鄧汝樂,高 鵬,賈 松,王雪飛

燃燒生成棕色碳的三維熒光光譜分析

鄧汝樂,高 鵬,賈 松,王雪飛*

(中國科學院大學化學科學學院,北京 100049)

模擬燃燒11 種常見物質(zhì)(秸桿、木材、煤和生活垃圾等),并收集煙氣中可溶于甲醇的有機產(chǎn)物,利用紫外-可見吸收光譜(UV-Vis)和三維熒光光譜(EEMs)對收集的甲醇可溶性有機物(MSOM)進行表征.進一步,結(jié)合非負矩陣分解法(NMF)提取三維熒光光譜主要組分的特征激發(fā)/發(fā)射光譜,根據(jù)熒光信號輪廓差異對不同種類物質(zhì)進行區(qū)分,旨在建立棕色碳溯源依據(jù).結(jié)果顯示,秸桿和木材燃燒源棕色碳在紫外-可見吸收光譜上呈現(xiàn)相似的譜形,均在265nm處存在肩峰;瓦楞紙板和塑料燃燒源棕色碳的吸收則隨波長增加單一下降.由于基本組分相同,各生物質(zhì)及紙板對應的棕色碳的EEM有著相似的輪廓,NMF解析結(jié)果表明,生物質(zhì)和紙板的MSOM存在3種主要熒光組分,分別為兩種類腐殖質(zhì)C1、C2和類蛋白質(zhì)C3;煤的EEM在長波處有較強的分布,可歸因于芳香類基團,由其EEM分解出M1、M2和M3熒光團,三者位置均較生物質(zhì)紅移.根據(jù)熒光團位置以及光譜信號輪廓特征,可對生物質(zhì)和煤進行區(qū)分;泡沫、塑料袋和塑料瓶屬于有機高分子材料,其EEM與生物質(zhì)有較大的差別,且三者之間也存在差異,泡沫和塑料袋的MSOM含有4種熒光成分,而從塑料瓶的MSOM中只可得到兩種熒光團,特征明顯.

燃燒；棕色碳；三維熒光光譜；紫外-可見吸收光譜；非負矩陣分解；來源

棕色碳(BrC)特指大氣氣溶膠中的一類有機物,特點是在紫外-可見光波段(200～700nm)有光吸收,且吸收強度由可見光到紫外波段快速增強[1-2].由于認識的不足,以前人們將大氣中能夠吸收大氣輻射的物質(zhì)劃歸為黑碳(BC),Andreae等[3]2006年首次提出棕色碳的概念,人們開始關(guān)注這類物質(zhì).BrC可以看作是介于強吸光的黑碳和不吸光的有機碳之間的具有一定吸光性的有機物[1].BC和BrC均可吸收或散射太陽輻射,它們的存在影響著氣候的變化.雖然相同含量的BrC對光的吸收比BC少,但由于BrC在大氣中含量豐富,其在大氣光吸收中的重要性越來越突出[4-5].

目前,文獻報道的關(guān)于棕色碳的來源主要包括兩種,燃燒產(chǎn)生的一次排放和大氣化學反應的二次生成.關(guān)于BrC的二次形成,從已有的報道分析主要解釋為醛類小分子發(fā)生氨化聚合形成的氮雜環(huán)物質(zhì),大部分形成芳環(huán)結(jié)構(gòu),共軛度高[5-7].由于二次反應受環(huán)境及前體物影響較大,使得BrC成分組成較為復雜,且具有不確定性.一次排放的BrC在短波處有較強吸收. Hoffer等[8]研究發(fā)現(xiàn)燃燒產(chǎn)生的顆粒物在300nm處對氣溶膠吸光貢獻達30%～50%.煤和生物質(zhì)燃燒是構(gòu)成一次排放的主因[2,9],近期報道,北京地區(qū)冬季BrC的來源組成:50%燃煤,17%生物質(zhì)燃燒[10].除了生物質(zhì)和化石燃料的燃燒,生活垃圾的焚燒處理也可能產(chǎn)生BrC.BrC對地球輻射平衡有著直接影響,加速積雪融化[11-12],并且對人類健康及農(nóng)作物生產(chǎn)質(zhì)量也造成一定危害[13],已成為大氣環(huán)境領(lǐng)域的研究熱點之一.對于BrC的研究,一方面要深入了解其本身性質(zhì)和演化過程,另一方面要加強對來源的追蹤,特別是人為源.目前,第一方面的研究廣泛開展,后者則相對滯后,主要是因為缺乏有效的BrC檢測和區(qū)分方法.

