Effects of pH on the release of metals from naturally‐occurring oxides of Mn and Fe

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Abstract
A study was made of the release of metals (Ca,Co,Cu,Zh,Cd,Ba,Pb) from two natural oxide samples, a 2:1 mixture of MnO2 and ferrihydrite (sample A) and a goethite (sample B). Release was inversely related to pH, and at pH 1–3 large proportions (up to 90%) of most of the metals came into solution, without dissolution of the oxides. For Ca,Cu,Zn,Cd and Ba the pH dependences of release from sample A were compatible with published adsorption data for synthetic MnO2. The pH values required for sample B to release Cu,Zn,Cd and Pb were considerably lower than expected on the basis of adsorption studies with synthetic goethite.