梁 文,何 維,李滿(mǎn)林,黃 輝,Wang Jim J,張?jiān)鰪?qiáng),李榮華,4*(.西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院,陜西 楊凌 700;.西北農(nóng)林科技大學(xué)理學(xué)院,陜西 楊凌 700;.School of Plant, Environmental and Soil Sciences, Louisiana State University Agricultural Center, Baton Rouge, LA 7080, USA;4.農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室,陜西 楊凌 700)
金屬氧化物納米顆粒對(duì)磷的吸附及回收潛力
梁 文1,何 維1,李滿(mǎn)林2,黃 輝1,Wang Jim J3,張?jiān)鰪?qiáng)1,李榮華1,4*(1.西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院,陜西 楊凌 712100;2.西北農(nóng)林科技大學(xué)理學(xué)院,陜西 楊凌 712100;3.School of Plant, Environmental and Soil Sciences, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA;4.農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室,陜西 楊凌 712100)
比較4種金屬氧化物納米顆粒(nMgO,nAl2O3,nTiO2和nFe2O3)對(duì)水體P的吸附性能,并探討了pH、吸附時(shí)間、共存離子等因素對(duì)nMgO吸附P的影響,用XRD和XPS分析了nMgO對(duì)P的吸附機(jī)制,最后通過(guò)小青菜盆栽實(shí)驗(yàn)法探討n(yōu)MgO從養(yǎng)殖廢水中吸附回收P的應(yīng)用潛力.結(jié)果發(fā)現(xiàn),在pH 3.0~8.0范圍內(nèi),nMgO對(duì)P的吸附量顯著高于nAl2O3、nTiO2和nFe2O3,4種納米顆粒對(duì)P的吸附量分別可達(dá)40、31.77、15.93和13.08mg/g.吸附后P的解吸可逆性較差.nMgO對(duì)P的吸附能在0.5h內(nèi)達(dá)到吸附平衡,P的吸附符合準(zhǔn)二級(jí)動(dòng)力學(xué)過(guò)程.體系共存的等量F-、Cl-、NO3-、SO42-、Na+、K+和NH4+離子對(duì)nMgO吸附P無(wú)顯著影響, Mg2+和Ca2+離子對(duì)P吸附具有促進(jìn)作用.nMgO對(duì)P的吸附等溫線(xiàn)可用Langmuir模型描述,最大吸附量達(dá)139.3mg/g. XRD和XPS分析表明,nMgO對(duì)P的吸附是伴有靜電吸引的以表面絡(luò)合沉淀反應(yīng)為主的吸附過(guò)程.nMgO能有效地從養(yǎng)殖廢水中吸附回收 P,吸附 P的 nMgO作為肥料,能使小青菜干物質(zhì)量從 0.31g/kg土增加到0.96g/kg土.
納米顆粒;nMgO;P;吸附回收;養(yǎng)殖廢水
磷(P)是重要的生命元素,對(duì)生物的生長(zhǎng)繁衍具有重要作用,也是農(nóng)作物種植過(guò)程不可缺少的植物影響元素.隨著人類(lèi)農(nóng)業(yè)種植水平的提高,磷礦開(kāi)采量持續(xù)增加和磷肥的盲目施用,導(dǎo)致全球水體的富營(yíng)養(yǎng)化問(wèn)題較為普遍[1].2012年美國(guó)地質(zhì)調(diào)查局在全球范圍內(nèi)開(kāi)展的調(diào)查指出,中國(guó)作為世界上最大的P肥生產(chǎn)國(guó),若對(duì)自身的P礦資源開(kāi)采規(guī)劃不當(dāng),以現(xiàn)有的開(kāi)采速度, P礦資源將在35a內(nèi)耗竭[2].因此,一方面要加強(qiáng)控制P使用量,合理施肥以提升作物養(yǎng)分供給水平[3],另一方面要注重從水體中回收 P,才是從根本上解決 P資源短缺、實(shí)現(xiàn)P資源可持續(xù)循環(huán)利用的最終途徑[4-5].
