污泥超高溫(65℃)厭氧消化系統啟動方案

戴曉虎,于春曉,李 寧,董 濱 (同濟大學環境科學與工程學院,污染控制與資源化研究國家重點實驗室,上海200092)

水污染與控制

污泥超高溫(65℃)厭氧消化系統啟動方案

戴曉虎,于春曉,李 寧*,董 濱 (同濟大學環境科學與工程學院,污染控制與資源化研究國家重點實驗室,上海200092)

以高溫(55℃)厭氧消化反應器的污泥為接種泥,以不同比例的牛糞和脫水污泥為基質,通過產甲烷潛力測試實驗,對污泥超高溫(65℃)厭氧消化系統的啟動策略進行了初步的探討.實驗結果表明:污泥超高溫(65℃)厭氧消化系統具有其可行性;65℃條件下,由于水解酸化過程加快,易發生 VFAs(尤其是乙酸和丙酸)的累積.同時,與中溫(37℃)和高溫(55℃)污泥厭氧消化系統相比,超高溫(65℃)系統的產氣量雖然較低,但所產沼氣中CH4含量明顯升高,可以達到79.0%.對系統細菌和古菌進行的多樣性分析結果表明:超高溫(65℃)條件下,反應器中的細菌以Coprothermobacter、Caldicoprobacter、Ruminiclostridium等極端嗜熱的蛋白質水解菌和木質纖維素水解菌為主,不同反應器之間細菌種群多樣性的差異是由所投加物料的不同造成的;所有反應器的古菌中,嗜熱的氫營養型產甲烷菌 Methanothermabactor成為絕對優勢菌群,占古菌的比例均超過96%.在超高溫反應器(65℃)的啟動初期,可適當提高投加基質中牛糞的比例,加快對嗜熱產甲烷菌(氫利用型產甲烷菌)的富集,同時避免系統中的VFAs的積累,保證反應器順利啟動.

超高溫；厭氧消化；啟動策略；甲烷嗜熱桿菌

厭氧消化技術作為一種在實現污泥穩定化同時可產生綠色能源-沼氣的技術,一直被認為是污泥處理處置的合適選擇之一[1].厭氧消化系統一般在中溫(37℃)和高溫(55℃)條件下運行.然而,中溫厭氧消化系統中由于受到水解步驟的限制,有機顆粒物的降解率較低,所需的 SRT較長[2].即便在高溫厭氧消化(55℃)系統中,微生物的降解潛能也仍沒有得到充分地釋放,有機物降解率也僅有 50%左右[3].因此,污泥厭氧消化的產氣潛能還有較大的提高空間[4],其關鍵是提高污泥中有機物的水解效率[5].Nielsen等[6]與Scherer等[7]通過污泥厭氧消化的對比研究發現,與普通的中溫(37℃)厭氧消化相比,高溫(55℃)和超高溫(超過 55℃)厭氧消化系統更有利于提高有機顆粒溶解性及沼氣產量.這主要是由于水解酸化菌生長的最適溫度為 55～70℃[8],當厭氧消化系統的溫度從55℃上升到65℃時,污泥中蛋白質等難降解有機物的溶解性會顯著提高,傳質速率加快[2].同時,嗜高溫的氫利用型產甲烷菌的最佳生存溫度是 55～70℃[9],如若將厭氧消化的溫度提升到65℃,氫利用型產甲烷途徑的效率將有可能達到最大[10].

污泥熱水解是一種能夠加快污泥水解速率,提高甲烷產率的前處理技術,近年來在實際工程中得到了廣泛的應用[11-12].熱水解后污泥溫度較高,利用熱水解污泥中的剩余熱量,進行超高溫厭氧消化,能提高顆粒狀污泥的溶解性和降解速率以及嗜高溫的水解酸化微生物活性,并富集嗜高溫的氫利用型產甲烷菌以提高產氣效率[4].而且,氫利用型產甲烷途徑的充分利用,能夠減少 CO2排放,節省以乙酸等短鏈脂肪酸形式存在的碳源,方便對碳源進行后續的綜合利用.此外,當溫度上升至60℃以上時,污泥中的膠體物質降解率提高,絲狀菌被殺死,能夠明顯提高污泥的脫水性[13];同時,污泥中的致病菌也能夠被殺滅,初步滿足污泥處理處置無害化的要求[14].

