Cameron Wood
email: cameron.wood1@ucalgary.ca

Anna L Harrison
email: anna.harrison@get.omp.eu

Ian M Power

Impacts of dissolved phosphorus and soil-mineral-fluid interactions on CO2 removal through enhanced weathering of wollastonite in soils

Keywords: wollastonite, enhanced rock weathering, CO2 removal, mineral-soil interactions, negative emissions technologies

The weathering of silicate minerals removes carbon dioxide (CO2) from the atmosphere over geologic timescales and is also investigated as an engineered strategy to mitigate climate change on decadal timescales. "Enhanced rock weathering" (ERW) is a carbon dioxide removal strategy that involves spreading of pulverized, highly reactive silicate rock at the Earth's surface such as within agricultural and natural soils. The rate and efficacy of ERW in agricultural soils to remove CO2 is difficult to quantify owing to the complex geochemical environment including biological-mineral-fluid-atmosphere interactions and human interventions that occur. Column experiments containing wollastonite [CaSiO3] and soils were conducted to investigate the geochemical processes that remove CO2 during ERW, and to determine the potential impact of P-bearing fertilizers on CO2 removal. Columns containing wollastonite, soil, or 8:1 soil-wollastonite mixtures were irrigated with de-ionized water or a 1 × 10 -4 M K3PO4 solution. Analysis of column effluent concentrations indicated that P was rapidly removed from solution and retained within the columns, with little difference in weathering rate between columns irrigated with water and those receiving the K3PO4 solution. Bulk wollastonite weathering rates were significantly enhanced in the presence of soils compared to columns with wollastonite alone. Cation concentrations in effluent from columns with soil-wollastonite mixtures were strongly impacted by adsorption to soil surfaces, complicating efforts to estimate CO2 capture. Based on wollastonite weathering rates estimated by Si release, a maximum of 255 kg CO2 t -1 amendment yr -1 was captured in the soil-wollastonite mixture columns. The majority of CO2 stored in the soil-bearing columns was most likely soil-generated rather than removed directly from the atmosphere. The results of this study indicate that adsorption to soils may limit the ability of P to enhance wollastonite weathering, that silicate weathering behaviour within soils differs substantially from that of pure mineral phases, and that CO2 budgets must be carefully constrained to assess the source of stored CO2. Moreover, leachate concentrations of cations and HCO3 -may not serve as accurate indicators of the extent of silicate weathering, at least over the short term, due to the numerous processes that control their concentrations in soils, such as sorption to soil components.

Introduction

Carbon dioxide removal (CDR) technologies will most likely be necessary alongside the reduction of CO2 emissions to limit the global temperature increase to < 1.5 °C above pre-industrial levels and avoid the worst consequences of climate change [START_REF] Ipcc | Global warming of 1.5 o C. An IPCC Special Report on the impacts of global warming of 1[END_REF]. Enhanced rock weathering (ERW) is a CDR technology that takes advantage of the natural process by which atmospheric CO2 is removed during silicate mineral weathering but accelerates natural reaction rates to provide CO2 removal on the much faster timescales that are necessary to address climate change (National Academies of Sciences, 2019; [START_REF] Schuiling | Enhanced weathering: an effective and cheap tool to sequester CO2[END_REF]. ERW involves the dissolution of pulverized, high surface area alkaline earth metal-bearing silicate or hydroxide minerals in the presence of CO2, which generates alkalinity, removes CO2 from the atmosphere in dissolved form, and releases cations like Ca and Mg [START_REF] Andrews | Combating climate change through enhanced weathering of agricultural soils[END_REF][START_REF] Beerling | Potential for large-scale CO2 removal via enhanced rock weathering with croplands[END_REF][START_REF] Daval | Carbon dioxide sequestration through silicate degradation and carbon mineralisation: promises and uncertainties[END_REF]National Academies of Sciences, 2019;[START_REF] Power | Carbon mineralization: from natural analogues to engineered systems[END_REF][START_REF] Schuiling | Enhanced weathering: an effective and cheap tool to sequester CO2[END_REF]. ERW captures CO2 in dissolved form as carbonate alkalinity that is transported eventually to the oceans, and in some cases also stores CO2 in solid form as carbonate minerals (e.g., calcite [CaCO3]) (National Academies of Sciences, 2019; [START_REF] Power | Carbon mineralization: from natural analogues to engineered systems[END_REF].

The application of alkaline silicate minerals to agricultural fields as soil amendments is one method for implementing ERW [START_REF] Zhang | Review and outlook for agromineral research in agriculture and climate mitigation[END_REF]. The acidic root exudates emitted by some crops may also promote silicate dissolution, releasing additional nutrients for plant consumption [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF]. The high dissolution rate, high Ca content per unit mass, and simple chemical composition of wollastonite [CaSiO3] make it an attractive option both for CO2 storage and for studies of silicate dissolution rates and mechanisms [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF][START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF]. In the case of wollastonite weathering, CO2 removal occurs according to Equation 1. 𝐶𝑎𝑆𝑖𝑂 3 + 2𝐶𝑂 2 + 3𝐻 2 𝑂 → 𝐶𝑎 2+ + 𝐻 4 𝑆𝑖𝑂 4 + 2𝐻𝐶𝑂 3 -Equation 1From the alkalinity and Ca released from the silicate minerals, carbonate minerals such as calcite can form (according to Equations 2 and 3):

𝐻𝐶𝑂 3 -↔ 𝐶𝑂 3 2-+ 𝐻 + Equation 2 𝐶𝑎 2+ + 𝐶𝑂 3 2-↔ 𝐶𝑎𝐶𝑂 3 Equation 3
In cases where silicate weathering occurs due to an acid other than CO2, such as nitric acid from N-bearing fertilizers, no atmospheric CO2 is stored [START_REF] Andrews | Combating climate change through enhanced weathering of agricultural soils[END_REF][START_REF] Hartmann | Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification[END_REF]. However, the impacts of ligands in soil pore waters, like dissolved organics and phosphorus, on CO2 removal during wollastonite dissolution are not well understood. Microbial activity within soils can also result in release of CO2 to the atmosphere as organic C is consumed and expelled as CO2, though the organic C may initially have been atmospheric CO2. Some of the CO2 released by biological processes could be retained as dissolved inorganic carbon (DIC), resulting in no net change in atmospheric CO2. The many interrelated processes that occur during ERW in soils make tracing the source and fate of CO2 and thus quantifying the efficacy of CDR during ERW in soils highly complex.

It has been estimated that by 2050, global agricultural adoption of ERW could remove up to 2.0 Gt of CO2 per year [START_REF] Beerling | Potential for large-scale CO2 removal via enhanced rock weathering with croplands[END_REF]. In addition to CO2 capture, silicate soil amendments could potentially provide further environmental and agricultural benefits. Silicate mineral amendments can combat soil acidification by raising soil pH and supplying Ca, Mg, Si, and various micronutrients to soils [START_REF] Harley | Factors influencing the release of plant nutrient elements from silicate rock powders: A geochemical overview[END_REF]. Soil Si depletion is a concern in areas of intensive agriculture and could necessitate direct Si fertilization [START_REF] Tubana | A review of silicon in soils and plants and its role in US agriculture: history and future perspectives[END_REF]. Silicate soil amendments thus have the potential to simultaneously enhance soil productivity and remove atmospheric CO2 [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF].

Experimental studies investigating the dissolution of silicate soil additives have used olivine [Mg2SiO4] [START_REF] Pogge Von Strandmann | The Dissolution of Olivine Added to Soil at 4°C: Implications for Enhanced Weathering in Cold Regions[END_REF][START_REF] Renforth | The dissolution of olivine added to soil: Implications for enhanced weathering[END_REF], basalt [START_REF] Anda | Increasing negative charge and nutrient contents of a highly weathered soil using basalt and rice husk to promote cocoa growth under field conditions[END_REF][START_REF] Anda | Improving chemical properties of a highly weathered soil using finely ground basalt rocks[END_REF][START_REF] James | Carbon Dioxide Removal via Enhanced Rock Weathering With Agriculture in Large-Scale Field Trials[END_REF][START_REF] Mcdermott | An investigation of waste basalt (quarry dust) as a soil amendment to sequester atmospheric CO2[END_REF][START_REF] Ten Berge | perenne L.): a pot experiment[END_REF][START_REF] Kelland | Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil[END_REF], dolerite [START_REF] Manning | Carbonate precipitation in artificial soils produced from basaltic quarry fines and composts: An opportunity for passive carbon sequestration[END_REF], dunite [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF], and wollastonite [START_REF] Haque | CO2 sequestration by wollastonite-amended agricultural soils-An Ontario field study[END_REF][START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF][START_REF] Peters | Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1[END_REF][START_REF] Shao | Long-term responses in soil solution and stream-water chemistry at Hubbard Brook after experimental addition of wollastonite[END_REF][START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF]. In a large-scale field experiment where pelletized wollastonite was applied to a 11.8 ha watershed, a 50% increase in stream water concentrations of Ca and Si over a period of 1 to 2 years was observed [START_REF] Peters | Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1[END_REF][START_REF] Shao | Long-term responses in soil solution and stream-water chemistry at Hubbard Brook after experimental addition of wollastonite[END_REF]). An upper estimate for the CO2 captured by wollastonite weathering in this watershed was calculated to be 0.22 kg CO2 t amendment -1 yr -1 [START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF]. Bench-scale experimental studies showed that in the presence of legumes, wollastonite weathering can result in CO2 storage by CaCO3 precipitation that is enhanced up to 10 times compared to soils without wollastonite [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF]. A field study revealed up to 224-4800 kg CO2 t amendment -1 yr -1 was sequestered as solid inorganic carbon (SIC) in a wollastoniteamended field growing leafy vegetables, with the greatest CO2 sequestration achieved in a field that had received both wollastonite and monoammonium phosphate fertilizer [START_REF] Haque | CO2 sequestration by wollastonite-amended agricultural soils-An Ontario field study[END_REF].

