The exper-iment was carried out on the column imitated a bore core of anaerobic aquifer with water phase containing FeII, MnII, AsIII concentration of 45.12 mg/L, 14.52 mg/L, 219.4 lg/L,
Trang 1O R I G I N A L P A P E R
Investigation of As, Mn and Fe fixation inside the aquifer
during groundwater exploitation in the experimental system
imitated natural conditions
Nguyen Thi Kim Dung•Tran Hong Con•
Bui Duy Cam•Yumei Kang
Received: 21 March 2011 / Accepted: 13 July 2011 / Published online: 9 August 2011
Ó Springer Science+Business Media B.V 2011
Abstract Water-dissolved oxygen was supplied
into anaerobic aquifer , which oxidized Fe(II), Mn(II)
and trivalent arsenic and changed them into
undis-solved solid matter through hydrolysis, precipitation,
co-precipitation and adsorption processes The
exper-iment was carried out on the column imitated a bore
core of anaerobic aquifer with water phase containing
Fe(II), Mn(II), As(III) concentration of 45.12 mg/L,
14.52 mg/L, 219.4 lg/L, respectively and other ions
similarly composition in groundwater After 6 days
of air supply, concentration of iron reduced to
0.38 mg/L, manganese to 0.4 mg/L, arsenic to
9.8 lg/L (equivalent 99.16% of iron, 97.25% of
manganese and 95.53% of arsenic fixed), and for other
ions, the concentration changed almost according to
general principles Ion phosphate and silicate strongly
influenced on arsenic removal but supported iron and
manganese precipitation from water phase Based on the experimental results, new model of groundwater exploitation was proposed
Keywords Arsenic Exploitation Groundwater manganese Iron fixation
Introduction
In recent years, arsenic contamination in groundwater and drinking water in Vietnam has received consid-erable attention Elevated levels of As were found in groundwater in the upper aquifers in several areas in Red River Delta (Berg et al.2001; Agusa et al.2006, 2009) Residents living in high As-contaminated groundwater area are exposed to As through con-sumption of rice and groundwater, suggesting potential health risk of As exposure (Agusa et al 2009) Symptoms for chronic exposure to As have not yet observed, but the continuous usage of tube wells might cause health effects of the residents Given high degree of exposure to As in a large area of Red River and Mekong River in Vietnam, investigations
on the mechanisms of arsenic releases into ground-water, as well as model for reduction in As in groundwater, are critically important to develop a simple and effective technology for arsenic removal
in groundwater and thus reduce health risk due to elevated and chronic exposure
N T K Dung ( &)
Haiphong Private University, 36 Dan Lap Street,
Le Chan District, Hai Phong, Vietnam
e-mail: dungntk10@gmail.com
T H Con B D Cam
Hanoi University of Science, Vietnam National
University, 334 Nguyen Trai Street, Thanh Xuan,
Hanoi, Vietnam
Y Kang
Laboratory of Soil Environmental Science, Faculty
of Agriculture, Kochi University, Monobe B200,
Nankoku City, Kochi 783-8502, Japan
DOI 10.1007/s10653-011-9401-7
Trang 2In the aquifer, under anaerobic conditions, iron
and manganese exist as divalent species and arsenic
as almost nondissociated trivalent arsenious acid
(Con et al.2002; Nriagu1994; Saha et al 1999) In
order to remove iron from underground water,
tra-ditional technology used aeration technique to
oxi-dize Fe(II) to Fe(III) and then separated it from water
in the form of insoluble Fe(III) For manganese
removal, normally use filtration through sand-coated
MnO2(Chang et al.2010) For groundwater
contam-inated by arsenic, there were many methods and
technologies for arsenic treatment, especially since
‘‘largest poisoning in the World’’ at Bangladesh
revealed in 1993 (Chang et al 2010; Chakraborti
et al.2010) Our recent study has showed that during
and after oxidation of Fe(II) to Fe(III), Fe(III)
immediately hydrolyzed to form almost insoluble
Fe(OH)3 and the iron hydroxide species strongly
adsorbed arsenate anions and partly co-precipitated
with manganese (Dung et al.2009) We investigated
fixation capabilities of iron, manganese and arsenic
on equipment imitated natural conditions of the
aquifer During the fixation of iron, manganese and
arsenic, the expected influencing factors such as
phosphate, silicate, NO3-, NH4?and SO42-
concen-trations were also investigated Experiment result
of this study was applied for fixation of iron,
manganese, arsenic inside aquifer and reduction in
ammonium concentration in exploited water The
underground water exploitation model was
estab-lished based on the idea that underground water after
pumped up was saturated by air oxygen Part of
the oxygen-saturated water was pumped back to
exploited layer to do fixation of iron, arsenic and
manganese, and other part was filtrated to do supply
Experiment
The experimental system was installed as described
in Fig.1 The research column was filled 50-mm
layer of weathering gravel in bottom, next was
600-mm layer of sand mixed with 0.001% of As in
the form of arsenate, 0.01% of Mn in the form of
MnO2and 0.1% of Fe in the form of Fe(OH)3(w/w
percentage) The main components in water phase are
listed in Table 1
Before fixation investigation, the experimental
system (Fig 1) was running with circulation of water
