There are various types of sealant for various purposes, as well as myriad manufacturers. One thing is standard – they all come in tubes made of quite sturdy plastic: how does one dispose of these?
Silicone sealant: Based on a derivative of sand (silicon dioxide), this is non-toxic but non-biodegradable (which is why it’s so routinely used). It’s also anti-fungal. The horrid smell that comes off it many sealants is some form of acetic acid (vinegar), which helps it cure. See here for other curatives.
Manufacturers: Many sealant manufacturers are large multi-nationals. One of the leaders is Dow Corning, which invented silicone. Dow Corning insists that it’s helping communities and using less energy and so on (you can read the report here), but it’s still environmentally questionable. We should remember that Dow Corning manufactured napalm and Agent Orange for the American Vietnam War. It also bought Union Carbide; this was after the Bhopal disaster, but the many Indian victims have yet to be properly compensated: given that DC is the second largest chemical manufacturer in the world, this is shameful. (I personally try to avoid DC products for this reason.)
C-Tec: this is a British-based manufacturer whose main product is CT-1 sealant. I presume it’s a relatively small and environmentally responsible company, but its website is singularly obscure about such matters. They’re very unwilling to say what CT-1 is made of, too. although the claim that it’s safe, non-toxic etc. I’ve used it a lot, and it’s good stuff, although expensive. And, although it has better flexibility than other sealants, it has still cracked and let in water on my Hawksbox! (And the shower still goes mouldy.)