Boosting Cu ion capture in high-salinity environments with amino-functionalized millispheres

High salinity in wastewater often hampers the performance of traditional adsorbents by disrupting electrostatic interactions and ion exchange processes, limiting their efficiency. This study addresses these challenges by investigating the salt-promoted adsorption of Cu ions onto amino-functionalized chloromethylated polystyrene (EDA@CMPS) millispheres. The adsorbent was synthesized by grafting ethylenediamine (EDA) onto CMPS, which significantly improved Cu adsorption, achieving nearly three times the capacity in saline solutions (1.65 mmol g^-1) compared to non-saline solutions (0.66 mmol g^-1). Mechanistic analysis showed that the presence of salts, such as NaCl, promoted the protonation of amino groups on EDA@CMPS, increasing their positive charge and enhancing their affinity for Cu ions. The solution's ionic strength further amplified this protonation, reducing electrostatic repulsion between the adsorbent and the Cu ions, thus improving binding efficiency. Additionally, the increased ionic strength altered Cu speciation, favoring the formation of Cu(NH₃)₄²⁺ complexes, which were more easily adsorbed. These synergistic effects resulted in faster adsorption kinetics, higher capacity, and improved Cu ion removal, particularly in saline environments. Overall, these findings bridge the gap between material design and functional performance in high-salinity wastewater, offering a promising strategy for efficient heavy metal removal and environmental remediation.