Optimization Study of the Capacity of Chlorella vulgaris as a Potential Bio-Remediator for the Bio-Adsorption of Arsenic (III) from Aquatic Environments

This study examined the ability of the green microalgae Chlorella vulgaris to remove arsenic from aqueous solutions. A series of studies was conducted to determine the optimal conditions for biological arsenic elimination, including biomass amount, incubation time, initial arsenic level, and pH values. At 76 min, pH 6, 50 mgL−1 metal concentration, and 1 gL−1 bio-adsorbent dosage, the maximum removal of arsenic from an aqueous solution was 93%. The uptake of As (III) ions by C. vulgaris reached an equilibrium at 76 min of bio-adsorption. The maximum adsorptive rate of arsenic (III) by C. vulgaris was 55 mg/gm. The Langmuir, Freundlich, and Dubinin–Radushkevich equations were used to fit the experimental data. The best theoretical isotherm of Langmuir, Freundlich, or/and Dubinin–Radushkevich for arsenic bio-adsorption by Chlorella vulgaris was determined. To choose the best theoretical isotherm, the coefficient of correlation was used. The data on absorption appeared to be linearly consistent with the Langmuir (qmax = 45 mgg−1; R2 = 0.9894), Freundlich (kf = 1.44; R2 = 0.7227), and Dubinin–Radushkevich (qD–R = 8.7 mg/g; R2 = 0.951) isotherms. The Langmuir and Dubinin–Radushkevich isotherms were both good two-parameter isotherms. In general, Langmuir was demonstrated to be the most accurate model for As (III) bio-adsorption on the bio-adsorbent. Maximum bio-adsorption values and a good correlation coefficient were observed for the first-order kinetic model, indicating that it was the best fitting model and significant in describing the arsenic (III) adsorption process. SEM micrographs of treated and untreated algal cells revealed that ions adsorbed on the algal cell’s surface. A Fourier-transform infrared spectrophotometer (FTIR) was used to analyze the functional groups in algal cells, such as the carboxyl group, hydroxyl, amines, and amides, which aided in the bio-adsorption process. Thus, C. vulgaris has great potential and can be found in eco-friendly biomaterials capable of adsorbing arsenic contaminants from water sources.


Introduction
Heavy metals are found randomly and naturally in the layers of the Earth and water sources, but they negatively affect human life if they exceed internationally permissible concentrations [1,2]. It is typical for living creatures on Earth to inhale these pollutants and thus influence the general health of these neighborhoods [3][4][5]. A population explosion worldwide has led to the increase and development of more industries, resulting in an increased release of pollutants into the environment [5][6][7]. One type of pollution is the

Preparation of Adsorbent Biomass
These steps were used to prepare the microalgal biomass. First, the algal biomass was deactivated in an autoclave at 121 • C for 10 min. Second, the deactivated biomass was collected by filtering cultured media using cellulose nitrate Whatman membranes with pore sizes of 45 m and washed with deionized water. Third, the biomass was sun-dried for three days before being oven-dried at 55 • C for 24 h and pulverized in a stone mortar and pestle to achieve a uniform particle size. For future usage, forty sieved materials were stored in an airtight container [31].

Elemental Analysis
The chemical constituents of the algal adsorbent (% of carbon, hydrogen, nitrogen, and oxygen) were analyzed using an elemental analyzer (Eltra GmbH, Retsch-Allee, Haan, Germany).

FTIR Analysis
The functional groups of the algal adsorbent and arsenic (III)-loaded algal adsorbent were further investigated using a Fourier-transform infrared (FTIR) spectrophotometer (Mattson Satellite 5000 FTIR, Warwickshire, UK) in the 400-4000 cm −1 wave number range.

Morphological Study of Adsorbent Surface
The untreated and arsenic-treated adsorbents were collected and prepared for the SEM (Zeiss, Sigma VP, Jena, Germany) examination [32].