目前,國內外關于超高溫厭氧消化的研究主要集中于牛糞或餐廚垃圾的可行性探索,對于系統中酸化和甲烷化過程的基礎研究較少[15],純污泥體系的超高溫厭氧消化系統的相關研究更是幾近空白.本研究主要通過產甲烷潛力測試實驗(BMP),對超高溫條件下反應器不同啟動方案中的產氣性能和各項指標進行分析對比,以確定超高溫厭氧消化反應器的最佳啟動條件,為污泥超高溫厭氧消化反應器的啟動提供理論依據.

1 材料與方法

1.1 實驗材料

接種牛糞取自上海某生態園,取回后為了保證微生物的活性在 35℃下保存,并于 2d內完成接種;脫水污泥取自上海某污水處理廠,于 4℃下保存;所使用的接種泥取自本實驗室穩定運行的高溫(55℃)厭氧消化反應器.上述物料的基本參數如表1所示.使用前用去離子水分別將牛糞和脫水污泥稀釋至含固率(TS)為8%.

表1 牛糞、脫水污泥和接種泥的相關參數Table 1 Characteristics of cow dung, dewatered sludge and inoculum

1.2 BMP實驗設計

實驗分2個批次進行.BMP實驗通過全自動產甲烷潛力測試儀(AMPTS II,bioprocess control,瑞典)完成.第一批次的BMP實驗主要為了完成超高溫甲烷菌的富集并初步探索超高溫厭氧消化反應器啟動的可行性,第二批實驗在第一批BMP實驗的基礎上,對超高溫厭氧消化反應器的啟動過程的穩定性和微生物種群分布特征進行了研究.

表2 實驗設計參數Table 2 Designed experimental parameters

第一批次的實驗以穩定運行的污泥高溫(55℃)厭氧消化反應器的出料為接種泥.第二批次的接種泥為第一批次超高溫(65℃)厭氧消化反應器的出料.第一批次實驗的基質為牛糞和污泥的混合物,接種比(接種泥:基質(VS:VS))為1:1,反應器為有效容積600mL的雙孔塞玻璃瓶.實驗設置空白對照組和實驗組,空白對照組的反應器中僅添加接種泥,實驗組所投加基質設置為不同比例的牛糞和污泥,具體參數如表2所示.

反應器用雙孔塞和止水夾密封,啟動前用真空泵抽真空后,用氮氣吹脫3min以進一步去除上層空間和溶液中的空氣.實驗設3組平行,結果分析取其平均值.反應器置于 65℃水浴鍋中,攪拌設置為5min工作,5min停止,在反應器運行過程中,采集累積甲烷產量和產甲烷速率的數據.

1.3 測試方法

全自動產甲烷潛力測試儀能測定并記錄累積甲烷產量和產甲烷速率.實驗過程中以集氣袋收集氣體,通過氣相色譜(GC112A,上海儀電,中國)測定氣體成分.實驗結束后反應器中樣品的pH 值通過pH計(S210,梅特勒,瑞士)測定.樣品取出并稀釋離心后,上清液經0.45 μm濾膜過濾,測定其中的揮發性脂肪酸 (Volatile Fatty Acids, VFAs) (GC 2010plus,島津,日本)和堿度(自動電位滴定儀 G20,梅特勒,瑞士).氨氮含量通過使用凱氏定氮儀(9860,海能,中國)由滴定法測定.

1.4 微生物種群結構分析

反應器出料于 2mL凍存管中,于-80℃冷凍保存.DNA 樣品使用土壤 DNA 提取試劑盒(PowerSoil,MO BIO, USA)提取.提取出的 DNA經過 PCR 擴增,采用高通量測序(Miseq4000, Illumina)分析其中細菌與古菌的種群結構.PCR產物先利用QuantiFLuorTM系統(Promega)測定濃度,隨后依據AxyPrep DNA 試劑盒(AXYGEN, USA)的凝膠回收的方法進行提純[16].提純的PCR 產物的質量通過凝膠電泳確定.引物設計根據Illumina 公司(San Diego, California, USA)的操作手冊,針對細菌和古菌的 16s rRNA基因的通用擴增引物對分別為 338f(5'-ACTCCTACGGGAGGCAGCA-3')/806r(5'-GGACTACHV GGGTWTCTAAT-3')[17]和 524f(5'-TGYCAGCCGCCGCGGTAA-3')/958r(5'-YCCGGCGTTGAVTCCAATT-3')[18].

