Effect of Short Carbonation on Element Mobility in Wood Ash

Abstract

During carbonation of wood ash, carbon dioxide reacts with alkali or alkaline earth components in the ash-forming carbonates. The impact of a short carbonation on the mobility of elements in wood ash was investigated. The aim was therefore to apply a sequential extraction approach to investigate the influence of a short carbonation process on the mobility of environmentally relevant metals and plant nutrients in wood ash. Results show that short carbonation results in differences in the chemical bonding form of ash-forming elements and thus their potential mobility. The mobility of almost all examined metals (with the exception of Zn) increased after short carbonation treatment compared to the untreated ash. The extractable fractions of Cd, Cu, and Ni increased from 16.5 to 21.2%, 14.7 to 18.1%, and 20.3 to 24.5%, respectively. Pb and Cr also exhibited slight increases in element mobility, while for Zn, the overall extraction efficiency decreased from 19.6% to 15.8%. Also plant nutrients (phosphorusmore oxidizable and magnesiummore acid-soluble) exhibited increased mobility after carbonation. Overall, the results demonstrate that short carbonation is not a purely stabilizing process but a dynamic transformation governed by changes in pH, carbonate formation, and mineral equilibria. While carbonation can contribute to immobilization, it may also temporarily increase element mobility. These findings improve the mechanistic understanding of ash–CO₂ interactions and provide a basis for targeted ash conditioning strategies.

1. Introduction

The use of wood ash as a secondary raw material is becoming increasingly important in the context of the energy transition and sustainable circular economy. In addition to plant nutrients, wood ash also contains environmentally relevant metals and other potentially problematic elements. Their bioavailability and environmental mobility must be critically assessed when wood ash is used as a material, for example as fertilizer. , Element release is a key criterion for evaluating both environmental risks under natural conditions and potential technical treatment options, such as wet chemical processing.While the availability of nutrients to plants may be desirable, for example in agricultural use, high mobility of heavy metals poses a risk to soil and groundwater. The chemical stability and leaching behavior of elements in wood ash are influenced by carbonation. During this process, the alkali and alkaline earth metals present in the ash react with carbon dioxide to form solid carbonates. These reactions can occur naturally through contact with the outside air, or be induced in technical processes. Certain patterns can occur within individual element groups. For instance, some heavy metals become firmly incorporated into the carbonate phases, whereas other element groups may become more mobile after carbonation.

Wood ash is the inorganic residue produced after the thermal conversion of solid biofuels and is a byproduct of the use of biomass for energy. Most of the trace elements contained in the biomass are heavy metals which largely remain in the ash after combustion, significantly impacting its quality. Ash composition depends mainly on fuel properties and combustion technology, including grate fired furnaces, fluidized bed boilers, pulverized fuel firing systems, and downstream flue gas cleaning. Accordingly, ash is typically categorized as grate ash, cyclone ash or filter ash, with each type differing in terms of its chemical composition and pollutant content. ,

Wood ash can be used as a secondary raw material, for example as a soil amendment, fertilizer component ,,, or construction material. , However, its use is restricted by national regulations and depends strongly on ash origin, composition and pollutant content. In particular, the mobility of environmentally relevant metals is a key factor for assessing potential risks to soil and groundwater.

In Germany, wood ash is generally classified as waste under the Circular Economy Act (Kreislaufwirtschaftsgesetz KrWG). Material recycling of wood ash generally takes precedence over disposal. If this is not possible, the wood ash is disposed of in suitable landfills. Currently, a significant proportion of the wood ash produced is still landfilled. Landfilling poses ecological risks, such as the possible release of harmful substances into groundwater and the dispersal of fine ash particles. Therefore, infrastructure that complies with legal requirements is necessary. , Ash utilization and disposal is subject to legal requirements including limit values for pollutants, specifications regarding ash origin and composition, and documentation requirements. Table provides an overview of the most important legal regulations and requirements for wood ash recycling.

1. Legal Regulations and Requirements for Wood Ash Recycling.

recycling or disposal route	legal framework
Utilization in agriculture or forestry as a soil amendment, fertilizer or source material for fertilizer products	Fertiliser Ordinance (Düngemittelverordnung DüMV); Fertiliser Act (Düngegesetz DüngG); Forestry law
Additive for compost	Ordinance on organic waste (Bioabfallverordnung BioAbfV)
Utilization in the construction industry	Landfill ordinance (Deponieverordnung DepV); Product-specific requirements, environmental impact assessment required
Use as backfill material in mining	Landfill ordinance (Deponieverordnung DepV); Underground disposal ordinance (Versatzverordnung VersatzV)
Landfilling (disposal)	Landfill ordinance (Deponieverordnung DepV)

Carbonation occurs when CO₂ reacts with alkaline ash components, particularly Ca- and Mg-bearing phases, leading to carbonate formation and changes in pH, mineral composition and element binding. These changes can influence the leaching behavior of both nutrients and potentially harmful metals. Carbonation can occur naturally over geological time scales, for example through the reaction of rocks with atmospheric CO₂, or as technical accelerated carbonation under controlled conditions, commonly applied to alkaline waste materials such as ash.

