The most commonly used method for the morphological analysis of nanoparticles is SEM imaging. Kalaiyan et al. synthesized particles between 1.5 µm and 4.5 µm by using Moringa oleifera leaf extract and examined the bactericidal activity of these particles [28]. The size of the particles which were synthesized by Narasaiah et al. by using Drypetes sepiaria leaf extract were determined as 298 nm in SEM and used for dye degradation [29]. However, Gültekin et al. recorded the size of the particles that they obtained by using the fruit extract of Erzincan Cimin grape as 25–50 nm [30]. In this study, two SEM images of the synthesized CuNPs, recorded at 10.0k and 40.0k magnification, have smooth spherical forms. The sizes of the particles are between 109 nm and 184 nm (Fig. 1). Although the obtained particles are bigger than the particles of chemical synthesis methods, they are acceptable if compared with the results of green synthesis methods.
EDX is an analytical method frequently used in nanoparticle studies to determine the relative abundance of the elements and chemical composition of the solid surface. The recorded distribution map of SEM images and EDX graph proves the presence of major elements of the copper oxide nanoparticle. According to the recorded EDX results, the mass fraction of copper in the product is 62.81%, while the mass fraction of oxygen is 22.97%. Except these two major elements, carbon is also detected as 14.22% (Fig. 2).
XRD analysis is an essential characterization toll in nanoparticle research. It is known that copper nanoparticles have two forms as Cupric Oxide (CuO) and Cuprous Oxide (Cu2O) [31]. Ahmad et al. recorded diffraction peaks at 32.8°, 35.9°, 39.1°, 46.3°, 49.1°, 52.9°, 58.7°, 66.6°, 68.3° and 72.6° for CuO; while they regarded as evidence of Cu2O that recorded at 29.4°, 36.8°, 42.1°, and 61.9° [32]. On the other hand, Akter et al. labeled the diffraction patterns recorded at 32.72°, 35.97°, 38.51°, 49.96°, 52.60°, 57.86°, 61.17°, 66.27°, 68.22°, 71.96°, and 75.01° as CuO, while 36.40°, 42.92°, 52.33°, 61.47°, and 73.94° patterns are presented as evidence of Cu2O [33]. In this study, the peaks at 29.94°, 36.82°, 42.74°, 62.83°, and 74.36° are attributed to the crystal planes (111), (200), (211), (222), and (400) respectively. These results recorded that the amount of Cu2O was greater than that of CuO. The relative higher intensity at 36.82° for Cu2O also indicate bulk proportion in the total mixture (Fig. 3).
UV-Vis spectra of CuNP, extract and CuSO4 solution were recorded for track the changes after NP synthesis by green synthesis. In the spectrum of the CuSO4 solution, only a slight shoulder is observed at 263 nm, except for the broad band at 595 nm, which is compatible of the solution color. For apricot extract, the band observed at 287 nm is in accordance with the pale-yellow extract. Although a very slight rise was recorded at 518 nm for the CuNP obtained from these two precursors, a clear band is not observed in the UV-Vis spectrum (Fig. 4-a). FTIR spectra of apricot extract and CuNP were also performed. The band recorded at 3371 cm− 1 is due to the O-H structure of the aqueous phase while 2924 cm− 1 can be labeled as C-H. The sharp peaks observed at 1574 cm− 1 for NP and 1743 cm− 1 for the extract could be attributed to C = O. The bands observed in 1300–1400 cm− 1 can be recorded as the presence of both C = C and C-N. On the other hand, the band observed at 1026 cm− 1 in both spectra can be interpreted as C-O in the structures. The most remarkable band on the IR spectrum of CuNP is the Cu-O band, which is prominently observed at 602 cm− 1, and this band could be approved as another proof of the copper oxide formation.
Shertate and Thorat investigated the decolorization effect of Marinabacter algicola MO-17, which was isolated from the natural marine environment, for MO-1 and noted that 800 µgmL− 1 dye was decolorized within 24 hours [9]. Hadibarata et al. have tried various species of T. Hazianum, a fungi strain isolated from a rain forest in Malaysia, for MO-1 decolorization; they noted that, among these isolated species, RY44 achieved the highest decolorization rate with 79% [34]. Vanlalmingmawia et al. tried to eliminate MO-1 by Ag0/TiO2 nanocomposite with using UV-A and LED light, and they achieved the highest elimination with UV-A as 60.61% [8]. Siddique et al., tested chemically synthesized CuS nanoparticle for removal of dye, including MO-1, in various environments [35]. They degraded 51% of MO-1 in 180 minutes with their method and also noted that they provided 100% removal of the dyestuff in the presence of H2O2. To the best of our knowledge, CuNP nanoparticles (synthesized chemically or green way) has not been studied for MO-1 removal so far.
