Catalytic ozonation of gaseous reduced sulfur compounds using wood fly ash

The feasibility of reusing wood ash as an inexpensive catalyst in a catalytic ozonation process has been demonstrated. Catalytic ozonation was demonstrated to oxidize H2S, methanethiol (MT), dimethyl sulfide (DMS), and dimethyl disulfide (DMDS) at low temperatures (23-25 degrees C). The process oxidized 25-50% of an inlet MT stream at 70 ppmv without the formation of DMDS (contrary to ash plus oxygen in air), oxidized 90-95% of an 85 ppmv stream of DMS, and oxidized 50% of a 100 ppmv DMDS stream using 2 g of wood ash at a space velocity of 720 h(-1) using ozone concentrations ranging from 100 to 300 ppmv. Similarly, 60-70% conversion of a 70 ppmv H2S stream was achieved with 2 g of ash in 1.1 s without catalytic deactivation (approximately 44 h). The overall oxidation rate of H2S, DMS, and DMDS increased with increasing ozone concentration contrary to the oxidation rate of MT, which was independent of ozone concentration. Dimethyl sulfoxide and dimethyl sulfone were identified as the primary end products of DMS oxidation, and SO2 was the end product of H2S and MT oxidation.     

Removal of hydrogen sulfide by sulfate-resistant Acidithiobacillus thiooxidans AZ11

Toxic H2S gas is an important industrial pollutant that is applied to biofiltration. Here, we examined the effects of factors such as inlet concentration and space velocity on the removal efficiency of a bacterial strain capable of tolerating high sulfate concentrations and low pH conditions. We examined three strains of Acidithiobacillus thiooxidans known to have sulfur-oxidizing activity, and identified strain AZ11 as having the highest tolerance for sulfate. A. thiooxidans AZ11 could grow at pH 0.2 in the presence of 74 g l(-1) sulfate, the final oxidation product of elemental sulfur, in the culture broth. Under these conditions, the specific sulfur oxidation rate was 2.9 g-S g-DCW (dry cell weight)(-1) d(-1). The maximum specific sulfur oxidation rate of A. thiooxidans AZ11 was 21.2 g-S g-DCW(-1) d(-1), which was observed in the presence of 4.2 g-SO4(2-) l(-1) and pH 1.5, in the culture medium. To test the effects of various factors on biofiltration by this strain, A. thiooxidans AZ11 was inoculated into a porous ceramic biofilter. First, a maximum inlet loading of 670 g-S m(-3) h(-1) was applied with a constant space velocity (SV) of 200 h(-1) (residence time, 18 s) and the inlet concentration of H2S was experimentally increased from 200 ppmv to 2200 ppmv. Under these conditions, less than 0.1 ppmv H2S was detected at the biofilter outlet. When the inlet H2S was maintained at a constant concentration of 200 ppmv and the SV was increased from 200 h(-1) to 400 h(-1) (residence time, 9 s), an H2S removal of 99.9% was obtained. However, H2S removal efficiencies decreased to 98% and 94% when the SV was set to 500 h(-1) (residence time, 7.2 s) and 600 h(-1) (residence time, 6 s), respectively. The critical elimination capacity guaranteeing 96% removal of the inlet H2S was determined to be 160 g-S m(-3) h(-1) at a space velocity of 600 h(-1). Collectively, these findings show for the first time that a sulfur oxidizing bacterium has a high sulfate tolerance and a high sulfur oxidizing activity below pH 1.     

Processing of arsenopyritic gold concentrates by partial bio-oxidation followed by bioreduction

Gold is commonly liberated from sulfide minerals by chemical and biological oxidation. Although these technologies are successful, they are costly and produce acidic waste streams. Removal of mineral-sulfur to overcome the mineralogical barrier could also be done by bioreduction, producing hydrogen sulfide (H(2)S). To make the sulfur within these minerals available for bioreduction, the use of partial bio-oxidation as a pretreatment to oxidize the sulfides to elemental sulfur was investigated in gas lift loop reactor experiments. Experiments at 35 °C using a refractory concentrate showed that at pH 2 arsenopyrite is preferentially partially oxidized over pyrite and that elemental sulfur can be subsequently converted into H(2)S at pH 5 via bioreduction using H(2) gas. A single partial bio-oxidation/bioreduction treatment increased the gold recovery of the concentrate from 6% to 39%. As elemental sulfur seems to inhibit further oxidation by covering the mineral surface, several treatments may be required to reach a gold recovery >90%. Depending on the number of treatments this method could be an interesting alternative to bio-oxidation.     

