The adsorption of AsV ions from aqueous solution on coated rice husk was then studied at varying pH, AsV concentrations, contact times, ionic strength, and adsorbent amounts.. of the ini
Trang 1Removal of As(V) from aqueous solutions by iron coated rice husk
E Pehlivana,⁎ , T.H Tranb, W.K.I Ouédraogoc, C Schmidtd, D Zachmannd, M Bahadird
a
Department of Chemical Engineering, Selcuk University, Campus, 42079 Konya, Turkey
b
Hanoi University of Science, Hanoi, Vietnam
c Laboratoire de Chimie Organique, Structure et Réactivité, UFR-SEA, Université de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso
d
Institute of Environmental and Sustainable Chemistry, Technische Universitaet Braunschweig, Germany
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 5 December 2011
Received in revised form 15 August 2012
Accepted 6 September 2012
Available online 30 September 2012
Keywords:
Adsorption
Rice husk
As(V)
Fe(III)
Isotherms
A lignocellulosic material extracted from rice husk (Oryza sativa), Vietnam, was modified as a new adsorbent for the removal of As(V) ions from aqueous solution Iron was coated onto this adsorbent by hydrolization of ferric nitrate while adding an alkaline solution drop wise into the batch type reactor The adsorption of As(V) ions from aqueous solution on coated rice husk was then studied at varying pH, As(V) concentrations, contact times, ionic strength, and adsorbent amounts The minimum contact time to reach equilibrium is about 6 h The adsorption of As(V) anions on the coated rice husk was found to be highly pH dependent due to Coulomb interactions between As(V) species in solution and positively charged surface groups RH-FeOOH, as well as formation of chelate complexes with naturally occurring carboxyl and carbonyl functional groups in the ma-trix As(V) adsorption on Fe(III)-coated rice husk (RH-FeOOH) from aqueous solution was studied in the pH range 2–10 The main effects of pH on adsorption are estimated by considering both the behavior of As(V) ions (hydrolysis and hydroxide precipitation) and the effect of pH on coordination A strong effect of pH was demonstrated at pH 4.0 with a maximum percentage for removal of As(V) ions 94% Although both Langmuir and Freundlich isotherms have been used to characterize the adsorption of As(V), the Langmuir modelfitted the equilibrium data better than Freundlich model and confirmed the surface homogeneity of adsorbent The maximum adsorption capacity is determined as 2.5 mg/g of adsorbent at pH 4.0 for the Fe(III)-coated rice husk It is concluded that initial As(V) concentration has an effect on the removal efficiency
of RH-FeOOH Higher adsorption of As(V) was observed at lower initial concentrations RH-FeOOH as a low cost material is effective for the removal of As(V) ions and may become a valuable adsorbent to improve the ground water quality in Vietnam
© 2012 Elsevier B.V All rights reserved
1 Introduction
Arsenic in water streams was reported from over 70 countries to
pose serious threat to an estimated 150 million people world-wide
[1] Water supply in many countries, e.g., in Bangladesh, India,
Taiwan, Vietnam, Burkina Faso, Mongolia, Mexico, Pakistan, France,
Italy, Chile, New Zealand and even in the United States contains
dissolved arsenic in excess amounts (>10μg/L), which is the
maxi-mum acceptable level recommended by the USEPA[2,3] Arsenic
oc-curs in water stream in several forms depending upon pH value and
redox potential The oxidation state of arsenic in dissolved phase
plays an important role since it determines the properties of the
relat-ed chemical species, i.e., toxicity, sorption behavior, and mobility in
the aquatic environment[4] Since the pH of the aqueous medium
de-termines the predominant species, it is one of the important
parame-ters for the arsenic removal from drinking and wastewater At typical
pH values of natural water (pH 5–8), the two predominant forms of
inorganic arsenic species in aqueous environments are the trivalent arsenite, As(III), and the pentavalent arsenate, As(V) Arsenite mainly exists as fully protonated form and arsenate remains as an anion, which could be found in various ionic forms in dissolved species[4,5]
A number of treatment methods have been applied for the
remov-al of arsenic from water such as precipitation, co-precipitation[6], coagulation-microfiltration [7], ion-exchange [8], reverse osmosis and nano-filtration [9] Adsorption has been paid more attention due to its high treatment efficiency and lower process costs compared with the above mentioned ones Industrial and agricultural by-products play an important role for improving arsenic removal after their simple and cheap chemical modifications that could be of partic-ular interest for developing countries Iron compounds are among the most popular adsorbents being used for the removal of arsenic from aqueous solutions Rice husk (RH) is a promising adsorbent that has interesting properties such as hydrophilic, porous, and high surface area as well as high resistivity Studies on applying RH as adsorbent for arsenic are still scarce Coating RH with iron can increase the re-moval capacity for As(V) from aqueous solutions Arsenic rere-moval through iron oxyhydroxide (FeOOH)-coated matrices primarily
Fuel Processing Technology 106 (2013) 511–517
⁎ Corresponding author Tel.: +90 332 2232127; fax: +90 332 2410635.
