There are vast amounts of groundwater extracted in conjunction with mining activities every year. Such groundwater, normally brackish and rich in iodine, can be managed with engineered pond systems coupled with downstream reverse osmosis (RO) water treatment facilities.
A microalgae biofilm that grows in the pond water was found to contain a significant amount of iodine, 350 ± 29 mg kg-1 iodine on a dry basis. This thesis investigated the role that microalgae could play on iodine cycling in the ponds, and considered potential stabilization strategies for the microalgae if they were to be harvested from the ponds, with the view of converting waste microalgae into an iodine-rich fertilizer.
Cultivation of the microalgae biofilm in extracted groundwater was carried out at bench scale. A new method was developed to quantify the fixed film growth. In parallel, iodine release from the iodine-rich microalgae biofilm kept in the dark was monitored. Iodine uptake and release were found to be proportional to algal growth and decay, with the maximum net growth rate and specific decay rate being 0.53 ± 0.05 g m-2 d-1 and 0.12 ± 0.02 d-1 respectively.
Iodine uptake and release in an engineered groundwater holding pond (1 m3: 1 m2 × 1 m) was simulated. The results indicated that bioaccumulation could cause non-steady state conditions in such ponds, leading to accumulation of iodine in the pond. In extreme circumstances decay events could elevate iodine concentrations to problematic levels.
The harvested microalgae were stabilized by composting and digestion, respectively. A high degree of organic matter degradation was achieved during both composting and (aerobic and anaerobic) digestion. The study found algal composts were very stable, but composting hardly proceeded if the harvested microalgae were not pre-treated by washing to reduce salinity.
In aerobic and anaerobic digestions, where microalgae stabilization proceeded after minimal rinsing, the mobilization of iodine was studied. The mobilization of iodine was found to be linearly correlated to carbon emission under both aerobic and anaerobic conditions which indicated iodine was probably in the form of organoiodine in the microalgae biofilm. By the end of the stabilizations, there was 0.22 ± 0.05 and 0.19 ± 0.01 mg g-1 VSadded iodine remaining in the solid phase in the aerobic and anaerobic Page III processed material respectively, meaning 38 ± 5.0 % (aerobic) and 50 ± 8.6 % (anaerobic) of the total iodine were mobilized, and consequently lost from the material. The results showed that aerobic digestion residues were of higher iodine content and lower heavy metals, while anaerobic digestion residues were of lower iodine content and higher heavy metals. But the anaerobic process was still attractive because of the considerable amount of bioenergy that could be recovered.
The project represents the start-up of a new research activity in the School of Chemical Engineering at UQ, and as such involved substantial method development. The project was sponsored by APLNG. The outcomes are directly relevant for management of algae in APLNG’s engineered holding ponds. But significantly, the outcomes have broader relevance for natural and engineered aquatic systems in which algal blooms occur. This thesis shows that biomass growth and retention in such systems can cause significant accumulation of elements that algae take up and store (e.g. phosphorus, metals and silicon as well as iodine), and that decay events can then result in a spike in the concentration of such elements. Harvesting biomass mitigates this, and opens the door for the development of a new resource, namely a biofertilizer.