Why is calcium salt of EDTA an effective detoxicant for lead poisoning ??

EDTA or ethylenediaminetetra acetic acid is a methal chelator, and it binds heavy metals quite well. It has a higher affinity for Pb than for Ca, so that is why it is an effective detoxicant for Pb poisoning. The main reason the salt is used (and many times it is disodium salt) is to make the EDTA more soluble.