Sono-synthesised algae-based magnetic mesoporous adsorbent for dye adsorption: Characterization, reusability and toxicity assessment

ABSTRACT In this work, filamentous algae-based activated carbon was composited with Fe3O4 nanoparticles and applied as a potential adsorbent to remove Basic Blue 41. AAC/Fe3O4 nanocomposite was synthesised by the impregnation method and characterised by several techniques including, FTIR, FESEM, EDX, TGA, XRD, VSM, and BET. The characterisation results confirmed the existence of Fe3O4 in the nanocomposite structure, which had uniformly dispersed over AAC with mesoporous texture. The effects of various operational parameters on removal efficiency were investigated. The maximum AAC/Fe3O4 nanocomposite adsorption capacity (141 mg g−1) and removal rate (96.76%) were determined under optimum conditions (initial concentration of 100 mg L−1, solution pH of 9, nanocomposite dose = 1 g L−1 at 25°C for 90 min). The obtained adsorption data fitted well with the pseudo-second-order Langmuir isotherm model. The reusability assessment of AAC/Fe3O4 nanocomposite (with acidic solution) revealed about 11% decreases in the removal efficiency after five consecutive runs. Finally, bioassay studies using D. Magna confirmed AAC/Fe3O4 nanocomposite could create low toxicity and acceptable quality effluents. These attractive features make it a potential adsorbent for practical application in actual textile wastewater treatment.


Introduction
At the outset of the new millennium, the significance of environmental pollution control has soared due to industrialisation and globally increasing population density [1,2].Synthetic dyes are chemical compounds widely employed in numerous industries like textiles, pulp and paper, leather, cosmetics, and food processing [3].According to statistics and estimates, about 700,000 tons of dyes are produced per year, and roughly 15% of the total production is discharged into water resources without appropriate treatment as effluent [4].Basic Blue 41 with Azo bonds is classified as a cationic Azo textile dye; these dyes are not only toxic and carcinogenic for humans, but they also cause imbalances in aquatic ecosystems and are considered as difficult-to-handle organic dyes because of their Azo functional groups (-N = N-) and aromatic rings.Thus, these dyes are very stable and even more tenacious to eliminate from the aquatic media than anionic dyes [4][5][6][7].Hence, the purification of wastewater containing dyes is a significant issue and has forced researchers to find new methods to tackle this challenging environmental problem [8].In the last few decades, diverse treatment systems have been utilised for this purpose, such as filtration [9], coagulation and flocculation [10,11], and chemical oxidation [12].However, these methods are likely to have drawbacks, including dependence on high energy, generating chemical by-products, and low efficiency, which disrupts their performance [13,14].Among several chemical and physical treatment methods, the adsorption process with easy operations, low cost, high removal efficiency, and no harmful by-products are superior to other techniques [15,16].Adsorption efficiency is directly related to adsorbate features and adsorbent properties like surface area and pore structure, so it is crucial to find new adsorbents to meet the process requirements and take full advantage of their capability [17,18].Activated carbon has received wide currency among adsorbents because of its high surface area and porosity, thermal stability, and reusability in wastewater treatment [19,20].Plenty of carbon-rich materials have been adopted as precursors to the preparation of activated carbon.
Nonetheless, its application is restricted because of the high cost, which has led to exploring low-cost alternatives [21].Inexpensive materials such as green algae have been developed due to their rich carbon, high uptake capacities (because of biopolymers on the surface), and particular channel-like microstructure [22][23][24].Green algae can give rise to environmental problems, especially the eutrophication of seas, and their employment as the potential carbon source can be a desirable choice for generating AC and controlling marine ecosystem degradation simultaneously.Among green algae, Filamentous species compared to unicellular forms are the easiest and less costly to harvest; and could dramatically promote the economics of AC production [7,25].Despite the positive attributes of algae-based powderactivated carbon, powdered absorbents are tenacious to separate from effluents after the treatment process [26].Separation using filtration and centrifugation methods may not completely isolate the absorbent from the effluent; hence, a magnetised activated carbon has been suggested as a suitable solution.The modification of activated carbon with iron oxide as a magnetic source provides a quick and easy separation method compared to conventional filtration and centrifuge methods for adsorbent from effluent [27,28].Among the iron oxide nanoparticles, Fe 3 O 4 magnetic nanoparticles (Fe 3 O 4 Magnetic Nano Particles (MNPS)) have been attracted attention due to their superior physical and chemical properties, mesoscopic effect, non-toxic, chemical stability, uniform particle size, and biocompatibility [29,30].Accordingly, magnetic activated carbon (algae-based) nanocomposite can possess invaluable merits such as a remarkable increase in the surface area followed by high adsorption capacity, eco-friendly, and cost-effectiveness separation after the adsorption process.
Numerous studies have been conducted on the removal of dyes from aqueous solutions using magnetic adsorbents.The efficiency of magnetic carrageenan-g-poly for Crystal Violet adsorption was investigated, which adsorption capacity was 28.24 mg g −1 [15].Polyaniline/Magnetite composites were reported for Basic Blue3 removal, and results illustrated that the maximum capacity is 78.13 mg g −1 [31].Fe 3 O 4 -silica composite for acidic dyes indicated a maximum adsorption capacity of about 61.33 mg g −1 [32].The CoFe 2 O 4 -montmorillonite composite showed 90 mg g −1 of maximum adsorption capacity in Methylene blue removal (Ai et al. 2011b), and graphene-magnetite composite was able to adsorption of Methylene blue with an adsorption capacity of 61.33 mg g −1 [33].According to this simple literature survey and best knowledge, there is no accessible information concerning the removal of Basic Blue 41 using Fe

Reagents, materials, and solutions
All chemicals and reagents such as Ammonia Solution (NH 4 OH), Phosphoric Acid (H 3 PO 4 ), Hydrochloric Acid (HCL), Sulphuric Acid (H 2 SO 4 ), Sodium Hydroxide (NaOH), and Iron Chloride (FeCl 2 .4H 2 O & FeCl 3 .6H 2 O) were purchased from Merck in analytical grade.Basic Blue 41 (purity ≥ 98) as a pollution model was supplied by Alvan Sabet Hamadan, and the Structure of the Basic Blue 41 dye is presented in Figure S1 [6].The initial pH of the solution was adjusted by H 2 SO 4 (0.1 M) or NaOH (0.1 M).Distilled water was used throughout all the experiments.

