Quantification and identification of microplastics and heavy metals in restored and natural soil - environmental and human risk assessments

Abstract

Background: Plastic pollution is a pressing concern with global plastic waste expected to increase in the coming years. Microplastics (MPs), measuring <5 mm in diameter, infiltrate aquatic, terrestrial and air environments. MPs can be primary (e.g microbeads in cosmetics, fibre discharge from textiles) or secondary (from the breakdown of larger plastics). Heavy metals (HMs) are toxic pollutants that stem from natural sources like volcanic eruption or anthropogenic sources like industrial activity. Both MPs and HMs can cause histopathological damage and inflammation in humans and tend to accumulate in the environment due to their non-degradable nature and long-range transport capacity. Using sewage sludge (SS) from municipal wastewater treatment plant (WWTP) as soil amendment, may be an entry pathway for MPs and HMs into the environment. MPs and HMs can have detrimental effects in soil, affecting various factors like soil quality, vegetation growth, soil flora and fauna. Aim: To quantify and identify MPs and HMs in soil, and to assess their associated human and environmental risks. Material and methods: A total of 18 soil samples were collected from a quarry restoration in Alcover, Spain, with SS amendment (AL-F), without SS (AL-T), and a nearby forest (AL-BO), across different soil horizons (A, A/C, C). MP analysis involved ZnCl2 density separation and Fenton oxidation, before being examined under the microscope (50x) and eventually analysed with FT-IR-spectroscopy. For HM analysis, microwave-assisted acid extraction was done (EPA method nr. 3051A), following analysis with inductively coupled plasma mass spectrometry (ICP-MS) or -atomic emission spectrometry (ICP-AES). Results: AL-F soil contained an average of 1600 MPs/kg dw, compared to 123 MPs/kg in AL-T and 40 MPs/kg in AL-BOB. Polyester fibre was the most prevalent polymer type. Excessive Zn- (146.5; 140.3; 116.1 mg/kg) and Pb-values (119.8 mg/kg) were observed in Technosol with sludge, exceeding the Catalonian reference value for the ecosystem and human health protection, with a significant difference in Zn concentrations (p = 0.04) Conclusion: Soil amended with SS showed significantly higher concentrations of MPs and HMs, indicating a potential threat to the soil, surrounding environment, and can affect human health. However, the bioavailability of HMs in soil is deemed low, suggesting a reduced risk of leaching and of affecting the soil ecosystem and humans. Considering that the soil stems from a quarry and not agricultural farmland, the chances of reaching humans is likely low. The persistence of MPs and HMs in soil even after 27 years underscores the importance of monitoring their levels in SS used as soil amendment due to their environmental impact.