Valorization of Alum Sludge Waste through Zeolite Synthesis for Sustainable Fertilizer Production

Abstract

Alum sludge (AS) from water treatment plants is an abundant, non-toxic residue with high Si and Al contents. These characteristics make it an ideal precursor for zeolite synthesis, not only reducing the environmental impacts from AS disposal but also adding value to the waste by promoting its reuse. Herein, we propose a route to prepare zeolitic materials from AS, aiming for its application as a component of slow-release fertilizers, using mild conditions and cost-effective hydrothermal synthesis. X-Ray Diffraction revealed the formation of sodalite (Na-SOD) in short times (5-10 h) and moderate temperatures (100-120ºC), with sodium silicate addition favoring Si-quartz consumption and zeolite crystallization, without a fusion step. The Na-SOD showed a desirable cation exchange capacity, reaching 117 mg g^-1 for K⁺. A K-SOD fertilizer was then prepared with an average 490 nm size, 2 m² g^-1 BET surface area, and slow-release behavior. Incubation in soil confirmed that Al was inactive in K-SOD and did not immobilize phosphate, in addition to promoting high K⁺ availability. The results support the potential of AS reuse for application in agriculture as an efficient and sustainable fertilizer.

Keywords:
Alum sludge; Waste; Zeolite; Sodalite; Potassium; Fertilizer

1. Introduction

Globally, water treatment plants generate an estimated amount of 10,000 tons of sludge every day, and its disposal is a significant challenge due to the cost and heterogeneity of this by-product. Alum sludge composition mainly depends on the origin of the water and chemicals applied in the treatment (i.e., flocculants, coagulants, etc.), consisting primarily of silicon (Si) and aluminum (Al) oxides¹,². Although it can be incorporated into cement and brick manufacturing, there is still a limit of adding no more than 15% of the total composition³. Considering the low pathogenicity of this residue and its high availability, finding novel eco-friendly solutions for its reuse in high-added-value products is an urgent task. Agricultural application as a soil conditioner could be a sustainable option for alum sludge reuse thanks to its clay content⁴. Nevertheless, the high amount of labile Al is a concern as it can present phytotoxicity to plants and immobilize phosphate in soil, reducing its availability to crops⁵,⁶.

An innovative strategy to transform the Al in an inert and functional form for agriculture is using alum sludge as a precursor for zeolite synthesis and incorporating this product in fertilizer formulation⁷. Zeolites are crystalline aluminosilicate materials with a microporous structure containing exchangeable cations and a high adsorption capacity, thus presenting a vast range of applications that are ideal for agriculture⁸-¹⁰. Zeolite water holding and cation exchange capacities (CEC) are desirable to improve water and nutrient availability for plants, for instance¹¹. These characteristics are especially interesting for application as slow-release fertilizers, which promote more efficient nutrient delivery to plants and reduce environmental impacts¹²,¹³. Macronutrients can be incorporated as cations in the zeolite microstructure, being slowly exchanged and released to plants.

In this study, we propose the reuse of alum sludge as Si and Al source in the synthesis of a zeolitic material for application as a slow-release potassium (K) fertilizer through a straightforward method, with no need for previous residue preparation to increase reactivity. Mild conditions were studied in a conventional hydrothermal synthesis. Different compositions of alum sludge, synthesis time and temperature, and the addition of sodium silicate were evaluated to improve the crystallization process. The results presented in this work suggest that it is possible to synthesize zeolite material in moderate time and temperature conditions, resulting in a material capable of improving nutrient delivery efficiency to plants.

2. Materials and Methods

2.1. Alum sludge sampling

Alum Sludge (AS) was collected from a water treatment plant (WTP) located in Hortolândia (São Paulo State, Brazil, latitude 22°50'52.52"S and longitude 47°11'59.07"W), in which aluminum polychloride is used as a coagulant. Three samplings were analyzed: in August (AS 1) and December (AS 2), 2022 and March (AS 1), 2023. The AS was dried at 40 ºC in an oven for four days and ground in a grinding mill (Pulverisette 14, Fritsch, Germany) to reduce particle size before being used in synthesis.