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide over TiO2

Photocatalytic oxidation of gaseous 2-chloroethyl ethyl sulfide (2-CEES, ClCH2CH2SCH2CH3) over TiO2 illuminated with UV light and maintained at 25 or 80 degrees C in air has been investigated. 2-CEES was found to suffer progressive oxidation to yield ethylene (CH2CH2), chloroethylene (ClCHCH2), ethanol (CH3CH2OH), acetaldehyde (CH3C(O)H), chloroacetaldehyde (ClCH2C(O)H), diethyl disulfide (CH3CH2S2CH2CH3), 2-chloroethyl ethyl disulfide (ClCH2CH2S2CH2CH3), and bis(2-chloroethyl) disulfide (ClCH2CH2S2CH2CH2Cl) as the main primary intermediates, and water (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), surface sulfate ions (SO4(2-)), and hydrogen chloride (HCl) as the final products. Trace concentrations of gaseous 2-chloroethanol (ClCH2CH2OH), ethanesulfonyl chloride (CH3CH2SO2Cl), ethyl thioacetate (CH3CH2SC(O)CH3), and considerable amounts of acetic acid (CH3C(O)OH), crotonaldehyde (CH3CHCHC(O)H), methyl acetate (CH3C(O)OCH3), and methyl formate (CH3OC(O)H) were also detected in the gas phase during the photooxidation conducted at 80 degrees C. Increase in temperature from 25 to 80 degrees C accelerates formation of gaseous ethanol, acetaldehyde, chloroacetaldehyde, diethyl disulfide, 2-chloroethyl ethyl disulfide, and bis(2-chloroethyl) disulfide but suppresses ethylene and chloroethylene production at initial stages of the process. Some aspects of the possible reaction mechanism leading to this wide array of intermediates and final products are discussed.     

Retention of arsenic and selenium compounds using limestone in a coal gasification flue gas

Volatile arsenic and selenium compounds present in coals may cause environmental problems during coal combustion and gasification. A possible way to avoid such problems may be the use of solid sorbents capable of retaining these elements from flue gases in gas cleaning systems. Lime and limestone are materials that are extensively employed for the capture of sulfur during coal processing. Moreover, they have also proven to have good retention characteristics for arsenic and selenium during combustion. The aim of this work was to ascertain whether this sorbent is also useful for retaining arsenic and selenium species in gases produced in coal gasification. The study was carried out in a laboratory-scale reactor in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. In these conditions, retention capacities for arsenic may reach 17 mg g(-1) in a gasification atmosphere free of H2S, whereas the presence of H2S implies a significant decrease in arsenic retention. In the case of selenium, H2S does not influence retention which may reach 65 mg g(-1). Post-retention sorbent characterization, thermal stability, and water solubility tests have shown that chemical reaction is one of the mechanisms responsible for the capture of arsenic and selenium, with Ca(AsO2)2 and CaSe being the main compounds formed.     

Performance of a sulfide-oxidizing, sulfur-producing membrane biofilm reactor treating sulfide-containing bioreactor effluent

Sulfide-containing waste streams are generated in mining, petrochemical plants, tanneries, viscose rayon manufacture, and the gasification of coal. Colorless sulfur bacteria can oxidize sulfide to elemental sulfur (S°), which can be recovered, when oxygen is their electron acceptor. This study evaluated sulfide oxidation and S° recovery in an oxygen-based membrane biofilm reactor (MBfR) treating the effluent from a sulfidogenic anaerobic baffled reactor. Sulfide oxidation efficiency (37-99%) and S° recovery (64-89% of oxidized sulfide) could be controlled by manipulating the sulfide loading, oxygen pressure to the fibers, and hydraulic retention time (HRT). For example, too-low oxygen pressure decreased S° recovery due to decreased sulfide oxidation, but too-high oxygen pressure lowered S° recovery due to its oxidation to sulfate. Most importantly, high sulfide oxidation (>98%) and conversion to S° (>75%) could be achieved together when the sulfide loading was less than 1.7 mol/m2·d and the O? pressure was sufficient to give an O? flux of at least 1.5 mol/m2·d. However, higher sulfide loading could be compensated by a higher O? pressure, and the best performance occurred when the sulfide loading was high (2 molS/m2·d), the O? pressure was high (～1 atm), and the HRT was short (1.9 h). Membrane fouling caused a low O? flux, which led to low sulfide-oxidation efficiency, but fouling could be reversed by mild acid washing.     

粘膜乳杆菌发酵乳清生产L-乳酸的研究

粘膜乳杆菌L-05是从猪小肠中分离的兼性厌氧革兰氏阳性细菌,是乳杆菌属中发现的新种。本文研究了粘膜乳杆菌L-05利用乳清粉生产L-乳酸的摇瓶发酵特性,并对其发酵动力学进行了初步探讨。研究结果表明,粘膜乳杆菌生长并合成L-乳酸的适宜条件为37℃,起始pH6.0,摇床转速为150r/min。在优化发酵培养基(g/L)乳清30,蛋白胨15,酵母提取物2,柠檬酸氢二铵2,CH3COONa2.5,K2HPO42,MgSO4·7H2O1.16,MnSO4·4H2O0.5中对乳清的转化率为88.1%,并对最适生长条件下的细胞生长模型和L-乳酸合成模型进行了拟合。     

The influence of light/dark cycle at low light frequency on the desulfurization by a photosynthetic microorganism

H2S dissolved in water can be converted to elementary sulfur or sulfate by the photosynthetic bacterium Chlorobium thiosulfatophilum. The effects of the light/dark cycle on cell growth and the rate of sulfide removal were investigated to develop an appropriate fermentation strategy. Dark fermentation was also studied without addition of H2S and CO2 as electron and carbon sources. Average specific growth rates of bacterial cultures with a continuous supply of H2S and CO2 both in light and dark conditions were occurred in the range of 0.008 to 0.009 h(-1), indicating little dependence on the light/dark cycle, but about 25% of the growth rate that was occurred only in the presence of light. Average H2S removal capacities for cultures grown under the light/dark cycles of 14/10 , 12/12 , and 9/15 h, respectively, with a continuous supply of feed gases, were 0.08, 0.07, and 0.04 micromol H2S.min(-1)/mg protein.l(-1) in the dark, and was slightly less than those in the light. H2S removal capacity with variation of the light/dark cycle was about 30-60% of that obtained in the continuously illuminated cultures. ATP concentration in the dark decreased from 0.43 to 0.37 mg ATP.mg protein(-1) as the daily dark duration decreased from 15 to 10 h. The production rate for lactic acid from a culture grown without a supply of mixtures of H2S and CO2 gases was 0.218 g lactic acid.l(-1).h(-1), much more than that grown with a supply of feed gas mixtures. Time-averaged concentrations of lactic acid produced overall during the light and dark periods were 13.7 g lactic acid.l(-1) during the light/dark cycle of 14/10 h without a supply of feed gas, and 3.1 and 2.4 g lactic acid.l(-1) during the cycles of 9/15 and 14/10 h, respectively, with a supply of feed gas.     

Catalytic ozonation of propanal using wood fly ash and metal oxide nanoparticle impregnated carbon

Catalytic ozonation of propanal at ambient temperatures (23-25 degrees C) was investigated by varying propanal and ozone concentrations and catalyst type. The catalysts tested included wood fly ash (WFA), magnetically separated ash, synthetic hematite and magnetite, and metal oxide nanoparticle impregnated activated carbon and peanut hull char. A power law model independent of ozone concentration for WFA (r(w), moles g(-1) s(-1)) and magnetite (r(m)) were, respectively, r(w) = k'(w) C(R(0.89)) and r(m) = k'(m)C(R(1.55)), where kw, and k'(m) were 2.36 x 10(-6) g(-1) s(-1) (moles)(-0.11) (m3)(0.89) and 6.5 x 10(-4) g(-1) s(-1) (moles)(-0.55) (m3)(1.55), respectively (5-15 ppmv). Magnetite and hematite present in the WFA were theorized to be the primary active sites, since magnetically separated WFA had a significantly higher reaction rate (approximately 12x, mol m(-2) s(-1)) than that of WFA. X-ray diffraction analysis demonstrated a qualitative increase in magnetite and hematite in the magnetically separated ash, and synthetic magnetite and hematite had reaction rates >80x and 200x that of WFA or activated carbon (surface area basis). Supercritical deposition of hematite on/in peanut hull char successfully generated a porous, pelleted catalystfrom an agricultural residue capable of oxidizing propanal at rates 12x activated carbon and similar to commercially available catalysts (per mass basis). Water vapor significantly increased the propanal reaction rate when using wood fly ash and activated carbon.     

