Sorption of trace organics to the organic matter present in the treatment system is thought to be the primary physicochemical mechanism of removal (USEPA, 1982a). The concentration of the trace organic that is sorbed relative to that in solution is defined by a partition coefficient Kp, which is related to the solubility of the chemical. This value can be estimated if the octanol-water partition coefficient (Kow) and the percentage of organic carbon in the system are defined, as shown by Equation 3.22:

where

Koc = Sorption coefficient expressed on an organic carbon basis equal to

KsorbOc.

Ksorb = Sorption mass transfer coefficient (cm/hr).

Oc = Percentage of organic carbon present in the system.

Kow = Octanol-water partition coefficient.

Hutchins et al. (1985) presented other correlations and a detailed discussion of sorption in soil systems.

Jenkins et al. (1985) determined that sorption of trace organics on an overland-flow slope could be described with first-order kinetics with the rate constant defined by Equation 3.23:

ksorb

where ksorb = Sorption coefficient (hr-1).

B3 = Coefficient specific to the treatment system, equal to 0.7309 for the overland-flow system studied. y = Depth of water on the overland-flow slope (1.2 cm). Kow = Octanol-water partition coefficient.

B4 = Coefficient specific to the treatment system = 170.8 for the overland-

flow system studied. M = Molecular weight of the organic chemical (g/mol).

In many cases, the removal of trace organics is due to a combination of sorption and volatilization. The overall process rate constant (ksv) is then the sum of the coefficients defined with Equations 3.19 and 3.23, and the combined removal is described by Equation 3.24:

where

C0 = Initial concentration at t equal to 0 (mg/L or |g/L).

ksv = Overall rate constant for combined volatilization and sorption equal to kvol + ksorb.

Table 3.5 presents the physical characteristics of a number of volatile organics for use in the equations presented above for volatilization and sorption.

### Example 3.5

Determine the removal of toluene in an overland-flow system. Assume a 30-m-long terrace; hydraulic loading of 0.4 m3-hr-m (see Chapter 8 for discussion); mean residence time on slope of 90 min; wastewater application with a low-pressure, large-droplet sprinkler; physical characteristics for toluene (Table 3.5) of Kw = 490, H = 515, M = 92; depth of flowing water on the terrace = 1.5 cm; concentration of toluene in applied wastewater = 70 |g/L.

Solution

1. Use Equation 3.20 to estimate volatilization losses during sprinkling: C

TABLE 3.5

Physical Characteristics for Selected Organic Chemicals

TABLE 3.5

Physical Characteristics for Selected Organic Chemicals

 Substance Ka Hb Vapor Pressurec Md Chloroform 93.3 314 194 119 Benzene 135 435 95.2 78 Toluene 490 515 28.4 92 Chlorobenzene 692 267 12 113 Bromoform 189 63 5.68 253 m-Dichlorobenzene 2.4 x 103 360 2.33 147 Pentane 1.7 x 103 125,000 520 72 Hexane 7.1 x 103 170,000 154 86 Nitrobenzene 70.8 1.9 0.23 123 m-Nitrotoluene 282 5.3 0.23 137 Diethylphthalate 162 0.056 7 x 10-4 222 PCB 1242 3.8 x 105 30 4 x 10-4 26 Naphthalene 2.3 x103 36 8.28 x 10-2 128 Phenanthrene 2.2 x 104 3.9 2.03 x 10-4 178 2,4-Dinitrophenol 34.7 0.001 — 184

a Octanol-water partition coefficient. b Henry's law constant, 105 atm-m3/mol at 20°C and 1 atm. c At 25°C.

d Molecular weight (g/mol).

2. Use Equation 3.19 to determine the volatilization coefficient during flow on the overland-flow terrace: