李方芳,鞠勇明,鄧東陽,賈文超,丁紫榮,雷國元

海綿鐵三金屬降解對硝基苯酚的影響因素及催化機理

李方芳1,2,鞠勇明2,3,鄧東陽2,賈文超2,丁紫榮2,雷國元1*

(1.武漢科技大學(xué)資源與環(huán)境工程學(xué)院,湖北 武漢 430000；2.生態(tài)環(huán)境部華南環(huán)境科學(xué)研究所,廣東 廣州 510655；3.生態(tài)環(huán)境部南京環(huán)境科學(xué)研究所,江蘇 南京 210042)

通過超聲置換反應(yīng)制備鈀銅共修飾海綿鐵三金屬催化劑(Pd-(Cu-s-Fe0)),研究了三金屬負(fù)載順序、金屬負(fù)載量、材料投加量以及重復(fù)利用對材料降解對硝基苯酚(PNP)的影響,并利用掃描電子顯微鏡(SEM)和X射線光電子能譜(XPS)表征材料表面結(jié)構(gòu)特征.結(jié)果表明,Pd-(Cu-s-Fe0)催化活性高于Cu-(Pd-s-Fe0)和(Cu-Pd)-s-Fe0;Cu和Pd的最佳負(fù)載量分別為5%和0.025%.在100mL初始濃度為100mg/L的PNP溶液中投加3g Pd-(Cu-s-Fe0)并超聲反應(yīng)30min,PNP的降解率超過80%,降解反應(yīng)基本符合一級動力學(xué)方程;Pd-(Cu-s-Fe0)材料循環(huán)利用4次表現(xiàn)出良好的循環(huán)利用性能.此外,PNP的主要催化還原產(chǎn)物是對氨基苯酚(PAP),主要的反應(yīng)路徑是催化還原反應(yīng).

海綿鐵;Pd-(Cu-s-Fe0)三金屬；對硝基苯酚(PNP)；降解機理

對硝基苯酚(PNP)被廣泛用作染料、農(nóng)藥和防腐劑的原料或中間體[1],具有溶解度高和結(jié)構(gòu)穩(wěn)定等特點,在自然條件下很難被降解[2].作為一種環(huán)境內(nèi)分泌干擾物,PNP具有顯著的高毒性和致癌性[3-5],長期接觸含PNP廢水能造成機體內(nèi)分泌系統(tǒng)功能紊亂[3].目前PNP的降解方法主要有微生物法[1-2]、化學(xué)氧化還原法[2-8]、微波輔助催化氧化[6]和電化學(xué)氧化[7]等.然而,微生物降解周期長,微波和電化學(xué)氧化對設(shè)備要求高且能耗大.零價鐵具有無毒、含量豐富、強還原性,在含PNP廢水處理的應(yīng)用成為研究熱點.納米零價鐵比表面積大、還原性能強,但在應(yīng)用過程中容易團聚而降低活性,且納米零價鐵的價格昂貴,成為限制其實際應(yīng)用的因素.因此探索高效、經(jīng)濟的降解材料處理PNP具有重要意義.

