天然红土与苦草联合控制沉积物中磷释放

张 瑜,尹月鹏,唐金勇,曹 熙,刘晶京,张 雯,3*

天然红土与苦草联合控制沉积物中磷释放

张 瑜1,2,尹月鹏1,2,唐金勇1,2,曹 熙1,2,刘晶京1,2,张 雯1,2,3*

(1.成都理工大学生态环境学院,四川 成都 610059；2.国家环境保护水土污染协同控制与联合修复重点实验室,四川 成都 610059；3.地质灾害防治与地质环境保护国家重点实验室,四川 成都 610059)

运用天然红土和苦草(RS +VS)的原位联合修复技术,探究单一及联合修复对污染底泥磷去除的影响.结果表明,RS与VS联合对沉积物P的去除能力高于两者单一修复,在37d的批量实验中,覆盖RS+VS组沉积物释放到上覆水中的磷被抑制了91%,与无覆盖RS+VS组的底泥释放磷相比,上覆水中溶解态活性磷(SRP)从1.41mg/L降至0.12mg/L. RS+VS联合修复对沉积物磷的固定作用显著,将不稳定的亚铁磷(Fe(II)-P)和铁铝结合磷(CDB-P)转变成惰性的钙磷(Ca-P),沉积物中的Ca-P含量增加了51%,Fe(II)-P ,CDB-P分别降低了1%和24%,有效降低了底泥磷释放到上覆水的风险.综上,RS+VS联合可以应用于处理富营养化水域的内部磷负荷,实现两者协同去除沉积物磷,同时RS和VS两者价廉且广泛分布可作为一种潜在高效益的磷酸盐吸附剂应用于实际工程中.

黑臭底泥；红土；苦草；磷形态；沉积物；影响因素

水体营养化是由于养分的过量输入而引起的全球性问题[1-3].沉积物被认为是养分的汇集地,在富营养化中起着至关重要的作用[4-6].磷在上覆水体和沉积物之间(即内部磷负荷)进行着持续循环,当外源磷输入得到一定控制后,沉积物中的内源磷释放仍然将可能维持数十年,延缓了受损水生态系统的恢复[7-10].因此通过抑制水体内源负荷的释放,降低水体中磷的浓度,对于防止水体富营养化的发生具有重要意义.

近年来,为控制富营养化开发了许多修复的技术和策略,这些包括减少外部污染源如:拦截点源污染、建设污水处理厂[11]、面源控制[12]、湿地处理技术[13]流域综合管理.内部负荷控制如:沉积物疏浚[14]、曝气、原位覆盖[15-17]、生物技术[18];生态修复如水生植物修复技术[19-21]生物操纵[22]等.由于底泥疏浚成本高[23],使用化学产品可能造成生态风险[24],原位覆盖和生态修复是应用最广泛的两个缓解策略.原位覆盖的吸附剂使用天然材料包括沸石[17]、凹凸棒土[25]、方解石[26]、红土[27-30]、膨润土[31]等,其中红土富含铝和铁的氧化物,而铁在磷酸盐吸附中有着关键作用.氧化铁具有低晶体尺寸的颗粒,会形成强键位点,用于磷酸盐物质的表面络合[32],使得红土的吸收能力高于其他天然矿物的吸收能力[29].且红土是一种价廉分布广泛的土壤[30],无生物毒性,能为沉水植物提供更好的生长环境.另一方面,沉水植物在减少沉积物再悬浮和控制内源磷的释放方面有着关键作用[20,33-34].苦草是一种一年或半年生的植物,具有大量根[35-36],它可以通过根部从沉积物中吸收大量养分[37],然后通过吸收养分、稳定沉积物、庇护过滤藻类的浮游动物以及与浮游植物竞争光和养分等方式来改善水质[38-39].但沉水植物对沉积物磷的处理效果往往受到生长周期和周围环境的限制,单一的修复技术不能有效和普遍的使用,迫切需要一种组合技术来处理沉积物磷.

基于此,为进一步增强沉水植物对沉积物磷的修复效果,弥补应用单一技术的不足,本研究开发了原位覆盖与沉水植物相结合的技术,采用天然红土和苦草联合修复作为一种生态环境材料,具有广泛的推广价值和应用前景[28],且原位覆盖和生物修复的联合作用[33,40-41]可能是下一代缓解富营养化商业产品的重要方向.

