Olivine Dissolution in Seawater: Implications for CO2 Sequestration through Enhanced Weathering in Coastal Environments.

Enhanced weathering of (ultra)basic silicate rocks such as olivine-rich dunite has been proposed as a large-scale climate engineering approach. When implemented in coastal environments, olivine weathering is expected to increase seawater alkalinity, thus resulting in additional CO2 uptake from the atmosphere. However, the mechanisms of marine olivine weathering and its effect on seawater-carbonate chemistry remain poorly understood. Here, we present results from batch reaction experiments, in which forsteritic olivine was subjected to rotational agitation in different seawater media for periods of days to months. Olivine dissolution caused a significant increase in alkalinity of the seawater with a consequent DIC increase due to CO2 invasion, thus confirming viability of the basic concept of enhanced silicate weathering. However, our experiments also identified several important challenges with respect to the detailed quantification of the CO2 sequestration efficiency under field conditions, which include nonstoichiometric dissolution, potential pore water saturation in the seabed, and the potential occurrence of secondary reactions. Before enhanced weathering of olivine in coastal environments can be considered an option for realizing negative CO2 emissions for climate mitigation purposes, these aspects need further experimental assessment.

Introduction

Climate engineering approaches that aim to deliberately and actively remove greenhouse gases from the atmosphere are categorized as carbon dioxide removal (CDR) or negative emission technologies (NETs).[1] CDR or NETs are seen as a possible future complement to current climate policies, which are presently only focused on the reduction of CO2 emissions. Model studies emphasize that the large-scale implementation of NETs will be needed to limit global warming to within a 2 °C increase with respect to preindustrial conditions.[2,3] The worldwide commitment to attain this target, thus avoiding a “dangerous” level of climate change, has been strengthened by the recent COP21 Paris Agreement.[4]

Enhanced silicate weathering (ESW) is a NET approach in which the natural process of (silicate) rock weathering is artificially stimulated.[5,6] The technique has been recognized as a potentially promising strategy for CO2 removal from the atmosphere while at the same time counteracting ocean acidification.[1,6−9] The implementation of ESW requires suitable source rock to be mined, ground to small grain sizes, and subsequently spread over suitable areas.[5] The mineral grains dissolve (i.e., chemical weathering), through which CO2 is eventually captured from the atmosphere.[6,10] Olivine (Mg2–FeSiO4) is an abundant and fast-weathering ultramafic silicate mineral and has been advanced as a prime candidate mineral for ESW application.[5,11] The dissolution of olivine in an aqueous environment consumes protons or equally increases alkalinity,[6,12,13] and so increases CO2 uptake by the aqueous medium (Supporting Information section 1).

In theory, ESW can be applied in terrestrial soils,[5,6] in the surface mixed layer of the open ocean,[13] or by spreading minerals onto sediments of the coastal zone and continental shelf.[14] The largest application domain for ESW would be the open ocean, but model analysis suggests that the olivine particles need to be ground to very small sizes to facilitate dissolution in the surface ocean.[13] As such, the high grinding costs and CO2 emissions during production potentially limit this approach. An alternative scheme is the application of ESW to coastal and shelf environments, where it could be integrated into existing coastal zone management practices, such as dredging operations, land reclamation, and beach nourishment. A theoretical examination of the concept of coastal ESW[14] has indicated advantages as well as challenges. One important knowledge gap is that detailed experimental investigations of olivine dissolution under natural conditions (i.e., realistic for coastal ESW) are lacking. A better understanding of the rate and mechanism of olivine dissolution in natural marine environments is needed to better evaluate the feasibility and potential of coastal ESW as a NET. Previous work has largely focused on olivine dissolution under laboratory conditions using artificial seawater solutions.[15−18] Such idealized approaches potentially exclude important geochemical and environmental influences that could be relevant under field conditions.

Here, we specifically address a number of questions related to the application of enhanced silicate weathering in natural coastal environments: (1) What is the rate of olivine dissolution in natural seawater and how does this differ from artificial seawater? (2) Does olivine dissolve stoichiometrically in natural seawater? (3) What dissolution products can be used to efficiently monitor the dissolution rate of olivine in coastal sediments, i.e., quantify the efficiency of enhanced silicate weathering? (4) To what extent does secondary mineral formation diminish the CO2 sequestration efficiency of olivine dissolution in seawater?

We present results from dissolution experiments with simulated grain–grain collisions, in which olivine was dissolved in natural filtered seawater as well as in artificial seawater media with modified cation composition. Potential proxies for quantifying the dissolution rate of olivine are analyzed and compared. Based on these results, we discuss a number of challenges for ESW in coastal environments.

Materials and Methods

Materials

Commercially available olivine sand (Mg2–FeSiO4) and lab-grade quartz (SiO2) were used in slurry dissolution experiments. The olivine sand (particle size quantiles: D10 = 91 μm, D50 = 143 μm, D90 = 224 μm) had a molar Mg-to-Fe ratio of 0.94:0.06, characterizing the olivine as forsterite-94 (Fo94). The Ni content was estimated at 0.0075 mol Ni mol–1 olivine. Further details on the chemical composition, grain-size distribution, and pre-experimental treatment are summarized in Tables S1 and S2.

Different reactive seawater media were used as supernatant. Filtered seawater (FSW) was collected as natural seawater from the Oosterschelde tidal basin (The Netherlands) and filtered over a Mahle amaGuard FP 0.2 [μm] woven cotton filter (Mahle Benelux, The Netherlands). In addition, three types of artificial seawater were prepared according to the ASTM Standard Practice D 1141–98[19] (Table S3): (1) plain artificial seawater (ASW), (2) artificial seawater with Ca2+ replaced by Na+ (hereafter named ASW-Ca), and (3) artificial seawater with both Ca2+ and Mg2+ replaced by Na+ (hereafter named ASW-CaMg).

Experiments

Specific amounts of olivine and quartz grains were added to a specific volume of seawater in 500 mL borosilicate glass bottles, which were then subjected to continuous rotating movements on a CH-4103 rotating shaking platform (INFORS AG, Switzerland) set at 155 rpm. The bottles were closed with membrane screw caps, equipped with a 0.5 mm thick silicone septum that prevented evaporation but allowed gas exchange. To prevent photosynthesis, the experiment took place in the dark, while the bottles were wrapped in aluminum foil (leaving the top of the membrane cap uncovered).

