Introduction:

Table 1. Toxicity of some dyes [1].

Table 2. Classification of Dyes based on their nature and application [1,3].

Properties:

Table 3. Different types and properties of dyes used in industries [3].

Figure 2. Classification of Dyes.

Figure 2. Classification of Dyes.

Table 4. Examples of some organic dyes and their chemical formulae.

Table 4. Examples of some organic dyes and their chemical formulae.

Class	Chromophores	Examples
Azo dyes		Reactive Black 5
Anthraquinone dyes		Reactive Black 4
Nitroso dyes		Acid Green 1
Nitro dyes		Acid Yellow 24
Indigoid dyes		Acid Blue 71
Triarylmethane dyes		Malachite Green

Figure 3. Top down and Bottom-up Synthesis Approach of Nanoparticles.

Figure 3. Top down and Bottom-up Synthesis Approach of Nanoparticles.

Figure 4. Different Morphologies of Au-NPs.

Figure 4. Different Morphologies of Au-NPs.

Figure 5. Dye Adsorption using Silk Fibroin-Gold Nanoparticles (SF-AuNPs).

Figure 5. Dye Adsorption using Silk Fibroin-Gold Nanoparticles (SF-AuNPs).

Table 5. Removal of dyes using variety of AuNPs.

Properties of gold nanoparticles:

Sources of Nanoparticles:

Formation of gold nanoparticles and their use for dyes removal:

Properties of dyes:

Congo red:

Congo red was discovered in 1884 and a significant direct dye, has a carcinogenic effect in human and animal health [70]. The big Congo red molecule has two azo linkages with 4-amino-1-naphtalenesulphonic acid groups on either end. It act as inhibitor to prevent the metal from corrosion [71]. It exists as a brownish-red crystal that is stable in air and dissolves in water at a rate of 1 g/30 mL. It is an anionic acid dye that is used in laboratories to diagnose amyloidosis, diagnose free hydrochloric acid in gastric contents, determine pH, and histologically stain for amyloid. Because of its great affinity for cellulose fibres, it is used in the textile industry and paper industry. It is a benzidine and napthoic acid derivative that breaks down into cancer-causing substances [72].

Figure 6. Structure of Congo Red Dye.

Figure 6. Structure of Congo Red Dye.

Methylene blue:

Methyl blue is crystalline, odor-free, deep green powder and when dissolve in water gives blue solution. The molecular weight of MB is 319.85 gmol^‾1 and is an aromatic heterocyclic basic dye. Due to its high water solubility, it can, create a stable solution with water. MB is a positively charged chemical that belongs to the class of poly-methine dyes containing an amino autochrome unit [73]. When exposed to MB, people have the danger of developing many health problems, including methemoglobinemia, nausea, vomiting, and ocular damage. Even a small amount of dye in water could be harmful and extremely noticeable removing MB from waste effluents becomes crucial for the environment [74].

Figure 7. Structure of Methylene Blue.

Figure 7. Structure of Methylene Blue.

Degradation of organic dyes using fabricated Gold Nanoparticles

Removal of Malachite Green and Ethyl violet using Graphene oxide Gold Nanoparticles (GeONPs):

Graphene oxide gold Nano-composites was efficiently used for the removal of cationic dyes from polluted water. It was prepared by using a sodium citrate as reducing agent to reduce Au-ions and then incorporating it on Graphene oxide (G-O) sheet. Then it was characterized by different techniques like FESEM, TEM, EDX, XRD, and FT-IR AND TGA. It was observed that GeO-Au Nano-composite showed efficient adsorption of Malachite green and ethyl violet dyes [7].

Figure 8. Thermal gravimetric analysis curves for nanomaterials made of graphene oxide and graphene oxide-gold nanocomposite [7] .

Figure 8. Thermal gravimetric analysis curves for nanomaterials made of graphene oxide and graphene oxide-gold nanocomposite [7] .

