Formatted Title
A New Method to Treat Fumigant Pesticides-Spent Granular Activated Carbon Utilizing Alkaline Hydrolysis
Background/Objectives
Addressing legacy fumigant pesticides in groundwater and soil is a continuous environmental challenge despite their discontinued application decades ago. Fumigant pesticides 1,2-dibromo-3-chloropropane (DBCP), 1,2-dibromoethane (EDB), and 1,2-dichloropropane (DCP) are frequently detected at or near agricultural sites in US and abroad. 1,2,3-Trichloropropane (TCP), a haloalkane with similar chemical structure, is typically detected at sites where these pesticides were once applied. These recalcitrant chemicals are believed to pose adverse health effects including male sterility and potential carcinogenicity; hence, effective methods are needed to successfully achieve their cleanup. The current approach often involves a pump and treat system with granular activated carbon (GAC) that is consequently thermally regenerated. Alternatively, a novel and promising method proposed of treating the pesticide-spent GAC is based on alkaline hydrolysis, a well-understood abiotic transformation mechanism, that offers a potentially greener approach to conventional thermal regeneration.
Approach/Activities
Laboratory-scale tests were designed and executed to determine optimal reaction conditions of pesticide-spent GAC regeneration through alkaline hydrolysis. Specific objectives were to determine the transformation kinetics of the pesticides in the aqueous alkaline solution (i.e., homogeneous, GAC-free), quantify the loss of pesticides in alkaline treated pesticide-spent GAC, assess the impact of alkaline treatment on GAC sorption properties, and to identify reaction byproducts as indicators of the reaction mechanism and potential products of the fumigant pesticides’ transformation.
Results/Lessons Learned
The hydrolysis of pesticide fumigant compounds under aqueous alkaline conditions (pH 12.0–12.4) occurred at significantly different rates (DBCP>>TCP>>EDB>>DCP). Under the alkaline conditions used in this study, transformation of the fumigant compounds was first-order and reaction rate constants were DBCP 11.7 d-1, TCP 0.595 d-1, EDB 0.072 d-1, and DCP 0.0048 d-1. Under heterogeneous conditions involving GAC, 94.95–99.98% loss of DBCP, TCP and EDB was measured within 30 d in both the bituminous (F400) and coconut (COC-L55) GAC. After a short reaction period of 5 d, the loss of DBCP at pH 11, 12.0, and 12.6 was 74%, 89%, and 99%, respectively, indicating that the DBCP hydrolysis reaction is pH dependent in heterogeneous systems. This result underscores the effect of aggressive alkaline conditions on pesticide hydrolysis rates. Based on the 50% pesticide breakthrough curves using rapid small-scale column tests, alkaline treatment of GAC exhibited limited impact on the post-treatment sorption of the pesticides, and in some cases enhanced sorption was measured. Conceptually, based on the kinetic parameters developed in this study and a simplified three-reactor GAC treatment system, an off-line (i.e., stand-by) unit could be treated using aggressive alkaline pH conditions (i.e., pH 12.0–12.4) to hydrolyze DBCP, TCP, and EDB within a practical timeframe (~2 months). In cases where DCP treatment was required, longer reaction periods are projected. Overall, fumigant pesticide destruction using the alkaline hydrolysis represents a new and novel method to treat pesticide-spent GAC.