巢湖沉積物磷鐵硫形態(tài)記錄及其環(huán)境變化指示

陳 茜,寧成武,汪 杰,黃 濤,孫慶業(yè)

巢湖沉積物磷鐵硫形態(tài)記錄及其環(huán)境變化指示

陳 茜,寧成武,汪 杰,黃 濤*,孫慶業(yè)

(安徽大學(xué)資源與環(huán)境工程學(xué)院,濕地生態(tài)保護(hù)與修復(fù)安徽省重點(diǎn)實(shí)驗(yàn)室,安徽 合肥 230601)

分析了巢湖表層和柱狀沉積物中磷(P)、鐵(Fe)和硫(S)元素的形態(tài)組成、分布、相互關(guān)系及其指示的湖泊環(huán)境變化.西半湖S3采樣點(diǎn)位柱狀沉積物總磷(TP)記錄表明,巢湖西半湖區(qū)自20世紀(jì)60年代開(kāi)始受人類(lèi)活動(dòng)影響逐步明顯,其中鈣磷(Ca-P)指示的流域徑流輸入增加早于鐵鋁磷(Fe/Al-P)指示的居民生活污水輸入;西半湖區(qū)沉積物15～0cm有機(jī)質(zhì)埋藏持續(xù)增加伴隨著pH值的逐步升高,指示了水體富營(yíng)養(yǎng)化導(dǎo)致藻類(lèi)生產(chǎn)力(光合作用)提高并顯著影響pH值;而東半湖S7采樣點(diǎn)位柱狀沉積物磷形態(tài)則記錄了東半湖區(qū)不同的環(huán)境變化特征.巢湖沉積物活性鐵組分以Fe(Ⅱ)為主,S3和S7沉積剖面Fe(Ⅲ)/Fe(Ⅱ)值整體均呈上升趨勢(shì)且與Fe(Ⅲ)同步變化,表明其比值由Fe(Ⅲ)變化驅(qū)動(dòng);Fe(Ⅲ)/Fe(Ⅱ)指示沉積物上層為弱氧化性,其余層位為還原性環(huán)境.沉積物還原性無(wú)機(jī)硫(RIS)以酸可揮發(fā)性硫(AVS)為主,沉積物高有機(jī)質(zhì)含量、低元素硫和還原條件降低了AVS向黃鐵礦硫(CRS)的轉(zhuǎn)化.巢湖沉積物中與P, S結(jié)合的Fe占比很小,高Fe/P和Fe/S比值會(huì)抑制沉積物磷的釋放,導(dǎo)致柱狀剖面P, Fe和S之間的相互作用關(guān)系整體上并不顯著.

磷形態(tài)；活性鐵；還原性無(wú)機(jī)硫；沉積物；富營(yíng)養(yǎng)化

沉積物是水域生態(tài)系統(tǒng)營(yíng)養(yǎng)鹽的儲(chǔ)存庫(kù),能夠記錄豐富的流域環(huán)境變化信息[1].磷(P)、鐵(Fe)和硫(S)是生物有機(jī)體的重要組成元素,對(duì)水域生態(tài)環(huán)境有重要的影響.外源磷輸入是湖泊水體富營(yíng)養(yǎng)化和水華形成的重要原因[2];然而在外源磷輸入得到控制的情況下,沉積物內(nèi)源磷釋放依然可造成水體富營(yíng)養(yǎng)化[3];因此,湖泊磷的沉積變化不僅可以反映流域污染源排放歷史,還對(duì)沉積物內(nèi)源磷遷移轉(zhuǎn)化具有指示意義[4].鐵是常見(jiàn)的氧化還原敏感元素,通常以鐵氧化物的形式參與各種生物地球化學(xué)反應(yīng)過(guò)程[5-6].還原性硫影響金屬的循環(huán)及生物有效性,沉積物剖面S(-Ⅱ)濃度是控制金屬元素有效性及毒性的重要因子[7].P, Fe和S耦合體系在湖泊內(nèi)源磷生物地球化學(xué)轉(zhuǎn)化、循環(huán)方面起著重要的調(diào)控作用[8].研究表明,還原條件下,沉積物中Fe(Ⅲ)被還原為Fe(Ⅱ)并導(dǎo)致鐵結(jié)合態(tài)磷向孔隙水乃至上覆水?dāng)U散,是湖泊內(nèi)源磷釋放的重要途徑[3].硫酸鹽還原產(chǎn)物S(-Ⅱ)可與Fe(Ⅱ)形成不溶性的FeS和FeS2,阻止Fe(Ⅱ)與磷結(jié)合,P, Fe和S之間具有相互制約或相互促進(jìn)的耦合機(jī)制[9-11],并共同參與沉積物的早期成巖過(guò)程.

