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Article

Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice

by
Oanamari Daniela Orbuleț
,
Cristina Modrogan
,
Magdalena Bosomoiu
,
Mirela Cișmașu (Enache)
,
Elena Raluca Cîrjilă (Mihalache)
,
Adina-Alexandra Scarlat (Matei)
,
Denisa Nicoleta Airinei
,
Adriana Miu (Mihail)
,
Mădălina Grinzeanu
and
Annette Madelene Dăncilă
*
Faculty of Chemical Engineering and Biotechnologies, National University of Science and Technology Politehnica Bucharest, Gheorghe Polizu Street, No. 1-7, 011061 Bucharest, Romania
*
Author to whom correspondence should be addressed.
Environments 2025, 12(8), 288; https://doi.org/10.3390/environments12080288
Submission received: 18 July 2025 / Revised: 13 August 2025 / Accepted: 15 August 2025 / Published: 21 August 2025

Abstract

Phosphates are key pollutants involved in the eutrophication of water bodies, creating the need for efficient and low-cost strategies for their removal in order to meet environmental quality standards. This study presents a comparative thermodynamic evaluation of phosphate ion adsorption from aqueous solutions using two sustainable and readily available materials: autoclaved aerated concrete (AAC) and pumice stone (PS). Batch experiments were conducted under acidic (pH 3) and alkaline (pH 9) conditions to determine equilibrium adsorption capacities, and kinetic experiments were carried out for the best-performing adsorbent. Adsorption data were fitted to the Langmuir and the Freundlich isotherm models, while kinetic data were evaluated using pseudo-first-order and pseudo-second-order models. The Freundlich model showed the best correlation (R2 = 0.90 − 0.97), indicating the heterogeneous nature of the adsorbent surfaces, whereas the Langmuir parameters suggested monolayer adsorption, with maximum capacities of 1006.69 mg/kg for PS and 859.20 mg/kg for AAC at pH 3. Kinetic results confirmed a pseudo-second-order behavior, indicating chemisorption as the main mechanism and the rate-limiting step in the adsorption process. To the best of our knowledge, this is the first study to compare the thermodynamic performance of AAC and PS for phosphate removal under identical experimental conditions. The findings demonstrate the potential of both materials as efficient, low-cost, and thermodynamically favorable adsorbents. Furthermore, the use of AAC, an industrial by-product, and PS, a naturally abundant volcanic material, supports resource recovery and waste valorization, aligning with the principles of the circular economy and sustainable water management.

Graphical Abstract

1. Introduction

Phosphate pollution in aquatic environments has become a critical concern worldwide due to its role in accelerating eutrophication. a process leading to excessive algal blooms, oxygen depletion, and overall degradation of water quality [1,2]. The presence of phosphates in wastewater originates primarily from domestic sewage, agricultural runoff, detergents, and industrial discharges. The presence of phosphates in wastewater originates primarily from domestic sewage, agricultural runoff, detergents, and industrial discharges. To prevent ecological imbalances and comply with environmental regulations, the efficient removal of phosphates from wastewater has become a priority in modern water treatment practices.
Under natural conditions, surface waters maintain phosphorus levels in a balanced state, with concentrations meeting but not greatly exceeding the needs of aquatic ecosystems. However, when external phosphorus inputs exceed what aquatic microorganisms and plants can assimilate, the surplus triggers rampant algal growth and eutrophication. This phenomenon not only diminishes water quality but also significantly increases water treatment costs for affected water bodies [3]. Even relatively small increases in phosphate concentration can stimulate nuisance algal blooms; accordingly, stringent effluent standards have been established to limit phosphorus in treated wastewater [4,5].
Phosphorus in wastewater is present in various forms, including organic phosphorus compounds, condensed inorganic phosphates (polyphosphates), and inorganic orthophosphate [4,5]. Organic and polyphosphate species—originating from sources like biological waste, fertilizers, and detergents—can undergo hydrolysis or biodegradation and eventually convert into orthophosphate, the form most readily taken up by algae. Effective treatment processes must therefore capture or transform all these species, ultimately removing the orthophosphate to prevent ecological impact. In addition to environmental protection, there is a resource recovery incentive: phosphorus is a non-renewable element essential for agriculture, so recovering it from waste streams aligns with the growing demand for sustainable mineral resources. This dual concern (eutrophication control and resource sustainability) has heightened interest in technologies for phosphorus removal and recovery from wastewater [6,7].
The recovery of phosphorus from wastewater is becoming an increasingly important issue, both to meet the growing demand for sustainable mineral resources and to prevent the eutrophication of water bodies.
Improving the efficiency of municipal wastewater treatment plants can be achieved by developing high-performance processes aimed at reducing phosphate ion concentrations in the effluent. The discharge of wastewater containing excessive phosphate concentrations into receiving waters leads to the occurrence of undesirable eutrophication phenomena.
Numerous methods have been developed and implemented to remove phosphate from wastewater. Conventional treatments include chemical precipitation using alum or iron salts and enhanced biological phosphorus removal in activated sludge systems. While these methods can effectively reduce phosphate levels, they have well-documented drawbacks: high reagent costs, the generation of large volumes of sludge requiring further handling, and limited opportunities for phosphorus recovery [8]. Achieving ultra-low residual phosphate concentrations with such methods can be challenging and often requires additional treatment steps or polishing units [9,10]. Furthermore, specialized processes for phosphorus recovery, such as controlled crystallization of struvite (magnesium ammonium phosphate) or calcium phosphate, have been explored to reclaim phosphorus as a reusable product [11]. These recovery processes can produce valuable fertilizers, but they necessitate strict operational control and can be costly to implement at scale [11,12].
In contrast, adsorption has emerged as an attractive and sustainable alternative for phosphate removal in water treatment [9]. Adsorption-based processes employ solid sorbent materials to capture phosphate ions from solution, and they offer several advantages over conventional methods. Notably, adsorption is relatively simple to operate and, in many cases, does not require the addition of harsh chemicals. This means that the process produces minimal secondary waste—no extensive sludge production—and does not significantly alter the pH or other quality parameters of the treated water [8]. These benefits translate into lower operational costs and fewer downstream disposal issues. Accordingly, developing high-performance adsorbents for phosphate removal is seen as a promising strategy to meet stringent effluent standards in an economical and environmentally friendly manner [9,13].
According to the Romanian Technical Norm for Water Protection NTPA 001/2002, the permissible limits for phosphorus load in industrial and municipal wastewater discharged into natural receptors apply to all categories of effluents, whether or not originating from treatment plants, as presented in Table 1 and Table 2 [4,5,6].
Globally, phosphorus pollution is a significant environmental issue, with municipal and industrial wastewater, as well as agricultural runoff, contributing millions of tons of phosphorus annually to aquatic ecosystems. For example, the European Environment Agency reports that in some EU river basins, average total phosphorus concentrations exceed 0.1 mg/L—the threshold above which eutrophication risks increase substantially. Current removal methods, such as chemical precipitation, can cost between 1–3 EUR/m3 of treated water, depending on the scale and technology used, which can be prohibitively expensive for small or resource-limited treatment plants [3]. In recent years, numerous low-cost adsorbents—including industrial by-products (e.g., fly ash, steel slag, red mud), agricultural wastes (e.g., rice husk biochar, coconut shell biochar), and natural minerals (e.g., zeolites, limestone, laterite)—have been investigated for phosphate removal, often achieving adsorption capacities between 5 and 50 mg P/g, depending on preparation and operational conditions. Comparative studies indicate that materials rich in calcium, magnesium, and aluminum tend to exhibit higher phosphate affinity through precipitation and complexation mechanisms, making them attractive for cost-effective and sustainable water treatment solutions.
Recent research has focused on identifying low-cost, natural, or waste-derived materials that can serve as effective phosphate adsorbents [10]. The use of such materials can reduce treatment costs and promote a circular economy approach by repurposing industrial by-products or natural minerals for pollution control. Among the promising candidates are autoclaved aerated concrete (AAC) and pumice stone (PS). AAC is a lightweight, highly porous form of concrete produced from silica-rich materials (sand, cement, lime, etc.) and an expanding agent, which results in a material rich in calcium, silica, and alumina. Crushed AAC, often obtained as waste from the construction industry, provides numerous binding sites for phosphates due to its rough, porous texture and alkaline surface chemistry [11]. Pumice stone, on the other hand, is a naturally occurring volcanic rock characterized by its low density and microporous structure. With a high specific surface area and a composition dominated by silica (along with significant alumina content), pumice has been utilized as an adsorbent for various contaminants in water. Both raw and modified pumice have demonstrated the capacity to remove pollutants—for example, natural and iron-coated pumice have been used to adsorb nutrients and even dyes from wastewater [13]. These attributes suggest that ACC and PS could serve as cost-effective sorbents for phosphate, an idea that this study investigates in detail.
The objective of this study is to evaluate the potential of AAC and PS as alternative adsorbent materials for phosphate removal from wastewater. Equilibrium batch adsorption experiments were conducted under varying pH conditions (3 and 9) to determine the phosphate uptake capacity of each material. The resulting adsorption data were analyzed using established isotherm models—Langmuir and Freundlich—as well as kinetic models, namely the pseudo-first-order and pseudo-second-order equations, to characterize the adsorption behavior and estimate key parameters. Through this investigation, we aim to gain a deeper understanding of the mechanisms involved in phosphate adsorption onto AAC and PS and to assess the feasibility of utilizing these readily available materials in sustainable wastewater treatment applications. Unlike previous studies that investigate only one of these materials in isolation or involve costly chemical modifications, this research provides a direct comparison of the two untreated materials under conditions relevant to wastewater treatment, thus contributing to the development of sustainable and applicable solutions within a circular economy context.
The sorption process of phosphorus is complex and involves several steps and retention mechanisms. The apparent adsorption of phosphorus can be viewed as a combination of successive processes, including a rapid (almost instantaneous) reversible sorption process on particle surfaces and various slower processes. These slower processes can be further divided into relatively fast and very slow components. The retention mechanisms include electrostatic interaction, formation of surface complexes, formation of hydrogen bonds, ion exchange, etc. Opinions differ about the degree to which these processes are reversible [18,19,20].
There are many nonlinear models that are used to describe the experimental isotherms for the adsorption of ions from aqueous phase. These include the Temkin, Freundlich, Langmuir models, the double layer Langmuir model, or the Elovich model. In their simplest form, these equations are defined to assume the instantaneous approach to equilibrium, but they can also be modified to represent a time-dependent approach to equilibrium [21,22,23,24,25].
The mechanisms governing phosphate adsorption onto AAC and PS are presumed to involve electrostatic attraction between negatively charged phosphate ions and positively charged surface sites, ion exchange with calcium or aluminum ions, and surface precipitation on alkaline substrates.
It has been previously reported, in the case of some adsorbents, that phosphorous forms strong bonds with metallic ions in the process of adsorption [26,27,28,29]. This increases the adsorption capacity of the material. Some metals may flow back into the current stream, causing secondary pollution [18,30]. For the materials, we tested the risk of releasing dangerous metal ions back into the water, which is minimized by the fact that both materials contain a large amount of SiO2, which contains very stable or harmless Ca2+ and Mg2+ ions. The Al3+ ions have been reported to be blocked by the adsorbed phosphate ions [12].
While several adsorbents have shown promising results, many rely on chemical modification to enhance performance, which increases cost and limits practical application. Therefore, there is a need to explore affordable, unmodified materials with sufficient efficiency. AAC and PS offer distinct advantages such as low cost, high porosity, and alkaline composition, as well as the potential—reported in previous studies [31]—for reuse or safe disposal. In this study, we focus on evaluating their adsorption performance and exploring their suitability as sustainable alternatives for tertiary wastewater treatment.

