酸化赤泥吸附環丙沙星的特征、機理及過程優化

史京轉,魏 紅*,周孝德,史穎娟,鄭佳欣

酸化赤泥吸附環丙沙星的特征、機理及過程優化

史京轉1,魏 紅1*,周孝德1,史穎娟2,鄭佳欣1

(1.西安理工大學,省部共建西北旱區生態水利國家重點實驗室,陜西 西安 710048；2.陜西水環境工程勘測設計研究院,陜西 西安 710021)

為提高赤泥的資源化利用及抗生素有機廢水的深度處理,以酸化赤泥為吸附劑、環丙沙星為目標污染物,研究了酸化赤泥吸附環丙沙星的條件、特征和機理.采用響應面法中Box-Behnken設計方法,以吸附溫度、溶液pH值、環丙沙星初始濃度、酸化赤泥投加量為自變量,吸附量為響應值建立4因素3水平優化模型,確定了最佳吸附條件,并對吸附過程的動力學模型、等溫線模型、熱力學特性及吸附機理進行了研究.結果表明,溶液pH值、環丙沙星初始濃度、酸化赤泥投加量為影響吸附量的顯著因素.酸化赤泥吸附環丙沙星的最佳條件為:溫度45℃、pH=3.04、環丙沙星初始濃度29.20mg/L,酸化赤泥投加量3.40g/L,預測最大吸附量為7.30mg/g.酸化赤泥吸附環丙沙星的過程遵循偽二級反應動力學模型及Langmuir-Freundilich吸附等溫線模型,經過擬合最大吸附量分別為7.90和7.35mg/g.根據Van Tehoff公式計算吸附熱力學狀態函數Δ0為-82.13~-94.37kJ/mol、Δ0為0.61J/(mol·K)、Δ0為100.25KJ/mol,吸附為自發進行的吸熱反應. FTIR表明環丙沙星分子中—COO與酸化赤泥的Al—O鍵發生絡合反應,C=O與Fe—O鍵發生微弱的靜電或內球面鍵合作用.研究表明,酸化赤泥是一種極具潛力的廉價吸附劑,可用于處理抗生素污染廢水.

酸化赤泥；環丙沙星；響應面優化；吸附動力學；吸附熱力學

赤泥是工業生產氧化鋁過程中產生的固體廢渣,主要包含Na、K、Al、Fe、Ni、Si、Cu、Mn、Ti 及 Zn等元素,不同地區鋁土礦及氧化鋁生產工藝不同而導致赤泥的成分有所差異[1].據統計,2014年世界范圍內赤泥的貯存量為35億t,并以每年1.2億t的速度增長[2]. 我國每年赤泥的產生量7000萬t,累積庫存約為6億t[3].目前,赤泥最普遍的處置方式是筑壩填埋[4],由此導致的環境污染及次生災害問題時有發生[5].因此,對赤泥進行資源化和無害化利用越來越受到人們的關注[6].近年來,國內外對赤泥的有效利用主要集中在建材生產[7]、稀有金屬回收[8]、赤泥基催化劑及吸附材料的制備[9-10]等方面.為了降低赤泥的高堿性危害,研究普遍對赤泥進行酸活化處理.不同酸活化可顯著提高赤泥對重金屬離子[11]、有機染料[12]及磷酸鹽[13]等污染物的吸附性能.但赤泥對新興污染物如抗生素的吸附還鮮見報道.

近年來,抗生素類有機污染物在環境介質中頻繁檢出,引起了學者們的廣泛關注[14].不僅在河流[15]、污水中[16]有抗生素檢出,飲用水中也檢測到痕量抗生素.抗生素在環境介質中的“假持久性”[17]和“耐藥性”[18]已經嚴重威脅到人類的健康.環丙沙星作為環境中檢出率最高的人畜共用類抗生素之一[19],傳統的水處理工藝對其去除效果不佳[20],采取更有效的處理方法已經迫在眉睫. 吸附法具有操作簡單、成本低、不添加任何氧化劑等特點而備受青睞[21].

本實驗根據響應面法[22]中Box-Behnken設計方法對酸化赤泥吸附環丙沙星的影響因素進行評價、優化, 研究吸附過程的動力學、吸附等溫線和吸附熱力學特性,并結合傅里葉變換衰減全反射紅外光譜(ATR-FTIR)對吸附機理進行分析,以期為赤泥的綜合利用和環丙沙星污染水體的修復提供一定科學依據.

