Pyrolysis of Waste Pharmaceuticals as a Novel Treatment for a Circular Economy: Analysis of Kinetics, Thermodynamics, and Products Characteristics.

Abstract

In this expanding pharmaceutical market, the issue of drug pollution constitutes a significant concern that needs to be widely addressed because of its global relevance. Sustainable waste management requires that industries transition from the existing linear model to a circular economy; that incorporates wastes as raw materials for producing new products. In this study, pyrolysis was suggested as an economically and environmentally sustainable treatment for pharmaceutical waste within the context of circular economy. Fast pyrolysis was conducted at 500℃ on each of immediate release tablets of 500 mg paracetamol and 600 mg ibuprofen drug products, with the liquid and gas products analyzed using GC-MS (gas chromatography-mass spectroscopy) and micro GC-TCD (gas chromatography-thermal conductivity detector), respectively. The pyrolytic behavior of each pharmaceutical product was studied through kinetic and thermodynamic analysis. The data obtained by thermogravimetric analysis (TGA) were used to estimate the activation energy (𝐸) through model-free isoconversional models of Kissenger-Akahira-Sunose (KAS) and Flynn-Wall-Ozawa (FWO). The pre-exponential factor (𝐴) was evaluated using Kissinger model and followed by thermodynamic parameters determination. The average 𝐸 values for the pyrolysis of the paracetamol drug product using KAS and FWO methods were found to be 125.9 and 128.8 𝑘𝐽.𝑚𝑜𝑙−1, respectively. For ibuprofen drug product pyrolysis, the average 𝐸 values were 77.9 and 82.0 𝑘𝐽.𝑚𝑜𝑙−1 for the first decomposition stage, and were found to be 136.0 and 138.9 𝑘𝐽.𝑚𝑜𝑙−1 for the second decomposition stage, using KAS and FWO models, respectively. The small difference between 𝐸 and Δ𝐻 values (~5 𝑘𝐽.𝑚𝑜𝑙−1) indicated the small energy barrier to overcome. The results showed that the pyrolysis liquids consisted of the active pharmaceutical ingredient, along with other compounds of commercial importance, while pyrolysis gas from both drugs consisted of carbon monoxide, methane, and ethylene. Methane was the most abundant in the pyrolysis gas of paracetamol drug product (39.16 mol%), while ethylene was the most prominent in that of ibuprofen drug product (49.99 mol%). Both pyrolysis liquid and gas products were high-value products that may be repurposed, as the active pharmaceutical ingredient (API) may be recycled to produce new tablets, and the other compounds may be utilized as fuels or chemical feedstocks.