Laboratory and field studies of the aeration of mangrove sediments as a bioremediation method

Brief description

Six blocks of mangrove sediments, measuring 30 cm x 30 cm x 15 cm, complete with crab burrows and root fragments were collected one metre within the edge of a Rhizophora stylosa forest adjacent to Cocoa Creek, Cape Cleveland, North Queensland. Sediment blocks were placed in individual plastic bins to form temporary mesocosms. Seawater was added to the mesocosms at 15 and 23 hours daily and siphoned off after one hour to simulate tidal cycles. Light conditions were kept low and shade cloth covers were used to inhibit growth of an algal mat which might alter oxygen conditions in the sediments.Magnesium peroxide powder, an oxygen release compound, (3.14 g dry weight) was evenly distributed along a 13 cm long and 4 cm deep cut into three of the mesocosms. After 40 hours, vertical oxygen profiles were measured down to the aerobic-anaerobic zone interface in 100 µm increments at three sites along the centre line of the buried compound in each mesocosm.In each of the three remaining mesocosms three pre-disturbance vertical oxygen profiles were measured along a 13 cm long and 4 cm deep cut in the sediment. An airstone was then placed into the 4 cm deep cut, with its flat upper surface approximately 2 cm below the surface and completely buried. Air was administered to the sediments via the airstone at a rate of 1L/min at 400 kPa. Following aeration treatments for 40 hours, the aerobic zone was described in reference to the airstone. Oxygen concentration of sediments was measured for several profiles taken vertically from the surface. The first profile was taken near the centre, immediately adjacent to the airstone, and 3.5 cm from the input end. Each successive profile was taken 5 mm further away from the previous, perpendicular to the airstone, until they resembled pre-disturbance profiles. Air flow continued whilst measurements were taken. To establish whether burial of the airstone might influence the oxygen profiles, an additional three profiles were measured after burial of the airstone and before aeration in one of the mesocosms.Field trials of forced aeration were conducted in a Rhizophora stylosa mangrove forest at Fishermans Landing, near Gladstone. The procedures, described earlier, for positioning the airstone in sediments and the depth of burial and measuring oxygen saturation, were repeated in this trial. The delivery of air was pulsed in field trials since this was considered sufficient to aerate sediments based on observations made during mesocosm trials. The pulse cycle used was 60 minutes airflow and 60 minutes off, repeated for around 40 hours prior to measurements being taken.
As the majority of hydrocarbons introduced to sediment in an oil spill initially reside on the surface, availability of oxygen in this layer is critical for biodegradation by the more active aerobic microbes. The objective of this series of experiments was to determine whether forced aeration and/or magnesium peroxide (an oxygen release compound) might provide significant increases in molecular oxygen saturation in the surface aerobic layer of sediments under common tropical mangrove trees.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Measurement of molecular oxygen concentration:Molecular oxygen concentration was measured using an oxygen microelectrode (MI-730, Microelectrodes Inc.) enclosed in a fixed teflon membrane. Pre-treatment of the electrode was carried out according to the instructions supplied with the electrode. The electrode was connected to a digital multimeter, measuring volts DC. Calibration of the electrode was made prior to each use, with dissolved sodium dithionite (Na2S2O4) to determine the zero oxygen reading and again in seawater aerated for thirty minutes at 1 L/min at 400 kPa to determine the 100% oxygen saturation reading. The oxygen electrode was mounted onto a manually operated micro manipulator. Vertical profiles were measured down to the aerobic-anaerobic zone interface in 100 µm increments to thoroughly describe the oxygen gradient through the surface aerobic layer. The aerobic-anaerobic zone interface was defined arbitrarily based on oxygen saturation levels around 15%.Forced aeration and pilot study:The airstone used (stock aquarium kind; dimensions: 13 cm long, 2 cm wide at base of triangular prism in cross-section) was chosen based on its availability, low cost, small size of the bubbles released, and its practical size and shape for use in muddy sediments amongst dense roots. To appraise the uniformity of outflow from the airstone, it was submerged in seawater and air from a cylinder of compressed air (industrial grade) was released through it at a constant flow rate of 1L/min at 400 kPa. Air appeared to be released from the airstone along the upper 'sharp' edge, so to maximise the potential zone of influence in this experiment it was placed horizontally with the broad surface upper-most.The uniformity of outflow from the airstone in sediments was quantified in several experiments. Prior to this, three predisturbance profiles were measured along a 13 cm cut in the sediment and the mean depth of the surface aerobic zone was identified at 300 µm. The airstone was carefully placed into a 4 cm deep cut in the sediment, with its flat upper surface approximately 2 cm below the surface. The sediments were then closed over the airstone, completely burying it beneath the sediments. Air was run through the airstone at a flow rate of 1 L/min at 400 kPa for forty hours, after which three profiles were measured from the surface toward the centre of the airstone. Profiles were not taken down to the airstone because the microelectrodes would be damaged if they contacted the hard surface. The depths of the aerobic layer of those three profiles were 1100 µm, 1500 µm and 800 µm, indicating an approximately consistent flow along the length of the airstone. Based on this assumption, the 3-dimensional zone of influence of the airstone was determined using oxygen concentrations measured perpendicularly out from a point on the airstone and multiplying these 2-dimensional results by the length of the airstone. Since the mesocosm dried notably during this pilot trial, it was decided to add water in subsequent trials to simulate a tidal cycle and to maintain the moist environment. Each profile required between 1-2 hours to measure, and a maximum of nine profiles could be taken in one day based on using one microelectrode. These restrictions required that replication be provided through repetition of the experiment with one measurement taken at each time point. All procedures involving pre-treatment measurements, burial of airstones, and duration of oxygenation treatment were followed in each subsequent experiment.

Notes

Credit
Burns, Kathryn A, Dr (Principal Investigator)