Introduction

Adsorption techniques are now widely regarded as the most effective method for removing various classes of organic pollutants from wastewater (Khan et al. 2019; Nipa et al. 2023). Methylene blue (MB), a deep blue dye, is a significant pollutant in industrial wastewater (Zuhara et al. 2023; Javed et al. 2024). Although commercial activated carbon is the preferred adsorbent for dye removal, its relatively high cost limits its widespread application (Rehman et al. 2024; Prasad et al. 2024). This has driven research into alternative, non-conventional, and cost-effective adsorbents. Among these, the use of lignocellulosic material for the adsorption of organic dyes has been identified as a particularly economical approach for wastewater purification.

Numerous lignocellulosic-based biomass sources have been utilized to produce efficient adsorbents such as the leaves of Gleditsia triacanthos L. and the needles of Pseudotsuga menziesii (Isinkaralar 2023, 2024). Significantly, extensive research has concentrated on assessing the adsorption efficiency of palm waste-derived lignocellulosic materials, such as biochar produced from palm petioles and palm bark, powdered palm leaves, and magnetic biochar derived from oil palm fronds, for dye removal applications (Sun et al. 2013; Chahinez et al. 2020; Khan et al. 2024; Oyekanmi et al. 2024). While these adsorbents demonstrate high dye removal efficiency, their preparation processes are costly and require substantial energy for pyrolysis.

Palm Peat (PP) is the world's first rich lignocellulosic medium derived from abundant and sustainable date palm agricultural residues using proprietary technology to extract lignin-rich media. It comprises up to 90% organic matter and is free from harmful pesticides and heavy metals, rendering it both environmentally friendly and biodegradable. Its porous structure and high surface area enhance pollutant adsorption, while its natural lignocellulosic composition provides functional groups (hydroxyl, carboxyl, and phenolic) that facilitate strong interactions with dyes, heavy metals, and organic contaminants (VALORIZEN R&I Center 2021a). The production of palm peat is economically feasible due to its widespread availability as an agricultural by-product, significantly reducing the cost of raw materials. In contrast to activated carbon, palm peat requires minimal processing, reducing production costs and energy consumption. Furthermore, its local sourcing in palm-producing regions minimizes transportation costs. Its biodegradable nature also facilitates low-cost disposal or repurposing as compost, aligning with the principles of a circular economy and sustainability. Moreover, the local production of PP from date palm waste minimizes dependence on imports, thereby reducing the carbon footprint associated with transportation (VALORIZEN R&I Center 2021b). These combined advantages position palm peat as a cost-efficient and sustainable alternative to conventional adsorbents. The selection of PP as an adsorbent is supported by its environmental benefits, cost-effectiveness, and alignment with global “waste-to-wealth” and “zero-waste discharge” initiatives.

This study innovates the use of a new lignocellulosic material as a novel adsorbent for the removal of MB dye, marking its first application in this context. The research explores the impact of various parameters such as adsorption time, temperature (T), stirring speed, adsorbent dose, solution pH, and initial dye concentration on the effectiveness of PP. Using Response Surface Methodology (RSM), the study optimizes these conditions to enhance performance. The novelty of PP is further highlighted by comparing its adsorption efficiency with that of previously reported adsorbents, demonstrating its unique advantages. The study also assesses the adsorbent’s recyclability across five consecutive cycles to gauge its viability for large-scale applications. Detailed analyses of adsorption kinetics, isotherms, and thermodynamics provide insight into the underlying mechanisms. Additionally, the performance of PP is tested across various water matrices to evaluate its versatility. The practical application is further validated through its use in treating real textile wastewater, showcasing PP’s potential for effective and sustainable wastewater management.

Materials and methods

Materials

All chemicals were used in their original form without any pretreatment or purification procedures. MB (C16H18ClN3S, ACS reagent grade, 90%) was obtained from Sigma Aldrich. Sodium hydroxide (NaOH) was acquired from Fisher Chemicals. Hydrochloric acid (HCl) with a concentration of 37%, and sodium chloride (NaCl) ACS reagent grade, 99% were obtained from Merck. PalmPeat® (PP) was supplied by the VALORIZEN Research and Innovation Center in Cairo, Egypt. It had a particle size range of 1–8 mm, an organic content of 91.6%, and a water-holding capacity of 600–700 mL/100 g. It was used as received without any further processing.

Raw wastewater

A real textile wastewater sample was obtained from a manufacturer in New Borg El-Arab City, Alexandria, Egypt. Additionally, five different water matrices, including deionized water (DW), tap water (TW), lake water (LW), seawater (SW), and drain water (DRW), were tested to evaluate the practical application of PP for MB decolorization through adsorption. DW, with an electrical conductivity below 0.055 μS/cm, was prepared using a laboratory purification system (WG251, Yamato, Japan). TW and DRW samples were collected from New Borg El-Arab City (Alexandria, Egypt), while LW and SW were sourced from Lake Mariout and the Mediterranean coast in Alexandria (Egypt), respectively. All water samples were filtered through 0.22-μm membrane filters to remove suspended solids, stored in a freezer, and equilibrated to ambient T before use. The pH of the wastewater samples was adjusted using 0.1 mol/L NaOH or H2SO4, and all experiments were conducted within 7 days of sample collection. The characteristics of the raw wastewater samples are outlined in Table S1.

Palm Peat characterization

Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy (JSM–IT200, JEOL, Japan) was used to elucidate the structure, morphology, and chemical composition of the PP. XRD analysis (XRD 6000, Shimadzu, Japan) was conducted to investigate the chemical structure and crystallinity of the adsorbent. Fourier transform infrared (FTIR) spectroscopy (FTIR–8400S, Shimadzu, Japan) was utilized to identify the chemical functional groups present on the surface of the adsorbent. Furthermore, the specific surface area and pore size distribution of the PP were measured using the automated analyzer (BELSORP–max, BEL, Japan), which utilized the Brunauer, Emmett, and Teller (BET) method as well as the Barrett–Joyner–Halenda (BJH) technique.

