耿 濤,趙立欣,姚宗路,申瑞霞,于佳動(dòng),羅 娟

水熱炭特性及強(qiáng)化厭氧發(fā)酵潛力研究進(jìn)展

耿 濤,趙立欣*,姚宗路,申瑞霞,于佳動(dòng),羅 娟

(中國農(nóng)業(yè)科學(xué)院農(nóng)業(yè)環(huán)境與可持續(xù)發(fā)展研究所,農(nóng)業(yè)農(nóng)村部華北平原農(nóng)業(yè)綠色低碳重點(diǎn)實(shí)驗(yàn)室,北京 10081)

厭氧發(fā)酵技術(shù)可以對(duì)作物秸稈,畜禽糞便,市政污泥等有機(jī)廢棄物進(jìn)行清潔轉(zhuǎn)化,生成的沼氣,沼渣和沼液在能源,農(nóng)業(yè),環(huán)保等領(lǐng)域有極大的應(yīng)用價(jià)值.但目前厭氧發(fā)酵技術(shù)仍存在停滯期長,發(fā)酵過程中微生物活性受氨氮和有機(jī)酸等代謝產(chǎn)物與微塑料,酚類等有毒物質(zhì)抑制,沼氣中甲烷含量低等問題.水熱炭是生物質(zhì)在高溫密閉環(huán)境以及亞臨界水的作用下發(fā)生熱化學(xué)反應(yīng)得到的固體產(chǎn)物,具有豐富的孔隙結(jié)構(gòu)和表面含氧官能團(tuán),是一種新興的多功能炭基材料.厭氧發(fā)酵體系中適量添加水熱炭能夠有效縮短停滯期,緩解不同物質(zhì)的抑制效應(yīng),促進(jìn)微生物種間電子傳遞,增強(qiáng)甲烷生成等,具有綜合的強(qiáng)化效果.對(duì)水熱炭理化特性以及水熱炭強(qiáng)化厭氧發(fā)酵的作用方式與機(jī)理進(jìn)行了解,有利于進(jìn)一步開展水熱炭強(qiáng)化厭氧發(fā)酵以及農(nóng)業(yè)廢棄物高效厭氧處理的研究與應(yīng)用.

農(nóng)業(yè)廢棄物；生物質(zhì)；厭氧發(fā)酵；水熱炭；產(chǎn)甲烷

厭氧發(fā)酵是一種用于處理作物秸稈,畜禽糞便,廚余垃圾等有機(jī)廢棄物的成熟技術(shù),有機(jī)物在微生物的協(xié)同代謝作用下經(jīng)過水解、酸化、產(chǎn)乙酸、乙酸裂解和氫營養(yǎng)型產(chǎn)甲烷實(shí)現(xiàn)穩(wěn)定化[1].發(fā)酵產(chǎn)物沼渣,沼液,沼氣,可用于清潔能源生產(chǎn),土壤改良等[2-4].厭氧發(fā)酵技術(shù)為實(shí)現(xiàn)能源的可再生與廢棄物的有效管理提供技術(shù)支持.作為一種以微生物為驅(qū)動(dòng)主體的生化反應(yīng),微生物代謝活性決定了厭氧發(fā)酵體系的運(yùn)行效率,發(fā)酵過程中底物降解難易程度,發(fā)酵環(huán)境,功能菌豐度等因素都會(huì)對(duì)厭氧發(fā)酵產(chǎn)生重要影響[5].

水熱炭是一種以生物質(zhì)材料為原料,制備工藝溫和簡單的新興炭基材料,具有一定的酸堿性,表面孔隙結(jié)構(gòu)復(fù)雜多樣,官能團(tuán)種類及數(shù)量豐富,在土壤改良,污染物吸附,儲(chǔ)能材料制備等領(lǐng)域有較大應(yīng)用潛力.厭氧發(fā)酵體系中添加水熱炭可以縮短停滯期,構(gòu)建功能菌種間電子傳遞途徑,有效增強(qiáng)厭氧發(fā)酵過程中產(chǎn)酸細(xì)菌與產(chǎn)甲烷古菌間的互營代謝,提高功能菌豐度,增強(qiáng)體系穩(wěn)定性,強(qiáng)化甲烷生成,對(duì)厭氧發(fā)酵體系有綜合強(qiáng)化效果[6-8].

本文主要聚焦廢棄生物質(zhì)制備水熱炭,對(duì)水熱炭理化特性以及水熱炭強(qiáng)化厭氧發(fā)酵的研究現(xiàn)狀進(jìn)行總結(jié)和梳理,進(jìn)一步指出目前水熱炭在厭氧發(fā)酵強(qiáng)化的局限性并對(duì)未來發(fā)展方向進(jìn)行展望,旨在對(duì)有機(jī)廢棄物的高效處理以及水熱炭在厭氧發(fā)酵強(qiáng)化方向的研究與應(yīng)用提供一定助力.