Adsorption Isotherm
The optimum concentration of chemical reagents (1 M NaCl for C. vulgaris) was utilized in the adsorption isotherm analysis under conditions such as an adsorbent dosage of 1 g, varied initial arsenic (III) concentrations (10 to 300 mgL −1 ), and agitation at 150 rpm and 25 • C for 8 h to ensure that the adsorption system attained an equilibrium, with the initial solution pH adjusted to 6.0.
The amount of equilibrium adsorption (qe mgg −1 ) of the As (III) was calculated following Equation (1): where V (L) denotes the solution volume, W (g) represents the dry adsorbent weight, and C 0 (mgL −1 ) represents the initial As (III) concentration. C e (mgL −1 ) represents the (III) concentration at the equilibrium. The Langmuir and Freundlich models [33] in Equations (2)-(5), respectively, and the Dubinin-Radushkevich (D-R) isotherms in Equations (6) and (7) were employed to evaluate the adsorption isotherms to assess the performance of the adsorbents. q e = q max K L C e 1 + K L C e . (2) In q e = 1 n In C e + InK F (5) where q max (mgg −1 ) represents the maximum adsorption capacity, q e (mgg −1 ) represents the equilibrium-adsorbed amount of As (III), and C e (mgL −1 ) denotes the solute's equilibrium concentration. The Langmuir constant for adsorption energy is represented by K L , whereas K F is the magnitude of bio-adsorption capacity; n is the bio-adsorption intensity, K D-R and β are D-R constants, and E is energy for the uptake of arsenic on the bio-adsorbate surface.

Bio-Adsorption Kinetics
The bio-adsorption kinetics were investigated in 50 mL of solution under conditions such as 30 mgL −1 As (III), 1 gL −1 adsorbent, pH 6, and agitation at a speed of 150 rpm and a temperature of 25 • C, with time intervals ranging from 5 min to 8 h, depending on the appropriate equilibrium time. The adsorption kinetic data on the removal of As (III) were assessed via pseudo-first-and second-order models. Equation (8) expresses the pseudo-first-order kinetic model.
Equation (9) shows the pseudo-second-order kinetic model: where q e and q t represent the equilibrium adsorption capacity (mgg −1 ) and the adsorption capacity at t (min), respectively, and k 1 and k 2 are the rate constants for the pseudo-firstorder and pseudo-second-order models, respectively. All the experiments were duplicated and the averages were used for the analysis.

