Biosorption Potential of Desmodesmus sp. for the Sequestration of Cadmium and Lead from Contaminated Water

Abstract

Industrialization, urbanization, and natural processes have potentially accelerated the pace and level of heavy metals in the aquatic environment. Recently, modern strategies for heavy metal treatment in wastewater have received the specific attention of the scientific community. The present study aimed to assess the amorphous biomass of Desmodesmus sp. as a low-cost adsorbent to remove the cadmium (Cd) and lead (Pb) from aqueous solutions. It involved the optimization of pH, contact time, initial concentration of metal ions, and the dosage of biosorbent. Data collation revealed that an optimum contact time for both metals was 60 min, with an adsorption capacity of 63% for Cd and 66% for Pb. Different models were applied to the equilibrium data. The pseudo 2nd order described the best adsorption of Cd and Pb. The equilibrium data were computed with various isotherms. Langmuir isotherms better suit the adsorption of the above-mentioned metals. Hence, the maximum adsorption capacity of Desmodesmus sp. for Cd and Pb was 64.1 and 62.5 mg/g, respectively. The mechanism of biosorption was validated through a comparative FT-IR and Scanning Electron Microscopy of raw and metal-loaded algal biomass based on cell morphological changes. In order to study the reusability of adsorbent, adsorption-desorption of Cd and Pb ions was repeated three times using HCl. These results did not noticeably change in adsorption capacity during the three cycles. Using HCl (0.1 M), desorption of both metals was achieved up to 90% in three cycles. This work presented a long-term bioremediation approach for heavy metal pollutants in wastewater. This research could be seen as an interdisciplinary approach to large-scale heavy metal remediation. In addition, growing microalgae in wastewater produces animal feed and biodiesel. When compared to other conventional methods for environmental remediation and the manufacture of valuable products, the use of microalgae is a more efficient and cost-effective method.

1. Introduction

Toxic (heavy) metals constitute the most hazardous moieties found persistently in the environment [1]. Heavy metals are the contaminants that impact humans and animals [2,3]. Heavy metals that are toxic to the surroundings include cadmium and lead [4]. Despite this, Cd has been excessively used in pesticides, fertilizers, plastics, mining, welding, and refining [5]. Similarly, lead is used in batteries, cables, steel, paint, and plastic industries [6,7]. Discharge of waste from these production units causes heavy metal contamination in the aquatic system. Prolonged exposure to Cd can cause hepatic injury, hypertension, lung damage, renal dysfunction, and teratogenic consequences in humans [8,9]. Likewise, the effects of lead exposure may damage the brain, kidneys, and liver, in addition to developing ancillary issues, such as loss of appetite, gastrointestinal, anemia, and mental defects in children [10,11,12,13,14]. Due to being non-degradable and accumulative, heavy metals persist in the food chain with biological consequences, as already mentioned [15].

To avoid adverse impacts of heavy metals, it is imperative to purify wastewater before it is discharged into the surroundings [16,17]. Heavy metals can be eliminated from wastewater through a variety of processes. These may include ion exchange, coupled processes of oxidation/reduction, lime precipitation, membrane technology, electrochemical treatment, filtration, and reverse osmosis [16,18]. There are some limitations of these methods, such as not being more efficient at low metal concentrations and producing toxic sludge [19,20]. Therefore, new techniques are needed that eliminate the heavy metals from the ecosystem, ideally in an economical and environmentally friendly way [21]. Biosorption, as an alternative, provides the advantage of safe and efficient means of heavy metal removal from aquatic bodies, including wastewater, either in catchment areas or in ponds [22]. Consequently, this research is focused on finding effective and economically viable biosorbents to deal with these persistent heavy metals for biosafe ecosystem.

The process of biosorption is complex and is affected by the type of microbes, either in the living or amorphous form [23]. Since 2009, algae and cyanobacteria have been recognized as a preferred means to eliminate metal pollutants [24,25]. Algae, bacteria, fungi, and yeast all exhibit the uptake capacity of metal ions either by bioaccumulation or biosorption [26,27]. The application of amorphous biomass as an adsorbent has been promising in removing or recovering pollutants from aquatic environments [28]. Besides accumulating pollutants, algae are a broad category of organisms in aquatic environments that contain chlorophyll and prepare their food, thus boosting the water quality [29,30,31]. Algae as a biosorbent are efficient even at low levels of metal ion concentration [32], thus offsetting disadvantages associated with commercial resins at lower metal concentrations [33].

