Effects of pH, Electrolyte Concentration and Organic Acid Coating on the Aggregation Kinetics of Hematite Nanoparticles
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Date
2018-12
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The Ohio State University
Abstract
Natural aqueous systems contain dissolved organic matter as well as colloidal mineral particles, and the interactions between them in different environmental conditions are important because results have wide-spread application in water treatment and filtration. Hematite nanoparticles exhibit a high level of stability and predictability, making them an ideal model to study the effect of organic acids on mineral surface charges and the subsequent effect on the aggregation kinetics of environmentally ubiquitous nanoparticles. The organic acids that were studied are citric acid and humic acid, sourced from the Suwannee River. I explored the effects of varying electrolyte concentrations on the aggregation kinetics of the hematite particles, tested with either a citric acid or humic acid coating in solutions of environmentally relevant pH’s.
The aggregation kinetics of bare hematite at pH’s 4, 5, 6, 8, and 9 were measured. In each condition, the kinetics of bare hematite nanoparticles were measured by combining a hematite solution with varying concentrations of NaCl. After measuring the behavior of bare hematite nanoparticles, studies with ranging ionic strengths were conducted with a citric acid coating ranging from 100 μmol citric acid: 1 g hematite to 20,000 μmol citric acid: 1 g hematite. The effect of humic acid on hematite nanoparticles was conducted by using 0.5 mg/L humic acid to 100 mg/L hematite. Measurements were made using the Dynamic Light Scattering (DLS) apparatus, which measures the effective diameter of solution particles as a function of time and indicates the aggregation of the solution over time. The data was analyzed to calculate the critical coagulation concentration (CCC).
Major findings with this project had to do with the effect of changing pH, varying citric acid levels, and type of acid on the stability of hematite. It was found that at low pH, bare hematite was stable and stability decreased with a citric acid coating. At a high pH, bare hematite was unstable and stability increased with the addition of citric acid. In addition, it was found that at a high pH, larger acid concentrations led to greater stability. However, at a lower pH, the acid coating led to less stability. It was also found that a humic acid coating had a comparable effect on hematite stability as a citric acid coating when the humic acid had a 70x lower concentration.
This research has expanded on knowledge regarding hematite behavior in aqueous settings. Future work should be conducted on the effects of varying humic acid on the stability of hematite nanoparticles.
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Keywords
hematite, nanoparticle aggregation, humic acid, citric acid