2 結果與討論

2.1 厭氧消化產氣性能

2.1.1 累積甲烷產量 圖1為第一批次BMP實驗中各反應器的產甲烷情況,從圖1可以看出:各反應器的產甲烷過程均順利進行,在第14～16d達到了累積甲烷產量的最大值,說明超高溫(65℃)厭氧消化反應器的啟動是可行的.且牛糞投加比例高的反應器的啟動速度和累積甲烷產量明顯高于污泥投加比例高的反應器,這主要由于牛糞中所存在的嗜熱甲烷菌(氫利用型產甲烷菌)能夠快速適應65℃環境[19],保證了厭氧消化產甲烷過程的順利啟動和進行.因此,在反應器啟動初期,適當增加牛糞的比例能夠加快嗜熱產甲烷菌的富集,保證反應器快速穩定啟動.

圖1 第一批次實驗累積甲烷產量Fig.1 Cumulative methane production of the first batch experiments

圖2為第二批次BMP實驗中各反應器的累積產甲烷情況.可以看出,和第一批次相比,各實驗組的產甲烷速率和累積甲烷產量均明顯上升,說明第一批次BMP實驗有效地實現了嗜熱產甲烷菌的初次富集,使體系中嗜熱產甲烷菌的含量上升,再次確認了超高溫厭氧消化反應系統啟動的可行性.但是,即使在第二批次BMP實驗中,各反應器的累積甲烷產量也只達到了600～800mL,低于相同條件下的中溫和高溫厭氧消化甲烷產量(約1200～1400mL).

2.1.2 氣體成分 在 65℃條件下,厭氧消化所產沼氣中,CH4含量在60%～80%之間(圖3),明顯高于中溫(37℃)和高溫(55℃)條件下的 CH4含量(48%～65%)[20].沼氣中的 CO2含量在 20%～26%,低于中溫(37℃)和高溫(55℃)條件下的CO2含量(36%～41%)[20],這證明:氫利用型產甲烷途徑在65℃條件下得到了強化,成為主要的產甲烷途徑.另外,雖然隨著投加物料中污泥比例的升高,產甲烷速率在一定程度上減慢,但CH4含量逐漸升高,純污泥作為基質的反應器中,CH4含量甚至可以達到 79.0%,表明超高溫條件下氣體成分的差異與牛糞和污泥的物質組成相關.

圖2 第二批次實驗累積甲烷產量Fig.2 Cumulative methane production of the second batch experiments

Fig.3 第二批次實驗各反應器產氣中CH4和CO2含量Fig.3 Methane and carbon dioxide content in biogas of each reactor of the second batch experiments

2.2 液相指標

2.2.1 VFAs 圖4為第一批次各樣品中VFAs的濃度和組成情況,可以看出,各反應器均發生了較嚴重的VFAs累積,達到了1900～4000mg/L.當溫度上升至 65℃時,達到水解酸化菌的最適溫度(55～75℃)[6],水解酸化過程加快,VFAs快速產生,但是,在此溫度條件下,乙酸利用型產甲烷菌的活性降低甚至喪失,導致乙酸向甲烷的轉化過程被抑制,造成了VFA快速累積[14].

圖4 第一批次各反應器中的VFAs濃度及組成(產氣終止時取樣)Fig.4 The concentrations and compositions of VFAs in each reactor of the first batch experiments (sampled when gas production stopped)

如圖5所示,與第一批次實驗中的反應器相比,第二批次實驗的反應器中 VFAs累積更加明顯(表3),說明反應器中的水解酸化過程繼續加強,其中丙酸的累積最明顯,且隨著污泥比例的升高,累積程度也相應略微升高.丙酸的嚴重累積可能是由于:在 65℃條件下,微生物種群結構發生改變,乙酸利用型產甲烷菌失活.與此同時,嗜熱的氫利用型產甲烷菌成為優勢菌群.在氫利用型產甲烷途徑為主的情況下,乙酸氧化菌將乙酸轉化為H2/CO2,在氫利用型產甲烷菌的作用下轉化為甲烷[21].乙酸氧化也會導致氫分壓上升,使得丙酸氧化微生物活性下降,從而導致丙酸降解過程被抑制[22].而且,乙酸是丙酸的降解產物[23],反應器中乙酸的累積也會影響丙酸的降解速率[24-25],并進一步出現丙酸累積抑制甲烷菌生長的情況[26].由于牛糞和污泥的組成成分不同,牛糞中含有較多難生物降解的木質纖維素類物質,升溫后仍不易降解,但污泥的主要成分是蛋白質,隨著溫度的上升,其溶解性和降解率提高[3],因此,投加了污泥的反應器(#2和#3)中,VFAs 累積更加嚴重,反應器的啟動速率慢,甲烷產量低.

