Fast and highly efficient catalytic degradation of dyes using κ-carrageenan stabilized silver nanoparticles nanocatalyst

Journal Pre-proof Fast and highly efficient catalytic degradation of dyes using κ-carrageenan stabilized silver nanoparticles nanocatalyst Sadanand Pandey, Jeong Yeon Do, Joonwoo Kim, Misook Kang

PII:

S0144-8617(19)31265-2

DOI:

https://doi.org/10.1016/j.carbpol.2019.115597

Reference:

CARP 115597

To appear in:

Carbohydrate Polymers

Received Date:

9 September 2019

Revised Date:

30 October 2019

Accepted Date:

9 November 2019

Please cite this article as: Pandey S, Do JY, Kim J, Kang M, Fast and highly efficient catalytic degradation of dyes using κ-carrageenan stabilized silver nanoparticles nanocatalyst, Carbohydrate Polymers (2019), doi: https://doi.org/10.1016/j.carbpol.2019.115597

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Fast and highly efficient catalytic degradation of dyes using κ-carrageenan stabilized silver nanoparticles nanocatalyst Sadanand Pandeya*, Jeong Yeon Doa, Joonwoo Kimb, Misook Kanga* a

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Department of Chemistry, College of Natural Sciences, Yeungnam University, 280 Daehak-

Ro, Gyeongsan, Gyeongbuk, 38541, Republic of Korea b

Particulate Matter Research Center, Research Institute of Industrial Science & Technology

Corresponding author. Email address: [email protected]; [email protected]

AUTHOR INFORMATION

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*

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(RIST), 187-12, Geumho-ro, Gwangyang-si, Jeollanam-do, 57801, Republic of Korea

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Corresponding Author *Phone: +82-10-6286-7573; e-mail: [email protected]; [email protected]

Sadanand

Pandey

(ORCID-0000-0003-2065-897X);

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[email protected] Misook Kang (ORCID-0000-0002-7199-9881)

Highlights (for review) 

κ-Carrageenan gum stabilized AgNPs nanocatalyst prepared via a greener approach.

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Physicochemical properties of nanocatalyst were investigated.

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κ-CG-s-AgNPs are spherical in shape, with average size of 12 nm.

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Nanocatalyst exhibit fast and superior catalytic performance against organic dyes

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The catalyst showed excellent stability and good reusability.

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

ABSTRACT

Herein, we have reported the synthesis of silver nanoparticles (AgNPs) nanocatalyst by using high-molecular-weight κ-carrageenan. The developed methodology was rapid, facile,

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ecofriendly and cost effective, which did not require subsequent processing for reduction or stabilization of AgNPs. The physico-chemical characterization was performed by FT-IR, zeta

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potential (ζ), XRD, TGA, XPS and TEM techniques. The TEM results revealed that the AgNPs were spherical in shape, with average size of 12 nm, and face centered cubic (FCC) structure throughout the polymer matrix and was stable without any protecting or capping

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reagents over two months. The synthesized nanocatalyst exhibited high catalytic degradation and mineralization of industrially important organic dyes such as Rhodamine B, and

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methylene blue, with a degradation efficiency of ∼100% in a very short interval. The fast kinetics of the dye degradation is quite unique compared to the reported literatures based on

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various catalyst systems where slow kinetics was reported.

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Keywords: κ-Carrageenan gum; silver nanoparticles; Rhodamine B dye; methylene blue dye; Catalytic Degradation

Chemical compounds: Kappa-Carrageenan (PubChem CID: 11966249) 2

Silver nitrate (PubChem CID: 24470) Methylene blue (PubChem CID: 6099) Rhodamine B (PubChem CID: 6694) Sodium borohydride (PubChem CID: 4311764)

1. INTRODUCTION Synthetic fabric dyes and other industrial dyestuffs symbolize crucial group of chemical

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compounds produced in the world. These dye effluents as soon as released in rivers, contaminate water as well as surrounding environment. It was discovered that these dyes when collected in water bodies’ results in several problem in aquatic system such as

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eutrophication, reduce sunlight penetration in water bodies that affects the photosynthetic activities of aquatic flora and in turn hindering the nutrition supply of aquatic life types. It

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also results in gradual increase in the biological oxygen demand (BOD) of water by

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enhancing the turbidity of water and destroying the food web (Faisal, Tariq, & Muneer, 2007). It is well known that dyes are highly complex, stable toxic pollutants within the atmosphere

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over an extended period and exhibits antimicrobial properties. Therefore, they are not readily degradable and not usually not aloof from water by wastewater treatments and also several

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other conventional methods like adsorption, ultrafiltration, chemical and electrochemical methods (Tang & An, 1995). Photocatalytic degradation and Fenton-like process have been

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widely applied in the treatment of organic pollutants (Nie et al. 2019; Chen et al. 2018; Liu & Zhao, 2000). But it was still found that photocatalysis or Fenton-like process has some drawbacks that limit their efficiency in the removal of organic pollutants. Such as issues of low quantum yield and demand of UV light, while Fenton-like process alone is often limited in working at low pH levels to avoid catalyst precipitation (Martín-Sómer et al., 2018; 3

Clarizia et al., 2017). These associated drawbacks prevent its usability in realistic remediation technology. Although in order to overcome this drawbacks, photocatalytic Fenton-like system is recently presented by the researchers to improve the catalytic performance for the removal of organic pollutants (Yi et al., 2018; Yi et al., 2019). But still they have certain limitation. Thus, there is high scope for exploring and developing varied strategies for the quick and economical degradation of dyes.

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Nanoparticle derived dye deterioration provides several benefits corresponding to rapid reduction but no development of polycyclic compounds. Later, several metal and metal oxide nanoparticles (NPs) like Ag, Au, CuO, ZnO and TiO2 have derived awareness on account of

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their potential to degrade varieties of dyes such as Congo red (CR), methylene blue (MB), gentian violet dye (GV), Methyl orange (MO, alizarin red (AR) etc (Bhatt et al. 2019; Su et al.