Carbonation reactions are mainly influenced by CO₂ concentration and pressure, water content, temperature, pH, ash properties and reactor design. Higher CO₂ partial pressure, suitable moisture and elevated temperature can accelerate carbonation, whereas excessive pressure, water content or temperature may limit CO₂ diffusion, solubility or carbonate formation. − The pH value affects both the dissolution of metal ions and the precipitation of carbonates. Low pH values promote the release of Ca²⁺, whereas high pH values promote precipitation as CaCO₃. ,, Carbonation efficiency depends heavily on the content of alkaline/alkali earth hydroxides in the ash, i.e., Ca­(OH)₂, KOH and NaOH. It also depends on physical properties such as particle size, porosity and morphology. Smaller particles with a larger surface area show higher reaction rates. −

Carbonation is used for waste stabilization, CO₂ storage and sustainable construction applications. In ash treatment, it can reduce the mobility of environmentally relevant metals by promoting carbonate formation and lowering leaching potential. At the same time, reactive ash phases can bind CO₂ as stable carbonates, and carbonated ash may be used as a secondary raw material in construction materials such as mortar or concrete. ,,

Element mobility in wood ash and other solid waste is central to assessing environmental risks, particularly the potential leaching of heavy metals under natural conditions which determines recyclability and disposal options. Therefore, the transition from a linear to a circular economy requires detailed knowledge of the mobility of elements in wood combustion ash. A suitable risk assessment method is sequential extraction. This method evaluates the ecological risk posed by the potential leaching of elements from environmental samples, categorizing them into five groups: ,− (I) Exchangeable fraction (water-soluble), (II) Carbonate-bound fraction (acid-soluble), (III) Fraction bound to iron and manganese oxides (reducible), (IV) Organically bound fraction (oxidizable) and (V) Residual fraction (bound in primary and secondary mineral structure, not expected to be released).

Elements in the exchangeable fraction are considered particularly mobile and environmentally relevant. They are weakly bound to the material and can therefore be easily released. In contrast, metals in the residual fraction are firmly bound to the mineral structure and are very difficult to dissolve and considered stable. Elements that remain after the final extraction step are generally not available to plants and can only be released from the ash matrix through long-term weathering processes. ,,, Sequential extraction has been successfully used on various materials, including soils, sediments, sewage sludge, and residues from waste incineration such as wood ash. , However, the interpretation of the results is always subject to methodological limitations. One key issue is that results from different materials are not easily comparable, as extraction methods are often adapted. ,,

Carbonation is considered an effective method of reducing the mobility of elements in various types of ash, particularly wood and waste incineration fly ash. The conversion of soluble heavy metal compounds into poorly soluble carbonates is accompanied by physical and chemical changes that significantly reduce the release of potentially environmentally hazardous elements. Table provides an overview of the possible effects of carbonation on element mobility, which will be explained in more detail below.

2. Overview of the Possible Effects of Carbonation on Element Mobility ,,,− .

underlying mechanism	effect on element mobility in carbonated ash
Formation of carbonates	Immobilization of various elements
Decrease of pH due to reaction with CO2	Promotes precipitation of amphoteric elements; reduces leaching since alkalinity is lowered
Removal of soluble salts	Improves stabilization, as chlorides can promote metal mobility; chlorides are removed with the liquid phase
Unfavorable process control (excessive CO2 supply, extreme pH values, insufficient chloride removal)	Can damage the mineral matrix, leading to element mobilization
Additives	Increased reaction efficiency and increased immobilization

A key principle of carbonation is the chemical binding of heavy metals in the form of poorly soluble carbonates, such as PbCO₃ or ZnCO₃. Several studies show particularly high immobilization rates for lead (Pb), zinc (Zn), barium (Ba), copper (Cu) and arsenic (As). ,, In, the leaching concentration of Pb from waste paper fly ash was reduced by over 99% – from 7.35 mg/kg to less than 0.25 mg/kg. Similar effects were also observed for Ba. Sequential extraction showed a clear shift of heavy metals from easily soluble fractions to carbonate-bound and residual fractions.

Carbonation significantly affects the pH value, which is a key factor in the solubility of many metals. The initial pH of many ashes is often above 12, which can result in the mobilization of amphoteric elements, such as zinc (Zn) and chromium (Cr). These elements are characterized by the fact that their oxides and hydroxides can react with both acids and bases. During carbonation, CO₂ reacts with the alkaline components of the ash (e.g., Ca­(OH)₂), usually shifting the pH value to the neutral range (pH 7–10). ,, This decrease in pH significantly decreases the solubility of many metals and creates conditions for stable mineral formation. However, extreme values must be avoided. Under acidic (<6) or strongly alkaline conditions (>10), the leaching of heavy metals such as Pb, Zn and Cd can increase again.

