負(fù)載過渡金屬顆粒電極催化降解焦化廢水尾水

尤欣雨,劉 雨,秦琳琳,楊 帥,張文文,梁文艷

負(fù)載過渡金屬顆粒電極催化降解焦化廢水尾水

尤欣雨,劉 雨,秦琳琳,楊 帥,張文文,梁文艷*

(北京林業(yè)大學(xué)環(huán)境科學(xué)與工程學(xué)院,北京 100083)

制備以活性炭為載體,負(fù)載Ni、Fe、Co單元素和Ni-Fe、Co-Fe雙元素的5種顆粒電極,用以降解模擬焦化廢水尾水.結(jié)果顯示,負(fù)載雙元素的顆粒電極對模擬廢水的處理效果要好于單元素,其中Ni-Fe/PAC降解性能最好,COD與TOC降解率可達(dá)70.1%和40.1%.Ni-Fe/PAC也具有最高的析氧電位和最低的Tafel斜率,分別為2.25V和86mV/dec;所負(fù)載的晶體結(jié)構(gòu)為單質(zhì)Ni和Fe,含有少量的鐵氧化物.顆粒電極的添加,會導(dǎo)致出水呈堿性,Co的摻入導(dǎo)致堿性增大,而Fe的摻入有利于降低出水pH值.模擬廢水中3類有機(jī)物在Ni-Fe/PAC上降解效果呈現(xiàn)為多環(huán)芳烴<雜環(huán)化合物<苯系物.Ni-Fe/PAC能催化·OH和新生態(tài)氫的產(chǎn)生,有機(jī)物的降解以間接氧化為主.

顆粒電極；過渡金屬；焦化廢水；電化學(xué)降解；高級氧化；影響因素

焦化廢水經(jīng)過生化處理后,其尾水中仍含有大量的有機(jī)物,如苯系物、稠環(huán)芳烴和雜環(huán)化合物等[1-2].由于他們大多含有共軛π鍵,表現(xiàn)出較強(qiáng)的生物降解毒性和惰性[3],難于在生化處理中得到有效去除,使焦化廢水尾水達(dá)不到排放或回用的要求.

目前,焦化廢水尾水處理的方法主要有絮凝法[4]、高級氧化法[5]和膜分離法[6].絮凝法操作簡單,運(yùn)行成本低,但去除效率低[7].膜分離法去除效果較好,但有機(jī)物易導(dǎo)致膜堵塞,增加了膜清洗次數(shù),縮短了膜組件壽命[8].因此,焦化尾水常采用高級氧化技術(shù)進(jìn)行處理[9].臭氧氧化單獨(dú)使用對有機(jī)物的氧化和礦化能力有限,通常與其他技術(shù)相結(jié)合以改善污染物降解效果[10].電催化氧化是利用電解產(chǎn)生強(qiáng)氧化性活性物質(zhì),其操作簡單、無二次污染.但傳統(tǒng)的二維板狀電極系統(tǒng)(2D系統(tǒng))的電流利用效率不高,傳質(zhì)速率低[11].三維電極系統(tǒng)(3D系統(tǒng))是在傳統(tǒng)極板間隙中填充顆粒電極,從而增大了比表面積和反應(yīng)活性位點(diǎn),提高了傳質(zhì)和電流效率[12].

顆粒電極常由載體和所負(fù)載的催化劑組成.由于活性炭比表面積較大、價(jià)格低廉,被廣泛用作顆粒電極的載體[13].目前,活性炭所負(fù)載催化劑的類型可分為單金屬型和復(fù)合型,Co、Fe和Ni等過渡金屬具有催化性能強(qiáng)、高穩(wěn)定性和易獲取的特點(diǎn),常作為顆粒電極催化改性的首選[14-15],復(fù)合型為常用單元素催化劑的組合,其中雙金屬之間的協(xié)同作用導(dǎo)致了有效的軌道雜化,可以優(yōu)化電極催化活性,并抑制催化劑團(tuán)聚,減少催化金屬的溶出[16-17].

