A highly efficient and multifunctional biomass supporting Ag, Ni, and Cu nanoparticles through wetness impregnation for environmental remediation

2.1 Chemicals and reagents

Salts of silver, nickel and copper nitrate and sodium borohydride were purchased from Sigma-Aldrich (St. Louis, Missouri). TP was purchased from the market and nitrophenols and dyes from BDH, London, UK. The deionized water used throughout the experiment was obtained from the Millipore-Q machine present in the chemistry department of King Abdulaziz University, Saudi Arabia.

2.2 Synthesis of Ag@TP, Ni@TP and Cu@TP NPs

Stock solutions of silver, copper and nickel nitrate (1 m) were prepared, and then diluted to 0.1 m. After that, 1 g adsorbent (TP) was added to 50 ml of a 0.1 m solution. All of the solutions were kept on an orbital shaker for 5 h to completely saturate the adsorption sites, and the reactions were carried out in triplicate; the adsorption capacity was determined with an anatomic adsorption spectrophotometer. After that, the TP was dried and washed with the distilled water in order to remove any remaining unwanted materials. Then 10 mg of the dried TP was treated with 1 m of the freshly prepared NaBH₄ solution, where it turns black, indicating the formation of metal NPs. The synthesized NPs were then used against the reduction of isomeric nitrophenols and dyes and the bactericidal potential was evaluated.

2.3 Instrumental characterization

2.4 Reduction of nitroarene and azo dyes

A concentration of 0.1 mmol PNP and 0.05 mmol of all the dyes, along with freshly prepared NaBH₄ (1 m) were used in this experiment. By adding NaBH₄, the nitrophenols shifted and increased in wavelength (red shift) due to the formation of more stable phenolate anions, which appeared at 400 nm, however, the azo dyes transformed to the hydrazine derivative and cationic dyes transformed to leuco derivatives after NaBH₄ addition. The modified TP (after adsorbing the metal ions) becomes black in color after treating with 0.5 ml of 1 m freshly prepared NaBH₄ solution. The black color clearly shows the formation of metallic NPs on the surface of TP. The synthesized NPs were dried and applied for the degradation of isomeric nitrophenols and dyes. Before the addition of NPs, 0.5 ml of 1 m freshly prepared NaBH₄ solution was added to a UV cuvette individually containing 3 ml of nitrophenols and dyes solution. The decrease in the concentration of the analyte was measured with UV-vis spectrophotometer.

2.5 Bacterial cell culturing

Escherichia coli O157:H7 was used as a model for evaluation of the antibacterial activities, and was supplied by King Fahad Hospital, Jeddah. For the antibacterial test, bacterial cells were grown on nutrient agar (Sigma-Aldrich-70148) comprising 1 g/l meat extract, 2 g/l yeast extract, 2 g/l sodium chloride, 5 g/l peptones, and 15 g/l agar in distilled water. Medium plates of nutrient agar were prepared, solidified and sterilized. Bacterial cultures were cleaned through solidification on these plates.

2.6 Antibacterial assessment

The colony forming units count method was used for the assessment of antimicrobial activities of the synthesized materials. About 0.15 g/ml of samples that contained 9 ml of liquid medium (Nutrient Broth) was added to all test tubes. Afterwards, 1 ml of E. coli O157:H7 was put in a 9 ml tube that contained liquid medium and incubated at 37°C for 24 h. The samples were diluted serially and later spread on nutrient agar. Using a colony counter, the number of colonies were counted and the experiment was performed in triplicate.

3.1 Synthesis and characterization

The percent adsorption of metal ions by TP was calculated by the decrease in the concentration of Cu²⁺, Ag¹⁺ and Ni²⁺ ions. The decreases in metal ion concentrations from their solution were calculated by comparison with their corresponding stock solution using Eq. (1):

where Cs is the concentration of metal ions (mg/l) and Ct is the residual metal ions concentration after 5 h of treatment with TP.

The percent adsorptions of the Cu²⁺ ions were highest among all the salts, which showed 44.26% adsorption capacity as compared to Ni²⁺ and Ag¹⁺, which were 41.67% and 37.49%, respectively.

The uptake of the metal salts is due to the interactions with -OH functional groups present in the chemical constituents of the TP, largely phenolic and terpenoids compounds. The % adsorption of metal ions are presented in Table 1.

