.. _example-predominance-phreeplot-01: As-O\ :sub:`2`\ -H\ :sub:`2`\ O Predominance ============================================ Arsenic speciation as a function of pH and oxygen fugacity: the diagram that says which arsenic species dominates a water, and therefore how the arsenic in it will behave. It is examples 8 and 9 of the PhreePlot manual, rebuilt here. Arsenic is worth this treatment because its two oxidation states behave differently in almost every respect that matters -- As(III) is more mobile and more toxic than As(V), and sorbs less readily to iron oxides. A diagram that shows where the boundary between them lies is a diagram about whether arsenic in an aquifer stays put. The example also shows something diagrams of this kind usually leave out: what is *second* largest. How the axes are imposed ------------------------ Both axes are held at a chosen value by equilibrating with a phase, which is PhreePlot's idiom and worth recognising because it is how most of these diagrams are built. .. raw:: html :file: study_01 - As-O2-H2O System.html ``Fix_H+`` is a dummy phase -- the reaction ``H+ = H+`` with log K 0 -- whose only purpose is to be something pH can be fixed against. Equilibrating with it at a saturation index of ``-pH`` pins the hydrogen ion activity, with NaOH as the reagent free to enter or leave. The negation in ``@{$-1*ph_param$}@`` is this definition, not an adjustment: a saturation index of -7 is pH 7. ``O2(g)`` at a given log fugacity fixes the redox state the same way. The solution itself is 1 mmol/l As in a 0.1 mol/l NaCl background at 20 °C, with chloride balancing the charge. The grid runs pH 2 to 12 and log fO2 from -90 to 0 -- which is far wider than water is stable over, and deliberately so: the diagram marks where it has left the field, rather than pretending the edges do not exist. The two diagrams ---------------- .. figure:: Dominantspecies_op_ex8_cp_.svg :alt: Predominance diagram of arsenic species against pH and oxygen fugacity :align: center The dominant arsenic species over the plane. The upper band is As(V) -- H3AsO4, H2AsO4-, HAsO4-2 and AsO4-3 in sequence as pH rises, the ordinary deprotonation of an acid. The lower band is As(III), H3AsO3 and H2AsO3-. The near-horizontal line between them is the redox boundary, and it is the one that matters: it sits at a fugacity most natural waters straddle, which is why arsenic speciation in groundwater is so often finely balanced. The two shaded strips are not species. ``O2(g) > 0.21 atm`` is above atmospheric oxygen and ``H2(g) > 1 atm`` below the stability of water: conditions outside them do not occur, and the regions there are drawn only to show the boundary continuing. .. figure:: Sub_minus_dominantspecies_op_ex9_cp_.svg :alt: The same plane showing the second most abundant arsenic species :align: center The same plane, now coloured by the **second** most abundant species. This is the same trace with its target level set to sub-predominant, over the same results -- no second simulation was needed. Two species appear here that are nowhere in the first diagram: H4AsO3+ and HAsO3-2. Neither ever dominates anywhere, so a predominance diagram alone would say they do not exist. It is also a map of how sharp the first diagram is. Where the sub-dominant species is the neighbour across a boundary, the two are comparable and the boundary is a gradual change; where it is something else entirely, the dominant species holds the field decisively. A predominance diagram draws lines, and the lines are not cliffs -- this is the plot that says how steep each one is. Try it ------ * Narrow the fugacity range to the water stability field and see how much of the diagram is actually reachable. * Change the background electrolyte from NaCl to something arsenic complexes with and watch new regions appear. * Switch the sub-dominant trace back to dominant to confirm the two plots read the same results. Source ------ * Kinniburgh, D. G. and Cooper, D. M. (2011). *PhreePlot: Creating graphical output with PHREEQC.* Examples 8 and 9 of the PhreePlot manual are the original of this diagram, including the ``Fix_H+`` construction and the water stability limits. See `the PhreePlot website `_. * Parkhurst, D. L. and Appelo, C. A. J. (2013). *Description of input and examples for PHREEQC version 3.* U.S. Geological Survey Techniques and Methods, book 6, chapter A43. * The thermodynamic data is ``wateq4f.dat``, distributed with PHREEQC.