Evidence of co-metabolic bentazone transformation by methanotrophic enrichment from a groundwater-fed rapid sand filter

Abstract

The herbicide bentazone is recalcitrant in aquifers and is therefore frequently detected in wells used for drinking water production. However, bentazone degradation has been observed in filter sand from a rapid sand filter at a waterworks with methane-rich groundwater. Here, the association between methane oxidation and removal of bentazone was investigated with a methanotrophic enrichment culture derived from methane-fed column reactors inoculated with that filter sand. Several independent lines of evidence obtained from microcosm experiments with the methanotrophic enrichment culture, tap water and bentazone at concentrations below 2 mg/L showed methanotrophic co-metabolic bentazone transformation: The culture removed 53% of the bentazone in 21 days in presence of 5 mg/L of methane, while only 31% was removed in absence of methane. Addition of acetylene inhibited methane oxidation and stopped bentazone removal. The presence of bentazone partly inhibited methane oxidation since the methane consumption rate was significantly lower at high (1 mg/L) than at low (1 μg/L) bentazone concentrations. The transformation yield of methane relative to bentazone normalized by their concentration ratio ranged from 58 to 158, well within the range for methanotrophic co-metabolic degradation of trace contaminants calculated from the literature, with normalized substrate preferences varying from 3 to 400. High-resolution mass spectrometry revealed formation of the transformation products (TPs) 6-OH, 8-OH, isopropyl-OH and di-OH-bentazone, with higher abundances of all TPs in the presence of methane. Overall, we found a suite of evidence all showing that bentazone was co-metabolically transformed to hydroxy-bentazone by a methanotrophic culture enriched from a rapid sand filter at a waterworks.

Microbial degradation pathways of the herbicide bentazone in filter sand used for drinking water treatment

“…The presence of carboxylic acid moieties in TP304, TP278, TP276, and TP234 clearly indicates the enzymatic cleavage of the aromatic ring in bentazone. This biodegradation reaction, which is common for aromatic compounds (27,28), involves hydroxylation reactions as primary biodegradation step (10,20). This is supported by previous studies showing the formation of 6-OH-, 8-OH-, isopropyl-OH-and di-OHbentazone during bentazone degradation by a methanotrophic enrichment culture enriched from the same rapid sand filter at Sjaelsø waterworks Plant II (20).…”

“…Based on the MS/MS spectra of carboxy-bentazone revealing the cleavage of a C3H4O2 group instead of C3H6, it is clear that this transformation product was formed by oxidation of one of the isopropyl CH3 groups to the corresponding carboxylic acid. This is further supported by the loss of a CO2 group as well as by results showing the hydroxylation of the isopropyl-moiety by methanotrophs as one main biotransformation pathway for bentazone (Figure 1) (20). For TP246 the obtained exact mass indicated the cleavage of two carbon atoms and the incorporation of two oxygen atoms.…”

“…Rapid degradation of bentazone was recently observed in filter sand from a biological rapid sand filter receiving methane-rich groundwater (19). Investigations of bentazone degradation with a methanotrophic culture, enriched from this rapid sand filter revealed transformation to several hydroxylated dead-end transformation products, more specifically 6-OH-, 8-OH-, isopropyl-OH-and di-OH-bentazone (20). As oppose to previous studies with enrichment cultures, methane is generally stripped off by aeration prior to sand filtration, and filter sand harbors complex microbial communities, including e.g.…”

“…By combining results from different methods it was possible to investigate bentazone degradation pathways at high concentrations and compare it to degradation at low concentration. Since methanotrophic degradation of bentazone can lead to accumulation of 6-OH-and 8-OH-bentazone (20), it was investigated at high concentrations (0.3-1.0 mg L -1 (HPLC-DAD)) whether these bentazone transformation products could be degraded in filter sand or be dead-end products (BM1 and BM2). Experiment FTP1 and BM1 (Table 1) were screening experiments, and were conducted at room temperature (18-23⁰C).…”

Recent Development on the Electrochemical Detection of Selected Pesticides: A Focused Review

“…Hence, it has been reported as one of the most important contaminants in terms of frequency of detection and maximum concentration in surface freshwater and groundwater [79]. For example, in Denmark, it was one of the pesticides detected in 49.5% of the groundwater monitoring wells in the period 1990-2015 [84].…”