Fluoride removal by Ca-Al-CO3 layered double hydroxides at environmentally-relevant concentrations

Calcined Bean Dregs-Hydrocalumite Composites as Efficient Adsorbents for the Removal of Ofloxacin

Calcined bean dregs-hydrocalumite composites were prepared through in situ self-assembly of hydrocalumite on the surface of bean dregs and used for the adsorption of ofloxacin from water. The adsorbents were characterized by scanning electron microscopy, X-ray powder diffraction, and N2 physical adsorption. The results showed that the adsorption performance of calcined bean dregs-hydrocalumite composites for ofloxacin was much better than that of a single bean dreg carbon or calcined hydrocalumite. The effects of preparation and adsorption conditions on the adsorption property of calcined bean dregs-hydrocalumite for ofloxacin were also investigated. The adsorption ratio of ofloxacin reached up to 99.93% using 4 g·L-1 adsorbent dosage with 20 mg·L-1 initial concentration of ofloxacin at 30 °C in 2 h. The adsorption process mainly occurred in the first 5 min. In addition, the adsorption of ofloxacin by calcined bean dregs-hydrocalumite was more in line with pseudo-second-order dynamics and the Langmuir isotherm model.

Kinetics, isotherm, mechanism, and recyclability of novel nano-sized Ce4+-doped Ni-Al layered double hydroxide for defluoridation of aqueous solutions

Excessive fluoride removal from aqueous solutions is of utmost importance as it has an adverse impact on human health. This study investigates the defluoridation efficiency of a novel nano-sized Ce+4-doped Ni/Al layered double hydroxide (Ni-Al-Ce LDH) for aqueous solutions. The synthesized Ni-Al-Ce LDH exhibited a well-defined nanoscale plate-like morphology and a high surface area with an average size of 11.51 nm, which contributed to its enhanced fluoride adsorption capacity. XRD, SEM, HRTEM, and BET studies confirmed these characteristics. XPS analysis confirmed the presence of Ce4+ ions within the Ni-Al LDH. The experimental results indicated that the process of defluoridation followed a pseudo-second-order model of kinetics, suggesting a chemisorption mechanism. The fluoride adsorption isotherms demonstrated well fits to the Freundlich, Langmuir, and Jovanovic models, indicating both monolayer and multilayer fluoride adsorption on the Ce-doped Ni-Al LDH. The maximum adsorption capacity was found to be 238.27 mg/g (Langmuir) and 130.73 mg/g (Jovanovic) at pH 6.0 and 25 °C. The proposed mechanisms for fluoride adsorption on the LDH include ion exchange, surface complexation, hydrogen bonding, and ligand exchange. The Ni-Al-Ce LDH nanomaterial exhibited good recyclability, maintaining 71% of the fluoride adsorption efficiency even after four consecutive cycles. This study highlights the significant role of Ce doping in improving the performance of Ni-Al LDH as a defluoridation adsorbent.

Reutilization of carbon of waste filter cartridge after its surface modification for the fluoride removal from water by continuous flow process

In the present study, the waste carbon cartridge of the water filter was modified and reutilized for defluoridation of water. The modified carbon was characterized by particle size analysis (PSA), Fourier transformed infrared spectroscopy (FTIR), zeta potential, pHzpc, energy-dispersive X-ray (EDS), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and X-ray crystallography (XRD). The adsorptive nature of modified carbon was investigated with pH (4-10), dose (1-5 g/L), contact time (0-180 min), temperature (25-55 °C), fluoride concentration (5-20 mg/L), and the effect of the competitive ions. Adsorption isotherm, kinetics, thermodynamics, and breakthrough studies were evaluated for fluoride uptake on surface-modified carbon (SM*C). Fluoride adsorption on the carbon accepted Langmuir model (R2 = 0.983) and pseudo-second-order kinetic (R2 = 0.956). The presence of HCO3- in the solution reduced the elimination of fluoride. The carbon was regenerated and reused four times; the removal percentage was decreased from 92 to 31.7%. This adsorption phenomenon showed exothermic behavior. The maximum fluoride uptake capacity of SM*C achieved 2.97 mg/g at 20 mg/L of initial concentration. The modified carbon cartridge of the water filter was successfully employed for fluoride removal from water.

