5. Discussion
5.2 Experiment B Discussion
5.2.1 Mass Balance-Speciation Modelling Discussion- Experiment B
111 also suggests that, during reactor operation, it is possible to have movement of ions between the phases.
112 kinetic and mass transfer effects and therefore the amount of metal ions that have been predicted to dissolve into the soluble phase (to replace each cycle of washed out soluble ions) from their precipitates may be smaller than predicted by the model. This in turn will cause the model to over- predict the rates at which the metals are being washed out of the system.
Since there are other phases (adsorbed and organically bound metals) that sequester metal ions in the reactor but are not accounted for in the model, the model may not predict the metal washout as it would occur in reality. When the system is at equilibrium, the metal soluble concentration dictates the amount of metals that would be sequestered in precipitates (through Ksp values), by the adsorbed phase (through adsorption isotherms) and by the organically bound phase (through some equilibrium/formation function). Once micro-metals are no longer added to the reactor and the soluble metal ions that have washed out of the system would need to be replaced, there is more than one source that could replenish the lost metal ions. These include the metals sequestered in the precipitates (which, in the model, are the sole source that replenishes the washed out metal ions), metals that are adsorbed and metals that are organically bound.
The rate at which these different phases move to the soluble phase is dependent on a number of factors. The organically bound metals are not considered mobile since they are believed to be associated with stable, high molecular weight organic substances that do not release metals in large quantities or at rapid rates (Filgueras et al., 2002). Therefore, the rate of washout of metals in this phase will most likely occur after metals sequestered by other phases have been released. This alone would result in metals being retained in the reactor for a longer time than what the model, which only considers precipitates as a sequestering phase, would predict. Based on the results from Experiment A (Figure 11) as well as indications from literature (Stover et al., 1976, van Hullebusch et al., 2005, Aquino and Stuckey, 2007, Filgueras et al., 2002), a small fraction of metals is expected in the adsorbed phase. Nevertheless, metal ions in this phase are in equilibrium with the soluble metal concentration which is controlled by the Ksp value. Therefore, if each cycle goes to ionic equilibrium, the amount of metals in this phase would remain the same, since the precipitates exert the same equilibrium soluble ion concentration. Consequently, the metals in this phase will only deplete once the precipitates have completely dissolved.
5.2.1.1 Precipitate formation
For the results of Experiment A, (sections 5.1.2.4 and 5.2.2), the extremely low Ksp values that metals have with their sulphide precipitates were discussed. Figure 18, which shows the predicted precipitates and their changes with the washout experiment for Experiment B, shows that most of
113 the precipitates are sulphides. The metals that are predicted to occur in sulphide precipitates include Cu, Co, Mo, Ni, Zn and Fe with only Mn and Ca predicted to preferentially precipitate with the phosphate ion.
Figure 18 also shows that the model predicts that from the very first cycle of the washout experiment, all the precipitate concentrations start decreasing. This is because from the first cycle that micro-metals are not dosed to the system (cycle 0), dissolution of the precipitates occur so that sufficient metal ions move to the soluble phase to replenish those metal ions lost through decanting, as discussed earlier. Once metal dosing was stopped, the reservoir of metals in the form of precipitates move from a potentially bioavailable phase to a soluble bioavailable phase, highlighting that changing reactor conditions can result in the movement of metals between the phases.
The rate at which the amount of precipitates in Figure 18 is lost is initially high, but as the cycles are modelled, the rate decreases. The different precipitates also wash out at different rates with pyrite (FeS2) being the highest followed by hydroxyapatite (Ca5(PO4)3OH). CoS followed by MoS2
have the lowest precipitate washout rates. The amount of precipitate that dissolves to replace the metal ions that are no longer being dosed to the system is a function of the Ksp value.
The concept of different metals having different rates of washout is shown in Figure 19 which shows the change in the percentage of ion found within precipitates. Since a soluble metal ion will only decrease once its associated precipitate has completely dissolved, the model predicts that soluble Ca ions will start washing out from cycle 0, soluble Mn ions will start washing out from cycle 3 and so on. However, due to the presence of other significant metal phases (such as the organically bound metals), the soluble metal ions are likely to start washing out later than what the model predicts.
In Figure 19, prior to the metal washout experiment, almost all the metals present in the system are predicted to occur 100% within precipitates. HS-1, on the other hand, is predicted to occur only 10%
in the precipitate phase even though sulphide precipitates dominate in the system as shown in figure 18. This indicates the high probability of over-dosing sources of HS-1. When revisiting the micronutrient recipe used to dose the FTRW fed reactors, a number of metal ions are dosed as sulphates, including Na, Fe, Mn, Zn and Cu (section 2.8.2, table 7). Changing the compound the metals are dosed as may help reduce the amount of excess sulphide in the system however, due to the low metal sulphide Ksp values this may not help with increasing the bioavailability of the metals. As recommended for Experiment A, the effect of chelating agents should be investigated.
114 5.2.1.2 Soluble Concentration Changes
Since Figure 19 suggested that most of the metals occur as precipitates, the expected soluble concentration will be very small; equal to the soluble concentration that corresponds to the Ksp
value. With the exception of Ca and Mg, this is shown in the scale of concentrations shown in Figures 21 to 24. In alignment with this are the experimental results where only Ca, Mg and Fe were determined in the soluble fraction. However, the model is able to show the pattern with which each of the metals is leaving the system once all their respective precipitates have dissolved.
A comparison of the experimentally determined and the predicted soluble concentrations for Mg, Ca and Fe, in Figure 20 shows that the Mg comparisons are the closest with the experimental values only slightly higher than the predicted values after the washout experiment. This may be attributed to the model predicting that all the Mg is contained within the soluble phase as well as to the generalized characteristic of Mg having a significant portion of ions within the soluble phase (Experiment A, Figure 11). For Ca (Figure 20) and Fe (Figures 22 and 23), prior to the wash out experiment, the experimentally determined values are higher than the predicted equilibrium soluble concentration suggesting that if the experimental data is correct, either equilibrium had not been reached prior to the metal washout experiment or the model is inadequate. After cycle 0, the experimental data show that Ca and Fe were washed out almost in proportion to the initial (experimental) soluble concentration, observing similar declining trends as those observed in the model. This suggests that although the model is still in its early stages of development, it is a valuable tool that could provide information that cannot, with the current experimental techniques and equipment, be found experimentally.