李 含,赵 雨,陈嘉超,许海民,朱雅娴,陈志辉,申萌萌,杨文澜,*

Ce-La双金属氧化物同步去除酸性废水中磷酸盐和氟的性能与机理

李 含1,赵 雨1,陈嘉超1,许海民2,朱雅娴1,陈志辉1,申萌萌1,杨文澜1,2*

(1.扬州大学环境科学与工程学院,扬州 225127；2.江苏启创环境科技股份有限公司,宜兴 214264)

采用共沉淀法制备出能同步吸附磷酸盐和氟的Ce-La双金属氧化物纳米吸附剂(CLBOs).结果表明,CLBOs主要以粒径20～50nm的纳米颗粒或纳米团簇的形式存在,比表面积为117.9m2/g.CLBOs在pH为4～12的范围内具有良好的稳定性,且酸性条件有利于CLBOs的除磷除氟;在pH = 4.0、磷酸盐和氟初始浓度为30、10mg/L的条件下,CLBOs最大磷、氟吸附量分别可达59.14,19.25mg/g.得益于静电吸引、配体交换和内配位络合的综合作用,CLBOs表现出优异的同步除磷除氟性能,且在高浓度竞争离子体系中能实现磷酸盐和氟的选择性吸附.CLBOs除磷除氟过程符合准二级动力学,且对氟的吸附速率显著快于磷酸盐,其中磷酸盐的吸附平衡时间约为240min,氟达到吸附平衡仅需100min.吸附饱和的CLBOs具有较好的再生性能,可长期重复使用,在含磷、氟酸性废水的深度处理领域具备良好的应用潜力.

Ce-La双金属氧化物；酸性废水；磷酸盐；氟；同步去除

水体中超负荷的磷和氟化物会导致生态系统恶化、危害人类健康.其中磷是引发水体富营养化的限制性污染因子[1-2],氟摄入过量会导致人体骨骼脆化甚至神经损伤[3-4].更为严重的是,水体中共存的磷和氟化物能够产生协同作用,对生态环境和人群健康造成更为严重的危害[5].

工业生产是水体磷、氟污染的重要来源,其中半导体、磷肥和矿冶等行业会产生大量含磷酸盐和氟化物的生产废水[6-7].此类废水具有水质复杂、毒性强,酸度高等特点[8-9],常规污水处理技术无法满足其稳定达标排放的需求,迫切需要开发稳定高效的污水磷、氟同步去除新技术.

当前磷、氟废水常用的处理技术包括混凝沉淀、离子交换、膜分离、吸附法等,其中吸附法因操作简单、效果稳定、投资运行费用低等优点,被广泛应用于污水中阴离子污染物的深度去除[10-11],但常见的商品化吸附剂在处理酸性含磷、氟废水过程中存在吸附选择性差、吸附容量低、易溶解流失等问题.近年来,铝、铁、锆、稀土等金属氧化物因其较高的比表面积和良好的目标污染物吸附选择性,在磷、氟废水深度处理领域受到研究人员的广泛关注[12-13].其中,稀土金属氧化物具有较强的碱度和较低的离子电位,在水中能够将表面羟基(-OH)解离为氢氧根(OH-),易与阴离子污染物发生配体交换实现其选择性吸附[14].铈(Ce),镧(La)是地壳中丰度较高的两类稀土金属,其氧化物化学性质稳定、表面羟基丰富,具有良好的抗酸碱溶出性能和较高的磷、氟吸附容量[15-16].近期研究表明,通过金属掺杂制备双金属氧化物,能够显著提高材料的表面羟基含量进而提升材料的吸附性能[17].例如,Zhang等[18]通过共沉淀法制备的Ce-Fe双金属氧化物(CFBOs)羟基含量达到30.8 %,远高于单金属氧化物CeO2的12.6 %和Fe3O4的19.6 %,使得CFBOs的除砷性能得到显著提高;Guo等[19]将Al-La双金属氧化物载入纤维素/石墨烯载体内,制备的复合纳米材料Al-La@CG表现出优异的磷酸盐、氟吸附性能.基于此,本研究拟采用共沉淀法制备Ce-La双金属氧化物(Ce-La bimetal oxides, CLBOs)纳米吸附剂,考察其对酸性废水中磷酸盐和氟的同步吸附性能,探究其理化性质及同步除磷除氟机制,以期为含磷、氟酸性废水的深度处理提供理论基础和技术支撑.