Controllable Doping of Mn into Ni0.075-xMnxAl0.025(OH)2(CO3)0.0125·yH2O for Efficient Adsorption of Fluoride Ions

Here, the structural, optical, and adsorptive behaviors of Ni_0.075-xMn_xAl_0.025(OH)₂(CO₃)_0.0125·yH₂O (Ni-Mn/Al) layered double hydroxides (LDHs) are investigated to capture fluoride from aqueous media. The 2D mesoporous plate-like Ni-Mn/Al LDHs are successfully prepared via a co-precipitation method. The molar ratio of divalent to trivalent cations is maintained at 3:1 and the pH at 10. The X-ray diffraction (XRD) results confirm that the samples consist of pure LDH phases with a basal spacing of 7.66 to 7.72 Å, corresponding to the (003) planes at 2θ of 11.47^o and the average crystallite sizes of 4.13 to 8.67 nm. The plate-like Mn-doped Ni-Al LDH consists of many superimposed nanosheets with a size of 9.99 nm. Energy-dispersive X-ray and X-ray photoelectron spectroscopies confirm the incorporation of Mn²⁺ into the Ni-Al LDH. UV-vis diffuse reflectance spectroscopy results indicate that incorporating Mn²⁺ into LDH enhances its interaction with light. The experimental data from the batch fluoride adsorption studies are subjected to kinetic models such as pseudo-first order and pseudo-second order. The kinetics of fluoride retention on Ni-Mn/Al LDH obey the pseudo-second-order model. The Temkin equation well describes the equilibrium adsorption of fluoride. The results from the thermodynamic studies also indicate that fluoride adsorption is exothermic and spontaneous.

A new basic burning raw material for simultaneous stabilization/solidification of PO43--P and F- in phosphogypsum

Phosphogypsum (PG) contains a lot of soluble phosphate (PO₄^3--P) and fluorine ion (F^-), which seriously has hindered the sustainable development of the phosphorous chemical industry. In this study, a new burning raw material (BRM) as an intermediate product in the cement production process was used for PO₄^3--P and F^- stabilize in PG. The stabilizing mechanism of PO₄^3--P and F^- were investigated by Fourier Transform infrared spectroscopy (FT-IR), X-ray diffractometer (XRD), Scanning Electron Microscopy (SEM), X-ray fluorescence (XRF) and X-ray spectroscopy system (XPS). The effect of PG and BRM weight ratio, solid-to-liquid ratio, reaction time, and reaction temperature on the concentrations of PO₄^3--P and F^- were studied. The results showed that the concentration of F^- in the PG leaching solution was 8.65 mg/L and the stabilizing efficiency of PO₄^3--P was 99.78%, as well as the pH of the PG leaching solution was 8.12 when the weight ratio of PG and BRM was 100:2, and the solid to liquid ratio was 4:1, reacting for 24 h at the temperature of 30 ℃. PO₄^3--P and F^- were mostly solidified as Ca₅(PO₄)₃F, CaPO₃(OH), Ca₅(PO₄)₃(OH), Ca₂P₂O₇·2H₂O, CaSO₄PO₃(OH)·4H₂O, CaF₂, and CaFPO₃·2H₂O. Leaching test results indicated that the concentrations of PO₄^3--P, F^- and heavy metals were less than the integrated wastewater discharge standard (GB8978-1996). This study provides a new harmless treatment method for PG.

Effective defluoridation of water using nanosized UiO-66-NH2 encapsulated within macroreticular polystyrene anion exchanger

Environmental concerns associated with the efficient defluoridation of contaminated water remain a substantial challenge. In this work, a new nanocomposite, UiO-66-NH₂@PS⁺, was successfully fabricated via in situ precipitation of a water-stable metal-organic framework (UiO-66-NH₂) inside a commercial polystyrene anion exchanger PS⁺. The as-formed nanocomposite UiO-66-NH₂@PS⁺ was characterized using various morphological methods, which demonstrated that nanosized UiO-66-NH₂ was homogenously dispersed within the inner pores of PS⁺. Batch adsorption experiments indicated that UiO-66-NH₂@PS⁺ exhibited outstanding adsorption performance for fluoride over a broad pH range of 3.0-8.0. The saturated adsorption capacity of fluoride at 298 K was 27.5 and 32.8 mg/g for pH 6.5 and 4.5 with the adsorbent dosage of 0.5 g/L and initial concentration of 5-80 mg/L. Moreover, the utilization rate of active adsorption sites of UiO-66-NH₂ was greatly improved after encapsulation. The XPS study indicated that the integrated effects of specific inner-sphere coordination and ligand exchange between fluoride and UiO-66-NH₂ might be the dominant adsorption mechanism. Fixed-bed tests indicated that the UiO-66-NH₂@PS⁺ column could successively produce clean water with bed volumes of 350 and 70 ([F^-] <1.5 mg/L) from simulated fluoride-pollution water at pH 4.5 and 8.0, with a liquid velocity of 20 mL/h, and an empty bed contact time (EBCT) of 15 min, which was higher than that of the other materials. In addition, the exhausted UiO-66-NH₂@PS⁺ was regenerated and reused for 5 times through complete regeneration, highlighting the potential feasibility of defluorination in practical applications.

