沉積物磷形態(tài)空間分布特征及釋放風(fēng)險(xiǎn)評(píng)估——以沱江流域?yàn)槔?/h1>
2022-09-20 07:07:30唐金勇尹月鵬
中國環(huán)境科學(xué) 2022年9期
唐金勇,尹月鵬,曹 熙,張 瑜,張 雯,3*
沉積物磷形態(tài)空間分布特征及釋放風(fēng)險(xiǎn)評(píng)估——以沱江流域?yàn)槔?/p>
唐金勇1,2,尹月鵬1,2,曹 熙1,2,張 瑜1,2,張 雯1,2,3*
(1.成都理工大學(xué)生態(tài)環(huán)境學(xué)院,四川 成都 610059；2.國家環(huán)境保護(hù)水土污染協(xié)同控制與聯(lián)合修復(fù)重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué)),四川 成都 610059；3.地質(zhì)災(zāi)害防治與地質(zhì)環(huán)境保護(hù)國家重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué)),四川 成都 610059)
為闡明沉積物磷賦存形態(tài)的空間分布特征及潛在釋放風(fēng)險(xiǎn),提供更準(zhǔn)確合適的風(fēng)險(xiǎn)評(píng)估指標(biāo), 分析了沱江干流及其支流12個(gè)樣點(diǎn)表層沉積物的磷賦存形態(tài),測定了水溶性磷(WSP)及磷平衡濃度(EPC0),計(jì)算沉積物磷吸附指數(shù)(PSI)、磷吸附飽和度(DPS)及其衍生的磷釋放風(fēng)險(xiǎn)指數(shù)(ERI).結(jié)果表明,沉積物5種形態(tài)磷含量順序?yàn)?鐵/鋁結(jié)合磷(CDB-P,60.63%)>鈣磷(Ca-P,30.84%)>有機(jī)磷(OP,3.92%)>亞鐵磷(Fe(Ⅱ)-P,3.48%)>松散態(tài)磷(Loosely-P,1.13%).CDB-P是沉積物磷的主要存在形態(tài)(0.468～2.287mg/g),由上游至下游逐漸降低,這主要與上游工業(yè)污染有關(guān).DPS、EPC0和PSI在空間分布上均呈現(xiàn)由上游至下游逐漸增大的趨勢,變化范圍分別為44.28%～80.39%、0.012～0.084mg/L和0.153～1.526L/g;上游大部分采樣點(diǎn)ERI均超過了25%;各指標(biāo)綜合表明:上游存在較高的磷釋放風(fēng)險(xiǎn).回歸分析與相關(guān)性表明,EPC0與上覆水磷、CDB-P、OP、有機(jī)質(zhì)(OM)以及粒徑均呈極顯著相關(guān)性,且相關(guān)性遠(yuǎn)高于其他指標(biāo)(ERI,DPS,PSI,WSP).因此,EPC0是評(píng)估沱江流域沉積物磷釋放風(fēng)險(xiǎn)潛力更準(zhǔn)確高效的指標(biāo),Fe/Al含量、粒徑的增加以及有機(jī)質(zhì)的減少會(huì)增加磷釋放風(fēng)險(xiǎn),因此應(yīng)控制工業(yè)污染以及農(nóng)業(yè)面源污染的輸入.
磷形態(tài)；空間分布；磷平衡濃度；磷釋放風(fēng)險(xiǎn)
沉積物是水生態(tài)系統(tǒng)中磷庫的重要組成部分,其作為營養(yǎng)鹽的“源”與“匯”[1],同時(shí)又是磷遷移轉(zhuǎn)化、再生的主要場所.磷是水體富營養(yǎng)化的重要限制因子[2-3],當(dāng)湖泊、河流等水體外源磷負(fù)荷減少,內(nèi)源磷負(fù)荷可能阻止其水質(zhì)好轉(zhuǎn)[4],并且成為水質(zhì)恢復(fù)延緩的重要原因之一[5].同時(shí),沉積物中磷的不同形態(tài)影響磷的生物有效性[6],因此研究沉積物中磷形態(tài)含量和變化特征,有助于了解河流污染程度及地球化學(xué)信息[7].改進(jìn)的SEDEX沉積物磷形態(tài)分離方法[8]將沉積物中的磷形態(tài)分為松散態(tài)磷(Loosely- P)、亞鐵磷(Fe(II)-P)、鐵/鋁結(jié)合磷(CDB-P)和有機(jī)磷(OP)5種形態(tài).該方法能將Fe(II)結(jié)合的磷單獨(dú)提取,研究表明,缺氧沉積物結(jié)合磷的潛力歸因于Fe(II)相的出現(xiàn),它們與磷直接相互作用形成Fe(II)磷酸鹽相,因此將Fe(II)-P分餾為一個(gè)獨(dú)立的磷組分,對(duì)獲取沉積物中磷的地球化學(xué)信息具有重要意義.
此外,國內(nèi)外學(xué)者對(duì)沉積物中磷的污染分布及風(fēng)險(xiǎn)評(píng)價(jià)進(jìn)行了大量的研究,而對(duì)于沉積物磷釋放風(fēng)險(xiǎn)評(píng)價(jià)指標(biāo)沒有統(tǒng)一的體系標(biāo)準(zhǔn)[9].目前,磷吸附指數(shù)(PSI)和磷吸附飽和度(DPS)常被用來表征土壤磷吸附容量[10-11].除此之外,水溶性磷(WSP)因其測試方法簡單快速也被用來評(píng)估土壤磷釋放風(fēng)險(xiǎn)[12].磷平衡濃度(EPC0)是指沉積物固相與周邊水溶液中的磷酸鹽(SRP)達(dá)到吸附與解吸平衡時(shí),水相中磷酸鹽的濃度[13-14].EPC0值越高,沉積物向水柱釋放磷的風(fēng)險(xiǎn)越大[13],廣泛用于沉積物而日益受到大家的關(guān)注.研究表明,EPC0和生物有效磷存在明顯的相關(guān)性,EPC0越高,往往生物有效磷也越高,對(duì)評(píng)價(jià)沉積物磷釋放風(fēng)險(xiǎn)有一定的參考價(jià)值[15],因此沉積物充當(dāng)水體磷“源”或“匯”的功能可由EPC0估計(jì).
本研究以四川省沱江流域(綿遠(yuǎn)河新源大橋至流灘壩水電站,104°25′～105°23′E,28°55′～31°13′N)為研究對(duì)象,系統(tǒng)分析了沉積物的磷賦存形態(tài)及其含量的空間分布特征,分析了EPC0和ERI等指標(biāo),旨在深入揭示沉積物磷吸附行為的變化特征及其環(huán)境意義,闡明磷釋放風(fēng)險(xiǎn),為其提供最佳的沉積物磷釋放風(fēng)險(xiǎn)評(píng)估指標(biāo).
1 材料與方法
1.1 樣品采集與處理
根據(jù)沱江流域水系特點(diǎn),選擇了12個(gè)均勻分布于沱江上中下游具有代表性的點(diǎn)位(圖1).采用彼得森抓斗式采泥器采取表層沉積物,采集后將樣品置于密封的聚乙烯塑料袋中,一部分于4℃左右儲(chǔ)存在實(shí)驗(yàn)室用于沉積物磷賦存形態(tài)及EPC0的測定,另一部分于-20℃下冷凍干燥研磨后儲(chǔ)存于干燥器以待后續(xù)實(shí)驗(yàn)分析.
圖1 采樣點(diǎn)分布
1.2 實(shí)驗(yàn)方法
1.2.1 理化性質(zhì)測定 上覆水理化指標(biāo),包括pH值、DO,在現(xiàn)場用YSI多參數(shù)水質(zhì)分析儀測定,流速采用流速儀測定,其他指標(biāo)低溫保存帶回實(shí)驗(yàn)室分析.沉積物pH值采用pH計(jì)測定,氧化還原電位(h)采用電極法測定;含水率(WC)于105℃下干燥測定[16];有機(jī)質(zhì)(OM)采用燒矢重法測定[17];粒徑采用激光粒度分析儀(Mastersizer 2000)測定,并將其分為3類:黏粒(0～0.002mm)、粉粒(0.002～0.02mm)和砂粒(0.02～2mm)[18].
