coagulacion con cloruro ferrico

8/2/2019 Coagulacion Con Cloruro Ferrico

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Wat. Res. Vo l. 28, No. 3, pp. 559-569, 1994Copyright 1994Elsevier ScienceLtdPergamon Printed in G reat B ritain. All rights reserved0043-1354/94 $6 .00+ 0.00

DYNAMICS OF COAGULATION OF KAOLIN PARTICLESWITH FERRIC CHLORIDE

HSIAO-WEICHING1, THEODO RES. TANAKA and MENACHEMELIMELECHI*OIDepartment of Civil and E nvironmental Engineering, University of California, Los A ngeles,CA 90024-1593 and 2Metropolitan Water D istrict of Southern California, 700 Moreno Avenue,La Verne, CA 91750, U.S.A.

(F irs t received O ctober 1992; accep ted in rev i sed fo rm June 1993)Ab strac t--A n optical monitoring technique is used to investigate the dynam ics of coagulation o f kaolinsuspensions with ferric chloride. Particular attention is given to the effects of coagulant dose, solution p Hand mix ing intensity on coagulation dynam ics. Results show that this monitoring method providesvaluable information on the dynamics of aggregates in coagulation with Fe(III) salts.K e y w o r d s - - c o a g u l a t i o n , fer ric chloride, metal salt coagulant, clay particles, mon itoring, coagulationdynamics, Fe(II I) solution chem istry

INTRODUCTION t ransm i t t ed l igh t me asu red by a de tec to r f luc tua tesa b o u t a m e a n l e ve l. I t c a n b e s h o w n ( G r e g o r y , 1 98 5)C o a g u l a t i o n i s a n e s se n t ia l p r o c e s s i n t h e o v e r a l l t h a t th e r a t i o o f t h e r o o t m e a n s q u a r e o f t h e f l u ct u -s o l i d - li q u i d s e p a r a t i o n s c h e m e i n w a t e r a n d a d - a t i n g s i gn a l t o t h e m e a n l ev el i s r o u g h l y p r o p o r t i o n a lv a n c e d w a s t e w a t e r t r e a tm e n t . H y d r o l y z i n g m e t a l to the s i ze o f the agg rega tes flowing th rou gh thes a lt s , s u c h a s a l u m i n u m a n d f e r r ic c o a g u l a n t s , a r e d e t e c t o r a n d t o t h e s q u a r e r o o t o f t h e ir c o n c e n -w i d e l y u s e d a s p r i m a r y c o a g u l a n t s t o p r o m o t e th e t r a t i o n . T h i s r a t i o , r e f e r r e d t o a s f l o c c u l a t i o n o rf o r m a t i o n o f a gg r e ga t es . F o r o p t i m a l p e r f o r m a n c e o f c o a g u l a t io n i n d e x , c a n g i v e i m m e d i a t e i n f o r m a t i o n o nsubsequ en t so l id - l iqu id separa t ion p rocesses , i t i s the s t a te o f agg re ga t ion o f par t i c le su spens ion overimpe ra t ive tha t agg rega tes o f a ce r ta in si ze , s t reng th the en t i re per iod o f coag u la t ion .a n d d e n s i t y b e f o r m e d i n t h e c o a g u l a t i o n p r o c e s s ( e. g. T h e t e c h n i q u e d e s c ri b e d a b o v e h a s b e e n s u c c es s -R a m a l e y e t a l . , 1 98 1; O ' M e l i a , 1 9 8 5) . I n c o a g u l a t i o n f u l ly a p p l ie d t o s t u d y t h e d y n a m i c s a n d m e c h a n i s m sw i t h h y d r o l y z i n g m e t a l s a lt s, t h e se p r o p e r t ie s a r e o f c o a g u l a t i o n o f d i l ut e a n d c o n c e n t r a t e d s u s p e n s io n sl a r g el y d e t e r m i n e d b y c o a g u l a n t d o s e , s o l u t io n p H w i t h p o l y m e r i c c o a g u l a n t s ( e. g. G r e g o r y , 1 98 8;a n d m i x i n g i n t e n si t y ( A m i r t h a r a j a h a n d O ' M e l i a , G r e g o r y a n d L e e , 1 9 90 ; L i a n d G r e g o r y , 1 99 1;1990 ; Den te l , 1991). G reg o ry and Li , 1991) . In th i s paper , i t i s show n tha tT h e s t a t e o f a g g r e g a t i o n a n d d y n a m i c s o f a g g r e - t h is t e c h n i q u e c a n b e a p p l ie d t o s t u d y t h e d y n a m i c sg a t e s i n a c o a g u l a t i n g s u s p e n s io n c a n b e m e a s u r e d b y o f c o a g u l a t i o n w i t h m e t a l s a lt c o a g u l a n t s ( f e rr i cpar t i c le coun te rs (e .g . Lawler e t a l . , 1983 ; F ranc o i s , ch lo r ide) . I t i s fu r the r dem ons t r a ted tha t th i s m etho d1 98 8; A m i r t h a r a j a h a n d O ' M e l i a , 1 99 0) . H o w e v e r , c a n g i v e v a l u a b l e i n si g h ts i n t o t h e m e c h a n i s m s o fb e c a u s e o f b r e a k a g e o f a g g r e g a t e s a n d f o r m a t i o n o f p a r t ic l e a g g r e g a t i o n w i t h fe r r ic c hl o r id e .g e l a t i n o u s h y d r o x i d e p r e c i p i t a t e s ( T r e w e e k a n dM o r g a n , 1 9 7 7; L a w l e r e t a l . , 1983), serio us difficultiesa r i se wh en par t i c le coun te rs a re u sed to me asu re the BACKGROUNDs i z e o f a g g r e g a t e s f o r m e d i n c o a g u l a t i o n w i t h h y - C h e m i s t ry o f F e ( l l I ) s a l t sdro lyz ing meta l s a lt s. In add i t ion , p ar t i c le coun te rsa r e e x p e n si v e a n d a r e n o t s u i ta b l e f o r o b t a i n i n g W h e n F e ( I I I ) s a lt s a r e d i s s ol v e d i n w a t e r , t h e m e t a li m m e d i a t e a n d c o n t i n u o u s i n f o r m a t i o n a b o u t t h e i o n h y d r a t e s , c o o r d i n a t i n g s ix w a t e r m o l e c u l e s a n ds t a te o f a g g r e g a t i o n a n d d y n a m i c s o f a g g r e g a t e s i n a f o r m i n g a n a q u o m e t a l i o n , F e ( H 2 0 ) ~ + . T h ec o a g u l a t i n g s u s p e n si o n , a q u o m e t a l i o n c a n t h e n h y d r o l y z e a n d f o r m

A s imp le bu t s ens it ive op t ica l t echn ique has re - m ono m er ic an d po lym er ic fe r r ic spec ies , the fo r -c e n t l y b e e n d e v e l o p e d to m o n i t o r t h e s ta t e o f ag g r e - m a t i o n o f w h i c h i s h i g h ly p H d e p e n d e n t ( S t u m m a n dg a t i o n o f c o l l o id a l s u s p e n s io n s ( G r e g o r y , 19 8 5 ). T h e M o r g a n , 1 98 1; O ' M e l i a e t a l . , 1989 ; More l andt e c h n i q u e is b a s e d o n m e a s u r e m e n t o f t u r b i d it y f lu c - H e r i n g , 1 9 9 3) . I t is p o s s ib l e t o d e sc r i b e t h e f o r m a t i o ntua t ions in f lowing suspens ions . W hen a f lowing o f severa l hyd ro lys i s spec ies th a t a re pos i t ive lysuspens ion i s i l lumina ted by a nar r ow l igh t beam , the charged : Fe O H 2+, Fe(O H)~ ' , Fe2(OH)~ + andF e 3 ( O H ) ~ + ; n e u t r a l : F e ( O H ) ; a n d n e g a t i v e l y*Author to whom all correspondence should be addre ssed, cha rged : Fe(O H)~-.

559


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560 HSlAO WEI CHING e t a l .I n g e n e r a l , t h e h y d r o l y s i s r e a c t i o n s o f F e ( I I I ) i n i n s o r p t i o n d e n s i t y o f t h e p r e c i p i t a t i n g c a t i o n , o c c u r s

a q u e o u s s o l u t i o n c a n b e w r i t t e n a s b e l o w s a t u r a t i o n i n t h e b u l k s o l u t io n . T h u s , a m -F e ( O H ) 3 ( s m a y b e f o r m e d a t t h e s u rf a c e o f m i n e r a lx F e 3 + y H 2 0 = ~3~ ~.)F e x ( O H ) , . - + y H ( 1 ) p a r t i c l e s a n d a ff e c t t h e i r s u r f a c e p r o p e r t i e s , e v e n

A t a n y p H , th e m a x i m u m d i s s o lv e d c o n c e n t r a t i o n o f w h e n t h e s o l u b i l it y p r o d u c t f o r a m - F e ( O H ) 3 ( s i s n o tF e ( I I I ) i n e q u i l i b r i u m w i t h t h e h y d r o x i d e s o l i d i s e x c e e d e d i n t h e b u l k s o l u t i o n .d e t e r m i n e d b y t h e s o l u b i l i t y o f t h e s o l i d p h a s e , i n t h i s I t s h o u l d b e e m p h a s i z e d t h a t t h e c h e m i s t r y o f f e r r icc a s e a m o r p h o u s f e rr i c h y d r o x i d e , a m - F e ( O H ) 3 ( s ) , a d s o r p t i o n a n d p r e c i p i t a t i o n i n n a t u r a l w a t e r s i sa n d b y t h e e x t e n t o f f o r m a t i o n o f m o n o m e r i c a n d m u c h m o r e c o m p l i c a t e d b e c a u s e o f t h e p r e se n c e o fp o l y m e r i c h y d r o l y s i s s p e c ie s i n s o l u t io n . T h e s o l u b il - h u m i c s u b s ta n c e s . I n t e r a c t i o n o f h u m i c s u b s t a n c e si t y d i a g r a m o f F e ( I I I ) , s u c h a s t h a t s h o w n i n M o r e l w i t h s u r f a c e s a n d d i s s o l v e d f e r ri c s p e c i e s c a na n d H e r i n g ( 1 99 3) o r J o h n s o n a n d A m i r t h a r a j a h m a r k e d l y i n fl u en c e c o a g u l a t i o n p r o c e ss e s . I t i s w e l l( 19 83 ), i s v e r y u s e f u l t o d e s c r i b e t h e c o n c e n t r a t i o n s o f k n o w n t h a t a c t u a l f e rr i c d o s a g e s i n w a t e r t r e a t m e n tt h e d i s s o l v e d f e r r ic s p e c i e s a s a f u n c t i o n o f p H , a t a r e c o n t r o l l e d b y t h e c o n c e n t r a t i o n o f d i s s o l v e de q u i l i b ri u m w i t h a m - F e ( O H ) 3 ( s ) . I t c a n b e s e e n f r o m n a t u r a l o r g a n i c m a t t e r ( A m i r t h a r a j a h a n d O ' M e l i a ,t h e s o l u b i l i ty d i a g r a m th a t a m o r p h o u s f e rr i c h y d r o x - 1 99 0). T h e s e a s p e c ts , t h o u g h b e y o n d t h e s c o p e o fi d e i s l e a s t s o l u b l e a t a p H c l o s e t o 8 . t h i s p a p e r , s h o u l d b e c o n s i d e r e d i n a c t u a l t r e a t m e n t