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2019; Kamarudin et al. 2018; Kanagamani et al. 2019; Yugandhar et al. 2019; Pandey, 2017; Ulaah, & Dutta, 2008; Liu et al., 2000). However, it has been scrutinized from the literature

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that the process of catalytic degradation of dyes is very slow compared to the waste generation of the dyes from industries. Most of the previous reported catalysts suffer from

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one or more than one drawbacks such as used of UV light, H2O2, elevated temperature, low concentration of dyes etc. It has been shown in Table 1 that rate of catalytic degradation of

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various dyes is very slow along with low concentration of dyes being used for the

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degradation as obtained from the literature. We believe that there can be scope for improved degradation kinetics even with high concentration of dyes for degradation and higher TOC value that can tie-up with industrial processes in the removal of waste dyes from the environment. Therefore, synthesis of catalyst NPs with better catalytic activities is necessary to achieve such targets. This is one of the major focuses of the current work that can be novel 4

in this field. NPs have distinctive chemical, electronic, magnetic and optical properties due to their small size and high surface area (Kamat, Flumiami, & Hartland, 1998; Pandey, 2016). From the literature, biological methods used for developing metallic NPs (MNPs) were found to be very useful in the biomedical field for purposes such as protection from harmful microorganisms, bio-imaging, cancer treatment, medical diagnosis etc. These applications of

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MNPs are possible because of their unique properties such as insulator, optics, antimicrobial, antioxidant, anti-metastasis, biocompatibility, and stability. Apart from this, MNPs are also used in several other applications such as sensor, catalysis, antimicrobial activity, drug

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delivery system, optoelectronics, magnetic devices etc. (Pandey, & Nanda, 2016; Pandey, & Mishra, 2014; Schmid, 2004). Therefore, a lot of efforts have been made recently in order to

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fabricate noble metal nanohybrids (MNHs) by different physical and chemical methods. However, poisonous reducing agents (such as sodium borohydride and hydrazine)

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are typically used within the synthesis of those MNHs (Zhang et al. 2015), that are undoubtedly toward phenomenon in the direction of progress of feasible and economically

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lucrative materials (Han et al. 2016).

Green synthesis has currently become an enormous developing area of new research groups.

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Therefore, bioreduction is known to be the safest and most convenient out of

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all accessible choices. There are numerous reviews devoted to this approach of synthesis (Tamuly et al. 2013a; Aromal, Vidhu, & Phillip, 2012; Arunachalam, Annamalai, & Hari, 2013). The phytochemicals found in plant extracts were used for the stabilization and reduction of NPs (Tamuly et al. 2013b). However NPs synthesized by all the previously mentioned approaches were released in the 5

environment that causes severe harm to the eco-system and ultimately to human being. On the other hand, supported or composites of MNPs are always been more secure as a result of their stability in the matrix especially polymer matrix. The various biopolymers such as guar gum, alginate, gum acacia, cellulose, gum tragacanth, gum acacia, starch, alginate, dextran as well as chitosan can acquire special attention. It is not because of their sustainable and biodegradable character but also because of their ability to boost the optical and mechanical

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properties in addition to delivering unique functions as per their use (Mittal and Ray, 2016; Manohar, Basak, & Singh, 2011).

One of such idea of the current work is to apply the accurate concepts of green chemistry for

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the synthesis of AgNPs by utilizing κ- carrageenan gum (κ- CG) as stand-alone reducing and simultaneously a capping agent. κ- CG is usually a seaweed‐ based polysaccharide and consists

of

3‐ o‐ linked

d‐ galactopyranose

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with

4‐ o‐ linked

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3,6‐ anhydro‐ d‐ galactopyranose (fig. S1). Essentially, there are actually three important

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types of this compound: lambda-λ (with three sulfate groups, non-gelling), iota-ι (with two sulfate groups, weak gelling) and kappa-κ (with one sulfate group, strong gelling). In the

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present research work, we have demonstrated for the first time the green-synthesis of AgNPs using a biopolymer κ- CG and studied their catalytic degradation capabilities in presence of

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organic dyes (Rhodamine B (RhD B) and methylene blue (MB)). κ- CG is abundantly

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available and also commercially viable for the production of stable and eco-friendly catalytically active AgNPs for the dye degradation process. It has been found that the kinetics of degradation of dyes was faster in the current work compared to the literature reports till date. The TOC after the degradation process was also found better compared to other reports so far. Therefore, the synthesized eco-friendly κ-CG-s-AgNPs nanocatalyst demonstrated 6

ideal catalyst which work at room temperature, show fast kinetics, degrade high concentration of dyes and reduce almost 77% of TOC from the toxic, mutagenic, and hazardous dyes at higher concentration in this novel work. 2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials Silver nitrate ≥ 99.0%, κ- CG, MB dye (C16H18ClN3S), RhD B dye (C28H31ClN2O3) and

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sodium borohydride (NaBH4 (98%)) were purchased from Sigma-Aldrich (US). All chemicals were use without any additional purification. Deionized water was use in the

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course of the experimental procedure.

2.2. Synthesis of k-Carrageenan gum stabilized AgNPs (κ-CG-s-AgNPs) nanocatalyst

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30 ml consisting of 0.12% w/v uniform solution of κ- CG was well prepared in an e

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rlenmeyer flask. After which, 10 ml of 2.5mM homogenous solution of silver nitrate was added to κ-CG solution under comprehensive stirring on magnetic stirrer at 60°C. The colour of the reaction mixture progressively transformed from transparent to light-

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yellow to finally dark brown within 100 min. A Reduction of Ag+ to Ag0 was established by the colour change of solution from colourless to dark brown. Its formation was also proven

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through the use of UV–visible spectroscopy and other techniques.

