Na2S脅迫/誘導(dǎo)下Bacillus vallismortis sp. EPS組分變化及其對(duì)銅鋅的吸附

李秋華,宋衛(wèi)鋒,孫夢(mèng)格,李家耀,余澤峰

Na2S脅迫/誘導(dǎo)下spEPS組分變化及其對(duì)銅鋅的吸附

李秋華,宋衛(wèi)鋒*,孫夢(mèng)格,李家耀,余澤峰

(廣東工業(yè)大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣東 廣州 510006)

為研究胞外聚合物(EPS)對(duì)重金屬吸附效果的影響,本文通過Na2S脅迫/誘導(dǎo)sp. EPS的化學(xué)組成變化來強(qiáng)化其對(duì)典型重金屬的吸附.結(jié)果表明,Na2S脅迫/誘導(dǎo)強(qiáng)度為20mg/L時(shí),S-EPS產(chǎn)量最高,達(dá)到105.58mg/gVSS,蛋白質(zhì)較Control-EPS提高了近一倍,其中巰基含量達(dá)到154.36μmol/L,比脅迫前提高了48.2%.在此條件下,S-EPS對(duì)Cu(Ⅱ)和Zn(Ⅱ)的吸附效果最好,對(duì)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(dá)到1428.57和979.09mg/g EPS,且吸附過程符合二級(jí)動(dòng)力學(xué)規(guī)律.三維熒光(3D-EEM)和紅外光譜(FTIR)分析表明,巰基、蛋白質(zhì)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(duì)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(yīng)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(lǐng)域研究的重點(diǎn)和難點(diǎn)[1].長時(shí)間飲用被重金屬污染的水,即使?jié)舛鹊鸵部赡芤饑?yán)重疾病[2],如高劑量的銅離子可能導(dǎo)致肝損傷或威爾遜病[3],過量的鋅會(huì)引起急性腸胃炎等癥狀,嚴(yán)重時(shí)甚至?xí)?dǎo)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(dāng)今眾多研究者的重要課題.與傳統(tǒng)的物理化學(xué)法相比,生物吸附技術(shù)具有高效、環(huán)境友好等優(yōu)點(diǎn)[5],生物吸附劑來源廣泛、豐富廉價(jià),具有廣闊的應(yīng)用前景.其中,胞外聚合物(EPS)成為多數(shù)研究者的焦點(diǎn).

EPS是細(xì)菌在不利于自身生存環(huán)境中自我保護(hù)的屏障之一,主要由多糖、蛋白質(zhì)、核酸和腐殖質(zhì)等組成,含有大量負(fù)電子基團(tuán),是一種良好的重金屬吸附劑[6].近年來國內(nèi)外的研究工作主要集中在EPS對(duì)重金屬的吸附和機(jī)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(jī)理主要是離子交換和絡(luò)合作用;Ye等[8]以ps-5.EPS為吸附劑,對(duì)Cu2+和Pb2+進(jìn)行吸附,提出對(duì)重金屬起作用的是EPS中O—H、C=O、C—O—C、C=O—C官能團(tuán),吸附過程以化學(xué)作用為主.但關(guān)于EPS中各組分與吸附性能之間的關(guān)系未進(jìn)行深入探討.

EPS的化學(xué)組成、官能團(tuán)種類及濃度對(duì)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(xué)特征研究,發(fā)現(xiàn)EPS中的蛋白質(zhì)和腐殖酸都是Cu2+的強(qiáng)配體,而與蛋白的交互作用要比腐殖質(zhì)強(qiáng);Wang等[11]通過. EPS吸附Zn2+和Cu2+的研究,發(fā)現(xiàn)EPS中蛋白質(zhì)對(duì)Zn2+和Cu2+均有較強(qiáng)的結(jié)合能力,通過三維熒光分析進(jìn)一步驗(yàn)證了Zn2+可以和蛋白質(zhì)及多糖作用而Cu2+只與蛋白質(zhì)作用;簡靜怡等[12]用微量的Cu2+為脅迫因子,對(duì)sp.進(jìn)行脅迫培養(yǎng),發(fā)現(xiàn)EPS中蛋白質(zhì)含量明顯上升,對(duì)Cu(Ⅱ)的吸附效果相比于脅迫前明顯提高.可見,蛋白質(zhì)對(duì)重金屬的去除起著極其重要的作用,表明了對(duì)EPS特定組分進(jìn)行定向調(diào)控,以此增強(qiáng)EPS對(duì)金屬離子吸附性能的重要性和必要性.但如何實(shí)現(xiàn)對(duì)EPS組分的定向調(diào)控,是當(dāng)前研究中面臨的一個(gè)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(jié)合成穩(wěn)定的化合物達(dá)到去除重金屬的目的[13].生物體內(nèi)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(shí)驗(yàn)通過對(duì)菌種的外源硫脅迫/誘導(dǎo)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(duì)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(chǔ)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(cè)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(jìn)行保存.

無機(jī)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(diào)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(diào)節(jié)pH值為(7.2±0.2),高壓滅菌20min后進(jìn)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(jìn)行.

