The role of conduction/valence bands and redox potential in accelerated mineral dissolution

Abstract

T h e dissolution kinetics of a number of m a n g a n e s e o x i d e s in acidic solutions were studied. Using a slurry r e a c t o r , r a t e l a w s were d e t e rmined for e a c h of t h e m a n g a n e s e o x i d e s in t h e hydrohalogen a c i d s . Significant variations in t h e reaction r a t e s (up t o five o r d e r s of magnitude over t h e HCI b a s e rate) were obtained with different a c i d s . T h e addition of anions a s neutral salts to acidic solutions is s h o w n t o p r o d u c e t h e same rate-accelerating effect as a c i d s containing t h a t anion. T h e effect of t h e different a c i d s on t h e o x i d e dissolution r a t e s is explained using semiconductor band theory a n d e n e r g y diagrams.

A Review of Electrical Assisted Photocatalytic Technologies for the Treatment of Multi-Phase Pollutants

“…Low E bg value of 2.2 eV allows the application of visible light energy [54,55]. Other potential promising semiconductor is MnO with an even low E bg value of 1.3 eV to be an interesting functional material with low cost, large surface, electrochemical stability [55], and innocuous character [56,57]. Further combining MnO with polyaniline can demonstrate the enhancement of PEC properties because of the interaction with the polyaniline's E bg of 2.8 eV, which could retard the recombination of photo-induced e cb − /h vb + [57].…”

Oxidative dissolution of lithium and manganese from lithium manganospinel (LiMn2O4): towards climate-smart processes for critical metal recycling

“…Moreover, the rate constant increases with temperature, as expected; this dependency enables the estimation of the activation energy for the oxidative dissolution of 22 kJ mol À1 (0.23 eV). This is significantly smaller than required by other treatments, 36,38,40 and is for slow activation kinetics and not mass transfer in solution. Since the activation energy for electronic conductivity in LiMn 2 O 4 semiconductors through electron hopping (small polaron) between Mn III and Mn IV sites has been given as between 56-58 0.16-0.34 eV, this indicates that it is transport within the solid of Mn III to the surface that rate limits the dissolution.…”

“…37 This activation energy is larger than that required for Mn 2+ dissolution using SO 2 to reduce MnOOH (16-65 kJ mol À1 ) under ambient conditions, 38 but comparable with those required for dissolution from manganese oxides by sulfuric acid: 39 68 AE 2 kJ mol À1 (MnO), 80 AE 2 kJ mol À1 (Mn 3 O 4 ), 86 AE 2 kJ mol À1 (Mn 2 O 3 ) and 90 AE 2 kJ mol À1 (MnO 2 ). It is noteworthy that LeBlanc and Fogler 40 observed that manganese dissolution from this oxide series using mineral haloacids is more rapid, as the overall manganese oxidation state is lowered, with MnO dissolution having sufficiently low activation energies (ca. 18-20 kJ mol À1 ) for the chemical dissolution to be mass-transfer controlled, with reductive dissolution of the higher oxidation states requiring activation energies of between ca.…”

“…As mentioned earlier, whilst the dissolution of most manganese oxides typically occurs under the control of surface kinetics, this is not always the case. 40 For surface redox reaction-controlled dissolution, a simple cubic law is typically followed. [50][51][52][53] However, the dissolution rate of irreversible chemical reactions occurring under mixed control by mass transfer and surface kinetics is given by:…”

“…As mentioned earlier, whilst the dissolution of most manganese oxides typically occurs under the control of surface kinetics, this is not always the case. 40 For surface redox reaction-controlled dissolution, a simple cubic law is typically followed. 50–53 However, the dissolution rate of irreversible chemical reactions occurring under mixed control by mass transfer and surface kinetics is given by:in which k mt is the mass transfer coefficient, k het is the surface reaction rate constant and the bulk concentration of the hypochlorite lixiviant is , and is formally derived in ESI3 (ESI†).…”