Behaviour of Persistent Organic Pollutants in Soil

MSC-E Technical Note 1/2002

G.Vassilyeva, V.Shatalov

ABSTRACT

The processes of accumulation in and volatilization from soil are of importance for some POPs such as PCBs, PCDD/Fs and others. In particular, according to model calculations [Shatalov et al., 2001], re-emission PCB flux from soil in Europe in late 90th is comparable with anthropogenic emissions and became in some countries a main source of air contamination together with transboundary transport.

The description of behavior of POPs in soil and of atmosphere/soil exchange process used in MSCE-POP model is rather simplified and does not take into account some processes having considerable influence on POP fate in soil. The present Note is aimed at refinement of model description of some processes governing the behavior of POPs in soils.

Among processes having essential influence on POP fate in soil we mention here vertical transport with dissolved organic carbon and redistribution of POPs between different phases in soil.

Transport with dissolved organic carbon. Dissolved organic carbon (DOC) is a component of soil solute for almost all soil types. The most interesting fraction of DOC is mobile dissolved organic carbon, which can be readily transported along soil micro- and macropores. DOC concentration in soil solute depends on soil type and, in particular, on fraction of organic content in soil of given type. As a first approximation the quantity of mobile organic carbon is about 1% of total organic carbon content in soil.

Redistribution between different soil phases. In the context of high storage capacity of the soil solid phase for some POPs, the processes of their sorption and desorption to soil solution are of a particular importance. At present in the model a concept of instantaneous equilibrium between dissolved POP fraction and that sorbed on solid soil organics is used. However, recently more and more evidences appear that real sorption-desorption exchange between POPs and soil differ drastically from equilibrium ones. It is connected with soil organics heterogeneity and low availability of POPs located in micropores. Due to a high volume of POP molecule and its continuous interaction with the hydrophobic surface while moving inside the soil organics micropores, the establishment of thermodynamic equilibrium between the dissolved and adsorbed phase requires considerable time (usually from one to six months). POP desorption in the inverse direction is also non-uniform and delayed.

Proposals for model refinement. On the basis of the above described investigations a scheme describing POP redistribution between different phases in soil and vertical transport with dissolved organics was included into MSCE-POP model description. This scheme includes the following POP phases in soil:

- gaseous (POP in the intercellular air);
- dissolved:
- sorbed on dissolved organic carbon;
- sorbed on readily accessible soil organics fraction;
- sorbed on potentially accessible soil organics fraction.

The model assumes the concept of instantaneous equilibrium between all phases but the last. The exchange between two last phases is described by a kinetic equation of first order.