1.2.2 沉積物磷賦存形態(tài)的測定 使用改進(jìn)的SEDEX法[8]將沉積物中的磷分為:松散結(jié)合磷(Loosely-P)、亞鐵磷(Fe(II)-P)、鐵/鋁結(jié)合磷(CDB-P)、鈣磷(Ca-P)和有機(jī)磷(OP)5類(圖2).提取液中溶解磷酸鹽使用鉬銻抗分光光度法測定[19].沉積物總磷(TPS)為5種形態(tài)磷之和.
1.2.3 等溫吸附實(shí)驗(yàn) EPC0的測定基于磷等溫吸附線.約1g(按干重)樣品分別與20mL含有濃度為0.0,0.5,1.0,5.0,10.0,15.0,25.0,50.0,75.0mg/L的磷酸鹽標(biāo)準(zhǔn)溶液(如KH2PO4)一起放入50mL聚乙烯離心管中(用于測定EPC0的新鮮沉積物需保證在7d內(nèi)分析,以減少樣品變質(zhì)[20])恒溫[(25±1)℃]下振蕩24h以達(dá)到平衡[21].通過測定濾液的磷濃度(EPC)與初始磷濃度的差值計(jì)算磷吸附量(SP).磷吸附等溫線由SP和EPC作圖得到.
圖2 SEDEX法流程
1.2.4 水溶性磷(WSP)濃度測定 稱取約1g(按干重)樣品于50mL含有25mL蒸餾水的聚乙烯離心管中,恒溫(25±1)℃下振蕩24h,離心后取上清液,測定濾液中磷酸鹽濃度,即為WSP[22].
1.2.5 磷吸附指數(shù)(PSI)[23]磷吸附指數(shù)計(jì)算公式如下:
式中:PSI表示磷吸附指數(shù),L/g;和分別表示1g沉積物于含有75mg P/L的20mL磷溶液在振蕩24h達(dá)到平衡后溶液中磷的吸附量和磷平衡濃度, mg/g,mg/L.
1.2.6 磷吸附飽和度(DPS)[24]磷吸附飽和度計(jì)算公式如下:
式中:DPS表示磷吸附飽和度,%;TPS為沉積物總磷含量, mg/g;SPmax為沉積物最大磷吸附量, mg/g,由1.2.3得出.
1.2.7 磷釋放風(fēng)險(xiǎn)指數(shù)(ERI) 用黃清輝等[25]提出的ERI來評(píng)估沉積物中磷的釋放風(fēng)險(xiǎn),計(jì)算公式如下:
式中: ERI為磷釋放風(fēng)險(xiǎn)指數(shù),%;DPS為磷吸附飽和度,%;PSI為磷吸附指數(shù),L/g.
1.3 數(shù)據(jù)處理
運(yùn)用Excel和Origin對(duì)數(shù)據(jù)進(jìn)行處理和圖表繪制,采用IBM SPSS Statistics22(IBM,USA)進(jìn)行數(shù)據(jù)的Pearson相關(guān)性分析.
2 結(jié)果與討論
2.1 上覆水與沉積物理化性質(zhì)分析
表1 上覆水和沉積物的理化性質(zhì)
注:DO為溶解氧;h為氧化還原電位;OM為有機(jī)質(zhì).
由表1可知,流域上覆水的pH值呈中性偏堿(7.78～9.27).水體DO含量變化范圍為4.60～8.30mg/ L.沉積物pH值整體呈中性偏弱堿性(6.79～9.27),氧化還原電位(h)變化范圍為-81～48mV,沉積物呈現(xiàn)弱缺氧狀態(tài).從上游至下游沉積物細(xì)顆粒(黏粒+粉粒)呈現(xiàn)增加的趨勢,其平均值分別為37.26%, 56.43%和61.89%,這可能因?yàn)樯嫌瘟魉?平均值為1.40m/s)比中下游(平均值分別為0.1, 0.19m/s)湍急.沉積物OM平均含量為7.41%(3.32%～12.16%),空間分布呈現(xiàn):上游(4.3%)<中游(7.87%)<下游(10.05%).上游OM含量分布均勻(3.32%～5.08%),中游S6處于干流上游,地處成都-資陽市界處,導(dǎo)致其OM含量高達(dá)12.16%.下游S12處OM含量出現(xiàn)最大值,高達(dá)12.14%,該處水庫水體流動(dòng)性較差,為OM的沉降和富集創(chuàng)造了有利條件.OM在空間上呈現(xiàn)出高度的差異性,與不同區(qū)域的污染程度以及沿程筑壩對(duì)OM的滯留有關(guān).
2.2 上覆水與沉積物磷形態(tài)空間分布特征
上覆水總磷(TPw)、溶解態(tài)總磷(DTP)和溶解反應(yīng)磷(SRP)均表現(xiàn)為從上游至下游降低的趨勢(圖3),變化范圍分別為0.08～0.23, 0.05～0.21和0.01～ 0.07mg/L.上游水體磷含量較高,與上游工業(yè)、城市點(diǎn)源污染以及農(nóng)業(yè)面源污染有關(guān),TPw最大值出現(xiàn)在上游S1點(diǎn)位,這可能是由于附近農(nóng)田以及污水處理廠的排放.此外,一般認(rèn)為,當(dāng)TPw濃度達(dá)到0.02mg/L時(shí),水體有可能會(huì)出現(xiàn)藻華[26].在本研究中,發(fā)現(xiàn)所有點(diǎn)位的TPw濃度均超過了這一臨界濃度,因此需要相關(guān)的措施來改善水中磷污染情況.
圖3 上覆水磷含量及空間分布
沉積物總磷(TPs)為1.84mg/g(0.84～2.85mg/g),空間上表現(xiàn)為:上游(2.45mg/g)>中游(1.12mg/g)>下游(1.12mg/g),TPs最大值出現(xiàn)在上游S1采樣點(diǎn),最小值出現(xiàn)在下游S9采樣點(diǎn)(圖4).由于流域上游土地利用類型以旱地、水田居多,沿河畜禽、水產(chǎn)養(yǎng)殖發(fā)達(dá),農(nóng)藥化肥及飼料使用頻繁,且污水收集處理設(shè)施不健全(如采樣點(diǎn)S2與S4均處于城鎮(zhèn)下游).與世界各地TPs含量(法國克魯茲河[27]:0.51～2.29mg/g;美國卡萊爾湖[28]:0.14～1.60mg/g;黃河[29]:0.20～0.75mg/g)相比,沱江流域TPs含量均高于以上研究區(qū)域,特別是上游河段.同時(shí),TPs濃度沿程降低,除河流水體磷的自然沉降過程外,梯級(jí)筑壩對(duì)水體磷污染物的攔截作用也不可忽略[30],Wang等[31]將沉積物的污染水平根據(jù)總磷的含量分為3級(jí),重污染水平:TPs>1.3mg/g;中等污染水平:0.5mg/g￡TPs￡1.3mg/g,輕度污染水平:TPs<0.5mg/g.本研究中TPs平均含量為1.84mg/g,遠(yuǎn)超沉積物重污染1.3mg/g水平線,并且TPs與TPw的空間分布相似,這表明沉積物中的磷有向上覆水體二次釋放的趨勢.