Application of solid ash based catalysts in heterogeneous catalysis

Solid wastes, fly ash, and bottom ash are generated from coal and biomass combustion. Fly ash is mainly composed of various metal oxides and possesses higher thermal stability. Utilization of fly ash for other industrial applications provides a cost-effective and environmentally friendly way of recycling this solid waste, significantly reducing its environmental effects. On the one hand, due to the higher stability of its major component, aluminosilicates, fly ash could be employed as catalyst support by impregnation of other active components for various reactions. On the other hand, other chemical compounds in fly ash such as Fe2O3 could also provide an active component making fly ash a catalyst for some reactions. In this paper, physicochemical properties of fly ash and its applications for heterogeneous catalysis as a catalyst support or catalyst in a variety of catalytic reactions were reviewed. Fly-ash-supported catalysts have shown good catalytic activities for H2 production, deSO(x), deNO(x), hydrocarbon oxidation,and hydrocracking, which are comparable to commercially used catalysts. As a catalyst itself, fly ash can also be effective for gas-phase oxidation of volatile organic compounds, aqueous-phase oxidation of organics, solid plastic pyrolysis, and solvent-free organic synthesis.     

Experimental study on the effect of SO2 on PCDD/F emissions: determination of the importance of gas-phase versus solid-phase reactions in PCDD/F formation

Cofiring coal in municipal solid waste incinerators (MSWls) has previously been reported to reduce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) emissions due to increasing the flue gas SO2 concentration. The present study was focused on understanding the primary mechanism responsible for the suppressant effect of SO2 on total PCDD/F formation and toxic equivalent (TEQ) emissions. The addition of SO2, simulating the effect of coal addition on the flue gas composition, resulted in significant reductions in the TEQ emissions due to reactions involving SO2 in the postcombustion zone. However, emissions of total PCDDs/Fs, unlike the TEQ value, were dependent upon the Cl2 and SO2 injection temperatures due to increases in non-TEQ correlated isomers. The conversion of metal chlorides in the fly ash to sulfates, thus reducing the sites responsible for chlorination/oxidation reactions, was concluded to be the main suppressant mechanism; proposed reactions for copper and iron are presented. This mechanism was found to be independent of combustion conditions and could have prolonged effects on PCDD/F emissions from deposits formed with high flue gas S/Cl ratios.     

粉煤灰/镧/壳聚糖复合材料对模拟染料废水的吸附效果

针对印染废水难处理的问题,将经镧改性的壳聚糖负载在粉煤灰上,制备出粉煤灰/镧/壳聚糖复合材料。考查了pH值、吸附时间、初始质量浓度、温度等因素对其吸附效果的影响,并利用吸附动力学、吸附热力学和等温吸附模型探讨其吸附机制。结果表明:粉煤灰/镧/壳聚糖复合材料对酸性橙溶液的吸附速率较快,60 min内达到吸附平衡,且pH值为6时,投加量为8 g/L时,25℃的条件下吸附效果最好,脱色率达到90%以上。粉煤灰/镧/壳聚糖复合材料对酸性橙溶液的吸附更加符合Freundlich等温模型和准二级吸附速率方程,其吸附过程由化学吸附和颗粒内扩散共同控制,吸附过程为自发进行。     

酿酒废酵母吸附Cr6+的动力学及吸附平衡研究

对酿酒废酵母吸附Cr6+的吸附动力学及等温吸附进行研究,结果表明:当Cr6+初始浓度为30mg/L时,Cr6+在酿酒废酵母上的吸附可分为2个阶段,第1阶段为物理吸附且在5min内达到平衡;Cr6+在酿酒废酵母上的吸附过程可用准二级动力学方程描述,动力学参数k2为0.0174g/(mg·min),qe为4.92mg/g;同时Cr6+在酿酒酵母上的生物吸附也可用Langmuir方程和Freundlich方程来描述,但以Langmuir方程较好,最大吸附量qmax为6.38mg/g,b为0.2331g/(mg·min)。     

