Removal of toxic metals from aqueous solutions by layered double hydroxides

The article has investigated the possibility of using layered double hydroxide intercalated by diethylenetriaminepentaactetic acid for the removal of toxic metals from aqueous solutions. It has been shown that sorption of Ni(II), Co(II), and Pb(II) from aqueous solutions on the indicated sorbent is determined by several mechanisms of sorption: complexation, ionic exchange and precipitation of metal hydroxides. Comparative research of the efficiency of removing toxic metals by carbonate and chelate forms of sorbents was conducted.     

Uptake of arsenite by synthetic layered double hydroxides

Two types of layered double hydroxides (LDHs), hydrotalcite and hydrocalumite with different composition of layers and interlayers were investigated for the removal of arsenite from solutions. The kinetics study showed that anion exchange process was very fast and apparently attained a steady-state in the range of 8–16 h. Arsenite removal was found to be 87.5% and 83.6% with the nitrate forms of hydrotalcite and hydrocalumite, respectively. Layered double hydroxides synthesized at room temperature showed higher uptake than those synthesized by hydrothermal method due to small crystal size (high surface area) of the former. As expected, calcined hydrotalcite showed higher uptake than uncalcined carbonate form of hydrotalcite. The uptake process was anion exchange in hydrotalcite-type LDH as confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) but possibly some dissolution–reprecipitation occurred with hydrocalumite-type LDH.     

Removal of the herbicide 2,4-dichlorophenoxyacetate from water to zinc-aluminium-chloride layered double hydroxides

Batch sorption studies were conducted to investigate the potential of [Zn-Al-Cl] layered double hydroxides (LDHs) for the removal of the herbicide 2,4-dichlorophenoxyacetate (2,4-D) from contaminated aqueous solutions. Experiments were performed at different pH values, initial pesticide concentration, solid/pesticide ratio and anion exchange capacity of LDHs. The LDH samples evaluated had very high retention capacity for 2,4-D whose removal was a rapid process, as a quasi-equilibrium state was reached after 1-h reaction time. The adsorption can be described by Langmuir-type isotherms, with an average affinity constant of 12.5 L mmol(-1). At initial 2,4-D concentrations between 0.08 and 4 m molL(-1), the solids removed up to 98% of the pesticide. Physicochemical characterization of the LDH solids, both fresh and after removal of 2,4-D, by X-ray diffraction, infrared spectroscopy and thermogravimetry, indicates that the retention of 2,4-D is done by adsorption on the surface of the solid for low 2,4-D concentrations. However, a combination of surface adsorption and interlayer ion exchange takes place when the 2,4-D concentration is high.     

Removal of U(VI) from water media by layered doubled hydroxides of magnesium and iron

The article has investigated the process of removing U(VI) compounds from water media by layered double hydroxides [Mg^II//[Fe^III] with different molar relationship in a brucite layer (2: 1; 3: 1; 4: 1) as well as by products of their heat treatment at 400°C. The paper has also shown high efficiency of purification of water of various composition of compounds U(VI) by the given sorbents within broad interval of the pH (2.5–9).     

Removal of phosphate ions from aqueous solutions Zn/Al- and Mg/Fe- by layered doubled hydroxides

The article has investigated processes of sorption removal of phosphate–ions from water media Zn/Al- and Mg/Fe by layered double hydroxides. It was established that sorption properties of these materials are substantially affected by their composition, heat treatment and the pH of the aqueous solution. It is shown that calcined forms of studied sorbents are more effective than their initial carbonate forms.     

Preparation of layered double hydroxides using boron mud and red mud industrial wastes and adsorption mechanism to phosphate

Phosphate removal was important for wastewater treatment, and adsorption was an efficient treatment process. In this study, the layered double hydroxide adsorbent (BR‐LDH), which was prepared under alkali conditions using industrial residues boron mud and red mud, was used to adsorb the phosphate. The prepared BR‐LDH was characterised by X‐ray diffraction, Scanning electron microscopy, Energy‐dispersive X‐ray spectroscopy, and Thermo‐gravimetric‐differential thermal analysis. Adsorption experiments were carried out as a function of dosage, contact time, temperature and initial pH of phosphate solution. The removal ratio of phosphate onto OBR‐LDH reached 93%. The adsorption data showed a good compliance with the pseudo‐second‐order kinetic model. In addition, the mineral composition, the functional groups, the valence of elements and zeta potentials of OBR‐LDH before and after adsorption were used to analyse the adsorption mechanism. The result of real wastewater suggested that OBR‐LDH was excellent adsorbent for phosphorus removal from actual wastewater.     

层状双金属氢氧化物在水泥混凝土中的应用研究进展

层状双金属氢氧化物(LDHs)是水滑石类化合物的统称,具有典型的阴离子层状结构特征和独特的性能,在医药、催化等行业已经得到广泛应用。由于起步较晚,LDHs在土木工程材料行业仅处于探索性研究阶段,还未实现工业化应用。在国内外文献分析的基础上,进行了如下工作:(1)介绍了LDHs的结构特征及主要性能;(2)分析总结了LDHs对混凝土物理力学性能及耐久性影响等方面的研究成果,表明LDHs作为外加剂,在混凝土中具有良好的应用前景;一定掺量范围内,LDHs对新拌混凝土的工作性影响不大;由于具有阴离子可交换性/"记忆效应",可改善混凝土的抗氯离子、抗硫酸盐侵蚀、抗碳化能力;(3)最后针对LDHs在土木工程材料行业的应用提出了几点建议。     

Application of layered double hydroxides for removal of oxyanions: a review

Layered double hydroxides (LDHs) are lamellar mixed hydroxides containing positively charged main layers and undergoing anion exchange chemistry. In recent years, many studies have been devoted to investigating the ability of LDHs to remove harmful oxyanions such as arsenate, chromate, phosphate, etc. from contaminated waters by both surface adsorption and anion exchange of the oxyanions for interlayer anions in the LDH structure. This review article provides an overview of the LDH synthesis methods, the LDH characterization techniques, and the recent advancement that has been achieved in oxyanion removal using LDHs, highlighting areas of consensus and currently unresolved issues. Experimental studies relating to the sorption behaviors of LDHs with various oxyanions, and the kinetic models adopted to explain the adsorption rate of oxyanions from aqueous solution onto LDHs, have been comprehensively reviewed. This review discusses several key factors such as pH, competitive anions, temperature, etc., that influence the oxyanion adsorption on LDHs. The reusability of LDHs is discussed and some mechanistic studies of oxyanion adsorption on LDHs are highlighted. The sorption capacities of LDHs for various oxyanions are also compared with those of other adsorbents. In addition, this review critically identifies the shortcomings in current research on LDHs, such as the common weaknesses in the adopted methodology, discrepancies among reported results and ambiguous conclusions. Possible improvement of LDHs and potential areas for future application of LDHs are also proposed.     

Treatment of high fluoride concentration water by MgAl-CO3 layered double hydroxides: kinetic and equilibrium studies

MgAl-CO(3) layered double hydroxides (LDHs) have been employed to treat high fluoride concentration solution. The influences of solution pH, initial fluoride concentration and other anions in the solution were investigated by a series of batch experiments. A marked decrease in the amount of adsorbed fluoride by LDHs is observed with increasing pH. The extent of fluoride removal in the presence of other anions decreases in the order HCO(3)(-)>Cl(-)>H(2)PO(4)(-)>SO(4)(2-). The equilibrium isotherm for fluoride uptake corresponds closely to the Langmuir-Freundlich (L-F) model. The maximum capacity of LDHs for fluoride ions and the Gibbs free energy (DeltaG(0)) for the defluoridation process were calculated to be 319.8+/-5.7mg/g and -9.0+/-0.66kJ/mol, respectively. The negative value of DeltaG(0) indicates the spontaneous nature of the treatment process. Four kinetic models have been evaluated in order to attempt to fit the experimental data, namely the pseudo-first order, the pseudo-second order, the modified multiplex and the double exponential models. It was found that the modified multiplex model, involving a rapid first order step and a slow second order step most closely described the kinetics. The activation energies for the two steps are 37.2+/-5.26 and 72.6+/-4.52kJ/mol, respectively, suggesting that the rapid step is controlled by diffusion processes, whilst the second step is controlled by the reaction of fluoride with the LDHs.     

