Bioaccumulation & Biodegradation

It is now accepted that the commonly used phthalates are all readily biodegradable and do not bio-magnify up the food chain.

Bioaccumulation
Bioaccumulation is the accumulation of a substance by an aquatic organism through exposure to that substance in its aqueous environment and the diet, as characterized by the bioconcentration factor (BCF) in the laboratory, or the bioaccumulation factor (BAF) in the field. Biomagnification is the bioaccumulation of a substance up the food chain through ingestion of lower level organisms containing the substance by higher level organisms.

Bioaccumulation can be assessed using different measures including BAF and BCF. Some quantitative modeled relationships between hydrophobicity as measured by log Kow, have been developed. Low hydrophobicity is associated with a low Log Kow and conversely high hydrophobicity is associated with high Log Kow. Log Kow criteria based on these models are being used as first tier criteria for bioconcentration.

A large number of bioconcentration factors (BCF) computed from several early studies are fundamentally flawed because radiolabelled phthalates were used and total radioactivity in the organism was divided by the radioactivity found in the water at the end of the experiment. The radioactivity in the water often declined by several orders of magnitude during the course of the experiment and this was not taken into consideration. Moreover, results were often based on total radioactivity without distinguishing between the phthalate and its metabolites which, as all living creatures process their food in this way, have been transformed by metabolic processes into fish tissue. 

Reliable bioconcentration studies that maintain constant exposure concentrations over the course of the experiment indicate a much lower, more consistent range of BCFs for low phthalates. Examples include around 120 for DEHP (level in fish is 120 times the level in water) in the carp and 9.4 for BBP in bluegill sunfish1.  A comprehensive critical review of phthalate bioaccumulation literature is provided in the review paper of Staples et al2, and supports that phthalates are not bioaccumulative. For substances like the high phthalates, in which aqueous studies are difficult to perform, studies where exposure through diet is analysed may be performed. However, field studies of biomagnification up the food chain still provide the most conclusive evidence of bioaccumulation potential(1).

A recent paper presents a field study measuring the concentration of phthalate esters (PEs) in organisms occupying different positions in the food chain (trophic levels) of a marine food web(2). This type of study allows for the assessment of whether or not a compound is biomagnifying from the bottom to the top of the food web. Biomagnification results in increasing concentrations of a substance in organisms of increasing levels in the food chain. Thus, if a compound biomagnifies, organisms higher in the food chain will display increasing body burdens of the compound. In this study, 13 different individual PEs, or isomeric PE mixtures, including DINP and DIDP, were analyzed in samples of plankton, macroalgae, benthic invertebrates, various fish species, and marine birds. In addition to PEs, polychlorinated biphenyls (PCBs) were also analyzed to provide benchmark data for a chemical class that is known to bioaccumulate, and enable comparison with the results of the PE analysis. PCBs were found to biomagnify up the food chain in a clear manner. That is, organisms higher in the food chain, such as fish, had higher concentrations of PCBs than organisms lower in the food chain, such as plankton.

Relationships between PE lipid equivalent concentrations (how much PE is in the organism) and trophic level were not statistically significant for PEs with alkyl chains of less than 8 carbons. The lipid equivalent concentration of several phthalates, including DEHP, DINP and DIDP, significantly declined with increasing trophic position in the food web. This indicates that these phthalates do not biomagnify up the food chain.

Biodegradation
Environmental fate and specifically the term ‘persistence’ refers to the length of time a substance remains in the environment as measured by its degradation rate in various compartments (i.e., air, water, soil, sediment). Persistence is quantitatively specified by a half-life, which is the time it takes a substance to degrade by 50%. Degradation can be mediated by biological and/or physical processes, but is typically quantified using standard testing procedures that measure the extent of biodegradation over time as mediated by microorganisms obtained from a wastewater treatment plant (WWTP).

A review encompassing 18 commercial phthalate esters2 shows that phthalates undergo >50% ultimate degradation within 28 days in standardised tests (e.g. OECD 301 series), and that primary degradation half-lives in surface and marine waters range from <1d to 2 weeks. Five European Risk Assessment Reports (DBP, BBP, DEHP, DINP and DIDP) report that each of these phthalates are readily biodegradable, confirming the data reported by Staples et al.(3)

References
1.    Gobas, FAPC, de Wolf, W. Burkhard, LP, Verbruggen, E, and Plotzke, K (2009), Revisiting Bioaccumulation Criteria for POPs and PBT Assessments. Integrated Environmental Assessment and Management. 5, 624-637.
2.    Mackintosh CE, Maldonado J, Hongwu J, Hoover N, Chong A, Ikonomou MG, Gobas FA. 2004. Distribution of phthalate esters in a marine aquatic food web: comparison to polychlorinated biphenyls. Environ Sci Technol 38:2011-2020.
3.    Staples, CA., Peterson DR, Parkerton TF and Adams WJ, 1997, “The Environmental Fate of Phthalate Esters : A literature Review”, Chemosphere 35, 667-749.