HAP誘導磷酸鈣結(jié)晶回收低磷污水中的磷

肖輝毅,聶小保,萬俊力,鄧權(quán)慶,王奕睿,隆院男,蔣昌波

HAP誘導磷酸鈣結(jié)晶回收低磷污水中的磷

肖輝毅,聶小保*,萬俊力,鄧權(quán)慶,王奕睿,隆院男,蔣昌波

(長沙理工大學水利與環(huán)境工程學院,洞庭湖水環(huán)境治理與生態(tài)修復湖南省重點實驗室,湖南省環(huán)境保護河湖疏浚污染控制工程技術(shù)中心,湖南 長沙 410114)

為提高磷酸鈣結(jié)晶產(chǎn)物回收價值和低磷適應性,以羥基磷酸鈣(HAP)為晶種,誘導Ca-P結(jié)晶回收模擬二級出水(初始PO4-P濃度1.0mg/L)中的磷,對比了誘導結(jié)晶與均相結(jié)晶的磷回收效果,考察了Ca/P比和晶種投加量對磷回收的影響,并結(jié)合產(chǎn)物SEM、EDS、XRD和FTIR分析,探討了HAP誘導Ca-P結(jié)晶機制.結(jié)果表明, HAP誘導Ca-P結(jié)晶在避免晶種材料對結(jié)晶產(chǎn)物純度和品質(zhì)影響的同時,還具有低磷適應性強和啟動快速的優(yōu)勢; Ca-P結(jié)晶模式包括構(gòu)晶離子在HAP表面的逐層結(jié)晶模式和在HAP顆?？障堕g的晶橋模式.實驗條件下,模擬二級出水磷回收率可達80%以上,產(chǎn)物包括HAP及其前體物ACP(無定形態(tài)磷酸鈣)和OCP(磷酸八鈣). Ca-P結(jié)晶過程伴隨發(fā)生CaCO3結(jié)晶,干擾磷結(jié)晶回收.研究成果為低磷污水磷回收率和回收產(chǎn)物品質(zhì)的提升提供了依據(jù).

HAP；低磷污水；磷回收；磷酸鈣結(jié)晶；晶種

磷是有限的不可再生資源.據(jù)預測,到2050年我國將面臨磷資源嚴重短缺[1],且磷是造成水環(huán)境污染的最主要污染物之一.從市政污水中回收磷,有望滿足10%～15%的磷資源需求[2].

市政污水磷回收位點包括生物污泥及其灰渣[3-6]、污泥濃縮脫水液[7-9]、厭氧消化液[10-12]和二級出水[13-14].生物污泥中重金屬、持久性有機污染物(POPs)和病原體濃度較高[15-17],不利于磷回收和回用;經(jīng)焚燒后灰渣中磷含量可達50～100g/kg,但植物可利用性較低[9],重金屬和鐵/鋁(污水混凝時加入)也會增加回收成本[9].污泥濃縮脫水液中腐殖質(zhì)含量較高,會降低磷回收產(chǎn)品品質(zhì)[18].厭氧消化液中磷濃度較高,且以PO4-P為主[19],便于結(jié)晶回收,但厭氧消化池和回收裝置結(jié)垢明顯,存在安全風險[20].據(jù)報道目前我國僅有不足50家市政污水廠厭氧消化裝置仍在運行.

針對二級出水的磷回收,可避免或減輕上述問題,但也存在自身弊端,主要是水量大和磷濃度低,直接回收困難[9].為此,常采用離子交換進行預處理以提高磷濃度[14,21].但離子交換對磷的選擇性不高,腐殖質(zhì)、POPs和病原體會干擾離子交換裝置運行,降低磷回收效率[22].目前鮮見針對二級出水的直接磷回收報道.

