Qili Qiu ,Xuguang Jiang,* ,Guojun Lü ,Zhiliang Chen ,Shengyong Lu ,Mingjiang Ni ,Jianhua Yan,Xiaobing Deng

1 State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

2 Hangzhou Lijia Environmental Services Co.,Ltd.,Hangzhou 311101,China

ABSTRACT In this work,microwave treatment was introduced to a hydrothermal treatment process to degrade PCDD/Fs (Polychlorinated dibenzo-p-dioxins and dibenzofurans) in municipal solid waste incineration(MSWI) fly ash.Three process additives (NaOH,Na2 HPO4,H2 O),temperatures (150°C,185°C,220°C)and reaction times(1 h,2 h,3 h)were investigated to identify their effect on the disposal of fly ash samples through orthogonal experiments.High-resolution gas chromatography-mass spectrometry (HRGC/MS) was applied to determine the PCDD/F concentrations in MSWI fly ash.The experimental results revealed that 83.7%of total PCDD/Fs was degraded.Reaction temperature was the most important factor for the degradation of the total PCDD/Fs.Both direct destruction and chlorination reactions (the chlorination degree of PCDFs increased) took part in the degradation of PCDD/Fs in fly ash,which was a new discovery.Several PCDD/F indexes determined by the concentration of indicative congeners were found to quantitatively characterize the dioxin toxicity of the fly ash.Furthermore,heavy metals in the fly ash sample were solidified using microwave-assisted hydrothermal treatment,which provided an experimental basis for the simultaneous disposal of dioxins and heavy metals.Thus,the microwave-assisted hydrothermal process should be considered for the future disposal of MSWI fly ash.

Keywords:Dioxins Degradation MSWI fly ash Hydrothermal treatment Microwave

1.Introduction

The fly ash from MSWI plants contains considerable highly toxic heavy metals and organic pollutants,such as polychlorinated dibenzo-p-dioxins (PCDDs),polychlorinated dibenzofurans(PCDFs)dioxin-like polychlorinated biphenyls(dl-PCBs),and other persistent halogenated hydrocarbons [1,2].It was reported that more than half of all dioxin from waste incineration was discharged into the environment in the form of fly ash [2].According to China Statistical yearbook (2017),249 MSWI plants have been constructed since 2016,and approximately 74 million tons of municipal solid waste (MSW) are treated per year in China.It has been indicated that incineration is widely accepted as a method of MSW management,and the disposal of the increasing amounts of MSWI fly ash is crucial[3].Dioxins in MSWI fly ash are produced during incineration,and historic dioxin sources persist in soil or sediment and waste reservoirs for decades.It has been reported that short-term human exposure to high levels of dioxins or other persistent organic pollutants (POPs) [4,5]may result in skin lesions,while long-term exposure damages the immune system,the developing nervous system,etc.[6].Thus,the decomposition of dioxins in MSWI fly ash needs to be taken seriously.

Recently,several technologies [7-11]have been applied to the degradation and dechlorination of dioxins in fly ash,especially the hydrothermal treatment [12,13].For the comparison of thermal treatment and hydrothermal process,it was reported that thermal treatment of dioxins at 300°C for 2 h resulted in 90%degradation of dioxins [14],while only 20 min was needed to decrease the dioxin concentration from 1100 ng·g-1to 0.45 ng·g-1by hydrothermal process with the solution of NaOH containing methanol at 300°C[15].Thus it was suggested that the hydrothermal process was superior to purely thermal treatment at the same temperature [15].To increase the decomposition rate of the hydrothermal process,much work has been done so far.It was found that the addition of alkaline reagents improved the degradation of PCDD/Fs (Polychlorinated dibenzo-p-dioxins and dibenzofurans) [16].Ma et al.[17]found that in the presence of oxygen,the degradation reaction of dioxins was significantly accelerated.The experiments showed that 88.31% of the total PCDD/Fs was removed at 150°C in 2 h,while only 38.45% was destroyed at the same temperature in 12 h under nonoxidative conditions.Hu et al.[13]observed that ferric and ferrous sulfate enhanced the dioxin decomposition procedure and worked as a catalyst and that reaction temperature was the most important factor.Xie et al.[12]reported the use of carbohydrazide as an auxiliary chemical during hydrothermal treatment to remove dioxins,which increased the dioxin decomposition rates to above 80%and 90%at 518 K and 533 K,respectively.Nevertheless,no significant change was found for the toxic PCDD/Fs,indicating that the treated fly ash still contained highly toxic PCDD/Fs.Peroxide oxidation in H2SO4/HNO3solution destroyed nearly 99% PCDD/Fs with microwave heating at 150°C in 2 h.At the same time,a few of the PCCD/Fs were dissolved into the acid solution,which produced more toxic waste water [1].

