Revue Frangal$e des SclEilcES llE L'tAU,4 lt985l tBt-t92
a v e c l e p o l y m d r e d ' h y d r o x y d e d ' a l u m i n i u m
The eflect 0l calcium and sulphate ions on the removal of fluoride
lrom fluoridated waste water
by precipitation with polymeric aluminium hydroxide
Effets des ions sulfate et
d e s e a u x r d s i d u a i r e s p a r c a l c i u m s u r la d 6 f l u o r a t i o n p r 6 c i p i t a t i o n
N . P A R T H A S A R A T H Y , J . B U F F L E * , a n d W . H A E R D I
S u m m a n y
P r e c i p i t a t i o n o f f l u o r i d e b y p o { . y m e r i c a t u r n i n i u m h y d r o x i d e , P , i n t h e p r e s e n c e o f f o r e . i g n i o n s s u c h a s c a l c i u m a n d s u L i h a - t e i o n s h a s b e e n i n v e s t i g a t e d . T h e r e s u L t s s h o w e d th a t t h e p n e s e n c e o f c a L c i u m . i m p r o v e s th e p r e c . i p i t a t . i o n w h e n e a s s u L p h a - t e i o n s n e d u c e s s o m e w h a t t h e f L u o r i d e r e m o v a t e f f i c i " n . y . i h "
a p p l i c a t i o n o f p o t y m e r i c a l u m i n i u m h y d r o x i d e t o f L u o r i d a t e d w a s t e w a t e r p n o d u c e d f r o m a t u m i n . i u m p L a n t w h i c h a L s o c o n t a i n e d s u L p h a t e ' i o n s s h o w e d th a t f L u o r . i d e w a s n e m o v e d e f f e c t i v e L y b y P t o l e v e I s b e t o w t h e S w i s s p e r m . i s s i b I e ti m i t s f o r w a s t e w a t e r d i s c h a n g e
R 6 s u m 6
N o u s a v o n s e t u d i e [ ' e f f e t d e s i o n s c a L c i u m e t s u I f a t e s u r L a p n 6 c i p i t a t i o n d u f t u o r u r e p a r u n e s o L u t i o n d ' a L u m i n i u m p o [ y m 6 - r i s 6 ( P ) , d a n s I e s c o n d i t i o n s d e s e a u x r - 6 s i d u a i r e s . N o u s a v o n s u t i t i s 6 d e s s o l u t i o n s d ' h y d r o x y d e d ' a l u m i n i u m p o t y m e r i s 6 a v e c u n r a p p o r t m o L a i n e d e 2 - 5 . E L L e s o n t 6 t 6 c a r a c t 6 r ^ i s 6 e s p a r u L t r a f i L t r a t i o n , 2 t A I R . M . N , e t p a r u n e 6 t u d e s p e c t r o p h o t o m 6 - t r i q u e a v e c L e " f e r r o n " . N o u s a v o n s o b t e n u t e s r 6 s u L t a t s s u i - v a n t s :
a ) P o u r L e s y s t d m e s i m p l e P - F , L e s c o n d i t i o n s o p t i m a L e s p o u r L e p 1 6 c i p i t a t i o n d u f L u o r u r e s o n t : r F = 0 . 5 4 ( r p = l F l t / l A L l t ) e t 4 < o H < 7 .
O e n a r i m e n f n f T n n r o : n i n A n : l r r f i n : ' l, r r r u r y L f u q f
l J n i v e r s i t y o f G e n e v a , 1 2 1 1 - G e n e v a
a n d A p p l i e d C h e m i . s t r y , 4 t S I ^ I I T Z E R L A N D I .
L 8 2 Sciences de L'eau 4, no2-3
L ' a n a L y s e c h i m i q u e d e L a p h a s e s o t i d e i n d i q u e q u ' i I s ' a g i t d ' u n p r e c i p i t e d e f L u o r u r e d ' a t u m i n i u m b a s i q u e .
b ) D a n s I e s y t d m e P - F - S O , * ( c o n d i t i o n s c o m m e s o u s a ) , t a c o n c e n - t r a t i o n 1 6 s i d u e L L e d u f I u o r u n e a u g m e n t e L o r s q u e I a c o n c e n t r a - t i o n d u s u t f a t e a u g m e n t e . L ' a n a L y s e m o n t r e q u ' i L s e f o r m e , d a n s c e c a s , u n s u L f a t e d ' a L u m i n i u m b a s i q u e .
c ) P o u r [ e s y s t d m e P - F - C a , a v e c r F = 0 . 5 4 e t 4 < p H < 7 , L e s r 6 s u L t a t s m o n t r e n t q u e L a c o n c e n t r a t i o n r 6 s i d u e L t e d u f L u o r u r e e s t p L u s fa i b l . e q u e p o u r I e s y s t d m e P - F . L ' a n a L y s e i n d i q u e t ' i n c o n p o n a t i o n d e c a I c i u m d a n s L e p r 6 c i p i t 6 .
d ) P o u r t e s y s t b m e P - F - C a - S O a , a v e c r F = O . 5 4 , t a c o n c e n t r a t i o n r 6 s i d u e L L e d u f L u o n u r e e s t p L u s 6 [ e v 6 e q u e p o u r L e s y s t d m e a v e c P - F - C a , c e c i p t o v e n a n t d e L ' e f f e t o p p o s 6 d u c a t c i u m e t d u s u l - f a t e d a n s [ a r 6 a c t i o n d e p r ' 6 c i p i t a t i o n .
C e s r 6 s u L t a t s m o n t r e n t e u e L t e f f i c a c i t 6 d e L ' 6 L i m i n a t i o n d u f L u o r u n e e s t r 6 d u i t e p a r L a p r 6 s e n c e d e s u L f a t e e t a u g m e n t 6 e p a r [ a p r 6 s e n c e d e L ' i o n c a t c i u m .