羥基磷酸鈣(HAP)和磷酸銨鎂(MAP)結(jié)晶可直接回收污廢水中的磷[23-28],前者對低磷水質(zhì)適應性更強[25].為進一步提高HAP結(jié)晶的低磷適應性,研究者以方解石[25-30]、改性方解石[27]、磷灰石(AP)[27]、石英砂[31]、牛骨粉[32-33]、轉(zhuǎn)爐渣[34-35]、磁粉[36]、貝殼粉[37]等為晶種,借助晶種誘導作用降低結(jié)晶反應能壘,縮短結(jié)晶誘導期,從而在較低過飽和度下實現(xiàn)磷結(jié)晶回收.目前HAP誘導結(jié)晶原水磷濃度最低可至3.5mg/L[32].本文前期研究以方解石為晶種,通過增加晶種投加量(10g/L)和減小晶種粒徑(45μm),對模擬二級出水(初始磷濃度1.0mg/L)獲得80%以上磷回收[30].研究表明,誘導結(jié)晶效果主要決定于晶種表面對構(gòu)晶離子的識別度[38],因此以HAP為晶種理論上可以獲得最佳誘導效果;同時在結(jié)晶產(chǎn)品純度、品質(zhì)和回收價值方面具有優(yōu)勢.如以石英砂為晶種的HAP結(jié)晶產(chǎn)物,只能作為肥料回收,而以HAP為晶種,結(jié)晶產(chǎn)物有望作為回收價值更高的工業(yè)和生物原材料[39].

為此,本文以HAP為晶種、模擬二級出水為對象,開展低磷污水Ca-P結(jié)晶磷回收研究.考察HAP對Ca-P結(jié)晶的誘導作用,分析Ca/P物質(zhì)的量比(Ca/P比)和晶種投加量對磷回收效果的影響,結(jié)合結(jié)晶產(chǎn)物SEM、XRD、EDS和FTIR分析,探討HAP作為晶種的除磷機制.本文首次提出采用HAP作為晶種回收低磷污水中的磷,可為低磷污水磷回收率和回收產(chǎn)物品質(zhì)的提升提供依據(jù).

1 材料和方法

1.1 實驗材料

CaCl2·2H2O、NaOH、HCl和KH2PO4均為分析純(國藥集團化學試劑有限公司).晶種粒徑選擇是誘導除磷的關(guān)鍵,減小粒徑有助提升除磷效果,但粒徑過小晶種容易流失導致出水濁度增加,為此采用100～150目HAP作為晶種,購自陜西飛米生物科技有限公司.溶液配制用水由Millipore Milli-Q Gradient水凈化系統(tǒng)(Billerica, MA)制備,電阻率18.2MΩ·cm,pH=6.6～6.8.配制1.0g/L的磷溶液(以PO4-P計,下同)作為貯備液.NaOH和HCl貯備液濃度均為1mol/L.

原水采用自來水配制.自來水先經(jīng)結(jié)晶去除鈣硬度,再經(jīng)離心分離(3000r/min)后,取離心液加入磷貯備液至預設初始磷濃度,最后采用NaOH和HCl貯備液調(diào)整pH值至9.0.水質(zhì)指標如表1所示.

表1 原水水質(zhì)指標

1.2 實驗方法

1.2.1 HAP的誘導結(jié)晶效果 通過對比均相結(jié)晶(未投加HAP晶種)與誘導結(jié)晶(投加HAP)的磷回收率,考察HAP誘導結(jié)晶效果.對比實驗采用六聯(lián)攪拌儀(ZR4-6,深圳中潤)進行.往燒杯中加入500mL原水(初始磷濃度分別為0.5、1.0、5.0、10.0、20.0和40.0mg/L),在快速攪拌條件下(300r/min)加入100目HAP,晶種濃度分別為0和10g/L.此后改為中速攪拌(150r/min),并投加Ca2+貯備液至Ca/P比為1.67,反應30min后再靜置10min,取上清液測定磷濃度和pH值(過0.45 μm濾膜).

