Optimizing biodegradable SAPs: A Systematic study of monovalent counterion effects on citrate-based networks

• Systematic study on how monovalent counterions impact SAP performance. • Larger counterions lead to lower cross-linking density and higher biodegradability. • Potassium-neutralized SAPs achieve the highest water absorption capacity. • Provides guidance for optimizing biodegradable SAPs via counterion selection. Developing biodegradable superabsorbent polymers (SAPs) offers an eco-friendly solution to microplastic pollution from current products. In this study, we synthesized a series of cross-linked, bio-based SAPs using citric acid, glycerol, and monovalent citrate salts of alkali metals (lithium to cesium). The primary goal was to systematically evaluate how the size of the counterion influences the material’s performance, including its chemical structure, cross-linking density, water absorption capacity, and biodegradability. Our findings reveal a clear trend: under identical synthesis conditions, SAPs with larger counterions exhibit a lower cross-linking density, which was confirmed by rheological analysis. Notably, SAPs neutralized with potassium achieved the highest water absorption capacity, reaching 30.48 g/g. We attribute this optimal performance to the interplay of four factors: the radial distribution of ions within the polymer network, the variations in cross-link density, the degree of carboxylic acid neutralization, and the free space available for water uptake. Additionally, SAPs with larger counterions demonstrated higher biodegradability, with K + , Rb + , and Cs + neutralized SAPs showing over 30 % biodegradation after 28 days. These insights provide a valuable guide for optimizing SAP synthesis through polycondensation, enabling the creation of materials with tailored properties by carefully selecting both monomers and counterions.