Fluoride removal from brackish groundwater by direct contact membrane distillation

The direct contact membrane distillation applied for fluoride removal from brackish groundwater was investigated. The self-prepared polyvinylidene fluoride membrane exhibited high rejection of inorganic salt solutes. The maximum permeate flux 35.6 kg/（m^2-hr） was obtained with the feed solution at 80℃ and the cold distillate water at 20℃. The feed concentration had no significant impact on the permeate flux and the rejection in fluoride. The precipitation of CaCO3 would clog the hollow fiber inlets and foul the membrane surface with increasing concentration factor when natural groundwater was used directly as the feed, which resulted in a rapid decline in the module efficiency. This phenomenon was diminished by acidification of the feed. The experimental results showed that the permeate flux and the quality of obtained distillate kept stable before concentration factor reached 5.0 with the acidified groundwater as feed. The membrane module efficiency began to decline gradually when the feed continued to be concentrated, which can be mainly attributed to the formation of CaF2 deposits on the membrane surface. In addition, a 300 hr continuous fluoride removal experiment of acidified groundwater was carried out with concentration factor at 4.0, the permeate flux kept stable and the permeate fluoride was not detected.

Fluoride removal by Al,Ti,and Fe hydroxides and coexisting ion effect

Batch experiments were conducted to evaluate fluoride removal by Al,Fe,and Ti-based coagulants and adsorbents,as well as the effects of coexisting ions and formation of aluminum–fluoride complexes on fluoride removal by co-precipitation with alum（Al_2（SO_4）_3·18H_2O）.Aluminum sulfate was more efficient than the other coagulants for fluoride removal in the pH range between 6 and 8.Nano-crystalline TiO_2 was more effective for fluoride removal than Al and Fe hydroxides in a pH range of 3–5.Coexisting anions in water decreased the removal of fluoride in the order：phosphate（2.5 mg/L） 〉 arsenate（0.1 mg/L） 〉 bicarbonate（200 mg/L） 〉sulfate（100 mg/L） = nitrate（100 mg/L） 〉 silicate（10 mg/L） at a pH of 6.0.The effect of silicate became more significant at pH 〉 7.0.Calcium and magnesium improved the removal of fluoride.Zeta-potential measurements determined that the adsorption of fluoride shifted the PZC of Al（OH）_3 precipitates from 8.9 to 8.4,indicating the chemical adsorption of fluoride at the surface.The presence of fluoride in solution significantly increased the soluble aluminum concentration at pH 〈 6.5.A Visual MINTEQ modeling study indicated that the increased aluminum solubility was caused by the formation of AlF^（2＋）,AlF^（＋2）,and AlF_3complexes.The AlF_x complexes decreased the removal of fluoride during co-precipitation with aluminum sulfate.

Performance and Mechanisms of Fluoride Removal from Groundwater by Lanthanum-aluminum-loaded Hydrothermal Palygorskite Composite

Lanthanum-aluminum loaded hydrothermal palygorskite（La-Al-HP） composite was prepared and selected as adsorbent for the fluoride removal from simulated groundwater. The adsorbent was characterized by scanning electron microscopy（SEM）, Brunauer-Emmet-Teller（BET） analysis, X-ray diffraction（XRD） analysis and X-ray pho- toelectron spectroscopy（XPS）. SEM visualization shows that the dense surface structure of raw HP appeared loose and presented micro canals after modification. The BET analysis also proved the specific surface area of La-Al-HP composite（95.58 m^2/g） was higher than that of the raw HP（34.31 m^2/g）. Subsequently, the adsorption capacity of La-Al-HP composite was demonstrated in adsorption experiments. The kinetics of fluoride ion adsorption into La-Al-HP composite followed the pseudo-second order with a correlation coefficient of 0.997. The isotherm data was well fitted with the Langmuir model. The monolayer adsorption capacity of La-Al-HP composite was 1.30 mg/g. The XRD and XPS results reveal that the La^3＋ and Al^3＋ ions were loaded on the surface of modified HP and the fluoride ion was adsorbed onto the La-Al-HP composite. A large amount of La-Al-O composite oxide existing on the surface of La-Al-HP composite might be the immanent cause for the excellent adsorption capacity of fluoride ions.

Batch and Column Experiments on Fluoride Removal from Waters Using Modified Zeolite

High fluoride groundwater occurs widely in China, presenting a quite serious environmental problem. Zeolite from Xinyang, China was tested as the fluoride removing adsorbent. Batch and column experiments on fluoride removal using modified zeolites treated with hydrochloric acid, sodium hydroxide, sodium chloride and ferric chloride, respectively show that 0.1 mol/L HCl modified zeolite can be used as an adsorbent for fluoride, with an adsorption capacity of 173.16 mg/kg.

Characterization and physicochemical aspects of novel cellulose-based layered double hydroxide nanocomposite for removal of antimony and fluoride from aqueous solution

A series of novel adsorbents composed of cellulose(CL)with Ca/Al layered double hydroxide(CC_(x)A;where x represent the Ca/Al molar ratio)were prepared for the adsorption of antimony(Sb(V))and fluoride(F^(-))ions from aqueous solutions.The CC_(x)A was characterized by Fourier-transform infrared spectroscopy(FTIR),Brunauer–Emmett–Teller(BET),elemental analysis(CHNS/O),thermogravimetric analysis(TGA-DTA),zeta potential,X-ray photoelectron spectroscopy(XPS)and scanning electron microscopy with energy dispersive Xray spectroscopy(SEM-EDX)analysis.The effects of varying parameters such as dose,pH,contact time,temperature and initial concentration on the adsorption process were investigated.According to the obtained results,the adsorption processes were described by a pseudo-second-order kinetic model.Langmuir adsorption isotherm model provided the best fit for the experimental data and was used to describe isotherm constants.The maximum adsorption capacity was found to be 77.2 and 63.1 mg/g for Sb(V)and F^(-),respectively by CC_(3)A(experimental conditions:pH 5.5,time 60 min,dose 15 mg/10 mL,temperature 298 K).The CC_(3)A nanocomposite was able to reduce the Sb(V)and F^(-)ions concentration in synthetic solution to lower than 6μg/L and 1.5 mg/L,respectively,which are maximum contaminant levels of these elements in drinking water according to WHO guidelines.

Defluoridation of groundwater by calcined Mg/Al layered double hydroxide

The present study evaluated calcined Mg/Al layered double hydroxide(CLDH)availability for the removal of fluoride from local groundwaters.The Mg/Al layered double hydroxide(LDH)was synthesized by coprecipitation method and characterized by XRD,FT-IR and TGA-TDA analyses.Batch defluoridation experiments were performed under various conditions such as calcination,solution pH,contact time,temperature,material dosage and reuse.Experimental results indicate that fluoride removal strongly increased after calcination of the LDH up to 600℃.The maximum fluoride removal was obtained at solution pH of 6.85.Kinetics of fluoride removal followed the pseudo-second order kinetic model.The rise in solution temperature strongly enhances the removal efficiency.The adsorption mechanism involved surface adsorption,ion exchange interaction and original LDH structure reconstruction by rehydration of mixed metal oxides and concomitant intercalation of fluoride ions into the interlayer region.The optimum dosages required to meet the national standard for drinking water quality were found to be 0.29 and 0.8 g/L,respectively,for Bejaad and Settat goundwaters.A decrease in the fluoride uptake with increasing the number of regeneration cycles was observed.