Plastic waste is accumulated in landfills and the environment at an exponentially increasing rate. Currently, about 350 million tons of plastic waste is generated annually while only 9% is recycled. Plastic waste and its degradation products, microplastics, pose a severe threat to the ecosystem and eventually human health. Polyolefin (Polyethylene (PE) and Polypropylene (PP)) waste is 63% of the total plastic waste. Converting polyolefin waste into useful products including clean gasoline, diesel, wax, and monomers, via hydrothermal processing (HTP) can help reduce the plastic waste accumulation. In this study, sorted PE waste was converted via supercritical water liquefaction (SWL) into gasoline blendstock, No.1 ultra-low-sulfur diesel, and clean waxes with high yields and high purities. Comprehensive reaction pathways for PE conversion were proposed based on detailed GC×GC analyses. Furthermore, a new low-pressure (~2 MPa) hydrothermal processing (LP-HTP) method was developed to convert mixed polyolefin waste. This new LP-HTP method can save 90% of the capital cost and energy compared to SWL. The oil products were distilled into clean gasoline and No.1 ultra-low-sulfur diesel. The reaction pathways of PE and PP were independent while the synergistic effects improved the fuel qualities. With this LP-HTP method, polyolefin waste can be converted into up to 190 million tons of fuels globally, while 92% of the energy and 71% of the GHG emissions can be saved compared to conventional methods for producing fuels. Overall, this method is robust, flexible, energy-efficient, and environmental-friendly. It has a great potential for reducing the polyolefin waste accumulation in the environment and associated risks to human health.<br>

<p dir="ltr">Annual rates for global polystyrene (PS) waste accumulation have reached 28 million tons, yet recycling rates remain around 1%. Conventional waste treatment methods have proven largely ineffective in reducing PS waste accumulation. As PS waste degrades, it generates microplastics and releases harmful chemicals that impact human health and ecosystems. This study developed batch and continuous low-pressure hydrothermal processing (LP-HTP) methods to convert PS into oils. In the batch LP-HTP study, the effects of temperature, time, and water loading on oil yield and composition were evaluated. The process converted PS to 96-99% oils with minimal char formation (1-2%) while requiring no catalyst, outperforming traditional pyrolysis. The LP-HTP methods also require lower energy inputs and pressures than supercritical water liquefaction. Co-processing PS with polyolefins resulted in oil yields of 87% and higher aromatic contents compared to polyolefin-only oils. Mono-aromatic (C₆-C₉) yields were limited by reversible reactions with poly-aromatics (C₁₀-C₂₄₊). Efficient continuous LP-HTP methods were then developed, achieving 95-99% oil yields at 0.2-1.2 kg/hr under atmospheric pressure. The oil contained styrene monomers, dimers, and trimers with a total yield of 88.5% at 391°C. A detailed kinetic model was constructed, with intrinsic parameters estimated from continuous conversion data to enable process optimization and scale-up. These LP-HTP methods show potential for reducing environmental impacts and achieving up to 4.7 times higher energy recovery than incineration. The resulting hydrocarbons, if separated into pure monomers, can be used as chemical feedstocks, supporting a circular hydrocarbon economy that incentivizes plastic waste conversion.</p>