Application of diffusive gradient in thin films technique for mercury bioavailability prediction in soil

Abstract

The technique of diffusive gradients in thin films (DGT) has been shown to be a promising tool to assess metal bioavailability in soils under laboratory conditions. however, its applicability is subject to the type of metal and organism involved. The theoretical aspect and application of DGT to measure in situ speciation and bioavailability of diverse metal pollutants in soils were introduced in the first chapter. In the second chapter, the accumulated masses of Hg in DGT probes and in the earthworm species Eisenia fetida were monitored for 10 days, to test if the DGT technique can be used as a predicting method for the bioavailability of soil Hg to earthworms. The Hg exposure tests were conducted using freshly prepared artificial soils with different peat moss concentrations of 5, 10, 15, and 20% and aged prepared soils with varying pH values of 4.6, 5.6, and 6.2. It is interesting to note that the Hg uptake rates by DGT and earthworms were considerably higher for fresh soils than for aged soils, while pore water (and acid-extractable) Hg concentrations were rather similar between the two types of soils. DGT-measured Hg flux used to assess bioavailability in earthworm showed a strong positive correlation with steady-state Hg concentration in earthworm ([earthworm Hg] = 354(DGT−Hg flux) − 34, R2 = 0.88). The overall results indicate that DGT-measured Hg flux is a better tool than the conventional methods for predicting Hg bioavailability for earthworms inhabiting diverse types of soil. In the third chapter, the critical soil characteristics affecting Hg bioavailability to the earthworm Eisenia fetida were evaluated using DGT technique in the metal-contaminated soils collected from Gumu Creek, a tributary of the Hyeongsan River. The correlation analysis showed water holding capacity is the key variable of soil properties related to Hg accumulation in the soil, earthworm, and binding gel. Indeed. the water-holding capacity played a dual role in the Gumu Creek deposits: increasing the soil Hg concentration and decreasing Hg bioavailability and leachability. DGT–Hg flux showed a positive correlated with the Hg concentration in earthworms (r = 0.93). The results of this chapter proved that the DGT method is promising for predicting soil Hg bioavailability to the earthworm Eisenia fetida, and the water-holding capacity simultaneously regulates Hg availability to the DGT and the earthworms. In the last chapter, the aging effect of Hg, one of the most important factors controlling Hg bioavailability in soil, was studied using diverse types of field soils. Surface soil samples were collected from forestry, agriculture, and riverbank sites (Youngsan and Hyeongsan river). The soil samples were spiked with inorganic Hg(II) and incubated for 1, 3, 5, 8, 15, 25, 40, 60 and 90 days. We found that, during the aging process, the proportions of Hg tended to migrate from mobile fraction to the stable binding fractions. Effective Hg concentration (CE) was estimated by DGT and DIFS model, as it represents bioavailable fraction of Hg in soils according to the previous literature. The effect of Hg aging on CE was evaluated by aging rate constant (k1) obtained by fitting the CE values using pseudo-first order kinetic model. Partial Least Square regression (PLSR) model was used to relate various soil properties to the variations of k1 in those soils. PLS model constructed by cross-validation and variable selection routines predicted 31% of k1 when applied to entire soil samples, but 73 – 92% of k1 was predicted when applied to specific soil type. The results showed that variation of k1 was mainly predicted by soil pH and organic matter content. The overall results indicate that the combination of DGT technique and PLSR method is a useful tool for evaluating the aging effects on bioavailability of Hg in various types of soils using selected soil properties.