1 材料与方法

1.1 试剂

实验使用化学试剂均为分析纯,购于国药集团化学试剂有限公司或南京化学试剂有限公司,实验用水为超纯水.分别采用KH2PO4和NaF配制1000mg/L的磷酸盐、氟化物储备液;使用Na2SO4和NaCl配置SO42-、Cl-竞争离子溶液.

1.2 材料制备

称取11.13g LaCl3·7H2O和12.12g Ce(SO4)2·4H2O置于烧杯中(物质的量比为1:1),加入100mL纯水充分搅拌至完全溶解后,将1mol/L的NaOH溶液缓慢滴入上述混合液中至pH值为8左右,匀速搅拌1h后离心得到沉淀物.用纯水多次洗涤沉淀物至出水为中性,置于70 ℃烘箱内干燥12h,研磨得到CLBOs纳米吸附剂.用同样方法制得水合氧化铈(HCO)和水合氧化镧(HLO)用于对比研究.

1.3 实验方法

如无特别说明,实验中吸附剂用量均为0.50g/L,反应在含有100mL溶液的锥形瓶中进行,磷酸盐和氟初始浓度分别为30mg/L和10mg/L,pH值为4.0 ± 0.1,温度为298K,吸附时间为12h,使用1mol/L的HCl或NaOH调节溶液pH值;吸附后用0.22 µm醋酸纤维膜过滤分离CLBOs,并测定滤液中磷酸盐和氟浓度.稳定性实验中,将0.05g CLBOs分别置于pH值为2～12的100mL溶液中,298K下恒温振荡168h后测定溶液中Ce、La浓度.吸附剂性能对比实验中,分别称取0.05g CLBOs、0.025g HCO+0.025g HLO、0.05g HLO和0.05g HCO于100mL混合溶液中,测定各吸附剂平衡吸附量.投加量实验中,分别称取0.01～0.06g的CLBOs加入到100mL混合溶液中,测定不同投加量下CLBOs的吸附量和磷酸盐、氟去除率.pH值影响实验中,分别调节不同锥形瓶中溶液的pH值至2.0～12.0,考察pH值对CLBOs除磷除氟性能的影响.加入不同浓度的共存离子(SO42−、Cl−),考察CLBOs的选择性吸附性能.吸附动力学实验中,将0.5g CLBOs放入含1000mL混合溶液的三口烧瓶中,每隔一段时间取样测定磷酸盐和氟浓度.采用1.0mol/L NaOH溶液对吸附后的CLBOs进行脱附再生,考察CLBOs的重复利用性能.

1.4 实验仪器及分析方法

磷酸盐浓度采用《水质总磷的测定钼酸铵分光光度法》(GB 11893-89)的方法通过紫外/可见分光光度计(UV-1100B,上海美普达)测定;氟离子浓度采用《水质氟化物的测定离子选择电极法》(GB 7484-87)的方法通过氟离子选择电极(PXS-270, INESA,上海仪电)测定;溶液中Ce、La含量通过ICP-AES(ICP-Optima 7300DV, PerkinElmer, USA)测定.CLBOs的比表面积采用Nova-3000氮气吸附仪(Quantachrome, USA)测定,使用扫描电子显微镜SEM(S-4800II, 17Hitachi, Japan)考察材料的表面形貌,使用透射电子显微镜TEM(Tecnai 12, Philips, Netherlands)和高分辨TEM(HRTEM)观测CLBOs的微观结构,使用X射线衍射仪XRD(D8Advance, Bruker-AXS, Germany)分析CLBOs的晶型.材料的表面化学键采用FTIR红外光谱仪测定(Cary 5000, Varia, USA),CLBOs与氟、磷的相互作用通过光电子能谱(XPS)(ESCALAB250Xi, ThermoFisher, USA)进行表征分析.

2 结果与讨论

2.1 CLBOs的理化性质

图1 CLBOs的SEM图(a), TEM图(b), HRTEM图(c), XRD图(d)

图2 不同pH值下CLBOs的Ce、La溶出率

CLBOs的比表面积为117.9m2/g,SEM图(图1a)表明CLBOs纳米颗粒具有不规则的表面形貌;由TEM图(图1b)可知,CLBOs主要以粒径为20～50nm的纳米颗粒或纳米团簇的形式存在;HRTEM图(图1c)中清晰的晶格衍射条纹表明CLBOs具有微晶特性[20].CLBOs的XRD衍射图(图1d)没有出现明显的衍射峰,说明CLBOs纳米颗粒主要为无定形形态[19].

