Qun-chao Wang,Shu-gen Liu*,Hua-ping Gao

School of Environmental Science and Engineering,Kunming University of Science and Technology,Kunming 650500,China

Received 4 May 2018;accepted 1 November 2018

Available online 30 March 2019

Abstract Hydroxyquinone compounds,such as 1,4-dihydroxyanthraquinone and alizarin sulfonate,are widely used in dye manufacturing,pharmaceutical manufacturing,and other industries.However,thetreatment of hydroxyquinone-containing wastewater hasseldom been examined.This study used a precipitation method with barium salt to treat nano-silver industrial wastewater.The results show that barium chloride was a suitable reagent for signif icantly degrading COD and color from nano-silver wastewater.When the initial pH valuewas 10.5,8 g of BaCl2·2H2O were added to 100 mL of wastewater.After reaction at 15°C for 1 h,the removal eff iciencies of COD and color in the nano-silver wastewater were 85.6%and 97.1%,respectively.Simulated wastewater containing sodium alizarin-3-sulfonate(ARS)or purpurin was used to further investigatethe removal mechanism of hydroxyquinone compounds.Fourier transform infrared spectroscopy,X-ray diffraction,and somerelated experiments showed that hydroxyquinone compounds can directly react with barium ions in the solution so as to transfer from wastewater to precipitate.In addition,the newly produced barium sulfate particles have positive surface charges,which can improve the removal eff iciency of hydroxyquinone compounds due to electrostatic attraction.

Keywords:Hydroxyquinone compounds;Wastewater;Barium salt;Precipitation;Electrostatic attraction

1.Introduction

With the rapid development of pharmaceutical,dye,and pigment industries,hydroxyquinone compounds,such as emodin,alizarin sulfonate,and 1,4-dihydroxyanthraquinone,havecomeinto wideuse(Furkan et al.,2017;Lemlikchi et al.,2014;Tehrani-Bagha et al.,2013)due to their special functional groups.In addition to anthraquinone derivatives,other typical hydroxyquinone compounds may be produced from nano-silver production,which adopts liquid-phase chemical reduction with hydroquinone as a reductant(Li et al.,2010,2015)to prepare nano-silver powder.During the aforementioned processes,wastewater containing hydroxyquinone compounds will inevitably be produced,resulting in potential environmental pollution due to the toxicity of hydroxyquinone compounds.In addition,the color and chemical oxygen demand(COD)concentration in the wastewater are relatively high.According to the National Catalogue of Hazardous Waste of China in 2016,hydroxyquinone-containing wastewater should be classif ied as hazardous waste,and the standards of its disposal should be more stringent.However,previous studies have paid little attention to the treatment of this kind of wastewater.

Inview of theseriousdetrimental effectsof azo dyeor textile wastewater on the environment and human health,a variety of innovative processeshave been developed to treat these kindsof wastewater,and some of them have been applied in practice.These processes can be classif ied into biological,chemical,or physical processes.Thebiological processhasanadvantageover otherprocessesduetoloweroperationcost,andithasbecomeone of themostcommonly used process(Pazetal.,2017;Ayed etal.,2011;Holkar et al.,2014;Rosenkranz et al.,2013).However,because most of the pollutants in dye or textile wastewater are non-biodegradable,the direct biological treatment is generally ineffective(Uygur and K¨ok,1999).In order to facilitate the degradation of those phenolic or aromatic compounds in the wastewater,many researchers have attempted to introduce chemical methods,which include processes using strong oxidizingagents(e.g.,H2O2,O3,and Fentonreagent)(Ayed etal.,2011;Humaetal.,2015;Fangetal.,2017)or advanced oxidation processes(e.g.,H2O2/UVoxidation and catalytic wet air oxidation)(Caiet al.,2015;Kim and Ihm,2011;Song et al.,2016a).Nevertheless,chemical treatments involve some disadvantages,such as complicated procedure,ineff iciency against some refractory matters,and high cost(Bilinska et al.,2016;Zangeneh et al.,2015).Compared with biological and chemical methods,physical methods have the characteristics of compact design,a simple process,and low energy consumption,and the typical processes include coagulation/f locculation,nanof iltration,and adsorption(Lietal.,2016;Liangetal.,2014;Tanetal.,2015;Zhu et al.,2007).Previousexperimentshave proven that hydroxyapatitecan reactwithorganic speciessuch asalizarin sulfonatedue tostronginteractionbetweencalciumionsand phenolic hydroxyl groups in organic molecules,and hydroxyapatite can be potentially used to remove textile dyes from industrial wastewater(Lemlikchiet al.,2014).However,physical methodsare usually noteffectiveincolor removal,or areexpensiveand lessadaptable to a wide range of dye wastewater(Anjaneyulu et al.,2005;Srinivasan and Viraraghavan,2010).

