Mechanistic insight into selective adsorption and easy regeneration of carboxyl-functionalized MOFs towards heavy metals.

The development of heavy metal adsorbents with high selectivity has become a research hotspot due to the interference of coexisting ions (e.g., Na+, Ca2+) in the actual wastewater, but the more difficult regeneration caused by high adsorption selectivity severely limits its practical applications. Herein, a carboxyl adsorbent, MIL-121, demonstrated high adsorption selectivity for heavy metals at 10,000 mg/L of Na+ (removal > 99% for Cu2+) as well as unexpected easy regeneration (desorption > 99%) at low H+ concentration (10-3.5-10-3.0 M), which is hundreds of times lower than that of ever reported selective adsorbents (> 10-1 M H+). X-ray photoelectron spectrometry (XPS), extended X-ray absorption fine structure (EXAFS) coupled with Density functional theory (DFT) simulation unveil that the -COOH groups in MIL-121 for heavy metals adsorption is specific inner-sphere coordination with higher binding energy (1.31 eV for Cu), and less energy required for regeneration (0.26 eV for H). Similar high selectivity and easy regeneration were also satisfied with other heavy metals (e.g., Pb2+, Ni2+), and removal of heavy metals remained > 99% in 10 consecutive adsorption-desorption cycles. For actual copper electroplating wastewater treatment, MIL-121 could produce ~ 3600 mL clean water/g sample, outperforming 300 mL that of the benchmark commercial adsorbent D-113. This study shows the potential of MIL-121 for heavy metal wastewater treatment and provides mechanistic insight for developing adsorbents with high selective adsorption and easy regeneration.