L e s c o n d i t i o n s o p t i m a L e s o o u r L a d 6 f L u o r a t i o n d e L ' e a u e n p r ' 6 s e n c e s i m u L t a n 6 e d e c a L c i u m e t d e s u L f a t e s o n t : r F = 0 . 5 4 ; 4 < p H < 7 . L ' a p p l i c a t i o n d e c e s c o n d i t i o n s a u x e a u x r 6 s i - d u a i r e s p r o v e n a n t d ' u s i n e s d e p r o d u c t i o n d ' a L u m i n i u m m o n t r e q u e L a c o n c e n t r a t i o n d u f L u o r u r e p e u t 6 t r e a b a i s s 6 i m o i n s d e 1 0 m g / [ , q u i e s t L a L i m i t e L 6 g a L e e n S u i s s e p o u r L a d 6 c h a n g e d e s e a u x r ' 6 s i d u a i n e s -
1 - I N T R o D U c r r o N
Lime and aluminium salts are wldely used as precipitants for removal
o f f l u o r i d e f r o m f l u o r i d a t e d w a s t e r ^ r a t e r s [ 1 - 8 ] . H o w e v e r t h e r e i s a v r i d e s p r e a d i n t h e o p t i m a l c o n d i t i o n s a s w e l l a s t h e r e m o v a l e f f i c i e n c i e s
reported in the literature outing to lack of understandinq of the mecha-
n i s m o f p r e c i p i t a t i o n r e a c t i o n s i n s u c h s y s t e m s . f t h a s r e c e n t l y b e e n
shown that aluminium hydroxide polymers ptay an important role in the
p r e c i p i t a t i o n r e a c t i o n s o f p h o s p h a t e [ 9 - 1 1 ] a n d f l u o r i d e U 2 ) , i n t n e r e m o v a l o f s u s p e n d e d m a t t e r s [ 1 3 ] a n a i n t h e c o a g u l a t i o n o f o r g a n i c l i g a n d s o f n a t u r a l w a t e r s s u c h a s h u m i c s u b s t a n c e s [ 1 4 ] . r n o u r e a r l i e r r e p o r t [ t Z ] i t w a s s h o w n t h a t f l u o r i d e f i x e s o n t o p o s i t i v e l y c h a r g e d
polymeric alumj-nium hydroxide, P, thereby neutralising its charge and
inducing precipitation. It was al-so shovtn that in the absence of inter-
ferinq components, the concentration of fluoride ions can be lowered in
t h i s w a y t o l e s s t h a n 1 0 m g / I i . e . b e l o w t h e S l , r i s s p e r m i s s i b l e l i m i t
for discharge of industrial waste \{aters. Ho\^rever \4aste L{aters in parti-
cular the effluent of aluminium manufacturing plants often contain largre
c o n c e n t r a t i o n s o f s u l p h a t e a n d c a l c i u m i o n s , t h e s e m a y i n f l u e n c e t h e p r e c i p i t a t i o n o f f l u o r i d e b y P . T h e p r e s e n c e o f d i v e r s e a n i o n s Particu-
Iarly sulphate and fluoride ions have been shown to affect the precipi-
t a t i o n o f a l u m i n i u m h y d r o x i d e I t S - t Z ] a n d t h e f o r m a t i o n o f a q u e o u s a l u - m i n i u m h y d r o x i d e c o m p l e x e s [ 19 ] .
RemouaL of fLuot',ide from fZuoridated uaste water 1 8 3
T h e r e f o r e i n t h i s t h e p r e c i p i t a t i o n o f
i n r z o q f i a : # a d
r e p o r t t h e e f f e c t o f f l u o r i d e u n d e r w a s t e
s u l p h a t e a n d c a l c i u m i o n s o n w a t e r c o n d i t i o n s h a s b e e n
2 - E x p r n r M E N T A L
For the sake of convenience the slzmbols ro and rf; wifl be used t h r o u g h o u t f o r t h e m o l a r r a t i o " j r l l Z l a r l . a ^ n a lrlrTlorl5 r e s p e c r i v e l y
in sampl-e preparation. lflt, ltrllg, respectively are the total concen-
trations of fluoride and alumirrium in the solution ana lr'15 ana lafl5 are the concentrations of fruor:ide and aluminium bound in the polymei.
s i m i l a r r y t g 1 1 = lonll/larla, w h L e r e loHls is the total concenrrarion of hydroxide ions added.
2 - L M a t e r i a l a n d m e t h o d s
R e a g e n t g r a d e c h e m i c a l s a n d d e m i n e r a r i - s e d w a t e r p u r i f i e d o n M i r l i p o r e M i l l i - Q s y s t e m s w e r e u s e d t h r o u r g h o u t .
A s t o c k s o l u t i o n o f p o l y m e r i . c a l u m i n i u m h y d r o x i d e , p , r{rith rgg = 2.5 w a s p r e p a r e d a s d e s c r i b e d e l s e l l ' h e r e I f g ] . I n t h e s t o c k s o l u t i o n
( l A l l t = 0 . 1 1 6 M ) a b o u t 8 0 B o f , a l u m i n i u m i s i n t h e f o r m o f p o s i t i v e l y
charged polymeric aluminium hydlroxide of average net^positive charge
0 . 5 e q p e r m o l e o f aluminium a n d s i z e b e t w e e n 1 0 - 2 0 [ . p w a s f o u n d t o b e e s s e n t i a l l y A l 1 3 0 * ( o H ) r , r 7 + . T h e r e m a i n d e r o f t h e a l u m i n i u m w a s f o u n d .
