Biosorption Potential of Sargassum for Removal of Aqueous Dye Solutions

Abstract

Sargassum muticum is an invasive species to the coasts of the British Isles, mainland Europe and North America, with negative ecological and socioeconomic impacts. Pelagic Sargassum inundations on the beaches of the Caribbean have also been causing adverse health, ecological and economic effects. The finding of commercial uses of these biomasses may alleviate the costs of removal and control. Both pelagic Sargassum and S. muticum could be low-cost biosorbents for removing aqueous cationic dyes but may not be suitable for anionic substances without modification. This study found that a Sargassum biomass could remove up to 93% of methylene blue and that the species, concentration and treatment (CaCl₂) were all statistically highly significant factors (p < 0.001) in its removal.

1. Introduction

Sargassum is a broad genus or family of brown seaweed with over 300 species [1]. Three species of Sargassum have recently been causing environmental and economic issues [2,3,4]. S. muticum, which is native to the northwest Pacific region [5], appeared on the shores of Europe in the 1970s, displacing native species and having negative environmental impacts and is classified in many European countries as an invasives species [6,7,8]. Pelagic Sargassum, particularly Sargassum fluitans and S. natans, is a tremendous ecological resource floating in the open ocean [4,9,10,11]. Small quantities arriving on the beach give environmental benefits, such as dune stabilisation [3,4,9,11,12]. Nevertheless, beaches across the Caribbean and the Gulf of Mexico have experienced massive inundations of pelagic Sargassum since 2011, negatively impacting the environment, human health and the local economies [3,13,14,15,16,17,18,19,20,21]. Removing and disposing of Sargassum is costly [3,9,11,22,23] and applications that generate revenue are being researched [3,4,22].

The living and dead cells of seaweeds can effectively remove heavy metals and other pollutants from wastewater [24,25,26,27]. Both S. muticum and pelagic Sargassum were suggested as biosorbents [2,9,26,28,29]. Synthetic dyes are found in the wastewater streams of many industries and can be highly resistant to conventional biological wastewater treatments, leaving behind highly coloured and toxic effluents [30,31]. Methylene blue (MB) is a cationic dye that is commonly used in the textile industry. Ingestion may cause respiratory issues, nausea, jaundice and skin irritation [32]. Some initial studies have shown that both S. muticum and pelagic Sargassum can effectively remove synthetic dyes, such as Methylene Blue [28,33]. However, these studies used freshwater washed and pretreated Sargassum with H₂O₂, CaCl₂, HCl and formaldehyde to improve the absorption capacity via chemical modification, including protonation and chemical crosslinking [28,33]. Enhanced biosorption capacity is attributed to the protonation of cell wall components and improved stability by the Ca²⁺ ions of alginate molecules via the formation of the characteristic alginate arrangement, known as the egg-box structure.

Azo dyes, which are synthetic colours containing an azo group (-N=N-), are the most used dyes (60–70%) in industrial applications, including printing, textiles, tanning, packaging and, to a rapidly reducing extent, in the food industry [34,35,36]. Congo Red (CR) is an anionic dye that is often used in experiments as a typical example of one of the hundreds of potential azo dyes [34,35,36]. Brilliant Blue R (BB), which is a disulfonated triphenylmethane dye, is an anionic dye that is widely used in biotechnology. It was effectively removed using a fungal biosorbent (A. tubingensis), with the highest removal efficiency at lower pH values (pH 2) where the nitrogen-containing functional groups of the fungus are positively charged, allowing for electrostatic interaction with the OH⁻ groups of the dye [37].

This study examined the biosorption of three dyes (MB, CR and BB) using two types of Sargassum with a minimum of pretreatment (S. muticum freshwater rinsed and freeze-dried and mixed pelagic Sargassum drained and freeze-dried). This research also studied the effect of CaCl₂ pretreatment of Sargassum on the biosorption of MB.

