密度分離法提取土壤中微塑料的優(yōu)化

林 婧,李振國(guó),余光輝,張 勇,宋 焱,王廣懷,姜曉倩

密度分離法提取土壤中微塑料的優(yōu)化

林 婧,李振國(guó)*,余光輝,張 勇,宋 焱,王廣懷,姜曉倩

(湖南科技大學(xué)地球科學(xué)與空間信息工程學(xué)院,湖南 湘潭 411201)

為探究一種簡(jiǎn)單、經(jīng)濟(jì)和可靠的微塑料分離提取手段,設(shè)立了11種(F1～F11)具有不同體積比的飽和NaCl和飽和NaI溶液的混合溶液,將混合液作為浮選液來(lái)提取土壤中的4種類型(聚乙烯,PE;聚苯乙烯,PS;聚氯乙烯,PVC;聚對(duì)苯二甲酸乙二醇酯,PET)微塑料,同時(shí)對(duì)浮選過(guò)程進(jìn)行了改進(jìn).結(jié)果表明:當(dāng)單獨(dú)使用NaCl溶液浮選時(shí),微塑料的總提取率為55.83%,但隨著NaI所占體積比的不斷增加,微塑料的總提取率基本呈上升趨勢(shì),在NaCl:NaI=1:1時(shí)總提取率超過(guò)了90%,在只有NaI的條件下總提取率高達(dá)96.67%;在F1～F11的任意條件下,PE和PS表現(xiàn)為較高的提取率,均超過(guò)了86.67%;PVC和PET在單獨(dú)使用NaCl溶液浮選時(shí)提取率極低,在NaCl:NaI=1:1條件下提取率分別為93.33%和90%,接近單獨(dú)使用NaI溶液浮選時(shí)的提取率; F1～F11任意條件下,低密度微塑料PE和PS的提取率之和均高于高密度微塑料PVC和PET的提取率之和,但在F6～F11條件下提取率差值不大.根據(jù)提取結(jié)果并結(jié)合經(jīng)濟(jì)成本等多方面因素考慮,建議使用NaCl:NaI=1:1的混合溶液來(lái)分離土壤中的微塑料.

密度分離法；微塑料；土壤；提??；浮選液

“微塑料”一詞由Thompson于2004年首次提出[1],它通常是指粒徑小于5mm的塑料顆粒[2].微塑料由于其體積小、難被降解等性質(zhì),能夠在環(huán)境中長(zhǎng)距離運(yùn)輸并持久存在[3].如今在水體[4]、大氣[5]、土壤[6]、食品[7]、飲用水[8-9]甚至人體[10]中都發(fā)現(xiàn)了微塑料的蹤跡,影響著生物生長(zhǎng)與人類的健康安全.此外,微塑料還可作為載體,吸附環(huán)境中的持久性有機(jī)污染物(多氯聯(lián)苯 (PCBs)、多環(huán)芳烴(PAHs)和二氯二苯基三氯乙烷(DDTs)等)[11-13]及重金屬[14-16],產(chǎn)生復(fù)合污染效應(yīng).因此,有必要對(duì)微塑料污染狀況進(jìn)行研究調(diào)查,以明確其潛在環(huán)境風(fēng)險(xiǎn).

有關(guān)于微塑料的研究最早開(kāi)始于海洋環(huán)境,研究者們對(duì)海洋及海岸環(huán)境中微塑料的污染特征及環(huán)境毒理學(xué)效應(yīng)進(jìn)行了分析[17-19].而后延續(xù)到陸地環(huán)境中,國(guó)內(nèi)外學(xué)者開(kāi)始對(duì)河流及湖泊等淡水水體和沉積物中微塑料的含量、分布、富集情況及生態(tài)風(fēng)險(xiǎn)評(píng)價(jià)等方面展開(kāi)研究[20-22].直到近幾年才逐漸把研究焦點(diǎn)放到土壤中來(lái).但由于土壤是一個(gè)復(fù)雜的三相體系,且易受到土壤質(zhì)地、有機(jī)質(zhì)及團(tuán)聚體結(jié)構(gòu)的影響,土壤中微塑料的分離與鑒定比水和沉積物中更加困難[23],因此土壤微塑料的研究還相對(duì)較少.土壤是人們生存和發(fā)展的基礎(chǔ),土壤所種植的糧食作物及蔬菜更是直接與人類的飲食和健康密切相關(guān),因此開(kāi)展土壤微塑料的研究工作具有重要意義.

開(kāi)展土壤微塑料的研究工作的關(guān)鍵在于如何將微塑料從土壤中分離提取出來(lái),才能進(jìn)一步進(jìn)行檢測(cè)和分析,而目前國(guó)際上在提取微塑料的方法學(xué)上還未形成統(tǒng)一的技術(shù)規(guī)范.研究者們最常使用的是密度分離法,通過(guò)利用浮選液和微塑料的密度差來(lái)達(dá)到分離效果.由于飽和NaCl溶液(1.2g/cm3)廉價(jià)易得且對(duì)環(huán)境無(wú)污染[24],成為了現(xiàn)有研究中使用最普遍的浮選液[25-26].但飽和NaCl溶液只能用于提取密度低于1.20g/cm3的聚合物,而在分離聚氯乙烯(PVC,1.22～1.50g/cm3)、聚對(duì)苯二甲酸乙二醇酯(PET, 1.29～1.40g/cm3)等高密度聚合物時(shí)效果很不理想.針對(duì)這一情況,不少研究人員采用KI[27]、ZnCl2[28-29]、NaI[30-31]、聚鎢酸鈉(3Na2WO4·9WO3·H2O, SPT)[32-33]和甲酸鉀(HCO2K)[34-35]等高密度浮選液成功提取了高密度聚合物.但高密度浮選液基本會(huì)對(duì)環(huán)境造成危害,而與其他高密度鹽溶液相比,飽和NaI溶液對(duì)環(huán)境的污染相對(duì)較小.Claessens等[36]使用自來(lái)水和NaI溶液進(jìn)行兩步萃取,該方法對(duì)中塑性顆粒和小微塑性顆粒的提取效果較好,但由于儀器體積大,需要大量溶液,而碘化鈉價(jià)格十分昂貴,導(dǎo)致分析成本較高.Nuelle等[37]采用兩步分選法,樣品首先在NaCl溶液中使用基于流態(tài)化的空氣誘導(dǎo)溢流法進(jìn)行預(yù)提取,然后再用更高密度的NaI溶液對(duì)樣品做后續(xù)浮選,雖減少了NaI使用成本,但該方法操作復(fù)雜.