as monomerj.c aluminium hydroxo species (3 10 ?) and inert hydrated alu-
m i n i u m o x i d e ( = 1 0 ?). ft must be mentioned that p is metastabre l r i t h r e s p e c t t o p r e c i p i t a t i o n b u t r e m a i n s v i r t u a l l y u n a r t e r e d f o r a t l e a s t
a few months when it is prepared. under carefully controrred conditions
[ t g ] - r r r i s s o r u t i o n w a s r e f t t o s t a n d f o r 6 d a y s for reasons given in r e f - 7 9 . s t a n d a r d f l u o r i d e s o l u t i o n s o v e r t h e c o n c e n t r a t i o n r a n g e 1 o - r t o 1 0 - 6 M w e r e p r e p a r e d b y s e r i - a ] d i l u t i o n o f o . 2 M s t o c k sodiurn fluo- r i d e s o r u t i o n . T h e s e s o r u t i o n s w e r e u s e d f o r c o n s t r u c t i n g c a r i b r a t i o n c u r v e f o r f l u o r i d e b y m e a n s o f f l u o r i d e i o n s e l e c t i v e e l e c t r o d e .
A l 1 m e a s u r e m e n t s were made at 25 + o-1"c in polyethylene c o n t a i n e r s
under stirring conditions and Lhe ionic strength of the solutions were
a d j u s t e d t o t h e d e s i r e d c o n c e n t r a t i o n s w i t h e i t h e r N a N O 3 o r N a z S O + .
2 - 2 M e t h o d s
2-2-1 Effect of sulphate
S o l u t i o n s c o n t a i n i n g c a . 2 - 2 1 . 7 0 - 3 M f l u o r i d e a n d 0 . 0 2 M N a 2 S O a w e r e p r a c e d i n a s e r i e s o f b e a k e r s . T h e s e c o n c e n t r a t i o n s c o r r e s p o n d t o t h o s e f o u n d i n t h e e f f l u e n t o f a l u m i r i i u m r n a n u f a c t u r i n g p l a n t ( s e e sec.5).
T h e p H o f t h e s o r u t i o n s w e r e a i L j u s t e d t o 5 . 5 a s i t w a s s h o w n earrier [ t g ] t f r a t f l - u o r i d e c a n r e a c t e f : f e c t i v e l y w i t h p around this pH. Then a r i q u o t s o f P w e r e a d d e d s u c h t h a t r p w a s c . a 0 . 5 i n e a c h c a s e and the s o r u t i o n s w e r e k e p t a t c o n s t a n t . p H for 30 mins under stirring c o n d i t i o n s .
T'hese were left to stand over nigrht and firtered in cascade throuqh
1 8 4 S c i e n c e s d e L ' e a u 4 , n o 2 - 3
S c h l e i c h e r a n d S c h u l l 2 5 m m d i a r n e t e r f i l t e r s o f p o r e s i z e ' : 8 p m , 0 . 4 5 p m , a n d 0 . 2 p m . r h e c o n c e n t r a t i o n s o f t o t a l f l u o r i d e , . f r e e f l u o r i d e a n d t o t a l a l u m i n i u m i n e a c h f i f t r a t e w h i c h a r e d e n o t e d l V l r l r e s ' l F l ' a n a l a f l r e s r e s p e c t i v e l y w e r e d e t e r m i n e d I Z O ] . t f r e r e a c t i o n p r o d u c t c o l l e c t e d o n t h e f i l t e r w a s w a s h e d o n c e w i t h w a t e r , t w i c e w i t h 7 0 % e t h a - n o l d r : i e d i n a d e s l c c a t o r o v e r s i l i c a g e l a n d a n a l y s e d f o r i t s c h e m i c a l c o m p o s i t i o n b y d i s s o l v i n g i t i n 1 M I I C I . T h e s o l i d w a s a l s o e x a n i n e d b y X - r a y d i f f r a c t i o n .
2 - 2 - 2 E f f e c t o f c a T c i u m
C a l c i u m c h l o r i d e s o l u t i o n l v a s a d d e d t o s o l u t i o n s c o n t a i n i n g 2 . 1 0 - 3 t ' l o f f l u o r i d e a n d 2 . 1 0 - 2 M s o d i u m n i t r a t e s u c h t h a t t h e f i n a l t o t a l c o n c e n - t r a t i o n w a s 8 . 1 0 - q 1 4 . T h e p H o f t h e s a m p l e w a s a d i u s t e d t o 5 ' 5 a n d a l i - q u o t s o f P w e r e a d d e d s u c h t h a t r F v a r i e d b e t w e e n O ' 1 a n d 3 ' T h e p H w a s m a i n t a i n e d c o n s t a n t ( p n = 5 . 0 ) f o i 3 0 m i n s u n d e r s t i r r i n g c o n d i t i o n s b y m e a n s o f a p H s t a t a n d t h e a m o u n t o f a c i d o r b a s e a d d e d w e r e r e c o r d e d .
T h e n , t t r e s o l u t i o n s w e r e l e f t t o s t a n d o v e r n i g h t a n d f i l t e r e d s u c c e s s t -
v e l y t h r o u g h S c h l e i c h e r a n d S c h u I I f i l t e r s o f . p o r e s i z e : 8 Ut' 0'45 luo' a n d 0 . 2 l J n . T h e t o t a l c a l c i u m ( d e n o t e d b y l c a l r . " ) , t o t a l f l u o r i d e a n d f r e e f l u o r i d e c o n c e n t r a t i o n s w e r e d e t e r m i n e d i n e a c h f i l t r a t e ' T h e s o l i d c o l l e c t e d o n t h e f i l t e r s w e r e a n a l y s e d a s d . e s c r i b e d a b o v e . c o n t r o l
e x p e r i m e n t s w e r e p e r f o r m e d u n d e r i d e n t i c a l c o n d i t i o n s w i t h o u t a d d i n g P ' F o r c o m p a r i s o n p u r p o s e s t h e s e m e a s u r e m e n t s w e r e a l s o m a d e i n s i m p l e P - F
s y s t e m u n d e r s i m i l a r c o n d i t i o n s .
