Bought used mattresses. Now we want to clear them. How to do it effectively and without chemicals?

You can try to steam it. Since ancient times, laundry has been steam cleaned.

You ask very correct questions, so that linen bugs do not start in the hostel, you need to clean the mattresses. Many students seek advice and instructions https://www.fortador-usa.com/blog/how-to-steam-clean-a-mattress-to-remove-urine-sweat-stains-dust-mites and can you steam clean a mattress with a steam cleaner. This is a simple procedure - first you need to vacuum it and preferably treat it with soda, then go through the steam cleaner and let it dry for more than 24 hours.