Sediment and Contaminant Transport in Surface Waters - Chapter 7 doc

279 7 The Sorption and Partitioning of Hydrophobic Organic Chemicals To understand and predict the transport and fate of hydrophobic organic chemicals (HOCs) in surface waters and bottom sediments, knowledge of the rates of sorp- tion to and from solid sedimentary particles and the partitioning of these chemicals between solid particles and water is necessary. In early work, it was often assumed that adsorption and desorption occurred rapidly and that chemical equilibrium between the solids and water was attained in a very short time (e.g., see reviews by Sawhney and Brown, 1989; DiToro et al., 1991; Baker, 1991). This equilibrium was quantied by means of a partition coefcient, K p (L/kg), dened as K C C p s w  (7.1) where C s (kg/kg) is the mass of HOC sorbed to the sediment divided by the mass of the sediment, and C w (kg/L) is the mass of HOC dissolved in the water divided by the volume of water. However, early sorption experiments were generally short term, hours to a few days, and were misleading; long-term experiments later dem- onstrated that both adsorption and desorption processes are often quite slow, with time scales of days to months or even longer before equilibrium is attained (e.g., Karickhoff and Morris, 1985; Coates and Elzerman, 1986). By comparison, the time of transport of a sediment particle in a river or lake may be as short as minutes to a few hours. Because of this, the assumption of chemical equilibrium in surface waters may not be a good approximation in many real situations, and therefore time-dependent sorption processes must be considered in detail. In the rst section of this chapter, experiments that illustrate basic and impor- tant characteristics of this time-dependent sorption as well as steady-state parti- tioning are presented and qualitatively analyzed. These experiments, as well as others, demonstrate that sorption times are long, that sorption processes depend on the HOC, and that these processes are signicantly modied by colloids from the water, colloids from the sediments, organic content of the sediments, and particle and oc size and density distributions. The effects of these parameters on © 2009 by Taylor & Francis Group, LLC 280 Sediment and Contaminant Transport in Surface Waters the steady-state partitioning of an HOC between sedimentary particles and water are discussed subsequently. In this rst set of experiments, linear isotherms (i.e., K p values, which are constant and independent of dissolved HOC concentration at constant temperature) were obtained. However, nonlinear and interactive effects on isotherms are often observed and have been reported in the literature; experi- ments and analyses that delineate these processes are also presented. For a quantitative understanding of sorption dynamics and also an accurate ability to predict the sorption, transport, and fate of HOCs in aquatic systems, quantitative models of the time-dependent sorption processes are needed. In Section 7.2, a quite general but complex model of time-dependent sorption is described rst; this model includes effects of particle and oc size and density distributions. A simpler, less accurate, but computationally efcient model is then presented. This model is sufcient to describe major characteristics of the sorp- tion experiments. However, for more accurate descriptions of the experimental results, the general model is needed. Results with this latter model are then com- pared with experimental results. The discussion in this chapter primarily concerns the sorption of HOCs to suspended particles. The sorption and ux of HOCs in bottom sediments are dis- cussed in Chapter 8. 