SAB 4973: HAZARDOUS WASTE TREATMENT TECHNOLOGIES

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SAB 4973HAZARDOUS WASTE TREATMENT TECHNOLOGIES
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Technologies

• Chemical methods
• Coagulation, flocculation, combined with
  flotation and filtration, precipitation, ion
  exchange, electroflotation, electrokinetic
  coagulation.
• Physical methods
• Membrane-filtration processes (nanofiltration,
  reverse osmosis, electrodialysis, . . .) and
  adsorption techniques.
• Biological treatments
• Biodegradation methods such as fungal
  decolorization, microbial degradation, adsorption
  by (living or dead) microbial biomass and
  bioremediation systems

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Advantages and disadvantages

• Chemical methods
• Advantages
• Rapid and efficient process
• Removes all pollutants types, produce a
  high-quality treated effluent
• No loss of sorbent on regeneration and effective
• Disadvantages
• Expensive, and although the pollutants are
  removed, accumulation of concentrated sludge
  creates a disposal problem
• High energy cost, chemicals required.

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Advantages and disadvantages

• Physical methods
• Advantages
• The most effective adsorbent, great, capacity,
  produce a high-quality treated effluent
• No sludge production, little or no consumption of
  chemicals.
• Disadvantages
• Economically unfeasible, formation of
  by-products, technical constraints

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Advantages and disadvantages

• Biological treatments
• Advantages
• Economically attractive, publicly acceptable
  treatment
• Disadvantages
• Slow process, necessary to create an optimal
  favorable environment, maintenance and nutrition
  requirements

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COAGULATION

• Definition
• Destabilisation of colloid particles by the
  addition of chemicals (coagulant)
• Applications
• Industrial waste containing colloidal and
  suspended solids (e.g. pulp and paper, textile)

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Coagulant type

• Metal coagulants aluminium-based coagulants,
  Fero-based coagulants magnesium chloride (MgCl2)
• Organic polymer coagulants Polyacrylamide,
  Chitosan, Moringa olifeira Alginates (brown
  seaweed extracts)

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Coagulant agent
Alum
Magnesium chloride
Polyacrylamide
Moringa oleifera
Chitosan
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Coagulant - Reaction

• Some of the coagulants used include
• Aluminium sulphate
• Ferric chloride
• Ferric sulphate
• Lime (not true coagulant)
• Polymer as coagulant aid eg cationic, anionic,
  non-ionic.
• PAC new types
• Al2(SO4)3.18H20 3Ca(HCO3) 2AI(OH)3
  3CaSO4 6C02 18H20
• AI(OH)3 or Al2O3 ( form as floc is the key
  element causing destabilisation of charge).

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Raw waste Floc Formation Settle floc
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Flocculation

• is a process of forming aggregate of flocs to
  form larger settleable particle. The process can
  be described as follows
• Mutual collision of small floc resulting in
  bigger size.
• Usually slow speed or gentle mixing is used so as
  not to break the large flocs due to shear.
• Polymer or large molecular wt compound is added
  to enhance floc build up. Most of them are
  proprietary chemicals.

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Flocculation mechanism
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Flocculation mechanism
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Flocculation mechanism
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Flocculation

• The benefits of flocculation are
• To improve settling of particles in sedimentaion
  tank
• To increase removal of suspended solids and BOD
• To improve performance of settling tanks

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Differences

• Coagulation is a chemical technique which is
  directed towards the destabilisation of the
  charged colloidal particals.
• Flocculation is the slow mixing technique which
  promotes the agglomeration of the stabilised
  particles.

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CHEMICAL PRECIPITATION

• Definition
• Removal of metal ions from solution by changing
  the solution composition, thus causing the metal
  ions to form insoluble metal complexes.

insoluble complexes
clean Water
chemical reaction
solution with soluble ions

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Natural methods of precipitation include settling
or sedimentation, where a solid forms over a
period of time due to ambient forces like gravity
or centrifugation
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CHEMICAL PRECIPITATION(Applications)

• Removal of metals from waste stream
• e.g. plating and polishing operations, mining,
  steel manufacturing, electronics manufacturing
• include arsenic, barium, chromium, cadmium, lead,
  mercury, silver
• Treatment of hard water removal of Mg2 and
  Ca2
• Phosphorus removal
• Making pigments
• Removing salts from water in water treatment

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CHEMICAL PRECIPITATION(Theoretical Background)

• Solubility equilibria
• A chemical reaction is said to have reached
  equilibrium when the rate of forward reaction is
  equal to the rate of the reverse reaction
• ABs ? A B-
• where ABs solid A, B- - ionic species

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CHEMICAL PRECIPITATION(Theoretical Background)

• Due to dilute concentration,
• Ksp A B-
• solubility product constant
• where refer to molar concentration
• Eg.

