Environ. Sci. Technol. 1994, 28, 1843-1852
Behavior and Fate of Chlorobenzenes in Spiked and Sewage Sludge-Amended Soil Mln-Jian Wangt and Kevin C. Jones‘ Institute of Environmental and Biological Sciences, Lancaster University, Lancaster, LA 1 4YQ, UK
The main fate and loss kinetic characteristics of chlorobenzenes (CBs) introduced into soil either by spiking or by addition of sewage sludge were investigated. Soils amended with sludge or spiked with CB solution were treated under different conditions. Volatilization was the main loss pathway of CBs from the soils, while the other transformation loss (Le., biodegradation and abiotic losses) processes were minor by comparison. Volatilization of the CBs from soils was influenced by compound properties, environmental conditions, soil composition, and structure. CBs spiked into soil were lost more quickly (with “general” half-lives of 11-181 days) than those applied in sewage sludge (with “general” half-lives of 13-622 days). Loss of the individual CBs followed two-step first-order processes. Subsequent to soil treatments, CBs are likely to volatilize to the atmosphere over short periods in the “first step” (e.g., 74 % of 287 pg CCBs k g l in sludge-amended soil was lost over 35 days, and 67% of 144 pg k g l in spiked soil over 20 days). During the “second step”, CB loss became much slower (e.g., only 8 5% of the CCBs in sludged soil was lost in 224 days and 20% in spiked soil in 239 days), implying that a certain proportion of the CBs added to soil is resistant to loss and much more persistent. Introduction
Organic chemicals introduced into agricultural soil by sewage sludge application are of current research interest, because it is common practice to dispose of sludge to agricultural land and because the organic matrix of sludge can affect compound behavior (1-3). The behavior and fate of chlorobenzenes (CBs) in soil systems were reviewed recently (31, highlighting that there has been little work reported on the CBs in sludge-amended soil. This is somewhat surprising, since CBs are ubiquitous in sludges, and their range of physicochemical properties provides interesting contrasts in compound behavior. It is important to know the main fate of compounds in sludgeamended soil systems, because of current concerns over the potential transfer of contaminants to the human food chain and to enable decisions about how the additions of sludge of land should be managed (1,3). This paper reports on an experiment designed to separate the various loss mechanisms and investigate their relative importance. The investigation was constructed to obtain information on the following: (1)the role of volatilization, biodegradation, photolysis, and other possible loss processes on the fate of CBs in sludge-amended soil; (2) the kinetic characteristics of these possible processes in the system; (3) possible differences in the behavior of CBs between spiked soil and sludge-amended soil. Materials and Methods
Materials. Soil for the experiments was collected adjacent to the Woburn Market Garden Experiment t Present address: Research Center for Eco-Environmental Sciences, Academia Sinica, P.O. Box 2871, Beijing, 100085, People’s Republic of China. 0013-938X/94/0928-1843$04.50/0
0 1994 American Chemical Society
managed by Rothamsted Experimental Station. The soil was taken from the 0-15-cm depth range of a plot with no history of sludge or farmyard manure applications. Subsequent to collection the soil was partially air-dried and passed through a 2-mm mesh size sieve. This soil, classified as a sandy loam with 9% clay, has also been used for experiments described elsewhere (4-6). Some properties of the soil are listed in Table 1, and further details can be found elsewhere (4-6). The anaerobically digested sewage sludge used in this study was collected from a sewage treatment works serving a municipal (- 60 % ) and industrial (-40 % ) catchment. The sludge was only 3% dry matter, of which 68% was organic matter. Sludge from this works is currently disposed of at sea, an operation which must cease by 1998 under new European Commission legislation. The concentrations of CBs in the soil and sewage sludge are listed in Table 2. Compared with the CB concentrations in treated soils, the CB content in the original soil was negligible. Experimental Design. Two series of soil treatments were adopted: one was freshly amended with sewagesludge (l),and the other was spiked with a mixed standard solution of CBs (2). For each treated series, four different experimental conditions, normal (I),sterilized [with 1% (wt) of sodium azide] (II), sterilized and shaded (with aluminium foil) (1111, sterilized, shaded, and sealed (using a Teflon-coated septum) (IV), were investigated. In addition there was a control, Le., the untreated soil (V). For the sludge-amended series, sludge was centrifuged, the supernatant discarded, and the wet solid added to the soil in weighed quantities at a rate of 69 g kg’ soil ( 190 t ha-l, dry weight). For the spiked series, CB stock solution in 18mL of hexane was first diluted with 350 mL of acetone and then the diluted solution was diluted further with distilled and deionized water to -2 L, and added to the soil. All the treated soils were saturated and mixed thoroughly. The initial water content of sludged soil was 35% (w/w) and that of spiked soil was 18% (w/w). The control soil was saturated by adding distilled and deionized water only. Some initial properties of the treated soils are listed in Table 1. Soils for the experimental conditions 1-111 and V were put into glass jars 20 cm tall and 10 cm in diameter as microcosms. Each microcosm held 1000 g (dry weight) of soil. The glass jars for condition I11 were surrounded with aluminium foil to keep the soil from direct solar radiation. Soil for condition IV was put in to 100mL glass bottles which were sealed by screwed caps with Teflon-coated silicone rubber seals and surrounded with aluminium foil. Each of the bottles for condition IV held -60 g (dry weight) of soil. All the microcosms were placed in a glass house where the temperature ranged from 20 to 30 OC over the 259 days of the experiment. Soils were sampled after 0, 0.6, 1.6, 3.6, 7.6, 18, 32, 67, 147, and 259 days. Before each sampling, the soils were mixed thoroughly to ensure typical representation of the samples. Each time soil for condition IV was sampled by opening
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Envlron. Scl. Technol., Vol. 28, No. 11, 1994
1843
Table 1. Initial Properties of t h e Treated Soils soil treatment
moisture' ( % )
soil (for V)e sludge amended soil [for I(l)]e sludge amended and disinfected soil [for II(1), III(1), and IV(l)]e standard spiked soil [for I(2)Ie standard spiked and disinfected soil [for II(2), 111(2),and IV(2)p
18 35 35 18 18
CECbSC pHb (mequiv of Na+/100 g) ESPbpd(%) 6.05 7.64 8.00 6.07 7.16
10.3 20.6 15.6 8.76 6.21
0.24 0.23 3.74 0.19 11.7
organic matter ( % ) 1.48 4.52 4.75 1.41 1.64
Moisture was measured by heating the samples a t 105 OC for 24 h. The properties were measured using the methods described in ref 35. CEC, cation exchange capacity. ESP, exchangeable sodium percentage (30).e I, 11,111, IV, (l), and (2) are experimental conditions and soil treatment methods adopted; see the section on Experimental Design.
