1 Introduction

Today, the presence of heavy metals uncontrollably discharged into water bodies as a result of anthropogenic activities and rapid industrialization has become a global threat [1]. Heavy metals in dissolved or particle forms such as lead (Pb), mercury (Hg), copper (Cu), cadmium (Cd), zinc (Zn), and nickel (Ni) harm ecological systems because they are non-biodegradable and remain stable [2, 3]. Pb is one of the most common toxic heavy metals found in industrial wastewater, owing to its widespread use in various industrial processes such as mining, automotive manufacturing, battery production, and metal plating [4]. However, the World Health Organization (WHO) has reported that the maximum allowable concentration of Pb2+ ions in wastewater and drinking water should be 0.05 mg L−1 and 0.01 mg L−1, respectively [5]. Exceeding these limits in water consumption can lead to various health issues in living beings, including kidney, brain, and liver damage, hypertension, anemia, as well as neurological and hematological disorders [6, 7]. Therefore, effectively removing Pb2+ ions pollution from water bodies is critical for human health [8, 9]. Adsorption is a low-cost, simple, and efficient method, offering higher selectivity than alternatives such as coagulation, membrane removal, ion exchange, and chemical precipitation [10]. However, adsorption efficiency may vary depending on factors such as the surface area of the adsorbent, pore volume and size, and functional groups present on the surface [11, 12].

Biomass (biosorbents) has attracted the attention of researchers as an environmentally friendly, biodegradable, and cost-effective option for removing low concentrations of Pb2+ ions from wastewater [13]. These adsorbents are sustainable and do not cause secondary pollution, making them a promising solution for this issue [14]. Biosorbents such as bacteria [15], algae [16], fungi [17], plant residues [18], activated sludge [19], biopolymers [20], and yeast [21] have been investigated in Pb2+ ions removal studies.

Fungi are organisms with robust structures found worldwide, possessing physical properties that render them suitable for use as biosorbents [22]. They are increasingly employed in removal studies due to the diverse array of functional compounds present in their cell walls, as well as their low cost, ease of growth, simple fermentation techniques, and industrial abundance [23]. Fungal cell walls contain polysaccharides, lipids, and proteins with metal-binding functional groups, such as hydroxyl, carbonyl, carboxyl, amine, and amide [24]. These compounds enable fungi to efficiently remove heavy metals. Macrofungi such as Phanerochaete chrysosporium [25], Lentinula edodes, Agaricus bisporus [26], and Ganoderma lucidum [27] have been investigated for their ability to remove Pb2+ ions via biosorption. This study explores the potential of using inactivated and dried P. papilionaceus as a sustainable, inexpensive, readily available, and environmentally friendly biosorbent for removing Pb2+ ions from aqueous media. P. papilionaceus is a small, brown-blackish colored mushroom with conical or bell-shaped caps from the Bolbitiaceae family, inedible, non-poisonous, with remarkable health benefits and a common species [28].

In the literature review, no study was found investigating the biosorption capacity of P. papilionaceus. Therefore, the current study is significant as it represents the first exploration into this subject. The surface morphology and chemical structure of P. papilionaceus before and after biosorption were examined using FTIR, SEM, and EDX characterization techniques. Studies were conducted under various parameters including solution pH, contact time, amount of biosorbent, temperature, and initial Pb2+ ions concentration to assess their effects on the biosorption process. In addition, the determination of the antimicrobial and antibiofilm properties of P. papilionaceus was also investigated in the study.

2 Material and method

2.1 Chemicals and instrumentation

Pb(NO)3, KNO3, HCl, HNO3, NaOH, ethyl alcohol, and all other chemicals were of analytical purity (Sigma Aldrich).

The functional groups on the fungal biosorbent before/after Pb2+ ions biosorption were determined using the FT-IR (ATR, Bruker Model: Tensor II) technique. The changes in surface morphology of the fungal biosorbent before/after Pb2+ ions biosorption, along with the identification of different elements, were analyzed by SEM–EDX (TESCAN MIRA3 XMU). The Pb2+ ion concentrations were determined using the PAR method [29] with a UV–vis spectrophotometer (UV-DR-6000; Shimadzu, China) at λ = 519 nm. Fatty acid methyl esters in the fungi sample were analyzed by gas chromatography using flame ionization detection (GC-FID) equipment (Shimadzu, QP2010) and DB FastFame capillary column [30].

