Dissolved organic matter-solution speciation

Dissolved Organic Matter-Solution Speciation Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on measured total dissolved Cd concentration and computation of free Cd2+ using chemical equilibrium models. The accuracy of the... 

Koopmans GF, Groenenberg JE. Effects of soil oven-drying on concentrations and speciation of trace metals and dissolved organic matter in soil solution extracts of sandy soils. Geoderma 2011 161 147-158. 

In the last section, a rough estimate for the solution speciation in the presence of a dissolved organic matter (DOM) was attempted. DOM is the MinteqAl equivalent of DOC. This involved the speciation of DOM as a function ofpH for two calcium concentrations. Subsequently the impact of DOM on calcium solubility and calcite precipitation was examined. Again, the calcium speciation was determined as a function of pH. 

Few studies report actual measurement of Cd + speciation in soil solutions. Many authors report Cd speciation based on the measured total dissolved Cd concentration and computation of free Cd using chemical equilibrium models. The accuracy of the speciation depends on the correctness of Cd-complex stability constants (Turner, 1995). For example, the proportion of free Cd (as Cd ) in solution varies from 16 to 82% depending on the log K values for Cd-(DOM) dissolved organic matter complexes (Table 4). The stability constants of Cd-humics are particularly uncertain. The reported high values of pCd + ranged... 

Groenenherg, J. E., G. F. Koopmans, and R. N. J. Comans. 2010. Uncertainty analysis of the nonideal competitive adsorption—Donnan model Effects of dissolved organic matter variahihty on predicted metal speciation in soil solution. Environmental Science Technology 44, no. 4 1340-1346. 

Filtration. To separate the dissolved from the particulate fraction filtration or centrifugation will be necessary. This causes a severe risk of contamination it is therefore often not carried out with open ocean samples, where the concentrations of suspended material are low. The presence of phytoplankton or a variable concentration of suspended matter affects the total concentration and a comparison of samples can thus become difficult. In speciation Studies the presence of particles may influence the results even more (complexation, adsorption), therefore filtration over acid washed membrane or screen filters in an appropriate filtration apparatus is recommended for all natural samples (Bewers et al., 1985). High pressure during filtration should be avoided ruptured (plankton-) cells will contribute organic matter, nutrients and trace metals to the solution. A pressure < 25 kPa is recommended (Florence and Batley, 1980). 

The hioavailability of metal to plants and soil organisms is mainly through the solution phase. However, soil solutions are in intimate contact with the soil solid phase and are therefore influenced by mineral equilibria and exchange and sorption processes that involve soil organic matter and mineral phases, as well as complexation by organic matter in both the solid phase and that dissolved in the soil solution. Le Chatelier s principle of chemical equilibrium would suggest that metal hioavailability in soils is thus controlled by both solution speciation and the metal associated with the solid phase. 

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