因此,在超高溫厭氧消化反應器的啟動初期,為避免 VFAs尤其是丙酸的大量累積,盡快完成嗜熱產甲烷菌的富集,保證反應器順利啟動,宜采用較高比例的牛糞作為進料.污泥:牛糞(VS: VS)=1:1作為啟動初期的投加方案是具有其可行性的.

表3 各反應器運行性能參數 (產氣終止時取樣)Table 3 The performance characteristics in each digester (sampled when gas production stopped)

圖5 第二批次各反應器中的VFAs濃度及組成(產氣終止時取樣)Fig.5 The concentrations and compositions of VFAs in each digester of the second batch experiments (sampled when gas production stopped)

2.2.2 氨氮在本實驗中,各反應器中的氨氮濃度均在 2000～2500mg/之間,遠低于氨抑制濃度(5880～6000mg/L),因此,在所實驗的 TS和溫度(65℃)條件下,氨抑制情況并不容易發生[27].而且,由于牛糞中木質纖維素的含量較高[3],牛糞投加比例較高的反應器中物料碳氮比較高,氨氮濃度相對較低.

2.2.3 pH值和堿度 兩個批次BMP實驗中的反應器的pH值分別為7.7～7.9和7.6～7.7,略高于產甲烷菌的最佳pH值范圍(6.8～7.2)[20].

堿度能夠反映厭氧消化系統的緩沖能力[28].如表3所示，BMP實驗中各組反應器中的堿度均在 4000～6000mg/L(以 CaCO3計)之間,且隨著污泥比例的增加,系統堿度(尤其是第一批 BMP實驗中)基本呈現上升趨勢.這主要由于與牛糞相比,污泥中的蛋白質含量較高,蛋白質降解過程中會產生較高濃度的游離氨,提高系統堿度[20].較強的緩沖性能使得各反應器在較高濃度的VFAs累積的情況下,沒有發生pH值明顯降低的現象.

VFAs/堿度是表征厭氧消化系統穩定性的參數:當 VFAs/堿度值小于 0.4時,系統穩定,當VFAs/堿度值在0.4～0.8之間時,系統可能會發生失穩現象;當VFA/堿度值大于0.8時,系統嚴重失穩[29-30].兩批BMP實驗中各反應器的VFAs/堿度值均在0.4～0.8之間,這說明,在目前的TS和實驗條件下,反應器是不穩定的,且隨著污泥投加量的增加,反應失穩的可能性增大.與第一批次 BMP實驗相比,第二批次實驗中,各反應器的穩定性有提高,這說明,對嗜熱微生物的富集是有效的,反應器啟動初期增加牛糞的投加量和富集次數,能夠提高反應器的穩定性,保證反應器穩定啟動.

