Competitive Sorption Phenomena 17

Given all of the above information, one of the least explored areas of hormone and soil transport processes is the impact that chemical co-contaminants may have on partitioning, biodegradation, and movement through the soil. Few published studies have examined the effects of co-contaminants on the fate and transport of hormones and other EASs through soil and water systems, though indications are that such effects may be quite significant in

transport processes and could lead to increased rates of transport. Colloidal organic matter has been shown to decrease aqueous free concentrations of hormones and NPs in batch studies where aqueous solutions of hormones were equilibrated with colloids in suspension (Holbrook et al., 2003, 2004, Yamamoto and Liljestrand, 2003, Zhou et al., 2007). The presence of dissolved organic carbon (DOC) at concentrations as low as 4 mg C/L was shown to decrease the partitioning coefficient of E2 and NPs to lipid membranes by 20 to 30% (Yamamoto et al., 2004). While lipid membranes are not the same as soils, the shift in equilibrium towards the aqueous phase (either as free or colloid-bound) could be indicative of the potential for facilitated transport through soil. It has been estimated that wastewater colloids could bind up to 60% of estrogens in an aquatic system, representing an extreme

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case (Holbrook et al., 2004), though that number is more likely in the range of 4 to 31% (Zhou et al., 2007).

Surfactants (chemicals present in soaps, detergents, and personal care products used largely to solubilize non-polar organic compounds) may also play a significant role in the transport of estrogens and NPs, though the extent of their impact is not clear. On the one hand, dodecylbenzenesulfonic acid [a linear alkylbenzene sulfonate anionic surfactant, found at concentrations of up to 18 mg/L in septic tanks and 1.5 mg/L in groundwater (Nielsen et al., 2002)] has been shown to lower the partitioning coefficient for E1 and E2 in sediment- water systems (thereby increasing their presence in the aqueous phase) (Bowman et al., 2002) and to increase the likelihood that those compounds would sorb to colloidal organic carbon (Zhou et al., 2007). On the other hand, nonionic surfactants such as alkylphenol polyethoxylates (APEOs) found at concentrations of up to 11 mg/L (sum of oligomers) in septage and 7.5 mg/L in untreated wastewater (Rudel et al., 1998) have been shown to bind via hydrogen bonding to charged sites on sediment particles, potentially providing additional surficial hydrophobic binding sites4 for non-polar compounds such as estrogens and

nonylphenols (Brownawell et al., 1997, Zhou and Zhu, 2005). Likewise, neutral pharmaceuticals, such as carbamazepine (100 – 300 ng/L in septic effluent) and

acetaminophen (up to 1 mg/L in septic effluent) (Godfrey et al., 2007), were shown to have increased sorption coefficients in the presence of nonionic surfactants, again potentially slowing their migration through soils(Hari et al., 2005). Given that surfactants and/or surfactant degradation byproducts have been consistently found in household waste (from aforementioned literature evidence and ubiquitous use in soaps and detergents), it is

4 _{Provided that the concentration does not exceed the critical micelle concentration of around 600 mg/L for} linear alkyl sulfonates (Goon et al., 1997) and 43 mg/L for nonylphenol 12-ethoxylate (Brix et al., 2001).

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important to evaluate the impact these compounds may have on partitioning and transport, especially during non-equilibrium transport conditions.

Other compounds present in wastewater may also impact partitioning and transport of EASs through competitive sorption mechanisms and may serve to force EASs to spend more time in the aqueous phase during transport (Doucette, 2000). For example, estradiol valerate (a synthetic estrogen with high KOW and not used pharmacologically) decreased sorption of

E1, E2, E3, and EE2 to sediments via competitive binding by as much as 75% (Lai et al., 2000). As another example, the presence of phenanthrene at aqueous concentrations of 100 µg/L has been shown to reduce the sorption coefficient (Kd) of EE2 by as much as 35% via

competitive sorption, forcing EE2 to spend more time in the aqueous phase and possibly increasing its rate of transport in a soil-water system(Yu and Huang, 2005). In the same phenanthrene-EE2 study, co-solutes/sorbates of higher hydrophobicity were found to out- compete those with lower hydrophobicity [e.g., phenanthrene (log KOW = 4.57) reduced the

sorption coefficient of EE2 (log KOW = 4.15) which in turn reduced the sorption coefficient

of naphthalene (log KOW = 3.30)] (Yu and Huang, 2005). The implication of these studies is

that co-solutes in a septic effluent with different KOW values may induce competitive sorption

effects, thereby changing the rate of transport of each chemical through the soil. The existing contradictions in findings indicate that significant work needs to be

undertaken with respect to co-solute transport phenomena in complex systems. For the target EASs in this study (pKa ~10.1 to 10.4, therefore protonated and neutral at typical wastewater

pH ranges of 5 to 8), pH and conductivity are not likely to have much effect on sorption coefficients(Doucette, 2000). However, DOC and co-solutes in solution may increase the “free” concentration of compounds found in the aqueous phase (thereby decreasing the

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sorption coefficients) as previously indicated. It is critical that both the solid and aqueous phases be considered as well as the role of biodegradation on partitioning and transport. Additionally, none of the competitive sorption studies have examined the phenomenon in non-equilibrium based systems (i.e., column studies or percolation through soils) and as such few conclusions can be drawn about the impact co-contaminants will have in the

environment. Where septic tank effluent or treated wastewater is concerned, one needs to consider a plethora of compounds including surfactants and their breakdown byproducts, other pharmaceuticals, dissolved/colloidal organic matter, and numerous hormones and metabolites, all of which coexist in septic effluents and have been found in groundwater plumes as previously indicated..