Permeable reaction barrier

The method or methods employed to remediate an aquifer vary, depending on the type, degree, and extent of contamination. Where pollution is shallow and dispersed over a small area, the sediments can sometimes be dug up and transported to a landfill designed especially to isolate the contaminants. Permeable reaction barriers can be installed to intercept a contaminant plume and strip pollutants from it, if the plume is shallow and narrowly focused. 

Yuan C, Chiang TS. (2007). The mechanisms of arsenic removal from soil by electrokinetic process coupled with iron permeable reaction barrier. Chemosphere 67 1533-1542. 

For permeable reactive barriers, the monitoring effort must consider the contaminant that exceeds the groundwater standards. In addition, monitoring of degradation products from reductive dehalogenation reactions or other contaminant transformation products may be required. Other groundwater parameters such as pH, alkalinity, specific conductance, and major compositions with respect to cations and anions must be included... 

Specific interactions with Ni (Wang et al. 2004) are reported to be at the base of the almost total conversion of CC14 to methane, thus suggesting that the preferred reaction path is not the sequential dechlorination, usually observed on other cathodes [e.g. Fe and glassy carbon (Liu et al. 2000 Costentin et al. 2003 Li et al. 2000)]. This is an important feature for the treatments based on permeable reactive barriers, because the intermediate species are also toxic and are much more slow reactants than carbon tetrachloride. A high yield in methane was also observed on vitamin B12 - modified silver electrode (Betterton et al. 1995). 

Morrison SJ. (2003). Performance evaluation of a permeable reactive barrier using reaction products as tracers. Environmental Science and Technology 37 2302-2309. 

Transfonnation processes are very important for the treatment of many organic contaminants with permeable reactive barriers. The components and their products undergo sorption reactions. This sorption reactions may be either in equilibrium or nonequilibrium. Desintegration of tetrachloroethene or trichloroethylene are two of many examples, that can be treated with this permeable reactive barriers. Khandelwal and Rabideau (1999) developed analytical and semi-analytical solutions for this problem. They consider the sorption reaction with a nonequdibrium model. We verified the numerical RF-RTM model with their analytical solution. 

This example shows the migration and degradation of TCE (trichloroethylene) and its reaction products cl2DCE (c-l,2-dichloroethene) and VC (vinyl chloride) in a zero-valent iron PRB (permeable reactive barrier) (Kandelwal and Rabideau, 1999) ... 

Figure 6.5 Predicted relative concentrations across a permeable reactive barrier exit for the migration of trichloroethylene and reaction products (equilibrium model).

The A value obtained from this expression is in mass-weighted coordinates, which enables the reduced mass to be dropped from the standard (76) Eckart formulae (41), resulting in the following expression for the permeability of the reaction barrier G(W) as a function of the energy W ... 

Barrier Layers. Depending on composition, barrier layers can function simply as spatial separators or they can provide specified time delays by swelling at controlled rates or undergoing reactions such as hydrolysis or dissolution. Suitable barrier materials include cellulose esters and water-permeable polymers such as gelatin and poly(vinyl alcohol) (see Barrier polymers). 

The functions of porous electrodes in fuel cells are 1) to provide a surface site where gas/liquid ionization or de-ionization reactions can take place, 2) to conduct ions away from or into the three-phase interface once they are formed (so an electrode must be made of materials that have good electrical conductance), and 3) to provide a physical barrier that separates the bulk gas phase and the electrolyte. A corollary of Item 1 is that, in order to increase the rates of reactions, the electrode material should be catalytic as well as conductive, porous rather than solid. The catalytic function of electrodes is more important in lower temperature fuel cells and less so in high-temperature fuel cells because ionization reaction rates increase with temperature. It is also a corollary that the porous electrodes must be permeable to both electrolyte and gases, but not such that the media can be easily "flooded" by the electrolyte or "dried" by the gases in a one-sided manner (see latter part of next section). 

---