Arsenic Capture Research Articles

Experiment and mechanism research on gas-phase As2O3 adsorption of Fe2O3/γ-Al2O3

Fe2O3/γ-Al2O3 sorbents that containing different quantities of iron (III) oxide were prepared by ultrasound-assisted wet impregnation method. Application of these sorbents for arsenic adsorption was tested on a fixed bed reactor and some factors, such as temperature, precursor solution concentration (PSC) and SO2/NO concentration were discussed. The results indicated that more than 65% of gas-phase arsenic was adsorbed at 600°C, while the efficiency decreased gradually when temperature over 600°C. The higher precursor solution concentration could increase the amount of active substance on carrier, but also cause structure deterioration of sorbent. SO2 in flue gas exhibited a promotion effect on arsenic adsorption across Fe2O3/γ-Al2O3; however, the effect of NO was not as notable as SO2. The adsorption of arsenic is proposed to follow the Mars–Maessen mechanism. In relative low oxygen condition, the oxidation of arsenic was mainly relied on lattice oxygen of sorbent. Introduction of oxygen in flue gas could reoxidize the reduced oxide and therefore facilitate the arsenic capture.

Findings of proper temperatures for arsenic capture by CaO in the simulated flue gas with and without SO2

Arsenic emission during fuel combustion is of great concern due to its high toxicity. CaO shows a great capacity for arsenic capture and the addition of CaO is a promising way to suppress arsenic emission during fuel combustion. SO2 in the flue gas could also react with CaO and might influence arsenic capture. It remains unknown which temperature is proper for CaO injection for arsenic capture considering the effect of SO2. And the reaction mechanism is still unclear at different temperature range. In the present study, the capture of arsenic by CaO was investigated in simulated flue gas with and without SO2 at temperatures ranging from 573 to 1323K. The results show that arsenic capture was predominantly through physical adsorption at 573 and 723K while was mainly carried out through chemical oxidation at higher temperatures. With temperature increasing, arsenic capture was enhanced. Nevertheless, the sintering of CaO particles occurred at 1173 and 1323K. It is sure that arsenic capture was remarkably suppressed at these temperatures owing to the particles sintering. On the other hand, SO2 competed with arsenic vapors to react with CaO. The enhancement of sulfate reaction strongly inhibited arsenic capture with temperature increased from 573 to 1023K. At higher temperatures, the formed CaSO4 was confirmed to be able to absorb arsenic vapors which could partly facilitate arsenic capture. Considering the sintering of CaO particles and the effect of SO2, the proper temperature for arsenic capture should be around 873K.

General and Controllable Synthesis of Novel Mesoporous Magnetic Iron Oxide@Carbon Encapsulates for Efficient Arsenic Removal

A facile ammonia-atmosphere pre-hydrolysis post-synthetic route that can uniformly and selectively deposit Fe(2) O(3) nanoparticles in the predefined mesopores (5.6 nm) of a bimodal (2.3, 5.6 nm) mesoporous carbon matrix is demonstrated. The mesoporous magnetic Fe(2) O(3) @C encapsulates show excellent performance for arsenic capture with remarkable adsorption capacity, fast uptake rate, easy magnetic separation, and good cyclic stability.

Surface characterization of palladium–alumina sorbents for high-temperature capture of mercury and arsenic from fuel gas

Coal gasification with subsequent cleanup of the resulting fuel gas is a way to reduce the impact of mercury and arsenic in the environment during power generation and on downstream catalytic processes in chemical production. The interactions of mercury and arsenic with Pd/Al 2O 3 model thin film sorbents and Pd/Al 2O 3 powders have been studied to determine the relative affinities of palladium for mercury and arsenic, and how they are affected by temperature and the presence of hydrogen sulfide in the fuel gas. The implications of the results on strategies for capturing the toxic metals using a sorbent bed are discussed.

