滲濾液中Na+對生物膜形成初期吸附特性的影響

劉 豐,王 前,吳華南,徐期勇

滲濾液中Na+對生物膜形成初期吸附特性的影響

劉 豐,王 前,吳華南,徐期勇*

(北京大學(xué)深圳研究生院環(huán)境與能源學(xué)院,廣東 深圳 518055)

為了探究滲濾液微生物生物膜形成初期吸附特性,設(shè)置4種Na+濃度采用耗散型石英晶體微天平(QCM-D),結(jié)合剛性模型和黏彈性模型擬合分析,探究吸附質(zhì)量和黏彈性隨時間變化.結(jié)果表明:生物膜初期吸附存在可逆吸附和不可逆吸附過程,150mmol/L的Na+促進了不可逆吸附的形成.Na+增加了溶液Zeta電位,細菌表面所帶凈電荷量減小;隨Na+濃度由2.5mmol/L增加至50mmol/L,表面吸附速率逐漸增加,符合靜電雙層理論,Na+繼續(xù)增加至150mmol/L,吸附速率無顯著變化.在不同濃度Na+溶液中,吸附膜柔性大小順序為:150mmol/L>2.5mmol/L>10mmol/L≈50mmol/L.模型擬合結(jié)果表明,膜黏彈性隨吸附過程逐漸增加;隨Na+濃度增加,吸附膜質(zhì)量增加,膜黏彈性先升高后不變.

微生物吸附；耗散型石英晶體微天平；頻率；耗散；黏彈性

滲濾液污染控制是保證填埋場正常運行的關(guān)鍵因素[1-3].滲濾液具有水質(zhì)復(fù)雜、鹽分濃度高、微生物群落豐富的特點[4-6].而微生物可通過相互黏附和生長繁殖在滲濾液收集和處理過程中形成生物膜[7-9].生物膜的形成會增加滲濾液的滲透阻力,甚至造成填埋場內(nèi)部淤堵,降低滲濾液收集效果;同時會增加滲濾液處理反應(yīng)器結(jié)垢風(fēng)險,降低滲濾液處理效率[10-11].

在生物膜形成初期,離子濃度是影響微生物吸附的重要因素之一[12].研究表明較高的離子濃度可能會影響?zhàn)じ轿镔|(zhì)的結(jié)構(gòu)或起到中和電荷作用,繼而減弱微生物和表面相互作用[13].目前已有研究主要針對Ca2+等二價陽離子對生物膜形成的影響,包括靜電作用、絡(luò)合作用等[14-16].但目前仍缺乏對Na+影響滲濾液中生物膜形成過程的解釋.在滲濾液中Na+為主要的陽離子之一,其濃度范圍為70～ 7700mg/L[17].研究表明Na+對微生物傳輸及吸附起重要作用.Jewett等[18]利用連續(xù)流實驗柱研究了Na+濃度對熒光假單胞菌P17的傳輸和遷移影響,發(fā)現(xiàn)離子濃度增大能提高細菌碰撞效率.但目前研究主要通過宏觀實驗檢測Na+離子濃度對微生物傳輸?shù)撵o態(tài)影響[19],而微生物吸附初期較短時間內(nèi)的動態(tài)變化過程尚未明確.Long等[20]采用耗散型石英晶體微天平(QCM-D)監(jiān)測Na+對單一菌種沉積效率的影響.但在混合菌種環(huán)境下Na+對微生物的動態(tài)吸附及解吸附過程仍不清晰.

因此,本研究選取模擬填埋柱滲濾液中微生物,使用QCM-D監(jiān)測不同Na+濃度中混合菌種初期吸附動態(tài)過程.分析沉積速率及吸附膜結(jié)構(gòu)特性,并采用2種不同模型擬合,比較吸附質(zhì)量和黏彈性變化,解析Na+離子濃度對滲濾液微生物生物膜初期吸附影響機制.研究結(jié)果可為實際滲濾液收集處理過程中生物膜防治提供參考.

1 材料與方法

1.1 材料

本實驗采用分析純級氯化鈉(NaCl,阿拉丁試劑有限公司),胰蛋白胨(北京拜爾迪生物技術(shù)有限公司),酵母提取粉(北京拜爾迪生物技術(shù)有限公司),無水乙醇(安徽安特食品股份有限公司).

