Y.L. Lau

Fate and ecotoxicity of the new antifouling compound irgarol 1051 in the aquatic environment

Residue analyses and ecotoxicity assessment were conducted on the new antifouling compound Irgarol 1051 (2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine) and its degradation product M1 (2-methylthio-4-tert-butylamino-6-amino-s-triazine) in order to delineate the environmental fate and impact of Irgarol 1051 on the aquatic ecosystem. For the first time, the Irgarol degradation product (M1) was positively identified in environmental samples. During the 1998 Irgarol survey, concentrations of M1 (up to 1870 ng/l) were generally higher than those of Irgarol in the coastal waters of the Seto Inland Sea in Japan, suggesting a greater environmental persistence for M1 than for the parent compound Irgarol 1051 in the aquatic ecosystem. Ecotoxicity testing revealed that Irgarol 1051 and M1 were moderately toxic to a marine bacterium and the four crustaceans tested, but were highly toxic to some algae and higher plants. In the root elongation inhibition bioassay, M1 showed a phytotoxicity at least 10 times greater than that of Irgarol and six other triazine herbicides (terbutryn, terbutylazine, terbumeton, simetryn, atrazine and simazine). These results strongly suggest that both Irgarol 1051 and its persistent degradation product M1 may potentially affect and/or damage the primary producer community in aquatic ecosystems. To safeguard the aquatic ecosystem from the damaging impact of micro contaminants, it is recommended that, besides monitoring for the target parent compound, major degradation products should also be included in environmental surveys. Otherwise, there is a risk of underestimating the ultimate impact of a particular toxicant on the environment.

Transformation of the new antifouling compound Irgarol 1051 by Phanerochaete chrysosporium

Irgarol 1051, 2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine, is a newly developed herbicidal additive for use in copper-based antifouling paints. It is intended to replace the antifouling agent tributyltin, which has been regulated internationally due to its severe impact on the aquatic ecosystem. However, there is no information in the open literature on the persistence and degradation of Irgarol, a fact that hinders the assessment of its ultimate impact on the environment. This study showed that the white rot fungus Phanerochaete chrysosporium was capable of biotransforming Irgarol 1051. It appears that the metabolism of Irgarol by the fungus proceeds mainly via partial N-dealkylation. Metabolic dealkylation occurs at the cyclopropylamino group resulting in metabolite M1, which has tentatively been identified as 2-methylthio-4-tert-butylamino-6-amino-s-triazine. M1 appeared to be a stable and/or terminal metabolite. No evidence of the heterocyclic ring cleavage of Irgarol 1051 was observed, thus implying a possibility of its degradation product(s) accumulating in the environment.

Effect of flow rate on biofilm accumulation in open channels

Abstract Experiments were carried out to study the effect of flow shear stress on the rate of accumulation of bottom-attached biofilms. Natural harbour water without special nutrient enrichment was circulated through an outdoor open channel, simulating natural river conditions. Biofilm development on small glass plates (15 × 25 mm) placed on the bottom of the channel was monitored through microscopic examination and determination of total carbohydrate by a recently developed biochemical method. Results indicate that biofilm biomass accumulation was substantially reduced as flow shear stress increased and that the maximum accumulation occurred under very low flow conditions.

Influence of antecedent conditions on critical shear stress of bed sediments

Experiments were conducted in an annular flume using kaolinite clay and bed sediment collected within an industrial boat slip of Hamilton Harbour to assess their stability and transport characteristics. The critical shear stresses required to erode the sediment were measured under different conditions for bed formation. It was discovered that the way in which the bed deposited (i.e. deposited under quiescent conditions or under shear) had a strong influence on the stability of the bed. The critical shear stress for beds deposited under shear was up to eight times larger than that for beds deposited under quiescent conditions. The results suggest that modeling efforts which do not take into account the history of the bed formation may seriously underestimate the bed strength and as such, can result in erroneous predictions of sediment and contaminant source, fate and effect.

Mercuric chloride-catalyzed hydrolysis of the new antifouling compound irgarol 1051

