Amendment Optimization to Enhance Lead Extractability from Contaminated Soils for Phytoremediation

Abstract

ABSTRACT Eight lead-contaminated soils and one background soil artificially contaminated with several lead compounds were examined to determine the factors that limit lead extractability and thus plant availability during phytoremediation, as lead must be in soluble form for plant uptake to occur. The effect of the chemical form of the lead as well as the association of the lead among the different soil chemical fractions on lead extractability was specifically addressed. Results indicate that all the added lead forms tested except PbCrO4 were readily extracted and believed to be available for plant uptake, operationally defined as EDTA-extractable lead, as EDTA is the primary soil amendment for phytoremediation of lead-contaminated soils. Sequential extraction of the eight lead-contaminated soils that previously had been extracted with EDTA shows that the EDTA-extractable or plant available lead corresponds to mainly the exchangeable and carbonate fractions of each soil. Lead associated with oxide, organic, and residual fractions were less effectively targeted and solubilized by EDTA and therefore are not as readily available for plant uptake. Attempts to increase the available pool of soluble lead included the combination of EDTA with organic acids, reducing agents, and surfactants. Results from these studies indicate that high concentrations or extremely low pH conditions are required to enhance the plant available pool of lead by the organic acids and reducing agents. Surfactants, particularly caprylic acid in combination with 0.25 mM EDTA, were shown to be as effective as 0.50 mM EDTA alone. An amendment formulation combining less EDTA with surfactants is attractive for phytoremediation because of the biodegradability and cost concerns commonly associated with using larger amounts of EDTA.