微生物獲取與培養(yǎng):本研究中混合微生物菌種提取于模擬填埋場反應(yīng)器(垃圾焚燒爐渣:模擬生活垃圾=1:9,wt%)滲濾液[21].微生物采用液體LB培養(yǎng)基(胰蛋白胨10g/L,酵母提取粉5g/L和氯化鈉10g/L),使用超純水溶解并在121℃下滅菌20min.微生物培養(yǎng)基置于35℃恒溫?fù)u床中培養(yǎng),轉(zhuǎn)速為150r/min.微生物初步生長至穩(wěn)定期后,被稀釋轉(zhuǎn)移至另一空白液體LB培養(yǎng)基,重復(fù)該操作多次,直至微生物生長周期達到穩(wěn)定.后續(xù)實驗采取處于生長穩(wěn)定期的微生物菌液,該階段細胞濃度約為1.2×107個/mL.微生物群落分析方法參考之前研究中的方法進行[22],組成的細菌(科級別)有Clostridiaceae(48%), norank_o__Peptostreptococcales- Tissierellales(27%), Dysgonomonadaceae(13%),Anaerovoracaceae(5%), Oscillospiraceae(2%),Morganellaceae (低于1%的忽略不計).

1.2 菌液處理及基本表征

用超純水配置2.5, 10, 50和150mmol/L的NaCl溶液(考慮實際滲濾液中Na+濃度[17]設(shè)計),取處于LB液體培養(yǎng)基中穩(wěn)定期的微生物溶液5mL于離心管中,1000離心15min后去上清液,加入不同濃度NaCl溶液至50mL,重復(fù)離心清洗操作3次[20,23],以獲得稀釋于不同濃度NaCl鹽溶液中的菌液.

對不同濃度NaCl溶液及對應(yīng)菌液進行Zeta電位測試(Nano-ZS90,英國),每個樣品測試3次取其平均值.

1.3 QCM-D實驗

采用耗散型石英晶體微天平(QCM-D,Q-Sense Analyzer,瑞典)考察不同濃度Na+溶液對填埋場微生物在金芯片(QSX 301,Biolin Scientific AB,直徑為14mm)表面的吸附影響.

用無水乙醇清洗QCM-D金芯片后,氮氣吹干使用.使用蠕動泵向QCM-D的樣品室中通入超純水10min,建立穩(wěn)定基線(頻率變化小于1Hz);再通入不同濃度NaCl溶液(背景溶液)40min至穩(wěn)定后,注入稀釋于對應(yīng)濃度鹽溶液中的菌液90min(本實驗中菌液吸附時間統(tǒng)一為90min),該過程微生物吸附于芯片表面;最后用同樣濃度NaCl溶液(背景溶液)清洗30min以上使得曲線穩(wěn)定,該解吸附過程目的是排除細菌與芯片表面的非特異性吸附[24].獲取不同頻率(3,5,7,9,11)和耗散(3,5,7,9,11)隨時間變化數(shù)據(jù).實驗過程中采用蠕動泵控制液體流速為0.1mL/min,反應(yīng)溫度為25℃,每組實驗重復(fù)2次.

式中:D為沉積質(zhì)量,(1, 3, 5, 7……)為泛音數(shù),Df為共振頻率變化,為晶體質(zhì)量靈敏度常數(shù)(當(dāng)=1,0=5MHz,=17.8ng/(Hz×cm2).

式中:0為芯片密度, kg/m3;0為芯片厚度, mm;3為背景溶液穿透深度, mm;3為背景溶液黏性模量, Pa·s;為吸附層密度, kg/m3;為吸附層黏性模量, Pa·s;為吸附層彈性模量, kPa;h為吸附層厚度, mm;為吸附層層數(shù),=1,2;為頻率, Hz; w是2p, s-1.

利用Qsense Dfind軟件(Biolin Scientific AB, Sweden)對所得數(shù)據(jù)進行模型擬合分析,包括Sauerbrey和Voight模型,基于吸附膜是均勻的假設(shè),以獲取定量分析數(shù)據(jù),包括吸附質(zhì)量和黏彈性等.

