). Nanofibrous membranes, fabricated using electrospinning, have attracted considerable attention as materials of choice for the effective adsorption of these unwanted species due to their ease of production, high porosity, economic feasibility, high permeability, ease of modification, small interfibrous pore sizes and high surface area (Huang et al., 2013). Such nanofibers can be produced from materials such as ceramics, polymers, and even natural materials.

Polyacrylonitrile (PAN), among other polymers, has been used to produce membranes and nanofibers for various applications such as filtration in water treatment due to its good mechanical properties, desirable chemical resistance, surface functionalization, thermal stability, non-toxic nature and commercial availability (Patel and Hota, 2016, Saeed et al., 2008, Neghlani et al., 2011). PAN can be blended with other materials such as natural cyclodextrin (CDs) to improve their properties due to their robustness. CD-based or CD modified nanofibers have become common materials in water treatment due to their high porosities, small fiber diameters and high surface area to volume ratio. CDs are environmentally friendly and have unique interactions with various pollutants such as inclusion complexation, hydrophobic interaction, weak van der Waals forces and charge transfer interactions due to their unique molecular structure (Nthunya et al., 2017, Fu et al., 2016), properties that the current study seeks to exploit. β-CDs have water solubility of 1.8 g/100 mL in its pure form, however, previous studies indicate that crosslinking β-CDs with citric acid makes it insoluble in water (Zhao et al., 2009a, Zhao et al., 2009b, Junthip, 2019) and has been used for the adsorption of organic substances and heavy metals (Huang et al., 2018, Zhao et al., 2009c). However, insoluble β-CDs blended with PAN and its use for the adsorption of bromophenol blue and/or atrazine has not been reported.

Foroozmehr et al. used PAN-CD nanofiber membranes for removing reactive red in wastewater and the composite membrane achieved a maximum of 31.4% reactive red removal (Foroozmehr et al., 2016). In another study, Kadam et al. produced electrospun PAN/β-CD nanofibers for the simultaneous air filtration and adsorption of volatile compounds. The nanofiber membranes demonstrated a filtration efficiency of 95% at low pressure drop (112 Pa) and achieved 66% and 73% for the adsorption of formaldehyde and xylene, respectively (Kadam et al., 2018). Li et al. used α-, β- and γ-CDs to modify PAN nanofibers for the efficient adsorption of Cu(II) ions (Li et al., 2014). In a different study, Wang et al. conducted antimicrobial studies using silver modified electrospun PAN-CD nanofibers on Staphylococcus aureus (gram positive) and Escherichia coli (gram negative). The silver containing nanofibers tested positive for antimicrobial activity as was indicated by a clear inhibition zone around the sample (Wang et al., 2012). Using a different polymer, Zhao et al. selectively removed methylene blue from water using heat treated electrospun poly(acrylic acid) blended with β-CD molecules. This materials achieved 826.45 mg/g in adsorption capacity according to Langmuir fitting (Zhao et al., 2015).

Several studies on the removal of various micropollutants via the adsorption process have been reported. Kirschhofer et al, 2016 reported the adsorption of various xenobiotics and their transformed products using activated carbon derived from sewage sludge by hydrothermal carbonization. Using this material, they achieved over 50% removal of sulfamethoxazole, diclofenac and bezafibrate from synthetic water samples. They also managed to remove 70% and 80% of carbamazepine and atrazine, respectively (Kirschhöfer et al., 2016). Yang et al. 2017 reported the adsorption of di-n-butyl phthalate, di-(2-ethylhexyl) phthalate, acetaminophen, caffeine, cefalexin and sulfamethoxazole using graphene adsorption coupled with electrocoagulation and electrofiltration. Using graphene alone, adsorption capacities between 35% and 85% were obtained. The combination of the two methods (graphene adsorption and electrocoagulation or electrofiltration) resulted in higher adsorption capacity of up to 99%. The enhanced removal efficiency was attributed to hydrophobic interactions between graphene and hydrophobic compounds as well as the ability of graphene to form π-π bonds with compounds containing benzene rings as well as C–C, and C=C bonds (Chen et al., 2014, Yang et al., 2017).

The purpose of the present study was first to fabricate electrospun PAN nanofiber membranes modified with β-CD crosslinked with citric acid and second, to apply these hybrid membranes in the adsorption of undesirable bromophenol blue and atrazine from aqueous systems. The study seeks to enhance the adsorption capacity of PAN nanofiber membranes through the introduction of new properties from the crosslinked PAN-CD molecules and via unique adsorption mechanisms. According to our knowledge, there is no reported study on the fabrication of electrospun PAN-CD nanofiber membranes through this unique crosslinking process and on their application in the removal of bromophenol blue and atrazine in aqueous solutions via the adsorption process while unravelling the mechanistic models responsible for such adsorption. Thus, the work discusses seldomly reported work of the mechanistic adsorption of bromophenol blue and atrazine through PAN-CD nanofibers matrix possessing enhanced adsorption properties induced by the grafting of citric acid crosslinked β-CD.

Section snippets

Materials

Polyacrylonitrile (PAN) MW: 150 000, β-cyclodextrin 97%, N,N-dimethylformamide (DMF) 99.8%, citric acid monohydrate (CA) 99.0%, bromophenol blue (BPB) ACS reagent, atrazine (ATR) analytical standard, sodium hydroxide (NaOH) anhydrous pellets 97% and sulfuric acid (H2SO4) 99.999% were all purchased from Sigma Aldrich (South Africa) and were used as received.

Fabrication of PAN and PAN-CD nanofiber membranes

PAN and PAN-CD solutions of 13 w/v% were prepared in DMF solution where the ration of PAN:CD was kept at 80:20. The PAN and PAN-CD solutions

SEM and AFM analysis

The electrospun PAN and PAN-CD nanofiber membranes were analysed using SEM to investigate their surface morphology (Fig. 2(a and b), respectively) and measure their average diameter. The membranes were found to be free from any deformation such as beads and breakage. The incorporation of CDs resulted in reduced fiber diameter by 10% when calculated using the ImageJ software from 557 to 497 nm for PAN and PAN-CD, respectively. This demonstrates that the addition and crosslinking of CDs with PAN

Conclusions

We have successfully demonstrated that natural cyclodextrin polymers enhance the adsorption capacity of polyacrylonitrile nanofiber membranes when applied in the removal of undesirable bromophenol blue and atrazine in water systems. The improved adsorption capacity of electrospun PAN nanofiber membranes is credited to the incorporation of β-CD monomers into the polymer matrix via a desirable crosslinking process using citric acid as a crosslinker. CDs resulted in reduced average fiber sizes (497

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

The authors would like to gratefully acknowledge The University of South Africa, in particular the Institute for Nanotechnology and Water Sustainability, Department of Physics (UNISA), and the Sultan Qaboos University (Oman) for allowing access to their facilities and for financially supporting this work. The National Research Foundation (South Africa) is also highly acknowledged for funding.

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