1 材料與方法

1.1 儀器與試劑

實驗所用試劑均為分析純;原始赤泥取自三門峽市義翔鋁業公司.赤泥浸出液pH值為11.04,經XRD(X-ray diffraction,X射線衍射)分析赤泥的主要組分為水化石榴石、鈣霞石、赤鐵礦、剛玉等,經BET(Brunauer-Emmett-Teller)多層吸附計算赤泥的比表面積10.96m2/g,平均孔徑40.93nm;環丙沙星購于日本東京化成工業株式會社,純度大于98 %,分子式: C17H18FN3O3, 相對分子量331.35.

1.2 實驗方法

1.2.1 酸化赤泥的制備 將5g原始赤泥溶于1.0L超純水中,調節pH值(6.0±0.2),以100r/min攪拌9h,靜置分離,將泥餅沖洗至中性,并收集于100℃烘干,過150目篩(孔徑106μm),記為酸化赤泥.

1.2.2 吸附量的測定 將酸化赤泥(3.0,4.0,5.0g/L)和200mL環丙沙星溶液(10,20,30mg/L)混合置于250mL棕色錐形瓶中.用0.1mol/L的HCl和NaOH調節溶液pH值(3.0, 5.0, 7.0, 9.0, 11.0,誤差±0.1),密封后放入250r/min的氣浴恒溫(25,35,45℃)搖床(HZ-8811K,常州德歐)中振蕩180min.一定時間間隔取一定混合液, 4000r/min離心10min,過0.22μm濾膜,用高效液相色譜儀(HPLC,Aglient 1200,美國)測定環丙沙星濃度.同時設置空白對照實驗(不加吸附劑)和3組平行實驗. 吸附量的計算如式(1)所示.

式中:q為時刻的吸附量,mg/g;0為環丙沙星的初始濃度,mg/L;c為吸附時刻環丙沙星的濃度, mg/L;為反應液體積,mL;為酸化赤泥的質量,g.

1.2.3 響應面優化吸附條件 通過前期大量單因素實驗分析,以吸附溫度、溶液pH值、環丙沙星初始濃度和酸化赤泥投加量為自變量,分別記為A、B、C、D;以酸化赤泥對環丙沙星的吸附量作為響應值,記為Y.使用Design Expert 10.0.7軟件中Box- Behnken響應面優化設計方法設計實驗方案,各自變量因素及其水平見表1.

表1 實驗自變量因素及其水平表

1.2.4 吸附動力學 將0.68g酸化赤泥加入200mL初始濃度為30mg/L的環丙沙星溶液,調節pH值為3.04, 在45℃、250r/min條件下振蕩,一定時間取混合液,方法同1.2.2,測定環丙沙星濃度,設置3組平行.

1.2.5 吸附等溫線 將不同初始濃度的環丙沙星溶液(10,20,30,50,100,500mg/L),在不同溫度(25,35, 45℃)下振蕩180min至吸附平衡,測定環丙沙星濃度,其他條件同1.2.4.

1.2.6 分析方法 HPLC色譜條件為: Eclipse XDB-C18色譜柱(150mm×4.6mm,5μm);流動相:乙腈與0.2%(體積分數)甲酸水溶液(體積比20:80);流速0.2mL/min;檢測波長λ=277nm;進樣量10μL;柱溫30℃.在此條件下,環丙沙星的保留時間R=9.768min.采用ATR法在傅里葉紅外光譜儀(VERTEX70, BRUKER,德國)上于400~4000cm-1范圍內對吸附前后酸化赤泥樣品進行掃描.

1.2.7 數據處理 響應面優化實驗數據采用Design Expert 10.0.7軟件進行處理,并得出最優吸附條件;吸附動力學、吸附等溫線、吸附熱力學數據均采用Origin Pro 8軟件進行處理和擬合.

2 結果與討論

2.1 吸附條件的響應面實驗結果分析

2.1.1 吸附預測模型建立 采用Box-Behnken實驗設計方法對酸化赤泥吸附環丙沙星的條件進行研究并優化,實驗結果見表2.