Phosphorus-bearing fertilizers are commonly applied in agricultural settings, where interactions between dissolved phosphorus and mineral soil amendments (such as wollastonite) may take place. In a study that supplied 118 kg P2O5 ha -1 yearly to soils, soil porewater P concentrations up to 0.170 mM were measured [START_REF] Sigua | Impacts of irrigation scheduling on pore water nitrate and phosphate in coastal plain region of the United States[END_REF]. This is near to or less than application rates that have been recommended for certain crops in some U.S.

states [START_REF] Rosen | Optimizing Phosphorus Fertilizer Management in Potato Production[END_REF]. This suggests that in many agricultural areas, soil porewater P concentrations may be in excess of the 0.03 mM H2PO4 2-shown to triple the dissolution rate of wollastonite at 25 °C and neutral pH [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. If soil porewater P significantly enhances the dissolution of wollastonite, it could represent an important factor affecting release rates of Ca and Si, impacting CO2 storage from ERW in soils. Although the potential impact of N-bearing fertilizers on ERW has been previously discussed [START_REF] Andrews | Combating climate change through enhanced weathering of agricultural soils[END_REF][START_REF] Hartmann | Enhanced chemical weathering as a geoengineering strategy to reduce atmospheric carbon dioxide, supply nutrients, and mitigate ocean acidification[END_REF], the impact of P-bearing fertilizers on ERW rates is yet to be fully understood. Prior research on the interactions between phosphorus and silicate minerals within soils has largely focused on the availability of P for uptake by plants [START_REF] Mkhonza | Effects of lime application on nitrogen and phosphorus availability in humic soils[END_REF][START_REF] Ipcc | Global warming of 1.5 o C. An IPCC Special Report on the impacts of global warming of 1[END_REF]. Silicon was found to compete with P for soil adsorption sites and to increase the availability of P in solution [START_REF] Lee | Reduction of phosphate adsorption by ion competition with silicate in soil[END_REF][START_REF] Obihara | Specific adsorption of silicate and phosphate by soils[END_REF][START_REF] Roy | Influence of calcium silicate on phosphate solubility and availability in Hawaiian Latosols[END_REF]. Geochemical modeling on the watershed scale has also indicated that wollastonite could potentially aid in remediation of P-rich agricultural runoff through formation of hydroxylapatite [Ca5(PO4)3(OH)] [START_REF] Hayder | Phosphorous runoff risk assessment and its potential management using wollastonite according to geochemical modeling[END_REF]. However, the potential of P-bearing fertilizers to accelerate the dissolution of silicates within soils and to contribute to ERW and CDR has not previously been investigated, to the best of our knowledge. In this study, a series of small-scale (10 cm tall by 1.95 cm diameter and 30 cm 3 volume) laboratory column experiments were conducted to evaluate the influence of soil-fluid-mineral interactions and the presence of dissolved P on the weathering of wollastonite and CO2 removal. The objectives of this study were: 1) to evaluate the influence of soil-fluid-mineral interactions on elemental fluxes during ERW, 2)

to quantify the weathering rates of wollastonite in soils, and the impact of the presence of P in particular, and 3) to contribute to the understanding of processes impacting CO2 removal during ERW in soils.

Materials and Methods

Experimental design

Wollastonite used in the column experiments was obtained from the Canadian Wollastonite Mine located 34 km northeast of Kingston, Ontario, Canada. The wollastonite was sieved to a grain size of 62.5 -125.0 µm and sonicated in ethanol to remove fine particles. Local potting soil with no added fertilizer was purchased from a garden center in Kingston, Ontario. This soil was an in-house blend which contained visible fragments of wood and plant matter. The soil was air dried prior to analysis and column construction.

After drying, the soil was homogenized by hand, and any visible large pebbles or pieces of plant matter were removed. Ten columns were constructed using 30 mL cylindrical syringes filled to the 30 mL mark (10 cm height with 1.95 cm diameter) (Error! Reference source not found.). Two of these columns were filled with wollastonite (32.8 and 34.6 g), two

with soil (12.44 and 12.19 g), and six with an 8 to 1 mixture of soil and wollastonite (approximately 13.81 g total). Columns containing the wollastonite-soil mixture were run in triplicate to assess impacts of heterogeneity in the soil-wollastonite mixture. An 8 to 1 ratio of soil to wollastonite employed in a previous study of wollastonite weathering in agricultural soils was calculated to result in a positive saturation index for calcite [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF]. To inhibit discharge of solids, 63 µm nylon mesh was placed at the base of each column. Tygon tubing with a 4.76 mm internal diameter was affixed to the bottom of each syringe to direct flow to a fluid collection container. Masses of the empty columns, columns filled with dry solids, and columns filled with water saturated solids were recorded. Water was added gradually using a transfer pipette. Columns were considered fully saturated when the mass of the column after added water was no longer present on top of the soil was equal to the column mass prior to the most recent addition of water. Column initial conditions are provided in Table 1. A schematic of the column experiment is provided in Figure . The following nomenclature is used throughout the manuscript: Wollastonite-Water is referred to as WW, Wollastonite-P as WP, Soil-Wollastonite as SW, Soil-P as SP, Mixture-Water replicates as MW (1, 2, 3), and Mixture-P replicates as MP (1, 2, 3).

Sampling Protocol

Effluent discharged rapidly as the columns were irrigated and the entire volume of effluent was collected during each irrigation period. Effluent vials were weighed before and after sample collection such that the mass of fluid discharged at each step could be determined. The mass of each column was also measured before and after each irrigation.

An aliquot of 6 mL of the irrigation effluent was filtered using a 0.20 µm polyethersulfone (PES) syringe filter and acidified to 2% HCl for analysis of cation and P concentrations.

The pH of the unacidified effluent was measured using a WTW portable pH meter model pH 3310 with a SenTix 41 probe which was calibrated daily using pH 4.01, 7.00, and 9.18 buffer solutions. Two milliliters of unacidified, unfiltered effluent was collected in tubes without head space for P and organic/inorganic carbon measurement. After the final irrigation, solids from each of the columns were collected and material from each of the upper, middle, and lower 3-4 cm of each column were separated. The solids in each of the upper, middle, and lower sections was then homogenised. The wet masses of each wollastonite, soil, and mixture segment were approximately 13, 5, and 6 g, respectively.

The solids were dried at room temperature and reweighed after drying to evaluate the saturated water content. Solids were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Solid Analyses

Brunauer-Emmett-Teller (BET) surface area of the cleaned wollastonite was analyzed by Kr adsorption using a Micromeritics Tristar II plus. Analysis of the wollastonite yielded a surface area of 0.142 m 2 /g. The mineralogical composition of the initial solids and select samples after the experiments was analyzed using XRD. Dry aliquots of samples were ground using an agate mortar and pestle with anhydrous ethanol and mounted as a slurry on zero-diffraction quartz plates. The XRD data were collected using a Bruker D2 Phaser diffractometer using a 0.02° step size over a range of 5-80° 2θ at 2 s per step. The patterns were collected using a Lynx Eye XE-T detector coupled with a 0.6 mm divergence slit, and incident and diffracted beam Soller slits. A fine-focus Cu X-ray tube, operated at 30 kV and 10 mA, was used to generate X-rays. Mineral phases were identified using DIFFRACplus EVA V4.3 a search-match software obtaining crystal structure data from the International Center for Diffraction Data PDF 2+ 2020. Organic and inorganic carbon content of solids was analyzed at the Agriculture and Food Laboratory at Guelph University, using a Leco CN828 apparatus. This instrument provides elemental analysis of nitrogen, total carbon, and inorganic carbon (IC) by ashing the sample at 475 °C for 3 h prior to carbon analysis using catalytic combustion (950 °C), separating out the desired measurement components from foreign gases, and analyzing using thermal conductivity detection and infra-red detection. Organic carbon is calculated from the subtraction of the inorganic carbon result from the total carbon result. The limit of carbon detection in soils was 0.005 % for inorganic and total carbon.

Initial solids and certain reacted solid samples were examined using an FEI Quanta 650 Field Emission Gun (FEG) Environmental Scanning Electron Microscope (ESEM).

Samples were analyzed under high vacuum mode with a working distance of 9-12 mm, voltage of 5.00 kV, and a secondary electron detector.

Fluid Analyses

The Ca, Si, and K concentrations in fluid samples from columns WW, WP, SW, SP, MW 1, and MP 1 were analyzed using an iCAP TM TQs triple quadrupole inductively coupled plasma mass spectrometer (TQ ICP-MS). Isotopes were measured in three modes:

single quadrupole with no cell gas, single quadrupole in KED mode (He collision gas) and triple-quadrupole with O2 as the reaction gas, utilizing mass shifting. Dissolved organic and inorganic carbon concentrations (DOC and DIC, respectively) were measured using a Shimadzu TOC-VCSN analyzer. Samples were first acidified to release CO2 from dissolved inorganic carbon. After DIC removal, the sample was heated to 680 °C to transform dissolved organic carbon to CO2, which was quantified as non-purgeable organic carbon with an infrared detector. The quantification limit for DIC was 0.57 ppm, and for DOC was 0.47 ppm. Blanks of two samples of filtered deionized water were analyzed and their average DIC and DOC concentration subtracted as background from samples. Uncertainty in DIC and DOC concentration was better than 1.25 ppm and 1.98 ppm, respectively. However, the presence of solid organic material observed in some samples may impact the precision of DOC results.

Speciation calculations

Aqueous speciation of column leachate and saturation indices of mineral phases that could dissolve or precipitate were determined using PHREEQC Interactive v. 3.4.0 with the LLNL database [START_REF] Parkhurst | Description of input and examples for PHREEQC version 3: a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF]. Data input to PHREEQC consisted of the pH and all measured elemental concentrations for each modeled timepoint, including DIC and concentrations measured by TQ ICP-MS, ICP-OES, and digital colorimetry.

Results

Characterization of Solids

Wollastonite was the only phase detected by XRD in the initial wollastonite samples. The initial wollastonite contained 0.026% inorganic and 0.033% organic C. The initial soil was composed of albite (NaAlSi3O8), quartz (SiO2), and minor amounts of Although it is possible that clay minerals were present in the initial soil, they were not present in sufficient quantity to be detected by XRD. Precise abundances of minerals could not be determined due to analytical limitations related to the large quantity of X-ray amorphous material. Organic and inorganic C contents of the initial wollastonite, initial soil, and initial mixture were each measured for triplicate aliquots of the initial material to assess heterogeneity in C content (Table 2). The soil was rich in organic C (31.1 ± 1.6%, n=3) (Figure 2) and contained 0.363 ± 0.067% (n=3) inorganic C prior to reaction. Less solid C was detected in the mixture than in the soil, owing to its wollastonite content, with average total solid C contents of 0.042 ± 0.012, 26.7 ± 0.8, and 31.4 ± 1.6% (n=3) for the initial wollastonite, initial mixture, and initial soil, respectively. Detection limit for inorganic and organic carbon was 0.005%.

b Values presented are an average (n = 3) with error values indicating standard deviations. Standard deviations of the triplicate measurements of initial materials are also used for final solids as there was insufficient final solid for triplicate analyses.

After the experiments, no secondary phases nor substantial changes to initial mineralogy were evident in the reacted solids based on XRD results (Figure S2 in SM).

Dolomite was detected in the reacted mixture columns, despite not being detected in the initial mixture, suggesting it may be heterogeneously distributed (Figure S2 in SM). Calcite remained in all soil and mixture columns after the experiments (Figure S2 in SM). A decrease in the relative intensity of wollastonite peaks in the reacted mixtures compared to the initial mixtures may reflect the dissolution of wollastonite within the column.

