With the exception of arsenic, most of these metals can be effectively immobilized as the sulfides. That's how they naturally occur, and the sulfides are so ine

With the exception of arsenic, most of these metals can be effectively immobilized as the sulfides. That's how they naturally occur, and the sulfides are so inert that early life never encountered the metal ions or adapted to them — which is probably why they are so toxic! The sulfides of lead, mercury, cadmium and copper are all very stable and insoluble. So the solution may be quite simple: once the lake pH is adjusted to be not too acidic, pumping hydrogen sulfide into it should entrain any heavy metals in their natural form. This is a dangerous process, though, because hydrogen sulfide is an extremely toxic gas.