目前,從水體回收P的主要方式是向含P水體中投加水溶性鈣鹽或鎂鹽的方法獲得沉淀形式的磷酸鹽[6].但這種方法的應(yīng)用要求是水體 P含量必須較高,才能推動(dòng)磷酸鹽難溶物的溶解-沉淀化學(xué)平衡移動(dòng);如果水體P含量不滿(mǎn)足沉淀產(chǎn)生反應(yīng)商要求時(shí),則必須投加大量的鈣鹽或鎂鹽滿(mǎn)足反應(yīng)商.但在富營(yíng)養(yǎng)化水體中P含量較低,低P含量體系(限值0.01mg/L)并不適用于鈣鎂鹽沉淀回收法[4-5,7].雖然已有大量的研究報(bào)道證明,生物富集法[8]、離子交換法[9]、膜分離法[10]、電動(dòng)富集法[11]、吸附法等均能有效從水體回收 P,但由于吸附法相比而言成本低廉、操作簡(jiǎn)單、適用于痕量污染物富集的優(yōu)點(diǎn),而被認(rèn)為是最有應(yīng)用前景的措施之一[4-5].在吸附法的應(yīng)用中,許多學(xué)者已經(jīng)在農(nóng)業(yè)秸稈[12]、生物炭[13-14]、工業(yè)廢棄物[15]、金屬氧化物[16-17]、殼聚糖[18]、天然礦物[19-20]等吸附劑方面做了大量卓有成效的研究工作,但尋找新型高效吸附劑仍是推進(jìn)吸附法應(yīng)用的關(guān)鍵[5].
隨著納米技術(shù)的飛速發(fā)展,納米材料因具有比表面積大、吸附能力強(qiáng)、易于功能化等優(yōu)點(diǎn),而被認(rèn)為是一類(lèi)潛在的污染物高效吸附材料[21].例如,ZrO2修飾CMK-3納米顆粒[22]、鑭氧化物負(fù)載Fe3O4@SiO2納米顆粒[23]、ZrO2納米顆粒[24]等均具有良好的水體P吸附性能,但目前的研究仍以模擬含P污水進(jìn)行吸附劑性能探討為主.為推進(jìn)吸附法的應(yīng)用,在吸附劑的篩選過(guò)程中,仍需要在相同試驗(yàn)條件下系統(tǒng)地比較不同吸附劑的吸附性能,同時(shí)需要加強(qiáng)吸附劑對(duì)實(shí)際污水中 P的吸附回收研究[7].但當(dāng)前有關(guān)不同納米材料吸附劑的P吸附性能系統(tǒng)比較研究仍鮮見(jiàn)報(bào)道,有關(guān)其在實(shí)際污水 P吸附回收方面的應(yīng)用潛力仍缺乏探討[4,7-8].
為此,本研究通過(guò)批處理實(shí)驗(yàn)法,系統(tǒng)的比較了金屬氧化物納米顆粒吸附劑(nMgO, nAl2O3, nTiO2和nFe2O3)對(duì)水體P的吸附性能,并進(jìn)一步探討了nMgO對(duì)P的吸附機(jī)制及其在養(yǎng)殖廢水P回收中的應(yīng)用潛力,以期為水體P回收提供新型納米材料吸附劑,豐富納米顆粒吸附劑在吸附法中的應(yīng)用.
1.1 主要試劑與儀器
1.1.1 試劑 nMgO、nAl2O3、nTiO2和nFe2O3納米顆粒(純度 99.9%),購(gòu)于阿拉丁試劑公司.鉬酸銨購(gòu)于陜西金堆城鉬業(yè)股份有限公司,酒石酸銻鉀、NaF、NaCl、KCl、NH4NO3、MgSO4、CaCl2和KH2PO4購(gòu)于天津化學(xué)試劑公司,均為分析純,實(shí)驗(yàn)用水為去離子水.
1.1.2 儀器 JEOL 200CX型高分辨透射電子顯微鏡(TEM),TD-3500型X射線(xiàn)衍射儀(XRD), TriStar II 3020型比表面積分析儀,Zetasizer Nano ZS型Zeta電位儀,Kratos AXIS Ultra DLD型X射線(xiàn)能譜儀(XPS),島津UV-VIS-1800型紫外可見(jiàn)分光光度計(jì).
1.2 實(shí)驗(yàn)方法
1.2.1 納米顆粒的表征 用JEOL 200CX型高分辨透射電子顯微鏡觀察材料形貌.77K下用Tristar II 3020型比表面積測(cè)定儀測(cè)定材料的N2吸附-解吸曲線(xiàn),通過(guò) Brumauer-Emmett-Teller (BET)法計(jì)算樣品的比表面積.用 Zeta電位儀測(cè)試納米顆粒的zeta電位.以Mg K 為X射線(xiàn)源進(jìn)行X射線(xiàn)能譜分析(XPS).
