Adsorption Properties For Metal Ions Research Articles

A nanocomposite paper comprising calcium silicate hydrate nanosheets and cellulose nanofibers for high-performance water purification.

Removal of soluble organic and inorganic contaminants from wastewater to produce clean water has received much attention recently. However, the simultaneous enhancement of water permeability and removal efficiency is still a challenge for filtration membranes. Here, we present a new kind of nanocomposite paper (CSH/CNF) consisting of calcium silicate hydrate (CSH) nanosheets and cellulose nanofibers (CNFs), and demonstrate the rapid water filtration and highly efficient contaminant (e.g., dyes, proteins, and metal ions) adsorption properties. The CNFs can serve as the bridging material to connect the CSH nanosheets to form a porous network structure and vital channels in the CSH/CNF paper for rapid water transportation. The weight ratio of CSH nanosheets in the paper is up to 75–85%. The weight ratio of CSH nanosheets has a significant effect on the water permeability and removal efficiency. The water permeability of the CSH/CNF paper with 82.5 wt% CSH nanosheets reaches as high as 312.7 L m−2 h−1 bar−1, which is about 14.7 times that of the CSH/CNF paper with 75 wt% CSH nanosheets. Because of the high specific surface area and abundant adsorption sites of CSH nanosheets, the CSH/CNF paper with 82.5 wt% CSH nanosheets exhibits high adsorption capacities and removal efficiencies for methyl blue (242.6 mg g−1, 97.3%), bovine serum albumin (289.2 mg g−1, 98.5%) and Pb2+ ions (366.2 mg g−1, 98.2%). The CSH/CNF nanocomposite paper holds great potential for application in environmental wastewater purification.

Synthesis of magnetic Fe-doped hydroxyapatite nanocages with highly efficient and selective adsorption for Cd2+

Fe-doped hydroxyapatite (HA) nanocages were firstly synthesized using cheap and readily available potassium mono-dodecyl phosphate as the micelle template agent. The incorporation of Fe into HA crystal lattices makes the hollow structure of Fe-doped HA nanocages resist the shrinkage caused by calcination. The obtained Fe-doped HA nanocages have a size of 80–100 nm, a mesoporous shell with a thickness of 12 nm, a specific surface area of 80.1973 m2/g and a pore volume of 0.43298 cm3/g. The nanocages have a highly efficient and selective adsorption for Cd2+ as well as a superparamagnetic property. The magnetic characteristic and the outstanding adsorption property for heavy metal ions of Fe-doped HA nanocages provide evidence that the material has a great promise for wastewater treatment.

Facile synthesis of amino-functional large-size mesoporous silica sphere and its application for Pb2+ removal

Amino-functional large-size mesoporous silica spheres (LMS-AP) were successfully synthesized through a one-step method with (3-aminopropyl) triethoxysilane (APTES) addition during the pseudomorphic transformation process. LMS-AP were characterized using thermogravimetry-differential thermal analysis, Nitrogen adsorption-desorption measurement, infrared spectroscopy, and X-ray photoelectron spectroscopy. The study found that -NH2 was grafted into LMS, and the LMS-AP had a better thermal stability than other samples. The Pb2+ removal properties of LMS-AP were investigated using the static and dynamic experiments in simulated and real wastewater solutions. The kinetic and equilibrium experiments indicated that the adsorption process of LMS-AP fitted the Langmuir adsorption model and the pseudo-second-order kinetics model (R2 > 0.98), respectively. The maximum Qe (mg/g) was about 100 mg/g in the static adsorption condition. The adsorption mechanism of removal of Pb2+ was also investigated. In fix bed column experiments, LMS-AP exhibited excellent Pb2+ adsorption ability for simulated wastewater, with the maximum qe (mg/g) of 48.7 mg/g for particle size under 1-3 mm. Meanwhile in actual industrial wastewater treatment process, LMS-AP had a better Pb2+, Zn2+ and Cr (VI) removal efficiency of 80% and As (V) of 30-40% removal efficiency at initial pH 4, suggesting selective adsorption property for different heavy metal ions.

