Hybrid thermo-electrochemical conversion of plastic wastes commingled with marine biomass to value-added products using renewable energy

Abstract

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.