Adsorption of Cu2+ Cations from Aqueous Solution by S-doped TiO2

S-doped TiO₂ as a novel adsorbent for Cu²⁺ cations removal from aqueous solutions was synthesized by simple sol-gel process. Removal of Cu²⁺ cations from aqueous solutions was investigated with particular reference to the effects of initial Cu²⁺ cations concentration, pH-value, adsorbent dosage, and temperature on adsorption. It was found that the maximum adsorption capacity was 96.35 mg g^?1 at 328 K. The adsorption equilibrium isotherms and the kinetic data were well described by the Langmuir and pseudo-second-order kinetic models, respectively. The high uptake capability of S-doped TiO₂ makes it a potentially attractive adsorbent for the removal of heavy metal pollutants from aqueous solution.     

Preparation of amine-modified SiO<Subscript>2</Subscript> aerogel from rice husk ash for CO<Subscript>2</Subscript> adsorption

Amine-modified SiO₂ aerogel was prepared using 3-(aminopropyl)triethoxysilane (APTES) as the modification agent and rice husk ash as silicon source, its CO₂ adsorption performance was investigated. The amine-modified SiO₂ aerogel remains porous, the specific surface area is 654.24 m²/g, the pore volume is 2.72 cm³/g and the pore diameter is 12.38 nm. The amine-modified aerogel, whose N content is up to 3.02 mmol/g, can stay stable below the temperature of 300 °C. In the static adsorption experiment, amine-modified SiO₂ aerogel (AMSA) showed the highest CO₂ adsorption capacity of 52.40 cm³/g. A simulation was promoted to distinguish the adsorption between the physical process and chemical process. It is observed that the chemical adsorption mainly occurs at the beginning, while the physical adsorption affects the entire adsorption process. Meanwhile, AMSA also exhibits excellent CO₂ adsorption–desorption performance. The CO₂ adsorption capacity dropped less than 10 % after ten times of adsorption–desorption cycles. As a result, AMSA with rice husk ash as raw material is a promising CO₂ sorbent with high adsorption capacity and stable recycle performance and will have a broad application prospect for exhaust emission in higher temperature.     

Adsorption of Cu2+ and Ni2+ from Aqueous Solution by Arabinoxylan Hydrogel: Equilibrium,Kinetic, Competitive Adsorption

In this work, arabinoxylan-graft-acrylic acid (AX-g-AA) hydrogel was prepared and used as an adsorbent to remove and recover Cu²⁺ and Ni²⁺ from aqueous solutions. The influences of pH, ligand content on the adsorption capacity of the hydrogel, adsorption equilibrium, and kinetic were studied in detail. The competitive adsorption and recovery of heavy metal ions, regeneration and reusability of the hydrogel were present. Furthermore, the relationship between the physiochemical properties of the adsorbent and its adsorption performance was also studied. The results showed that a more expanded network favored the diffusion and adsorption of metal ions. Cu²⁺ and Ni²⁺ uptake by this hydrogel was pH and concentration dependent with the maximum loading of 330.1 mg/g for Cu²⁺ and 248.7 mg/g for Ni²⁺. The pseudo-second-order kinetics suggested that the ion exchange process was chemisorption-controlled. The Langmuir equation could well describe the isotherm data. Cu²⁺ and Ni²⁺ adsorbed on the hydrogel could be effectively recovered in a diluted HNO₃ solution (0.01 M) in 30 min. AX-g-AA hydrogel also exhibited highly efficient reusability, and thus could be used repeatedly.     

Supporting of Potassium Copper Hexacyanoferrate on Porous Activated Carbon Substrate for Cesium Separation

Abstract

Activated carbon (AC) was chosen for carrying potassium copper hexacyanoferrate (KCuCF) to prepare a new Cs-selective material. The adsorbent was prepared by repetitious batch precipitation reaction of Cu²⁺ with [Fe(CN)₆]^4?. Characterization analyses identified the backbone formula of the activated carbon-supported KCuCF with K₂Cu[Fe(CN)₆], and shown the successful loading of KCuCF microcrystals on the porous carbon substrate with a loading percentage of 12.5 wt%, and with BET specific surface area and a total pore volume of 527.8 m²/g and 0.38 cm³/g for AC and 160.8 m²/g and 0.16 cm³/g for KCuCF-loaded activated carbon (KCuCF-AC), respectively. The optimal Cs adsorption capacity of about 0.46 mmol·g^?1 was observed between the HNO₃ concentration of 0.5–1.5 M, and the capacity even reached to 0.38 mmol·g^?1 in the presence of a large amount of competing cations. The results indicate that activated carbon has no significant effect on the selectivity of the as-synthesized composite.     

