Investigating the chemistry of alumina based defluoridation using safe H+ source

The existence of high levels of fluoride in drinking water is a worldwide problem and activated alumina is an established adsorbent for fluoride removal. This article investigates factors governing rate of fluoride uptake, mechanical stirring as well as interference from competing ions, of the defluoridation employing activated alumina. Pre-treatment with aqueous solution of sulphuric acid has shown faster uptake in a small setup was demonstrated recently. An attempt to understand the phenomena by profiling ionic species on the active surface of activated alumina, native and acid treated. Presence of carbonate species was found, in Raman spectra, on native surface which can explain effectiveness of pre-treatment. Broadening of hydroxyl species profiles, in infrared and solid-state NMR spectra, suggests incorporation of water into new sites. Fluoride treatment of the pre-treated activated alumina leads to narrowing of hydroxyl species profiles suggesting incorporation of fluoride into new sites. A mechanism is being proposed wherein H⁺ ions in the presence of SO₄^2- (from KHSO₄ or NaHSO₄) enables the energetics for faster defluoridation and regeneration of activated alumina.     

Effect of cation doping on the structure of hydroxyapatite and the mechanism of defluoridation

The structure of three cation-doped hydroxyapatite filter materials with different cation radii and valence states, aluminum-doped hydroxyapatite (Al-HA), magnesium-doped hydroxyapatite (Mg-HA) and lanthanum-doped hydroxyapatite (La-HA), were studied to explore the performance and possible mechanism of defluoridation by means of Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscope (SEM)/Energy-dispersive X-ray spectroscopy (EDS), BET and Zeta potential analysis. The results demonstrated that cation doping with different radii could weaken lattice structure of hydroxyapatite, increase content of hydroxyl group and improve exchange ability with fluoride ions. However, the decisive factor affecting the capacity of fluoride removal was valence state of doped ion, because defluoridation capacity of Al-HA and La-HA with trivalent cations were larger than Mg-HA with bivalent cation. In addition, other studies related to the process of defluorination were carried out. The mechanism of defluoridation mainly included physical adsorption and chemical adsorption. First, F^- was deposited on the surface of the filter material by physical adsorption. After F^- diffused into the voids of the filter interval, and underwent ion exchange and complexation with -OH functional group in the lattice. Cation-doped hydroxyapatite structure and mechanism of fluoride removal were of great significance in the field of environmental governance.     

Defluoridation from aqueous solutions by granular ferric hydroxide (GFH)

This research was undertaken to evaluate the feasibility of granular ferric hydroxide (GFH) for fluoride removal from aqueous solutions. Batch experiments were performed to study the influence of various experimental parameters such as contact time (1 min-24 h), initial fluoride concentration (1-100 mg L⁻¹), temperature (10 and 25 °C), pH (3-12) and the presence of competing anions on the adsorption of fluoride on GFH. Kinetic data revealed that the uptake rate of fluoride was rapid in the beginning and 95% adsorption was completed within 10 min and equilibrium was achieved within 60 min. The sorption process was well explained with pseudo-first-order and pore diffusion models. The maximum adsorption capacity of GFH for fluoride removal was 7.0 mg g⁻¹. The adsorption was found to be an endothermic process and data conform to Langmuir model. The optimum fluoride removal was observed between pH ranges of 4-8. The fluoride adsorption was decreased in the presence of phosphate followed by carbonate and sulphate. Results from this study demonstrated potential utility of GFH that could be developed into a viable technology for fluoride removal from drinking water.     

Defluoridation of drinking water using chemically modified bentonite clay

Adsorption potential of metal oxide (lanthanum, magnesium and manganese) incorporated bentonite clay was investigated for defluoridation of drinking water using batch equilibrium experiments to gain insight of adsorption behavior, kinetics and mechanisms of adsorption of fluoride ion. The effect of various physico-chemical parameters such as pH, adsorbent dose, initial fluoride concentration and the presence of interfering co-ions on adsorption of fluoride has been investigated. The 10%La-bentonite shows higher fluoride uptake capacity for defluoridation of drinking water as compared to Mg-bentonite, Mn-bentonite and bare bentonite clay. The uptake of fluoride in acidic pH was higher as compared to alkaline pH. The equilibrium adsorption data fitted reasonably well in both Langmuir and Freundlich isotherm models. It was also observed that in the presence of certain co-existing ions can have positive effect on removal of fluoride, while carbonate and bicarbonate anions show deleterious effect. The rate of adsorption was reasonably rapid and maximum fluoride uptake was attained within 30 min. The modified adsorbent material shows better fluoride removal properties for actual field water, which could be due to the positive effect of other co-ions present in the field water.     

