Chemical behavior of phthalates under abiotic conditions in landfills

The phthalates comprise a family of phthalic acid esters that are used primarily as plasticizers in polymeric materials to impart flexibility during the manufacturing process and to the end product. It is estimated that the annual worldwide production of phthalate esters exceeds five million tons. Plasticizers are one of the most prominent classes of chemicals, but unfortunately, they possess endocrine-disrupting chemical properties. As endocrine-disrupting chemicals, plasticizers have produced adverse developmental and reproductive effects in mammalian animal models.Phthalates are easily transported into the environment during manufacture, disposal,and leaching from plastic materials, because they are not covalently bound to the plastics of which they are a component. Because of their fugitive nature and widespread use, the phthalates are commonly detected in air, water, sediment/soil, and biota, including human tissue. Large amounts of phthalic acid esters are often leached from the plastics that are dumped at municipal landfills.Phthalate esters undergo chemical changes when released into the environment.The primary processes by which they are transformed include hydrolysis, photolysis,and biodegradation. It is noteworthy that all of these degradation processes are greatly influenced by the local physical and chemical conditions. Hence, in the present review, we have sought to ascertain from the literature how the phthalate esters undergo transformation when they are released into lower landfill layers.Within the upper landfill layers, biodegradation prevails as the major degradation mechanism by which the phthalates are dissipated. Generally, biodegradation pathways for the phthalates consist of primary biodegradation from phthalate diesters to phthalate monoesters, then to phthalic acid, and ultimately biodegradation of phthalic acid to form C02 and/or CH4• We have noted that the phthalate esters are also degraded through abiotic means,which proceeds via both hydrolysis and photolysis. Photodegradation generally involves reactions of the phthalates in the atmosphere with hydroxyl radicals. The hydrolysis of phthalate diesters produces the corresponding monoesters, which are subsequently converted to phthalic acid. Phthalic acid has been observed to accumulate within landfill zones where phthalate contamination exists.Hydrolysis is usually not an important fate process for phthalate esters in the environment, including in upper landfill layers. However, the conditions prevalent at lower landfill layers are generally suitable for phthalate transformation via hydrolysis.The conditions in this zone include high temperatures and pressures, presence of chemical catalysts, as well as wide pH fluctuations. Such conditions foster hydrolysis that may be either acid- or base-catalyzed by metal ions, anions, or organic materials catalysts. In addition, research indicates that the propensity for ongoing hydrolysis increases as landfill depth increases.We can be emphatic in asserting that hydrolysis of phthalate esters in lower landfill layers is the dominant process for transforming these esters; in contrast,biodegradation is the predominant process in the upper landfill layers.We recommend that future research be performed to expand the understanding of what influence each reaction condition (high temperature, presence of chemical catalysts, etc.) has on the rate of chemical transformation of the phthalates in lower landfill zones. We also recommend that the combined effects of all conditions on the rate of chemical transformation at lower landfill layers be assessed for the phthalates.Such research could be achieved under simulated conditions.