海綿鐵(s-Fe0)作為一種新型零價鐵材料,具有不易團聚、價格低廉等優(yōu)點[8],已成功應(yīng)用于含鹵代有機物、芳香族硝基化合物等各類廢水的處理. 通過在s-Fe0表面負(fù)載貴金屬形成雙金屬和三金屬催化劑能夠顯著提高材料的催化活性[9].已有研究表明,零價鐵粉(ZVI)、鐵銅雙金屬(Fe/Cu)和鐵銅銀三金屬(Fe-Cu-Ag)能有效催化還原降解PNP[8-11].然而,零價鐵粉(ZVI)的腐蝕產(chǎn)物容易沉積在零價鐵表面上形成鈍化膜并顯著降低零價鐵表面催化降解PNP的活性,在￡200W的超聲功率下,零價鐵粉(ZVI)幾乎無法降解PNP[8].鐵銅雙金屬(Fe/Cu)[10]和鐵銅銀三金屬(Fe-Cu-Ag)[11]降解PNP的反應(yīng)活性與鐵基材料上銅銀金屬負(fù)載量、負(fù)載順序和Na2SO4溶劑濃度顯著相關(guān).例如:Fe/Cu雙金屬在Cu負(fù)載量為6%時,幾乎不能降解PNP. Fe-Cu-Ag三金屬降解PNP需要0.75%的Ag負(fù)載量,材料成本顯著增加.貴金屬鈀(Pd)作為一種優(yōu)良的產(chǎn)氫材料,其催化產(chǎn)氫性能遠(yuǎn)高于Cu和Ag等金屬.目前,暫未檢索到利用Cu-Pd雙金屬修飾零價鐵進(jìn)行催化降解PNP的研究.

本研究通過超聲輔助還原法制備微量Cu、Pd負(fù)載的鐵銅雙金屬和Fe-Cu-Pd三金屬材料.考察制備過程中調(diào)控不同參數(shù)對三金屬催化活性的影響,研究三金屬在超聲條件下(200W)催化降解PNP的主要影響因素,探索了三金屬的催化反應(yīng)機理,為海綿鐵材料在去除酚類有機物的應(yīng)用提供理論基礎(chǔ).

1 材料與方法

1.1 試劑和儀器

試劑:對硝基苯酚(PNP;天津大茂化學(xué)試劑廠),硝酸銅(天津市大茂化學(xué)試劑廠),氯鈀酸鉀(上海麥克林生化科技有限公司),海綿鐵(河南西爾環(huán)保科技有限公司),實驗用水為超純水(Milli-Q 超純水系統(tǒng)).

儀器:紫外可見分光光度計(UV-2450,日本島津公司);掃描電子顯微鏡(SEM;S-3400N,日本HITACHI);X射線光電子能譜(XPS; ESCALAB250Xi,美國Thermo公司);超聲波清洗機(KQ5200E,昆山市超聲清洗有限公司);高速離心機(SIGMA,4K15),pH計(FE20,上海梅特勒托利多有限公司),便攜式溶解氧測定儀(雷磁JPB-607A,上海儀電科學(xué)儀器股份有限公司).

1.2 實驗方法

1.2.1 材料制備 Cu-s-Fe0和Pd-s-Fe0雙金屬材料的制備:雙金屬的制備參照之前的制備方法[12-13],將s-Fe0投加到裝有適量2M Cu(NO3)2溶液中進(jìn)行置換反應(yīng),溶液由藍(lán)色變?yōu)辄S色后結(jié)束反應(yīng),用過量超純水反復(fù)沖洗,得到Cu-s-Fe0雙金屬顆粒;將s-Fe0投加到裝有適量濃度為500mg/L的氯鈀酸鉀溶液中進(jìn)行置換反應(yīng),溶液由橘黃色變?yōu)闊o色后結(jié)束反應(yīng),用過量超純水反復(fù)沖洗,得到Pd-s-Fe0雙金屬顆粒.

Pd-(Cu-s-Fe0)三金屬材料的制備:將制備好的Cu-s-Fe0雙金屬投加到裝有適量濃度為500mg/L的氯鈀酸鉀溶液中進(jìn)行置換反應(yīng),溶液由橘黃色變?yōu)闊o色后結(jié)束反應(yīng),用過量超純水反復(fù)沖洗,得到Pd-(Cu-s-Fe0)三金屬顆粒.

Cu-(Pd-s-Fe0)三金屬材料的制備:將制備好的Pd-s-Fe0投加到裝有適量2mol/L Cu(NO3)2溶液中進(jìn)行置換反應(yīng),溶液由藍(lán)色變?yōu)辄S色后結(jié)束反應(yīng),用過量超純水反復(fù)沖洗,得到Cu-(Pd-s-Fe0)三金屬顆粒.

