電場(chǎng)對(duì)污泥堆肥富里酸結(jié)構(gòu)特征的影響

譚知涵,孫曉杰*,席北斗,盧雪霜,李秋虹,莫晶晶,張 軍

電場(chǎng)對(duì)污泥堆肥富里酸結(jié)構(gòu)特征的影響

譚知涵1,2,孫曉杰1,2*,席北斗1,3,盧雪霜1,2,李秋虹1,2,莫晶晶1,2,張 軍1,2

(1.桂林理工大學(xué),廣西環(huán)境污染控制理論與技術(shù)重點(diǎn)實(shí)驗(yàn)室,廣西 桂林 541004；2.桂林理工大學(xué),巖溶地區(qū)水污染控制與用水安全保障協(xié)同創(chuàng)新中心,廣西環(huán)境污染控制理論與技術(shù)重點(diǎn)實(shí)驗(yàn)室科教結(jié)合科技創(chuàng)新基地,廣西 桂林 541004；3.中國(guó)環(huán)境科學(xué)研究院,國(guó)家環(huán)境保護(hù)地下水污染模擬與控制重點(diǎn)實(shí)驗(yàn)室,北京 100012)

為了研究電場(chǎng)對(duì)污泥堆肥富里酸結(jié)構(gòu)特征及電子轉(zhuǎn)移能力(ETC)的影響,以市政污泥和米糠為原料,設(shè)置施加5V的直流電場(chǎng)(T1)和不施加電場(chǎng)(CK)的兩個(gè)堆肥進(jìn)行對(duì)比實(shí)驗(yàn).通過(guò)三維熒光-平行因子分析(EEM-PARAFAC)、紫外可見(jiàn)光譜(UV-vis)、傅里葉紅外光譜(FTIR)結(jié)合二維相關(guān)光譜(2DCOS)分析堆肥過(guò)程中富里酸結(jié)構(gòu)和組成變化.通過(guò)電化學(xué)方法測(cè)定了堆肥富里酸的電子接受能力(EAC)和電子供給能力(EDC).結(jié)果表明,類色氨酸、類富里酸和類胡敏酸是堆肥FA的主要組分,隨著堆肥的進(jìn)行,FA中類色氨酸物質(zhì)減少,類富里酸和類胡敏酸含量增加,電場(chǎng)能夠促進(jìn)FA在堆肥過(guò)程中類色氨酸的降解和類胡敏酸的形成,提高了FA的芳香性、分子量和腐殖化程度.電化學(xué)分析結(jié)果表明,FA的ETC呈現(xiàn)先增后減的趨勢(shì).相比CK處理,施加電場(chǎng)降低了FA的EDC,但同時(shí)增強(qiáng)了其EAC,使得堆肥后期的FA具有更強(qiáng)的ETC.結(jié)果將有助于進(jìn)一步了解污泥堆肥過(guò)程中FA的形成過(guò)程及其氧化還原特性.

污泥堆肥；電場(chǎng)；富里酸；二維相關(guān)光譜；平行因子分析；結(jié)構(gòu)組成；電子轉(zhuǎn)移

堆肥是一種處理市政污泥的有效手段,其本質(zhì)是在微生物作用下有機(jī)物的礦化和腐殖化過(guò)程.腐殖質(zhì)作為堆肥過(guò)程的主要產(chǎn)物,不僅可以改善土壤理化性質(zhì),而且具有吸附和絡(luò)合污染物的功能[1].此外,腐殖質(zhì)還具有氧化還原活性,能夠影響環(huán)境中重金屬和有機(jī)污染物的降解和轉(zhuǎn)化.因此,腐殖質(zhì)的形成過(guò)程和腐殖質(zhì)的質(zhì)量是決定堆肥產(chǎn)品土地應(yīng)用價(jià)值的關(guān)鍵因素[2].

不少研究表明,施加電場(chǎng)是一種促進(jìn)堆肥腐殖化的有效手段.已有研究提出在堆肥中施加一個(gè)低壓直流電場(chǎng),達(dá)到提高堆肥溫度,加速腐殖化過(guò)程的目的[3].還有研究表明,施加電場(chǎng)增強(qiáng)了堆肥過(guò)程微生物的代謝,堆肥產(chǎn)品中腐殖質(zhì)和胡敏酸含量相比對(duì)照分別提高了19%和69%[4].電場(chǎng)還能夠加速溶解性有機(jī)質(zhì)的芳香化和腐殖化,有效縮短堆肥腐熟時(shí)間[5].

