Degradation of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) using biodegradable polymer modified nZVI, and platinum and gold catalysts in hydrogen-saturated water

The extensive use of the explosive compound hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by the military has caused environmental concerns due to the high toxicity of RDX and its persistence in groundwater and soil media. In this study, RDX degradation was assessed using aged biodegradable polymer-coated nZVI and noble metal catalysts in hydrogen saturated water.;The polymers investigated included guar gum, potato starch, alginic acid (AA), and carboxymethyl cellulose (CMC). The RDX degradation experiments were conducted using iron samples aged for 1, 2, and 4 weeks. The results indicated that the RDX degradation rates by bare nZVI and starch coated nZVI suspensions were least affected by aging although these suspensions exhibited the least favorable suspension stability. Conversely, the aged CMC and AA samples achieved the best nZVI suspension stabilization but significantly lower RDX degradation rates at longer aging periods. Guar gum with loadings rates one order of magnitude lower than that of CMC and alginate achieved good iron stabilization but significantly higher RDX degradation rates. The reduced RDX degradation efficiency of the polymer coated nZVI is attributed to cluster formation and the change in polymer confirmation caused in part by the interaction of the polymer with the released ferrous upon aging.;Catalytic degradation of RDX was assessed using nano scale gold (Au), nano scale platinum (Pt1), micro scale platinum (Pt2), and alumina supported nano scale platinum (Pt/Al2O3) in hydrogen saturated water under different catalyst loadings and reaction temperatures. The catalytic activity of RDX degradation of the four catalysts decreased in the following order Pt/Al2O3>Pt1>Pt2>Au. The degradation rates of RDX increased with catalyst loading, temperature and smaller catalyst particle size. RDX and its by-products were completely removed in the presence of Pt1 and Pt/Al2O3 at the experimental reaction conditions while Pt2 and Au achieved significantly lower RDX removal rates. The catalytic reactions activation energies were low, in the order of less than 50 kJ/mol, whereas Au was most sensitive to the temperature changes. Pt/Al2O 3, with only 5% platinum doping, demonstrated high catalytic activity and low activation energy, which would make it a good candidate catalyst for field RDX degradation.