Point of zero charge

The point of zero charge (pHpzc) of the PP adsorbent was measured using the powder addition (pH drift) (Oseghe et al. 2015). The method involves mixing 0.15 g of the adsorbent with 50 mL of sodium chloride solution (0.1 M) and adjusting the solution pH to be within 2 and 12 using NaOH and HCl solutions of 0.1 M. The mixture was then stirred at 25 °C for 24 h using a magnetic stirrer hotplate. After this period, the dispersion was filtered, and the final pH values were measured with a digital pH meter. The pHpzc was estimated by plotting the relationship between the initial and final pH values (Samy et al. 2020; Mensah et al. 2022).

Batch adsorption procedures

Initially, batch experiments were used to evaluate the PP adsorption capacity employing 100 mL of the MB solution in a glass beaker (250 mL) placed on a magnetic stirrer hot plate. The experiment was conducted at conditions (initial dye concentration = 30 mg/L, PP dose = 1 g/L, T = 30 °C, pH = 7, stirring speed = 900 revolutions per minute (rpm), and contact time = 120 min). The concentration of MB dye was measured at a wavelength of 660 nm using a UV–visible spectrophotometer (Amoh et al. 2024; Gaber et al. 2024c). From this experiment, the equilibrium reaction time was determined to be used in the succeeding experiments. To ensure accuracy and reliability, each experiment was performed in triplicate, and the results were reported as average values, with error bars in the figures representing the associated standard deviations. Under the same conditions at the predetermined reaction time, the influence of T (values of 30, 50, 70, and 90 °C) and stirring speed (values of 600, 750, 900, 1050, and 1200 rpm) were also investigated. The optimum values for the pH of the solution, initial PP dose, and initial MB concentration were determined through a design of experiment (DOE) approach using RSM coupled with a central composite design (CCD) conducted using Minitab©21 software. The RSM model was validated by measuring the MB removal efficiency under optimal conditions and comparing it with the expected ratio calculated from the model's polynomial equation using the optimum values of the independent parameters. The optimum values of all operating parameters which achieved the highest MB removal efficiency (RE%), were subsequently employed in the following experiments. Further, the mechanism of MB dye adsorption was investigated through the study of isotherms, kinetics, and thermodynamics, as detailed in Texts (S1-S3). Additionally, the reusability of PP over five consecutive cycles was examined under optimal conditions. The dye removal percentages were measured and evaluated throughout the five successive runs. Moreover, the weight loss percentage of the dried PP particles collected after each run was investigated to reconfirm the adsorbent stability. The XRD and FTIR findings of the dried adsorbent particles obtained after the final run were compared with those of the original PP particles to reaffirm the PP recyclability. Furthermore, the impact of organic and inorganic matter on PP adsorption performance was examined under optimal conditions. This involved evaluating dye and total organic carbon (TOC) removal across various real water matrices. Finally, the decontamination of real industrial textile wastewater, focusing on the removal of MB (the target pollutant) and TOC, was explored under optimal conditions to evaluate the potential utility of the novel PP adsorbent for integration into practical wastewater treatment plants.

Experimental design

A DOE consisting of 20 trials was conducted using RSM coupled with CCD. This approach investigated the effects of independent parameters (initial PP dose, starting MB concentration, and pH of the solution) on the dependent parameter (dye removal percentage) and to optimize the independent parameters to maximize dye removal. The ranges for the independent parameters were 0.5 to 1.5 g/L for the PP dose, 10 to 50 mg/L for the initial dye concentration, and 3 to 11 for the pH value. The CCD levels are depicted in Table 1. DOE provides significant advantages over one-factor-at-a-time (OFAT) methods. It allows for the simultaneous study and optimization of multiple factors and their interactions, resulting in improved performance outcomes. Additionally, DOE requires fewer experiments than OFAT and enhances predictive accuracy by accounting for interactions that OFAT might overlook (Lamidi et al. 2023; Gaber et al. 2024a). The quadratic equation linked the dye RE% and the independent factors is expressed by (Eq. 1) (El-Bestawy et al. 2023; Gaber et al. 2024d).

$${\text{RE}}\% = \beta_{0} + \mathop \sum \limits_{i = 1}^{n} \beta_{1} X_{i} + \mathop \sum \limits_{i = 1}^{n} \beta_{2} X_{i}^{2} + \mathop \sum \limits_{1 < i < j}^{n} \beta_{ij} X_{i} X_{j } + \varepsilon$$
(1)

where the coefficients β₀, β₁, β₂, and βij correspond to the model's intercept, linear, quadratic, and interaction terms, respectively. Xi and Xj represent the independent factors, n denotes the number of dependent parameters, and ε signifies the residual term. Analysis of Variance (ANOVA) was conducted to assess the model's fitness and significance (Gaber et al. 2024b, c).

Table 1 Values and ranges of operational conditions
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Analytical methods

Samples (3 mL) were collected every 5 min using a syringe, then centrifuged at 4000 rpm for 15 min followed by filtration through a 0.45-μm nylon syringe filter to remove the spent adsorbent before analysis. The T and stirring speed were modified utilizing a magnetic stirrer hot plate. Sodium hydroxide (0.1 M) and hydrochloric acid (0.1 M) were used to adjust the initial pH of the solution, followed by pH measurement with a digital pH meter. All analytical methods and equipment used for water analyses are summarized in Table S2. The removal efficiency was computed using Eq. (2) (Abdel Azim et al. 2024). TOC was measured using a TOC analyzer. The adsorption capacity of PP was determined using Eq. (3) (Yang et al. 2022). In the recyclability experiment, the PP particles were retrieved after each cycle through filtration and centrifugation at 3500 rpm for 20 min. Desorption of MB was performed by washing the collected PP particles with a 1 M hydrochloric acid solution, followed by rinsing with DW (Daneshvar et al. 2017). The eluted PP was subsequently dried at 100 °C for 3.5 h and reused in the subsequent run. The weight loss percentage of the dried PP particles collected after each consecutive run in the recyclability experiment was calculated using Eq. (4).

$${\text{RE}}\% = \frac{{C_{0} - C_{t} }}{{C_{0} }} \times 100$$
(2)
$$q_{t} = \frac{{V\left( {C_{0} - C_{t} } \right)}}{W}$$
(3)
$${\text{Weight}} {\text{loss}}\% = \frac{{m_{0} - m}}{{m_{0} }} \times 100$$
(4)

where (t) is the contact time (min), (Ct) indicates the dye concentration at time t, (Ao) represents the initial dye absorbance (a.u.), (A) is the dye absorbance after treatment (a.u.), (Co) represents the initial dye concentration (mg/L), (V) is the initial solution volume (L), (qt) is the adsorption capacity at time t (mg/g), (W) is the mass of the PP used (g), (mo) is the initial PP dose (g/L) at zero time, and m is the PP dose (g/L) at the end of each successive run.

