Selective Co(ii) removal from aqueous media by immobilizing silver nanoparticles within a polymer-matrix through a formaldehyde cross linking agent

Abstract

Silver nanoparticles (AgNPs) were synthesized by a biological reduction method using Cyperus rotundus grass extract (CRGE) and were found to have a regular diameter of 1–100 nm. Polymer nanocomposites (PFR–AgNPs) were then prepared by encapsulating the green synthesized AgNPs within a phenol–formaldehyde resin (PFR) as a cross linking agent using a polycondensation method. The composites were then applied to the sorption of Co(II) from aqueous solution in a batch adsorption system with an initial concentration of 30–150 mg L⁻¹, a contact time of 10–50 min and a temperature of 303–333 K. The non-diffusible negatively charged sulfonic acid groups bound to the PFR template should significantly develop the penetration and preconcentration of the goal metal cations from the aqueous solution to the interior plane of the polymeric matrix and also create favorable conditions for Co(II) removal by the AgNP particles. Then, column adsorption studies were carried out for the Co(II) retention in the presence of alkali and alkaline earth metals (Na⁺, Ca²⁺ and Mg²⁺). Compared to PFR, PFR–AgNPs demonstrated extremely penetrative cobalt removal from wastewater in the presence of competing Ca²⁺, Mg²⁺, and Na⁺ ions, which were present in larger amounts than the target heavy metal. Pseudo-first order reaction, pseudo-second order reaction and Weber–Morris intraparticle diffusion models were used to analyze the data. The adsorption equilibrium data was explored by the Freundlich and Langmuir adsorption isotherm models and the reaction was found to show a good relationship with the Freundlich adsorption model. Free PFR–AgNPs and Co²⁺ loaded PFR–AgNPs were characterized using FT-IR spectroscopy, scanning electron microscopy (SEM), energy-dispersive microanalysis, thermogravimetric analysis (TGA), and differential thermal analysis (DTA). The regenerant used for the regeneration of the cation-exchange resin was 5% (w/w) NaCl. The experimental results verified that the PFR–AgNP cation exchange resin can be applied effectively for the removal of Co(II) from aqueous media.