A total of three types of agitation experiments were conducted: A1, A2, and A3 (Supporting Information section 3 and Table S4). In both the A1 and A2 experiments, 0.1 mol of either olivine (OLI) or quartz (QUA) were added to 300 mL of FSW, with one control treatment (i.e., only seawater; SW). A total of three replicates were conducted per treatment (n = 3). Experiment A1 was conducted under ambient conditions, i.e. without regulation of temperature (range: 13.5–20 °C) and pCO2 (range: 445–525 ppmv), and lasted for 88 days. The second agitation experiment, A2, had essentially the same setup as A1 except for a few modifications. First, natural seawater was bubbled with air prior to the experiment to ensure CO2 equilibration with the surrounding atmosphere. Second, the experiment had a much shorter duration (20 days), and it was conducted under stable temperature and pCO2 conditions. The third experiment, A3, was designed to specifically investigate the effect of the composition of seawater on the dissolution rate of olivine (quartz was not investigated). Agitation experiment A3 was designed to examine the effect of the composition of seawater on the dissolution rate. A3 was also conducted under stable temperature and pCO2 conditions and used atmosphere-equilibrated reactive fluids, bubbled with air. The dissolution of olivine (OLI) was monitored in four reactive fluids (FSW, ASW, ASW-Ca, and ASW-CaMg; see the Supporting Information section 2) and compared to control treatments (respective solution media without olivine). Instead of 0.10 mol olivine used in A1 and A2, and 0.03 mol was used in A3 (Table S3).

To test the impact of agitation, a layer of olivine sand was placed also in a nonmoving cylindrical container with FSW (n = 1). In this nonagitated treatment, the olivine itself was not agitated. Rather, the overlying water was stirred, and only samples for solid-phase analysis were collected.

Water and Solid-Phase Analysis

The overlying water of the slurry batch reaction experiments was sampled at regular time intervals and analyzed for temperature, salinity, pH, total alkalinity (TA), dissolved inorganic carbon (DIC), dissolved silicate (Si), dissolved nickel (Ni), and dissolved magnesium (Mg) using standard analytical procedures[20,21] (see section 3 of the Supporting Information). All solute concentrations are reported as μmol per kg of seawater (μmol kg–1).

Upon completion of the A3 experiment, olivine grains were recovered from the agitated and nonagitated treatments, inspected for dissolution features, and analyzed for carbonate precipitates, according to Nieuwenhuize et al.,[20] to yield the mass percentage of inorganic carbon (mass% Cinorg). The elemental composition of the olivine particle surfaces were investigated using scanning electron microscope energy-dispersive X-ray spectroscopy (SEM–EDX). Additional details on both water and solid-phase analyses can be found in section 3 of the Supporting Information.

Olivine Dissolution Rate Calculations and Simulations

The accumulation over time of the reaction products in the reactor vessels is reported as excess concentration values, ΔCi(t) = Ctreatment(t) – Ccontrol(t). In this, the control refers to the treatment without the addition of any solid minerals. A total of three empirical mathematical models were implemented to describe ΔCi(t) as a function of the incubation time, and from these model fits, the accumulation rate Ri [μmol kg–1 day–1] of compound i and the associated area-specific dissolution rate constant ki [μmol m–2 day–1] were derived (section 4 of the Supporting Information). Note that in the case of stoichiometric dissolution, the rate constants ki should be, at least in theory, identical for all olivine dissolution products.

The accumulation of weathering products and the change of solution chemistry during the batch dissolution experiments was also mechanistically simulated using the geochemical software package PHREEQC v2[22]· In these simulations, the solution chemistry (and, hence, the saturation states) were free to evolve with time as a consequence of mineral dissolution (kinetic rate equations specified in section 4 of the Supporting Information). Solubility constants were taken from the MINTEQ.dat and LLNL.dat databases to calculate the saturation states of solid phases in the solution. The measured initial composition of the solution, which was specific for each treatment, was used as the starting conditions for the PHREEQC simulations.

All mathematical analyses, apart from the PHREEQC analyses, and plotting were done using the open source R framework for statistical computing.[23]

Results

Olivine and Quartz Dissolution in Natural Filtered Seawater

In the A1 and A2 experiments, we investigated the dissolution of olivine and quartz in natural filtered seawater. In both A1 and A2, there was a clear ΔSi signal in the quartz treatment (QUA), most likely caused by dissolution of very fine quartz particles (Figure ). ΔSi increased until ∼18 μmol kg–1 within the first week of the experiments, after which it remained constant. There was no discernible Ni release in the A1 and A2 quartz treatment (Figure ), and hardly any response from the carbonate system. The ΔpH increased by 0.05 within the first 2 weeks but then decreased again to its initial value by the end of the experiment. While ΔTA remained constant with time, ΔDIC decreased with 22 μmol kg–1 during the first 15 days, likely caused by CO2 outgassing, as the initial solution in A1 may not have been in equilibrium with the atmosphere (sections 2 and 3 of the Supporting Information). In the A2 experiment, the experimental procedure was improved, and the FSW medium was bubbled with ambient air at the start of the experiment. As a result, the carbonate system variables ΔDIC, ΔTA, or ΔpH did not change significantly over time (linear regression, p = 0.35, p = 0.28, and p = 0.696, respectively).

Figure 1

Temporal development of olivine dissolution response variables in experiments A1 and A2. Symbols denote mean seawater-corrected values (see the Materials and Methods section), with error bars denoting standard error of the mean (SEM). Circles: values from experiment A1; triangles: values from experiment A2. The values for both experiments are plotted with the olivine (OLI) and quartz (QUA) treatments plotted alongside on the same vertical scale for comparison. The reported units are μmol/kg of seawater, except for pH, which is in pH units on the total scale.