2.3. Catalytic test for reductive degradation of dyes In order to assess the catalytic activity of κ-CG-s-AgNPs nanocatalyst for reductive degradation of organic dyes in the presence of NaBH4 solution was led in a standard quartz 7

cuvette and recorded utilizing an UV-vis spectrophotometer to examine the catalytic activity of κ-CG-s-AgNPs at room temperature and atmospheric pressure. The control reactions were also carried out without κ-CG-s-AgNPs. We have determined the value of λmax for MB and RhD B dye at 665nm and 554nm respectively (Fig.S2) for calibrations. Stock solutions of organic dyes (MB and RhD B) (500mg/L), κ-CG-s AgNPs (40μL of 2.5mM), and NaBH4 (0.001 M) were prepared in distilled water. Test solutions were prepared by placing 3mL of the dyes stock solutions and an appropriate aliquot of fresh NaBH4 stock solution in cuvettes,

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adding a suitable aliquot of the as-prepared κ-CG-s-AgNPs stock. Eventually, the solution was instantly subjected to the UV-vis measurements; later on, the absorbance of the solution was in situ at every minutes in the wavelength range of 450–750 nm to detect progressive

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changes in the reaction. The catalyzed reaction was followed by measuring the time-

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dependent fall in absorbance (Abs) at 665nm and 554nm for MB and RhD B dyes

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respectively, and the reductive degradation of dyes were calculated as following Eqn. 1.

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The reaction kinetics was assessed by expecting the concentration of dyes obeying the

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pseudo-first order reaction, expressed as follows Eqn. 2,

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where A0 is the absorbance at zero time, At is the absorbance at t time and k is the rate constant.

The mineralization of MB and RhD B dyes has been measured by using the disappearance of the total organic carbon (TOC). Estimation of TOC had been done by standard methods using TOC analyzer (Elementar, Vario TOC Cube). The efficiency of dye mineralization was 8

estimated using the following Eqn. 3, (Aleboyeh et al. 2008):

Where, TOC correspond to TOC at time t and TOC0 correspond to TOC at initial conditions. After reduction was complete, the catalysts were separated, washed and the process was repeated to investigate the recyclability of the catalyst. 2.4. Characterization of κ-CG-s-AgNPs nanocatalyst

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Surface chemical characterization of κ-CG-s-AgNPs was conducted using Thermoscientific Model: K-Alpha XPS system (USA) with a monochromatic Al Kα source operated with a power of 280 W. The HRTEM images were taken with a Titan G2 ChemiSTEM Cs Probe

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(FEI Company, Hillsboro, OR, USA) TEM, operating at 200 kV. Thermal gravimetric

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analysis (TGA) was carried out by using TGA N-1000 - SCINCO Co. Ltd. (South Korea) from 35 to 850 °C at a heating rate of 10 °C min−1 under N2 atmosphere. Surface interaction

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study between biopolymer and MNPs was carried out with Fourier transform infrared spectroscopy (FTIR) spectra obtained on Thermo Scientific™ Nicolet™ iS10 FTIR

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spectrometer (US). The particle size distribution and zeta potential of the κ-CG-s-AgNPs were studied by using a Zeta-potential & Particle Size Analyzer (ELS-Z from PHOTAL

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OTSUKA Electronic, Korea). Hydrodynamic diameters and polydispersity index (PDI) were analyzed at 25 °C. As a reference, a dispersive medium of pure water with a refractive index

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of 1.332, and a viscosity of 0.8878 was used. Phase purity and crystalline patterns for AgNPs were studied using fifth generation (Gen 5) MiniFlex600 desktop XRD, Rigaku (USA). The total organic carbon (TOC) of the dyes solution was analyzed with the TOC analyzer (Elementar, Vario TOC Cube, Germany). The pH measurements were made with accumet AB15 Basic, Fisher Scientific (Pittsburgh, US). Dye standard solution of 100 mgL-1 9

was used for preparation of calibration standards and for the catalytic reductive degradation for RhD B and MB dyes at the wavelength of λ554 and λ665 were analyzed using the UV-Vis SCINCO Co. Ltd. (South Korea). 3. RESULTS AND DISCUSSION 3.1. UV–Vis spectroscopic study

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Optical spectroscopy is used as a primary tool for the exploration of MNPs. UV-Vis spectral profiles for AgNPs were recorded with time. It is well known that optical properties of MNPs changes proportional to the shape and size of NPs. The UV-vis spectra is one in all the vital MNPs formation, and also the

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technique to point out strong indication of colloidal

productivity growth within the synthesis medium was referred by the gradual increase in the

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absorbance values (Bogireddy, Kiran Kumar, & Mandal, 2016; Pandey, Goswami, & Nanda, 2012). Kinetics of the synthesis reaction process of κ-CG-s-AgNPs was furnished in Fig.1a.

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The change in color with different reaction time intervals also indicates the formation of κCG-s-AgNPs (Fig.1b). Broad bell-shaped spectrum curve was obtained from UV–Vis

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analysis. It had been ascertained that presence of many useful functional groups in κ-CG imposed to solution made the plasmon band broad. SPR of κ-CG-s-AgNPs appeared at 435

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nm (Fig.1a). It can be ascertained that AgNPs absorb radiation intensely at a wavelength of

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435 nm because of the transition of electrons. This peak was founded to extend with time up to a 100 min. In agreement with Mie theory, spherical nanoparticles showed solely one SPR band.

The amount of peaks will increase by increasing diversity of particles shapes

(Banerjee et al. 2014). Thus, it can be concluded that κ-CG-s-AgNPs were altogether spherical in nature. UV−vis spectra of Ag nanoparticle solution stand consistent over several 10

months, indicating the AgNPs size distribution in water was highly stable. Primarily, κCG within this study acted as a reducing, and capping agent for the κ-CG-s-AgNPs synthesis. The macromolecular chains present in the κ-CG molecules acted like templates in the synthesis of AgNPs by supporting in their distribution inside the matrix, and inhibit the development of aggregates. FTIR spectroscopy offer very important information regarding surface interactions. In the

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present work, we have analyzed the FTIR spectra of synthesized κ-CG-s-AgNPs and compared the result with pure κ-CG to examine the potentially functional groups of κ-CG liable for reduction and stabilization of the AgNPs (Fig. S3, Text S1.). Based on the FTIR

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results, it was confirmed that O-H, carbonyl and C-O groups not only reduced the Ag metals

3.2. X-ray diffraction (XRD) analysis

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but also worked as stabilizing agents.

