Impact of microalgal biomass and microplastics on the sorption behaviour of pesticides in soil: a comparative study

Abstract

The sustainability of resource recovery and agricultural practices can be enhanced by utilising microalgal biomass from a high-rate algal pond for wastewater treatment to improve soil quality and food yields. However, certain factors should be considered first. The addition of fertilisers and the presence of contaminants such as microplastics (MPs) can modify the behaviour of pesticides, applied to such fields. A sorption study of three model pesticides with different octanol–water partitioning coefficient (log K_ow): acetamiprid (ACE, log K_ow 0.80), chlorantraniliprole (CAP, log K_ow 2.76), and flubendiamide (FLU, log K_ow 4.20), was carried out in soils amended with microalgal biomass in the presence and absence of MPs. The surface of the sorbents in the study was characterised by attenuated total reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), water contact angle (CA), and point of zero charge (pH_PZC). Overall, the sorption of model pesticides increased with their hydrophobicity: ACE > CAP > FLU. The addition of microalgal biomass to soil increased the sorption of ACE (0.72 ± 0.05 µg g⁻¹), compared to soil only (0.08 ± 0.08 µg g⁻¹). The greater sorption capacity of ACE can be attributed to electrostatic interactions, hydrogen bonding and π–π interactions between ACE moieties and the negatively charged surface of microalgal biomass containing polar functional groups. The presence of MPs (3% w/w, mixture of polyethylene terephthalate (PET), polyethylene (PE), and polystyrene (PS)) did not affect the sorption of ACE, CAP or FLU, regardless of the presence or absence of microalgal biomass. However, FLU sorption was enhanced in the presence of PE and PS in soil, spiked with these individual MP polymers. Microalgal soil amendment can, therefore, influence the behaviour of hydrophilic compounds in soil.