Density Functional Theory-Guided Molecular Level Design of Biomaterials for Efficient, Cost-effective, and Sustainable Adsorption of Hexavalent Chromium from Wastewater.

The low-carbon strategy mandates the sustainable remediation of hexavalent chromium (Cr(VI)) contamination, driving the demand for efficient eco-adsorbents. However, current research prioritizes adsorption performance, neglecting environmental trade-offs and quantum chemical mechanisms of Cr(VI) adsorption. Here, we pioneered the first density functional theory (DFT) exploration of Cr(VI) adsorption mechanisms across chitosan (CS), polydopamine (PDA), UiO-66-NH, and polyethylenimine. Results identify PDA with the highest Cr(VI) affinity (Δ = -16.66 eV). Additionally, CS/PDA nanocomposites reduce the HOMO-LUMO gap by 53% (from 5.343 to 2.531 eV), markedly enhancing the reactivity. Critically, protonation-induced surface charge rearrangement triggers covalent Cr-N bonding via O p (hydroxyl)/N p (amine)-Cr d/O p (HCrO) orbital coupling, resulting in the highest adsorption strength (Δ = -20.41 eV). This mechanism synergizes with the intrinsic reactivity of HCrO (2.942 eV compared to CrO at 3.295 eV), explaining the enhanced adsorption efficiency at an acidic pH, as validated experimentally. The Langmuir model predicts a maximum adsorption capacity of 268.9 mg/g, which is 22.9% to 580.8% higher than previously reported values. Life cycle assessment (LCA) then exposed energy-intensive processes as the dominant carbon source, directly motivating our low-energy design: a one-pot citric acid synthesis eliminates thermal drying, utilizing natural cross-linking and multisite chemisorption to achieve an ultralow carbon footprint (4.72 kg of CO eq/kg, 90% reduction) at scalable cost (74.9 CNY/kg, 69% reduction).