Different processes, such as coordination, electrostatic attraction, complexation, and microprecipitation, participate in the binding mechanism of heavy metal in the process of adsorption. At the same time, ion exchange has significantly influenced the binding of adsorbates to biosorbents [34]. Therefore, algal biomass is a reliable system for eliminating heavy metals offering dual capacity for biosorption and bioaccumulation. Biosorption is a passive phenomenon that essentially involves the chemical interaction of adsorbate and adsorbent [35], while bioaccumulation is an active process that occurs when there is intracellular incorporation of metal ions [36,37].

No research work has been done on the adsorption of cadmium and lead on Desmodesmus sp. collected from Poonch AJK. Desmodesmus sp. has good characteristics and is not toxic to the ecosystem. Cadmium and lead were used in this study because both metals have been discussed in the literature due to their high potential risk to human health and the environment. Hence, as a novelty, Desmodesmus sp. was used for the first time as a sorbent for the removal of heavy metals from an aqueous solution. The present study aimed to assess the capacity of the amorphous state biomass of Desmodesmus sp. as a sorbent of cadmium and lead. The experimental validations, including optimization of medium pH, contact time, the initial concentration of cadmium and lead ions, and adsorbent dose on the efficiency of adsorption of metals from mixed solutions and desorption of loaded (metal) biosorbents, were also investigated. The adsorption mechanism was evaluated through adsorption kinetics and isotherms, and finally, the evidence was assessed through FT-IR analysis and SEM.

2. Materials and Methods

2.1. Preparation of Sorbent

Planktonic algal samples were collected from a freshwater stream in Hajira Poonch Azad Jammu and Kashmir (AJK). The adsorbent preparation was described following the procedure with slight modifications [38]. The sample was streaked on agar plates with BBM with 1.5% agar and incubated at 25 °C under constant illumination at a light intensity using 1500 lux white LED light for 15 days. Upon emergence, colonies were picked and reinoculated in agar plates to obtain axenic cultures. The isolate was subjected to morphological and molecular identification revealing this to be a member of Desmodesmus sp. The axenic inoculum was subjected to mass culturing in 200 mL of BBM and kept in a shaker for 3 days under constant illumination of 1500 lux of white light at 25 °C. The biomass of Desmodesmus sp. was harvested at the exponential phase of growth and centrifuged at 5000 rpm (10 min) to obtain a pellet. Finally, it was washed with sterilized deionized water, dried at 40 °C, ground, sieved (100 µm), and stored (4 °C) for future use [39]. A schematic presentation of production and processing of biomass is shown in Figure 1.

2.2. Reagents Preparation

The stock solutions of Cd and Pb and serial dilutions were prepared by dissolving CdCl₂ and Pb (NO₃)₂ (1000 mg/L) in deionized water. In the next step, the pH (827 Metrohm benchtop pH meter, USA) of both solutions was adjusted between 2.0 and 8.0 [40].

2.3. Point of Zero Charge

To determine the point of zero charge (PZC) of algal sorbent, the addition of salt method was used with some modifications [41]. In nine different flasks, 40 mL of NaNO₃ (0.1 M) were collected. Acid and base solution were used to attain the varied pH (2–10) levels. The algal biosorbent was then added to each flask and kept at 150 rpm for 24 h at room temperature. The mixtures were filtered, and their pH was noted one by one. The PZC value of algal biomass was obtained by plotting the graph between initial pH and change in pH.

