應(yīng)用鈰-活性炭催化劑的臭氧催化氧化工藝處理水中的芘和熒蒽

戚 霽,鄧慧萍,劉 浩

(1.同濟大學(xué) 環(huán)境科學(xué)與工程學(xué)院,上海 200092;2.西安科技大學(xué) 建筑與土木工程學(xué)院,西安 710054)

多環(huán)芳烴類化合物(polycyclic aromatic hydrocarbons,PAHs)是一種典型的持久性有機污染物(persistent organic pollutants,POPs),是煤、石油和煤焦油等在還原氣氛中熱分解和不完全燃燒時的產(chǎn)物,具有致畸、致癌、致突變和難生物降解等特性,國家飲用水衛(wèi)生標(biāo)準中PAHs的總量上限為0.002 mg/L.芘和熒蒽是典型的PAHs,帶有4個苯環(huán),其分子量較大，沸點較高，不易揮發(fā)，水溶性較低，遺傳毒性較高且不易降解[1],傳統(tǒng)水處理工藝無法將其有效去除.

目前去除PAHs的方法主要有生物[2-6]、物理[7-12]和高級氧化[13-23]等方法,但上述方法都存在不同程度的缺點.將O3工藝和活性炭(AC)吸附工藝相結(jié)合,由于AC可快速吸附水中的有機污染物,有利于O3的氧化降解,因此提高了O3的利用率.此外,AC和O3接觸可形成羥基自由基(·OH),由于·OH的氧化還原電位較高,對有機物的氧化選擇性較差,因此其去除PAHs的效果較好.目前研究人員制備了多種負載型催化劑,如Fe2O3-AC,MnO-AC,MnO2-AC,TiO2-AC和Cu-AC等.

本文研發(fā)一種新型負載型催化劑(鈰-活性炭),并對其進行表征.通過將其應(yīng)用于O3氧化工藝中,對比單獨O3,AC+O3,Ce-AC+O3工藝對芘和熒蒽的處理效果,并考察Ce-AC的催化性能.

1 實 驗

1.1 實驗方法

圖1 靜態(tài)實驗流程Fig.1 Processs chart of static test

采用靜態(tài)實驗法,工藝流程如圖1所示.O3由純氧經(jīng)O3發(fā)生器產(chǎn)生,產(chǎn)量為3 g/h,產(chǎn)生的O3通入4 ℃的去離子水中,尾氣由KI溶液吸收,約20～30 min后形成高質(zhì)量濃度的O3儲備液,對其質(zhì)量濃度進行測試.取出適量的O3儲備液加入反應(yīng)器(容積為1.15 L)中,開始計時,定時取樣,在水樣中加入少量Na2S2O3溶液去除樣品中殘留的O3后,通過0.45 μm濾膜,待測.

1.2 檢測方法

利用粉末投加法測定零電荷點pH(pHZPC)；采用美國Thermo Fisher公司生產(chǎn)的NitonXL3t600型X射線熒光分析儀分析X射線熒光光譜.

利用高效液相色譜檢測芘和熒蒽的質(zhì)量濃度，用C18反相色譜柱(填料粒徑5 μm,柱長250 mm,柱直徑4.6 mm).芘檢測條件：流量0.8 mL/min,流動相為V(甲醇)∶V(水)=7∶3,柱溫40 ℃,進樣量50 μL,檢測器為熒光檢測器,λex=338 nm,λem=375 nm.熒蒽檢測條件：流量0.8 mL/min,流動相為V(甲醇)∶V(水)=8∶2；柱溫40 ℃；進樣量50 μL,檢測器為熒光檢測器,λex=290 nm,λem=490 nm.

1.3 鈰催化劑的制備

先用去離子水浸泡AC并攪拌3 h,再用100 ℃去離子水反復(fù)沖洗,在100 ℃烘箱中烘干并稱質(zhì)量.重復(fù)上述程序,直至AC的質(zhì)量不變,將其置于玻璃樣品瓶中備用.采用浸漬法制備催化劑：將煤質(zhì)AC浸泡在質(zhì)量分數(shù)為1%的硝酸鈰溶液中,調(diào)節(jié)pH=8±0.5,振蕩24 h,在25 ℃靜置24 h,于100 ℃烘箱中烘干.制得的AC和Ce-AC質(zhì)量分數(shù)的X射線熒光光譜分析(XRF)結(jié)果列于表1.

