使用浒苔提取物作为传统混凝剂在黄河水处理混凝行为和絮状物特性的助凝剂的影响

Influence of using Enteromorpha extract as a coagulant aid on coagulation behavior and floc characteristics of traditional coagulant in Yellow River water treatment
Shuang Zhao,Baoyu Gao ⇑,Xiangzuo Li,Min Dong
Shandong Key Laboratory of Water Pollution Control and Resource Reuse,School of Environmental Science and Engineering,Shandong University,Ji’nan 250100,Shandong,People’s Republic of China
h i g h l i g h t s
"An effective way of the recycle of hazard wastes-Enteromorpha was found.
"A new kind of coagulant aid-Ee was found and applied in the water treatment process."Coagulation performance could be obviously enhanced when appropriate Ee was dosed."Floc characteristics were significantly improved due to Ee addition.
a r t i c l e i n f o Article history:
Received 24April 2012
Received in revised form 21June 2012Accepted 21June 2012
Available online 30June 2012Keywords:Coagulant aid
Enteromorpha extract Coagulation performance Floc characteristic
a b s t r a c t
A new coagulant aid,Enteromorpha extract (Ee),was used together with traditional coagulants in water treatment process to assess its effect on coagulation behavior and floc characteristics.Four kinds of alu-minum and ferric coagulants were studied in this article,with the results indicating that aluminum sul-fate (AS)was the optimal coagulant due to its high efficiency and perfect cooperation with Ee for given test water.Then coagulation behavior and floc characteristics of AS coagulation system were studied.Turbidity,UV 254(ultraviolet absorbance at 254nm wavelength)and DOC (dissolved organic carbon)removal efficiencies were used to evaluate coagulation effects;and floc characteristics were investigated in terms of floc size,strength and recovery ability.The results obtained in this study indicated that the coagulation effect of AS could be significantly improved by around 30%when appropriate proportion of Ee was dosed.When AS was used in conjunction with Ee,the generated flocs had bigger sizes and faster growth rate than those generated by AS alone.Meanwhile,the former were also stronger and had better recovery abilities than the latter.The charge neutralization was the dominant mechanism in AS coagula-tion,and Ee displayed good adsorption bridging effect.Consequently,coagulation efficiency could be enhanced significantly due to the combination of two advantages.
Ó2012Elsevier B.V.All rights reserved.
1.Introduction
It is well known that coagulation is the most common process used for particles and organic matter removal in drinking water treatment [1].In this process,coagulants are added to aggregate destabilized dissolved organic matter (DOM)and colloidal particles into larger-sized flocs,and then the flocs can be effectively re-moved in sedimentation processes [2].Due to the effectiveness in treating a wide range of waters at relatively low lost,traditional coagulants (aluminum and ferric salts)become the most com-monly used coagulant in water treatment [3–5].However,recent studies have discovered a number of drawbacks of using tradi-tional coagulants.Alum salts may cause human body disease and ferric salts may cause the unsightly brown staining of equipments [6–8].Therefore,it is necessary to decrease the dosage of tradi-tional coagulants to reduce negative effects.Applying coagulant aids is the most common method to solve above problems,and meanwhile,the water treatment cost can also be cut down by dos-ing appropriate coagulant aids.Coagulant aids can accelerate the flocculation process or strengthen the floc to make it easier to fil-ter.They can be roughly divided into two categories based on their mechanism of action.Synthetic polymers are the most common coagulant aids,which bind to particles much like coagulants.Oth-ers,generally the inorganic and natural polymers,act as sites of nucleation to speed the formation of floc.However,it has been
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Corresponding author.Tel.:+8653188364832;fax:+8653188364513.
E-mail address:baoyugao_sdu@ (B.Gao).
reported that synthetic polymers contain contaminants from the manufacturing process that may threaten human health[8,9]. Meanwhile,Synthetic polymers can also react with other chemi-cals added to the water treatment process to form undesirable sec-ondary products[10–12].Natural polymers,which are extracted from plant or animal life,can be workable alternatives to synthetic polymers[13–16].They are biodegradable and also have a wider effective dosage range offlocculation for various colloidal suspen-sions.Therefore,it is indispensable to develop high efficient and low cost natural polymers which were used as coagulant aids for water and wastewater treatment[17–19].