In the olivine treatment (OLI) of experiment A1, increases with time of both ΔSi and ΔNi were observed, suggesting olivine dissolution (Figure ). While ΔNi leveled off at 3.2 μmol kg–1, suggesting that an equilibrium was reached, ΔSi increased almost linearly over the 88 day incubation period, displaying a much-stronger dissolution than in the QUA treatment. Experiment A2 showed a similar temporal evolution of ΔSi and ΔNi (Figure ). ΔTA showed a pronounced increase over the first 5 days (Figure ), leveling off thereafter at ΔTA = 103 μmol kg–1. ΔDIC increased in a similar way, albeit over a slightly longer period of ∼15 days, leveling off at ΔDIC = 93 μmol kg–1. The pH in the olivine treatment increased rapidly over the first 5 days by ∼0.1, after which it decreased again and leveled off at ΔpH = 0.02 (Figure ). The carbonate system in A2 showed a similar pattern, with ΔTA and ΔDIC leveling off at 104 and 74 μmol kg–1, respectively (Figure ). Similar to experiment A1, the ΔpH in A2 increased strongly within the first 6 days by ∼0.15, after which it decreased again to ΔpH = 0.06 (Figure ).

Olivine Dissolution in Artificial Seawater Media with Different Cation Composition

In experiment A3, large differences in the release of dissolution products were observed between the different seawater media. All four media displayed a quasi-linear ΔSi response with time (Figure ). The ΔSi attained at the end of the experiment was lowest in the natural seawater (FSW: 68 μmol Si kg–1) and artificial seawater (ASW: 82 μmol Si kg–1) and increased markedly when Ca2+ and Mg2+ were replaced by Na+ in the medium (ASW-Ca: 122 μmol Si kg–1; ASW-CaMg: 162 μmol Si kg–1).

Figure 2

Temporal development of olivine dissolution response variables in experiment A3. Symbols denote mean seawater-corrected values (see the Materials and Methods section) for the olivine treatment (OLI), with error bars denoting standard error of the mean (SEM). Circles: values from filtered seawater treatment (FSW); triangles: values from artificial seawater treatment (ASW); squares: values from artificial seawater without calcium treatment (ASW-Ca); diamonds: values from artificial seawater without calcium and magnesium treatment (ASW-CaMg). The reported units are μmol/kg of seawater (corrected for control values), except for pH, which is in pH units on the Total scale.

The Ni release showed a saturation-type response, which varied between media. In the FSW, ASW, and ASW-Ca treatments (Figure ), the ΔNi concentration showed a comparable accumulation (plateau concentrations between 1.19 and 1.68 μmol Ni kg–1). In contrast, the ASW-CaMg treatment showed hardly any Ni accumulation, apart from a small initial release, which was taken up again by the end of the experiment.

The carbonate system (TA, pH, and DIC) responded very differently in the Ca- and Mg-free seawater compared to the three other treatments. Although the shape of the response curves were similar, the overall accumulation of alkalinity (ΔTA = 340 ± 14 μmol kg–1) and dissolved inorganic carbon (ΔDIC = 317 ± 11 μmol kg–1) was substantially higher in the ASW-CaMg compared to the other treatments (range ΔTA = 41–69 μmol kg–1 and ΔDIC = 31–66 μmol kg–1 in FSW, ASW, and ASW-Ca). The ΔTA and ΔDIC in ASW-CaMg quickly increased over the first 10 days, after which the increase rate slowed down and the accumulation became linear. The long-term accumulation, i.e. between 30 and 137 days, was higher for DIC (168 ± 50 μmol kg–1) than for TA (127 ± 12 μmol kg–1).

The ΔpH showed an “overshoot” response, with a sharp initial increase in the first 5 days, reaching maximum between 4 and 7 days, after which the ΔpH gradually decreased and tended toward an asymptotic equilibrium value at the end of the experiment (Figure ). Consistent with the stronger alkalinity accumulation in the Ca- and Mg-free seawater, the long-term pH (pHt137 minus pHt0) increase was much higher in the Ca- and Mg-free seawater (ΔpH = 0.06), compared to the other three treatments (range in ΔpH = −0.01 to 0.035 at 137 days).

Olivine Dissolution Stoichiometry and Rates

PHREEQC kinetic modeling of the ASW, ASW-Ca, and ASW-CaMg treatments suggested that all reactive fluid media in experiment A3 were undersaturated for forsterite (Ωforsterite = 10–5 for FSW, ASW, and ASW-Ca and Ωforsterite = 10–10 for ASW-CaMg).

The accumulation rates R were determined by the best model fit[23] (see Figure S6 for representative examples) to the response curves of the olivine dissolution products ΔSi, ΔNi, ΔMg, and ΔTA and the ensuing CO2 sequestration ΔDIC (Table ). As emphasized above, different dissolution products tended to have different response curves within the same treatment, thus indicating nonstoichiometric dissolution (Table ). In the case of ΔSi, the accumulation response was generally best described by a linear model (model 1, Table S5), while for ΔTA, ΔDIC, ΔMg, and ΔNi, the profiles were typically best fitted with a saturation model (model 2, Table S5) or a combination of short-term saturation with a long-term linear increase (model 3, Table S5). Only in the ASW-CaMg treatment (experiment A3) could all variables be described best by model 3 (Table S5)

Table 1

Release Rate Rimax (μmol kg–1 day–1) of Each of the Measured i Variables ΔSi, ΔNi, ΔMg, ΔTA, and ΔDIC within Each Experimenta

exp	medium	.RΔSi	RΔNi	RΔMg	RΔTA	RΔDIC
A1	FSW	0.91	0.23		39.92	6.32
A2	FSW	2.81	0.42		52.32	13.62
A3	FSW	0.62	0.13		11.52	7.92
	ASW	0.61	0.23		332	2.12
	ASW-Ca	0.91	0.22		1.62	1.72
	ASW-CaMg	3.83	0.23	35.43	32.43	22.73

The number next to each of the Rimax values corresponds to the model that best fit the data (for the significance of parameters, see the Materials and Methods section), where 1 = linear model, 2 = saturation model, 3 = combined model according to the equations in Table S6.

To further illustrate the absence of stoichiometric dissolution in either natural or artificial seawater in the presence of magnesium, Figure S8 plots the accumulation of response variables in experiment A3 normalized for stoichiometry (Table S1: ΔSi/1, ΔTA/4, ΔNi/0.0075, and ΔMg/1.87). Only in the ASW-CaMg treatment did olivine dissolution tend to become stoichiometric, as shown by the similar responses for ΔMg and ΔTA (Figure S8).