富里酸(FA)是腐殖質(zhì)的重要組分之一[6],是一種分子量較小的芳香性有機(jī)酸,具有良好的溶解性和流動(dòng)性,已被廣泛用于改善土壤功能,如促進(jìn)養(yǎng)分吸收,改善土壤結(jié)構(gòu),增強(qiáng)微生物活性等[7-9].同時(shí),與腐殖質(zhì)其他組分相比,FA 中含有更多具有氧化還原活性的官能團(tuán)(如羧基、羥基等)[10-11],這使得FA具有良好的污染物親和力和電子轉(zhuǎn)移能力(electron transfer capacity,ETC),能夠影響土壤中的重金屬和有機(jī)污染物的氧化還原和轉(zhuǎn)化[12].而這些特性主要由FA的結(jié)構(gòu)和組成決定[13].在堆肥過(guò)程中,FA的結(jié)構(gòu)特征會(huì)發(fā)生轉(zhuǎn)化,導(dǎo)致其電子轉(zhuǎn)移能力的改變.有研究表明,腐殖質(zhì)的電子接受能力與芳香性呈正相關(guān)關(guān)系[14].也有研究表明,污泥堆肥中溶解性有機(jī)質(zhì)轉(zhuǎn)移電子的能力與有類腐殖質(zhì)物質(zhì)有關(guān)[15-16].因此,揭示 FA 在堆肥過(guò)程中結(jié)構(gòu)特征及其電子轉(zhuǎn)移能力的變化是有必要的.

已開展的研究主要從光譜學(xué)角度研究電場(chǎng)對(duì)堆肥有機(jī)物結(jié)構(gòu)特征的影響,而很少?gòu)碾娀瘜W(xué)角度研究堆肥有機(jī)物演化規(guī)律,施加電場(chǎng)是否會(huì)影響腐殖質(zhì)組分的氧化還原特性仍不清楚.基于此,本文假設(shè)在污泥堆肥中施加電場(chǎng)能夠促進(jìn)腐殖化進(jìn)而影響FA的結(jié)構(gòu)特征及氧化還原特性.本研究通過(guò)設(shè)置施加電場(chǎng)和不施加電場(chǎng)的污泥堆肥進(jìn)行對(duì)照實(shí)驗(yàn),并測(cè)定了堆肥各個(gè)階段FA的ETC,從電化學(xué)角度分析有機(jī)物演化規(guī)律.采用紫外-可見(jiàn)光譜,三維熒光光譜,紅外光譜等多種光譜學(xué)技術(shù)對(duì)堆肥FA結(jié)構(gòu)和組成進(jìn)行表征,結(jié)合二維相關(guān)光譜和平行因子分析等化學(xué)計(jì)量學(xué)方法,揭示了電場(chǎng)對(duì)堆肥FA結(jié)構(gòu)特征及電子轉(zhuǎn)移能力的影響,分析施加電場(chǎng)強(qiáng)化堆肥腐殖質(zhì)電子轉(zhuǎn)移能力的可行性.結(jié)果將有助于進(jìn)一步了解污泥堆肥過(guò)程中FA的形成過(guò)程及其氧化還原特性.

1 材料與方法

1.1 堆肥實(shí)驗(yàn)

堆肥反應(yīng)器如圖1所示.堆肥在2個(gè)體積為60L的反應(yīng)器(直徑 45cm,高度52cm)中進(jìn)行,反應(yīng)器的外壁包裹玻璃棉(厚度約5cm)用于保溫.反應(yīng)器底部連接轉(zhuǎn)子流量計(jì)和空氣泵,底部鋪有兩層直徑為5cm的塑料空心球,空心球上部覆蓋一層紗網(wǎng).參考付濤等[5]的方法,采用直流電源(UTP1306S,優(yōu)利德科技)進(jìn)行供電,在反應(yīng)器的內(nèi)壁周圍放置一塊圍繞成圓形的304不銹鋼板作為正極(長(zhǎng)120cm,寬40cm,厚0.3cm),將石墨棒(直徑5cm,高40cm)放入堆體的中心作為負(fù)極,兩個(gè)反應(yīng)器具有相同的構(gòu)造.設(shè)置2個(gè)堆肥施加的直流電壓分別為0V(CK)和5V(T1).

