Aspects of the chemistry of cobalt(iii) in aqueous perchlorate solution

Degradation of phosphonates in Co(II)/peroxymonosulfate process: Performance and mechanism

The increased release of phosphonates to natural waters causes global concern due to their potential threat to the aquatic environment. It is curial to mineralize phosphonates to orthophosphate (PO₄^3-) before they are thoroughly removed from wastewater via conventional biological treatment. In this study, we systematically investigated the performance and mechanism of degradation of phosphonates in Co(II)-triggered peroxymonosulfate (PMS) activation process. The degradation efficiency of various phosphonates is highly dependent on their coordination with Co(II). Using 1-hydroxyethane 1,1-diphosphonic acid (HEDP) as a target pollutant, the Co(II)/PMS process is effective in a broad solution pH range from 5.0 to 10.0. Multiple experimental results imply that Co(II)-PMS complex is the primary reactive species, while hydroxyl radicals (HO^•), sulfate radicals (SO₄^•-), singlet oxygen (¹O₂) and Co(III) play as the secondary reactive species for the degradation of HEDP. The presence of Cl^-, HCO₃^-, and natural organic matters (NOM) inhibits the degradation of HEDP. However, in real water samples, the selectivity and efficiency for HEDP removal in the Co(II)/PMS process are higher than that in free radicals-mediated advanced oxidation processes. This study not only sheds new lights on the mechanism of Co(II)-triggered PMS activation process, but also provides feasible technology for the degradation of phosphonates in wastewater.

Protonation effects on dynamic flux properties of aqueous metal complexes

The degree of (de)protonation of aqueous metal species has significant consequences for the kinetics of complex formation/dissociation. All protonated forms of both the ligand and the hydrated central metal ion contribute to the rate of complex formation to an extent weighted by the pertaining outer-sphere stabilities. Likewise, the lifetime of the uncomplexed metal is determined by all the various protonated ligand species. Therefore, the interfacial reaction layer thickness, μ, and the ensuing kinetic flux, Jkin, are more involved than in the conventional case. All inner-sphere complexes contribute to the overall rate of dissociation, as weighted by their respective rate constants for dissociation, kd. The presence of inner-sphere deprotonated H2O, or of outer-sphere protonated ligand, generally has a great impact on kd of the inner-sphere complex. Consequently, the overall flux can be dominated by a species that is a minor component of the bulk speciation. The concepts are shown to provide a good description of experimental stripping chronopotentiometric data for several protonated metal–ligand systems.

Kinetics of Formation and Dissociation of Aquocobalt(III) Complexes with Some Carboxylic Acids in Acid Perchlorate Solution

The rates of formation and dissociation of monocarboxylic complexes of aquocobalt(III) cations with propionic, malonic, and 2-ethylmalonic acids have been measured with the stopped-flow method over a range of concentrations and temperatures in acid perchlorate media at an ionic strength 3.0 M. Although the rate constants for reactions of CoOHaq2+ with neutral ligands cover only a small range, indicating a dissociative mechanism, the associated activation parameters change cooperatively. These variations are discussed in terms of differences in the structure, proton distribution, and rates of water loss in the ion-pair precursors for the different ligands. Similar activation enthalpies of dissociation indicate a common mode of coordination, and the positive activation entropies for dissociation are consistent with a neutral leaving group.

First Hydrolysis Constants of Hexaaquacobalt(III) and -manganese(III): Longstanding Issues Resolved

The acid dissociation constants, Kh, of the hexaaquacobalt(III) and -manganese(III) ions are reported under conditions which minimize the problems of instability of these species at modest acidities (pH 1-4). A rapid and convenient preparation of aqueous Co(III) has been used, and the latter has been reacted with aqueous Mn(II) to form aqueous Mn(III). The noncoordinating buffer H2PIPPS (1,4-bis(3-sulfopropyl)piperazine) has been used to react with H+ and provide some control of the pH. Spectrophotometric changes, after stopped-flow mixing for Co(III) and conventional mixing for Mn(III), were used to determine the Kh values. The pKh values are as follows: Co(III), 3.10 +/- 0.07 (6.0 degrees C) and 2.92 +/- 0.04 (20.0 degrees C) in 1.0 M H+, Na+, ClO4-; Mn(III), 0.22 +/- 0.04 (7.6 degrees C) and 0.08 +/- 0.04 (20.0 degrees C) in approximately 4 M H+, Mn2+, ClO4-. Global fits of the data for each metal have been used to estimate the DeltaH degree and DeltaS degree values for Kh. It is suggested that the unusually high acidity of Mn(OH2)6(3+) may be due to a Jahn-Teller distortion effect.