表1 AC和Ce-AC質(zhì)量分數(shù)的XRF結(jié)果Table 1 Results of XRF analysis of AC and Ce-AC

2 結(jié)果與討論

2.1 不同工藝對水中芘和熒蒽的處理效果

當(dāng)芘和熒蒽的初始質(zhì)量濃度均為0.15 mg/L,pH=3,催化劑的投加量為1.5 g/L,O3的初始質(zhì)量濃度為2 mg/L,溫度為 25 ℃時，O3,AC,O3+AC,O3+Ce-AC工藝處理水中芘和熒蒽的效果如圖2所示.

圖2 不同工藝對水中芘(A)和熒蒽(B)的去除效果Fig.2 Decomposition efficiencies of pyrene (A) and fluoranthene (B) in different processes

由圖2可見:芘和熒蒽的降解率隨反應(yīng)時間的增加而增加,芘和熒蒽的降解導(dǎo)致反應(yīng)物質(zhì)量濃度降低,中間產(chǎn)物的質(zhì)量濃度發(fā)生變化,其降解速度逐漸降低;O3+AC和O3+Ce-AC工藝明顯高于O3工藝對芘和熒蒽的去除率：O3工藝對芘和熒蒽的去除率均小于15%,O3+AC和O3+Ce-AC工藝對芘和熒蒽的去除率均大于70%,表明催化劑Ce-AC對O3有較好的催化作用;AC和Ce-AC對芘和熒蒽單純吸附作用的去除率約為50%,這與芘和熒蒽的憎水性有關(guān);Ce-AC略低于AC對有機污染物的吸附,這與負載Ce后比表面積及孔徑的變化有關(guān)[24];在Ce-AC吸附作用較差的情況下,O3+Ce-AC工藝優(yōu)于O3+AC工藝,表明在臭氧催化氧化工藝中,Ce-AC比AC有更好的催化性能.

2.2 羥基自由基捕獲劑對水中熒蒽處理效果的影響

圖3 不同工藝中叔丁醇對熒蒽去除效果的影響Fig.3 Effect of tertiary butanol on the removal of fluoranthene in different processes

采用叔丁醇作為·OH捕獲劑,考察·OH捕獲劑對實驗的影響,結(jié)果如圖3所示.由圖3可見,加入叔丁醇后,O3+AC工藝對熒蒽的去除率由約70%降低至50%；O3+Ce-AC工藝對熒蒽的去除率由80%降低至50%,表明污染物的去除率受到顯著抑制.

圖4 不同初始質(zhì)量濃度對O3+Ce-AC工藝去除芘(A)和熒蒽(B)的影響Fig.4 Effect of intial concentration on the remomval of pyrene (A) and fluoranthene (B) in the O3+Ce-AC processes

2.3 初始質(zhì)量濃度對芘和熒蒽處理效果的影響

在其他實驗條件不變的條件下,當(dāng)芘和熒蒽的初始質(zhì)量濃度分別為0.1,0.15,0.2 mg/L時,O3+Ce-AC工藝的處理結(jié)果如圖4所示.由圖4可見,在不同的初始質(zhì)量濃度下,O3+Ce-AC工藝對芘和熒蒽的去除率均大于75%,這是吸附與氧化共同作用的結(jié)果,表明芘和熒蒽在此質(zhì)量濃度范圍內(nèi)的輕微波動對吸附效果影響較小.