Compound Solubility (mg/L) Ksp
CaCO3 18 5 x 10-9
CaCl 745000 159 x 106
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CHEMICAL PRECIPITATION(Basic Principles)

1. Add chemical precipitants to waste stream
2. Mix thoroughly
3. Allow solid precipitates to form floc by slow
   mixing
4. Allow floc to settle in clarifier

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CHEMICAL PRECIPITATION(Types of Precipitation)

• Heavy metals removal
• Hydroxide precipitation (OH-)
• Sulphide precipitation (S2-)
• Carbonate precipitation (CO32-)
• Phosphorus removal
• Phosphate precipitation (PO42-)

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CHEMICAL PRECIPITATION(Hydroxide Precipitation)

• Add lime (CaO) or sodium hydroxide (NaOH) to
  waste stream to precipitate heavy metals in the
  form of metal hydroxides.
• Cd2 Ca(OH)2 ? Cd (OH)2 ? Ca2
• CaO in the form of slurry (Ca(OH)2) while NaOH in
  the form of solution.
• NaOH is easier to handle but is very corrosive.
• Will form floc and settle in clarifier

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CHEMICAL PRECIPITATION(Sulphide Precipitation)

• Use of sulphide in the form of FeS, Na2S or NaHS
• Better metal removal as sulphide salt has low
  solubility limit
• Cu2 FeS ? CuS ? Fe2
• Limitation can produce H2S (g) at low pH
• 2H FeS ? H2S Fe2
• At low pH, reaction will proceed to the right.
  Thus, require pH gt 8 for safe sulphide
  precipitation.

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CHEMICAL PRECIPITATION
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Reaction rate

• Reaction rate is a measure of how fast a reaction
  occurs, or how something changes during a given
  time period.
• Consider the oxidation of glucose, C6H12O6
• C6H12O6(s) 6 O2(g) ? 6 CO2(g) 6 H2O(g)
• One of the things that happens during this
  reaction is simply that glucose gets used up as
  it reacts with oxygen in the air, and carbon
  dioxide and water start to form.

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• A common measure of reaction rate is to express
  how the concentration of a reaction participant
  changes over time. It could be how the
  concentration of a reactant decreases, or how the
  concentration of a product increases. This is the
  standard method we will be using.
• Now that we have something that changes to
  measure, we must consider the second key aspect
  of determining rate - time. Rate is a measure of
  how something changes over time.
• Change in concentration
• Change in time

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Chemistry Notation

• In chemistry, we typically represent
  concentration by using square brackets around the
  chemical formula of the substance. For example to
  indicate the concentration of SO2(g) in the
  following reaction we would write it as SO2.
• Also, the delta symbol, ? is used to indicate a
  change. ?T, for example, means "the change in
  temperature."
• Therefore, if we wanted to express the rate of
  the following reaction
• SO2(g) NO2(g) ? SO3(g) NO(g)

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• Let's try an example of calculating a reaction
  rate. Consider the following reaction
• A ? B
• The following data were obtained for how the
  concentration of these substances changed during
  the experiment.
• Time A B
• (min) mol/L mol/L
• 0.0 1.000 0.000
• 3.0 0.400 0.600
• 6.0 0.250 0.750

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We could measure the rate of the reaction either
by measuring how the concentration of reactant A
changes or how the concentration of product B
changes. Let's measure A's average rate of change
first



Compare this rate to the rate of just the first
three minutes of the reaction
If we calculate the average rate based on the
production of product B
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Factors that Affect the Chemical Reaction Rate

• Concentration of Reactants
• A higher concentration of reactants leads to more
  effective collisions per unit time, which leads
  to an increasing reaction rate (except for zero
  order reactions).
• Temperature
• Usually, an increase in temperature is
  accompanied by an increase in the reaction rate.
  Temperature is a measure of the kinetic energy of
  a system, so higher temperature implies higher
  average kinetic energy of molecules and more
  collisions per unit time.

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Factors that Affect the Chemical Reaction Rate

• Medium
• The rate of a chemical reaction depends on the
  medium in which the reaction occurs. It may make
  a difference whether a medium is aqueous or
  organic polar or nonpolar or liquid, solid, or
  gaseous.
• Presence of Catalysts and Competitors
• Catalysts (e.g., enzymes) lower the activation
  energy of a chemical reaction and increase the
  rate of a chemical reaction without being
  consumed in the process. Catalysts work by
  increasing the frequency of collisions between
  reactants, altering the orientation of reactants
  so that more collisions are effective, reducing
  intramolecular bonding within reactant molecules,
  or donating electron density to the reactants.

