In Situ Chemical Oxidation (ISCO):

Stephen Rosansky, P.E.

Abraham Chen, Ph.D.,P.E.

December, 2009

In Situ ^Chemical Oxidation (ISCO):

Current Advancement

Principles of Common Oxidation Processes

• The target contaminant is oxidized (loses electrons) and is transformed to a non-toxic or less-toxic

product

• Oxidation potential is a measure of the oxidative power of an oxidant. The higher the oxidation potential (volts), the greater the oxidative power.

An oxidant is a chemical that has a tendency to

accept electrons from other chemicals (preferably

target contaminants in groundwater and soil)

Oxidation Potential of Select Oxidants

0 1 2 3 4

Oxidation Potential (volts) Hypochlorous acid (HOCl)

Oxidant

Fluorine (F) 3.03

Hydroxyl radical (OH•) 2.80

Ozone (O

) 2.07

Hydrogen peroxide (H

O

) 1.78 Permanganate (MnO

) 1.68

Chlorine dioxide (ClO

) 1.57 1.49 Chlorine (Cl) 1.36

Bromine (Br) 1.09

Persulfate radical (SO

•) 2.60

Persulfate (S

O

) 2.1

ISCO Conceptual Injection Process

Injection

Well Monitoring

Well Reagent

Screened Section

Reagent is injected and occupies set volume

Spread of Reagent

Contaminated

Water Table Residual

DNAPL MnO₄

H₂ O₂ S₂ O₈

ISCO Definitions

• Can be used to treat dense, nonaqueous-phase liquid (DNAPLs) as well as dissolved-phase

contaminants

• Does not rely on biological processes

• May not require aboveground treatment, as with

pump and treat (P&T) systems, thermal heating,

and surfactant flushing

Advantages and Limitations

• Advantages

–

Can destroy contaminants in

flushing technologies)

–

Reagents relatively inexpensive (e.g., KMnO

at $1 to $1.50/lb)

Is potentially effective with

many different types of organic contaminants in sorbed and DNAPL states

–

Is cost-effective for contaminant source zones or "hot spots"

• Limitations

–

Has some handling hazard (e.g., hydrogen peroxide)

–

As with any in-situ technology, reagent delivery to the target regions may be challenging

Strong oxidants may oxidize

other (naturally occurring) reduced species in the subsurface

–

An injection permit may be required

–

May not be cost-effective for

treating very dispersed, dilute

Common Oxidant Chemistries

• Permanganate oxidation

2KMnO

+ C

HCl

Æ 2CO

+ 2 MnO

+ 2K

+ H

+ 3Cl

• Fenton’s Reagent

H

O

+ Fe

Æ Fe

+ OH• + OH

• Persulfate

3NaS

+ C

HCl

+4H

O → 9H

+ 2CO

+ 3Na + 3Cl

+ 6SO

S

O

→ 2 SO

S

O

+ Fe

→ Fe

+ SO

+ SO

Permanganate – Target Contaminants

• Permanganate has been shown to oxidize:

–

Chloroethenes (e.g., TCE)

PAHs

–

Chlorinated pesticides (e.g., aldrin and dieldrin)

High explosives

–

Some chlorophenols

• Permanganate is ineffective with:

–

Chlorinated alkanes (e.g., TCA, dichloroethane)

–

Aromatic hydrocarbons (e.g., benzene and chlorobenzene)

• MTBE – is oxidized to TBA

–

Functional group is oxidized, but not the parent structure

Permanganate Reagents

• Sodium permanganate

– Supplied as dark purple liquid (40% min. conc.)

– No concerns with dust

– More expensive than KMnO

• Potassium permanganate

– Supplied as crystalline solids – Mixed onsite

– Concerns with dust

– Derived from mined potassium ores; hence contains small

amounts of impurities, such as metals,

K, etc.

KMnO₄ Solution

Factors Affecting Permanganate Application

• Soil oxidation demand (SOD) often exceeds the oxidant demand of the COC, sometimes by 2 or 3 orders of magnitude

-

Reduced solid species (e.g., sulfides, ferrous iron minerals, etc.)

-

Natural organic matter

-

Aqueous species (dissolved iron, etc.)

