Silver nanoparticles stabilized in polymer hydrogels for catalytic degradation of azo dyes

Highlights

• Smart colloidal hydrogels were successfully loaded with silver nanoparticles.

• Hybrid system was found as an effective catalyst for catalytic degradation of azo dyes.

• Novel hybrid system has ability to degrade various dyes present in water simultaneously.

• Catalyst can be recovered easily and reused for many cycles without significant loss of activity.

Abstract

Fabrication of poly-(N-isopropylmethacrylamide-co-methacrylic acid) [p(NMA)] microgels to be utilized as microreactors to synthesize stable Ag nanoparticles for catalytic reductive degradation of dyes has been addressed in this work. Both p(NMA) microgel and Ag-p(NMA) hybrid microgel systems have been analyzed by Fourier transform infra-red and Dynamic light scattering, Ultraviolet–Visible spectroscopy, X-ray diffraction and Transmission electron microscopy. Catalytic activity of Ag-p(NMA) towards reductive degradation of Congo Red (CR), Methyl Orange (MO) and Alizarin Yellow (AY) was investigated under different operating conditions. Spectrophotometry was employed to check the progress of reaction while the rate constant (k_app) value of degradation reaction was determined under various conditions to optimize reaction parameters for rapid and economical degradation of these dyes. An increase in k_app value was observed by increasing feed content of dye up to a certain value that decreases again by further increment in dye concentration which reflects that catalysis follows Langmuir–Hinshelwood mechanism. A gradual increase in the k_app value was also observed with increasing quantity of hybrid microgel used as a catalyst. By comparing k_app values of degradation of aforementioned dyes, it was found that Ag-p(NMA) hybrid microgel gives better activity for MO dye degradation in comparison to catalytic degradation of CR and AY.

Introduction

The dyes containing nitrogen-nitrogen double bond (NN) are known as azo dyes. These are organic compounds widely utilized in textile industries. Azo dyes have advantages over other dyes in fabric industries due to their easy preparation and good ability to adhere well onto fabric without any significant fading over the time. But azo dyes have also some serious drawbacks. These are now become major source of water pollution because an enormous amount of dyes are discharged through industrial water effluents into our environment and has bad effect on ecological systems especially on aquatic life. These dyes are resistant towards oxygen and sunlight and also cause bioaccumulation (Naseem et al., 2018; Sreedharan and Rao, 2019). About 60–70% of the dyes used in textile industries are azo dyes and more than 70% of their annual production is consumed by these industries and a huge amount of these dyes is released into our environment through textile effluents (Tarkwa et al., 2019).

To overcome environmental and ecological problems caused by azo dyes, their degradation is necessary. Various methods had been applied for their degradation such as chemical (Tan et al., 2016), biological (Sun et al., 2016), photo-catalysis (Rezaie et al., 2018; Tarkwa et al., 2019) and metal nanoparticles (NPs) based catalytic reduction (Shah et al., 2016; Shahid et al., 2018; Weng et al., 2018). Metal nanoparticles (NPs) based catalytic reduction method is considered the best one among all of above reported methods for degradation of toxic azo dyes because of provision of their large surface area, good catalytic activity, high efficiency and excellent electron transfer abilities (Naseem et al., 2018; Shah et al., 2016). Metal NPs facilitate the transfer of electrons between reducing agent and dye like nano-electrode of negative potential (Naseem et al., 2019).

However, naked metal NPs have some disadvantages over the stabilized metal nanoparticles. They agglomerate with each other owing to their high surface energy. Agglomeration induces remarkable decrease in their catalytic activity. Secondly, naked nanoparticles cannot be easily recycled by simple centrifugation technique (Naseem et al., 2019). Both problems may be overcome by immobilization of metal NPs on some solid support. Different supporting materials are used to immobilize metal NPs such as dendrimers (Rajesh et al., 2014), polymeric microgels (Farooqi et al., 2016; Khan et al., 2013), bulk hydrogels (Ai and Jiang, 2013) and some inorganic material like reduced graphene oxide (Weng et al., 2018). Smart polymeric microgels are excellent stabilizers of metal nanoparticles because of their crosslinked network. The hybrid microgel system is considered as the best catalytic system due to its good stability, synergistic properties, easy preparation method, multi-sensitive behavior and sieve-like structure (Naseem et al., 2019).

A number of researchers have applied different polymeric microgels incorporated with different metal nanoparticles (NPs) as a catalyst for reductive degradation of organic dyes (Farooqi et al., 2017a, 2017b). But special interest has been given to N-isopropylacrylamide (NIPAM) based microgels loaded with metal nanoparticles (NPs) because of their thermally tunable catalytic activity. Such kind of hybrid microgels have been widely reported as catalysts for reductive degradation of dyes (Begum et al., 2016; Shahid et al., 2018). For example, Shah et al. have used poly(N-isopropylacrylamide-co-methacrylicacid-co-2-hydroxyethylmethacrylate) microgel network filled with silver nanoparticles (NPs) for degradation of Methylene blue, Congo red and 4-nitrophenol using NaBH₄ as reductant in aqueous medium (Shah et al., 2016). Farooqi and coworkers have applied silver NPs fabricated inside poly(N-isopropylacrylamide-co-acrylic acid) microgels for catalytic hydrogenation of 4-nitroaniline (4-NA) into p-phenylenediamine using different concentrations of hybrid microgels, dye and NaBH₄ (Farooqi et al., 2016). Titanium dioxide (TiO₂) nanoparticles loaded into the network of Poly-(N-isopropylacrylamide-co-acrylic acid) microgel system has been reported by Wang et al. for catalytic reductive degradation of methyl orange (MO) under different conditions of pH and temperature (S. Q. Wang et al., 2011). Only few reports are available on catalytic degradation of organic dyes in the presence of metal nanoparticles loaded into N-isopropylmethacrylamide (NIPMAM) based microgels (Naseem et al., 2019) that are similar to NIPAM based microgels in responsive behavior but are more appropriate catalytic systems particularly for degradation of dyes. NIPMAM has an additional methyl group in its structure that makes NIPMAM based network relatively less dense in comparison to NIPAM based polymer network. The wider sieve size of NIPMAM based hybrid microgels loaded with metal nanoparticles facilitates the diffusion of bulky dye molecules towards the catalyst surface and enhances rate of degradation of dyes. Some of us have imbedded silver nanoparticles in the polystyrene-poly(N-isopropylmethacrylamide-acrylic acid) core shell microgels and have used this system as catalyst for fast degradation of Methylene Blue and Brilliant Blue (Naseem et al., 2019) but more than one steps were involved in protocol of fabrication of core shell microgels. To the best of our knowledge, catalytic degradation studies of azo dyes in the presence of homogeneous NIPMAM based hybrid microgels has not been reported in literature. In present work, poly-(N-isopropylmethacrylamide-co-methacrylic acid) [p(NMA)] microgels loaded with silver nanoparticles were synthesized and used as catalysts for reductive degradation of various azo dyes.

The p(NMA) microgels have been prepared by free radical precipitation polymerization in aqueous medium. Prepared microgels have been utilized as microreactors to synthesize and stabilize Ag-nanoparticles. Catalytic activity of Ag-p(NMA) hybrid system towards reductive degradation of dyes including Congo Red, Methyl Orange and Alizarin Yellow under various reaction conditions was evaluated to investigate the process of their reductive degradation.