(Cu-Pd)-s-Fe0三金屬材料的制備:將活化后的s-Fe0投加到同時裝有適量500mg/L的氯鈀酸鉀和2mol/L Cu(NO3)2溶液中進(jìn)行置換反應(yīng),當(dāng)溶液變?yōu)辄S色后結(jié)束反應(yīng),用過量超純水反復(fù)沖洗,得到(Cu- Pd)-s-Fe0三金屬顆粒.置換反應(yīng)方程式如下:

1.2.2 PNP降解實驗 以250mL燒杯為反應(yīng)器,每個反應(yīng)器中加入一定濃度的PNP溶液100mL和一定量的海綿鐵材料,置于超聲反應(yīng)器中進(jìn)行降解反應(yīng),每隔一定時間取樣離心后,再取上清液上機測量.

1.3 雙金屬和三金屬催化劑的表征

利用掃描電子顯微鏡(SEM)和X射線光電子能譜(XPS)對5%-Cu-s-Fe0雙金屬和Pd負(fù)載量為0.025%的三金屬顆粒的表面形貌結(jié)構(gòu)進(jìn)行分析.

1.4 PNP濃度檢測

PNP采用紫外分光光度計檢測,檢測波長為316nm.本實驗采用相對濃度/0表示不同影響因素對PNP的去除效果,表示時刻溶液中PNP的剩余濃度,0則表示溶液的初始濃度.采用一級動力學(xué)模擬PNP的降解,方程式為:

2 結(jié)果與討論

2.1 雙金屬和三金屬顆粒的表征

不同負(fù)載順序三金屬的SEM圖如圖1所示.由Cu-s-Fe0雙金屬的Cu元素XPS分峰處理(圖1(a)插圖)可知,超聲置換法成功將Cu沉積到s-Fe0表面,但Cu沉積到s-Fe0表面過程中生成大量CuO和Cu2O,這是由于s-Fe0、Cu-s-Fe0能有效還原溶液中的NO3-[14],從而導(dǎo)致雙金屬表面上負(fù)載新生成的Cu單質(zhì)被氧化.根據(jù)圖1所示的SEM可知,相對于Cu-s-Fe0雙金屬材料,三金屬表面疏松,并且出現(xiàn)一定量的微小顆粒,有利于增大材料比表面積,促進(jìn)材料與污染物接觸.而Cu-(Pd-s-Fe0)和(Cu-Pd)-s-Fe0相對于Pd-(Cu-s-Fe0)表面則出現(xiàn)部分團聚,因此Pd-(Cu-s-Fe0)表面結(jié)構(gòu)有利于和污染物的接觸,理論上具有更高的催化反應(yīng)活性.

圖1 不同材料掃描電鏡(SEM)圖片(′2000)

圖2 3種海綿鐵三金屬XPS表征

圖2展示了不同負(fù)載順序三金屬的XPS數(shù)據(jù).由圖2(a)可知,3種三金屬表面Fe主要為Fe的氧化態(tài)[15];由圖2(b)可知,3種三金屬表面Cu的結(jié)合能約為932.58和952.38eV,分別代表Cu(0)和Cu(Ⅰ)[16], (Cu-Pd)-s-Fe0和Pd-(Cu-s-Fe0)均在943.78和962.28eV處的特征峰代表Cu(Ⅱ)[17].由圖2(c)可知,(Cu-Pd)-s-Fe0和Pd-(Cu-s-Fe0)表面Pd的結(jié)合能主要約為335.4和340.8eV,為Pd(0)[18].Cu-(Pd- s-Fe0)由于Cu將Pd覆蓋,并未有效檢測出Pd.