E-mail address: erolpehlivan@gmail.com (E Pehlivan).
0378-3820/$ – see front matter © 2012 Elsevier B.V All rights reserved.
Contents lists available atSciVerse ScienceDirect Fuel Processing Technology
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / f u p r o c
Trang 2of the initial solution, As(V) concentration, contact time, amount of
RH-FeOOH, and ionic strength should be investigated in easy to run
batch experiments that could be easily applied in those countries in
order to diminish the arsenic pollution in drinking and process
water, e.g., in rural areas
2 Materials and methods
2.1 Treatment of rice husk (RH)
Air-dried RH was purchased from the vicinity of Hanoi, Vietnam, and
ground in a vibratory mill (BLB Braunschweig) The size fraction of
125–200 μm was collected, washed with deionized water, and
air-dried in an oven at 60 °C for one day before use All chemicals and
re-agents used were purchased from Merck (Darmstadt, Germany) and
were of analytical grade Aqueous solutions of 0.2 M NaOH and HCl
were used to adjust pH MgSO4.7H2O (Fluka, Seelze, Germany), Na
3-PO4.12H2O (Sigma-Aldrich, Seelze, Germany), and NaNO3 (Merck)
were used for ionic strength studies Dissolved As(V) stock solution (X
mg/L) was purchased from Merck Iron Nitrate Fe(NO3)3was used for
the modification of adsorbent All glassware was cleaned by soaking in
diluted nitric acid and washing with deionized water
2.2 RH treatment with ferric nitrate Fe(NO3)3solution
The milled and air-dried RH (30 g) was pretreated (1) with 2 mol/L
H2SO4(1:1 w/w of dry matter at 80 °C for 0.5 h) for removing starch,
proteins, and carbohydrates, and (2) with 0.5 mol/L NaOH (RH/NaOH
5:1, stirring for 24 h at room temperature) for removing the low
molec-ular weight lignin type compounds Afterfiltration, the adsorbent was
air-dried in an oven at 50 °C for 24 h
The prepared adsorbent was then coated with a ferric nitrate
Fe(NO3)3 solution A mixture of 800 mL of 0.05 M ferric nitrate
Fe(NO3)3in ultrapure water and 25 g RH was given in a 2 L beaker
A 1 mol/L NaOH solution was slowly added into the reaction vessel
at a velocity 10 mL/h under continuous stirring The mixture was
left for 1 day during this procedure Thereby, pH was kept between
2.8 and 3.5 and readjusted if necessary The suspension was then
fil-tered using a cellulose acetate membrane of 0.45μm pore size and
washed with ultrapure water several times until pH neutral,
air-dried in an oven at 50 °C, and stored in closed bottles at room
temperature until use
2.3 Determination of iron amount loaded onto RH
The content of iron loaded onto RH, an important factor in
fluenc-ing the As(V) adsorption capacity, was determined by soakfluenc-ing RH in
1 M HCl for recovering all the loaded iron, and the solution was
ana-lyzed for iron concentration using HG-AAS The amount of iron loaded
in RH was calculated as 1.2 mg/g of adsorbent
served to be decreased at 1263 cm−1 This can be ascribed to a partial oxidation of lignin by Fe3+ions[10]
2.5 Preparation of standards, reagents and analyses Ultrapure water was used for all experiments All chemicals used for the coating process and batch equilibrium studies were of analytical grade and purchased from Merck (Darmstadt, Germany) The AAS stan-dard solution of 1000 mg/L As(V) was prepared by transferring the con-tents of a Titrisol ampoule with As2O5in H2O (Merck, Germany) into a
1 L volumetricflask, which was filled up to the mark at 20 °C according
to the instructions by Merck Arsenic solutions with different concentra-tion used in the batch studies were prepared by diluting the main stock solution The As analyses were performed with a Hitachi Atomic Absorption Spectrophotometer (Series Z-2000; Hitachi Corporation, Japan) which was connected to a hydride formation system (model HFS-3; Hitachi) For hydride generation the following solutions were used: (i) 1.2 M HCl (p.a., Merck); (ii) NaBH4–NaOH solution: solute 10 g NaBH4(p.a., Fluka) in 1 L H2O (Seralpure) by adding
