Subsurface aqueous environment

The effect of pH on transformation in the subsurface aqueous environment is considered here for the case of two highly toxic biocides (methyl parathion and acrolein) and for an industrial semivolatile organic pollutant (tribromoneopentyl-alchohol, or TBNPA). The examples are based on the work of Guo and Jans (2006), Oh et al. (2006) and Ezra et al. (2005). 

Reduction and oxidation reactions in the subsurface environment lead to transformation of organic and inorganic contaminants. We consider chromium (Cr) as an example of an inorganic toxic chemical for which both oxidation and reduction processes may transform the valence of this element, in subsurface aqueous solutions, as a function of the local chemistry. 

The most stable oxidation states of chromium in the subsurface environment are Cr(III) and Cr(VI), the latter being more toxic and more mobile. The oxidation of Cr(III) in subsurface aqueous solutions is possible in a medium characterized by the presence of Mn(IV) oxides. Eary and Rai (1987), however, state that the extent of Cr(III) oxidation may be limited by the adsorption of anionic Cr(VI) in acidic solutions and the adsorption and precipitation of various forms of Cr(OH). These authors also report a rapid quantitative stoichiometric reduction of aqueous Cr(VI) by aqueous Fe(ll), in a pH range covering the acidity variability in the subsurface even in oxygenated solutions. 

SRB are essentially ubiquitous in aqueous environments that contain organic carbon and sulfate (e.g., subsurface aquifers and lake sediments). Moreover, analysis of a key gene associated with sulfate reduction (dissimilatory sulfite reductase) indicates that microbial sulfate reduction is an ancient trait, suggesting that organisms may have contributed to sulfide mineral formation throughout much of Earth history (Wagner et al. 1998). SRB are tolerant to environmental extremes of heat (some are hyperthermophiles) and salinity (some are halophiles). 

A system is homogeneous when the intensive properties are not a function of position, while a system is heterogeneous when the composition of a given mixture varies as a function of position. For example, the subsurface liquid phase usually comprises an aqueous solution incorporating a number of solutes in contaminated subsurface environments, nonaqueous phase liquids also may be present. The air phase of the subsurface includes gases with various partial pressures, and the solid phases comprise a mixture of minerals and organic compounds. 

Entropy-related adsorption, denoted hydrophobic sorption (or solvophobic interaction) is the partitioning of nonpolar organics out of the polar aqueous phase onto hydrophobic surfaces. Fig. 5.6 shows a schematic model of forces that contribute to the sorption of hydrophobic organics, relevant to the subsurface environment. 

Acrolein (CHj=CHCHO, also known as 2-propenal) is a a,P-unsaturated aldehyde that can be transformed reducfively to saturated or unsaturated alcohols by reduction of the C = 0 or C = C double bonds (Claus 1998). In addition, a,P-unsaturated aldehydes may undergo hydration reactions in aqueous solutions. It was observed that, under acidic (pH12) conditions, acrolein is hydrated to 3-hydroxypropanal (Jensen and Hashtroudi 1976). In a natural subsurface environment, where pH may range from 6.5 to 8.5, the hydration rate of acrolein increases with the pH and its half-life decreases. Based on an experiment to analyze effects of iron on acrolein transformation, Oh et al. (2006) note that, under acidic conditions (e.g., pH = 4.4), acrolein disappears rapidly from solution in the presence of elemental iron (Fig. 16.1). Moreover, the formation of... 

In summary, we can conclude that at moderate salt concentrations typical for seawater ( 0.5 M), salinity will affect aqueous solubility (or the aqueous activity coefficient) by a factor of between less than 1.5 (small and/or polar compounds) and about 3 (large, nonpolar compounds, n-alkanals). Hence, in marine environments for many compounds, salting-out will not be a major factor in determining their partitioning behavior. Note, however, that in environments exhibiting much higher salt concentrations [e.g., in the Dead Sea (5 M) or in subsurface brines near oil fields], because of the exponential relationship (Eq. 5-28), salting-out will be substantial (see also Illustrative Example 5.4). 

Detailed experimental procedures have been previously reported (Ko, 1998 Ko et al., 1998a,b) therefore, they are only briefly described here. Phenanthrene (Aldrich, 99.5+%), naphthalene (Aldrich, 99+%), SDS (Sigma, 99.5+%), and Tween 80 (Aldrich, no purity reported) were used as received selected physicochemical properties for these compounds are shown in Table 1. Kaolinite, a nonswelling 1 1 layer phyllosilicate clay and common constituent of many subsurface environments, was used as received from Sigma. Solution pH and ionic strength were adjusted as necessary with 0.5 M HC1 and/or 0.5 M NaOH and NaCl, respectively. Aqueous phenanthrene and naphthalene concentrations were quantified by fluorescence (PTI, Inc.) at the excitation/emission wavelengths of 250/364 and 278/322 nm, respectively. A total organic carbon (TOC) analyzer (Shimadzu Model 5050) was used to determine aqueous SDS concentrations and Tween 80 concentrations were determined by UV absorbance at 234 nm. 

In certain environments, localized anomalously low concentrations of soil O2 have been used by exploration geologists to indicate the presence of a large body of chemically reduced metal sulfides in the subsurface. Oxidation of sulfide minerals during weathering and soil formation draws down soil gas po below regional average. Oxidation of sulfide minerals generates solid and aqueous-phase oxidation products (i.e., sulfate... 

Thus, natural attenuation studies of hydrocarbons often focus on the availability of electron acceptors (e.g.. Barker and Wilson, 1997 Cozzarelli et al., 1995 Gieg et al., 1999 McGuire et al., 2002 Skubal et al., 2001). Numerous studies have focused on the availability of electron acceptors in both the sediment (e.g., Baedecker et al., 1993 Bekins et al., 2001a Chapelle et al., 2002 Cozzarelli et al., 2001a Heron and Christensen, 1994 Heron and Christensen, 1995 Heron and Christensen, 1995) and the aqueous phase (e.g.. Ball and Reinhard, 1996 Cozzarelli et al., 1999) as a key control of the fate of hydrocarbons in subsurface environments. In most sediment, Fe(III)s, as iron oxides, is abundant... 

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