Wet carbonation can remove soluble salts, particularly chlorides and sulfates, thereby improving environmental compatibility and reducing the risk of metal release. Carbonation may also alter ash particle morphology, for example by forming smoother and more compact surfaces that contribute to pollutant encapsulation. ,

The effect of carbonation varies for different metals. While lead (Pb), zinc (Zn), barium (Ba) and copper (Cu) have been effectively immobilized in many studies, elements such as cadmium (Cd), nickel (Ni), chromium (Cr) and selenium (Se) exhibit inconsistent or even opposite reactions. Under unfavorable process conditions, such as an excessive CO₂ supply, extreme pH values (for amphoteric elements) or insufficient chloride removal, remobilisation can occur. , The ecological relevance of an element is also dependent on its binding state. It is not the total content of a metal that determines its mobility, but its chemical speciation. Studies such as and demonstrate that carbonation can occur naturally, resulting in reduced heavy metal leaching and stabilized pH levels in the long term. In agricultural applications in particular, this can result in a reduced “shock effect” on soils. At the same time, certain plant nutrients, such as phosphorus (P) and boron (B), can be released in a form that is more readily available to plants. Although carbonation of biomass ashes has been widely investigated, the early stage effect of mild CO₂ exposure on the redistribution of elements among operationally defined extractable fractions remains less well resolved. This is particularly relevant because short carbonation may occur during technical ash conditioning or initial aging, before equilibrium carbonation and long-term stabilization are reached.

This work investigates the effect of short carbonation on the mobility of elements in wood ash. In contrast to studies focusing on long-term or equilibrium carbonation, the present study targets the interactions between CO₂ and ash occurring during short carbonation treatment or through initial aging. To this end, wood ash samples were subjected to a controlled, short-duration CO₂ exposure in a laboratory experiment. Subsequently, element mobility in treated and untreated ash samples was assessed using sequential extraction. The objective is to determine how short carbonation alters the distribution of elements among operationally defined extractable fractions, with particular emphasis on whether short-term CO₂ exposure promotes mobilization or stabilization through phase transformation and redistribution. By resolving these processes, the study aims to improve the mechanistic understanding of ash–CO₂ interactions and to identify how short carbonation can be leveraged to selectively influence element mobility, providing a basis for targeted ash conditioning and treatment strategies.

2. Material and Methods

All samples were analyzed at the laboratory at the University of Applied Sciences in Rottenburg. Figure shows sampling, sample preparation, lab-scale carbonation, extraction procedures and analysis methods described in the following sections.

1.

Overview of sampling, sample preparation, lab-scale carbonation, extraction procedures and analysis methods. Created in BioRender. Endriss, F. (2026) https://BioRender.com/fgbz0sr.

2.1. Ash Origin and Sample Preparation

The ash samples examined originate from a 180 kW grate-fired boiler (Schmid AG Energy Solutions, Eschlikon, Switzerland). The samples were taken from the collection container for grate ash. The boiler is installed at Rottenburg University and is fuelled by natural wood chips.

The grate ash sample was homogenized using a sample divider, reducing the total sample volume (ca. 20 L) and producing two representative samples. One of them was used as the laboratory sample for further analysis and the other was retained according to DIN EN 14778:2011–09 and DIN EN ISO 14780:2020–02. The laboratory sample was then finely ground in a planetary mono mill (Fritsch, Idar-Oberstein, Germany) for 5 min at 300 rpm.

The total element concentrations in the ash sample were determined using aqua regia digestion.

For the analysis, 0.1 ± 0.0015 g of ash was digested in six replicates using aqua regia with H₂O₂ according to DIN EN 13650:2002–01. After digestion, the samples were diluted with aqua bidest. to 50 mL. The elemental composition of the ash was then determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) according to DIN EN ISO 16967:2015 and DIN EN ISO 16968:2015.

2.2. Lab-Scale Carbonation Setup

The lab-scale carbonation setup consisted of a glass gas scrubber bottle connected to a gas mixing station (IAS, Oberursel/Taunus, Germany) on the inlet side. The gas mixing station has two thermal mass flow controllers. These are factory calibrated for N₂, CO, CO₂, CH₄ and H₂ gases. A mixture of CO₂ and N₂ (Westfalen AG, Münster, Germany) was used for the experiments. The scrubber bottle contained a glass tube insert with frit to ensure fine gas distribution. The reaction vessel, which contained a magnetic stir bar, was placed on a magnetic stirrer (Phoenix Instrument, Garbsen, Germany). For carbonation, 20 g of ash were mixed with 200 mL of deionized water. The suspension was mixed for 20 min without gas supply to enhance the formation of hydroxides. Then, the gas flow for CO₂ supply was started and carried out for 15 min. The carbonation duration was limited to 15 min in order to represent early stage carbonation under mild, practically relevant conditions. The aim was not to maximize CO₂ uptake or reach carbonation equilibrium, but to isolate the initial mineralogical and chemical transformations that may occur during short technical treatment or early aging of ash. The gas mixture consisted of 15% CO₂ and 85% N₂. A CO₂ concentration of 15% was chosen to simulate practical conditions, based on typical CO₂ levels in the exhaust gases of wood-fired heating plants. N₂ was used as an inert diluent for CO₂ because the employed gas mixing station is calibrated by the manufacturer for CO₂/N₂ mixtures. This enables precise control of the CO₂ concentration while N₂ remains chemically inert under the applied conditions and does not participate in the carbonation reaction. The volume flow was set to 2900 mL/min, as this was the maximum flow rate for the selected gas mixture. After completion of the gas treatment, the suspension was placed in a drying oven (Heraeus, Hanau, Germany) for drying at 105 °C for at least 24 h.

