Sustainable Restoration of Depleted Quarries by the Utilization of Biomass Energy By-Products: The Case of Olive Kernel Residuals

Abstract

The combustion of biomass has a neutral atmospheric CO₂ fingerprint, because the overall produced CO₂ emissions are balanced by the CO₂ uptake from the plants during their growth. The current study evaluates the environmental impact of the biomass ash wastes originating from the combustion of olive-kernel residuals for electricity production in accordance with Directive EE/2003. Additionally, the study investigates the potential use of such waste in the restoration of depleted calcareous aggregate quarries in the frame of the circular economy, as a substrate or as a soil amendment. Olive-kernel residual ash, obtained from a 5 MW operating electricity power plant, was mixed with soil and tested for its adequacy for use as a substrate or soil amendment in a depleted calcareous aggregate quarry. The positive effects of the olive-kernel residual bottom ashes in the availability and the mobility of major and trace elements were assessed in both batch and column experiments. The effect of biomass ash in soil amelioration was assessed via pot experiments, by examining the growth of two plant species Cupressus sempervirens (cypress) and Dichondra repens (alfalfa). The environmental characterization of the olive-kernel residual bottom ash indicates that the water-leachable concentrations of controlled elements are, generally, within the acceptable limits for disposal as inert waste in landfills. However, the bottom ash was found to contain significant amounts of K, Ca and Mg, which are macro-nutrients for the growth of plants, serving as a slow-release fertilizer by adding nutrients in the soil. The application of bottom ash in the alkaline soil had a minor positive effect in plant growth while the addition of the ash in the acidic soil exhibited considerable effect in the growth of Dichondra repens and Cupressus sempervirens due to the release of nutrients and to the pH conditioning. Olive-kernel residual bottom ash has been proved to be appropriate as a soil amendment, and as a soil substrate for the restoration of depleted quarries, decreasing the requirement for commercial inorganic fertilizers.

1. Introduction

The use of biomass in the production of electric energy has been gaining ground over the last decades, under the obligation of the EU’s 2030 and 2050 directives. By the year 2030, renewable energy resources should contribute at least 32% of the gross energy consumption, with the ambition to increase this share to 90% by 2050 [1]. Within this framework the use of biomass in the energy sector has been promoted (17%), mainly in the production of electricity (70%) [2]. In Greece, renewable energy resources are aimed to contribute up to 55% of total electrical energy produced. It is estimated that by 2030 about 1700 GWh will be produced from biomass and biogas [3]. In 2019, 655 TWh of global electricity was generated from biomass combustion, accounting for 10% of total renewable energy production.

The combustion of biomass has a neutral atmospheric CO₂ fingerprint, since overall produced CO₂ emissions are balanced by the CO₂ uptake from the plants during their growth. Woody biomass is the main feedstock in EU power-plants, comprising mainly bark, woodchips, and sawdust. However, biomass combustion generates large amounts of by-products mainly bottom and fly ash. It is estimated that ~170 Mt/yr of biomass ash are produced annually from the combustion of biomass in power-plants. which is potentially going to increase up to ~1000 Mt/yr [4]. The chemical composition of biomass ashes depends on the type of feedstock and the combustion conditions. Woody biomass ashes may contain up to 68% wt SiO₂, 84% CaO, 15% wt Al₂O₃, and 9% wt Fe₂O₃. These ashes are also rich in plant nutrients such as MgO (< 19% wt), P₂O₅ (< 17% wt) and K₂O (< 35% wt), while Na₂O and Cl₂O may reach 4% wt and 8% wt respectively. Finally a majority of trace elements are also found in solid biomass ashes, such as Cr, Cu, Ni, Mn, Fe, Pb, Zn and Se [4,5,6]. In a recent published work comparing the properties of biomass (sawdust, olive kernel residuals and reed) and coal fuels (lignite) from Greece, the researchers reached the conclusion that all studied biomass samples exhibited higher calorific values than the coal used in electricity production in Greece [7].

