Na2S脅迫/誘導下Bacillus vallismortis sp. EPS組分變化及其對銅鋅的吸附

李秋華,宋衛(wèi)鋒,孫夢格,李家耀,余澤峰

Na2S脅迫/誘導下spEPS組分變化及其對銅鋅的吸附

李秋華,宋衛(wèi)鋒*,孫夢格,李家耀,余澤峰

(廣東工業(yè)大學環(huán)境科學與工程學院,廣東 廣州 510006)

為研究胞外聚合物(EPS)對重金屬吸附效果的影響,本文通過Na2S脅迫/誘導sp. EPS的化學組成變化來強化其對典型重金屬的吸附.結果表明,Na2S脅迫/誘導強度為20mg/L時,S-EPS產(chǎn)量最高,達到105.58mg/gVSS,蛋白質較Control-EPS提高了近一倍,其中巰基含量達到154.36μmol/L,比脅迫前提高了48.2%.在此條件下,S-EPS對Cu(Ⅱ)和Zn(Ⅱ)的吸附效果最好,對重金屬離子的吸附與Langmuir等溫式擬合較好,擬合理論最大吸附量分別達到1428.57和979.09mg/g EPS,且吸附過程符合二級動力學規(guī)律.三維熒光(3D-EEM)和紅外光譜(FTIR)分析表明,巰基、蛋白質酰胺I帶與酰胺Ⅱ帶在吸附Cu(Ⅱ)和Zn(Ⅱ)中起到了主要作用,尤其是對Zn(Ⅱ)的吸附.說明外加硫源提高了巰基蛋白含量,大大提高了重金屬去除效果.該生物吸附劑在重金屬污染防治中顯示出極大的應用前景.

胞外聚合物；重金屬；硫化鈉；巰基蛋白；吸附

目前,水環(huán)境中重金屬的污染問題已經(jīng)成為我國環(huán)境污染治理領域研究的重點和難點[1].長時間飲用被重金屬污染的水,即使?jié)舛鹊鸵部赡芤饑乐丶膊2],如高劑量的銅離子可能導致肝損傷或威爾遜病[3],過量的鋅會引起急性腸胃炎等癥狀,嚴重時甚至會導致死亡[4].因此,如何高效、低耗地去除水中重金屬,最大限度降低重金屬污染是當今眾多研究者的重要課題.與傳統(tǒng)的物理化學法相比,生物吸附技術具有高效、環(huán)境友好等優(yōu)點[5],生物吸附劑來源廣泛、豐富廉價,具有廣闊的應用前景.其中,胞外聚合物(EPS)成為多數(shù)研究者的焦點.

EPS是細菌在不利于自身生存環(huán)境中自我保護的屏障之一,主要由多糖、蛋白質、核酸和腐殖質等組成,含有大量負電子基團,是一種良好的重金屬吸附劑[6].近年來國內外的研究工作主要集中在EPS對重金屬的吸附和機理方面.Harish等[7]在EPS吸附Cr6+的研究中指出,C=O、C—N、—COOH在吸附過程中起了主要作用,吸附機理主要是離子交換和絡合作用;Ye等[8]以ps-5.EPS為吸附劑,對Cu2+和Pb2+進行吸附,提出對重金屬起作用的是EPS中O—H、C=O、C—O—C、C=O—C官能團,吸附過程以化學作用為主.但關于EPS中各組分與吸附性能之間的關系未進行深入探討.