巢湖P, Fe和S相關(guān)研究多集中以沉積物-水界面為對(duì)象,主要從藻類(lèi)降解對(duì)界面P, Fe和S動(dòng)態(tài)變化及耦合關(guān)系等角度開(kāi)展.如室內(nèi)模擬研究顯示藻類(lèi)降解會(huì)同時(shí)釋放P和S,導(dǎo)致沉積物和上覆水具有雙向擴(kuò)散通量,沉積物是上覆水中不穩(wěn)定Fe的主要來(lái)源,富營(yíng)養(yǎng)化水體中形成黑色硫化物沉淀的Fe和S分別來(lái)自沉積物和降解的藻類(lèi)[12];南淝河和巢湖西半湖區(qū)沉積物-水界面有效態(tài)P與S自界面向下明顯增加,部分點(diǎn)位二者存在同步釋放現(xiàn)象,表明內(nèi)源磷的釋放伴隨著硫酸鹽的還原過(guò)程;巢湖P與Fe的原位分析和室內(nèi)模擬驗(yàn)證了Fe還原是內(nèi)源P釋放的關(guān)鍵過(guò)程[13].研究表明,巢湖沉積剖面可還原態(tài)磷(BD-P)和鐵鋁結(jié)合態(tài)磷(NaOH-P)與間隙水不同形態(tài)磷顯著相關(guān),它們隨深度由下及上的增加指示了近年來(lái)巢湖磷負(fù)荷受人類(lèi)活動(dòng)影響加重[14].鐵鋁結(jié)合態(tài)磷(NaOH-Pi)和活性有機(jī)磷(NaOH-Po)是巢湖沉積磷增加的主要形態(tài),西部湖區(qū)沉積物向上覆水釋放磷的風(fēng)險(xiǎn)高于中東部湖區(qū)[15].沉積剖面總磷分布指示巢湖自20世紀(jì)60年代以來(lái)受人類(lèi)活動(dòng)影響逐步增加[1].目前關(guān)于湖泊沉積剖面P, Fe和S體系賦存形態(tài)分布及其耦合關(guān)系的研究還很有限.本文對(duì)巢湖表層和柱狀沉積物開(kāi)展P, Fe和S各形態(tài)分析,研究P, Fe和S沉積指示的湖泊環(huán)境變化及其耦合作用關(guān)系,為認(rèn)識(shí)湖泊沉積演化和內(nèi)源污染治理提供科學(xué)依據(jù).

1 材料與方法

1.1 研究區(qū)概況

巢湖位于安徽省中部、江淮丘陵南部,屬長(zhǎng)江下游水系,湖區(qū)面積約780km2,東西向長(zhǎng)54.5km,南北向長(zhǎng)15.1km,平均水深為3.06m[16].巢湖屬于亞熱帶季風(fēng)氣候區(qū),年平均氣溫為15～16℃,年平均降雨量為1100mm,是我國(guó)著名的五大淡水湖泊之一[17].受合肥市生活污水,工業(yè)廢水和流域農(nóng)業(yè)面源持續(xù)排放的影響,20世紀(jì)70年代末以來(lái),巢湖已逐漸發(fā)展成為典型的富營(yíng)養(yǎng)化湖泊[2,18].

1.2 樣品采集和預(yù)處理

參照巢湖流域主要入湖河流以及水質(zhì)國(guó)控監(jiān)測(cè)點(diǎn)位分布,本研究在全湖湖面共布設(shè)9個(gè)點(diǎn)位(圖1),其中S1～S5位于巢湖西半湖區(qū),S6～S9位于東半湖區(qū).2019年12月開(kāi)展9個(gè)點(diǎn)位表層和柱狀沉積物樣品采集.利用彼得森采泥器采集各點(diǎn)位0～10cm的表層沉積物,裝入密封袋并抽真空;使用重力式柱狀采樣器采集各點(diǎn)位柱狀沉積物樣品,用橡膠塞密封端口.所有樣品置于放冰袋的保溫箱內(nèi)于4℃冷藏保存運(yùn)回實(shí)驗(yàn)室.本研究選擇全湖9個(gè)點(diǎn)位表層沉積物樣,西半湖S3點(diǎn)位和東半湖S7點(diǎn)位兩個(gè)柱狀沉積物樣品開(kāi)展分析.柱狀沉積物在實(shí)驗(yàn)室內(nèi)按照1cm間隔進(jìn)行分層,每層樣品取部分裝入密封袋抽真空在-20℃冰箱中冷凍保存;剩余樣品冷凍干燥后剔除貝殼及動(dòng)植物殘?bào)w,用瑪瑙研缽充分研磨混勻過(guò)200目篩,編號(hào)放置干燥器中待分析.

