Microplastics Influence Phosphate Adsorption in Volcanic Ash Soil

Purpose: There is a debate whether microplastic particles released into soils can modify phosphorus bioavailability by altering the soil surface properties. Here, we aim to explore the impact of polyethylene microplastics (PE–MPs) on the adsorption–desorption of inorganic phosphate anions (P) on a volcanic ash soil (VAS). Methods: Batch P adsorption-desorption experiments were conducted in a Chilean VAS with and without 1% (w/w) PE-MPs addition taking P concentrations (KH₂PO₄ dissolved in 0.01 mol L⁻¹ NaCl background solution) 0.02–6.47 mmol L⁻¹, solid (g):liquid (mL) ratio 1:40, and at a pH range of 4.5 to 10.5 at 20 ± 1 °C temperature. The VAS and VAS/PE–MPs systems were characterized and kinetic and isotherm adsorption data were modelled to predict mechanisms. Results: The Elovich model described the kinetics P adsorption data on VAS with and without 1% PE–MPs (r² ≥ 0.985 and χ² ≤ 12). Adsorption isotherms fitted well to the Freundlich model (r² ≥ 0.994 and χ² ≤ 6.39), indicating a high heterogeneous surface for both systems. The Freundlich model indicated an increase in P adsorption capacity from 49.55 (mmol kg⁻¹) (L mmol⁻¹)^1/n for VAS to 54.66 (mmol kg ⁻¹) (L mmol ⁻¹)1/n for VAS + 1% PE–MPs. Desorption of P was higher in the VAS + 1% PE–MPs system compared to VAS alone. For both systems, solution pH showed no significant changes in P adsorption on VAS. Scanning electron microscopy–energy dispersive X–ray spectroscopy and Fourier transform infrared spectroscopy results showed that P was bound to PE–MPs through a weak van der Waals force and/or pore–filling mechanism. Conclusion: This study demonstrated that PE–MPs in VAS could modify surfaces available for P adsorption and act as a carrier to enhance P mobility.