表2 Box-Behnken實驗設計及其實驗結果

對表2中的結果用Design Expert 10.0.7進行方差分析,結果見表3. 吸附量與4個變量(溫度、溶液pH值、環丙沙星初始濃度和酸化赤泥投加量)的二次多項式見式(2):

由表3可知,模型中B、C、D、BC、B2參數的值<0.05,說明溶液pH值、環丙沙星初始濃度、酸化赤泥投加量、溶液pH值與環丙沙星初始濃度交互作用、溶液pH值的平方效應對吸附量的影響具有顯著性;其他參數的值>0.05,說明其他因素對吸附量的影響不顯著.模型的值為40.08,<0.0001,說明模型非常顯著.

表3 方差分析

圖1 吸附量實測值與預測值比較

圖1表明,建立的二次多項式模型計算出的預測值與實測值服從正態分布,決定系數2=0.9629,表明96.3 %的變異能夠被解釋.校正系數2Adj= 0.9389(接近1),說明模型的擬合程度很高;模型的信噪比為25.92(大于4視為合理),說明模型的可信度高,數據合理[23].因此,該模型建立的方程式[式(2)]能準確合理的反映吸附條件與吸附量之間對應關系.

2.1.2 響應面分析及模型驗證 由表3可知,溶液pH值與環丙沙星初始濃度交互作用顯著,圖2為其對吸附量的三維曲面和對應的等高線圖.

由圖2可知,酸化赤泥對環丙沙星的吸附量隨環丙沙星初始濃度的升高而升高;隨溶液pH值的升高而降低.這是因為在不同pH值條件下,環丙沙星的解離形態不同(圖3所示).當溶液pH值較低時,環丙沙星的解離形態主要為H4CIP3+,由于H4CIP3+的化學活性較強于H3CIP2+離子,有利于與酸化赤泥中的活性物質發生吸附作用[24];當溶液偏堿性時,酸化赤泥中的活性成分主要以沉淀的形式存在,從而降低了酸化赤泥對環丙沙星的吸附性能.

2.1.3 最佳吸附條件確定 根據模型預測出酸化赤泥對環丙沙星的最佳吸附條件為:吸附溫度45℃,溶液pH值3.04,環丙沙星初始濃度29.20mg/L,酸化赤泥投加量為3.40g/L,最大吸附量為7.30mg/g.

圖3 不同pH值環丙沙星的解離形態

2.2 吸附動力學

吸附動力學主要研究吸附劑吸附溶質的速率快慢.通過動力學模型對實驗數據進行分析擬合,從而探討其吸附機理.常用的吸附動力學模型有:偽一級動力學模型、偽二級動力學模型、顆粒內擴散模型、Elovich動力學模型[式(3)~(6)].

式中:e是達到吸附平衡時的吸附量, mg/g;q是時刻的吸附量,mg/g;1是偽一級吸附動力學反應速率常數,min-1;2是偽二級反應動力學速率常數, g/(mg·min);是涉及到厚度、邊界層的常數;k為粒子內擴散常數, mg/(g·min1.5);是Elovich常數,表示初始吸附速率,mg/(g·min2);是Elovich常數,表示解吸脫附系數,mg/(g·min).

圖4(a)為酸化赤泥對環丙沙星的吸附結果;(b)~(e)分別為不同動力學模型擬合情況,具體擬合結果見表4.

由圖4(a)可知,吸附初期,酸化赤泥吸附環丙沙星非常迅速,5min的吸附量達到6.54mg/g; 100min后,反應體系達到動態吸附平衡,最大吸附量為7.84mg/g.出現這一現象的原因可能是,反應剛發生時,體系內酸化赤泥有足夠多的吸附點位,環丙沙星的濃度也很高,反應速度非常快,隨著吸附點位和環丙沙星濃度的降低,吸附反應相對緩慢,最后達到動態平衡.

圖4 吸附結果(a)及吸附動力學模型(b~e)擬合情況

表4 吸附動力學模型擬合參數

由圖4(b)、(c)、(e)和表4的擬合結果可知,酸化赤泥對環丙沙星的吸附過程遵循偽二級吸附動力學模型,2=0.9998,且擬合的e值和實驗值較吻合,這表明酸化赤泥吸附環丙沙星是一個化學吸附過程[25].這可能是由于赤泥中含有多種過渡金屬元素,容易和環丙沙星中含富電子,N、O元素的官能團配位.為進一步了解吸附過程機理,用顆粒內擴散模型進行擬合分析,結果見圖4(d).整個吸附過程包括快速吸附、慢速吸附、顆粒內擴散3個階段,多種吸附機理同時參與反應;擬合曲線不經過原點進一步說明除了顆粒內擴散外,還有其他的速控步驟參與[26].