Synthesis of Fe 3 O 4 nanoparticles
A precipitation correlation method was employed to synthesise magnetic Fe 3 O 4 nanoparticles: First: FeCl 3 .6H 2 O and FeCl 2 .4H 2 O with the ratio of Fe 3+ /Fe 2+ = 2:1 were dissolved in deionised water (200 mL) in an ultrasonic bath under N 2 gas.The suspension was kept in an ultrasonic bath under nitrogen flow for 45 min at a temperature of 60°C.Then, NH 4 OH (25 mL) solution was added dropwise at the same temperature until the suspension pH reached the value of 9 to precipitate the iron oxide.After 45 min completed the reaction, the black precipitate was collected by an external magnetic field and then washed several times with deionised water and ethanol.Finally, the attained nanoparticles were heated at 70°C in the oven, kept for 12 h, and then allowed to cool to room temperature [29].The precipitation correlation method with Fe 2+ and Fe 3+ ions is performed under alkaline conditions with magnetic formation [34], according to Equations 1-6.

Giving an overall reaction:
First, the Fe 3+ and Fe 2+ hydroxides precipitated; this reaction occurs at a rapid rate.Next, the Fe 3+ hydroxide degraded to FeOOH as the low water activity of the sodium chloride solution in a slower reaction.Eventually, a solid-state reaction occurs between FeOOH and Fe (OH) 2 due to the low water activity of the solution, which results in magnetite production.It takes between 10 and 30 minutes for a solid-state reaction to happen.Generally, the reaction mechanism is a dynamic equilibrium equation in which the concentrations and size of Fe 3 O 4 nanoparticles are affected by Fe 2+ , Fe 3+ , OH − , and the water activity of the solution [34].

Preparation of adsorbent
The Filamentous algae were collected from Shurabil Lake in Ardabil city, Iran.After that, it was washed several times with distilled water to remove contamination and dust.In the next step, the algae were dried in the oven at 60°C for 72 h, then their particle size reduced to about 100 meshes; the chemical activation process of the precursors was performed with phosphoric acid (H 3 PO 4 ).Powdered algae mixed with 30 wt% diluted H 3 PO 4 at a ratio of 3: 1, and the acid suspension was transferred in an ultrasonic bath for 45 min.According to the mesoporosity features of prepared nanocomposite from raw material, cavitation can turn to suspension [35].The H 3 PO 4 and algae powder mixture was placed in a furnace at 650°C for 3 h under a nitrogen flow rate of 94.4 ml min −1 .Finally, to remove residual mineral and organic matter, the samples were washed with hydrochloric acid (0.5 M), warm water (80°C), and deionised water, respectively, and were dried at 105°C for 2 h.The prepared activated carbon samples were crushed and then sieved to 100 meshes to obtain a homogeneous particle size [36].

Synthesis of magnetic activated carbon nanocomposite by impregnation
Activated carbon and magnetic Fe 3 O 4 nanoparticles with a ratio of 10:1 were added separately to distilled water (200 mL) and sonicated for 45 min.The Fe 3 O 4 nanoparticles suspension mixed with the activated carbon suspension, and sonicated for 45 min for more dispersion.Then the suspension was maintained for 2 hours under vigorous stirring to more mixture (300 rpm).The obtained products were washed several times with distilled water until the pH of filtrate water reached 7 ± 0.2, and then separated by an external 1.3-tesla magnet.The prepared nanocomposite was dried at 80°C for 12 h and stored in desiccators for the subsequent experiments [29].

Experimental procedure and analysis
Batch experiments for the adsorption of BB 41 onto AAC/Fe 3 O 4 nanocomposite were performed in 250 mL Erlenmeyer flasks containing different BB 41 concentrations and a certain amount of adsorbent.The solutions were stirred at room temperature at a constant speed of 250 rpm to reach equilibrium time.The adsorbent was immediately separated from the solution using a magnetic field upon reaching the adsorption equilibrium time.The residual dye concentration in solution was measured using a UV-Visible spectrophotometer at a wavelength of 609 nm.After equilibrium, the removal efficiency, RE (%), adsorption capacity, and q e (mg g −1 ) were calculated using Equations 7 and 8, respectively [37]: where C 0 and C e are the initial dye concentration and the dye concentration at the time of equilibrium (mg L −1 ), V is the volume of suspension (L), and M is the dry weight of AAC/Fe 3 O 4 nanocomposite in suspension (g).Kinetics studies were carried out using BB 41 solutions (250 mL) with concentrations of 50, 100, 150, and 200 mg L −1 at pH 9.These solutions were placed in contact with AAC/Fe 3 O 4 (1 g L −1 ) and shaken for times ranging between 5 and 160 min at 25°C.Adsorption isotherm study determined in initial dye concentration of 50, 100, 150, 200, 250, 300, 350, 400, and 450 mg L −1 ) in 250 mL solution with adding 1 g L −1 of the adsorbent at pH 9.