2. Materials and Methods

2.1. Chemical Reagents

All reagents used in this study were of analytical grade and prepared in accordance with the international standard protocol SR ISO 6878:2005 [32]. The chemicals included 63% nitric acid (ρ = 1.41 g·cm−3), vanadomolybdate color reagent, and KH2PO4 solutions with concentrations ranging from 10 to 100 mg·L−1. Sodium hydroxide (2 M) was used for pH adjustment, and the phosphate ion (PO43−) was expressed as phosphorus (P) throughout the study.
All reagents were purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany) and used without further purification. Sulfuric acid, NaOH, and nitric acid (63%) were also supplied by Merck. Distilled water was used for the preparation of the adsorbent, standard solutions, and synthetic wastewater.
The raw pumice stone was collected from the volcanic area of the Călimani Mountains, Romania, while the AAC material was obtained from construction waste generated at a local construction site in Iași. Both materials were used in their natural form, without chemical or thermal pre-treatment.

2.2. Pumice Stone Material

Pumice stone material (PS) is an igneous rock formed by the rapid cooling and solidification of volcanic lava at the Earth’s surface. Igneous rocks, in general, include both those formed deep underground through the solidification of magma, as well as those resulting from the cooling of lava at the surface. The most common types of magmas are silicate-based (containing more than 30% silicon dioxide—SiO2), though smaller amounts of carbonate, sulfuric, or oxide magmas may also occur.
Silicate magmas are classified according to their silica content: acidic magmas—containing more than 63% SiO2; intermediate (neutral) magmas—between 52% and 63% SiO2; basic magmas—below 52% SiO2.
Basic magmas contain higher amounts of volatile substances, which gives them greater fluidity compared to acidic magmas. Pumice is a type of volcanic rock known as rhyolite. It is characterized by a high silica content and low levels of iron and magnesium, which result in its low hardness and distinctive light color. Although rhyolite is of volcanic origin, its chemical composition is similar to that of granite. Thus, pumice contains minerals such as feldspar, quartz, and biotite.
The typical chemical composition of pumice is as follows: 70–77% silicon dioxide (SiO2), 11–14% aluminum oxide (Al2O3), 3–5% potassium oxide (K2O), 3–5% sodium oxide (Na2O), 1–3% ferrous oxide (FeO), 1–2% ferric oxide (Fe2O3), 0.5–1% magnesium oxide (MgO), and approximately 0.03% water [7,8,9,10,11,12,13]. This rock is extremely lightweight due to its porous structure—porosity can reach up to 85% of the total volume. The microporous structure of pumice results in a high specific surface area, making it suitable for adsorption applications.
Depending on its composition, pumice can exhibit either basic or acidic characteristics, but generally it has a high silicon content (60–75% SiO2), which imparts abrasive properties. Due to its high porosity and significant content of silicon and aluminum, pumice is considered a promising material for use as an adsorbent.
Moreover, some studies have shown that pumice represents an ideal support for metal impregnation due to its structure [11]. Therefore, research into the ability of this rock to remove pollutants from wastewater is of particular interest.
In order to characterize the composition of pumice, an analysis was carried out using certified reference materials (CRM) of igneous rocks, and the chemical composition of pumice was determined using X-ray analysis, as shown in Table 3:

