Removal of RDX and HMX from an artificial groundwater by granular activated carbon

Abstract

Granular activated carbon (GAC) adsorption was efficient at removing high explosive contaminants such as Hexahydro-1,3,5-trinitro-1,3,5-tririne (RDX) and Octahydro-1,3,5,7-tetrritro-1,3,5,7-tetruocine (HMX) from an artificial groundwater (AGW). A completely mixed batch reactor (CMBR) system was selected for all rate and isotherm experiments. A number of rate and isotherm experiments were conducted to measure performance in the removal of RDX and HMX using GAC depending on dissolved oxygen, natural organic matter (NOM) preloading and GAC pretreatment. The investigation of competitive adsorption of RDX and Cr(VI) on GAC was conducted. In addition, IAST model predictions were made for RDX and HMX bisolute isotherms. When oxygen was excluded in the experimental system, there was enhancement in the removal of RDX from AGW using Fe[] pretreated GAC. However, in spite of this enhancement, it was still lower than the level of removal using virgin GAC as discussed below. According to the screening experiment results, dissolved Fe[]alone could not chemically reduce RDX. Despite expectations to the contrary based on the screening results, there was indeed some increase in the removal of RDX by Fe[] pretreated GAC under anaerobic conditions. It was suspected that in regard to the removal of RDX using GAC, there may have been chemical reactions occurring between RDX and the chemically reduced GAC surface under anaerobic conditions. Overall, the sorption capacity of GAC for RDX and HMX decreased as preloaded humid acid concentration increased. GAC procreated with a strong reluctant such as Fe[] or dithionite did not result in the enhancement of RDX removal from AGW compared with isotherms of virgin GAC under both aerobic and anaerobic conditions. Experimental values showed that for RDX, the single-solute isotherm data, bisolute isotherm data in the presence of Cr(VI) exhibited similar results. This suggests that the presence of Cr(VI) had negligible effect on RDX removal by GAC, indicating that competitive effects between RDX and Cr(VI) were minimal. Also, Cr(VI) removal was not significantly affected by the presence of RDX. IAST model predicted that the presence of HMX would reduce the adsorption of RDX compared to the single-solute isotherm of RDX. In the same manner, similar effects were obtained for HMX.