Complexation of DTPA and EDTA with Cd2+: stability constants and thermodynamic parameters at the soil-water interface

摘要

Two alkaline soils collected from the surface horizon (0-15 cm) of two agricultural fields Lakshmikantapur (LKP; 22 degrees 06' 03 '' N and 88 degrees 18' 19 '' E) and Diamond Harbour (DHB; 22 degrees 11' N and 88 degrees 14' E) of West Bengal, India were studied to observe the stability of cadmium (Cd) chelate complexes with diethylenetriaminepentaacetatic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA), removing organic matter (OM). The objective of the present study is "determination of the stability constants and the thermodynamic parameters of Cd-DTPA and Cd-EDTA complexes at different pH and temperatures at the soil-water interface". Complex formation of soil Cd with DTPA and EDTA at the soil-water interface was studied under different ligand-to-metal ratios, pHs and temperatures. Apparent conditional stability constants (log k') were calculated from the concentrations of Cd chelates and free Cd2+, estimated by solid phase extraction with an ion exchanger. Standard Gibbs energy (Delta G degrees), standard enthalpy (Delta H degrees) and standard entropy (Delta S degrees) of formation were calculated at three different temperatures. The higher stability constants of Cd-DTPA than Cd-EDTA indicated longer persistence of Cd-DTPA at the soil solution interface than Cd-EDTA complex. Increase of Delta G degrees, Delta H degrees and Delta S degrees with progress of temperature revealed that Cd-complex formation was facilitated by temperature. Highly negative Delta G degrees and positive Delta H degrees for Cd-complex formation indicated the reaction spontaneous and exothermic. In general, both ligands complexed high percentages of cadmium signalling their role in enhancing remobilization of Cd present in soil and preventing exchange of contaminated Cd from external source with soil mineral matrix; these phenomena may greatly reduce hazard for environment and human health. The result of this study support that DTPA increases solubility and more persistence of Cd in acidic soils within the range of temperature and mole fraction (MF = moles of Cd2+ /sum of the moles of Cd2+ and chelating agent) than that of EDTA due to higher capability of complex formation with Cd2+. Therefore, DTPA enhanced Cd toxicity in acid soils and ground-water. Complex formation in the presence of DTPA at acidic pH decreases with temperature and increases with pH. The higher per cent of Cd complexed in the presence of DTPA revealed that DTPA is a stronger chelating agent than EDTA at acidic pHs. Whereas, the capability of complex formation by EDTA is lower at lower pH but higher at higher pH.