Facile activation of sludge-based hydrochar by Fenton oxidation for ammonium adsorption in aqueous media

Lately, there has been a growing interest in converting low-cost biomass residuals, including wastewater sludge, into char-like materials for various applications. In this research, ammonium (NH₄⁺) adsorption and desorption potential of hydrochar activated via Fenton oxidation were systematically investigated. Hydrochar was prepared from domestic wastewater treatment plant sludge and activated by Fenton oxidation using different H₂O₂ concentrations, H₂O₂/Fe²⁺ ratios, and activation times. The activated hydrochars (AHs) were characterized by ATR-FTIR, high-resolution XPS, BET specific surface area, and SEM, and their NH₄⁺ adsorption capacity was analyzed. The NH₄⁺ adsorption isotherms and kinetics, adsorption in the presence of competing ions (with and without humic acid), and NH₄⁺ desorption were investigated. The results show that following hydrochar activation, the acidic groups' concentration and the BET surface area increased, but the morphology remained essentially unchanged. It was also found that the activation occurs within a few minutes when using a relatively low concentration of reagents, and without extensive post-treatment steps. The NH₄⁺ adsorption onto AH at equilibrium fitted the Langmuir isotherm model, with a maximum adsorption capacity of 30.77 mg g^-1, and the NH₄⁺ adsorption kinetics fitted the pseudo-second-order model. NH₄⁺ adsorption in the presence of competing ions decreased by up to 33 ± 3%. NH₄⁺ desorption experiments demonstrated that NH₄⁺ recovery can reach 33 ± 5% with ultrapure water and 67 ± 2% with 2 M KCl. The results of this study indicate that Fenton oxidation is a promising alternative for hydrochar activation, and can be used as an adsorbent for NH₄⁺ remediation in wastewater treatment processes.