Diffusion in the Aquarium

BACKGROUND

Anybody who has ever watched smoke rise, fade, and eventually “disappear”, or has noticed that the aromatic effects of passing gas eventually fade, has experienced diffusion first-hand. In scientific terms, diffusion is the movement of molecules/atoms down a concentration gradient from a more concentrated to a less concentrated area. Diffusion occurs any time there is a concentration gradient — two areas with a different concentrations of some gas or liquid. It is one of the ubiquitous phenomena of nature.

There has been some spirited debate involving the topic of diffusion, both on the Aquatic Plants mailing list and the Planted Tank’s public forums. Although the debate has taken many twists and turns, it originated with a theory involving a method of CO₂ injection to create a mist of micro-bubbles in the aquarium, and ensuing improvements in plant growth — a theory that is sometimes referred to as “CO₂ misting.” One side of the debate theorizes that the micro-bubbles seen when employing the “CO₂ misting” method are composed primarily of CO₂, and that the observed improvement in plant growth is because the plants are more easily able to utilize the gaseous CO₂ in the micro-bubbles when they attach to plant leaves. The other side believes that it is unlikely that micro-bubbles composed primarily of CO₂ would be able to persist more than a few moments, and that the cause(s) of the observed improvement in plant growth are not due to increased availability of CO₂, but instead to other causes which need further investigation.

You might be wondering what all of that has to do with diffusion. There are essentially two issues concerning diffusion at the heart of the debate:

1. The rate at which CO₂ diffuses into water — Proponents of the “CO₂ misting” theory hold that when CO₂ concentrations reach levels around 30 mg/L more time is required for the CO₂ to diffuse, and this explains the persistence of the micro-bubbles. Opponents of the theory argue that, given the extremely high solubility of CO₂, the micro-bubbles are not likely to be composed of “stubborn CO₂.” It is much more likely, opponents contend, that the micro-bubbles persist because they are no longer primarily CO₂. This brings about the second issue concerning diffusion;
2. The rate at which gases diffuse from the water into a CO₂ bubble — Is the rate of diffusion taking place at the surface of a CO₂ bubble is sufficient to allow in significant amounts of gases other than CO₂? If significant amounts of other gases are entering CO₂ bubbles before they have time to fully dissolve, opponents of the misting theory argue, then the rest of the theory falls apart.

The center of the debate really boils down to, as one poster put it, “rates, types and causes of diffusion.”

The debate is not about whether there are improvements in plant growth to be seen when using the “CO₂ misting” method. A number of people have tried it and reported improvements. Although no one has yet made public a quantitative analysis of the increase in growth, this does not necessarily suggest that no increase can be seen. At this time there just aren't data on how much improvement can be expected and how such improvements correlate with other variables.

ARGUMENTS

PRO

The proponents of the theory that the increased plant growth seen with “CO₂ misting” is due to micro-bubbles of CO₂ adhering to the plant leaves make the following arguments:

• One

CON

Those who believe that there is some factor other than CO₂ micro-bubbles responsible for the observed improvements in plant growth make the following points:

• One

ANALYSIS

Before I start out, I want it to be clear that I am not unbiased in this controversy. I am attempting to be fair in presenting both sides, but I cannot guarantee full impartiality.