Fast Sorption Rate Research Articles

Encapsulation of Bamboosa vulgaris culms derived activated biochar into hierarchical permeable, phosphate rich and functionalized alginate aerogel composites and its contribution in U(VI) adsorption

In this study, a facile methodology was designed to encapsulate Bamboosa vulgaris culms derived activated biochar (BVC) in a variable mass ratio, into a three-dimensional hierarchical porous and permeable and amino-thiocarbamated alginate (TSC) to prepare hybrid biosorbents (BVC-MSA). These ultralight and lyophilized phosphate rich macroporous sorbents were rationally characterized through FTIR, XRD, BET, SEM-EDS, elemental mapping, XPS techniques and employed for efficient UO22+ adsorption from aqueous solutions. The phytic acid (PA) was found to be a suitable hydrophilic and phosphorylating agent for the TSC matrix through hydrogen-bonded crosslinking when employed in a correct mass ratio (1:3). The SEM-EDS and XPS analyses confirmed the UO22+ sorption onto BVC-MSA-3 (the most suitable composite with a BVC/TSC mass ratio of 30.0 % w/w) and provided evidence of heteroatom involvement in developing the physico-chemical interactions. The BCV-MSA-3 exhibited the best response as a sorbent during kinetics/sorption process, therefore, it was selected to study the equilibrium sorption studies. The BCV-MSA-3 removal efficiency increased from 12.1 to 94.2 % using 0.2 to 1.8 g/L sorbent dose at pH (4.5). The mentioned sorbent displayed a significant maximum sorption capacity qm (309.55 mg/g at 35 °C) calculated through the best-fitted Langmuir and Temkin models (R2 ≈ 0.99). The sorption kinetics followed the pseudo-second-order (PSORE) model and exhibited fast sorption rate teq (180 min). Thermodynamic parameters clarified that the sorption process is feasible ΔGo (−25.3 to −27.6 kJ/mol kJ/mol), endothermic ΔHo (27.17 kJ/mol), and proceeds with a positive entropy (0.176 kJ/mol.K). The study shows that BCV-MSA-3 could be an alternative and auspicious sorbent for uranium removal from aqueous solution.

Adsorption kinetics of organic phosphates on goethite and aluminium oxide: The equation used to describe the reaction

AbstractAdsorption kinetics of three organic phosphate compounds (OPs) with varying molecular sizes and structures and inorganic phosphate (Pi) were investigated on α‐Al2O3 and poorly crystalline goethite. The organic phosphates were inositol hexaphosphate (IHP), glycerol phosphate (GlyP) and glucose‐6‐phosphate (G6P), and the inorganic phosphate was KH2PO4. Batch adsorption experiments were performed at 25°C. We tested the sorption kinetic data using various non‐linear models/equations and on their transformed linear forms by applying appropriate statistics. Besides, we also used a modified non‐linear equation having four parameters to this effect. Data were found to fit best with the modified equation and described the whole sorption process satisfactorily. For sorption of compounds on to the surface of these minerals, the equation with four parameters may be used in contrast with many standard equations applied for kinetic studies in soils. Sorption was described to take place in two processes: a fast one that takes place in less than 45 min and a slow one that takes place in several hours or more. The rate of the slow process did not depend directly on the concentration of phosphate compounds in solution, but depended linearly on the amount of phosphate that was adsorbed during the fast process. These initially adsorbed ions carrying some amount of negative charge likely hindered the movement of subsequent adsorbate ions to the solid surface due to decreased surface potential. This caused the variation in fast and slow sorption rate constants. Sorption densities increased in the order, Pi >Gly P >G6P >IHP, which revealed that the sorption density and initial sorption rate of OPs decreased with increasing molecular weights of OPs.