圖4 沉積物各形態(tài)磷的相對(duì)含量
圖5 沉積物磷含量及空間分布
圖6 各評(píng)價(jià)指標(biāo)空間分布
沉積物各形態(tài)磷在采樣點(diǎn)之間存在顯著差異(圖5),說明人類活動(dòng)對(duì)流域的影響.各形態(tài)磷含量占總磷百分比為:CDB-P(60.63%)>Ca-P(30.84%)>OP (3.92%)>Fe(Ⅱ)-P(3.48%)>Loosely-P (1.13%).CDB- P是沉積物磷形態(tài)的主要組分,主要指由鐵、鋁等金屬(氫)氧化物結(jié)合的磷,并能與OH-及有機(jī)配體進(jìn)行交換.因此,CDB-P很容易在堿性條件或還原環(huán)境下轉(zhuǎn)化為溶解態(tài)磷[32-33]而釋放到上覆水中,最終使水質(zhì)惡化.本研究中.CDB-P含量為0.468～2.287mg/g,從上游(1.648mg/g)至中游(1.268mg/g)、下游(0.557mg/g)平均含量逐漸降低,推測上游來水磷負(fù)荷輸入(S2、S3和S4點(diǎn)處的工業(yè)和生活污水)是影響沱江沉積物CDB-P含量的主要原因.
Ca-P主要是由自生磷灰石或碎屑巖等陸源輸入形成的一種穩(wěn)定態(tài)磷,由沉積物向上覆水釋放的可能性較小,受人類活動(dòng)影響不大.沉積物Ca-P含量空間變化與TPs一致,由上游(0.688mg/g) 逐漸降低至下游(0.422mg/g).沱江水體pH值總體偏堿性(7.78～9.27),上覆水中鈣離子容易吸附上覆水中的磷酸鹽從而形成Ca-P[34],導(dǎo)致在外部磷輸入較高的上中游沉積物中Ca-P含量比較高.
沉積物中的OP一般認(rèn)為來自水生動(dòng)植物殘?bào)w和農(nóng)業(yè)化肥,而其中一部分OP為不穩(wěn)定磷,容易隨沉積物有機(jī)物的分解礦化轉(zhuǎn)化成無機(jī)磷向上覆水中釋放.本研究沉積物OP含量變化范圍為0.019～ 0.076mg/g,其空間變化與OM一致,這與水生植物及周邊農(nóng)作物殘?bào)w沖入至中下游區(qū)域有關(guān).
Fe(Ⅱ)-P指沉積物中與磷相互作用形成的Fe(II)磷酸鹽相,是沉積物中重要的磷組分.沉積物中Fe(II)-P的穩(wěn)定性對(duì)pH值、h、離子強(qiáng)度和天然配體等環(huán)境因素高度敏感[35-36].盡管沱江沉積物中Fe(Ⅱ)-P含量為0.034～0.111mg/g,在整個(gè)磷組分占比較低(3.48%),但隨著pH值和h的降低,Fe(II)-P極易向上覆水中釋放,特別是在表層沉積物中[37-38],因此它在沉積物當(dāng)中也是一種不可忽略的磷組分.
Loosely-P作為無機(jī)磷重要的形態(tài)之一,用于浮游植物的消耗和生產(chǎn)循環(huán),極易由沉積物向上覆水體釋放,其釋放量主要取決于沉積物中的環(huán)境條件(如pH值、溫度、水動(dòng)力學(xué)和h).Loosely-P是本研究中含量最低的磷組分,變化范圍為0.007～ 0.030mg/g,在溫度升高或擾動(dòng)條件下容易向上覆水中釋放.由于各種形態(tài)磷(Loosely-P、Fe(Ⅱ)-P、CDB-P)的生物不穩(wěn)定性,因此需要高度關(guān)注這些形態(tài)的磷.
2.3 流域沉積物磷釋放風(fēng)險(xiǎn)評(píng)估
當(dāng)EPC0大于SRP時(shí),沉積物表現(xiàn)出釋放磷的趨勢,反之為吸附磷[39].本研究EPC0含量為0.012～ 0.084mg/L,上中下游平均值依次為0.069, 0.033和0.024mg/L.從空間分布來看,上游EPC0較高(圖6),并且其值均高于SRP,上游水體磷含量較高,導(dǎo)致沉積物-水界面達(dá)到磷吸附/解吸平衡時(shí)的EPC0值就越高.
ERI可將富營養(yǎng)化風(fēng)險(xiǎn)細(xì)分為高度風(fēng)險(xiǎn)(ERI>25%)、較高風(fēng)險(xiǎn)(20%
圖7 上覆水磷含量與各指標(biāo)線性回歸分析(n=12)
DPS是反映沉積物中磷吸附量占總磷吸附量百分比的指標(biāo),可用于評(píng)價(jià)沉積物對(duì)磷的吸附能力[42].一般認(rèn)為,較低的DPS表明沉積物中的磷吸附位點(diǎn)尚未飽和,沉積物具有較高的磷吸附能力[43],同時(shí)表明沉積物作為“匯”的可能性較大.本研究沉積物的磷吸附飽和度(DPS)為44.28%～80.39%,平均值為上游(73.71%)>中游(63.92%)>下游(54.17%),最大值在S1,最小值在S11,從空間分布上來看,上游DPS高于中下游,是由于上游污染源較多,沉積物累積了大量的外源磷輸入.其次,與我國閩江[41](7.30%～18.29%)相比,沱江流域DPS總體偏高,因此表明其沉積物具有較小的磷吸附能力和潛在的磷釋放風(fēng)險(xiǎn).
PSI表示可溶性磷酸鹽的固定能力,代表沉積物對(duì)磷的緩沖能力,一般隨著沉積物含量的增加而增加[44-45].本研究中,沉積物PSI為0.153～1.526L/g,空間變化與DPS相反,由上游至下游依次增加(平均值分別為0.208, 0.389, 0.727L/g),最大值出現(xiàn)在S9,最小值出現(xiàn)在S2,這是由于上游細(xì)顆粒相對(duì)較少,對(duì)磷的吸附能力較弱.另一方面,上游OM含量相對(duì)較低,而OM在一定程度上能很好的與磷絡(luò)合,并將其固定在沉積物中.
WSP主要是用蒸餾水可提取的磷,其值越高,表明磷的釋放風(fēng)險(xiǎn)就越高[46].已有研究將該指標(biāo)運(yùn)用于土壤磷的流失風(fēng)險(xiǎn)分析,對(duì)沉積物而言,WSP作為生物可利用磷而極易向上覆水中釋放.本研究WSP含量為19.294mg/L(9.496～26.190mg/L),表現(xiàn)出上游 >中游>下游的空間分布特征.上游細(xì)顆粒和OM含量占比較低,導(dǎo)致沉積物吸附的磷呈弱吸附態(tài)[47],極易向上覆水體釋放.
通過以上各指標(biāo)的綜合分析,表明沱江流上游沉積物具有較高的磷釋放風(fēng)險(xiǎn).
如圖7所示,相比ERI、DPS、PSI和WSP,EPC0與水中各形態(tài)磷相關(guān)性均最好,且EPC0與SRP的相關(guān)性(2=0.918,<0.001)顯著高于TPw(2=0.591,<0.05)和DTP(2=0.716,<0.05),表明EPC0能有效并準(zhǔn)確地評(píng)估沱江流域沉積物-水界面磷吸附-解吸狀態(tài),而DPS、PSI由于從沉積物飽和度和吸附能力去表征,沒有考慮磷的釋放風(fēng)險(xiǎn)[48],并且ERI作為磷釋放風(fēng)險(xiǎn)評(píng)價(jià)指標(biāo)在國內(nèi)外應(yīng)用并不廣泛.此外,通過比較EPC0和SRP,可以估算沉積物向水柱釋放SRP的潛能,當(dāng)水中SRP的濃度高于EPC0值時(shí),沉積物表現(xiàn)出吸附磷的趨勢,反之為釋放磷.EPC0越大,則釋放風(fēng)險(xiǎn)越高[49],進(jìn)一步對(duì)比分析上覆水中的SRP和EPC0濃度(圖8),上游EPC0均大于SRP,可能暗示上游沉積物磷釋放風(fēng)險(xiǎn)更大.
圖8 EPC0風(fēng)險(xiǎn)評(píng)估示意
黑色箭頭表示沉積物向上覆水釋放磷:EPC0>SRP,灰色箭頭表示沉積物吸附磷: EPC02.4 影響內(nèi)源磷釋放的沉積物組分
如表2所示:EPC0與CDB-P、OP、OM、粉粒和砂粒均存在極顯著的相關(guān)性,且相關(guān)性高于其他指標(biāo),表明沉積物磷釋放行為與這些組分密切相關(guān).