A novel starch-enhanced melamine-formaldehyde aerogel with low volume shrinkage and high toughness

A new hybrid aerogel with low volume shrinkage and high toughness was prepared by aqueous condensation of melamine, starch and formaldehyde and extracting the solvent from the wet gel with carbon dioxide under the supercritical condition. The sizes of the wet gel and aerogel were measured by vernier calipers and the volume shrinkage of the aerogel was as low as 1.8%. The results from BET and SEM indicated that the hybrid aerogel belonged to the mesoporous material, which showed a typical three-dimensional porous structure with a specific surface area about 366.2 m²/g and pore diameter about 12.9 nm. To study the mechanical properties of the hybrid aerogel, we measured the compressive stress as a function of strain and the hybrid aerogel demonstrated excellent elasticity and mechanical durability.     

Removal of metal ions using lignite in aqueous solution—Low cost biosorbents

Turkish lignite can be used as a new adsorption material for removing some toxic metals from aqueous solution. The adsorption of lignite (brown young coals) to remove copper (Cu²⁺), lead (Pb²⁺), and nickel (Ni²⁺) from aqueous solutions was studied as a function of pH, contact time, metal concentration and temperature. Adsorption equilibrium was achieved between 40 and 70 min for all studied cations except Pb²⁺, which is between 10 and 30 min. The adsorption capacities are 17.8 mg/g for Cu²⁺, 56.7 mg/g for Pb²⁺, 13.0 mg/g for Ni²⁺ for BC₁ (Ilg?n lignite) and 18.9 mg/g for Cu²⁺, 68.5 mg/g for Pb²⁺, 12.0 mg/g for Ni²⁺ for BC₂ (Beysehir lignite) and 7.2 mg/g for Cu²⁺, 62.3 mg/g for Pb²⁺, 5.4 mg/g for Ni²⁺ for AC (activated carbon). More than 67% of studied cations were removed by BC₁ and 60% BC₂, respectively from aqueous solution in single step. Whereas about 30% of studied cations except Pb²⁺, which is 90%, were removed by activated carbon. Effective removal of metal ions was demonstrated at pH values of 3.8–5.5. The adsorption isotherms were measured at 20 °C, using adsorptive solutions at the optimum pH value to determine the adsorption capacity. The Langmuir adsorption isotherm was used to describe observed sorption phenomena. The rise in temperature caused a slight decrease in the value of the equilibrium constant (K_c) for the sorption of metal ions. The mechanism for cations removal by the lignite includes ion exchange, complexation and sorption. The process is very efficient especially in the case of low concentrations of pollutants in aqueous solution, where common methods are either economically unfavorable or technically complicated.     

Separation and Recovery of Silver(I) Ions from Base Metal Ions by Melamine‐formaldehyde‐thiourea (MFT) Chelating Resin

Abstract

Melamine‐formaldehyde‐thiourea (MFT) chelating resin were prepared using melamine (2,4,6‐triamino‐1,3,5‐triazine), formaldehyde, and thiourea and this resin has been used for separation and recovery of silver(I) ions from copper(II) and zinc(II) base metals and calcium(II) alkaline‐earth metal in aqueous solution. The MFT chelating resin was characterized by elemental analysis and FT‐IR spectra. The effect of pH, adsorption capacity, and equilibrium time by batch method and adsorption, elution, flow rate, column capacity, and recovery by column method were studied. The maximum uptake values of MFT resin were found as 60.05 mg Ag⁺/g by batch method and 11.08 mg Ag⁺/g, 0.052 mg Zn²⁺/g, 0.083 mg Cu²⁺/g and 0.020 mg Ca²⁺/g by column method. It was seen that MFT resin showed higher uptake behavior for silver(I) ions than base and earth metals due to chelation.     

