Batch Metal Removal by Peat

8/6/2019 Batch Metal Removal by Peat

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Wat. Res. Vo l. 20, No. 1, pp. 21- 26. 1986 0043-1354/86 $3.00 + 0.00Printed in Gr ea t Britain . All rights reserved Copyrigh t ~-~ 1986 Pergam on Press Ltd

BATCH METAL REMOVAL BY PEATK I N E T I C S A N D T H E R M O D Y N A M I C S

THIERRY GOSSET , JEA N- Luc TRANCART an d DANIEL R. THI~VENOT '*~Agence F inanci6 re de Bassin Seine-No rmand ie , C .R.E .A.T .E . , 5 ,7 ,9 , Bou leva rd Lou is Segu in , 92700Colom bes and 2Laborato i re de Bio61ectroch imie e t Analyse du Mil ieu , L .A. 329 du C .N.R.S . , U .E .R. deSciences , Un ivers i t6 Par is-Val de Marne, Avenue du G6n6ral de Gau l le , 94010 Cr6 tei l Cedex , F rance

( R e c e i v e d S e p t e m b e r 1 9 8 4 )

Ab strac t - -Pe at m oss , a natu ral inexpensive mater ia l , i s ab le to p lay an importan t r r le in t reatmen tp rocesses o f me tal -bear ing indust r ia l ef f luen ts s ince i t adso rbs , complexes o r exchanges va r ious metalcat ions . Th is paper p resen ts k inet ics and thermo dynam ics o f batch metal rem oval react ions by 50 g l -(d ry wt) eu troph ic o r o l igo troph ic pea t par t ic les us ing Cu 2+, Cd , Zn 2 and Ni 2 concen trat ions rang ingfrom 0 .01 to 100raM.Metal cat ion removal react ions are moderately rap id in l0 mM metal unbuffered so lu t ions: the fo rwardk inet ic consta n t ranges between 0 .005 and 0 .17 M -I s- ~ , and equ i l ib r ium is reached wi th in abo u t 1 h .Un der these cond i t ions o f pH (2 .2 -4.2) and conce n trat ions , appare n t b ind ing equ i l ib r ium constan ts werefound to range between 2 and 3150 M -~ depend ing up on the peat o r ig in and the m etal cat ion .In 0 -6 .5 pH-buffered metal cat ion so lu t ions , the fou r cat ions b ind ing react ions behaved d i fferen tlydem onstrat ing that metal b ind ing equ i l ib r ium con stan t decrease in the o rder N i 2+ > Cu 2+ > Cd 2+ = Zn 2Wh en pH is h igher than 6 .7 , mor e than 90% of a 10 mM metal cat ion so lu t ion is removed by 50 g 1 tpeat par t ic les and metal b ind ing capaci t ies equal 200 mm ol kg - ~ d ry wt , wha tever the me tal natu re andthe peat o r ig in . Excep t fo r n ickel cat ion which is very s t rong ly bound to peat , a l l metal cat ions arecompletely released when pH is fixed below 1.5.K e y w o r d s - - p e a t , heavy metals , ion exchange, complexat ion , k inet ics , thermodynamics

INTRODUCTIONS u r f a c e w a t e r i s o f t e n p o l l u t e d b y t h e i m p r o p e rd i s p o s a l o f m e t a l - b e a r i n g i n d u s t r i a l e f fl u e nt s . N u m e r -o u s a p p r o a c h e s h a v e b e e n s t u d i e d f o r t h e d e v e l -o p m e n t o f m e t a l t r a p p i n g m a t e r i a l s . B e s i d e s t h eh i g h l y e f f e c ti v e a rt i fi c i al c h e l a t i n g p o l y m e r s c o n t a i n -i n g , f o r e x a m p l e , t h i o l f u n c t i o n s ( D e r a t a n i a n dS 6 b i l l e , 1 9 8 l ) , c h e a p n a t u r a l p o l y m e r i c m a t e r i a l s ,s u c h a s h u m i c s u b s t a n c e s o r p e a t , h a v e b e e n e x t e n -s i v el y s t u d ie d . T h e y i n d e e d s t r o n g l y a d s o r b , e x c h a n g eo r c o m p l e x v a r i o u s m e t a l c a t i o n s u s i n g t h e i r c a r b o x -y l ic , p h e n o l i c a n d h y d r o x y l i c f u n c t i o n a l g r o u p s( S m i t h e t a l . , 1 9 7 7 ; W o l f e t a l . , 1 9 7 7 ; T a k a m a t s u e ta l . , 1 9 7 8 ; S ip o s e t a l . , 1 9 7 8 ; M ei se l e t a l . , 1 9 79 ; B l o o ma n d M c B r i d e , 1 97 9) . F o l l o w i n g C o u p a l a n d L a l -a n c e t t e ( 1 9 76 ) , i t h a s b e e n s u g g e s t e d b y s e v e r a lr e s e a r c h g r o u p s t o u s e p e a t m o s s i n b e d s o r c o l u m n sf o r m e t a l - c o n t a i n i n g e f f l u e n t t r e a t m e n t ( P o o t s e t a l . ,1 9 7 8 ; P o o t s a n d M c K a y , 1 9 8 0 ; C h a n e y a n dH u n d e m a n n , 1 97 9; M c K a y 1 98 0; D i s s a n a y a k e a n dW e e r a s o o r i y a , 1 98 1) . B e f o r e a t t e m p t i n g t o d e v e l o ps u c h a p e a t t r e a t m e n t p r o c e s s i n c o l u m n s , w e d e c i d e dt o s t u d y t h e m e t a l r e m o v a l r e a c t i o n s i n b a t c h a n dd e t e r m i n e t h e i r k i n e t i c s ( G o s s e t e t a l . , 1 9 8 4 ) an dt h e r m o d y n a m i c s . W e p r e s e n t h e r e r e s u l t s o b t a i n e d*To whom al l co rrespondence shou ld be addressed .