1 廢棄生物質(zhì)水熱炭化

水熱炭化指將廢棄生物質(zhì)原料(作物秸稈,畜禽糞污,市政污泥等)投入到密封反應(yīng)裝置中,生物質(zhì)在相對(duì)溫和的溫度條件(180～250℃),自生壓力(2～8MPa)以及亞臨界水的作用下經(jīng)過水解,解聚,縮合等關(guān)鍵步驟實(shí)現(xiàn)炭化[9-10].亞臨界狀態(tài)下,水的極性,酸堿性,黏度發(fā)生明顯改變,傳熱與傳質(zhì)能力顯著增強(qiáng),可對(duì)生物質(zhì)原料進(jìn)行深度滲透與炭化[11].與傳統(tǒng)炭化技術(shù)比較,水熱炭化無需對(duì)反應(yīng)原料進(jìn)行提前干燥,反應(yīng)介質(zhì)簡單易得,溫和的反應(yīng)條件在降低能耗的同時(shí)使得材料表面官能團(tuán)種類與數(shù)量更為豐富,具有較大的應(yīng)用潛力.

反應(yīng)溫度,停留時(shí)間,升溫速率,反應(yīng)物濃度等因素都會(huì)對(duì)生物質(zhì)水熱炭化過程,最終產(chǎn)物分布與品質(zhì)產(chǎn)生影響[12].溫度是其中的關(guān)鍵參數(shù),對(duì)反應(yīng)過程與產(chǎn)物特性起著主導(dǎo)作用.隨反應(yīng)溫度升高,水熱炭產(chǎn)率顯著降低,材料炭化程度,熱穩(wěn)定性,芳香化程度與能量密度皆隨溫度升高逐漸提高[13-16].

種植廢棄物是水熱炭制備的重要原料來源,木質(zhì)纖維素是種植廢棄物的主要成分,由木質(zhì)素(9%～29%),纖維素(33%～47%)和半纖維素(21%～ 32%)組成[17].其中纖維素與半纖維素?zé)岱€(wěn)定性較差,在較低的溫度條件(200℃)即開始炭化.木質(zhì)素作為一種非晶態(tài)雜聚物,其復(fù)雜成分與結(jié)構(gòu)需要300℃以上的溫度才開始炭化,并且木質(zhì)素的組成分子廣泛交聯(lián)形成植物細(xì)胞壁的穩(wěn)定骨架,對(duì)纖維素和半纖維素形成包裹,阻礙木質(zhì)纖維素的炭化反應(yīng)[18].木質(zhì)纖維素材料這種組成與結(jié)構(gòu)特性使得水熱炭化過程中不同成分與反應(yīng)途徑相互作用,水熱炭的形成機(jī)理更加復(fù)雜(圖1).

圖1 木質(zhì)纖維素生物質(zhì)水熱炭化演化路徑[19]

2 水熱炭理化特性

2.1 表面形貌與孔隙結(jié)構(gòu)

木質(zhì)纖維素水熱炭比表面積一般在1.09～ 52.86m2/g,孔隙度0.02～0.97cm3/g,平均孔徑0.086～ 17.48nm,主要以介孔為主.木質(zhì)纖維素原料在水熱反應(yīng)過程中,表面成分不斷溶出并與其他物質(zhì)反應(yīng),對(duì)材料表面形成剝蝕,覆蓋并產(chǎn)生碳微球結(jié)構(gòu),塑造水熱炭表面形貌[20];水熱反應(yīng)過程中揮發(fā)性物質(zhì)產(chǎn)生氣體排放效應(yīng)使得水熱炭表面形成孔隙[21].水熱炭的表面形貌與孔隙特征主要受反應(yīng)溫度與停留時(shí)間的影響(圖2).隨著反應(yīng)溫度升高,炭材料表層會(huì)逐漸破碎,材料孔隙度增大并且表面碳微球增多[15,22];碳微球的直徑隨溫度升高先增大后減小,隨溫度升高或停留時(shí)間的延長水熱炭表面碳微球發(fā)生黏連,形態(tài)也逐漸不規(guī)則[23].高溫下生物油中組分沉積會(huì)掩埋材料表面孔隙,阻礙材料表面孔隙結(jié)構(gòu)發(fā)展,使水熱炭表面變得平滑致密[24].

圖2 松樹皮,秸稈,纖維素水熱炭SEM圖像[10,16,23]

2.2 表面含氧官能團(tuán)

