Modelling the complexation of Cd in soil solution at different temperatures using the UV-absorbance of dissolved organic matter

It is well established that the soil temperature impacts on the mobility of Cd in soil but the role of dissolved organic matter (DOM) quality in this process is still unclear. During a 42-day period, soil solutions were sampled from a Cd-contaminated soil incubated at 10, 20 or 30 °C. The quantity and the quality of DOM were monitored over time as well as the concentration and the speciation of dissolved Cd. The fraction of Cd complexed by ligands (fCdL) increased over time and with the rise in soil temperature and exhibited a significant correlation with the absorbance of DOM at 254 nm. In contrast, the quantity of DOM did not much vary between soil solutions. These results suggest that, during incubation, the composition of DOM shifted to more aromatic compounds, assimilated to humic substances (HS), with higher Cd affinity. The kinetics of DOM aromatization and Cd complexation were described using thermal time. A first order model adequately fitted all the experimental data, suggesting that similar microbial processes responsible for the aromatization of DOM occurred at 10, 20 and 30 °C. The fraction of Cd complexed by ligands was predicted by Visual MINTEQ using the solution properties measured at 140 day-degrees and the specific UV-absorbance of DOM (SUVA) monitored over thermal time. SUVA values were used to adjust the reactivity ratio of DOM (R). The model predictions were highly correlated with the measured values but they overestimated fCdL. It is concluded that the aromaticity of DOM should be taken into account to predict the impact of soil temperature on the complexation of Cd in rewetted soils. The relation between SUVA values and the HS fraction of DOM still needs, however, to be worked out.