Fabrication of nanofibers using sodium alginate and Poly(Vinyl alcohol) for the removal of Cd(2+) ions from aqueous solutions: adsorption mechanism, kinetics and thermodynamics

Nowadays, separation of heavy metals from polluted wastewater is one of the most important environmental issues, and various methods have been investigated for treating polluted water and industrial wastewater. Surface adsorption using an inexpensive, biodegradable and environmentally consistent adsorbent can be considered an efficient and cost-effective method. One of these adsorbents is sodium alginate (SA). The purpose of this study was to fabricate composite nanofibers using poly (vinyl alcohol) (PVA) and sodium alginate to remove cadmium metal ion from aqueous solutions. For this purpose, polymer solutions consisting of poly (vinyl alcohol) (10% wt)/sodium alginate (2% wt) with three volume ratios of 0/100, 20/80 and 40/60 were first made, and then nanofibers were produced from the resulting solutions by electro-spinning process. The prepared nanofibers were examined by scanning electron microscopy (SEM) and the synthetic poly (vinyl alcohol)/sodium alginate nanofibers at a ratio of 40/60 were selected as adsorbent. The obtained nanofibers were characterized by Fourier-transform infrared spectroscopy (FTIR). The synthesized adsorbent was used to remove the aqueous solution of cadmium metal; the effect of various parameters such as changes in initial metal ion concentration, pH, temperature, contact time and stirring speed on the adsorption process were investigated, and optimum values of the parameters were obtained. The maximum amount of equilibrium adsorption under optimum experimental conditions was 67.05 mg/gr. The Taguchi experiment design method was used to optimize the three effective factors in the cadmium ion adsorption process. The results of the adsorption process were adapted to different adsorption isotherms such as Langmuir and Freundlich isotherms. The fit of the laboratory data to the Langmuir model was better, and the maximum adsorption capacity through this model was obtained equal to 93.163 mg/g of the adsorbent. Since the performance of an adsorbent depends on the duration of the adsorption process, the kinetics of the adsorption process were investigated by pseudo-first-order equation and pseudo-second-order equation. Moreover, the results indicated that the laboratory data showed a better fit to the pseudo-second-order model. Finally, the thermodynamic perspective was examined, and the process was found to be endothermic and spontaneous. The results showed the optimum values for maximum cadmium uptake.