已知有機物對紫外光和可見光的吸收是因為其分子中含有共軛結(jié)構(gòu),BrC亦是如此.空氣中含有大量無機物、VOC小分子,它們對可見光沒有吸收,也不具備發(fā)光能力.而BrC含有的多環(huán)芳烴類化合物、類腐殖質(zhì)物質(zhì)、焦油類物質(zhì)和連有含氧、含氮官能團的高分子量化合物等官能團均具有高度共軛結(jié)構(gòu),不但吸收可見光還呈現(xiàn)較好的熒光特性[14-16].因此熒光光譜法可作為“指紋”識別工具測定BrC.目前,將激發(fā)掃描與發(fā)射掃描相關(guān)聯(lián)的三維熒光光譜(EEM),因其靈敏度高、選擇性強、樣品量小等優(yōu)點,在水體檢測、廢水處理、大氣監(jiān)測等環(huán)境分析中得到廣泛應用[17-18].與普通的熒光分析法相比,三維熒光技術(shù)在多組分混合物的分析中有著顯著優(yōu)勢,能同時獲得熒光強度隨激發(fā)波長和發(fā)射波長變化的關(guān)系,信息采集量大,對組分分析更為準確有效[19-20].因BrC可發(fā)射熒光,通過測定不同激發(fā)波長下的發(fā)射波長,可對BrC的不同組分進行定性分析[21].

在利用熒光光譜分析BrC組分方面,盡管已有學者對生色團進行分類,但來源不同特征可能不同,在根據(jù)不同的光譜特征對BrC進行來源解析方面,目前仍鮮有報道.本研究通過秸稈、木材、化石燃料和可回收干垃圾4類物質(zhì)的燃燒,利用甲醇提取法收集BrC成分,獲得各物質(zhì)的三維熒光光譜.為避免信號堆疊的干擾,采用非負矩陣分解法(NMF),對數(shù)據(jù)維數(shù)進行約減,從三維光譜中獲得熒光組分的激發(fā)發(fā)射特征圖譜.以期基于各物質(zhì)EEM呈現(xiàn)的輪廓及其特征峰所在區(qū)域的不同,對BrC的來源作進一步區(qū)分.

1 材料與方法

1.1 樣品采集

從來源上將燃燒的物質(zhì)分為:玉米、小麥、大豆秸桿和竹子,代表草本植物;柳樹和松木,代表木本植物;原煤(神華煙煤),代表化石燃料;瓦楞紙板、泡沫、食品塑料袋、一次性塑料瓶,代表生活垃圾(可回收干垃圾).研究表明,在發(fā)達國家垃圾成分構(gòu)成中,紙和塑料的占比為40%～60%,我國可回收干垃圾含量逐年增長[12],可利用價值增大,研究其燃燒十分有意義.雖然濕垃圾在我國生活垃圾中占比高,但主要種類為易腐爛生物質(zhì)、纖維等,其中部分物質(zhì)成分與本實驗所選的植物代表成分相同且不易收集和處理,因此未納入考慮范圍內(nèi).