Thermodynamics of As (III) Bio-Adsorption
The uptake of arsenate ions by Chlorella vulgaris was investigated by varying the temperature from 10 to 50 • C at optimum conditions and evaluating the temperature effect using the following equations: where Kc is the equilibrium constant; ∆H, ∆S, and ∆G are the standard enthalpy, entropy, and Gibbs free energy, respectively; T is the temperature (kelvin); and R is the gas constant. The bio-adsorption capability was highly dependent on pH, as shown in Figure 1. The optimal pH for adsorption at 100 gL −1 arsenic, 1 g bio-adsorbent, and a 120 min incubation was determined using a pH range of 1-8. The bio-adsorption rate was limited at an initial pH of 3.0 and reached a maximum at an optimal pH of 6.0, with a bio-adsorption rate of 58 mgg −1 ; the removal percentage was 63%. According to Abdel-Raouf and colleagues, determining the optimal pH has a significant impact on regulating the metal aqueous and functional groups of adsorbent surfaces, thereby influencing heavy metal adsorption in an aqueous environment [34]. Bio-adsorption has been discovered to be relatively stable above a pH of 6.0 [35]. Due to the high concentration of H + hydrogen ions, the adsorption capacity diminishes when the pH falls below 3. In this instance, the rate of arsenic ions in the solution was lower than the concentration of H + ions, increasing the possibility of competition with them for surface binding sites of adsorption due to repulsive forces between metal ions and proteins [36]. The absorbent material's surface was fully covered by hydrogen ions at low pH values, thus preventing the AS (III) ions from binding to the adsorption sites. Additionally, the functional groups on the absorbent's surface were influenced by the pH. Surface charge changes on these functional groups played a crucial role in determining the activity of the absorbent material. The increased positive charge (proton) density on the functional groups at low pH values created a barrier that prevented the AS (III) ions' approach due to repulsive forces. Nonetheless, as the pH level increased, the competition from hydrogen ions decreased, with a greater proportion of absorbent functional groups becoming negatively charged. This led to a decrease in electromagnetic repulsion, facilitating the attraction of metal cations and adsorption on the absorbent material's surface. Figure 2 shows the bio-adsorptive capacity and removal percentage of As (III) onto the surface of C. vulgaris at various contact times (20,40,60,80, 100, 120, 140, and 160 min). After 76 min of bio-adsorption, C. vulgaris' uptake of As (III) ions reached an equilibrium. The time it takes for heavy metals in a solution to reach a constant value is known as the adsorption equilibrium time. The maximum bio-adsorption rate for As (III) uptake was 55 mgg −1 and the removal percentage was 64%. According to a similar study, a rapid absorption rate was followed by a slower rate and, eventually, an equilibrium [37]. The rate of metal removal was initially rapid due to the abundance of binding sites on the biomass surface, which eventually became saturated. The rapid uptake of As (III) by the biomass binding sites demonstrated an adsorption process with no energy-mediated reactions occurring via surface binding. Due to the rapid metal bio-adsorption, the application was simplified and more efficient.  Figure 2 shows the bio-adsorptive capacity and removal percentage of As (III) on the surface of C. vulgaris at various contact times (20, 40, 60, 80, 100, 120, 140, and 160 min After 76 min of bio-adsorption, C. vulgaris' uptake of As (III) ions reached an equilibrium The time it takes for heavy metals in a solution to reach a constant value is known as th adsorption equilibrium time. The maximum bio-adsorption rate for As (III) uptake was 5 mgg −1 and the removal percentage was 64%. According to a similar study, a rapid absorp tion rate was followed by a slower rate and, eventually, an equilibrium [37]. The rate metal removal was initially rapid due to the abundance of binding sites on the bioma surface, which eventually became saturated. The rapid uptake of As (III) by the bioma binding sites demonstrated an adsorption process with no energy-mediated reactions o curring via surface binding. Due to the rapid metal bio-adsorption, the application wa simplified and more efficient.

Bio-Adsorption Capacity at Varying Bio-Adsorbent Doses
The results depicted in Figure 3 show that increasing the bio-adsorbent dose from 0.25 to 1 gL −1 led to a rise in the As (III) ion removal percentage, from 8 to 100%. The removal percentage remained unchanged after the concentration reached 1 gL −1 . In contrast, increasing the bio-adsorbent dosage between 1 and 5 mgg −1 reduced the bio-adsorption rate from 55 to 20 mgg −1 . Consequently, the optimal bio-adsorbent dose was found to be 1 gL −1 , with the highest rate of arsenic bio-adsorption by C. vulgaris being 55 mgg −1 and removal percentage of 93%. It should be noted that the adsorption rate decreased as the adsorbent dose increased due to the initial metal ion concentration being insufficient to match all the active sites on the adsorbent's surface. This was an indicator of the attraction between the functional groups present on the adsorbent's surface and the arsenic particles [38,39].

