Anaerobic Reactions

[Randy Holmes-Farley]

In the past there have been many questions about anaerobic reactions. Most recently, Tatu asked about the degradation of benzoic acid and in a different thread, commented on Mn oxidation.

Here is a list compiled by L. Y. Young and M. M. Haggblom of Rutgers of the anaerobic reactions that are most ecologically significant (in fresh and/or salt water) and are mediated by microbes. They are listed in order of energy release, and the first, of course, is aerobic oxidation:

1. O2 + 4H+ + 4e- ------> 2H2O

2. 2NO3- + 12H+ + 10e- ----> N2 + 6H2O

3. NO3- + 10H+ + 8e- -----> NH4+ + 3H2O

4. MnO2 + 4H+ + 2e- ----> Mn++ + 2H2O

5. Fe+++ + e- -----> Fe++

6. SO4-- + 10H+ + 8e- ----> H2S + 4H2O

7. HCO3- + 9H+ + 8e- -----> CH4 + 3H2O

8. 2HCO3- + 10H+ + 8e- -----> CH3COOH + 4H2O


Each of these is a half reaction. To get a balanced equation, it is coupled to another half reaction, like the microbial oxidation of acetate:

CH3COO- + 2H2O -----> 2CO2 + 7H+ + 8e-

These may not look familiar, but they explain many of the processes that we observe and worry about in our tanks.

For example, one is normal aerobic oxidation that fish use. Two represents the conversion of nitrate to nitrogen gas that we hope happens in our sand and rock. Six is the dreaded hydrogen sulfide generator, etc..

[tatuvaaj]

Randy,

Now that most people have DSB in their reef aquarium maybe someone should write a series of articles about these processes ;););)

Here is a list I have for oxidation of marine organic matter. These might be easier to understand for those (like me) who are not so good at chemistry...

Org. = (CH2O)80 (CH2)42 (NH3)16
(This composition of organic matter is developed by the author from Redfield ratios)

org + 175 O2 --> 122 CO2 + 16 NO3- + 16 H+ + 138 H2O
org + 114.4 NO3- + 114.4 H+ --> 122 CO2 + 57.2 N2 + 16 NH3 + 179.2 H2O
org + 286 MnO2 + 572 H+ --> 122 CO2 + 286 Mn++ + 16 NH3 + 408 H2O
org + 286 Fe2O3 + 1144 H+ --> 122 CO2 + 572 Fe++ + 16 NH3 + 694 H2O
org + 71.5 SO4-- + 71.5 H+ --> 122 CO2 + 16 NH3 + 71.5 HS- + 122 H2O
org --> 40 CO2 + 40 CH4 + 16 NH3 + (C43H84)

Author gives following energy yields (Gibbs free energy):
O2 -73.2 MJ
NO3- -65.5 MJ
MnO2 -65.2 MJ
Fe2O3 -33.7 MJ
SO4- -12.8 MJ
org -6.0 MJ

[Randy Holmes-Farley]

Thanks, Tatu.

For the benefit of others, here's what someone might use these equations for:

In another thread, some people were interested in why sand dissolves. These reactions show why. The reactions that Tatu posted make this especially clear:

His first reaction is a huge source of acid (both H+ and CO2)., The sulfate reaction and the methane generation reaction also produce net acid.

The second reaction is less clear. Is that a producer of acid? You use up 114.4 H+, and you create 122 CO2 and 16 NH3. Is that a net production of acid? Let's look.

122 CO2 = 122 H+ + 122 HCO3- = 244 H+ + 122 CO3--

The NH3 is a base and will combine with acid to form:

16 NH3 + 16 H+ <---> 16 NH4+

Let's assume that reaction goes all the way to NH4+.

So overall we have a net production of acids and bases of:

244-(114.4 + 16) = 113.6 H+
and
122 CO3--

If we recombine all of the protons with carbonate, we have

113.6 HCO3- and 8.4 CO3--

At what pH are those in equilibrium? Well, not far from normal seawater pH of 8.2.

