3. Research Methodology
3.3 Mass Balance-Chemical Speciation Modelling
3.3.3 Assumptions
During the development of this model, certain assumptions had to be made. Due to the novel nature of this steady-state mass-balance model, it was decided to begin with a simple model as a basis which may be built upon at a later stage. For this reason, many simplifying assumptions were made,
64 some of which may be questionable in other scenarios. The following list provides these assumptions:
1. At the end of each cycle, the system and its reactions reach a state of equilibrium.
2. Growth of microorganisms did not affect the concentrations of the ions.
3. The increase in mass of the sludge due to bacterial growth was 10 g wet sludge and was equal to the mass of sludge lost during decanting.
4. The electrical potential and reduction/oxidation reactions of the system were not taken into account.
5. Metals may be found as free or complex inorganic soluble ions in solution or in a solid precipitate form. Metal adsorption and the formation of organic complexes were not accounted for.
6. The pH of the system was not an input into the model; it was determined by the VM speciation calculations.
7. Only certain precipitates were allowed to form. Table 15 that follows provides a list of all the potential precipitates according to VM, i.e. minerals that were predicted to be supersaturated at the end of the ASBR cycle, whether they were allowed to precipitate or not and the reason/s for that decision.
8. The precipitates that were formed during the VM modelling are only lost via the sludge. It is assumed that the amount of precipitates lost via the supernatant was 0.3% of the volume of sludge lost.
9. For the setup of Experiment A, the partial pressure of both CH4 and CO2 for the glucose- fed reactors was 0.5 atm. For the ethanol-fed reactors, the partial pressures were 0.65 atm for CH4 and 0.35 atm for CO2. These were based on the following equations:
For Glucose:
For Ethanol:
10. For the setup of Experiment B, the partial pressures of CH4 and CO2 that were used corresponded to the average values determined experimentally. These partial pressures were 0.72 and 0.28 atm for CH4 and CO2 respectively.
65
Table 16: Precipitates likely to form under Experimental Conditions.
Mineral Formation Reason Reference
Sphalerite(ZnS) Yes Formation has been demonstrated previously under lab conditions. Formation also
occurred at low temperatures, in dilute solutions in presence of sulphate reducing bacteria.
Labrenz et al., 2000
Wurtzite(Zn/Fe) S
No High temperature polymorph of Sphalerite. Precipitated in saturated hydrothermal solutions at 250-350°C.
Deer et al., 1992, Beaudoin, 2000 Covellite(CuS) Yes In nutrient solutions contaning copper sulfate and sulfate reducing bacteria, covellite was
formed.
Gramp et al., 2006
Chalcopyrite(Cu FeS2)
No Widely occuring natural copper mineral. Synthetic formation occurs through fusion or heating of other precipitates together.
Deer et al., 1992
Mackinawite(Fe/
Ni)xS
Yes Under simulated ground water conditions, pH ranging from 7 to 9 and Fe:S molar ratios of 1:1,2:1 and 1:2, mackinawite was formed.
Hyun and Hayes, 2009
Pyrite(FeS2) Yes Pyrite formation was found in an anaerobic H2S and FeS aqueous system. Drobner et al., 1990
Malachite(Cu2C O3(OH)2)
Yes Occurs in cupric ion solutions at PCO2 values lower than atmospheric and a pH range of 7 to 8.
Vink, 1986
Azurite(Cu3 (CO3
)2(OH)2)
Yes Occurs in cupric ion solutions at PCO2 values higher than atmospheric and pH range of 6 to 7.
Vink, 1986
Siderite(FeCO3) Yes Can be produced artificially by heating (NH4)2CO3 with FeCl. Deer et al., 1992 Rhodochrosite
(MnCO3)
Yes It is made by adding sodium carbonate solution to a solution of manganous salt. Wadley and Buckley, 1997 Magnesite(MgC
O3)
No Formation was limited by magnesite nucleation formation which required sufficient time and a critical degree of supersaturation.
Giammar et al., 2005
66
Mineral Formation Reason Reference
Aragonite (CaCO3)
Yes Likely to form below 100°C. Synthesized by natural reaction of calcium salts with alkali carbonates.
Wadley and Buckley, 1997 Calcite(CaCO3) Yes Precipitates were formed in solutions of calcium nitrate and sodium carbonate at 30 and
40°C.
Wray and Daniels, 1957 Dolomite
(CaMg(CO3)2)
Yes Formation occurred in 25 and 35°C aqueous solutions containing organic and was aided by bacteria.
Sanchez-Raman et al., 2009
Hydroxylapatite (Ca5(PO4)3OH)
Yes Precipitation occurred from solutions of calcium salts and ammoniacal phosphate solutions.
Deer et al., 1992
Huntite No Occurs as an alteration of dolomite or magnesite bearing rocks. Deer et al., 1992
Vivianite No Found within iron, copper and tin ores. Gribble, 1988
67 The mass balance part of the model was done using Excel. This accounts for the loss of metals via the effluent and sludge and for the addition of metals through the feed. The first cycle was modelled by taking the initial ion concentrations from the WEST model and adding the ions in the feed as follows:
( ) ( ) ( )
The final concentration of each ion found within the reactor from the equation above then formed the input concentrations for the VM speciation modelling. After the model was executed, the output from VM included the total soluble ion concentrations (in the sludge and supernatant) and the total concentration of ions within precipitates (which is the concentration of ions in the sludge since the precipitates are associated with the solid phase). This output of concentrations from the speciation model represents the concentrations in the reactor at the end of the first cycle.
Mass balances to thereafter account for the loss and gain of ions from decanting and feeding for the next cycle were required. The following equations provide these
( )
( )
Where:
(
) ( ) (
) ( )
(
) ( )
(
) ( )
( )
68 The final concentration in the reactor found for each ion was then used as an input into the speciation model and the output represented the ion concentrations at the end of the cycle. This combination of mass balance equations and speciation modelling was repeated until the ion concentrations reached a quasi-steady state. For the washout experiment in Experiment B, the same mass balance equations applied with the exception that no moles of micronutrients were added in the feed. An illustration of the mass balance for both Experiment A and B is provided in Appendix D.
69