Because of nonstoichiometric dissolution, the olivine dissolution rate constant ki showed a dependence on the response variable (ΔSi, ΔNi, ΔMg, ΔTA, and ΔDIC; Table ). The rate constant based on ΔNi (kΔNi) is the highest of all response variables and is similar across all treatments (31–74 μmol of olivine m–2 day–1). In the ASW-CaMg treatment, kΔMg (63 μmol of olivine m–2 day–1) was consistent with kΔNi values, while kΔSi were an order of magnitude lower than kΔNi values in the reactive fluid media containing Mg2+ (Table and 2). The exclusion of Mg2+ in the Mg-free reactive fluid (ASW-CaMg) increased kΔSi by 1 order of magnitude. The values of both kΔ and kΔDIC show substantial variation between treatments, and are highest in the ASW-CaMg treatment. The temperature-normalized[24] (to 25 °C) mean values for ki (where i = ΔSi, ΔNi, ΔTA, or ΔDIC), for the FSW and ASW cases are shown in Figure (the ASW-Ca and ASW-CaMg treatments are considered unrealistic for ESW and are thus excluded).

Table 2

Olivine Dissolution Constant ki (μmol m–2 day–1) Based on the Rimax (Table ) of Each of the i Variables ΔSi, ΔNi, ΔMg, ΔTA, and ΔDIC within Each Experiment

exp	solvent	kΔSi	kΔNi	kΔMg	kΔTA	kΔDIC
A1	FSW	1	31		16	2
A2	FSW	3	56		13	3
A3	FSW	2	60		10	7
	ASW	2	74		28	2
	ASW-Ca	3	65		2	1
	ASW-CaMg	13	65	63	27	19

Figure 3

Olivine dissolution rate constant k, calculated as the mean (± SD) value of the different response variables measured in the three agitation experiments A1, A2, and A3 (Table S4). To obtain the most-realistic estimates for olivine dissolution in seawater, only values from the FSW and ASW treatments were considered. For comparison, the estimated value by Hangx and Spiers[14] from previous studies (literature, H&S 2009) is given in the same units as the rates obtained in this study. The literature value and range are denoted by the gray circle and the gray area for clarity. The gray triangles represent the values obtained in this study at 17 °C but recalculated to 25 °C, the same standard temperature as the literature estimates.

SEM–EDX

SEM–EDX analyses of mineral grains from fresh, unreacted olivine were generally angular, with sharp edges (Figure A). In contrast, olivine grains that had been rotating during the entire experiment (137 days) were generally subrounded (Figure B), suggesting abrasion due to grain–grain collisions. The Mg-to-Si atomic ratios (Mg/Si) at the surface of the unreacted particles were significantly higher (mean ± SEM Mg/Si = 2.11 ± 0.02, ngrains = 6; Figure S9) than for grains that were agitated in solution (mean ± SEM Mg/Si 1.7 ± 0.04–2 ± 0.03, ngrains = 3–10; Figure S9). This suggests preferential mobilization of Mg during dissolution, consistent with the higher dissolution rates obtained for Mg and Ni compared to Si. The preferential leaching of Mg2+ (lowest Mg-to-Si ratio) was most prominent in the ASW-CaMg treatment (Figure S10), where areas with Mg/Si ≤ 1 and lower were observed. No carbonate minerals were observed on any of the analyzed olivine grains.

Figure 4

(A) SEM–EDX micrograph of unreacted olivine (substrate material) with very clear angular features and sharp edges. The Mg-to-Si atomic ratio in area 1 typically lies between 2 and 2.5. (B) SEM–EDX micrograph of an olivine particle after being subjected to continuous movement in FSW during 137 days (experiment A3). On the surface of the same olivine particle, abrupt changes in Mg-to-Si atomic ratios can be observed within small distances. Areas denoted with 1 are characterized by Mg-to-Si atomic ratios of 2–2.5, while Mg-to-Si atomic ratios in areas denoted with 2 showed values of around 1. Such locations, where the Mg-to-Si ratio decreases well below 2, are considered local weathering sites.

Potential for Carbonate Precipitation

The inorganic carbon content (Cinorg) in the solid mineral phase recovered from experiment A3 was very low (mean Cinorg < 0.005%) and was not significantly different between the four treatments (one-way ANOVA, p = 0.112; Figure S9). Small changes in the solid-phase carbonate content (which are difficult to measure) could nevertheless be associated with substantial changes in the alkalinity of the supernatant. Although not significantly different, the difference in Cinorg content between FSW and ASW-CaMg was 0.003 mass percent. If this difference would be real and caused by carbonate precipitation, this would imply that the FSW contained 37 μmol kg–1 of CaCO3 in excess to the ASW-CaMg, when expressed per unit volume of fluid. The absence of this precipitation would hence cause the alkalinity to be 74 μmol kg–1 higher in ASW-CaMg. However, the measured ΔTA difference between the ASW-CaMg treatment and the FSW and ASW was much higher, amounting to ca. 300 μmol TA kg–1 at the end of experiment A3 (Figure ). Accordingly, carbonate precipitation cannot explain the difference in alkalinity between the FSW and ASW-CaMg treatments, and so it was likely that more olivine dissolution took place in the ASW-CaMg treatment.

Discussion

The dissolution experiments here demonstrate several features regarding olivine weathering in seawater and its potential applications for ESW in coastal settings. First, the basic principle underlying ESW in seawater appears to work. Olivine dissolution in natural seawater under nonsterile laboratory conditions consistently causes alkalinization, followed by CO2 invasion from the atmosphere into the seawater, at rates in agreement with those estimated by previous studies.[14,24,25] Second, apparent nonstoichiometric dissolution complicates the experimental determination of the rate and extent of olivine dissolution within the seabed, making it more challenging to assess of the efficiency of ESW. Therefore, the quantification of the actual olivine dissolution rate under realistic in situ conditions will require a multiparameter approach, combining flux measurements of dissolved silicate, dissolved metals (nickel and iron), and alkalinity, with appropriate experimental controls. Third, the rate of olivine dissolution within the seabed can be limited by saturation effects, which could decrease the efficiency of ESW applications. Fourthly, the extent to which secondary reactions impact the CO2 sequestration efficiency of olivine dissolution under in situ conditions remains unresolved and is an important issue to address in further studies on coastal ESW. We will now discuss each of these aspects in more detail.

