Nghiên cứu cơ chế giải phóng Asen ra nước ngầm - mô phỏng các quá trình khử yếm khí tự nhiên trong các tầng ngậm nước

NGHIEN
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,
NGUY£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
962
con 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 20-35 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.
963
Qua 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

964
De 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.
10
';::,;.'
J,
•,;,•.,••
'
,;. •^^^„i <
>N.
•vm
•
n
-
•
,
•
/
•
• •
«
.
n
•
"

^

•
. • •
•
• •
•
.•
Mi
•
**
•
:-—-
••••
—
—
«
• «
^
-
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
965
xud'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
966
nhifin)
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
967
ket 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.
968
Bang 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
HLK.loai^^^^
(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.
969
Arsenic 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).
970
12.
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.
971