曾堅(jiān)賢,鄭立鋒,孫霞輝,賀勤程 (湖南科技大學(xué)化學(xué)化工學(xué)院,湖南 湘潭 411201)

聚合物強(qiáng)化超濾技術(shù)分離Hg2+和Cd2+的研究

曾堅(jiān)賢*,鄭立鋒,孫霞輝,賀勤程 (湖南科技大學(xué)化學(xué)化工學(xué)院,湖南 湘潭 411201)

以聚丙烯酸鈉為絡(luò)合劑,研究Hg2+和Cd2+混合溶液的絡(luò)合超濾分離行為.考察了pH值、負(fù)載比(LR)及外加鹽濃度對(duì)混合體系分離的影響.結(jié)果表明,當(dāng)pH值從5增大到7.5時(shí),2種離子分離系數(shù)(S)逐漸下降,最適pH值為5;當(dāng)LR從0.01增大到2時(shí),S逐漸升高到最大值,此后迅速降低,LR=1.5時(shí)S達(dá)到最大值;Na2SO4的加入影響S值.控制混合溶液pH=5及LR=1.5,當(dāng)體積濃縮因子為15時(shí),截留液汞濃度(Cr,Hg)從30mg/L線性遞增至444.9mg/L,截留液鎘濃度(Cr,Cd)僅從30mg/L升高至35.4mg/L,S值約為227.濃縮液隨著洗滌水體積增大,Cr,Hg基本不變,而Cr,Cd下降至5.24mg/L.收集各滲透液,調(diào)整至pH=6及LR=0.033予以濃縮,濃縮13倍時(shí),Cr,Cd從27.37mg/L升高到354.7mg/L.

Hg2+；Cd2+；聚丙烯酸鈉；絡(luò)合；超濾

Abstract：Polymer-enhanced ultrafiltration was investigated to separate selectively Hg2+and Cd2+from aqueous solutions by using poly (acrylic acid) sodium salt as a complexing agent. Effects of pH, loading ratio (LR) and the added salt on separation coefficient (S) were investigated. When pH increased from 5 to 7.5, S decreased gradually. Controlling pH=5 was helpful to achieve the selective separation. At LR between 0.01 and 2, S increased with LR, reached a maximum value, and then decreased rapidly. It was preferable for the selective separation to control LR=1.5. S changed in the present of sodium sulfate. A concentration experiment was carried out at pH=5 and LR=1.5. When volume concentration factor (VCF) reached 15, mercury concentration in the retentate (Cr,Hg) increased linearly from 30 mg/L to 444.9 mg/L, whereas cadmium concentration in the retentate (Cr,Cd) changed only from 30 mg/L to 35.4 mg/L. S was about 227. Then, the final concentrated solution was used to study the process of washing. With increasing the washing water volume, Cr,Hgdid not change, but Cr,Cddecreased rapidly to 5.24 mg/L. Further, another concentration experiment was performed at pH=6 and LR=0.033 by using all permeate solutions. At VCF=13, Cr,Cdincreased from 27.37 mg/L to 354.7 mg/L.

Key words：Hg2+；Cd2+；poly (acrylic acid) sodium salt；complexation；ultrafiltration

從廢水中分離重金屬離子具有實(shí)際意義[1-4],聚合物強(qiáng)化超濾技術(shù)在該領(lǐng)域中展示了良好應(yīng)用前景[5-9],該技術(shù)先以大分子水溶性聚合物絡(luò)合目標(biāo)金屬離子,再借助超濾膜予以截留,使得被“綁定”在聚合物上的金屬離子獲得分離濃縮[10-13].近年來,一些研究者嘗試采用該技術(shù)選擇性分離金屬離子[14-16],但大多基于操作條件的優(yōu)化,鮮見以單一溶液與混合溶液的對(duì)比研究,混合溶液濃縮及進(jìn)一步洗滌分離更少見報(bào)道.

聚丙烯酸鈉(PAASS)對(duì)Hg2+和Cd2+絡(luò)合容量分別為1.0g Hg2+/g PAASS和0.033 g Cd2+/g PAASS,且Hg2+截留系數(shù)在較低pH值時(shí)才下降,Cd2+截留系數(shù)在pH5～6變化明顯,Hg2+對(duì)PAASS具有更強(qiáng)的親和能力[17-18].本研究以PAASS為絡(luò)合劑,考察操作參數(shù)對(duì)Cd2+和Hg2+混合溶液分離特性的影響,并與單一溶液比較,研究混合溶液濃縮行為,考察濃縮液洗滌及滲透液濃縮過程.