生物質(zhì)原料不完全炭化以及水熱反應(yīng)過程中物質(zhì)的生成使得水熱炭表面官能團(tuán)的種類和數(shù)量得到豐富,底物類型與工藝條件都對(duì)水熱炭表面官能團(tuán)起影響作用(表1).水熱炭化后材料表面的含氧官能團(tuán)數(shù)目較原材料增加,并且含氧官能團(tuán)的數(shù)量隨溫度升高呈現(xiàn)先增加后減少的趨勢[25-28].Liu等[29]發(fā)現(xiàn)松木經(jīng)過300℃水熱炭化后,水熱炭表面含氧官能團(tuán)可增加95%.炭化過程中,纖維素,半纖維素?zé)岱€(wěn)定性較差,在較低溫度下即發(fā)生分解并進(jìn)行反應(yīng),隨著脫水脫羧發(fā)生,—OH,—CO等官能團(tuán)減少;木質(zhì)素?zé)岱€(wěn)定性較強(qiáng),在高溫下才開始反應(yīng),形成水熱炭骨架,隨著芳構(gòu)化反應(yīng)的進(jìn)行,材料表面C=C,C=O官能團(tuán)增多[24].水熱炭表面官能團(tuán)在污染物吸附,土壤結(jié)構(gòu)調(diào)控,材料儲(chǔ)能等方面都發(fā)揮重要作用,選取合適原料與炭化工藝實(shí)現(xiàn)功能性材料的獲取是水熱炭應(yīng)用的關(guān)鍵.

2.3 pH值

木質(zhì)纖維素材料在水熱炭化過程中生成有機(jī)酸與碳酸,同時(shí)高溫下木質(zhì)素分解產(chǎn)生酚類物質(zhì)也會(huì)影響水熱炭的pH值.水熱炭的pH值受原料特性及制備溫度影響[30].水熱炭化過程中,溫度升高導(dǎo)致材料脫水脫羧作用加劇,同時(shí)高溫下水相中OH-與水熱炭表面酸性官能團(tuán)加速結(jié)合,導(dǎo)致水熱炭表面酸性含氧官能團(tuán)脫落轉(zhuǎn)移,液相中的酸性物質(zhì)的合成增加,水熱炭pH值升高.李金銘等[31]發(fā)現(xiàn)水熱炭表面弱酸性與材料表面含氧官能團(tuán)含量有關(guān).Liu等[29]認(rèn)為水熱炭的pH值與材料表面豐富的羧基官能團(tuán)有關(guān).

2.4 水熱炭改性

為了更好地增強(qiáng)水熱炭的功能性,可采用不同改性手段對(duì)水熱炭理化特性進(jìn)行優(yōu)化.目前,水熱炭改性的方式按照改性步驟可分為一步法或兩步法,按照改性方法可分為物理改性,化學(xué)改性和生物改性.

2.4.1 物理法 物理改性具有廉價(jià),不使用化學(xué)藥劑,沒有二次污染的優(yōu)點(diǎn),改性方法包括高溫氣體活化,球磨,高溫或凍融循環(huán)等.謝偉玲,劉宇等[27]和謝偉玲[32]發(fā)現(xiàn)水熱炭經(jīng)高溫活化后比表面積與孔隙度顯著增大且孔徑縮小,含氧官能團(tuán)豐度一定程度上降低. Huang等[33]發(fā)現(xiàn)水熱炭經(jīng)過混合氣體高溫活化以后,水熱炭材料微孔與比表面積增加,比表面積從7.5m2/g增至618.02m2/g.球磨法則通過物料間研磨與碰撞使炭材料粒徑細(xì)化至納米級(jí)別,增大材料比表面積并且通過破壞或者拉伸大分子化學(xué)鍵,在材料表面增加新的官能團(tuán)[34].He等[35]使用球磨法對(duì)水熱炭改性后,發(fā)現(xiàn)表面O—H,C—O官能團(tuán)增加.關(guān)俊杰[36]將稻殼和玉米秸稈分別在70℃和-25℃進(jìn)行高溫和凍融循環(huán)老化,發(fā)現(xiàn)兩種老化方法都可以使水熱炭表面—OH,C=C,C—OH顯著增多,其中凍融老化效果更加顯著.

2.4.2 化學(xué)法 化學(xué)改性按照使用化學(xué)試劑類型可以分為酸堿改性,金屬鹽改性,有機(jī)物改性,過氧化物改性;按照處理方式則可以分為化學(xué)處理,水熱添加劑,水熱后處理等.化學(xué)改性可以有效改變水熱炭材料表面形貌,官能團(tuán)特性并且實(shí)現(xiàn)金屬,氮,磷等元素的摻雜,顯著增強(qiáng)水熱炭功能性.

(1)酸堿改性.酸改性可以溶解水熱炭表面金屬鹽等無機(jī)成分,降低水熱炭灰分,在材料表面引入酸性官能團(tuán)并對(duì)表面形貌有一定改良作用.謝偉玲[32]通過H3PO4對(duì)水熱炭進(jìn)行一步改性,使得水熱炭表面官能團(tuán)種類與數(shù)量得以豐富,水熱炭對(duì)水體中Pb(Ⅱ)的去除率提高28.3%;分別以鹽酸,硝酸,硫酸和毛竹共混合進(jìn)行水熱炭化,發(fā)現(xiàn)改性水熱炭孔隙結(jié)構(gòu)并未優(yōu)化但是在材料表面發(fā)現(xiàn)氯元素,氮元素和硫元素.Li等[41]用硝酸對(duì)楊木屑水熱炭進(jìn)行浸漬改性后,材料比表面積最高提高5.6倍且含氧官能團(tuán)豐度提高.堿改性通過強(qiáng)堿物質(zhì)腐蝕,清除水熱炭表面覆蓋物,促進(jìn)材料孔隙發(fā)育,增強(qiáng)材料芳構(gòu)化程度.劉宇等[27]使用KOH對(duì)水熱炭進(jìn)行改性,優(yōu)化材料表面形貌與孔隙結(jié)構(gòu),改性后水熱炭對(duì)水體中PFOS的吸附量最高提升近1000倍.

