4. Results
4.1 Experiment A Results
4.1.4 Mass Balance-Speciation Modelling Results- Experiment A
A combination of Visual Minteq (VM) and Microsoft Excel was used for the mass balance- speciation modelling. For each cycle, the effect of decanting and feeding, as well as growth were
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Al (1)
Al (2) RI
Al (3)
Al (1)
Al (2) RII
Al (3)
Zn (1)
Zn (2) RI
Zn (3)
Zn (1)
Zn (2) RII
Zn (3)
Fe (1)
Fe (2) RI
Fe (3)
Fe (1)
Fe (2) RII
Fe (3)
Mg (1)
Mg (2) RI
Mg (3)
Mg (1)
Mg (2) RII
Mg (3)
Metal Mass (mg)
Acid Digestion Sequential Extraction
75 accounted for using mass balances in Excel. VM was used to determine the chemical speciation of the system at the end of each cycle just before decanting. Results from VM were used as an input to the mass balances in Excel and once decanting, feeding and growth were accounted for, resultant values were used as an input to VM. This process was performed until the system reached steady- state with respect to ion concentrations.
Since the initial conditions as well as the assumptions made for reactor I and II were similar (with the exception of gas partial pressures), the results obtained were similar. For the sake of avoiding repetition, only results obtained for reactor I will be discussed, however, results for reactor II are presented in Appendix E. The number of cycles modelled does not represent experimental cycles, but is the number required until the soluble ion concentrations at the end of each modelled cycle are approximately constant.
4.1.4.1 Soluble Concentration Changes
At this stage, only modelling of the precipitation phase has been performed to see whether the precipitation is controlling and if just modelling this phase is sufficient to describe the data. Since only precipitation is considered, for an ion that does not precipitate, the concentration of the supernatant will tend towards the concentration at which the ion is being added before each cycle.
If the initial supernatant concentration is higher than the concentration at which the ion is added, the supernatant will get diluted each cycle when decanting and adding the feed is performed until the ion reaches the concentration it has in the feed. Similarly, for an ion where the initial supernatant concentration is lower than in the feed, the supernatant concentration will increase with each cycle, tending towards the concentration it has in the feed.
Figure 14 shows the model prediction of concentration of ions in the dissolved phase for those ions in the mg/l range and how they change with each successive cycle modelled. The graph also displays the comparison between the model predicted value (over 20 hypothetical cycles from the assumed initial condition) and the soluble concentration determined experimentally for the magnesium and iron ions. The experimental values correspond to the supernatant soluble metal concentration associated with the cycle A initial sludge, cycle A final sludge/cycle B initial sludge and the cycle B final sludge.
76
Figure 14: Concentration of ions in the dissolved phase for Ca, Fe, K and Mg ions, and their changes with each successive cycle modelled for Reactor I, including comparisons to experimental values for Fe and Mg (mg/l).
The model predicted graphs represent a number of points where each point represents one cycle modelled. At a glance, the system reaches a pseudo-steady state whereby the species reach a constant concentration. For all the ions the concentrations are predicted to increase from the initial point until they plateau. This is due to the selected hypothetical initial concentrations being lower when compared to the concentration that the species tend to. As a function of the model assumptions, the species concentrations should tend to the concentration that is added via the feed for each cycle if that species is not being precipitated. This is observed for the potassium, magnesium and calcium ions where the model feed concentrations were 3.41, 1.08 and 0.34 mg/l respectively, matching the final predicted supernatant concentrations. For iron, the feed concentration was 0.52 mg/l but when compared to the final modelled dissolved concentration 0.25 mg/l, it would indicate that the model predicts that this metal is being precipitated.
With regards to the comparison between the predicted and experimentally determined values, the experimentally determined values for both iron and magnesium are higher than the model predictions. The largest discrepancy exists with magnesium where the experimental values are approximately three times higher than the model predicted values. Since the experimentally
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0 5 10 15 20 25
Soluble ion concentration (mg/l)
Cycle
Fe2+ Exp Mg2+ Exp Ca2+ Fe2+ K+ Mg2+
77 determined values are higher, it suggests the possibility that there are other processes or mass transfer effects that may be involved.
Figure 15 displays the predicted dissolved ion concentrations for species in the µg/l range, and their changes with modelling each successive cycle. The graph also displays the comparison between the predicted value and the concentration determined experimentally for copper and zinc.
Figure 15: Concentration of ions in the dissolved phase for Mn, Cu and Zn ions (µg/l), and their changes with each successive cycle modelled for Reactor I, including comparisons to experimental values for Cu and Zn.
While most ions have reached a level dissolved concentration, the manganese ion displays a trend that is increasing continually at an increasing rate. This trend suggests that manganese, which has a feed concentration of 28 µg/l, was initially predicted to precipitate but then started to dissolve into the soluble phase in the latter cycles, causing the predicted soluble concentration to increase. If no further precipitation is predicted as more cycles are modelled, the soluble concentration would increase and tend towards the feed concentration of 28 µg/l. This is further elaborated on in the following section on precipitate formation (section 4.1.4.2).
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0
0 5 10 15 20 25
Concentration (μg/l)
Cycle
Mn2+ Cu2+ Exp Zn2+ Exp Cu2+ Zn2+
78 The feed concentration for both copper and zinc was 5 µg/l. Since the final model predicted values were significantly lower for both copper (0 µg/l) and zinc (0.73 µg/l) the model suggests that all of the copper ions and most of the zinc ions will be sequestered in precipitates. This is somewhat reflected in the experimental analysis where both zinc and copper were not found in the soluble phase (figures 11 and 12 respectively). Although the model predicts that some zinc will occur in the soluble phase, the discrepancy between the model predicted and experimentally determined values could be attributed to the difficulty and limitations associated with determining metal concentrations in the micro ranges experimentally.
The graph that follows displays the dissolved ion concentrations for ionic species Co2+ and HS-1, in the μg/l range, as well as their changes with modelling each successive cycle.
Figure 16: Concentration of ions in the dissolved phase for Co2+ and HS-1 (μg/l), and their changes with each successive cycle modelled for Reactor I.
The two ions above are predicted to occur in minute concentrations. The model predicts that these ions reach concentrations of about 0.08 and 0.15 μg/l for the cobalt and hydrogen sulphide ions respectively. Although sulphur is regarded as a macronutrient, since it is required by microorganisms and dosed into the system in macro concentrations, it is found in the soluble micro concentration range (µg/l range). Since the metals would precipitate with S2- ion and the amount of S2- ion is dictated by the HS- ion (depending on the pH), this suggests that bulk of the S2- lies in one of the other metal phases. For cobalt, the feed concentration was 37 µg/l, and it was not detected experimentally, possibly because the concentration in the soluble phase was below the detection
0.00 0.05 0.10 0.15 0.20 0.25 0.30
0 5 10 15 20 25
Concentration (μg/l)
Cycle
Co2+ HS-1
79 limit. This suggests that, like the bi-sulphide ion (S2-), a significant portion of this ion is predicted to occur in phases other than the soluble phase.
4.1.4.2 Precipitate Formation
VM and the mass balance-speciation model predicts the formation of a number of possible precipitates. From this list, precipitates most likely to occur under reactor conditions were allowed to precipitate. A key result from this was the percentage of a metal ion that was found within precipitates. The following graph displays the changes in the amount of ion within precipitates with each cycle modelled. To compare this percentage to the experimental data, it would be necessary to separate the experimental data into either the soluble or precipitate phase since the model only predicts the soluble and precipitate phases. Therefore, for this comparison, the ions from the supernatant analysis are regarded as soluble ions and the ions from the total acid digestion minus the ions from the supernatant analysis are regarded as within precipitates.
Figure 17: Percentage of ions that are within precipitates as predicted for each successive cycle modelled together with the values obtained from experimental data for Mg, Ca, Fe, Cu and Zn.
0 20 40 60 80 100 120
0 5 10 15 20
% of ion within precipitates
Cycle
Ca2+ Co2+ Cu2+ Fe2+ HS-1
K+ Mg2+ Mn2+ Zn2+ Mg2+ Exp
Fe2+ Exp Zn2+ Exp Cu2+ Exp
Cu2+, Zn2+, Fe2+ Exp
Mg2+ Exp HS-1, Cu2+, Co2+
Zn2+
Mg2+, K+, Ca2+
Mn2+
Fe2+
80 A significant result from the graph above is that the model predicts that certain metals occur completely within precipitates from the start of the experiment. These metals include Co2+, Cu2+
and also the anion HS- (not measured experimentally). This suggests the possibility that these metals may never occur in a bioavailable phase for micronutrient absorption. Mn2+ is initially predicted to occur mostly within precipitates, however, as conditions change with each cycle modelled, the amount found within precipitates decreases. This was reflected in Figure 15 where an increase in the Mn2+ soluble concentration was observed through the cycles. A similar trend for Zn2+ and Fe2+ are observed where the amount of ion predicted to occur within precipitates are initially higher and then decrease through the cycles as conditions within the reactor change. For Mg2+, K+ and Ca2+, the model predicts that these metals occur within the soluble phase only.
Since the experimental values representing precipitates are taken as the total ions from the acid digestion minus the soluble ions from the supernatant analysis, it is anticipated that the amount of precipitates will be over-represented. When comparing the predicted values to the experimentally determined values, a large discrepancy exists and part of this is due to the over-representation of precipitates for the experimental values. With the exception of Cu2+ which compares well to the model predicted value, the experimental values for the rest of the ions are higher than the model predicted values.