21

w i t h m e t a l s c a t i o n s p l a y i n g a n i m p o r t a n t r 6 1 e i nw a t e r q u a l i t y , i . e . c o p p e r , c a d m i u m , z i n c a n d n i c k e l .

M A T E R I A L S A N D M E T H O D SP e a t a n d r e a g e n t s

Tw o types o f peat were s tud ied : an o l igo troph ic peatca l led "F lo ra to r f " an d an eu t ro p h ic o n e cal l ed "H eu r t eau -v i l le peat" , bo th being comm ercial ly avai lab le in F ranc e fo ragr icu l tu ral pu rposes .The metal l ic sal ts used were cupr ic n i t ra te , cadmiumacetate, zinc and nickel chlorides dissolved in slightlyacidified distilled water. All metallic salts and chemicalswere o f analy t ical g rade (Pro labo P .A. ) .P e a t t r e a t m e n t

In o rder to homogen ize peat samples and to c lear themfrom metal l ic cat ions which they cou ld have p rev iouslyfixed, we pretreated peat in five steps:(a) 24h d ry ing at 100C;(b) dry sieving with a shaker (Prolabo) to 0 .5-1.25 or

1 . 2 5 - 5 mm par t ic le s ize;(c) acid i ficat ion o f the s ieved samples, I0 g o f d ry peatbeing tho rough ly shaken fo r 2 h w i th 100 ml o f l M HCl;(d ) wash ing wi th deion ized water un t i l f i l t ra te reachesp H 4 ;(e) 24 h dryi ng a t 70C.Whe n the effect o f acid i f icat ion p rocedure o f peat sampleswas studied, steps (c) and (d) were avoided. In particle sizedependence s tud ies , pea t t reatme n t p rocedure a vo ided s teps(b), (c), (d) and (e).


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22 THIERRY GOSSET eta/.

All kinetic and thermodynamic batch experiments usedthe same concen trat ion of peat particles, i.e. 50 g l ~ (drywt).K i n e t i c c o n s t a n t s d e t e r m i n a t i o n

Five grams of formerly acidified or non-acidif ied peat, i.e.including or omitt ing step (c) in peat t reatment procedure,were thoroughly mixed into 100ml of 10raM solution ofone of the following metallic cations: Cu 2+, Cd 2+ , Zn 2~ andNIX: peat suspensions were shaken during 2 h at roomtemperature and 1 ml samples were collected every 15 minand centrifuged during 5min at 12,000 rpm. Supernatantswere analysed using either differential pulse polarography orflame atomic absorption.The electrochemical assembly contained a static mercurydrop electrode (EGG PAR 303) connected to a differentialpulse polarograph (EGG PAR 364); 0.2 ml samples weretaken from the supernatant and added to 10 ml of deionizedwater formerly acidified with 0.2 ml of 1 M perchloric acid.Metal de terminations were performed by direct differentialpulse polarography with initial potentials of + 140, -740and -42 0mV/Ag CI respectively for copper, zinc + nickeland cadmium and potential sweep rate fixed to - 5 mV s-~.The superimposed constant amplitude pulse was 50 mV andthe mercury dr op period fixed at 1 s. Heavy metals weremeasured, element per element, using standard additionprocedure and recording 2-3 polarograms per solution.The analysis by acetylene-ai r flame atomic absorptionspectrophotometry was performed with a Perkin-Elmer2380 and single element hollow cathode lamps. All sampleswere filtered through a 0.45 iLm membrane filter (Millipore)before metal determination. Metal concentrations were cal-culated by averaging 3-5 determinations with the samesolution and using calibration curves taking into account ablank and 3 standards.T h e r m o ( I v n a m i c c o n s t a n t s d e t e r m i n a t i o n

