Synthesis of silver nanoparticle from Vigna unguiculata stem as adsorbent for malachite green in a batch system

This study describes a simple and cost effective method for the synthesis of silver nanoparticles from locally available Vigna unguiculata L. stem extract, as alternative method to the expensive and toxic chemicals used, and its application as adsorbent for malachite green (MG) in a Batch system. The AgNPs were synthesized using AgNO3 as precursor and stem extract as reducing and capping agent. The AgNPs were characterized using UV-Vis, SEM-EDX, FTIR and XRD. The effects of contact time, pH of solution, initial dye concentration, adsorbent concentration and temperature were studied and they proved useful in the description of the adsorption process. AgNPs showed SPR bands at 455 nm. The XRD micrographs showed face centered cubic crystal structure with average crystalline size of the synthesized nanoparticle of similar to 25 nm. Alkaloids present in Vigna unguiculata L. stem played a role in the reduction of AgNO3. An optimum pH of adsorption was achieved at 9.0. The adsorption process was observed to be exothermic. Equilibrium isotherm models showed Langmuir isotherm with R-2 of 0.99194 and residual sum of squares (RSS) of 5.23541 x 10(-5), indicating monolayer adsorption. Kinetic studies revealed Pseudo-first order kinetics best fitted the rate of adsorption and intra-particle diffusion revealed that the adsorption process was controlled by surface phenomenon and intra-particle diffusion. The results showed the applicability of AgNPs in adsorption. However, the relatively low percentage removal (21.6% at 200 ppm) suggests that Silver nanoparticle synthesized from Vigna unguiculata L. stem, alone may not be sufficient in the complete adsorption of MG dye.