因此,本文選擇Ni、Fe、Co過渡元素,以活性炭為載體,制備了Ni、Fe、Co單一負(fù)載的、以及Ni-Fe和Co-Fe雙元素負(fù)載的顆粒電極,對其降解模擬焦化廢水的性能進(jìn)行了研究,分析了3種過渡金屬的催化作用,通過電化學(xué)性能、自由基測試等探究了有機(jī)物降解去除機(jī)制.

1 材料與方法

1.1 試劑與儀器

活性炭購于北京科誠光華公司;Ti網(wǎng)陰極和Ti/RuO2網(wǎng)陽極尺寸為50mm×120mm,購于北京恒力鈦公司;5,5-二甲基-1-氧化吡咯啉(DMPO)購于麥克林公司;其他試劑均購于國藥公司,為分析純.TOC/TN分析儀(TOC-VCSN,日本島津有限公司),COD快速測定儀(CTL-12,承德華通環(huán)保有限公司),傅里葉紅外光譜儀(Vertex 70,德國布魯克光譜儀器公司),X射線衍射分析儀(XRD-7000s,日本島津有限公司),電子順磁共振儀(EMX plus,Bruker),電化學(xué)工作站(CHI660E,上海辰華公司).

1.2 模擬焦化尾水的配制

如表1所示,模擬廢水由氮雜環(huán)化合物、多環(huán)芳烴和苯系物配制而成.雜環(huán)化合物(A類)包括吡啶、喹啉和異喹啉;多環(huán)芳烴類化合物(B類)由萘、苊、芴、菲、蒽、芘、1-甲基萘和熒蒽組成;苯系物(C類)由苯、苯酚、苯胺、2,4-二甲基苯酚和甲苯組成.配制A、C、A+C、B+C、A+B+C組成的5種模擬廢水,模擬廢水的TOC均為300mg/L左右,加入10mmol/L Na2SO4作為電解質(zhì).

1.3 顆粒電極制備及表征

采用Ni(NO3)2、Co(NO3)2和Fe(NO3)3配制浸漬溶液,包括只含單一金屬鹽的溶液,以及物質(zhì)的量比為1:1的Ni+Fe和Co+Fe溶液,金屬離子總濃度為0.5mol/L.將粉末活性炭浸漬于上述溶液中,恒溫25℃振蕩8h,離心烘干,用5%(/)聚乙烯醇進(jìn)行造粒,粒徑為4～6mm.在氮?dú)獗Ｗo(hù)和600℃中焙燒4h,顆粒電極表達(dá)為Ni/PAC、Co/PAC、Fe/PAC、Ni-Fe/PAC和Co-Fe/PAC.使用X射線衍射分析儀(XRD)對物相組成進(jìn)行分析.采用傅里葉變換紅外光譜儀(FTIR)對電極表面官能團(tuán)進(jìn)行測定,光譜范圍為400～ 4000cm-1,分辨率為4cm-1.顆粒電極比表面積測試之前,先在高溫下真空脫氣8h,檢測溫度為80K,采用Brunauer-Emmett-Teller(BET)公式計(jì)算.

表1 5種模擬焦化廢水尾水組成

1.4 電化學(xué)裝置及實(shí)驗(yàn)操作

如圖1所示,電解槽的長×寬×高為60mm× 50mm×120mm.顆粒電極填充于兩極板間,高度為70mm.廢水以連續(xù)流方式泵入反應(yīng)裝置中,反應(yīng)后直接排放.實(shí)驗(yàn)中,先在不通電條件下,以7mL/min流速通入模擬廢水12h;然后施加電流0.10,0.15,0.20和0.30A,每個(gè)電流處理2h;水力停留時(shí)間設(shè)置為60min,每間隔30min取樣測定COD、總有機(jī)碳(TOC)和pH值.