2.2 Preparation of biosorbent

Fungal biosorbent was collected from Sivas, Turkey. The sample was photographed in the field and then transported to the laboratory by being carefully wrapped in individual paper parcels to prevent damage. The samples were diagnosed based on field data and microscopic features observed in laboratory examinations. Then, the biosorbent was washed several times with deionized water and then ıt was dried at 25 °C. It was then stored in a polystyrene container to be used in biosorption studies.

2.3 Biosorption procedure

Pb2+ ions biosorption was investigated using the batch biosorption method. The main factors examined are pH, contact time, adsorbent amount, temperature, and initial Pb2+ concentration. In each experimental set, a fixed dosage of 50 mg of biosorbent was added to 10 mL solutions consisting of 1000 mg L−1 Pb2+ with stirring for 24 h at room temperature. Then, the supernatant concentration and maximum absorbance of 519 nm for Pb2+ ions were measured. Pb2+ ion concentration was determined by the absorbance measurement. The batch adsorption conditions are given in Table 1. Q (mg g−1) and Biosorption% were calculated using (1) and (2) [31];

Table 1 Batch biosorption experiment conditions
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$$Q=\left[\frac{{C}_{i}-{C}_{f}}{m}\right]xV$$
(1)
$$Biosorption\%=\left[\frac{{C}_{i}-{C}_{f}}{{C}_{i}}\right]x100$$
(2)

2.4 Preparation of the fungi extract for biological activity

The fungi extract, intended for use in antibacterial and antibiofilm activity studies, was prepared using the Soxhlet extraction method. Fungi sample was dried and pulverized into a fine powder by grinding them in a mortar. The extraction process was conducted for 12 h in 400 mL methanol. Following the extraction, the methanol was removed at 40 °C using an evaporator, resulting in the isolation of a dry, pure substance.

For the preparation of the main stock extract solution, 0.15 g of extract was dissolved in 3 mL of methanol. After sterilization with a 0.22 µm diameter syringe filter, the main stock solution was immediately used for bioactivity analysis. The rest of the pure substance was stored at + 4 °C.

2.5 Assessment of antibacterial activity

The antibacterial activity of fungi extract was assessed using the micro-broth dilution method, following the guidelines developed by the Clinical and Laboratory Standards Institute (CLSI) (Clinical and Laboratory Standards Institute, 2019). The study used a total of 6 different indicator bacteria, comprising 3 g-positive and 3 g-negative species. Indicator microorganisms include Listeria monocytogenes ATCC 7644, Bacillus subtilis (ATCC 6633) and Staphylococcus aureus (ATCC 43300), Escherichia coli (ATCC 25922), Klebsiella pneumoniae (ATCC 10031) and Pseudomonas aeruginosa (ATCC 27853). Ciprofloxacin was used as a control drugs. The sample and inoculum were prepared as follows.

For inoculum preparation, the indicator bacteria were cultured in Mueller Hinton Broth (MHB) at 37 °C for an overnight period. Subsequently, the density was adjusted to McFarland no 0.5 standard, and an additional dilution of 100 fold was performed in MHB to obtain an inoculum at a concentration of 106 colony-forming units per milliliter (CFU/mL). For sample preparation, the fungi extract was diluted in MHB within a concentration range of 2 mg/mL to 0.0312 mg/mL.

To analyze, 100 µl of the prepared dilutions of fungi sample and 100 µl inoculum were transferred to sterile ELISA plates. The plates were incubated at 37 °C for 24 h. Following the incubation period, the absorbance values of the plates were read at 600 nm by BMG LABTECH’s ultra-fast UV/vis spectrophotometer (SPECTROstar Nano, Germany). Thus, in the final analysis, antibacterial activity was evaluated in the concentration range of 1 mg to 0.0156 mg. The analysis was repeated twice. The well in which the minimum inhibitory concentration (MIC) was seen was identified, and the corresponding MIC value was computed.