圖1 堆肥反應(yīng)器示意

1:直流穩(wěn)壓電源2:導(dǎo)線 3.石墨棒4:保溫材料(玻璃棉) 5:紗網(wǎng) 6:塑料空心球7:通氣導(dǎo)管 8:不銹鋼板9:轉(zhuǎn)子流量計(jì) 10:空氣泵

堆肥主料為桂林市雁山污水處理廠的板框壓濾脫水污泥,作為堆肥輔料的米糠購(gòu)自雁山鎮(zhèn)田園米廠.表1顯示了堆肥物料的初始理化性質(zhì).將市政污泥和米糠按質(zhì)量比為2:1的比例混合均勻,將含水率控制在60%左右后裝入兩個(gè)反應(yīng)器中.采用連續(xù)強(qiáng)制曝氣方式,曝氣流量均設(shè)置為0.2L/(kg干物質(zhì)×min).分別在第0,3,6,9,12,16,24,32,40d對(duì)堆體上、中、下3個(gè)不同的深度采集各100g樣品后混合均勻,每次取樣前進(jìn)行翻堆.

表1 初始物料理化性質(zhì)

1.2 堆肥富里酸的提取與純化

采用國(guó)際腐殖酸協(xié)會(huì)(IHSS)提供的方法提取FA:稱取20g冷凍干燥過(guò)100目篩后的堆肥樣品與浸提液(0.1mol/L Na4P2O7和0.1mol/L NaOH以1:1混合所得)以比例1:10 (g/mL)混合后200r/min振蕩提取24h,后將混合液10000r/min高速離心20min,將離心后的上清液通過(guò)0.45μm纖維濾膜,濾液用6mol/L的HCl將pH值調(diào)節(jié)至1,靜置12h出現(xiàn)沉淀,4000r/min離心20min,收集上清液.將上清液通過(guò)XAD-8型樹脂并棄去出水,用超純水淋洗樹脂柱至中性.后用 0.1mol/L NaOH沖洗樹脂柱并收集洗脫液,洗脫液隨后通過(guò)IR120氫型陽(yáng)離子交換樹脂,得到FA溶液.使用總有機(jī)碳分析儀(Multi N/C 3100,德國(guó)耶拿)測(cè)定FA溶液的DOC濃度,將一部分FA樣品DOC濃度調(diào)整到10mg/L用于紫外-可見(jiàn)光譜和熒光光譜分析,另一部分經(jīng)冷凍干燥后用于紅外光譜分析.

1.3 光譜分析

采用熒光光度儀(Aqualog,法國(guó)HORIBA JY)測(cè)定三維熒光光譜,激發(fā)波長(zhǎng)(x)掃描范圍為200～ 450nm,發(fā)射波長(zhǎng)(m)掃描范圍為280～550nm;狹縫寬帶均設(shè)置為5nm.所有樣品測(cè)定時(shí)均需要扣除超純水空白以減少瑞麗散射和拉曼散射的影響[17].

紫外可見(jiàn)光譜采用紫外-可見(jiàn)分光光度計(jì)(UV– 6100,上海元析)測(cè)定,掃描范圍為200～800nm,掃描間隔為1nm.

將1mg冷凍干燥后的FA樣品與300mg烘干后的KBr(光譜純)混合,然后將混合物在100MPa下壓片后使用傅里葉變換紅外光譜儀(Nicolet iS20,美國(guó)賽默)測(cè)定紅外光譜,掃描波長(zhǎng)范圍為4000～450cm-1,分辨率為4cm-1,掃描次數(shù)為32次.

