Facile preparation of an injectable carboxymethyl chitosan-silica sol persulfate controlled-release gel coupled with micro zero-valent iron for sustained nitrobenzene removal in groundwater

Nitrobenzene (NB) is a highly toxic and persistent groundwater contaminant that poses significant challenges for in situ remediation. Herein, an injectable persulfate (PS) controlled-release gel (CSG) was synthesized from environmentally friendly precursors via a facile room-temperature sol-gel method. CSG exhibited stable PS release under a wide pH range and representative groundwater matrix conditions. In dynamic column experiments, CSG sustained PS release over 42 pore volumes, approximately 17 times longer than free PS injection. The CSG was further coupled with micro zero-valent iron (mZVI) for the remediation of NB-contaminated groundwater. Compared with the conventional mZVI + PS system, controlled PS release in the mZVI + CSG system improved PS utilization and achieved a 2.4-fold higher NB removal efficiency. The mZVI + CSG system maintained high NB removal efficiency under different pH conditions, coexisting ions, and organic matter. Mechanistic analyses suggested that CSG regulated PS flux, while mZVI supplied reactive Fe⁰/Fe²⁺ species for reductive transformation and PS activation. NB removal proceeded through both direct oxidation and reduction-oxidation coupling pathways. SO₄ • − and •OH were identified as the dominant reactive oxygen species responsible for NB degradation. Overall, the mZVI + CSG system offered a feasible strategy for in situ remediation of NB-contaminated groundwater.