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4. Highly Efficient Cd 2+ Removal Using Tobermorite with pH Self-Adjustment Ability from Aqueous Solution.

Author

Qin, Juan, Yuan, Sujuan, Córdova-Udaeta, Mauricio, Oyama, Keishi, and Tokoro, Chiharu

Subjects
AQUEOUS solutions, LANGMUIR isotherms, ADSORPTION capacity, ION exchange (Chemistry), HEAVY metals, DISEASE incidence
Abstract

Cadmium (Cd), as a type of heavy metal, can increase the incidence of many diseases, even in low concentrations. In this study, tobermorite was hydrothermally synthesized and then applied to adsorb Cd2+ from an aqueous solution. The physicochemical characteristics of the synthesized tobermorite were detected, and the results indicated that the well-crystallized tobermorite had a lot of mesopores and a large specific surface area of 140.92 m2/g. It acquired a pH self-adjustment ability via spontaneously releasing Ca2+ and OH- into the aqueous solution. The effects of different factors on Cd2+ removal were investigated. For Cd2+, the removal efficiency could reach 99.71% and the maximum adsorption capacity was 39.18 mg/g using tobermorite. The adsorption data was best fitted with the pseudo-second-order kinetic and Langmuir isotherm models. In addition, there was no strict limit on the solution pH in Cd2+ adsorption because the tobermorite could adjust the solution pH to an alkaline atmosphere spontaneously. The efficient removal of Cd2+ using tobermorite was a result of surface complexation and ion exchange. [ABSTRACT FROM AUTHOR]

Published
2023
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5. Insights on the Mechanism of Manganese Oxide Precipitation from Mn2+Solutions Using NaClO Under Highly Acidic Conditions, via Experimental Observations and Numerical Fitting of the Kinetic ODE System

Author

Córdova-Udaeta, Mauricio, Cheng, Bowen, Fuchida, Shigeshi, Takaya, Yutaro, Oyama, Keishi, and Tokoro, Chiharu

Abstract

AbstractManganese (Mn2+) oxidation is an important phenomena in environmental and industrial processes. This study aims to shed light on the mechanism by which NaClO oxidizes Mn2+to precipitate MnO2in the presence of HCl. Experiments at pH = 1.5 indicated that the autocatalytic formation and the BET surface area of the MnO2precipitates are influenced by NaClO addition speed, while further tests suggested that highly acidic conditions may promote the redissolution of the initial MnO2crystals formed after oxidant addition. Based on these observations, a completely novel kinetic mechanism for Mn2+oxidation with NaClO in HCl solutions was proposed. Namely: the formation of MnO2crystal nuclei by direct oxidation, the formation of an intermediary of the MnO2•HOCl type, and the disappearance of said intermediary, which can take two competing reaction pathways. The first pathway is thought to be the autocatalytic formation of MnO2which ultimately precipitates, whereas the second one is proposed to be the redissolution of MnO2back into Mn2+. To validate the proposed mechanism, the experimental setup was modeled as a semi batch reactor with pulsed NaClO addition, and the ensuing system of ordinary differential equations (ODEs) was solved numerically, yielding a reasonable fitting with experimental data.

Published
2024
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