Adsorption of Phosphate and Ammonium on Waste Building Sludge

Abstract

Two selected waste building sludges (WBS) were used in this study: (i) sludge from the production and processing of prestressed concrete pillars (B) and (ii) sludge from the production of technical stone (TS). The materials were used in their original and Fe-modified forms (B_Fe/TS_Fe) for the adsorption of NH₄⁺ and PO₄³⁻ from contaminated waters. The experiments were performed on a model solution simulating real wastewater with a concentration of 1.7 mmol·L⁻¹ (NH₄⁺) and 0.2 mmol·L⁻¹ (PO₄³⁻). The adsorption of PO₄³⁻ had a high efficiency (>99%) on B, B_Fe and TS_Fe, while for TS, the adsorption of PO₄³⁻ was futile due to the high content of available P in the raw TS. The adsorption of NH₄⁺ on all sorbents (B/B_Fe, TS/TS_Fe) had a lower efficiency (<60%), while TS proved to be the most effective. Leaching tests were performed according to the CSN EN 12457 standard for B/B_Fe and TS/TS_Fe before and after NH₄⁺ and PO₄³⁻ sorption when the contents of these ions in the leachates were affected by adsorption experiments in the cases of B and TS. For B_Fe and TS_Fe, the ion content in the leachates before and after the adsorption experiments was similar.

1. Introduction

In developed countries, the construction industry can create environmental issues, such as the depletion of natural resources and the production of several tons of construction waste [1,2]. Construction waste in various branches of the construction industry (the production of concrete, artificial stone, etc.) also includes dried powder building sludge (WBS), which is defined as a very fine material that is dispersed in water [1,2].

Concrete is one of the most widely used building materials, with an annual global consumption of 25 billion tons [3]. At present, various separation and recycling processes are used in its production, enabling the reuse of water and coarse aggregates [2,4,5]. The remaining concrete sludge (fine aggregates and cement particles) can be used in the production of ceramic materials [6], synthesis of geopolymers [7], or in the production of new concrete to reduce the required amount of cement [1,3,5].

All of these processes are relatively effective but insufficient for modern sustainable development. This is because the remaining concrete sludge (B) is landfilled without further use, along with several other wastes from the construction industry, such as powder waste from the production and treatment of technical stone (TS), which currently has no other application [4]. However, sewage sludges have a large specific surface area (S_BET), suitable structural properties and chemical composition (Si, Ca, Al and Fe content), which predetermine their possible applications in environmental technologies, for example, as adsorbents for removing toxic ions from contaminated waters [4,8,9].

Nitrogen and phosphorus in NH₄⁺ and PO₄³⁻ ionic forms are an integral part of living organisms and plants [3,8,10]. Both elements are important for good plant growth and development and are often applied in the form of fertilizers to satisfy the growing requirement for food, but high concentrations of NH₄⁺ and PO₄³⁻ in water result in excessive algae growth, which consumes dissolved oxygen and kill fishes and other organisms living in the water (water eutrophication) [3,8,10,11,12]. High concentrations of NH₄⁺ and PO₄³⁻ enter into natural streams from various sources, such as agricultural effluents, industrial wastewater and domestic wastewater [3,8,10,11,12]. Addressing the issue of declining reserves of mineable phosphate ore requires new solutions for capturing and reusing phosphates from wastewater [3,10,11,12]. Several types of absorbents (e.g., biochar, fly ashes, iron-enriched zeolites, etc.) have been developed for the regeneration of phosphates from wastewater [3,10,11,12]. The adsorption of NH₄⁺ was studied, for example, on a polyurethane film prepared from ball-milled algal polyol particles to maintain low concentrations of this ion in fish and shrimp breeding tanks [13]. The coadsorption of NH₄⁺ and PO₄³⁻ in wastewater was not discussed in these studies.

