A model for the removal of Cr(VI) in water by nanoscale zero-valent iron: quantifying the reaction process and calculating the electron efficiency

Nanoscale zero-valent iron (nZVI) has received more attention due to its high reactivity in removing contaminants. However, assessing reactivity requires time and cost savings to improve the potential applicability of nZVI. In this study, a model for Cr(VI) removal by nZVI was established to simulate the changes of various ions during the reaction. The aim was to elucidate the reduction mechanism and provide a new method for calculating electron efficiency. And we calibrated the parameters through experimental data and characterization analysis, which improved the reliability of the model. In addition, we discussed the specific effects of the initial concentration and initial pH of Cr(VI) on electron efficiency and reactivity. The results indicated that the initial concentration and initial pH primarily affected the electron efficiency by altering the utilization efficiency of active sites and the corrosion degree of nZVI. At pH values of 3, 5, 7, 9, and 11, the electron efficiencies were 39.18%, 47.56%, 58.81%, 59.12%, and 84.19%, respectively. We found that both nZVI and Fe(II) had reduction effects, but which one played the major role depended on the reaction conditions. Finally, the possible reaction mechanisms were analyzed by combining experiments, models, and characterizations. The whole reaction process was divided into three stages: pollutant adsorption, electron transfer process, and mass transfer process.