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Cation Exchange Behavior of Bauxite Refining Residues from Western Australia

Dep. of Biology, Hong Kong Baptist Univ., 224 Waterloo Rd., Kowloon, Hong Kong;

G. E. Ho

School of Biology and Environmental Sciences, Murdoch Univ., Murdoch, WA 6150, Australia.

^* Corresponding author (jwcwong{at}ctsc.hkbc.hk).

ABSTRACT

Over 60 million tonnes per year of bauxite refining residue (red mud) is produced worldwide. Its high Na concentration inhibits plant growth and hence reclamation. The cation-exchange capacity (CEC) of red mud and the cation exchange equilibria between Na⁺ and several other cations were measured to elucidate the mechanism of Na release from red mud. The CECs obtained by using K⁺ and NH⁺₄ were significantly higher than those obtained using Ca²⁺ and Ba²⁺. This unusual cation exchange phenomenon can be attributed to the presence of zeolitic minerals in red mud. Cation exchange equilibria show that Na⁺ originally present in red mud was preferentially adsorbed by the mud over other cations. At high cation fractions in red mud (>0.1), the mud selectively sorbed monovalent over divalent cations, with the following order of selectivity: K⁺ > Li⁺ > NH⁺₄ > Ba²⁺ Ca²⁺ > Mg²⁺. The exchange of Na⁺ has been found to have significant negative correlations with the radius of hydration and Debye-Huckei parameter. Divalent cations have little ability to exchange Na⁺ from zeolitic exchange sites. Incremental extraction of Na⁺ in red mud shows that K⁺ and NH⁺₄ displaced 99 (63%) and 57 (44%) cmol_c kg^–1 red mud whereas Ca²⁺ and Mg²⁺ could only displace 33 and 29 cmol_c kg^–1 red mud, respectively, out of a total of 99 cmol_c Na kg^–1 red mud. In a reverse process 95% of K⁺ adsorbed on red mud was readily replaced by Na⁺. The experimental evidence suggests that the release of Na⁺ from red mud is due to cation exchange.

This research was supported by funds from the Alcoa of Australia Pty. Ltd.

Received for publication January 11, 1994.

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