Ve ea che ciia su giai phdng asen ra nude ngam, mot so tac gia da dua ra cac gia thuyet nhu su oxi hda eae khoang asenopyrit ed trong cac tang da't ngam nude [5, 6, 7]; su trao doi gifla
Trang 1NGHIEN CQU CO CHE GIAI PHONG ASEN RA NUOC NGAM - MO PHONG CAC QUA TRINH KHLJ YEM KHI TU NHIEN TRONG CAC TANG NGAM NaOC
IRAN HONG CON, N G U Y £ N THI HANH
Khoa Hoa hoc, Dai hoc KHTN, DHQG Hd Ngi
Summary
In recent several, years there were publications of arsenic contamination in groundwater This event actuated us to study and explain the mechanism of arsenic releasing into groundwater of aquyfers under alluvial plains Understanding of this mechanism could make clear of the relations between solid phase (arsenic-bearing material) and water phase (arsenic dissolvable) in earth crust layers The investigation was implemented at the laboratory model imitated natural anaerobic conditions similar those in the ac/uyfers with present ofFe(OH)j, Mn02 and AsO/' as co-precipitated compounds The results were obtained after 56 days continues study Following the running time, Mn^* was released firstly and then Fe'* and arsenic were done about some days later when mainly part ofMn02 was reduced The decrease of iron and arsenic concentration in water phase was determined as consequence of the forming of FeS and FeAsS in experimental system The results also showed the relations between species of concerning elements and their interaction in the system
Mci dau
Nam 1993, Dang Van Can [1] trong mot chuyen khao sat tai viing niii thugng nguon sdng Ma da phat hien tha'y trong nude ciia mot vai
Trang 2con sud'i ham lugng asen cao khae thudng Cac khao sat tiep theo eho tha'y day la vung cd su phan bd kha nhieu loai khoang chfla asen -asenopyrit va pyrit giau asen Qua trinh phong hda eae loai quang giau asen da eung ea'p asen eho eae ddng sud'i chay qua khu vue nay San ph^m asenat sau phong hda mot ph^n tao cac ket tua, mot phan bi hap phu len nhung hat set phu sa va cac hat hidroxit sat (III) vfla hinh thanh Cac hat chat ran nay se duge cac ddng sdng mang ve boi dap nen cac ddng bang Day ehinh la nguyen nhan tich luy asen trong eae tSng phu sa bdi tich va giai phdng ra asen trong nude ngam [2] 6 Viet Nam, dac biet la khu vue phia Bae ed md pyrit ra't Idn d phia Tay Bdc each Ha Ngi khoang 60 km va ra't nhieu eae diem md vang rai rae; trong dd khoang arsenopyrit phan bd kha day Trong nhieu nam gan day vice khai thae vang diln ra rat manh va tren mot dien rdng khap khu vue vung niii phia Bae Cac khoang giau asen duge dua len va phan tan rdng tren mat da't Neu qua trinh phong hda nhu da nhac tdi d tren la nguyen nhan that su eung ea'p asen eho cac ddng sdng thi day qua thuc se la mot tham hoa Nam 1999, khi tien hanh phan tich ham lugng asen trong nude sdng Hong eho tha'y ehiing dao ddng trong khoang 2 0 - 3 5 ag/L; cd khi len tdi 90 ag/L va trong bun phu sa len tdi
38 mg/kg [3]
Nam 1997, sau khi phat hien tha'y ngudn nude ngam ciia mot so gieng khoan kieu UNICEF tai Ha Ngi cd ham lugng asen kha cao, tfl nam 1998 chiing tdi da tien hanh khao sat tuong ddi he thd'ng va da