2. Materials and Methods

2.1. Sample Collection and Pretreatment

Sargassum muticum was collected in Broadstairs, Kent (UK) (54.3602° N, 1.4320° E), in Spring 2020. The seaweed was rinsed with deionised water and frozen at −20 °C, then freeze-dried for 72 h (ScanVac, Coolsafe, Laboscene freeze drier running at −50 °C). Mixed sargassum samples (Sargassum fluitans, Sargassum natans I, Sargassum natans VIII) were collected from Shark Bay, South Caicos, the Turks and Caicos Islands 55 (21.491° N, 71.503° W), between September 2020 and May 2021. The mixed samples were frozen at −40 °C. (Harvest Right HRFD-PMed-SS, Salt Lake City, Utah USA). Samples were frozen to −40 °C. A vacuum established <66 Pa in the chamber. During the drying phase, trays were warmed to 52 °C at <66 Pa for 26 h. At the end of this process, samples were double-bagged and shipped via air to the University of Greenwich, UK. Seaweed (2.5 g) was incubated in a CaCl₂ solution (0.2 mol L⁻¹), pH 5.0, for 24 h while stirring. The biomass was then filtered (70 mm qualitative filter paper) and washed twice with deionised water to remove excess calcium. The treated biomass was then dried in an oven at 60 °C for 24 h.

2.2. Methylene Blue, Brilliant Blue and Congo Red Dye Solutions

Methylene blue (C₁₆H₁₈ClN₃S·3H₂O), brilliant blue (C₄₅H₄₄N₃NaO₇S₂) and congo red (C₃₂H₂₂N₆Na₂O₆S₂) were purchased from Sigma Aldrich (Gillingham, UK). The chemical structures are shown in Figure 1. Different stock solutions were prepared from a 1000 mg L⁻¹ stock solution. Solutions for calibration curves were created by diluting the stock solution (0–312.5 mg L⁻¹).

2.3. Biosorption Study

Experiments using different amounts of biomass (0.01–0.4 g) and dye solutions of 10 mg L⁻¹ (80 mL) were conducted in Erlenmeyer flasks (250 mL) at room temperature (25 ± 2 °C) while stirring with contact times between 0–120 min. Samples were centrifuged at 5000 rpm for 2 min. All experiments were carried out in triplicates. The supernatant was analysed using a UV spectrophotometer (Jenway 6305, Fisher Scientific, Loughborough, UK) at wavelengths of 664 nm (MB), 554 nm (BB) and 497 nm (CR) to determine the dye concentrations. The removal efficiency (η) was calculated using the initial and residual MB concentrations as follows:

$$\mathsf{\eta }=\frac{C_0-C_e}{C_0}\times 100\%$$

(1)

where C₀ is the initial concentration of the dye in solution and C_e is the equilibrium concentration of dye.

The equilibrium biosorption capacity (Q_e) was calculated according to Equation (2).

$$Q_e=\frac{\left(C_0-C_e\right)\times V}{m}$$

(2)

where Q_e (mg g⁻¹) is the amount of dye adsorbed by the biomass, C₀ and C_e (mg L⁻¹) are the initial and equilibrium concentration of the dye solution, and m (g) is the amount of dried biomass.

A first-order kinetic equation was fitted to the MB adsorption by Sargassum (Equation (3)) [38] using the nonlinear regression function for parameter estimation in IBM SPSS Statistics (v27) (IBM Corp, Armonk, NY, USA).

$$q\left(t\right)=q_e\left(1-e^{-kt}\right)$$

(3)

where q(t) is the amount of dye adsorbed at time t, q_e is the amount of dye adsorbed at equilibrium and $k$ is the adsorption rate constant.