Enteromorpha is a kind of green algae,which is known as a dominant species in saline coastal wetlands with high nitrogen levels[20].Enteromorpha can tolerate salinities varying from freshwater to seawater.As a fast growing and opportunistic macro-algae,this species can proliferate in a wide range of abiotic and biotic conditions[21–23].In the summer of2008,Enteromorpha broke out in Yellow Sea and East Sea on a large scale,which posed a threat to the Sailing Competition of the2008Beijing Olympics. The local government had to spend much manpower andfinancial resources to carry out the emergency treatment measures against the disaster.But Enteromorpha management is supposed to be a long termfight rather than an episode during the29th Olympic Games for China,since it has been gaining in scale during the past 3decades in both marine and estuary environment all over the world.As we all know that waste is a kind of resource that was put in the wrong place.The comprehensive utilization of Enteromorpha is the ultimate way to settle the problem rather than piled them on the beach.But there has not been too much literature about the recycle used method of Enteromorpha at pres-ent since most researches focused on the Enteromorpha biological characteristics[24,25].It was once reported that Enteromorpha can be successfully transformed into biomass oil under appropriate conditions[26],but the cost was higher than the low-level fuel oil existing on market.The higher production cost limits its application and this technology is not suitable for the actual mass production.Enteromorpha is rich in macromolecular substances which can play an positive role in water supply and wastewater treatment,and also it has advantages of containing many active groups,various natural polymers and non-toxic character. Considering all the advantages of Enteromorpha,it is worth trying to apply the extracting solution of Enteromorpha(compound of many kinds of natural polymers)as a coagulant aid in coagulation process of water treatment.It is a tentative try since there is no relevant literature so far.By doing this,we attempt to provide a new solution to the Enteromorpha disaster,on the other hand,a new-style,cheap and non-toxic coagulant aid may be found.
In this research,several kinds of aluminum and ferric salts were selected to use in conjunction with Ee in the treatment process of test water.Traditional coagulants were addedfirstly at the start of rapid mixing in coagulation process,and then Ee was dosed after 30s.That was denoted as dual-coagulant,which can be displayed in form of coagulant–coagulant aid.The coagulation aid perfor-mance of Enteromorpha extract(Ee)forflocculation–sedimenta-tion process was studied through a series of contrast test. Coagulation effects of traditional coagulants and dual-coagulants were comparatively investigated in terms of turbidity,UV254 (ultraviolet absorbance at254nm wavelength)and DOC(dissolved organic carbon)removal efficiency.Zeta potential was also researched in order to study the coagulation mechanism. Furthermore,the growth,breakage and re-growth properties of flocs produced in optimal condition were conducted.Based on the coagulation performance andfloc properties,the coagulation aid effect of Ee can be found,meanwhile,the coagulation aid mechanism can be discussed.2.Materials and methods
2.1.Chemicals and raw water
The chemicals used in this study including FeCl3Á6H2O,FeS-O4Á7H2O,AlCl3Á6H2O and Al2(SO4)3Á18H2O,which were all pur-chased from Sinopharm Chemical Reagent Co.,Ltd.,Beijing.All reagents used were of analytical grade.Deionized water was used to prepare all solutions.
The raw water investigated in the study was sampled from Quehua water works of Jinan city in China in October2011.The properties of the test water were:temperature=19.5–22.5°C, pH=8.17–8.45,turbidity=2.18–2.76NTU,UV254=0.038–0.045cmÀ1,DOC=3.636–4.284mg/L.
2.2.Preparation of coagulants
Fresh Enteromorpha were gathered fromfirst bathing beach of Qingdao,China.They were put into the vacuum drying oven for5h at the constant temperature of70°C after being washed.The dry sample were shattered into powder by magnetic crusher and then passed through100mesh sieve.The screening Enteromorpha and deionized water were mixed at the mass ratio of1:75and then that compound was heated in water bath for4h at the constant tem-perature of90°C[27].The extract of Enteromorpha were centrifu-galized for30min at the speed of5000r/min.Supernatant were collected to measure volume and then diluted with equal volume of deionized water.That was used as the coagulant aid and the dos-age of Ee was metering by volume.