1.2.2 金屬氧化物納米顆粒對(duì)P的吸附 采用批處理實(shí)驗(yàn)法,分別研究體系pH、吸附時(shí)間、離子強(qiáng)度、濃度、溫度等因素對(duì)P吸附的影響.即將 20mL一定濃度的 P溶液(KH2PO4配制)于75mL聚乙烯離心管中控制好實(shí)驗(yàn)條件后,加入一定量的金屬氧化物納米顆粒吸附劑,室溫振蕩一定時(shí)間后,經(jīng)12000r/min高速離心分離,0.10μm濾膜過(guò)濾,吸取5mL濾液,用島津UV-VIS-1800型紫外可見(jiàn)分光光度計(jì)以鉬藍(lán)法測(cè)定P濃度.所有實(shí)驗(yàn)重復(fù) 3次.參考文獻(xiàn)[5,25]的方法,根據(jù)吸附前后溶液中P的濃度,按qe=V(c0-ce)/m計(jì)算P吸附量 qe(mg/g).用準(zhǔn)一級(jí)動(dòng)力學(xué)模型 qt= qe?(1-e-k1t)和準(zhǔn)二級(jí)動(dòng)力學(xué)模型qt= k2?qe2?t/(1 + k2?qe?t)對(duì)吸附動(dòng)力學(xué)數(shù)據(jù)進(jìn)行擬合.式中: qt為 t時(shí)刻的吸附量, mg/g; k1和k2分別為一級(jí)和二級(jí)動(dòng)力學(xué)速率常數(shù).用 Langmuir模型 qe= qmax?KL?ce/[(1 + KL?ce)]和 Freundlich模型 qe= KF?ce1/n對(duì)吸附等溫線(xiàn)進(jìn)行擬合.式中: c0和ce分別為吸附前后溶液P的濃度, mg/L; V為P溶液的體積, L; m為吸附劑的質(zhì)量, g; KL為L(zhǎng)angmuir常數(shù); qe和 qmax分別為吸附量和最大吸附量, mg/g; ce為吸附平衡時(shí)溶液P的濃度, mg/L; KF與n分別為Freundlich模型常數(shù); T為溫度, K; R為氣體常數(shù),取值8.314J/(mol·K).
1.3 養(yǎng)殖廢水中P的回收應(yīng)用研究
為評(píng)價(jià)nMgO納米顆粒從含P水體吸附回收P的農(nóng)業(yè)應(yīng)用潛力,研究中首先自楊凌本香農(nóng)業(yè)產(chǎn)業(yè)集團(tuán)某大型肉豬養(yǎng)殖場(chǎng)(揉谷鎮(zhèn))采集養(yǎng)殖廢水,經(jīng)過(guò)濾除去懸浮固體后備用.該廢水 pH 7.86,CaCO3堿度 1377.2mg/L,COD 2080.4mg/L, NH4+-N 218.8mg/L,總P 113.3mg/L,K+103.7mg/ L, Ca2+20.2mg/L,Na+13.3mg/L.然后將200mL養(yǎng)殖廢水裝入500mL聚乙烯硬質(zhì)塑料瓶中(pH未調(diào)節(jié)),同時(shí)加入 0.25g nMgO 納米顆粒,室溫振蕩2h后,用0.10μm濾膜過(guò)濾,吸取5mL濾液用鉬藍(lán)法測(cè)定總P濃度然后將濾膜小心取出,并投入盛有 500mL 去離子水的燒杯中,超聲波振蕩30min 使粘附在濾膜上的固體顆粒分散在液相中,獲得乳白色懸液備用.將采集自楊凌某地的廢棄窯洞周邊的表層0~30cm黃褐土 (該區(qū)域土壤貧瘠,幾乎寸草不生,土壤樣品理化性質(zhì)分析表明,pH 8.02,粘粒46.25%,EC 119.79 μS/cm,全K 4.03g/kg,全 N 0.25g/kg,全 P 0.53g/kg,有機(jī)質(zhì)4.16g/kg)挑去碎石塊后,實(shí)驗(yàn)室自然風(fēng)干,用木棒壓碎,過(guò)2mm尼龍篩.參考文獻(xiàn)[26]的方法進(jìn)行小青菜盆栽實(shí)驗(yàn),具體為稱(chēng)取土壤樣品 2kg裝入塑料花盆中,加入吸附P后的乳白色懸液500mL并充分混勻,對(duì)照處理中僅加入 500mL 去離子水,室溫靜置2 周后,每盆播入10粒小青菜種子,等種子萌發(fā)至4個(gè)葉片后間苗至每盆5株,每一處理重復(fù)3盆,持續(xù)盆栽實(shí)驗(yàn)35d后收獲小青菜地上部分,裝入紙袋在120℃溫度下殺青, 60℃烘干至恒重,計(jì)算小青菜地上部分干生物量(g/kg土).試驗(yàn)期間,不施用其他肥料,僅用去離子水保持土壤含水率約80%左右田間持水量.
