Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
7699329 | Marine Chemistry | 2015 | 7 Pages |
Abstract
The soluble porphyrin ligand [α, β, γ, δ-tetrakis(4-carboxyphenyl)porphine (T(4-CP)P)] was used as a competitive complexing ligand to study Mn speciation in the sediment porewaters of four stations in the Laurentian Trough. Both Mn(II) and Mn(III) concentrations were determined simultaneously. The rate of recovery of Mn(II) complexes by the porphyrin shows similar kinetics, but Mn(III)-ligand complexes give different values for the kinetics of recovery, which can be used along with the steady-state approximation to determine the dissociation rate constant, kd, of the Mn(III)L complex(es). Dissociation rate constants for the recovery of inorganic Mnâ² as the Mn(III)-porphyrin complex ranged from 2.28 Ã 10â 3 to 7.20 Ã 10â 3 sâ 1. The kd values reflect the dissociation of the ligand from natural Mn(III)L complexes, which are slower to dissociate than Mn(III) bound to pyrophosphate but faster than desferrioxmaine-B bound to Mn(III). A comparison of the kd values of natural Mn(III)L and Fe(III)L complexes from previous studies indicates that Mn(III)L complexes dissociate faster than Fe(III)L complexes. Because Kcond = kf / kd, we estimate Kcond at 1.39 to 4.35 Ã 1011 Mâ 1 for these Mnâ²L complexes [where Mnâ² is defined as all inorganic forms of Mn(III)] from the diffusion-controlled rate of water exchange (kex) for Mn(H2O)63 + and its hydroxo species, which is the upper limit for the value of kf for Mnâ² reacting with ligands. These Kcond values for Mnâ²L complexes are similar to those found for Feâ²L in open ocean waters.
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Chemistry (General)
Authors
George W. III, Andrew S. Madison, Alfonso Mucci, Bjørn Sundby, Véronique E. Oldham,