BRONOPOL - ENVIRONMENTAL SAFETY

The increasing awareness and concern over the impact of chemicals on the environment has created a demand for well researched and safe antimicrobial products.  The development of Bronopol from its original pharmaceutical and cosmetic uses into the industrial sector as Myacide AS (95% Bronopol) led to BASF MicroCheck Limited being awarded the Pollution Abatement Technology award in 1985 in the UK.  In order to maintain these high safety standards, BASF MicroCheck Limited is continually updating its environmental data package on the activity of Bronopol, also known as 2-bromo-2-nitropropane-1,3-diol or BNPD.

Abiotic Degradation

Bronopol at recommended use rates is sufficiently stable to provide effective control of microbial problems in Industry but has been shown to be readily broken down in the environment.  Several factors such as higher temperatures, higher pH and light are known to increase the rate of degradation of Bronopol.

In an early study, the decomposition of aqueous solutions of Bronopol was found to be more rapid at elevated temperatures and under alkaline conditions.  The half life of Bronopol at 22°C was found to be 4 months at pH8 compared to >5 years at pH6 and pH4.  However, this study was carried out under darkened conditions and at a high concentration of 300ppm in distilled water.  The results can be seen in Table 1 below.

Table 1 - Half-lives of Bronopol (300ppm)

	pH 4	pH 6	pH 8
5°C   22-25°C   30°C   40°C   60°C	> 5 Years   > 5 Years   -   > 2 Years      2 Weeks	> 6 Years     6 Years   > 4 Months     4 Months          36 Hours	6 Months   4 Months         14 Days           8 Days           3 Hours

In a recent study nominal Bronopol concentrations of 5 and 15ppm at pH7 were stored at ambient temperature and 40°C.  Both concentrations gave reduced half-lives of approx 20 hrs at ambient temperature and approximately 2 hrs at 40°C.

The increase in Bronopol degradation at lower concentrations was confirmed in a marine degradation study.  Bronopol at 300ppm showed rapid initial degradation which reached a near equilibrium state, before reaching 50% degredation. In contrast, lower concentrations of 50 and 20ppm showed half lives of 13 and 8 hours respectively.

The importance of light on the decomposition of Bronopol was demonstrated when 0.02% aqueous solutions (200ppm) of Bronopol at pH6 were stored for one month at 30°C.  In darkened conditions 72% activity was retained but in the light only 5% activity remained.  The degradation of Bronopol was also found to be more rapid in tap water than in distilled water.

In a photodegradation study, a dilute (5ppm) aqueous solution of Bronopol showed a short half life of approximately 24 hours under conditions of continuous light, at a pH of 4 and at 25°C.  Only 13% of the Bronopol remained after 3 days and degradation was complete at 7 days.  The main breakdown products were tris(hydroxymethyl)nitromethane, carbon dioxide and an unidentified, relatively polar component.

In a study by Challis and Yousaf (1990), 12 breakdown products were identified from four degradation pathways (see page 5).  Although the route of decomposition is complicated, all the end products are simple, naturally occurring substances.

Heavy Metal Content

Tests on the trace metal content of Bronopol supplied by BASF MicroCheck Limited have shown levels below the limits of detection for lead and antimony (3ppm) and for other heavy metals such as cadmium and mercury (1ppm).  A combined contamination of total heavy metals is unlikely to exceed 10ppm which at typical use rates of 200ppm of Bronopol would result in less than 2ppb of total heavy metals.

Adsorbable Organic Halogen (AOX)

The adsorbable organic halogen (AOX) content was determined for Bronopol according to the German method DIN 38409 (H14).  An aqueous solution of 20mg/l of Bronopol resulted in a calculated AOX value of 1.6mg/l.  At typical application rates, Bronopol is unlikely to contribute significantly to the AOX threshold levels set by government authorities.

In addition, the DIN 38409 (H14) procedure stipulates the use of acidic conditions thus preventing the normal abiotic degradation of Bronopol where organically bonded bromine is converted to inorganic bromide.  Consequently the AOX value produced by this method is likely to be an over-estimate, as demonstrated in a recent analysis of a cooling tower system, which showed no AOX contribution from Bronopol.  (See PI – PBP 09)

Bronopol and Dioxins

The term “dioxins” encompasses a large group of closely related compounds known as polychlorinated dibenzofuran’s (PCDF’s) and polychlorinated dibenzo-p-dioxins (PCDD’s).  They are by-products in the manufacture of commercial chemical products such as chlorinated phenols and polychlorinated biphenyls and may be formed during the incineration of industrial waste.  Dioxins are potent teratogens and carcinogens in animals and may cause chloracne, a severe and persistent skin condition.

A study to investigate the possible existence of trace dioxin levels in Bronopol, showed that sample material analysed contained no dioxins at a limit of detection of 0.5ppb.

Biodegradation

In the standard 28 day closed bottle, ready biodegradability test (OECD guideline 301 D), Bronopol at 3 and 6ppm was not biodegraded by the bacteria used in the test.  This is not unexpected as this test procedure was not designed to deal with bactericides.

In a more meaningful activated sludge study, Bronopol at concentrations up to 50ppm showed no suppression of oxygen uptake by a wide range of organisms present in the sludge flocs.  Higher concentrations of 100 to 150ppm led to a suppression of the rate of removal of organic carbon from 80% to 20% over a three week period.  The lower rate of removal of organic carbon reflected a reduction in the number of sludge organisms.

In an additional test carried out by the North West Water Authority in England, Bronopol at 40ppm showed no significant suppression of oxygen uptake, sludge floc formation or the microscopic animal population.  A higher rate of 100ppm led to complete suppression of the activated sludge process.  In a further test, Bronopol at up to 15ppm had no significant effect on the anaerobic digestion of sewage sludge.  More recently, the effect of Bronopol on the inhibition of activated sludge respiration has been examined using OECD guideline 209.  The respiration rate of micro-organisms was measured under defined conditions, to different concentrations of Bronopol, over a 2.5 hour exposure period.  The EC₅₀ of Bronopol to activated sludge respiration was estimated to be 43mg/l with the EC₂₀ and EC₈₀  estimated  at 2 and 887mg/l respectively.

In conclusion, the amount of Bronopol reaching sewage treatment plants is considerably diluted by other sources of effluent resulting in a concentration of Bronopol that provides a safety factor to the sewage treatment process of more than 1000 times.

The inherent biodegradability of Bronopol was clearly demonstrated in a modified Zahn-Wellens test where 50% degradation of Bronopol had occurred by day 45 (see Figure 1).  However, the inoculum (100mg/l) did not undergo prior acclimatisation.  The apparent, initial negative degradation was due to the increase in dissolved organic carbon resulting from inoculum cell lysis.  The surviving inoculum was able to use this lysed cell material as well as Bronopol.

Figure 1 - Percent Degradation Levels for the Test Substance (Zahn-Wellens Test)

In an aerobic aquatic degradation study (modified OECD 304A and 302B) using ¹⁴C Bronopol, Bronopol at 1 ppm was shown to be completely utilised by the microorganisms.  At 21 days, over 80% of the ¹⁴C was present either as CO₂ or in the biomass.  Bronopol itself was completely degraded by day 3 and one major metabolite (probably trishydroxymethylnitromethane or 2-nitropropan-1,3-diol) was formed, which was also completely degraded by day 17.  The results can be seen in Figure 2.

Figure 2 - Time-course of Metabolism of ¹⁴C-Bronopol - Non-sterile Treated

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