Catalytic Degradability of p-Nitrophenol Using Ecofriendly Silver Nanoparticles

In an effort to produce non-toxic and economically viable “green” protocols for waste water treatment, researchers are actively involved to develop versatile and effective silver nanoparticles (SNPs) as nano-catalyst from bio-based techniques. Since, p-nitrophenol (PNP) is one of the anthropogenic contaminants, considerable attention has been focused in catalytic degradability of PNP in wastewater treatment by curtailing serious effect on aquatic fauna. Ingestion of contaminants by aquatic organisms will not only affect the aquatic species but is also a potential threat to human health, especially if the toxic contaminants are involved in food chain. In this short report, we provided a comprehensive insight on few remarkable nanocatalysts especially based on SNPs and its biopolymer composites synthesized via ecofriendly “green” route. The beneficiality and catalytic performance of these silver nanocatalysts are concisely documented on standard model degradation reduction of PNP to p-aminophenol (PAP) in the presence of aqueous sodium borohydride. The catalytic degradation of PNP to PAP using SNPs follows pseudo first order kinetics involving six-electrons with lower activation energy. Furthermore, we provided a list of highly effective, recoverable, and economically viable SNPs, which demonstrated its potential as nanocatalysts by focusing its technical impact in the area of water remediation.


Introduction
Organic pollutants from industries are the major source for water pollution; these synthetic toxins are extremely harmful to the environment and influences health risks to human [1,2]. Knowledge of the health effects of organic contaminants at the low levels found in industrial wastewater supplies are very limited [3]. However, the water source is known to contain significant amounts of industrial effluent and toxic impurities, this has given rise to concern before releasing to the aquatic stream. As anthropogenic contaminants removal from water is essential before domestic use, the cost effectiveness for the water treatment is also to be considered. There is an imperative need to improve the technical knowledge and methods/protocols that should be environmental friendly and anticipated its impact on marine pollution [4][5][6].
Apart from its immense thermal and electrical conduction, silver metal plays a vital role in catalysis of organic reactions. In recent years, organic reactions in aqueous medium were paid much

Phyto-Synthesis of Silver Nanoparticles and Its Biopolymer Nanocomposites
Besides commensurable to the environmental safety research, development of new nano-technological "green" protocols for the synthesis of metal nanoparticles imparts implementing solution to technical challenges in the field of nanocatalysis [36][37][38][39]. The researchers for the reduction of nitroarenes explore varieties of reducing agent, while Fe-HCl is one of the most common reducing agents for nitro compounds reduction. The environmental hazard caused by this Fe-HCl reagent is the massive production of hazardous Fe-FeO sludge with unsatisfactory reaction yields [40][41][42][43][44]. Even though many researchers study SNPs supported variety of templates, plant-mediated green synthesis of SNPs is always a prime choice in environmental remediation. The main advantages of designed nanocatalysts from the plant-mediated "green" route are being inexpensive and specifically providing a controlled size and surface morphology. Another key consideration of plant-mediated SNPs are potent antioxidant, antimicrobial, and cytotoxic activity on cancerous cells in biological systems [45][46][47][48]. A systematic study of plant extract mediated synthesis of SNPs with comparative parameters are reported in Table 1.
Recently, a comprehensive review by Iravani [49] highlights the sustainable and eco-friendly synthesis of various nanoparticles along with SNPs of different dimensions using plant gum (an adhesive substance usually obtained as exudate from the tree bark). Since the metal nanoparticles are successfully applied in close proximity of biomedical fields such as pharmaceuticals, tissue engineering, and drug delivery, hazardous toxic substances should be avoided. Concerning this, plant gum polysaccharides plays a vital role with appealing cost effectiveness and biodegradability advantages [50,51]. Thakur et al. and Velusamy et al. reported effective antibacterial SNPs against Bacillus subtilis, Bacillus cereus, Salmonella enteritidis, and Pseudomonas aeruginosa from gums of Acacia Arabica with particle sizes in the range 30.0-35.0 nm [52].
