Mineral-driven pyrolysis chemistry and temperature-dependent pyrolysis of textile wastewater sludge: kinetics, reaction pathways, and char functionality

Abstract

This study investigated the pyrolysis behaviour, conversion-dependent kinetics, and char production potential of textile wastewater sludge (TXT) for sustainable waste valorisation. The feedstock contained 45.84 wt% volatile matter, 24.09 wt% fixed carbon, 20.25 wt% ash, and exhibited a significantly higher heating value (HHV) of 11.98 MJ kg−1. Thermogravimetric analysis at 10, 20, and 30 °C min−1 identified three major degradation stages, with increasing heating rate shifting decomposition towards higher temperatures. The kinetic analysis over α = 0.1–0.8 using KAS, OFW, DAEM, Tang, and VZ methods yielded average activation energies of 213.03, 218.61, 208.81, 157.72, and 170.14 kJ mol−1, confirming the multistep and conversion-dependent nature of TXT decomposition. Criado master-plot analysis indicated predominantly diffusion-controlled behaviour at lower conversion, with increasing contributions from reaction-order and nucleation-related mechanisms at higher conversion. Char production at 450–800 °C reduced the yield from 51.20 to 43.53 wt% while generally increasing aromaticity, alkalinity, and pore development. Among the tested temperatures, char produced at 700 °C showed the most favourable properties, with 44.40 wt% carbon, HHV of 18.21 MJ kg−1, a BET surface area of 55.23 m2 g−1, and a water-holding capacity of 64.82 wt%. These findings demonstrate the potential of pyrolysis for converting TXT into a stable, mineral-rich carbonaceous material with relevance to environmental remediation, adsorption, and resource recovery.