Adsorption Isotherm Curves Research Articles

Enhanced adsorption of Orange II on bagasse-derived biochar by direct addition of CTAB

Surface charge properties of an adsorbent always play an important role for the removal of contaminants from water. A cationic surfactant hexadecyl tri-methyl ammonium bromide (CTAB) was involved into adsorptive removal of Orange II (ORII) by bagasse biochars to realize an in-situ modification and an enhanced adsorption capability. Adsorption capacity of biochar (BC600) improved significantly from 1.66 mg/g in the absence of CTAB to 4.42 mg/g in the presence of 2.0 mg/L CTAB. A more hydrophobic surface of bagasse biochar was favorable for the dye uptake in the presence of CTAB. Linear pseudo-second-order kinetic model fitted the kinetics data better at three pH conditions than pseudo-first-order kinetic model, whether in the presence and absence of CTAB. Both nonlinear pseudo-first-order and pseudo-second-order kinetic models were suitable to describe the experimental data. The maximal adsorption capacity in the absence of CTAB was very limited (41.4 mg/g), while the adsorption isotherm curve in the presence of CTAB was almost linear, indicating a strong adsorption capability due to the introduction of CTAB. Direct addition of CTAB into wastewater is a potential technique for the enhanced removal of negatively-charged pollutants by bagasse biochar.

Seed bank longevity and germination ecology of invasive and native grass species from Mediterranean wetlands

Most Mediterranean wetlands are overexploited and degraded by different factors and are especially vulnerable to alien and invasive species. Currently, there is still a lack of knowledge on certain aspects of plant invasions, especially from an applied point of view, as key tools in restoration and conservation programmes. In this sense, Cortaderia selloana has showed a rapid expansion in the western Mediterranean Basin, threatening habitats of special interest such as wetlands. A comparative study between C. selloana and Saccharum ravennae, a representative plant of Mediterranean marshlands, was carried out to characterize the biological pattern that has made C. selloana a potential threat. The role of these plant species in soil seed banks and the potential competitive displacement of S. ravennae were evaluated through seed germination and viability tests under accelerated ageing conditions and seedling root development, considering seed morphological characteristics, seed maturity and plant sexuality as influential factors. Additionally, the potential allelopathic effects of C. selloana leaves on S. ravennae seed germination were evaluated. The study found differences between the species in all the tested parameters, seed morphology, germination response and longevity, adsorption isotherm curves, and allelopathic effects. All these factors establish a complex interaction that contributes to the proven invasive character of C. selloana. These results provide useful data for eradication and management programmes.

Risk assessment of dynamic disasters induced by gas injection displacement in coal seams

Gas injection displacement technology not only helps to increase gas production, but also serves to store greenhouse gases by reducing CO2 emissions. However, coal seam dynamic disasters caused by gas (CO2) injection displacement are rarely reported. In order to explore the change in coal seam outburst risk after gas injection, the indexes (ΔP) of initial velocity of gas diffusion and the isothermal adsorption curves of coal with different metamorphic degrees were experimentally measured under different concentration ratios of CO2/CH4 mixed gas, and the experimental process of gas injection displacement and the change in pore structure were also investigated. The experiment results indicate that the ΔP of CO2 is nearly three times of that of CH4. Therefore, the accuracy of safety situation judgment of a coal seam would be reduced if outburst risk of the coal seam is only assessed through the ΔP in the pure CH4 environment. The results of isothermal adsorption experiment show that the gas adsorption capacity of three kinds of coal increases with the rise of the CO2 concentration in the mixed gas. The results of gas injection displacement experiment reveal that CO2 plays a significant role in replacing CH4, but a large amount of CO2 has been adsorbed into the coal seam before CH4 can be completely desorbed, which in turn raises outburst risk of the coal seam. After the displacement, the internal pore structures of the three kinds of coal samples become tinier and tinier. Finally, a new comprehensive index K′=αBET×ΔP/f was established to assess the risk of dynamic disasters in a coal seam. Compared with the single index ΔP, the new comprehensive index K′ can more accurately grasp the possibility of outburst danger in a coal seam subjected to gas injection displacement.