Graphene oxide Au-NPs was added as adsorbent in the water containing cationic dyes (Malachite green and ethyl violet) and anionic dyes (methyl orange). The aqueous solution of Methyl Orange was orange, MG was light blue and MV was dark blue. When all three dyes were mixed together giving aqueous solution of green color. After the adsorption green color turned to pale yellow. By using UV-Visible Spectrophotometer the removal of dyes was monitored from water. Two peaks were shown by UV-Visible spectrophotometer one at 600nm for cationic and other at 463nm for anionic dyes. Both peaks disappeared after 15 minutes that confirmed the removal of dyes from water [7].

The ability of GeO and GeO-Au nanocomposite to adsorb MG and EV was investigated. For the removal of these dyes, 50ml of 10Mm solution of each dye was taken in a separate container, and GeO and GeO-Au Nanoparticles were then added as an adsorbent. It was observed that GeO-Au had a greater adsorption capacity than GeO, due to the increased adsorption of Au that was integrated on GeO sheets. The adsorption capacity of GeO-Au NPs was higher. The higher surface energy of gold nanoparticles imparted them a greater capacity for adsorption. Although this increase was not particularly significant, gold nanoparticle integration on GeO has some additional advantages, including preventing GeO sheets from aggregating upon reduction, making it simpler to separate from the adsorption medium, extending shelf life, and adding antimicrobial and catalytic properties to GeO-based composites. [7].

Figure 9. Impact of initial dye concentration on graphene oxide-gold nanocomposites' adsorption of (a) malachite green and (b) ethyl violet. Reaction Conditions: Adsorbent weight is 0.005g, and the dye solution volume is 50ml, 100rmp, and 25°C. [7].

Figure 9. Impact of initial dye concentration on graphene oxide-gold nanocomposites' adsorption of (a) malachite green and (b) ethyl violet. Reaction Conditions: Adsorbent weight is 0.005g, and the dye solution volume is 50ml, 100rmp, and 25°C. [7].

The amount of adsorbed dye was increased by enlarging the initial concentration. This increase was explained with the help of concentration gradient. By enhancing the concentration of dyes the concentration gradient was increased due to which large amount of molecule of dyes diffuse from surrounding to the surface of adsorbent. Due to the presence of large amount of adsorbate molecule in adsorption medium the total adsorption increases. By increasing the concentration gradient between the adsorbent and adsorbate and the presence of large active sites of GeO-Au Nano-composites the adsorption rate was higher at very start and then slow afterward [7].

Temperature effect: For a 50 ml, 10μM solution of each dye containing 0.005 g adsorbent and a temperature range of 25 to 45℃, the impact of temperature on the adsorption process was examined. The results Indicates that adsorption capacity was increased by enhancing temperature in case of both the dyes. The dyes molecule gain energy by interaction with active sites by raising the temperature on the adsorbent surface [7].

pH effect: pH is another factor that has an impact on the adsorption process. Different functional groups are present on the adsorbent surface. At different pH values the adsorbate surface can be protonated and deprotonated to form charged species in the medium. As a result charges on the adsorbent and the adsorbate surfaces exert electrostatic attractive or repulsive forces. The pH of dye aqueous solution was raised from 2 to 9 using 0.1 M NaOH and 0.1 M HCl aqueous solutions. It was observed that both dyes (malachite green and ethyl violet) minimal and maximum levels of adsorption occurred at pH 7 and pH 2 respectively. In the case of cationic dyes, the adsorption capability was increased by raising the pH. This was caused by growing electrostatic contact between ions and increased π-π staking between negatively charged GeO sheets and cationic dyes. As a result, the percentage of dye removal was increased [7].

Removal of Methylene Blue using AuNPs from Sesbenia grandiflora

Methylene blue is used in aquatic samples to analyze trace level of sulphide ions. In aqua industry the cationic form of methylene blue dye is used as anti- Malarial agent and chemotherapeutic agent. For the reduction of MB dye biogenic gold nanoparticles were prepared from Sesbenia grandiflora extract. It was characterized by UV-Visible spectroscopy and observed that the absorbance maxima value of MB decrease with respect to time. The decrease in absorbance showed that the phyto-extract lowered the pH value. A drop in absorbance was seen at 666 and 292 nm with the addition of glass beads coated with Au-NPs, which typically implied a progressive conversion of methylene blue (MB) to leucomethylene blue (LMB). It was noticed that in the absence of catalyst the absorbance value decrease gradually in the reduction reaction. The decrease in absorbance indicated the reduction of MB. At 298k and different time interval the recorded absorbance spectra was observed for the reduction of MB [86].

