Metal mobilization in soil by two structurally defined polyphenols Michael A. Schmidt a , Javier M. Gonzalez b , Jonathan J. Halvorson c , Ann E. Hagerman a,⇑ a Department of Chemistry & Biochemistry, Miami University, Oxford, OH 45056, United States b National Soil Erosion Research Laboratory, USDA-ARS, West Lafayette, IN 47907, United States c Northern Great Plains Research Laboratory, USDA-ARS, Mandan, ND 58554, United States highlights " The larger polyphenol (oenothein B) binds more Fe and All than the smaller EGCg. " Oenothein B mobilized a significant amount of Fe and Al from soil. " Micelle-mediated separation was utilized to study tannin-metal binding. " Langmuir model for competitive binding was used to predict multiple metal binding. article info Article history: Received 25 April 2012 Received in revised form 26 September 2012 Accepted 15 October 2012 Available online 11 November 2012 Keywords: Phytochelation Metal mobilization Tannin Polyphenol Phytoremediation abstract Polyphenols including tannins comprise a large percentage of plant detritus such as leaf litter, and affect soil processes including metal dynamics. We tested the effects of tannins on soil metal mobilization by determining the binding stoichiometries of two model polyphenols to Al(III) and Fe(III) using micelle- mediated separation and inductively coupled plasma optical emission spectroscopy (ICP-OES). By fitting the data to the Langmuir model we found the higher molecular weight polyphenol (oenothein B) was able to bind more metal than the smaller polyphenol (epigallocatechin gallate, EGCg). For example, oenothein B bound 9.43 mol Fe mol 1 , while EGCg bound 4.41 mol of Fe mol 1 . Using the parameters from the bind- ing model, we applied the Langmuir model for competitive binding to predict binding for mixtures of Al(III) and Fe(III). Using the parameters from the single metal experiments and information about poly- phenol sorption to soils we built a model to predict metal mobilization from soils amended with polyphe- nols. We tested the model with three natural soils and found that it predicted mobilization of Fe and Al with r 2 = 0.92 and r 2 = 0.88, respectively. The amount of metal that was mobilized was directly propor- tional to the maximum amount of metal bound to the polyphenol. The secondary parameter in each model was the amount of weak organically chelated Fe or Al that was in the soil. This study provides the first compound-specific information about how natural polyphenols interact with metals in the envi- ronment. We propose a model that is applicable to developing phytochelation agents for metal detoxifi- cation, and we discuss how tannins may play a role in metal mobilization from soils. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Metals accumulate in soils by natural, industrial, or agricultural activity (Galloway et al., 1982; Bradford et al., 2008; Ngah and Hanafiah, 2008). After oxygen and silicon, iron and aluminum are the most abundant elements in soils, comprising about 3% and 7%, respectively (Sposito, 1989), and mainly found in the primary and secondary minerals. Natural sources of metals in soil include weathering of the underlying bedrock, soil dust, and volcanic ash (Galloway et al., 1982; Schutzendubel and Polle, 2002). Agricul- tural practices are also potential sources of metal input into soils. Fertilization with animal manure from livestock fed diets enriched in heavy metals can increase metal concentrations in wastewater and soil (Bradford et al., 2008). Industrialization and fossil fuel utilization have increased the amount of metal released into the atmosphere, which increases atmospheric deposition to soil (Galloway et al., 1982). Anthropogenic inputs of metals from the atmosphere account for large portions of the total yearly input with the two highest metals being Fe and Al (Lantzy and Mackenzie, 1979). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.10.010 Abbreviations: EGCg, epigallocatechin gallate; X m , maximum amount of metal bound; b, isotherm model parameter; HPLC, high performance liquid chromatog- raphy; ICP-OES, inductively coupled plasma optical emission spectroscopy; TFA, trifluoroacetic acid. ⇑ Corresponding author. Tel.: +1 513 529 2827; fax: +1 513 529 5715. E-mail address: hagermae@muohio.edu (A.E. Hagerman). Chemosphere 90 (2013) 1870–1877 Contents lists available at SciVerse ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere