The Study On The Potential-pH Diagram Of As-Pb-S-H₂O System

The potential-p H diagram refers to a relational diagram between system potential and p H in the reaction process. In a limited plane, it clearly and concisely reflects the equilibrium relationship of elements’ complex, dynamic, heterogeneous chemical reaction and non-heterogeneous chemical reaction in the aqueous solution. In addition, it can intuitively and visually judge materials’ redox ability, position and size of predominance diagram, direction of reaction, p H conditions, etc.According to the principles of simultaneously balance, this thesis made a thermodynamic analysis on the As-Pb-S-H₂ O system and related sub-systems when the temperature is 25 ℃ and it is a normal atmospheric pressure, established a mathematic model of the species concentration and E, p H, and drew the predominance diagrams of dissolved species and pourbaix diagrams of As-H₂ O, Pb-H₂ O, S-H₂ O, As-Pb-H₂ O, As-S-H₂ O, Pb-S-H₂ O, As-Pb-S-H₂ O. After comparing predominance diagrams with pourbaix diagrams, it found that the species’ predominance diagrams are closely related to species concentration, E, p H and system temperature, gaseous phase partial pressure and so on. The research shows that:In the predominance diagram of the Pb-H₂ O system, lead dissolved components mainly exist in high potential areas in a form of +2 valence. With the increase of total lead concentration in the system, species Pb OH+ and Pb O had a polymerization reaction and generated Pb4（OH）44+、Pb6（OH）84+, but the predominance diagram of dissolved components Pb2+ and HPb O2- in the system was basically unchanged; In pourbaix diagram, the stability region of solid-phase components Pb O, Pb12O17 and Pb12O19 significantly reduced with the increase of total lead concentration, and the region variation of solid-phase Pb and Pb O2 was small.In the predominance diagram of S-H₂ O system, high-valence species were in oxidation areas with a higher potential value, and low-valence species distributed in the reducibility areas with a lower potential. With the increase of total sulfur concentration in the system, the predominance of dissolved state S（aq.） increased obviously, and the changes of the rest of components were smaller; In pourbaix diagram, the gas-phase components and dissolved components have a liquid phase equilibrium, and the area of the stable region of gas-phase components S8, H₂ S, H（H3）S significantly increased with the increase of the total sulfur concentration in the system.In the predominance diagram of the As-H₂ O system, the distribution of dissolved-state species was associated with the species’ valence state. Low-valence arsenic mainly distributed in low potential areas, while high-valence arsenic distributed in higher potential areas. The size of predominance diagram of dissolved constituents was insignificantly influenced by the total arsenic concentration in the system; In pourbaix diagram, solid-phase components mainly exists in the form of mononuclear, so the increase of total arsenic concentration was beneficial to the increase of solid-phase stable regions.In the predominance diagram of As-Pb-H₂ O system, the high-valence arsenic, lead dissolved species and Pb3（As O4）2（aq.） mainly distributed in the high potential areas. While the low-valence arsenic, lead species mainly distributed in reducibility areas. H3 As O4, H₂ As O4-, Pb2+, HAs O2 distributed in acidic areas; HAs O42-, As O43-, HPb O2- distributed in alkaline areas; The stability region of Pb3（As O4）2 was in the area where p H was between 4 and 11, and it was affected along with the change of environment p H. While in pourbaix diagram, solid-phase components mostly existed in alkaline areas, and their forms were mononuclear and multinuclear.In the predominance diagram of As-S-H₂ O system, pentavalent arsenic, hexavalent sulfur dissolved species mainly distributed in the oxidizability areas; Sulfur mainly distributed in low potential areas in the form of negative bivalent dissolved species. As O43-, HS-, S2- mainly distributed in alkaline areas; And H3 As O4, HAs O2, H₂S（aq.）, HSO4- mainly distributed in the acidic regions. While in pourbaix diagram, sulfur species mainly existed in the gas-phase form, and arsenic species existed in the solid-phase form, and arsenic sulfur species were As2S3 and As2S2.In the predominance diagram of Pb-S-H₂ O system, in high potential areas, the species were mainly bivalent lead, hexavalent sulfur dissolved species; In electrical neutral areas, lead sulfur dissolved species were Pb（HS）2、Pb（HS）3-;In low potential areas, there were mainly low-valence sulfur dissolved species. In pourbaix diagram, lead sulfur species were Pb SO4, 2Pb O*Pb SO4, 3Pb O*Pb SO4, Pb S, but their nature had a greater difference.In the predominance diagram of As-Pb-S-H₂ O system, in high potential areas, the species were mainly hexavalent sulfur, pentavalent arsenic, bivalent lead and arsenic lead dissolved species; In low potential areas, there were mainly low-valence sulfur and lead and sulfur dissolved species; In pourbaix diagram, arsenic sulfur species As2S3 and As2S2 distributed in strongly acidic areas; Lead and arsenic lead series solid-phase species mainly distributed in the alkaline regions; Lead sulfur species distributed in the entire p H range.Under the condition of constant temperature and atmospheric pressure, comparing the experimental results with theoretical results of S-H₂ O system with the same concentration, it found that the experimental results and theoretical results are basically the same. Hexavalent sulfur distributed in the high potential areas above the curve; And negative bivalent sulfur distributed in low potential areas below the curve. With the increase of total sulfur concentration in the system, the steady area of hexavalent sulfur reduced, and the steady area of negative bivalent sulfur increased; Through comparing the measured results of various forms of sulfur and total sulfur in the solution, it speculated that it generated elemental sulfur in this process.