Effect of Humic Acid on Metal Uptake Measured by Diffusive Gradients in Thin Films Technique

Impact of the presence of humic acid (HA) on the metal uptake of cadmium, copper, nickel, and lead in the diffusive gradients in thin films technique (DGT) was studied. Both the diffusive and resin gel were composed of the agarose polyacrylamide hydrogel, the resin one also with embedded Chelex-100 ion exchanger. A restrict gel, excluding high-molecular species, was used in comple-mentary experiments. Metal concentrations of 50 µg dm−3 of Cd, Cu, Ni, and Pb, various mass concentrations of HA substance of ρ/(mg dm−3): 0, 0.3, 1, 3.1, 10, 31, 100, and 316, and concen-tration of 0.05 mol dm−3 sodium nitrate were adjusted in experimental solutions (pH = 6.8). The DGT probes were deployed in solutions at (26 ± 2)◦C for up to 32 h. After elution, the overall DGT-accumulated masses of metals were determined by electrothermal atomic absorption spec-trometry and apparent diffusion coefficients of individual metals were evaluated. The HA substance at approximately equimolar (1 mg dm−3) and higher concentrations with respect to metals ions considerably reduces the DGT-measured metals uptake. This suppression effect is more pronounced at higher concentrations of HA substance, it is element-specific and increases in the sequence Cd, Ni, Pb, Cu. HA species diffuse through the permeable gel and affect predominantly the interac-tion of metal ions with specific iminodiacetic groups of the resin by competitive reactions. Thi