._
Table 2. Initial Concentrations of Chlorobenzenes in Sludge a n d Soile (pg k g l ) sludge
soil
compd
mean
SD
1,3-DCB 1,CDCB 1,P-DCB 1,3,5-TCB 1,2,4-TCB 1,2,3-TCB 1,2,3,5-TeCB 1,2,4,5-TeCB 1,2,3,4-TeCB PeCB HCB ECBs
535 1120 2310 53.7 276 228 27.7 1.5 37.6 47.1 223 4860
19.2 54 70.1 0.22 18.9 5.85 3.32 0.27 2.31 1.59 2.26 108
mean 1.38
SD 0.86
Results and Discussion 0.11
0.01
0.036
0.004
0.050 0.036 0.068 1.68
0.002 0.002 0.0005 0.85
The blank space indicates below detection limit.
two (numbered as 1and 2) of the sealed bottles. All the other conditions were conducted in triplicate (numbered as 1,2, and 3). However, only samples numbered as 1for each condition were used for this investigation. Soils for conditions 1-111 and V were watered when necessary. During the experimental period, the moisture of sludged soil was 20-30% (wiw) and that of spiked and control soils was 10-15% (wiw). All of the samples were analyzed in duplicate. Wet soil (10-20 g) was mixed with the same amount of anhydrous sodium sulfate, transferred to a preextracted cellulose extraction thimble, and Soxhlet extracted for 6 h with a mixture of hexane and acetone (2:1, v/v) in full-glass systems using a boiling water bath. Activated copper powder was put into the flask of the Soxhlet system to desulfur the sewage sludge extract (7). The extract was then concentrated to -2 mL in a Buchi RE 121Rotorapor (Switzerland) without using the vacuum system. Cleanup of this extract was accomplished by eluting the concentrated extract with hexane from a hexane prewashed column (i.d., 7 cm) packed with 2-3 g of Florid and collecting the first 10-15 mL in a glass vial. The clean extract was evaporated to 500 p L under the gentle flow of nitrogen and dosed with 1pL of 88 pg mL-l TBB solution in hexane. The dosed extracts were analyzed by gas chromatography (GC) with an electron capture detector (ECD) on DB Wax and Ultra-2 columns. Details of the GC analysis have been reported previously (8). Recovery of monochlorobenzene (MCB) was -38% on average; MCB also gave a poor response on the ECD, so that the MCB detection limit (700pg kg-l) was much higher than the other CBs. Recoveries of the other CBs were 82-97 % , with coefficients of variation (% CV) less than 10%. The detection limits of the method were estimated as 0.008 (hexachlorobenzene, HCB), 0.01 (pentachlo1844
Envlron. Scl. Technol., Vol. 28, No. 11, 1994
robenzene, PeCB), 0.02-0.03 (tetrachlorobenzenes,TeCBs), 0.04-0.06 (trichlorobenzenes, TCBs), and 0.3-0.5 pg k g l (dichlorobenzenes, DCBs) respectively. It was impossible to investigate MCB quantitatively in this work because of its poor response on ECD and low recovery by the method used in this study.
Kinetic Characteristics of CB Loss. CB Loss and Modeling. Tables 3 and 4 present the CB concentrations in sludge-amended and standard spiked soils, respectively, under the normal conditions (condition I) through the experimental tinie course. In addition to MCB, 1,2,4,5TeCB was always below the detection limit in the sewage sludge-amended soil and 1,2,3,5-TeCBwas not added into the spiked soil. Each of the two treatment series therefore had 10 CBs quantitatively monitored in this study. The standard deviations (SD) of CB concentrations in Tables 3 and 4 are generally acceptable, although the SD values were relatively large for some of the early samples, possibly because of the imperfect homogenization of the treated soils. From Table 3, it can be seen that the major loss of most CBs in sludged soil occurred within the first 32 days; i.e., ECBs declined from 287 to 82.8 pg kgl. This was even more marked in the standard spiked soil: Table 4 shows that the CCBs under this treatment decreased from 144 to 51.5 pg k g l in the first 18 days. The loss processes of organic pollutants in the environment are usually complicated, but for the trace concentrations of chemicals, a quasi- (or pseudo-) first-order process model has often been used to describe the process kinetic characteristics, to predict the possible environmental behavior of the chemicals, and to assess the potential impact of a compound on ecosystems (9). The most useful way of judging whether aprocess could actually be treated as first order is to plot the relationship of the logarithmic