EPS的化學組成、官能團種類及濃度對重金屬去除具有很大的影響[9].Fang等[10]通過EPS吸附Cu2+的熱力學特征研究,發(fā)現(xiàn)EPS中的蛋白質和腐殖酸都是Cu2+的強配體,而與蛋白的交互作用要比腐殖質強;Wang等[11]通過. EPS吸附Zn2+和Cu2+的研究,發(fā)現(xiàn)EPS中蛋白質對Zn2+和Cu2+均有較強的結合能力,通過三維熒光分析進一步驗證了Zn2+可以和蛋白質及多糖作用而Cu2+只與蛋白質作用;簡靜怡等[12]用微量的Cu2+為脅迫因子,對sp.進行脅迫培養(yǎng),發(fā)現(xiàn)EPS中蛋白質含量明顯上升,對Cu(Ⅱ)的吸附效果相比于脅迫前明顯提高.可見,蛋白質對重金屬的去除起著極其重要的作用,表明了對EPS特定組分進行定向調控,以此增強EPS對金屬離子吸附性能的重要性和必要性.但如何實現(xiàn)對EPS組分的定向調控,是當前研究中面臨的一個重要問題.

此外,巰基具有良好的吸附重金屬性能,可以與重金屬結合成穩(wěn)定的化合物達到去除重金屬的目的[13].生物體內重要的重金屬解毒因子是含有巰基的分子蛋白,如巰基蛋白.故本實驗通過對菌種的外源硫脅迫/誘導培養(yǎng),定向提高EPS特定組分,尤其是巰基蛋白的含量,提高EPS對典型重金屬Cu(Ⅱ)、Zn(Ⅱ)的去除效果,為生物吸附法去除重金屬提供數(shù)據(jù)基礎和參考意義.

1 材料與方法

1.1 菌種的活化與培養(yǎng)

本文所用菌株為苯胺黑藥高效降解菌,經(jīng)16S rRNA測序確定其為死谷芽孢桿菌(sp.)[14],從廣州市瀝滘污水處理廠沉淀池回流污泥中選育并經(jīng)過分離純化獲得,并用甘油法進行保存.

無機鹽培養(yǎng)基:稱量磷酸二氫鉀1.6g,磷酸氫二鉀0.4g,硫酸鎂0.06g,氯化鈣0.001g,氯化銨1.0g,苯胺黑藥0.1g,定容到1000mL容量瓶,未調節(jié)pH值.高壓滅菌20min,冷卻后將解凍的菌株按2%()接種于該培養(yǎng)基中,在35℃、150r/min條件下振蕩培養(yǎng)48h.

LB培養(yǎng)基:稱量蛋白胨10g,酵母粉5g,氯化鈉10g,定容到1000mL容量瓶,調節(jié)pH值為(7.2±0.2),高壓滅菌20min后進行冷卻,然后將培養(yǎng)后的菌液按6%(/)接種至LB培養(yǎng)基中,在35℃、150r/min條件下缺氧培養(yǎng)2h,外加硫源以Na2S溶液形式加入,濃度范圍為0~80mg/L,在相同條件下培養(yǎng)22h.以上接種過程均在無菌條件下進行.

1.2 EPS的提取與測定

取30mL活化培養(yǎng)后的菌液在4℃,5000r/min條件下離心15min,收集菌體,用0.9%NaCl溶液重復清洗2遍,制備成菌懸液.用改進的EDTA法提取EPS[15],之后取上清液進行過濾,透析24h后保存待用.

EPS產(chǎn)量用多糖、蛋白質、核酸之和表示,單位為mg/g VSS.測定方法分別為苯酚硫酸法、考馬斯亮藍法[16]和二苯胺法[17],—SH測定方法為DTNB法[18].實驗結果均取3次平行試驗的平均值.

1.3 吸附實驗

將脅迫/誘導前后的sp.EPS分別注入濃度均為20mg/L的Cu(Ⅱ)、Zn(Ⅱ)溶液中,在pH=5.0,35℃,150r/min條件下,振蕩吸附2h.然后放入經(jīng)過預處理的透析袋中,將透析袋放入盛有250mL蒸餾水的燒杯中,室溫下在磁力攪拌機上透析12h,然后用火焰原子吸收分光光度計測定透析液中的金屬離子濃度.每個樣品均設3個平行樣,實驗數(shù)據(jù)取其平均值.

EPS吸附量公式如下:

式中:為EPS單位吸附量,mg/g EPS;0為金屬離子初始濃度,mg/L;0為吸附前溶液體積,L;C為某時刻金屬離子濃度,mg/L;V為透析液體積,L;為EPS質量,g.