? - 3 A n a 1 y s i s
f n g e n e r a l a l l t h e a n a l y s e s w e r e d o n e b y t h e s a m e p r o c e d u r e s a s d e s c r i b e d p r e v i o u s l y I Z O ] . t f r e t o t a l a l u m i n i u m a n d c a l c i u m c o n c e n t r a - t i o n s \ ^ r e r e d e t e r m i n e d b y f l a m e a t o m i c a b s o r p t i o n s p e c t r o m e t r y ( A - A . S . ) .
Aluminium and fluoride ionS were found to interfere in the determina-
t i o n o f c a l c i u m b y A . A . s . c a l c i u m r e c o v e r i e s t r e r e f o u n d t o b e a s 1 o w a s 2 0 Z i n t h e p r e s e n c e o f h i g h c o n c e n t r a t i o n s o f a l u m i n i u m i o n s a n d t h e i n t e r f e r e n c e e f f e c t o f f l u o r i d e w a s o f s i m i l a r o r d e r o f m a g n i t u d e . I n t h e a b s e n c e o f a l u m i n i u m i o n s q u a n t i t a t i v e r e c o v e r i e s o f c a l c i u m i n t h e p r e s e n c e o f f l u o r i d e c a n b e o b t a i n e d b y a c i d i f y i n g t h e s a m p l e s w i t h h y d r o c h l o r i c a c i d ( p H c a . 1 ) . T h e a d d i t i o n a l i n t e r f e r e n c e d u e t o
aluminium was overcome by making the measurements in lanthanum nitrate
s o l u t i o n ( 7 Z 'i/v) a c i d i f i e d w i t h h y d r o c h l o r i c a c i d t o p t t 2 .
The total and free fluoride concentrations were determined by means
o f f l u o r i d e i o n s e l e c t i v e e l e c t r o d e ( I . s . E . ) . T h e t o t a l f l u o r i d e c o n c e n - t r a t i o n w a s d e t e r m i n e d b y d i l u t i n g t h e s a m p l e s w i t h t o t a l i o n i c s t r e n g t h b u f f e r c o n t a i n i n g C . D . T . A . a s t h e d e c o m p l e x i n g a g e n t ( T I S A B ) a n d t h e
f r e e f l u o r i d e b y d i r e c t m e a s u r e m e n t h r i t h t h e e f e c t r o d e .
3 - Resulrs
3 - 1 E f f e c t o f s u l p h a t e
o n l y t h e r e s u l t s o b t a i n e d f o r t h e 0 . 2 u m f i l t r a t e s a r e r e p o r t e d h e r e
RemouaL of fluol.ide from fluor,idated uaste uater L 8 5
a s j . d e n t i c a l r e s u l t s w e r e o b t a L n e d w i t h t h e o t h e r f i l t r a t e s . A t r p = g . 5 4
which is close to the optimal value for removal of fluoride (see
f i g u r e t ) , l r ' [ . . " i n c r e a s e s w i - t h i n c r e a s e i n t h e t o t a l s u r p h a t e c o n c e n -
l - r A f i ^ n l e n . l .
r ' - s r t u P t o m o l a r
t r a t i o n , l S O u l * u p t o m o
r a r i o o f l s o - l i Z l e r l s o r
about 1 (figure 2) and then
becomes independent of
applied sulphate concenrra-
tion. The aluminium in all
c a s e s i s v i r t u a l l y a s s o c i a - ted with the precipitate
( t a b ] e 1 ) . T h u s t h e p r e s e n c e
of sulphate j.ons reduces the
f l u o r i d e r e m o v a l e f f i c i e n c v by a factor of two.
Figut,e 1 P l o t s o f l 7 l y n " u s r p ( o l = c u r v e a r l c a l l = o ; T , = o . D 2 M N a N O : ;
l F l t = 4 2 m g / I t ( a ) = c u r v e b l c u l t = B . B . i o - 4 t u l ;
I = 0 , 0 2 l v l N a N O g ;
l r l t = 4 2 n g / r .
T h e h o r i z o n t e l l i n e i n d i c a t e s t h e f l u o r i d e c o n c e n t r a t i o n b e r o r e t r e a t i n g t h e s a m p l e w i t h P .
Figure 2
Effect of sulphate on the y,esiduaL fluo r i de e o ne entr at t o n c o n d i t i o n s u s e d :
l r l t = 2 . 2 . 1 0 - ' . y ( q z n g / r ) ; r F = O . 5 4 [ r g b = D . 7 ) ;
l s o 1 l t = v a r i e d ; l A l l t = 4 . 1 0 - ' t Y ; T - =
2 5 6 C ;
e n C l f l . " = = r e s i d u a l f l u o r i d e . lFlrr, ( nqll )
l F l . r , ( m q l l )
l S 0 4 l 1 r ' 1 , { 1 1 1
1 8 6 Seienees de Ltectt 4, no2-3
TabLe 1
Effect ctf suLphate on the pteei'pi'tati'on of fLuoride by .P r p
' = "
c o n s t l n t = ' 0 . 5 4 , r e s = i e s i d u a l f l u o r i d e c o n c e n t r a t l o n 1 n 0 . 2 U m fi l t r a t e .