In this work,microwave heating and hydrothermal treatment were combined to study their effect on the degradation of PCDD/Fs.Microwave heating was intended to increase the molecular diffusion rate and degrade or dissociate more dioxins in a shorter treatment time.In our previous research,microwave-assisted heating was applied to accelerate the stabilization of heavy metals in MSWI fly ash during hydrothermal process.For the equivalent stabilization effect,it is found that the disposal time(30 min)with microwave heating is significantly shorter than that under traditional heating (several hours).Furthermore,with the assistance of microwave,the dissolution of silica and aluminum minerals in fly ash can be accelerated,shortening the crystallization period from hours to a few minutes meanwhile [18].Based on these observations,it is rational to suppose that the degradation of PCDD/Fs can also be accelerated with microwave heating during hydrothermal treatment.Compared with traditional hydrothermal process,microwave-assisted hydrothermal process decreased the energy consumption and disposal cost.In addition,so far there are few literatures to study the hydrothermal conditions that make a difference on both dioxin degradation and heavy metal stabilization.In our work,the leaching concentration of heavy metals in raw and treated fly ash was detected to get the stabilization effect and it indicated that dioxins and heavy metals were simultaneously detoxified in microwave-assisted hydrothermal process,which provided a new understanding of hydrothermal treatment.Orthogonal experiments were designed to evaluate the significance of different factors (additives,reaction temperatures and reaction times).The distribution of PCDD/F homolog groups and chlorination degree of fly ash samples treated under different conditions were studied to analyze the degradation pathways of PCDD/Fs with the effect of microwave.

Since we have studied in detail the solidification of heavy metals in MSWI fly ash by microwave-assisted hydrothermal treatment in our previous work [19-21],we report here a comparison of PCDD/F concentrations in fly ash leachate after different treatment conditions.The primary objective is to provide a new treatment for the simultaneous disposal of PCDD/Fs and heavy metals in MSWI fly ash.Although PCDD/F degradation efficiency is not sufficiently high,the process nonetheless provides a new route to reduce the toxicity of MSWI fly ash.

2.Materials and Methods

2.1.Materials

The MSWI fly ash used in this study was sampled from the baghouse of a circulating fluidized bed (CFB) boiler at Xiaoshanincinerator plant,in which only MSW was combusted.The samples were dried at 105°C for 24 h before and after treating.The chemical reagents were all of reagent grade,including sodium hydroxide(NaOH)and sodium phosphate(Na2HPO4).The elemental composition was determined by an X-ray fluorescence (XRF) spectrometer(ThermoFisher,IntelliPower 4200).

