my son burned chemically treated wood in my grill for a bonfire. Can I still cook in it, is it safe? or do I need to buy another grill?
Timber is treated using CCA (chromated copper arsenate). It is not safe to burn or eat food cooked over this timber. The main risk with this compound is that the smoke and ash will contain arsenic ("As" is the abbreviation for the element), which is a risk for acute and chronic arsenic poisoning as a result of short or long-term exposures respectively.
These results indicate that the open burning of CCA-treated wood can lead to significant air emissions of the more toxic trivalent form of As in particle sizes that are most respirable.
I only found one paper on the use of CCA in cooking which showed a weak positive correlation between the use of CCA treated wood and urinary output of copper, chromium and arsenic, so the risk is real!
I believe that these compounds will not accumulate in the grill, so I think if you (wearing respirator and PPE) were to clean the ash and soot out of the grill and dispose of it according to local regulations (check with your council/state/country), as well as washing out the interior of the grill, you should be safe. I would still advise that you get some expert advice on this before using the grill again.
Your grill (i.e. barbecue) is fine to use. I wouldn't want to eat food cooked over treated wood (although it probably isn't a big deal), but it won't leave any lasting effects. Just empty the ashes and give it a good clean. By clean I mean to use water, cleansers and a scouring pad to clean off all the soot and residue from the inside and the metal grills.
Some of the chemicals used in pressure treated wood are quite nasty. They recommend good ventilation and respirators to even cut it. It can also leave your grill imparting an off taste. Clean it well, then remove the impossible-to-remove residue with (much) higher temps than you use to cook. That way, cooler yet still-hot temps of cooking can't vaporize anything bad that cleaning missed.
How? Load it with extra charcoal, light as normal, wait until all fuel is glowing. Spread the coals around the perimeter of the kettle. Then hit it with air from a fan, leaf blower, or even a hair drier, blowing the flames against the side of the grill. It can get red-hot, but you don't need to go beyond the point where the walls no longer smoke on their own right after the air is removed. Once all the smoking grime is burned-off, re-clean and enjoy peace of mind with your chops.