新型水泥基材料富鎂C3A對氮磷的共去除

鄒友琴,李勇麗,郝鵬飛,歐陽思達,朱衷榜,章 萍

新型水泥基材料富鎂C3A對氮磷的共去除

鄒友琴,李勇麗,郝鵬飛,歐陽思達,朱衷榜,章 萍*

(南昌大學資源環(huán)境與化工學院,江西 南昌 330031)

將鎂摻雜進水泥基材料鋁酸三鈣(C3A)制得新型富鎂鋁酸三鈣(Mg@C3A)應用于水體氨氮(NH4+-N)和磷(PO43-)的共去除.通過批量實驗,考察了Mg@C3A投加量、氮磷濃度、溶液pH值、溫度等因素對NH4+-N、PO43-共去除的影響,并闡述了共去除機制.結果表明:Mg@C3A是由Mg摻雜C3A同構體和表面MgO組成,其中Mg的引入未改變C3A晶體結構和基本形貌.Mg@C3A材料對NH4+和PO43?具有良好的共去除效果.當Mg@C3A的投加量為3g/L,NH4+和PO43?的最大去除量分別為38.4,78.9mg/g;溫度升高有利于Mg@C3A 對NH4+和PO43?的共去除,而高pH值可促進NH4+的去除.Mg@C3A材料對NH4+的去除主要是OH-的中和作用和鳥糞石的沉淀作用主導,PO43-主要是與Mg2+或Al3+結合形成鳥糞石或磷酸鋁被去除.

富鎂鋁酸三鈣(Mg@C3A)；銨根(NH4+)；磷酸根(PO43-)；共去除

鳥糞石(MgNH4PO4·6H2O)結晶法是脫氮除磷的傳統(tǒng)工藝之一[1-3],其是通過外加鎂源將廢水中的NH4+和PO43-與Mg2+形成鳥糞石晶體(MgNH4PO4·6H2O)沉淀而實現(xiàn)氮、磷共去除[4-6].然而,鳥糞石結晶法應用中受多種因素如氮磷濃度、溶液pH值、溫度等影響.研究表明當NH4+和PO43-的摩爾配比小于1.2:1,溶液pH值較低,環(huán)境溫度較高均不利于鳥糞石的生成,導致NH4+的去除率降低[7-9].因此,尋找合適的含鎂化合物,且探明其去除氮磷的影響機制,對提高鳥糞石結晶法在水體氮磷共去除的應用性尤為重要.

鋁酸三鈣(Ca3Al2O6,簡稱C3A),水泥熟料的主要礦物之一.C3A易發(fā)生水化反應,可釋放大量Ca2+、Al3+及OH-[10-12],目前已作為環(huán)境材料被應用于含氨氮、鉛及鋅等水體污染修復中[13-15].研究發(fā)現(xiàn),C3A晶格中的鈣可被鎂等離子取代而形成金屬摻雜的C3A復合材料,且所摻雜的金屬不影響C3A的水化行為[16-17].因此,若將鎂摻雜進C3A制得的鎂摻雜水泥基材料Mg@C3A投加至氮、磷廢水,其水化過程中能釋放大量Mg2+、Ca2+、Al3+與OH-,通過誘導Mg2+與NH4+、PO43-生成鳥糞石,OH-中和部分NH4+,Ca2+、Al3+與PO43-結合形成含磷化合物,從而有望實現(xiàn)水體氮磷的共去除.然而,目前利用鎂摻雜水泥基材料Mg@C3A對水體氮磷共去除的研究尚未報道.

本研究采用固相反應合成了Mg@C3A復合材料,通過X射線衍射(XRD)、拉曼光譜(Raman)、及掃描電鏡(SEM)等表征探究Mg@C3A的晶相組成與形貌特征;通過批量實驗,考察Mg@C3A投加量、氮磷初始濃度、溶液pH值、溫度等因素對其共去除氮磷效果的影響,并深入闡述共去除機制,以期為拓寬水泥基材料應用于水污染修復提供理論與技術支持.

1 材料與方法

1.1 Mg@C3A的制備

采用固相反應法,將氫氧化鎂、氫氧化鋁和碳酸鈣以MgO/(CaO+Al2O3)質量比為15%進行稱量,均勻混合后進行壓片處理.將壓片后的樣品置于1623K的馬弗爐內煅燒3.5h.煅燒后的樣品經球磨粉碎均勻后,在333K條件下烘干.重復上述步驟2～3次即可獲得Mg@C3A.