由于上述浮選液及提取過(guò)程存在提取率不高、成本昂貴、操作復(fù)雜等方面問(wèn)題,鑒于此,本研究基于密度分離法對(duì)浮選液和浮選過(guò)程進(jìn)行了改進(jìn)和優(yōu)化,擬探討一種操作簡(jiǎn)單、經(jīng)濟(jì)、準(zhǔn)確度高的土壤微塑料的提取方法.將飽和NaCl溶液和飽和NaI溶液按照不同體積比混合,共設(shè)立11種不同組合的浮選液.通過(guò)人工模擬微塑料的方法,探究11種不同組合的NaCl和NaI混合液對(duì)土壤中的4種類型(聚乙烯, PE; 聚苯乙烯, PS;聚氯乙烯, PVC;聚對(duì)苯二甲酸乙二醇酯, PET)微塑料的提取效果.根據(jù)提取率結(jié)果,并結(jié)合經(jīng)濟(jì)成本等因素考慮,選出最佳浮選溶液,以期為土壤中微塑料的分離提供方法學(xué)參考和基礎(chǔ)數(shù)據(jù)信息.

1 材料與方法

1.1 供試材料

本試驗(yàn)購(gòu)買了四種常見(jiàn)類型的塑料商品:土工膜(聚乙烯, PE)、快餐泡沫盒(聚苯乙烯, PS)、塑料水管(聚氯乙烯, PVC)和飲料瓶(聚對(duì)苯二甲酸乙二醇酯, PET),分別代表密度較低(PE, 0.94～0.97g/cm3; PS, 1.04～1.08g/cm3)和密度較高(PVC, 1.22～1.50g/ cm3; PET, 1.29～1.40g/cm3)的塑料.人工切割、剪碎和研磨這些塑料商品后過(guò)1mm標(biāo)準(zhǔn)篩,制得不同形狀、大小的微塑料標(biāo)準(zhǔn)品.在連續(xù)變倍體視顯微鏡 (SZN71,舜宇光學(xué)科技(集團(tuán))有限公司)和掃描電子顯微鏡(日立,SU3500)下觀察微塑料的粒徑及形狀特征.微塑料原產(chǎn)品、微塑料標(biāo)準(zhǔn)品、微塑料標(biāo)準(zhǔn)品的顯微鏡及電鏡掃描圖見(jiàn)圖1.每種微塑料標(biāo)準(zhǔn)品具有不同的顏色,以便區(qū)分計(jì)數(shù)并計(jì)算提取率.表1列出了塑料顆粒的性質(zhì)及其原產(chǎn)品.

試驗(yàn)介質(zhì)土壤均采自湘潭市河?xùn)|沿江風(fēng)光帶的表層砂土(0～5cm).土壤于105℃烘箱中烘干、冷卻至室溫后篩選出肉眼可見(jiàn)的動(dòng)植物殘?jiān)?、塑料碎片以及石塊等,再經(jīng)研磨后過(guò)1mm標(biāo)準(zhǔn)篩.研磨后的土壤依次經(jīng)蒸餾水、飽和NaCl溶液(密度為1.20g/cm3)和飽和NaI溶液(密度為1.80g/cm3)多次浮選后去除了土壤樣品本身所含的微塑料,得到了不含微塑料的清潔土壤,經(jīng)80℃烘干后放入預(yù)先清洗過(guò)的棕色玻璃瓶中保存?zhèn)溆?

圖1 試驗(yàn)所用不同類型的微塑料

(a)微塑料原產(chǎn)品; (b)微塑料標(biāo)準(zhǔn)品; (c)微塑料標(biāo)準(zhǔn)品的顯微鏡圖;(d)微塑料標(biāo)準(zhǔn)品的電鏡掃描圖

表1 塑料顆粒的性質(zhì)及其原產(chǎn)品

1.2 試驗(yàn)設(shè)計(jì)及過(guò)程

為探究按照不同體積比混合的NaCl和NaI的浮選液對(duì)微塑料提取率的影響,試驗(yàn)選擇PE、PS、PVC、PET四種微塑料,設(shè)置11組不同組合進(jìn)行試驗(yàn):

F1: NaCl;F2: NaCl:NaI=9:1;F3: NaCl:NaI=8:2; F4: NaCl:NaI=7:3;F5: NaCl:NaI=6:4;F6: NaCl:NaI= 5:5;F7: NaCl:NaI=4:6;F8: NaCl:NaI=3:7;F9: NaCl: NaI=2:8;F10: NaCl:NaI=1:9;F11: NaI.

飽和氯化鈉溶液和飽和碘化鈉溶液由過(guò)量的氯化鈉(NaCl, AR, 99.5%)和碘化鈉(NaI, AR, 99%)顆粒溶解于蒸餾水中制得.F1～F11每組平行3次,共計(jì)33次試驗(yàn).具體步驟如下:

①用分析天平稱取10g清潔土壤于100mL錐形瓶中,并向其中添加4種類型(PE、PS、PVC、PET)的微塑料標(biāo)準(zhǔn)品各10個(gè),共計(jì)40個(gè).

②用量筒量取50mL 浮選液(F1～F11),加入錐形瓶中.

③將錐形瓶置于磁力攪拌器上以200rpm的轉(zhuǎn)速搖勻30min,然后靜置沉降24h,待溶液分層.

④分離上層懸浮液:將上清液倒入配備有微孔濾膜(尼龍,孔徑0.45μm)的布氏漏斗中進(jìn)行抽濾.并用少量蒸餾水沖洗錐形瓶?jī)?nèi)壁和布氏漏斗內(nèi)壁,以盡可能避免微塑料粘附在瓶壁上.

上述浮選過(guò)程重復(fù)3次.

⑤將濾膜轉(zhuǎn)移至玻璃培養(yǎng)皿中,加蓋風(fēng)干.

圖2 微塑料的提取過(guò)程

⑥待濾膜干燥后,在連續(xù)變倍體視顯微鏡 (SZN71,舜宇光學(xué)科技(集團(tuán))有限公司)和掃描電子顯微鏡(JSM-6380LV,JEOL)下觀察微塑料的表面微觀形態(tài),記錄微塑料的數(shù)量并計(jì)算提取率.

以上提取過(guò)程如圖2所示.

⑦消解:將濾膜上的濾渣轉(zhuǎn)移至30mL 30% H2O2中,在水浴恒溫振蕩器中70℃連續(xù)振蕩24h.

⑧消解后將液體真空抽濾至孔徑0.45μm尼龍膜上,并用少量蒸餾水沖洗容器內(nèi)壁,隨后將濾膜轉(zhuǎn)移至玻璃培養(yǎng)皿中加蓋風(fēng)干.

⑨在連續(xù)變倍體式顯微鏡和掃描電子顯微鏡下觀察消解后的微塑料,對(duì)比消解前后微塑料的數(shù)量及表面形態(tài)特征.

1.3 數(shù)據(jù)分析

在本研究中,提取率進(jìn)行了均值計(jì)算和標(biāo)準(zhǔn)偏差核算,結(jié)果用“平均值±標(biāo)準(zhǔn)偏差(=3)”表示.相關(guān)圖表制作在Origin8.0與Excel 2016軟件中完成.

1.4 質(zhì)量控制

為了避免試驗(yàn)過(guò)程中空氣沉降、衣物合成纖維等帶來(lái)的數(shù)據(jù)失真問(wèn)題,采取了以下防護(hù)措施:

①在取樣和實(shí)驗(yàn)過(guò)程中,避免使用一切塑料制品,均采用不銹鋼產(chǎn)品或玻璃制品;

②試驗(yàn)人員始終穿著純棉衣物,而非化纖衣服.

③對(duì)所需用到的儀器預(yù)先進(jìn)行清洗,清洗后立即用鋁箔覆蓋,以避免空氣中可能的微塑料進(jìn)入容器中.

④在每個(gè)單獨(dú)步驟后,所有材料和容器均用鋁箔紙覆蓋開(kāi)口.

⑤在打開(kāi)和分析儲(chǔ)存微塑料的玻璃培養(yǎng)皿之前,對(duì)連續(xù)變倍體視顯微鏡分析的工作場(chǎng)所進(jìn)行清潔.

⑥在試驗(yàn)中設(shè)置空白對(duì)照,除不加入土樣外,其余操作均一致,以監(jiān)控實(shí)驗(yàn)室大氣中可能存在的微塑料污染,但均未觀察到.

2 結(jié)果與分析

2.1 不同浮選液下微塑料的總提取率

在不同浮選液下微塑料的總提取率有所差異,微塑料的總提取率與浮選液的變化關(guān)系如圖3所示.當(dāng)單獨(dú)使用NaCl溶液浮選時(shí)(即F1條件下),微塑料的總提取率最低,僅為55.83%.隨著NaI在混合液中比例的增加,F1àF6條件下微塑料的總提取率大幅上升.當(dāng)浮選液比例為NaCl:NaI=1:1(即F6條件下)時(shí),微塑料的總提取率超過(guò)了90%.F6àF11總提取率緩慢上升并趨于穩(wěn)定,F10和F11條件下微塑料的總提取率高達(dá)96.67%.總體而言,隨著NaI所占體積比的不斷增加,微塑料的總提取率基本呈上升趨勢(shì).

圖3 微塑料的總提取率與浮選液的變化關(guān)系

2.2 不同類型微塑料的提取率

分析結(jié)果顯示,不同類型的微塑料的提取率有所差異(圖4).針對(duì)PE和PS這兩種低密度微塑料,在F1～F11的任意條件下,都具有很高的提取率.PE提取率最低的一組為F2條件下的86.67%,其余條件下均超過(guò)90%;PS提取率最低的一組為F3條件下的90%,F11條件下PS完全被分離出來(lái),提取率為100%.

PVC和PET這兩種高密度微塑料在單獨(dú)使用NaCl溶液浮選時(shí)(即F1條件下)提取率很低,僅為13.33%和23.33%;在F2～F4條件下提取率雖在不斷提高,但也均未超過(guò)80%.當(dāng)NaCl:NaI=1:1(即F6條件下),PVC的和PET的提取率分別達(dá)到了93.33%和90%,與單獨(dú)使用NaI溶液浮選時(shí)(即F11條件下)兩種微塑料提取率分別為96.67%和93.33%的結(jié)果十分相近.