To ascertain the absence of any impact of the solvent itself on bacterial growth, a control experiment was conducted. This control involved the use of inoculated broth supplemented solely with methanol, at the same dilutions employed in the experimental procedures.

2.6 Assessment of antibiofilm activity

The assessment of antibiofilm activity was conducted using a modified version of the Christensen method, as originally suggested by Christensen et al. [32]. In parallel with the antibacterial activity study, a total of 6 bacteria species (identical to the aforementioned in the antibacterial activity study) were used as indicator bacteria.

For inoculum preparation, the indicator bacteria were cultured in Tryptic Soy Broth (TSB) supplemented with 2% glucose at 37 °C for an overnight period and, the density was adjusted to McFarland no 0.5 standard to obtain an inoculum at a concentration of 108 CFU/mL. For sample preparation, the fungi extract was diluted in TSB supplemented with 2% glucose within a concentration range of 2 mg/mL to 0.0312 mg/mL.

To biofilm formation, 100 µl of the prepared dilutions of fungi sample and 100 µl inoculum were transferred to sterile ELISA plates. Then, the plates were incubated at 37 °C for 48 h. Thus, in the final analysis, antibiofilm activity was evaluated in the concentration range of 1 mg to 0.0156 mg. To assess the biofilm inhibition, after the incubation period, the medium present in the wells was discarded and, the plates were washed by using phosphate buffered saline solution. Following the washing step, plates were kept at 25 °C for 2 h. Then 1% crystal violet solution (200 µl) was added the each well, after waiting 15 min, and then washed by using phosphate-buffered saline solution. Finally, 200 µL absolute ethanol was added to the wells to dissolve the crystal violet and the absorbance values of the plates were read at 600 nm by BMG LABTECH’s ultra-fast UV/vis spectrophotometer (SPECTROstar Nano, Germany). Results were calculated as % biofilm inhibition concentration. In the analyses, Indicator bacterial suspensions (200 µl) were used as negative control and TSB containing 2% glucose was used as positive control.

3 Results and discussion

3.1 Effect of pH on adsorption and pHpzc for fungal biosorbent

One of the vital parameters affecting biosorption efficiency in the biosorption process is the initial pH value of the medium. The solution for fungal biosorbent Pb2+ ion biosorption was investigated in the pH range of 1.0–5.0 and the results obtained are shown in Fig. 1. When Fig. 1 is examined, it was seen that there was an increase in biosorption efficiency from 26 to 56% as the pH value increased from pH:1.0 to pH:4.0. After pH: 4.0, biosorption efficiency decreased relatively.

Fig. 1
figure 1

Effect of pH on biosorption {[Pb2+]0: 500 mg L−1, biosorbent dosage:10 g L−1, pH:1.0–5.0, contact time:24 h, temperature: 25 °C}

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At low pH values, biosorption efficiency decreases due to partial protonation of surface functional groups of the fungal biosorbent and strong competition between H+ ions and Pb2+ ions. Since there is competition between H+ ions and Pb2+ ions for binding sites at low pH values, biosorption efficiency is less at low pH values. With increasing pH values, the surface functional groups became deprotonated, causing strong electrostatic interactions with Pb2+ ions, and biosorption increased. It was observed that Pb2+ ion biosorption reached its maximum at pH: 4.0. After pH: 4.0, biosorption was observed to decrease relatively.

At high pH values, metal hydroxides of Pb2+ ions begin to precipitate. At pH > 5.5, Pb2+ ions undergo a series of hydrolysis reactions and as a result, hydroxide species are formed [33]. In this case, the repulsion forces between the functional groups on the adsorbent surface and the Pb2+ ions in the solution decrease, and the number of Pb2+ ions removed from the solution decreases. For this reason, Pb2+ ions were not studied since they tend to precipitate at pH > 5.5.

The zero charge point (pHpzc) represents the pH value at which the biosorbent surface is electrically neutral. The surface charge solid addition technique of fungal biosorbent was found by adding the same amount of solid to a series of solutions with the same ionic strength at different pH values [34]. The pHpzc value for the fungal biosorbent was found to be 4.49 (Fig. 2).

