Nickel speciation in several serpentine (ultramafic) topsoils via bulk synchrotron-based techniques

Authors: SIEBECKER, MATTHEW - University Of Delaware; Chaney, Rufus; SPARKS, DONALD - University Of Delaware

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2017
Publication Date: 3/23/2017
Citation: Siebecker, M.G., Chaney, R.L., Sparks, D.L. 2017. Nickel speciation in several serpentine (ultramafic) topsoils via bulk synchrotron-based techniques. Geoderma. 298:35-45. https://doi.org/10.1016/j.geoderma.2017.03.008.
DOI: https://doi.org/10.1016/j.geoderma.2017.03.008

Interpretive Summary: Serpentine soils naturally contain high concentrations of nickel and may become locations of commercial phytomining of nickel using nickel hyperaccumulator plants. The chemistry of nickel in soils is complex and the high levels of nickel in serpentine soils have caused phytotoxicity at locations where soil pH fell. At lower soil pH, soil nickel is more soluble and hence possibly phytotoxic. Our previous research indicated that nickel phytotoxic soils could be remediated by raising pH which promoted formation of insoluble forms of nickel in the soil. In all cases of high levels of soil nickel, understanding the fundamental chemistry and mineral species of nickel present in such soils is needed to achieve environmental protection and effective phytomining. The present studies were undertaken to use newly available technologies such as X-ray Absorption Spectroscopy (XAS) and Extended X-ray Analysis of Fine Structure (EXAFS) coupled with soil chemical measurements to characterize the mineral and chemical species of nickel normally present in serpentine soils. Serpentine soils from Oregon, California and Maryland were separated into size separates (clay, silt and sand), and iron oxides removed to allow cleaner identification of nickel chemical species present. Specimens of known nickel minerals were used as standards to calibrate the evaluation of EXAFS spectra. One chemical form of nickel, the nickel-aluminum-Layered Double Hydroxide (Ni-LDHs) was found to be formed when nickel phytotoxic soils were limed to remediate nickel phytotoxicity, so existence of the Ni-LDHs was examined. Previous research on the major minerals in serpentine soils, the magnesium silicates, had found that nickel entered into the magnesium silicate minerals during their formation from parent rocks and nickel substituted for magnesium in the minerals. Because silicate enters into the Ni-LDH mineral over time to form more stable minerals, Ni-LDH is transformed to a "serpentine" mineral (magnesium-nickel-silicate) in which nickel is substituted for magnesium within the Mg-silicate mineral. In previous research we had shown that roots of nickel hyperaccumulator plants could absorb nickel from the Ni-LDH minerals, and these plants have been shown to absorb nickel from many serpentine soils. Thus these new findings clarify the forms and phytoavailability of Ni in serpentine soils, indicating that the nickel-magnesium-silicate serpentine minerals which are the dominant Ni mineral in diverse serpentine soils support Ni phytomining or phytoremediation of nickel rich serpentine soils.

Technical Abstract: Serpentine soils are extensively studied because of their unique soil chemical properties and flora. They commonly have high magnesium-to-calcium ratios and elevated concentrations of trace metals including nickel, cobalt, and chromium. Several nickel hyperaccumulator plants are native to serpentine soils. Although nickel hyperaccumulation is well documented, the mechanisms of hyperaccumulation, for example nickel dissolution from various solid phase minerals, are not understood. The chemical forms (species) of nickel in the soil influence how much nickel the plant can accumulate. Nickel species are directly related to soil weathering, local climate, topography, and bedrock of the specific site. We used synchrotron-based techniques to directly speciate nickel in our topsoils. We developed a sonication method for this study that was useful to identify nickel species in separate particle size fractions. In the clay fractions, nickel was associated with iron oxides and primary serpentine minerals, such as lizardite. In these topsoils, nickel commonly persisted in primary serpentine parent material and was associated with layered-silicate phases, and iron and manganese oxides. Thus, to some degree, nickel must be bioavailable from these species. Linear combination fitting (LCF) of X-ray data reveals the importance of choosing appropriate standards based not only on statistical fit results but also on sample mineralogy and particle size. Using the statistical F-test is beneficial to determine the appropriate number of standards for LCF of X-ray data from soils.