Thermal decomposition pathways of extracellular polymeric substances recovered from wastewater sludge using TG-FTIR with Gaussian deconvolution and 2D-COS analysis

Abstract

The current research attempts to elucidate fundamental mechanistic correlation between the complex chemical architecture of wastewater-derived biopolymers – EPS (extracellular polymeric substances) and their inherent thermal properties for fire-safety applications. By integrating thermogravimetric-infrared spectroscopy with two-dimensional correlation spectroscopy, we resolve intricate mass-loss profiles into three pseudo-components (PCs), each characterised by kinetic signatures and functional group transformations. PC1 (150–350 °C, activation energy (AE) = 140–150 kJ/mol), is primarily governed by the degradation of polysaccharides and release of early-stage volatiles (H₂O, CO₂, CH₄, NH₃, and HNCO). PC2 (210–450 °C, AE = 160–175 kJ/mol), represent the transition stage dominated by proteinaceous and lipid cross-linking, which produces nitrogenous species essential for promoting condensed-phase char development. PC3 (290–600 °C, AE > 180 kJ/mol) corresponds to the decomposition of humic-like substances and subsequent aromatic condensation of stable residues. Furthermore, comparative analysis reveals that EPS extracted from activated sludge exhibits higher thermal stability and a significantly increased char yield (33.5 %) than aerobic counterpart, attributed to higher AE during the middle decomposition stage. The persistent detection of C-O-C/P–O–C and aromatic C=C vibrations up to 700 °C confirms the formation of a phosphorus-rich aromatic char structure. This multi-dimensional analytical framework moves beyond conventional TG-based pseudo-component fitting, providing high resolution interpretation of the sequential evolution of volatile species and early-stage charring mechanisms of EPS.