氨氮濃度對(duì)雞糞中高溫甲烷發(fā)酵的影響

喬 瑋,畢少杰,熊林鵬,鞠鑫鑫,董仁杰*

氨氮濃度對(duì)雞糞中高溫甲烷發(fā)酵的影響

喬 瑋1,2,畢少杰1,熊林鵬1,鞠鑫鑫3,董仁杰1,2*

(1.中國(guó)農(nóng)業(yè)大學(xué)工學(xué)院,國(guó)家能源生物燃?xì)飧咝е苽浼熬C合利用技術(shù)研發(fā)(實(shí)驗(yàn))中心,北京 100083；2.中國(guó)農(nóng)業(yè)大學(xué)煙臺(tái)研究院,山東 煙臺(tái) 264670；3.山東中農(nóng)三月環(huán)?？萍脊煞萦邢薰?山東 煙臺(tái) 264670)

為探究氨氮濃度對(duì)雞糞中高溫甲烷發(fā)酵的影響,采用固定水力停留時(shí)間(HRT,20d),提高進(jìn)料總固體濃度(TS,5%、7.5%和10%)的方式增加氨氮濃度,通過(guò)265d的長(zhǎng)期甲烷發(fā)酵試驗(yàn),比較了不同氨氮濃度條件下雞糞中高溫甲烷發(fā)酵效果和污泥的比產(chǎn)甲烷活性.結(jié)果顯示,TS由5%增至10%,中高溫反應(yīng)器中氨氮濃度由2.1~2.5g/L增至6.1~6.5g/L,對(duì)應(yīng)的比產(chǎn)甲烷活性分別降低了44%和100%,中溫反應(yīng)器中揮發(fā)性脂肪酸由0.4g/L增至7.6g/L,甲烷產(chǎn)率由253mL/gTS降至203mL/gTS;高溫反應(yīng)器中揮發(fā)性脂肪酸由0.4g/L增至26.1g/L,甲烷產(chǎn)率由181mL/gTS降至18mL/gTS.氨氮濃度對(duì)高溫甲烷發(fā)酵系統(tǒng)的抑制作用更加明顯.

氨氮濃度；進(jìn)料濃度；雞糞；甲烷發(fā)酵

目前,我國(guó)每年產(chǎn)生約38億t畜禽糞污,其處理和綜合利用率較低,既污染了環(huán)境又浪費(fèi)了資源[1].雞糞富含有機(jī)質(zhì),較為適合采用甲烷發(fā)酵的方法處理,產(chǎn)甲烷潛力達(dá)到296~377mL/gVS[2].但是,雞糞中含有大量的氮元素,過(guò)高的氨氮(TAN)在甲烷發(fā)酵過(guò)程中會(huì)對(duì)微生物產(chǎn)生抑制作用,引起有機(jī)酸積累和產(chǎn)氣量下降等一系列問(wèn)題[3].通常,TAN超過(guò)1.5g/L就可能阻礙甲烷發(fā)酵的進(jìn)行,超過(guò)3.0g/L會(huì)對(duì)中高溫甲烷發(fā)酵產(chǎn)生明顯抑制[4].研究發(fā)現(xiàn),抑制豬糞中溫甲烷發(fā)酵的TAN為2.5g/L[5].經(jīng)過(guò)馴化后,雞糞中溫甲烷發(fā)酵在5.0g/L的TAN條件下才會(huì)受到抑制[6].目前,大多數(shù)的沼氣工程在中溫(~37℃)條件下運(yùn)行.但是,高溫(~55℃)發(fā)酵在產(chǎn)氣效率和滅殺病原菌方面更具優(yōu)勢(shì)[7-8].然而,與中溫甲烷發(fā)酵相比,高溫發(fā)酵對(duì)TAN、揮發(fā)性脂肪酸(VFAs)和其它毒害物質(zhì)的變化更敏感[9].TAN達(dá)到1.7g/L就會(huì)抑制牛糞高溫甲烷發(fā)酵[10].經(jīng)過(guò)馴化后,雞糞高溫甲烷發(fā)酵能夠在4.0g/L的TAN條件下穩(wěn)定運(yùn)行[11].盡管關(guān)于氨氮濃度對(duì)甲烷發(fā)酵影響的研究已經(jīng)較多,但不同研究報(bào)道的中高溫甲烷發(fā)酵氨氮抑制濃度差異較大.甲烷發(fā)酵不僅受TAN影響,而且與總固體濃度(TS)和負(fù)荷等因素密切相關(guān)[6].此外,以往的研究多采用添加氯化銨增加氨氮濃度[11-12],而采用提高雞糞進(jìn)料濃度的方式增加氨氮濃度,更加符合實(shí)際沼氣工程運(yùn)行狀態(tài),對(duì)工程實(shí)踐更具有指導(dǎo)意義.目前,關(guān)于氨氮濃度對(duì)雞糞中高溫甲烷發(fā)酵影響的對(duì)比研究還鮮有報(bào)道.

