Is there any way to convert waste plastic recovered floating in the ocean to a Basic Alkaline substance that could be then dumped into the Ocean to lower the PH of the Ocean?

If it was burned would it turn to an ash that would be Alkaline?

Additionally to this the trash would be converted by burning to carbondioxide and other gases HCl, HF, etc.  and this is also a problem, how we know.

Could you dissolve the plastic in a solvent, then ad a compound to change it back to a non toxic material that could then be incinerated?

The problem is if you incinerate it you get CO₂ and others. The solvent step makes no sense , it increases the waste material.

It would be far better to use a ship (perhaps an unmanned one) to collect the plastic and bring it back for recycling. Most plastics can be economically recycled. Ideally, this ship would be solar powered.

Don't know who's going to pay for it though.

Good idea, payment no problem for each platic container or bag the product should be 10 cent more expensive. This money can be used.