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OXIDATION

• a method by which wastewater is treated by using
  oxidizing agents.
• Generally, two forms viz.
• Chemical oxidation and
• UV assisted oxidation using chlorine, hydrogen
  peroxide, fentons reagent, ozone, or potassium
  permanganate are used for treating the effluents,
  especially those obtained from primary treatment
  (sedimentation)

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CHEMICAL OXIDATION(Oxidants)

• Rapid and efficient process
• High energy cost, chemicals required

REDOX Oxidation and reduction in terms of oxygen
transfer Definitions Oxidation is gain of
oxygen. Reduction is loss of oxygen.

• Fe2O3 3CO ? 2Fe 3CO2

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Another definition

• Oxidation and reduction in terms of hydrogen
  transfer
• These are old definitions which aren't used very
  much nowadays. The most likely place you will
  come across them is in organic chemistry.
• Definitions
• Oxidation is loss of hydrogen.
• Reduction is gain of hydrogen.
• CH3CH2OH ?CH3CHO
• Oxidation by loses of hydrogen

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Another definition

• Oxidation and reduction in terms of electron
  transfer
• This is easily the most important use of the
  terms oxidation and reduction at A' level.
• Definitions
• Oxidation is loss of electrons.
• Reduction is gain of electrons.
• OIL RIG ? oxidation is loss, reduction is gain
• CuO Mg ? Cu MgO
• Cu2 Mg ? Cu Mg2

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OXIDATION STATES (OXIDATION NUMBERS)

• Oxidation state shows the total number of
  electrons which have been removed from an element
  (a positive oxidation state) or added to an
  element (a negative oxidation state) to get to
  its present state.
• Oxidation involves an increase in oxidation state
• Reduction involves a decrease in oxidation state

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Some elements almost always have the same
oxidation states in their compounds

• Group 1 metals always 1
• Group 2 metals always 2
• Oxygen usually -2 except in peroxides and F2O
• Hydrogen usually 1 except in metal hydrides
  where it is -1
• Fluorine always -1
• Chlorine usually -1 except in compounds with O
  or F

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Example 1

• This is the reaction between magnesium and
  hydrochloric acid or hydrogen chloride gas
• Mg 2HCl ?MgCl2 H2
• 0 1 -1 2 -1 0
• The magnesium's oxidation state has increased -
  it has been oxidised. The hydrogen's oxidation
  state has fallen - it has been reduced. The
  chlorine is in the same oxidation state on both
  sides of the equation - it hasn't been oxidised
  or reduced.

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Example 2

• The reaction between sodium hydroxide and
  hydrochloric acid is
• NaOH HCl ? NaCl H2O
• 1 -2 1 1 -1 1 -1 1 -2
• Nothing has changed. This isn't a redox reaction.

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Example 3

• The reaction between chlorine and cold dilute
  sodium hydroxide solution is
• 2NaOH Cl2 ? NaCl NaClO H2O
• 1 -2 1 0 1 -1 1 1 -2 1 -2
• One atom has been reduced because its oxidation
  state has fallen. The other has been oxidised.

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Symbols
European Union chemical hazard symbol for
oxidizing agents
Dangerous goods label for oxidizing agents
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Common oxidizing agents

• Hydrogen peroxide and other inorganic peroxides
• Nitric acid and Nitrates
• Chlorites, chlorate, perchlorate, and other
  analogous halogen compounds
• Hypochlorite and other hypohalite compounds such
  as bleach
• Fluorine and other halogens
• Ozone
• Nitrous oxide(N2O)
• Silver oxide
• Permanganate salts

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Hydrogen peroxide

• In acidic solutions H2O2 is one of the most
  powerful oxidizers knownstronger than chlorine,
  chlorine dioxide, and potassium permanganate.
• Also, through catalysis, H2O2 can be converted
  into hydroxyl radicals (.OH), which are highly
  reactive.
• H2 O2 ? H2O2
• It is used as a disinfectant, antiseptic,
  oxidizer, propellant in rocket. Hydrogen peroxide
  is naturally produced in organisms as a
  by-product of oxidative metabolism. Nearly all
  living things (specifically, all obligate and
  facultative aerobes) possess enzymes known as
  peroxidase.