1 材料與方法

1.1材料

2種中空纖維超濾膜由天津膜天膜工程技術(shù)有限公司提供,型號(hào)為UEIP-503和UEOS-503,截留分子量分別為2×104和6×103,前者僅用于預(yù)處理PAASS,后者用于其他實(shí)驗(yàn).膜的其他特性參數(shù)見文獻(xiàn)[19].

所有藥品均采用分析純.PAASS(日本和光純藥工業(yè)株式會(huì)社),平均分子量2.5×105;硝酸汞(江蘇泰興化學(xué)試劑廠);硝酸鎘(沈陽化學(xué)試劑廠);硫酸鈉(上海金山化工廠);1mol/L硝酸和氫氧化鈉用于調(diào)節(jié)溶液pH值.

1.2實(shí)驗(yàn)裝置與儀器

實(shí)驗(yàn)裝置如圖1所示.原料液加入料液罐中,經(jīng)泵驅(qū)動(dòng)進(jìn)入膜組件,在壓差推動(dòng)下,滲透液徑向透過超濾膜,截留液返回料液罐.跨膜壓差由水銀壓差計(jì)測(cè)定,其變化范圍為5～50kPa;以電子天平測(cè)量一定時(shí)間內(nèi)滲透?jìng)?cè)流出的液體質(zhì)量,并換算成膜通量[L/(m2·h)].未作說明時(shí),超濾體系參數(shù)均為:壓差為22.2kPa,流量為60L/h,溫度為25℃,外加鹽濃度為0.

圖1 實(shí)驗(yàn)裝置示意Fig.1 Schematic diagram of the experimental apparatus 1.恒溫槽;2.料液罐;3.泵;4.閥門;5.水銀壓差計(jì);6.膜組件

Mettler Toledo 320-S型pH計(jì)(梅特勒托利多儀器有限公司),測(cè)定各溶液pH值;AA-670型原子吸收分光光度計(jì)(日本島津株式會(huì)社),測(cè)定Cd2+濃度[20];752N型紫外可見分光光度計(jì)(上海精密科學(xué)儀器有限公司),用于PAASS溶液預(yù)處理過程及雙硫腙分光光度法測(cè)定Hg2+濃度[20].

1.3實(shí)驗(yàn)方法

以硝酸汞和硝酸鎘的1種或2種配制單一或混合金屬離子溶液,混合溶液Hg2+和Cd2+質(zhì)量濃度相等,未說明時(shí)Hg2+和Cd2+濃度均為10mg/L.負(fù)載比(LR)定義為金屬離子濃度總和與聚合物濃度的比值,即:單一金屬離子溶液LR=(gHg2+或gCd2+)/g PAASS;混合金屬離子溶液LR=(gCd2++gHg2+)/g PAASS.PAASS使用前需預(yù)處理,以除去低分子量部分,其過程為:以UEIP-503膜過濾較高濃度PAASS溶液,加入去離子水洗滌,移走滲透液,采用752N紫外可見分光光度計(jì)在波長(zhǎng)207nm定時(shí)測(cè)定滲透液PAASS濃度[19],至檢測(cè)不到溶質(zhì),即預(yù)處理結(jié)束,截留液用于UEOS-503膜實(shí)驗(yàn).操作參數(shù)對(duì)金屬離子截留系數(shù)和分離系數(shù)影響:在特定條件下,將一定量PAASS與單一或混合金屬離子溶液充分混合后攪拌1h,然后進(jìn)行超濾實(shí)驗(yàn).混合溶液濃縮、截留液洗滌及滲透液濃縮:控制pH=5,Hg2+和Cd2+離子濃度均為30mg/L,PAASS濃度為40mg/L,料液總體積為30L,最大濃縮因子為15,最終濃縮液為2L;將該濃縮液用于洗滌研究,洗滌液為pH=5硝酸溶液,每次加入0.5L洗滌液,獲得0.5L滲透液后再加入下批洗滌液,洗滌液總用量為4.5L;收集各階段滲透液,調(diào)整pH=6和LR=0.033,超濾濃縮Cd2+.