2.3. Autoclaved Cellular Concrete

Autoclaved aerated concrete (AAC) is a construction material that belongs to the category of lightweight concrete. It is produced by mixing components such as sand, cement, lime, water, silica, and an expanding agent—typically aluminum powder—which, through a chemical reaction, releases gases, generating its characteristic porous structure.
AAC was first produced in Yxhult, Sweden, by architect and inventor Johan Axel Eriksson. It is a prefabricated, lightweight material widely used in construction due to its excellent thermal insulation, fire resistance, mold resistance, and load-bearing capacity. Additionally, it contributes to structural rigidity while maintaining a much lower weight compared to conventional concrete—up to five times lighter.
The internal structure of Autoclaved Aerated Concrete (AAC) is characterized by uniformly distributed, small, closed spherical pores, typically ranging from 0.1 to 1 mm in diameter. This porous microstructure results from the chemical reaction between calcium hydroxide (Ca(OH)2), produced during binder hydration, and aluminum powder. The reaction generates hydrogen gas, which causes the mixture to expand, forming the characteristic cellular architecture during the autoclaving process.
The basic mixture for producing AAC includes fine sand (with a grain size comparable to that of cement), cement, lime, gypsum, water, and the expanding agent. After the mixture is prepared and the porous structure develops, the material undergoes thermal treatment in an autoclave, where high pressure and temperature finalize the hardening process and stabilize the structure [18,21,22].
Due to its porous structure and large specific surface area, AAC is not only an excellent construction material but also a potential efficient adsorbent, suitable for use in adsorption processes aimed at removing pollutants from aqueous or gaseous environments.
The chemical composition of the ACC sample as determined by XRF analysis is given in Table 4. Like in the case of PS, the main oxide is SiO2, followed by CaO and Al2O3.

2.4. Characterization of PS and AAC Materials

The surface morphology of the samples (initial and final PS/AAC with phosphate) was analyzed using a Scanning Electron Microscope (SEM) Quanta Inspect F50 operating at an acceleration voltage of 30 kV, with magnifications ranging from 1000× to 5000×. A secondary electron detector (ETD) was used in SE mode for topographical contrast. Elemental composition of selected areas was analyzed using Energy Dispersive X-ray Spectroscopy (EDX) having 133 eV resolution at Mn Kα, coupled to the SEM (all from FEI Company, Eindhoven, The Netherlands,).
The crystallographic structure of the samples was analyzed by X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer (Tokyo, Japan) equipped with a Cu-Kα radiation source (λ = 1.5406 Å) and calibrated with an Al2O3 standard (SRM 676A) [33]. The measurements were performed in the 2θ range of 10–70°, with a scan step of 0.02° and a counting time of 100 s per step.
Specific surface area, pore volume, and pore size distribution of the adsorbents were determined by nitrogen adsorption–desorption isotherms using the BET method. Measurements were carried out with an instrument capable of performing multi-point BET analysis and BJH pore analysis, using adsorption at liquid nitrogen temperature (77 K). The analysis has been performed using a Micromeritics TriStar II Plus (Norcross, GA, USA) surface area and porosity analyzer.
The elemental composition of pumice stone (PS) and autoclaved aerated concrete (AAC) was determined using an energy-dispersive X-ray fluorescence spectrometer (MiniPal PW4025, PANalytical, Almelo, The Netherlands). The instrument is equipped with a rhodium anode X-ray tube (operating voltage: 5–30 kV; current: up to 1 mA) and a high-resolution silicon drift detector (SDD), allowing qualitative and quantitative determination of elements from sodium (Na, Z = 11) to uranium (U, Z = 92) in solid samples. Prior to analysis, samples were oven-dried at 105 °C for 24 h, finely ground to pass a 100 µm sieve, and pressed into pellets (32 mm diameter) under a load of 20 t using a hydraulic press. Calibration was performed using certified reference materials, and measurements were carried out under vacuum to improve detection sensitivity for light elements. Results are expressed as weight percentages (wt%) of oxides, as calculated by the instrument’s software package.

2.5. Adsorption Experiments

The batch adsorption studies were performed by mixing the solution containing phosphate ions and the adsorbent, using a magnetic stirrer. The size of the adsorbent material granules was between 0.063 and 2 mm.
The retention of phosphate ions from the solution was calculated as the difference between the initial concentration and the concentration at different contact times. The amount removed per unit mass of adsorbent (at, mg/kg) at time “t” results from:
a t = C 0 C t     V m .
The removed percentage (RP) of phosphate ions was calculated from:
R P = C 0 C t C 0 100 ,
where at, mg/kg is the amount of phosphate ions removed per unit mass of the adsorbent at a given time, t; C0, mg/L is the initial concentration of phosphate ions in the aqueous solution, Ct, mg/L is the concentration of phosphate ions at time, t; V, L is the volume of the solution, m, g, is adsorbent mass.

2.5.1. Adsorption Equilibrium Study

The equilibrium study aimed to obtain adsorption isotherms in order to determine the degree of phosphorus retention on the granules of the adsorbent materials (PS and AAC). To characterize the adsorption process, Langmuir and Freundlich isotherms were used, at a mass ratio of solid to liquid phases of 1:10, and pH values of 3 and 9.
In the thermodynamic study, the adsorbent samples were kept in contact with the standard phosphorus solution (KH2PO4) of different concentrations: 10, 20, 40, 60, 80, and 100 mg/L, for one hour to achieve equilibrium. Afterwards the suspension was filtered on a filter paper with pore size of 0.45 μm. The resulting aqueous extract was analyzed for phosphorus content with an X-ray spectrometer, as described in Section 2.4.
The description of the equilibrium state of the adsorbed-adsorbent system was made through the Langmuir and Freundlich models presented below. These models represent the most used method to describe retention of ions from liquid solutions onto the surface of porous solids.
  • The Langmuir equation
The Langmuir model assumes that the adsorbent surface has active centers that can hold the adsorbate ions by physisorption (by Vand der Waals forces) or chemisorption. He applies the hypothesis that physical and chemical bonds are formed between the surface of the adsorbent and the molecules of the adsorbed component, and there are no interactions between the molecules of the adsorbed component. The adsorption is monolayer and there are no interactions among the adsorbate species.
The Langmuir isotherm model is described by the following formula:
a e = K C a max 1 + K C ,
which gives by linearization of
1 a e = 1 K C a max + 1 a max .
Performing the variable changes y e = 1 a e and x = 1 C the equation of a straight line is obtained,
y e = A x + B   A = 1 K a max ;                 B = 1 a max ,
where ae, amax are the phosphorus content at equilibrium and the maximum adsorption capacity (at saturation, mg P/kg dry adsorbent; C is the concentration of adsorbate (phosphate) solution after adsorption equilibrium is reached, mg/L; K is the adsorption coefficient, L/mg [27,28].
  • The Freundlich equation
The Freundlich model starts from the hypothesis of reaching chemical equilibrium when there is a dynamic exchange between adsorbed molecules and those remaining in solution. The adsorbent surface is heterogeneous, and the adsorbate species are distributed in multilayer at the adsorbent surface. This implies that all the adsorption sites have different affinities. Freundlich isotherm is expressed by the equation:
a e = K C 1 / n .
Equation (5) is linearized by logarithmization obtaining:
ln a e = ln K + 1 n ln C .
Replacing y = ln a e and x = ln C, the equation of a straight line is obtained y c = A x + B ; A = 1 n ;     B = ln K .

