南京大学学报(自然科学), 2024, 60(1): 141-150 doi: 10.13232/j.cnki.jnju.2024.01.014

非离子及两性型碳氢表面活性剂对全氟辛酸在土壤中迁移的影响

张庆泉1,2, 赵燕鹏1, 蔡兴1, 张琦3,4, 郭红岩,2, 孙媛媛,4

1.江苏环保产业技术研究院股份公司,南京,210019

2.南京大学环境学院,南京,210023

3.南京市浦口区水务局,南京,211800

4.南京大学地球科学与工程学院,南京,210023

Effects of nonionic and amphiphilic hydrocarbon surfactants on the transport of perfluorooctanoic acid in soils

Zhang Qingquan1,2, Zhao Yanpeng1, Cai Xing1, Zhang Qi3,4, Guo Hongyan,2, Sun Yuanyuan,4

1.Jiangsu Academy of Environmental Indurstry and Technology Corp, Nanjing, 210019, China

2.School of Environment, Nanjing University, Nanjing, 210023, China

3.Pukou District Water Bureau, Nanjing, 211800, China

4.School of Earth Sciences and Engineering, Nanjing University, Nanjing, 210023, China

通讯作者: E⁃mail:sunyy@nju.edu.cn

收稿日期: 2023-10-25  

基金资助: 国家自然科学基金.  42077109

Received: 2023-10-25  

摘要

碳氟型表面活性剂全氟辛酸(Perfluorooctanoic Acid,PFOA)的大量使用导致其与各类碳氢表面活性剂广泛共存于土壤⁃地下水环境中,对人体健康和生态安全存在潜在威胁.通过柱实验探究不同离子强度条件下(1.5 mmol·L-1和30 mmol·L-1),非离子型碳氢表面活性剂烷基糖苷(Alkyl Polyglycoside,APG)及两性型碳氢表面活性剂十二烷基二甲基甜菜碱(Dodecyl Dimethyl Betaine,BS⁃12)对PFOA在两种饱和土壤中迁移行为的影响和作用机制.结果表明,APG能促进PFOA的迁移行为,其主控机制为APG与PFOA在土壤表面的竞争吸附作用;BS⁃12能通过对土壤表面负电荷的中和作用及疏水作用增强PFOA在土壤中的阻滞,抑制其迁移行为.因此,预测和评估PFOA在土壤中的迁移行为时,需考虑共存碳氢表面活性剂的影响.

关键词: 全氟辛酸 ; 非离子表面活性剂 ; 两性表面活性剂 ; 土壤性质

Abstract

The extensive use of perfluorooctanoic acid (PFOA) leads to its widespread coexistence with kinds of hydrocarbon surfactants in the subsurface environment,which has posed potential risks to human health and ecological security. In this study,column experiments were conducted to explore the effect of nonionic hydrocarbon surfactant alkyl polyglycoside (APG) and amphiphilic surfactant dodecyl dimethyl betaine (BS⁃12) on the transport of PFOA in two saturated soils under different ionic strength. The results showed that APG could promote the transport of PFOA due to its competitive adsorption with PFOA on the soil surface. BS⁃12 could inhibit the transport of PFOA by neturalizing the negative charge on soil surface and hydrophobic interaction.Therefore,the effects coexisting hydrocarbon surfactants should be considered when predicting and evaluating the transport behavior of PFOA in soils.

Keywords: perfluorooctanoic acid ; nonionic surfactant ; amphiphilic surfactant ; soil properties

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张庆泉, 赵燕鹏, 蔡兴, 张琦, 郭红岩, 孙媛媛. 非离子及两性型碳氢表面活性剂对全氟辛酸在土壤中迁移的影响. 南京大学学报(自然科学)[J], 2024, 60(1): 141-150 doi:10.13232/j.cnki.jnju.2024.01.014

