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Title: Determining breakthrough of the soil fumigant chloropicrin from 120 mg XAD-4 sorbent tubes

Author
item Ashworth, Daniel
item ZHENG, WEI - UC RIVERSIDE
item Yates, Scott

Submitted to: Atmospheric Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/27/2008
Publication Date: 7/1/2008
Citation: Ashworth, D.J., Zheng, W., Yates, S.R. 2008. Determining breakthrough of the soil fumigant chloropicrin from 120 mg XAD-4 sorbent tubes. Atmospheric Environment. Vol 42(21):5483-5488

Interpretive Summary: Fumigant pesticides, such as chloropicrin, are used to eliminate pests (e.g. weeds, nematodes, bacteria and fungi) prior to the planting of high cash crops. This helps to ensure high yield and maximizes economic return. However, because fumigants are gaseous, they are easily lost from the soil to the atmosphere. Their presence in the atmosphere is linked to potential human health effects through direct inhalation, and the formation of near-surface ozone (smog). In order to research the loss of fumigants from soil to air, satisfactory methods for determining their concentration in air are required. Usually, this involves pumping air through a sorbent tube that chemically retains the fumigant. However, the rate at which air is pumped through the tube, and the mass of fumigant in the air may influence the retention ability of the sorbent. For example, at high flow rate and mass loading, the fumigant may ‘breakthrough’ the sorbent tube, i.e. not all the fumigant entering the tube is retained. This would lead to an underestimate of fumigant concentration in the air. Therefore, we tested the potential for chloropicrin breakthrough from 120 mg XAD-4 sorbent tubes (a commonly used tube in this type of study). The effects of chloropicrin loading (0.33 and 3.3 mg) and air flow rate (50 mL min-1 and 1000 mL min-1) on the transport of chloropicrin through six XAD-4 tubes (connected in series) were examined over time periods ranging from 1 to 360 minutes. The higher flow rate led to rapid and high breakthrough of the chloropicrin. At 360 minutes, all six tubes together retained only 46-54 % (depending on initial loading) of the added chloropicrin. At the lower flow rate, essentially all of the added chloropicrin was always retained on the first two tubes. The effect of flow rate was much greater than that of initial chloropicrin loading. It is hypothesized that at the higher flow rate, the chain of sorbent tubes acted like a chromatography column; retarding the passage of chloropicrin, rather than irreversibly adsorbing it. It is concluded that when 120 mg XAD-4 tubes are used in soil fumigant emission studies, it should be at low flow rates only and always with at least one back-up tube.

Technical Abstract: The emission to the atmosphere of soil fumigants such as chloropicrin represents a potentially important human exposure pathway. Commonly, determining the air concentration of fumigants is carried out by pumping air through sorbent tubes which chemically retain the fumigant. In order to obtain an accurate measurement, it is essential that the fumigant does not breakthrough the sorbent tubes, since this would result in an underestimation. Using simple apparatus, we tested the potential for chloropicrin breakthrough from 120 mg XAD-4 sorbent tubes. The effects of chloropicrin loading (0.33 and 3.3 mg) and air flow rate (50 mL min-1 and 1000 mL min-1) on the transport of chloropicrin through six XAD-4 tubes (connected in series) were examined over time periods ranging from 1 to 360 minutes. The higher flow rate led to rapid and high breakthrough of the chloropicrin. At 360 minutes, all six tubes together retained only 46-54 % (depending on initial loading) of the added chloropicrin. At the lower flow rate, essentially all of the added chloropicrin was always retained on the first two tubes. The effect of flow rate was much greater than that of initial chloropicrin loading. It is hypothesized that at the higher flow rate, the chain of sorbent tubes acted like a chromatography column; retarding the passage of chloropicrin, rather than irreversibly adsorbing it. It is concluded that when 120 mg XAD-4 tubes are used in soil fumigant emission studies, it should be at low flow rates only and always with at least one back-up tube.