Two grams of formerly acidified or non-acidified peat, i.e.including or omitting step (c) and (d) in the peat treatmentprocedure, were thoroughly mixed into 40 ml metal solutionduring 2 h at room temperature. The initial concentrationsof metallic cations were fixed to values ranging between 0.01and 100 mM. For high free metal concent rations, i.e. using1 100 mM initial metal concentrations, analysis procedurewas direct differential pulse polarography. For deter-minations of free metal at lower concentrations, we usedanodic stripping differential pulse polarography with apreconcentrat ion step of 90 s at - 1100 mV/AgC1.p H d e p e n d e n c e

Two and half grams of natural peat, i.e. dried 24 h at100 ('. unsieved and non acidified, were suspended in 50 mtof 10mM metal cation solutions, pH was adjusted byaddition o f I M perchloric acid or of 1M sodium hy-droxyde, pH was measured at the beginning of reactions andafter 1 h of mixing and then readjusted if necessary. Peatparticles werc stirred for 3 h and centri fuged 5min at12.000 rpm. Concentra tion of metallic cations were deter-mined by flame atomic absorption spectrophotometry onsupernatants as previously mentioned.

KINETICS OF METAL BINDING

In order to optimize the residence time of industrialwaste water in peat columns, we studied the kinetics withof t 0m M metal cation removal by 50gl '~ peatparticles in batch experiments. Figure I shows theevolution of metal amounts bound to eutrophic peat.As these first experiments were made without wherebuffering pH, solutions in contact with peat presentedsome pH variations: pH decreases were found to

200 --

~ t 6 0 - -. ~ - - C / " _ . _ _

~ 80 a/

~ 40 . " "~ ~o _

I 1 I I I" 0 . 0 0 . 5 I 0 1 . 5 2 0 2 5

T i m e ( h )

Fig. 1. Kinetics of metal binding reactions on eutrophicpeat. 5 g acidified (dry wt) 0.5 1.25 mm peat shaken in100 ml 10 mM ( ll ) copper, (+ ) cadmium, ([Z) zinc and (A )nickel unbuffered solutions: final pH ranged from 2.2 to 4.2.

range between 0.2 and 0.6pH units whatever themetal and the peat origin. Although we do not intendin such experiments to identify the chemical or phys-ical nature of peat- metal cation interactions, it seemsclear that, under these conditions, complexing oradsorption reactions are more important than ionexchange reactions in metal removal processes bypeat. Indeed if 1 or 2 protons were released duringeach metal binding reaction on peat, one shouldobserve a 10 or 20 mM prot on conce ntrat ion increaseand a pH shift from 3-4 to ap prox. 2. Table 1 showsthat H/M ratios, i.e. the number of proton releasedper metal cation bound to peat is always lower than0.25. Furthermore, the pH variations observed whenpeat was suspended into 10mM initial metal ionsolutions were similar to those observed when equalamounts of peat were suspended in deionized water,in absence of any metal. Thus pH variations encoun-tered when peat particles are mixed with metal cationsolutions result probably from:

the acidic properties of carboxylic and phenolicfunctional groups present in humic substances(Bloom and McBride, 1979; Boyd e t a l . , 1981):

some ion exchange reactions, i.e. proton releasewhen metal cations bind to peat (Bunzl e t a l . , 1976;Bloom and McBride, 1979; Meisel e t a l . , 1979; Ahoand Tummavuori, 1984);

the pH buffering capacity of peat weak acidgroups, limiting possible pH variations related topreviously mentioned proton release (Attal e t a l . ,1985).

