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Table 1 Summary of BOD5 and COD results for tested deicers. Values as Neat Values as Primary Source Formulation of Oxygen Demand % COD BOD5 Primary COD BOD5 Formulation FPD (mg/kg) (mg/kg) Source (mg/kg) (mg/kg) Ethylene glycol Type I 92 1,180,000 492,000 EG 1,280,000 535,000 Ethylene glycol Type IV 64 826,000 331,000 EG 1,290,000 517,000 Propylene glycol Type I 88 1,420,000 990,000 PG 1,610,000 1,130,000 Propylene glycol Type IV 50 842,000 539,000 PG 1,680,000 1,080,000 Potassium acetate (liquid) 50 315,000 247,000 Acetate 1,050,000 821,000 Sodium acetate (solid) 96 700,000 571,000 Acetate 1,010,000 826,000 Sodium formate (solid) 98 242,000 --a Formate 373,000 --a a BOD test results for sodium formate deicer were not considered reliable estimates of potential BOD exertion in environmental situations due to apparent toxicity of the formulation to BOD seed organisms. at 20°C and 5°C in marine water and freshwater. findings in a format useful to chemical manufac- BOD5 and COD results are presented in Table 1. turers in producing more environmentally friendly Biodegradability was examined by comparing deicing formulations and to airport operators in eval- BOD and COD results. The degradation percentage uating alternatives to meet environmental compli- was between 40% and 82% for six of the formula- ance requirements. tions. Decay rates in the 40-day test indicate that degradation occurred more rapidly in the first 15 days Research Approach than during the rest of the test period. At least 78% degradation occurred for the six formulations over A tiered approach was taken to achieve this the 40-day test period. Results from sodium formate objective. Pure candidates and simple mixtures with PDM testing are not included owing to apparent water were tested in tier 1. Candidates that survived toxicity of this formulation to organisms in the tier 1 testing were subjected to tier 2 tests involving BOD seed. more complex mixtures. Freshwater and marine water test results were In tier 1, comparable in the 40-day tests conducted at 20°C. 1. Candidate FPDs, thickeners, surfactants, and Results of the 40-day freshwater tests at 5°C indicate corrosion inhibitors were identified with im- lower degradation than those at 20°C with 2355% proved environmental qualities compared to degradation for ethylene glycol products, 6177% components of commercial aircraft deicers degradation for propylene glycol products, and and anti-icers. 8694% for acetate-based products. Results of the 2. Laboratory analysis of the candidate compo- 5°C marine tests indicated that degradation was sig- nents were conducted for BOD and aquatic nificantly less in low temperatures with less than 10% toxicity. degradation for the propylene glycol ADF and AAF 3. Candidate components were down-selected and 69% degradation for the potassium acetate PDM. to identify a subset for use in building candi- date formulations. EVALUATION OF ALTERNATIVE ADF In tier 2, COMPONENTS AND FORMULATIONS 1. A series of testing and down-selecting of pro- The primary objective of this phase of the research gressively more complex mixtures was con- was to identify and characterize alternative deicer ducted to arrive at a final formulation that is formulations with reduced aquatic toxicity and BOD equivalent to, or better than, commercial for- that would be operationally and commercially viable. mulations in current use in terms of deicing per- ACRP also stressed the need to present the research formance and environmental characteristics. 5
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Table 2 Number of components and deicer formulations tested in tiers 1 and 2. Deicer Component Tier No. FPD Surfactant/Antifoam Corrosion Inhibitor Thickener Deicer Formulations 1 26 19 14 6 -- 2 2 3 2 3 Type IV AAF 1a Runway PDM a Evaluated with two anti-caking materials. 