Table 1 Orthogonal experiments design

Table 2 Experimental conditions

2.2.The microwave experiments

The experiments were performed using a microwave apparatus(Sineo MDS-6,China).The dried fly ash was mixed with different chemical reagents and deionized water in a liquid/solid (L/S) ratio of 3 ml·g-1.The reagent dosage was 5 wt% of dried MSWI fly ash.The mixture was sealed in a modified poly(tetrafluoroethylene)(TFM) container with the assistance of a frame structure,which is a matching product of the microwave device.The maximum operating temperature of the microwave system is 250°C.The reaction temperature and reaction time were controlled by the microwave system.An orthogonal experimental design was adopted in this study.The experimental conditions performed are shown in Tables 1 and 2.Parallel experiments were carried out.The treated MSWI fly ash was dried before the analysis of dioxins content.The degradation efficiency (η) of PCDD/Fs and WHO-TEQ is calculated as:

where C0is the PCDD/F or WHO-TEQ concentration of raw MSWI fly ash;Ctis the PCDD/F or WHO-TEQ concentration of the treated MSWI fly ash.

2.3.Dioxin analytical methods

Approximately 1.0 g sample to be tested was treated in 40 ml hydrochloric acid(HCl)solution for 4 h,and the HCl concentration was 2 mol·L-1.Then,the mixture was filtered to separate the solid and liquid parts.The solid phase was washed with distilled water several times until the aqueous layer was pH 7.The separated liquid phase was extracted twice with 30 ml dichloromethane,and after shaking vigorously for 20 min,the dichloromethane layer was dried.The dried solid phase was transferred to a Soxhlet extractor and spiked with a mixture of13C-labeled PCDD/PCDF internal standards for quantification before Soxhlet extracting for 24 h with 250 ml toluene.The Soxhlet extraction solution was mixed with the dried dichloromethane layer.A multistage silica gel column and an alumina column were assembled for stepwise purification of the concentrated solution mixture.The purified extract was evaporated to near-dryness under a stream of nitrogen and then dissolved in 25 μl nonane for PCDD/F analysis.Highresolution gas chromatography-mass spectrometry (HRGC/MS,JMS-800D,JEOL Co.,Japan) was applied to determine the PCDD/F concentrations.The recoveries of the PCDD/F standard solution met the standard requirements of US EPA method 1613B (EPA Method 1613).Replicates were conducted for each trial.

2.4.Leaching test

The solid waste extraction procedure for leaching toxicity,namely,the acetic acid buffer solution method (HJ/T 300-2007),is the accepted method for assessing the stability of heavy metals in fly ash.According to the standard,the acetic acid(pH=2.64±0.05) extraction buffer was selected as the leaching solution,and the liquid-to-solid ratio was 20 ml·g-1.The mixture of the leaching solution and fly ash was subsequently extracted for(18±2)h at a speed of(30±2)r·min-1,after which the mixture was filtered to collect the leachate.Finally,the concentration of heavy metals in the leachate was detected by inductively coupled plasma-atomic emission spectroscopy (ICP-AES;Thermo Scientific XII,USA).Replicates were conducted as well.

3.Results and Analysis

3.1.Characteristics of MSWI fly ash

The raw fly ash is collected from the circulating fluidized bed(CFB) of an incinerator plant (Hangzhou,China) with only MSW combustion.An air pollution control (APC) system is equipped,including selective noncatalytic reduction (SNCR),denitration,a semidry scrubber,activated carbon injection,and a fabric filter.Therefore,the fly ash sample,collected from the fabric filter,is representative.The sample was dried at 105°C in an oven for 24 h before use in this study.The elemental composition of raw MSWI fly ash is shown in Table 3.Elemental analysis indicated thatCaO,SiO2and Al2O3were the main oxides,accounting for 35.0%,22.4% and 9.7% of the total oxides,respectively.WHO-TEQ means the toxic equivalency(TEQ)of dioxins is calculated using the toxic equivalency factors (TEF) evaluated by World Health Organization(WHO).The total polychlorinated dibenzo-p-dioxin (PCDD) and polychlorinated dibenzofuran (PCDF) concentration in the raw MSWI fly ash was 180-882 pg·g-1,and the WHO-TEQ value (seventeen individual toxic 2,3,7,8-substituted PCDD/Fs) was 2601.4 pg of WHO-TEF·g-1(as shown in Table 4).As reported[3,22],OCDD is usually the dominant component of dioxins in MSWI fly ash;the major source of PCDD/F toxicity comes from the 2,3,4,7,8-PeCDF homolog,which accounts for 26.8% of 17 PCDD/PCDF congeners.For the toxic congeners (2,3,7,8-substituted) in raw fly ash,the concentrations of low-chlorinated congeners were higher,but highly chlorinated congeners contributed more to the WHO-TEQ value[23].The total concentration of PCDFs is slightly higher than PCDDs at the ratio of 1.1:1.The concentration distribution of dioxins in raw MSWI fly ash samples is shown in Fig.1.