2.5.2. Description of Models for Kinetic Study

The kinetic experimental determinations were carried out on the PS (pumice stone) material at two pH values, 3 and 9, maintaining a solid-to-liquid phase mass ratio of 1:10. The pH was adjusted using 2 M NaOH and H2SO4 solutions. The samples were brought into contact with KH2PO4 solutions of varying concentrations (10, 20, 40, 60, 80, and 100 mg/L) for different contact times: 2, 5, 10, 15, 20, 30, 40, and 60 min, under stirring at 200 rpm. After contact, the suspensions were vacuum filtered, and the resulting aqueous extract was analyzed according to the SR ISO 6878:2005 standard [32]. To understand the adsorption mechanism of phosphorus ions on the adsorbent, a kinetic study was conducted based on the pseudo-first-order and pseudo-second-order models, analyzing efficiency as a function of pH.
(a) The pseudo-second-order adsorption kinetic model is mathematically described according to Equation (7) [23]:
d a t d t = k ( a e q a t ) 2 ,
where: k = pseudo-second-order rate constant of the adsorption process (g/min·mg); aeq = phosphate ion loading of the adsorbent at equilibrium (mg/g); at = adsorbent loading at time t (mg/g).
Applying the boundary condition: t = 0, t = t şi at = 0, at = at by integrating Equation (7) we obtained:
0 a t d a t ( a e q a t ) 2 = k 0 t d t 1 a e q a t 1 a e q = k t .
(b) The pseudo-first-order adsorption kinetic model is mathematically described according to Equation (8) [23]:
d a t d t = k ( a e q a t ) .
Applying the boundary condition: t = 0, t = t şi at = 0, at = at by integrating Equation (9), we obtained: log ( a e q a t ) = log ( a e q ) k 2303 t , where k and aeq can be determined from the slope of the line.
The model used to determine the evolution of the load over time is given by the balance Equation (10)
a 1   = V a q ( C 0 C ) m P S + a 0 ,
where a 1 = a + a 0 , at and C—have the same meaning as in Equation (7), mg/g; C0 = initial concentration of phosphate in the aqueous phase, mg/L; Vaq = volume of the aqueous phase, mL; a0 = initial phosphate content in the pumice stone, mg/g; mPS = mass of the solid (pumice stone), g.

3. Results and Discussion

3.1. Characterization of PS and AAC Materials

3.1.1. X-Ray Diffraction (XRD) Analysis for PS

The XRD patterns for initial PS sample and for PS after being kept at pH = 3 are given in Figure 1 and confirm the crystalline structure of the material presenting sharp peaks. The XRD spectra of initial PS sample shows three main components (xCaO SiO2 zH2O, SiO2 and K2Ca6Si4O15). The sample that was kept at pH = 3 (Figure 1b) in the presence of phosphate shows same phase rearrangements and the presence of five components (xCaO SiO2 zH2O, SiO2, Mn2O3, K3(MnH4)H, Mg2P2O7), as shown in Figure 1.

3.1.2. X-Ray Diffraction (XRD) Analysis for AAC

The XRD patterns for initial ACC sample and for AAC after being kept at pH = 3 are given in Figure 2. The sharp peaks in the XRD patterns confirm the crystalline structure of the AAC material. The XRD spectra of initial ACC sample (Figure 2a) shows four components (Ca2SiO4, SiO2, Mg0.06Ca0.94CO3, and Al2O3). The sample that was kept at pH = 3 (Figure 2b) in the presence of phosphate ions shows same phase rearrangements and the presence of five crystalline components (Ca2SiO4, SiO2, Mg0.06Ca0.94CO3, MgHPO4 H2O, and Ca0.05Al0.1Si1.9O4).
The specific surface of initial ACC sample used in our experiments is 30.99 m2/g and average pore size 9.2 nm, which is in agreement with previously reported values [19,20,30]. The value of the sample at acidic pH (3) is slightly lower than the initial sample, 23.86 m2/g and average pore size 9.6 nm, indicating a slight reduction of number of pores at lower pH.

3.1.3. SEM Analysis for PS

The SEM analysis of the control sample highlights the heterogeneous nature of the pumice stone, with particles of varying sizes and morphologies. The bright areas in the image indicate the presence of elements with higher atomic numbers, particularly calcium, while the darker areas likely correspond to compounds rich in silicon and oxygen. To identify the local chemical composition, two EDX analyses were performed in distinct regions of the sample (Figure 3) [13,23].
In the EDX1 area (Figure 3), a high concentration of calcium was observed, accompanied by silicon and small amounts of aluminum and magnesium (at least on the surface), suggesting the presence of calcareous phases. In contrast, the EDX2 spectrum shows a predominance of silicon with a lower calcium content, probably associated with silicates or quartz. Correlating these results with the overall chemical analysis indicates that the sample consists mainly of SiO2 (51.26%) and CaO (28.08%), along with Al2O3 (14.84%) and small amounts of Fe2O3, MgO, and other oxides. Overall, the pumice stone exhibits a heterogeneous mineralogical structure, consisting of alternating silicate-rich and calcium-rich zones, which is characteristic of volcanic-origin materials [34].
The SEM analysis of pumice stone (PS) after contact with phosphate solution at pH 3 (Figure 4) reveals a granular morphology with a rough surface and visible porosity, characteristic of aluminosilicate volcanic materials. The analyzed areas (EDX1, EDX2, EDX3) display fine deposits on the particle surfaces, suggesting the adsorption or precipitation of phosphate species. The porous structure remains apparent, indicating that the acidic treatment did not cause extensive dissolution, although partial leaching of certain mineral components is likely to have occurred, leading to surface modification.
EDX analysis confirms that silicon (Si) and oxygen (O) are the major elements, corresponding to the aluminosilicate matrix, with aluminum (Al) consistently present (Figure 4). Potassium (K), calcium (Ca), and magnesium (Mg) are also detected in smaller proportions, typical of the pumice mineral composition. The presence of phosphorus (P) in all analyzed points indicates the retention of phosphate from solution, either via adsorption onto the mineral surface or through the formation of calcium or aluminum phosphate precipitates. Variations in phosphorus intensity between points suggest a heterogeneous distribution, likely related to local mineralogical composition and surface reactivity [35].
Overall, the morphological and compositional data indicate that acidic conditions at pH 3 promote partial leaching of exchangeable cations (Ca, Mg, K), generating active sites for phosphate retention while preserving the inherent porosity of the pumice stone.
The SEM images of the initial pumice stone (PS) show a highly porous (Figure 5), rough surface with irregularly shaped particles, typical of aluminosilicate volcanic material. After contact with the phosphate solution at pH 3, the surface morphology exhibits visible deposits partially covering the pores, suggesting phosphate adsorption or precipitation. Despite these surface changes, the porous structure is still evident, indicating that the acidic conditions did not cause severe dissolution of the matrix.
EDX analysis of the initial PS reveals silicon (Si) and oxygen (O) as the predominant elements, with aluminum (Al), potassium (K), calcium (Ca), and magnesium (Mg) also present. After treatment at pH 3, phosphorus (P) is clearly detected, confirming phosphate retention. Additionally, a slight reduction in Ca and Mg signals suggests partial leaching under acidic conditions, potentially generating new active sites for phosphate binding.

