Cation-Exchange Properties of Zeolite Synthesized from Coal Fly Ash by Hydrothermal Reaction

摘要

The cation-exchange properties of monovalent (K~+, NH_4~+ and Li~+) and divalent (Ca~(2+), Mg~(2+), Cd~(2+), Pb~(2+) and Ni~(2+)) cations has been investigated for the Na-zeolite synthesized from coal fly ash by a hydrothermal reaction. The amount of NH_4~+ exchanged with the Na-zeolite was compared to that with a commercial product, A-4 zeolite. The effect of cations (H~+, K~+, Na~+ and NH_4~+) on the removal of PO_4~(3-) was studied for the solution containing phosphate ions.The cation-exchange capacity of the Na-zeolite synthesized from coal fly ash is as much as that of an A-4 zeolite. The monovalent and divalent caion exchange selectivity for Na-zeolite is shown to increase in the order of Pb~(2+)>Cd~(2+)>Ca~(2+)>K~+>NH_4~+>Ni~(2+)>Li~+>Mg~(2+). It is found that the cation-exchange selectivity is closely concerned with an electric charge and a hydration ion radius of the cation. Namely, the cation having a small hydration ionic radius might exchange much more with Na-zeolite, while the cation having a large hydration ionic radius might hardly exchange.Phosphate ions can be removed by a quantitative precipitation formation with Ca~(2+) released from Ca-zeolite. In phosphate solutions such as K_3 PO_4, Na_3 PO_4 and (NH_4)_3 PO_4, Ca~(2+) is released by an ion-exchange reaction between a cation in the solution and Ca~(2+) on the zeolite, while in H_3 PO_4 solution Ca~(2+) is supplied in the aqueous solution from the zeolite surface destroyed by the action of an acid. It is possible to remove simultaneously NH_4~+ and PO_4~(3-) in (NH_4)_3 PO_4 solution through an ion-exchange reaction of NH_4~+ with Ca-zeolite and a precipitation reaction of PO_4~(3-) with Ca~(2+) released from Ca-zeolite.