金屬離子截留系數(shù)(Ri)及分離系數(shù)(S)是描述離子分離程度的重要指標(biāo),采用如下公式計(jì)算:

式中:Cp,i、Cr,i分別為滲透液和截留液中金屬離子 (Hg2+或Cd2+)的濃度; Cp,Cd、Cp,Hg分別為滲透液中Cd2+和Hg2+濃度;Cr,Cd、Cr,Hg分別為截留液中Cd2+和Hg2+濃度;RCd、RHg分別為Cd2+和Hg2+的截留系數(shù).S越大意味著RCd越小而RHg越大,表明2種離子分離程度越明顯.

2 結(jié)果與討論

2.1操作參數(shù)對(duì)金屬離子截留系數(shù)和分離系數(shù)的影響

2.1.1pH值的影響 pH值顯著影響金屬離子絡(luò)合特性,因而對(duì)混合體系離子分離起重要作用.由圖2可見,在低LR下,當(dāng)pH值從5增大到7.5時(shí),混合溶液與單一溶液RHg相同,表明離子混合不影響PAASS對(duì)Hg2+絡(luò)合行為.當(dāng)pH值從5增大到6時(shí),混合溶液較單一溶液有更高RCd;pH值超過6時(shí),混合溶液與單一溶液RCd相同且接近1.這一現(xiàn)象可解釋為,由于LR值遠(yuǎn)低于Hg2+在pH5時(shí)的臨界負(fù)載比(LRCr=1)[17],過量PAASS能完全絡(luò)合Hg2+,混合溶液與單一溶液絡(luò)合情形相同.Cd2+卻有不同現(xiàn)象,混合體系LR為0.033,對(duì)每單一金屬離子為0.0165,盡管Hg2+存在已占據(jù)部分絡(luò)合位,但Hg2+高臨界負(fù)載比使得未參與絡(luò)合的PAASS濃度較單一Cd2+時(shí)高,故混合溶液較單一溶液有更高RCd.在高LR下,混合溶液較單一溶液有更高RHg,且前者RHg接近1,不隨pH值變化;混合溶液較單一溶液RCd低.表明Hg2+加入削弱了Cd2+與PAASS的絡(luò)合作用,這可能是高LR使得PAASS不能完全“捕獲” Hg2+和Cd2+,此時(shí)發(fā)生2種離子競(jìng)爭(zhēng)絡(luò)合位的反應(yīng).Hg2+對(duì)PAASS具有更強(qiáng)親和能力,優(yōu)先與絡(luò)合位結(jié)合,Cd2+絡(luò)合剩余絡(luò)合位,故RHg升高且接近1,而RCd下降.由表1可見,在LR=0.033下,pH＞6時(shí)S非常低,此時(shí)PAASS不能將Hg2+和Cd2+分離,只能同時(shí)濃縮;當(dāng)LR=1.5時(shí),S隨pH值增大而下降,表明升高pH值不利于混合體系分離.選取pH5適宜Hg2+和 Cd2+分離.

圖2 不同LR下pH值對(duì)混合溶液及單一溶液Ri的影響Fig.2 Effect of pH value on rejection coefficients for mixture and single solutions at different LR values

表1 不同LR下pH值對(duì)S的影響Table 1 Effect of pH value on separation coefficient at different LR values

2.1.2LR值的影響 LR值是影響Ri的另一重要因素,當(dāng)溶液中有不同離子共存時(shí),改變LR使得絡(luò)合效應(yīng)發(fā)生變化,進(jìn)而影響分離程度.

