3.3 Results and discussion
3.3.4 Influence of DOC under oxidative bottom sediment
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In the case of low NO3-N, a comparison between cases 1 and 2 showed that the rate of increase in sulfide by means of high DOC was significant. In contrast, a comparison between cases 3 and 4 did not confirm differences in the temporal rate of change in sulfide due to DOC conditions. For case 4, these factors also involved low concentrations of initial SO42, and there-fore, the effect of high DOC on the rate of sulfide increase is assumed to be minor.
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Fig. 3.8 Continuous measurements of DO and ORP for 20 days in case 4 and case 5.
A five-step process of decline in ORP is represented by points a, b, c, and d with each step having the following characteristics: Step 1: a linear decline from a to b; Step 2: a point of inflection at approximately 0 mV; Step 3: a steep decline from b to c; Step 4: a gradual decline from c to d; and Step 5:
state of equilibrium. The red broken line (case 4) and the black broken line (case 5) indicate results of linear regression formula for the drop from pos-itive value to negative value in ORP. S is the slope of the linear line. R2 is the coefficient of determination.
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Fig. 3.9 Periodic measured results of NO3-N, NH4-N, PO4-P, sulfide, SO42-, TFe, and E254 during the experimental period in case 4 and case 5. Error bar stand-ards are one standard error from the mean. The red solid line (case 4) and the black solid line (case 5) indicate the approximately straight line for tem-poral change of concentrations, except for PO4-P in case 5, which can be approximated by the logarithmic trend line. The values in parentheses de-note the regression coefficients of logarithmic trend line for PO4-P in case 5. S is the slope of the linear line. R2 is the square of the correlation coeffi-cient.
0 0.2 0.4 0.6
0 1 2 3
0 0.2 0.4 0.6
0 200 400
0 5 10 15
0 4 8
0 10 20 30 40 50 60 70
0 0.4 0.8 1.2 NO3-N (mg/L)PO4-P (mg/L)NH4-N (mg/L)Sulfide (μg/L)SO42- (mg/L)TFe (mg/L)E254
Lapsed times from the experiment start (d) Case 4: = 0.016 mg/L/d; = 0.61
Case 4: = 0.0028 mg/L/d; = 0.58 Case 5: = 0.021 mg/L/d; = 0.86
Case 5 (0.288 mg/L); = 0.90
Case 4 ( NH -DH -R) Case 5 ( NH -DH -O)
Case 4: = 2.71μg/L; = 0.64 Case 5: = 10.4μg/L; = 0.94
Case 5: = -0.256 mg/L/d; = 0.84 Case 4: = -0.051 mg/L/d; = 0.94 Case 5: = -0.037 mg/L/d; = 0.94
Case 4: = -0.035 mg/L/d; = 0.94S
S R2
R2
R2 S
S R2
S R2
R2
R2 S
S R2
S R2
R2 S
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the other hand, PO4-P in case 5 suddenly began to rapidly increase about 6 days after the DO concentration reached zero, and then switched to a moderate increase after 35 days. The PO4-P increase in case 5 could be divided into two periods, in which each period was described by a linear regression. While they have different slopes, the time-dependent change through the en-tire period of increase could well be approximated by a logarithmic function. Iron concentra-tions in case 5 maintained a constant level, not fluctuating according to the decrease in DO.
However, approximately 6 days after DO reached zero (the same time at which PO4-P began to increase) total iron ion increased rapidly exceeding that of case 4. As indicated by these results, the initial bottom sediment oxidative state created a time lag when PO4-P eluted and when DO reached zero. Furthermore, as the rate of anaerobic respiration of iron-reducing bacteria in-creased, it significantly affected the PO4-P release from the bottom material.
In case 5, although the sulfide increased linearly from the point when ORP had shifted to Step 5 and NO3-N had reached zero, it started to show a slight decline after 28 days from the start of the experiment. It was found that in both cases 4 and 5 the point at which the sulfide began increasing correlated with the point at which ORP shifted in Step 5. In addition, similar-ities between cases 4 and 5 could be found in the monotonous increase of sulfide, which can be represented by a linear regression, and in the increasing trend that changed into a moderate decline after reaching the maximum value. Furthermore, the rates during the period of linear increases in cases 4 and 5 were quite different (2.71 µg/L/d and 10.4 µg/L/d, respectively).
Knowing the magnitude of the rate of decrease in sulfate (case 4: 0.051 mg/L/d, case 5: 0.256 mg/L/d), which exhibited a declining trend during the same period, it is suggested that oxida-tion-reduction reactions were promoted in case 5 because sulfide production was elevated by sulfur-reducing bacteria due to the oxidative conditions of the bottom sediment.