Results and discussion

Palm Peat characterization

The SEM micrographs of the PP (Figs. 1a, b) reveal its distinctive spongy cellular structure with 53.06 µm as the average particle size and confirm the presence of a porous structure within the adsorbent with an average pore size of 10.96 µm. The adsorbent porosity is crucial in enabling the deeper diffusion of adsorbate molecules into the active sites within the adsorbent material. This ensures a more efficient adsorption process by maximizing the contact between adsorbate molecules and the adsorbent surface (Wen et al. 2023; Eshghabadi and Javanbakht 2024). Additionally, the adsorbent porous morphology provides an expanded surface area, fostering enhanced interaction between adsorbate and adsorbent, consequently bolstering the adsorption capacity (Chen et al. 2024; Brahmi et al. 2024). The EDX pattern of the PP (Fig. 1c) reveals the mass ratios of its constituents, with carbon and oxygen comprising 49.07 and 45.25%, respectively. Additionally, several trace elements were detected in the EDX spectra of the PP, including chlorine (Cl), calcium (Ca), potassium (K), sulfur (S), magnesium (Mg), aluminum (Al), sodium (Na), and silicon (Si), with mass ratios of 2.42, 1.1, 0.59, 0.51, 0.35, 0.26, 0.22, and 0.22%, respectively.

Fig. 1
figure 1

a, b SEM micrographs, and c EDX pattern of the palm peat before treatment

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The FTIR bands of the PP are depicted in Fig. 2a. The prominent peak noted at 3452.53 cm−1 corresponds to the − OH group within lignin. This finding aligns with previous research by Fan et al. (2022) and Zhang et al. (2020), who reported similar functional groups at 3409 and 3421 cm−1, respectively. The peaks identified at 2922.29 and 2854.17 cm−1 correspond to the asymmetric and symmetric –CH stretching in the methyl and methylene groups of lignin molecules, respectively (Zhang et al. 2020; Perera et al. 2022). The signal at 2353 cm−1 corresponds to the vibration of the –COOH functional group (Shokry et al. 2019; Mensah et al. 2022). The peak observed at 1638.58 cm−1 was attributed to the stretching vibration of the C = C bond, which is likely associated with the benzene ring framework in lignin, primarily derived from aromatic structures (Suhaimi et al. 2022; Taha and Daffalla 2023). Bands observed at 1384.15 and 1257.73 cm−1 are attributed to the C–O stretching of syringyl and guaiacyl units, respectively (Sammons et al. 2013; Fan et al. 2022). Ju et al. (2024) observed the aromatic –CH deformation of syringyl units at 1125 cm−1 which closely aligns with the value of 1121.48 cm−1 observed in the current investigation. Carbonyl group vibrations were detected at 1166.62 cm−1, consistent with observations by Wittner et al. (2023) who reported values of 1156 cm−1. Additionally, bands at 1044.32 and 622.44 cm−1 are attributed to the C–O stretching of primary alcohols and aromatic –CH stretch vibration, respectively. Misson et al. (2012) reported C–O stretching of primary alcohols at 1038.98 cm−1. Furthermore, Ahmad et al. (2016) and Md Salim et al. (2021) identified the aromatic –CH stretch vibration at 690–700 and 542 and 618 cm−1, respectively. Table S3 presents a summary of the FTIR peak positions observed in PP samples, along with their respective assignments as documented in the existing literature.

Fig. 2
figure 2

a FTIR spectrum and b XRD pattern of the Palm Peat before treatment

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The major diffraction peaks (2θ°) in the PP’s XRD (Fig. 2b) are indexed to quartz (SiO2) and calcite (CaCO3) according to ICSD#01 − 076 − 0941 and JCPDS#00 − 001 − 083, respectively. Zubair et al. (2020) identified the diffraction peaks of biochar derived from date palm fronds as SiO2 and CaCO3. Similarly, Taha et al. (2023) attributed the XRD peaks of palm waste raw material to calcium carbonate and silicon dioxide. The XRD patterns for standard cards of quartz and calcite are shown in Fig. S1. Table S4 presents a comparison of the crystalline peaks of the PP with the corresponding crystal planes (hkl) from the reference cards and d-spacing (nm) at each diffraction angle. The average d-spacing of PP before adsorption was recorded as 102.23 nm.

Figure 3a and b shows the N₂ adsorption/desorption isotherms and the pore size distribution of PP, respectively. The texture properties of PP are summarized in Table S5. The findings revealed that the PP exhibited a surface area of 16 m2/g. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, PP exhibits type III isotherms (Fig. 3a) (Hassan et al. 2023). The pore size distributions (Fig. 3b) demonstrate that PP possesses an average pore size of 9.36 nm with a total pore volume of 1.02 cm3/g. These pores fall within the 2–50 nm range, indicating that mesopores constitute most of the total pore volume in the adsorbent, following IUPAC guidelines (Tohdee et al. 2024).

Fig. 3
figure 3

a N2 adsorption/desorption isotherms and b pore size distribution of the palm peat

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Adsorption performance of the Palm Peat

The adsorption efficiency of the PP for MB dye was evaluated under the following conditions: an initial dye concentration of 30 mg/L, a PP dose of 1 g/L, a T of 30 °C, a pH of 7, a stirring speed of 900 rpm, and an adsorption time of 120 min. The removal ratios of MB dye after 90 and 120 min were recorded as 68.26 and 70.42%, respectively, as depicted in Fig. 4a. Additionally, the PP adsorption capacities after 90 and 120 min amounted to 20.48 and 21.12 mg/g, respectively, as depicted in Fig. 4b. The relatively high MB removal ratio and PP adsorption capacity confirm the promising adsorption performance of the PP. At early stages, the MB removal ratios and PP adsorption capacity were high due to the adsorption sites availability. As the reaction progressed, most binding sites became occupied by dye molecules which led to minimal changes in dye removal percentages and adsorption capacity after 90 min, indicating equilibrium had been reached. Therefore, the following experiments were employed within 90 min as the reaction time. Nouioua et al. (2023) observed a similar trend in their study on the adsorption of crystal violet dye over 300 min, noting that adsorption capacity did not significantly change once equilibrium was reached at 120 min.

