The experiment described in this chapter differed from the carbonation experiment described in Chapter 2 in that a dissolved source of inorganic carbon was not provided. As a result, dissolving CO2 from the atmosphere into the bioreactor water became an
additional step required for carbonate precipitation to take place. The results of this study demonstrate that atmospheric CO2 can act as a carbon source for carbonate precipitation
under the right water chemistry conditions. Such a demonstration is important because dissolution of atmospheric CO2 into water is often the rate-limiting step in many mineral
carbonation experiments (Power et al. 2011). Based on the PHREEQC results, the water was still undersaturated in CO2 and never reached equilibrium with the atmosphere
throughout the duration of the experiment. It is possible that some of the CO2 used in
carbonate precipitation was generated through biological processes. Heterotrophic oxidation of photosynthetically derived organic compounds such as acetate (CH3COO-)
contribute to DIC by producing dissolved CO2 or HCO3- (Von Knorre et al. 2000; Power
et al. 2011; Sánchez-Román et al. 2011). In microbial mat systems, organics oxidised by heterotrophs are largely derived from atmospheric carbon fixed by phototrophic
organisms, acting as an indirect pathway for transferring atmospheric CO2 into the water.
Heterotrophic activity, therefore, may have been partially responsible for the patterns observed in the alkalinity and DIC measured in the bioreactor. Both of these parameters exhibited increases immediately following the addition of the BG-11 solution at 0 m and 5 m. These locations had the highest concentrations of nutrients, likely resulting in a localised increase in heterotrophic activity. This increase likely caused a downstream increase in heterotrophy-generated DIC, which was observed immediately after the nutrient addition from 1 m to 2 m, and 6 m to 8 m. The pH drop at 5 m was likely also caused by the addition of nutrient at that location. Not only was the pH of the BG-11 solution much lower than that of the water in the bioreactor, but dissolution of
heterotroph generated CO2 at that location would have caused a decrease in pH. Beyond
remained relatively constant for the remaining length of the bioreactor. The difference is important because in the current experiment, the bioreactor was able to remove
magnesium continually from solution, instead of being limited to carbonate formation only in the upper few metres of the system. This is particularly notable because the concentration of magnesium being added was up to five times higher than in the experiment described in Chapter 2. At leach solution inflow rates of 1 - 3 L/day, the system was able to precipitate carbonate minerals at a rate which reduced the
conductivity to the value measured in the system prior to starting the experiment. At inflow rates of 4 and 5 L/day, although magnesium was being removed from the water demonstrating that the carbonation potential of the bioreactor had not yet been exhausted, the magnesium concentration and conductivity values began to increase. It is quite likely that if the length of the bioreactor was greater, perhaps 20 m, the conductivity and magnesium concentration would have decreased further. In this experiment, it appears that the system was able to manage the lower flow rates, as indicated by the way the magnesium concentration and the conductivity were reduced to equal or lower than the respective time zero values.
Based on the reduction of the magnesium concentration, the rate of magnesium carbonate precipitation can be deduced for specific times during the carbonation experiment, as outlined below. Mineral precipitation is closely linked to the state of mineral saturation in the bioreactor water. The conditions in the bioreactor were able to increase the SI for hydromagnesite from the -10.07 value determined for the leach solution, to above zero (<4). These values are not as high as those achieved in Chapter 2, likely because of the lack of a soluble inorganic carbon source in the current experiment. Another cause of this difference is that alkalinity, as opposed to a direct DIC measurement, was used to
represent carbon when determining the SI values reported in Chapter 2. Although a large portion of the alkalinity was represented by DIC, it was likely an overestimate of the
Hydromagnesite precipitation was still successful in the present experiment, due to the higher magnesium concentration used and the higher pH achieved as a result of the higher concentration of BG-11 supplied to the microorganisms. These results indicate that saturation index cannot be used as a rigid guide for determining if a solution will be able to produce magnesium carbonate minerals. For instance, the carbonation experiment described in Chapter 2 yielded many hydromagnesite SI values greater than 4 that did not result in precipitation. This is in contrast to the current experiment, which did not produce any SI values above 4, although hydromagnesite was observed at all sampling locations in the bioreactor. It appears that although the balance between pH, the availability of dissolved inorganic carbon, and magnesium concentration can result in a wide range of possible saturation indices, different combinations of these factors can result in successful magnesium carbonate precipitation.
With the above observations in mind, the following interpretations can be made regarding the relationship between water chemistry, saturation index, and mineral precipitation. The changes in magnesium concentration in the system indicated that the greatest carbonation rate was achieved between day 60 and day 67, when the leach solution was being added to the system at the highest implemented flow rate of 5 L/day. Based on the magnesium concentration and the conductivity measurements documented on day 67, carbonate precipitation was taking place throughout the entire bioreactor. At this time, the hydromagnesite SI values were among the lowest observed at any point during the experiment, ranging from -0.56 at 0 m, to 3.07 at 9 m. These values, particularly those from the first few metres of the system, do not seem appropriate for a high rate of carbonate precipitation. The inconsistency would likely be resolved if more frequent samples had been taken throughout the last week of the experiment. The system average DO concentration on day 60 was an experiment high of 21.0 mg/L and the average pH was 9.59, indicating that the bioreactor was in good ‘health’ at this time. In the following week, the DO and the pH dropped at the start of the bioreactor, corresponding to the region of the system with the lowest SI values. It is likely that with the good state of the
less than half of what it was on day 60, which also indicates fast precipitation of carbonate and removal of DIC from solution. It is likely that if the experiment had continued beyond day 67, the rapid carbonate mineralisation would have been detrimental to the cyanobacteria, even though it resulted in the greatest amount of precipitation. The addition of 5000 ppm magnesium at a rate of 5 L/day was likely approaching the upper limits of the system’s ability to consume magnesium through carbonate precipitation. In order to maintain the biogeochemical conditions required for successful carbonation, reducing either the flow rate or the magnesium concentration would likely be necessary if this process is to be implemented over a longer period of time.