Degradation of Polyethylene Terephthalate Microplastics by Mineral Acids: Experimental, Molecular Modelling and Optimization Studies

Mineral acids in the atmosphere breakdown on the action of oxygen and release acid gases into atmosphere causing acid rain, which can chemically weather materials. We hypothesized that the chemical weathering caused by these mineral acids may also influence the rate of degradation or cracking of MPs in the environment. However, studies focusing on the chemical weathering process of mineral acids in the environment on polyethylene terephthalate (PET) is not available in literature. In the present work, PET microplastics (MPs) (1000 µm) were artificially degraded by acids such as hydrochloric acid (HA), nitric acid (NA) and sulphuric acid (SA) under the effect of contact time (30, 60, 120, 720 and 1440 min), temperature (10, 25, 40, 70,100 and 130 °C), and shaking speed (100, 150, 200 and 250 rpm). Degradation was monitored by weight loss, Variable Pressure Scanning Electron Microscope (VP-SEM) and Attenuated total reflectance-Fourier-Transform Infrared (ATR-FTIR) and optimized by Response Surface Methodology (RSM). Results showed that the weight loss were slow but increased with increasing contact time, temperature and shaking speed. For all three acids, the PET MPs spectra peak loss was similar for the different effects. However, the speed of degradation based on the ATR-FTIR and weight loss results followed temperature > shaking speed > contact time. The surface interactions between the PET MPs and acids were investigated using Monte Carlo (MC) simulations and Density functional theory (DFT) Studies. Overall, ATR-FTIR analysis and DFT studies suggested that the breakdown took place through the parts of the PET MPs structure containing oxygen atoms (–OH, C=O) and the aromatic ring. However, optimization results from RSM showed optimal weight loss of 33%, 30% and 22% for HA, SA and NA respectively. Therefore, these acids can be employed in PET MPs degradation and higher rate of degradation will require longer time, high temperature and shaking speeds.