In-situ measurement of free trace metal concentrations in a flooded paddy soil using the Donnan Membrane Technique

• Published in: Geoderma 2015-03, Vol.241-242, p.59-67
• Main Authors: Pan, Yunyu, Koopmans, Gerwin F., Bonten, Luc T.C., Song, Jing, Luo, Yongming, Temminghoff, Erwin J.M., Comans, Rob N.J.
• Format: Article
• Language: English
• Subjects: Anaerobic conditions; Concentration gradient; copper redox transformation; Dissolution; dissolved organic-matter; Donnan Membrane Technique; Free trace metal concentration; heavy-metals; higher-plants; humic substances; ion activity model; Lead (metal); Mathematical models; natural-waters; Oryza sativa; Paddy soil; ray-absorption spectroscopy; Redox potential; reduced sulfur groups; sandy soil; Soil (material); Soil solution; Speciation model; Trace metals
• ISSN: 0016-7061; 1872-6259
• DOI: 10.1016/j.geoderma.2014.11.003

The field Donnan Membrane Technique (DMT) has been used successfully to measure in-situ free trace metal concentrations in surface waters. However, it has not been applied previously in submerged soil systems including flooded paddy rice fields. We tested this technique in a column experiment with a flooded paddy soil contaminated with trace metals and compared the DMT measurements with predictions from a geochemical speciation model. Flooding led to a strong gradient in the redox potential (Eh) along the soil column, and the pH and concentrations of Mn, Fe, and dissolved organic carbon increased with decreasing Eh. Total dissolved trace metal concentrations decreased from the overlying water layer to the soil layers, which might be ascribed to the elevated pH outweighing the effect of the increased DOC concentrations under anaerobic conditions. Also, free trace metal concentrations were lower in the soil solution of the upper and bottom soil layers than in the overlying water layer, largely due to the increased pH under anaerobic conditions. The DMT measurements and model predictions were similar in the aerobic water layer, except for Pb. In both anaerobic soil layers, however, Cu and Pb were poorly predicted. The applied geochemical modeling approach, which is frequently being used and performs well under aerobic conditions, does not fully capture all the chemical processes occurring under anaerobic conditions. Overall, the field DMT proved to be a useful tool for the in-situ measurement of free trace metal concentrations in flooded paddy soils. •The field DMT is a useful tool to measure free trace metals in-situ in flooded paddy soils.•DOM mobilized under anaerobic conditions has a lower reactivity towards complexation of trace metals.•Speciation model predictions agreed reasonably well with field DMT measurements.