Bio-Adsorption Capacity at Varying Bio-Adsorbent Doses
The results depicted in Figure 3 show that increasing the bio-adsorbent dose from 0.25 to 1 gL −1 led to a rise in the As (III) ion removal percentage, from 8 to 100%. The removal percentage remained unchanged after the concentration reached 1 gL −1 . In contrast, increasing the bio-adsorbent dosage between 1 and 5 mgg −1 reduced the bioadsorption rate from 55 to 20 mgg −1 . Consequently, the optimal bio-adsorbent dose was found to be 1 gL −1 , with the highest rate of arsenic bio-adsorption by C. vulgaris being 55 mgg −1 and removal percentage of 93%. It should be noted that the adsorption rate decreased as the adsorbent dose increased due to the initial metal ion concentration being insufficient to match all the active sites on the adsorbent's surface. This was an indicator of the attraction between the functional groups present on the adsorbent's surface and the arsenic particles [38,39].
Toxics 2023, 11, x FOR PEER REVIEW 8 of 16 Figure 3. Determination of optimal algal adsorbent doses for arsenic adsorption by C. vulgaris. The initial concentration was 100 mgL −1 , the pH was 6.0, and the contact time was 76 min.

Bio-Adsorption of As (III) at Different Metal Concentrations
We evaluated the highest concentration of arsenic ions that could be adsorbed by the algal absorbent at a 1 g bio-adsorbent dose with pH of 6.0 and a contact time of 76 min. Figure 4 displays the arsenic bio-adsorption rate by C. vulgaris at different concentrations of arsenic ions. The bio-adsorption capacity of As (III) grew proportionally with the initial concentration, registering a 50 mgL −1 bio-adsorption capacity with a 55 mgg −1 removal percentage of 100%. The minimum and maximum As (III) bio-adsorptive capacities were 24 mgg −1 and 55 mgg −1 , respectively, with As (III) concentrations of 25 and 50 mgL −1 . An increase in the initial As (III) concentration may have resulted in inconsistent metal ions being in contact with the algal absorbent surface, which could have led to mass transfer resistance between the aqueous and absorbent phases. Nonetheless, increasing the As (III) ion concentration from 50 to 200 mgL −1 did not increase the rate of uptake capacity, with the capacity remaining constant at 55 mgg −1 . This indicated that near-constant saturation was achieved and that there were limited binding sites available for adsorption [17].

Bio-Adsorption of As (III) at Different Metal Concentrations
We evaluated the highest concentration of arsenic ions that could be adsorbed by the algal absorbent at a 1 g bio-adsorbent dose with pH of 6.0 and a contact time of 76 min. Figure 4 displays the arsenic bio-adsorption rate by C. vulgaris at different concentrations of arsenic ions. The bio-adsorption capacity of As (III) grew proportionally with the initial concentration, registering a 50 mgL −1 bio-adsorption capacity with a 55 mgg −1 removal percentage of 100%. The minimum and maximum As (III) bio-adsorptive capacities were 24 mgg −1 and 55 mgg −1 , respectively, with As (III) concentrations of 25 and 50 mgL −1 . An increase in the initial As (III) concentration may have resulted in inconsistent metal ions being in contact with the algal absorbent surface, which could have led to mass transfer resistance between the aqueous and absorbent phases. Nonetheless, increasing the As (III) ion concentration from 50 to 200 mgL −1 did not increase the rate of uptake capacity, with the capacity remaining constant at 55 mgg −1 . This indicated that near-constant saturation was achieved and that there were limited binding sites available for adsorption [17].

Elemental Analysis of Algal Adsorbent
The percentages of the most critical element on the algal adsorbent surface (Table 1) were oxygen (39.87%), carbon (43.65%), hydrogen (8.51%), nitrogen (6.98%), and sulfur (1.11%), which formed the functional groups (carboxyl, amines, alcohol, phenol, etc.)  Figure 5A,B demonstrate the surface morphology of blank and arsenic-treated algal cells. The prevalence of a heterogeneous porous structure and its preceding structures with coarse holes, which were significant for the prospective adsorbent, can be noticed in the blank picture ( Figure 5A). As (III) confinement caused the treated algal cells ( Figure  5B) to assume a tight and smooth structure throughout the image [40].   Figure 5A,B demonstrate the surface morphology of blank and arsenic-treated algal cells. The prevalence of a heterogeneous porous structure and its preceding structures with coarse holes, which were significant for the prospective adsorbent, can be noticed in the blank picture ( Figure 5A). As (III) confinement caused the treated algal cells ( Figure 5B) to assume a tight and smooth structure throughout the image [40].