表1 水熱炭官能團(tuán)特性變化

(2)金屬鹽改性.使用金屬鹽對(duì)水熱炭進(jìn)行改性,不僅優(yōu)化材料表面形貌及官能團(tuán)特性,而且金屬元素的附著可以增強(qiáng)水熱炭的功能性,例如通過鐵鹽改性實(shí)現(xiàn)水熱炭表面鐵元素附著,增強(qiáng)材料在污水處理,厭氧強(qiáng)化等方面的效果,并且在外加磁場的作用下實(shí)現(xiàn)材料的分離與再利用[42].段佳男[43]以FeCl3溶液為反應(yīng)介質(zhì)制備稻殼改性水熱炭,鐵鹽改性水熱炭比表面積和孔容明顯增加.孫暢[44]以FeCl3溶液為反應(yīng)介質(zhì)進(jìn)行米糠改性水熱炭制備,材料比表面積和孔隙度分別提高60.49%與32.71%,表面碳微球更加豐富同時(shí)實(shí)現(xiàn)了鐵元素附著.嚴(yán)偉等[45]發(fā)現(xiàn)KMNO4改性后水熱炭表面存在錳氧化物.王曦等[46]使用NH4H2PO4溶液作水熱反應(yīng)介質(zhì)進(jìn)行改性水熱炭制備,改性后,水熱炭表面N元素增加,總孔體積顯著縮小;—COOH,C=O, C=C,C—O官能團(tuán)增多,—CH減少,在水熱炭表面摻入含氮官能團(tuán).

(3)有機(jī)物改性.有機(jī)物改性使水熱炭表面形貌發(fā)生改變,增強(qiáng)水熱炭芳香性,增加材料碳元素含量并且可以通過交聯(lián)等方式引入新的元素.孫迎超[47]以檸檬酸溶液為反應(yīng)介質(zhì)對(duì)玉米芯與松子殼進(jìn)行水熱炭化,改性后材料含氧官能團(tuán)與表面碳微球都有所增加.楊正武[48]以丙酮溶液為反應(yīng)介質(zhì)對(duì)小麥秸稈進(jìn)行水熱改性,改性后水熱炭孔隙結(jié)構(gòu)得到優(yōu)化.段佳男等[43]以葡萄糖對(duì)水熱炭進(jìn)行改性,改性后水熱炭表面C=C,C—O官能團(tuán)增加.謝偉玲[32]以聚乙烯亞胺對(duì)水熱炭進(jìn)行改性,兩者發(fā)生交聯(lián)作用,在水熱炭表面引入含氮官能團(tuán).

(4)過氧化物改性.過氧化物具有強(qiáng)氧化性,對(duì)水熱炭材料改性可以增加水熱炭的氧含量并且豐富水熱炭表面含氧官能團(tuán).Xue等[49]使用H2O2對(duì)花生殼水熱炭進(jìn)行浸漬改性,發(fā)現(xiàn)水熱炭表面羧基,羰基等含氧官能團(tuán)增多且氧元素含量增加.關(guān)俊杰等[50]發(fā)現(xiàn)H2O2改性可以在水熱炭表面引入大量含氧官能團(tuán),增加材料中氧含量并降低碳含量.王曦[46]對(duì)木屑水熱炭進(jìn)行H2O2浸漬改性,改性水熱炭C元素含量顯著減少,O元素含量明顯增加,材料親水性與極性增強(qiáng)且內(nèi)部形成大量不規(guī)則孔道,表面積與平均孔徑增加,表面官能團(tuán)種類與數(shù)量得到豐富.

2.4.3 生物法 生物改性也稱“微生物陳化”,指微生物吸收利用水熱炭表面有機(jī)物質(zhì)并產(chǎn)生分泌物,使水熱炭表面官能團(tuán)與表面形貌發(fā)生變化.花昀[51]將麥稈和楊樹鋸末水熱炭加入到厭氧發(fā)酵體系中陳化后發(fā)現(xiàn)水熱炭孔隙結(jié)構(gòu),pH值,表面含氧官能團(tuán)增加,水熱炭的碳元素減少,而氧元素所占比例增加.胡子瑛[52]使用好氧細(xì)菌枯草芽孢桿菌對(duì)水熱炭進(jìn)行生物改性,發(fā)現(xiàn)改性后水熱炭表面形成了許多非均質(zhì)的裂縫和空洞,同時(shí)碳微球萎縮甚至消失,C—O—C,C=O等含氧官能團(tuán)減弱.