Quantification of the Olivine Dissolution Rate

To be implemented as a negative emission technology for climate change mitigation,[7] the carbon sequestering potential of marine olivine dissolution needs to be quantified. In other words: How much olivine dissolution occurs within the seabed? What is the time frame in which olivine particles react? How much CO2 is eventually taken up by the seawater as a consequence of ESW?

The overall CO2 sequestration rate (RCO; mol CO2 per m2 of seabed per unit of time) can be expressed asTo determine the effectiveness of coastal ESW, both the factors γCO and ROLI need to be accurately constrained. The CO2 sequestration efficiency γCO specifies the net amount of CO2 that is taken up from the atmosphere during the dissolution of 1 kg of olivine within the seafloor (this parameter will be further discussed below). For a given amount of finely ground olivine distributed onto the seafloor (COLI; mol of olivine m–2 of seabed), the olivine dissolution rate (ROLI; mol of olivine per m2 of seabed per unit of time) determines over what time frame the ESW application will be effective (dissolution period τ = COLI/ROLI). The olivine dissolution rate ROLI further depends on the specific surface area of the mineral grains (Asurface; m2 g–1) and the intrinsic dissolution rate constant (kOLI; mol of olivine per m2 of grain surface area per unit of time).

In practical ESW applications, the olivine dissolution rate (ROLI) within the seabed can be determined experimentally by monitoring the release of olivine dissolution products from the seabed. This poses the question as to which dissolution product (e.g., Mg2+, Si, and TA) should be monitored as a reliable proxy for the olivine dissolution rate in field-type experiments. The use of both dissolved silicate and alkalinity is nontrivial as these are generated in sediments by other processes than olivine dissolution.[26] Any observed sediment efflux of dissolved silicate and alkalinity can thus not be exclusively attributed to olivine dissolution. Furthermore, Mg2+ cannot be used as a dissolution proxy due to the high background concentration in seawater (∼50 mmol of Mg2+ kg–1 of seawater), and thus, its accumulation in the overlying water cannot be reliably measured.

Our experiments suggest that Ni2+ could be a suitable dissolution proxy, generating a dissolutive accumulation, which substantially supersedes the ambient seawater concentration[27,28] (0.002–0.16 μmol Ni kg–1). This way, pore water accumulation and sediment fluxes of Ni can be accurately measured using standard analytical techniques for trace metals (e.g., inductively coupled plasma–mass spectrometry). However, to qualify as a good proxy for sedimentary dissolution of olivine, two important conditions need to be fulfilled. First, the efflux of the olivine proxy (Ni) from the sediment should also match the release rate of the weathering products in the pore solution. In this regard, Ni2+ seems an advantageous proxy. The natural cycling of Ni2+ in coastal sediments is restricted, and so the observed Ni2+ efflux from the sediment in olivine addition experiments can be fully attributed to olivine dissolution. Still, in future studies, it should be verified whether the Ni2+ release is modulated by diagenetic effects within the sediment (e.g., sorption onto minerals).

A second important condition is that stoichiometric dissolution of olivine occurs, so that the Ni2+ release can be properly rescaled to the overall olivine dissolution rate ROLI by means of the Ni content of the olivine source rock that is used[25]· However, our experiments suggest that this not the case (Tables 1 and 2). The experiments A1–A3 show that, when assessed over short-time scales, the dissolution of olivine in seawater is nonstoichiometric, implying incongruent dissolution under Earth surface conditions[29]· In the absence of secondary precipitation reactions, nonstoichiometric dissolution cannot continue indefinitely. Experiments of longer time scales should therefore clarify to what extent the Ni2+ release from sediments can be a valid proxy for in situ olivine dissolution.

In our dissolution experiments, the k value for Si (determined at the initial time t0) was ca. 30 times lower compared to that of Ni (or Mg). This suggests a preferential release of divalent cations, respective to silicate. The preferential release of metal cations (Ni2+ and Mg2+) compared to Si, and the observed quasi-linear increase in ΔSi are typical for solid-state diffusion in silicate minerals,[29] which facilitates the formation of a “surface (altered) layer”.[30] The crystal ionic radius of nickel (83 pm) is only slightly smaller than that of magnesium[31] (86 pm), implying that both metal ions will have a similar rate of solid-state diffusion. Both Pokrovsky and Schott[25] and Palandri and Kharaka[22] already suggested that for slightly alkaline solutions (e.g., seawater), forsterite dissolution at steady-state is controlled by the decomposition of a protonated surface complex, which is silica-rich and magnesium-deficient. Maher et al.[32] postulate that olivine dissolution occurs as a series of boundary layer processes, in which primary dissolution of cations is followed by dissolution of silicic acid ions, which may subsequently repolymerize at the surface. This implies that the measured dissolved silica release rate is a net value, which may not serve as the sole proxy for olivine dissolution. Although in some SEM–EDX images -particularly those from the ASW-CaMg treatment (Figure S10), the particle surface did look as if flakes of surface material had been detaching, and the examined olivine grains did not show any evidence of secondary silicate precipitates.[17,33−35]

A significant finding here is that SEM–EDX analyses show decreasing Mg-to-Si atomic ratios of the forsterite surface between initial substrate and reacted material (Figure S9). Rather than a buildup of thick silica formations, these decreasing Mg-to-Si ratios corroborate the mechanism of a cation-leached, surface altered layer formation by preferential dissolution and subsequent repolymerization processes sensu Hellmann et al.[30] and Maher et al.[32] The time scale on which the weathering takes place in this study is much longer than in high-temperature and high-pressure studies or studies in which an elevated pCO2 is employed.[29] This so-called “unstrained dissolution”,[36] combined with physical disturbances, such as grain abrasion, does not allow for the buildup of a conspicuous passivating layer or thick silicate precipitates.