Although many of those technologies,such as advanced oxidation and biological treatment,have been used to treat dye wastewater or refractory phenolic wastewater,there are still no documented application cases of removing organic pollutants and color in hydroxyquinone-containing wastewater.With the aim of developing a highly eff icient technique,typical wastewater,i.e.,nano-silver industrial wastewater,wasselected inthis study.Themethod of bariumsaltprecipitationwasintroduced to remove chromophore groups from wastewater.The collected sediments could be roasted in a muff le furnace to cause the adsorbed organic materialsto burn,and allow for easier disposal of thef inal slag.Using practical industrial wastewater,thisstudy investigated the effects of technical parameters on the removal eff icienciesof COD and color from hydroxyquinone-containing wastewater.The simulated wastewater containing sodium alizarin-3-sulfonate(ARS)or purpurin was then used to further elucidate the removal mechanism of hydroxyquinone compounds.The obtained results can provide a valuable reference for the treatment of hydroxyquinone-containing wastewater.

2.Materials and methods

2.1.Preparation of wastewater

The industrial wastewater was sampled from a nano-silver production company in Yunnan,China.Due to adopting liquid-phase chemical reduction with hydroquinone as a reductant,the produced nano-silver wastewater had a high COD concentration as well as a high level of color.The indicators,including COD concentration,color,and pH value,are listed in Table 1.An ultraviolet-visible(UV/Vis)spectrophotometer(HACH-DR6000,USA)was used to determine the main organic component of the wastewater.Samples of nano-silver wastewater,hydroquinone solution,and benzoquinone solution presented characteristic adsorption peaks at wavelengthsof 312,288,and 245 nm,respectively,revealing that there was no hydroquinone or benzoquinone in the nanosilver wastewater,and that the organic compound in the wastewater should be a stable hydroxyquinone-containing complex.

With the aim of investigating the removal mechanism of hydroxyquinone compounds,two kinds of simulated wastewater were prepared by adding analytical-grade ARS or purpurin.Their mass concentrations were 4000 and 2850 mg/L,respectively,and their molar concentrations were the same.

2.2.Experimental method

The pH of the wastewater was adjusted to the design level with 5 mol/L of NaOH or HCl solution.Then,a certain amount of soluble barium salt was added to the 100 mL of wastewater.After uniform mixing in a 100-mL colorimetric tube,the tube was placed in a water bath to cause the mixture to precipitate at a stable temperature for a period of time.Finally,the reaction mixture was centrifuged for 15 min at a centrifugal acceleration of 10000g(where g isthegravitational acceleration),then f iltered through a 0.45-μm mixed cellulose ester membrane.The collected f iltrate and precipitates could be used for further analyses.

2.3.Analytical methods

The pH of the wastewater was measured with a pH meter(PHS-3C,Yidian Scientif ic Instruments Co.,Ltd.,China).The COD concentration was measured through the standard ref lux titrimetric method(SEPAC,2002),whereas theconcentration was measured through ion chromatography(ICS-3000,Dionex,USA).The color was determined using a HACH-DR6000 UV/Vis spectrophotometer according to the platinum-cobalt scale(Zhang,1994),and the absorbance of f iltrate was measured at 370 nm with a 5.0-cm quartz cell against water.With comparisons of the variations of COD concentrations and color before and after treatment,the removal eff iciency could be obtained.

我国对问题银行进行早期干预、接管、重组、破产、撤销等相关法律规定散见于《存款保险条例》《银行业监督管理法》《企业破产法》等法律法规。相关具体条款主要是明确各监管机构相应的风险处置职责，且非常原则性，没有清晰地界定出问题银行是否需要救助、是否需要关闭，怎样重组、怎样清算退出等一整套处置规则，且有的法规之间缺少有效衔接。

The precipitate was transferred to an electric oven,dried at 60°C for 24 h,and then ground to less than 75μm.The resulting powder was subjected to X-ray diffraction analysis(Empyrean,PANalytical Inc.,USA).In order to analyze the organic functional groups in the precipitate,the Fourier transform infrared spectroscopy(FTIR)spectrum of KBr powder-pressed pelletswere recorded with a Bruker Tensor 27 spectrometer.

