Could human stomach acid be bioengineered to break down microplastics?

I don't know, but I don't think hydrochloric acid would be able to break down plastics. However, maybe we could find enzymes that would be able to.

I found this article that says we already have but they don't work at ambient temperature : https://news.utexas.edu/2022/04/27/plastic-eating-enzyme-could-eliminate-billions-of-tons-of-landfill-waste/

Also this one that talks about the subject : https://www.sciencedirect.com/science/article/pii/S0167779921000408