燃燒前將切割長度均為3cm的原料置于烘箱60℃下6h進行烘干.分別取3g置于燃燒爐中(圖1),爐底裝有電熱絲,使爐內(nèi)升溫.以10L/min的進氣速率,于450℃下燃燒10min,燃燒產(chǎn)生的煙氣通過煙道冷卻后,進入到緩沖箱中,在風扇的作用下混勻.在真空泵的作用下,利用玻璃濾膜(于馬弗爐400℃預灼燒4h以除去可能吸附的碳雜質(zhì))收集煙氣中的顆粒物,每種燃料采集3～4個平行樣品,采集流速為3L/ min.另外收集一組空白樣品,作為空白背景.

圖1 燃燒和煙霧收集裝置

1.2 BrC的提取

利用刻刀截取等面積小塊濾膜(約78mm2)于3mL甲醇溶液(99.9%,百靈威)中,使用數(shù)控超聲波清洗機(功率150W,舒美KQ3200DE)在25℃下超聲1h.將超聲后的溶液用PTFE有機系濾頭(13mm,0.22μm,津騰)進行過濾,獲得甲醇可溶性有機物(MSOM).

1.3 吸收光譜與EEM光譜

所有MSOM根據(jù)收集所得的含量不同,用甲醇稀釋10～50倍,使用紫外-可見分光光度儀(UV-2600, Shimadzu)測定MSOM的紫外可見吸收光譜,掃描范圍為200～800nm,間隔為1nm,純甲醇作為背景進行矯正.使用三維熒光光譜儀(Aqualog,HORIBA)測定三維熒光光譜,激發(fā)和發(fā)射波長分別為240～600nm和250～700nm,間隔為1nm.同樣方法測量空白樣品,以對樣品信號進行矯正.對所得數(shù)據(jù)進行背景扣除和內(nèi)濾效應校正.

1.4 數(shù)據(jù)分析

吸收波長指數(shù)(AAE)用于表征BrC光吸收強度隨波長變化的依賴性[22-23],計算公式為:

式中:abs代表波長為時的吸光度,為常數(shù),擬合范圍為340～400nm.

三維熒光光譜信號的堆疊,給光譜解析帶來困難,將數(shù)據(jù)進行降維,從低維中探究物質(zhì)特征相關(guān)性是獲取成分信息有效手段.Lee等[24-25]提出的非負矩陣分解法(NMF)可高效處理矩陣數(shù)據(jù)信息,其表達式為:

式中:為各BrC熒光組分的熒光強度.和分別為′和′維的正交矩陣,其中的元素值均為正值,兩者可分別理解為個組分的激發(fā)光譜和發(fā)射光譜.NMF是進行數(shù)據(jù)分解、提取特征信息的有效方法,相對于平行因子分析,其結(jié)果可直接理解為激發(fā)和發(fā)射光譜,較為直觀,其在EEM分解中的應用已有報道[26].NMF作為有效的多維數(shù)據(jù)處理工具之一,被廣泛應用于人臉檢測、細胞分析、語言建模等研究中,已逐漸滲透于信號處理[27]、生物醫(yī)學工程[28]、語音識別[29]、計算機視覺和圖像工程[30]等研究領(lǐng)域.本研究利用NMF對測量所得的EEM進行分解,提取特征激發(fā)和發(fā)射光譜.

2 結(jié)果與分析

2.1 紫外-可見光吸收特征

如圖2所示,所有物質(zhì)燃燒生成的MSOM的光吸收呈現(xiàn)從可見光區(qū)到近紫外區(qū)指數(shù)上升的趨勢,與大氣氣溶膠的BrC吸收光譜相似.運用AAE指數(shù)對波長依賴性大小進行表征,結(jié)果如下:大豆(7.80±0.02)、小麥(7.39±0.08)、玉米(6.38±0.02)、竹子(8.17±0.03)、柳樹(5.85±0.08)、松木(5.39±0.03)、紙(6.53±0.06)、煤(7.84±0.03)、泡沫(9.52±0.29)、塑料袋(7.39±0.06)、塑料瓶(8.16±2.94),AAE值均較大,具有明顯的波長依賴性.