2.3 吸附等溫線

吸附等溫線是在一定溫度下,溶質在兩相界面上達到吸附平衡時的濃度關系曲線.常用的等溫線模型有Langmuir、Freundlich和Langmuir-Freundlich [式(7)~(9)].

式中:e為溶質的平衡濃度,mg/L;e是平衡吸附量, mg/g;m是理論上的最大單分子層吸附量,mg/g;L是與吸附能相關的Langmuir吸附常數,L/mg;F是Freundlich常數,與溶質的移除效率有關;是表征吸附強度的常數;LF是Langmuir-Freundlich常數.

圖5為Langmuir-Freundlich模型的擬合結果,其他擬合見表6.

根據式(7)中L得到一個表征吸附分離難易的無因次分離因子L[式(10)]:

L是一個無量綱常數,L>1說明過程不利于吸附;L=1是線性的;0

式(8)中是表征吸附強度的常數,一般在1~10之間說明有利于吸附;值更高或者1/更小,說明吸附劑和溶質之間有很強的相互作用;=1時是線性吸附,說明所有的位點都有相同的吸附量[27].根據計算可知(表5)值為1~10,說明該吸附有利于進行.

圖5 Langmuir-Freundlich吸附等溫線

根據Langmuir-Freundlich模型的計算結果,最大吸附量7.35mg/g與實測值較符合,2為0.9999,因此該模型能更好地擬合該吸附過程.

根據文獻報道,高嶺土對環丙沙星的最大吸附量為0.143mg/g[28];湖泊底泥吸附環丙沙星的最大吸附量為6.13mg/g[29];微波輔助合成的磁性生物炭對環丙沙星的吸附量為8.30mg/g[30].本實驗中酸化赤泥對環丙沙星的最大吸附量為7.84mg/g,稍低于磁性生物炭材料.這可能與酸化赤泥比表面積不高有關,后續將圍繞優化赤泥結構,提高比表面積和暴露活性點位開展進一步研究.

表5 不同吸附等溫線模型的擬合參數

2.4 吸附熱力學

吸附熱力學通過Van Tehoff方程依據不同溫度下Langmuir吸附等溫線常數計算[式(11~12)].

式中:L是Langmuir吸附平衡系數.

lnL對1/作圖,結果見圖6,根據直線斜率和截距求算的結果見表6.

由表6可知,不同溫度下,Δ0均為負值,這說明酸化赤泥吸附環丙沙星的過程是自發進行的.隨著溫度升高|Δ0|增大,說明溫度升高吸附劑的吸附性能增大[31].物理吸附Δ0一般認為在0~20kJ/mol,化學吸附在-80~400kJ/mol之間[32],因此環丙沙星在酸化赤泥上的吸附為化學吸附.Δ0>0,則說明吸附過程為吸熱反應[33].

圖6 溫度對吸附平衡常數的影響

表6 吸附熱力學狀態函數

2.5 ATR-FTIR分析

通過FTIR(圖7)分析,1614cm-1處的吸收峰為環丙沙星中C=O的伸縮振動峰;1372和1588cm-1為環丙沙星的—COO的對稱和反對稱伸縮振動峰[34]; 1498cm-1為O—H的伸縮振動峰.

圖7 環丙沙星及酸化赤泥吸附環丙沙星前后的紅外光譜

(before)表示吸附環丙沙星之前的酸化赤泥;(after)表示吸附環丙沙星之后的酸化赤泥

酸化赤泥的FTIR有幾個重要的吸收峰,其中1628cm-1是Fe—O鍵(赤鐵礦)的伸縮振動峰; 1394cm-1為Al—鍵(一水軟鋁石)的伸縮振動峰; 1504cm-1處為O—H鍵的彎曲振動,說明酸化赤泥中有吸附水的存在[35].