圖1 巢湖沉積物采樣點(diǎn)分布

1.3 樣品分析方法

磷形態(tài)分析采用SMT法[19],包括總磷(TP)、無(wú)機(jī)磷(IP)、鐵/鋁結(jié)合態(tài)磷(Fe/Al-P)、鈣結(jié)合態(tài)磷(Ca-P)和有機(jī)磷(OP).總硫(TS)使用HCS-218型高頻紅外碳硫分析儀測(cè)定[20].燒失量(LOI)采用灼燒法在550℃下灼燒3h測(cè)定質(zhì)量差;pH值按照沉積物:水=1g:5mL采用PHS-3C型pH計(jì)測(cè)定.

活性鐵組分具體分析如下:稱(chēng)取0.5g濕沉積物樣品于離心管中,加入25mL 1mol/L鹽酸后密封,室溫振蕩24h,離心(6000r/min, 10min),取部分上清液用鄰菲啰啉分光光度法測(cè)定活性Fe(Ⅱ);向部分剩余上清液中加入10%的鹽酸羥胺溶液加熱煮沸,以保證全部的Fe(Ⅲ)還原為Fe(Ⅱ),再次測(cè)定,即為總活性鐵Fe(T).活性Fe(Ⅲ)的含量為Fe(T)與Fe(Ⅱ)的差值.實(shí)驗(yàn)室內(nèi)樣品重復(fù)測(cè)定誤差小于5%.

采用改進(jìn)的冷擴(kuò)散法提取了沉積物中的酸可揮發(fā)性硫(AVS)和黃鐵礦硫(CRS)[21-22].具體分析步驟如下:稱(chēng)取約3g的濕沉積物樣品迅速置于反應(yīng)瓶中,先后加入25mL 6mol/L鹽酸和20mL 1mol/L還原態(tài)氯化鉻將沉積物中的AVS和CRS轉(zhuǎn)化為硫化氫(H2S)氣體,并用氮?dú)鈱2S吹入裝有20%的堿式醋酸鋅的吸收瓶中固定為硫化鋅(ZnS)沉淀,在實(shí)驗(yàn)過(guò)程中通過(guò)90℃水浴加熱將上述反應(yīng)時(shí)間分別縮短為1.5, 2h[23],最后通過(guò)亞甲基藍(lán)分光光度法測(cè)定其含量.實(shí)驗(yàn)中通過(guò)加入固體硫化鋅進(jìn)行回收實(shí)驗(yàn),回收率為91.89%～102.74% (=3).本研究中元素硫(ES)低于檢測(cè)限,因此定義還原性無(wú)機(jī)硫(RIS)=AVS＋CRS.

1.4 數(shù)據(jù)分析

采用Origin 2019和Arcgis 10.2繪制數(shù)據(jù)圖表.利用SPSS 25.0進(jìn)行Pearson相關(guān)性分析,其中<0.01為極顯著相關(guān),而<0.05為顯著相關(guān).

2 結(jié)果與討論

2.1 巢湖表層沉積物磷鐵硫形態(tài)、LOI和pH值的空間分布

圖2 巢湖表層沉積物磷鐵硫形態(tài)及LOI和pH值的空間分布

如圖2a所示,TP含量呈現(xiàn)西半湖區(qū)高,東半湖區(qū)較低的空間分布特征,與先前相關(guān)報(bào)道結(jié)果[24-25]一致,主要原因?yàn)槲靼牒^(qū)受合肥市污水排放影響嚴(yán)重,東半湖區(qū)受到的污染相對(duì)較輕.靠近南淝河入湖口的S1點(diǎn)TP含量相比西半湖區(qū)其它點(diǎn)位較低,且與溫勝芳等[15]2010年的結(jié)果相比明顯偏低,可能與該點(diǎn)位附近多次底泥疏浚有關(guān)[26].巢湖表層沉積物磷以IP為主,其占TP比例范圍為65.3%～86.5%.西半湖區(qū)沉積物IP以Fe/Al-P為主,Ca-P占比較低,而東半湖區(qū)沉積物IP中Fe/Al-P和Ca-P占比相當(dāng),S8和S9點(diǎn)位Ca-P含量超過(guò)Fe/Al-P,這主要是由于西半湖區(qū)受以Fe/Al-P為主要組分的生活污水污染嚴(yán)重,而東半湖區(qū)更多受到東北岸高Ca-P含量的土壤地表徑流輸入影響[17].巢湖表層沉積物OP含量空間變化不大,但西半湖OP/TP比例明顯小于東半湖;西半湖區(qū)污染來(lái)源主要是合肥市生活污水和工農(nóng)業(yè)排放,IP占比高,相應(yīng)的OP占比較低,而東半湖區(qū)污染來(lái)源主要是周邊農(nóng)業(yè)面源污染[27].研究表明,對(duì)于富營(yíng)養(yǎng)化水體,以工業(yè)廢水和生活污水為主要污染源的沉積物中OP濃度低于以農(nóng)業(yè)面源污染為主要污染源的沉積物[28-29].

如圖2b所示,Fe(T)含量在S6點(diǎn)位最低而在S9點(diǎn)位最高;S6點(diǎn)位位于巢湖南部,靠近兆河入湖口區(qū),主要接受農(nóng)業(yè)面源污染輸入,而S9點(diǎn)位靠近污染嚴(yán)重的雙橋河,相關(guān)研究結(jié)果[30]表明,絮凝沉淀作用可導(dǎo)致沉積物中活性鐵的明顯富集.S4和S9點(diǎn)位Fe(Ⅱ)含量低于Fe(Ⅲ),其余點(diǎn)位Fe(Ⅱ)含量高于Fe(Ⅲ).表層沉積物中Fe(Ⅱ)含量與Fe/Al-P呈顯著正相關(guān)(=0.84,<0.01),表明其主要受污染輸入影響.

表層沉積物TS含量為409.3～720.4mg/kg (均值為524mg/kg),S3、S8和S9點(diǎn)位TS含量較高,其余點(diǎn)位含量變化較小(圖2c).AVS和CRS的含量范圍分別為55.7～142.2, 5.8～36.3mg/kg,平均值為94.7, 18.5mg/kg,表明沉積物中還原硫以AVS為主.研究表明AVS的形成和累積主要受有機(jī)質(zhì)、活性鐵以及氧化還原環(huán)境的共同影響[9].巢湖富營(yíng)養(yǎng)化乃至藍(lán)藻水華使得表層沉積物的有機(jī)質(zhì)含量高,為硫酸鹽還原微生物提供了充足的碳源,進(jìn)而形成AVS累積;盡管AVS可向CRS轉(zhuǎn)化,但這是一個(gè)較為緩慢的過(guò)程,導(dǎo)致AVS的生成速率可能大于其轉(zhuǎn)化速率[31],因此,表層沉積物AVS得以積累,RIS以AVS為主.RIS占TS比例較低,范圍為11.7%～35.8%,平均比值為22.5%.表層沉積物L(fēng)OI含量范圍為4.9%～7.4% (均值為5.8%);而pH值范圍在6.92～7.11之間,變化很小(圖2d).TS和LOI呈顯著正相關(guān)(r=0.83,<0.01),指示表層沉積物中硫主要以有機(jī)硫形式存在.AVS、CRS與Fe(Ⅱ)均不存在顯著相關(guān)性,指示巢湖表層沉積物中Fe和S之間的相互關(guān)系較弱.

