Journal of Nanomaterials
Volume 2019 (2019), Article ID 7232715, 11 pages
https://doi.org/10.1155/2019/7232715

Research Article

Borax-Cross-Linked Guar Gum-Manganese Dioxide Composites for Oxidative Decolorization of Methylene Blue

Rohan S. Dassanayake,1,2 Erandathi Rajakaruna,¹ and Noureddine Abidi¹

¹Fiber and Biopolymer Research Institute, Department of Plant and Soil Science, Texas Tech University, Lubbock, TX 79409, USA
²Department of Chemistry, Ithaca College, Ithaca, NY 14850, USA

Correspondence should be addressed to Noureddine Abidi; noureddine.abidi@ttu.edu

Received 16 July 2018; Accepted 13 November 2018; Published 31 January 2019

Academic Editor: Paulo Cesar Morais

Copyright © 2019 Rohan S. Dassanayake et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Borax-cross-linked guar gum-manganese dioxide (GGB-MnO₂) composite was synthesized using an environmentally friendly synthesis route and investigated for its efficiency of decolorizing methylene blue (MB) dye solution by an ultraviolet-visible (UV-Vis) spectrophotometric study. The GGB-MnO₂ composite was characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray (EDX) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The composite (1.2 g/L) exhibited excellent oxidative decolorization of MB (30 mg/L, 50 mL) solution to over 99% in 6, 13, and 40 min at pH 4, 7, and 10, respectively. The complete decolorization of MB occurred via a catalytic adsorption-oxidation-desorption mechanism. The GGB-MnO₂ composite showed very good reusability and was stable after ten successive cycles with negligible losses of the decolorization efficiency.

1. Introduction

Industries including textile, paper, printing, leather, and paint use excessive amounts of water for processing and have the potential to contaminate waterways through the discharge of organic dyes into natural water resources and water treatment systems. Over 10000 different textile dyes with an estimated annual production of over tons are available commercially [1]. Most of these organic dyes are excessively used and 10–20% are directly discharged as aqueous effluents into different water bodies [2]. Discharging of organic dyes into water bodies has raised both acute and chronic concerns to the ecosystems and human health. For example, the release of those organic dyes can give rise to eutrophication, nonaesthetic pollution, and imbalance in the aquatic biological systems and also causes chronic toxicity, carcinogenicity, and neurotoxicity towards humans and animals. In fact, the presence of a very low concentration of dye in water (>1 mg/L for some dyes) is highly visible and enough to introduce an aesthetic problem [3].

Organic dyes are mainly categorized as anionic, cationic, and nonionic dyes [4]. The major anionic dyes are divided into direct, reactive, and acid dyes, and the most problematic ones are the brightly colored, water-soluble reactive, and acid dyes as these dyes cannot be removed through conventional treatment systems. The cationic dyes are basic dyes including anthraquinone disperse, azo, and reactive dyes. Anthraquinone-based dyes are bioaccumulative and resistant to degradation due to their fused aromatic ring structure. Nonionic dyes are disperse dyes that do not ionize in the aqueous environment. Organic dyes consist of chromophores and auxochromes where chromophores determine the color of the dye while the auxochromes determine the intensity of the color [5]. Methylene blue (MB) is a heterocyclic cationic dye, used extensively in textile, leather, and paper industries. The existence of MB in wastewater can cause vomiting, profuse sweating, nausea, diarrhea, permanent burn effects of the eye, mental health disorder, and methemoglobinemia [6, 7]. Hence, the remediation of industrial wastewater is imperative before it becomes detrimental to the biological systems. At present, there is a critical need of research that enables new means of inexpensive, reusable, environmentally, and energetically sustainable wastewater management systems for wastewater treatment.