2.3 微生物種群結構分析

2.3.1 細菌種群結構 圖 6為各個反應器中細菌多樣性分析結果:在各反應器中,細菌種群均呈現明顯的多樣性,但各反應器中細菌種群的分布差異比較明顯.#1反應器中,Caldicoprobacter、Ruminiclostridium、 OPB54_norank、 D8A-2_norank和Coprothermobacter五種細菌含量均較高,分別達到了細菌總量的16.93%、13.90%、13.10%、9.90%和 9.62%.#2 反應器的優勢菌種為Caldicoprobacte和Coprothermobacter,分別占細菌的 18.74%、17.51%.#3 反應器中, Coprothermobacter成為了最主要的優勢菌種,所占比例達到了 21.85%.Coprothermobacter、Caldicoprobacter、Ruminiclostridium都是極端嗜熱的水解菌[15,31-32],而多存在于中溫和高溫反應器中的水解酸化菌,如:Bacteroidetes 和Proteobacteria[33]在超高溫反應器中沒有檢測到,Firmicutes所占的比例很低,僅1.5%～3.5%,這說明,與中溫和高溫反應器相比,超高溫條件下,細菌的種群結構發生了明顯的改變,極端嗜熱的細菌成為優勢菌種.隨著反應器中所投加污泥比例的升高,Coprothermobacter的種群優勢逐漸增大,而其比例的變化與牛糞和污泥的組成成分相關.Coprothermobacter是一種極端嗜熱的蛋白質水解菌,在厭氧消化過程中主要參與蛋白質和多肽的降解,在70℃仍能存活[15,34-35],在牛糞厭氧消化反應器[36]和污泥高溫厭氧消化反應器[37-38]中均有發現,牛糞厭氧消化反應器中Coprothermobacter的含量較低[39].與牛糞相比,污泥中蛋白質含量較高,因此,在#3反應器中,Coprothermobacter的所占細菌的比例要明顯高于其在#1號反應器中的比例.

Caldicoprobacter是一種嚴格厭氧的嗜熱菌,最適生長溫度為 65℃(55～75℃),最適生長的 pH值為6.9(6.2～8.3),是污泥高溫厭氧消化反應器重要的水解菌[31,40].Caldicoprobacter能夠產生耐高溫的木聚糖酶,一般參與半纖維素(木聚糖等)的降解[41].Ruminiclostridium是一種嗜熱厭氧菌,最佳的生長溫度為55 ～60℃,pH值為 7.3～7.5,能夠合成纖維素降解酶,降解木質纖維素類物質(纖維素、半纖維素和植物細胞壁多糖等),普遍存在于纖維素類物質的堆肥中[32,42-44].牛糞中纖維素和半纖維素等物質的含量較高,因此,在投加了牛糞的#1和#2反應器中,Caldicoprobacter、Ruminiclostridium所占比例均較高.

由于Coprothermobacter、Caldicoprobacter、Ruminiclostridium的水解終產物均為乙酸、H2和CO2等,這些的細菌的存在有利于促進氫利用型產甲烷菌的活性,從而促進 CO2/H2產甲烷途徑[36,40,44].

另外,Intrasporangiaceae、Tepidimicrobium、Proteiniphilum等均存在于3個反應器中,這些細菌大多是嗜熱的蛋白質和碳水化合物水解菌[45-47],能利用多種碳源,因此這些細菌的含量在各個反應器中的差別并不明顯.

圖6 各反應器中的細菌相對豐度對比Fig.6 the relative abundance of bacteria in each digester

綜上所述,在超高溫(65℃)條件下,3個反應器中細菌主要為極端嗜熱的蛋白質和木質纖維素水解菌,不同反應器中細菌種群多樣性的差異部分來源于牛糞和污泥中本身所含細菌種類的不同,本質上是由牛糞和污泥的組成成分的不同決定的.

2.3.2 古菌種群結構 圖 7為第二批次實驗中各反應中的古菌多樣性分析結果.可以看出,與細菌相比,各反應器中古菌的種類較少,且分布情況相近,經過二次富集后,各反應器中的古菌均以嗜熱產甲烷菌Methanothermabactor為主,所占比例超過 96%.這說明,Methanothermabactor在牛糞和高溫厭氧消化污泥中都存在,這與Chachkhiani[48]和Zeikus[49]等的結論一致.且在超高溫條件(65℃)下,Methanothermabactor能夠被富集,最終成為優勢菌群.Methanothermabactor是一種氫利用性產甲烷菌,利用H2和CO2途徑產甲烷,最佳pH值范圍為7.2～7.6,最適溫度范圍在65～70℃[49-50].各反應器終止時的 pH 值接近Methanothermabactor的最適 pH值范圍,均在7.6～7.7之間,保證了過程中Methanothermabactor的生長.而中溫和高溫厭氧消化反應器中普遍存在的乙酸利用型產甲烷菌 Methanosaeta[51-52]所占的比例很低,分別只占 0.38%、1.86%和2.61%,3個反應器中均未檢測到Methanosarcina.這為反應器中以氫利用型產甲烷過程為主的甲烷化過程提供了直接證據,而乙酸利用型產甲烷途徑幾乎終止,從而導致乙酸等揮發性脂肪酸大量累積.另外,與投加生污泥的反應器相比,投加牛糞的反應器中Methanosaeta的含量更低. Methanothermabactor的成功富集保證了反應器中產甲烷過程的順利進行,為超高溫厭氧消化系統順利啟動提供了條件.