The total elemental compositions of ashes have been used in various classification systems in order to assess their efficiency in different applications [5,8,9,10] so as to minimize the amounts of wastes that end up in landfills [11]. However, the water-leachable elemental concentrations are more appropriate to assess both the potential environmental hazard and efficiency of the ashes. Biomass ashes have a higher water-leachable fraction compared with coal ashes (up to 61% wt), due to the presence of soluble alkali salts (KCl, K₂SO₄, etc.). The water-leachable concentrations of Cl, S, K, Na, and Mg vary from 0 to 99% wt. They also exhibit a wider pH range, from 4.5 to 13.4, owing to the different combustion temperature and storage conditions [12]. Consequently, the mobility of trace elements also varies. The mean water-leachable concentrations of these elements do not exceed 10%wt of the total concentrations [12,13]. Furthermore, the high water-soluble content of macro- and micro-nutrient in solid biomass ashes makes them appropriate as soil fertilizers and amendments. Specifically, the high content in K, P, S, Ca, etc. aids the growth of plants, whilst their alkaline character enhances the biological activities of soils and improves the texture aeration and water-holding capacity of the soils. Despite these advantages, few countries (Denmark, Finland, Germany) have established regulations for the recycling of biomass ashes back into soils as fertilizers [10].

In southern Europe, biomass originates mainly from the by-products of olive-oil production, such as kernels, peels, and wastewater [14]. The world average yield of olive groves is about 11.44 million ha, whereas Europe has 4.6 million ha [15]. Moreover, as in Greece, olive residues display a significant allocation among different agro-industrial residues, with an annual (2020 data) share of ca. 9.5 Mt [16,17]. The physicochemical, chemical and morphological characteristics of these ashes have been studied for their utilization in the construction industry [18,19,20] and in civil engineering [21]. Most studies focus on the bulk chemical composition of the biomass ashes for the evaluation of their utilization in various applications. Few studies have been conducted regarding the leachability of major and trace elements from olive by-product biomasses and their effect on the respective amended soils [13].

On the other hand, extraction activities for aggregates, usually, come with negative effects on the various environmental components of the quarry areas. Extraction activities may lead to a complete loss of topsoil, compaction of the remaining soil, nutrient depletion and high pH (e.g., in the case of a limestone quarry) that makes natural establishment of vegetation very unlikely [22]. Therefore, revegetation requires the addition of amendments for soil improvement. It is noted that both EU and Greek legislation require the environmental restoration and rehabilitation of any exhausted quarry.

The current study investigates the potential use of biomass ash originating from the combustion of olive-kernel residuals for electricity production in the restoration of depleted quarries as substrate or soil amendment in the frame of the circular economy. In earlier studies lignite fly ash has been successfully utilized for depleted mine remediation and clean up [e.g., [23]]. Olive-kernel residuals are used in Mediterranean regions as feedstock for power-plant boilers to produce electricity. Such a power plant exists in central Greece that provides energy up to 5 MW, using olive-kernel residuals imported from north Africa (due to logistics issues) and the environmental management of waste from the combustion is required by EU and Greek regulations. For the scope of this study, the produced bottom ash will be mixed with soil and used for the backfilling of a depleted aggregates quarry that is located nearby the power-plant. The specific objectives of the study are: (a) the environmental characterization of the bottom ash regarding its disposal in a landfill, (b) the determination of the leachability rate of trace and major elements (inorganic nutrients) from the bottom ash and the respective amended soil and (c) the effect of the bottom ash application on the growth of different plant species suitable for the restoration and rehabilitation of a depleted aggregates quarry.

2. Materials and Methods

2.1. Preparation and Characterization of Solid Samples

The bottom ash (BA) samples were collected from a combustion power plant with a capacity of 5 MW that uses olive-kernel residuals as a feedstock for the production of electricity. In order to ensure the representativeness of the samples, sampling took place from the power-plant boilers (operating in their maximum capacity of 5 MW at approximately 1000 °C) on a daily basis for a week during a month. At the end of the month 17 L of bottom ash were collected. This bottom ash was homogenized manually by coning and quartering, and crushed to obtain particles with a size < 2 cm. A representative subsample was subtracted for the subsequent experiments and analysis.

