Thermally treated mango seed kernel as an adsorbent for removal of methylene blue: surface area and adsorption studies

Water is an essential resource for life on earth because it enables the body to absorb and assimilate minerals through biological processes (Cheuvront et al. 2020). However, anthropogenic activities in the industrial sector led to the discharge of various dyes, metals, and toxic chemicals into water channels. This contamination can alter the physical and chemical properties of the soil, degrade water bodies, and harm essential microorganisms by inhibiting their growth (Some et al. 2021). Furthermore, developments in the industrial and agricultural sectors have increased pollutants in both water and air, disrupting entire aquatic ecosystems (Bharathi & Ramesh 2013). Currently, continuous monitoring is essential for identifying contaminants and reducing the risks associated with water pollution (Salehi 2022).

The textile industry uses dyes containing chromophoric groups to color fabric. These dyes are soluble in water and are often discharged into water channels without proper treatment, leading to pollution (Lellis et al. 2019). Approximately 15% of these dyes are released directly into water sources. Water pollutants containing dyes can cause severe diseases in humans and other organisms, including dysentery, polio (infantile paralysis), trachoma (eye infection), typhoid fever, cholera, and diarrhea (Fazal-ur-Rehman 2019). Some of the disease-causing dyes include acid dyes, reactive dyes for wool, pre-metalized dyes, azoic dyes, vat dyes, and sulfur dyes (Pandey et al. 2008). Major dyes such as azo, anthraquinone, indigoid, and triphenylmethane dyes are known to cause skin irritation, eye itching, zootoxicity, and hepatotoxicity (Yang et al. 2018). In addition to dyes, other pollutants contribute to dangerous human diseases, affecting bones, glands, the nervous system, and the endocrine system (Robinson et al. 2002).

Methylene blue (MB) is a cationic dye and a derivative of phenothiazine (Khadieva et al. 2021). The presence of this dye in natural water has a severe impact on both aquatic and human life. It can cause urinary tract infections, leading to kidney, ureter, and bladder issues. Additionally, it may result in abdominal and precordial pain, headaches, nausea, and, in some cases, excessive sweating (drizzling) (Buzga et al. 2022). While MB is safe as a drug at doses of 2 mg/kg or less, higher levels can have serious effects on the human body (Dabhokar et al. 2021).

Highly porous materials were found to be the best candidates for water treatment and removal of impurities (Li et al. 2023). They can also be used for drug delivery, gas storage, separation, magnetism, catalysis, and molecular sensing (Li et al. 2014). Such materials can remove water impurities and control their quality and purity physically and chemically. However, techniques such as adsorptive bubble separation, electrocoagulation, photocatalysis, nanofiltration, reverse osmosis, ozonation, microbial decomposition, photolytic decolorization, and ultrafiltration have been used for water purification (Saravanan et al. 2021).

Compared with other water treatment techniques, the adsorption method is inexpensive and straightforward. It is considered one of the most effective techniques for removing harmful contaminants due to its simple design, low cost, ease of operation, and ability to handle a wide range of pollutants (Kordbacheh & Heidari 2023). For organic molecules, adsorption is associated with higher aromaticity and hydrophobicity (with a lower O/C ratio). However, it is less suitable for ionic dyes and situations where noncarbonized materials are required for adsorption (Choi & Kan 2019).

Specific adsorbents and biosorbents are used for water treatment and have the characteristics of high abrasion resistance, small pores diameters, and high thermal stability. Adsorbents are chemically inert and may be organic, inorganic, and biosorbents including significant types such as silica gel, activated carbon (charcoal), molecular sieve carbon, alumina, and biochar (Grégorio et al. 2018). Biosorbents such as apricot shell, rubber wood sawdust, clay, a mixture of both (sawdust and clay), oil palm fiber, mango seed kernel (MSK), and sunflower waste biomass are also used for impurity removal and considered as cheapest and finest methods (Singh & Arora 2011). Other compounds such as zeolites, porous coordination polymers (PCPs), and metal–organic frameworks (MOFs) have high porosity and extended structures due to organic linkers are being used for water treatment recently (Yang & Sun 2007; Busch et al. 2012; Furukawa et al. 2013). These compounds are usually nontoxic and can be used in pharmaceuticals and cosmetics industries because of their availability at a very cheap rate (Busch et al. 2012). Recent studies have investigated the potential of mango seed char synthesized by the pyrolysis method. Mango seed char has been utilized for the removal of dyes and phenol with high efficiency due to its polarity and surface area. The effect of mango seed char, using ZnCl₂ activation, in the removal of MB was also examined and the study demonstrated the highest percentage of dye removal under optimal conditions. Mango seed char was further used to remove phenol due to its smaller size, proving useful in water purification. These findings highlight the potential of mango seed char for removing dyes and other pollutants from water (Dávila-Jiménez et al. 2009; Akpen et al. 2014; Razali et al. 2022).

