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 an d desorption of seven organic compounds with water solubilities rangin g from 0.005 to 517 mg/L have been studied in natural sediments. In ev ery 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 combina tion exhibited a fixed maximum irreversible adsorption, q(max')(irr) w hich could be filled in one or several steps and which is related to c ommon molecular properties and sediment organic carbon content (OC). F or most compounds, q(max)(irr) (mu g/g) approximate to 10(3.8)OC. Furt hermore, the OC-normalized partition constant for this irreversible co mpartment is essentially constant for the compounds and sediments stud ied with K-OC(irr) = 10(5.53+/-0.48) mL/g. After about 1-3 days of con tact time, all laboratory adsorption and desorption data could be mode led using a single isotherm equation, based upon commonly measured che mical and sediment parameters. The isotherm equation consists of two t erms, 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 i mpact of this model on sediment quality criteria (SQC) and remediation are discussed.