2. Redox processes have a significant influence on the mobility of arsenic within groundwater systems. These proce...
2. Redox processes have a significant influence on the mobility of arsenic within groundwater systems. These processes affect not only As speciation, but also the stability of iron (hydr)oxide minerals, which are principal sorbents of As in soils, sediments, and aquifers. a) Write out a balanced equation for the reduction of H2ASO to HaAsO° coupled with the oxidation of Fe2 to Fe(OH)se(6 marks) b) The pe - pH diagram for As is developed on pp. 425-427 in Appelo and Postma (2007). However, H2ASO. and AsO were not included in this diagram. Using the following information and the provided diagram, re-construct the pe - pH diagram to include these species. Provide the equations that define all boundaries on the diagram and make sure to thoroughly label your diagram. (18 marks) Eq. 1: H,Aso+H,0 H, AsO; +3H +2e pK 21.14 H,ASOH,AsO; +H* Eq. 2: pKa1 2.24 H,ASOHASO +H pKaz 7.00 Eq. 3: Eq. 4: HASO» AsO +H pK 11.50 H,AS0H,AsO, +H* Eq. 5: pKa1 9.23 c) Using the Nenst equation, calculate the [H3AsO] for the given conditions. What does this tell you about pe -pH stability diagrams (consider your diagram from part b)? (6 marks) H,AsO+H.0H,AsO+2H +2e ; E0 = +560 mv Assume 25°C and 1 atm; Eh +200 mV; pH 2; [HsAsO ] 103 M 1 of 3