CHN elemental analysis (LECO, Mönchengladbach, Germany) was used to determine the carbon content of the ash before and after carbonation. For this purpose, 80 ± 3.4 mg of sample material was weighed before and after carbonation in quadruplicate in tin foil cups (LECO, Mönchengladbach, Germany; Sartorius, Göttingen, Germany) and pressed into closed droplets. The samples were then placed in the sample magazine of the device and analyzed (DIN EN ISO 16948:2015–09).

2.3. Sequential Extraction Procedure

The sequential extraction of elements from the grate ash before and after the carbonation treatment as shown in Figure was carried out in accordance with. ,, Initially, 1 ± 0.002 g of the ground, carbonated ash sample and the ground, untreated ash sample were weighed in six replicates each. The solvents used for the sequential extraction were: double-distilled water (H₂O bidest.), acetic acid (CH₃COOH, 0.1 M), hydroxylamine hydrochloride (NH₂OH-HCl, 0.1 M) and ammonium acetate (CH₃COONH₄, 2 M) with hydrogen peroxide (H₂O₂, 30%).

2.

Sequential extraction procedure.

The exposure time for each extraction solution was 24 h under continuous shaking with circular motion on a shaking machine (GFL Gesellschaft für Labortechnik, Burgwedel, Germany) at 350 rpm. After each extraction step, the solid and liquid phases were separated by centrifugation (20 min at 4000 rpm; Hettich GmbH & Co. KG, Tuttlingen, Germany). The obtained leachate was decanted and its pH value was determined using a pH meter (WTW, Weilheim, Germany). Then, 0.2 mL of HNO₃ was added to the leachate for stabilization. The remaining ash was washed with 20 mL of H₂O bidest. for 20 min at 350 rpm on the shaking machine before the reduction and oxidation step. The resulting wash water was discarded.

2.4. Data Evaluation

All samples were analyzed using ICP-OES. As described in, the sample’s total element content (TEC) was set at 100%. The TEC describes the concentration c of element i determined via aqua regia digestion in mg kg^–1 (eq ).

$${\mathrm{TEC}}_i={\mathrm{c}}_{i,\mathrm{aquaregia}}[\mathrm{mg}{\mathrm{kg}}^{-1}]$$ 1

To compare the element leachability from carbonated and untreated ash samples, the TEC was compared with the element content in the leachates from the successive extraction steps. For each extraction step, the soluble fraction (SF) indicates the concentration of an element i in the leachate in mg kg^–1 relative to the TEC in the sample (eqs –).

$${\mathrm{SF}}_{i,\mathrm{water}}={\mathrm{c}}_{i,\mathrm{water}}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 2

$${\mathrm{SF}}_{i,\mathrm{aceticacid}(\mathrm{I})}={\mathrm{c}}_{i,\mathrm{aceticacid}(\mathrm{I})}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 3

$${\mathrm{SF}}_{i,\mathrm{aceticacid}(\mathrm{II})}={\mathrm{c}}_{i,\mathrm{aceticacid}(\mathrm{II})}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 4

$${\mathrm{SF}}_{i,\mathrm{reducingagent}}={\mathrm{c}}_{i,\mathrm{reducingagent}}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 5

$${\mathrm{SF}}_{i,\mathrm{oxidisingagent}}={\mathrm{c}}_{i,\mathrm{oxidisingagent}}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 6

The total SF is defined as the sum of the element concentrations across all extraction steps (eq ).

$${\mathrm{SF}}_{i,\mathrm{total}}={\mathrm{SF}}_{i,\mathrm{water}}+{\mathrm{SF}}_{i,\mathrm{aceticacid}(\mathrm{I})}+{\mathrm{SF}}_{i,\mathrm{aceticacid}(\mathrm{II})}+{\mathrm{SF}}_{i,\mathrm{reducingagent}}+{\mathrm{SF}}_{i,\mathrm{oxidisingagent}}[\%]$$ 7

The amount of elements that cannot be extracted is referred to as the residual fraction (RF). This is calculated from the TOC and SF _i,total (eq ).

$${\mathrm{RF}}_i={\mathrm{TEC}}_i-{\mathrm{SF}}_{i,\mathrm{total}}[\%]$$ 8

The extraction efficiency (EE) of an element i through sequential extraction is defined as the ratio of exctracted elements to the total element content in the raw ash (eq ).

$${\mathrm{EE}}_i={\mathrm{c}}_{i,\mathrm{\Sigma }\mathrm{water},\mathrm{aceticacid}(\mathrm{I}),\mathrm{aceticacid}(\mathrm{II}),\mathrm{reducingagent},\mathrm{oxidisingagent}}/{\mathrm{c}}_{i,\mathrm{aquaregia}}[\%]$$ 9

3. Results and Discussion

3.1. Lab-Scale Carbonation of the Ash

The CHN elemental analysis demonstrates the impact of the laboratory-scale carbonation process on the carbon content of the ash. In this context, the carbon content serves as a proxy for the degree of CO₂ fixation and thus for the effectiveness of carbonation. The untreated, ground ash exhibited an average carbon content of 3.0 ± 0.05 m.-%, which was taken as the baseline for evaluating carbonation performance. Following carbonation, the carbon content increased to 5.0 ± 0.16%. This increase confirms the successful sequestration of CO₂ in the form of carbonates within the ash matrix. It corresponds to a CO₂ equivalent of ca. 73.3 g CO₂ per kg ash immobilized through the carbonation process.