Improving Atmospheric Water Harvesting in Carbon‐Based Sorbents Through CO2 Activation

AbstractAtmospheric water harvesting (AWH) can serve as an alternative fresh water source in regions with scarce access to drinking water. While many techniques are geographically or seasonally limited, sorbent‐based AWH shows the potential for wider application. Low‐cost, carbon‐based sorbents named nanoporous sponges (NPS) are recently developed, exhibiting fast and repeatable water uptake of 0.14 g g−1 at 90% relative pressure. While useful from a cost and daily water yield point of view, there is room for improvement. This article presents an improved production process combining pyrolysis and activation under CO2 in a single step, yielding improved NPS capable of reproducibly reaching a water uptake of 0.47 g g−1 at 95% relative pressure while maintaining fast sorption rates. The NPS show significant endurance and are able to maintain stable performance over numerous humidity cycles. An unexpected time‐dependant sorption behavior is also identified for NPS produced with a modified synthesis formulation, due to an increase in sodium carbonate residual content.

Enhanced removal of short- and long-chain per- and poly-fluoroalkyl substances from aqueous phase using crushed grafted chitosan beads: Performance and mechanisms

The widespread use of per- and poly-fluoroalkyl substances (PFASs), environmentally persistent halogenated hydrocarbons, in various industrial and commercial applications has caused significant concerns owing to their contamination of soil and groundwater. Chitosan is a biopolymer substance with abundant amine and hydroxyl functional groups, making it a good candidate for adsorption of PFASs. This study aimed to increase chitosan's adsorption capacity by grafting additional amine functional groups on its surface for the removal of long- and short-chain PFASs from an aqueous phase. Two types of chitosan-based sorbents were developed: crushed chitosan beads (CBs) and polyethyleneimine-grafted CBs (GCBs). Batch adsorption tests assessed the adsorption capacities of the sorbents in terms of the sorption kinetics, isotherms, selectivity, and reusability. Based on the results, the GCBs had significant potential for adsorbing PFASs. These capacities were significantly higher than those demonstrated by the CBs. The sorption kinetics data revealed that the GCBs had a fast sorption rate. Furthermore, the GCBs demonstrated a high adsorption affinity, with log Kd values ranging from 1.5 to 2.5 for PFASs at environmentally relevant concentrations (1000 ng L−1). They also demonstrated excellent selectivity sorption for these compounds, even in the presence of other organic and inorganic pollutants.

Green synthesis of bio-functionalised copper nanoparticles using cinnamon extract and its application as a fast and high-efficiency sorbent for the removal of Janus Green B

ABSTRACT In this study, a biocompatible sorbent based on copper nanoparticles was synthesised from the extract of cinnamon. The sorbent was characterised by X-ray diffraction, field emission electron microscopy, and Fourier transform infrared spectroscopy. Then, the capability of the prepared sorbent was evaluated for the removal of the Janus Green B (JGB) azo dye from the aqueous media. Initially, the sorbent showed a very fast sorption rate that gradually became slower. Thus, with an initial dye concentration of 200 mg L−1, pH of 6.0, and 10 mg of sorbent, about 84% of the dye is removed in 2 minutes, and the maximum removal efficiency is achieved after 15 minutes (>95%). The sorption data were best fitted with the Langmuir isotherm model with a maximum sorption capacity of 193.62 mg g−1. A pseudo-second-order kinetic model was suitable for explaining the JGB dye sorption rate on the sorbent.

High efficient removal of U(VI) by covalent organic frameworks fabricated by mechanochemical method: Adsorption process, mechanism and surface complexation modeling

Herein, covalent organic frameworks (i.e., TpBD) were fabricated by mechanical delamination of 1, 3, 5-triformylphloroglucinol (Tp) and benzidine (BD) using NaHCO3 as foaming agent. The batch characterizations revealed that flower-like TpBD nanosheets exhibited the large porosity, high BET specific area (139 m2/g), small pore size (∼ 14 Å), remarkable stability and keto form. The adsorption process of U(VI) removal by TpBD displayed the fast sorption rate (equilibrium within 0.5 h), high adsorption capacity (167 mg/g) and good re-usability (no significant difference after five cycles). According to analysis of X-ray photoelectron spectroscopy (XPS) and surface complexation modeling, the removal mechanism of U(VI) by TpBD was chemisorption and inner-sphere surface complexation. These findings are of great importance for the application of covalent organic frameworks fabricated with rapid and easy-to-operate mechanochemical method in actual environmental cleanup.