1 材料与方法

1.1 采集与处理

表1 沉积物上覆水的理化性质

实验所用红土来自四川省龙泉驿区(30.55658°N,104.27462°E).先去除其中较大颗粒植物残渣,再将天然红土自然风干,粉碎机粉碎到颗粒直径为3～5mm.沉水植物苦草采自湖北荆州,采集的苦草长势好并且新鲜,将苦草冲洗干净,移除无脊椎食草动物,并在实验室里培育7d,均剪到20cm长以符合实验要求的高度.采集了四川省成都市一条典型的城市黑臭水体十陵河的沉积物和上覆水,同时进行相关理化性质分析,分析结果见表1.

1.2 覆盖实验设计

1.2.1 吸附实验 研究红土剂量、反应温度和pH值对磷释放的影响以探究红土覆盖的最佳条件.在锥形瓶中进行动态吸附实验,其中装有150mL,0.02mol/L氯化钾溶液,10g沉积物样品和不同剂量(0.5,2,4,5,7,9g)的RS .恒温调节pH值(5,6,7,8,9,10),不同温度(10,20,30,40,50℃),加入两滴0.1%氯仿抑制细菌活性的条件下进行实验研究.控制振动速率为200r/min下振荡24h后离心,测量上覆水的溶解态活性磷(SRP)浓度.每个处理由3个重复组成,数据均表示为平均值.

1.2.2 RS和VS对底泥磷释放的影响 放置10cm厚的沉积物到实验桶(聚丙烯材质的塑料收纳桶,长70.5cm、宽52cm、高43cm),桶壁上设有取样小孔.设置4组大桶作为实验室单元,每组包括3个平行容器.空白组S:无覆盖红土的沉积物10cm; RS:在沉积物上覆盖厚5cm的红土; VS:沉积物上移植16株苦草; RS+VS:沉积物上覆盖厚度为5cm红土,并移植16株苦草.实验进行37d,在试验期间,不包含沉积物和红土的高度保持水位为30cm,沉积物及上覆水来自于成都大学十陵河,在实验过程中从不同反应器中收集15mL上覆水,测定上覆水SRP浓度.同样,每次取样后将等量的河水加入反应器.

图1 SEDEX法提取沉积物各形态磷示意

1.2.3 RS+VS联合对沉积物中磷形态的影响 实验进行37d后收取表层沉积物(0～2cm),覆盖前后沉积物中磷形态提取按照革新的SEDEX方法[40]进行,沉积物磷形态分别为:松散结合态磷Loosely-bound P,亚铁磷Fe(II)-P,铁/铝结合态磷 CDB-P,钙结合态磷Ca-P和有机磷O-P,具体提取方法如(图1).金属氧化物结合CDB-P和Fe(II)-P通常被认为具有潜在的移动性和生物可利用性,碳酸钙沉淀的Ca-P具有很高的化学和生物稳定性[42-43].根据所分类进行分析检测,测定每个实验组沉积物磷形态的含量,所有分析均基于3个重复,未添加天然红土的沉积物为空白组.

1.3 实验方法

用扫描电子显微镜(SEM,JSM-5610LV,日本)表征观察RS+VS覆盖前后的表面形貌特征及微观结构,使用便捷参数式水质检测仪(Hach Senstion1,Colorado,USA)对水体pH值、温度和氧化还原电位进行现场测定;使用电极法测沉积物的氧化还原电位,上覆水用0.45μm滤膜过滤后测SRP浓度,使用过硫酸钾进行消解测TP,通过紫外-可见分光光度计(UV-2102PCS;上海的Unico仪器有限公司,中国)测定沉积物磷形态的含量,所有分析均基于3个重复. RS、VS和RS+VS对P的去除量()和去除效率()由方程式计算:

式中:0为溶液中初始磷浓度,mg/L,e为天后溶液中的磷浓度,mg/L.每个样本重复3份测量,结果为平均值.使用Origin Pro 8.0 (Origin Lab Corp.)处理数据,使用SPSS 18.0(SPSS软件,IBM,美国)进行统计分析.