2.2 PNP降解的影響因素

2.2.1 不同材料對PNP降解的影響 如圖3(a)所示,當(dāng)反應(yīng)30min后,s-Fe0體系中PNP的去除率為62.4%,而Cu-s-Fe0、Pd-s-Fe0和Pd-(Cu-s-Fe0)對PNP降解率分別為26.9%、63.5%和78.8%.s-Fe0負(fù)載Cu材料對PNP的降解效率明顯下降,負(fù)載Pd材料對PNP降解效率無明顯提高.這是由于本實驗選用Cu(NO3)2作為銅源,Cu-s-Fe0可有效還原硝酸根[19],從而生成大量銅氧化物使材料鈍化(詳見圖1(a)插圖).而Pd作為一種高效產(chǎn)氫材料[20],能產(chǎn)生大量活性氫原子[H]abs并還原銅氧化物,從而使Pd- (Cu-s-Fe0)三金屬材料具有更高催化還原活性.因此,結(jié)果推測Pd-(Cu-s-Fe0)降解PNP的催化位點主要在銅上.

2.2.2 金屬負(fù)載順序?qū)NP降解的影響 如圖3(b)所示,反應(yīng)30min后,(Cu-Pd)-s-Fe0、Cu-(Pd-s-Fe0)和Pd-(Cu-s-Fe0)對PNP的降解率分別為34.9%、26.8%和78.8%.Pd-(Cu-s-Fe0)具有最高活性,與已有的三金屬活性研究一致[21].這是由于Fe/Pd氧化還原電位(θ(Fe/Pd) =1.398V)大于Fe/Cu氧化還原電位(θ(Fe/Cu) =0.7889V)[11].當(dāng)先負(fù)載Cu后再負(fù)載Pd,Pd產(chǎn)生活性氫原子[H]abs還原材料表面的各種氧化物,形成Fe/Pd最大電位差,調(diào)整負(fù)載順序則無法有效形成原電池催化體系,降低了催化反應(yīng)效率.由表1可知,三金屬降解PNP的過程遵循一級動力學(xué),且動力學(xué)常數(shù)Pd-(Cu-s-Fe0)>(Cu-Pd)-s-Fe0>Cu-(Pd-s-Fe0).

2.2.3 Cu負(fù)載量對PNP降解的影響 如圖3(c)所示,當(dāng)Pd負(fù)載量為0.025%時,Cu負(fù)載量從1%增加到5%,PNP去除率由67.5%增加至78.8%;當(dāng)Cu負(fù)載量由5%增加至10%,PNP去除率降低為63.0%,這與此前鐵銅雙金屬的規(guī)律基本一致[12].由表1可知,一級動力學(xué)常數(shù)obs也隨著Cu負(fù)載量增加呈現(xiàn)先增后減的趨勢.這是由于Cu在s-Fe0表面形成原電池促進(jìn)s-Fe0腐蝕失去電子;而繼續(xù)增加Cu負(fù)載能在s-Fe0表面形成致密的Cu層,抑制內(nèi)部s-Fe0和溶液接觸,從而抑制s-Fe0的腐蝕、降低材料活性[10].

表1 不同降解條件下的擬一級動力學(xué)參數(shù)變化規(guī)律

表2 水溶液中PNP:O2:H+的物質(zhì)的量比

2.2.4 Pd負(fù)載量對PNP降解的影響 如圖3(d)和表1所示,隨著Pd負(fù)載量不斷增加,材料催化活性和obs也呈現(xiàn)出先增加后減少的規(guī)律.Pd負(fù)載量為0.015%、0.025%、0.5%、0.075%和0.1%的三金屬對PNP的降解率分別為47.5%、78.8%、77.5%、69.3%和67.8%.材料表面負(fù)載Pd后能顯著提高材料產(chǎn)氫能力并提高催化效率,而當(dāng)Pd負(fù)載超過0.025%時,進(jìn)一步提高負(fù)載量會導(dǎo)致材料產(chǎn)氫過快并在材料表面形成一層氫氣膜[22]阻止目標(biāo)物和材料表面接觸,從而抑制PNP的降解.