F e r r i c a d s o r p t i o n o n t o p a r t i c l e s u r f a c e s c a n a f f e c t p r a c t i c e s .b o t h t h e s p e c i a t i o n o f f e r ri c in s o l u t i o n a n d t h es u r f a c e p r o p e r t i e s o f t h e p a r t i c l e s . A d s o r p t i o n o f P r i n c i p le s o f t u r b i d it y f lu c t u a t i o n sf e r r ic o n t o c l a y p a r t i c l e s c a n o c c u r b y f o r m a t i o n o f T h e p r i n c i p l e s o f t h e o p t i c a l t e c h n i q u e u s e d i n t h i ss u r f a c e c o m p l e x e s b e t w e e n f e r ri c a n d s u r f a c e h y - s t u d y a n d i t s a p p l i c a t i o n t o c o a g u l a t i o n p r o c e s s e sd r o x y l g r o u p s o f t h e m i n e r a l . F o r k a o l i n i t e , c a t i o n h a v e b e e n d e s c r i b e d p r e v i o u s l y ( e .g . G r e g o r y , 1 9 85 ;a d s o r p t i o n c a n i n v o l v e b o t h i o n - e x c h a n g e ( p a r t i c u - G r e g o r y a n d N e l s o n , 1 98 6) . A s h o r t d e s c r i p t i o n o fl a r l y a t l o w p H ) a n d s u r fa c e c o m p l e x f o r m a t i o n . T h i s t h i s t e c h n i q u e i s g i v e n b e l o w .l a t t e r r e a c t i o n i s t h o u g h t t o o c c u r p r e d o m i n a n t l y a t W h e n a f l ow i n g s u s p e n s i o n i s i l l u m i n a t e d b y at h e b a s a l g i b b s i t e a n d e d g e s u r f a c e s o f t h e c l a y n a r r o w l i g h t b e a m , t h e t r a n s m i t t e d l i g h t i n t e n s i t y ,m i n e r a l ( S t u m m , 1 99 2). R e a c t i o n w i t h m o n o m e r i c m o n i t o r e d by a p h o t o d e t e c to r , f l u c tu a t e s r a n d o m l yf e rr i c h y d r o l y s i s s p e c ie s m a y b e w r i t t e n in a g e n e r a l a b o u t s o m e m e a n v a l u e . T h e o u t p u t f r o m t h e p h o t of o r m a s d e t e c t o r c o n s i s t s o f a s t e a d y ( D . C . ) s i g n a l a n d a

= S O H + F e ( O H ) ~3-~)+ = -=SOFe(OH )~31n) f l u c t u a t i n g ( A . C . ) c o m p o n e n t . T h e D . C . v a l u e i s am e a s u r e o f t h e a v e r a g e t r a n s m i t t e d l i g h t i n t e n s it y a n d+ H 2 0 ( 2) d e p e n d s o n th e t u r b i d i t y o f t h e s u s p e n si o n . T h e

f l u c t u a ti n g ( A . C . ) c o m p o n e n t i s a r e s u l t o f r a n d o mw h e r e = S i n d i c a t e s a s u r f a c e s p e c ie s . N o t e t h a t t h e v a r i a t i o n i n t h e n u m b e r o f p a r t i c l e s i n t h e i l l u m i n a t e ds u r f a c e - b o u n d f e r r i c c a n a l s o u n d e r g o a c i d - b a s e r e a c - v o l u m e . I t c a n b e s h o w n ( G r e g o r y , 1 98 5) t h a t t h et i o n s. I n t h e s u r f a c e c o m p l e x a t i o n m o d e l , t h e s u r f a c e r o o t m e a n s q u a r e ( R M S ) v a l u e o f th e f l u c tu a t i n gc h a r g e o f t h e m i n e r a l i s a t t r i b u t e d t o t h e p r e se n c e o f ( A . C . ) s i gn a l i s r e l a t e d t o t h e a v e r a g e n u m b e r c o n c e n -c h a r g e d s u r fa c e s p e c ie s ( = S O H f a n d = S O - , i n t h e t r a t i o n a n d t h e s i ze o f th e s u s p e n d e d p a r t i c l e s . I na b s e n c e o f a n y a d s o r b i n g s p e c ie s o t h e r t h a n p r o to n s ) , p r a c t ic e , it is c o n v e n ie n t t o d i v i d e t h e R M S v a l u e b yT h u s , f o r m a t i o n o f f e r r ic s u r f a c e c o m p l e x e s c a n a l t e r t h e s t e a d y D . C . v a l u e t o g i v e a d i m e n s i o n l e s s te r ms u r fa c e c h a r g e ( S t u m m a n d M o r g a n , 19 81 ; S t u m m , R = R M S / D . C . W i t h t h e u s e o f t h i s r a t i o , o n e1 99 2) . A d s o r p t i o n o f p o l y m e r i c f e r r ic s p e c i e s , e s - c a n a v o i d t h e e f fe c t s o f o p t i c a l s u rf a c e f o u l i n g a n dp e c i a l l y h i g h l y c h a r g e d s p e c i e s, c a n d r a m a t i c a l l y e l e c t r o n i c d r i f t .a f f e c t s u r f a c e c h a r g e ( T a n g a n d S t u m m , 1 98 7a , b ; I t c a n b e s h o w n ( G r e g o r y a n d N e l s o n , 1 98 6) t h a t ,O ' M e l i a e t a l . , 1 98 9) . I n p r a c t i c e , h o w e v e r , t h e a d - f o r a h e t e r o d i s p e r s e s u s p e n s i o n , t h e r a t i o R c a n b es o r p t i o n o f p o ly m e r i c s p e c ie s m a y b e i n d is t i n g u is h - e x p r e s se d a sa b l e f r o m p r e c i p i t a t i o n o f a m - F e ( O H ) 3 ( s ) a t t h em i n e r a l s u r f a c e . W i t h i n c r e a s i n g f e r r i c a d s o r p t i o n ( L ' ~ L 2a n d s u r fa c e c o v e r a g e , t h e a c i d - b a s e p r o p e r t i e s o f t h e R = \ A J ( Y . N i C 2 )~ /2 (3 )s u r f a c e w i l l b e i n c r e a s i n g l y c h a r a c t e r i s t i c o f a m -F e ( O H ) 3 ( s ) . w h e r e L i s t h e o p t i c a l p a t h l e n g t h , A i s t h e e f f ec t i v e

A c o n t i n u u m b e t w e e n s u r f a c e a d s o r p t i o n ( c o m - c r o s s - se c t i o n a l a r e a o f t h e l i g h t b e a m a n d N i a n d C~p l e x a t i o n ) a n d p r e c i p i t a t i o n i s d e s c r i b e d b y th e su r - a r e t h e n u m b e r c o n c e n t r a t i o n a n d s c a t t e ri n g c r o s s -f a c e p r e c i p i t a t i o n m o d e l . T h i s m o d e l a l l o w s f o r t h e s e c t i o n o f p a r t i c l e s o f s i ze i , r e s p e c t i v e l y . T h i sf o r m a t i o n o f a s u r fa c e p h a s e w i t h a c o m p o s i t i o n t h a t e q u a t i o n d e m o n s t r a t e s t h a t t h e f l u c t u a ti n g s i g n a lv a r ie s c o n t i n u o u s l y b e tw e e n t h a t o f t h e o ri g i n a l s o l id d e p e n d s o n t h e s q u a r e r o o t o f p a r t i c l e c o n c e n t r a t i o na n d t h a t o f a p u r e p r e c i p i t a te o f th e a d s o r b i n g c a t i o n a n d o n t h e fi rs t p o w e r o f t h e s c a t te r i n g c r o s s s e c t io n .( F a r l e y e t a l . , 1 98 5). F o r m a t i o n o f t h e p r e c i p i t a t e a t T h e l a t t e r i s h i g h l y d e p e n d e n t o n t h e s i z e o f t h et h e m i n e r a l s u r f a c e , e v i d e n c e d b y a g r a d u a l i n c r e a s e s u s p e n d e d p a r ti c l e s .


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D ynami cs o f coagu l a t i on o f kao l i n 561A n a n a l y s is o f t h e t e r m (Y ,NiC 2)1/2 i n eq ua t i o n ( 3 ) C oagu lan t

r e v e a l s t h a t s m a l l e r p a r t i c l e s h a v e a n e g l i g i b l e e f fe c to n R , a n d t h a t , in a c o a g u l a t i n g s u s p e n s i o n , t h e la r g e ra g g r e g a t e s h a v e a d r a m a t i c i n f l u e n c e o n R ( G r e g o r ya n d N e l s o n , 1 98 6) . T h i s m e a n s t h a t a s c o a g u l a t i o n = i i ~ - ~ m ~ J,i - ~ _ ~ ~p r o g r e s s e s , t h e v a l u e o f R i n c re a s e s . A l t h o u g h t h er a t io R d o e s n o t p r o v i d e q u a n t i t a t iv e i n f o r m a t i o n o na g g r e g a t e s i z e, t h e r e l a t i v e i n c r e a s e i n t h e R v a l u e i s ~a u s e f u l i n d i c a t o r o f t h e d e g r e e o f c o a g u l a t i o n . F o r ~ i i l i ! i i li i i l i i i iia g i v e n s u s p e n s i o n , i t c a n b e a s s u m e d t h a t l a r g e r Rv a l u e s i m p l y l a r g e r a g g r e g a t e s i z e . T h i s r a t i o w i l l b er e f e r r e d t o a s t h e " c o a g u l a t i o n i n d e x " i n t h i s s t u d y . S usp en sio n

M A T E R I A L S A N D M E T H O D SSynthetic kaolin suspension Peristaltic Pum p