2 结果与讨论

2.1 吸附研究

2.1.1 RS剂量 如表2所示,覆盖5g天然红土的上覆水平均SRP浓度(1.12mg/L)显著低于空白组(2.65mg/L),其最大的去除效率为58%.并且随着RS的剂量增加,上覆水中平均SRP浓度减少并趋于稳定,这可能是由于RS上吸附位点的饱和以及颗粒聚集、干扰或相互排斥,降低了天然红土的吸附作用,进而导致了去除率的降低.可以看出天然红土的覆盖显著减少了沉积物向上覆水释放的磷,因此根据去除效果和经济成本选择剂量,在最佳的用量底泥:天然红土质量比为2:1时,探讨其他影响因素的实验.

表2 不同的RS剂量对上覆水平均SRP浓度(mg/L)的影响

2.1.2 pH值和温度的影响 如图2所示,总体来看上覆水SRP浓度随pH值增加呈现“U”型趋势.当pH值从5增加到7时,水体沉积物向上覆水释放量减少了1.07mg/L,随后上覆水SRP浓度又随pH值增加而增加,达到了1.56mg/L,相对酸性和碱性条件下,水体中性条件下红土抑制沉积物向上覆水释放磷更佳,去除效率为78%.而沉积物磷的释放发生在酸性和碱性条件下,这可能是由于OH-电荷和磷酸盐阴离子的竞争效应.

红土覆盖修复实验中沉积物向上覆水平均SRP释放量随温度升高而增加,但温度为20℃时,水体SRP的释放量是最低的,为0.89mg/L,抑制效果最佳去除效率为66%,相对于低温10℃(1.01mg /L)和高温50℃(1.64mg/L)的平均SRP释放量有所降低,去除效率分别降低了4%和28%.温度较高时沉积物中磷的释放增加,可能是由于生物活动增加而产生了间接影响,高温下水体底部形成厌氧区域导致某些氧化还原控制磷的释放.

2.2 RS和VS对沉积物磷的去除效果

2.2.1 RS和VS单一修复对沉积物中磷释放的影响 如图3所示,20d上覆水SRP浓度与空白组相比(1.56mg/L),VS和RS分别降低了0.81和0.84mg/L,去除效率分别为52%和54%.实验结束后S中上覆水SRP浓度变化趋于稳定,且37天RS去除效率(68 %)相比VS(32%)有增加.VS去除效率逐渐降低可能是由于植物修复对沉积物磷的处理能力往往受到植物生长周期和周围环境的限制,在修复后期植物苦草可能会由于水底缺氧,透明度降低等因素逐渐腐烂而成为水体沉积物的源,因此实际应用应注意定期收割,以防根部腐烂使得底泥环境成为厌氧环境,减弱抑制的效果.而RS对于沉积物磷释放仍处于抑制状态,上覆水SRP浓度逐渐减低达到了0.45mg/L,去除效率为68%.在两者修复实验中RS对磷释放的抑制效果是高于VS的,两者最大去除量相差0.51mg/L.

2.2.2 RS和VS联合修复对沉积物中磷释放的影响 如图4所示,在37d时,VS,RS和RS+VS联合组都显著降低上覆水中平均SRP的浓度,对比S组(1.41mg/L)苦草VS,天然红土RS和RS+VS联合组去除量分别为0.45,0.96,1.29mg/L,去除效率分别为32%、68%、91%.RS+VS中观察到明显的磷浓度降低,上覆水中平均SRP的浓度从初始浓度1.01mg/L降低到0.12mg/L,上覆水平均SRP去除效率分别从单一VS和RS组的32%、68%达到RS+VS组的91%,去除效率增幅分别为59%,23%.

图3 RS和VS单一修复对上覆水中SRP的去除效果

图4 不同覆盖方式对上覆水中SRP的去除效果

2.3 RS+VS对沉积物中磷形态的影响

原沉积物中的磷形态以CDB-P(53%)为主(图5),活跃态的磷(loosely-P、Fe(II)-P、CDB-P和O-P的总和)占TP的74%,易从沉积物中释放[27],活跃态的磷也是水体中SRP的主要来源[31].表明沉积物内部磷负荷不稳定,从而增加了磷释放的潜在风险.

