Designing Safer Chemicals - ACS Publications - American Chemical

Chapter 11

Designing an Environmentally Safe Marine Antifoulant G. L. Willingham and A. H. Jacobson

Downloaded by GEORGETOWN UNIV on August 27, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0640.ch011

Rohm and Haas Company, 727 Norristown Road, Spring House, PA 19477

Tin-based antifoulants came under regulatory advisement in the 1980's and we began to examine isothiazolones as an environmentally safe alternative to tin compounds. Our search generally followed the paradigm that environmental risk is a function of toxicity and exposure. We realized that it would be extremely difficult to design a marine antifoulant that was toxic to fouling organisms (e.g. tube worms and barnacles) but non-toxic to their related non-target organisms (e.g. mussels and oysters). Thus we looked for a compound that would have reduced exposure - a short environmental half-life and/or partition rapidly into a matrix of limited bioavailability. The results of this examination yielded 4,5dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI), a very efficacious compound with a half-life in a marine environment of less than one hour and limited bioavailability of metabolites due to their strong association with sediment.

Fouling is the result of the growth of a variety of marine plants and animals on submerged structures. The fouling of a ship's hull causes increased hydrodynamic drag, resulting in increased fuel consumption, decreased ship speed, increased costs to clean and service the vessel, and increased time out of service. The total annual cost due to fouling is estimated to significantly exceed a billion dollars with a major expense being the increased consumption of fuel (estimated at several hundreds of thousands of gallons) used to overcome the hydrodynamic drag. Organotin biocides are effective at preventing fouling, but environmental concerns have been raised regarding their use (7,2). Since the discovery of their ecotoxicological problems in the mid to late 1970's, many countries have banned the use of tin in antifoulants on ships less than 25 meters in length. Japan has effectively banned the use of tin as an antifoulant in all marine applications. In the United States, tin usage on vessels longer than 25 meters is restricted to formulations with a certifiable leach rate of less than 4 ng/cm /day. Worldwide, 2

0097-6156/96/0640-0224$15.00/0 © 1996 American Chemical Society

In Designing Safer Chemicals; DeVito, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

11.

WILLINGHAM & JACOBSON

Environmentally Safe Marine Antifoidant

more stringent restrictions and/or registration cancellations are certain to follow. New antifoulants are needed which are efficacious at preventing fouling, but which meet stringent ecotoxicological requirements. The ideal antifoulant must prevent fouling from a wide variety of marine organisms while causing no harm to non-target organisms. The environmental risk from the introduction of a new marine antifoulant active ingredient can be described by the following function:


Environmental Risk = f (Toxicity / Exposure) It would be extremely difficult to design an effective marine antifoulant that was efficacious against a wide range of organisms but non-toxic to the systematically related non-target organisms. Thus we sought to minimize environmental risk by developing an active ingredient with reduced environmental exposure. This reduced exposure could be expressed by rapid degradation/metabolism of the active ingredient and/or partition into a matrix such as the sediment which would limit the bioavailability. Experimental Biological Testing Against Fouling Organisms. Compounds were evaluated for their control of fouling by algae and barnacles and water solubility was estimated by membrane screening as described in Miller and Lovegrove (3). Biological testing against marine algae, bacteria, diatoms, and barnacle larvae was carried out as previously described (4). Aquatic Microcosms. The procedures used in these studies have been described in detail elsewhere (5). Briefly, sediment and seawater from the Chesapeake Bay (York River) were placed into Erlenmeyer flasks. In the anaerobic microcosm, anaerobic conditions were assured by pretreating the sediment and seawater with glucose in a stoppered flask for 30 days prior to dosing. Flasks were dosed with x! DCOI at a nominal dose of 0.05 ppm. The seawater and sediment were separated by centrifugation or filtration. The sediment was Soxhlet extracted with dichloromethaneimethanol (9:l/v:v) for 48 hours and then methanol for an additional 24 hours. The Soxhlet insoluble residue was characterized by separation into humin, humic acid and fulvic acid (6). Radiocarbon compounds detected in the Soxhlet solutions were chromatographically analyzed by HPLC as described previously (5). Structures of isolated metabolites were confirmed by GC-MS (Hewlett-Packard, Model 5985) or DCI-MS (Finnigan, Model TSQ 46). l3/1

Photolysis and Hydrolysis. Hydrolysis and photolysis studies were performed following the EPA prescribed methods (7). Fish Bioconcentration. The procedure used in determining the bioconcentration factor and quantitation of parent compound in bluegill sunfish has been presented previously (4).


