Standard Methods for Examination of Water & Wastewater_13 ppt

Removal of Phosphorus by Chemical Precipitation Phosphorus is a very important element that has attracted much attention because ofits ability to cause eutrophication in bodies of water.

Removal of Phosphorus

by Chemical Precipitation

Phosphorus is a very important element that has attracted much attention because ofits ability to cause eutrophication in bodies of water For example, tributaries from asfar away as the farmlands of New York feed the Chesapeake Bay in Maryland andVirginia Because of the use of phosphorus in fertilizers for these farms, the bay receives

an extraordinarily large amount of phosphorus input that has triggered excessive growths

of algae in the water body Presently, large portions of the bay are eutrophied Withoutany doubt, all coves and little estuaries that are tributaries to this bay are alsoeutrophied Thus, it is important that discharges of phosphorus be controlled in order

to avert an environmental catastrophe In fact, the eutrophication of the ChesapeakeBay and the clogging of the Potomac River by blue greens are two of the reasonsfor the passage of the Federal Water Pollution Control Act Amendments of 1972.This chapter discusses the removal of phosphorus by chemical precipitation Itfirst discusses the natural occurrence of phosphorus, followed by a discussion on themodes of removal of the element The chemical reactions of removal, unit operations

of removal, chemical requirements, optimum pH range of operation, and sludge duction are all discussed The chemical precipitation method employed uses alum,lime, and the ferric salts, FeCl3 and Fe2(SO4)3

pro-14.1 NATURAL OCCURRENCE OF PHOSPHORUS

The element phosphorus is a nonmetal It belongs to Group VA in the PeriodicTable in the third period Its electronic configuration is [Ne]3s23p3 [Ne] meansthat the neon configuration is filled The valence configuration represented by the

3, the M shell, shows five electrons in the orbitals: 2 electrons in the s orbitals and

3 electrons in the p orbitals This means that phosphorus can have a maximumoxidation state of +5 The commonly observed oxidation states are 3−, 3+, and 5+ Phosphorus is too active a nonmetal to be found free in nature Our interest

in its occurrence is the form that makes it as fertilizer to plants As a fertilizer, itmust be in the form of orthophosphate Phosphorus occurs in three phosphateforms: orthophosphate, condensed phosphates (or polyphosphates), and organicphosphates Phosphoric acid, being triprotic, forms three series of salts: dihydrogenphosphates containing the ion, hydrogen phosphate containing the ion, and the phosphates containing the ion These three ions collectively arecalled orthophosphates As orthophosphates, the phosphorus atom exists in itshighest possible oxidation state of 5+ As mentioned, phosphorus can cause eutroph-ication in receiving streams Thus, concentrations of orthophosphates should becontrolled through removal before discharging the wastewater into receiving bodies

14

PO43−

of water The orthophosphates of concern in wastewater are sodium phosphate(Na3PO4), sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate(NaH2PO4), and ammonium hydrogen phosphate [(NH4)2HPO4] They cause theproblems associated with algal blooms

When phosphoric acid is heated, it decomposes, losing molecules of water formingthe P–O–P bonds.The process of losing water is called condensation, thus the termcondensed phosphates and, since they have more than one phosphate group in themolecule, they are also called polyphosphates Among the acids formed from thecondensation of phosphoric acid are dipolyphosphoric acid or pyrophosphoric acid

(H4P2O7, oxidation state = 5+), tripolyphosphoric acid (H5P3O10, oxidation state = 5+),and metaphosphoric acid [(HPO3)n, oxidation state = 5+] Condensed phosphatesundergo hydrolysis in aqueous solutions and transform into the orthophosphates Thus,they must also be controlled Condensed phosphates of concern in wastewater aresodium hexametaphosphate (NaPO3)6, sodium dipolyphosphate (Na4P2O7), and sodiumtripolyphosphate (Na5P3O10)

When organic compounds containing phosphorus are attacked by microorganisms,they also undergo hydrolysis into the orthophosphate forms Thus, as with all the otherphosphorus species, they have to be controlled before wastewaters are discharged

Figure 14.1 shows the structural formulas of the various forms of phosphates Notethat the oxygen not bonded to hydrogen in orthophosphoric acid, trimetaphosphoric

FIGURE 14.1 Structural formulas of various forms of phosphates.