Adsorption of Dyes Using Multi-walled Carbon Nanotube Hydrogel

The hydrophilic multi-walled carbon nanotube(MWCNT) hydrogel was prepared using acrylic acid, acrylamide and hydrophilic MWCNT. The orthogonal experiment was applied to optimize the synthetic conditions. The MWCNT hydrogel was characterized by Fourier transform infrared spectrophotometer(FTIR) and scanning electron microscopy(SEM) analysis. The MWCNT hydrogel was used as the adsorbent to adsorb water-soluble cationic dye. This study evaluated the adsorption performance of hydrogels on four dyes of safranine T, crystal violet, malachite green and methylene blue in water. The effects of the amount of hydrogel, the size of hydrogel, pH, and the temperature on the adsorption performance were investigated. The adsorption kinetic and adsorption isotherm curves were measured. The experimental results show that the MWCNT hydrogel can be easily separated from water and the adsorption capacity is much greater compared to the hydrogel without MWCNT. The MWCNT hydrogels can be used in wastewater treatment with a great potential.

Prediction Model for Isothermal Adsorption Curves Based on Adsorption Potential Theory and Adsorption Behaviors of Methane on Granular Coal

In this study, adsorption tests of methane on granular coal were performed to investigate the effects of various factors (temperature, particle size, pressure, and water content) on the adsorption ...

One pot-economical fabrication of molecularly imprinted membrane employing carbon nanospheres sol coagulation bath with specific separation and advanced antifouling performances

In this paper, one pot-economical approach for in-situ modification of enoxacin (EXC) imprinted nanocomposite membrane was developed by employing carbon nanosphere sol (CNS) as coagulation bath during phase inversion process for increasingly antifouling property. Using polydopamine (pDA) as hydrophilic coating layer and second-reaction platform, the CNS based biomimetic molecularly imprinted nanocomposite membranes (CBMIMs) were then obtained which used EXC as the target molecule for specific adsorption and separation EXC from the sewage. The imprinting polymer layer was highly hydrophilic, which advanced antifouling performance to avoid the adsorption of contaminant protein. Using EXC as template, the optimal fabrication conditions on performances and properties of CBMIMs were studied. Isothermal adsorption curves, adsorption kinetics and rebinding selectivity were investigated to well explain the selectivity adsorption mechanism of imprinted membranes on EXC. The CBMIMs presented highly selective specificity, short kinetic equilibrium time and preferable rebinding capacity for EXC. The rebinding ability and imprinted factor (β) of CBMIMs could reach 32.10 mg g−1 and 3.15, respectively, which presented various highly favorable features, including high rebinding strength and excellent selective performance. Furthermore, the adsorption of actual environmental sewage by CBMIMs was investigated, which highlighted the advantage of CBMIMs. Several favorable performances of CBMIMs also provided further application for sewage treatment in the future.

Understanding HCN heterogeneous adsorption on CaO(1 0 0) surface for the pyrolysis of sludge: A first-principles study and GCMC simulation

Pyrolysis and combustion of the sludge can eliminate the secondary pollution, and achieve the recovery of resource. However, it will produce large amount of nitrogen oxide. HCN is one of the major NOx precursors, and calcium oxide, the dehydrating agent and desulfurizer, have positive effect for HCN emission. Thus, the HCN on the perfect and defect CaO(1 0 0) surface heterogeneous adsorption is investigated through a first-principles calculation and Grand Canonical Monte Carlo simulation. Compared with single molecular model in our previous work, the CaO slab model with periodical boundary conditions has a better effect on describing the solid phase surface. The adsorption of HCN molecule on perfect CaO(1 0 0) surface proves to be chemisorption, and the most stable configuration is H-downward at O top site with the binding energy of −1.09 eV. Furthermore, HCN molecule adsorbs more tightly on to the O vacancy and Ca vacancy surface, and the binding energy increases to −2.76 eV and −2.79 eV, respectively. With the electron population and PDOS analysis, it reveals that the more electrons transfer from defect calcium oxide surface to HCN molecule. In the GCMC results, it is obvious that multiple HCN molecules easily adsorb on Ca vacancy surface. Meanwhile, HCN molecules are inclined to adsorb on the hollow site of the defect CaO(1 0 0) surface. From the isothermal adsorption curves, the adsorption ability decreases with the temperature increasing, and the fitted equation is coincide with the Freundlich isothermal adsorption equation. The variation of adsorption equilibrium constant K in the fitted equation indicates that the Ca vacancy surface is the most activity surface and the temperature increase is negative for the HCN adsorption, conforming to the adsorption rule calculated in the first-principle study. Thus, the point vacancy surface caused by uneven temperature can increase the adsorption ability of HCN on CaO(1 0 0) surface.