As part of this study, selective, simultaneous and additional adsorptions of NH₄⁺ and PO₄³⁻ were monitored. The experimental data were fitted by the Langmuir and Freundlich adsorption isotherm to determine the sorption parameters (q_max.—maximum equilibrium adsorption capacity, Q_t—theoretical adsorption capacity, K_L—Langmuir adsorption constant, R²—correlation factor, 1/n—heterogeneity factor, K_F—Freundlich constant indicating adsorption capacity). The Langmuir adsorption isotherm is the simplified sorption model, which assumes the equivalence and even distribution of the active sites, to which only one series of non-interacting molecules can be bound [14,15,16]. The Freundlich adsorption isotherm is the first known model describing reversible multilayer adsorption with a different distribution of active sites [17]. Kinetic measurements were performed for NH₄⁺ and PO₄³⁻ adsorption, and the data for systems that could be described by the Langmuir model (NH₄⁺—TS, PO₄³⁻— B, PO₄³⁻—B_Fe and PO₄³⁻—TS_Fe) were processed by pseudo-first- and pseudo-second-order formal kinetic models to find appropriate rate constants (k₁ for pseudo-first-order formal kinetic and k₂ for pseudo-second-order formal kinetic) [18].

The goal of this study was to find new possible applications of B and TS in their original and surface-modified forms (B_Fe and TS_Fe) for the coadsorption of NH₄⁺ and PO₄³⁻ ions from wastewater and their subsequent use for improving the quality and nutritional values of agricultural soils.

2. Materials and Methods

2.1. Characterization of Used Building Waste Sludge

The WBS from the production of concrete (B) and artificial stone (TS) with a particle size of <0.1 mm was used. The B is formed during the production and abrasion of prestressed concrete columns, with a high cement content of ~21%. The TS is created during the production and processing of technical stone from Technistone, Czech Republic. The mineralogical and elemental composition of both materials were determined using X-ray powder diffraction (XRD) and X-ray fluorescence analysis (XRF), and the results are discussed further in Section 3.1.

For the selective sorption of anions, the surfaces of B and TS were modified with Fe²⁺ ions (B_Fe, TS_Fe) according to the verified method [19,20,21,22]. The surface modification was performed with 0.6 M FeSO₄ solution for 24 h at the laboratory temperature (20 °C) upon stirring the mixture with a shaker. Then, the suspension was filtered, and the obtained modified sludge was washed with distilled water, dried (60 °C) and homogenized.

2.2. Model Solution

The ion concentrations in the model solutions were chosen according to the real values in the wastewater (pond from the contaminated area) in the Havlíčkův Brod vicinity (Czech Republic—Highlands).

Model solutions of selected ions and their mixture were prepared in the concentration of 1.7 mmol·L⁻¹ NH₄⁺ and 0.2 mmol·L⁻¹ PO₄³⁻. The solutions were prepared from analytically pure inorganic salts NH₄Cl, K₂HPO₄ and distilled water at the original pH (~7.5).

Distilled water, tap water and 0.1 M KCl were used for leaching experiments.

2.3. Adsorption Experiments

The suspension of a defined amount of sorbent (5–40 g·L⁻¹) and 50 mL of model solution was shaken in 100 mL sealed polyethylene containers for 24 h (chosen based on preliminary experiments) at laboratory temperature (20 °C), pH of the model solution (~7.5) and at a speed of 280 rpm. Subsequently, vacuum filtration was performed on 0.6 μm pore size filters. The residual NH₄⁺ and PO₄³⁻ concentrations in the obtained filtrates were analyzed.

The experimental data were fitted by the Langmuir and Freundlich adsorption isotherm to determine the sorption parameters (q_max., Q_t, K_L, 1/n, K_F, R²). The accuracy of fitted data was supported by the triple measurement of the adsorption series.

The Langmuir isotherm is defined by Equation (1) [14,15,16]:

$$\mathrm{q}=\frac{\mathrm{QKc}}{1+\mathrm{Kc}} ,$$

(1)

and its linearized form by Equation (2) [14,15,16]:

$$\frac{1}{\mathrm{q}}=\frac{1}{\mathrm{Q}}+\frac{1}{\mathrm{QKc}} ,$$

(2)

where q is an equilibrium concentration of an adsorbed ion in the solid phase [mmol·g⁻¹], c is an equilibrium concentration of an adsorbed ion in the solution [mmol·L⁻¹], Q_t is the theoretical adsorption capacity [mmol·g⁻¹], and K_L is a Langmuir adsorption constant [L mmol⁻¹].