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The results in reference to XRD evaluation data for κ-CG as well as κ-CG-s-AgNPs are shown in Fig.1(c) and (d). XRD of κ-CG exhibited amorphous nature, whereas κ-CG-s-

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AgNPs indicated crystallinity. All of the peaks of the patterns of the κ-CG-s-AgNPs may be conveniently indexed to face centered-cubic silver (JCPDS 41-1402), where the diffraction

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peaks at 2θ values of 32.40⁰, 38.20⁰, 46.40⁰, 67.56⁰, and 77.18° might be ascribed to the reflection of (100), (111), (200), (220), and (311) planes of face-centered cubic (fcc) lattice.

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Few unassigned peaks (marked with stars) have been noticed recommending that the crystallization of bio-organic phase takes place on the surface of the κ-CG-s-AgNPs. Other researchers (Philip, 2009; Shankar, Ahmad, & Sastry, 2003) also found comparable results. Interactions between these κ-CG (biopolymer) and silver ions caused the reduction of silver nitrate and synthesis of AgNPs. Negatively charged groups present in the κ-CG such as 11

carboxylate (COO−) as well as polar groups such as OH and CO have a strong tendency to attach on the surface of the Ag+. Therefore, these groups contributed in both reduction and stabilization of Ag ions. Thermal stability of κ-CG-s-AgNPs was dependent on the decomposition temperature of its diversified functional groups Comparison of TGA results of κ-CG-s-AgNPs and κ-CG were shown in fig. S4 and Text S2. TGA plot depicted that the weight loss in the temperature of 30

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⁰ C to 121 ⁰ C be commonly because of moisture. Degradation pattern of organic compounds was usually between 80 ⁰ C to 480 ⁰ C. There was no degradation above 500 ⁰ C that

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confirmed the high thermal stability of κ-CG-s-AgNPs.

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3.3. X-ray photoelectron spectroscopy (XPS) measurement of κ-CG-s-AgNPs Figure. 2 revealed the XPS results of the obtained κ-CG-s-AgNPs. The overall spectrum indicates the existence of strong C1s, O1s, N1s, as well as Ag 3d core levels. Figure 2(a) exhibited the XPS survey spectra of the κ-CG-s-AgNPs where the atoms of C, N, O and Ag

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were detected and no diverse obvious peaks were found, demonstrating the high purity of the sample. The XPS peak for O, C and N was obviously refer to organic structure as a result of

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existence of κ-CG, whereas the new peak for Ag 3d was on account of the generated AgNPs. In addition, more details could be extracted from high resolution reports shown in fig. 2 (b-d).

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Curve fitting analysis of the XPS O1s peaks centered at 532.2 eV were attributed to the C– OH group (fig. 2b). The O 1 s spectrum was recorded for κ-CG capped AgNPs. The binding

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energies at 285.6 eV and 398.6 eV emerged from C1s (fig.2 (c)) and N1s respectively (Fig. S5). Fig. 2(c) showed the detailed C 1s spectra of κ-CG capped AgNPs and can be

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deconstructed into three components centered at binding energies of 284.2, 285.8 and 286.4 eV, which correspond to C–C, C–O and C=O, respectively (Balachandramohan, Anandan, &

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Sivasankar, 2018). Comparison to the C 1 s spectrum of κ-CG (fig.S6), it was observed that the binding energies are decreased after the reduction of Ag+ to Ag0 in case of κ-CG-s-

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AgNPs. This is in agreement with the observations made by FTIR (Fig. S3). The peaks for C– O and C=O can be assigned to hydroxyl and carboxylate groups present in κ-CG. The N1s peak was resolved into two peaks at 398.4 and 406 eV respectively (Fig.S5). The 398.4 eV peak suggested the existence of charged nitrogen atoms, indicating an electrostatic interaction with the silver. Peaks in the region of 402–406 eV were assigned to oxygen chemisorbed on 13

nitrogen in the XPS N1s spectra of examined surface (Pietrzak, & Wachowska, 2006). In fig. 2(d), the Ag 3d spectra of κ-CG-s-AgNPs consist of two individual peaks at ∼367.9 and ∼373.9 eV, which could be attributed to Ag 3d5/2 and Ag 3d3/2 binding energies, respectively (Xu et al.2013). The splitting of the 3d doublet was found to be 6 eV, demonstrating the metallic nature of silver compared with pure metallic Ag (368.2 and 374.3 eV) (Lin et al. 2009). The peaks ascribed to Ag 3d in κ-CG-s-AgNPs exhibited an evident

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shift. This shift was probably because of the interactions between AgNPs and κ-CG. Presence of C1s and N1s spectra displayed that the NPs were stabilized by the κ-CG macromolecules. The results of the XPS spectroscopy confirmed that κ-CG stabilized the

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AgNPs from aggregation and presented corroborating evidence for the κ-CG-capped AgNPs

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structure.

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3.4. DLS and zeta potential analysis DLS analysis was carried out to check the NPs size distribution and zeta potential in order to find out their mean NPs size (hydrodynamic diameter) and possible surface charge on NPs surface, which serve their stability (fig. S7). The hydrodynamic diameter found for κ-CG-s-

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AgNPs was 127 nm and polydispersity index (PDI) of 0.208 indicates the monodispersed pattern of nanoparticles (fig. S7). The stability of colloidal solutions revolved around zeta potential (ζ) of -35.75mV for κ-CG-s-AgNPs (fig.S8). High negative zeta potential (ζ) values

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are symbolic of NPs that raise adequate surface charge to be electrostatically stabilized and resistant to spontaneous aggregation (Sadeghi, & Gholamhoseinpoor, 2015). Thus negative

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zeta potential (ζ) value of −35.75 mV indicates excellent stability without any sign of

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coagulation or precipitation for months. This negative value is said to the existence of electronegative functional group from the κ-CG.