Fig. 2
figure 2

pHpzc for fungal biosorbent

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3.2 Effect of biosorbent dose

The effect of fungal biomass amount on Pb2+ ion biosorption was (Fig. 3). Figure 3 shows that the adsorption rate increases from 28 to 85% with the increase of the fungal dosage from 1 to 20 g L−1, while the quantity adsorbed decreases from 75 to 40 mg g−1. The amount of Pb2+ removed increases with the biosorbent mass because more active sites can be accessed to take part in the biosorption process [35]. Fungal cell walls contain various functional groups such as carboxyl, hydroxyl, amino, sulfhydryl, and phosphate [36]. These functional groups are also of interest for Pb2+ ions. This reveals the excellent biosorption properties of fungi.

Fig. 3
figure 3

Effect of biosorbent dose on biosorption {[Pb2+]0: 500 mg L−1, biosorbent dosage: 1, 3, 5, 10 and 20 g L−1, natural pH:4.5, contact time:24 h, temperature: 250C}

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3.3 Biosorption isotherms

Biosorption equilibrium isotherms, whether graphical or mathematical, play a crucial role in understanding and describing biosorption processes. They provide valuable information about how a biosorption system reaches equilibrium and the characteristics of the biosorbent involved in the process. These isotherms are used to elucidate the fundamental nature of biosorption, the mechanisms behind it, the optimum biosorption capacity, and the affinity of the biosorbent for the target substances in a solution [37].

Langmuir [38], Freundlich [39], and D-R [40] isotherm models were used to explain the concentration effect on biosorption. Nonlinear biosorption isotherms for Pb2+ ions into the fungal biosorbent are presented in Fig. 4, and the equations and determine parameters of these isotherms are presented in Table 2. The optimum biosorption capacity (QL) determined for the fungal biosorbent from the Langmuir biosorption isotherm model was calculated as 31.2 mg g−1. The KL was determined as 0.745 L mg−1. When the Freundlich and Langmuir isotherm models are compared, it is understood that the R2 value of the Freundlich model is higher and it is the model that describes the biosorption process better. This shows that biosorption follows multilayer biosorption behavior [41]. Freundlich capacity (XF) was determined as 3.89 and heterogeneity factor (β) was determined as 0.332. Since the EDR value was found as 11.9 kJ mol−1, it was concluded that the biosorption process proceeded chemically. The maximum biosorption capacity of the fungal biosorbent determined for the removal of Pb2+ ions and the isotherm model they are compatible with are compared with various sorbents in Table 3. As a result of this comparison, it was determined that the maximum biosorption capacity of fungal biomass was high enough to be an alternative to other sorbents. This indicates that the fungal biosorbent is a competitive and potentially cost-effective alternative to other sorbents for the removal of Pb2+ ions from solutions.

Fig. 4
figure 4

Biosorption isotherms {[Pb2+]0: 50–1000 mg L−1, biosorbent dosage:10 g L−1, natural pH:4.5, contact time:24 h, temperature: 25 °C}

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Table 2 Parameters calculated from isotherm models
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Table 3 Isotherm modeling on the biosorption of Pb2+ ions onto another sorbent in literature
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3.4 Biosorption kinetics

The mechanism of the biosorption process depends on the chemical and physical properties of the biosorbent as well as the system conditions. The biosorption process must reach equilibrium in a short time. Because increasing contact time may cause higher treatment costs in wastewater treatment plants. When Fig. 5 was examined, it was observed that Pb2+ ion biosorption increased with contact time. It was observed that biosorption was rapid until the first 240 min and reached equilibrium after that. The high biosorption rate in the first 240 min is initially due to the number of empty active centers present in the fungal biosorbent. Therefore, increasing concentration gradients exist between Pb2+ ions in solution and the surface of the fungal biomass. This increase in the concentration gradient causes an increase in the removal rate of Pb2+ ions in the initial stages. Over time, this concentration decreases due to Pb2+ ions adhering to empty active centers, which causes the adsorption rate to decrease.

