Cementitious binders modified with halloysite nanotubes for enhanced lead immobilization

Stabilization of Pb/Zn mine tailings by modified fly ash: characterization, performance, and mechanism

The presence of heavy metals in mine tailings poses a serious threat to the surrounding environment. In this study, we aimed to stabilize Pb/Zn-containing mine tailings using modified fly ash (FA) with various alkali solutions. Notably, the modification of FA with Na2SiO3 (NaSi-FA) resulted in the most significant structure changes. To understand the adsorption mechanism of Pb and Zn by modified FA, batch adsorption experiments were conducted. Doubling the adsorption capacity for both Pb and Zn was observed in the modified FA samples compared to unmodified samples. These results could be attributed to the enhanced surface area and porous structure, providing more anchor sites for the heavy metal ions. Additionally, the adsorption of Pb and Zn was found to follow the Langmuir isotherm and pseudo-second order kinetic. Molecular dynamics simulations further supported the notion that Pb and Zn ions could effectively exchange with Na ions within the N-A-S-H gel network, ultimately solidifying them in its structure. Stabilizing Pb/Zn tailings with NaSi-FA resulted in a significant decrease in the leaching of Pb and Zn. Specifically, the leading amount decreased by 55.2% for Pb and 35.3% for Zn, showcasing the superior performance of this stabilization method. This reduction in leaching indicates effective compliance with environmental regulations regarding the containment of Pb and Zn.

Coupling effect of superfine zeolite and fiber on enhancing the long-term performance of stabilized/solidified Pb-contaminated clayey soils

The focus of the present study was on the applicability of superfine zeolite (SZ) and polypropylene fibers in improving the geo-environmental parameters as well as the durability of cement-based stabilized/solidified low plasticity clay containing different dosages of Pb. The leaching data revealed that while adding a low range (≤ 7.5%) of sole cement even in the severely polluted soils could fully eliminate the Pb bioavailability, the metal retention capacity might portray a marked sensitivity to the acid-washing process. A major reduction was also observed in the mechanical/leaching performance of those samples after undergoing the wetting-drying (w-d) cycle, especially at a high proportion of Pb, which could weaken the cementation bonding dramatically; hence, much more cement was needed to pass the required stabilization/solidification (S/S) standards. Besides, the micro level tests indicated that the application of SZ (with 25% cement replacement) would alleviate the Pb declining impact on the S/S reactions and modify the porous network of soil. As a result, the specimens amended by cement-SZ (CSZ) were more functional (~ 1.4 times) in immobilizing the toxic ions than the cement alone was. However, the CSZ admixture might not perfectly restrain the w-d forces/deteriorations. Such a potential drawback was found to be solvable by the insertion of fiber, in which case, an enhancement in the ductility and the metal capsulation could be also manifested. In fact, the CSZ/fiber treatment could form a well-intertwined matrix, showing high success rates in stabilizing/solidifying the contaminated soils alongside a significant decrease (~ 2-folds) in the quantity of needed amount of cement to give the S/S satisfactory operation under the harsh environmental conditions.