2.2 巢湖有機(jī)質(zhì)、磷沉積記錄及其指示的湖泊環(huán)境變化

如圖3所示,LOI是沉積物中有機(jī)質(zhì)含量的替代性指標(biāo),可反映湖泊的營(yíng)養(yǎng)化水平[32].S3點(diǎn)位沉積物剖面LOI含量為3.2%～7.2% (均值為4.6%),其中,33～15cm沉積物L(fēng)OI較低且變化較小,15～0cm沉積物L(fēng)OI持續(xù)顯著地增加,反映了巢湖西半湖區(qū)早期營(yíng)養(yǎng)鹽輸入逐步增加導(dǎo)致水體富營(yíng)養(yǎng)化和藻類(lèi)生產(chǎn)力提高并形成有機(jī)質(zhì)持續(xù)快速的埋藏.S7點(diǎn)位沉積物L(fēng)OI含量低于S3點(diǎn)位,含量范圍為2.9%～ 4.6% (均值為3.7%),這是由于S7點(diǎn)位營(yíng)養(yǎng)鹽含量和藻類(lèi)生產(chǎn)力相對(duì)較低,有機(jī)質(zhì)形成和埋藏相應(yīng)較少.研究表明,藻類(lèi)光合作用和有機(jī)質(zhì)降解均會(huì)影響湖泊水體pH值,光合作用吸收CO2將提高水體pH值,而有機(jī)質(zhì)降解產(chǎn)生CO2和小分子酸導(dǎo)致pH值降低[33-34].S3點(diǎn)位沉積物剖面pH值范圍在6.55～7.21之間,其垂直分布特征與LOI相似,且在15～0cm沉積物pH值和LOI持續(xù)協(xié)同升高,指示S3點(diǎn)位水體富營(yíng)養(yǎng)化促進(jìn)了湖泊藻類(lèi)生產(chǎn)力提高(光合作用增強(qiáng)),導(dǎo)致有機(jī)質(zhì)埋藏增加和水體pH值升高;S7點(diǎn)位柱狀沉積物營(yíng)養(yǎng)鹽負(fù)荷相對(duì)較低,pH值與LOI在29～7cm層位具有整體相反的變化趨勢(shì),指示S7點(diǎn)位pH值更多受到有機(jī)質(zhì)降解的控制,而受藻類(lèi)光合作用影響有限,這與朱瑾燦等[35]對(duì)太湖表層沉積物的相關(guān)研究結(jié)果類(lèi)似.巢湖東西半湖柱狀沉積物pH值和LOI關(guān)系的差異可能主要與2個(gè)點(diǎn)位營(yíng)養(yǎng)鹽高低及其導(dǎo)致的藻類(lèi)光合作用與有機(jī)質(zhì)降解差異有關(guān),但這還需要進(jìn)一步的研究證實(shí).

Fe/Al-P是流域人為磷污染源排放的重要指標(biāo),可指示生活污水和工業(yè)廢水等污染磷源的輸入[19,24]; Ca-P主要來(lái)自于陸源風(fēng)化產(chǎn)物和土壤,可以指示陸源徑流輸入[19,36].巢湖西半湖S3點(diǎn)位沉積物剖面TP、IP和Fe/Al-P呈同步變化(圖3),即低位穩(wěn)定(33～21cm)-緩慢持續(xù)增加(21～11cm)-快速增加(11～5cm)-略微降低(5～0cm)的整體趨勢(shì);而Ca-P在23～5cm呈持續(xù)增加的趨勢(shì),且比TP、IP和Fe/Al-P提前開(kāi)始增加,指示了以Ca-P為主的自然的陸源徑流輸入增加早于以Fe/Al-P為主的人為的生活污水和工業(yè)廢水等輸入;TP、IP和Fe/Al-P從21cm向上開(kāi)始持續(xù)增加,而LOI只在15cm以上才開(kāi)始增加,表明早期營(yíng)養(yǎng)鹽輸入的緩慢增加并未立即提高湖泊生產(chǎn)力,直到15cm營(yíng)養(yǎng)鹽輸入達(dá)到較高水平,湖泊藻類(lèi)生產(chǎn)力才得到提高,有機(jī)質(zhì)埋藏開(kāi)始持續(xù)升高.S7點(diǎn)位沉積剖面TP、IP和Fe/Al-P自下而上呈持續(xù)降低-波動(dòng)增加-降低的同步變化趨勢(shì)(圖3);不同的是TP在25cm向上開(kāi)始增加,而IP和Fe/Al-P在23cm向上開(kāi)始增加,TP的提前增加來(lái)自于高OP的貢獻(xiàn).近年來(lái),對(duì)巢湖流域主要入湖河流污染治理力度逐步加大,巢湖水體污染得到部分改善[37],這可能是S3點(diǎn)位沉積剖面5～0cm和S7點(diǎn)位沉積剖面3～0cm中TP、IP和Fe/Al-P下降的主要原因.