Zhang Qingquan, Zhao Yanpeng, Cai Xing, Zhang Qi, Guo Hongyan, Sun Yuanyuan. Effects of nonionic and amphiphilic hydrocarbon surfactants on the transport of perfluorooctanoic acid in soils. Journal of nanjing University[J], 2024, 60(1): 141-150 doi:10.13232/j.cnki.jnju.2024.01.014

作为一种碳氟表面活性剂,全氟辛酸(Per⁃fluorooctanoic Acid,PFOA)具有极强的耐酸碱性、耐氧化性和高表面活性,被广泛用于皮革制品、纺织品、灭火剂、洗涤剂等各类生活用品和工业产品中1-2.通过生产、销售、使用、处理等环节,PFOA进入各类环境介质中,并通过大气沉降、地表水、垃圾渗滤液等进入土壤⁃地下水系统3.据报道,PFOA在地下水和土壤中的浓度普遍为ng·L-1和μg·kg-1数量级4-6,如Hepburn et al7发现澳大利亚某废弃垃圾填埋场周围地下水中PFOA浓度为1.7~74 ng·L-1;Bao et al8发现我国阜新氟化工园区地下水和土壤中的PFOA浓度分别为105~2510 ng·L-1和1.2~6.3 μg·kg-1;许静等9发现我国湖北省孝昌县某化工厂周边土壤中PFOA的最高检出浓度达142 µg·kg-1.由于PFOA稳定、不易降解,是一种持久性有机污染物,具有生物累积性和多重毒性,对人体健康和生态安全存在诸多威胁10-13.因此,深入研究、探明PFOA在土壤中的迁移转化行为至关重要.

生产生活中,碳氢⁃碳氟型混合表面活性剂因其性能好、成本低而得到广泛使用(如常见的消防泡沫灭火剂),这导致碳氟型与碳氢型表面活性剂在环境介质中的广泛共存14-15,尤其在受废水排放和污泥堆放影响的土壤中,各类表面活性剂总浓度可达mg·kg-1量级16-17.研究发现不同类型的碳氢表面活性剂能改变土壤特性18,并显著影响全氟和多氟烷基化合物在环境介质中的吸附与运移行为19-20.Pan et al19研究发现阳离子表面活性剂十六烷基三甲基溴化铵(Cetyltrimethylammonium Bromide,CTAB)能显著增大沉积物对全氟辛磺酸(Perfluorooctane Sulfonate,PFOS)的吸附量,而阴离子表面活性剂十二烷基苯磺酸钠(Sodium Sodecylbenzene Sulfonate,SDBS)对PFOS吸附作用的影响受其浓度控制,高浓度时抑制PFOS的吸附,低浓度时则促进其吸附.Guelfo and Higgins20研究发现阴离子表面活性剂十二烷基硫酸钠(Sodium Decyl Sulfate,SDS)能显著增加全氟庚酸在土壤上的吸附作用,但对PFOA在土壤中的吸附几乎没有影响.Ji et al21研究表明,高浓度的SDS通过促进PFOA的吸附而促进其在石英砂中的运移行为.Lyu et al22研究发现SDBS能抑制PFOA在非饱和带中运移,其主控因素为SDBS与PFOA在气⁃液界面的竞争吸附作用.但以上研究主要针对阴离子和阳离子型碳氢表面活性剂,有关非离子型及两性型碳氢表面活性剂对PFOA迁移行为的影响及作用机制尚无报道.

烷基糖苷(Alkyl Polyglycoside,APG)是一类常见的非离子型表面活性剂,多与阴离子表面活性剂复配用于餐具洗涤等领域,十二烷基二甲基甜菜碱(Dodecyl Dimethyl Betaine,BS⁃12)是一种复配性良好的两性型表面活性剂,可用作土壤修饰剂增强土壤对污染物的吸附能力23-26.因此,本文拟采用APG和BS⁃12两种碳氢表面活性剂,通过室内饱和土柱实验,探究不同离子强度条件下其对PFOA在不同类型土壤中迁移行为的影响及作用机制.