Commonly investigated technologies for removing organic dyes from wastewater include chemical precipitation and adsorption [8, 9], electrochemical oxidation and reduction [10], aerobic and anaerobic treatment [11, 12], coagulation and flocculation [13, 14], membrane separation [15], ultrafiltration [16], H₂O₂/ultraviolet (UV) and photocatalysis [17–19], ion exchange [20], sonochemical degradation [17], Fenton and heterogeneous Fenton-like catalysis [21–23], and electrolysis [24]. However, many of these methods are accompanied by various constraints such as sludge generation, adsorbent regeneration, membrane fouling, high operational cost, long operational time, and ineffectiveness in the complete removal of polycyclic aromatic compounds. Recently, new oxidation processes including advanced oxidation processes (AOPs) have attracted interests as emerging alternative techniques for the treatment of industrial effluents [25, 26]. These processes are involved in highly reactive species which have high redox potentials and, hence, can cause complete destruction of organic pollutants from wastewater [25]. Moreover, oxidation processes also show many advantages over conventional techniques including no sludge disposal, one-step decolorization and complete degradation of organic pollutants into small ions, large reactivity, low cost, minimal handling, and reusability.

Metal oxide nanoparticles (NPs) such as titanium dioxide (TiO₂) [18, 27], zinc oxide (ZnO) [28], nickel oxide (NiO) [29], cerium dioxide (CeO₂) [30], copper oxide (CuO) [30, 31], and manganese oxides (MnOx) [32] have been widely studied in oxidation processes. Among those, MnOx NPs have attracted special attention for the removal of organic dyes due to their unique structure and physicochemical properties [32, 33]. MnOx are ubiquitous, nontoxic, highly reactive catalysts present in soils and sediments, desert varnishes, rock dendrites, freshwater bodies, and ocean floors [32]. MnOx are among the strongest oxidants in natural environment with high reducing potential between 1.27 and 1.50 V [34]. Hence, MnOx are capable of oxidizing inorganic compounds [35, 36] and a wide range of natural and xenobiotic organic contaminants including proteins [37], catechol [38], quinines [39], polyphenols [40], aromatic amines [41], and phosphonates [42]. However, the use of MnOx particles has several problems such as self-aggregation and precipitation of NPs due to small size and large surface area, leaching of NPs with treated effluent, and difficulty in separating from the solution [43]. To evade these limitations and to facilitate their recovery and reuse, the assembly of NPs onto various solid organic supports has been proven to be an effective strategy. Due to synergetic effects, these organic-inorganic composites exhibit multifunctionalities compared to the free nanomaterial counterparts. Natural biopolymers including cellulose [44, 45], chitin [46], and chitosan [47, 48] have been investigated as excellent solid organic supports for MnOx NPs due to their high surface area, low density, environmental friendliness, relative abundance, and high fiber strength. Recently, biopolymer-based inorganic-organic composites have been utilized for oxidation processes in dye removal [45, 46]. Polysaccharides such as guar gum (GG) and starch have also been studied in wastewater treatments as flocculants and adsorption agents [49–52]. In contrast to long-chain polymers such as cellulose, chitin, or chitosan, polysaccharides have very high swelling capacities because of the hydrophilic nature of repeating polysaccharide units. Interestingly, polysaccharides can form water-induced interlinked networks during the adsorption permitting fast diffusion of pollutants. However, the utilization of polysaccharides as supporting matrices for NPs has not been extensively investigated.

GG is a naturally occurring polysaccharide derived from the seeds of the guar plant Cyamopsis tetragonolobus (cluster bean) which is made up of a linear backbone of β-(1,4)-linked D-mannose units and with the presence of randomly attached α-1,6-linked galactose units as side chains (see Figure 1). These galactose units in the GG facilitate its solubility in water. GG is also a biodegradable, renewable, nontoxic, low-cost, and readily abundant material and has been used in a range of applications including water treatment [49], food [53], pharmaceutical [54], drug delivery [55], hydraulic fracturing [56], and textile printing [57]. Borax is an effective cross-linker for polysaccharides having hydroxyl functional groups where the borax ions hold the polymer chains together by both physical and chemical linkages [58]. Consequently, the polysaccharide networks form a three-dimensional hydrogel structure extending throughout the solution. Cross-linked polymer exhibits much higher adsorption capacities compared to its native form. In the current study, we report on the use of borax-cross-linked guar gum (GGB) as a host for MnO₂ NPs and its ability to remove the model dye MB. GG and GG-based composites have been mainly investigated for adsorptive decolorization in dye wastewater treatments. To the best of our knowledge, this may be the first attempt to utilize GGB-MnO₂ composite for the oxidative decolorization of the organic dye MB.