In absence of stoichiometric data, the simplest wayto describe these metal removal reactions by peat is:

P+ M~,~-PM

(PM)K' - - kl/k :(P)(M)

(P) = the concentration of peat binding sites (M),


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Batch metal removal by peatTable 1. Metal binding kinetics and equilibrium constants using 50 g 1 I peat in unbuffered 10 mM metal cationsolutions

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Particle Cor.Peat size k kz coeff. (P)i, K' pHtype (ram) Metal (M Is i) (s i) (r 2) (m M) (M -I) (final) H/Mac. olig. 0.5-1.25 Cu 0.03 10.7 E-4 0.60 28 30 3.2 0.03ac. olig. 1.25-5 Cu 0.005 1.8 Eo4 0.75 27 30 3.0olig. nat. Cu 0.013 3.2 E4 0.85 41 26 2.6 0.03ac. olig 0.5-1.25 Cd 0.026 5.1 E-3 0.31 5.1 39 2.8 0.23ac. olig. 0.5-1.25 Zn 0.055 9.7 E-3 0.77 5.6 30ac. olig. 0.5-1.25 Ni 0.041 1.6 E-2 0.74 2.6 49 0.25ac. eutr. 0.5-1.25 Cu 0.118 3.3 E-4 0.96 355 25 2.5ac. eutr. 1.25 5 Cu 0.061 1.3 E-4 0.98 483 27 3.0eutr. nat. Cu 0.170 0.5 E-4 0.98 3150 20 4.2 0.0005ac. eutr. 0.5-1.25 Cd 0.036 4.6 E-3 0.63 7.8 46 4.0 0.01ac. eutr. 0.5 1.25 Zn 0.060 1.7 E-3 0.98 34 52ac. eutr. 0.5 1.25 Ni 0.050 1.2 E-3 0.74 41 55 2.2 0.004(eutr.) eutrophic, (olig.) oligotrophic, (ac.) acidified and (nat.) unsieved and unacidified pea t, (H/M)proton/metal exchange ratios, ( P ) i n metal binding capacities estimated by batch metal binding experimentsusing 100 mM metal cation solutions buffered at pH equal to final pH in this table (Gangneuxe t a l . , 1985).

(M) = the concentration of free metal in solution(M),

(PM) = the concentration of metal bound to peat(M )K' = the apparent conditional s tability constant at

experimental pH (M ~),kl (M -t s ~) and k2(s ~) are the forward and re-

verse kinetic constant, respectively.Apparent conditional s tability constants K' were

calculated taking into account equilibrium concen-trations of free (M) and bound (PM) metal and theamount (P)m of metal binding sites on peat. Thesemetal binding capacit ies (P)~, were estima ted f rombound metal amounts, using higher metal cationconcen tratio ns, i.e. 100 mM, in solutions whose p Hwas buffered between 0 and 6 (Gan gneux e t a l . , 1985).Apparent binding constants at pH2.2~, .2 werefound to range between 2 and 3150 M- ~, the highestvalues being obtained for copper removal by eu-trophic peat (Table 1).

Binding kinetic cons tant k~ was obta ined as sumingthat:

reverse reaction 2 was negligible;peat-metal cation stoichiometry was constant for

all experimental conditions and equal to one metalcation per peat binding site;

overall reaction kinetic was limited by the bindingreaction itself and not by diffusion of species;

and by plotting calculatedl (M)i n (PM)lim - (P M)k t " t I n - -(PM)lim - (M)i n (PM)lim (M)~. -- (PM)

vs timewhere

(PM) and (PM)lim are the variable and equilibriumbound metal concentrations (M),

(M)~, is the initial free metal concentration (M).Kinetic plots similar to those presented on Fig. 2

were found to be linear with correlation coefficientshigher than 0.7 on 9 curves over 12. When other

assumptions than those listed above were tested, wewere not able to obtai n such good fit of experimentaldata. Slopes of these strai ght lines, giving k~, allowe dus to calculate the reverse kinetic constant k 2 usingpreviously mentioned apparent s tability constant K'(Table 1). Whereas k~ was found to range between0.005 an d 0.17 M-~ s-~ , with the highest values forcoppe r removal by eut rophic peat, k 2 values werealways s maller tha n 0.01 s -~. On the few exampl estested, unsieved and non acidified oligotrophic oreutrophic peat samples seemed to bind copper morerapidl y and efficiently tha n sieved and acidifie d ones.This result may be interpreted as a structuremodification of this natural matter when acid pre-treated.

Using these values for k~ and K', we compa red theexperimental evolution of bound metal (PM) andcorresponding calculated curves (where reverse reac-tions were neglected):

(PM) =exp( [(M)~. - (PM),~m] "k, "t ) - 1(M)i. (M)in

(PM),im- - ' exp ([(M),n - (PM),~m " k," t) - 1

1 2 5 01 o o o

7 5 0

5 O O

2 5 O0O 0

i ~.. /." i /+ . . /

/

0 5 1 .0 1 .5 2 0T i m e ( h )

I25Fig. 2. Eutrophic peat pretreatment and size dependence ofcopper binding reaction kinetics: determina tion of forwardkinetic constant k I 5 g dry wt (. ) unsieved and unacidifiedor acidified ( +) 0.5 1.25 mm or (A) 1.25-5 mm peat sus-pended in 100 ml l0 mM copper unbuffered solutions. Lineswere obtained by least square regression: slopes of theselines and correlation coefficients are given in Table 1.