7.1 EXPERIMENTAL RESULTS AND ANALYSES 7.1.1 B ASIC EXPERIMENTS Results of several experiments are presented here that illustrate the basic char- acteristics of time-dependent adsorption, desorption, and short-term adsorption followed by desorption processes for one HOC, and the effects of different HOCs on adsorption and desorption (Jepsen et al., 1995; Tye et al. 1996; Borglin et al., 1996). From these experiments, sorption rates as well as partitioning of the HOC to suspended sedimentary particles can be determined as a function of time. All experiments were long term and were usually continued until chemical equilib- rium was attained. All sediments used in this set of experiments were subsam- ples of the same batch of natural sediments from the Detroit River. The median particle size was 7 µm, and the organic carbon content was 1.42%. All HOCs used were carbon-14 labeled in order to simplify the analytical procedures and to enhance sensitivity. Filtration of the sediment-water mixtures separated opera- tionally dened sediment-sorbed and dissolved fractions of the HOC at a particle size of 1 µm. Details of the experimental procedures and additional results can be found in the articles referenced. The adsorption experiments were batch-mixing experiments. To initiate the experiments, a dissolved HOC and clean sediments (i.e., no sorbed HOC) at con- centrations from 2 to 10,000 mg/L were mixed together with water in amber Qor- pak glass jars. The jars were then rotated on a rolling table to ensure continuous mixing of the contents until they were sampled. The experiments were conducted for different periods of time up to 6 months. One sample jar was prepared for each © 2009 by Taylor & Francis Group, LLC The Sorption and Partitioning of Hydrophobic Organic Chemicals 281 sample point. Results of the experiments were reported as the logarithm of the partition coefcient as a function of time. Typical results of experiments with hexachlorobenzene (HCB), sediments, and Optima pure water are shown in Figure 7.1, where log K p is plotted as a function of time for sediment concentrations of 2, 10, 100, 500, 2000, and 10,000 mg/L. As the HOC is adsorbed to the sediment, log K p varies relatively rapidly at small time but more slowly as time increases and a steady state is approached. The time to steady state varies from less than 1 day (for a sediment concentration of 2 mg/L) to about 30 days (for a sediment concentration of 10,000 mg/L). The mea- sured steady-state partition coefcient depends somewhat on sediment concentra- tion; that is, log K p = 4.0 for a sediment concentration of 2 mg/L and decreases monotonically to 3.78 at a sediment concentration of 10,000 mg/L, a factor of 1.66 for K p . Experiments of this type were performed with Optima pure water as well as with ltered tap water, with different-size fractions of the original sediments, with organically stripped sediments, and with three PCB congeners. All gave results qualitatively similar to those shown in Figure 7.1. The desorption experiments used a purge-and-trap procedure. Sediments to which an HOC was sorbed, typically for 3 to 12 months (a time sufcient for sorp- tion equilibrium to be attained), were mixed with water in a ask. The mixture was kept suspended and continuously purged with water-saturated compressed air. The air bubbled through the suspension and exited through a resin column that trapped the HOC. The HOC then was extracted from this column using a methanol solution and sonication. The experiments continued until essentially all the HOC had desorbed from the sediments. Typical results of desorption experiments with HCB and pure water at different sediment concentrations are presented in Figure 7.2. The percent of 020 4.5 4.0 3.5 3.0 2.5 Time (days) Log K p 40 60 mg/L 2 10 100 500 2000 10000 FIGURE 7.1 Adsorption experiments with HCB and pure water. Log K p as a function of time with sediment concentration as a parameter. (Source: From Jepsen et al., 1995. With permission.) © 2009 by Taylor & Francis Group, LLC 282 Sediment and Contaminant Transport in Surface Waters the HCB initially adsorbed to the particles that has subsequently desorbed is shown as a function of time. Within experimental error (a few percent), all of the HCB initially sorbed to the sediments is desorbed with time, indicating that the adsorption and desorption processes are reversible. The desorption rate is greatest at the beginning and then decreases as the HCB is desorbed and its concentration goes to zero. The rate is slightly dependent on sediment con- centration; the time for 90% desorption is on the order of 50 days (at 100 mg/L) to 100 days (at 10,000 mg/L). The desorption times for this type of experiment are signicantly longer than the adsorption times for the adsorption experi- ments described above (compare Figures 7.1 and 7.2). However, it should be noted that the desorption experiments (purge-and-trap) are inherently different from the adsorption experiments (batch-mixing). Hence, there is no direct and/ or simple relation between adsorption and desorption times for these experi- ments. This is discussed further below and in the next section. Experiments of this type have been performed with pure water as well as with ltered tap water, with different-size fractions of the original sediment, with organically stripped sediments, and with two PCB congeners. Qualitatively similar results were obtained in all cases. In addition to adsorption and desorption experiments, several short-term adsorption followed by long-term desorption experiments were performed. In these experiments, the HOC was adsorbed to sediments for either 2 days or 5 days (batch-mixing experiments); in this short period of time, sorption equilibrium was not attained. This was followed by desorption, which lasted until essentially 100 80 Percent of HCB Desorbed 60 40 20 0 0 50 100 150 200 Time (Days) 100 mg/L 500 mg/L 2000 mg/L 10000 mg/L 250 300 FIGURE 7.2 Desorption experiments with HCB and pure water. Percent of the initially sorbed HCB that has desorbed as a function of time. Sediment concentration as a param- eter. (Source: From Borglin et al., 1996. With permission.) © 2009 by Taylor & Francis Group, LLC The Sorption and Partitioning of Hydrophobic Organic Chemicals 283 all the HOC had desorbed (purge-and-trap experiments). For experiments with HCB in pure water and at a sediment concentration of 500 mg/L, results for desorption are shown in Figure 7.3 and are there compared with the standard desorption experiment (where HCB had been adsorbed for 120 days and there- fore equilibrated before desorption began). For 2- and 5-day adsorption times, the desorption times are proportional to the adsorption times but are longer. For each experiment, essentially all the HCB that was sorbed to the sediment during the adsorption phase of the experiment is desorbed, again indicating reversibility of the processes. Experiments also were performed at sediment concentrations of 100 and 10,000 mg/L; the results were similar in character. The results shown in Figures 7.1 through 7.3 are all for HCB. For other HOCs, the results are qualitatively the same but depend quantitatively on the partition coefcient of the HOC. To illustrate this, results of adsorption experiments are shown here for three PCB congeners: a monochlorobiphenyl (MCB), a dichloro- biphenyl (DCB), and a hexachlorobiphenyl (HPCB). For each of these HOCs and for HCB, log K p is shown in Figure 7.4 as a function of time at a sediment concen- tration of 2000 mg/L. The times to steady state increase as the steady-state value of K p increases. For the PCBs, it was shown that the times to steady state depend on the sediment concentration in a similar manner as for HCB. Results of desorption experiments (percent desorbed as a function of time) are shown in Figure 7.5 for HCB, MCB, and HPCB at a sediment concentration of 2000 mg/L. Desorption times increase as K p increases; for each HOC, they are proportional to adsorption times (Figure 7.4) but are longer. All the MCB 100 80 Percent of HCB Desorbed 60 40 20 0 0 50 100 150 200 Time (Days) 2 Day adsorption 5 Day adsorption 120 Day adsorption 250 300 FIGURE 7.3 Short-term adsorption followed by desorption experiments. Sediment con- centration is 500 mg/L. Percent of initially sorbed HCB that has desorbed as a function of time. (Source: From Borglin et al., 1996. With permission.) © 2009 by Taylor & Francis Group, LLC 284 Sediment and Contaminant Transport in Surface Waters and HCB desorbed completely during the experiments; HPCB was at 80% des- orption at 200 days and was still desorbing when the experiment concluded at 230 days. 2.0 2.5 3.0 3.5 4.0 0 204060 Time (days) Log K p MCB DCB HCB HPCB 4.5 5.0 FIGURE 7.4 Partition coefcients for the adsorption of HCB and three PCB congeners (MCB, DCB, and HPCB). Log K p as a function of time. Experimental data are shown as open and closed symbols, whereas the modeling results are shown as solid lines. (Source: From Lick et al., 1997. With permission.) Percent Desorbed Time (Days) 100 80 60 40 20 0 0 50 100 150 200 250 300 MCB HCB HPCB FIGURE 7.5 Desorption experiments with HCB, MCB, and HPCB. Percent desorbed as a function of time. (Source: From Borglin et al., 1996. With permission.) © 2009 by Taylor & Francis Group, LLC The Sorption and Partitioning of Hydrophobic Organic Chemicals 285 7.1. 