Figure 1: Structure of guar gum.

2. Experimental

2.1. Materials

Guar gum (GG) and borax were purchased from Sigma-Aldrich (Saint Louis, MO). Sodium hydroxide (NaOH, ≥98%), hydrochloric acid (HCl, 37%) potassium permanganate (KMnO₄, ≥99%), acetone (C₃H₆O, ≥99.5%), and manganese dioxide (MnO₂, 85%) were purchased from Fisher Scientific (Pittsburgh, PA). Methylene blue was purchased from ACROS (Morris Plains, NJ). Distilled water was purchased from AquaOne (Amarillo, TX).

2.2. Synthesis of Borax-Cross-Linked Guar Gum Composite

Borax-cross-linked guar gum (abbreviated as GGB) composite was prepared using a previously described method [49]. In a typical synthesis, 1 g of guar gum powder was completely dispersed in 80 mL of distilled water. Then, the solution was kept at room temperature (RT) for 2 h to hydrate and achieve maximum viscosity. 0.2 g of cross-linker borax (20% weight of GG) was separately dissolved in 60 mL of distilled water and added to the GG solution. The mixture was mechanically stirred at RT for 30 min and left for another 4 h without any mechanical stirring. The resulting yellow-color material is the borax-cross-linked guar hydrogel. This synthesized hydrogel was crushed and gently washed with distilled water to remove unreacted borax. Then, the crushed hydrogel was soaked in acetone solution overnight for dehydration. Dehydrated hydrogels were filtered and kept in the oven at 45°C until a constant weight is attained.

2.3. Synthesis of Borax-Cross-Linked Guar Gum-Manganese Dioxide (GGB-MnO₂) Composite

GGB-MnO₂ composite was synthesized using a modified literature method [46]. First, 6.75 mL of 0.25 mol·L⁻¹ alkaline permanganate ( KMnO₄ : NaOH) solution was mixed with 100 mL of distilled water. GGB (1 g) was added to the solution and mechanically stirred with a stirring speed of 500 rpm at RT overnight to obtain in situ-grown GGB-MnO₂ composite. The brown-color composite was thoroughly washed with distilled water to remove unreacted residuals. Finally, the composite was dispersed in distilled water by constant stirring with a stirring speed of 500 rpm. The sample was kept wet to avoid irreversible agglomeration of the hybrid [45]. The solid content of the wet slurry was 60% (w/w).

The mechanism for the formation of MnO₂ from the reaction between various polysaccharides and KMnO₄ has been previously reported [59, 60]. During this reaction, permanganate ion acts as a strong oxidizing agent and involves the oxidation of GG [59, 60]. The overall reaction mechanism involves the formation of keto-GG and MnO₂ nanoparticles as final products. The generation of MnO₂ occurs via two steps: (1) formation of a transient manganese (VI) species and its gradual build up in the solution and (2) a slow decay of the transient species to generate MnO₂ nanoparticles [60]. These particles then deposit and form a thin layer of MnO₂ on the surface of guar gum [60]. The final color of the GGB-MnO₂ composite is brown.

2.4. Ultraviolet-Visible (UV-Vis) Spectrophotometry

UV-Vis spectrophotometric measurements were conducted using a PerkinElmer Lambda 650 spectrophotometer running with PerkinElmer UV WinLab software. All data were analyzed using Microcal Origin version 8.0.

2.5. pH Measurements

pH measurements were carried out at RT using a Fisher Scientific™ accumet™ AB15 Plus pH meter equipped with benchtop stirring probe. The electrode was filled with a 3 M saturated KCl solution (pH 7) and standardized with RICCA precision reference standard solutions at pH 4, 7, and 12. Solution pH was adjusted using HCl (0.10 M) or NaOH (0.05 M) solutions as necessary.

