Adsorption Of VOCs Research Articles

Experimental design methodology for the preparation of carbonaceous sorbents from sewage sludge by chemical activation––application to air and water treatments

The objective of this study is to optimize experimental conditions of sorbent preparation from sewage sludge using experimental design methodology. Series of carbonaceous sorbents have been prepared by chemical activation with sulfuric acid. The sorbents produced were characterized, and their properties (surface chemistry, porous and adsorptive properties) were analyzed as a function of the experimental conditions (impregnation ratio, activation temperature and time). Carbonaceous sorbents developed from sludge allow copper ion, phenol and dyes (Acid Red 18 and Basic Violet 4) to be removed from aqueous solution as well as VOC from gas phase. Indeed, according to experimental conditions, copper adsorption capacity varies from 77 to 83 mg g −1, phenol adsorption capacity varies between 41 and 53 mg g −1 and VOC adsorption capacities (acetone and toluene) range from 12 to 54 mg g −1. Each response has been described by a second-order model that was found to be appropriate to predict most of the responses in every experimental region. The most influential factors on each experimental design response have been identified. Regions in which optimum values of each factor were achieved for preparation of activated carbons suitable for use in wastewater and gas treatments have been determined using response surfaces methodology. In order to have a high mass yield and to minimize the energetic cost of the process, the following optimal conditions, 1.5 g of H 2SO 4 g −1 of sludge, 700 °C and 145 min are more appropriate for use of activated carbon from sludge in water and gas treatments.

Sensing Mechanism of Conducting Polymer Sensor for Volatile Organic Compounds

In this study, the chemically polymerized PPy and PANi films with different selectivity by controlling dedoping time were fabricated. Additionally, the sensing properties and mechanism of VOCs adsorption to conducting polymers were investigated with contact angles measurement, a scanning probe microscope (SPM) and a UV-Vis-NIR spectrophotometer. The thin sensor had higher sensitivity compared to the thick one, which the dedoped sensor at 1-minute had the highest ; sensitivity. Upon gas absorption, polypyrrole exhibited positive sensitivity while polyaniline had negative sensitivity. PPy film showed hydrophilic property and PANi film showed hydrophobic property. After gas absorption, the sensitivity increased as a function of the polarity of the absorbed molecules. These behaviors are due to the polar molecules absorbed with the movable polaron or free carrier, which then interrupts or generates the movement of polaron and carrier, and then changes the conductivity of the polymer. We found that conducting polymer sensors are very sensitive to the difference in polarity of gas molecules.

Gas adsorption gates based on ultrathin composite polymer films.

High surface area alumina coatings were prepared on surface acoustic wave (SAW) mass balances. These coatings were fabricated by anodic etching of evaporated aluminum films. The coatings consisted of roughly collinear pores penetrating through the monolithic alumina film. The nanoporous (NP) coatings were characterized by scanning electron microscopy, and the pore number density and diameter were found to be (3.8 +/- 0.5) x 10(3) pores/microm2 and 6.8 +/- 4.8 nm, respectively. The mass of volatile organic compounds that adsorbed onto naked and chemically modified NP alumina coatings was measured using SAW mass balances and compared to the mass absorbed onto SAW devices having planar aluminum coatings. Thirty-four times more heptane adsorbed to the naked NP coating than to the planar coating. The mass loading response was also measured after modification with organic thin films (3-12 nm thick) that spanned the pores of the NP coating. These organic thin films were composed of sixth-generation, amine-terminated poly(amido amine) dendrimers and poly(maleic anhydride)-c-poly(methyl vinyl ether) (Gantrez). The key result of this study is that these organic thin films modulate adsorption of VOCs onto the pore walls of the NP alumina. Specifically, a single 3-nm-thick monolayer of the dendrimer reduces permeability of the VOCs by approximately 17%, whereas a 12-nm-thick G6-NH2/Gantrez composite reduces permeability by 100%. Thus, the polymer composite acts as a nonselective gate that controls access of VOCs to the underlying surface area of the pores.