Feather-weight cryostructured thiourea-chitosan aerogels for highly efficient removal of heavy metal ions and bacterial pathogens

Designing of economically feasible and recyclable polysaccharide-based materials with thiourea functional groups for removal of specific metal ions such as Ag(I), Au(I), Pb(II) or Hg(II) remains a major challenge for environmental applications. Here, we introduce ultra-lightweight thiourea-chitosan (CSTU) aerogels engineered by combining successive freeze-thawing cycles with covalent formaldehyde-mediated cross-linking and lyophilization. All aerogels exhibited outstanding low densities (0.0021–0.0103 g/cm3) and remarkable high specific surface areas (416.64–447.26 m2/g), outperforming the common polysaccharide-based aerogels. Benefitting from their superior structural features (honeycomb interconnected pores and high porosity), CSTU aerogels demonstrate fast sorption rates and excellent performance in sorption of heavy metal ions from highly-concentrated single or binary-component mixtures (1.11 mmol Ag (I)/g and 0.48 mmol Pb(II)/g). A remarkable recycling stability was observed after five sorption-desorption-regeneration cycles when the removal efficiency was up to 80 %. These results support the high potential of CSTU aerogels in the treatment of metal-containing wastewater. Moreover, the Ag(I)-loaded CSTU aerogels exhibited excellent antimicrobial properties against Escherichia coli and Staphylococcus aureus bacterial strains, the killing rate being around 100 %. This data points towards the potential application of developed aerogels in circular economy, by employing the spent Ag(I)-loaded aerogels in the biological decontamination of waters.

Influence of non-equilibrium and nonlinear sorption of 137Cs in soils. Study with stirred flow-through reactor experiments and quantification with a nonlinear equilibrium-kinetic model

This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical Kd approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2–0.3 h), a slow desorption rate (half-time of 3–9 days) and a very low sorption capacity (0.02–0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the Kd model to predict 137Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach.

Functionalization of metal organic frameworks by [BMIM][CH3COO] ionic liquid for toluene capture

Indoor exposure to volatile organic compounds (VOCs) is detrimental to the health of the occupants, and their removal is crucial in maintaining good air quality. Novel adsorbents prepared by modifying Metal-Organic Frameworks (MOFs), MIL101 (Cr), UiO-66, and UiO-66-NH2 with [BMIM][CH3COO] ionic liquid, were characterized and tested for toluene adsorption. [BMIM][CH3COO]/MIL101 performed best with a fast sorption rate, large sorption capacity, and good regenerability. It displays synergistic interactions between the IL and MOF. Adding one weight percent [BMIM][CH3COO] to UiO-66 and UiO-66-NH2 has a synergistic effect with respective 14% and 5% enhancements sorption over calculated values. The strong interactions between IL and UiO-66 and UiO-66-NH2, as observed in their thermogravimetric data, results in poor toluene sorption for 10 wt% [BMIM][CH3COO] loadings. This work provides a basis for IL modification of MOFs for enhanced sorption of VOCs for air treatment.