An alkaline soil (pH 9.3) was collected from a nearby depleted calcareous aggregates quarry that was to be restored for use in batch, column and pot experiments. Moreover, an acidic soil (pH 5.7) was collected from Kyparissia town (SW Greece) and used only in pot experiments for comparison. Both soils were homogenized by quartering and coning and sieved to obtain the <2 mm grain fraction. Mixtures with bottom ash and soil were then prepared at a ratio 1:1 w/w.

The solids (bottom ash and soils) were characterized regarding their mineralogical composition using X-ray diffraction in randomly oriented samples (SIEMENS D5005, CuKa, 40 mA, 40 mV). The samples were scanned from 5° to 65° (2theta) at a scanning rate of 0.18°/min. Zincite (ZnO) had been added as internal standard at a ratio 1:4 w/w. The mineralogical phases of the samples were identified using EVA v.2.2 (Bruker, Billerica, MA, USA) and the Rietvield quantitative analysis was performed using Topas V.5.0 (Bruker, Billerica, MA, USA).

The morphology of the bottom ash samples was examined using scanning electron microscopy (SEM) on graphite-coated free surfaces. EDS spot analysis was also used to identify the chemical character of selected grains (JEOL JSM-5600 equipped with Oxford Link ISIS 300, with a beam diameter of 3 μm, operating at 0.5 nA and 20 kV).

2.2. Leaching Experiments

A series of batch and column leaching experiments were performed to assess the mobility of nutrients and potentially toxic elements (PTEs) in bottom ash amended soils. Duplicates of bottom ash, alkaline soil, acidic soil and their mixtures were mixed with deionized water in 10 L/kg ratio for 24 h in a rotary shaker, according to the EN 12457-4 protocol [24]. The suspensions were vacuum filtered using 0.45 cellulose membranes.

The column experiments were set using glass burets filled with 300 g of bottom ash, alkaline soil, and their mixtures in duplicates. Glass wool was added at the bottom of the column to support the solids bed. The solids were kept at saturation by adding 250 mL of water once per week from the top of the column. The leachates (150–250 mL) were collected from the bottom of the column in the 1st, 7th, 14th and 28th day of the experiment and were immediately vacuum filtrated using 0.45 μm cellulose membranes. The pH and total dissolved solids were measured in the batch and column solutions immediately after their collection.

2.3. Pot Experiments

To investigate the efficiency of bottom ash as a substrate for quarry restoration, pot experiments were carried out using Cupressus sempervirens (cypress) and Dichondra repens (alfalfa) plant species. Cupressus sempervirens is suitable for the restoration of the quarry area while the rapid growth rate of Dichondra repens gives the opportunity to assess the results of the experiment in a relatively short time. Bottom ash was added in different pots containing alkaline and acidic soil in ratio 1:1 w/w. The plants of Cupressus sempervirens used for the experiments were about 12-months old. Pots containing only each different soil were also used as controls. The pots (containing about 2.5 kg of solids) were watered daily for a week using deionized water. After a week, an amount of cypress plants or 1.5 g of alfalfa grains were planted in the pots. Then, the pots were watered in a daily basis and the development of the plants was monitored.

2.4. Methods of Chemical Analysis

Bulk chemical analysis of the solids was carried out using X-ray fluorescence (XRF) after fusion with lithium tetraborate (SGS labs., Mississauga, ON, Canada). Sulfur and total carbon content were estimated using LECO analyzer (SGS labs., Mississauga, ON, Canada). Trace elements were determined after digestion with Na₂O₂/NaOH using ICP-AES/MS (SGS labs., Mississauga, ON, Canada). The concentration of Cl⁻ and SO₄²⁻ ions in batch and column leachates was determined by Mercuric Thiocyanate (Method 8113) and SulfaVer4 (Method 8051) using HACH DR6000 UV-Vis spectrophotometer in the Laboratory of Economic Geology and Geochemistry of the National and Kapodistrian University of Athens. Aliquots were oxidized and stored at 4 °C for chemical analysis by ICP-AES/MS techniques (Bureau Veritas Lab., Vancouver, BC, Canada). The obtained analytical relative percent difference between the duplicates was < 10%.