Analysis and characterisation
The UV-vis absorption of the dye samples was recorded using a UV-vis spectrophotometer (model DR 5000, HACH) at 617 nm and ultrasonic used to synthesise adsorbent (Elma Ultrasonic, 120 Hz).The X-ray diffraction (XRD) analysis was performed using the Philips and PW 1730 (40 kV and 30 mA) Holland, Netherland.The Fourier transform infrared spectroscopy (FTIR, Perkin Elmer) was prepared using potassium bromide (KBr) as a reference and recorded at 1 cm −1 of resolution in the wavenumber range 450-4000 cm −1 .FESEM and EDX technique (FE-SEM, MIRA III TESCAN) was applied to evaluate surface morphology and elements under an acceleration voltage of 10 KV.The N 2 adsorption-desorption isotherms were determined by BET analysis (II BELSORP mini) at 77 K to determine the specific surface area and pore volume of the AAC/Fe 3 O 4 nanocomposite.The TGA test was performed using a Shimadzu TG-50A instrument in a temperature range of 25-1000°C, and the ramping rate was 15°C/min.
For pHpzc determination, 100 mL of 0.01 M NaCl was transferred to a series of Elnermayer flasks, then 0.2 g of the adsorbent was added to the solutions, and its pH was adjusted in the range of 2-12 by adding 0.1 M H 2 SO 4 or NaOH.These Elnermayer flasks were kept for 48 hours under agitation at room temperature, and the final pH of the solutions was measured by a pH metre.Finally, the diagrams plotted of pH final vs. pH initial [38].

FESEM and EDX analysis
The AAC, Fe 3 O 4 , and AAC/Fe 3 O 4 surface morphology was investigated before dye adsorption using FESEM.The FESEM images of AAC, Fe 3 O 4 , and AAC/Fe 3 O 4 nanocomposite are shown in Figure 1(a-c), respectively.As can be seen, Figure 1(a) demonstrates the irregular and porous adsorbent morphology due to removing volatile compounds and impurities during the activation process with H 3 PO 4 .The algae-based activated carbon achieved at the temperature of 650°C had a porous surface, showing a high surface area.

FT-IR analysis
FT-IR spectroscopy presents compositional and structural information on the functional groups present in the adsorbent samples [39].FT-IR spectra of the AAC, Fe 3 O 4 , and AAC/Fe 3 O 4 nanocomposite are shown in Figure 3(a).The broad peak between 3200 and 4000 cm -1 and the bands at low wavenumbers (≤700 cm -1 ) was attributed to the stretching vibrations of hydrogen-bonded -OH and the Fe-O bonds in iron oxide, respectively.The presence of Fe 3 O 4 nanoparticles can be demonstrated by the appearance of a two-band absorption spectrum at 598 and 526 cm -1 .The absorption bands around 1600 corresponding to aromatic stretching vibrations of C = C and the stretching vibrations of C-O were observed at 980 and 916 cm -1 .The peak in 870 cm -1 was relative to C-H out-of-plane bending adsorption in the aromatic ring [40,41].Briefly, the FT-IR analysis confirms that OH, C = C, C-O, and C-H functional groups are present on the AAC/Fe 3 O 4 nanocomposite surface.It is worth noting that at high pH values, the whole -OH and C-H groups are destroyed, followed by the surface of AAC/Fe 3 O 4 nanocomposite is negatively charged, and the positively charged dye molecules can interact strongly with O -and C -sites present in the adsorbent.In general, this result demonstrates that cationic dye molecules are absorbed by electrostatic attraction onto AAC/Fe 3 O 4 nanocomposite [40,41].similar to other authors' researches [15,43].As seen in Figure 3

BET analysis
The specific surface areas (S BET ) and pore size distributions of adsorbents were calculated by using the multipoint Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods.Figure 3(c-d) shows the nitrogen adsorption/desorption isotherms of the AAC/Fe 3 O 4 nanocomposite and AAC.The results clearly show that H 3 PO 4 , ultrasonic process, and pyrolysis (at a temperature of 650°C) on the filamentous algae helps create mesoporous structure in the AAC/Fe 3 O 4 nanocomposite and AAC.In the IUPAC classification, the isotherms are classified into Type IV, indicating that the AAC/Fe 3 O 4 nanocomposite and AAC are mesoporous.The hysteresis loops (in the relative pressure range of 0.45-0.95)depict a capillary condensation step between p/pº of 0.5 to 0.9 and show the presence of mesoporous structures in both absorbents [40,44].According to Figure 3(c-d), it can be found that the AAC/Fe 3 O 4 nanocomposite has S BET than the AAC.The results BET surface area and pore volume of AAC and AAC/Fe 3 O 4 are presented in Table 1.The obtained results showed that the S BET and total pore volume for the AAC/Fe 3 O 4 nanocomposite and AAC are 588.83m 2 g −1 , 549.92m 2 g −1 , and 0.4927 cm 3 g −1 , 0.466cm 3 g −1 , respectively.The study results are similar to other studies, which first describe chemical activations that generally produce high surface area carbonaceous materials, and secondly show that chemical activation with acids results in the formation of porous carbon with more mesoporous particles [40,45].Similar results were observed for activated carbon prepared from potato Peels [46].The average pore diameter was about 2.4 nm for the AAC/Fe 3 O 4 nanocomposites and AAC, which play pivotal roles in the adsorption properties.On the one hand, materials' mesoporosity feature allows ions to penetrate easier into their porous [40,47].This result indicates that combining the Fe 3 O 4 nanoparticles with ACC could make a greater specific area of the new, enhancing the adsorption capacity.
According to pore size distribution results obtained by the DFT-based method (Figure S2), the pores in the AAC and AAC/Fe 3 O 4 nanocomposites were recognised to be in the range of 3-4.5 and 3.2-6.5 nm, respectively.The results illustrate that both AAC and AAC/Fe 3 O 4 nanocomposites lack 2-2.5 nm mesoporous, and great deals of them are <3 nm.The difference between BJH and DFT results can be due to instrumental analysis and their calculations.However, both methods confirm that the AAC and AAC/Fe 3 O 4 nanocomposites structure is mesoporous (2-50 nm).