3.2. Ash Composition

To enable a comparison of the sequential extraction results between untreated and carbonated ash, the total elemental content (TEC) of the ash was first determined by aqua regia digestion (Figure ). In total, 28 elements were analyzed. Of these, gallium (Ga), indium (In), magnesium (Mg), antimony (Sb), and selenium (Se) were not detected, as their concentrations were below the method detection limit of 0.001 ppm. Trace elements, including several heavy metals, are mainly present in concentration ranges below 100 mg kg^–1 and between 100 and 10′000 mg kg^–1. Among these, arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), copper (Cu), and zinc (Zn) are considered particularly relevant to the environment. In the investigated ash, copper and lead were detected at approximately 138 mg kg^–1 and 260 mg kg^–1, respectively. Essential plant nutrients, such as phosphorus (P), calcium (Ca), potassium (K), iron (Fe), magnesium (Mg), and sodium (Na), were also identified, with concentrations predominantly exceeding 10,000 mg kg^–1. Calcium was the most abundant element, with about 275,500 mg kg^– 1, whereas sodium exhibited the lowest concentration at around 2300 mg kg^–1.

3.

Element concentrations of untreated, ground ash after aqua regia digestion measured with ICP-OES; categorized into elements <100 mg kg^–1, elements 100 – 10,000 mg kg^–1 and elements >10,000 mg kg^–1; box plots with 25% and 75% quartiles, mean, maximum and minimum values.

3.3. Element Leaching through Sequential Extraction

To assess the environmental mobility of the ash forming elements, a sequential extraction was carried out in order to differentiate the chemical binding forms within the ash matrix. The elements were fractionated into the fractions water-soluble, acid-soluble, reducible, oxidizable, and residual. Figure illustrates the percentage leaching of the elements in the successive steps of the sequential extraction for both the untreated and the carbonated ash.

4.

Extractability [%] of the investigated elements in the water-soluble (blue), acid-soluble (yellow, orange), reducible (red), oxidizable (green), and residual fractions (gray) obtained from the sequential extraction. Results are shown for untreated and carbonated ash.

Extraction efficiency (EE), derived from the sequential extraction procedure, was used as an operational indicator of element mobility. It should be noted that sequential extraction does not directly represent leaching behavior under real environmental conditions; rather, it provides a standardized approach to assess the relative mobility of elements under defined chemical conditions. Consequently, EE cannot be directly translated into leaching risk in natural systems. Sequential extraction should be interpreted as an operational dissolution sequence, rather than a strictly phase-specific method. Despite this limitation, the results allow conclusions regarding the mineral phases or binding forms that are preferentially attacked in the individual extraction steps.

A subsequent interpretation of the operational dissolution sequence is based on the following assumptions for the individual extraction steps. In the first step, water removes readily water-soluble salts, primarily highly soluble alkali and alkaline earth compounds such as KCl and K₂SO₄. The second step using acetic acid targets acid-soluble and neutralizable phases, commonly referred to as the carbonate-bound fraction, including minerals such as CaCO₃ and dolomite. In the third step, hydroxylamine hydrochloride is applied to dissolve reducible phases, particularly Fe and Mn oxyhydroxides as well as poorly crystalline oxides. The fourth step, involving H₂O₂ followed by ammonium acetate, is conventionally interpreted as releasing elements associated with organic matter and sulfides. However, as wood ash typically contains negligible amounts of organic matter, this step is instead understood to disrupt oxidizable and complexing domains, thereby mobilizing elements from phosphate-rich and amorphous matrix phases. Finally, all material not dissolved in the preceding steps is assigned to the residual fraction. This fraction is generally considered to represent elements strongly bound within crystalline mineral structures or highly refractory phases. In wood ash, this includes quartz, stable silicates, glassy matrix components, and crystalline Fe oxides such as hematite or magnetite.

3.4. Assessment of Element Mobility before and after Carbonation

The pH value is a key indicator for assessing the mobility of individual elements. While a decrease in pH generally leads to an increased solubility of many metals, under extreme conditions it can also have the opposite effect (amphoteric elements). Figure shows the pH values of the successive extraction steps for both untreated and carbonated ash. Examining the pH values following the initial water extraction step reveals that the untreated ash is strongly alkaline, with a pH value of around 12.5. After carbonation, the pH value decreased to around 11.8. While this decrease indicates that carbonation has occurred, it is relatively minor compared to other studies likely due to short treatment not leading to full carbonation. Chen at al. reported a decrease in pH from over 12 to values between 7 and 10 in waste incineration ash. In Ottosen & Sigvardsen, the pH value decreased from 12.8 to 10.8 (wood fly ash), while Rehl et al. achieved a decrease from 12.2 to around 10 (biomass ash). The pH values of the carbonated ash were 7.9 and 7.0 in the first and second steps of the acid treatment, respectively. Compared to the untreated ash (pH 7.2 in the first and 5.6 in the second acid treatment step), the carbonated ash maintained a higher pH during acid treatment, especially in the second step, suggesting greater resistance to acidification. This may be related to the formation of carbonate phases during carbonation, such as CaCO₃, which can contribute to acid neutralization. In the final two steps of sequential extraction, i.e., under reducing and oxidizing conditions, the pH values of the untreated ash increased again, reaching 7.6 and 7.1 respectively. In contrast, the pH values of the carbonated ash stabilized at slightly lower levels of 6.8 and 6.9 under reducing and oxidizing conditions, respectively.