The equilibrium ion concentration in the solid phase was calculated from the experimental data according to Equation (3) [14,15,16]:

$$\mathrm{q}=\frac{{\mathrm{V}}_0\left({\mathrm{c}}_0-\mathrm{c}\right)}{\mathrm{m}} ,$$

(3)

where V₀ is the volume of solution [L], c₀ is the initial concentration of adsorbate in solution [mmol·L⁻¹], and m is the mass of the solid phase [g].

The Freundlich isotherm is defined by Equation (4) [17]:

$$\mathrm{q}={ \mathrm{K}}_{\mathrm{F}}·{\mathrm{c}}^{1/\mathrm{n}}$$

(4)

and its linearized form by Equation (5) [17]:

$$\log \left(\mathrm{q}\right)=\log ({\mathrm{K}}_{\mathrm{F}})+\frac{1}{\mathrm{n}}·\log \left(\mathrm{c}\right)$$

(5)

where q is an equilibrium concentration of an adsorbed ion in the solid phase [mmol·g⁻¹], c is an equilibrium concentration of an adsorbed ion in the solution [mmol·L⁻¹], 1/n is the heterogeneity factor relating to adsorption intensity, and K_F is a Freundlich adsorption constant [mmol·g⁻¹]. The kinetic measurements were performed for NH₄⁺ and PO₄³⁻ adsorption with 1.7 mmol·L⁻¹ NH₄⁺ and 0.2 mmol·L⁻¹ PO₄³⁻ model solutions, the dosages of 10 g·L⁻¹ (NH₄⁺ adsorption) and 2.5 g·L⁻¹ (PO₄³⁻ adsorption) and time intervals of 0.2, 1, 3, 5, 18.5, 24, 28 and 48 h.

Kinetic data for the systems that could be described by the Langmuir model (NH₄⁺—TS, PO₄³⁻—B, PO₄³⁻—B_Fe and PO₄³⁻—TS_Fe) were processed by the pseudo-first- and the pseudo-second-order formal kinetic models to find rate constants (k₁ and k₂) [18].

The pseudo-first-order kinetic model is described by Equation (6) [18]:

$$\frac{{\mathrm{dq}}_{\mathrm{t}}}{\mathrm{dt}}={ \mathrm{k}}_1\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)$$

(6)

Integrated Equation (4) and substituted the boundary conditions from t = 0 to t = t and q_t = 0 to q_t = q_t, a linearized equation was obtained (Equation (7) [18]:

$$\ln \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\ln \left({\mathrm{q}}_{\mathrm{e}}\right)-{\mathrm{k}}_1\mathrm{t}$$

(7)

The pseudo-second-order kinetic model is described by Equation (8) [18]:

$$\frac{{\mathrm{dq}}_{\mathrm{t}}}{\mathrm{dt}}={ \mathrm{k}}_2{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2$$

(8)

Integrated Equation (6) and substituted the boundary conditions from t = 0 to t = t and q_t = 0 to q_t = q_t, a linearized equation was obtained (Equation (9) [18]:

$$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{\mathrm{h}}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}\mathrm{t}$$

(9)

$$\mathrm{h}={ \mathrm{k}}_2{\mathrm{q}}_{\mathrm{e}}^2$$

where t is time [h], q_t is a concentration of an adsorbed ion in the solid phase at time t [mmol·g⁻¹], qe is an equilibrium concentration of an adsorbed ion in the solid phase [mmol·g⁻¹], k₁ is pseudo-first-order formal kinetic rate constant [h⁻¹] and k₂ is a pseudo-second-order formal kinetic rate constant [g·mmol⁻¹·h⁻¹].

2.4. Leaching Tests

The leaching of both ions from the original and saturated WBS was performed according to the CSN EN 12,457 standard [23]. The defined amounts of B and TS before and after the sorption of NH₄⁺ and PO₄³⁻ were poured with the appropriate leaching solution (Section 2.2) at the solid–liquid ratio of 1:10.