2.4. Biosorption Experiments

Adsorption experiments were performed to examine the impact of pH, contact time, initial concentration of cadmium and lead ions, and sorbent dosage in the aqueous solutions. The optimum pH was studied using 100 mg/L Cd and Pb concentration at a contact time of 60 min. The impact of contact time was examined by sample collection at specified intervals of time (5, 10, 15, 20, 30, 60, 90, and 120 min) at the 100 mg/L of metal concentration, pH 6 for cadmium, 5 for lead and biomass dosage 1 g/L. After optimizing the pH of the medium and contact time, the tests were performed with different initial concentrations of metal ions, keeping the time interval (60 min) constant. 1 g/L of amorphous algal biomass was mixed with a metal ion solution with a concentration ranging between 20 and 120 mg/L at optimized pH for cadmium and lead in a batch experiment at 23 °C. Another experiment was performed at optimized conditions to know the influence of biosorbent dosage, 0.5–2 g/L. Biosorption tests were performed in 10 mg/100 mL of metal solutions within a specific pH range and then mixed with algal biomass and agitated at 150 rpm with two replicates. Metal solution mixed with biosorbent was filtered, and the final concentration level of metals in the medium was assessed by atomic absorption spectrophotometry [42] (Agliant Technologies, MC187906, Petaling Jaya, Selangor, Malaysia). The following formulas were used to compute the amount of sorbate adsorbed on the biosorbent, which described the efficiency of metal adsorption (as mg/g and removal percentage) [40,43].

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{V}\frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}}{\mathrm{M}}$$

(1)

$$\mathrm{R}\%=\frac{{\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{i}}}\times 100$$

(2)

Here, in Equation (1), q_e quantifies the adsorbed ions on the biosorbent, where C_i is an initial and C_e is the final level of metal concentration, V denotes the solution volume, whereas M represents the biomass of sorbent (g).

2.5. Analysis of Adsorption Kinetics

To decipher the biosorption mechanism, the sorption data were assessed against pseudo 1st order [44] and pseudo 2nd order [45]. Finally, the intraparticle diffusion model [46] was applied to determine the rate constants and order of reactions for the processes. The equations of adsorption kinetics were tabulated in

Table 1. The linearized form of kinetics model and isotherms.

Kinetic Model	Linearized Equation	Graphical Dependence
Pseudo 1st order	$\mathrm{L}\mathrm{o}\mathrm{g}\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)=\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{q}}_{\mathrm{e}}-\frac{{\mathrm{k}}_1\mathrm{t}}{2.303}$	log (qe − qt) vs. time
Pseudo 2nd order	$\frac{\mathrm{t}}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{k}}_2\mathrm{q}{\mathrm{e}}^2}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}\mathrm{t}$	t/qt vs. time
Intra-Particle diffusion	${\mathrm{q}}_{\mathrm{t}}={\mathrm{k}}_{\mathrm{i}}{\mathrm{t}}^{0.5}+\mathrm{I}$	qt vs. time 0.5

.

k₁ and k₂ are constants for the 1st and 2nd order of adsorption. In Table 1, k_i denotes the rate constant for intraparticle diffusion.

2.6. Adsorption Isotherms

The sorption isotherm models provide vital knowledge on the sorption process. Here, sorption data are evaluated using Langmuir, Freundlich, and Temkin isotherms. The Langmuir isotherm presumptively involves monolayer adsorption on a solid surface with a predetermined number of identical sites [47]. Langmuir adsorption model [48] is specified by Equation (3).

$$\frac{1}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}}+\frac{1}{\mathrm{b} {\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}{\mathrm{C}}_{\mathrm{e}}}$$

(3)

where q_e is the sorbate content adsorbed onto the amorphous biomass of algae (biosorbent) at equilibrium, q_max is maximum adsorption capacity, b is the constant (Langmuir). b and q_max can be obtained from slope (1/q_maxb) and intercept (1/q_max) of the plot 1/q_e verses 1/C_e.

In contrast to the above, the Freundlich adsorption isotherms account for the heterogeneous surfaces and multilayer adsorption process [49]. The Freundlich adsorption isotherm [48] in linear form is defined in Equation (4).

$$\mathrm{In}{\mathrm{q}}_{\mathrm{e}}=\mathrm{I}\mathrm{n}{\mathrm{K}}_{\mathrm{F}}+\left(\frac{1}{\mathrm{n}}\right)\mathrm{I}\mathrm{n}{\mathrm{C}}_{\mathrm{e}}$$

(4)

where q_e represents the quantity of biosorbate adsorbed on the biosorbent (algal biomass), K_F is Freundlich isotherm constant from a linear plot of ln q_e vs. ln C_e.

The interaction between adsorbate molecules with sorbent sites may impact the adsorption properties. The Temkin adsorption, which asserts that as the concentration of sorbate molecules adsorbed on the sorbent rises, the adsorption heat of all molecules in the adsorption layer decreases linearly, suggests that the adsorbate and adsorbent may interact mutually. The Equation of Temkin isotherm model is presented in Equation (5) [50].

$${\mathrm{q}}_{\mathrm{e}}=\mathrm{B} \mathrm{l}\mathrm{n}\left({\mathrm{K}}_{\mathrm{t}}{\mathrm{C}}_{\mathrm{e}}\right)$$

(5)

B (mg/g) and k_t (L/mg) are the constants of isotherm.

2.7. Desorption of Cd and Pb Ions from Metal Loaded Adsorbent

To effectively desorb the metal ions and restore the adsorbent for reuse, the reusability of the adsorbent was assessed by following the described techniques. To conduct the consecutive experiment of adsorption and desorption, the repetition of cycles (3 cycles) was done using the mentioned above preparations. Desorption studies were conducted by mixing loaded sorbent (metal) with desorption medium (0.1 M HNO₃, 0.1 M HCl, 0.1 M H₂SO₄) and were agitated at 150 rpm for 60 min. The amount of metal ions was determined by AAS [51]. The percentage of desorption efficiency was calculated by using Equation (6) [52].

$$\mathrm{D}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{p}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}\%=\frac{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{d}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}}{\mathrm{A}\mathrm{m}\mathrm{o}\mathrm{u}\mathrm{n}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{m}\mathrm{e}\mathrm{t}\mathrm{a}\mathrm{l} \mathrm{a}\mathrm{d}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}}\times 100$$

(6)

2.8. Characterization

FT-IR analysis (PerkinElmer Spectrum 65) was performed to figure out the functional groups involved during the adsorption of metal ions in Desmodesmus sp. from aqueous solutions. Three samples (R3 control, R3 Cd, and R3 Pb) were assessed. SEM (JSM5910, JEOL Japan) was performed to analyze the morphology before and after cadmium treatment and lead in Desmodesmus sp.

3. Results

3.1. Effect of pH on Metal Adsorption

pH is considered a main factor that affects the biosorption of metal ions in mixed solutions. The pH of the mixture impacts the sites (binding sites) that exist on the algal surface and the chemistry of the adsorbate present in the medium. The effect of pH on the adsorption of Cd and Pb onto Desmodesmus sp. was investigated in the pH range of 2–8, as shown in Figure 2. No notable effect of pH was observed on Cd and Pb adsorption at pH 2–4. Figure 2 clearly shows an increase in the biosorption process as the pH was increased and leveled off at pH 6.0 and 5.0 for Cd and Pb, while at pH > 6 for Cd and pH > 5, adsorption of metal ions started decreasing. However, the optimum pH for maximum metal uptake for each of the two metals differed, i.e., 6.0 and 5.0 for cadmium and lead, with the removal efficiency of 62% and 65%, respectively. As a result, all further experiments were performed at pH 6 for cadmium and pH 5 for lead.

The charge on the surface of biomass is positive when pH < pH_PZC, whereas the negative charged surface is represented by pH > pH_PZC. In this study, the pH_PZC of algal sorbent is 3.9, as shown in Figure 3, and the optimum pH values of sorbent and sorbate were 5.0 and 6.0 for Cd and Pb. Based on the pH_PZC values of the adsorbents, pH is above pH_PZC, the sorbent will be negatively charged.

3.2. Effect of Contact Time on Biosorption of Metals Ions

The effect of contact time greatly influences the adsorption process. Figure 4 depicts the effect of contact time on the adsorption of Cd and Pb onto the amorphous biomass of Desmodesmus sp. The results depict that the adsorption of both metals was rapid in the first 20 min and then gradually increased till the equilibrium was reached after 60 min of contact with the adsorption efficiency of 58 and 60% for Cd and Pb, respectively, and then it remained almost constant.

