Hazard Assessment of Liquid Organic Hydrogen Carriers (LOHCs) in Terrestrial Environment

Abstract

Liquid Organic Hydrogen Carriers (LOHCs) are part of a novel energy system that can efficiently and relatively safely store and transport hydrogen, which is a clean, high energy density fuel. LOHCs have high hydrogen holding capacities and are superior to most current energy sources, such as fossil fuels, because they have the potential to reduce CO2 emission and have advantages in operational and handling safety; they are also adaptable to renewable energy. Therefore, massive integration and circulation of hundreds of thousands of tons of LOHCs in the market are anticipated in the near future. However, LOHC chemicals will likely be released into the environment during the process of producing and circulating the large forecasted volumes of these compounds. In addition to the interest in developing LOHC systems to improve the technological performance, increasing attention has been focused on the behavior, fate and toxicity of LOHCs in the environment. When LOHCs enter the environment, the behavior and subsequent toxic effects of LOHCs on organisms are critical metrics used to evaluate and predict the environmental hazards of LOHCs. However, limited data are available to perform such comprehensive predictions. To help fill this gap, this thesis was conducted to characterize the potential adsorption and mobility behavior of different LOHC candidates in soils. Specifically, the organic carbon-water partition coefficient (Koc), soil-water partition coefficient (Kd) and leaching capacity of LOHCs were investigated via instrumental analyses and software predictions. These outcomes were correlated to the physicochemical properties of the compounds to determine their potential adsorption mechanisms. In general, the Koc values were correlated to hydrophobicity in the following order: indoles quinaldines carbazole derivatives benzyltoluenes dibenzyltoluene. When ionizable LOHC structures were investigated, the Kd and leaching capacity (with quinaldines as examples) revealed adsorption governed by ionic interactions. In such a case, ionization corrected octanol-water partition coefficient (log D) was found a propriate indicator for the prediction of adsorption. Ecotoxicity tests for acute and chronic toxicity were conducted in soil (quinaldines) and aquatic (quinaldines and carbazole derivatives) test scenarios. Data were compiled and integrated with the adsorption and mobility of LOHCs in soils to interpret the extent of the exposure, bioavailability and mode of toxic action. The toxicity of LOHCs appeared to be dominated by hydrophobicity. With the deduction of toxicity classification and predicted no-effect concentrations (PNECs) as well as the evaluations for the behavior and fate, a proactive assessment of the potential environmental hazards of these chemicals was ultimately conducted in the context of realistic environmental conditions and potential application quantities. A comparative analysis showed that LOHCs presented fewer potential environmental hazards than traditional energy systems (e.g., gasoline, diesel, and oil) and analogous compound structures, such as typical N-PAHs (nitrogen-substituted polycyclic aromatic hydrocarbon, which share a similar structure with certain LOHCs). Although LOHCs seem not appear to present the same hazards as these equivalent energy sources, further studies regarding several key considerations are recommended for a better understanding of the potential environmental hazards of LOHCs. The careful application of LOHCs under appropriate monitors is suggested because of the likelihood that these compounds will be used in large application volumes in the future.