Fig. 5
figure 5

Biosorption kinetics {[Pb2+]0: 500 mg L−1, biosorbent dosage:10 g L−1, natural pH:4.5, contact time:2–1440 min, temperature: 25 °C}

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Experimental data were applied to pseudo first order (PFO) [48], pseudo second order (PSO) [49], and intra particle diffusion (IPD) [50] models to predict the mechanism involved in the biosorption process [51, 52]. The applicability of PFO and PSO kinetic models in the removal of Pb2+ ions using fungal biomass was evaluated by comparing their correlation coefficients and the most appropriate model was selected. According to Table 4, it can be seen that the R2: 0.777 value of the PFO model is smaller than the R2: 0.834 value of the PSO model. In addition, it has been determined that the theoretical Qt value and the experimental Qe values of the PSO model are in agreement with each other in explaining the experimental data. It was seen that the PSO model described the biosorption process better. The multicollinearity of the Q-t0.5 graph drawn using experimental data indicates that the biosorption mechanism follows two or more steps (Fig. 6). When Fig. 6 is examined, it is seen that the IPD model graph is divided into two linear parts. The first linear part of the IPD model plot shows a boundary layer effect, while the second linear part represents intra-particle diffusion. When all this information was evaluated together, it was concluded that the biosorption process followed the PFO and IPD models.

Table 4 The calculated parameters from kinetic models
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Fig. 6
figure 6

Biosorption thermodynamics {[Pb2+]0: 500 mg L−1, biosorbent dosage:10 g L.−1, natural pH:4.5, contact time:24 h, temperature: 5 °C, 25 °C, ve 40 °C}

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3.5 Biosorption thermodynamics

To explain the thermodynamic behavior of the biosorption process, the studies were carried out at 5 °C, 25 °C, and 40 °C. Thermodynamic parameters were derived from data obtained from temperature effect studies on biosorption (Eq. 3–6). The following equations were used to determine the ΔHo, the ΔSo, and the ΔGo parameters [53].

$${K}_{d}= \frac{Q}{{C}_{e}}$$
(3)
$${\Delta G}^{^\circ }=-RTln\left({K}_{d}\right)$$
(4)
$$ln{K}_{D}= \frac{{\Delta S}^{^\circ }}{R}-\frac{{\Delta H}^{^\circ }}{RT}$$
(5)
$${\Delta G}^{o}={\Delta H}^{o}-T{\Delta S}^{o}$$
(6)

As seen in Fig. 6, a 1/T graph was drawn against ln Kd for Pb2+ removal on the fungal biosorbent. Figure 6 represents a graph where 1/T is plotted against ln Kd (the natural logarithm of the distribution coefficient) for the removal of Pb2+ ions on the fungi. ΔSo was calculated from the intersection point of this obtained graph, and ΔHo was determined from the slope of the ln Kd vs. 1/T graph. The obtained parameters are given in Table 5. The positive ΔHo value of 92.9 kJ mol−1 determined for Pb2+ biosorption indeed indicates an endothermic nature of the biosorption process. The positive ΔSo value of 380 kJ mol−1 determined for the biosorption process indicates that randomness increases at the biosorbate-biosorbent interface during the process and ion exchange reactions occur [54]. Additionally, high negative ΔGo values indicate that biosorption occurs chemically [55].

Table 5 Thermodynamic parameters
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3.6 FT-IR and SEM–EDX analyses