由圖3可見,當(dāng)混合溶液LR低于單一Hg2+溶液臨界負(fù)載比(LRCr=1)時(shí),混合體系RHg維持在1左右,而RCd較單一Cd2+體系更高;反之,混合體系RHg升高而RCd降低.這些現(xiàn)象可解釋為,低LR下混合溶液中聚合物濃度較高,盡管Hg2+占據(jù)了部分絡(luò)合位,但剩余絡(luò)合位仍高于單一Cd2+體系,故RCd升高;高LR下聚合物濃度較低,Hg2+和Cd2+競(jìng)爭(zhēng)絡(luò)合位,前者對(duì)PAASS具有強(qiáng)的親和能力,使得RHg升高而RCd降低.此外,在高LR下,當(dāng)pH值從5增大到6時(shí),RCd下降幅度逐漸增大.在LR=2下,當(dāng)pH值為5時(shí),RCd由單一溶液0.105降低至混合溶液0.052,下降50.5%;當(dāng)pH值為6時(shí),RCd由單一溶液0.236下降至混合溶液0.067,下降71.6%.從表2可以看出,當(dāng)LR從0.01增大到1.5時(shí),S逐漸升高并達(dá)到最大值,表明適當(dāng)提高金屬離子濃度有利于分離,進(jìn)一步增大LR至2,S迅速降低.這可解釋為,當(dāng)LR＜1.5時(shí),混合溶液RHg維持在1左右,RCd隨LR增大而下降,因而S不斷升高.當(dāng)LR增大到2時(shí),RCd不斷降低的同時(shí)RHg也下降,故S迅速變小.因此,控制LR = 1.5可使Hg2+和Cd2+分離效果良好.2.1.3外加鹽濃度的影響 外加鹽干擾金屬離子與聚電解質(zhì)競(jìng)爭(zhēng)反應(yīng),改變離子間分離程度.由圖4a可見,隨著鹽濃度增大,混合及單一溶液RHg下降相對(duì)平緩,最大下降2%,表明汞絡(luò)合物對(duì)鹽濃度變化不敏感.單一溶液RCd隨鹽濃度增大迅速降低,最大下降11.5%,混合溶液RCd下降較為緩慢,最大下降3.5%.可見,鹽濃度增大導(dǎo)致RHg和RCd的下降幅度不一樣,這將影響兩者的分離,從表3可以看出,此時(shí)S隨Na2SO4濃度增大而略有升高,意味著在較高pH值和較低LR下,外加鹽在一定程度上有助分離.由圖4b可見,混合及單一溶液RHg最大下降分別為3.6%和5.5%,鹽濃度對(duì)RHg的影響較圖4a略微增大;混合溶液較單一溶液有稍低RCd值,鹽濃度對(duì)Cd2+的影響程度降低.可見,在此條件下,外加鹽使RHg下降幅度稍微增大,RCd下降幅度變小,S隨Na2SO4濃度增大而下降較快,外加鹽不利于分離.2.2 混合溶液的濃縮

圖3 不同pH值下LR對(duì)混合溶液及單一溶液Ri的影響Fig.3 Effect of loading ratio on rejection coefficients for mixture and single solutions at different pH values

表2 不同pH值下LR對(duì)S的影響Table 2 Effect of loading ratio on separation coefficient at different pH values

圖4 不同pH值和LR下Na2SO4濃度對(duì)混合溶液及單一溶液Ri的影響Fig.4 Effect of Na2SO4concentration on rejection coefficients for mixture and single solutions at different pH and LR values

表3 不同pH值和LR下Na2SO4濃度對(duì)S的影響Table 3 Effect of Na2SO4concentration on separation coefficient at different pH and LR values

為提高金屬離子分離程度,在pH=5和LR=1.5下將Hg2+和Cd2+混合溶液超濾濃縮,考察體積濃縮因子(以VCF表示,定義為原料液體積與截留液體積之比)對(duì)Cr,Hg、Cr,Cd、S、Ri及膜通量(J)的影響.由圖5可見,隨著VCF增大,Cr,Hg線性遞增,當(dāng)VCF為15時(shí),Cr,Hg高達(dá)444.9mg/L,表明Hg2+已被有效濃縮.Cr,Cd隨VCF增大略有升高,料液濃縮15倍時(shí)Cr,Cd僅為35.4mg/L,即Cd2+濃縮程度非常小.2種金屬離子在一定程度上實(shí)現(xiàn)了分離.S基本不變,約為227,濃縮操作不影響2種離子分離程度,這從聚電解質(zhì)絡(luò)合機(jī)理可獲得解釋.在整個(gè)濃縮過程中,RHg非常接近1,而RCd很低,這與Cr,Hg和 Cr,Cd變化一致.另外,隨著VCF增大, J略有下降,VCF＝15時(shí)J下降率為19.4%,表明在此研究條件下膜污染輕微.