S7點(diǎn)位沉積剖面Ca-P含量較高,自下而上呈降低-上升的兩段式變化趨勢(shì),即在31～19cm呈持續(xù)降低的趨勢(shì),而在19～0cm呈持續(xù)增加的趨勢(shì);Ca-P主要是由變質(zhì)巖或巖漿巖碎屑態(tài)磷灰石等陸源輸入而形成的一種沉積磷[36],對(duì)沉積物向水體釋放的促進(jìn)作用較小,較難被生物利用[25].Ca-P在S3點(diǎn)位剖面23cm以上和S7點(diǎn)位剖面19cm以上逐漸增加,指示巢湖流域逐漸增強(qiáng)的陸源徑流輸入.OP可反映有機(jī)質(zhì)沉積埋藏水平,包括水生生物遺體的可降解磷和陸源難降解磷,其中部分OP通過(guò)化學(xué)和生物轉(zhuǎn)化釋放磷酸根進(jìn)入上覆水,從而對(duì)湖泊富營(yíng)養(yǎng)化產(chǎn)生影響[38-39].S3點(diǎn)位剖面7～0cm OP含量明顯高于中下層,反映了湖泊富營(yíng)養(yǎng)化、生產(chǎn)力升高并導(dǎo)致有機(jī)質(zhì)埋藏增多.S7點(diǎn)位剖面表層OP低于S3點(diǎn)位,與東西半湖受藻華影響程度差異有關(guān),這與劉國(guó)峰等[40]對(duì)太湖的研究結(jié)果相似.

巢湖西半湖區(qū)沉積物的歷史記錄結(jié)果[1,18]顯示,在新中國(guó)成立后至1960年,巢湖經(jīng)歷了低生產(chǎn)力和穩(wěn)定的低營(yíng)養(yǎng)鹽輸入階段;之后隨著人口增長(zhǎng)和農(nóng)業(yè)發(fā)展巢湖營(yíng)養(yǎng)鹽物質(zhì)輸入開(kāi)始增加,同時(shí)巢湖閘的建設(shè)使得湖泊沉積環(huán)境更加穩(wěn)定,助推了有機(jī)質(zhì)和營(yíng)養(yǎng)鹽的埋藏累積;20世紀(jì)70年代末,隨著國(guó)家改革開(kāi)放政策的實(shí)施,合肥地區(qū)人口、工業(yè)和農(nóng)業(yè)快速發(fā)展,巢湖開(kāi)始了高營(yíng)養(yǎng)鹽、高污染物輸入和高生產(chǎn)力及有機(jī)質(zhì)沉積階段.本研究西半湖區(qū)S3點(diǎn)位沉積剖面中磷形態(tài)和LOI記錄與上述研究呈同步變化趨勢(shì),因此,本研究推斷S3點(diǎn)位剖面TP開(kāi)始增加的21cm處對(duì)應(yīng)于20世紀(jì)60年代初,而TP開(kāi)始快速增加的11cm處對(duì)應(yīng)于20世紀(jì)70年代末.

2.3 巢湖活性鐵沉積記錄及其指示的湖泊環(huán)境變化

活性鐵是指能被1mol/L鹽酸提取的鐵組分,主要包括無(wú)定型或弱晶型Fe(Ⅲ)氧化物和Fe(Ⅱ) (黃鐵礦除外),以及Fe3S4(含量很少,可忽略不計(jì))[41];Fe(Ⅲ)/Fe(Ⅱ)值可用來(lái)指示沉積物氧化還原條件,Fe(Ⅲ)/Fe(Ⅱ)>3為氧化環(huán)境,1

2.4 巢湖硫形態(tài)沉積記錄及其指示的湖泊環(huán)境變化

S3點(diǎn)位沉積剖面TS含量為129.6～790.7mg/kg (均值為313.8mg/kg),33～11cm變化較小,而11cm以上快速增加并在7～5cm達(dá)到最高值區(qū)間;TS與TP的快速增加趨勢(shì)相似,指示硫的升高主要來(lái)自于改革開(kāi)放以來(lái)高污染排放及其引起的富營(yíng)養(yǎng)化乃至藻華的有機(jī)質(zhì)埋藏貢獻(xiàn).S7點(diǎn)位沉積剖面TS含量為162.4～420.9mg/kg,自下而上整體呈上升的趨勢(shì),在9cm處達(dá)到最高值;相較于S3點(diǎn)位沉積剖面,S7點(diǎn)位沉積剖面TS整體偏低且垂直變化范圍不大,指示其受到的污染輸入和富營(yíng)養(yǎng)化程度均較輕.

