Ca2+在飲用水銅綠微囊藻控制中的應用

易 晉,聶小保*,王奕睿,肖輝毅,隆院男,蔣昌波

Ca2+在飲用水銅綠微囊藻控制中的應用

易 晉1,2,3,聶小保1,2,3*,王奕睿1,2,3,肖輝毅1,2,3,隆院男1,2,3,蔣昌波1,2,3

(1.長沙理工大學水利與環境工程學院,湖南 長沙 410114；2.洞庭湖水環境治理與生態修復湖南省重點實驗室,湖南 長沙 410114；3.湖南省環境保護河湖疏浚污染控制工程技術中心,湖南 長沙 410114)

為改善飲用水藻類的混凝去除效果,以銅綠微囊藻為研究對象,考察了單獨投加Ca2+、Ca2+與PAC聯用、Ca2+與CO32-原位結晶三種方法的除藻效果,并對Ca2+和結晶產物CaCO3的除藻機制進行探討.結果表明,單獨采用Ca2+時,Ca2+在低濃度下對藻細胞具有吸附電中和作用,高濃度時同時還有架橋作用,但兩者均無法實現對銅綠微囊藻的去除.Ca2+與PAC聯用,Ca2+可以通過吸附電中和顯著提高PAC 的除藻效果,最大去除率可達98.0%,同時Ca2+與溶解性藻源有機物(dAOM)的絡合可將殘余鋁降低50%以上.含藻水中原位CaCO3結晶對銅綠微囊藻的去除率最高可達83.5%,其產物為帶正電荷、粒徑2～4 μm 左右的球型球霰石.球霰石對藻細胞的去除機制包括球霰石與藻細胞的互絮凝,以及球霰石團聚物對藻細胞的卷掃絮凝,同時球霰石還可以作為加重劑促進藻晶產物沉降分離.自來水廠采用CaCO3原位結晶與PAC聯用除藻,可望降低PAC投加量和殘余鋁風險,并解決CaCO3原位結晶導致的濁度和pH偏高問題.研究成果為飲用水除藻提供了新思路.

Ca2+；銅綠微囊藻；飲用水；CaCO3原位結晶；電中和

地表飲用水水源容易遭受季節性藻華影響.藍藻高發期,自來水廠將面臨混凝除濁效果降低、濾池頻繁堵塞泄露、色嗅味超標、消毒副產物風險增加,以及藻毒素等一系列問題[1-4].

通過混凝和后續的沉淀/氣浮,大部分藻細胞及部分藻源有機物(AOM)可以得到有效去除[5].然而,由于藻細胞電負性高、比表面積大、具有一定活動性,加上AOM的空間位阻效應,以及部分藻細胞具有的膠質鞘,導致鐵鹽和鋁鹽混凝劑對其混凝效果遠低于無機顆粒[6].自來水廠除藻時往往不得不大幅提高混凝劑用量,這又會引發殘余鋁(鋁鹽混凝劑)或色度 (鐵鹽混凝劑) 超標的問題[7-8],且產生大量濃縮脫水性能較差的含藻污泥.為改善混凝除藻效果,研究者嘗試采用預氧化強化混凝[9-11]、磁絮凝[8,12]、生物絮凝劑助凝[7,13-14]和礦物質共絮凝(co- coagulation)[15]除藻.研究和應用結果表明,上述技術措施均能明顯提高自來水廠控藻效果,但也存在各自瓶頸.預氧化可能會導致藻細胞破裂,大量釋放包括藻毒素在內的胞內有機物(IOM)[16];磁絮凝需在常規混凝基礎上增加磁分離和磁核回收裝置[17],操作相對復雜;生物絮凝劑制備成本較高[18];黏土等礦物質雖價格低廉,但投加量大[19],會增加自來水廠含藻污泥處理負荷.

在藻類養殖中,Ca2+被廣泛應用于藻細胞絮凝采收[20-22].一般認為, Ca2+對藻細胞的絮凝主要是通過Ca2+的吸附電中和、二價陽離子架橋,以及與多糖等胞外多聚物(如海藻酸)絡合來實現[23-24].但也有研究者認為[25], Ca2+的藻水分離效果并非是Ca2+自身作用所致,而是由Ca2+的結晶產物如Ca3(PO4)2和CaCO3引起.由于藻類養殖采用的含藻水與飲用水源水在pH值、含鹽量、PO4-P和無機碳(CI)濃度等方面差異較大,目前關于Ca2+在飲用水除藻中的應用研究較少.吳昊瀾等[26]的研究表明,單獨投加Ca2+對銅綠微囊藻不具去除效果,但可顯著提高聚合氯化鋁(PAC)的除藻和除濁效果,當PAC為0.02mmol/ L時,投加6mmol/L的Ca2+可將藻細胞去除率由27.3%提高至93.7%.Ca2+應用于飲用水除藻,具有來源廣泛、價格低廉和不破壞細胞結構的明顯優勢,此外Ca2+及其結晶產物還被證實有助于改善含藻污泥的濃縮脫水性能[27].