All wollastonite column segments exhibited a decrease in organic C relative to the initial wollastonite at the end of the experiments (Table 2). The upper section (upper 3-4 cm) of column WW showed a marked increase in solid inorganic C (SIC) from 0.026 % ± 0.002% to 0.064 % ± 0.002%, but an increase was not observed in either the lower section (lower 3-4 cm) of WW or in solids from WP, for which a modest decrease in SIC was observed. Organic C content of the mixture columns varied, with the upper portions of MW 1 and MP 1 containing 2.5-2.7 % more organic C than the initial mixture and the lower portions within uncertainty of the initial mixture (Table 2). This could be caused by heterogeneity in the initial soil used in the mixtures, although the difference in the upper column portions exceeds the uncertainty of the triplicate analyses. The only significant change in SIC in a mixture column segment was observed in the lower portion of MW 1, which showed an increase from 0.294 to 0.955 % (Table 2), which suggests either some accumulation of SIC or potentially large heterogeneity in initial soil carbonates resulting in different initial SIC between mixture columns. However, such substantial heterogeneity was not observed in the measurement of triplicate aliquots of either the initial soil or the initial mixture.

Scanning electron microscopy showed that the initial wollastonite had smooth surfaces (Figure 3

Trends in fluid composition

In all cases, effluent pH values were higher than the input solution pH value of 6 (Figure 5). Effluent pH values were consistently 1.0 to 1.7 units higher in the wollastonite columns compared to mixture columns and 1.4 to 2.3 units higher than the soil columns. A difference in pH between columns receiving water and K3PO4 was only apparent in the soil columns. Effluent pH from the WW and WP columns was between 8.91 to 9.40 and remained fairly constant throughout the experiment (Figure 5 A). The pH values of effluent from column WP were slightly higher than for WW at all but one time point. The effluent from the SW column had a consistently higher pH than the SP column effluent, with each beginning near pH 8 at the beginning of the experiment before declining to approximately stable values of ~7.5 and 6.9, respectively after ~180 h (Figure 5 Calcium concentrations in the wollastonite column effluent were initially 0.57 and 0.90 mM after ~ 1 h but declined to a stable value between 0.36 and 0.58 mM in the WW and WP column effluents, respectively. Effluent from column WP consistently displayed higher Ca concentrations than effluent from column WW, in contrast to the behaviour of Si between these columns (Figure 5 C). Initial Ca concentrations in the soil column effluent were 0.43 and 0.49 mM for columns SW and SP, respectively, but declined to ~0.2 mM within 100 h before gradually increasing to a fairly stable plateau at 0.27-0.33 mM by 500 h. Calcium concentrations in the mixture columns showed a similar trend as the soil columns over time, with a rapid decline to a minimum followed by a more gradual increase after 100 h. The Ca concentrations in the mixture columns were generally greater than those in the soil columns and reached higher values than in the pure wollastonite columns. In both the soil and mixture columns, there was no clear impact of irrigation with water versus K3PO4 on Ca concentrations. The total moles of Ca and Si exported from the wollastonite columns was comparable to the quantity exported from the mixture columns despite a much smaller amount of wollastonite present in the mixture columns (Table 3). Despite differences between the wollastonite columns receiving water and the K3PO4 solution being apparent, the effects on Ca and Si were opposite, making it difficult to discern whether dissolution of wollastonite was enhanced in the presence of P. The impact of P on the mixture columns also differed for Ca and Si, with more Ca discharged from the mixture columns receiving water and more Si discharged from the mixture columns receiving the K3PO4 solution. This impact is opposite from that observed in the wollastonite columns. Column WW discharged 43 % more Si and 13 % less Ca than column WP normalized to initial wollastonite mass.

On average, mixture columns receiving water discharged 9 % more Ca than mixed columns receiving K3PO4, but the mixture columns with P discharged 13 % more Si than those with water. The mixture columns receiving water and K3PO4, respectively, discharged an average of 202 % and 332 % more Ca and an average of 243 % and 196 % more Si, normalized to initial wollastonite mass, than the respective wollastonite columns receiving the same inlet fluid. In all columns containing wollastonite, the Si:Ca ratio of column effluent was > 1 throughout the experiment, except in samples from column MW 2 after 600 h. The Si:Ca ratio of the mixture columns and of column WP declined over time, approaching 1:1 by the end of the experiment (Figure S3 in SM). In all columns receiving K3PO4, less than 22 % of P added to columns through irrigation was discharged. In all experiments, the effluent remained well below the influent concentration throughout the experimental duration (Figure 6). There was some initial P discharge from the MW 2 and 3 columns, and P concentration in the MP 2 and 3 column effluent gradually increased over time. Phosphorus detected in effluent from soil columns was near the detection limit. Phosphorus concentrations for the MW 2 and 3 and MP 2 and 3 columns were measured by TQ ICP-MS as opposed to digital colorimetry, which was used for P concentrations in columns WW, WP, MW 1, and MP 2. The detection limit for P is lower with ICP-MS, at 0.00016 mM as opposed to 0.0032 mM for digital colorimetry.

This is likely why P was detected in the effluent from the mixture columns but not the soil columns. The greatest amount of DIC was exported from the mixture columns compared to the wollastonite and soil columns (Figure 8). The maximum amount of DIC exported was from MP 3, with 1.10 mmol total DIC exported over the experimental duration (Figure 8).

There were variations in total DIC export between triplicate mixture columns, which ranged from 0.68 to 0.96 mmol and 0.84 to 1.10 mmol in the water and K3PO4 columns, respectively. No substantial difference in DIC export was documented between mixture columns irrigated with K3PO4 and those irrigated with water (Figure 8). Export of DIC from column WP was 0.002 % greater than column WW, but differences in DIC export between the mixture columns was within measurement uncertainty (Figure 8). Both wollastonite columns exhibited the least DIC export overall (Figure 8). The calculated pCO2 assuming equilibrium with the measured DIC was significantly greater in the soil and Columns therefore received approximately 0.056 mmol of DIC due to irrigation throughout the experiment. This equates to 16.45, 12.48, 10.14, 12.96, 6.87, and 6.11 % of the DIC discharged from the WW, WP, SW, SP, MW average, and MP average columns respectively.

For the majority of the duration of the experiment, the concentrations of most elements were the same within experimental uncertainty for the mixture-water and mixture-P columns based on the standard deviation between measurements from the triplicate experiments. The implication is that the P-treatment at the applied rate and concentration had no significant impact on wollastonite weathering rates in the wollastonite-soil mixture columns over the month-long experimental duration. The lack of replicates for the wollastonite only columns means that no concrete statistics can be calculated pertaining to the effect of P on wollastonite in the wollastonite only columns. However, there was little difference in effluent concentrations between the wollastonite column receiving the Ptreatment and that receiving water, consistent with the lack of impact of P on weathering observed in the mixture columns.

Because the columns received high levels of irrigation, with some columns receiving up to 35% more irrigation relative to their pore volume than others, effluent samples from the wollastonite and soil columns may have experienced dilution relative to the mixture columns. However, the degree of potential dilution and its relevance could not be robustly quantified for the present experiment. The average loss in column mass between irrigation sessions, which may be considered equivalent to the evaporation that took place between each irrigation, was equal to 4% ± 0.46% of the water content of the columns.

Changes in water content and thus water-filled pore volume and evapoconcentration are thus not expected to have substantially affected measured concentrations or the mineralfluid interface.

Trends in mineral saturation states

The fluids were undersaturated with respect to wollastonite throughout the experiment in all columns, indicating dissolution of wollastonite was possible throughout the experiment in the bulk fluid (Figure S6 in SM). Calcite was supersaturated in both pure wollastonite columns throughout the entire experimental duration. Column WP was slightly more supersaturated with respect to calcite than column WW (SIcalcite = ~0.9 compared to ~0.6). Calcite was consistently undersaturated in both soil columns, indicating antecedent calcite in the soil was favoured to dissolve rather than precipitate. In most of the mixture columns (except MW 1), calcite was undersaturated at the beginning of the experiment, but became supersaturated after 200 h and its degree of supersaturation continually increased throughout the experiment. Magnesium concentrations were only available for the mixture columns and the SI of dolomite displayed a similar trend to that of calcite and remained supersaturated throughout the experiments, with no clear difference between columns receiving water and K3PO4 solution. Amorphous silica was undersaturated for the entirety of the experiment in both the wollastonite and soil columns. In all mixture columns, amorphous silica remained near equilibrium for the duration of the experiment, with moderately higher SIs in the MP than the MW column effluent.

Hydroxylapatite reached supersaturation in all columns that contained wollastonite and received the K3PO4 solution. Hydroxylapatite was supersaturated for the entirety of the experiment in column WP. Quartz was initially supersaturated in the soil columns but became undersaturated by 100 h, where it is initially present. In the mixture columns, quartz was supersaturated at all time points. Polymorphs of SiO2 (chalcedony, coesite, cristobalite, and tridymite) were indicated to be supersaturated in the wollastonite and mixture columns.

However, formation of coesite, cristobalite, and tridymite requires elevated temperature/pressure conditions [START_REF] Swamy | A thermodynamic assessment of silica phase diagram[END_REF]. Although formation of chalcedony has been documented at temperatures < 100 °C, it is typically associated with more elevated temperatures than the experimental temperature in the present study (25 °C) [START_REF] Heaney | A proposed mechanism for the growth of chalcedony[END_REF].

Discussion

Processes controlling elemental fluxes

Several processes may impact the release of elements during enhanced weathering of wollastonite, including dissolution of wollastonite itself, dissolution of the antecedent minerals in the soil, adsorption and desorption from mineral and organic surfaces, and precipitation of secondary phases. To evaluate the efficacy of wollastonite weathering for CO2 storage, it is necessary to quantify the extent of weathering of wollastonite and resultant element fluxes (Ca, Si, DIC). The trends in release of elements from the soil and wollastonite columns can shed light on the extent to which elements detected in the mixture column effluent may have originated from the soils themselves, and to what extent the soils impacted the weathering of wollastonite. However, the geochemical conditions evolved differently in the wollastonite and soil end members compared to the mixtures, implying that the soil endmember cannot be treated simply as "background," with element fluxes subtracted from those from the mixtures.

