Kinetics and thermodynamic studies of organic dyes removal on adsorbent developed from Viola tricolor extract and evaluation of their antioxidant activity

Eco-friendly synthesis of nanoparticles is gaining attention due to its applications in all fields of science. By controlling different parameters (size, composition and shape, etc.), these nanomaterials behave differently and show a lot of biological activities. In the present study, synthesis of silver nanoparticles (Ag-NPs) was performed by using aqueous extract of Viola tricolor and AgNO3 as a precursor. UV–Vis spectrum displayed a broad peak of phyto-mediated synthesis of Ag-NPs in the range of 375–500 nm. Energy-dispersive X-ray (EDX) spectra exhibited a strong peak of silver. It was observed from scanning electron microscopy (SEM) spectra that Ag-NPs were roughly spherical in shape. Results of XRD studies indicated that average size of Ag-NPs was 42.01 nm. The presence of various functional groups in aqueous extract of Viola tricolor was confirmed by fourier transform infrared (FTIR) spectra. Synthesized Ag-NPs showed good total antioxidant activity 1.178 ± 0.07 against BHT as reference as well as good total phenolic contents 231.55 ± 1.21 GAE mg/g and moderate DPPH scavenging activity 75.73% at 500 mg/ml concentration. Adsorption capacity of Ag-NPs was checked with methylene blue (MB) and congo red (CR) dyes. Different factors like pH, adsorbent concentration, initial concentration of adsorbate, contact time and temperature were optimized for the utmost removal of methylene blue and congo red dyes from solution. Lagergren’s model was used to study the kinetics of the system. Kinetic studies revealed the reaction was pseudo-first order for both dyes. Various thermodynamic factors i.e., entropy (ΔS°), Gibb’s free energy (ΔG°) and enthalpy (ΔH°) of the system were also calculated.