Orthophosphoric acid Trimetaphosphoric acid

Tripolyphosphoric acid

Organic backbone

Organic phosphate Dipolyphosphoric acid or

pyrophosphoric acid

acid, dipolyphosphoric acid, and tripolyphosphoric acid has a single bond with the

central phosphorus atom Oxygen has six electrons in its valence shell, therefore,

this indicates that phosphorus has shared two of its own valence electrons to oxygen

without oxygen sharing any of its electrons to phosphorus The acceptance by oxygen

of these phosphorus electrons, completes its valence orbitals to the required eight

electrons for stability

All of the hydrogen atoms are ionizable This means that the largest negative

charge for the complete ionization of orthophosphoric acid is 3−; that of

trimet-aphosphoric acid is also 3− Dipolyphosphoric acid will have 4− as the largest

negative charge and tripolyphosphoric acid will have 5− as the largest negative

charge The charges in organic phosphates depend upon the organic backbone the

phosphates are attached to and how many of the phosphate radicals are being

attached

The phosphorus concentration in domestic wastewaters varies from 3 to 15 mg/L

and that in lake surface waters, from 0.01 to 0.04 mg/L all measured as P These

values include all the forms of phosphorus

14.2 MODES OF PHOSPHORUS REMOVAL

Again, as in previous chapters, the best place to investigate for determining the

modes of removal is the table of solubility products constants as shown in Table 14.1

A precipitation product that has the lowest K sp means that the substance is the most

insoluble As shown in the table, the phosphate ion can be precipitated using a

calcium precipitant producing either Ca5(PO4)3(OH)(s) or Ca3(PO4)2 Of these two

precipitates, Ca5(PO4)3(OH)(s) has the smaller K sp of 10−55.9; thus, it will be used as

the criterion for the precipitation of phosphates Ca5(PO3)3(OH)(s) is also called

calcium hydroxy apatite

As shown in the table, the other mode of precipitation possible is through

precipitating the phosphate ion as AlPO4(s) and FePO4 The precipitant normally used

in this instances are alum and the ferric salts (ferric chloride and ferric sulfate),

respectively

TABLE 14.1 Solubility Product Constants for Phosphate Precipitation

Precipitation Product Solubility Product, K sp at 25°C

Ca 5 (PO 3 ) 3 (OH) (s) (10−55.9)

Ca3(PO4)2 (10−25) AlPO 4(s) (10−21)

14.3 CHEMICAL REACTION OF THE PHOSPHATE ION

WITH ALUM

To precipitate the phosphate ion as aluminum phosphate, alum is normally used

The chemical reaction is shown next:

(14.1)

As shown in these reactions, the phosphorus must be in the phosphate form

The reaction occurs in water, so the phosphate ion originates a series of equilibrium

orthophosphate reactions with the hydrogen ion This series is shown as follows

(Snoeyink and Jenkins):

(14.2)

(14.3)

(14.4)

Let represent the species in solution containing the PO4 species of the

orthophosphates, using alum as the precipitant Therefore,

(14.5)

Express Equation (14.5) in terms of using Eqs (14.2) through (14.4) This

will enable to be expressed in terms of [Al3+] using Equation (14.1) and

Proceed as follows:

(14.6)

(14.7)

Al3++PO43−
AlPO4 s( ) ↓AlPO4 s( )↓
Al3+PO43− K sp,AlPO4 = 10−21

phos-[Al3+] needs to be eliminated for the equation to be expressed solely in terms of[H+] When alum is added to water, it will unavoidably react with the existing naturalalkalinity For this reason, the aluminum ion will not only react with the phosphate ion

to precipitate AlPO4(s), but it will also react with the OH− to precipitate Al(OH)3(s) Also,

in addition to the Al(OH)3(s) All these interactions complicate our objective

of Al3+ needed to precipitate AlPO4 is 3.0(10−11) gmol/L These two concentrations

H3PO4[ ] {H3PO4} {H2PO4−} H{ }+

PO43−

[ ] H[ ]+ 3

KH3PO4KH2PO4KHPO4

+

=

γH 3

are so close to each other that it may be concluded that Al(OH)3 and AlPO4 arecoprecipitating This finding allows us to eliminate [Al3+] Thus,

Substituting into Equation (14.10),

(14.12)Equation (14.12) portrays the equilibrium relationship between and [H+]

con-tains 140 mg/L of dissolved solids

+

+

µ 2.5 10( 5) 140( ) 3.5 10( 3) γPO4 10

0.5 3 ( ) 2

– [ 3.5 10 ( 3) ] 1+1.14 3.5 10 3

–

The answer of 9.17(10+6) mg/L emphasizes a very important fact: phosphoruscannot be removed at alkaline conditions It will be shown in subsequent discussionsthat the solution conditions must be acidic for effective removal of phosphorus usingalum

