Sorption kinetics of low-molecular-weight hydrophobic organic compounds on surface-modified silica

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The rate of sorption and desorption of substituted benzenes was investigated in a series of batch and column experiments using surface-modified silica of known chemical composition. Porous silicas with patchy surface coatings of aliphatic-chain groups (C 1, C 8 and C 18) and an aliphatic chain terminating in a phenyl group were prepared; these were less hydrophobic and more easily wetted by water than similar, commercially available, chromatography materials. The surfaces were chosen to represent important functional groups found in soil and aquifer materials. Batch desorption of p-nitrobenzene, nitrotoluene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene, and 1,2,4,5-tetrachlorobenzene required up to 100 hours to reach equilibrium. Slow sorption and desorption of 1,4-dichlorobenzene in column experiments was evidenced by tailing in breakthrough curves. Results were interpreted considering three possible rate-limiting steps: diffusion through immobile liquid, diffusion through a bound organic phase, and slow chemical binding and release. Diffusion through immobile fluid caused some but not all of the observed slow sorption, as evidenced by experiments with different particle size and pore size (toruosity). Desorption on C 8- and C 18- modified silicas was at least one order of magnitude faster than on phenyl and C 1 surfaces, illustrating the importance of the nature of the surface organic phase. Simple diffusion through the bound organic phase is not an adequate explanation for the behavior, due to the small distances. Slow binding and release at strong sites for sorption of 1,4-dichlorobenzene on the phenyl-polymer-modified silica apparently caused most of the observed tailing. First-order rate coefficients observed for all modified surfaces were in the range of 10 −4-10 −1 s −1. As equilibrium is reached within hours, slow sorption is observed at the laboratory scale, but local equilibrium applies to most field situations.