Mn release behaviors from electrolytic manganese residue-based slow-release fertilizer using acid/alkali-activated geopolymers as binders: A comparative study

Abstract

In this study, acid/alkali-activated geopolymers and electrolytic manganese residue (EMR) were used as a binder and Mn source, respectively, to prepare EMR-based slow-release fertilizers (SRFs) and Mn release behaviors from SRFs were comparatively investigated. Mn release from SRFs prepared by acid/alkali-based geopolymers follows first-order kinetics and the Ritger-Peppas model. For alkali activator attack, the easily leachable manganese salt (MnSO4·H2O) in EMR is converted to stable fractions (such as MnO2, Mn4Si8O20, and Ca4Mn4Si8O24) in the SRFs. The K-(A)-S-H gel in SRFs prepared using alkali-based geopolymers (SRFs-alGo) restricts rapid Mn release through physical encapsulation and ion exchange. In contrast, SRFs prepared using acid-based geopolymers (SRFs-acGo) mainly limit Mn release through pediocratic NH4MnPO4·H2O formation and physical adsorption/encapsulation of phosphoric acid-activated geopolymer gel (Si–O–Si–O–P–O). Phosphoric acid attack did not significantly changed the proportion of the exchangeable fraction of Mn in SRFs, which was maintained at approximately 40%. Moreover, Mn in SRFs-alGo was uniformly distributed on the surface and bulk phases (SEM-EDS and XPS co-analysis), while most of the Mn in SRFs-acGo was distributed on the surface of particles, leading to readily released Mn and even higher Mn release concentrations of SRFs-acGo. This study provides a theoretical basis for strategically applying EMR-based SRFs to plants with different Mn requirements and guides subsequent field experiments.