2.6. Decolorization of Methylene Blue (MB)

Decolorization of MB was studied using UV-Vis spectroscopy. In a typical experiment, 100 mg of wet GGB-MnO₂ composite (60% (w/w) solid) was added to a 100 mL beaker containing 50 mL of 30 mg/L MB solution (note that the composite was kept in the wet form in order to prevent the aggregation of small nanoparticles during the drying). The solution was magnetically stirred with a stirring speed of 500 rpm and at predetermined time intervals (30 sec, 1 min, and 2 min at pH 4, 7, and 10, respectively); an aliquot of MB solution (3 mL) was suctioned off and filtered through 0.20 μm syringe filter. Then, the concentrations of the solutions were measured at absorbance maxima,  nm. For comparison purposes, control experiments were also conducted in parallel with 100 mg of GGB and 100 mg of bare MnO₂ under the same experimental setup as described above. The final concentration of MB was determined using a calibration plot (see Figure S1, Supplementary Materials). The same experiment was repeated three times. Reported decolorization efficiencies are the average values of at least three independent measurements.

2.7. Recycling Experiment

Recycling experiments were conducted at pH 4. After the first cycle, the GGB-MnO₂ composite was filtered and thoroughly washed with distilled water several times and then dried in the oven at 60°C for 5 h. The dried composite was reused for decolorization with a MB concentration of 30 mg/L.

2.8. Fourier-Transform Infrared (FTIR) Spectroscopy

FTIR spectroscopy of the GG, GGB, and dry GGB-MnO₂ samples was recorded using a PerkinElmer Spectrum-400 FTIR spectrometer equipped with a universal attenuated total reflectance (UATR) accessory (PerkinElmer, Waltham, MA). All FTIR spectra were collected at a spectral resolution of 4 cm⁻¹, with 32 coadded scans in the wavenumber range of 4000 to 650 cm⁻¹. The spectra were analyzed using PerkinElmer Spectrum software.

2.9. X-Ray Diffraction (XRD) Analysis

The wide-angle powder X-ray diffraction patterns of the samples were recorded on a SmartLab XRD system (Rigaku Corporation, Model HD2711N) with CuKα radiation ( Å). The accelerating voltage and tube current used were 40 kV and 44 mA, respectively. A continuous scanning was performed at a scan speed of 2°/min, and the 2θ ranged from 10° to 70°.

2.10. Thermogravimetric Analysis (TGA)

Thermogravimetric (TG) analysis of GG, GGB, and GGB-MnO₂ were obtained using a PerkinElmer Pyris1TGA instrument (PerkinElmer, Waltham, MA) equipped with a 20-sample autosampler. TG profiles were recorded in an inert nitrogen atmosphere (20 mL/min) from 37°C to 600°C with a constant heating rate of 10°C/min using a high-resolution mode. All data were analyzed using Pyris Data Analysis software.

2.11. Scanning Electron Microscopy (SEM)

The morphology of GG, GGB, and GGB-MnO₂ was studied on a Hitachi S-4700 field emission scanning electron microscope (FESEM) equipped with a GENE-SIS4000 energy-dispersive X-ray (EDX) spectroscopy. Resolutions of 1.5 nm at 15 kV and 2.5 at 2 kV can be achieved at 12 mm and 3 nm working distance, respectively.

All samples were conditioned in an environmentally controlled laboratory maintained at a relative humidity of % and temperature of °C for at least 48 h prior to their characterization.

3. Results and Discussion

3.1. XRD Studies

XRD studies were performed to investigate the crystalline properties of GG, GGB, and GGB-MnO₂ samples. All samples display different behaviors in the crystalline structure. XRD patterns of GG, GGB, and GGB-MnO₂ are shown in Figure 2. The GG sample showed a characteristic peak at , and this is in agreement with the diffraction patterns reported for pure GG at and 72.2° [61]. GGB exhibited slightly shifted broad peak at 18.2° with no distinguishable peaks confirming the formation of cross-linked GG. The GGB-MnO₂ displayed a different XRD pattern with significantly shifted and a reduced main peak. As shown in Figure 2, GGB is low in crystallinity with a broad and weak XRD peak. Upon deposition of MnO₂, the diffraction peak of GGB became much weaker and obscured leading to a highly amorphous structure.

Figure 2: XRD patterns of GG, GGB, and GGB-MnO₂.