VSM analysis
VSM measured the magnetic properties of the AAC/Fe 3 O 4 nanocomposite and Fe 3 O 4 nanoparticles.The magnetic hysteresis curves of AAC/Fe 3 O 4 nanocomposite and Fe 3 O 4 nanoparticles are shown in Figure 3(e).As can be seen, both adsorbents are magnetic at room temperature, and no hysteresis loop is observed.The mass saturation magnetisation S meso (m 2 g −1 ) V Total (cm 3 g −1 ) V micro (cm 3 g −1 ) V meso (cm 3  respectively [48].Since the weights of all particles applied for magnetic properties measuring were constant, mass saturation magnetisation declined due to the soared amount of AAC incorporated in the magnetite nanocomposite.The decrease of the saturation magnetisation is most likely ascribed to the span-80 existence on the surface of Fe 3 O 4 nanoparticles, which may create a magnetically quenching layer.With a remarkable fraction of surface atoms, any crystalline disorder within the surface layer may also cause a considerable reduction in the saturation magnetisation of nanoparticles.However, with decreasing of Ms in the AAC/Fe 3 O 4 nanocomposite still can be attracted towards the external magnetic field within 2 min, and it shows an acceptable magnetic response.Figure 3(f) shows the separation ability of AAC/Fe 3 O 4 nanocomposite after dye adsorption using Tesla magnets 1.3.The Ms of AAC/Fe 3 O 4 nanocomposite is sufficient for fast separation in the adsorption studies [49].

Thermogravimetric analysis (TGA)
TGA analysis was applied to the weight variations determination and thermal stability behaviour of AAC/Fe 3 O 4 nanocomposite as a function of temperature under a controlled atmosphere.TG curve of AAC/Fe 3 O 4 nanocomposite presented in Figure S3.It can be observed from the thermogram that active carbon and nanocomposite had three distinct stages of weight loss.In the first step of the plot that occurred at roughly 215°C, both samples indicated the weight loss of about 4% and 6.47% for AAC and AAC/Fe 3 O 4 nanocomposite, respectively.The weight loss in the first phase can be attributed to desorption and evaporation of the physisorbed water along with rigidly bound water onto materials [50].The higher weight loss of composite than AAC can be related to more water molecules in its structure.The weight loss second step happened approximately at the 215°C to 860°C range.It may be caused by the breakdown of low volatile organic compounds in the samples and their exit in gas [51].The decomposition process continued during the third phase (860-1000°C), but samples experienced a slight weight loss.
It is worth noting that at above 900°C, carbon began to decompose.Hence, the weight loss can be due to the degradation of the carbon in the structure of materials.Generally, AAC and AAC/Fe 3 O 4 nanocomposite lost about 26% and 17.7% of their weights, respectively.According to TGA results, the residual weight for AAC/Fe 3 O 4 nanocomposite was higher than AAC.It illustrates high thermal stability for nanocomposite due to the successful embedding of Fe 3 O 4 nanoparticles on its structure.

Effect of pH PZC
The pH PZC demonstrates the pH at which the surface charge of the adsorbent is zero.The pH PZC values are essential to investigate the effect of solution pH on the adsorbent's capacity.The pH PZC value in the AAC/Fe 3 O 4 nanocomposite was obtained at 7.49 (Figure 4 (a)).In the AAC/Fe 3 O 4 nanocomposite, the surface will be positively charged at pH<7.49, negatively charged at pH>7.49, and neutral for pH = 7.49.Therefore, AAC/Fe 3 O 4 nanocomposite is a suitable adsorbent for cationic dye adsorption from the aqueous solution.
The obtained pH PZC value for AAC/Fe 3 O 4 nanocomposite is similar to the data obtained from FTIR analysis; both suggest that the AAC/Fe 3 O 4 nanocomposite have acidic functional groups on their surfaces.

Effect of pH
The pH of the solution is one of the significant controlling parameters in the adsorption process.The pH range varied between 3 and 11 to assess the effect of initial pH on the BB41 adsorption; other parameters remained constant (reaction time of 60 min, the adsorbent dosage of 1 g L −1 , and BB41 initial concentration of 100 mg L −1 ).The effect of the initial pH variation on the adsorption capacity (q e ) of the AAC/Fe 3 O 4 nanocomposite is shown in Figure 4(b).The results showed that the adsorption capacity of BB 41 dye using AAC/Fe 3 O 4 nanocomposite was better at higher pH than lower pH.As the pH increased from 3 to 9, the rate of BB 41 adsorption capacity increased from 40.15 to 66.55 mg g −1 .At pH ≥ 10, the BB 41 becomes unstable; hence, no studies were performed above this value.The observed trend can be interpreted by the effect of pH PZC and adsorbent surface charges.The hydroxyl groups on AAC/Fe 3 O 4 nanocomposite confirmed by FTIR spectra play a vital role in the BB 41 dye adsorption.The numbers of OH-increase at higher pH in the solution and are adsorbed on the nanocomposite surface, followed by increased negative electrical charges on the adsorbent.Therefore, electrostatic attraction between the positive charge of dye and the negative charge of the adsorbent surface causes increases in cationic dye adsorption [27].A decrease of adsorption capacity in acidic conditions is probably related to the reduction in electrostatic affinity between positive surfaces onto AAC/Fe 3 O 4 nanocomposite with cationic dye.In other words, at low pH values, there is a competition between the extra H + ions with the cationic dye and repulsive force between the BB 41 dye and AAC/Fe 3 O 4 nanocomposite surface with a positive charge.Such results have been previously reported [52].Moreover, the existence of −COOH and Fe-OH functional groups in AAC and AAC/Fe 3 O 4 structure was proven by FTIR analysis.These functional groups in alkaline pHs can be converted to Fe−O − and -COO − and enhance the removal efficiency through interaction with cationic dyes.These claims are consistent with the results of other studies.Atar et al. reported that the maximum adsorption capacity of Basic Blue 41 using Bacillus maceran was found to be 89.2 mg g −1 under alkaline conditions (pH = 10) [53].Noreddine Boudechiche et al. [54] have investigated two basic dye removals using activated carbon from Ziziphus lotus stone; results indicated that high adsorption capacity for both dyes occurred at pH ~ 8.