2 PARAMETERS THAT AFFECT STEADY-STATE SORPTION AND PARTITIONING The experiments described above, as well as similar ones, demonstrate that (1) sorption times are long (days to months or even longer), (2) desorption times are longer than adsorption times (but adsorption and desorption experiments are inherently different), (3) adsorption and desorption are reversible processes, (4) sorption times and the measured partition coefcients depend on the sediment concentration, and (5) sorption times depend on the partition coefcient. These statements have been difcult to quantify and interpret because of seemingly con- tradictory experimental results and analyses reported in the literature. To assist in the clarication and quantication of these statements, various factors that affect the steady-state sorption and partitioning processes are reviewed and discussed here. The most signicant of these factors are colloids from the sediments, col- loids from the water, and the organic content of the sediments (Lick and Rapaka, 1996). The dynamics of sorption, including the effects of particle and oc size and density distributions, are discussed in Section 7.2. 7.1.2.1 Colloids from the Sediments Colloids are here operationally dened as particles or ocs less than 1 µm in diam- eter. In pure water, no colloids are present. However, they are always present in natural waters but vary widely in amount and character. In addition, because there is a wide distribution of particle sizes in natural sediments (inevitably including some particles less than 1 µm in diameter), colloids are inherently present in any sample of natural sediments; they are a natural part of the sediments, and their amount in the water is more or less proportional to the amount of sediments in suspension. In the adsorption experiments with results as shown in Figure 7.1, pure water was used and there were therefore no colloids from the water. However, because natural sediments were used, colloidal particles from the sediments were present. In these experiments, HCB was truly dissolved in the water and also adsorbed to the solid sedimentary particles greater than 1 µm in diameter, to the colloidal particles, and to colloidal particles that had occulated such that the oc diameter was greater than 1 µm (i.e., no longer colloidal). For these experiments, it was demonstrated that the mass of HCB adsorbed to the occulated colloidal matter was generally small in comparison with the HCB adsorbed to the solid particles, and it was therefore ignored. To interpret and quantify the steady-state partition coefcients as shown in Figure 7.1, especially the dependence of K p on sediment concentration, consider the following. During ltration to separate C s and C w and hence to determine K p from Equation 7.1, the amount of HCB retained on the lter consists of the HCB sorbed to the sediment particles greater than 1 µm, m Hs , whereas the amount of HCB in the ltrate consists of the truly dissolved HCB, m Hd , plus the amount of © 2009 by Taylor & Francis Group, LLC 286 Sediment and Contaminant Transport in Surface Waters HCB sorbed to the colloidal matter from the sediments, m Hdc . It follows that the measured partition coefcient in this case is given by K m m mm V m m m V m pm Hs d Hd Hdc Hs sed Hd Hd    ¤ ¦ ¥ ³ µ ´  se 1 cc Hd s w Hdc Hd p Hdc Hd m C C m m K m m ¤ ¦ ¥ ³ µ ´   ¤ ¦ ¥ ³ µ ´   1 1 (7.2) where m sed is the mass of sediments, V is the volume of water, and K p is the true partition coefcient as dened in Equation 7.1. A partition coefcient for the colloidal matter can be dened as K m m m V c Hdc dc Hd  (7.3) where m dc is the mass of colloidal particles from the sediments. In general, m dc should be proportional to the mass of the sediments; that is, m dc = Bm sed , where B is the fraction of colloidal particles in the sediments. It follows from the above that m m K m V K m V KC Hdc Hd c dc c sed c   A A (7.4) where C is the sediment concentration. By substituting this