2.5. Analytical Methods

X-ray powder diffraction (XRD) of solid samples was measured using a 2D Phaser (Bruker s.r.o., Billerica, MA, USA). A current of 10 mA, a voltage of 30 kV, a step size of 0.02° and a range of angles (6–80 2θ) were used for the measurements.

The semi-quantitative chemical composition was determined by X-ray fluorescence analysis (XRF), which was performed using a NEX QC instrument (Rigaku Company, Tokio, Japan), where the powder sludge was measured at 50 kV using an SDD detector.

Zero-charge pH (pH_zpc) was measured using the Stabino^®, Version 2.0 (Particle Metrix GmbH, Inning am Ammersee, Germany). The stabilized suspensions of the solid sample and 0.1, 0.01 and 0.001 M KCl (solid: liquid ratio of 1:100) were dynamic with 0.1 M solution of NaOH or HCl to the isoelectric point (IEP). The resulting pHzpc value is the average of three pH values corresponding to the zero potential.

The Micromeritics ASAP 2020 (accelerated surface area and porosimetry) analyzer (Micromeritics^®, Norcross, GA, USA) was used to measure the specific surface area (S_BET) of the sludge used, which uses gas sorption (N₂) to study macropores and micropores using the Horvath–Kavazoe method (BJH method) bath at −195.8 °C. Prior to measurement, the samples were degassed at 313 K for 1000 min.

NH₄⁺ and PO₄³⁻ concentrations were determined by UV/Vis spectrophotometry using an Evolution 220 instrument (Thermo Scientific^®, Waltham, MA, USA) at 425 nm for NH₄⁺ using potassium sodium tartrate and Nessler reagent [24], and at 820 nm for PO₄³⁻ using the molybdenum blue method [25].

3. Results and Discussion

3.1. Characterization of Original and Modified B/BFe and TS/TSFe

From the XRD diffractograms (Figure 1) of the original waste building sludge, B (Figure 1a) is characterized by portlandite and calcite. The aggregate used in the concrete was granite; the filler used in TS (Figure 1b) was quartz.

The XRD diffractograms for B_Fe and TS_Fe were identical to their original forms of B and TS (Figure 1) because Fe oxides were bound to the silicate skeleton of B or TS by chemisorption in an amorphous form during the modification of Fe²⁺ ions when hydrated metal particles formed on the surface of the sorbents (B_Fe, TS_Fe) in reactive, ion-exchangeable positions and there were no changes in mineralogical composition [17,18]. The chemical and surface properties were changed by the modification with Fe²⁺ ions; the B_Fe and TS_Fe significantly differed in S_BET, Fe and alkali content (

Table 1. Chemical and surface properties of B/B_Fe and TS/TS_Fe.

Sample	Chemical Composition (% wt.)							SBET (m2·g−1)	pHZPC
	SiO2	Al2O3	Fe2O3	TiO2	CaO	MgO	P2O5
B	32.3	6.6	1.3	<0.1	46.9	1.8	0.2	38.2	10.3
BFe	26.6	4.3	29.8	0.4	18.7	2.1	0.1	118.2	7.5
TS	85.3	35.0	0.01	0.0	3.6	1.8	0.6	2.1	6.2
TSFe	75.6	28.9	5.4	0.06	2.8	1.9	0.4	14.9	6.7

), which affected PO₄³⁻ and NH₄⁺ adsorption. The chemical and surface properties of B, TS, B_Fe and TS_Fe are listed in Table 1.

3.2. Adsorption of the Selected Ion (NH₄⁺ or PO₄³⁻) on Original and Modified B/B_Fe and TS/TS_Fe

All adsorption experiments were performed under the same conditions described in Section 2.3. Figure 2 shows the dependence of adsorption efficiencies ε (%) on the weight m (g·L⁻¹) of B/B_Fe and TS/TS_Fe for NH₄⁺ or PO₄³⁻.

Table 2. Adsorption parameters for NH₄⁺ and PO₄³⁻ on B, TS, B_Fe and TS_Fe.