Fig. 4
figure 4

a MB removal efficiency and b Palm Peat adsorption capacity (conditions: initial MB concentration = 30 mg/L, PP dose = 1 g/L, pH = 7, T = 30 °C, stirring speed = 900 rpm, and reaction time = 120 min)

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Effect of temperature

The impact of T on the removal of MB was investigated at an initial MB concentration of 30 mg/L, PP dosage of 1 g/L, stirring speed of 900 rpm, pH 7, and reaction time of 90 min. As depicted in Fig.  5a, the percentage of MB removal exhibited values of 68.26, 69.85, 70.38, and 70.71%, while the corresponding adsorption capacities were recorded as 20.48, 20.96, 21.12, and 21.21 mg/g at T values of 30, 50, 70, and 90 °C, respectively. The findings indicate that the MB removal efficiency and PP adsorption capacity increase with higher temperatures. This is likely due to a decrease in mass transfer resistance and an increase in the diffusion rate of MB molecules from the solution to both the external surface and internal pores of the PP at higher temperatures (Wolski et al. 2023). Abdel Azim et al. (2024) observed a similar influence of T on MB dye removal ratios and adsorption capacities, employing biochar prepared from mint waste as the adsorbent. Their investigation revealed that as the T was raised to 90 °C, the removal efficiency increased to 98.1% and adsorption capacity reached 4.9 mg/g compared to values of 93.6% and 4.7 mg/g, observed at 30 °C. Due to the relatively minor variance in MB removal ratios observed across the tested T values, the ambient T at 30 °C was selected for subsequent experiments to evaluate adsorption efficiency at room T and to eliminate energy cost.

Fig. 5
figure 5

MB removal ratios (%) and Palm Peat adsorption capacities (mg/g) at a different temperature values and b different stirring speeds

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MB dye removal under different stirring speeds

The impact of stirring speed on MB removal ratios and adsorption capacities was investigated across a range of agitation speeds from 600 to 1200 rpm. The examination was conducted at an initial dye concentration of 30 mg/L, pH of 7, T = 30 °C, an adsorbent dose of 1 g/L, and a reaction time of 90 min. The MB dye removal efficiencies were 50.68, 57.57, 68.26, 63.25, and 60.72%, while the corresponding adsorption capacities were 15.21, 17.27, 20.48, 18.97, and 18.22 mg/g, respectively, at 600, 750, 900, 1050, and 1200 rpm, as depicted in Fig. 5b. The results indicate that both MB dye removal ratios and adsorption capacities exhibited an upward trend from lower stirring speeds (600 and 750 rpm) until reaching their peak values at 900 rpm. Above 900 rpm (1050 and 1200 rpm), the adsorption efficiency slightly declined. The increase in MB dye removal ratios and adsorption capacities between 600 and 900 rpm could be ascribed to enhanced mass transfer rates of MB molecules toward the adsorbent surface which results from improved mixing and contact between dye molecules and the adsorbent surface, thereby achieving more effective fixation of dye molecules onto the PP surface (Maleki et al. 2020; Bingül 2022). However, excessively high stirring speeds might introduce shear forces that could potentially damage the adsorbent contact with the adsorbate or induce the desorption of previously adsorbed dye molecules, thereby diminishing the overall adsorption capacity and dye removal percentage (Yogeshwaran and Priya 2020; Hasan et al. 2024). Hamitouche et al. (2017) observed a comparable impact of stirring speed on MB dye removal ratios when utilizing biomass derived from Anethum graveolens as an absorbent. They noted that the removal efficiency increased to reach the maximum (80.6%) at 300 rpm and then decreased when increasing the stirring speed to 400 rpm because of the generation of a thick outermost layer surrounding the adsorbent which may decrease the diffusion of dye molecules.

The stirring speed of 900 rpm, at which maximum dye removal and adsorption capacity were observed, was selected for subsequent experiments.

Analysis of RSM model data for MB dye adsorption

Detailed experimental conditions, expected dye removal ratios, and observed MB removal percentages across the designated 20 experiments are tabulated in Table 2. The correlation between the MB removal efficiency (surface response) and the parameters X (initial MB concentration, mg/L), Y (pH value of the solution), and Z (initial PP dose, g/L) was delineated in the polynomial.

Table 2 Measured and model-derived MB dye removal ratios (%) under different operating parameters
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model presented in Eq. (5).

$$RE\% = - 7.2 - 0.527X + 6.06Y + 66.6Z - 0.00643X^{2} - 0.129Y^{2 } - 20.29Z^{2} + 0.1125XY - 0.2XZ - 0.5YZ$$
(5)

The significance of the RSM model was substantiated by the low P–values and high F–values derived from the ANOVA analysis (Table 3). Additionally, a high coefficient of determination (R2) value of 0.9761 and a high adjusted R2 value of 0.9546 affirmed the model's significance.

Table 3 Assessment of the significance of the model
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Effect of independent parameters on MB removal

Figure 6 presents contour plots illustrating the interactions among the three independent parameters (initial MB concentration, solution pH, and initial PP dose) and their combined influence on MB dye removal efficiency. The F-values for initial dye concentration, solution pH, and PP dosage were 18.28, 351.41, and 29.45, respectively (Table 3). These results indicate that solution pH has the greatest impact on MB removal efficiency, followed by PP dosage, with initial dye concentration having the least effect.

