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CHAPTER
3
Physical Adsorption
Dissolved organic carbon (DOC) can be removed from aquarium water by physical adsorption using either granular activated carbon or foamfractionation. The inorganic nutrients ammonia, nitrate, and phosphate can be reduced in solution by the use of ion exchangers. Ion exchange i~ ordin-arily not considered a physical adsorption process, but ithas bee I! included here for convenience. Granular activated carbon and foam frae tionation work well in freshwater, brackish water, and seawater; ion ex changers are useful only in freshwater.Physical adsorption follows biolog ical and mechanical filtration in a water management scheme, and precede •• disinfection, as illustrated in Fig. 3-1. Ion exchangers should follow tilt' other physicaladsorption methods in series.
Adsorption is defined as the collection of DOC on a suitable interface.
The interface is the boundary between two phases, one being the aquarium water. The otherphase can be a gas, such as an air bubble in a foam fractionation contactor, or a solid, such as an activated carbon grain. During adsorption, the substance collected (called the adsorbate) leaves thewatv and becomes bound to the surface of the gas or solid (called the adsorbcnts, If the chemical bond is strong, the adsorption process is irreversible am! chemical adsorption is said to haveoccurred. If the bonds are weak, Sill II as those formed by van der Waals forces, the process is one of pAy-,il'lll adsorption. A weak bond means that the adsorption process is rcversibk-. often as theresult of a change in adsorbate concentration. Desorption is tlureverse phenomenon by which adsorbates that have been adsorbed p:I'l" back across the phase boundary and reenter solution.3.1 ACTIVATED CARBON
Some of the DOC can be removed from aquarium water by adsorplioll on activated carbon. Activated carbon is manufactured ill two steps. 'III( first is char formation in which a...