2. Candidate anti-caking agents were identified Two candidate anti-caking additives were also and tested in solid FPD formulations to eval- identified for use in sodium formate runway deicers, uate performance. and one candidate defoamer was identified for use in 3. The environmental characteristics of the final deicing formulations. formulations were determined. Upon review of the tier 1 results, there did not ap- pear to be substantial potential to improve BOD and Tables 2 through 4 summarize the tiered testing. aquatic toxicity in Type I fluids, nor did there appear The results of the tier 2 tests yielded a final selec- to be potential to improve upon BOD in PDMs as tion of components for complete Type IV formu- compared to the products in current use with the most lation development. Tests needed to certify the favorable environmental characteristics. For this rea- down-selected formulations, including deicing and son, ACRP directed the research team to focus the anti-icing performance and aircraft materials compat- tier 2 testing on Type IV aircraft anti-icing formula- ibility, were not undertaken as part of this research. tions and anti-caking additives for sodium formate runway deicers. Tier 1 Results A combination of molecular modeling, database Tier 2 Results searches, and literature searches were used to identify Type IV Aircraft Anti-Icing Formulations candidates for each of four major functional cate- gories of deicer components. The following numbers Table 5 lists the FPDs, surfactants, thickeners, of candidates were identified for further experimental and corrosion inhibitors that were evaluated during evaluation based on their performance characteristics tier 2 testing of Type IV aircraft anti-icing formula- related to their function in deicer formulations and tions. Based on the results of tier 1 testing, the FPDs their contributions to BOD and toxicity: that were selected showed improvements in COD, BOD, aquatic toxicity, or all three over propylene gly- · FPD: 27 candidates (5 aircraft, 12 runway, and 10 for both aircraft and airfield pavement). · Thickeners: 6 candidates. · Surfactants: 19 candidates. Table 4 Tier 2 tests for Type IV and runway PDM. · Corrosion Inhibitors: 14 candidates. Key Area Test/Evaluation Type IV AAF Table 3 Tier 1 deicing and anti-icing formulation tests. Deicing Surface tension (contact angle), performance viscosity, foaming Key Area Test/Evaluation Environmental Biological oxygen demand, impact aquatic toxicity Deicing performance Freezing point depression, Materials Total immersion and sandwich viscosity, contact angle compatibility corrosion testing (aluminum Environmental impact Biological oxygen demand, clad and anodized aluminum) aquatic toxicity Safety properties Flash point Runway Deicer Cost Supplier cost estimates Deicing performance Water absorption, anti-caking 6
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Table 5 Candidate components of Type IV aircraft anti-icing fluids evaluated in tier 2. FPD Surfactants Thickeners Corrosion Inhibitors Glycerol Tergitol L-64 Kalzan HP TEA Diethylene glycol (DEG) Tergitol TMN-10 K1A96 Mazon RI 325 Triton CG-110 with Carbopol EZ-4 with 10% Ridafoam NS 221 triethanolamine (TEA) col. Many surfactants tested in tier 1 had improve- mation of residues. For example, at 20°C a 90 wt% ments in toxicity over currently used surfactants. glycerol solution would partially freeze into a slurry Surfactants were selected to take advantage of these or highly viscous solution whereas a 90 wt% DEG toxicity improvements as much as possible while solution would still be completely liquid. Concentra- reducing the contact angle and surface tension to en- tion of FPD in this manner could potentially result sure that the formulations completely coat the air- in the accumulation of highly viscous glycerol craft surfaces. Thickeners were selected based on residues on the aircraft surface. For this reason, their aquatic toxicity and their ability to shear in a DEG was selected as the preferred FPD for the final manner similar to commercial Type IV anti-icing formulation. agents. Corrosion inhibitors were down-selected based on aquatic toxicity. Additive Selection Tier 2 experiments involved the testing of pro- Additives in the final formulation include Tergi- gressively more complex mixtures as compared to tol L-64 as the surfactant, triethanolamine (TEA) as tier 1: the corrosion inhibitor, and Carbopol EZ-4 as the · FPD + water + thickeners; FPD:water = 1:1 by thickener. These additives were chosen by consider- weight. ation of performance, aquatic toxicity, and cost. · FPD + water + surfactants; FPD:water = 1:1 by weight. Toxicity Results · FPD + water + thickeners + surfactants; FPD: Results from the stepwise toxicity testing show water = 1:1 by weight. · FPD + water + thickeners + surfactants + corro- how toxicity changed as each additional component was included in the formulation (Table 6). These sion inhibitors; FPD:water = 1:1 by weight. results were compared to theoretical results calcu- lated from the toxicity of the individual components FPD Selection and their