Magnetic carbon aerogel pyrolysis from sodium carboxymethyl cellulose/sodium montmorillonite composite aerogel for removal of organic contamination

In this paper, we present a facile approach for the synthesis of polysaccharide-based carbon aerogel by sol–gel processing, freeze-drying, and pyrolysis of a sodium carboxymethyl cellulose/sodium montmorillonite composite aerogel. The as-prepared carbon aerogel was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffractometry, nitrogen adsorption measurements, and vibrating sample magnetometry. The carbon aerogel obtained in this study possessed low density (0.064?±?0.0029 g/cm³), a high surface area (185 m²/g), and flame retardance. Measurements of the magnetic properties indicated that the carbon aerogel exhibited typical ferromagnetic characteristic at room temperature. The absorption capacity of the carbon aerogel for oils and organic solvents is as much as 10–20 times its own weight. Moreover, a method of combustion could be employed to recycle the carbon aerogel. The results imply that the carbon aerogel is a potential cost-effective adsorbent for oil and organic pollutants from aqueous solutions in environmental pollution cleanup.     

Reusability and selective adsorption of Pb<Superscript>2+</Superscript> on chitosan/P(2-acrylamido-2-methyl-1-propanesulfonic acid-<Emphasis Type="Italic">co</Emphasis>-acrylic acid) hydrogel

Reusability and selective adsorption toward Pb²⁺ with the coexistence of Cd²⁺, Co²⁺, Cu²⁺ and Ni²⁺ ions on chitosan/P(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylic acid) [CS/P(AMPS-co-AA)] hydrogel, a multi-functionalized adsorbent containing –NH₂, –OH, –COOH and –SO₃H groups was studied. The CS/P(AMPS-co-AA) was prepared in aqueous solution by a simple one-step procedure using glow discharge electrolysis plasma technique. The reusability of adsorbent in HNO₃, EDTA-2Na and EDTA-4Na was investigated in detail. The competitive adsorption of the metal ions at the initial stage was compared between their equal mass concentration and equal molar concentration. In addition, the adsorption mechanism of the adsorbent for adsorption of Pb²⁺ was also analyzed by XPS. The results showed that the optimum pH of adsorption was 4.8, and time of adsorption equilibrium was about 180 min. Adsorption kinetics fitted well in the pseudo second-order model. The equilibrium adsorption capacities of Pb²⁺, Cd²⁺, Co²⁺, Cu²⁺, and Ni²⁺ at pH 4.8 were obtained as 673.3, 358.3, 176.7, 235.0 and 171.7 mg g^?1, in their given order. The adsorbent displayed an excellent reusability using 0.015 mol L^?1 EDTA-4Na solution as the eluent, and the desorption ratio could not correctly reflect the true characteristics of adsorption/desorption process. Moreover, the adsorbent showed good adsorption selectivity for Pb²⁺. The molar adsorption capacity at the initial stage with equal molar concentration was more reliable than the mass adsorption capacity during the study of selective adsorption. According to the XPS results, the adsorption of Pb²⁺ ions by the CS/P(AMPS-co-AA) absorbent could be attributed to the coordination between N atom and Pb²⁺ and ion-exchange between Na⁺ and Pb²⁺.     

Peanut Shell Activated Carbon： Characterization,Surface Modification and Adsorption of Pb^2＋ from Aqueous Solution

Metal ion contamination of drinking water and waste water, especially with heavy metal ion such as lead, is a serious and ongoing problem. In this work, activated carbon prepared from peanut shell （PAC） was used for the removal of Pb^2＋ from aqueous solution. The impacts of the Pb25 adsorption capacities of the acid-modified carbons oxidized with HNO3 were also investigated. The surface functional groups of PAC were confirmed by Fourier transform infrared （FTIR） spectroscopy, X-ray photoelectron spectroscopy （XPS）, Boehm titration. The textural properties （surface area, total pore volume） were evaluated from the nitrogen adsorption isotherm at 77 K. The experimental results presented indicated that the adsorption data fitted better with the Langmuir adsorption model. A comparative study with a commercial granular activated carbon （GAC） showed that PAC was 10.3 times more efficient compared to GAC based on Langmuir maximum adsorption capacity. Further analysis results by the Langmuir equation showed that HNO3 [20% （by mass）] modified PAC has larger adsorption capacity of Pb^2＋ from aqueous solution （as much as 35.5 mg·g^-1）. The adsorption capacity enhancement ascribed to pore widening, increased cation-exchange capacity by oxygen groups, and the promoted hydrophilicity of the carbon surface.     

