Zero-valent iron (ZVI) removal of
†
arsenic in a spouted vessel
J.M. Calo
(JMCalo@brown.edu)
J. Kirchner, L. Madhavan, and E.J. Bain
Brown University
School of Engineering
Providence, RI 02912, USA
Arsenic - Health and Remediation Applications
NIEHS Webinar, April 15, 2013
†
Project 5, Brown University Superfund Research Program, NIEHS
1
Overview
 What is a spouted bed?
 Spouted bed applications to heavy metals removal
 Our recent related work with arsenic removal
 Arsenic ZVI in a hybrid spouted bed/filter system
- Experimental
- Batch experiment results
- Continuous flow system results
- Results of arsenic ZVI in a hybrid spouted bed/filter
- Summary
- Continuing ZVI work
 Conclusions
2
What is a Spouted Bed?
“Freeboard”
“Fountain”
Dense
Moving
Bed
Dilute,
Entrained
“Spout”
Inlet Jet
• A fluid-solid contacting device in which internal circulation of a bed of particles
is maintained by imparting momentum with a flowing fluid
- Particles are entrained upwards in the “spout” and move downwards by gravity
in the peripheral annulus
- “Freely spouted” – no vessel internal - fluid jet “carves out” channel(s) through
dense bed of particles and entrains them upwards
- “Draft tube or duct” – particles entrained by a fluid jet that enters a
central tube or duct which stabilizes the flow
3
Spouted Bed Applications to Heavy Metals Removal
Spouted Particulate Electrodes (SPE)
 Metal electrowinning
- Direct electrochemical reduction of metal ions to solid metal on
cathodic particles using feeder current
- Typically necessitates the use of additional internal elements: inverted
cone distributor, an opposing anode (or cathode), etc.
- Initially applied to “point source” recovery of single metals present at
relatively high concentrations
- Key element of our Cyclic Electrowinning/Precipitation (CEP) system
for removal of low concentration, heavy metal mixtures
 Advantages
- High current efficiencies on active particles
- High quality of deposited metal due to randomized particle
motion/mechanical action
- Non-fouling particulate electrode (self-cleaning)
- Hydrodynamic particle introduction/withdrawal for continuous
operation
4
Spouted Particulate Electrode (SPE)
5
Spouted Particulate Electrode (SPE)
6
Cyclic Electrowinning/Precipitation (CEP) System
(Chemical Engineering Journal 2011, 175, 103–109)
100
Cu
Ni
Cd
Metal Ion Concentration /ppm
80
60
40
20
0
0
200
400
600
800
1000
Time/min
• CEP System
- Designed to remove multiple metal contaminants from aqueous solutions at low
concentrations
- Automated system that combines electrowinning on cathodic spouted particles with
in-process precipitation/redissolution of toxic precipitate
- Metals removed on solid particles; precipitate formed and redissolved cyclically
within the process to produce final effluent clean water
- Results also show that it is possible to separate and recover single metals from
mixtures on specific circulating particle sets
7
ZVI Contactor
 ZVI Contactor (Hybrid Spouted Vessel/Fixed Bed Filter System)
Corrosion on circulating ZVI particles continuously generates colloidal iron
corrosion products via “self-polishing” action in moving bed on conical vessel
bottom
- Intrinsic “surface renewal” mechanism continuously exposes fresh ZVI
material for good utilization
- Allows facile for facile hydrodynamic introduction/withdrawal of ZVI
particles for continuous operation
-
8
Our Recent Related Work With Arsenic
Arsenic Electrosorption/Electrodesorption on Carbons
(Energy & Fuels, 2010, 24(6), 3415–3421)
 Electrosorption - adsorption
under the influence of an electric
field
Effective for removal of heavy
metal anions (As, Cr)
- Increases adsorption rate and
sorbent capacity
- Reversing the field desorbs As
quantitatively (electrodesorption) provides for adsorbent
regeneration
-
Fe, etc.) can also enhance As
removal via various mechanisms
- electro-precipitation,
complexation, etc., on carbon
adsorbent active sites
9
As Electroadsorption/Electrodesorption
110
100
90
As (µg/l)
 Co-contaminant metals (e.g., Cr,
Electrochemical Cell
80
70
Darco GAC (w/o field)
1.25 V Anodic
1.50 V Anodic
1.50 V Cathodic
60
50
0
20
40
60
Time (h)
80
100
120
9
Cyclic Arsenic Removal on a Carbon Electrode
(Electrochimica Acta, 2009, 54, 3996–4004)
• Arsenic redox on porous