無(wú)論是在何種條件下,低密度微塑料PE和PS的提取率之和均高于高密度微塑料PVC和PET的提取率之和,但在F6～F11條件下低密度微塑料跟高密度微塑料的提取率差值不大.

2.3 微塑料的粒徑及形狀特征

對(duì)制備出的微塑料標(biāo)準(zhǔn)品進(jìn)行了粒徑和形狀表征(圖1(c)～(d)、圖6).

2.3.1微塑料的粒徑特征微塑料的粒徑范圍為58.97～986.43μm,平均粒徑為453.33μm.將微塑料按尺寸大小的不同劃分為<200μm、200～400μm、400～ 600μm、600～800μm以及800～1000μm這5類,微塑料不同尺寸所占百分比如圖5所示.

從總體來(lái)看,微塑料的粒徑范圍以400～600μm居多,占比35.60%,其次分別為200～600μm和600～800μm.而粒徑范圍在<200μm和800～1000μm較少,分別占比7.59%和4.62%.

不同類型的微塑料的粒徑也有所差異.PE與PVC這兩種類型微塑料的粒徑所占百分比最高的均為400～600μm,但所占百分比最低的粒徑范圍有所不同,PE為粒徑<200μm的最低,PVC則是粒徑范圍在400～600μm的最低.而對(duì)于PS和PET這兩種微塑料,粒徑分布較為集中,均有超過(guò)50%的微塑料位于同一粒徑范圍內(nèi),PS有53.88%的微塑料的粒徑位于600～800μm,而PET有55.73%微塑料的粒徑為200～400μm.

圖5 微塑料不同尺寸所占百分比

圖6 不同形狀的微塑料掃描電鏡圖

(a)碎片類; (b)泡沫類; (c)顆粒類; (d)纖維類; (e)薄膜類

2.3.2 微塑料的形狀特征 微塑料的形狀主要包括顆粒、碎片、泡沫、薄膜和纖維五種(圖6及圖7).不同形狀的微塑料所占數(shù)量的百分比依次遞減的順序?yàn)樗槠?顆粒>泡沫>纖維>薄膜.碎片狀的微塑料主要以PE及PET為主,而泡沫狀的微塑料全部為PS.顆粒狀及纖維狀的微塑料主要由PVC構(gòu)成.在四種微塑料中,PET表現(xiàn)的形狀最多,有顆粒、薄膜和碎片三類.

圖7 不同形狀微塑料的百分比

3 討論

3.1 浮選過(guò)程對(duì)微塑料提取率的影響

在分離提取土壤中微塑料的過(guò)程中,由于土壤本身含有懸浮顆粒物、細(xì)小的植物體等雜質(zhì),在分離上清液時(shí)易將雜質(zhì)與微塑料一同分離出去,從而導(dǎo)致濾膜上有較多雜質(zhì),可能會(huì)堵塞濾膜.

Nuelle等[37]在使用NaI溶液浮選時(shí),首先將微塑料與樣品混合于容量瓶中,靜置沉淀分出上清液于燒杯中,然后在燒杯中再次靜置后再傾倒至過(guò)濾裝置中進(jìn)行過(guò)濾;Liu等[38]和Lü等[39]在用飽和NaCl溶液浮選時(shí),均是先在燒杯中浮選出上清液,然后將上清液于錐形瓶中再次靜置再于過(guò)濾器中過(guò)濾.這些研究人員在浮選時(shí)都采取了梯次靜置的措施以達(dá)到減少雜質(zhì)的目的.但在上述過(guò)程中溶液轉(zhuǎn)移2次,有可能造成微塑料丟失的情況,從而使微塑料的提取率數(shù)值略低.因此,為了減少溶液轉(zhuǎn)移次數(shù)從而減少微塑料丟失的可能性,本研究直接將錐形瓶中上清液倒入過(guò)濾裝置中.而為了避免出現(xiàn)濾膜堵塞的情況,試驗(yàn)過(guò)程中采取了勤換濾膜和增加靜置沉降時(shí)間的措施.

在以往的研究中,研究人員們大多選擇靜置沉降1h[40]、6h[41]、24h[38-39]或48h[42]不等.本研究對(duì)比了使用NaCl:NaI=1:1浮選液(F6條件下)浮選時(shí)不同時(shí)間段的沉降效果(圖8).

圖8 NaCl:NaI=1:1浮選液下不同時(shí)間段靜置沉降效果對(duì)比

根據(jù)效果對(duì)比圖,當(dāng)靜置時(shí)間為1h,浮選液十分渾濁;靜置時(shí)間為6h時(shí),體系組分基本分開(kāi),但上清液不夠澄清;靜置時(shí)間為24h時(shí),溶液沉淀完全,上清液十分澄清;靜置時(shí)間為48h時(shí),肉眼下與靜置沉降24h時(shí)效果區(qū)別不大.因此,為盡可能減少進(jìn)入到濾膜上的雜質(zhì)、增大提取率,同時(shí)縮短實(shí)驗(yàn)時(shí)間、提高實(shí)驗(yàn)效率,本研究選擇靜置沉淀24h后再分離上清液.