Adsorption and desorption properties of the chelating membranes prepared from the PE films

The chelating membranes for adsorption of metal ions were prepared by the photografting of glycidyl methacrylate (GMA) onto a polyethylene (PE) film and the subsequent modification of the resultant GMA‐grafted PE (PE‐g‐PGMA) films with disodium iminodiacetate in an aqueous solution of 55% DMSO at 80°C. The adsorption and desorption properties of the iminodiacetate (IDA) group‐appended PE‐g‐PGMA (IDA‐(PE‐g‐PGMA)) films to Cu²⁺ ions were investigated as functions of the grafted amount, pH value, Cu²⁺ ion concentration, and temperature. The amount of adsorbed Cu²⁺ ions increased with an increase in the pH value in the range of 1.0–5.0. The time required to reach the equilibrium adsorption decreased with an increase in the temperature, although the degree of adsorption stayed almost constant. The amount of Cu²⁺ ions desorbed from the (IDA‐(PE‐g‐PGMA)) films increased and the time required to reach the equilibrium desorption decreased with an increase in the HCl concentration. About 100% of Cu²⁺ ions were desorbed in the aqueous HCl solutions of more than 0.5M. The amounts of adsorbed and desorbed Cu²⁺ ions were almost the same in each cyclic process of adsorption in a CuCl₂ buffer at pH 5.0 and desorption in an aqueous 1.0M HCl solution. These results indicate that the IDA‐(PE‐g‐PGMA) films can be applied to a repeatedly generative chelating membrane for adsorption and desorption of metal ions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99:1895–1902, 2006     

Preparation of Activated Carbons from Tobacco Stems by Potassium Hydroxide Activation and Phosphine Adsorption

Activated carbon preparation from tobacco stems by KOH activation at different activation temperatures and KOH/char mass ratios were investigated in this study. The effects of preparation parameters on activated carbon pore structure, morphometrics, microcrystallinities, and surface functional groups were characterized by N₂ adsorption, SEM, XRD, and FTIR technologies, respectively. The optimum preparation condition of activated carbon was activation temperature of 850°C, and KOH/char mass ratio of 2. Under this condition, the BET surface area of 2215 m²/g, and the pore volume of 1.343 cm³/g can be obtained. Prepared activated carbon showed clearly honeycomb holes, and a predominated amorphous structure. With increase of activation temperature and KOH/char mass ratio, decrease of surface oxygen functional group, and aromatization of the carbon structure was found. The activated carbon was subject to PH₃ purification, and the maximum PH₃ adsorption capacity of 253 mg/g can be realized based on well prepared KOH-AC with modification of 2.5% Cu. It seems that the activated carbon produced from chemical activation of tobacco stem would be an effective and alternative adsorbent for PH₃ adsorption because of its high surface area, adsorption capacity, and low cost.     

测试——聚乙烯亚胺改性磁性膨润土对Pb2+和Cu2+的吸附性能(测试稿件)

在磁性膨润土(MBent)表面接枝聚乙烯亚胺(PEI)制备了聚乙烯亚胺改性磁性膨润土(PEI/KH560/MBent),采用FTIR、VSM、XRD、TGA、EA、SEM和EDS对其进行了表征,考察了其对水溶液中Pb_²⁺和Cu_²⁺的吸附性能。结果表明,聚乙烯亚胺已成功接枝于磁性膨润土表面,并有效提高其对Pb_²⁺和Cu_²⁺吸附量；溶液初始pH对吸附量影响较大,随着pH的增大,吸附量增加。在pH=5,溶液初始质量浓度为300 mg/L,PEI/KH560/MBent对Pb_²⁺和Cu_²⁺吸附量分别为96.21和61.08 mg/g；吸附过程符合准二级动力学模型,吸附行为符合Langmuir吸附等温模型。热力学研究表明,吸附为自发吸热过程。经过5次循环利用后,其吸附容量仍保持初始的60%以上,表明PEI/KH560/MBent具有一定的重复利用性。     

Preparation of carbon aerogels with different pore structures and their fixed bed adsorption properties for dye removal