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24 THIERRY GOSSET ' t al .2 0 0

1 6 0

~ 12o

Ei2

. . . . . . . . . . : - 7 :

, 5 " o

, > /~ /

o I i I 1 lO 0 0 5 t 0 ] 5 2 . 0 2 5

T i m e ( h )Fig. 3. Calcula ted and exp er imental evolu t ion of copp erf ixed on peat : eut rophic peat pre t reatment and s ize de-pendence. Lines correspond to ca lcula ted f ixed copperamou nts , taking in to accoun t k inet ic constants valuespresented in Table 1. Sam e symbo ls as Fig. 2 for experi-mental data .

A s s h o w n o n F i g . 3 w h e r e s u c h a c o m p a r i s o n i sp r e s e n t e d , a g r e e m e n t i s g e n e r a l l y g o o d , d i f f e r e n c e sb e t w e e n e x p e r i m e n t a l a n d c a l c u l a t e d d a t a b e i n gu s u a l l y sm a l l e r th a n 6 % . S u c h a g r e e m e n t s u p p o r t so u r s e t o f a s s u m p t i o n s c o n c e r n i n g t h e s e k i n e ti c s a n de s p e c i a ll y th e a s s u m p t i o n o f a c h e m i c a l r a t e l i m i t in gs t ep : f l o w - t h r o u g h e x p e r i m e n t s i n c o l u m n s s h o u l d b eu s e d t o c o n f i r m s u c h k i n e t i c b e h a v i o u r .

K i n e t i c s o f m e t a l i o n b i n d i n g b y p e a t h a s b e e np r e v i o u s l y s t u d i e d b y B u n z l (1 9 7 4 a , b) a n d B u n z l e ta l . ( 1 9 7 6 ) u s i n g e i t h e r c o n t i n u o u s o r d i s c r e t e m e t a lc a t i o n a d d i t i o n t o p r e a c i d i f i e d s p h a g n u m p e a t i nb a t c h e x p e r i m e n t s . R e a c t i o n k i n e t ic s w e r e m u c hm o r e r a p i d , r e a c t i o n h a l f - t i m e s r a n g i n g b e t w e e n 5a n d 1 5 s ( i n s t e a d o f 1 5 - 3 0 m i n i n t h i s p a p e r ) . S u c h ad i f fe r e n c e o f m a g n i t u d e i n k i n e t ic c o n s t a n t s m a y b er e l a te d t o t h e l a r g e d i f fe r e n c e s in e x p e r i m e n t a l c o n d i -t i o n s : B u n z l u s e d t o s h r e d a n d s i e v e p e a t s a m p l e s i nw a t e r t o a p a r t ic l e s i ze o f 0 .2 - 0 . 7 m m , a n d b e f o r ee a c h e x p e r i m e n t 0 . 5 o r 5 g I t ( w e t w t ) p e a t p a r t i c l e sw e r e a l l o w e d t o e s t a b l is h s w e l l in g e q u i l i b r i u m i nw e l l - s ti r r e d d e i o n i z e d w a t e r f o r s e v e r a l h o u r s . O u r2 4 h d r y i n g p r o c e d u r e o f n o n - s h r e d d e d p e a t s a m p l e sm a y h a v e s i g n i f ic a n t l y d e c r e a s e d s w e l l in g a n d m e t a lb i n d i n g k i n et ic s o f m u c h m o r e c o n c e n t r a t e d p e a ts u s p e n s i o n s ( 5 0 g I ' d r y w t ) .

M ETA L R EM O V A L TH ER M O D Y N A M IC SI n o r d e r t o u s e p e a t f o r e n v i r o n m e n t a l a p p l i c a -

t i o n s , w e f o u n d i t n e c e s s a r y t o d e t e r m i n e t h ep e a t m e t a l c a t i o n b i n d i n g i s o t h e r m s f o r d if f e r e n tu n b u f f e r e d m e t a l s o l u t i o n s e q u i l i b r a t e d w i t h t w ot y p e s o f p e a t s a m p l e s ( F i g s 4 a n d 5 ). W h a t e v e r t h ep e a t o r i g i n a n d t h e m e t a l c a t i o n , n o n e o f t h e s e c u r v e si s l i n e a r , s l o p e s r a n g i n g b e t w e e n 0 . 5 a n d 2 . 0 ; t h i ss e e m s t o i n d i c a te t h a t t h e p e a t - m e t a l c o m p l e xs t o i c h i o m e t r y a n d t h e r m o d y n a m i c a r e p r o b a b l y d e -p e n d e n t o n t h e f r e e m e t a l c o n c e n t r a t i o n a n d o n p Hw h i c h , i n t he s e e x p e r i m e n t s , m a y v a r y d u e t o

.'