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DESIGNING SAFER CHEMICALS

Dissipation Model. A model was employed in order to assess the fate and maximum concentration of DCOI that might theoretically occur in an aquatic marine environment following implementation of the compound as a marine antifoulant. Three harbors were chosen as example environments: New York harbor, San Diego bay, and Norfolk harbor. Modeling was accomplished using the Exposure Analysis Modeling System, EXAMS (Version 2.92) (8). Toxicology Testing. Detailed procedures of the toxicological testing appeared previously (9).


Results and Discussion Biological Testing Against Fouling Organisms. The isothiazolin-3-one class of compounds has been shown to have high activity against bacteria, fungi, and algae, and has demonstrated biological activity in some aquatic organisms. Additionally, biological degradation of these compounds was rapid and they showed a strong affinity for soil and sediment (10). Based on these results, we began to search for an isothiazolone that would provide broad spectrum control of marine fouling organisms, have sufficiendy low water solubility to provide long term control, bind tightly to the sediment to limit bioavailability, and degrade rapidly in the environment. A screening program was initiated to identify appropriate candidates which had excellent biological efficacy. In order to provide broad spectrum control, the antifoulant must have activity against both soft fouling organisms (e.g., algae, diatoms) and hard fouling organisms (e.g., barnacles). The screening model we used is shown in Figure 1. Drop No Antifouling Candidates

Aquatic Toxicology Tests

Selection of Lead Candidates

Lab/Field Tests (Algae, Diatoms, Barnacles)

Environmental Fate Tests • Degradation • Sediment Partitioning • Bioaccumulation

Yes

Test in Other Areas

Figure 1. Screening model for new antifoulants


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Environmentally Safe Marine Antifoulant

Membrane screening was carried out at Newton Ferrers, Devon, England. This technique evaluated control of algae and barnacle fouling and estimated the water solubility of the compounds. From this screening, the basic potential of a test compound as an antifoulant was assessed. Activity was noted as:


1) Effective - prevents major fouling organisms from settling and/or persisting; 2) Some - allows organisms to settle but in considerably reduced quantity compared to the non-toxic control; and 3) None - settlements are very similar to those of the non-toxic control. Over 140 isothiazolones were screened in field exposure tests. Test data was used to determine the structure-activity relationships for substitution at the 4 and 5 positions in the isothiazolone ring. The influence of halogenation at these positions on activity, particularly at the 5 position, was apparent (3). In addition, halogenation lowers the water solubility, which is essential to prevent rapid leaching from the antifouling paints. Further testing revealed the 2-(n-alkyl)-substituted isothiazolones to be particularly promising candidates. Results in Table I show the effect of the n-alkyl chain length on biological activity and water solubility of 4,5-dichloro-2-n-alkyl-4isothiazolin-3-ones. Based on the superior biological activity and low water solubility, 4,5-dichloro-2-n-octyl-4-isothiazolin-3-one (DCOI) was selected for further studies. Table I. Effect of n -alkyl Chain Length on the Biological Activity and Water Solubility of 4.5-dichloro-2-/i-alkvl-4-isothiazolin-3-ones a

Estimated Aqueous Alkyl Barnacle^ Solubility (ppm) Algae** -(CH ) -H ++ x=4 0 1000 ++ ++ x=6 120 ++ ++ x=8 2 ++ x=10 0 1 + x=12 0 1 x=14 0 0 1 Reproduced with permission from ref. 3. Activity; (++) Effective; (+) Some; (0) None. Solubility measured at 25°C by ultraviolet absorption analysis. 3

9

0

T

a

b

c


227


228


DCOI was tested against several major fouling organisms. Data has been presented elsewhere (4). Data against three of the major marine fouling organisms is presented in Table II. The marine diatom, Amphora coffeaeformis, is a major fouling organism. Many diatoms contribute to the formation of a slime layer and are major sources of ship fouling. DCOI was shown to be highly efficacious against Amphora coffeaeformis. The most ubiquitous marine fouling algae is the species Enteromorpha intestinalis. DCOI showed excellent activity against Enteromorpha intestinalis. The major hard fouling species are barnacles. Data shown in Table H demonstrate that DCOI activity against barnacle larvae was considerable, although it was significantly less than the activity against slime formers and marine algae. Based on the promising activity shown in these initial results, studies on the environmental fate and toxicology of DCOI were conducted. Table II. Efficacy of DCOI Against Marine Fouling Organisms Organism Amphora coffeaeformis Enteromorpha intestinalis Balanus amphitrite

