Formation of a Transient Magnesium-Aluminum Double Hydroxide in Soils of Southeastern Ohio
- N. E. Smeck ,
- J. M. Bigham and
- H. T. Saif
To assess the potential for Mg-Al double hydroxide formation in acid, Al-rich soils of southeastern Ohio under native conditions, laboratory studies were conducted to determine if Mg-Al double hydroxide could be produced with Al-saturated clays and soils of mixed mineralogy common to the region. Aluminum-saturated clays suspended in dilute Mg solutions and titrated with NaOH to pH 9.5 yielded pronounced buffer regions between pH 8.3 and 8.5 attributable to the formation of Mg-Al double hydroxide. Equilibration of the clays titrated to pH 9.5 for 55 d showed a gradual decline to pH 8.0 to 8.4. The decline was attributed to the precipitation of additional Mg-Al double hydroxide caused by the slow release of interlayer hydroxy Al, which showed a correlation with the rate of pH decline. X-ray diffractograms of the equilibrated clays yielded peaks at 0.78 and 0.39 nm that were indicative of Mg-Al double hydroxide. Aluminum-saturated soils equilibrated with Ca solutions at pH 8.4 retained only 50 to 70% as much added cation as soils equilibrated with solutions of combined but equal total quantities of Ca and Mg. Furthermore, the soils equilibrated with Ca and Mg retained 2.4 to 4 times more Mg than Ca. The enhanced retention of Mg was attributed to Mg-Al double hydroxide formation. Sequential leaching of the equilibrated soils with NaOAc at pH 8.2, 7.0, and 5.0 and 0.05 M HCl indicated that Mg-Al double hydroxide was not readily solubilized until the pH dropped below neutral. It was concluded that the pedogenic formation of an ephemeral Mg-Al double hydroxide was a probable consequence of calcareous loess deposition on acid, residual soils during the Wisconsinan glaciation.
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