2.2. Synthesis of zeolitic concentrates

Zeolitic concentrates were prepared with a conventional hydrothermal method using AS 2 as a precursor for both Si and Al. Synthesis with sodium silicate (Na₂O₃Si; Exodo, Brazil) as an additional Si source was compared with the use of only the alum sludge, using Si/Al ratios of 1.7 and 1.1.

First, an alkaline solution (2.2 mol L^-1) was prepared by dissolving NaOH (Synth, Brazil) in deionized water, followed by Na₂O₃Si and/or AS addition. The mixture was kept under constant agitation of 500 rpm for 30 min at room temperature, resulting in a homogeneous reaction mixture. This reaction mixture was then transferred to a Teflon bottle and placed in a sealed steel container for conventional hydrothermal treatment at different times and temperatures. This crystallization step was performed under static conditions. At the end of the synthesis, the resulting material was washed with deionized water until pH 9 and dried in an oven overnight at 100 ºC. Table 1 summarizes all the studied experimental conditions.

Table 1
Experimental conditions for the synthesis of zeolite concentrates using alum sludge as Si and Al source.

Experiment	Si/Al molar ratio	Additional Si source	Temperature (ºC)	Time (h)
1	1.1	-	100	5
2			100	8
3			100	10
4			120	8
5			120	10
6	1.7	sodium silicate	100	5
7			100	8
8			100	10
9			120	8
10			120	10
11			100	16
12			100	24
13			120	16
14			120	24

• All experiments were carried out with 2.2 mol L^-1 NaOH.

2.3. Materials characterization

The chemical composition of the alum sludge and zeolitic materials was determined by X-Ray Fluorescence (XRF) (Malvern Panalytical miniPaI4, United Kingdom) and CHN Elemental Analysis (Perkin Elmer 2400 analyzer, USA). Results for zeolitic materials can be seen in Table S1 in Supplementary material. A diffractometer (Shimadzu 6000, Japan) with Cu Kα radiation (λ=1.54178 Å), in a range 4 to 60º and scan speed of 2º/min was used for obtaining of X-Ray Diffraction (XRD) patterns. The morphology of the material was analyzed by Scanning Electron Microscopy (SEM), using a microscope equipped with a secondary electron detector (JEOL JSM6510, Japan). For the analysis, the samples were coated with gold in an ionization chamber (BalTec Med. 020, Switzerland). The size distribution of the zeolitic materials was analyzed with Zetasizer Advance equipment (Malvern Panalytical, United Kingdom). The external surface area of zeolitic materials was determined according to Brunauer–Emmet–Teller (BET) 5-point method from the N₂ adsorption-desorption isotherms using an ASAP (Micromeritics Corporation 2020, United States America).

2.4. Cation exchange capacity (CEC)

The CEC of the obtained sodium-based zeolitic material (Na-SOD) was evaluated with a concentrated potassium solution. Briefly, 0.4 g of Na-SOD was added in beakers containing 100 mL of 1 mol L^-1 KCl (Exodo, Brazil) solution. The beakers were agitated at 300 rpm for 120 minutes at room temperature. Aliquot parts were taken in 0, 5, 10, 20, 30, 40, 60, 80, 100 and 120 minutes. This experiment was performed in triplicate. The concentration of K and Na in the aliquots parts was determined by induced coupled plasma optical emission spectrometry, ICP OES (Agilent Technologies 5110, Australia).

The CEC of Na-SOD was calculated according to the Equation 1:

$CEC=\frac{\left(C_0-C_f\right)V_{sol}}{m}$ (1)

Where C₀ and C_f are the initial and final K concentrations (mg L^-1), V_sol is the solution volume (L), and m is the zeolite material mass (g).

