Amendment-induced immobilization of lead in a lead-spiked soil: Evidence from phytotoxicity studies

Lead immobilization was evaluated on soils spiked with increasing concentrations of Pb (as Pb-acetate) using the following soil amendments: bentonite, zeolite, cyclonic ash, compost, lime, steelshot, and hydroxyapatite. The immobilization efficacy of the amendments was evaluated according to the following criteria: Ca(NO3)(2)-extractable Pb as an indicator of Pb phytoavailability, morphological and enzymatic parameters of bean plants (Phaseolus vulgaris) as indicator of phytotoxicity, and Pb concentration in edible tissue of lettuce (Lactuca sativa). The lowest reductions in Ca(NO3)(2)-extractable soil Pb occurred when bentonite and steelshot were applied. Phytotoxicity from application of steelshot was confounded by toxic amounts of Fe and Mn released from the by-product which killed the lettuce seedlings. Addition of zeolite induced poor plant growth independent of Pb concentration due to its adverse effect on soil structure. Substantial reductions in Ca(NO3)(2)-extractable Pb were observed when cyclonic ash, lime, compost and hydroxyapatite were applied. In general, these amendments reduced Pb phytotoxicity concomitant with reduced Pb concentration in lettuce tissue. Cyclonic ash, lime and compost further improved plant growth and reduced oxidative stress at low soil Pb concentrations due to soil pH increase mitigating Al or Mn toxicity.