1.2 EPS的提取與測(cè)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(fù)清洗2遍,制備成菌懸液.用改進(jìn)的EDTA法提取EPS[15],之后取上清液進(jìn)行過濾,透析24h后保存待用.

EPS產(chǎn)量用多糖、蛋白質(zhì)、核酸之和表示,單位為mg/g VSS.測(cè)定方法分別為苯酚硫酸法、考馬斯亮藍(lán)法[16]和二苯胺法[17],—SH測(cè)定方法為DTNB法[18].實(shí)驗(yàn)結(jié)果均取3次平行試驗(yàn)的平均值.

1.3 吸附實(shí)驗(yàn)

將脅迫/誘導(dǎo)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(yù)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(jī)上透析12h,然后用火焰原子吸收分光光度計(jì)測(cè)定透析液中的金屬離子濃度.每個(gè)樣品均設(shè)3個(gè)平行樣,實(shí)驗(yàn)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(shí)刻金屬離子濃度,mg/L;V為透析液體積,L;為EPS質(zhì)量,g.

1.4 表征分析

脅迫/誘導(dǎo)因子對(duì)sp. EPS主要成分的影響以及EPS主要官能團(tuán)的變化分別用三維熒光(3D-EEM)和傅里葉紅外光譜(FTIR)進(jìn)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(diào)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(jìn)行吸附及透析實(shí)驗(yàn),然后測(cè)定透析液中金屬離子濃度,按照式(1)計(jì)算吸附量,對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)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(jī)制、吸附層結(jié)構(gòu)和吸附劑的宏觀表面結(jié)構(gòu)[19].

Langmuir吸附等溫線的形式如下所示:

Freundlich模型是一條經(jīng)驗(yàn)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(dòng)力學(xué)研究

分別配制濃度為20mg/L的Cu(Ⅱ)、Zn(Ⅱ)溶液,并調(diào)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(cè)定透析液中金屬離子濃度,按照式(1)計(jì)算吸附量,對(duì)實(shí)驗(yàn)數(shù)據(jù)進(jìn)行動(dòng)力學(xué)模型擬合.

EPS吸附金屬離子的過程,分別用準(zhǔn)一級(jí)動(dòng)力學(xué)模型和準(zhǔn)二級(jí)動(dòng)力學(xué)模型[21]對(duì)時(shí)間影響因素的實(shí)驗(yàn)數(shù)據(jù)進(jìn)行擬合,2種動(dòng)力學(xué)模型的方程式如下:

準(zhǔn)一級(jí)動(dòng)力學(xué)模型:

準(zhǔn)二級(jí)動(dòng)力學(xué)模型:

式中:e為平衡吸附量,mg/g;t為時(shí)刻吸附量,mg/g;1為準(zhǔn)一級(jí)吸附速率常數(shù),min-1;2為準(zhǔn)二級(jí)吸附速率常數(shù),g/(mg·min);為吸附時(shí)間,min.

2 結(jié)果與討論

2.1 不同濃度Na2S脅迫誘導(dǎo)對(duì)EPS組分及其吸附性能影響

本實(shí)驗(yàn)用不同濃度的Na2S作為脅迫/誘導(dǎo)因子(前期嘗試了不同價(jià)態(tài)的硫源,Na2S效果最好.脅迫/誘導(dǎo)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(zhì)在Na2S濃度為20mg/L時(shí)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(shí)EPS含量達(dá)到105.58mg/g VSS.從整體來看,EPS產(chǎn)量隨著Na2S的脅迫強(qiáng)度呈先增加后減少的趨勢(shì),原因可能是EPS是微生物為適應(yīng)外部環(huán)境變化而產(chǎn)生的,而Na2S對(duì)菌株而言是有害物質(zhì),會(huì)形成對(duì)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(hù)自己[5].但當(dāng)Na2S濃度過高時(shí),微生物活性下降,導(dǎo)致EPS產(chǎn)量下降.

圖1 不同強(qiáng)度Na2S脅迫下EPS產(chǎn)量

圖2 不同強(qiáng)度Na2S脅迫/誘導(dǎo)下EPS對(duì)Cu(Ⅱ)和Zn(Ⅱ)的吸附

按照上述脅迫/誘導(dǎo)強(qiáng)度,提取相應(yīng)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(shí),EPS對(duì)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(jié)合前面實(shí)驗(yàn)的結(jié)果,脅迫/誘導(dǎo)因子強(qiáng)度為20mg/L時(shí), S-EPS中蛋白質(zhì)含量相比于Control-EPS提高幅度較大,故推測(cè)是蛋白質(zhì)在吸附兩種重金屬時(shí)起了較大作用,其中的—SH、C=O、C—N/N—H等基團(tuán)數(shù)量/濃度增加,使EPS暴露出的點(diǎn)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(dǎo)菌株產(chǎn)生特異性EPS.以下實(shí)驗(yàn)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(shí)驗(yàn)采用三維熒光光譜進(jìn)一步研究了Na2S脅迫因子對(duì)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(tuán),故2種EPS的官能團(tuán)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(qiáng)度的參數(shù)見表1.

