Surface plastics and microplastics commingled with biomass are emerging pollutants in the marine environment. With the projected demand for plastics sharply increasing, innovative methods of abating these end-of-life (EOL) marine plastic wastes are necessary. Thermochemical methods to convert plastics and biomass are potential candidates for this task, although anthropogenic CO₂ emissions are often inevitable. Alkaline thermal treatment (ATT) is a promising thermochemical conversion technology, running at moderate conditions (<600 °C, 1 atm) and fixing carbon in the form of stable carbonate salts (e.g., Li₂CO₃ and K₂CO₃). Carbonate salts can be electrochemically converted to high purity carbon nanotubes (CNTs) using renewable energy (e.g., offshore wind energy). Herein, the integration of these two technologies is studied in the context of a novel tandem thermo-electrochemical (elecATT) process to treat and upcycle marine wastes. This study shows that our novel elecATT of polyethylene and salty brown seaweed performed at 500–600 °C can produce hydrogen at high purity (85%), and these reactions can be enhanced by the presence of both heterogeneous in situ and ex situ y-zeolite and Ni/ZrO₂ catalysts. The majority of carbon from seaweed and plastics was converted to carbonate ions in a molten electrolyte composed of Li₂CO₃/K₂CO₃/LiOH and converted to carbon nanotubes via electrosplitting of carbonate ions with near 100% Coulombic efficiency. Carbon analysis was performed to analyze the fate of carbon throughout the elecATT reactions, and to evaluate the recyclability of LiOH/KOH electrolytes for the ATT reaction. Overall, this study presents an innovative method for the treatment and upgrading of marine plastic pollutants by producing high purity H₂ and purified polymer intermediates for upcycling, and capturing carbon via a molten salt which can then be electrochemically converted to produce high-value CNTs using renewable energy.

In recent decades, economic development and population growth has been accompanied by the generation of billions of tonnes of solid residues or municipal “wastes”, a substantial portion of which is composed of plastics and biomass materials. Combustion-based waste-to-energy is a viable and mature method of extracting calorific value from these end-of-life post-recyclable materials that are otherwise landfilled. However, alternative thermochemical methods, such as gasification, are becoming attractive due to the ability to synthesize chemical precursors for supply chain recirculation. Due to the infancy of gasification technology deployment, especially in the context of anthropogenic CO₂ emission reduction, additional systems engineering studies are necessary. Herein, we conduct an attributional life cycle analysis to elucidate the syngas production and environmental impacts of advanced thermochemical gasification methods for the treatment of biomass and plastic wastes obtained from municipal solid wastes, using a comprehensive thermodynamic process model constructed in AspenTech. Feedstock composition, process parameters, and gasification methods are varied to study the effects on syngas quality, yield, power generation potential, and overall greenhouse gas emissions. Steam-based gasification presents up to 38% reductions in CO₂ emissions when compared to conventional thermochemical methods. Using gasifier-active materials, such as metal hydroxides, can also further reduce CO₂ emissions, and realizes a capture load of 1.75 tonnes of CO₂ per tonne of plastic/stover feedstock. This design alteration has implications for reductions in CAPEX due to the mode of CO₂ capture utilized (e.g., solid sorbent vs. liquid SELEXOL). The use of renewable energy to provide a method to generate steam for this process could make the environmental impact of such MSW gasification processes lower by between 60–75% tonnes of CO₂ per tonne of H₂. Overall, these results can be used to inform the guidance of advanced waste gasification methods as a low-carbon transition towards a circular economy.

The synthesis of high-purity precipitated calcium carbonate (PCC) is responsible for approximately 30% of the global annual production of calcium carbonates. As increasing carbon capture utilization and storage (CCUS) facilities come online to mitigate anthropogenic emissions, PCC production is expected to grow through carbon mineralization production pathways. Tuning the PCC morphological and crystal structure properties will largely dictate the potential downstream uses of the carbonate products; thus, increased research is needed into process design considerations for crystal habit modification of PCC. Of the three anhydrous polymorphic forms of PCC, aragonite shows great promise in applications as a filler material or flow-modifier due to its unique needle-like structure. Synthetic aragonite is generally produced at elevated reaction temperatures (60–80 °C), however alternative crystallization methods can be applied to reduce this energy barrier and better control crystal production. Herein, we compare the crystallization of aragonite in both a batch and semi-continuous crystallizers using a model system (e.g., CaCl₂ and K₂CO₃) to elucidate the effect of mixing and saturation in relation to PCC crystal morphology. Seeding of aragonite is also utilized as a method to better control the PCC production process and lower the required synthesis temperature to 25–40 °C. Finally, these methods are extended to a Ca-rich leachate from waste hydrated cement paste (HCP) for comparison with the model system. The produced aragonite is reincorporated into new cement and tested for its potential hydration and rheological advantages. Overall, this work motivates the use of alternative crystallization methods to promote polymorph control for niche CCUS applications, especially cement decarbonization.

The use of organic carboxylic ligands derived from CO ₂ can enhance the material recovery and upcycling potential of construction and demolition wastes via carbon mineralization.

An indirect carbonation process combining extraction and CO2 mineralization was developed to upcycle waste cement paste into supplementary cementitious materials (SCMs) containing highly amorphous Ca-Si-Al-Fe phases and calcium carbonate. The method employed a pH swing process to extract key elements prior to carbonation, yielding two reactive SCMs termed ”residue” and ”precipitate”. Their reactivity was evaluated using model cement systems (R3 tests) to assess both latent hydraulic and pozzolanic properties. Despite the distinct formation mechanisms, both residue and precipitate exhibited strong pozzolanic reactivity, enhancing 28-day compressive strength by around 25% when substituting 10 wt% cement. X-ray diffraction (XRD) was employed to track time-dependent quantitative phase content in cement pastes with these SCMs at various hydration stages, while 29Si Nuclear Magnetic Resonance (NMR) was employed to analyze the transformation of silicate structures during the entire cyclic process, elucidating the mechanisms driving the high reactivity of the upcycled materials.