3.1.4. SEM Analysis for AAC

The initial material AAC sample (Figure 6a) was analyzed morphologically using scanning electron microscopy (SEM), with images captured at magnifications of 1000× and 5000×. The micrographs revealed a non-homogeneous morphology, characterized by randomly oriented acicular lamellar formations. Elemental composition analysis was performed by energy-dispersive X-ray spectroscopy (EDX) at multiple points. At EDX points 1 and 4, high concentrations of calcium (Ca) and silicon (Si) were recorded, along with traces of magnesium (Mg) and aluminum (Al). At EDX point 2, significant amounts of Si, Al, and Ca were identified, while Mg was present in lower proportions. EDX point 3 was dominated by silicon, most likely in the form of silicon dioxide (SiO2), with other elements being weakly represented [19,20,21].
In the case of the AAC sample treated at pH 3 (Figure 6b), SEM images were taken at magnifications of 1000× and 2000×. Compared to the reference sample, significant surface modifications were observed: over the initial lamellar formations, compact, angular deposits resembling granules appeared. The EDX analysis (Figure 7) indicated, at points 1 and 3, a predominant presence of Ca and Si, distributed in the form of submicron-sized lamellar compounds. At EDX point 2, silicon was dominant, associated with the compact granular formations also observed in the SEM images.
By comparing the two samples, it can be concluded that treating the AAC with an acidic solution (pH 3) induces visible morphological changes, suggesting chemical rearrangements and/or the formation of new phases. Although the distribution of the dominant elements, Ca and Si, remains relatively constant, the mode of aggregation and surface structure differ substantially. These changes may directly influence the adsorption capacity of the analyzed material [36,37,38].

3.1.5. BET Method for PS and AAC

The specific surface of the initial PS sample used in our experiments is 16.38 m2/g, which is slightly higher than previous values reported in the literature [13,23,30]. The value of the sample at acidic pH (3) is slightly higher than the initial sample, 20.96 m2/g, indicating some pore structure modifications at lower pH, as shown in Table 5.
Pumice exhibits a lower specific surface area than the reference AAC but increases significantly under acidic pH, suggesting pore activation. In contrast, AAC initially has the highest specific surface area, but this decreases at pH 3, likely due to pore clogging or structural changes.
Pore volume follows the same trend: higher in reference AAC and lower in pumice. pH variations influence the porous structure of both adsorbents.

