Fluoride removal from water by chitosan derivatives and composites: A review

Fluoride is an essential element, indispensable for maintenance of dental health. Nevertheless, fluoride concentrations in drinking water above 1.5 mg L⁻¹ may be detrimental to human health. Many methods have been developed for removing excessive fluoride from drinking water, adsorption seems to be an effective, environmentally friendly and economical one. Since the sorption capacity of fluoride below 2 mg L⁻¹ on most conventional adsorbents is not satisfactory, much effort has been devoted to develop new and cost-effective fluoride adsorbents. This review reports the recent developments in the F⁻ removal in water treatment, using chitosan derivatives and composites in order to provide useful information about the different technologies. When possibly the adsorption capacity of chitosan derivatives and composites under different experimental conditions is reported to help to compare the efficacy of the fluoride removal process. A comparison with the adsorption capacity of other low cost adsorbents is also tabled.     

Removal of copper from aqueous solution by aminated and protonated mesoporous aluminas: kinetics and equilibrium

A novel adsorbent, aminated and protonated mesoporous alumina, was prepared and employed for the removal of copper from aqueous solution at concentrations between 5 and 30 mg/l, in batch equilibrium experiments, in order to determine its adsorption properties. The removal of copper by the adsorbents increases with increasing adsorbent dosages. The adsorption mechanism is assumed to be an ion exchange between copper and the hydrogen ions present on the surface of the mesoporous alumina. The adsorbent was characterized by XRD, TEM, SEM, and BET methods. The sorption data have been analyzed and fitted to linearized adsorption isotherm of the Freundlich, Langmuir, and Redlich-Peterson models. The batch sorption kinetics have been tested for first-order, pseudo-first-order, and pseudo-second-order kinetic reaction models. The rate constants of adsorption for all these kinetic models have been calculated. Results also showed that the intraparticle diffusion of Cu(II) on the mesoporous catalyst was the main rate-limiting step.     

Studies on fluoride adsorption capacities of amorphous Fe/Al mixed hydroxides from aqueous solutions

This study evaluated the effectiveness of amorphous iron and aluminum mixed hydroxides in removing fluoride from aqueous solutions. A series of mixed Fe/Al samples were prepared at room temperature by co-precipitating Fe and Al mixed salt solutions at pH 7.5. The compositions (Fe:Al molar ratio) of the oxides were varied as 1:0, 3:1, 2:1, 1:1 and 0:1 and the samples were characterized by XRD, BET surface area and pH_ZPC. The XRD studies indicated the amorphous nature of the samples and Al(III) incorporation on Fe(III) hydroxides. Batch adsorption studies for fluoride removal on these materials showed that the adsorption capacities of the materials were highly influenced by solution pH, temperature and initial fluoride concentration. The rate of adsorption was fast and equilibrium was attained within 2 h. The adsorption followed first-order kinetics with intraparticle diffusion as the rate determining step for all the samples. The experimental data fitted well to both Langmuir and Freundlich adsorption isotherms. All samples exhibited very high Langmuir adsorption capacities; the sample with molar ratio 1 has shown maximum adsorption capacity of 91.7 mg/g. The thermodynamic parameters were determined to study the feasibility of the adsorption process.     

Synthesis of La-incorporated chitosan beads for fluoride removal from water

In this study, lanthanum incorporated chitosan beads (LCB) were synthesized using precipitation method and tested for fluoride removal from drinking water. The effect of various parameters like complexation and precipitation time, lanthanum loading and ammonia strength on fluoride removal have been studied. It is observed that the parameters for the synthesis of LCB have significant influence on development of LCB and in turn on fluoride removal capacity. The optimal condition for synthesis of LCB includes lanthanum loading: 10 wt%, complexation time: 60 min, precipitation time: 60 min, drying temperature: 75 °C for 72 h. The maximum fluoride adsorption capacity of LCB was found to be 4.7 mg/g and negligible release of lanthanum ion was observed. XRD analysis shows the presence of lanthanum hydroxide and amorphous nature of LCB. SEM of LCB shows the presence of oval lanthanum hydroxide particles spread over the chitosan matrix. Fluoride adsorption capacity has been calculated by applying Langmuir and Freundlich isotherms. The comparative study suggests that LCB shows four times greater fluoride adsorption capacity than the commercially used activated alumina.     