3.2. FESEM and EDX Analysis

FESEM analysis was carried out to observe the surface morphological characteristics of the sample studied. Figure 3 shows the FESEM micrographs of GG, GGB, and GGB-MnO₂ composites and the EDX analysis of GGB-MnO₂ composite. GG powder exhibits a rough surface morphology with no crosslinking between the GG granules, see Figure 3(a). However, GGB composite has a distinct flake-like surface morphology with a smoother surface (see Figure 3(b)). The morphology of GGB-MnO₂ surface is significantly different from GGB. As shown in Figure 3(c), the MnO₂ nanoparticles are completely dispersed and anchored intimately on the surface of the GGB-MnO₂ composite with a broad distribution of the particle size. This suggests strong interactions between the GGB surface and MnO₂ nanoparticles. The particle size of the MnO₂ nanoparticles is in the range of 10–300 nm. The reason for the large particle sizes is due to the formation of aggregates because of irreversible agglomeration of small nanoparticles when the sample is dry. Typically, smaller-sized MnO₂ nanoparticles are expected to exhibit higher catalytic activity as compared to large aggregates. EDX analysis was performed on the GGB-MnO₂ and GGB composites to confirm the presence of B and Mn. As it can be seen in Figure 3(d), the EDX spectrum clearly displays the B, C, O, and Mn peaks at values 0.18, 0.27, 0.52, and 5.8 KeV corresponding to boron, carbon, oxygen, and manganese, respectively. The EDX spectrum of the GGB composite is shown in Figure S2, Supplementary Materials. The EDX data further reveal the formation of borax-cross-linked guar gum (GGB) composite and the deposition of MnO₂ nanoparticles on the surface of the composite.

(a)

(b)

(c)

(d)

Figure 3: FESEM images of (a) GG, (b) GGB, and (c) GGB-MnO₂ composites. (d) The EDX spectrum of GGB-MnO₂ composite.

3.3. FTIR Analysis

FTIR spectra of GG, GGB, and GGB-MnO₂ were recorded to study and compare the changes in their chemical structure. The FTIR spectra of GG, GGB, and GGB-MnO₂ are shown in Figure 4. The presence of a broadband at 3308 cm⁻¹ is attributed to –OH stretching band of the polymer, and the peak at 2917 cm⁻¹ is assigned to C–H asymmetric stretching vibrations. The peak at 1643 cm⁻¹ is associated with –OH bending modes of adsorbed water molecules [49]. The bands at 1015, 1059, and 1147 cm⁻¹ are assigned to CH₂ twists, C–O(H) stretching, and C–O–C asymmetric stretching vibrations, respectively [49, 57]. The peak observed at 868 cm⁻¹ is due to the skeletal stretching vibrations of galactose and mannose units [61, 62]. The –OH stretching band at 3308 cm⁻¹ has reduced in both GGB and GGB-MnO₂ composites suggesting the presence of covalent bonding between borax and –OH groups of the galactomannan backbone. The peak 868 cm⁻¹ has also decreased in intensity for both GGB and GGB-MnO₂ composites and slightly shifted towards higher wavelength in the GGB-MnO₂ spectrum. This is due to the physical and chemical interaction of –OH groups with borax. The similar spectral pattern for all samples indicates that the chemical structure of guar gum is retained during the synthesis of both GGB and GGB-MnO₂ composites.

Figure 4: FTIR spectra of GG, GGB, and GGB-MnO₂ samples.

3.4. TG Analysis

The thermal stability of the GG, GGB, and GGB-MnO₂ samples was studied by TGA (see Figure 5(a)). TG profiles were obtained to determine the thermal stability of the samples studied and identify their applicability at higher temperatures. Two distinct weight loss regions were observed for pure GG. The initial weight loss region (35–150°C) is attributed to the removal of physically adsorbed water. The second region (150–600°C) is related to the thermal decomposition of the polysaccharide. Both GGB and GGB-MnO₂ samples exhibited three distinct weight loss regions. In the first temperature range of 35–150°C, the weight loss is attributed to the removal of adsorbed moisture. The major weight loss in the region (150–310°C) is assigned to the decomposition of the polysaccharide backbone. The small weight loss above 310°C (310–600°C) is due to the degradation of the cross-linked bonds between borax and hydroxyl groups of the guar side chains. The presence of a third weight loss region for both GGB and GGB-MnO₂ is an indication of the cross-linking between guar and borax. The differential thermogravimetric (DTG) profiles generated form TG results are shown in Figure 5(b). The DTG curves for GG, GGB, and GGB-MnO₂ show peaks at 302, 281, and 277°C, respectively. Our results suggest that the thermal stability is similar for both GGB and GGB-MnO₂; however, it is slightly lowered than pure GG due to the decrease in the crystallinity upon dissolution and crosslinking with borax.