2.4 催化劑投加量對水中芘和熒蒽處理效果的影響

在其他實驗條件不變的條件下,考察催化劑投加量對O3+Ce-AC工藝去除芘和熒蒽的影響,結(jié)果如圖5所示.由圖5可見,O3+Ce-AC工藝對芘和熒蒽的去除率隨催化劑投加量的增加而增加.當(dāng)反應(yīng)時間為12 min,催化劑投量為1.5 g時,對芘和熒蒽的去除率分別約為90%和83%,比O3工藝分別提高了68%和66%.繼續(xù)加入催化劑,O3+Ce-AC工藝對芘和熒蒽的去除率增加較小.因此,催化劑的最佳投加量為1.5 g/L.

圖5 催化劑投加量對O3+Ce-AC工藝去除芘(A)和熒蒽(B)的影響Fig.5 Effect of catalyst loading on the romoval of pyrene (A) and fluoranthene (B) in the O3+Ce-AC processes

2.5 催化劑的循環(huán)使用及Ce的穩(wěn)定性測試

圖6 催化劑多次使用對芘去除率的影響Fig.6 Stability experiment of catalyst for the decomposition of pyrene

催化劑多次使用對水中芘去除率的影響如圖6所示.每次使用后的催化劑不變,將反應(yīng)液換為初始質(zhì)量濃度的污染物,連續(xù)使用5次為一個循環(huán).由圖6可見,經(jīng)過5個循環(huán)后(每個循環(huán)催化劑應(yīng)用5次),O3+Ce-AC工藝對芘的去除率約為85%,O3+AC工藝對芘的去除率約為70%.即Ce-AC具有更好的催化穩(wěn)定性.

對O3催化氧化工藝處理后的水樣進行多次檢測,均未檢測出Ce離子,表明AC上負載的金屬Ce較穩(wěn)定.

2.6 芘和熒蒽共存對去除率的影響

圖7 O3+Ce-AC工藝中芘和熒蒽單一組分溶液與混合液去除率的對比Fig.7 Degradation of pyrene,fluoranthene and the mixtures of pyrene and fluoranthene in the O3+Ce-AC process

將芘和熒蒽兩種有機物制備成混合溶液并進行O3多相催化氧化研究.實驗條件：ρ(芘)=ρ(熒蒽)=75 μg/L,pH=3,催化劑的投加量為1.5 g/L,ρ(O3)=2 mg/L,反應(yīng)時間為12 min,溫度為25 ℃,實驗結(jié)果如圖7所示.由圖7可見,O3+Ce-AC工藝處理混合后有機物中芘和熒蒽的去除率分別為95.11%和87.06%,分別比單組分條件下提高了4.12%和2.18%.表明O3在多相催化氧化降解多種PAHs過程中,各PAHs間彼此促進,即存在協(xié)同作用.產(chǎn)生協(xié)同作用的原因為有機污染物在降解過程中形成中間產(chǎn)物有利于O3的分解,相當(dāng)于自由基的引發(fā)劑,其結(jié)果表現(xiàn)為有機物去除率的提高.

此外,當(dāng)多種PAHs共存時,O3催化氧化工藝對芘的去除率>熒蒽的去除率,與單組分的去除率大小順序一致.可見O3多相催化氧化工藝更易氧化降解芘.

2.7 pH值對水中芘和熒蒽及其混合液去除率的影響

在其他實驗條件不變的條件下,對比O3+Ce-AC工藝在不同pH值下對芘、熒蒽及芘和熒蒽混合有機物的去除率,結(jié)果如圖8所示.由圖8可見:當(dāng)pH值較低時,去除率隨pH值的升高而增大,這是因為水中OH-離子濃度的增加有利于O3分解產(chǎn)生·OH,即OH-可引發(fā)鏈反應(yīng),從而加速·OH等活性物質(zhì)的生成,提高去除率；當(dāng)pH值較高時,去除率隨pH值的升高而減小,這是因為隨著OH-離子濃度的增加,使得短時間內(nèi)水中·OH濃度激增,過多的·OH彼此相互淬滅,導(dǎo)致·OH總量降低,從而去除率下降.

