Metabolic fate studies of $^{14}$C-radiolabeled pesticides in water-sediment / soil systems under different conditions : (supplemented by a case study with societal aspect)

  • Metabolismus-Studien zu radioaktiv mit $^{14}$C markierten Wirkstoffen in Wasser-Sediment- bzw. in mit Wasser überschichteten Boden-Test-System unter verschiedenen Bedingungen : (ergänzt um eine Fallstudie mit sozialem Aspekt)

Yuan, Ye; Schäffer, Andreas (Thesis advisor); Hollert, Henner (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2021


Profound knowledge on the behavior of anthropogenic compounds is not only a prerequisite for risk assessment of related ecosystems, but also of great importance for the human beings living therein and interacting directly or indirectly with them. Since such an integrative concept was rarely investigated so far, the so-called "Fate" module has been extended in the current study with societal aspects on agricultural activities and the use of plant protection products (PPP) in an area of concern by qualitative open interviews with farmers, small pesticide shop retailers and big wholesalers / dealers. Since realistic situations are influenced by various social interactions between all related stakeholders and relevant risk-benefit-cost estimations of a PPP use, in-depth assessments should give best advice to improve decision-making for getting practicable win-win solutions. From the chemistry part of this work we learned for specimen active ingredients, i.e. clodinafop-propargyl (CfP), the organochlorine insecticide DDT, the herbicide propanil and its degradation product 3,4-DCA, that the route and rate of degradation of such test compounds in water-sediment / soiliment systems can be adequately investigated using radiolabeled test compounds under aerobic dark laboratory conditions. Such techniques enable to establish material balances, including the quantitation of volatile degradation products (such as 14CO2), radioactivity in water phase, extractable and non-extractable residues in sediment / soiliment at each sampling interval. On the other hand, the following experimental results confirmed that an adequate simulation of real-world agricultural conditions is essential. Main route of [phenyl-UL-14C]clodinafop-propargyl (14C-CfP) degradation in a water- sediment system from River Rhine was formation of NER (approx. 34% of AR at DAT-28, days after treatment), mainly associated with fulvic acids (FA) and humin / minerals (HU) fractions, whereas portions of 14C in non-humics (NH) and humic acids (HA) were low. Mineralization of respective radiolabel exhibited a minor route of degradation, exceeding approx. 5% of AR, only. In the water phase, radioactivity decreased to 18.5% of AR until DAT-28. Almost the entire radioactivity detected in the systems consisted of clodinafop (Cf, >99% of AR) after 2 days, already, indicating a rapid ester hydrolysis of CfP. Cf was the only primary metabolite unequivocally identified in water phase and sediment extracts (max. 53% of AR at DAT-28). DT50 of CfP and Cf was below 1 day and slightly above 28 days, respectively. Respective investigation of [phenyl-UL-14C]DDT in water and sediment from River Rhine resulted in very low mineralization to 14CO2 (< 0.02% of AR) and moderate transformation to NER (to 8.65% of AR) throughout the entire study period of 14 days. However, radioactivity measured in the water decreased rapidly, i.e. to 1.9% of AR at DAT-14, while sediment associated radioactivity increased to 56.7% and 93.8% of AR after 4 hours and 14 days, respectively. It was shown that DDT dissipated quickly from water to the sediment phase (84.4% of AR at DAT-14). Metabolites were found in minor amounts, only. In conclusion, persistency of DDT in aquatic environment and sediment phase as major sink and matter of interest for risk assessment were confirmed by the current investigation. The respective investigation of [phenyl-UL-14C]propanil in water and sediment from River Rhine demonstrated moderate adsorption to sediment, formation of NER (20.1% of AR), and a very low mineralization to 14CO2 of 0.12% of AR until study end (DAT-28). It was confirmed that propanil, a slightly-moderately mobile and moderately soluble compound in water, degrades fast in water-sediment system, forming other major metabolites found in sediment phase. In the water phase, propanil decreased linearly from 75.6% to 0.12% of AR at study end. Portions of the major metabolite 3,4-DCA were detected from DAT-4 on. It increased rapidly to 27.3% of AR at DAT-7, and then decreased to 9.3% of AR at DAT-28. An incomplete material balance determined at all incubation intervals (between 65 and 82% of AR), indicated loss of radioactivity from the system during incubation or during