1.4 電化學(xué)分析

使用電化學(xué)工作站(CHI660D,上海辰華)測(cè)定FA的電子轉(zhuǎn)移能力.采用的三電極電解池系統(tǒng)如唐朱睿等[16]所述,玻碳電極作為工作電極,鉑柱電極作為對(duì)電極,Ag/AgCl電極作為參比電極.采用Amperomentric i-t Curve法測(cè)定,整個(gè)測(cè)定過(guò)程中持續(xù)向電解池中通入氮?dú)庖员３秩毖鯛顟B(tài).首先將2.5mL磷酸鹽緩沖溶液(0.2mol/L)和2.5mL KCl (0.2mol/L)作為電解質(zhì)加入電解池中.將工作電極電位設(shè)置為+0.61V,ABTS(2,2-聯(lián)氨-雙(3-乙基-苯并噻唑-6-磺酸)二銨鹽)作為介導(dǎo)劑測(cè)定電子供給能力(EDC),工作電極電位設(shè)置為-0.49V,DQ (1,1￠-乙撐-2,2￠-聯(lián)吡啶二溴鹽)作為介導(dǎo)劑測(cè)定電子接受能力(EAC).加入1mL樣品(DOC濃度統(tǒng)一調(diào)整為50mg/L)穩(wěn)定運(yùn)行1000s后獲得電流—時(shí)間曲線,計(jì)算其積分面積,使用He等[18]描述的公式計(jì)算EDC和EAC值,ETC為EDC和EAC之和.

1.5 數(shù)據(jù)統(tǒng)計(jì)及處理

將CK和T1共18個(gè)FA樣品的三維熒光光譜數(shù)據(jù)轉(zhuǎn)化為三維矩陣,使用MATLAB 2018a (Mathworks,Natick,MA)中的DOMFluor工具箱進(jìn)行平行因子分析[19].腐殖化指數(shù)(HIX)為x在254nm下,m在435～480nm和300～345nm兩區(qū)間內(nèi)的熒光積分值之比.x在370nm 波長(zhǎng)下,Em在470和520nm波長(zhǎng)處熒光強(qiáng)度的比值為熒光指數(shù)(FI).x在310nm時(shí),m在380與430nm處的熒光強(qiáng)度比值記為生物源指數(shù)(BIX).

在紫外-可見(jiàn)光譜中波長(zhǎng)254和280nm處的吸光度與DOC比值的100倍分別記為SUVA254和SUVA280.253與203nm處吸光度的比值記為253/203,226～400nm區(qū)間內(nèi)的吸光度面積積分記為226～400;275～295nm及350～400nm波長(zhǎng)范圍內(nèi)曲線斜率的比值記為R[20].

以堆肥時(shí)間為外界擾動(dòng),波數(shù)為變量,對(duì)兩個(gè)處理750～1800cm-1波長(zhǎng)的紅外光譜進(jìn)行二維相關(guān)分析,使用Origin2018繪制紅外光譜圖和二維相關(guān)光譜圖.

2 結(jié)果與討論

2.1 結(jié)合三維熒光-平行因子分析FA組分變化

兩個(gè)處理共18個(gè)樣品的熒光光譜數(shù)據(jù)經(jīng)平行因子分析后分離出3個(gè)組分(圖2),組分C1(x=250/ 320nm,m=438nm)為類富里酸物質(zhì)組分[8].組分C2(x=275nm,m=334nm)位于傳統(tǒng)的T峰(x= 270～280nm,m=320～350nm)區(qū)域,為類色氨酸熒光峰,屬于類蛋白物質(zhì),該組分與微生物活動(dòng)密切相關(guān)[21].組分C3(x=260/400nm,m=500nm)表現(xiàn)為一強(qiáng)一弱的雙峰,均為類胡敏酸物質(zhì).C1和C3均為類腐殖質(zhì)物質(zhì).

圖2 通過(guò)平行因子分析得到FA的3個(gè)組分及其熒光載荷

通過(guò)最大熒光強(qiáng)度(Fmax)來(lái)反映堆肥不同階段各組分的相對(duì)含量.FA各組分的占比變化如圖3所示.C1和C3在堆肥前期均略有下降,這可能是由于一部分可溶性的腐殖質(zhì)類物質(zhì)被微生物分解導(dǎo)致,12d以后,C1和C3的相對(duì)含量穩(wěn)步增加,說(shuō)明逐步生成分子量大、結(jié)構(gòu)復(fù)雜、共軛程度高的類腐殖質(zhì)物質(zhì).FA中C2在堆肥前期有一定增長(zhǎng),這可能是由于堆肥高溫期微生物生長(zhǎng)代謝活動(dòng)劇烈,產(chǎn)生大量可溶性的代謝產(chǎn)物和殘?bào)w.隨著堆肥的進(jìn)行,微生物活性降低,微生物代謝產(chǎn)物減少,類色氨酸等結(jié)構(gòu)簡(jiǎn)單、易降解的蛋白類物質(zhì)被降解,逐漸生成結(jié)構(gòu)更為穩(wěn)定的類富里酸物質(zhì)和類胡敏酸物質(zhì),C2的相對(duì)含量下降,C1和C3的相對(duì)含量增加.堆肥結(jié)束時(shí),CK和T1處理的C1含量相差不大,C2含量分別為27%和24%,C3含量分別為19%和22%,這說(shuō)明T1促進(jìn)了堆肥過(guò)程中C2的降解和C3的形成.該結(jié)果與付濤等[5]的研究結(jié)果相似,該研究證明了電場(chǎng)能夠促進(jìn)雞糞堆肥有機(jī)物中類蛋白物質(zhì)的分解和類腐殖質(zhì)物質(zhì)的形成.