Fouling class Marine diatoms Marine algae Barnacles (larvae)

L D (ppm) 3.4 x 102.0 x 103.4 x 105 0

3

3

1

Half-life in the Environment. The half-lives of DCOI in various environmental matrices are listed in Table HI. 3

Table III. Summary of Half-lives in Various Environmental Matrices Matrix Photolysis Hydrolysis pH5 pH7 pH9 Aerobic aquatic microcosm Anaerobic aquatic microcosm Water Seawater Synthetic seawater Synthetic seawater + algae Reagent water Reproduced with permission from ref.4.

Half-life (hrs) 322 216 >720 288 1 1 720 720

Biological degradation, as demonstrated in the aquatic microcosms, is over 200 times faster than hydrolysis or photolysis. The predominance of biological degradation is further demonstrated in the water samples. DCOI is essentially stable in synthetic seawater, whereas it degrades rapidly in seawater samples


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obtained from the Atlantic and Pacific Oceans, or synthetic seawater spiked with algae. DCOI was also stable in laboratory reagent water (from a Millipore Milli-QWater System) which is sterile. In addition, the half-life of DCOI was determined in natural seawater samples collected from coastal sites in New Jersey and California. The half-lives were all less than 24 hrs and were directly proportional to the microbial concentration (reference 9 and unpublished data from A. Jacobson). The faster half-life observed in microcosm compared to water is due to the sediment being naturally enriched in microbial activity (5). 14

Environmental Partitioning. The distribution of applied C-activity in the microcosms between the seawater and sediment (both Soxhlet soluble and insoluble fractions) has been documented elsewhere (5). After separation of the treated sediment and seawater phases over 92% of the C-activity, for both aerobic and anaerobic microcosms, was in the sediment (Soxhlet soluble and insoluble). A majority of the activity was associated with the Soxhlet insoluble residue. In the aerobic and anaerobic studies, C 0 increased throughout the study and by day 30 comprised 24% and 6% of the applied dose, respectively. The recovery of C label averaged 103.6 ± 10.8% and 88.1 ± 9.6% for the aerobic and anaerobic studies, respectively. Since the Soxhlet insoluble residue contained a majority of the activity, it was further analyzed by exhaustive extraction with 0.25 N HCl, followed by 1 N NaOH. The results appear in Table IV. Treatment with acid had virtually no effect on the bound residues. Overnight extraction with NaOH released a portion of the bound residues with practically all the base soluble residues being associated with the humic acid fraction rather than the fulvic acid fraction. However, even with the severe base treatment, over 50% of the bound residues were associated with the insoluble humin. Thus a large percentage of the applied C-activity is tightly bound to the sediment and probably not bioavailable. The activity is not parent compound since the application of DCOI to sterilized sediment demonstrated that over 90% of the applied activity could be removed by Soxhlet extraction (5).


14

14

2

14

14

Table IV. Exhaustive Extraction of Sediment Bound Residues 14

Day

Percent of Applied C Detected Soxhlet HCl Humin Insoluble Soluble

Anaerobic 0 21.0 Aerobic 0 53.4 30 60.3

Humic Acid Fulvic Acid

0.1

14.5

13.4

0.2

0.1 0.1

40.7 29.3

22.1 25.0

0.3 0.5

Metabolic Pathway of DCOI in the Aquatic Sediment. Metabolite identification has been presented elsewhere (4,5). A metabolic pathway is presented in Figure 2. The initial reaction has been shown to be cleavage of the N S bond by either nucleophiles (11,12) or photoisomerization (13). The N-methyl isothiazolones were shown to have a similar metabolic pathway in the environment (10).


229

230


O C 1

•

N

" ^N-(CH ) CH 2

C 1

^~

7

3

S

C H NHC(0)CH C0 H 8

17

2

2

N-(w-octyl) malonamic acid

DCOI

C Hi NHC(0)CH 8

7

3


N-(w-octyl) acetamide

• C H NHC(0)C0 H 8

17

2

N-(«-octyl) oxamic acid

•

C H NHC0 H 8

17

2

N-(w-octyl) carbamic acid

Figure 2. Metabolic pathway for DCOI in aquatic sediment/ Reproduced with permission from ref.4.