3. Results

3.1. Characterization of Solids

The morphology of bottom ash particles as investigated by SEM-EDS and representative images are shown in Figure 1. These particles are mainly amorphous (glassified) material (77% wt). having a size > 1 mm and consisting of Si Ca Na and K. Moreover, organic remnants (> 2.5 mm diameter) of the olive-kernel feedstock were also identified. The mineral components of bottom ash were quartz (12.5% wt) sanidine (3.9% wt) wollastonite (1.9% wt) cristobalite (1.9% wt) and sylvine (< 1% wt). The amorphous phase was found to comprise 78.8% wt of the bottom ash. Figure A1 in the Appendix presents the X-ray diffraction pattern of the bottom ash, showing its mineralogical composition. The alkaline soil consisted mainly of calcite (36% wt), micas (20.5% wt) and quartz (13.2% wt). Illite (9.1% wt), plagioclase (4.7% wt), chlorite (4.6% wt), actinolite (1.6% wt) and dolomite (1.7% wt) were also identified. while 8.6% wt of this soil comprised its amorphous phase.

The bulk chemical composition of the bottom ash prior to and after leaching with deionized water (following the EN 12457-4 protocol) is presented in

Table 1. Bulk chemical composition (% wt) of primary bottom ash and water-leached bottom ash samples.

Element (% wt)	Bottom Ash	Leached Bottom Ash
SiO2	59.64	61.20
Al2O3	2.97	2.71
TiO2	0.19	0.18
Fe2O3	1.55	1.45
CaO	14.98	13.98
MgO	2.34	2.24
Na2O	1.18	1.20
K2O	8.18	8.28
Cr2O3	<0.01	0.02
MnO	0.03	0.03
P2O5	1.65	1.54
L.O.I	7.07	6.77
Total	99.78	99.60

.

No significant differences were observed in the chemical composition of the bottom ash after washing it with deionized water. Silica and calcium are the main elements comprising the bottom ash particles, followed by K, A, Fe, Mg and Na. Moreover, the total elemental concentrations of selected trace elements in BA samples are low, compared with the Finnish legislation limits for the application of biomass ash in agriculture and forestry (

Table 2. Chemical composition of bottom ash (BA) in relation with the Finnish legislation limits (FLL) for the application of biomass ash in agriculture and forestry [10,25].

Element	Unit	BA	FLL for Agricultural Ash-Based Fertilizers	FLL for Forestry Ash-Base Fertilizers
Ca	%	10.90	>10	>6
P + K	%	7.54	>2	>2
As	mg/kg	<5	<25	<40
Cd	mg/kg	<0.2	<2.5	<25
Cr	mg/kg	85	<300	<300
Cu	mg/kg	87	<300	<700
Pb	mg/kg	<5	<100	<150
Ni	mg/kg	30	<100	<150
Zn	mg/kg	61	<1500	<4500

).

3.2. Chemical Composition of Suspensions and Leachates

The elemental water-leachable concentrations of the solid suspensions and their respective maximum acceptable limits for waste characterization according to Directive 33/2003 [26] are presented in

Table 3. Physicochemical characteristics (in mg/kg dry basis) of the solid suspensions and comparison to the maximum acceptable limits for waste characterization according to Directive 33/2003 (EU Council, 2003) [26].

				EC 33/2003
Parameters	Bottom Ash	Soil	Bottom Ash/Soil	Inert Wastes	Non-Hazardous Wastes	Hazaradous Wastes
pH	12.5	9.7	11.6
TDS	4975	4300	1150	4000	60,000	100,000
Ca	772.2	157.3	255.8
K	2018.8	5.9	312.1
Mg	2.3	18.5	1.2
Na	156.4	25.2	64.3
SO4−2	676	60.0	306	1000	20,000	50,000
Cl−	205	65	20	800	15,000	25,000
Si	92.23	23.77	101.13
Al	23.53	0.37	0.97
Fe	0.08	0.19	0.26
Mn	0.08	0.00	0.01
As	0.02	0.00	0.10	0.50	2.00	25.00
Ba	0.31	0.10	0.20	20	100	300
Cd	bdl	bdl	bdl	0.04	1.00	5.00
Cr	1.47	0.00	1.10	0.50	10.00	70.00
Cu	0.07	0.03	0.09	2.00	50.00	100.00
Mo	0.08	0.01	0.04	0.50	10.00	30.00
Ni	0.03	0.01	0.05	0.40	10.00	40.00
Pb	0.003	bdl	0.002	0.50	10.00	50.00
Sb	bdl	0.01	0.01	0.06	0.70	5.00
Se	bdl	bdl	bdl	0.10	0.50	7.00
Zn	0.32	0.10	0.20	4.00	50.00	200.00