Traditional coagulants and dual-coagulants,were compara-tively investigated in this study.The conventional coagulants, FeCl3Á6H2O and FeSO4Á7H2O,were used to prepare iron solutions with deionized water at a concentration of1.0g/L as Fe.Similarly, AlCl3Á6H2O and Al2(SO4)3Á18H2O were used to prepare alum solu-tions at a concentration of1.0g/L as Al.
2.3.Jar-test
At a room temperature of20±2°C,Coagulation experiments were conducted by a jar-test apparatus(ZR4–6,Zhongrun Water Industry Technology Development Co.Ltd.,China).The test water of1000mL was poured into each of the1400mL plexiglass beakers and a six-paddled stirrer was used for mixing.The coagulation pro-cedure was as follow:predetermined amount of coagulant or dual-coagulant was dosed after30s rapid stirring of200rpm;after 1.5min,the stirring speed was changed to40rpm with a duration of15min;then after30min of quiescent settling,sample was col-lected from1cm below the surface for measuring.Part of sample was directly used for measuring residual turbidity and zeta poten-tial by using a2100P turbidimeter(Hach,USA)and zeta sizer 3000HSa(Malvern Instruments,UK),respectively.The remaining samples wasfiltrated by0.45l mfiber membrane and then was used to test UV254absorbance and DOC.UV254absorbance was analyzed with an UV-754UV/VIS spectrophotometer(Precision Scientific Instrument Co.Ltd.,Shanghai,China),and DOC was mea-sured by a TOC analyzer(TOC-VCPH,Shimadzu,Japan).
2.4.On-line monitoring offloc formation,breakage and re-growth measurement
A laser diffraction instrument Mastersizer2000(Malvern,UK) was used to monitor the evolution offloc size in coagulation pro-cess.The suspendedfloc was monitored through the sample cell of the Mastersizer and then transferred back into the jar by a peri-staltic pump(LEAD-1,Longer Precision Pump,China)using a5mm
570S.Zhao et al./Chemical Engineering Journal200–202(2012)569–576
internal diameter tube at aflow rate of2.0L/h.The pump was de-signed to be located at the downstream of Mastersizer to prevent disturbing theflocs prior.The inflow and outflow tubes were posi-tioned opposite each other at a depth just above the impeller in the holding ports.Size measurements of thefloc were taken every half minute in the process of coagulation and the results were recorded automatically by computer.
Floc characteristic experiments were carried out under different dosage of(aluminum sulfate)AS and Ee.The coagulation procedure were conducted as follows:(1)a rapid mix step at200rpm for 1.5min;(2)a slow stir step at40rpm for15min;(3)a breakage step for5min200rpm;and(4)a slow stirring at40rpm for a fur-ther15min forflocs re-growth.
2.5.Characteristic representation offloc
In this study,the median volumetric diameter(d50)was used to denote thefloc size.The growth rate was an important representa-tion offloc characteristic.It was denoted as the slope of rapid growth region.The larger the growth rate,the fasterfloc generated. It was calculated as follow[28,29]:
growth rate¼D size
ð1Þ
For comparing the strength and re-growth ability offlocs in differ-entflocculated systems,floc strength factor(S f)and recovery factor (R f)were induced.They could be calculated by the following equa-tions[30,31]:
S f¼d2
d1
Â100ð2Þ
R f¼
d3Àd2
12
Â100ð3Þ
where d1(l m)and d2(l m)represent the stablefloc size before and after breakage,while d3represents thefloc size after re-growth to the new steady step.The strength factor value indicates the ability offlocs to withstand shear,such that a higher value suggests that theflocs are stronger.Meanwhile,the recovery factor is an indica-tive offloc recoverability,such that a higher value implies that theflocs have better re-growth ability after breakage.All the exper-iments were carried out three times and very little variation was observed.
3.Results and discussion
3.1.Coagulation effect of Ee
Although the Enteromorpha was non-toxic,but the outbreaks of it could block sunshine just as the red tides,and meanwhile the death Enteromorpha would consume oxygen in the sea,so the floating Enteromorpha may affect the algae growth in seabed.Fur-thermore,the outbreak of Enteromorpha will seriously affect land-scape and the proceeding of water sports.That is the biggest disadvantage that the people want to try to eliminate.Therefore, it is of great significance to explore an effective way for compre-hensive utilization of Enteromorpha,as well as for the restoration and improvement of the ecological environment and development of a recycling economy.In this section,Ee was used as a kind of coagulant.Fig.1showed the variation of turbidity,UV254and DOC removal efficiency with various Ee dosages.