2.2.5 材料投加量對PNP降解的影響 如圖3(e)所示,隨著三金屬投加量從10g/L增加到30g/L, 100mg/L PNP反應(yīng)30min后降解率從32.1%顯著增加到83.2%;當(dāng)投加量從30g/L繼續(xù)增加到50g/L, PNP降解率僅增加0.7%.這表明當(dāng)PNP濃度一定時,增加三金屬投加量能顯著增加有效活性位點和體系中原電池數(shù)量;當(dāng)三金屬投加量達(dá)到30g/L后,體系中腐蝕電池數(shù)量已經(jīng)接近飽和狀態(tài),繼續(xù)提高投加量無法顯著提高PNP的降解效率.因此,三金屬催化劑投加量選擇為30g/L.

圖3 不同單因素對材料降解PNP的影響

2.2.6 重復(fù)利用對PNP降解的影響 如圖3(f)所示.當(dāng)Pd-(Cu-s-Fe0)三金屬材料在最優(yōu)反應(yīng)參數(shù)下持續(xù)循環(huán)4個周期,PNP去除率分別為83.2%、82.0%、79.9%和77.2%.結(jié)果表明,在4個循環(huán)利用周期內(nèi),三金屬對PNP的去除率保持在75%以上,表明材料具有良好的循環(huán)利用穩(wěn)定性.與納米零價鐵nZVI相比(8000元/kg),s-Fe0的價格低廉(5000元/t)[23],并且在制備三金屬過程中不需要氮氣的保護(hù).因此,海綿鐵催化劑具有相對較高且穩(wěn)定的活性,在含PNP廢水處理中具有良好的應(yīng)用前景.

2.3 三金屬催化機理

零價鐵催化降解PNP主要包括3條路線:(1)由三金屬表面轉(zhuǎn)移的電子和生成的活性氫原子[H]abs將PNP中的硝基(-NO2)還原為氨基(-NH2)[24];(2)溶液中溶解氧得到電子生成羥基自由基(·OH),將PNP氧化降解為NO3-和小分子酸[25-26];(3)先由活性氫原子[H]abs將硝基(-NO2)還原為氨基(-NH2),再由羥基自由基(·OH)將氨基(-NH2)氧化為NO3-和小分子酸[27].圖4(a)插圖呈現(xiàn)了降解反應(yīng)過程中DO、pH值和鐵離子濃度的變化.反應(yīng)30min后,溶液中DO由7.8mg/L降至0.5mg/L,pH值無明顯變化,鐵離子在前6min迅速增加為12.89mg/L,隨后略微下降,這是由于材料在降解過程中不斷釋放Fe2+,極易被氧化為Fe3+隨后生成Fe(OH)3沉淀,從而維持溶液pH值和鐵離子濃度基本不變.如表2所示,溶液中PNP : O2: H+由884.56 : 299.88 : 1降低86.35 : 8.01 : 1,這表明, DO能夠有效競爭Pd-(Cu-s-Fe0)轉(zhuǎn)移電子并引發(fā)一系列降解反應(yīng).

紫外-可見光吸收波譜表明,316nm處吸收峰主要是由苯環(huán)和硝基(-NO2)的共軛引起[10],227nm處吸收峰是由于單環(huán)芳香烴苯環(huán)的π-π*躍遷引起[28].圖4(a)記錄了PNP降解過程中的全波掃描波譜變化.在30min降解過程中,316nm處峰的強度隨反應(yīng)時間增加而逐漸下降,并藍(lán)移至297nm.如圖4(b)可知,對氨基苯酚(PAP)的濃度隨降解反應(yīng)時間逐漸增加,反應(yīng)30min后PAP濃度為79.75mg/L,這表明Pd-(Cu-s-Fe0)能有效催化還原-NO2為-NH2,與已有研究一致[29].其次,由圖4(a)可知,227nm處吸收峰強度略有下降,這表明少量苯環(huán)被羥基自由基(·OH)氧化破壞[30],這與圖4(b)中TOC數(shù)據(jù)略有下降相符.因此,Pd-(Cu-s-Fe0)材料催化降解PNP是還原為主,氧化為輔的過程,催化降解機理如圖5.