EPC0與CDB-P的極顯著正相關(guān)性(=0.78,<0.01)表明CDB-P是可能增加沉積物磷釋放風(fēng)險(xiǎn)的主要磷組分.一般來說,(氫)氧化物對(duì)鐵、鋁有很強(qiáng)的吸附能力,CDB-P可與OH-及其他在堿性條件下可溶解的無機(jī)磷化合物交換.CDB-P為本研究主要的磷形態(tài)之一,因此很容易受到環(huán)境的改變向上覆水中釋放.而鐵結(jié)合磷會(huì)隨著pH值的降低更多地向上覆水中釋放,隨后被初級(jí)生產(chǎn)者消耗[50].此外,當(dāng)氧氣充足時(shí),磷會(huì)被三價(jià)鐵吸收,但如果水處于缺氧狀態(tài),三價(jià)鐵會(huì)還原為二價(jià)鐵,導(dǎo)致磷和鐵從沉積物中釋放出來[38].
EPC0與OP、OM的極顯著負(fù)相關(guān)性(分別為-0.803、-0.746,<0.01)表明,OM對(duì)沉積物OP的吸附釋放有很大的影響.OP因分解緩慢而較為穩(wěn)定,與大型植物、浮游植物和陸生有機(jī)碎屑的沉積關(guān)系更密切[51],OM中的腐殖質(zhì)可以形成膠膜粘覆在粘土礦物、鐵、鋁氧化物以及碳酸鈣等無機(jī)物內(nèi)外表面,形成無機(jī)有機(jī)復(fù)合體,成為沉積物-水界面對(duì)磷遷移轉(zhuǎn)化的重要自然膠體.此外,OM釋放的氫離子可使礦物表面基團(tuán)質(zhì)子化而有利于磷的吸附,并且腐殖質(zhì)能和鐵/鋁等礦物形成有機(jī)或無機(jī)復(fù)合體,提供了重要的磷吸附位點(diǎn),從而增強(qiáng)了對(duì)磷的吸附[52],不易向上覆水中釋放.綜上表明:OM組分的增加將降低沱江沉積物OP的釋放風(fēng)險(xiǎn)[53].
此外,EPC0與粉粒、砂粒也呈極顯著相關(guān)性(分別為-0.727、0.719,<0.01).粒徑是影響沉積物磷釋放的重要因素,砂粒具有典型的很低的磷吸附點(diǎn)位,因?yàn)樗姆蔷B(tài)鐵和鋁濃度較低[54],吸附在上面的磷也容易向上覆水中釋放.
表2 理化參數(shù)間的相關(guān)性分析(n=12)
注:**在0.01級(jí)別(雙尾),相關(guān)性顯著;*在0.05級(jí)別(雙尾),相關(guān)性顯著.
3 結(jié)論
3.1 研究區(qū)TPw、DTP和SRP含量為分別為0.08～ 0.23, 0.05～0.21和0.01～0.07mg/L.水體污染程度呈現(xiàn)上游>中游>下游的空間分布特征,這可能是由于上游污染源較多,外源磷輸入導(dǎo)致水體總磷含量較高.
3.2 研究區(qū)TPs含量為0.84～2.85mg/g,空間分布呈現(xiàn)由上游向下游遞減的趨勢,上游TPs污染表現(xiàn)為重度污染(2.45mg/g).沉積物各形態(tài)磷含量為: CDB-P(60.63%)>Ca-P(30.84%)>OP(3.92%)>Fe (Ⅱ)-P(3.48%)>Loosely-P(1.13%),在水平空間分布上,CDB-P與TPs空間變化趨勢一致,這主要與上游工業(yè)污染有關(guān),表明沉積物吸附的磷主要以CDB-P的形式貯存在沉積物中,并且其含量越高,越容易向上覆水中釋放.
3.3 由EPC0、ERI、DPS、PSI和WSP綜合評(píng)估,沱江流域上游沉積物表現(xiàn)為較高的磷釋放風(fēng)險(xiǎn). EPC0與SRP、CDB-P、OP、OM以及粒徑均呈極顯著相關(guān)性(2分別為0.918, 0.780, -0.803, -0.746),且相關(guān)性遠(yuǎn)高于其他指標(biāo)(ERI,DPS,PSI, WSP),因此,EPC0是評(píng)估沱江流域沉積物磷釋放風(fēng)險(xiǎn)潛力更準(zhǔn)確高效的指標(biāo).同時(shí),EPC0與CDB-P、砂粒的極顯著正相關(guān)性,并與OP、OM、砂粒的極顯著負(fù)相關(guān)性表明了沱江流域沉積物磷主要受到Fe/Al金屬(氫)氧化物、OM及粒徑的控制.當(dāng)Fe/Al含量和砂粒增加以及OM含量減少時(shí),磷釋放風(fēng)險(xiǎn)會(huì)顯著增加,因此應(yīng)控制工業(yè)污染以及農(nóng)業(yè)面源污染的輸入.
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Spatial distribution characteristics and release risk assessment of phosphorus forms in sediments: A case study of the Tuojiang River Basin.
TANG Jin-yong1,2, YIN Yue-peng1,2, CAO Xi1,2, ZHANG Yu1,2, ZHANG Wen1,2,3*
(1.College of Environment and Ecology, Chengdu University of Technology, Chengdu 610059, China；2.State Environmental Protection Key Laboratory of Synergetic Control and Joint Remediation for Soil and Water Pollution (SEKL-SW), Chengdu University of Technology, Chengdu 610059, China；3.State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China)., 2022,42(9)：4264～4273
In order to clarify the spatial distribution characteristics of the phosphorus (P) in sediments and provide more accurate and suitable indicators for assessing P release risk, samples of surface sediments were taken from 12 sites in the main stream of the Tuojiang River and its tributaries for determining water-soluble phosphorus (WSP), equilibrium P concentration (EPC0), sediment P adsorption index (PSI) and adsorption saturation (DPS), and the derived P release risk index (ERI). The results show that the order of the P contents in the five forms of sediments is as follows: iron/aluminum combined P (CDB-P,60.63%) > calcium P (Ca-P, 30.84%) > organic P (OP, 3.92%) > ferrous P (Fe(Ⅱ)-P), 3.48%) > loosely P (Loosely-P, 1.13%). CDB-P is the main form of sediment P (0.468～2.287mg/g)and decreases gradually from the upstream to the downstream, which is mainly related to upstream industrial pollution. The spatial distribution of DPS, EPC0and PSI tends to gradually increase with downstream, varying from 44.28% to 80.39%, 0.012 to 0.084mg/L and 0.153 to 1.526L/g, respectively. ERI exceeded 25% at the most upstream sampling sites, indicating a higher risk of P release in the upstream. Regression analysis and correlation show that EPC0 and the overlying water P, CDB-P, OP, OM, and particle size were significantly correlated. Therefore, EPC0can be thought to be a more accurate and efficient indicator for assessing the potential of P release from sediments in the Tuojiang River Basin. An increase in Fe/Al content, particle size, and the reduction of organic matter will elevate the P release risk, so the input of industrial pollution and agricultural non-point source pollution should be controlled.
phosphorus forms；spatial distribution；phosphorus equilibrium concentration；phosphorus release risk
X171
A
1000-6923(2022)09-4264-10
2021-12-31
國家自然科學(xué)基金資助項(xiàng)目(42007148)
*責(zé)任作者, 教授, zhangwen2014@cdut.edu.cn
唐金勇(1995-),男,四川南充人,成都理工大學(xué)碩士研究生,主要從事水體與沉積物磷污染治理的研究.發(fā)表論文2篇.
