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61 <br /> 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 <br />• <br />