Fig. 6
figure 6

Contour plots showing the effect of operating conditions on MB dye removal

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The solution pH significantly influences the MB adsorption because of the adsorbent surface charge. With a pHpzc of 3.5 for PP (Fig. S2), it exhibits a predominantly positive net charge at pH values below 3.5 and a predominantly negative net charge at pH values above 3.5 (Kooh et al. 2018; Mosoarca et al. 2023). Since MB is a cationic dye, it interacts favorably with the predominantly negatively charged surface of PP at pH levels above its pHpzc, thereby enhancing MB removal efficiency (Rahmani et al. 2020). Moreover, increasing the solution pH beyond 3.5 promotes the deprotonation of oxygen functional groups on PP, further enhancing its predominantly negative surface charge and facilitating electrostatic interactions with MB molecules (Zhou et al. 2023; Lyu et al. 2024). Conversely, at pH values below the pHpzc of PP, MB removal efficiency decreases due to electrostatic repulsion between the positively charged MB molecules and the positively charged surface of PP (Uddin et al. 2017). Additionally, under acidic conditions, competition between MB molecules and hydrogen ions for binding sites on the predominantly negatively charged PP surface becomes pronounced, inhibiting effective MB adsorption (Chen et al. 2021). The same results were reported by Abdel Azim et al. (2024) and El-Bery et al. (2022).

Increasing the dosage of PP resulted in higher MB dye removal ratios but lower adsorption capacities. The improvement in MB removal percentages was attributed to the higher mass of adsorbent, which increased the total number of available adsorption sites (Elzahar and Bassyouni 2023). Conversely, the decrease in adsorption capacities may be caused by the accelerated agglomeration of adsorbent particles due to the increased adsorbent mass. This agglomeration saturates the binding sites, reducing the active surface area of the adsorbent and extending the diffusion path length (Khamseh et al. 2023). These findings are consistent with previous research by Mousavi et al. (2022) and Maleki et al. (2020).

At the beginning of the adsorption process, employing a high initial dye concentration can decrease the MB removal efficiency due to the increased mass transfer rate resulting from the higher collision frequency between dye molecules and the adsorbent surface (Harja et al. 2022; Gürses et al. 2023). Subsequently, the dye removal ratio decreases over time due to the rapid saturation of active sites on the adsorbent surface. This saturation reduces the number of available adsorption sites relative to the influx of MB molecules to the adsorbent surface, which impedes the transfer of MB molecules and thereby reduces the dye removal ratio (Strebel et al. 2024). In contrast, complete adsorption is more likely with a low initial dye concentration. This is because a lower initial dye concentration results in slower saturation of adsorption sites on the PP surface, because of the greater number of available locations relative to migrating MB molecules’ count. In contrast, complete adsorption is more likely with low initial dye concentrations. (Wong et al. 2020; Soltani et al. 2021). The present outcomes aligned with previous investigations reported by (Al-Ghouti and Al-Absi 2020) and (Poornachandhra et al. 2023).

In conclusion, the optimal values for the independent operating parameters, as determined by the RSM model, were as follows: initial dye concentration of 34.6 mg/L, PP dose of 1.3 g/L, and pH of 11, as summarized in Table 4.

Table 4 Optimal operating conditions as well as the predicted and real MB removal rates
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Validation of the RSM model

Under the obtained optimum conditions (initial dye concentration = 34.6 mg/L, PP dose = 1.3 g/L, pH = 11, T = 30 °C, stirring speed = 900 rpm, and reaction duration = 90 min), the measured MB removal ratio and adsorption capacity were 97.89% and 26.05 mg/g, respectively, as shown in Fig. 7. The expected MB removal efficiency, calculated using Eq. (5) with values X = 34.6 mg/L, Y = 11.3, and Z = 1.3 g/L, was determined to be 100%. The minimal discrepancy between the measured and expected MB removal ratios highlights the consistency and reliability of the model.

Fig. 7
figure 7

a MB removal efficiency and b Palm Peat adsorption capacity (conditions: initial MB concentration = 34.6 mg/L, PP dose = 1.3 g/L, pH = 11, T = 30 °C, stirring speed = 900 rpm, and reaction time = 90 min)

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Comparison with other adsorbents for the removal of various dyes

The adsorption performance of PP for MB dye in this study achieved a removal efficiency of 97.98% under optimized conditions of [MB]o = 34.6 mg/L, [PP]o = 1.3 g/L, pH = 11, T = °C, and a contact time of 90 min. This result compares favorably with other palm waste-derived adsorbents reported in the literature, as presented in Table 5. For instance, palm bark-derived biochar exhibited a slightly higher removal efficiency of 99.3% for MB at a lower dye concentration of 5 mg/L but required a larger adsorbent dose (8 g/L), a longer time (180 min), and a higher T (40 °C) (Sun et al. 2013). In contrast, palm petiole-derived biochar removed 72.34% of crystal violet dye under less favorable conditions of pH 9 and lower T (25 °C) with a contact time of 180 min (Chahinez et al. 2020). Palm leaves powder showed a 90.6% removal efficiency for Rhodamine-B dye at pH 3, but this required a much higher adsorbent dose of 30 g/L and an extended adsorption time of 2880 min (Khan et al. 2024). Similarly, Oil palm frond magnetic biochar achieved 87.2% removal of Sunset Yellow dye at an acidic pH of 1, with a contact time of 90 min, but with a relatively lower adsorbent dose (1 g/L))Noorisepehr et al. 2020(. The high removal efficiency of PP demonstrated in this study, coupled with moderate adsorption conditions, highlights its potential as an effective and sustainable adsorbent for dye removal compared to other palm waste-derived materials.

Table 5 Comparison of Palm Peat with other palm waste-derived adsorbents in the literature for removing dyes
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Possible adsorption mechanism