圖7 各反應器中的古菌相對豐度對比Fig.7 The abundance of archaea in each digester

在污泥超高溫(65℃)厭氧消化系統中,CO2/ H2產甲烷途徑成為主要的產甲烷途徑,乙酸產甲烷途徑幾乎不再進行,在實現CO2減排的同時,能夠節省以短鏈脂肪酸為代表的有機碳源.這些富含短鏈脂肪酸的厭氧消化沼液可以作為碳源,補充到污水處理的反硝化池中,促進脫氮除磷,節省污水處理的成本,提高污水處理效果,實現碳源的綜合利用.

3 結論

3.1 研究證實了污泥超高溫(65℃)厭氧消化反應器啟動的可行性,在進料中混合投加含有嗜熱產甲烷菌(氫利用型產甲烷菌)的牛糞能夠加快反應器的啟動速率.

3.2 65℃條件下,污泥中蛋白質等有機顆粒的溶解性提高,水解酸化菌的活性提高,易造成VFAs累積,反應器啟動初期,宜通過逐步降低牛糞比例的方式來保證反應器啟動過程的穩定性,研究中所采用的方案-牛糞:污泥(VS:VS)=1:1可以作為啟動的初始投加方案.

3.3 與中溫(37℃)和高溫(55℃)污泥厭氧消化相比,超高溫(65℃)的產氣量較低,但所產生物氣中CH4含量高79.0%,CO2含量低(20.0%).

3.4 超高溫(65℃)條件下,各反應器中的細菌以嗜熱的木質纖維素和蛋白質的水解菌為主,包括 Coprothermobacter、Caldicoprobacter、Ruminiclostridium,不同反應器之間細菌種群多樣性的差異主要由所投加物料的不同造成.氫利用型產甲烷菌Methanothermabactor成為主要的產甲烷菌,占各反應器的甲烷菌的總量中占 96%以上,而乙酸利用型產甲烷菌 Methanosaeta、Methanosarcina等已經幾乎不存在了.

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Start-up strategy for hyperthermophilic anaerobic digestion system of sewage sludge.

DAI Xiao-hu, YU Chun-xiao, LI Ning*, DONG Bin (State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 20092, China). China Environmental science, 2017,37(7)：2527～2535

The feasibility of hyperthermophilic anaerobic digestion system of sewage sludge is evaluated in the study, which is conducted with biochemical methane potential experiments using thermophilic anaerobic digestate as inoculum and cow dung and dewatered sludge as substrate. The results show that:①the start-up of hyperthermophilic sludge anaerobic digestion system is practicable;②the accumulation of VFAs (particularly propionic acid) is easy to occur due to the acceleration of hydrolysis and acidification process under 65℃;③compared with mesophilic(37℃) and thermophilic (55℃) anaerobic digestion systems, the total gas production of hyperthermophilic (65℃) system is relatively lower, while the methane content is elevated significantly, even reaching 79.0%;④under hyperthermophilic (65℃) condition, the extremely thermophilic bacteria responsible for lignocellulose and protein degradation such as Coprothermobacter、Caldicoprobacter、Ruminiclostridiumwere dominant, the difference of which is due to the different substrate addition; hydrogenotrophic methanogens-Methanothermobacter accounted for nearly 96% of archaea in all the digesters. Thus, in the start-up period, the addition of cow dung can not only accelerate the accumulation of hyperthermophilic methanogens (Hydrogenotrophic methanogens), but also avoid the accumulation of VFAs, particularly the accumulation of propionic acids, ensuring successful start-up of the system.

hyperthermophilic；anaerobic digestion；start-up strategy；Methanothermobacter

X703.1

A

1000-6923(2017)07-2527-09

戴曉虎(1962-),男,江蘇鎮江人,教授,博士,主要研究方向為城市有機廢棄物的處理與處置.發表論文120余篇.

2016-11-31

國家自然科學基金項目(51678429,51308402,51538008)

* 責任作者, 博士, lining@tongji.edu.cn