Residual Fluoride Concentration Research Articles

Influence of sulfate ion on fluoride removal from flue gas scrubbing wastewater using lanthanum salts

Batch experiments were conducted to remove fluoride from simulated flue-gas scrubbing wastewater using La(NO3)3·6H2O. The effects of SO42− concentration and addition sequence on residual fluoride concentration and precipitates properties (mass, morphology, phase structure and composition) were investigated using fluorine ion-selective electrode, SEM, EDS and XRD. The results show that precipitates obtained in the presence of SO42− mainly consisted of LaF3 in irregular bulk shape with high crystallinity. However, amorphous particles were observed in the precipitates, which were speculated to be La[SO4]F. Due to the smaller stoichiometric coefficient of F/La in La[SO4]F compared to that in LaF3, the residual fluoride concentration increased as the SO42− concentration ascended. Subsequent addition of SO42− after lanthanum salt could inhibit the formation of amorphous particles, and further reduce the residual fluoride concentration. At a La/F molar ratio of 1:3.0, the subsequent addition of SO42− (0.02 mol/L) after La(NO3)3∙6H2O could obtain a residual fluoride concentration of 6.9 mg/L, lower than the required fluoride emission level. Further increasing the La/F molar ratio could reduce residual fluoride concentration to below 2 mg/L, while the risk of retaining La3+ would emerge.

Removal of fluoride ions from aqueous solution by metaettringite.

Fluoride contamination is a major problem in wastewater treatment. Metaettringite (which has previously shown enhanced anion adsorption) was investigated as a possible adsorbent to remove fluoride from low-concentration solution (25 mg-F/L). The fluoride removal properties of ettringite and metaettringite were first compared at pH 10, and metaettringite was found to be more effective. The dominant reaction mechanism for fluoride adsorption in metaettringite was found to be recrystallization of metaettringite by rehydration; this was accompanied by precipitation of calcium fluoride. The adsorption kinetics followed the pseudo-second order model. Metaettringite was also able to remove fluoride effectively in low pH environment (i.e., at pH 3.5). The influence of coexistence of sulfate ions in solution on the fluoride removal performance was investigated, and a small decrease in performance was noted. The residual fluoride concentrations obtained with higher doses of metaettringite were lower than those specified by the Japanese effluent standard (non-coastal areas: 8 mg-F/L; coastal areas: 15 mg-F/L). The fluoride removal capacity of metaettringite was compared with those of other solid materials. The observed maximum capacity was 174.7 mg-F/g-metaettringite. In the case of high fluoride concentration solution, the main removal mechanism will be changed to calcium fluoride precipitation. In general, metaettringite is regarded as promising material for fluoride removal in wastewater treatment.

An integrated separation technology for high fluoride-containing wastewater treatment: Fluoride removal, membrane fouling behavior and control

The high concentration of fluoride in aqueous, which exacerbated by the discharged wastewater from semiconductor, electroplating, metallurgical and ceramics industries, has gone far beyond the World Health Organization (WHO) standard and seriously affected the health of human life. This study aims to validate the deep removal of fluoride from high fluoride-containing wastewater by using an integrated precipitation/crystallization/ultrafiltration (UF)/reverse osmosis (RO) system. The system enables an extremely low residual fluoride concentration (down to 0.25 mg L−1) qualified WHO standard, which was much lower than those of other reported membrane-based processes. In particular, the application of crystallization rather than conventional coagulation/flocculation was demonstrated to be effective in reducing fluoride, turbidity and sludge water content. Membrane fouling analysis indicated that UF membrane scaling was attributed to the CaF2 particles deposition at recovery rates of 70%–90%. RO membrane fouling behaviors were identified at recovery rate of 50% (organic fouling), 70% (inorganic scaling occurred and organic fouling) and 80% (inorganic scaling and organic fouling). Meanwhile, UF/RO membrane scaling/fouling could be removed by pure water cleaning (UF) and Na2CO3/NaOH/HCl cleaning (RO). Results of this work shall provide useful information on the application of a precipitation/crystallization/UF/RO system to purify and recover high fluoride-containing wastewater.