3.2. Adsorption Equilibrium Study

The experimental results establish the equilibrium curves in order to determine the degree of retention of phosphorus on two types of adsorbent materials (PS and ACC) using the Langmuir and the Freundlich isotherms, at a mass ratio of the adsorbent material: water phases of 1:10 at two pH values (3 and 9) for one hour. The experiments were performed in triplicate. The results showed standard deviations lower than 15% of the average measurements. The experiments were conducted at pH 3 and 9, corresponding to conditions where phosphate species differ significantly (H2PO4 in acidic and HPO42−/PO43− in alkaline media). Neutral pH (around 7) was avoided to prevent overlapping of multiple species, which could obscure the interpretation of adsorption mechanisms. Langmuir and the Freundlich model parameters were calculated by linear regression.
The relevant parameters obtained using the Langmuir and the Freundlich models are presented in Table 6 and Table 7.
The Langmuir isothermis characterised by a dimensionless constant called the separation factor, RL, or the equilibrium parameter. This is used to predict whether an adsorption system is favorable or not [22]. Based on RL value, the represented isotherm can indicate favorable adsorption, when RL is between zero and one; unfavorable when RL is higher than one; in the linear model for adsorption, RL equals one; irreversible adsorption, RL = 0. RL is calculated using Equation (9):
RL = 1/(1 + K C0),
where K in L/mol is the Langmuir constant and C0 in mol/L is the highest initial phosphate concentration. The calculated values for PS are 0.069 (pH = 3) and 0.19 (pH = 9), while for ACC are 0.103 (pH = 3) and 0.147 (pH = 9). These values indicate that phosphate adsorption on PS or ACC is favorable.
The Freundlich model provides the best approximation of phosphorus retention isotherms (R2 = 0.90 ÷ 0.97) for all adsorbent material types compared to the Langmuir model (R2 = 0.82 ÷ 0.92).
To quantify the quality of fit of the Langmuir and Freundlich adsorption models to the experimental data, two error indicators were calculated: RMSE (Root Mean Square Error) and χ2 (chi-squared). RMSE represents the average squared deviation of the estimated values from the experimental values (the square root of the mean of squared differences); the lower the RMSE, the better the model approximates the experimental data. The χ2 statistic was determined using a formula commonly applied in adsorption studies, namely the sum of squared errors normalized by the predicted values, following common practice in adsorption studies.
It can be observed from the table that the Freundlich model yields significantly lower errors than the Langmuir model in all analyzed cases. For each material and pH, both RMSE and χ2 values are lower for Freundlich compared to Langmuir, suggesting a superior fit of the Freundlich model to the experimental data. This indicates that the adsorbent surface exhibits a heterogeneous character, which favors the applicability of the empirical Freundlich model (which assumes the presence of multiple types of binding sites and possibly multilayer adsorption), whereas the Langmuir model (based on homogeneous surfaces and monolayer adsorption) cannot describe the studied systems as accurately.
In the case of the Freundlich model, the influence of the pH value is also felt on the intensity of the anion exchange. Thus, the slope A = 1/n gives us indications on the intensity of adsorption, and B = lnK gives indications about the adsorption capacity. The values of A and B calculated based on the Freundlich model are presented in Table 8.
The phosphate anions in the solution are attracted to the positively charged areas on the surface of the adsorbent material particles, according to the anion exchange capacity.
Using the determined parameters, the Langmuir and the Freundlich isotherms for phosphates sorption were calculated and are presented in Figure 8, Figure 9, Figure 10 and Figure 11. The calculated adsorption isotherm data shown in these figures were fitted to experimental data using the least squares method (R2 = 0.82 ÷ 0.97). The graph of the isotherms, for the Langmuir and the Freundlich model, represents the loading a (amount of adsorbed phosphate) depending on the concentration of phosphates in the solution, Ce. Generally, the Langmuir isotherm presents three regions: a steep one given by the intense adsorption process, a flat third region characterized by a low intensity of the adsorption due to the saturation of the adsorbent with adsorbate, and the intermediate region. The shape of the Langmuir isotherm is characterized in this case by a steep, practically linear initial region and a curved middle portion; the lack of a horizontal segment demonstrates the high availability of the adsorbent material for the adsorption process. This allure is specific to the phenomena of chemosorption and physical adsorption, where the adsorption is carried out in a monomolecular layer [24,25,26,27,28,29,39,40,41,42].
The extent of phosphorus removal (percentage removed) as a function of aqueous solute concentration was also studied. In Figure 12, the percentage of phosphorus removed is shown; the removal efficiency increases from 5% (pH = 9) to 90% (pH = 3) with increasing solute concentration. It is also observed that at an acidic pH, the studied adsorbent material presents a higher selectivity and adsorption capacity.
Phosphorus adsorption on the surface of adsorbent materials is determined by the solid surface charge and the protonation state of phosphorus in the water solution [18]. The results of this study show that AAC and pumice are effective adsorbents for the removal of phosphate ions from aqueous solutions. Laboratory studies carried out on PS and ACC have shown that phosphorus sorption varies with the pH value. Based on previous studies [36,42], phosphate adsorbed onto AAC could potentially be recovered and reused in agriculture as a phosphate-based fertilizer.
Although PZC (point of zero charge) or IEP (isoelectric point) measurements were not conducted experimentally in this study, literature values were included to support the interpretation of pH influence on adsorption. For pumice stone, PZC values are generally reported in the range of 6.0–7.5, depending on the mineralogical composition and origin of the material [31,34]. For autoclaved aerated concrete (AAC), PZC values are reported around 10.0–10.5, due to its high calcium content and alkaline nature [38,43]. These values are consistent with the pH conditions we selected (3 and 9) and support the observed behavior that phosphate retention is favored under acidic pH for both materials.
Although the bulk chemical composition of AAC and PS is broadly similar (both being rich in SiO2, Al2O3, and CaO), the XRD analysis revealed distinct mineralogical phases, which directly influence their adsorption mechanisms. Pumice contains species such as Mg2P2O7 and Mn2O3, while AAC exhibits phases like Mg0.06Ca0.94CO3, Ca2SiO4, and MgHPO4·H2O after phosphate adsorption at acidic pH. These differences suggest that specific crystalline structures, especially those containing Ca, Mg, or Al, may serve as active sites for phosphate immobilization.
The phosphate adsorption efficiency is strongly influenced by the pH-dependent speciation of phosphate ions. At acidic pH (≈3), the dominant species in solution is H2PO4, while at alkaline pH (≈9), HPO42− and PO43− prevail. Simultaneously, the surface charge of the adsorbent materials changes with pH due to protonation/deprotonation reactions, which impacts the electrostatic interaction between phosphate anions and the adsorbent surface.
This behavior can be better understood by considering the point of zero charge (PZC) of each material. Thus, at pH 3, both materials are positively charged, enhancing electrostatic attraction for phosphate anions (especially H2PO4), which explains the higher removal efficiency observed in acidic conditions. Conversely, at pH 9, both materials are at or beyond their PZC and may carry a negative surface charge, leading to weaker attraction or even repulsion of phosphate species like HPO42− and PO43−.
Additionally, the presence of Mg, Ca, and Al-based species on the surface promotes inner-sphere complexation or precipitation mechanisms, especially under acidic pH, which increases phosphate fixation. For example, Ca2+ and Mg2+ can form insoluble phosphates (e.g., MgHPO4, Ca3(PO4)2), and Al3+ may participate in ligand exchange reactions with phosphate, forming strong Al–O–P bonds.
Therefore, the observed differences in adsorption performance between PS and AAC, despite similar oxide compositions, are attributed to their distinct mineralogical phases and surface charge behavior at varying pH levels, which modulate their affinity for different phosphate species.
In addition to the crystalline phases identified, it is important to consider the contribution of amorphous structures, which are characteristic of volcanic materials such as pumice. These may play a significant role in phosphate adsorption, as discussed below.
The chemical analysis of the PS (pumice stone) material revealed a significant concentration of Al2O3 (14.84%); however, this oxide was not detected as a distinct crystalline phase in the XRD pattern. This discrepancy suggests that aluminum is predominantly present in amorphous phases or incorporated into aluminosilicate structures, a characteristic feature of volcanic rocks such as pumice [34,44].
Amorphous phases play an important role in adsorption processes due to their high porosity and the presence of unsaturated reactive groups (e.g., –OH, structural defects). These features provide an extended active surface that facilitates interactions with anions such as phosphate, either through ion exchange mechanisms or by complexation with available cations (e.g., Al3+, Ca2+, Fe3+) present in the disordered network [27,31].
Furthermore, aluminum and calcium ions in amorphous phases are often more chemically available than those in well-defined crystalline phases, which favors the formation of low-solubility compounds such as AlPO4 or Ca3(PO4)2 [38,45]. These interactions contribute to the efficient retention of phosphorus from solution, even in the absence of crystalline phases detectable by XRD.
Therefore, the experimental results obtained in this study, which indicate a high phosphate adsorption capacity for the PS material, can be explained, at least in part, by the presence and activity of these amorphous structures.