Preparation and characterization of nanocomposite,silica aerogel,activated carbon and its adsorption properties for Cd (II) ions from aqueous solution

A novel composite adsorbent, silica aerogel activated carbon was synthesized by sol-gel process at ambient pressure drying method. The composite was characterized by Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and Nitrogen adsorption/desorption isotherms (BET).In the present study, the mentioned adsorbent was used moderately for the removal of cadmium ions from aqueous solutions and was compared with two other adsorbents of cadmium, activated carbon and silica aerogel. The experiments of Cd adsorption by adsorbents were performed at different initial ion concentrations, pH of the solution, adsorption temperature, adsorbent dosage and contact time. Moreover, the optimum pH for the adsorption was found to be 6.0 with the corresponding adsorbent dosage level of 0.1 g at 60 °C temperature. Subsequently, the equilibrium was achieved for Cd with 120 min of contact time.Consequently, the results show that using this composite adsorbent could remove more than 60% of Cd under optimum experimental conditions. Langmuir and Freundlich isotherm model was applied to analyze the data, in which the adsorption equilibrium data were correlated well with the Freundlich isotherm model and the equilibrium adsorption capacity (q_e) was found to be 0.384 mg/g in the 3 mg/L solution of cadmium.     

Kinetic,mechanism and equilibrium studies on removal of Pb(II) using <Emphasis Type="Italic">Citrus limettioides</Emphasis> peel and seed carbon

The sorption behaviour of Pb(II) ions onto activated carbon prepared from Citrus limettioides peel (CLPC) and seed (CLSC), which is a novel waste material, was evaluated as a function of contact time, pH, adsorbent dose, ionic strength, initial metal ion concentration and temperature in batch adsorption processes with raw Citrus limettioides peel (CLP) and seed (CLS). The maximum uptake of lead(II) ions was obtained at pH range 4.0–6.0 for CLPC, CLSC and 5.0–6.0 for raw materials (CLP, CLS). The optimal contact time was found to be 3 h. Surface morphology and functionality of the adsorbent were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) analysis and Fourier-transform infrared (FT-IR) spectroscopy. The equilibrium data fit well with the Langmuir isotherm, confirming monolayer coverage of lead(II) ions onto CLP, CLPC, CLS and CLSC. The Langmuir monolayer adsorption capacity of CLP, CLPC, CLS and CLSC was found to be 123.60, 166.67, 15.32 and 142.86 mg/g. The calculated thermodynamic parameters showed that the sorption process was feasible, spontaneous and exothermic in nature. Kinetic studies demonstrated that adsorption of lead(II) ions followed a pseudo-second-order equation, suggesting that the adsorption process is presumably chemisorption. The adsorbents were tested for removal of Pb(II) from electroplating wastewater in connection with the reuse and selectivity of the adsorbents.     

Effective adsorption of Hg(II) ions by new ethylene imine polymer/β-cyclodextrin crosslinked functionalized magnetic composite

A new effective magnetic composite material was prepared successfully for adsorption Hg(II) ions by introducing β-cyclodextrin/ethylene imine polymer to the mesoporous silica. The morphology and structure of EIP-β-CD magnetic adsorbents were characterized by FT-IR, XR, DTG, XPS and SEM technologies. The effect of many factors were discussed detailedly such as adsorption time, initial concentration, pH, different composition of adsorbent and adsorption temperature. It was found that EIP-β-CD showed excellent adsorption capacity, high selectivity, good reutilization and fast adsorption rate. The maximum adsorption capacity was 248.72 mg/g and the best removal rate was 99.49 % under the optimized experimental conditions. The kinetic and thermodynamic study showed typical characteristic of chemical adsorption, exothermic and spontaneous. The best mass proportion of β-cyclodextrin, ethylene imine polymer and glutaraldehyde was 1.0:0.4:0.2, and proper β-cyclodextrin can develop the adsorption capacity for Hg(II) ions in this adsorbent. The possible adsorption mechanism was investigated in detail. After the fifth cycle experiment, this new adsorbent still showed excellent adsorption capacity which indicated that it has great potential for Hg(II) ions cleanup in water solution.     