• Near potential receptor (purple water)

BUSINESS SENSITIVE

Advantages and Limitations:

Fenton’s Reagent

• Advantages

– Hydroxyl free radical is much more reactive than permanganate and can therefore oxidize many more COCs

– Reactions that generate multiple free radicals destroy nearly all organic contaminants

– Ability to treat strongly sorbed and DNAPL contaminants

– Chemicals involved do not appear to contain trace impurities of

concern

– Color is not a concern

– No significant generation of solids that could clog the aquifer

• Limitations

– Peroxide and hydroxyl free radicals are extremely short-lived and this could limit distribution (reaction rate is diffusion controlled). Other

reactive species generated are more long-lived.

– Safety issues with H₂O₂

9Chemical fires and explosions 9Chemical burns

9 Reaction is highly exothermic and higher peroxide concentrations can cause steaming and volatilization of COCs

Compounds Not Reactive with Hydroxyl Radicals (OH•)

• Halogenated Alkanes

– Carbon tetrachloride

– Hexachloroethane

– Chloroform

Modified Fenton’s Reagent Applications

• Use of relatively high hydrogen peroxide concentrations

(typically 2%-12% H

O

)

• A range of different materials can catalyze the generation of free hydroxyl and other reactive radicals

– Soluble Iron (II): Most common so far, with the addition of FeSO

– Iron (III)

– Naturally occurring minerals

– Iron chelates

Modified Fenton’s Reagent:

Formation of Other Reactive Species

H₂ O₂ Fe²⁺

Fe²⁺ OH•

Fe³⁺

Fe³⁺

OH• OH^–

OH^– H₂ O₂

H₂ O₂

H₂ O₂

O₂ •^– O₂ •^–

HO₂^– H₂ O₂ H₂ O₂

HO₂^–

Superoxide Anion -reductant

-long-lived in water

-reduce CCl₄, TCE, PCE

Hydroperoxide Anion -reductant

-short-lived, recombines with water

Catalysis by Iron Chelates

• Iron-EDTA (Ethylenediaminetetraacetic Acid)

• Iron-NTA (Nitrilotriacetic Acid)

• Iron-Citrate

Advantage:

– Promote Fenton’s reactions at neutral pH

Disadvantages:

– High potential for metals mobility

– Chelate is oxidized

Catalysis by Iron Minerals

• Natural soil minerals can catalyze the reaction and form reactive radicals

• At many sites, there may not be a need to add ferrous compounds

• pH 3 to 4 required (acid addition)

• pH rebounds after treatment

• Releases carbonates as CO

• Provides highest degree of H

O

stability

• Addition of an iron catalyst not required

Factors Affecting Fenton’s Reagent Application – Presence of COCs as DNAPL

•

Evidence from the field has indicated DNAPL destruction by Fenton’s reagent

•

DNAPL destruction by Fenton’s reagent has been documented through laboratory research and occurs more rapidly than any other treatment process (up to 50x the rate of natural dissolution)

•

DNAPL destruction most likely does not involve hydroxyl radicals, but is likely superoxide

•

Even when dissolved COC concentrations do not show a significant decrease, considerable DNAPL mass may have been oxidized

– Exothermic reaction may cause higher desorption

– DNAPL destruction may cause improved advective flow and higher dissolved concentrations

Peroxide Distribution and Dosing

• Peroxide instability limits distribution

– Primary catalysts for the

unproductive decomposition of hydrogen peroxide in

subsurface are manganese oxides

• Formation of large volume of vapor

Implication : Treatability tests should be conducted to monitor

peroxide decomposition rates,

Peroxide Distribution and Dosing (Cont.)