2.5. Nutrient release in solution

A K-SOD fertilizer material was prepared by cation exchange of Na-SOD with KCl. The material was weighed (2.3 g) and added to 200 mL of KCl solution (74.6 g L^-1, i.e., 1 mol L^-1) and kept under constant stirring for 2 hours. The sample was centrifuged and added again to 200 mL of the KCl solution. This process was repeated two more times, with the last one being stirred over 24 hours. The resulting material was dried in an oven (100 ºC) and macerated.

The release pattern of potassium from K-SOD was then evaluated by adding the material (200 mg K L^-1) to beaker flasks containing 300 mL of miliQ water, with aliquots being taken at different times over seven days. A positive control of a highly soluble K fertilizer (KCl) was also studied with the same concentration. The materials were tested in triplicates, and the flasks were kept in an incubator under constant agitation (100 rpm) at 30 ºC. Potassium was quantified by ICP OES.

2.6. Soil incubation with phosphorus

The prepared materials were incubated in soil with triple superphosphate (TSP), a highly soluble phosphate fertilizer, to verify if aluminum was structurally immobilized in the zeolitic material and confirm that it would not fixate available phosphate from the soil. The potassium-based sodalite, obtained after cation exchange with KCl, was tested to study its potential as a K fertilizer simultaneously.

For the incubation experiment, an Oxisol soil was collected from the top layer (0-20 cm) of an agricultural region from São Carlos, Brazil. Prior to use, the soil was dried in an oven at 40ºC for 24 hours and sieved (< 2.0 mm). The acidity was then corrected with limestone powder (3:1 wt%) application¹⁴. Soil characterization can be seen in Table S2 in Supplementary information.

Polyethylene screw-cap bottles with perforated lids were filled with 50 g of soil, followed by the incorporation of phosphorus fertilizers and zeolitic material (K-SOD). The following treatments were studied: Control (bare soil), TSP, and TSP+K-SOD. The phosphate source was added with a dose of 200 mg of P per kg of soil, and K-SOD was added to achieve approximately 198 mg K kg^-1.

The experiment was carried out in an incubator with a controlled humidity and temperature of 25 °C. Soil humidity was checked periodically and maintained with water addition, according to the estimated water weight loss. After each incubation time (7, 14, 21, and 28 days), the soil samples were immediately dried in an oven at 40ºC. Available phosphate was extracted in water with an anionic resin, according to van Raij et al.¹⁵. The concentration was then determined with colorimetry in a UV-spectrophotometer (FEMTO 600 Plus)¹⁶. Potassium was extracted with an ion exchange resin and quantified by flame atomic absorption spectrometry (Perkin Elmer 2380, USA). Exchangeable aluminum in soil was extracted with KCl and estimated by titration with NaOH. The values obtained in the soil control were discounted from the results of the other treatments. The final results were subjected to one-way statistical analysis (ANOVA) using Tukey's test with a level of significance of p < 0.05.

3. Results and Discussion

3.1. Alum sludge composition and zeolitic material synthesis

Water treatment plants commonly use a conventional process to assure the quality and potability of drinking water, composed of coagulation, flocculation, sedimentation, filtration, and disinfection steps. The AS generated is an inherently heterogeneous residue, in which the chemical composition depends on the source water quality and different factors related to the water treatment process. Season changes can significantly affect the amount of coagulant applied, for instance. Moreover, the quality and type of chemical products vary among facilities, such as the use of polymer flocculants and the addition of active carbon for taste and smell control⁵,¹⁷.

Elemental composition of the AS samples in the form of oxides is presented in Table 2. The residue is mainly composed of SiO₂, Al₂O₃, and Fe₂O₃, similar to the alum sludge collected in other countries¹⁸. According to Espejel-Ayala et al.¹⁹ and Nguyen et al.², SiO₂, Al₂O₃, and Fe₂O₃ contents are mainly related to aluminosilicate sediments carried from rivers. It is important to highlight that this type of residue has low levels of pathogens and toxic metal impurities, thus being safe for agricultural applications.

