天然紅土與苦草聯(lián)合控制沉積物中磷釋放

張 瑜,尹月鵬,唐金勇,曹 熙,劉晶京,張 雯,3*

天然紅土與苦草聯(lián)合控制沉積物中磷釋放

張 瑜1,2,尹月鵬1,2,唐金勇1,2,曹 熙1,2,劉晶京1,2,張 雯1,2,3*

(1.成都理工大學生態(tài)環(huán)境學院,四川 成都 610059；2.國家環(huán)境保護水土污染協(xié)同控制與聯(lián)合修復重點實驗室,四川 成都 610059；3.地質災害防治與地質環(huán)境保護國家重點實驗室,四川 成都 610059)

運用天然紅土和苦草(RS +VS)的原位聯(lián)合修復技術,探究單一及聯(lián)合修復對污染底泥磷去除的影響.結果表明,RS與VS聯(lián)合對沉積物P的去除能力高于兩者單一修復,在37d的批量實驗中,覆蓋RS+VS組沉積物釋放到上覆水中的磷被抑制了91%,與無覆蓋RS+VS組的底泥釋放磷相比,上覆水中溶解態(tài)活性磷(SRP)從1.41mg/L降至0.12mg/L. RS+VS聯(lián)合修復對沉積物磷的固定作用顯著,將不穩(wěn)定的亞鐵磷(Fe(II)-P)和鐵鋁結合磷(CDB-P)轉變成惰性的鈣磷(Ca-P),沉積物中的Ca-P含量增加了51%,Fe(II)-P ,CDB-P分別降低了1%和24%,有效降低了底泥磷釋放到上覆水的風險.綜上,RS+VS聯(lián)合可以應用于處理富營養(yǎng)化水域的內部磷負荷,實現兩者協(xié)同去除沉積物磷,同時RS和VS兩者價廉且廣泛分布可作為一種潛在高效益的磷酸鹽吸附劑應用于實際工程中.

黑臭底泥；紅土；苦草；磷形態(tài)；沉積物；影響因素

水體營養(yǎng)化是由于養(yǎng)分的過量輸入而引起的全球性問題[1-3].沉積物被認為是養(yǎng)分的匯集地,在富營養(yǎng)化中起著至關重要的作用[4-6].磷在上覆水體和沉積物之間(即內部磷負荷)進行著持續(xù)循環(huán),當外源磷輸入得到一定控制后,沉積物中的內源磷釋放仍然將可能維持數十年,延緩了受損水生態(tài)系統(tǒng)的恢復[7-10].因此通過抑制水體內源負荷的釋放,降低水體中磷的濃度,對于防止水體富營養(yǎng)化的發(fā)生具有重要意義.

近年來,為控制富營養(yǎng)化開發(fā)了許多修復的技術和策略,這些包括減少外部污染源如:攔截點源污染、建設污水處理廠[11]、面源控制[12]、濕地處理技術[13]流域綜合管理.內部負荷控制如:沉積物疏浚[14]、曝氣、原位覆蓋[15-17]、生物技術[18];生態(tài)修復如水生植物修復技術[19-21]生物操縱[22]等.由于底泥疏浚成本高[23],使用化學產品可能造成生態(tài)風險[24],原位覆蓋和生態(tài)修復是應用最廣泛的兩個緩解策略.原位覆蓋的吸附劑使用天然材料包括沸石[17]、凹凸棒土[25]、方解石[26]、紅土[27-30]、膨潤土[31]等,其中紅土富含鋁和鐵的氧化物,而鐵在磷酸鹽吸附中有著關鍵作用.氧化鐵具有低晶體尺寸的顆粒,會形成強鍵位點,用于磷酸鹽物質的表面絡合[32],使得紅土的吸收能力高于其他天然礦物的吸收能力[29].且紅土是一種價廉分布廣泛的土壤[30],無生物毒性,能為沉水植物提供更好的生長環(huán)境.另一方面,沉水植物在減少沉積物再懸浮和控制內源磷的釋放方面有著關鍵作用[20,33-34].苦草是一種一年或半年生的植物,具有大量根[35-36],它可以通過根部從沉積物中吸收大量養(yǎng)分[37],然后通過吸收養(yǎng)分、穩(wěn)定沉積物、庇護過濾藻類的浮游動物以及與浮游植物競爭光和養(yǎng)分等方式來改善水質[38-39].但沉水植物對沉積物磷的處理效果往往受到生長周期和周圍環(huán)境的限制,單一的修復技術不能有效和普遍的使用,迫切需要一種組合技術來處理沉積物磷.