Comparing cases 4 and 5 indicates that under high NO3-N conditions and high DOC conditions the initial bottom sediment oxidation-reduction state is an environmental factor that strongly affects the dynamics of water quality. It specifically affected PO4-P and sulfide under anaerobic conditions. In conclusion, the oxidative state of the bottom sediment delayed the in-itiation of PO4-P and the sulfide increases, while elevating the increase rates in these concen-trations. The aerobic conditions in the oxidation zone on the surface of the bottom sediment
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suppressed the activity of the iron-reducing bacteria (Melton et al., 2012) and the sulfate-re-ducing bacteria (Sørensen et al., 1979), which are anaerobic bacteria. It is speculated that this led to prolonging the period of time required for the initiation of anaerobic respiration of the bacteria. Furthermore, it is believed that the oxidative state of the initial bottom sediment in-creased the aerobic respiration and the amount of easily decomposable organic matter (Nedwell, 1984). This, it is believed, was connected to the surge in respiration substrate for anaerobic bacteria. Promoting anaerobic respiration of iron-reducing bacteria and sulfate-reducing bacte-ria drastically increased PO4-P and sulfide. E254 increased in case 5 at the same time that PO4 -P and total iron ion began increasing. Compared to case 4, E254 had a much greater rate of increase in case 5. If this rise in E254 is interpreted as a release of the organic matter in the bottom sediment that increased due to anaerobic respiration, then it can be considered that the initial oxidative state of the bottom sediment mainly promotes organic matter decomposition via iron-reducing bacteria, which supports the above mentioned idea.
Finally, the influence of DOC on water quality dynamics under anaerobic conditions was considered, when the initial concentration of the NO3-N is high and the bottom sediment is in an oxidative state. By comparing changes in DO and ORP in cases 5 and 6, under high DOC conditions in case 5, it was observed that the time required for DO to reach zero in this case was shorter than that in case 6 (Fig. 3.10). Furthermore, the starting point of the decline in ORP in case 5 came earlier in compared with case 6, which resulted in its reaching the equilib-rium in case 5 earlier than case 6. On the other hand, there was no specific difference determined for the time-dependent changes in NO3-N, NH4-N, PO4-P, and sulfide between high and low DOC conditions (Fig. 3.11). As for NO3-N, it decreased in both cases at about the same rate along with the decline in DO, i.e. the concentrations in both cases took almost the same time to reach zero. Furthermore, the course of increases in NH4-N could well be represented by linear approximations and the increases in PO4-P were described by log approximations. The regres-sion coefficients in the approximations indicated the same level in both cases. In particular, the start of the PO4-P increase had almost the same timing in cases 5 and 6. Moreover, it was con-firmed that iron, sulfate, and E254 were also similar in both cases.
It can be concluded from these results that if the initial bottom sediment is in an oxidative state,
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Fig. 3.10 Continuous measurements of DO and ORP for 25 days in case 5 and case 6. A five-step process of decline in ORP is represented by points a, b, c, and d with each step having the following characteristics: Step 1: a linear decline from a to b; Step 2: a point of inflection at approximately 0 mV;
Step 3: a steep decline from b to c; Step 4: a gradual decline from c to d;
and Step 5: the state of equilibrium. The red broken line (case 5) and the black broken line (case 6) indicate results of linear regression formula for the drop from positive value to negative value in ORP. S is the slope of the linear line. R2 is the coefficient of determination.
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Fig. 3.11 Periodic measured results of NO3-N, NH4-N, PO4-P, sulfide, SO42-, iron ion, and E254 during the experimental period in case 5 and case 6. Error bar standards are one standard error from the mean. The red solid line (case 5) and the black solid line (case 6) indicate the approximately straight line for NH4-N and the logarithmic trend line for PO4-P. The regression coef-ficients of logarithmic trend lines of PO4-P were shown in parentheses. S is the slope of the linear line. R2 is the coefficient of determination.
0 0.2 0.4 0.6
0 1 2 3
0 0.2 0.4 0.6
0 200 400
0 5 10 15
0 4 8
0 10 20 30 40 50 60 70
0 0.4 0.8 1.2 NO3-N (mg/L)PO4-P (mg/L)NH4-N (mg/L)Sulfide (μg/L)SO42- (mg/L)TFe (mg/L)E254
Lapsed times from the experiment start (d) Case 5: = 0.021 mg/L/d; = 0.86
Case 6: = 0.026 mg/L/d; = 0.96
Case 5 (0.288 mg/L); = 0.90 Case 6 (0.217 mg/L); = 0.88
Case 5 ( NH -DH -O) Case 6 ( NH -DL -O)
S R2
S R2
R2 R2
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the impact of DOC conditions on decomposition of anaerobic organic matter (such as denitrification, iron reduction, and sulfate reduction) is quite small. In other words, the state of oxidation and reduction of the bottom sediment significantly influences water quality dynamics under anaerobic conditions. By boosting the quantity of the sulfide generated and increasing the elution amount of E254, which is an index of NH4-N, PO4-P, and humic acid, it could be observed that the oxidative state of the bottom sediment leads to eutrophication and organic contamination problems. These results reveal important information about the mechanisms of aqueous environmental deterioration in closed water bodies where organic contamination occurs at accelerating rates.