3.3. Effect of Initial Concentration of Cd and Pb on Metal Adsorption

The impact of the initial concentration of metal ions on the adsorption capacities of metal ions in the presence of amorphous biomass of Desmodesmus sp. is depicted in Figure 5. It was noted that the biosorption of Cd and Pb ions increased with an increase in the initial concentration of metal ions, and then it attained an equilibrium. The maximum adsorption capacity of metal ions (61 and 64 mg/g for Cd and Pb) on the amorphous biomass of Desmodesmus sp. was obtained approximately at an initial metal ion concentration of 100 mg/L.

3.4. Effect of Biomass on Adsorption

The graphical form (Figure 6) clearly indicates that the removal efficiency of Desmodesmus sp. improved when there was an increase in adsorbent dosage. At 1 g/L of the sorbent dose, the optimum removal percentage was 63% and 66% for cadmium and lead, respectively. Once equilibrium was reached (at 1 g), no further increase in removal efficiency was observed.

3.5. Adsorption Kinetics

The determination coefficients (R²) for pseudo 1st order were observed to be significantly lower for Cd and Pb, as shown in

Table 2. A comparison among rate constants of different kinetics models.

Metal	Pseudo 1st Order			Pseudo 2nd Order			Intraparticle Diffusion
	K1 (min−1)	qe (mg/g)	R2	K2 (g mg−1 min−1)	qe (mg/g)	R2	Ki(mg g−1 min−1)	R2	qe exp (mg/g)
Cd	−0.034545	27.510	0.8313	27,297.3	62.5	0.9952	3.9189	0.7834	58
Pb	−0.029478	22.819	0.7135	36,090.7	62.8	0.9956	3.6035	0.7554	60

. The experimental and calculated values of q_e (Table 2) were not close to each other, as also depicted from the R² values, pointing to the fact that the biosorption of these metals onto Desmodesmus sp. did not follow pseudo 1st order kinetics [53]. The R² (determination coefficient) values of pseudo 2nd order is close to unity, i.e., 0.9952 for Cd and 0.9956 for Pb, respectively. The calculated values of q_e for Cd and Pb are 62.5 and 62.8 mg/g, showing agreement with q_e, _exp (experimental values) in pseudo 2nd order of kinetics (Table 2). The intraparticle diffusion model helped to define the mechanism of adsorption. The values of R² for cadmium and lead were noted as 0.7834 and 0.7554, respectively. The coefficient values were not close to unity. The kinetics models are presented in Figure 7.

3.6. Adsorption Isotherms

The parameters of the adsorption isotherm displayed in

Table 3. Adsorption isotherm parameters for the biosorption of metal ions using biomass of Desmodesmus sp. pH 6 and 5 for Cd and Pb, contact time 60 min, biosorbent dosage 1 g/L.

Metal	Langmuir Constants			Freundlich Constants			Temkin Constants
	1/b	qmax (mg g−1)	R2	KF (mg g−1)	1/n	R2	B	R2
Cd	5.153	64.1	0.9941	572.137	0.361	0.9475	13.203	0.9742
Pb	2.275	62.5	0.9864	1250.55	0.286	0.9258	11.001	0.9628

explain the biosorption of metal ions is followed by the Langmuir sorption isotherm due to higher R² values of cadmium and lead when compared to other isotherms. The value of q_max, calculated by Langmuir isotherm, was observed at 64.1 and 62.5 mg/g for Cd and Pb, respectively, as shown in Table 3. The 1/n values (Table 3) obtained were 0.361 and 0.286 for Cd and Pb, respectively. The plots of different isotherms are shown in Figure 8.

In addition,

Table 4. A comparison of adsorption capacity between different sorbents and Desmodesmus sp.

Biosorbent	Biosorption Capacity (mg/g)
	Cd	Pb	References
U. lactuca	29.2	34.7	[54]
Rhizopus arrhizus	27.0	56.0	[55]
Chlorella minutissima	11.1	9.74	[56]
Caulerpa lentillifera	4.7	28.7	[57]
Mucor rouxii	8.5	35.7	[58]
Desmodesmus sp	64.1	62.5	Current study

compares the biosorption capacity of Desmodesmus for Cd and Pb with that of other biosorbents. Desmodesmus sp. has a better biosorption capacity for these metal ions than other adsorbents in Table 4. Therefore, it is noteworthy that the Desmodesmus has a high potential for removing metal ions from an aqueous solution.