Controlled porosity carbon aerogels are prepared by a sol–gel polymerization method and the prepared materials are used as fixed bed adsorbents for dye removal. The influences of operating conditions and preparation factors on the adsorption of C. I. Reactive Red 2, as a model compound, from aqueous solution were investigated. Many column parameters were estimated at different stages and the Bed-Depth-Service-Time model was used to analyze the experimental data. The results showed that the adsorption bed capacity increased with increasing bed height, decreasing liquid flow rate and decreasing initial dye concentration. Moreover, the work indicated that the adsorption ability of the carbon aerogel could be controlled by adjusting the molar ratio of resorcinol to surfactant and carbonization conditions. In addition, the adsorption capacity could be improved when the carbon aerogel was activated by CO₂. The reasons for the difference in adsorption ability could be related to the different pore structure characteristics of various samples.     

The efficient removal of organic pollutant over magnetic mesoporous polymer

Nanoporous polymers due to their abundant pores and organic frameworks are very suitable for the adsorption of organic pollutants in aqueous solution. However, the difficulty in separation and recovery limit their widely practical applications. Herein, polydivinylbenzene was first synthesized by solvothermal method, then magnetic Fe₃O₄ precursors were incorporated into the polymer framework to obtain magnetic polydivinylbenzene, designated as Fe₃O₄/PDVB. The as-prepared magnetic mesoporous polymer possessed rich porous structure, high specific area of 444.7 m²/g, pore size of 8.9 nm and pore volume of 0.82 cm³/g, which were fully verified by various characterization techniques. Noticeably, Fe₃O₄/PDVB showed efficient and fast adsorption rate to dye Rhodamine B, and the adsorption capacity reached 85.81 mg/g within 15 min. Moreover, the magnetic polymer can be easily separated by external magnetic field after adsorption and facilely recovered by ethanol extraction, which benefit its large-scale applications in the environmental protection, especially in the chemical accident remediation.     

Evaluating Cu(II) Removal From Aqueous Solutions with Response Surface Methodology by Using Novel Synthesized CaCO3 Nanoparticles Prepared in a Colloidal Gas Aphron System

Removal of heavy metals from water and wastewaters has recently gained a great deal of attention due to their serious environmental problems. In this study, novel synthesized calcium carbonate nanoparticles, prepared in a colloidal gas aphron (CGA) system, were used as adsorbents for the removal of Cu²⁺ ions from aqueous solutions under different conditions. A developed pseudo-second-order (PSO) model well described the adsorption kinetics of the process. Langmuir and Freundlich adsorption isotherms have been examined and the maximum adsorption capacity from the Langmuir isotherm equation was found to be 666.67?mg Cu/g adsorbent. The effects of temperature, Cu²⁺ initial concentration, and CaCO₃ dosage on the removal capacity were also investigated using the three-level Box–Behnken experimental design method. The response surface modeling results demonstrated that under certain experimental conditions (i.e., T?=?26°C, [Cu²⁺]?=?200?mg/L, and [CaCO₃]?=?0.5?g/L), maximum removal capacity value (393.52?mg/g) was achieved.     

Preparation and characterization of magnetic carbon aerogel from pyrolysis of sodium carboxymethyl cellulose aerogel crosslinked by iron trichloride

Multifunctional carbon aerogel was prepared by pyrolysis of sodium carboxymethyl cellulose aerogel after sol–gel processing and freeze-drying. Ferric iron and D-(+)-gluconic acid-lactone were used as cross-linker and releasing agent, respectively. The as-prepared carbon aerogel was characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffractometry, thermogravimetric analysis, nitrogen adsorption measurements, vibrating sample magnetometry and water contact angle tests. The carbon aerogel had a high specific surface area of 742.34 m²/g and contained mostly mesopores. Its density was as low as 0.062 g/cm³. The carbon aerogel morphology has an uneven three-dimensional structure and presented a type-IV adsorption isotherm. Carbonization resulted in the destruction of the carboxymethyl cellulose crystalline structure, damage to the oxygen-containing functional groups and provided the carbon aerogel with good magnetic properties. The carbon aerogel displayed good hydrophobicity and flame retardance, which has potential application in adsorption, hydrophobic materials and fireproofing.     