Figure 10. UV-Visible spectra of methylene blue reduction by S.grandflora leaf extract [86].

Figure 10. UV-Visible spectra of methylene blue reduction by S.grandflora leaf extract [86].

Figure 11. UV-Visible spectra of methylene blue reduction by S.grandflora leaf extract in the presence of glass bead coated with AuNPs [86].

Figure 11. UV-Visible spectra of methylene blue reduction by S.grandflora leaf extract in the presence of glass bead coated with AuNPs [86].

Figure 12. Reduction Mechanism of Dyes using Gold Nanoparticles.

Figure 12. Reduction Mechanism of Dyes using Gold Nanoparticles.

pH effect:

By increasing the PH from 7 to 9 the removal of MB was increased along with reaction rate by increasing the pH further from 11 to 13 respectively. And the removal of MB was decreased by decreasing the pH. The pH investigation showed that the phenolic OH group on the nanoparticle surface was essential for the adsorption of MB [8].

By absorption spectrophotometer the adsorption of MB on Au-NP particles was studied within the range of 400-800nm [8]. The characteristic absorption peaks of the aqueous MB containing samples obtained at various times after adsorption showed that the MB concentration profile decreased with an increase in the adsorption time. Concentration of dye was examined by photometric method. According to CCD (central composite design) method the efficiency of MB removal was examine at different experimental conditions [8].

Figure 13. Absorption spectrum of methylene blue on Au-NPs [8].

Figure 13. Absorption spectrum of methylene blue on Au-NPs [8].

Reduction of Methyl orange and Congo red from Dalbergia coromandeliana Au-NP:

From the root of Dalbergia coromandeliana Au-NP was synthesized by using 5, 7-dihydroxy-6-metoxy-3, 4methylenedioxyisoflavone (Dalspinin). The average particle size of Au-NPs' was 10.5 nm. It was discovered that the prepared Au-NPs remained stable for more than 5 months without aggregating. The Au-NPs showed good activity for the reduction of Congo red and methyl orang dye. Initially a light yellow color of the gold salt solution changed to a wine red color when Dalspinin was added. That showed the synthesis of DLP-Au-NPs. The synthesized DLP-Au-NPs work well as a catalyst to degrade CR in the presence of NaBH₄.

Figure 14. Reduction of Methyl orange and Congo Red from Dalbergia coromandeliana using gold nanoparticles.

Figure 14. Reduction of Methyl orange and Congo Red from Dalbergia coromandeliana using gold nanoparticles.

In an aqueous media, CR displayed an absorption band at 497 nm for the π -π*-transition and 350 nm for the n-π*-transition. When the Nano-catalyst was not present the (DLP-Au-NPs) showed wavelength maxima at 497nm for CR that remained un-changed after 2hours. The reaction mixture was changed after the addition of catalytic amount of DLP-Au-NPs. Within 10 minutes, the color changed from vivid red to colorless. This confirmed the considerable catalytic impact with DLP-Au-NPs [87].

Figure 15. UV-visible spectra of Congo red [87].

Figure 15. UV-visible spectra of Congo red [87].

Figure 16. UV-Visible spectra of Congo Red reduction by dalbergia coromandeliana Au-NP in the presence of NaBH₄ [87].

Figure 16. UV-Visible spectra of Congo Red reduction by dalbergia coromandeliana Au-NP in the presence of NaBH₄ [87].

