Potential effects of rainwater-borne H2O2 on competitive degradation of herbicides and in the presence of humic acid
Introduction
Herbicides are present in open water environments that receive agricultural runoff (Murray et al., 2010, Davis et al., 2013, Hijosa-Valsero et al., 2016). Understanding the chemical behaviour of herbicides in receiving water environments is essential for developing management strategies to minimize the ecological impacts of herbicides in aquatic ecosystems. Herbicides undergo decomposition via microbial degradation, photodegradation and other chemical degradation (Cullington and Walker, 1999, Salvestrini et al., 2002, Fenoll et al., 2013). Diuron, butachlor and glyphosate are among the most common herbicides in open water environments (Solomon and Thompson, 2003, Yu et al., 2003, Giacomazzi and Cochet, 2004). Diuron (N-(3,4-dichlorophenyl)-N,N-dimethyl-urea) has moderate water solubility (Cabrera et al., 2010). Diuron can be decomposed by microbes both aerobically and anaerobically (Giacomazzi and Cochet, 2004). However, in surface water environments, anaerobic biodegradation is most likely to be limited to sediment-water interface (Attaway et al., 1982). The rate of microbial degradation of diuron may be affected due to the toxicity of diuron to the microbes involved (Guérit et al., 2008). The hydrolysis and photodegradation rates of diuron in natural water under circumneutral pH conditions are relatively low (Salvestrini et al., 2002, Salvestrini et al., 2004). Like diuron, the hydrolysis rate of butachlor (N-butoxymethyl-2-chloro-2, 6-diethyl acetanilide) at circumneutral pH conditions is low. However, photodegradation of butchlor is very rapid (Zheng and Ye, 2001). Glyphosate (N-(phosphonomethyl)glycine) is highly soluble (Schuette, 1998) but it tends to be immobilized by adsorption to organic matter (Piccolo et al., 1996). Hydrolysis and photodegradation are unlikely to be major degradation pathways for glyphosate in natural water environments (Rueppel et al., 1977). While microbially mediated degradation is effective (Zaranyika and Nyandoro, 1993), the availability of glyphosate-degraders in natural water environments may limit the effect of microbial path on the degradation of water-borne glyphosate (Ghassemi, 1982).
Hydrogen peroxide (H2O2) is commonly present in rainwater (Willey et al., 1996, Gonçalves et al., 2010). For example, our monitoring data collected at a location in Guangzhou, southern China (unpublished) shows that H2O2 was detected in the rainwater samples taken on any sampling occasions (n = 103) with the concentration of rainwater-borne H2O2 ranged from 1 to 93 μM. Trace amount of ferrous iron (Fe2+) is also encountered in open water especially stagnant water environments (Mackey and Mackay, 1996, Díez et al., 2007, Sanders et al., 2012). For example, our unpublished data shows that Fe2+ concentration in canal water in Manchester and Leeds in the UK ranged from 0.2 to 0.5 mg/L. Testa et al. (2002) found a concentration of Fe2+ up to nearly 20 μM in the estuarine water of Waquoit Bay. Consequently Fenton reaction may take place in open water environments during heavy rainfall events. The hydroxyl radical (OH) generated from Fenton reaction is likely to act as a powerful oxidant to decompose water-borne herbicides.
In a previous piece of work (Qin et al., 2013), we reported the preliminary experimental results showing that hydrogen peroxide at a concentration range frequently encountered in rainwater could lead to degradation of diuron, butachlor and glyphosate. However, the work was limited to the observation on the effects of Fenton process on the individual herbicides. In field conditions, different types of herbicides along with other organic molecules may occur concurrently. Since different organic molecules have different composition of functional groups, it is unclear how they compete for the available hydroxyl radical. In this study, further laboratory experiments were conducted to observe the changes in herbicides in the scenarios where multiple herbicides or humic acid are present. The objective was to obtain further insights into the interactive processes of hydroxyl radical, various herbicides and humic acid.
Section snippets
Materials
Three commonly used herbicides (diuron, butachlor and glyphosate) were selected for the experiments. The analytical standards of these selected herbicides were purchased from the Shanghai Anpel Scientific Instrument Co., Ltd. The purity of the diuron, butachlor and glyphosate standards was 98%, 98% and 97%, respectively. The humic acid was purchased from the Shanghai Jufeng Scientific and Chemical Supplies Ltd. (purity: 97.4%).
Experiment 1: concurrent presence of herbicide and humic acid
The experiment was to observe the change in each of the three
Herbicide removal in the presence of humic acid
For all the scenarios, there was a generally consistent trend showing the solution TOC in the following decreasing order: Ck > H20 > H50 > F20 > F50 (Table 2).
For the control (Ck, no added H2O2 or Fenton reagent), there was a general trend that herbicide in the solution decreased with increasing dose of humic acid. However, the magnitude of herbicide removal differed among the three herbicides: for diuron, significant difference was only observed between HA0 and the higher doses of humic acid
Discussion
It has been long recognized that humic acid is an effective sorbent for herbicides (Khan, 1973, Martin-Neto et al., 1994, Arroyave et al., 2016). The results obtained from Experiment 1 suggest that butachlor had a stronger affinity towards the humic acid, as compared to diuron and glyphosate. For the control (no added H2O2 or Fenton reagent), at a humic acid dose greater than 20 mg/L, >70% of the water-borne butachlor disappeared after 1 h (Fig. 1a). Since H2O2- or Fenton-driven degradation of
Conclusion
Under the set experimental conditions, the presence of humic acid could impede degradation of the diuron and butachlor by hydroxyl radical. However, humic acids had no significant effects on reducing glyphosate removal rate. Glyphosate could compete strongly with the humic acid for the available hydroxyl radical in the reaction systems. The reactivity of glyphosate with hydroxyl radical was much higher than those of diuron and butachlor due possibly to its relatively simpler chemical structure,
Acknowledgements
This work was partly supported by the Natural Science Foundation of China (Project No. 41271469). The authors would like to thank the two anonymous reviewers for their constructive comments and suggestions that contributed to the improvement of the manuscript.
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