因此,本文以我國營養型湖庫水源中常見藍藻銅綠微囊藻為研究對象,開展Ca2+在飲用水藻類控制中的應用研究,考察了單獨投加Ca2+、Ca2+與PAC聯用、Ca2+與CO32-原位結晶三種應用模式的除藻效果,并對Ca2+和結晶產物CaCO3的除藻機制進行探討.本文首次提出將CaCO3結晶應用于飲用水除藻,將為自來水廠除藻提供新思路.

1 材料和方法

1.1 實驗材料

CaCl2·2H2O、NaOH、HCl和Na2CO3均為分析純,國藥集團化學試劑有限公司產品.PAC購自天津市光復精細化工研究所,Al2O3含量28%.溶液配制用水由Millipore Milli-Q Gradient水凈化系統(Billerica, MA)制備,電阻率18.2MΩ·cm,pH=6.6～6.8. NaOH和HCl貯備液濃度均為1mol/L.含藻水配制采用湘江水,濁度4.1NTU,CODMn=1.33mg/L,pH= 7.8,OD680=0.007.

銅綠微囊藻購自中科院水生生物研究所淡水藻種庫,編號FACHB-905.藻種于25℃、2000lx條件下采用BG11培養基進行實驗室擴培.

1.2 實驗方法

1.2.1 銅綠微囊藻的培養和含藻水制備 在2000mL燒瓶中加入1000mL經滅菌鍋高溫滅菌的BG-11培養液,于無菌環境下接種銅綠微囊藻藻種,然后置于光照培養箱中進行培養.培養箱控制溫度25℃、光暗比12h:12h,光照強度2000lx.每天定時搖瓶3次,以減小光照不均帶來的影響.采用湘江水稀釋藻細胞培養液,模擬飲用水源藍藻爆發.取處于對數增長末期藻細胞進行稀釋,最終含藻水藻細胞密度2.0×106cells/mL,濁度24.1NTU, pH=8.03,Zeta電位-62.8mV,OD680=0.099.

1.2.2 單獨加Ca2+除藻 采用六聯攪拌儀(ZR4-6,深圳中潤)進行.往6個1000mL燒杯中分別加入500mL含藻水,通過加HCl或NaOH控制pH在7.0左右.Ca2+投加量0.5～16mmol/L.反應程序為300r/ min 1min、100r/min 30min、靜置30min.靜置結束后于液面下2cm處取樣,分別測定Ca2+濃度(過0.45μm濾膜) 、OD680、pH值和Zeta電位.Ca2+濃度與初始Ca2+濃度之差視為藻細胞吸附鈣量.

1.2.3 Ca2+與PAC聯合除藻 采用六聯攪拌儀進行.往5個1000mL燒杯中分別加入500mL含藻水,通過加HCl或NaOH控制pH在7.5左右.Ca2+投加量0～4mmol/L,PAC投加量5mg/L(折合Al 0.03mmol/L).首先加入Ca2+,300r/min攪拌1min,然后加入PAC進行反應,反應程序為100r/min 1min、50r/min 30min、靜置30min.靜置結束后取上清液分別測定殘余鋁濃度(過0.45μm濾膜)、OD680、pH值和Zeta電位.

1.2.4 Ca2+與CO32-原位結晶除藻 采用全自動電位滴定儀(907Titrando,瑞士萬通)進行.往500mL燒杯中加入300mL含藻水,再依次加入Ca2+和CI各2～4mmol,控制Ca2+/CI物質的量比為1:1,其中CI為HCO3-與CO32-之和.反應時間30min,磁力攪拌器轉子轉速1000r/min,整個反應過程pH控制為9.反應結束后靜置30min.靜置結束后取上清液分別測定OD680、pH值、Zeta電位;取結晶產物進行電鏡(S4800,日本Hitachi)觀測和XRD(Smartlab,日本理學)分析.

另取等體積上清液2份,一份采用HCl調整pH值至5.0,玻璃棒攪拌5min后過0.45μm濾膜,測定濾液Ca2+濃度,記為Ca1;另一份由電位滴定儀鈣電極測定Ca2+濃度,記為Ca2,反應體系初始Ca2+濃度記為Ca0,則結晶鈣濃度=(Ca0-Ca1),結合鈣濃度=(Ca1-Ca2),游離鈣濃度=Ca2.