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3.5. FE-TEM, and elemental mapping analysis

The results acquired from the TEM study presents a transparent explanation regarding the

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shape and size of the NPs. The AgNPs synthesized in the current study were in the range of

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9–15 nm and distributed with spherical structures (fig. 3a). This spherical shape designated that the produced NPs seemed to have minimum surface energy and high thermodynamic stability, and that verified the high value of the zeta potential of the produced AgNPs (Mohammadlu, ,Jafarizadeh-Malmiri, & Maghsoudi, 2016). TEM evaluation additionally proven the NPs to own narrow size distribution ranges with average mean particle sizes of 15

∼12 nm (fig. S9). The lattice spacing calculated via HR-TEM used to be 0.23 nm (fig. 3b). The acquired values of d-spacing between the lattice of AgNPs were in good agreement with Ag (111) being reported (Mishra et al. 2015). Further, AgNPs were also characterized by elemental mapping. HAADF-STEM image of the κ-CG-s-AgNPs (fig.3c), and (d) Ag (yellow), elemental mapping images obtained from image in (fig3c) revealed that, the distribution of Ag element was the predominant (fig.3d). The

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obtained results were similar to the diffraction pattern of AgNPs synthesized by utilizing heparin and hyaluronan, being both reducing and stabilizing agents (Kemp et al. 2009).

3.6.1 Catalytic degradation of Rhodamine B

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3.6. Catalysis

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MNPs with high reactivity and unique surface area can speed up the reduction rate of dyes, thus intensifying the reducing efficiency. Therefore the green synthesis of κ-CG-s-AgNPs

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nanocatalyst was exploited for the hazardous and harmful textile dye removal. Structure of RhD B dye is shown in fig. 4a. Fig. 4b summarizes the analyzed absorption spectra of

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degradation of RhD B by NaBH4. It was experimentally observed that the rate of reaction in the event of the absence of the nanocatalyst in reducing RhD B and MB was extremely slow,

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and it was observed that the reaction has never been completed even after 60 min (Fig.S10). However, after the addition of κ-CG-s-AgNPs nanocatalyst a fast catalytic degradation of

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RhD B dye can be observed. In the presence of AgNPs, the catalytic degradation of RhD B with time is presented in (fig. 4). The catalytic degradation process used to be governed through the change in intensity of the attributed absorption peak of RhD B at 554 nm. The attributed absorption peak of RhD B dye at 554 nm promptly reduced in intensity with time and it disappeared completely in 2.5 min (fig. 4b) and the color of the solution transformed 16

with the original pink-red to nearly colorless (shown in the inset of fig. 4b). The κ-CG-s-AgNPs nanocatalyst prevail to demonstrate higher catalytic efficiency in direction of degradation reaction of RhD B. The acquired % degradation of RhD B dye was found to be ∼100% at 100 ppm in 15s. Fig.5 (a-c) shows the normalized kinetic trace of the absorbance at 554 nm during the reduction of RhD B at three concentrations (100ppm, 300ppm and 500ppm).

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Plot of ln(At/A0) vs time in the direction of catalytic degradation of RhD B at three different dye concentrations by κ-CG-s-AgNPs nanocatalyst are showcased in (fig.5d-f). The closeness of the correlation coefficient (R2) to unity indicates that the degradation process followed

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pseudo first-order kinetics and determined degradation rate constant value of RhD B(100 ppm, 300 ppm and 500 ppm) are 7.87 min−1, 3.8 × 10−1 min−1 and 8.89 × 10−2 min−1

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respectively. Overall, the degradation rate constant of κ-CG-s-AgNPs nanocatalyst for the

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degradation of RhD B dye prevail to be better than the rate constants reported in literature. The enhanced degradation noticed in case of these NPs may be attributed to numerous factors,

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such as spherical shape, small size of generated nanoparticles, and existence of a number of functional group of biopolymer.

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3.6.2 Catalytic degradation of methylene blue (MB) The catalytic activity of κ-CG-s-AgNPs nanocatalyst is scrutinized by utilizing them on the

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reduction of MB to LMB by way of excess of NaBH4. Structure of MB dye is shown in (fig. 6a). The absorption peaks in the interest of MB in water was found to be centered at 665 nm in the visible region corresponding to the n–π* transition of MB (Shahwan et al., 2011). The UV–Vis spectrum of the catalytic character of κ-CG-s-AgNPs nanocatalyst in the reduction of MB by NaBH4 is shown in (fig. 6b) and the color of the solution transformed with the 17

original blue to nearly colorless (shown in the inset of fig. 6b).

(a)

In the presence of κ-CG-s-AgNPs nanocatalyst, absorption spectrum confirmed the reduced peaks for MB with varying time intervals. The kinetic studies at various

concentrations of MB indicates catalytic reduction of MB in the existence of κ-CG-s-AgNPs nanocatalyst with the significant decrease of the absorbance value of dye impending the

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baseline (fig. 7a-c). Ultimately it disappeared with the raise of reaction time, which signifies that the degradation of MB dye occurs. The κ-CG-s-AgNPs nanocatalyst were also discovered to reveal higher catalytic efficiency in the direction of degradation reaction of MB

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dye. The acquired % degradation of MB dye encounter to be ∼100% in the vicinity of 100 ppm MB in 70s. Whereas, the control experiments carried out with κ-CG and NaBH4 did not

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exhibit any catalytic activity in case of MB dye.

