Bone‐derived biochar and magnetic biochar for effective removal of fluoride in groundwater: Effects of synthesis method and coexisting chromium

Abstract

The presence of fluoride in groundwater in excess of 1.5 mg L−1 is a major environmental health concern, and biochar is a promising low‐cost adsorbent for the treatment of such water. In the present study, pristine and magnetic biochars were synthesized by peanut hull and bovine bone for the adsorption of fluoride. The biochars were systematically characterized by SEM‐EDS, BET, XRD, VSM, FT‐IR, and XPS. The experiment results showed that the magnetic biochar prepared by soaking biomass in FeCl3 solution and then pyrolyzing (“prepyrolysis”) had a higher adsorption capacity than that prepared by mixing pristine biochar with Fe2+/Fe3+ solution and then treating with NaOH (“postpyrolysis”). The bone‐derived biochar and magnetic biochar exhibited high adsorption capacity of fluoride (>5 mg g−1) due to the presence of hydroxyapatite (HAP) and γ‐Fe2O3. The 0.1 M NaOH solution could be optimal desorption agent, and the adsorption–desorption experiments indicated the bone biochars maintained the reasonable adsorption capacity after several cycles. Moreover, the coexisting Cr(VI) and fluoride could be removed simultaneously by bone‐derived biochars. It is suggested that bovine bone‐derived pristine and magnetic biochars can be used as preferential adsorbents for fluoride removal from contaminated groundwater.Practitioner points Bone‐derived pristine and magnetic biochars exhibit high adsorption capacity for fluoride in weakly alkaline solution. The presence of hydroxyapatite and γ‐Fe2O3 in bone‐derived biochars plays an important role for fluoride adsorption. Magnetic biochars prepared by soaking biomass in FeCl3 solution and then pyrolyzing (“prepyrolysis”) perform better. The coexisting Cr(VI) and fluoride can be simultaneously removed in groundwater by bone biochars.

Recent Developments in Understanding Biochar’s Physical–Chemistry

“…In particular, a cooling agent used in scientific instruments commercially known as Galden ® -IUPAC name is oxidized poly(1,1,2,3,3,3-hexafluoroprop-1-ene)-with formula CF 3 − (OCF(CF 3 )CF 2 ) n − (OCF 2 ) m − OCF 3 and an average molecular weight of 610 Da, can be adsorbed on biochar's surface by (i) charge transfer bond with biochar metal impurities; (ii) H-bonds with biochar surface hydrophilic groups; (iii) Van der Waals interactions with the biochar C-skeleton; (iv) outer-sphere interactions where water molecules bridge the Galden ® with the electronpoor biochar metal impurities. Biochar can also trap anionic pollutants such as fluoride (F − ) in groundwater [214]. In this case, the presence of biochar surface cation impurities is considered responsible for F − electrostatic entrapment.…”

“…However, the effect of these impurities reduces as pH increases. In fact, Zhou et al [214] observed that the higher the pH, the lower the amount of fluoride retained on the biochar surface. This can be explained by the progressive deprotonation of the surface acidic functional groups which allow negative charge to increase and electrostatic repulsions.…”

“…This can be explained by the progressive deprotonation of the surface acidic functional groups which allow negative charge to increase and electrostatic repulsions. In addition, when magnetic biochars are applied for groundwater remediation, the presence of paramagnetic systems modifies biochar z-potential, thus allowing enhancement of the capacity of magnetic biochar surfaces to attract fluoride ions [214].…”

Engineered Magnetic Carbon-Based Adsorbents for the Removal of Water Priority Pollutants: An Overview

“…Adsorption mechanism of both adsorbates (i.e., fluoride and nitrate) implied the hydrogen bond formation with the participation of protonated surface hydroxyls on the iron oxides. BCs and their magnetic versions were prepared with bovine bone and peanut hull for fluoride adsorption [162]. XPS, FTIR, XRD, SEM-EDX, and VSM were used to analyze the surface chemistry and magnetic properties of these adsorbents.…”

Evaluation of fluoride adsorption in solution by synthetic Al2O3/CeO2: A fixed‐bed column study

“…A variety of conventional methods have been employed for the removal of fluoride, including electrolysis (Amor et al, 2001), ion exchange (R. X. Liu et al, 2002), chemical precipitation (M. Yang et al, 2001), membrane filtration (Tahaikt et al, 2007), and adsorption (Z. L. He et al, 2013; P. Li et al, 2014). Among them, adsorption method is commonly viewed as the most promising method for removal of fluoride due to its advantages of high efficiency, low cost, and easy operation (Jagtap et al, 2012; Zhou et al, 2019). As an adsorbent, metal oxides can be used as good fluoride capture agents because of their positive electric properties.…”