• Optimum hydrogen peroxide concentrations are usually 0.5%-12% and are highly site specific

– Lower concentrations (0.5%-1%) are most effective when contaminants are not sorbed and DNAPLS are not present – Higher concentrations (2%-12%) are usually required to

treat sorbed and DNAPL contaminants

– Concentrations >12% are problematic because of highly

exothermic reactions and rapid decomposition of hydrogen

peroxide

Advantages and Limitations of Persulfate

• Advantages

– Sulfate free radical is much more reactive than permanganate and can therefore oxidize many more COCs

– Less oxidant demand than permanganate

– More stable than the hydroxyl free radical

– Ability to treat strongly adsorbed and DNAPL contaminants

– Color is not a concern

– Minimal safety issues (primarily dust)

• Limitations

– Requires on site mixing

– As with all oxidants, application can mobilize metals

– Newer, less understood than other reagents

– Forms sulfuric acid, can lower aquifer pH

– May degrade soft metals (copper, brass)

– Requires an activation agent for best results

Available Forms of Persulfate

• Ammonium persulfate

• Potassium persulfate

– Limited solubility in water (6%)

• Sodium persulfate

– Most commonly used

– Supplied as crystalline yellow solid, mixed on site

– Relatively high solubility in water (40%)

Persulfate Activation Methods

• Iron

– Requires low pH or chelating agent (similar to Fenton’s) – Efficiency decays with time and distance from injection – Optimum loading of 100 to 250 mg/L

– May not need to add at some sites

• Heat (steam or resistive heating)

• Alkaline (pH >10)

– sodium hydroxide

– Shown to oxidize chloromethane, chloroform, and TCA

Reagent Application Methods

• Direct injection - The reagents are injected directly into the subsurface in a specified volume of water from an external source, displacing groundwater corresponding to the volume of reagent injected

• Pull-Push: A set volume of groundwater is extracted,

amended with reagents above ground and then reinjected into the subsurface through the same well

• Recirculation: In a closed system, groundwater is extracted from a set of extraction wells, amended with the reagents and then reinjected into a different series of injection wells.

Multiple injections may be required!!!

Oxidant Application - Direct Injection

• Injection Only (currently more practiced, viewed as "easy“)

• Injection wells have to be arranged in a way that makes use of natural gradient to distribute the oxidant

• Or, use multiple temporary injection points to inject the oxidant in several locations and depths in the target treatment zone

• Higher injection pressures may be required in tighter soils (possibility of spreading the COCs)

•

Mn (discoloration) and trace metals could migrate downgradient

Oxidant Application - Recirculation

• Better hydraulic control – better distribution of oxidant, less chance of COC migration

• Reinjection may need to meet stricter guidelines

• More elaborate aboveground

equipment required

• Typically lower oxidant dosing

• Minimizes potential for

surfacing

PID Drawing ISCO System

Example System Layout

GAC Groundwater

Mix Tank Injection Well

Extraction

Well Extraction Skid

with Pumps Injection

Pump

Chemical Amendment Skids

Health and Safety Considerations

•Most safety concerns associated with application of hydrogen peroxide

•Proper personal protective equipment, including face shields, glasses, Tyvek^® aprons, reinforced-toed boots, hard hats, are worn while working in the work zone

•Chemical oxidants must be stored within secondary containment pads

•Carefully monitor temperatures and pressures

•Injections discontinued if surfacing of reagents (“daylighting”) occurs

•Seals equipped with pressure relief valves installed on all injection points and monitoring wells to control release of fluids from wells

Process Monitoring

•

Performed during application

•

Injection pressures, flowrates, and temperatures

•

Groundwater quality parameters (very useful)

•

Reagent levels in monitoring wells and extracted and injected water using real time analytical techniques

•

Aboveground destruction of ethenes (in the case of recirculation)

Target Area

Volume Extracted^(a)

Volume Injected^(b,c)

(gal)

Sodium Persulfate

Activator Solution^(b,e)

(gal) Volume (gal)

% Pore Volume^(f)

(gal)

Volume solution^(d)

(gal)

Mass (lbs)

Conc.

(g/L)

1 60,400 60 63,700 2,600 6,600 12 727

2 34,600 46 38,500 3,060 6,600 21 839

3 41,800 41 45,000 2,440 6,600 18 782

Totals 136,800 147,200 8,100 19,800 2,348

Performance Monitoring

• Performed after concluding reagent injections

• May be performed quarterly or semi- annually

• VOCs and metals

• Groundwater quality measurements (pH, ORP, DO, temperature, and conductivity)

– Is aquifer geochemistry

returning to baseline conditions?

• Groundwater levels