Modeling VOCs adsorption onto activated carbon

The activated carbon adsorption process is affected by the characteristics of adsorbent, adsorbate and environmental conditions. In this study, both adsorption and desorption processes are assumed to occur simultaneously and a numerical model was developed with a non-linear driving force in conjunction with the Langmuir model for predicting the overall adsorption process. The numerical model provides both adsorption and desorption rate constants and activation energies. The resultant equilibrium constants are of the same order of magnitude as reported by other studies. Results show that the model could well predict the adsorption isotherms and breakthrough curves under various conditions.

Dynamical adsorption and temperature-programmed desorption of VOCs (toluene, butyl acetate and butanol) on activated carbons

The elimination of solvent vapors from paint shop atmospheres and their recovery for possible re-use is a current environmental requirement. We have studied the dynamical adsorption at 23 °C of typical car paint solvents, i.e. toluene, butylacetate and butanol, on two microporous activated carbons, and the thermal regeneration of these carbons with hot air at 150 °C. A sequence of seven adsorption–desorption cycles with a mixture of these solvents left the carbons with some textural changes but the adsorption capacity remained virtually unaffected. Heavy volatile compounds, proceeding from the possibly unstabilized binder, are eliminated in the course of the first adsorption–desorption runs. Differential scanning calorimetry applied to the desorption in argon of pure solvents showed that the desorption enthalpies Δ H des are close to, or slightly larger than, the evaporation enthalpies Δ H vap except for butanol, which exhibited a particularly high Δ H des value. Temperature-programmed desorption experiments, obtained with carbon samples heated in a helium flow containing oxygen traces, evidenced the desorption of numerous oxidation products. This finding may have consequences for hot air regeneration processes.

Usefulness of chemically activated anthracite for the abatement of VOC at low concentrations

A previous paper showed a very effective and simple chemical activation method for the preparation of activated carbons from an anthracite that allowed to obtain different ranges of porosity. In addition, these materials were suitable for the VOC adsorption because this activation method renders AC with high volumes of narrow micropores. In this paper, the method has been successfully extended to some other precursors, such as subbituminous and lignite coals and lignocellulosic materials. The developed method permits the easy preparation of AC with very well-developed porosities and with high narrow micropore volumes. These characteristics of the AC allow to reach high adsorption capacities to VOC (benzene and toluene) at low concentrations, which are much higher than previously reported in the literature.

Effects of pore structure and temperature on VOC adsorption on activated carbon

Effects of pore structure and temperature on VOC adsorption on activated carbon

Integrated emissions management for automotive painting operations

Abstract An integrated scheme has been studied to reduce gaseous (volatile organic compounds [VOCs]), liquid (scrubber water), and solid (paint sludge) emissions from automotive painting operations by converting an existing spray-booth scrubber system to a bioreactor to capture and degrade VOCs and pyrolyzing paint sludge to produce an adsorbent. This paper describes the experimental results of 1) a pilot-scale study conducted at an assembly plant to evaluate the biological VOC removal using activated-sludge bioreactors and 2) a bench-scale study on VOC adsorption on paint-sludge adsorbents. The results indicate that the biological VOC removal is technically feasible (comparable VOC removal and less energy usage as compared to the widely used, vapor phase-activated carbon adsorption/thermal oxidation process) and economically feasible (one order of magnitude cheaper) and that paint-sludge adsorbents exhibited appreciable adsorption capacity. Implementation of the scheme requires more than technical and economical feasibility. The issues to be overcome include the inertia of past practices, overall systemic thinking, and moving targets (processes and regulations).

Effects of pore structure and temperature on VOC adsorption on activated carbon

This research was undertaken to investigate the pore structure of three activated carbons and determine the temperature dependence of the adsorption of VOCs onto activated carbon. Three kinds of activated carbon made of different raw materials and four VOC species were chosen. The microporosity of activated carbon was assessed by the pore size distribution. The adsorption of VOCs showed that only C 6H 6 exhibited the activated entry effect. The VOC adsorption capacity of peat-derived carbon was less dependent on temperature. A characteristic curve was observed for the peat-derived carbon. Benzene adsorption was the most preferable compared to other three VOCs because of higher heats of adsorption and lower entropy change. Results indicate physical adsorption played a critical role during adsorption processes in this study system.

