Raw and Calcined Eggshells as P-Reactive Materials in a Circular Economy Approach

Abstract

Phosphorus (P) is a crucial factor influencing both plant growth and the enrichment of the aquatic environment. Agriculture is the primary sector of the economy where the demand for phosphorus is the highest. Due to the depletion of P, more and more attention is being paid to the possibility of recovering and reusing P through the idea of a circular economy (CE). The main objective of this study was to compare raw eggshells (R-ESs) and calcined eggshells (C-ESs) for P removal from wastewater and assess the possible use of agro-waste materials according to CE requirements in non-flow conditions. A synthetic indicator, the zeroed unitarization method, was calculated to evaluate the critical aspects of materials according to the CE. The sorption of R-ESs and C-ESs equaled 0.90 and 1.66 mgP-PO₄/g for an initial concentration of 17.3 mgP-PO₄/L. The C-ESs were characterized by an almost two times higher reduction rate than R-ESs. The calculated indicator for the CE requirements equaled 0.89 and 0.72 for R-ESs and C-ESs, respectively. This means that R-ESs are more sustainable than C-ESs. Although C-ESs potentially have a more significant environmental impact, it is worth considering that this method of P elimination is beneficial from an ecological perspective.

1. Introduction

The circular economy (CE) idea requires maintaining resources and materials in the environment as long as possible and turning waste into a product for other uses. Such an approach is essential to the EU’s development of a sustainable, low-carbon, resource-efficient, and competitive economy [1]. The concept of a CE has gained significant attention in recent years as the world grapples with pressing environmental challenges. It represents a paradigm shift from the traditional linear economy, characterized by “take, make, and dispose,” to a more sustainable and regenerative model [2]. Such an approach along the life of products minimizes wastes and maximizes the value of products and materials. In a CE, the life cycle of products and resources is extended, limiting resource consumption.

Phosphorus (P) is one of the most crucial nutrients for living beings and plants [3]. P is a component of DNA and RNA molecules and ATP, which is regarded as the energy center of cells, and is an essential component of bones and teeth [4]. The primary P sources in water come from agriculture run-off and insufficiently treated wastewater from households and industries [5,6]. In 2005, it was estimated that the loss of P into wastewater amounts to 15% of the total imported P in EU-27 [7]. P is found mainly in aqueous solutions such as untreated wastewater and urine, treated effluent from wastewater treatment plants, and solid forms such as sewage sludge or ash [4]. For these reasons, wastewater seems to be a primary source of P nutrient recovery.

The main requirement for P comes from the agriculture sector, through fertilization. This sector consumes about 90% of the total phosphate rock [8]. Moreover, depending on factors influencing the demand and supply, P rock resources, like political moods, are estimated to last 40 to 400 years [9]. It is also worth noting that P is a substance that no other material can replace. On the other hand, P is also listed as a critical raw material for the EU economy, and the supply risk is threatened [10].

Surpluses of P that are not used by plants or that come from sewage and their treatment products enter surface waters, causing anthropogenic eutrophication, a severe environmental problem. P is considered a limiting factor in this process, and even small concentrations exceeding 0.025 mg/L (according to the United States Environmental Protection Agency [11]) or even 0.01 mg/L (Water Framework Directive [12]) can damage the aquatic environment.

The gap between P overflow in wastewater and the P agricultural demand may be solved using waste P-reactive calcareous materials. The sorption properties of selected Ca-based materials are presented in

Table 1. Sorption of selected Ca-based P-reactive materials.

P-Reactive Material	Sorption [mg/g]	Specification	Reference
Limestone	4.95 1	CaO—29.3%; 10 g/L; 1–1000 mg/L; pH = 8.8	[41]
Opoka	0.1 2	CaCO3—50%; 20 mg/L; 5–25 mg/L; pH = 8.5	[42]
Calcined opoka	100.26 1	CaO—23.86%; 10 g/L; 1–1000 mg/L; pH = 10.8	[41]
Raw eggshells	0.57 1	10g/L; 1–15 mg/L; pH = 10	[43]
Calcined eggshells	72.87 1	CaO—96%; 10 g/L; 6–978 mg/L; pH = 12.5	[14]
	26 1	Ca—26.1%; 40 g/L; 0–150 mg/L; pH = 12	[27]
	39.0 3	0.1g/L; 10–600mg/L; pH = 11	[16]
Marl	43.89 1	CaO—14.62%; 20 g/L; 1.95–1610 mg/L	[44]
Calcined marl	80.44 1	CaO—30.66%; 20 g/L; 1.95–1610 mg/L	[44]
Travertine	140.48 4	CaO—35.84%; 20 g/L; 1.95–1610 mg/L	[44]
Calcined travertine	282.34 5	CaO—33.98%; 20 g/L; 1.95–1610 mg/L	[44]
Shell sand	3.50 2	CaO—40%; 3 g/75 mL; 5–1000 mg/L	[45]
Apatite	4.7 1	Ca—37%; 40 g/L; 0–500 mg/L; pH = 7.0	[46]

Sorption calculation: ¹ Langmuir isotherm; ² observed; ³ Langmuir–Freundlich isotherm; ⁴ Marczewski–Jaroniec isotherm; ⁵ Tóth isotherm.