3.3. Kinetic Study

The kinetics of the phosphate adsorption process were quantified through laboratory assessments conducted using batch experiments.
Following the mathematical analysis of the experimental data using pseudo-first-order and pseudo-second-order kinetic models, the rate constants describing the retention of phosphate ions on the surface of the adsorbent material (PS) were determined. The variation in the amount of phosphate adsorbed as a function of contact time was studied under controlled conditions: the pH of the solution was set at 3 and 9, the initial phosphate ion concentration ranged from 10 to 100 mg/L (10, 40 și 100 mg/L), and the stirring speed was maintained at 200 rpm. The results showed that pH had a significant impact on the adsorption efficiency; as the pH increased from 3 to 9, the average amount of phosphate ions adsorbed decreased. The kinetic parameters calculated by pseudo-first-order and pseudo-second-order kinetic models are presented in Table 9.
The data in Table 6 show that the pseudo-second-order model describes the adsorption process with higher accuracy, having R2 values closer to 1 compared to the first-order model. Furthermore, the equilibrium adsorption capacity (aeq) increases proportionally with the initial concentration and is significantly higher at pH 3 than at pH 9. This behavior can be explained by the stronger electrostatic attraction between phosphate ions and the protonated surface of the adsorbent in an acidic medium.
The graphs in Figure 13, Figure 14, Figure 15 and Figure 16 illustrate the kinetic behavior of phosphate ion adsorption onto PS (pumice stone) at two distinct pH values (3 and 9) and for three initial concentrations (10, 40, and 100 mg/L).
In Figure 13 and Figure 15, a rapid decrease in phosphorus concentration in solution is observed during the first 10–15 min, followed by a tendency toward stabilization, suggesting that the adsorption process reaches equilibrium relatively quickly. This behavior is typical of a process controlled by mass transfer and surface chemical reactions. The efficiency is noticeably higher at pH 3, where the residual phosphate concentration decreases much more sharply compared to pH 9.
Figure 14 and Figure 16 show the evolution of the adsorbent loading (a, mg/kg) over time. The experimental data (points) are very well fitted by the lines corresponding to the pseudo-second-order model, supporting the hypothesis of a chemisorption-type kinetic (the adsorption phenomenon is controlled by chemical reactions between phosphate ions and the active surface of PS). It is noteworthy that at the initial concentration of 100 mg/L, aeq reaches values above 700 mg/kg at pH 3 and above 600 mg/kg at pH 9—consistent with the values presented in Table 9, confirming the high efficiency of pumice stone in retaining phosphates.
This difference between pH values suggests that an acidic environment favors adsorption, probably due to the positive surface charge of PS, which attracts phosphate anions.
In addition, the shape of the curves in both graphs (a versus time) indicates rapid initial adsorption, followed by a gradual attainment of equilibrium—typical of processes governed by surface reactions rather than solely by diffusion.
The analysis of the correlation between the experimental data and the kinetic models showed that the pseudo-second-order model describes the adsorption process with superior accuracy, indicating that chemical interactions play a dominant role in the mechanism of phosphate retention on PS. The high determination coefficient values (R2 > 0.98) for this model confirm the hypothesis of a higher-order kinetic, influenced by the availability of active sites on the adsorbent surface and the electrostatic nature of the interactions between the solid phase and phosphate ions.
These results are consistent with observations from the literature: Fetene et al. (2020) demonstrated that phosphate adsorption onto pumice reaches equilibrium in approximately 60 min, and the data fit best to the pseudo-second-order model, suggesting a chemisorption mechanism [28]. Furthermore, their study reported an efficiency of up to 94% at pH 7—a value comparable to the performance observed in our study at pH 3, where adsorption was even more efficient, suggesting an increased potential of PS under acidic conditions [28].
Another earlier study, Onar and Ozturk (1993) [46], reported an adsorption efficiency above 80%, with a reversible mechanism combining first- and second-order reactions, as well as good conformity to the Freundlich model at various pH values (3, 5, 7, and 9). Our observations regarding the strong influence of pH, with lower adsorption in basic media, are thus also confirmed by previous findings [35].
Table 10 provides a comparison of the adsorption capacities of several materials.
Although the adsorption capacities obtained in this study are lower compared to those reported for chemically or thermally modified materials, the use of unmodified, low-cost, and readily available adsorbents such as natural pumice and construction waste-derived AAC offers significant economic and environmental advantages. Furthermore, the study provides a comprehensive analysis of adsorption behavior and mechanisms, which is less commonly addressed for raw materials. These findings support the potential practical applicability of AAC and PS in cost-effective and sustainable wastewater treatment systems.
Thus, pumice stone demonstrates promising potential as an adsorbent material for phosphate removal from aqueous solutions, particularly under acidic conditions where its efficiency is maximized. The kinetic study provides a solid basis for understanding the adsorption mechanism and for optimizing future processes for the treatment of phosphate-contaminated waters.
Regeneration studies involving at least five adsorption–desorption cycles will be carried out in future work to provide a comprehensive evaluation of the adsorbents’ potential for practical applications.

4. Conclusions

This study investigated the efficiency of two low-cost adsorbent materials—pumice stone (PS) and autoclaved aerated concrete (AAC)—for phosphate removal from wastewater, as a function of solution pH and contact time.
PS demonstrated a superior phosphate adsorption capacity (up to ~1000 mg/kg at pH 3), due to the presence of amorphous aluminosilicate phases and reactive Al3+, Ca2+, and Mg2+ species, as identified by XRD and XRF analyses.
The removal efficiency was strongly influenced by the solution pH, with optimal performance in acidic medium, where phosphate species (especially H2PO4) interact favorably with the positively charged surfaces of the adsorbents. This behavior was supported by the differences in the point of zero charge (PZC) of the materials.
Isotherm studies showed that the Freundlich model best describes the adsorption process. The presence and role of amorphous phases were confirmed as key factors, especially in the case of PS, significantly contributing to phosphate retention through complexation and precipitation mechanisms (e.g., formation of MgHPO4, AlPO4, Ca3(PO4)2).
Kinetic modeling indicated a pseudo-second-order mechanism, confirming that the dominant adsorption mechanism is chemisorption, accurately described by the pseudo-second-order model. The observed performances, including the increased efficiency at lower pH, are consistent with the literature, reinforcing the validity of the method and the potential of pumice stone as an efficient adsorbent for phosphate removal from aqueous solutions.
From an environmental perspective, these results support the sustainable use of natural materials and construction waste for phosphorus removal from domestic and industrial effluents, reducing the risk of eutrophication and enabling nutrient recycling in agriculture.
Although the adsorption capacities determined for AAC and pumice are relatively modest compared to those reported for chemically or synthetically modified materials, these materials offer significant advantages in terms of practical applicability. Their low cost (a synthetic material with a capacity of 10 mg/g can be up to 10 times more expensive than a raw material with 1 mg/g capacity), wide availability, and environmental compatibility make them ideal candidates for use in decentralized wastewater treatment systems or tertiary treatment stages, where phosphate concentrations are low, but effluent quality standards are strict. Moreover, the absence of additional activation or chemical modification steps reduces process complexity and operational costs. Therefore, the use of unmodified materials such as AAC and PS can represent an efficient and sustainable solution within the framework of the circular economy and sustainable phosphorus management [47,48].
In conclusion, this work makes a relevant contribution to the current knowledge, highlighting the importance of mineralogical composition and surface charge behavior in controlling phosphate adsorption, and emphasizing the potential of PS and AAC materials as efficient, reusable, and low-cost solutions for wastewater treatment.