Due to its unique structural properties and active functional groups, PP can interact with the MB dye molecules through multiple adsorption mechanisms. Evaluating these mechanisms allows for designing and modifying adsorbent surfaces to enhance dye molecules’ interaction and diffusion pathways, thereby improving adsorption capacity (Rai and Singh 2018; Mensah et al. 2022). The adsorption mechanism of MB dye is elucidated through changes observed in the FTIR spectra of the adsorbent after adsorption, as presented in (Fig. S3a) and Table S3. These changes included peak shifts, variations in band intensities, the appearance and disappearance of new bands, and initial peaks, respectively. The new peak at 3653.27 cm−1 in the PP after MB adsorption is attributed to the bending vibration of the C − S − C group in the dye (Khan et al. 2019). The peak of OH groups (3452.53 cm−1) shifted to 3542.64 cm−1 and its intensity decreased after treatment. This depicts that dye adsorption occurs by displacing physically adsorbed water molecules (Mashkoor and Nasar 2020). It also suggests dye adsorption due to H bonding between the dye’s [NH]+ group and hydroxyl groups on the PP surface (Wong et al. 2016). Additionally, the electrostatic attraction between deprotonated OH groups on the PP surface and the [NH]+ group of MB plays a crucial role in the adsorption mechanism (Herrera-González et al. 2017). The initial peak of the − COOH functional group (2318.27 cm−1) shifted to 2305.31 cm−1 and decreased in intensity, depicting a potential engagement between the − COOH groups on the PP surface and the C − H and N − H and groups of MB (Shokry et al. 2019). In a higher pH environment, the OH and COOH groups on the PP adsorbent (with a pHpzc of 3.5) deprotonate quickly, forming [O]− and [COO]− ions. These negatively charged ions exhibit electrostatic attraction toward the positively charged [NH]+ groups of the MB dye, thereby enhancing the adsorption of the cationic MB dye by the adsorbent (Wong et al. 2016). After dye adsorption, the peak at 1638.58 cm−1, corresponding to the C = C groups, shifted to 1654.31 cm−1 and increased in intensity. This shift suggests that the π electrons in the C = C bonds of MB molecules interact with the π electrons in the aromatic C = C groups of the adsorbent through π–π stacking (Gao et al. 2013; Shokry et al. 2019). Kooh et al (2017) and Kua et al (2020) attributed the interactions between the nonpolar aromatic groups on the PP’s surface and the nonpolar groups of dyes to hydrophobic-hydrophobic interactions. The shift in the peak of C–O of syringyl units from 1384.15 cm−1 to 1376.74 cm−1, and in the peak of C–O of guaiacyl units from 1257.73 cm−1 to 1247.88 cm−1, after dye adsorption, can be attributed to interactions with the C = S groups of MB (Mensah et al. 2023; Amoh et al. 2024). The 1166.62 cm−1 peak of C = O shifted to 1175.65 cm−1 due to the C–N stretching, C = S, and the vibrations of the N–H group in the dye (Abdel Azim et al. 2024). The disappearance of the peak at 1044.32 cm−1, associated with the C–O of primary alcohols, suggests the potential oxidation of these groups after adsorption (Khairnar et al. 2020). The new peaks at 885.09 and 771.82 cm−1, along with the shift and splitting of the peaks of the aromatic –CH groups from 622.44 cm−1 to (595.35 – 427.09 cm−1), are attributed to the C–H vibration in the dye ring and alkyl group (Geng et al. 2018). The significant changes in the FTIR spectra of PP after MB adsorption confirm the efficient interaction of the OH, COOH, C = C, C–O, C = O, and C–H functional groups on the surface of the adsorbent with the MB molecules during the adsorption process. A schematic diagram illustrating the potential mechanism of the MB dye adsorption is depicted in Fig. 8.

Fig. 8
figure 8

Schematic of proposed adsorption mechanism of methylene blue dye on palm peat

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Additionally, the solution pH can significantly influence the surface chemistry and interactions between MB dye and the PP, providing valuable insights into the underlying adsorption mechanism (Zhou et al. 2023). The PP adsorbent (pHpzc = 3.5) carries a predominantly positive surface charge at pH values below 3.5 and a predominantly negative surface charge above 3.5 (Chen et al. 2021). In basic solutions, the electrostatic attractions between the negatively charged adsorbent surface and the cationic MB dye molecules enhance the adsorption efficiency (Rahmani et al. 2020). Conversely, the electrostatic repulsion between the dye molecules and the positively charged adsorbent surface in acidic solutions diminishes the MB removal efficiency (Abdel Azim et al. 2024). Furthermore, the occupation of pores on the PP surface was confirmed by the decrease in its porosity observed in the SEM micrographs after adsorption, Fig. S3a.

Adsorption kinetics

The adsorption plots for the PFO and PSO kinetic models are presented in Fig. 9. The kinetics study was conducted under the following conditions: pH = 7, PP dose = 1.3 g/L, stirring speed = 900 rpm, T = 30 °C, and reaction time = 90 min. The three tested MB concentrations were 30, 40, and 50 mg/L. Table 6 summarizes the parameter values for the MB adsorption kinetics models. The PSO model exhibited higher R2 values (0.9983, 0.995, 0.9946) compared to the PFO model (0.9622, 0.9337, 0.9181) at initial dye concentrations of 30, 40, and 50 mg/L, respectively. These results indicate that the MB dye adsorption on the PP surface was best described by the PSO model (Revellame et al. 2020). The (qe, cal) values derived from the PFO kinetic models were 25.58, 34.6, and 43.1 mg/g, with Δqe (%) error values error values of 3.14%, 3.83%, and 3.97% at initial MB concentrations of 30, 40, and 50 mg/L, respectively. The corresponding RMSE value was 4.74. The PSO model demonstrated calculated equilibrium adsorption capacities of 20.35, 26.82, and 32.75 mg/g, with normalized standard deviation values of 2.39, 2.42, and 2.74%, respectively, and an RMSE value of 3.32. The lower Δqe (%) and RMSE values, along with the better agreement between experimental and calculated adsorption capacities at equilibrium for the PSO model compared to the PFO model, confirm the superior fitting of the PSO model for MB adsorption on PP (Nouioua et al. 2023). This behavior of the PSO model suggests that adsorption might be chemisorption, primarily depending on the q values rather than the MB concentration. Moreover, the adsorption process involves various mechanisms, including chemical interactions between MB and oxygen functional groups on the PP surface, and electrostatic attraction between reactive binding centers and the PP surface (Elboughdiri et al. 2024). The findings align with previous studies on the adsorption of MB using oil palm shells (Foo and Hameed 2013) and date palm seeds (Din et al. 2024).

