Thermal plasma processing of Moringa oleifera biochars: adsorbents for fluoride removal from water

Abstract

Anthropogenic activities accelerate fluoride contamination in groundwater, which largely affects public health. Though biochars have been explored for defluoridation, the plasma technology-based production of biochars has not received as considerable attention as other methods and it is also important that biochars be tested on groundwater samples. In the present study, for the first time, we report the preparation of biochars from different parts of Moringa oleifera using thermal plasma processing and demonstrate fluoride adsorption in both synthetic and contaminated groundwater. Water samples were collected from different locations in Nuapada district of Odisha such as Kotamal-Makardampada (20°24'46''N 82°37'19''E), Pandrapathar (20°34'41''N 82°39'25''E), Karlakot-Kadobhata (20°22'52''N 82°37'24''E), Kotamal-Jhakarpada (20°24'35''N 82°37'20''E), and Dohelpada (20°33'50''N 82°38'57''E). The Moringa leaf samples are processed at 1600 °C for 3 min in an inert atmosphere under a continuous flow of argon to get suitable biochars. The plasma-synthesized biochars contain larger exposed surfaces, which are efficient for the adsorption of fluoride. The prepared biochars were highly porous, amorphous, and contain > 72% carbon, which increases the efficiency of defluoridation due to the surface adsorbate site exposed. XRD of the samples showed the presence of calcium hydroxide, magnesium oxide, and calcium oxide, and large peaks of carbon. Raman data showed the double bond of carbon with oxygen in the form of carbonyl bonds, thioether, and sulfhydryl bonds, which contribute to the protonated site for the adsorption of fluoride, and assist in water penetration and swelling of biochars. The biochar of Moringa oleifera is very efficient for the adsorption of fluoride from standard samples as well as groundwater samples up to a concentration of 6 ppm. Conclusively, the present investigation shows that Moringa oleifera leaves are a good alternative adsorbent that could be used for the removal of fluoride from groundwater samples with > 85% removal in 18 h using 1 g biochar for 100 mL or 10 g biochar for 1 L water containing 4 ppm fluoride. To our knowledge, this is the first report on the thermal plasma-based production of Moringa biochars for the removal of fluoride from drinking water.

Fig. 1. Fluoride adsorption in a standard fluoride solution using biochars of different parts of Moringa oleifera (a). Drumstick biochar adsorbed 10%, bark-free oldest stem adsorbed 17%, bark-free younger stem adsorbed 19.8%, bark-free older stem adsorbed 24%, bark adsorbed 27%, leaf biochars adsorbed 44%, younger stem adsorbed 49%, the youngest stem with leaves adsorbed 55%, while seed biochars adsorbed 100%. The fluoride content in the filtrate after biochar-based fluoride removal (b). Removal of fluoride from contaminated ground water samples collected from Nuapada district of Odisha. Fluoride adsorbed using Moringa oleifera seed and leaf biochars in 18 sh (c). Level of fluoride in the groundwater samples before and after treatment with leaf biochars (d).

Fig. 2. FESEM images of the Moringa leaf biochar before fluoride adsorption. All images show a higher surface area and porosity (a, b and c). EDX spectra of the leaf biochar showing the presence of Ca, C, O, Al, Mg, and S (d).

Fig. 3. FESEM images of the Moringa leaf biochar after fluoride adsorption (a, b and c). EDX spectra of the Moringa leaf biochar showing the presence of Ca, C, O, F, Al, and Mg (d).

Fig. 4. XRD spectra of the leaf biochar of Moringa oleifera before (black spectrum) and after (red spectrum) fluoride adsorption. XRD spectra of the seed biochar of Moringa oleifera before (blue spectrum) and after (green spectrum) fluoride adsorption (a). RAMAN spectra of the leaf biochar of Moringa oleifera before (black spectrum) and after (red spectrum) fluoride adsorption. RAMAN spectra of the seed biochar of Moringa oleifera before (green spectrum) and after (pink spectrum) fluoride adsorption (b).

Fig. 5. Antibacterial activity of the Moringa oleifera biochar on Escherichia coli. Pure cultures of E. coli were mixed with autoclaved MQ water in the presence of different concentrations of leaf biochars, 0%, 0.01%, 0.1%, and 1%. After 6 h of incubation, suitably diluted samples were spread into LB agar plates (a) 0%, (b) 0.01%, (c) 0.1% and (d) 1%. The disappearance of colonies with the increase in concentrations of biochar implies the antibacterial activity of Moringa biochars (e).