Relevance of PCDD/PCDF formation for the evaluation of POPs destruction technologies--review on current status and assessment gaps

One important criterion for assessment of a POPs destruction technology is the potential formation of new POPs and other toxic by-products, in particular whether the highly toxic PCDDs/PCDFs are formed and under which operation conditions their formation is relevant. For incineration processes the formation mechanisms of PCDDs/PCDFs have been investigated thoroughly and strategies and technologies were developed to minimize their formation and emission. A detailed assessment of non-combustion technologies with respect to PCDD/PCDF formation is, however, lacking to date. A comparison of reaction conditions for PCDD/PCDF formation from precursor formation studies and actual applied conditions of a broad range of POPs destruction technologies in the present paper indicates that the operation conditions for a number of destruction technologies have the potential to generate high concentrations of PCDDs/PCDFs if dioxin precursors are present and that also PCDD/PCDF de novo formation can take place. Therefore a strategy and regulations for a more profound assessment and monitoring of the fate of PCDD/PCDF formation and emission is essential for the evaluation of POP destruction technologies and for a sound risk management of POPs. The present paper aims to provide a critical impulse in this respect, discusses the relevant formation pathways with respect to POPs destruction technologies and proposes a basic framework on how evaluations may be performed.     

Establishing the propensity for dioxin formation using a plume temperature model for medical waste incinerator emissions in developing countries

Air pollution control devices (APCDs) are not compulsory for medical waste incinerators (MWIs) in developing countries. In South Africa, combustion gases are usually vented directly to the atmosphere at temperatures greater than the formation temperature of dioxin. The possibility of dioxin formation outside the incinerator stack has been hypothesized. A plume model has been developed and tested in the wind tunnel with a scale model of an incinerator stack. The plume temperature and trajectory predictions of the plume model were verified within a +/- 3% experimental accuracy. Using South African data, the plume model predicts that the residence time of gases in the temperature range of 150-450 degrees C in a plume is 1.3 sec on average for 5% of a year (18 days) at meteorological conditions resulting in wind speeds of less than 1 m/sec. Two published dioxin formation models were used to assess the probability of dioxin formation in the plume. The formation models predict that the average polychlorinated dibenzodioxins/furans (PCDD/Fs) formed in the plume will exceed the stack emission regulations in South Africa of 0.2 ng/Nm3 toxic equivalent quotient (TEQ) by between 2 and 40 times. The calculated concentrations do not include additional gaseous PCDD/F compounds that may be formed at high-temperature post-combustion zones through pyrosynthesis mechanisms.     

Rapid formation of polychlorinated dioxins/furans in the post combustion region during heterogeneous combustion

The formation of chlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) downstream from the combustion zone of a heterogeneous combustor occurred within seconds in the temperature range of 430-380°C, but not at substantially lower temperatures. A spouted bed reactor was used with sand as the heterogeneous medium and a fuel mixture of 1,2-dichlorobenzene/heptane. Total PCDD/F-concentrations were in the range of 100 ng m⁻³; chlorobenzenes other than the starting compound and chlorophenols were formed. The formation of PCDD/F may be mediated by high surface area particles originating from the bed due to attrition. Homogeneous gas phase mechanisms cannot account readily for the levels observed.     

PCDD/DF formations by the heterogeneous thermal reactions of phenols and their TiO2 photocatalytic degradation by batch-recycle system

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/DFs) formation by the thermal reactions of phenols with CuCl2 under oxygen flux were carried out in relation to their formation mechanisms. To evaluate the effect of photocatalytic degradation of titanium dioxide (TiO2) thin film prepared by the sol-gel method, the photocatalysis of PCDD/DFs in acetonitrile/water solution by batch-recycle system was conducted. For the thermal reaction system of powder mixtures of 2,4,5-trichlorophenol (2,4,5-TCP) and CuCl2, the formation rates were 8.1 microg/g-2,4,5-TCP/min for total PCDD/DFs and 6.9 microg/g-2,4,5-TCP/min for PCDDs, and total PCDD/DF rate was higher by approximately 40 fold compared to phenol vapor/oxygen/CuCl2 powder system. For the system of 2,4,5-TCP, PCDDs were mainly formed via ortho-phenoxyphenols (POP) intermediate by the condensation of 2,4,5-trichlorophenate. For PCDD/DF photocatalytic degradations, most PCDD congeners photodecomposed rapidly and the rates presented more than 70% (as dechlorination rates of 76% for PCDDs) at 24 h after irradiation, using PCDD/DFs formed with 2,4,5-TCP. The rate constants were in the order of 4.8-6.1 x 10(-3) min(-1), assuming the pseudo-first-order reactions for their low levels.     