From the aqueous extracts of plant leaves, fruit peels, roots, rhizomes, etc., plentiful research has been undertaken from several decades for the bio-green synthesis of SNPs. The biosynthesized SNPs with sizes 4.0-35.0 nm from Albizia adianthifolia leaves were reportedly effective against A549 lung cell line with viability data of 21% (10 µg/mL) [53]. In contrast, the leaf extract of Alternanthera sessilis Linn. (Amaranthaceae) acts as remarkable capping and reducing agent for silver ions; which shows superior stabilizing behavior with potent antioxidant activities including antimicrobial properties [54]. The SNPs reported by Vivek et al. from Annona squamosa leaf extract were found to be in the range of 20.0 to 100.0 nm and exhibited a dose-dependent cytotoxic effect on human breast cancer cell (MCF-7) with inhibitory concentration (IC50) of 30 µg/mL at 48 h incubation period [55]. The outer peel extract of Ananas comosus (L.) was effective to synthesize antioxidant, antibacterial, and antidiabetic SNPs with cytotoxicity potential towards HepG2 cells [56]. A very fine size of SNPs < 25 nm were reported by Azadirachta indica and Boerhaavia diffusa extracts, which show characteristic surface plasmon resonance of SNPs at around 420.0 nm, and detailed antibacterial assay reveals that these biosynthesized SNPs are active against Gram-positive bacteria Bacillus subtilus and demonstrated highest sensitivity toward Flavobacterium branchiophilum [58,59]. The environmentally amenable SNPs produced from leaf extracts of Brassica oleracea, Caesalpinia pulcherrima, and Cassia auriculata shows potential cytotoxic efficacy towards MCF-7, HeLa, and PC-3 cells, respectively, proves advantageous in biomedical techniques, especially in cancer therapy [60][61][62]. Balashanmugam et al. reported phytogenically synthesized SNPs from Cassia roxburghii aqueous leaf extract at ambient conditions, showing noteworthy in vitro antifungal activity against human and plant pathogens [63]. Roasted Coffea arabica seed extract facilitated SNPs shows diminished bacterial growth of E. coli and S. aureus [64]. The various cancer cell lines, such as SW480, J-774, MCF-7, MDA-MB-231, HepG2, A549, MCF7, HeLa, SiHa, B16F10, PC3, COLO205, HEp-2, U-87, LoVo, RKO, MDA-MB-231, and HT-29, etc. were studied in detail with varieties of SNPs using phyto-synthetic green routes . In combination with biopolymers, these biogenic SNPs are immobilized in the polymer matrix and show greater stability (see Figure 1, for the graphical representation of phyto-synthesis and stability of SNPs in combination with biopolymers); we can also notice superior mechanical and physical properties of biopolymers such as chitosan, agar, and pectin silver nanocomposites [66,77]. The reported biopolymer based silver nanocomposite films shows potential applications in food packaging [77,[90][91][92]. combination with biopolymers, these biogenic SNPs are immobilized in the polymer matrix and show greater stability (see Figure 1, for the graphical representation of phyto-synthesis and stability of SNPs in combination with biopolymers); we can also notice superior mechanical and physical properties of biopolymers such as chitosan, agar, and pectin silver nanocomposites [66,77]. The reported biopolymer based silver nanocomposite films shows potential applications in food packaging [77,[90][91][92]. The presence of the various functional entities such as amine, hydroxyl, carbonyl and carboxyl groups with carbohydrate polymer frameworks are responsible for the synthesis of SNPs by bioreduction process [100,101]. Some researchers synthesized SNPs successfully using cyanobacterium algae [102]. Additionally, the synthesized SNPs are stabilized by functional groups of phytochemicals present in the plant extract. We can notice plentiful research on phyto-synthesis of various nanomaterials, such as gold, platinum, copper, gold, titanium, and zinc, etc., but SNPs The presence of the various functional entities such as amine, hydroxyl, carbonyl and carboxyl groups with carbohydrate polymer frameworks are responsible for the synthesis of SNPs by bioreduction process [100,101]. Some researchers synthesized SNPs successfully using cyanobacterium algae [102]. Additionally, the synthesized SNPs are stabilized by functional groups of phytochemicals present in the plant extract. We can notice plentiful research on phyto-synthesis of various nanomaterials, such as gold, platinum, copper, gold, titanium, and zinc, etc., but SNPs have proved their efficiency as a potent antimicrobial agent with unique optical, electrical, thermal, and catalytic properties [103,104]. The increasing popularity of phyto-synthesis, the so-called "green route" for SNPs offer many advantages over routine chemical synthesis. Plant extracts (from leaf, gum, roots, stems, rhizomes, seeds, flowers etc.) have a rich source of active phytochemicals such as catechins, anthraquinones, phenolics, terpenoids, flavonoids, tannins, enzymes, proteins, polysaccharides, and organic acids, etc. These biomolecules took active part in the complex mechanism of reduction and stabilization of SNPs from silver ions [105,106]. (See Figure 1 for phyto-synthesis and stability of SNPs).