Table 2
Chemical composition of AS samples determined by XRF and CHN.

Oxides	Concentration - % (m/m)
	AS 1	AS 2	AS 3
SiO2	20.43	27.10	31.16
Al2O3	23.13	23.89	16.81
Fe2O3	8.51	9.26	8.02
TiO2	0.59	0.73	0.94
SO3	1.85	0.67	0
P2O5	1.07	0.57	0.26
K2O	0.48	0.53	1.06
CaO	0.31	0.35	0.39
MgO	0.23	0.15	0.42
MnO	0.31	0.10	0.09
C	13.7	5.30	3.03
N	1.39	0.48	0.26

Table 2 results confirm the presence of a considerable amount of aluminum from the use of coagulants (aluminum polychloride) in the water treatment process⁵. Thus, the studied AS shows, in general, a suitable composition for reuse as a Si and Al source in zeolite synthesis, with great potential as a material to enhance agricultural efficiency²,¹¹,¹⁸. Comparing the AS collected in different periods (Table 2), it is possible to notice a considerable variation in Si and Al contents. AS 1 shows a higher aluminum concentration than silicon, which is not proper for zeolite synthesis. In contrast, AS 2 and AS 3 samples feature Si/Al ratios above 1 (1.1 and 1.7, respectively), ideal for the synthesis of low silica zeolites like linde type A (LTA or zeolite A), cancrinite (CAN), faujasite (FAU), philipsite (PHI), and sodalite (SOD)²⁰.

Figure 1 shows the X-ray pattern of the AS samples. Characteristic peaks of kaolinite, quartz, and aluminum compounds (i.e., Al(OH)₃ and Al₂O₃.H₂O) can be identified in all samples, consistent with the literature¹⁹,²¹,²². Kaolinite and quartz are common minerals from clay and sand found in raw water, and the identified Al phases are compatible with these sediments and with reagents added in water treatment. AS 3 presents more quartz compared to the other two AS samples, which may reduce silicon availability for zeolite formation. Thus, considering the low silicon content of AS 1, AS 2 was selected as a model alum sludge for the zeolite synthesis.

Figure 1
XRD patterns of AS samples with different silicon and aluminum concentrations. Where: K=kaolinite and Q=quartz.

The optimal condition to synthesize the zeolitic concentrate was then evaluated, using mild temperatures (100 ºC and 120 ºC) and short times (5, 8, and 10 hours). Conventional hydrothermal synthesis was carried out in two ways: using only AS 2 as a silicon and aluminum source and with sodium silicate as an additional silicon source (Table 1).

Since the Si/Al ratio (i.e., 1.1) of AS 2 is already appropriate for the synthesis of low silica zeolites, its direct use in the synthesis has the advantage of a lower cost. Figure 2a features the XRD patterns of products obtained with only AS. Although Al₂O₂.H₂O peaks disappeared after treatments at 100 ºC, quartz, kaolinite, and Al(OH)₃ phases remained unreacted at all times tested. In contrast, characteristic peaks for sodalite zeolite were identified in the products from hydrothermal synthesis at 120 ºC. Still, intense peaks related to kaolinite and quartz were observed, revealing that the Si phase is highly stable and unreactive under the tested conventional hydrothermal conditions. The nucleation and crystallization stages of zeolite formation depend on the availability of amorphous Si in solution, and, therefore, Si mineral composition in AS was limiting for the efficiency of the zeolite synthesis¹⁹,²²-²⁴.

Figure 2
XRD patterns of AS compared with the products from conventional hydrothermal synthesis (a) with only the AS and (b) with sodium silicate addition. Where: K=kaolinite, Q=quartz, S=sodalite.

