Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium

• Published in: Environmental science & technology Vol. 58; no. 5; pp. 2564 - 2573
• Main Authors: Park, Jaeseon, Bandstra, Joel Z., Tratnyek, Paul G., Harvey, Omar R., Bae, Jong-Seong, Lee, Giehyeon
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06-02-2024
• Subjects: Adsorption; Chromate; Chromates; Chromates - chemistry; Chromium; Chromium - analysis; Chromium - chemistry; Contaminants; Correlation analysis; Hydrogen-Ion Concentration; Iron - chemistry; Kinetics; Magnesium; Metals; Oxidants; Oxidizing agents; pH effects; Physico-Chemical Treatment and Resource Recovery; Time series; Water Pollutants, Chemical - analysis; pH dependence; comprehensive kinetic model; corrosion; zerovalent magnesium (ZVMg); reduction
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.3c08367

The role of pH in sequestration of Cr­(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr­(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr­(VI), Cr­(III), and Mg­(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr­(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr­(VI) and Mg­(II) concentrations were fully predicted based on the Langmuir–Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively.