Biosorption of Cadmium and Lead by Dry Biomass of Nostoc sp. MK-11: Kinetic and Isotherm Study

Cadmium (Cd) and lead (Pb) are global environmental pollutants. In this study, Nostoc sp. MK-11 was used as an environmentally safe, economical, and efficient biosorbent for the removal of Cd and Pb ions from synthetic aqueous solutions. Nostoc sp. MK-11 was identified on a morphological and molecular basis using light microscopic, 16S rRNA sequences and phylogenetic analysis. Batch experiments were performed to determine the most significant factors for the removal of Cd and Pb ions from the synthetic aqueous solutions using dry Nostoc sp. MK1 biomass. The results indicated that the maximum biosorption of Pb and Cd ions was found under the conditions of 1 g of dry Nostoc sp. MK-11 biomass, 100 mg/L of initial metal concentrations, and 60 min contact time at pH 4 and 5 for Pb and Cd, respectively. Dry Nostoc sp. MK-11 biomass samples before and after biosorption were characterized using FTIR and SEM. A kinetic study showed that a pseudo second order kinetic model was well fitted rather than the pseudo first order. Three isotherm models Freundlich, Langmuir, and Temkin were used to explain the biosorption isotherms of metal ions by Nostoc sp. MK-11 dry biomass. Langmuir isotherm, which explains the existence of monolayer adsorption, fitted well to the biosorption process. Considering the Langmuir isotherm model, the maximum biosorption capacity (qmax) of Nostoc sp. MK-11 dry biomass was calculated as 75.757 and 83.963 mg g−1 for Cd and Pb, respectively, which showed agreement with the obtained experimental values. Desorption investigations were carried out to evaluate the reusability of the biomass and the recovery of the metal ions. It was found that the desorption of Cd and Pb was above 90%. The dry biomass of Nostoc sp. MK-11 was proven to be efficient and cost-effective for removing Cd and especially Pb metal ions from the aqueous solutions, and the process is eco-friendly, feasible, and reliable.


Introduction
Heavy metal pollution is a severe global issue because of its non-biocompatibility, concealment, and high toxicity [1]. In general, mining, metal manufacturing, electronic electroplating, agricultural activities, and wastewater discharge from metallurgy are the major anthropogenic factors that contribute to the rising level of harmful metals in the environment [2][3][4]. A long-term exposure to heavy metals can cause irreversible damage to many biological systems [5].  Figure 2 depicts the FTIR spectra of Nostoc sp. MK-11 biomass before and after the biosorption of the Pb and Cd ions. The surface of the biosorbent consisted of numerous functional groups. The core absorption bands in the cyanobacterial biomass at 3525 cm −1 are related O-H and N-H stretching in the hydroxyl and amine functional groups, respectively. The stretching distribution of CH2 and CH3 is displayed by the bands between 2800 and 3000 cm −1 of the symmetric and asymmetric aliphatic chain (C-H) stretching vibrations. The band at 1652 cm −1 showed the C=O functional group's asymmetric vibration and the presence of the SO (sulfoxide group). In general, biomass characterization is helpful for deciphering the process of metal binding on the surface of biomass and assessing the efficacy of biosorbents for use in the biosorption process [25]. The FTIR results suggest that the dry biomass of Nostoc sp. MK-11 is useful for the biosorption of Cd and Pb ions. Figure 2 depicts peak shifting after Cd and Pb ions biosorption. The shifting of the peaks related to functional groups such as hydroxyl, amines, sulfoxide, and carbonyl is an indication of an interaction between metals and the biomass functional groups, which confirms the involvement of these functional groups in the removal of Pb and Cd ions from the aqueous solution. Similar FTIR results were described in the previous studies, which presented the involvement of functional groups in the removal of metals by the dry biomass of algae [26][27][28][29].  Figure 2 depicts the FTIR spectra of Nostoc sp. MK-11 biomass before and after the biosorption of the Pb and Cd ions. The surface of the biosorbent consisted of numerous functional groups. The core absorption bands in the cyanobacterial biomass at 3525 cm −1 are related O-H and N-H stretching in the hydroxyl and amine functional groups, respectively. The stretching distribution of CH 2 and CH 3 is displayed by the bands between 2800 and 3000 cm −1 of the symmetric and asymmetric aliphatic chain (C-H) stretching vibrations. The band at 1652 cm −1 showed the C=O functional group's asymmetric vibration and the presence of the SO (sulfoxide group). In general, biomass characterization is helpful for deciphering the process of metal binding on the surface of biomass and assessing the efficacy of biosorbents for use in the biosorption process [25]. The FTIR results suggest that the dry biomass of Nostoc sp. MK-11 is useful for the biosorption of Cd and Pb ions. Figure 2 depicts peak shifting after Cd and Pb ions biosorption. The shifting of the peaks related to functional groups such as hydroxyl, amines, sulfoxide, and carbonyl is an indication of an interaction between metals and the biomass functional groups, which confirms the involvement of these functional groups in the removal of Pb and Cd ions from the aqueous solution. Similar FTIR results were described in the previous studies, which presented the involvement of functional groups in the removal of metals by the dry biomass of algae [26][27][28][29].

SEM
The morphology of non-treated and Cd and Pb treated biomass was investigated using SEM.
As shown in Figure 3A, the biomass surface appeared dull and smooth before the biosorption of Cd and Pb ions. However, the biosorbent surface changed after the biosorp-

SEM
The morphology of non-treated and Cd and Pb treated biomass was investigated using SEM. As shown in Figure 3A, the biomass surface appeared dull and smooth before the biosorption of Cd and Pb ions. However, the biosorbent surface changed after the biosorption process ( Figure 3B,C); it became brilliant, rough, and sharp ended. These observed changes in the biomass morphology were caused by the precipitation of Cd and Pb ions onto the surface of the dried biomass. Similar results were described for Parachlorella sp. when Cd was removed from the aqueous solution [30]. The changes in the morphology of the biomass may be credited to the cross-linking of functional groups and metal ions [25].

SEM
The morphology of non-treated and Cd and Pb treated biomass was investigated using SEM.
As shown in Figure 3A, the biomass surface appeared dull and smooth before the biosorption of Cd and Pb ions. However, the biosorbent surface changed after the biosorption process ( Figure 3B,C); it became brilliant, rough, and sharp ended. These observed changes in the biomass morphology were caused by the precipitation of Cd and Pb ions onto the surface of the dried biomass. Similar results were described for Parachlorella sp. when Cd was removed from the aqueous solution [30]. The changes in the morphology of the biomass may be credited to the cross-linking of functional groups and metal ions [25].

Effect of pH
The biomass of algae possesses a rich carboxyl group content from guluronic and mannuronic acids on the polysaccharide cell wall, which suggests that variation in the pH of the solution could affect the biosorption [31]. Figure 4A depicts the pH effect on the biosorption of Pb and Cd ions onto Nostoc sp. MK-11 dry biomass. The highest biosorp-

Effect of pH
The biomass of algae possesses a rich carboxyl group content from guluronic and mannuronic acids on the polysaccharide cell wall, which suggests that variation in the pH of the solution could affect the biosorption [31]. Figure 4A depicts the pH effect on the biosorption of Pb and Cd ions onto Nostoc sp. MK-11 dry biomass. The highest biosorption was found at pH 5 for Cd and at pH 4 for Pb. Therefore, all further experiments were conducted at pH 4 and 5. Metal biosorption began to decline when the pH level crossed from 5 for Cd and 4 for Pb, as shown in Figure 4A. At a low pH (2-3), a rise in the positive charge on the biomass surface restricted the metal cations approach because of the repulsion force. A decrease in the biosorption at a low pH was due to a higher number of hydrogen ions, which form ligands with metal ions at the biosorbent surface [32]. According to the findings of this study, the sorption of Cd and Pb ions occurred at two different pH values, which is due to differences in the metal properties such as the electronegativity, size, or the availability of extra metal ions. charge on the biomass surface restricted the metal cations approach because of the repulsion force. A decrease in the biosorption at a low pH was due to a higher number of hydrogen ions, which form ligands with metal ions at the biosorbent surface [32]. According to the findings of this study, the sorption of Cd and Pb ions occurred at two different pH values, which is due to differences in the metal properties such as the electronegativity, size, or the availability of extra metal ions.

Effect of Contact Time
One of the most essential components in the biosorption process is contact time. Figure 4B represents the biosorption of Cd and Pb ions as a function of the contact time and describes how biosorption increases up to 60 min contact time. After that, a gradual decrease in metal ion biosorption occurred. The decrease in the biosorption after the 60 min contact time may be due to the discharge of metal ions from the surface of the biomass into the solution [33]. Consequently, 60 min was chosen as the optimum contact time for further investigations. The findings of our study are consistent with those of an earlier study in which the maximum biosorption of lead and cadmium occurred at 60 min of contact time [34,35]. Figure 4C represents the effects of initial metal concentrations (ranging from 20 mgL −1 to 120 mgL −1 ) on the biosorption of Pb and Cd ions by dry Nostoc sp. MK-11 biomass. The biomass showed an improved ability to absorb Cd and Pb ions up to 100 mgL −1 . The rise in the metal concentrations enhances the metal ions driving force from the bulk solution to the biomass and improves the biosorption capacity [29]. However, at 120 mgL −1 of metal ion concentrations, a slight decrease in the biosorption capacity of the biomass occurred. It might be brought on by more metal ions remaining unabsorbed in

Effect of Contact Time
One of the most essential components in the biosorption process is contact time. Figure 4B represents the biosorption of Cd and Pb ions as a function of the contact time and describes how biosorption increases up to 60 min contact time. After that, a gradual decrease in metal ion biosorption occurred. The decrease in the biosorption after the 60 min contact time may be due to the discharge of metal ions from the surface of the biomass into the solution [33]. Consequently, 60 min was chosen as the optimum contact time for further investigations. The findings of our study are consistent with those of an earlier study in which the maximum biosorption of lead and cadmium occurred at 60 min of contact time [34,35]. Figure 4C represents the effects of initial metal concentrations (ranging from 20 mgL −1 to 120 mgL −1 ) on the biosorption of Pb and Cd ions by dry Nostoc sp. MK-11 biomass. The biomass showed an improved ability to absorb Cd and Pb ions up to 100 mgL −1 . The rise in the metal concentrations enhances the metal ions driving force from the bulk solution to the biomass and improves the biosorption capacity [29]. However, at 120 mgL −1 of metal ion concentrations, a slight decrease in the biosorption capacity of the biomass occurred. It might be brought on by more metal ions remaining unabsorbed in the solution as the biomass binding sites become saturated at higher ion concentrations. This agrees with the results of previously described studies [36,37].

Effect of Biomass Dosage
The biomass dosage effect was investigated using biomass in the range of 0.5 to 2.5 g L −1 ( Figure 4D). The results show that metal ion biosorption hinges on the dosage of biomass in the solution. Maximum biosorption of the Cd and Pb ions were obtained at 1 g/L of biomass in solution, and the biosorption efficiency decreased when the concentration of the biomass was more than 1 g L −1 . This might be because of aggregate formation at an increased biomass dosage, which ultimately decreases the surface area and metal binding sites. This finding is similar to the results of a previously described study in which the biosorption of metals enhanced as the dosage of the biomass increased [38,39].

Kinetic Study
It is quite interesting to use experimental data in kinetic studies to figure out mechanisms, such as chemical reactions and mass transfer, that influence the rate of biosorption [40,41]. To study the kinetics of biosorption, pseudo-first, and pseudo-second order models, (Figure 5A,B) were applied. L −1 ( Figure 4D). The results show that metal ion biosorption hinges on the dosage of biomass in the solution. Maximum biosorption of the Cd and Pb ions were obtained at 1 g/L of biomass in solution, and the biosorption efficiency decreased when the concentration of the biomass was more than 1 g L −1 . This might be because of aggregate formation at an increased biomass dosage, which ultimately decreases the surface area and metal binding sites. This finding is similar to the results of a previously described study in which the biosorption of metals enhanced as the dosage of the biomass increased [38,39].

Kinetic Study
It is quite interesting to use experimental data in kinetic studies to figure out mechanisms, such as chemical reactions and mass transfer, that influence the rate of biosorption [40,41]. To study the kinetics of biosorption, pseudo-first, and pseudo-second order models, (Figure 5A,B) were applied. Based on the calculated qe, experimental qe, and correlation coefficients (R 2 ), the rate of the Pb and Cd ions' biosorption was found to be well suited to pseudo-second order kinetics. The qe obtained from the pseudo-first order does not fit well to the qe experimental and the R 2 values are also not up to the mark (Table 1), which made it clear that the pseudo-first order does not fit to the biosorption data. Previous studies also showed similar results [42]. In pseudo-second order kinetic modelling, the experimental qe agree with the qe calculated, and R 2 is quite close to unity, as described in Table 1. It indicates that pseudo-second order is well suitable to the biosorption of Pb and Cd ions. The well-fitting of pseudo-second order shows that the rate of biosorption is limited by chemical sorption [41,43]. Previous studies used the dry biomass of Chlamydomonas reinhardtii and Oedogonium sp. to characterize the Cd and Pb biosorption kinetics using pseudo-second order [35,44]. Based on the calculated qe, experimental qe, and correlation coefficients (R 2 ), the rate of the Pb and Cd ions' biosorption was found to be well suited to pseudo-second order kinetics. The qe obtained from the pseudo-first order does not fit well to the qe experimental and the R 2 values are also not up to the mark (Table 1), which made it clear that the pseudo-first order does not fit to the biosorption data. Previous studies also showed similar results [42]. In pseudo-second order kinetic modelling, the experimental qe agree with the qe calculated, and R 2 is quite close to unity, as described in Table 1. It indicates that pseudo-second order is well suitable to the biosorption of Pb and Cd ions. The wellfitting of pseudo-second order shows that the rate of biosorption is limited by chemical sorption [41,43]. Previous studies used the dry biomass of Chlamydomonas reinhardtii and Oedogonium sp. to characterize the Cd and Pb biosorption kinetics using pseudo-second order [35,44].

Isotherm Study
The isotherm represents the correlation between the metal ion concentrations in the solution at equilibrium and the concentrations of metal ions adsorbed on the surface of the biomass. In the current study, three equilibrium isotherm studies, Langmuir, Freundlich, and Temkin, were performed ( Figure 6A-C).

Isotherm Study
The isotherm represents the correlation between the metal ion concentrations in the solution at equilibrium and the concentrations of metal ions adsorbed on the surface of the biomass. In the current study, three equilibrium isotherm studies, Langmuir, Freundlich, and Temkin, were performed ( Figure 6A-C).  Table 2 shows the computed parameters of isotherm models as well as their R 2 coefficient of correlation. Consequently, the R 2 values confirmed that the biosorption data followed Langmuir's isotherms. Langmuir's isotherm model calculated the qmaxas as 75.757 and 83.963 mg/g for Cd and Pb ions, respectively, which corresponded to the experimental qe 67.45 for Cd and 78 mg/g for Pb. The biosorption of Pb and Cd ions by Nostoc sp. MK-11 dry biomass took place as a homogeneous monolayer on the dry biomass surface [45,46]. Filamentous algae from the Chlorophyta (Cladophora calliceima, Pithophora oedogonium, Hydrodictyon reticulatum, and Spirogyra neglecta) and cyanobacteria (Aulosira fertilissima) non-living biomass have shown Langmuir isotherm fitting for Cd and Pb biosorption [32].  Table 2 shows the computed parameters of isotherm models as well as their R 2 coefficient of correlation. Consequently, the R 2 values confirmed that the biosorption data followed Langmuir's isotherms. Langmuir's isotherm model calculated the q maxas as 75.757 and 83.963 mg/g for Cd and Pb ions, respectively, which corresponded to the experimental qe 67.45 for Cd and 78 mg/g for Pb. The biosorption of Pb and Cd ions by Nostoc sp. MK-11 dry biomass took place as a homogeneous monolayer on the dry biomass surface [45,46]. Filamentous algae from the Chlorophyta (Cladophora calliceima, Pithophora oedogonium, Hydrodictyon reticulatum, and Spirogyra neglecta) and cyanobacteria (Aulosira fertilissima) non-living biomass have shown Langmuir isotherm fitting for Cd and Pb biosorption [32].

Desorption
It is important to assess the reusability of dried biomass to ensure that the biosorption process is economical. To determine the ratio of desorption to biosorption (D/B), three cycles of sorption and desorption tests were carried out. Changes in the desorption to biosorption ratio were used to evaluate the likelihood of reusing the dry biomass [47,48]. The metal biosorption results indicated that the Nostoc sp. MK-11 biomass was efficient for the biosorption of Pb and Cd ions. Using 0.1 M HCL, over 90% of both metals underwent desorption even though the sorption capability of the cyanobacterial biomass decreased after the first cycle ( Table 3).
The metal binding regions in the cyanobacterial cells may have been destroyed by the contaminated solution, which decreased the biosorption effectiveness during sorption and desorption. However, by using a suitable chemical agent, the process of regeneration might be enhanced without reducing the potential of the biosorbent [49]. Both the loss of functional groups and the mass of the biosorbent were most likely responsible for the abrupt drop. The surface functional groups of the biomass have been found to be destroyed by high acid solutions, which disrupted the process of desorption. However, it is important to carry out the desorption because it enables the recovery of absorbed metals using an easy approach (such as electrolysis) for other applications [50,51]. This demonstrated that Nostoc sp. MK-11 have the potential to be used as a dual agent for heavy metal removal from the environment and heavy metal recovery from biosorbent for useful industrial applications.

Cyanobacterial Strain and Biomass
The cyanobacterium, Nostoc sp. MK-11, was isolated from the polluted aqueous environment of Islamabad, Pakistan. The BG11 solid media [52] was prepared with 1.5% agar. In total, 20 mL of this media was poured in each Petri plate and allowed to solidify under UV illumination in the Laminar flow hood. A homogenized sample of approximately 500 µL was poured in each Petri plate and gently spread with a glass rod. The samples of inoculated Petri plates were stored at 25 ± 2 • C under continuous LED white light illumination with a 2000 lux intensity. After 10 days, the agar plates were observed under a light microscope and colonies were picked and reinoculated onto new agar Petri plates. After carefully picking, the axenic culture was cultivated in test tubes and conical flasks containing BG11 liquid media.
Morphological and molecular analysis were used for the identification of the isolated cyanobacterium. Morphological characters were observed using light microscopy. For a molecular identification, the genomic DNA was extracted using the CTAB method and a mortar and pestle. The sequences of the PCR-amplified DNA fragments encoding 16S ribosomal RNA region were achieved using cyanobacteria specific primers CYA 106 F as a forward primer and CYA 781 R (a) as a reverse primer [53]. NCBI BLAST-based similarity search and phylogenetic analysis were carried out during the molecular identification. Phylogenetic analyses were performed through the neighbor-joining method using MEGA-X software [54,55].
For producing biomass, the axenic cultures of the cyanobacterium were cultivated in multiple 100, 250, and 500 mL Erlenmeyer flasks containing 70, 200, and 350 mL of BG-11 media, respectively [52]. The cultures were kept at 25 ± 2 • C under 2000 lux white LED light. The cultures were harvested at the maximum growth by centrifugation at 4000 rpm for 15 min. After washing with double distilled water, the pellet was left to air dry. The dry biomass was crushed finely and sieved through a 100 µm mesh to achieve the amorphous state. The powdered form was then stored in a sealed bottle to avert rehydration.

Batch Experiments
In batch experiments, the effects of initial metal concentrations, solution pH, contact time, and biomass dose on the biosorption of the Pb and Cd ions from the synthetic aqueous solutions were studied. The effect of pH was studied with a contact time of 60 min, 1 g L −1 of biomass, and concentrations of metals 100 mg L −1 . The optimal contact time was found at 100 mg L −1 of metal concentrations, pH 4 and 5 for Pb and Cd, respectively, and 1 g L −1 of biomass. By adding NaOH or HCL, the liquids' initial pH was adjusted, and during the experiments, the pH of the solutions was not controlled. After optimizing the pH and contact time, the effect of the initial metal concentrations with a range of 20-120 mg L −1 was studied at 60 min contact time, 1g L −1 biomass at pH 4 and 5 for Pb and Cd ions, respectively. In the last study, the biomass impact on the biosorption of Pb and Cd ions was investigated at metal concentrations of 100 mgL −1 , pH 4 and 5, and contact time of 60 min.
Batch studies were conducted with 100 mL of 100 mg L −1 Pb and/or Cd ion concentrations in 250 mL Erlenmeyer flasks at pH 4 and 5 and mixed with a specific quantity of cyanobacterial biomass. Metals and biomass mixtures were stirred at 150 rpm using an orbital shaker for a predetermined duration of the contact time at 25 • C with three replications. The final and initial metal concentrations of the solutions were determined using a flame atomic absorption spectrometer (FAAS) (Agilent, Santa Clara, CA, USA) after the suspension's filtration with Whatman 40 ash-free paper. Equations (1) and (2) were used to calculate the biosorption capacity (mg/g) and efficiency (%) of biomass, respectively.
C i represents the initial concentration of the metals, C e is the metal concentrations at equilibrium (mg L −1 ), V represents the volume of the solution (L), and W shows the weight of the biomass (g).

Biosorbent Characterization
Dry cyanobacterial biomass (before and after biosorption) was characterized using Fourier transform infrared spectroscopy (FTIR) and scanning electron microscope (SEM). The surface functional groups of the biomass were identified through FTIR [56], and the morphology of the raw and metal-treated biomass was investigated using SEM (JSM5910 JEOL, Tokyo, Japan).

Kinetics Study
The biosorption rate of the Pb and Cd ions onto the Nostoc sp. MK-11 dry biomass was studied using pseudo-first and pseudo-second order kinetic models. Equation (3) represents the pseudo-first order as follows: K 1 is a pseudo-first order equilibrium rate constant, and qt is the biosorption capacity at time t.
The pseudo-second order is described in Equation (4) as follows: K 2 is the pseudo-second order rate constant determined by plotting t/q t vs. t.

Isotherm Study
The processes of the adsorption isotherm's features were assessed using the Langmuir, Freundlich, and Temkin isotherm models. The parameters of the isotherms were determined using the following equations: Langmuir isotherm equation: K L is Langmuir's isotherm constant, q max is the maximum adsorption capacity, and R L is the separation factor. Freundlich isotherm equation: K f shows the Freundlich's constant, and 1/n is the adsorption intensity. Temkin isotherm equation: A T is the Temkin equilibrium binding constant, b T shows the Temkin isotherm constant, R 2 is the coefficient of the correlation constant, B is a constant related to the heat of sorption, and T is the temperature.

Desorption Study
To discover the reusability potential of the biomass, biosorption-desorption experiments were carried out three times using the same preparations of dried biomass. In total, 0.1 M of hydrochloric acid (HCL) was used for the desorption of the metals. The desorption potential was investigated by shifting metal-loaded biomass to the desorption medium, and this was shaken at 100 rpm for 1 h at 25 • C. After each cycle of sorption and desorption, saline solution was used to wash and recondition the biomass [35]. FAAS was used to calculate the final concentration of metals in the aqueous phase, as explained above. The quantity of adsorbed metal ions onto the biomass surface and the final metals concentration in the sorption medium were used to compute the desorption ratio. Equation (7) was used to evaluate the efficiency of desorption (ED).
q D is the quantity of desorbed metal ions and q B is the amount of adsorbed metal ions.

