Remobilization characteristics and diffusion kinetic processes of sediment zinc (Zn) in a tidal reach of the Pearl River Estuary, South China

Abstract

Exploration of the remobilization mechanism of trace metals in estuarine sediments remain challenging because of dynamic hydrochemical conditions. This study integrated a chemical sequential extraction procedure (BCR), the diffusive gradient in thin films (DGT) and high-resolution dialysis techniques, and Visual MINTEQ ver.3.1 to identify the seasonal mobilization characteristics of sediment Zn within a tidal reach, South China. The mobility of sediment Zn based on the BCR procedure contradicted the results of DGT analysis. In summer, reductive dissolution of Fe/Mn oxides was the key driver of sediment Zn remobilization; during winter, cation exchange reactions facilitated the mobilization of Zn in the brackish water zone. The time-dependence ratios of DGT-labile Zn and dissolved Zn concentrations (mean: 0.34–0.81) indicated the sediment solid phase had partially sustained capacity to resupply Zn to the porewater in both seasons. Sediments generally functioned as a source of Zn in the freshwater zone with organically complexed Zn being diffusively released into the water column at rates of 0.3–15.5 μg·m−2·d−1. In the brackish water zone, the dominant Zn species were transformed into free Zn ions and Zn–inorganic complexes and migrated into sediment, with respective influxes of 18.9–70.7 μg·m−2·d−1 and 18.9–68.3 μg·m−2·d−1, which shifted to a sink of Zn.