3.2 浮選溶液對(duì)微塑料提取率的影響

密度分離法被認(rèn)為是分離微塑料的有效方法,并且采用的鹽溶液密度越高則可以提取的微塑料種類范圍就越廣.單獨(dú)使用NaCl溶液浮選時(shí),對(duì)PE、PS、PVC、PET四種類型微塑料的提取率分別為93.33%±5.77%、93.33%±5.77%、13.33%±11.55%、23.33%±15.28%.對(duì)PVC和PET的提取效果不好的原因是飽和NaCl溶液的密度低于PVC和PET,無(wú)法將這兩種微塑料全部從土壤分離開(kāi)來(lái).Liu等[38]使用飽和NaCl溶液對(duì)上海市農(nóng)田土壤中的微塑料進(jìn)行浮選提取,PP、PE、PA、PC、ABS、PMMA和PS顆粒的回收率均在90%以上,而未成功提取出PVC和PET.PE與PS的提取結(jié)果與本實(shí)驗(yàn)相似,而PVC和PET的提取率比本實(shí)驗(yàn)略低,這可能是由于Liu等在轉(zhuǎn)移微塑料的過(guò)程中進(jìn)行了兩次轉(zhuǎn)移,造成了部分微塑料的損失,且Liu等[38]的提取對(duì)象跟本實(shí)驗(yàn)不同,提取對(duì)象間存在特征差異可能會(huì)導(dǎo)致提取率結(jié)果存在差異.

當(dāng)混合液體積比為NaCl:NaI=1:1時(shí),PE、PS和PVC的提取率的提取率均為93.33%,而PET也增至90%.相比使用NaCl溶液浮選,PVC及PET兩種微塑料的提取率大幅增高,這是由于隨著混合液中NaI的體積增加,混合液的密度逐漸增大,因而提取率也在逐漸上升.

當(dāng)浮選液全部為NaI溶液時(shí),溶液密度達(dá)到最大值,此時(shí)PE、PS、PVC、PET的提取率分別96.67%±5.77% 、100.00%±0.00%、96.67%±5.77%、93.33%±5.77% .此結(jié)果與Nuelle等[37]所得出的PE、PP、PVC、PET、PS、PUR的回收率為91%～99%的結(jié)論相一致.

PE、PS、PVC和PET四種類型的微塑料在NaCl、NaCl:NaI=1:1和NaI三種不同浮選液下的提取率結(jié)果對(duì)比見(jiàn)圖9.相較使用NaCl溶液浮選,使用NaCl:NaI=1:1的混合液或NaI溶液浮選對(duì)高密度微塑料PVC和PET的提取效果好,三種溶液下PE與PS的提取率值相差不大.

圖9 NaCl、NaCl:NaI=1:1、NaI三種不同浮選液下PE、PS、PVC和PET的提取率

3.3 消解對(duì)微塑料提取率及表面形態(tài)特征的影響

土壤中有機(jī)質(zhì)的密度通常介于1.0～1.4g/cm3,與PET和PA等微塑料的密度相近[43].因此簡(jiǎn)單的密度分離法并不能很好地實(shí)現(xiàn)微塑料的分離[44],還需進(jìn)行消解處理.

表2 消解前后微塑料的提取率變化

圖10 消解前后PE的掃描電鏡圖

(a)消解前; (b)消解后

目前,H2O2是大多數(shù)研究者們常用的有機(jī)質(zhì)消解液[38,45-46],但有關(guān)H2O2消解的最佳濃度、消解時(shí)間和溫度還未統(tǒng)一,各項(xiàng)研究采取的參數(shù)都不相同,且對(duì)于 H2O2是否會(huì)破壞微塑料的結(jié)構(gòu)還存在爭(zhēng)議[37,46-47].

試驗(yàn)對(duì)比了在70℃下經(jīng)30% H2O2消解24h前后的微塑料,發(fā)現(xiàn)消解過(guò)程對(duì)微塑料的數(shù)量未造成影響,11種(F1～F11)浮選液組合下微塑料的提取率均沒(méi)有發(fā)生變化(表2).而通過(guò)掃描電子顯微鏡觀察到經(jīng)30% H2O2處理后會(huì)導(dǎo)致PE表面輕微溶解(圖10),消解過(guò)程對(duì)于其余三種類型微塑料表面形態(tài)的影響則未觀察到.由于此消解方案對(duì)土壤有機(jī)質(zhì)的去除效果良好,且對(duì)微塑料表面雖有輕微溶解但不會(huì)造成微塑料丟失.因此,使用30% H2O2于70℃下消解24h可作為一個(gè)優(yōu)良的消解方案用于土壤有機(jī)質(zhì)的去除.

3.4 土壤中微塑料分離提取的最佳浮選液

目前,國(guó)內(nèi)外研究中大多使用單一的浮選液,而將高密度鹽溶液和低密度鹽溶液的混合液用來(lái)分離提取的研究還很罕見(jiàn).本研究探究了NaCl和NaI的混合液對(duì)土壤中微塑料的提取效果,根據(jù)實(shí)驗(yàn)結(jié)果,若單獨(dú)選用NaCl溶液浮選或NaCl溶液體積少于NaCl和NaI混合液一半時(shí),提取效果不太理想.當(dāng)NaCl:NaI=1:1及NaI溶液體積多于NaCl溶液體積時(shí),浮選液能夠?qū)⑼寥乐?0%以上不同密度不同種類的微塑料成功提取出來(lái).由于NaI價(jià)格昂貴,而比較使用NaCl:NaI=1:1浮選液與NaI浮選液的兩種提取結(jié)果,提取率數(shù)值相差不大,且由圖11可明顯得出使用NaCl:NaI=1:1浮選的效果十分穩(wěn)定,因此從經(jīng)濟(jì)成本角度出發(fā),建議使用NaCl:NaI=1:1的混合液用于土壤中微塑料的分離提取.