. As can be seen, the sorption capacity is widely justified, even for the same materials (e.g., calcined eggshells) and different laboratory protocols (mass-to-volume ratio, ranges of concentrations, pH, etc.) [13]. Also, different kinds of shells are regarded as valuable P sorption materials [14,15,16,17]. Apart from chicken eggshells, calcined sepiolite [17], eggshell-modified peanut shell biochar [18], oyster shells [19], marsh clamshells, and waste mussel shells [20] have also been tested. Because of their relatively high and stable contents of CaCO₃, such materials have been widely tested as reactive materials to remove P from aqueous solutions [21]. However, CaCO₃ has a lower specific surface area and porosity [22] and requires a longer contact time at a concentration lower than 10 mg/L to remove phosphate from a solution [23]. For these reasons, there are different ways of increasing the sorption, including calcination, acidification, and hydrothermal treatment [22]. The calcination process of expanding the sorption capacity seems to be the most popular [14,15,16,24,25,26,27,28,29,30,31,32] and significantly increases the sorption capacity of the material (Table 1). Raw eggshells can remove P because of their CaCO₃ (aragonite) content, with CaCO₃ being less reactive than CaO [22]. During the calcination process, an increase in temperature to 500–600 °C causes CaCO₃ to turn into calcium carbonate calcite [22]. A further rise in temperature to 800–900 °C causes it to turn into CaO, which has a higher affinity for phosphate and a higher removal rate [28]. On the other hand, the reactive material of saturated shells can be used in agriculture as a slow-release phosphate fertilizer [33,34]. Both raw and calcined shell waste are alkaline. It can also be a valuable liming material and soil conditioner for acidic soils, thus contributing to the CE and environmental sustainability [19,35]. Furthermore, using Ca-based reactive P material is non-toxic to the environment, as opposed to using materials based on, e.g., Al or Fe [36,37]. Another advantage of using shells is the minimal waste in landfills and agro-waste disposal. It is estimated that the global annual production of eggshells is 87 million tons [38], of which the FAO estimates that 6.4 million tons ends up in landfills [39]. The use of waste materials, such as various types of shells, is consistent with the assumptions of a CE, where waste or byproducts are reprocessed into valuable resources with economic and environmental benefits [19,40].

Several methods of removing P from wastewater include physical, biological, chemical, ion exchange, and adsorption methods [13,17]. Microfiltration, reverse osmosis, electrodialysis, and magnetic separation are physical methods of P removal from wastewater. However, their main disadvantages are high operational costs and a low efficiency [13]. Chemical P treatment methods include those that use metal salts and advanced oxidation processes such as chlorination, ozonation, and the Fenton reaction [47], which are effective for P removal. Still, their main limitations are producing large amounts of sludge and releasing chemicals into the environment [17]. Instead of being effective, the main disadvantage of biological methods is their unreliability due to their sensitivity to environmental conditions [48].

When considering the limitations of various methods for P removal, the most important benefits of using ESs include low costs to obtain the material (waste material), a high removal efficiency, no toxic pollutants released into the environment, and the possibility of reusing the used material in agriculture [16,19,35].

In most previous research, the sorption properties of such materials were tested with synthetic solutions, not with real wastewater. This study addresses the knowledge gap between comparing the sorption ability of raw and calcined material made from hen eggshell waste with real wastewater and assessing the materials’ environmental impact for a CE approach. Eggshells may be proposed as an additional filter to support an on-site wastewater treatment plant in P removal. For this reason, these studies were carried out using real wastewater.

This study aimed to (i) compare raw and calcined eggshells as potential reactive materials to remove phosphorus from wastewater and (ii) assess the possible use of biowaste materials to remove phosphorus according to circular economy requirements.

2. Materials and Methods

2.1. Eggshell Sourcing

The eggshell (ES) waste samples were sourced domestically. The residuals of the egg white, yolk, and double egg membrane were removed to obtain raw eggshells (R-ESs). Then, the eggshells were ground (IKA A10) to obtain a powder fraction (>50 μm). A part of the obtained material was calcined (C-ESs) in a furnace (P330, Nabertherm, Lilienthal, Germany) at 900 °C for 3 h. The remaining material was used as raw eggshells (R-ESs). The differences between the surfaces of R-ESs and C-ESs are presented in Figure 1. The main chemical composition of both materials is provided in

Table 2. The main chemical composition of raw and calcined eggshells.

