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Materials (Basel)
2020 Nov 02;1321:. doi: 10.3390/ma13214926.
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Transformation Kinetics of Burnt Lime in Freshwater and Sea Water.
Justnes H
,
Escudero-Oñate C
,
Garmo ØA
,
Mengede M
.
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Calcium oxide (CaO), also known as burnt lime, is being considered as a possible treatment to reduce the negative impact of sea urchins on tare forests in northern coastal waters and blue-green algal blooms in the surrounding of fish-farms. In this respect, the reaction kinetics of burnt lime in contact with sea water has been elucidated and compared to its behaviour in fresh water. In the first minutes of contact between burnt lime and water, it "slaked" as CaO reacted with water to yield calcium hydroxide (Ca(OH)2). Subsequently, calcium hydroxide reacted with magnesium, sulphate and carbonate from the sea water to yield magnesium hydroxide (Mg(OH)2), calcium sulphate dihydrate (gypsum, CaSO4·2H2O) and calcium carbonate (CaCO3), respectively. In a closed system of 1% CaO in natural sea water (where the supply of sulphate, magnesium and carbonate is limited), more than 90% reacted within the first 5 h. It is foreseen that in an open system, like a marine fjord, it will react even faster. The pH 8 of sea water close to the CaO particle surface will immediately increase to a theoretical value of about 12.5 but will, in an open system with large excess of sea water, rapidly fall back to pH 10.5 being equilibrium pH of magnesium hydroxide. This is further reduced to <9 due to the common ion effect of dissolved magnesium in sea water and then be diluted to the sea water background pH, about 8. Field test dosing CaO particles to sea water showed that the pH of water between the particles stayed around 8.
Figure 1. An electron microscopy image of the pore walls of an unhydrated light-burnt lime particle [7].
Figure 2. pH for neutral water as a function of temperature (t) in the range 0–50 °C plotted from tabular data [12]. The regression line follows formula neutral pH = 8 × 10−5 × t2 − 0.0208 × t + 7.4692 with regression factor r2 = 0.9999.
Figure 3. Distribution of solute species versus pH for a closed aqueous carbonate system at 25 °C and zero ionic strength plotted based on their equilibrium constants [13].
Figure 4. Dissolved CO2 versus temperature (°C) for carbon dioxide pressure P (CO2) = 1 atm plotted from tabular data [14].
Figure 5. Physical appearance of fine CaO to the left and coarse CaO to the right.
Figure 6. Sieving curves for fine and coarse CaO (burnt lime).
Figure 7. Concentration of magnesium in sea water at T = 5 °C (dashed line) and 15 °C (solid line) as function of time after adding 1% (w/v) fine CaO.
Figure 8. Concentration of calcium in sea water at T = 5 °C (dashed line) and 15 °C (solid line) as function of time after adding 1% (w/v) fine CaO.
Figure 9. Concentration of sulphate in sea water at T = 5 °C (dashed line) and 15 °C (solid line) as function of time after adding 1% (w/v) fine CaO.
Figure 10. Relative concentration of carbonate in sea water at T = 5 °C (dashed line) and 15 °C (solid line) as function of time after adding 1% (w/v) fine CaO.
Figure 11. pH in sea water at T = 5 °C (dashed line) and 15 °C (solid line) as function of time after adding 1% (w/v) fine CaO.
Figure 12. The thermogravimetry curves (TG) in black and its derivative (DTG) in red for fine CaO as received.
Figure 13. The thermogravimetry curves (TG) in black and its derivative (DTG) in red for the solids of 1% CaO dispersed in sea water for 2 days.
Figure 14. Remaining content of CaO as a function of time after 1% fine CaO have been mixed in sea water at 5 °C.
Figure 15. The heat flow (top) and cumulative heat (bottom) under isothermal conditions at 15 °C when 17.5 g CaO is mixed with 1 litre sea water.
Figure 16. The heat flow (top) and cumulative heat (bottom) under isothermal conditions at 15 °C when 17.5 g CaO is mixed with 1 litre distilled water.
Figure 17. A selection of images from the SEM investigation as a function of time for lime particle; (a) 5 min, (b) 15 min, (c) 2 days, (d) 4 days and (e) 7 days, as well as (f) after 3 days showing gypsum crystals.
Figure 18. Photo of the boat spraying lime suspension of coarse CaO over the sea.
Figure 19. pH measured in open sea in the area where a suspension of coarse CaO was sprayed.
Figure 20. (a) Sketch of the «open system» sea water with temperature profile and different ions; (b) Sketch of an agglomerate of burnt lime particles; (c) Phase 1: Immediate reaction between burnt lime and sea water—a surface layer of calcium hydroxide is formed; (d) Phase 2: The surface reaction forming calcium hydroxide continues and magnesium hydroxide is precipitated; (e) Phase 3: Structural compression and formation of crystals; (f) Phase 4: All burnt lime particles are completely reacted and transformed to other solids.
References :
Brooks,
Determining the risk of calcium oxide (CaO) particle exposure to marine organisms.
2020, Pubmed,
Echinobase
Brooks,
Determining the risk of calcium oxide (CaO) particle exposure to marine organisms.
2020,
Pubmed
,
Echinobase