Ping-Chieh Hsieh

Effects of ionic strength on the binding of phenanthrene and pyrene to humic substances: three-stage variation model.

This study compared the effects of ionic strength on the binding constants (K(doc)) of selected polycyclic aromatic hydrocarbons (PAHs) (phenanthrene and pyrene) and a terrestrial humic acid (Leonardite Humic Acid) in different electrolyte solutions (KCl, KBr, MgCl(2) and MgSO(4)). Distinct trends were found in K(doc) variation depending upon the range of ionic strength resulting from added electrolytes. These trends demonstrated similar shapes for all the systems studied, while degree of variation increased with hydrophobicity of the PAHs. Furthermore, different types of electrolytes had different effects on the interactions between humic acid (HA) and the PAHs. These differences were primarily caused by types of cation, not anion. To describe the complicated effects of ionic strength on K(doc), we developed a three-stage variation model that encompasses increasing and decreasing trends and plateaus in K(doc) associated with ionic strength, as well as the mechanisms behind these trends, including the variation of HA structure configuration, HA aggregation and the salting-out effect. This model illustrated the importance of sufficient experimental data when interpreting the influence of ionic strength on the trends in K(doc) variation.

pH dependence of binding benzo[h]quinoline and humic acid and effects on fluorescence quenching

The binding constant (K(DOC)) between humic acid and the nitrogen-containing polycyclic aromatic compound (N-PAC), benzo[h]quinoline, was measured at varying pH levels using fluorescence quenching (FQ). Because fluorescence characteristics of benzo[h]quinoline change with pH, determination required two optimum sets of excitation and emission wavelength pairs. A simple mixing model was used to eliminate the inherent fluorescence interference between benzo[h]quinoline and its protonated form, benzo[h]quinolinium, when estimating binding constants. Hydrophobic interaction is likely to control the binding between humic acid and benzo[h]quinoline and benzo[h]quinolinium, in lower and higher pH ranges (pH <3, pH >6). In contrast, cation exchange seems to control the binding affinity of benzo[h]quinolinium in the middle range of pH. The estimates of K(DOC) were up to 70% smaller after elimination of interference. This indicates that the contribution of the minor form influences estimates of the K(DOC)-pH trend for benzo[h]quinoline, and potentially explains the large discrepancy reported in the literature between results based on using FQ and those based on equilibrium dialysis methods. Previous FQ measurements overestimate K(DOC) at some pH values and lead to an underestimation of bioavailability in an aquatic environment. The application of our models appears to be necessary when using FQ for determining the K(DOC)-pH trend for organic compounds with acid-base pair analogs.

Mixing of dissolved organic matter from distinct sources: using fluorescent pyrene as a probe.

Binary mixtures of humic substances (HSs) from three distinct sources (two soil humic acids and one natural organic matter) were used to investigate the mixing behavior of dissolved organic matters in the aquatic environment. Much longer time was required for equilibration of a solution of mixed HSs with pyrene as a probe, than for a solution comprising a single HS. Restructuring processes were responsible for the difference in equilibration kinetics and included conformational changes and aggregation. A major factor found controlling these processes was intermolecular interactions between components of HSs, although the concentration and hydrophobicity of the HSs were also important. In addition, the fluorescence quenching method yielded the binding constant (KDOC) of the HSs with pyrene. The KDOC of pyrene with binary humic mixtures was found to follow the conservative mixing rule. The findings of this study are applicable to studies of other hydrophobic organic pollutants and HSs and, will assist in modeling of the transport and fate of pollutants in aquatic environments.

The role of the characteristics of humic substances in binding with benzo[h]quinoline

The binding constants (K(DOC)) of the mixture of benzo[h]quinoline and its protonated analog, benzo[h]quinolinium, to four types of humic substances obtained from the International Humic Substances Society were determined by the fluorescence quenching method. A simple mixing model was used to eliminate the fluorescent interference from the minor analog in the solution and to deduce K(mix), which represents the overall binding as the sum of that for the individual analogs. The characteristics of humic substances, especially their hydrophobicity and aromaticity, established by principal component analysis of structural and elemental compositions, were the main determinants of the binding affinity with both benzo[h]quinoline and benzo[h]quinolinium (K(BQ) and K (BQH+) across a range of pH values. The strongest overall affinity of benzo[h]quinoline for humic substances is observed near pH 4 and with more hydrophobic humic substances, which suggests possible choices in attempts at remediation of benzo[h]quinoline containing particles with humic substances.

Desorption kinetics of naphthalene from sediment particles: batch and stepwise desorption approach

We investigated three major parameters in simulating desorption behaviours: the pre-contamination period, dilution by clean water mass (dilution ratio) and the length of the time interval between dilutions, in both batch and stepwise desorption experiments using particles pre-contaminated with a polycyclic aromatic hydrocarbon, naphthalene (NAPH). We found that the dilution ratio plays a major role in batch desorption, whereas the pre-contamination period plays a major role in stepwise desorption (SD). In addition, the diffusion process was found to be the rate-determining step for our study of SD kinetics. Based on the results of our batch experiments, we were able to derive a mathematical expression capable of correlating the SD rate constant with the length of time interval between dilutions. This expression can be applied in numerical modelling of various naturally occurring processes such as the dispersion and dilution of a heavily contaminated water mass in a marine environment due to an oil spill incident.

Extraction of Polycyclic Aromatic Hydrocarbons from: A Sediments Comparison between Accelerated Solvent Extraction and Soxhlet Extraction Techniques

The methods of simultaneous extraction of polycyclic aromatic hydrocarbons (PAHs) from field sediments using Soxhlet extraction and accelerated solvent extraction (ASE) were established, and the extraction efficiencies were systemically compared from procedural blank, limits of detection, spiked method recovery, and other factors. The results showed that the values obtained in this study were comparable with the values reported by other studies. In the respects of method recovery and analysis parameter there were no significant differences between ASE and Soxhlet methods for the extraction of PAHs. In this study, under the ASE operational parameters of 100 ℃, 1500 psi, and 5 minutes, and extraction solvents of acetone and hexane, it receives a reliable extraction effectiveness compared to Soxhlet extraction. Overall, considering solvent consumption and extraction time, ASE is preferable to Soxhlet extraction.