Sunlight-Driven Chlorate Formation during Produce Irrigation with Chlorine- or Chloramine-Disinfected Water

Abstract

The increasing use of chlorine-or chloraminecontaining irrigation waters to minimize foodborne pathogens is raising concerns about the formation and uptake of disinfection byproducts into irrigated produce. Chlorate has received particular attention in the European Union. While previous research demonstrated the formation of chlorate from dark disproportionation reactions of free chlorine and uptake of chlorate into produce from roots, this study evaluated chlorate formation from solar irradiation of chlorine-and chloramine-containing irrigation droplets and uptake through produce surfaces. Sunlight photolysis of 50 μM (3.6 mg/L as Cl 2 ) chlorine significantly enhanced the formation of chlorate, with a 7.2% molar yield relative to chlorine. Chlorate formation was much less significant in sunlit chloramine solutions. In chlorinated solutions containing 270 μg/L bromide, sunlight also induced the conversion of bromide to 280 μg/L bromate. Droplet evaporation and the resulting increase in chlorine concentrations approximately doubled sunlight-induced chlorate formation relative to that in the bulk solutions in which evaporation is negligible. When vegetables (broccoli, cabbage, chicory, lettuce, and spinach) were sprayed with chlorine-containing irrigation water in a sunlit field, sunlight promoted chlorate formation and uptake through vegetable surfaces to concentrations above maximum residue levels in the European Union. Spraying with chloramine-containing waters in the dark minimized chlorate formation and uptake into the vegetables.

Electrochemical Dechlorination of Municipal Wastewater Effluent

“…After 110 min, no chlorine residual was detected. The chlorate concentration measured in a control containing only 100 μM sodium hypochlorite contained 21 μM chlorate, comparable to concentrations measured previously in chloramine solutions and attributable to background chlorate concentrations that occur in sodium hypochlorite stock solutions . After electrochemical treatment, the chlorate concentration was 19 μM (range 17–21 μM), indicating that chlorate formation was not important.…”

Purified Chlorine Dioxide as an Alternative to Chlorine Disinfection to Minimize Chlorate Formation During Postharvest Produce Washing

“…The minimum detection limits for chlorate ranged from ∼0.48–1.20 μM in the vegetable matrices, which corresponded to ∼0.48–1.20 nmol/g wet weight of vegetable, for ∼1 g vegetable samples. Our previous application of this produce processing method demonstrated overall recoveries of 80–85% for chlorate applied directly to produce surfaces, with ∼30–50% recovered in rinsate and the remainder from within the produce . In a control experiment to evaluate the effect of the 10 g/L thiosulfate quenching agent applied here, there were no observable differences in chlorate concentrations measured in lettuce matrices in the presence and absence of 10 g/L thiosulfate, confirming that both the lettuce matrix and thiosulfate had minimal interferences with chlorate measurements using ion chromatography (Figure S4).…”

“…Based on our previous work indicating that ∼30–50% of chlorate applied to produce surfaces can be rinsed from the surface, we rinsed vegetable samples to distinguish chlorate taken up into produce . To measure chlorate levels in vegetable samples, the quenched samples were passed over a sieve.…”

“…Based on our previous work indicating that ∼30−50% of chlorate applied to produce surfaces can be rinsed from the surface, we rinsed vegetable samples to distinguish chlorate taken up into produce. 10 To measure chlorate levels in vegetable samples, the quenched samples were passed over a sieve. The ∼1 g vegetable samples were rinsed briefly under deionized water, and then were placed in a −20 °C freezer overnight and then freeze-dried for a minimum of 24 h. The dry mass of the freeze-dried samples was measured and recorded; vegetable samples showed a 92−95% reduction in mass on average with freeze-drying.…”

“…Research also has documented the formation of high concentrations of low molecular weight DBPs (e.g., 312 μg/L dichloroacetic acid and 1069 μg/L dichloroacetonitrile) in authentic washwaters, although these products predominantly partitioned to the washwater, limiting consumer exposure. However, chlorate (ClO 3 – ) has raised particular concerns because, although it is highly soluble, research has documented uptake from water through leaves, , and the European Union has set very low temporary Maximum Residue Levels (MRLs) for various foods (e.g., 0.7 mg/kg for leafy vegetables) . Chlorate forms via disproportionation of chlorine (eq ); while Cl 2 gas does not contain ClO 3 – , ClO 3 – accumulates slowly in high pH sodium hypochlorite stock solutions and more rapidly when application to circumneutral pH waters increases the concentration of HOCl. − 2 HOCl + OCl − → ClO 3 − + 2 Cl − + 2 H + …”