Introduction
Describing pesticide degradation
using Monod kinetics: informationfrom batch and column data
Karlien Cheyns

Introduction

Experiments

ModelConclusions• Crucial for modeling transport ofpesticides through the unsaturated zone
towards the groundwater are:– Sorption kinetics– Degradation kinetics• In this study, the parameters to describe
this processes are estimated from:

Division of Soil and Water Management– Batch mineralization– Pesticide leaching data through columns

KULeuven

Pesticide degradation
Introduction

Experiments

ModelSCEM
Conclusions• Pesticide degradation is commonlydescribed using a first-order approach• However: a lag phase is observed often
in literature (also in both experiments inthis study)• A first order approximation of theMonod kinetics is usedIntroduction

Experiments

ModelConclusions• To identify the model parameters (andtheir uncertainty) the Shuffled Complex
Evolution Metropolis (SCEM) algorithm
is used in this study.

1

Objectives
Introduction

Experiments

Design of the experiments

ModelConclusions• Comparing the optimized model
parameter distributions when optimized
against:– Only batch mineralization data
– Only column leachate data

IntroductionExperiments

ModelConclusions• Soil, treated with AT for > 30years, depth 30-45 cm(0.47 % C)
• Column: 6.5 cm, Ø 3 cm
• BD ~ 1.47 g/cm³• Investigate the suitability of theoptimized batch parameters to predict
the observed column leachate data

Batch mineralization
Introduction

1.7 mm / day

8 µg 14C-Atrazine/L+ 20 mg Atrazine/L

ModelConclusions• Columns
leached with 2 pore volumes of a70
14C-atrazine containing solution
60• Incubated with a NaOH-trap (14C-CO250
periodically analysed)% Atrazine broken down-100 cm H2OExperiments

4030

NaOH trap
2014C-atrazine

1000containing soil
102030Time (days)

40502Column experimentAtrazine concentration (mg cm -3 )Introduction

12 mg Atrazine/LExperiments

ModeldX= (q m mod * S )X − k decay X
dtdS
1= − (qm mod * S ) X
dt
YConclusions

0.015

0.005

Lag phase00

Monod kinetics
Introduction

Model• Step application of atrazine (20 pore volumes)
Inlet concentration• Atrazine concentration measured
in leachates(SPE-HPLC)
0.01

1.7 mm / day-100 cm H2OExperiments

51015Time (days)

202530

Monod kinetics
Conclusions

X = Biomass concentration
S = Substrate concentration
Ks= half saturation constant
kdecay = decay constantY = yield factor
qmmod = modified specific growth rate

Introduction

Experiments

ModelConclusions• Implemented in Matlab -> simulation of batchmineralization• Implemented in Hydrus-1D -> describing
pesticide transport and degradation in soil columns• Assumptions in both experiments:– Y = 0.4 -> assuming all atrazine is respired as CO2 orincorporated in biomass(from sensitivity analysis: no sensitive parameter)– kdecay = 0 -> no information in these experiments +limited time span

3Batch simulation
Introduction

Experiments

Column simulation

ModelConclusions

Introduction

6050403020x1000102030

40

Observed
Simulated

Atrazine concentration (mg cm-3 )% Atrazine broken downExperiments

0.005

x0050

Experiments

0.5

Model

1

1.5

K f (cm3 g -1 )1Conclusions

122

2.5

33-1

3.5

4-1log(qmmod ) (cm mg day )0.5
0-13-12-11-1015202530log(xini) (mg cm-3 )-9-8Introduction

Experiments

ModelConclusions

0.015
• Can
we use the optimized batch parameters topredict the observed column leachate data?

0.51.5

10Prediction column data

Batch fit--- Column fit

Atrazine concentration (mg cm-3 )Cumulative distribution

Introduction

5

Observed
Simulated

Time (days)

Optimized parameter distributions

01Conclusions

0.01

Time (days)

10.5
0Model0.015

700.01

Observed column
Simulated column--- Predicted column
x0.0050050

100150Time (days)

4Prediction column dataIntroduction

Experiments

ModelConclusions• Can we use the optimized batch parameters to
predict the observed column leachate data?
• NO!
• Possible explanations:– difference between the mineralization process(batch) and the degradation process (column)– liquid flow in the soil columns versus the single
atrazine application in the batch experiment

Conclusions
Introduction

Experiments

ModelConclusions• Batch mineralization was described well withthe model using the Monod kinetics• Atrazine transport through soil columns waswell described by Hydrus-1D with
implemented Monod kinetics to describe
degradation• predicting atrazine breakthrough throughcolumns using optimized batch parameters for
growth rate and initial biomass concentration
is not possible!

5