5.

pH values of the successive extraction steps for untreated and carbonated ash.

As described by Eichermüller et al., metals such as arsenic (As), lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), copper (Cu) and zinc (Zn) are particularly relevant to the environment. The most important plant nutrients are phosphorus (P), calcium (Ca), potassium (K), iron (Fe), magnesium (Mg) and sodium (Na). Table shows the EE for these elements across all extraction steps in the sequential extraction of untreated and carbonated ash.

3. Extraction Efficiency (EE) of Environmentally Relevant Metals and Plant Nutrients across All Steps of Sequential Extraction of Untreated and Carbonated Ash in %.

	plant nutrients [%]			metals [%]
extraction step	element	carbonated	untreated	element	carbonated	untreated
Water-soluble	Ca	0.8	7.6	As	0.0	0.0
Acid-soluble I		8.4	7.0		0.2	0.9
Acid-soluble II		7.0	8.9		0.6	0.4
Reducible		7.0	7.3		0.0	0.0
Oxidisable		8.0	6.7		7.9	5.7
Total EE		31.2	37.4		8.6	7.0
Water-soluble	Fe	0.0	0.0	Cd	0.0	0.0
Acid-soluble I		0.0	0.0		0.9	4.8
Acid-soluble II		0.0	0.1		4.0	3.5
Reducible		0.0	0.1		8.9	4.9
Oxidisable		0.0	0.0		7.4	3.3
Total EE		0.1	0.2		21.2	16.5
Water-soluble	K	15.5	26.8	Cr	5.1	6.4
Acid-soluble I		6.4	13.0		5.3	3.4
Acid-soluble II		10.9	12.0		5.5	7.1
Reducible		8.9	9.1		5.1	4.9
Oxidisable		7.0	6.1		28.1	24.3
Total EE		48.7	67.1		49.0	46.0
Water-soluble	Mg	2.9	2.7	Cu	0.0	0.0
Acid-soluble I		14.6	10.1		0.0	3.5
Acid-soluble II		18.1	12.7		0.2	2.4
Reducible		19.0	19.8		7.8	3.0
Oxidisable		0.0	0.0		10.1	5.7
Total EE		54.6	45.4		18.1	14.7
Water-soluble	Na	9.6	12.1	Ni	3.1	0.0
Acid-soluble I		9.9	16.1		3.8	7.3
Acid-soluble II		18.7	16.9		5.9	5.3
Reducible		13.3	14.2		3.3	2.7
Oxidisable		19.0	19.1		8.4	5.0
Total EE		70.5	78.5		24.5	20.3
Water-soluble	P	0.0	0.1	Pb	0.0	0.2
Acid-soluble I		2.5	3.8		2.1	2.8
Acid-soluble II		2.6	4.3		2.5	2.2
Reducible		5.9	1.7		2.3	2.2
Oxidisable		26.3	12.3		21.7	16.2
Total EE		37.3	22.1		28.6	23.5
Water-soluble				Zn	2.6	2.7
Acid-soluble I					2.9	5.6
Acid-soluble II					2.6	5.9
Reducible					3.0	2.1
Oxidisable					4.7	3.3
Total EE					15.8	19.6

Results in Table show that carbonation results in differences in the chemical bonding form of ash forming elements and thus their potential mobility. Overall, the EE for the individual metals varied significantly, ranging from 7.0% for As in untreated ash to 49.0% for Cr in carbonated ash. The EE of almost all examined metals (with the exception of Zn) increased after carbonation treatment, indicating increased mobility of these elements in the carbonated ash compared to the untreated ash. The extractable fractions of Cd, Cu and Ni increased from 16.5 to 21.2%, 14.7 to 18.1% and 20.3 to 24.5%, respectively. Pb and Cr also exhibited slight increases in element mobility, while for Zn, the overall EE decreased from 19.6% to 15.8%.

The influence of pH on leaching behavior of metals from waste materials has been investigated. − Leaching patterns are typically classified as cationic (decreased leaching with increasing pH), amphoteric (enhanced leaching under acidic or alkaline conditions, minimal leaching near neutral pH) and oxyanionic (increased leaching with increasing pH). As shown in Figure , the leachates from untreated ash exhibited lower pH values compared to those from carbonated ash, which theoretically favors the release of amphoteric elements. Such behavior has previously been reported for As, Cu, Pb, and Zn. − In our experiments, these elements also show a higher EE under rather acidic conditions (untreated ash, pH 7.2–5.6) compared to near-neutral conditions (carbonated ash, pH 7.9–7.0). A cationic leaching pattern has been described for Cd and Ni, where leaching is predominantly expected under acidic conditions. Our findings are consistent with this: Cd showed substantially lower extraction from carbonated ash (0.9%) than from untreated ash (4.8%) during the first acid treatment step, while differences in the second step were negligible. Ni displayed a similar pattern, with 3.8% extracted from carbonated ash and 7.3% from untreated ash in the first step (5.9 and 5.3% EE, respectively, after the second step).