The nonstoichiometric dissolution as observed in the experiments here emphasizes that the proper quantification of olivine dissolution in field-type ESW experiments requires a careful experimental design. Overall, the nonstoichiometric dissolution of olivine makes the experimental assessment of ESW more challenging. One cannot simply measure one dissolution proxy (e.g., Ni2+) and estimate the release of other reaction products by application of reaction stoichiometry. Moreover, both dissolved silicate and alkalinity are generated in sediments by other processes than olivine dissolution.[26] From a biogeochemical perspective, it is crucial to know how olivine dissolution stimulates the efflux of dissolved silicate and alkalinity from the seabed because the efflux of alkalinity is the ultimate driver of CO2 uptake,[37] while silicate could stimulate primary productivity by marine diatoms. Hence, a multiparameter assessment, combining flux measurements of Ni2+, dissolved silicate, and alkalinity with appropriate experimental controls, seems to provide the best strategy to confidently determine the olivine dissolution rate under in situ conditions.

Impact of Saturation

The values for the dissolution rate constant k of olivine in seawater obtained in this study are consistent with literature values. For the temperature ranges used in FSW and ASW, the dissolution rate constant varied between 1.9 ± 0.8 μmol of olivine m–2 day–1 for kSi and 56 ± 18 μmol of olivine m–2 day–1 for kNi (mean ± SD values). Normalized for temperature differences, these k value ranges corresponded well with the mean value of 14 μmol of olivine m–2 day–1, as compiled by Palandri and Kharaka[22] and Hangx and Spiers,[14] of which the latter had an order of magnitude of variation around the mean (Figure ).

The nonstoichiometric dissolution in the seawater media FSW and ASW, together with the saturation behavior observed in the TA, Mg2+, and Ni2+ results (Figure ), suggest that the olivine dissolution approached thermodynamic equilibrium, thus slowing down the reaction. Only by using a lower solubility product for forsterite (log K = 26.448) than those found in the PHREEQC databases did model simulations indeed show a slowing of the dissolution reaction by saturation, mirroring our experimental observations. Furthermore, the DIC accumulation followed that of alkalinity perfectly in all cases, albeit with a time lag. This lag is due to the relatively slow process of CO2 invasion[38] and is also observed in the pH response, which first increases to reach a maximum and then subsequently decreases again. This pH response reflects the initial removal of protons through olivine dissolution, followed by a replenishment of the proton pool by lagged CO2 transfer.

The observed time response of the reaction products in our experiments provide a first idea about the possible influence of saturation effects under in situ conditions. Our experiments show that saturation occurs within a time frame of about 20 days (Figures 1 and 2), for an experimental setup with 15 g of olivine in 300 mL of seawater (i.e., 20 mL of solution g–1 of olivine). Assuming the same dissolution rate occurs under in situ pore water conditions, the ratio of pore solution to olivine will be lower. For example, if 10–20% of the solid sediment consists of olivine (mixing a 1–2 cm olivine layer into the top 10 cm of sediment) and assuming a porosity of 0.8 and an olivine particle density of 3.3 g mL–1, we obtain a ratio of 6–12 mL of solution g–1 of olivine. Based on our results, we determined that such a pore solution will be saturated within 4.5 to 9 days, after which olivine dissolution will slow down and cease. However, the pore water of coastal sediments is also regularly refreshed through physical, advective pore water flow induced by waves and currents[39] and biological irrigation by burrowing macrofauna.[40,41] Coastal sediments subject to moderate and high bioirrigation show flushing rates in the range of 10–100 L m–2 day–1,[39] implying that the pore solution of the first 10 cm would be refreshed on a time scale of 0.5–8 days (assuming a porosity range from 0.5 to 0.8). In these sediments, irrigation appears sufficient to counteract the saturation of olivine dissolution in the pore water. However, in more cohesive (muddy) coastal sediments with flushing rates <10 L m–2 day–1, saturation effects can be expected, which could decrease the efficiency of enhanced olivine weathering applications. Therefore, a judicious choice of the application location seems warranted.[42]

CO2 Sequestration Efficiency

The experiments in this study were performed in a setup that allowed free gas exchange with the atmosphere, while internal biological processes affecting the DIC pool were excluded (i.e., primary production and microbial degradation of organic matter).[26,37] Accordingly, the observed DIC increase in the experiments can be entirely attributed to CO2 invasion induced by olivine dissolution, illustrating the proof-of-principle that ESW enhanced silicate weathering works as a NET.

The CO2 sequestration efficiency expresses the amount of CO2 transferred across the air–sea interface per unit mass of silicate rock that dissolves within the seabed and can be written asThis formulation reflects the two consecutive steps in the process of CO2 sequestration. In a first step, olivine dissolution takes place (rate ROLI), which increases alkalinity in the pore solution (rate RTA). This alkalinity increase will then shift the acid–base equilibrium from dissolved CO2 to bicarbonate and carbonate, thus stimulating a CO2 uptake from the atmosphere across the air–sea interface (rate RCO).[26,37] The CO2 sensitivity(∂ΣCO2/∂TA)pCO specifies how much CO2 is taken up from the atmosphere for each mole of alkalinity that is released from the seabed. This thermodynamic factor is evaluated at a given partial pressure of CO2 in the atmosphere and is dependent on the local salinity, temperature, and chemical composition of the coastal seawater.[43] Calculating the CO2 sensitivity over the entire experimental period, and for all the experiments that had full ionic strength of seawater (A1, A2, A3: FSW and A3: ASW), we obtain a CO2 sensitivity of 0.84 ± 0.1 (mol of DIC mol–1 of TA), which is in close agreement with the theoretical value 0.854 for seawater at the experimental conditions employed (T = 17 °C, S= 33, TA = 2400 μmol L–1, and pCO2 = 400 ppmv).[44]