2.1 納米顆粒的表征
圖1 金屬氧化物納米顆粒的TEM照片F(xiàn)ig.1 The TEM images of metal oxides nanoparticles (a)nMgO; (b)nAl2O3;(c)nTiO2;(d)nFe2O3
納米顆粒在無(wú)水乙醇中分散后,對(duì) 4種納米顆粒進(jìn)行的透射電鏡觀察見(jiàn)圖 1.由圖 1可知, nTiO2納米顆粒的球形結(jié)構(gòu)清晰,顆粒均勻且分散良好,粒徑介于15~28nm之間;nMgO、nAl2O3和nFe2O3納米顆粒的粘連較為嚴(yán)重,顆粒分散度相對(duì)較差,3種納米顆粒的粒徑介于 18~89nm、13~92nm和17~112nm 之間(圖1), nTiO2納米顆粒分散度較好,顆粒相對(duì)較為均勻,粒徑約 40nm左右.4種納米顆粒材料對(duì)N2的吸附呈現(xiàn)出IV型吸附-脫附曲線(xiàn)(圖2(a)),這表明4種材料的孔隙結(jié)構(gòu)較為均勻. nMgO、nAl2O3、nTiO2和nFe2O3納米顆粒的比表面積分別為93.8, 101.2, 126.4和86.5m2/g.除nFe2O3納米顆粒外,nTiO2、nMgO 和nAl2O3納米顆粒的孔徑分布較為均勻,分別集中在2.77nm、3.78nm和4.56nm(圖2(b)).進(jìn)一步采用X射線(xiàn)衍射儀對(duì)4種納米顆粒進(jìn)行了晶型分析,結(jié)果見(jiàn)圖2(c).由圖2(c)可知, 4種納米顆粒的XRD衍射峰清晰對(duì)稱(chēng),晶型良好,其衍射峰分別和粉晶數(shù)據(jù)庫(kù)中的 JCPDS 75-0447、JCPDS 10-0425、JCPDS 71-1187和JCPDS 33-0664標(biāo)準(zhǔn)卡片相對(duì)應(yīng),說(shuō)明研究的4種納米顆粒分別是由MgO、TiO2、Al2O3和Fe2O3所組成.這和Li等[5]制備的 MgO-生物炭、Cheng等[27]制備的TiO2-碳納米管、Xia等[28]制備的 Al2O3微管和Asfaram等[29]制備的Fe2O3吸附材料等納米顆粒衍射峰位基本一致.Zeta電位分析表明,nMgO、nAl2O3、nTiO2和nFe2O34種納米顆粒的零電荷電位pHzpc分別為>11、8.83、7.82和8.06.
圖2 金屬氧化物納米顆粒的N2吸附-脫附等溫線(xiàn)、孔徑分布、XRD譜圖和Zeta電位Fig.2 The N2adsorption-desorption isotherms、pore size distribution、XRD patterns and zeta potential of metal oxides nanoparticles
2.2 pH值對(duì)P吸附的影響及P的解吸
將一系列 20mL 濃度 50mg/L的 P溶液用0.1mol/L HNO3及NaOH 調(diào)節(jié)pH 3.0 ~11.0后,分別加入25mg nMgO、nAl2O3、nTiO2和nFe2O3,室溫振蕩12h. 4種納米顆粒在不同pH值下對(duì)P吸附的影響見(jiàn)圖3(a).由圖3(a)可知,在pH 3.0~8.0 的范圍內(nèi), nAl2O3、nTiO2和nFe2O3對(duì)P的吸附量基本維持平衡,分別可達(dá) 31.77mg/g (P被吸附79.42%)、15.93mg/g (P 被吸附 39.83%)和13.08mg/g (P被吸附32.70%),此后隨著pH值的繼續(xù)增大,吸附量逐漸降低.相比而言,在 pH 3.0~8.0的范圍內(nèi), nMgO對(duì)P具有極高的吸附親和力,吸附量基本維持 40mg/g (P全部被吸附),此后隨著pH值增大至10.94,吸附量逐漸降低至24.42mg/g.一般而言,影響吸附劑吸附量的因素主要包括吸附劑的比表面積大小、吸附劑顆粒分散度、吸附劑表面活性點(diǎn)位密度和介質(zhì)pH等因素[30].本研究中, 4種納米顆粒對(duì) P的吸附量表現(xiàn)出 nMgO>nAl2O3>nTiO2>nFe2O3的順序.由此可見(jiàn),對(duì)于本研究中的4種納米顆粒而言,其對(duì)P的吸附量與材料本身的比表面積大小和顆粒分散度關(guān)系不大.但介質(zhì)pH值會(huì)影響吸附劑表面的電荷特征和P在溶液中的存在形態(tài)[22,24].一般而言,水溶液中 P主要以 PO43-、HPO42-和 H2PO4-等形態(tài)存在[5,13].當(dāng)pH<8.0時(shí),溶液中P主要以帶負(fù)電荷的H2PO4-和HPO42-為主[5,24],而此時(shí) nAl2O3、nTiO2和 nFe2O3的表面會(huì)被質(zhì)子化帶正電荷,從而能通過(guò)靜電吸引維持較高的吸附量[23].但當(dāng)體系 pH值較高時(shí),溶液中P主要以HPO42-和PO43-為主,吸附劑的表面質(zhì)子化變?nèi)?所帶正電荷密度逐漸減少,靜電吸引力減小,同時(shí)體系中較多的 OH-能與 HPO42-和PO43-發(fā)生競(jìng)爭(zhēng)吸附,從而導(dǎo)致吸附量較低[23-24].
