Gravel sludge as an immobilizing agent in soils contaminated by heavy metals; A field study

Field plot studies were conducted on sandy loam soils in Dottikon, Rafz and Giornico contaminated by Zn, Cu and Cd to investigate the effectiveness of gravel sludge as an in situ immobilization additive. In addition to NaNO3- extractable heavy metal concentrations in the soil, metal concentrations in ryegrass (Lolium perenne L.) and lettuce (Lactuca sativa L.) were analyzed at two application doses of gravel sludge. Application of gravel sludge in creased pH in all three topsoils by up to 0.6 units and reduced NaNO3-extra ctable Zn concentrations by more than 65%. No effect was seen for the NaNO3 -extractable Cu concentrations in Rafz and even an increase resulted in Gio rnico. In Dottikon, gravel sludge additions decreased the NaNO3-extractable Cu concentrations from 0.97 to 0.7 mg kg(-1) and reduced the concentration s of Zn and Cu by more than 35% in ryegrass. For lettuce grown in Giornico and Dottikon, we found reductions of Zn and Cd concentrations by 22 to 48%, whereas in Rafz, no significant variations were found. Immobilization by g ravel sludge was most efficient in soils with high total/NaNO3-extractable heavy metal ratio and more effective for ryegrass than for lettuce. Part of the immobilization effect was due to a pH increase, caused by CaCO3 in gra vel sludge.