Chlorinated ethene reduction by cast iron: Sorption and mass transfer

Tetrachloroethylene (PCE) and trichloroethylene (TCE) exhibited significant nonlinear sorption to nonreactive sites when exposed to four cast irons. Cast iron is a reactive material that promotes reductive dechlorination and has recently been used for in-situ remediation of chlorinated solvent contaminated ground water. Comparisons between PCE sorption to cast iron, graphite, and iron-containing minerals indicate that nonreactive sorption is due to exposed graphite inclusions in the cast iron. Sorption of the homologous series of chloroethenes to a cast iron adheres to Traube's rule; thus, the extent of sorption is related primarily to compound hydrophobicity. An analytical model incorporating rate-limited sorption/desorption to nonreactive sites was used to assess sorption nonequilibrium. Effective sorption and desorption rate coefficients determined how significant mass transfer limitations to nonreactive sorption sites exist for PCE and not for TCE. The nonreactive sorption observed indicates that flow-through cast iron treatment systems will exhibit significant delayed attainment of steady-state conditions for chlorinated ethenes, particularly PCE and TCE.