1.4 表征分析

脅迫/誘導因子對sp. EPS主要成分的影響以及EPS主要官能團的變化分別用三維熒光(3D-EEM)和傅里葉紅外光譜(FTIR)進行分析.

1.5 吸附等溫模型

分別配制濃度為10,20,30,50,80,120mg/L的Cu(Ⅱ)、Zn(Ⅱ)溶液,并調節(jié)pH=5.0,分別移取20mL于50mL錐形瓶中,加入適量EPS溶液,放于搖床中,在與1.3中同樣的條件下進行吸附及透析實驗,然后測定透析液中金屬離子濃度,按照式(1)計算吸附量,對實驗數(shù)據(jù)進行等溫吸附模型擬合.

Langmuir型和Freundlich型吸附等溫線是EPS吸附重金屬研究中最常見的2種模型,用來反映吸附機制、吸附層結構和吸附劑的宏觀表面結構[19].

Langmuir吸附等溫線的形式如下所示:

Freundlich模型是一條經(jīng)驗公式,用于非理想條件下的表面吸附和多分子層吸附過程.其等溫方程可表示為:

1.6 吸附動力學研究

分別配制濃度為20mg/L的Cu(Ⅱ)、Zn(Ⅱ)溶液,并調節(jié)pH=5.0,分別移取20mL于50mL錐形瓶中,加入適量EPS溶液,放于搖床中,在pH=5.0,35℃, 150r/min條件下吸附0~180min,混合溶液透析12h后測定透析液中金屬離子濃度,按照式(1)計算吸附量,對實驗數(shù)據(jù)進行動力學模型擬合.

EPS吸附金屬離子的過程,分別用準一級動力學模型和準二級動力學模型[21]對時間影響因素的實驗數(shù)據(jù)進行擬合,2種動力學模型的方程式如下:

準一級動力學模型:

準二級動力學模型:

式中:e為平衡吸附量,mg/g;t為時刻吸附量,mg/g;1為準一級吸附速率常數(shù),min-1;2為準二級吸附速率常數(shù),g/(mg·min);為吸附時間,min.

2 結果與討論

2.1 不同濃度Na2S脅迫誘導對EPS組分及其吸附性能影響

本實驗用不同濃度的Na2S作為脅迫/誘導因子(前期嘗試了不同價態(tài)的硫源,Na2S效果最好.脅迫/誘導下的EPS稱為S-EPS,空白樣稱為Control-EPS). EPS各組分及含量如圖1所示.

可以看出,脅迫前后EPS組分多糖和核酸變化不大,而蛋白質在Na2S濃度為20mg/L時含量相比脅迫前增加了近一倍,從21.36增加至36.08mg/g VSS,此時EPS含量達到105.58mg/g VSS.從整體來看,EPS產(chǎn)量隨著Na2S的脅迫強度呈先增加后減少的趨勢,原因可能是EPS是微生物為適應外部環(huán)境變化而產(chǎn)生的,而Na2S對菌株而言是有害物質,會形成對菌株生長不利的環(huán)境,為了抵御這種不利條件,菌株需要產(chǎn)生更多的EPS保護自己[5].但當Na2S濃度過高時,微生物活性下降,導致EPS產(chǎn)量下降.

圖1 不同強度Na2S脅迫下EPS產(chǎn)量

圖2 不同強度Na2S脅迫/誘導下EPS對Cu(Ⅱ)和Zn(Ⅱ)的吸附

按照上述脅迫/誘導強度,提取相應的EPS,分別吸附Cu(Ⅱ)和Zn(Ⅱ),吸附量如圖2所示.可以看出,脅迫濃度為20mg/L時,EPS對Cu(Ⅱ)和Zn(Ⅱ)的平衡吸附量最高,分別為584.80和519.02mg/g EPS,較未脅迫前吸附量分別提高了35.6%和43.8%.結合前面實驗的結果,脅迫/誘導因子強度為20mg/L時, S-EPS中蛋白質含量相比于Control-EPS提高幅度較大,故推測是蛋白質在吸附兩種重金屬時起了較大作用,其中的—SH、C=O、C—N/N—H等基團數(shù)量/濃度增加,使EPS暴露出的點位數(shù)量增加,提高了重金屬去除效果.表明了外源硫能通過脅迫/誘導菌株產(chǎn)生特異性EPS.以下實驗及表征均用濃度為20mg/ L的Na2S.