Additionally, the catalytic activity of Au-NPs for the degradation of MO was investigated the results revealed a maximum absorbance at 465 nm (Figure).When DLP-Au-NPs and NaBH₄ were present, the dye colour changed from orange to colorless. Methyl orange was not reduced affectively only by NaBH₄ in the absence of Au-NPS. But in the presence of DLP-Au-NPs MO was efficiently reduced within 8 minutes [87].

Figure 17. UV-Visible spectra of Methyl Orange [87]_.

Figure 17. UV-Visible spectra of Methyl Orange [87]_.

Figure 18. UV-Visible spectra of methyl orange reduction by DLP-Au-NPs [87].

Figure 18. UV-Visible spectra of methyl orange reduction by DLP-Au-NPs [87].

Removal of Malachite green using Activated Carbon Gold Nanopartices (Au-NP-AC):

Malachite green is a basic, synthetic water-soluble N-methyl-di-amine-nitro-phenyl methane dye that is used in many industries such as silk, wool cotton, paper and leather industry for coloring purpose. It is also widely used in aquaculture across the globe eradicate fish parasites and mold growth. Activated carbon (AC) seems to be universal adsorbent for dye removal because it contain various reactive sites and having porous structure and large surface area. For the removal of MG an easy, affordable, quick, sensitive and aided adsorption technique with UV detection has been devised. With the help of central composite design and response surface methodology the effect of significant variables including time, pH, initial MG concentration, temperature and amount of adsorbent were examined and optimized. Gold nanoparticles loaded with activated carbon was prepared and characterized by different techniques. The adsorption kinetics and isotherms of MG removal on this adsorbent were then examined with ultrasound assistance serving a straight forward sensitive method. Ultrasound was used to examine dye solution clearance. In this experiment MG chemical was mixed with Au-NP-AC. The experiment was carried out by taking known amount of adsorbent dye and specified amount of dye solution in a flask at pH 7. The flask was then kept at required temperature for some time. The sample was centrifuged and examined after adsorption test. At maximum wavelength 619nm and working concentration the concentration of dye was examined by photometric method. On the activated carbon surface the activated sites were deprotonated by increase in pH and the OH group or COOH absorb MG by electrostatic interaction or hydrogen bonding. It was confirmed that with increase in time maximum absorption was observed. [78].

Figure 19. Degradation of Malachite green using Activated Carbon Gold Nanopartices (Au-NP-AC) [78].

Figure 19. Degradation of Malachite green using Activated Carbon Gold Nanopartices (Au-NP-AC) [78].

Synthesis of Gold Nanoparticles:

1. Synthesis of gold nanoparticles by using marine alga (Sargassum wightii):

Characterization Techniques:

The pattern of light absorption lies between 300nm and 800 nm. The band was seen in the UV-visible spectra at 527 nm. [80].

Figure 20. UV-Visible spectra of absorption of Au-NPs synthesized by marine alga [80].

Figure 20. UV-Visible spectra of absorption of Au-NPs synthesized by marine alga [80].

TEM micrograph

A large number of gold nanoparticles measured in the TEM micrograph ranged in size from 8 to 12 nm, and the shape of gold nanoparticles was thin planner structures rather than spherical structures. X-ray diffraction was used to verify the particle's crystalline nature and the resulting XRD pattern. [80].

Figure 21. X-ray diffraction of Au-NPs synthesized by marine alga [80].

Figure 21. X-ray diffraction of Au-NPs synthesized by marine alga [80].

2. Synthesis of gold nanoparticles by using Scutellarin barbata:

Figure 22. UV-Visible spectra of Au-NPs synthesized by Scutellarin barbata [81].

Figure 22. UV-Visible spectra of Au-NPs synthesized by Scutellarin barbata [81].

Figure 23. Average Size of Au-NPs by DLS [81].

Figure 23. Average Size of Au-NPs by DLS [81].

Figure 24. Energy dispersive X-ray (EDX) and analysis of Scutellaria barbata-derived AuNPs [81].

Figure 24. Energy dispersive X-ray (EDX) and analysis of Scutellaria barbata-derived AuNPs [81].

Figure 25. Presence of different functional group on the surface of synthesized Au-NPs from Scutellaria barbata [81].