carbon electrode deposited on
a quartz crystal microbalance
(continuous electrode mass
measurement)
• Observed increase in arsenic
uptake with each cycle
- Due to creation of additional
active carbon sites for As
adsorption/reduction produced
by carbon electrogasification
over redox cycle
- Cyclic adsorption/desorption could
be used to increase adsorbent
capacity and effectiveness (during
reduction), as well as providing
for adsorbent regeneration
(during oxidation)
10
10
Zero-Valent Iron (ZVI) Removal of Arsenic
(Chem. Eng. J. 2012, 189 – 190, 237– 243)
• Removal of aqueous arsenic species with zero-valent
iron (ZVI) is a well-known, abiotic process that occurs
on ZVI materials as they corrode in water
- Arsenic species removed irreversibly from solution via a
mechanism involving adsorption, co-precipitation, and
surface complexation with ZVI-generated iron oxides,
hydroxides, and oxide-hydroxide corrosion products
- Some typical problems include:
+ On colloidal corrosion products - relatively low rates due to low
encounter frequencies between arsenic species and colloidal
particles in solution
+ On bulk ZVI - limited ZVI material utilization due to diffusion
barrier formation
11
Experimental
 Iron particles - Grade 100, 1/8” (3.175 mm) diameter, carbon steel
spheres (98.2 - 99.2 wt % iron) - Salem Specialty Ball Company, Inc.
 Cylindrical (5 in. long, 2.5 in. O.D.) 20 µm (FOS-398 304) stainless steel,
in-line filter (ISC Sales, Inc.) installed in solution reservoir of spouted
vessel system
 Arsenic solutions prepared as 1000 mg/L standard solutions of either
As(III) or As(V)
As2O3 standard prepared with sodium hydroxide, and then subsequently
acidified with nitric acid (TraceSELECT)
+ Resultant As(III) 30 mg/L stock solution was buffered at pH 4 with
potassium hydrogen phthalate (KHP) and sodium hydroxide (1.492 g/L
KHP, 8 mg/L NaOH)
- As(V) 30 mg/L stock solution, including 1.5 g/L of NaCl, was prepared from
the 1000 mg/L As(V) standard solution initially made up from 99.9%
(arsenic basis) arsenic (V) pentoxide (Alfa Aesar)
-
 Total arsenic concentrations determined with Perkin Elmer 4100ZL
Zeeman Effect Graphite Furnace Atomic Absorption Spectrometer
(GFAAS)
12
Batch Experiment Results
• To simulate ZVI particle behavior in
packed column (immobilized) and
spouted vessel (agitated)
1.2
•
Results
- Immobilized: no arsenic removal
initially; no visible corrosion products
on particles until ~ 198.5 h; ultimately
~10% arsenic removal
- Agitated: arsenic removal ~45% in 24
h; corrosion products visible after ~ 1 h
• Conclusions
- Apparent arsenic removal kinetics
orders of magnitude more rapid for the
agitated samples
- Results consistent with “surface
renewal” mechanism hypothesis
Relative Arsenic Concentration, [As]/[As]
solution
- Immobilized: spheres in PP mesh bags
suspended in stirred solution
- Agitated: spheres in sample tubes
mounted on mechanical rotator
0
- 100 ppb As(III) in pH 4 buffered
1
0.8
0.6
0.4
0.2
0
Unagitated
Agitated
Estimated Error Bounds
0
2
4
6
8
10
Time/h
13
Continuous Flow System Experiment Results
• Packed column and spouted vessel
contactors
- Packed column – ¾” diameter tube
- Each charged with same amount (270.1 g) of
ZVI spheres
• Results
- Particles in spouted vessel exhibit about an
order of magnitude greater mass loss than in
comparable fixed bed experiments; i.e., 24.3
vs. 2.5 g over a 10 h run (pH 4)
 Conclusions
- Packed column promotes the formation of a
heavy coating of corrosion products on ZVI
sphere surfaces; eventually occluding liquid
flow in the column
- Spouted vessel promotes the formation of
colloidal corrosion products in the bulk
liquid phase due to the self-polishing surface
renewal mechanism
- Results consistent with “surface renewal”
mechanism hypothesis
14
Arsenic ZVI in a Hybrid Spouted Bed/Filter System
• Arsenic removal by ZVI in a spouted vessel
- “Rust” (iron oxides and oxyhydroxides) is generated on circulating
ZVI particles
- “Self-polishing” action of the particles in the dense moving bed on
the conical vessel bottom, continuously removes surface rust,
exposing fresh ZVI to corrosion
- “Surface renewal” mechanism creates a continuous source of active
“rust” generation
• Multiple reservoirs of corrosion products in the spouted
