Estudo da labilidade de espécies metálicas empregando difusão em filmes finos por gradiente de concentração (DGT) em água superficial e solo da região de Ponta Grossa, Paraná

Abstract

The quality of water and soil resources is fundamental for sustainability and human well- being, being directly affected by the presence of trace metals, which can be non-degradable

environmental contaminants, toxic, and bioaccumulative. Assessing the chemical speciation of these metals is essential as their mobility, toxicity, and bioavailability critically depend on the forms and chemical species present. The Diffusion in Thin Films with a Gradient of Concentration technique allows for in situ determination of labile metal fractions in natural waters, soils, and sediments. DGT (Diffusive gradients in thin films) offers advantages such as: passive sampling, pre-concentration capability, and minimal sample alteration due to collection. However, traditional DGT devices use toxic synthetic materials and have high export costs. In this context, an alternative DGT device was developed using agarose hydrogel, a more environmentally benign material, for the diffusive phase and a Chelex®-100 resin-impregnated phase for studying the behavior of metal species in environmental water and soil samples from Ponta Grossa (Paraná, Brazil), an area with limited scientific data on these species in environmental samples. Initial studies were conducted with agarose hydrogels for the diffusive and binding phases with homemade devices. After determining the diffusion coefficient (D) and elution coefficient (ƒe) values for Cd, Cu, and Cr for the prepared DGT devices, indicating their adequacy and good elution efficiency, accuracy tests were performed with aqueous solutions containing Cd(II), Cu(II), and Cr(III) without organic ligands. The recovery values obtained for these metals were 100.36% ± 0.25% for cadmium, 86.14% ± 2.73% for copper, and 81.85% ± 1.20%, respectively. These values fall within the 100 ± 20% range considered acceptable for commercial DGT devices, indicating the precision and accuracy of the device in determining Cd, Cu, and Cr under conditions similar to those of a natural water sample. The DGT devices were applied for 24 hours in surface water samples collected from the Cará-Cará River in Ponta Grossa - Paraná. The total concentrations determined in the samples were 0.57 μg L-1

for Cd, 2.2 μg L-1

for Cu, and 2.9 μg L-1 for Cr,

and the labile concentrations were 0.182 μg L-1

, 0.4 μg L-1

, and 0.39 μg L-1

for Cd, Cu, and Cr, respectively. The bioavailable percentages in the water were 31.93% for Cd, 18.18% for Cu, and 13.45% for Cr. The distribution of labile species in the Cará-Cará River revealed a greater lability for Cd, followed by Cu and Cr(III). The results suggest that DGT devices with agarose can be effectively used for speciation analysis. Subsequently, studies were conducted on soil near the Cará-Cará River. This soil is classified by EMBRAPA as Latossolo Vermelho Distrófico, with a major fraction of sand totaling 67.28%. The soil also had high values of Silicon, Aluminum, and Iron oxides. Additionally, the soil pH is acidic (5.44), and its cation exchange capacity is relatively low (8.16 cmolc dm-3

). The pseudototal metal concentrations were determined using USEPA 3050B methodology, and the values obtained were 0.053 ± 0.27 and 20.25 ± 0.77 μg g-1

for Cd and Cu, respectively, which are below the levels established by CONAMA Resolution 420/2009. However, the concentration determined by DGT to predict the bioavailability of these elements showed that Cd (CDGT – 0.0053 μg g−1 )

is more available in the soil than Cu (CDGT – 0.0026 μg g−1

), with bioavailable percentages in the soil being 10.53% for Cd and 0.015% for Cu. The R ratio, representing the relationship between the bioavailable concentration estimated by DGT (CDGT) and the concentration in the soil’s liquid phase (Csol), was 0.27 for Cadmium and 0.21 for Copper, indicating that Cadmium, even in lower pseudototal concentrations, tends to be more mobile and bioavailable in the soil from the Ponta Grossa region. Additionally, a study was conducted on soil enriched with these metal species to assess the performance of the homemade DGT device in

determining the lability of Cd and Cu in contaminated soil samples. A linear relationship was observed for increasing concentrations of Cd, whereas for Cu, which is naturally less available, concentrations above 100 μg g−1

resulted in an R value exceeding 90%, indicating that artificially contaminated soil can influence the accuracy of DGT measurements.