phase and air tightening for 50 days in order to create anaerobic condition in inner system similar to condition in natural aquifer
The investigation started when air was continually supplied to the regulation tank with a rate of 0.5 L/min Samples were taken daily at fixed time from valve (6)
(2)
(1)
(3) (4) (5) (6)
1
9
2
7
6
4
5
10
Fig 1 Schematic diagram of the research system, 1 Column head, 2 Thermoisolation cover, 3 Layer of sand, MnO2, Fe(OH)3, undissolved As(V) and other components, 4 Weath-ering gravel layer, 5 Porous membrane, 6 (1)-(6) Sampling valves, 7 Peristaltic pump, 8 Regulation tank, 9 Thermostat, 10 Air supply device
Table 1 Main composition of water phase (Berg et al 2001 ) Component Concentration (M)
Ca2? 1.0 9 10-3 HCO3- 2.4 9 10-3
NO3- 3.0 9 10-4
SO42- 5.2 9 10-4
PO43- 3.0 9 10-5
Mg 2? 6.0 9 10 -5
Digestible organic matter 1.2 9 10 -3 (glucose)
Trang 3at the experimental column, and parameters were
analyzed triplicate by the methods listed in Table2
Results and discussion
Composition of water phase in anaerobic state
The composition and main parameters of water phase
in anaerobic system (after 50 days air absent running)
are analyzed (samples were taken from valve number
(6) at the experimental column and result is shown in
Table3)
The variation in Fe, Mn and As concentrations
under influence of oxygen present
When air oxygen was supplied into system, dissolved
oxygen concentration (DO) increased along with
bubbling time and reached near 8 mg/L after 10 days
(the system changed into almost aerobic condition)
Changing nature of the system from anaerobic to
aerobic caused variation in almost all constituents in
the system (Fig.2)
Together with increasing in DO, ORP of water
phase also increased regularly It was inevitable
In case of arsenic and iron, the variations were
different In the beginning hours of oxygen
supply (about first day), the concentrations of
both elements increased The reason of this
phenomenon could be oxidation by DO yielding
dissolved forms of iron(II) and arsenic(III) from fresh and unstable precipitate species of iron arsenide and sulfide (Dung et al 2010) In the following days, the system was in oxygen-rich condition, iron(II) oxidized into iron(III) This species started to hydrolyzed and precipitated as undissolved Fe(OH)3 That is why iron concen-tration decreased In this condition, arsenite species also slowly oxidized into arsenate in the form of anions hydoarsenate and started to co-precipitate with iron(III) hydroxide or to adsorb onto surface of iron(III) hydroxide particles So total arsenic concentration in the system also decreased (Dung et al.2009) When system was almost in the aerobic condition, the concentra-tions of iron as well as arsenic decreased to meet limited concentrations of 0.50 mg/L and 0.010 lg/L, respectively The results of samples collected from valves 1–5 showed the similar law of elements’ concentration variation and transformation but the time to reach aerobic condition was earlier from valves 1–5
For manganese(II) ion, its concentration was decreased continuously from beginning to the end The reason could be that slow oxidation of Mn(II)
to Mn(IV) by DO in neutral environment formed undissolved MnO2 This process was less influenced
Table 2 Analysis methods APHA, AWWA, WEF ( 1995 )
Parameter Analysis method
As AAS-HVG method
Fe, Mn F-AAS method
NO3-, Cadmium reduction method
Phosphate Stannous chloride method
Silicate Molybdosilicate method
SO42- Methylthymol blue method
NH4? Phenate method
Table 3 Composition of water phase in anaerobic state
Parameters DO
(mg/L)
ROP (mV)
Fe (mg/L)
Mn (mg/L)
As (lg/L)
NO2 -(mg/L)
NH4? (mg/L)
SO4 2-(mg/L)
PO4 3-(mg/L)
SiO3 2-(mg/L) Value 0.8 -45 45.12 1.45 219.4 1.24 48.20 13.76 0.95 2.82
Fig 2 Variation in Fe, Mn, As concentrations, ORP and DO versus air supply time (sampling from valve 6)
Trang 4by chemical and physicochemical processes of iron
and arsenic species in the system
The variation in sulfate, phosphate and silicate
concentrations
For investigation of sulfate, phosphate and silicate
variations, samples were taken after interval of 2 days
each other The result for 20-day survey is shown in
Fig.3 There were different variations in
concentra-tions between ions While concentraconcentra-tions of phosphate
and silicate were almost unchanged, concentration
of sulfate increased during survey time However, in
beginning 4 days, the increase rate was low in
com-parison with the following time
Increasing in sulfate ion in the system was result of
oxidation process of sulfide together with arsenide in
the precipitate created before in anaerobic period
The low increasing rate of sulfate concentration in the
system at beginning days could be consequence of
competitive oxidation reactions of iron(II),
arseni-c(III) and other easier oxidation species present in the
system
The variations in NH4?, NO2-, NO3
-concentrations
When system in anaerobic condition, concentration
of nitrate was almost limited to analyze, nitrite was
1.28 mg/L and ammonium was 48.20 mg/L
Supply-ing oxygen from air supplied changed concentration
of all those nitrogen formations Ammonium
con-centration slowly decreased, nitrite concon-centration