Eggshells	CaCO3	CaO	MgO	SO3	S	P2O5	Cl	Al2O3	Fe2O3	Fe
R-ESs	97.08	-	0.85	0.76	-	0.18	0.1	0.055	0.02	-
C-ESs	-	95.0	2.13	-	0.165	0.724	0.131	0.694	-	0.027

.

2.2. Methods

The non-flow condition tests used R-ESs and C-ESs, with wastewater as the adsorbate. Triplicate samples comprising 0.5 g of material were shaken with 50 mL of wastewater (mass-to-volume ratio of 10g/L) at various contact times ranging from 5 to 2880 min at room temperature (20 °C). All the samples had their P-PO₄, pH, oxidation–reduction potential (ORP), electrical conductivity (EC), and total suspended solids (TDS) determined. The wastewater was obtained from an on-site wastewater treatment plant outflow using low-loaded activated sludge technology.

The wastewater (WW) was analyzed to determine its P-PO₄ (mg/L), biological oxygen demand (BOD) (mgO₂/L), pH (−), total solids (TS) (mg/L), electrical conductivity (EC) (µS/cm), total suspended solids (TDS) [ppm], oxidation–reduction potential (ORP) (mV), turbidity (NTU), and color [PtCo]. The P-PO₄ concentrations were measured with the ammonium-molybdate method in a flow injection analysis using an FIAstar 5000, FOSS (Hilleroed, Denmark) analyzer for 5–1000 µg P-PO₄/L and 1–10 mg P-PO₄/L. The BOD was measured using respirometry and the OxiTop kit, WTW, Germany. The pH and ORP were measured with an model CPR-411 meter, Elmetron, Zabrze, Poland. The total solids were indirectly measured by spectrophotometrically examining the unfiltered suspension in the 0–750 mg/L range. The EC and TSD were measured with an Ultimar conductometer, China, and the turbidity was measured with TurbiDirect, Lovibond, Dortmund, Germany. The color was measured spectrophotometrically in the range of 0–500 PtCo. The characteristics of wastewater used in this study are shown in

Table 3. The composition of wastewater used for the tests.

P-PO4 [mg/L]	BOD [mgO2/L]	pH [−]	TS [mg/L]	EC [µS/cm]	TDS [ppm]	ORP [mV]	Turbidity [NTU]	Color [PtCo]
17.3	25	6.41	24	1190	595	224	10.0	302

.

Before the P-PO₄ analysis, all the samples were filtered through a hard paper filter and a syringe filter with a 0.45 µm pore size.

The P-PO₄ removal ratio (R) was calculated based on the following equation:

$$\mathrm{R}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}\right)}{{\mathrm{C}}_0}}·100 [\mathrm{\%}]$$

where R is the P-PO₄ removal ratio (%) and C₀ and C_t are the P-PO₄ concentrations (mg/L) initially and after time t.

The sorption capacity (S) was calculated from the following equation:

$$\mathrm{S}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}\right)·\mathrm{V}}{\mathrm{m}}}\left[{\displaystyle \frac{\mathrm{m}\mathrm{g}}{\mathrm{g}}}\right]$$

where S is the sorption capacity (mg/g), V is the volume of wastewater (L), m is the mass of the material (g), and C₀ and C_t are the P-PO₄ concentrations (mg/L) initially and after time t.

2.3. Kinetic Models

The obtained results of the non-flow condition tests were modeled using four kinetic models: pseudo-first-order kinetic order (PFO), pseudo-second-order kinetic order (PSO), Elovich, and the intraparticle diffusion kinetic model (IPD). The formulas of the following models are presented in the below equations.

Pseudo-first-order (PFO) kinetic model [17]:

$$log\left(q_e-q_t\right)=log\left(q_e\right)-{\displaystyle \frac{k_1}{2.303}}$$

where q_t (mg/g) represents the amount of P adsorbed at any time t (min); k₁ (min⁻¹) is the constant rate of sorption of the pseudo-first-order kinetic model; and q_e is the amount adsorbed at equilibrium (mg/g).

Pseudo-second-order kinetic model [17]:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{{k_{2}q}_e^2}}+{\displaystyle \frac{1}{q_e}}$$

where q_t (mg/g) represents the amount adsorbed at any time t (min); k₂ (g/mg·min) is the constant rate of sorption of the pseudo-second-order kinetic model; and q_e is the amount adsorbed at equilibrium (mg/g).