圖5 VCF對(duì)Cr,Hg、Cr,Cd、S、Ri及J的影響Fig.5 Effect of volume concentration factor on metal concentrations in the retentate, separation coefficient, metal rejection coefficient and permeate flux

2.3截留液洗滌及滲透液的濃縮

超濾濃縮后得2L截留液,其Hg2+和Cd2+濃度分別為444.9,35.4mg/L,為了進(jìn)一步分離金屬離子,需對(duì)截留液予以洗滌;另外,滲透液中含大量Cd2+需進(jìn)行濃縮.考察洗滌過程對(duì)Cr,Cd、Cr,Hg和J的影響.由圖6可見,隨著洗滌水體積增大, Cr,Cd不斷下降,Cr,Hg則基本不變.當(dāng)洗滌水體積為3.5L,即為初始截留液體積1.75倍時(shí),Cr,Cd下降幅度明顯變小,洗滌結(jié)束后為5.24mg/L,這部分Cd2+被PAASS捕獲,不以游離態(tài)存在.另外,體系膜通量在此過程中基本不變,大致與濃縮結(jié)束時(shí)相等.為了濃縮Cd2+,收集濃縮及洗滌過程中滲透液,得料液體積32.5L,其Cd2+濃度為27.37mg/L,調(diào)整pH6及LR=0.033,絡(luò)合平衡后超濾濃縮,考察滲透液體積對(duì)RCd和J的影響.由圖7可見,整個(gè)過程中RCd接近1,當(dāng)滲透液體積為30L,截留液體積為2.5L,即濃縮13倍時(shí),截留液鎘濃度為354.7mg/L,鎘獲得有效濃縮.此外,該過程膜通量變化規(guī)律與混合溶液的濃縮過程類似.

圖6 洗滌水體積對(duì)Cr,Cd、Cr,Hg和J的影響Fig.6 Effect of washing water volume on metal concentrations in the retention and permeate flux

圖7 滲透液體積對(duì)RCd和J的影響Fig.7 Effect of permeate volume on cadmium rejectioncoefficient and permeate flux

3 結(jié)論

3.1當(dāng)pH值從5增大到7.5時(shí),在低LR下,混合溶液較單一溶液RCd高,RHg相等且接近1;在高LR下,混合溶液較單一溶液有更高RHg,而RCd降低;降低pH可以提高S值,選用pH5有利于分離.3.2當(dāng)LR從0.01增大到2時(shí),如果混合溶液LR低于Hg2+臨界負(fù)載比,混合溶液較單一溶液RCd為高,RHg不變;反之,混合溶液較單一溶液RHg升高而RCd降低;適當(dāng)增大LR可提高S,當(dāng)LR=1.5時(shí)S達(dá)到最大值.

3.3在pH=6和LR=0.033下,當(dāng)Na2SO4濃度從0增大到0.1mol/L時(shí),RHg和RCd下降幅度不同,S略有升高;在pH=5和LR=1下,RHg和RCd下降幅度隨鹽濃度增大分別變大和變小,S下降.

3.4在pH=5和LR=1.5下濃縮Hg2+和Cd2+混合體系,VCF＝15時(shí)Cr,Hg從30mg/L呈線性遞增至444.9mg/L, Cr,Cd僅從30mg/L升高至35.4mg/L, RHg接近1而RCd非常低,S約為227,膜通量衰減緩慢.

3.5將濃縮液洗滌,隨著洗滌水體積增大,截留液中Hg2+濃度基本不變,而Cd2+濃度下降至5.24mg/L.收集各滲透液,調(diào)整至pH=6及LR= 0.033予以濃縮,截留液中鎘可濃縮至354.7mg/L.通過控制適宜pH及LR,Hg2+和Cd2+混合體系可有效分離.

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Separation of Hg2+and Cd2+by using polymer-enhanced ultrafiltration.

ZENG Jian-xian*, ZHENG Li-feng, SUN Xia-hui, HE Qin-cheng (College of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China). China Environmental Science, 2010,30(6)：780～785

X703.5

A

1000-6923(2010)06-0780-06

曾堅(jiān)賢(1970-),男,湖南漣源人,副教授,博士,主要從事膜法水處理技術(shù)的研究.發(fā)表論文21篇.

2009-11-12

國(guó)家自然科學(xué)基金資助項(xiàng)目(20976040);湖南省科技計(jì)劃資助項(xiàng)目(2009SK3036)

* 責(zé)任作者, 副教授, zengjianxian@163.com