AVS是硫化物中生物有效性最高的形態(tài)[31],主要受到氧化還原條件、有機(jī)質(zhì)及活性鐵含量等多種因素影響[9].沉積物中CRS的主要組成為黃鐵礦,在一定程度上指示活性鐵氧化物的硫化程度,是還原性環(huán)境中鐵硫的最終存在形態(tài)[43].S3和S7點(diǎn)位沉積剖面還原性無(wú)機(jī)硫均以AVS為主,其中S3點(diǎn)位沉積剖面AVS含量為3.71～126.73mg/kg (均值為33.98mg/kg),高于S7點(diǎn)位沉積剖面含量(均值為26.97mg/kg).兩個(gè)剖面AVS變化均與TS相似,分別在9～5cm, 11～9cm處達(dá)到高值區(qū)間(圖4),均對(duì)應(yīng)于Fe(Ⅲ)/Fe(Ⅱ)指示的較強(qiáng)還原環(huán)境層位,表明巢湖沉積物AVS含量主要受到硫的來(lái)源沉積和還原轉(zhuǎn)化控制.S3點(diǎn)位沉積剖面CRS與TS、AVS均在9～5cm層位出現(xiàn)高值,指示其主要受到硫的來(lái)源沉積控制;而S3點(diǎn)位沉積剖面15～13cm和S7點(diǎn)位沉積剖面19～15cm CRS的高值與TS、AVS無(wú)明顯關(guān)系,指示這些層位CRS受到其它轉(zhuǎn)化條件影響.Gagnon等[44]研究得出AVS/CRS比值大于0.3時(shí),AVS不能有效地轉(zhuǎn)化為CRS;本研究中S3和S7點(diǎn)位沉積剖面沉積物AVS/CRS比值均大于0.3,表明巢湖沉積物中AVS向CRS的轉(zhuǎn)化率較低.AVS是形成CRS的重要前體[43,45],它主要通過(guò)多硫化物、亞鐵損失和硫化氫結(jié)合三條途徑[43,46-47]轉(zhuǎn)化為CRS.本研究S3和S7點(diǎn)位沉積剖面元素硫(ES)低于檢測(cè)限,導(dǎo)致多硫化物途徑生成CRS(FeS與ES反應(yīng))過(guò)程受限;同時(shí)S3和S7點(diǎn)位沉積剖面中下層整體較強(qiáng)的還原環(huán)境也不利于亞鐵損失途徑生成CRS.研究表明沉積物高有機(jī)質(zhì)可有效限制AVS向CRS的轉(zhuǎn)化[22],巢湖沉積物高含量有機(jī)質(zhì)有利于形成還原微環(huán)境并能夠?yàn)榱蛩猁}和鐵還原提供充足的活性有機(jī)質(zhì),促進(jìn)S(-Ⅱ)與Fe(Ⅱ)及其他金屬離子在沉積物中形成AVS并快速地沉積埋藏,降低了AVS向CRS的轉(zhuǎn)化.

2.5 巢湖沉積物P, Fe和S相互關(guān)系

Fe/Al-P是流域人為污染磷源排放的指示性指標(biāo),巢湖表層沉積物中Fe(Ⅱ)與Fe/Al-P呈顯著正相關(guān)(=0.84,<0.01),指示其主要受到污染源輸入控制,受鐵的氧化還原轉(zhuǎn)化過(guò)程影響較弱.而在S3和S7點(diǎn)位沉積剖面中,Fe(Ⅲ)與Fe/Al-P之間均呈顯著正相關(guān)(=0.73,<0.01;=0.52,<0.05),指示沉積物中高含量三價(jià)鐵對(duì)磷吸附作用明顯.Jensen等[48]對(duì)歐洲眾多湖泊的調(diào)查研究發(fā)現(xiàn),沉積物Fe/P比值控制著磷的遷移釋放能力,比值越高沉積物磷穩(wěn)定性越強(qiáng).Fe(Ⅱ)與AVS、CRS整體上無(wú)顯著相關(guān)性,指示其與硫結(jié)合的占比很小;與尹洪斌等[49]關(guān)于太湖沉積物的相關(guān)研究結(jié)果類(lèi)似,巢湖沉積物中的鐵可能也是主要以與鐵的氧化物/氫氧化物結(jié)合的形式存在.部分層位P, Fe和S呈現(xiàn)明顯的相互作用關(guān)系.S3點(diǎn)位沉積剖面7cm處沉積物中Fe(Ⅱ)和AVS呈同步升高表明同時(shí)發(fā)生了鐵和硫酸鹽還原過(guò)程,指示該層位沉積物較強(qiáng)的還原環(huán)境;而Fe/Al-P并沒(méi)有呈現(xiàn)相應(yīng)地降低,反而出現(xiàn)上升,這主要與巢湖沉積物具有顯著高的Fe/P比值有關(guān),過(guò)量的Fe會(huì)抑制沉積物磷的釋放,這與孫清清等[50]在研究阿哈水庫(kù)沉積物P, Fe和S分布特征相似.

