skip to main content


Title: New insights into the mechanisms of carbon dioxide mineralization by portlandite
Abstract

Portlandite (Ca(OH)2; also known as calcium hydroxide or hydrated lime), an archetypal alkaline solid, interacts with carbon dioxide (CO2) via a classic acid–base “carbonation” reaction to produce a salt (calcium carbonate: CaCO3) that functions as a low‐carbon cementation agent, and water. Herein, we revisit the effects of reaction temperature, relative humidity (RH), and CO2concentration on the carbonation of portlandite in the form of finely divided particulates and compacted monoliths. Special focus is paid to uncover the influences of the moisture state (i.e., the presence of adsorbed and/or liquid water), moisture content and the surface area‐to‐volume ratio (sa/v, mm−1) of reactants on the extent of carbonation. In general, increasing RH more significantly impacts the rate and thermodynamics of carbonation reactions, leading to high(er) conversion regardless of prior exposure history. This mitigated the effects (if any) of allegedly denser, less porous carbonate surface layers formed at lower RH. In monolithic compacts, microstructural (i.e., mass‐transfer) constraints particularly hindered the progress of carbonation due to pore blocking by liquid water in compacts with limited surface area to volume ratios. These mechanistic insights into portlandite's carbonation inform processing routes for the production of cementation agents that seek to utilize CO2borne in dilute (≤30 mol%) post‐combustion flue gas streams.

 
more » « less
Award ID(s):
1922167
NSF-PAR ID:
10452740
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley Blackwell (John Wiley & Sons)
Date Published:
Journal Name:
AIChE Journal
Volume:
67
Issue:
5
ISSN:
0001-1541
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    A large part of the hydrated oceanic lithosphere consists of serpentinites exposed in ophiolites. Serpentinites constitute reactive chemical and thermal systems and potentially represent an effective sink for CO2. Understanding carbonation mechanisms within ophiolites are almost exclusively based on studies of outcrops, which can limit the interpretation of fossil hydrothermal systems. We present stable and radiogenic carbon isotope data that provide insights into the isotopic trends and fluid evolution of peridotite carbonation in ICDP Oman Drilling Project drill holes BA1B (400‐m deep) and BA3A (300‐m deep). Geochemical investigations of the carbonates in serpentinites indicate formation in the last 50 kyr, implying a distinctly different phase of alteration than the initial oceanic hydration and serpentinization of the Samail Ophiolite. The oldest carbonates (∼31 to >50 kyr) are localized calcite, dolomite, and aragonite veins, formed between 26°C and 43°C and related to focused fluid flow. Subsequent pervasive small amounts of dispersed carbonate precipitated in the last 1,000 years. Macroscopic brecciation and veining of the peridotite indicate that carbonation is influenced by tectonic features allowing infiltration of fluids over extended periods and at different structural levels such as along fracture planes and micro‐fractures and grain boundaries, causing large‐scale hydration of the ophiolite. The formation of dispersed carbonate is related to percolating fluids withδ18O lower than modern ground and meteoric water. Our study shows that radiocarbon investigations are an essential tool to interpret the carbonation history and that stable oxygen and carbon isotopes alone can result in ambiguous interpretations.

     
    more » « less
  2. Abstract

    We use geochemical and petrographic data from anoxic sequences of the Nicobar Fan to document extensive marine silicate weathering (MSiW) in the input sediment of the Sumatra subduction zone and the conditions that result in authigenic minerals originating from this reaction: precipitation of authigenic carbonate—which sequesters carbon—and formation of authigenic clay—which releases CO2. Increase in87Sr/86Sr in pore fluids from International Ocean Discovery Program Expedition 362 (Site U1480 to 0.71376 and Site U1481 to 0.71296) reveals a radiogenic strontium contribution from alteration of the Himalayan continental sediment that dominates the Nicobar Fan. Peaks in the dissolved strontium isotope data coincide with zones of methane presence, consistent with MSiW reactions driven by CO2generation during methanogenesis. Later‐stage fan sequences from 24 to 400 mbsf (meters below seafloor) contain only minor carbonate with87Sr/86Sr ratios that deviate only slightly from the co‐eval seawater values (0.70920–0.70930); geochemical data in this zone point to a contribution of authigenic clay formation. In contrast, microscopy and elemental mapping of the carbonate‐cemented zones in the earliest fan deposits (>780 mbsf) show replacement of feldspars and dense minerals by carbonate, which ranges in volume from a few percent of the grain to near total grain obliteration. This deeper authigenic carbonate is significantly enriched in radiogenic87Sr (0.71136–0.71328). Thus, MSiW leads to distinct products, likely in response to a weathering‐derived supply of silica in the younger setting versus calcium enrichment via diffusion from oceanic basement in the older sequence.