In the wollastonite columns, dissolution of wollastonite was the only source of Si and Ca. Although stoichiometric dissolution of wollastonite results in equivalent moles of Si and Ca being released, the ratio of Si to Ca exported was consistently > 1 for columns WW and WP, respectively. Because amorphous silica was undersaturated in the bulk fluid throughout the experimental duration in the wollastonite columns, and aqueous Si is predominantly present as a neutrally charged species (H4SiO4) at the experimental pH, it is likely that Si behaved conservatively in the wollastonite columns. Stoichiometric dissolution of wollastonite is expected over the pH range measured in the experiments [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF], which implies that Ca was significantly retarded within the wollastonite columns. Retention of Ca in the wollastonite columns could be due to formation of calcite or due to sorption of Ca. Calcite was supersaturated throughout the experimental duration in the wollastonite columns, indicating that calcite formation was possible. Although calcite was not detected by XRD in the wollastonite columns, the SIC content of the upper portion of column WW was observed to increase, suggesting that there was likely some precipitation of calcium carbonate in the column. The increase from 0.025 to 0.064 % inorganic carbon in the upper portion of column WW, if applied to the entire column could represent up to 0.13 mmol of Ca incorporated into solids as CaCO3.

Precipitation of hydroxylapatite could also incorporate Ca and P in columns receiving P, although no phosphate minerals were detected by XRD or by SEM imaging. The fact that precipitation of calcium carbonate does not account for 100% of the difference in Ca and Si export suggests that sorption of Ca could also have occurred in WW. Wollastonite surfaces are negatively charged at pH > 2.6, favouring cation adsorption [START_REF] Panday | Use of wollastonite for the treatment of Cu (II) rich effluents[END_REF], and cations have been observed to adsorb to wollastonite surfaces at circumneutral pH [START_REF] Sharma | Use of wollastonite in the removal of Ni (II) from aqueous solutions[END_REF], comparable to the pH range in the columns of 6.85 to 9.35. Adsorption of Ca to wollastonite surfaces could in part account for its retention in the wollastonite columns.

The removal of Ca relative to Si was not observed in the wollastonite dissolution mixedflow experiments of [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF], but this may be due to the much lower solid:fluid ratio in mixed-flow experiments compared to columns, and the continuous refreshing of inlet solution, compared to the periodic refreshment of the inlet solution in the present study. Although the aqueous P species at the experimental conditions are negatively charged or neutral, P may still form surface complexes with wollastonite, as occurs when H2PO4 2-promotes the dissolution of wollastonite at pH 2 to 9 [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. The formation of surface complexes could limit the mobility of P in solution in the wollastonite columns. A sorption capacity of 123.31 mg P kg -1 wollastonite [START_REF] Fondu | Phosphate sorption capacities of different substrates in view of application in water treatment systems for ponds[END_REF] could potentially capture > 5 mmol P, which exceeds the 1.24 mmol P estimated to be retained within the column. If all P in column MP 2 were removed through precipitation of hydroxylapatite, it would correspond to a mass of 0.623 g, or 4.36% of the dry column mass. This mass would be detectable by XRD, suggesting that sorption rather than precipitation was the dominant process controlling P retardation.

The results of PHREEQC calculations indicate that HCO3 -was the dominant species at all time points in the wollastonite columns and mixture columns (Figure S7 in SM). If every mole of DIC, which is dominantly HCO3 -, corresponds to 0.5 moles of wollastonite dissolved (Equation 1), 0.17 and 0. An integrated calculation of Si and Ca release from the soil columns indicates that the soils exported up to 0.101 and 0.052 mmol of Ca and Si, respectively during the experiment. This soil Si and Ca could have originated from the dissolution of Si and Cabearing phases in the soils, such as albite and calcite, or from release of Si and Ca ions adsorbed to mineral and organic surfaces. Although Si is present primarily as a neutrally charged complex (excluding organic Si-complexes) under these pH conditions, its potential to adsorb to and desorb from soils [START_REF] Haynes | Silicate sorption and desorption by a Si-deficient soil-Effects of pH and period of contact[END_REF] and competition with phosphate for adsorption sites has been documented [START_REF] Lee | Reduction of phosphate adsorption by ion competition with silicate in soil[END_REF][START_REF] Obihara | Specific adsorption of silicate and phosphate by soils[END_REF][START_REF] Roy | Influence of calcium silicate on phosphate solubility and availability in Hawaiian Latosols[END_REF]. Quartz was undersaturated for the majority of the experiment in the soil columns, indicating it could dissolve and release the observed Si. The SI of albite could not be determined because Al concentrations of the column effluent were not measured. The rapid decline in effluent Si concentrations suggests that aqueous Si was derived primarily from ion desorption and the available exchange sites were quickly exhausted. The only Ca-bearing mineral phases detected by XRD in the soils were minor amounts of calcite and dolomite, and possibly albite if Ca were present in the crystal structure. Calcite was undersaturated throughout the entirety of the experiment in the soil columns, but the SI of dolomite could not be determined as Mg concentrations were not measured. The decrease in effluent Ca concentrations over time is consistent with desorption of Ca ions from soil surfaces and/or a decreasing amount of dissolution of soil calcite over time. The removal of P in the soil column to below the limit of detection (0.00656 M) and the fact that hydroxylapatite was consistently undersaturated, suggests that P was removed from the solution through adsorption. Notably, it is possible for P to adsorb to clay minerals at neutral to alkaline pH despite negatively charged soil surfaces favouring adsorption of cations [START_REF] Lee | Reduction of phosphate adsorption by ion competition with silicate in soil[END_REF][START_REF] Obihara | Specific adsorption of silicate and phosphate by soils[END_REF][START_REF] Roy | Influence of calcium silicate on phosphate solubility and availability in Hawaiian Latosols[END_REF].

Inorganic C fluxes from the soil columns may have been sourced from dissolution of calcite or dolomite, dissolved atmospheric CO2, or organic C converted to DIC. The much higher pCO2 in the soil columns compared to atmospheric pCO2 suggests the DIC was derived primarily from the soils.

Silicon released to solution in the mixture columns may have been derived from the above-described processes documented in the soil columns, in addition to dissolution of wollastonite. Quartz was supersaturated in all mixture columns, thus would not contribute to the Si released. The saturation index of albite could not be determined, so it is unknown to what extent albite contributed to the Si release. However, the dissolution rate constant of albite is > 600-fold lower than wollastonite at the pH conditions of the experiment [START_REF] Palandri | A compilation of rate parameters of water-mineral interaction kinetics for application to geochemical modeling[END_REF], so the majority of Si was likely sourced from wollastonite dissolution.

Moreover, the Si concentrations in the mixture columns were much higher than in the soil columns. Precipitation of crystalline Si-bearing phases is not expected in the columns over the short timescale of the experiments. Amorphous silica remained close-to-equilibrium throughout most of the experimental duration in the mixture columns, suggesting precipitation of amorphous silica was possible. However, the high Si:Ca ratio relative to stoichiometric wollastonite dissolution suggests significant precipitation of Si is unlikely.

Therefore, the main process controlling Si release in the mixture columns is most likely the dissolution of wollastonite.

Although initially undersaturated within the mixture columns, the fluid became supersaturated with respect to calcite within approximately 200 h and the calcite saturation index continued to increase throughout the experimental duration. The SIC content of MW 1 and MP 1 (other mixture columns were not measured) was observed to increase between the initial and final solids, suggesting calcite may have precipitated. Calcite was therefore likely a more significant sink than source of Ca in the mixture columns. Adsorption to soil surfaces was also likely a sink for Ca within the mixture columns. Similar to what was observed in the wollastonite columns, more Si than Ca was discharged in all mixture columns for at least the first 600 h of the experiment. After 600 h, Si and Ca concentrations began to approach parity. The rising Ca concentrations between 100 h and 400 h followed No clay minerals were identified by XRD in the soils used in the columns.

However, the initial soil material contained 29.0 % organic carbon. With this abundance of organic C, a CEC of 1015 meq/kg for the soil is calculated using Equation 4. For an average mixture column soil mass, there is 12.46 meq of CEC, which equates to an adsorption potential of 6.23 mmol of Ca. This adsorption capacity is approximately 10-times greater than the maximum quantity of Si exported (0.632 mmol), which may provide an indication of the amount of Ca released through stoichiometric dissolution, implying that the exchange capacity was likely in excess of that required to account for the non-stoichiometric Ca and Si release. The increase in Mg concentrations towards the end of the experiment is also consistent with ion exchange sites becoming filled over time, in addition to the observed increase in P and K concentration in some column fluids. Although a small increase in SIC was observed in columns MW 1 and MP 1, it is insufficient to account for the gap between Ca and Si concentrations, except for the anomalously high SIC increase observed in column MW 1. Sorption is thus sufficient to explain all Ca retardation within the mixture columns and was likely a more significant sink than precipitation of Ca-bearing minerals over the experimental timescales. The lesser export of Ca than DIC within the wollastonite and mixture columns is consistent with sorption being an important mechanism for the retention of Ca (Figure 9). However, the lower Ca and DIC export relative to stoichiometric wollastonite weathering based on Si concentrations suggests removal of DIC by precipitation may also occur and contribute to Ca removal from solution (Figure 9). The wollastonite and mixture columns approached steady-state conditions based on pH and release of Ca and Si within 600 h. Bulk dissolution rates of wollastonite can be estimated based on the concentrations of Si. After 600 h, concentrations of Si in the columns irrigated with water were 0.88-1.19 mM for the wollastonite column and 1.04-1.38 mM for the mixture columns, respectively. After 600 h in the columns irrigated with K3PO4, Si concentrations were 0.56-0.71 mM for the wollastonite column and 1.09-1.43 mM for the mixture columns. Because Si in the mixture column effluent primarily originates from dissolution of wollastonite, this suggests that bulk wollastonite dissolution rates in the mixture columns receiving K3PO4 were up to twice the dissolution rate of wollastonite in the wollastonite column receiving K3PO4, and dissolution rates of wollastonite columns receiving water were within the range of mixture columns receiving water. However, the 22-times greater mass of wollastonite in the wollastonite column compared to mixture columns implies the surface or mass-normalized dissolution rate of wollastonite was in fact ~22-times greater in mixture columns.

Previous experiments have shown that H2PO4 -enhances the rate of wollastonite dissolution [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF], however, no consistent difference in wollastonite weathering rate in columns irrigated with K3PO4 was evident in any of the columns in the present study. Release of Si was more rapid in the first 400 h of the experiment in the mixture columns receiving K3PO4 compared to the mixture columns receiving water. However, this early Si release may be attributable to lower initial pH in the MP columns compared to the MW columns. The only evidence of enhanced dissolution in column WP compared to WW was the slightly higher pH and greater Ca concentration in column WP than WW. However, Si release was greater in the WW column, inconsistent with a P-enhanced wollastonite dissolution rate. Similarly, despite a ~38% higher total Si export (0.16 mmol) for the average of the triplicate mixture columns receiving K3PO4 compared to those receiving water, Ca release was approximately equal. Although Ca-bearing precipitates may have formed in the mixture columns receiving K3PO4, evidence of their presence was not observed. The similar concentrations of elements measured for the MW and MP columns, within experimental uncertainty, suggests that the P treatment did not significantly enhance the dissolution of wollastonite within the mixture columns. Although differences in concentration of Si, Ca, and DIC for the wollastonite columns with and without P supplied are outside of analytical uncertainty, it cannot be concluded whether these small differences were the result of P, due to the lack of replicate experiments for these conditions. However, because the mixture columns display no substantial impact of P on weathering rate, coupled with the low P concentrations relative to those required to significantly accelerate wollastonite dissolution [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF], it is unlikely that P significantly affected weathering rates in the wollastonite columns.

