I really wish I could do pic manipulation like that, my theory is different. I hope I can explain this so it's clear the way I think it would work, in principle rather like an undergravel filter.
Turn Werner's pic upside down. I agree the CO2 enters at the top of the straight tube. When the bubbles get to the bottom "U" , they would naturally make the U-turn and head upwards through the spiral. The gas travelling upwards will create an upwards current, pulling new water in through the short extension at the bottom of the "U" , and the bubbles should have enough contact with water through all that spiral to be absorbed (adsorbed?) before the top of the spiral.
I hope that makes sense!
To me, as soon as I equated the principle to a UG filter uplift tube, that's when my theory came to me and seemed to be correct.
I won't be surprised if I'm wrong, but I'm now really curious to see how it actually works, and if I'm correct.
(Edit. If I owned that, I just know I would break it!)