attempt to recycle them? and where can they be recycled?
They are supposed to come with a green plastic cap that has a pin that opens the valve. So after they are empty you are supposed to push that cap on, pin down. That opens the valve and 'proves' that they are empty. Then put them in recycle / garbage depending on your local rules.
That green cap with the pin must be a state rule, I have never had one come with a cap like that. Green plastic cap that slips over the threads, yes, but no pin.
In addition, every one I have ever had has a Schroder valve (tire valve) near the top that you can press on the valve pin to insure it's empty.
It's against federal law to refill them but many people do it anyway at home with a special valve adapter that's easily available.
Whoops... That is, "Schrader" valve above... Not Schroder valve
That valve, while looking like a regular Schrader valve, is actually a pressure release valve. If you press on the stem, it will not release pressure like a tire. You actually have to pull it out with a small pair of needle-nose pliers or similar. You can use a removal tool and take it out, but only for disposal. I understand it may mess with the release pressure removing/installing it. Not sure, I use the needle-nose pliers. . When refilling, the liquid propane escaping from the opened valve indicates full.

RE: green cap - ""In 2009, The Coleman Company included a Green Key® tool with each of
our propane canisters. We used the Green Key® program as a way to
increase empty propane canister recycling. Unfortunately, the program
was not accepted by many recycling agencies across the country. As a
result, the Green Key® tool will no longer be included with the purchase of
our Coleman® propane cylinders. ""
https://www.maricopa.gov/DocumentCenter/View/3602/Disposal-Instructions-for-Coleman-Propane-Cylinders---Green-Key-PDF?bidId=Also - pretty sure that refilling the 1lb is legal, the federal law prohibits transport when refilled. So if you completely empty them after refilling, they are legal to transport again. But then again... a refilled bottle can look just like a new one.

Not sure how they would tell the difference, unless the bottles are beat up. And if they are looking beat up, scratched or rusted, DONT REFILL THEM.

There are a few warnings about wall thickness and lack of bleed valve that can put them in the 'dangerous after refill' to many. If you really want to refill/transport 1lb cylinders right and legal, get one of these -
https://www.amazon.com/Flame-King-Refillable-Propane-Cylinder/dp/B07F9LCYRT
A little pricey, but doing it the right way always seems cost more.