基于此,為進一步增強沉水植物對沉積物磷的修復效果,彌補應用單一技術的不足,本研究開發(fā)了原位覆蓋與沉水植物相結合的技術,采用天然紅土和苦草聯(lián)合修復作為一種生態(tài)環(huán)境材料,具有廣泛的推廣價值和應用前景[28],且原位覆蓋和生物修復的聯(lián)合作用[33,40-41]可能是下一代緩解富營養(yǎng)化商業(yè)產品的重要方向.

1 材料與方法

1.1 采集與處理

表1 沉積物上覆水的理化性質

實驗所用紅土來自四川省龍泉驛區(qū)(30.55658°N,104.27462°E).先去除其中較大顆粒植物殘渣,再將天然紅土自然風干,粉碎機粉碎到顆粒直徑為3～5mm.沉水植物苦草采自湖北荊州,采集的苦草長勢好并且新鮮,將苦草沖洗干凈,移除無脊椎食草動物,并在實驗室里培育7d,均剪到20cm長以符合實驗要求的高度.采集了四川省成都市一條典型的城市黑臭水體十陵河的沉積物和上覆水,同時進行相關理化性質分析,分析結果見表1.

1.2 覆蓋實驗設計

1.2.1 吸附實驗 研究紅土劑量、反應溫度和pH值對磷釋放的影響以探究紅土覆蓋的最佳條件.在錐形瓶中進行動態(tài)吸附實驗,其中裝有150mL,0.02mol/L氯化鉀溶液,10g沉積物樣品和不同劑量(0.5,2,4,5,7,9g)的RS .恒溫調節(jié)pH值(5,6,7,8,9,10),不同溫度(10,20,30,40,50℃),加入兩滴0.1%氯仿抑制細菌活性的條件下進行實驗研究.控制振動速率為200r/min下振蕩24h后離心,測量上覆水的溶解態(tài)活性磷(SRP)濃度.每個處理由3個重復組成,數據均表示為平均值.

1.2.2 RS和VS對底泥磷釋放的影響 放置10cm厚的沉積物到實驗桶(聚丙烯材質的塑料收納桶,長70.5cm、寬52cm、高43cm),桶壁上設有取樣小孔.設置4組大桶作為實驗室單元,每組包括3個平行容器.空白組S:無覆蓋紅土的沉積物10cm; RS:在沉積物上覆蓋厚5cm的紅土; VS:沉積物上移植16株苦草; RS+VS:沉積物上覆蓋厚度為5cm紅土,并移植16株苦草.實驗進行37d,在試驗期間,不包含沉積物和紅土的高度保持水位為30cm,沉積物及上覆水來自于成都大學十陵河,在實驗過程中從不同反應器中收集15mL上覆水,測定上覆水SRP濃度.同樣,每次取樣后將等量的河水加入反應器.

圖1 SEDEX法提取沉積物各形態(tài)磷示意

1.2.3 RS+VS聯(lián)合對沉積物中磷形態(tài)的影響 實驗進行37d后收取表層沉積物(0～2cm),覆蓋前后沉積物中磷形態(tài)提取按照革新的SEDEX方法[40]進行,沉積物磷形態(tài)分別為:松散結合態(tài)磷Loosely-bound P,亞鐵磷Fe(II)-P,鐵/鋁結合態(tài)磷 CDB-P,鈣結合態(tài)磷Ca-P和有機磷O-P,具體提取方法如(圖1).金屬氧化物結合CDB-P和Fe(II)-P通常被認為具有潛在的移動性和生物可利用性,碳酸鈣沉淀的Ca-P具有很高的化學和生物穩(wěn)定性[42-43].根據所分類進行分析檢測,測定每個實驗組沉積物磷形態(tài)的含量,所有分析均基于3個重復,未添加天然紅土的沉積物為空白組.