3.7. Desorption Studies

To demonstrate the reusability of the sorbent, the desorption of Cd and Pb ions (adsorbed) from the sorbent was examined using different desorption solutions. Results show that HNO₃ and HCl are effective agents for desorption, as shown in Figure 9. However, HNO₃ damages the adsorbent at high concentrations. The process of desorption was repeated thrice by using HCl. However, the adsorption-desorption processes did not significantly alter the adsorption capability of the biosorbent under study, and desorption of both metals was reached by more than 90% (

Table 5. Reusability of amorphous biomass of Desmodesmus sp. (0.1 M, 50 mL HCl with contact time of 60 min at 150 rpm).

Metal	Cycles	% Sorption	% Desorption
Cd	1	61 ± 0.57	93.4 ± 0.63
	2	61 ± 0.57	90.1 ± 0.63
	3	60 ± 0.57	91.6 ± 0.57
Pb	1	64 ± 0.57	95.3 ± 0.57
	2	63 ± 0.57	93.6 ± 0.57
	3	63 ± 0.57	92 ± 0.57

Values are expressed as mean ± SEM.

).

3.8. Adsorption Mechanism

A comparative analysis of FTIR spectra in the case of pre-adsorption and post-adsorption samples revealed a possible mode of adsorption mechanism for Desmodesmus sp. Figure 10 depicts the functional groups that are involved in the process of adsorption. The control represents the peak (black) at 3306 cm^−1, demonstrating the presence of N-H or O-H functional groups [59]. The C-H stretching vibrations are in 2932–2842 cm⁻¹ [60]. The C=O stretching vibration band appears at 1754 cm⁻¹ for carbonyl functional groups. The absorption band of C=O of amide linkage appeared in the range of 1646 and 1539 cm⁻¹. The absorption band located near 1243 cm⁻¹ corresponds to C-N functionality, and the band at 1012 cm⁻¹ indicates the existence of C-O stretching band.

The FTIR spectrum shows metal ions bound to the functional groups of sorbents, and many changes were noted (red and blue peaks) from the original spectrum (black peak, Figure 10). The N-H/O-H band is shifted from 3306 to 3189 cm⁻¹ (Cd) and 3277 cm⁻¹ (Pb), which depicts the interaction of metal ions with amide and hydroxyl groups. The band’s disappearance at 1754 cm⁻¹ indicates metal binding to the carbonyl group in both metals. The complex band of amide C-N is shifted from 1243 to 1290 cm⁻¹ in Cd. Similarly, the C-O stretching band shifts from 1012 to 1042 cm⁻¹ and 1059 cm⁻¹ for Cd and Pb, respectively. The results support a sufficient efficiency of metal surface binding capability.

Scanning electron microscopy (SEM) of the raw biomass of Desmodesmus sp. (Figure S1) revealed the porous nature of the cell surface with a transparent layer on the outer surface of the cell wall. The porous surface of the cell wall increases the surface area of the sorbent and improves the biosorption of metal ions. Possible mechanisms for the biosorption of metal ions from the aqueous system are shown in Figure 11.