Similarly, the concentrations of metal ions in the solution were calculated by using Eq. (2):

were Cr is the atomic adsorption reading, 50 is the amount of sample taken for adsorption from 1000 ml stock solution containing 1 g of adsorbent, and Cs is the concentration of metal ions in the stock.

The amounts of metal ions adsorbed on 1 g of adsorbent from the stock solution were calculated by using Eq. (3):

Cr is the concentration of metal ions in the mixture and Pa is the percent of metal ions adsorbed on 1 g of adsorbent, as shown in Figure 1A.

Figure 1:

Atomic adsorption studies for the percent uptake of different metal ions by turmeric powder (TP) (A); mg of metal ions adsorbed on 1 g of the adsorbent (B).

The amounts of the metal adsorbed by TP were calculated using Eq. (3). The amounts of Ag¹⁺, Cu²⁺ and Ni²⁺ adsorbed in 1 g of TP were 113.4 mg, 96.26 mg and 65.21 mg, respectively, as shown in Figure 1B.

Figure 2A depicts the diffraction pattern of TP and NPs. A broad peak at 2θ=20° appeared in the XRD data of all samples, due to the presence of organic constituents present in the TP and NPs. The Ag@TP displayed diffraction peaks at 2θ=38.33° (111), 2θ=64.7° (220) and 2θ=77.7 (311) [38], [39]. The Cu@TP NPs displayed a sharp peak at 2θ=43.1° (111). The Ni@TP displayed peaks at 2θ=43.6° (111), and 2θ=48.8° (200) [40], [41].

Figure 2:

(A) X-ray diffraction (XRD), (B) Fourier transform infrared (FTIR) and (C) thermal gravimetric analysis (TGA) spectrum of pure turmeric powder (TP), Ag@T, Ni@T and Cu@T nanoparticles (NPs).

The functional groups and particle interactions with the materials were determined using ATR-FTIR. All of the samples indicated a broad peak at approximately 3390 cm⁻¹ which is assigned to the -OH stretching vibration. The broadness of the peak indicates the presence of H-bonding, which was clearly found in all the stated samples. Similarly, another common peak in all of the NPs, including TP, is the C-H asymmetric stretching vibration, which appeared at 2933 cm⁻¹; this is because the organic compound contains C-H bonds. All of the samples also contain a peak at 1643 cm⁻¹, which might be due to the presence of a conjugated carbonyl group. This group is present in the TP, which mainly contains curcumin and curcuminoids with conjugated carbonyl groups. The absorption peaks at 1036 cm⁻¹ in all of the samples are due to the C-O vibrations. The metal NPs peaks were clearly displayed at 657 cm⁻¹ in Ag@TP, Ni@TP and Cu@TP NPs. No change in the chemical composition of TP was observed, which suggested that TP acts only as support to the NPs. The FIR-ATR data are depicted in Figure 2A.

TGA was performed to explore the stability of NPs. The TGA thermogram of the stated NPs displayed two weight losses. The first weight loss which occurred at 246°C is due to the TP, while the second weight loss, which occurred at 335°C, is attributed to the presence of the uptake metal ions as shown in Figure 2C.

The FESEM images were taken at low and high resolutions. The FESEM images of the pure TP were recorded at 10 μm and 100 nm, and showed a highly rough increase surface of the TP, while the Ag@TP NPs at 100 nm which displayed 15–20 nm particle size are supported on the surface of the TP, as well as adsorbed inside the TP as shown in the inset of Figure 3C and D.

Figure 3:

(A) Low and (B) high resolution field emission scanning electron microscope (FESEM) spectrums of turmeric powder (TP) and (C, D) high resolution spectrum of Ag@T nanoparticles; the white spot in the spectrum clearly indicated the presence of Ag nanoparticles.

The energy-dispersive X-ray spectroscopy (EDS) spectrum is depicted in Figure 4. The EDS spectrum showed that the NPs are supported by the plant materials and give a strong peak at 3 KeV, which confirms the formation of AgNPs [42]. The C, O and Ag appeared in the EDS spectrum at 65.02 weight%, 24.78 weight% and 10.20 weight%, respectively. The presence of C and O elements in the EDS spectrum supported the stabilization of Ag NPs by the TP.

Figure 4:

Energy-dispersive X-ray spectroscopy (EDS) spectrum of Ag@TP nanoparticles.

