Author ORCID Identifier

https://orcid.org/0000-0002-4191-4015

Year of Publication

2022

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Arts and Sciences

Department/School/Program

Chemistry

First Advisor

Dr. David A. Atwood

Abstract

A thiol molecule, 2,6-pyridinediamidoethanethiol (PB9), was synthesized based on the pyridine-2,6-dicarboxamide scaffold with appended cysteamine groups. PB9 acts as an effective chelator for Pb(II) due to multiple binding sites (N3S2) through irreversible binding precipitating Pb(II). Removal of aqueous Pb(II) from solution was demonstrated by exploring the effects of time, initial PB9:Pb(II) ratios, pH, exposure time, and solution temperature. After 15 min the Pb(II) concentrations were reduced from 50.00 ppm to 0.30 ppm (99.4%) and 0.25 ppm (99.5%) for PB9:Pb ratios of 1:1 and 2:1, respectively. Removal of > 93% Pb(II) was observed over multiple pH values with negligible susceptibility for leaching over time. The thermodynamic studies reveal that the removal of Pb(II) from solution is an entropically driven, spontaneous process. Solution-state (UV−vis, 1H-NMR, 13C- NMR) along with solid-state (IR, Raman, and thermal) studies of PB9/Pb(II) compounds were performed. UV-vis displays a global maximum at 274 nm and a local maximum at 327 nm for ligand-to-metal charge transfer S- 3p to Pb2+ 6p, and intraatomic Pb2+ 6s to Pb2+ 6p transitions. FT-IR absorption spectra show significant absorption bands corresponding to amide I (C=O stretching) and amide II bands (C-N stretching, NH bending). The spectral shifting due to coordination of the amidic and pyridinic N to Pb(II) and further covalent bonding with sulfur was observed. Probable PB9 + Pb(II) interactions are proposed based on the techniques above mentioned. The molecular structure was designed as PB9 behaving like a bis-deprotonated ligand with an N3S2 donor set to give Pb(II) a trigonal bipyramidal environment with non-stereochemically active s electrons. The existence of a cyclic oligomeric (PB9)4(Pb)4 or polymeric (PB9)(Pb) structure is evidenced by broad melting point, insolubility in most common solvents, and amorphous powder XRD. Moreover, PB9 also exhibits high sensitivity and selectivity towards Fe(III) over other metal ions by fluorescent quenching. Theoretical studies comprising Benesi- Hildebrand, and studies such as Job’s plot, Stern-Volmer (S-V), and detection limits illustrate higher sensing abilities, possible dynamic and static quenching, and reversibility of binding. The quenching efficiency found by S-V is 7.42 ± 0.03 × 103 M−1. Job’s plot indicates the molar binding ratio of PB9: Fe(III) as 1:1 with a higher apparent association constant of 9.537 × 103 M–1 from the Benesi- Hildebrand plot. A linear range of Fe(III) (0 – 80 µM) with a detection limit of 0.59 µM (0.003 ppm) was found. The obtained detection limit was much lower than the maximum allowable limit of Fe(III) (0.3 ppm) regulated by EPA in drinking water. PB9-sensor exhibits visible color change from colorless to yellow acting like a naked-eye detector for Fe(III). In a separate study, 2,2'-(isophthalolybis(azanediyl))bis-3-mercaptopropanoic acid (AB9) was coupled to amine-functionalized silica and silica-coated magnetic nanoparticles (with magnetite, Fe3O4, core). This exploration was conducted for achieving > 15 ppb (EPA level in drinking water) by a previously established method in the lab. The impact of initial concentration, pH, exposure time, and adsorbent dosage on the adsorption properties of Pb(II) from an aqueous solution was studied and optimized. Characterization was performed with ICP, FT-IR, Raman, XRD, TEM, and SEM. Results revealed successful fabrication of AB9 on mesoporous silica and MNP surfaces without introducing crystalline impurities. Indeed, an added advantage for AB9-MNP over AB9-silica is its magnetic nature, whereby a magnet was used to isolate the Pb(II)-containing (solid) composite from the treated water. The > 99.9% removal of Pb(II) was obtained by AB9-MNP with detectable Pb(II) dropping below 15 ppb EPA level. The obtained equilibrium results were inserted in various adsorption isotherm models, including Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich. The data was in agreement with the Langmuir model, suggesting a dominant chemical adsorption mechanism on mesoporous AB9- silica and AB9-MNP with monolayer coverage. Maximum adsorption capacities were 22.05, 24.80, 35.57, and 56.40 mg/g, respectively, for silica, AB9-silica, MNP, and AB9-MNP. This demonstrates that a thiol group improves the adsorption capacity of Pb(II). This is an eco-friendly modification with rapid magnetic separation and chemicals utilizing HSAB to form stable compounds. Lack of complicated operations, extensive reaction times, high temperatures or high pressures, and toxic/ harmful reaction media make these AB9-MNP a good candidate for aqueous Pb(II) removal.

Digital Object Identifier (DOI)

https://doi.org/10.10323/etd.2022.165

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Chemistry Commons

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