The lack of impact of P on wollastonite weathering is likely due to the rapid removal of P from solution in all columns, limiting its capacity to enhance weathering rates (Figure 6).

Moreover, the speciation of P in solution is likely to govern its influence on dissolution rate, as not all dissolved P species enhance dissolution rates depending on the type of surface complex that forms [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF] documented that H2PO4 -enhanced the dissolution of wollastonite but suggested that other dissolved P species may not equally enhance, or could inhibit, dissolution based on the interactions that have been documented with brucite and goethite. Surface complexes that bond to only one metal are predicted to promote wollastonite dissolution to a far greater extent than bi-or multidentate complexes with more than one nucleus [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. The species H2PO4 - forms monodentate surface complexes that promote wollastonite dissolution. Speciation calculations indicate that despite declining concentrations at elevated pH, approximately 16% of P would still have been present as H2PO4 -in the mixture columns receiving K3PO4, potentially enhancing the dissolution rate of wollastonite (Figure S8 in SM). In the wollastonite columns, in which pH was consistently > 9, less than 2% of the P would be present as H2PO4 -, limiting its impact on rate (Figure S8 in SM). The pH of soil pore water will therefore play a strong role in dictating the effect of dissolved P on wollastonite dissolution in enhanced weathering scenarios through its impact on both aqueous speciation and removal from solution by adsorption. The mixture columns showed that P removal during ERW may occur by adsorption, however, over longer timescales P concentrations could increase towards the influent concentrations due to saturation of adsorption sites. It is possible that over longer timescales, P addition may exert a greater impact on weathering rates. Moreover, at higher concentrations of P added, the impact on weathering may have been greater. The impact of P on silicate weathering during ERW therefore merits further study over a broader range of conditions and over longer timescales.

The median pH of the wollastonite columns exceeds the mixture column pH by as much as 1.47 pH units. The lower pH in the mixture columns is partially attributable to less reactive surface area of wollastonite in the mixture columns compared to the wollastonite columns, and the higher soil pCO2. The wollastonite columns had a much higher wollastonite-fluid ratio, with a greater potential for pH increase through wollastonite-fluid interaction, but this higher pH would engender a reduction in wollastonite dissolution rate. It is feasible that microbial activity also played a role in promoting dissolution in the mixture columns. The soils used in this experiment were rich in organic matter, with an organic carbon content of 29.0% prior to reaction. In addition to organic acids, carbon dioxide released by bacteria dissolves in porewaters and forms carbonic acid, leading to lower porewater pH (Eq. 5):

𝐶𝑂 2 + 𝐻 2 𝑂 ↔ 𝐻 2 𝐶𝑂 3 ↔ 𝐻𝐶𝑂 3 -+ 𝐻 + ↔ 𝐶𝑂 3 2-+ 2𝐻 + Equation 2
The rate and mechanism of the dissolution of wollastonite depend strongly upon the pH of the fluid [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF]. When pH ≥ 4.8, as it was in all the present experiments, the dissolution rate can be described by (Eq. 6; [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF]:

𝑟 +,𝑆𝑖,𝑝𝐻≥5 = 10 -11.41 𝑎 𝐻 + 0.18 Equation 6Where, r+, Si is the dissolution rate (mol/cm 2 /s) expressed for release of Si, and aH+ is the activity of H + in solution. Calculated bulk dissolution rates (i.e., for the total surface area of wollastonite in each column) based on the median pH value and Equation 6for each wollastonite-containing column are provided in Table 4. Due to the lower pH, the surface area-normalized mixture column dissolution rates for the average of the triplicate columns is approximately 38 and 35 times greater in the MW and MP columns, respectively, than in the wollastonite-water column due to the difference in pH between columns alone. Thus, pHdependence of the dissolution rate can largely account for the higher dissolution rate in the mixture compared to the wollastonite columns. b pH values represent the median pH of effluent samples for each column.

c SA represents the total surface area of wollastonite in the column based on BET analysis.

d pH-dependent dissolution rate was calculated based on the median pH for each column and surface area (SA) using the rate law of [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF]. Dissolved CO2 can also promote wollastonite dissolution through formation of HCO3 - ligands. Under acidic conditions (pH ~4), the concentration of CO2 is insignificant to the overall dissolution rate, but at circumneutral pH, HCO3 -was shown to slightly increase the dissolution rate of wollastonite [START_REF] Golubev | Experimental determination of the effect of dissolved CO2 on the dissolution kinetics of Mg and Ca silicates at 25 °C[END_REF][START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. However, the concentrations of HCO3 -in the present experiment were notably lower than required to double the dissolution rate of wollastonite, at < 5mM compared to 10 mM [START_REF] Golubev | Experimental determination of the effect of dissolved CO2 on the dissolution kinetics of Mg and Ca silicates at 25 °C[END_REF]. Several organic ligands have also been shown to enhance the dissolution of wollastonite [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF]. Maximum total DOC concentrations of ~ 8 mM were in excess of the concentrations of EDTA, catechol, gallic acid, and 8-hydroxychinolin necessary to triple the dissolution rate of wollastonite at neutral pH [START_REF] Pokrovsky | Effect of organic ligands and heterotrophic bacteria on wollastonite dissolution kinetics[END_REF].

The impact of these additional ligands could account for some of the gap between the theoretical dissolution rates predicted by the pH-dependent rate law [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF] and the measured release rates of Ca and Si in the mixture columns.

Quantification of CO2 Storage by Wollastonite Weathering

There are numerous geochemical processes operating in the columns that could influence the cycling of carbon. Some of these processes would result in carbon storage, some in carbon release, and some are net neutral with respect to the carbon balance. Before quantifying carbon storage within the columns, these reactions, and their effects on the fluxes of C from various sources must be defined. The degradation of soil organic matter and DOC can result in release of CO2 to the atmosphere, which is consistent with the greater than atmospheric pCO2 calculated in the soil and mixture columns (Figure S5 in SM). Conversely, the weathering of wollastonite is the primary mechanism by which atmospheric CO2 is sequestered in the columns. At circumneutral pH, wollastonite weathering proceeds according to Equation 1 and brings CO2 into solution as dissolved inorganic carbon. If this CO2 is sourced from the atmosphere, the result of wollastonite dissolution is net removal of CO2 from the atmosphere. On the other hand, if the CO2 driving wollastonite weathering is from the oxidation of soil organic matter, wollastonite weathering may instead represent mitigated CO2 emissions of soil-generated CO2 rather than net removal of modern atmospheric CO2. The effect of wollastonite weathering in the latter case is to reduce the flux of CO2 to the atmosphere from the soils. Similarly, formation of solid carbonate from DIC would result in modern CO2 removal or avoided CO2 emissions if the CO2 source were the atmosphere or soil generated CO2, respectively.

Dissolution of antecedent carbonate in the soil removes or avoids CO2 in the same way as wollastonite dissolution (Equation 7),

𝐶𝑎𝐶𝑂 3 + 𝐶𝑂 2 + 𝐻 2 𝑂 ⟷ 𝐶𝑎 2+ + 2𝐻𝐶𝑂 3 - Equation 7
but precipitation of SIC from carbonate dissolution results in no net loss or gain of CO2. In this study, CO2 storage is considered to encompass both formation of carbonates and eventual CO2 storage in the oceans as alkalinity. Thus, in the wollastonite columns, DIC and SIC represent net modern atmospheric CO2 storage, as the atmosphere is the only DIC source. In contrast, the high pCO2 in the mixture columns suggests most CO2 in the mixture columns was derived from soils rather than directly the atmosphere. Measurement of DIC concentrations in the irrigation water indicates that atmospheric CO2 introduced through irrigation water comprised approximately 6 to 7% of DIC detected in mixture column effluent. Although carbon respired by microbes ultimately originates from the atmosphere, the degree of decomposition of the soil, and therefore the timescale of carbon cycling cannot be determined in the present study. The extent to which the wollastonite mineral amendment affected different carbon pools is unknown. For the purposes of this study, a distinction is therefore made between modern CO2 directly removed from the atmosphere through ERW, representing negative emissions, and fluxes of soil-generated CO2 to the atmosphere that have been mitigated as a result of ERW, though both represent CO2 storage.

In the soil columns, calcite remained undersaturated and may have contributed to the DIC flux in addition to organic C degradation, however in the mixture columns, calcite and dolomite were supersaturated for the majority of the experiment, indicating they were unlikely to dissolve substantially and contribute to the DIC flux (Figure S6 in SM). Thus, the main CO2 source for DIC in the mixture columns is likely degradation of organic C. The higher pH in the mixture columns compared to the soil columns due to wollastonite dissolution would encourage lower CO2 loss to the atmosphere due to the greater solubility of CO2 at higher pH.

Because of the complex set of geochemical interactions impacting DIC, the efficiency of CO2 removal by weathering is often determined based on cation or Si fluxes [START_REF] Gaillardet | Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers[END_REF][START_REF] Kelland | Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil[END_REF][START_REF] Peters | Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1[END_REF]

𝑅 𝐶𝑅 = 2 • 𝑀 𝑆𝑖,𝑡𝑜𝑡𝑎𝑙 • 𝑚 𝐶𝑂 2 (𝑚 𝑤𝑜𝑙 • 𝑡 𝑒𝑥𝑝 )
Equation 8

Where RCR is the CO2 removal rate in kg CO2 t amendment -1 yr -1 , MSi, total is the total moles of Si exported from the column (multiplied by 2 as each mole of Si is balanced by 2 moles of HCO3 -per Equation 1), 𝑚 𝐶𝑂 2 is the molar mass of carbon dioxide (kg/mol), mwol is the mass of wollatonite in the column (t), and texp is the duration of the experiment (yr). Similar equations were used to calculate CO2 removal rates based on total fluxes of Ca and DIC, for comparison of different approaches of CO2 removal calculation (Table S2 in SM). The total mass of CO2 stored in each column was extrapolated to 1 ha of land area (Table 5, Figure 10) for ease of comparison with previous studies. [START_REF] Manning | Carbonate precipitation in artificial soils produced from basaltic quarry fines and composts: An opportunity for passive carbon sequestration[END_REF] documented an SIC accumulation of 17.6 t CO2 ha -1 yr -1 with a blend of compost and basalt/dolerite quarry fines, but mesocosm experiments with dunite amended soils captured much less, with an estimated 0.05 t CO2 ha -1 yr -1 as DIC [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF] [START_REF] Kelland | Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil[END_REF]. At the higher end of carbon storage rates, [START_REF] Washbourne | Rapid Removal of Atmospheric CO2 by Urban Soils[END_REF] showed that weathering of urban soils with a high waste concrete content resulted in capture of 85 t CO2 ha -1 yr -1 as SIC [START_REF] Washbourne | Rapid Removal of Atmospheric CO2 by Urban Soils[END_REF]. An upper estimate of CO2 removal rate by wollastonite amendment in the Hubbard Brooke watershed was 0.77 t CO2 ha -1 yr -1 [START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF]. Laboratory [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF] and field [START_REF] Haque | CO2 sequestration by wollastonite-amended agricultural soils-An Ontario field study[END_REF] experiments with wollastonite amended soils and crops produced strikingly different estimates of CO2 storage with 39.3 t CO2 ha -1 yr -1 and 0.28 to 2.4 t CO2 ha -1 yr -1 , respectively, with the difference possibly due to lower wollastonite loading in the field study.

