Distinctive Nature of Ca and Cu Adsorption by Halloysite

We measured calcium and copper adsorptions from a 0.01 mol L-1 CaC12 solution by three halloysite samples that were purified by size-fractionation and dithionite treatments. An allophanic clay and noncrystalline silica were used as references. The Ca and Cu adsorptions increased as pH increased and all of the added Cu was adsorbed at and above pH 6.3. The halloysite samples adsorbed significant amounts of Ca and Cu even at pHs<4, suggesting that these halloysites have surface negative charge arising from isomorphous substitution. The values of the natural logarithm of the selectivity coefficient for Ca-Cu exchange were around 5 at pHs<4 and exponentially increased to about 7 at pH6.5. The values for halloysite samples were markedly higher than the values reported for 2: 1 type layer silicates. Comparison of the adsorption percentage vs. pH curves of the halloysites and allophanic clay, and silica suggested that halloysite is superior to allophane and oxide minerals as an adsorbent in heavy metals-immobilization technology.