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Plot of Ln(A/A0) vs time for catalytic degradation of MB at three different dye concentrations by κ-CG-s-AgNPs nanocatalyst are featured in fig. 7(d-f). The closeness of the correlation

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coefficient (R2) to unity indicates that the degradation process follows pseudo firstorder kinetics and determined degradation rate constant value of MB(100 ppm, 300ppm and

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500ppm) are 1.7 min−1, 4.6 × 10−1 min−1 and 2.1 × 10−1 min−1 respectively. Since, compared to the catalytic degradation at different MB dye concentration (100 ppm, 300 ppm and 500

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ppm), the faster degradation kinetics and higher removal efficiencies continues to follow the following order (MB(100 ppm)〉300 ppm〉500 ppm). In the existing reduction reaction κ-CGs-AgNPs nanocatalyst is known to demonstrate an excellent performance as a catalyst. The high rate constant value deduced the high catalytic nature of κ-CG-s-AgNPs nanocatalyst 18

toward MB and RhD B dyes, which are much higher than the reported literatures. Comparison of our work with literature for the catalytic degradation performance of MB and RhD B dye with κ-CG-s-AgNPs nanocatalyst is manifested in Table 1. 3.7 Total Organic Carbon (TOC) analysis It is generally perceived that total decolourisation of MB doesn't really imply that the dye is totally mineralized into CO2 and H2O. Thus, it is also significant to examine the mineralized

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process. TOC values have been identified with the total concentration of organic in solution and the decrease of TOC reflects the level of mineralization toward the end of the catalytic process. Mineralization of MB was well examined by observing amount of TOC loss in the

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dyes solution. The TOC removal efficiency results of MB and RhD B dyes are plotted in Fig. 8(a). It confirms that with the decrease in TOC content take place with increase in time,

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which shows the mineralization of the MB and RhD B dye. Despite the fact that the UV–vis

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spectra shows ∼95% decolouration, however the TOC reports reveal the maximum TOC removal efficiency of ∼77%, particularly TOC removal efficiency of MB and RhD B are

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77%, and 71% respectively. The observed outcomes reveal that the κ-CG-s-AgNPs nanocatalyst shows decent mineralization ability and can be used for mineralization of MB

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and RhD B dyes.

3.8 Stability of the κ-CG-s-AgNPs nanocatalyst

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The practical applicability, stability and recyclability measure a crucial role in the catalytic degradation process. As seen in Fig. 8(b), the reuse, stability and recyclability of κ-CG-sAgNPs nanocatalyst were tested and the result shows that there is no significant decrease in MB and RhD B dyes degradation even after three consecutive cycles under identical 19

condition. The results showed that, after three consecutive cycles, the MB and RhD B dye removal rate was maintained at about 90%. The obtained nanocatalyst performed well in terms of stability and recyclability. In fourth regeneration cycle MB and RhD B dye degradation percentage rate dropped to about 80%. In addition, the reduction of MB degradation could be attributed to the loss (during handling and centrifugation process), and aggregation of the catalyst in the recycling process, as well as the adsorption of MB/RhD B

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or intermediates. The EDX spectrum and TEM images of the κ-CG-s-AgNPs nanocatalyst after being reused showed no chemical and morphological alterations indicating that the

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catalyst is stable and effective for the degradation of organic pollutants in water.

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3.9 Plausible mechanism for catalytic degradation of dyes

The process in reference to degradation could be interpreted by means of electron transfer

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mechanism. The reductant molecules as well as dye molecules most likely become adsorbed over the large surface of AgNPs devoid of hindering their activity because of the existence of

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biopolymer (fig. 8). It appears earlier that the metallic NPs (MNPs) can make the method kinetically viable via lowering the activation energy (Gupta, Singh and Sharma, 2011). To

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behave as an excellent catalyst, the redox potential of AgNPs ought to be discovered between the redox potential of donor (NaBH4) and the acceptor (MB) system (Mallick, Witcomb, &

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Scurrell, 2011; Jana, Sau, & Pal, 1999). The electrode potential of Ag/Ag+ is +0.80 V. Immediately upon addition of reducing agent (NaBH4), it was adsorbed over the NPs, its reductive strength decreases. As NaBH4 is a strong nucleophile, it exhibited high ability to supply electrons to the degradation reaction (Nemanashi, & Meijboom, 2013). On the other hand, when dye molecules get adsorbed on NPs, their reduction potential increases, as the 20

molecules are electrophilic in nature and hence, when both the species are adsorbed on NPs they become more negative (-ve) for NaBH4 molecules and more positive (+ve) for dye molecule. The electron transfer happened from reducing agent to dye molecule because of AgNPs. AgNPs help in the electron relay from BH4− (donor) to MB/RhD B (acceptor). BH4− ions are nucleophilic, while MB/RhD B are electrophilic in nature with respect to AgNPs, where the AgNPs accept electrons from BH4− ions and conveys them to the MB. Consequently it may result in the destruction of the dye chromophore structure to form small

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species such as SO4- 2, CO2, H2O etc. The uptake of electrons easily led to the degradation of organic dyes (RhD B and MB) by an oxidation–reduction reaction (Li, Liu, & Liu, 2012). In order to validate the mechanism, we have performed the experiment in presence of NaBH4

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(without nanocatalyst) and it was observed that NaBH4 alone could not degrade the dye even

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after 60 min (Fig.S10). Similarly, in the absence of NaBH4 (with nanocatalyst) showed rather slower degradation efficiency. This suggests that neither NaBH4 nor the nanocatalyst alone is

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viable to catalyze the rapid degradation of dye and the reaction could only be finished instantaneously when both the nanocatalyst and NaBH4 were used together. Hence, it could

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be concluded that the nanocatalyst efficiently catalyzed the reduction of dye by relaying of electrons from BH4− species to MB by means of the AgNPs. Similar finding was also reported

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by other researchers (Ali et al., 2017; Azad et al. 2011). Ali et al. represented the electron transfer from the BH4− ions onto the dyes molecules are responsible for the reduction of dyes,

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which take place with assistance of Fe/CC-CH catalyst. Azad et al. showed that in aqueous environment, the BH4− ions create a negatively charged layer around the Au NPs which facilitates the cationic dye MB to be adsorbed on the surface of Au NPs through electrostatic interaction. Several researchers (Ghosh et al., 2015; He et al., 2013, Azad et al., 2011, Wang et al., 2015, Khan et al.2014) also claimed the similar tendency of MNPs in the presence of 21

BH4− ion.