Thermal wet oxidation improves anaerobic biodegradability of raw and digested biowaste

Anaerobic digestion of solid biowaste generally results in relatively low methane yields of 50-60% of the theoretical maximum. Increased methane recovery from organic waste would lead to reduced handling of digested solids, lower methane emissions to the environment, and higher green energy profits. The objective of this research was to enhance the anaerobic biodegradability and methane yields from different biowastes (food waste, yard waste, and digested biowaste already treated in a full-scale biogas plant (DRANCO, Belgium)) by assessing thermal wet oxidation. The biodegradability of the waste was evaluated by using biochemical methane potential assays and continuous 3-L methane reactors. Wet oxidation temperature and oxygen pressure (T, 185-220 degrees C; O2 pressure, 0-12 bar; t, 15 min) were varied for their effect on total methane yield and digestion kinetics of digested biowaste. Measured methane yields for raw yard waste, wet oxidized yard waste, raw food waste, and wet oxidized food waste were 345, 685, 536, and 571 mL of CH/g of volatile suspended solids, respectively. Higher oxygen pressure during wet oxidation of digested biowaste considerably increased the total methane yield and digestion kinetics and permitted lignin utilization during a subsequent second digestion. The increase of the specific methane yield for the full-scale biogas plant by applying thermal wet oxidation was 35-40%, showing that there is still a considerable amount of methane that can be harvested from anaerobic digested biowaste.     

AgNO3-lnduced photocatalytic degradation of odorous methyl mercaptan in gaseous phase: mechanism of chemisorption and photocatalytic reaction

In this study, AgNO3 films prepared by a simple dip-coating method were used to remove gaseous methyl mercaptan (CH3SH) for odor control. The AgNO3 films were characterized by means of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy/energy-dispersive X-ray spectrometry(SEM/EDX), and X-ray photoelectron spectroscopy (XPS) before and after the reaction, and as- obtained products were identified by means of gas chromatography/mass spectrometry (GC/MS) and ion chromatography. The experiments demonstrated that the AgNO3 film can induce a quick chemisorption of gaseous CH3SH to form AgSCH3 and other intermediate products such as alpha-Ag2S, Ag4S2, and AgSH on its surface. Under UVA illumination, these sulfur products can be photocatalytically oxidized to AgSO3CH3 and Ag2SO4. Then AgSO3CH3 and Ag2SO4 will continue the chemisorption of gaseous CH3SH, similar to AgNO3, to form AgSCH3 again and release two final products, HSO3CH3 and H2SO4. Hence it is a AgNO3-induced photocatalytic reaction for odorous CH3SH degradation in gaseous phase. This fundamental research about the mechanism of chemisorption and photocatalytic reaction provides essential knowledge with potential to further develop a new process for gaseous CH3SH degradation in odor control.     

Arsenic and selenium capture by fly ashes at low temperature

Arsenic and selenium compounds may be emitted to the environment during coal conversion processes, although some compounds are retained in the fly ashes, in different proportions depending on the characteristics of the ashes and process conditions. The possibility of optimizing the conditions to achieve better trace element retention appears to be an attractive, economical option for reducing toxic emissions. This approach requires a good knowledge of fly ash characteristics and a thorough understanding of the capture mechanism involved in the retention. In this work the ability of two fly ashes, one produced in pulverized coal combustion and the other in fluidized bed combustion, to retain arsenic and selenium compounds from the gas phase in coal combustion and coal gasification atmospheres was investigated. To explore the possible simultaneous retention of mercury, the influence of the unburned coal particle content was also evaluated. Retention capacities between 2 and 22 mg g(-1) were obtained under different conditions. The unburned coal particle content in the fly ash samples does not significantly modify retention capacities.     