Equation (14.12) shows that is a function of the hydrogen ion concentration.This means that the concentration of the species containing the PO4 species of theorthophosphates is a function of pH If the equation is differentiated and the resultequated to zero, however, an optimum value cannot be guaranteed to be found A range

of pH values can, however, be assigned and the corresponding values of lated By inspection of the result, the optimum range can be determined Tables 14.2

calcu-and 14.3 show the results of assigning this range of pH calcu-and values of calculatedusing Equation (14.12) These tables show that optimum removal of phosphorus usingalum results when the unit is operated at pH values less than 5.0

Note: The dissolved solids content has only a negligible effect on the resulting

concentrations

14.4 CHEMICAL REACTION OF THE PHOSPHATE

ION WITH LIME

Calcium hydroxy apatite contains the phosphate and hydroxyl groups Using calciumhydroxide as the precipitant, the chemical reaction is shown below:

( ) 0.94( )3

10–10[ ]3

- 10

21 –

( ) 10– 14

( )3

0.77( ) 10– 12.3

( ) 10– 33

( ) 0.94( )2

10–10[ ]2

+

-=

10–21( ) 10– 14

( )3

0.94( ) 10– 7.2

-10–21( ) 10– 14

( )3

10–2.1( ) 10– 7.2

( ) 10– 12.3

( ) 10( −33) -+

1.0 10( –63)4.65 10( –64) - 1.0 10

63 –

3.41 10( –66) - 1.0 10

63 –

2.79 10( –63) - 1.0 10

63 –

2.51 10( –55) -

=295.76 gmols/L 9.17 10( 6) mg/L as P

TABLE 14.2

pH, Dissolved Solids = 140 mg/L (mg/L)

These equations also show that the phosphorus must be in the phosphate form.

As in the case of alum, the phosphate ion produces the set of reactions given byEqs (14.2) through (14.4) Let be the species in solution containing the PO4species, using the calcium in lime as the precipitant will be the same asgiven by Equation (14.5) which, along with Eqs (14.2) through (14.4), can bemanipulated to produce Equation (14.9) From Equation (14.13),

Substituting in Equation (14.9) and simplifying,

is much, much greater than K sp,apatite = 10−55.9

and calcium hydroxide will not beprecipitating along with Ca5(PO4)3OH(s) The other possible precipitate is CaCO3that is produced when lime reacts with the natural alkalinity of the water; however,

calcium carbonate has a K sp value of 4.8(10−9) Again, this value is much, much

greater than K sp,apatite and calcium carbonate will not be precipitating along with

Ca5(PO4)3OH(s), either We will, therefore, let the equation stand and express

as a function of [Ca2+], along with [H+] and the constants

Assume the water contains 140 mg/L of dissolved solids and that [Ca2+] = 130 mg/L