An improved zeolite crystallization was observed when sodium silicate was added to the reaction mixture at a Si/Al ratio of 1.7 (Figure 2b) for all the studied conditions. Sodalite peaks were predominantly identified, appearing more intense and well-defined than for synthesis with only AS. Moreover, quartz peaks were significantly reduced, and the kaolinite phase completely disappeared, suggesting a higher degree of Si consumption from AS to form the zeolite material. The highest signal intensity of sodalite peaks and relatively lower proportion of unreacted quartz correspond to the synthesis product at 120ºC for 8 hours.

The addition of an amorphous and soluble Si source proved to be highly favorable to the crystallization process. Sodium silicate was readily available to react with the soluble aluminum sources from AS, lowering the energy barrier needed to initiate the nucleation and crystal growth⁷,²²,²⁵. In other words, sodium silicate addition allowed a faster nuclei formation, favoring the crystallization kinetics in a similar way to the use of zeolite crystals as seeds²²,²⁵. These results suggest that, with sodium silicate addition, both AS 1 and AS 3 could also be successfully used in zeolite synthesis, as it can respectively correct the Si content and compensate for the high Si-quartz concentration.

Changes in the temperature and time of synthesis did not seem to influence the zeolite type or crystallinity. Higher hydrothermal synthesis times were tested to investigate if other zeolite phases could be produced from the AS, namely at 16 and 24 hours, under the same 100 ºC and 120 ºC. Figure S1 shows that XRD patterns are comparable in all the studied conditions, with sodalite zeolite being successfully obtained.

Sodalite framework is formed by cubic β-cages with tiny pore sizes (around 2.8 Å)²⁶. Because of its microporosity and high cation exchange capacity, sodalite has been studied as a membrane for the separation of small molecules, as an adsorbent for contaminants elements, and as a catalyst²⁶,²⁷. Thus, the obtained sodalite-based material has great potential to act as a slow-release K fertilizer. Although the zeolitic concentrate still has some quartz and Al₂O₃, these impurities are not critical for agricultural applications since they do not affect nutrient fixation or promote toxicity. Indeed, they are soil components widely seen in many different soil types.

3.2 Morphology, particle size, external surface area, and CEC of the materials

Based on the results discussed, the sodalite-based zeolitic materials from conventional hydrothermal synthesis with sodium silicate were further characterized to be tested as a potential fertilizer material.

Morphology was investigated with SEM, as shown in Figure 3. The alum sludge (Figure 3a) consists of a heterogeneous mixture of particles with irregular shapes and various sizes, with aggregates ranging from 10 µm to up to 100 µm. Overall, the agglomerates have an uneven and rough surface, consistent with different phases adhered to each other, like the quartz, kaolinite, and Al(OH)₃ identified in XRD. In contrast, the material obtained from the hydrothermal synthesis, identified as sodalite (Na-SOD), shows smaller and uniform particles (Figure 3b), both isolated and in aggregates. The particles exhibit a round thread-ball-like shape, with sizes from 0.87 µm to 0.56 µm. Although sodalite morphology may vary depending on the synthesis conditions (i.e., temperature, alkali type, concentration, impurities, etc.), this type of structure has been previously identified, further confirming the XRD results²⁶. Other phases can be seen in lower amounts in the agglomerates (<30 µm) with sodalite crystals, probably from residual quartz and the Al₂O₃ formed.

Figure 3
SEM images of (a) the alum sludge and (b) the zeolitic material (Na-SOD).

The zeolitic materials prepared with sodium silicate were then compared regarding their particle size distribution (Figure 4a), aiming to verify if the different synthesis conditions had any influence on the zeolite dimension. Overall, the average particle size increases with reaction temperature and time. The synthesis at 100 ºC and 5h produced particles with diameters ranging from 489 to 661 nm, for instance. In harsher conditions, like the 120 ºC/ 10h product, particle size was found between 489 and 769 nm. The dimensions are consistent with the pure sodalite synthesized by Li et al.²⁶, for instance, which showed around 0.5 μm. Two regions of similar distribution were observed in the 120 ºC/8h material (at around 450 nm and 770 nm), suggesting the presence of both smaller and larger particles. The group with smaller particles may correspond to the sodalite phase, while the larger particles could be from agglomerates containing residual quartz and aluminum oxide. This is consistent with the diameters observed in the SEM images. It is worth noting that the product from this synthesis achieved the highest sodalite peak intensity in XRD, further indicating that the higher percentage of smaller particles is attributed to the zeolite formation.