Ion	Sorbent	qmax. (mmol·g−1)	Langmuir Model				Freundlich Model
			Qt * (mmol·g−1)	KL * (L·mmol−1)	R2 *	RMSE	1/n *	KF * (mmol·g−1)	R2 *	RMSE
NH4+	B	0.06	-**	-**	-**	-**	-**	-**	-**	-**
	BFe	0.04	-**	-**	-**	-**	0.95	0.008	0.565	0.001
	TS	0.06	0.09	0.62	0.897	0.004	0.62	0.032	0.894	0.002
	TSFe	0.01	-**	-**	-**	-**	0.96	0.005	0.496	0.001
PO43−	B	0.06	0.03	1922.39	0.944	0.011	0.69	2.36	0.935	0.006
	BFe	0.07	0.07	1868.00	0.966	0.007	0.80	8.89	0.952	0.004
	TS	-***	-**	-**	-**	-**	-**	-**	-**	-**
	TSFe	0.06	0.04	1439.24	0.978	0.007	0.68	2.33	0.956	0.004

* Calculated adsorption parameters based on the adsorption model (Section 2.3.); ** did not follow adsorption model; *** PO₄³⁻ was released into the solution instead of sorption.

shows the sorption parameters (theoretical adsorption capacities—Q_t; adsorption constants—K_L and K_F; heterogeneity factor—1/n; root mean squared error—RMSE) calculated using the Langmuir and Freundlich model [14,15,16].

PO₄³⁻ adsorption occurred with high efficiency (<99%) on modified forms B_Fe and TS_Fe (Figure 2b). Due to its high alkalinity, B did not primarily support the adsorption of anions. The high efficiency of PO₄^3- adsorption on B can be explained by the precipitation of PO₄³⁻ into a poorly soluble amorphous form or as apatite (Ca₅(PO₄)₃ (OH)). Modified forms of B_Fe (Figure 2b, orange line) and TS_Fe (Figure 2b, red line) achieved high sorption efficiencies with PO₄³⁻ because they were enriched with hydrated metal particles in reactive, ion-exchangeable surface positions (Section 3.1). These available Fe ions are sufficient for the adsorption of an oxyanion such as PO₄³⁻ onto Fe oxy(hydroxides). The TS released PO₄³⁻ into the solution, where the concentration of this ion increased by more than 50% at the highest dosage of sorbent (Figure 2b, blue line).

NH₄⁺ adsorption occurred with lower efficiency (<60%), whereas the TS proved to be most effective (Figure 2a, blue line). The sorption efficiency of NH₄⁺ adsorption on B decreased with the increase in sorbent dosage (Figure 2a, green line) due to the alkaline nature of B. The pH of the solution increased more rapidly when the dosage of B increased, and the solution became alkaline (~12) very quickly at the highest dose of B. The NH₄⁺ ion in an alkaline environment is converted to NH₃ and cannot be absorbed onto the surface of the sorbent.

The adsorption of PO₄³⁻ on B, B_Fe and TS_Fe and NH₄⁺ on TS corresponded to both the Freundlich and Langmuir models, but the worse correlation of experimental data for the Freundlich model (R²: 0.496–0.956 versus 0.897–0.999, Table 2) indicated the Langmuir isotherm more appropriate for investigated systems. The NH₄⁺ adsorption on B_Fe a TS_Fe followed the Freundlich model but with very low correlation factors.

Kinetic experiments were performed under the same conditions described in Section 2.3. The dependence of concentration q_t (mmol·g⁻¹) of an adsorbed ion (NH₄⁺ or PO₄³⁻) in the solid phase on the contact time t (h) is shown in Figure 3.

The PO₄³⁻ and NH₄⁺ adsorption equilibrium was reached around 19 h (Figure 3).

The obtained rate constants (k₁ and k₂) and correlation factors (R²) for the pseudo-first- and the pseudo-second-order formal kinetic models, which were used for the systems that could be fitted by the Langmuir model (Section 2.3), are reported in

Table 3. Correlation factors (R²) and velocity constants (k₁ and k₂) of the pseudo-first-order kinetics model and the pseudo-second-order kinetics model.

Adsorption System	Pseudo-First-Order Kinetics Model		Pseudo-Second-Order Kinetics Model
	R2	k1 (h−1)	R2	k2 (g·mmol−1·h−1)
PO43−—B	0.782	0.11	0.999	124.8
PO43−—BFe	0.934	0.20	0.999	39.3
PO43−—TSFe	0.983	0.21	0.999	20.3
NH4+—TS	0.983	0.31	0.999	11.6

.