1.2.2 Ca/P比和晶種投加量的影響 往燒杯中加入500mL初始磷濃度1.0mg/L的原水,采用與1.2.1中相同的程序進行誘導結(jié)晶磷回收. Ca/P比取1.67、5、10、20、50和100;晶種投加量取0、0.5、1、2、5、10和20g/L.靜置結(jié)束后, 取上清液測定磷濃度和pH值(過0.45 μm濾膜),再取一份上清液測定濁度(如有需要).

1.2.3 結(jié)晶產(chǎn)物表征 在流化床中(有效高度1.5m,內(nèi)徑2cm)投放10g/L晶種,流化床進水初始PO4-P濃度1.0mg/L、Ca/P比20、上升流速20m/h.穩(wěn)定運行48h后,取晶種進行SEM、EDS、XRD和FTIR分析.

1.3 分析方法

pH值測定采用pH電極(雷磁PHSJ-3C,上海儀電科學);濁度測定采用濁度儀(2100P,美國哈希);磷濃度測定采用鉬酸銨分光光度法,見《水和廢水監(jiān)測分析方法》(第四版)[40].

結(jié)晶產(chǎn)物40℃烘干后進行SEM與EDS分析(S4800,日本Hitachi)、XRD分析(D8-Advance,德國布魯克)和FTIR分析(Nicolet iS50,美國賽默飛).將樣品涂抹在電鏡底座上,獲得樣品微觀照片(SEM)和微觀局部區(qū)域的元素分析數(shù)據(jù)(EDS),以射線能量的不同繪制X軸,能量強度為縱坐標繪制EDS分析圖.對樣品進行X射線衍射,得到樣品的XRD譜圖與標準圖譜對照即可確定物質(zhì)的組成相.根據(jù)紅外光照射到樣品上表面發(fā)生的電荷分布變化獲得波數(shù)為橫坐標,強度為縱坐標的紅外光譜圖,確定樣品的官能團,對樣品定性分析.

2 結(jié)果與討論

2.1 HAP晶種的誘導效應

晶種的誘導效應是通過降低結(jié)晶體系固液界面自由能水平,達到類似催化結(jié)晶的效果,如減小結(jié)晶反應能壘和縮短結(jié)晶誘導期.

由圖1(a)可知,初始PO4-P濃度小于10mg/L時,HAP晶種顯著提高除磷率(<0.05)，且結(jié)晶速率快.在實驗中觀察到,當初始PO4-P為10mg/L時反應第1min去除率就達40%以上.而相同條件下Song等[27-28]以AP和方解石為晶種,去除40%以上PO4-P所需時間分別為2h和2d.這是因為磷結(jié)晶產(chǎn)物為HAP及其前體物,HAP作為晶種對其分子識別度顯然最高.而且,有研究證實[31]其它材質(zhì)晶種只有在表面逐漸被結(jié)晶產(chǎn)物覆蓋后,誘導效應才開始發(fā)揮.因此,采用HAP作為晶種回收磷,在保證回收產(chǎn)物純度同時,還有助于結(jié)晶回收裝置的快速啟動.

當初始PO4-P濃度超過10mg/L后,誘導結(jié)晶與均相結(jié)晶PO4-P回收率無顯著差異(>0.05),說明均相結(jié)晶可順利實現(xiàn),無需外投晶種.Caddarao等[41]針對高磷廢水(>500mg/L)設計的流化床均相結(jié)晶器(FBHC),無需外投晶種即可實現(xiàn)磷回收,且回收產(chǎn)品純度高.Pahunang等[42]研究表明,初始PO4-P濃度100mg/L時均相結(jié)晶已能獲得滿意磷回收效果. Vasenko和Qu[19]研究證實,高磷廢水(155mg/L)中晶種的加入,甚至會改變產(chǎn)物晶型,降低磷回收效率.