2.8 O3投加量對水中芘、熒蒽及其混合液去除率的影響

在其他實驗條件不變的條件下,對比O3+Ce-AC工藝在不同O3投加量對芘、熒蒽及芘和熒蒽混合有機物的降解率,結(jié)果如圖9所示.由圖9可見:當(dāng)O3質(zhì)量濃度較低時,去除率隨O3質(zhì)量濃度的升高而增大,這是因為水中O3質(zhì)量濃度增加,使得在單位時間形成的·OH增多,從而去除率增加；當(dāng)O3質(zhì)量濃度較高時,去除率隨O3質(zhì)量濃度的升高而減小,這是因為O3質(zhì)量濃度過大使得短時間內(nèi)水中·OH濃度激增,過多的自由基彼此相互淬滅,導(dǎo)致·OH總量降低,從而去除率下降.

圖8 O3+Ce-AC工藝中pH值對芘和熒蒽去除率的影響Fig.8 Effect of pH on the removal of pyrene and fluoranthene in the O3+Ce-AC process

圖9 O3+Ce-AC工藝中ρ(O3)對芘和熒蒽去除率的影響Fig.9 Effect of ρ(O3) on removal of pyrene，fluoranthene in the O3+Ce-AC process

2.9 不同水質(zhì)對芘和熒蒽及其混合液去除率的影響

在其他實驗條件不變的條件下,對比O3+Ce-AC工藝催化氧化對自來水和去離子水配置的溶液中芘、熒蒽及芘和熒蒽混合有機物的去除率,結(jié)果如圖10所示.由圖10可見,芘和熒蒽在自來水中比去離子水中的去除率均略有提高.與去離子水相比,自來水中各種無機離子、有機物和腐殖酸的濃度較大,有利于催化O3分解產(chǎn)生自由基.表明水中存在的微量無機物和有機物可促進O3催化氧化工藝.

2.10 O3+Ce-AC工藝催化氧化芘和熒蒽的毒理學(xué)

采用發(fā)光細菌對水樣進行毒理學(xué)研究[25-26],結(jié)果如圖11所示.

圖10 水質(zhì)本體條件對O3+Ce-AC工藝中芘和熒蒽去除率的影響Fig.10 Effect of water background on the removal of pyrene and fluoranthene in the O3+Ce-AC process

圖11 芘和熒蒽的毒理學(xué)變化Fig.11 Comparison of combined toxictity changes of pyrene and fluoranthene

由圖11可見,反應(yīng)前芘和熒蒽對發(fā)光細菌的抑制為80%,毒性均較大；隨著反應(yīng)時間的增加,水樣對發(fā)光細菌的抑制作用減弱,即在O3多相催化氧化工藝降解芘和熒蒽過程中,水樣對生物的毒性不斷變小,表明芘和熒蒽降解產(chǎn)物的毒性減小了.

3 結(jié) 論

1) 本文選用浸漬法,以活性炭為載體,制備了新型負載型Ce-AC催化劑.催化劑參與的O3多相催化氧化工藝對芘和熒蒽均具有較好的去除效果,其去除率比O3、活性炭以及活性炭和O3組合催化氧化工藝分別提高了50%,30%和10%,實驗條件最優(yōu)為ρ(污染物)=0.15 mg/L,催化劑投量為1.5 g/L.

2) 通過對毒理學(xué)測試可知,芘比熒蒽的毒性大,在O3+Ce-AC工藝去除芘和熒蒽過程中,水樣的毒性逐漸降低.Ce-AC多次循環(huán)使用不影響催化效果,未析出金屬Ce,表明該工藝處理芘和熒蒽安全可靠,可應(yīng)用于飲用水的處理.

3) 過量投加叔丁醇降低了O3+Ce-AC工藝對熒蒽的去除率,表明Ce-AC有助于O3分解產(chǎn)生·OH等自由基.

4) 隨著pH值和O3質(zhì)量濃度的增加,污染物的去除率先升高后降低.當(dāng)pH值和O3質(zhì)量濃度升高時,·OH的濃度升高,促進反應(yīng)進行;·OH的濃度過高會相互淬滅,使得其濃度降低,導(dǎo)致污染物去除率降低.

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