Influence of oxalic acid formed on the degradation of phenol by Fenton reagent

The objective of this work is to examine the influence of oxalic acid formed on the degradation of phenol by Fenton reagent. Oxalic acid formed at initial stage within 30 min significantly suppresses the reduction of ferric ion, thus terminating degradation reaction. The yield of oxalic acid is dependent on the amount of ferrous ion dosed since the minimal amount of oxalic acid is formed after the degradation reaction terminates. Mineralization of phenol by Fenton reagent stagnates after 120 min under the conditions used in this study. The reason why the mineralization stagnated can be assumed to be following two mechanisms other than the depletion of H₂O₂. In the case where a small amount of ferrous ions is dosed, the reduction of ferric ions is minimal by oxalic acid formed. In the case where a large amount of ferrous ions is dosed, the amount of degradable organic compounds is insufficient owing to preferential conversion to oxalic acid. The mineralization can be enhanced by the intermittent dosing of ferrous ions, which facilitates the suppression of oxalic acid formation during the degradation by Fenton reagent.     

Effects of copper chloride on formation of polychlorinated dibenzo-p-dioxins in model waste incineration

Combustion experiments in a laboratory-scale fluidized-bed reactor were conducted to elucidate the effects of copper chloride as a catalyst on polychlorinated dibenzo-p-dioxins (PCDDs) formation in municipal waste incineration. We used model wastes with and without copper chloride (CuCl₂ · 2H₂O), both of which contained polyvinyl chloride as a chlorine source. Combustion temperature was set to 900 °C, and the amount of air supplied was twice the stoichiometric ratio. The experimental setup was carefully planned to suppress the influences of experimental conditions except the waste composition. Results of these experiments showed that copper chloride in the waste increased the amount of PCDDs formed and made the homologue profile to shift towards the highly chlorinated species. Copper chloride contributes to the PCDDs formation by promoting chlorination, whereby the reaction is important in that organic matter is chlorinated directly by copper compounds. Copper chloride did not exert a great influence on the isomer distribution patterns of PCDDs, while there appeared a significant difference in the case of PCDFs. This points out the difference of the major formation mechanisms between PCDDs and PCDFs. PCDDs are less formed by the catalytic reactions from carbon/polycyclic aromatic hydrocarbons than PCDFs in our experimental conditions.     

Comparison of different gas-phase mechanisms and aerosol modules for simulating particulate matter formation

The effects of two gas-phase chemical kinetic mechanisms, Regional Atmospheric Chemistry Mechanism version 2 (RACM2) and Carbon-Bond 05 (CB05), and two secondary organic aerosol (SOA) modules, the Secondary Organic Aerosoi Model (SORGAM) and AER/EPRI/Caltech model (AEC), on fine (aerodynamic diameter < or =2.5 microm) particulate matter (PM2.5) formation is studied. The major sources of uncertainty in the chemistry of SOA formation are investigated. The use of all major SOA precursors and the treatment of SOA oligomerization are found to be the most important factors for SOA formation, leading to 66% and 60% more SOA, respectively. The explicit representation of high-NO, and low-NOx gas-phase chemical regimes is also important with increases in SOA of 30-120% depending on the approach used to implement the distinct SOA yields within the gas-phase chemical kinetic mechanism; further work is needed to develop gas-phase mechanisms that are fully compatible with SOA formation algorithms. The treatment of isoprene SOA as hydrophobic or hydrophilic leads to a significant difference, with more SOA being formed in the latter case. The activity coefficients may also be a major source of uncertainty, as they may differ significantly between atmospheric particles, which contain a myriad of SOA, primary organic aerosol (POA), and inorganic aerosol species, and particles formed in a smog chamber from a single precursor under dry conditions. Significant interactions exist between the uncertainties of the gas-phase chemistry and those of the SOA module.     

Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) isomer patterns from municipal waste combustion: formation mechanism fingerprints

Polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) byproducts can be formed in combustion systems by a variety of mechanisms. While total PCDD/F emissions and, to a lesser extent, homologue distributions from incinerators have been found to vary widely depending on combustion conditions, PCDD/F isomer distributions do not. Formation mechanisms can be grouped into two general categories: condensation of precursors, such as chlorinated phenols, and formation from particulate carbon, termed de novo synthesis. In addition to these mechanisms, chlorination and dechlorination reactions may affect isomer patterns. In this work, isomer patterns from field and laboratory municipal waste combustion samples are compared with computed thermodynamic distributions and those from the following experimental investigations: both gas-phase and metal-catalyzed condensation of chlorinated phenols, chlorination of dibenzo-p-dioxin and dibenzofuran, and dechlorination of octachlorodibenzo-p-dioxin and octachlorodibenzofuran. PCDD/F isomer patterns produced by different formation mechanisms in controlled experiments are distinct and robust, largely unaffected by combustion conditions. PCDD isomer patterns from municipal waste combustion are most similar to those produced by CuCl(2)-catalyzed phenol condensation from 10 chlorinated phenols. PCDF isomer patterns are most similar to those produced by chlorination and dechlorination.     

Photoinduced addition of the fungicide anilazine to cyclohexene and methyl oleate as model compounds of plant cuticle constituents

Photoreactions, initiated by sunlight irradiation, between organochlorine pesticides and olefinic compounds of plant cuticles have been postulated. Concerning the formation of bound residues, which so far have not been detectable by common analytical techniques, photoaddition reactions are of main interest. In order to study the photochemical behavior of chlorinated fungicides, anilazine was irradiated in cyclohexene and methyl oleate as model compounds for olefinic plant cuticle constituents. Anilazine extensively reacted with the cis-double bond of both model compounds via radical mechanisms. In addition to a dechlorinated photoproduct several addition products were formed showing plausible reaction pathways for the formation of bound residues in plant cuticles. Photoproducts were isolated by preparative HPLC and analyzed by HPLC, MS, 1H-, and 13C-NMR.     

Analysis of TCDD and TCDF on the ppq-level in milk and food sources

Analysis of 2378-TCDD and 2378-TCDF on the ppq-level in foods with high fat content is described. The migration over twelve days of 2378-TCDD, 2378-TCDF and 1278-TCDF from bleached paperboard cartons into whole milk was found to be exponential with time. A survey of selected fatty foods shows the presence of 2378-TCDD and 2378-TCDF in foodstuff.     

Mechanisms of molecular product and persistent radical formation from the pyrolysis of hydroquinone

Hydroquinone is considered to be one of the major, potential molecular precursors for semiquinone-type radicals in the combustion of complex polymeric and oligomeric structures contained in biomass materials. Comprehensive product yield determinations from the high-temperature, gas-phase pyrolysis of hydroquinone in two operational modes (rich and lean hydrogen conditions) are reported at a reaction time of 2.0s over a temperature range of 250-1000 degrees C. Below 500 degrees C, p-benzoquinone is the dominant product, while at temperatures above 650 degrees C other products including phenol, benzene, styrene, indene, naphthalene, biphenylene, phenylethyne, dibenzofuran and dibenzo-p-dioxin are formed. Hydrogen-rich conditions initially inhibit hydroquinone decomposition (below 500 degrees C) but promote product formation at higher temperatures. The decomposition process apparently proceeds via formation of a resonance stabilized p-semiquinone radical. Detailed mechanisms of formation of stable molecular species as well as stable radicals are proposed.     

Effects of copper chloride on formation of polychlorinated dibenzo-p-dioxins in model waste incineration

Combustion experiments in a laboratory-scale fluidized-bed reactor were conducted to elucidate the effects of copper chloride as a catalyst on polychlorinated dibenzo-p-dioxins (PCDDs) formation in municipal waste incineration. We used model wastes with and without copper chloride (CuCl₂ · 2H₂O), both of which contained polyvinyl chloride as a chlorine source. Combustion temperature was set to 900 °C, and the amount of air supplied was twice the stoichiometric ratio. The experimental setup was carefully planned to suppress the influences of experimental conditions except the waste composition. Results of these experiments showed that copper chloride in the waste increased the amount of PCDDs formed and made the homologue profile to shift towards the highly chlorinated species. Copper chloride contributes to the PCDDs formation by promoting chlorination, whereby the reaction is important in that organic matter is chlorinated directly by copper compounds. Copper chloride did not exert a great influence on the isomer distribution patterns of PCDDs, while there appeared a significant difference in the case of PCDFs. This points out the difference of the major formation mechanisms between PCDDs and PCDFs. PCDDs are less formed by the catalytic reactions from carbon/polycyclic aromatic hydrocarbons than PCDFs in our experimental conditions.     

