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59 <br />0 If sulfur and iron oxidizing bacteria are present, their most <br /> likely habitat will be in abandoned mines and tailings. It <br /> takes on the order of years for the biological catalysts to <br /> operate at peak efficiency. In active operations, their <br /> effect can be reduced by not allowing water to remain in <br /> contact with sulfide minerals. In abandoned operations, <br /> the microbiologic organisms can work to their greatest extent. <br /> Mixing abandoned and active residues will allow the biologic <br /> weathering of sulfides to increase in the current refuse. <br /> PROCESS WATER <br /> For this category of water, the operations of neutrali- <br /> zation and settling which are currently used in the industry <br /> are reasonable. However, some recent observations on what <br /> happens upon addition of base to the water may help to make <br /> these processes more efficient. There are some properties <br /> of the hydroxide precipitates that are formed which can <br /> affect how well the water is cleaned. These properties have <br /> only recently been investigated (41, 42,°43), so that an <br /> explanation of how they affect the removal of heavy metals <br /> from waters is in order. <br /> The raising of the pH of water causes the formation of <br /> heavy metal hydroxide precipitates which settle from the <br /> water. Figure 1 shows the sequence of removal as the pH is <br /> raised. There are also two other mechanisms for removal <br /> of heavy metals, specifically, adsorption of ions onto the <br /> surfaces of solids and occlusion of ions into the structure