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);Zeta電位測定采用Zeta電位儀(Zetasizer Nano ZS90,英國馬爾文);OD680采用紫外可見分光光度計(DR5000,美國哈希);鈣離子濃度(除Ca2)和殘余鋁濃度測定采用ICP-MS(Agilent 7850,美國安捷倫).

預實驗表明,藻細胞密度與含藻水OD680有良好的線性關系(2=0.99),因此采用OD680作為藻細胞密度度量標準.藻細胞去除率=(初始OD680-上清液OD680)/初始OD680′100%.

2 結果與討論

2.1 單獨投加Ca2+除藻

Ca2+明顯降低了含藻水pH(圖1),這是由Ca2+的水解引起的.作為陽離子,Ca2+對電負性的藻細胞具有吸附電中和作用,當Ca2+投加量低于4mmol/L時,隨著投加量的增加,藻細胞的Ca2+吸附量和Zeta電位同步提高,且呈現一定的線性正相關(2=0.678),說明此時藻細胞Zeta電位的提高是由Ca2+吸附電中和引起.但Ca2+吸附量和Zeta電位的提高并非嚴格線性相關,這是因為低低濃度時Ca2+將優先吸附藻細胞表面羧基,只有在高濃度情況下才會吸附羥基,且鍵合較弱[28].

當Ca2+投加量超過4mmol/L后,藻細胞對Ca2+的吸附量急劇增加,而Zeta電位的提升有限,說明此時Ca2+對藻細胞除吸附電中和外,還存在其他作用形式.二價陽離子架橋理論(DCB)認為[24],Ca2+對藻細胞還有架橋作用,這同樣會增加藻細胞對Ca2+的吸附量.結合圖1,有理由認為在高Ca2+濃度時,Ca2+架橋作用將占主要.此外,藻細胞Zeta電位沒有出現鐵鹽或鋁鹽混凝時電位反轉的現象[26],也說明此時吸附電中和作用不占主導.

圖1 單獨采用Ca2+時投加量對藻細胞Zeta電位和Ca2+吸附量及含藻水pH的影響

圖2 單獨采用Ca2+時投加量對藻細胞去除率的影響

單獨投加Ca2+對藻細胞幾乎不具去除效果(圖2),說明僅依靠Ca2+的吸附電中和和架橋作用難以實現藻水分離. Schlesinger等[15]和吳昊瀾等[26]的研究也得到類似結論.盡管在藻類養殖的藻細胞采收中,單獨投加Ca2+可以實現藻細胞的高效自動絮凝(Auto-flocculation),但這更多是與培養液的高pH、高PO4-P和CO32-濃度有關[29],且藻細胞光合作用會引起pH顯著提高[30],這為Ca(OH)2、Ca3(PO4)2和CaCO3結晶創造了有利條件,藻細胞得以在上述結晶產物的電中和和網捕卷掃作用被高效采收.而在飲用水源水中,上述條件均不具備,因此單獨投加Ca2+難以去除藻細胞.

2.2 Ca2+與PAC聯用除藻

上述分析表明,Ca2+濃度小于4mmol/L時對藻細胞的作用以吸附電中和為主,因此Ca2+與PAC聯用時控制Ca2+濃度不超過4mmol/L. PAC濃度為5mg/L(折合Al 0.03mmol/L)時,Ca2+濃度對上清液pH值、Zeta電位和除藻率的影響見圖3.Ca2+顯著提高了PAC除藻效果(<0.05),除藻率由0mmol/L時的(19.5±3.5)%提高到4mmol/L時的(98.0±1.4)%.PAC單獨除藻效果有限,說明PAC對藻細胞的脫穩作用不明顯.飲用水除藻中,藻細胞Zeta電位在-10～ +5mV范圍內,被認為脫穩效果良好[8],而本研究中單獨PAC混凝后藻細胞電負性仍較高,Zeta電位為(-19.8±2.3)mV.PAC對藻細胞脫穩效果不佳,一方面與藻細胞表面負電荷密度較高有關,另一方面也與含藻水中溶解性藻源有機物(dAOM)對PAC的消耗有關.

圖3 Ca2+與PAC聯用時Ca2+濃度對上清液pH值、Zeta電位和藻細胞去除率的影響(PAC:25mg/L)

預投Ca2+后,dAOM首先與Ca2+絡合,同時藻細胞電負性也被Ca2+降低(圖1).此后投加PAC,藻細胞電負性在鋁鹽的電中和作用下進一步降低,Zeta電位升至-3.7～+3.5mV(圖3),達到良好脫穩,相應除藻率也顯著提高. Gregor等[28]分析對比了AOM對Ca2+和Al3+的結合能力,發現AOM中的羧基對兩者的結合能力非常接近,分別為0.143和0.125,低pH值時羧基對Al3+的結合能力更低,因此在鋁鹽混凝之前投加Ca2+,可以消除AOM對鋁鹽混凝除藻除濁的影響.