T h e p e r c e n t a g e s o f A 1 a n d F w e r e e v a l u a t e d w i t h r e s p e c t t o l A I l l a n d I r l a r e s p e c t i v e l Y .
l l l : ! l o r l . l . l . l o ' 1 , " "
l A r lt
molar ratio (mM) (mM) (mM)
l a r l l r l l n l
' ' r e s ' ' r e s ' r e s
( r ) ( r )
(mM)0 . 0 0 . 2 s o . 2 5
L . Z b
2 . 5 3
R 1 1
9 . 6 9
t q . o
t 9 . 4 2 4 . 8 4 9 . r
o . 2 3
U . Z J
o . 2 5
u - z >
o . 2 6 n r q 0 - 2 2
o . 2 7
i ) a
0 . 1 5
n 1 q
0 . 1 5
5 . 8 5 . 5
t . )
4 . 2
7 . O
1 n 2 Q
3 . 9 9 2 . r 9 3 . 9 5 2 - 1 6 3 . 9 5 2 . 1 6 3 . 9 5 2 . 1 5 3 . 9 5 2 . 1 6 3 . 9 3 2 . 1 6
J . v f z . L o
? q q t 1 A
3 . 9 5 2 . 1 6 3 . 9 s 2 - 1 6 3 . 9 5 2 . 1 6 3 . 9 5 2 . L 6
J . y f , z . L 0
0 . 3 9 8 1 8 . 2 o . 6 3 9 2 9 . 5 0 . 6 8 5 3 1 .7 0 . 6 8 5 3 1 .7 o . 7 2 5 3 3 . 6 0 . 8 1 4 3 7 . 7 o . 7 6 3 5 . 2 0 . 8 5 3 9 . 4 0 . 7 5 3 6 . 0 o . ' 1 5 3 4 . 7 o . 7 7 8 3 6 . 0 0 . 9 0 4 r - 6 0 . 8 1 8 3 7 . 8 0
In order to maintain the pH constant it was necessary to add acid to
the sample which indicates that hydroxide ions are being released during
the reaction. fhe amount of
h y d r o x i d e r e l e a s e d , l o u l r " 1 , t u t
evaluated from the amount of
a c i d a d d e d . A P I o t o f
l o n | ," r . ' " I s o u I s / l e r lg i s
s h o w n i n f i g u r e 3 . T t r e s e r e s u l t s a r e i n p a r a l l e l w i t h t h o s e
obserVed in figure 2. The frac-
tion of hydroxide released is
much smaller than that bound to
P ( l o H l b ) .
The chemical composition of
the solid reaction product for-
med between P and F in sulphate
medium was found to be : A l ( o H ) 2 . , * r F o - s ( S O q ) s . 6 a . T t r i s
compound was X-raY amorPhous and
hence its structure could not be
determined. Neverthless the
chemical composition of this
product suggests that sulphate
0 2 4 6
Figure 3
A p l o t o f l ) H l o n t r t s c o n d i t i o n s u s e d :
8 r 0 1 2
l s o , l
" t l a t l ,
r F = 0 . 5 4 ;
p i r = s . s , l n r lt = 4 . 1 o - 3 Y t ;
l r l t = 2 . 2 . 1 D - ' + Y t ( 4 2 n g / r ) t
f - t E o a
l n H l , Y t n ' - " ' r e t ' l M \
l s O 4 1 1 / t l t l 1 o o l a r r a t i o
RenouaL of fluoride fron fluoridated uaste aater 1 8 7
i o n s d o n o t i n t e r f e r e b y c o m p e t i t i o n w i t h f r u o r i d e . T h e w e a k c o m p r e x i n g
ability of sulphate is confirmecl by the fact that addition of sulphate to
P i n t h e a b s e n c e of fluoride i o r r s p r o d u c e s n o r e l e a s e o f h y d r o x i d e i o n s .
3 - 2 E f f e c t o f c a l c i u m
The effect of calcium on t.he precipitation of fluoride by p is shown
i n f i g u r e 1 ( c u r v e a ) a n d t a b l e 2 . f h e s e r e s u l t s r e v e a l s t h a t t h e r e s i -
dual concentration fluoride are lower in the presence of calcium. The
c h e m j . c a l a n a l y s i s o f t h e r e a c t i o n p r o d u c t a t r p = 0 . 5 , y i e l d e d t h e f o l l o w i n g e m p i r i c a l f o r m u l a : A 1 ( O H ) z . z z F z . o e C a o . T s - A s i n t h e c a s e o f
sulphate, this compound was amorphous to X-rays, and hence its structure
could not be determined without aribiguity. Neverthless the stoichiometry
of the precipitate as well as the fact that basic alumini.um fluoride is
f o r m e d i n t h e a b s e n c e o f c a l c i u m [ t Z ] t e n a t o s u g g e s t t h e s o l i d o b t a i n e d
to be either a mixture of basic aruminiurn fruoride and ca.rcium fruoride
o r m i x e d c o m p l e x . f t m u s t b e n o t e d t h a t i f t h e s e q u e n c e o f a d d i t i o n o f c a l c i u m i o n s a n d P a r e r e v e r s e d i . . e p i s a d d e d t o s o l u t l o n s c o n t a i n i n g f l u o r i d e i o n s p r i o r c a l c i u m i c n s , t h e n t h e r e s u l t s o b t a i n e d a r e s i m i l a r t o t h o s e o b t a i n e d i n t h e a b s e n c e o f c a l c i u m .
3 - 3 E f f e c t o f a m i x t u r e o f c , a l c i u m a n d s u l D h a t e i o n s
The combined effect of calcium and sulphate ions on the precipitation
o f f l u o r i d e b y P a t r F = 0 . 5 4 i s s h o w n i n f i g u r e 4 . I t i n d i c a t e s t h a t
resj.dual- fluoride concentraticns are higher than the Ca-F-p system.
Hence calcium and sulphate io:ns seem to have opposing actions in the
system under study. T'he chemical analysis of the precipitate gave the
f o l l o w i n g e m p i r i c a l f o r m u l e : A 1 ( O H ) z . + z B o . s c a o . : s ( S O , * ) 9 . 6 7 . O w i n g t o
TaL'Le 2
Effeet of eaLcitun ions on the pz,eeipi.tation of fLuoride bg P
.F .f lollt
( m M )
l r l t i " u l " f o t l r . " l o r l . " " 1 " 1 . . "
(Ev,) (trll'l) (mM) (*) (nM)
1.1,." | .. i ,., l.u 1...