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沉積物磷形態(tài)空間分布特征及釋放風(fēng)險(xiǎn)評(píng)估——以沱江流域?yàn)槔?/p>
唐金勇1,2,尹月鵬1,2,曹 熙1,2,張 瑜1,2,張 雯1,2,3*
(1.成都理工大學(xué)生態(tài)環(huán)境學(xué)院,四川 成都 610059；2.國家環(huán)境保護(hù)水土污染協(xié)同控制與聯(lián)合修復(fù)重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué)),四川 成都 610059；3.地質(zhì)災(zāi)害防治與地質(zhì)環(huán)境保護(hù)國家重點(diǎn)實(shí)驗(yàn)室(成都理工大學(xué)),四川 成都 610059)
為闡明沉積物磷賦存形態(tài)的空間分布特征及潛在釋放風(fēng)險(xiǎn),提供更準(zhǔn)確合適的風(fēng)險(xiǎn)評(píng)估指標(biāo), 分析了沱江干流及其支流12個(gè)樣點(diǎn)表層沉積物的磷賦存形態(tài),測定了水溶性磷(WSP)及磷平衡濃度(EPC0),計(jì)算沉積物磷吸附指數(shù)(PSI)、磷吸附飽和度(DPS)及其衍生的磷釋放風(fēng)險(xiǎn)指數(shù)(ERI).結(jié)果表明,沉積物5種形態(tài)磷含量順序?yàn)?鐵/鋁結(jié)合磷(CDB-P,60.63%)>鈣磷(Ca-P,30.84%)>有機(jī)磷(OP,3.92%)>亞鐵磷(Fe(Ⅱ)-P,3.48%)>松散態(tài)磷(Loosely-P,1.13%).CDB-P是沉積物磷的主要存在形態(tài)(0.468～2.287mg/g),由上游至下游逐漸降低,這主要與上游工業(yè)污染有關(guān).DPS、EPC0和PSI在空間分布上均呈現(xiàn)由上游至下游逐漸增大的趨勢,變化范圍分別為44.28%～80.39%、0.012～0.084mg/L和0.153～1.526L/g;上游大部分采樣點(diǎn)ERI均超過了25%;各指標(biāo)綜合表明:上游存在較高的磷釋放風(fēng)險(xiǎn).回歸分析與相關(guān)性表明,EPC0與上覆水磷、CDB-P、OP、有機(jī)質(zhì)(OM)以及粒徑均呈極顯著相關(guān)性,且相關(guān)性遠(yuǎn)高于其他指標(biāo)(ERI,DPS,PSI,WSP).因此,EPC0是評(píng)估沱江流域沉積物磷釋放風(fēng)險(xiǎn)潛力更準(zhǔn)確高效的指標(biāo),Fe/Al含量、粒徑的增加以及有機(jī)質(zhì)的減少會(huì)增加磷釋放風(fēng)險(xiǎn),因此應(yīng)控制工業(yè)污染以及農(nóng)業(yè)面源污染的輸入.
磷形態(tài)；空間分布；磷平衡濃度；磷釋放風(fēng)險(xiǎn)
沉積物是水生態(tài)系統(tǒng)中磷庫的重要組成部分,其作為營養(yǎng)鹽的“源”與“匯”[1],同時(shí)又是磷遷移轉(zhuǎn)化、再生的主要場所.磷是水體富營養(yǎng)化的重要限制因子[2-3],當(dāng)湖泊、河流等水體外源磷負(fù)荷減少,內(nèi)源磷負(fù)荷可能阻止其水質(zhì)好轉(zhuǎn)[4],并且成為水質(zhì)恢復(fù)延緩的重要原因之一[5].同時(shí),沉積物中磷的不同形態(tài)影響磷的生物有效性[6],因此研究沉積物中磷形態(tài)含量和變化特征,有助于了解河流污染程度及地球化學(xué)信息[7].改進(jìn)的SEDEX沉積物磷形態(tài)分離方法[8]將沉積物中的磷形態(tài)分為松散態(tài)磷(Loosely- P)、亞鐵磷(Fe(II)-P)、鐵/鋁結(jié)合磷(CDB-P)和有機(jī)磷(OP)5種形態(tài).該方法能將Fe(II)結(jié)合的磷單獨(dú)提取,研究表明,缺氧沉積物結(jié)合磷的潛力歸因于Fe(II)相的出現(xiàn),它們與磷直接相互作用形成Fe(II)磷酸鹽相,因此將Fe(II)-P分餾為一個(gè)獨(dú)立的磷組分,對(duì)獲取沉積物中磷的地球化學(xué)信息具有重要意義.
此外,國內(nèi)外學(xué)者對(duì)沉積物中磷的污染分布及風(fēng)險(xiǎn)評(píng)價(jià)進(jìn)行了大量的研究,而對(duì)于沉積物磷釋放風(fēng)險(xiǎn)評(píng)價(jià)指標(biāo)沒有統(tǒng)一的體系標(biāo)準(zhǔn)[9].目前,磷吸附指數(shù)(PSI)和磷吸附飽和度(DPS)常被用來表征土壤磷吸附容量[10-11].除此之外,水溶性磷(WSP)因其測試方法簡單快速也被用來評(píng)估土壤磷釋放風(fēng)險(xiǎn)[12].磷平衡濃度(EPC0)是指沉積物固相與周邊水溶液中的磷酸鹽(SRP)達(dá)到吸附與解吸平衡時(shí),水相中磷酸鹽的濃度[13-14].EPC0值越高,沉積物向水柱釋放磷的風(fēng)險(xiǎn)越大[13],廣泛用于沉積物而日益受到大家的關(guān)注.研究表明,EPC0和生物有效磷存在明顯的相關(guān)性,EPC0越高,往往生物有效磷也越高,對(duì)評(píng)價(jià)沉積物磷釋放風(fēng)險(xiǎn)有一定的參考價(jià)值[15],因此沉積物充當(dāng)水體磷“源”或“匯”的功能可由EPC0估計(jì).
本研究以四川省沱江流域(綿遠(yuǎn)河新源大橋至流灘壩水電站,104°25′～105°23′E,28°55′～31°13′N)為研究對(duì)象,系統(tǒng)分析了沉積物的磷賦存形態(tài)及其含量的空間分布特征,分析了EPC0和ERI等指標(biāo),旨在深入揭示沉積物磷吸附行為的變化特征及其環(huán)境意義,闡明磷釋放風(fēng)險(xiǎn),為其提供最佳的沉積物磷釋放風(fēng)險(xiǎn)評(píng)估指標(biāo).
1 材料與方法
1.1 樣品采集與處理
根據(jù)沱江流域水系特點(diǎn),選擇了12個(gè)均勻分布于沱江上中下游具有代表性的點(diǎn)位(圖1).采用彼得森抓斗式采泥器采取表層沉積物,采集后將樣品置于密封的聚乙烯塑料袋中,一部分于4℃左右儲(chǔ)存在實(shí)驗(yàn)室用于沉積物磷賦存形態(tài)及EPC0的測定,另一部分于-20℃下冷凍干燥研磨后儲(chǔ)存于干燥器以待后續(xù)實(shí)驗(yàn)分析.
圖1 采樣點(diǎn)分布
1.2 實(shí)驗(yàn)方法
1.2.1 理化性質(zhì)測定 上覆水理化指標(biāo),包括pH值、DO,在現(xiàn)場用YSI多參數(shù)水質(zhì)分析儀測定,流速采用流速儀測定,其他指標(biāo)低溫保存帶回實(shí)驗(yàn)室分析.沉積物pH值采用pH計(jì)測定,氧化還原電位(h)采用電極法測定;含水率(WC)于105℃下干燥測定[16];有機(jī)質(zhì)(OM)采用燒矢重法測定[17];粒徑采用激光粒度分析儀(Mastersizer 2000)測定,并將其分為3類:黏粒(0～0.002mm)、粉粒(0.002～0.02mm)和砂粒(0.02～2mm)[18].
1.2.2 沉積物磷賦存形態(tài)的測定 使用改進(jìn)的SEDEX法[8]將沉積物中的磷分為:松散結(jié)合磷(Loosely-P)、亞鐵磷(Fe(II)-P)、鐵/鋁結(jié)合磷(CDB-P)、鈣磷(Ca-P)和有機(jī)磷(OP)5類(圖2).提取液中溶解磷酸鹽使用鉬銻抗分光光度法測定[19].沉積物總磷(TPS)為5種形態(tài)磷之和.