A s toc k suspens ion of c lay par t ic les was prep ared by Fig. 2 . Schem at ic descr ipt ion of the exper imen ta l se tup fordispers ing pur i f ied kaol in in pow der form (Fisher Scienti fic , mo ni tor ing the dynamics of coagu la t ion by the PD A 2000.Pi t t sburgh, Pa , U.S.A.) in deionized water to a concen-t ra t ion of 10 g/ l . The tes t suspensions were prepared by coag ulant solut ion of 0 .3 M was prepared f rom the concen -di lut ing the s tock suspension to a des ired concen t ra t ion. Al l t ra ted s tock solut ion before every se t of exper iments . In a l lt es t suspensions conta in ed 1 mM of sodium bicarb onate to coag ula t ion exper iments , ferr ic chlor ide was added di rect lyprovid e a lkal ini ty . The s ize di s t r ibut ion o f the kaol in sus- f rom the 0 .3 M coagulant solut ion. Pre l iminary exper imen tspens ion was measu red by a par t ic le cou nter (Mul t i s izer I I , showed that the dosage of coag ulant needed to neut ra l izeCo ul ter E lect ronics , Hia leah , Fin , U.S.A .) wi th a 30-#m the negat ive charge of the par t ic les i s l arger when mo reaper ture tub e (effect ive range 0 .6-18 #m ) . The par t ic les in di lute fer r ic chlor ide s tock solut ions are used. Th e 0 .3 Mthe suspens ion had a mean volu me diameter of 2 .1 # m wi th coag ulant solut ion can be c lassi fied as type A based on thea re la t ively nar row s ize di s t r ibut ion. The nu mb er and analys is of Tang a nd Stum m (1987a, b). They a l so repor tedvolum e dis t r ibut ions of the c lay part ic les , presented in the that fer r ic coag ulant solut ions wi th conce nt ra t ions higherform discussed by Am ir thara ja h and O 'M el ia (1990) , are than 0 .1 M are mo re ef fective than those of lower concen-shown in Fig. 1 . E lect ropho ret ic mobi l i t i es of the par t ic les t ra t ion.w er e measur ed by a L aze r Z ee Mode l 501 appa r a t us ( PenKern Inc . , Bedford Hi l l s , N.Y. , U.S.A) . The c lay suspen- Experimental setup for monitoring coagulation dynamicss ions used in the e lec t rophoret ic mobi l i ty measurementswere t rea ted s imi lar ly to those used in the coagu la t ion A l ight scat ter ing ins t rument wi th a f low- through detectorexper iments , and the resul t s were easi ly reproduced. I t was (PD A 2000 , Ran k Brothers L td, Cam br idge, U .K.) wasf ound t ha t t he pa r t i c le s a r e nega t ive l y cha r ged a t a l l pH used t o mon i t o r the dynami cs o f coagu l a t ion . A s chema t icvalues exam ined ( f rom 3.4 to 10.0) . d iagra m of the exper ime nta l se tup i s shown in Fig. 2 . Thecoagula t ion vessel was a l - l i t er beaker , mixed by aPreparation of ferric coagulant solution 762 x 254-mm rectangular f l a t b lade located 45 mm above

A concen t r a t ed s t ock so lu t i on o f fe r r ic ch l o r ide w as t he base o f t he beake r. T he b l ade w as d r iven by anprepared by di ssolving analyt ica l reagent -grade adjus table speed mo tor via a th in spindle , cent ra l ly locatedFeC I3 6H 20 (Fisher Scient if ic , Pi t t sburgh, Pa , U .S.A.) in in the beaker . A 3- ram ( i .d. ) Tyg on tube, located 30 mmdeionized water to a conc ent ra t ion o f 2 .25 M. A f resh below the l iquid level, convey ed the suspens ion to thef l ow - th r ough de t ec t o r o f t he PD A 2000 mon i t o r . T he14 5 suspension was drive n by a peristal t ic pu m p (Masterflex,C o l e Pa l mer I ns t r umen t C omp. , C h i cago , I l l . , U .S .A . ) l o -ca t ed dow ns t r eam o f t he mon i t o r . T he f l ow r a t e i n t he, - , 12 / . , - ,, 4 '~ convey ing tube was 22 cm3/min, and the t ime for the sample, ~ , .~ to pass f rom the coagu la t ion beaker to the f low- through,'~ 10 , -~ detector was approx. 10 s . The abov e cond i t ions ( i.e . the8 3 ~ = . f l ow t h r ough t he s ampl ing t ube and t he l oca ti on o f t heu ~ sampl ing tube) were s t r ic t ly mainta ine d throug hou t a l l theo expe r imen t s , so t ha t t he coagu l a t i on i ndex dynami cs behav-e 6 2 ~ iors unde r var ious chemical and physical cond i t ions couldbe compar ed t o each o t he r .4 z"~ 1 C oa gu l a t i onxperiments

2 A l - l i t er kaol in suspension, wi th 10 -3 M Na H CO 3 and a0 . . . . . . , a ' "~ : . . . . . 0 so l id concen t r a t i on o f 50 mg/ l , w as used i n t he coagu l a t i on0.01 0 .1 1 10 100 exper iments . The exper iments were cond ucted a t roo md p (p .m ) temp erature (average of 20C). In these exper iments , thet a r ge t pH w as ad j us t ed by add i ng p r ede t e r mi ned amoun t sof KO H o r HCI to the suspens ion, fol lowed by the addi t ionFig. 1 . Nu mb er and vo lum e dis t r ibut ions , dN /d( log do) of fer r ic chlor ide wi th a micropipet te , d i rec t ly f rom the(sol id l ine) and d V/d( log de) (broken l ine) as a funct ion of 0 .3 M fer r ic coagu lant solut ion. Rapid mixing ( for 2 min) bylog d 0 , of the kaol in suspens ion used in th i s work (N i s the mag net ic s t i r r ing a t h igh speed was ini t i a ted af ter thenum ber o f par t ic les per uni t volum e of suspens ion, V the addi t ion of fer r ic chlor ide . At the end of the rapid mixing,volu me of par t ic les per uni t volu me of suspens ion and dp i s a smal l sample was taken by a 25-ml pipet te for e lec t rophor-the par t ic le diameter ) , e t i c mo bi l i ty measuremen ts . Fol low ing rapid mixing, the

WR 28/3--E


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562 HS]AO WEI CHING et al.suspe nsion was mixed gently by a single flat blade at a 4 0desired rpm for a per iod of 15 min . The s tate of aggrega tion - - o - - p H 6 .0of the c lay par tic les was mo ni tored cont in uous ly by the ~ 3 5PD A 2000, as described above. Th e susp ensio n was allowed ---q3----p H 7 . 8to sett le for 15m in after the slow mixing. Samples for ~ 3 0residual tu rb id i ty measurem ents (Ra t io /X R Turbid im ete r , "-"Hach Chemical Com p. , Ames, Iowa , U.S.A .) were then . ~taken by a 25-ml p ipet te , f rom abou t 25 mm be low the wate r ~ " 2 5level. ~ ,2 O ' ,RESULTS AND DISCUSSION ~ ~ ,

T h e r e s u lt s o f o u r e x p e r i m e n t s a r e p r e s e n t e d i n ~ 1 5t e r m s o f t h e e ff e ct o f f e rr i c c h l o r i d e c o n c e n t r a t i o n a n d " 4 1 0s o l u t i o n p H o n ( 1) t h e e l e c t r o p h o r e ti c m o b i l i t y o f ~ , o ,p a r t i c l e s , ( 2 ) t h e r e s i d u a l t u r b i d i t y a f t e r c o a g u l a t i o n 5a n d s e t tl i n g ; a n d ( 3) t h e d y n a m i c s o f t he c o a g u l a t i o nin d e x d ur i n g t h e c oa gu la t i o n p ro c e s s . F u r th er m o re , 0 ~ , , , I , , ~ , I , , v t . . . .t h e u s e o f t h e o p t i c a l t e c h n i q u e t o s t u d y t h e e f fe c t o f 0 1 0 2 0 3 0 4 0 5 0 6 0m i x in g i n te n s it y o n c o a g u la t io n d y n a m i c s is d e m o n - F e r r i c C h l o r i d e ( t t M )s t r a t e d . Fig. 4. Residu al turbid ity (after sett ling) as a fun ction ofE f f e c t o f f e r r i c d o s e o n e l e c t r o p h o r e t i c m o b i l i t y a n d fe rr ic ch lor ide con centra t ion for suspensions wi th d i ffe ren tr e s id u a l tu r b id i t y pH . The following chemical an d physical cond ition s wereused in th e experiments: c lay conc entr atio n = 50 rag/l, co-Th e e f f e c t o f f e r r i c c h lo r id e c o n c e n t ra t i o n o n t h e a g u l a t i o n m ix in g i n t e n si t y = 2 6 s -~ , r a p id -m ix in ge l e c t r o p h o r e t i c m o b i l i t y ( E M ) o f t h e k a o l i n p a r t i c l e s t im e = 2 m i n , c o a g u la t io n t im e = 15 m i n , s e tt li n ga t t w o p H v a l u e s ( 6. 0 a n d 7 . 8 ) i s s h o w n i n F i g . 3. A t t im e = 1 5 m i n a n d b ic a r b o na t e c o n c e n tr a t io n = 1 m M .p H 6 .0 , t h e E M o f t h e k a o l i n p a r t i cl e s i n c re a s e ss i g n i f i c a n t l y a s f e r r i c c h l o r i d e i s a d d e d , a n d c h a r g e 1 9 9 0; S t u m m , 1 9 92 ), a n d , a s a r e s u l t , c h a r g e r e v e r s a lr e v e r s a l i s o b s e r v e d a t f e r r i c c h l o r i d e c o n c e n t r a t i o n s w a s o b s e r v e d a t f e rr i c c h lo r i d e c o n c e n t r a t i o n s l a r g e rl a r g e r t h a n 1 0 t i M . A t p H 7 .8 , o n t h e o t h e r h a n d , t h e t h a n a b o u t 1 0 # M . O n t h e o t h e r h a n d , t h e c ha r g e o fp a r t i c l e s r e m a i n n e g a t i v e l y c h a r g e d a s f e r ri c c h l o r i d e f e r r ic h y d r o x i d e p r e c i p i t a t e s i s c l o s e t o z e r o a t p H 7 . 8i s a d d e d , a n d n o c h a r g e r e v e r s a l i s o b s e r v e d . ( D z o m b a k a n d M o r e l , 1 99 0; S t u m m , 1 99 2) , a n d .A t h i g h f e rr i c c h l o r i d e c o n c e n t r a t i o n s , t h e p a r t ic l e s t h e r ef o r e , t h e E M o f t h e p a r t i cl e s d o e s n o t c h a n g ea r e c o a t e d w i t h f e rr i c h y d r o x i d e p r e c i p i t a t e s , w h i c h s i g n i f i c a n t l y a t h i g h f e r r ic c h l o r i d e c o n c e n t r a t i o n s . A tt h e n d o m i n a t e t h e s u r f a c e a c i d - b a s e p r o p e r t ie s , s u r - l o w e r f e rr ic c h l o r i d e c o n c e n t r a t i o n s , t h e s u r f a c ef a ce c h a r g e a n d E M o f t h e p a r t ic l e s ( J a m e s a n d c h a r g e o f t h e k a o l i n p a r t ic l e s r e fl ec ts t h e f o r m a t i o n o fH e a l y , 1 9 72 ). A t p H 6 . 0 , t h e f e r r ic h y d r o x i d e p r e c i p i - f e r r ic s u r f a c e c o m p l e x e s a n d t h e a c i d - b a s e p r o p e r t i e st a te s a r e p o s i t iv e l y c h a r g e d ( D z o m b a k a n d M o r e l , o f t h o s e c o m p l e x e s ( S t u m m , 1 99 2).