the sample processing procedures. By method improvement this mass balance deficiency could be overcome for following study, testing propanil more closely to agricultural conditions. The herbicide propanil, which is widely used in rice cultivation in Asia, was selected as project model substance for the lab-scope simulation of a realistic fluctuation zone situation in the TGR area of China. Radiolabeled propanil was applied into solid soiliment phase (which is different in a water-sediment study treating the water) and incubated in a lab system with water and soiliment collected from rice paddy area of the TGR area at Yangtze during dry season. Flooding conditions were simulated and applied during the study. Adequate recoveries of radioactivity well confirmed the validity of used methods. Extractable residues (ER) of soiliment phase steadily declined from 75.7 (at 4 h) to 22.4% of AR at study end (DAT-92), while NER steadily increased to a rather high proportion of 59.3% of AR. Mineralization was low throughout the study, probably because the selected radiolabel promoted a NER formation via its radiolabeled aniline moiety, which is known to be strongly bound to soiliment matrix. Interestingly, via High-Performance Size-Exclusion Chromatography (HPSEC) it was elaborated that the radioactivity associated with the isolated FA fraction of NER was supposed to be bound in a covalent manner to the organic soiliment components. In conclusion, it was confirmed that propanil is very low to low persistent, since a DT50 of between 4 hours to 2 days was observed. Thereby, the major sink was NER, and the respective results of simulating wet periods of flooding (at 30 - 60 days) proofed that remobilization of significant amounts of parent compound or metabolites from the soiliment is not occurring. Rather small amounts of propanil (max. 3.6% of AR), 3,4-DCA (approx. 1.4% of AR), and other minors of <1% of AR in sum were detected in the water phase, but the major portion of residues remained in the soiliment phase. Therefore, after impoundment of supernatant water from rice paddy area, a contamination of the aquatic environment is regarded as minimal. This enables the overall conclusion that the fate of propanil metabolite 3,4-DCA, which had been investigated as 14C label in a water-sediment metabolism study from a rice paddy area in Northern Italy (a traditional rice-growing area), is not that important for an overall risk assessment of using propanil, since the data on simulating more practical conditions for rice production were available, now. This learning from the chemistry part of propanil work, i.e. that it should be carefully avoided that rather extreme worst-case assumptions are dominating risk-benefit evaluations, was connected to a supplementary social part perspective of agricultural activities and PPP use in the researched region in TGR area in China. There is no doubt that serious damage could occur to both environmental and human health if pesticides in rice production are misused. In consequence, proper education, understandable information and practical training for the end-users (farmers or professionals) and not just for the dealers or retailers is extraordinary important. In addition to economical evaluation, this includes knowledge on proper use indication parameter, label time tables instructions for doses, protecting measures for the workers (e.g. masks, gloves), safety protection of bystanders and the entire environment. All this holds true not just for the use, but for disposal and storage condition of PPPs in order to avoid any cross contamination and severe accidents, as well. Interviews like such performed in this work are highly valuable tools to record and evaluate where knowledge and education gaps exist, and to understand the reasons for farmer’s decisions and behaviors in the longer run. Altogether, in addition to environmental fate studies performed under lab, semi-field or field conditions, which mainly describe the intrinsic active ingredient properties regarded as key basic knowledge, a better holistic understanding of economic, risk-benefit and social PPP use parameter of most relevant stakeholders such as consumers, end-users farmers, retailers closely connected to farmers, and bigger wholesalers / dealers in a relevant geographical area is needed (human dimension for risk assessment). This clearly shows that multidisciplinary co-operations from various diverse aspects, combination of respective natural, economic and social sciences should be applied more frequently, and improved or newly developed in future, if not yet adequately available. In this rather quickly changing economic and social environment of agriculture, a lot of future work is anticipated in this area.