2.4.4 混合法 除以上3種單一的改性手段,諸多研究將不同改性手段結(jié)合使得水熱炭材料理化特性明顯改善.混合改性可以在增強(qiáng)材料特性的同時(shí)減小改性造成的環(huán)境或成本負(fù)擔(dān).嚴(yán)偉[45]將毛竹與不同堿性試劑共水熱后進(jìn)行高溫?zé)峤?材料比表面積與孔徑顯著增大,混合改性在減少強(qiáng)堿使用量的同時(shí)得到優(yōu)質(zhì)吸附性炭材料.郭大川[10]使用尿素溶液為反應(yīng)介質(zhì)進(jìn)行松樹皮水熱炭制備后進(jìn)行高溫?zé)峤饣罨?所得改性材料平均孔徑與氮元素明顯增加,在表面氧元素含量基本不變的情況下將部分C—O鍵轉(zhuǎn)化為C—N鍵,進(jìn)一步增強(qiáng)了材料對(duì)溶液中鎘的吸附性能.

3 水熱炭強(qiáng)化厭氧發(fā)酵

近年來,水熱炭在生物領(lǐng)域的應(yīng)用成為新興熱點(diǎn).厭氧發(fā)酵體系中添加適量水熱炭可以縮短厭氧發(fā)酵的啟動(dòng)期,提高厭氧發(fā)酵沼氣中甲烷的含量,增強(qiáng)發(fā)酵系統(tǒng)對(duì)酸,氨抑制等不良發(fā)酵環(huán)境的耐受性,有明顯的強(qiáng)化厭氧發(fā)酵產(chǎn)甲烷效果(表2).Shi等[7], Leithaeuser等[53],Xu等[54]分別以玉米秸稈,污泥等為原料制備熱解炭與水熱炭進(jìn)行比較研究,發(fā)現(xiàn)水熱炭強(qiáng)化厭氧發(fā)酵產(chǎn)甲烷效果優(yōu)于熱解炭.為了探明水熱炭強(qiáng)化厭氧發(fā)酵的作用機(jī)理,有研究借助代謝組學(xué)分析,宏基因組分析,元基因組分箱算法,同位素標(biāo)記,熒光標(biāo)記等技術(shù)和手段進(jìn)行探索,提出水熱炭對(duì)厭氧發(fā)酵的強(qiáng)化作用與炭材料表面含氧官能團(tuán)豐度,供受電子能力等特性相關(guān).目前水熱炭強(qiáng)化厭氧發(fā)酵的主要作用途徑包括:(1)緩解厭氧發(fā)酵體系物質(zhì)抑制;(2)促進(jìn)互營微生物間電子傳遞;(3)富集厭氧發(fā)酵功能微生物.

表2 水熱炭對(duì)厭氧消化性能的影響

3.1 緩解厭氧發(fā)酵體系物質(zhì)抑制

水熱炭孔隙結(jié)構(gòu)復(fù)雜且表面官能團(tuán)豐富,可以和發(fā)酵體系中氨氮,酚類,微塑料等物質(zhì)發(fā)生作用并進(jìn)行吸附,同時(shí)刺激微生物產(chǎn)生胞外聚合物,減少微生物與抑制因子的接觸,削弱其對(duì)發(fā)酵微生物的毒害,緩解發(fā)酵體系中的抑制效應(yīng)(圖3).

圖3 水熱炭對(duì)厭氧體系抑制效應(yīng)的緩解作用

3.1.1 緩解氨抑制 畜禽胴體以及糞便等高含氮量底物在發(fā)酵過程中產(chǎn)生大量銨離子(NH4+)和游離氨(NH3),不僅對(duì)發(fā)酵過程中酶活性有抑制作用,而且游離氨進(jìn)入細(xì)胞內(nèi)改變細(xì)胞代謝過程,會(huì)對(duì)微生物產(chǎn)生毒害[55].水熱炭可以對(duì)發(fā)酵體系中氨氮進(jìn)行吸附并促進(jìn)微生物對(duì)氨氮的轉(zhuǎn)化.徐杰等[6]發(fā)現(xiàn)發(fā)酵體系中每克水熱炭吸附約1.0～17.5mg氨氮,促進(jìn)微生物對(duì)氨氮的轉(zhuǎn)化同時(shí)提高發(fā)酵體系氨氮抑制的閾值. Usman等[5]發(fā)現(xiàn)厭氧發(fā)酵體系中水熱炭對(duì)氨氮的吸附量達(dá)到40.98mg/g. Leithaeuser等[53]發(fā)現(xiàn)氨抑制體系中水熱炭添加組甲烷產(chǎn)量較對(duì)照組增加17.30%.Wang等[56]發(fā)現(xiàn)KOH改性水熱炭降低發(fā)酵體系中NH4+-N的含量并促進(jìn)含氮化合物的降解.