為此,本文分別開(kāi)展雞糞中溫和高溫甲烷發(fā)酵的長(zhǎng)期連續(xù)實(shí)驗(yàn),通過(guò)提高進(jìn)料濃度的方式增加氨氮濃度,解析在不同氨氮濃度下中高溫甲烷發(fā)酵的性能.

1 材料和方法

1.1 試驗(yàn)材料及處理方法

雞糞取自中國(guó)農(nóng)業(yè)大學(xué)西校區(qū)蛋雞養(yǎng)殖基地,取回后放置于4℃冷藏室中保存.使用之前,用自來(lái)水分別將雞糞稀釋至總固體濃度(TS)約5%、7.5%和10%.接種污泥分別為北京密云石匣村玉米秸稈高溫(50~60℃)沼氣工程和北京順義北郎中豬糞中溫(37℃)沼氣工程的出料,取料時(shí)兩座沼氣工程均常年連續(xù)運(yùn)行.雞糞和接種污泥的性質(zhì)見(jiàn)表1.

表1 雞糞和接種污泥的性質(zhì)

注:TS:總固體;VS:揮發(fā)性固體;“/”表示未檢測(cè);*:元素質(zhì)量分?jǐn)?shù)以干物質(zhì)計(jì).

1.2 長(zhǎng)期連續(xù)發(fā)酵試驗(yàn)設(shè)計(jì)

圖1 試驗(yàn)裝置示意

在中溫(37℃)和高溫(55℃)條件下,采用2個(gè)相同的全混式反應(yīng)器(CSTR)進(jìn)行雞糞發(fā)酵試驗(yàn),試驗(yàn)裝置見(jiàn)圖1.反應(yīng)器的容積為15L,有效容積為12L.反應(yīng)器采用連續(xù)機(jī)械攪拌,轉(zhuǎn)速設(shè)置為100r/min;設(shè)有水浴夾層,采用水浴循環(huán)加熱,分別維持反應(yīng)器溫度為55和37℃.基質(zhì)罐的容積為8L,溫度設(shè)置為4℃.采用蠕動(dòng)泵(BT100N,保定申辰)自動(dòng)進(jìn)出料,通過(guò)定時(shí)器(DJ-B14M,深圳定時(shí)寶)控制4次/d進(jìn)出料,每次進(jìn)出料量為0.15L.水力停留時(shí)間(HRT)設(shè)置為20d,梯度提高進(jìn)料TS(5%、7.6%和10%)和有機(jī)負(fù)荷(OLR,2.5,3.75,5gTS/(L·d)),每個(gè)階段的維持時(shí)間分別為93,87,85d.每天測(cè)定產(chǎn)氣量、pH值和沼氣成分;每4d測(cè)定堿度、VFAs和TAN.

1.3 污泥比產(chǎn)甲烷活性測(cè)試

污泥的比產(chǎn)甲烷活性(SMA)是指污泥生成甲烷的能力,是表征污泥品質(zhì)和產(chǎn)甲烷菌活性的重要指標(biāo)之一[13].以乙酸鈉為基質(zhì),采用批次試驗(yàn)進(jìn)行污泥產(chǎn)甲烷活性測(cè)試.接種污泥取自反應(yīng)器穩(wěn)定運(yùn)行階段(第75,160,240d)的新鮮出料,去除溶解性成分并恢復(fù)活性后使用[3].經(jīng)測(cè)定,高溫接種污泥的VSS分別為10,24,52gVSS/L,中溫接種污泥的VSS分別為9,15,31gVSS/L.試驗(yàn)共6組,每組3個(gè)平行試驗(yàn).