殘余鋁超標風險是飲用水PAC除藻的主要瓶頸之一[26].低藻期PAC投加量相對較低,dAOM將與氫氧化鋁發生配合,阻止氫氧化鋁交聯成簇(cross linking and clusting)和沉降[6],dAOM和Al都得不到有效去除,增加殘余鋁風險;高藻期PAC投加量大,氫氧化鋁的交聯成簇占主導,dAOM和Al的同步去除,殘余鋁風險相對較低.然而,當藻細胞去除以氫氧化鋁網捕卷掃為主時,藻泥絮體結構松散,容易再懸浮,這又增加了殘余鋁風險[8].本研究證實,預投Ca2+可以顯著降低PAC除藻的殘余鋁風險(圖4).單獨PAC除藻時殘余鋁濃度為119.5±10.6μg/L,雖低于《活飲用水衛生標準》生(GB 5749-2006)[31]要求,但相對較高.預投Ca2+后,殘余鋁濃度顯著降低,降幅達50%以上.如前所述, Ca2+和Al3+與AOM結合能力接近,兩者對dAOM的吸附存在競爭,減小了Al3+與dAOM的結合幾率,從而降低了殘余鋁風險.此外, Ca2+還被證實可以提高含藻鋁鹽污泥的致密性[27],這可以抑制藻泥再懸浮,同樣起到降低殘余鋁風險的效果.

圖4 Ca2+與對PAC除藻時殘余鋁的控制效果(PAC: 5mg/L)

2.3 Ca2+與CO32-原位結晶除藻

當Ca2+投加量為2～4mmol/L, Ca2+/CI物質的量比為1:1時,均觀察到明顯的CaCO3結晶. SEM和XRD分析結果表明, CaCO3結晶產物晶型為球霰石,粒徑在2～4μm左右(圖5).靜沉階段上清液pH值均有明顯降低(圖6),也表明CaCO3結晶(或晶型轉化)仍在繼續.經計算,當Ca0=2、3和4mmol/L時,結晶鈣濃度分別為0.12±0.06、1.02±0.15和1.75± 0.06mmol/L(圖7),各自占總鈣(CaT)的6±2.8%、33.8±4.9%和43.6±1.6%,說明構晶離子Ca2+和CO32-濃度越高,體系過飽和度就越大,相應結晶率也越高.

陳辛未等[32]研究表明, CaCO3晶體顆粒的等電點為10左右.當結晶體系pH值低于等電點時, CaCO3晶體顆粒表面正電荷密度較高,因此對藻細胞具有吸附電中和作用,可以作為藻細胞絮凝劑[33].本研究中,結晶鈣濃度分別為0.12、1.02和1.75mmol/L左右時,藻細胞Zeta電位從-62.8mV分別提高至-41.8、-23.8和-20.5mV左右,對應藻細胞去除率分別為29.0%、70.5%和83.5%(圖6),證實了原位生成的球霰石可以實現藻細胞去除,并且去除效果與球霰石的吸附電中和作用有關.

本研究中原位結晶生成的帶正電荷球霰石顆粒,密度在2.7g/mL左右,作為藻細胞絮凝劑具有以下優勢.首先帶正電荷的球霰石顆粒和帶負電荷的藻細胞可能具有互絮凝作用(mutual flocculation),即球霰石顆粒有助于藻細胞的脫穩,藻細胞顆粒促進了球霰石的團聚;其次,球霰石的密度較高,可以作為良好的加重劑改善藻泥(藻晶混合物)的沉降性能;再次,球霰石團聚物的沉降過程還可能對藻細胞起到網捕卷掃作用,進一步提高藻細胞去除效果.圖6中Ca2+投加量為4mmol/L時,藻細胞Zeta電位為-20.5mV,并未達到良好的脫穩范圍(-10～+5mV),但藻細胞去除率高達83.5%,這也說明球霰石對藻細胞的去除并非單一由電中和作用引起,網捕卷掃可能也是主要作用機制之一. Schlesinger等[15]在藻類養殖的藻細胞采收中也曾報道過CaCO3對藻細胞的卷掃絮凝作用.

