# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 1 Introduction  
Nature is exposed to a variety of human influences. Part of these influences is the input of foreign substances and or pollutants into a wide variety of ecosystems worldwide. Similar to all other ecosystems, soils are particularly affected by such human impacts. Soils form the basis of human life, with 95% of global food being produced directly or indirectly on soils [1]. In addition, soils are important ecological systems, integrated into all global material cycles (e.g., C-cycle), water filters (groundwater treatment), and much more [2, 3]. The intensive use of soils worldwide makes them vulnerable towards human utilization and the input of pollutants [4]. Soils are intensively researched concerning the enrichment, mobility, and fate of heavy metals and metalloids. These naturally occurring metals and metal compounds--in the case of heavy metals, defined as metals with a density > 5.0 g/cm\({}^{3}\)--show geogenic background levels in all soils, as they are released into the environment through rock weathering [4]. However, human activity during the industrial revolution (e.g., construction and transport), mining for ores, and the resource use of modern society have led to heavy metals contamination through anthropogenic inputs [4, 5, 6]. However, through intensive research, the ecotoxicological consequences of heavy metals are well studied, which has led to national legislation in many countries, as well as monitoring of contaminated sites and measures to reduce or improve this soil pollution [2, 4]. Conversely, a new foreign substance and potential pollutant has been detected in a wide variety of ecosystems for a few years now. Plastics, and in particular microplastics with a size below \(<\)5 mm, occurring in the environment as particles consisting of man-made polymers, now pose a new environmental challenge [7, 8, 9, 10]. In addition to the worldwide detection of microplastics in all ecosystems [8], it is now possible to detect microplastics in compost [11], garden [12], agricultural [13, 14, 15, 16, 17], and floodplain soils [18, 19, 20]. Furthermore, it has also been shown that plastic can be transported in soils [21, 22], and can have an impact on soil structure (aggregates) [15], plants, and organisms [23, 24, 25]. Even if it has not yet been conclusively clarified whether there is an extensive risk to soils, soil organisms, or humans (uptake into the food chain), plastic is fundamentally a foreign substance in soils and, thus, corresponds to a contamination whose pollution status is still discussed [26, 27, 28]. The simultaneous occurrence of metals and plastic in soils raises the question of whether there are connections and interactions between these two groups of pollutants. In the case of heavy metals and plastic particles, studies from marine and aquatic environments demonstrated that interactions between heavy metals and microplastics take place in an aqueous environment [29]. Thereby, it was shown that, in marine environments, microplastics can desorb (e.g., Cd, Zn) and adsorb (e.g., Cu, Pb) heavy metals [30]. The sorption of heavy metals was demonstrated in laboratory experiments for Cu and Zn to polystyrene (PS) and polyvinyl chloride (PVC) fragments in seawater [31]. Plastics can, therefore, be seen as a transport vehicle of metals in aquatic environments [29]. In addition to the adsorption of heavy metals from the surrounding medium, further studies indicate the release of heavy metals into the environment through particle degradation [32, 33]. Significantly, metals and heavy metals can enter into polymer products and, thus, into microplastic particles through additives. In this context, the number of potential additives is extremely diverse. Examples are flame retardants (Al and Zn), heat stabilizers (mostly Cd, Pb, Ba, and Sn in PVC), slip agents (Zn), inorganic pigments (Cr, Co, and Pb), or fillers (clay minerals and metal powders) [34].