Screening of Low Cost Sorbents for Arsenic and Mercury Capture in Gasification Systems

A novel laboratory-scale fixed-bed reactor has been developed to investigate trace metal capture on selected sorbents for cleaning the hot raw gas in Integrated Gasification Combined Cycle (IGCC) power plants. The new reactor design is presented, together with initial results for mercury and arsenic capture on five sorbents. It was expected that the capture efficiency of sorbents would decrease with increasing temperature. However, a commercial activated carbon, Norit Darco “Hg”, and a pyrolysis char prepared from scrap tire rubber exhibit similar efficiencies for arsenic at 200 and at 400 °C (70% and 50%, respectively). Meta-kaolinite and fly ash both exhibit an efficiency of around 50% at 200 °C, which then dropped as the test temperature was increased to 400 °C. Activated scrap tire char performed better at 200 °C than the pyrolysis char showing an arsenic capture capacity similar to that of commercial Norit Darco “Hg”; however, efficiency dropped to below 40% at 400 °C. These results suggest that the capture mechanism of arsenic (As4) is more complex than purely physical adsorption onto the sorbents. Certain elements within the sorbents may have significant importance for chemical adsorption, in addition to the effect of surface area, as determined by the BET method. This was indeed the case for the mercury capture efficiency for all four sorbents tested. Three of the sorbents tested retained 90% of the mercury when operated at 100 °C. As the temperature increased, the efficiency of activated carbon and pyrolysis char reduced significantly. Curiously, despite having the smallest Brunauer−Emmet−Teller (BET) surface area, a pf-combustion ash was the most effective in capturing mercury over the temperature range studied. These observations suggest that the observed mercury capture was not purely physical adsorption but a combination of physical and chemical processes.

Simultaneous Removal of SO2 and Trace As2O3 from Flue Gas: Mechanism, Kinetics Study, and Effect of Main Gases on Arsenic Capture

Sulfur dioxide (SO2) and trace elements are pollutants derived from coal combustion. This study focuses on the simultaneous removal of S02 and trace arsenic oxide (As2O3) from flue gas by calcium oxide (CaO) adsorption in the moderate temperature range. Experiments have been performed on a thermogravimetric analyzer (TGA). The interaction mechanism between As2O3 and CaO is studied via XRD detection. Calcium arsenate [Ca3(AsO4)2] is found to be the reaction product in the range of 600-1000 degrees C. The ability of CaO to absorb As2O3 increases with the increasing temperature over the range of 400-1000 degrees C. Through kinetics analysis, it has been found that the rate constant of arsenate reaction is much higher than that of sulfate reaction. SO2 presence does not affect the trace arsenic capture either in the initial reaction stage when CaO conversion is relatively low or in the later stage when CaO conversion is very high. The product of sulfate reaction, CaS04, is proven to be able to absorb As2O3. The coexisting CO2 does not weaken the trace arsenic capture either.

Sampling methods to determine the spatial gradients and flux of arsenic at a groundwater seepage zone

Sampling techniques with centimeter-scale spatial resolution were applied to investigate biogeochemical processes controlling groundwater arsenic fate across the groundwater-surface water interface at a site characterized by fine sediments (40% sand, 46% silt, 14% clay). Freeze-core sediment collection gave more detailed and depth-accurate arsenic and iron contaminant and microbial distributions than could be obtained with the use of a hand auger. Selective chemical extractions indicated that greater than 90% of the arsenic was strongly sorbed to very amorphous iron oxyhydroxides. These solids accounted for more than 80% of the total iron in the sediments. Microbial enrichments indicated that iron-oxidizing bacteria (IOB) were up to 1% of the total bacterial abundance, whereas iron-reducing bacteria (IRB) were about two orders of magnitude less abundant than IOB. The abundance of IRB mirrored the IOB depth profile. Push-point pore-water sampling captured large amounts of sediment fines, even with controlled (20 ml/min) water withdrawal, thereby necessitating filtration before water quality analysis. Bead columns containing glass media enabled short-term (29 d) characterization of pore water-to-sediment transfer of arsenic and iron. Bead columns indicated quantitative capture of groundwater arsenic and iron during 2003, suggesting that freeze-core inventories corresponded to 2 to 20 years of accumulation, depending on location.