Fig.1showed that the turbidity and UV254removal gradually increased with the increasing Ee dosage,while the removal trend of DOC showed a parabola shape.The optimum point was at the dosage of0.1mL/L,in which condition the DOC removal rate was 20.5%.Zeta potential decreased with the increasing Ee dosage and below zero all the time within the whole dose ranges investigated. This indicated that charge neutralization is not the dominate mechanism of Ee.Due to the repulsive force between negative col-loids,thefloc was quiet small and hardly to settle down.Therefore, the removal of organic matter was in low levels.Overall,Ee exhib-ited some potential in removing organics,but good coagulation efficiency could not be achieved when Ee was used as a coagulant. So the performance of using Ee as a coagulant aid in coagulation process was studied as follow.
3.2.Determination of the optimal coagulant
Appropriate coagulant will not only improve the effluent water characteristics,but also can decrease the cost of water treatment. In order to ascertain the optimal coagulant for given test water, experiments were performed with afixed coagulant aid(Ee)in conjunction with four traditional coagulants(FeCl3,FeSO4,AlCl3 and Al2(SO4)3).Coagulants doses varied within a range of 2–12mg/L while coagulant aid dose was constant at0.1mL/L. Turbidity,UV254and DOC removal efficiency were taken to esti-mate coagulation performance.Results were shown in Fig.2.
It can be seen from thefigure that the coagulation performance of four coagulants showed significant pared with iron salts coagulants,aluminum salts had better coagulation ef-fects with fasterflocs growth rates and largerflocs sizes.For DOC removal:the removal rates were rather low when iron salts were used and the maximal value was21%,while the removal of DOC can reach34%when aluminum salts were used.The conditions of turbidity and UV254removal efficiencies are similar to that of DOC.For aluminum salts coagulants,both of them could provide relatively high performances even at low dosages.Meanwhile the efficiencies of coagulation increased with coagulants doses increasing.In case of AlCl3,the application of coagulant aid could improved the performance,but only slightly effect could be ob-served,while the addition of Ee led to a significant increase of coagulation performance in Al2(SO4)3system.Consequently, Al2(SO4)3was chosen to carry out the following experiments as the optimal coagulant for the test water.
3.3.Determination of the optimal dose of coagulant aid
In this section,the tests were performed with afixed coagulant (AS)based on Section3.2.Optimization tests were conducted to ascertain the optimum Ee dosage for particles and organic matter
S.Zhao et al./Chemical Engineering Journal200–202(2012)569–576571
removal under the raw water pH conditions.Traditional coagulant (AS)and dual-coagulant(aluminum sulfate–Enteromorpha extract (AS–Ee))were comparatively evaluated in terms of coagulation performance.The variation of coagulation efficiencies and zeta po-tential were presented in Fig.3with the coagulant dosages2–12mg/L as Al.
When AS was used alone,Fig.3showed that UV254and DOC re-moval efficiency increased with alum dosage increasing,but the turbidity removal increased rapidly at a low alum dosage and then decreased slightly as the alum dose further increased.The coagulation with AS has been studied for decades and the coagula-tion mechanisms of it include charge neutralization,precipitation, bridge-aggregation,adsorption and sweep-flocculation[32,33].As shown in Fig.3,the organic matter removal efficiency increased as AS dosage increasing,which was in accordance with the increas-ing zeta potential(Fig.3d).The highest organics removal was achieved at the dosage of12mg/L with zeta potential of0.3mV, which indicated that charge neutralization was the dominant mechanism of AS coagulation system.The alkaline test water could produce precipitation of amorphous hydroxide,so the charge neu-tralization was likely to be achieved by adsorbed precipitate. Therefore,efficient turbidity removal still occurred at low dosages, even though the zeta potentials were below zero.