Table 3 Elemental content of raw MSWI fly ash (wt%)

Fig.1.The concentration distribution of dioxins in raw MSWI fly ash sample(pg·g-1).

3.2.Degradation of PCDD/Fs under the microwave-assisted hydrothermal process

A comparison of PCDD/F concentrations and WHO-TEQ values in raw and treated fly ash under different microwave-assistedhydrothermal conditions from orthogonal experiments is summarized in Table 5.For ease of understanding,it must be clarified that“degradation”is defined as the general term of “destruction”and“dechlorination”.In this study,destruction means that the main structure of dioxins is destroyed,while dechlorination means that chlorine atoms are replaced by hydrogen atoms during the process.The experiments were carried out with a fixed liquid-to-solid ratio of 3 ml·g-1(water/fly ash) and fixed reagent dosage of 5 wt%.The effect of different reagents,temperatures and time was evaluated.The degradation rate was selected as the evaluation criteria.Here,the total degradation rates of the three different influencing factors were termed I,II and III.The difference between the maximum and minimum of these degradation rates under the same factor-related conditions was termed as the “range”,which indicates the degradation effect of different influencing factors.

Table 4 WHO-TEQ and PCDD/F concentrations in raw MSWI fly ash (pg·g-1)

Table 5 Degradation efficiency of PCDD/F concentrations and WHO-TEQ values

Among the nine trials,test 3 performed best,with a degradation rate of 83.7% (PCDD/Fs) and 68.5% (WHO-TEQ).According to the range analysis,the reaction temperature was the most important factor for the degradation of the total PCDD/Fs,followed by reaction time and reagents.Nevertheless,in terms of the decrease in the WHO-TEQ values,the reaction temperature and time both played crucial roles,followed by the reagent.Comparing the values of I,II and III,it was found that the best reagent was NaOH.Similarly,it was concluded that increasing the reaction temperature and time also increased the degradation efficiency,which matches the conclusion of previous literature reports [12,17].Based on the degradation efficiency of PCDD/Fs,the optimal condition from orthogonal design is NaOH,3 h and 220°C,which is the same with test 3.The degradation rate of WHO-TEQ at 185°C and 220°C was 145.02%and 151.22%,respectively.That is,no obvious impact was found with increasing temperature,thus suggesting that microwave heating reduced the degradation temperature of toxic dioxins.Since the range difference of temperature and time was relatively close to the degradation of WHO-TEQ,the degradation efficiency of dioxins could be improved by prolonged reaction time,instead of increasing reaction temperature.However,for the total PCDD/F concentration,reaction temperature was still the dominant factor.This phenomenon led one to the conclusion that the nontoxic dioxins were more sensitive than toxic dioxins to the reaction temperature.

To determine the degradation pathways,the PCDD/F distribution over homolog groups and its average degree of chlorination were used,which are summarized in Tables 6 and 7.The degree of chlorination (dc,average number of chlorine substituents) was calculated as follows:where wiis the mass percentage of all PCDD/F congeners,niis the number of hydrogen atoms substituted by chlorine.