MO-1 gives two bands at 272 nm and 375 nm in UV-Vis. absorption spectra, also have two shoulders at 196 nm and 224 nm. In this study, the processes were carried out by using the band at 375 nm. It was determined that the recorded absorbance versus the different molar concentration of MO-1 was linear, the regression value of the obtained calibration graph was calculated as 0.9999, and the equation was figured out as y = 16.15438 x [MO-1] (Fig. 5).
Before examining the removal capacity of CuNP, MO-1 solution was prepared and the stability of the continuously stirring (1500 rpm) solution under ambient light was analyzed for 120 minutes at constant pH and temperature. No significant change was observed in the absorbance of the dye versus time, except small fluctuations (Fig. 6-a). On the other hand, 5 mL of the 25 ppm stock MO-1 solution was put in glass container and 2.5 mg of green CuNP was added. The solutions were also sonicated 10 min. After stirring at 1500 rpm for 0 min., 10 min., 20 min., 30 min., 60 min., 90 min., and 120 min., the latter solution was centrifuged at 7000 rpm and the concentration of the remaining MO-1 was determined with UV-Vis absorption measurement. Effective MO-1 adsorption was determined and analyzed kinetically (Fig. 6b and 6c).
The kinetic constants of dye adsorption are tried to estimate by applying a pseudo-first order and second-order reaction rate [36, 37]. The adsorption followed second order reaction rate which suits the equation given below:
$$\frac{1}{{\text{C}}_{\text{t}}}-\frac{1}{{\text{C}}_{0}}=\text{K}.\text{t}$$
Where t (hr) is the reaction time, C0 is the initial concentration, Ct is the concentration at time t, K represents the second order reaction rate constant. The rate constant (K) of was obtained by plotting [(1/Ct)-(1/C0)] against the reaction time t as shown in Fig. 6d which is showed that the adsorption of dye is completely dose dependent and fits perfectly with second order kinetic since R2 (R2 = 0.998) value of second-order reaction is higher than first order reaction for all doses. The rate constant was recorded as 2.4381 min− 1.
The changes in adsorption at different pH values was also investigated. The pH value of the stock MO-1 solution prepared without any adjustment was measured as 7.65 and it must be noted here that the addition of CuNP did not make any differences on the pH of the solution. 5 mL of the 50 ppm stock MO-1 solution was put in glass container and 0 mg, 2.5 mg, 5.0 mg, 10,0 mg, and 15,0 mg of green CuNP was added. The solutions were also sonicated 10 min. After stirring at 1500 rpm for 30 min., the latter solution was centrifuged at 7000 rpm and the concentration of the remaining MO-1 was determined with UV-Vis absorption measurement. The removal efficiency was calculated by equation below [38],
$$\text{R}\text{e}\text{m}\text{o}\text{v}\text{a}\text{l} \text{E}\text{f}\text{f}\text{i}\text{c}\text{i}\text{e}\text{n}\text{c}\text{y}\left(\text{%}\right)=\frac{{\text{C}}_{0}-{\text{C}}_{\text{t}}}{{\text{C}}_{0}}\text{x}100$$
C0 is the initial concentration and Ct is the concentration at time t of MO-1 dye. It was observed that the dye adsorption reached 77% with the addition of 5 mg CuNP at pH = 7.65, while the graph was reached a plateau after the removal efficiency exceeded 90% with the addition of 10 mg CuNP. Additionally, it should be noted that the addition of 2.5 mg of CuNP adsorbed reasonable amount of the dye.
The dye stock solution was adjusted to pH = 5.00 with 0.1 M HCl and pH = 10.00 with 0.1 M NaOH, and the removal effect of different initial amounts at these pH values was investigated. Effective and nearly linear adsorption was recorded at pH = 5.00. More than 70% dye adsorption was detected 10 mg of CuNP at acidic pH, while over 90% removal efficiency was measured with 15 mg of CuNP was added. On the other hand, it was determined that the adsorption quickly reached above 70% with 5 mg of CuNP at pH = 10.00.