圖1 HAP誘導結(jié)晶與均相結(jié)晶除磷效果對比

圖1(b)表明均相與誘導結(jié)晶出水pH值無顯著差異(>0.05),這與原水HCO3-堿度較高有關(guān).雖然磷結(jié)晶回收率的提高會降低體系pH值,但由于CO32-～HCO3-緩沖體系的緩沖作用,降低并不顯著.

2.2 Ca/P比對誘導結(jié)晶效果的影響

Ca/P比改變磷結(jié)晶體系SI,影響結(jié)晶速率和磷回收率.由圖2a可知,初始PO4-P濃度1.0mg/L、Ca/P比1.67～100時,均相結(jié)晶磷回收率均不足10%,說明均相結(jié)晶動力SI不足.投加晶種HAP后,磷回收率顯著增加,且隨Ca/P比的增加而增加, Ca/P比超過20后,磷回收率達70%以上,上清液PO4-P濃度滿足GB3838-2002的Ⅳ類標準[43].可見,通過提高Ca/P比增加SI,可有效提升誘導結(jié)晶速率和磷回收率.

圖2(b)表明,盡管均相結(jié)晶時磷結(jié)晶率不足10%,但上清液pH值隨Ca/P比明顯降低,這可能是伴隨的CaCO3結(jié)晶所致.原水中無機碳會發(fā)生 CaCO3結(jié)晶,引起pH值降低.相關(guān)研究[10,27]也證實Ca-P結(jié)晶往往伴隨CaCO3結(jié)晶,兩者競爭構(gòu)晶離子(Ca2+)和晶種表面結(jié)晶位點,從而干擾磷回收.初始PO4-P濃度越低,HAP過飽和度越低,干擾可能越嚴重.考慮到HAP熱力學溶度積常數(shù)(sp=10-58.52,25℃)要遠小于CaCO3(sp,方解石= 10-8.48,25℃),控制結(jié)晶體系pH值至9以下,適度降低HAP和CaCO3的過飽和度(即結(jié)晶動力),有望減小無機碳對HAP結(jié)晶磷回收的干擾.

圖2 Ca/P比對結(jié)晶除磷和上清液pH值的影響

初始PO4-P=1.0mg/L

2.3 晶種投加量對誘導結(jié)晶效果的影響

初始PO4-P濃度1.0mg/L、初始pH值8.0、Ca/P比20時,磷回收率和pH值隨HAP投加量變化見圖3.回收率和pH值變化均表明提高晶種投加量會促進磷的結(jié)晶回收.當HAP晶種投加量為20g/L時,出水磷濃度甚至滿足GB3838-2002的Ⅲ類標準[43].

圖3 晶種投加量對磷回收效果的影響

表2給出了相關(guān)研究中晶種使用情況.石英砂和轉(zhuǎn)爐渣投加量最大[31,34-35],說明兩者對Ca-P分子識別度最低.參考文獻[34]中轉(zhuǎn)爐渣投加量0.6g/L遠小于[35]10g/L,與其粒徑較小(15μm)相關(guān).其余材質(zhì)晶種投加量在1～30g/L間,與本研究接近.對方解石,初始PO4-P濃度越低,晶種粒徑越小,可以提供更多結(jié)晶活性位點[44-45].但也有研究認為[25,29],結(jié)晶活性位點的增加會引起對構(gòu)晶離子的競爭,導致晶種表面構(gòu)晶離子濃度降低,反不利于低磷污水的磷結(jié)晶回收.本文磷回收率隨晶種投加量持續(xù)升高,說明不存在構(gòu)晶離子的競爭.分析認為,在低磷情況下,構(gòu)晶離子不足以在整個晶種表面結(jié)晶,此時誘導結(jié)晶將優(yōu)先在相鄰晶種的空隙中進行,對晶種起到晶橋作用[46].晶橋模式產(chǎn)生的固液界面面積和界面自由能更小,更有利于低磷的磷結(jié)晶回收.

