Interactions of geochemical processes

The geochemical processes discussed in previous sections are interrelated and interdependent. If they are properly understood and utilized, they can enhance the efficiency of electrochemical remediation. Examples are given to demonstrate the importance of these interactions of geochemical processes in electrochemical remediation of contaminated fine-grained soils. 

Another innovative application of the interactions of geochemical processes during electrochemical remediation of soils is demonstrated by Faulkner, Hopkinson,... 

Trace elements are useful tracers of geochemical processes mostly because they are dilute their behavior depends primarily on the trace element-matrix interaction (e.g., Rb-host feldspar, Sr-calcite) and very little on the trace-trace interaction (e.g., Rb-Rb, Sr-Sr). Consequently, the distribution of trace elements among natural phases largely obeys the linear Henry s law. The modeling of trace elements in various geological environments (magmas, hydrothermal fluids, seawater,...) relies on three different aspects... 

Many trace element studies of archaeological samples have used neutron activation analysis (NAA). Although this technique is not useful for all elements, it is very sensitive for many of those that have proved to be valuable indicators of geochemical processes (e.g., the rare earth elements). The precision of the actual measurements is usually high and easy to determine. Samples can be irradiated with little or no sample preparation, so there are few chances of contamination during the analysis. However, the limited number of nuclear reactors severely limits access to this type of analysis. When samples are sent to a distant laboratory for analysis, the critical interaction between archaeologist and analyst can be lost. 

The Mo concentration measured in a soil also is affected by the influence of geochemical processes on its mobility, transport, and deposition. The extent of its mobilization and transport from source rocks is determined by mineral stability, which in turn is affected by the weathering environment. Once it is mobilized from the mineral source, the transport, deposition, and availability of Mo to organisms are dependent on its interactions with other soil components (such as clays, organic matter, microbes, and Fe and Mn oxyhydroxides) and the chemistry (pH, Eh, and other ion concentrations) of the soil solution. Molybdenum associated with clay minerals, oxyhydroxides, and organic matter represents the available fraction. 

The complex interactions amongst geological, biological, and geochemical processes at the land-sea margin control the delivery and fate of radionuclides, contaminants, and other natural elements in coastal environments (Swarzenski et al. 2003). For many such constituents, there is at least a fundamental understanding of major source and sink functions and their potential estuarine transformation reactions. For example, rivers can be monitored quite easily for discharge rates into estuaries as well as for elemental... 

We have studied the interaction of geologic, geochemical, geomorphic, hydrologic, and anthropogenic processes in the Sacramento Valley of north-central California, U.S.A. 

In most natural situations, physical and chemical parameters are not defined by a unique deterministic value. Due to our limited comprehension of the natural processes and imperfect analytical procedures (notwithstanding the interaction of the measurement itself with the process investigated), measurements of concentrations, isotopic ratios and other geochemical parameters must be considered as samples taken from an infinite reservoir or population of attainable values. Defining random variables in a rigorous way would require a rather lengthy development of probability spaces and the measure theory which is beyond the scope of this book. For that purpose, the reader is referred to any of the many excellent standard textbooks on probability and statistics (e.g., Hamilton, 1964 Hoel et al., 1971 Lloyd, 1980 Papoulis, 1984 Dudewicz and Mishra, 1988). For most practical purposes, the statistical analysis of geochemical parameters will be restricted to the field of continuous random variables. 

Interface and colloid science has a very wide scope and depends on many branches of the physical sciences, including thermodynamics, kinetics, electrolyte and electrochemistry, and solid state chemistry. Throughout, this book explores one fundamental mechanism, the interaction of solutes with solid surfaces (adsorption and desorption). This interaction is characterized in terms of the chemical and physical properties of water, the solute, and the sorbent. Two basic processes in the reaction of solutes with natural surfaces are 1) the formation of coordinative bonds (surface complexation), and 2) hydrophobic adsorption, driven by the incompatibility of the nonpolar compounds with water (and not by the attraction of the compounds to the particulate surface). Both processes need to be understood to explain many processes in natural systems and to derive rate laws for geochemical processes. 

Edwards, K.J., Bach, W., and McCollom, T.M., 2005, Geomicrobiology in oceanorgraphy Microbe-mineral interactions at and below the seafloor, Trends in Microbiol. 13 449-456 Schrenk, M.O., Kelley, D.S., Delaney, J.R., and Baross, J.A., 2003, Incidence and diversity of microorganisms within the walls of an active deep-sea sulfide chimney, Appl. Environ. Microbiol. 69 3580-3592 Schrenk, M.O., Kelley, D.S., Bolton, S., and Baross, J.A., 2004, Low archaeal diversity linked to sub-seafloor geochemical processes at the Lost City Hydro-thermal Field, Mid-Atlantic Ridge, Environ. Microbiol. 6 1086-1095. 

Considering the pH range 4—10 as the geochemical range, the redox potential due to the breakdown of water due to redox processes, the rare earths are predominantly present as Ln(III). Since the anions OH- and CO2 are present in natural environments, rare earths combine with these anions to form insoluble hydroxides and carbonates and hence immobilized. At lower pH, rare earth ions are adsorbed on clays, which are natural ion exchangers. The interactions of rare earth ions with humic and fulvic acids in soils, and Fe/Mn oxides are so strong, that they become immobile. 

Figure 7 Potential physical and chemical processes occurring in a magmatic-hydrothermal system, including the influence of magma dynamics in the chamber-conduit plumbing system, and interactions between magmatic fluids and the crust. These can strongly modulate the speciation and flux of various magmatic components emitted into the atmosphere, compheating the interpretation of geochemical measurements of surface emissions.

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