Adsorption mechanism of carboxymethyl cellulose onto mesoporous mustard carbon: Experimental and theoretical aspects

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Abstract

Carboxymethyl cellulose (CMC) is a versatile polymer for several industrial applications such as oil drilling, detergents, food and beverage, papers, ceramics, coatings, and other promising utilization. However, the deficiency for exploring the binding mechanism of the CMC onto the solid surfaces in aqueous system retards its applications. Herein, the binding aspect of CMC onto mesoporous mustard carbon (MMC) was illuminated where the BET analysis reveals that MMC had a surface area (SBET) = 16.576 m2 g−1, which was primarily contributed by mesopores (average pore diameter and total pore volume of MMC were found to be 12.432 nm and 0.051 cm3 g−1, respectively). The electrokinetic analysis demonstrated that the surface of the MMC is negatively charged with point of zero charge value 9.8 which favor the adsorption of CMC at particular pH value (pH = 3.0). The adsorption free energy of CMC was found as -22.561 kJ mol−1 which was in close agreement with H-bond energy suggesting hydrogen bonding as a governing parameter for CMC L2 type adsorption (also confirmed by high-resolution X-ray photoelectron spectrophotometer, attenuated total reflection Fourier transformation infra-red and urea test). A theoretical model of adsorbent-adsorbate complex (MMC-CMC) was considered in order to probe the interaction between the both. The structural, stability, electronic, and charge transfer features have been explained on the basis of binding energy, HOMO-LUMO gap, natural population analyses, Bader’s QTAIM analysis using DFT approach. The molecular electrostatic potential surface is calculated in probing the reactive sites of the system. Different forms of CMC aggregates stabilized by the HB(s)/π…π stacking, demonstrate varied structural and electronic properties. Such variations could have a significant assessment on the performance of food and pharmaceutical industries. Thus, an apparent overview of the binding aspects of CMC onto the MMC can endorse CMC as a potential adsorbate for various industrial applications.

Introduction

Carboxymethyl cellulose (CMC), carboxymethyl groups conjugated to hydroxyl groups of the glucopyranose on the cellulose backbone which is naturally occurring anionic polysaccharide with the characteristics properties for example, water-solvent, non-harmful, biocompatible and biodegradable [1]. It has used as a viscosity regulator, thickening agent, sizing agent and coating agent and emulsion stabilizer, electrode binder in various industries [[2], [3], [4]]. It is extensively used as a depressant, dispersant as well as flocculants in mineral processing especially in the flotation of sulfides ores [5]. An important industrial, analytical derivative of the CMC is its sodium salt, called as cellulose gum [[6], [7], [8]]. The polymeric structure of CMC has several repeating units in which three OH groups present on each unit of CMC are responsible for H-bonding [9]. The research on the binding mechanism of the CMC onto metallic surfaces is continue to improve year by year as significant work has been devoted to adsorption of CMC onto the mineral surfaces. Several binding phenomenon have been developed, but as yet no common mechanism has received general acceptance. Thus, the associated problems have received adequate attention. Raju reported the adsorption of polysaccharides (dextrin) onto graphite is mainly due to hydrophobic-hydrophobic interaction [10]. Laskowski’s investigation on adsorption of CMC on graphite reported that pure high-quality graphite (Ceylon) adsorbs CMC strongly [11]. However, the same graphite which was purified by leaching processes experimentally does not show the adsorption of CMC almost at all. Pugh, Healy and Rath suggested that adsorption of CMC at mineral water surfaces is due to the complex interplay of electrostatic, hydrophobic, hydrogen bonding and chemical interactions, although, the selective adsorption behavior of CMC on to the minerals has not been demonstrated clearly [[12], [13], [14], [15], [16]]. The binding mechanism of CMC on minerals was also proposed by Jenkins and according to them adsorption occurs mainly at the basal planes via hydrophobic force [[17], [18], [19]]. Contrary to this, Rath and Jucker proposed the binding mechanism of polysaccharides onto solids is mainly due to hydrogen bond [20,21]. Based on infrared spectroscopy Bakinov observed that there are fundamental changes in the carboxyl group upon adsorption on a solid surface and reported that these spectral changes due to the interaction of carboxyl group with metal ions present on the surface of adsorbent (talc) [22]. However, the infrared spectra were observed using a transmission with dried samples causing any deduction on polymer binding mechanisms to be overshadowed by the substantial alteration to the polymer’s environment during dehydration. Furthermore, no infrared spectra were actually presented in this work. Wang also used infrared spectroscopy using a dry sample to support hydrogen bonding hypothesis [23]. On the basis of in situ film ATR FTIR spectroscopy (in liquid form) Cuba suggested adsorption of polymer on solid surfaces occurs via two different types of interactions i.e. chemical complexation and hydrophobic interaction [24]. Fujimoto also studied adsorption behaviour of CMC on amino-terminated surfaces and reported that CMC adsorbs onto amino-terminated surfaces driven by electrostatic interactions [25].