. All solid suspensions were highly alkaline with a pH > 9. Total dissolved solids were also elevated in the bottom ash and soil samples, exceeding the limits for inert wastes (> 4000 mg/kg). However, TDS values decreased in the bottom ash/soil mixture, falling below the legislation limits. Accordingly, the concentrations of Ca, K, Na, SO₄²⁻ and Cl⁻ decreased when the bottom ash was mixed with soil. The same trend was also observed in the concentrations of trace elements such as Al, Mn, Ba, Cr, Mo, Pb and Zn.

The major ion concentrations of column leachates (mg/L) with respect to time are presented in Figure 2. The pH of the leachates remained stable at ~11 during the leaching experiment in BA columns, whereas it increased from 8.7 to 10.4 in soil and from 9.1 to 11.1 in BA/soil columns. Moreover, the TDS decreased with time in all columns from 3525 to 600 mg/L in BA, from 191 to 76 mg/L in soil and from 305 to 153 mg/L in BA/soil columns respectively. The BA columns exhibited high leachability rates for all the elements except Mg (Figure 2,

Table A1. Concentrations of ions in the leachates of BA columns versus time and their % difference between the 1st and 28th day.

Sample		BA				% Difference
days		1	7	14	28
Ca	mg/L	76	40	32	27	65
Mg	mg/L	1	1	1	0	82
K	mg/L	3398	963	479	316	91
Na	mg/L	201	78	48	32	84
Cl	mg/L	200	20	5	5	98
SO4	mg/L	1061	212	91	47	96
Fe	μg/L	2700	820	325	65	98
Mn	μg/L	66	31	11	3	95
Co	μg/L	6	2	1	0	97
Cr	μg/L	855	134	87	12	99
Cu	μg/L	215	180	185	34	84
Ni	μg/L	90	20	12	13	86
Se	μg/L	13	5	3	0	98
Zn	μg/L	333	141	98	31	91

), followed by the BA/soil columns (

Table A3. Concentrations of ions in the leachates of BA/soil mixture columns versus time and their % difference between the 1st and 28th day.

Sample		BA/Soil				% Difference
days		1	7	14
Ca	mg/L	65	13	10	11	84
Mg	mg/L	8	1	0	0	94
K	mg/L	73	45	48	53	27
Na	mg/L	64	27	21	17	74
Cl	mg/L	27	1	1	1	98
SO4	mg/L	200	30	18	15	92
Fe	μg/L	89	192	153	51	43
Mn	μg/L	29	10	7	3	90
Co	μg/L	4	2	1	1	80
Cr	μg/L	214	29	15	3	98
Cu	μg/L	15	19	12	10	35
Ni	μg/L	95	51	21	8	92
Se	μg/L	2	1	1	1	68
Zn	μg/L	177	176	22	21	88

). About 44% of Ca, 43% of Mg, 66% of K and 56% of Na were released within the first day of elution from BA columns. The respective percentages in BA/soil columns were 66% for Ca, 81% for Mg, 33% for K and 49% for Na. Chloride and sulfate also decreased in BA columns, but no significant differences were observed in soil and BA/soil columns.

The variation in the elemental concentrations of trace elements with time are presented in Figure 3. Trace elements, such as Fe, Cr, Cu, Mn, Zn and Se were rapidly leached from BA columns (Table A1) but were retained in the BA/soil columns (Table A3). Approximately 69% of Fe, 59% of Mn, 72% of Co, 79% of Cr, 35% of Cu, 67% of Ni, 62% of Se and 55% of Zn were released from BA columns. Most of the elements were retained in BA/soil columns. The leachabilities were 18% for Fe, 59% for Mn, 48% for Co, 27% for Cu, 54% for Ni, and 45% for Se and Zn.

