Small Scale Heterogeneity of Soil Chemical Properties. I. A Technique for Rapid Aggregate Fractionation

Difference of lead, copper and zinc concentrations between interiors and exteriors of peds in some contaminated soils

Solute transport of elements in soils depends on the soil structural and hydraulic properties, and it is controlled by sorption and diffusion, which both limit the mobility and distribution of elements in soils. This study was conducted to compare lead (Pb), copper (Cu) and zinc (Zn) concentrations between ped exteriors and interiors of some contaminated soils. The results show that the differences of the heavy metals between exteriors and interiors decreased in the order clayey soil, clayey loam soil, loam soil. For same soils, the differences decreased from Pb to Cu to Zn. The differences in readily extractable concentrations of the three metals between ped exteriors and interiors were much larger than the differences in their total metals, this may indicate that extractable metals were more recently deposited. The higher Pb and Cu concentrations in the ped exteriors than interiors may additionally be explained by anthropogenic input, movement and downward through preferential flow.

Combining selective sequential extractions, X-ray absorption spectroscopy, and principal component analysis for quantitative zinc speciation in soil

Selective sequential extractions (SSE) and, more recently, X-ray absorption fine-structure IXAFS) spectroscopy have been used to characterize the speciation of metal contaminants in soils and sediments. However, both methods have specific limitations when multiple metal species coexist in soils and sediments. In this study, we tested a combined approach, in which XAFS spectra were collected after each of 6 SSE steps, and then analyzed by multishell fitting, principal component analysis (PCA) and linear combination fits (LCF), to determine the Zn speciation in a smelter-contaminated, strongly acidic soil. In the topsoil, Zn was predominately found in the smelter-emitted minerals franklinite (60%) and sphalerite (30%) and as aqueous or outer-sphere Zn2+ (10%). In the subsoil, aqueous or outer-sphere Zn2+ prevailed (55%), but 45% of Zn was incorporated by hydroxy-Al interlayers of phyllosilicates. Formation of such Zn-bearing hydroxy-interlayers, which has been observed here for the first time, may be an important mechanism to reduce the solubility of Zn in those soils, which are too acidic to retain Zn by formation of inner-sphere sorption complexes, layered double hydroxides or phyllosilicates. The stepwise removal of Zn fractions by SSE significantly improved the identification of species by XAFS and PCA and their subsequent quantification by LCF. While SSE alone provided excellent estimates of the amount of mobile Zn species, it failed to identify and quantify Zn associated with mineral phases because of nonspecific dissolution and the precipitation of Zn oxalate. The systematic combination of chemical extraction, spectroscopy, and advanced statistical analysis allowed us to identify and quantify both mobile and recalcitrant species with high reliability and precision.

PAH-pools in soils along a PAH-deposition gradient

The objectives of this study are to characterize different PAH-pools (soil horizons, microsites influenced by stem flow, aggregate core and surface fractions, particle size fractions) in soils affected by depositions. Three forest soils affected by the emissions of an aluminium plant near Ziar/Central Slovakia were sampled to analyze 20 PAHs. The organic layers have high concentrations of PAHs (40-200 mg kg(-1)), decreasing as soil depth and distance from the aluminium plant increases. At the two sites nearest to the plant PAH-concentrations are higher in microsites affected by stem flow than in microsites not influenced by stem flow. They are also higher in aggregate surface fractions than in aggregate core fractions and in bulk soil samples than in aggregates. Sand- and siltsize particles contain decreasing percentages of the sum PAH-concentrations as distance from the plant increases. This microscale heterogeneity affects PAH-availability and has to be considered when assessing environmental risks.