Catalytic Degradation of p-Nitrophenol Using Silver Nanoparticles
The extent of toxic compounds impact on the environment leads to a significant effect on exposed organisms. We can encounter the aggregation of the toxic components in the natural environment especially to air, soil, and water. The environmental pollution is a serious problem and has a devastating effect on nature because of the chain of events that ensue the toxic contaminants and eventually enter into the natural environment [107,108]. In spite of other sources of pollution, industries are the worst generators of toxic contaminants. The contaminants finally enters the environment and sequentially contaminate water by degrading the water quality, rendering toxicity to aquatic life and humans [109][110][111]. The major effluent from pesticides, explosives, and dye industries are nitrophenols; these toxic organic anthropogenic pollutants can easily enter into the aquatic stream if the appropriate precautionary measures are not implemented on effluent treatment. Due to the remarkable demonstration and significant catalytic activity of SNPs, many researchers documented the efficient catalytic degradation of nitroarenes from industrial wastewater [112][113][114][115][116][117][118]. The main criteria of the developed nanocatalyst are environmentally friendly, economically viable, biodegradable, and non-toxic with multifunctional behavior such as high adsorption, removal ability, and reusability.
We can find plentiful applications of sodium borohydride in various catalytic reactions. The use of sodium borohydride as a reducing agent is reported in various organic and inorganic reactions [119][120][121]. In contrast, the reduction of PNP is not possible by sodium borohydride alone. The ratio of potential difference for PNP to PAN is −0.76 V and H 3 BO 3 / BH 4¯i s −1.33 V at ambient temperature. Even though the reaction of PNP to PAP is thermodynamically favorable, the conversion reaction is kinetically too slow [122]. This is due to the presence of kinetic barrier and potential difference between borohydride (donor) to p-nitrophenolate ions (acceptor). The SNPs overcome this kinetic barrier by catalyzing the reaction and facilitating the relay of electrons from the donor to acceptor molecules. The interaction of aqueous sodium borohydride with SNPs, quickly generates hydrogen gas and adsorb on the SNPs surface, which further enables the reduction process by interactive adsorption of p-nitrophenolate ions on the SNPs surface [123]. The reaction involves various organic intermediates and finally desorption of p-aminophenolate ions from the SNPs surface (for graphical representations, see Figure 2). In accordance with the catalyst, the analyte PNP in presence of sodium borohydride follows pseudo-first-order kinetics [124,125]. The electron transfer reaction from borohydride ions to pnitrophenolate ions will transpire after the adsorption of donor-acceptor molecules on SNPs surface. The reaction proceeds by diminishing the activation energy and SNPs catalyst play a vital role in the catalytic reaction (For graphical representations, see Figure 3). It is believed that the conversion of PNP to PAP is a six-electron transfer reaction in the presence of sodium borohydride; the conviction was also supported by the reaction intermediates isolated and studied via mass-spectrometric techniques [126,127] (see Figure 4).        For brief understanding, the reduction of PNP is not possible by using sodium borohydride alone. After adding SNPs catalyst to PNP in presence of aqueous sodium borohydride, the formed p-nitrophenolate shows the maximum absorption (λ max ) in UV-vis spectrum in the range of 400-410 nm. The time dependent UV-vis spectrum is to be recorded to check the progress of the catalytic reaction. The diminishing peak of λ max = 400-410 nm was observed with the appearance of new λ max peak at around 300-320 nm, which is due to the formation of p-aminophenolate ions in the reaction mixture. Further, decrease in the pH of the solution was observed due to the addition of sodium borohydride, which enables the conversion of PNP to PAP [127,129]. The use of sodium borohydride concentration is significantly higher than the concentration of PAP, and the reduction rates are independent of the sodium borohydride concentration, accordingly the reaction follows the pseudo-first order reaction [130,131], the equation can be written as follows: where k app (k 1 S = according to Langmuir-Hinshelwood mechanism) is the apparent rate constant; t is the reaction time; C 0 is the relative concentration of PNP at time zero (initial concentration); C t is the concentration of the PNP at time 't' (different interval of time during the catalytic reaction). From this equation, it is obvious that the higher the value of apparent rate constant (k app ) for the catalytic reaction, the more efficient is the used catalyst [132,133]. Various researchers investigated the catalytic efficacy of ecofriendly SNPs prepared from several plants of different source. The comprehensive list was reported in Table 2. These effective SNPs catalysts show remarkable catalytic degradation efficiency against PNP, one of the main mutagenic organic pollutants. Researchers successfully carried out catalytic reactions by removing PNP in aqueous media using biogenic SNPs. The obtained SNPs are spectro-chemically characterized using different advanced analytical techniques such as Ultraviolet-Visible spectroscopy (UV-Vis), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), dynamic light scattering (DLS), and X-ray photoelectron microscopy (XPS), etc.