HIX可反映有機(jī)物中腐殖化程度[22].如圖4(a), T1處理的HIX在0～3d低于CK處理,而在6～40d高于CK處理,在堆肥結(jié)束時(shí),CK和T1處理的HIX分別達(dá)到1.180和1.513,這說(shuō)明施加電場(chǎng)顯著提高了FA的腐殖化程度,高腐殖化程度的FA相比低腐殖化程度的FA具有更強(qiáng)的污染物吸附和絡(luò)合能力,因此T1處理中的FA在土壤污染治理方面具有更高的應(yīng)用價(jià)值[23].FI反映了芳香物質(zhì)和非芳香物質(zhì)對(duì)腐殖質(zhì)熒光強(qiáng)度的相對(duì)貢獻(xiàn)率,可區(qū)分腐殖質(zhì)的來(lái)源.堆肥過(guò)程中兩個(gè)處理FA的FI 略有上升,這可能是由于FA作為一種不穩(wěn)定的小分子芳香物質(zhì),堆肥前期兩個(gè)處理FA的FI均小于1.4,說(shuō)明堆肥過(guò)程中的FA主要來(lái)源于市政污泥-米糠物料中自身的FA.隨著堆肥的進(jìn)行,兩個(gè)處理FA的FI逐漸增加到1.4左右,說(shuō)明堆肥后期的FA來(lái)源同樣受到自身源作用的影響,可能由微生物分解有機(jī)物料產(chǎn)生[24].BIX反映了新產(chǎn)生的腐殖質(zhì)組分在整體腐殖質(zhì)組分中所占的比例.BIX在0.6～0.7之間時(shí),FA具有較少的自生組分,在0.7～0.8之間時(shí)則具有中度新近自生源特征[25].堆肥過(guò)程中兩個(gè)處理的BIX在0.6～0.75之間波動(dòng),CK處理的BIX值有所下降,而T1處理的BIX呈緩慢增加的趨勢(shì),這說(shuō)明T1處理中新生成的FA含量更高.FI和BIX的結(jié)果表明,堆肥FA的來(lái)源以外源輸入為主,其次為堆肥過(guò)程中微生物活動(dòng)產(chǎn)生.

圖3 堆肥不同階段形成的FA組分占比變化

圖4 堆肥過(guò)程中FA的HIX, FI和BIX的變化

2.2 堆肥富里酸的紫外-可見(jiàn)光譜變化特征

SUVA254和226～400通常可反映有機(jī)物中芳香化合物及不飽和C=C雙鍵的變化,與FA的芳香性成正比[26].SUVA254在堆肥過(guò)程中呈現(xiàn)逐漸增加的趨勢(shì),且T1處理的SUVA254顯著高于CK處理,說(shuō)明施加電場(chǎng)可以提高堆肥過(guò)程中FA的芳香化程度.兩個(gè)處理的226～400隨著堆肥的進(jìn)行逐漸上升,但兩個(gè)處理間的差異不大.253/203可評(píng)價(jià)FA中苯環(huán)上取代基的種類,253/203的值增大說(shuō)明取代基上的羰基、羧基、羥基、酯類等含氧活躍基團(tuán)含量增加,該值減小說(shuō)明取代基上的脂肪鏈等穩(wěn)定基團(tuán)增多.兩個(gè)處理的253/203值變化相近,總體上呈增加的趨勢(shì),這說(shuō)明堆肥過(guò)程中FA芳香環(huán)上的脂肪鏈降解生成羥基、羰基等官能團(tuán),這些官能團(tuán)能夠配位絡(luò)合重金屬,因此堆肥過(guò)程能夠提高FA絡(luò)合重金屬的能力[27].兩個(gè)處理的253/203值在12～16d都出現(xiàn)了明顯的下降,這可能是由于部分含氧官能團(tuán)(如羧基)轉(zhuǎn)化為CO2釋放到環(huán)境中[28].R和SUVA280可表征FA的分子量大小[18].R與分子量具有負(fù)相關(guān)關(guān)系,而SUVA280與分子量具有正相關(guān)關(guān)系.兩個(gè)處理的R隨著堆肥的進(jìn)行逐漸下降,而SUVA280逐漸增加,說(shuō)明FA從結(jié)構(gòu)簡(jiǎn)單的小分子物質(zhì)逐步向結(jié)構(gòu)復(fù)雜的大分子物質(zhì)轉(zhuǎn)化子物質(zhì)轉(zhuǎn)化,堆肥結(jié)束時(shí),T1處理的R小于CK處理而SUVA280大于CK處理,這說(shuō)明T1處理中的FA具有更高的分子量.結(jié)果表明,相對(duì)CK處理,T1處理中FA的分子量較大,芳香性和腐殖化程度更高.

