Environ. Sci. Technol. 1982, 16, 428-432
Adsorption of Phthalic Acid Esters from Seawater Kevin F. Sullivan, Elllot L. Atlas, and Choo-Seng Giam"
Department of Chemistry, Texas A&M University, College Station, Texas 77843 Controlled laboratory experiments have shown the influence of several factors on the adsorption of two phthalic acid esters. Di-n-butyl phthalate and bis(2-ethylhexyl) phthalate dissolved in seawater rapidly adsorb onto and desorb from three clay minerals, calcite, a sediment sample, and glass test tubes. The adsorption of phthalates is inversely related to the aqueous solubility of the two phthalates studied. An increase in the lipophilic character of the adsorbent or the salinity of the solution increases the amount of phthalate bound. The probable binding mechanisms include van der Waals forces and hydrophobic interactions. Introduction Phthalic acid esters (PAEs) are prevalent industrial chemicals that have been found in a variety of environmental samples. The annual production of PAEs in the United States more than doubled from 1960 to 1970 and has since remained in excess of one billion pounds ( I ) . Bis(2-ethylhexyl) phthalate (BEHP), which accounts for nearly one-third of the production, and di-n-butyl phthalate (DBP) are the most frequently identified PAEs in diverse environmental samples. These environmental samples include ground water (2),river water (3),drinking water ( 4 ) ,open ocean water ( 5 ) ,urban and suburban air ( 6 ) ,marine air (7), fish (8, 9),crustaceans (IO),seal ( I I ) , soil humates ( I 2 ) ,lake sediments (€9,and marine sediments (5). Of particular importance is the fact that these compounds are being transported to the Oceans where their fate is largely unknown. The distribution and fate of a compound in the oceans is largely controlled by transport mechanisms within phases and across phase boundaries. Adsorption of a compound from solution is one mechanism that can significantly influence the transport and distribution in the ocean environment. To our knowledge, however, no studies have been reported on the adsorption of the ubiquitous phthalate esters. Several studies have been done concerning the adsorption from aqueous solutions of various pesticides onto clay minerals (I3-I6), humates (16, 17), and sediments (17-19). Weber (20) reviewed the factors affecting adsorption and the mechanisms of adsorption for numerous classes of pesticides. The adsorption reactions of the persistent polychlorinated biphenyls also have been studied experimentally (21,22)and theoretically (22). Representative biogenic compounds demonstrate adsorption behavior that is similar to that of the synthetic organics (23-25). In particular, the adsorption of different classes of compounds is influenced by the mineralogy of the adsorbing particle and by the presence of other naturally occurring organics. The desorption process is as important as adsorption in understanding the distribution of organic compounds. The desorption of DDE and lindane from settling particles once they were below a freshwater thermocline caused the pesticides to reach uniform aqueous concentrations prior to the autumn breakdown of the thermal stratification (26). Sediments contaminated with DDT and polychlorinated biphenyls by a submarine outfall acted as a source of the pollutants to the surrounding water and biota well after the abatement of the initial source (27). Whether sedi428
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ments act as a final repository or as a secondary source of pollutant is determined by the adsorption and desorption reactions. For help in understanding the fate of phthalic acid esters in the marine environment, an investigation of their adsorption and desorption reactions was done. To examine these reactions, we chose to first characterize the adsorption/desorption behavior of phthalates on common inorganic components of marine sediments. This behavior is then compared to adsorption reactions on a natural marine sediment. Methods Apparatus. Two electron-capture gas chromatographs were used for quantitation of the nonradioactive phthalates and for checking the cleanliness of the glassware. A Tracor MT-220 gas chromatograph was fitted with 6-ft glass columns packed with 3% SE-30 on Gas-chrom Q (100-120 mesh) or 1.95% QF-1/1.5% OV-17 on Supelcoport (100-120 mesh). A Hewlett-Packard 5700 A gas chromatograph was fitted with a 6-ft glass column packed with 1.5% SP-2250/1.95% SP-2401 on Supelcoport (100-120 mesh). The carrier gases for the instruments were nitrogen at 60 mL/min and 95% argon/5% methane at 40 mL/ min, respectively. External standards of the phthalates were injected along with the samples and formed a linear calibration curve for the quantitation of samples. The radioactive phthalates were quantitated on one of three liquid scintillation counters: a Beckman LS-lOOC, a Beckman LS-7OO0, or a Beckman LS-9000. The samples were counted for a minimum of 20 min or until enough data had been accumulated so that there was only a 2.0% probability that the actual counts per minute (cpm) were greater than twice the sample standard deviation away from the measured cpm. All the samples for the adsorption studies had negligible and constant quench. Other apparatus included a centrifuge and a constant temperature bath. The centrifuge was an Adams analytical angle-head centrifuge with a specified speed of 3400 rpm (1550g). The constant-temperature bath consisted of a Haake heat bath and circulator regulated against a Polyscience KR-30 refrigerated chiller. Glassware. The glass containers for the adsorption experiments and for the analysis of phthalates were made of borosilicate glass and had Teflon-lined caps. The vessels for the adsorption experiments were 16 X 125 mm screwcap culture tubes. The extracts of the various phases for nonradioactive phthalates were stored in 3-dram vials. Meticulous cleaning of glassware was required to reduce the background contamination by the ubiquitous phthalates. The test tubes and vials were placed in a 300 OC oven overnight and, once cool, were rinsed twice with acetone and repeatedly with petroleum ether. The glassware was acceptable when contamination corresponding to less than 1 ng of phthalate per piece of glassware was measured, which corresponds to calcite N calcium montmorillonite) could be due to differences in the surface areas for the first three adsorbents. The K , values for montmorillonite, 12.7 f 0.8, and kaolinite, 12.1 f 1.8, are not statistically different; however, the K , value for calcite, 1.8 f 0.4, is significantly smaller. The specific
surface area for calcite particles was estimated to be 2.5 m2/g by treating the particles as spheres with a minimum diameter of 0.1 pm, which was determined via microscopic examination. This estimate of the surface area of the calcite particles is considerably less than characteristic surface areas of clay minerals (50-750 m2/g (20)). The relatively small adsorption onto calcium montmorillonite could be explained if the interlamellar surfaces of the calcium montmorillonite were inaccessible to BEHP. Experiments have shown that the extent of adsorption correlated with the available surface area (15, 33), while other researchers have found the correlation to be poor (14, 23). In our experiments the organic content of the adsorbent also influences the extent of adsorption, which substantiates hydrophobic interactions as an adsorption mechanism. Solvent-extractedmontmorillonite adsorbed 3% less DBP than the unextracted montmorillonite, while cleaned calcium montmorillonite adsorbed 14% less BEHP than the unextracted calcium montmorillonite. The greatest difference in adsorption behavior was seen with the actual sediment sample and DBP. The sediment sample is predominantly sand-size particles and so has less surface area per gram than the clay minerals; however, the natural organic component of the sediment was left intact. The lipophilic character of the sediment contributed to the K, for DBP (0.149 f 0.017), which is an order of magnitude greater than for DBP on any other adsorbent. An increase in the lipophilicity of the solid adsorbent has been reported to increase the adsorption of polychlorinated biphenyls (21), DDT (17), 2,4-D (16), and lindane (19). Hydrophobic and van der Waals adsorption on clay minerals involves nonspecific interactions that should allow mass transfer to and from the surface of the clay minerals. For the BEHP studies, between 2% and 7% of the total phthalate desorbed into the unspiked seawater that replaced the phthalate-spiked seawater solutions. After equilibrium with the solid adsorbents, the spike solutions retained between 3% and 22% of the total BEHP with five of the seven studies falling between 2% and 7%. The aqueous phases accommodated comparable amounts of BEHP regardless of the direction of mass transfer. All of the studies with BEHP exhibit extensive desorption, and most of the studies show completely reversible adsorption. If adsorption is completely reversible, one would expect the K , and Kd values to be the same. There is no statistically significant difference between the K , and Kd values for the BEHP studies with calcium