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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Yeung, Ken.

2nd Supervisor

Berruti, Franco.

Co-Supervisor

3rd Supervisor

Klinghoffer, Naomi.

Co-Supervisor

Abstract

Biochar, a carbon-rich by-product of the pyrolytic processing of lignocellulosic biomass in a zero or low-oxygen environment, has the potential application as a promising adsorbent for the elimination of a variety of pollutants. This work proposes and outlines a detailed method to investigate the adsorption mechanisms of biochar towards organic compounds in water, which was used to investigate a range of biochars with the objective of determining what compounds biochars are best suited to adsorb in real-world applications. Ibuprofen, acetaminophen, methyl orange and methylene blue were selected as four test compounds that are representative of organic pollutants and were used to investigate biochars produced by four woody and four non-woody biomasses under different pyrolysis temperatures and flow gases, N2 and CO2. While biochars that had mainly hydroxyl functional groups showed good adsorption of aromatic compounds with electron-withdrawing groups, biochars with carbonyl groups were found to have greater adsorption capacities for aromatic compounds with electron-donating groups. This suggests that the electrostatic interactions and - EDA interactions are both heavily influenced by the same functional groups, resulting in possible competition between the mechanisms. Subsequently, further investigation of the effect of the physical properties on adsorption mechanisms of biochar was performed where HNO3, H2O2, and KOH, post-treatment on different woody biochars prepared under CO2 at 800˚C were chosen for further study. While the mineral content was not seen to have a significant role in these interactions, oxygen-functional groups dominated in contributing to negatively charged sites and thus electrostatic interactions of biochar. It was also found that OH groups increased electrostatic interactions for all the biochars, while increasing - electron donor-acceptor interactions with compounds with electron-withdrawing groups, as was seen for biochars activated with CO2. Carbonyl functional groups increased these interactions with compounds containing electron-donating groups. Overall, five types of adsorption mechanisms were identified. Five of these biochars were selected that displayed each of these five main mechanisms, which were investigated for adsorption with complex mixtures of aromatic compounds, which is more representative of wastewater. The oxygen-containing functional groups had the largest influence on competitive adsorption, as they often provide the greatest number of adsorption sites for organic compounds via both electrostatic and - electron donor-acceptor interactions. In general, the biochars with different oxygen-containing functional groups, carbonyl and hydroxyl in particular, had greater synergetic adsorption when combing compounds. Finally, a bio-magnetic adsorbent was produced from different biomasses via co-pyrolysis with iron-rich red mud, a waste product of the aluminum industry that is rich in iron oxide, for easier removal post adsorption. The resulting biochar-red mud composite material demonstrated good adsorption and had magnetic properties. Mixing ratios of biomass and RM were also tested to determine optimal composition for magnetic and adsorptive properties of the composites, where better adsorption performance was found with a ratio of 1:2 for RM/biomass. A threshold of biomass to RM ratio was also found to maintain magnetic properties, where the magnetic properties of the adsorbents decreased gradually when a greater ratio of biomass to RM was used, with the 1:9 RM:DF having the optimal properties of all composites tested in this study.

Summary for Lay Audience

Biochar is a charcoal-like product that is made by decomposing farm, food, and other waste products in high-heat environments in the absence of oxygen to prevent combustion. This process is referred to as pyrolysis, and the biochar that is produced can be used to adsorb a variety of pollutants in water sources including drugs and other small molecules that may have detrimental effects on health and the environment. Since biochar may have different chemical bonds on its surface, as well as different pore sizes and surface areas depending on what is used to produce the biochar, many biochars can adsorb different molecules better than others. This thesis first proposes a method to investigate what makes biochar able to adsorb aromatic molecules, which are common pharmaceutical compounds in water sources. This is done by using common drugs such as Ibuprofen and acetaminophen, as well as common dyes including methyl orange and methylene blue as test molecules to see how biochars made from woods and grasses behave differently in terms of adsorption. It was found that biochars with bonds to oxygen atoms tended to have higher adsorption capacities of aromatic molecules (containing carbon ring structures), attributed to a charge transfer between the adsorbing molecule and the biochar. This was because oxygen groups can both donate or withdraw electrons from the biochar, allowing it to attract molecules of opposing charge. This highlights the importance of both aromatic and oxygen groups on biochar surfaces in terms of adsorption. This was confirmed when the biochars were purposefully altered to have more of these oxygen groups by post-treatment with oxidizing acidic and basic chemicals. It was then found that when the oxygen groups were bonded to hydrogens there was a decrease in adsorption of negatively charged molecules, as these were repelled by the oxygen groups. Overall, five types of adsorption were identified, all of which were investigated for their adsorption in mixtures of two molecules together. In general, it was found that the biochars with different kinds of oxygen groups had the most favourable adsorption results as molecules did not compete for adsorption sites with both types of groups. Finally, a bio-magnetic adsorbent was produced from biomasses mixed with iron-rich red mud, a waste product of the aluminum industry rich in iron oxide. The processes were shown to be successful in converting all biomass species and demonstrated good adsorption and magnetic properties of the produced composites, with the best being the 1:9 RM: DF composite.

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