圖11 NaCl:NaI=1:1浮選液下微塑料的提取率

此外,NaCl:NaI=1:1的混合液相較于其他常用的高密度鹽溶液具有一定的優(yōu)越性,如:CaCl2溶液會(huì)促進(jìn)土壤有機(jī)質(zhì)結(jié)塊,干擾后續(xù)微塑料的識(shí)別[44]; ZnCl2溶液具有很強(qiáng)的毒性和高腐蝕性,可能會(huì)造成環(huán)境風(fēng)險(xiǎn)[48];NaI雖具有一定的氧化性[49],但在實(shí)驗(yàn)過(guò)程中是可控的,不會(huì)對(duì)測(cè)量結(jié)果造成影響.因此,可將NaCl:NaI=1:1作為浮選液用于分離提取微塑料,且為了節(jié)約成本可進(jìn)行回收與重復(fù)利用.

為了驗(yàn)證NaCl:NaI=1:1的混合液作為浮選液的合理性,試驗(yàn)中測(cè)定了所配置的NaCl:NaI=1:1的混合液的密度,得出結(jié)果為1.56g/cm3,而環(huán)境介質(zhì)中發(fā)現(xiàn)的微塑料的密度通常為0.8～1.4g/cm3 [50],符合密度分離法的原理,將NaCl:NaI=1:1的混合液用于提取現(xiàn)實(shí)環(huán)境中的土壤樣品具有理論依據(jù).

使用NaCl:NaI=1:1浮選液對(duì)各種微塑料的提取率均高達(dá)90%及以上,說(shuō)明該浮選液用于提取微塑料是有效的,但由于密度分離法本身的局限性,它可能不適合分離粒徑小于10μm的塑料顆粒[34],而小塑料微粒(<50μm)含量很高,約占總塑料微粒的35%～90%[19,51],如何有效從土壤中分離小顆粒微塑料是今后需要關(guān)注的問(wèn)題.

4 結(jié)論

4.1 本研究對(duì)土壤微塑料檢測(cè)中的浮選過(guò)程進(jìn)行了改進(jìn):通過(guò)減少溶液的轉(zhuǎn)移次數(shù)從而避免了微塑料丟失的可能性,采取了增加靜置沉降的時(shí)間與頻換濾膜以達(dá)到減少雜質(zhì)的目的.

4.2 采用NaCl和NaI混合液浮選時(shí),隨著NaCl和NaI混合液中NaI所占比例不斷增加,微塑料的提取率不斷上升.由于使用NaCl:NaI=1:1溶液浮選時(shí)的對(duì)各類微塑料的提取率均在90%以上,且接近于使用NaI溶液浮選的提取率,從經(jīng)濟(jì)成本角度出發(fā),建議使用NaCl:NaI=1:1的混合液用于土壤中微塑料的分離提取.

4.3 NaCl:NaI=1:1的混合液相較于其他常用的浮選液具有一定優(yōu)越性:比CaCl2準(zhǔn)確、比ZnCl2環(huán)保、比NaI經(jīng)濟(jì),將NaCl:NaI=1:1的混合液用于分離提取土壤中的微塑料是合理、準(zhǔn)確且高效的.

[1] Thompson R C, Olsen Y, Mitchell R P, et al. Lost at Sea: where is all the plastic? [J]. Science, 2004,304(5672):838-838.

[2] Claessens M, De Meester S, Van Landuyt L, et al. Occurrence and distribution of microplastics in marine sediments along the Belgian coast [J]. Marine pollution bulletin, 2011,62(10):2199-2204.

[3] Andrady A L. Microplastics in the marine environment [J]. Marine Pollution Bulletin, 2011,62(8):1596-1605.

[4] Alimi O S, Farner Budarz J, Hernandez L M, et al. Microplastics and nanoplastics in aquatic environments: aggregation, deposition, and enhanced contaminant transport [J]. Environmental Science & Technology, 2018,52(4):1704-1724.

[5] 田 媛,涂 晨,周 倩,等.環(huán)渤海海岸大氣微塑料污染時(shí)空分布特征與表面形貌 [J]. 環(huán)境科學(xué)學(xué)報(bào), 2020,40(4):1401-1409.

Tian Y, Tu C, Zhou Q, et al. The temporal and spatial distribution and surface morphology of atmospheric microplastics around the Bohai Sea [J]. Acta Scientiae Circumstantiae, 2020,40(4):1401-1409.

[6] Nizzetto L, Langaas S, Futter M. Pollution: Do microplastics spill on to farm soils? [J]. Nature, 2016,537(7621):488-488.

[7] EFSA Panel on Contaminants in the Food Chain (CONTAM). Presence of microplastics and nanoplastics in food, with particular focus on seafood [J]. Efsa Journal, 2016,14(6):e04501.

[8] Kosuth M, Mason S A, Wattenberg E V. Anthropogenic contamination of tap water, beer, and sea salt [J]. PloS one, 2018,13(4):e0194970.

[9] Schymanski D, Goldbeck C, Humpf H U, et al. Analysis of microplastics in water by micro-Raman spectroscopy: release of plastic particles from different packaging into mineral water [J]. Water Research, 2018,129:154-162.

[10] Wright S L, Kelly F J. Plastic and human health: a micro issue? [J]. Environmental science & technology, 2017,51(12):6634-6647.

[11] Antunes J C, Frias J G L, Micaelo A C, et al. Resin pellets from beaches of the Portuguese coast and adsorbed persistent organic pollutants [J]. Estuarine, Coastal and Shelf Science, 2013,130:62-69.

[12] Frias J, Sobral P, Ferreira A M. Organic pollutants in microplastics from two beaches of the Portuguese coast [J]. Marine Pollution Bulletin, 2010,60(11):1988-1992.

[13] Oliveira M, Ribeiro A, Hylland K, et al. Single and combined effects of microplastics and pyrene on juveniles (0+ group) of the common goby(Teleostei, Gobiidae) [J]. Ecological Indicators, 2013,34:641-647.