3.1.2 緩解微塑料毒害作用 污水中微塑料(粒徑<5mm)會(huì)抑制顆粒污泥胞外聚合物的分泌并導(dǎo)致污泥顆粒破碎,使發(fā)酵體系中微生物總量與功能菌相對(duì)豐度降低,發(fā)酵系統(tǒng)COD去除率與甲烷產(chǎn)率下降[63-64].水熱炭可以吸附并累積發(fā)酵體系中的微塑料,促進(jìn)厭氧微生物分泌腐殖酸,增加胞外聚合物生成,減少微塑料與微生物的直接接觸,阻止微塑料對(duì)厭氧顆粒污泥中微生物產(chǎn)生毒性,增強(qiáng)發(fā)酵體系穩(wěn)定性[65].

3.1.3 緩解酚類物質(zhì)毒害作用 酚類有機(jī)物對(duì)厭氧發(fā)酵體系的抑制作用是當(dāng)今有機(jī)廢棄物高效處理的一個(gè)發(fā)展障礙.酚類物質(zhì)具有毒性和腐蝕性,可以破壞微生物細(xì)胞膜并改變細(xì)胞通透性,抑制微生物活性.酚濃度過高甚至?xí)?dǎo)致體系中甲烷生成完全停止.He等[60]發(fā)現(xiàn)水熱炭對(duì)厭氧發(fā)酵體系中酚類物質(zhì)有明顯的吸附作用并且可以刺激微生物分泌胞外聚合物,增強(qiáng)細(xì)胞結(jié)構(gòu),減少酚類物質(zhì)對(duì)微生物的毒害,同時(shí)水熱炭加強(qiáng)微生物對(duì)苯酚的降解作用,緩解發(fā)酵體系中酚抑制.

3.2 促進(jìn)互營微生物間電子傳遞

厭氧發(fā)酵產(chǎn)甲烷是多種微生物協(xié)同實(shí)現(xiàn)的復(fù)雜生化反應(yīng),提高微生物胞外電子傳遞效率可以有效增強(qiáng)厭氧發(fā)酵過程中底物轉(zhuǎn)化與甲烷生成(圖4).發(fā)酵初期或體系有機(jī)負(fù)荷過高時(shí),丙酸,丁酸等揮發(fā)酸無法及時(shí)消耗形成累積,發(fā)酵體系pH值下降,產(chǎn)甲烷功能菌活性受到抑制進(jìn)而破壞整個(gè)體系的穩(wěn)定性.水熱炭可以增強(qiáng)發(fā)酵體系中微生物種間電子傳遞,促進(jìn)微生物對(duì)底物的降解與轉(zhuǎn)化,緩解厭氧發(fā)酵酸抑制.Shi等[7-8]發(fā)現(xiàn)水熱炭對(duì)發(fā)酵體系中揮發(fā)酸降解與轉(zhuǎn)化有促進(jìn)效果并增強(qiáng)甲烷的生成,對(duì)發(fā)酵體系酸抑制有一定的緩解作用.Xu等[54]發(fā)現(xiàn)水熱炭有效促進(jìn)發(fā)酵體系中丙酸鹽降解.

圖4 厭氧發(fā)酵體系中微生物種間電子轉(zhuǎn)移方式[72]

種間氫轉(zhuǎn)移(IHT)曾被認(rèn)為是厭氧發(fā)酵種間電子傳遞的主要方式,即在極低的氫分壓(約0.1～10Pa)環(huán)境中,產(chǎn)甲烷菌消耗發(fā)酵性細(xì)菌產(chǎn)生的H2,還原環(huán)境中CO2并生成甲烷[66-67].但發(fā)酵體系中H2分子主要依靠擴(kuò)散進(jìn)行轉(zhuǎn)移,電子轉(zhuǎn)移效率極低,通過IHT產(chǎn)生的甲烷僅占總甲烷的4.7%[68].

厭氧發(fā)酵體系中微生物種間電子直接轉(zhuǎn)移(DIET)是目前厭氧發(fā)酵領(lǐng)域的研究熱點(diǎn).已知的DIET構(gòu)建途徑主要有3種:①借助微生物導(dǎo)電鞭毛(Electrically conductive pili)與細(xì)胞外表面相關(guān)色素(Cytochrome C);②向體系中添加導(dǎo)電物質(zhì);③構(gòu)建微生物電解池[1,5,69-70].DIET的電子傳遞效率可達(dá)IHT的106～108倍,是更加高效且穩(wěn)定的電子傳遞方式,可以有效增強(qiáng)厭氧發(fā)酵體系性能[71].