1.4 數(shù)據(jù)分析方法

對所得結(jié)果進行顯著性分析(SPSS Inc.,USA),采用單因素方差分析(ANOVA)評價其統(tǒng)計學(xué)差異,值小于0.05為統(tǒng)計學(xué)差異.

2 結(jié)果與分析

2.1 不同濃度Na+溶液中微生物Zeta電位

如表1所示,在不同濃度Na+溶液(2.5, 10, 50, 150mmol/L)中,微生物Zeta電位都為負(fù)值,且隨Na+濃度升高電位絕對值減小,與已有研究結(jié)果一致[20,29].一般認(rèn)為細菌帶有凈的負(fù)電荷,靜電雙層理論認(rèn)為當(dāng)陽離子濃度增加時,靜電雙層壓縮導(dǎo)致體積減小,溶液電位負(fù)值減小,顆粒物之間排斥力繼而減小,因此細菌接近表面時,范德華吸引力可克服兩個表面之間的排斥能壘,會導(dǎo)致細菌更易黏附在表面增加生物絮凝[30-31],因此Na+的添加可能會通過靜電雙層壓縮作用影響微生物的表面吸附和沉積.

表1 不同離子濃度Na+溶液中的菌液表面Zeta電位

2.2 QCM-D結(jié)果分析

2.2.1 Na+影響微生物吸附行為分析 選取頻率3隨時間變化曲線,以直觀說明Na+對微生物吸附形成生物膜行為的影響,如圖1所示.

超純水穩(wěn)定基線10min后,通入背景溶液(不同濃度的Na+溶液),該階段頻率先降低后穩(wěn)定,不同濃度背景溶液頻率降低值|D|存在明顯差異.按Na+濃度從低到高(2.5, 10, 50, 150mmol/L), |D|分別為0.1, 1.7, 1.8和6.6Hz,即|D|隨濃度增大而增大,說明Na+自身也可以吸附在金芯片表面,且隨濃度增大吸附量增大.通入稀釋于對應(yīng)濃度鹽溶液中的菌液后,頻率持續(xù)下降,且無穩(wěn)定趨勢,說明微生物持續(xù)吸附在芯片表面逐漸形成生物膜.微生物吸附階段, |D|隨濃度增加依次為10.9, 16.1, 19.1和19.7Hz,在150mmol/L的Na+背景溶液中|D|為最低濃度2.5mmol/L溶液的1.8倍.隨Na+濃度增加,D逐漸增加,但高濃度(50,150mmol/L)時頻率變化較小,說明吸附過程并不是受靜電雙層壓縮單一因素影響[29].

圖1 頻率(f3)隨時間變化曲線

隨后重新通入對應(yīng)濃度背景溶液以觀察微生物解吸附過程,結(jié)果表明4種濃度的f都有所上升, |D|按Na+濃度增加趨勢分別為2.0, 0.8, 0.7和0.2Hz,分別占吸附階段的18%, 5%, 4%和1%,說明在Na+溶液洗脫下都存在解吸附現(xiàn)象,但吸附過程不是完全可逆,即微生物表面吸附行為都存在可逆吸附和不可逆吸附,且以不可逆吸附為主導(dǎo).在低濃度Na+(2.5mmol/L)作用下,可逆吸附比例更高,高濃度Na+會促進生物膜不可逆吸附的形成.Olsson等[32]認(rèn)為細菌和吸附表面鍵的成熟是可逆吸附轉(zhuǎn)變?yōu)椴豢赡娴脑?而鍵的成熟可能與界面水的去除或細菌代謝導(dǎo)致胞外聚合物EPS的分泌有關(guān).

常用|D|/D值評估吸附速率[33-34],由圖1可知,該階段頻率隨時間下降趨勢并不是線性變化,斜率絕對值逐漸減小,說明沉積速率隨時間逐漸下降.由此可知,當(dāng)表面吸附微生物時,隨著時間的推移,微生物聚集體增長形成生物膜,進一步降低了溶液中微生物向芯片表面的傳輸速率,吸附速率會逐漸降低[33].

