Production and characterisation of char derived from pyrolysis of waste residue for removal of nitrates from wastewater

Abstract This study presents an innovative valorisation pathway for two challenging waste streams, Refuse-Derived Fuel (RDF) and biosolids, by converting them into highly efficient adsorbents for nitrate removal from wastewater. Chars produced via pyrolysis at 500 °C and 700 °C were structurally modified using ZnCl 2 at varying impregnation ratios (1:1, 2:1, 3:1), significantly enhancing their adsorption uptake capacity and functional groups, as confirmed by surface area determination by Brunauer-Emmett-Teller (BET, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). analysis. The batch adsorption experiments were conducted under varying conditions of pH, temperature, initial nitrate concentration, and adsorbent dosage. Among the samples, RDF-derived char (RDF500-2) demonstrated a maximum adsorption capacity of 160 mg g −1 at optimal conditions (pH 7, 35 °C, adsorbent dose 5 g l −1 ), outperforming various conventional low-cost adsorbents. Adsorption followed pseudo-second-order kinetics and was best described by the Sips isotherm model (R 2 = 0.99), indicating a mixed monolayer-multilayer chemisorption mechanism. Thermodynamic parameters confirmed the process as spontaneous and exothermic. Importantly, the modified RDF and biosolid chars were enriched with additional −COOH and −C=O groups, which were responsible for nitrate binding, demonstrating tailored surface chemistry for environmental applications. The present study introduces sustainable, circular-economy-based adsorbents derived from problematic urban and industrial wastes and offers a low-cost solution for decentralised nitrate removal. These results contribute to a new direction in waste-to-resource innovation, aligning with environmental remediation and waste minimisation policies.