圖3 pH值對(duì)P吸附的影響及P的解吸Fig.3 Effect of pH on phosphate adsorption andphosphate desorption
為考察 P的解吸情況,試驗(yàn)中,將一系列20mL 濃度 50mg/L的 P溶液用 0.1mol/L HNO3及NaOH 調(diào)節(jié)pH=7.0,并分別加入25mg nMgO、nAl2O3、nTiO2和nFe2O3,室溫振蕩12h,經(jīng) 12000r/min高速離心分離,測(cè)定上清液中殘余的P濃度,計(jì)算出P的吸附量.然后,將固體殘?jiān)⌒挠萌ルx子水洗滌3次后,分別加入10mL pH值為 4.0和 9.0的去離子水,室溫振蕩 12h,測(cè)定液相中的P含量,計(jì)算P的解吸量,結(jié)果見(jiàn)圖 3(b).由圖 3(b)可見(jiàn),在 pH=7.0時(shí),nMgO、nAl2O3、nTiO2和nFe2O3對(duì)P的吸附量分別可達(dá)39.96、31.82、15.97和13.03mg/g,這一研究結(jié)果和圖 3(a)具有一定的吻合性.然而,在pH=4.0的去離子水中,從nMgO、nAl2O3、nTiO2和nFe2O3中解吸的P僅占吸附量的40.49%、4.23%、12.02%和 2.92%;而在 pH=9.0時(shí),從nMgO、nAl2O3、nTiO2和 nFe2O3中解吸的 P僅占吸附量的43.04%、9.11%、25.34%和5.99%.這表明對(duì)4種納米顆粒吸附P后,P的解吸性較差.這一研究結(jié)果和前人分別用磁性MgO生物炭[5]、介孔 α-Fe2O3[31]和納米氧化鋁[32]進(jìn)行 P吸附時(shí)P的解吸結(jié)果相類(lèi)似,暗示了4種納米顆粒對(duì)P的吸附可能與形成內(nèi)層表面金屬-P絡(luò)合物有關(guān)[5,31-32].由于nMgO對(duì)P的吸附量遠(yuǎn)高于nAl2O3、nTiO2和 nFe2O3,因而后續(xù)試驗(yàn)僅以nMgO為吸附劑進(jìn)行研究.
2.3 吸附時(shí)間對(duì)P吸附的影響
圖4 吸附時(shí)間對(duì)P吸附的影響Fig.4 Effect of contact time on phosphate adsorption
在吸附時(shí)間影響研究中,將 200mL濃度50mg/L的P溶液調(diào)節(jié)pH=7.0后,加入250mg nMgO,室溫振蕩12h.結(jié)果發(fā)現(xiàn), nMgO對(duì)P的吸附很迅速,能在 0.5h內(nèi)達(dá)到吸附平衡(圖 4).這和前人用氧化鑭修飾 Fe3O4@SiO2[23], ZrO2[24]等納米顆粒吸附 P的研究結(jié)果相類(lèi)似.為保證吸附劑對(duì) P的吸附效率,后續(xù)試驗(yàn)中吸附時(shí)間選用 2h.進(jìn)一步對(duì)吸附數(shù)據(jù)進(jìn)行動(dòng)力學(xué)擬合發(fā)現(xiàn),準(zhǔn)一級(jí)動(dòng)力學(xué)模型擬合結(jié)果為 k1= 0.5673h-1,決定系數(shù) R2=0.8802;準(zhǔn)二級(jí)動(dòng)力學(xué)模型擬合結(jié)果為 k2=3.0308g/(mg·h),決定系數(shù)R2=0.9993.說(shuō)明nMgO對(duì)P的吸附動(dòng)力學(xué)過(guò)程可用二級(jí)動(dòng)力學(xué)模型描述,暗示了其對(duì) P的吸附是化學(xué)吸附[5].
