Citation: Schmidt, M.A., Gonzalez, J.M., Halvorson, J.J., Hagerman, A.E. 2013. Metal mobilization in soil by two structurally defined polyphenols. Chemosphere. 90:1870-1877. Interpretive Summary: In this study we investigate chemical interactions between tannins and metals, and the implication of those interactions on metals in soil systems. Tannins are produced by plants and are present in plant tissues such as leaves, bark, and roots. Tannins enter the soil during rain storms when water washes through leaves, or during autumn when leaves start to decompose. Once in the soil tannins can change metal dynamics or function as a possible source of bioremediation. Investigating the amount of metal that binds to tannins is the first step in understanding tannin effects on metals in the environment. This study used two model compounds that represent the different classes of tannins to investigate the maximum binding of metals to tannins, how multiple metals systems change binding, and how tannins change metal concentrations in soils. Using this system we found that the larger of the two tannins was able to bind more metal then the smaller tannin. This is due to the larger molecule having more functional groups that can bind to metal then the small molecule. Using the information from the tannin-metal binding to the individual metals we also were able to model the changes in metal binding to tannins when more than one metal is present in solution. Finally we investigated how tannins change metal concentrations in soil. To accomplish this we added known amounts of tannins to soil and measured increases of metals in water extracts. We found that the tannin that could bind more metal was also able to increase the amount of metal in solution. This work is useful for soil scientists assessing potential impacts tannins have on agricultural and ecosystem processes.
Technical 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 effect 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 binding model, we applied the Langmuir model for competitive adsorption to predict binding for mixtures of Al(III) and Fe(III). By using the parameters from the single metal experiments and information about polyphenol sorption we built a model to predict metal mobilization from soils amended with polyphenols. The model predicted mobilization of Fe and Al with r2=0.86 and r2 =0.79, respectively. The amount of metal that was mobilized was directly proportional to the maximum amount of metal bound to the polyphenol and inversely proportional to the amount of polyphenol that sorbed to the soil.