为进一步考察CLBOs在酸碱废水中长期使用的稳定性,实验测定了CLBOs在不同溶液pH值条件下Ce、La的溶出率.由图2可知,在pH值为3.0的条件下,La的溶出率为4.63%,Ce的溶出率仅为0.55%;当pH³4.0时,CLBOs均未检测到Ce、La的溶出,表明CLBOs在pH值4～12的范围内具有良好的稳定性,可用于酸性废水的除磷除氟.

2.2 吸附性能实验

为评估CLBOs的同步除磷除氟性能,选用CLBOs、HLO、HCO以及HCO+HLO混合材料(质量比为1:1)在相同条件下进行了对比实验.如图3所示,吸附剂对磷酸盐和氟的吸附性能表现为CLBOs > HLO > HCO+HLO > HCO,表明HLO对磷酸盐和氟的亲和力优于HCO.总体而言,CLBOs对磷酸盐和氟的吸附性能优于单金属氧化物及其混合材料,这是由于通过金属掺杂制备的双金属氧化物相比其单金属氧化物具有更高的表面羟基含量,从而获得更多的活性位点用于磷酸盐和氟的吸附[17-19].

CLBOs投加量对磷酸盐和氟同步去除性能的影响如图4所示.当吸附剂用量从0.1g/L增加到0.5g/L时,磷酸盐和氟的去除率分别从24.06%、40.5%提高到98.6%、95.8%,当CLBOs用量进一步增加到0.6g/L时,其对磷酸盐和氟的去除率没有明显提升,综合考虑处理效果和经济性,后续实验均将CLBOs投加量设定为0.5g/L.

2.3 pH值影响实验

溶液pH值是影响吸附性能的主要因素.由图5可知,酸性条件下CLBOs的吸附性能显著优于中性和碱性条件,当pH值为4.0时,CLBOs对磷酸盐和氟的吸附量最大,分别达到59.14mg/g和19.25mg/g.这是因为酸性条件下,CLBOs表面质子化带正电荷,有利于CLBOs通过静电吸引作用吸附磷酸盐和氟离子;同时,溶液中的H+也能够促进CLBOs与磷酸盐和氟的配体交换反应.当pH值降低至2时,溶液中磷酸盐主要以H3PO4分子存在,静电吸引和配体交换作用受到抑制,导致磷酸盐吸附量显著下降;当pH值介于2.12～7.21之间时,磷酸盐主要以H2PO4-的形式存在;而当pH值在7.21～12.31时,HPO42-占主导地位[21],研究表明H2PO4-相比HPO42-具有更低的吸附能,在配体交换方面更具优势[22].当pH值小于3.18时,氟主要以HF存在,此时氟的吸附主要依赖于CLBOs与HF的内配位络合作用;而当pH值大于3.18时,氟主要以F-存在,CLBOs主要通过静电吸引和配体交换作用吸附F-[23].随着pH值升高至碱性环境,CLBOs表面去质子化呈负电性,与HPO42-和F-之间产生静电排斥效应;同时溶液中高浓度的OH-会与HPO42-和F-竞争CLBOs的吸附位点,导致CLBOs的吸附性能显著下降.

2.4 共存离子实验

废水中常见的共存阴离子(如:SO42−、Cl−、NO3−等)对磷酸盐和氟的吸附存在显著的抑制作用,且SO42-对吸附性能的影响远高于其它一价阴离子[24-25].本研究中选择SO42-和Cl-为代表性共存离子,考察其对CLBOs除磷除氟的影响,并选用HLO和HCO进行对比研究.如图6所示,SO42-对CLBOs吸附磷酸盐和氟化物的抑制作用均大于Cl-.随着共存离子浓度从0增加到100mg/L,吸附剂对磷酸盐和氟的吸附性能都有所下降,进一步提高共存离子浓度至500mg/L, CLBOs的吸附性能能够保持相对稳定.此外,无论是否存在竞争离子,CLBOs均表现出比HLO和HCO更优的吸附性能,这是由于Ce-La双金属氧化物相比单金属氧化物具有更多的表面羟基,有利于CLBOs通过配体交换或内配位络合作用实现对磷酸盐和氟的选择性吸附[18-19].