通過計算通入菌液階段(約60～150min)的平均|D|/D值得平均沉積速率,按Na+濃度從低到高的順序依次為(0.118 ± 0.004), (0.179 ± 0.001), (0.216 ± 0.005)和(0.211 ± 0.011)Hz/min, ANOVA分析結(jié)果證明其具有顯著性差異(=0.001).Na+濃度從2.5mmol/L增加至50mmol/L,微生物平均吸附速率明顯加快,因為Zeta電位表明濃度越高電位(負(fù))絕對值越小,顆粒物與芯片表面壁壘越小,越易吸附,符合靜電雙層理論[20,33],這與Long等[20]研究結(jié)果一致;但當(dāng)Na+濃度繼續(xù)增加至150mmol/L時,平均吸附速率基本不變,這可能與Na+濃度過高有關(guān),Chen等[33]的研究表明當(dāng)電解液濃度高于臨界聚沉濃度時會發(fā)生明顯的聚集形成,從而減少表面的對流擴散傳輸.因此Na+在一定濃度范圍增大可加快微生物沉積速率,但當(dāng)濃度高于臨界濃度時,表面對流擴散傳輸可能被抑制導(dǎo)致沉積速率不再加快.

2.2.2 Na+影響微生物吸附膜結(jié)構(gòu)特征 耗散3隨時間變化曲線如圖2所示.通入背景鹽溶液后,耗散均有所上升,D分別為0.20×10-6, 0.24×10-6, 0.60×10-6和2.2×10-6;通入菌液后,4組曲線耗散繼續(xù)上升,耗散變化存在明顯差異,不同濃度的D差別隨時間而增大,隨Na+濃度增加,表面吸附導(dǎo)致上升的D分別為1.3×10-6,1.7′10-6,1.9′10-6和3.2′10-6,依次增加,D均大于1′10-6說明微生物形成的吸附層都具有耗散性[27],150mmol/L的Na+溶液中耗散增加顯著.使用對應(yīng)背景溶液解吸附后,3有輕微下降或基本不變,3的下降可能由液體流動狀態(tài)下微生物的運動引起[35],也可能與吸附層水含量下降有關(guān)[36].

圖2 耗散D3隨時間變化曲線

常用|D/D|考察單位質(zhì)量增加引起的能量耗散,消除時間作為顯式參數(shù)的影響[35,37-39].|D/D|值越小,表明吸附膜剛性程度越強,反之吸附膜越松散柔軟[40].

圖3 耗散(D3)隨負(fù)頻率(-f3)變化曲線

表2 不同濃度Na+溶液中的平均DD/Df值(′10-6Hz-1)

2.3 模型擬合吸附質(zhì)量和黏彈性變化

圖4 Sauerbrey和Voight模型擬合下的吸附膜質(zhì)量

2.3.1 剛性模型擬合 Sauerbrey模型擬合質(zhì)量分別為(196.57 ± 7.82), (296.13 ± 5.17), (349.69 ± 8.49), (332.70 ± 39.13)ng/cm2,具有顯著性差異.隨Na+濃度增加,吸附質(zhì)量逐漸增加,至高濃度(50mmol/L)時質(zhì)量增加不再明顯.相較于2.5mmol/L的Na+環(huán)境下微生物吸附質(zhì)量,150mmol/L條件下吸附質(zhì)量增加了69%.

圖5 Voight模型擬合黏彈性隨時間變化

此外,黏彈性模型還可對吸附層的物理特性進行擬合,包括黏性和彈性.如圖5所示,通入微生物溶液后,隨表面微生物的吸附,吸附膜黏性和彈性不斷增加.菌液流通90min后,隨Na+濃度增加,黏性模量依次為(1143.9±17.3), (1366.5±1.4), (1348.1±49.1), (1268.8±137.1)Pa×s,彈性模量分別為(2.8±0.7), (9.0± 0.9), (7.6±1.0), (11.0±5.6)kPa,說明Na+會加大微生物吸附膜黏性和彈性程度.低濃度2.5mmol/L的Na+溶液中吸附膜雖然呈柔性,但其黏彈性程度最低,也說明了其柔性主要和耦合水有關(guān),而受Na+自身影響不大.隨Na+濃度增大至10mmol/L,膜黏彈性有明顯提高,后續(xù)濃度繼續(xù)增大導(dǎo)致的黏彈性變化不顯著.在150mmol/L的Na+溶液環(huán)境中,膜黏彈性隨時間上升變化最快;研究表明Na+在一定范圍內(nèi)增加會提高細菌EPS、SMP(溶解性微生物產(chǎn)物)的含量[44-46],結(jié)合其形成的不可逆且較疏軟的膜結(jié)構(gòu),其黏彈性變化可能與微生物代謝產(chǎn)物的分泌有關(guān).