Imidazolium-based ionic liquid confined into ordered mesoporous MCM-41 for efficient dehumidification

Composite sorbents with hygroscopic salts (like LiCl and CaCl2) impregnated porous matrixes obtained special attention in the past years, especially for solid desiccant dehumidification. However, the drawback of strong corrosivity limited their practical implementation. Alternatively, ionic liquid (1-ethyl-3-methylimidazole acetate) with less corrosivity is confined into ordered mesoporous MCM-41 to fabricate composite sorbent in this study, overcoming the poor mass transfer or limited adsorption capacity of single sorbent. Comprehensive characterizations including SEM, TEM, XRD, FTIR and N2 adsorption prove that the ionic liquid is encapsulated in ordered channels of MCM-41. Sorption isotherms and kinetics indicate that the composite sorbent has superior water sorption performance (0.5 g/g at 20 °C/70 % RH), fast sorption rate under wide humidity range and potential of low temperature-driven regeneration. Moreover, the leakage test and corrosion experiment reveal that the composite sorbent has long-term stability and is corrosion-free to metal components. Further, the dehumidification performance using desiccant coated heat exchanger with reported composite sorbent is evaluated, showing 5 times higher dehumidification capacity than that of MCM-41 and 2 times higher than traditional silica gel (typical ARI summer, 35 °C, 40 % RH). This study identifies a feasible way of using ionic liquids to develop energy-efficient, safety-ensured and low-cost composite sorbents for water sorption-related applications.

Efficient and Fast Removal of Aqueous Tungstate by an Iron-Based LDH Delaminated in L-Asparagine.

High concentrations of tungstate in aqueous systems pose a severe threat to the environment and human health. This study explored the potential of iron-based LDHs to remove tungstate from water. To improve its tungstate uptake capacity, environment-friendly L-asparagine was used to delaminate iron-based LDH synthesized via a coprecipitation method. The successful delamination was proved by AFM, revealing that the thickness of the obtained nanoparticles was approximately 1–2 times that of a single LDH layer. XRD, TEM, and XPS analyses confirmed that the delaminated LDHs were amorphous and ultrathin and had surface defects within their nanosheets that acted as active sites, leading to a very fast tungstate sorption rate and superior tungstate uptake capacity. Notably, the original layered structure of the L-asparagine-treated LDH was recovered upon its reaction with tungstate-bearing solutions, and therefore, the high availability of aqueous tungstate to the interlayer regions during the structural restoration of the delaminated iron-based LDH contributed to its excellent capability of tungstate removal as well. In addition, the tungstate uptake by the delaminated iron-based LDH was not affected substantially by the presence of coexisting anions, implying that the strong inner-sphere complexation between the tungstate and LDH layers with defects (i.e., Fe-O bonds) was the primary mechanism responsible for the tungstate removal. The delamination process described in this paper was validated to be an effective way to enhance the immobilization of tungstate by iron-based LDHs without inducing secondary pollutions, and delaminated iron-based LDHs are promising to be used extensively in the practice of treating tungstate-rich waters.

Characteristics of MgO-based sorbents for CO2 capture at elevated temperature and pressure

MgO-based sorbents are promising candidates for precombustion CO2 capture performed at elevated temperature and pressure in thermodynamics, where the development of efficient sorbents is of great importance. Herein, we report MgO-Na2CO3-KNO3 sorbents with fast sorption kinetics, high capture capacity and good cyclic stability at elevated conditions. The effects of nitrate species, Na2CO3 doping amounts and sorption conditions on the MgO conversion were investigated during cyclic test. In comparison with LiNO3 and NaNO3, the sample with KNO3 possesses the highest MgO conversion, which increases from 0.78 to 0.86 after 30 cycles with sorption at 400 °C, 2 MPa. For nitrate-promoted MgO, Na2CO3 plays an essential role in the initial fast sorption rate. The sorbent with 60 mol%Na2CO3 possesses the highest MgO conversion of 0.89 after 30 cycles. Moreover, during the operation range of 400 ℃-480 ℃, 2 MPa, the sorbent exhibits an excellent cyclic stability with porous structure. Density functional theory calculations are further conducted to investigate the mechanism of performance improvement brought by Na2CO3. We find dissolved CO32– ions will provide chemisorption sites through the formation of C(CO2)-O(CO32–) bonds and can serve as the CO2 carrier in molten nitrate salts, thus improving both sorption and diffusion rates of CO2.

