Applicability of Montmorillonite Immobilized in Hydrogel for the Determination of Labile Cd, Pb, Mn, and Zn in Water Using Diffusive Gradient in Thin Films (DGT)

Diffusive gradient in thin films (DGT) is one of useful techniques regarding in situ metal speciation in water environment. It has attracted significant research interest aimed at finding new binding phases in order to diversify DGT application. In this study, montmorillonite (MMT), the natural clay mineral with high ion-exchange capacity and trace metal adsorbability, was immobilized in hydrogel and investigated for its applicability as a binding phase in the DGT technique. The structure of the new binding material before and after the acid treatment was characterized using X-ray Powder Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Brunauer Emmet Teller (BET) specific surface area testing, and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The montmorillonite embedded in DGT (DGT-MMT) was tested for the determination of labile Cd, Pb, Mn, and Zn in standard solutions. Various parameters affecting the working performance of DGT-MMT including deployment period, pH, ionic strength, and complexing agent (EDTA) were examined. The results showed that after the acid treatment, most of trace metals such as Cd, Pb, Mn, and Zn in MMT were removed to levels of under the detection limits of the analytical instrument, while the major structure of MMT remained stable. The MMT K10 exhibited high adsorbability to all tested trace metals and Langmuir adsorption isotherm was considered the best fit. In each standard solution containing 20 μg L−1 of tested trace metal, it took only 6 h to recover 85.7–93.5% of labile Cd, Pb, Mn, and Zn by the DGT-MMT. Better working performance of the DGT-MMT was observed at pH, ionic strength, and complexing agent conditions similar to those found in natural water environments. Moreover, we found no significant difference in working efficiency between the new DGT-MMT and traditional DGT-Chelex100 techniques.