Accordingly, the CO2 uptake in our experiments appears entirely congruent with the standard acid–base thermodynamics of the carbonate system in seawater. Nevertheless, the alkalinity increase during olivine dissolution, RTA/ROLI, was less than expected. Traditionally, olivine dissolution is described by the reaction equation:Because 4 moles of protons are consumed per mole of olivine dissolved, and hence 4 mol of alkalinity are produced, a ratio (ΔTA/ΔSi = 4 (RTA/ROLI = 4)) is expected. Only in the ASW-CaMg treatment, the ΔTA/ΔSi approached the expected value of 4, while it was substantially less in the FSW, ASW, and ASW-Ca treatments. These observations suggests that the Equation does not provide a complete description of the overall olivine dissolution process, but that secondary reactions could be active. Overall, CO2 sequestration efficiency can be formulated asHere, 4 denotes the theoretical stoichiometry between olivine dissolution and CO2,[5,14] and x denotes a reduction in the CO2 sequestration efficiency due to secondary reactions. Because our slurry experiments were conducted with oxygenated seawater, one such possible reaction is the aerobic oxidation of ferrous iron:This reoxidation process produces free protons, thus consuming again the alkalinity generated during dissolution of the Fe-component of olivine. The olivine employed here contains 6% of Fe (Table S1), which would reduce the alkalinity release by an equal percentage. However, this reduction is not enough to explain the observed ΔTA/ΔSi values. Another possibility to reduce the ΔTA-to-ΔSi ratio is calcium carbonate precipitation:Although the supernatant in the FSW and ASW was saturated with respect to calcite and aragonite, magnesium is known to act as an inhibitor for CaCO3 nucleation in seawater, limiting its precipitation[45,46]· Moreover, exclusion of Ca2+ and Mg2+ from the ASW-CaMg (section 3 of the Supporting Information) implied a strong undersaturation with respect to calcium and magnesium carbonate, thus preventing carbonate precipitation. Additionally, our SEM–EDX analyses did not reveal carbonate minerals on the surface of olivine grains, while at the end of the dissolution experiment, no significant increase in the inorganic carbon (carbonate) content of the solid phase was observed. Accordingly, we consider carbonate precipitation unlikely in the batch experiments performed here, and hence, the cause of the ΔTA/ΔSi < 4 remains unexplained and requires further investigation.

Thermodynamic modeling in Griffioen[47] suggests that precipitation of the hydrated phyllosilicate sepiolite (Mg4Si6O15(OH)2·6H2O) could reduce ΔTA/ΔSi values, thus inducing a lower CO2 sequestration efficiency of enhanced olivine weathering in seawater. However, no sepiolite was found in the XRD analyses. The extent to which secondary reactions impact the CO2 sequestration efficiency of olivine dissolution under in situ conditions within the seabed remains an important issue to address in further studies on coastal ESW.

Due to the exclusion of Mg2+ and Ca2+, the ionic strength of the ASW-CaMg medium was lower than that of the ASW (Table S3). The ionic strength of the ASW was 0.72 mol kg–1, while that of ASW-Ca was 0.015 mol kg–1 (or 2%) lower than ASW. Equally, the ionic strength of ASW-CaMg was 0.0975 mol kg–1 (or 13%) lower than that of ASW. Ionic strength impacts the activity coefficients of aqueous species and has been found to impact dissolution kinetics, particularly at lower pH.[16] Still, other factors (i.e., pCO2, pH, and saturation state) exhibit a much larger influence on dissolution kinetics[48,49]· Given the relatively high pH in the reactive fluids (pH 7.9–8.2) and the fact that all solutions were highly undersaturated with respect to fosterite, the impact of the lower ionic strength of the ASW-CaMg was likely to be very small.

The rate at which CO2 is sequestered due to olivine dissolution in seawater can thus be formulated as the following relation:Here, 4 denotes the theoretical stoichiometry between olivine dissolution and CO2,[5,14]ROLI is the olivine dissolution rate, γCO is the reaction efficiency of the CO2 sequestration in seawater, and x is the molar fraction of Fe in the olivine source material.

Olivine Application in a Coastal Geo-Engineering Framework

To place coastal ESW in a broader perspective, a real-world example illustrates its carbon-capturing potential. The Netherlands is a densely populated, industrialized country, with a GDP of ca. 850 billion USD (2013) and ca. 50% of its surface area below sea level.[50] To protect the coastal region of the country where ca. 60% of the GDP is produced,[51,52] continuous large-scale sand nourishments are needed. Between 2000 and 2010, ca. 12 million m3 (Mm3) sand per year have been deployed along The Netherlands’ coast, which is expected to increase due to predicted climate change-induced sea level rise[51,52] (https://www.noordzeeloket.nl/en/functions-and-use/surface-mining-and-quarrying/).

In a thought experiment, the sand used in these coastal nourishments is replaced by finely ground olivine, as used in the experiments described here. In a hypothetical one-time application of 12 Mm3 (≈ 26 Mt) of olivine sand, parameter values for kΔTA obtained in our experiments (Table ) were implemented in the Olsen[53] shrinking core model for olivine carbonation (assuming the measured olivine particle size distribution; see section 2 of the Supporting Information). This model has been previously implemented in ten Berge et al.,[54] describing total mass of olivine weathered and consequential CO2 captured (section 8 of the Supporting Information). Our simulations showed a cumulative weathering of 4% of the olivine after the first year, 12% after 5 years, 35% after 25 years, 57% after 50 years, and 84% after 100 years (Figure A). After 200 years, 98% of the initially applied 12 Mm3 olivine will be dissolved. These values are in accordance with those presented by Hangx and Spiers,[14] in which 100 μm (median diameter: D50) olivine grains would take >100 years to dissolve.

Figure 5

(A) Model results of both absolute and relative cumulative dissolution over time (using dissolution rate constant values as obtained from the experiments in this study) of a one-time hypothetical coastal olivine application of 12 Mm3, or 26.4 Mton, of olivine sand with the same characteristics as that used here. (B) Model results of the yearly CO2 uptake rate as a consequence of hypothetical repeated (multiyear) olivine application as a substitute for yearly coastal sand nourishments during periods of 1, 5, 10, and 25 years.

