Removal Of Metal Contaminants And Radionuclides With Natural Zeolites - Competitive Sorption And Effects Of Zeolite Modifications

Abstract

Metal contaminants including radionuclides found in wastewaters must be reduced to acceptable levels before discharging the wastewater into receiving waters. These cations can accumulate in aquatic organisms causing toxicity and death, hence strict regulatory limits for their allowable discharge levels have been established over the years. This has also prompted the development of cost effective technologies that will permit the efficient removal of contaminants, while concentrating the contaminants in a form suitable for immobilization, storage and disposal. Removal requirements can be met by some inorganic zeolites, which can be natural or synthetic. The zeolites have a strong affinity for transition metal cations, and their cage-like structures offer large internal and external surface areas for ion exchange and adsorption.

In view of these considerations, investigations with two low-cost natural zeolites (clinoptilolite and chabazite) were started, with the objective of developing a cost effective technology for column operations. Sorption tests of Cs⁺, Sr2⁺ and Cu2⁺ ions on chabazite and clinoptilolite were carried out using batch experiments in 125 mL vessels under various test conditions. Competitive sorption was studied in mixed cation solutions containing the commonly occurring non-toxic cations Ca2⁺ and Na⁺. The presence of these non-toxic cations inhibits removal of the targeted cations. Additional pretreatments and modifications of the “as received” sorbent particles were also investigated, which are important in multiple cation systems, to enhance removal selectivity for the target contaminant cations.

Sorption tests with chabazite and clinoptilolite showed that these two natural zeolites can be effectively used to remove cesium, strontium and copper ions from aqueous solutions. The performance can be controlled by maintaining the zeolite to waste volume ratio. The contaminant removal performance of chabazite was approximately 20% superior to that of the clinoptilolite. The presence of calcium ions has an adverse effect on the removal of copper and strontium ions, and this effect is more pronounced with clinoptilolite. The role of the hydrated radius and energy of hydration in the sorption process offered an explanation of the differing removal efficiencies for the various cations by the zeolites. Conditioning of chabazite in a concentrated sodium chloride solution improved its performance by about 10%, while there was little improvement attained by conditioning with calcium or ammonium ions.

This paper also describes a research plan for optimizing field-scale applications in column operations. Considerations such as fluidization and hydraulic behavior, mass transfer and mixing in the zeolite beds, and safety considerations will be investigated. These investigations should lead to the selection of appropriate particle sizes, operating modes and the design of sorptioncolumn internals.