FT-IR analysis was performed to identify surface functional groups of the fungus as well as groups that would assist in Pb2+ biosorption. FT-IR spectra of the fungus before and after Pb2+ biosorption are given in Fig. 7 (a, b). The broad and strong band in the range of approximately 3000–3600 cm−1 observed in the FT-IR spectrum of fungus without Pb2+ loading can be explained by the stretching vibration of –OH and N–H groups [56]. The observed peak at 2926 cm−1 can be attributed to the symmetric and asymmetric vibration of the CH3 and CH2 groups [57], and the peak at 1638 cm−1 can be attributed to the stretching vibration of the carboxyl group (-COOH) [22]. In addition, the peaks at 1539 cm−1, 1410 cm−1 and 1040 cm−1 can be associated with the amide (II) vibration, bending vibration of the C-H group, and stretching vibrations of the C-O-C and C-O groups, respectively. The peak observed at 609 cm−1 can be explained as the vibration of phosphate groups (P = O) [58]. In line with this information obtained from the FTIR spectrum, it was determined that the fungus has functional groups that help metal biosorption such as carboxyl, hydroxyl, amino, and amide. When looking at the FT-IR spectrum of the fungus after Pb2+ biosorption, a shift in some characteristic absorption peaks and an increase or decrease in their intensity were detected. It is observed that the peaks where the vibrations of –CH, -COOH, C-O, and phosphate groups detected in the structure of the fungus are determined to have shifted to 2916 cm−1, 1623 cm−1, 1310 cm−1, 732 cm−1, respectively. The changes observed before and after biosorption can be attributed to surface complexation and electrostatic interaction reactions between these functional groups and Pb2+ ions [58].

Fig. 7
figure 7

FT-IR spectra of fungal biomass (a) Pb2+ biosorbed fungal biomass (b)

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SEM images of the fungal biosorbent before and after Pb2+ ion biosorption are given in Fig. 8. In the SEM image of the fungal biomass, the structure consisting of sporangiospore mass is seen (Fig. 8 (a)). The SEM image of the fungal biomass (Fig. 8 (a)) was observed to be obviously different from that of the Pb2+ ions biosorbed (Fig. 8 (b)). It was seen that the sporangium surface of the fungus became irregular and rough after the biosorption of Pb2+ ions. As shown in Fig. 8 (c-d), EDX analysis results showed that there were changes in percentage by weight in the inorganic components after Pb2+ ions biosorption, and there were increases and decreases indicating ion exchange. Based on this information, it was thought that more than one mechanism was involved in Pb2+ ion biosorption onto fungal biomass, such as surface complexation and electrostatic interactions.

Fig. 8
figure 8

SEM images of fungal biomass (a) Pb2+ biosorbed fungal biomass (b)

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3.7 Biological activity of the fungal extract

Certain species belonging to the Panaeolus genus have garnered attention due to their notable psilocin production, as well as their diverse range of biological activities, including antioxidative, anti-inflammatory, and antibacterial capabilities. However, there is limited data available regarding the antibacterial properties of P. papilionaceus, a species known for its capability to produce psychoactive compounds. Although Coprophilous mushroom species are generally considered to have a high potential for producing antimicrobial compounds, the limited number of studies conducted on this subject indicate the absence of antimicrobial activity by this species [59]. The current investigation aimed to evaluate the antibacterial efficacy of the methanol extract derived from P. papilionaceus against a selected set of six distinct bacterial strains. However, it was shown that there was no notable impact on any of the bacteria examined when exposed to concentrations ranging from 1 to 0.0156 mg.

In the current study, were examined not only the antimicrobial effects of the fungal extract but also its ability to inhibit biofilm formation. Biofilms are complex structures that result from the attachment of microbes to one other and a surface, accompanied by the release of extracellular polymeric substrates (EPS) into the surrounding environment. Microorganisms engage in the formation of biofilms as a means to enhance their resilience against antimicrobial substances and various environmental stressors present in natural settings. The production of biofilms is considered unfavorable in terms of hygiene and safety, especially within the food industry, due to its propensity to facilitate the attachment of pathogenic microbes or those responsible for food spoiling onto food and related surfaces. Therefore, understanding the formation of biofilms on surfaces and their removal is of great importance [60]. There is a lack of available evidence related to the antibiofilm capabilities exhibited by Panaeolus species, including P. papilionaceus. To the best of our current understanding, this study represents the initial documentation of the anti-biofilm properties of P. papilionaceus. The findings of our investigation revealed that P. papilionaceus could prevent biofilm formation at varying rates depending on the type of biofilm-forming bacteria (Table 6). It was determined that 1 mg of fungal extract noteworthy inhibitory effect on the formation of biofilms by indicator bacteria, particularly S. aureus and Klebsiella, with reductions of 87.85% and 72.73% respectively. However, no discernible impact was observed on the biofilm formation of P. aeruginosa and B. subtilis. Upon analysis of the fungal sample, it was determined that the predominant fatty acid component was linoleic acid (%80.6) (Fig. 9). In addition to linoleic acid, the presence of palmitic acid, stearic acid, and oleic acid in small amounts has also been observed.