γCa 1/3

γCa 1/3

-=

K sp,apatite1/3 γH

7/3

H+[ ]7/3

γH2PO4KH

2 PO4KHPO

4γCa 1/3

Ca2+

[ ]5/3K w

1/3

+

-K sp,apatite1/3 γH

10/3

H+[ ]10/3

-spPO4Ca

spPO4Ca

γCa 1/3

γHPO4KHPO

4γCa 1/3

Ca2+

[ ]5/3K w

1/3

+

-=

K sp,apatite1/3 γH

7/3

H+[ ]7/3

γH2PO4KH

2 PO4KHPO

4γCa 1/3

Ca2+

[ ]5/3K w

1/3

+

-K sp,apatite1/3 γH

10/3

H+[ ]10/3

3.24 10( 3)[ ]5/3(10–14)1/3(0.56) -

=

10–55.9( )1/3(0.94)4/3

10 8[ ]4/30.77

( ) 10– 12.3

( ) 0.77( )1/3[3.24 10( 3)]5/3(10–14)1/3 -+

10–55.9( )1/3(0.94)7/3

108[ ]7/30.94

( ) 10– 7.2

( ) 10– 12.3

( ) 0.77( )1/3[3.24 10( 3)]5/3(10–14)1/3 -+

10–55.9( )1/3(0.94)10/3[108]10/3

10–2.1( ) 10– 7.2

( ) 10– 12.3

( ) 0.77( )1/3

3.24 10( 3)[ ]5/3(10–14)1/3 -+

4.91 10( –22)7.85 10( –10) - 4.62 10

30 –

5.41 10( –22) - 4.34 10

38 –

4.16 10( –29) - 4.08 10

46 –

3.52 10( –31) -

=9.58 10( 9) gmol/L

=6.25 10( –13) 8.54 109

=

14.4.1 D ETERMINATION OF THE O PTIMUM pH AND THE O PTIMUM

Analyzing Equation (14.14) shows that [H+]’s are all written in the numerator Thismeans that if the equation is differentiated, the result will give positive terms onone side of the equation which will then be equated to zero to get the minimum.This kind of equation cannot guarantee a minimum Thus, the optimum and theoptimum pH range will be determined by preparing a table as was done in the case

of alum Tables 14.4 through 14.6 show the results This table was prepared usingEquation (14.14)

Tables 14.4 and 14.5 reveal very important information The efficiency ofremoval of phosphorus increases as the concentration of calcium increases from 0 to

130 mg/L Remember that we disallowed the use of the K sp of Ca(OH)2 because itwas too large, and we concluded that the hydroxide would not be precipitatingalongside with the apatite It is not, however, preventable to add more lime in order

to increase the concentration of the calcium ion to the point of saturation and, thus,

be able to use in calculations In theory, before the hydroxide precipitationcan happen, all the apatite particles would have already precipitated, resulting,indeed, in a very high efficiency of removal of phosphorus Would this really happen?The answer would be yes, but this could be a good topic for applied research

A concentration of zero for the calcium ion is, of course, nonexistent Thus,Table 14.4 may be considered as purely imagined Also, from the tables there is nosingle optimum pH; the range, however, may be determined by inspection as ranging

from pH 7.0 and above The effect of dissolved solids has reduced this range to

8 and above; however, a concentration of 35,000 mg/L of total solids is alreadyvery high and would not be encountered in the normal treatment of water andwastewater This concentration is representative of the dissolved solids concentra-tion of sea water

14.5 CHEMICAL REACTION OF THE PHOSPHATE ION

WITH THE FERRIC SALTS

The chemical reaction to precipitate the phosphate ion as ferric phosphate is shownnext:

γFeIII is the activity coefficient of the ferric ion

Investigate the possibility of eliminating [Fe3+] from the denominator of theabove equation This is, indeed, possible through the use of ferric hydroxide Thedissociation reaction is

(14.17)

Fe3++PO43−
FePO4↓FePO4↓
Fe3+ PO43− K sp,FePO

=

γH 2

K sp,FePO4[ ]H+ 2

γH2PO4KH2PO4KHPO4γFeIII[Fe3+] - γH

3

K sp,FePO4[ ]H+ 3

KH3PO4KH2PO4KHPO4γFeIII[Fe3+] -

Fe OH( )3 s( )
Fe3+ 3 OH( −) K sp,Fe OH( )

3

From = 1.1(10−36

), the concentration of Fe3+ needed to precipitate Fe(OH)3can be calculated to be equal to 4.5(10−10) gmol/L And, from = 10−21.9

,the concentration of Fe3+ needed to precipitate FePO4 can also be calculated to beequal to 1.1(10−11) gmol/L Because these two concentrations are practically equal,Fe(OH)3(s) will definitely precipitate along with FePO4 Therefore,

Substituting in Equation (14.16),

(14.18)

Assume the water contains 140 mg/L of dissolved solids

+

+

( ) 0.94( )3

10 8[ ]3

-=

10–21.9( ) 10– 14

( )3

0.77( ) 10– 12.3

( ) 1.1( ) 10– 36

( ) 0.94( )2

108[ ]2

+

-14.5.1 D ETERMINATION OF THE O PTIMUM pH AND THE O PTIMUM

TABLE 14.7

pH, Dissolved Solids ==== 140 mg/L (mg/L)

( )3

0.94( ) 10– 7.2

-10–21.9( ) 10– 14

( )3

10–2.1( ) 10– 7.2

( ) 10– 12.3

( ) 1.1( ) 10– 36

+

-1.26 10( –64)5.12 10( –61) -

= 1.26 10( –64)

3.75 10( –65) - 1.26 10

64 –

3.07 10( –64) - 1.26 10

64 –

2.76 10( –58) -