4. CONCLUSION

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For a healthy way forward for nanotechnology, green synthetic approach needs to be adopted for NPs synthesis by way of eco-friendly and sustainable molecules to get rid of risks developing out from the practice of chemical reducing agents and organic solvents. Colloid

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dependent nanotechnology has been evolved to govern the size, shape, uniformity and functionality. It can be clarified from the UV−Vis, FTIR, TEM, XPS and zeta potential

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reports that NPs form on reduction of AgNO3 by κ-CG and have always been stable. The

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green synthesis of κ-CG-s-AgNPs nanocatalyst displayed an extremely strong degradation activity under visible light source. In the existing article, we have established the synthesis of κ-CG-s-AgNPs nanocatalyst. The current synthesis approach demonstrated to be beneficial to

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reduced synthesis of AgNPs and their reduction properties. The κ-CG-s-AgNPs nanocatalyst presented extraordinary degradation characteristics in a reduction of organic dyes (MB and

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RhD B). Dyes decolourization was observed to be faster than its mineralization. It was also

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found that the κ-CG-s-AgNPs nanocatalyst is a highly active and recyclable catalyst for related reactions and can be recovered and reused several times without significant loss of its catalytic activity. This new green chemistry holds numerous beneficial landmarks as well as offers a quick, powerful and economic way to environmental safety.

22

Contributions Conceived and designed the experiments: S.P. Execution of experiments: S.P. Data analysis: S.P. Data interpretation: S.P and J.Y.D. Results discussion of project (S.P and M.K). Author S.P wrote the main manuscript text. All authors (S.P., J.Y.D, M.K and J.K.) reviewed the manuscript thoroughly before submission.

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Notes

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The authors declare no competing financial interest.

Acknowledgments: This study was supported by the National Research Foundation of Korea

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(NRF) grant funded by the Korea government (MSIT) (No. 2018R1A2B6004746). This work

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List of Table and figures caption

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Nonradical Reaction, Applied Catalysis B: Environmental., 261, 118238

Table 1. Comparison of the catalytic degradation performance of MB and RhD B with κ-CG-

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s-AgNPs nanocatalyst with those reported in the literature Figure 1. (a) UV-vis spectra for κ-CG-s-AgNPs recorded as function of time of reaction of

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2.5mM aqueous solution of AgNO3 with k-Carrageenan gum; (b) Color change with different reaction time interval indicates the formation of κ-CG-s-AgNPs nanocatalyst; (c) XRD

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pattern of κ-CG and (d) κ-CG-s-AgNPs nanocatalyst Figure 2 (a)XPS survey spectra of κ-CG-s-AgNPs nanocatalyst; XPS spectra of κ-CG-sAgNPs showing (b) binding energy spectrum for O 1s, (c) binding energy spectrum for C 1s, and (d) binding energy spectrum for Ag 3d.

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Figure 3. (a) HRTEM image of silver nanoparticles, average 12 nm; (b) Fringe spacing of AgNPs (c) HAADF-STEM image of the κ-CG-s-AgNPs, and (d) Ag (yellow), elemental mapping images obtained from image in (b) reflecting the uniform distribution of Ag elements Figure 4 (a) Structure of RhD B dye (C28H31ClN2O3). (b) UV–visible spectra showing decrease in RhD B absorbance at 554 nm over time using κ-CG-s-AgNPs as catalyst in the presence of NaBH4. The inset shows RhD B dye before and after catalytic degradation.

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Figure 5 Catalytic properties of κ-CG-s-AgNPs. (a-c) normalized kinetic trace of the absorbance at 554 nm during the reduction of RhD B at three concentrations (100ppm, 300ppm and 500ppm). (d-f) Plot of Ln(A/A0) vs time for catalytic degradation of RhD B dye

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at three concentrations (100ppm, 300ppm and 500ppm).

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Figure 6 (a) Structure of MB dye (C16H18ClN3S); (b) UV–visible spectra showing decrease in MB absorbance at 665 nm over time using κ-CG-s-AgNPs as catalyst in the presence of

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NaBH4. The inset shows MB dye before and after catalytic degradation. Figure 7 Catalytic properties of κ-CG-s-AgNPs. Normalized kinetic trace of the absorbance

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at 665 nm during the reduction of MB at different MB concentrations (100ppm, 300ppm and 500ppm) (a-c). Kinetic rate constant. at different MB concentrations (100ppm, 300ppm and

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500ppm) (d-f).

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Figure 8 (a) Percent mineralization efficiency (% TOC removal) for MB and RhD B dye with NaBH4 and κ-CG-s-AgNPs nanocatalyst; (b) Catalytic reusability of κ-CG-s-AgNPs nanocatalyst in RhD B and MB dye reduction at room temperature. Figure 9 Schematic illustration for plausible mechanism of reduction of dyes in presence of κ-CG-s-AgNPs nanocatalyst using NaBH4 as a reducing agent 34

10 min 30 40 60 70 80 90 100

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900

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30

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100

200

300

400

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(b)

lP

C-O 285.8eV

536

re

Intensity (a.u)

C1s

C=O 286.4eV

532

Binding energy (eV)

Binding energy (eV) (b)

ro of

(a)

36

378

ro of

-p

re

lP

na

ur

Jo

Figure. 3

37

(b)

RhD B RhD B+NaBH4

500 550 600 Wavelength (nm)

Jo

ur

na

lP

re

Figure. 4

450

ro of

400

Adding CG-s-AgNP

-p

Absorbance (au)

(a)

38

650

(a)

(d)

100ppm RhD B

0.0

R2=0.983 (100ppm)

