Grantee Research Project Results
Final Report: Redox Status and Degradation Kinetics of Representative Triazine and Urea Herbicides in Soil-Water Systems
EPA Grant Number: R824008Title: Redox Status and Degradation Kinetics of Representative Triazine and Urea Herbicides in Soil-Water Systems
Investigators: Jayaweera, Gamani R. , Rolston, Dennis E. , Biggar, James W. , Spurlock, Frank
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1995 through September 1, 1998
Project Amount: $350,653
RFA: Exploratory Research - Chemistry and Physics of Water (1995) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
The objectives of this research project were to: (1) determine the degradation kinetics of atrazine and diuron under transient redox conditions from aerobic to anaerobic redox levels and steady state redox conditions under alternate electron acceptors: nitrate reducing (denitrifying), manganese reducing, iron reducing, and sulfate reducing (sulfidogenic) conditions; (2) evaluate the role of soluble organic carbon and acclimation in influencing the microbial transformations; (3) decouple the microbial and chemical transformations; and (4) propose possible transformation pathways and estimate the half lives associated with various redox transformations.Summary/Accomplishments (Outputs/Outcomes):
This research project on atrazine and diuron can be summarized as:(1) studies on degradation kinetics under transient and steady (specific) state redox levels; and (2) studies on adsorption, extraction, and means to enhance degradation.
Triazine and urea derivatives together comprise the most widely used group of modern herbicides. In the United States, the most produced and used triazine herbicide is atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) with an aromatic heterocyclic ring structure. Diuron (3-(3,4-dichlorophenyl)-1,1-dimethylurea) is the urea herbicide with a homocyclic aromatic ring. Atrazine has been banned in Germany since 1991 and the use is restricted in France and Italy. High usage means high probability of contaminating soil and groundwater systems with these compounds. Both atrazine and diuron are considered as having high potential to leach into groundwater systems. The U.S. EPA considers both of these herbicides as pesticides of highest priority.
Atrazine has been detected in groundwater and surface water systems in many states in the United States and it also is the most commonly detected pesticide. Diuron also was detected in groundwater systems in states, including California. The groundwater contamination is a significant problem for more than 50 percent of the population and nearly 97 percent of rural residents in the United States as they use groundwater as their drinking water source.
The question we may need to address is how these pesticides move into water systems. To evaluate this, we need to understand the transformations of these pesticides in a natural soil environment. Natural soil environment can be aerobic or anaerobic. There are many reports to indicate that even in well-aerated soils, there are numerous anaerobic microsites. Irrigation and drying events in upland agricultural soils may create continuous shifts in anaerobic and aerobic conditions in the vadose zone. Groundwater fluctuations also may create similar situations.
In the past, the majority of degradation research on pesticides was focused on aerobic biological processes, whereas anaerobic degradation has received only a little attention. Until recently, however, very few investigations have been done on the organic reactions characteristic of reduced environments. There are several reports to indicate the lack of understanding of the relative importance of redox reactions for the transformation of organic compounds in soils. Presently, there is growing evidence to indicate the importance of anaerobic biodegradation of aromatic and nonaromatic hydrocarbons.
When an atrazine or diuron molecule moves through the vadose and saturated zones in the soil, it may be subjected to subsurface microenvironments of: (1) transient redox conditions that represent the changing redox conditions from aerobic to anaerobic; or (2) steady state redox conditions that represent the activity of high concentrations of alternate electron acceptors under anaerobic conditions (e.g., steady denitrification due to excess of nitrate in soil-water systems).
The redox condition is a measure of aerobic or anaerobic status of the soil and it is measured by redox potential. In aerobic soils, the redox potential is in the range of +700 to +400 mV and the redox potential decreases as the system is reduced?it may go as low as -75 to -150 mV at sulfate reducing condition. The term "anaerobic condition" encompasses a wide range of redox conditions from denitrifying (redox values: +300 to +200 mV) to sulfate reducing (redox values: -75 to -150 mV). The microbial populations at different redox potentials are significantly different in soil-water systems and the rate and/or pathway of degradation of a chemical also may be significantly different. Therefore, it is important to establish specific anaerobic conditions and to specify the redox status that has been used. However, in other studies in the past, there has been little mention of the various anaerobic redox levels.
