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Challenge the future
DelftUniversity ofTechnology
Investigating subsurface iron and arsenic removal: Anoxic column experiments to explore efficiency parametersGraduation Harmen van der Laan | 18 September 2009
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Contents
i. Introductioni. Arsenic problem
ii. Subsurface iron and arsenic removal
iii. Problem description and objectives
iv. Research setup
v. Experimental procedure
ii. Theoretical backgroundiii. Results and discussioniv. Conclusions and recommendations
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Arsenic contamination in drinking water
Arsenic problem Naturally in ground water Chronic exposure: higher
rates of lung, bladder and skin tumors
Big social impact (ostracism) WHO guideline: < 10 μg/L
Bangladesh 30 million people are exposed
to concentrations > 50 μg/L Rural areas: no centralized
systems (10 million tube wells)
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Subsurface iron and arsenic removal
Ground water level
Ground water with Fe(II) and As
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Subsurface iron and arsenic removal : injection phase
Ground water level
O2 front
Injected water front
Ground water with Fe(II) and As
Injection water without oxygen
Injection water with oxygen
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Subsurface iron and arsenic removal: abstraction phase
Ground water level
Ground water with Fe(II) and As
Injection water without oxygen
Oxidation zone withfreshly formed ferric oxides
iron oxidewith adsorbed Fe(II) and As
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Subsurface iron and arsenic removal: efficiency ratio
Volume [m3]
Iron c
once
ntr
ati
on [
mg/L
]
4
2
0
VinjectionV
VVi
Efficiency ratio Typically increasing over successive cycles
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Problem description and objective
Problem description There is a lack of insight in (i) the dominant mechanisms responsible for the (increasing) sorption of iron and arsenic(ii) operational factors how to optimize the removal efficiency
The objective of this study To obtain reliable experimental data to investigate the parameters affecting the removal efficiency
The primary goal is to gain a better understanding of the dominant sorption mechanisms and the increasing efficiency, in order to optimize the operation of this technology in the field.
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Research setupAnoxic column experiments to simulate several injection/abstraction
cycles in Bangladesh
Experimental setup 4 columns diameter 36mm, height 308mm
2 types of soil material Virgin Sand 0.6-1.2mm Aquifer Sand 0.12-2.5mm Fe: 2.7 and 2.5 mg/g . As: 2 and 0.5 µg/g
‘average Bangladesh’ Synthetic Ground Water 4 mg/L Fe2+ 200µg/L As(III) pH 6.9 buffers: 5mM NaHCO3 1.64mM NaCl
Ionic Strength 2·10-2
Four experiments, 10 injection/abstraction cycles
Experiment I: Investigation increasing capacity over successive cycles (cycle 1 – 5)
Experiment II: Influence pH: 6.5, 6.9 and 7.5 (cycle 6 – 8)
Experiment III: Influence injection volume (cycle 9)
Experiment IV: Influence increase ionic strength (0.1M NaNO3) (cycle 10)
Monitoring Fe, As, pH, Eh, Conductivity and Oxygen
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Experimental procedure: the story of one data point
How does one data point at the graph come into existence? What is ‘the story of one ‘data point’
A short movie shows the experimental procedure
11Met dank aan: Samuël (cameraman) en Ruben (camera én microfoon)
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Contents
i. Introductionii. Theoretical background
iii. Results and discussioniv. Conclusions and recommendations
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Adsorption is influenced by: Surface charge Chemical affinity
Adsorption capacity of a material: Number of sites (sites/nm2) Surface area (m2/g)
Furthermore, Competing ions Inner/outer-sphere complexation
Fe2+ and As(III) form inner-sphere complexes; their adsorption is fairly insensitive to ionic strength changes
Theoretical background
Fe2+
OH OFe
+
H+
OH OH
OH
OH
Sand grain surface
M2+
M2+ M2+
M2+
M2+
Example: adsorption Fe2+
Iron: Fe2+ and Fe3+
Arsenic: As(III) and As(V)Arsenite
Arsenate
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Contentsi. Introductionii. Theoretical backgroundiii. Results and discussion
i. Experiment I : Influence successive cycles
a. High adsorption capacities
b. Increasing retardation As
c. Stable retardation Fe2+
ii. Experiment IV: Effect of ionic strength
iii. General discussion
iv. Conclusions and recommendations
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Results experiment I: successive cycles
Expectation, based on other experiments and literature:Retardation factor between 5 and 20, slightly increasing
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Results experiment I: successive cycles
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Results experiment I: successive cycles
Three main findingsa. High adsorption capacities (in absolute
values)b. Increasing adsorption As(III)c. Stable adsorption Fe2+
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High adsorption capacities
Sand grain surface
Fe2+
OH OFe
+
H+
OH OH
OH
OH
OH OH
OH
OH
OH OH
OH
OHOH OH
OH
OH
OH OH
OH
OH
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High adsorption capacitiesHypothesized mechanism
Ion exchange mechanism
Ion exchange capacity determined by a.o. clay particles, in Cation Exchange Capacity (CEC).
Surprisingly, a low CEC value can result in a high retardation!
2 meq/kg Retardation factor 30! (normal sandy aquifer is 10 meq/kg)
Yet, ion exchange in virgin sand?!