Removal of fluoride from aqueous solution using protonated chitosan beads

In the present study, chitosan in its more usable bead form has been chemically modified by simple protonation and employed as a most promising defluoridating medium. Protonated chitosan beads (PCB) showed a maximum defluoridation capacity (DC) of 1664mgF-/kg whereas raw chitosan beads (CB) possess only 52mgF-/kg. Sorption process was found to be independent of pH and altered in the presence of other co-existing anions. The sorbents were characterized using FTIR and SEM with EDAX analysis. The fluoride sorption on PCB follows both Freundlich and Langmuir isotherms. Thermodynamic parameters, viz., DeltaG degrees , DeltaH degrees DeltaS degrees and Ea indicate that the nature of fluoride sorption is spontaneous and endothermic. The sorption process follows pseudo-second-order and intraparticle diffusion kinetic models. 0.1M HCl was identified as the best eluent. The suitability of PCB has been tested with field samples collected from a nearby fluoride-endemic area.     

Economic evaluation of fluoride removal by electrodialysis

An economic study was conducted to evaluate the costs of fluoride removal by electrodialysis on the basis of industrial and economic data. The investment and operating costs were estimated for an industrial plant with a capacity of 2200 m³/d water consumption for 50,000 per capita according to Moroccan standards for rural areas. The capital cost was estimated to be € 833,207 and the calculated operating cost to be € 0.154/m³.     

Defluoridation behavior of nanostructured hydroxyapatite synthesized through an ultrasonic and microwave combined technique

The absorption performance of a nano-structured hydroxyapatite produced from a combined ultrasonic and microwave technique was examined for the removal of fluoride from contaminated water. The effect of physical and chemical parameters such as initial pH, contact time, initial fluoride concentration and temperature were investigated. The results indicated that the equilibrium adsorption data followed both the Langmuir and Freundlich isotherms, with a maximum monolayer adsorption capacity of 5.5mg/g at 298K. In addition, the kinetic studies have shown that the fluoride adsorption data followed a pseudo-second order model and that the intra-particle diffusion process played a significant role in determining the rate. The thermodynamic analysis also established that the adsorption process was endothermic and spontaneous. The initial and final fluoride loaded nano-hydroxyapatite samples were characterized using FESEM, TEM, XRD, FTIR and XPS methods. The analysis revealed that structural changes to the adsorbent had taken place.     

杭锦2#土处理含高氟水的初探

本文研究了杭锦2^#土经酸改性后，在不同温度下，进行焙烧后处理高氟水。探讨焙烧温度、吸附时间对吸氟作用的影响，以此确定最佳工艺条件。     

Defluoridation chemistry of synthetic hydroxyapatite at nano scale: equilibrium and kinetic studies

This study describes the advantages of nano-hydroxyapatite (n-HAp), a cost effective sorbent for fluoride removal. n-HAp possesses a maximum defluoridation capacity [DC] of 1845 mg F⁻/kg which is comparable with that of activated alumina, a defluoridation agent commonly used in the indigenous defluoridation technology. A new mechanism of fluoride removal by n-HAp was proposed in which it is established that this material removes fluoride by both ion-exchange and adsorption process. The n-HAp and fluoride-sorbed n-HAp were characterized using XRD, FTIR and TEM studies. The fluoride sorption was reasonably explained with Langmuir, Freundlich and Redlich–Peterson isotherms. Thermodynamic parameters such as ΔG°, ΔH°, ΔS° and E_a were calculated in order to understand the nature of sorption process. The sorption process was found to be controlled by pseudo-second-order and pore diffusion models. Field studies were carried out with the fluoride containing water sample collected from a nearby fluoride endemic area in order to test the suitability of n-HAp material as a defluoridating agent at field condition.     

High yield combustion synthesis of nanomagnesia and its application for fluoride removal

We describe a novel combustion synthesis for the preparation of Nanomagnesia (NM) and its application in water purification. The synthesis is based on the self-propagated combustion of the magnesium nitrate trapped in cellulose fibers. Various characterization studies confirmed that NM formed is crystalline with high phase purity, and the particle size varied in the range of 3-7 nm. The fluoride scavenging potential of this material was tested as a function of pH, contact time and adsorbent dose. The result showed that fluoride adsorption by NM is highly favorable and the capacity does not vary in the pH range usually encountered in groundwater. The effects of various co-existing ions usually found in drinking water, on fluoride removal were also investigated. Phosphate was the greatest competitor for fluoride followed by bicarbonate. The presence of other ions studied did not affect the fluoride adsorption capacity of NM significantly. The adsorption kinetics followed pseudo-second-order equation and the equilibrium data are well predicted by Frendlich equation. Our experimental evidence shows that fluoride removal happened through isomorphic substitution of fluoride in brucite. A batch household defluoridation unit was developed using precipitation-sedimentation-filtration techniques, addressing the problems of high fluoride concentration as well as the problem of alkaline pH of the magnesia treated water. The method of synthesis reported here is advantageous from the perspectives of small size of the nanoparticle, cost-effective recovery of the material and improvement in the fluoride adsorption capacity.