圖4 30g/L Pd-(Cu-s-Fe0)降解100mL 100mg/L PNP過程中各因素隨時間的變化

圖5 Pd-(Cu-s-Fe0)催化降解PNP機理

3 結(jié)論

3.1 Pd-(Cu-s-Fe0)比Cu-s-Fe0和Pd-s-Fe0對PNP具有更好的降解效果.相同條件下,Pd-(Cu-s-Fe0)比Cu-s-Fe0和Pd-s-Fe0的降解效率分別高51.9%和15.3%,且循環(huán)利用4次后, 其降解效率僅下降6%.

3.2 三金屬材料的催化活性與金屬負(fù)載順序有關(guān),最優(yōu)三金屬為Pd-(Cu-s-Fe0),且最佳負(fù)載量為5%Cu和0.025%Pd,投加量為30g/L.

3.3 Pd-(Cu-s-Fe0)催化降解PNP的反應(yīng)過程遵循一級動力學(xué)規(guī)律,PNP的主要降解途徑為還原催化降解,主要降解產(chǎn)物為PAP.

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Study on influencing factors and catalytic mechanism of p-nitrophenol degradation with sponge iron-based tri-metals.

LI Fang-fang1,2, JU Yong-ming2,3, DENG Dong-yang2, JIA Wen-chao2, DING Zi-rong2, LEI Guo-yuan1*

(1.Department of Resources and Environmental Engineering, Wuhan University of Science and Technology, Wuhan 430000, China；2.South China Institute of Environmental Science, Ministry of Ecology and Environment of the People’s Republic of China, Guangzhou 510655, China；3.Nanjing Institute of Environmental Science, Ministry of Ecology and Environment of the People’s Republic of China, Nanjing 210042, China)., 2021,41(10)：4670～4676

Pd-(Cu-s-Fe0) trimetals were synthesized adopting with displacement reactions under ultrasonic conditions, and the surface structure of the aforementioned materials was further characterized with scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Moreover, the effects of noble metal loading sequence, the loading amount, input dosage and recycling reuse for the degradation of p-nitrophenol (PNP) was studied in detail. The experimental results show that the catalytic activity of Pd-(Cu-s-Fe0) was higher than that of Cu-(Pd-s-Fe0) and (Cu-Pd)-s-Fe0. The loading amounts of Cu and Pd were optimized as 5% and 0.025%, respectively. Under the optimized conditions including 30g/L of Pd-(Cu-s-Fe0), the removal content of PNP (100mL, initial concentration of 100mg/L) reached more than 80% after 30min of ultrasonic reactions, and the degradation reactions conformed to a pseudo-first-order kinetics equation. Furthermore, after 4times of recycling tests, Pd-(Cu-s-Fe0) showed good recycling performance. Based on the UV-visible spectral variations and high-performance liquid chromatography, we proposed the degradation mechanism mainly via catalytic reductions of PNP into p-aminophenol (PAP).

sponge iron；Pd-(Cu-s-Fe0) trimetal；p-nitrophenol (PNP)；degradation mechanism

X703.5

A

1000-6923(2021)10-4670-07

李方芳(1997-),女,湖北荊州人,武漢科技大學(xué)碩士研究生,主要從事海綿鐵材料降解機理研究.發(fā)表論文1篇.

2021-02-04

國家重點研發(fā)計劃(2019YFE0111100);廣東省國際合作項目(2018A050506045);廣東省基礎(chǔ)與應(yīng)用基礎(chǔ)研究基金資助項目(2020A1515010969);公益性科研院所專項項目(GYZX210301)

* 責(zé)任作者, 教授, leiguoyuanhit@126.com