Adsorption of RDX and its Nitroso Metabolites onto Activated Carbon

Groundwater contaminated with the high explosive Demolition eXplosive, or hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) may also contain the nitroso-RDX metabolites hexahydro-1-nitroso-3,5-dinitro-1,3,5-triazine (MNX), hexahydro-1,3-dinitroso-5-nitro-1,3,5-triazine (DNX), and hexahydro-1,3,5-trinitroso-1,3,5-triazine (TNX). Although adsorption to activated carbon effectively removes RDX from contaminated water, the treatability of the nitroso-RDX compounds by adsorption has not been previously reported. Single-component isotherms were completed and the Freundlich parameters (K and 1/n) were calculated for adsorption of these compounds to Calgon F400 from unbuffered high purity water under aerobic and anoxic (<1 mg/L dissolved oxygen) conditions. Under both conditions, MNX was the most adsorbable, followed by RDX, DNX, and TNX. The presence of dissolved oxygen slightly increased the Freundlich parameters for all compounds except TNX. Under aerobic conditions, Freundlich capacity parameters (K) ranged from 46.4 (mg/g)(L/mg)1/n for TNX to 164.2 (mg/g)(L/mg)1/n for MNX, while under anoxic conditions, K ranged from 47.1 (mg/g)(L/mg)1/n for TNX to 133.6 (mg/g)(L/mg)1/n for MNX. Additional testing showed that 7 days were sufficient for adsorption equilibration in most cases. Simplified granular activated carbon (GAC) column modeling demonstrated that the presence of these compounds in groundwater can reduce GAC service life.