Optimizing the recovery process of ceramic grade calcium fluoride from hydrofluoric/hexafluorosilicic acid wastewater

Chemical precipitation using calcium salts has been widely applied in hydrofluoric acid wastewater treatment due to its simplicity, flexibility and economy. Coagulants/flocculants are used to improve solid-liquid separation in a fine precipitate of calcium fluoride (CaF2). As a result, a large amount of sludge is generated by CaF2 precipitation process. In order to move towards a circular economy, the current study developed an environment-friendly and cost-effective approach to treat concentrated hydrofluoric/hexafluorosilicic acid wastewater (∼117 g F−/L) obtained from an optoelectronic industry in Taiwan and to recover fluoride as the high purity CaF2, as well as to reduce the amount of final waste disposal. The influence of three independent variables, including calcium-to-fluoride molar ratio ([Ca2+/F−], silicon concentration ([Si4+]), and polymer dosage applied, on the purity of recovered CaF2 products, was investigated and optimized by combining response surface methodology (RSM) with central composite design (CCD). The ceramic grade CaF2 of 85.6–87.9% was achieved under the optimum conditions as [Ca2+/F−] of 0.618–0.645, [Si4+] of 1.48 g/L, and polymer concentration of 17.2–17.8 mg/L. The final turbidity, residual fluoride concentration in the supernatants and fluoride removal efficiency were 14.8–16.2 NTU, 62.2–67.6 mg/L and greater than 99%, respectively. The value of R2 = 0.9498 and adjusted R2 = 0.9319 indicated an adequate correlation between experimental results and predictions by the quadratic model. In addition, a new insight into the CaF2 purification method has been proposed based on the idea of pH control.

The influence mechanism of HCO3− on fluoride removal by different types of aluminum salts

In order to study the influence mechanism of HCO3− on the fluoride removal by AlCl3 and Al13, the residual fluoride concentration, residual aluminum concentration and pH value in coagulation effluent were measured at different HCO3− concentrations. Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) was used to analyze the aluminum species in the supernatant after coagulation, and the flocs collected were analyzed by nuclear magnetic resonance (NMR) and X-ray photoelectron spectra (XPS). The results show that AlCl3 is used as a coagulant, when the concentration of HCO3− increases from 100 mg/L to 300 mg/L, Al3+ and HCO3− are double hydrolyzed to form Al(OH)3 alum flower after adding AlCl3, and the removal rate of fluoride rapidly increases from 9.55 % to 78.0 %. When the concentration of HCO3− increases from 300 mg/L to 1000 mg/L, the precipitation of Al(OH)3 will gradually dissolve to form Al(OH)4− and the removal rate of fluoride drops from 78.0 % to 56.2 %. When Al13 is used as a coagulant, with the increase of HCO3− from 100 mg/L to 1000 mg/L, its double hydrolysis with Al-F complexes is enhanced, resulting in the formed Al6F2, Al7F2 and Al9F3 gradually dissociate, and the removal rate of fluorine decreases from 78.8 % to 59.4 %.

Fluoride removal studies using virgin and Ti (IV)-modified Musa paradisiaca (plantain pseudo-stem) carbons.

The preparation of carbons in virgin and Ti-modified forms under controlled conditions at low temperature from plantain pseudo-stem (Musa paradisiaca) was achieved. These prepared carbons were characterized for instrumental studies such as BET, FTIR, XRD, SEM with EDS and TGA to understand the chemistry and modification. The determination of IEP and pHZPC established the presence of positive surface sites on the virgin (VMPC) and Ti-modified (TiMPC) carbons to facilitate the sorption of fluoride. The fluoride removal efficiency as a function of time, pH, dose, initial fluoride concentration, temperature, and co-ion intervention was studied. The maximum fluoride removal of about 81.2 and 97.7% was achievable with VMPC and TiMPC, respectively, after 20min at the pH of 2.04 and continued for the equilibrium of 60min. Temperature was found to be influential both by way of initial increase followed by a decrease in the fluoride uptake of MPCs. Regeneration was very consistent up to 7cycles with the residual fluoride concentration below the WHO guide line of 1.5mgL-1. Highest intervention due to hydrogen carbonate ions was observed during the fluoride removal process. Kinetic (pseudo-first-order, pseudo-second-order, and intra-particle diffusion) and isotherm models (Langmuir, Freundlich, and DKR) were checked for their compliance with the present sorption system. These low temperature synthesized MPCs are found to be effective candidates in the process of fluoride abatement in water.