1 材料与方法

1.1 化学试剂及溶液配制

实验用14C⁃PFOA(放射性浓度为2.035 GBq·(mmol·L-1-1,99%)购自美国放射性化学品公司;未标记PFOA购自德国克曼公司;APG和BS⁃12均购自南京晚晴化玻仪器有限公司.表面活性剂性质见表1.配制单一PFOA工作溶液时,准确量取361.4 μL 14C⁃PFOA母液(浓度为5.76 KBq·mL-1)、125.2 μL未标记PFOA母液(浓度为10 mg·L-1)和一定量NaCl母液(浓度为3.0 mol·L-1,用于控制离子强度)于250 mL容量瓶中,用超纯水定容,得到浓度为6.80 μg·L-1的PFOA工作溶液.配制PFOA⁃APG混合溶液时,同样在250 mL容量瓶中依次加入上述14C⁃PFOA、未标记的PFOA、NaCl母液、1 mL或10 mL APG母液(浓度为250 mg·L-1),定容得到PFOA⁃APG混合液(其中PFOA浓度为6.80 μg·L-1,APG浓度为1 mg·L-1或10 mg·L-1).PFOA⁃BS混合溶液配制方法同PFOA⁃APG混合溶液.此外,分别配制离子强度为1.5和30 mmol·L-1的NaCl溶液作为柱实验背景溶液.

表1   表面活性剂基本性质

Table 1  Properties of surfactants

CAS分子式相对分子质量(g·mol-1)pKaCMCa (mg·L-1
PFOA335⁃67⁃1C7F15COOH414.07-0.2~3.8[27-29]15696.0[30]
BS⁃12682⁃20⁃3C16H33NO2271.44-488.59[31]
APGb68515⁃72⁃2----

a临界胶束浓度(critical micelle concentration);b这里使用的APG(0810)为C8~C10的混合物.

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1.2 土柱实验

研究选用的两种土壤分别采自我国新疆(85°34′ E,44°54′ N,简称Soil⁃1)和江西(117°58′ E,28°26′ N,简称Soil⁃2)农田表层土,取样深度为0~20 cm,均无表面活性剂及PFOA污染.采集的土壤经风干、过筛至粒径小于2 mm备用.土壤的基本性质引自本课题组前期研究结果(见表232.从粒径分布来看,Soil⁃1为砂质土,Soil⁃2为粉黏质土,Soil⁃2的孔隙度、比表面积均大于Soil⁃1,且两种土壤的有机质含量都较低.从矿物组成来看,两种土壤中主要矿物类型均为石英,主要组成元素均为Fe和Al,其中Soil⁃2的Fe/Al氧化物含量(Fe2O3:5.66%,Al2O3:16.21%)高于Soil⁃1 (Fe2O3:3.02%,Al2O3:12.51%)32.此外,使用Zetasizer Nano Z (Malvern Instrument Ltd.,英国)测定不同离子强度条件下土壤的ζ电位,结果见表3.

表2   土壤主要理化性质[32]

Table 2  Physicochemical properties of soils[32]

土壤类型粒径分布孔隙度比表面积(m2·g-1有机碳含量pH
黏粒粉粒砂粒
Soil⁃10.86%2.97%96.17%0.346.120.19%8.55
Soil⁃216.65%68.60%14.75%0.5132.340.51%4.30