[14] Ashton K, Holmes L, Turner A. Association of metals with plastic production pellets in the marine environment [J]. Marine Pollution Bulletin, 2010,60(11):2050-2055.

[15] 范秀磊,常卓恒,鄒曄鋒,等.可降解微塑料對(duì)銅和鋅離子的吸附解吸特性 [J]. 中國(guó)環(huán)境科學(xué), 2021,41(5):2141-2150.

Fan X L, Chang Z H, Zou Y F, et al. Adsorption and desorption properties of degradable microplastic for Cu2+and Zn2+[J]. China Environmental Science, 2021,41(5):2141-2150.

[16] Brennecke D, Duarte B, Paiva F, et al. Microplastics as vector for heavy metal contamination from the marine environment [J]. Estuarine, Coastal and Shelf Science, 2016,178:189-195.

[17] La Daana K K, G?rdfeldt K, Lyashevska O, et al. Microplastics in sub-surface waters of the Arctic Central Basin [J]. Marine Pollution Bulletin, 2018,130:8-18.

[18] Zhao S, Zhu L, Wang T, et al. Suspended microplastics in the surface water of the Yangtze Estuary System, China: first observations on occurrence, distribution [J]. Marine Pollution Bulletin, 2014,86(1/2): 562-568.

[19] Eriksen M, Mason S, Wilson S, et al. Microplastic pollution in the surface waters of the Laurentian Great Lakes [J]. Marine Pollution Bulletin, 2013,77(1/2):177-182.

[20] Anderson P J, Warrack S, Langen V, et al. Microplastic contamination in lake Winnipeg, Canada [J]. Environmental Pollution, 2017,225: 223-231.

[21] 王志超,竇雅嬌,周 鑫,等.岱海冰封期微塑料與環(huán)境因子的關(guān)系及風(fēng)險(xiǎn)評(píng)價(jià) [J]. 中國(guó)環(huán)境科學(xué), 2022,42(2):889-896.

Wang Z C, Dou Y J, Zhou X, et al. Relationship between microplastics occurrence and environmental factors and risk assessment during ice-covered period of the Daihai Lake. China Environmental Science, 2022,42(2):889-896.

[22] He B, Goonetilleke A, Ayoko G A, et al. Abundance, distribution patterns, and identification of microplastics in Brisbane River sediments, Australia [J]. Science of the Total Environment, 2020, 700:134467.

[23] Rillig M C. Microplastic in terrestrial ecosystems and the soil? [J]. Environmental Science & Technology, 2012,46(12):6453-6454.

[24] Li J, Liu H, Chen J P. Microplastics in freshwater systems: A review on occurrence, environmental effects, and methods for microplastics detection [J]. Water research, 2018,137:362-374.

[25] Lots F A E, Behrens P, Vijver M G, et al. A large-scale investigation of microplastic contamination: abundance and characteristics of microplastics in European beach sediment [J]. Marine Pollution Bulletin, 2017,123(1/2):219-226.

[26] Zhao J, Ran W, Teng J, et al. Microplastic pollution in sediments from the Bohai Sea and the Yellow Sea, China [J]. Science of the Total Environment, 2018,640:637-645.

[27] Phuong N N, Poirier L, Lagarde F, et al. Microplastic abundance and characteristics in French Atlantic coastal sediments using a new extraction method [J]. Environmental Pollution, 2018,243:228-237.

[28] Liebezeit G, Dubaish F. Microplastics in beaches of the East Frisian islands Spiekeroog and Kachelotplate [J]. Bulletin of environmental contamination and toxicology, 2012,89(1):213-217.

[29] 韓麗花,李巧玲,徐 笠,等.大遼河沉積物中微塑料的污染特征 [J]. 中國(guó)環(huán)境科學(xué), 2020,40(4):1649-1658.

Han L H, Li Q L, Xu L, et al. The pollution characteristics of microplastics in Daliao River sediments [J]. China Environmental Science, 2020,40(4):1649-1658.

[30] Wang J, Wang M, Ru S, et al. High levels of microplastic pollution in the sediments and benthic organisms of the South Yellow Sea, China [J]. Science of the Total Environment, 2019,651:1661-1669.

[31] Van Cauwenberghe L, Vanreusel A, Mees J, et al. Microplastic pollution in deep-sea sediments [J]. Environmental Pollution, 2013,182:495-499.

[32] Corcoran P L, Biesinger M C, Grifi M. Plastics and beaches: A degrading relationship [J]. Marine pollution bulletin, 2009,58(1): 80-84.

[33] Ballent A, Corcoran P L, Madden O, et al. Sources and sinks of microplastics in Canadian Lake Ontario nearshore, tributary and beach sediments [J]. Marine Pollution Bulletin, 2016,110(1):383-395.

[34] Zhang K, Xiong X, Hu H, et al. Occurrence and characteristics of microplastic pollution in Xiangxi Bay of Three Gorges Reservoir, China [J]. Environmental Science & Technology, 2017,51(7):3794- 3801.

[35] Zhang K, Su J, Xiong X, et al. Microplastic pollution of lakeshore sediments from remote lakes in Tibet plateau, China [J]. Environmental Pollution, 2016,219:450-455.

[36] Claessens M, Van Cauwenberghe L, Vandegehuchte M B, et al. New techniques for the detection of microplastics in sediments and field collected organisms [J]. Marine Pollution Bulletin, 2013,70(1/2):227- 233.

[37] Nuelle M T, Dekiff J H, Remy D, et al. A new analytical approach for monitoring microplastics in marine sediments [J]. Environmental Pollution, 2014,184:161-169.