取120mL玻璃發(fā)酵瓶,加入接種污泥10mL,添加乙酸鈉和營(yíng)養(yǎng)液至100mL,形成乙酸濃度為4gCOD/L的發(fā)酵液.添加NH4Cl調(diào)節(jié)TAN與反應(yīng)器運(yùn)行階段的一致,對(duì)應(yīng)濃度分別為2.5,5.5,6.5g/L.向發(fā)酵液上方充入氮?dú)?min,形成厭氧環(huán)境,密封.水浴(HH-60,常州國(guó)華)保持37,55℃恒溫,每天手動(dòng)震蕩3次混合料液.發(fā)酵過(guò)程中測(cè)定產(chǎn)氣量和沼氣成分.

SMA采用公式(1)進(jìn)行計(jì)算[14]:

式中:(CH4)為累積產(chǎn)甲烷量,mL;R為接種污泥添加量,L;為COD與甲烷產(chǎn)量的轉(zhuǎn)化系數(shù),350mL/ gCOD;VSS是接種污泥的懸浮揮發(fā)性固體質(zhì)量濃度,代表微生物的含量,gVSS/L;為發(fā)酵時(shí)間,d.

1.4 分析方法

TS、VS和VSS采用質(zhì)量法測(cè)定.pH值采用Orion 5-Star pH計(jì)測(cè)定.雞糞中的碳、氫、氧和氮的元素質(zhì)量百分含量采用Vario Macro型元素分析儀測(cè)定.沼氣成分、堿度、VFAs和TAN的測(cè)定參照文獻(xiàn)[15].

1.5 數(shù)據(jù)處理

利用 Excel 2010進(jìn)行數(shù)據(jù)處理和制圖.

2 結(jié)果與討論

2.1 雞糞中高溫甲烷發(fā)酵性能

雞糞中高溫甲烷發(fā)酵試驗(yàn)進(jìn)行了265d,不同進(jìn)料TS和OLR條件下的產(chǎn)氣率和甲烷含量的變化情況見(jiàn)圖2.發(fā)酵開(kāi)始至第93d,進(jìn)料TS為5%,OLR為2.5gTS/(L·d),中溫反應(yīng)器的甲烷產(chǎn)率為253mL/gTS,比高溫反應(yīng)器的高41%(180mL/TS).中高溫反應(yīng)器的甲烷濃度相同,為67%~68%.發(fā)酵第94~180d,TS為7.5%,OLR為3.75gTS/(L·d).與TS5%階段相比,中溫反應(yīng)器的甲烷產(chǎn)率為243mL/gTS,甲烷產(chǎn)率變化不明顯;高溫反應(yīng)器的甲烷產(chǎn)率顯著下降,僅為112mL/gTS.此階段,高溫反應(yīng)器的甲烷產(chǎn)率比中溫反應(yīng)器低54%.同時(shí),高溫的甲烷濃度(56%)也顯著低于中溫反應(yīng)器(67%).發(fā)酵第181~265d,TS為10%,OLR為5gTS/(L·d).中高溫反應(yīng)器的甲烷產(chǎn)率分別為203和18mL/gTS,分別是TS5%時(shí)的80%和10%,高溫反應(yīng)器的甲烷產(chǎn)率下降更加明顯.此時(shí)高溫反應(yīng)器的甲烷濃度僅為36%.

圖2 反應(yīng)器在不同進(jìn)料濃度和負(fù)荷下的甲烷產(chǎn)率和甲烷含量

表2顯示,OLR由2.5gTS/(L·d)增至3.75,5g/(L·d)的過(guò)程中,中高溫反應(yīng)器的TAN濃度基本相同,由2.1~2.5g/L增至5.5,6.1~6.5g/L.高濃度的TAN為發(fā)酵系統(tǒng)提供了充足的堿度.OLR為2.5gTS/(L·d)時(shí),中高溫反應(yīng)器的碳酸氫鹽堿度相近,分別為7.9, 8.6gCaCO3/L.OLR為3.75,5gTS/(L·d)時(shí),由于高溫反應(yīng)器中VFAs大量累積,造成碳酸氫鹽堿度(8.1, 3.7gCaCO3/L)低于中溫反應(yīng)器(15.6,20.5gCaCO3/L), pH值(8.1和6.9)低于中溫反應(yīng)器(8.5和8).