RS+VS覆盖后RS(图6)中的CDB-P含量减少了265.44mg/kg(减少了66%),而Ca-P却增加了87.60mg/kg(增加了55%),这意味着天然红土与苦草联合覆盖后沉积物磷形态从不稳定的CDB-P转化为了惰性Ca-P,从而降低了磷沉积物向上覆水中释放,降低了藻华爆发的风险.

此外,早期红土对loosely-P和Fe(II)-P增加起主导作用,红土中loosely-P从8.88mg/kg增加到13.55mg/kg及Fe(II)-P从34.09mg/kg增加到82.80mg/kg,植物可以吸收这些生物可利用的磷作为养分,导致loosely-P和Fe(II)-P逐渐下降,在15天时分别下降到6.14mg/kg (去除效率55%)和19.81mg/kg (去除效率76%),这表明红土中生物可利用的磷增加不会增大沉积物释放磷的风险,反而为后期植物的生长创造了有利的营养条件.

图5 单一及联合修复对沉积物各形态磷的影响

图6 RS+VS联合修复对RS中各形态磷的影响

如图5所示,空白组实验中Ca-P的含量为170.48mg/kg,占沉积物中磷总量的26%.在RS+VS联合修复下Ca-P含量有所增加,最大含量为280.28mg/kg,是空白组S的1.9倍,占了沉积物总磷的一半多(51%). RS+VS的沉积物中Ca-P浓度(280.28mg/kg)明显高于VS组(213.51mg/kg)和RS组(245.30mg/kg),相比之下沉积物中Ca-P的增幅分别达到了14%,9%.

对比空白组(S)Loosely-bound P占沉积物总磷1%,RS+VS联合覆盖后沉积物中Loosely-bound P减少从9.47mg/kg到7.08mg/kg,对比空白组(S)低了2.39mg/kg.沉积物中Fe(Ⅱ)-P浓度有所减少,在空白组中Fe(Ⅱ)-P的浓度为50.74mg/kg,占总磷的7%.在RS+VS覆盖下Fe(Ⅱ)-P的浓度下降到39.62mg/kg,占沉积物中总磷的6%.沉积物中CDB-P和O-P有下降,在沉积物总磷中的占比分别从53%和12%降低到29%和7%,RS-VS联合修复下,沉积物样品中的Ca-P的浓度有所增加,Fe(Ⅱ)-P、CDB-P和O-P均有下降,活跃态磷量显著低于空白组.

可以看出天然红土可以吸附一部分底泥中的P,并将不稳定的Fe(II)-P,CDB-P转化为稳定的Ca-P,从而减少沉积物向上覆水中的释放.同时植物可以将一部分沉积物中磷(Ex-P)转化为生物利用磷或摄取Ex-P和Fe(II)-P.沉积物中的微生物可矿化和降解OP,并消耗孔隙水中的溶解氧,从而导致较低的氧化还原电势,并将Fe(III)还原为Fe(II),从而导致Fe(II)-P释放,该生物可利用磷的增加促进了天然红土对沉积物P的吸附过程和植物的生长,从而进一步提高了植物和红土对磷的利用.最后沉积物内部的P可以固定在天然红土中,并且可以通过植物收获将部分沉积物P从系统中去除,从而降低沉积物P的浓度.

2.4 RS+VS联合修复技术降低内源磷负荷的机制

天然红土的主要成分为氧化铁,氧化铝和石英等,如图7所示,RS+VS表面更粗糙,获得了更多的孔隙,RS覆盖前后和RS+VS联合的表面形态之间的差异显著,天然红土表面有微小的颗粒,并具有粗糙的表面,而大量的较大颗粒出现在覆盖后的天然红土表面.RS+VS联合覆盖后,天然红土的颗粒比表面积增大,形成大量片状薄片,颗粒之间间隙更紧密. 由表3可知,原始天然红土比表面积26.26m2/g.这种粗糙度有助于天然红土的比表面积和孔体积的增加,活性位点较多而增强沉积物对磷的吸附,从而增强RS+VS对沉积物磷的吸附能力,使得沉积物磷倾向于在红土颗粒上吸附成为一个汇,而不是释放到上覆水成为源.