3.Results and discussion

3.1.Effects of pH on removal eff iciencies of COD and color

In view of the fact that pH signif icantly affects many physico-chemical processes,such as adsorption(Song et al.,2016b),coagulation,and redox treatment,the effects of pH on the precipitation were investigated in this study.The pH of the nano-silver industrial wastewater was regulated to the design level,and 8 g of BaCl2·2H2Owere added to the100 mL of wastewater.When the pH value increased from 8.5 to 10.5,both the COD concentration and color decreased signif icantly after reaction at 15°Cfor 1 h,and their corresponding removal eff iciencies reached 85.6%and 97.1%,respectively.However,the removal eff iciencies of COD and color only showed a minor increase in the pH range of 10.5-11.5(Fig.1).Since the pH value of the raw wastewater was 10.5,it was not necessary for the next experimental cases to regulate the pH value.

Ge and Jin(1995)investigated the recovery of phenols from coal tar by precipitation,and found that the removal of total phenol presented an ascending tendency when the pH value increased from 7.2 to 10.3,which was consistent with the results obtained in this study.As a metal cation(Mn+)is added to the wastewater containing phenolic compounds(R-OH),the pH value may decrease due to the accumulation of hydrogen ions,based on the following reaction:

Thus,increasing the pH valuemay facilitate theremoval of phenolic compounds,due to the fact that the chemical reaction described above goes forward.

Fig.1.Effects of pH valueon removal eff icienciesof COD and color.

3.2.Effects of temperature on removal eff iciencies of COD and color

This study investigated the variations of COD concentration and color at different reaction temperatures(Fig.2).When the reaction temperature was 15°C,the removal eff iciencies of COD and color reached 85.6%and 97.1%,respectively.However,with the increase of the temperature,the removal eff iciencies of both COD and color presented a gradual decrease,and their values at 55°C dropped to 73.8%and 89.0%,respectively.It was clear that raising reaction temperature was not conducive to the removal of the organic components in hydroxyquinone-containing wastewater.

3.3.Effectsof bariumsalt dosage on removal eff iciencies of COD and color

A certain amount of chemical reagent BaCl2·2H2O was added to 100 mL of nano-silver wastewater at a pH value of 10.5.The removal eff iciencies of COD and color after settling for 1 h at 15°C in different treatment systems are shown in Fig.3.As the dosage of BaCl2·2H2O increased from 4 to 8 g,the removal eff iciencies of COD and color improved signif icantly.When the dosage was higher than 8 g,there was no clear improvement in the removal of hydroxyquinone compounds,and the removal eff iciencies of both COD and color increased less than 1.5%.Therefore,the suitable dosage of BaCl2·2H2O was 8 g per 100 mL of wastewater,and the mass ratio of barium versus COD in the solution was 3.12:1.Under this condition,the concentrations of Ba2+and Cl-in the eff luent were 795 and 1537 mg/L,respectively.Generally speaking,the nano-silver wastewater treated by barium salt may be discharged into the municipal wastewater system for f inal treatment,and may bediluted at least 500 times.It isthen clear that adding BaCl2should not cause secondary pollution in the wastewater treatment system.

Fig.2.Effects of temperature on removal eff iciencies of COD and color.

Fig.3.Effects of barium salt dosage on removal eff iciencies of COD and color.

3.4.Variations of removal eff iciency at different reaction times

Fig.4 shows the removal eff iciencies of COD and color at different reaction times.When the reaction time was set to 30 min,the treatment system achieved a relatively high COD removal eff iciency of 85.8%.However,the removal eff iciency at 5 h decreased to 83.9%.Similarly,theremoval eff iciency of color decreased from 97.6%at 30 min to 94.3%at 5 h.Both the COD and color removal eff iciencies exhibited a gradual decrease with the increase of settling time,indicating that the treatment system only required a short period of time to remove those hydroxyquinone compounds from the wastewater.When the reaction time was 1 h,the removal eff iciencies of COD and color were 85.6% and 97.1%,respectively.This is a minor decrease compared with the removal eff iciency at 30 min.On the other hand,f locculation settlement of the treatment system improved with the increase of reaction time.Therefore,the suitable reaction time was 1 h for the treatment of nano-silver wastewater.