Elemental Analysis of Algal Adsorbent
The percentages of the most critical element on the algal adsorbent surface (Table 1) were oxygen (39.87%), carbon (43.65%), hydrogen (8.51%), nitrogen (6.98%), and sulfur (1.11%), which formed the functional groups (carboxyl, amines, alcohol, phenol, etc.)  Figure 5A,B demonstrate the surface morphology of blank and arsenic-treated algal cells. The prevalence of a heterogeneous porous structure and its preceding structures with coarse holes, which were significant for the prospective adsorbent, can be noticed in the blank picture ( Figure 5A). As (III) confinement caused the treated algal cells ( Figure  5B) to assume a tight and smooth structure throughout the image [40].

FTIR Analysis
FTIR spectroscopy was used to identify several functional groups present on the surface of the algae. The FTIR spectra of the algal adsorbent revealed several types of vibrational frequencies due to distinct functional groups. The FTIR graphs of the original and chemically transformed algal samples are shown in Figure 6A,B. The FTIR spectroscopic graph revealed an intensity peak at 3300 cm −1 , indicating the existence of O-H and N-H [41]. According to Gupta et al. [42], a prolonged rise was seen to be displaced in a treated algal biomass compared with an untreated algal biomass. Peaks at 3000 cm −1 correlated with the C-H stretching vibrations in cellulose and lignin [43]. Peaks near 1653 cm −1 were also linked to C=O asymmetric stretching. More significant shifting revealed a tight connection between the metal ions (As (III)) and the algal surface. Plazinski et al. [44] reported that the peak at 1200.10 cm −1 was due to -SO 3 with aliphatic amine stretching (C-N). Compared with unmodified C. vulgaris, the strength of the O-H, C-H, and C=O stretching bands for modified C. vulgaris increased, indicating the increase in these functional groups. The FTIR graph showed that the C. vulgaris surface was occupied by the hydroxyl, carboxylate, amino, and amide groups, which had a remarkable role during the adsorption process [45].
face of the algae. The FTIR spectra of the algal adsorbent revealed several types of vibrational frequencies due to distinct functional groups. The FTIR graphs of the original and chemically transformed algal samples are shown in Figure 6A,B. The FTIR spectroscopic graph revealed an intensity peak at 3300 cm −1 , indicating the existence of O-H and N-H [41]. According to Gupta et al. [42], a prolonged rise was seen to be displaced in a treated algal biomass compared with an untreated algal biomass. Peaks at 3000 cm −1 correlated with the C-H stretching vibrations in cellulose and lignin [43]. Peaks near 1653 cm −1 were also linked to C=O asymmetric stretching. More significant shifting revealed a tight connection between the metal ions (As (III)) and the algal surface. Plazinski et al. [44] reported that the peak at 1200.10 cm −1 was due to -SO3 with aliphatic amine stretching (C-N). Compared with unmodified C. vulgaris, the strength of the O-H, C-H, and C=O stretching bands for modified C. vulgaris increased, indicating the increase in these functional groups. The FTIR graph showed that the C. vulgaris surface was occupied by the hydroxyl, carboxylate, amino, and amide groups, which had a remarkable role during the adsorption process [45]. The mechanisms of adsorption can be generally categorized as physisorption and chemisorption. The physisorption mechanism involves adsorption between the surface and adsorbate, which includes a range of factors such as diffusion, van der Waals interactions, hydrogen bonding, electrostatic interactions, and exothermal phenomena [46]. On the other hand, the chemisorption mechanism involves electronics, valence forces that prevail between the absorbent and adsorbate, chemical bonding to the adsorbent's surface, complex formation, chelation, proton displacement, redox reaction, and covalent bonding [47].