values of the ratio of the chemical concentration (C) to its initial concentration (CO),i.e., In (CICo) or log (C/Co), against time. If the relationship is linear, the process can safely be treated as first-order. Figures 1and 2 give these relationships for different CBs in sludgeamended soil and CB standard spiked soil respectively, under the normal conditions (condition I). Obviously, none of the relationships shown in Figures 1and 2 is linear, but all of them can be divided into two linear portions. This suggests that the loss processes of CBs in soils occurs in two-steps, each of which is a firstorder process. The length of time for the first step generally increased with increasing compound chlorination level in both the sewage sludge-amended soil and the CB standard spiked soil. Losses of lower chlorinated CBs therefore occur more rapidly than for higher chlorinated
Table 3. Concentrations of CBs in Sludge Amended Soil under Normal Conditions (1)s (pg k g l , n = 2) time (days)
compd
0
1,3-DCB 28.2 f 0.97 1,4-DCB 72.8f14.1 1,P-DCB 126 f 5.88 1,3,5-TCB 3.44 f 0.053 1,2,4-TCB 19.0 f 0.788 1,2,3-TCB 15.1 f 0.234 1,2,3,5-TeCB 2.23 f 0.019 1,2,3,4-TeCB 4.66 f 0.129 PeCB 2.94 f 0.031 HCB 12.8 f 0.154 ICBs 287 f 5.85 0
0.6
1.6
3.6
7.6
17.5
31.6
67
147
259
27.5 f 0.09 72.6i3.64 119 f 4.05 3.27 f 0.050 18.7 f 0.920 14.2 f 0.222 2.12 f 0.002 4.38 f 0.040 2.90 f 0.045 12.0 f 0.349 277 f 9.41
27.5 f 1.33 68.8f5.39 115 f 18.1 3.27 f 0.105 18.0 f 1.72 13.8 f 0.559 2.09 f 0.120 4.36 f 0.260 2.82 f 0.011 11.7 f 0.201 267 f 27.4
21.8 f 1.67 59.1f3.73 92.0 f 6.02 2.73 f 0.102 15.3 f 0.443 12.9 f 0.520 1.93 f 0.012 3.51 f 0.050 2.82 f 0.113 11.0 f 0.189 223 f 12.1
18.1 f 0.66 508f2.71 78.7 f 0.07 2.46 f 0.079 14.4 f 0.610 11.4 f 0.029 1.83 f 0.022 3.30 f 0.078 2.78 f 0.165 10.6 f 0.048 194 f 4.38
13.1 f 0.14 41.1f1.08 56.2 f 0.91 2.14 f 0.077 12.8 f 0.193 9.81 f 0.203 1.76 f 0.028 2.60 f 0.129 2.73 f 0.374 10.4 f 0.021 153 f 1.28
4.52 f 0.48 22.9f0.49 27.5 f 3.64 1.50 f 0.260 6.23 f 0.376 4.95 f 0.322 1.30 f 0.067 1.85 f 0.031 2.51 f 0.083 9.57 f 0.175 82.8 f 4.95
3.89 f 0.10 20.1f3.19 23.1 f 0.20 0.506 f 0.015 3.62 f 0.095 2.33 f 0.097 0.750 f 0.013 1.68 f 0.012 2.29 f 0.052 9.45 f 0.016 67.8 f 3.34
3.37 f 0.15 13.7f0.41 20.4 f 2.24 0.391 f 0.031 2.77 f 0.269 1.11f 0.159 0.273 f 0.019 0.589 f 0.017 1.53 f 0.018 9.05 f 0.068 53.1 f 3.21
3.10 f 0.43 13.3f1.58 19.9 f 2.75 0.292 f 0.018 2.76 f 0.283 1.05 f 0.149 0.257 f 0.085 0.529 f 0.021 1.46 f 0.038 8.94 f 0.078 51.6 f 5.36
Mean f SD.
Table 4. Concentrations of CBs in Standard Spiked Soil under Normal Conditions (I)a ( p g k g * , n = 2) time (days) compd
0
0.6
1.6
3.6
7.6
17.5
31.6
67
147
259
1,3-DCB 1,4-DCB 1,2-DCB 1,3,5-TCB 1,2,4-TCB 1,2,3-TCB 1,2,4,5-TeCB 1,2,3,4-TeCB PeCB HCB ICBs
22.5 f 0.06 36.6 f 0.09 26.8 f 0.14 8.50 f 0.027 12.2 f 0.039 10.3 f 0.024 8.74f0.011 10.5 f 0.104 4.55 f 0.051 2.96 f 0.021 144 f 0.17
22.0 f 0.47 35.1 f 0.79 26.8 f 0.35 8.40 f 0.046 12.1 f 0.172 10.3 f 0.033 8.61f0.138 10.5 f 0.002 4.52 f 0.029 2.90 f 0.012 141 f 1.62
20.4 f 1.29 32.7 f 1.53 24.2 f 1.23 8.01 f 0.465 11.5 f 0.451 9.82 f 0.331 8.45f0.150 10.3 f 0.227 4.49 f 0.084 2.85 f 0.141 133 f 5.89
13.8 f 0.82 21.4 f 1.34 18.0 f 1.31 6.14 f 0.371 8.95 f 0.437 9.66 f 0.573 8.20f0.563 10.0 f 0.585 4.39 f 0.220 2.80 f 0.092 103 f 6.31
12.3 f 0.88 20.5 f 1.08 15.1 f 0.88 5.36 f 0.139 8.02 f 0.230 7.55 f 0.185 7.50f0.144 9.05 f 0.213 4.11 f 0.026 2.65 f 0,019 92.1 f 3.76
6.01 f 0.22 9.82 f 0.37 6.90 f 0.22 2.66 f 0.072 4.81 f 0.149 4.00 rt 0.108 5.01f0.204 6.47 f 0.301 3.36 f 0.161 2.45 f 0.140 51.5 f 1.95
5.54 f 0.28 9.47 f 0.34 6.21 f 0.18 2.24 f 0.151 3.89 f 0.238 2.80 f 0.170 3.66f0.251 4.75 f 0.366 2.70 f 0.199 2.30 f 0.074 43.5 f 2.26
4.84 f 0.22 7.95 f 0.56 5.39 f 0.41 2.01 f 0.123 3.62 f 0.196 2.79 f 0.143 3.46f0.182 4.20 f 0.265 2.61 f 0.157 2.13 f 0.142 39.0 f 1.27
3.58 f 0.04 5.39 f 0.60 3.88 f 0.04 1.52 f 0.028 2.28 f 0.040 1.89 f 0.008 2.4110.044 2.77 f 0.027 1.77 f 0.027 1.70 f 0.012 27.2 f 0.33
2.53 f 0.09 2.38 f 0.09 2.82 f 0.18 1.19 f 0.006 1.93 f 0.004 1.47 f 0.019 2.26f0.254 1.98 f 0.033 1.20 f 0.009 1.05 f 0.036 18.8 f 0.33
a
Mean f SD.
3 1,3-DCB
2
Measured data - Two step model ----- One step model
Y
h
w $1 Y
ci C
0
0
I! 0
0
‘*.
I
100 200 Time (days)
i
300
’. %
.I -
8.