1.3 實驗方法

用掃描電子顯微鏡(SEM,JSM-5610LV,日本)表征觀察RS+VS覆蓋前后的表面形貌特征及微觀結構,使用便捷參數式水質檢測儀(Hach Senstion1,Colorado,USA)對水體pH值、溫度和氧化還原電位進行現場測定;使用電極法測沉積物的氧化還原電位,上覆水用0.45μm濾膜過濾后測SRP濃度,使用過硫酸鉀進行消解測TP,通過紫外-可見分光光度計(UV-2102PCS;上海的Unico儀器有限公司,中國)測定沉積物磷形態(tài)的含量,所有分析均基于3個重復. RS、VS和RS+VS對P的去除量()和去除效率()由方程式計算:

式中:0為溶液中初始磷濃度,mg/L,e為天后溶液中的磷濃度,mg/L.每個樣本重復3份測量,結果為平均值.使用Origin Pro 8.0 (Origin Lab Corp.)處理數據,使用SPSS 18.0(SPSS軟件,IBM,美國)進行統(tǒng)計分析.

2 結果與討論

2.1 吸附研究

2.1.1 RS劑量 如表2所示,覆蓋5g天然紅土的上覆水平均SRP濃度(1.12mg/L)顯著低于空白組(2.65mg/L),其最大的去除效率為58%.并且隨著RS的劑量增加,上覆水中平均SRP濃度減少并趨于穩(wěn)定,這可能是由于RS上吸附位點的飽和以及顆粒聚集、干擾或相互排斥,降低了天然紅土的吸附作用,進而導致了去除率的降低.可以看出天然紅土的覆蓋顯著減少了沉積物向上覆水釋放的磷,因此根據去除效果和經濟成本選擇劑量,在最佳的用量底泥:天然紅土質量比為2:1時,探討其他影響因素的實驗.

表2 不同的RS劑量對上覆水平均SRP濃度(mg/L)的影響

2.1.2 pH值和溫度的影響 如圖2所示,總體來看上覆水SRP濃度隨pH值增加呈現“U”型趨勢.當pH值從5增加到7時,水體沉積物向上覆水釋放量減少了1.07mg/L,隨后上覆水SRP濃度又隨pH值增加而增加,達到了1.56mg/L,相對酸性和堿性條件下,水體中性條件下紅土抑制沉積物向上覆水釋放磷更佳,去除效率為78%.而沉積物磷的釋放發(fā)生在酸性和堿性條件下,這可能是由于OH-電荷和磷酸鹽陰離子的競爭效應.

紅土覆蓋修復實驗中沉積物向上覆水平均SRP釋放量隨溫度升高而增加,但溫度為20℃時,水體SRP的釋放量是最低的,為0.89mg/L,抑制效果最佳去除效率為66%,相對于低溫10℃(1.01mg /L)和高溫50℃(1.64mg/L)的平均SRP釋放量有所降低,去除效率分別降低了4%和28%.溫度較高時沉積物中磷的釋放增加,可能是由于生物活動增加而產生了間接影響,高溫下水體底部形成厭氧區(qū)域導致某些氧化還原控制磷的釋放.

2.2 RS和VS對沉積物磷的去除效果

2.2.1 RS和VS單一修復對沉積物中磷釋放的影響 如圖3所示,20d上覆水SRP濃度與空白組相比(1.56mg/L),VS和RS分別降低了0.81和0.84mg/L,去除效率分別為52%和54%.實驗結束后S中上覆水SRP濃度變化趨于穩(wěn)定,且37天RS去除效率(68 %)相比VS(32%)有增加.VS去除效率逐漸降低可能是由于植物修復對沉積物磷的處理能力往往受到植物生長周期和周圍環(huán)境的限制,在修復后期植物苦草可能會由于水底缺氧,透明度降低等因素逐漸腐爛而成為水體沉積物的源,因此實際應用應注意定期收割,以防根部腐爛使得底泥環(huán)境成為厭氧環(huán)境,減弱抑制的效果.而RS對于沉積物磷釋放仍處于抑制狀態(tài),上覆水SRP濃度逐漸減低達到了0.45mg/L,去除效率為68%.在兩者修復實驗中RS對磷釋放的抑制效果是高于VS的,兩者最大去除量相差0.51mg/L.