Highly efficient removal of Cr(VI) from wastewater via adsorption with novel magnetic Fe3O4@C@MgAl-layered double-hydroxide

Novel magnetic Fe₃O₄@C@MgAl-layered double-hydroxide (LDH) nanoparticles have been successfully prepared by the chemical self-assembly methods. The properties of surface functional groups, crystal structure, magnetism and surface morphology of magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal gravity-differential thermal gravity (TG-DTG), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The adsorption studies of the novel adsorbent in removing heavy metals Cr (VI) from waste water showed that the maximum absorption amount of Cr(VI) was 152.0 mg/g at 40℃ and pH 6.0. The excellent adsorption capacity of the Fe₃O₄@C@MgAl-LDH nano-absorbents plus their easy separation, environmentally friendly composition and reusability makes them more suitable adsorbents for the removal of metal ions from waste water.     

Removal of heavy metal ions from aqueous solution using Fe3O4-SiO2-poly(1,2-diaminobenzene) core-shell sub-micron particles

In this work, Fe₃O₄-SiO₂-poly(1,2-diaminobenzene) sub-micron particles (FSPs) with high saturated magnetization of ∼60-70 emu/g were developed and utilized for the removal of As(III), Cu(II), and Cr(III) ions from aqueous solution. The isothermal results fitted well with the Freundlich model and the kinetic results fitted well with the two-site pseudo-second-order model, which indicated that multilayer adsorption of As(III), Cu(II), and Cr(III) ions on FSPs occurred at two sites with different energy of adsorption. The maximum adsorption capacities followed the order of As(III) (84 ± 5 mg/g, pH = 6.0) > Cr(III) (77 ± 3 mg/g, pH = 5.3) > Cu(II) (65 ± 3 mg/g, pH = 6.0). And the chelating interaction was considered as the main adsorption mechanism. The as-prepared materials were chemically stable with low leaching of Fe (?1.7 wt.%) and poly(1,2-diaminobenzene) (?4.9 wt.%) in tap water, sea water, and acidic/basic solutions. These metal-loaded FSPs could be easily recovered from aqueous solutions using a permanent magnet within 20 s. They could also be easily regenerated with acid. The present work indicates that the FSPs are promising for removal of heavy metal ions in field application.     

Novel Fe3O4/hydroxyapatite/β‐cyclodextrin nanocomposite adsorbent: Synthesis and application in heavy metal removal from aqueous solution

The increased global concern on environmental protection has made researchers focus their attention on new and more efficient methods of pollutant removal. In this research, novel nanocomposite adsorbents,i.e., magnetic hydroxyapatite (Fe₃O₄@HA) and magnetic hydroxyapatite β‐cyclodextrin (Fe₃O₄@HA‐CD) were synthesized and used for heavy metal removal. The adsorbents were characterized by FTIR, XRD, TGA, VSM, and SEM. In order to investigate the effect of β‐cyclodextrin (β‐CD) removal efficiency, adsorption results of nine metal ions were compared for both adsorbents. β‐CD showed the most increasing effect for Cd²⁺ and Cu²⁺ removal, so these two ions were selected for further studies. The effect of diverse parameters including pH, contact time, initial metal ion concentration and adsorbent dosage on the adsorption process was discussed. The optimum pH was 6 and adsorption equilibrium was achieved after 1 hr. Adsorption kinetic data were well fitted by pseudo‐second‐order model proposing that metal ions were adsorbed via chemical reaction. Adsorption isotherm was best described by the Langmuir model, and maximum adsorption capacity for Cd²⁺ and Cu²⁺ was 100.00 and 66.66 (mg/g), respectively. Desorption experiment was also done, and the most efficient eluent used for desorption of metal ions was EDTA (0.001 M) with 91% and 88% of Cd²⁺ and Cu²⁺ release, respectively. Recyclability studies also showed a 19% decrease in the adsorption capacity of the adsorbent after five cycles of regeneration. Therefore, the synthesized adsorbents were recognized as potential candidates for heavy metal adsorption applications.     