I
d
1 1 0
I
I
100 200 Time (days)
I
300
processes in soil. This is consistent with reports that retention of CBs in soil increases with the organic content of the soil (IO) and that PCB volatilization from soil decreases with sewage sludge addition (11). The CB loss rates in the second step were all much slower than in the first steps. A proportion of the chemical added in sludge will therefore stay in the soil for a comparatively long Environ. Sci. Technol., Voi. 28, No. 1 1 ,
1994
1845
lo
* Measured data Two step model
1
1,3,5-TCB
---_. One step model
1 I
I
1
I
300
100 200 Time (days)
0
0
100 200 Time (days)
300
0
100 200 Time (days)
300
’. 1
I
0
100 200 300 Time (days) Flgure 2. Kinetic characteristics of CB loss in standard spiked soil.
Table 5. Half-Lives of Chlorobenzenes in the Sewage Sludge-Amended Soil in Microcosm Experiments first step compd
half-life (days)
1,3-DCB 1,4-DCB 1,2-DCB 1,3,5-TCB 1,2,4-TCB 1,2,3-TCB 1,2,3,5-TeCB 1,2,3,4-TeCB PeCB HCB CCBs
12.4 17.4 13.2 23.7 22.5 22.2 42.6 31.4 170 74.7 16.5
second step
“general”
R2 period (days) loss ( % ) half-life (days) R2 period (days) loss ( % ) half-life (days) R2 loss (% ) 0.989 0.989 0.985 0.976 0.981 0.994 0.984 0.822 0.966 0.452 0.985
0-35 0-35 0-35 0-67 0-75 0-94 0-125 0-132 0-153 0-25 0-35
85.1 70.5 79.9 85.3 83.2 88.6 77.1 74.0 48.0 22.2 73.7
579 294 892 239 49500 1370 1250 721 1550 2230 467
0.944 0.762 0.799 0.996 1.000 1.000 1.000 1.000 1.000 0.899 0.774
35-259 35-259 35-259 67-259 75-259 94-259 125-259 132-259 153-259 25-259 35-259
3.93 11.3 4.29 6.22 2.26 4.45 11.4 14.6 2.38 8.18 8.28
13.0 22.1 14.4 24.4 23.3 22.5 45.4 34.5 219 622 19.7
0.966 0.897 0.925 0.973 0.955 0.990 0.984 0.901 0.940 0.508 0.903
89.0 81.7 84.2 91.5 85.5 93.0 88.5 88.7 50.4 30.4 82.0
Table 6. Half-Lives of Chlorobenzenes in the Standard Spiked Soil in Microcosm Experiments first step compd
half-life (days)
1,3-DCB 1,4-DCB 1,P-DCB 1,3,5-TCB 1,2,4-TCB 1,2,3-TCB 1,2,4,5-TeCB 1,2,3,4-TeCB PeCB HCB ZCBs
8.42 8.57 8.63 10.5 12.5 15.0 24.4 26.6 41.1 66.5 11.6
second step
R2 period (days) loss ( % ) half-life (days) R2 period (days) loss ( % ) half-life (days) R2 loss ( % ) 0.958 0.941 0.984 0.981 0.972 0.979 0.985 0.989 0.994 0.971 0.977
0-18 0-17 0-19 0-20 0-22 0-29 0-32 0-32 0-29 0-16 0-20
73.3 73.2 75.6 71.2 64.5 73.0 58.1 54.9 40.7 17.3 67.0
time. Furthermore, a proportion of the residue may become bound in the soil (12-14). The half-lives of CB loss from the sludged and spiked soils were obtained by regressingthe concentrations shown in Tables 3 and 4 using the two-step first-order kinetic model. CB half-lives, the step periods, and the loss percentage in each of the steps for the sludged and spiked soils are listed in Tables 5 and 6, respectively. For the purpose of comparison and overall discussion, the pa1846
“general”
Envlron. Scl. Technol., Vol. 28, No. 11, 1994
189 131 191 238 194 222 280 168 187 215 181
0.995 0.990 0.993 0.990 0.942 0.948 0.895 0.988 0.975 0.987 0.995
18-259 17-259 19-259 20-259 22-259 29-259 32-259 32-259 29-259 16-259 20-259
15.4 20.4 13.9 14.8 19.8 12.8 16.0 26.3 33.0 47.2 20.0
10.6 11.2 10.5 13.6 19.4 17.6 46.9 47.2 103 181 16.9
0.892 0.895 0.921 0.897 0.867 0.909 0.829 0.896 0.908 0.964 0.879
88.8 93.5 89.5 86.0 84.3 85.8 74.1 81.2 73.7 64.5 86.9
rameters for the so-called “general” first-order process, i.e., the one-step kinetic model, were also calculated and are listed in these tables. From Tables 5 and 6, it can be seen that the half-lives of CBs with high chlorination levels are generally longer than those with lower chlorination levels, except for HCB in sludge-amended soil. The regression coefficients (95 % confidence) for all the kinetic modeling are good, again with the exception of HCB in sludged soil. HCB has the
greatest tendency of all the CBs to sorb to soil. Sewage sludge addition increases the sorption capacity of soil, which can affect the behavior of the chemicals which are strongly sorbed (11)more than others (15). The loss of HCB in sludged soil (only 30%) was the least of all the CBs; the “special” behavior of HCB in sludge-amended soil might therefore have resulted partially from the higher relative errors (experimental errors over the decrease in HCB concentration). The CB loss percentage in the first step decreased with the increase of chlorine atom number in the molecules. For example, in the sludge-amended soil, 85.1% of the 1,2-DCB was lost during the first step and only 3.9% in the second step while only 22.2% of HCB was lost in the first step and 8.2% in the second step. In CB standard spiked soil, this effect was even more dramatic, with first step losses of 73.3% and 17.3% for 1,2-DCB and HCB, respectively, and second step losses of 15.4% and 47.2%; Le., HCB loss in the second step was 2-fold more than that in the first step. This observation indicates that, on the one hand, the quick loss of organic compounds in the first step had less effect on the persistent chemicals than on the nonpersistent ones. On the other hand, a certain proportion of the chemicals which tended to be lost very quickly in the first step remained in the soil over quite a long period. The half-lives of CBs from the “general” firstorder kinetic model were similar to those for the first step model and much shorter than those for the second step. The one-step model was therefore not suitable for adequately describing CB loss processes, especially for the purpose of discussion and prediction of CB behavior under field conditions, where long-term persistence may be of concern. Although the CB loss rates in sewage sludge-amended soil were slower than those in the CB standard spiked soil, the total percentage lost from the sludged soil after 259 days was nearly the