A homogeneous thiol-functionalized chitosan membrane as a DGT binding phase for trace Hg(II) determination in seawater

Mercury is highly toxic. Hg(II) methylation produces methylmercury, which biomagnifies in aquatic food webs and poses neurological risks. Grab sampling may not represent average labile Hg(II) concentrations during deployment in estuarine and coastal aquaculture waters. Diffusive gradients in thin films (DGT) provides time-integrated measurements, but matrix interference and ion-dependent corrections limit some binding phases. Reliable monitoring requires a matrix-tolerant binding phase.

A homogeneous thiol-functionalized chitosan membrane (CS-MSA/PVA) was fabricated as a DGT binding layer. The membrane exhibited a maximum Hg(II) capacity of 639.12 mg/g and high affinity at trace levels, with S- and N-containing sites jointly participating in binding. CS-MSA/PVA@DGT achieved 99.0-100.5% elution within 20 min using HCl-thiourea. Hg(II) was measured by hydride generation-atomic fluorescence spectrometry (HG-AFS). The method blank and LOD were 0.45 ± 0.094 ng and 0.28 ng/disc. Hg(II) accumulated linearly in ultrapure water, river water and seawater, with effective diffusion coefficients of 17.9, 16.8 and 16.2 × 10^-6 cm²/s, respectively. C_DGT/C_SOL remained within 0.90-1.10 across pH 3.6-9.6, ionic strength 0-800 mmol/L, fulvic acid below 25 mg/L, major seawater ions, and Mg²⁺/Cl^- gradients, requiring no Mg²⁺ or Cl^- correction. Static and stepwise dynamic spike-recovery tests confirmed time-weighted average Hg(II) determination in river water and seawater (C_DGT/C_SOL = 0.91-1.08). After temperature and diffusive boundary layer correction, field DGT-labile Hg(II) concentrations were 1.24-2.62 ng/L, agreeing with purge-trap HG-AFS measurements within 10% deviation.

The homogeneous membrane combines efficient Hg(II) capture and quantitative elution with tolerance to coastal water matrices. CS-MSA/PVA@DGT supports labile Hg(II) monitoring and assessment of Hg exposure and aquaculture water quality.