2.4 共存離子和P濃度對(duì)P吸附的影響
為考察共存離子對(duì) P吸附的影響,將 10mL濃度100mg/L的P溶液分別與10mL 100mg/L的NaF、NaCl、KCl、NH4NO3、MgSO4、CaCl2溶液混合后,調(diào)節(jié)pH=7.0并加入25mg nMgO,室溫振蕩2h.共存離子對(duì)P吸附的影響見(jiàn)圖5(a).從圖 5(a)可知,與對(duì)照(CK)相比,雖然體系共存的F-、Cl-、NO3-、SO42-、Na+、K+、NH4+等離子對(duì)nMgO吸附P沒(méi)有顯著影響, nMgO對(duì)P的吸附量基本維持在40mg/g左右,這暗示了nMgO對(duì)P的吸附具有較高的專(zhuān)一性[13,24].但Mg2+及Ca2+的存在會(huì)使P全部被吸附,這可能是由于Mg2+、Ca2+與P之間存在較高的化學(xué)親和力所致[5,24].
圖5 共存離子和P濃度對(duì)nMgO吸附P的影響Fig.5 Effect of coexist ions (a) and phosphate concentration (b) on phosphate adsorption onto nMgO
表1 不同吸附劑對(duì)P的吸附比較Table 1 Comparison of P adsorption with other reported adsorbents
表1 不同吸附劑對(duì)P的吸附比較Table 1 Comparison of P adsorption with other reported adsorbents
吸附劑 pH值吸附劑用量(g/L )平衡時(shí)間(h) 吸附模型 qmax(mg/g) 文獻(xiàn)Zr4+修飾殼聚糖膜 3 2 0.33 Langmuir 71.68 [10] Fe3O4納米顆粒 2~6 0.6 2 Redlich–Peterson 5.03 [17] ZrO2修飾CMK-3 6.5 0.25 13 Freundlich 70 [22]鑭氧化物Fe3O4@SiO2 5~9 1 0.17 Langmuir 27.8 [23] ZrO2納米顆粒 6.2 0.1 8 Langmuir 99 [24] MgO納米顆粒 3~8 1.25 0.5 Langmuir 139.3 本研究
在濃度影響研究中,將 20mL 濃度 5~200mg/L的P溶液調(diào)節(jié)至pH=7.0后,加入25mg nMgO,室溫振蕩2h. P初始濃度對(duì)吸附的影響見(jiàn)5(b).從圖5(b)可知, nMgO對(duì)P的吸附量隨著P的初始濃度的增加而增大并逐漸趨于平衡,這是因?yàn)槲絼┍砻娴奈轿稽c(diǎn)在濃度低時(shí)并未完全吸附,隨著濃度增加,吸附位點(diǎn)逐漸飽和,吸附的量則逐漸趨于定值[18].用 Freundlich 及Langmuir模型對(duì)圖5 (b)中的吸附數(shù)據(jù)進(jìn)行擬合可知, Freundlich模型的KF=15.94mg1-1/n·L1/n/g, n = 2.16. Langmuir模型的qmax= 139.3mg/g, KL= 6.68L/mg, Langmuir模型的決定系數(shù)R2(0.9882)大于 Freundlich模型的決定系數(shù) R2(0.8813), Langmuir模型更適合于描述nMgO對(duì)P的吸附等溫線(xiàn),說(shuō)明nMgO對(duì)P的吸附可能是單層吸附過(guò)程.這一結(jié)果和前人用不同吸附劑進(jìn)行P吸附的研究結(jié)果相類(lèi)似(表1).但本研究中, nMgO對(duì)P的最大吸附量高于文獻(xiàn)報(bào)道的諸多吸附劑(表1),這可能與吸附劑的化學(xué)組成有關(guān),也有吸附劑和及其與 P的作用機(jī)制有關(guān)[4]. 有關(guān)這一問(wèn)題,本研究將進(jìn)一步開(kāi)展探討.