Bio-Adsorption Isotherm
The bio-adsorption isotherm at a given condition represents the equilibrium relationship between the adsorbate concentration in an aqueous solution and that on the adsorbent's surface of microalgae C. vulgaris. Langmuir, Freundlich, and Dubinin-Radushkevich models were used to describe the equilibrium relationships. Table 2 depicts the equilibrium adsorption data and summarized isotherm models. According to the regression coefficients, the Langmuir isotherm model fitted the experimental equilibrium data better than the Freundlich isotherm and Dubinin-Radushkevich isotherm models. The As (III) bio-adsorption data (R 2 ; As = 0.9894) suited the Langmuir isotherm model better for The mechanisms of adsorption can be generally categorized as physisorption and chemisorption. The physisorption mechanism involves adsorption between the surface and adsorbate, which includes a range of factors such as diffusion, van der Waals interactions, hydrogen bonding, electrostatic interactions, and exothermal phenomena [46]. On the other hand, the chemisorption mechanism involves electronics, valence forces that prevail between the absorbent and adsorbate, chemical bonding to the adsorbent's surface, complex formation, chelation, proton displacement, redox reaction, and covalent bonding [47].

Bio-Adsorption Isotherm
The bio-adsorption isotherm at a given condition represents the equilibrium relationship between the adsorbate concentration in an aqueous solution and that on the adsorbent's surface of microalgae C. vulgaris. Langmuir, Freundlich, and Dubinin-Radushkevich models were used to describe the equilibrium relationships. Table 2 depicts the equilibrium adsorption data and summarized isotherm models. According to the regression coefficients, the Langmuir isotherm model fitted the experimental equilibrium data better than the Freundlich isotherm and Dubinin-Radushkevich isotherm models. The As (III) bio-adsorption data (R 2 ; As = 0.9894) suited the Langmuir isotherm model better for C. vulgaris than the Freundlich and Dubinin-Radushkevich isotherm models (R 2 ; As = 0.7227 and 0.951, respectively). The Langmuir maximum bio-adsorption capacity (q max ) was 45 mgg −1 whilst in the case of the Dubinin-Radushkevich model, it was 8.7 mgg −1 . The As (III) adsorption behavior approximated the Langmuir isotherm better (R 2 ; As = 0.986), implying that the nature of adsorption was homogeneous via monolayer adsorption [48][49][50]. This suggested that C. vulgaris could successfully remove As (III) from aqueous solutions.

Bio-Adsorption Kinetics
Experiment data were evaluated with pseudo-first-order and pseudo-second-order models to investigate the As (III) bio-adsorption kinetics on C. vulgaris. The values of the constant kinetic parameters for As (III) bio-adsorption are shown in Table 3. The R 2 for As (III) calculated from the pseudo-first-order model was significantly higher (0.9801) than the values calculated from the pseudo-second-order kinetic model for C. vulgaris. The observed qe values (3.130 mgg −1 ) were confirmed by the computed qe values for As (III), derived using the pseudo-second-order model with the adsorbent [33]. Figure 7A shows a linear plot of As (III) and the correlation coefficient values (R 2 = 0.9801 for an initial concentration of 50 mgL −1 ). This indicated that the bio-adsorption of arsenic ions was best through pseudo-first-order kinetics. The slope and intercept of the plot in Figure 7B were used to calculate the bio-adsorption parameters q e and k 2 . Table 3. Arsenic ion (III) bio-adsorption kinetics parameters of C. vulgaris as a bio-adsorbent.

Parameter
Pseudo-First-Order Pseudo-Second-Order C. vulgaris than the Freundlich and Dubinin-Radushkevich isotherm models (R ; As = 0.7227 and 0.951, respectively). The Langmuir maximum bio-adsorption capacity (q max ) was 45 mgg −1 whilst in the case of the Dubinin-Radushkevich model, it was 8.7 mgg −1 . The As (III) adsorption behavior approximated the Langmuir isotherm better (R 2 ; As = 0.986), implying that the nature of adsorption was homogeneous via monolayer adsorption [48][49][50]. This suggested that C. vulgaris could successfully remove As (III) from aqueous solutions.

