C.15 Wetterbericht ANTXVIII/I
12.4 Production of iron fertilizer batches in seawater
The production of iron-batches in seawater was designed jointly by AWI- NIOZ-UEA with the objective of safe and enclosed delivery of both the iron- sulphate powder and hydrochloric acid. None of the fine powder should be allowed in Open air as to avoid it to enter the two nearby positioned clean air analytical laboratory containers for Fe measurements. Production of new tanks of iron in seawater solution should also be possible in heavy seas at foul weather conditions, such that any decision on a new iron release should not be hampered by inability of production of new batches. The iron-sulphate is not toxic but the fine powder upon excessive inhalation would affect the respiratory system. Hydrochloric acid was purchased in the non fuming 50 % diluted 6N HCI concentration, any inadvertent spills would easily be diluted with seawater, the very dilute acid is safe for both personal health and the environment.
Two large plastic tanks with 215 cm diameter and 21 7 cm height were constructed according to AWI directions and placed on the working deck within a strong steel scaffolding structure. The stirring motors and propellors were provided and installed by the UEA. For delivery a closed system was deisgned at NIOZ (dr. Timmermans). One NIOZ container van had been placed on the upper helicopter deck. Inside the van was a large funnel of about one Square metre, connected by about 10 metres length of 13 cm diameter steel-reinforced transparent pvc tubing feeding into one of the tanks at the working deck. Delivery teams of three scientists were working inside the container van, dressed in protective pvc overalls, rubber gloves and rubber boots and wearing integral face masks with air filters as to avoid inhaling any fine dust. One more scientist adjacent to the door of the container was supervising while in contact by talkie-walkie with a fifth scientist serving as tank inspector at the below working deck. The fire hose of the helicopter deck with running seawater supply was used. The complete procedure was verified jointly with the ships medical doctor as to ensure there were no risks of health and safety.
First one 25 litre jerrycan of 6N HCI was placed inside the funnel, opened, the contents delivered and washed down with seawater. This was followed by the contents of 31 bags of iron sulphate powder also washed down with seawater
and one final jerrycan of another 25 litres 6N HCI. The tanks had a capacity of 8000 litres but were topped up to only the 6000 litres level, as to avoid any spillage of tank contents over some openings in the top due to ships
movements. Upon 90 minutes stirring the pH was verified with pH paper and always found to be well below 2. Within the tanks the liquid was green due to the high abundance of Fe(ll) ions, and turbid due to the amount of about 0.5 percent ground rock within the salt powder, an addition commonly used in commercial salt powders to ensure proper flow of fine salt grains. One sarhple bottle of 100 ml would become clear green upon about two days storage, with visible powder deposit On the bottom.
Overall fifieen tanks of the iron in acidified seawater solution had been prepared for three consecutive releases of five tanks each, During each release the net weight of released pure Fe was 780 kg. One release woultl take about fifteen hours, for delivering five tanks of 6000 litres each at a flow rate of about 2000 litres per hour. During the initial release this flow was mixed with a flow of about 200 litres per hour from the two SFc enriched seawater tanks.
The tank contents were in accordance with the SOIREE recipe which had been verified by the below rationale. In each 6000 liters tank 31 bags of 25 kg each dry FeS04.7H20 powder was added. Given the approximate atomic weights (Fe = 56, S = 32, 0 = 16, H 2 0 = 18) one derives that 1 mole dry FeS04.7H20 equals 278 grammes. Therefore an amount of (31 X 25 X
1000)/278 = 2788 moles per tank was added. For the 600 litres volume per tank this leads to a final concentration of 2788 moles 16000 litres = 0.4644 moles1L. Upon dilution with a factor 100 million in ambient seawater this would yield 0.4644 X 10" = 4.644 nanomoles1L in central patch. This is about 100- fold higher than the 'typical' ambient dissolved Fe concentration of 0.05 nM.
When using 5 tanks for one release to be distributed uniformly over an assumed 30 metres deep wind mixed layer in an area of 10 km by 10 km one would closely approach the assumed 100 million-fold dilution. For an assumed 60 metres deep layer the corresponding area would be 7 km by 7 km Square.
Acidification to pH at about 2 is required to maintain the Fe in its dissolved Fe (11) state. At higher pH levels the Fe will become oxidized to Fe(lll) which is highly insoluble, i.e. leading to colloidal and larger particulate Fe-oxides in the tanks.
Titration to endpoint at pH 3 of the typical Alkalinity of about 2300 umoles L-1 of seawater would require 6000 X 2300 pmoles L-1 = 13.8 moles acid.
Moreover at the very low pH 2 some of the 2788 moles sulphuric anion from the added FeS04.7H20 powder will associate with a proton
For this reaction the conditional stability constant
in ambient Antarctic seawater (S=35, T=2 ¡C is about Ks*=0.25 such that at final pH=2 the ratio [SO:'] / [HSO4"] is about 25. Because of the very larg&
amounts of both the dissolved FeS04.7H20 salt and the added HCI the ionic strength l of the tank solution will be much higher at about 1=4.5 as compared to already quite high 1=0.73 in seawater at S=35 salinity. This leads to major changes in the activity coefficients hence the conditional stability constants as used above would still need to be readjusted for the calculations of both Alkalinity titration and sulphate. Exact calculations will be done afterwards following our procedures for Mediterranean brines (Schijf et al., 1995). For the time being we reckon that the sulphate requires about 107 moles acid such that we end up with (2788-1 07)=2681 moles SO4'' and about 107 moles HS04' per tank with the prescribed ratio of about 2681/107=25.
Finally one needs acid to bring pH to the required pH 2 value:
pH = -log [H"] thus [H"] = 10-2 moles per liter
Therefore 6000 liters X 10'2 moles per liter = 60 moles acid is required.
Summation of all acid requirements yields 13.8
+
107+
60 = 123.8 = 180.8 moles acid per tank. Two 25 L jerrycans of 6N HCI = 2x25 X 6 = 300 moles would be in excess, by choice as to compensate for uncertainty due to incorrect activity coeffcients in above calculations. The free Protons or excess acid of (300-1 3.8-1 07)= 180 moles acid in 6000 litres would yield a proton concentration of 0.03 moles L' corresponding to pH 1.5 which is in good agreement with the actual readings of the pH lest Papers.Upon release the above mentioned 100 million fold dilution would add 10'10 moles acid to each liter ambient seawater. This is 10'4 pmoles per liter but will be mostly buffered by the natural CO2 System buffer capacity. Even for a
tenfold higher acid addition of 10'3 pmoles per Iiter (corresponding to extreme limit of pH at 1 in each tank) the impact on the CO, system in ambient seawater will be negligible. Using CO, in seawater calculation models we reckon the pH change due to additions of 1 0"1 pmoles or even 10'3 pmoles per liter is much less than 0.0001 pH unit and therefore not detectable with the underway pH Sensor with reproducibility of 0.001 pH unit. More importantly one may conclude that the addition in itself of the very dilute acid does not give rise to artefacts in either the dissolved CO, system or the viability of the plankton organisms.
The complete shipboard scientific party is most grateful to the officers and Crew for their assistance in setting up and operating the delivery of iron.
Special thanks are to the chief engineer for his advice and support.
Schijf, J. H.J.W. de Baar and F.J.Millero. (1995) Vertical distributions and speciation of dissolved rare earth elements in the anoxic brines of Bannock Basin, eastern Mediterranean. Geochim et Cosmochim. Acta, 59: 3285-3299.
12.5 Depletion of essential metals Zn, Mn, Co and Cd