Transport of sequentially decaying reaction products influenced by linear nonequilibrium sorption

Predicting the migration of sequentially degrading reaction products is important for the accurate analysis of subsurface treatment walls and other environmental systems. Recently, it has also been realized that consideration of rate-limited sorption may be necessary to quantify the transport of contaminants in reactive treatment walls. In this paper, analytical and semianalytical solutions are developed for the one-dimensional transport of a parent compound and three sequentially decaying reaction products influenced by nonequilibrium sorption. The solutions are based on the Laplace transformation technique and incorporate a different sorption distribution coefficient and mass transfer rate for each reaction product. Two types of exit boundary conditions are considered: semi-infinite and finite zero-gradient. The developed solutions are used to simulate the migration of trichloroethylene and its reaction products in a zero-valent iron treatment wall. The effect of nonequilibrium sorption on the time to reach steady state is investigated, and it is shown that a zero-valent iron treatment wall may exhibit a significant transient period. However, concentrations reach 80% of the steady state values within the first 100 pore volumes. It is also shown that neglecting dispersion may lead to nonconservative predictions of the effluent contaminant concentrations.