1.2.3 等溫吸附實(shí)驗(yàn) EPC0的測定基于磷等溫吸附線.約1g(按干重)樣品分別與20mL含有濃度為0.0,0.5,1.0,5.0,10.0,15.0,25.0,50.0,75.0mg/L的磷酸鹽標(biāo)準(zhǔn)溶液(如KH2PO4)一起放入50mL聚乙烯離心管中(用于測定EPC0的新鮮沉積物需保證在7d內(nèi)分析,以減少樣品變質(zhì)[20])恒溫[(25±1)℃]下振蕩24h以達(dá)到平衡[21].通過測定濾液的磷濃度(EPC)與初始磷濃度的差值計(jì)算磷吸附量(SP).磷吸附等溫線由SP和EPC作圖得到.
圖2 SEDEX法流程
1.2.4 水溶性磷(WSP)濃度測定 稱取約1g(按干重)樣品于50mL含有25mL蒸餾水的聚乙烯離心管中,恒溫(25±1)℃下振蕩24h,離心后取上清液,測定濾液中磷酸鹽濃度,即為WSP[22].
1.2.5 磷吸附指數(shù)(PSI)[23]磷吸附指數(shù)計(jì)算公式如下:
式中:PSI表示磷吸附指數(shù),L/g;和分別表示1g沉積物于含有75mg P/L的20mL磷溶液在振蕩24h達(dá)到平衡后溶液中磷的吸附量和磷平衡濃度, mg/g,mg/L.
1.2.6 磷吸附飽和度(DPS)[24]磷吸附飽和度計(jì)算公式如下:
式中:DPS表示磷吸附飽和度,%;TPS為沉積物總磷含量, mg/g;SPmax為沉積物最大磷吸附量, mg/g,由1.2.3得出.
1.2.7 磷釋放風(fēng)險(xiǎn)指數(shù)(ERI) 用黃清輝等[25]提出的ERI來評(píng)估沉積物中磷的釋放風(fēng)險(xiǎn),計(jì)算公式如下:
式中: ERI為磷釋放風(fēng)險(xiǎn)指數(shù),%;DPS為磷吸附飽和度,%;PSI為磷吸附指數(shù),L/g.
1.3 數(shù)據(jù)處理
運(yùn)用Excel和Origin對(duì)數(shù)據(jù)進(jìn)行處理和圖表繪制,采用IBM SPSS Statistics22(IBM,USA)進(jìn)行數(shù)據(jù)的Pearson相關(guān)性分析.
2 結(jié)果與討論
2.1 上覆水與沉積物理化性質(zhì)分析
表1 上覆水和沉積物的理化性質(zhì)
注:DO為溶解氧;h為氧化還原電位;OM為有機(jī)質(zhì).
由表1可知,流域上覆水的pH值呈中性偏堿(7.78～9.27).水體DO含量變化范圍為4.60～8.30mg/ L.沉積物pH值整體呈中性偏弱堿性(6.79～9.27),氧化還原電位(h)變化范圍為-81～48mV,沉積物呈現(xiàn)弱缺氧狀態(tài).從上游至下游沉積物細(xì)顆粒(黏粒+粉粒)呈現(xiàn)增加的趨勢,其平均值分別為37.26%, 56.43%和61.89%,這可能因?yàn)樯嫌瘟魉?平均值為1.40m/s)比中下游(平均值分別為0.1, 0.19m/s)湍急.沉積物OM平均含量為7.41%(3.32%～12.16%),空間分布呈現(xiàn):上游(4.3%)<中游(7.87%)<下游(10.05%).上游OM含量分布均勻(3.32%～5.08%),中游S6處于干流上游,地處成都-資陽市界處,導(dǎo)致其OM含量高達(dá)12.16%.下游S12處OM含量出現(xiàn)最大值,高達(dá)12.14%,該處水庫水體流動(dòng)性較差,為OM的沉降和富集創(chuàng)造了有利條件.OM在空間上呈現(xiàn)出高度的差異性,與不同區(qū)域的污染程度以及沿程筑壩對(duì)OM的滯留有關(guān).
2.2 上覆水與沉積物磷形態(tài)空間分布特征
上覆水總磷(TPw)、溶解態(tài)總磷(DTP)和溶解反應(yīng)磷(SRP)均表現(xiàn)為從上游至下游降低的趨勢(圖3),變化范圍分別為0.08～0.23, 0.05～0.21和0.01～ 0.07mg/L.上游水體磷含量較高,與上游工業(yè)、城市點(diǎn)源污染以及農(nóng)業(yè)面源污染有關(guān),TPw最大值出現(xiàn)在上游S1點(diǎn)位,這可能是由于附近農(nóng)田以及污水處理廠的排放.此外,一般認(rèn)為,當(dāng)TPw濃度達(dá)到0.02mg/L時(shí),水體有可能會(huì)出現(xiàn)藻華[26].在本研究中,發(fā)現(xiàn)所有點(diǎn)位的TPw濃度均超過了這一臨界濃度,因此需要相關(guān)的措施來改善水中磷污染情況.
圖3 上覆水磷含量及空間分布
沉積物總磷(TPs)為1.84mg/g(0.84～2.85mg/g),空間上表現(xiàn)為:上游(2.45mg/g)>中游(1.12mg/g)>下游(1.12mg/g),TPs最大值出現(xiàn)在上游S1采樣點(diǎn),最小值出現(xiàn)在下游S9采樣點(diǎn)(圖4).由于流域上游土地利用類型以旱地、水田居多,沿河畜禽、水產(chǎn)養(yǎng)殖發(fā)達(dá),農(nóng)藥化肥及飼料使用頻繁,且污水收集處理設(shè)施不健全(如采樣點(diǎn)S2與S4均處于城鎮(zhèn)下游).與世界各地TPs含量(法國克魯茲河[27]:0.51～2.29mg/g;美國卡萊爾湖[28]:0.14～1.60mg/g;黃河[29]:0.20～0.75mg/g)相比,沱江流域TPs含量均高于以上研究區(qū)域,特別是上游河段.同時(shí),TPs濃度沿程降低,除河流水體磷的自然沉降過程外,梯級(jí)筑壩對(duì)水體磷污染物的攔截作用也不可忽略[30],Wang等[31]將沉積物的污染水平根據(jù)總磷的含量分為3級(jí),重污染水平:TPs>1.3mg/g;中等污染水平:0.5mg/g￡TPs￡1.3mg/g,輕度污染水平:TPs<0.5mg/g.本研究中TPs平均含量為1.84mg/g,遠(yuǎn)超沉積物重污染1.3mg/g水平線,并且TPs與TPw的空間分布相似,這表明沉積物中的磷有向上覆水體二次釋放的趨勢.
圖4 沉積物各形態(tài)磷的相對(duì)含量
圖5 沉積物磷含量及空間分布
圖6 各評(píng)價(jià)指標(biāo)空間分布
沉積物各形態(tài)磷在采樣點(diǎn)之間存在顯著差異(圖5),說明人類活動(dòng)對(duì)流域的影響.各形態(tài)磷含量占總磷百分比為:CDB-P(60.63%)>Ca-P(30.84%)>OP (3.92%)>Fe(Ⅱ)-P(3.48%)>Loosely-P (1.13%).CDB- P是沉積物磷形態(tài)的主要組分,主要指由鐵、鋁等金屬(氫)氧化物結(jié)合的磷,并能與OH-及有機(jī)配體進(jìn)行交換.因此,CDB-P很容易在堿性條件或還原環(huán)境下轉(zhuǎn)化為溶解態(tài)磷[32-33]而釋放到上覆水中,最終使水質(zhì)惡化.本研究中.CDB-P含量為0.468～2.287mg/g,從上游(1.648mg/g)至中游(1.268mg/g)、下游(0.557mg/g)平均含量逐漸降低,推測上游來水磷負(fù)荷輸入(S2、S3和S4點(diǎn)處的工業(yè)和生活污水)是影響沱江沉積物CDB-P含量的主要原因.