Elovich model [17]:

$$q_t={\displaystyle \frac{1}{\beta }}ln\left(\alpha \beta \right)+{\displaystyle \frac{1}{\beta }}ln\left(t\right)$$

where q_t (mg/g) represents the amount adsorbed at any time t (min); α (mg/g·min) is the initial sorption rate; and parameter β (g/mg) is related to the extent of surface coverage and the activation energy for chemisorption.

Intraparticle diffusion model [49]:

$$q_t=k_d{t}^{{\displaystyle \frac{1}{2}}}+C$$

where q_t (mg/g) represents the amount adsorbed at any time t (min); k_d (mg/g·min^0.5) is the constant rate of sorption of the intraparticle diffusion model; and C is the boundary layer thickness. The higher the value, the greater the effect.

Non-linear regression was performed by considering the following kinetic models to estimate the fitting parameters. The Excel solver optimized the correlation between the experimental data and the models.

2.4. Statistical Analysis

The P-PO₄, pH, ORP, and EC results for the R-ESs, C-ESs, and WW were compared statistically. A lack of a normal distribution (by the Shapiro–Wilk test) was detected for all the tested results, so a non-parametric test was used. The values were subjected to an analysis using the Kruskal–Wallis test. As the test was statistically significant, multiple rank mean comparisons were used for all the trials, which is the post hoc equivalent of a non-parametric test. Also, a Spearman rank order correlation table was calculated for the tested parameters. Both the tests were performed with a level of significance of p < 0.05. All the statistical analyses were carried out using the STATISTICA 13.3 software by StatSoft, Krakow, Poland.

2.5. Reactive Eggshell Materials from the Perspective of a Circular Economy

Based on the existing literature on ESs and other P-reactive materials in the CE approach [4,10,19,33,36,50], we created a list of limitations and benefits called the critical aspect of future development when using ESs (

Table 4. The critical aspects relating to the future development of ESs (own study).

Benefits and Limitations of Implementation; xi	Reference Value	Normalization
Material availability	+++	Stimulant
Technological treatment (grinding, calcination)	+	Destimulant
Impact on the environment (pH, CO2)	+	Destimulant
P-removal efficiency (sorption)	+++	Stimulant
Connection durability (adsorption, precipitation)	+++	Stimulant
Economic importance (technology vs. P removal)	+++	Stimulant
Social acceptance of technology	+++	Stimulant
Toxicity to the environment	+	Destimulant
Reuse as a fertilizer	+++	Stimulant

Scale of weighted indicators: +, low; ++, fair; +++, high.

). We used a scale to weigh the benefits and limitations as + low, ++ fair, and +++ high.

The values of the critical aspects of both R-ESs and C-ESs were standardized by zeroed unitarization, i.e., by converting destimulants into stimulants [51]. Depending on the nature of the critical aspects (x_i), the formulas calculated the benefits or limitations as stimulants or destimulants [52].

Formula for stimulants:

$$X_i={\displaystyle \frac{x_i-x_{min}}{x_{max}-x_{min}}}$$

Formula for destimulants:

$$X_i={\displaystyle \frac{x_{max}-x_i}{x_{max}-x_{min}}}$$

where x_i is the score of the key aspect; x_min is the minimum value of the key aspect (+); and x_max is the maximum value of the key aspect (+++).

The values of the synthetic variable for the degree of achievement of the circular economy requirements by R-ESs and C-ESs were determined according to the formula in [53], which ranges from 0 to 1.

$$Q={\displaystyle \frac{1}{n}}\sum _{i=1}^n{X}_i$$

where Q is the synthetic indicator [−]; n is the number of key aspects; and X_i is the normalized value of the x_i key aspect.

3. Results

3.1. Non-Flow Condition Tests

Figure 2 shows the changes in the P-PO₄ concentrations, pH, redox (ORP), and EC of the R-ESs and C-ESs according to the reaction time. The horizontal line indicates this study’s wastewater (WW) values.

The P-PO₄ concentrations decreased in both R-ESs and C-ESs (Figure 2). The concentration in R-ESs gradually reduced over time, and at the end of the experiment, it was 7.665 mg/L. In the case of C-ESs, the removal of P-PO₄ was very rapid and stable during the experiment. For C-ESs, even after 5 min of contact time, the initial concentration of 17.300 mg/L was reduced to 0.253 mg/L. At the end of the experiment, the concentration was under the detection level (<5 µg/L). The C-ES results differed significantly at p < 0.05 from WW, but were similar to those of R-ESs (Figure 3).

The pH of WW was 6.41. The observed pH values of R-ESs ranged from 6.75 to 8.35. On the contrary, the pH of C-ESs was higher and ranged from 11.76 to 12.27 (Figure 2). No significant difference between R-ESs and C-ESs at p < 0.05 was detected, but they differed significantly (p < 0.05, Figure 3) from the WW.