Author Contributions

Conceptualization, O.D.O. and C.M.; methodology, O.D.O.; software, E.R.C., M.B. and A.M.D.; validation, C.M., A.-A.S. and M.C. and D.N.A.; formal analysis, M.G. and O.D.O. investigation, M.C.; resources, A.M.; data curation, A.M.D. and O.D.O.; writing—original draft preparation, O.D.O. and C.M.; writing—review and editing, E.R.C., M.B., C.M. and A.-A.S.; visualization, O.D.O. and D.N.A.; supervision, O.D.O. and C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns of PS (a) initial material and (b) at pH = 3.
Figure 1. XRD patterns of PS (a) initial material and (b) at pH = 3.
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Figure 2. XRD patterns of AAC (a) initial material and (b) at pH = 3.
Figure 2. XRD patterns of AAC (a) initial material and (b) at pH = 3.
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Figure 3. SEM and EDX images for initial material PS.
Figure 3. SEM and EDX images for initial material PS.
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Figure 4. SEM and EDX images for PS at pH = 3.
Figure 4. SEM and EDX images for PS at pH = 3.
Environments 12 00288 g004aEnvironments 12 00288 g004b
Figure 5. SEM and EDX analysis of PS before and after at pH 3.
Figure 5. SEM and EDX analysis of PS before and after at pH 3.
Environments 12 00288 g005aEnvironments 12 00288 g005b
Figure 6. SEM and EDX images for (a) initial material AAC, (b) AAC at pH = 3.
Figure 6. SEM and EDX images for (a) initial material AAC, (b) AAC at pH = 3.
Environments 12 00288 g006
Figure 7. SEM and EDX images for AAC at pH = 3.
Figure 7. SEM and EDX images for AAC at pH = 3.
Environments 12 00288 g007
Figure 8. Isotherms for phosphorus adsorption on PS for pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 8. Isotherms for phosphorus adsorption on PS for pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g008
Figure 9. Isotherms for phosphorus adsorption on PS for pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 9. Isotherms for phosphorus adsorption on PS for pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g009
Figure 10. Isotherms for phosphorus adsorption on ACC for pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 10. Isotherms for phosphorus adsorption on ACC for pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g010
Figure 11. Isotherms for phosphorus adsorption on ACC for pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 11. Isotherms for phosphorus adsorption on ACC for pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g011
Figure 12. Phosphorus removal rate on PS as a function of solution concentration.
Figure 12. Phosphorus removal rate on PS as a function of solution concentration.
Environments 12 00288 g012
Figure 13. Variation of phosphorus concentration over time for PS at pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 13. Variation of phosphorus concentration over time for PS at pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g013
Figure 14. Variation of phosphorus loading over time for PS at pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 14. Variation of phosphorus loading over time for PS at pH = 3. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g014
Figure 15. Variation of phosphorus concentration over time for PS at pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 15. Variation of phosphorus concentration over time for PS at pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g015
Figure 16. Variation of phosphorus loading over time for PS at pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Figure 16. Variation of phosphorus loading over time for PS at pH = 9. Note: The points represent experimental data, and the line represents the calculated data according to the mentioned model. Concentrations and adsorption capacities are expressed as phosphorus (P); 1 mg P/L = 3.23 × 10−5 mol/L; 1 mg P/kg = 3.23 × 10−5 mol/kg.
Environments 12 00288 g016
Table 1. Permissible phosphorus loading limits for industrial and municipal wastewater [5].
Table 1. Permissible phosphorus loading limits for industrial and municipal wastewater [5].
Quality IndicatorM.U.Limit ValuesReference
Total phosphorusmg/L1 (2)SR EN 1189:1999 [14]
Synthetic detergentsmg/L0, 5SR ISO 7825/1—1996 [15]
SR ISO 7825/2—1996 [16]
Table 2. Permissible phosphorus loading limits for industrial and municipal wastewater discharged into municipal sewer networks [6].
Table 2. Permissible phosphorus loading limits for industrial and municipal wastewater discharged into municipal sewer networks [6].
Quality IndicatorM.U.Limit ValuesReference
Total phosphorusmg/L5STAS 10064—75 [17]
Table 3. Chemical composition of PS.
Table 3. Chemical composition of PS.
CompoundSiO2Al2O3Fe2O3CaOMgOOthers
Concentration, %51.2614.843.1628.082.040.61
Table 4. Chemical composition for ACC.
Table 4. Chemical composition for ACC.
CompoundSiO2Al2O3Fe2O3CaOMgOOthers
Concentration, %61.0113.052.9321.731.010.26
Table 5. Specific surface area and pore volume.
Table 5. Specific surface area and pore volume.
MaterialBET
(m2/g)
Langmuir (m2/g)Adsorbed Pore Volume (cm3/g)Observations
initial material PS16.3724.670.04315Low specific surface area
PS pH = 320.9731.020.04519Significant increase at acidic pH
initial material AAC30.9945.570.06761Highest BET surface area
AAC pH = 323.8635.610.05407Decrease at acidic pH
Table 6. Values of thermodynamic parameters for pumice stone (PS).
Table 6. Values of thermodynamic parameters for pumice stone (PS).
pHLangmuirFreundlich
K
(L/mg)
amax (mg/kg)R2RMSEχ2K1/nR2RMSEχ2
30.1341006.690.91740.1690.131141.9390.5970.97550.0710.032
90.042924.140.92040.1190.06067.820.5940.95400.0870.034
Table 7. Values of the thermodynamic parameters for ACC.
Table 7. Values of the thermodynamic parameters for ACC.
pHLangmuirFreundlich
K
(L/mg)
amax (mg/kg)R2RMSEχ2K1/nR2RMSEχ2
30.087859.200.82710.2250.22488.660.61880.93040.1270.088
90.058630.500.82090.1760.13668.3690.51530.90470.1220.072
Table 8. Parameters of the interpolation line A and B for the Freundlich model.
Table 8. Parameters of the interpolation line A and B for the Freundlich model.
pHPSACC
A (Slope)B (Ordinate)A (Slope)B (Ordinate)
30.5974.95540.61884.4848
90.5944.21690.51534.2249
Table 9. Kinetic parameters for PS.
Table 9. Kinetic parameters for PS.
pHC0
(mg/L)
Pseudo First OrderPseudo Second Order
k′
(min−1)
R2k″
(kg/(min·mg)
v0
(mg/(g·min))
aeq
(mg/kg)
R2
3100.02710.98396.639 · 10−45.864493.98150.9778
400.02840.97301.855 · 10−426.4275377.40780.9869
1000.0450.93442.536 · 10−4139.0584740.44790.9909
9100.02400.95319.796 · 10−42.960354.97130.9949
400.02590.97222.361 · 10−413.9092242.71180.9848
1000.0360.92542.070 · 10−482.5609631.51990.9974
Table 10. Comparison of phosphate adsorption capacities of pumice and AAC materials.
Table 10. Comparison of phosphate adsorption capacities of pumice and AAC materials.
MaterialAdsorption Capacity (mg/g)Reference
Pumice (natural)7.4–12.5[28]
Pumice (natural)1.114[46]
Pumice (Fe-coated)18.9–29.6[34]
Crushed AAC (construction waste)7.3–15.8[8,43]
Modified AAC (calcined)16.4[10]
Pumice (this study)1.006 (pH 3)This work
AAC (this study)0.859 (pH 3)This work
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Orbuleț, O.D.; Modrogan, C.; Bosomoiu, M.; Cișmașu, M.; Cîrjilă, E.R.; Scarlat, A.-A.; Airinei, D.N.; Miu, A.; Grinzeanu, M.; Dăncilă, A.M. Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice. Environments 2025, 12, 288. https://doi.org/10.3390/environments12080288

AMA Style

Orbuleț OD, Modrogan C, Bosomoiu M, Cișmașu M, Cîrjilă ER, Scarlat A-A, Airinei DN, Miu A, Grinzeanu M, Dăncilă AM. Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice. Environments. 2025; 12(8):288. https://doi.org/10.3390/environments12080288

Chicago/Turabian Style

Orbuleț, Oanamari Daniela, Cristina Modrogan, Magdalena Bosomoiu, Mirela Cișmașu (Enache), Elena Raluca Cîrjilă (Mihalache), Adina-Alexandra Scarlat (Matei), Denisa Nicoleta Airinei, Adriana Miu (Mihail), Mădălina Grinzeanu, and Annette Madelene Dăncilă. 2025. "Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice" Environments 12, no. 8: 288. https://doi.org/10.3390/environments12080288

APA Style

Orbuleț, O. D., Modrogan, C., Bosomoiu, M., Cișmașu, M., Cîrjilă, E. R., Scarlat, A.-A., Airinei, D. N., Miu, A., Grinzeanu, M., & Dăncilă, A. M. (2025). Efficient and Sustainable Removal of Phosphates from Wastewater Using Autoclaved Aerated Concrete and Pumice. Environments, 12(8), 288. https://doi.org/10.3390/environments12080288

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