图7 覆盖前后天然红土的SEM图像

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

表3 天然红土的理化性质

2.5 RS+VS联合控制内源磷的实际应用潜力

在实际工程中红土和苦草两种修复材料成本低,易于培养且见效快,是很好的沉积物原位修复材料.采取RS+VS联合修复对污染水体的修复比单独的RS或VS修复更佳. 如图8所示,沉水植物的种植可以稳定沉积物再悬浮,另外根部能从沉积物进一步吸收磷,植物根际分泌营养激素来刺激微生物活动.此外,根系释放有机酸有利于提高沉水植物苦草对磷的吸收[44],还避免RS被沉积物淹埋失去吸附能力,天然红土的覆盖可以改善植物生长环境,将易释放磷转变成稳定性磷(Ca-P增加了174.26mg/kg,52%),加大对底泥磷的固定能力.尽管在实验初期,RS的覆盖对VS的修复产生了一定的胁迫作用,但长期观察红土能为苦草提供有利的生存环境并促进整个生态系统的构建.

图8 RS+VS联合技术控制沉积物磷示意

2.6 RS+VS联合控制内源磷的工程应用可行性

RS+VS联合技术中,天然红土可以有效控制磷的释放且有较好的吸附效果,并且能为沉水植物创造良好的生长环境.而沉水植物苦草也能在水体富营养化恢复过程中发挥巨大作用[45],提高了水体生产力,减轻沉积物再悬浮和释放磷.因此,将RS+VS联合技术应用于表层污染沉积物的治理是一种相对经济和效果最大化的方法,能抑制沉积物磷释放,可以为富营养化水体的原位修复提供一个新的思路和方法.

图9 RS+VS控制沉积物磷释放的工程应用示意

进行实地应用可设计一种三层垫子生态一体箱(图9),该设备被投放在浅湖的底部.生态一体箱上层用于植物的生长,中间层用于种植沉水植物,而下层用于填充修复材料天然红土,网格层孔径小于天然红土粒度,各层之间安装有穿孔的管道,以使水流通过.网格层的主要作用:1)将沉水植物苦草固定在网中,并使苦草根层种植在天然红土中以吸收养分;2)封盖的天然红土接触底泥以固定磷,上层的植物可防止沉积物重悬浮并吸收沉淀物或天然红土吸附的磷,且避免了天然红土层被掩埋;3)方便设备从湖中取出进行苦草收割,避免植物腐烂成为污染的源.

然而,实际应用RS与VS联合技术治理某一区域污染水体时,实际条件往往很难满足实验室设计的最佳控制条件,考虑到自然水体中更复杂(水体波动大、污染物复杂、水生动植物活动等)可能难以实现预期的联合效果,因此选用工程设计将苦草种植在红土上培育一段时间,再定点投放红土-苦草联合生态一体箱,有利于苦草生长同时减少受到的环境干扰.其次,沉水植物生长周期也是一个不可忽略的问题,即苦草随着衰败腐烂释放磷并重新进入上覆水体,因此在红土-苦草联合生态一体箱中应定期收割苦草,但对苦草实际的收割周期不确定,有可能让苦草成为水体营养物的来源,后期在进行实际工程应用时应观察苦草生长周期,以确定最佳收割时间,从而降低二次污染的风险.在保证预期治理效果前提下,原位覆盖和植物修复联合技术在未来一段时间可能是主要治理手段,除此之外,基于长期修复过程的经济考虑,建议后续研究可以将天然红土进行改性,增加其颗粒的比表面积以加大磷的吸附,改性材料的应用还需进一步研究并探讨覆盖材料的回收,能多次使用覆盖材料也是后期研究的重点方向之一.

3 结论

3.1 RS的添加对沉积物中磷的固定作用显著,在静态吸附实验中RS剂量,pH值,温度等是影响RS对河道底泥P吸附效果的主要因素.

3.2 VS、RS及RS+VS修复均对沉积物磷释放有明显的抑制作用,去除效率分别为32%、68%、91%. RS+VS联合修复下沉积物中CDB-P、Fe(II)-P、OP向稳定态Ca-P转化,Ca-P的比例增加了52%,意味着RS+VS联合后沉积物中惰性磷比例升高从而降低了内源磷释放的风险.

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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

国家自然科学基金青年科学基金资助项目(42007148)

* 责任作者,教授,zhangwen2014@cdut.edu.cn

张 瑜(1997-),女,贵州铜仁人,成都理工大学硕士研究生,主要从事湖泊水库底泥污染物释放研究.发表论文1篇.