The ORP value of WW was 224 mV, whereas the values of R-ESs ranged from 88 to 159 mV and increased during the experiment. The ORP of C-ESs ranged from −42 to −74 mV, indicating the material’s reducing properties. The lower the ORP value, the stronger the material is as a reducer [54]. Such observations did not find confirmation in statistical tests. The calcined eggshells differed significantly (p < 0.05, Figure 2) only from the WW.

Because of a similar course for the TDS and EC, only the results for the EC are shown. In the case of the EC, the values were generally stable (Figure 2) for the raw material during the experiment, ranging from 1163 to 1728 µS/cm. Moreover, the values for R-ESs did not differ significantly from the median value noted for WW, which was 1190 µS/cm. (Figure 2 and Figure 3). In the case of C-ESs, the course of the EC curve looked different (Figure 2). During the first 5 h of contact between the material and the solution, an increase in the EC values was observed. After that, the concentrations started to decrease. The EC for C-ESs differed significantly from the values of WW and R-ESs, with p < 0.05 (Figure 3).

Considering both sorption (S) and reduction (R) (Figure 4) for R-ESs and C-ESs, the first one is characterized by slowly increasing the sorption and reduction values during the experiment. After 5 min of contact time for the R-ESs with the wastewater, the sorption value equaled 0.07 mg/g and corresponded to a reduction of 4.5%. Ultimately, these values equaled 0.90 mg/g and 55.7%, respectively. Completely different behavior was observed in the case of C-ESs. From the first minutes of contact of the material with the wastewater, a sharp and rapid reaction was observed. Even after 5 min, a sorption of 1.62 mg/g and a 99% reduction in phosphates were observed. These values were stable among all the experiments and equaled 1.72 mg/g and 100%, respectively.

When the results of both materials were compared, C-ESs were characterized by an almost two times higher phosphate sorption and reduction than R-ESs.

The linear relationship between the P-PO₄, pH, TDS, and ORP was determined based on the Spearman correlation (

Table 5. The Spearman rank order correlation table.

Variable	R-ESs				C-ESs
	pH	ORP	EC	P-PO4	pH	ORP	EC	P-PO4
pH	1.000000	0.928571 *	1.00000	−0.854381 *	1.000000	−0.261905	−0.738095 *	−0.380952
ORP		1.000000	0.928571 *	−0.904762 *		1.000000	0.071429	0.071429
EC			1.000000	−0.833333 *			1.000000	0.738095 *
P-PO4				1.000000				1.000000

* The correlation coefficients significant with p < 0.05.

). For R-ESs, the P-PO₄ concentrations were correlated with the rest of the tested parameters at a significant level (p < 0.05). All the correlation coefficients indicated a strong relationship among the tested parameters (>0.83). However, the strongest was for the ORP (−0.904762). In the case of C-ESs, a significant relationship was seen only for P-PO₄ and the EC, but the strength of the relationship was relatively moderate (0.738095).

3.2. Kinetic Model Results

The kinetic results for P sorption by R-Ess and C-Ess were studied using the previously mentioned models. The non-linearized forms of the PFO, PSO, Elovich, and IPD models were plotted and correlated with the experimental data to describe the P-sorption processes shown in Figure 5 and

Table 6. Results of examined kinetic models.

Specification	R-ESs	C-ESs
Pseudo-first-order kinetic model (PFO)
k1 (min−1) qe (mg/g)	0.001663 0.946797	0.861739 1.719434
R2 (−)	0.8410	0.9986
Pseudo-first-order kinetic model (PFO)
k2 (g/mg·min)	0.002052	6.625683
qe (mg/g)	1.081489	1.722558
R2 (−)	0.8617	0.9995
Elovich
α	0.009608	1∙1018
β	5.586314	29.07358
R2 (−)	0.9201	0.8878
Intraparticle diffusion kinetic model
kd (mg/g·min 0.5)	0.016895	0.000489
C (mg/g)	0.11496	1.707202
R2 (−)	0.9674	0.9990

.

Considering the results of the analyzed kinetic models, the highest degree of fit was achieved by the IPD for both the R-ESs (R² = 0.9674) and C-ESs (R² = 0.9990) (Table 6). On the other hand, the second highest kinetic models differed in the case of R-ESs and C-ESs. The Elovich model obtained a fit level of R² = 0.9201 for the raw material. Both the PFO and PSO yielded a level of fit lower than 0.86. In the case of C-ESs, the best fit followed the following order: PSO (R² = 0.9995) > IPD (R² = 0.9990) > PFO (R² = 0.9986).