Adsorption systems that could be fitted to the Langmuir model (PO₄³⁻—B, PO₄³⁻—B_Fe, PO₄³⁻—TS_Fe and NH₄⁺—TS) proceeded by chemisorption, according to the pseudo-second-order kinetic model (Section 2.3). The other studied systems did not correlate sufficiently with any of the applied adsorption models, and prevailing physical adsorption could be assumed.

3.3. Additional Adsorption of NH₄⁺ and PO₄³⁻ on Original and Modified B/B_Fe and TS/TS_Fe

In order to determine the possible accumulation of NH₄⁺ or PO₄³⁻ and the effect of adsorbed NH₄⁺ or PO₄³⁻ on the possibility of further sorption, the most effective systems of selected adsorption (Section 3.2) were saturated with the oppositely charged ion. Figure 4 compares the sorption efficiencies of the ions adsorbed in the selective sorption (Sec.) and in the additional sorption (Add.) on the oppositely charged ion captured on the sorbent surface during the prior selective sorption (Section 3.2). Additionally, the PO₄³⁻—B, PO₄³⁻—B_Fe and PO₄³⁻—TS_Fe systems were used for NH₄⁺ adsorption, while for the PO₄³⁻ adsorption, only the NH₄⁺—TS system was used.

During the additional adsorption, the sorption efficiency increased from 6% for adsorption NH₄⁺ on PO₄³⁻—TS_Fe system (Figure 4b) to 60% for adsorption of PO₄³⁻ on the NH₄⁺—TS system (Figure 4a) because active sites formed on the surfaces of the formerly saturated sorbents with NH₄⁺ or PO₄³⁻, causing the additional binding of oppositely charged ions, whereby the adsorption yield of additional adsorption increased. These active sites also supported the accumulation of nutrients in the sorbents for possible applications in agricultural soils.

3.4. Simultaneous Adsorption of NH₄⁺ and PO₄³⁻ on Original and Modified B/B_Fe and TS/TS_Fe

The tested ions can usually coexist in real water systems; therefore, their simultaneous sorptions (Sim.) on B, TS, B_Fe and TS_Fe were performed. Figure 5 shows the dependence of the adsorption efficiencies on the dosage of B/B_Fe and TS/TS_Fe for NH₄⁺ (Figure 5a) and PO₄³⁻ (Figure 5b) adsorption when the data obtained in this sorption experiment are compared with the sorption efficiencies of selective ion sorption (Sec.) mentioned in Section 3.2.

The simultaneous sorption of PO₄³⁻ on B, B_Fe and TS_Fe (Figure 5b) was very efficient (>99% adsorption efficiency) in the presence of NH₄⁺ in the solution. The efficiency of the simultaneous sorption for PO₄³⁻ is very similar to the efficiency of the selective adsorption of the PO₄³⁻ ion. The simultaneous sorption of PO₄³⁻ on TS remained ineffective (light and dark blue lines in Figure 5b).

The efficiency of adsorption of NH₄⁺ in the simultaneous presence of PO₄³⁻ in the solution was higher for all sorbents when compared to the adsorption efficiency of the selective adsorption of the NH₄⁺ ion (Figure 5a); it is possible that a similar situation occurred, as in the case of additional sorption experiments (Section 3.3).

3.5. Leaching Experiments

The leaching experiments (described in Section 2.4) were performed to determine the possible use of both sludges (B and TS) as additives to agricultural soils to improve their quality. Figure 5 and Figure 6 show the amounts of NH₄⁺/PO₄³⁻ ions leached from the individual sludges (B/B_Fe and TS/TS_Fe) before (Figure 6) and after (Figure 7) the adsorption of selected ions.

The leaching experiments revealed a relatively high release of PO₄³⁻ (Figure 6b and Figure 7b) and NH₄⁺ (Figure 6a and Figure 7a) from saturated and original sorbents, B and TS. For the B and TS, the leaching tests also showed that the leaching of PO₄³⁻ and NH₄⁺ was affected by the saturation of the PO₄³⁻ or NH₄⁺ on the sorbent surface (PO₄³⁻ and NH₄⁺ adsorption is discussed in Section 3.2, Section 3.3 and Section 3.4).

