Simultaneous removal of lead, copper, cadmium, nickel, and cobalt heavy metal ions from the quinary system by Abies bornmulleriana cones

Water is the primary matter for the presence of life, and for humanity, which improved on their civilizations by the side of water resources to promote the consumption of water (Vuorinen et al. 2007). Deterioration of water resources originating from heavy metal ions is a continual problem due to the inadequate discharging of industrial effluents such as printing processes, metallurgy, agricultural, and mining activities to the receiving environments (Munagapati et al. 2010).

Heavy metal ions can create many problems for the human body due to the distinctive physicochemical properties of each ion. Pb²⁺, Cu²⁺, Cd²⁺, Co²⁺, and Ni²⁺ metal ions originate from various industries. Some of them are micronutrients required for living organisms at trace amounts. But, excess intake of these heavy metal ions can give rise to health problems, including kidney and liver failure, gastrointestinal disturbance, birth defects, liver damage, hypertension, cancer, mental retardation, reduction in haemoglobin production, neurologic signs and depression (Kurniawan et al. 2006; Han et al. 2009; Laus & de Favere 2011). At present, the uptake of metal ions originating from wastewater is a primary matter for environmental organizations. Thus, it is required to keep under control the fraction of heavy metal ions in wastewaters before their discharge into the receiving environment.

The treatment of wastewater, including heavy metal ions, is a difficult task for environmental engineers, policymakers, and municipalities. Several technologies such as adsorption, catalysis, coagulation, electro/chemical reduction, advanced oxidation, biological treatment, and membrane separation processes are widely used in the removal of hazardous pollutants from wastewater (Ahsan et al. 2019, 2020b, 2020c; Islam et al. 2019). However, the adsorption process is one of the most effective, simple and low-cost techniques for the removal of organic and inorganic substances from wastewater (Hyder et al. 2014; Begum et al. 2016). Adsorbents obtained from polymeric materials, industrial solid wastes, and agricultural by-products have gained widespread attention in recent years for the removal of organic substance from wastewater (Begum et al. 2016; Peng et al. 2016; Islam et al. 2017; Liu et al. 2017).

The biosorption has been known to be a cost-effective alternative in the treatment of heavy metal ions (Volesky 2007). Usually, the biosorption technique can diminish operational costs by 36%, capital costs by 20% and total treatment cost by 28% compared with the standard methods (Loukidou et al. 2004). Therefore, the interest in the application of low-cost alternatives in recent years has considerably augmented. Biosorption is the most investigated process for such kinds of waters containing heavy metal ions at trace dosages. It can be practiced on the treatment of wastewaters using inexpensive biosorbent materials such as wood waste, weeds and sawdust (Asadi et al. 2008; Pereira et al. 2010), sunflower biomass (Oguz & Ersoy 2010, 2014), yellow squash (Oguz & Utku 2019), sugarcane bagasse (Homagai et al. 2010), pulp and seeds (Liu et al. 2012; Torab-Mostaedi et al. 2013), wheat straw (Pehlivan et al. 2012), olive pomace and stone (Martin-Lara et al. 2012), etc.

In general, the evaluation of biosorption in the quinary system is more complicated than a single system due to the competitive interactions between heavy metal ions for the biosorbent sites. Multi-metal biosorption investigations are significant to evaluate the level of interference caused by co-ions available in the water and wastewater. Several studies have been conducted about the selectivity sequences of heavy metal ions in multi-component systems using different biosorbents; for example, biosorption of Pb²⁺, Cu²⁺ and Ni²⁺ by native grapefruit (Bayo 2012), biosorption of Cu²⁺, Pb²⁺, Zn²⁺ and Cd²⁺ by cabbage waste (Hossain et al. 2014), biosorption of Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, and Zn²⁺ by black gram husk (Saeed et al. 2005), and the biosorption of Cu²⁺ and Ni²⁺ by peat (Ho & McKay 1999a).

