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-NH₂, and polyethylenimine. Results identify PDA with the highest Cr(VI) affinity (ΔE_ads = -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 (HCrO₄^-) orbital coupling, resulting in the highest adsorption strength (ΔE_ads = -20.41 eV). This mechanism synergizes with the intrinsic reactivity of HCrO₄^- (2.942 eV compared to CrO₄^2- 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 CO₂ eq/kg, 90% reduction) at scalable cost (74.9 CNY/kg, 69% reduction).