vessel system:
- On surfaces of circulating primary ZVI particles
- As colloidal corrosion products in the bulk liquid phase
- As colloidal corrosion products adsorbed onto vessel surfaces
- The observed arsenic removal behavior is dependent on the relative
importance of the various corrosion product “reservoirs”
15
Arsenic ZVI in Spouted Vessel (w/o Filter)
Relative Arsenic Concentration, [A*]/[A*]
0
1.2
1
0.8
0.6
0.4
0.2
0
10 µg/L
0
100
200
300
400
500
600
Time/min
16
Arsenic ZVI in Spouted Vessel With Filter)
Relative Arsenic Concentration, [A*]/[A*]
0
1.2
1
0.8
0.6
0.4
0.2
0
10 µg/L
0
100
200
300
400
500
600
Time/min
17
Comparison of Arsenic Removal Rates
Relative Arsenic Concentration, [A*]/[A*]
0
1.2
Data (w/filter)
Data (w/o filter)
Fit (w/filter)
Fit (w/o filter)
1
0.8
0.6
0.4
0.2
10 µg/L
0
0
100
200
300
400
500
600
Time/min
18
Summary of Results
• Arsenic was removed, but only very slowly, with just the
ZVI particles circulating in the spouted vessel
- From 100 µg/L, the As concentration still exceeded 10 µg/L (ppb)
after 10 h
- A numerical bimodal fit to the data indicated approximately half the
arsenic was complexed by iron corrosion products adsorbed onto
vessel surfaces (faster mode), and the other half by colloidal corrosion
particles circulating in the bulk liquid solution (slower mode)
• Operation with the filter installed on one of the two
reservoir drain lines dramatically accelerated arsenic
removal to below the USEPA MCL in less than one hour
- Kinetic behavior of arsenic concentration controlled by increased
“encounter frequency” of arsenic species with colloidal corrosion
particles concentrated in the filter
19
Continuing ZVI Work
 Extension of spouted bed ZVI
to removal of multiple heavy
metals - As, Hg, Cr, etc.
 Scoping batch experiments
conducted with Hg and As and
different iron and iron oxide
materials
Fe3O4
Fe2O3
ZVI
20
Continuing ZVI Work – Some Results
 Arsenic Removal
- Rates of As(III) and As(V) removal considerably greater
on iron oxides than ZVI
- Removal of As is more pH dependent for ZVI than the
oxides – viz., coupled to corrosion rate
 Hg(II) Removal
− ZVI material significantly outperformed Fe2O3 and
Fe3O4 in 500 ppm NaCl and Na2SO4 solutions
+ Equilibrium uptake: 52 and 26 mg Hg/g, respectively
- Effective rates considerably greater on ZVI as well
- Large differences in removal capacity of ZVI, Fe3O4
and Fe2O3 indicate that Hg reduction-precipitation
plays a major role in the mechanism
21
Conclusions
 The hybrid spouted vessel/fixed bed filter system was
demonstrated to be very effective for arsenic removal from
aqueous solutions at low concentrations (e.g., to meet
drinking water standards)
- Effective treatment times can be reduced by one or two orders of
magnitude, proportional to the increase in the effective water
treatment rate
- This performance can be improved upon, since the current system,
as tested, has not been optimized with respect to the filter properties
or the geometry/operating conditions of the spouted vessel system
 The novel hybrid nature of the system lies in its dual-
function character, whereby colloidal material is
continuously generated in the moving bed on the spouted
vessel bottom, and arsenic removal/complexation occurs in
the fixed bed filter
22
Conclusions
This method of generating colloidal corrosion products maximizes
utilization of ZVI material in comparison to other methods, such as
with fixed bulk iron sources
- Operation of the hybrid system is simple and cost effective
-
+ Requires little attention - periodic backwashing of the filter to remove
collected colloidal material
+ With the current system under the operating conditions tested, the filter
reaches capacity after about 28 h on stream
This is sufficient to remove essentially all the arsenic at an initial
concentration of 100 µg/L to less than detectable limits (<<10µg/L) from
about 800 L or 213 gal of water, or 7.8 gal/h, utilizing only about 7.5% of
the initial charge of ZVI material in a relatively small device
-
ZVI particles can be introduced hydrodynamically as required
-
ZVI particles do not have to be continuously recirculated – periodic or
pulsed circulation
-
The system can be readily scaled-up (e.g., with greater filter
and liquid capacities, etc.)
23