montmorillonite, kaolinite, and calcite. Substantial irreversibility is observed with the sediment sample, whose K,, 5.1 f 1.0, is notably smaller than its Kd,13.9 f 2.2. The larger Kd value indicates that the sediment is retaining proportionately more BEHP than expected for a reversible equilibrium. However, we do not know if this apparent irreversible behavior would be smaller if the sediment/ water mixture were allowed more time to equilibrate. It is evident though, that the adsorption of BEHP by the sediment involves factors other than merely equilibrium with clay minerals. The long-term significance of such factors on sorption processes in the marine environment has yet to be determined. These factors might include the extensive van der Waals forces and hydrophobic interactions of an organic complex. Varying degrees of irreversibility for the adsorption of DDT (17,18) and polychlorinated biphenyls (21)also have been attributed to the organic material in the adsorbents. To determine how representative of environmental processes the partition coefficients for the phthalates are, Environ. Scl. Technol., Vol. 16, No. 7, 1982
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we calculated partition coefficients for environmental samples and then compared them to the laboratory partition coefficients. By division of the average phthalate contents of sediment samples by the average concentrations of phthalates in surface water for the Mississippi Delta (5),approximate partition coefficients for DBP, 0.14, and for BEHP, 1.00, are obtained. Surprisingly these partition coefficients are close to the K , values for the sediment adsorption studies with DBP, 0.149 f 0.017, and BEHP, 5.1 f 1.0, indicating that laboratory experiments can approximate environmental processes. However, caution should be used in applying laboratory-determined partition coefficients to field situations. Kinetic factors, as well as the quantity and character of the organic components in natural sediments, may significantly influence the long-term behavior of phthalates in marine sediments. Conclusions Adsorption experiments for di-n-butyl phthalates and bis(2-ethylhexyl)phthalate on three clay minerals, calcite, and a sediment sample were done to elucidate some of the factors and mechanisms controlling the transfer of the phthalic acid esters between the aqueous and pqticulate phases in the oceans. A kinetic study showed that most of the adsorption of BEHP occurred within the first hour and that after 12 h the solid adRorbent and aqueous phase were essentially at equilibrium. The adsorptive behavior was described adequately by partition coefficients relating the amount of phthalate in the aqueous phase to the amount of phthalate bound to the solids at equilibrium. Both phthalates adsorbed onto the solid adsorbents and the glass test tubes. The effect on adsorption of the solubility of the phthalates, salinity, the surface area of the adsorbent, and the organic content of the adsorbent were studied. The extent of adsorption increased with an increase in salinity or a decrease in the solubility of the phthalate. The expected positive correlation between the adsorption of BEHP and the surface area of the adsorbent was apparent for calcite in comparison to some of the clay minerals but not for comparison among the clay minerals. An increase in the lipophilicity of the adsorbent increased the adsorption of both phthalates. Extensive desorption of BEHP occurred for all the adsorbents, and most of the adsorbents exhibited completely reversible adsorption. Comparable amounts of BEHP were extracted from adsorption and desorption aqueous phases for all the adsorbents. For kaolinite, calcite, and calcium montmorillonite the adsorption and desorption partition coefficients for BEHP were statistically identical, indicating complete reversibility. The sediment sample showed significant irreversibility of adsorption, which suggests that marine sediment may act as a final repository of phthalic acid esters. Acknowledgments We would like to express our appreciation to J. B. Dixon and M. V. Fey for the X-ray analyses. Literature Cited (1) US.International Trade Commission, “Synthetic Organic Chemicals”; U.S. Production and Sales, 1960-1978. (2) Robertson, James M. Water Sewage Works 1976,123,58.
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Received for review June 23,1981. Revised manuscript received March 5,1982. Accepted March 14,1982. This work was supported by the National Science Foundation, Grant Nos. OCE76-14148 and OCE77-12482, and in part by the Robert A. Welch Foundation.