Applications of Nano Hydroxyapatite as Adsorbents: A Review

Nano hydroxyapatite (Ca10(PO4)6(OH)2, HAp) has aroused widespread attention as a green and environmentally friendly adsorbent due to its outstanding ability in removing heavy metal ions, radio nuclides, organic pollutants and fluoride ions for wastewater treatment. The hexagonal crystal structure of HAp supports the adsorption mechanisms including ionic exchange reaction, surface complexation, the co-precipitation of new partially soluble phases and physical adsorption such as electrostatic interaction and hydrogen bonding. However, nano HAp has some drawbacks such as agglomeration and a significant pressure drop during filtration when used in powder form. Therefore, instead of using nano HAp alone, researchers have worked on modificationsand composites of nano HAp to overcome these issues and enhance the adsorption capacity. The modification of cationic doping and organic molecule grafting for nano HAp can promote the immobilization of ions and then increase adsorption capacity. Developing nano HAp composite with biopolymers such as gelatin, chitosan and chitin has proven to obtain a synergetic effect for improving the adsorption capacity of composites, in which nano HAp fixed and dispersed in polymers can playmuch more of a role for adsorption. This review summarizes the adsorption properties and adsorbent applications of nano HAp as well as the methods to enhance the adsorption capacity of nano HAp.

Fluoride ions sorption using functionalized magnetic metal oxides nanocomposites: a review

Fluoride is an anionic pollutant found superfluous in surface or groundwater as a result of anthropogenic actions from improper disposal of industrial effluents. In drinking water, superfluous fluoride has been revealed to trigger severe health problems in humans. Hence, developing a comprehensive wastewater decontamination process for the effective management and preservation of water contaminated with fluoride is desirable, as clean water demand is anticipated to intensify considerably over the upcoming years. In this regard, there have been increased efforts by researchers to create novel magnetic metal oxide nanocomposites which are functionalized for the remediation of wastewater owing to their biocompatibility, cost-effectiveness, relative ease to recover and reuse, non-noxiousness, and ease to separate from solutions using a magnetic field. This review makes an all-inclusive effort to assess the effects of experimental factors on the sorption of fluoride employing magnetic metal oxide nanosorbents. The removal efficiency of fluoride ions onto magnetic metal oxides nanocomposites were largely influenced by the solution pH and ions co-existing with fluoride. Overall, it was noticed from the reviewed researches that the maximum sorption capacity using various metal oxides for fluoride sorption was in the order of aluminium oxides >cerium oxides > iron oxides > magnesium oxides> titanium oxides, and most sorption of fluoride ions was inhibited by the existence of phosphate trailed by sulphate. The mechanism of fluoride sorption onto various sorbents was due to ion exchange, electrostatic attraction, and complexation mechanism.

Capacitive Removal of Fluoride Ions via Creating Multiple Capture Sites in a Modulatory Heterostructure

Fluoride pollution has become a major concern because of its adverse effects on human health. However, the removal capacity of defluorination agents in traditional methods is far from satisfactory. Herein, capacitive removal of F^- ions via creating multiple capture sites in a modulatory heterostructure has been originally demonstrated. The heterostructure of uniformly dispersed Al₂O₃ coating on hollow porous nitrogen-doped carbon frameworks was precisely synthesized by atomic layer deposition. An exceptional F^- ion removal efficiency at 1.2 V (95.8 and 92.9% in 5 and 10 mg/L F^- solutions, respectively) could be finally achieved, with a good regeneration ability after 20 consecutive defluorination cycles. Furthermore, we investigated the removal mechanisms of F^- ions by in situ Raman, in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy measurements. The promotional removal capacity was realized by the multiple capture sites of the reversible conversion of Al-F species and the insertion of F^- ions into the carbon skeleton. This work offers an important new pathway and deep understanding for efficient removal of F^- ions from wastewater.

Effect of Preparation Methods on the Adsorption of Glyphosate by Calcined Ca-Al Hydrotalcite

Calcined Ca-Al hydrotalcites were prepared by the clean method (Ca-LDO-1) and traditional co-precipitation method (Ca-LDO-2), respectively. The effect of the preparation method on the adsorption of glyphosate by calcined Ca-Al hydrotalcites was investigated. The adsorbents were also characterized by X-ray diffraction (XRD), thermogravimetric (TG) analysis, inductively coupled plasma optical emission spectroscopy (ICP-OES), and low-temperature N2 adsorption-desorption, respectively. Compared with Ca-LDO-2, Ca-LDO-1 had higher specific surface area and pore volume, which caused it to show better adsorption performance and reusability for the adsorbing of glyphosate. In addition, the kinetics and thermodynamics of the adsorption of glyphosate by Ca-LDO-1 were studied. The results showed that it was more consistent with the pseudo-second-order kinetic equation and Langmuir isotherm equation.

