Investigation of microplastics behavior in the soil environment

Detectability, leaching, and impact on soil physical functions

authored by

Leila Shafea

supervised by

Stephan Peth

Abstract

In the modern lifestyle, plastics are ubiquitous materials, and their production and usage often need to be followed by appropriate management worldwide. As a result, plastic litter accumulates in the environment as microplastics (MP: 0.001–5 mm). Organisations such as UNEP (United Nations Environment Programme) recently ranked MP among the most relevant topics for marine biodiversity conservation on a global scale. Nevertheless, the majority of plastics that end up in the oceans were originally produced, used, and discarded on land. Terrestrial systems have received less attention than their aquatic counterparts, so a more detailed analysis of the hazards of MP contamination of soil is required. The currently available research results suggest that MP contamination might trigger significant changes in soil and plant function, including the potential for detrimental effects on plant tissue and the soil's essential physical functions. Understanding the behaviour of MP in the soil environment is imperative due to their substantial impact on the physical and chemical characteristics of the soil, as well as their detectability and the dynamics of movement and transport within the soil. In this dissertation, three work packages (WP) have been designed and investigated. In work package one the wettability, pore sizes, and saturated hydraulic conductivity of silt loam topsoil were investigated. In this work package, the two most common types of plastic polymers in agricultural soils, polyethylene terephthalate (PET) and polystyrene (PS), at three various sizes (between approx. 0.5- and 3-mm diameter) and three concentrations from 0.5 to 2% (w/w), were used to evaluate the impact of MP particles on soil physical properties. The results of this study indicated a significant effect of plastic particles on soil water repellency (increased), soil hydraulic conductivity (decreased), and water retention (decreased). In the second work package, the recovery rate of both pristine and UV-aged PET and PS at three sizes (500 µm, 500-630 µm, and 630 µm -1mm) and 0.5% concentration in silt loam and sandy loam soils was assessed based on a newly developed protocol using saturated NaCl (1.2 g cm-3) and NaI (1.8 g cm-3) solutions, followed by H2O2 (33% treatment) to remove soil organic matter. Recovered MP particles were analysed by Mid-FTIR for changes in surface functional groups and by contact angle (CA) determination for changes in wetting properties due to ageing. This investigation involved subjecting materials to UV ageing via UVC irradiation, resulting in noticeable alterations in the colour of PS and the emergence of oxygen-containing functional groups on the surfaces of both PS and PET. The CA of aged particles decreased significantly, coinciding with the detection of OH peaks in ATR-FTIR spectra, yet exhibited an increase after recovery for UV-aged MP. The recovery of UV-aged MP particles exhibited reduced wettability, potentially attributed to the absorption of amphiphilic compounds,while treatment with H2O2 led to an increase in CA, suggesting the potential removal of oxidized layers. Despite the physiochemical changes in MP, the recovery rate from both soils remained unaffected, with approximately 81% recovery observed from both silt loam and sandy loam soils. In the third work package, the transport behaviour of pristine and UV-aged MP particles of 1 µm size fraction was analysed in a column experiment by percolating MP particles suspended in CaCl2 solution (ionic strength of 7.5 mM) through a quartz sand matrix (0.3 mm). The MP particles were aged with UV irradiation using an irradiance of 2.05 W/m2 (ultraviolet A, UVA; 365 nm) and 5.58 W/m2 (ultraviolet C, UVC; 254 nm) and exposure times of 24, 48, 168, 336, 504, 672, 840, 1008, and 1176 hours. To assess the chemical and physical changes on the polymer surface, ATR-FTIR, zeta potential, and particle size analyses were performed. The results showed increased oxygen-containing functional groups in UV-aged particles compared to pristine ones and reduced particle sizes by increasing time exposure. The transportation of UV-aged MP particles intensified with longer exposure durations, accentuating the ageing effects compared to those not subjected to UV exposure. UVC irradiation exhibited more pronounced effects on the ageing process of particles compared to UVA. In summary, the results of this study suggest notable effects of UV irradiation on MP transport in the soil environment. Hence, further evaluation of the potential risk of MP particles in soil and plant systems to evaluate their impact on soil and human health through the food chain is recommended. In conclusion, this thesis provides a thorough analysis of the impact of MP on soil ecosystems, delving into the influence of ageing on their physiochemical characteristics, detectability, and transportation dynamics within the soil. Moreover, the current dissertation proposes innovative research approaches to evaluate the effects of UV-induced ageing on the recovery and identification of MP in soil samples, elucidating the implications of ageing on the detectability and movement of MP in soil matrices.

Organisation(s)

Institute of Soil Science

Type

Doctoral thesis

No. of pages

167

Publication date

10.10.2024

Publication status

Published

Sustainable Development Goals

SDG 3 - Good Health and Well-being, SDG 14 - Life Below Water, SDG 15 - Life on Land

Electronic version(s)

https://doi.org/10.15488/18010 (Access: Open)