Soil Adsorption Phenomena
Soil Adsorption Phenomena
Adsorption experiments involving soil particles typically are performed in a sequence of three steps: (1) reactio of an adsorptive (ion or molecule) with a soil contacting an aqueous solution of known composition under controlled temperature and applied pressure for a prescribed period of time, (2) separationof the wet soil slurry from the supernatant aqueous solution, and (3) quantitationof the ion or molecule of interest, both in the aqueous solution and in the separated soil slurry along with its entrained soil solution. The reaction step can be performed in either a closed system (batch reactor) or an open system (flow-through reactor), and it can proceed over a time period that is either relatively short (to investigate adsorption kinetics) or very long (to investigate adsorption equilibration). The separation step is similarly open to choice, with centrifugation, filtration, or gravitational settling being conventional methods to achieve separation. The quantitation step, in principle, should be designed not only to determine the moles of adsorbate and unreacted adsorptive, but also to verify whether unwanted side reactions, such as precipitation of the adsorptive or dissolution of the adsorbent, have influenced the experiment. After reaction between an adsorptive i and a soil adsorbent, the moles of i adsorbed per kilogram of dry soil is calculated with the standard equation ni ≡ niT − Mwmi where niT is the total moles of species i per kilogram dry soil in a slurry (batch process) or a soil column (flow-through process), Mw is the gravimetric water content of the slurry or soil column (measured in kilograms water per kilogram dry soil), and mi is the molality (moles per kilogram water) of species i in the supernatant solution (batch process) or effluent solution (flow-through process). Equation 8.1 defines the surface exces, ni, of an ion or molecule adsorptive that has become an adsorbate. Formally, ni is the excess number of moles of i per kilogram soil relative to its molality in the supernatant solution. As mentioned in Section 7.2, this surface excess may be a positive, zero, or negative quantity.
Keywords: Langmuir equation, Marcus process, adsorption edge, boron adsorption, distribution coefficient, exclusion volume, ferrihydrite, ligand exchange, metal cation adsorption, molality, negative adsorption
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