隨著進(jìn)料TS的增加,中高溫反應(yīng)器的TAN和VFAs開(kāi)始累積,導(dǎo)致甲烷產(chǎn)率迅速降低.在未經(jīng)高TAN馴化的厭氧反應(yīng)器中,2.0g/L的TAN濃度就能完全抑制產(chǎn)甲烷菌的生長(zhǎng),而馴化后的產(chǎn)甲烷菌能夠耐受超過(guò)6.0g/L的TAN,但甲烷產(chǎn)量會(huì)降低[12].研究發(fā)現(xiàn),4.0g/L的TAN將導(dǎo)致雞糞高溫發(fā)酵產(chǎn)甲烷產(chǎn)量降低17%[11];TAN達(dá)到5.0g/L,雞糞高溫產(chǎn)甲烷產(chǎn)量降低80%[16].與高溫發(fā)酵相比,雞糞中溫發(fā)酵耐受TAN的能力較強(qiáng),4.8g/L的TAN才會(huì)抑制雞糞中溫發(fā)酵[17].在穩(wěn)定運(yùn)行的反應(yīng)器中,產(chǎn)酸率和嗜酸速率一致,VFAs保持在較低的水平[18].任南琪等[19]研究發(fā)現(xiàn),當(dāng)厭氧發(fā)酵體系中乙酸濃度高于2.3g/L,丙酸濃度高于0.3g/L或丁酸濃度高于2g/L時(shí),產(chǎn)甲烷菌的活性將受到抑制.如果產(chǎn)酸速率增高或有機(jī)酸降解被抑制,VFAs將大量積累,造成產(chǎn)甲烷菌的活性降低與VFAs累積的惡性循環(huán).

根據(jù)表1中的元素組成,雞糞可用化學(xué)式C7.9H12.2O4.8N表達(dá).通過(guò)Boswell發(fā)酵方程可建立雞糞甲烷發(fā)酵的化學(xué)計(jì)量[20]:

C7.9H12.2O4.8N+8.9H2O→4.65CH4+3CO2+NH4HCO3(2)

因此,理論上雞糞完全降解的甲烷產(chǎn)率是360mL/gTS,甲烷濃度為60%,同時(shí)產(chǎn)生53mgTAN/ gTS.進(jìn)料TS為5%、7.5%和10%時(shí),中溫反應(yīng)器的甲烷產(chǎn)率分別為理論值的70%、68%和56%,均高于高溫反應(yīng)器的50%、31%和5%.因此,中溫反應(yīng)器的產(chǎn)甲烷效果優(yōu)于高溫反應(yīng)器.

表2 雞糞中高溫甲烷發(fā)酵試驗(yàn)的運(yùn)行情況

2.2 固體去除率

有機(jī)污染物的去除效果通常用TS和VS去除率表示.圖3中,進(jìn)料濃度為5%,TAN為2.1~2.5g/L,中溫反應(yīng)器的TS和VS去除率分別為46%和70%,高于高溫反應(yīng)器的44%和59%,處于Nizami等[21]報(bào)道的CSTR處理能源作物40%~70%的VS去除率范圍內(nèi).隨著TS升至10%,TAN增至6.1~6.5g/L,中溫反應(yīng)器的TS和VS去除率(35%和61%)降低了22%和13%;高溫反應(yīng)器的TS和VS去除率(24%和30%)降低了45%和50%,進(jìn)料TS和TAN提高,高溫反應(yīng)器的固體去除率下降更明顯.TAN濃度隨進(jìn)料濃度的增加而提高,能夠抑制甲烷發(fā)酵的水解,酸化和甲烷化進(jìn)程,造成雞糞中有機(jī)質(zhì)的去除率降低[11].與中溫相比,高溫條件下TAN抑制甲烷發(fā)酵的效果更明顯,導(dǎo)致了高溫發(fā)酵的固體去除率更低.

圖3 反應(yīng)器在不同OLR下的TS和VS去除率

2.3 氨氮對(duì)VFAs累積和產(chǎn)氣率的影響

雞糞中高溫發(fā)酵過(guò)程中TAN與VFAs和甲烷產(chǎn)率間均具有較好的線性關(guān)系(圖4).圖4a中,中溫發(fā)酵的值為-12.16(甲烷產(chǎn)率/TAN),2為0.67;高溫發(fā)酵的值為-34.49(甲烷產(chǎn)率/TAN),2為0.88,TAN累積對(duì)雞糞高溫甲烷產(chǎn)率的降低效果更明顯.圖4b中,中溫發(fā)酵的值為1.48(VFAs/TAN),2為0.89;高溫發(fā)酵的值為5.55(VFAs/TAN),2為0.89,TAN累積對(duì)雞糞高溫VFAs的累積效果更明顯.