4 g NaOH (p.a., Merck); the solution was prepared immediately be-fore use; (iii) KI-solution as a reduction agent; 20% (w/v; reduction
to As(III)) All standards, reference solutions, and sample solutions were adjusted to 0.24 N HCl and 2% KI The reduction agent was added at least 30 min before analysis In general a 5-point calibration was run before starting the analyses (0–20 μg/L) Argon was used as carrier gas with a flow rate of 0.3 L/min for constant transfer of As-hydride from the reaction cell to the cuvette The 193.7 nm emis-sion line of the As-hollow-cathode lamp was used For the reduction
of As(V) into As(III), 2.5 mL of 30% HCl and 2.5 mL of 20% (w/v) KI was added to 25 mL of the standard or sample solution and left for
15 min
2.6 Batch adsorption experiments Batch adsorption experiments were carried out in order to evalu-ate the performance of the adsorbent for As(V) removal Batch exper-iments were performed in triplicate in sealed glass beakers by adding RH-FeOOH in 50 mL of aqueous As(V) solution of desired initial pH, As(V) ion concentration, and temperature The pH of working solu-tions was controlled and adjusted by adding 0.2 M HCl or 0.2 M NaOH as required The beakers were shaken on a horizontal shaker
at 200 rpm for certain periods (15 min–24 h) The adsorbents were then separated throughfiltration and the remaining filtrates were an-alyzed for As(V) concentration by hydride generation atomic absorp-tion spectrometry with a Zeeman correcabsorp-tion (HGAAS-Hitachi Z-2000 AAS) The amount of As(V) adsorbed per unit mass of the RH-FeOOH (mg/g) was calculated using following Eq.(1):
Trang 3where Ciand Ceare the As(V) concentrations in mg/L initially and at
equilibrium, respectively; V is volume of the arsenic solution in mL;
and W is the weight of RH-FeOOH in mg
For studying the effect of initial pH (2–10) on arsenic uptake by
RH-FeOOH, adsorption experiments were performed using 50 mL of
so-lution with initial As(V) concentration of 5 mg/L and adsorbent dose of
4 g/L at 23 °C Effect of variation of initial As(V) concentration was
stud-ied with different initial arsenic concentration of 1, 3, 5, 7.5, 10, 15, 20,
30, 50, and 75 mg/L; adsorbent dose of 4 g/L; pH 4; temperature
23 °C Effect of contact time (15 min–24 h) and adsorbent amount
(0.1–0.3 g) was studied with initial As(V) concentration of 2 mg/L; pH
4; temperature 23 °C
3 Results and discussion
3.1 Effect of pH on As(V) removal
Iron oxides have been considered already as effective materials for
removal of As(V) in water streams and sediments[21] The
adsorp-tion process of As(V) on these materials takes place at the hydrous
oxide/water interface The adsorption capacity for As(V) strongly
de-pends on the chemical species and characteristics of the solid
supports
RH consists of cellulose, hemicellulose and lignin The cellulose and
hemicellulose are bound to lignin both by hydrogen and covalent
bonds Cellulose is a common material in plant cell walls Hemicellulose
consists of different monosaccharide units such as glucose, xylose,
man-nose, galactose, and arabinose Lignin comprises a variety of functional
groups including aliphatic and phenolic hydroxyl-, methoxyl-, and
car-bonyl groups, which are able to transfer electron pairs from oxygen
atoms and forming coordination complexes with toxic metals Raw
RH also contains some polar functional groups such as alcoholic,
car-bonyl, carboxylic and phenolic ones, which are potentially able to
com-plex As(V)
pH value is considered as an important parameter in arsenic
re-moval[22] The effect of pH on adsorption process was studied at
22 °C in the range from 2.0 to 10.0 It was observed that the initial
pH of all solutions increased slightly after shaking the samples for
6 h Fig 2 shows the effect of equilibrium pH on the As(V) ion