Scalable synthesis of Li4SiO4 sorbent from novel low-cost orthoclase mineral for CO2 capture

A novel Li4SiO4 sorbent has been synthesized using low-cost and naturally occurring mineral (orthoclase) as silicon source for high-temperature CO2 sorption. This new CO2 sorbent O-Li4SiO4 exhibited looser microstructures with enhanced surface area and porosity compared to the sorbent using conventional SiO2 powder as raw materials (S-Li4SiO4) perhaps due to the existence of the support oxides and alkali impurities. As a result, the CO2 sorption rate and cyclic capacity of the new sorbent were significantly enhanced. Over 20 cycles under low CO2 sorption conditions (15%), O-Li4SiO4 still showed high sorption capacity of ∼ 0.255 g CO2/g sorbent, nearly 8 times higher than the last-cycle capacity of S-Li4SiO4. The new sorbent powder was further granulated into spherical pellets and it was demonstrated that the mechanical granulation process led to the densification of sorbent structures and, thus, the reduction of sorption capacity. However, the reduced cyclic CO2 sorption performance could be regained by adding microcrystalline cellulose as pore-forming materials. In addition to the good chemisorption performance, the pellets also presented excellent mechanical properties, i.e., high compressive strength and attrition resistance, to satisfy the requirements for realistic application. Thanks to its fast sorption rate, high CO2 capture capacity, good stability, excellent mechanical strength, and characterizations of low cost and scalable synthesis, orthoclase-derived Li4SiO4 sorbent is a promising candidate for realistic CO2 removal.

Dual Effect of Acetic Acid Efficiently Enhances Sludge-Based Biochar to Recover Uranium From Aqueous Solution.

Excess sludge (ES) treatment and that related to the uranium recovery from uranium-containing wastewater (UCW) are two hot topics in the field of environmental engineering. Sludge-based biochar (SBB) prepared from ES was used to recover uranium from UCW. Excellent effects were achieved when SBB was modified by acetic acid. Compared with SBB, acetic acid-modified SBB (ASBB) has shown three characteristics deserving interest: 1) high sorption efficiency, in which the sorption ratio of U(VI) was increased by as high as 35.0%; 2) fast sorption rate, as the equilibrium could be achieved within 5.0 min; 3) satisfied sorption/desorption behavior; as a matter of fact, the sorption rate of U(VI) could still be maintained at 93.0% during the test cycles. In addition, based on the test conditions and various characterization results, it emerged as a dual effect of acetic acid on the surface of SBB, i.e., to increase the porosity and add (−COOH) groups. It was revealed that U(VI) and −COO− combined in the surface aperture of ASBB via single-dentate coordination. Altogether, a new utilization mode for SBB is here proposed, as a means of efficient uranium sorption from UCW.

Biomass poplar catkin fiber-based superhydrophobic aerogel with tubular-lamellar interweaved neurons-like structure

Superhydrophobic aerogels are attractive candidates in controlling oil spills. The major challenges for existing aerogels are the construction of mechanical endurance as well as accessible of building materials. Herein, a newfangled biomass superhydrophobic aerogel (M-PCF/CS) with both superior compressibility and oil caption speed is fabricated by assembling poplar catkin fiber (PCF) hollowed-out shell of 330 nm and chitosan (CS) into tubular-lamellar interweaved neurons-like structure. The resultant aerogels (porosity ~ 96.12%), with flexuous PCF as the elastic buffer and second-pore capillaries, exhibit large longitudinal and transverse compressibility, endurable fatigue tolerance, fast oil sorption rate with a capacity of 28.8–78.1 g/g at 5–25 s. In parallel, the aerogels are tolerant of NaCl, UV radiation, and organic solvents without superhydrophobic variation and a case of oil spill remediation via pump-supported experiment shows that the aerogels facilely achieve continuous oil recycling from seawater by 23052–43956 L·m–2·h–1. Furthermore, the resultant M-PCF/CS, with assistance of an oscillator, can be applied to separate oil/water emulsions with efficiency of 98.07–99.11%. The successful fabrication of this material provides a new design strategy for the construction of mechanically robust aerogels for speedy and economical cleanup of oil pollutants from water.