水熱炭借助表面含氧官能團(tuán)發(fā)揮電子介體作用,構(gòu)建厭氧發(fā)酵體系中功能菌DIET途徑,加速產(chǎn)酸細(xì)菌與厭氧發(fā)酵功能菌對(duì)揮發(fā)酸的互營代謝,促進(jìn)厭氧發(fā)酵體系甲烷生成[68].Shi等[7-8]結(jié)合微生物轉(zhuǎn)錄與代謝組學(xué)分析發(fā)現(xiàn)水熱炭添加促進(jìn)了厭氧發(fā)酵體系內(nèi)維生素B6的代謝,使得微生物群落的代謝過程和活動(dòng)受到了干擾;通過以基因組為中心的元轉(zhuǎn)移組學(xué)對(duì)水熱炭構(gòu)建厭氧發(fā)酵DIET途徑的方式進(jìn)行探索,發(fā)現(xiàn)水熱炭添加體系中和富集程度最高,這兩種微生物可以參與DIET強(qiáng)化甲烷生成.Ren等[57]通過無標(biāo)記蛋白質(zhì)組學(xué)分析揭示參與DIET,進(jìn)一步提出水熱炭強(qiáng)化效果厭氧發(fā)酵與炭材料表面含氧官能團(tuán)豐度呈正相關(guān).He等[35]發(fā)現(xiàn)水熱炭進(jìn)行球磨改性后,材料表面C—O,O—H含氧官能團(tuán)進(jìn)一步豐富,對(duì)甲烷生成的促進(jìn)效果優(yōu)于未改性水熱炭并且對(duì)和等可能參與DIET的微生物有富集作用.

3.3 富集厭氧發(fā)酵功能微生物

表3 水熱炭對(duì)厭氧發(fā)酵體系中功能微生物群落演替作用

水熱炭表面孔隙與官能團(tuán)可以作為結(jié)合位點(diǎn)對(duì)厭氧發(fā)酵體系中功能微生物進(jìn)行固定與富集,增強(qiáng)體系穩(wěn)定性(表3).Xu等[62]探究水熱炭對(duì)厭氧發(fā)酵不同環(huán)節(jié)的強(qiáng)化效果,發(fā)現(xiàn)水熱炭顯著富集與有機(jī)物水解,產(chǎn)酸,產(chǎn)甲烷相關(guān)的功能菌,明顯改變發(fā)酵體系中微生物群落結(jié)構(gòu).He等[60]通過宏基因組分箱手段發(fā)現(xiàn)水熱炭富集厭氧發(fā)酵功能微生物并促進(jìn)甲烷生成的相關(guān)基因表達(dá).Hurst等[61]發(fā)現(xiàn)水熱炭使得厭氧發(fā)酵體系中Bacteroidetes與Firmicutes顯著富集.Usman等[58]通過三維熒光激發(fā)發(fā)射矩陣分析發(fā)現(xiàn)水熱炭添加后厭氧發(fā)酵體系中腐殖質(zhì),黃腐酸等難降解物質(zhì)的濃度降低并推測水熱炭改變厭氧發(fā)酵體系中微生物群落結(jié)構(gòu),實(shí)現(xiàn)對(duì)這些物質(zhì)的降解.Yang等[72]發(fā)現(xiàn)水熱炭使得發(fā)酵體系中Bacteroidetes,Firmicutes和Proteobacteria等微生物一定程度富集.

4 結(jié)語與展望

作為一種厭氧發(fā)酵外源添加劑,水熱炭原料來源豐富,較納米零價(jià)鐵,活性炭等材料,制備成本低,工藝對(duì)環(huán)境影響小;而且水熱炭表面孔隙,含氧官能團(tuán),電導(dǎo)率等特性在厭氧發(fā)酵體系中能夠吸附毒害物質(zhì),富集功能微生物,提高微生物種間電子傳遞效率,有效縮短發(fā)酵停滯期,增強(qiáng)體系穩(wěn)態(tài)并改善厭氧發(fā)酵產(chǎn)甲烷效果,有利于實(shí)現(xiàn)有機(jī)廢棄物的高效消納與利用.深入了解水熱炭強(qiáng)化厭氧發(fā)酵的作用方式,探索厭氧發(fā)酵過程中物質(zhì)轉(zhuǎn)化路徑與關(guān)鍵微生物間互營機(jī)制,實(shí)現(xiàn)對(duì)發(fā)酵過程及產(chǎn)物分布的有效調(diào)控,促進(jìn)有機(jī)廢棄物向甲烷等產(chǎn)物轉(zhuǎn)化,提高厭氧發(fā)酵技術(shù)對(duì)自然環(huán)境中復(fù)雜混合物和毒害廢棄物的處理水平,擴(kuò)大厭氧發(fā)酵技術(shù)的應(yīng)用規(guī)模,增強(qiáng)有機(jī)廢棄物消納與利用能力.生物質(zhì)厭氧發(fā)酵與水熱炭化一體化工藝具有良好的經(jīng)濟(jì)可行性和環(huán)境效益,有助于構(gòu)建有機(jī)廢棄物清潔處理的循環(huán)經(jīng)濟(jì)模式,沼氣,水熱炭等產(chǎn)物在環(huán)境,能源等發(fā)面都有良好的應(yīng)用價(jià)值,能夠?qū)崿F(xiàn)有機(jī)廢棄物的高效清潔處理與多元高值利用.

