In situ long-acting ammonium removal from groundwater using a modified sustained-release candle-constructed SO4•-–Cl- system

Ammonium in groundwater hampers sustainable water use, calling for efficient, stable in-situ remediation technologies. In this study, we design a novel coated sustained-release candle (CSRC) that contains peroxymonosulfate (PMS) and chloride ions (Cl − ). The aim is to construct an in situ long-acting SO 4 • − –Cl − oxidation system for groundwater ammonium removal. The outer coating effectively addressed the issue of “burst release” of paraffin-based sustained-release materials during the initial release stage. It also prolonged the effective release period and guaranteed the synchronous PMS and Cl − release. Systematic static release experiments revealed the critical roles of the CSRC diameter and reagent-to-paraffin mass ratio in regulating the release profiles. Column experiments demonstrated that CSRC deployment within an injection well significantly enhanced the reagent release compared to direct embedding. In addition, the groundwater flow rate exerted no influence on the CSRC release. The long-term operation confirmed the sustained generation of Cl•/ClO• and effective ammonium removal along the flow path. Moreover, three-dimensional tank experiments under varying hydraulic gradients and screen lengths showed that increases in the hydrodynamic conditions and well-screen length markedly improved the radial penetration and vertical dispersion of reagents in the aquifer and enabled the effective ammonium removal for up to 56 days. Collectively, this work presents a feasible and engineered strategy by adjusting the reagents ratio, diameter, and CSRC length that achieves the stable, long-term in-situ chemical oxidation of ammonium in groundwater. This work offers theoretical insights and practical guidance for applying sustained-release material-based remediation systems in the field.