圖2 不同物質(zhì)燃燒排放顆粒物中MSOM的吸收光譜

草本植物與木本植物的主要成分均為纖維素、半纖維素(合稱綜纖維素)和木質(zhì)素[31],從而使兩者MSOM的吸收曲線走勢大致相同.但兩類物質(zhì)在成分含量上存在差異,區(qū)別主要在于兩者的莖部:與草本植物相比,木本植物擁有更發(fā)達的木質(zhì)部,因此木質(zhì)素含量較高,而綜纖維素含量占比較低[32-33].各組分含量不同對生物質(zhì)燃燒有著較大影響.綜纖維素易于熱解,產(chǎn)物主要為揮發(fā)性物質(zhì),木質(zhì)素較難分解,所需時間長,產(chǎn)物芳香化程度高,因此木本植物的MSOM具有更強的光吸收能力[31,34].

煤屬于化石燃料,燃燒時易產(chǎn)生黑炭、有機碳、多環(huán)芳烴類、醛類等物質(zhì)[35].煤的MSOM在紫外區(qū)有較高的吸收值,且250～280nm波段出現(xiàn)的肩峰通常表現(xiàn)為C=C和C=O雙鍵的π-π*電子躍遷[14,36].

瓦楞紙板的基本成分與生物質(zhì)相同,吸收光譜相似.但紙板在制作過程中去除了植物纖維中含有的木質(zhì)素、果膠、樹脂等其他成分,僅保留綜纖維素等成分,纖維素由多糖組成,生成的BrC在短波長處不存在肩峰.泡沫、塑料袋和塑料瓶燃燒產(chǎn)生多種共軛烯烴或芳香烴化合物,因此也能看到提取物具有BrC的光吸收特性.

2.2 三維熒光光譜分析

對于有色混合物的成分分析,三維熒光是繼質(zhì)譜之后最有力的測量方法[37],因為它能在激發(fā)和發(fā)射的兩個維度上對生色團進行區(qū)分.對上述原料的MSOM分別測量EEM光譜(圖3).生物質(zhì)和紙板有3個明顯的熒光特征峰,根據(jù)文獻可分別歸因于類腐殖質(zhì)F1(x/m=230～250nm/350～400nm),和相對較弱的類腐殖質(zhì)F2(x/m=280～300nm/340～370nm)、類蛋白質(zhì)F3(x/m=270～280nm/300～310nm)[38-40].盡管三維熒光能根據(jù)熒光團的吸收和發(fā)射波長的不同,在光譜上以強度峰的形式體現(xiàn),但是有機官能團的吸收和發(fā)射帶通常較寬,范圍在十幾到幾十納米,導致在EEM上相鄰的兩個或多個峰呈現(xiàn)一定程度的重疊,單憑肉眼難以精確區(qū)分.

本研究利用NMF程序?qū)λ萌S熒光光譜進行分解,獲得主要發(fā)色團的激發(fā)和發(fā)射特征光譜(圖3).結(jié)果表明,生物質(zhì)(分別對應光譜A～F)的MSOM包含3種主要熒光團,由其激發(fā)/發(fā)射特征光譜可見,熒光組分C1(x/m=250nm/355nm、x/m=310nm/ 355nm)和C2(x/m=240nm/400nm、x/m=325nm/ 400nm)均含有2個明顯的熒光峰,與相關(guān)文獻進行比較,兩者屬于類腐殖質(zhì),對應上述F1峰和F2峰,C3(x/m=275nm/325nm)為單峰,屬類蛋白質(zhì)(類色氨酸),與F3峰對應[41-43].C1和C2同為類腐殖質(zhì),但所含的熒光團不同,從而顯現(xiàn)不同的熒光特性.C1的最大吸收和發(fā)射波長均較C2藍移,說明C1含有較少的芳香或共軛結(jié)構(gòu)或含有硝基等吸電子基團[21].紙板主要成分為植物纖維,由圖3可看出,其BrC熒光特征光譜(光譜G)與生物質(zhì)的光譜有著相似的輪廓,說明紙板燃燒產(chǎn)生的BrC中也存在C1、C2和C3三種熒光成分.