Influence of sample treatment after bomb digestion on determination of trace elements in rock samples by ICP-MS

In the present work, the influence of chemical preparation on the determination of Li, Be, Sc, V, Cr, Co, Ni, Cu, Ga, Rb, Sr, Y, Zr, Nb, Mo, Sn, Cs, Ba, rare earth elements, Hf, Ta, W, Tl, Pb, Th and U in rock samples by inductively coupled plasma - mass-spectrometry (ICP-MS) is studied. The conditions of a treatment stage after a rock bomb digestion, leading to significant (>30%) underestimation in Ta, Nb, Mo, Sn and W determination, are refined. The erroneously low results are most likely due to hydrolysis of the compounds of these elements during evaporation of an excess amount of hydrofluoric acid. It is found that the degree of underestimation depends on the total concentration of the residual fluoride ion in the final rock solution for an ICP-MS measurement. The significant losses in Ta and Nb in the nephelinite NKT-1 take place when the residual fluoride concentration becomes <80 and 70 μg ml−1, respectively, but it is different for other types of rocks. The fluoride concentration is related to the kinds of acids that are used for the rock treatment after a rock bomb digestion, as well as the degree of solution evaporation and the bulk rock composition. A correlation between the residual fluoride ion total concentration in the prepared solution and the total content of aluminium, R = 0,887, p < 0,001, and of calcium, R = −0.699, p 0,011, in the analysed rock samples is found. The optimized conditions of an acid treatment of rock samples after a bomb digestion to avoid hydrolysis and to obtain the correct results for determination of studied trace elements are described.

Defluoridation of Groundwater Using Raw Material of Autoclaved Aerated Concrete

A batch process of defluoridation using raw material of autoclaved aerated concrete (AAC) was studied under optimum conditions. The parameters of adsorption method, such as pH, adsorbent dose and contact time were optimized under batch experiments. SPADNS method was used to quantitatively evaluate the residual fluoride concentration. Under optimum conditions, pH of solution, adsorbent dose and contact time was to be 7, 0.1 g/10 mL and 60 min, respectively. The amount of defluoridation was found to be 3.23 mgF-/gACC. Deflouridation capacity of AAC can be explained on the basis of the chemical interaction of fluoride with the metal oxides under suitable pH conditions. The adsorption process was found to follow first order rate mechanism as well as Freundlich isotherm.

Comparison of residual salivary fluoride retention using amine fluoride toothpastes in caries-free and caries-prone children

AimThis was to compare the salivary fluoride levels following tooth brushing with amine fluoride toothpastes containing three different concentrations of F (250 ppm F, 500 ppm F and 1250 ppm F) and to evaluate the effect of rinsing with water on the oral fluoride levels up to 90 min.MethodsA double blind randomised six-arm crossover study was conducted with 32 child participants. Patients were divided into two groups depending on their caries experience with caries-free group (n = 17, mean age = 72.9 months) and caries-prone group (n = 15, mean age = 69.6 months, mean dmfs = 12.3). Each participant brushed their teeth with a smear of dentifrice containing (250 ppm, 500 ppm and 1250 ppm F toothpastes) for 60 s. After spitting out the dentifrice/saliva slurry, participants either rinsed with water or did not rinse at all. Samples of whole mixed unstimulated saliva were collected at 0 (baseline), 1, 15, 30, 45, 60 and 90 mins post-brushing/rinsing.ResultsAfter completing the study on residual fluoride concentration it was found that caries was not a significant variable (p = 0.567) while every other variable was (all p values <0.001). Time, toothpaste F concentration and rinse had significant effects (p < 0.001). In general, higher residual salivary F concentrations were found with increased F concentration in toothpastes and when no rinsing was performed after brushing.ConclusionThe results of this study support the current recommendation of using toothpastes with >1000 ppm F concentration in children with an increased caries risk in addition to spitting excess toothpaste with no rinsing following brushing.

