The role of photooxidation and organic matter in Cr(III) and Cr(VI) interactions with poly(lactic acid) microplastics in aqueous solution

Abstract

There is limited research on the influence of environmental variables on the interactions of biodegradable microplastics with chromium. This study reports the results of adsorption experiments with Cr and poly(lactic acid) (PLA) in synthetic aqueous solutions. It addresses the influence of the initial oxidation state, Cr(III) or Cr(VI), the effects of UV irradiation and the presence of organic matter. The results indicate that pristine PLA has a low affinity for Cr (between 0.05 and 46 μg/g) across varying pH levels, ionic strengths, and microplastic concentrations. However, the presence of organic matter (OM), represented by humic and tannic acids, resulted in 5.2-fold and 620-fold increases for Cr(III) and Cr(VI) adsorption, respectively. Possible mechanisms for that behavior are discussed, including Cr-OM complexation and formation of surface coatings. Noteworthy, we demonstrate that Cr(VI) adsorption involves a coupled redox-complexation process that appears to be related to the antioxidant potential of OM. Indeed, the ratio of (poly)phenol content of tannic and humic acids (6.23) is consistent with the ratio of Cr(VI) reduction in presence of both acids (6.97). Finally, photooxidation of PLA enhanced Cr(III) and Cr(VI) adsorption by a factor of 60 and 15, respectively. This is primarily attributed to UV-induced changes in surface chemical groups (increased oxygen content), rather than a change in surface area. This research provides key insights into the behavior of PLA as a potential Cr carrier, revealing the importance of organic matter and the photoaging of microplastics in the mobility of trace metal pollutants in the environment.