1.2 去除實驗

將已知量的Mg@C3A和20mL已知濃度的氮磷共存溶液置于50mL錐形瓶中,在攪拌速度為150r/min的氣浴恒溫振蕩器中進行去除反應.考察Mg@C3A投加量(1～5g/L)、初始NH4+濃度(10～ 1000mg/L)、初始PO43-濃度(10～1000mg/L)、反應溫度(298～318K)、pH值(4.0～10.0)等參數(shù)對共去除氮磷效果的影響.實驗設置3個平行.反應結束后,將混合物離心分離,取上清液測定溶液的剩余銨根和磷酸根的濃度.

Mg@C3A對氨氮和磷酸根的去除量與去除率的計算方法按照式(1)與式(2)進行.

e=(0-e)/(1)

=(0-e)/0(2)

式中:e為Mg@C3A對氨氮和磷酸根的去除容量, mg/g;為Mg@C3A對氨氮和磷酸根的去除率,%;0和e分別為氨氮或磷酸根的初始和剩余濃度, mg/L;為反應溶液體積,L;為Mg@C3A質量,g.

1.3 固體產物表征

樣品的物相結構采用德國BRUKER公司D8ADVANCE型X射線衍射儀(XRD)進行檢測,測試條件為Cu靶Ka輻射源,管電壓40kV,= 0.15418nm,管電流50mA,掃描速度6o/min;樣品的微觀形貌和晶體結構采用日本Hitachi公司Nano- S450型場發(fā)射掃描電子顯微鏡(SEM),工作電壓為15kV;樣品的物質組成采用Virsa型拉曼光譜儀(英國Renishaw公司)分析,激發(fā)波長633nm,波數(shù)范圍100～1000cm-1,激發(fā)能9mW.溶液中NH4+-N與PO43-濃度采用紫外–可見光分光光度計(上海元析UV-6100型)測定;溶液pH值采用精密酸度計(上海雷磁PHS-3C型)測定;溶液金屬離子濃度采用電感耦合等離子體發(fā)射光譜分析儀(ICP-OES,DV 2000, PerkinElmer公司,美國)測定.

1.4 數(shù)據(jù)處理

實驗數(shù)據(jù)用Excel 2016進行數(shù)據(jù)處理、分析和統(tǒng)計,并采用Origin進行圖表繪制.

2 結果與討論

2.1 Mg@C3A物相表征

如圖1所示, Mg@C3A在2=33.30°、47.62°和59.27°處出現(xiàn)強且尖銳的衍射峰,分別對應于C3A標準卡片(JCPDS No. 38-1429)衍射峰的(440)、(800)和(844)晶面[14-15,18],表明鎂的引入未改變材料的立方相晶體結構[21].將樣品的(440)晶面特征衍射峰放大,鎂引入后,(440)晶面的衍射峰峰位向高衍射角度偏移,說明Mg以同晶取代方式替換C3A的晶格中的Ca2+[17].

同時,鎂引入后的(440)特征峰峰形明顯寬化,且晶面間距((440))從0.2705nm降至0.2701nm,這可能是由于Mg2+的離子半徑(0.066nm)小于Ca2+的離子半徑(0.099nm)[20].此外,在Mg@C3A樣品的圖譜中還觀測到了位于42.92°和62.29°的衍射峰,對應于四方相MgO的(200)和(220)晶面,表明Mg2+不僅以同晶取代方式摻雜進C3A晶體,還以MgO的形式存在.

如圖2所示,在506和752cm-1處的特征峰分別為n1[AlO45-]彎曲振動峰和n3[AlO45-]非對稱伸縮振動峰[21],表明樣品中存在構成C3A的AlO45-四面體[22].鎂引入后,AlO45-基團特征峰峰形明顯寬化,說明鎂的引入增加了C3A晶格結構的無序度[27].同時,353cm-1處出現(xiàn)Ca-O鍵發(fā)生藍移,進一步表明Mg以同晶取代的形式替換Ca摻雜進C3A的晶格中,與XRD分析的結果一致[24].此外,位于271和581cm-1處的特征峰為Mg-O鍵的振動峰,證實了樣品中存在MgO[25-26].

圖2 Mg@C3A和C3A樣品的拉曼圖譜

圖3 純C3A和Mg@C3A樣品的SEM照片

從圖3a可以看出,純C3A呈塊狀結構,顆粒尺寸為12.3mm.由圖3b可知,鎂引入后,Mg@C3A保持塊狀結構,粒徑增至21.4mm,其表面凸顯出圓球形小顆粒,表明材料表面可能存在MgO顆粒.綜上所述,Mg@C3A是由Mg摻雜C3A同構體和表面MgO組成,其中Mg的引入不改變C3A晶體結構和基本形貌.

