Relationship between microplastics and heavy metals in Tam Giang-Cau Hai lagoon sediments

This study investigates microplastic (MP) and heavy metal distribution and interactions in Tam Giang - Cau Hai Lagoon sediments. We assessed sediment properties (pH, TOC, TN, grain size), concentrations of ten metals (including Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Pb, Hg), and MP abundance in sediment (MP/s) and water (MP/w) using Spearman correlation and PCA. Sediments were predominantly sandy (mean 86.7 %), acidic (mean pH 6.43), with moderate TOC (mean 0.61 %). The lagoon is moderately contaminated with MPs (MP/s mean 345.7 ± 135.8 items.kg^-1; MP/w mean 150.6 ± 98.2 items.m^-3). Metal concentrations were generally low (Zn, Pb, Cd below ERLs), though Arsenic (max 13.46 mg.kg^-1) exceeded the ERL at localized hotspots. Correlation and PCA revealed a clear partitioning of contaminants. MP/s showed no association with MP/w, indicating sediments are a long-term sink decoupled from transient water concentrations. Furthermore, MP/s lacked correlation with TOC, TN, or grain size but showed a negative correlation with heavy metals. Conversely, metals were strongly and positively correlated with TOC (e.g., Mn vs. TOC, r = 0.71) and TN (Fe vs. TN, r = 0.87). PC1 (53.48 % variance) confirmed this, linking metals directly to TOC, TN, and silt, independent of MP distribution. Findings suggest a competitive sorption dynamic: abundant natural organic matter (TOC) outcompetes MP surfaces for metal binding, possibly enhanced by acidic conditions. This study concludes that water column data is an invalid proxy for benthic risk and that metal monitoring strategies (based on TOC) are ineffective for predicting MP accumulation, necessitating compartment-specific management.