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Richard L Johnson

Publications and source records attributed to Richard L Johnson.

4 recordsLinked to original sources
PubMed2005

Reduction of 2,4,6-trinitrotoluene by iron metal: kinetic controls on product distributions in batch experiments.

The reaction kinetics and product distributions for the reduction of 2,4,6-trinitrotoluene (TNT) by granular iron metal (Fe0) were studied in batch experiments under a variety of initial concentrations of TNT and Fe0. Although the kinetics of TNT disappearance were found to behave in accord with the standard theory for surface-mediated reactions, a complex relationship was found between the initial concentrations of TNT and Fe0 and the appearance of the expected nitro reduction product, 2,4,6-triaminotoluene (TAT). TNT was completely converted to TAT only when the initial concentration of TNT was low and/or the initial concentration of Fe0 was high. Mathematical analysis of a range of generic reaction schemes that produce stable end products in addition to TAT showed that (i) surface complexation of TAT is insufficient to describe all of our data and (ii) polymerization reactions involving TAT and/or various reaction intermediates are the likely source of the incomplete conversion of TNT to TAT at high initial TNT concentration and low Fe0 concentration. The relationship between TAT production and reaction conditions is shown to imply that passivation due to reaction products is more likely when the ratio of initial TNT concentration to Fe0 concentration is high and, therefore, that passivation rates observed at the laboratory scale are likely to be faster than those which would be observed at the field scale.

Environmental Pollutants↗
PubMed2003

Effects of daily precipitation and evapotranspiration patterns on flow and VOC transport to groundwater along a watershed flow path.

MTBE and other volatile organic compounds (VOCs) are widely observed in shallow groundwater in the United States, especially in urban areas. Previous studies suggest that the atmosphere and/or nonpoint surficial sources could be responsible for some of those VOCs, especially in areas where there is net recharge to groundwater. However, in semiarid locations where annual potential evapotranspiration can exceed annual precipitation, VOC detections in groundwater can be frequent. VOC transport to groundwater under net discharge conditions has not previously been examined. A numerical model is used here to demonstrate that daily precipitation and evapotranspiration (ET) patterns can have a significant effect on recharge to groundwater, watertable elevations, and VOC transport. Ten-year precipitation/ET scenarios from six sites in the United States are examined using both actual daily observed values and "average" pulsed precipitation. MTBE and tetrachloroethylene transport, including gas-phase diffusion, are considered. The effects of the precipitation/ET scenarios on net recharge and groundwater flow are significant and complicated, especially under low-precipitation conditions when pulsed precipitation can significantly underestimate transport to groundwater. In addition to precipitation and evapotranspiration effects, location of VOC entry into the subsurface within the watershed is important for transport in groundwater. This is caused by groundwater hydraulics at the watershed scale as well as variations in ET within the watershed. The model results indicate that it is important to consider both daily precipitation/ET patterns and location within the watershed in order to interpret VOC occurrence in groundwater, especially in low-precipitation settings.

Carcinogens↗