Insights into dual functions of amino acid salts as CO₂ carriers and CaCO₃ regulators for integrated CO₂ absorption and mineralisation

Highlights

• Nearly 100% CO₂ desorption from amino acid solutions was triggered by CaCl₂.

• Amino acids regulated the morphology and polymorph of CaCO₃ products during carbonation reactions.

• Potassium argininate provided the smallest particle size with sharp crystal edges.

Abstract

Integrated CO₂ absorption and mineralisation (IAM) is a promising technology for rapidly CO₂ absorption and sequestration. However, the traditional IAM using alkanolamines as CO₂ solvents and solid wastes as carbonation feedstocks still suffers from the utilization difficulty of low value products and environmental risks of amine degradation. In this study, the green solvent, amino acid salts (AAS), including potassium l-argininate (ArgK), potassium glycinate, potassium sarcosinate, potassium l-alaninate, potassium β-alaninate, were selected for IAM using rejected brine as the carbonation feedstock. Results shows that compared to amines (monoethanolamine, diethanolamine, N-methyldiethanolamine, triethanolamine, piperazine, 2-amino-2-methy-1-propanol), the selected AASs in IAM were not only more effective CO₂ carriers that passed CO₂ to calcium ions to form CaCO₃, but also better CaCO₃ growth regulators to tune morphology properties of products. CO₂ desorption efficiencies in CO₂-loaded AASs (nearly 100 %) were much higher than those of in CO₂-loaded amines especially than in MEA (38 %). ArgK stood out in regulating the morphology of crystal with smallest particle size. Then ArgK was selected for optimization experiments at various conditions (temperature, Ca²⁺/CO₂ ratio, reactant concentration and chelating agent) to further investigate effects of operating conditions on CaCO₃ properties and to provide some basic information for the polymorph and morphology regulation of CaCO₃. Ethylene diamine tetraacetic acid (EDTA) as a chelating agent was found a notable influence on controlling the polymorph of CaCO₃, where the amorphous CaCO₃ and new structures of calcite appeared. However, the effect of EDTA on CO₂ absorption step was not clear and required further investigation.

Introduction

Carbon dioxide (CO₂) is the major greenhouse gas contributing to global warming which is becoming a threat to human survival. Determining how to deal with the anthropogenic CO₂ emissions has become an issue that awaits an urgent solution. Amine-based CO₂ absorption is recognized as a leading technology for large-scale CO₂ emission reduction because of its excellent CO₂ removal efficiency and reaction reversibility. However, due to the huge energy input required for absorbent regeneration, the traditional amine-based scrubbing process is always energy-intensive and economically unfeasible [1]. The subsequent CO₂ transport and geological sequestration results in further energy consumption and additional cost. On the other hand, CO₂ mineralisation technology using industrial by-products, such as fly ash [2], biomass ash [3], and steel slag [4], can effectively store CO₂ and convert it into thermodynamically stable carbonates. In this way, CO₂ capture and permanent sequestration can be realized simultaneously [5]. However, the reaction kinetics of traditional CO₂ mineralisation was too slow and the economic feasibility of wide-spread application was not fully known [6], which made the traditional CO₂ mineralisation unsuitable for direct CO₂ separation from realistic gas streams. Such problems raise a significant demand for an alternative technology, in which CO₂ capture and storage/utilization can be achieved in a single process with fast absorption rate and without huge energy penalty.

Considering the high CO₂ removal efficiency of amine absorption method and low energy consumption of mineralisation method, integrated CO₂ absorption and mineralisation process (IAM) was developed to fulfill advantages and eliminate disadvantages of the abovementioned two technologies [7]. In IAM, CO₂ was firstly absorbed by an alkanolamine to form a CO₂-loaded solution. Alkaline sources such as calcium oxide (CaO) and fly ash were then added into the CO₂-rich solution to launch carbonation reactions for achieving amine regeneration and calcium carbonate (CaCO₃) formation by hydroxyl ions (OH⁻) and calcium ion (Ca²⁺) released by CaO or fly ash, respectively. Previously, CO₂-loaded solutions of primary and secondary amines have been experimentally confirmed to transfer CO₂ to CaO and enable the complete conversion of CaO into CaCO₃ at 40 °C, in which the traditional thermal triggered amine regeneration was replaced by CaO or fly ash triggered chemical regeneration [2]. However, if CaO was used as alkaline source, the cost was relatively high, which would restrict the industrial applications of IAM. If fly ash was used as the mineralisation feedstock, the further utilization of carbonated fly ash might hinder the industrial applications of IAM because of the low value of carbonated products. Additionally, the input of metal ions into the solution might make the amine degradation worse, leading to amine loss and environmental concerns. Therefore, the development of IAM requires alternative carbonation feedstock to produce high value-added products and green solvent to minimize solvent degradation.