Cu2+ removal from aqueous solution by modified chitosan hydrogels

BACKGROUND: The adsorption of Cu²⁺ from aqueous solution using crosslinked chitosan hydrogels impregnated with Congo Red (CR) by ion‐imprint technology was systematically investigated with particular reference to the effects of contact time, pH, and initial concentration on adsorption. RESULTS: The adsorption capacity of the crosslinked chitosan without impregnation was only 68.68 mg g^?1 for Cu²⁺. However, the adsorption capacity increased from 77.42 (without imprint ion) to 84.54 mg g^?1 (imprint ion content 0.5 mmol) after the chitosan was impregnated with a ratio of 1/12 of CR to chitosan. The as‐prepared adsorbents were found to be pH‐dependent and the process of adsorption agreed well with the Freundlich isotherm. The loaded adsorbents could be regenerated and reused without the appreciable loss of capacity. CONCLUSION: Chitosan hydrogels impregnated with CR showed higher Cu²⁺ adsorption capacities compared with those prepared conventionally without imprint ion, and thus developed a good approach to increase Cu²⁺ adsorption efficiency in the treatment of waste‐water. Copyright © 2012 Society of Chemical Industry     

Adsorption of Pb2+, Cu2+ and Co2+ by polypropylene fabric and polyethylene hollow fiber modified by radiation-induced graft copolymerization

Cation-exchange adsorbents were prepared by radiation-induced grafting of glycidyl methacrylate (GMA) onto polypropylene (PP) fabric and polyethylene (PE) hollow fiber and subsequent phosphonation of epoxy groups of poly(GMA) graft chains. The adsorption characteristics of Pb²⁺, Cu²⁺ and Co²⁺ for the two cation-exchange adsorbents were studied. In the grafting of GMA onto PP fabric, the degree of grafting (%) increased with an increase in reaction time, reaction temperature, and pre-irradiation dose. The maximum grafting yield was observed around 60% GMA concentration. In 50, 130 and 250% GMA-grafted PP fabric, the content of phosphoric acid was 1.52, 3.40 and 4.50 mmol/g at 80 °C in the 85 % phosphoric acid aqueous solution for 24 h, respectively. The adsorption of Pb²⁺, Cu²⁺ and Co²⁺ by PP fabric adsorbent was enhanced with an increased phosphoric acid content The order of adsorption capacity of the PP fabric adsorbent was Pb²⁺>Co²⁺>Cu²⁺. In adsorption of Pb²⁺, Cu²⁺ and Co²⁺ by PE hollow fiber, the amount of Pb²⁺ adsorbed by the PE hollow fiber adsorbent containing 1.21 mmol/g of -PO₃H wasca. 54.4 g per kg. The adsorption amount of Cu²⁺ and Co²⁺ in the same PE hollow fiber wasca. 21.0 g per kg andca. 32.1 g per kg, respectively. The order of adsorption of the PE hollow fiber adsorbent was Pb²⁺>Co²⁺>Cu²⁺.     

Effect of Preparation Conditions on the Property Cu/AC Adsorbents for Phosphine Adsorption

Copper-based activated carbon adsorbents (Cu/AC) were prepared and used to investigate the effects of various copper precursors, impregnation solution concentration, and calcination temperature on phosphine (PH₃) adsorption removal from yellow phosphorus tail gas. N₂ adsorption isotherm and X-ray Diffraction (XRD) were used for characterizing the Cu/AC adsorbents. It can be seen that the Cu(N)/AC adsorbent prepared from the Cu(NO₃)₂ precursor has higher PH₃ breakthrough adsorption capacity than other three adsorbent because the surface copper status of it is mainly CuO. Fresh activated carbon requires an optimal impregnation solution concentration (0.05 mol/L) to reach this optimal PH₃ breakthrough adsorption capacity (78.62 mg/g). The result shows that the surface chemical characteristics (Cu content) of activated carbon is more important than the physical ones (specific surface or pore volume) for the PH₃ adsorption performance. When the calcination temperature is 350°C, the Cu(N)/AC adsorbent has the biggest PH₃ breakthrough adsorbed amount of 112.38 mg/g. The present study confirmed that the Cu/AC adsorbents would be one of the candidates for PH₃ adsorption removal from yellow phosphorus tail gas.