The variation of the amount of MB dye adsorbed by SNCM as a function of the adsorption time was studied at three different temperatures—293, 308 and 323 K—and the experimental results are shown in

Figure 8. It is clear from the figure that the amount of MB dye removed from solution by SNCM reached equilibrium within 30 min: 18.5, 19.6, and 19.9 mg MB/g SNCM at 20, 35, and 50 °C, respectively, and no significant improvement in the amount of MB dye removed was observed with a further increase in time. Additionally, it was clear that the adsorption was endothermic in nature, as the amount of MB dye removed was enhanced with increasing solution temperature: 18.8, 19.8, and 20.0 mg MB/g SNCM at 20, 35 and 50 °C after 60 min, respectively.

The experimental results of the effect of contact time on the removal of MB dye by SNCM were used to study the adsorption kinetics using the most used kinetic models—namely, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model, as presented in Equations (3) and (4), respectively:

where

q_{e} and

q_{t} are the values of the amount MB dye adsorbed per unit mass of SNCM at equilibrium and at any time

t, respectively;

k_{1} (min

^{−1}),

k_{2} (g/(mg∙min)) are the pseudo-first-order adsorption rate coefficient, and pseudo-second-order rate coefficient, respectively. Applying the pseudo-first-order kinetic model (Equation (3)) to the experimental results (

Figure 9), the plot of ln(

q_{e} −

q_{t}) vs.

t for MB dye at different temperatures did not converge well and did not give straight lines (as is clear from

Figure 9), with unacceptable regression coefficients. Additionally, the estimated values of the amount adsorbed at equilibrium (

**q**_{e}_{,}_{calc}) were far from the experimental values (

**q**_{e}_{,}_{exp}). This indicated that pseudo-first-order kinetic model is not appropriate for the description of MB dye removal by SNCM from water. Applying the pseudo-second-order kinetic model (Equation 4) to the experimental data, the plot of

t/q_{t} vs.

t converged well, with straight lines and an excellent regression coefficient higher than 0.99, as presented in

Figure 10. In addition, there was an excellent correlation between the calculated amount of MB adsorbed by the SNCM (

**q**_{e}_{,calc}) and the experimental values (

**q**_{e}_{,exp}). These findings confirmed the suitability of the pseudo-second-order kinetic model for describing the removal of MB dye from the model solution by SNCM.

There are many previous studies that also showed the suitability of the pseudo-second-order kinetic model for the description of MB dye from water by the different solid adsorbents, such as Fe

_{3}O

_{4}[email protected] SiO

_{2} nanocomposites [

45], zeolite synthesized from electrolytic manganese residue [

46], zinc oxide nanorods loaded on activated carbon [

47], activated carbon [

48], poly(sodium p-styrene sulfonate)/poly(methyl methacrylate) particles [

49], and many other adsorbents. The thermodynamic parameters include the enthalpy change (Δ

H), free energy change (Δ

G), and entropy change (Δ

S), and were calculated to evaluate the thermodynamic feasibility and the spontaneous nature of the MB dye removal by SNCM according to the following equations:

where

D is the distribution coefficient;

R is the gas constant (8.314 J·mol

^{−1}·K

^{−1}); and

T is the temperature (K). The values of Δ

H and Δ

S are determined from the slope and the intercept of the plots of ln

D versus 1/

T, which is associated with a good correlation coefficient; R

^{2} equal 0.961, as is shown in

Figure 11. The removal of MB dye using SNCM from water associated with Δ

H value of +144.0 kJ/mol, indicating the adsorption process was endothermic in nature. This finding confirmed the above-mentioned result that the adsorption is fast and obeys the pseudo-second-order kinetic model. The Δ

S value of +523 J/mol∙K indicates the increase in the degree of disorder upon the adsorption of the MB dye molecules by the SNCM. The Δ

G value was calculated based on Equation (7) at 20 °C, and the value was found to be negative (–9.18 kJ/mol), indicating that the process was spontaneous, and this value became more negative by raising the solution temperature: –16.9 kJ/mol and −24.6 kJ/mol at 35 °C and 50 °C, respectively. It could be concluded here that the more negative the Δ

G value, the more spontaneous the removal, which was accompanied by higher values of MB dye uptake by the SNCM. Additionally, the negative values of Δ

G, the positive value of Δ

H, and the positive value of Δ

S indicated that the removal of MB dye by SNCM is an entropy-driven process. According to the kinetics and thermodynamics study, the MB dye removal by SNCM could be described by the pseudo-second-order kinetic model, and was spontaneously endothermic and chemical in nature.

According to the above results, the adsorption capacity of MB by SNCM at ambient temperature is 18.8 g MB/g within 30 min. In comparison with other adsorbents, SNCM could be considered as a potential and promising adsorbent for the removal of organic dyes, such as MB from water. This adsorption capacity is much higher and better compared with spent rice biomass: 8.13 mg MB/g [

50]; activated carbon prepared from rice husk: 9.73 mg MB/g [

51]; natural Jordanian Tripoli: 16.6 mg MB/g [

52]; and lower compared with manganese oxide nanocorals: 41.26 mg MB/g [

46]; and surface hydroxyl group-enriched TiO

_{2} nanotubes: 57.14 mg/g [

53]. In general, based on the above results, it could be stated that SNCM is a promising and potentially competitive adsorbent for the removal of organic dyes such as MB from solution.