Dual-responsive copolymer hydrogel as broad-spectrum adsorbents for metal ions

A copolymeric hydrogel (PEGMA) with poly(ethylene glycol) methyl ether acrylate (PEGA), 2-methoxyethyl acrylate (MEA) and acrylic acid (AA) was synthesized in this study. The obtained PEGMA hydrogel exhibited sensitive response to pH and temperature and showed excellent adsorption properties for many metal ions, such as Fe(Ⅲ), Pb(Ⅱ), Zn(II), Ni(II), Mn(II) and Co(II), etc. Fe(Ⅲ) and Pb(Ⅱ) were selected to study thermodynamics and kinetics. The adsorption process followed pseudo-second-order kinetics equation. The adsorption isotherms could be described by Freundlich isotherm models. The maximum adsorption amount was calculated to be 195.43 (Fe3+) and 176.06 (Pb2+) mg/g. Besides, using the thermal and pH responsiveness, the PEGMA hydrogels can quickly release the adsorbed molecules with control. The heavy metal ions loaded PEGMA hydrogels could also be easily regenerated by diluted hydrochloric acid and still keep a major adsorption capacity. After five cycles, the adsorption ability remained more than 90% of the original PEGMA. Thus, the PEGMA hydrogels may be new high-performance and environment friendly material for metal pollution treatment with promising developmental potential.

Preparation of Polymeric Adsorbent and Study of Its Adsorption of Heavy Metal Ions in Water

Liu, Y. and Zhang, Z., 2018. Preparation of polymeric adsorbent and study of its adsorption of heavy metal ions in water. In: Liu, Z.L. and Mi, C. (eds.), Advances in Sustainable Port and Ocean Engineering. Journal of Coastal Research, Special Issue No. 83, pp. 414–417. Coconut Creek (Florida), ISSN 0749-0208.The physicochemical properties and structural characteristics of heavy metal ion adsorbents, such as inorganic adsorbents, organic macromolecule adsorbents and composite adsorbents, are analyzed. The adsorption properties of metal ions commonly used in the present stage are studied, it is found that large heavy metal sludge is produced during processing of these materials. Therefore, the preparation of polymeric adsorbents and adsorption of heavy metal ions in water are studied and reviewed. The research status and development trend of novel heavy metal ion adsorbents are also discussed. Through the combination of modern detection techniques and analytical tools, experiments are carried out to test the preparation of polymeric adsorbents and the adsorption properties to heavy metal ions in water. The experimental results show that there is interaction between the adsorption capacity of polymeric adsorbents and heavy metal ions.Thus, the use of polymer adsorbents can better enhance the adsorption of heavy metal ions in water. This has important theoretical and practical significance for expanding the adsorption mechanism and developing the adsorptive material with excellent performance.

Microfiltration membrane possessing chelation function and its adsorption and rejection properties towards heavy metal ions

AbstractBackgroundRemoving toxic heavy metal ions from water bodies is one of the most important areas of water treatment, and using porous membranes with a chelating function is a good route for this. Graft‐type porous membranes with a chelating function are innovate functional membranes. In this work, the main aims are to prepare such a functional membrane and evaluate its adsorption and rejection properties for heavy metal ions in water through adsorption isotherm experiments and permeation experiments.ResultsGlycidyl methacrylate (GMA) was graft‐polymerized on the surface of polysulfone (PSF) microfiltration membrane through a surface initiating system, getting the grafted membrane PSF‐g‐PGMA. Then the ring‐opening reaction of the epoxy group of the grafted PGMA was carried out with iminodiacetic acid (IDAA) as reagent, obtaining PSF‐g‐PGMA/IDAA membrane, on which a great deal of IDAA as chelating ligand was bonded. PSF‐g‐PGMA/IDAA membrane possesses a strong adsorption ability towards heavy metal ions via the chelating action and electrostatic interaction. The order of adsorption capacity was Pb2+>Cu2+>Cd2+, and the adsorption capacity of Pb2+ ion reached 4.5 µmol cm−2. When an injection filter with a membrane area of 1.77 cm2 was used, the rejection rate of PSF‐g‐PGMA/IDAA membrane for the three heavy metal ions with concentrations of 0.1 mmol L−1 were maintained at a level of 95% until the permeation solution volume reached 100 mL.ConclusionThe grafted microfiltration membrane PSF‐g‐PGMA/IDAA can be easily prepared and possesses excellent rejection and removing performance towards heavy metal ions © 2018 Society of Chemical Industry