2.2 Mg@C3A對NH4+與PO43-的共去除性能

2.2.1 Mg@C3A投加量的影響 在氨氮濃度為160mg/L,磷酸根濃度為160mg/L的條件下反應12h,考察不同投加量(1～5g/L)對Mg@C3A去除氨氮與磷酸根的效果影響.結果如圖4所示,當Mg@C3A的投加量從1g/L增至3g/L時,NH4+的去除量由20.0mg/g左右上升到26.3mg/g.而當投加量增至4和5g/L,去除量呈下降趨勢,這可能是由于高劑量引起Mg@C3A釋放大量的離子不能被有效利用所致[27].隨著投加量增加,NH4+的去除率由10.6%增至64.0%.而PO43?的去除趨勢不同,隨著投加量增加,PO43?的去除量顯著降低,從128.0mg/g降至31.1mg/g. Mg@C3A對PO43?的去除率隨著投加量增加由80.0%升至97.2%.結果表明,Mg@C3A對NH4+和PO43?的去除行為不同.從水處理成本和效率角度考慮,Mg@C3A的投加量設為3g/L.

2.2.2 初始NH4+濃度的影響 在Mg@C3A投加量為3g/L,磷酸根濃度為160mg/L的條件下反應12h,考察氨氮初始濃度(10～1000mg/L)對Mg@C3A去除氨氮與磷酸根的效果影響,結果如圖5所示.當初始NH4+濃度從10mg/L增至200mg/L時,Mg@C3A對NH4+的去除量由2.1mg/g升至28.3mg/g,而當濃度繼續(xù)增大,NH4+的去除率增速變緩,在初始NH4+濃度為1000mg/L時達38.4mg/g.而PO43?的去除量呈現(xiàn)出緩慢的增長趨勢,其在初始NH4+濃度為10～1000mg/L的范圍內e由46.8mg/g增至54.0mg/g,表明高NH4+濃度有利于PO43?的去除.這可能是由于高NH4+濃度提高了溶液的過飽和度,有利于鳥糞石生成[28].

圖5 初始NH4+濃度對Mg@C3A去除NH4+與PO43-的影響

2.2.3 初始PO43-濃度的影響 在Mg@C3A投加量為3g/L,氨氮濃度為160mg/L的條件下反應12h,考察磷酸根初始濃度(10～1000mg/L)對Mg@C3A去除氨氮與磷酸根的效果影響,結果如圖6所示.當PO43?初始濃度從10mg/L增至200mg/L時, Mg@C3A對PO43?的去除量由3.3mg/g升至62.5mg/g,而當濃度繼續(xù)增大,PO43?的去除量增加緩慢,在濃度為1200mg/L時達78.9mg/g.而NH4+的去除量保持在26.9mg/g左右,表明PO43?對NH4+的去除無明顯的影響.

圖6 初始PO43-濃度對Mg@C3A去除NH4+與PO43-的影響

2.2.4 反應溫度的影響 圖7為NH4+和PO43?初始濃度160mg/L,反應時間12h時,Mg@C3A在298～ 318K下與NH4+和PO43?的反應結果.由圖可見,溫度對NH4+的去除影響較大.溫度從298K升高至308K時,Mg@C3A對NH4+的去除量迅速增加,由26.3mg/g提高至48.2mg/g.隨后,溫度升至318K,其去除量緩慢增至51.6mg/g.PO43?的去除量隨溫度升高由51.9mg/g緩慢升至53.3mg/g.結果表明,Mg@C3A與NH4+和PO43?的反應為吸熱反應,溫度升高有利于NH4+和PO43?的共去除.此外,這一現(xiàn)象與文獻中高溫會抑制鳥糞石法對NH4+和PO43?去除的現(xiàn)象[13]不一致,說明除鳥糞石沉淀外,部分NH4+和PO43?還可通過另一種方式被去除,將在機理部分進行深入分析.

2.2.5 溶液pH值的影響 圖8為NH4+和PO43?初始濃度為160mg/L,反應時間12h時,Mg@C3A在溶液pH值為4～10的條件下與NH4+和PO43?的反應結果,可以看出,隨著pH值的升高,Mg@C3A對NH4+的去除量呈現(xiàn)緩慢的增長趨勢,說明pH值的升高有利于Mg@C3A對NH4+的去除.而PO43?的去除量受pH值影響較為微弱,其隨pH值的增大由51.9mg/g降至48.2mg/g.由此可見,Mg@C3A對NH4+和PO43?的去除行為及機制不同.