Table 6 PCDD/F homolog groups and degree of chlorination (pg·g-1)

Table 7 PCDD/F homolog distribution (%)

Table 8 Internal variance of the individual congeners

Table 9 Toxic PCDD/F concentration distribution (pg·g-1)

Previous literature [11,24]has suggested that dechlorination and destruction of PCDD/Fs (or other chlorinated aromatics)occurred simultaneously and that dechlorination usually dominated the degradation of PCDD/Fs,which exhibited a decreased degree of chlorination after treatment.In our study,a strange phenomenon was observed;the degree of chlorination of PCDFs increased,except in test 3.It was the formation of OCDF that increased the chlorination degree,according to PCDD/F homolog group distribution.For example,the concentration of OCDF in test 2 was approximately 21600 pg·g-1(nearly ten times that of the concentration in the raw MSWI fly ash),while the chlorination degree increased from 5.46 to 6.67.In terms of the percentages of homolog groups,the percentages of HpCDF and OCDF under most disposal conditions were much higher than those in the raw sample.The chlorine atoms in PCDFs more actively took part in dechlorination and chlorination reactions compared with those in PCDDs [25].Generally,after hydrothermal treatment,the concentration of PCDD/Fs decreased as a function of dechlorination and destruction,and dechlorination resulted in an increase in low-chlorinated PCDD/Fs and a decrease in highly chlorinated PCDD/Fs [1].However,based on the results of this work,it was hypothesized that a transformation from lower chlorinated PCDFs to higher chlorinated PCDFs occurs.The microwave degradation of polychlorinated furan was accompanied by a strong chlorination reaction in addition to the direct destruction.For PCDDs,the chlorination degree of most trials basically remained unchanged,and that of test 3 decreased to 5.37.Thus,it was concluded that destruction of PCDDs was the main pathway of degradation.In terms of the degradation rate of PCDD/F and WHO-TEQ (83.7%and 68.5%,from the experimental results),the dechlorination was not negligible.However,the chlorination obviously occurred,based on the increased chlorination degree.In total,the microwave-assisted hydrothermal degradation included both direct destruction and dechlorination.On the other hand,the existence of chlorination,caused by microwave heating,weakened the PCDD/F degradation.Comparing tests 1,2 and 3,it was speculated that part of the degradation mechanism during the microwave hydrothermal process was the destruction of the dioxin structure,first by a chlorination reaction and then by a direct destruction reaction.At higher temperatures,the destruction of 2,3,7,8-PCDDs/Fs occurred much more when CaO was present [23].As Chen et al.[11]reported,the presence of Cl in fly ash reacted with dioxin precursors,thus inhibiting dioxin degradation.Chlorination resulted from the existence of chlorine in the MSWI fly ash.The fly ash sample in this experiment,which was not washed,had a Cl content of approximately 6 wt%.In addition,the concentration of PCDFs was higher than that of PCDDs,both in the raw fly ash and in the treated fly ash.The proportion of PCDDs/PCDFs in fly ash decreased after the reaction,indicating that the microwave hydrothermal degradation effect was greater on PCDDs than on PCDFs.This result was also opposite with other hydrothermal degradation effects,which was speculated to be caused by the microwave process.In conclusion,microwave treatment promoted the PCDD/F degradation process,reduced the reaction time,and led to chlorination due to higher intermolecular reactivity [26].With the existence of Na+in the solution,it was reported that the dechlorination will be accelerated,and OH-will be beneficial to the hydrolysis of PCDD/Fs [27].The heavy metals in fly ash tend to play a role of catalyst in PCDD/F degradation [13,28].The additives used in this study will also influence the PCDD/F degradation by impacting heavy metal behaviors in fly ash.In future work,inhibiting chlorination should be researched,which may increase the degradation efficiency.

