Pesticide Degradation

The
pesticide concentrations in the effluent samples taken from the batch and
continuous bioreactors overtime are shown in Figure 4. The pesticide
concentration started to decrease with time under the batch mode of operation
reaching zero after 120 hours from the start. The concentration of captan in
the effluent under the continuous mode of operation also decreased with time
reaching a constant value of 15 mg/L after 288 hours. Thus, a removal
efficiency of 89.6% was achieved after 10 days with the continuous bioreactor
compared to a removal efficiency of 100% after 5 d with the batch bioreactor. Megadi
et al. (2010) achieved 100% degradation of the fungicide captan after 6 days of
operation by the growth of Bacillus
circulans in mineral salt medium (MSM) containing 0.1% captan. Buyanovsky
et al. (1988) achieved 33% degradation efficiency of captan (at initial
concentration of 50 mg/L) after 2 weeks (including lag phase of 2 days) of
incubation with soil bacteria, no further degradation of captan was detected
after the 2 weeks operation. The maximum permissible
value for captan and metabolites in livestock water is set 13 µg/L. The batch
bioreactor used in the study achieved 100% removal of captan, while the
effluent from continuous bioreactor contained 15 mg/L which is not acceptable
for livestock water.

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Figure 4. Concentration of captan in the batch and continuous
bioreactors

Biological degradation of pesticides can be defined as the use of
microorganisms to convert those pesticides either in solid or liquid wastes to
a harmless by product. The biological treatment mainly depends on the microbial
activity and aeration. In this study, microbes that naturally exist in soil
increased significantly in number and began to biodegrade pesticide. The microorganisms
used the pesticide as a carbon source for obtaining energy in addition to the synthesis
of new cells according to the following equations:

Energy

                          Organic matter + O2                 

           CO2
+ H2O + other products +Heat  
                   (2)

Synthesis

                          Organic matter + NH4             

           more
microbes                                            (3)

The proposed pathway for the degradation of captan is shown in
Figure 5. The soil microbial population used in this study contained
microorganisms that was able to oxidieze the carbon, chloride, nitrogen and sulphur
in captan (C9H8Cl3NO2S) and convert
them to carbon dioxide (CO2), water (H2O), chloride (Cl),
nitrate (NO3) and sulphate (SO4) and thus obtaining the
energy required for microbial synthesis according to the following equations (Swanner
and Templeton, 2011; Megadi et al., 2010; Munch et al., 1996).

C9H8Cl3NO2S
+8.5O2

      9CO2
+ H2O + 3HCl + NH3 + H2S+?E                                         (4)

The captan degradation
process takes place in several steps. In the first step, captan is converted
into cis-1,2,3,6-tetrahydro
phthalimide, thiocarbonyl chloride and hydrochloric acid. In the second, step cis-1,2,3,6-tetrahydro pthalimide is
converted into cis-1,2,3,6-tetrahydro
pthalimidic acid and thiocarbonyl chlorides converted into H2S, CO2
and H2O. In the third step, the cis-1,2,3,6-tetrahydro
pthalimidic acid is converted into O-phthalic acid and ammonia. In the forth
step, the O-phthalic acid is converted into protocatechuic acid. In the fifth
step, the protocatechuic acid is converted into 3- carboxy-cis, cis muconic acid which is oxidized to CO2 and H2O.
Nitrifying bacteria is responsible for the conversion of NH3 to NO3
under aerobic conditions while the H2S is converted into SO4
by hydrogen sulphide reducing bacteria.

NH3
+ 2O2        

          NO3-
+ H2O + H+ + ?E                                                                              (5)

 H2S + 2O2       

          SO4-2
+ 2H+ + ?E                                                   (6)

The biodegradation of organic substrates such as pesticides in a
batch system can be described by the following equation.

 

Pt =Po e-kt                                                                     
                                                                                   (7)

 

Where:

Pt  = Concentration of pesticide at the time t
(mg/L)

Po
= Initial concentration of pesticide (mg/L)

k  =  Rate
constant (h-1)

t  =  Time
(h)

 

Figure
5. Pathway of fungicide captan degradation under aerobic condition (adopted
from Megadi et al., 2010).

A
plotting of ln (Pt/P0) versus time (t) yields straight
line with a slope equals k. However, a straight line could not be obtained for
continuous bioreactor when plotting the data for the entire period. The results
(Figure 6) indicated different degradation rates for the lag phase (0.0025 h-1)
and exponential growth phase (0.71 h-1). It is apparent from the
results that the microorganisms were able to use captan as a carbon source for
obtaining energy for maintenance during the lag period. Karpouzas et al. (2005)
reported 25% removal efficiency of cadusafos (12 mg/L initial concentration)
during 30 h lag period of 30 h using Flavobacterium
sp. and Sphingomonas sp. (isolated
from contaminated soil) followed by a gradual decline in bacterial populations (reaching
3´106
and 8´106
cells/mL for the Flavobacterium and
the Sphingomonas sp. in 72 h ,
respectively) and resulted in complete degradation of cdusafos by both isolated
bacteria after 78 h. Karpouzas and Walker (2000) reported a 30% degradation of ethoprophos (initial concentration of 100 mg/L) after inoculation of Pseudomonas
putida (isolated
from ethoprophos contaminated soil) with a mineral salts medium supplemented with nitrogen (MSMN) in the
first 33 h and
observed complete degradation after 50 h. In this study,
8.2% (12 mg/L) of the captan in the batch bioreactor was degraded during the
lag period of 22 h and complete degradation was achieved in 120 h.

Figure
6. Determination of rate constant k.

The captan half-life observed for the batch
bioreactor in this study was 52 h. Leoni et al. (1992) reported a captan half-life
of 3.6 days in an activated sludge system, Hermanutz et al. (1973) reported captan
half-life of 7 h at 12 ºC and 1 h at 25 ºC in Lake Superior. Ghaly et al. (2007)
reported 25 h in a composting system for the pesticide primiphos-methyle at a
temperature of 50-60ºC. In this study, a captan half life of 52 h was observed
for the batch bioreactor.

 

CONCLUSIONS

The initial cell number (30.1´106 cells/mL) in the soil water mixture was first decreased
as time increased for the first 24 h. The maximum cell number reached 15.6´106 and 11.1´106
cells/mL for the batch and continuous bioreactors, respectively. This was the
result of inhibitory effect of captan on some of the soil microbes that are
less tolerant to the pesticide at initial concentration of 144 mg/L. The
results showed maximum microbial population after 5 and 12 days of incubation from
the start of the experiment in batch and continuous bioreactors, respectively.
Lag period and specific growth rate of 22 h and 0.096 h-1, respectively
were achieved for the batch mode of operation. Captan degradation of 89.6% was obtained
after 10 days for the continuous mode of operation compared to complete removal
(100% degradation efficiency) after 5 days for the batch operation. A half life of 52 h was
observed in the batch bioreactor. This study showed that the batch mode of
operation completely removed captan while the effluent from the continuous
bioreactor had a captan concentration of 12 mg/L which is not acceptable for
livestock drinking water.

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