草本植物(A～D):大豆、小麥、玉米、竹子;木本植物(E、F):柳樹、松木;紙板(G);煤(H);生活垃圾(I～K):泡沫、塑料袋、塑料瓶

圖4 聚苯乙烯、聚乙烯、聚對苯二甲酸乙二醇酯分子結(jié)構(gòu)

泡沫和塑料的組成成分為高分子聚合物,與上述物質(zhì)成分不同,測定的EEM光譜亦呈現(xiàn)較大差異,且不同材質(zhì)的塑料的光譜也表現(xiàn)出各自的光譜分布輪廓,辨識度高.泡沫、塑料袋和塑料瓶的BrC熒光光譜較為復雜(分別對應光譜I～K),經(jīng)NMF處理,泡沫和塑料袋MSOM含4種熒光特征組分,而塑料瓶僅含有2種.泡沫主要成分為聚苯乙烯(PS)、塑料袋為聚乙烯(PE)、塑料瓶為聚對苯二甲酸乙二醇酯(PET)(圖4).單體組成不同,燃燒過程中發(fā)生分解和結(jié)構(gòu)重組的產(chǎn)物相應不同,從而在光譜上呈現(xiàn)出各自熒光特性.由圖3可見,與塑料袋相比,泡沫MSOM熒光團的強度較高,這是由于泡沫主成分PS的芳環(huán)結(jié)構(gòu)具有比塑料PE更強的吸附多環(huán)芳烴的能力[44].泡沫和塑料袋含有相似的熒光峰,兩者的P1和P2與生物質(zhì)的兩個類腐殖質(zhì)熒光團處于同一區(qū)域.P3為酪氨酸類芳香族蛋白質(zhì),與生物質(zhì)有所區(qū)別.根據(jù)文獻報道,在266nm激發(fā)波長下,PS和PE發(fā)射峰位于340nm附近[45-46],與之對比,可認為泡沫和塑料袋的P4即為兩者對應的主成分PS和PE,但本研究結(jié)果顯示P4發(fā)射峰相對紅移,可能是由于兩種成分對芳香族化合物的吸附所造成[47].相對于泡沫和塑料袋,塑料瓶的光譜成分簡單.

與生物質(zhì)不同,煤的MSOM熒光峰出現(xiàn)在M1(x/m=250nm/365nm、x/m=320nm/365nm)、M2(x/m=255nm/400nm、x/m=340nm/405nm)、M3(x/m=285nm/375nm)以上3個熒光位置,均較生物質(zhì)沿激發(fā)和發(fā)射波長紅移,表明煤燃燒源MSOM具有更多的芳香性物質(zhì)[9],且可根據(jù)兩者熒光團位置及所呈現(xiàn)的輪廓的差異,對生物質(zhì)和煤進行區(qū)分.

進一步,根據(jù)各MSOM的x/m光譜數(shù)據(jù)可計算各個組分的三維熒光并通過積分獲得各組分所對應的熒光強度,如圖5所示.總體來看,對于生物質(zhì)和紙板,C1為最主要熒光團,其貢獻在總熒光強度中約占42.8%,C2次之約36.3%,C3在各類MSOM中的貢獻均相對較小,僅占22.4%.有機大分子樣品MSOM主要成分數(shù)為2個或4個,泡沫和塑料原材料成分雖然單一,但燃燒過程中經(jīng)過重組,得到多種復雜產(chǎn)物,組成上與生物質(zhì)有一定區(qū)別.其余未列出組分熒光強度占比甚小,可視為噪音,忽略其貢獻.