Synthesis of bitter gourd-shaped nanoscaled hydroxyapatite and its adsorption property for heavy metal ions

As an eco-friendly adsorption material, hydroxyapatite (HA) exhibits different adsorption characteristics depending on its morphology. In the present work, nano HA with different morphologies was successfully synthesized by controlling the addition of mono-dodecyl phosphate potassium (MDP-K). The particle size, specific surface area and pore volume of the as-prepared HA powders are inclined to be on the rise with the increase of MDP-K addition. The bitter gourd-shaped nano HA, synthesized with addition of 4 g MDP-K, possesses with a specific surface area of 77.25 m2/g, a pore volume of 0.1825 cm3/g as well as a high adsorption capacity of heavy metal ions. The outstanding adsorption property for heavy metal ions of nano HA provides evidence that the material has a great potential in application of water treatment procedure.

Adsorption properties of heavy metal ions in landfill leachate by Na-bentonite

Abstract As a result of the rapid development of industry, the heavy metal contamination of landfill leachate has become more serious. Adsorption is an effective method that can remove heavy metal ions from landfill leachate. Na-bentonite was selected as the adsorbent for the adsorption of heavy metal ions (lead, cadmium, copper) from landfill leachate in both single and competitive systems. Batch experiments were conducted to study the effects of contact time, pH value, temperature, and initial concentration on adsorption efficiency. Adsorption behavior was investigated by means of adsorption isotherms, adsorption kinetics, and adsorption thermodynamics. The study results show that: 1. The amount of heavy metal ions adsorbed by Na-bentonite increases with increasing contact time, pH value, temperature, and initial concentration of heavy metal ions. 2. Adsorption behavior is suitable for modeling with the Langmuir isotherm and a pseudo second-order kinetic model. 3. The adsorption of lead and cadmium are thermodynamically spontaneous, the adsorption of copper is not spontaneous, and the adsorption processes of lead, cadmium and copper are all endothermic using thermodynamic analysis. 4. Competitive adsorption between lead, cadmium, and copper occurs in competitive systems.

Fabrication of electrospun chitosan/cellulose nanofibers having adsorption property with enhanced mechanical property

Chitosan/cellulose (CS/CL) nanofibers were fabricated through electrospinning with a mixture of chitosan (CS) and cellulose acetate (CA) in a co-solvent system (trifluoroacetic/acetic acid) and afterward Na2CO3 treatment was followed. The treatment induces the neutralization of CS and deacetylation of CA, converted into cellulose (CL). The CS/CA nanofiber webs maintained the fibrous structure after treatment (converted into CS/CL nanofibers), which cannot be achieved by CS nanofibers. In addition, the combination of CS and CA enhanced the mechanical properties of the resultant nanofibers up to approximately 17 MPa in tensile strength and 5.5% in elongation at break. More importantly, the resulting nanofibers showed adsorptive characteristics; whereas, CL nanofibers showed no adsorption behavior. The incorporation of CS with CL offers the metal ion adsorption property to the composite nanofibers and gives them a waterproof property, which could be utilized in wastewater purification. The adsorption capacity of CS/CL nanofibers for As(V), Pb(II) and Cu(II) ions reached up to 39.4, 57.3 and 112.6 mg/g. Therefore, this nanofiber system showed effective removal behavior in aqueous solution with reasonable mechanical strength, unattainable with pure CS or CL nanofibers.

Ar/O2 plasma treatment of carbon nanotube membranes for enhanced removal of zinc from water and wastewater: A dynamic sorption-filtration process

In this study, pristine multi-walled carbon nanotubes (MWCNTs) were functionalized by using Ar/O2 plasma treatment technique, which enhanced adsorptive membrane filtration of zinc ions from water and wastewater. The XPS analysis showed that plasma treatment largely increased the surface oxygen groups content of MWCNTs from 2.78% to 6.79%. This change increased the surface negative charged, dispersion and adsorption properties of MWCNTs without causing any damages to the integrity of the nanotube pattern. Pressure-driven filtration of plasma-treated MWCNT (P-CNT) dispersion formed a stable layer inside the lumen of a hollow fiber membrane. The contact angle analysis demonstrated that after incorporation of P-CNT into the HF membrane increased membrane hydrophilicity. The P-CNT membrane effectively removed almost 100% of zinc from synthetic waters and approximately 80% of zinc from a wastewater effluent by surface complexation reaction. A follow-up regeneration study demonstrated that the adsorptive removal of zinc by the CNT membrane was reversible under selected conditions, thus making it possible to repeatedly use the membrane for long-term zinc filtration. This study suggests that HF membranes modified with P-CNTs possess superb adsorption properties for metal ions, allowing the operation of CNT membranes for water treatment.