目前,水熱炭強(qiáng)化厭氧發(fā)酵的作用機(jī)制與相關(guān)應(yīng)用工藝仍無明確定論,建議在以下幾點(diǎn)對(duì)水熱炭強(qiáng)化厭氧發(fā)酵進(jìn)行研究:

(1)從水熱炭表面形貌,孔隙結(jié)構(gòu),表面官能團(tuán),供受電子能力,可降解性等角度對(duì)水熱炭強(qiáng)化厭氧發(fā)酵的關(guān)鍵理化特性進(jìn)行探索,進(jìn)一步明確實(shí)現(xiàn)并強(qiáng)化該類特性的相關(guān)工藝與手段,為開發(fā)低成本高效率的厭氧發(fā)酵外源添加劑提供支持.

(2)結(jié)合水熱炭添加體系中功能微生物群落演變情況與微生物種間電子傳遞不同途徑對(duì)甲烷生成的貢獻(xiàn)程度深入探索水熱炭強(qiáng)化厭氧發(fā)酵產(chǎn)甲烷機(jī)制.

(3)結(jié)合厭氧發(fā)酵體系中水熱炭添加量,材料老化與可重復(fù)利用性,沼液沼渣營養(yǎng)成分變化等方面對(duì)水熱炭強(qiáng)化厭氧發(fā)酵的可持續(xù)利用性與后端產(chǎn)物應(yīng)用途徑進(jìn)行探索.

(4)現(xiàn)有研究主要針對(duì)實(shí)驗(yàn)室規(guī)模水熱炭強(qiáng)化批式厭氧發(fā)酵效果進(jìn)行探究,需加強(qiáng)對(duì)水熱炭強(qiáng)化中試規(guī)模以及實(shí)際沼氣工程中連續(xù)厭氧發(fā)酵的效果研究以及水熱炭結(jié)合其他厭氧發(fā)酵調(diào)控策略的耦合效果及作用方式研究.

(5)綜合經(jīng)濟(jì)利益與環(huán)境效益等角度借助生命周期,技術(shù)經(jīng)濟(jì)評(píng)估等手段對(duì)生物質(zhì)水熱炭化與厭氧發(fā)酵技術(shù)結(jié)合處理有機(jī)廢棄物的效果進(jìn)行充分的評(píng)估,為水熱炭強(qiáng)化厭氧發(fā)酵技術(shù)的推廣應(yīng)用提供充足理論支撐.

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Characteristics and potential to strengthen anaerobic digestion of hydrochar.

GENG Tao, ZHAO Li-xin*, YAO Zong-lu, SHEN Rui-xia, YU Jia-dong, LUO Juan

(Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Key Laboratory of Low-carbon Green Agriculture in North China, Ministry of Agriculture and Rural Affairs P. R. China. Beijing 100081, China)., 2023,43(10)：5170～5180

Anaerobic digestion (AD) offers an attractive method to reduce the negative impact of organic waste existing in the environment such as stalk, animal manure and municipal sludge. The biogas, digestate and slurry generated have great application value in energy, agriculture, environmental protection and other fields. However, AD has some problems such as the lag phase, the inhibition effect caused by ammonia nitrogen, VFA and so on, and low methane content in biogas. Hydrochar is the solid product of hydrothermal carbonization of biomass which is converted to porous multi-functional carbon-based material with the presence of subcritical water in the sealed equipment under the effect of heat and pressure. The potential benefits and applications of hydrochar have received significant attention with complex pore structures and high density of oxygen-containing functional groups. There is a comprehensive strengthening effect on AD including shortening the lag phase, alleviating the inhibition, favoring electron transfer between microbes, and enhancing the CH4production with the addition of hydrochar. Specifically, it was brought together recent advances made in the area through a systematic and critical review of the characteristics, modification, and strengthening effect on AD of hydrochar. The potential and limitations involved in the hydrochar application on AD were pointed out with suggestions for further research to exploit the great potential of AD on treating agricultural wastes.

agricultural wastes；biomass；anaerobic digestion；hydrochar；methanogenesis

X705;S216.4;TK6

A

1000-6923(2023)10-5170-11

2023-02-23

國家重點(diǎn)研發(fā)計(jì)劃(2022YFD2002100);中央級(jí)公益性科研院所基本科研業(yè)務(wù)費(fèi)專項(xiàng)(BSRF202221);中國農(nóng)業(yè)科學(xué)院科技創(chuàng)新工程

* 責(zé)任作者, 研究員, zhaolixincaae@163.com

耿 濤(1995-),男,山西太原人,中國農(nóng)業(yè)科學(xué)院研究生院碩士研究生,主要從事農(nóng)業(yè)廢棄物厭氧生物處理技術(shù)研究.發(fā)表論文1篇.gt0502@126.com.

耿 濤,趙立欣,姚宗路,等.水熱炭特性及強(qiáng)化厭氧發(fā)酵潛力研究進(jìn)展 [J]. 中國環(huán)境科學(xué), 2023,43(10):5170-5180.

Geng T, Zhao L X, Yao Z L, et al. Characteristics and potential to strengthen anaerobic digestion of hydrochar [J]. China Environment Science, 2023,43(10):5170-5180.