溴化铵改性膨润土脱除气态单质汞的特性及机理分析

采用溴化铵对钠基膨润土进行改性制得脱汞吸附剂,在固定床实验装置上对所制备的吸附剂进行脱汞性能测试。脱汞实验结果表明,钠基膨润土较钙基膨润土在脱汞性能上提高不大,而溴化铵改性的钠基膨润土(Br-Ben/Na)脱汞性能得到明显提高,脱汞效率达到97.7%。吸附温度的升高有利于对Hg0的脱除,在140℃下,10%Br-Ben/Na吸附剂的脱汞率能长时间保持在90%以上,说明在此吸附过程中化学吸附占主导性作用。通过N2吸附/脱附、X射线衍射(XRD)、元素分析仪和傅里叶变换红外光谱(FT-IR)分析等结果表明,改性后的膨润土比表面积下降,平均孔径增大;铵根离子进入到膨润土的层间置换出层间钠离子,煅烧活化过程中层间的铵根离子并未分解,而在层间与膨润土结合为某吸附活性组分协助Br-与Hg0反应,提高了膨润土的脱汞性能。     

磷酸铝吸附除水中氟的研究

采用静态吸附法研究了比表面为308m2/g的无定形磷酸铝吸附除氟性能,研究了接触时间、pH值、吸附剂量等对吸附的影响。结果表明,磷酸铝吸附除氟高效、迅速,30min内可以接近最大吸附量。对含氟50mg/g的溶液,优化条件下的最大除氟率约93%。研究了吸附与溶液pH的关系,得到了优化pH值并解释了吸附机理。吸附的最佳pH值约为5.5。用拟二级动力学方程描述了吸附速率并计算了速率常数。用Langmuir方程拟合了吸附等温线,计算的饱和吸附量为53.5mg/g。吸附剂量对分配系数的影响表明吸附剂表面是不均匀的。     

Fast Removal and Recovery of Methylene Blue by Activated Carbon Modified with Magnetic Iron Oxide Nanoparticles

A magnetic adsorbent was synthesized by modification of activated carbons with magnetic iron oxide nanoparticles (AC‐MIONs). The preparation method is fast and could be carried out in an ordinary condition. The AC‐MIONs were used as quite efficient adsorbents for separation of methylene blue (MB) from aqueous solution in a batch process. The effect of different parameters such as pH, temperature, electrolyte concentration, contact time and interfering ions on the removal of MB were studied. The adsorption data were analyzed by Langmuir and Freundlich isotherm models and a maximum adsorption amount of 47.62 mg g^‐1 and a langmuir adsorption equilibrium constant of 3.0 L mg^‐1 were obtained. The obtained results revealed that AC‐MIONs were effective adsorbents for fast removal of MB from different aqueous solutions. This adsorbent was successfully used for removal of MB from Karoon River water.     

Cellulose/poly(ethylene imine) composites as efficient and reusable adsorbents for heavy metal ions

A series of Cellulose/poly-ethylene imine (PEI) composites were prepared by grafting hyperbranched PEI onto cellulose chains in alkali/urea aqueous solvent system through “one step” method. The SEM results showed that the Cellulose/PEI composite maintained porous structure. The Cellulose/PEI composites were tested as Cu(II) adsorbents through thermodynamics and kinetics study. The adsorption process followed pseudo-second-order kinetics equation. The adsorption isotherms could be described by both Langmuir and Freundlich isotherm models. The maximum adsorption amount was calculated to be 285.7 mg/g. The composites showed good stability so that they could be used in a wide range of pH and temperature. Besides, the Cu(II) loaded Cellulose/PEI composite could also be easily regenerated by dilute sulfuric acid and still keep a major adsorption capacity. Finally, the adsorption capacities of Celluloes/PEI composite towards other metal ions, such as Zn(II), Ni(II), Cr(III) and Pb(II), were also demonstrated. It will be a new high-performance and environmental friendly material for sewage disposal and metal pollution treatment with promising developmental potential.     

