As a plastics engineer, I would be more concerned with the heavy metals in Himalayan pink salt. Also, any microplastics wouldn’t be “absorbed”. If anything the salt would abraid the container through shaking which could scrape the walls and grind out some small particles over time. That being said, the plastics used for these types of applications are relatively virgin and frequently don’t contain any additives aside from possible colorants or glass fill or something line that.