Effect of AAC/Fe 3 O 4 dose
The effect of AAC/Fe 3 O 4 nanocomposite dosage on the adsorption process was investigated by changing the absorbent dosage from 0.1 to 2 g L −1 .Figure 4(c) presents adsorption capacities (q e ) and removal percentage versus adsorbent dosage.The results indicated that the dye removal rate enhanced from 14.95% to 96.11%, increasing adsorbent dose from 0.1 to 2 g L −1 .The number of active sites and hydroxyls on the absorbent surface soared by increasing the adsorbent dose.Thus the dye removal rate increases noticeably [53,55].It is worth noting that the highest removal rate occurs at 1 g L −1 of adsorbent dose; with increasing dosage until 2 g L −1 , the removal efficiency remains roughly stable.The adsorption of almost all the BB41 molecules onto nanocomposite surfaces and establishing equilibrium circumstances between the pollutant model and adsorbent can cause such a phenomenon.However, the adsorption capacity declined from 151.2 mg g −1 to 48.5 mg g −1 with soring adsorbent dosage.To clarify, when the nanocomposite dose is low in the solution, the dye molecules are in touch with all existing active sites.Accordingly, these molecules became adsorbed, and the nanocomposite surface was saturated.In the opposite situation, these molecules adsorbed quickly due to exposure to active sites requiring less energy to adsorb.The active sites needing high-energy levels to adsorb contaminants may not contact dye molecules, resulting in dropped adsorption capacity [56].Moreover, it can be attributed to overlapping or aggregating adsorbent particles, resulting in a decrease in the total adsorbent surface area available to the dye and an enhancement in diffusion path length.Similar results were provided in previous research.In a study done by Kamaraj et al. [19] for dye removal onto groundnut shell-activated carbon, the results showed the dye's removal efficiency with increasing adsorbent dose (from 0.2 to 1 g L −1 ) increased to a certain amount, and then its amount was constant.The obtained results from Gülşah Mersin et al. [57] investigation indicating with increasing magnetic clinoptilolite dose, the adsorption capacity of BB41 soared significantly (370.37 mg g −1 ).

Effect of dye concentration
The experiments were conducted using initial BB41 concentrations ranging from 50 to 450 mg L −1 under optimum conditions (pH = 9, nanocomposite dosage = 1 g L −1 , and equilibrium time = 120 min) to assess the effect of initial dye concentration on adsorption efficiency.As depicted in Figure 4(d), the adsorption capacity rate (q e ) ascended from 49.15 mg g −1 to 148.88 mg g −1 with a rising initial dye concentration from 50 to 450 mg L −1 .However, but the removal percentage of the dye descended from 98.3% to 20.66%.
Increasing the amount of q e could be due to the promotion of interactions and the chance of contact between adsorbent surfaces and dye molecules [41].The formation of hydrogen bonding between BB41 molecules and AAC/Fe 3 O 4 nanocomposite maybe happen.This phenomenon, alongside π-π stacking and electrostatic interaction, can positively affect adsorption capacity in high dye concentrations.Also, the driving force between the dye solution and the nanocomposite surface at high concentrations is higher; this factor facilitates external mass transfer [29,58].Simultaneously, the internal mass transfer can occur quickly [41].However, declining the removal efficiency with an increase in BB41 concentration could be justified by binding all dye molecules with nanocomposite surfaces at low concentrations.While at high concentration, the available adsorption positions are reduced, and the build-up of BB41 molecules on the surface of AAC/Fe 3 O 4 .It prevents the diffusion of more dye molecules into the adsorbent pores.The low diffusion rate is related to pores, similar to the diffusing molecules [59,60].Similar results have been reported in several studies.Bagheri et al. assessed the dye adsorption by magnetic-activated carbon; they stated that the adsorbent efficiency declines with increasing dye concentration.The best performance was seen in 15 mg L −1 of methylene blue concentration [29].Moreover, our claim was proven by a study conducted by Niyaz Mohammd Mahmoodi et al. [61] on Direct Red 23 using zeolite nanoparticles.This research showed that as the initial dye concentration increased, the adsorbed amount of DR23 increased, and this happened while the dye removal decreased.

Effect of contact time
Under optimum operational conditions, the exposure time effect on the adsorbing BB41 was evaluated at diverse dye concentrations (50, 100, 150, and 200 mg L −1 ).As presented in Figure 4(e), dye adsorption is quick at the first 45 minutes.After that, it slightly increases until it reaches equilibrium time.Adsorption dye by AAC/Fe 3 O 4 nanocomposite is almost constant after the contact time of 90 minutes.The acute jump of dye sorption at beginning times can be related to a shorter equilibrium time, owing to plenty of active sites available on absorbent surfaces for adsorbing dye molecules, succeeding in external adsorption.Besides, many accessible free functional groups such as -OH and Fe-O on the nanocomposite surface can cause fast adsorption in initial times.After 45 minutes, the remaining empty sites were filled gradually by dye molecules; at this stage, adsorption is complex due to less attraction force between adsorbent and adsorbate and cause aggregation of dye molecules around the adsorbent surface [58].The intra-particle diffusion in this phase may also be corresponding to a slower adsorption rate.A similar trend was observed previously in another work.Lamia Dali Youcef et al. [62] investigated the cationic methylene blue dye adsorption using Algerian palygorskite; they found that 97% of dye uptake was obtained after 5 min contact time, adsorption process was not significant.Also, Adeela Kanwal et al. [63] reported that maximum basic dye adsorption using Clay/MnFe 2 O 4 composite occurred at the first 30 min of contact time and after this time adsorption rate became constant.