Figure 4
(a) Particle size distribution of zeolite concentrates synthesized with sodium silicate; (b) K concentration over time in cation exchange capacity (CEC) experiment with Na-SOD.

The CEC of the synthesized zeolitic material (Na-SOD) was evaluated for potassium, an essential macronutrient for plant development that could be carried by the zeolite and delivered as a fertilizer. Figure 4b displays K⁺ adsorption pattern over time. It is possible to verify the continuous reduction of K⁺ concentration in the solution, indicating Na-SOD was efficiently exchanging its Na⁺ content with K⁺. Figure S2 further confirms this process, with Na⁺ concentration increasing over time. An equilibrium was reached at 120 minutes, and, according to Equation 1, the CEC of Na-SOD was estimated as 117 mg g^-1, a similar value reached by pure sodalite octahydrate in Baccouche et al.²⁸ and Makgabutlane et al.²⁹ (i.e., 111 and 114 mg g^-1, respectively). Since the material consists of a mixture of phases instead of pure sodalite, K⁺ adsorption was probably limited by the amount of exchangeable Na⁺ present only in the zeolitic structure.

In order to add agronomic value to the zeolitic material, and based on the CEC test results, K-SOD was prepared via Na-SOD cation exchange with KCl. The obtained material was then characterized to verify possible chemical and structural changes and differences. Figure 5a shows the XRD pattern for K-SOD obtained after cation exchange. It is possible to observe peaks at 2θ 14.1° and 24.6º, corresponding to the sodalite form of K₈(AlSiO₄)₆Cl₂, in addition to signals from quartz and Al₂O₃. It reveals that not only Na⁺ was exchanged from the zeolite structure for K⁺, but also that Cl^- replaced H₂O sorbate in the network. K-SOD morphology is very similar to the sodium-based SOD, as seen in SEM (Figure 5b). The crystal agglomerates observed are generally smaller, but the sodalite particle size appears larger on average, with around 0.8-0.9 µm.

Figure 5
(a) XRD pattern of the zeolitic material before (Na-SOD) and after K⁺ cation exchange (K-SOD); (b) SEM image of K-SOD.

Particle size distribution (Figure S3) shows that K-SOD diameter is mostly around 490 nm, which is, on average, smaller than the original sodalite. Despite this, external surface area from BET equation for Na-SOD was significantly higher than K-SOD (14 and 2 m² g^-1, respectively; estimated values for total surface areas and pore volumes in Na-SOD and K-SOD were 20 and 6 m² g^-1 and 0.003 and 0.002 cm³ g^-1, respectively). Given the range of external surface external, one can conclude that the variation is due to the higher agglomeration promoted by cation exchange, rather than some effect in mesoporosity. In Figure S4 the N₂ adsorption-desorption isotherms can be classified according to IUPAC as Type IV, with hysteresis loop H3, typical for mesoporous materials such as zeolites²¹,³⁰-³². Ji et al.³⁰, Yoldi et al.³² and Sousa et al.²¹ found comparable surface areas of 2, 3, and 8.5 m² g^-1 for Na-SOD in the hydrothermal synthesis process. These differences are related to synthesis conditions (temperature, time, and starting material used as silicon and aluminum source) that can affect the purity of zeolite and its superficial area.

