Copper remediation from salinization-impacted water by iron nanoparticles: insights into post-sequestration remobilization and polymer-enhanced heavy metal stabilization.

Abstract

Anthropogenic freshwater salinization is a growing global concern, often accompanied by heavy metal contamination from saline discharges such as industrial effluents and brines. Nanoscale zerovalent iron (nZVI) and its carboxymethylcellulose-functionalized form (CMC-nZVI) are widely proposed for redox-based heavy metal remediation. However, the post-sequestration stability of immobilized metals, particularly under saline conditions, remains poorly understood. This poses critical risks to long-term treatment efficacy and environmental fate. This study systematically examines both the initial sequestration and post-sequestration stability of the heavy metal Cu²⁺ by bare and CMC-nZVI across a wide salinity range (0.1-1 M NaCl) and in a representative saline wastewater matrix containing co-ions (Ca²⁺, Mg²⁺, SO₄^2-). While > 98% Cu²⁺ was reductively sequestered as Cu(I/II) oxides across all conditions within 5 min, salinity significantly destabilized copper sequestered by bare nZVI. Remobilization kinetics, modeled using Hill's equation, revealed that increasing salinity from 0.1 to 1 M delayed the remobilization time (t_1/2) from 148 to 272 min. Conversely lowering nZVI dosages accelerated t_1/2 (47-93 min) whereas lowering Cu^2⁺ loading influenced remobilization extent by suppressing it to 52-65% versus 78-86% at higher loading. Detailed characterization revealed that chloride-induced pitting facilitated oxidative dissolution of sequestered copper, while concurrent iron-oxide buildup modulated timing and extent of Cu²⁺ remobilization. In contrast, CMC coating on nZVI enhanced post-sequestration stability by suppressing pitting, limiting remobilization, and enabling re-capture of transiently released Cu²⁺, while preserving rapid initial sequestration. Background co-ions in the representative saline matrix moderately accelerated t_1/2 and increased remobilization extent for bare nZVI, whereas CMC-nZVI maintained stable performance.