Defluoridation Performance Comparison of Nano-hydrotalcite/Hydroxyapatite Composite with Calcined Hydrotalcite and Hydroxyapatite

Fluoride retention from water is nowadays a serious health problem. This study reports the potential of a newly developed nano-hydrotalcite/hydroxyapatite (n-HT/HAp) composite, and its constituent materials, hydrotalcite (HT) and hydroxyapatite (HAp), in fluoride removal. Calcined hydrotalcites (cHT) showed a remarkable fluoride removal ability from water through memory effect mechanism. HAp, the mineral compound of bones, adsorbs fluoride as well but through ion exchange mechanism. Fluoride substitutes hydroxyls to produce fluorapatite. Among the tested calcined hydrotalcites, cHT Mg-Al (4:1) sample, composed of magnesium divalent cation to aluminum ratio of 4, was identified as the best-performing hydrotalcite. The differences among cHT samples in fluoride removal capacities are attributed to hydrotalcite composition as well as to particle size. The performance of these materials is compared with that of n-HT/HAp composite whose main features are basic acidic material and not yet tested in fluoride retention. Interestingly, n-HT/HAp also performs best, 98 %, slightly higher than the best cHT Mg-Al (4:1) sample with 97 % fluoride removal efficiency from such a high initial fluoride solution of 20 mg/L at 10 g/L dose, yielding the final residual fluoride concentrations of 0.36 and 0.6 mg/L, respectively; both meet the WHO standard for drinking water. Besides, the uncalcined hydrotalcite constituent added virtue to the advantage of using n-HT/HAp in fluoride removal as the efficiency was compensated by the nanometric size of the hydrotalcite particle.

Arsenic and fluoride removal from groundwater by electrocoagulation using a continuous filter-press reactor.

We investigated simultaneous arsenic and fluoride removal from ground water by electrocoagulation (EC) using aluminum as the sacrificial anode in a continuous filter-press reactor. The groundwater was collected at a depth of 320 m in the Bajío region in Guanajuato Mexico (arsenic 43 µg L(-1), fluoride 2.5 mg L(-1), sulfate 89.6 mg L(-1), phosphate 1.8 mg L(-1), hydrated silica 112.4 mg L(-1), hardness 9.8 mg L(-1), alkalinity 31.3 mg L(-1), pH 7.6 and conductivity 993 µS cm(-1)). EC was performed after arsenite was oxidized to arsenate by addition of 1 mg L(-1) hypochlorite. The EC tests revealed that at current densities of 4, 5 and 6 mA cm(-2) and flow velocities of 0.91 and 1.82 cm s(-1), arsenate was abated and residual fluoride concentration satisfies the WHO standard (CF < 1.5 mg L(-1)). Spectrometric analyses performed on aluminum flocs indicated that these are mainly composed of aluminum-silicates of calcium and magnesium. Arsenate removal by EC involves adsorption on aluminum flocs, while fluoride replaces a hydroxyl group from aluminum aggregates. The best EC was obtained at 4 mA cm(-2) and 1.82 cm s(-1) with electrolytic energy consumption of 0.34 KWh m(-3).

Fluoride removal by induced crystallization using fluorapatite/calcite seed crystals

The removal of fluoride from water by induced crystallization using fluorapatite (FAP)/calcite seed crystals was investigated. Batch experiments were conducted to determine optimal operating parameters. The effect of FAP/calcite mass ratio of the seed crystals, initial seed crystal concentration, molar ratios of Ca2+:PO43−:F− and contact time were examined. FAP/calcite with any mass ratio reduced fluoride concentrations from 9.5mg/L (0.5mM) to less than 1mg/L. The use of FAP alone and calcite alone was not effective. Use of initial seed crystal concentrations greater than 4.0g/L were correlated to residual fluoride concentrations less than 1.0mg/L. The optimal parameters for fluoride removal were an initial seed crystal concentration of 6g/L (FAP 4.8g/L, calcite 1.2g/L), Ca/P/F molar ratio of 8:4:1, and contact time of 1h. The final pH was always close to neutral. The presence of natural organic matter (NOM) in the raw water was found to not affect fluoride removal. SEM–EDS and XRD analysis showed that fluoride removal was due to crystallization of Ca5(PO4)3F on the surface of FAP. Phosphate, but not fluoride, accumulated on calcite by adsorption. The seed crystal could be regenerated easily, and used repeatedly, and used seed crystals could also be used as resources in phosphorus fertilizer production. Induced crystallization using FAP/calcite as a seed crystal is an economically and environmentally friendly process, and has the potential to remove fluoride from groundwater.

