The sorption mechanism of Zn on gibbsite and montmorillonite exchanged with Al{sup}(3+) (Al-mont) or Keggin Al{sub}13 polymer (Al{sub}13-mont) was probed by powder and polarized EXAFS spectroscopy as a function of pH (5.85-7), reaction time (1-65 days), and sorbate to sorbent ratio (50-200 μM Zn/2 g montmorillonite). For all Al-mont samples, Zn is octahedrally coordinated to oxygens at ～2.08(2) A, and surrounded in-plane by six Al atoms at 3.02-3.06(2) A, and another six at 6 A. No out-of plane Si neighbors are detected. These results are interpreted as Zn incorporation in vacant octahedral sites of gibbsite-like layers at the basal and/or interlayer surface of montmorillonite particles. Zinc sorbed on the edges of gibbsite layers would give a split first oxygen shell with bond distances of 2.00(2) and 2.16(3) A, and 2.1(8) nearest Al at 3.02 A with no second-nearest Al, none of which were observed in Al-mont. The binding environment of Zn on Al{sub}13-mont after 1 day is similar to that on the edges of gibbsite, and is interpreted as Zn complexation at the surface of Al polymers. After 28 days, the Zn environment resembles that of Zn-sorbed Al-mont, indicating the progressive buildup of Zn-containing gibbsite-like layers parallel to montmorillonite layers. The results of this work clarify the incorporation mechanism of Zn in hydroxy-Al interlayered phyllosilicate and provide insight on the formation mechanism of this common Zn species in soil.
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