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60 <br />of newly formed precipitates. These processes enable the <br />removal of heavy metal.cations even though the pH may not <br />be high enough to cause removal through precipitation. <br />Diagrammatic reactions for the three processes are shown <br />below: <br />1. M2+(aq) + 20H (aq) -? M(OH)2 Precipitation <br />2 <br />2., 2M +(aq) + <br />solid M <br />solid Adsorption <br />M <br />3. M2+(aq) + solid solid solid solid <br />solid ?' M <br />so id Occlusion <br />For adsorption and occlusion to be significant processes, <br />. the solid particles have to be small so that they possess <br />high relative surface areas. In natural waters, the hydroxides <br />of aluminum, silicon, manganese and iron form colloidal <br />sized precipitates. These gelatinous solids have been found <br />to have a high propensity for adsorbing and occluding other <br />ions (6, 17). Since Mn and Fe are present in mine drainage <br />systems it has been speculated that the hydroxides of these <br />two metals have been just as important to the removal of <br />cations such as Pb2+, Cd2+, Zn2+, Ag+, and Cu2+ as the preci- <br />pitation of the respective hydroxides (43, 44). Of the two <br />processes, adsorption appears to be more important than <br />occlusion (43, 44). A discussion of the adsorptive capacities <br />of Mn and Fe hydroxides and how this relates to process waters <br />• <br />