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Adsorption Of Naturally-Occurring Dicarboxylic Acids At The Hematite/Water Interface

Hwang, Yu Sik

Abstract Details

2008, Doctor of Philosophy, Ohio State University, Civil Engineering.

Numerous naturally-occurring organic acids (low- or high-molecular weight organic acids) are commonly found in the natural environment, often at significant concentrations. These acids adsorb strongly to mineral surfaces and affect the fate and transport of contaminants and nutrients as well as other basic biogeochemical processes (e.g., mineral dissolution). An accurate assessment of the impact the adsorbed organic acid has on these processes hinges on developing, at the molecular-level, an understanding of the interactions occurring at the mineral-water interface.

The objectives of this investigation were 1) to evaluate how differences in the molecular structure of organic acids can affect adsorption behavior at the mineral-water interface as a function of environmental conditions; 2) to develop a surface complexation modeling strategy to successfully integrate the molecular-level information with observations made at the macroscopic results; 3) to elucidate how competitive interactions between multiple organic acids and the mineral-water interface alters adsorption behavior in advective systems.

To meet these objectives, a systematic investigation of the adsorption of four low- molecular weight (LMW) dicarboxylic acids (phthalic acid, maleic acid, succinic acid, and fumaric acid) on the hematite surface was performed. These acids were chosen to determine the influence of simple structural differences, in particular the orientation of the carboxylic groups, on adsorption. Results are presented for a variety of systems studied using traditional equilibrium batch adsorption experiments, in-situ attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), theoretical molecular orbital calculations, potentiometric titration experiments, surface complexation models (SCMs), and column experiments.

From the batch adsorption experiment, it was found that the orientation of the carboxylic groups in each dicarboxylic acid and their pKa values significantly influences the adsorption density as well as the position and characteristics of the pH adsorption edges. In the circum-neutral pH range, the highest adsorption was measured for phthalic acid and maleic acid, followed by succinic acid. In particular, the adsorption of fumaric acid, an acid with two carboxylic groups in a trans-configuration, was much lower than that observed for the other organic acids. Through ATR-FTIR spectroscopic results, it was also established that the adsorption structures and modes at the interface strongly depend on the position of the carboxylic function groups. Phthalic acid with ortho-position, maleic acid with cis-configuration, and succinic acid with higher flexibility of the two carboxylic groups formed similar two fully deprotonated surface complexes at the hematite/water interface: an outer-sphere complex and a bidentate inner-sphere complex likely involving both carboxylic groups. Fumaric acid with trans-configuration, however, seems to bind to hematite primarily as a deprotonated outer-sphere complex. Unlike phthalic acid, maleic acid, and succinic acid, both carboxylic groups on the trans configured fumaric acid molecule are oriented opposite to one another and likely fail to anchor to the same surface, which results in different adsorption behavior at macroscopic and molecular level. In addition, ATR-FTIR spectroscopic results showed that the relative importance of both inner- and outer- sphere complexes varies with the solution pH as well as the types of organic acids. At high pH, the outer-sphere complex appeared to dominate, whereas the inner-sphere complex was favored by low pH.

The results of a surface complexation model (SCM) that includes one deprotonated bidentate inner-sphere complex and one deprotonated bidentate outer-sphere complex was qualitatively consistent with the spectroscopic results, exhibiting the importance of the outer-sphere complex through all pH ranges and the presence of the inner-sphere complex at low pH. However, the resulting surface speciation was highly influenced by model parameter choice, with the concentration of inner-sphere complex at low pH values significantly increasing with decreasing inner-layer capacitance and site density. These results suggest the reliability of SCMs to predict bulk adsorption amounts and surface speciation relies on the selection of appropriate surface complexation reactions and adjustable model parameters as constrained by specific direct knowledge of surface speciation.

The results of column experiments revealed that an organic acid with a higher adsorption affinity (e.g., phthalic acid) can competitively displace an organic acid with a lower adsorption affinity (e.g., fumaric acid). Adsorption mode (inner- vs. outer- sphere complex) also likely plays a role in competitive adsorption. In addition, it was found that an accurate prediction of organic acid transport requires a through understanding of proton reactions on mineral or in solution that in turn evolve upon the adsorption of the organic acid on minerals surfaces.

John Lenhart, PhD (Advisor)
Harold Walker, PhD (Committee Member)
Linda Weavers, PhD (Committee Member)
Nicholas Basta, PhD (Committee Member)
245 p.

Recommended Citations

Citations

  • Hwang, Y. S. (2008). Adsorption Of Naturally-Occurring Dicarboxylic Acids At The Hematite/Water Interface [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1226256959

    APA Style (7th edition)

  • Hwang, Yu Sik. Adsorption Of Naturally-Occurring Dicarboxylic Acids At The Hematite/Water Interface. 2008. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1226256959.

    MLA Style (8th edition)

  • Hwang, Yu Sik. "Adsorption Of Naturally-Occurring Dicarboxylic Acids At The Hematite/Water Interface." Doctoral dissertation, Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1226256959

    Chicago Manual of Style (17th edition)