Fluoride Removal from Aluminium Finishing Wastewater by Hydroxyapatite

The hexavalent chromium toxicity forces the aluminium finishing industry to use fluoride-containing chemicals in order to improve aluminium passivation. Disposal of industrial wastewaters containing excess concentration of fluoride to surface and ground waters can cause several health hazards. Effluents containing fluoride need treatment to reduce their concentration to disposable values before being discharged into public sewage. In this study, fluoride-containing wastewater from the aluminium finishing industry was firstly treated by precipitation at pH 12.5, achieving residual fluoride concentrations of 8.69 ± 1.84 mg/L from an initial concentration of 74.5 ± 4.6 mg/L, and then by adsorption of the remaining solution onto hydroxyapatite (HAP), resulting in fluoride effluent concentrations of 4.09 ± 0.16 mg/L. The total fluoride removal efficiency was 94.5 %. The sorption onto HAP data fitted well the Freundlich (r2 = 0.992) and Langmuir (r2 = 0.994) models. The adsorption capacity indicator (KF) was equal to 0.723. The dimensionless constant RL of 0.62 indicates favorable sorption of fluoride onto HAP. It is concluded that the adsorption step using HAP can be used as a post-treatment to remove fluoride from aluminium finishing industry wastewater.

Multiobjective optimization for minimum residual fluoride and specific energy in electrocoagulation process

In the present paper, 24 factorial design method was employed for optimization of two responses: residual fluoride and specific energy consumed. The study was based on interaction effect of four factors namely, current, initial fluoride concentration, time, and inter-electrode distance. A regression model equation was developed which was validated by high R2 values of 98.78 and 99.80% for residual fluoride and specific energy consumed, respectively. Optimization was targeted for residual fluoride concentration less than 1 mg/l and minimum specific energy consumption. The optimized conditions as suggested by the model for initial fluoride concentration 6 mg/l were: applied current—0.27 A, inter-electrode distance—20 mm, treatment time—46.70 min, specific energy—5.85 J/mg, and residual fluoride—0.77 mg/l. These results were used for experimental verification, which was in good agreement with the predicted results.

Fluoride recovery from spent fluoride etching solution through crystallization of Na3AlF6 (synthetic cryolite)

Fluoride removal by precipitation of calcium fluoride is the most common practice in Taiwan to treat fluoride-containing wastewater in semiconductor or optoelectronic industries. Due to very fine CaF2 precipitates (∼0.1μm), coagulants/flocculants are needed to facilitate sedimentation of CaF2. In turn, large amount of sludge is produced by CaF2 precipitation/sedimentation process. In this study, removal of fluoride from spent fluoride etching solution by cryolite crystallization was investigated. Experimental and chemical equilibrium modeling results show that a good control of reaction pH and Al:F molar ratio is the key to form cryolite successfully. Formation of cryolite is exothermic reaction and formation of cryolite is less favor under elevated temperature. The highest residual fluoride concentration is found at reaction temperature of 90°C while those at 20 and 55°C are similar. Under Al/F molar ratios of 1:3 and 1:6 and pH of 5.5, the precipitates produced at room temperature are very similar to the commercial cryolite according to XRD, SEM, and EDX analysis. Particle size of cryolite precipitates ranges from 3μm to 15μm and is much larger than CaF2 precipitates of 0.1μm, resulting in rapid sedimentation. Cryolite crystallization process produces much less sludge volume than does by CaF2 precipitation.

Defluoridation of Algerian Drinking Water by Precipitation and Adsorption Methods

The aim of the study was to investigate performance of precipitation and adsorption methods in removing excessive fluoride from water samples collected from Algerian North-Eastern Sahara. Defluoridation processes were conducted using one the one hand lime and on the other hand a raw and activated bentonite from western Algeria. Results showed that high lime dosage was needed to achieve treatment but the presence of high levels of magnesium was beneficial for the defluoridation. Activated bentonite had higher fluoride removal capacity than raw bentonite and resulted in residual fluoride concentrations well within recommended standards. Both defluoridation methods were dependent on inorganic quality of tested waters.