Fig. 9
figure 9

a Pseudo-first-order and b pseudo-second-order models for the adsorption of MB on palm peat surface

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Table 6 Constants and correlation coefficients for the PFO and PSO kinetic models
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Adsorption isotherms

The adsorption isotherms for MB dye adsorption by PP for both the Langmuir and Freundlich isotherm models are depicted in Fig. 10. This study was conducted at five different initial dye concentrations (10, 20, 30, 40, and 50 mg/L) under the following conditions: pH 7, PP dose 1.3 g/L, stirring speed 900 rpm, T = 30 °C, and reaction time 90 min. The results of the two models and their corresponding correlation coefficients are summarized in Table 7. For the Langmuir isotherm model, the maximum adsorption capacity, Langmuir constant, and the correlation coefficient were found to be 46.729 mg/g, 1.829 L/mg, and 0.985, respectively. In the case of the Freundlich isotherm model, the Freundlich constant, the heterogeneity factor, and the R2 value were determined to be 28.563, 0.5258, and 0.949, respectively. The R2 value for the Langmuir isotherm model was higher than that for the Freundlich model, indicating that the adsorption system was better described by the Langmuir isotherm (Samadi Kazemi and Sobhani 2023). This good alignment with the Langmuir model suggests that MB adsorption on PP occurred as a monolayer, with no interactions between the adsorbed dye molecules. Additionally, the binding locations were uniformly spread across the surface of the adsorbent and were the same for every dye molecule, indicating that each active site binds to only one dye molecule (de Oliveira et al. 2023). The values of the separation factor for initial MB concentrations of 10, 20, 30, 40, and 50 mg/L were 0.0518, 0.0266, 0.0179, 0.0135, and 0.0108, respectively. Since the RL values lie between zero and one, this indicates that the adsorption of MB dye onto PP is a favorable process (Allahkarami et al. 2024). Durrani et al. (2022) reported that the Langmuir adsorption isotherm provided an excellent fit for the adsorption of MB dye onto date palm-based activated carbon-alginate membranes. Furthermore, the Langmuir isotherm model was effectively applied to the adsorption of MB dye using oil palm frond magnetic biochar, as reported by Oyekanmi et al. (2024).

Fig. 10
figure 10

a Langmuir and b Freundlich isotherm models for the adsorption of MB on the palm peat

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Table 7 Isotherm models’ parameters
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Adsorption thermodynamics

The MB adsorption on the PP was investigated at different T values (30, 50, 70, and 90 °C) under optimal conditions (initial dye concentration = 34.6 mg/L, pH = 7, PP dose = 1.3 g/L, stirring speed = 900 rpm, and reaction time = 90 min). The results are presented in Fig. 11 and Table 8. The ΔH° value was determined to be 13.6142 kJ/mol. This positive value indicates that the adsorption of MB dye onto PP is an endothermic process (Fisli et al. 2020). Additionally, the ΔG° values were determined to be 23.0917, –24.6168, –26.1419, and –27.667 kJ/mol at 30, 50, 70, and 90 °C, respectively. The ΔG° values were negative at all tested temperatures indicating that the MB dye adsorption on the PP is a spontaneous and thermodynamically favorable process (Bhavyasree and Xavier 2021). Moreover, the decrease in ΔG° values with increasing T indicates that the adsorption process becomes more spontaneous at higher temperatures, highlighting the feasibility and favorability of the process under these conditions (Khalid et al. 2022). Further, the positive value of ΔS° (76.2552 J/(mol. K)) indicates an increase in disorder and spontaneity at the point where the adsorbent and adsorbate solution meet throughout the adsorption of MB dye onto PP (Khairnar et al. 2020). This suggests that adsorption involves replacing water molecules with MB molecules on the adsorbent surface (Sudrajat et al. 2021). The current study's findings are consistent with previous research on the adsorption of MB using synthesized biochar derived from date palm kernels (Elkatory et al. 2024) and biochar produced from palm bark (Sun et al. 2013).

Fig. 11
figure 11

Adsorption thermodynamic of methylene blue dye onto the palm peat adsorbent

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Table 8 Thermodynamics factors for methylene blue adsorption of MB onto the PP surface at temperatures ranging from 30 to 90 °C
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Reusability of Palm Peat

The reusability of PP was assessed over five consecutive cycles under the optimum conditions (PP dose of 1.3 g/L, initial dye concentration of 34.6 mg/L, T of 30 °C, pH of 11, stirring speed of 900 rpm, run time of 90 min). The MB removal percentages across the five successive runs were 97.89, 95.75, 93.75, 90.05, and 88.35%, respectively, as explained in Fig. 12a. The adsorption capacities were 26.05, 25.48, 24.95, 23.97, and 23.52 mg/g (Fig. 12b). The slight decrease in MB removal ratios and adsorption capacities across the consecutive runs indicates high adsorbent recyclability. The dye removal efficiencies decreased throughout the runs due to the saturation of binding sites by MB molecules (Amoh et al. 2024). The PP weight loss percentages were 6.73% after the 2nd run, 8.95% after the 3rd run, 11.02 after the 4th run, and 13.27% after the last cycle, as illustrated in Fig. S3b. The low loss of PP particles during collection after each run indicates its stability. However, this loss may reduce the number of active sites and decrease MB removal ratios across the runs. The collection efficiency of PP particles after each cycle can be enhanced by magnetization (Mensah et al. 2022).

Fig. 12
figure 12

a MB removal ratios (%) and b Palm Peat adsorption capacities (mg/g) through five consecutive runs under the optimum conditions (conditions: initial MB concentration = 34.6 mg/L, PP dose = 1.3 g/L, pH = 11, T = 30 °C, stirring speed = 900 rpm, and run time = 90 min)

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The SEM image of PP after five adsorption cycles (Fig. S3a) reveals no significant morphological changes compared to the SEM images before adsorption (Figs. 1a, b), confirming the adsorbent stability. The only morphological alteration observed is a reduction in the number of pores due to dye adsorption. Additionally, the chemical composition of PP, indicated by the EDX pattern, after five dye adsorption cycles (Fig. S4a) remains nearly identical to its pre-adsorption composition (Fig. 1c), further confirming the adsorbent's stability. As shown in Table S4, the chemical structure of PP, indicated by XRD peaks, remains unchanged after and before treatment, as depicted in Fig. S4b and Fig.  2b, respectively. However, variations in FTIR band wavenumbers and intensities, as well as the appearance and disappearance of characteristic bands, were observed in PP after dye adsorption (Fig. S3a) compared to the pre-adsorption bands (Fig. 2a). These changes are attributed to MB dye adsorption and do not represent alterations in the adsorbent's chemical composition, thereby confirming its stability. In conclusion, the insignificant deviations in morphology, chemical composition, and chemical structure between the raw PP and PP after five adsorption cycles reaffirm the stability of the material.