When adequate Ee(0.1mL/L,0.3mL/L)was used in conjunction with AS,the turbidity,UV254and DOC removal were higher than that of AS used alone.Especially when Ee dosage was0.3mL/L,the organics removal efficiency was significantly improved:the UV254and DOC removal rate exceeded47%and44%at the alum dosage of12mg/L,while the removal obtained by AS alone were only37%and35%in the same test condition.Even though higher organic matter removal was obtained,zeta potential was still be-low zero(Fig.3d).That suggested that charge neutralization may be not the dominant mechanism for organics removal by AS–Ee. When AS was dosedfirstly,it quickly adsorbed on the surface of the microflocs and then neutralized the negative charge on it.So repulsion forces between the colloids became very weak.When Ee was dosed30s after AS,the adsorption bridging ability of it could play a significantly positive effect on theflocculation process, which could generate largerflocs those had preferably settling ability.Therefore,better coagulation efficiency could be achieved. But further increase in the dosage of Ee did not cause further ele-vation in the removals of organics:when Ee dosage was0.5mL/L, the coagulation performance was poor and even showed lower le-vel than that of AS used alone.The turbidity removal was about8% lower than that of AS.Since Ee itself was a complicated organic matter,the overdosed Ee in the coagulation system caused the de-crease of organics removal.Meanwhile,in such a situation,zeta potential was underÀ10mV due to larger dose of negatively charged Ee(À35.0mV).Therefore,the stronger inter-particle repulsion and the restrainedflocs growth resulted in poor coagula-tion efficiency.Consequently,based on thefigure and discussion above,it is easy for us to draw the conclusion that the optimal
572S.Zhao et al./Chemical Engineering Journal200–202(2012)569–576
coagulant aid dosage was 0.3mL/L.Considering the coagulants cost and coagulation performance,the dosage of Ee was constant at 8mg/L for the next floc characteristic experiments.
3.4.Effect of coagulant aid on floc size and growth rate
The growth profiles of flocs were investigated and the results were shown in Fig.4,where the floc size was represented by med-ian equivalent diameter.Floc growth rate in steady state were adopted to examine the floc formation process,and the growth rates were shown in Table 1.The dosages of AS were constant at 8mg/L based on Section 3.3,and all the floc characteristic tests were taken in situation of raw water pH.
Fig.4showed that the floc in the AS flocculation process dis-played a sharp increase in size during the first 5min,achieving the largest floc size of 338.1l m,followed by a steady-state period during the next 15min.The sharp growth of floc size in the first 5min is likely due to the aggregation of particles and the stable phase indicated that the floc growth and floc breakage reached
Table 1
Flocs growth rates for various doses.
Ee dosage (mL/L)0.0
0.10.30.5Growth rate (l m/min)
45.87
87.78
122.20
123.54
S.Zhao et al./Chemical Engineering Journal 200–202(2012)569–576
573
appropriate balance.The sizes of flocs generated by AS–Ee and AS showed the similar variation tendency but the former were larger than the latter.As shown in Fig.4:the floc sizes at the steady stage
were 334.7l m,570.4l m,717.3l m,743.3l m for 0.0mL/L,0.1mL/L,0.3mL/L,0.5mL/L Ee dosage,respectively.Meanwhile,Table 1indicated that flocs produced by AS–Ee gave faster aggrega-tion than that of AS:at constant alum dose of 8mg/L,the flocs growth rate could achieve 122.20l m /min when 0.3mL/L Ee was used,while the growth rate of flocs produced by AS was only 45.87l m/min.
The phenomenon mentioned above may be related to the dom-inate coagulation mechanism.The prime mechanism of AS was charge neutralization [2]while extra adsorption bridge were dom-inating when AS–Ee was used.Ee could promote the growth of flocs by adsorption bridge ability due to natural polymers con-tained in it.This result was in agreement with the observation of Ray and Hogg [33],who had reported that flocs produced by bridging flocculation and charge neutralization can be much larger
Table 2
Strength and recovery factors of flocs for different Ee dosages.Parameter
Ee dose (mL/L)0.0
0.10.30.5Strength factor 37.0840.6469.3975.33Recovery factor
19.26
25.73
46.22
55.63
574
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than those formed simply by charge neutralization.As we can see, when0.3mL/L Ee was used,the generatedflocs sizes and growth rate were much lager than those of0.1mL/L Ee applied,but there was no sharp improvement when Ee dosage further increased.The growth curves were almost overlapping when Ee dosage larger than0.5mL/L.This can be explained as follow:The bridge role of Ee can led to the increase of aggregation at the initial aggregation. There were not enough macromolecule matter particles when Ee dosage was small,so the adsorption bridge effect would be enhanced as the increase dosage of Ee.But all particles were positively aggregated and precipitated when Ee dosage larger than 0.5mL/L.Therefore,the due role of extra Ee couldn’t play out. Accordingly,thefloc sizes were hardly to further increased.