same as in the spiked soil, except for PeCB and HCB, because the first step lasted longer in sludged soil than in spiked soil. Volatilization was the main loss pathway of CBs in the soils, as will be discussed in the following section. According to Jury et al. (16-18), the predominant resistance to volatilization of CBs from soil lies in the retardation of compound movement toward the soil surface. Presumably the slower desorption of CBs in the sludged treatment resulted in slower but more prolonged volatilization than in the spiked soil. The general half-lives reported here are likely to be shorter than measured in field situations, since the soils in this study would have been aerated more often and more thoroughly and kept at higher temperature than field soils in Britain. Although the measured data in Figures 1 and 2 show that the CB losses in the soil systems were two-step firstorder processes, the regression coefficients for the onestep first-order kinetic model were also quite high (see Tables 5 and 6). To compare these two models directly, CB concentrations predicted by both models are shown in Figures 1 and 2 for the sludged and spiked soils separately. These curves clearly show that the two-step models fit the experimental data much better than the one-step models. The one-step model generally fits the first step of CB loss with low precision. For HCB and PeCB in spiked soil, and HCB in sludged soil, the onestep model gave quite a close fit to the second step, since the concentrations did not change much during the first
steps in these cases. When the number of samples is big enough, a regression usually gives good coefficients,though sometimes the model used may not actually fit the data very well (19). It is therefore always worthwhile to present the curves in graphical form, rather than relying only on regressing the data with proposed models. It is worth stressing that these model fits point to differences in loss process mechanisms over time. Clearly, if the sampling is not frequent enough during the early period (first step), or if the experimental time is too short, the two-step phenomenon will not be observed. Equally, it is possible that losses may actually occur over more than two-steps in some situations and that a greater sampling frequency than that used here would be necessary to detect it. These comments have general relevance to studies of the persistence of organic chemicals in soils. Comments on the Two-step Loss Process. Two-step first-order loss kinetics of organic chemicals in soils have been observed previously by other authors. PCBs were reported to be lost from sludge-amended soils in a twostep first-order process mainly through volatilization (11). Foxall and Maroko (20) reported that DDT volatilized from soil following its application to cotton crops, showing an initial very rapid step, followed by a much slower one. These cases were very similar to the observations presented here. LaFleur and co-workers (21, 22) reported and discussed a two-step first-order process for the loss of prometryne, carbaryl, trifluralin, and metribuzin from a sandy loam. In these processes, the compound loss rates during the first steps were all significantly higher than those in the second steps. O’Connor et al. (23)investigated the behavior of 2,4-dinitrophenol in sludge-amended soils and found that the degradation was characterized by three steps. One possible explanation for this observation could be that an initial acclimatization period for microorganisms may have resulted in a slow first step, which was followed by a quick second step and then a much slower third step. The influence of sorption-desorption processes can be most readily used to interpret the two-step behavior of organic chemical loss in soils. Organic pollutants introduced into a soil may be initially “free” (nonsorbed or labile), but a proportion (sometimes large) will become sorbed (loosely and/or slowly reversibly sorbed and/or irreversibly sorbed) (24,25). Loss of the sorbed fraction of the chemicals is likely to be restricted (26). Initially, therefore, there is continued loss and increased sorption of compound in the “free” chemical reservoir (first step). Sorption of nonionic chemicals in soils is thought to be a continuum of possible interactions starting with spontaneous adsorption on fixed sites and ending with a slow diffusion of true partition between three-dimensional phases (27, 28). In materials with high organic matter content, partitioning is thought to dominate the sorption process for compounds with high sorbability or high octanol-water partition coefficient (Kow)(29). Desorption principally reverses the process: slow outward diffusion from the three-dimensional matrix and equilibrating release at the matrix-water interface (22). Obviously,the CB loss kinetic characteristics in the second step will be affected by sorption-desorption processes, especially for compounds (like HCB) with a high propensity to sorb on soils, because CBs must be present in a “free” or available form to be lost. Hamaker and Goring (21)proposed a model for pesticide disappearance from soil with time, which was used to Environ. Sci. Technol., Vol. 28, No. 11, 1994
1847
-
28
--P-
h
en
Y
\
1
-
IV Ill
m j 18
--+- I1 - 1
v
- v
C
6
8 -2
T " 0
m
100 200 Time (days)
300
-1
P
:
1
100 200 Time (days)
0
300
1.9
f
1,2,3,5-TeCB
en
I v
g 0.9
PeCB
0
0
-
- I
0
100 200 Time (days)
300
I
0
100 200 Time (days)
1
300
Flgure 3. Comparison of results from sludge-amended soil under different experimental conditions: I-V. See section on Experimental Design.
interpret the two-step first-order process: "Free"chemicals
Loss of the sorbed chemicals is usually much restricted (26).