Different interactions have been studied previously in removing organic and inorganic pollutants using biochar, involving the specific and nonspecific interaction between the absorbent and the adsorbate (Madani et al. 2015). In physical adsorption, the polarity of target molecules impacts the adsorbent surface as it depends upon various factors such as π–π stacking, hydrophobic and electrostatic interaction, and van der Waals forces. Chemical adsorption occurs due to the formation of chemical bonds through electron sharing between the biochar and the adsorbate. This process is selective and involves stronger, more specific forces. As a result, monolayer adsorption is typically observed in such cases (Naja & Volesky 2011).

In this study, the pyrolysis method was used to synthesize biochar using MSK, which significantly helped to remove dyes from aqueous solutions. The study assessed the effects of important variables such as contact time, pH, MSK dosage, and MB concentration to optimize adsorption. Reported pyrolysis studies greatly expand the surface area of MSK and add functional groups that contain oxygen, which gives MB molecules many places to bind (Ganeshan et al. 2016). Compared with other raw kernels, this improves the adsorption capacity and diversity. Mango seed char is far more affordable than other low-cost adsorbents. The results point to promising opportunities for using this easily accessible agricultural waste as an effective and long-term remedy for MB remediation in wastewater treatment.

The experiment utilized MB dye from BDH Gurr with a purity of 82%. Activated carbon was purchased from Riedel-de Haen and employed in a series of experiments. The research equipment included a WHL-25 electrothermal constant temperature drying oven (Tianjin Taisite Instrument, China) and a Ney VULCAN D-550 furnace (Trendtop Scientific Crop. China) to synthesize the adsorbent. The separation process was conducted using a centrifugal machine, which separated substances based on their density by spinning the vessel around its axis. A Model-16 high-speed centrifugal machine with a speed of 16 × 10³ (Changzhou Guohua Electric Appliance Co., Ltd, China) was utilized. A 721 UV–Vis spectrophotometer (TBT Scietech, China) with a wavelength range of 340–1,000 nm was employed. A&D Gulf's weight balance, capable of weighing approximately 300 g of substance, and a pH meter were used to study the pH effect.

MSK 600 (mango seed kernel pyrolyzed at 600 °C) was prepared by deshelling the mango kernel. After deshelling, the mango kernel was grinded and dissolved in ethanol and the mixture was transferred to a separatory funnel. The mixture for subjected to liquid–liquid extraction using hexane. By gently shaking the mixture in the funnel, the fats dissolved in hexane and were removed once the solution settled. This extraction process was repeated until the fats were sufficiently removed, followed by further processing through microwave heating. For neutralization, it was immersed in a 0.1 M solution of KOH for 12 h and then dried at 70 °C. Subsequently, it was introduced into the furnace for 2 h at a temperature of 600 °C, where the pyrolysis of the adsorbate occurred. MSK was then immersed in a 1 M solution of KOH for 6 h and neutralized using 0.1 M HCl.

MSK 600 with a 150-mesh size (0.105 mm) was obtained using laboratory sieve shakers. To study the potential of the synthesized biochar for adsorption isotherm studies, a stock solution of MB of approximately 1,000 ppm was prepared. Removal of the dye using biochar by considering various parameters, including changes in the concentration of MB and the effect of biochar dosage on MB removal was investigated. The pH of the MB solution was adjusted using KOH, NaOH, and HCl, and adsorption parameters at different time intervals were also studied. The mixture of the biochar and MB was centrifuged, and the adsorption process was monitored using UV–visible spectrometry by measuring the absorbance. Calibration curves for MB were generated by measuring the absorbance at various concentrations using UV–Vis spectrometry. After the adsorption process, the residue of the char and MB was filtered using filter paper. Clean water was collected in another vessel, while the residue was left on the filter paper.

Specific surface area is one of the important physical properties of porous materials. There are many ways to determine the specific surface area, which is vital for numerous responses. Commonly used methods include the gas adsorption method (such as the Brunauer–Emmett–Teller (BET) method) (Mohmmadkhani et al. 2016; Han et al. 2017), direct physical measurement by assuring the gas adsorption, NMR technique, SAXS curve, and X-ray diffraction. Surface area studies are helpful in water treatment, energy storage, gas adsorption, cation exchange capacity (CEC), and catalyst activity (Trickett et al. 2017).

For surface area studies, the adsorption capacity (Q_max) of the biochar is determined by monitoring the number of MB molecules adsorb onto the MSK surface. By measuring the adsorption capacity, the efficiency of the biochar was determined. A_x was the cross-sectional area of MB occupied by a single MB molecule on the biochar surface. The molecular weight of the MB was used to ensure that the adsorption sites were covered by the dye molecule.

