Occurrence and distribution of microplastics in long-term biosolid-applied rehabilitation land: An overlooked pathway for microplastic entry into terrestrial ecosystems in Australia

Abstract

Wastewater treatment plants (WWTPs) efficiently eliminate over 98% of microplastics (MPs) from wastewater discharge, subsequently accumulating them in sludge. This sludge is frequently employed as fertilizer in agricultural practices or land rehabilitation. While there is significant research on biosolid application in agriculture, the discussion regarding its application in rehabilitating industrial zones and MPs contamination is limited. The current study investigates the abundance, distribution, and composition of MPs in rehabilitation land with long-term biosolid-application in Australia. Three minesite fields (designated 1–3), each with distinct biosolid application histories since 2011, 2012, and 2017, and a control field without any biosolid application history, were chosen for this study. The abundances of MPs in biosolid-applied fields 1–3 (6.04 ± 1.92 x 102 MP kg−1; 4.94 ± 0.73 x 102 MP kg−1; 2.48 ± 0.70 x 102 MP kg−1) were considerably higher compared to non-biosolid-applied field (0.70 ± 0.63 x 102 MP kg −1). This indicates that the application of biosolids significantly contributes to the presence of MPs in the soil. Moreover, the results suggest that with each successive application, the abundance of MPs increases. The abundance and size of MPs in both biosolid and non-biosolid soils decreased as the soil depth increased. Microbeads were dominant in soils where biosolids were applied (up to 61.9%), while fibres were dominant in non-biosolid soils (accounting for 85.7%). The distribution of plastic polymer types varied among fields and soil depths. Most MPs were microbeads of polyamide (PA), fragments of polyethylene (PE), foam of polystyrene (PS), and fibres of rayon. This research presents evidence that the extended utilization of biosolids results in elevated MP pollution in minesite rehabilitation land, highlighting a frequently overlooked origin of MP contamination in terrestrial settings. Additional evaluations needed to understand ecological risks of MPs in soil ecosystems affected by biosolid application.