Fig.4.Removal eff iciencies of COD and color at different reaction times.

4.Removal mechanism of hydroxyquinone-containing wastewater

4.1.Characterization analysis of precipitate

FTIR spectra were used to characterize the property of the precipitate.As shown in Fig.5,the standard sample,quinhydrone,presented a noticeable absorption band at wavenumbers of 3394,1604,1380,and 1034 cm-1,respectively,corresponding to vibration bands of four kinds of groups:Ar-OH in phenolic hydroxyl,C=O in quinone compounds,the aromatic ring skeleton,and C-C in quinone compounds(Moriguchi et al.,2003;Weng and Xu,2016).However,some characteristic peaks at wavenumbers of 2924 and 2422 cm-1were weak,and other vibrations,such as asymmetric and symmetric deformation,could not be easily detected,due to molecular symmetry of quinhydrone.For the precipitate derived from the optimal condition,clear absorption bands appeared at wavenumbers of 3423,2446,1596,1424,and 1033 cm-1.These were very close to the characteristic peaks of quinhydrone.In addition,the existence of sulphonate groups was conf irmed by the strong absorption bands at wavenumbers of 1152 and 856 cm-1,which could be respectively assigned to axial symmetric bending vibration of the S=O group and stretching vibrations of the S-O group(Marletta et al.,2017;Weng and Xu,2016).FTIR analysis revealed that the main chemical groups in the precipitate also contained sulphonate in addition to hydroxyquinone.The characteristic absorption bands of Ar-OH in phenolic hydroxyl group,and C=O and C-C groups in quinone compounds,all affected by sulphonate groups,presented minor differences.

XRD analysis was carried out to conf irm the structure of wastewater precipitate.Fig.6 illustrates the XRD patterns for the precipitate and the mixture of precipitate and BaSO4.The precipitate showed six major diffraction peaks with diffraction angle(2θ)values at 23.9°,27.7°,34.4°,42.0°,44.2°,and 46.8°,which were respectively indexed to(111),(002),(022),(221),(202),and(113)crystal planes of BaCO3(Joint Committee on Powder Diffraction Standards(JCPDS)card no.71-2394)(Fig.6(a)).As the same amount of BaSO4was added to the precipitate,the characteristic diffraction peaks presented signif icant changes.Two very sharp,strong peaks appeared at 29.1°and 31.8°,corresponding to(211)and(112)crystal planes of BaSO4(JCPDS card no.05-0448)(Fig.6(b)).The concentrations ofbefore and after the precipitation treatment were 1710 and 350 mg/L,respectively.The maximum mass of BaSO4in the wastewater should have been 0.33 g according to the reaction equation+Ba2+→BaSO4↓,and the produced BaSO4was only 5.4%of the total mass of the precipitate.Consequently,the characteristic diffraction peaks of BaSO4were so weak that they could not be detected at levels that would allow them to appear in Fig.6(a).

After BaCl2·2H2Owasadded to thenano-silver wastewater,the COD concentration decreased signif icantly,indicating that the precipitation method with barium salt facilitates the transfer of chromophore groups from wastewater to sediment.However,Fig.6 does not show the characteristic peaks of hydroxyquinone compounds;it only shows the diffraction peaks of BaCO3appearing at their corresponding locations.Accordingly,a speculative conclusion can be obtained that most of the characteristic peaks of hydroxyquinone compounds may overlap with the bands of BaCO3.

Fig.5.FTIR spectra of wastewater precipitate and quinhydrone.

4.2.Removal pathway of hydroxyquinone

In order to elucidate the removal mechanism of hydroxyquinone compounds,the model compounds ARS(PⅠ)and purpurin(PⅡ)(Fig.7)were used to prepare the simulated wastewater,with massconcentrations of 4000 and 2850 mg/L,respectively.Then,the pH of the solution wasregulated to the design level(shown in Table 2),and barium salt or Na2SO4wasadded to the simulated wastewater.After settling for 1 h at 15°C,all the treatment systems achieved relatively high removal eff iciencies of COD and color.For Case 1,when 2.44 g of BaCl2·2H2O were added to 100 mL of ARScontaining wastewater at a pH value of 3.8,the removal eff iciencies of COD and color were 92.3%and 96.2%,respectively,indicating that producing phenolic-barium precipitate(PⅠ-Ba)was the main way that hydroxyquinone pollutants were removed from wastewater.