Use of Simplified Surface Tension Measurements To Determine Surface Excess: An Undergraduate Experiment

The effect of surfactant adsorption on surface tension, as well as associated thermodynamic concepts are introduced in a laboratory experiment designed for undergraduate students. Using a reliable and accessible method, students measure the surface tension of aqueous solutions at different concentrations of sodium dodecyl sulfate. Students collect data to estimate the critical micelle concentration and quantitatively determine the maximum surface excess using the Gibbs adsorption equation. Students subsequently determine the surface area per molecule of this surfactant at the liquid–air interface and learn how to generate adsorption isotherm curves.

Investigation of caffeine taste mechanism through a statistical physics modeling of caffeine dose-taste response curve by a biological putative caffeine adsorption process in electrophysiological response

This work is a contribution to understand the taste mechanism of caffeine molecule using a modeling of a putative adsorption process by model expressions established by a statistical physics treatment. A physicochemical and a gustative parts are the main constituents of this work. We start with a physicochemical investigation of the adsorption process of caffeine molecule, as adsorbate in liquid phase, onto β-cyclodextrin as adsorbent. Experimental adsorption isotherm curves of caffeine onto β-cyclodextrin coated onto Quartz crystal are carried out at three different temperatures. The Hill model expression with three parameters n, NM and C1/2, established by statistical physics formalism investigated in part I, is the best fitting model of the experimental data. Thermodynamic potential functions that govern the adsorption process, such as entropy, internal energy and Gibbs free enthalpy are investigated. PSD and AED are derived by a steric and energetic derivatives of the Hill model. In part II the same method of fitting is applied to the taste electrophysiological dose-response curve by a caffeine putative adsorption on gustative nerve in caterpillar. All the physicochemical parameters introduced in the fitting Hill model expression, serve first, to analyze the taste mechanism of the bitter caffeine taste. Secondly, all these stereographic and energetic parameters will be considered henceforward for an objective characterization of caffeine molecule taste in both its two aspects, intensive and qualitative aspects of caffeine taste.

Mechanisms of shale gas adsorption: Evidence from thermodynamics and kinetics study of methane adsorption on shale

Studies on the mechanisms of shale gas adsorption are of great significance for shale gas accumulation and reserves evaluation. In order to investigate the mechanisms of shale gas adsorption from the perspective of methane adsorption thermodynamics and kinetics, high-pressure methane adsorption and adsorption kinetics experiments were measured at 40.6 °C, 60.6 °C, 75.6 °C and 95.6 °C at pressures up to 52 MPa for the Lower Silurian Longmaxi shale sample collected from the Southern Sichuan Basin, China. The adsorption isotherms and kinetics curves of methane were obtained and a detailed analysis was performed. The results indicate that (1) Under the condition of 0–52 MPa, the absolute adsorption isotherm of methane on shale has the characteristics of type I adsorption isotherm. Temperature has an important effect on the maximum excess and absolute adsorption of methane. At the same temperature, the absolute adsorption amount of methane on shale increases slower at a higher pressure, which suggests that the methane adsorption rate decreases at a higher pressure. (2) The average isosteric heat of adsorption of methane on shale is 21.06 kJ/mol, indicating that the dominant adsorption process of methane on shale may be physical adsorption. The isosteric heat of adsorption increases with increasing absolute methane adsorption amount, indicating that the adsorption heat is mainly affected by the interaction between the adsorbed methane molecules. (3) Bangham kinetic model can be used to describe the dynamic adsorption process of methane on shale. Higher temperature and pressure lead to a lower Bangham adsorption rate constant, which makes it more difficult to adsorb methane molecules for shale. This is consistent with the conclusion drawn from the thermodynamics study of absolute adsorption isotherms of methane on shale.

