The application of waste Leucaena leucocephala seed pods as a bio-based adsorbent for decolourization of solution

Nowadays, water contamination with organic and inorganic contaminants is one of the most significant environmental harms and needs global action to stop or limit its serious effects (Vakili et al. 2014). When the total water demand increases, the amount of wastewater always creates a global increase. This makes a requirement for effective purification techniques. Among organic contaminants, synthetic dyes and colouring agents are the main types of organic compounds with many aromatic rings and are widely used in various industries. The release of coloured effluents from these industries without treatment, into streams and lakes or penetrating aquifers, may present an ecotoxic hazard, being carcinogenic, mutagenic, teratogenic and stable during aerobic degradation, introducing the potential danger of bioaccumulation (Gupta & Ali 2013). Very low strengths of dyes containing industrial effluents decrease light diffusion through the water's surface, affecting aqueous plants’ photosynthesis. So, reducing these dyes is necessary for wastewater treatment before release. Many chemicals, physical and biological treatment techniques are currently applied to eliminate concentrations of dyes from coloured wastewater, such as the adsorption technique (Ghasemi et al. 2016; Osamah et al. 2021), electrochemical degradation (Steter et al. 2014; Hashim et al. 2021b), biological treatment (Paz et al. 2017), photocatalytic degradation (Magdalane et al. 2017; Karaghool et al. 2022), and decolourization and detoxification (Maalej-Kammoun et al. 2009). For example, chemical coagulation is applicable for dye removal from solution, and it can be used to remove high concentrations of the dyes (Mcyotto et al. 2021). The severe effects of the chemicals (coagulants) on the environment and the production of intermediates are the main barriers to using chemical coagulation for dye removal. The electrocoagulation method is the safest form of chemical coagulation because it is the indirect addition of chemicals to a solution that reduces the effects of chemicals on the environment (Abdulhadi et al. 2019), and it was demonstrated that the electrocoagulation method can remove high loads of dyes from solutions (Hashim et al. 2019; Sadoon & M-Ridha 2020). However, electrocoagulation methods face some challenges, such as the passivation of electrodes and other operating problems (Yasri et al. 2022).

Among these techniques, adsorption is considered a preferable and efficient way of eliminating contaminants from wastewater. The adsorption approach is favourable because it depends on low-cost and available materials, ease of use and excellent removal efficacy (Hashim et al. 2021a). Agricultural biosorbents have advantages as they are readily available, cost-effective, need less processing time and are renewable, giving appropriate adsorption potential (Nguyen et al. 2013). Therefore, many studies have considered the development of non-conventional and low-cost agricultural wastes such as Psidium guajava leaves and Solanum tuberosum peels (Rehman et al. 2015), Pinus roxburghii leaves (Rehman et al. 2019) and Indian neem leaves (Das et al. 2020). The main goal of this investigation was to assess the potential biosorption of Leucaena leucocephala seed pods for sequestration of JGBD from aqueous effluents. Leucaena leucocephala, previously known as L. glauca, is a type of rapidly growing tropical leguminous tree which can rise to heights of 7–18 m. Leucaena leucocephala belongs to the family of Fabaceae and the subfamily Mimosoideae. The ‘miracle tree’ is a type of multiuse tree for wide-ranging procedures such as pulp and paper manufacturing, timber and firewood (Vijay et al. 2017). It is also used for animal feed as a protein source (Loaiza et al. 2017).

LLSP texture properties, including Brunauer–Emmett–Teller (BET) surface area and pore size distribution, were found using a Micromeritics ASAP 2020 absorption analyzer (Micro meritics Instrument Company, USA) based on adsorption–desorption isotherms measured using N₂ over the relative pressure (p/p^o) range from 0.01 to 0.991. The specific surface area was calculated using the BET method the pore diameter, the size distribution of the pore and the overall pore volume were found using the Barrett–Joyner–Halenda (BJH) method. The surface functional groups on the LLSP sample before and after biosorption of JGBD were analyzed using Fourier transform infrared (FTIR) spectroscopy (SHIMADZU, IRPRESTIGE-21, Japan) in the range 4,000–400 cm⁻¹ with KBr tablets. Scanning electron microscopy (SEM; TESCAN, Mira3, France) was based on determining the surface morphology, crystalline structure and orientation of the LLSP sample and the energy distribution spectroscopy (EDS). The LLSP topography was examined by scanning probe microscopy (SPM-AA300, Modern Angstrom Inc., USA, with an AFM connection). The LLSP surface charge was neutral at the point of zero charges (pH_pzc), and the final pH value and initial pH were equal, which is an essential property that aids in knowing which ionic species may be absorbed using LLSP at the favored pH value. The pH_pzc of LLSP was determined as described previously (Mohseni-Bandpi et al. 2016). This method has been elucidated as subsequent: add 0.5 g of LLSP to 50 ml of 0.1 M NaCl adjusted to various initial pH (2–10) at ambient temperature (30 ± 2 °C). The suspensions were agitated for 24 h and removed, so the final pH values of each residual solution were tested. The pH_pzc value for LLSP was found by the point of intersection at zero with the curve, achieved through comparing the ΔpH vs. pH_initial.

All reagents used in this investigation were of analytical grade. For Janus Green B dye (JGBD), this adsorbate is a cationic dye which possesses the chemical name [3-diethylamino-7-(4-dimethylaminophenylazo)-5-phenylphenaziniumchloride,3-(diethylamino)-7-((p(dimethylamino)phenyl)azo)-5-phenylphenazinium chloride]; C.I. 11050; molecular mass 511.07 g/mol with λ_max of 611 nm; chemical formula C₃₀H₃₁ClN₆. The dye stock solution was prepared by dissolving a suitable mass of powdered dye in double-distilled water to produce a solution of 10³ mg/l. The dilution method was based on preparing different concentrations of JGBD solutions. The pH values of dye solutions were adjusted using 0.1 M HCl or NaOH as needed.