Aforesaid findings tried to illustrate the interaction mechanism of CMC with solid surfaces (hydrophobic and hydrophilic minerals) talc, metal particles, or hydrophobic self-assembled monolayers [[26], [27], [28], [29], [30], [31], [32]]. Except for adsorption on inorganic materials, the binding mechanism of CMC with cellulosic surfaces was investigated in detail to increase the paper strength, to inhibit vessel picking in papermaking, to modify textile fibres or to irreversibly attach functional molecules [[33], [34], [35], [36], [37], [38], [39], [40]]. A number of those surface modification strategies with CMC are primarily based on the assumption that there may be a selective CMC − cellulose interaction which originates from structural similarities among the polyelectrolyte and the biopolymer's surfaces. Some other interaction studies of CMC, for instance, cellulose fibres or spin coated regenerated cellulose model films from trimethylsilyl cellulose by applying a quartz crystal microbalance (QCM-D) and surface plasmon resonance (SPR) Pure cellulose (CE), cellulose acetate (CA), partially deacetylated cellulose acetate (DCA), polyethene terephthalate (PET), and cycloolefin polymer (COP) model films were studied by several researchers [[41], [42], [43], [44], [45], [46], [47]]. Also, the hydrogen bonding structure in cellulose was studied intensively by Chanzy, Kondo and Ozaki [[48], [49], [50]]. In contrast to the structure of cellulose, the structure of the carboxymethyl cellulose sodium salt (NaCMC) was studied by Wu [51].

The present literature shows notable adsorption phenomenon of CMC on different adsorbents but substantial mechanistic aspects of the interactions between CMC and solid surface (MMC in the present case) is lacunae in the literature. Therefore, the present work illustrates in-depth study of binding aspect of CMC onto mesoporous mustard carbon (MMC) using various characterization techniques.

Section snippets

Materials

The de-oiled mustard cake (DOMC) material was purchased from a local mill, Shyam Enterprises, Lucknow, India. CMC was purchased from Akshar Exim Company Private Limited, Kolkata. Hydrogen peroxide, cetrimide, sodium chloride, sodium hydroxide, potassium chloride, urea were purchased from Bionic Enterprises, India. The DOMC was crushed and washed with distilled water followed by drying in sunlight. This material was treated with 20 wt% H2O2 at 60 °C for 24 h to oxidize the adhering organic

Charge measurement of MMC

The results obtained from the electrokinetic study shown in Fig. 1, illustrated that the surface of the MMC is negatively charged. Since PZC of adsorbent has been found 9.8, therefore at any pH below pHZPC, the surface is positively charged and at pH above pHZPC, the surface is negatively charged. When the pH of the solution is lowered than pHZPC, the repeat units of polysaccharides are more easily attracted by the positively charged surface of MMC, favouring the adsorption of CMC on the MMC

Kinetic studies

Some of the important widely used kinetic models such as first order, second order, pseudo-first order, and pseudo-second-order were applied to the adsorption kinetic study in order to observe the behaviour of CMC adsorption onto MMC. It was found that initially, the amount of adsorption percentage of CMC occurs fast and at equilibrium time, becomes slow because in initial time huge numbers of unoccupied surface sites of MMC were available for CMC adsorption.

Computational details

Computational experiments rooted in the density functional theory (DFT) play an important role in understanding the detailed atomistic features governing the performance of interface study of two interacting species. In order to fill this void, here we present a quantum chemical study about the intermolecular interaction(s) between adsorbent (functionalized coronene acquired from MMC) and adsorbate [dimer unit (α-D glucose) of the carboxymethyl cellulose (CMC)] nanostructure by choosing a model

Conclusions

The present study leads to important conclusions that the BET analysis reveals that MMC had a surface area (SBET) = 16.576 m2 g−1, which was primarily contributed by mesopores. The average pore diameter and total pore volume of MMC were found to be 12.432 nm and 0.051 cm3 g−1, respectively confirm the mesoporous nature of the MMC accordance with IUPAC classification of materials. It was observed that the adsorption of CMC on MMC affected by changes in pH and ionic strength. These observed

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are thankful to Indian Institute of Technology, Kanpur for XPS, XRD and computational facilities. Sophisticated Instrumental Lab, Department of Chemistry, Babasaheb Bhimrao Ambedkar University, Lucknow for BET surface characterization facility. The financial support to (A.K.) from University Grants Commission, New Delhi (173/Conf./DAC/BBAU, 30/09/2016) is gratefully acknowledged.

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