Uptake of chloride ion from aqueous solution by calcined layered double hydroxides: equilibrium and kinetic studies

Layered double hydroxides (LDH) calcined within a certain temperature range (denoted as CLDH) have been shown to recover their original layered structure in the presence of appropriate anions. In the light of this so-called "memory effect", uptake of chloride ion from aqueous solution by calcined MgAl-CO3 LDH was investigated in batch mode. The equilibrium isotherm showed that the uptake of chloride ion by CLDH was consistent with the Langmuir and Freundlich equations and that the Langmuir model gave a better fit to the experimental data than the Freundlich model. The maximum uptake capacity of CLDH for chloride ion was 149.5 mg/g, close to the stoichiometric uptake (168 mg/g). The influence of varying pH of solution, initial chloride concentration, adsorbent quantity, and temperature on the kinetics of chloride removal has also been explored. Four kinetic models were used to fit the experimental data, and it was found that the pseudo-second-order kinetics model could be used to describe the uptake process satisfactorily. The calculated value of Ea was found to be 56.8 kJ/mol, which suggests that the process of uptake of chloride ion is controlled by the rate of reaction of chloride ion with the CLDH rather than diffusion. A mechanism for removal of chloride ion has been confirmed by X-ray diffraction, FT-IR spectroscopy and TG-MS measurements.     

Synthesis,anion-exchange properties,and hydrolytic stability of Mg-Fe(III) layered double hydroxides

Composite materials having the hydrotalcite structure and based on iron and magnesium dou ble hydroxides have been synthesized and investigated. The sorption of phosphate and chromate anions on these materials was studied after their heat treatment at different temperatures. The magnesium washing-out from the heat treated samples under discussion was also studied. The temperature of the heat treatment of Me and Fe(III) double hydroxides was found to produce a major impact on their hydrolytic stability and sorption of anions that generally increased with the temperature rise.     

从“四维对接法”看环境史研究中的资料分析

环境史研究的对象,在绝大多数情况下都不是纯粹意义上的自然环境,而是经过历史上不同民族在不同地区用不同的方法加工、改造后的人造次生产物,也就是生态民族学所称的"民族生境"。在未从事环境史研究前,具体的民族生境发源过程当然不得而知,但必然会对今天的资源利用和环境保护产生不容忽视的影响。问题在于,学人们无论从考古材料、文献资料,还是在田野调查中所获取的证据,其间的关联性总会表现得纷繁复杂,千头万绪。以至于如何探寻其间存在的客观因果关系,自然成了当代环境史研究的重大难题,弄清楚其间的发展脉络和过程机制,也成了环境史研究的目的和意义所在。为此,在整合分析来自不同学科资料时,显然需要确保"时间""空间""环境"和"文化"4个维度的对接重合,由此而得出的因果分析才可望接近历史的真相。其间的真相又表现为,是人类有意识或者无意识活动的结果,并且对人类社会和生态环境而言都必然会发挥重大影响,若能辨明这样的作用机制,今天的生态维护对策也就包含在其中了。     

Sorbents with the structure of layered double hydroxides for aquatic media treatment from U(VI)

Sorbents representing the layered double hydroxides of composition [Mg₄Fe₂(OH)₁₂] ? CO₃ ?nH₂O, [Zn₄Al₂(OH)₁₂] ? CO₃ ? nH₂O, and also the products of their thermal treatment at 400°C intended for the aquatic media treatment from U(VI) have been investigated. It is shown that the high sorption capacity of these sorbents makes it possible to recommend them for the effective removal of U(VI) from waters having different mineral composition.     