Previously, we reported the biosynthesis of SNPs from Rhubarb stem extract (RS extract) as bioreductant. In brief, the chopped Rhubarb stems were suspended in hot double distilled water for about 1 h and filtrate was collected and stored under <5 • C for further use. The RS extract was mixed stoichiometrically with silver nitrate solution at different interval of time to get ecofriendly SNPs [designated here as RS-AgNPs (Rhubarb stem extract-Silver nanoparticles)] within 15 min. The RS extract was lyophilized to get in powder form to compare the morphology with RS-AgNPs [91].  It is evident from the Figure 5 that, we can easily distinguish RS extract powder with RS-AgNPs, the adopted synthetic process does not involve any harmful chemicals. The morphology of RS-AgNPs demonstrate SNPs capped with various phytochemical groups of RS extract. Recently, by varying the amount of guar gum biopolymer, we formulated biopolymer silver nanocomposites (designated here as AgNC@PAAG1). The developed silver nanocomposites demonstrate its efficacy as nanocatalysts against model reduction reaction of PNP to PAP by aqueous sodium borohydride with apparent rate constant of 121.8 × 10 −3 min −1 at ambient temperature. In addition, silver nanocomposite hydrogels prepared from RS extract showed potent antimicrobial activity against B. subtilis and E. coli. We also proposed drug delivery application of these silver nanocomposite hydrogels [155]. The morphology of developed guar gum based silver nanocomposite hydrogels are portrayed in Figure 6, which signpost uniform distribution of SNPs throughout the hydrogel networks.
Previously, we reported the biosynthesis of SNPs from Rhubarb stem extract (RS extract) as bioreductant. In brief, the chopped Rhubarb stems were suspended in hot double distilled water for about 1 h and filtrate was collected and stored under <5 °C for further use. The RS extract was mixed stoichiometrically with silver nitrate solution at different interval of time to get ecofriendly SNPs [designated here as RS-AgNPs (Rhubarb stem extract-Silver nanoparticles)] within 15 min. The RS extract was lyophilized to get in powder form to compare the morphology with RS-AgNPs [91].
It is evident from the Figure 5 that, we can easily distinguish RS extract powder with RS-AgNPs, the adopted synthetic process does not involve any harmful chemicals. The morphology of RS-AgNPs demonstrate SNPs capped with various phytochemical groups of RS extract. Recently, by varying the amount of guar gum biopolymer, we formulated biopolymer silver nanocomposites (designated here as AgNC@PAAG1). The developed silver nanocomposites demonstrate its efficacy as nanocatalysts against model reduction reaction of PNP to PAP by aqueous sodium borohydride with apparent rate constant of 121.8 × 10 −3 min −1 at ambient temperature. In addition, silver nanocomposite hydrogels prepared from RS extract showed potent antimicrobial activity against B. subtilis and E. coli. We also proposed drug delivery application of these silver nanocomposite hydrogels [155]. The morphology of developed guar gum based silver nanocomposite hydrogels are portrayed in Figure 6, which signpost uniform distribution of SNPs throughout the hydrogel networks.   Previously, we reported the biosynthesis of SNPs from Rhubarb stem extract (RS extract) as bioreductant. In brief, the chopped Rhubarb stems were suspended in hot double distilled water for about 1 h and filtrate was collected and stored under <5 °C for further use. The RS extract was mixed stoichiometrically with silver nitrate solution at different interval of time to get ecofriendly SNPs [designated here as RS-AgNPs (Rhubarb stem extract-Silver nanoparticles)] within 15 min. The RS extract was lyophilized to get in powder form to compare the morphology with RS-AgNPs [91].