Effects of temperature
Temperature is a significant parameter to obtain knowledge of the thermal nature of the adsorption process.Therefore, the effect of temperature on the adsorption of BB 41 dye was investigated in optimum conditions at different contact times by changing temperature from 10 to 45°C.As shown in Figure 4(f), the dye removal percentage increases significantly with increasing temperature.The removal rate grew from 75 to 96% with rising temperature from 10 to 40°C after 60 min; it affirmed the endothermic character of the process.The adsorptions increased with temperature, maybe because of enhancing adsorbate molecules diffusion towards the external boundary layer.The chemical interaction between sorbate ions and AAC/Fe 3 O 4 surface functionalities increases with elevated temperature [31].
Meanwhile, some water molecules can desorb from nanocomposite as temperature improves, which causes more pores to open to BB41.The results of our study are consistent with the findings of other researches.Adsorption of methyl orange dye onto multiwalled carbon nanotubes was assessed by Donglin Zhao et al. [64]; they found that MO adsorption increases with increasing temperature until 293 K.In another study, Murat Akgül et al. investigated dye adsorption using desilicated natural zeolite.The temperature influence on adsorption processes indicated that increasing the temperature positively affected adsorption efficiency, and 323 K was reported as optimum temperature [65].

Adsorption kinetics
Adsorption kinetic models are suitable for interpreting experimental data to increase insight into the adsorption efficiency and the nature of interactions between BB 41 molecules and the AAC/Fe 3 O 4 nanocomposite surface.For adsorption kinetic study, dye solutions in various concentrations were investigated in optimum conditions.The kinetic models of pseudo-first-order and pseudo-second-order adsorption are frequently used to examine the experimental data obtained.These models can be expressed as below Equations ( 9) - (10) [66]: where t, q t , and q e are the contact time (min), the sorption capacity at any time t, and equilibrium (mg g −1 ), respectively; k f and k S are the pseudo-first-order rate constant (min −1 ) and the pseudo-second-order constant rate (mg g −1 min −1 ), respectively.
The nonlinear models for pseudo-first-order, pseudo-second-order kinetics, and calculated parameters for kinetic models are presented in Figure 5 and Table 2, respectively.
According to the results presented in Table 2, it is observed that the adsorption of BB 41 dye onto AAC/Fe 3 O 4 nanocomposite adsorbent follows the pseudo-second-order kinetic model with the highest coefficient of determination and the lowest standard deviation (SD).It implies that the rate-limiting step on the system may be chemisorption, which involves valency forces through the sharing or exchange of electrons between the AAC/Fe 3 O 4 nanocomposite hydrophilic sites and the BB 41 dye molecules [67].The k S values of the pseudo-second-order model decrease with the increasing initial concentration of BB 41 dye (Table 2), which may be due to competition between higher levels of ions and limited adsorption active sites on the adsorbent [40].Pseudo-second-order kinetic model was observed that better fit for the adsorption of methylene blue on activated lignin-chitosan [59], and activated carbon for adsorption of tetracycline antibiotic [68].

Adsorption isotherms
Adsorption isotherms studies indicate the relationships between the adsorbate and absorbent allow determine the theoretical maximum adsorption capacity of an adsorbent to a given adsorbate [68].In the present study, Langmuir and Freundlich isotherms were used to analyse adsorption data at different dye concentrations [52].The Langmuir isotherm describes the monolayer adsorption on the surface of porous materials Table 2. Comparison of pseudo-first-order and pseudo-second-order model's parameters, calculated q e (cal), and experimental q e (exp) values for different initial BB 41 concentrations.

Pseudo-first-order
Pseudo-second-order C O (mg L −1 ) q e, exp (mg g 1 ) q 1, cal (mg g −1 ) q 2, cal (mg g 1 ) k S (g mg − where q e , Ce, and Q max are the amount of adsorbate (BB 41) adsorbed at the equilibrium (mg g −1 ), the equilibrium concentration of adsorbate (mg L −1 ), and the maximum adsorption capacity of adsorbent (mg g −1 ), respectively; K L and K F are the Langmuir equilibrium constant (L mg −1 ), Freundlich equilibrium constant [(mg g −1 ) (L mg −1 ) 1/n ], n is the dimensionless exponents of the Freundlich.The adjusted determination coefficient (R 2 adj ), determination coefficient (R 2 ), and the standard deviation (SD) were used to compare the suitability of the evaluated models.The standard deviation indicates the difference between the theoretical q e values predicted with the actual q e value measured experimental.The respective mathematical equations of R 2 , R 2 adj , and SD are respectively given below by Equations 13 and 15: [66].In these equations, q i , exp represents unique values of q e measured experimentally; qi, model represents individual values of q e predicted by the fitted model, q exp represents the average value of all q e measured experimentally; n is the number of experiments performed, and p represents the number of fitted model parameters.In the Langmuir isotherm, the dimensionless equilibrium parameter, the Langmuir R L separation factor is calculated using the following equation: where C o is the highest initial dye concentration (mg L −1 ).The R L <1 value indicates the favourable isotherm type when the model is fit [69].
Figure 5 presents the experimental data and their compatibility with the Langmuir and Freundlich models.The predicted parameters through the nonlinear adsorption models are displayed in Table 3.According to the results, the Langmuir isotherm model showed the highest R 2 value and the lowest SD value, and also, the adsorption process of BB 41 dye occurs in monolayer.The result suggests that the mesoporous particle supported the adsorption process in the AAC/Fe 3 O 4 nanocomposite.The calculated R L values in different dye concentrations were much lower than 1, indicating the favourable adsorption of BB 41 dyes by AAC/Fe 3 O 4 nanocomposite.
Additionally, the Langmuir model determined the maximum monolayer adsorption capacity (141 mg g −1 ); it confirms that this adsorbent is effective for the BB 41 dye removal.As a result, the Langmuir model best describes the equilibrium data for this adsorption process.The obtained results are agreed with the other studies such as adsorption of Basic Blue 41 dye on Raw and Modified Rice Husk by Faraji et al. [70], V.K. Gupta et al. in the adsorption of Cadmium on orange peel and Fe 2 O 3 nanoparticles [43], and Crystal Violet dye adsorption onto magnetic particles by Mostafa Gholami et al. [15].Table S1 shows a comparison of the adsorption capacity of different adsorbents for BB41 dye.It is noteworthy that the AAC/Fe 3 O 4 nanocomposite showed higher adsorption capacity for BB 41 dye than many other adsorbents.