Data Analysis
OriginPro 8.5 and Microsoft Office Excel (2010) software were used for the data analysis. The results were described as the mean of 3 (n = 3) replicates. The biosorption of the Pb and Cd ions employing Nostoc sp. MK-11 as a biosorbent is illustrated in a series of batch studies (Figure 7).

Data Analysis
OriginPro 8.5 and Microsoft Office Excel (2010) software were used for the data analysis. The results were described as the mean of 3 (n = 3) replicates. The biosorption of the Pb and Cd ions employing Nostoc sp. MK-11 as a biosorbent is illustrated in a series of batch studies (Figure 7).

Conclusions
Cadmium and lead are environmental and industrial heavy metal pollutants classified as human carcinogens. Using Nostoc sp. MK-11 dry biomass as a biosorbent, batch biosorption experiments were conducted to optimize the conditions for the removal of Pb and Cd ions from synthetic aqueous solutions. The optimum conditions obtained for the maximum removal of Cd and Pb ions were initial metal concentrations of 100 mg L −1 , Nostoc sp. MK-11 dry biomass of 1 g L −1 , a contact time of 60 min, and pH 4 and 5 for Pb and Cd, respectively. The primary groups involved in the biosorption of Cd and Pb ions have been identified as hydroxyl, amines, sulfoxide, and carbonyl. After the biosorption process, SEM micrographs of the Nostoc sp. MK-11 biomass show that it can adsorb and remove Pb and Cd ions from an aqueous environment. The biosorption of Cd and Pb ions onto Nostoc sp. MK-11 dry biomass followed the pseudo-second order kinetic model. The Langmuir isotherm mode was well fitted to biosorption equilibrium, which showed the homogenous monolayer biosorption of both metals onto the dry biomass of Nostoc sp. MK-11. The biomass maximum biosorption capacities were calculated as 75.757 and 83.963 mg −1 for Cd and Pb ions, respectively, by the Langmuir isotherm model. The desorption of Pb and Cd from the Nostoc sp. MK-11 dry biomass was greater than 90% using 0.1 M of HCL. The approach adopted for the biosorption of Cd and Pb is affordable,

Conclusions
Cadmium and lead are environmental and industrial heavy metal pollutants classified as human carcinogens. Using Nostoc sp. MK-11 dry biomass as a biosorbent, batch biosorption experiments were conducted to optimize the conditions for the removal of Pb and Cd ions from synthetic aqueous solutions. The optimum conditions obtained for the maximum removal of Cd and Pb ions were initial metal concentrations of 100 mg L −1 , Nostoc sp. MK-11 dry biomass of 1 g L −1 , a contact time of 60 min, and pH 4 and 5 for Pb and Cd, respectively. The primary groups involved in the biosorption of Cd and Pb ions have been identified as hydroxyl, amines, sulfoxide, and carbonyl. After the biosorption process, SEM micrographs of the Nostoc sp. MK-11 biomass show that it can adsorb and remove Pb and Cd ions from an aqueous environment. The biosorption of Cd and Pb ions onto Nostoc sp. MK-11 dry biomass followed the pseudo-second order kinetic model. The Langmuir isotherm mode was well fitted to biosorption equilibrium, which showed the homogenous monolayer biosorption of both metals onto the dry biomass of Nostoc sp. MK-11. The biomass maximum biosorption capacities were calculated as 75.757 and 83.963 mg −1 for Cd and Pb ions, respectively, by the Langmuir isotherm model. The desorption of Pb and Cd from the Nostoc sp. MK-11 dry biomass was greater than 90% using 0.1 M of HCL. The approach adopted for the biosorption of Cd and Pb is affordable, eco-friendly, practical, and safe. The dry biomass of Nostoc sp. MK-11 might be used as a cheap and efficient biosorbent for the removal of Cd and Pb ions from contaminated water.