大量研究就TAN對(duì)甲烷發(fā)酵的影響進(jìn)行了論證,但是不同原料不同溫度下產(chǎn)生抑制的氨濃度不同.研究發(fā)現(xiàn)1.7g/L的TAN濃度就可能抑制甲烷發(fā)酵[22].Andrew等[23]發(fā)現(xiàn),TAN達(dá)到2.5g/L時(shí),牛糞中高溫甲烷發(fā)酵受到抑制.目前普遍認(rèn)為TAN抑制中高溫甲烷發(fā)酵的下限濃度均為3.0~4.0g/L[11].本研究發(fā)現(xiàn),進(jìn)料濃度為5%時(shí),中高溫反應(yīng)器的TAN濃度均低于3.0g/L,VFAs濃度在1.0g/L以內(nèi),甲烷產(chǎn)率波動(dòng)不明顯.隨著TAN濃度的升高,中高溫反應(yīng)器內(nèi)出現(xiàn)VFAs累積和產(chǎn)氣率降低.中溫反應(yīng)器的TAN濃度提升至5.5g/L左右,VFAs濃度在2.0~6.0g/L范圍波動(dòng),甲烷產(chǎn)率雖略有下降,但不顯著;TAN濃度進(jìn)一步提升至6.5g/L左右,VFAs濃度在6.7g/L波動(dòng),甲烷產(chǎn)率下降明顯,為203mL/gTS,比TAN濃度2.5g/L時(shí)下降20%,產(chǎn)氣效果受到明顯抑制.TAN對(duì)高溫反應(yīng)器的抑制更明顯,TAN濃度在5.5g/L左右,VFAs濃度約19.2g/L,產(chǎn)氣率出現(xiàn)明顯降低.TAN濃度提升至6.1g/L左右,VFAs累積至26.1g/L,甲烷產(chǎn)率比TAN濃度2.5g/L時(shí)下降約90%,僅為18mL/gTS.因此,本研究中雞糞中高溫甲烷發(fā)酵氨抑制的TAN閾值分別為~5.5g/L和~2.5g/L,中溫發(fā)酵耐受TAN的能力遠(yuǎn)高于高溫發(fā)酵.

圖4 氨氮累積對(duì)VFAs累積和TS甲烷產(chǎn)率的影響

2.4 污泥比產(chǎn)甲烷活性分析

圖5中,進(jìn)料TS為5%,TAN為2.5g/L時(shí),中高溫反應(yīng)器的SMA分別為204和151mLCH4/(gVSS·d).隨著TAN濃度升高至5.5和6.5g/L,中溫反應(yīng)器的SMA分別下降了27%和44%,分別為176和134mLCH4/(gVSS·d);高溫反應(yīng)器的SMA分別下降了67%和100%,分別為51和0mLCH4/(gVSS·d).TAN濃度的提高導(dǎo)致SMA的降低.在4.0g/L的TAN濃度下,雞糞和豬糞中溫混合發(fā)酵的SMA為0.08gCOD/(gVSS·d)(約28mLCH4/(gVSS·d))[24];在6.3g/L的TAN濃度下,雞糞中溫發(fā)酵的SMA僅為0.03gCOD/(gVSS·d)(約11mLCH4/(gVSS·d))[25].TAN累積明顯降低了中高溫甲烷發(fā)酵的SMA,是導(dǎo)致中高溫甲烷發(fā)酵產(chǎn)氣量下降的主要原因.同時(shí),TAN對(duì)高溫甲烷發(fā)酵的抑制效果更加明顯[26].本文各進(jìn)料濃度階段,中溫反應(yīng)器的比產(chǎn)甲烷活性(240,176, 134mLCH4/(gVSS·d))均高于高溫反應(yīng)器的比產(chǎn)甲烷活性(151,51,0mLCH4/(gVSS·d)).與Yin等[27]報(bào)道的結(jié)果一致,在2.1g/L的TAN條件下,中溫反應(yīng)器的比產(chǎn)甲烷活性(196mLCH4/(gVSS·d))明顯高于高溫反應(yīng)器(161mLCH4/(gVSS·d)).因此,隨著進(jìn)料TS的增加,在相同的TAN(2.5~6.5g/L)條件下,中溫甲烷發(fā)酵利用乙酸產(chǎn)甲烷的能力均高于高溫發(fā)酵,這也是雞糞中溫甲烷發(fā)酵產(chǎn)氣率高于高溫發(fā)酵的主要原因.

