Alternative Aircraft Anti-Icing Formulations with Reduced Aquatic Toxicity and Biochemical Oxygen Demand (2010)

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6-1 SECTION 6 DEGRADATION PATHWAYS FOR DOWN- SELECTED DEICER COMPONENTS Although each candidate component was carefully screened for environmental impact, it is possible that their degradation products could have significant effects. For example, nonylphenol ethoxylate surfactants have a much lower environmental impact than does nonylphenol, one of their degradation products. Possible degradation pathways and degradation products for the down-selected components of the final Type IV formulation were examined to evaluate the potential for significant environmental effects. DEG DEG is the proposed FPD. As such, its mixture with water will constitute more than 90 percent of the weight of the final deicing or anti-icing product. The molecular structure of DEG is: HO-CH2-CH2-O-CH2-CH2-OH Pearce and Heydman (39) examined the biodegradation of DEG by various strains of the Psedomonas bacteria species. They proposed that the biodegradation of DEG by Acinetobacter S8 begins with a dehydration reaction that forms ethylene glycol monovinyl ether: HO-CH2-CH2-O-CH=CH2 + H2O (6-1) This glycol ether product further degrades to ethylene glycol and vinyl alcohol: HO-CH2-CH2-OH + HO-CH=CH2 The ethylene glycol product undergoes an additional dehydration reaction, similar to Equation 1, to yield water plus another vinyl alcohol molecule. These vinyl alcohol molecules then react to form acetaldehyde: O=CH-CH3 Acetaldehyde degrades into ethanol and acetic acid: CH3-CH2-OH + CH3-C(O)-OH Both of these products are consumed by common bacterial metabolism and are thus not expected to persist in the environment. The other intermediate degradation products are fairly reactive and are thus also not expected to persist in the environment. The University of Minnesota Biocatalysis/Biodegradation Database (UM-BBD) (40) does not contain a degradation pathway for DEG. However, its rule-based expert system was able to suggest the following possible pathway: In the first step, one of DEG’s primary alcohols is converted by dehydrogenation to an aldehyde: HO-CH2-CH2-O-CH2-CH=O

ALTERNATIVE AIRCRAFT ANTI-ICING FORMULATIONS 6-2 The aldehyde is then converted to a carboxylate: HO-CH2-CH2-O-CH2-C(O-)=O These two reaction steps are repeated for DEG’s remaining hydroxide to produce a dicarboxylate: O=(-O)C-CH2-O-CH2-C(O-)=O This dicarboxylate degrades at the ether linkage into an alcohol and an aldehyde, specifically a glycolate and a glyoxylate: O=(-O)C-CH2-OH + O=CH-C(O-)=O Both of these products are consumed by common bacterial metabolism and are thus not expected to persist in the environment. The other intermediate degradation products are fairly reactive and are thus also not expected to persist in the environment. Tergitol L-64 Surfactant biodegradation is typically classified into primary degradation and ultimate degradation. Primary degradation occurs when the surfactant’s molecular structure has been sufficiently changed such that it no longer exhibits surface activity. Ultimate biodegradation occurs when the surfactant’s molecular structure has been changed into carbon dioxide, water, mineral salts, and biomass. The focus in this section is on the reaction pathways that lead to the ultimate biodegradation of Tergitol L-64. Tergitol L-64 is a non-ionic ethylene oxide/propylene oxide copolymeric surfactant. It is marketed by Dow Chemical as being readily biodegradable. An example structure is shown in Equation 6-2: [(-CH2-CH2-O)n-(CH(CH3)-CH2-O-)m] (6-2) The values of n and m are proprietary but probably range from 6 to 20. The ethylene oxide portion of the surfactant degrades by forming shorter glycol ethers, including ethylene glycol and DEG [5]. The degradation pathways and products should thus be the same as those described previously for the degradation of DEG. The degradation mechanism for the propylene oxide portion of the surfactant’s molecular structure is less certain. There is general agreement that the branching of this portion inhibits biodegradation [5]. One possible mechanism is the oxidation of an end group to propionaldehyde [6]: CH3-CH2-CHO Propionaldehyde is then further oxidized to propanoic acid [6]: CH3-CH2-CO-OH An alternative mechanism would result in the production of acetone [6]: CH3-CO-CH3 Table 6-1 lists the aquatic toxicities for these possible degradation products to the rainbow trout (oncorhynchus mykiss).

SECTION 6—5BDEGRADATION PATHWAYS FOR DOWN-SELECTED DEICER COMPONENTS 6-3 TABLE 6-1. Toxicity of Tergitol L-64 degradation products. Compound 96-hour LD50 Rainbow Trout 1 Propionaldehyde 5 mg/L a 2 Propanoic Acid 51 mg/L 3 Acetone 4.4 mg/L aThis value is for 24 hours. Table 6-1 indicates that the degradation products are fairly toxic to rainbow trout. It is also possible that these products are toxic to bacteria, which could partly account for the reported poor biodegradation. TEA TEA is the proposed corrosion inhibitor. It would typically be used in formulations at less than 2 percent by weight. TEA’s molecular structure is: N(CH2-CH2-OH)3 Frings et al. showed that TEA can be completely degraded to acetate and ammonia under anaerobic conditions (41). UM-BBD also lists a degradation pathway for TEA (40). The product of the first degradation step is the unstable hemiaminal intermediate: (HO-CH2-CH2)2N(CH(OH)-CH3) This intermediate will degrade into diethanolamine and acetaldehyde: (HO-CH2-CH2)2NH + O=CH-CH3 Diethanolamine will undergo the same degradation steps to produce ethanolamine and acetaldehyde: HO-CH2-CH2NH2 + O=CH-CH3 UM-BBD reports that both of these products are consumed by common bacterial metabolism and are thus not expected to persist in the environment. Table 6-2 shows the aquatic toxicity of TEA and its degradation products to Pimephales promelas. The data indicate that the aquatic toxicity of the degradation products is much higher than that of TEA. Carbopol EZ-4 Carbopol, the proposed thickener, is a lightly crosslinked poly(acrylic acid) polymer. Studies conducted by Lubrizol found that Carbopol does not biodegrade (42). These studies also showed that Carbopol does not pass through municipal waste water treatment facilities into lakes and rivers but adsorbs onto biomass and is therefore removed with the biomass during treatment.

ALTERNATIVE AIRCRAFT ANTI-ICING FORMULATIONS 6-4 TABLE 6-2. Toxicity of TEA and its degradation products. Compound 96-hour LD50 Pimephales Promelas 1 TEA 11,800 mg/L 2 Diethanolamine 1,370 mg/L 3 Ethanolamine 2,070 mg/L 4 Acetaldehyde 36.8 mg/L