Skip to main content
ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Adaptive Cropping Systems Laboratory » Research » Publications at this Location » Publication #329065
Title: Micro-scale investigations on soil heterogeneity: Impacts on Zn retention and uptake in Zn contaminated soils

Author
item ROSENFELD, CARLA - Pennsylvania State University
item Chaney, Rufus
item TAPPERO, RYAN - Brookhaven National Laboratory
item MARTINEZ, CARMEN - Pennsylvania State University

Submitted to: Biogeochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2016
Publication Date: 3/17/2017
Citation: Rosenfeld, C.E., Chaney, R.L., Tappero, R.V., Martinez, C.E. 2017. Micro-scale investigations on soil heterogeneity: Impacts on Zn retention and uptake in Zn contaminated soils. Biogeochemistry. 46:373-383. doi: 10.2134/jeq2016.05.0184.

Interpretive Summary: The chemical and physical species of Zn in long term mineralized or contaminated soils has been shown to affect the plant availability and potential phytotoxicity of soil Zn. To better characterize the chemical species of Zn in such soils in relation to the phytoavailability to Zn hyperaccumulator plant species, soils were obtained from four locations where Zn was substantially enriched by either geological or anthropogenic factors. Three of the soils were rich in Zn from long term or long ago application of biosolids which increased soil Zn to levels which would be phytotoxic if the soils were acidic, but the soils were near neutral pH which prevented Zn phytotoxicity. It has been reported that the Zn in anaerobic biosolids is nearly all ZnS (sphalerite), but that the ZnS is rapidly transformed to new equilibria in aerobic soils. This work is unique because it used both micro-X-ray Fluorescence Spectroscopy (µ-XRF) and micro-X-ray-absorption-near-edge-structure (µ-XANES) and density gradient separation of soil fractions to improve the identification and quantitation of the solid phases of Zn present. These methods can identify the specific mineral forms of Zn present in a few cubic millimeters of sample by comparing the absorption and fluorescence spectra for a sample with known mineral and adsorption complex specimens. Separating the soil solids by density gradient centrifugation gave 3 fractions, a light fraction with < 1.6 g/cm3, a medium fraction with 1.6-2.8 g/cm3, and a heavy fraction with >2.8 g/cm3. The highest Zn soil was a mineralized peat soil from New York; much of the Zn was associated with the organic matter in the low density organic matter fraction and that Zn had low phytoavailability because the organic matter chelated the Zn strongly. The heavy fraction contained the ZnS mineral sphalerite and appeared to have higher phytoavailability than the other fractions. In the biosolids amended mineral soils, Zn was mostly associated with the iron oxide minerals and other adsorption and co-precipitation minerals. Some Zn was in the mineral franklinite or Zn-coprecipitated ferrihydrite. Zn phyllosilicate minerals were also present in the mineral soils. The XAS of the intact soil indicated different chemical species of Zn than found in the separated density fractions, which had not previously been reported. The present work identified new information about long term Zn speciation in contaminated soils and showed that simple approaches to characterize Zn species gave incomplete results compared to this combined density separation combined with XAS.

Technical Abstract: Metal contaminants in soils can persist for millennia, causing lasting negative impacts on local ecosystems. Long-term contaminant bioavailability is related to soil pH and the strength and stability of their solid phase associations. We combined physical density separation with synchrotron-based microspectroscopy to reduce solid-phase complexity and clarify understanding of zinc-speciation in multiple field-contaminated soils obtained from Salinas Valley, California (bedrock contaminated, mineral soil), Chicago, Illinois (biosolids contaminated, mineral soil), St. Mary’s, Pennsylvania (biosolids contaminated, mineral soil), and Manning Peatlands, New York (bedrock contaminated, organic soil), USA. We also investigated Zn uptake in two Zn-hyperaccumulating ecotypes of Noccaea caerulescens (Ganges and Prayon). Soils represented a gradient of Zn contamination, from moderate (500-800 mg/kg) to grossly contaminated (26,000 mg/kg). We separated the soils using sodium polytungstate into three fractions: light fraction (LF: <1.6 g/cm3), medium fraction (MF: 1.6 – 2.8 g/cm3), and heavy fraction (HF: >2.8 g/cm3) and analyzed each fraction using wet chemical and synchrotron microprobe techniques (µ-XRF, Zn-µ-XANES). Approximately 45% of total Zn was associated with MF in the mineral soils (Chicago and St. Mary’s soils). From this, we infer substantial redistribution from the HF and LF to the MF in these soils, as the Zn within biosolids is principally associated with the HF and LF. Uptake by Zn-hyperaccumulator plants generally increased with increasing soil Zn; however, a high prevalence of soft ligand-complexes appeared to inhibit uptake from the St. Mary’s LF. In contrast to reduced uptake from the LF, our results also suggested that greater amounts of Zn contained within the HF could result in greater Zn uptake by plants. Combining micro-scale spectroscopic tools with density fractionation offered a more detailed look into highly complex soil systems, one that does not necessarily match with bulk scale observations of the soils. This information furthers our understanding of the dominant solid phases interacting Zn in field-contaminated soils and can improve predictions of Zn mobility and/or bioavailability in contaminated ecosystems.