Table 10 Correlation between indicative congener and TEQ value of PCDD/Fs

Table 11 Leaching concentration of heavy metals in raw and treated MSWI fly ash (mg·L-1)

3.3.Internal variance of the congeners

The internal variance of the individual congeners is shown in Table 8,which was evaluated on the basis of standard deviation(SD,%).TCDD(32%)and PeCDF (31%)showed the highest variance for the arithmetic average of all the isomers,whereas HpCDD showed a negligible variance of 4%.These findings were consistent with the results of Chen et al.[11].The most variable (＞50%) congeners were 1,2,3,6-TCDD (96%),1,2,3,4+1,2,6,9-TCDD (84%),1,4,6,7,8-PeCDF (90%),1,2,3,6,8+1,3,4,7,8+1,2,4,7,8-PeCDF (76%),1,3,4,6,8+1,2,4,6,8-PeCDF (70%),2,3,7,8-TCDF (65%).At the same time,the most invariant congeners were 1,2,3,7,9-PeCDD,1,2,3,4,6,7,8-HpCDF,1,2,3,4,6,7,9-HpCDF,1,2,6,7-TCDF and 2,3,6,7+3,4,6,7-TCDF,whose standard deviations were below 5%.The value of standard deviation showed the sensitivity of each congener to the microwave-assisted hydrothermal process.

3.4.PCDD/F index

A PCDD/F index determined by the concentration of the indicative congener is useful to quantitatively characterize the dioxin toxicity of fly ash,and the representative congener content of 2,3,4,7,8-PeCDF has been widely used[22].Fiedler et al.[29]found that the concentration of 2,3,4,7,8-PeCDF was proportional to the ITEQ of PCDD/Fs in stack emission gas samples,with highly linear correlations (R2＞0.99).Yun et al.[3]reported that the concentration of 1,2,3,4,7,8-HxCDF +1,2,3,4,7,9-HxCDF showed a strongest correlation to the WHO-TEQ of fly ash samples.As shown in Table 10,the findings in our study were summarized based on the concentrations of toxic congeners and the TEQ values (presented in Table 9).The results revealed that the concentrations of 2,3,4,7,8-PeCDF and 1,2,3,7,8+2,3,4,7,8-PeCDF were strongly correlated with the total toxicity of fly ash,regardless of the different reaction temperatures,reaction times and reagents.This conclusion([I-TEQ of PCDD/Fs]=1.4[2,3,4,7,8-PeCDF])was similar to previous research conclusions [3,29].

3.5.Heavy metal solidification

The solidification of heavy metals was studied in detail in our previous work [21];thus,the results of this study on this aspect are simply presented (Table 11).The leaching concentrations of Cd (0.6025 mg·L-1) and Pb (0.4639 mg·L-1) in raw fly ash were above the permissible regulatory limits based on the Chinese national standard GB16889-2008.In the previous experiments[21],heavy metals were perfectly solidified under the microwave heating condition of 1 mol·kg-1(Na2HPO4/fly ash),150°C,and 20 min.After treatment,in this study,the leaching results of test 6 reached the standard.Although the heavy metal leaching results under these conditions were not very ideal because of the small amount of additive added,it provided experimental basis for the disposal of dioxins and heavy metals using microwave-assisted hydrothermal treatment.As long as the appropriate amount,dosage,reaction temperature and time are selected,ideal disposal results can be achieved.

4.Conclusions

Microwave heating and hydrothermal treatments were applied to degrade PCDD/Fs of MSWI fly ash in this work.The effects of additives,reaction temperature and reaction time were studied through orthogonal experiments.The results are listed as follows:

(1) Reaction temperature was the most important factor for the degradation of the total PCDD/Fs;in terms of WHO-TEQ value,both reaction temperature and reaction time dominated the effect of toxic PCDD/Fs.

(2) The microwave-assisted hydrothermal process accelerated the degradation of PCDD/Fs,and more than 80% PCDD/Fs were removed in 3 h treatment at a relatively low temperature.In the presence of Cl,the degradation efficiency might be reduced under the microwave conditions.

(3) Several PCDD/F indexes by the concentration of indicative congener were found to quantitatively characterize the dioxin toxicity of fly ash.For example,[I-TEQ of PCDD/Fs]=1.4[2,3,4,7,8-PeCDF].

(4) Microwave-assisted hydrothermal treatment provided an experimental basis for the simultaneous disposal of dioxins and heavy metals.