圖7 反應溫度對Mg@C3A去除NH4+與PO43-的影響

圖8 pH值對Mg@C3A去除NH4+與PO43-的影響

2.3 機理分析

為進一步探究Mg@C3A共去除NH4+和PO43-的機理,通過XRD對Mg@C3A與NH4+和PO43-反應后產物進行分析.圖9為Mg@C3A與NH4+和PO43-反應前后的XRD圖譜.反應后,圖譜中未觀察到C3A的衍射峰,說明Mg摻雜C3A同構體與NH4+和PO43-的反應完全;鈣鋁雙金屬氫氧化物(鈣鋁LDH)和AlPO4的特征衍射峰出現(xiàn),分別與各自的標準圖譜(JCPDS No.78-2051和No.47-0608)相對應,其匹配度值(FOM)分別為19.2和31.7.此外,反應產物中也存在少量鳥糞石(MAP,No.15-0762)的特征峰,結合Mg@C3A在純水中鈣、鋁、鎂離子的釋放情況(表1),可見,Mg@C3A投加至水體后發(fā)生水化,其釋放的一部分鈣、鋁、鎂離子分別與NH4+和PO43-結合生成鳥糞石(MAP)和AlPO4沉淀,而另一部分與溶液中的Cl-組裝形成了鈣鋁LDH.

圖9 Mg@C3A去除NH4+與PO43-前后的XRD圖譜

表1 Mg@C3A在純水中的金屬離子釋放量及pH值

表2 Mg@C3A與NH4+和PO43-反應前后的pH值以及在純水水化后的pH值

表3 反應后固相中NH4+和PO43-

為進一步研究NH4+與PO43-的去除機制, Mg@C3A反應前后的溶液pH值變化如表2所示.由表2明顯看出,反應前后溶液pH值高于反應前,但較水化后低,表明反應過程中Mg@C3A消耗了溶液中OH-.此外,將生成的產物進行溶解,以深入探究反應過程,結果如表3所示,通過生成MAP去除NH4+的量為0.15mmol/g.而NH4+的總去除量為1.92mmol/g(圖5),可以推測大部分NH4+(1.77mmol/g)通過與OH-結合生成游離NH3被去除.PO43-去除則主要是AlPO4(0.42mmol/g)> MAP(0.15mmol/g).

基于上述討論,Mg@C3A與NH4+和PO43-的反應機制如圖10所示.Mg@C3A去除NH4+的機制主要是OH-的中和作用和鳥糞石的沉淀作用;PO43-主要通過與Mg或Al結合形成鳥糞石或磷酸鋁沉淀被去除.

圖10 Mg@C3A共去除NH4+與PO43-反應機制示意

3 結論

3.1 Mg@C3A呈塊狀結構,表面粗糙,且有球形小顆粒均勻分布在材料表面.Mg@C3A是由Mg摻雜C3A同構體和表面MgO組成,其中Mg的引入不改變C3A晶體結構和基本形貌.

3.2 當Mg@C3A的投加量為3g/L,NH4+和PO43?的最大去除量分別為38.4mg/g和78.9mg/g.Mg@C3A對NH4+和PO43?的去除均是吸熱反應;溶液pH值的升高有利于NH4+的去除.

3.3 Mg@C3A對NH4+的去除主要是以OH-的中和作用為主導及鳥糞石的沉淀作用主導,PO43-主要是與Mg2+或Al3+結合形成鳥糞石或磷酸鋁被去除.

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The co-removal of ammonium and phosphate by novel cement-based material magnesium-rich tricalcium aluminate and its mechanism investigation.

ZOU You-qin, LI Yong-li, HAO Peng-fei, OUYANG Si-da, ZHU Zhong-bang, ZHANG Ping*

(School of Environmental and Chemical Engineering, Nanchang University, Nanchang 330031, China)., 2021,41(6)：2639～2645

Magnesium-rich C3A (Mg@C3A) was successfully prepared via doping magnesium into cement-based material tricalcium aluminate (C3A) for the co-removal of ammonium (NH4+-N) and phosphorus (PO43-). The effects of Mg@C3A dosage, contaminants concentration, solution pH value, and temperature as well as the mechanism were investigated. Mg@C3A was composed of the C3A isomorphism formed by the replacement of Ca with Mg and the surface MgO, in which the Mg doping does not affect the cubic symmetric structure and morphology of C3A. The maximum removal capacity of NH4+and PO43?were 38.4, 78.9mg/g at the Mg@C3A dosage of 3g/L, respectively. The increase of temperature was conducive to the co-removal of NH4+and PO43?by Mg@C3A. High pH value enhanced the NH4+removal. The removal mechanism of NH4+was mainly neutralization and the precipitation; PO43?was removed via the precipitation combining with Mg2+or Al3+to form struvite or aluminum phosphate.

magnesium-rich tricalcium aluminate (Mg@C3A)；ammonium (NH4+)；phosphate(PO43-)；co-removal

X703.1

A

1000-6923(2021)06-2639-07

鄒友琴(1973-),女,江西奉新人,副教授,博士,主要從事水資源與環(huán)境研究.發(fā)表論文20余篇.

2020-11-02

國家自然科學基金資助項目(21767018);江西省重點研發(fā)計劃項目(20192BBG70054)

* 責任作者, 副研究員, zhangping@ncu.edu.cn