14. Hazardous Waste Treatment

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Hazardous Waste

Treatment

Chemical TreatmentPhysical/Chemical Treatment


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Chemical Treatment It does not make the hazardous substances

disappear

It converts the hazardous substance into to

another form that is at least less toxic than

the original substance


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Chemical TechniquesNEUTRALIZATION

OXIDATION

PRECIPITATION

REDUCTION


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NEUTRALIZATION A waste is considered hazardous if,

among other things, the pH is less

than 2 or greater than 12.5

Good treatment practice requires that

the final pH should be at 6-8 toprotect the natural biota


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Sulfuric (H2SO4) or hydrochloric

(HCl) acid is added to basic solutions

to reduce the pH

Caustic (NaOH) or slaked lime

[Ca(OH)2] is added to acidicsolutions to increase pH

NEUTRALIZATION


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OXIDATION Via the use of chlorine and ozone

as oxidizing agents

Via electrolytic oxidation

Via wet air oxidation (also known

as the Zimmerman Process)


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Wet Air Oxidation It is potentially the most widely

applicable of all chemical oxidation

methods

It operates on the principle that most

organic compounds can be oxidizedby oxygen given sufficienttemperature and pressure


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It is the aqueous phase oxidation of

dissolved or suspended organic particles at

temperatures of 175 to 325C and

sufficiently high pressure to preventnt

excessive evaporation

Air is bubbled through the liquid

Wet Air OxidationWet Air Oxidation


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The process is fuel efficient

Once oxidation has begun, the

process is usually self-sustaining

In many instances, the addition ofmetal salt catalysts can increase the

destruction efficiency or allow theprocess to run at lower temperatureor pressure

Wet Air Oxidation


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PRECIPITATION The goal of precipitation is to remove

hazardous substances from solution by

reducing the solubility and precipitating the

substances

This is often applied to metals


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REDUCTION

Although most heavy metals readily

precipitate as hydroxides, hexavalentchromium (Cr+6) must be reduced to its

trivalent form (Cr+3) before it will

precipitate

Reduction is usually done with sulfur

dioxide (SO2) or sodium sulfite (NaHSO3)


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3SO2 + 2H2CrO4 Cr2(SO4)3 +2H2O

Because the reaction proceeds rapidly at

low pH, an acid is added to control the

pH

REDUCTION


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Chemical/Physical Treatment The waste is not detoxified but only

concentrated for further treatment or

recovery

Several treatments are used to separate

hazardous waste from aqueous solution.


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Chemical/Physical Techniques CARBON ADSORPTION

DISTILLATION ION EXCHANGE

ELECTRODIALYSIS

REVERSE OSMOSIS

SOLVENT EXTRACTION


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Adsorption is a mass-transfer process in

which gas vapors or chemicals in solution

are held to a sold by intermolecular forces

(e.g. hydrogen bonding and van der

Waals interactions)

It is a surface phenomenon

CARBON ADSORPTION


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Pressure vessels having a fixed bed are

used to hold the adsorbent

Common adsorbents:

o Activated carbon

o Molecular sieves

o Silica gel

o Activated alumina

CARBON ADSORPTION


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The active sites become saturated at

some point in time

If the adsorbed organic material hascommercial value the bed is

regenerated by passing stream through

it

The vapor-laden stream is condensed

and the organic fraction is separated

from water

CARBON ADSORPTION


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If the organic compounds have no

commercial value, the carbon may either be

incinerated or shipped to the manufacturer

for regeneration

CARBON ADSORPTION


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It is the separation of more volatile

materials from less volatile ones by a

process of vaporization When a liquid mixture of two or more

components is brought to the boiling

point of the mixture, a vapor phase is

created above the liquid phase

DISTILLATION


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If the vapor pressure of the purecomponents are different then the

constituents are different having thehigher vapor pressure will becomemore concentrated in the vapor phasethan the constituents having the lowervapor pressure

If the vapor phase is cooled, a partialseparation of the constituents willresult

DISTILLATION


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ION EXCHANGE In ion exchange, the waste stream

containing the ion to be removed is

passed through a bed of resin The resin is selected to remove either

cations or anions

In the exchange process, ions of likecharge are removed from the resin

surface in exchange for ions in solution


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When the bed becomes saturatedwith the exchanged ion, it is shut

down and the resin is regenerated bypassing a concentrated solutioncontaining the original ion (typicallyhydrogen or sodium) back throughthe bed

The exchanged pollutant is forcedoff the bed in a concentrated form

that can be recycled

ION EXCHANGE


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During the ion exchange, the normal

flow pattern is downward through the

bed Because the surface of the bed acts like a

filter, the regeneration is often

countercurrent, that is, the regeneration

solution is pumped into the bottom of

the column and the flow is upward

ION EXCHANGE


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The electrodialysis unit uses amembrane to selectively retain or

transmit specific molecules The membranes are thin sheets of

ion exchange resin reinforced by a

synthetic fiber backing Cation membranes are placed

alternate with anion membranes

ELECTRODIALYSIS


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An electric potential is applied across

the ion to provide the motive force for

ion migration

Cation membranes permit passage of

only positively charged ions, while

anion membranes only allownegatively charge ions to pass

ELECTRODIALYSIS


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Osmosis is the spontaneoustransport of a solvent from a dilute

solution to a concentrated solutionacross an ideal semipermeablemembrane that impedes thepassage of the solute but allows thesolvent to flow

Solvent flow can be reduced byexerting pressure on the solution

side of the membrane

REVERSE OSMOSIS


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If the pressure is increased above the

osmotic pressure on the solution

side, the flow reverses

Pure solvent will then pass from the

solution into solvent

REVERSE OSMOSIS


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Also called liquid extraction

In the solvent extraction process,

the solvent and the waste stream are

mixed to allow mass transfer of the

constituents form the waste to the

solvent

SOLVENT EXTRACTION


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The wastewater is contacted with a

solvent having a greater solubility

for the target contaminants than thewastewater

The contaminants will tend migrate

from the wastewater into the solvent

SOLVENT EXTRACTION

14. Hazardous Waste Treatment

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