綜上,采用2種不同的模型擬合,其結(jié)果表明:除Voight模型擬合的2.5mmol/L環(huán)境下吸附質(zhì)量可能被高估以外,吸附膜質(zhì)量都隨Na+濃度增加而增加;2種模型擬合質(zhì)量差別進一步反應(yīng)了吸附膜的疏軟結(jié)構(gòu)差異;高Na+濃度下,膜黏彈性快速增加可能與促進細菌代謝產(chǎn)物的分泌有關(guān).由于高濃度(150mmol/L)Na+會促進細菌吸附在表面,形成黏彈性較高且不可逆的生物膜,因此建議減少高鹽分垃圾(如餐廚垃圾)直接填埋,以減少膜污染結(jié)垢的形成.

3 結(jié)論

3.1 滲濾液微生物群落在不同Na+濃度下,初期吸附過程包括可逆吸附和不可逆吸附,以不可逆吸附為主導(dǎo),且高濃度Na+會促進微生物不可逆吸附的形成.

3.2 當(dāng)Na+濃度升高,靜電雙層壓縮增強,溶液Zeta電位凈值降低,表面吸附速率提高,但是濃度升高至150mmol/L時吸附速率不再提高,可能存在臨界聚沉濃度現(xiàn)象.

3.3 隨Na+濃度增加,吸附膜柔性先減小后增大,低濃度下生物膜柔性主要和水合作用有關(guān),高濃度(150mmol/L)Na+明顯提高膜柔性,可能與Na+促進細菌分泌具有黏彈性的代謝產(chǎn)物有關(guān).Na+濃度增加會導(dǎo)致微生物吸附質(zhì)量增加,膜黏彈性先升高后變化不再明顯.

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Effects of Na+in leachate on initial adsorption of biofilm.

LIU Feng, WANG Qian, WU Hua-nan, XU Qi-yong*

(School of Environment and Energy, Shenzhen Graduate School, Peking University, Shenzhen 518055, China)., 2022,42(1)：213～219

A quartz crystal microbalance with dissipation (QCM-D) was used to investigate the initial adsorption behavior of biofilm under four kinds of Na+concentrations. The rigid model and viscoelastic model were fitted to explore the changes in adsorption mass and viscoelasticity with time. The results indicate that both reversible and irreversible adsorption processes occurred in the initial adsorption of biofilm. The formation of irreversible adsorption was promoted when Na+concentration was 150mmol/L, due to increased zeta potential and net charge reduction in bacterial surfaces. With an increase in Na+concentration from 2.5mmol/L to 50mmol/L, the adsorption rate increased gradually, which is consistent with the electrostatic double-layer theory. No significant change in the adsorption rate was observed when the concentration was increased to 150mmol/L. The softness of the absorbed layer in the 150mmol/L group was the highest, followed by the 2.5, 10 and 50mmol/L groups. An increase in the viscoelasticity of the layer was also observed during the early adsorption process. With an increase in Na+concentration, the adsorption mass kept increasing and the viscoelasticity first increased and then remained unchanged.

bacteria adsorption；QCM-D；frequency；dissipation；viscoelasticity

X705

A

1000-6923(2022)01-0213-07

劉 豐(1997-),女,湖北黃岡人,北京大學(xué)深圳研究生院碩士研究生,主要從事固體廢棄物資源化處理研究.發(fā)表論文4篇.

2021-06-03

國家重點研發(fā)計劃(2018YFC1902903);深圳市科技計劃(JSGG20170822164024506)

* 責(zé)任作者, 副教授, qiyongxu@pkusz.edu.cn