In contrast to other anaerobic degradation studies of atrazine and diuron, our research has specifically addressed the degradation under transient and steady state (specific) redox conditions (denitrifying, sulfidogenic, etc.) with the use of flow-through methodology developed in our laboratory.
Degradation Kinetics of Atrazine and Diuron. In this project, we studied the degradation kinetics of atrazine and diuron under transient and steady state (specific) redox levels as a continuos function of time. The steady state redox levels include:
1. Aerobic (redox potential, Eh = +700 to +400 mV) 2. Denitrifying (Eh = +300 to +200 mV) 3. Iron reducing (Eh = +120 to 0 mV) 4. Sulfate reducing (Eh = -75 to -150 mV)
This study was conducted at transient and steady state redox conditions with the use of flow-through experimental techniques. The aerobic condition was achieved by bubbling a stream of humidified air through the solution reservoir. Denitrifying, iron reducing, and sulfate reducing conditions of a particular soil were established and maintained by bubbling humidified N2 gas through the solution reservoir in the presence of excess of alternate electron acceptors nitrate, ferric ion, and sulfate, respectively. The pH and Eh data were recorded in an IBM compatible computer by using in-line pH and Eh probes. This technique has several unique features: it mimics the porous condition in natural soil; low soil to solution ratio, which is the situation under field condition; redox transformations (transient or steady state) of a chemical can be studied as a continuous function of time; steady state redox levels for specific microbial transformations (e.g., denitrifying, sulfidogenic, etc.), which are characteristic of a particular soil, can be established and maintained by using O2 and N2/alternate electron acceptors.
The atrazine or diuron were pre-adsorbed onto the soil by shaking the soil with respective herbicide solution with a soil to solution ratio of 1:5. The soil solution was filtered and the pre-adsorbed soil (soil samples used were taken from 0-15 cm depth of the dry bed of Pond 2 at the Kesterson Reservoir, CA) was air dried, ground, sieved, and thoroughly mixed. The soil was then packed in a stainless steel cylindrical test chamber and was connected to the flow-through system.
The test solution was prepared by using a solution extract of the same soil used for pre-adsorption. This extract was prepared by treating the soil as it was treated for herbicide adsorption but using deionized water instead of herbicide solution. The objective of using this extract to prepare the test solution is to replace any soluble carbon that has been removed from the soil during atrazine pre-adsorption process. After wetting the soil column with the test solution, the solution was circulated through the soil column by a metering pump. The solution samples were collected into vacutainers as a function of time for analysis.
A. Atrazine Degradation Kinetics
a. Transient Redox Conditions
i. Native Soil The objective of this experiment was to study the degradation kinetics of atrazine as a continuous function of time when the soil system was exposed to progressive biochemical reduction from aerobic to slightly reduced to highly reduced redox potentials. This progressive reduction was achieved by bubbling N2 gas through the solution reservoir. Under transient redox conditions, it was possible to identify two phases with respect to degradation of atrazine. Initially a slow phase (half life of 72.9 d), which may coincide with aerobic to slightly reduced denitrification state, and a final rapid phase (half life of 44.4 d) which may coincide with the more reduced iron reducing to sulfate reducing conditions.
ii. Sodium Azide Amended Soil The objective of this experiment was to study the microbiological and chemical degradation of atrazine under transient redox conditions. Sodium azide was used to minimize microbiological activity to quantify the chemical degradation of atrazine. Atrazine was desorbed in both treatments (with and without sodium azide) at the beginning of the experiment, but the desorption was more significant in the presence of sodium azide. This difference may be due to lack of microbiological activity in the presence of sodium azide relative to the treatment without sodium azide. This initial phase may be the region of aerobic to mild reducing condition. During the latter part of the experiment, which may represent highly reduced iron and sulfate reducing conditions, atrazine concentration dropped sharply in both treatments indicating that sodium azide was ineffective in controlling the microbial activity when the system was highly reduced.
