Utilizing the ozonation pathway for enhanced conversion of manganese dithionate to manganese dioxide from acid leaching solution: Insights into mechanism and kinetics

Abstract

In response to the fact that the presence of manganese dithionate (MnS2O6) leads to a series of adverse impacts, especially lower purity of manganese sulfate (MnSO4) and disruption of its recovery, advanced oxidation methods such as ozonation system are used to manage MnS2O6 in the leaching solution, replacing conventional methods. To ascertain the conversion rate and kinetics of MnS2O6 during the ozonation process, we explored the factors influencing its removal rate, including ozone dosage, manganese dithionate concentration, sulfuric acid concentration, and reaction temperature. Batch experiments were conducted to determine the reaction rate constant of ozone (k) and activation energy (Ea) obtained from intermittent experimental data fitting, revealing a least-squares exponential conversion relationship between k and the MnS2O6 removal amount, wherein an increase in the aforementioned factors led to an enhanced MnS2O6 conversion rate, exceeding 99.3%. The formation mechanism of the ozone products proposed during the experiment was summarized and proposed as follows: 1) Mn2+ was directly oxidized to MnO2, and 2) SO4^2− was obtained by the catalytic oxidation of S2O6^2− with HO• from O3 decomposition. According to the kinetics analysis, the pre-exponential factor and total activation energy of the ozonation kinetics equation were 1.0×10^23 s−1 and 177.28 kJ/mol, respectively. Overall, the present study demonstrates that O3 as an oxidizing agent can effectively facilitate MnS2O6 disproportionation while preventing the release of the secondary pollutant, SO2 gas.

1. Introduction

Due to its potent oxidation and disinfection potentials, ozone (O3) has recently received considerable attention in water treatment technology. Its merits include robust oxidation ability, rapid reaction rates, straightforward processes, and absence of secondary pollution [1–4]. Recently, as the oxidation process (whether biological, chemical, or physical) is pivotal in pollutant remediation, ozone application has expanded into wastewater treatment. Ozone oxidation also has common applications in processes such as decolorization, deodorization, disinfection and sterilization, improvement of biodegradability for refractory organic compounds, and removal of the oxidation residues from inorganic contaminants [5]. However, its application of ozone in water treatment technology is limited due to its relatively low solubility and stability in aqueous environments [6–9]. While the economic feasibility of ozonation application might pose challenges, the undeniable oxidizing potential and effectiveness in pollutant removal can help mitigate these limitations [10].

Manganese is recognized as a “strategic and green” benign metal and is extensively used in steel production, metallurgy, agriculture, electronics, and many other fields [11–13]. To date, over 70 manganese salts have been identified, predominantly comprising inorganic salt types like manganese sulfate, manganese carbonate, permanganate, exceeding 80%. With the rapidly growing demand for high-grade manganese sulfate, considerable efforts have intensified towards recovering manganese from low-grade ores, such as pyrolusite (MnO2) and rhodochrosite (MnCO3) [14–18]. This approach is an important technology for obtaining manganese sulfate by wet flue gas desulfurization with MnO2 or leaching pyrolusite with SO2. In addition, the resource recycling method of pyrolusite for manganese sulfate is regarded as an economical and environmentally friendly...