expression into Equa- tion 7.2, one obtains K K KC pm p c  1 A (7.5) It can be seen that K pm depends on the sediment concentration and reduces to K p as the sediment concentration decreases to zero. From this and Figure 7.1, it fol- lows that log K p is approximately equal to 4.0 ± 0.1, or K p = 10,000 L/kg. If it is further assumed that the partition coefcient for the colloidal matter is approximately the same as that for the sediments, the above equation reduces to K K KC pm p p  1 A (7.6) © 2009 by Taylor & Francis Group, LLC The Sorption and Partitioning of Hydrophobic Organic Chemicals 287 This expression is similar to that derived by Wu and Gschwend (1986). From this equation and the data in Figure 7.1, B can be estimated and is approximately 0.005, a reasonable number for the fractional mass of colloidal particles in natural sediments. With this, the above equation is consistent with the results in Fig- ure 7.1. By comparison of K p for 2 mg/L and 10,000 mg/L, the maximum effect of colloids from the sediments on K p in these experiments is a factor of about 1.66. As with these experiments, most sorption experiments have been performed with suspended sediments at relatively low concentrations of 10 4 mg/L or less. The extension of these results to high concentrations, such as may occur in sur- face waters during large oods or storms but especially in consolidated bottom sediments, was questionable. Because of this difculty, adsorption experiments with HCB and pure water were done at sediment concentrations from 10 2 mg/L up to 6.25 × 10 5 mg/L, concentrations approaching those of consolidated sedi- ments (Deane et al., 1999). Measured partition coefcients for HCB and Detroit River sediments (with 3.2% organic carbon and different from that above) are shown in Figure 7.6(a) as a function of time and for sediment concentrations of 10 2 , 10 3 , 10 4 , 5×10 4 , 10 5 , and 6.25 × 10 5 mg/L. At the largest sediment concentration, the effect on K p is a factor of about 5 compared with K p at 100 mg/L. For each of these concentra- tions, the colloidal fraction, B, was determined by means of a submicron particle sizer and also by the difference in mass between the ltrates from a 1-µm and a 0.1-µm lter (Table 7.1). With this data, Equation 7.6 then was used to determine 4.2 4.0 3.8 3.6 3.4 Log K pm (L/kg) 3.2 3.0 2.8 2.6 0 50 100 150 200 10 2 mg/L 10 3 mg/L 10 4 mg/L 10 5 mg/L 5 × 10 4 mg/L 6.25 × 10 5 mg/L Time (days) FIGURE 7.6(a) Partition coefcients as a function of time during adsorption at different sediment concentrations: measured partition coefcients. (Source: From Deane et al., 1999. With permission.) © 2009 by Taylor & Francis Group, LLC 288 Sediment and Contaminant Transport in Surface Waters 4.2 4.0 3.8 3.6 3.4 Log K pm (L/kg) 3.2 3.0 2.8 0 50 100 Time (days) 150 200 10 2 mg/L 10 3 mg/L 10 4 mg/L 10 5 mg/L 5 × 10 4 mg/L 6.25 × 10 5 mg/L FIGURE 7.6(b) Partition coefcients as a function of time during adsorption at differ- ent sediment concentrations: partition coefcients corrected for colloidal effects. (Source: From Deane et al., 1999. With permission.) TABLE 7.1 Colloidal Fractions in Highly Suspended Sediment Concentration Experiments Sediment Concentration (mg/L) Colloidal Fraction Measured by Submicron Particle Sizer Colloidal Fraction Measured by 0.1 µm Filtration Detroit River 10 2 0.00057 0.00062 10 3 0.00046 0.00039 10 4 0.00039 0.00035 5 r 10 4 0.00008 0.00012 10 5 0.00011 0.00017 6.25 r 10 5 0.00006 0.00016 Stripped Detroit River 10 3 0.00572 0.00613 10 4 0.00540 0.00500 5 r 10 4 0.00470 0.00430 10 5 0.00325 0.00415 6.25 r 10 5 0.00070 0.00150 Santa Barbara Mountain 10 2 0.00232 0.00236 10 3 0.00221 0.00248 10 4 0.00190 0.00215 5 r 10 4 0.00057 0.00071 10 5 0.00040 0.00036 6.26 r 10 5 0.00027 0.00037 © 2009 by Taylor & Francis Group, LLC [...]... model is quite complex and requires considerable auxiliary data and computer time Because of this, a general calculation of the transport © 2009 by Taylor & Francis Group, LLC 298 Sediment and Contaminant Transport in Surface Waters of contaminants in surface waters with this model is impracticable A simplified model is therefore needed (Lick et al., 19 97) and is described in Section 7. 2.2 This simplified... convenient and * * can be defined from Equation 7. 