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61 <br />0 follows. <br />The diagrammatic reaction for adsorption shows an impor- <br />tant feature of the process. The surface of the solid is <br />charged and the.force of attraction of the ion for the solid <br />surface is electrostatic in nature (45). Now the amount of <br />surface charge as well as the sign of the charge can be changed <br />by changing the conditions of precipitation or by strongly <br />adsorbing some other ions onto the surface of the solid. For <br />hydroxide precipitates in water, the pH of the system strongly <br />dictates whether the surface is positive or negative. At <br />low pH's, H+ ions are strongly adsorbed and the hydroxide <br />surface is positive; at high pH's an excess of OH ions occur <br />• on the surface and thus it is negative. The pH at which the <br />surface changes from positive to negative is called the iso- <br />electric point (45). For manganese oxide precipitates, the <br />isoelectric point is at about a pH of 2, whereas for iron <br />hydroxide precipitates, the isoelectric point varies between <br />pH 5 and 9 (41, 44). What this means is that if a tailings <br />pond suspension is at pH of about 6, then Mn hydroxides have <br />a negative surface and Fe hydroxides have a positive or neutral <br />surface. Consequently, Mn oxides will more strongly adsorb <br />trace metal cations than will Fe oxides. This situation <br />has been substantiated by the recent studies of trace metal <br />associations with Fe and Mn oxides (41, 42). <br />What this implies regarding the removal of obnoxious <br />. cations from mine and mill waters is that one cannot rely on