Fe2+
Na
+
OH
Na
+
Sand grain surface
Na
+
Na
+
Na
+
Fe2+
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Contentsi. Introductionii. Theoretical backgroundiii. Results and discussion
i. Experiment I : Influence successive
cycles
a. High adsorption capacities
b. Increasing retardation As
c. Stable retardation Fe2+
ii. Experiment IV: Effect of ionic strength
iii. General discussion
iv. Conclusions and recommendations
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Retardation As(III) increasing
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Stable Fe2+ capacity
Non-increasing capacity Fe2+
Very remarkable! Increase iron content, thus in adsorption sites yet no increase in adsorption
In accordance with other studies and experiments
Ion exchange provides explanation: Exchange Capacity remains constant.
Fe2+
Na
+Na
+
Na
+
Na
+
Fe2+
Fe2+
Na
+Na
+
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Contentsi. Introductionii. Theoretical backgroundiii. Results and discussion
i. Experiment I : Influence successive cycles
a. High adsorption capacities
b. Increasing retardation As
c. Stable retardation Fe2+
ii. Experiment IV: Effect of ionic strength
iii. General discussion
iv. Conclusions and recommendations
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Effect of the ionic strength (0.1M NaNO3)
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Effect of the ionic strength (0.1M NaNO3)
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Effect of the ionic strength (0.1M NaNO3)
Main finding Adsorption As(III) is increasing with increasing ionic strength, while Ferrous iron adsorption is decreasing
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Decrease Fe2+ with high ionic strength
Decrease Fe2+ -70% (average)
Ionic strength influenced adsorption iron? Remember: Inner-sphere complexes,
thus rather insensitive for ionic strength!
The ion exchange mechanism provides a clear explanation.
High Na+ concentration (0.1M vs. 7 mM) results in shift exchanger composition (98% Na+ / 2% Fe2+ vs. 37% Na+ / 63% Fe2+)
Fe2+
Na
+
Na
+Na
+
Sand grain surface
Na
+
Na
+
Na
+
Fe2+
Na
+
Na
+Na
+
Na
+Na
+
Na
+
Na
+
Na
+Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
Na
+Na
+
Na
+
Na
+
Na
+
Na
+
Na
+
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Increase As(III) with high ionic strength
Increase As(III) 8 – 43 in one cycle (438%)
Other studies: increasing adsorption with increasing ionic strength. But, there with negative surface charge. Here, As(III) is uncharged and positive charge.
Hypothesis : ionic strength causes surface charge of zeroSurface charge and potential becomes 0 (“point-of-zero-charge”) thus no electrostatic repulsionwhich favors adsorption of the uncharged As(III)
Compare: experiment I: 10 – 50 in 5 cycles
As(III)0
++0 0
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Contentsi. Introductionii. Theoretical backgroundiii. Results and discussion
i. Experiment I : Influence successive cycles
a. High adsorption capacities
b. Increasing retardation As
c. Stable retardation Fe2+
ii. Experiment IV: Effect of ionic strength
iii. General discussion
iv. Conclusions and recommendations
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General discussion
Ion Exchange mechanism
Pro’sNot possible to describe with surface sites theoryStable retardation Fe2+
Decrease Fe2+ adsorption with high ionic strength
Results in adsorption capacity similar to other studies
Con’s / remaining questionsExchange capacity (virgin) sand?!Why no increase adsorption for ferrous iron?Why not all Fe2+
accessible for adsorption?
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Contentsi. Introductionii. Theoretical backgroundiii. Results and discussion
iv. Conclusions and recommendationsi. Iron removal mechanism
ii. Arsenic removal mechanism
iii. (Practical) implications
iv. Recommendations
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Conclusions sorption mechanism of iron
I. High capacity!Much more as ‘theoretically’ possible
II. No increasing efficiency with increasing amount iron oxide.
III. Surprisingly, the ion exchange mechanism played a dominant role
Disclaimer: under laboratory circumstances
Fe2
+
Na
+
OH
Na
+
Sand grain surface
Na
+
Na
+
Na
+
Fe2
+
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Conclusions sorption mechanism of arsenic
I. High capacity!Much more sites accessible as expected
II. The efficiency is increasing (by iron oxides)1 day injection = 1 month 50% arsenic removal!
III. Higher ionic strength, higher efficiencyHypothesis: surface charge becomes zero
Disclaimer: under laboratory circumstances
As(III)0
++0 0
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(Practical) implications
I. Measure ionic strength and ‘point-of-zero-charge’ for site selectionWhere to apply subsurface arsenic removal
II. Honestly, more research is required for more practical implications
III. Biggest implication for future research
if ion exchange mechanism is true, it has a large influence on interpretation results
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Recommendations
I. More column experimentsvarying water quality, sand materials, experiment run times
II. Verify the ion exchange mechanismMeasure Cation exchange capacity, apply cation free injection water, more sampling
III. Focus on soil chemistrydetailed surface analyses: charge, potential, surface area (BET), X-ray diffraction
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General conclusion
Subsurface treatment has a large potential for iron and arsenic removal.
Study results illustrate the theoretical possibilities under ideal circumstances
More research is required to optimize the operational efficiency in the field
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Thank you for your attention
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DE FILIPPIJNENGretha (tropen)verpleegkundigeGezondsheidstraining in communities (niet in kliniek)
Harmen drinkwater ingenieurFaciliteren bij implementatie drinkwatersysteem in dorp
Lokaal team, Filippijnse NGOFebruari 2010 - 1 tot 1.5 jaarWonen in plattelandsdorp‘onbetaald’ – op basis van giftenAvontuur, concrete vraag, drive vanuit God
Nieuwsgierig? Harmenengretha.wordpress.com
www.watervoorfilippijnen.nl