The binary gas sorption in the bituminous coal of the Huaibei Coalfield in China

Knowledge of the gas sorption characteristics of a coal not only helps to explain the mechanism of enhanced coalbed methane recovery but also provides an important basis for simultaneous coal and gas extraction. In consequence, the pure and binary gas excess sorption capacity of methane, carbon dioxide, and nitrogen of bituminous coal samples derived from the Xutuan Coal Mine in Huaibei coalfield, in Anhui Province in China, was measured using the volumetric method. The fitting analysis of the pure gas Langmuir adsorption model was carried out. The binary gas excess sorption measurement showed that the final sorption capacity of bituminous samples was the same no matter what the gas adsorption order of competitive adsorption and displacement adsorption. Hence, coal gas adsorption is physical adsorption, i.e. the different adsorption and desorption process of gas molecules does not affect the final adsorption amount of coal to each component of gas. Using the fitting parameters obtained by the Langmuir equation, the extended Langmuir equation was used to predict the adsorption capacity for each component of the binary gas. The comparison between predicted adsorption capacity and measured adsorption capacity showed that the extended Langmuir equation can better describe the trend of the adsorption isotherm curves of a binary gas under different pressures. The separation coefficient and displacement coefficient were defined from Langmuir adsorption theory. The separation coefficient involves the proportion of each component in the free phase and the proportion of each component in the adsorption phase. The displacement coefficient involves the displacement ability of gas molecules at adsorption sites by free gas molecules.

Selective recognition and fast enrichment of anthocyanins by dummy molecularly imprinted magnetic nanoparticles

In this work, a novel synthetic strategy for separation media which quickly specific recognize anthocyanins was developed by dummy molecular imprinting technology and magnetic separation technology. Rutin, similar to cyanidin-3-O-rutinoside structure, was selected as a virtual template. The appropriate molecular imprinting system was selected by NIP library screening method, 4-vinyl pyridine and acetonitrile as functional monomer and solvent respectively. The molecularly imprinted layer was formed on the surface of the magnetic carrier to prepare dummy magnetic molecularly imprinted polymers. The microstructures of prepared composites were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TGA) and X-ray diffraction (XRD). Adsorption kinetics, isothermal adsorption curves and selective specificity were investigated to further reveal the specific recognition mechanism of the adsorbents on anthocyanins. The dummy molecularly imprinted polymers showed a short kinetic equilibrium time, high selectivity (comparing with quercetin and naringin), and satisfying adsorption capacity for anthocyanins. The binding capacity and the imprinting factor of dummy molecularly imprinted polymers can reach 15.69 mg g−1 and 2.05, respectively. In addition, the purity of cyanidin-3-O-rutinoside in the crude extract from Lonicera caerulea increased to 89% after the adsorption of dummy magnetic molecular imprinted polymers. Many favorable capabilities of the prepared molecularly imprinted polymers also provided the basis for further application for separation and purification of anthocyanins in the future.

Experimental study of hygroscopic equilibrium and thermodynamic properties of sewage sludge

A good knowledge of the hygroscopic behavior of sewage sludge is one of the keys to a better perception of the subsequent management of Sewage sludge such as drying, storage or other valorization processes. The present work aims to determine, experimentally, the sorption isotherms of sewage sludge generated at the wastewater treatment plant of Marrakesh city in Morocco by using saturated salt solutions method at different temperatures.The obtained results show that, at constant water activity, the increase in temperature leads to a decrease of the sorption curves. Seven statistical models have been used for modeling and analyzing the experimental data of sorption. The Peleg model seems to be the most appropriate to describe the desorption and adsorption isotherm curves of sewage sludge with the highest value of correlation coefficient.Thermodynamic properties of sewage sludge were determined by using the hygroscopic equilibrium data at different temperatures, the net isosteric heat of sorption and entropy of sorption decreased considerably with the increase in equilibrium moisture content. The enthalpy–entropy compensation suggested that the sorption mechanism involved was enthalpy given. Gibbs free energy for adsorption and desorption are respectively 58.87 kJ/mol, and -445.21 kJ/mol, indicating two different processes.