An in situ study of the role of surface films on granular iron in the permeable iron wall technology

Permeable walls of granular iron are a new technology developed for the treatment of groundwater contaminated with dissolved chlorinated solvents. Degradation ofthe chlorinated solvents involves a charge transfer process in which they are reductively dechlorinated, and the iron is oxidized. The iron used in the walls is an impure commercial material that is covered with a passive layer of Fe2O3, formed as a result of a high-temperature oxidation process used in the production of iron. Understanding the behaviour of this layer upon contact with solution is important, because Fe2O3 inhibits mechanisms involved in contaminant reduction, including electron transfer and catalytic hydrogenation. Using a glass column specially designed to allow for in situ Raman spectroscopic and open circuit potential measurements, the passive layer of Fe2O3 was observed to be largely removed from the commercial product, Connelly iron, upon contact with Millipore water and with a solution of Millipore water containing 1.5 mg/l trichloroethylene (TCE). It has been previously shown that Fe2O3 is removed from iron surfaces upon contact with solution by an autoreduction reaction; however, prior to this work, the reaction has not been shown to occur on the impure commercial iron products used in permeable granular iron walls. The rate of removal was sufficiently rapid such that the initial presence of Fe2O3 at the iron surface would have no consequence with respect to the performance of an in situ wall. Subsequent to the removal of Fe2O3 layer, magnetite and green rust formed at the iron surface as a result of corrosion in both the Millipore water and the solution containing TCE. The formation of these two species, rather than higher valency iron oxides and oxyhydroxides, is significant for the technology. The former can interfere with contaminant degradation because they inhibit electron transfer and catalytic hydrogenation. Magnetite and green rust, in contrast, will not inhibit the mechanisms involved in contaminant reduction, and hence their formation is beneficial to the long-term performance of the iron material.     

加药量和水力搅拌速度对雨水混凝效果的影响

为降低分流制雨水中悬浮颗粒物及其他污染物浓度,减轻城市景观河道的水体富营养化程度,对取自泵站的雨水进行混凝沉淀工艺优化实验。以PAC为混凝剂,采用Zeta电位仪、激光粒度仪和iPDA在线监测技术对混凝过程进行监测,考察了混凝剂投加量和水力搅拌速度对絮体形成和分流制雨水处理效果的影响,结果表明,混凝剂投加量和混合水力搅拌速度直接影响絮体Zeta电位和聚沉特性;混合搅拌速度控制混凝反应速率,絮凝速度梯度影响絮体形成粒径。FI曲线特征参数对控制混凝工艺具有指导意义。PAC投加量为35 mg/L,混合阶段搅拌速度800 r/min,搅拌30 s,絮凝阶段采用150、108和60 r/min的转速各自搅拌5 min,沉后水中剩余颗粒总数最少,浊度、COD和总磷去除效果最佳。     

无纺布动态膜生物反应器处理碱减量印染废水

应用无纺布动态膜生物反应器（DMBR）处理碱减量印染废水,对该反应器处理碱减量印染废水过程中的实验结果表明,动态膜形成过程、影响因素和系统的运行效果进行了研究。通过测定临界通量得到反应器稳定运行的亚临界条件。实验结果表明,反应器运行70 min后,形成了稳定的动态膜。动态膜的形成速度与曝气量有关,当曝气量过大时,使得膜面错流速度过大,从而影响动态膜的形成。孔径对动态膜形成的初期影响较大,小孔径膜基材比大孔径膜基材更容易形成动态膜,而当动态膜稳定形成后,小孔径膜基材形成的动态膜性能略好,对污染物的去除效果更好。实验条件下,动态膜生物反应器对COD、UV254、色度和浊度的去除率分别为74%~85%、74%~79%、79%~86%和96.8%~98.6%,出水水质达到《纺织染整工业水污染物排放标准》（GB4287-2012）排放要求。     

Simplified spectrophotometric method using methylene blue for determining anionic surfactants: applications to the study of primary biodegradation in aerobic screening tests

In the present work, we propose a simplified spectrophotometric method for determining anionic surfactants, based on the formation of the ionic pair anionic surfactant-methylene blue (AS-MB). This method, in relation to the conventional analytic procedure, considerably reduces not only the quantity of chloroform used in extracting the ionic pair formed, but also the time and the quantity of sample necessary to perform the assay, eliminating the filtration stage. The method has been simplified by displacing the transfer equilibrium of the ionic pair AS-MB towards the organic phase, augmenting the volumetric relationship of chloroform/sample. The method proposed has been applied in the study of primary biodegradation kinetics of linear alkylbenzenesulfonate (LAS).     