圖6 CaCO3原位結晶除藻時Ca2+投加量對除藻率和上清液pH與Zeta電位的影響(Ca2+/CI物質的量比1:1)

圖7 CaCO3原位結晶除藻時Ca2+投加量鈣形態的影響(Ca2+/CI物質的量比1:1)

由圖7還可知,在各Ca2+投加量情況下,有一部分Ca2+以結合鈣形式存在,并且濃度不會隨Ca2+投加量的變化而明顯變化.推測認為這部分結合鈣主要是以Ca2+與AOM的絡合態形式存在.因此,在實際應用中,可以采用CaCO3原位結晶與PAC聯用模式除藻,一方面CaCO3原位結晶可以大幅降低PAC投加量,以及AOM引起的殘余鋁風險,另一方面PAC混凝可以解決CaCO3原位結晶可能導致的濁度和pH偏高問題,進一步保障出水水質.

3 結論

3.1 Ca2+對銅綠微囊藻具有吸附電中和和架橋作用.投加量低于4mmol/L時以吸附電中和為主,藻細胞Zeta電位明顯降低,超過4mmol/L后以架橋作用為主,Zeta電位提高幅度降低.單獨投加Ca2+不足以實現藻細胞的去除.

3.2 Ca2+通過吸附電中和和與dAOM絡合,可以顯著提高PAC對銅綠微囊藻的去除效果.PAC投加量為5mg/L時,投加4mmol/L的Ca2+可將除藻率由(19.5±3.5)%提高到(98.0±1.4)%.通過與dAOM絡合,Ca2+還可降低殘余鋁50%以上.

3.3 含藻水中原位CaCO3結晶產物為帶正電荷、粒徑2～4μm左右球霰石.球霰石對藻細胞的去除機制包括球霰石與藻細胞的互絮凝和球霰石團聚物對藻細胞的卷掃絮凝.球霰石密度為2.7g/mL左右,可以作為加重劑促進藻晶產物的沉降.

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Application of calcium ions for the control ofin drinking water treatment works.

YI Jin1,2,3, NIE Xiao-Bao1,2,3*, WANG Yi-Rui1,2,3, XIAO Hui-Yi-jiang1,2,3, LONG Yuan-nan1,2,3*, JIANG Chang-bao1,2,3

(1.School of Hydraulic and Environmental Engineering, Changsha University of Science & Technology, Changsha 410114, China；2.Key Laboratory of Dongting Lake Aquatic Eco-Environmental Control and Restoration of Hunan Province, Changsha 410114, China；3.Engineering and Technical Center of Hunan Provincial Environmental Protection for River-lake Dredging Pollution Control, Changsha 410114, China)., 2021,41(11)：5187～5193

In order to improve the flocculation removal effect of algae in drinking water treatment works, takingas the research object, the algae removal effects of three methods: Ca2+alone, Ca2+combined with poly-aluminum chloride (PAC), and Ca2+and CO32-in-situ crystallization were investigated. The algae removal mechanism of Ca2+and the crystalline product CaCO3are discussed. The results show that when Ca2+was used alone, Ca2+had an adsorption and electric neutralization effect on algae cells at low concentrations, and a bridging effect at high concentrations, but both can not achieve the removal of. Ca2+combined with PAC, Ca2+could significantly improve the algae removal effect of PAC through adsorption and electric neutralization, and the maximum removal rate could reach 98.0%. At the same time, Ca2+could aid in the decrease of residual aluminum of PAC coagulation by complexation with dissolved algae organic matter (dAOM), which could reduce the residual aluminum by more than 50%. The removal rate ofby in-situ CaCO3crystals in algae-containing water could reach up to 83.5%, and the product was positively charged vaterite with a particle size of about 2 to 4μm. The removal mechanism of vaterite to algae cells includes mutual flocculation of vaterite and algae cells, as well as the sweeping flocculation of vaterite aggregates to algae cells. At the same time, vaterite could also be used as a weighting agent to promote the sedimentation and separation of algae crystal products. The use of in-situ crystallization of CaCO3and PAC to remove algae in water plants was expected to reduce the dosage of PAC and the risk of residual aluminum, and solve the problem of turbidity and high pH caused by in-situ crystallization of CaCO3. The research result provides new ideas for algae removal in drinking water.

Ca2+；；drinking water；CaCO3crystallization；charge neutralization

X703

A

1000-6923(2021)11-5187-07

易 晉(1995-),男,江西萍鄉人,長沙理工大學碩士研究生,主要從事飲用水安全保障方面研究.

2021-03-29

湖南省自然科學基金項目(2020JJ4609); 湖南省研究生科研創新項目(CX20200855);長沙市科技計劃項目(kq2005005)

* 責任作者, 副教授, niexbcslg@163.com