T h e r e s i d u a l t u r b i d i t i e s , m e a s u r e d a f t e r c o a g u -0 . 5 o - o . . . . . . . . . . . - o l a t i o n a n d s e t t li n g o f t h e s u s p e n s i o n s d e s c r i b e d

~ " a b o v e , a r e s h o w n i n F i g . 4 . A t t h e se p H v a l u e s ( i. e.0 5 6 . 0 a n d 7 . 8) , t h e r e s i d u a l t u r b i d i t y c u r v e s a r e c o m p a r -

" -" 9 a b l e . T h e r e s i d u a l t u r b i d i t y d e c r e a s e s c o n t i n u o u s l y a s- 0 . 5 t h e f e r r ic c h l o r i d e d o s e i n c r e a s e s , a n d i t r e a c h e s a

,,9 "~3 c o n s t a n t v a l u e a t f e r r i c c h l o r i d e c o n c e n t r a t i o n s l a r g e r

- 1 " ' t h a n a b o u t 3 5 /~ M . T h e s e r e s u lt s d e m o n s t r a t e t h a t t h e"l 0 c l a y p a r t i c l e s a g g r e g a t e e v e n a t l o w f e r r ic d o s a g e s .. |~ - 1 . 5 ~ / . . . . o - - - . . . . . _ _ _ w h e r e t h e p a r ti c le s a r e n e g a ti v e ly c h a rg e d ." -" ~ ~ C o a g u l a t i o n i n t h e e x p e r i m e n t s d e s c r i b e d i n F i g . 4

i s i n d u c e d b y f o r m a t i o n o f f e r r i c h y d r o x i d e p r e c i p i -/ - - - - p H 6 .0 t a te s i n s o l u t i o n a n d b y a d s o r p t i o n o f f e rr ic p r e c ip i -t a t e s a n d d i s s o l v e d s p e c ie s o n t o t h e s u r f a c e o f t h e c l a y- 2 . 5 - - o - - - p H 7 .8 p a r ti c le s . O n e s h o u l d n o t e t h a t t h e s o l u t i o n i s s u p e r -

- 3 , , , I , , , , I , , , , I . . . . I . . . . I . . . . s a t u r a te d w i t h r es p e ct t o f er ri c h y d r o x i d e u n d e r a ll0 1 0 2 0 3 0 4 0 5 0 6 0 t h e c h e m i c a l c o n d i t i o n s u s e d i n t h e e x p e r i m e n t s

d e s c r i b e d a b o v e ( S t u m m a n d M o r g a n , 1 98 1) . A t p HF e r r i c C h l o r i d e ( ~ t M ) 6 a n d l o w f e rr ic c h l o ri d e c o n c e n t r a t io n s , a d s o r p t i o no f p o s i t i v e l y c h a r g e d f e r r i c h y d r o x i d e p r e c i p i t a t e sFig. 3. Electrophoretic m o b i l i t y ( E M ) o f t h e k a o l i n p a r t i c le sa s a fu n c tio n o f a d d e d f er ri c c h lo rid e fo r two d if fe re n t p H a n d d i s so lv e d sp e c i e s o n t o t h e c l a y p a r t i c l e s r e d u c e s

v a l u e s. A l k a l i n i t y w a s p r o v i d e d b y a d d i n g 1 m M s o d iu m t h e i r n e g a t i v e c h a r g e a n d t h u s c a n p r o m o t e a g g r e g a-b i ca r b o na t e, t i o n a t f e r r ic d o s a g e s w h e r e t h e p a r t i c l e s h a v e n o n e t


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Dynam ics of coagulation of kaolin 563charge . Th i s mec han i sm, however , cann ot exp la in the The coagu la t ion index fo r the exper iments a t pHcoa gulat io n of the clay suspen sion at pH 7.8, s ince 6.0 [Fig. 5(a)] increases grad ual ly with t ime in thethe par ti c l es a t th i s pH are h igh ly charged a t a l l fe r r ic fe r r ic ch lo r ide concen t ra t ion range o f 1 .5 -9.0 # M . Atdosages (Fig . 3) . As wil l be descr ibed l a t er , fo rm at ion fer r ic dosages l a rger than 9 .0 #M , the ini ti al ra t es o fof pa tches o f pos i t ive ly charged ferr i c p rec ip i t a tes on the increase in the coagu la t ion ind ices are g rea ter ,the negat ive ly charged sur face o f the c lay par t i c les imply ing fas ter aggregat ion ra tes . S imi lar behav ior i sm ay a l so induce coagu la t ion . At bo th pH values , observed in the exper iments a t pH 7 .8 [Fig . 5(h)] ," sweep -f loe" coagu la t ion i s the p re dom inan t aggre- where the ini ti al s lope o f the coagu la t ion index curvesgat ion mec han i sm a t h igh fer r ic ch lo r ide conce n- increases wi th fer r ic coag u lan t dose . Fo r bo th suspen-t ra t ions . In th is mec han i sm, the l a rge fer r ic hydrox ide s ions , the curves o f the coagu la t ion index a t h ighprec ip i t a tes enm esh suspended par t ic l es an d ef fec- fe rr ic dosages pass th rough a m axim um fo l lowed byt ive ly remove the m by se tt ling . The coagu la t ion a g radual decrease o f the coagu la t ion index. Th i smec han i sm s in the exper iments wi th the abov e sus - phen om enon wi l l be d iscussed l a t er in th i s sec tion .pens ions are di scussed in m ore de ta il in the fo llowing Fur the rm ore , i t shou ld be no ted tha t , in the absencesect ion . I t i s shown tha t me asurem ents o f coagu la t ion o f Fe( I I I ) , the coagu la t ion index in a l l the exper-index o f the aggregat ing suspens ions by the f low- iments repor ted here remained unchanged , as shownthrough m oni to r can p rov ide m ore ins igh t in to the in Fig. 5 (b) .mec han i sm s of coagu la t ion wi th fer r ic ch lo r ide . S ince the EM o f the c lay par t i c les a t pH 6 .0increases (becom es l ess negative) and passes th roughEf fec t of fer ric chloride concentration on coagulation zero as fer r i c ch lo r ide i s added , one m igh t concludeindex curves tha t charge neu t ra l i za t ion i s the p redominan t coagu-

The change o f the coagu la t ion index wi th time fo r l a t ion mec han i sm in those experiments ; however , thethe coagu la t ion exper imen ts descr ibed abov e i s increase in the coagu la t ion index a t a ll fe r r ic dosagesshown in Fig . 5 . As descr ibed before , the coagu la t ion (1 .5 #M and l a rger ) fo r the exper iments a t pH 6 i sindex, me asured con t inuous ly by the op t i ca l mo ni to r , mu ch grea ter than tha t expected fo r coagu la t ion byi s very sens it ive to the s t a te o f aggregat ion o f par t i cu- charge neu t ra l i za t ion . The num ber conc en t ra t ion o flate suspensions. Th e data prese nted in Fig. 5 are for part icles larger than 0.6 #m in the clay suspension, asthe per iod o f s low mix ing , tha t is , fo l lowing the 2 min me asured by the Cou l ter co un ter , i s 4 .02 x l0 s perof rap id mix ing , cm 3. Calcu la t ions o f coagu la t ion ra te based on the

7 L [. p H 6 .0 ; 1 .5 gM6 - - - - o - - 3 . 0 ~ M5 " ~ A 6 .0~M

~ - - - - -o - -9 .0 ~3 = 1 0 . 5glVl

~ 12.0 gMO 2 I ~ * 15.0gMO 1 q - - - -a- - -18 .0FM

o b . p H 7 .8~ 5 = 0 g M 6 g M

4 ~ 9 . M~O ) ~. 12

) - - - - o - - - 1 5 g M~ 2 ------cY-- 1 gMo ~ A 30p_M~J 1 * 4 S p _ M

O 0 5 1 0 1 5T i m e ( r a i n )

Fig. 5. Coagulation index as a function of time for various concentrations of ferric chloride for thecoagu lation experimen ts described in Fig. 4.


