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Feature Article

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Washington University Undergraduate Research Digest: WUURD 10


Faculty Mentor: Jeffrey Catalano

Organic acids, such as the complexant oxalate and the reductant ascorbate, and iron oxides, like goethite, occur naturally in soil systems. Metals, including zinc (Zn), also exist in the soil and serve as a micronutrient in small quantities and a contaminant in larger ones. The relative quantities of these substances differ based upon the climate, soil horizons present, the plant and animal life that lives in a particular system, and human use of the land. This study seeks to determine how the presence of oxalate and ascorbate influences the adsorption of Zn onto goethite. After synthesizing goethite, five sets of samples were prepared at either pH 5 or 7 and in the presence of oxalate or ascorbate or without any organic acid. Each sample had a varying initial Zn concentration. The samples were reacted for five days at which time the goethite was removed. The final concentration of Zn in each sample was determined using an Inductively Coupled Plasma – Optical Emission Spectrometer (ICP-OES). The ICP-OES results indicate that at pH 7, the presence of organic acid reduces the adsorption of Zn onto goethite, but at pH 5, the reverse process is observed. This indicates that varying pH and the organic acid present have an effect on adsorption. By including a variety of pH values and organic acids, the experiments more closely resemble a naturallyoccurring soil system where pH is inconsistent and a multitude of organic acids are present. Furthermore, it is important to study Zn binding to soil minerals because its concentration does not necessarily reflect its availability or mobility which can either beneficially or detrimentally impact quantities available to plants.


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