The CO2 removal rates in this study were greater than those documented in many previous experimental ERW studies (Table 4;[START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF][START_REF] Dietzen | Effectiveness of enhanced mineral weathering as a carbon sequestration tool and alternative to agricultural lime: An incubation experiment[END_REF][START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF][START_REF] Kelland | Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil[END_REF][START_REF] Manning | Carbonate precipitation in artificial soils produced from basaltic quarry fines and composts: An opportunity for passive carbon sequestration[END_REF][START_REF] Pogge Von Strandmann | The Dissolution of Olivine Added to Soil at 4°C: Implications for Enhanced Weathering in Cold Regions[END_REF][START_REF] Ten Berge | perenne L.): a pot experiment[END_REF]. The average CO2 removal rate in the mixture columns of 50 t CO2 ha -1 yr -1 is only exceeded by artificial concrete soils [START_REF] Washbourne | Rapid Removal of Atmospheric CO2 by Urban Soils[END_REF], and is of similar order of magnitude to the pot experiments by [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF] in which plants contributed to wollastonite weathering. However, CO2

removal rates for the present study when normalized to material loading were closer to 5). The material loading normalized CO2 removal rate for the present study is of similar magnitude to that reported by [START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF] for which wollastonite was applied at the watershed scale. For the present study, if assuming a 1 C: 1

Ca: 1 Si ratio with carbonate precipitation being the primary CO2 sink rather than DIC, then CO2 removal rates of 9.1 and 6.4 t CO2 ha -1 yr -1 are calculated for columns WW and WP respectively, and 22 and 28 t C ha -1 yr -1 for the MW and MP columns, respectively (Figure 982 S9). time strongly influence weathering rates and CO2 uptake from the atmosphere; thus, the irrigation rate could significantly influence CO2 storage rates [START_REF] Andrews | Combating climate change through enhanced weathering of agricultural soils[END_REF][START_REF] Cipolla | The role of hydrology on enhanced weathering for carbon sequestration I. Modeling rock-dissolution reactions coupled to plant, soil moisture, and carbon dynamics[END_REF][START_REF] Harrison | Influence of surface passivation and water content on mineral reactions in unsaturated porous media: Implications for brucite carbonation and CO2 sequestration[END_REF][START_REF] Harrison | The impact of evolving mineral-water-gas interfacial areas on mineral-fluid reaction rates in unsaturated porous media[END_REF]. Soil moisture correlates strongly to enhanced weathering of silicate soil amendments due to the increase in accessible reactive surface area of minerals [START_REF] Cipolla | The role of hydrology on enhanced weathering for carbon sequestration I. Modeling rock-dissolution reactions coupled to plant, soil moisture, and carbon dynamics[END_REF]. Columns in the present experiment received approximately 15,900 mm yr -1 of irrigation, substantially greater than the 1400 mm average annual precipitation of the Hubbard Brooke Experimental Forest [START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF]. The use of pure wollastonite in the present experiment may have resulted in greater CO2 capture rates than application of bulk silicate rock. Furthermore, the shorter experimental length could influence the CO2 removal rate, as wollastonite dissolution rates may slow over time, causing a lower time-averaged rate over longer periods compared to the approximately 700 h of the present experiment.

Implications for ERW

The overall rates of CO2 removal for the average of the triplicate mixture columns receiving H2O and K3PO4 were 200 ± 31 kg CO2 t amendment -1 yr -1 and 255 ± 44 kg CO2 t amendment -1 yr -1 , respectively, overlapping within uncertainty. The addition of P had little influence on the net dissolution of wollastonite in the experiments, largely due to the removal of P from solution by adsorption to mineral and organic surfaces. In soils with lower cation exchange capacity, or over a longer time and/or higher added P concentrations, the removal of P by adsorption may become less significant, but precipitation as Caphosphate phases may occur. No evidence of phosphate mineral precipitation was found in any of the experiments suggesting it was not a major P sink compared to adsorption; however, this may not be the case over longer timescales in the field. In addition, pH changes from the dissolution of wollastonite or other geochemical processes may limit the effectiveness of P for enhancing the dissolution of wollastonite due to alteration of aqueous speciation. In field settings, where P may be taken up by plants or consumed by microbes, its availability in solution to enhance dissolution may be further limited. With so many factors that can reduce the impact of aqueous P on the dissolution of wollastonite, the most significant way in which the application of P-bearing fertilizers is likely to support enhanced weathering is by promoting plant growth. Plants enhance wollastonite dissolution in soils [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF] and are themselves a carbon sink. The greatest CO2 capture in a field-scale trial of wollastonite soil amendment was observed in a field that received a Pbearing fertilizer [START_REF] Haque | CO2 sequestration by wollastonite-amended agricultural soils-An Ontario field study[END_REF]. However, the nature of interactions between plants, mineral amendments, and fertilizers has not been fully resolved [START_REF] Mkhonza | Effects of lime application on nitrogen and phosphorus availability in humic soils[END_REF][START_REF] Jariwala | Mineral-Soil-Plant-Nutrient Synergisms of Enhanced Weathering for Agriculture: Short-Term Investigations Using Fast-Weathering Wollastonite Skarn[END_REF][START_REF] Ipcc | Global warming of 1.5 o C. An IPCC Special Report on the impacts of global warming of 1[END_REF]. Direct interactions between minerals and P-fertilizers are less likely to represent a significant factor in governing CO2 removal rates.

The physical and chemical properties of soils used in ERW may have crucial implications regarding the dissolution of any mineral soil treatment and resulting element release trends, which cannot be predicted based on dissolution rates and behaviour of pure mineral phases. Our experiments demonstrate that even in fairly simple systems with a pure mineral amendment in soil without plant growth, the complex and coupled interactions between living and abiotic soil components and rock amendments substantially complicate the quantification of CO2 capture. Differences in surface-area normalized weathering rates between the wollastonite and mixture columns reveal that the behaviour of pure minerals or background soils do not adequately reflect the conditions of a mineral-soil mixture; elemental releases from individual components are not simply additive. Crucially, because the pH was lower in the mixture columns, more wollastonite dissolved and therefore, more CO2 was captured or retained than with wollastonite alone. These lower pH values may have resulted from the lower wollastonite:fluid ratio, or from the release of CO2 from soils due to microbial activity, promoted by the high organic content of the soils. This soil CO2

was also likely key to the increasing DIC concentrations over time in the mixture columns, but it only became apparent in the presence of wollastonite, not in the soil columns.

Silicon concentrations exceeding those of Ca are atypical in the case of wollastonite dissolution, which is generally stoichiometric at the experimental pH conditions [START_REF] Schott | Formation, growth and transformation of leached layers during silicate minerals dissolution: The example of wollastonite[END_REF]), yet Si concentrations in excess of Ca concentrations were observed in both mixture and wollastonite columns (Figure 5). However, an increase in Ca concentrations over time was observed only in mixture columns as it most likely resulted from reduced exchange site availability for Ca over time. Calcite went from undersaturated to oversaturated in the mixture columns, potentially resulting in carbonate mineral formation.

This stands in contrast to the wollastonite columns, in which less dissolution of wollastonite occurred, but calcite was supersaturated throughout the experiment. Similarly, several soil mineral constituents were undersaturated in the soil columns but were supersaturated in the mixture columns. For example, despite being undersaturated in the soil columns, quartz and calcite were oversaturated in the mixture columns (>200 h). This underscores the fact that elemental release from soils themselves may differ when treated with a soil amendment.

The implication is that the presence of soil amendments substantially alters weathering of pre-existing soil minerals. The different trends in DIC for the mixture columns compared to both the wollastonite or soil columns also highlight the importance of evaluating soil treatments within the soils to be amended. Elements released from the soil amendment (Ca and Si in this case), as well as DIC and SIC are key indicators of ERW and CO2 capture.

The importance of considering the effects of soils is supported by complications observed in previous enhanced weathering experiments [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF]. In a mesocosm experiment in which soils were amended with dunite, calculated CO2 consumption by weathering was up to three orders of magnitude lower than in similar studies [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF]. The calculation was based on effluent Mg concentrations, which were speculated to have been retarded by processes such as adsorption, mineralization, uptake by plants, and preferential flow [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF]. The present study, along with the previous study of [START_REF] Amann | Enhanced Weathering and related element fluxes-a cropland mesocosm approach[END_REF], demonstrate the potential for soils to complicate estimates of CO2 capture and nutrient release in ERW scenarios. To account for the influence of soils in evaluations of field-deployed ERW, site-specific characterization of soil-mineral mixtures is necessary, such as characterizing organic carbon and clay content, mineralogical composition, and sorption capacity. Evaluation of the microbial populations in soils would also be valuable, as microbe-soil and microbe-mineral interactions are strong controls on the evolution of the fluid composition and CO2 stored. In future field experiments or largescale deployment of ERW, it is vital to consider the contribution of soils to elemental trends observed, as well as how that contribution may be influenced through interactions with additives. Reactive transport modeling, at both the laboratory and field scale, is important to define pathways of elemental transport and transformation. For example, the watershedscale modeling of [START_REF] Hayder | Phosphorous runoff risk assessment and its potential management using wollastonite according to geochemical modeling[END_REF] suggests that interactions between wollastonite and P may be important not only for CO2 capture but also for remediation of agricultural pollution.

Conclusion

The capacity of dissolved P to accelerate the dissolution of wollastonite was severely limited by its rapid removal from solution in all columns. This removal was driven by adsorption of P to mineral and organic surfaces and the change in speciation due to the pH increase caused by wollastonite dissolution. These mitigating factors indicate that the capacity of P to accelerate wollastonite dissolution was limited, leading to little impact of added P on observed wollastonite weathering rate.The high pCO2 of soils suggests that the majority of CO2 present in the soil/mixture columns, and the majority of CO2 stored in those columns was released through degradation of organic matter and represents a flux of carbon from soil to the atmosphere that was mitigated, rather than direct removal of modern atmospheric CO2. Material loading normalized rates of silicate weathering in the mixture columns were similar to those in previous ERW studies, suggesting comparable CO2 capture/retention to wollastonite application at the watershed scale [START_REF] Taylor | Increased carbon capture by a silicate-treated forested watershed affected by acid deposition[END_REF].