For the first time by this study, Abies bornmulleriana cones were utilised to define the biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Co²⁺, and Ni²⁺ metal ions in the quinary system. The present investigation aimed to evaluate the ability of Abies bornmulleriana cone to treat multi-metal ions in the quinary system taking into account the competitive interactions of heavy metal ions. The influence of the various experimental parameters, including multi-metal ion concentration, pH, temperature, biosorbent quantity, particle size, and different salt types on the uptake of heavy metal ions was researched. Besides, the mechanism of multi-metal removal was illustrated in terms of FTIR results. Electrophoretic mobilities of Abies bornmulleriana cones were determined to gain information about the biosorption process. Pore volume and BET surface area of Abies bornmulleriana cones before and after the biosorption were defined to interpret the biosorption mechanism. Different kinetic studies were practiced to explain the experimental data. The selectivity of heavy metal ions in the competitive biosorption resulted in the magnitude order of Pb²⁺ > Cu²⁺ > Cd²⁺ > Ni²⁺> Co²⁺.

Abies bornmulleriana cones were obtained from Azdavay district of Kastamonu province in Turkey. The cones collected in August of 2016 were washed more than once with deionized water so that the ordinary impurities on the biosorbent surface could be removed. Afterwards, biosorbent particles were dried in the open air during 120 hours and were broken into small pieces, ground in a blender, and screened to classify by various particle sizes (0.25–0.250, 0.250–0.5, 0.5–1, and 1–2 mm).

All chemicals used in this investigation were of analytical grade from Sigma-Aldrich (USA) and were used as obtained. The stock solutions containing 200 mg L⁻¹ of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions were prepared by dissolving lead, copper, cadmium, nickel and cobalt nitrate salt, Pb²⁺ [(Pb(NO₃)₂], Cu²⁺ [Cu(NO₃)₂.3H₂O], Cd²⁺[Cd(NO₃)₂.4H₂O], Ni²⁺[Ni(NO₃)_2.6H2O], and Co²⁺ [Co(NO3)₂.6H2O]. Additionally, to evaluate the influence of Ca²⁺, Na⁺ and K⁺ ions on Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺ and Co²⁺ biosorption, 200 mg L⁻¹ analytical grade stock solutions of Ca²⁺[(CaNO₃)], Na⁺[(NaNO₃)] and K⁺[(KNO₃)] were prepared and diluted to the desired concentration of 0.01 M. Nitrate salt of the metal ions was utilised as the counter ion because it has a low tendency to produce metal complexes All the heavy metal ions exist in their free form. The Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ solutions were prepared by diluting a stock solution of 200 mg L⁻¹ with pure water to a required concentration range between 20 and 60 mg L⁻¹.

Biosorption investigations for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions in the multi-system were conducted at the concentrations of 20, 30, 40, 50 and 60 mg L⁻¹ to obtain the biosorption efficiencies and capacities. The pH of the solution was maintained by the addition of 0.1 M NH₃ and 0.1 M HNO₃. A known dosage of biosorbent (100 mg) was added to a series of 100 mL Erlenmeyer flasks containing 50 ML of heavy metal ions, which were located in the temperature-controlled shaker (Edmund Bühler Incubator HoodTH15) during 1 h. After equilibrium, filter paper (Whatman No. 42) was utilised to separate the biosorbent particles from solutions, and the final concentration of the ions was defined by ICP-MS (Agilent 7800).

The cones surface area and pore volume were defined using a Quantachrome QS-17 model apparatus 35. Barrett-Joyner-Halenda (BJH) method was utilised to estimate the pore size distribution of the biosorbent particles. The total pore volume of the cones was defined at a relative pressure of 0.99. BET surface area and pore volume of cones before and after competitive biosorption were determined as (5.05 m² g⁻¹ and 0.0018 cm³ g⁻¹) and (0.97 m² g⁻¹ and 0.00032 cm³ g⁻¹), respectively. The average pore width of the biosorbent before and after competitive biosorption was calculated to be 9.34 and 13.04 Å, respectively.

A Perkin-Elmer 1720 spectrometer was utilised to take down Fourier transform infrared (FTIR) spectra of Abies bornmulleriana cones. In this investigation, the target of FTIR analysis is the observation of whether Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ heavy metal ions adsorbed on Abies bornmulleriana cones alter the peaks of functional groups on the biosorbent.