Figure 25. Presence of different functional group on the surface of synthesized Au-NPs from Scutellaria barbata [81].

3. Synthesis of gold nanoparticles by Alpinia nigra:

Removal of Rhodamine B and Methyl Orange dyes by using ANL-AnNPs:

200mg/L -1-solution of both methyl orange and Rhodamine B were treated with 5mg of ANL- AuNPs (Alpinia nigra). Stirred the solution for 30 minutes in the dark for absorption or desorption and then carried these mixtures in the sunlight to continue reaction. 5ml of each mixture collected at the interval of 15 minutes, 45 minute, 60 minute, 90 minutes, and 120 minutes and Nano particles were removed after centrifugation. The supernatant was observed for UV-Visible analysis for Rhodamine B (553 nm) and for Methyl orange (464 nm). Percentage of degradation was collected by the formula

$$\% \mathbf{degradation}=\frac{{\mathbf{A}}^{\mathbf{o}}-\mathbf{A}}{{\mathbf{A}}^{\mathbf{o}}}\times 100$$

Where A° is the absorbance of a dye solution that has not been added with ANL-AuNPs and was exposed to sunlight, and A was the absorbance of the mixture [82].

Rhodamine B and methyl orange degradation was tracked with the use of a UV-visible spectrometer. Methyl orange's absorption maxima was observed at around 464 nm, while Rhodamine B absorption maxima was observed at about 553 nm. Rhodamine B and methyl orange have rate constants of 1.494 10-2 min-1 and 1.443 10-2 min-1, respectively. The regression coefficient values for methyl orange and rhodamine B are 0.985 and 0.965, respectively, indicating that the pseudo-first order kinetic model matches the data from the experiment. The presence of sunlight speed up the degradation process and led to the formation of low-energy electrons in gold atoms.

Figure 26. Percentage degradation of Rhodamine B [82].

Figure 26. Percentage degradation of Rhodamine B [82].

4. Synthesis of graphene/Au-NPs:

5. Adsorption of MB dye using Polyaniline coated gold aryl particles:

Figure 27. UV-Vis monitoring of (a) 6 mg L^-1 MB adsorption in 80 mg L^-1 and 480 mg L^-1 Au-COOH NPs for 24 hours in the presence of NS (nano sand) (b) Au-COOH/PANI (100 and 200 mg PANI) [77].

Figure 27. UV-Vis monitoring of (a) 6 mg L^-1 MB adsorption in 80 mg L^-1 and 480 mg L^-1 Au-COOH NPs for 24 hours in the presence of NS (nano sand) (b) Au-COOH/PANI (100 and 200 mg PANI) [77].

Table 7. Pseudo first and second order was applied to analyze the adsorption process.

Comparison of Reaction kinetics for both Au-COOH NPs and Au-COOH NPs/PANI

Pseudo second order rate constants has more correlation coefficient than first order. After analyzing result, it is concluded that more adsorption sites were available in nano composites than in NPs. After MB adsorption, FTIR of nano sand has been run, and it confirmed the MB adsorption by NPs. The adsorption peak of dye moved to 827, 883, and 1593 cm^-1 from 828, 885, and 1596cm^-1, these shifts symbolizes the π-π interaction in dye and ability of adsorption by NPs [77].

Figure 28. UV–vis spectrum of (a) 6 mg L⁻¹ MB +5 mg NS soaked in Au−COOH NPs (480 mg L−1 ) for 24 h, (b) Pseudo-second order [77].

Figure 28. UV–vis spectrum of (a) 6 mg L⁻¹ MB +5 mg NS soaked in Au−COOH NPs (480 mg L−1 ) for 24 h, (b) Pseudo-second order [77].

Figure 29. UV–vis spectrum of (a) 6 mg L⁻¹ MB +5 mg NS soaked in 5 mg of Au−COOH NPs/PANI for 24 h, (b) Pseudo-second order. Conditions: synthesis in PANI (200 mg) at 0.2 V [40].

