The long-term sequestration of CO

2

in solid form: the application of nesquehonite

VINCENZO FERRINI, CATERINA DE VITO, SILVANO MIGNARDI Dipartimento di Scienze della Terra, Università di Roma “La Sapienza”, P.le A. Moro, 5 – 00185 Roma, Italy [email protected], [email protected], [email protected]

Our option…

We describe here encouraging results of our experiments on the synthesis of nesquehonite [MgCO

3

·3H

2

O]* by flushing CO

2

in a MgCl

2

solution with a view to investigate a possible role for nesquehonite in a “CO

2

sequestering process”.

Our option could result a cost-effective niche CO 2 sequestration process mineral

* Nesquehonite is a rare low-temperature carbonate encountered in alkaline soils and in cave deposits. It generally forms euhedral prismatic crystals, but also is found in fibroradial and botryoidal arrays. Ideal formula contains 29.13% MgO, 39.06% H 2 O, 31.81% CO 2

2

Sources of magnesium available

: Potential magnesium sources involve •

seawater

•

artificial saltpans

•

evaporitic saline deposits

They locally can represent point sources for small-scale industrial applications of the proposed method of carbonation.

 A massive supply of magnesium could be provided by saline aqueous wastes as a by-product of oil and gas production, the so-called barrels worldwide ) ,

produced water

as well as ( PW, 70 billion

reject brines

from the desalination process.

The experiments in the first step:

 using both doubly distilled and tap water, compressed CO 2 SAPIO (Italy) from  analytical grade reagents (MgCl 2 ·6H 2 O and NH 3 , Merck p.a.)  the suitable range of pH for the optimum formation of nesquehonite in our experimental conditions (7.8

–8.2) was adjusted by adding about 2% of ammonia solution  in thirty-two experiments, we synthesized nesquehonite by sparging CO 2 at a rate of ~100 mL/min through 200 mL of a MgCl 2 ·6H 2 O solution (~ 7 g/L of Mg) at 20 ± 2 ºC  the suspension was filtered using 0.20

µm Nucleopore polycarbonate membrane filters, and washed with doubly distilled water and dried in air

Results

The kinetics of the formation of the solid products were followed by sampling the solution at appropriate time intervals and measuring the concentration of Mg 100 80 60 40 20 0 0 20 40 60 80 100 Time (min) 120 140 160 180 200 Fig. 2 Fig. 3b Fig. 3a The reaction rate is rapid, with carbonate deposition almost complete in about 10 minutes (Fig. 2).

Nesquehonite exhibits well-formed needles up to 0.5 mm in length and 30  m Ø (Fig. 3a,b).

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…..

results

Fig. 4 shows a typical XRD pattern (sample AC12); the precipitate has a very high degree of crystallinity. All patterns are in agreement with those reported in JCPDS card 20 –669 for nesquehonite.

6000 5000 4000 3000 2000 1000 0 -1000 0 10 20 30 40

2 theta (°)

50

Sample AC 12 card 20.669

60 70 The TG-DTG curves (Fig.

5) document the thermal decomposition of nesquehonite during gradual heating, proceeding

via

dehydration at low temperature (below 350 °C) and, above that threshold, complete loss of CO 2 (427 °C).

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Fig. 5

The efficiency of the CO

2

mineralization process

 On the basis of these experimental data, 81.7

sparged CO 2 was captured to form nesquehonite  0.7% of the  About 5% of the starting concentration of Mg was left in the solution after the carbonate formation 7

Thermal behavior of nesquehonite

 The results of our study,

by in situ using real-time laboratory parallel-beam X ray powder diffraction

, show that nesquehonite appears to be stable up to 373 K suggesting that its storage as “sequestering medium of CO 2 ” remains

stable under the temperature conditions that prevail at the Earth's surface



At temperature above 373 K

the process of thermal decomposition of nesquehonite (

via

intermediate hydrated magnesium carbonate phases) ultimately

produces magnesite in the range 423 – 483 K.

 This sequence involves the formation of carbonate minerals more stable than nesquehonite,

resulting in a CO 2

thermodynamically

storage stable for millions of years.

 Therefore, if the

decomposition

storage facilities, this process

disposal.

of nesquehonite would occur into underground

further on would increase the safety of CO 2

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Possible uses for the nesquehonite and by-products

 The sequestration of CO 2

via

carbonation produces a solid material that can be utilized as aggregate in bricks, blocks, mortars, and other building materials.

 This mineral can be used in the production of eco-cement* concretes because it contributes to strength of the concrete.

 The ammonium chloride solution produced in the process could be treated for recoverying the salt or decomposed by heat (~ 350 °C) to obtain NH 3 and HCl.

* F. Pearce, Green Foundations, New Scientist 175 (2002) 39-40.

J. Harrison, Tececo eco-cement masonry product update. www.tececo.com.

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Why nesquehonite option merits further research ?

 our method as complementary solution could be applied in the countries where the other solutions are not applicable and in those where suitable MgCl 2 sources exist, also as by-product of several industrial processes  the process is rapid, simple and environmentally friendly  nesquehonite is a light, thermodynamically stable solid product allowing for the long-term storage of CO 2  the starting reactants are easy to be found  nesquehonite can be used for industrial and agricultural purposes, and its near surface or underground disposal involves limited environmental risks  by-products of the process are sought for a large number of industrial applications 10

Work in progress



Aqueous saline wastes having different salinities and pollutants

(e.g., Fe, Pb, Cu, Zn, etc.) 

CO 2 fluxes

with different amount of the greenhouse gas 

Tests

of the process at power plants 11

“Nesquehonite solution”



Uses

 hazardous wastes (CO 2 , saline wastewaters, PW) 

Produces

 solid products stable for millions of years 

Recycles

 by-products of other industrial processes 12

…our idea in a “naïve image

”

Main relevant products of the research

Ferrini V., De Vito C., Mignardi S. (2009) Synthesis of nesquehonite by reaction of gaseous CO 2 with Mg chloride solution: Its potential role in the sequestration of carbon dioxide,

J.

Hazard. Mater.

168, 832-837.

Ballirano P., De Vito C., Ferrini V., Mignardi S. (2010) The thermal behavior and structural stability of nesquehonite, MgCO 3 ·3H 2 O, evaluated by in situ laboratory parallel-beam X-ray powder diffraction: New constraints on CO 2

J. Hazard. Mater.

178, 522 –528.

sequestration within minerals.

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