( t ) ( n l 4 ) ( s )
1 . 9 5 2 . 4 3 1 . 1 4 o . 9 ' t | . 2 r 2 . 2 5 a ) 0 . 7 0 0 . 8 8 3 . 1 3 0 . 5 4 0 . 6 8 4 . O 4
2 . 2 4 0 . A 1 0 . 2 1 1 8 . 4 1 . 2 s 5 5 .9 0 . s 3 6 0 . 9 3 9 . 2 0 . 4 8 . : 5 , 2 2 2 , 4 0 . 4 9 5 8 . 6 r 4 . 2 0 . 4 9 5 6 . 3 2 . 2 2 0 . 8 7 0 . 2 1
2 , 2 2 0 . 8 7 0 . 2 1 2 . 2 2 0 . S ? A . 1 2
2 . 1 9 3 . 8 6 0 . 3 5 2 . 1 9 4 . A 2 0 . 2 6 2 . 1 9 1 0 . 0 0 . l 8
9 . 3 0 . 8 7 5 . 0 O . 4 9 2 . 9 0 . 3 1
_ 0 _ . ! l _ _ 0 _ . ! 8 _ _ _ - 1 . _ o _ _ _ _ _ 2 : r _ e _ _ _ _ o _ . ! 2 . _ _ _ ! : 2 1 _ _ _ _ ! j _ _ _ _ 0 _ . _ 2 2 _ _ _ 1 3 _ . 1 _ _ _ _ o : 6 j _ _ _ _ r _ o . r _ _
0 . 5 4 0 , 5 8 4 , 0 2 . 1 9 t . 4 6 0 . 3 4 8 . 8 0 . 2 6 r 2 . o 1 . 0 3 7 0 . 5 0 . 5 4 0 . 6 8 4 . 0
0 . 5 4 0 , 6 a 4 . o 0 . 5 4 0 . 6 8 4 . O
8 . 8 0 . 2 6 I 2 . O 2 . 3 0 5 9 . 6
6 . 5 0 . 2 6 1 2 . 0 3 . 5 7 2 . 6
4 . 5 0 . 1 8 8 . 0 ' t
. ' 7 2 ' / ' 7 . 2
l X l . r e s i r h r a l c n n c e n t r a t i c r n o f X i n O . 2 p m f i l t r a t e ,
" r e s
a ) r O . v a r l e d , l C u l t = c o n s t a n t b ) r O o c o n s t a n t , l C u lt - v a r . l e d
A I I p e r c e n t a g e s o f l X l a r e c a l , c u l a t e d w i t h r e s p e c t t o t o t a l
t n l t l a l c o n c e n t r a t i o n o f X .
f a o
Scienees de L'eau 4, no2-3
the amorphous nature of the preciPitate no information on its structure
c o u l d b e o b t a i n e d .
40 35
FLqure 4
Effeet of caLclwn on the rLsiduaT fluoride' eoneentnation' p L o t s o f l r l ' T e s u s l C a l . ' t
C o n d i t i o n s u s e d :
r F = 0 . 5 4 ; i r l t = 4 2 n g / I ; ' 1 . c u r v e ( a l ( V l l s o 4 l t = o ; T = - 2 . c u r v e ( b ) ( x l l s o u l t = 4 ' 9 . 1 0 - '
0 . 0 2 N N a N 0 3 ;
I t ; ( o l l s o u l l = 9 . 8 ' 1 0 - 2 r'
l F l r r , ( a r g l l )
4 - D r s c u s s r o N
4 - 7 I n t e r o r e t a t i o n o f d a t a
A c c o r d i n g t o H s u r s m o d e t I f t ] f o r t h e s i m p l e p h o s p h a t e - a l u m i n i u m
hydroxy polymer system, the preclpitation of phosphate occurs tthen the
p o s i t i v e c h a r g e o n t h e p o l y m e r i c s p e c i e s i s n e u t r a l i s e d b y t h e c o u n t e r
ions in the solution. rf inadequate amounts of phosphate ions $'ere pre-
sent then soluble phosphate complex with the hydroxy species is formed'
This model was used fo-r interpreting the data obtained in this study'
I n s i m p l e F - p s y s t e m i t w a s p r e v i o u s l y s h o w n [ 1 2 ] t t r a t t h e p r e c i p i t a t ' i o n
of fluoride foll-o\4's the same pathway as phosphate with the exception
that some of the hydroxide bound to the polymer are replaced by fluorirle'
T h i s d i f f e r e n c e v r a s a t t r i b u t e d t o t h e s t r o n g e r a f f i n i t y o f f l u o r i i l e f o r a l u m i n i u m t h a n p h o s p h a t e ' A t r F = o ' 5 o r r p b = o ' 7 ' f r u o r i d e w a s c o m p r e - t e l y p r e c i p i t a t e d b y P a s P F r o a c c o r d i n g t o t h e r e a c t i o n :
A l r : O + ( o H ) z s + 1 0 F * A l t a o + ( o H ) z z F r o ( s ) + 3 o H ( 1 )
Remoual of fluo:nide fron fluorida.ted uaste uater 1 8 9
D u r i n g t h e p r e c i p i t a t i o n o f f l u o r i d e b y P i n t h e p r e s e n c e o f s u l p h a t e ,
it was found that sulphate ions were not incorporated in the precipitate
indicating that sulphate ions do not compete with ffuoride ions to bind
c h e m i c a l l y b / i t h P . H e n c e t h e h i g h e r r e s i d u a l f l u o r i d e c o n c e n t r a t i o n s
observed in the presence of sulphate cannot be accounted by such a reac-
t i o n . I n t h i s c a s e i t i s p r o b a b l e t h a t s u l p h a t e i o n s i n c r e a s e t h e s o l u - b i l i t y o f P F t o . T h e c a u s e f o r t h i s i n c r e a s e m a y b e d u e t o t w o r e a s o n s : 1 ) t h e i o n i c s t r e n g t h o f t h e m e d i u m i n c r e a s e s a s t h e s u l p h a t e i o n c o n c e n - t r a t i o n i n c r e a s e s w h i c h i n t u r n d e c r e a s e s t h e a c t i v i t y c o e f f i c i e n t i . e t h e s o l u b i l i t y o f f l u o r i d e i n c r e a s e s ,