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表3   不同离子强度下的土壤ζ电位

Table3  Zeta potential of soils under different ionic strength

土壤类型ζ电位(mV)
IS=1.5 mmol·L-1IS=30 mmol·L-1
Soil⁃1-27.77-26.95
Soil⁃2-6.63-0.25

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土柱实验装置如图1所示.聚丙烯柱(内径2.5 cm,长12 cm)两端设置300目不锈钢滤网支撑柱内土壤并分散水流.采用干法装填土柱,通过蠕动泵(BT100⁃1F,保定兰格恒流泵有限公司,中国)以0.05 mL·min-1的流速,自下而上将土柱缓慢饱和超纯水.根据添加的土壤和超纯水的克重,测定柱内总孔隙体积(PV)和孔隙度.饱水结束后,将NaCl背景溶液以0.2 mL·min-1的流速(Darcy流速为0.59 m·d-1)自上而下预冲洗土柱(约10 PV),使体系达到稳定状态,随后将3 PV工作溶液以同样0.2 mL·min-1的流速自上而下注入土柱.待工作液注入完成后,换背景溶液继续冲洗土柱直至流出液中无PFOA检出.实验过程中,利用自动部分收集器(BS⁃100A,上海沪西分析仪器有限公司)每隔20 min (Soil⁃1)/30 min(Soil⁃2)收集流出液样品4 mL(Soil⁃1)/6 mL(Soil⁃2).按1∶1比例向流出液样品中加入闪烁液(Gold Star multipurpose,Meridian Biotechnologies Ltd.,英国),混合均匀后通过液体闪烁计数仪(LS6500,Beckman Coulter,美国)测量样品中的14C放射量并计算得到流出物中的PFOA浓度.所有实验均重复两次,数据为平行实验测定结果的平均值.

图1

图1   柱实验装置示意图

Fig.1   Column experimental equipment


2 结果与讨论

2.1 非离子表面活性剂对PFOA在土壤中迁移行为的影响

表4为单一PFOA和PFOA⁃APG混合体系分别在Soil⁃1和Soil⁃2中的运移柱实验条件及结果,图2为对应条件下PFOA的穿透曲线.结果表明,非离子型表面活性剂APG能促进PFOA的迁移,减弱土壤对PFOA的滞留作用,且APG浓度越高,对PFOA迁移的促进效应越显著.

表4   单一PFOA和PFOA⁃APG混合体系柱实验条件及结果

Table 4  Experimental conditions and results summary of column experiments of single PFOA and PFOA⁃APG mixed systems

土壤类型

离子强度

(mmol·L-1

CAPG (mg·L-1PFOA回收率
Soil⁃11.50100.2%
1.096.9%
10.099.3%
30097.0%
1.0100.2%
10.0100.1%
Soil⁃21.5093.6%
1.090.9%
10.093.4%
30097.7%
1.094.9%
10.095.6%

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图2

图2   APG作用下PFOA在土壤中的穿透曲线:(a,b) Soil⁃1,(c,d) Soil⁃2,误差线表示两次重复实验的偏差

Fig.2   Breakthrough curves of PFOA in the absence and presence of APG in columns packed with (a,b) Soil⁃1,(c,d) Soil⁃2


在Soil⁃1中,离子强度为1.5 mmol·L-1时(图2a),相比于单一PFOA体系,加入APG后PFOA穿透曲线显示出向左偏移和峰值升高的趋势;同样,当离子强度为30 mmol·L-1时(图2b),加入APG后PFOA的穿透曲线峰值从0.98分别升高至1.06 (CAPG=1 mg·L-1)和1.27 (CAPG=10 mg·L-1).在Soil⁃2中,当离子强度为1.5 mmol·L-1时(图2c),加入APG后PFOA穿透曲线的峰值从0.56分别增大到0.66 (CAPG=1 mg·L-1)和0.73(CAPG=10 mg·L-1);当离子强度为30 mmol·L-1时(图2d),加入APG后PFOA穿透曲线呈现轻微的左移和峰值升高趋势.这些穿透曲线的左移和峰值升高现象,表明PFOA在土壤中的阻滞减弱、迁移性增强,即共存的APG能促进PFOA在土壤中的迁移行为,且相较于加入1 mg·L-1的APG,当体系中APG浓度为10 mg·L-1时,穿透曲线的峰值变化更大,对PFOA迁移性的促进效应更显著.

