New insights into release behavior and catalytic degradation of microplastic additives in aquatic environment

Abstract

This thesis primarily focuses on the release behaviors of additive molecules from microplastics and aims to develop effective, cost-efficient, and eco-friendly methods for additive removal in aquatic environment. Based on laboratory and molecular dynamics (MD) simulations, it explored the additive release behaviors from microplastics at a macroscopic level and the additive diffusion mechanisms within microplastic particles. Furthermore, the dissertation developed functional composite materials to activate peroxymonosulfate (PMS) molecules, effectively degrading the harmful additives leached from microplastics in aquatic environment. Hence, grounded in this objective and research, we derived the following principal conclusions.

The additive release behavior from microplastics is a complex process, particularly under the complex natural environment. To elucidate the release kinetics, UV irradiation experiments were conducted on additive-containing (DBP, DEHP, BPA and BDE-209) microplastics (PVC, PE and ABS). The kinetic results indicate a generally consistent initial release pattern, transitioning into more complex dynamics due to additive degradation and microplastic aging. By integrating diffusion models, UV-mediated additive degradation and microplastic aging led to changes in the decisive parameters Dp (intraparticle diffusion coefficient) and Kp/w (microplastic-water partition coefficient), thereby affecting the additive release kinetics from microplastics.

Related findings indicate that the additive release from microplastics is mainly controlled by the additive diffusing within microplastic particles, making it essential to investigate the intraparticle diffusion behavior of these molecules. Interparticle diffusion of BPA, DBP, and DEHP molecules within PE and PVC was investigated by MD simulations, which elucidates how shorter polymer chains, smaller microplastic particle size, smaller additive molecule size (radius and volume), higher temperatures, and saline conditions promote the additive release. By examining the motion trajectories of the additive molecules' centroids, the migration mechanism was further identified: additive molecules migrate by continuously overcoming the energy barriers among polymer voids with thermal motion.

Transition metal oxides, particularly bimetallic spinels, have been proven to activate PMS molecules; however, the aggregation behavior reduces their activation performance. Thus, a composite of CoFe2O4 dispersed and coupled with halloysite (HNTs) was developed to efficiently degrade the BDE-209. CoFe2O4@HNTs achieved an impressive 91% BDE-209 degradation efficiency, significantly surpassing 75% of the standalone CoFe2O4 nanoparticles, suggesting that HNTs greatly enhance the activation performance of CoFe2O4. DFT calculations confirmed the micro-mechanism of PMS activation, highlighting adsorption and electron transfer as key processes, and helped identify vulnerable sites of BDE-209, outlining possible degradation pathways, including debromination, hydroxyl substitution, and attacks on ether bonds.

Transition metal oxides combined with ceramic membranes for organic pollutant degradation by PMS activation is a promising strategy. Conventionally, functional ceramic membranes are typically produced by loading transition metal oxides onto the membrane surfaces, yet their naked catalysts can be easily peeled off under harsh conditions. In response, a sediment-based ceramic membrane embedded with multimetallic Cu(FexAl2-x)O4 spinels was developed via a one-step, low-temperature sintering technique. This low-cost and eco-friendly membrane achieved a 98.3% degradation efficiency for BPA. DFT studies proved that Al doping in Cu(FexAl2-x)O4 increases the adsorption energy and electron transfer rate between the spinel and PMS than CuFe2O4 spinels, enhancing the PMS activation efficiency.