Synthesis of starch graft-copolymers via reactive extrusion and their ammonium sorption properties

Abstract

The limiting features of existing ammonium-recovery technologies, such as pH-dependency, inefficient manufacturing, and low removal capacity of ammonium sorbents, limit their commercial application potential. To overcome these technical limitations, we employed reactive extrusion (REx) as a sustainable alternative synthesis method to prepare cost-effective ammonium sorbents based on starch-copolymers.This paper describes the graft copolymerization of starch with acrylamide and 2-acrylamido-2-methylpropane-sulfoacid monomers using twin-screw extrusion technology and evaluates the resulting products’ structural features and ammonium uptake capacity. The relationship between the starch molecular structure and effectiveness of the REx process was studied by monitoring the specific mechanical energy, die pressure and torque during the extrusion process. The impact of the starch features on the product’s chemical structure, thermal resistance, and swelling capacity was also determined. The ATR-FTIR, 1H NMR, and TG-DSC and elemental analysis confirmed the success of the proposed REx method for producing starch-copolymers, with an average graft efficiency of ∼65% across all samples. The starch structure and chemical composition controlled the grafting reaction efficiency, but had limited influence on monomer conversion. Cationic starch produced amphoteric copolymers that behaved similarly to acrylamide-based polyampholytes in electrolyte solutions. The starch- copolymer’s adsorption capacity increased proportionally with the initial ammonium concentration and remained constant within the pH range of ∼5.5–8.5. A monolayer adsorption capacity of ∼23 mg/g was recorded for the as-prepared starch-copolymer at 25 °C, comparable with those of the hydrogel sorbents at ambient temperature. This study highlights the potential of the as-prepared starch-copolymers as a cost-effective material solution for achieving sustainable ammonium removal.