The morphologic structure of the biosorbent surface was observed by using a scanning electron microscope with EDS spectroscopy (FE-SEM: Zeiss Sigma 300, USA).

Zeta potentials of Abies bornmulleriana cones were obtained to access advanced knowledge on the multi-component biosorption. The zeta potential measurements were carried out using a micro-electrophoresis cell (ZETAMETER 3:0-542, USA).

N₂ adsorption/desorption isotherms of Abies bornmulleriana cones before and after biosorption at 77 K are given in Figure 1(a). It is seen that the first part of the isotherms takes place at a lower relative pressure (P/P^o). The uptake of N₂, which increased quickly with the increase in the relative pressure, confirmed the availability of the meso and macropore structures. BJH technique was utilised to estimate the pore size distribution and volume of the biosorbents, and they are demonstrated in Figure 1(b). In Figure 1(b), the biosorbent before biosorption included macro and mesoporous volumes while the biosorbent after the biosorption included mainly macroporous volumes, as seen in Figure 1(b). The mesoporous volume of the biosorbent considerably decreased at the pore size of 20 nm. In contrast, the other mesoporous and macroporous volumes of the biosorbent generally increased after the biosorption, as presented in Figure 1(b). The BET surface area and pore volume of the biosorbent before and after the biosorption were defined to be (5.05 m² g⁻¹ and 0.0018 cm³ g⁻¹) and (0.97 m² g⁻¹ and 0.00032 cm³ g⁻¹), respectively. The average pore width of the biosorbent before and after the biosorption was calculated to be 9.34 and 13.04 Å, respectively.

FTIR spectroscopy was used to interpret the interchanges in the major functional groups of the biosorbent, and the interchanges are given in Figure 1(c). The FTIR spectra of Abies bornmulleriana cones describe many bands in the structure of the biosorbent. The changes in wavenumber and band intensity after the biosorption point out the functionalities of the biosorbent surface. Figure 1(c) indicates that some functional groups on the biosorbent are inclined to bind with the heavy metal ions. For instance, the peak at 3,139 cm⁻¹ was due to the vibration of the C-H stretching in 3-substituted pyrroles on the biosorbent. This peak shifted to 3,141 cm⁻¹ after the uptake of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions and shrank.

The peaks at 2,928 and 2,856 cm⁻¹ were related to OH stretching in the carboxylic acid groups and shifted to 2,929 and 2,858 cm⁻¹ after the biosorption. These changes in the biosorption bands illustrate that OH in the carboxylic acid was affected by binding with Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions and shrank after the biosorption. The peaks appearing at 1,637 and 1,516 cm⁻¹ were associated with NH stretching vibration in the secondary amides (-CONH-) and shifted to 1,638 and 1,515 cm⁻¹ after the biosorption. The peaks at 1,262, 1,060 and 872 cm⁻¹ were related to aryl O stretching in the aromatic ethers, asymmetric C-O-C stretching in the aliphatic ethers, and symmetric C-O-C stretching in the vinyl ethers and shifted to 1,267, 1,064 and 874 cm⁻¹ after the biosorption. The peak at 616 cm⁻¹ was related to N-C = S deformation in the tertiary thioamides and shifted to 619 cm⁻¹ and shrank after the biosorption. It is estimated that there is an electrostatic and chemical attraction between the biosorbent sites and the metal ions. This interaction altered the charges on the biosorbent. Thus, the FTIR spectra of the functional groups on the biosorbent modified.

Figure 2(a) and 2(b) show the surface morphology of Abies bornmulleriana cones before and after the biosorption, respectively. The biosorbent not containing heavy metal ions (Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺) appears to have an intact structure with groove rooms and an irregular surface. After the competitive biosorption, the pores were blocked and saturated due to the adsorption of the heavy metal ions (Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺). Moreover, the cone's initial structure disappeared due to the metal ions' accumulation. Figure 2(c) shows the chemical composition of the cones before biosorption. This is qualitatively and quantitatively determined by the EDS analysis and given in Figure 2(c). The EDS spectrum confirmed that the cones are chemically composed of carbon, oxygen, and potassium. After the biosorption, the amount (wt.%) of C, O and K decreased from (52.2, 45.7, and 2.1 (wt.%)) to (50.4, 45.2, and 0 (wt.%)). The heavy metals (Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺) were qualitatively determined by EDS analysis and are shown in Figure 2(d). This condition suggested that the biosorption of the metal ions might include an ion-exchange mechanism.

The biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions according to the concentration are given in Figure 3(a). The experimental investigations were conducted using mixed solutions (20–60 mg L⁻¹). The biosorption efficiency decreased with the increase in the concentration. It was considered that the active sites saturated on the biosorbent caused a reduction in the biosorption efficiency. But, the number of metal ions removed per unit mass of the biosorbent increased with the increase in concentration. The ratio of the number of metal ions to the present sites augmented due to the augmentation of the driving force between the metal ions and solid phase (Wang et al. 2018). The increase in the biosorption capacity could arise from the increase in electrostatic interaction between the metal ions and sites (Alasheh & Duvnjak 1995). The maximum biosorption efficiency and the minimum biosorption capacity for the metal ions were obtained when the metal ion concentration was 20 mg L⁻¹. After a biosorption contact time of 60 min, the biosorption efficiency and capacity for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions were defined as (95, 83.2, 52.3, 40.8 and 33%) and (4.8, 4.15, 2.61, 2.1 and 1.7 mg g⁻¹), respectively. But, the minimum biosorption efficiency and the maximum biosorption capacity for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions were obtained as (76.1, 83, 43.6, 25.3, 15.6 and 13.6%) and (11.4, 6.4, 3.8, 2.4 and 2 mg g⁻¹), respectively when the concentration of each ion was 60 mg L⁻¹. The maximum biosorption efficiency and capacity for the metal ions resulted in the magnitude order of Pb²⁺ > Cu²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺.

A possible interpretation for the outcomes is that the ionic radius of Pb²⁺ is bigger than others. The ionic radius of metal ions is in the magnitude order of Pb²⁺ (1.19 Å) > Cd²⁺ (0.95 Å) > Cu²⁺ (0.73 Å) > Ni²⁺ (0.71 Å) > Co²⁺ (0.65 Å). It is thought that the biosorbent sites have a stronger physical affinity for Pb²⁺ ions. Due to Brownian motion, the possibility of collision between Pb²⁺ ions and the biosorbent sites increased with the increase of metal ionic radius. Thus, the biosorption capacity related to Pb²⁺ increased. Similar results published in the literature concerning the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ on the different biosorbents show that Pb²⁺ ion has a higher biosorption affinity than the other metal ions in multi-ion systems (Nourbakhsh et al. 2002; Hawari & Mulligan 2007; Bayo 2012).

In this study, it was defined that the biosorption efficiency and capacity for Cu²⁺ ion were bigger than those of Cd²⁺ ions. Therefore, the selectivity order did not correlate precisely the ionic radius order of the metal ions. The electronegativity (1.95 Pauling scale) of Cu²⁺ ion is higher than the electronegativity (1.69 Pauling scale) of Cd²⁺ ion. But, the hydrated radius (4.19 Å) of Cu²⁺ ion is smaller than the hydrated radius (4.26 Å) of Cd²⁺ ion. It was thought that these two criteria were more effective than the magnitude order of the ionic radius of both metal ions. In the competitive biosorption, the complicated interactions, including ionic radius, hydrated radius, the electron affinity of the ions, and functional properties of the biosorbent, would affect removal of the Cd²⁺ ions, and almost suppressed its biosorption capacity. The metal ions with higher electronegativity are strongly attracted by the biosorbent. Additionally, it is thought that the functional groups (OH, -CONH- and C-O-C) availability on the biosorbent have more affinity to bind Cu²⁺ ions than Cd²⁺ metal ions.

