In situ pE-pH Analysis on the oxidation kinetics of manganese bearing solution

Open Access
Author:
Ethier Colon, William Camilo
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 14, 2014
Committee Members:
  • Hiroshi Ohmoto, Thesis Advisor
Keywords:
  • kinetics
  • manganese oxidation
  • rate equation
  • feitknechtite
  • groutite
  • hausmannite
  • pH
  • pE
Abstract:
The study of manganese(II) oxidation in oxygenated solutions has been well documented over long timescales (> 30 days) and over varying temperature ranges. Experimental setup for this study involved the addition of MnCl2∙4H2O solid to a basic (pH 9 - 10.25) aqueous solution to produce an initial MnII concentration ranging 1 – 100 mM. We apply a method of in situ pH and pE analysis to study the major reactions occurring during Mn oxidation. The experimental system is not run under equilibrium conditions. pH is neither titrated nor buffered, where doing so would interfere with the pH and pE of the solution. This type of analysis has not been previously performed. Calculation of an experimental rate constant of the system confirmed that changes in pE/pH slope of the solution reflected changes in the rate of the reaction. Changes in rate of reaction are due to alteration of the major reaction taking place. From solution chemistry, two major reaction stages were noted. Stage 1 can be further broken down into stages 1A and 1B, with 1A lasting one minute and consisted of the MnII hydration reaction to Mn(OH)2. Stage 1B continued with Mn hydration as well as the slower 1 electron oxidation Mn(OH)2 to Mn(OH)3. Both manganese hydroxides are amorphous phases, confirmed by both XRD and elemental analysis. However with increasing extent of reaction, the crystallinity of the solid increased. Since rate of reaction decreases with decreasing pH, and Mn hydration is fast, the expected transformation pathway for the production of observed groutite, feitknechtite and hausmannite is by oxidation of amorphous phases Mn(OH)2 to Mn(OH)3, and transformation to crystalline phases. Specifically rate of production of feitknechtite is slightly fast compared to groutite, but groutite is metastable and its rate of transformation to hausmannite is faster than its rate formation. Comparatively, the transformation of feitknechtite to iv hausmannite is slower, therefore feitknechtite transformation primarily dictates the rate of reaction. [OH-] was found to be second order with respect to the rate of Mn oxidation, which is in accordance with the rate law for Mn oxidation given by Stumm and Morgan, 1996: -d[MnII(aq)]/dt = k1 [MnII(aq)][OH-(aq)]2[O2 (aq)] + k2[MnII(aq)][MnOx (s)][OH-(aq)]2[O2 (aq)]. However, experimental MnII(aq) concentration data over time, gathered from this study revealed [MnII] to be second order with respect to rate of reaction, which countered Stumm and Morgan, (1996). [OH-] was noted to be the major contributor to the Mn oxidation. From this, an initial approximation of the rate constant was calculated as Mn oxidation reaction to be pseudo-second order with respect to [OH-], written as –d[MnII]/dt = kOH[O2][OH-]2 . kOH was then calculated and used to iteratively solve for the apparent rate constant for the overall rate equation, kapp = [MnII]2[O2][OH-]2. kapp for the oxidation of Mn was determined to be 6.2*1015 M-4 Hr-1. Complete comparison to the Stumm and Morgan, (1996) equation was not possible as BET surface area studies were not performed during this study. However, [MnII] over the course of the experiments was never reduced more than by 33%, suggesting Mn underwent autocatalytic oxidation over the course of the experiment.