176 



ANALYSIS OF THE ENVIRONMENT 



ADSORPTION 



Much that has been written about ad- 

 sorption has indirect rather than direct 

 appUcation to ecology. This subject is dis- 

 cussed at length in physical and colloidal 

 chemistry, and the interested student must 

 consult such sources to get a general view 

 of the subject. Adsorption has been httle 

 studied from the ecological point of view; 

 here again general physiology is much in 

 advance of ecology. 



Any process that diminishes the free 

 energy at the boundary between a solid or 

 immiscible or slowly miscible fluid, and an 

 aqueous solution, tends to proceed toward 

 an equilibrium, whatever may be the energy 

 involved— chemical, mechanical, or electri- 

 cal. A process that diminishes the surface 

 electrical charge is an example. The con- 

 centration of a substance in an aqueous 

 solution on the surface of another phase of 

 matter is called adsorption. The relation be- 

 tween adsorption and the surface energy is 

 knovm as the principle of Gibbs. In addi- 

 tion to stating that the concentration of a 

 substance will be increased if it lowers sur- 

 face energy, Gibbs' principle implies the 

 opposite process: If a substance raises sur- 

 face energy, its interphase concentration 

 will be decreased, a process that has been 

 called negative adsorption. 



Finely divided materials— powdered char- 

 coal, suspensions of clay in water, or col- 

 loids in general— present large amounts of 

 surface per unit of sohd matter and are 

 hkely to be potent adsorbing agents. Char- 

 coal in water carries a negative surface 

 charge and adsorbs positively charged col- 

 loidal particles or the cations of dissociated 

 chemicals; the surface of most solids, in- 

 soluble in water, have a negative charge 

 when immersed in it. Clay is an electro- 

 negative colloid; egg albumin carries an op- 

 posite charge. Electrical charges opposite in 

 sign diminish, neutralize, or even reverse 

 the surface charge. Alkalies and acids act- 

 ing through OH" and H* ions affect the 

 surface charge so that the same substance 

 may adsorb now anions and later cations 

 from the same solution, depending on its 

 pH. Among the ecological effects produced 

 by adsorption, we may mention the follow- 

 ing: 



The adsorption of water on dust or salt 

 particles is the basis of condensation of 

 water droplets in the atmosphere. 



Inorganic salts raise the surface tension 

 at the water-air interphase and produce 

 negative adsorption there. 



Gases are adsorbed by soUds; the action 

 of charcoal is well known in this respect 

 and may have ecological importance in the 

 soil atmosphere of burned-over areas. Char- 

 coal may also reduce the free cations in 

 soil water and, if present in sufficient 

 amount, may affect the reaction of the soil. 



Clay suspensions in water also adsorb 

 cations and tend to lower the pH. Such 

 suspensions are common in the muddy 

 waters of flooded streams. The aluminum 

 content of clay is high, and aluminum is a 

 particularly effective adsorbing agent. In 

 keeping with its positive charge, unlike clay, 

 it adsorbs negative ions. Its effectiveness as 

 an adsorbent illustrates the rule that adsorb- 

 ing power increases rapidly with valence, 

 and aluminum has a valence of three. Gen- 

 eral information concerning adsorption in 

 relation to valence is not easily come by be- 

 yond the fact that trivalent ions are more 

 potent adsorbing agents than are ions with 

 a valence of two or one. Recent books on 

 surface chemistry should be consulted for 

 further pertinent details. 



To continue with the general ecological 

 effects of adsorption: 



The solutes present in a mixed solution, 

 and the solvent as well, are adsorbed in defi- 

 nite proportions. 



Adsorbed material may be eluded either 

 by changing the electrical charge on the 

 interface or by the presence of another sub- 

 stance with greater ability to lower surface 

 energy. In the latter instance, the more po- 

 tent substance tends to replace the less 

 potent. 



Many soil characteristics are determined 

 by the adsorbing power of soil particles; for 

 example, adsorption allows the soil to re- 

 tain soluble mineral nutrients so that they 

 are not all carried away by percolating 

 water. 



Micro-organisms may be poisoned in pro- 

 portion to the amount of adsorption of 

 certain poisons on their body surfaces, and 

 cell processes in general can be strongly 

 affected by substances that remain on the 

 surface. In ecology, such processes have 

 direct and obvious importance with micro- 

 organisms. Bacteria may themselves be ad- 

 sorbed on soil particles and so rendered 

 inactive. 



Adsorption plays an intimate role in 