In order to investigate the effect of sulphate on the removal of chromophore groups,4.88 g of BaCl2·2H2O and 1.42 g of Na2SO4were added to the simulated wastewater at the same time(Case 2),and the COD removal eff iciency reached 93.9%,slightly higher than that of Case 1.Theoretically,half of the added barium ions should have been consumed by the formation of BaSO4,and the remaining 2.44 g of BaCl2·2H2O could have reacted with ARS to form phenolic-barium precipitate,which is exactly what happened in Case 1.According to the obtained results,the newly formed BaSO4in the liquidphase reaction system usually consistsof small-sized particles(Watanabe et al.,2004)that have the characteristic of a large surface area.As barium ions in the solution are abundant,the f ine BaSO4particles are positively charged(Wang et al.,2015),and they can adsorb those electronegative groups due to electrostatic force.The obtained f indings and the results in thisstudy revealed that the formation of BaSO4may aid in the removal of hydroxyquinone compounds from the wastewater.Furthermore,the treatment system exhibited a stronger settlement performance under these conditions.

Fig.6.XRD patterns of precipitate and mixture of precipitate and BaSO4.

Fig.7.Structures of PI and PII.

Table 2Results of simulated experimental cases using hydroxyquinone compounds.

As chemical reagent BaCl2·2H2O was added to the hydroxyquinone-containing wastewater,in addition to the formation of phenolic-barium precipitate,the precipitate BaCO3could also be generated due to the following reaction:

As Case 5 was carried out with nitrogen as the protection gas,the removal eff iciencies of COD and color reached 98.2%and 99.7%,respectively,higher than that of Case 4(performed in the air).It was evident that producing BaCO3may lead to adverse effects on the treatment system.

Previous studies have proposed the structures of Ca-ARS complexes.Misra (1992)pointed out that the monodeprotonated ARS forms the complex Ca(ARSH)2,and the doubly deprotonated ARS forms the complex Ca(ARS).Moriguchietal.(2003)investigated mechanismof adsorptionof ARS on hydroxyapatite and their research indicated that two kinds of bonding of ARS could be observed:one involving diphenolate or catecholate salt with two phenolic hydroxyl groups,and the other akin to ketophenolate chelate with a phenolic hydroxyl group and adjacent quinone oxygen.It can thenbeconcluded thatonly twocomplexesmay existinthe ARS wastewater:a pentacyclic compound Ba(ARS)and a barium chelate of Ba(ARSH)2(Fig.8).As for purpurin-containing wastewater at a pH value of 10.5,on the one hand,Ba2+can react with 1,2-O-of purpurin to form astable f ive-membered ring compound;on the other hand,Ba2+may combine with 1-O-and 9-C=Ogroupstoformasix-membered ringstructure,and a long-chain chelate can be generated for the reason that Ba2+may continue to react with 4-O-and 10-C=O groupsof another purpurin molecule(Fig.9).

According to the aforementioned FTIR analysis and the scanning results of the UV/Vis spectrophotometer,the organic compound in the nano-silver wastewater wasdetermined to be a hydroxyquinone-containing complex with sulphonate group,which was similar to quinhydrone to some degree.As BaCl2was added to the nano-silver wastewater,Ba2+could react with the phenolic hydroxyl group and adjacent quinone oxygen to produce ketophenolate chelate.Furthermore,the newly produced BaSO4particles could adsorb those electronegative hydroxyquinone compounds.Thus,the COD concentration and color could decrease signif icantly after the addition of barium salt.

Fig.8.Possible structures of Ba-ARScomplexes.

Fig.9.Possible structure of barium-hydroxyquinone complexes.

5.Conclusions

(1)The precipitation method with barium salt was suitable for the treatment of hydroxyquinone-containing wastewater.Under the optimal conditions such asa reaction temperature of 15°Cand an initial pH value of 10.5,when 8 g of BaCl2·2H2O were added to the 100 mL of nano-silver wastewater to react for 1 h,the removal eff iciencies of COD and color reached 85.6%and 97.1%,respectively.

(2)Producing phenolic-barium precipitate was the main way that hydroxyquinone pollutants were removed.Deprotonating of phenolic hydroxyl groupsmay result in thereleaseof hydrogen ions,and raising the pH value can facilitate the removal of hydroxyquinone compounds.