2.5 nMgO對(duì)P的吸附機(jī)制
為考察nMgO對(duì)P的吸附機(jī)制,在離心管中,將50mg吸附P后的nMgO顆粒分散于10mL無(wú)水乙醇中,然后離心 5min后,傾去上清液,再加入2mL無(wú)水乙醇,劇烈震蕩至固體顆粒分散后,于60℃真空干燥箱中烘干 8h后,進(jìn)行 XRD,結(jié)果見(jiàn)圖6(a).將圖2(c)和圖6(a)進(jìn)行比較可知,對(duì)吸附P后的nMgO顆粒中除了含有MgO外,還出現(xiàn)了一些新的衍射峰,這些新出現(xiàn)的衍射峰來(lái)自Mg(OH)2、Mg(H2PO4)2和 MgHPO4.為了分析新生成物質(zhì)的比例,本研究進(jìn)一步對(duì)吸附P前后的nMgO顆粒進(jìn)行了XPS分析,結(jié)果如圖6(b)~6(d)所示.從圖6(b)可知,與吸附P前的nMgO相比,吸附P后的nMgO顆粒表面在135.1eV處出現(xiàn)了P 2p峰,證明了有P被吸附.進(jìn)一步對(duì)P 2p峰進(jìn)行分析可見(jiàn),吸附P后的nMgO顆粒的P 2p峰中含有PO43-(135.5eV)、MgHPO4和 Mg(H2PO4)2(圖 6(c));與此同時(shí), 1304.6eV處的 Mg 1s峰中也存在 MgHPO4, Mg(H2PO4)2, Mg(OH)2和MgO(圖6(d)).Mg 1s峰中MgHPO4和 Mg(H2PO4)2的峰面積比為0.96, P 2p峰中MgHPO4和 Mg(H2PO4)2的峰面積比為0.94,二者較為接近.結(jié)合XRD和XPS分析中Mg(OH)2的出現(xiàn)可知,雖然 nMgO在水中的溶解性很小,但nMgO水合后可能會(huì)被體系中共存的 HPO42-和H2PO4-離子促進(jìn)溶解,通過(guò)化學(xué)反應(yīng)MgO + H2O→ Mg(OH)2→ Mg2++2OH-,Mg2++ HPO42?→MgHPO4(s),Mg2++ 2H2PO4?→ Mg(H2PO4)2(s)等生成了Mg(H2PO4)2和MgHPO4[5].這一研究結(jié)果與前人在用磁性MgO修飾甘蔗葉生物炭[5]、富含鎂的西紅柿秸稈生物炭[33]及 MgO負(fù)載的鋰電池碳粉[34]吸附P的研究結(jié)果基本一致.
圖6 吸附P后nMgO的XRD圖譜和吸附P前后nMgO的XPS圖譜Fig.6 XRD pattern of nMgO sorbed phosphate and XPS pattern of nMgO before and after phosphate adsorption
基于以上研究結(jié)果,可認(rèn)為, nMgO對(duì)P的吸附是以表面絡(luò)合沉淀反應(yīng)過(guò)程為主,同時(shí)伴有靜電吸引過(guò)程,這一過(guò)程可示意為圖7.
圖7 nMgO對(duì)P的吸附機(jī)制Fig.7 Schematic illustration of the phosphate adsorption onto nMgO
2.6 nMgO對(duì)養(yǎng)殖廢水中P的回收應(yīng)用潛力
為探討n(yōu)MgO對(duì)養(yǎng)殖廢水中P的回收潛力,向200mL養(yǎng)殖廢水中投加了250mg nMgO 納米顆粒,結(jié)果發(fā)現(xiàn),經(jīng)過(guò) nMgO吸附后,養(yǎng)殖廢水中幾乎檢測(cè)不到 PO43-,說(shuō)明 nMgO 納米顆粒能有效地從養(yǎng)殖廢水中通過(guò)吸附回收 P.進(jìn)一步將吸附P的nMgO 納米顆粒作為肥料用于小青菜盆栽實(shí)驗(yàn)后發(fā)現(xiàn),由于研究所選取的土壤樣品養(yǎng)分十分貧瘠,對(duì)照處理中小青菜的地上部分干生物量?jī)H為0.31g/kg土,而施入吸附P的nMgO 納米顆粒后的小青菜地上部分干生物量則高達(dá)0.96g/kg土,這表明從養(yǎng)殖廢水中吸附 P后的nMgO具有較好的P肥應(yīng)用潛力.同時(shí),由于養(yǎng)殖廢水中除了含有P外還含有NH4+,因此從化學(xué)角度來(lái)看, nMgO還有望能從養(yǎng)殖廢水通過(guò)化學(xué)反應(yīng)Mg2++NH4++PO43-→ MgNH4PO4(s),同時(shí)實(shí)現(xiàn)對(duì)P和N的回收[35].