a. Steady State Redox Conditions (Comparison of Atrazine Degradation at Different Steady State Redox Conditions)
i. Native Soil Atrazine degradation occured in native soil (i.e., without any amendments) under aerobic, nitrate, iron, and sulfate reducing conditions. At any particular time, the degradation of atrazine under aerobic and denitrifying conditions was very similar and degradation under sulfate and iron reducing conditions was very similar as well. Generally, the degradation of atrazine was twice as fast under highly anaerobic conditions (sulfate and iron reducing) than under slightly anaerobic (denitrifying) to aerobic conditions. This is well reflected in their half lives. The half lives for atrazine degradation in native soil in the increasing order are as follows:
Iron reducing (25.1 d) < Sulfate reducing (26.5 d) < Aerobic (58.4 d) < Denitrifying (59.0 d)
The rapid degradation of atrazine under sulfate and iron reducing conditions may be due to the use of nitrogen in the heterocyclic ring of atrazine as a N-source by the microorganisms in these environments. There are several reports to indicate high mineralization of triazine ring structure when atrazine was supplied as a N-source. Various pure and mixed cultures that are capable of utilizing s-triazines as N sources also have been isolated in the past by several researchers.
Another observation is that the initial desorption of atrazine from soil surfaces was evident only under aerobic and denitrifying conditions where they have slower degradation rates compared to the sulfate and iron reducing conditions. Most likely, under sulfate and iron reducing conditions any atrazine desorbed may have degraded so fast it may have not appeared in soil solution. There are some contradictory reports on atrazine degradation under anaerobic conditions. Some reported that the persistence of atrazine decreased under saturated soil moisture conditions and they mentioned that this is of significant interest in situations involving flooded soils or elevated water tables. This report confirms our finding that atrazine undergoes rapid degradation under anaerobic conditions. However, our data contradict the findings of another group of researchers. According to them, atrazine degradation was much slower at redox levels depicting anaerobic conditions and their results showed that atrazine is more persistent under anaerobic conditions such as found in wetlands or subsurface environments than in more oxidizing environments such as surface agricultural soils. In our study, however, the atrazine degraded much faster under more anaerobic or reducing conditions. This difference may be due to different experimental techniques used to study the system. It also is interesting to note that some of the breakdown products appeared only in sulfate and iron reducing conditions.
ii. Carbon Amended Soil In atrazine experiments, sucrose is added as the carbon amendment. The half lives for atrazine degradation in sucrose amended soil in increasing order are as follows:
Sulfate reducing (5.2 d) < Iron reducing (14.4 d) < Denitrifying (15.4 d)
In general, it shows that addition of carbon amendment enhanced atrazine degradation under aerobic, nitrate, iron, and sulfate reducing conditions. Especially carbon (sucrose) had a significant effect on degradation of atrazine under highly reduced sulfate reducing condition and the degradation rate increased nearly five times with carbon. In the presence of carbon (sucrose), the rate of degradation of atrazine is nearly three times faster under sulfate reducing condition relative to the iron reducing condition. This is shown by the half lives of 5.2 and 14.4 days under sulfate reducing and iron reducing conditions, respectively. In other words, if we supply organic carbon to soil systems under slight to highly reduced conditions, atrazine can be degraded much faster than in native soil. It also is interesting to note that in sucrose amended soils some of the breakdown products appeared only under sulfate and iron reducing conditions as in native soils.
B. Diuron Degradation Kinetics
a. Comparison of Diuron Degradation at Different Steady State Redox Conditions
i. Native Soil Diuron experimental runs were conducted similar to atrazine runs. Diuron degraded in native soil (i.e., without any amendments) under aerobic, nitrate, iron, and sulfate reducing conditions. In all experimental runs, initially there was an increase in diuron concentration in the solution due to desorption; then, diuron concentration decreased as the degradation proceeded. The degradation rate seemed to be different at the redox levels we studied. The half lives for diuron degradation in native soil in decreasing order are as follows:
Iron reducing (72.5 d) > Aerobic (62.6 d) > Denitrifying (51.0 d) > Sulfate reducing (35.6 d)
According to these values, in native soils diuron degrades much faster under sulfate reducing conditions and slower under iron reducing conditions. Different microbial populations at aerobic, denitrifying, iron reducing, and sulfate reducing levels may breakdown diuron at entirely different rates.