1.5 自由基的檢測

使用50mmol/LDMPO為捕獲劑,以10mmol/L Na2SO4為電解質(zhì),采用電子順磁共振儀(ESR)測定自由基,條件為:中心場3520G、微波頻率9.87GHz、掃描時(shí)間63s、功能衰減10dB.

圖1 電催化降解反應(yīng)裝置

1.6 電化學(xué)性能測試

電化學(xué)測試在三電極體系中進(jìn)行,鉑絲為對電極,Ag/AgCl為參比電極,玻碳電極為工作電極.工作電極的制備:取5mg顆粒電極,碾磨為粉末,加入50μL質(zhì)量分?jǐn)?shù)為5% Nafion和1mL乙醇,混勻充分;滴加20μL于玻碳電極表面.

線性掃描伏安(LSV)和循環(huán)伏安曲線(CV)測試均在1mol/L KOH溶液中進(jìn)行,LSV掃描速度為5mV/s,CV掃速為15～150mV/s,穩(wěn)定性實(shí)驗(yàn)中CV掃速為50mV/s,共1000圈.測試前,向待測溶液中充15min氮?dú)?測試電位Ag/AgCl均轉(zhuǎn)換為可逆氫電極電位RHE.

1.7 數(shù)據(jù)處理

利用Excel 2019軟件對實(shí)驗(yàn)數(shù)據(jù)進(jìn)行分類整理,曲線圖均通過Origin 2018軟件繪制,采用Microsoft PowerPoint 2019軟件繪制污染物降解機(jī)理圖.

2 結(jié)果與討論

2.1 5種顆粒電極在不同電流條件下的降解效能

顆粒電極降解模擬廢水的組成為A+B+C,如圖2(a～b)所示,2D系統(tǒng)對COD和TOC的去除都顯著低于3D系統(tǒng),其對COD和TOC的去除主要來自Ti/RuO2的陽極氧化,顆粒電極的加入極大地提高了處理效率.對于5種顆粒電極,當(dāng)電流為0.1A時(shí),Ni-Fe/PAC對COD的降解去除效果最好,Co/PAC的效果最差.隨著電流逐步提高,5種顆粒電極之間的差異縮小,表現(xiàn)為Ni-Fe/PAC>Co-Fe/PAC≈Ni/ PAC≈ Fe/PAC>Co/PAC,Ni-Fe/PAC對COD的去除率可達(dá)70.1%.與COD降解去除的規(guī)律不同,隨著電流的升高,5種顆粒電極在TOC去除方面的差異逐步增大.當(dāng)電流提高至0.3A時(shí),Co-Fe/PAC和Ni-Fe/ PAC的TOC去除率分別達(dá)到36.7%和40.1%,遠(yuǎn)優(yōu)于單元素負(fù)載顆粒電極,表現(xiàn)為Ni-Fe/PAC> Co-Fe/ PAC>Ni/ PAC>Fe/PAC>Co/PAC.

圖2c可以看出,2D系統(tǒng)的電壓最低,但當(dāng)添加顆粒電極后,電壓明顯升高,說明顆粒電極的填充增加了極板之間的電阻.負(fù)載元素不同,所帶來的電阻大小也不同,電壓呈現(xiàn)Co/PAC>Co-Fe/PAC>Ni/ PAC≈Fe/PAC>Ni-Fe/PAC的規(guī)律.正因?yàn)镃o帶來的電阻較大,使Co-Fe/PAC的電壓明顯高于Ni-Fe/ PAC.圖2d顯示,2D系統(tǒng)出水的pH值在6.9～7.8間,與進(jìn)水6.9～7.2相近.但是,5種顆粒電極處理后的出水均呈現(xiàn)堿性,pH值表現(xiàn)為Ni-Fe/PAC≈Fe/PAC< Ni/PAC

Ni/PAC、Fe/PAC、Co/PAC、Ni-Fe/PAC和Co-Fe/ PAC顆粒電極的比表面積分別為:778.9,786.5,782.5,776.3和779.7m2/g,顆粒電極的比表面積相差較小,而其降解效果不同,說明顆粒電極比表面積對降解性能影響不大.5種顆粒電極吸附飽和后(即0min)的COD與TOC值之間沒有明顯差異,說明顆粒電極對廢水的降解性能主要受負(fù)載元素影響.