Adsorption mechanism
According to the above surveys and obtained results, the physical process through van der Waals force occurs mainly during the adsorption process of BB41 by AAC/Fe 3 O 4 nanocomposite.This phenomenon can be attributed to well-developed porosity and large surface area of AAC/Fe 3 O 4 , which could be garbed via BB41 [71].Simultaneously, chemical interactions also take place in the adsorption process.The benzene groups and nitro in BB41 including the π-electron-acceptors with strong electron-withdrawn ability; hence, there is an intense interaction between BB41 and AAC/Fe 3 O 4 , which possesses graphite surfaces of rich polarised π-electron by π-π electro-donor acceptor interacting on the adsorption process [72].Additionally, other interaction types can exist between -N = N-,-N = C = C = C-N and O-containing groups such as (-OH, C-O and Fe-O) on the AAC/Fe 3 O 4 surface.

Regeneration and reusability studies
The recycling and regeneration capabilities of the adsorbent are crucial for their practical applications.The spent adsorbents are of no benefit and are disposed of as wastes, creating severe environmental problems.Therefore, to survey the reusability of AAC/Fe 3 O 4 nanocomposite, sequential tests (under optimum conditions) were carried out up to five times in the presence of HNO 3 (0.1 M) and NaOH (0.1 M) as a regenerator.For each test, AAC/Fe 3 O 4 nanocomposite (1 g) was added in 100 mg L −1 of BB 41 dye solution and separated from the solution after treatment.Afterwards, it was washed by regenerators, followed by drying at 100°C for 12 hours for the next cycle.Figure 6(a) illustrates good reusability and high removal efficiency (83.78%) for nanocomposite after five cycles with regeneration by HNO 3 (0.1 M).Nevertheless, nanocomposite regeneration using NaOH was not successful, and the removal rate after five cycles decreased by about 53%.Since acid produces positive and hydrated hydrogen ions, dissolving in water and reducing the solution pH are likely to protonate functional groups and desorption of cationic dye molecules [73].

Comparison of adsorption capacity of AAC/Fe 3 O 4 , AAC and Fe 3 O 4
The adsorption capacity of AAC/Fe 3 O 4 , AAC, and Fe 3 O 4 were compared under the same optimum conditions.) C e (mg L -1 ) q e (mg g -1 ) Freundlich Langmuir analysis showed that the specific surface area of AAC/Fe 3 O 4 nanocomposite is higher than the AAC adsorbent; this result confirms that combining the Fe 3 O 4 with the ACC adsorbent could make a greater specific area.Since the Fe 3 O 4 nanoparticles possess limited adsorption ability and specific surface area, they cannot perform dye adsorption efficiently.As a result, magnetise of AAC by Fe 3 O 4 can diminish its adsorption capacity to some extent.

Toxicity assessment
In full-scale application, after treatment, treated wastewater is released to receiving streams; therefore, the toxicity evaluation is one of the appropriate methods to investigate the potential of discharging it into the environment.Thus, in this study, Daphnia Magna was the tasted-organism since they are sensitive [25,74] for toxicological implications.Accordingly, the toxicity trend of BB41 adsorption was appraised on samples taken from treated sewage several times (30, 90, and 150 min) by exposure of D. Magna to the solution (8, 24, 48, 72 hours).As is indicated in Figure 7, the toxicity rose in the first 90 min; after that increasing the time of adsorption, the mortality rate was declined remarkably, and the highest detoxification rate was seen after 150 min.Based on these results, the effluent toxicity after adsorption is at least several times lower than the non-treated effluent toxicity.

Dye adsorption from natural wastewater
To investigate the industrial-scale efficiency of AAC/Fe 3 O 4 nanocomposite the natural textile wastewater treated under optimum adsorption conditions (initial dye concentration of 100 mg L −1 , equilibrium contact time, the adsorbent dosage of 1 g L −1 , and pH of 9).Physico-chemical characteristics of textile wastewater are listed in Table 4.
According to attained results, the removal rate for synthetic and natural wastewater was 93.66% and 87.56%, respectively.In natural textile wastewater, the presence of different impurities may reduce the mass transfer of the dye molecules onto the adsorbent, so it needs more time to reach equilibrium conditions than the synthetic type.Nevertheless, the applicability of AAC/Fe 3 O 4 nanocomposite in actual textile wastewater treatment is perfectly justifiable.