Membrane filtration-based recovery of extracellular polymer substances from excess sludge and analysis of their heavy metal ion adsorption properties

Excess sludge production by wastewater treatment plants (WWTPs) represents a serious environmental problem that necessitates the development of effective sludge treatment/disposal methods. Excess sludge contains valuable materials such as extracellular polymer substances (EPSs), and their recovery is of great practical importance. However, conventional recovery methods afford EPS solutions with moisture contents of ∼100% and the dehydration of these solutions via direct drying methods is considerably energy consuming. Here, we investigate the separation of EPSs extracted from excess sludge by microfiltration (0.5 μm) and ultrafiltration (10 kDa), revealing that Ca2+ addition reduces the extent of membrane fouling during EPS isolation since the interactions between Ca2+ and EPS carboxylate groups increase the size and reduce the concentration of colloids/polymers in EPS suspensions. Additionally, we establish an ultrafiltration-based EPS separation protocol, demonstrating that it allows highly efficient EPS recovery (>90%) irrespective of the Ca2+ concentration, and show that EPSs extracted from excess sludge samples derived from two different WWTPs exhibit similar filtration resistances at Ca2+ concentrations of 0–4 mM. Finally, we demonstrate that the EPSs recovered from excess sludge can be used as adsorbents for the removal of diverse heavy metal ions (Pb2+, Cd2+, Cu2+) from wastewater, exhibiting performances that are on par with those of commercial adsorbents and are unaffected by the employed extraction procedure. Based on the obtained results, Ca2+ addition is confirmed to be a viable method for reducing filtration resistance during the membrane recovery of EPSs, since this ion does not affect the adsorption capacity for heavy metal ions.

Bacterial exopolysaccharides as a modern biotechnological tool for modification of fungal laccase properties and metal ion binding

Four bacterial EPSs extracted from Rhizobium leguminosarum bv. trifolii Rt24.2, Sinorhizobium meliloti Rm1021, Bradyrhizobium japonicum USDA110, and Bradyrhizobium elkanii USDA76 were determined towards their metal ion adsorption properties and possible modification of Cerrena unicolor laccase properties. The highest magnesium and iron ion-sorption capacity (~ 42 and ~ 14.5%, respectively) was observed for EPS isolated from B. japonicum USDA110. An evident influence of EPSs on the stability of laccase compared to the control values (without EPSs) was shown after 30-day incubation at 25 °C. The residual activity of laccases was obtained in the presence of Rh76EPS and Rh1021EPS, i.e., 49.5 and 41.5% of the initial catalytic activity, respectively. This result was confirmed by native PAGE electrophoresis. The EPS effect on laccase stability at different pH (from 3.8 to 7.0) was also estimated. The most significant changes at the optimum pH value (pH 5.8) was observed in samples of laccase stabilized by Rh76EPS and Rh1021EPS. Cyclic voltamperometry was used for analysis of electrochemical parameters of laccase stabilized by bacterial EPS and immobilized on single-walled carbon nanotubes (SWCNTs) with aryl residues. Laccases with Rh76EPS and Rh1021EPS had an evident shift of the value of the redox potential compared to the control without EPS addition. In conclusion, the results obtained in this work present a new potential use of bacterial EPSs as a metal-binding component and a modulator of laccase properties especially stability of enzyme activity, which can be a very effective tool in biotechnology and industrial applications.

Advanced Modified Polyacrylonitrile Membrane with Enhanced Adsorption Property for Heavy Metal Ions

Advanced modified polyacrylonitrile (PAN) membrane with high adsorption property for heavy metal ions was designed and fabricated for the first time. The introduced diazoresin-ethylenediaminetetraacetic acid (DR-EDTA) layer could effectively absorb the metal ion, such as Cu2+, Pb2+, Hg2+ in the waste water. The effects of layers, metal ion concentration, pH, temperature and cycle time were investigated. The results showed that the adsorption isotherms for Cu2+ were well fitted by Langmuir model. The maximum adsorption capacity of the modified membrane for Cu2+ was approximately 47.6 mg/g. In addition, the prepared PAN-(DR-EDTA)3 membrane could be regenerated more than 720 h based on their adsorption/desorption cycles. The results demonstrated that the modified PAN membrane could be used as effective adsorbents for heavy metal removal from waste water.