6/11

564 HSlAO WEI CHINGet al.t he o r y o f o r t hok i ne t i c c oa gu l a t i on ( I ves , 1990 ) f o r a gg r e ga t e s ( " s w e e p - f loe " c oa gu l a t i on ) . A t h i gh f e r ri ct h is s u s pe ns ion , a s s um i ng f a vo r a b l e c he mi c a l c on - c h l o r i de c onc e n t r a t i ons , t he f o r m a t i on o f hyd r o x i dedi t ions , shows tha t the res idua l (normal ized ) par t i c l e prec ip i t a tes is ve ry fas t . Thi s is ev idenced by the h ighc onc e n t r a t i on a f t e r 15 mi n o f c oa gu l a t i on a t i n it ia l va l ue s o f t he c oa gu l a t i on i nde x , i mme d i a t e lyG = 26 s - ~ i s 0 .93 . Thi s me ans tha t , a l tho ug h the a f t e r the rapid mixing [F ig . 5(h) for fe r r ic dosag es ofpa r t ic l e s a r e des t a b il iz e d , mo s t o f t he m w i ll re ma i n 30 a nd 45 #M ] .una ggr e g a t e d be c a us e o f t he l ow i n t e rpa r t ic l e c o l - A n i n te r e s ti ng f e a tu r e in the c oa gu l a t i on inde xl is ion r a t e . C oa gu l a t i on e xpe r i me n t s a t h igh e le c t r o - c u r ve s s how n i n F i g . 5 i s t he ma x i m um obs e r ve d a tl y te ( K C I a nd C a C12) c onc e n t r a t i on , unde r c ond i t i ons f e r ri c c h l o r i de c onc e n t r a t i ons l a r ger t ha n a bo u tw he r e t he pa r t i cl e s a r e a s s ume d t o be de s t a b il iz e d , 10 #M . T he c oa g u l a t i on i nde x c u r ve s pa s s t h r ou gh aw e r e c ond uc t e d to t e st t h is hypo t he s is . I t w a s f oun d ma x i m um f o l l ow e d by a c on t i nuou s de c re a s e. T het ha t t he c oa g u l a t i on i nde x i n t hos e e xpe r i me n t s ma x i m um oc c u r s a t a n e a r li e r t ime a s t he f e r ri cc ha ng e d ve r y l it tl e ( by l es s t ha n a f a c t o r o f 2 ) a f t e r c h l o r i de c onc e n t r a t i on i nc r ea s e s. T h i s ph e no me non15 mi n o f c oa gu l a t i on , t hus s uppo r t i ng t he c on - a ppa r e n t l y s ugge st s t ha t t he a gg r e ga t e s iz e de c r e a se sc l u s ion t ha t t he c oa gu l a t i on r a t e s i n the e xpe r i me n t s a f t e r t he t ime c o r r e s pond i ng t o t he pe a k i n t he c u r ve .s how n in F i g . 5 ( a ) a r e l a r ge r t ha n thos e e xpe c t e d f o r A c l o se i n s pe c t i on o f t h i s phe n om e non , how e ve r ,c ha r ge ne u t r a l iz a t i on , s ugge s ts t ha t t he de c r e a s e i n t he va l ue o f t he c oa gu -

At pH 6 .0 and low fe r r i c dosages , fe rr i c hyd roxid e l a t ion index resul ts f rom the se t t ling of l a rge aggre-prec ip i t a tes and d i s solved spec ies a re pos i t ive ly ga tes ( flocs ) dur in g the coa gula t ion process . At lowcha rge d, wh ereas the k aol i n part icles are ne gat ive ly m ixing intensit ies , i t i s diff icul t to keep the large f loesc ha r ge d (s ee F i g . 3 ). U n de r t he se c ond i t i ons , a ds o r p - s u s pe nde d i n t he w a t e r , a nd a s i gn if i ca n t num be r o ft ion of smal l fe r r i c hyd roxid e prec ip i t a tes or sur face these f locs se tt le . Thu s , the n um be r of par t i c l esprec ip i t a t ion (Far ley e t a l . , 1985) mi gh t f o r m s ma l l s a mp l e d by t he T y gon t ube l e a d ing t o t he pho t opos i ti ve ly c ha r ge d pa t c he s on the ne ga t ive l y c ha r ge d d i s pe r si on a na l yz e r de c r ea s e s. S i nc e the c oa g u l a t i onsur faces of the c lay par ti c l es . As a resul t, a t t rac t ive index i s pro po r t ion a l to par t i c l e s ize and to the squar ef o r c e s c ou l d de ve l op be t w e e n a pa t c h a nd a n opp o- r oo t o f pa r t i cl e c onc e n t r a t i on ( G r e go r y , 1985 ), thes it ely charg ed bare sur face as par t i c l es col l ide dur ing coa gu la t ion index decreases as fewer par t i c l es ente rc oa gu l a t i on . T h i s " e l e c t r o s ta t i c pa t c h " mod e l c a n t he s a mpl i ng t ube . T h i s hypo t he s i s w a s f u r t he r ve r -e xp l a in t he c oa gu l a t i on o f ne ga t ive l y c ha r ge d pa r - i fi ed by low e r i ng t he s a mpl i ng po i n t in the c oa gu -t ic le s w i th c a t i on i c po l ym e r s obs e r ve d at c oa gu l a n t l a t ion ve ss el du r i ng a gg r e ga t i on a t h igh dos a ge s o fc onc e n t r a t i ons i n su f f ic i en t f o r c ha r ge ne u t r a l iz a t i on , f e r ri c c h lo r i de . I t w a s f ou nd t ha t t he c oa gu l a t i oni .e . when the E M of the par t i c l es i s subs tant i a l ly index increased s igni f icant ly soo n a f t e r the samp l ingne ga t ive ( G r e g o r y , 1973 , 1976 ) . A s d i s cus s e d be f o r e , t ube w a s l ow e r e d . I t s ho u l d be no t e d t ha t h i ghe rt he c oa gu l a t i on r a te s a t l ow f er r ic c onc e n t r a t i ons a r e c onc e n t r a t i ons o f fl oes w e re v i sua l ly obs e r ve d be l owh i ghe r tha n t hos e f o r c ha r ge ne u t r a li z a t ion . I t ha s t he s a mpl i ng po i n t du r i ng t hos e c oa gu l a t i on e xpe r -be e n s how n t ha t c oa gu l a t i on r a t e s o f pa t c he d s u r - i me n t s. I n a dd i t i on , a s w il l be show n l a te r i n th i sf a ce s c a n be f a s te r t ha n t hos e obs e r ve d in a f u ll y pa pe r , t h is m a x i m um d i s a ppe a r s a t h i gh m i x i ngdestab i l ized suspe nsion , i .e . by ch arg e neu tral iza t ion intensit ies .(Gre gory , 1976, 19 88) . As the fe r r i c dose increases ,t he c onc e n t r a t i on o f f e r ri c hyd r ox i de p r e c i p i ta t e s E f f e c t o f s o lu t i o n p H o n e le c t r o p h o r e t i c m o b i l i t y a n dinc reases , resul t ing in h igher coagula t ion ra tes and, r e s i d u a l t u r b i d i t yhence , l a rger coa gula t ion indices . Th e e f fec t of so lu t ion pH on the e lec t roph ore t i c

At p H 7 .8, the kaol in par t i c l es a re nega t ive ly mo bi l i ty of the kaol in par t i c l es a t d if fe rent fe r r icchar ged (F ig . 3) , whi le the fe rr i c hyd rox ide prec ip i - ch lor ide conce nt ra t io ns is show n in F ig . 6 . W i th not a te s ha ve a c ha r ge c l o s e t o z e r o ( D z om ba k a nd f e r ri c c h l o r i de in s o l u t ion , t he ka o l in pa r ti c le s a reM o r e l , 1990; S t um m, 1992) . A t l ow fe r r ic dos a ge s , ne ga t i ve ly c ha r ge d a t a ll pH va l ues . A t pH va l ue sthe par t i c l es have a l a rge nega t ive charge ; never the- smal le r than a bo ut 8 , the EM o f the par t i c l es in-l es s, the agg rega t io n ra te i s subs tant i a l . Thi s , fo r c reases (becom es l es s nega t ive) as fe r r ic chlor ide i sins tance , can be seen f rom the coag ula t io n index add ed; the par t i c l es reverse the i r cha rge ( f rom nega-curve for a fe r r ic d ose of 6 g M [F ig . 5(b) ]. T he t i re to pos i t ive ) a t low pH va lues . Thi s is a t t r ibu teda gg r e ga t i on a t t he s e c ond i t i ons c a n be pa r t l y e x - t o a ds o r p t i on o f f e rr i c s pe ci es a nd f e rr i c hyd r o x i depla ined by the e lec t ros ta t i c pa tch m odel d i scussed prec ip i t a tes on to the sur face of the c lay par t i cl es .abov e . In addi t ion , col l i s ion be tween prec ip i t a tes a t Fer r i c hyd rox ide prec ip i t a tes a re pos i t ive ly char gedpH 7 .8 is f a vo r a b l e , r e s u lt ing i n l a r ge r p r e c i p it a te s a t pH va l ue s s ma l le r t ha n a b ou t 8 ( D z o m ba k a n da nd , t he r e f o r e, h i ghe r a gg r e ga t i on r a te s . M o r e l , 1990 ) . T he i s oe l e ct r ic po i n t s ( pH w he r e t heA t h i gh f e rr i c dos a ge s , t he f o r m a t i on o f f e r ri c E M is z e r o ) , c o r r e s po nd i ng t o f e rr ic c h l o r i de dos a ge shydr ox i de p r e c i p it a te s d r a ma t i c a l l y i nc re a s e s t he c o - o f 3 .0 , 10 . 5 a nd 36 . 0p M a r e 4 . 5 , 6 . 0 a n d 6 .5 ,agu la t ion ra te by ( ! ) inc reas ing the so l ids conc en- respec t ive ly .t r a t i on a nd t hus t he r a te o f a gg r e ga t i on i n t he R e s i dua l t u r b i d i t y c u r ve s f o r t he c oa g u l a t i on e x -suspens ion and (2) enm eshing c lay par t i c l es in l a rger per im ents un der the so lu t ion ch emis t r i es descr ibed


7/11

Dynamics of coagulat ion of kaol in 5653 c h a r g e ( s e e F i g . 6 ) , i s s m a l l e r t h a n t h a t a t h i g h e r p H

~ " ~ - - 0 - - 3 .0 p .M v a l u e s , w h e r e t h e p a r t i c l e s a r e n e g a t i v e l y c h a r g e d . A s2 " .. '~ d e s c r i b e d e a r l i e r , c o a g u l a t i o n r a t e s h i g h e r t h a n t h o s e\ - - v - - 1 0 . 5 ~ f o r c h a r g e n e u t r a l i z a ti o n a r e p o s s i b l e w h e n t h e e l ec -1 " " "~ - - ~ - - 3 6 . 0 ~ M t r o s t a t ic p a t c h m o d e l i s i n v o k e d . T h e c o a g u l a t i o n. ~ " '. 4 i n d e x a t p H 8 .7 r e m a i n s u n c h a n g e d d u r i n g t h e e n t i r e, c o a g u l a t i o n p e r i o d . A t t h is p H , t h e p a r t ic l e s a n d t h e1 " ' o , , ~ s m a l l f e r ri c h y d r o x i d e p r e c i p it a t e s a r e n e g a t i v e ly~ . .~ A , . ~ c h a r g e d , s o t h a t p r e c i p i t a t e - p r e c i p it a t e , p a r t i c l e -, ~ ~ " p a r t ic l e a n d p a r t i c l e - p r e c i p it a t e i n t e r a c ti o n s a r e

- 2 ~_ ~ 'ox h, . ~ p r o b a b l y u n f a v o r a b le . A s a re s u l t , g r o w th o f a g g re -- 3 g a t e s o r s o l i d p r e c i p i t a t e s i s n o t f e a s i b le , r e s u l t i n g i nn e g l i g i b l e c o a g u l a t i o n r a t e s .

