Soil organic matter (SOM) is expected to play a key role in the in-situ bioremediation of both inorganic and organic pollutants because it regulates pollutant transport and bioavailability and can be readily manipulated by organic amendments and/or plant growth. Lignosulfonates (LS) may be well-suited as organic amendments for immobilizing metal ions in contaminated soils. They are relatively resistant to microbial degradation, interact readily with metal ions, are available at low cost, and appear to be "friendly" in terrestrial environments. However, the chemical mechanisms underlying LS functions in these applications are unclear, and it is not known how LS aging or humification can be exploited for metal stabilization. This basic knowledge is crucial to designing and engineering LS or other organic amendments for metal sequestration and to assessing the long-term stability of the sequestered metals.
This one-year proof-of-concept project will address whether LS amendment and subsequent humification lead to the sequestion of heavy metals in soils and explore the chemical sequestration mechanisms that are involved.
Two sets of benchscale soil-aging experiments will be conducted. The first will be performed by incubating for 4 months different types and amounts of LS with metalcontaminated soils from the former McClellan Air Force Base (MAB). Soil leachates will be collected periodically and analyzed for a broad spectrum of heavy elements by inductively coupled plasma mass spectroscopy (ICP MS). The leachability of LS and structures of humified products will be analyzed by a suite of spectroscopic techniques, including nuclear magnetic resonance (NMR), pyrolysis gas chromatography mass spectroscopy (GC MS), fourier transform infrared (FT IR), three-dimensional fluorescence, and photon correlation spectroscopy. The types and amounts of LS that exhibit different metal leaching properties will be chosen for incubation with 13C and 15N-labeled MAB soils in the second set of aging experiments. LS humification should result in a dilution of 13C and 15N labels in various humic substructures of MAB soils. By following the time course of label reduction, the lability of humic substructures will be deduced and linked to the metal-leaching characteristics. From this, the humic substructures that are both recalcitrant to turnover and involved in metal sequestration may be identified.
Improved understanding of the fundamental mechanisms of metal ion-SOM interactions is vital to soil bioremediation since it is the organic portion of soil that is amenable to rapid, engineered manipulation by organisms or amendments such as LS to achieve desired long-term stability of the sequestered metals. When the bench-scale work is complete, the tools developed will be readily applied to pilot- or field-scale studies that can lead to mechanism-based choice of organic amendments and design of processes for superior long-term immobilization of pollutant metal ions.