Efficiency of almond gum as a low-cost adsorbent for methylene blue dye removal from aqueous solutions
Graphical abstract
Introduction
Dyes are mainly chemical compounds constituted of two important components: chromophores; which are responsible for producing the color, and auxochromes; which enhance the affinity of the dye toward the fibers (Gupta and Suhas, 2009). Numerous dyes are linked to surfaces to impart color and are known to be resistant to the action of detergents. Synthetic dyes are widely used in several industries such as textile, paper, leather tanning, plastic, carpet, food, and cosmetics to provide color to their products (Ivanov et al., 1996, Kabdaşli et al., 1999, Sokolowska-Gajda et al., 1996). These dyes are always released in industrial waste, leading to disposal problems (Crini, 2006, Forgacs et al., 2004, Muthukumar and Selvakumar, 2004, Ong et al., 2007). Their discharges into effluents involve a significant source of pollution due to their recalcitrance nature. Furthermore dyes may impart toxicity to aquatic and plants biota and may be mutagenic and carcinogenic, causing severe damage to human life; such as dysfunction of kidneys, reproductive system, liver, brain, and central nervous system (Dinçer et al., 2007, Kadirvelu et al., 2003, Shen et al., 2009). The removal of dye pollutants from waste effluents becomes environmentally of great importance because even a small quantity of dye in water could be toxic and highly visible (Chiou et al., 2004).
Numerous removal methods such as adsorption, coagulation, advanced oxidation, membrane separation, aerobic, and anaerobic microbial degradation are used to eliminate dyes from waste water (Gupta and Suhas, 2009). Among all of these methods, adsorption is the most used regarding its cheapness and efficiency as advanced treatment employed in industries to reduce hazardous organic and inorganic pollutants from the effluents (Kant, 2012). Activated carbon is the most commonly used adsorbent regarding its high capacity of adsorption and microporous structure (McKay, 1982). However, due to its high price, its industrial use is still limited (EL-Geundi, 1997). Recently, several agricultural waste and residues have been tested as adsorbent, to remove dyes from waste waters, because of their low cost, low toxicity, and easy availability (Hameed, 2009). In fact, peach gum (Zhou et al., 2014), rice husk (Vadivelan and Kumar, 2005, Zou et al., 2011), yellow passion fruit waste (Pavan et al., 2008), grass waste (Hameed, 2009), banana and orange peels (Annadurai et al., 2002), and wheat shells (Bulut and Aydın, 2006) were used for dyes removal from aqueous solutions. However, most of the reported bioadsorbent materials show low adsorption capacity, limiting thus their industrial applications. Other economical, easily available, eco-friendly, and highly efficient adsorbents are still needed.
Almond gum exudate is produced by almond tree (Prunus amygdalus, Rosaceae family) after a mechanical injury followed by a microbial attack. Almond gum represents an abundant natural biomass worldwide and is an acidic polysaccharide composed of galactose, arabinose, xylose, mannose, rhamnose, and glucuronic acid, with molar ratios of 45, 26, 7, 10, 1, and 11, respectively (Bouaziz et al., 2015). Due to its high molecular weight and highly branched molecular structure, almond gum is insoluble in aqueous solutions, and could be a promising adsorbent for the removal of methylene blue cationic dye from waste water. Indeed, almond gum contains numerous negatively carboxylic groups that can adsorb cationic dyes such as methylene blue through strong electrostatic interactions. For this purpose, this work is devoted to study for the first time the efficiency of almond gum as a bioadsorbent to remove methylene blue dye from aqueous solutions. The influence of adsorption parameters (initial dye concentration, methylene blue dose, contact time, and optimal pH and temperature), using almond gum, was investigated. The adsorption isotherms, kinetics, and thermodynamic properties of almond gum were then discussed.
Section snippets
Materials
Almond gum was collected from almond trunks (Achaak’s variety) in the suburb of Sfax city (Tunisia) on April 2014. The dried almond gum was grinded to fine powder (Fig. 1) and sieved to obtain particle sizes below 100 μm, 110–250 μm, and 260–350 μm. Methylene blue was purchased from Sigma–Aldrich (SdnBhd, Malaysia) and was used as adsorbate. A stock solution of 1 g L−1 methylene blue was prepared in deionized water and used to generate the working solutions at different concentrations of methylene
Characterization of the adsorbent
Scanning electron microscopy has been the primary tool used to characterize the surface morphology and fundamental physical properties of the adsorbent. Pictures of almond gum surface were taken before and after methylene blue adsorption process and were shown in Fig. 2a and b, respectively. Fig. 2a shows a rough surface of almond gum with the presence of pores and cavities, whereas Fig. 2b shows that these pores at the surface of almond gum were completely filled with methylene blue dye.
Fourier transform infrared spectroscopy
Almond
Effect of adsorbent’s dose on dye adsorption
The almond gum dose impact on methylene blue adsorption was conducted using an initial methylene blue concentration of 100 mg L−1, and by varying the amount of almond gum as adsorbent. Results (Fig. 4b) show that the dye removal increased rapidly from 67% to 88.5% over the range of 0.05–0.2 g of almond gum dissolved in 100 mL methylene blue solution. Beyond 0.2 g of almond gum, the dye removal percentage remains unchanged. Thus, the dose of adsorbent was fixed to 0.2 g for the subsequent experiments.
Conclusion
Almond gum is a low-cost and efficient adsorbent for the removal of methylene blue from aqueous solutions. It represents a promising feedstock to replace the activated carbon; which is although expensive, is commonly used. In the batch model of the study, the equilibrium adsorption was reached after 60 min contact time. The equilibrium data fitted very well in Freundlich isotherm equations (R2 = 0.99). The maximum adsorption capacity obtained was 500 mg g−1 at 303.3 K. The rate of adsorption was
Conflict of interest
The authors declare no conflict of interest.
Acknowledgment
This work was funded by the Ministry of Higher Education and Scientific Research of Tunisia.
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These authors contributed equally in this work.