! , ~ _ J . . . . . J L t I- 8 5 L- 6 - 5 - 4 - 3 - 2 -1 0lo g ( M } ( m o l [ - ~ )

F i g . 4 . M e t a l b i n d i n g i s o t h e r m s o n e u t r o p h i c p e a t i nu n b u f f e r e d s o l u t i o n s . 2 g ( d r y w t ) a c i d i fi e d 0 . 5 - 1 . 2 5 m m p e a te q u i l ib r a t e d w i t h 4 0 m l d e i o n i z e d w a t e r c o n ta i n i n g m e t a lc a t i o n s in t h e 0 .0 1 l O O m M i n i t i a l c o n c e n t r a t i o n r a n g e .

Same symb o ls as F ig . 1 .

u n b u f f e r e d c o n d i t i o n s . F u r t h e r m o r e , n o s o r p t i o n o rb i n d i n g s a t u r a t i o n w a s o b s e r v e d i n t h e s e u n b u f f e r e ds o l u t i o n s , e v e n w h e n t o t a l m e t a l c o n c e n t r a t i o nr e a c h e d 0 .1 M i n 5 0 g l ' p e a t s u s p e n s i o n s . W e f r e -q u e n t l y o b s e r v e d , e s p e c i a l l y f o r o l i g o t r o p h i c p e a t ,t h a t t h e b o u n d t o f re e m e t a l r a ti o ( P M ) / ( M ) w a sm a x i m u m f o r to t a l m e t a l c o n c e n t r a t i o n s i n t h e0 . 1 - 1 m M r a n g e : s u c h a c o n c e n t r a t i o n r a n g e s h o u l dc o r r e s p o n d t o t h e m a x i m u m e f f ic i e n cy o f a w a s t ew a t e r t r e a t m e n t p r o c e s s u s i n g p e a t c o l u m n s o rb a t c h e s .

I n o r d e r t o e v a l u a t e t h e o c c u r r e n c e o f s o l u b l ef o r m s o f c o m p l e x e d m e t a l , su c h i s o t h e r m s w e r ed r a w n u s i n g e it h e r p o l a r o g r a p h i c d a t a , i .e . fr e e a n dl a b il e s p e ci e s, o r a t o m i c a b s o r p t i o n d a t a a f t e r0 . 4 5 / ~ m f i l t r a t i o n , i . e . t o t a l s o l u b l e c o n c e n t r a t i o n s .D i f f e r e n c e s w e r e g e n e r a l l y w i t h i n t h e r a n g e o f r e -p r o d u c t i b i l i t y o f s u c h h e t e r o g e n e o u s e x p e r i m e n t s , i .e .a b o u t 5 % , i n d i c a t i n g t h e n e g li g i b l e i m p o r t a n c e o fs o l u b l e h u m i c m e t a l c o m p l e x e s u n d e r o u r e x p e r i -m e n t a l c o n d i t i o n s . T h i s r e s u l t is o f e x t r e m e i m -p o r t a n c e i f o n e t h i n k s o f t h e p o ss i b l e a p p l i c a t io n s o fp e a t c o l u m n s i n w a s t e w a t e r t r e a t m e n t : m e t a l b i n d i n gf u n c t i o n s s h o u l d n o t d i s s o l v e i n t o t h e f l o w i n g

- 2 5- 3 5

~ - 4 5

E - 5 5n~ - 6 5

- 7 5

- 8 5

. 7 6 /

/

I

I [ I I I I-5 -4 -3 -2 -4 0log (M ) (mol t 1)

Fig. 5 . Metal b inding i sotherms on ol igot rophic peat inunbuffered solutions. 2 g (dry wt) acidified 0.5-1.25 mm peatequi l ibra ted wi th 40 ml deionized wa ter conta ining metalcat ions in the 0 .01 -10 0ram init ia l concen tra t ion rangeSame sym bols as Figs 1 and 4 .


