Effect of chloride ions and water chemistry on copper(II) adsorption on functionalized and pristine carbon nanotubes compared to activated carbon F-400

The objective of this study was to investigate the effect of chloride ions (Cl^-) on Cu²⁺ adsorption to carbon nanotubes (CNT). The isotherms showed a significant decrease in adsorption capacity on F-400, pristine, and acid-functionalized CNT in the presence of Cl^-, but had little effect on alcohol-functionalized CNT. Several inductively coupled plasma (ICP) analyses measured the impurities concentration of (1) aqueous-phase isotherm solute, (2) as-received, and (3) acid-washed CNT solutions. Chemical-equilibrium-modeling software MINEQL⁺ calculations were applied to compare ICP results to complexes formation. The model suggested that some solid-phase residual-catalytic metals, such as Cr²⁺, after released in water from as-received CNT, formed aqueous-phase complexes and were readsorbed. The 18-metal ICP results were more than two orders of magnitude lower (<4 μM/g-adsorbent) than the lowest isotherm Cu²⁺ concentration (157 μM) without significant impact on the isotherm results. The reduced adsorptive capacity of acid-functionalized CNT was related to the mechanisms of water molecule displacement followed by deprotonation during Cu²⁺ sorption in the CNT-surface hydration layer and its interaction with other species, generating different ion exchange forces. Brunauer-Emmett-Teller and pore-distribution measurements defined bulk water structure within CNT bundles. Zeta-charge and pHpzc measurements compared as-received and hybrid-CNT indicating copper chemisorption. Functionalized CNT remained negatively charged above pH 2.7, suggesting consistent adsorptive capacity at pH>5.1, when less Cu²⁺ ions are present in solution. scanning electron microscopy-energy dispersive X-ray spectroscopy analysis showed impurities on as-received F-400 and positively charged surface at pH 5.1 (pHpzc 7.1) explaining possible electrostatic attraction of Cl^- ions, blocking adsorptive sites, reducing its adsorptive capacity for Cu ²⁺.