CO ket qua ve su nhidm asen trong nude ngam khu vue Ha Ngi [4]
Ve ea che ciia su giai phdng asen ra nude ngam, mot so tac gia da dua ra cac gia thuyet nhu su oxi hda eae khoang asenopyrit ed trong cac tang da't ngam nude [5, 6, 7]; su trao doi gifla anion photphat va asenat [8] hay su khii asenat [9, 10] nhung cQng cdn nhieu tranh luan xung quanh va'n de nay [2]
Dflng trudc su de doa ve hiem hga ciia tinh trang nhiem asen trong nude sinh boat va an udng [4, 6, 11]; ben canh viec nghien eflu tim ra cac giai phap cdng nghe nham loai bd an toan asen, ehiing tdi eung quan tam tdi viec tim hieu ea che thuc te giai phdng asen ra mdi trudng nude trong dieu kien mot nude nhiet ddi nhu d Viet Nam
Trang 3Qua trinh thuc nghiem
De nghien cflu ca che ciia qua trinh giai phdng asen ra nude ngam, ehiing tdi da thiet ke va lap dat mot thiet bi thir nghiem khep kin nhu tren hinh 1
Hinh I Sa do thiet bi nghien cAu
Binh chiia nude va cac vi cha't Cot yem khi
Khoa la'y mSu
Bo khir oxy khong khi Bom nhu d6ng - tudn hoan May khuay tir
Bom b6 sung dinh duong va vi chat
Cot yem khi cao 700 mm, dudng kinh 45 mm duge ea'u tao bdi 3 Idp Ldp thfl nha't day khoang 50 mm chira cha't mun duge la'y tfl t^ng da't mat nham tao ngudn vi sinh vat va mot phan cha't huu ca Ldp thii hai day 50 mm la mot ldp sdi hat to dudng kinh 2 - 5 mm Ldp thfl ba day 600 mm chfla cat sach duge trgn vdi 0,001%As (so vdi khd'i lugng eat khd) dudi dang ASO4' ddng ket tua vdi 0,1% Fe d dang Fe(0H)3
va 0,005% Mn d dang Mn02 va duge bam tren be mat cua cac hat cat Pha nude duge md phdng theo thanh phan gan dung ciia nude tu nhien va ed thanh phan nhu tren bang 1 [12]
Bang 1 Thanh phan cua pha nu6c
Thanh phan
Canxi
HCO3-NO3
SO42-Na-glutamat
PO4'-Mg^^
Nong do (M) 1,0.10-' 2,4.10-' 3,0.10' 5,2.10' 1,2.10' 6,0.10-' 6,0.10-'
Bang 2 Phuong phap phan tich
Thanh phdn Asen
Fe, Mn
PP Phan tich AAS-HVG AAS
NO3-, PO4'-, NH4^ Colorimetry
SO4'-Oxi hda tan
Do do due Colorimetry (xanh metylen) lodometry
Trang 4De qua trinh yem khi duge bao dam, ehung tdi su dung bam nhu dgng bom nude tfl binh chfla (1) vao cot yem khi (2) theo Id'i tfl day cot, qua tang mun, tang sdi, tang eat chfla As, FeOOH, Mn02 rdi quay trd lai binh chfla
Mau nude duoc la'y deu dan sau nhung khoang thdi gian lien tiep
each nhau la 24 gid d phia dau cot nhd mot khoa ba thong (3) va duge dem phan tich cac chi tieu can thiet Lugng miu la'y ra se lam giam ap sua't trong he thd'ng Ap suat duge dieu hda bang each eho khdng khi vao sau khi da khu het oxi qua bd khu (4) Cac chi tieu phan tich duge thuc hien theo cac phuang phap duge cdng bd trong "Standard Methods for Examination of Water and Waste water" [13] va TCVN [15]
Ket qua va thao luan
Thi nghiem duge nghien cflu lien tue trong vdng 56 ngay dem Su bien thifin ndng do theo thdi gian trong pha nude ciia Fe, Mn, As duge the hien tren hinh 2 va ciia S04^-, S^', As the hien tren hinh 3
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^