Conclusions
In the present research, Filamentous algae were utilised for activated carbon preparation, then Fe 3 O 4 magnetic nanoparticles embedded on AAC adsorbent by impregnation method for removing Basic Blue 41 dye from aqueous solution.The prepared nanocomposite was characterised by various techniques such as SEM, EDX, TGA, XRD, VSM, FTIR, and BET.The EDX and XRD results confirmed the correct formation of AAC/Fe 3 O 4 nanocomposite.DFT calculation showed AAC and AAC/Fe 3 O 4 nanocomposite with pore distribution <3 nm possessed mesoporous textures.According to the VSM test, saturation magnetisation for nanocomposite acquired 5.53 emu g −1 , favourable for simple separation by external magnet field.The adsorption studies of BB41 on AAC/Fe 3 O 4 nanocomposite revealed that under optimum conditions (pH = 9, nanocomposite dosage = 1 g L −1 , and equilibrium time = 120 min) with increasing initial dye concentration from 50 to 450 mg L −1 , the adsorption capacity rate (q e ) ascended from 49.15 mg g −1 to 148.88 mg g −1 ; in contrast, the removal percentage of the dye descended from 98.3% to 20.66%.The kinetic studies depicted that the pseudo-second-order model can well describe dye adsorption behaviour.Also, the adsorption isotherm was fitted with a Langmuir isotherm.The natural textile wastewater was effectively treated (removal rate = 87.56%)by nanocomposite.Besides, the regeneration and toxicity assessments approved that the AAC/Fe 3 O 4 nanocomposite had appealing features for field application and provided effluents with less toxicity from a health perspective.
Figure 1(b) demonstrates that the Fe 3 O 4 nanoparticles are uniform, homogeneous, and spherical.The particle size distributions were determined to approximate size around 30 nm.Figure 1(c) indicates that some Fe 3 O 4 nanoparticles tend to get aggregated onto the AAC surface, and a strong aggregation is evident [30].There were uneven surfaces of the Fe 3 O 4 magnetic nanoparticles in Figure 1(c).Rough and uneven surfaces of the Fe 3 O 4 nanoparticle have been increased the adsorption sites of AAC.Moreover, EDX analysis, including spectrum and elemental composition (Figure 2), illustrating AAC/Fe 3 O 4 nanocomposite contains peaks responding to the C, Fe, and O atoms.The peaks of Fe (37.67) and O (18.83%) are related to Fe 3 O 4 , while the C (43.5%) atom is associated with AAC.The results of EDX confirm the deposition of Fe 3 O 4 nanoparticles on the activated carbon.
(b), the XRD pattern of the AAC/Fe 3 O 4 composite demonstrates the primary phase of the Fe 3 O 4 nanoparticles and AAC, which indicated the presence of Fe 3 O 4 nanoparticles and AAC on the AAC/Fe 3 O 4 nanocomposite.The results of the XRD analysis exhibited that Fe 3 O 4 nanoparticles have been successfully synthesised and coated onto activated carbon.In addition, there is no change in the structure of Fe 3 O 4 nanoparticles and AAC during the nanocomposite synthesis.

Figure 3 .
Figure 3. FTIR spectra (a), typical XRD patterns (b), N 2 adsorption -desorption isotherm (c) and the pore size distribution (d) of AAC, Fe 3 O 4 and AAC/ Fe 3 O 4 nanocomposite before dye adsorption and Magnetic field of Fe 3 O 4 nanoparticle (e) and AAC/Fe 3 O 4 nanocomposite and the separation of AAC/Fe 3 O 4 nanocomposite from solution under an external magnetic field (f).

Figure 4 .
Figure 4. pH PZC (a), Effect of initial pH (b), adsorbent dosage (c) and the initial dye concentration (d), contact time (e) and different temperatures (f) on the adsorption of BB 41 by AAC/Fe 3 O 4.

Figure 5 .
Figure 5. Kinetic plots for the adsorption of BB 41 onto AAC/Fe 3 O 4 at different dye concentration (pseudo-first-order and pseudo-second-order) and Langmuir and Freundlich isotherms.

Figure 6 (
b) demonstrates that AAC/Fe 3 O 4 nanocomposite and AAC adsorbent have better adsorption ability than Fe 3 O 4 , mainly because of their porous surface.By comparing the AAC absorbent and the AAC/Fe 3 O 4 nanocomposite, it can be seen that the absorption capacity (q e ) has been slightly decreased by adding Fe 3 O 4 .This slight decrease was probably due to the non-porous Fe 3 O 4 .Also, obtained results from BET

Figure 6 .
Figure 6.Recycling and regeneration of AAC/Fe 3 O 4 nanocomposite in five cycles (a) and comparison of adsorption capacity on three adsorbents (b).
3 O 4 /Alga Activated Carbon nanocomposite.This study evaluated the applicability of AAC/Fe 3 O 4 nanocomposite for Basic Blue 41 adsorption as a contaminant model in textiles industries.The chief purposes of this study were to (A) synthesis of Fe 3 O 4 /Alga Activated Carbon (AAC/Fe 3 O 4 ) nanocomposite by the impregnation method; (B) characterisation of structural and morphological properties of prepared nanocomposite by various techniques; (C) investigate the influence of different operational parameters on removal efficiency; (D) determination of the kinetic rate and equilibrium parameters; (E) estimate the stability and reusability of the nanocomposite; and (F) evaluation of plausible adsorption mechanism and toxicity assessment.

Table 1 .
The S BET and pore volume of AAC and AAC/Fe 3 O 4 nanocomposite.
Fe 3 O 4 nanocomposite and Fe 3 O 4 nanoparticles found 5.53 and 59.59 emu g −1 , [40]er of homogeneous active sites.The Freundlich isotherm is an experimental model showing that the adsorption process occurs in multilayers on the heterogeneous surface formed by active sites with different energies[40].The nonlinear equation of the Langmuir and Freundlich isotherm models are given in Equations 11 and 12, respectively.

Table 3 .
Fitting results of Langmuir and Freundlich adsorption isotherm models.