Synthesis of hierarchical hollow MIL-53(Al)-NH2 as an adsorbent for removing fluoride: experimental and theoretical perspective

The MIL-53(Al)-NH₂ was designed to remove fluoride with hierarchical hollow morphology. It was used as an adsorbent for fluoride removal at a wide pH range (1-12) due to the positive zeta potential of MIL-53(Al)-NH₂. The pH did not significantly influence the fluoride adsorption into MIL-53(Al)-NH₂. However, the adsorbent indicated good adsorption capacity with maximum adsorption of 1070.6 mg g^-1. Different adsorption kinetic and thermodynamic models were investigated for MIL-53(Al)-NH₂. The adsorption of fluoride into MIL-53(Al)-NH₂ followed the pseudo-second-order model and a well-fitted Langmuir model indicating chemical and monolayer adsorption process. When mass transfer model was used at initial concentrations of 100 ppm and 1000 ppm, the rates of conversion were 8.4 × 10^-8 and 4.7 × 10^-8 m s^-1. Moreover, anions such as [Formula: see text], [Formula: see text], [Formula: see text], Cl^-, and Br^- also had less effect on the adsorption of fluoride. Also, experimental and theoretical calculations on adsorption mechanism of MIL-53(Al)-NH₂ revealed that the material had good stability and regenerative capacity using alum as regenerant. In a nutshell, the dominant crystal face (1 0 1) and adsorption sites Al, O, and N combined well with F^-, HF, and HF₂^- through density functional theory. It opens a good way of designing hollow MOFs for adsorbing contaminants in wastewater.

Mechanism of fluoride removal by AlCl3 and Al13: The role of aluminum speciation

Coagulation is an important defluorination process. However, because of the poor sedimentation properties, conventional coagulants often result in limited defluorination performance and excessive residual aluminum. In this study, AlCl₃ and the highly-positively-charged molecule [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺ (Al₁₃) was utilized to treat fluoride-containing water. By comparison, the role of aluminum speciation in fluoride removal was elucidated. Under initial pH of 6.0, 7.0 and 8.0, the highest defluorination efficiencies of high-fluoride water ([F^-]₀ = 8.0 mg/L) were 78.2%, 71.6% and 83.2% at Al₁₃ dosage of 20 mg/L, 40 mg/L and 50 mg/L. Combined with detailed investigations of the chemical compositions of flocs, along with electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) analysis of residual aluminum, the defluorination mechanisms of both coagulants were discussed. In acidic conditions, both coagulants hydrolyzed and formed various Al clusters, among which transient Al (Al_ts) was the intermediate of the other clusters. The coprecipitation of high-polymerized Al and F^- contributed most of the defluorination rate. While under neutral and alkaline conditions, hydrogen bonding and ion exchange together with coprecipitation were the main roles for Al₁₃. The effects of AlCl₃ were merely physical actions, and were affected by the decreased pH. This work provides new insights into the coagulation and defluorination process.

Role of polyurethane-modified layered double hydroxides on SCFAs extraction from waste activated sludge fermentation liquid for elevating denitrification: Kinetics and mechanism

Extraction of short-chain fatty acids (SCFAs) from fermentation liquid of waste activated sludge (WAS) is the key bottleneck hindering its application as electron donor in denitrification. This study explores the feasibility of polyether-type polyurethane (PU)-modified layered double hydroxides (LDHs, prepared using eggshell waste as calcium source) in SCFAs adsorbing from WAS fermentation liquid (SFL). The adsorption parameters were first optimized by adsorption tests using artificial fermentation liquid (AFL). Then, adsorption kinetics, thermodynamic and isotherms were explored to further understand the adsorption mechanism. It revealed that SCFAs absorption by PU-LDHs from SFL was an endothermic and spontaneous process with positive enthalphy (ΔH^◦) values and negative Gibbs free energy (ΔG^◦) values. In addition, the maximum adsorption capacity of 208.0 mg SCFAs/g PU-LDHs was obtained from the Langmuir isotherm. Noting that both soluble carbohydrates and soluble proteins were simultaneously extracted, with efficiencies of 30.9%, 6.2%, respectively, compared with 62.9% SCFAs. The reuse tests confirmed that the prepared PU-LDHs can be used at least three times with high adsorptive capacity. With PU-LDHs-loaded SFL as external carbon source in the biodenitrification process, a denitrification rate of 0.014 mg NO₃^--N/mg mixed liquid suspended solids (MLSS)·d was recorded. This study provided a sound basis for the preparation of cost-effective biodenitrification carbon source from SFL by a novel adsorbent.