Irgarol 1051, 2-methylthio-4-tert-butylamino-6-cyclopropylamino-s-triazine, is a newly developed herbicidal additive for use in copper-based antifouling paints. It is intended to replace the antifouling agent tributyltin, which has been regulated internationally due to its severe impact on the aquatic ecosystem. However, there is no information in the open literature on the abiotic degradation of Irgarol, a fact that hinders the assessment of its ultimate impact on the environment. This study showed that mercuric chloride was capable of rapidly catalyzing the hydrolysis of Irgarol 1051 in distilled water and buffer solutions. The degradation appeared to follow the reaction of a catalyzed hydrolysis and was not significantly affected by the pH tested (5 to 9). All other 5 heavy metal salts tested (AgNO3, CdCl2, CuSO4, PbCl2 and ZnCl2) had practically no catalytic property on Irgarol hydrolysis, implying the involvement of a specific activity for Hg2+ in this reaction. The mechanism for the catalyzed hydrolysis may be the formation of bidentate chelation through nitrogen (No. 5) on the ring and the nitrogen on the cyclopropylamino side chain in Irgarol 1051 with the Hg2+ ion. The resulting four-member chelate complex would weaken the cyclopropyl-amino bond considerably, thus facilitating the hydrolysis reaction. Ultraviolet spectroscopy of the reaction mixtures and the identification of Irgarol hydrolysis product M1 (2-methylthio-4-tert-butylamino-6-amino-s-triazine) by GC-MS and LC-MS provided the basis for the proposed mechanism on the HgCl2-catalyzed hydrolysis of Irgarol 1051. M1 appeared to be more stable than the parent compound Irgarol 1051, thus implying its possible accumulation in the environment. One practical aspect of this work is that HgCl2 should not be used in preserving water samples in Irgarol 1051 monitoring programs.

Characterization of biofilm development on artificial substratum in natural water

Abstract In aquatic systems with large specific areas (bottom surface area/volume of water), such as shallow streams or rivers, the growth of biofilms often determines the rate at which environmental contaminants are removed and degraded. Thus, understanding biofilm growth is critical in predicting the ultimate fate of chemicals in the aquatic environment. Through sequential acid hydrolysis and HPLC analysis of biofilms, bacteria and algae, our study provided the first direct biochemical evidence that the origin and mass of a biofilm was mainly derived from bacterial activity, thus justifying the approach of using the major carbohydrate component of microorganisms to approximate the mass of a biofilm. Our data also demonstrated the potential applicability of the carbohydrate method for biofilm mass estimation in various aquatic environments, as biofilm mass can be readily determined in the presence of seawater and hard water.

Modelling the consumption of dissolved contaminants by biofilm periphyton in open-channel flow

Abstract An analytical model of the decrease in dissolved contaminant concentration in an open channel flow through the consumption by a botton biofilm is presented. This idealized model considers the flux of the contaminant by diffusion through the concentration boundary layer into the biofilm, with diffusion and reaction within the biofilm. Solutions for zero-order as well as first-order kinetics are derived which show that the rate of change of the concentration in the main flow is directly related to the kinetics in the biofilm. The thickness of the concentration boundary layer is shown to have a significant influence on the rate of decrease of concentration in the main flow.

Sequential erosion/deposition experiments: Demonstrating the effects of depositional history on sediment erosion

Experiments on the erosion of a bed of kaolinite were carried out in a rotating circular flume. Each experiment was carried out using the stratified bed which resulted from the previous experiment. Changes in suspended sediment concentrations during the experiments were explained by the history of the deposition. The sequence of experiments showed how the rate of erosion and the amount eroded reflected the structure of the bed and that of the individual flocs which created it. Results suggest that modelling of sediment/contaminant transport needs to account for the manner in which deposition took place.

Factors affecting chemical biodegradation

Microbial degradation is one of the most important processes responsible for the removal of chemical contaminants from the environment. Since the aquatic compartment is frequently the ultimate depository for many man‐made substances, there is a need to understand factors that control and/or affect the rate of biodegradation for chemical substances in the aquatic environment. In this study, several priority chemicals encompassing various biocides (2,4‐dichlorophenoxy acetic acid, carbaryl, fenitrothion, pentachlorophenol) and a nitroaromatic (2,4‐dinitrotoluene) were assessed for their biodegradability in cyclone fermentors under aerobic and anaerobic conditions, with and without co‐metabolites. Among those factors investigated, aerobic and anaerobic conditions, availability of co‐metabolites, and pre‐exposure of microorganisms to the test chemical were found to be the most significant elements in controlling the rate of biodegradation. Other factors (e.g., acclimation period) requiring attention in calculating the rate of biodegradation were also discussed. © 2000 John Wiley & Sons, Inc. Environ Toxicol 15: 476–483, 2000

Volatilization of metolachlor from water

Abstract The volatilization of metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl)‐N‐(2‐methoxy‐l‐methylethyl)acetamide]from water was studied in the laboratory and in an outdoor open‐channel experiment. As expected, volatilization was not significant at temperatures ≤ 25 °C. However, at temperatures ≥ 30 °C, there was significant volatilization (e.g., half‐life of 20 days at 40 °C in unstirred solutions). This increased volatility reflected the rapid increase of the Henrys law constant with temperature. Additional experiments indicated that aeration of water significantly accelerated volatilization losses. Such air‐stripping may be important in very turbulent streams and rivers and when water flows over hydraulic structures such as weirs. The experiments reported here indicate the importance that ecosystem‐specific characteristics can have on the persistence of environmental contaminants.