APG促进PFOA的迁移可能是因为PFOA和APG分子在土壤表面的竞争吸附作用.此前研究表明,吸附作用是控制PFOA在土壤中迁移和滞留的主要原因33-34.作为一种阴离子表面活性剂,PFOA溶解、电离后带负电荷,而APG兼具非离子和阴离子型表面活性剂的性质,溶于水后表面呈现负离子特性2435-36,二者均能吸附于带电荷的极性土壤表面.当体系中PFOA和APG共存时,两种分子将竞争土壤表面有限的吸附位点,导致PFOA在土壤表面吸附量减少,同时由于体系中APG具有绝对的浓度优势,其与PFOA分子中疏水碳链之间的斥力可能进一步导致PFOA在土壤表面的吸附被抑制,即PFOA在土壤中的迁移性增强.碳氟表面活性剂之间、碳氢⁃碳氟表面活性剂之间存在竞争吸附现象37-38,如SDBS能通过竞争吸附作用促进PFOA在饱和土壤及非饱和石英砂介质中的迁移行为2232,SDS在土壤⁃水界面通过和PFOS的竞争吸附作用降低其在土壤上的吸附20.

土壤性质影响混合体系中PFOA的迁移行为,在Soil⁃1中APG对PFOA迁移性的促进效应比Soil⁃2更显著.对比图2a,2b和图2c,2d可知,在Soil⁃1中加入APG后,PFOA穿透曲线的左移及峰值升高的变化更显著,当体系中的CAPG为10 mg·L-1时,PFOA在Soil⁃1中的穿透曲线峰值分别为1.14 (IS=1.5 mmol·L-1)和1.27 (IS=30 mmol·L-1),存在C/C0>1的“过穿透”现象;同样条件下,在Soil⁃2中,PFOA穿透曲线仅存在一定程度的峰值变化.PFOA在Soil⁃1中的“过穿透”现象可以描述为Soil⁃1对APG的吸附亲和力较强,APG分子能取代PFOA分子并占据土壤表面有限的吸附位点,导致实验早期刚刚吸附在土壤表面的PFOA分子快速解吸脱附,并随后续通入的PFOA一起穿透出来,造成流出液中的PFOA浓度超过原工作溶液的初始浓度,即C/C0>1.Montalvo and Smolders39在重金属的吸附迁移研究中也曾报道过类似的“过穿透”现象,其主导机制同样是竞争吸附作用.

导致PFOA和PFOA⁃APG混合体系在两种土壤中的迁移行为不同的主要原因是土壤表面电性、粒度等土壤特性的差异.

(1)土壤表面电性:由于Fe/Al氧化物的零电荷点pH (pHPZC)较高(Fe:pHPZC=8.1~8.5,Al:pHPZC=9.4)40-41,实验中碱性土壤Soil⁃1中的Fe/Al氧化物倾向于带负电荷,而酸性土壤Soil⁃2中的Fe/Al氧化物更倾向于带正电荷,ζ电位测量结果同样表明Soil⁃1的表面负电性强于Soil⁃2(见表3).Soil⁃1表面吸附位点有限,且负电荷间的静电斥力导致PFOA的吸附作用较弱,加入APG后,二者对有限的吸附点位竞争激烈,最终APG取代PFOA并使其快速解吸穿透.而Soil⁃2表面负电性较弱,对PFOA和APG的吸附作用相对较强,二者间的竞争吸附不如在Soil⁃1中显著.

(2)土壤粒径:研究表明PFOA的吸附作用受介质粒径的影响,介质颗粒越细,对PFOA的吸附能力越强,其主要原因在于介质的孔隙尺寸会影响有机物在多孔材料中的扩散行为3742-44.Soil⁃1为砂质土,而Soil⁃2为粉黏质土,因此粒径较大的Soil⁃1对PFOA的吸附作用较弱,与APG间竞争吸附作用较为激烈,而粒径较小的Soil⁃2对PFOA的吸附作用较强,同时体积较大的APG分子难以进入Soil⁃2的狭窄孔隙,从而减弱了二者的竞争吸附作用.此外,单一PFOA在Soil⁃1中的吸附解吸速率更快,迁移性更强,而在Soil⁃2中穿透曲线拖尾明显,阻滞更显著,其主要原因为Soil⁃2表面正电荷较多,对PFOA的静电吸附作用较强45.

