A proposed reporting framework for microplastic-metal mixtures research, with emphasis on environmental considerations known to influence metals.

In recent years, there has been an increase in research to understand the consequences of microplastic contamination. A subset of this research assesses the interaction of microplastics with metals and the subsequent effects of the resulting microplastic-metal complexes in freshwater environments. While our understanding of how microplastics behave in freshwater remains largely unknown, our knowledge of metal behavior in those same environments is well-established. The behavior (partitioning, speciation, bioavailability) of metals is highly dependent on environmental characteristics, including water quality variables such as hardness, pH, and dissolved organic matter. This study reveals that despite our understanding of metal behavior, there is little consideration for these influential factors in the current body of microplastic-metal research. Multiple instances highlighted throughout this study show that even when similar plastic, metal, and biota are utilized, there are conflicting observations as to whether the mixture is toxic; we stress that without adequate reporting of environmental conditions, these contradictions are likely to persist without explanation. Through justification of water quality characteristics known to influence metal behavior, this study proposes a framework of reporting requirements for all future microplastic-metal research.