Adsorption of phenol by activated carbon in rotating packed bed: Experiment and modeling

This work assessed the feasibility of adsorbing phenol in aqueous solution by activated carbon in rotating packed bed (RPB). The effects of high gravity factor, liquid spray density and initial phenol concentration on the adsorption amounts of phenol in RPB were experimentally investigated, which was about 1.4 times of that in convention packed bed. Furthermore, the experimental adsorption isotherms curves were simulated by varies of models to assist the selection of important and characteristic parameters for the designing process of RPB, including Langmuir, Freundlich, Elovich, Temkin, Fowler–Guggenheim and Kiselev. Among the above six models, Freundlich model exhibited the best performance with the correlation coefficient (R2) of 0.9970 and average percentage errors (APE) of 0.241. Besides, the intraparticle diffusion model with the correlation coefficient (R2) above 0.990 could well simulate the adsorption performance for variety of operation conditions in RPB. The intraparticle diffusion coefficient (kr) in RPB could be improved 1.48 times of that in convention packed bed at high gravity factor of 44.68.

Molecular dynamics simulations of aluminum nanoparticles adsorbed by ethanol molecules using the ReaxFF reactive force field

The adsorption of ethanol molecules on annealed aluminum nanoparticles was investigated by ReaxFF molecular dynamics simulations. The force field used in this paper has been validated by comparing the obtained adsorption process results with the results of quantum mechanical (QM) calculations. First, the case of single molecule adsorption was investigated and was shown to be a kind of chemisorption. We also simulated the processes of coating the aluminum nanoparticles with ethanol molecules at different temperatures and pressures. The results indicate that two zones were formed around the surface of aluminum nanoparticles by different interactions during the adsorption processes. The radial distribution function plots and isothermal adsorption curves were used to describe the behaviors. The results show that hydrogen-bonding formed by the ethanol molecules in the solution is a constituent of the coating layer and that the adsorption rate is proportional to the pressure. The cycle-coating method is adopted to produce a fully coated particle, and the oxidation resistance test shows that at 300 K, the organic layer can adsorb the oxygen atom rather than decompose by oxidation. Our simulation results are in good agreement with the results of experimental observations.

Adsorption and removal of organophosphorus pesticides from environmental water and soil samples by using magnetic multi-walled carbon nanotubes @ organic framework ZIF-8

In this study, the functional adsorbent metal–organic framework ZIF-8/magnetic multi-walled carbon nanotubes (M-M-ZIF-8) was successfully prepared for removal of eight organophosphorus pesticides from environmental water and soil samples. The M-M-ZIF-8 was characterized and their adsorption capacity was evaluated based on their isothermal adsorption curves. The results suggest that ZIF-8 particles are deposited on the surface of the magnetic MWCNTs via coordination–polymerization strategy. In addition, M-M-ZIF-8 has high static adsorption capacities for organophosphorus pesticides because of its high specific surface area and porous structure. The static adsorption data fit the Freundlich adsorption model better than the Langmuir model. Under the optimized conditions, M-M-ZIF-8 was successfully applied to remove the eight organophosphorus pesticides from environmental water and soil samples. A possible mechanism is valence-electron-driven adsorption by sharing or exchanging of electrons between the organophosphorus pesticide molecules and the vacant active sites of M-M-ZIF-8. Therefore, M-M-ZIF-8 is a promising hybrid adsorbent for adsorption and removal of organic pollutions from the environment.

Physical properties and adsorption kinetics of silica-gel/water for adsorption chillers

The choice of a suitable adsorbate/adsorbent pair is critical for an adsorption cooling cycle. The surface characteristics and thermo-physical properties of the adsorbent, and the adsorption rate of adsorbate are key parameters in making the choice. Through literature review, it is found that there are disagreements among the experimental measurements and various equations/models used to describe adsorption isotherms and surface diffusivity of water in silica-gel. In this work, an experimental set-up is built to measure the isotherms and kinetics of vapor adsorption for any working pair. Using the newly measured data, those from the manufacturers and from the literature, these inconsistencies are eliminated by utilizing the Dubinin-Astakhov (D-A) model to fit the entire adsorption isotherm curve. The Brunauer-Emmett-Teller(BET) method is used to calculate the surface area, pore volume and pore diameter of two different types of silica-gel. Based on the adsorption rate and the adsorbent temperature measured simultaneously, a new approach is proposed to measure the surface diffusivity in the temperature and pressure ranges typical of those during the operating conditions of adsorption cooling systems. Analysis of the results indicates that the surface diffusivity follows the Arrhenius-form equation. The calculated activation energy at different adsorption conditions varies from 40.0 to 41.2 kJ/mol and the pre-exponential factor varies from 2.5 × 10−4 to 2.8 × 10−4 m2/s. These values are close to those previously reported in the literature. Thus, the proposed approach can be used to measure the surface diffusivity in nanoporous materials.

