Alan R. Bowers

Determination of acute Hg emissions from solidified/stabilized cement waste forms

Abstract The chemical form of mercury in wastes to be solidified/stabilized may lead to volatile losses from the finished solidified/stabilized monolith. Elemental mercury vapor (Hg vapor) was detected in the headspace of batch reactors that contained solidified/stabilized ordinary Portland cement doped with mercuric oxide (HgO) or liquid elemental mercury [Hg°(1)]. Vapor concentrations increased as a function of time and temperature; the headspace over the HgO samples was saturated in about one hour, while the samples containing Hg°(1) reached approx. 20% of saturation in about two hours. Increased temperatures due to cement hydrolysis lead to increased Hg vapor evolution. Mercury solidified/stabilized as mercuric sulfide (HgS, black) emitted no Hg vapor. Data for the HgO and Hg°(1) experiments was fit to a reversible first-order rate expression. Samples containing HgO displayed the greatest volatility as a result of the rapid dissolution of HgO and the subsequent formation of a strong driving force across the air-water interface. The evolution of Hg vapor from samples solidified/stabilized as Hg°(1) is limited by mass transfer resistances that kinetically limit the dissolution of Hg°(1) into the aqueous phase. The inert character of HgS (extremely low solubility and resistance to oxidative dissolution) prevents the evolution of detectable Hg in wastes solidified/stabilized as HgS. The findings of these studies may be important when considering treatment and disposal scenarios for Hg-containing wastes.

Role of Fe (III) in metal complex adsorption by hydrous solids

Abstract The effect of Fe(III) on the adsorptive behavior of complexed Ni(II), Pb(II) and Zn(II) by hydrous solids (γ-Al 2 O 3 , SiO 2 and mordenite) has been examined in the laboratory. Ethylenediaminetetraacetic acid (EDTA) and 1, 2-dihydroxybenzene-3, 5 disulfonic acid (Tiron) were used as strong organic complex formers. Iron(III) was found to have stronger complex formation constants than the divalent metals; however, the pH-dependent conditional constants indicated that the organic ligands would favor Fe(III) in the low pH range (∼ 6). The partitioning of organic ligands between Fe(III) and the divalent metals as a function of pH was shown to play a significant role in the adsorptive behavior of the complexed divalent metals. When present at low concentration ( −6 M), the divalent metals adsorbed in a “metal-like” manner when Fe(III) was present, compared to a “ligand-like” behavior when Fe(III) was not available to compete for the organic ligand. At high concentrations (10 −4 M), Ni(II) behaved in a “ligand-like” manner due to the comparatively low solubility of the Fe 3+ species, while Pb(II) and Zn(II) exhibited a transitory behavior, somewhere between the “free” and the complexed metal species.

Effects of surface oxidation and oxygen on the removal of trichloroethylene from the gas phase using elemental iron

The reduction of trichloroethylene (TCE) in the gas phase byFe° was examined under water vapor saturatedconditions (relative humidity (RH) = 100%). The reactionconformed to first-order rate kinetics under anaerobic(N2 atmosphere) conditions and acid-washedFe°exhibited a faster TCE removal rate than unamended (partially oxidized) Fe°, i.e., kobs = 0.015 h-1 versus0.012 h-1. Analysis of the two types of Fe° showedthat 40.3% of the unamended Fe° surface was nonreactive. Experiments with iron oxides, which form commonly on the surface of Fe° exposed to humid air (magnetiteand maghemite), showed that these solids were nonreactive with TCE under anaerobic conditions. Under aerobic conditions (air),TCE reduction occurred in two distinct phases. There was a fastinitial rate followed by a slower later rate of reduction when the oxide layer was formed. Further experiments showed that theFe° surface was saturated with TCE at higher concentrations (K1/2 = 5,397.4 ± 345.4 ppmv) and thatoxygen acted as an irreversible inhibitor of TCE reduction(maximum rate of reaction decreased when oxygen was present).

Two‐dimensional groundwater transport of reactive solutes with competitive adsorption

Chemical contamination of groundwater is typically associated with multicomponent solutions, the migration of which is affected by preferential adsorption and solution reactions. Transport models should, therefore, account for advection and dispersion and for the reactions in the solution and on the solid phase. The two-dimensional model METLI has been developed for the personal computer (PC) simulation of groundwater transport with competitive adsorption of a three-species system consisting of a metal, a ligand, and their dominant complex at a given pH. The model structure, which employs spatial and process splitting algorithms, is described. One- and two-dimensional application examples are used to demonstrate how chemical reactivity and affinity for adsorption of a species, adsorption capacity of soil, and the number of the adsorbing species affect pollutant plumes in groundwater. The model executes rapidly on PCs using grids of several hundred nodes.