Converting polyethylene-based fishing net waste into ethylene via catalytic pyrolysis over oyster shell waste

Fishing net waste (FNW) is one of the most persistent and harmful marine plastic pollutants. Yet most recovered FNW is landfilled or incinerated, causing secondary environmental pollution. Herein, polyethylene (PE)-based FNW was converted into ethylene, the monomeric building block of PE, via CO₂-assisted pyrolysis. Conventional pyrolysis mainly yielded waxy condensable products. Extending volatile residence time at elevated temperature promoted secondary cracking toward lighter products, more markedly under CO₂, yet ethylene remained limited. Oyster shell waste (OSW, 98.3 wt% CaCO₃), another abundant marine waste, was therefore employed as a Ca-rich catalyst in the downstream zone, promoting cracking of condensable hydrocarbons into light gases. Combining OSW and CO₂ increased the ethylene yield to 24.37 wt%. The higher CO yield under CO₂ than N₂ (2.24 vs. 0.45 wt%) is consistent with CO₂-mediated gas-phase reactions of OSW-generated intermediates. Superior ethylene production despite less CaO formation indicates that the enhancement is most consistent with CO₂ participation rather than catalyst basicity. Techno-environmental assessment estimated gross benefits of 1.05 t CO₂-eq (0.28 t as permanent fossil displacement) and US$442 per tonne of FNW, excluding process burdens. Overall, this closed-loop strategy co-utilizes two marine wastes and CO₂ to recover ethylene for virgin PE production while mitigating disposal-derived pollution.