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June 18, 2003 Chemical Forms of Zinc in a Smelter-Contaminated SoilA.C. Scheinost1, R. Kretzschmar2, S. Pfister2, D.R. Roberts3, and D.L. Sparks4 Zinc is contained in many objects of our daily life, from baby care products to anti-corrosive coatings of cars. Concomitant of its frequent use, zinc is released into the environment and accumulates in soils. While zinc is benign to human health, it is phytotoxic at relatively modest concentrations, thereby reducing plant growth. In spite of its importance, relatively little is known on the chemistry of zinc in soils. We have used a combination of microscopic, spectroscopic, wet chemistry, and chemometric tools to investigate the chemical forms of zinc in a strongly acidic soil near the historic zinc smelter at Palmerton, Pennsylvania.
Previous investigations have shown that Zn-bearing soil clay minerals, called phyllosilicates, may form after raising the soil pH to neutral values. Consequently, the bare soils in the vicinity of a zinc smelter in France could be re-vegetated by applications of lime and fly ash. On the other hand, a similar approach to remediate the Palmerton Superfund Site in Pennsylvania largely failed (Figure 1). The intention of our work was to determine the specific problems of this site by determining the chemical forms of zinc in a soil profile.
Our results are summarized in Figures 2 and 3. In the topsoil with an extremely acidic pH of 3.1, most zinc atoms occur in franklinite, a zinc-iron oxide spinel-type mineral (Figure 2). Zinc sulfide (the mineral sphalerite) constitutes the second largest fraction. Divalent zinc cations dissolved in the soil water or exchangeably bound to clay minerals are the smallest fraction. The topsoil contains 6200 mg kg-1 zinc, however, this easily plant-available fraction still constitutes 620 mg kg-1, a definitively phytotoxic concentration.
In the subsoil, the zinc concentration is lower (900 mg kg-1). Due to the protective cover of the topsoil, no smelter-emitted minerals are present. Since the pH is still quite acidic (3.9), relatively stable inner-sphere sorption complexes and known zinc-bearing soil minerals cannot form. Therefore, we would have expected that all zinc is dissolved or easily exchangeable. Surprisingly, however, we found that 45% of zinc was bound by Al-hydroxide interlayers sandwiched between phyllosilicate layers (Figure 3). This clay mineral (HIM) is quite common in acidic soils, and may effectively scavenge a range of transition metals at low pH, thereby reducing their ecotoxicity.
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Due to the burning of coal and other fossil fuels, zinc ore
smelting and processing, and the application of sewage sludge and
agrochemicals, soils are increasingly contaminated with zinc. In
Switzerland, and most likely also in other industrialized countries,
zinc concentrations in soils may soon reach levels which could
significantly reduce the production of food, fiber, and renewable
energy sources. The problem of such predictions is, however, that
the toxicity of plants for zinc, called phytotoxicity, in a given
soil cannot be simply predicted from its concentration. For
instance, aqueous zinc cations in soil solution are readily taken up
by plant roots and are therefore very phytotoxic, while the
incorporation of zinc into the crystal structure of recalcitrant
(i.e. very insoluble) soil minerals may drastically reduce its
solubility and hence phytotoxicity. Knowing the chemical forms of
zinc is therefore a prerequisite for risk assessments and the
development of effective remediation strategies. 
The zinc speciation was based on extended x-ray fine-structure
absorption (EXAFS) spectroscopy performed at beamline X11A. While
EXAFS is perhaps the only tool capable of identifying the various
species possibly present in soils, it was complemented by additional
methods to achieve unequivocal results. First, we made use of the
non-homogeneous distribution of species at the micrometer scale,
employing micro-focused x-rays for elemental mapping and
microspectroscopy. Second, we made use of the different solubilities
of species with respect to various solvents. We employed a sequence
of six wet-chemical extraction procedures for a step-wise removal of
increasingly recalcitrant species, while monitoring the remaining
species with EXAFS spectroscopy. Third, we used statistical methods
like principal component analysis to extract the single components
hidden in the experimental spectra of species mixtures. 
Both franklinite and sphalerite were part of the zinc ores
smelted in the Palmerton furnaces. The deposition of these minerals
by smelter-emitted dust initiated the geochemical cycle of zinc in
the surrounding soils. While franklinite has a low solubility,
sphalerite dissolves in the presence of oxygen releasing sulfuric
acid. This ongoing release of protons explains the difficulty of
raising the soil pH for longer than just a few years by applying lime
and fly ash.