Performing all the experiments for JGBD using LLSP depended on a batch mode at room temperature (30 ± 2 °C), using several 250 ml conical flasks filled with 100 ml dye solution. The effects of the factors on the biosorption procedure rate were studies experimentally to find the best conditions for dye removal and obtain sufficient equilibrium data that were suitable for finding the studied material activity and analysis of biosorption isotherms. Table 1 includes these factors.

Table 2 presents the physical and chemical characteristics values of natural LLSP. The BET approach was based on calculating the S_BET of LLSP according to the linear portion of the isotherm of N₂ adsorption–desorption.

The LLSP sample presents a very high percentage of carbon content compared with other elements (Table 3). Therefore, the sample is carbonaceous in nature, and it is suitable for biosorption. Furthermore, the very low quantity of S indicated the presence of a minor amount of impurities.

In order to determine the mechanism of JGBD biosorption and classify the functional groups existing on the LLSP surface, which can biosorb dye ions, FTIR analysis was performed. The analysis of LLSP before and after biosorption of JGBD is shown in Figure 3(a) and 3(b). The biosorbent material was lignocellulosic. It shows the characteristic broad and strong peaks at 3,851.20 and 3,401.23 cm⁻¹, ascribed to the presence of the carboxylic acids group (–COOH), which overlapped with the O–H stretching vibration of the hydrogen-bonded hydroxyl groups in the cellulose molecule and –NH groups (Mansur et al. 2020). The band at 2,922.77 cm⁻¹ corresponded to C–H asymmetric stretching vibrations in aromatic methoxyl groups in the methyl and methylene groups of the side chains (Ospina Álvarez et al. 2014). The bands at 2,356.51 and 2,326.94 cm⁻¹ were allocated to the C = C stretching vibrations in the alkyne group. The peak observed at 2,028.96 cm⁻¹ was assigned to the C = O stretching vibrations. The esters or carboxylic acids described at 1,732.86 cm⁻¹ appeared to be C = O stretching vibrations of carboxylic groups (–COOH, COOCH₃) (Patil & Shrivastava 2012). The peaks at 1,652.11 and 1,557.78 cm⁻¹ were attributed to the stretching of C = O or C = C of aromatic bond and –COO⁻ groups (Hanafiah et al. 2015). The peak observed at 1,538.54 cm⁻¹ was assigned to the N–H bending of secondary amines (Hanafiah et al. 2015). At 1,505.26, 1,489.31 and 1,463.69 cm⁻¹, the olefin v(C = C) vibrations can be qualified. Whereas those shown at 1,436.21 and 1,423.41 cm⁻¹ were assigned to the presence of O–H bending (lactonic, ether, phenol, etc.). The peaks at 1,403.78, 1,319.42 and 1,292.07 cm⁻¹ were associated with the C–H₂ rocking vibration, where these peaks are referred to as the cellulose form of the carbohydrate (Ilyas et al. 2018). The stretching vibrations of the C–O (ether) group occurred at around 1,090.29–1,196.90 cm⁻¹, among others; whereas that shown at 1,046.34 cm⁻¹ was allotted to the stretching vibrations of the C–OH bond (Cimá-Mukul et al. 2019). The appearance of bands at 667.96, 603.09, 588.01 and 568.35 cm⁻¹ were attributed to the presence of the phosphate and sulphur functional groups (Munagapati et al. 2010). After dye-loaded LLSP, most absorption peaks were shifted, others were absent and new peaks appeared. These changes may be attributed to changes in counter ions associated with group anions, suggesting that the dye removal was organized by carboxyl, hydroxyl, carbonyl and other cited groups. Thus, the biosorption of JGBD onto LLSP may be attributed to (i) chemical interaction between JGBD molecules and surface functional groups, (ii) electrostatic interaction between the electron-rich places on the biosorbent surface and JGBD molecules and (iii) weak physical forces, van der Waals interactions and hydrogen bonding between the biosorbent and JGBD molecules. Therefore, the JGBD biosorption onto the LLSP surface occurs by the interaction between the negative groups of the biosorbent and the positively charged JGBD molecule. This means that a contribution pattern of physical and chemical adsorption contributes to JGBD biosorption.

Finally, the authors would like to highlight that the obtained results in this study agreed with the literature; for example, Shrivastava (2012) and Hishamudin et al. (2022), respectively, successfully removed Congo red and malachite green using the Leucaena leucocephala.

For future approaches, the use of the Leucaena leucocephala for the removal of heavy metals and the regeneration of the biosorbent is recommended.

The results confirmed that the biosorbent prepared from LLSP-low-cost agricultural wastes is promising for removing JGBD from aqueous solutions because of its availability as waste and eco-friendly adsorbents. The operating factors, initial pH of dye solution, contact time, the dosage of LLSP and primary concentration of JGBD were effective in removing JGBD using LLSP in batch experiments. The equilibrium data were simulated best by the Langmuir isotherm model with a maximum capability of 142.85 mg/g. All findings demonstrated that both benefits could be seen simultaneously using LLSP, including recycling LLSP wastes and treating coloured wastewater. In addition, LLSP could remove other pollutants, such as heavy metals, suitable for future research.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.