Figure 29. UV–vis spectrum of (a) 6 mg L⁻¹ MB +5 mg NS soaked in 5 mg of Au−COOH NPs/PANI for 24 h, (b) Pseudo-second order. Conditions: synthesis in PANI (200 mg) at 0.2 V [40].

6. Synthesis of Asp-AuNPs:

Figure 30. Degradation of Organic Dyes using Asp-AuNPs synthesize from L-Aspargine.

Figure 30. Degradation of Organic Dyes using Asp-AuNPs synthesize from L-Aspargine.

6.(. a) Degradation of Acid Red using Asp-AuNPs

Figure 31. Structure of Acid Red.

Figure 31. Structure of Acid Red.

Figure 32. (A) Structure of amaranth dye; (B) UV–visible absorption spectra of (a) amaranth, (b) amaranth in presence of NaBH4; (C) absorption spectra of amaranth at different times in presence of Asp-AuNPs and NaBH4; (D) pseudo first-order kinetics of degradation of amaranth. [9].

Figure 32. (A) Structure of amaranth dye; (B) UV–visible absorption spectra of (a) amaranth, (b) amaranth in presence of NaBH4; (C) absorption spectra of amaranth at different times in presence of Asp-AuNPs and NaBH4; (D) pseudo first-order kinetics of degradation of amaranth. [9].

7. Annona squamosa Au-NPs and its degradation activity:

Figure 33. Synthesis of Au-NPs from Annona squamosa and their catalytic activity for Congo Red, 4-Nitrophenol and Methylene Blue.

Figure 33. Synthesis of Au-NPs from Annona squamosa and their catalytic activity for Congo Red, 4-Nitrophenol and Methylene Blue.

8. Amylopectin-TiO2- Au Nanocomposite and its degradation activity:

Synthesis of Nano-composite:

10. Gold nanoparticles synthesize on activated carbon:

11. IPEI coated GNPs:

Method-II

10mg of IPEI was dissolved in 1 ml of deionized water, which was then added to a warmed oil bah at at 95⁰C. The effects of pH on the IPEI coated GNPs after was also studied [89].

Table 6. MW, pH, and mass ratio effects on the characteristics of GNPs.

Table 6. MW, pH, and mass ratio effects on the characteristics of GNPs.

Sample Name	PEI Mw(kDa)	lPEI: Au (weight ratio)	Sizeα[nm]	PDIb	Zeta potential [mV]
Particle formation Method I
Control-I	25	1:21	31.53	0.33	+44
lPEI2.5-Au	2.5	1:21	24.12	0.22	+51
Method II
Control-II	25	1:21	25.55	0.23	+52
pH 3.5	25	1:21	52.42	0.43	+53
pH 5	25	1:21	24.32	0.45	+47
lPEI2.5-Au	2.5	1:21	51.42	0.25	+50
Post-synthetic particle properties Method I
pH 3	25	1:21	51.55	0.44	+34
pH 5	25	1:21	46.10	0.47	+31
pH 7	25	1:21	49.27	0.47	+16
Method II
pH 3	25	1:21	50.50	0.46	+24
pH 5	25	1:21	41.62	0.41	+38
pH 7	25	1:21	50.97	0.52	+14

12. Biosynthesis gold nanoparticles using Bacillus marisflavi:

Figure 34. Formation and stabilization of gold nanoparticles.

Figure 34. Formation and stabilization of gold nanoparticles.

3). Catalytic activities of Asp-AuNPs in degradation of organic dyes:

A) Rhodamine B (RB) Degradation

Figure 35. Structure of Rhodamine B.

Figure 35. Structure of Rhodamine B.

Figure 36. Absorbance maxima value (553 nm) vs time in presence of (a) 20 μL, (b) 15 μL and (c) 10 μL of Asp-AuNPs; NaBH4 concentration: 5 mM, inset shows the pseudo first-order kinetics of degradation for 20 μL of Asp-AuNPs, (D) absorbance spectra of RB at different times in the presence of Asp-AuNPs (15 μL) and NaBH4 [9].

