Submitted to: Geochimica et Cosmochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2007
Publication Date: 2/20/2007
Citation: Mcnear, D.H., Chaney, R.L., Sparks, D.L. 2007. The Effects of Soil Type and Chemical Treatment on Nickel Speciation in Refinery Enriched Soils: A Multi-Technique Investigation. Geochimica et Cosmochimica Acta. 71:2190-2208. Interpretive Summary: A nickel smelter and refinery operated at Port Colborne, Ontario, Canada from 1918 to 1980. During operation, stack emissions caused contamination of regional soils with Ni and some other elements present in the ores. If these soils were acidic, soil Ni was severely phytotoxic to sensitive vegetable crops, while if pH remained high little evidence of Ni phytotoxicity was observed. Pot and field studies of the acidic phytotoxic soils have shown that addition of limestone plus needed fertilizers could fully remediate soils with high Ni contamination (6000 mg Ni/kg). The present research was conducted to identify the chemical species of Ni in the long-term contaminated soils and to learn if the soil amendments caused change in the chemical speciation of Ni in the soils in relation to reduction of the phytotoxicity of the soil Ni. Individual particles rich in Ni were investigated by electron probe microanalysis, and X-ray Absorption Spectroscopy with quantitation of the fraction of Ni in different forms using Extended Fine Structure Analysis of the XAS data. In untreated soils, Ni was present both as unchanged Ni oxide emitted from the smelter, Ni bound to organic matter, Ni in the form of a layered double hydroxide with aluminum and Ni in the form of phylosilicates. With treatment, much less Ni was water soluble and more of the Ni-Al double hydroxide were present in the soils. Results show that high soil Ni can be remediated with inexpensive soil amendments. Further, this research identified the layered double hydroxide of Ni and Al in a field soil for the first time; previous knowledge of this form of Ni from studies of the reaction of Ni with soils in short term laboratory studies. Thus entirely new chemical forms of Ni are generated in smelter contaminated soils helping to explain why long-term contaminated soils are much less phytotoxic than soils freshly amended with soluble Ni salts which is knowledge critical to valid risk assessment for Ni contamination of soils.
Technical Abstract: Aerial deposition of Ni from a refinery in Port Colborne, Onatrio, Canada has resulted in the enrichment of 29km2 of land with Ni concentrations exceeding 200 mg/kg. The Canadian Ministry of the Environment has established the latter value as a phytotoxicity threshold above which remedial action is required. Several studies on these soils have shown that making the soils calcareous was effective at reducing the DTPA or SrNO3 extractable Ni fraction and subsequently alleviating Ni phytotoxicity symptoms in vegetable crops grown in the vicinity of the refinery. Conversely, dolomitic limestone additions resulted in the increased uptake of Ni in the Ni hyperaccumulator Alyssum murale ‘Kotodesh’; a plant whose use was proposed as an alternative or concomitant method of remediation for this area. None of the previous studies have directly assessed the speciation of Ni in the soils around the refinery, or what role soil type and lime treatments play in altering the soil Ni speciation. Therefore, the objectives of this paper are to examine the influence of soil type and treatment on the speciation of Ni in a Welland loam and Quarry muck soil and relate these findings to Ni mobility and bioavailability. The speciation and mobility of Ni was assessed using conventional and cutting-edge techniques. Stirred-flow dissolution experiments using pH 4 HNO3 showed that Ni release from the treated Quarry Muck and Welland Loam soils was reduced (~0.10%) relative to the untreated soils (~2.0%). Electron probe microanalysis (EMPA) identified approximately spherical NiO and Ni metal particles, which are associated with no other metals, and range from 5-50 m in diameter. Synchrotron micro X-ray absorption fine structure and X-ray fluorescence spectroscopy’s showed that Ni Al-LDH phases were present in both the treated and untreated mineral soils, with a tendency towards more stable (e. g., aged-LDH and phyllosilicate) Ni species in the treated soil, possibly aided by the solubilization of Si with increasing pH. In the muck soils, Ni-organic complexes (namely fulvic acid) dominated the speciation in both treated and untreated soils. The results reported herein show that both soil type and treatment have a pronounce effect on the speciation of Ni in the soils surrounding the Port Colborne refinery. We demonstrate for the first time that Ni Al-LDH phase can form in anthropogenically enriched mineral field soils at circumneutral pH, and can lead to a reduction in Ni mobility. In the organic soils, Ni is strongly complexed by soil organic matter, a property enhanced with liming. In both systems, liming was effective at reducing Ni mobility by both the formation and secondary precipitates and strong complexation with soil organic matter.