Absorbance (a.u)

-0.2 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8

10

(b)

20

30

40

time (s)

50

0.00

(e)

Absorbance (a.u)

300ppm RhD B

0.05

0.10

-p

0

ro of

ln (At/A0)

-0.4

0.15

0.20

time (min)

0.25

0.0

R2=0.933 (300ppm)

50

100

150

time (s)

200

ur

0

na

lP

ln (At/A0)

re

-0.2

(c)

250

-0.8 -1.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

time (min)

(f)

0.00

R2=0.859 (500ppm)

-0.05

ln (At/A0)

Jo

-0.6

-1.2

500ppm RhD B

Absorbance (a.u)

-0.4

-0.10 -0.15 -0.20 -0.25 -0.30

0

100

200

time (s)

300

39

0

1

2

3

4

time (min)

5

Figure. 5

Jo

ur

na

lP

re

Figure. 6

600 700 Wavelength (nm)

-p

500

ro of

Absorbance (a.u)

(b)

40

(a)

(d)

100 ppm MB

Absorbance (a.u)

-0.2

-0.8 -1.0 -1.2 -1.4 -1.8

40

60

80

time (s)

100

(e)

300ppm MB

0.2

0.4

0.6

150

time (s)

ln (At/A0)

lP na 100

ur

50

-0.4 -0.6 -0.8 -1.0 -1.2

200

1.0

(f)

500ppm of MB

1.5

2.0

2.5

ln (At/A0)

3.0

100

150

200

time (s)

250

300

3.5

time (min)

4.0

0.0 R2=0.82 (500ppm)

-0.2

Jo 50

1.2

R2=0.983 (300ppm)

-0.4 -0.6 -0.8 -1.0 -1.2

0

1.0

0.0

-0.2

0

0.8

time (min)

re

Absorbance (a.u)

0.0

-p

20

ro of

ln (At/A0)

-0.6

-1.6

(b)

Absorbance (a.u)

2

R =0.9864 (100ppm)

-0.4

0

(c)

0.0

41

0

1

2

3

time (min)

4

5

Figure. 7

(b) 100

80

60 50 40 30 20

60 40 20

50

100

150

200

250

300

0

0

1

2

3

4

Number of cycles

lP

time (s)

re

10

na

Figure. 8

ur

0

80

MB RhD B

-p

% Degradation

70

ro of

MB dye RhD B dye

Jo

TOC removal (%)

(a)

42

ro of

-p

re

lP

na

ur

Jo Figure. 9

43

Table

Table 1. Dye degradation % 98.2% 48% 99% 99.8%

Dye concentration (ppm) 8 ppm 8 ppm 20 ppm 20 ppm

Ag/g-C3N4

120 min

96%

10 ppm

Fe-Mn composite metal oxide (650℃) SMG capped AuNPs

50 min

100%

100 ppm

9 min

-

320 ppm

0.241 min-1

SnS2-SiO2@α-Fe2O3

100 min

96%

5 ppm

0.0196 min−1

SnO2 NPs

50 min

90%

10 ppm

0.0438 min−1

CoSiOx/PMS system

9 min (25°C) 1 min (55°C)

-

50 ppm

0.387 min−1

-

50 ppm

2.025 min-1

∼100% 95% 93 93%

100 ppm

7 min−1

300 ppm 500 ppm 10 ppm

0.46 min−1 0.21 min−1 0.1018 min−1

120 min

-

0.00959 min−1

κ-CG-s-AgNPs nanocatalyst

RhD B

-

90 min

98%

30ppm

0.0064min−1

Bi2Mo3O12 NR Bi1.9Eu0.1Mo3O12 NR

270 min 135 min

2% 15%

14.4 ppm 14.4 ppm

0.012 min−1 0.024 min−1

Bi1.8Pr0.2Mo3O12 NR

140 min

14%

14.4 ppm

0.026 min−1

Au/CeO2-TiO2 NH

10 min

-

23 ppm

0.223 min−1

Jo

90%

Ag2S–ZnS loaded on cellulose (AZCE)

ur

Ag3PO4/MIL101/NiFe2O4 [APO/MOF/NFO(20%)] SnO2 NPs

〈1min 3.7 min 5 min 30 min

0.158 min−1

-p

MB

re

Polyaniline/Bi2SnTiO7 N-doped TiO2 1.0 % Ag-TiO2 Fe–Ni/SiO2 catalysts

Degradation rate constant k (min-1) 0.01504 min− 1 0.00333 min− 1 0.034 min-1 -

Reference

Yang & Luan (2012). Çifçi (2016) Ahmed, Yaakob & Akhtar (2016). (Fu et al. 2015) (Liu et al. 2015) Ganapuram et al. 2015 (Balu et al. 2018) (Li et al. 2018) (Zhu et al. 2020)

ro of

Reaction time (min) 200 min 200 min 90 min 60 min

lP

Target dye

na

Catalysts

Present work (Zhou et al. 2018) (Li et al. 2018) Prashantha Kumar & Ashok kumar (2019) (Mandlimath et al. 2016)

(Saikia et al. 2017) 44

H-Bi 2 WO 6 catalyst using SSL/H-Bi 2 WO 6 /H 2 O 2 process. 10Co10Mg/SBA-15– oxone 10Co/SBA-15–oxone κ-CG-s-AgNPs nanocatalyst

60 min

99.5%

10 ppm

-

(Yi et al. 2018)

5 min

100%

5 ppm

1.065 min-1

(Hu et al., 2013)

60 min 〈1min 4.7 min

87% 99%

5 ppm 100ppm

0.063 min-1 7.87 min−1

93.3%

300 ppm

0.38 min−1

5 min

89%

500 ppm

0.089 min-1

Present work

ro of

MB: Methylene Blue; RhD B: Rhodamine B; NR: Nano rod; NPs: Nanoparticles; NH: Nanohybrid

Jo

ur

na

lP

re

-p

\

45