Irreversible sorption of neutral hydrocarbons to sediments: Experimental observations and model predictions

Contaminants of environmental concern commonly reside in the sediment or solid phase. The extent and rate of desorption has heretofore been particularly unpredictable. In the present research, the adsorption and desorption of seven organic compounds with water solubilities ranging from 0.005 to 517 mg/L have been studied in natural sediments. In every case, a fraction of the adsorbate was adsorbed irreversibly (i.e., desorption was not the opposite of adsorption, yet the sorbate is not covalently bonded to the sediment). Each sediment-contaminant combination exhibited a fixed maximum irreversible adsorption, q(max')(irr) which could be filled in one or several steps and which is related to common molecular properties and sediment organic carbon content (OC). For most compounds, q(max)(irr) (mu g/g) approximate to 10(3.8)OC. Furthermore, the OC-normalized partition constant for this irreversible compartment is essentially constant for the compounds and sediments studied with K-OC(irr) = 10(5.53+/-0.48) mL/g. After about 1-3 days of contact time, all laboratory adsorption and desorption data could be modeled using a single isotherm equation, based upon commonly measured chemical and sediment parameters. The isotherm equation consists of two terms, a linear term to represent reversible sorption and a Langmuirian-type term to represent irreversible sorption. This combined isotherm is used to interpret numerous published field studies. The potential impact of this model on sediment quality criteria (SQC) and remediation are discussed.