從上述結(jié)果可以看出,3種單元素顆粒電極中,Ni/PAC降解性能最好.由于在顆粒電極表面產(chǎn)生大量的·OH,進(jìn)而消耗了大量的H+,使系統(tǒng)出水的pH值增高,但Ni可以在堿性溶液中表現(xiàn)出高穩(wěn)定性和耐腐蝕性,而Co和Fe催化劑雖具有較高的催化活性,但存在不耐強(qiáng)堿的問題,二者在堿性溶液中易鈍化[18],所以Ni呈現(xiàn)出更好的處理效果.負(fù)載雙元素顆粒電極的處理效果整體好于單元素負(fù)載顆粒電極,這是因?yàn)殡p元素之間的電子相互作用以及幾何效應(yīng)導(dǎo)致電子結(jié)構(gòu)的適當(dāng)修飾,提高了活性物種的流動性,從而使其催化活性優(yōu)于單一金屬的催化活性[19].但Co-Fe/PAC的出水pH值過高,抑制了氧化性物質(zhì)(如·OH)的形成,不利于有機(jī)物的降解[20],使其降解性能弱于Ni-Fe/PAC.

2.2 顆粒電極的電化學(xué)性能

由于顆粒電極的出水pH值呈堿性,因此在1mol/L KOH中進(jìn)行線性伏安析氧極化曲線測試.如圖3a所示,析氧極化曲線的切線與水平線的交點(diǎn)為析氧電位(OEP),呈現(xiàn)Ni-Fe/PAC>Co-Fe/ PAC>Ni/PAC>Fe/PAC>Co/PAC,分別為2.25,2.11,1.97,1.65和1.59V.Ni-Fe/PAC的OEP最高,較高的析氧電位可以減少析氧副反應(yīng)的發(fā)生,增強(qiáng)·OH的產(chǎn)生和電極的催化性能,進(jìn)而提高有機(jī)物的降解效率[21].而其余4種顆粒電極的析氧電位較低,易發(fā)生生成氧氣的副反應(yīng),與污染物的降解形成競爭,降低電流效率的利用.如圖3b所示,Tafel斜率呈現(xiàn)Ni-Fe/PAC

圖3 5種顆粒電極的析氧極化曲線和Tafel斜率

2.3 Ni-Fe/PAC顆粒電極的表征

由于Ni-Fe/PAC性能表現(xiàn)最好,對其進(jìn)行了XRD和FTIR表征測定.圖4a顯示,衍射角(2)為44.12°,52.85°和76.87°時(shí),分別對應(yīng)單質(zhì)Ni的(111)、(200)和(220)晶面(PDF#04-0850).2為44.12°和64.53°的特征衍射峰為單質(zhì)鐵的(400)和(440)晶面(PDF#85-1410).在Ni-Fe/PAC使用后,XRD圖譜上仍可以觀測到很強(qiáng)的晶面峰,說明催化劑具有很好的穩(wěn)定性.FTIR的測定結(jié)果顯示(圖4b),3332,2398和1069cm-1處的吸附峰分別為-OH、C-H與C=C的振動吸收峰,572cm-1歸屬Fe-O鍵伸縮振動峰[23].羥基的振動峰較弱可能是由于顆粒電極制備過程中的高溫造成了部分-OH官能團(tuán)的分解.Ni-Fe/ PAC上除單質(zhì)Fe和Ni外,還有少量鐵氧化物.圖4b還顯示,吸附前后圖譜變化不大,有機(jī)物的吸附對Ni-Fe/PAC沒有影響,而反應(yīng)后衍射峰強(qiáng)度有所降低.使用后的FTIR吸收峰位置基本不變,表明Ni-Fe/ PAC穩(wěn)定性較好,但是C-O、C-H與-OH的振動峰強(qiáng)度明顯變?nèi)跎踔料?說明其參與了反應(yīng),可能是由于被中間產(chǎn)物所取代造成的.