Under reducing conditions, As could not be extracted from either the untreated or the carbonated ash. For Cr, Ni, Pb and Zn, there were no discernible differences in extraction efficiency between the two types of ash. However, a noticeable increase in EE was observed for Cd (8.9%) and Cu (7.8%) in the carbonated ash compared to the untreated ash (4.9% and 3.0%, respectively).

Extraction under oxidizing conditions notably increased the overall EE after carbonation, with comparatively large proportions of individual elements released in this step. For all investigated metals, carbonated ash exhibited consistently higher extraction rates than untreated ash. However, when the oxidizing step is excluded from the total EE calculation, As, Cr, Cu, Pb, and Zn displayed lower EE in carbonated ash compared to untreated ash, while only Cd and Ni remained more strongly mobilized after carbonation. This shows that carbonation generally promotes metal immobilization, but particularly under short-term conditions it can temporarily increase extractable fractions due to mineral transformation and redistribution among binding phases.

Carbonation had mixed effects on the mobility of plant nutrients. The total EE decreased in carbonated ash compared to untreated ash for Ca (31.2 vs 37.4%), K (48.7 vs 67.1%) and Na (70.5 vs, 78.5%). In contrast, phosphorus and magnesium exhibited increased mobility after carbonation, with extractable P rising from 22.1% in untreated ash to 37.3% in carbonated ash, and Mg from 45.4 to 54.6%.Iron, by comparison, showed minimal solubility across all extraction steps and could not be extracted in significant amounts from either the untreated or carbonated ash.

In the water extraction step, carbonated ash exhibited lower EE for nearly all plant nutrients compared to untreated ash. Mg was the only exception, with a marginal increase in EE from 2.7% in untreated ash to 2.9% in carbonated ash. The decreased EE of carbonated ash in general may be attributed either to its prior contact with water during carbonation or to the immobilization of plant nutrients within the water-soluble fraction.

In the acid extraction step, the EE of Ca was similar for both ash types. In contrast, K, Na and P were extracted in smaller quantities from the carbonated ash compared to the untreated ash, suggesting that carbonation reduced the mobility of these elements under acidic conditions. Conversely, Mg exhibited a higher EE in the carbonated ash, which may indicate that carbonation enhanced its solubility in acidic environments.

Under reducing conditions, the EE of all examined plant nutrients, except P, were comparable between untreated and carbonated ash. In contrast, P exhibited a marked difference: only 1.7% was extracted from untreated ash, whereas 5.9% was extracted from carbonated ash. Among all nutrients, Mg showed the highest extraction proportions under reducing conditions, with an EE of 19.8% for untreated ash and 19.0% for carbonated ash.

Under oxidizing conditions, the EE for Ca and K was slightly higher in carbonated ash compared to untreated ash, while Na showed comparable levels in both ash variants. The most pronounced difference was observed for P, with EE 12.3% in untreated ash and 26.3% in carbonated ash. Results from previous work suggest that oxidation effectively mobilizes P from wood ash. This can be attributed to the chemical forms in which P is stabilized within the ash, often as stable mineral forms such as Ca-, K- or Mg-phosphates that are either present as discrete minerals or occluded within oxides. These compounds are resistant to dissolution under mild acidic or reducing conditions but can be released under oxidizing conditions. Results from this study show that for carbonated ash, the fraction of P that can be mobilized during the applied oxidation treatment is higher than for untreated ash. The majority of P in wood ash is typically retained in relatively stable Ca-rich phosphates. Ash hydration and carbonation is known to convert CaO/Ca­(OH)₂ into CaCO₃, thereby reducing alkalinity and lowering pH. Because P solubility and plant availability are maximized near neutral pH and markedly decrease at high pH due to precipitation with Ca, we hypothesize that carbonation shifts P from highly stable high-pH Ca phases and modifies mineral phases that previously occluded P, resulting in more P being mobilized. To verify this mechanism, further investigations using mineralogical techniques such as XRD and SEM-EDS are recommended.

Although carbonation is commonly reported to reduce the mobility of metals in alkaline residues, the present results suggest that mild carbonation of ash is not a simple immobilization process but rather a dynamic redistribution process governed by coupled changes in pH, carbonate availability, and mineral equilibria. The sequential extraction data indicate that the extractable fractions of several metals can increase during carbonation in the short term, even though the overall long-term trend according to literature may favor immobilization.

Carbonation simultaneously affects (i) pH, (ii) dissolved inorganic carbon, (iii) competition between mineral phases for precipitation, and (iv) the binding forms of metals. Fresh ash is typically highly alkaline, and metals are often associated with hydroxides, oxides, amorphous phases, or sorbed to particle surfaces. As CO₂ is introduced, alkalinity is consumed and the pH decreases from very high values toward moderately alkaline conditions. This shift can initially destabilize hydroxide-bound or surface-associated metals, leading to partial remobilization.