Production of hydrogen sulfide from tetrathionate by the iron-oxidizing bacterium Thiobacillus ferrooxidans NASF-1

When incubated under anaerobic conditions, five strains of Thiobacillus ferrooxidans tested produced hydrogen sulfide (H2S) from elemental sulfur at pH 1.5. However, among the strains, T. ferrooxidans NASF-1 and AP19-3 were able to use both elemental sulfur and tetrathionate as electron acceptors for H2S production at pH 1.5. The mechanism of H2S production from tetrathionate was studied with intact cells of strain NASF-1. Strain NASF-1 was unable to use dithionate, trithionate, or pentathionate as an electron acceptor. After 12 h of incubation under anaerobic conditions at 30 degrees C, 1.3 micromol of tetrathionate in the reaction mixture was decomposed, and 0.78 micromol of H2S and 0.6 micromol of trithionate were produced. Thiosulfate and sulfite were not detected in the reaction mixture. From these results, we propose that H2S is produced at pH 1.5 from tetrathionate by T. ferrooxidans NASF-1, via the following two-step reaction, in which AH2 represents an unknown electron donor in NASF-1 cells. Namely, tetrathionate is decomposed by tetrathionate-decomposing enzyme to give trithionate and elemental sulfur (S4O6(2-)-->S3O6(2-) + S(o), Eq. 1), and the elemental sulfur thus produced is reduced by sulfur reductase using electrons from AH2 to give H2S (S(o) + AH2-->H2S + A, Eq. 2). The optimum pH and temperature for H2S production from tetrathionate under argon gas were 1.5 and 30 degrees C, respectively. Under argon gas, the H2S production from tetrathionate stopped after 1 d of incubation, producing a total of 2.5 micromol of H2S/5 mg protein. In contrast, under H2 conditions, H2S production continued for 6 d, producing a total of 10.0 micromol of H2S/5 mg protein. These results suggest that electrons from H2 were used to reduce elemental sulfur produced as an intermediate to give H2S. Potassium cyanide at 0.5 mM slightly inhibited H2S production from tetrathionate, but increased that from elemental sulfur 3-fold. 2,4-Dinitrophenol at 0.05 mM, carbonylcyanide-m-chlorophenyl- hydrazone at 0.01 mM, mercury chloride at 0.05 mM, and sodium selenate at 1.0 mM almost completely inhibited H2S production from tetrathionate, but not from elemental sulfur.     

Fenton高级氧化法处理化机浆废水研究

采用高级氧化法处理了某BCTMP化机浆制浆综合废水二级生化出水。研究结果表明：化机浆综合废水经厌氧-好氧处理后,再用Fenton氧化法进行深度处理,COD可降至50mg/L以下。用稀硫酸调初始pH到5.8,100ml废水投加5%的FeSO4溶液6.4ml后快速搅拌2min,加12%的H202溶液0.15ml,加Ca（OH）2溶液调节pH到6.0,整个高级氧化段COD去除率达85%,SS去除率达到90%以上。Ca（OH）2不仅调节了pH,而且还起到了絮凝的作用。与此同时,加PAM来改善污泥沉降性能,PAM的助凝作用,改变了化学污泥的沉降性。本研究结果为福建某企业化机浆废水的深度处理工程设计提供了重要的基础数据。     

Micrometeorological measurements of methane and carbon dioxide fluxes at a municipal landfill

Continuous and area-integrating monitoring of methane (CH4) and carbon dioxide (CO2) emissions was performed for 6 and 9 months, respectively, at a municipal landfill in Finland with the micrometeorological eddy covariance (EC) method. The mean CH4 emission from June to December was 0.53 mg m(-2) s(-1), while the CO2 emission between February and December averaged 1.78 mg m(-2) s(-1). The CH4 emissions from the summit area of the landfill, where active waste deposition was going on, were 1.7 times as high as from the slope area with a better surface cover. The variation in emissions over the source area of the measurement was high. Significant seasonal variation, linked to air and soil temperature, was only seen in the CO2 release rates. Results obtained with the EC method were comparable to those measured with closed static chambers. According to the EC measurements, the gas recovery system decreased CH4 fluxes by 69-79%. The ratio of the measured CH4 and CO2 emissions roughly indicated the route of the landfill gas emission, resembling the ratio of the gases measured in the gas wells (1.24) when the emission originated from the area with no oxidizing cover layer and being smaller when CH4 oxidation had taken place.