圖5 不同進(jìn)料濃度,OLR和TAN對(duì)SMA的影響

3 結(jié)論

3.1 高溫發(fā)酵對(duì)TAN的增加更為敏感,當(dāng)TAN達(dá)到5.5g/L時(shí),VFAs開(kāi)始出現(xiàn)明顯積累,到6.5g/L時(shí)產(chǎn)氣率僅為18mL/gTS.相比之下,中溫發(fā)酵的TAN耐受性較高,當(dāng)氨氮濃度達(dá)到6.1g/L時(shí),甲烷產(chǎn)率仍可以達(dá)到203mL/gTS.

3.2 TAN累積導(dǎo)致雞糞中高溫發(fā)酵體系利用乙酸產(chǎn)甲烷的能力降低.在4gCOD/L乙酸濃度下,TAN濃度達(dá)到5.5g/L,中高溫反應(yīng)器的SMA分別降低27%和67%;TAN濃度達(dá)到6.5g/L,中高溫反應(yīng)器的SMA分別降低44%和100%.因此,高溫反應(yīng)器受到TAN的抑制效果更明顯.

3.3 在各進(jìn)料TS和TAN階段,中溫反應(yīng)器的SMA均高于高溫反應(yīng)器,中溫甲烷發(fā)酵利用乙酸產(chǎn)甲烷的能力更強(qiáng),雞糞中溫甲烷發(fā)酵產(chǎn)氣率更高.

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Effects of ammonium on methane fermentation of chicken manure under mesophilic and thermophilic conditions.

QIAO Wei1,2, BI Shao-jie1, XIONG Lin-peng1, JU Xin-xin3, DONG Ren-jie1,2*

(1.R&D Center for Efficient Production and Comprehensive Utilization of Biobased Gaseous Fuels, Energy Authority, National Development and Reform Committee, College of Engineering, China Agricultural University, Beijing 100083, China；2.Institute of Yantai, China Agricultural University, Yantai 264670, China；3.Shandong Zhongnong Sanyue Environmental Protection Technology Corporation Limited, Yantai 264670, China)., 2019,39(7)：2921~2927

In order to explore the effect of total ammonium nitrogen (TAN) on mesophilic and thermophilic anaerobic digestion of chicken manure, the performances of anaerobic digestion of chicken manure under mesophilic and thermophilic conditions were compared through 265 days’ experiment with the increased TAN when the feed total solid (TS) increased from 5% to 7.5% and 10%. During the operation, the specific methanogenic activity (SMA) test was carried out with sodium acetate. The TAN in the mesophilic and thermophilic reactors increased from 2.1~2.5to 6.1~6.5g/L when TS increased from 5% to 10%. The SMAs of the mesophilic and thermophilic reactors reduced by 44% and 100%, respectively, resulting in a decrease in the ability of the fermentation system to produce methane by acetic acid. In the mesophilic reactor, the methane yield reduced from 253 to 203 mL/gTS, associated with the accumulation of volatile fatty acids (VFAs) from 0.4 to 7.6g/L. The methane yield decreased from 181to 18mL/gTS when the VFA increased from 0.4 to 26.1g/L in the thermophilic reactor. The effect of TAN on reducing methane yield was more obvious under thermophilic condition.

ammonium nitrogen；total solid of feed；chicken manure；methane fermentation

X705

A

1000-6923(2019)07-2921-07

喬 瑋(1979-),男,內(nèi)蒙古赤峰人,副教授,博士,主要從事廢棄物和廢水的厭氧生物處理方面研究.發(fā)表論文40余篇.

2018-12-18

北京市自然科學(xué)基金資助項(xiàng)目(6182017)

* 責(zé)任作者, 教授, rjdong@cau.edu.cn