Cr(III) and Hg(II) retention on zeolites. Zeolite nature and process variables influence.

The rate and relative capacity of retention of Cr(III) and Hg(II) ions from aqueous media in some synthetic (4A, 5A and NaX) and natural zeolites (cli noptilolite, erionite and mordenite) has been studied. The exchange rate of Cr(III) in the zeolites is controlled by the size of the pores and by the exchange capacity of the materials, and the exchange rate of Hg(II) is main ly controlled by the Si/AI ratio of the zeolites. The partial exchange of N a+ for NH4+ in the NaX zeolite improves the exchange rate of Cr(III), but t his exchange rate is not affected by the partial exchange of Na+ for La(III ), probably due to the La(III) ions being located in sites non accessible t o Cr(III).