2.2 兩種EPS的三維熒光分析

本實驗采用三維熒光光譜進一步研究了Na2S脅迫因子對sp. EPS主要成分的影響.

(a), Control-EPS; (b), S-EPS

如圖所示,由于含—SH蛋白無發(fā)色基團,故2種EPS的官能團種類未發(fā)生變化.脅迫前后EPS熒光光譜位置和最大熒光強度的參數(shù)見表1.

已有研究指出熒光強度與EPS 的含量具有密切的關系[25].結合3圖和表1可看出,峰A和峰B的蛋白質類熒光強度較脅迫前增強最為明顯,原因在于部分官能團如C=O、—NH2、—COOH含量增加,這些負電子基團更容易與重金屬離子結合,達到去除重金屬的目的.以上結果說明,外加硫源脅迫/誘導可以改變EPS成分的含量,使目標物質-蛋白質類含量大幅增加.

表1 脅迫/誘導前后EPS熒光光譜譜峰信息

2.3 吸附重金屬前后EPS的紅外光譜分析

spEPS受外加硫源脅迫/誘導前后關鍵基團峰值和峰位的移動情況可用傅里葉紅外光譜進行分析,從而研究其與重金屬相互作用機理.脅迫/誘導前后的EPS以及吸附Cu(Ⅱ)和Zn(Ⅱ)后的S-EPS紅外譜圖見圖4.

圖4 脅迫/誘導前后和吸附重金屬后EPS的紅外光譜圖

Cu-EPS代表吸附Cu(Ⅱ)之后的EPS,Zn-EPS代表吸附Zn(Ⅱ)之后的EPS

Cu(Ⅱ)和Zn(Ⅱ)之后發(fā)生了不同程度的紅移, Cu-EPS的紅移程度更大,說明C=O參與了對Cu(Ⅱ)的吸附.Zn-EPS的S—O特征峰相較于S-EPS發(fā)生了很大程度的紅移,說明S—O基團在Zn(Ⅱ)的吸附中發(fā)揮了部分作用,猜測可能是外加硫源在此起了部分作用.Cu-EPS在蛋白質酰胺Ⅱ帶較S-EPS發(fā)生了紅移,而在Zn-EPS中酰胺Ⅱ帶消失,說明N—H和C—N基團均參與了對Cu(Ⅱ)和Zn(Ⅱ)的吸附.除了個別基團峰發(fā)生了漂移和消失外,部分特征峰的強度也存在著明顯的不同,說明脅迫后的EPS中部分官能團仍存在某些變化.Jiang等[30]認為譜峰強度與樣品所含官能團濃度存在著密切關系,Xu等[31]利用紅外光譜研究sp. EPS時,指出峰值強度的大小能反映對應官能團的相對濃度,可看出Cu-EPS和Zn-EPS的特征峰強度均比S-EPS特征峰強度強,驗證了前面結論.另外,與糖有關的基團也發(fā)生了小幅度的紅移.

2.4 吸附等溫模型研究

實驗采用Langmuir模型和Freundlich模型對Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)進行擬合,各模型擬合系數(shù)見表2.

表2 脅迫/誘導前后EPS吸附等溫模型擬合

2.5 吸附動力學研究

本研究采用一級動力學模型和二級動力學模型對Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)進行擬合,各模型擬合系數(shù)見表3.