The "free" chemical concentrations in soils are relatively high and variable in the first step and relatively low and stable in the second step (21), which results in rapid compound loss rates with bigger experimental errors during the first step and the slow loss rates with smaller errors in the second step. This is consistent with the observations from these experiments. Main Fate of CBs in the Soils. In addition to showing the loss kinetics, the experiments were also designed to investigate the main fate of CBs in soil. As described in the experimental section, five different conditions were adopted: I, allowing loss by all the possible processes; 11, allowing loss by all the possible processes except biodegradation; 111, allowing loss by all the processes except biodegradation and photolysis; IV, allowing loss by all the processes other than biodegradation, photolysis, and volatilization; and V, the control (untreated). On the basis of the differences between the results obtained from the different conditions, it was possible to estimate the relative importance of CB loss processes. The contents of CBs in the soils under these conditions were monitored during the time course. Some CB concentrations through the experiment for all the conditions are shown in Figures 3 and 4 for the sludge-amended soil and standard spiked soil, respectively. In Figures 3 and 4,it is clear that, for both the sludgeamended soil and the CB standard spiked soil, the most significant difference occurred between conditions I11and 1848
Environ. Scl. Technol., Vol. 28, No. 11, 1994
IV. The only difference between these two conditions was that (IV) was sealed, while condition I11 was open, like conditions I, 11,clearly indicating that volatilization was the dominant loss process for all compounds. This result is essentially consistent with the discussion based on the literature review (3). Under condition IV, CB concentrations basically remained unchanged during the experimental period. [The concentrations were lower at some points, especiallyduring the early period of the incubation, presumably because of compound volatilization during the sampling and/or sample treatment procedures. After -20 days, the CBs in the soils were sorbed more firmly and the losses caused by volatilization declined. However, this should not have significantlyaffected the other conditions because the most mobile proportion of CBs in these (open) conditions was already lost before the samples were collected.] This implies that biodegradation and photolysis, which were limited by this condition, and the other abiotic losses, such as chemical hydrolysis and oxidation, were also minor in affecting the CB content of soils. In the control soil, Le., condition V, all the CB concentrations were not detectable or were negligible compared to the other treated soils. Sludge- Amended Soil. In the original experimental design, differences between conditions I and I1 should be caused by biodegradation and differences between conditions I1 and I11 should result from photolysis. However, in Figure 3, it can be seen that during the second-step loss process, there was little difference between the concentrations of DCBs and TCBs in sludge-amended soils under conditions 1-111, implying that neither biodegradation nor photolysis is a significant loss process for these compounds. The differences in these compound concentrations during the first step could not be caused primarily by biodegradation and photolysis,because such interpretation would not be consistent with the disappearance of the differences in the second loss step. It is most logical to interpret the
B
11.51
19 h
m
4
-
-Y-
Y
1,3-DCB
Ill II
- 0 - 1
- v
0
0
1.5
-0.5
0
100
200
0
300
Time (days)
L
100
200 Time (days)
300
100 200 Time (days)
300
4.8
9.5
. h
3 7.5
m
2 2.8
1,2,3,4-TeCB
m
3 5.5
g
Y
$
3.5
0
3.8
X
1.8
0
1.5
- .-0.5 )k 0
0.8
-
I
100
d I
I
200
300
Time (days)
-0.2
0
Flgure 4. Comparison of results from standard spiked soil under diffei*ent experimental conditions: I-V. See section on Experimental Design.
differences as resulting from different volatilization rates, affected by the different experimental conditions and soil components. This will be discussed in the following section. There were some differences between the concentrations of TeCBs, PeCB, and HCB in the sludgeamended soils under conditions 1-111. These basically occurred during the first-step-loss processes and were maintained into the second step. It is possible that the same factors that affected the loss processes of DCBs and TCBs resulted in these differences among the losses of TeCBs, PeCB, and HCB as well. However, in summary, even if biodegradation and photolysis did occur in the soils,they were minor processes compared to volatilization. Spiked Soil. In the CB standard spiked soils, the results were dramatically different. The concentrations of CBs in condition I remained higher than those under conditions I1 and 111. This implies that biodegradation in spiked systems was negligible compared to volatilization and that other factors affected CB loss from these soils, resulting in the low loss rates of CBs under condition I. Another interesting observation was that the concentrations of CBs in condition I11were close to those under condition I during the first step, but close to those under condition I1 during the second steps. This implies that experimental conditions were probably not the only factor affecting the CB loss processes. Again, the differences between the CB concentrations under conditions I1 and I11could not have been caused by photolysis, since the differences became smaller during the second steps and it was not possible for photolysis to become a negative factor affecting the CB loss rates during the second steps. Obviously, photolysis, biodegradation, and other possible reactions were not major processes in either the sludge-amended or spiked soils compared with volatilization. Differences between the CB loss rates in soils under the different conditions occurred basically during the first steps. These differencescould not result from any process other than volatilization itself.
Factors Affecting CB Loss Processes in Soils. To evaluate the factors which influenced CB loss rates under the different conditions, the relationship between some CB loss rate constants (KcB)and water evaporation rate constants (Kwakr)for the first steps under the different experimental conditions are shown in Figure 5. Under identical soil conditions, it is reasonable to expect that water evaporation linearly correlates to compound volatilization. From Figure 5, it can be seen that the relationships were basically linear except for HCB, which gave no apparent trend. This gives further support to the assumption that CB loss from soils was mainly caused by volatilization. Soils under conditions 11-IV were all sterilized by sodium azide and would have had essentially the same properties. The only difference was in their contact with the surrounding environment: condition I1 was open to the air and light; condition I11 was open to the air but shaded; and condition IV was both shaded and sealed from the air. Soil under condition I11 would not have received as much sunlight as that under condition 11, so presumably the CBs and water would not have volatilized from the soil under condition I11 as quickly as under condition 11. CBs and water could not volatilize from the soil under condition IV. The application of sewage sludge increased the soil organic matter content and the amount and sizes of clusters in the soil domains and aggregates. Therefore, on the one hand, the diffusion rates of water and CBs in sludged soil became slower, and on the other hand, water and CBs had to move greater distances to reach the soil surfaces. Both of these factors would result in the CBs and water evaporating to the atmosphere more slowly than in spiked soil. In Figure 5, all the points for condition I (i.e., with all loss processes possible), especially those for the spiked soil, gave lower ratios of KCBover Kwater, implying there was something different in the soil under condition I from those under other conditions. One possible explanation is that the sodium azide may have affected the surface Environ. Sci. Technol., Vol. 28, No. 11, 1994
1849
h
$ 0.2
4 -
1,2 DCB y = 3.20e-3 + 1 . 2 0 ~ RY? = 0.864
II(2)
1,3,5-TCB y = 1.93e-3 + 0.992~ RA2 = 0.838
y
0.00
0.02 0.04 0.06 0.08 0.10 K of water loss (first step, llday)
n
g 0.08-
. U
”
0.12
y
~(2) Q
I
0.00
g 0.04
0.02 0.04 0.06 0.08 0.10 K of water loss (first step, l/day)
0.12
h
. 1 5
1,2,3,4-TeCB
l-
y = 1.20e-4 + 0 . 4 5 7 ~ RA2 = 0.809
U
W2)
PeCB
?-
EJ
0.03-
c
y = - 1.06e-3
+ 0.265~ RA2 = 0.841
I
e
0.02-
In
y
0.00
0.02 0.04 0.06 0.08 0.10 K of water loss (l/day)
0.12
y
0.00
0.02 0.04 0.06 0.08 0.10 K of water loss (first step, l/day)
0.12
Flgure 5. Relationship between loss rate coefficients of CB and water: I-V. (1) sludged soil, and (2) spiked soil. See section on Experimental Design.