adsorption of RH-FeOOH The percentage of adsorption increases slightly at the pH range of 2.0–4.0 and maximizes at pH 4.0 which shows high selectivity of the modified adsorbent for As(V) After pH 6.0, the adsorption of As(V) decreases significantly until pH 10.0 The mechanism for the removal of As(V) from solution phase is at-tributed to Coulomb interactions and formation of chelate complexes between the As(V) ions and the charged functional groups on the adsorbent's surface At lower pH, the positive charge density on sur-face sites is rising which results in higher electrostatic attraction be-tween (FeOH2+) and As(V) ions This provides a higher adsorption capacity for As(V) In contrast, while increasing the pH, the electro-static repulsion is increasing due to the decrease of positive charge density of proton on the adsorption sites Moreover, the electrostatic attraction between the positively charged surface groups (FeOH2+) and As(V) species H2AsO4 −and HAsO4 −decreases and hinders the formation of surface complexes resulting in lower adsorption capacity for As(V)
The pH also affects the presence of various As(V) species in aque-ous solution It was reported that As(V) occurs in solution at different
pH in form of H3AsO4, H2AsO4 −, HAsO4 −, and AsO4 −oxo anions The predominance of various As(V) species as a function of pH is shown in
Fig 3 The different species of As(V) are present in solution based on the following three equilibriums and their respective stability con-stants[22](Eqs.(2)–(4)):
H3AsO4↔H2AsO−4 þ Hþ; PK1 ¼ 2:3 ð2Þ
H2AsO−4↔HAsO2−4 þ Hþ; PK2 ¼ 6:7 ð3Þ
HAsO2−4 ↔AsO3−4 þ Hþ; PK3 ¼ 11:6 ð4Þ The pH influences the protonation or deprotonation of the adsor-bent surface The interactions of As(V) ions with the coated RH sur-face are due to Coulomb interactions, ligand exchange phenomena, and formation chelate complexes Narasimhan reported that goethite, ferrihydrite and FeOOH loaded bio-sorbent have the same adsorption mechanisms for As(V) [23] Iron oxides can form oxy-hydroxides
Fig 1 FTIR spectra of raw RH and coated RH-FeOOH (black: raw RH, red: RH-FOOH).
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E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517
Trang 4which can be protonated or deprotonated depending on the pH value
of solutions and as a result, a positive or negative surface can be
formed as the following[24](Eqs.(5) and (6)):
In the pH range 2–7, the major arsenate species in aqueous
solu-tion is H2AsO4 − An adsorption takes place by the reaction between
the active hydrolyzed form of`FeOH on the surface of RH-FeOOH
and As(V) ions that leads to the formation of surface complexes
according to the following equilibriums[24](Eqs.(7) and (8)):
2`FeOHþ2 þ H2AsO−4→½ð`FeOÞ2\AsOðOHÞþ H3Oþþ H2O ð7Þ
`FeOHþ2 þ H2AsO−4→½`Fe\ðOÞ2\AsOðOHÞ−þ H3Oþ ð8Þ
In the first reaction, non-specific Coulomb interactions
(outer-sphere adsorption) or ligand exchange reaction on the surface
(inner-sphere adsorption) with As(V) can occur[25]as the following
(Eqs.(9)–(13)):
`Fe\OHþ2 þ H2AsO−4↔`Fe\OHþ2…−O4AsH2 ð9Þ
`Fe\OH þ H2AsO−4↔`Fe\OAsO3H2þ OH− ð10Þ
`Fe\OHþ2 þ H2AsO−4↔`Fe\OAsO3H2þ H2O ð11Þ
`Fe\OH þ HAsO2−4 ↔`Fe\OAsO3H−þ OH− ð12Þ
2`Fe\OH þ HAsO2−4 ↔`Fe2\AsO2H þ 2OH− ð13Þ
The protonation of active surface sites at low pH plays a significant
role in reducing the free As(V) ions in the dissolved phase Above pH
7.0, adsorption of As(V) decreases because of the competition between
HAsO−ions and OH−ions for the reactive sites of RH-FeOOH[26,27]
3.2 Effect of agitation time on the removal of As(V) The results of time effects on As(V) adsorption process on RH-FeOOH matrix are given inFig 4 This graph demonstrates that the As(V) adsorption reached an equilibrium 96–99% after shaking for 6 h It seems that there is not much difference in adsorption per-centage thereafter The adsorption of As(V) takes place at a pH below the pHpzc that was found to be about 5.8–6.3 (Fig 5) For that reason, the experiments were carried out at pH below pHpzc of adsorbent
