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Waste Treatment, Chemical

ENVE 649

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Why Treat Waste

• Have a RCRA Waste– TSDS– Treat instead of disposal, landfill– Treat before disposal

• Or treat in process stream– eliminate waste

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Typical Treatment

• Remove hazardous constituent from matrix

• Frequently a chemical from liquid– Solubility

• Main “chemical” techniques:– Precipitation– Neutralization– Coagulation and flocculation

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Solubility

• Polar vs. Non-polar

• Water is polar- -

= Oxygen= Hydrogen

+-

- -

+-

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N-Octane, non-polar

= Hydrogen= Carbon

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Like Dissolves Like

• Polar substances are hydrophilic– Ions are very hydrophilic

• Non-polar are hydrophobic and poorly soluble

• Some are both, ethanol

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Ethanol

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1-octanol

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Hydrophobic in water

• Remove the hydrophobic layer

• In petroleum cleanups this is the “free product”

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Neutralization

• Watery wastes• May make non-hazardous directly• Makes waste amenable to other processes

• pH = -log[H+]• pH = 7, neutral• pH > 7, base or “alkaline”• pH < 7, acid

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Neutralizing

• Acid + Base = Salt +Water

• For acid water, use bases

– soda ash Na2CO3

– caustic soda NaOH

– slaked lime Ca(OH)2

• For alkaline water, use acids

– H2SO4, HCL, CO2

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Weak Acids and Bases

• Chemistry of weak acids

• Strong acids are 100% ionized

• The ionization of weak acids depends on pH

• Most organic acids are weak

• Changes in pH may change solubility

• H-Aweak in low pH (acid) solution

• Aweak- (anion) in high pH (basic) solution

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Oxidants

• Chlorine Cl2

• Ozone, O3

• Hydrogen Peroxide, H2 O 2

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Precipitation

• Not all salts are soluble

• Some metals (Pb) form insoluble hydroxides as high pH (alkaline)

•

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Solubility of copper

0.00010.001

0.010.1

110

100

6 7 8 9 10 11 12

pH

Lo

g1

0 s

olu

bili

ty

mg

/L Solubility ofcopper

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Precipitation• Temperature is important• Oxygen content• Valence state of metal• Example• Raw well water has Fe (II) or ( Fe++ or Ferrous)in

water as Fe(OH)2 which is soluble

• But at surface Fe++ goes to Fe+++ or Ferric)which forms Fe(OH)3 which is insoluble

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Sulfide formation

• Add Na2S or NaHS -> S--

• Metal, M+++ S-- -> metal sulfide, MS

• Most metal sulfides are insoluble

• (at same pH where metal hydroxide is soluble)

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Leaching

• Opposite of precipitation

• CN forms complexes Fe+++(CN-)6

• But also gold (Au)

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Size Scales

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• Solids– Evaporate water and get– Total Solids– Filter for Suspended Solids

• Define, 1 micron filter typically

– Also, Imhoff cone• What settles in 60 minutes

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Colloids

• Very small

• typically charged and will not agglomerate

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+++

++++

++++

++++

+

+++

+++

Stable Suspended Particles

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+++

++++

++++

++++

+

+++

+++

Add Flocculent and mix rapidly

----

--

--

--

- --

--

-

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++

+++

++

++++

++

+++

Micro-flocs

----

--

--

--

- --

--

-

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++

+++

++

++++

++

+++

Flocs aggregate

----

--

--

--

- ---

-

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Most common coagulants

• Inorganic

• Alum Al2(SO4)3

• Ferric chloride FeCl3

• Ferric sulfate Fe2(SO4)3

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Organic polymers

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• Oil-water emulsion– emulsion breakers

Stable Emulsion Flocculation

Creaming

Breaking

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Ion Exchange

• Water softener

• “Zeolites”

• Remove low level metals

• recharge cycles

• Can’t use if suspended solids, organic material, oxidants