A Study on Biochar Properties Toward their Effects on Adsorption Mechanisms for Aromatic Organic Compounds in Water

Griffin Loebsack, Western University

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.