Determination of Ni(II) uptake mechanisms on mordenite surfaces: A combined macroscopic and microscopic approach

The uptake mechanisms of Ni(II) on mordenite were investigated by macroscopic experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. The results demonstrated that Ni(II) could be retained via different mechanisms, depending on pH, ionic strength, temperature, etc. At low pH, the uptake of Ni(II) is primarily dominated by cation exchange and/or outer-sphere surface complexation. The interatomic distances of Ni-O (∼2.05Å) and the coordination number (∼6.0) are similar to those of Ni(II)(aq) reference sample, suggesting that Ni(II) is present in an [Ni(H₂O)₆]²⁺ octahedral environment at low pH. With increasing pH, the uptake of Ni(II) on mordenite tends to form inner-sphere surface complexes and precipitation/co-precipitation at high pH. The uptake of Ni(II) on mordenite becomes more favorable with increasing temperature, and the EXAFS spectra analysis show a trend from outer-sphere surface complexation at low temperature to inner-sphere surface complexation at high temperature. The addition of HA/FA increases Ni(II) uptake at low pH and decreases Ni(II) uptake at high pH. Enhanced Ni(II) uptake is attributed to the formation of "ligand-bridging" ternary surface complexes that are adsorbed on mordenite surface, while reduction is attributed to the formation of soluble Ni(II)-HA/FA complexes in solution that compete with uptake processes. The findings presented in this study are important toward a molecular-level description of Ni(II) uptake processes at the water-mineral interface.