2.5 吸附动力实验

吸附动力学是评价吸附速率的重要指标.由图7可知,CLBOs对磷酸盐和氟的吸附速率均较快,相比较而言CLBOs对氟的吸附速率显著快于磷酸盐,其中氟达到吸附平衡仅需100min,而磷酸盐的吸附平衡时间约为240min.这是由于F-(0.133nm)与OH-(0.133nm)具有相同的离子半径,而磷酸根的离子半径较大(0.200nm)[26],使得CLBOs在羟基配体交换过程中对氟的亲和力更强,吸附速率更快,在磷酸盐和氟共存条件下表现出对氟的优先吸附[8].

为更好的解析整个吸附过程,本研究采用准一级和准二级动力学模型对实验数据进行了拟合[27].由表1可知,准二级动力学模型能较好的拟合CLBOs对磷酸盐和氟的吸附过程,拟合所得平衡吸附量与实验结果较为接近.

表1 CLBOs对磷酸盐和氟的吸附动力学参数

2.6 “吸附-再生”循环实验

吸附剂的再生性能是评价其实际应用潜力的重要指标,本研究通过连续5批次的“吸附-再生”循环实验考察了CLBOs的脱附和重复利用性能.由图8可知,整个循环实验中CLBOs对氟的吸附量能保持基本稳定,而对磷酸盐的吸附量在第2批次和第3批次较第1批次分别下降了23.79 %和37.97 %,但从第3批次吸附循环开始吸附量能保持稳定.整个“吸附-再生”循环实验中CLBOs对磷酸盐的吸附量下降约43.57 %,对氟的吸附量没有明显变化,这可能是因为初始吸附批次中CLBOs的部分吸附位点被磷和氟永久占据,而F-与OH-有相似的离子半径[26],后续吸附过程中CLBOs对氟表现出优先吸附从而导致磷酸盐吸附量的降低[8].综上,CLBOs在吸附磷酸盐和氟后能够实现高效再生,多批次循环吸附性能保持相对稳定,是一种具有良好再生性能的除磷除氟吸附剂.

图8 CLBOs同步除磷除氟的“吸附−再生”循环实验

2.7 吸附机理

图9 CLBOs吸附磷酸盐和氟前后的FTIR光谱

为了进一步探究CLBOs潜在的除磷除氟机理,本研究通过FTIR红外光谱和XPS能谱对吸附磷酸盐和氟前后的CLBOs进行了表征.如图9所示, 3470cm-1处的宽峰代表结合水的弯曲振动[28], 600cm-1处的吸收峰代表M—O或O—M—O(M= Ce或La)的伸缩振动[29-30],吸附后该处峰强度明显下降并偏移至590cm-1,同时在651cm-1处出现一个新峰,可能是由于CLBOs吸附磷酸盐和氟后形成了M-O-P和M-F配位键[31-34].1370和1510cm-1处的吸收峰代表金属氧化物表面羟基(M-OH)的弯曲振动,吸附后M-OH峰强度明显减弱,这是由于CLBOs的表面羟基与磷酸盐和氟发生了配体交换所致[24-25].

由图10a可知,吸附后在结合能684.45eV(F 1s)和133.0eV(P 2p)处出现两个新的谱峰,表明磷酸盐和氟化物已被CLBOs成功吸附.F 1s峰的结合能与NaF标准F 1s峰(684.9eV)相比向低结合能方向偏移了0.45eV(图10b),表明CLBOs与氟之间产生了较强的结合作用[35-36].P 2p高分辨扫描图谱可分解为La-P(133.75eV)和Ce-P(132.95eV)两个特征峰(图10c)[37],峰面积占比分别为59.58 %和40.42 %,表明La对磷酸盐的亲和力略强于Ce.CLBOs吸附前的O 1s高分辨扫描图谱可分解为532.20eV、531.35eV和529.25eV三个特征峰(图10d),分别代表结合水(H2O)、金属羟基(M-OH)和金属氧化物(M-O)[37-38];吸附后(图10e)M-O峰的位置向高结合能方向发生偏移,这与CLBOs吸附磷酸盐后形成M-O-P配合物有关[39];M-OH的峰面积占比从吸附前的46.43%降至吸附后的29.02%,表明CLBOs的表面羟基通过配体交换被磷酸盐和氟取代[40-41],这与FTIR红外光谱的分析结果一致.

综上所述,CLBOs同步吸附磷酸盐和氟的机制包括:质子化CLBOs纳米颗粒对磷酸盐和氟的静电吸引作用(非选择性),CLBOs与磷酸盐和氟的羟基配体交换和内配位络合作用(选择性).