2 ) t h e p r e c i p i t a t i o n o f d e i o n s a t t h e s u r f a c e
^ h a r d a ^ f D 7 L' ^ n 6 h i c
l - r l - l
t f t = t t t . e x D
w h e r e IFI = fluoride c o n c e n t r a , t i o n i n t h e b u l k o f t h e s o l u t i o n ,
z = - 7 = c h a r g e o f f l u o r i d e i o n s , a n d Y = s u r f a c e p o t e n t i a f o f P . F r o m e q . ( 2 ) , iL follows t h a t t h e c o n d i t i o n a l s o l u b i l i t y p r o d u c t ( r e l a t e d t o
l r l ) i s s m a l " I e r t h a n t h e i n t r i . n s i c o n e ( r e l a t e d t o l F l o ) . t t o w e v e r , w h e n s u l p h a t e i o n s a r e p r e s e n t , t h e : i o n i c s t r e n g t h o f t h e m e d i u m i n c r e a s e s a n d Y d e c r e a s e s [ 2 1 ] . H e n c e t h L e c o n d i t i o n a l s o l u b i l i t y p r o d u c t a n d
l _ l
c o n s e q u e n c l Y | ! l r e s r n c r e a s e s .
I n c o n t r a s t ! q g u l n h a f e i . . = - l r ' l d e c r e a s e s i n t h e n r e s e n c e o f f a r g e c o n c e n t r a t i o n s o f c a l c i u m i o n s . I n d e e d , c a l c i u m i o n s m a y r e a c t w i t h f l u o r i d e t o f o r m i n s o l u b f e c a f c i u m f l u o r i d e w h i c h c o - p r e c i p i t a t e s w i t h P F l e ( s ) a n d a c c o u n t s f o r t h e I o w e r r e s i d u a l f l u o r i d e c o n c e n t r a t i o n o b s e r v e d e x p e r i m e n t a l l y .
5 - A p p t - r c A T r o N T o W A S T E w A T E R T R E A T M E N T
r t w a s s h o w n e a r l i e , l t Z l t h a t i n t h e a - b s e n c e o f i n t e r f e r i n g c o m p o - n e n t s f l u o r i d e c a n b e p r e c i p i t a t e d w i t h P e i t h e r a s b a s i c a l u m i n i u m f l u o r i d e o r a s c r y o l i t e d e p e n d i n g o n t h e f l u o r i d e c o n c e n t r a t i o n p r e s e n t i n t h e s a m p l e . A t h i g h f l u o r i d e c o n c e n t r a t i o n s ( 1 0 0 rnq,/I) it c a n b e p r e c i p i t a t e d a s c r y o l i t e u s i n g m o l - a r r a t i o r a b ) 3 w h e r e a s a t l o w f l u o - r i d e c o n c e n t r a t i o n s ( < 1 0 0 mg/I) i E c a n b e p r e c i p i t a t e d a s b a s i c a l u m i - n i u m f L u o r i d e w i t h r p b e q ' r a l t o a b o u t 0 . 7 . T h e r e s u l t s o f t h e p r e s e n t
study have shown that the elimination of fluoride is influenced by the
p r e s e n c e o f f o r e i g n i o n s a n d h e n c e t h e c o m p o s i t i o n o f t h e w a s t e w a t e r m u s t b e d e t e r m i n e d b e f o r e t r e a t m e n t w i t h P . T h e h i n d r a n c e o f s u l p h a t e i n t h e p r e c i p i t a t i o n o f P - F c a n b e o v e r c o m e b y c a l c i u m u n d e r c o n d i t i o n s w h e r e l s o + l s l l c a l t > 1 . H o w e v e r , t h e u s e o f h i q h l c . l a r i r r g i v e r i s e t o l a r g e r e s i d u a l c a l c i u m c o n c e n t r a t i o n s w h i c h i s u n d e s i r a b f e b e c a u s e
of subsequent rrater hardness problem. Consequently a compromise has to
be made with regards to the concentration of calciurn to be ernployed to
o b v i a t e t h i s p r o b l e m . T h u s t h e o p t i m a l c o n d i t i o n s i n w a t e r s c o n t a i n i n g f s o u l t o - ' M , a r e a s f o r l o w s ' f o r l F l . < 5 . 1 0 - 3 y ( 1 0 0 n g / r ) - ; r r b = 0 . 5 ; l c " l t = 1 o - 2 t t ; a n d p H = 5 - 7 . F o r l r l t t 5 . 1 0 - 3 M , t r b ) 3 c a n b e u s e d . , b u t i n t h a t c a s e d i r e c t p r e c i p i t a t i o n b y C a F 2 i s p r e f e r a b l e b e c a u s e i t i s l e s s e x p e n s i v e [ 2 2 ] .
P F ' . . d e n e n d s o n t h e c o n c e n t r a t i o n o f f r e e
^ € + h 6 h ^ r r ^ 6 , I r l . D u e t o t h e p o s i t i v e : - : " - ' r - r o
l 2 r I - .