Possible factors contributing to differences in weathering rate with previous studies include greater mineral loading and irrigation, the use of a more reactive amendment, and a shorter experimental duration. The substantially greater wollastonite dissolution and CO2 capture in the mixture columns than with wollastonite alone demonstrate that soils exert an important impact on dissolution rate and elemental release in ERW scenarios. In experiments conducted over longer timescales and in the field with soils that have been routinely exposed to Pbearing fertilizers, the extent of P removal from solution and thus the impact of P on ERW may differ. The impacts of ERW on plants and on CO2 removal rates will likely depend on the mineral loading rates used. Interactions with plants will likely impact CO2 removal rates and further complicate quantification of CO2 removal [START_REF] Haque | Co-Benefits of Wollastonite Weathering in Agriculture: CO2 Sequestration and Promoted Plant Growth[END_REF][START_REF] Kelland | Increased yield and CO2 sequestration potential with the C4 cereal Sorghum bicolor cultivated in basaltic rock dust-amended agricultural soil[END_REF], suggesting this is an area requiring further detailed study.

The outcomes observed in laboratory and field scale ERW initiatives will depend heavily upon the soils that are used and how they interact with mineral treatments.

However, the addition of mineral treatments may significantly alter soil geochemical behaviour. Because neither mineral-only nor soil-only experiments accurately represent the conditions of enhanced weathering in soils, only mixtures using site-specific soils can be relied upon to provide accurate estimates of weathering rates and CO2 sequestration capacity.

a

  Soil-water (SW), Soil-P (SP), Wollastonite-Water (WW), Wollastonite-P (WP), Mixture-Water (MW), Mixture-P (MP) where mixture indicates an 8:1 mixture of soil and wollastonite.b De-ionized water was applied to columns receiving water and a 1 × 10 -4 M K3PO4 (3.096 mg L -1 P) solution was applied to columns receiving K3PO4.c Dry and wet masses are initial masses with Pore Volume = Wet Mass -Dry Mass.Irrigation of the columns was carried out daily during the first week, then every other day for the remainder of the month-long experimental period. Transfer pipettes were used to simulate rain and avoid disturbing the solids. A volume of 21 mL, equivalent to at least one pore volume, was used for each irrigation event. A separate aliquot of fluid was used for each column, with the mass of the container measured before and after irrigation to verify the mass of fluid added to each column. Half of the columns (one wollastonite, one soil, three mixture) were irrigated with de-ionized water. The other half of the columns were irrigated with a 1 × 10 -4 M K3PO4 (3.096 mg L -1 P) solution, which was acidified to pH 6 (equivalent to the pH of the deionized water) with < 1 µL 1 N HCl.

Figure 1 .

 1 Figure 1. Column experiment schematic. The masses of wollastonite present in columns were 32.8 g (WW), 34.6 g (WP), 1.47 g (MW 1), 1.42 g (MW 2), 1.54 g (MW 3), 1.61 g (MP 1), 1.59 g (MP 2), and 1.58 g (MP 3).

  calcite (CaCO3) and dolomite [CaMg(CO3)2] in addition to organic matter. Calcite and dolomite were not detected by XRD in the soil-wollastonite mixture, suggesting they were present below the detection limit (~ 0.5 wt.%). Wollastonite, albite, and quartz were the only minerals detected by XRD in the initial soil-wollastonite mixture (Figure S1 in (SM)).

Figure 2 .

 2 Figure 2. Solid inorganic (SIC) and organic carbon (SOC) contents of initial and reacted column contents. Error bars indicate standard deviation (n = 3) of initial sample measurements for each material. Abbreviations are defined as: Wollastonite-Water (WW), Wollastonite-P (WP), Mixture-Water (MW), and Mixture-P (MP) where mixture indicates an 8:1 mixture of soil and wollastonite. Upper and lower refers to the upper and lower 3-4 cm of each column, respectively.

  A). Soils were of irregular texture with a range of constituents amalgamated into clumps (Figure 3 B). Wollastonite from columns irrigated with water (Figure 4 A) or K3PO4 (Figure 4 B) displayed rougher surfaces than the initial material, indicative of dissolution. The wollastonite surfaces were observably rougher in the mixture columns, with more fractures and pits (Figure 4 D, E, F), than in the wollastonite columns after reaction (Figure 4 A, B, C), likely reflecting a greater extent of weathering. Surface features consistent with secondary precipitates were visible in the upper portion of MP 1 (Figure 4 E) and in the lower portion of MW 1 (Figure 4 D). The precipitates could not be identified based on the images, however, and were not of sufficient quantity to be detected by XRD although an increase in the SIC content of MW 1 was observed.

Figure 3 .

 3 Figure 3. Scanning electron microscope images of initial wollastonite (A), initial soil (B), and soil-wollastonite mixture (C).

Figure 4 .

 4 Figure 4. Scanning electron microscope images of Wollastonite-Water (WW) lower (A), Wollastonite-P (WP) lower (B), Wollastonite-P lower (C), Mixture-Water 1 (MW 1) upper (D), Mixture-P 1 (MP 1) upper (E), and Mixture-P 1 lower (F), where mixture indicates an 8:1 mixture of soil and wollastonite. Upper and lower refer to the upper and lower 3-4 cm of the columns, respectively.

  A). Mixture column pH values were ~8 throughout the experiment for the columns receiving water (Figure 5 A).

Figure 5 .

 5 Figure 5. pH values of column effluent (A), Si concentrations of column effluent (B), Ca concentrations of column effluent (C). Mixture column data is presented as an average of concentrations from three replicate columns. Error bars for non-replicated columns indicate the maximum standard deviation (SD) of triplicate samples (maximum SD was used where only one sample was analyzed for a time point) and error bars for replicated columns indicate the SD of concentrations in samples taken at the same time point for replicate columns. WW (Wollastonite-Water), WP (Wollastonite-P), Soil-Water (SW), Soil-P (SP), MW (Mixture-Water), MP (Mixture-P), where mixture indicates an 8:1 mixture of soil and wollastonite.

  Sodium and Mg concentrations were only measured for columns MW 2, MW 3, MP 2, and MP 3. Magnesium concentrations in the mixture columns exhibited similar trends to Ca concentrations though concentrations were lower overall. Magnesium concentration in all measured columns declined to ~0.2-0.4 mM in the first 100 h, then increased to a stable value of 0.6 mM by 400 h. Sodium concentrations decreased from ~2 mM at the start of the experiment to < 0.1 mM by 400 h.

Figure 6 .

 6 Figure 6. Dissolved P concentrations column effluent. Mixture column data is presented as an average of three replicate columns. Error bars for non-replicated columns indicate the maximum standard deviation (SD) of triplicate samples (maximum SD was used where only one sample was analyzed for a time point) and error bars for replicated columns indicate the SD of concentrations from column samples taken at the same time point for replicate columns. WW (Wollastonite-Water), WP (Wollastonite-P), Soil-Water (SW), Soil-P (SP), MW (Mixture-Water), MP (Mixture-P), where mixture indicates an 8:1 mixture of soil and wollastonite.

Figure 7 :

 7 Figure 7: Dissolved inorganic carbon concentrations of column effluent (A) and dissolved organic carbon concentrations of column effluent (B). Mixture column data is presented as an average of three replicate columns. Error bars for non-replicated columns indicate the maximum standard deviation (SD) of triplicate samples (maximum SD was used where only one sample was analyzed for a time point) and error bars for replicated columns indicate the SD of concentrations in samples taken at the same time point for replicate columns. WW (Wollastonite-Water), WP (Wollastonite-P), Soil-Water (SW), Soil-P (SP), MW (Mixture-Water), MP (Mixture-P), where mixture indicates an 8:1 mixture of soil and wollastonite.

Figure 8 .

 8 Figure 8. Cumulative dissolved inorganic carbon (DIC) exported from columns normalized to initial total solid mass in each column. Error bars indicate the propagated error for the sum of DIC concentrations and fluid mass. WW (Wollastonite-Water), WP (Wollastonite-P), Soil-Water (SW), Soil-P (SP), MW (Mixture-Water), MP (Mixture-P), where mixture indicates an 8:1 mixture of soil and wollastonite.

  in the wollastonite columns(Figure S5 in SM). The calculated pCO2 declined over time in the mixture columns, stabilizing at approximately 2000 ppm by 200 h, yet pCO2 in the soil and wollastonite columns did not display any clear trend. The calculated pCO2 in the wollastonite columns was between 24 to 74 ppm throughout the experiment, indicating that the leachate was not at equilibrium with an atmospheric pCO2 of ~400 ppm. The average concentration of DIC in the influent solution was 0.16 mM.

  by stabilization in the mixture columns are consistent with Ca adsorption sites being filled, and thus less available, over time. Adsorption of some of the Ca released through wollastonite dissolution to soils could lead to a lower Ca concentration in outlet fluids compared to Si concentrations. If wollastonite continued to dissolve at a similar rate over time, as adsorption sites became less available the Ca concentration in the effluent would increase and approach the Si concentration. Sorption of a significant portion of the Ca released would necessitate a sufficient cation exchange capacity (CEC) to accommodate the Ca ions. The CEC of a soil can be estimated based on its clay and organic content (

Figure 9 .

 9 Figure 9. Export of Ca and DIC as a percent of stoichiometric release from wollastonite. Stoichiometric release was calculated relative to Si concentrations assuming reaction stoichiometry of Equation 1. Wollastonite-Water (WW), Wollastonite-P (WP), Mixture-Wollastonite (MW), Mixture-P (MP) where Mixture indicates an 8:1 mixture of soil and wollastonite.

e

  Rates are normalized to the calculated pH-dependent WW or measured WW value. Measured Ca Rate/WW and Measured Si Rate/WW are based on the average rate of Ca and Si discharge from each column within the last 400 h of the experiments, relative to the measured rates in WW during the same period.

  . Calcium concentrations are unreliable tracers of the extent of wollastonite weathering in the present experiments due to adsorption (and possible precipitation) processes. Solid and fluid inorganic carbon are also unreliable indicators of wollastonite weathering within the mixture columns in the present study due to the presence of SIC in soils and the generation of DIC through processes other than wollastonite weathering. Silicon export directly provides a weathering rate that can be related to the CO2 removal rate though Equation 1, but the possibility for subsequent fluidsoil interactions such as sorption and precipitation creates uncertainty in the mass balance. Despite this limitation, effluent Si concentrations were considered the most precise tracer of wollastonite weathering of those measured in the column experiments. The CO2 removal rate in the column experiments was therefore calculated using the total Si flux as follows:

Figure 10 .