2.4. Statistical Analyses

IBM SPSS Statistics 25 SPSS was used for a three-way ANOVA on the effects of species (Pelagic Sargassum or S. muticum), pretreatment (CaCl₂), Sargassum concentration and their interactions on the final concentration of MB. Excel 2021 (Microsoft Corporation 2021)) was used for t-tests to compare the statistical significance of the effects of species (Pelagic Sargassum or S. muticum), pretreatment (CaCl₂) and Sargassum concentration on the final concentration of MB.

3. Results

Effect of CaCl₂ Treatment on the Biosorption of Methylene Blue

At a biosorbent dose of 0.1 g, removal efficiencies of 89.98% and 82.55% were achieved in the initial 30 min of biosorption for CaCl₂-treated and untreated S. muticum, respectively. The difference between the treated and untreated biomass after 30 min was found to be significant (p = 0.003, t-test, unequal variance, two-tailed). When the biosorbent was increased to 0.4 g, the removal efficiency increased to 95.48% within the initial 30 min for CaCl₂-treated S. muticum but remained fairly constant at 81.19% for the untreated S. muticum, indicating that the pretreatment may have some effect on the biosorption capacity of methylene blue (Figure 2). For the untreated S. muticum, no significant difference was observed between biosorbent doses of 0.1 g and 0.4 g after 30 min (p = 0.762) and 60 min (p = 0.299); however, after 120 min at 0.4 g, the concentration in the solution increased, indicating desorption of the dye, decreasing the removal efficiency to 76.09% compared to 82.91% at 0.1 g, which was a significant difference (p = 0.048).

The three-way ANOVA showed that species, Sargassum concentration and treatment (CaCl₂) were statistically highly significant factors (p < 0.001) in the MB removal. The interactions between the species and Sargassum concentration (p < 0.05) and Sargassum concentration and CaCl₂ (p < 0.001) were also statistically significant factors in the removal of MB.

To describe the adsorption of MB on Sargassum, a first-order kinetic model was fitted with a high coefficient of determination (

Table 1. Kinetic parameters q_e is the amount of dye adsorbed at equilibrium, $k$ is the adsorption rate constant and R² is the coefficient of determination between calculated and average experimental results.

	qe	k	R2
0.1 S. muticum CaCl2	93.641	0.107	0.999
0.4 S. muticum CaCl2	94.538	-	0.999
0.1 Mixed Sarg CaCl2	88.700	0.099	1.000
0.4 Mixed Sarg CaCl2	91.892	0.165	0.999

). It was generally seen that the amount of dye adsorbed at equilibrium was higher in CaCl₂-treated seaweed compared to untreated seaweed, with a maximum difference of 17.29% between the 0.4 g CaCl₂-treated and untreated mixed Sargassum. For the CaCl₂-treated seaweed samples (0.1 g biomass), the adsorption constant differed by only 0.008 between S. muticum and mixed Sargassum. There was a relatively higher difference in the adsorption constants between using 0.1 g or 0.4 g of CaCl₂-treated mixed Sargassum (difference of 0.066). The decrease in adsorption over time limited the fitting of this model for 0.4 g CaCl₂-treated S. muticum.

Pretreatment of the mixed pelagic Sargassum (S. fluitans, S. natans I, S. natans VIII) biomass also showed better biosorption capacity when the biomass was treated; after 120 min, CaCl₂-treated samples had removed 89.81% (0.1 g biomass) and 93.20% (0.4 g biomass) of the dye compared to 77.19% and 71.32% for the equivalent untreated biomass quantities (Figure 3). The difference between the treated and untreated biomass was significant at all times analysed (30 min, p = 0.011, 60 min p = 0.018, 120 min p = 0.0009).

At low additions of pelagic Sargassum (0.1 g), the treated (p < 0.05) and untreated (p < 0.01) samples removed statistically more significant amounts than S. muticum. However, there was no significant statistical difference (p > 0.05) in MB removal between pelagic Sargassum and S. muticum at 0.4 g, even though the adsorption rate constant was considerably lower for CaCl₂-treated S. muticum (k = 0.099) compared to the treated pelagic Sargassum (k = 0.165).

