A New Colorimetric Using Zr Oxide DGT Technique for High-Resolution Measurement of Labile Mo(VI) in Sediment Porewater

Understanding the distribution of labile molybdenum (Mo(VI)) in sediment porewater is essential for managing Mo-contaminated environments. This study developed a two-dimensional (2D) high-resolution method combining diffusive gradients in thin films (DGT) with sodium thiocyanate coloration and computer-imaging densitometry (CID) to measure labile Mo(VI). Optimal color development occurred at 25 °C for 36 min using a reagent volume that was 100 times that of the binding gel. The method proved to be robust against variations in pH, ionic strength, and phosphate level. It was applied to sediment from the Luhun Reservoir, revealing increased Mo(VI) concentrations in overlying water, a decline near the sediment–water interface, a subsequent increase, and then a decrease with depth. These patterns suggest Mo(VI) originates from overlying water, binds to Fe/Mn (hydr)oxides below the interface, and interacts with dissolved sulfide in deeper layers. The DGT-CID technique offers a simple, field-applicable approach for accurate, high-resolution measurement of labile Mo(VI) and enhances our understanding of its spatial distribution in sediments, aiding effective environmental monitoring and remediation.