Boron Removal from Aqueous Solutions by Adsorption on Waste Sepiolite and Activated Waste Sepiolite Using Full Factorial Design

In the present paper, boron removal from aqueous solution by adsorption was investigated and 2³ full factorial design was applied. Non activated waste sepiolite (NAWS) and HCl activated waste sepiolite (AWS) were used as adsorbents. Regression equation formulated for boron adsorption was represented as a function of response variables. The results obtained from the study on parameters showed that as pH increased and temperature decreased boron removal by adsorption increased. Adsorbed boron amount on AWS was higher than that of NAWS. Maximum boron removal was obtained at pH 10 and 20°C for both adsorbents. Adsorption data obtained from batch adsorption experiments carried out with NAWS and AWS fitted to the Langmuir equation. The batch adsorption capacities were found in mg/g: 96.15 and 178.57 for NAWS and AWS, respectively. The capacity value for column study was obtained by graphical integration as 219.01 mg/g for AWS. The Thomas and the Yoon-Nelson models were applied to experimental data to predict the breakthrough curves and to determine the characteristic parameters of the column useful for process design.     

由单层薄水铝石制备的活性氧化铝处理水中氟离子

The preparation of high-efficiency and low-cost adsorbents for the defluoridation of drinking water remains a huge challenge. In this study, single-layer and multi-layer boehmite were first synthesized via an organic-free method, and active alumina used for fluoride removal from water was obtained from the boehmite. The advantage of a single layer is that more aluminum is exposed to the surface, which can provide more adsorption sites for fluoride. The active alumina adsorbent derived from single-layer boehmite exhibits a high specific surface area and excellent adsorption capacity. The high surface area ensures a high adsorption capacity, and the organic-free synthesis method lowers the preparation cost. The as-prepared adsorbent was characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FTIR) and nitrogen adsorption-desorption analysis. The single-layer structure of boehmite was determined from the simulated XRD diffraction pattern of single-layer boehmite. The disappearance of the (020) diffraction peak of boehmite illustrates that the dimensions in the b direction are extremely small, and according to the XRD simulation results, the single-layer structure of boehmite could be determined. Single-layer boehmite with a surface area of 789.4 m²·g^-1 was formed first. The active alumina obtained from the boehmite had a surface area of 678.4 m²·g^-1, and the pore volume was 3.20 cm³·g^-1. The fluoride adsorption of the active alumina was systematically studied as a function of the adsorbent dosage, contact time, concentration, co-existing anions, and pH. The fluoride adsorption capacity of the active alumina obtained from the single-layer boehmite reached up to 67.6 mg·g^-1, which is higher than those of most alumina adsorbents reported in the literature. The adsorption capacities of the active alumina are related to the specific surface area and the number of hydroxyl groups on the surface. Dosages of 0.6, 1.0, and 2.6 g·L^-1 of active alumina were able to lower the 10, 20, and 50 mg·L^-1 fluoride solutions, respectively, below the maximum permissible limit of fluoride in drinking water in China (1.0 mg·L^-1), suggesting that the active alumina synthesized in this work is a promising adsorbent for defluoridation of drinking water. In addition, the fluoride adsorption is applicable in a wide pH range from 4 to 9 and is mainly interfered by SO₄^2- and PO₄^3-. Further investigation suggested that the fluoride adsorption of the active alumina follows the pseudo second-order model and Langmuir isotherm model     

Development of Composite Adsorbents of Carbon and Intercalated Clay for N2and O2Adsorption: A Preliminary Study