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Nitric acid

• Nitric acid is made by reacting nitrogen dioxide
  (NO2) with water.
• 3 NO2 H2O ? 2 HNO3 NO
• Nitric acid reacts with most metals.
• 3 Cu 8 HNO2 ? 3 Cu2 2 NO 4 H2O 6 NO3-
• Cu 4 H 2 NO3-? Cu2 2 NO2 2 H2O

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ION EXCHANGE

• Definition
• Ion exchange is basically a reversible chemical
  process wherein an ion from solution is exchanged
  for a similarly charged ion attached to an
  immobile solid particle.
• Removal of undesirable anions and cations from
  solution through the use of ion exchange resin
• Applications
• Water softening
• Removal of non-metal inorganic
• Removal or recovery of metal

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ION EXCHANGE(Medium - resin)

• Consists of an organic or inorganic network
  structure with attached functional group
• Synthetic resin made by the polymerisation of
  organic compounds into a porous three dimensional
  structure
• Exchange capacity is determined by the number of
  functional groups per unit mass of resin

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ION EXCHANGE(Type of Resin)

1. Cationic resin - exchange positive ions
2. Anionic resin exchange negative ions

(a)
(b)
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ION EXCHANGE(Exchange Reactions)

• Cation exchange on the sodium cycle
• Na2 R Ca2 ? Ca R 2Na
• where R represents the exchange resin. When all
  exchange sites are substantially replaced with
  calcium, resin is regenerated by passing a
  concentrated solution of sodium ions (5-10)
  through the bed
• 2Na Ca R ? Na2 R Ca2

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ION EXCHANGE(Exchange Reactions)

• Anion exchange replaces anions with hydroxyl
  ions
• SO42- R (OH)2 ? R SO4 2OH-
• where R represents the exchange resin. When all
  exchange sites are substantially replaced with
  sulphate, resin is regenerated by passing a
  concentrated solution of hydroxide ions (5-10)
  through the bed
• R SO4 2OH- ? SO42- R (OH)2

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ION EXCHANGE(Basic Principles)
H, CN-
H, OH-
Clean water
Cation Resin
Anion Resin
Cr3, CN-
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ION EXCHANGE(Selectivity)

• Cations
• Ra2 gt Ba2 gt Sr2 gt Ca2 gt Ni2 gt Cu2 gt Co2 gt
  Zn2 gt Mn2 gt Ag gtCs gt K gt NH4 gt Na gt Li
• Anions
• HCRO4- gt CrO42- gt ClO4- gt SeO42- gt SO42- gt NO3-
  gt Br- gt HPO4- gt HAsO4- gt SeO32- gt CO32- gt CN- gt
  NO2- gt Cl- gt H2PO4-, H2AsO4-, HCO3- gt OH- gt
  CH3COO- gt F-
• Note The least preferred has the shortest
  retention time, and appears first in the effluent
  and vice versa for the most preferred.

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Ion exchange-electrochemistry

• During redox reactions, electrons pass from one
  substance to another. Electrochemistry is the
  branch of chemistry that deals with the
  conversion between chemical and electrical
  energy.
• The fact that different substances are oxidized
  more readily than others is the driving force
  behind electrochemical cells, and it is this
  force that forces electrons through the external
  circuit from the anode (site of oxidation) to the
  cathode (site of reduction). This force is known
  as the potential difference or electromotive
  force (emf or E). Potential difference is
  measured in volts (V), and thus is also referred
  to as the voltage of the cell. Voltage is a
  measure of the tendency of electrons to flow. The
  higher the voltage, the greater the tendency for
  electrons to flow from the anode to the cathode.

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• For example, if copper and hydrogen half-cells
  are joined together we find that the copper
  half-cell will gain electrons from the hydrogen
  half-cell. Thus the copper half-cell is given a
  positive voltage and given a relative value of
  0.34 V
• Cu2(aq) 2e- ? Cu(s)    E 0.34 V
• Since both half-reactions cannot undergo
  reduction, we must reverse the equation of the
  reaction that will undergo oxidation. This will
  give us an electrochemical cell voltage of 0.34
  V
• E
• Cu2(aq) 2e- ? Cu(s)    0.34
  V
• H2 (g)  ? 2H(aq) 2e- 0.00 V
• Cu2(aq) H2 (g) ? 2H(aq) Cu(s)    0.34 V

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• We see in the Table of Standard Reduction
  Potentials that zinc has a negative E indicating
  that it is not as good at competing for electrons
  as hydrogen.
•  Zn2(aq) 2e- ? Zn(s)    E -0.76 V
• Therefore if zinc and hydrogen are paired
  together in an electrochemical cell, the hydrogen
  would be reduced (gain the electrons) and zinc
  would be oxidized (losing electrons). To
  determine the net redox reaction as well as the
  voltage of the electrochemical cell we reverse
  the zinc equation, and also reverse it's sign
  before adding the equations and E together
• E
• Zn(s)  ? Znu2(aq) 2e-   0.76 V
• 2H(aq) 2e- ? H2 (g)  0.00 V
• Zn(s) 2H(aq) ? Zn2(aq) H2 (g)    
  0.76 V