If I wanted gut feelings I could ask anyone.
I suspect my gut feelings on the properties of chemicals are better than average.
It might turn your rock brown with MnO2.
How come?
Because it is dark brown? I'm not saying it will, just that it might. Fresh CaCO3 surfaces bind many things and MnO2 is not all that soluble..
Why are oxidizing agents used for bleaching rather than reducing agents?
I discuss that in this ozone article:
Ozone and the Reef Aquarium, Part 1: Chemistry and Biochemistry
http://reefkeeping.com/issues/2006-03/rhf/index.php
from it:
Oxidation of Organics by Ozone: Decoloration
The oxidation of organics is, it turns out, the primary reason that reef aquarists use ozone because it is the organic material in seawater that causes clarity and color issues. Its impact on organic materials is also why ozonation impacts skimming. While most organic compounds that are exposed to enough ozone for a long enough period will be oxidized in some way, some are very much more sensitive than others. In fact, at the levels of ozone attained in a typical reef aquarium contact chamber (less than about 0.3 ppm ozone) or even disinfection applications where the doses are much higher, the total dissolved carbon does not appreciably change during the ozone exposure (although it may later if bacteria find the newly oxidized organics more bioavailable; see below).
In a marine mammal pool,18 for example, it was found that disinfection with 4 ppm ozone with a 30 minute contact time (a disinfection level much higher than is typically used in reef aquaria) did not reduce the pool's total organic carbon (TOC) (~13 ppm TOC), while the use of granular activated carbon (GAC) did reduce it by 37%. Interestingly, the combination of ozone and GAC was even more effective, removing 60-78% of the TOC, suggesting that the ozonation may have altered some of the molecules in a way that made them bind more strongly (or more rapidly) to GAC. An alternative explanation that cannot be ruled out involves biological transformations of the organic compounds taking place on the GAC surface as it became colonized with bacteria).
One research group19 studying the reaction between a variety of organic compounds and ozone concluded:
"…comparisons of rate constants with chemical structures of the reacting groups show that all reactions of O3 are highly selective…"
Fortunately, many of the organic compounds that are most reactive with ozone coincidently are those that aquarists want to eliminate from aquaria. As seawater ages in marine aquaria, the water often becomes yellow as a wide variety of different organic pigments build up. Because of the ozone's reaction with many natural pigments, it is often used in drinking water purification for the purpose of "decoloration;" not organic removal per se, but decoloration.20
In order to understand this effect, it is first instructive to understand which organic molecules lead to coloration, because not all of them do. In fact, most organic molecules are not colored. That is, they do not absorb visible light. Looking through bottles of purified organic compounds, the vast majority are white powders. Organisms, however, have a significant need to absorb light, for example, to photosynthesize or to see.
In order to generate molecules that absorb visible light, natural systems often turn to conjugated carbon-carbon double bonds. Figures 1 and 2, for example, show the structures of chlorophyll and b-carotene. Both of these molecules are widespread in organisms, and both contain conjugated double bonds that lead to the absorption of visible light. These figures do not show the hydrogen atoms (there are dozens of them), but all of the other atoms are shown, and there is a carbon at each intersection of two or more lines. This is how chemists often show structures, allowing the important features to stand out and not get lost in a clutter of atomic letters. What is important here is each segment with a C═C (shown in red). Without going into ridiculous chemical detail for a reef article, having a bunch of C═C bonds arranged together with a single C─C bond between them can lead to the absorption of visible light. That is why organisms have developed such chemical structures for the absorption of light despite their instability toward oxidation (see below).
It is just that instability, however, that aquarists take advantage of when employing ozone. Figure 3 shows, for example, where ozone first attacks oleic acid (a dietary fatty acid).21,22 It is attacked at its double bond, breaking it apart into smaller fragments that no longer have a C═C bond. Consequently, while a huge dose of ozone lasting a very long time will break down these bits even more, even a small dose will remove the C═C bond.
Translating that reactivity to the pigments shown in Figures 1 and 2 makes it apparent why ozone is so good at reducing seawater's coloration and increasing its clarity: it reasonably selectively targets many of the structures that nature uses to absorb light, and converts them to nonabsorbing chemical structures.
A second type of colored organic compound that accumulates in seawater (in both the ocean and aquaria) is one of the functional groups in humic and fulvic acids (the compounds often identified as the yellowing agents in aquaria).20 These "compounds" are complex mixtures of many compounds, but among them is the phenol functional group (Figure 4). Phenol can be attacked by ozone,23,26 with breakdown products shown in Figure 4. It is the Ring-OH group that is colored when in the Ring-O- ionized form, and many of these breakdown products lack such a functional group. Hence the oxidation of such phenolates in humic acids with ozone will reduce color in aquarium water.
The various chemical products described in this section are, of course, not the only reaction products of ozone, hypobromous acid and hypobromite with organic compounds. Other products include brominated organic compounds and many other chemical structures. These have not been fully elucidated, a fact which is not surprising since even in the absence of ozone, the nature of all of the organics in natural seawater or reef aquarium water remains poorly defined.