Synthesis,characterisation of ternary layered double hydroxides (LDH) for sorption kinetics and thermodynamics of Cd2+

Kinetic and thermodynamic studies were carried out on the sorption of Cd²⁺ on three synthesised and characterised layered double hydroxides, namely: Ni-Co-Ti, Ni-Co-La and Mn-Ni-Al-CO₃. Sorption kinetics were investigated through time-dependent studies. Thermodynamics were determined by the effect of temperature on adsorption. Kinetic modelling of the adsorption of Cd(II) ions on the LDHs was determined using the first-order, pseudo-first order, pseudo-second order and Elovich kinetic models, as well as additional diffusion models. The results showed that the pseudo-second-order model best correlated the adsorption data with R² values of 1.000, 0.998 and 1.000 for NiCoTi, NiCoLa and MnNiAl-LDH respectively. Diffusion of the adsorbate was best correlated by the Weber-Morris intraparticle diffusion model with R² values of 0.9753, 0.8472 and 0.9412 for NiCoTi, NiCoLa and MnNiAl-LDH respectively. Thermodynamic studies showed that the adsorption was spontaneous and exothermic with a high probability of sticking.     

Extraction of U(VI) from aqueous media by layer double hydroxides intercalated by chelating agents

The comparison research is made of the efficiency of removing U(VI) compounds from wastewaters synthesized by hydrotalcite-like sorbents: intercalated by hexamethylenediaminetetracetic and ethylene diaminetetracetic acids. It has been shown that the efficiency of sorbents for extracting U(VI) from wastewaters at pH 8 is determined by stability of its complexes with chelate ligands located in the interlayer space of layered double hydroxides.     

Suppression of phosphate liberation from sediment by using iron slag

In order to suppress the phosphate liberation from sediment to sea water, several suppressing materials such as four kinds of iron slag, river and sea sands, and glass bead were tested. The sediment and sea water were sampled from Hiroshima Bay which seemed to be a eutrophicated area. The suppressing materials were covered on the surface of sediment, and sea water was introduced in the aquarium. The concentrations of phosphate, sulfide and dissolved oxygen were measured during the incubation period under the condition of anaerobic state. The suppressing effects of slags, sea and river sands were compared with that of glass bead used as control materials, and the suppression efficiencies were measured. The suppression efficiency of glass bead was 54.1%, and those of slag-2, -3 and -4 were 97.3, 96.1 and 98.8%, respectively. The suppression efficiency depended on the amounts of slag used, and about 85% of liberated phosphate were suppressed by slag-3 of 7.5 kg m⁻². The sulphide ion generated under anaerobic state reacted with some metals on the surface of slags and sands, and the dissolution of precipitated phosphate (Fe-P, Al-P and Ca-P) was suppressed by formation of metal sulfide on the surface of slags and sands. The suppression mechanisms on phosphate liberation, such as covering, chemical and adsorption effects were proposed and discussed. The suppression by slag was mainly carried out by the adsorption effect, in addition to the covering effect.     

Removal of phosphate from aqueous solution by thermally treated natural palygorskite

