Are these drums stainless steel? The wikipedia site states that dimethyl carbonate is fairly soluble in water (13.9 g/100 ml) so it should be easily rinsed out. If the drums are plastic (HDPE) its not worth the risk as there are plenty of cheap plastic drums available. If it is stainless steel, I think you could easily clean them. When I got my SS drum, I wasn't sure what it had been used for. I cleaned it out with oxiclean. I then boiled solutions of both base (NaOH) and acid (H3PO4) in the drum. I felt pretty confident that I wouldn't have to worry about any toxic exposures.