properties of the soil solids and reduced the soil adsorption capacity for organic compounds, making the soils under conditions I1and 111less able to retain organic compounds than the corresponding unsterilized soils. The sludgeamended soil had a higher buffering capacity against salinization than the spiked soil, which made the effect of sodium azide on the former less. Exchangeable sodium percentage (ESP) values are usually used to indicate soil salinization levels, which has a deleterious effect on the chemical and physical properties of soils (30).From Table 1,it can be seen that, after the application of sodium azide, the ESP in spiked soil increased from 0.19% to 11.7% while the value in sludge-amended soil only changed from 0.23% to 3.74% so that the property of spiked soil was changed much more than that of sludged soil. For soils with low organic matter content, organic chemicals are mainly adsorbed on the active sites on the soil particulates, which can be affected seriously by the ionic inorganic materials occupying the active sites. For soils with high organic matter content, organic compounds will mainly distribute into the organic matter so that the effect from the inorganic materials is “buffered” significantly. Sorption of nonionic organics in soils could decrease with increasing soil pH (31). The addition of sodium azide increased the pH values of the soils by different amounts (seeTable 1). This may also have made some contribution to the lower sorption capacity of the sterilized soils. These factors (ESP and pH) may explain why, when compared with the situation in sterilized soils, volatilization of CBs from the unsterilized soils was slower, and the ratios of KCBover Kwaterin the unsterilized soils appeared to be relatively low. As discussed in the previous section, a large proportion of the CBs probably became sorbed to the soil during the second step and would have had to be desorbed from the soil to allow movement to the surface and volatilization. Loss rates are therefore thought to be dominated by the 1850
Environ. Sci. Technol., Vol. 28, No. 11, 1994
kinetics of desorption. In the standard spiked soil, the addition of sodium azide affected the soil properties so that CB concentrations under condition 111changed from being close to condition I during the first step to being close to condition I1 in the second step. This implies that soil components (soil under condition I11 was the same as that under condition I1 and different from that under condition I) play an important role in determining loss during the second step. Sodium azide addition affected the evaporation process from the sludged and spiked soils in apparently opposite ways. The sludge-amended soil became very blocky after being sterilized by sodium azide, which made evaporation from the soil slower than that from the unsterilized one. Volatilization of CBs from soil is thought to be controlled by compound diffusion in soil (16-18), and the stickiness and radii of the aggregates would influence diffusion time through soils (32,33). In contrast, sodium azide made the spiked soil looser, so that evaporation in the sterilized soil increased. I t is likely that different components in the soils, especially the different organic matter contents caused by sludge application, resulted in the opposite response to the same disinfectant. Relationships between CB Half-Lives and Properties. It is useful to relate the half-lives of CBs in soils to some of their physicochemical properties. Figure 6 shows the relationships between the mean “general” half-lives and molecular volume, boiling point, log KO,,log vapor CB half-life increased pressure (V,), andlog solubility (8). with molecular volume (R2of 0.952; see Figure 6a), boiling point (R2 of 0.924; see Figure 6b), and log KO,(R2of 0.890; see Figure 6c), whereas it decreased when log vapor pressure ( R of 0.912; see Figure 6d) and log solubility ( R of 0.920; see Figure 6e) increased. These relationships support the point of view that the main fate of CBs in the soils was volatilization.
500 1
500
I
4oo
y = 4.5e-2
* lo"(0.68~) RA2= 0.89
300
_ .
150
200 250 300 350 a) Molecular Volume (AA3/molecule)
4oo
~
1 k=
0.70* lo"(4.46~) R"2 = 0.91
5.
3
S
350
250
4.0 c) Log Kow
3.0
b) Boiling Point ("C)
-=$
500
5.0
y = 1.0 * 1OA(-O.34x) R"2 = 0.92
400
U
300 9nn-l
-6.5
I \
T
-5.5
-4.5
0
-3.5
-2.5
d) Log Vapor Pressure (atm)
e) Log Solubility @/I)
Flgure 6. Relationship between means of "general" half-lives and chlorobenzene physicochemical properties.
It has been suggested that volatilization of organic chemicals from soil would be controlled by the value of log (HIK,,) (Henry's law constant/octanol-water partition coefficient; Prof. D. Mackay, University of Toronto, 1991, personal communication). Figure 7a gives the relationship between CB half-lives and log (H/Kow). As mentioned above, the principle resistance to volatilization of all the CBs is thought to be within the soil phase (3,16-18). Molecular volume could have an inverse effect on movement through the soil. Boiling point is actually dependent on the vapor pressure. By combining Vp, S, and KO,(i.e., solubility in octanol/S) together, the potential of CB volatilization from soil can be expressed as
m
5001
E 400
I
y = 0.283 * 1OA(-O.512x) R"2 = 0.905
-6
a)
3
-5 -4 Log(H/Kow) (atm Ilmole)
y = 0.391
400
* 10"(-0.157~)
-3
R"2 = 0.937
volatilization potential = (V,S)/K,,
or as volatilization potential = Vp/Kow These empirical expressions indicate the importance of water solubility in the CB volatilization process, besides those factors suggested by Laskowski et al. (341, which have been discussed previously (3). Panels b and c of Figure 7 show the relationships between CB half-lives in soils and their "volatilization potential". Interestingly, they give better correlation coefficients (>0.93) than those obtained in Figure 6c-e (0.89-0.92) and Figure 7a (0.91).
