# Spatial Connections between Microplastics and Heavy Metal Pollution within Floodplain Soils
## 1 Introduction
Although the main focus has been on the interrelationships between heavy metals and plastic particles in aquatic environments, since 2019, a limited number of studies have also been conducted on these interrelationships in soils (Figure 1). For soils in which heavy metals and plastic particles are embedded in a mineral-organic matrix, partly with water (pore water), ad- and desorption processes comparable to those in aquatic environments have been obtained so far. [PERSON] et al. (2019) reviewed  
Figure 1: Web of Science (Clarivate) hits for different search term combinations (microplastic + heavy metal + “term”) (date: 27 November 2021). the attachment of heavy metals to microplastics via adsorption considering the influence of polymer type and shape of the plastic particle [35]. The assumption is that metal ions can adsorb on microplastic surfaces and that the adsorption rate increases over time due to the presence of biofilms and a larger surface area resulting from degradation of the plastic particles [35]. Other studies suggest an adsorption or desorption of heavy metals depending on the specific metal and the concentration of both pollutants in the soil [36]. In addition, the influence of the presence of microplastics on heavy metals has been investigated: The presence of microplastics seems to reduce the exchangeable, carbonate-bound, and Fe-Mn-oxide-bound fraction of the metals, while it increases the organic-bound fraction [37]. Furthermore, the bioavailability of Cu, Cr, and Ci seems to be affected by the presence of microplastics, whose presence also affects critical factors (e.g., DOC and pH) of the chemical behavior of heavy metals in soils [38]. From the research to date, it can be concluded that relationships and interactions exist between heavy metals and microplastics in soils, as well as in the aquatic environment. However, the question arises as to whether these findings also apply to other types of soils and what role spatial factors play in the connection? One suitable category of environmental space and landscape to study those interactions is floodplains and their soils. Both substances are present within floodplain soils, due to the sink function of floodplains [19; 20; 39]. Rivers and their floodplains interact with each other through flood water dynamics, material flows, and their context as a part of a semi-terrestrial ecosystem [40]. Sediments and sediment-bound substances from the entire catchment of a river system can temporally be deposited in floodplains, which are, therefore, an important part of global material flows [40; 41]. Similar dynamics can also be assumed for plastic particles within the environment [41; 42; 43; 44]. Assuming that this is the main source (besides direct sources, e.g., agriculture) of pollutants in floodplain soils, both pollutants can be studied under comparable input conditions in floodplain soils. Previous research has documented levels of mass-based microplastics concentrations from 0 to 55.5 mg kg\({}^{-1}\) for Swiss floodplain soils [18], or particle-based concentrations ranging from 0 to 186.3 p kg\({}^{-1}\) in different floodplain soils of the Lahn and Inde River (Germany) [19; 20; 45]. Furthermore, a wide-ranging, heterogeneous spatial distribution down to depths of two meters and associated in-situ vertical displacement has been demonstrated [19; 20]. Morphological factors, land use, and flood dynamics (sedimentation dynamics) appear to influence the spatial distribution of microplastics in floodplain soils [19; 20]. Despite this knowledge, many questions about the spatial distribution of microplastics in soils in general, and especially in floodplain soils, remain unanswered, which is not least due to the few studies with a representative spatial contexts [46]. Considering the few studies conducted so far on the relationships and interactions between heavy metals and microplastics in soils, even more questions remain about interrelationships, processes, and potential environmental risks.