Effect of nitrogen-doped carbon derived from ZIF-8 on the hydration and Cr(VI) immobilization behavior of cement containing heavy metals

The cement solidification method serves as an effective approach for the immobilization of heavy metal-containing waste. However, Cr(VI), a highly toxic substance, is inadequately stabilized and solidified within cement pastes, posing a significant risk of leaching. This study innovatively incorporated nano-porous nitrogen-doped carbon (NC), derived from ZIF-8 through carbonization, into the cement containing heavy metals for collaborative disposal mixing with K 2 CrO 4 (HMC-Cr). Subsequently, the hydration behavior, mechanical properties, and solidification mechanism of Cr(VI) in the composite cement pastes were examined. The experimental results revealed that, compared to ZIF-8, NC demonstrated significantly greater positive impacts on the hydration kinetics, compressive strength, and Cr(VI) immobilization capabilities of HMC-Cr cement. Compared with pure HMC-Cr pastes, the 28-day compressive strength of HMC-Cr/NC increased by 16.7 %, and the leaching concentration of heavy metal Cr decreased by 52.5 %. The HMC-Cr/NC pastes exhibited superior Cr(VI) solidification capacity, attributed to the appropriate pore size, excellent nucleation effect, and reduction properties of NC. The primary immobilization forms of Cr in HMC-Cr/NC pastes included CrO 4 2- -AFm, Cr(OH) 3 , and coordination structures involving C-NH. Consequently, NC nanomaterials derived from ZIF-8 serve as effective additives for enhancing the stable immobilization of Cr(VI) in the cement solidification treatment of heavy metal-containing waste.