- Series 1 -Series2 Series3 Series4
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53
Hinh 2 Subie'n thien nong do theo thdi gian ciia Fe, Mn vd As trong pha nudc Series 1: Tong Fe (mgIL); Series 2: Mn (IQ-'mglL); Series 3: Tdng As (IQ-^mg/L);
Series 4: DO (lO'mgOJL)
Tfl do thi tren hinh 2 ta cd the thay rang qua trinh khu Mn02 ve
Mn^"" da bat dau xiy ra sdm nhat; ngay tfl ngay thfl 3 (khi DO da giam
Trang 5xud'ng chi cdn 2,4 mg/L) va sau dd ham lugng Mn^"" trong pha nudc tang dan do Mn02 van tiep tuc bi khu Den ngay thfl 31 trd di thi ham lugng mangan hau nhu khong tang nfla Day cd the la lugng mangan dua vao tfl dau da duge chuyen het ve Mn^*
Ndng do cua Fe nhflng ngay dau tang rat cham; chflng to qua trinh giai phdng Fe dudi dang Fe^* trong di6u kien van cdn mot ham lugng Idn MnOj ben canh la rat ban che mac du d dd mdi trudng da la mSi trudng yem khi (DO < 0,2 mg/L) Su khu Fe(in) bat dau tang rd rft sau khoang 14 ngay; khi dd lugng Mn^^ da duge giai phdng ban 80% ra moi trudng nude So vdi qua'trinh giai phdng Fe^^ thi hSu nhu trong vong 2 tuan dau qua trinh khfl yem khi uu tien eho su giai phdng Mn^^ Dieu nay phu hgp ca ve mat the oxi hda cua cap Mn02/Mn^^ Idn han nhieu so vdi cap Fe(OH)3/Fe^^ trong mdi trudng nudc cd pH xung quanh 7 [12]
va ea ve mat ddng hge cua qua trinh phan flng oxi hda khu khi c6 mat
ca Fe(in) va MnOz Qua giai doan tang nhanh tfl ngay thfl 14 den ngay thfl 31, ham lugng Fe^^ sau dd lai giam dan rdi hau nhu khdng ddi d giai doan cud'i Khi ndng do Fe^* bat dau giam thi ddng thdi trong he thd'ng eung xua't hien ket tua mau den ciia FeS Tong ndng do sunphua trong pha nude xae dinh duge tai thdi diem nay la xung quanh gia tri 0,25.10"*
M Cac CO che chuyen hda trong mdi trudng khu yem khi ciia hgp cha't sat, mangan va luu huynh trong thi nghiem tren cd the minh hga mot phdn bang cac phan flng hda hge sau:
Chat hflu CO va SO4'- ^^ ^^""^ ^^^ • CH4 + COj + NH4" + S'" + MnOj + 2e + 4W = Mn'^ + 2H2O
Fe(0H)3 + e + H^ = Fe(0H)2 + H2O
Fe(0H)2 + 2CO2 = Fe^" +
2HCO3-,2+ ,
c2-Fe'^ + S'- = FeS
'2 + ~r" — 1 c -r LYiii -r ^n.2
Fe^^ -I- Mn02 + 4W = Fe^^ + Mn^^ + 2H,0
Fe^^ + 3H2O = Fe(OH)j + 3H*
S^ +H2O <=>HS +W
Su giai phdng asen nhu tren hinh 2 eho thay nd x^y ra g^n nhu song song vdi su giai phdng cua Fe(ll) Gia thiet eho rang khdi ngudn ciia hau het asen trong pha ran (nhu trong cac tang da't phu sa bdi tich tU
Trang 6nhifin) tdn tai dudi dang asenat va ehii yen duge hap phu tren eae hat sat(III) hydroxit Khi Fe(III) bi khu ve Fe(II) va tan ra dudi dang mudi bieacbonat thi ddng thdi asen(V) eung bi khu ve asen(III) va tan vao nudc la hoan toan hgp ly Ddng thdi eung nhu ddi vdi Fe^*, khi ham lugng Mn02 trong mdi trudng cdn cao thi qua trinh oxi hda trd lai cua ASO3'- tao thanh ASO4'- va tai ket tua asenat eung hoan toan ed the x^y ra
100
90
- Series 1 -Series2 Series3
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52
Hinh 3 Subie'n thien nong do siinphat, sunphua vd asen trong pha nudc theo thdi gian