ii. Carbon Amended Soil We conducted flow-through experiments to quantify the influence of organic carbon on diuron degradation under sulfate reducing condition, as it is the most effective redox level in degrading diuron. We used sucrose and glucose as the organic carbon sources. Both sucrose and glucose were beneficial in the degradation of diuron. Initially, there was a diuron desorption phase followed by degradation. The half lives for diuron degradation in decreasing order are as follows:
Native soil (41.8 d) > Glucose amended soil (30.9 d) > Sucrose amended soil (28.9 d)
This shows that carbon amendments independent of the carbon source (glucose or sucrose) enhanced diuron degradation under highly reduced sulfate reducing condition. This implies that carbon may limit diuron degradation under sulfate reducing condition in soil systems.
iii. Sodium Azide/Carbon Amended Soil We conducted flow-through experiments in the presence of sodium azide to study the chemical degradation of diuron under sulfate reducing condition. The degradation in native soil and the NaN3/sucrose amended soil were similar with half lives of 35.6 d and 31.3 d, respectively. This behavior is similar to the behavior of atrazine under sulfate reducing condition.
C. Comparison of Atrazine and Diuron Degradation at Different Redox Levels Under aerobic and denitrifying conditions in native soils, the rates of degradation of atrazine and diuron seem to be similar and the rates are generally slower than under more reduced conditions, except for diuron under iron reducing condition.
The half lives for atrazine and diuron degradation under aerobic condition are 58 d and 62.6 d, respectively, and under denitrifying condition are 59 d and 51 d, respectively. In fact, the denitrifying condition is the least reduced state under anaerobic condition. When the system is under more reduced iron and sulfate reducing conditions, the degradation rates of atrazine and diuron differ somewhat. This is reflected in their half lives. The half lives for atrazine and diuron degradation under iron reducing condition are 25 d and 72.5 d, respectively, and under sulfate reducing condition are 26 d and 35.6 d, respectively. It is interesting to note that under iron reducing condition, atrazine degradation is the fastest and diuron degradation is the slowest.
Studies on Adsorption, Extraction and Means To Enhance Degradation. Adsorption coefficient of atrazine and its degradation products in a soil with 2.7 percent organic carbon decreased in the order of:
Hydroxyatrazine > atrazine > deethylhydroxyatrazine > deisopropylatrazine > deethylatrazine > deethyldeisopropylatrazine
In extraction studies, it was found that heat treatment was the best method out of the three methods tested to extract atrazine and its degradation products from soils. The percent recovery by this method in the decreasing order is as follows:
atrazine = deethylhydroxyatrazine = deethyldeisopropylhydroxyatrazine> deethyldeisopropylatrazine > deethylatrazine > hydroxyatrazine > deisopropylatrazine
In studies of ways to enhance degradation, it was found that microbiological degradation was the dominant process at temperatures of 25 C and 35 C, whereas chemical breakdown was dominant at 45 C. It also was observed that atrazine degradation increased with increasing humic acid concentrations from 0.25 percent to 1 percent.
Significance of the Study. The information from this study is useful to the scientific community and general public. This study provided half lives for degradation of atrazine and diuron under transient (transition from aerobic to anaerobic conditions) and steady state redox condition (nitrate, iron, and sulfate reducing conditions) in a porous soil matrix that mimics the natural soil environment. These data may be useful to the scientists who are interested in understanding the transformations of these pesticides in soil systems, especially under anaerobic conditions as there is very little information available.
The computer models are an important tool in predicting the contamination of water systems by atrazine and diuron. The model inputs are important to make fairly accurate predictions. The half life data provided by this study will be useful for modelers who are developing chemical pesticide models.
When the scientists and the modelers understand the nature and the rate of atrazine and diuron degradation under many different conditions, they can make reliable predictions as well as strategies to prevent contamination and/or to minimize the damage caused by such contamination to benefit the public.
Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
triazine, urea herbicide, persistence, redox potential, pesticides, biodegradation, environmental chemistry., Scientific Discipline, Waste, Water, Ecosystem Protection/Environmental Exposure & Risk, Environmental Chemistry, Physics, Chemistry, Fate & Transport, Engineering, Chemistry, & Physics, fate and transport, aqueous impurities, soil redox status, microbial degradation, triazine, triazine compounds, degradation kinetics, aquifer remediation design, electron accpter conditions, groundwater contaminationProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.