Mechanistic insights into carbon release and denitrification in sandwich-structured hydrogel composites for low C/N wastewater denitrification

• Sandwich-structured hydrogel particles with CC shell were designed for low C/N denitrification. • The new particle simultaneously enabled controlled carbon release and served as a biofilm carrier. • Optimized skeleton and crosslinking stabilized the hydrogel network and preserved hydrophilicity. • CC shell increased porosity and roughness, enhancing carbon release and microbial attachment. • The dynamic coupling between carbon release and biofilm enrichment was investigated. The efficiency of solid carbon sources (SCSs) in low C/N wastewater denitrification is critically governed by their carbon release dynamics and microbial interactions. Here, a novel sandwich-structured hydrogel composite particle was designed, consisting of a hydrogel skeleton encapsulating mixed corncob (CC) and polyhydroxybutyrate (PHB) as the inner composite carbon core and further coated with an outer CC shell. The resulting particle exhibits dual functionality by providing a continuous carbon supply while simultaneously serving as a biofilm carrier for denitrification. Systematic screening identified a PVA-SA skeleton crosslinked with H 3 BO 3 -5% CaCl 2 as the optimal matrix. Carbon release from the optimum particle followed a two-phase pattern, characterized by an initial rapid release and a subsequent diffusion-controlled slow-release process, and could be satisfactorily described by a second-order kinetic model. With microorganisms, the particle achieved a total nitrogen removal efficiency of 54% and a maximum specific denitrification rate of 16.41 mg TN/(g MLSS·d). With continued operation, carbon release and biological consumption became progressively synchronized, leading to a carbon utilization efficiency exceeding 80% during the stable denitrification. SEM and 16S rRNA gene sequencing further confirmed enhanced biofilm attachment and denitrifier enrichment, with Proteobacteria and Actinobacteria increasing from 40% to 48% and 4% to 8%, respectively. The dynamic coupling between carbon release behavior and biofilm enrichment in sandwich-structured hydrogel composites was thus investigated, highlighting their potential as an effective and controllable SCS for low C/N wastewater denitrification