Ca-P主要是由自生磷灰石或碎屑巖等陸源輸入形成的一種穩(wěn)定態(tài)磷,由沉積物向上覆水釋放的可能性較小,受人類活動(dòng)影響不大.沉積物Ca-P含量空間變化與TPs一致,由上游(0.688mg/g) 逐漸降低至下游(0.422mg/g).沱江水體pH值總體偏堿性(7.78～9.27),上覆水中鈣離子容易吸附上覆水中的磷酸鹽從而形成Ca-P[34],導(dǎo)致在外部磷輸入較高的上中游沉積物中Ca-P含量比較高.
沉積物中的OP一般認(rèn)為來自水生動(dòng)植物殘?bào)w和農(nóng)業(yè)化肥,而其中一部分OP為不穩(wěn)定磷,容易隨沉積物有機(jī)物的分解礦化轉(zhuǎn)化成無機(jī)磷向上覆水中釋放.本研究沉積物OP含量變化范圍為0.019～ 0.076mg/g,其空間變化與OM一致,這與水生植物及周邊農(nóng)作物殘?bào)w沖入至中下游區(qū)域有關(guān).
Fe(Ⅱ)-P指沉積物中與磷相互作用形成的Fe(II)磷酸鹽相,是沉積物中重要的磷組分.沉積物中Fe(II)-P的穩(wěn)定性對(duì)pH值、h、離子強(qiáng)度和天然配體等環(huán)境因素高度敏感[35-36].盡管沱江沉積物中Fe(Ⅱ)-P含量為0.034～0.111mg/g,在整個(gè)磷組分占比較低(3.48%),但隨著pH值和h的降低,Fe(II)-P極易向上覆水中釋放,特別是在表層沉積物中[37-38],因此它在沉積物當(dāng)中也是一種不可忽略的磷組分.
Loosely-P作為無機(jī)磷重要的形態(tài)之一,用于浮游植物的消耗和生產(chǎn)循環(huán),極易由沉積物向上覆水體釋放,其釋放量主要取決于沉積物中的環(huán)境條件(如pH值、溫度、水動(dòng)力學(xué)和h).Loosely-P是本研究中含量最低的磷組分,變化范圍為0.007～ 0.030mg/g,在溫度升高或擾動(dòng)條件下容易向上覆水中釋放.由于各種形態(tài)磷(Loosely-P、Fe(Ⅱ)-P、CDB-P)的生物不穩(wěn)定性,因此需要高度關(guān)注這些形態(tài)的磷.
2.3 流域沉積物磷釋放風(fēng)險(xiǎn)評(píng)估
當(dāng)EPC0大于SRP時(shí),沉積物表現(xiàn)出釋放磷的趨勢,反之為吸附磷[39].本研究EPC0含量為0.012～ 0.084mg/L,上中下游平均值依次為0.069, 0.033和0.024mg/L.從空間分布來看,上游EPC0較高(圖6),并且其值均高于SRP,上游水體磷含量較高,導(dǎo)致沉積物-水界面達(dá)到磷吸附/解吸平衡時(shí)的EPC0值就越高.
ERI可將富營養(yǎng)化風(fēng)險(xiǎn)細(xì)分為高度風(fēng)險(xiǎn)(ERI>25%)、較高風(fēng)險(xiǎn)(20% 圖7 上覆水磷含量與各指標(biāo)線性回歸分析(n=12) DPS是反映沉積物中磷吸附量占總磷吸附量百分比的指標(biāo),可用于評(píng)價(jià)沉積物對(duì)磷的吸附能力[42].一般認(rèn)為,較低的DPS表明沉積物中的磷吸附位點(diǎn)尚未飽和,沉積物具有較高的磷吸附能力[43],同時(shí)表明沉積物作為“匯”的可能性較大.本研究沉積物的磷吸附飽和度(DPS)為44.28%～80.39%,平均值為上游(73.71%)>中游(63.92%)>下游(54.17%),最大值在S1,最小值在S11,從空間分布上來看,上游DPS高于中下游,是由于上游污染源較多,沉積物累積了大量的外源磷輸入.其次,與我國閩江[41](7.30%～18.29%)相比,沱江流域DPS總體偏高,因此表明其沉積物具有較小的磷吸附能力和潛在的磷釋放風(fēng)險(xiǎn). PSI表示可溶性磷酸鹽的固定能力,代表沉積物對(duì)磷的緩沖能力,一般隨著沉積物含量的增加而增加[44-45].本研究中,沉積物PSI為0.153～1.526L/g,空間變化與DPS相反,由上游至下游依次增加(平均值分別為0.208, 0.389, 0.727L/g),最大值出現(xiàn)在S9,最小值出現(xiàn)在S2,這是由于上游細(xì)顆粒相對(duì)較少,對(duì)磷的吸附能力較弱.另一方面,上游OM含量相對(duì)較低,而OM在一定程度上能很好的與磷絡(luò)合,并將其固定在沉積物中. WSP主要是用蒸餾水可提取的磷,其值越高,表明磷的釋放風(fēng)險(xiǎn)就越高[46].已有研究將該指標(biāo)運(yùn)用于土壤磷的流失風(fēng)險(xiǎn)分析,對(duì)沉積物而言,WSP作為生物可利用磷而極易向上覆水中釋放.本研究WSP含量為19.294mg/L(9.496～26.190mg/L),表現(xiàn)出上游 >中游>下游的空間分布特征.上游細(xì)顆粒和OM含量占比較低,導(dǎo)致沉積物吸附的磷呈弱吸附態(tài)[47],極易向上覆水體釋放. 通過以上各指標(biāo)的綜合分析,表明沱江流上游沉積物具有較高的磷釋放風(fēng)險(xiǎn). 如圖7所示,相比ERI、DPS、PSI和WSP,EPC0與水中各形態(tài)磷相關(guān)性均最好,且EPC0與SRP的相關(guān)性(2=0.918,<0.001)顯著高于TPw(2=0.591,<0.05)和DTP(2=0.716,<0.05),表明EPC0能有效并準(zhǔn)確地評(píng)估沱江流域沉積物-水界面磷吸附-解吸狀態(tài),而DPS、PSI由于從沉積物飽和度和吸附能力去表征,沒有考慮磷的釋放風(fēng)險(xiǎn)[48],并且ERI作為磷釋放風(fēng)險(xiǎn)評(píng)價(jià)指標(biāo)在國內(nèi)外應(yīng)用并不廣泛.此外,通過比較EPC0和SRP,可以估算沉積物向水柱釋放SRP的潛能,當(dāng)水中SRP的濃度高于EPC0值時(shí),沉積物表現(xiàn)出吸附磷的趨勢,反之為釋放磷.EPC0越大,則釋放風(fēng)險(xiǎn)越高[49],進(jìn)一步對(duì)比分析上覆水中的SRP和EPC0濃度(圖8),上游EPC0均大于SRP,可能暗示上游沉積物磷釋放風(fēng)險(xiǎn)更大. 圖8 EPC0風(fēng)險(xiǎn)評(píng)估示意 黑色箭頭表示沉積物向上覆水釋放磷:EPC0>SRP,灰色箭頭表示沉積物吸附磷: EPC0 如表2所示:EPC0與CDB-P、OP、OM、粉粒和砂粒均存在極顯著的相關(guān)性,且相關(guān)性高于其他指標(biāo),表明沉積物磷釋放行為與這些組分密切相關(guān). EPC0與CDB-P的極顯著正相關(guān)性(=0.78,<0.01)表明CDB-P是可能增加沉積物磷釋放風(fēng)險(xiǎn)的主要磷組分.一般來說,(氫)氧化物對(duì)鐵、鋁有很強(qiáng)的吸附能力,CDB-P可與OH-及其他在堿性條件下可溶解的無機(jī)磷化合物交換.CDB-P為本研究主要的磷形態(tài)之一,因此很容易受到環(huán)境的改變向上覆水中釋放.而鐵結(jié)合磷會(huì)隨著pH值的降低更多地向上覆水中釋放,隨后被初級(jí)生產(chǎn)者消耗[50].此外,當(dāng)氧氣充足時(shí),磷會(huì)被三價(jià)鐵吸收,但如果水處于缺氧狀態(tài),三價(jià)鐵會(huì)還原為二價(jià)鐵,導(dǎo)致磷和鐵從沉積物中釋放出來[38]. EPC0與OP、OM的極顯著負(fù)相關(guān)性(分別為-0.803、-0.746,<0.01)表明,OM對(duì)沉積物OP的吸附釋放有很大的影響.OP因分解緩慢而較為穩(wěn)定,與大型植物、浮游植物和陸生有機(jī)碎屑的沉積關(guān)系更密切[51],OM中的腐殖質(zhì)可以形成膠膜粘覆在粘土礦物、鐵、鋁氧化物以及碳酸鈣等無機(jī)物內(nèi)外表面,形成無機(jī)有機(jī)復(fù)合體,成為沉積物-水界面對(duì)磷遷移轉(zhuǎn)化的重要自然膠體.此外,OM釋放的氫離子可使礦物表面基團(tuán)質(zhì)子化而有利于磷的吸附,并且腐殖質(zhì)能和鐵/鋁等礦物形成有機(jī)或無機(jī)復(fù)合體,提供了重要的磷吸附位點(diǎn),從而增強(qiáng)了對(duì)磷的吸附[52],不易向上覆水中釋放.綜上表明:OM組分的增加將降低沱江沉積物OP的釋放風(fēng)險(xiǎn)[53]. 此外,EPC0與粉粒、砂粒也呈極顯著相關(guān)性(分別為-0.727、0.719,<0.01).粒徑是影響沉積物磷釋放的重要因素,砂粒具有典型的很低的磷吸附點(diǎn)位,因?yàn)樗姆蔷B(tài)鐵和鋁濃度較低[54],吸附在上面的磷也容易向上覆水中釋放. 表2 理化參數(shù)間的相關(guān)性分析(n=12) 注:**在0.01級(jí)別(雙尾),相關(guān)性顯著;*在0.05級(jí)別(雙尾),相關(guān)性顯著. 3.1 研究區(qū)TPw、DTP和SRP含量為分別為0.08～ 0.23, 0.05～0.21和0.01～0.07mg/L.水體污染程度呈現(xiàn)上游>中游>下游的空間分布特征,這可能是由于上游污染源較多,外源磷輸入導(dǎo)致水體總磷含量較高. 3.2 研究區(qū)TPs含量為0.84～2.85mg/g,空間分布呈現(xiàn)由上游向下游遞減的趨勢,上游TPs污染表現(xiàn)為重度污染(2.45mg/g).