Multifunctional Ternary NLP/ZnO@ l -cysteine- grafted-PANI Bionanocomposites for the Selective Removal of Anionic and Cationic Dyes from Synthetic and Real Water Samples

Abstract

The development of competent adsorbents based on agro-waste materials with multifunctional groups and porosity for the removal of toxic dyes from aqueous solutions is still a challenge. Herein, a bionanocomposite made up of neem leaf powder (NLP), zinc oxide (ZnO), and amino acid (l-cysteine)-functionalized polyaniline (PANI), namely, NLP/ZnO@l-cysteine-grafted-PANI (NZC-g-PANI), has been prepared by an in situ polymerization method. The as-prepared bionanocomposite was tested for the adsorptive removal of three anionic dyes, namely, methyl orange (MO), amido black 10B (AB 10B), and eriochrome black T (EBT), as well as three cationic dyes, namely, brilliant green (BG), crystal violet (CV), and methylene blue (MB), from synthetic aqueous medium. The morphological and structural characteristics of the NZC-g-PANI nanocomposite were examined with the help of HR field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Raman spectroscopy. FTIR and Raman studies show that the formulated NZC-g-PANI have an ample number of functional moieties such as carboxyl (-COOH), hydroxyl (-OH), amines (-NH2), and imines (-N═), thus demonstrating outstanding dye removal capacity. C-S linkage helps to attach l-cysteine with polyaniline. Moreover, the predominance of chemisorption via ionic/pi-pi interaction and hydrogen bonding between the NZC-g-PANI nanocomposite and dyes (BG and MO) has been realized by FTIR and fitting of kinetics data to the PSO model. For both BG and MO dyes, the biosorption isotherm was precisely accounted for by the Langmuir isotherm with qmaxvalues of up to 218.27 mg g-1for BG at pH 6 and 558.34 mg g-1for MO at pH 1. Additionally, thermodynamic studies revealed the endothermic and spontaneous nature of adsorption. NZC-g-PANI showed six successive regeneration cycles for cationic (MO: from 96.3 to 90.4%) and anionic (BG: from 94.7 to 88.7%) dyes. Also, batch adsorption operations were validated to demonstrate dye biosorption from real wastewater, such as tap water, river water, and laundry wastewater. Overall, this study indicates that the prepared NZC-g-PANI biosorbent could be used as an effective adsorbent for the removal of various types of anionic as well as cationic dyes from different aqueous solutions.