Experimental and theoretical evidence of zinc structurally bound in vermiculite from naturally metal-enriched soils

Abstract

AbstractThis paper provides evidence of appreciable amounts of Zn (up to 0.52 a.p.f.u.) residing in the crystal structure of trioctahedral vermiculite; the results were obtained by a combination of experimental methods (X-ray diffraction, energy dispersive X-ray spectrometry, electron microprobe analysis and selective chemical extractions) and theoretical studies based on density functional theory (DFT). Vermiculite occurs in size fractions ranging from 10 to 40 μm of near-neutral soils naturally enriched in Zn (up to 8110 mg kg–1) due to the weathering of carbonate-hosted Zn-Pb deposits in SW Spain. Zinc was partitioned among the various pedogenic phases during soil formation, but the larger part (>80%) was concentrated in the residual silicate fraction. The soil exchangeable Zn pool was found to be virtually negligible. The DFT calculations support the hypothesis that the role of vermiculite is an effective sink for geogenic Zn; this is consistent, in terms of energetic stability, with the assumption that most Zn is structurally bound in octahedral sheets rather than in the interlayer exchange sites. The findings conclusively indicate that the potential mobility of Zn should remain low under the present soil conditions.