2.2.2 RS和VS聯(lián)合修復對沉積物中磷釋放的影響 如圖4所示,在37d時,VS,RS和RS+VS聯(lián)合組都顯著降低上覆水中平均SRP的濃度,對比S組(1.41mg/L)苦草VS,天然紅土RS和RS+VS聯(lián)合組去除量分別為0.45,0.96,1.29mg/L,去除效率分別為32%、68%、91%.RS+VS中觀察到明顯的磷濃度降低,上覆水中平均SRP的濃度從初始濃度1.01mg/L降低到0.12mg/L,上覆水平均SRP去除效率分別從單一VS和RS組的32%、68%達到RS+VS組的91%,去除效率增幅分別為59%,23%.

圖3 RS和VS單一修復對上覆水中SRP的去除效果

圖4 不同覆蓋方式對上覆水中SRP的去除效果

2.3 RS+VS對沉積物中磷形態(tài)的影響

原沉積物中的磷形態(tài)以CDB-P(53%)為主(圖5),活躍態(tài)的磷(loosely-P、Fe(II)-P、CDB-P和O-P的總和)占TP的74%,易從沉積物中釋放[27],活躍態(tài)的磷也是水體中SRP的主要來源[31].表明沉積物內部磷負荷不穩(wěn)定,從而增加了磷釋放的潛在風險.

RS+VS覆蓋后RS(圖6)中的CDB-P含量減少了265.44mg/kg(減少了66%),而Ca-P卻增加了87.60mg/kg(增加了55%),這意味著天然紅土與苦草聯(lián)合覆蓋后沉積物磷形態(tài)從不穩(wěn)定的CDB-P轉化為了惰性Ca-P,從而降低了磷沉積物向上覆水中釋放,降低了藻華爆發(fā)的風險.

此外,早期紅土對loosely-P和Fe(II)-P增加起主導作用,紅土中l(wèi)oosely-P從8.88mg/kg增加到13.55mg/kg及Fe(II)-P從34.09mg/kg增加到82.80mg/kg,植物可以吸收這些生物可利用的磷作為養(yǎng)分,導致loosely-P和Fe(II)-P逐漸下降,在15天時分別下降到6.14mg/kg (去除效率55%)和19.81mg/kg (去除效率76%),這表明紅土中生物可利用的磷增加不會增大沉積物釋放磷的風險,反而為后期植物的生長創(chuàng)造了有利的營養(yǎng)條件.

圖5 單一及聯(lián)合修復對沉積物各形態(tài)磷的影響

圖6 RS+VS聯(lián)合修復對RS中各形態(tài)磷的影響

如圖5所示,空白組實驗中Ca-P的含量為170.48mg/kg,占沉積物中磷總量的26%.在RS+VS聯(lián)合修復下Ca-P含量有所增加,最大含量為280.28mg/kg,是空白組S的1.9倍,占了沉積物總磷的一半多(51%). RS+VS的沉積物中Ca-P濃度(280.28mg/kg)明顯高于VS組(213.51mg/kg)和RS組(245.30mg/kg),相比之下沉積物中Ca-P的增幅分別達到了14%,9%.

對比空白組(S)Loosely-bound P占沉積物總磷1%,RS+VS聯(lián)合覆蓋后沉積物中Loosely-bound P減少從9.47mg/kg到7.08mg/kg,對比空白組(S)低了2.39mg/kg.沉積物中Fe(Ⅱ)-P濃度有所減少,在空白組中Fe(Ⅱ)-P的濃度為50.74mg/kg,占總磷的7%.在RS+VS覆蓋下Fe(Ⅱ)-P的濃度下降到39.62mg/kg,占沉積物中總磷的6%.沉積物中CDB-P和O-P有下降,在沉積物總磷中的占比分別從53%和12%降低到29%和7%,RS-VS聯(lián)合修復下,沉積物樣品中的Ca-P的濃度有所增加,Fe(Ⅱ)-P、CDB-P和O-P均有下降,活躍態(tài)磷量顯著低于空白組.

可以看出天然紅土可以吸附一部分底泥中的P,并將不穩(wěn)定的Fe(II)-P,CDB-P轉化為穩(wěn)定的Ca-P,從而減少沉積物向上覆水中的釋放.同時植物可以將一部分沉積物中磷(Ex-P)轉化為生物利用磷或攝取Ex-P和Fe(II)-P.沉積物中的微生物可礦化和降解OP,并消耗孔隙水中的溶解氧,從而導致較低的氧化還原電勢,并將Fe(III)還原為Fe(II),從而導致Fe(II)-P釋放,該生物可利用磷的增加促進了天然紅土對沉積物P的吸附過程和植物的生長,從而進一步提高了植物和紅土對磷的利用.最后沉積物內部的P可以固定在天然紅土中,并且可以通過植物收獲將部分沉積物P從系統(tǒng)中去除,從而降低沉積物P的濃度.