2.4 不同模擬廢水組成的降解效果

為了解模擬廢水中3類組成對降解效能的影響,使用Ni-Fe/PAC顆粒電極,分別對含有A、C、A+C、B+C和A+B+C的模擬廢水進(jìn)行降解處理.如圖5所示,雖然5種模擬廢水起始TOC都為300mg/L,但其所對應(yīng)的COD值卻相差很大,說明部分組份難于被K2Cr2O7氧化.在顆粒電極催化處理中,組份C的降解去除效果最好,COD與TOC去除率分別為58.6%和67.3%,說明苯系物易被電催化降解.當(dāng)組份C中部分替換為A類物質(zhì)時(shí)(即A+C),COD和TOC去除率降至52.4%和47.0%,當(dāng)組成只有A類時(shí),其降解效果也略低于A+C,說明Ni-Fe/PAC降解含氮雜環(huán)的效率低于苯系物.原因在于氮雜原子的電負(fù)性較強(qiáng),使雜環(huán)有機(jī)物苯環(huán)上電子云密度降低,羥基化反應(yīng)速率變慢[24].當(dāng)組分C引入B類時(shí)(即B+C),COD與TOC去除率繼續(xù)下降至51.5%和20.1%,且組分B+C的降解效率也低于A+B+C,說明顆粒電極降解多環(huán)芳烴的效率最低.由于稠環(huán)化合物的共平面結(jié)構(gòu)使其能夠形成較大的超共軛體系,因而分子穩(wěn)定結(jié)構(gòu),難于降解[25].

2.5 ESR測定和降解機(jī)制

自由基的測定結(jié)果如圖6a所示,2D系統(tǒng)中幾乎沒有檢測到·OH的產(chǎn)生,這也是2D系統(tǒng)降解效果較差的原因.而對于Ni-Fe/PAC顆粒電極,觀測到峰高為1:2:2:1的DMPO-OH四重峰(a(N) = a(H) = 14.9G),加入模擬廢水(A+B+C)后,出現(xiàn)DMPO-H加合物信號(1:1:2:1:2:1:2:1:1,a(N)=15.5G,a(H)=20.6G).結(jié)果說明,電解過程中Ni-Fe/PAC可將H2O催化分解成·OH,污染物的加入導(dǎo)致羥基自由基被消耗,使新生態(tài)活性氫的信號增加.

羥基自由基具有很強(qiáng)的電負(fù)性和電子親和力.苯系物中的苯環(huán)有較高的電子云密度,所以·OH可對其進(jìn)行降解[26].對于雜環(huán)化合物,吡啶的環(huán)裂可以直接在C-N鍵處產(chǎn)生,之后或經(jīng)脫氨作用生成戊二酸,然后開裂再繼續(xù)降解.帶有苯環(huán)的含氮雜環(huán)化合物,由于受環(huán)上氮原子的影響,雜環(huán)化合物的苯環(huán)上的電子云密度高于吡啶環(huán)上的電子云密度,因此,親電反應(yīng)總是優(yōu)先在苯環(huán)部分的碳原子發(fā)生[27].·OH可以取代多環(huán)芳烴苯環(huán)結(jié)構(gòu)上比較活潑的H原子,所形成的羥基會活化其苯環(huán)結(jié)構(gòu)上的鄰位H原子,進(jìn)而實(shí)現(xiàn)中間產(chǎn)物的轉(zhuǎn)化和苯環(huán)的開環(huán)反應(yīng),最終產(chǎn)生CO2和H2O(圖7)[28].

插圖是1000次循環(huán)伏安掃描曲線

循環(huán)伏安掃描結(jié)果顯示(圖6b),當(dāng)加入模擬廢水(A+B+C)后,反應(yīng)電流增加,但沒有觀察到明顯的氧化還原峰,說明廢水中有機(jī)物的降解主要是依靠間接氧化過程完成.另外,在1000次循環(huán)掃描后,Ni-Fe/PAC顆粒電極的電流密度只略有下降,表現(xiàn)出了優(yōu)異的長期穩(wěn)定性.