Table 6 Reduction in biofilm formation on 1 mg mL−1 concentration
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Fig. 9
figure 9

Fatty acid profile of fungal (Panaeolus papilionaceus) biosorbent

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3.8 Biosorption mechanism

The biggest challenge in a biosorption study is to elucidate the biosorption mechanism. To elucidate the biosorption mechanism, the surface properties and structure of the biosorbent need to be examined. In this context, it should be noted that; Fungi are biological materials that can remove pollutants such as heavy metals and dyes [2]. The binding mechanism of heavy metals to fungi can be explained by the strong attraction forces of cell wall components. Fungi have great advantages over other biosorbent materials in terms of biosorption and bioaccumulation activities due to their cell wall materials showing metal binding properties [61]. The fungal cell wall consists of glycoproteins, chitin, and glucans, as well as chitosan and other components [62]. The complex structure of fungal cell walls containing various functional groups such as hydroxyl (OH−), carboxyl (COOH−), amino (NH2), phosphate (PO43−), and sulfhydryl (SH−) reveals the excellent biosorption properties of fungi (confirmed by the FT-IR spectrum, SEM images, and fatty acid profile). The biosorption mechanism is assumed to include the following steps [63]; i) bulk solution transport to the surface of the biosorbent, ii) diffusion of Pb2+ ions across the boundary layer to the surface of the fungal biosorbent (film diffusion), iii) accumulation of increasing amount of Pb2+ ions to the surface of the fungal biosorbent (biosorption). Electrostatic interactions occur between biological groups such as OH−, COOH−, NH2, PO43−, SH−, and Pb2+ ions in the fungal biosorbent cell wall, which is one of the active centers for biosorption. Additionally, during the process of biosorption to the fungal surface, chelation occurs between one or more Pb2+ ions in the solution and the active functional groups on the fungal cell wall surface. In addition, solution pH supports ion exchange because at the natural pH of Pb2+ ions: 4.5, the competitive effect of H+ ions decreases, and negative ion gaps are formed on the fungal cell wall surface that can biosorb more Pb2+ ions. iv) Pb2+ ions into the internal pores and capillary spaces of the fungal biosorbent (intra-particle diffusion). Each of these four mechanisms are independent process (Fig. 10). When all this information is evaluated together, it is thought that the biosorption process of Pb2+ ions into fungal biomass is accompanied by the steps of surface biosorption, intra-particle diffusion, and film diffusion on the fungal cell walls. In addition, during the biosorption process, protons or light metal cations such as Na+, K+, Mg2+, and Ca2+ attached to the functional groups on the biosorbent surface are exchanged with metal cations in the aqueous solution (ion exchange) [64].

Fig. 10
figure 10

Biosorption mechanism of fungal biomass

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4 Conclusion

In this study, to effectively remove Pb2+ ions from aqueous solutions fungal (P. papilionaceus) biomass was used. The optimum biosorption of Pb2+ ions was achieved in 240 min at 25 °C at a natural pH of 4.5. It was observed that biosorption isotherms fit the Freundlich model better. The optimum biosorption capacity calculated from the Langmuir model was found to be 31.2 mg g−1. The biosorption energy found from the D-R model showed that the biosorption process was chemical. Negative values of the Gibbs free energy change, one of the thermodynamic parameters, showed that the biosorption process was spontaneous, and positive enthalpy values showed that the biosorption process was endothermic. Biosorption kinetics showed that the biosorption process followed PSO and IPD patterns. Biosorption of Pb2+ ions into fungal biomass was observed to involve surface biosorption, film diffusion, and intra-particle diffusion mechanisms. In light of all this information, it was seen that the fungal biosorbent is a low-cost, natural, effective, and potentially promising biosorbent in the removal of Pb2+ ions from aqueous solution.