Heavy metal adsorption of a novel membrane material derived from senescent leaves: Kinetics, equilibrium and thermodynamic studies

Copper pollution around the world has caused serious public health problems recently. The heavy metal adsorption on traditional membranes from wastewater is limited by material properties. Different adsorptive materials are embedded in the membrane matrix and act as the adsorbent for the heavy metal. The carbonized leaf powder has been proven as an effective adsorbent material in removing aqueous Cu(II) because of its relative high specific surface area and inherent beneficial groups such as amine, carboxyl and phosphate after carbonization process. Factors affecting the adsorption of Cu(II) include: adsorbent dosage, initial Cu(II) concentration, solution pH, temperature and duration. The kinetics data fit well with the pseudo-first order kinetics and the pseudo-second order kinetics model. The thermodynamic behavior reveals the endothermic and spontaneous nature of the adsorption. The adsorption isotherm curve fits Sips model well, and the adsorption capacity was determined at 61.77 mg/g. Based on D-R model, the adsorption was predominated by the form of physical adsorption under lower temperatures, while the increased temperature motivated the form of chemical adsorption such as ion-exchange reaction. According to the analysis towards the mechanism, the chemical adsorption process occurs mainly among amine, carbonate, phosphate and copper ions or other surface adsorption. This hypothesis is confirmed by FT-IR test and XRD spectra as well as the predicted parameters calculated based on D-R model.

11B-MAS NMR approach to the boron adsorption mechanism on a glucose-functionalised mesoporous silica matrix

Boron chemistry has raised much interest because, despite the difference between necessities and toxicity being very narrow, it is still widely used in industrial processes. In a previous work we reported an adsorbent for boron extraction from water by the functionalisation of a UVM-7 mesoporous silica matrix with gluconamide moieties. The ability of this material to adsorb boron is based on its well-known affinity for coordinating the cis-diols present in attached saccharide. Although much research on the formation of boron esters with sugars and sugar derivatives in solution has been done, very few reports have dealt with the adsorption mechanism of boron onto functionalised materials in the solid state. Hence an in-depth study of the adsorption process was conducted in this paper. For this purpose, several solids with increasing boron contents were prepared from the gluconamide-functionalised UVM-7 material and an isothermal boron adsorption curve was obtained. The 11B and 13C MAS NMR techniques were used to characterise the obtained solids, and the simulation of the boron NMR and isothermal adsorption combined data was accomplished to enlighten the boron adsorption process. A model with three different coordination environments, two possible adsorption sites and the presence of oligomeric boron species allowed us to reproduce not only the isothermal boron adsorption curve, but also the evolution of the integrated areas for the signals in the 11B MAS NMR spectra obtained for the different boron-containing gluconamide-functionalised UVM-7 materials.

Shale high pressure isothermal adsorption curve and the production dynamic experiments of gas well

The high pressure static adsorption curves of shale samples from Silurian Changning-Weiyuan Longmaxi Formation were tested by using high pressure isothermal adsorption equipment. The physical modeling of depletion production was tested on single cores and multi-core series by using self-developed shale gas fluid-solid coupling experiment system. The adsorption and desorption laws were summarized and a high pressure isothermal adsorption model was established. The calculation formula of gas content was corrected, and the producing law of adsorption gas was determined. The study results show that the isothermal adsorption law of the shale reservoir under high pressure was different from the conventional low pressure. The high pressure isothermal adsorption curve had the maximum value in excess adsorption with pressure change, and the corresponding pressure was the critical desorption pressure. The high pressure isothermal curve can be used to evaluate the amount of adsorbed gas and the producing degree of adsorption gas. The high pressure isothermal adsorption model can fit and characterize the high pressure isothermal adsorption law of shale. The modified gas content calculation method can evaluate the gas content and the proportion of adsorbed gas more objectively, and is the theoretical basis of reserve assessment and production decline analysis. The producing degree of adsorption gas is closely related to the pressure, only when the reservoir pressure is lower than the critical desorption pressure, the adsorption gas can be produced effectively. In the process of gas well production, the pressure drop in the near-well area is large, the production of adsorption gas is high; away from the wellbore, the adsorption gas is low in production, or no production.