2.4 RS+VS聯(lián)合修復技術降低內源磷負荷的機制

天然紅土的主要成分為氧化鐵,氧化鋁和石英等,如圖7所示,RS+VS表面更粗糙,獲得了更多的孔隙,RS覆蓋前后和RS+VS聯(lián)合的表面形態(tài)之間的差異顯著,天然紅土表面有微小的顆粒,并具有粗糙的表面,而大量的較大顆粒出現在覆蓋后的天然紅土表面.RS+VS聯(lián)合覆蓋后,天然紅土的顆粒比表面積增大,形成大量片狀薄片,顆粒之間間隙更緊密. 由表3可知,原始天然紅土比表面積26.26m2/g.這種粗糙度有助于天然紅土的比表面積和孔體積的增加,活性位點較多而增強沉積物對磷的吸附,從而增強RS+VS對沉積物磷的吸附能力,使得沉積物磷傾向于在紅土顆粒上吸附成為一個匯,而不是釋放到上覆水成為源.

圖7 覆蓋前后天然紅土的SEM圖像

a-S,b-VS,c-RS,d-RS+VS

表3 天然紅土的理化性質

2.5 RS+VS聯(lián)合控制內源磷的實際應用潛力

在實際工程中紅土和苦草兩種修復材料成本低,易于培養(yǎng)且見效快,是很好的沉積物原位修復材料.采取RS+VS聯(lián)合修復對污染水體的修復比單獨的RS或VS修復更佳. 如圖8所示,沉水植物的種植可以穩(wěn)定沉積物再懸浮,另外根部能從沉積物進一步吸收磷,植物根際分泌營養(yǎng)激素來刺激微生物活動.此外,根系釋放有機酸有利于提高沉水植物苦草對磷的吸收[44],還避免RS被沉積物淹埋失去吸附能力,天然紅土的覆蓋可以改善植物生長環(huán)境,將易釋放磷轉變成穩(wěn)定性磷(Ca-P增加了174.26mg/kg,52%),加大對底泥磷的固定能力.盡管在實驗初期,RS的覆蓋對VS的修復產生了一定的脅迫作用,但長期觀察紅土能為苦草提供有利的生存環(huán)境并促進整個生態(tài)系統(tǒng)的構建.

圖8 RS+VS聯(lián)合技術控制沉積物磷示意

2.6 RS+VS聯(lián)合控制內源磷的工程應用可行性

RS+VS聯(lián)合技術中,天然紅土可以有效控制磷的釋放且有較好的吸附效果,并且能為沉水植物創(chuàng)造良好的生長環(huán)境.而沉水植物苦草也能在水體富營養(yǎng)化恢復過程中發(fā)揮巨大作用[45],提高了水體生產力,減輕沉積物再懸浮和釋放磷.因此,將RS+VS聯(lián)合技術應用于表層污染沉積物的治理是一種相對經濟和效果最大化的方法,能抑制沉積物磷釋放,可以為富營養(yǎng)化水體的原位修復提供一個新的思路和方法.

圖9 RS+VS控制沉積物磷釋放的工程應用示意

進行實地應用可設計一種三層墊子生態(tài)一體箱(圖9),該設備被投放在淺湖的底部.生態(tài)一體箱上層用于植物的生長,中間層用于種植沉水植物,而下層用于填充修復材料天然紅土,網格層孔徑小于天然紅土粒度,各層之間安裝有穿孔的管道,以使水流通過.網格層的主要作用:1)將沉水植物苦草固定在網中,并使苦草根層種植在天然紅土中以吸收養(yǎng)分;2)封蓋的天然紅土接觸底泥以固定磷,上層的植物可防止沉積物重懸浮并吸收沉淀物或天然紅土吸附的磷,且避免了天然紅土層被掩埋;3)方便設備從湖中取出進行苦草收割,避免植物腐爛成為污染的源.

