Post on 13-Jan-2016
description
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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