Create a pe ph diagram for the following solid and aqueous iron species:

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Jonah Powell
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1 Assignment 6 Crete pe p digrm for the following solid nd queous iron species: (O) 3 ferrihydrite (O) 3 (O) (O) 3 (O) 4 (O) solid O For solid queous species rections, plot stbility field boundries for queous ctivities of 6 moles/l. For queous-queous rections, plot boundries for ctivity rtios of 1. First, plot the stbility field for wter, ccording to the two hlf rections: O e O e G log K= r = log 0 K= O O e P = P e = 1 e = O PO K 1 e = P log e = log O ¼log P O log 0.78 log e = log P log O pe= 0.78 p ¼log P O pe = log P p For tmospheric prtil pressures of 1 for O nd, : pe = 0.78 p pe = p These two lines cn be plotted on the pe p digrm to delinete the stbility field of wter. Within this field we cn now define the fields for the different iron species. Strting with the ferric iron species: (O) 3 ferrihydrite (O) O (O) = 0.78 log (O) = 0.78 p For (O) = 6, the stbility boundry occurs t p = = 5.. As there is no pe considertion in this rection, it is verticl line. From the nture of the rection, ferrihydrite is stble on the right side of the line t higher p nd the stbility field for (O) is on the left. If we chose to drw the line for higher vlues of (O), sy 5, the line would be one p unit to the left decresing the field for dissolved (i.e. more extreme p conditions re required to mintin higher ctivity of (O) in solution). If we consider nother ferrihydrite dissolution rection:

2 (O) 3 ferrihydrite (O) O (O) =.70 log (O) =.70 p The stbility field boundry for (O) = 6 occurs t p = 6.7 = 3.3, which would extend the field for ferrihydrite too fr to the left, through the (O) field. Rther, we hve to consider the boundry for rection between (O) nd (O). This will be the boundry for n ctivity rtio of one for these two species. (O) (O) O (O) = 3.48 (O) Thus, t p 3.48, the ctivity rtio, (O) (O) is equl to 1. At lower p vlues, the uncomplexed ferric iron ion, 3 becomes importnt, so we should consider the rection: (O) 3 O 3 =.19 (O) Thus, t p.19, the ctivity rtio, 3 (O) is equl to 1. Now looking t the ferric iron rections t higher p: (O) 3 ferrihydrite (O) log (O)3 = 7.67 This rection is controlled strictly by the solubility of ferrihydrite, with no p control. The ctivity of (O) 3 in equilibrium with ferrihydrite is 7.67, which is less thn our constrint of 6 for dissolved iron species. Although (O) 3 will be present in solution, it s concentrtion is too low to occupy field on our pe p digrm. The next rection we could consider is: (O) 3 ferrihydrite O (O) 4 (O)4 p = log (O)4 = p For (O)4 = 6, the ferrihydrite-(o) 4 boundry is t p.71 Now we cn estblish the boundries for the different ferrous iron species: (O) solid O K= O p = 18.83

3 log O = p For O = 6, p = = 1.83 And next, we determine the boundry between (O) solid nd O (O) (s) O O O p = 1.58 log O = 1.58 p For (O) = 6, p = 4.4 And for the finl ferrous iron rection: For O O =.8 O O.8 = log O = 1, p =.8 p Inspection of these lst two rections shows tht the field for solid (O) in equilibrium with O extends to p 4.4, much below the p of.8 where nd O re t equl concentrtions. This indictes tht O cn be present only t very low concentrtions, nd the rection controlling ferrous iron solubility t neutrl to low p is between nd (O) solid : (O) (s) O = 1.38 p log = 1.38 p And for = 6, p = 9.19 Plotting these p - boundries for these different hydrolysis products of ferrous nd ferric iron gives the lines observed in Fig. 1. Note tht the re drwn to terminte t the upper nd lower limits of stbility of wter. Their termintions within the wter field, however, re yet to be defined, ccording to the relevnt redox equtions.

4 O O (O) 3 ferrihydrite 5 (O)4 pe 0-5 (O) (s) p Fig. 1 Boundry lines for mjor ferric nd ferrous iron species, considering hydrolysis rections lone. At this point, redox rections between ferric nd ferrous iron species must be considered. The intercting species cn be determined from the digrm t this point. Working from low to high p, these rections, nd their pe p equtions re clculted s: 3 e log log 3 pe = 13.0 pe = 13.0 (O) e O log log p pe = 15.1 pe = 15.1 p (O) (O) e O log log p pe = (O) pe = p (O) 3 ferridydrite e 3 3 O log log 3p pe = For = 6, pe = p (O) 3 ferrihydrite e (O) (s) O

5 log pe p = 5.53 pe = p (O) 4 e (O) (s) O log pe p log (O)4 =.4 For (O)4 = 6, pe = 16.4 p (O) 4 e O O O log log pe p = 3.41 (O) 4 pe = 3.41 p Plotting these rection boundries on the pe - p digrm defines their stbility fields (Fig. ) O (O) 3 ferrihydrite 5 (O)4 pe (O) (s) p Fig. pe-p digrm showing stbility fields for ferric nd ferrous iron species considering hydrolysis nd redox rections.

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Click to edit Master title style Interntionl Conference Click to edit Mster title style STABILISATION/SOLIDIFICATION TREATMENT AND REMEDIATION Advnces in S/S for Wste Click to nd edit Contminted Mster Lnd text styles 12-13 April 2005