Gravimetric Studies on VOC Adsorption by Indoor Materials under Near-Ambient Conditions

Adsorption and desorption of volatile organic com pounds (VOCs) by indoor materials can have a signifi cant impact on indoor air quality. Investigations with oth er environmental matrices than building materials have shown that the presence of water has a strong impact on VOC adsorption. The purpose of the present study was to develop and validate a microbalance set-up aimed at investigating VOC adsorption by indoor materials under controlled relative humidity at ambient pressure. The experimental set-up is described and the adsorption iso therms and partition coefficients obtained for combina tions of m-xylene and n-octane with several indoor mate rials are reported. Linear or almost linear adsorption iso therms were found for most combinations of com pounds and materials. n-Octane is in all cases less strongly adsorbed than m-xylene. Comparison with data originating from a different experimental technique gave acceptable agreement between the methods for most of the materials tested.

The influence of relative humidity on the adsorption of toluene by soils — Interpretation with the adsorption energy distribution functions

It has been shown that the VOC adsorption capacity of soil decreases significantly with increasing relative humidity levels in the vapor phase. It might be due to the competition of adsorption sites by water molecules on the soil surfaces. Generally, the surface of nature adsorbents, such as soil and clay mineral, is highly heterogeneous owing to their complex chemical composition and irregularities of physical structure. It is necessary to describe the adsorption phenomena of nature adsorbent microscopically. Based on gas-solid adsorption isotherm, the energy distribution function of sorbing sites on soil surface were constructed. The variation of adsorption energy distribution function reveals that the adsorption energy for water was higher than that for toluene and, therefore, the soil surface would prefer to adsorb water vapor rather than toluene vapor. In addition, the adsorption energy of water vapor shifts towards the lower side at higher relative humidity level. It appears that the water vapor is condensed onto the surface. It is found that the previously adsorbed water molecules modified the soil surface and changed the energy spectrum of the adsorption sites. The shift of the adsorption energy spectrum to lower adsorption energy is the reason of the suppression of the soil uptake of organic vapors by water vapor.

Hydrophobic Zeolites as Adsorbents for Removal of Volatile Organic Compounds from Air

Adsorption isotherms of 1,1,1-trichloroethane, trichloroethylene and ethanol on dealuminated Y zeolite (DAY) and silicalite were measured at 25, 100 and 175°C. The isotherms for the VOCs on the zeolites are well correlated with the Langmuir model. The mechanism of VOC adsorption is by volume filling of zeolite micropores by physical adsorption, at relatively low pressures. DAY zeolite has 60–150% higher VOC adsorption capacity than silicalite due to larger pore volume of the former. Regeneration by temperature swing is thermodynamically more favorable for DAY zeolite. Due to its larger pore size, the rates of adsorption of VOC in DAY zeolite are up to one order of magnitude higher than in silicalite. The adsorption equilibrium and rate constants derived from the batch gravimetric experiments are used to predict the VOC breakthrough curves in zeolite packed adsorbers. The simulation model predicts longer breakthrough time and steeper breakthrough curves for DAY zeolite as compared to silicalite.

Adsorption of VOCs from the Gas Phase to Different Minerals and a Mineral Mixture

Vapor adsorption is an important process for the environmental fate of many organic pollutants by influencing the transport and degradation processes in the environment. This work is part of an attempt to understand vapor adsorption by systematically studying the influence of adsorbent, adsorbate, temperature, and relative humidity (RH) on the adsorption equilibrium. Here adsorption to three well-defined minerals (hematite, corundum, and lime) was studied by a chromatographic method. An exponential decrease of the adsorption coefficients of all tested compounds on all adsorbents was found between 30 and 90% RH. Below 30%, the influence of relative humidity was even stronger. The same functional relationship had been found for quartz and clay minerals tested earlier (1, 2). We therefore hypothesize that this relationship may be valid for all minerals in general. Differences in the surface area-normalized adsorption on different mineral surfaces were found, and they were most distinct at low relative humidi...

Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 11. Synthesis, Characterization, and Chemical Sensitivity of Self-Assembled Polydiacetylene/Calix[n]arene Bilayers

We report the synthesis and characterization of self-assembled polydiacetylene (PDA)/4-tert-butylcalix[n]arene (where n = 4 and 6, abbreviated as CA[4] and CA[6], respectively) bilayers on Au surfaces. The PDA/CA[n] bilayers act as chemically sensitive interfaces that are resistant to high thermal stresses and extreme electrochemical potentials, both of which may be encountered in technologically significant sensing applications. Moreover, these simply prepared bilayer materials are excellent protosystems for studying a range of important sensor-related issues. Here we discuss the linking chemistry that leads to calixarene immobilization and examine the role calixarenes play in vapor-phase sensing applications using surface acoustic wave (SAW) mass balances and Fourier transform infrared−external reflectance spectroscopy (FTIR-ERS). FTIR-ERS spectra indicate that CA[n] reacts with the acid chloride-terminated PDA self-assembled monolayers (SAMs) to form ester linkages. Using irreversible adsorption of vapor-phase n-butylamine onto the CA[n] bilayer-coated SAW devices as an indirect nanogravimetric probe of surface coverage, we calculate that CA[4] and CA[6] cover 58% and 61%, respectively, of the PDA surface. The irreversibly bound n-butylamine serves a secondary purpose by effectively filling voids between calixarenes, which hinders access to them by the vapor-phase molecules. Dosing the bilayer-coated SAW devices with VOCs after exposure to n-butylamine permits a better understanding of the role of the calixarene cavities relative to nonspecific adsorption at, for example, defect sites between CA[n] molecules. Additional control experiments involving PDA/CA[n] analogs, such as phenyl-terminated PDA, show that VOC adsorption is enhanced only on the CA[n] surfaces. This strongly suggests the presence of a specific interaction between the CA[n] cavities and VOCs rather than simple nonspecific adsorption.

RELATIONS BETWEEN SPECIFIC SURFACE AREA AND SOIL PHYSICAL AND CHEMICAL PROPERTIES

The total specific surface area (SA) is a factor that can relate grain-scale properties to macro-scale physical and chemical properties of a porous medium. It is, therefore, advantageous to establish the relationships between SA and general soil physical properties. In this study we investigated the relationship between SA and properties such as soil texture, cation exchange capacity (CEC), volatile organic vapor adsorption, water retention, and saturated hydraulic conductivity. Total specific surface area was highly correlated to the clay-size fraction of the soil and negatively correlated to the soil organic matter (SOM) of the soil. Both the CEC and the soil-water content at −15 bars (the wilting point) were highly correlated to SA. However, a steadily decreasing degree of correlation between SA and water contents at seven increasing matric potentials was seen, suggesting that only the tightly bound water is directly related to SA. Using the measured specific surface area, the soil-water contents at −15 bars were converted into equivalent molecular layers of water coverage. This revealed that the soil-water content at −15 bars was consistently between four and eight molecular layers of water coverage for most soils. This equals the water coverage below which volatile organic chemical vapor/solid partitioning becomes significant. The soil-water content at the wilting point, therefore, seems promising as a simple measure for the lower water content at which Henry's Law and the aqueous/solid partition coefficient can be used to predict VOC adsorption. The Campbell water retention model parameter, b, was highly correlated to the volumetric specific surface area, S0, potentially making S0 useful for predicting the soil-water characteristic curve. The Kozeny-Carman equation was applied to describe the relationship between S0 and the saturated hydraulic conductivity. The equation did not relate well to the measured data. This may be explained by the heterogeneity of all of the investigated soils, whereas the Kozeny-Carman equation was originally developed for structureless soils.