圖5 堆肥過(guò)程中紫外-可見(jiàn)光譜參數(shù)的變化

2.3 堆肥富里酸的紅外光譜變化特征

堆肥過(guò)程FA的紅外光譜變化如圖6所示.在3300, 2940, 1720, 1660, 1540, 1450, 1410, 1225和1070cm-1處都觀察到吸收峰.3300cm-1處的吸收峰來(lái)自于酚類化合物—OH官能團(tuán)的伸縮振動(dòng),通常在碳水化合物中出現(xiàn)[29];2940cm-1處的吸收峰來(lái)自于脂肪族化合物的C—H伸縮振動(dòng)[30];1720cm-1處的吸收峰由羧基C=O伸縮振動(dòng)引起[31];1660cm-1附近的強(qiáng)吸收峰由芳香環(huán)上的C = C伸縮振動(dòng)引起[32],在堆肥結(jié)束時(shí)(40d),T1處理的該峰明顯強(qiáng)于CK處理;1570cm-1處吸收峰由酰胺II型化合物的N—H變形和C=N伸縮引起;1225cm-1處的吸收峰代表酰胺 III 或芳香醚的C—O—C鍵[33].1070cm-1處的吸收峰代表多糖類物質(zhì),一般由植物中的淀粉纖維類物質(zhì)引起,可能來(lái)源于堆肥的原料米糠.

由于使用了完全相同的物料進(jìn)行堆肥,兩個(gè)處理FA的紅外吸收光譜都非常相似,因此可用主峰間強(qiáng)度的比值來(lái)評(píng)估不同官能團(tuán)的分解程度.兩個(gè)處理的1660/2940(芳香碳/脂肪碳)都呈現(xiàn)先下降后上升的趨勢(shì),這可能是由于堆肥初期脂肪族化合物和FA分解為小分子有機(jī)質(zhì),而堆肥中后期微生物利用難降解的木質(zhì)素產(chǎn)生芳香族結(jié)構(gòu)物質(zhì),使得芳香碳含量增加[34].CK組的1660/1540(芳香碳/酰胺II)在堆肥過(guò)程中略有增加,而T1處理出現(xiàn)下降,這說(shuō)明CK處理對(duì)酰胺化合物的降解效果更好.CK處理的1660/1070(芳香碳/多糖碳)在堆肥過(guò)程中略有下降,而在T1處理中從1.60增加到2.09,顯著高于CK處理,這說(shuō)明電場(chǎng)可能促進(jìn)FA中多糖類物質(zhì)的降解.

