Don't collect mercury in a plastic container. Mercury vapour will penetrate virtually all "plastic" material within a relatively short time no matter how thoroughly the element is buried in sand, soil, cat litter or anything else for that matter.
Ideally, you really need to have this collected professionally and some councils do operate such a facility. However, as the guys tend to don hazmat suits for the job, the councils don't do it for free.
Mercury spillage often occurs in laboratories and my university laboratories are cleared of staff before the technicians recover the stuff using specialised vacuum equipment and glass containers with air tight seals. Laboratory ventilation is switched to maximum. Plastic equipment is totally forbidden. The spillage area is drenched in specialised chemicals before staff are allowed to return.