SOIL-WASHING DESIGN METHODOLOGY FOR A LEAD-CONTAMINATED SANDY-SOIL

Soils located near high traffic roadways, particularly where renovatio ns have taken place, typically exhibit elevated levels of trace metals in the upper soil horizon. Regulators are currently seeking an effici ent method of site characterization and treatment system design which will lend itself to timely and environmentally efficacious clean-up. T he soil investigated in this study was a silty sand collected near a b ridge abutment for a major interstate highway. The soil had a total le ad content of 1392 mg/kg. In addition, the soil contained a considerab le fraction of organic carbon (approximately 6.3%). A sequential chemi cal extraction indicated that a fraction of contaminants were in labil e soil phases and thus amenable to chemical extraction (soil-washing). A soil washing design methodology is presented based on surface chemi stry and equilibrium stage operation. In this work, a double layer sur face complexation model was used to describe equilibrium sorption beha vior and a preliminary design of an ex-situ counter-current equilibriu m stage extraction process is presented. Model calibration was conduct ed using sorption data obtained from a 1:40 solid to liquid ratio (s/l ), adsorption edge. Model validation was accomplished with batch titra tion data and a 1:20 s/l, adsorption edge. The model accurately predic ted leachable lead concentrations over a wide pH range. The required n umber of ideal equilibrium stages was highly sensitive to pH. (C) 1997 Elsevier Science Ltd.