Formation characteristics of PCDD and PCDF during pyrolysis processes

In recent years, pyrolysis processes have become technologies developed to industrial scale and discussed as alternatives to the existing waste combustion technology. However, little information is published regarding PCDD/F formation characteristics during pyrolysis processes. Two common shredder fractions – industrial light shredder (ILS) and refrigerators (REF) – both with high chlorine and copper content were pyrolysed for this pyrolysis study using a pilot plant with a capacity of 100 kg/h. At oxygen concentrations below 2% and temperatures between 430°C and 470°C, considerable amounts of PCDD/F were formed during the pyrolysis. More than 90% of total TEQ was found in the oil fraction (gas phase). The PCDD/PCDF ratio and the homologue pattern differed significantly from those formed during waste incineration. Considering mono- to octachlorinated congeners, up to 400 times more PCDF were formed compared to PCDD. For the investigated pyrolysis conditions, the formation of low chlorinated congeners was highly favoured. The distribution of TEQ within the individual congeners were very similar in all investigated runs. More than 80% of total TEQ stem from 2,3,7,8-substituted T₄CDF and P₅CDF. The isomer pattern, however, did not show significant differences compared to the common waste incineration pattern suggesting that the basic formation routes are similar.     

混凝强化形成好氧颗粒污泥

为了缩短好氧颗粒污泥形成的时间,通过在颗粒污泥形成初期投加混凝剂的方式,研究强化造粒条件下颗粒污泥的形成过程及其特性。结果发现,投加混凝剂后,反应器在第8天出现了颗粒污泥,并在第25天时实现完全颗粒化,比未加混凝剂反应器颗粒形成的时间缩短了2 d,完全颗粒化时间缩短了10 d。强化造粒条件下形成的颗粒污泥,颗粒强度、比重分别为99.03%和1.1892,分别比对照高出3.28%和0.1539。可以看出,在混凝强化造粒条件下培养出的颗粒污泥,与常规方法相比具有颗粒化进程快,颗粒强度大,比重大等优点。     

Generation and stability of bentonite colloids at the bentonite/granite interface of a deep geological radioactive waste repository

The possible mechanisms of colloid generation at the near field/far field interface of a radioactive repository have been investigated by means of novel column experiments simulating the granite/bentonite boundary, both in dynamic and in quasi-static water flow conditions. It has been shown that solid particles and colloids can be detached from the bulk and mobilised by the water flow. The higher the flow rate, the higher the concentration of particles found in the water, according to an erosion process. However, the gel formation and the intrinsic tactoid structure of the clay play an important role in the submicron particle generation even in the compacted clay and in a confined system. In fact, once a bentonite gel is formed, in the regions where the clay is contacted with water, clay colloids can be formed even in quasi-static flow conditions. The potential relevance of these colloids in radionuclide transport has been studied by evaluating their stability in different chemical environments. The coagulation kinetics of natural bentonite colloids was experimentally studied as a function of the ionic strength and pH, by means of time-resolved light scattering techniques. It has been shown that these colloids are very stable in low saline (approximately 1 x 10(-3) M) and alkaline (pH > or = 8) waters.     

Formation of PCDF, PCDD, PCB, and PCN in de novo synthesis from PAH: mechanistic aspects and correlation to fluidized bed incinerators

The difference of polychlorinated dibenzofurans (PCDF) isomer patterns between stoker type incinerators and some fluidized bed incinerators (FBI) is a key to understand the formation mechanisms in both types of incinerators. The total yield and the isomer patterns of PCDF, polychlorinated biphenyls (PCB), polychlorinated naphthalenes (PCN), and polychlorinated benzenes (PCBz) formed via de novo synthesis from polycyclic aromatic hydrocarbons (PAH) indicate that chlorinated aromatics in the FBI are formed as a result of PAH breakdown. The detailed analysis of the isomer patterns of PCDF, PCB and PCN gives a first insight into the transformation mechanism of the PAHs and the sequence of degradation, chlorination and oxygen insertion. The major chlorination takes part at the position of the C-C cleavage during degradation of the PAHs. Further chlorination of the hydrogen position of the former PAH takes part preferably in ortho-position to this chlorination or is directed by incorporated oxygen. A perylene structure in soot is proposed as basis for the observed PCDF pattern in the FBI. Polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated phenols (PxCP) were formed in lower concentrations from the de novo experiments indicating an additional formation pathway for these compounds in the FBI.