Passive regenerating systems

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

In addition, the substrate and product should be transported through the cell membrane, either passively or actively, and necessary cofactors should be regenerated. Finally, the specific organism used should function well in an optimized bioreactor system. All of these requirements can be met by using strains that contain the desired enzyme in question. 

The vendor states that savings associated with the EnviroMetal Process are due to the low operation and maintenance costs for treatment walls. Because contaminants are destroyed rather than removed, process monitoring costs are negligible, and little or no waste products require disposal or regeneration on a regular basis. The vendor also states that since the technology is a passive treatment, there is no need for manual labor to operate, monitor, and maintain the system (D14522C, pp. 19, 20). 

The film decomposition begins after the iron potential rises above a certain threshold value. This is accompanied by a gradual extension of the film-free surface. This process is accelerated as iron dissolution progresses. Subsequently the accumulated nitrous acid promotes the regeneration of the passivating oxide film and then the system returns to its original state. 

In addition to exchanging structural electrons, ferric oxyhydroxide minerals also act to mediate electron exchange from surface bound Fe to several reducible pollutants of environmental concern (52). In this case, the redox capacity of the mineral is not limited by the formation of a passivating layer because the bulk reductant is aqueous ferrous iron and reactive surface sites are continually regenerated. Haderlein and Pecher (55) review the environmental factors affecting the reactivity of surface bound Fe(II) in heterogeneous systems. 

Passivated systems can then be regenerated in situ by re-reduction/nitridation. Within the literature, a number of approaches toward the preparation of nitrides have been reported. A few of the main ones are very briefly outlined below. 

Continuous Regenerable Trap. Another approach is to generate NO2 upstream of an uncatalyzed DPF for combusting the particulate matter. This so-called continuous regenerable trap (or CRT) is based on phenomenon first published in 1989 by Cooper and Thoss (143), in which NO2 oxidized the dry carbon soot held within the trap at temperatures below that of similar oxidation with the O2 in the air. This technology is considered passive in contrast to this active system. 

The possibility of electrochemical oxidation of the ion beam-produced carbonaceous layer at the surface of the conducting polymers (polyaniline, polypyrrole, polythiophene) was also demonstrated [99,100,112]. In this case the anodic oxidation and dissolution of the implanted layer result in regeneration of the electrode surface, accompanied by a substantial increase of the electrochemical activity [99,100,112]. On the other hand, an increase of the surface resistance of the conducting polymers by several orders of magnitude upon implantation with low doses (<10 ions cm [99]), originating in the destruction of the initial rr-electron system, leads to passivation of the polymer electrodes with respect to further electropolymerization [112] and galvanic deposition of metals [99,1(K)]. In particular, this offers the possibility of metal pattern formation with a resolution of several micrometers at the surface of the conducting polymers [99,1(K)]. 

Electrochemical methods, such as cathodic protection and chloride extraction, can be used as a part of a repair strategy. Cathodic protection techniques, described above, provide alkalinity. The impressed current transports alkalies to the reinforcing bar and allows alkalinity to be retained. The chloride extraction system removes chloride ions (Cl ) from the concrete electro-chemically, and does not allow the breaking of passive layers. In yet another process, called re-alkalization, alkaline metal ions penetrate concrete from an external source of a suitable electrode to re-alkaline the concrete and regenerate the hydroxyl ions. 

Materials that rely on a passive layer are particularly sensitive to triboconosion. Once the passive layer is removed, the bare metal surface will be exposed to the corrosive medium and it will be susceptible to corrosion damage. If the passive layer could not regenerate quickly, then the material would imdeigo significant corrosion damage. It was observed that material removal in a triboconosion system usually exceeds the sum of mechanical and conosion contributions measured separately (Goodman, 1994 Meunier and Sedel, 1998). 

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