T h e c o a g u l a t i o n i n d e x c u r v e s a t h i g h e r f e r r i c- 4 . n o F e C l 3 d o s a g e s [ 10 .5 a n d 3 6 . 0 / I M i n F i g . 8 ( b ) a n d ( c ),- $ " , , , , I , . , , t , , , . I . . . . t . . . . t . . . . t . . . . t . . . . r e sp e ct iv e ly ] i n c r ea s e m u c h f a s te r t h a t t h o s e a t

3 4 5 6 7 8 9 1 0 1 3 . 0 # M . I n a d d i t i o n , l a r g e r c o a g u l a t i o n i n d i c e s a r ep H r e a c h e d , i m p l y i n g l a r g e r a g g r e g a te s . A t t h e s e f e r ri cc o n c e n t r a t i o n s , f o r m a t i o n o f l a rg e h y d r o x i d e p r e c i p i-F ig . 6 . E le ct rophore ti c mob i l it y (EM) o f the ka o lin suspen - t a t e s i s m or e f a v o ra b le in the pH ra n ge o f 6 -8 . T he s es ion a s a funct ion o f so lu t ion pH fo r d i ff e re n t f e rr ic c h lo r ide p re c ip i t a t e s inc re a s e the so l id s c on c e n t ra t i on in su s -c once n tra tions . A lka lin ity i s p rovided by a dd ing I mM of pe n s ion , a nd , a s a r e su l t , t h e c o a g u la t ion ra t e in -sodium bicarbonate. c r e a s e s . A t v e r y l a r g e f e r r i c d o s a g e s [ F i g . 8 ( c ) ] , t h ef o r m a t i o n o f l a rg e h y d r o x i d e p r e c i p i ta t e s i s v e r y f a s t,a b o v e a r e s h o w n i n F ig . 7. A t l o w fe r r ic d o s e ( 3 / ~ M ) a n d s w e e p - f lo e c o a g u l a t i o n d o m i n a t e s . T h e s e p re c i p i-a n d l o w s o l u t i o n p H , c o a g u l a t i o n i s in d u c e d b y t a te s a r e q u i t e l a r g e a f t e r 2 r a i n o f r a p i d m i x i n g , a sa d s o r p t i o n o f f e r ri c s p e c i es a n d h y d r o x i d e p r e c i p i- e v i d e n c e d b y t h e h i g h i n it ia l v a l u e s o f t h e c o a g u l a t i o nr a t e s w h i c h c a n f o r m p o s i t iv e l y c h a r g e d p a t c h e s o n i n d e x a t th i s p H r a n g e . T h e m a x i m u m a n d s u b -t h e s u r f a c e o f t h e c la y p ar t ic l e s, a n d t h u s i n d u c e s e q u e n t d e c r ea s e o f t h e c o a g u l a t i o n i n d e x a t h i g ha g g r e g a t i o n ( d i s c u s s e d i n t h e p r e v i o u s s e c t i o n ) . I n f e r r i c d o s a g e s i s a t t r i b u t e d t o t h e s e t t li n g o f l a r g ea d d i t i o n , f o r m a t i o n o f h y d r o x i d e p r e c i p i ta t e s c a n a g g r e g a t e s , a s d i s cu s s e d p r e v i o u s l y i n t h is p a p e r .i n c r e a s e t h e s o l i d c o n c e n t r a t i o n a n d , h e n c e , t h e i n t e r -p a r t i c l e c o l l i s i o n r a t e . A t l o w p H v a l u e s , i t i s n o t Effe ct o f mi xing in ten si ty on residual turbidi ty an de x p e c t e d t h a t s i g n if i ca n t a m o u n t s o f l a r g e p r e c ip i t a t es coagulation ind ex curvesw o u l d b e f o r m e d a t a f e rr i c c o n c e n t r a t i o n o f 3 /z M . T h e e f fe c t o f m i x i n g i n t e n si t y ( e x p r es s e d i n t e r m s o fT h e r e f o r e , b e c a u s e o f l o w i n t e r p a r t i c l e c o l l i s i o n r a t e s , t h e v e l o c i t y g r a d i e n t G ) o n t h e r e s i d u a l t u r b i d i t ya g g r e g a t e s d o n o t i n c r e a s e i n s i z e s i g n i f i c a n t l y a n dr e m o v a l o f t u r b i d i t y b y s e t t l i n g i s l o w .

A t h i g h f e r ri c c h l o r i d e c o n c e n t r a t i o n s , a g g r e g a t e s 4 0 K o , ~ xi n cr e as e i n s iz e b e c a u se o f t h e h i g h er c o a g u l a ti o n r a te , ~ 3 5 ~ ~ Fa n d r e s i d u a l t u r b i d i t i e s a f t e r s e t t l i n g c a n r e a c h l o w e rv al ue s. A s d e sc ri be d p r ev io u sl y, t h e p r e d o m i n a n t " " 3 0 . ~ /m e c h a n i s m o f c o a g u l a t i o n a t h i g h f e r ri c c o n c e n - . .~ N , t~ ' ~ -~ , / ,

i - J i "t r a t i o n s i s t h e f o r m a t i o n o f f e r ri c h y d r o x i d e p r e c i p i- 2 5 Ar a t e s t h a t i n c r e a s e t h e p a r t i c l e c o l l i s i o n r a t e . I na d d i t i o n , a t h i g h f e r r i c d o s a g e s , l a r g e p r e c i p i t a t e s 2 0 ~ ; O fIe n m e s h t h e s m a l l e r p a r t ic l e s a n d r e m o v e t h e m f r o m ' ~ ~: It h e s u s p e n s i o n b y s e t tl i n g . E f f i c ie n t c o a g u l a t i o n a n d , ~ 1 5 q ' ~ I ,s e tt li n g o f a g g r eg a t es i s o b s e r v e d o v e r b r o a d e r p H "~ ~ , ~ . . . . . ~ '- - < y - 3 . 0 g Mr a n g e s a t h i g h e r t o t a l f e r r i c c o n c e n t r a t i o n s , c o n s i s t e n t ~ 1 0 ~ w i t h t h e s o l u b i l i t y d i a g r a m o f F e ( I I I ) ( S t u m m a n d - o - 1 0 . 5 I~ M ",, , 'M o r g a n , 1 9 81 ). 5 - - ~ - - 3 6 . 0 o_M ~ . . . . . ~ - , ~

, . , I . , . , I , , . , I , , , , I , , , . 1 , , . , I , , . , lEffec t o f solut ion p H on coagulat ion index curves 0 3 4 5 6 7 8 9 1 0T h e c h a n g e o f t h e c o a g u l a t i o n i n d e x a t d i f fe r e n t p Hp H v a l u e s i s s h o w n i n F i g . 8 f o r t h r e e f e r r i c d o s a g e s .A t l o w f e r r i c d o s a g e s [ 3 ~ t M i n F i g . 8 ( a )] , th e c o a g u - Fig. 7. Residual turbidity (after settling) of the kaolinl a t ion inde x inc re a s e s c on t inu ou s ly a t pH va lue s suspe ns ions a s a funct ion o f so lu t ion pH fo r d i ff e re n t f e rr icsm a l l e r t ha n 8 . The s lop e o f t he c oa gu la t io n inde x c h lo r ide c once n tra tions. The phys ic a l c ondi tions a re s imil a rc u r ve a t pH 4 .7 , w h e re the pa r t i c l e s ha v e no ne t t o those de scr ibe d in F ig . 4 .


8/11

566 HslAO WE! CHING e t a l .6 as

- - -O- -pH 8 .7I~ - - -o - - pH 7.4

- -O- - -p H 6 .4- - A - - p H 5 .8;0D 2 pH 4.7U

1{

0 I I b

5, ~ - - - O - -pH 8.24 - - O - - p H 7 .7

~D pH7.1~ 3 ~ p H 6 .4 pH 6.0

2 & pH 5 .3- - -O- -pH 4 .0

! !; ' ' ' ' ' I ' ' ' ' ' ' ' ' ' I ' ' ' ' ' ' ' ' l

4, ~ - - - O - - p H 9 .0- - -CF-pH 8.1

I~ 3 + p H 7.6------A--- H 6.4" ~ 2 - - @ - - p H 5 .5- - . o - - p H 4 . 0

"1 I I I I I I I I I I l I I I I I I I I I l I I I I I I I0 0 5 10 5T i m e ( r a in )

Fig. 8. Coagulation index as a function of time for different solution pH at three ferric chlorideconcentrations: (a) 3.0 pM , (b) 10.5 pM , an d (c) 36.0 #M . The coagulation indices were measured duringthe coagulation experiments reported in Fig: 7.

c u r v e s i s s h o w n i n F i g . 9 . T h e m i x i n g i n t e n s i t y i n c o a g u l a t i o n i s t h e p r e d o m i n a n t m e c h a n i s m , a s d i s -t h e s e e x p e r i m e n t s w a s c o n t r o l l e d b y a d j u s t i n g th e c u s s e d e a r l i e r i n t h i s p a p e r .s p e e d o f t h e f i a t - b l a d e m i x e r ( F i g . 2 ). T h e c o a g u l a t i o n A t a f e r r i c c h l o r i d e c o n c e n t r a t i o n o f 1 0 /~ M , t h ee x p e r i m e n t s r e p o r t e d i n t h i s f ig u r e w e r e c o n d u c t e d a t r e s i d u a l t u r b i d i t y d e c re a s e s c o n t i n u o u s l y a s t h e m i x -p H 6 . 0 w i t h t w o d i f f e r e n t f e rr i c c h l o r i d e c o n c e n - i n g i n t e n s i t y i n c r e a s e s f r o m 5 u p t o 57 s t . A t t h i st r a t i o n s ( 10 a n d 3 6 /~ M ) . T h e s e c o n c e n t r a t i o n s o f r a n g e o f m i x i n g in t e n si t ie s , th e c o a g u l a t i o n r a t ef e r r ic c h l o r i d e w e r e s e l e c t e d s o t h a t t h e i n f lu e n c e o f i n c r e a s e s w i t h G b e c a u s e o f t h e i n c r e a s e i n t h em i x i n g i n t e n s i t y c o u l d b e i n v e s t i g a t e d u n d e r d if f e r e n t c o l l i s i o n r a t e o f t h e s o l i d p r e c i p i t a t e s a n d t h e u n -c o a g u l a t i o n m e c h a n i s m s . A t t h e l o w e r f e rr ic c o n c e n - c h a r g e d p a r t ic l e s . A s t h e c o a g u l a t i o n r a t e i n c r e a se s ,t r a t i o n ( I 0 /~ M ) , c o a g u l a t i o n i s i n d u c e d b y f o r m a t i o n l a r g e r a g g r e g a t e s a r e f o r m e d , r e s u lt i n g i n h i g h e ro f f e r ri c p r e c i p i t a t e s i n s o l u t i o n ( i. e . i n c r e a s i n g p a r - s e t t l i n g r a t e s a n d c o n s e q u e n t l y l o w e r r e s i d u a l t u r b i d -t i c l e c o l l i s i o n r a t e ) a n d b y a d s o r p t i o n o f t h e p o s i t i v e l y i t ie s . A t G v a l u e s l a r g e r t h a n 5 7 s - ~, s t r o n g s h e a rc h a r g e d f e r r i c p r e c i p i t a t e s a n d d i s s o l v e d s pe c i e s o n f o r c e s d e v e l o p , c a u s i n g b r e a k a g e o f a g g r e g a t e s . A s at h e c l a y p a r t i c l e s . P a r t i c l e - p a r t i c l e i n t e r a c t i o n s u n d e r r e s u l t , th e r e s i d u a l t u r b i d i t y d o e s n o t c h a n g e a s Gt h e s e c o n d i t i o n s a r e f a v o r a b l e , s i n c e t h e p a r t i c l e s i n c r e a s e s f r o m 5 7 u p t o 1 37 s - k .h a v e n o n e t c h a r g e ( s e e F i g . 3) . O n t h e o t h e r A t h i g h e r f e r r ic c h l o r i d e d o s a g e s ( 3 6 # M ) , la r g eh a n d , a t t h e h i g h e r f e r r i c d o s e ( 3 6 / ~ M ) , s w e e p - f lo c a g g r e g a t e s a r e f o r m e d b y p r e c i p i t a t i o n o f f e r r i c


