Mechanisms of arsenic and lead release from hydrothermally altered rock.

This paper describes the effects of pH, dissolved oxygen (DO), redox conditions, and mixing ratio of different rocks on the leaching behaviors of As and Pb from hydrothermally altered rock as well as the functional groups incorporating As and Pb in the rock. Most of As and Pb were incorporated in the residual or crystalline phase although significant amounts were also determined to be exchangeable, with carbonates and with Fe-Mn oxides. Under oxic conditions, As and Pb showed similar leaching behaviors at similar pH values, a higher mobilization in the acidic and alkaline regions and a minimum at circumneutral pH. The absence of DO restricted the oxidation of sulfide minerals that also contained significant quantities of As resulting in a lower As release under these conditions. Strongly reducing conditions favored the release of As by the reductive dissolution of Fe-Mn oxides and prevention of carbonate precipitation while the same conditions immobilized Pb because of its re-precipitation under reducing conditions. In general, depending on the pH, DO, and redox conditions, the major modes of As and Pb release from these sources could be either one or more of the following mechanisms: acid dissolution, reductive dissolution, ion exchange, desorption and sulfide oxidation processes.