The synthesis of functionalized graphene oxide by polyester dendrimer as a pH-sensitive nanocarrier for targeted delivery of venlafaxine hydrochloride: Central composite design optimization

In this study, a new type of polyester dendrimer was synthesized through direct esterification reaction between phloroglucinol (PG) and terephthalic acid (TPA) in the presence of a catalyst. Graphene oxide (GO) functionalized with polyester dendrimer, decorated with 3,4-dihydroxybenzoic acid (DHB), was used to sustain delivery of venlafaxine hydrochloride (VH) as an antidepressant drug. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Brunauer–Emmett–Teller (BET), and thermogravimetric analysis (TGA) confirmed the attachment of polyester dendrimer onto the GO. Different variables, including pH, contact time, and temperature were optimized to evaluate the adsorption of VH on the PAH-GO/PGTPA/DHB Dendr using a central composite design (CCD). The highest adsorption percentage was obtained at pH of 7, contact time of 20 min, and temperature of 30 °C. The sorption mechanism was determined via the Langmuir isotherm model (R2 = 0.9965), representing the monolayer adsorption with the adsorption capacity of 19.72 mg g−1. PAH-GO/PGTPA/DHB Dendr displayed a fast sorption rate, and the pseudo-second-order kinetic model confirmed the best correlation between experimental data with R2 = 0.9995. The release of the VH from nanocarrier in simulated intestinal fluid (SIF) and in simulated gastric fluid (SGF) was studied. The release mechanism was considered by applying various mathematical models to the in vitro release curves. Nearly 40% and 99% of VH were released in SIF (pH = 7.4) and SGF (pH = 1.2) at 6 h, respectively. The cytotoxicity of PAH-GO/PGTPA/DHB Dendr was investigated via 3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) and apoptosis methods using PC-12 cell lines.

Synthesis of phosphorylated hyper-cross-linked polymers and their efficient uranium adsorption in water

Uranium (U) is hazardous and radioactive, wastewater containing U(VI) should be treated before being discharged. Here, two novel uranium adsorbents, phosphorylated hyper-cross-linked bisphenol A (PHCP-1) and fluorene-9-bisphenol (PHCP-2) were separately synthesized via Friedel-Crafts reaction followed by phosphorylation using phosphorus oxychloride. PHCPs had a BET surface area (up to 564 m2/g) with pore sizes of 2.2–2.8 nm. These adsorbents were used for the first time for uranium adsorption from water and demonstrated outstanding adsorption performance. PHCP-2 had a great uranium adsorption capacity (297.14 mg/g) and a very fast sorption rate (85% removal rate within 5 min). The adsorption data were well fitted with Freundlich isotherm and the pseudo-second-order kinetic model. PHCPs displayed selective adsorption capacity for U(VI) from solution that including a variety of competing metal ions. The reusability was confirmed through three regeneration cycles. Based on a series of spectroscopic analyses, the mechanism of action between PHCPs and U(VI) is primarily derived from the complex between phosphate functional groups and U (VI). The sorption performance of PHCPs is attributed to their huge specific surface area and the strong complex between phosphate groups and U(VI). These findings suggest that PHCPs could be useful in the effective adsorption of uranium from water.