3.3. Circular Economy Results

The critical aspect (x_i) results for both R-ESs and C-ESs were obtained during the experiment and assessment of materials. Based on these, the normalized values of stimulants and destimulants were calculated (X_i), and the synthetic variable for the degree of achievement of the circular economy requirements Q was calculated. All the results are shown in

Table 7. The results for the critical aspects of the future development of ESs; xi (own study).

Benefits and Limitations of ES Implementation	R-ESs		C-ESs
	xi	Xi	xi	Xi
Material availability	+++		+++
Technological treatment (grinding, calcination)	+		+
Impact on the environment (pH, CO2)	+		+
P-removal efficiency (sorption)	++		++
Connection durability (adsorption, precipitation)	++		++
Economic importance (technology vs. P removal)	+++		+++
Social acceptance of technology	+++		+++
Toxicity to the environment	+		+
Reuse as a fertilizer	+++	1.0	+++	1.0
Q		0.89		0.72

Scale of weighted indicators: +, low; ++, fair; +++, high.

.

The Q indicator ranges from 0 to 1. The higher its value, the more sustainable and consistent with the assumptions of a circular economy the material is. For R-ESs and C-ESs, the Q value equaled 0.89 and 0.72, respectively. Despite the high sorption possibilities of C-ESs, which is the main advantage, R-ESs seemed more sustainable. The most important limitation in the case of C-ESs, which influences the final score, is the technological and environmental treatment of the material, particularly by the calcination process. Mainly for these reasons, the score for R-ESs was 17% higher than for C-ESs.

4. Discussion

4.1. Sorption Influence Parameters

Comparing the removal ratio of raw Ca-rich natural materials with those that are thermally processed showed positive results. The calcination temperature (>600 °C) converted CaCO₃ to CaO, changed the morphological structure of the material, and made it more soluble and reactive for P [13]. Park et al. [28] also tested raw eggshells (ESs) and eggshells calcined at 900 °C (C-ES900) for a synthetic solution with an initial P concentration of 100 mg/L and a volume ratio of 0.05 g to 25 mL. After 24h of contact time, they indicated around a 50–60% (no exact data, own calculation based on figure) higher removal ratio in the case of C-ES900 compared to the ESs. Such an observation is similar to the results obtained in this study with wastewater, where we noticed 55 and 99% removal for R-ESs and C-ESs, respectively. Different kinds of raw (marsh clamshells—MCSs) and calcined (waste mussel shells—WMSs and eggshells—ESs) shells were tested [20]. During kinetic tests that lasted from 95 (for WMSs and ESs) to 5760 (for MCSs) min, and for a mass-to-volume ratio of 2g to 0.1L and an initial P concentration of 10 ppm, they observed a 94 and 96% removal ratio for WMSs and ESs, respectively. However, for MCSs, they noted a removal ratio of 65% only after 5760 min. In contrast, Kim et al. [25] observed no removal tendency by natural eggshells for a P synthetic solution with a 50mg/L concentration, a dosage of 0.1–1.0g to 500 mL, and a contact time of 0.25–4 h. They presumed that P removal is more effective in a CE because of a higher alkaline nature than natural ESs. Also, Bańkowska-Sobczak et al. [55] tested the sorption possibility of milled raw calcareous material (limestone aggregate). They obtained a P removal ratio of up to 67% for a synthetic solution with an initial concentration of 1.0 mg/L.

The influence of the pH value has been previously widely tested, and its effect on the P sorption is crucial in controlling that process [17]. Fritzen et al. [27] made a correlation analysis to assess the relationship between the pH and P removal. The obtained non-parametric Spearman’s coefficient was 0.92, indicating a strong correlation. Similar observations to those of this study regarding pH values were noted [56], with an effect of the final pH on phosphate removal by sepiolite calcined at 950 °C. The highest sorption, 172.3 $\mathrm{m}\mathrm{g}{\mathrm{P}-\mathrm{P}\mathrm{O}}_4^{-3}/\mathrm{g}$, was observed for a pH higher than 13. Such an observation was also noted in [28] with raw eggshells and eggshells calcined at 900 °C. The authors reported pH values of 7.99 and 11.47 for raw and calcined eggshells, respectively.

In contrast, some studies have examined the influence of the initial pH on sorption properties [15,56], and have reported that the initial pH of the aqueous solution has essentially no effect on phosphate removal by calcined waste ESs. Moreover, the authors of [56] noted the impact of the initial solution pH on phosphate removal by sepiolite calcined at 950 °C. With an increase in pH from 3 to 11, the phosphate adsorption decreased from 207.9 mg-PO43-/g to 91.7 $\mathrm{m}\mathrm{g}{\mathrm{P}-\mathrm{P}\mathrm{O}}_4^{-3}/\mathrm{g}$.