Bio-Adsorption Kinetics
Experiment data were evaluated with pseudo-first-order and pseudo-second-order models to investigate the As (III) bio-adsorption kinetics on C. vulgaris. The values of the constant kinetic parameters for As (III) bio-adsorption are shown in Table 3. The R 2 for As (III) calculated from the pseudo-first-order model was significantly higher (0.9801) than the values calculated from the pseudo-second-order kinetic model for C. vulgaris. The observed q e values (3.130 mgg −1 ) were confirmed by the computed q e values for As (III), derived using the pseudo-second-order model with the adsorbent [33]. Figure 7A shows a linear plot of As (III) and the correlation coefficient values (R 2 = 0.9801 for an initial concentration of 50 mgL −1 ). This indicated that the bio-adsorption of arsenic ions was best through pseudo-first-order kinetics. The slope and intercept of the plot in Figure 7B were used to calculate the bio-adsorption parameters q e and k 2 .

Thermodynamics of As (III) Bio-Adsorption
The temperature parameters during the sorption process are of immense significance. An increase in temperature generally accelerates the rate of chemical reactions. As part of the investigation, the Van' t Hoff equation and other relations were used to calculate the activation energy of arsenic sorption. Table 4 shows the values of ∆H = 31.214 kJ mol −1 , ∆S = 104.320 kJ mol −1 K −1 , and ∆G = −1.76 to −4.98 kJ mol −1 K −1 . Endothermic adsorption was indicated by the positive value of ∆H, whilst the negative value of ∆G implied spontaneous adsorption. The low activation energies of arsenate bio-adsorption on C. vulgaris implied that the sorption of arsenic on the algae could involve both physical and activated processes. However, the positive heat of adsorption of the bio-sorbent indicated that endothermic adsorption occurred [51]. Based on the experimental findings, increasing the temperature raised the arsenic bio-adsorption on C. vulgaris, indicating an endothermic process. Moreover, all free energy values at various temperatures were negative, indicating that arsenic adsorption on the adsorbents evaluated occurred spontaneously.   (20.9 mgg −1 at pH 6) had a potential adsorption efficiency for arsenic (III) from aqueous solutions when compared with Chlamydomonas sp. (53.8 mgg −1 at pH 4), C. vulgaris (13 mgg −1 at pH 6), C. vulgaris modified with NaCl (20.9 mgg −1 at pH 6), and Spirulina platensis modified with ZnCl2 NaCl (24.8 mgg −1 at pH 6). The results in Table 5 show that C. vulgaris (SAG 211-11b) as a bio-adsorbent was the best in the bio-adsorption of arsenic (III) from aqueous environments.

Conclusions
The study aimed to investigate the ability of green algae C. vulgaris to bio-adsorb As (III) simultaneously. To achieve the bio-adsorption of As (III) ions from aqueous solutions, C. vulgaris was employed. The findings indicated that the adsorption rate and removal percentage of As (III) ions were subject to the pH, initial metal concentrations, biomass dosage, and contact time as well as a rougher surface structure and larger specific surface area of bio-adsorbents. The optimal adsorptive ability of As (III) was found to be 55 mgg −1 with a 100% removal percentage. Among the models used, Langmuir was found to be the most precise model for As (III) bio-adsorption on the bio-adsorbent at an uptake of 45 mgg −1 compared with the Freundlich and Dubinin-Radushkevich isotherm models. The research concluded that the bio-adsorption process was endothermic and spontaneous, as per the thermodynamic studies. Consequently, the study proposed an efficient and environmentally friendly solution to simultaneously eliminate As (III) by C. vulgaris (SAG 211-11b).

Data Availability Statement:
The datasets obtained and analyzed in the current study are available from the corresponding author on reasonable request.