Figure 36. Absorbance maxima value (553 nm) vs time in presence of (a) 20 μL, (b) 15 μL and (c) 10 μL of Asp-AuNPs; NaBH4 concentration: 5 mM, inset shows the pseudo first-order kinetics of degradation for 20 μL of Asp-AuNPs, (D) absorbance spectra of RB at different times in the presence of Asp-AuNPs (15 μL) and NaBH4 [9].

Figure 37. Structure of Methyl orange.

Figure 37. Structure of Methyl orange.

Figure 38. A) Structure of MO dye; (B) UV–visible absorption spectra of (a) MO, (b) MO in presence of NaBH4, (c) MO dye in presence of Asp-AuNPs and NaBH4; (C) optimization of NaBH4 and Asp-AuNPs concentration for degradation of MO; (D) absorbance spectra of MO at different times in the presence of optimized concentration of Asp-AuNPs and NaBH4; (E) pseudo first order kinetics of degradation [9].

Figure 38. A) Structure of MO dye; (B) UV–visible absorption spectra of (a) MO, (b) MO in presence of NaBH4, (c) MO dye in presence of Asp-AuNPs and NaBH4; (C) optimization of NaBH4 and Asp-AuNPs concentration for degradation of MO; (D) absorbance spectra of MO at different times in the presence of optimized concentration of Asp-AuNPs and NaBH4; (E) pseudo first order kinetics of degradation [9].

4. Degradation of 4-NP by Annona Squamosa Au-NPs:

Figure 39. UV-Visible spectra of degradation of 4-Aminophenol by Annona squamosa Au-NPs along with NaBH₄ [85].

Figure 39. UV-Visible spectra of degradation of 4-Aminophenol by Annona squamosa Au-NPs along with NaBH₄ [85].

Degradation of MB and CR:

Au-NPs were utilized as a catalyst in the reduction of MB and CR in the presence of NaBH₄. The reaction mixture was diluted to 10ml with double distilled water and agitated for five minutes in accordance with standard technique. It was made up to 2 ml of 1mM MB solution and 1 ml of 10mM NaBH₄ along with Annona squamosa Au-NPs. UV-Visible spectra was taken after 1 min. [85].

Figure 40. UV-visible spectra of Methylene Blue reduction by Annona squamosa Au-NPs.

Figure 40. UV-visible spectra of Methylene Blue reduction by Annona squamosa Au-NPs.

The absorption peaks of MB in aqueous media was recorded at 614 nm and 665 nm. The absorption peak of MB slightly decreases in the presence of NaBH₄ after 60 mint. When AU-NPs were added in the reaction mixture MB dye was completely reduced after few minutes. For the reduction of MB dye the temperature used was 15-30 ⁰C. A plot was drawn between Ink verses 1/T. It was shown that with the increase in temperature reaction rate increases [85].

Figure 41. Kinetics of Degradation Reaction of Methylene Blue.

Figure 41. Kinetics of Degradation Reaction of Methylene Blue.

Two absorption peaks may be seen in the aqueous CR's UV-Vis spectrum at 495 nm and 342 nm. When NaBH₄ Is used alone the CR was not reduced effectively after 120minutes but when AU-NPs was added in it the CR was completely reduced after 150 sec [85].

Figure 42. UV-visible spectra of Congo Red reduction by Annona squamosa Au-NPs [85].

Figure 42. UV-visible spectra of Congo Red reduction by Annona squamosa Au-NPs [85].

While maintaining the other parameters constant, the impact of temperature (15 to 30 C) on the lowering of CR was examined. A plot was drawn by Ink verses 1/T. by increasing the temperature the rate of reaction increase [85].

Figure 43. Kinetics of Degradation Reaction of Congo Red.

Figure 43. Kinetics of Degradation Reaction of Congo Red.

5. Removal of methylene blue by using Au-NPs formed from activated carbon:

Figure 44. Schematic representation of MO and MB degradation by cationic GNPs:.

Figure 44. Schematic representation of MO and MB degradation by cationic GNPs:.

Conclusion:

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