3 结论

3.1 采用共沉淀法制备了Ce-La双金属氧化物纳米吸附剂(CLBOs),其比表面积为117.9m2/g,并以粒径20～50nm的纳米颗粒或纳米团簇的形式存在;与HCO、HLO及其混合材料相比,CLBOs具有最佳的同步除磷除氟性能.

3.2 CLBOs具有良好的抗酸碱溶出性能;溶液pH值对CLBOs同步除磷除氟的性能有较大影响,在pH = 4.0、磷酸盐和氟初始浓度为30mg/L、10mg/L的条件下,CLBOs最大磷、氟吸附量分别可达59.14mg/ g、19.25mg/g.

3.3 pH值影响实验以及FTIR和XPS表征分析表明,CLBOs同步吸附磷酸盐和氟的机制主要包括静电吸引作用、配体交换作用和内配位络合作用;其中内配位络合作用有助于CLBOs在高浓度竞争离子环境中实现磷酸盐和氟的选择性吸附.

3.4 吸附饱和的CLBOs能通过碱液高效再生,再生后其除磷除氟性能保持相对稳定,可长期稳定的用于酸性废水中磷酸盐和氟的同步去除.

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Simultaneous removal of phosphate and fluoride from acid wastewater by Ce-La bimetal oxides: Performance and mechanism.

LI Han1, ZHAO Yu1, CHEN Jia-chao1, XU Hai-min2, ZHU Ya-xian1, CHEN Zhi-hui1, SHEN Meng-meng1, YANG Wen-lan1,2*

(1.School of the Environmental Science and Engineering, Yangzhou University, Yangzhou 225127；2.Jiangsu Qichuang Environmental Science and Technology Co., LTD., Yixing 214264)., 2023,43(10)：5148～5156

A novel Ce-La bimetal oxides nano-adsorbent (CLBOs) capable of simultaneous phosphate and fluoride removal from water was successfully synthesized by coprecipitation method. The CLBOs existed primarily as nanoparticles or nanoclusters, with a particle size range of 20～50nm and a specific surface area of 117.9m2/g. Notably, the CLBOs displayed excellent chemical stability across a wide pH range (4～12), with acidic conditions proving beneficial for the adsorption of phosphate and fluoride. Under experimental condition of pH 4.0and initial concentrations of 30mg/L for phosphate and 10mg/L for fluoride, the CLBOs exhibited a remarkable maximum adsorption capacity of 59.14mg/g for phosphate and 19.25mg/g for fluoride. This outstanding adsorption performance was attributed to the combined effects of electrostatic attraction, ligand exchange, and inner-sphere complexation. Furthermore, the presence of competing anions had minimal impact on the removal efficiency of CLBOs. The adsorption process of phosphate and fluoride onto CLBOs followed a pseudo-second-order kinetic model, with fluoride being adsorbed significantly faster than phosphate. Equilibrium was achieved in approximately 100 minutes for fluoride and 240 minutes for phosphate. Importantly, the exhausted CLBOs could be efficiently regenerated through a simple alkaline treatment, enabling their cyclic utilization while maintaining consistent adsorption performance. In conclusion, the results demonstrate that CLBOs is a highly efficient adsorbent with significant potential for practical applications in the simultaneous removal of phosphate and fluoride from wastewater.

Ce-La bimetal oxides；acid wastewater；phosphate；fluoride；simultaneous removal

X703

A

1000-6923(2023)10-5148-09

2023-03-05

国家自然科学基金资助项目(52070160);江苏省重点研发计划(社会发展)项目;扬州大学高端人才支持计划;宜兴市“陶都英才”创新创业人才项目(CX202011C);宜兴市科技创新专项资金重点研发项目(Y2022002);江苏省大学生创新创业训练计划项目(X20220563)

* 责任作者, 教授,wlyang@yzu.edu.cn

李 含(1998-),女,河南南阳人,扬州大学硕士研究生,主要从事环境功能材料及其在污水深度处理中的应用方面研究. 2731278155@qq.com.

李 含,赵 雨,陈嘉超,等.Ce-La双金属氧化物同步去除酸性废水中磷酸盐和氟的性能与机理 [J]. 中国环境科学, 2023,43(10):5148-5156.

Li H, Zhao Y, Chen J C, et al. Simultaneous Removal of Phosphate and Fluoride from Acid Wastewater by Ce-La Bimetal oxides: Performance and mechanism [J]. China Environmental Science, 2023,43(10):5148-5156.