concentrations in the mixtures, and assum- Diethylene glycol (DEG) and glycerol were con- ing no synergistic or antagonistic toxicity inter- sidered to be equally promising FPD candidates actions. In most cases, it was valid to assume that through the first three stages of the tier 2 down- individual component toxicity could be used to deter- selection process. The properties of the two are very mine formulation toxicity. In the instances where similar: glycerol's theoretical oxygen demand and this was not true, toxicity evaluation was more com- aquatic toxicity, as measured by Microtox® testing, plicated, requiring empirical observations to under- are lower than that of DEG, but the aquatic toxicity of stand which components were responsible for final DEG toward C. dubia and P. promelas is lower than formulation toxicity. Addition of the anti-foaming glycerol's. Mammalian toxicity is nearly identical for agent to the first formulation indicated a synergistic both candidates. What proved to be a differentiator interaction for all three organisms (the formulation was the freezing point characteristics for mixtures of was more toxic than the theoretical value). Simi- each candidate with water that suggested the higher larly, synergistic interactions in the Microtox and freezing point of glycerol solutions could cause oper- fathead minnow tests were observed from addition ational problems. Specifically, asymmetric evapora- of TEA in the final formulation. In addition, dif- tion of water from applied Type IV formulations may ferent interactions were observed depending on the result in concentrated FPD solutions on aircraft sur- FPD. Of particular interest was the difference be- faces. In the case of glycerol, this could result in for- tween addition of the thickener to DEG as opposed 7
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Table 6 Comparison of theoretical values with measured test results for stepwise Type IV anti-icing formulation construction. Theoretical Values Test Results Percent of Fathead Fathead Component Microtox C. dubia minnow Microtox C. dubia minnow Added to EC 50a LCb50 LC50 EC50 LC50 LC50 Added Component Mixture (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) (mg/l) DEG formulation Water 50 -- -- -- -- -- -- DEG 50 130,000 110,000 110,000 130,000 110,000 110,000 Carbopol EZ4 with TEAc 0.0763 130,000 66,000 110,000 140,000 71,000 120,000 (thickener) Tergitol L-64 (surfactant) 0.25 130,000 66,000 110,000 110,000 57,000 140,000 Ridafoam 0.025 12,000 66,000 110,000 25,000 25,000 59,000 (anti-foaming agent) TEA (corrosion inhibitor, 0.2 110,000 66,000 110,000 43,000 53,000 89,000 does not include Ridafoam)-- final formulation Glycerol formulation Water 50 -- -- -- -- -- -- Glycerol 50 260,000 70,000 92,000 260,000 70,000 92,000 Carbopol EZ4 with TEA 0.0552 260,000 70,000 92,000 43,000 66,000 71,000 (thickener) Tergitol L-64 260,000 70,000 92,000 18,000 75,000 63,000 (surfactant) Ridafoam 12,000 91,000 480,000 18,000 53,000 63,000 (anti-foaming agent) Screening toxicity results are only approximations. Screening toxicity procedures include fewer replicates, non-renewal of test solutions, shorter exposure duration for Pimephales promelas, and other procedural variances from definitive toxicity tests. Compounds are organized by least toxic to most toxic endpoint, determined by the most sensitive species. a The Microtox EC50 is the statistically determined concentration that would result in a 50% reduction in light emission compared to a labora- tory control. b The LC50 is the statistically determined concentration that would cause death in 50% of the population exposed. c TEA is triethanolamine. to for glycerol. In the DEG formulation, results were (Table 7). Acute toxicity endpoints ranged from similar to theoretical values, but a synergistic inter- 219 to 13,800 mg/l in tests with currently used for- action for toxicity was present in this step for the mulations, with each of the four tested products hav- glycerol formulation. This indicated that synergistic ing one or more of the three endpoints at least as low interactions for the same component were different as 528 mg/l. The lowest of the acute toxicity end- depending on the composition of the rest of the points in the final DEG formulation in this research formulation. In this case, it was only a difference in was 32,700 mg/l. FPD that caused a difference in the synergistic Chronic toxicity endpoints ranged from 79.4 to interaction. 1,350 mg/l in tests with currently used formulations, Results of definitive aquatic toxicity tests on the with each of the four tested products having one final formulation indicate that acute and chronic tox- or more of the three endpoints at least as low as icity endpoints were one to three orders of magni- 130 mg/l. The lowest of the chronic toxicity endpoints tude greater (i.e., the toxicity was lower) than results in the final DEG formulation from this research was previously published for formulations in current use 8,970 mg/l (Table 7). 8