圖7 污染物降解反應(yīng)機(jī)理

3 結(jié)論

3.1 5種顆粒電極中,雙元素負(fù)載電極的催化性能要好于單一元素負(fù)載,呈現(xiàn)Ni-Fe/PAC>Co-Fe/ PAC>Ni/PAC>Fe/PAC>Co/PAC的趨勢.

3.2 5種顆粒電極中,Ni-Fe/PAC具有最高的析氧電位和最低的Tafel斜率,分別為2.25V和86mV/dec,所負(fù)載的晶體結(jié)構(gòu)為單質(zhì)Ni和Fe,含有少量的鐵氧化物.

3.3 顆粒電極添加后,會導(dǎo)致板間電壓升高,但是Ni元素的摻入有利于導(dǎo)電,Co元素的摻入不利于導(dǎo)電;Ni-Fe/PAC的導(dǎo)電性最佳.顆粒電極的添加,會導(dǎo)致出水呈堿性,且Co的摻入導(dǎo)致堿性最大,而Fe的摻入有利于降低出水pH值.

3.4 Ni-Fe/PAC對模擬尾水中苯系物的降解效率最高,TOC去除率可達(dá)67.3%,氮雜環(huán)化合物次之,對多環(huán)芳烴降解效率最低.Ni-Fe/PAC能催化·OH和新生態(tài)氫的產(chǎn)生,污染物質(zhì)的降解去除以間接氧化為主.

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Catalytic degradation of coking tail wastewater by particle electrodes loaded with transition metals.

YOU Xin-yu,LIU Yu,QIN Lin-lin,YANG Shuai,ZHANG Wen-wen,LIANG Wen-yan*

(College of Environmental Science and Engineering,Beijing Forestry University,Beijing 100083,China).,2022,42(8)：3683～3689

Large quantities of refractory organic matters still leave in the coking wastewater after biological treatment,making wastewater not meet the requirements of discharge or recycling. In the present study,activated carbon was applied as carrier. Five kinds of particle electrodes that were loaded with single elements of Ni,Fe and Co and binary elements of Ni-Fe and Co-Fe,were prepared and used to degrade the simulated coking tail-wastewater. The results showed that the particle electrodes loaded with binary elements achieved better treatment effects than those loaded with single elements. Ni-Fe/PAC possessed the best degradation performance among the electrodes,with COD and TOC removal efficiency of 70.1% and 40.1%,respectively. Ni-Fe/PAC also possessed the highest oxygen evolution potential and the lowest Tafel slope,namely 2.25V and 86mV/dec,respectively. The crystal structure was consisted by metallic Ni and Fe and small amount of iron oxide. The addition of particle electrodes resulted in the effluent to be alkaline. The loading Co increased the alkalinity,while the Fe was beneficial to reduce the pH value of effluent. The degradation effects of the three category organics in simulated wastewater were in the order of polycyclic aromatic hydrocarbon < heterocyclic compound < benzene series during Ni-Fe/PAC processes. Ni-Fe/PAC can catalyze the generation of ·OH and new atomic hydrogen. The degradation of organic matters was mainly fulfilled through indirect oxidation.

particle electrode；transition metal；coking wastewater；electrochemical degradation；advanced oxidation；influencing factors

X703.5

A

1000-6923(2022)08-3683-07

2021-12-11

國家自然科學(xué)基金資助項(xiàng)目(51672028);國家水專項(xiàng)(2013ZX07209001-003)

*責(zé)任作者,教授,lwy@bjfu.edu.cn

尤欣雨(1997-),女,黑龍江哈爾濱人,北京林業(yè)大學(xué)碩士研究生,主要從事難降解有機(jī)污染物去除研究.發(fā)表論文1篇.