The potential of activated palygorskite was assessed for sorption of phosphate from aqueous solution. The natural palygorskite used was treated by thermal activation over 100-1000 °C for 2 h. The thermal activation increased the phosphate sorption capacity and the highest phosphate sorption capacity occurred at 700 °C. H700 (palygorskite heated at 700 °C) showed higher sorption rate than natural palygorskite (NPAL), and the removal was favorable in acidic media. The sorption data were described using Freundlich isotherm equation over the concentration range (5-1000 mg/L) (25 °C). Calcium bound phosphorus was the main fraction of the adsorbed phosphorus, about 98.0% in NPAL and 58.2% in H700, but the extractive Ca-P species varied greatly, Ca₂-P was 87.7% in NPAL and 3.0% in H700, Ca₈-P was 10.1% in NPAL and 54.5% in H700, and metal bound phosphorus was less than 2% in NPAL but more than 41.4% in H700, respectively. The dependence of the phosphate sorption capacity in the heating samples on thermal activation appears to be related to major changes in the crystal structure of palygorskite, and more calcium, iron and aluminum were released from the crystal matrix at 700 °C, which promoted phosphorus sorption.     

两级过滤去除地下水中铁锰的试验研究

通过锰砂和硅碳素两级过滤的方式,研究了含锰、含铁及高铁低锰三种水质条件下铁锰的去除效果,进一步探讨了不同水质中铁锰的去除方法。结果表明,对于单独含锰的水质,可仅采用二级硅碳素滤柱去除;对于单独含铁的水质,可仅采用一级锰砂滤柱去除;对于高铁低锰的水质,可通过两级过滤同时去除铁锰。锰砂和硅碳素两级过滤处理含铁锰地下水是可行的,根据不同的水质,可以采用不同的处理方法。     

Removal of atrazine by nanoscale zero valent iron supported on organobentonite

The organobentonite (CTMA-Bent) was prepared from Na⁺-saturated bentonite (Na-Bent) by intercalation with cetyltrimethylammonium bromide (CTMA), and used as a carrier of nanoscale zero valent iron (NZVI) for the removal of atrazine. The NZVI/CTMA-Bent composite was characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM), and good dispersion of nanoscale iron particles on the carrier was observed. The removal efficiency of atrazine by this composite was compared with that by commercial iron powder and NZVI itself. For both treatments by NZVI and NZVI/CTMA-Bent, the removal efficiency increased as the pH of the solution decreased, and the removal percentage of atrazine by NZVI/CTMA-Bent reached 63.5% at initial pH = 5.0 after 120 min. It is not only much higher than that (26.6%) by NZVI containing the same amount of iron, but also superior to the sum (32.1%) of reduction by NZVI plus adsorption by CTMA-Bent (5.5%). Besides, the NZVI/CTMA-Bent has a good long-term stability, and the carrier CTMA-Bent could prevent the iron ions (the byproduct of dechlorination) from leaching into the solution.     

Absorption of the selenite anion from aqueous solutions by thermally activated layered double hydroxide

The presence of selenite or selenate in potable water is a health hazard especially when consumed over a long period of time. Its removal from potable water is of importance. This paper reports technology for the removal of selenite from water through the use of thermally activated layered double hydroxides.Mg/Al hydrotalcites with selenite in the interlayer were prepared at different times from 0.5 to 20 h through ion exchange. X-ray diffraction of the MgAlSeO₃ hydrotalcites indicates that the selenite anion entered the interlayer spacing of Mg/Al hydrotalcite and MgAlSeO₃ hydrotalcite was formed. Raman spectra proved the presence of selenite anion in the hydrotalcite interlayer as the counter anion. The band intensity and width of MgAlSeO₃ hydrotalcite in the region of 3800-3000 cm⁻¹ increase with the adsorption of selenite by the Mg/Al hydrotalcite. The characteristic bands of free selenite anions in the MgAlSeO₃ hydrotalcites are located between the region between 850 and 800 cm⁻¹. The Raman spectra of the lower wave number region of 550-500 cm⁻¹ show a shift toward higher wave numbers with adsorption of the selenite. An estimation of the amount of selenite anion removed by the thermally activated layered double hydroxide was obtained through the measurement of the intensity of the selenite Raman bands at 814 and 835 cm⁻¹ resulting from the amount of selenite anion remaining in solution. Thermally activated LDHs provide a mechanism for removing selenite anions from aqueous solutions.