-20
-18 -16 -14 -12 b) log(Vp WKow) (atm mole/l)
-12
-11
-10
-8
-7
-6
Conclusions The main loss process of CBs from soil is volatilization to the atmosphere. In comparison, biodegradation, photolysis, and all the other possible processes were insignificant. CB volatilization from soil could be affected by environmental conditions and the soil texture and structure, besides the compound's physicochemical properties. The kinetic characteristics of the CB loss processes from soil can be described by two-step first-order processes. During the first step, volatilization rates were high and a substantial proportion of the CBs was lost. The second step was much slower than the first and was presumably
-10 -9 -8 c) Log(Vp/Kow) (atm)
Figure 7. Relationship of CB properties with their half-lives in soils.
controlled by the rate of compound desorption from soil. Most of the CB applied onto field soils in sewage sludge is therefore likely to evaporate into the atmosphere over relatively short periods, but a proportion will stay in the soil for much longer periods, especially for HCB and PeCB. Environ. Sci. Technol., Vol. 28, No. 11, 1994
1851
The CBs spiked into soil were lost more quickly than those applied with sewage sludge, so that care should be taken when the behavior of organic compounds in spiked soils is extrapolated to the case of sludge application in the field. CBs with high chlorination levels, such as HCB and PeCB, are significantly more persistent in the field soil amended with sewage sludge than in spiked soil experiments. Given the importance of volatilization, if sewage sludge is injected into the soil rather than sprayed onto the surface of the field, CB loss rates from agricultural soils will presumably be much slower. Acknowledgments
We are grateful to the UK Ministry of Agriculture, Fisheries and Food for financial support to investigate the fate and effects of organic contaminants from sewage sludge applied to agricultural land, and to Dr. S. P. McGrath for supplying the Woburn soil. Literature Cited (1) Jacobs, L. W.; O’Connor, G. A,; Overcash, M .A.; Zabik, M.
J.; Rygiewicz, P. In Land Application of Sludge-Food Chain Implications; Page, A. L., Logan, T. J., Ryan, J. A., Eds.; Lewis Publishers: Chelsea, MI, 1987; pp 101-143. (2) Wild, S.R.; Jones, K. C. Sci. Total Environ. 1992,119,85119. (3) Wang, M.-J.; Jones, K. C. Chemosphere 1994, 28, 13251360. (4) Wang, M.-J.; Jones, K. C. Environ. Sci. Technol. 1994,28, 1260-1267. (5) McGrath, S. P. In Pollutant Transport and Fate in
Ecosystems; Coughtrey, P. J., Martin, M. H., Unsworth, M. H., Eds.; British Ecological Society Special Publication No. 6; Blackwell Scientific Publications: Oxford, UK, 1987; pp 301-317. (6) McGrath, S.P.; Lane, P. W. Environ. Pollut. 1989,60,235256. (7) Jones, K. C.; Stratford, J. A.; Waterhouse, K. S.; Furlong,
E. T.; Giger, W.; Hites, R. A.; Schaffner, C.; Johnston, A. E. Environ. Sci. Technol. 1989, 23, 95-101. (8) Wang, M.-J.; Jones, K. C. Chemosphere 1991,23,677-691. (9) Asher, S. C.; Lloyd, K. M.;Mackay,D.; Paterson, S.;Roberts, J. R. A Critical ExaminationofEnvironmentalModellingModelling the Environmental Fate of Chlorobenzenes Using the Persistence and Fugacity Models; NRCC No. 23990; National Research Council Canada: Ottawa, ON, Canada, 1985. (10) IPCS Chlorobenzenes other than Hexachlorobenzene; World Health Organization: Geneva, 1991. (11) Fairbanks, B. C.; O’Connor, G. A.; Smith, S. E. J. Enuiron. Qual. 1987, 16, 18-25.
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(12) Scheunert, I.; Topp, E.; Schmitzer, J.; Klein, W.; Korte, F. Ecotoxicol. Environ. Saf. 1985, 9, 159-170. (13) Pignatello, J. J.; Huang, L. Q. J. Enuiron. Qual. 1991,20, 222-228. (14) Harmsen, J. In On-Site Bioreclarnation; Hinchee, R. E.,
Olfenbuttel, R. F., Eds.; Butterworth-Heinemann: Oxford, UK, 1991; pp 255-272. (15) Jin, Y.; O’Connor, G. A. J. Environ. Qual. 1990, 19, 573579. (16) Jury, W.A.; Spencer, W. F.;Farmer, W. J. J. Environ. Qual. is83,12,55a-564. (17) Jury, W. A.; Farmer, W. J.; Spencer, W. F. J. Environ. Qual. 1984,13, 567-572. (18) Jury, W. A.; Spencer, W. F.; Farmer, W. J. J. Environ. Qual. 1984,13, 573-579. (19) Wold, S. Quant. Struct.-Act. Relat. 1991, 10, 191-193. (20) Foxall, C. D.; Maroko, J. B. M. In Proceedings of a
International Conference on Environmental Contamination; CEP Consultants Ltd: Edinburgh, 1984; pp 91-95. (21) LaFleur, K. S. Soil Sci. 1980, 130 (2), 83-87. (22) LaFleur, K. S.; McCaskill, W. R.; Gale, G. T., Jr. Soil Sci. 1978, 126 (5), 285-289. (23) O’Connor, G. A.; Lujan, J. R.; Jin, Y. J. Environ. Qual. 1990, 19, 587-593. (24) Pignatello, J. J. Environ. Toxicol. Chem. 1990, 9, 11071115. (25) Pignatello, J. J. Enuiron. Toxicol. Chem. 1990, 9, 11171126. (26) Weber, J. B.; Coble, H. D. J. Agric. Food Chem. 1968,16, 475-468. (27) Mingelgrin, U.; Gerstl, Z. J. Enuiron. Qual. 1983,12, 1-11. (28) Paya-Perez, A. B.; Riaz, M.; Larsen, B. R. Ecotoxicol. Enuiron. Sa/. 1991, 21, 1-17. (29) Wang, L. P.; Govind, R. Enuiron. Sci. Technol. 1993, 27, 152-158. (30) Marshall, T. J.; Holmes, J. W. Soil Physics; Cambridge University Press: Cambridge, UK, 1988; pp 196-223. (31) Gerstl, Z.; Kliger, L. J. Environ. Sci. Health 1990, B25, 729-741. (32) Wu, S.-C.; Gschwend, P. M. Environ. Sci. Technol. 1986, 20, 711-725. (33) Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25, 1237-1249. (34) Laskowski, D. A.; Goring, C. A. F.; McCall, P. J.; Swann, R.
L. In Environmental Risk Analysis for Chemicals;Conway, R. A., Ed.; Van Nostrand Reinhold Co.: New York, 1982; pp 198-240. (35) MAFF The Analysis of Agricultural Materials, Reference Book 427; Her Majesty’s Stationery Office: London, 1986.
Received for review December 20, 1993. Revised manuscript received June 1, 1994. Accepted June 7, 1994.’ ~~~
0
Abstract published in Advance ACS Abstracts, July 15, 1994.