Adsorption performance on various water matrices

The adsorption performance of PP for MB decolorization and TOC removal was evaluated with different real water matrices: SW, DRW, DW, TW), and LW. Table S1 presents the characteristics of raw water samples. The experiments were utilized under the optimum conditions (MB initial concentration = 34.6 mg/L, PP dosage = 1.3 g/L, T = 30 °C, pH = 11, mixing speed = 900 rpm, t = 90 min). DW exhibited the highest MB removal ratio at 97.89%. However, this ratio decreased in other matrices, with TW at 90.03%, LW at 63.25%, and SW at 52.72%. The lowest removal ratio was observed for DRW at 44.11% (Fig. 13a). The adsorption capacities were 26.05, 23.96, 16.83, 14.03, and 11.74 mg/g for DW, TW, LW, SW, and DRW, respectively (Fig. 13b). The TOC removal efficiencies were 97.91, 78.02, 54.42, 35.4, and 26.45% for DW, TW, LW, SW, and DRW, respectively (Fig. S5). The significant decrease in removal percentages and adsorption capacities for MB and TOC removal in SW, LW, and DRW compared to TW and DW can be attributed to the presence of salts and other inorganic ions (Table S1). These ions potentially influence the electrostatic interactions between the adsorbent and the adsorbate, reducing the PP adsorption capacity (John et al. 2018; Wu et al. 2020). Additionally, the dissolved organic matter in water matrices competes with the MB molecules for adsorption sites, further reducing MB and TOC removal percentages (Li et al. 2022; Hu et al. 2024). Despite the challenges posed by different water matrices, PP demonstrated notable MB and TOC removal ratios, indicating its high adsorption efficiency.

Fig. 13
figure 13

a MB removal ratios and b Palm Peat adsorption capacities for MB (mg/g) with different water matrices under the optimum conditions (initial MB concentration = 34.6 mg/L, PP dose = 1.3 g/L, pH = 11, T = 30 °C, stirring speed = 900 rpm, and t = 90 min)

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Palm Peat adsorption performance in real textile wastewater

Real textile wastewater sample with initial MB and TOC concentrations of 113.38 and 274.16 mg/L, respectively (Table S1), was subjected to adsorption utilizing PP under optimized conditions (PP dosage = 1.3 g/L, T = 30 °C, agitation speed = 900 rpm, pH = 11, and t = 90 min). After 60 min, the removal percentages for MB and TOC were recorded at 67.75 and 44.69%, respectively, as shown in Fig. 14a. The corresponding q values were 59.09 mg/g and 94.24 mg/g for TOC (Fig. 14b). At 90 min, the removal percentages increased to 71.51% for MB and 48.16% for TOC, with adsorption capacities of 62.36 mg/g for MB and 101.57 mg/g for TOC. As indicated in (Table 9), both removal efficiencies and adsorption capacities for MB and TOC significantly increased up to 60 min. Beyond 60 min, only a marginal increase was observed, suggesting equilibrium was reached at 60 min. Wang et al. (2021) conducted a study on the adsorption of real shale gas flowback water using a carbon-nanotube-nested diatomite adsorbent. Their findings revealed that the removal rate of TOC exhibited a rapid increase within the initial 10 min of contact with the adsorbent. Subsequently, the rate of increase gradually declined, eventually stabilizing to a smooth trend until the end of the experiment at 40 min. The higher MB removal ratio in synthetic wastewater sample (97.89%) than in real dyeing wastewater (71.51%) after 90 min under the optimum conditions can be attributed to the complex composition of the real wastewater that may contain other contaminants which can compete with MB for adsorption sites on the adsorbent, reducing the overall efficiency of MB removal (Amrutha et al. 2023). Additionally, real wastewater often contains interfering surfactants such as oils, surfactants, and other organic compounds that can block adsorption sites or create a coating on the adsorbent surface, hindering the adsorption of MB (Boubakri et al. 2018). Further, the lower initial concentration of MB in synthetic wastewater (34.6 mg/L) than in real wastewater (113.38 mg/L) can cause rapid saturation of the active sites reducing the MB removal efficiency (Fito et al. 2023). Moreover, real wastewater is more likely to cause fouling and clogging of the adsorbent material due to the presence of suspended solids and other particulate matter that can reduce the effective surface area and porosity of the adsorbent, leading to lower MB removal (Sounthararajah et al. 2015). In conclusion, the significant removal of MB and TOC from real textile wastewater through adsorption onto PP underscores the potential of the adsorbent for application in wastewater treatment plants. This potential extends to the possibility of reusing treated wastewater for various purposes or safely discharging it into water bodies.

Fig. 14
figure 14

a MB and TOC removal ratios and b adsorption capacities (mg/g) of MB and TOC, for the real textile wastewater under the optimum conditions

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Table 9 Residual concentrations, removal efficiencies, and adsorption capacities of MB and TOC, in real textile effluent using the Palm Peat adsorbent under optimal conditions at various exposure times
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Conclusions

This study introduces PP as a novel adsorbent, rich lignocellulosic medium, demonstrating its unique potential with a porous, spongy structure, a surface area of 16 m2/g, and 49.07% carbon content. PP achieved 68.26% MB adsorption efficiency and a capacity of 20.48 mg/g within 90 min under optimized conditions (initial MB concentration of 34.6 mg/L, PP dose of 1.3 g/L, pH 11), with an experimental MB removal efficiency of 97.89%, closely aligning with the RSM model’s prediction. The adsorption process, characterized by chemisorption and monolayer formation, was found to be spontaneous and endothermic. PP exhibited high stability across five cycles, with only a slight decrease in efficiency from 97.89% to 88.35%. Its performance varied with water matrices, showing decreasing efficiency from distilled water to drain water. Notably, PP achieved 71.51% MB removal and 48.16% TOC mineralization in real industrial textile wastewater. Future research should explore PP’s large-scale application, its potential as a co-catalyst for emerging pollutant degradation, and its capacity to activate oxidants for treating refractory pollutants.