1886 Environmental Toxicology and Chemistry, Vol. 24, No. 8, pp. 1886–1892, 2005  2005 SETAC Printed in the USA 0730-7268/05 $12.00 + .00 RAOULT’S LAW–BASED METHOD FOR DETERMINATION OF COAL TAR AVERAGE MOLECULAR WEIGHT DERICK G. BROWN,*² L OVLEEN GUPTA,² H ORACE KEITH MOO-YOUNG,² and A NDREW J. COLEMAN‡ ²Department of Civil and Environmental Engineering, Lehigh University, 13 East Packer Avenue, Bethlehem, Pennsylvania 18015, USA ‡Electric Power Research Institute, 3412 Hillview Avenue, Palo Alto, California 94304, USA ( Received 21 September 2004; Accepted 28 January 2005) Abstract—A Raoult’s law–based method for determining the number average molecular weight of coal tars is presented. The method requires data from two-phase coal tar/water equilibrium experiments, which readily are performed in environmental lab- oratories. An advantage of this method for environmental samples is that it is not impacted by the small amount of inert debris often present in coal tar samples obtained from contaminated sites. Results are presented for 10 coal tars from nine former manufactured gas plants located in the eastern United States. Vapor pressure osmometry (VPO) analysis provided similar average molecular weights to those determined with the Raoult’s law–based method, except for one highly viscous coal tar sample. Use of the VPO-based average molecular weight for this coal tar resulted in underprediction of the coal tar constituents’ aqueous con- centrations. Additionally, one other coal tar was not completely soluble in solvents used for VPO analysis. The results indicate that the Raoult’s law–based method is able to provide an average molecular weight that is consistent with the intended application of the data (e.g., modeling the dissolution of coal tar constituents into surrounding waters), and this method can be applied to coal tars that may be incompatible with other commonly used methods for determining average molecular weight, such as vapor pressure osmometry. Keywords—Nonaqueous-phase liquid Manufactured gas plant Vapor pressure osmometry Solubilility INTRODUCTION Coal tar is a byproduct of the manufactured gas process, which was used throughout the United States and Europe from the early 1800s through the mid-1900s [1–6]. It is a dense nonaqueous- phase liquid (DNAPL) that is composed of hundreds to thousands of organic compounds, mostly polycyclic aromatic hydrocarbons (PAHs) [7–9]. Due to past handling and storage practices, it is believed there is coal tar contamination at most former manu- factured gas plant (MGP) sites [1,2,4,5,10], with estimates of well over 1,500 sites in the United States alone [2–4]. This contam- ination is of concern, as the U.S. Environmental Protection Agen- cy (U.S. EPA) and the National Institute of Environmental Health Sciences have classified a number of PAHs as probable human carcinogens ([11]; http://ntp.niehs.nih.gov/index.cfm?objectid= 72016262-BDB7-CEBA-FA60E922B18C2540; [12,13]). Addi- tionally, a number of monocyclic aromatic hydrocarbons (MAHs) are present in coal tar, including benzene, which is a known human carcinogen ([11,13]; http:/www.epa.gov/iris). Environmental remediation of coal tars poses many chal- lenges due to the wide range of physical, chemical, and tox- icological properties of the individual constituents that make up coal tar [3,10,14,15]. For example, as seen in Table 1 the aqueous solubilities of common coal tar constituents span many orders of magnitude. Similar spans are observed for other fate and transport properties, such as biodegradation rates [14–16] and octanol/water partition coefficients [17,18]. Be- cause of the large number of compounds with such a wide range of properties, it is important to understand how the com- position of coal tar changes over time when developing re- mediation schemes. Many fate and transport modeling efforts that account for * To whom correspondence may be addressed (dgb3@lehigh.edu). the chemical composition of coal tar have been reported in the literature. Peters et al. examined the compositional dynamics [14–16,19,20] and phase stability [21,22] of coal tar and PAH NAPL systems. Brown et al. developed a fraction approach that addresses the inability to determine the complete com- position of coal tars [15]. Numerous researchers have modeled the aqueous-phase concentrations of coal tar constituents [10,23,24]; mass transfer of MAHs and PAHs from coal tar to water [25–30]; and enhanced solubility of coal tar com- ponents via solvents [7,20,31] and surfactants [32]. A common feature of these fate and transport models is that they are based on application of Raoult’s law, which relates the equilibrium aqueous concentration of the coal tar constituents to their mole fraction within the coal tar. A key requirement for application of Raoult’s law to coal tar systems is knowledge of the coal tar’s average molecular weight. The average molecular weight of a coal tar (or any mixture) can be defined a number of ways, such as number average and weight average molecular weights [33] n MW  j j j MW = (1) n n  j j m MW  j j j MW = (2) w m  j j where n j is the number of moles of compound j in the coal tar; m j is the mass of compound j in the coal tar (g); MW j is the molecular weight of compound j (g/mole); and n and MW w are the number and weight average molecular weights MW of the coal tar, respectively (g/mole). One other often-used definition is the viscosity average molecular weight, which is