To test the effect of the biosorbent dose on the biosorption efficiency, the experiment for untreated S. muticum was repeated, confirming that a higher biosorbent dose resulted in significant desorption of the dye solution after 120 min. This experiment also confirmed that biosorption occurred within the initial 15 min of contact (Figure 4) and that Sargassum biomass as low as 0.05 g was equally efficient in removing MB in the initial 15 min compared to higher dosages and down to 0.01 g if left for longer than 120 min. Similar results were obtained with the mixed Sargassum biomass (Figure 5) with 0.05 g biosorbent in 10 mg L⁻¹ (80 mL) of dye, indicating optimal conditions.

There was no removal of BB from the solution when using S. muticum at three concentrations studied (<16.80%). However, there was some slight removal of BB by the mixed Sargassum biomass (<26.99%) (results not shown). Likewise, the removal of congo red from the solution by either S. muticum or the mixed Sargassum samples showed little biosorption (<16% after 120 min) (Figure 6).

4. Discussion

Methylene blue (MB) is a cationic thiazine dye that is largely used as a model of cationic dyes for adsorption studies. Algae were shown to be a low-cost and efficient alternative biomaterial to remove dyes due to the functional groups present. FTIR analysis of the brown seaweed, S. muticum, ascribed effective biosorption to the cell wall structure containing functional groups such as amino, hydroxyl, carboxyl and sulphate, which can act as binding sites via electrostatic attraction, ion exchange and complexation [39]. Both anionic dyes investigated showed no or little affinity for the Sargassum biomasses. In addition to the interactions mentioned above, hydrophobic attractions, chemical bonding, hydrogen bonds and physical adsorption interactions are also likely to occur between a biosorbent and dyes. Hence, the ionic charge of the dye will directly affect the adsorption capacity of the biomass.

Chemical characterisation of many Sargassum species showed that metal content (in particular arsenic) severely hampers the prospects of using Sargassum species for food or feed. However, this seaweed’s high metal sorption ability could offer a feasible and economical approach for removing industrial heavy-metal-bearing wastewaters that require efficient and cost-effective treatment [40]. Other non-conventional low-cost adsorbents were reviewed by Rafatullah et al. [41]. The effect of biosorption dosage on methylene blue dye removal was investigated (0.01⁻¹ g) using an 80 mL dye solution (10 mg L⁻¹, room temperature).

Optimal biosorption dosages can be used to predict the overall cost of biomass per unit of the dye solution to be treated. The optimal biosorbent dose for either S. muticum or mixed Sargassum species was 0.05 g/80 mL, corresponding to Q_e values of 157 mg g⁻¹ (120 min) for S. muticum and 115 mg g⁻¹ (120 min) for the mixed Sargassum sample. Likewise, contact times over 120 min were investigated, indicating that biosorption (>75% for mixed Sargassum and >78% for S. muticum) occurred in the initial 15 min. The maximum adsorption capacity of MB on S. muticum at the optimal pH was reported to be 279 mg g⁻¹ [28]. The pH was not adjusted in this study, which could have caused the lower values obtained in this study. Chemical treatment with CaCl₂ improved the adsorption capacity, which is in line with those previously reported for other types of biomaterial [42].

5. Conclusions

Both pelagic Sargassum and S. muticum, either untreated or treated with CaCl₂, were effective in the biosorption of MB from solution. However, Sargassum did not remove the anionic dyes, namely, CR and BB. Brown seaweed’s primary cell wall polysaccharides are negatively charged [43,44] and algal cells are typically negatively charged [45,46,47]. This negative surface charge may make Sargassum suitable for removing cationic pollutants but may not be suitable for anionic substances without modification. Still, the results are encouraging, offering some potential benefits for commercial purposes for this brown, invasive seaweed as a low-cost adsorbent. Future work could include investigations into the number of cycles the Sargassum samples will be effective for MB biosorption.