A series of contact times from 0 to 60 minutes for the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions was examined at 40 mg L⁻¹ and 20 °C. It is seen from Figure 3(b) that the biosorption efficiency and capacity of metal ions increased very fast within the first 20 minutes due to the sufficient availability of binding sites. However, an increase over 20 minutes describes an inconsiderable improvement in the biosorption performance. The biosorption approached the equilibrium state within approximately 60 min. The biosorption efficiency and capacity of metal ions at various contact times are shown in Figure 3(b). The biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions for a time of 60 minutes were determined as (85.5, 56.4, 35.4, 21.8 and 18.9%) and (8.5, 5.6, 3.5, 2.2 and 1.9 mg g⁻¹), respectively. The variation in the biosorption of the metal ions was interpreted with the variation in the ionic size of the metal ions and the nature of active groups on the biosorbent surface (Okieimen et al. 1985). The findings were in good agreement with the first researches, in which biosorption took place between 10 and 20 min. (Bourliva et al. 2013). Extra progress in the time did not end in an excessive increase in the biosorption efficiency and capacity.

The influence of the biosorbent quantity (BQ) on efficiency and capacity was examined at the various quantities ranging from 0.25 to 0.75 g L⁻¹, and the conclusions were given in Figure 3(c). The efficiency of biosorption for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions increased with the increase of biosorbent quantity while the biosorbent capacities of these ions decreased. The increase in the efficiency arising from the biosorbent quantity caused a rise in the surface area and binding sites of the biosorbent. Thus, the efficiency of biosorption for the metal ions augmented, respectively. The maximum biosorption efficiency was obtained at the biosorbent amount of 300 mg. Besides, as shown from Figure 3(c), the quantity of metal ions removed per unit biosorbent decreased with the increase of biosorbent quantity. It was thought that the low concentration of metal ions caused the low biosorption capacity, as seen in Figure 3(c). Additionally, the high biosorbent quantity enhanced the possibility of collision between the biosorbent particles. Thus, the aggregation of the biosorbent particles induced a reduction in the biosorbent surface area and an expansion in the diffusional pathway. It was thought that these reasons contributed to a decrease in biosorption capacities (Chen et al. 2012). After a time of 60 min, the minimum biosorption efficiency and maximum biosorption capacity for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions were obtained as (73.4, 40.3, 15.1, 13.1 and 6.8%) and (14.7, 8.1, 3.1, 2.6 and 1.4 mg g⁻¹) respectively. In contrast, the maximum biosorption efficiency and the minimum biosorption capacity for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co⁺² metal ions were defined as (93.6, 73.6, 31.1, 26.4 and 20.9%) and (6.2, 4.9, 2.1, 1.7 and 1.4 mg g⁻¹), respectively.

The influence of particle size (P.S) on the biosorption efficiency and capacity were studied in the range of 0.125–0.25 to 1–2 mm. The biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions increased with the decrease of particle size, as given in Figure 4(a). It was thought that this increase was due to the high mass transfer in the particles of small size. The biosorption efficiency and capacity for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions decreased from (91.8, 64.8, 40.1, 27.1 and 23.1%) and (9.2, 6.5, 4.1, 2.7 and 2.3 mg g⁻¹) to (64.2, 35.7, 19.7,13.1 and 7.9%) and (6.4, 3.6, 1.9, 1.3 and 0.8 mg g⁻¹), respectively when the particle size increased from 0.125–0.250 to 1–2 mm. The BET surface area and pore volume of the biosorbent before and after the biosorption were defined to be (5.05 m² g⁻¹ and 0.0018 cm³ g⁻¹) and (0.97 m² g⁻¹ and 0.00032 cm³ g⁻¹), respectively. The average pore width of the biosorbent before and after the biosorption was calculated to be 9.34 and 13.04 Å, respectively. The decrease in the surface area and pore volume of the biosorbent and the increase in the particle size after biosorption emphasizes that the metal ions were retained on the biosorbent.

