3.1 Degradation behaviors of HCB and coexisting chlorobenzene congeners under different initial pH
Figure 1 is plot of residual concentrations of HCB and three coexisting chlorobenzene congeners versus reaction time under different initial pH. As shown in Fig. 1 (a), the residual concentrations of HCB decreased with the increase of initial pH. The lower residual concentrations of HCB were found under acidic conditions (pH = 3) and slightly acidic conditions (pH = 5), whereas higher residual concentrations were found under alkaline conditions (pH = 9 and pH = 11), within the initial pH range of 3.0–11.0. These indicate that the higher degradation rate of HCB can be obtained under acidic and slightly acidic reaction system and the degradation rate of HCB is lower under alkaline reaction system.
The degradation activity of three coexisting chlorobenzene congeners in real HCB-contaminated soil was investigated with the initial pH in the range of 3.0–11.0. As depicted in Fig. 1 (b), the residual concentrations of PeCB decreased rapidly, within reaction time of 23 days, and then increased slowly, following with decreasing under all initial pH condition, which is lower acidic conditions and slightly acidic conditions. The same results are also observed for 1,2,4,5-TeCBs and 1,2,4-TCB. The reason for the discontinuous decreasing and the different degradation behaviours of three chlorobenzenes is that there is accumulation, depending on reaction rate under different initial pH, existed in the process of degradation by nanoscale zero-valent iron.
Figure 2 shows the reaction rate of four coexisting chlorobenzene congeners HCB, PeCB, 1,2,4,5-TeCB and 1,2,4-TCB under different initial pH. The reaction rate can be described as the following equation:
ν= -dC/dt
Where ν is reaction rate per unit biomass, C is the chlorobenzene concentration, and t is the reaction time.
As shown, the reaction rate of three coexisting chlorobenzene congeners had a “V” type change. For PeCB, 1,2,4,5-TeCB and 1,2,4-TCB, the calculated value of reaction rate may not be “positive” or “negative” invariably, because that the production and degradation of three coexisting chlorobenzene congeners proceed simultaneously (Jiang et al. 2014). So, when the calculated value of ν is positive, the degradation of low-toxic chlorinated benzenes is predominant and the concentrations of chlorobenzenes decrease constantly, that is, there is no accumulation in the degradation. Otherwise, when the calculated value of ν is negative, the production of low-toxic chlorinated benzenes is predominant and the concentrations of chlorobenzenes increase constantly and there is accumulation. It is evident that the reaction rate of HCB is higher than other three coexisting chlorobenzene congeners, especially in the acidic (pH = 3) and slightly acidic conditions (pH = 5), and PeCB comes second.
For every single curve, the figure area under the “0” calibration can be calculated with the method of calculus and results are shown in Table 1. The size of the area reflects the amount and extent of accumulated coexisting chlorobenzenes. It is also evident that the accumulative amount and the extent of 1,2,4-TCB is the most in the acidic and alkaline condition, especially, acidic conditions (pH = 3) and slightly acidic conditions (pH = 5). The results indicate that the degradation reaction rate of high chlorinated benzenes is higher in the acidic condition during the degradation process with nanoscale zero-valent iron. The reason is that the dissolution of the passivating hydroxide layer on nZVI was facilitated at low pH, which increased the reactivity, and at higher pH, ferrous and ferric hydroxides were formed, which could have resulted in hydroxide covering the Fe0 surfaces and hampering the electron transfer (Wei et al. 2014). So, the degradation of HCB by nZVI is predominant under acidic condition, and the role of native microbial is predominant under alkaline condition.
Table 1 the accumulation of three coexisting chlorobenzene congeners (mg/kg)
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pH = 3
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pH = 5
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pH = 7
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pH = 9
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pH = 11
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PeCB
|
0.0083
|
0.0054
|
0.03136
|
0.04963
|
0.0205
|
1,2,4,5-TeCB
|
0.0122
|
0.0336
|
0.02761
|
0.05063
|
0.0135
|
1,2,4-TCB
|
0.0473
|
0.0682
|
0.02458
|
0.05223
|
0.0396
|
3.2 Degradation behaviors of HCB and coexisting chlorobenzene congeners under different reaction temperature
Figure 3 is plot of residual concentrations of HCB and three coexisting chlorobenzene congeners versus reaction time under different reaction temperature.
It is obvious that the residual concentrations of HCB decrease with increasing reaction temperature, within a temperature range of 15 − 55 ◦C. When the reaction temperature was up to 45◦C, the residual concentration of HCB was almost the lowest during the degradation of 23 days. This implies that the degradation rate of HCB was almost the highest with a reaction temperature of 45◦C. Similarly, it can be concluded that the lower degradation rate of HCB in room temperature is due to accumulation of the large amount of 1,2,4-TCB, according to the degradation behaviors of coexisting chlorobenzene congeners under different reaction temperatures. The degradation behavior of HCB is similar to Jiang (2014). It is observed from Fig. 3 that the concentrations had a “wave” type change within a temperature range of 15 − 55 ◦C for two coexisting chlorobenzene congeners 1,2,4,5-TeCB and 1,2,4-TCB. The residual concentrations were almost the lowest when the reaction temperature is 45 ◦C, comparing with other reaction temperature. The reason of increasing reaction temperature could facilitate the degradation of HCB is that higher temperature is helpful to the desorption of organic pollutants from soil (Jiang et al. 2018) and sterilize the microbes that influence decomposition of pollutants in the soil (Liu et al. 2014).
The plots of the reaction rate of three coexisting chlorobenzene congeners under different reaction temperature are presented in Fig. 4. It can be observed that the reaction rate of three coexisting chlorobenzene congeners had a “V” type change similarly and indicate that the accumulation exists in the process of degradation within reaction temperature ranging from 15 to 55 ◦C. The accumulation period of 1,2,4-TCB was longer, compared with the that of PeCB and 1,2,4,5-TeCB. Additionally, the size of the area calculated and shown in Table 2 reflects that the amount of 1,2,4-TCB is more than the other two chlorobenzenes under any reaction temperature.
Table 2 the accumulation of three coexisting chlorobenzene congeners (mg/kg)
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15 ◦C
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25 ◦C
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35 ◦C
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45 ◦C
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55 ◦C
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PeCB
|
——
|
——
|
——
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0.0019
|
0.0077
|
1,2,4,5-TeCB
|
——
|
0.0496
|
0.0146
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0.0339
|
0.0486
|
1,2,4-TCB
|
——
|
0.1280
|
0.1471
|
0.1274
|
0.1070
|