4. Discussion

The effect of different factors on the adsorption of heavy metals is usually investigated in batch experiments. The observations mentioned above in Figure 2 could be described by the fact that at lower pH, hydrogen ions and metal ions compete to form ligands at the surface of the sorbent (because of the charge repulsion) [61]. The increase in the sorption of metal ions is due to the increasing values of pH because the proton concentration decreases and the algal surface gets negatively charged, maximizing the biosorption of metal ions [39]. The maximum adsorption efficiency of metals occurred at different values of pH due to the different characteristics of the metals, e.g., electronegativity and the availability of metal ions that were better adsorbed on the binding sites of the sorbent [62]. Similar results have been described concerning the effect of pH on the adsorption of heavy metals by the biomass of various algal species [51]. The charge on the surface of biomass is necessary as it determines the interaction between sorbent and sorbate, therefore, it is compulsory to know the charge on the surface of the sorbent while doing the sorption tests. When the solution pH is above pH_PZC, the adsorbent will be negatively charged, and due to the electrostatic force of interaction, it binds to the more positive charge [63]. The obtained contact time in this study for both metals was 60 min. In the first 20 min, the reaction rate was fast, gradually decreasing and finally leveling off with no significant changes beyond 60 min. Similar outcomes have been noted for the green algal biomass of Chlorella vulgaris [43]. Previously, 60 min were also reported as an optimum contact time for the adsorption of cadmium [39] and lead [40] by the dried biomass of blue-green and green algae. After that, no significant changes were observed because of the saturation of binding sites with the metal ions on the surface of the biosorbent [64]. An increase in the initial concentration of heavy metal ions decreased the mass transfer resistance between the sorbate and sorbent. This, consequently, assisted the approaching adsorbate to active sites, thus improving the capacity to adsorb. At that point, the saturation of binding sites was achieved in the stipulated time with no further adsorption. The process of adsorption is related to the dosage of biomass in the aqueous solution because of the presence of binding sites on the adsorbent surface [65]. Figure 6 revealed the behavior of metal ions at various doses of biosorbent, and removal efficiency was not increased after 1 g of biomass dosage. This performance could be described by the fact that the formation of aggregates of the adsorbent decreases the surface area and biosorption process of metal ions [54].

The collected data revealed that the adsorption of Cd and Pb on the amorphous biomass of Desmodesmus sp. follows the pseudo 2nd order of kinetics [66,67]. It could be presumed from the results mentioned above (Table 2) that the metal-Desmodesmus biosorption mechanism was likely controlled by the chemosorption process involving covalent bonds or ion exchange between the sorbent and sorbate until all active sites were engaged, then metal ions were diffused into the adsorbent for further interactions. Similar findings were described by [68] in the material deployed to eliminate pollutants. The trend line in intra-particle diffusion was not observed to pass through the origin, which clearly indicated that this model is not supported by the data [69,70].

In order to study the interaction between adsorbate and adsorbent and design the adsorption process, adsorption equilibrium data were described by various isotherm models. The R² values of cadmium and lead were higher in Langmuir sorption isotherm when compared to other isotherms [71]. The Langmuir model indicates the homogeneous surface of the biosorbent and the monolayer adsorption of positive ions on the surface of microalgae. The 1/n values (Table 3) obtained ranged between 0 and 1, indicating Cd and Pb adsorption is favorable [72]. Therefore, this investigation demonstrates that the Langmuir model is well fitted to the observed data as manifested through various isotherms [73,74]. Under acidic conditions, the metal desorption caused protonation of the biosorbent surface, allowing the desorption of ions (positively charged) from the biosorbent [75]. Some recently performed experiments reported HCl as a desorbing agent [43,76]. In addition, an effective prospective sorbent to eliminate heavy metal ions must have good adsorption and recovery ability of metal ions [77].

The spectrum (Figure 10) explains the existence of main functional groups like carbonyl, hydroxyl, amide, and amines primarily involved in metal adsorption. Similar interpretations were put forth by [43], describing Cd ion adsorption in green alga Chlorella vulgaris. The monographs (Figure S1) demonstrate that the algae-based biosorbents reduce the porosity of the surface when brought in contact with metal ions, forming a flat structure [40]. Our results further revealed the morphological changes that have occurred due to the interlinking of Cd and Pb and the functional groups present algal cell wall [61].

5. Conclusions

Experiments on Desmodesmus sp. are concluded as follows: Removal efficiency of Cd and Pb was 63% and 66% at optimized conditions. The biosorption process correlated with the pseudo 2nd order kinetics model, and adsorption equilibrium data showed the best fit with the Langmuir model. The value of the q_max of amorphous biomass of Desmodesmus sp. for Cd and Pb was 64.1 and 62.5 mg/g, respectively. Desorption of metal ions from the biosorbent was more than 90% by using hydrochloric acid. FTIR data showed the role of carbonyl, hydroxyl, amine, and amide groups present on the surface of the amorphous biomass of Desmodesmus sp. in the adsorption process. SEM data showed morphological changes in the treated and untreated amorphous biomass of Desmodesmus sp., which revealed the efficient adsorption capacity for the elimination of metal ions from wastewater. Our research provides a foundation for future studies at the molecular level to develop a system that detects and eliminates heavy metals from the environment.