Initial fluoride concentration and loading capacity in defluoridation with bauxite, gypsum, magnesite and their composite

One of the parameters that impact on fluoride sorption capacities of materials in water defluoridation is initial concentration. Water from various sources will have different fluoride concentrations; as such the employment of a particular media in defluoridation requires information on how the media will perform in different concentrations. Consequently, research was carried out to determine how initial fluoride concentrations in raw water affect capacity in defluoridation with bauxite, gypsum, magnesite and their composite, and to investigate the possibility of predicting loading capacity of their composite filter through initial fluoride concentration to optimize their application in defluoridation. The results showed that sorption capacities increased with increase in initial concentrations. However, higher starting concentrations resulted in larger residual fluoride concentrations; as such the benefit of large sorption capacities obtained was overshadowed. Polynomial relations of capacity (Cs, mg/g) and initial concentration (Co, mg/l), were obtained for bauxite and gypsum. Magnesite obtained a logarithmic relation for Cs and Co. A power relation was obtained between Cs and Co for their composite, Cs = 0.0328C1.20160. Approximation of capacities of this composite from initial fluoride concentrations was feasible. Water defluoridation has become important as a result of dental and skeletal fluorosis.

Titanium(IV) hydrate based on chitosan template for defluoridation from aqueous solution

A cost effective bio-adsorbent, titanium(IV) hydrate based on chitosan template (Ti-CHI), was synthesized using Ti(SO4)2 and chitosan for defluoridation from aqueous solutions. The Ti-CHI material was characterized using FTIR, SEM with EDX, XRD, XPS and Zeta potential. The adsorption kinetics of fluoride onto Ti-CHI followed pseudo-second-order and intra-particle diffusion models. The Ti-CHI was effective for defluoridating systems with low fluoride concentrations (4.52mg/L), exhibiting a 87.50% removal rate and permitting a residual fluoride concentration (0.55mg/L). The most stable pH range for fluoride removal was from 4 to 9. After combining the results of pH effects, FTIR and Zeta potential, the NH groups and TiOH groups were revealed as the fluoride adsorption sites on Ti-CHI. In addition, the carbonate ions present in the aqueous solutions adversely impact the fluoride adsorption. The Langmuir, Freundlich, and Langmuir–Freundlich models were used to illustrate the isotherms of the adsorption process. The adsorption of fluoride onto Ti-CHI fits the Langmuir and Langmuir–Freundlich isotherm models well. The maximum adsorption capacity for fluoride on the Ti-CHI was 16.12mg/g. Thermodynamic studies revealed that the adsorption process was spontaneous.

Research on Coagulation/Sedimentation Process for Fluoride Removal in Simulation Wastewater

Experiments were carried out with the simulated fluoride-containing wastewater to study the fluoride removal. The effects on the residual fluoride concentration were investigated using lime-sedimentation and coagulation/sedimentation. The results showed that the fluoride contents were among 30mg/L～50mg/L, with lime alone. The residual fluoride concentrations of 600mg/L, 900mg/L and 1200mg/L solution decreased to below 10mg/L, using Ca(OH)2+FeCl3+PAM. The fluoride concentrations of 300mg/L, 600mg/L, 900mg/L, 1200mg/L fluoride wastewater remained below 10mg/L by Ca(OH)2+PFS+PAM. Comparing two methods, the supernatant was relatively clear with Ca(OH)2+PFS+PAM, which had better settling performance. It suggested PFS and PAM has an important increase effect on the fluoride removal, and the optimum conditions: PFS was 0.1mL, fluoride-calcium ratio was 2:1.15.

Research on Coagulation/Sedimentation Process for Simulation of Fluorine-Containing Wastewater Treatment

Over discharge of fluoride will cause the environment pollution and human harm. Experiments were performed with the simulated fluoride wastewater to study the removal of fluoride. The effects on the residual fluoride concentration were investigated using lime-sedimentation and coagulation/sedimentation. The results showed that the fluoride contents were among 30mg/L-50mg/L, with lime alone. The fluoride concentration decresde to 30mg/L-40mg/L, using the combination of Ca(OH)2+PAM. The residual fluoride concentrations of 600mg/L, 900mg/L, 1200mg/L fluoride wastewater remained below 10mg/L by the combination of Ca(OH)2+PAM+PAC. It suggested PAM and PAC has an important increase effect on the fluoride removal. The suitable conditions for defluoridation with Ca(OH)2+PAM+PAC: PAC was 0.2mL, fluoride-calcium ratio was 2:1.1.