The effect of the solution’s initial pH on sorption has been examined [16]. Eggshells calcined at 800 °C promoted a slight pH variation (∆pH = pH_final − pH_initial), which showed a pH between 1.6 and 3.5 and a removal efficiency close to 80%. In our cases, the ∆pH was higher for C-ESs and equaled 5.86, with a removal ratio of 99% for C-ESs. In the case of R-ESs, the ∆pH was much lower and equaled 1.94, with a removal ratio of 55%.

The dependence of shell material sorption on the pH is explained by CaCO₃’s conversion to CaO + CO₂ during the high-temperature pyrolysis process [57], and the dissolution of CaO in the aqueous phase caused by the high alkalinity [28]. Additionally, the formation of CO₂ can broaden a material’s pore size as an activation substance [18,37]. Raw eggshells are mainly composed of CaCO₃, have a relatively smooth surface before calcination (Figure 1 and Table 2), and are poorly soluble in an aqueous solution. After calcination, CaCO₃ is converted into another crystalline form—CaO—which is rougher, has irregular surface structures, and is easily soluble [25]. To sum up, P removal in the presence of C-ESs is influenced by Ca²⁺ dissolved in the water. Therefore, P removal may happen through precipitation compared to phosphorous adsorption on CaCO₃, which is likely the case when raw eggshells are used as the primary P-removal mechanism [18,25].

Another factor indicating the increased solubility of Ca²⁺ dissolved in water in the case of calcined eggshells is the values of the EC and TDS. Both indicators are closely correlated. The TDS represents the dissolved form of inorganic salts (calcium, magnesium, sodium, bicarbonates, chlorides, and sulfates) and small amounts of organic matter (BOD and COD). The EC measures the water’s capacity to conduct an electrical current [58,59]—the higher the values of the EC and TDS, the more ions are dissolved in the solution.

The ORP is a water quality index used to characterize the redox capacity. Liquid, which has a positive redox potential, is characterized by oxidizing properties. The higher the index, the more oxidizing the solution. On the other hand, the opposite of oxidation is reduction, which is characterized by a negative redox potential [60]. R-ESs are characterized by a positive redox potential, so the reaction of CaCO₃ with phosphorus follows an oxidation reaction. The opposite condition represents calcined eggshells. The negative redox potential values in the case of a response by CaO to P indicate the reducing properties of the material. The lower the ORP value, the more substantial the reduction of the material is [54]. The P-removal ratio results for raw and calcined eggshells were 55 and 99%, respectively. Other papers have also stated that the ORP could be used as a simple indicator to control the process of phosphorus removal in the anaerobic zone based on the relationships between the ORP, nitrate, and phosphate in an activated sludge system with a denitrification-enhanced biological P-removal process [61].

The IPD model is widely and successfully used in modeling the adsorption kinetics of metal ions, organic pollutants, and non-organic pollutants on reactive materials [49]. The IPD adsorption kinetics include three mass transfer processes: (1) external diffusion (or film diffusion)—the transfer of adsorbate in the liquid film around the adsorbent; (2) internal diffusion (or intraparticle diffusion)—the transfer of adsorbate in the pores of the adsorbent; and (3) adsorption onto the active sites [49]. In the case of the IPD model, a rate-limiting step may be either liquid film diffusion, intraparticle diffusion, or both [62]. In our case, the regression coefficient of IPD had a high fit (0.9674 and 0.9990 for R-ESs and C-ESs, respectively), and the kinetic adsorption process may be controlled by intraparticle diffusion as the only rate-limiting step.

4.2. Potential Limitations of and Recommendations for Using ESs

It should be noted that using any reactive material may have potential adverse environmental impacts and various limitations. At the moment, obtaining the appropriate amount of material is problematic. No legal regulations require the recovery of eggshells from the general waste stream [63]. Also, the alkaline pH of the leachate from C-ESs may hurt fragile water bodies and processes. Because this material is mainly composed of Ca, there is no risk of toxicity to the environment like with other P-removing reactive materials, including Al, Fe, and La [13]. For these reasons, the ES technology is mainly dedicated to use in on-site wastewater treatment plants as an additional step for P removal. However, organic compounds and other nutrients may necessitate pre-treatment or combined treatment methods to maintain efficiency [36].

Our previous study [14] recommended mixing the C-ESs with sand to protect the filter against fast clogging and the environment against the alkaline pH. We tested the column experiment’s C-ESs using additions of 1.0, 2.5, and 5.0%. For a 3 m³ P filter supporting the septic tank system, we needed 49, 122, and 244 kg of C-ESs to represent 1.0, 2.5, and 5.0% of the filter mass, respectively.

4.3. Circular Economy Approach

The P circular economy represents a sustainable approach to managing this nutrient, which is essential for agricultural productivity and food security [33]. The core idea behind the P CE is to reduce waste and optimize the use of this valuable resource throughout its lifecycle [19]. Table 5 sets out the potential benefits and limitations of using R-ESs and C-ESs as P-reactive materials for P removal from wastewater, according to aspects of a circular economy.