Uptake of heavy metal Cd(II) from aqueous solutions using brown algae Sargassum myriocystum

The uptake of brown marine algae, Sargassum myriocystum has been utilized for the exclusion of Cd(II) metal from wastewater. Various parameters such as solution pH, optimum temperature, biomass concentration, the contact time and agitation speed have been analyzed for the effective biosorption of Cd(II). Desorption studies of Cd(II) have been performed with various desorbent such as H3PO4, HNO3, HCL, H2SO4, NaOH, NaCl, and H2O among which 0.1M/L HCl is found to be the better desorbing agent. SEM-EDX and FTIR analyses were utilized for metal-algal interaction study. The thermodynamic parameters such as free energy change (ΔG°), enthalpy change (∆Hº) and entropy change (∆Sº) have been calculated through the Van’t Hoff plot. A positive value of 14.72 ∆S Jmol-1K-1, a negative value of free energy (ΔG°) and 42.21 kJ/mol of ∆H kJmol-1 at all the temperatures indicated that the process is feasible and spontaneous. Hence, S. myriocystum is an effective and prosperous cost-effective biosorbent approach for Cd(II) from various industrial wastes due to its fast sorption rate, high selectivity and great uptake capacity.

High efficiency removal of heavy metals using tire-derived activated carbon vs commercial activated carbon: Insights into the adsorption mechanisms

In this study, activated carbon was derived from pulverized waste tires using carbonization and chemical activation techniques. Single and competitive batch adsorption experiments for the removal of three synthetic heavy metal ions (Pb2+, Cu2+ and Zn2+) from an aqueous solution were performed to benchmark the efficiency of the Tire-derived Activated Carbon (TAC) in comparison to that of commercial activated carbon (CAC), which was used as the reference material. The sorbents physicochemical properties with corresponding adsorption mechanisms were evaluated by different experimental techniques. TAC exhibited great potential to adsorb heavy metals, with monolayer adsorption capacities as high as 322.5, 185.2, and 71.9 mg g−1 for Pb2+, Cu2+ and Zn2+, respectively, which were significantly higher than the adsorption capacities exhibited by CAC, which were 42.5, 15.0, and 14.0 mg∙g-1 for Pb2+, Cu2+ and Zn2+, respectively. Competitive adsorption results demonstrated the adsorption ability of sorbents is restricted by presence of other ions, and was decreased compared to the single sorption. Sorption kinetics data, with better fit to the pseudo-second order kinetics model, revealed that TAC had faster sorption rate in comparison to CAC. The adsorption capacities of TAC and CAC were reduced to half of their initial capacities after three successive adsorption-desorption cycles. Zeta potential, FT-IR, and XPS analyses revealed that electrostatic attraction and surface complexation mechanisms, as two metal-adsorbing mechanisms, were more influential for TAC. For CAC, a higher cation exchange capacity (CEC) value indicated that the removal of heavy metals by ion exchange was the predominant mechanism.

Super-stable mineralization of cadmium by calcium-aluminum layered double hydroxide and its large-scale application in agriculture soil remediation

Cadmium (Cd) contaminated cropland has posed severe risks to human health. While in-situ Cd immobilization by stabilizer has been widely adopted in practice, there is still a huge challenge to develop cost-effective stabilizers with strong and long-term immobilization ability. In this work, we report a CaAl-layered double hydroxide (CaAl-LDH) as an efficient stabilizer for Cd2+in-situ immobilization both in lab scale and practice remediation. This material presents fast sorption rate (ca. 5 min) and high capture capacity (592 mg/g) towards Cd2+ in solution, and high immobilization efficiency (up to 96.9%) in soil remediation. Detailed investigations suggest that Cd2+ induces the reconstructing of CaAl-LDH to CdAl-LDH, with Cd being stabilized in a super stable state. In a large-scale and long-term application (in soil of 10 ha for continuous three years’ test), we observe that the CaAl-LDH enables a low Cd content in wheat grain (0.06 mg/kg) that is below the national standard (0.1 mg/kg). By employing a scaling-up production in 3000 ton per year, we demonstrate the CaAl-LDH would be a promising competitive candidate for facile and cost-efficient remediation of Cd contaminated soil.