Coagulation-Softening

# Coagulation-Softening - ALUM SUMMARY General reaction(1...

This preview shows pages 1–4. Sign up to view the full content.

Coagulation-Softening 1 ALUM SUMMARY General reaction: (1) Al 2 (SO 4 ) 3 . 14H 2 O + 6H 2 O 2 Al(OH) 3 + 3H 2 SO 4 + 14H 2 O so 1 mol Alum destroys 6 mol AlK via (2) 3H 2 SO 4 + 6HCO 3 - 6H 2 CO 3 + 6H 2 O Alum produces Al(OH) 3 whose solubility is governed by pH via reactions (3) and (4): (3) Al(OH) 3 Al +3 + 3OH - K sp =10 -33 (4) Al(OH) 3 + OH- Al(OH) 4 - K =10 +1.3 can plot both (3) and (4) on a pC - pH diagram. This plot will show the solid and ionic or soluble phases for aluminum species. When from (3): K sp =10 -33 = (Al +3 )(OH - ) 3 taking negative logs (pK) pK sp = 33 = pAl + 3pOH from K w for water (pKw = pH + pOH) and substituting for pOH then 33 = pAl + 3(14 - pH) 33 = pAl + 42 - 3 pH and simplification gives: (5) pH = (pAl + 9)(1/3) = (pAl/3) + 3 Plot pH = 6 pAl = 9 pH = 8 pAl = 15 From (4), K = Al(OH - )4/OH - pK = p(Al(OH - )4) - pOH = -1.3 (6) p(Al(OH - ) 4 ) - 14 + pH = -1.3 pH = -p(Al(OH - ) 4 ) + 12.7 Plot pH = 6 p(Al(OH - ) 4 ) = 7.7 pH = 8 p(Al(OH - ) 4 ) = 4.7 Graphing (6) and (5) shows area for Al(OH) 3 formation and predicts optimum coagulation at pH= 5.5 for which pC is minimum at 10 -7.5 for optimum aluminum insolubility which is maximum coagulation or floc production. The figure shown on the following page shows other aluminum species than Al +3 and Al(OH) 4 - but is representative of the versatility of a pC - pH diagram, the amphoteric nature of aluminum species and the zone of solid formation or coagulation.

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
Coagulation-Softening 2 FIGURE 4.1 Equilibrium Concentrations of Hydroxo Aluminum (III) Complexes 0 2 4 6 8 10 12 14 024681 0 1 2 1 4 pH - log concentration, pC [Al 3+ ] [Al 13 (OH) 34 5+ ] [Al 2 (OH) 2 4+ ] [Al(OH) 2+ ] [Al(OH) 4 - ] [Al 7 (OH) 17 4+ ] Al(OH) 3(s) Can predict effect on pH by Alum or required acid or base addition to get desired pH for reaction. Example: Assume alkalinity of 50 mg/L CaCO 3 , alum dose is 200 mg/l as Alum , original pH is 2.0. What is final pH, how much acid or base meq. are required to adjust pH to 5.5. Rx Al 2 (SO 4 ) 3 .14H 2 O + 6H 2 O 2Al(OH) 3 + 6H + + 3(SO 4 - ) 2 6H + + 6HCO 3 - 6H 2 CO 3 H 2 CO 3 H + + HCO 3 - pK = 6.3 200 594 6 1 10 0002020 10 3 269 mg LAlum mM mg Hm o l Alummol M mM mL H /. / . +− −+ == pH is 7.0, all AlK is HCO 3 - 50 50 10 10 3 3 3 mg L meq mg mmol meq M mM m LHCO // −− = free H + = 10 -2.69 - 10 -3 = 10 -3.00 with no base addition pH = 3.0 For pH=5.5 must know how much of carbonate species, C T , are in H 2 CO 3 , HCO 3 - , CO 3 -2 forms using α values. Remember (is fraction C T that is H 2 CO 3 ) For pH = 5.5,
Coagulation-Softening 3 ) 10 ( 10 ) 10 ( 10 ) 10 ( ) 10 ( ) ( ) ( 3 . 10 3 . 6 5 . 5 3 . 6 2 5 . 5 2 5 . 5 2 1 2 1 + + + + + = + + = K K H K H H o α 0 10 10 16 . 1 10 ) 10 ( 10 ) 10 ( 10 ) 10 ( 14 . 0 10 16 . 1 10 ) 10 ( 10 ) 10 ( 10 ) 10 ( 86 . 0 7 . 5 11 6 . 16 3 . 10 3 . 6 5 . 5 3 . 6 2 5 . 5 2 1 2 11 8 . 11 3 . 10 3 . 6 5 . 5 3 . 6 2 5 . 5 1 1 = = + + = = + + = + x K K x H K o Since HCO 3 - = α 1 C T + C T = H 2 CO 3 + HCO 3 - + CO 3 -2 know HCO 3 - = AlK at pH 7 OriginalHCO mg LCaCO meg mg mmol meg M mmol ML C HCO pH x T 32 3 3 3 1 3 3 50 50 10 10 7 10 083 12 10 == = // (). . α (NOTE: if HCO 3 (alk) at pH 7 is used, must use α 1 at pH 7 for C T calculation) So at pH 3 all C T is H 2 CO 3 , (calculate α o C T ) (At pH 3) C T = H 2 CO 3 = 1.2 x 10 -3 m/L At pH 5.5 need H 2 CO 3 = α o C T = 0.86x10 -3 x 1.2 m/L ~ 1 x 10 -3 m/L HCO 3 - = α 1 C T = 0.14 x 10 -3 x 1.2 m/L = 0.20 x 10 -3 H 2 CO 3 ) Neutralized = 1.2 x 10 -3 m/L pH 3 - 1.0 x 10 -3 m/L pH 5.5 _____________________________ 0.2 x 10 -3 m/L Rx H 2 CO 3 + OH

This preview has intentionally blurred sections. Sign up to view the full version.

View Full Document
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

### Page1 / 13

Coagulation-Softening - ALUM SUMMARY General reaction(1...

This preview shows document pages 1 - 4. Sign up to view the full document.

View Full Document
Ask a homework question - tutors are online