然而,實際應用RS與VS聯(lián)合技術治理某一區(qū)域污染水體時,實際條件往往很難滿足實驗室設計的最佳控制條件,考慮到自然水體中更復雜(水體波動大、污染物復雜、水生動植物活動等)可能難以實現預期的聯(lián)合效果,因此選用工程設計將苦草種植在紅土上培育一段時間,再定點投放紅土-苦草聯(lián)合生態(tài)一體箱,有利于苦草生長同時減少受到的環(huán)境干擾.其次,沉水植物生長周期也是一個不可忽略的問題,即苦草隨著衰敗腐爛釋放磷并重新進入上覆水體,因此在紅土-苦草聯(lián)合生態(tài)一體箱中應定期收割苦草,但對苦草實際的收割周期不確定,有可能讓苦草成為水體營養(yǎng)物的來源,后期在進行實際工程應用時應觀察苦草生長周期,以確定最佳收割時間,從而降低二次污染的風險.在保證預期治理效果前提下,原位覆蓋和植物修復聯(lián)合技術在未來一段時間可能是主要治理手段,除此之外,基于長期修復過程的經濟考慮,建議后續(xù)研究可以將天然紅土進行改性,增加其顆粒的比表面積以加大磷的吸附,改性材料的應用還需進一步研究并探討覆蓋材料的回收,能多次使用覆蓋材料也是后期研究的重點方向之一.

3 結論

3.1 RS的添加對沉積物中磷的固定作用顯著,在靜態(tài)吸附實驗中RS劑量,pH值,溫度等是影響RS對河道底泥P吸附效果的主要因素.

3.2 VS、RS及RS+VS修復均對沉積物磷釋放有明顯的抑制作用,去除效率分別為32%、68%、91%. RS+VS聯(lián)合修復下沉積物中CDB-P、Fe(II)-P、OP向穩(wěn)定態(tài)Ca-P轉化,Ca-P的比例增加了52%,意味著RS+VS聯(lián)合后沉積物中惰性磷比例升高從而降低了內源磷釋放的風險.

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Combined control of phosphorus release from sediment by red soil and.

ZHANG Yu1,2,YIN Yue-peng1,2,TANG Jin-yong1,2,CAO Xi1,2,LIU Jing-jing1,2,ZHANG Wen1,2,3*

(1.College of Environment and Ecology,Chengdu University of Technology,Chengdu 610059,China；2.National Key Laboratory for Coordinated Control and Remediation of Water and Soil Pollution in Environmental Protection (SEKL-SW),Chengdu 610059,China；3.State Key Laboratory of Geological Disaster Prevention and Geological Environment Protection,Chengdu 610059,China).,2022,42(8)：3728～3735

The effect of single and combined remediation on phosphorus removal from polluted sediment was studied by using the in situ combined remediation technology of red soil and(RS +VS). The results showed that the removal capacity of sediment P by RS and VS combined remediation was higher than that by single remediation. In the 37d batch experiment,the phosphorus released from sediment in the RS+VS group was inhibited by 91%. Compared with the phosphorus released from sediment in the RS+VS group without coverage,the dissolved active phosphorus (SRP) in the overlying water decreased from 1.41mg/L to 0.12mg/L. RS+VS combined remediation had a significant effect on the fixation of sediment phosphorus,The unstable ferrous phosphorus (Fe(II)-P) and iron aluminum bound phosphorus (CDB-P) were transformed into inert calcium phosphorus (Ca-P). The content of Ca-P in sediment increased by 51%,while Fe(II)-P and CDB-P decreased by 1% and 24% respectively,effectively reducing the risk of phosphorus release from sediment to overlying water. In conclusion,RS+VS combination can be applied to treat the internal phosphorus load in eutrophic waters to realize the coordinated removal of sediment phosphorus. At the same time,RS and VS are cheap and widely distributed,and can be used as a potential high-benefit phosphate adsorbent in practical projects.

black odorous sediment；red soil；v；phosphorus form；sediment；influencing factors

X52

A

1000-6923(2022)08-3728-08

2022-01-04

國家自然科學基金青年科學基金資助項目(42007148)

* 責任作者,教授,zhangwen2014@cdut.edu.cn

張 瑜(1997-),女,貴州銅仁人,成都理工大學碩士研究生,主要從事湖泊水庫底泥污染物釋放研究.發(fā)表論文1篇.