Series 1: Siinphat, (mg/L); Series 2: Sunphua, (10'mg/L);
Series 3.: Asen, (lO'^mg/L)
FeAs04 + 3e + 2H^ <=> Fe'^ + ASO3'- + H2O
Me3(As04)2 + 4e + 4H^ o 3Me2^ + 2ASO3'- + 2H2O
AsO/- + Mn02 + 2W = AsO/' + Mn^^ + / / , 0
3Me^^ + 2 AsO/- = Me/AsO J,
Sau ngay thfl 31, ndng do cua asen giam trong khi ndng do Fe(II) eung giam, d day nhu chflng tdi da nhac tdi, hoan toan khdng phai do qua trinh khfl Fe(III) dflng lai ma ed mdi lien quan den sir tao thanh FeS va Fe2AsS Ket qua phan tich dinh tinh phan ket tua mau den hinh thanh trong he thd'ng eho tha'y cd ea ba thanh phan Fe, As va S va theo
Trang 7ket qua phan tich ban dinh lugng thi ham lugng S trong mau trung binh ga'p khoang 1000 Mn so vdi As
Nhung qua trinh tren chflng to su gan bd hihi ea gifla cac hgp cha't cua asen, sat va mangan neu chflng ed mat ddng thdi trong mdt m6i trudng gidi han Cac mdi quan he gifla ehung da quyet dinh su tdn tai
va hanh vi cua cac dang tdn tai ciia chflng trong mdi trudng
Khae vdi Fe, Mn va As, trong pha nudc ndng do ion sunphat giam ngay sau ngay dau, sau dd cfl giam dan gan nhu tuyen tinh Ddng thdi tdng sunphua eung bat dau tang sau ngay thfl nha't va tang deu eho tdi khoang ngay thfl 31 Mdt dieu rat trflng hgp dd la khi ndng do Fe(II) giam thi ddng thdi tdng asen va tdng sunphua eung giam Su hinh thanh sat sunphua da duge chflng minh rd rang qua phan tich ket tiia mau den cua FeS Song su giam ham lugng asen trong pha nudc thi do tao thanh asenua sat hay asen, nhu da ndi den d tren, chua duge chflng minh cu the
Nhu vay su giai phdng asen ra pha nudc trong dieu kien c6 su phan buy yem khi cac hgp cha't hflu ca nhu trong Idng da't d cac tang ngam nudc (aquyfer) lien quan mat thiet vdi ham lugng Mn02 trong da't, vdi qua trinh khu Fe(III) ve Fe(II) va ham lugng sunphua hinh thanh Khi ham lugng Mn02 trong dat cao thi qua trinh khfl sat eung nhu viec giai phdng asen bi flc che rd ret Mdt thuc te ehung tdi da gap khi khao sat mflc do nhiSm kim loai nang trong nudc ng^m khu vue Ha Ngi dd la gan nhu cd mdt quy luat la nai nao ham lugng sat cao thi ham lugng asen trong nudc eung cao va nai nao ham lugng mangan cao thi ham lugng sat thap va ham lugng asen eung tha'p
(tham khao tren bang 3) Dieu nay gin nhu phu hgp vdi nhflng ket
qua nghien cflu cua ehung tdi Day mdi chi la gia thuye't; thuc te' con ra't nhieu cac mdi quan he va nhflng dieu kien phfle tap khae anh hudng de'n nhflng can bang gifla cac pha trong cac ting ngap nude Chinh nhung can bang nay da quye't dinh su tdn tai cua cac ion trong nude ngam
Trang 8Bang 3 Ham lugng trung binh ciia sat, mangan va asen
trong nudc ngam tai 8 bai gieng chinh a Ha Noi
(so lieu phan tich nam 1999)
^ ^ ^ ^ Bai gieng
H L K l o a i ^ ^ ^ ^
(mg/L)
Tong Fe
Mn'^
Tdng asen
MD
0.310 0.951 0.036
NH
0.622 1.627 0.041
NSL
1.203 0.858 0.402
LY
2.149 0.366 0.061
YP
4.776 0.379 0.396
PV
4.496 0.112 0.342
HD
8.329 0.128 0.264
TM
5.640 0.264 0.062
Ghi chii: MD: Mai Dich; NH: Nggc Hd; NSL: Ngd Sy Lien; LY:
Luang Yen; YP: Yen Phu; PV: Phap Van; HD: Ha Dinh; TM: Tuang Mai