离子强度影响单一及混合体系中PFOA在土壤中的迁移行为.在Soil⁃1中,高离子强度条件下APG对PFOA迁移的促进作用更显著,而在Soil⁃2中,低离子强度条件下APG对PFOA迁移的促进作用更显著.由表3可知,随着离子强度的增加,土壤表面负电性减弱,Soil⁃1和Soil⁃2的ζ电位分别从-27.77 mV变为-26.95 mV、从-6.63 mV变为-0.25 mV.这是由于在双电层压缩作用下,更多反离子进入土壤吸附层,中和了部分土壤表面的负电荷46.对于吸附位点相对较少的Soil⁃1,土壤表面负电荷减少使其吸附位点稍有增多但程度有限,从而导致PFOA与APG间的竞争吸附更加显著;在吸附位点相对充足的Soil⁃2中,离子强度增大使土壤表面正电性增强,导致其对PFOA和APG的吸附能力显著增强,从而减弱了PFOA与APG间的竞争吸附作用.

2.2 两性表面活性剂对PFOA在土壤中迁移行为的影响

表5图3为单一PFOA及PFOA⁃BS混合体系在Soil⁃1和Soil⁃2中的运移柱实验结果和PFOA的穿透曲线.分析可知,在Soil⁃1和Soil⁃2中,BS⁃12均能增强土壤对PFOA的滞留作用,抑制PFOA的迁移行为,且BS⁃12浓度越高,PFOA在土壤中的阻滞越强.

表5   单一PFOA及PFOA⁃BS混合体系柱实验条件及结果

Table 5  Experimental conditions and results summary of column experiments of single PFOA and PFOA⁃BS mixed systems

土壤类型离子强度(mmol·L-1

CBS⁃12

(mg·L-1

PFOA回收率
Soil⁃11.50100.2%
1.092.1%
10.089.7%
30097.0%
1.092.2%
10.090.1%
Soil⁃21.5093.6%
1.071.6%
10.065.1%
30097.7%
1.078.0%
10.069.0%

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图3

图3   BS⁃12作用下,PFOA在土壤中的穿透曲线:(a,b) Soil⁃1,(c,d) Soil⁃2,误差线表示两次重复实验的偏差

Fig.3   Breakthrough curves of PFOA in the absence and presence of BS⁃12 in columns packed with (a, b) Soil⁃1,(c, d) Soil⁃2


图3a可知,当体系中加入BS⁃12时,PFOA的穿透曲线均呈现右移趋势,且峰值由原来的1.01分别降低为0.93 (CBS⁃12=1 mg·L-1)和0.87 (CBS⁃12=10 mg·L-1);同样,在Soil⁃2中(图3c),随着BS⁃12的加入,PFOA穿透曲线的峰值由0.56分别降低到0.49 (CBS⁃12=1 mg·L-1)和0.42 (CBS⁃12=10 mg·L-1).可见体系中共存的BS⁃12能增强PFOA在两种土壤中的阻滞现象,抑制PFOA的迁移行为,并且高浓度(10 mg·L-1)的BS⁃12对PFOA迁移性的抑制效应更加显著.

两性型表面活性剂BS⁃12的分子结构中包含有机碳链疏水基团和阴、阳离子亲水基团25,而且有研究表明BS⁃12能通过静电作用和疏水作用增强土壤对有机污染物的吸附作用26,BS⁃12在Soil⁃1和Soil⁃2中抑制PFOA迁移的主要机理可能如下.

(1)在Soil⁃1中,由于BS⁃12的pHPZC=5.1~6.126,而Soil⁃1的pH=8.55,因此体系中BS⁃12和Soil⁃1均呈负电性.吴琼等26研究发现在pH=10时,带负电的BS⁃12能吸附于同样带负电的针铁矿表面,主要机理为电性吸附、范德华力和疏水键作用.在Soil⁃1中,由于土壤表面电荷具有不均匀性,BS⁃12可通过静电相互作用(与土壤表面的局部正电荷)、范德华力和疏水键26等吸附于Soil⁃1表面,增强土壤表面的疏水特性及有机碳含量25,进一步通过疏水作用与体系中的PFOA结合,促进PFOA在土壤表面的滞留.此外,BS⁃12的CMC为488.59 mg·L-1[31,而当表面活性剂的浓度达到其CMC的0.1%~1%时可形成半胶束46,故本研究中,BS⁃12在1 mg·L-1和10 mg·L-1的浓度条件下可能在土壤表面形成半胶束,并通过疏水作用与溶液中的PFOA结合形成PFOA⁃BS混合半胶束,促进PFOA在土壤表面的滞留,从而抑制PFOA的迁移行为.

