Moringa oleifera: a sustainable method to treat fluoride-contaminated water

Higher fluoride (F) dosages in underground water have been noted in many countries, both developed and developing countries, including the USA, Africa, and Asia. It has been discovered that 17 different Indian states, particularly, Chhattisgarh, Kerala, Delhi, Haryana, Jharkhand, etc. are negatively impacted by excessive fluoride concentration (Baboo et al. 2022). Fluoride is an electronegative toxic element and its toxicity can cause adverse health effects for both human beings and animals (Bhaumik et al. 2017). World Health Organization (WHO) has already set a permissible limit of 1.5 mg/L. Fluoride ions are consumed in large quantities through drinking. Excessive fluoride in the body leads to a problem called fluorosis. This fluorosis affects the teeth, the skeleton, and the non-skeletal system(George & Tembhurkar 2019a). At concentrations above the permissible level, fluoride is a very hazardous ion and may be a factor in several health issues, including skeletal fluorosis, dental fluorosis, neurological problems, osteoporosis, parathyroid damage, Alzheimer's disease, male infertility, thyroid, kidney issues, and liver injury (Duvva et al. 2022).

As for purpose of eliminating too many fluoride ions from drinking water and industrial effluent, traditional treatment technologies such as precipitation, electrolysis, membranes, adsorption, ion exchange, electrodialysis, coagulation/precipitation, dialysis, reverse osmosis, nanofiltration, ultrafiltration, and others have been developed and tested (Jamwal & Slathia 2022). However, these procedures have several drawbacks, like a failed removal, an arduous treatment method, high operating costs, high energy costs, chemical usage, and the formation of hazardous sludge or other waste products, which must be disposed of again (Peng et al. 2017). The effectiveness of fluoride removal is always dependent on the qualities of raw water, including the original fluorine level, pH, heat, preparation time, and adsorbent dosage, other factors that affect adsorption include capacity, specificity for fluoride, renewability, price, granule and pores size, and adaptability (Mondal & Roy 2018). Therefore, the goal is to discover a practical, affordable, environmental friendly method that an average person could use to remove fluoride from drinking water.

Among all the plant materials examined over the years, a powder of Moringa seed, in comparison to alumina, a conventional chemical coagulant (Laney et al. 2020), exhibited an efficiency for water treatments. Moringa oleifera is known as a ‘multipurpose tree’ because of all these benefits. Properties of adsorption, demagnetizing, and interparticle bridging were suggested as the coagulation mechanisms of the M. oleifera coagulant protein. The high molecular weight polyelectrolytes are more prone to flocculation due to interparticle bridging. Recently, Khader and team reported M. oleifera seed beads mixed with sodium alginate for the adsorption of ions of copper, nickel, manganese, lead, and fluoride from polluted water (Khader et al. 2022). Importantly, a recent review on the application of M. oleifera by Gomes and others highlights its potential application for the removal of emerging contaminants in water (Gomes et al. 2022). Several portions of M. oleifera, including the seeds, leaves, and bark, are promising biosorbents, according to analytical methods like scanning electronic microscopy (SEM), as they exhibit optimal surface area and porosity in their morphological structure for adsorption of pollutants (Jiraungkoorskul & Jiraungkoorskul 2016). Specifically, these seeds are lignocellulosic materials that show themselves as an ecological, cheap, and rugged alternative for water treatment. Therefore, all plant residue is considered lignocellulosic material and can be used as a biosorbent.

Hence, the current work sought to determine whether M. oleifera seeds could effectively adsorb fluoride from aqueous solutions under a variety of operating circumstances such as pH, adsorbent dosage, contact time, initial fluoride concentration, and temperature. The prepared adsorbents were further examined by SEM and FTIR to gain knowledge of the metal ion loading procedure and the fluoride adsorption mechanism. The ultimate goal of this effort is to create large-scale water treatment procedures that provide the general public with safe drinking water and make use of natural and eco-friendly components that are widely available.

In the current work, analytical grades of chemicals, such as SPADNS solution, zirconyl acid reagent, sodium arsenite solution, sodium fluoride, stock, and standard solution were used. M. oleifera is used as a natural adsorbent.

The fluoride standard sample was made by diluting the standard fluoride solution in the range of 1–11 mg/L with the necessary volumes of distilled water. After that, 5 mL of the zirconyl acid solution and SPADNS solution were taken into each standard, and thoroughly combined. The spectrophotometer was calibrated using the standard solution and setting the value of absorbance to zero. As a blank solution, the reference solution was employed. As per the standard method procedure, a spectrophotometer with a wavelength of 540 nm was employed (Nazri et al. 2020).

For this experiment, M. oleifera pods were collected from local trees in villages J. V. Colony (Kolar). The seeds were then collected and dried for up to 8 h in a hot air oven at 60 °C. The seed coat was removed after collecting the seeds. The seeds of M. oleifera were then grounded into a fine powder, as shown in Figure 1. Then sieved to obtain particles in micro length which is then used as an adsorbent. To prevent spoilage, the M. oleifera powder was finally gathered and kept in an airtight container. Figure 2 depicts the schematic representation of the defluoridation of drinking water using a natural adsorbent.

SEM and FTIR were utilized in this experiment to analyze the morphological structure of M. oleifera. To study the morphological structure, the sieved M. oleifera powder was taken.

The FTIR method was employed to research the functional groups discovered in M. oleifera seeds. This approach enables the identification of several essential functional groups capable of adsorbing metal ions. The below graph shows the FTIR spectra of M. oleifera seed powder.

The spectra all have an identical profile: a wide band's center is 3,953.36 cm^–1 which is ascribed to O–ZH extending. This operational unit is mostly found in the proteins and fatty acid structures found in M. oleifera seeds. Because of the high protein composition of seeds, the N–ZH stretching of amide groups also contributes to this area. Peaks at 559.25 and 987.37 cm^–1 are attributed to symmetric and asymmetric stretching, respectively (Manna et al. 2015). Three powerful bands are ascribed to the C–O bond expanding in the area between 1,315 and 1,454 cm⁻¹. The carbonyl group can be found in fatty acid and protein structures. This example shows the spectra indicate double groups connected with fatty acids at 1,531 and 1,654 cm⁻¹ and a band related to the protein's amide group at 1,754 cm⁻¹. This band's existence validates the structure of proteins in Moringa seeds (Amin et al. 2015).

SEM analysis is also another important method for analyzing an adsorbent's surface morphology. Figure 4 shows the clear structures of M. oleifera powder and the material's morphology depicts a fragmented and mostly porosity core (Mihayo et al. 2021). This morphological composition enhances fluoride ion absorption in water. It is because of the intercellular spaces and, most significantly, the proteins element of the seed. Based on these properties, it is possible to conclude that this substance has a suitable structural character for holding fluoride ions in the water sample (Li et al. 2016).

In summary, M. oleifera seeds offered a unique advantage in fluoride elimination from aqueous solution in terms of its easy, inexpensive, and carbon-neutral impact. SEM and FTIR validated the internal features of the adsorbent. The primary operating parameters like pH, initial fluoride concentration, adsorbent dose, and contact time were optimized. The maximum removal efficiency was obtained at pH of 7, 1 mg/L of initial fluoride concentration with 120 min of contact time, and an adsorbent dose of 200 mg/L. Hence, this work is the potential for real-time application.

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

The authors declare there is no conflict.