The effect of biochar on dye removal was demonstrated by changing the pH of MB, which influenced the interaction of biochar with MB. By changing the pH, we found out that, at slightly acidic and basic conditions, the adsorption process was a little bit changed due to the change in the nature of the dye. It was observed that the increase in the pH caused a slight change in the adsorption. pH was maintained using 0.1 M of KOH, NaOH, and HCl. The pH of the MB solution was monitored using a pH meter, and varying concentrations of KOH, NaOH, and HCl were added to adjust the pH as required. However, significant change was observed at different pH values. Results indicated that the pH changes had different effects on the percentage removal. The maximum removal was observed at slightly acidic and neutral pH, and it almost remained the same as absorbance in a slightly basic zone. The percentage removal was about 98% at pH 4, 97% at pH 7, and 94% at pH 10. So, the adsorbent–adsorbate interactions seem to have been least influenced by a change in pH. The minimum percentage removal occurs at the highest acidic pH, about 38% at pH 2, as shown in Figure 2(c). The percentage removal values decrease as the pH value increases from neutral to basic. The results indicated that the neutral pH condition, without any modifications, provided optimal performance. Furthermore, the findings suggested that the interaction between the adsorbent and the adsorbate varied across different pH levels, leading to enhanced dye removal.

To study the effect of biochar for the adsorption of MB at different time intervals, the 100-ppm solution of MB was taken and MSK-600 (0.1 g) was added in it. After 20 min of adding adsorbent, the first absorbance was checked. After different time intervals, i.e. 40, 60, and 80 min, further readings were taken. The more time allowed, the more collisions occurred, resulting in increased dye adsorption. However, after a certain period, the adsorption process stopped due to the saturation of all MB molecules on the biochar and equilibrium was achieved. The results indicated that as contact time increases, the percentage removal of MB also increases. The maximum removal percentage was about 30% at 80 min (Figure 3(b)). In contrast, the minimum removal percentage of MB was 12% after 20 min. So, overall, the results indicated that as contact time increased, the value of percentage removal of the methylene also increased. The maximum time for the interaction of the dye molecules with the adsorbent effectively promotes the diffusion of dye molecules onto biochar.

Different values for isotherm parameters were calculated using the adsorption data of MB on MSK-600. The obtained values for these isotherm parameters were utilized to construct graphs for isotherm studies, elucidating the adsorption behavior of MB by MSK-600. The logarithmic transformations of C_e (equilibrium concentration) and q_e (amount of adsorbate adsorbed at equilibrium) were calculated using Equations (1)–(3). Tables 1 and 2 represent the different values calculated for the Langmuir, Freundlich, and Temkin isotherms using linear regression.

From Equation (1), the Freundlich isotherm equation showed the maximum adsorption capacity. The calculated value of q_max was 67.56 mg/g, reflecting the adsorption ability of MB under the specific conditions of MSK. Applying the BET equation for the surface area by adding the cross-sectional area (⁠⁠) and the molecular weight of MB, the surface calculated was 85.19 m²/g, indicating the adsorption sites for MB adsorption. The calculated adsorption capacity for MSK-600 was then compared with other biochar used for the removal of various dyes, as represented in Table 4. MSK-600 can be used for MB removal and other dyes, providing better adsorption capacity and making it competitive with other biochars.

To draw a comparison between charcoal and MSK methylene was adsorbed on the charcoal in the MB to study its surface area and adsorption studies through the isotherm. was calculated from Equation (1) which was 836 mg/g and by applying the cross-sectional area and molecular weight of MB, the surface calculated was 1,053 m²/g. The calculated surface area for charcoal was the same as reported in previous studies (Cao et al. 2006). Compared with charcoal, the difference in surface area and adsorption capacities of mango seed char proved to be the most potential candidate for the adsorption of the MB and the process could be repeated for the removal of other dyes for water treatment.

In this study, biochar derived from MSK pyrolyzed at 600 °C was used for the removal of MB. The adsorption capacity of 67.56 mg/g was determined by the Langmuir isotherm fitting. The adsorption and isotherm studies were compared with those of activated carbon (charcoal), highlighting improvements in water treatment through adsorption capacity and surface area. Despite the relatively low surface area of the mango seed char (85.19 m²/g), our findings suggest that surface area played a significant role in the adsorption process for MSK-600, effectively removing MB from wastewater and offering advantages over activated carbon. The results indicate that MSK-600 has potential as an alternative (bio)sorbent for water treatment applications.

The authors are grateful to the Department of Chemistry, University of Sahiwal, Pakistan, for granting access to their experimental facilities.

M.U.S.: Designed the material, performed the characterization, analyzed the data, and wrote the original manuscript. M.A.: Writing – review & editing and supervision.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.