Novel multifunctional cheese-like 3D carbon-BN as a highly efficient adsorbent for water purification

In this paper, a novel three dimensional carbon boron nitride (3D C-BN) was successfully prepared. The obtained material has porous cheese-like structure and pore size ranging from 2 nm to 100 nm. Attractively, the 3D C-BN, which combines the adsorption advantages of BN and carbon together, exhibits excellent adsorption properties for organic dyes, oils and heavy metal ions. The maximum removal capacities of 3D C-BN for methyl blue (MB) and congo red (CR) are 408 mg g−1 and 307 mg g−1, respectively. Furthermore, 3D C-BN can quickly and efficiently remove oils (salad oil, gasoline and pump oil) and heavy metal ions (Cr3+, Cd2+ and Ni2+) from waste water. The macro bulk 3D C-BN, which is more convenient to use than powdered materials, can be reused by burning or heating in air and still maintains high adsorption capacity. Significantly, these superior performances can find practical application in water purification.

Highly Productive Synthesis, Characterization, and Fluorescence and Heavy Metal Ion Adsorption Properties of Poly(2,5-dimercapto-1,3,4-thiadiazole) Nanosheets.

Poly(2,5-dimercapto-1,3,4-thiadiazole) (PBT) nanosheets were synthesized by chemical oxidative synthesis under mild conditions. The media, oxidant species, monomer concentrations, oxidant/monomer molar ratio, and temperature were optimized to achieve higher yields and better performance. The molecular structure, morphology, and properties of the nanosheets were analyzed by Fourier transform infrared (FT-IR), ultraviolet-visible (UV-Vis), and fluorescence spectroscopies, wide-angle X-ray diffraction (WAXD), matrix-assisted laser desorption/ionization/time-of-flight (MALDI-TOF) mass spectrometry, X-ray photoelectron spectroscopy (XPS), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), and simultaneous thermogravimetry and differential scanning calorimetry (TG-DSC). It was found that the polymerization of 2,5-dimercapto-1,3,4-thiadiazole occurs via dehydrogenation coupling between two mercapto groups to form the –S–S– bond. PBTs show the highest polymerization yield of up to 98.47% and form uniform nanosheets with a thickness of 89~367 nm. poly(2,5-dimercapto-1,3,4-thiadiazole) polymers (PBTs) exhibit good chemical resistance, high thermostability, interesting blue-light emitting fluorescence, and wonderful heavy metal ion adsorption properties. Particularly, the PBT nanosheets having a unique synergic combination of three kinds of active –S–, –SH, and =N– groups with a moderate specific area of 15.85 m2 g−1 exhibit an ultra-rapid initial adsorption rate of 10,653 mg g−1 h−1 and an ultrahigh adsorption capacity of up to 680.01 mg g−1 for mercury ion, becoming ultrafast chelate nanosorbents with a high adsorption capacity. With these impressive properties, PBT nanosheets are very promising materials in the fields of water treatment, sensors, and electrodes.

Modification of PPTA-based polymeric waste and adsorption properties for metal ions

Novel composite adsorbents PPTA-AOx were synthesized by grafting polyacrylonitrile onto poly(p-phenylene terephthalamide) (PPTA) followed by converting the acrylonitrile into the amidoxime (AO) groups. Their structures were characterized by Fourier transform infrared spectroscopy, thermogravimetric analysis, elemental analysis, transmission electron microscopy, etc. Scanning electron microscopy analysis and pore structure analysis manifested that PPTA-AOx adsorbents are all composed of nanoparticles aggregation. The as-synthesized PPTA-AOx adsorbents showed good adsorption capacity for Hg2+ with a maximum adsorption capacity of 2.50 mmol g−1. The pseudo-second-order model can reasonably describe the adsorption kinetics of the three adsorbents for Hg2+. Langmuir model provided better fit for the isothermal adsorption of Hg2+ on PPTA-AO1 and PPTA-AO2, while the Freundlich model was better for PPTA-AO3. The adsorption process might involve both chemisorption and physisorption. According to the calculated thermodynamic parameters, it can be concluded that the adsorption is an endothermic, spontaneous and entropy-driven process.