3.3. Nutrient release in aqueous solution and soil

Potassium release in water from K-SOD was studied to evaluate its potential as a controlled-release fertilizer (Figure 6a). Over the first 2 hours, nearly 85% of K from KCl was released, showcasing its high solubility. In contrast, K-SOD delivered around 32% of K in the same period. Thereafter, the zeolitic material maintained a slow-release pattern, reaching almost 40% at the end of 7 days. It is important to highlight that K-SOD release behavior is highly influenced by the availability of exchangeable cation on the medium, while KCl delivery is mainly governed by the salt dissolution. It is possible that, without a counter ion to favor K⁺ release, K-SOD entered an equilibrium. K-SOD performance is probably improved under soil-plant dynamics.

Figure 6
(a) Potassium release patterns over time of K-SOD and KCl; (b) Available phosphate in soil over the incubation period.

A soil incubation experiment was conducted to verify if the aluminum present in the zeolitic material would interact and fixate available phosphate in the soil. The results showed that not only no exchangeable Al was detected in soil (Table S3) but also that phosphate released from TSP with and without the presence of K-SOD was statistically identical (Figure 6b), confirming that Al was immobilized in the sodalite structure and was inactive towards phosphate. After 21 days of incubation, available P decreased due to soil fixation but remained similar among the treatments. Over the experiment duration, potassium concentration remained around 160 mg/dm³ in the presence of K-SOD (Table S3). Nevertheless, K⁺ is possibly slightly overestimated, with cations still retained in zeolite in soil being extracted by the ionic resin.

4. Conclusion

Reusing alum sludge as a silicon and aluminum source for zeolite material synthesis is a great and sustainable alternative compared to landfill site disposal. Sodalite zeolite was successfully obtained in different conditions of conventional hydrothermal synthesis with the AS residue. When only AS was used as a Si and Al precursor, a lower crystallization degree was observed, with considerable amounts of unreacted minerals (quartz, kaolinite, and Al(OH)₃). The addition of sodium silicate as a soluble supplementary Si source significantly improved the consumption of AS mineral phases and favored the crystallization process. Moreover, the sodalite-based zeolitic material showed excellent CEC, which should be attractive for agricultural applications.

K-SOD potential as a slow-release fertilizer was evaluated, compared to a highly soluble K source. While KCl delivered around 85% of K⁺ in the first 2 hours, only 32% of K⁺ was released from K-SOD, showing a slow-release behavior over time. Most importantly, results obtained from the soil incubation experiment indicated the immobilization of Al in the zeolite material structure, as it did not interact with available phosphate from TSP, confirming its suitability for application in agriculture. Overall, the results demonstrated the reuse potential of alum sludge residues in zeolite manufacturing for application in agriculture, especially as a slow-release fertilizer.

Supplementary material

The following online material is available for this article:

Table S1 XRF chemical composition of Na-SOD and K-SOD.

Table S2 Characterization of the Oxisol soil studied.

Table S3 K and Al concentration in soil over time.

Figure S1 XRD patterns of the zeolitic materials obtained from hydrothermal synthesis with alum sludge and sodium silicate under longer times (16-24 hours). Where: K = kaolinite; Q = quartz; S = sodalite.

Figure S2 Na⁺ release curve from Na-SOD in the cation exchange capacity (CEC) test.

Figure S3 K-SOD particle size distribution.

Figure S4 N₂ adsorption-desorption isotherms for Na-SOD and K-SOD.

5. Acknowledgements

The authors thank FAPESP (São Paulo State Research Foundation, project number 2020/12210-3, and 2023/01549-8 and grant 2022/09773-1) and FINEP (project number 01.22.0274.00, 01.22.0080.00 Ref. 1219/21 and grant 14716(6545)) for the financial support. The authors are thankful for the institutional support by Basic Sanitation Company of the State of São Paulo (SABESP) and support facilities provided by the Agronano Network (Embrapa Research Network), the Agroenergy Laboratory, the National Nanotechnology Laboratory for Agribusiness (LNNA).

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Publication Dates

• Publication in this collection
  11 Oct 2024
• Date of issue
  2024

History

• Received
  17 June 2024
• Reviewed
  25 July 2024
• Accepted
  16 Aug 2024