The scanning electron microscopic pictures (SEM) reveal the sur-face textures and porosities of RH and RH-FeOOH (Fig 6) It shows veryfine particle sizes having pores within the particle of varying size
3.3 Effect of initial As(V) concentration The influence of the initial concentration on the adsorption on RH-FeOOH was studied at pH 4.0 after 6 h shaking with As(V) with the concentrations in the range of 1–75 ppm (Fig 7) The results demonstrated that at higher concentrations, more As(V) ions remained in dissolved phase due to the saturation of binding sites (`Fe\OH) towards As(V) As(V) uptake by RH-FeOOH was 99.6 %
at a concentration of 3 ppm
The data obtained from equilibrium isotherm that provides infor-mation on the sorption capacity are the most important factor in any
0 20 40 60 80 100 120
Contact time (hours)
As (V)
Fig 4 Sorption isotherm of As(V) on RH–FeOOH as a function of contact time (initial As(V) concentration: 2 ppm; solvent volume: 50 mL; pH: 4; adsorbent amount: 0.2 g; temperature: 22 ± 2 °C).
Trang 5adsorption system The As(V) adsorption capacity of RH-FeOOH was
calculated using Langmuir and Freundlich models[28] These
iso-therms are related to As(V) uptake per unit weight of RH-FeOOH,
qe, and the equilibrium As(V) ion concentration in the dissolved
phase, Ce
The Langmuir isotherm has been widely applied for adsorption
pro-cesses to separate an analyte from aqueous solutions[29] Langmuir
iso-therm model assumes that adsorption process forms a monolayer and
occurs at specific homogeneous adsorption sites Intermolecular forces
decrease rapidly with the distance from the surface The Langmuir
model is more popular since it contains the two reasonable parameters
(Kband As) that are easy to interpret the adsorption[30–32]
The general form of Langmuir model is:
Langmuir equation :
Ce
qe¼Ce
AsþA1
where As(mol/g) and Kb(L/mol) are the coefficients, qeis the weight
adsorbed per unit weight of adsorbent and Ceis the analyte
concen-tration in solution at equilibrium
Freundlich equation can be used as another model for
determina-tion of As(V) adsorpdetermina-tion capacity as shown below:
Freunlich equation :
x
m
where 1/n is the intensity of adsorption; k is the adsorption capacity,
x/m is the weight adsorbed per unit weight of adsorbent and Ceis the
analyte concentration at equilibrium in solution The modified
formu-la of this equation, Eq.(16)was also obtained
log x m
Langmuir and Freundlich constants and correlation coefficients (R2) are shown inTable 1 For the determination of these coefficients,
R2value was calculated from the linear form of Langmuir isotherm as 0.995 for As(V) ion adsorption This result indicates that the As(V) ion adsorption onto RH-FeOOHfits well the Langmuir model Thereby, the adsorption of As(V) ions onto RH-FeOOH is considered forming
a monolayer that takes place at the functional groups or binding sites on the sorbent surface The maximum adsorption capacity (mg/g) of RH-FeOOH for As(V) was found to be 2.5 mg/g (Table 1) Comparison of As(V) adsorption (mmol As/g adsorbent) of differ-ent adsorbdiffer-ents reported in the literature is given inTable 2 It appears that RH-FeOOH has a reasonable potential as adsorbent for the re-moval of As(V) from aqueous solutions
3.4 Desorption studies The desorption of As(V) ion from the adsorbent (RH-FeOOH) was investigated as well Desorption studies can help to regenerate the adsorbents for further reuse Desorption efficiency of As(V) ions from RH-FeOOH was studied with 30% HCl and 1 M NaOH It was con-cluded that the desorption percentage of As(V) from adsorbent is higher when using NaOH than HCl.Table 3shows that desorption of As(V) enhanced by the increase of pH Maximum desorption was
0
2
4
6
8
10
12
Fig 5 The pH point zero of charge (pHpzc) of RH-FeOOH.
