Distribution and transport of microplastics and metals in sewage sludge-amended agricultural soil : a Comparative study between microplastics copper, cadmium, and nickel

Abstract: This thesis investigated the vertical transport of microplastics and metals in agricultural soil after sewage sludge application. Sewage sludge is commonly used in agriculture for adding nutrients and carbon to improve the soil quality. From a bio-based economy perspective, it is considered an important strategy to re-use this nutrient-rich by-product from wastewater treatment plants. However, the application of sewage sludge can also lead to unintentional spreading of unwanted elements to the soil, such as microplastics and metals. While extensive studies have been carried out to examine the environmental fate of metals in different soils, little is known about the emerging pollutant microplastics. Hence, the aim of this study was to investigate the distribution and transport of microplastics in direct comparison to more investigated metals. Moreover, this study compared microplastic and metal distributions in field measurements and in a laboratory-based process study where plastic and metal transport were studied under the influence of either earthworm bioturbation or simulated rainfall using intact soil cores, given that transferring results from controlled experiments to field conditions can often be difficult. The initial hypothesis was that there might be similarities between the transport of microplastics and particulate-bound metals, whereas dissolved metals were expected more mobile in the soil profile compared to microplastics. Secondly, bioturbation activity was suspected to contribute significantly to plastic transport. The distribution of microplastics showed spatial similarities to the distribution of copper and cadmium in depth profiles of agricultural soils after long-term sewage sludge application. Since copper and cadmium tend to bind to soil organic matter or clay particles, the results indicate that similar transport processes may affect microplastic transport in the field. Limited transport of microplastics was observed under controlled conditions in the laboratory in the presence of earthworms, but no significant metal transport could be detected in the intact soil columns. Overall, the results from the process study indicate that transport of microplastics and metals was limited compared to previous findings. The dense clay soil used in the intact soil columns and an apparent lack of macro-pores may have prevented advective transport and caused a high mechanical resistance preventing transport via earthworm mediated bioturbation. Thus, suggesting that translocation of MPs in clayey soil may not be as fast as earlier bioturbation studies have observed. This means that the MPs are likely accumulated within the upper part of the soil profile and retained within the soil. However, the generalisation of this finding is limited, since only one soil was examined, and the process study was short in time and consisted of few replicate soil columns. Continued research on this topic needs to be done to establish the transport processes and fate of MPs in terrestrial ecosystems. Keywords: Bioturbation, advective transport, earthworm, Aporrectodea longa