Impact of accelerated UV aging on chemical structure and sorption behavior of low-density polyethylene microplastics

This study investigates the effect of UV aging on the chemical structure of low-density polyethylene (LDPE) microplastics (MPs) and its impact on the sorption of organic compounds using malachite green (MG) as the model compound. Fourier Transform Infrared (FTIR) analysis revealed discernible structural changes in UV-aged LDPE MPs, including the formation of new functional groups such as -C=O, -OH, and unsaturation, along with shifts in band positions, changes in intensity, and an increase in carbonyl index (CI) values with higher UV doses. Exposure to progressively higher UV doses also caused a rise in melting temperature, while the crystallinity initially increased and then decreased but still surpassed that of the pristine sample. Surface alterations and fragment formation, increased surface area, changes in zeta potential, and contact angle values were also observed. LDPE MPs subjected to only thermal aging, on the other hand, exhibited the highest crystallinity, with no evidence of formation of new functional groups, as indicated by their FTIR spectra. Isotherm experiments with MG revealed that sorption increases ten-fold as LDPE MPs age. Furthermore, sorption which follows a linear trend with pristine LDPE, becomes non-linear with the aged sorbent. It is proposed that MG sorption is dominantly a partitioning process within the amorphous regions of pristine LDPE, while increased hydrophilic sorption sites of aged LDPE promoted adsorption in addition to partitioning. Our findings underscore the significant role of UV aging in altering the properties of MPs and their interactions, potentially modifying their impact on ecosystems.