Influences of isolated fractions of natural organic matter on adsorption of Cu(II) by titanate nanotubes

Abstract

With different functional groups and hydrophobic/hydrophilic properties, natural organic matters (NOMs) displayed different combining capacities with metal ions. By using XAD-4 and DAX-8 resins, NOMs in natural lake were isolated into three fractions, i.e., HoB (hydrophobic base), HoA (hydrophobic acid) and HiM (hydrophilic matter). Afterwards, influences on Cu(II) adsorption onto titanate nanotubes (TNTs) were compared with varying NOMs and initial pH. As results, HoB can significantly control Cu(II) adsorption at pH 5, with the adsorption capacity increased 15% for 0.5 mg L−1 of HoB (ca. 120 mg g−1), which could be attributed to the formation of HoB-Cu complexation and electrostatic bridge effect of HoB with optimal concentration. Due to the easier ionization and complexation with Cu(II) at lower pH, HoA showed more obvious impaction on Cu(II) adsorption at pH 2. While HiM can influence Cu(II) adsorption at all pH ranges due to its hydrophilic groups and weak affinity to both TNTs and Cu(II). Furthermore, HoB dramatically changed the Langmuir model, with sharp increase of adsorption capacity as equilibrium Cu(II) increased, suggesting its significant involvement in Cu(II) adsorption. X-ray photoelectron spectroscopy (XPS) analysis revealed the absorbed Cu(II) existed in the form of TNTs‑OCu, TNTs‑COOCu and Cu(OH)2, proving Cu(II) adsorption mechanism including both direct adsorption by TNTs and bridging connection with NOMs. Moreover, the CO and OCO groups content ranked as HiM > HoB > HoA, while TNTs‑COOCu content ranked as HoA > HoB > HiM, suggesting HoB had the moderate connection with both TNTs and Cu(II), thus the impact on Cu(II) adsorption was remarkable.