Soil solution speciation of lead(II): Effects of organic matter and pH

The effects of adjusting the levels of soil organic matter in a Pb-contaminated soil on the solubility and free Pb2+ speciation were studied within the pH range 3 to 8. A contaminated orchard soil containing 284 mg Pb kg(-1) was treated with leaf compost to increase soil organic matter and with H2O2 to decrease it, yielding six soil organic matter levels between 25.6 and 83.7 g C kg(-1). The equilibrated solutions were then analyzed for total dissolved Pb by graphite furnace atomic absorption spectrometry and for labile Pb by differential pulse anodic stripping voltammetry (DPASV). The labile Pb values were used to calculate the free Pb2+ activity based on the assumption that organo-Pb complexes are not DPASV-labile. The data showed that 30 to 50% of dissolved Pb is present as soluble OM complexes at low pH and up to 80 to 99% at near-neutral pH. The solubility of Ph shows a linear decrease from pH 3 to 6.5 and is independent of soil organic matter in that pH range. From pH 6.5 to 8, higher pH promotes the formation and dissolution of organo-Pb complexes, which increase Pb solubility. In this pH range, higher organic matter content results in higher concentrations of dissolved and labile Pb.