The B_Fe and TS_Fe were able to leach significantly lower contents than their original forms B and TS, and due to their affinity for oxyanions, PO₄³⁻ was almost not leached (Figure 6b and Figure 7b, yellow and grey lines).

The content of NH₄⁺ in the leachates decreased in the following order: TS_sorption > B > B_{Fe sorption} ≅ B_Fe > B_sorption > TS > TS_{Fe sorption} ≅ TS_Fe. The content of PO₄³⁻ in the leachates decreased in the following order: TS >> TS_sorption > B_sorption > B > TS_{Fe sorption} ≅ TS_Fe > B_{Fe sorption} ≅ B_Fe.

4. Conclusions

B, B_Fe and TS_Fe proved to be promising sorbents for the sorption of PO₄³⁻ when such adsorptions were successfully fitted by the Freundlich and Langmuir adsorption models, with better parameters for the Langmuir fit. The TS spontaneously released PO₄³⁻ into the solution, and no adsorption occurred.

The adsorption of NH₄⁺ had a lower efficiency compared to the sorption of PO₄³⁻, while the TS was found to be the most efficient sorbent. The adsorption of NH₄⁺ on the TS could be fitted by the Freundlich and Langmuir adsorption models when better correlation factors were achieved for the Langmuir fit. The NH₄⁺ adsorption on B_Fe and TS_Fe followed the Freundlich model but with very low correlation factors. Adsorption of NH₄⁺ proceeded with a lower sorption robustness compared to the PO₄³⁻ adsorption.

The kinetic equilibrium for PO₄³⁻ and NH₄⁺ adsorption was reached around 19 h. For the selected adsorption systems that could be fitted by the Langmuir model (PO₄³⁻ adsorption on B, B_Fe and TS_Fe and NH₄⁺ adsorption on TS), the pseudo-second-order kinetic model was the most suitable, and these adsorption systems proceeded by chemisorption.

During the adsorption of oppositely charged ions on the sorbents formerly saturated with NH₄⁺ or PO₄³⁻ (i.e., the NH₄⁺ adsorption on B, B_Fe and TS_Fe saturated with PO₄³⁻, and the PO₄³⁻ adsorption on TS saturated with NH₄⁺) the efficiency increased compared to the adsorption on the original sorbents due to the creation of new active sites on the sorbent surface. The simultaneous sorption of PO₄³⁻ and NH₄⁺ was more efficient when compared with the efficiency of selective ion adsorption.

The leaching experiments proved to have a relatively high release of PO₄³⁻ and NH₄⁺ from saturated sorbents, which made it possible to apply the saturated sorbents to agricultural soils, for example, to increase their nutritional values. The content of NH₄⁺ in the leachates decreased in the following order: TS_sorption > B > B_{Fe sorption} ≅ B_Fe > B_sorption > TS > TS_{Fe sorption} ≅ TS_Fe; the content of PO₄³⁻ in the leachates decreased in the following order: TS > TS_sorption > B_sorption > B > TS_{Fe sorption} ≅ TS_Fe > B_{Fe sorption} ≅ B_Fe.

The waste concrete sludge B was found to be an effective PO₄³⁻ sorbent. It is unsuitable for NH₄⁺ sorption due to its high alkalinity, which can be considered a major disadvantage for its possible use as a soil additive. Waste from the production of artificial stone TS was found to be a relatively good sorbent for the NH₄⁺, but a high dosage is necessary to achieve an acceptable sorption efficiency. For the sorption of PO₄³⁻, the TS is completely unsuitable because of the spontaneous release of this ion into the solution. Due to significantly lower alkalinity, the TS represents a promising candidate for application to agricultural soils. The modified B_Fe and TS_Fe forms proved to be selective and efficient sorbents of PO₄³⁻ ions, while the adsorption of NH₄⁺ on B_Fe and TS_Fe was almost ineffective. The use of B_Fe and TS_Fe as soil additives was possible.