Iron-modified cement hydration regulates DOM transformation and carbon stabilization in soil-concrete systems during...

Rainfall runoff from engineered slopes can mobilize substantial amounts of soil-derived dissolved organic matter (DOM), contributing to soil carbon loss and downstream water pollution. Cement hydration in vegetated concrete (VC) creates highly alkaline conditions that strongly influence soil structure, DOM mobilization, and vegetation establishment. Incorporating iron (Fe³⁺) during cement hydration may regulate these coupled processes, yet the underlying molecular mechanisms remain unclear. Here, controlled curing and rainfall simulation experiments were conducted to investigate the effects of Fe addition on mechanical performance, alkalinity regulation, and DOM behavior in VC systems. Vegetated concrete with and without Fe₂(SO₄)₃ (3 wt%) was examined after 28 days using excitation-emission matrix spectroscopy coupled with parallel factor analysis (EEM-PARAFAC) and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Results showed that moderate Fe addition enhanced compressive and shear strength while mitigating excessive alkalinity, and simulated rainfall revealed at least a 42% reduction in dissolved organic carbon (DOC) release from Fe-amended VC. Spectroscopic and molecular analyses indicated that Fe preferentially retained aromatic, humic-like DOM within the solid matrix rather than inducing direct transformation among DOM fractions. FT-ICR-MS further demonstrated that Fe oxy(hydro)oxides selectively immobilized aromatic and oxygen-rich organic molecules through surface complexation, ligand exchange, and π-d electron interactions involving carboxyl and phenolic groups, thereby suppressing the leaching of nitrogen- and sulfur-containing species and enhancing soil organic carbon stabilization. Vegetation experiments showed improved plant performance under Fe-amended, moderately alkaline conditions. Overall, Fe incorporation during cement hydration enables a synergistic optimization of mechanical stability, alkalinity regulation, soil organic matter retention, and vegetation performance in vegetated concrete.