The reject brine, a by-product from saline water desalination, may be an ideal metal source for IAM because of its high concentration of alkaline earth metals, especially Ca²⁺ [8]. Both two reactants, brine and CO₂-loaded amine solution were in liquid phase, which could provide a stable reaction environment for CaCO₃ crystal formation and growth. Compared to solid wastes, using brine for IAM could obtain high value-added products such as CaCO₃, which may reduce the cost of the process [[9], [10], [11]]. The key reactions involved in brine-based IAM include the CO₂ absorption by an amine solution, CO₂ desorption from the amine solution and formation of water insoluble CaCO₃, and the amine recovery as shown in Fig.1. CaCl₂ solution was widely used as the simulated brine and added into CO₂ loaded amine solutions for CO₂ mineralisation. Hong et al. reported a >80 % CaCO₃ purity obtained from CO₂-loaded monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) by adding CaCl₂ as a Ca²⁺ source [9]. However, reaction pathways involved were not clear, and the CO₂ desorption efficiency of amine solutions and the method of amine recovery was not discussed in that study. Ji et al. found that by adding CaCl₂ to CO₂-loaded MEA solution, CO₂ was desorbed from MEA solution and converted into CaCO₃ after reacting with Ca²⁺ ion [7]. They also found that MEA carbamate (MEACOO⁻) was converted into protonated MEA (MEAH⁺) after carbonation reactions. Previous studies indicated several potential methods for amine recovery from protonated amine [11], which required the input of additional alkaline source to grab proton (H⁺) from protonated amine. Amine also participated in CaCO₃ crystal formation and crystal grown as a regulator. Generally, CaCO₃ was formed from the reaction between Ca²⁺ and CO₃^2- through two main steps, i.e., nucleation and crystal growth. The crystal growth was normally initiated with amorphous calcium carbonate (ACC) formation. Thermodynamically unstable ACC was then rapidly converted to metastable vaterite or aragonite and then transformed further to the most stable calcite. Previous studies had confirmed that when amine groups were adjacent to the surface of CaCO₃ particles, which can change the surface energy, crystal growth, and thus tune the morphology of the newly formed CaCO₃ [9]. For example, CaCO₃ obtained from CO₂-loaded MEA solution exhibited vaterite as its major polymorph, while calcite was the dominating phase in the cases of DEA, MDEA, and AMP solutions at 20 °C [9]. Controlled synthesis of CaCO₃ with specific morphology and mineralogy has attracted much attention due to its potential applications in various industrial fields such as additives for engineering plastics, paints additives or paper coatings [12,13].

Although the abovementioned studies have confirmed the technical feasibility of brine-based IAM, most amines have several drawbacks such as high volatility, degradation, and corrosion of the equipment as well as the environmental and health risks. The inorganic absorbents, such as potassium hydroxide (K₂CO₃) and sodium hydroxide (Na₂CO₃) are potential alternatives to amines for IAM [14]. For example, high CO₂ removal efficiency (about 80 %) was achieved by using Na₂CO₃ for CO₂ capture while high quality CaCO₃ and NaCl products were obtained after adding CaCl₂ into the CO₂-loaded solution [14]. However, the inorganic absorbents normally have slow CO₂ absorption rate compared to amines. Additionally, even if amine could adsorb to the surface of CaCO₃ particles during the mineralisation step and thus affected the morphology of the newly formed CaCO₃ crystal, the regulating effect of amines on CaCO₃ crystal was very limited [[9], [10], [11]]. Amino acid salts (AASs) have advantages including minimal absorbent losses, greenness, nontoxic and fast CO₂ absorption, and thus could be more promising solvents for IAM [15]. However, the utilization of AASs for brine-based IAM has not been reported in open literatures. AASs were widely reported in naturally existing biomineralization [16]. Non-collagenous matrix macromolecules contain aspartic acid, glutamic acid in bone tissue have been deduced that might be involved in the control of nucleation and growth of the mineral phase [17], which implies AASs may have a significant influence on the morphology of the synthetic hydroxyapatite [18]. However, whether AASs were effective for controlled synthesis of CaCO₃ with specific morphology and mineralogy in brine-based IAM requires further research.

In this study, we investigated the technical feasibility of five typical AASs in IAM, potassium l-argininate (ArgK), potassium sarcosinate (SarK), potassium l-alaninate (AlaK), potassium β-alaninate (β-AlaK), and potassium glycinate (GlyK), using simulated brine (CaCl₂) as a metal source and verified the dual functions of AAS as a CO₂ carrier and CaCO₃ crystal regulator for IAM. Mineralisation experiments were carried out by introducing CaCl₂ to CO₂-loaded AAS solutions. CO₂ desorption efficiency of each solution was measured to confirm the capability of AAS as a CO₂ carrier in IAM while six typical amines including MEA, DEA, MDEA, triethanolamine (TEA), piperazine (PZ) and AMP were selected for comparisons. The mineralogy and morphology of CaCO₃ products obtained from different AAS or amine solutions were then analyzed by scanning electron microscopy (SEM), particle size analyzer and X-ray powder diffraction (XRD) to explore the underlying mechanism of CaCO₃ formation tuned by AASs or amines. To further investigated effects of operating parameters on CO₂ desorption efficiency and CaCO₃ crystal formation, CO₂-loaded ArgK was selected for mineralisation experiments at various temperatures (room temperature, 40 °C, 60 °C, and 80 °C), absorbent concentrations (0.5 mol/L, 1.0 mol/L, and 2.0 mol/L), Ca²⁺/CO₂ ratio (0.1, 0.2, 0.5, and 1.0) and chelating agents (ethylenediamine tetraacetic acid (EDTA) and magnesium chloride (MgCl₂)).