Purpose Gardening (especially food growing) in urban areas is becoming popular, but urban soils are often very contaminated for historical reasons. There is lack of sufficient information as to the bioavailability of soil heavy metals to plants and human in urban environments. This study examines the relative leachability of Cr, Ni, As, Cd, Zn, and Pb for soils with varying characteristics. The speciation and mobility of these metals can be qualitatively inferred from the leaching experiments. The goal is to use the data to shed some light on their bioavailability to plant and human, as well as the basis for soil remediation. Materials and methods Selective and sequential chemical leaching methods were both used to evaluate the speciation of Cr, Ni, As, Cd, Zn, and Pb in soil samples collected from New York City residential and community gardens. The sequential leaching experiment followed a standard BCR four-step procedure, while selective leaching involved seven different chemical extractants. Results and discussionThe results from selective and sequential leaching methods are consistent. In general, very little of the heavy metals were found in the easily soluble or exchangeable fractions. Larger fractions of Cd and Zn can be leached out than other metals. Lead appears predominantly in the organic or carbonate fractions, of which ∼30-60% is in the easily soluble organic fraction. Most As cannot be leached out by any of the extractants used, but it could have been complicated by the ineffective dissolution of oxides by ammonium hydroxylamine. Ni and Cr were mostly in the residual fractions but some released in the oxidizable fractions. Therefore, the leachability of metals follow the order Cd/Zn>Pb>Ni/Cr. Conclusions Despite of the controversy and inaccuracy surrounding chemical leaching methods for the speciation of metals, chemical leaching data provide important, general, and easy-to-access information on the mobility of heavy metals in soils, which in turn relates to their potential bioavailability to plant uptake and human health risk. Such data can be used to guide risk assessment of different metals and develop effective remediation strategies.

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Nanotubes are a class of host cavities increasingly used to encapsulate unstable molecules, yet none have been exploited to host reactive sulfur species, such as thiozone (S3). In this paper, density functional theory and (ONIOM) calculations were used to compute single-walled carbon nanotube (SWNT)–thiozone combinations for the inclusion of S3 into the hollow nanotube space and to rationalize when 1,2,3-thiozonide formation can take place. Nanotube diameter selectivity for the isomerization of the C2v form of S3 to the D3h form proved to be elusive. Acyclic C2v S3 was ~6 kcal/mol more stable than cyclic D3h S3 whether it was free or encapsulated within an SWNT. 1,2,3-Thiozonide formation took place on the convex side of nanotubes of low tube radii, such as the armchair (4,4) and (5,5) SWNTs. In terms of the reaction mode of C2v S3, the 1,3-dipolar addition reaction was preferred compared with the [2 + 2] cycloaddition and chelotrope paths.