 10 Figure 10. Comparison of CO2 storage rate in enhanced rock weathering studies. WW (Wollastonite-Water), WP (Wollastonite-P), MW (Mixture-Water), MP (Mixture-P), where mixture indicates an 8:1 mixture of soil and wollastonite. Averages of triplicate MW and triplicate MP columns were used for this study, with error bars indicating standard deviation of the average.

  other studies and exceeded by those of Dietzen et al. (2018), Haque et al. (2019), and Haque et al. (2020) (Table

Table 1 . Medium, solution, mass, and pore volume data for column experiments

 1

				Dry Mass		Pore Volume
	Column a Medium	Solution b	(g) c	Wet Mass (g) c	(mL) c
	WW	Wollastonite	Water	32.80	48.37	15.57
	WP	Wollastonite	K3PO4	34.60	50.52	15.92
	SW	Soil	Water	12.44	30.3	17.86
	SP	Soil	K3PO4	12.09	30.75	18.66
	MW 1	Soil-Wollastonite Water	13.20	32.91	19.71
	MW 2	Soil-Wollastonite Water	12.81	34.37	21.56
	MW 3	Soil-Wollastonite Water	13.81	34.36	20.55
	MP 1	Soil-Wollastonite K3PO4	14.52	34.47	19.95
	MP 2	Soil-Wollastonite K3PO4	14.28	34.06	19.78
	MP 3	Soil-Wollastonite K3PO4	14.26	34.77	20.51

Table 2 . Inorganic and organic carbon contents of solids

 2

		% Inorganic
	Sample a	Carbon	% Organic Carbon % Total Carbon
	Initial Wollastonite b 0.025 ± 0.002	0.018 ± 0.011	0.042
	Initial Soil b	0.363 ± 0.067	31.1 ± 1.6	31.4
	Initial Mixture b	0.367 ± 0.064	26.4 ± 0.9	26.7
	WW Upper	0.064 ± 0.002	0.018 ± 0.011	0.083
	WW Lower	0.012 ± 0.002	0.030 ± 0.011	0.043
	WP Upper	0.023 ± 0.002	0.011 ± 0.011	0.035
	WP Lower	0.023 ± 0.002	0.016 ± 0.011	0.039
	MW 1 Upper	0.355 ± 0.064	29.1 ± 0.9	29.5
	MW 1 Lower	0.955 ± 0.064	26.1 ± 0.9	27.0
	MP 1 Upper	0.293 ± 0.064	28.9 ± 0.9	29.2
	MP 1 Lower	0.399 ± 0.064	26.5 ± 0.9	26.9
	a Wollastonite-Water (WW), Wollastonite-P (WP), Mixture-Water (MW), Mixture-P (MP) where mixture
	indicates an 8:1 mixture of soil and wollastonite. Upper and lower refers to the upper and lower thirds of
	columns, respectively, which were each ~3.3 cm depth. Samples not labelled "initial" indicate final values.

Table 3 . Total calcium and silicon fluxes from the columns

 3

	Column a	mmol Ca Exported	% Ca Exported b	mmol Si Exported	% Si Exported b
	WW	0.155	0.06	0.423	0.15
	WP	0.183	0.06	0.312	0.11
	SW	0.086	N/A	0.010	N/A
	SP	0.101	N/A	0.052	N/A
	MW 1	0.260	2.06	0.349	2.77
	MW 2	0.283	2.31	0.465	3.80
	MW 3	0.240	1.82	0.431	3.26
	MP 1	0.226	1.63	0.466	3.36
	MP 2	0.272	1.99	0.622	4.56
	MP 3	0.333	2.44	0.632	4.63

a Wollastonite-Water (WW), Wollastonite-P (WP), Soil-Water (SW), Soil-P (SP), Mixture-Water (MW), Mixture-P (MP) where mixture indicates an 8:1 mixture of soil and wollastonite.

b Percent values are reported relative to initial moles of Ca or Si in wollastonite present in the column.

  retained through SIC or lost by degassing, meaning DIC concentrations do not necessarily represent the true extent of silicate weathering. The lower amount of wollastonite dissolution estimated using Ca is more similar to the DIC method, which could indicate retention of both Ca and C within SIC. However, if the SIC content of the upper portion of column WW is assumed to apply to the entire column, then up to 0.10 millimoles of C are estimated to be contained within SIC. A similar value of 0.13 millimoles of Ca are estimated to be contained in SIC in column WP. For both WW and WP, this estimated Ca removal through SIC formation is not enough to account for all of the difference between Si and Ca release, suggesting sorption of Ca at the mineral surface.

	23 millimoles of wollastonite are estimated
	to have dissolved in columns WW and WP respectively. For column WW, effluent
	concentrations of Si and Ca indicate 0.42 and 0.16 millimoles of wollastonite dissolution
	respectively, while Si and Ca concentrations in column WP correspond to 0.31 and 0.18

millimoles of wollastonite dissolution. The difference between wollastonite dissolution predicted by DIC (or alkalinity) and by effluent Si concentrations indicates that alkalinity may have been

Table 4 . Calculated wollastonite dissolution rate based on median column pH and measured elemental release rates

 4

				pH-dependent	pH-	Measured
			SA c	Dissolution Rate	dependent	Ca	Measured Si
	Column a pH b	(cm 2 )	(mol/cm 2 /s) d	Rate/WW e	Rate/WW e	Rate/WW e
	WW	9.17 46607	1.87E-18	1.0	1.0	1.0
	WP	9.23 49165	1.73E-18	0.93	1.1	0.6
	MW 1	7.87 2084	7.15E-17	38.3	50.9	20.1
	MW 2	7.93 2022	7.19E-17	38.5	60.6	32.8
	MW 3	7.86 2181	6.86E-17	36.8	50.5	26.4
	MP 1	7.87 2292	6.50E-17	34.8	40.5	20.2
	MP 2	7.76 2255	6.92E-17	37.1	52.1	32.2
	MP 3	8.07 2251	6.09E-17	32.7	64.9	31.2
	a Wollastonite-Water (WW), Wollastonite-P (WP), Mixture-Water (MW), Mixture-P (MP) where mixture
	indicates an 8:1 mixture of soil and wollastonite.

  . Other olivine experiments produced upper estimates of 2.69 t CO2 ha -1 yr -1 (Ten Berge et al., 2012), 4.16 t CO2 ha -1 yr -1[START_REF] Dietzen | Effectiveness of enhanced mineral weathering as a carbon sequestration tool and alternative to agricultural lime: An incubation experiment[END_REF], and 0.3 t CO2 ha -1 yr -1 (Pogge von[START_REF] Pogge Von Strandmann | The Dissolution of Olivine Added to Soil at 4°C: Implications for Enhanced Weathering in Cold Regions[END_REF], and basalt mesocosms capturing an estimated 3.01 t CO2 ha -1 yr -1

Table 5 . Comparison of CO2 removal rate in enhanced rock weathering studies

 5 Although the present study indicates a higher CO2 capture rate on a per area basis than in previous studies, much of the discrepancy can be eliminated by normalizing to the material loading rate. Other factors that may have increased the CO2 capture rate in the present study beyond what might be expected in field-scale ERW settings include the frequent irrigation and large volume of water used relative to the mass of soil, the purity of the wollastonite amendment, and the experimental length. Porewater content and residence

	Study	Material	Study Scale	Plant Presence	Material Loading (t /ha)	CO2 Capture Metric	kg CO2 /t amendment /yr	t CO2 /ha /yr
	Ten Berge et al. (2012)	Olivine	Pot	Yes	204	Mg Balance	132	2.69
	Manning et al. (2013)	Dolerite	Field	Yes	Not stated SIC	N/A	17.6
	Washbourne et al. (2015) Concrete	Field	No	Not stated SIC	N/A	85
	Amann et al. (2018)	Olivine	Mesocosm Yes	220	Mg Balance	2.23	0.05
	Dietzen et al. (2018)	Olivine	Column	No	50	Mg Balance	832	4.16
	Haque et al. (2019)	Wollastonite	Pot	Yes	221	SIC	1778	39.3
	Haque et al. (2020)	Wollastonite	Field	Yes	1.25-5.0	SIC	224-4800	0.28-2.4
	Kelland et al. (2020)	Basalt	Mesocosm Yes	100	Mg Balance	30.1	3.01
	Taylor et al. (2021)	Wollastonite	Watershed Yes	3.44	Ca Balance	223	0.77
	Pogge von Strandmann et
	al. (2022)	Olivine	Column	No	127	Mg Balance	2.4	0.30
	Wollastonite-Water a	Wollastonite	Column	No	1989	Si, Ca, HCO3 -3.36-9.13 b	6.67-18.2 b
	Wollastonite-P a	Wollastonite	Column	No	1989	Si, Ca, HCO3 -3.75-6.39 b	7.46-12.7 b
	Mixture-Wollastonite 1 a	Wollastonite	Column	No	221	Si, Ca, HCO3 -126-211 b	27.7-46.6 b
	Mixture-Wollastonite 2 a	Wollastonite	Column	No	221	Si, Ca, HCO3 -141-239 b	31.1-52.9 b
	Mixture-Wollastonite-3 a	Wollastonite	Column	No	221	Si, Ca, HCO3 -111-199 b	24.5-43.9 b
	Mixture-P 1 a	Wollastonite	Column	No	221	Si, Ca, HCO3 -99.3-205 b	21.9-45.2 b
	Mixture-P 2 a	Wollastonite	Column	No	221	Si, Ca, HCO 3 -122-278 b	26.8-61.4 b
	Mixture-P 3 a	Wollastonite	Column	No	221	Si, Ca, HCO3 -149-283 b	32.8-62.4 b
	a this study
	b range of CO2 removal rate calculated using either Si, Ca, or HCO3 -concentrations

Acknowledgments

Funding for this research was provided by an NSERC Discovery Grant and a Queen's University Research Initiation grant awarded to ALH. Wollastonite used in this study was graciously provided by Bob Vasily of Canadian Wollastonite. We would like to acknowledge Vasileios Mavromatis, Heather Jamieson, Matthew Leybourne, and the editor, Michael Kersten, and two anonymous reviewers for their insightful comments on this research. The authors would also like to thank Adriana Taylor for assistance in the laboratory, Alexandre Voinot and Marissa Valentino of Queen's University Facility for Isotopic Research for assistance with ICP-OES and TQ-ICP-MS analysis, Agatha Dobosz of Queen's University for assistance with SEM analysis, Karla Newman of the Trent University Water Quality Center for assistance with TQ-ICP-MS analysis, the University of Guelph Agriculture and Food Laboratory for their assistance with solid carbon analysis, and Carole Causserand for assistance with the DIC/DOC analyses.