3.1 納米顆粒對(duì)P的吸附受體系pH值控制,在pH<8的條件下, 4種金屬氧化物納米顆粒對(duì)P均能保持穩(wěn)定的吸附量,但nMgO對(duì)P的吸附量遠(yuǎn)高于nAl2O3、nTiO2和nFe2O3, nMgO對(duì)P的吸附能在0.5h內(nèi)達(dá)到平衡.
3.2 體系共存的F-、Cl-、NO3-、SO42-、Na+、K+、NH4+等離子對(duì) nMgO吸附 P沒(méi)有顯著影響,Mg2+和Ca2+對(duì)P吸附具有促進(jìn)作用.nMgO對(duì)P的吸附等溫線(xiàn)可用Langmuir模型描述,最大吸附量可達(dá)139.3mg/g. nMgO對(duì)P的吸附是伴有靜電吸引以表面絡(luò)合沉淀反應(yīng)為主的吸附過(guò)程.
3.3 nMgO納米顆粒能有效地從養(yǎng)殖廢水中通過(guò)吸附回收P,吸附P后的nMgO具有較好的P肥應(yīng)用潛力,能顯著促進(jìn)小青菜的生長(zhǎng).
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Phosphate adsorption from solution by metal oxide nanoparticles and the potential on phosphate capture.
LIANG Wen1, HE Wei1, LI Man-lin2, HUANG Hui1, Wang Jim J3, ZHANG Zeng-qiang1, LI Rong-hua1,4*(1.College of Natural Resources and Environment, Northwest A&F University, Yangling 712100, China;2.College of Science, Northwest A & F University, Yangling 712100, China;3.School of Plant, Environmental and Soil Sciences, Louisiana State University Agricultural Center, Baton Rouge, LA 70803, USA;4.Key Laboratory of Plant Nutrition and the Agri-environment in Northwest China, Ministry of Agriculture, Yangling 712100, China). China Environmental Science, 2017,37(7):2557~2565
Effects of four different metal oxide nanoparticles (nMgO, nAl2O3, nTiO2and nFe2O3) on phosphate adsorption were compared in the batch experiment. The influences of solution pH, contact time and coexist ions on phosphate adsorbed by nMgO were examined. Further, the phosphate adsorption mechanism onto nMgO was evaluated by XRD and XPS analysis. And the potential of recovered phosphate by nMgO from pig breeding wastewater on fertilzer was assessed by pot experiment. The results showed that nMgO had higher phosphate adsorption ability than nAl2O3, nTiO2and nFe2O3. The phosphate amount reached 40、31.77、15.93 and 13.08mg/g in the range of pH 3.0 to 8.0 for nMgO, nAl2O3, nTiO2and nFe2O3, respectively. The phosphate adsorption was a inreversible process. The phosphate sorption onto nMgO could reach equilibrium within 0.5h, the adsorption process fitted the pseudo-second order kinetic model. The equal content of coexisted F-, Cl-, NO3-, SO42-, Na+, K+and NH4+ions had no negative influence on phosphate adsorbed onto nMgO, while the existence of Mg2+and Ca2+ions could promote the phosphate adsorption. Langmuir model could be used to describe the adsorption isotherm, by which the maximum phosphate adsorption capacity was around 139.3mg/g. Based on the results of XRD and XPS analysis, it can be concluded that phosphate adsorption was dominated by chemical precipitation reaction combined with electrostatic attraction process. nMgO particles effectively recovered phosphate from piggery wastewater, in turn the phosphate loaded nMgO nanoparticle could be used as a potential substitute for phosphate-basedfertilizer, which significantly improved the cabbage dry biomass from 0.31 to 0.96g/kg soil.
nanoparticle;nMgO;phosphate;adsorption recovery;piggery wastewater
X703
A
1000-6923(2017)07-2557-09
梁 文(1991-),女,陜西楊凌人,碩士研究生,主要研究方向?yàn)楣腆w廢棄物資源化利用與污染控制.
2016-11-21
中央高?;究蒲袠I(yè)務(wù)費(fèi)項(xiàng)目(2014YB064);美國(guó)農(nóng)業(yè)部環(huán)保署項(xiàng)目(USDA-NRCS#69-3A75-10-156)
* 責(zé)任作者, 副教授, rh.lee@nwsuaf.edu.cn