對比環丙沙星和吸附前后酸化赤泥的FTIR可知,1394cm-1處的Al—O鍵特征峰位移至1380cm-1處,說明赤泥的Al—O鍵與環丙沙星的—COO發生表面絡合作用[36]. 1504cm-1處的O—H鍵發生位移,說明赤泥中的吸附水被消耗[37]; 1628cm-1處的Fe—O鍵發生的位移較小,說明環丙沙星的C=O與赤泥的Fe—O鍵發生微弱的靜電作用或與赤泥的內球面相鍵合[38].這也印證了前面吸附動力學和熱力學認為吸附是一個化學吸附過程.

3 結論

3.1 基于Box-Behnken設計方法建立的響應面優化模型對酸化赤泥吸附環丙沙星進行預測模擬.得到響應面優化的最佳吸附條件為:吸附溫度45℃,溶液pH=3.04,環丙沙星初始濃度29.20mg/L,酸化赤泥投加量3.40g/L,預測最大吸附量7.30mg/g.

3.2 酸化赤泥對環丙沙星的吸附過程遵循偽二級反應動力學模型(2=0.999),擬合的最大吸附量為7.90mg/g,與實測最大吸附量7.84mg/g相符.

3.3 酸化赤泥對環丙沙星的吸附等溫線遵循Langmuir-Freundlich模型(2=0.9998),擬合得到最大吸附量為7.35mg/g.

3.4 吸附熱力學表明,酸化赤泥吸附環丙沙星的過程是自發進行的吸熱反應.

3.5 ATR-FTIR分析表明,酸化赤泥吸附環丙沙星的機理為環丙沙星的—COO與酸化赤泥發生表面絡合作用,同時C=O可能與酸化赤泥發生微弱的靜電作用或內球面鍵合作用.

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Adsorption of ciprofloxacin by acidified red mud: characteristic, mechanism and process optimization.

SHI Jing-zhuan1, WEI Hong1*, ZHOU Xiao-de1, SHI Ying-juan2, ZHENG Jia-xin1

(1.State Key Laboratory of Eco-Hydraulics in Northwest Arid Region, Xi’an University of Technology, Xi’an 710048, China；2.Shaanxi Reconnaissance Design & Research Institute of Water Environmental Engineering, Xi’an 710021, China)., 2019,39(11)：4689~4696

In this paper, the adsorption conditions, characteristic and mechanism of ciprofloxacin on the acidified red mud were studied.A four-factor and three-level optimization model based on Box-Behnken design method was established to determine the optimum adsorption condition, andadsorption temperature, solution pH, ciprofloxacin initial concentration and acidified red mud dosage were as arguments and adsorption capacity as the response value. The kinetic model, isotherm model, thermodynamic property and mechanism of the adsorption process were discussed as well. The results showed that solutionpH, ciprofloxacin initial concentration, acidified red mud dosage had significant effect on the adsorption process.The predicted maximum adsorption reached 7.30mg/g under the optimized conditions of 45℃, pH 3.04, ciprofloxacin initial concentration of 29.20mg/L, and acidified red mud dosage 3.40g/L. The adsorption was well fitted the pseudo-second-order reaction kinetics and Langmuir-Freundilich isotherm model, with the maximum adsorption capacity were 7.90 and 7.35mg/g, respectively. Δ0, Δ0and Δ0were calculated by Van Tehoff equation as -82.13~94.37kJ/mol, 0.61J/(mol·K) and100.25kJ/mol, respectively. Ciprofloxacinadsorption on acidified red mud was a spontaneous endothermic process.Infrared spectrum showed that the complexation between carboxylate group of ciprofloxacin and Al-O bond of acidified red mud, and the weak electrostatic or inner-sphere bonding between keto group in ciprofloxacin and Fe-O in acidified red mud were attributed to the adsorption. This study showed that acidified red mud is a potentially low-cost absorbent for the treatment of antibiotic-contaminated wastewater.

acidified red mud；ciprofloxacin；response surface optimization；adsorption kinetics；adsorption thermodynamics

X703

A

1000-6923(2019)11-4689-08

史京轉(1985-),女,陜西渭南人,西安理工大學博士研究生,主要從事生態環境修復及有機污染有效控制研究.發表論文5篇.

2019-04-19

國家自然科學基金資助項目(51979223);陜西省自然科學基金資助項目(2017JM5082);陜西省水利科技項目(2013slkj-07)

* 責任作者, 教授, weihong0921@163.com