3.5.Effect of coagulant aid onfloc breakage and re-growth
The effect of Ee onfloc breakage and re-growth was investi-gated in this section.Fig.5showed that in all cases,theflocs sizes immediately decreased when the shear was introduced by increas-ing the mixing speed up to200rpm.After the shear period of 5min,d50offlocs was about131l m for AS.When AS–Ee were used,theflocs sizes were230l m,493l m,591l m for0.1mL/L, 0.3mL/L,0.5mL/L Ee dosage,respectively.As the shear was re-duced again,theflocs began to re-grow.However,irreversible breakage was usually observed.The initialflocs sizes were not recovered after breakage:flocs formed by AS re-grew up to 172l m size and when Ee was used besides AS,the corresponding floc sizes after the re-growth period was312l m,579l m and 682l m for0.1mL/L,0.3mL/L and0.5mL/L Ee dosage,respectively. In order to investigatefloc breakage and re-growth in detail, the strength and recovery factors were used to interpret thefloc strength and recoverability,and the results were shown in Table2.In parallel,theflocs particle size distributions were also analyzed in this section,and the results were shown in Fig.6.
Table2indicated that when AS–Ee was used in the coagulation process,the strength of generatedflocs were much larger than those of AS used alone:when0.5mL/L Ee was applied in conjunc-tion with AS,thefloc strength factor achieved75.33,which is twice larger than that of AS used alone(37.08).Meanwhile,For Fig.6,an apparent shift in the major peak after breakage could be seen for AS,while,comparatively,small extent of shift in the major peak was found of AS–Ee.Since the shift degree represented the ability of resisting shear,large excursion indicated poorflocs strength. Therefore,the results of Fig.6was in accordance with the conclu-sion of Table2obtained above.This results agreed with the conclu-sion by Li et al.[30]who found thatflocs generated by bridging were stronger than those generated simply by charge neutraliza-tion.Table2also showed that the strength factors increased as the rising dose of Ee with the constant AS dosage.Since there were not enough polymers between the particles when Ee dosage was low,the producedflocs were weak.But when the dose of Ee was large,the polymers were adequate to generate strongerflocs,so the strength factors became larger.For recovery ability,Table2 showed a gradual uptrend as the Ee dosage increased.The result was consist with the conclusion of Fig.6,which showed that:for AS–Ee,the small peak showed a comparatively apparent shift to the right of the major peak,while minor change could be observed between thefloc particle size distributions before and after re-growth of AS.Thefloc recovery factors of AS–Ee were larger than that of AS,which may be the result from Ee,which could undergo scission under high shear rate.Meanwhile,the renewed adsorbent polymer could reform on the particle surface whileflocs generated by AS were immediately broken when the high shear rate was introduced and then could not recover[33].4.Conclusion
In this study,the coagulation aid effect of Ee was investigated and it was found that Ee could be used as a new kind of coagulant aid due to its notable aid effects.Meanwhile,applying Ee in the water treatmentfield can be a new recycle way of disaster wastes–Enteromorpha.In this paper,AS was selected as the opti-mum coagulant from different traditional coagulants due to its high efficiency and perfect cooperation with Ee.The coagulation ef-fect of AS could be improved by around30%when0.3mL/L Ee was dosed.It was also found thatflocs generated by AS–Ee had bigger sizes and faster growth rates than that of AS.Meanwhile,the for-mer were also stronger and had better recovery ability than the lat-ter.The results of coagulation andflocs characteristic test showed that:the main coagulation mechanism of AS was precipitate charge neutralization,and Ee can play a positive aid role by adsorp-tion bridging in water treatment process.Therefore,better coagu-lation performance of AS–Ee could be achieved by the combination of two advantages.
Acknowledgments
This study is supported by The Scientific Technology Research and Development Program of Shandong,China(No. 2010GZX20605)and a grant from the National High Technology Research and Development Program of China(863Program)(No. SQ2009AA06XK1482412).The kind suggestions from the anony-mous reviewers are highly appreciated.
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