I used hydrochloric acid and water to remove concrete from my tools (in a plastic bucket) but did not know how dangerous it was and how complicated the disposal process can be--first time using it obviously. It is pretty late at night and there's not much I can do but I am scared to leave it outside overnight uncovered and I don't have a lid for it
You can neutralise the acid by mixing with alkali. However, be careful.
You will need quite a lot of something like baking powder. Or if you have any left over - cement powder.
Add a small amount to the acid - it will foam and release gas (Carbon Dioxide). Don't add too much at once or the bucket will overflow and you will get acid everywhere.
Keep going, small amount at a time until it no longer foams when you add more. You now have salty water that can be poured down the drain.
I repeat, be careful because of the foaming and you will need a lot of powder.