3.2 Catalytic reduction of nitrophenol

Initially, the TP and its adsorbed Ag, Ni and Cu ions were evaluated for their adsorption capacity. Some 10 mg of the pure TP and the modified TP were used against 3 ml of 0.05 mmol methyl orange (MO) solution for the adsorption or degradation phenomena. However, we observed no such physical or chemical change after 4 h, which showed that the materials were inactive for adsorption or degradation. After this, 10 mg of the TP and TP adsorbed Ag, Ni and Cu ions were treated with 0.3 ml of 1 m aqueous NaBH₄ solution, which imparts a black color to the adsorbed metal ions on TP, suggesting the formation of the respective metal NPs; however, the pure TP was not changed. We observed that the TP is not reduced to NPs after the addition of NaBH₄, which reflects that the pure sample has no inherent properties for NPs formation, and it is only the adsorbed metal ions, chelated with the TP, which are reduced to metallic NPs after treating with the reducing agent.

After the formation of metal NPs, the catalysts were screened against 3 ml of 0.05 mmol solution of MO dyes; the fastest catalytic performance was achieved with Ag@TP NPs followed by Cu@TP, and the slowest performance was obtained with Ni@TP, while the TP had no role in this reduction reaction, as shown in the inset of Figure 5A. After screening the catalysts, the Ag@TP catalyst was largely studied for the reduction of isomeric nitrophenols (ONP, MNP and PNP), two cationic dyes (methylene blue [MB] and rhodamine B [RB]) and two anionic dyes (MO and Congo red [CR]).

Some 10 mg of the Ag@TP NPs was employed for the reduction of 3 ml of 0.1 mmol of ONP and MNP and PNP and monitored in a time-dependent UV-visible spectrophotometer. The phenolate anions of ONP and MNP were exhibited at 415 nm and 395 nm and 400 nm respectively, and gradually decreased with the passage of time, as shown in Figure 5B and C. The conversion of PNP to p-aminophenol (PAP) is a benchmark reaction to check the catalytic activity of a catalyst, because it is a green and atom-economic reaction, where all the reactants are completely converted into the desired product without any side reactions. Furthermore, PAP is an extremely important class of organic compounds, which are mainly used at the industrial level for the development of hair color, photographic developer, and dye production [43]. The pure PNP exhibited a peak at λ_max 318 nm, however, after addition of the reducing agent (NaBH₄), the PNP shifted to a red shift at λ_max 400 nm. This is because the reducing agent takes the proton from the OH group of phenol, thereby creating a phenolate anion. The phenolate anion of PNP is more stable than the pure PNP due to the negative charge on the oxygen atom. The oxygen atom itself is highly electronegative, thus efficiently delocalizing the electrons onto the benzene ring, which is why the phenolate anions move toward the red shift (longer wavelength). Briefly, during this hydrogenation reaction, 10 mg of the Ag@TP NPs was employed for the reduction of 3 ml of 0.1 mmol PNP to PAP and monitored using the time-dependent UV-visible spectrophotometer, as shown in Figure 5D. The phenolate anions gradually decreased at 400 nm, labelled as point (b) in the inset of Figure 5D, and a gradual increase was observed at approximately 290 nm. The peak observed at 290 nm is largely reported in the literature for PAP, labelled as point (a) in the inset of Figure 5D. The % reductions are shown in the inset of Figure 5F, where 95% of ONP is converted in 9 min and 72% of MNP in 8 min, while 85% of PNP is converted in 26 min. The linear fit plots are depicted in Figure 5E, which show 0.935, 0.926 and 0.955 R values for ONP, MNP and PNP, respectively.

Figure 5:

Comparing the Ag@TP, Ni@TP and Cu@TP for methyl orange (MO) reduction (A), time-dependent UV-vis spectrum for the reduction of o-nitrophenol (ONP) (B), m-nitrophenol (MNP) (C) and p-nitrophenol (PNP) (D) to aminophenol, rate of reaction deduced from lnCt/Co vs time (E) percent reduction of the isomeric nitrophenols (F). The points a, and b in the spectrum (D) indicate the p-aminophenol peak and phenolate anion.