Competitive adsorption of vocs and bom-oxic and anoxic environments

The effect of the presence of molecular oxygen on the adsorption of volatile organic compounds (VOCs) in distilled Milli-Q water and in water supplemented with background organic matter (BOM) is evaluated. Experiments are conducted under conditions where molecular oxygen is present in the test environment (oxic adsorption), and where oxygen is absent from the test environment (anoxic adsorption). Adsorption isotherms for tetrachloroethylene (PCE) and trichloroethylene (TCE) in Milli-Q water showed no impact of the presence of oxygen on their adsorption behavior, while adsorption isotherms for cis-1,2-dichloroethane (DCE) showed higher capacities under oxic conditions. VOC adsorption isotherms in BOM are conducted for three initial concentration sets of VOCs and two initial concentration sets of BOM. VOC isotherms collected under oxic and anoxic conditions in water supplemented with BOM showed that by keeping the initial concentration ratio of VOCs to BOM constant, the adsorption behavior of the VOC is very similar. VOC isotherms in BOM conducted under oxic conditions generally showed lower capacities than the anoxic isotherms. The ideal adsorbed solution theory (IAST) successfully predicted the VOCs anoxic adsorption isotherms in BOM. However, the IAST model did not predict the VOCs oxic adsorption isotherms in BOM. Analysis of the data was conducted using the isotherm parameters of the VOCs in Milli-Q water, and correlation parameters using a four fictive component approach for the BOM. The poor agreement between experimental data and model predictions obtained for the oxic isotherms is attributed to the presence of molecular oxygen which promotes conglomeration of BOM on the surface of carbon.

Comparison of adsorption characteristics for VOCs on activated carbon and oxidized activated carbon

AbstractIn this investigation, the selected activated carbon is treated in NaOH solution or gas fluid with ozone from an ozone generator. FTIR, BET specific surface area and element analysis instruments were used to identify the difference in the chemical and physical characteristics between the activated carbon and oxidized activated carbon. The system of adsorption apparatus consists of a fixed adsorption column, quartz pan, electronic balance, and data acquisition system to measure the adsorption capacities and adsorption rates for VOCs under various operating conditions. VOCs, n‐hexane, methyl‐ethyl‐ketone, and benzene were selected to evaluate the effect of adsorption characteristics on the activated carbon and oxidized activated carbon.The results of chemical analysis with the FTIR instrument indicate the absorption bands and peaks measured at 1632 cm−1 on the oxidized activated carbons (AOHO3 and AO3) are much stronger than on the activated carbon (AC). This shows that carbonyl (conjugated CO) of the functional group is predominantly found to increase on the oxidized activated carbons (AO3 and AOHO3). Significant changes were observed for BET‐specific areas for AOHO3 and AO3 but the carbon contents of AOHO3 decreased slightly. Moreover, the oxidized activated carbons showed greater affinity for polar adsorbate than did AC, and the overall effective mass transfer coefficients, kD, of AO3 for various VOC adsorption were larger than those of AC and AOHO3.

Design and testing of a moving bed VOC adsorption system

AbstractA 0.28‐2.8 m3/min (10‐100 cfm) moving bed carbon adsorption pilot plant has been developed for testing of low (10–200 ppm) concentration VOC emissions. The pilot plant uses a cross‐flow arrangement with the air flowing horizontally through the adsorption bed while the carbon adsorbent particles flow vertically downward. At the bottom of the system, the “used” activated carbon is regenerated using a direct contact heat exchanger. The regenerated carbon is then transported to the top of the moving bed via a screw feeder and pneumatic transport tube. A small gas flow containing the VOCs removed from the used adsorbent is emitted continuously from the adsorbent regenerator. When a carbon flow rate of 2.3 kg/hr (5 lbs/hr), a face velocity of 0.32 m/sec (63 ft/ min), and an emissions stream containing 30 ppm ethanol were used, the observed collection efficiency was approximately 90%. The ability to regenerate the carbon was tested by first operating the system in a static mode, that is air being drawn into the system while the adsorption bed was not moving. This was continued until approximately 70% of the inlet gas VOCs passed through the system without being adsorbed. The carbon flow was then started and the VOC concentration in the off gases of the absorbent regenerator was measured as the partially saturated carbon passed through the system. The VOC concentration in the gases emitted from the adsorbent regenerator was about a factor of ten higher than the VOC concentration in the air entering the adsorption bed.

Use of carbon adsorption processes in groundwater treatment

AbstractOne of the leading treatment technologies for reduction of organic contaminants in groundwater has become the adsorption process utilizing granular activated carbon (GAC).This paper presents a methodology for selecting the proper adsorption system design. The use of the isotherm test for evaluation of carbon adsorption is discussed, with new data on adsorption of VOCs presented. The use of the dynamic column study for system design and its relationship to the isotherm test is reviewed using actual case studies. Design and economics for full‐scale GAC systems are discussed. The use of adsorption to remove organic contaminants from vapor sources resulting from groundwater treatment is also reviewed.