Assessing benzene and toluene adsorption with peat depth: Implications on their fate and transport

After a hydrocarbon spill in a peatland, dissolution of water-soluble compounds including benzene and toluene introduces a dissolved-phase plume to the peatland groundwater system, while the adsorption of these solutes onto the peat matrix restrains their distribution velocity. The adsorption of benzene and toluene and its dependency on peat depth, thus degree of decomposition, are investigated. The batch adsorption experiments revealed that benzene and toluene adsorption isotherms in peat are linear, with adsorption coefficients ranging from 16.2 to 48.7 L/kg and 31.6-48.7 L/kg, respectively. In a vertical peat profile benzene adsorption decreased with depth, while toluene adsorption increased. Considering toluene adsorption onto cellulose is significantly less than toluene adsorption onto humic substance, the increase in toluene adsorption was attributed to decreasing cellulose and increasing humic substances with depth. Negligible competition for adsorption was observed between benzene and toluene at the measured concentrations. The retardation factors of benzene and toluene ranged respectively from 3.5 to 10.7 and from 5.4 to 17.7, both increasing with depth. Higher retardation in deeper peat coupled with lower hydraulic conductivity will lead to a weaker solute velocity in deeper peat, thus preferential migration of these dissolved-phase contaminants in shallow layers. The results can help predict the behavior of dissolved hydrocarbons in peatlands after a hydrocarbon spill.