Undoubtedly, one of the main benefits of eggshells is their availability. This agro-waste material is widely distributed worldwide [64]. Given that eggshell waste has been ranked as the 15th major food pollution industry problem by the Environmental Protection Agency [65], reusability seems to be a required solution. On the other hand, the production of calcined eggshells may pose a potential problem due to the necessary calcining infrastructure. However, the calcination process is the main factor of environmental impacts associated with the processing and treatment of eggshell waste into highly effective P-reactive material, and is estimated to be more than 99% [19]. The recycling and reusing of 60g of eggshell waste accounts for nearly 0.187 kg CO₂ eq of carbon emissions [19]. In contrast, the carbon footprint for processed, packaged eggs at eggshell-processing facilities is estimated to be less than 5.2 kg CO₂ eq/kg [66].

To overcome such difficulties and negative impacts/pressures, the industrial production of calcined eggshells may accompany the production of quicklime, where an operating temperature of around 1000 °C in the industrial kilns is also required. On the contrary, a positive aspect of using eggshells involves using them for agricultural land applications to reduce soil acidification [19]. This global problem is caused by many factors, mainly by acidic precipitation and the deposition of acidifying gases or particles from the atmosphere, such as sulfur dioxide, ammonia, and nitric acid [67]. Considering the above, eggshells seem to be a valuable agro-waste product aimed at facilitating pH adjustments and promoting a circular economy and environmental sustainability [35]. For example, from oyster shells, the calcium supplement CIPCAL-500 is made, which is commercially available in markets [65].

Additionally, due to its low environmental toxicity, the P-saturated reactive material may be a valuable fertilizer that can be successfully used in agriculture [35]. Andreia F Santos et al. [15] determined the possibility of using eggshells calcined at 700 °C (CES700) as a P fertilizer. In this case, they conducted germination tests (GI) with Lepidium sativum seeds and CES700 loaded with P as a fertilizer. For CES700, the GI ranged from 119.5 to 123.6%. Because the GI results exceeded 100%, CES700 demonstrated its ability as a potential fertilizer. Also, the authors of [34] tested P-loaded hydroxyl eggshells (ES-OH) as a prospective P fertilizer. A pot experiment conducted with Urochloa brizantha showed that ES-OH was as effective as triple superphosphate (TSP) on the grass yield, with the advantage of maintaining nearly four-fold more available P and a higher soil pH than TSP after cultivation.

Using ESs provides many environmental, social, and economic benefits in a circular economy. The array of environmental benefits of ESs is wide. They help mitigate environmental pollution by preventing the accumulation of waste in landfills [68]. ESs may be a reactive material for P [14,20,27,66] and heavy metal removal [69], which would help environmental remediation. Also, ESs, which are rich in CaO, can be used as a raw material for producing bio-CaO, reducing the need for virgin materials [70]. P-reactive materials may also be used in agriculture as a fertilizer [16,19,35]. Job creation and social justice are the primary social benefits. The circular economy model, which includes the recycling and repurposing of ESs, can create new job opportunities in the waste-management and recycling industries, thus addressing unemployment issues [71]. The circular economy framework integrates social justice by ensuring that recycling and waste-management benefits are equitably distributed, promoting inclusive growth [72]. The economic benefits include reducing waste-disposal costs, providing a low-cost raw material for agriculture [63,68,70], incorporating ESs into the circular economy, supporting sustainable business practices, and enhancing brand reputation and compliance with environmental regulations, potentially leading to economic advantages [73].

Although ES reuse and recycling can potentially have a more significant environmental impact than conventional end-of-life disposal [19], it is worth considering that this method of managing ESs eliminates the waste problem and is beneficial from an ecological perspective.

5. Conclusions

Using chicken eggshells for phosphorus removal offers several advantages, including affordability, accessibility, and ease of implementation. The calcined material has a two times higher P-PO₄ removal rate than raw material and can remove 99% of P-PO₄ during the first 5 min of reaction time. Additional filters filled with eggshells as a P-reactive material can particularly benefit on-site wastewater treatment facilities or developing regions with limited resources.

However, further research is needed to optimize the eggshell-based treatment method and to ensure its efficiency over the long term. Despite the potential, it should be integrated into a holistic wastewater treatment strategy alongside other established techniques to effectively manage P and protect aquatic ecosystems. Also, implementing a law requiring the recovery of such Ca-rich base material from the waste stream is strongly desired and guarantees the reuse of the material. In this way, eggshells could play a significant role in sustainable and eco-friendly sewage management and its associated environmental challenges.