The removal of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co⁺² metal ions were researched at the various pH values ranging from 3 to 5.8, and conclusions are given in Figure 4(b). The experimental studies were not conducted above pH 5.8 to avoid any metal hydroxide precipitation. It is known that metal ions [M(II) = Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺] exist in species of M²⁺, M(OH)⁺and M(OH)_2(s) in deionised water. The biosorption efficiency and capacity of the metal ions augmented when pH augmented from 3 to 5.8. In general, the biosorption efficiency and capacity displayed a downward trend with the decrease of pH. The abundance of H⁺ ions in the low pH values surpasses the number of metal ions, and the biosorbent surface is controlled by H⁺ ions (Sen Gupta & Bhattacharyya 2008). More H⁺ ions leaving the biosorbent with the increase of pH make the biosorbent sites more suitable for the metal ions, which are bound on the biosorbent surface by the ion exchange mechanism (H⁺/Pb²⁺, H⁺/Cu²⁺, H⁺/Cd²⁺, H⁺/Ni²⁺and H⁺/Co²⁺) (Jiang et al. 2010; Bourliva et al. 2015). Biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions increased from (48.8, 20.1, 14.7, 11.3 and 5.6%) and (4.8, 2.1, 1.5, 1.2 and 0.6 2.3 mg g⁻¹) to (85.4, 56.4, 35.4, 21.7 and 18.9%) and (8.5, 5.6, 3.5, 2.2 and 1.9 mg g⁻¹), respectively with the increase of pH from 2 to 5.8.

In Figure 4(c), zeta potentials of the biosorbent particles at pH 3, 4, 5, and 5.8 in deionised water were distinct from those of the biosorbent particles containing Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions. The changes in the zeta potential of the charged biosorbents show the uptake of the metal ions from the quinary system. It was thought that Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions attracted by the loads on the biosorbent were adsorbed on the biosorbent surface. Thus, a positive increase in the zeta potentials occurred, as seen in Figure 4(c). The variations in the zeta potentials were evidence of the biosorption of metal ions.

Influence of the temperature on the removal of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions was investigated at temperatures ranging from 20 to 50 °C and are presented in Figure 5(a). The biosorption efficiency and capacity for the metal ions augmented with the augmentation of the temperature. This result is reasonably assigned to the swelling impact in the internal structure of the biosorbent due to the high temperatures. It was thought that a specific increase in the temperature facilitated the penetration of metal ions through the biosorbent pores to bind the functional groups on the biosorbent (Zhang et al. 2016). The biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions increased from (85.4, 56.3, 35.4, 21,7 and 18.9%) and (8.5, 5.6, 3.5, 2.2, 1.9 mg g⁻¹) to (92.2, 68.4, 47.6, 31.2 and 26.9%) and (9.2, 6.8, 4.3, 3.1 and 2.7 mg g⁻¹), respectively with the increase of temperature from 20 to 50 °C.

The effect of various salts having a concentration of 0.01 M on the biosorption was examined to understand the importance of ionic strength, and findings are presented in Figure 5(b). The decrease of the biosorption capacity for the metal ions displays that biosorption of the metal ions depends on ionic strength. It was believed that the salt addition varied the electrical double layer feature. This process limited the metal ions' transfer from solution to the biosorbent surface (Krishnan & Anirudhan 2002). Moreover, the salt ions compete with Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co⁺² species for binding to the biosorbent sites. Their binding to the biosorbent sites causes a reduction in Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ biosorption. Furthermore, the various types of salt also exhibit various effects on the biosorption of metal ions at equal salt concentration. Ca²⁺ ions have twice the ionic strength compared to Na⁺ and K⁺ ions. Thus, its impact on the electrical double layer in the solution is more effective than Na⁺ and K⁺. However, the Na⁺ and K⁺ ions, having an equal number of charges, display a similar influence on the electrical double layer. The ionic radius is effective in controlling the competitive biosorption. The average pore width of the biosorbent used in this study was determined to be 9.34 Å, which is bigger than the ionic radius of the salt ions. Na⁺ ions, having higher ionic diameter than K⁺ ions, can more easily compete with the metal ions for the sites present on the biosorbent. As a consequence, the addition of the different types of salts into the solution reduced the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ ions in the smallness order of K⁺ < Na⁺ <Ca²⁺ as shown in Figure 5(b). The biosorption efficiency and capacity of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions in the presence of Ca²⁺, Na⁺ and K⁺ ions were determined as [(80.2, 49.3, 11.5, 9.5 and 5.8%) and (8.1, 4.9, 1.2, 0.9 and 0.6 mg g⁻¹)], [(84.5, 55.6, 17.4, 13.6 and 11.6%) and (8.5, 5.6, 1.7, 1.4 and 1.2 mg g⁻¹)] and [(89.2, 69.1, 37.1, 34.9 and 32.4%) and (8.9, 6.9, 3.7, 3.5 and 3.2 mg g⁻¹)], respectively when the salt concentration for Ca²⁺, Na⁺ and K⁺ was 0.01 M.