3 結(jié)論

3.1 巢湖S3點(diǎn)位柱狀沉積物TP自21cm向上開(kāi)始增加顯示西半湖區(qū)自1960s開(kāi)始受人類(lèi)活動(dòng)影響逐步明顯;Ca-P自23cm向上逐步增加,提前于Fe/Al-P的增加,指示陸源徑流輸入增加早于城市污水排放輸入.

3.2 巢湖S3點(diǎn)位柱狀沉積物L(fēng)OI和pH值在15～0cm協(xié)同升高,而它們?cè)赟7點(diǎn)位柱狀沉積物的29～7cm呈整體相反趨勢(shì),指示西半湖S3點(diǎn)位pH值受藻類(lèi)光合作用影響較大而東半湖S7點(diǎn)位pH值主要受有機(jī)質(zhì)降解影響.

3.3 巢湖沉積物中與P, S結(jié)合的Fe占比很小;沉積物中還原性無(wú)機(jī)硫以AVS為主,高有機(jī)質(zhì)含量、低元素硫和還原條件降低了AVS向CRS的轉(zhuǎn)化.

3.4 巢湖沉積剖面部分層位P, Fe和S相互作用關(guān)系明顯,顯著高的Fe/P比值抑制沉積物磷的釋放.

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Fractions of phosphorus, iron and sulfur in lake sediments of Chaohu and its implication for environmental changes.

CHEN Xi, NING Cheng-wu,WANG Jie,HUANG Tao*, SUN Qing-ye

(Anhui Province Key Laboratory of Wetland Ecosystem Protection and Restoration, School of Resources and Environmental Engineering, Anhui University, Hefei 230601, China)., 2021,41(6)：2853～2861

Fractions of phosphorus, iron and sulfur in the surface sediments and two sediment profiles from Chaohu lake were analyzed to study their implication for the spatio-temporal environmental changes. Sedimentary records of total phosphorus (TP) in S3 sediment profile indicated an increasing pollution inputs from anthropogenic activities in the western lake since 1960s. The increase of watershed terrestrial runoff inputs indicated by calcium-bound phosphorus (Ca-P) was earlier than the domestic sewage inputs indicated by iron/aluminum-bound phosphorus (Fe/Al-P) in sediments. The synchronous increasing of organic matter and pH in the layer of 15～0cm in S3 sediment indicated an increasing lake primary productivity as well as the intensity of photosynthesis. Fractions of phosphorus in S7 profile, however, indicated different environmental changes in eastern lake. Fe(II) was the main fraction of total active iron in Chaohu lake sediments; the ratios of Fe(III)/Fe(II) in S3 and S7 profiles show an synchronous trend with Fe(III), indicating that the change of Fe(III)/Fe(II) was driven by Fe(III). Ratios of Fe(III)/Fe(II) indicated a weak oxidation condition for the upper sediment layer and reduction for the down layer in both of the S3 and S7 profiles. Acid volatile sulfur (AVS) was the main form of the reduced inorganic sulfur (RIS) in Chaohu lake sediments. High organic matter, low elemental sulfur and strong reducing condition in Chaohu lake sediments restricted the transformations of AVS to pyrite sulfur (CRS). The P- and S-bound iron proportioned only a very small part for the total iron in the sediments of Chaohu lake. The high ratios of Fe/P and Fe/S in the sediments of Chaohu lake weakened the release of internal phosphorus, and thus the correlation of P, Fe and S was insignificant on the whole.

phosphorus fractions；active iron；reduced inorganic sulfur；sediment；eutrophication

X524

A

1000-6923(2021)06-2853-09

陳 茜(1995-),女,安徽懷寧人,安徽大學(xué)碩士研究生,從事生態(tài)工程與環(huán)境修復(fù)技術(shù)研究.

2020-11-18

安徽省高校自然科學(xué)研究重點(diǎn)項(xiàng)目(KJ2019A0042);國(guó)家自然科學(xué)基金資助項(xiàng)目(41476165)

* 責(zé)任作者, 副教授, huangt@ahu.edu.cn