沉積物各形態(tài)磷含量為: CDB-P(60.63%)>Ca-P(30.84%)>OP(3.92%)>Fe (Ⅱ)-P(3.48%)>Loosely-P(1.13%),在水平空間分布上,CDB-P與TPs空間變化趨勢一致,這主要與上游工業(yè)污染有關(guān),表明沉積物吸附的磷主要以CDB-P的形式貯存在沉積物中,并且其含量越高,越容易向上覆水中釋放. 3.3 由EPC0、ERI、DPS、PSI和WSP綜合評(píng)估,沱江流域上游沉積物表現(xiàn)為較高的磷釋放風(fēng)險(xiǎn). EPC0與SRP、CDB-P、OP、OM以及粒徑均呈極顯著相關(guān)性(2分別為0.918, 0.780, -0.803, -0.746),且相關(guān)性遠(yuǎn)高于其他指標(biāo)(ERI,DPS,PSI, WSP),因此,EPC0是評(píng)估沱江流域沉積物磷釋放風(fēng)險(xiǎn)潛力更準(zhǔn)確高效的指標(biāo).同時(shí),EPC0與CDB-P、砂粒的極顯著正相關(guān)性,并與OP、OM、砂粒的極顯著負(fù)相關(guān)性表明了沱江流域沉積物磷主要受到Fe/Al金屬(氫)氧化物、OM及粒徑的控制.當(dāng)Fe/Al含量和砂粒增加以及OM含量減少時(shí),磷釋放風(fēng)險(xiǎn)會(huì)顯著增加,因此應(yīng)控制工業(yè)污染以及農(nóng)業(yè)面源污染的輸入. [1] Sun C, Xiong W, Zhang W, et al. New insights into identifying sediment phosphorus sources in river-lake coupled system: A framework for optimizing microbial community fingerprints [J]. Environmental Research, 2022,209:112854. [2] Hamlin Q F, Kendall A D, Martin S L, et al. Quantifying landscape nutrient iinputs with spatially explicit nutrient source estimate maps [J]. Journal of Geophysical Research: Biogeosciences, 2020,125(2):5134. [3] Huang C, Lin Y, Hao Y, et al. Variation pattern of particulate organic carbon and nitrogen in oceans and inland waters [J]. 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Phosphorus sorption dynamics in soils and coupling with surface and pore water in riverine wetlands [J]. Soil Science Society of America Journal, 2001,65(2):577-588. Spatial distribution characteristics and release risk assessment of phosphorus forms in sediments: A case study of the Tuojiang River Basin. TANG Jin-yong1,2, YIN Yue-peng1,2, CAO Xi1,2, ZHANG Yu1,2, ZHANG Wen1,2,3* (1.College of Environment and Ecology, Chengdu University of Technology, Chengdu 610059, China；2.State Environmental Protection Key Laboratory of Synergetic Control and Joint Remediation for Soil and Water Pollution (SEKL-SW), Chengdu University of Technology, Chengdu 610059, China；3.State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu 610059, China)., 2022,42(9)：4264～4273 In order to clarify the spatial distribution characteristics of the phosphorus (P) in sediments and provide more accurate and suitable indicators for assessing P release risk, samples of surface sediments were taken from 12 sites in the main stream of the Tuojiang River and its tributaries for determining water-soluble phosphorus (WSP), equilibrium P concentration (EPC0), sediment P adsorption index (PSI) and adsorption saturation (DPS), and the derived P release risk index (ERI). The results show that the order of the P contents in the five forms of sediments is as follows: iron/aluminum combined P (CDB-P,60.63%) > calcium P (Ca-P, 30.84%) > organic P (OP, 3.92%) > ferrous P (Fe(Ⅱ)-P), 3.48%) > loosely P (Loosely-P, 1.13%). CDB-P is the main form of sediment P (0.468～2.287mg/g)and decreases gradually from the upstream to the downstream, which is mainly related to upstream industrial pollution. The spatial distribution of DPS, EPC0and PSI tends to gradually increase with downstream, varying from 44.28% to 80.39%, 0.012 to 0.084mg/L and 0.153 to 1.526L/g, respectively. ERI exceeded 25% at the most upstream sampling sites, indicating a higher risk of P release in the upstream. Regression analysis and correlation show that EPC0 and the overlying water P, CDB-P, OP, OM, and particle size were significantly correlated. Therefore, EPC0can be thought to be a more accurate and efficient indicator for assessing the potential of P release from sediments in the Tuojiang River Basin. An increase in Fe/Al content, particle size, and the reduction of organic matter will elevate the P release risk, so the input of industrial pollution and agricultural non-point source pollution should be controlled. phosphorus forms；spatial distribution；phosphorus equilibrium concentration；phosphorus release risk X171 A 1000-6923(2022)09-4264-10 2021-12-31 國家自然科學(xué)基金資助項(xiàng)目(42007148) *責(zé)任作者, 教授, zhangwen2014@cdut.edu.cn 唐金勇(1995-),男,四川南充人,成都理工大學(xué)碩士研究生,主要從事水體與沉積物磷污染治理的研究.發(fā)表論文2篇.2.4 影響內(nèi)源磷釋放的沉積物組分
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