Synthesis of polyamine‐bridged polysilsesquioxanes and their adsorption properties for heavy metal ions

Four types of polyamine‐bridged polysilsesquioxane adsorbents were prepared via a facile sol‐gel method using pre‐synthesized ethanediamine (EDA)‐ and diethylenetriamine (DETA)‐bridged monomers containing soft organic bridge. Infrared spectra (IR), scanning electron microscope (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TG), elemental analysis, and nitrogen gas adsorption–desorption were used to characterize their structures. Porogen hexadecyl tyimethyl ammonium bromide (CTAB) was found to be efficient in increasing the porosity of the absorbents. Adsorption experiments at different temperatures showed that the obtained absorbents exhibited relatively high adsorption capacities for Hg(II). Pseudo second‐order model is more suitable for describing the adsorption kinetics in the temperature range of 15–35°C. Compared with Freundlich models, Langmuir isotherm was better to describe the adsorption process of Hg(II) for all adsorbents. Finally, the thermodynamic parameters demonstrated that the adsorption reaction was endothermic and spontaneous. © 2017 American Institute of Chemical Engineers Environ Prog, 36: 1089–1099, 2017

Structure–property relationships of elementary bamboo fibers

Natural fibers are promising alternatives to synthetic fibers because of their sustainability, low environmental footprint and specific properties desirable for a wide range of technical engineering applications. The industrial implementation of fine grade natural bamboo fibers, including technical (100–200 microns) and elementary fibers (<30 microns) has been of increasing interest in recent times because these fibers offer a unique set of properties including high tensile strength, antibacterial and UV absorption. However to date, very little scientific effort has been devoted to fully understand the inter-correlation between their mechanical, physico-chemical, microstructural and morphological properties. In this paper, we report for the first time the structure–property relationship of elementary bamboo fibers. The impact of the inner microstructural organization of fibers (including the micro-fibrils angle) and physico-chemical factors such as the cellulose content and crystallinity index, on the tensile performance of these fibers is discussed in detail. This work also provides an insight into the application of bamboo fibers as natural and low-cost sorbent material for the removal of Cu2+ metal ions from model industrial wastewater. The metal ion adsorption properties of the fibers are correlated to surface energy analysis obtained from inverse gas chromatography.

Synthesis of a Metal Ion Adsorbent from Banana Fibers and Its Adsorption Properties for Rare Metal Ions

The synthesis of metal ion adsorbents from modified banana fibers comprising a Schiff base was investigated. The adsorption properties of a metal ion adsorbent were evaluated, and banana fiber (BF)-g-(glycidyl methacrylate)(GMA)(BF-GMA) were synthesized. A Schiff base 2,2’-{iminobis[ethane-2,1-diylnitrilo(E)methylidine]}bisphenol was synthesized via a reaction of diethylenetriamine and 5-bromo-salicyialdehyde in methanol. A metal ion adsorbent (BF-GMA(Br)) was synthesized via a reaction of the Schiff base with BF-GMA. Over 90% of La3+, Gd3+, Tm3+, Pr3+, Tb3+, Yb3+, Nd3+, Dy3+, Lu3+, Sm3+, Ho3+, Eu3+, and Er3+ were adsorbed in a pH range of 6.0–6.6 by the synthesized metal ion adsorbent. Almost 100% of In3+ at a pH value of 3.0–4.0 and 95% of Ga3+ at a pH value of 3.9 were absorbed, thereby showing the maximum adsorption percentage. Almost 15% and 25% of In3+ was adsorbed at a pH value of 1.4–1.6 and 1.8, respectively. Adsorption of Ga3+ was barely observed in the pH range of 1.4–1.8. These results show that banana fibers can be utilized as metal ion adsorbents.

Hierarchical flowerlike magnesium oxide hollow spheres with extremely high surface area for adsorption and catalysis

3D hierarchical flowerlike MgO hollow spheres with extremely high surface area showed excellent adsorption properties for heavy metal ions and catalytic properties for the Claisen–Schmidt condensation reaction.