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Dynam ics of coagulation of kaolin 5673 0 increase in the par t i c l e col l i s ion ra te . A t G va lues

- - o - - 10 . 51a M l a r ge r t ha n 57 s - 1 , t he c oa g u l a t i on i nd ic e s r e a c h a2 5 - - v- - 36 .0 I tM co m par able va lue . In addi t ion , a t these mixin g in ten-

s i t i es , the coagula t ion indices reach the i r maximumva l ue a f t e r on l y a bo u t 5 r a i n o f c oa gu l a t i on . A t l a r ge r 2 0 c oa g u l a t i on t ime s , t he c oa g u l a t i on i nd ic e s do no tc ha nge , t hus i nd i c a t i ng t ha t t he a gg r e ga t e s a t t a i n a n

1 5 equi l ibr ium ( l imi t ing) s ize . Th e aggre ga tes can no tf u r t he r i nc r e a s e i n siz e due t o b r e a ka ge o f a gg r e ga t e sc a us e d by t he h i gh s he a r f o r c e s . A t ve r y l ow mi x i ng

1 0 [] in tens it i es (e.g . the curve for 5 s - 1), the coa gu la t ion" , i nde x i nc r ea s e s mo no t o n i c a l l y ove r t he e n t i re pe r i od

5 o f c oa gu l a t i on . B e c a us e o f t he l ow pa r t ic l e c o l l is i onO Ob . . . . . . . v - . . . . . . . ~ . . . . . . . . . . o r a t e a t t h is m i x i n g i n te n s it y , a g g r e g at e s d o n o t a t t a in| 1 i i i i i l | l | l l | l l | l l | l i i i i i | |0 their l imit ing s ize.0 5 0 1 0 0 1 5 0 T he c oa g u l a t i on i nde x c u r ve s f o r h i gh f e rr i c dos e

G (a "1) (36 /zM ) a re show n in F ig . 10(b) . A t th i s fe r r ic dose ,Fig. 9. Residual turbidity (after sett ling) as a function of large ferric pr ecipi ta tes are fo rm ed with in a s ho rtcoagula tion mixing in tensi ty (G) for d i fferent fer ri c dosag es , pe r io d of t ime; sweep -f loe coa gula t ion i s the p red om -Al l experiments were conduc ted a t pH 6 .0 and I mM inan t coa gula t ion me chani sm . A t low mix ing in tens i -sodium bicarbonate, t ies (G = 26 s-~), large f loes sett le an d the

c o a g u l a t i o n i n de x c u r v e p a ss e s t h r o u g h a m a x i m u mhyd roxid e . T he res idua l tu rb id i ty decreases as the (d i scussed ear l i e r in th i s paper ) . A t mixin g in tens it i esmi x i ng in t e ns i ty ( G ) i nc re a s e s up t o a l i m i ti ng va l ue l a r ge r t ha n 26 s - l , t he c oa gu l a t i on i nde x c u r ve s a r ebe tween 26 and 57 s - ~. At grea te r m ixing in tens i ti es , com para ble . The coa gu la t ion indices a t t a in a l imi t inga sl ight inc rease in the res idua l turb id i ty is observ ed, va lue wi th in les s than 3 min . A t h igh mix ing in tens i -T h i s m a y be a t t r i bu t e d t o t he f o r m a t i on o f s ma l l t ie s, t he l a r ge a gg r e ga t e s do no t s et tl e, a nd t hepa r t ic l e s up on a g g r e ga t e b r e a ka ge ; s ma l l pa r t ic l e s c oa g u l a t i on i nd ic e s r e ma i n unc ha n ge d a f te r a gg r e -c on t r i bu t e mo r e t o t he t u r b i d i t y t ha n l a rge a gg r e ga t e s ga t e s r e a c h t he i r l i m i ti ng s iz e.(van de Huls t , 1957; Kerker , 1969) .C oa gu l a t i on i nde x c u r ve s f o r t he e xpe r i me n t s de - Analysis of the shear in the f low through detectorsc r ibed abo ve a re presen ted in F ig . 10. A t a fe r r ic Du e to the na tu re of the s igna l ana lys i s in thedose of l0 /~ M [F ig . 10(a) ], the in it i a l s lope of the opt i ca l m on i tor used in th i s research , the f requ encyc oa gu l a t i on i nde x c u r ve s i nc re a s e s a s t he mi x i ng o f t he f l uc tua t i ng si gna l s hou l d be a bou t a f e wi n t e nsi t y i nc re a s es , i nd i c a t ing h i ghe r a gg r e ga t i on hun dr e d H z ( G r e go r y , 1985 ) . W i t h l ow e r f r e quenc i e s,r a te s . A s d i s c us s e d p r e v ious l y , t h is i s c a us e d by t he s ome o f the s igna l w i ll be l o s t, a nd t he R M S ou t pu tw i ll be low e r t ha n t he t r ue R M S va l ue. T o a vo i d t h is

p r ob l e m , t he a ve r a ge ve l oc i t y o f t he p a r t ic l e s f l ow i ng87 . a t h r ou gh t he de t e c t o r s hou l d be a f e w c m/ s . A f l ow, ~ ~ : ~ / ~ l r a te o f 22 m l/m in w as u sed in th e ex pe rim en ts re -6 C (s 1) po r ted in th i s pape r . Fo r a 3 m m ( i.d. ) tube , th i s f low

. ~ 5 - - -o - a a r r a t e c o r r e s pon ds t o a n a ve r a ge ve l oc i ty o f 5 c m/ s .~1 zl --C]---94 Sim ilar velo ci ty wa s used in al l pre vio us co ag ula t io n- - - o - s t s t ud ie s e m p l oy i ng t h is op t i c a l t e c hn i que ( e. g. G r e -t~ 3 ~ 2 62 - - 0 - 5 go r y , 1988 ; G r e go r y a nd L e e , 1990 ; G r e go r y a nd L i ,

1 1991)." ' " W h e n a pa r t ic l e s u s pe ns i on pa s se s t h r ou gh a" o na r r o w t ube unde r l a mi na r f l ow c ond i t i ons , a s he a r

6 G ( s - 1 ) ( ve l oc it y g r a d i e n t ) i s de ve l ope d . T he a ve r a ge s he a r5 - - -o- la7 ra te in the tube (G ) i s g iven by (Gre gory , 1981)4 ~ 8Q3 L - - ~ 2 6 G - 31rR 03 (4)

Ot9 2 wh ere Q i s the volum et r i c f low ra te , and R0 i s the

5 1 ' 0 ' " " i 5 i nne r r a d i u s o f t he t ube . E qu a t i on ( 4 ) a pp l i es t oT i me ( ra in ) l a mi na r f low c ond i t i ons . T he 5 c m/ s a ve r a ge ve l oc it yused in th i s inves t iga t ion i s wi th in the l aminar f lowFig. 10. Coagulation index as a function of time for diff ere nt reg ime .mixing intensities (G): (a) ferric do se = 10/~M ; (b) ferricdose =36 /aM . Al l exper iments were conduc ted a t pH 6 .0 Fo r the f low con di t ions used in ou r wo rkand 1 mM sodium bicarbonate. (Q = 22 ml /m in , Ro = 1 .5 ram) , the averag e shear ra te