(a)

(b)

Figure 5: (a) TG and (b) DTG profiles of GG, GGB, and GGB-MnO₂ samples.

3.5. Decolorization of MB

The decolorization of MB by GGB-MnO₂ was investigated using UV-Vis spectroscopy. Figure 6 shows the UV-Vis spectra as a function of time for MB solution (30 mg/L) treated with GGB-MnO₂ composite (1.2 g/L) at pH 7 (RT). The absorbance of the MB decreases rapidly as a function of time over the entire wavelength range. Interestingly, no additional peaks with any isosbestic points or shift in the peak positions were observed. This suggests that no intermediate or additional species were formed during the reaction between MnO₂ nanoparticles and MB. Therefore, these spectral changes are associated with the loss of conjugation in the heterocyclic aromatic structure, which is responsible for intense to electronic transitions. A similar experiment was conducted in parallel with GGB and bare MnO₂ particles. These control experiments show that the decrease in the absorbance of MB is significantly less as compared to GGB-MnO₂ composite (see Figures S3 and S4, Supplementary Materials).

Figure 6: UV-Vis spectra for MB solution (30 mg/L) in the presence of GGB-MnO₂ composite (1.2 g/L) as a function of time at pH 7 (RT).

The absorbance at 664 nm was used to evaluate the degree of decolorization. Figure 7 shows a plot of the normalized concentration () of MB as a function of time at pH 7 (RT) and also compares the efficiency of MB decolorization by GGB-MnO₂, GGB, and bare MnO₂. As expected, the decolorization process of MB solution is extremely fast in the presence of GGB-MnO₂ composite. The MB (30 mg/L) solution is % decolored over 3 min and almost completely decolored after a 13 min contact time, showing its effectiveness and efficiency. However, the degree of MB decolorization is % for GGB and is % for bare MnO₂ in 13 min under stirring. As can be seen in Figure 7, the GGB-MnO₂ hybrid exhibited a remarkably higher performance compared to GGB and bare MnO₂. This is due to the high surface area of the composite material providing more active sites for dye adsorption. We previously showed increased surface area for MnO₂-based composite materials as compared to the starting materials using nitrogen adsorption studies [46].

Figure 7: Plot of normalized concentration () versus time for the decolorization of MB (30 mg/L) in the presence of GGB-MnO₂ composite (1.2 g/L), GGB composite (2 g/L), and MnO₂ (2 g/L) at pH 7 (RT), respectively.

3.6. Decolorization Mechanism

The mechanism for the decomposition of the organic compounds by MnO₂ has been well established [46, 63]. Generally, the degradation is an adsorption-oxidation-desorption process which occurs via a surface mechanism [63]. The organic compounds are first adsorbed onto the surface of MnO₂ to form a surface precursor complex. Then, an electron transfer reaction occurs between the organic compound where Mn(IV) is reduced to Mn(II) while the organic reductant is being oxidized. During the reduction process of MnO₂, strong oxidizing radicals such as hydroxyl (OH∙) are also formed [39, 64]. These radicals undergo a cascade of reactions, leading to complete destruction of the chromophore. Moreover, this oxidative degradation is much favorable at low pH conditions [33]. Importantly, Mn(II) can be oxidized to Mn(IV) and precipitated as manganese hydrated oxide; hence, MnOx nanoparticles act as an active catalyst during the oxidative decolorization [63]. The main products of MB mineralization include mainly SO₄²⁻ and other inorganic ions, such as Cl⁻, NO₃⁻, CO₂, and H₂O [46].