Isotherm models are the equilibrium relations between the concentration of biosorbate on the biosorbent and its concentration in the liquid phase. Isotherm constants ensure the knowledge of the ability of the biosorbent or the quantity needed to uptake a unit mass of heavy metal ions under the experimental conditions. The experimental data were adapted to four different isotherms: Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich.

D-R constants and E values are given in Table 1. The size of E is vital for determining the character of biosorption. E values estimated for Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions were 14.43, 15.67, 14.14, 16.36, and 15.47 kJ mol⁻¹, respectively, which are between 8 and 16 kJ mol⁻¹. E values support that the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions occurred by an ion-exchange mechanism. The parameters related to Dubinin-Radushkevich isotherm are given in Table 1. The D-R isotherm for the biosorption was determined as an adaptable isotherm due to the high correlation coefficients (R²), as seen in Table 1.

R² values related to the pseudo-first-order equation are shown in Figure 6. R² values obtained from the pseudo-first-order equation for all the heavy metal ions were low. Thus, the biosorption did not fit to the pseudo-first-order model.

R² values related to the pseudo-second-order equation for all heavy metal ions were approximately 0.99. Thus, the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions was most adequately interpreted by the pseudo-second-order equation, as seen in Figure 6. The linear lines in the plots display an excellent collaboration among the experimental data. R² values related to the pseudo-second-order model are exhibited in Figure 6.

The pseudo-first-order model did not provide an excellent fit related to the experimental data. However, the pseudo-second-order model had the most suitable correlation value (R² 0.99) for all of the metal ions. The R² values indicate that the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions follows pseudo-second-order kinetic.

The mesoporous volume of the biosorbents decreased at the pore size of 20 nm while the other mesoporous and macroporous volumes of the biosorbents increased after the biosorption. The BET surface area and pore volume of the biosorbents decreased after the biosorption. The average pore width of the biosorbent after the biosorption was calculated as 9 13.04 Å. The changes in wavenumber and band intensity of FTIR spectrums related to the biosorbent after the biosorption point out the functionalities of the biosorbent surface.

The biosorption efficiency was influenced by surface loading regardless of the kind of ions, and the biosorption efficiencies decreased with the increase of metal ion concentration. But, the quantity of metal ions removed per unit mass of the biosorbent increased with the increase of the multi-metal concentration. The biosorption efficiency and capacity of the metal ions increased very fast within the first 20 minutes due to the sufficient availability of binding sites. However, an increase over 20 minutes emphasizes an inconsiderable improvement in the biosorption performance and the approach to the equilibrium state. The biosorption efficiency increased with the increase of biosorbent quantity while the biosorbent capacity decreased. The biosorption efficiency and capacity of the metal ions increased with the decrease of particle size. The biosorption efficiency and capacity of the metal ions augmented with the augmentation of the pH from 3 to 5.8. The changes in the zeta potential of the charged biosorbents exhibit the removal of metal ions. The biosorption efficiency and capacity of the metal ions increased with the increase of temperature. The biosorption capacity decreased with the availability of the salt concentration, which confirmed that the biosorption depended on ionic strength. The biosorption efficiency and capacity for the metal ions followed the magnitude order of Pb²⁺ > Cu²⁺ > Cd²⁺ > Ni²⁺ > Co²⁺.

The D-R isotherm for the biosorption was determined as an adaptable isotherm model due to the high (R²) values. Isotherm studies suggested that the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ metal ions could not involve various isotherm models. For all the ions, the correlation coefficients estimated from the pseudo-second-order equation were approximately 0.99. Thus, the biosorption of Pb²⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Co²⁺ heavy metal ions is most appropriately represented by this model.

Author is grateful to the research council of Atatürk University for providing financial support under the project no: FHD/2019/7080. In addition, I would also like to thank East Anatolia High Technology Application and Research Center (DAYTAM) for their support in the heavy metal analysis.

All relevant data are included in the paper or its Supplementary Information.