DFT Analysis of Frontier Orbitals (HOMO-LUMO) of Polylactic Acid Functionalized with N-Hydroxysuccinimide and N-Sulfosuccinimide for the Adsorption of the Heavy Metals Nickel, Arsenic, and Lead

Polylactic acid (PLA) is a biopolymer made from starch that is both sustainable and low-cost. But its chemical inertness limits its application in the removal of heavy metals from aqueous environments. This study addresses the limitations by functionalizing PLA with N-hydroxysuccinimide (NHS) and N-sulfosuccinimide (S-NHS). It is hypothesized that introducing the sulfonate group using S-NHS increases the electron-donating capabilities of PLA, optimizing its adsorption capabilities for heavy metals. Density Functional Theory (DFT) calculations of energy, optimization, frequencies and NBOs in Gaussian 16 (M05-2X/LanL2DZ) and Multiwfn 4.0 were used for the electronic properties of the pristine and functionalized polymer and their interactions with a simplified system of hexahydrated ions of nickel (Ni2+), arsenic (As3+), and lead (Pb2+) cations were analyzed. The results indicated that PLA-S-NHS has an energy gap (Egap) of 3.31 eV, being lower than that of PLA (5.51 eV) and PLA-NHS (4.42 eV), signaling an increase in its adsorption capabilities. Its total dipole moment (TDM) reached 196.16 Debye. The metal–polymer complexes exhibit high TDMs, such as 1104.78 Debye with Pb in PLA-S-NHS, confirming greater interactions. The NBO analysis shows that S-NHS functionalization strengthens the donor–acceptor interactions with the sulfonate group oxygens acting as a primary donor, enhancing the adsorption of heavy metals; this is shown by the adsorption energies (Eads), confirming that functionalization with S-NHS enhances the interaction with metal ions, with negative Eads values observed for all complexes, especially for Pb2+, indicating thermodynamically favorable adsorption. The functionalization with S-NHS optimizes the electronic properties of PLA for heavy-metal adsorption, thereby validating the hypothesis and providing a molecular basis for the rational design of advanced bioadsorbents. These results indicate the potential application of these functionalized PLA polymers, especially as membranes, for the selective extraction of heavy metals from aqueous solutions.