decreased to detection limit and nitrate concentration
increased (Fig 4)
Influence of phosphate concentration
For investigation of influence of phosphate concen-tration on immobilization of arsenic, iron and man-ganese, phosphate solution was putted on into the system to meet designed concentration range The samples were collected 8 h each after other, and arsenic, manganese, iron were analyzed
Based on the results presented in Fig 5, we can see that increasing phosphate concentration only lightly influenced on dissolved iron and manganese With concentration of 20 mg/L phosphate, concentration
of total iron dropped from 0.80 to 0.54 mg/L and manganese from 1.38 to 1.12 mg/L The lightly decreasing concentration of iron and manganese could
be the result of precipitation of iron and manganese phosphate in the system For arsenic, this was different Phosphate ion strongly influenced on arsenic immobi-lization With the concentration of phosphate lower than 10 mg/L, the concentration of arsenic was almost uninfluenced; but when phosphate concentration was higher than 10 mg/L, the adsorptive competition
Fig 3 Variations in sulfate, phosphate and silicate
concen-trations
Fig 4 Variations in NH4? , NO2- and NO3- concentrations
Fig 5 Influence of phosphate concentration on fixation of As,
Fe and Mn
Trang 5between phosphate and arsenate ions on solid phase
appeared; therefore, concentration of arsenic sharply
increased
Influence of silicate concentration
Investigation of influence of silicate concentration on
immobilization of arsenic, iron and manganese was
implemented similarly as case of phosphate
The result presented in Fig.6showed that soluble
species of iron and manganese in the system were
almost uninfluenced by concentration of silicate But
for arsenate ion, the situation was similar to
phos-phate interaction However, the competitive power was
weaker than phosphate Those expressed by affected
concentration of silicate was higher than phosphate
(15 mg/L vs 10 mg/L), and angular coefficient of line
segment slope in the graph of silicate was less than
phosphate (2.150 vs 5.342) So, in any case, the
presence of phosphate or silicate or both with high
enough concentration raised difficulties for
immobili-zation of arsenic, iron and manganese in oxygen-rich
(aerobic) condition
Proposal model of fixation of As, Fe and Mn
in the aquifer during groundwater exploitation
The aquifer is a water-saturated layer of sand and
gravel Horizontal water flow rate in the aquifer is
normally 10–15 m per day So the aquifer is
under-ground water resource and also can play as a good
water filter Based on our results presented above
together with exploitability of the aquifer, we have
had idea to bring some stages of groundwater
treatment process down to the aquifer These are
aeration, iron precipitation, filtration, arsenic and
manganese treatment stages The oxygenation and sterilization stages are kept on ground The schema of underground water exploitation is described in Fig.7 The production process could be that: Groundwa-ter firstly was pumped up from first tube well to oxygenation basin A portion of oxygen saturation water was pumped back to the aquifer though second tube well Where second well is located in front of first well along to groundwater flow direction Other portion of oxygen-saturated water was used for supply The proportion of supply and fit-back water portions depends on iron concentration in groundwa-ter and oxygen saturation possibility The distance between exploitation and fit-back wells depends on groundwater flow rate and exploitation capacity Based on the result of our study, when iron concentration in water phase C10 mg/L, more than 98% arsenic was remained in solid phase despite total arsenic concentration was up to 0.3 mg/L and the part
of it was oxidized from arsenide formation and dissolved into water phase
Conclusion
Supplying oxygen in the form of dissolved oxygen in water into anaerobic system imitated natural aquifer oxidized almost Fe(II) into Fe(III), partly Mn(II) into Mn(IV) and arsenite into arsenate Hydrolysis of Fe(III) and Mn(IV) helped co-precipitation and adsorption process of arsenate together with iron hydroxide and manganese dioxide The result of these processes reduced concentration of iron, manganese
Fig 6 Influence of silicate concentration
Groundwater current up
O 2 rich current
back
Exploitation well Fit-back well
Treatment system/Oxygenation
Ground water flow direction
Supply water
Oxidation zone
Fig 7 The schema of underground water exploitation
Trang 6and arsenic in water phase and retained them among
sand/gravel layer in the system Fixation of the
elements, especially arsenic, influenced by phosphate
and silicate concentrations in water phase However,
ammonia, nitrite, nitrate and sulfate showed almost no
affect Other oxidation processes of sulfide, ammonia
even organic mater increased supplied water quality
Acknowledgments The authors acknowledge the financial
support from the sub-project TRIG A from Hanoi University of
Science and Dr Michael Berg, ESTNV Manager & Scientific
Advisor Department of Water Resources and Drinking Water
to facilitate the implementation process.
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