Fluoride Sorption Using Zirconium ion-impregnated Macadamia Nutshell-Derived Biochar from Aqueous Solution: Adsorption Isotherms and Kinetic Modeling

Abstract Zirconium ion-impregnated macadamia nutshell biochar (ZMNB) was synthesized and tested for its fluoride sorption capacity by exploiting batch experiments. Four temperatures (300, 400, 500, and 600 ℃) were used to pyrolyze the macadamia nutshells to yield macadamia nutshell biochar (MNB) which was separately impregnated with Zr (IV) aqueous solution. Both non-modified and chemically modified MNB were assessed by X-ray diffraction (XRD), attenuated total reflection Fourier Transform Infra-red (ATR-FTIR), field emission scanning electron microscope-energy dispersive X-ray (FESEM-EDS), thermogravimetric analysis, and Brunauer-Emmett-Teller (BET) surface area analyzer. The sequence of F− adsorption capacities for the synthesized biosorbents was ZMNB 300 > ZMNB 400 > ZMNB 500 > ZMNB 600, which revealed the influence of pyrolysis temperature on sorption process. ZMNB 300 had a maximum percentage fluoride removal effectiveness of 99.05% using 1.0 g L− 1 adsorbent lot, at neutral pH for an interaction period of 1 h. The adsorption studies suited perfectly with the pseudo-second-order kinetic model and the linear Langmuir isotherm, affirming a chemisorption process. Thermodynamic studies revealed that the calculated ΔG° value (-5.593 kJ mol− 1) ascribed the rapidity and spontaneity of the sorption action, ΔH° value (30.102 kJ mol− 1) elucidated the endothermic, irreversible and chemisorption process whereas the ΔS° value (118.55 J mol− 1 K− 1) signified the F− adsorption’s random trait on the solid or liquid interface of the ZMNB 300. Results from the sorption experiments using fluoride-contaminated natural water removed the F− ions below the curbed WHO standard (1.5 mg L− 1) for drinking water and therefore, a potential material for practical applications.