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Table 7 Results from definitive aquatic toxicity testing of final Type IV DEG formulation (95% confidence interval). Acute Toxicity Chronic Toxicity Microtox C. dubia P. promelas C. dubia P. promelas S. capricorutum EC50a (mg/l) LC 50b (mg/l) LC50 (mg/l) IC 25c (mg/l) IC25 (mg/l) IC25 (mg/l) 54,900 32,700 126,000 8,970 60,200 42,100 (53,70056,100) (28,60037,400) (116,000136,000) (4,73013,800) (56,60062,500) (40,100-44,000) a The Microtox EC50 is the statistically determined concentration that would result in a 50% reduction in light emission compared to a labora- tory control. b The LC50 is the statistically determined concentration that would cause death in 50% of the population exposed. c The IC25 is the statistically determined concentration that would cause a 25% inhibition in growth (Fathead minnow) or reproduction (C. dubia). In currently used formulations, surfactants were were corroded and partially dissolved after two days identified as the component with the greatest influence in the environmental chamber, which eliminated this on toxicity. In the final formulation developed from candidate additive from further testing. Further this research, toxicity results indicated that the chosen experiments with sodium formate and mixtures of surfactant had little or no influence on toxicity. sodium formate and tripotassium citrate were con- tinued. The results showed that although tripotas- Biochemical Oxygen Demand Results sium citrate picked up a negligible amount of water, the mixtures of tripotassium citrate and sodium for- The concentration of COD in the final formula- mate picked up more water than the sodium formate tion was 752 g/kg with a relative standard deviation by itself, indicating that the tripotassium citrate pro- of 0.96%. These results are consistent with tier 1 test- duced a synergistic water absorption effect. No ready ing results on neat DEG (COD = 1,500 g/kg), con- explanation for these observations is available, and sidering that the final formulation contains 50% further testing was discontinued. DEG. The concentration of BOD5 could not be de- termined. Difficulties with seed acclimation to DEG encountered in tier 1 testing remained in tests with Degradation Pathways for Down-Selected the final formulation. Deicer Components Possible degradation pathways and degradation Runway Deicers products for the down-selected components of the The effectiveness of potassium carbonate and final Type IV formulation were examined to evaluate tripotassium citrate to prevent the caking of sodium the potential for significant environmental effects. formate granules was experimentally tested. The pro- cedure involved drying the components and mixtures Diethylene Glycol of the components in a desiccator, weighing the Diethylene glycol is the proposed FPD. Its mix- individual components and mixtures, and placing ture with water constitutes more than 90% of the sets of sample dishes in an environmental chamber at weight of the final deicing or anti-icing product. An constant temperature (30oC) and humidity (50%) for evaluation of potential biodegradation pathways different periods of time. The percentage of powder indicated that intermediate biodegradation products passing through a sieve was the metric used to eval- are fairly reactive and are thus are not expected to uate additive effectiveness. persist in the environment. No satisfactory anti-caking solution was found for sodium formate. The experimental results indi- Tergitol L-64 cated that mixtures containing the anti-caking addi- tives and sodium formate absorbed more moisture Tergitol L-64 is a nonionic ethylene oxide/ than the samples of sodium formate alone. Tests of propylene oxide copolymeric surfactant marketed as the individual components found that aluminum readily biodegradable. The degradation pathways sample dishes containing the potassium carbonate and products for the ethylene oxide portion of the 9