3.7. Effect of pH on the MB Decolorization

The pH of the solution plays a vital role in the degradation of organic dyes. Therefore, the influence of pH on the degree of decolorization of MB (30 mg/L) by GGB-MnO₂ (1.2 g/L) was investigated at pH 4 and pH 10, and the data are shown in Figures S5 and S6, Supplementary Materials. Plots of the normalized concentration () of MB as a function of time at pH 4 and 10 (RT) are shown in Figures S5 and S6, Supplementary Materials. Our data suggest that 97.5% of MB was decolored in 30 s and was completely decolored within 6 min at pH 4, whereas 99.1% of MB was decolored within 40 min at pH 10. The degree of decolorization for GGB composite and bare MnO₂ at pH 4 and 10 is 6.4, 37.5% within 6 min, and 30.4, 49.7% within 40 min, respectively (see Figures S5 and S6, Supplementary Materials). The reaction time for the decolorization of MB by GGB-MnO₂ increases with the increase in the pH. At alkaline pH, the cross-linked guar gets more negative charge, facilitating higher dye adsorption due to the electrostatic attraction between the cationic MB⁺ and the borate ions and the electron-rich hydroxyl groups of GG. The pH has been found to exert double-edged effects on the decolorization of organics by MnOx [34]. The zero point of charge (pHzpc) of MnO₂ is 4.7 [63]. Therefore, the oxidizing ability of MnO₂ increases with decreasing pH. Theoretically, at lower pH (pH<pHzpc) the surface layer of MnO₂ is positively charged due to protonation. The positive charge on the surface layer would not favor the interaction between cationic MB⁺ ions and MnO₂ due to the electrostatic repulsion and increased competition between H⁺ and MB⁺ ions. Hence, the increase in the pH (pH>pHzpc) would enhance the dye adsorption because of the ionization and increased electrostatic interaction. Oppositely, the increase in pH suppresses the oxidizing power of MnO₂ [45]. Our results show that decolorization at pH 4 is more effective than that at pH 10. The reason could be that the acidic pH enhances the reduction potential of MnO₂ and thus could compensate and even predominate over MB adsorption via electrostatic effects.

3.8. Recycling Experiment

The reusability of the GGB-MnO₂ composite was evaluated over a number of adsorption-oxidation-desorption cycles. Figure 8 shows the cycle stability of GGB-MnO₂ composite on the decolorization of MB at pH 4 (RT). Our results show an extremely high degree of decolorization of above 98% for 10 cycles. This reveals a long-term stability, high performance, and efficacy of the GGB-MnO₂ composite for effective removal of MB. One of the main reasons for the high stability and reusability of this material is due to the catalytic oxidative decolorization mechanism where the MnO₂ nanoparticles that regenerated during the dye degradation process continued to react with the dye. Another reason is the stability of MnO₂ particles anchored on the borax-cross-linked guar gum (GGB) composite. Evidently, the GGB composite plays an imperative role in stabilizing MnO₂ nanoparticles and preventing the agglomeration which suppresses the catalytic activity during the repeated oxidative decolorization process. Besides, the rich hydroxyl groups in GG with high chelating ability also help to stabilize the nanoparticles and keep them from leaching out.

Figure 8: Recyclability of the GGB-MnO₂ composite at pH 4 (RT).

Several studies have been reported on guar gum-based composites for the removal of organic dyes from aqueous solutions. For instance, Shruthi et al. studied the efficiency of the guar gum-grafted acrylic acid/nanoclay composite on the adsorption of crystal violet dye and reported that it requires at least three hours to reach the maximum dye adsorption capacity of 12.5 mg/g [65]. Saxena and Sharma investigated the adsorption and kinetic studies on the removal of methyl red dye from aqueous solution using guar gum powder, which showed a monolayer dye adsorption capacity of 66.66 mg/g in three hours at pH 4.2 [66]. Pal and coworkers synthesized guar gum graft-poly(acrylamide)/silica hybrid nanocomposite and showed adsorption capacities of 579.01 mg/g within 40 min and 233.24 mg/g within 30 min for reactive blue and Congo red dyes [67]. Magnetic guar gum-grafted multiwall carbon nanotube iron oxide (GG-MWCNT-Fe₂O₃) composite was investigated for the removal of neutral red (NR) and methylene blue (MB) by Yan and coworkers [68]. They reported maximum adsorption of NR, and MB reached 89.85 mg/L at 20 min and 61.92 at 120 min [68]. Sharma et al. synthesized guar gum-polyacrylic acid-based cross-linked (cl) hydrogels (GG-cl-poly AA) with interpenetrating networks (IPNs) of polyaniline and investigated their percent adsorption of MB with respect to contact time, adsorbent dosage, pH, and temperature [51]. Gupta and coworkers studied the removal of MB by guar gum-cerium(IV) tungstate nanocomposite (GG/CTNC) and showed that an optimum adsorption of MB onto GG/CTNC is 16 mg/g over 120 min [69]. Borax-cross-linked guar gum hydrogels were developed and investigated for the removal of aniline blue dye by Thombare et al. [49] where they showed a maximum decolorizing efficiency of 94% within 60 min [49].