Qua 56 ngay dem nghien cflu tren he thd'ng thuc nghiem md phdng mdt each dan gian qua trinh khfl yem khi trong cac tang ngam nudc tu nhien dudi long da't; bang cac ket qua phan tich eae thanh phan ehinh va eae thanh phan lien quan, ehung tdi manh dan neu ra mdt each so luge cac ca che' khu cac hgp chat oxit, hydroxit hay mudi it tan cua sat, mangan va asen thanh cac hgp chat de tan hon trong nudc Ddng thdi cac qua trinh ket tua, tai tao eung nhu cac mdi quan he gifla ehung eung duge thao luan Chung tdi by vong day la nhung tu lieu bude dau hflu ieh eho nhflng ai quan tam de'n va'n de nay
Tai lieu tham khao
1 Dang van Can Preliminary assessment of the distribution,
removal and accumulation of arsenic in hydrothermal depond
bearing high arsenic content Scientific Technical Communication on Geology Department of Geology and
Minerals of Vietnam, Hanoi 2001 (53-57)
2 J M McArthur, P Ravenseoft, S Safiulla, M F Thirlwall
Trang 9Arsenic in groundwater: Testing pollution mecnamsms lor
sedimentary aquyfers in Bangladesh, Water Resources Research, Vol 37, No 1, January 2001 (109-117)
3 Le Van Hdi Luan an tot nghiep, Khoa Hda hge, Dai hge Khoa
hge Tu nhien, Dai hge Qude gia Ha Ngi, 2000
4 Michael Berg, Hong Con Tran et al Arsenic Contamination of Groundwater and Drinking Water in Vietnam: A Human
Health Threat, Environmental Science & Technology, Vol 35,
No 13, 2001 (2621-2626)
5 S Mallick, N R Rajagopal Groundwater development in the arsenic-affected alluvial belt of West Bengal - Some question
Curr Sci., 70, 1966 (956-958)
6 B K Mandal, T R Chowdhury, G Samanta, D Mukherjee, C
R Chanda, K C Saha, D Chakraborti Impact of safe water for
drinking on five families for 2 years in West Bengal, India, Sci Total Environ., 218, 1998 (185-201)
7 S K Acharyya Comment on Nickson et al Arsenic poisoning
of Bangladesh groundwater Nature, 401, 1999 (545)
8 P Bhattacharya, D Chatterjee, G Jacks Occurrence of arsenic-contaminated groundwater in alluvial aquyfers from the Delta Plain, eastern India: Options for a safe drinking water
supply Water Res Dev., 13, 1997 (79-92)
9 R T Nickson, J M McAthur, W G Burgess, P Ravenscroft,
K Z Ahmed, M Rahman Arsenic poisoning of Bangladesh
groundwater Nature, 395, 1998 (338)
10 R T Nickson, J M McAthur, W G Burgess, P Ravenscroft,
K Z Ahmed Mechanism of arsenic poisoning of groundwater
in Bangladesh and West Bengal, App Geochem., 15, 2000
(403-413)
11 Kris Christen The arsenic threat worsens, Environmental Science & Technology, July 1, 2001 (286 A-291 A)
Trang 1012 Urs von Gunten, Jurg Zobrist Biochemical changes in
groundwater-infiltration systems: Column studies Geochimica
et Cosmochimica Acta, Vol 57, 1993 (3895-3906)
13 Mary Ann H Franson (Managing Editor) Standard Methods for the Examination of Water and Waste water, Amer Pub
Health Association, Washington DC, 1995
14 Beat Myller ChemEQL Manual, Limnological Research
Center EAWAG/ETH Switzerland, 1996
15 TCVN Cac tieu chudn nhd nudc Viet nam ve moi trudng, Tap 1:
Chat lugng nude Ha Ngi 1995