Attenuation mechanisms/processes natural

Downstream from the sources, the geochemical evolution of AMD is usually controlled by (1) oxidation of Fe(ll) to Fe(in), (2) progressive pH increase and dilution of metal concentrations by mixing with pristine waters, (3) hydrolysis and precipitation of different metal cations as pH increases, and (4) sorption of different trace elements (As, Pb, Cr, Cu, Zn, Mn, Cd) onto the solid surfaces of precipitated metal hydroxides/hydroxysulfates [14]. The pH-dependent sequences of precipitation and adsorption are very similar and follow the order Fe(III) > Pb > A1 > Cu > Zn > Fe(II) > Cd [5, 23]. The overall result of these processes represents a mechanism of natural attenuation with environmental benefits for the water quality [12-14, 44]. 

Abstract Contribution of the Jahn-Teller system to the elastic moduli and ultrasonic wave attenuation of the diluted crystals is discussed in the frames of phenomenological approach and on the basis of quantum-mechanical theory. Both, resonant and relaxation processes are considered. The procedure of distinguishing the nature of the anomalies (either resonant or relaxation) in the elastic moduli and attenuation of ultrasound as well as generalized method for reconstruction of the relaxation time temperature dependence are described in detail. Particular attention is paid to the physical parameters of the Jahn-Teller complex that could be determined using the ultrasonic technique, namely, the potential barrier, the type of the vibronic modes and their frequency, the tunnelling splitting, the deformation potential and the energy of inevitable strain. The experimental results obtained in some zinc-blende crystals doped with 3d ions are presented. 

The Spittelwasser example (see Section 8.3.2) indicated that, unlike problems related to conventional polluted sites, the hazards here are primarily connected with the transport and deposition of contaminated solids in a catchment area, especially in downstream regions. Any problem solution strategy for such sites, therefore, has to consider both the chemical stabilization - for example, by processes of (enhanced) natural attenuation - and an increase in mechanical stability (reduced erodibility). 

Natural attenuation processes are always site-specific, so every site needs to be evaluated individually. The accepted mechanisms for inorganics, including radionuclides, are immobilization and/or biotransformation. For tritium, decay is acceptable. In most of the cases listed in Table 10.2, the attenuation process is only moderately understood. Although recognized as being important. 

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