It is evident from the Figure 5 that, we can easily distinguish RS extract powder with RS-AgNPs, the adopted synthetic process does not involve any harmful chemicals. The morphology of RS-AgNPs demonstrate SNPs capped with various phytochemical groups of RS extract. Recently, by varying the amount of guar gum biopolymer, we formulated biopolymer silver nanocomposites (designated here as AgNC@PAAG1). The developed silver nanocomposites demonstrate its efficacy as nanocatalysts against model reduction reaction of PNP to PAP by aqueous sodium borohydride with apparent rate constant of 121.8 × 10 −3 min −1 at ambient temperature. In addition, silver nanocomposite hydrogels prepared from RS extract showed potent antimicrobial activity against B. subtilis and E. coli. We also proposed drug delivery application of these silver nanocomposite hydrogels [155]. The morphology of developed guar gum based silver nanocomposite hydrogels are portrayed in Figure 6, which signpost uniform distribution of SNPs throughout the hydrogel networks.   Gavade et al. synthesized biogenic SNPs catalyst from Acacia nilotica gums and its catalytic efficiency towards PNP was reported with different catalytic quantities, the catalytic loading of 15.0 mg·mL −1 of SNPs shows better performances in converting PNP to PAP within 12.0 min with k app value 0.03651 min −1 [134]. In contrast, SNPs synthesized form stem extracts of Acacia nilotica shows comparatively higher catalytic efficiency, if we consider the concentration of sodium borohydride used in the reaction [135]. Using Actinodaphne madraspatana bedd leaves, Priya et al. synthesized and reported the size dependent catalytic activity of SNPs. The SNPs of different sizes (60, 35, and 20 nm) were prepared at different pH (6.0, 9.0, and 12.0), respectively. The authors noticed the catalytic activity, it was found to increase with decrease in SNPs size. For the SNPs size of ≈ 20.0 nm, authors testified significant k app value of 13.25 × 10 −3 s −1 with conversion time within 1.5 min [127]. Manjari et al. documented the facile synthesis of SNPs using Aglaia elaeagnoidea flower extract. The authors mentioned the conversion time of PNP to PAP is around 15.0 with k app value 22.5 × 10 −2 min −1 [136]. IN contrast, the leaf extract of Aglaia elaeagnoidea produce average 12.0 nm size SNPs inside the sodium alginate gel network and shows extraordinary conversion efficiency. The prepared SNPs incorporated alginate gel beads shows superior catalytic recyclability up to 10.0 consecutive cycles with ≈80% conversion efficacy. The authors claim that the minimum loss of catalyst (<4%) was observed during the course of catalytic reaction throughout 10 successive cycles [137].

Conclusive Remarks
Currently, our ecosystem is becoming extremely unpleasant due to the release of anthropogenic pollutants from different industries to the environment. Due to this, pollutants are contaminating our natural water resources across the world. To interpret the experimental kinetic data of an organic pollutant, PNP from industrial wastewater, it is reasonable to utilize these phytochemical-induced, unhazardous SNPs and its biopolymer conjugates in catalytic processes. These biogenic SNPs demonstrate advantageous surface chemistry, because these SNPs are stabilized by phytochemical functional groups. It was noticed that, they exhibit remarkable antimicrobial properties and potent cytotoxic responses on cancerous cells.
In line with several benefits of "green" SNPs and its biopolymer composites, various functional parameters-particularly surface area and porosities of SNPs incorporated gel networks-are also to be expected for the favorable catalytic activity, which can enhance the interaction between the supported biopolymer templates and SNPs, and predict apparent rate constant (k app ) of the nanocatalysts. Despite this, the nature of phyto-synthesized SNPs and its biopolymer composite materials have been extensively studied along with some congruent reports. A number of advantageous points have been agreed upon as follows: • A series of well-stabilized SNPs can be achieved with tunable size distribution using plant-mediated protocols.

•
Biodegradable and non-toxic polymers in combination with ecofriendly SNPs always play an important role in medicinal and food-based industries.

•
Chemical synthesis of SNPs involve the usage of toxic reducing agents and are the subject of environmental concern, so it should be avoided.

•
Dynamic tunability of antimicrobial activity of plant-mediated SNPs toward various bacterial strains and several human viral pathogens were observed. • Ecofriendly SNPs demonstrate extraordinary and unique optical, thermal, and electrical properties of SNPs attracted researchers to utilize in diverse technical fields from photovoltaics to chemical sensors.
• Fabrication of ecofriendly SNPs and its non-toxic biopolymer composites with multi-functional properties are owing to superior catalytic degradability of PNP and wide range of applications in nanocatalysis.