3.3 Catalytic reduction of methyl orange

Dyes are an extremely important class of organic compounds which are largely used in the textile industries, and it is believed that the dye industries played a key role in the development of a country [44]. However, unfortunately, the dye industries dump their remaining dyes to the water resources, which pollute the water and even at trace amounts, the dyes colorize the water and produce a thick foam-like layer on the surface of water which stops the diffusivity of sunlight and oxygen, thus creating a huge problem for aquatic flora and fauna and affecting the environment. Therefore, the complete eradication of dyestuff from the environment is extremely important. In the present study, we applied the synthesized Ag@TP NPs for the reduction of two anionic and two cationic dyes. Some 3.5 ml of the solution containing 3 ml of 0.05 mmol of the respective dyes and 0.5 ml of 1 m NaBH₄ solutions were placed in a cuvette and 10 mg of the Ag@TP NPs were added to the cuvette. MO dyes have λ_max at ~464 nm and CR at ~500 nm. After the addition of NaBH₄ solution, both dyes are converted to hydrazine derivatives [45], [46]. The reduction reactions started after the addition of the catalyst, and then periodically investigated through the time-dependent UV-vis spectrophotometer, as shown in Figure 6A and B. During the course of the reaction, 90% of MO was converted in 13 min and approximately 94% of CR dye was reduced in 26 min, as depicted in Figure 6A and B. The kinetics for the Ct/Co and % reduction of the dyes are depicted in Figure 6C and D, respectively.

Figure 6:

Time-dependent UV-vis spectrum for the degradation of (A) methyl orange (MO), (B) Congo red (CR); the points a and b in the spectrum of MO indicate the -NH2 and MO hydrazine derivatives and points a–d in the spectrum of CR are due to -NH2, naphthalene and hydrazine derivative of CR, respectively. The kinetics due to the decrease in the original concentration with respect to time appeared in (C) and % reduction of dyes (D).

Keeping the same experimental conditions of the cationic dyes, 3.5 ml of solution containing 3 ml of 0.05 mmol MB and RB dyes and 0.5 ml of 1 m NaBH₄ solution were placed in a cuvette along with 10 mg of the Ag@TP NPs. The MB dyes appeared at λ_max~664 nm and RB appeared at 554 nm, which gradually decreased with the passage of time, as depicted in the inset of Figure 7A and B. After the addition of NaBH₄ solutions, both the cationic dyes, MB and RB, are converted to leuco MB and leuco RB. The kinetics of the cationic dyes are displayed in Figure 7C and D, respectively. Approximately 99% of MB and RB dyes are reduced in 11 min and 18 min, respectively.

Figure 7:

Time-dependent UV-vis spectrum for the reduction of (A) methylene blue (MB) and (B) rhodamine B (RB); the points a and b in the spectra of MB and RB indicate the -NH2 and MB luco derivatives. The kinetics due to the decrease in the original concentration with respect to time appeared in (C) and % reduction of dyes (D).

3.4 Antibacterial activity

The synthesized NPs were evaluated for antibacterial activity. All the mentioned NPs exhibited a stronger performance then pure TP. Figure 8 and Table 2 show the development properties of the cell viability of E. coli (O157:H7) after treatment with TP, Ag@TP, Ni@TP and Cu@TP. The results identify that TP, after 12 h and 24 h, does not exhibit development prevention against E. coli. It was seen that Ag@TP, Ni@TP and Cu@TP exhibited improved prevention activity against the designated pathogenic bacteria. However, Ag@TP NPs indicated the greatest prevention rate with E. coli. Since E. coli is a gram-negative bacteria containing a thin peptidoglycan layer and has a complex cell wall, comprised of two cell membranes [47]. The cell membrane is negatively charged, which may work together with the positively charged Ag@TP NPs. The permeability of cell membranes is changed due to the interaction of Ag@TP NPs which stopped the intake of nutrients into the cell. Apart from the procedure, these NPs can also develop species that are highly reactive, such as hydrogen peroxide, superoxides or hydrogen radicles. For this reason, the feasibility and development of these pathogenic bacteria is reduced by using the stated NPs as compared with TP. This results clearly demonstrate that Ag@TP NPs can reduce the organic pollutant effect as well the growth of E. coli, which reflects their applications in the environmental and biological sectors.

Figure 8:

Nutrient agar plates showing antibacterial potential of (A) turmeric powder (TP), (B) Ag@TP, (C) Ni@TP and (D) Cu@TP.

Ag@TP indicated the strongest antibacterial activity, while the nanoparticles (NPs) showed good activity, then turmeric powder (TP).