圖6 FA的紅外光譜圖

表2 堆肥過(guò)程中FA的紅外光譜主要吸收峰的強(qiáng)度比值

如圖7所示,在兩個(gè)處理的同步圖中共出現(xiàn)了6個(gè)正的自動(dòng)峰:1720/1720, 1660/1660, 1570/1570, 1450/1450, 1225/1225和1070/1070,其中1660/1660處的峰強(qiáng)度明顯高于其他峰,說(shuō)明FA的芳香族化合物在堆肥過(guò)程中的變化最為顯著.同步圖中出現(xiàn)的交叉峰均為正峰,這意味著各官能團(tuán)在堆肥過(guò)程中變化方向一致,可能均為降解過(guò)程.根據(jù)Noda規(guī)則[35],有機(jī)物轉(zhuǎn)化次序?yàn)?1570cm-1>1070cm-1> 1450cm-1>1225cm-1>1720cm-1,故CK處理堆肥過(guò)程中FA轉(zhuǎn)化順序?yàn)?酰胺II類>多糖>酚類>酰胺III類或芳香醚>羧基.對(duì)于T1處理,轉(zhuǎn)化次序?yàn)? 1720cm-1>1225cm-1>1570cm-1>1450cm-1>1070cm-1,即羧基>酰胺III類或芳香醚>酰胺II類>酚類>多糖.相比之下,T1處理中羧基結(jié)構(gòu)的降解先于蛋白質(zhì)和多糖,而羧基是形成腐殖質(zhì)的重要前體[36].以上結(jié)果表明,兩個(gè)處理堆肥過(guò)程中FA的轉(zhuǎn)化順序存在差異,施加電場(chǎng)可能會(huì)促進(jìn)羧基結(jié)構(gòu)的優(yōu)先降解,從而加速了FA的腐殖化進(jìn)程.

2.4 堆肥富里酸的電子轉(zhuǎn)移能力變化

FA具有電子穿梭體特性,可加快還原性電子供體和氧化性污染物之間反應(yīng)的速率,從而加速污染物的轉(zhuǎn)化和降解[37].堆肥過(guò)程FA的電子轉(zhuǎn)移能力的變化如圖8所示.兩個(gè)處理的EDC在堆肥過(guò)程中呈現(xiàn)先增加后減少的趨勢(shì),T1處理的EDC在171.85～294.49μmol/gC范圍內(nèi)變化,CK處理的EDC在173.43～382.2μmol/gC范圍內(nèi)變化,這說(shuō)明CK處理的FA可能含有更多的供電子基團(tuán).CK處理的EAC在158.06～242.971μmol/gC之間波動(dòng),T1處理的EAC在175.49～385.14μmol/gC之間波動(dòng),這說(shuō)明T1處理中的FA具有更強(qiáng)的得電子能力.電場(chǎng)可能促進(jìn)了堆肥過(guò)程中電子向O2的流動(dòng),加強(qiáng)了堆肥過(guò)程中的好氧反應(yīng),使得FA的官能團(tuán)被氧化,從而具有更強(qiáng)的得電子能力[3].如圖8所示,兩個(gè)處理FA的ETC呈現(xiàn)先增加后降低的趨勢(shì),在堆肥結(jié)束時(shí)兩個(gè)處理的ETC均高于堆肥初始值,這說(shuō)明堆肥過(guò)程提高了FA的氧化還原能力,CK處理在堆肥前期具有更強(qiáng)的ETC,而T1處理在堆肥后期ETC更強(qiáng).說(shuō)明FA在堆肥的中期階段具有更強(qiáng)的氧化還原活性,而在堆肥后期腐殖化過(guò)程中,FA逐漸降解或轉(zhuǎn)化為腐殖質(zhì)前體或更為穩(wěn)定的類胡敏酸物質(zhì),導(dǎo)致其氧化還原活性降低[38].

圖8 FA在堆肥過(guò)程中電子轉(zhuǎn)移能力的變化

(a)電子供給能力和電子接受能力;(b)電子轉(zhuǎn)移能力

2.5 堆肥富里酸各參數(shù)之間的相關(guān)性分析

相關(guān)性分析結(jié)果顯示(圖9),兩個(gè)處理的HIX與C1,C3呈現(xiàn)極顯著正相關(guān)(<0.01),說(shuō)明類富里酸,類胡敏酸的形成和芳香性的增強(qiáng)有利于FA腐殖化程度的提高.兩個(gè)處理的C1和C3與SUVA254呈極顯著正相關(guān),T1處理的253/203與C1和C3呈顯著正相關(guān),而在CK處理中相關(guān)性不明顯.說(shuō)明FA中腐殖質(zhì)組分的增加和芳環(huán)上極性含氧官能團(tuán)的增多有關(guān).SUVA254和A226～400與HIX,C1, C3呈現(xiàn)不同程度的正相關(guān)關(guān)系,這說(shuō)明FA的腐殖化程度,類富里酸和類胡敏酸的形成受到芳香族苯環(huán)結(jié)構(gòu)的影響.在T1處理中R與HIX,C1,C3呈負(fù)相關(guān)關(guān)系,這說(shuō)明小分子聚合成大分子能夠促進(jìn)腐殖化過(guò)程.兩個(gè)處理中各參數(shù)和電化學(xué)指標(biāo)之間沒(méi)有觀察到顯著的相關(guān)性,其原因還需要進(jìn)一步探討.