( - q p \ ! ! / p + t > ! r . l
t _ |
f l u o r i - e l e c t r i c
( 2 )
1 9 0 Scienees de L'ean 4 n " 2 - 3
_IAD Le ,t
Cornposition of Daste uater
S a m p l e IYpe of water pH
i r " l . l c r l t
( n M ) ( m M )
l a r l . l r l t
(mM) (mM)
l c " l . l s o + l s
( n M ) ( D M )
Waste water f raw
W a s t e w a t e r I c l e a n
Waste water II lime treated
3
6 . 8
1 3 . 0 1 3 . 0 1 2 . 6 1 2 . 6 0 . 5 0 . 5
0 . 0 1 3 - 4 9 0 . 0 1 2 . 4 0 0 . 0 1 2 . 1
o . 6 3 2 0 .- 7 0 . 8 4 2 0 . 4 t . 2 0 . 0 8 R a w a n d c l e a n w a t e r s a r e w a s t e w a t e r p r i o r t o t r e a t m e n t a n d a f t e r t r e a t m e n t w i t h 1 l m e r e s D e c t i v e l v ,
TabLe 4
Treatment of uaste L)ater uith P
Type of sampfe
r-
I
r r added
l C a l . , ' t
(mM)
l p l
l ^ l r e sl p l
l - l r e s(mM) ns/r
R a w w a t e r + R a w w a t e r + c l e a n w a t e r + c l e a n v ; a t e r W a s t e w a t e r ' I I
n q
u . f , n q
0 . 5
0 6 . 6 5 0
v . o f ,
0
o . 6 4 o . 4 4
U . f , I O
o -424 o . 2 5 6
I Z . L
4 . 4 q R
A 1
4 . 9
p H = 5 . 5 ; l C . l ; " " ' " = c o n c e n t r a t i o n o f c a l c i u m i o n s a d d e d .
* W a s t e w a t e r I
F o r d e f i n i t i o n o f r a w a n d c l e a n w a s t e w a t e r s s e e t a b l e 3 .
The above conditions were tested with recircutated water of smelter
c e l l s f r o m a n a l u m i n i u m m a n u f a c t u r i n g p l a n t ( A l u s u i s s e s . A . ) . T h e c o m -
position of the water samples are qiven in table 3. T'he results obtained
after treating them !,rith P (table 4) show that addition of calcium ions
prior to P improves fluoride removal efficiency. In addition these
r e s u l t s a g r e e w e I I w i t h t h o s e o b t a i n e d w i t h s y n t h e t i c s a m p l e s . T h e r e s i -
dual fluorid.e concentration of these samples are brought down to Iess
t h a n t h e s t a t u t a r y l i m i t o f 7 0 n g / I s e t b y S w i s s a u t h o r i t i e s ' B u t i t
must be mentioned however that a fairty high concentration of calcium
ions is necessary to achieve this when large amounts of sulphate ions
a r e p r e s e n t i n t h e s a m p l e . T h i s i s p a r t i a t l y d u e t o t h e i n c r e a s e d s o l u - b i l i t y o f t h e p r e c i p i t a t e i n s u c h m e d i a .
RemouaL of fLuoz,ide fz,om fluoridated aaste uater jgl
Cottct-us t orus
Ttre above resurts show that the effici-ency of fruoride removal using
polymeric aruminium hydroxide is governed by the nature and concentration
o f i o n s p r e s e n t i n v / a t e r . T h e e f f e c t i v e n e s s o f f l u o r i d e r e m o v a l i s
reduced by sulphate but improved by the presence of calcium at the opti-
m a l r p o f 0 . 5 4 . T t l e e f f e c t o f s u p h a t e i s n o t d r a s t i c a s e v e n a t h i g h s u l p h a t e c o n c e n t r a t i o n s ( 1 M ) the residuar f r u o r i d e c o n c e n t r a t i o n i s r e d u - c e d b y a f a c t o r o f t w o . T h e o p t i m a r c o n d i t i o n s f o r d e f l u o r i d a t i o n o f w a t e r , i n . t h e p r e s e n c e o f b o t h s u r p h a t e a n d c a f c i u m i o n s w e r e f o u n d to b e : l c a l t = 1 0 - ' M r p H = 5 - 6 ; a n d r F = 0 . 5 . H e n c e t h e c o m p o s i t i o n o f w a t e r m u s t b e d e t e r m i n e d t o a c h i e v e m a x i r n u m e f f i c i e n c y . T h e a p p l i c a t i o n
of these conditions to waste .naters of alurninium manufacturing plants
revealed that fluoride level-s can be brought dovm to ress than ro ng/r,
this value being the maximum alrowable l-evel set by swiss government fcr
t h e d i s c h a r g e o f f t u o r i d a t e d w a s t e w a t e r s . T h e p r e c i p i t a t i o n o f f r u o r i d e b y c a l c i u m s a r t s h a s b e e n d e s , : r i b e d e l s e w h e t e l z z J . r t l r a s f o u n d t h a t
rapid precipitation with resirfuar fluoride concentrations of less than
70 mg/r can be achieved under same conditions by innoculating cal-cium
f l u o r i d e c r y s t a l s e e d . s . r n t h . i s c a s e s m a r l c o n c e n L r a t i o n s o f p o r l a n e r i c
aluminium hydroxide should be used as a coagulant. The choice between
P and carcium sarts for deflur:ridation would depend on the composition
o f t h e w a t e r t o b e t r e a t e d ( p a r t l c u l a r l y o n t h e i n i t i a l f l u o r i d e c a r c i u m a n d s u i p h a t e i o n c o n c e n t r a t i o n s ) . t h i s a s p e c t h a s a l s o b e e n d e a l t w i t h L n r e x . l 2 z J .
R r r r n r n c e s
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