已有研究指出熒光強(qiáng)度與EPS 的含量具有密切的關(guān)系[25].結(jié)合3圖和表1可看出,峰A和峰B的蛋白質(zhì)類熒光強(qiáng)度較脅迫前增強(qiáng)最為明顯,原因在于部分官能團(tuán)如C=O、—NH2、—COOH含量增加,這些負(fù)電子基團(tuán)更容易與重金屬離子結(jié)合,達(dá)到去除重金屬的目的.以上結(jié)果說明,外加硫源脅迫/誘導(dǎo)可以改變EPS成分的含量,使目標(biāo)物質(zhì)-蛋白質(zhì)類含量大幅增加.

表1 脅迫/誘導(dǎo)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(dǎo)前后關(guān)鍵基團(tuán)峰值和峰位的移動(dòng)情況可用傅里葉紅外光譜進(jìn)行分析,從而研究其與重金屬相互作用機(jī)理.脅迫/誘導(dǎo)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(dǎo)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(duì)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(tuán)在Zn(Ⅱ)的吸附中發(fā)揮了部分作用,猜測(cè)可能是外加硫源在此起了部分作用.Cu-EPS在蛋白質(zhì)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(tuán)均參與了對(duì)Cu(Ⅱ)和Zn(Ⅱ)的吸附.除了個(gè)別基團(tuán)峰發(fā)生了漂移和消失外,部分特征峰的強(qiáng)度也存在著明顯的不同,說明脅迫后的EPS中部分官能團(tuán)仍存在某些變化.Jiang等[30]認(rèn)為譜峰強(qiáng)度與樣品所含官能團(tuán)濃度存在著密切關(guān)系,Xu等[31]利用紅外光譜研究sp. EPS時(shí),指出峰值強(qiáng)度的大小能反映對(duì)應(yīng)官能團(tuán)的相對(duì)濃度,可看出Cu-EPS和Zn-EPS的特征峰強(qiáng)度均比S-EPS特征峰強(qiáng)度強(qiáng),驗(yàn)證了前面結(jié)論.另外,與糖有關(guān)的基團(tuán)也發(fā)生了小幅度的紅移.

2.4 吸附等溫模型研究

實(shí)驗(yàn)采用Langmuir模型和Freundlich模型對(duì)Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)進(jìn)行擬合,各模型擬合系數(shù)見表2.

表2 脅迫/誘導(dǎo)前后EPS吸附等溫模型擬合

2.5 吸附動(dòng)力學(xué)研究

本研究采用一級(jí)動(dòng)力學(xué)模型和二級(jí)動(dòng)力學(xué)模型對(duì)Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)進(jìn)行擬合,各模型擬合系數(shù)見表3.

表3 脅迫/誘導(dǎo)前后EPS吸附重金屬動(dòng)力學(xué)參數(shù)

從表3可以看出,對(duì)于2種重金屬離子的吸附,Control-EPS和S-EPS的準(zhǔn)二級(jí)動(dòng)力學(xué)擬合效果均比準(zhǔn)一級(jí)動(dòng)力學(xué)高,2達(dá)到0.99以上,說明2種EPS吸附金屬離子的過程采用準(zhǔn)二級(jí)動(dòng)力學(xué)描述更為準(zhǔn)確,其得出的平衡吸附容量e也與實(shí)驗(yàn)值接近.準(zhǔn)二級(jí)動(dòng)力學(xué)模型認(rèn)為化學(xué)反應(yīng)是控速步驟,可用于多種吸附研究[33-34],表明S-EPS對(duì)Zn(Ⅱ)和Cu(Ⅱ)的吸附過程仍由化學(xué)反應(yīng)控制.

3 結(jié)論

3.1 在一定范圍內(nèi),隨著脅迫因子的強(qiáng)度增大,spEPS含量總體上呈先上升后降低趨勢(shì),并影響其組分含量變化:多糖、核酸含量變化不明顯,而對(duì)蛋白質(zhì)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(dǎo)濃度為20mg/L時(shí),S-EPS對(duì)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(jié)果分析表明,巰基蛋白在S-EPS對(duì)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(fù)電子基團(tuán)達(dá)到去除重金屬的目的.

3.4 采用Langmuir模型和Freundlich模型對(duì)Control-EPS、S-EPS分別吸附Cu(Ⅱ)和Zn(Ⅱ)數(shù)據(jù)進(jìn)行擬合,發(fā)現(xiàn)Langmuir模型有更好的模擬效果,對(duì)Cu(Ⅱ)和Zn(Ⅱ)的最大吸附量可分別達(dá)到1428.57和979.09mg/g EPS.動(dòng)力學(xué)模擬結(jié)果顯示,二級(jí)動(dòng)力學(xué)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(xué)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(jì)劃項(xiàng)目(2014A020209077)

* 責(zé)任作者, 教授, weifengsong@gdut.edu.cn