Relative Effects of CaCO3 and Ca(OH)2 Amendments On the Solubility, Localization and Potential Bioavailability of Mn, Zn and Cu In An Aerated Dredge Spoil Material.

Three parameters especially important in the control of metal bioavailability in contaminated soils are: (i) pH, (ii) the presence of adsorptive surfaces (including Fe oxide and CaCO₃), and (iii) organic matter content (which impacts the sequestration of most cationic metal species). The current study was conducted to compare CaCO₃ and Ca(OH)₂ as amendments for pH control in the environmental management and revegetation of a Mn, Zn and Cu contaminated dredge sediment, specifically to evaluate the relative impacts of these treatments on sediment mineralogical composition, localization of Mn, Zn and Cu, and metal solubility and potential bioavailability. Sediment samples were analyzed before, during and after oxidative incubation for mineralogical composition (by x-ray diffraction); localization of metals (by SEM, energy dispersive x-ray spectroscopy and selective extraction procedures); Mn, Zn and Cu solubility (by ICP-OES and ICP-MS); and mineralogical control of metal solubility (by chemical equilibrium and speciation modeling).  The sediment was composed principally of quartz, feldspars (labradorite, anorthite), amphiboles, phyllosilicates (micas, smectites, kaolinite, vermiculite, chlorite), gypsum, trace sulfide minerals, and 3% organic matter. With the CaCO₃ treatment, pH gradually increased from approximately 7.0 to 7.5, compared to the Ca(OH)₂ treatment which resulted in an initial pH > 11, subsequent gradual decrease to pH 8-8.5, and the concomitant precipitation of CaCO₃ and Ca-silicate phases.  The CaCO₃ treatment resulted in slight decreases in Mn and Zn solubility, compared to the Ca(OH)₂ treatment that resulted in significant decreases in Zn and Mn solubility to < 0.1 µg mL^-1 and slight increases in Cu solubility, which can be explained by reactions with precipitated mineral phases and organic matter, respectively.  An understanding of these processes is important in optimizing remediation and revegetation strategies.