圖9 堆肥富里酸各參數(shù)之間的相關(guān)性分析

*表示顯著相關(guān),<0.05, **表示極顯著相關(guān),<0.01

3 結(jié)論

光譜學(xué)表征顯示,類色氨酸、類富里酸和類胡敏酸是堆肥FA的主要組分,隨著堆肥的進(jìn)行,FA中類色氨酸物質(zhì)減少,類富里酸和類胡敏酸含量增加,電場(chǎng)能夠促進(jìn)FA在堆肥過(guò)程中類色氨酸的降解和類胡敏酸的形成,提高了FA的芳香性、分子量和腐殖化程度,其中羧基結(jié)構(gòu)的快速降解可能是關(guān)鍵的結(jié)構(gòu)變化.電化學(xué)分析結(jié)果表明,堆肥過(guò)程能夠提高FA的ETC,堆肥過(guò)程中FA的ETC呈現(xiàn)先增后減的趨勢(shì).相比對(duì)照處理,施加電場(chǎng)降低了FA的EDC,但同時(shí)增強(qiáng)了其EAC,使得堆肥后期的FA具有更強(qiáng)的ETC.

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Effect of electric field on structure of fulvic acid during sludge composting.

TAN Zhi-han1,2, SUN Xiao-jie1,2*, Xi Bei-dou1,3, LU Xue-shuang1,2, LI Qiu-hong1,2, MO Jing-jing1,2, Zhang Jun1,2

(1.Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541004, China；2.Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology for Science and Education Combined with Science and Technology Innovation Base, Guilin 541004, China；3.State Environmental Protection Key Laboratory of Simulation and Control of Groundwater Pollution, Chinese Research Academy of Environmental Sciences, Beijing 100012, China)., 2023,43(1)：244～254

Fulvic acid (FA) is an important component of compost-derived humic substances, and its structure and electron transfer capacity are vital factors causing environmental effects. In order to investigate the influence of electric field on structural characteristics and electron transfer capacity (ETC) of FA that was formed during sludge composting, municipal sludge and rice bran were applied as raw materials, and two composting experiments, i.e.,one with a 5V direct current electric field and the other with no electric field (CK),were set up. Excitation-emission matrix florescence spectra coupled with parallel factor analysis, ultraviolet and visible spectroscopy, and Fourier transform infrared spectroscopy combined with two-dimensional correlation analysis were employed to investigate the evolution of the structure and composition of FA during composting. In addition, electrochemical methods was applied to determine the electron accepting capacities (EAC) and electron donating capacities (EDC) of the compost-derived FA. The results revealed that the major components of compost-derived FA were tryptophane-like, fulvic-like, and humic-like substances. Tryptophane-like substances in FA decreased during composting, whereas fulvic-like and humic-like substances increased during the process. Notably, the electric field promoted the degradation of tryptophan-like substances and the formation of humic-like substances, which inceased the aromatic degree, molecular weight, and humification of the FA. Results obtained from electrochemical analysis showed that the ETC of compost-derided FA increased initially and then decreased, Compared to the CK treatment, electric field application enhanced the EAC of compost-derided FA, though it reduced the EDC of FA, resulting the ETC of the FA from the later stage of composing was the highest. These results facilitated to elucidate the formation process of the FA and its redox properties during sludge composting.

sludge compost；electric field；fulvic acid；two-dimensional correlation spectrum; parallel factor analysis；structural composition；electron transfer

X705

A

1000-6923(2023)01-0244-11

譚知涵(1998-),男,貴州安順人,碩士研究生,主要從事固體廢物處理與資源化研究.發(fā)表論文2篇.

2022-05-31

廣西自然科學(xué)基金資助項(xiàng)目(2018GXNSFGA281001);國(guó)家自然科學(xué)基金資助項(xiàng)目(51868011);廣西科技計(jì)劃項(xiàng)目資助(AD18126018);廣西研究生教育創(chuàng)新計(jì)劃項(xiàng)目資助(YCSW2022310)

* 責(zé)任作者, 教授,sunxiaojie@glut.edu.cn