According to the observed trends in Figure 2 and Figure 3, K and Na were retained by the soil components in BA-amended columns, at the expense of Ca and Mg. By the end of the experiment, 91% K and 84% Na, were released from the BA columns, whilst 27% K and 74% Na were released from the BA/soil columns respectively. On the contrary, 82% Mg and 65% Ca were eluted from BA columns, which increased to 94% Mg and 84% Ca in BA/soil columns. Furthermore, the trace elements, such as Fe, Mn, Co, Cu, Ni, Se and Zn were also adsorbed by the clay fraction of the soil (micas and illite), as well as the organic matter. More than 85% of these elements was eluted from the BA columns (Table A3) by the 28th day, which decreased to 35–92% in BA/soil columns, Specifically, high retention was observed for Fe (from 98% to 43%), Cu (from 84% to 35%), and Se (from 98% to 68% Therefore, BA can act as a slow fertilizer agent, providing a majority of inorganic nutrients to the soils.

3.3. Effect of Bottom Ash in the Development of Plants

The effect of bottom ash composition in the development of Cupressus sempervirens (cypress) and Dichondra repens (alfalfa) is presented in Figure 4 and Figure 5. The addition of bottom ash in the alkaline soil had no significant effect in the growth of alfalfa or cypress plants (

Table 4. Effect of bottom ash (BA) added to alkaline and acidic soil in the growth (height in cm) of Cupressus sempervirens (cypress).

Weeks	Alkaline Soil	BA/Alkaline Soil	BA/Acidic Soil
0 *	119.0	123.5	120.0
2	119.0	123.5	120.0
6	119.5	124.0	120.5
10	119.5	124.0	121.0
17	120.0	124.5	122.5

* Initial plant height (pot included).

). The generally low growth rate of the Cupressus sempervirens is attributed to the youth of the initial plants; Cupressus sempervirens grows rapidly (more than 25 cm annually) when planted in loam, after its second-year age.

On the other hand, the alfalfa stems showed exceptional growth in the acidic soil amended with biomass ash.

4. Discussion

In earlier studies the energetic valorization of olive biomass (Olive-tree pruning, Olive stones and pomaces) have been discussed [28] and the possibilities of energy generation from olive tree residue by-products and waste in Greece [7,29].

A power plant that provides electrical energy up to 5 MW by combusting olive-kernel residual biomass produces more than five tons of bottom ash daily and the environmental management of those wastes is required.

The obtained results for the composition of the olive-kernel bottom ashes studied are in line with those reported in earlier works on biomass ash mineralogy and chemistry [e.g., [7,30]].

The obtained BA samples are highly alkaline, typical of biomass ashes found in the literature, as compiled by Maresca et al. [31].

4.1. Evaluation of Biomass Ash Wastes in Accordance with Directive EE/2003

The high pH and TDS values observed in the water leachates are attributed to the high amounts of alkalis (mainly K), rather than sulfate and chloride ions (Table 3). About 3.0% of the total K present in BA samples was in water-soluble form (Table 1 and Table 3). All elements exhibit slightly lower water leached concentrations in BA (Table 3) compared with the bulk material (Table 1), implying that the majority of the elements are hosted in non-soluble forms, such as silicate minerals (quartz, chrostobalite, sanidine) calcium-silica oxide minerals (wollastonite CS) and amorphous (glassy) phases (see Figure A1 in Appendix A).

The biomass ashes also exhibit low concentrations of trace elements (As, Ba, Cd, Cu, Ni, Pb, Sb, Zn), chloride and sulfate ions that comply with regulations for inert wastes to be disposed in landfills (Table 3). Chromium concentrations fall marginally within the category of non-hazardous wastes and are higher than those found in the literature for woody ashes [31]. Elevated Cr concentrations have been reported in olive biomass bottom ashes in previous works [13,19].

4.2. Utilization of Biomass Ash as Quarry Substrate and as Soil Amendment

The effect of the olive-kernel residual bottom ashes in the mobility of major and trace elements was assessed in both batch and column experiments. Biomass ash was enriched in Ca, Mg, Na and K compared with the soil as depicted in the water-extractable concentrations of these elements (Table 3). By adding biomass ash to the soil, the extraction of cations decreased, showing that these elements were sorbed and retained by the soil components (Table 3).