To better understand these mechanisms, PHREEQC simulations (Figure ) were performed to calculate saturation indices (SI) of relevant mineral phases as a function of CO₂ concentration at pH 12, 11, and 7. These calculations provide a thermodynamic framework to interpret the experimental observations.

6.

Saturation indices (SI) according to PHREEQC simulations for relevant mineral phases as a function of CO₂ concentration at pH 12, 11, and 7.

The PHREEQC results clearly show that the saturation of metal carbonate phases strongly depends on both pH and CO₂ concentration. At high pH (pH 12), carbonate mineral saturation is dominated by phases such as hydrotalcite, followed by otavite, strontianite, dolomite, and rhodochrosite. On an elemental level, this corresponds approximately to the sequence Mg, Al > Cd > Sr > Ca > Mn in terms of precipitation tendency.

At slightly lower alkalinity (pH 11), the dominant phases shift, with hydrotalcite still showing the highest SI, followed by hydrozincite, hydrocerussite, otavite, and rhodochrosite. This corresponds to Mg, Al > Zn > Pb > Cd > Mn. The increased importance of Zn- and Pb-carbonate phases suggests that moderate carbonation conditions favor their incorporation into secondary carbonate minerals.

At near-neutral conditions (pH 7), the system changes markedly. Hydrocerussite and azurite exhibit the highest saturation indices, followed by cerussite, hydrozincite, and otavite. This indicates a precipitation sequence dominated by Pb > Cu > Zn > Cd.

Taken together, these trends indicate that the metals most likely to precipitate as carbonate phases during carbonation are Al, Mg, Zn, Cd, and Pb, although their relative importance depends strongly on pH. Assuming carbonation predominantly occurs under alkaline conditions typical for ash systems, a generalized precipitation tendency can be proposed as Mg > Al > Zn > Cd ≈ Pb > Mn > others.

It is important to emphasize that PHREEQC results represent thermodynamic saturation states and do not account for kinetic limitations. A positive SI indicates that precipitation is thermodynamically favorable but does not guarantee immediate or complete mineral formation. In real ash systems, factors such as reaction kinetics, nucleation barriers, particle surface area, and the presence of amorphous phases can significantly delay or alter the precipitation process.

Overall, the combined experimental and modeling results highlight that carbonation induces a transient phase of increased metal mobility followed by potential reimmobilization. This underscores the importance of considering carbonation as a time-dependent geochemical transformation rather than a purely stabilizing treatment under oxidizing conditions.

Figure illustrates the material flows in the sequential extraction procedure for untreated and carbonated ash. The elements are grouped into environmentally relevant metals (As, Cd, Cr, Ni, Cu, Zn, Pb) and plant nutrients (P, Ca, K, Fe, Mg, Na). In both ash types, the majority of elements were present in the acid-soluble fraction (17.7% in untreated ash and 17.2% in carbonated ash), followed by the water-soluble fraction (10.1% and 3.7%, respectively). Overall, lower amounts of elements were extracted from carbonated ash, which can be attributed to prior water contact during carbonation and the formation of poorly soluble carbonate compounds. Under reducing conditions, the EE was 8.8% for untreated ash and 8.6% for carbonated ash, while under oxidizing conditions, 5.7% of elements were extracted from untreated ash compared with 7.1% from carbonated ash. The most pronounced differences between the two ash types were therefore observed in the water-soluble and oxidizable fractions.

7.

Sankey diagram of material flows for untreated ash (top) and carbonated ash (bottom). The elements are grouped into environmentally relevant metals (As, Cd, Cr, Ni, Cu, Zn, Pb) and plant nutrients (P, Ca, K, Fe, Mg, Na).

4. Conclusion

The investigations carried out show that the carbonation of grate ash from wood chip combustion has a notable impact on the chemical bonding form and mobility of various elements. Rather than acting solely as a stabilization process, carbonation induces a redistribution of elements among operationally defined fractions, driven by coupled changes in pH, carbonate formation, and mineral phase transformations. The results show that, although carbonation can reduce the mobility of certain elements, the extractable fraction of most metals increases under short-term carbonation conditions. This indicates a transient phase of enhanced mobility before potential long-term immobilization. For plant nutrients, contrasting effects were observed: Ca, K, and Na were partially immobilized, while P and Mg became more accessible, with P redistributed from residual to more extractable fractions. This behavior highlights that short carbonation modifies the ash matrix and liberates elements from the ash matrix, followed by reassociation with secondary phases. As the present study was based on one grate ash sample and one selected short-duration carbonation condition, the results should be interpreted in this context. They provide initial insight into possible changes in element distribution and extractability during mild CO₂ exposure. Further investigations using additional ash types and varying carbonation conditions would be valuable to assess the general applicability of these findings and to clarify the relationship between short-term and longer-term carbonation effects. Overall, short-term carbonation should be understood as a dynamic, time-dependent geochemical process rather than a purely stabilizing treatment. These findings improve the mechanistic understanding of ash–CO₂ interactions and provide a basis for developing targeted carbonation strategies to control element mobility in wood ash.

The authors declare no competing financial interest.