Among those studies reported on guar-based materials, adsorption method has been preferred for the decolorization of organic dyes because of its simplicity and low cost. However, the adsorption method is associated with many drawbacks including high regeneration cost, loss of adsorption efficiency after regeneration, the hazardous nature of the used adsorbent, and require longer times. Albeit oxidative decolorization processes have distinct advantages over adsorptive methods, there is scant literature about the utilization of guar gum-based materials for oxidative decolorization processes. Importantly, our results show that GGB-MnO₂ composite has higher decolorization efficiency and fast reaction time as compared to previous studies reported on GG-based materials used for adsorption. Therefore, due to fast response time, zero accumulation of organics, reusability, and simplicity of the synthesis route, the low cost and green nature of GGB-MnO₂ composite make this material a promising candidate for large-scale dye removal from industrial wastewater.

4. Conclusions

This work reports the synthesis of borax-cross-linked guar gum-manganese dioxide (GGB-MnO₂) composite for efficient oxidative decolorization of methylene blue (MB) dye using UV-Vis spectroscopy. The presence of MnO₂ on the GGB composite showed superior decolorization activity and degradation of MB. The reaction times for the complete oxidative decolorization efficiency (<99%) of MB (30 mg/L) onto GGB-MnO₂ (1.2 g/L) are 6, 13, and 40 min at pH 4, 7, and 10, respectively. The reusability studies showed that the GGB-MnO₂ is stable even after 10 adsorption-oxidation-desorption cycles. Because of the environmentally benign synthesis route and dual adsorptive and catalytic oxidative decolorization mechanism and efficiency, GGB-MnO₂ composite exhibits potential for the remediation of industrial dye wastewater.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that they have no conflict of interest.

Acknowledgments

The authors would like to thank Dr. Yang Hu for conducting the EDX analysis and the Fiber and Biopolymer Research Institute (FBRI) at Texas Tech University.

Supplementary Materials

Figure S1: the calibration plot for methylene blue (MB) at 664 nm. The concertation of the MB solution was determined using the calibration plot. Figure S2: the EDX spectrum of borax-cross-linked guar gum (GGB) composite in the absence of MnO₂. The EDX spectrum clearly displays the peaks at 0.18, 0.27, and 0.52 KeV corresponding to boron, carbon, and oxygen, respectively. Figure S3: the UV-Vis spectra over 13 min for MB solution (30 mg/L) treated with GGB (1.2 g/L) at pH 7.00 (RT). The absorbance of the MB decreases as a function of time over the entire wavelength range. Figure S4: the UV-Vis spectra over 13 min for MB solution (30 mg/L) treated with bare MnO₂ particles (1.2 g/L) at pH 7.00 (RT). The absorbance of the MB decreases as a function of time over the entire wavelength range. Figure S5: a plot of the normalized concentration (C/C₀) of MB as a function of time at pH 4.00 (RT) and also a comparison of the efficiency of MB decolorization by GGB-MnO₂, GGB, and bare MnO₂. The degree of decolorization for GGB-MnO₂, GGB, and bare MnO₂ at pH 4.00 after 6 min is 99.9, 6.4, and 37.5%, respectively. Figure S6: a plot of the normalized concentration (C/C₀) of MB as a function of time at pH 10.00 (RT) and also a comparison of the efficiency of MB decolorization by GGB-MnO₂, GGB, and bare MnO₂. The degree of decolorization for GGB-MnO₂, GGB, and bare MnO₂ at pH 4.00 after 40 min is 99.1, 30.4, and 49.7%, respectively. (Supplementary Materials)

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