表3 脅迫/誘導前后EPS吸附重金屬動力學參數(shù)

從表3可以看出,對于2種重金屬離子的吸附,Control-EPS和S-EPS的準二級動力學擬合效果均比準一級動力學高,2達到0.99以上,說明2種EPS吸附金屬離子的過程采用準二級動力學描述更為準確,其得出的平衡吸附容量e也與實驗值接近.準二級動力學模型認為化學反應是控速步驟,可用于多種吸附研究[33-34],表明S-EPS對Zn(Ⅱ)和Cu(Ⅱ)的吸附過程仍由化學反應控制.

3 結論

3.1 在一定范圍內,隨著脅迫因子的強度增大,spEPS含量總體上呈先上升后降低趨勢,并影響其組分含量變化:多糖、核酸含量變化不明顯,而對蛋白質影響較大,其含量增加了接近一倍,巰基含量增加48.2%.

3.2 脅迫/誘導濃度為20mg/L時,S-EPS對Cu(Ⅱ)和Zn(Ⅱ)的平衡吸附量最高,分別為584.80和519.02mg/g EPS,較未脅迫前吸附量分別提高了35.6%和43.8%.

3.3 三維熒光和紅外光譜結果分析表明,巰基蛋白在S-EPS對Cu(Ⅱ)和Zn(Ⅱ)的吸附中發(fā)揮了極為重要的作用,聯(lián)合其他負電子基團達到去除重金屬的目的.

3.4 采用Langmuir模型和Freundlich模型對Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)數(shù)據(jù)進行擬合,發(fā)現(xiàn)Langmuir模型有更好的模擬效果,對Cu(Ⅱ)和Zn(Ⅱ)的最大吸附量可分別達到1428.57和979.09mg/g EPS.動力學模擬結果顯示,二級動力學更符合EPS重金屬離子的吸附過程.

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Influence of the stress of Na2S on component changes and sorption behavior on Cu (Ⅱ) and Zn (Ⅱ) ofsp. EPS.

LI Qiu-hua, SONG Wei-feng*, SUN Meng-ge, LI Jia-yao, YU Ze-feng

(School of Environmental Science and Engineering of Guangdong University of Technology, Guangzhou 510006, China)., 2019,39(11)：4858~4864

In order to study the mechanism of enhanced adsorption of heavy metals by extracellular polymeric substances (EPS), adsorption of typical heavy metals by the EPS fromsp. induced by Na2S was investigated. The results showed that the maximum EPS production of 105.58mg/g VSS coupling with doubled increase in protein in which the contant of -SH increased by 48.2% from 104.15 to 154.36μmol/L were recorded in the presence of 20mg/L Na2S, As a coinstantaneous process, the maximum adsorption of Cu (Ⅱ) and Zn (Ⅱ) by the S-EPS was observed in the presence of 20mg/L Na2S. The kinetics of the adsorption process of Cu (Ⅱ) and Zn (Ⅱ) by the S-EPS can be well fitted by the Langmuir isotherm and the pseudo-second-order model mode and the theoretical maximum adsorption amount of 1428.57 and 979.09mg/g EPS could be obtained, respectively. The results of 3D-EEM and FTIR analyses indicated that the -SH, protein amide I and amide Ⅱ played a major role in the adsorption of Cu (Ⅱ) and Zn (Ⅱ) by the S-EPS, especially for the adsorption of Zn (Ⅱ). The results obtained in this study demonstrated that the addition of sulfur source could increase the content of sulfhydryl protein, and effectively regulate the content of chemical composition, expecially for the sulfhydryl of EPS, and thereby greatly improving the removal efficiency of heavy metals, which showed a great application prospect in the prevention and control of heavy metal pollution.

EPS；heavy metals；Na2S；sulfhydryl protein；asorbent

X172

A

1000-6932(2019)11-4858-07

李秋華(1992-),女,河南商丘人,廣東工業(yè)大學碩士研究生,主要從事水中重金屬去除研究.發(fā)表論文1篇.

2019-04-11

廣東省科技計劃項目(2014A020209077)

* 責任作者, 教授, weifengsong@gdut.edu.cn