[38] Liu M, Lu S, Song Y, et al. Microplastic and mesoplastic pollution in farmland soils in suburbs of Shanghai, China [J]. Environmental Pollution, 2018,242:855-862.

[39] Lv W, Zhou W, Lu S, et al. Microplastic pollution in rice-fish co-culture system: A report of three farmland stations in Shanghai, China [J]. Science of the Total Environment, 2019,652:1209-1218.

[40] Fries E, Dekiff J H, Willmeyer J, et al. Identification of polymer types and additives in marine microplastic particles using pyrolysis-GC/MS and scanning electron microscopy [J]. Environmental science: Processes & Impacts, 2013,15(10):1949-1956.

[41] Shim W J, Song Y K, Hong S H, et al. Identification and quantification of microplastics using Nile Red staining [J]. Marine Pollution Bulletin, 2016,113(1/2):469-476.

[42] Li Q, Wu J, Zhao X, et al. Separation and identification of microplastics from soil and sewage sludge [J]. Environmental Pollution, 2019,254:113076.

[43] Bl?sing M, Amelung W. Plastics in soil: Analytical methods and possible sources [J]. Science of the Total Environment, 2018,612:422- 435.

[44] Scheurer M, Bigalke M. Microplastics in Swiss floodplain soils [J]. Environmental science & technology, 2018,52(6):3591-3598.

[45] Zhang K, Shi H, Peng J, et al. Microplastic pollution in China's inland water systems: a review of findings, methods, characteristics, effects, and management [J]. Science of the Total Environment, 2018,630: 1641-1653.

[46] Liebezeit G, Dubaish F. Microplastics in beaches of the East Frisian islands Spiekeroog and Kachelotplate [J]. Bulletin of Environmental Contamination and Toxicology, 2012,89(1):213-217.

[47] Avio C G, Gorbi S, Regoli F. Experimental development of a new protocol for extraction and characterization of microplastics in fish tissues: first observations in commercial species from Adriatic Sea [J]. Marine environmental research, 2015,111:18-26.

[48] 顧偉康,楊國(guó)峰,劉 藝,等.環(huán)境介質(zhì)中微塑料的處理與檢測(cè)方法研究進(jìn)展 [J]. 土木與環(huán)境工程學(xué)報(bào)(中英文), 2020,42(1):135-143.

Gu W K, Yang G F, Liu Y, et al. Treatment and detection methods of microplastics from environmental media: A review [J]. Journal of Civil And Environmental Engineering (Chinese and English), 2020,42(1): 135-143.

[49] Hidalgo-Ruz V, Gutow L, Thompson R C, et al. Microplastics in the marine environment: a review of the methods used for identification and quantification [J]. Environmental Science & Technology, 2012, 46(6):3060-3075.

[50] 李昇昇,李良忠,李 敏,等.環(huán)境樣品中微塑料及其結(jié)合污染物鑒別分析研究進(jìn)展 [J]. 環(huán)境化學(xué), 2020,39(4):960-974.

Li S S, Li L Z, Li M, et al. Study on identification of microplastics and the combined pollutants in environmental samples [J]. Environmental Chemistry, 2020,39(4):960-974.

[51] Imhof H K, Laforsch C, Wiesheu A C, et al. Pigments and plastic in limnetic ecosystems: A qualitative and quantitative study on microparticles of different size classes [J]. Water Research, 2016,98: 64-74.

Optimization of microplastics extraction from soil by density separation.

LIN Jing, LI Zhen-guo*, YU Guang-hui, ZHANG Yong, SONG Yan, WANG Guang-huai, JIANG Xiao-qian

(School of Earth Sciences and Spatial Information Engineering, Hunan University of Science and Technology, Xiangtan 411201, China)., 2022,42(7)：3285～3294

To explore a simple, economic and reliable method for separation and extraction of microplastics from soil, eleven mixed solutions comprising saturated NaCl and NaI solutions with progressively changing volume ratios (F1～F11) were prepared. These mixed solutions were used as the flotation liquid to extract four types of microplastics including polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET) absorbed by soil with an improved protocol for flotation separation. The results showed that the total extraction rate of microplastics was only 55.83% when used purely NaCl saturated solution, while this extraction rate increased with an growing volume ratio of NaI in the mixture. The total extraction rate exceeded 90% when the volume ratio of saturated NaCl solution to NaI solution was 1:1and reached as high as 96.67% when using purely NaI solution. For all the eleven mixed solutions used, the extraction rates of PE and PS exceeded 86.67%, relatively higher than that of the other two types of microplastics. When using saturated NaCl solution alone, the extraction rates of PVC and PET were extremely low compared with that of 93.33% and 90% respectively in case the volume ratio of NaCl to NaI solution was 1:1, which were close to those obtained using NaI solution alone. Sum of the extraction rates of lower-density microplastics PE and PS was constantly higher than that of higher-density microplastics PVC and PET for all the eleven mixed solutions. However, the difference between extraction rates was neglectable among the mixed solutions F6 to F11. Based on the abovementioned results and considering factors such as economic cost, it is recommended that the mixture of saturated NaCl to NaI solution with a volume ratio of of 1:1 could be the optimal to extract microplastics in soil samples.

density separation；microplastics；soil；extraction；flotation liquid

X53

A

1000-6923(2022)07-3285-10

林 婧,(1998-),女,湖南湘潭人,湖南科技大學(xué)碩士研究生,主要從事土壤環(huán)境污染生態(tài)修復(fù)研究.

2021-12-08

湖南教育廳科研項(xiàng)目(16A068);湖南省自然科學(xué)基金項(xiàng)目(2018JJ3157)

* 責(zé)任作者,副教授, lizhenguo@hnust.edu.cn