Aluminum ion catalyzed proton transfer: Mechanism on promoting highly stable passivation of Cr by soil organic matter

Abstract

Although biochar can passivate chromium (Cr³⁺) in soil, the low stability is still a challenge to be overcome since the passivation mechanism is dominated by weak interactions (complexation, electrostatic attraction, etc.). In this study, a highly stable passivation of Cr³⁺ was achieved in soil based on the strategy that the low-energy sp hybridisation orbitals of aluminum (Al³⁺) induced a decrease in the HOMO energy level, leading to the enrichment of off-domain electrons in carbon-based conjugated systems. It can promote the proton transfer and the ion exchange, facilitating the strong chemical binding of organic matter to Cr³⁺. It suggested that the introduction of Al³⁺ significantly enhanced the passivation efficiency, maintaining a growth over 42 days of aging. To achieving a high stable passivation, the key is promoting a higher proportion of organic matter-bound Cr³⁺ contributing by the introduction of Al³⁺. DFT calculations further validated thermodynamically that, only Al³⁺ had the catalytic effect on both proton transfer and Cr³⁺ passivation compared with K⁺, Na⁺, Ca²⁺, Mg²⁺, Fe³⁺, Zr⁴⁺. These findings can provide important insights for developing a new generation of passivators which can efficiently stabilize heavy metal.