What about ridding the oceans of mercury so we can all eat more fish without getting sick?
That would be worth something.
Beaches of thorium bearing monazite sand just waiting for extraction.
That's probably a yes/no answer. We probably can devise a way to extract mercury. However, there is so much mercury in the ocean, on the sea floor, and coming in from rivers that any attempt probably wouldn't make a difference. It is an environmentalist scare that mercury comes from coal plants, which it does. However, it also is contained in the rocks and soil, and leaches out into water and is carried by rivers into the ocean. So mercury is added to the ocean whether or not we burn coal. Better pollution controls on power plants may help some, but I think it is just there naturally as well, and the effort to get rid of it is not economical.
“Can analogous techniques be used to get thorium from the water?”
Yes it can. The use of biological materials to adsorb materials is called Biosorption and I worked on this process as a new hire at a Corporate Research lab >30 years ago. I was testing the ability of bacteria, chitin and other bio-materials to remove U238 from seawater. It was very effective and very rapid. The literature at the time indicated that the process worked best with higher Atomic weight elements, including thorium. I predict the researchers will run into the same problems I did. The absorption is not very specific, the material doesn’t just bind Uranium, but a wide variety of other elements reducing the effectiveness of the absorbent. They will also find that putting the absorbent into non-sterile water will result in rapid bio-fouling of the membrane that will destroy its effectiveness.