Application of diffusive gradients in thin films on the mobility and bioavailability assessment of arsenic and copper

Abstract

Bioavailability is a dynamic process involving lability and bioaccessibility, however, the conventional measurements were shown to overestimate the bioavailable metal concentrations in soil. In water environment, external force was applied in the colloidal separation but it could be time-consuming and result in changes in colloidal metal concentrasion in water. The diffusive gradients in thin films (DGT) was developed as monitoring tool for metals in water and soils environments. In order to better understand the mobility of metals in both soils and water, we studied the mobility of As and compared the uptake and toxicity with earthworm, Eisenia fetida with the use of artificial soil. In addition, we examined the potential use of DGT to understanding Cu speciation along the salinity gradients of Mekong River Delta. Our results demonstrated that cation and anion tend to affect As mobility more significantly as compared to organic carbon and phosphate. High peat moss content soils gave more porosity and the presence of hydrophobic organic carbon that preferably sorbed onto the soil complexed with As by the formation of As–metal–organic complex (with Ca) thereby limited the mobility of As, while phosphate was shown to compete with DOC and As for sorption site on the soils surface. The toxicity data were significantly correlated with As in DGT and body burden of E. fetida, suggesting that DGT can be ususful for As toxicity prediction. Other than that, the effective concentration (AsE) predicted using DIFS model (12 to 253% of total As) was shown to give closer prediction on E. fetida body burden (42 to 460% of total As). In water environment, Cu, DGT390 and DGT45 were deployed in the Mekong River. The colloidal fraction, determined by the difference between dCu and Cu accumulated in DGT45, ranged from 67 to 77% of dCu at the low-salinity sites but from 8.7 to 9.1% of dCu at the brackish sites. A large amount of particle suspension typically found at the salinity front of the Mekong River seemed to play a critical role in the transport of Cu by providing dCu and organic colloids from suspended sediment. Based on our test results, employing DGT probes is likely a promising method for bioavailable prediction for as in soils and colloidal Cu separation in water. This study offers an insight into the application of DGT in soils and waters as a metal toxicity and speciation monitoring tool, although further multi-elements geochemical test on in situ field soils and colloidal separation in water is recommended.