Total organic carbon-based proxy for QuEChERS estimation of microplastic mass in soils

Total organic carbon (TOC) analysis is a simple and rapid proxy for quantifying microplastic (MP) contamination, though its application to soils remains limited. This study proposes a QuEChERS (Quick, Easy, Cheap, Effective, Rugged, Safe) approach to estimate MP mass from its carbon mass in soils, involving organic matter removal using 6 % NaClO (commercial bleach), density separation with K₂CO₃ (1.54 g/mL), TOC quantification, and MP mass estimation. Digestion using 6 % NaClO removed 96.8 ± 3.5 % of organic matter (peatmoss), significantly higher than H₂O₂ and Fenton, with polymer carbon loss below 3 %. The method achieved 90.0 ± 2.0 % recovery of a mixture of five polymers commonly found in soils. The TOC-based protocol was applied to field soils across different land uses (forest, agricultural, roadside, residential, and industrial) and validated against pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Based on soil MP carbon content of 65-90 %, MP mass in 17 field soils was estimated at 22-2020 mg/kg by TOC and quantified at 21-134 mg/kg by Py-GC/MS, with both methods showing consistent spatial trends and a strong correlation (R = 0.82). However, TOC may overestimate MP mass in soils enriched with non-plastic anthropogenic particles (e.g., black carbon). Nonetheless, if pollutant persistence is prioritized over polymer specificity, TOC provides a more inclusive measure of anthropogenic particles, supporting broader environmental risk assessments beyond MPs. A multicriteria analysis of method recovery rate, reagent cost, and Analytical GREEness score indicates that the proposed TOC-based approach is effective, practical, and suitable for both intensive laboratory studies and routine monitoring.