As(V)
0 0,4 0,8 1,2 1,6 2
Arsenate concentration Ce (ppm)
Fig 7 Sorption isotherm of As(V) on Fe-loaded rice husk (RH) as a function of initial As(V) concentration (initial As(V) concentration: 1–75 ppm; solvent volume: 50 mL; pH: 4; adsorbent amount: 0.2 g; temperature: 22 ± 2 °C; contact time: 6 h).
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E Pehlivan et al / Fuel Processing Technology 106 (2013) 511–517
Trang 6observed at a pH range of 12–14 90% of As(V) is recovered under
these conditions These results demonstrate that adsorbed As(V)
can be desorbed from the RH-FeOOH using 1 M NaOH and thus
suc-cessfully applied for the regeneration of the RH-FeOOH while shaking
for 20 h
3.5 Effect of adsorbent amount on the As(V) removal
The amount of RH-FeOOH used for adsorption experiments was
varied from 0.1 to 0.3 g in 50 mL volume at an initial As(V)
concen-tration of 2 ppm; contact time of 6 h at 22 ± 2 °C and pH 6 The
As(V) equilibrium concentration in dissolved phase decreased when
increasing adsorbent quantity (Fig 8) Thus the optimum adsorbent
amount (RH-FeOOH) was found as 0.25 g by the given As(V) content
3.6 Effects of ionic strength on As(V) removal
Ionic strength is one of the important factors influencing aqueous
phase equilibrium The effects of the interfering sulfate, phosphate
and nitrate anions were evaluated for the As(V) adsorption
Adsorp-tion process decreases when increasing ionic strength of the
dissolved phase The results showed that there was significant
de-crease in As(V) adsorption when 50 ppm phosphate ions were
contained together with As(V) in the solution However, the
adsorp-tion of As(V) was slightly decreased by addiadsorp-tion of 50 ppm nitrate
and sulfate ions The competition of phosphate with arsenate for the
same sorption sites is very likely due to similar molecular structures
of the two anions in the periodic system of elements This fact has
to be considered if one wants to remove As(V) from natural waters
that contains also phosphate residues from wastewater or fertilizers
4 Conclusion
The RH-FeOOH adsorbent was prepared using rice husk from
Vietnam The characteristics of RH and RH-FeOOH were identified by
using FTIR technique The adsorption capacity of prepared RH-FeOOH was investigated by the batch adsorption experiments which revealed that RH-FeOOH adsorbent was effectively removing As(V) The As(V) removal capacity of the prepared RH-FeOOH material is 99.6% at pH 4 This proves that the coated adsorbent has a remarkable capacity for re-moving As(V) from aqueous solutions
The kinetic studies indicated that equilibrium of As(V) adsorption
on RH-FeOOH was reached after 6 h As(V) adsorption increased with
an increase of As(V) in the solution The optimum pH corresponding
to the maximum adsorption rates was found to be about pH 4 for RH-FeOOH The As(V) adsorption on RH-FeOOH was best described using Langmuir isotherm model only It was found that the desorp-tion percentage of As(V) from adsorbent is high at pH above 12 The presence of high phosphate concentrations decreases the As(V) ad-sorption due to the competition for the same ad-sorption sites Mean-while, the adsorption capacity for As(V) was not affected when adding nitrate and sulfate ions to the solution at the same amounts This laboratory investigation was performed under conditions that can easily be scaled up and applied for the removal of As(V) in devel-oping countries of tropics and subtropics suffering from high As con-tents in ground and drinking water and producing rice as main agricultural crop at the same time (e.g., Vietnam and Burkina Faso)
Acknowledgements This investigation was performed at the Guest Chair within the project“Exceed – Excellence Center for Development Cooperation – Sustainable Water Management in Developing Countries” at the Technische Universitaet Braunschweig, Prof Pehlivan being the visit-ing professor, and Ms Hien and Mr Ouedraogo bevisit-ing the
internation-al exchange staff members The Exceed Project is granted by the German Federal Ministry for Economic Cooperation and Development (BMZ) and German Academic Exchange Service (DAAD), and their fi-nancial support we gratefully acknowledge
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