So this reaction is not a big producer of acid or base, and isn't likely responsible for the dissolution of sand.

[Randy Holmes-Farley]

As a follow up post, I would point out that Tatu's second reaction showing conversion of nitrate to N2 isn't necessarily the one (or only) reaction for that conversion. And others have a different acid/base balance.

For example, the ammonia produced in that reaction has been sugggested to potentially undergo the following additional transformation with some new nitrate:

3NO3- + 5NH4+ ----> 4N2 + 9H2O + 2H+

That reaction is a net production of acid. If all of the ammonia produced in Tatu's second reaction went down this path to nitrogen gas (or the whole thing was written that way as a single reaction) it would be a net producer of acid in the sand.

Tatu's second reaction:

org + 114.4 NO3- + 114.4 H+ --> 122 CO2 + 57.2 N2 + 16 NH3 + 179.2 H2O

And the combined reaction:

org + 124 NO3- + 124 H+ --> 122 CO2 + 70 N2 + 208 H2O

This reaction uses up 124 H+ and produces 122 CO2, so for net you get:

120 HCO3- and 2 CO3--

That solution is substantially more acidic than seawater (i.e., has a higher proportion of HCO3- and a lower proportion of CO3--).

[tatuvaaj]

All,

I failed to mention that these reactions are from "An introduction to the chemistry of the sea" by Michael E. Q. Pilson !

It is a great book for hobbyists since it is easy to understand but still contains a lot of information not found in traditional aquarium books! For example, it has 54 pages about carbon dioxide.

Randy,
Thank you for your reply!

What books would you recommend for someone interested in seawater chemistry (both inorganic and organic)? Something cheaper than Spotte... :)

[Randy Holmes-Farley]

Tatu:

Sounds like a good book. I don't have that one.

Other books...cheaper than Spotte?

You just had to throw that in, didn't you!

Here's a few in order of preference:

Chemical Oceonography by Millero
http://www.amazon.com/exec/obidos/ASIN/0849384230/ref=ed_oe_h/103-2353268-2739046

Aquatic Chemistry Concepts by Pankow
http://www.amazon.com/exec/obidos/ASIN/0873711505/qid=1009400381/sr=1-5/ref=sr_1_8_5/103-2353268-2739046

I just got this one
Aquatic Chemistry : Interfacial and Interspecies Processes (Advances in Chemistry Series, No 244)
It's a symposium collection, so it is not comprehensive, but has some very good chapters (one on iodine especially)
http://www.amazon.com/exec/obidos/ASIN/084122921X/qid%3D1009400500/ref%3Dsr%5F11%5F0%5F1/103-2353268-2739046

I haven't read this other one by Millero, but it sounds good!
The Physical Chemistry of Natural Waters : By Frank J. Millero (Wiley-Interscience Series in Geochemistry)
http://www.amazon.com/exec/obidos/ASIN/0471362786/qid=1009400231/sr=1-1/ref=sr_1_8_1/103-2353268-2739046

[simonh]

Originally posted by Randy Holmes-Farley
Six is the dreaded hydrogen sulfide generator, etc..

Hi Randy,

I have a question related to the dreaded hydrogen sulfide.

Is it the hydrogen sulfide itself which is poisonous to life or is it that when it is released into the aquarium it rapidly consumes available oxygen (as it oxidises into sulfate) and therefore suffocates life ?

[Randy Holmes-Farley]

Simon:

It is toxic itself! When I get a chance (tomorrow), I'll look up what causes the toxicity.

[Randy Holmes-Farley]

Simon:

Here's an interesting link that describes the toxicity of hydrogen sulfide, and how some animals can live with it:

http://www.accessexcellence.com/BF/bf01/arp/bf01p1.html

[simonh]

Randy:

Many thanks for the link. It clears up my understanding of how hydrogen sulfide works.

I noticed when I read Yuri Sorokin's Reef Ecology book he mentions a layer of Thiobacilli above the sulfate reduction layer which oxidises the hydrogen sulfide...