Carbon dioxide sequestration using NaHSO4 and NaOH: A dissolution and carbonation optimisation study

Our plain-English summary of Carbon dioxide sequestration using NaHSO4 and NaOH: A dissolution and carbonation optimisation study, by A. Sanna, L. Steel, M.M. Maroto-Valer (Journal of Environmental Management, 2017).

The study

• Authors: A. Sanna, L. Steel, M.M. Maroto-Valer
• Journal: Journal of Environmental Management
• Published: 2017;189:84-97
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

The use of NaHSO4 to leach out Mg fromlizardite-rich serpentinite (in form of MgSO4) and the carbonation of CO2 (captured in form of Na2CO3 using NaOH) to form MgCO3 and Na2SO4 was investigated. Unlike ammonium sulphate, sodium sulphate can be separated via precipitation during the recycling step avoiding energy intensive evaporation process required in NH4-based processes. To determine the effectiveness of the NaHSO4/NaOH process when applied to lizardite, the optimisation of the dissolution and carbonation steps were performed using a UK lizardite-rich serpentine. Temperature, solid/liquid ratio, particle size, concentration and molar ratio were evaluated. An optimal dissolution efficiency of 69.6% was achieved over 3 h at 100 °C using 1.4 M sodium bisulphate and 50 g/l serpentine with particle size 75-150 μm. An optimal carbonation efficiency of 95.4% was achieved over 30 min at 90 °C and 1:1 magnesium:sodium carbonate molar ratio using non-synthesised solution. The CO2 sequestration capacity was 223.6 g carbon dioxide/kg serpentine (66.4% in terms of Mg bonded to hydromagnesite), which is comparable with those obtained using ammonium based processes. Therefore, lizardite-rich serpentinites represent a valuable resource for the NaHSO4/NaOH based pH swing mineralisation process.

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In plain English

This was a laboratory chemistry study; it did not measure fibers, asbestos exposure or any health effect. The authors tested locking carbon dioxide into minerals using serpentinite rock from Ballantrae, Scotland. Citing earlier work, they described the rock as 92.3% lizardite and 7.5% chrysotile. Magnesium was leached from the rock with sodium bisulphate, then reacted with sodium carbonate to form solid hydromagnesite. The best leaching, 69.6% of the magnesium, came after 3 hours at 100 °C with 1.4 M sodium bisulphate, 50 g/l of rock and 75 to 150 micrometre particles. Leaching removed 24% with 0.35 M sodium bisulphate and 200 g/l of rock, against 61% with 1.4 M and 50 g/l. Carbonation reached 95.4% with the real leaching solution, but 83.08% (about 12 percentage points lower) at 100 rather than 50 g/l. From the magnesium converted, the authors calculated 223.6 g of carbon dioxide stored per kg of rock; the capture step was not tested. They concluded this was comparable to ammonium-based processes, but said further investigation and full cost and life cycle assessments were needed.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory experimental study (chemical process optimisation of rock dissolution and carbonation for carbon dioxide storage); no human or animal subjects
• Who was studied: No people were studied. The material was lizardite-rich serpentinite rock from the Ballantrae area, West of Scotland, supplied by the British Geological Survey. It was tested under 10 dissolution conditions and 5 carbonation conditions.
• Exposure: No human exposure was studied. The rock was treated with sodium bisulphate (0.35 to 1.4 M) at 70, 100 and 140 °C, with solid/liquid ratios of 50 to 200 g/l and particles under 250 or 75 to 150 micrometres. Magnesium sulphate solutions were then reacted with sodium carbonate at 60 or 90 °C. Citing Styles et al. (2014), the authors described the rock as 92.3% lizardite and 7.5% chrysotile.
• Main results:
  • The best dissolution efficiency (magnesium extracted from the rock) was 69.6% (Table 4: 69.63%). This was achieved over 3 hours at 100 °C with 1.4 M sodium bisulphate, 50 g/l rock and 75 to 150 micrometre particles (experiment 12).
  • In the concentration series (experiments 3 to 6), the amount of rock rose as the sodium bisulphate concentration fell. Magnesium extraction was 24% (24.36%) with 0.35 M sodium bisulphate and 200 g/l of rock, compared with 61% (61.47%) with 1.4 M and 50 g/l. The intermediate tests gave 42.02% (0.7 M, 100 g/l) and 34.88% (0.47 M, 150 g/l).
  • Raising the temperature to 140 °C did not increase dissolution: 60.09%, against 61.47% at 100 °C and 56.43% at 70 °C, all with particles under 250 micrometres. The authors suggested a magnesium precipitate may have formed at 140 °C.
  • Using the real (non-synthesised) leaching solution at 90 °C and a 1:1 magnesium to sodium carbonate molar ratio, carbonation efficiency was 95.43% after 30 minutes at 50 g/l. At 100 g/l it was 83.08%, a drop of about 12 percentage points (the article calls it approximately 12%).
  • With synthesised (pure) magnesium sulphate solutions, carbonation efficiency was 98.89% at 90 °C and 98.07% at 60 °C. Using extra sodium carbonate (1:1.5 molar ratio) gave 99.35%.
  • Using the real solutions, the authors calculated a carbon dioxide storage of 66.4%, based on magnesium conversion. Converting this through the magnesium content of the rock and the composition of hydromagnesite gave 223.6 g of carbon dioxide per kg of serpentine.
  • The capture step, in which sodium hydroxide reacts with carbon dioxide to form sodium carbonate, was not tested in this study. Sodium carbonate was used directly as the carbon dioxide source.
• The authors' conclusion: The authors concluded that the sodium-salt process stored an amount of carbon dioxide comparable to ammonium-based and other pH swing processes. They added that whether it can be used effectively needs further investigation and complete techno-economic and life cycle assessments.
• Limitations the authors state:
  • The behaviour of trace elements (manganese, copper, zinc) was not assessed. The authors said this is needed to judge water contamination risks and wastewater treatment costs.
  • Some impurities remained in the real leaching solutions after the pH swing step. The authors linked this to lower carbonation efficiency than with pure synthesised solutions.
  • Only a limited volume of real leaching solution was available, so only 100 ml was used in those carbonation experiments.

Topics

Medical terms in this study, explained in our Lexicon: Serpentinite, Lizardite, Serpentine Group.

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