表2 相關(guān)研究中晶種的使用情況

磷結(jié)晶回收出需考慮出水濁度.圖4中出水濁度與晶種投加量線性正相關(guān),說明濁度主要由晶種引起.本文中由于初始PO4-P很低,磷的誘導結(jié)晶不會大規(guī)模生成難以沉降的微晶,濁度物質(zhì)主要還是晶種中少量的細碎顆粒.可以預見,在實際生產(chǎn)中隨著回收工藝的運行,出水濁度將逐漸降低.

圖4 晶種投加量對上清液濁度的影響

初始PO4-P=1.0mg/L,Ca/P=20

2.4 結(jié)晶產(chǎn)物顯微觀測和表征分析

HAP晶種呈現(xiàn)規(guī)則的球形,表面光滑(圖5a).相較于晶種,結(jié)晶產(chǎn)物粒徑略有增加;同時晶體顆粒出現(xiàn)明顯的兩兩團聚現(xiàn)象(圖5b),包括大顆粒與小顆粒 (實線框所示)和粒徑接近顆粒之間(虛線框所示)兩種類型,分別對應表面逐層結(jié)晶和晶橋模式.

(a)HAP晶種 (b)結(jié)晶產(chǎn)物

初始PO4-P=1.0mg/L,Ca/P=20,晶種投加量10g/L

EDS分析表明,晶種主要元素為Ca、P和O,Ca/P比1.83(圖6(a)),位于1.33～1.95之間,說明晶型以HAP為主.結(jié)晶反應后,晶種表面結(jié)晶產(chǎn)物中C含量明顯增加,顯然是磷酸鈣結(jié)晶過程中伴隨的CaCO3結(jié)晶所致.考慮CaCO3結(jié)晶,晶種表面結(jié)晶產(chǎn)物Ca/P比由1.83降低至1.41,表明結(jié)晶產(chǎn)物包括HAP及其前體物. 結(jié)晶產(chǎn)物XRD結(jié)果證實,晶種表面結(jié)晶產(chǎn)物包括HAP及其前體物無定型磷酸鈣(ACP)和磷酸八鈣(OCP),以及伴隨的CaCO3結(jié)晶產(chǎn)物(圖6(b)).

結(jié)晶產(chǎn)物FTIR譜圖中(圖6(c)),3570cm-1處特征峰對應HAP中O-H的伸縮振動峰[47],在1092, 1036,603和565處檢測到PO43-特征吸收峰[10,48],說明產(chǎn)物中含HAP;1636cm-1附近特征峰為結(jié)晶水中O-H的伸縮振蕩吸收峰[49],表明存在含結(jié)晶水的ACP.874cm-1處特征峰為CO32--特征吸收峰,進一步證實磷酸鈣結(jié)晶過程伴隨CaCO3結(jié)晶.

2.5 經(jīng)濟分析

僅以藥耗對除磷/磷回收進行經(jīng)濟分析.某污水廠二級出水磷濃度約1mg/L,采用聚合氯化鐵混凝除磷,平均藥耗0.08元/m3.采用HAP誘導結(jié)晶回收磷,藥耗來自氫氧化鈉和氯化鈣(晶種可循環(huán)再生,不計藥耗);Ca/P=20時,兩者投加量分別為10mg/L和72mg/L,市售價格分別為2000和700元/t,藥耗0.07元/m3. HAP誘導磷結(jié)晶磷回收率以80%計,每m3污水可回收HAP 4.8g,市售工業(yè)HAP價格以140元/kg計,回收效益0.67元/m3,則污水廠采用HAP誘導結(jié)晶回收磷可收益0.6元/m3.采用轉(zhuǎn)爐渣回收磷,市售轉(zhuǎn)爐渣磷肥600元/t,在相同藥劑投加量下,即便100%回收磷,效益也僅為0.038元/m3,藥耗為0.032元/m3.可見,HAP誘導結(jié)晶磷回收經(jīng)濟優(yōu)勢明顯.