31 as kt d = 1 (t d is the time for Cs to desorb to −1 = 0.368 of its initial value) Equation 7. 34 indicates that this time is given by e 0.0165 d 2 1 td 1 s * © 2009 by Taylor & Francis Group, LLC Dm Kp (7. 35) 302 Sediment and Contaminant Transport in Surface Waters FIGURE 7. 10 Desorption of HCB from suspended sediments as calculated by the diffusion and. .. X k and Yk are related by Xk = k Yk (7. 27) where k = (2.6 − 1.6 k)/(2.6 − 1.6 ) and it has been assumed that the density of the particles composing the floc is 2.6 g/cm3 For the cases considered here, the © 2009 by Taylor & Francis Group, LLC 300 Sediment and Contaminant Transport in Surface Waters size and density distributions are not wide, and the variation in k is less than 2 Because of this and. .. 0.995 0. 97 0 .74 0. 57 0.25 The Sorption and Partitioning of Hydrophobic Organic Chemicals 305 TABLE 7. 4 Steady-State Floc Size Distributions: Initial Disaggregated Size Distribution and Size Distributions during Adsorption and at End of Desorption Average diameter ( m) Size range ( m) Initial disaggregated size 3 1–5 0.295 7 5–10.5 0.243 17 10.5–33.5 0.395 40 >33.5 0.0 67 0.0 37 0.125 0.112 0. 076 0.143... mg/L, and 2 = 0 .74 and is the porosity of flocs at 500 mg/L The modified diffusion coefficient is now 1 .7 × 10 −13 cm2/s, a factor of about 10 greater than that for 10,000 mg/L For the calculated results with this value for D and with the initial size distribution for © 2009 by Taylor & Francis Group, LLC 308 Sediment and Contaminant Transport in Surface Waters desorption of X k = 0.0 37, 0. 076 , 0.282, and. .. (Ghosh et al., 2003; Accardi-Dey and Gschwend, 2002, 2003; Lohmann et al., 2005) have indicated that different types of organic matter may sorb HOCs in different amounts and at different rates In particular, investigators have identified two types of organic matter with differing sorption © 2009 by Taylor & Francis Group, LLC 292 Sediment and Contaminant Transport in Surface Waters properties: (1) amorphous... 294 Sediment and Contaminant Transport in Surface Waters 100 10–1 Organic Carbon (%) and Sediment Concentration (mg/L) 3.3%, 1000 mg/L 3.3%, 100 mg/L 1.95%, 100 mg/L Coc (kg/kg) 10–2 10–3 10–4 10–5 10–6 10–9 10–8 10 7 10–6 Cw (kg/L) 10–5 10–4 10–3 FIGURE 7. 8 Tetrachloroethylene isotherm Coc as a function of Cw for different organic carbon contents and sediment concentrations The solid line is the linear... Equations 7. 15 and 7. 16 are shown in Figures 7. 9(a) and (b), respectively Reasonably good agreement between theory and experiments is demonstrated A better assumption for is that 1 e Coc For low values of Coc, this is equivalent to 7. 17 into Equation 7. 13 leads to Coc © 2009 by Taylor & Francis Group, LLC K o Cw e oc (7. 17) = Coc Substitution of Equation Cw (7. 18) The Sorption and Partitioning of Hydrophobic... Figures 7. 1 and 7. 7 all reduce to a single value, independent of sediment concentration For these experiments, the effects of colloids from the tap water, Equation 7. 9, were greater than the effects of colloids from the sediments, Equation 7. 6 This is probably true for most natural waters at low sediment concentrations At high sediment concentrations, as in Figure 7. 6, the effects of colloids from the sediment. .. been developed and is as follows The major simplifications are that (1) the diffusion of Cs within a particle is approximated as a mass transfer process between the particle and the surrounding water, and (2) changes in particle and floc size and density distributions are not included For sediments consisting of single-size particles, the time-dependent change of the average value of Cs within the particle, . 279 7 The Sorption and Partitioning of Hydrophobic Organic Chemicals To understand and predict the transport and fate of hydrophobic organic chemicals (HOCs) in surface waters and bottom sediments,. Group, LLC 280 Sediment and Contaminant Transport in Surface Waters the steady-state partitioning of an HOC between sedimentary particles and water are discussed subsequently. In this rst set. Sediment and Contaminant Transport in Surface Waters the mass of PCE sorbed to the sediment is approximately 10 −2 of the mass of the organic carbon associated with the sediments) and continually