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568 Hs lno WEI CHING et al .c a l c u l a t e d f r o m e q u a t i o n ( 4 ) i s a b o u t 9 0 s ~. U n d e r R EF ER EN C ESt h i s s h e a r r a t e , i t i s l i k e l y t h a t s o m e a g g r e g a t e s w o u l db e d i s r u p t e d o n p a s s a g e t h r o u g h t h e t u b e b e f o r e A m i r th a r a ja h A . a n d O ' M e l i a C . R . ( 1 9 9 0) C o a g u l a ti o nprocesses: destab ilization, mixing, and flocculation. Inr e a c h i n g t h e o p t i c al f l o w - t h r o u g h d e t e c t or . H o w e v e r , Wat e r Q ua l i t y and Tr e a t me n t . M c G r a w - H i l l, N e w Y o r k .a s wa s d e m o n s t r a t e d i n F ig . 1 0 , t h e c o a g u la t i o n De n te l S . K . (1 99 1 ) Co a g u la t i o n c o n t ro l i n wa te r t r e a tm e n t .i n d i ce s s ti ll i n c r e a s e d w h e n t h e m i x i n g i n t e n s i t y w a s Crit. Rev. Envir. Control 21, 41-135.g r e a t e r t h a n 9 0 s - I , t h u s i n d i c a t i n g t h a t a g g r e g a t e s D z o m b a k D . A . a n d M o r el F. M . M . (1 9 9 0 ) Sur f ac ew e r e s t i l l g r o w i n g i n s i z e, r e g a r d l e s s o f t h e h i g h s h e a r . Compl e x a t i on Mode l i ng : Hy dr ous Fe r r i c Ox i de .W iley-In te rsc ience , New York .N o t e t h a t f o r t h e e x p e r i m e n t s a t h i g h m i x i n g i n t e n s i - F a r le y K . J. , D z o m b a k D . A . a n d M o r el F . M . M . 0 9 8 5 )t i e s (G > 9 0 s - l ) , t h e a v e ra g e s h e a r r a t e i n t h e t u b e i s A su r fa c e p re c ip it a t io n m o d e l for t h e so rp t i o n o f c a t i o n ss m a l l e r t h a n t h a t i n t h e s t i r r e d v e s s el . T h e s e a r g u - o n m e t a l o xi de s. J . Col lo id In ter face Sci . 106, 226-242.m e n t s m a y su g g e s t t h a t t h e e x t e n t o f a g g re g a t e F ra n q o i s R . J . (1 9 8 8 ) Gro w th k in e ti cs o f h y d ro x id e f lo c s.J . Am. Wa t . W k s Ass . 80, 92-96.b r e a k - u p i n t h e t u b e i s n o t v e ry l a rg e . In a d d i t i o n , Gre g o ry J . (1 97 3 ) Ra t e o f f lo c cu la ti on o f l at ex p a rt ic le s b yt h e a v e r a g e G t v a l u e ( " C a m p n u m b e r " ) i n t h e c a ti o n ic p ol ym e rs . J . Col lo id In ter face Sci . 42, 448--456.t u b e d u r i n g t h e e x p e r i m e n t i s s m a l l c o m p a r e d t o G r e g o r y J . ( 1 9 76 ) T h e e f fe c t o f c a ti o ni c p o ly m e r s o n t h eth a t i n t h e s t i r r e d v e ss e l ( t h e d e t e n t io n t im e in t h e c o l lo id a l s t a b il i ty o f l at ex p a rt ic le s . J . C ol lo id In ter facet u b e is a b o u t 1 0 s c o m p a r e d t o 1 5 m i n i n th e j a r) . Sci . 5 5 , 35-44.Greg ory J . (1981) Floccu la t ion in lamina r tube f low. Chem.H e n c e , t h e e x t e n t o f c o a g u l a t i o n i n t h e t u b e i s al s o Engng Sci . 36 , 1789-1794.neg l ig ib le . Greg ory J . (1985) Tu rb id i ty f luc tua t ions in f lowing suspen-sions. J . C ol lo id In ter face Sci . 105, 357-371.Greg ory J . (198 8) Polym er adsorp t ion and f loccula t ion insheared suspensions. Colloids Surfaces 31, 231--253.G rego ry J. and Lee S. Y. (1990) The effect of charge densityCONCLUSIONS and molecula r mass of ca t ion ic polymers on f ioccula t ionkine t ics in aqueous so lu t ion . J . W a t . S R T - A q u a 39 ,T h e c o a g u l a t i o n i n d e x m e a s u r e d b y t h e f lo w 2 65 -2 74 .th r o u g h o p t i c a l t e c h n iq u e i s v e ry se n s i t i v e t o t h e s t a t e Gre g o ry J . a n d Ne l so n D. W. (19 8 6) Mo n i to r in g o f a g gre-o f a g g r e g a t i o n o f p a r t i c l e s u s p e n s i o n s c o a g u l a t i n g g at es i n f lo w in g s us pe ns io ns . Col loids Sur faces 18 ,w i t h F e ( I I I ) c o a g u l an t s . M e a s u r e m e n t s o f c o a g u - 1 7 5 - 1 8 8 .l a t i o n i n d e x d y n a m i c s c o m p l e m e n t t h e d a t a o b t a i n e d G r e g o r y J . a n d L i G . ( 1 9 9 1) E f fe ct s o f d o s in g a n d m i x in gcondi t ion s on polym er f loccula t ion of concentra ted sus-f r o m r e s i d u a l t u r b i d i t y a n d e l e c t r o p h o r e t i c m o b i l i t y p en si on s. Che m. Engng Commun . 10a, 3-21.m e a s u r e m e n t s a n d t h u s c a n h e l p i n s e l ec t i ng o p t i m a l v a n d e H u l st H . C . 0 9 5 7 ) Li gh t Sc a t t e r i ng by Sma l lc o a g u l a n t d o s e . S u c h m e a s u r e m e n t s , t o g e t h e r w i t h Particles. Dover, New York.e l e c t ro p h o r e t i c m o b i l i t i e s , c a n p ro v id e a d d i t i o n a l i n - Iv es K. J. (1 9 90 ) Co a g u la t i o n a n d fl oc cu la ti on Pa r t l I - -O rthok inetic flocculation. I n So l i d - L i qu i d Se par a t i ons i g h t s i n t o t h e m e c h a n i s m s o f c o a g u l a t i o n w i t h h y - ( Ed it ed b y S v ar ov sk y L . ) . B u t t e rw o r t h - H e i n em a n n ,d r o l y z i n g m e t a l s a lt s . T h e s h ap e o f t h e c o a g u l a t i o n O x fo rd .i n d e x c u r v e s i s d e t e r m i n e d i n l a r g e p a r t b y t h e J am e s R . O . a n d H e a ly T . W . ( 1 9 7 2 ) A d s o r p t i o n o f h y -m e c h a n i s m s o f c o a g u la t i o n . In g e n e ra l , l a rg e r v a lu e s d ro ly za b l e m e ta l i o n s a t t h e o x id e -wa te r i nt er fa ce . 11.Charge reversal of SiO 2 and TiO 2 collo ids by ad sorbedo f t h e c o a g u la t i o n i n d e x im p ly l a rg e r a g g re g a t e s . Co 0 1 ), La ( l l l ) , a n d Th (IVI a s m o d e l sy st em s . J . Col lo idC a u t i o n m u s t b e t a k e n , h o w e v e r , i n c o a g u l a t i o n In ter face Sci . 40 , 5 3 ~ 4 .e x p e r i m e n t s a t l o w m i x i n g i n t e n s i t i e s w h e n t h e a g g r e - J o h n s o n P . N . a n d A m i r th a r aj a h A . ( 1 98 3 ) F e rr ic ch l or id eg a t e s a re v e ry l a rg e ( a s i n t h e c a se o f sw e e p - f lo c a n d a lu m a s s in g le a n d d u a l c o a g u la n ts . J . Am. Wa t . Wk sAss. 75, 232-239.c o a g u l a t i o n ) . U n d e r t h e s e c o n d i t i o n s , t h e c o a g u - Kerker M . (1969) The Sc a t t e r ing o f L i gh t and Ot he r E l e c t r o -l a t i o n i n d e x c a n d e c r e a s e , d u e t o s e t t l i n g o f l a r g e magnetic' Radiation. Academic Press, New York.a g g re g a t e s . Th e r e su l t s r e p o r t e d i n t h i s p a p e r a l so La wle r D. F ., I zu ie ta E . a n d Ka o C . -P . (1 98 3 ) Ch a n g e s i ni l l u s t r a t e t h a t m e a su re m e n t s o f t h e c o a g u la t i o n i n d e x p a rt ic le s i z e d i s t r ib u t io n s i n b a t c h f lo e cu la ti on . J . Am.c a n b e u s e d t o i n v e s t i g a t e t h e e f fe c t o f m i x i n g i n t e n - Wat . Wk s As s. 75, 604-612.Li G. a nd G regory J . (1991 ) F loccula t ion and sed im enta t ions i ty o n c o a g u l a t i o n d y n a m i c s w i t h h y d r o l y z i n g m e t a l of h igh- turb id i ty wate rs . Wat. Res . 25, 1137-1143.s a l t s . S u c h m e a s u r e m e n t s d e m o n s t r a t e t h a t , a t a M o re l F . M . M . a n d H e r in g J . G . ( 1 9 9 3) Principles andg i v e n m i x i n g i n t e n s i t y , a n e q u i l i b r i u m a g g r e g a t e s iz e Applications t~f Aq uatic C hem istry. Wiley- ln te rsc ience ,i s r e a c h e d . T h e s i ze o f t h e a g g r e g a t e i s d e t e r m i n e d b y N e w Y o rk .two c o m p e t in g p ro c e sse s : (1 ) g ro w th o f a g g re g a t e s O 'M e l i a C . R . (1 98 5 ) Pa rt ic le , p re t re a tm e n t , a n d p e rfo rm -ance in water filtration. J. envir. Engng, Div. Am. Soc. Cir.i n d u c e d b y p a r t i c l e c o ll i s io n s a n d ( 2 ) b r e a k a g e o f Engrs I l l , 8 7 4 -8 9 0 .a g g re g a t e s b y sh e a r fo rc e s . O 'M e l i a C . R . , Gra y K. A. a n d Ya o C . (1 9 8 9 ) Pol y me r i cInorganic' Coagulants. Am e r i c an Wa te r W o rk s A ssoc i-a t ion Research F oun da t ion , Denver, Colo .A c k n o w l e d g e m e n t s - - T h i s research was supported by the Ram aley B. L. , Lawler D. F . , W right W. C. andM etropol i tan W ater Dist r ic t o f Southern Ca l i forn ia O'M el ia C. R. (1981) In tegra l ana lysis of wate r p lan t(MW D) . We th a n k Ke v in W a t t ie r a n d Ma rk Be u hler o f p e rfo rm a n ce . J. envir. Engng, Div. Am. Soc. Cir. EngrsM W D for the i r assis tance , encouragem ent and ideas. The 107 , 547-562.f ind ings repor ted in th is paper do not necessar ily re flect the Stum m W . (1992) Che mi s t r y o f the So l i d - W at e r I n t e r f ac e :views of MWD, and no off ic ia l endorsement should be Pr oc e s s e s a t t he Mi ne r a l - Wa t e r and Par t i c l e - Wat e rinferred. I n t e r fac e i n N a t ur a l Sy s t e ms . Wiley , New York .


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Dynamics of coagulation of kaolin 569Stumm W. and O'Mefia C. R. (1968) Stoichiometry of behavior of FeOlI) polymeric species--II. Preformedcoagulation. J . Am . W at . W ks As s c . 60 , 514-539. polymers in various concentrations. Wat . Res . 21,Stumm W. and Morgan J. J. (1981) Aquatic Chemistry. 123-128.Wiley, New York. Trcwcck G. P. nd Morgan J. J. (1977) Size distributions ofTang H.-X. and Stumm W. (1987a) The coagulating behav- flocculated articles: applications of electronic particleiors of F(III) polymeric spccics---I.Preformed polymers counters. Envir. Sci. Technol. 11, 707-714.by base additions. Wat . Res . 21, 115-121. van de Hulst H. C. (1957) Light Scattering by SmallTang H.-X. and Stumm W. (1987b) The coagulating Particles. Dover, New York.

coagulacion con cloruro ferrico

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