5/6

B atch m e ta l r em ova l by pea t 25t h r o u g h s o l u t io n s ( C h a n e y a n d H u n d e m a n n , 1 97 9;A h o a n d T u m m a v u o r i , 1 98 4).

p H D E P E N D E N C E O F T H E M E T A LR E M O V A L T H E R M O D Y N A M I C S

A s e q u i l i b r i u m p H i n 5 0 g 1 I p e a t s u s p e n s i o n s h a db e e n f o u n d t o r a n g e b e t w e e n 2 . 2 a n d 4 . 2 i n t h ep r e v i o u s l y m e n t i o n e d u n b u f f e r e d s o l u t i o n s ( T a b l e 1 ),w e d e c id e d t o s t u d y m o r e p r e c i s el y t h e p H d e -p e n d e n c e o f m e t a l r e m o v a l th e r m o d y n a m i c s . C o m -p e t i t io n b e t w e e n p r o t o n a n d m e t a l io n e x c h a n g e o rc o m p l e x a t i o n i s a n i n d i re c t m e t h o d t o c o m p a r e t h e i re n e r g e ti c s ( S t u m m a n d M o r g a n 1 98 1) . W e f o u n d t h a ts u c h e x p e r i m e n t s n e e d s e v e ra l p H a d j u s t m e n t s w i t hs m a l l a d d i t i o n s o f s t r o n g a c i d o r b a s e: i n d e e d s e v e r a lh o u r s a r e n e c e s s a r y t o o b t a i n a s t a b l e p H i n as o l u t i o n i n c o n t a c t w i t h p e a t ( A t t a l e t a L , 1986) ; t hea d d i t i o n o f m e t a l c a t io n s a n d / o r s t r o n g a c i d o r b a sed o e s n o t s e e m t o d e c r e a s e s i g n i f ic a n t l y t h e t i m e w h i c hi s n e c e s s a r y f o r p e a t t o e q u i l i b r a t e i t s a c i d o - b a s i cf u n c t i o n s .

W e h a v e o b s e r v e d t h a t p H p r e s e nt s a s tr o n gi n f l u e n ce o n m e t a l i o n b i n d i n g e q u i l i b r i a w h e n t h et o t a l c o n c e n t r a t i o n o f m e t a l c a ti o n e q u a l s 1 0 m M .I n d e e d t h e p e r c e n t a g e o f m e t a l e x t r a c t i o n , i .e .( P M ) / [ ( P M ) + ( M ) ] r a t i o , v a r i e s f r o m 0 t o a l m o s t1 0 0% w i t h i n 4 - 5 p H u n i t s ( F i g s 6 a n d 7 i n T a b l e 2 ) .T h i s s tu d y w a s p e r f o r m e d a t p H l o w e r t h a n 6 . 5 i no r d e r t o p r e v e n t p r e c i p i t a ti o n o f m e t a l h y d r o x i d e s.I n d e p e n d e n t l y o f p e a t o r ig i n t h r e e i m p o r t a n t r e su l tsw e r e o b t a i n e d i n r e l a t i o n t o m e t a l c a t i o n :

n i c k e l i s t h e m o s t s t r o n g l y f i x ed , e v e n i n v e r y a c i d i cm e d i a ;

a b o v e p H 3 c o p p e r b i n d i n g i s v e r y s i m i la r t o n i c k e l,b u t b e l o w p H 3 c o p p e r m a y b e c o m p l e t e l y r el e a se df r o m p e a t ;

c a d m i u m a n d z i nc p r es e n t a si m i la r p H d e p e n d e n c ea n d a r e l es s s t r o n g l y f i x e d t h a n t h e t w o o t h e r c a t i o n s .

T a k i n g i n t o a c c o u n t p H v a l u e s f o r 5 0% m e t a lb i n d i n g c a p a c it i es , e q u i l i b r iu m c o n s t a n t s m a y b ec o m p a r e d :

N i 2+ > C u 2+ > Zn 2+ = C d 2+ .E x c e p t f o r n ic k e l a n d c o p p e r r e s p e c t i v e p o s i t i o n s ,

t h e se r e su l ts a r e in g o o d a g r e e m e n t w i t h m e t a l - h u m i cs u b s t a n c e s p r e v i o u s r e s u l t s ( B u n z l e t a l . , 1 9 7 6 ; G i e s y ,1983).

1 0 0 -

o

A 8 0 -_ / / / / o~, ~ o - . - / /

402o - , . 3 / " /

o I/% I I I I I I0 1 2 3 4 5 6 7

pHFig . 6. pH dependence o f t he m e ta l up t ake o f eu t roph i cpeat . 2 .5 g unsieved and unacidi fied peat eq ui l ibra ted w i th50 ml 10 mM metal ca t ion so lut ions e i ther ac idif ied by 1 Mperch lo r ic ac id o r a l ca li n i sed by I M sod ium hydrox ide : ( l l )copper , ( + ) cadm ium, ( I -1) z inc or (A) nickel solut ions .