     
    more » « less
  3. Abstract

    Vertical profiles of benthic foraminiferal oxygen and carbon isotopes (δ18O and δ13C) imply the volume of southern source water (SSW) in the Atlantic basin expanded during the Last Glacial Maximum. Shoaling of the boundary between SSW and northern source water (NSW) may reduce mixing between the two watermasses, thereby isolating SSW and enhancing its ability to store carbon during glacial intervals. Here we test this hypothesis using profiles of δ18O and δ13C from the Brazil Margin spanning the last glacial cycle (0–150 ka). Shoaling of the SSW‐NSW boundary occurred during Marine Isotope Stage (MIS) 2, 4, and 6, consistent with expansion of SSW and greater carbon sequestration in the abyss. But the watermass boundary also shoaled during MIS 5e, when atmospheric CO2levels were comparable to MIS 1. Additionally, we find there was little change in watermass structure across the MIS 5e‐d transition, the first major decline in CO2of the last glacial cycle. Thus, the overall pattern in glacial‐interglacial geometry is inconsistent with watermass mixing acting as a primary control on atmospheric pCO2. We also find that δ13C values for MIS 5e are systematically lower than MIS 1, with the largest difference (∼1‰) occurring in the upper water column. Low δ13C during MIS 5e was most likely due to a long‐term imbalance in weathering and deposition of calcium carbonate or input of13C‐depleted carbon from a reservoir external to the ocean‐atmosphere system.

     
    more » « less
  4. Abstract

    Drylands occupy nearly 40% of the land surface and comprise a globally significant carbon reservoir. Dryland‐atmosphere carbon exchange may regulate interannual variability in atmospheric CO2. Quantifying soil respiration rates in these environments is often complicated by the presence of calcium carbonates, which are a common feature of dryland soils. We show with high‐precision O2measurements in a laboratory potted soil experiment that respiration rates after watering were similar in control and carbonate treatment soils. However, CO2concentrations were up to 72% lower in the carbonate treatment soil because CO2was initially consumed during calcite dissolution. Subsequently, CO2concentrations were over 166% greater in the carbonate treatment soil as respiration slowed and calcite precipitated, releasing CO2. Elevated δ13C values of soil CO2(>6‰ higher in the treatment than control) confirm that observed differences were due to calcite dissolution. These findings demonstrate that calcite dissolution and precipitation can occur rapidly enough to affect soil gas compositions and that changes in soil CO2are not always directly related to changes in soil respiration rates. Studies of local soil respiration rates and carbon exchange are likely to be influenced by dissolution and precipitation of calcium carbonates in soils. We estimate that one fifth of global soil respiration occurs in soils that contain some amount of soil carbonate, underscoring the need to account for its obscuring effects when trying to quantify soil respiration and net ecosystem exchange on a regional or global scale.

     
    more » « less
  5. Abstract

    The deep ocean releases large amounts of old, pre‐industrial carbon dioxide (CO2) to the atmosphere through upwelling in the Southern Ocean, which counters the marine carbon uptake occurring elsewhere. This Southern Ocean CO2release is relevant to the global climate because its changes could alter atmospheric CO2levels on long time scales, and also affects the present‐day potential of the Southern Ocean to take up anthropogenic CO2. Here, year‐round profiling float measurements show that this CO2release arises from a zonal band of upwelling waters between the Subantarctic Front and wintertime sea‐ice edge. This band of high CO2subsurface water coincides with the outcropping of the 27.8 kg m−3isoneutral density surface that characterizes Indo‐Pacific Deep Water (IPDW). It has a potential partial pressure of CO2exceeding current atmospheric CO2levels (∆PCO2) by 175 ± 32 μatm. Ship‐based measurements reveal that IPDW exhibits a distinct ∆PCO2maximum in the ocean, which is set by remineralization of organic carbon and originates from the northern Pacific and Indian Ocean basins. Below this IPDW layer, the carbon content increases downwards, whereas ∆PCO2decreases. Most of this vertical ∆PCO2decline results from decreasing temperatures and increasing alkalinity due to an increased fraction of calcium carbonate dissolution. These two factors limit the CO2outgassing from the high‐carbon content deep waters on more southerly surface outcrops. Our results imply that the response of Southern Ocean CO2fluxes to possible future changes in upwelling are sensitive to the subsurface carbon chemistry set by the vertical remineralization and dissolution profiles.

     
    more » « less