圖5 11種物質(zhì)各熒光組分的相對含量

3 結(jié)論

3.1 由于基本成分相同,各生物質(zhì)燃燒產(chǎn)生的BrC的吸收曲線有相近的輪廓,相比草本植物,木本植物較多難熱解的木質(zhì)素使得其燃燒產(chǎn)物中含有更多芳香類化合物,在紫外區(qū)具有較強的吸光性;煤燃燒排放的BrC在短波處也具有很強的光吸收能力;泡沫塑料等生活垃圾燃燒產(chǎn)物在光譜上也呈現(xiàn)出BrC的吸收特性.

3.2 生物質(zhì)和紙板燃燒源MSOM的熒光峰所在區(qū)域十分貼近,在EEM光譜分布中大致分為三類:包括兩種類腐殖質(zhì)成分C1和C2及一種類蛋白質(zhì)成分C3,其中C1為最主要熒光團,分布在x/m=250nm/ 355nm、x/m=310nm/355nm;C2次之,主要分布在x/m=240nm/400nm、x/m=325nm/400nm附近,C3貢獻相對較小,位于x/m=275nm/325nm處.煤燃燒產(chǎn)生的BrC中3種熒光組分M1、M2和M3的發(fā)射峰較生物質(zhì)紅移,說明煤的 BrC 中含有更多共軛芳香結(jié)構(gòu).塑料和泡沫燃燒產(chǎn)生的BrC成分與生物質(zhì)和煤存在差異,且不同材質(zhì)呈現(xiàn)不同的熒光特性.

3.3 根據(jù)3類物質(zhì)的熒光團位置及信號輪廓差異,三維熒光可有效地區(qū)分生物質(zhì)、煤以及多種塑料燃燒生成的棕色碳,且其測量過程便捷,設備穩(wěn)定廉價,在建立完備的光譜數(shù)據(jù)庫和標準的前提下可能發(fā)展為棕色碳的高效測量,尤其是在線監(jiān)測.非負矩陣分解能快速地從三維光譜提取主成分的特征光譜和相對含量信息,可進行實時數(shù)據(jù)分析并自動溯源.

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Comparisons of three-dimensional fluorescence spectra of brown carbon from combustion.

DENG Ru-le, GAO Peng, JIA Song, WANG Xue-fei*

(School of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, China)., 2022,42(9)：3983～3990

11 common substances, including straw, wood, coal, waste paper and waste plastics, were employed to simulate combustion in the laboratory and the methanol soluble organic matters (MSOM) in smoke were collected. The brown carbon in MSOM was characterized by UV-Vis and three-dimensional fluorescence spectroscopy (EEMs). Furthermore, non-negative matrix decomposition (NMF) was used to extract the characteristic excitation/emission spectra of main components of EEM, aiming to establish the basis of brown carbon traceability. The results show that the UV-vis spectra of straw and wood burning source brown carbon were similar with a shoulder peak at 265nm. The absorption of brown carbon from corrugated board and plastic combustion sources decreases singly with increasing wavelength. Due to the same basic components, the EEM of brown carbon corresponding to biomass and paperboard had similar profiles. NMF analysis showed three main fluorescent components in MSOM of biomass and paperboard, which were two humus C1, C2 and protein-like C3, respectively. The EEM of coal has a strong distribution in the long wave, which can be attributed to aromatic groups. From its EEM, the M1, M2 and M3 fluorophores can be decomposed and their positions are redshifted compared to those of biomass. According to fluorophore position and spectral signal profile, biomass and coal can be distinguished. Foam, plastic bag and plastic bottle are organic polymer materials and their EEM is quite different from that of biomass. The MSOM of foam and plastic bag contains four fluorescent components, while the MSOM of plastic bottle can only get two fluorophore groups with obvious characteristics.

brown carbon；burning；excitation emission matrix；UV-vis spectrum；non-negative matrix factorization；source

X513

A

1000-6923(2022)09-3983-08

2022-02-28

北京市自然科學基金資助項目(8222074)

**責任作者, 副教授, wangxf@ucas.ac.cn

鄧汝樂(1998-),女,廣東湛江人,中國科學院大學科研助理,主要研究方向為大氣氣溶膠的棕色碳特征.