Concentration of DCOI in Fish. Since the logarithm of the octanol/water partition coefficient (log P) for DCOI is 2.85, very little bioaccumulation would be expected. In bluegill sunfish, less than 1% of the bioaccumulated C-activity in viscera and fillets was parent compound^). 14

Characterization of Fish Metabolites. As described previously (4), the metabolites are ring-cleaved compounds with a significant quantity being associated with proteins, especially cysteine residues. Toxicology in Nontarget Aquatic Organisms. A detailed discussion of the aquatic toxicology of DCOI has been presented previously (9). Summaries of the acute and subchronic results appear in Tables V and VI, respectively. From these two tables it can be concluded that DCOI is an acute toxin to marine organisms but not a chronic or reproductive toxin. The acute toxicity of DCOI is not surprising since the organisms tested are closely related to those which would foul the bottom of a ship's hull. It would be expected that an efficacious marine antifoulant would show such acute toxicities. Table V . Acute Toxicology of DCOI in Aquatic Organisms Species Bluegill sunfish Daphnia magna Bay mussel - adult American oyster - embryo Fiddler crab Mysid shrimp S. costatum

Test 96 hrLC50 48 hrLC50 96 hrLC50 48 hrLC50 96 hr LC50 96 hrLC50 96 hrLC50

ppb 14 9 850 24 1312 5 20


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Table VI. Subchronic Toxicology of DCOI in Aquatic Organisms Species Sheepshead minnow (Early life stage) Daphnia magna (21 day chronic)


a

ppb 6 14 9 1 0.63 1 0.83

a

Test NOEL LOEL MATC EC 0 NOEL LOEL MATC 5

NOEL, No Observable Effect Level; LOEL, Lowest Observable Effect Level; and MATC, Maximum Allowable Toxic Concentration.

Modeling Study. EXAMS, a computerized mathematical modeling program (8), was employed to study the environmental fate and dissipation of DCOI contained in a coating painted on ship bottoms. The three harbors chosen, New York harbor, San Diego Bay and Norfolk harbor, represent some of the busiest harbors in the world. The environmental parameters appearing in Table VII were taken from previous publications (4,5,9). Table VII. Environmental Fate Inputs for EXAMS Model Parameter Water Solubility Degradation Seawater Sediment Photolytic Bioconcentration Factor Adsorption Coefficient Vapor Pressure

End Point 4.7 ppm 24hrs 1 hr 322 hrs 13.6 (DCOI) 1666 7.4 x 10" torr 6

The results of the modeling study are presented in Table VflX The leach rate of DCOI from a painted surface was determined from submersion of painted panels. A representative high end value was 1.0 u\g DCOI/cm painted surface/day. The values obtained in the water column were at least 1.5 orders of magnitude less than the MAEC (Maximum Acceptable Environmental Concentration) of 0.63 ppb based on the Daphnia magna subchronic study. The major contributing factor to the variation in DCOI concentration predicted in the harbor is due to hydraulic flushing. San Diego Bay is an enclosed harbor with very little fresh water influx, and flushing is accomplished primarily by tidal action. New York harbor, on the other hand, has a tremendous amount of fresh water influx coming from the Hudson and East rivers. 2


232


Table VIII. DCOI Harbor San Diego


New York Norfolk

Results from the Environmental Dissipation Model for

Leach Rate |Ltg/cm7day 0.1 1.0 5.0 0.1 1.0 5.0 0.1 1.0 5.0

DCOI Detected Water (ppt) 0.9 9.4 47.0 0.07 0.7 3.6 0.5 5.0 25.0

Sediment (ppb) 1.8 0.2 4.6

Comparison of DCOI and Tributyltin oxide (TBTO). TBTO has been used as an antifouling agent for decades. It has recendy come under regulatory scrutiny due to toxicological concerns (14). The results in Table DC present a comparison of the environmental fate and toxicology of DCOI and TBTO. The values for TBTO are representative averages from a World Health Organization publication (14). The results in Table IX demonstrated that unlike DCOI, TBTO is persistent in the environment due to its long degradation time and will bioaccumulate. While both compounds are toxic to nontarget marine organisms, TBTO has been shown to be a reproductive toxin. The predicted concentration of TBTO in the environment was determined by the EXAMS model described above. The predicted concentration of TBTO in the environment, 0.01 to 1 ppb, could be in excess of the MAEC of 20 ppt based on a United Kingdom water quality standard (14). Table IX. Comparison of the Environmental Fate of DCOI and TBTO Property DCOI Half-life Seawater

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