Composite adsorbents of carbon and alumina intercalated montmorillonite were prepared and characterized by adsorption of N₂and O₂at various temperatures. The effects of pyrolysis, temperature, heating rate, subsequent degassing, and doping of cations and anions were investigated. The adsorption capacities of the composite adsorbents developed at higher temperatures (0 and −79°C) are found to be larger than those of normal alumina pillared clays. The experimental results showed that the framework of these adsorbents is made of alumina particles and clay sheets while the pyrolyzed carbon distributes in the space of interlayers and interpillars. The pores between the carbon particles, clay sheets, and alumina pillars are very narrow with very strong adsorption forces, leading to enhanced adsorption capacities at 0 and −79°C. The composite adsorbents exhibit features similar to those of carbonaceous adsorbents. Their pore structures, adsorption capacities, and selectivities to oxygen can be tailored by a controlled degassing procedure. Meanwhile, ions can be doped into the adsorbents to modify their adsorption properties, as usually observed for oxide adsorbents like zeolite and pillared clays. Such flexibility in pore structure tailoring is a potential advantage of the composite adsorbents developed for their adsorption and separation applications.     

可再生纤维素/环糊精聚合物对水中亚甲基蓝吸附行为

利用甘蔗渣提取纤维素修饰环糊精聚合物,成功制备可再生纤维素/环糊精聚合物（SUG-EPI-CDP）吸附剂。采用傅利叶红外光谱仪（FT-IR）与热重分析仪（TGA）对材料进行表征,同时考察了该材料对水中亚甲基蓝（MB）吸附特性和机理的影响。结果表明：在溶液pH值为7、温度为30 ℃的条件下,SUG-EPI-CDP可在120 min内有效去除MB,去除率达80.9%。通过模型拟合发现,SUG-EPI-CDP对MB的吸附是自发且吸热的过程,符合准二阶动力学方程和Langmuir等温线模型。该吸附剂实验最大吸附量达8.1 mg/g,远高于其他废料所制备的吸附剂。结果表明,利用可再生纤维素修饰可有效提高环糊精聚合物的吸附性能,同时为甘蔗渣资源化利用提供了新途径。     

Adsorption of fluoride, phosphate, and arsenate ions on a new type of ion exchange fiber

A new type of ion exchange fiber for the removal of fluoride, phosphate, and arsenate ions has been developed. A batch adsorption technique for investigating adsorption kinetic and equilibrium parameters and determining pH adsorption edges is applied. It is shown that the adsorption properties of the ion exchange fiber for fluoride, phosphate, and arsenate ions depend on the pH value and anion concentration. The adsorption of arsenate on the sorbent reaches a maximum of 97.9% in the pH value range of 3.5 to 7.0. The adsorption percentage of phosphate is more than 99% in the pH range of 3.0 to 5.5. The adsorption of fluoride on the ion exchange fiber is found to be 90.4% at pH 3.0. The Freundlich model can describe the adsorption equilibrium data of fluoride, arsenate, and phosphate anions. The sorption of the three anions on the ion exchange fiber is a rapid process, and the adsorption kinetic data can be simulated very well by the pseudo-second-order rate equation. The column performance is carried out to assess the applicability of the ion exchange fiber for the removal of fluoride, phosphate, and arsenate ions from synthetic wastewaters with satisfactory removal efficiency. The desorption experiment shows that fluoride ion sorbed by the fiber column can be quantitatively desorbed with 5 mL of 0.50 mol/L NaOH at elution rate of 1 mL/min, and 30 mL of NaOH is necessary for the quantitative recovery of phosphate and arsenate ions.     

负载金属镧的壳聚糖对氟离子的吸附动力学

利用负载金属镧的壳聚糖对含氟水进行吸附处理.该除氟剂的最佳吸附条件:pH值为7,温度为323K,吸附时间为60min.吸附剂对F的吸附过程符合Langmuir等温线方程,对F的饱和吸附容量为476.190mg/g.吸附动力学符合拟二级速率方程,颗粒内扩散过程和液膜形成的边界层是吸附过程的主要限速步骤.