Making use of the earlier derived relationship, eq , with γCO = 0.84 and x = 0.06 as discussed above, the amount of carbon dioxide taken up can be estimated. With annual 12 Mm3 applications, for periods of 1, 5, 10, and 25 years, the CO2-capturing rate would increase from ca. 2.5 Mton CO2 year–1 to a peak value of ca. 9 Mton CO2 year–1 after 25 years of coastal olivine application (Figure B). This would be the equivalent of 5% of The Netherlands’ yearly 170 Mton CO2 emissions (2013 value; http://data.worldbank.org/indicator/). Once the application stops, the remaining olivine will dissolve in about 250 years, with decreasing yearly CO2 uptake rates (Figure B). The long time scale over which ESW is effective has two important implications. First, the process of issuing and validating carbon credits for ESW will need to take into account that CO2 sequestration is not immediately realized at once but stretched out over a century-scale time window. Second, given the long-lasting effects, any potential ecosystem impacts need to be properly assessed and evaluated upfront in small-scale field trials before large-scale ESW application can start.

Environmental Implications

From an ecological perspective, the potential secondary effects of (large-scale) olivine dissolution are a critical issue. Although dilution processes in marine coastal environments will likely prevent accumulation to toxic levels of dissolution products, it is important (and obligatory in e.g. the European Union) to perform upscaling calculations of dissolution product concentrations and their conceivable effects on the marine ecosystem. The main consequences of forsteritic olivine dissolution are increases in Mg2+, Si, TA, DIC, Fe2+, and Ni2+, and their ecosystem effects should be thoroughly assessed. In addition, the geophysical consequences of olivine distribution in coastal ecosystems should be assessed, such as the increase in suspended particulate matter, sediment pore space clogging and smothering effects due to the higher specific density of olivine. While increases in alkalinity and DIC are a desired effect for climate engineering purposes, the increase in Mg2+ is not expected to pose a significant threat because of the high background concentration in seawater. Increases in dissolved Si and dissolved Fe can stimulate primary production and thus lead to additional CO2 sequestration, as recently assessed by model analysis[55]· However, the ultimate impacts on coastal foodwebs of fertilizing by olivine dissolution are uncertain and need further investigation.

The impact of increased nickel flux on marine ecosystems is a matter of potential concern, and has only been scarcely touched upon. Nickel leaches from the olivine mineral matrix in its ionic Ni(II) form. Dissolved nickel occurs in trace concentrations in seawater (0.03–0.16 μmol kg–1;[27]), as low as 0.002–0.006 μmol kg–1 in the central southern North Sea and up to 0.04 μmol kg–1 in the Rhine delta area.[28] In comparison, background nickel concentrations in the control treatments ranged between 0.14 μmol kg–1 in the FSW in experiment A3 and 0.45 μmol kg–1 in experiment A1, while background Ni concentrations in the artificial seawater media in experiment A3 (ASW, ASW-Ca, and ASW-CaMg) were an order of magnitude lower, between 0.017 and 0.032 μmol kg–1.

The ecotoxicology of nickel in marine organisms and ecosystems is summarized on the Web site of the UK Marine Special Areas of Conservation (http://www.ukmarinesac.org.uk/) and established for the UK at a chronic concentration of 0.25 μmol L–1. Nickel toxicity has been reported in a number of cases:[56−58] negative effects on spawning in mysiid shrimps at 2.4 μmol L–1, DNA damage with associated physiological and cytotoxic effects in the blue mussel Mytilus edulis at 0.3 μmol L–1, disrupting ionoregulatory functions in the green crab Carcinus maenas between 8.5 and 51 μmol L–1 in very low-salinity seawater (0.006 PSU), and organ oxidative stress in the killifish Fundulus heteroclitus, also mainly in freshwater. However, one of the conclusions of Blewett et al.[57] and Blewett and Wood[58] is that higher, seawater-like salinities (e.g., 30–38) seem to be negatively correlated with Ni-induced effects. In general, higher salinities are inversely correlated with Ni2+ seawater concentrations.[59] Although bioaccumulation of nickel in individual organisms occurs, there seems to be little evidence of biomagnification throughout (marine) foodwebs,[27] although Kumblad et al.[60] present results that suggest the contrary. The potential toxicity of nickel, combined with rather large uncertainties about the magnitude and direction of its response effects, make it paramount to further investigate its ecotoxicological effects within the framework of large-scale application of olivine in coastal environments.

Containment is not an issue for ESW. Before any field-scale application, there should be proper field trials in quasi-contained conditions, such as mesocosm setups, which can be upscaled in, e.g., tidal harbor basins. In the case that a mesoscale field trial (∼100 m2) would be undertaken, common dredging equipment would be used to apply the olivine into the (coastal) environment. The same equipment and expertise can be used to remove the olivine sand, should any acute unforeseen situation develop.

The CO2 sequestration induced by ESW is governed by the acid–base thermodynamics of seawater, which are well-understood,[26,37,38] therefore rendering the containment of CO2 in the ocean highly predictable. The central premise of ESW is that it increases the ocean’s alkalinity, enabling more CO2 to be dissolved into seawater at any given pCO2 compared to the situation in which no alkalinity is added to the ocean. This CO2 will be stay dissolved in the ocean (or contained) as long as no other process changes the alkalinity of the ocean. In the ESW, the CO2 storage reservoir (the ocean) is an open system, as CO2 can be freely exchanged between atmosphere and ocean across the air–sea interface. Because leakage cannot occur in an open system, storage of CO2 in the ocean is therefore leakage-proof. The evidence that CO2 will be contained for long periods of time is given by observations on the long-term (>1000 year) carbon cycle and the impact of natural silicate weathering: the long-term fate of fossil CO2 is to be absorbed in the ocean.[61]

If ESW is applied to coastal systems in a geo-engineering framework, it will be crucial to determine in situ olivine dissolution rates to determine the efficiency of the method.[42] Once in the natural sediment, the olivine will be subject to very different biogeochemical and geophysical conditions. Microbial mineralization processes could greatly increase the CO2 concentration in the sediment’s pore waters,[62] while benthic macrofauna process vast quantities of sediment for their sustenance and mobility.[63,64] These processes are likely to speed up the dissolution process within marine sediments. Large-scale sediment transport and wave action are expected to cause constant particle abrasion and faster mechanical weathering, in turn facilitating faster chemical weathering. If ESW is ever to be applied in a geo-engineering framework, it is of paramount importance to investigate the effects of all of these natural processes on the dissolution of olivine in coastal environments.