初始PO4-P=1.0mg/L,Ca/P=20,晶種投加量10g/L

3 結(jié)論

3.1 采用HAP作為晶種,利用磷酸鈣結(jié)晶回收低磷污水中的磷,避免了晶種材料本身對結(jié)晶產(chǎn)物的純度和品質(zhì)的影響.HAP對磷酸鈣結(jié)晶產(chǎn)物的分子識別度高,有利于回收裝置的快速啟動.

3.2 HAP晶種表面的磷酸鈣結(jié)晶模式,包括構(gòu)晶離子在晶種表面逐層結(jié)晶模式和在晶種空隙間的晶橋模式.后者產(chǎn)生的固液界面面積和界面自由能更小,對低磷適應性更強.實驗條件下,初始PO4-P濃度為1.0mg/L,磷回收率可達80%以上.

3.3 低磷濃度下,HAP誘導磷酸鈣結(jié)晶的產(chǎn)物為HAP及其前體物ACP和OCP.堿度物質(zhì)存在情況下,還會伴隨CaCO3結(jié)晶, 干擾磷結(jié)晶回收.

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Phosphorous recovery from wastewater with low phosphorous concentration by means of HAP-seeded crystallization of calcium phosphate.

XIAO Hui-yi1,2,3, NIE Xiao-bao*, WAN Jun-li, DENG Quan-qing, WANG Yi-rui, LONG Yuan-nan, JIANG Chang-bo

(Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Hunan Province, Engineering and Technical Center of Hunan Provincial Environmental Protection for River-lake Dredging Pollution Control, School of Hydraulic & Environmental Engineering, Changsha University of Science & Technology, Changsha 410114, China)., 2022,42(4)：1681～1687

Phosphorous recovery from secondary treated effluent by crystallization of calcium phosphate (Ca-P), is one of effective strategies for phosphorous recovery from municipal wastewater. However, this strategy still needs further improvement, such as enhancing the quality of final products and improving the adaptability to low PO4-P concentration. In this paper, phosphorous in simulated secondary treated effluent with initial PO4-P concentration of 1.0mg/L was recovered by Ca-P crystallization using hydroxyapatite (HAP) as seeds. The recovery effects of induced crystallization were compared to that of homogeneous one, and the influence of both Ca/P mole ratio and dosage of seeds were investigated. Moreover, the induced crystal mechanism of Ca-P under low PO4-P concentration was studied based on phosphorous recovery effects and SEM, EDS, XRD and FTIR of crystal products. The results showed that the disturb of seed materials to the quality and purity of final products was minimized, since HAP was the major polymorph of final products. For simulated secondary treated effluent, HAP-induced Ca-P crystallization possessed high phosphorous recovery performance and rapid start-up. The induced crystal models of Ca-P crystallization included layer-by-layer crystallization on the surface of seeds, and crystal bridge model of which crystallization occurred within the space among seeds particles. Under experimental conditions, phosphorous recovery efficiency over 80% was obtained, with HAP and its precursors, ACP and OCP, the main polymorphs of final products. CaCO3crystallization was proved to take place simultaneously with Ca-P, which disturbs the recovery of phosphorous. The result provides new ideas for simultaneous improvement of recovery efficiency and the quality and purity of final products for low concentrated PO4-P wastewater.

hydroxyapatite (HAP)；low concentrated PO4-P wastewater；phosphorous recovery；calcium phosphate crystallization；seed

X703.1

A

1000-6923(2022)04-1681-07

肖輝毅(1998-),男,湖北鄂州人,長沙理工大學碩士研究生,主要從事污水污泥資源化利用方面研究.發(fā)表論文1篇.

2021-10-08

湖南省自然科學基金(2020JJ4609,2020JJ4631);長沙市科技計劃項目(kq2005005);長沙理工大學水利與環(huán)境工程學院實踐創(chuàng)新與創(chuàng)業(yè)能力提升計劃項目(大型電站水庫底棲動物的遷移運動特性與驅(qū)動機制)

*責任作者, 副教授, niexbcslg@163.com