(2)在Soil⁃2中,由于土壤pH=4.30,而BS⁃12的pHPZC=5.1~6.126,因此体系中BS⁃12主要带正电,而Soil⁃2表面呈微弱负电性,大量带正电的BS⁃12分子能通过静电相互作用中和土壤表面部分负电荷,增强其表面正电性,削弱PFOA与土壤表面负电荷间的静电斥力,从而促进PFOA在Soil⁃2中的吸附滞留,抑制PFOA的迁移行为.此外,通过阳离子吸附于Soil⁃2表面的BS⁃12疏水基团向外伸展,可进一步通过疏水作用与PFOA结合或形成混合半胶束,从而促进PFOA在Soil⁃2中的滞留,抑制其迁移行为.

单一及PFOA⁃BS混合体系中PFOA的迁移行为同样受土壤性质和离子强度的影响.Soil⁃1表面负电性较强且粒径较大,对PFOA的吸附作用较弱,BS⁃12的加入虽然增强了土壤对PFOA的吸附作用,但总体而言PFOA的穿透曲线仍然保持较好的对称性,且吸附和解吸速率较快.而Soil⁃2本身表面负电荷少且粒径较小,PFOA在其中的阻滞效应较强,吸附和解吸速率缓慢,BS⁃12的加入进一步增强了PFOA在土壤表面的吸附作用,导致PFOA的穿透曲线相比于原单一PFOA体系,峰值降低且拖尾现象仍然显著.此外,离子强度增大能增强PFOA在Soil⁃1中的阻滞现象.比较图3a和图3b可知,在Soil⁃1中,当离子强度由1.5 mmol·L-1增大至30 mmol·L-1且保持其他条件不变时,PFOA穿透曲线均呈现出一定的右移趋势,其主要机理为离子强度增大引发双电层压缩作用,削弱了PFOA与土壤表面负电荷的静电斥力,进而促进PFOA的吸附滞留.但在Soil⁃2中,当离子强度变化时,PFOA穿透曲线无显著变化趋势,这可能与体系中PFOA的吸附迁移受静电作用与疏水作用的双重影响有关.

3 结论与展望

本研究通过室内饱和柱实验,系统研究了非离子及两性型碳氢表面活性剂烷基糖苷(APG)和十二烷基二甲基甜菜碱(BS⁃12)对PFOA在两种饱和土壤中迁移行为的影响效应及作用机制.研究表明,非离子型碳氢表面活性剂APG能削弱PFOA在土壤中的阻滞,促进其迁移行为,且APG浓度越高,对PFOA迁移性的促进效应越显著,其主控机制为APG与PFOA在土壤介质表面的竞争吸附作用;两性型碳氢表面活性剂BS⁃12能增强PFOA在土壤中的阻滞,抑制其迁移行为,且BS⁃12浓度越高,PFOA的阻滞效应越显著,其主控机制为BS⁃12分子对土壤表面负电荷的中和作用和疏水作用.此外,土壤的表面电性及粒度大小影响单一及混合体系中PFOA的迁移行为,土壤表面电负性越强、粒径越大,对PFOA的吸附作用越弱,PFOA在其中的迁移性越强.

然而,自然界中的土壤类型众多、PFOA的传输距离较远,下一步仍需在真实环境条件及复杂类型土壤中开展相关研究,以更加准确地刻画PFOA在真实土壤⁃地下水环境中的赋存及迁移状况.

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