Review of disposal technologies for chromated copper arsenate (CCA) treated wood waste, with detailed analyses of thermochemical conversion processes

Several alternative methods for the disposal of chromated copper arsenate (CCA) treated wood waste have been studied in the literature, and these methods are reviewed and compared in this paper. Alternative disposal methods include: recycling and recovery, chemical extraction, bioremediation, electrodialytic remediation and thermal destruction. Thermochemical conversion processes are evaluated in detail based on experiments with model compounds as well as experimental and modelling work with CCA treated wood. The latter category includes: determination of the percentage of arsenic volatilised during thermal conversion of CCA treated wood, identification of the mechanisms responsible for arsenic release, modelling of high temperature equilibrium chemistry involved when CCA treated wood is burned, overview of options available for arsenic capture, characterisation of ash resulting from (co-)combustion of CCA treated wood, concerns about polychlorinated dibenzo- p-dioxins/furans (PCDD/F) formation. Finally, the most appropriate thermochemical disposal technology is identified on short term (co-incineration) and on long term (low-temperature pyrolysis or high-temperature gasification).

Reaction of arsenic vapor species with fly ash compounds: kinetics and speciation of the reaction with calcium silicates

In coal combustion systems, the partitioning of arsenic between the vapor and solid phases is determined by the interaction of arsenic vapors with fly ash compounds under post-combustion conditions. This partitioning is affected by gas–solid reactions between the calcium components of the ash particles and arsenic vapors. In this study, bench scale experiments were conducted with calcium compounds typical of coal-derived fly ash to determine product formation, the extent of reaction and reaction rates when contacted by arsenic oxide vapors. Experiments conducted with arsenic trioxide (As 4O 6(g)) vapors in contact with calcium oxide, di-calcium silicate and mono-calcium silicate over the temperature range 600–1000 °C indicated that these solids were capable of reacting with arsenic vapor species in both air and nitrogen. Calcium arsenate was the observed reaction product in all the samples analyzed. Maximum capture of arsenic occurred at 1000 °C with calcium oxide being the most effective of the three solids over the range of temperatures studied. Using a shrinking core model for a first order reaction and the results from intrinsic kinetic experiments conducted in air, the reaction rate constants were found to be 1.4×10 −3exp(−2776/ T) m/s for calcium oxide particles, 7.2×10 −3exp(−3367/ T) m/s for di-calcium silicate particles and 5.5×10 −3exp(−3607/ T) m/s for mono-calcium silicate particles. These results therefore suggest that any calcium present in fly ash can react with arsenic vapor and capture the metal in water-insoluble forms of the less hazardous As(V) oxidation state.

Capture of gas-phase arsenic oxide by lime: kinetic and mechanistic studies.

Trace metal emission from coal combustion is a major concern for coal-burning utilities. Toxic compounds such as arsenic species are difficult to control because of their high volatility. Mineral sorbents such as lime and hydrated lime have been shown to be effective in capturing arsenic from the gas phase over a wide temperature range. In this study, the mechanism of interaction between arsenic oxide (As2O3) and lime (CaO) is studied over the range of 300-1000 degrees C. The interaction between these two components is found to depend on the temperature; tricalcium orthoarsenate (Ca3As2O8) is found to be the product of the reaction below 600 degrees C, whereas dicalcium pyroarsenate (Ca2As2O7) is found to be the reaction product in the range of 700-900 degrees C. Maximum capture of arsenic oxide is found to occur in the range of 500-600 degrees C. At 500 degrees C, a high reactivity calcium carbonate is found to capture arsenic oxide by a combination of physical and chemical adsorption. Intrinsic kinetics of the reaction between calcium oxide and arsenic oxide in the medium-temperature range of 300-500 degrees C is studied in a differential bed flow-through reactor. Using the shrinking core model, the order of reaction with respect to arsenic oxide concentration is found to be about 1, and the activation energy is calculated to be 5.1 kcal/mol. The effect of initial surface area of CaO sorbent is studied over a range of 2.7-45 m2/g using the grain model. The effect of other major acidic flue gas species (SO2 and HCl) on arsenic capture is found to be minimal under the conditions of the experiment.

Capture of arsenic by pyrite in near-shore marine sediments

Evidence of pyrite formation during diagenesis of suboxic sediments in the Laurentian Trough, Canada, is presented. The process is slower than in salt marsh sediments probably due to the low sulfate reduction rate of 2–3 nmol cm −3 day −1 measured in the St. Lawrence sediments. A strong correlation is observed between arsenic and pyrite, suggesting the capture of this element in the lattice sites of pyrite with a Fe/As ratio of 10 3. It is important to consider this association of As with pyrite in the study of the marine geochemistry of Fe and As.