The release rate of major and trace elements in the biomass ash amended soils compared with biomass ash and the control was also investigated. Ca, Na, K, Cl⁻, and SO₄²⁻ were eluted from the columns within the first five days. This implies that these elements are present in biomass ash as water-soluble salts and compounds [13,32,33]. About 1.7 g of K, 0.13 g of Na, and 0.08 g of Ca, were eluted from BA columns. The respective elemental mass from the columns containing the BA/soil mixture was 0.12 g of K, 0.05 g of Na and 0.04 g of Ca due to the fixation of these elements by the soil components [33]. This fixation was also observed in a majority of trace elements, such as Fe, Cr, Cu, Mn, Se and Zn.

Recirculation of biomass ashes into soil as fertilizers has been adopted in northern Europe, as an efficient way to both manage the large amounts of wastes and increase the fertility of forest and agricultural soils [10,31]. The total elemental concentrations of major and trace elements fall within the regulatory limits establish by Finland for the utilization of biomass ashes as agricultural and forestry fertilizers (Table 2). Especially for the micro-nutrients, the BA columns released 0.3 mg Cu, 0.006 mg Se, and 0.2 mg Zn, whilst the BA/soil columns released 0.03 mg Cu, 0.002 mg Se, and 0.14 mg Zn, respectively. This fixation is attributed not only in the soil components that may sorb the trace elements (such as organic matter, clays and Fe-oxides) but also to the alkaline pH of the solid leachates (pH > 9.5) [13,33]. Due to the high availability in nutrients and the low concentrations of toxic elements bottom ash could be used as a soil substrate.

According to Vasilatos et al. [7] the differences in the composition of biomass ashes lead to different environmental management and/or potential industrial applications. The same authors have indicated that the abundance of nutrients such as potassium, calcium and sulfur in the biomass ashes, may be beneficial for plants.

The application of bottom ash in the alkaline soil had a positive effect in plant development which is mainly attributed to the release of nutrient from the bottom ash and their retention by the soil components. Bottom ash served as a slow-release fertilizer [34] by adding nutrients to the soil. However, the most profound growth of the plants was observed in the acidic soil due to the concomitant increase of soil pH (pH conditioner). The significant amounts of K, Mg, Ca, Se and Zn helped in plant growth compared with the control soil (Figure 4 and Figure 5) [35]. As a result, the studied biomass ashes could aid the growth of cypress species that may be planted in the restored depleted quarry area.

5. Conclusions

During this study we evaluated the biomass ash wastes originating from the combustion of olive-kernel residuals for electricity production in accordance with Directive EE/2003 and investigated the potential use of this waste in the restoration of depleted calcareous aggregate quarries as substrate or soil amendment, in the frame of the circular economy.

The results of the chemical and environmental characterization of the olive-kernel residuals bottom ash, based on Directive 33/2003, indicate that the water-leachable concentrations of controlled elements (except Cr) are, generally, within the acceptable limits for the disposal of that ash, as inert waste in landfills.

However, bottom ash from electricity production through combustion of olive-kernel residuals was found to contain significant amounts of K, Ca and Mg, which are macro-nutrients for the growth of plants.

Both batch and column experiments showed that the leachability of major and trace elements decreased in biomass ash-amended soils, implying the retention of these elements by the soil components.

Bottom ash serves as a slow-release fertilizer by adding nutrients in the soil.

The application of bottom ash in the alkaline soil had a minor positive effect in plant growth which is mainly attributed to the release of nutrients from the bottom ash and their retention by the soil components.

The addition of the bottom ash in the acidic soil exhibited considerable effect in the growth of Dichondra repens and Cupressus sempervirens due to the release of nutrients from the bottom ash and due to the pH conditioning by the amendment.

Therefore, olive-kernel residual bottom ash is appropriate as a soil amendment and is suitable to be used as a soil substrate for the restoration of depleted quarries decreasing the requirement for commercial inorganic fertilizers.