1 o o . . ~ . e ~ . . . - ~

eo , / / / /_ / ; / o

/4 o

2 0

o I I I I I I I0 1 2 3 4 5 6 7

DHFig . 7 . pH dependence o f t he m e ta l up t ake o f o l i go t roph i cpeat . Same symbols as Fig . 6 .

A l t h o u g h t h e se ex p e r i m e n t s e n a b l e th e c o m p a r i s o no f th e r e s p e c t i v e m e t a l - p e a t b i n d i n g e n e r g e t i c s , t h e ya l so s h o w c l e ar l y t h a t t h e m a x i m u m b i n d i n g c a p a c i -t ie s in 1 0 m M m e t a l c a t i o n s o l u t i o n s a r e v e r y s i m i l a rf o r t h e d i f f e r e n t m e t a l s a n d p e a t s : a l l v a l u e s r a n g eb e t w e e n 1 80 a n d 2 0 0 m m o l k g - ~ d r y w t ( T a b l e 2 ).S u c h a r e s u lt is n o t o b v i o u s w h e n m e t a l b i n d i n ge x p e r i m e n t s a r e a c h i e v e d in u n b u f f e r e d m e d i a o f p Hr a n g i n g f r o m 2 . 2 t o 4 . 2 ( F i g s 1 - 5 ) . I t d e m o n s t r a t e st h a t m e t a l - p e a t i n t e r a c t i o n s p r e s e n t t h e s a m e s t o i-c h i o m e t r i e s a n d t h a t t h e a v a i l a b l e b i n d i n g c a p a c i t i e sa r e s im i l a r f o r e u t r o p h i c a n d o l i g o t r o p h i c p e a t . T h es a m e p H d e p e n d e n c e o f m e t a l d i s tr i b u t io n c o e ff i ci e n tw a s d e m o n s t ra t e d b y A h o a n d T u m m a v u o r i ( 19 84 )

Table 2. p H depend ence of m etal removal by 50 g l i dried, unsieved and unacidified peatequilibrated with 10 mM metal cation solutions: pH values for 10, 50 and 90% extraction andmaximum capacities observedEutrophic peat Oligotrophic peat

pH values forMetal 10% 50%cat ion ext r . extr .C u . . . 0 . 2 2 . 2Cd 1.5 3.1Zn 1.5 3.1Ni 0 1.2-1.6

pH values forMaximum Maximum90% cap ac ity 10% 50% 90% ca pac ityex t r . (m m ol g - i ) extr. extr. ex tr. (retoo l g- t )4.0 190 1.8 2.5 3.6 1905.6 180 2.0 3.3 4.7 2006.7 170 2.0 3.3 4.7 2004.5 190 0 1.6-2.0 4.0 200


6/6

26 THIERRY GOSSETel al.w h o w o r k e d w i t h p ea t c o l u m n s : th e y f o u n d c o p p e rb i n d i n g c a p a c i t ie s r a n g i n g b e t w e e n 2 0 0 a n d3 0 0 m m o l k g ' u s i n g 0 .0 0 2 5 m M C u 2+ s o l u t i o n s . P r e -v i o u s l y , B u n z l et al. ( 1 9 7 6 ) h a d f o u n d a p p a r e n t i o ne x c h a n g e c a p a c i ti e s o f s p h a g n u m p e a t a t p H 4 e q u a lt o 5 0 0 m m o l Z w ' * k g ~ d r y w t , 6 0 0 C d 2+ a n d6 5 0 C u 2~ u s i n g 0 .0 1 t o 0 . 5 m M m e t a l s o l u t i o n s . A l lt h e se m e t a l b i n d i n g c a p a c it i e s ar e s o m e w h a t s m a l l e rt h a n t h o s e o b t a i n e d r e c e n t ly b y G a n g n e u x et al.( 1 9 8 5 ) u s in g 1 00 m M m e t a l c a t i o n b u f f e r e d s o l u t i o n sa n d 5 0 g l t d r y w t e u t r o p h i c a n d o l i g o t r o p h i c p e a ts u sp e n s io n s : 6 5 0 - 7 8 0 m m o l C u Z + g ~ d r y w t a n d1 0 0 0 - 1 3 0 0 m m o l C d 2 +, Z n 2+ o r N i 2+ k g t.

F i n a l ly , s o m e m e t a l r e m o v a l a n d r e c o v e r y ex p e r i-m e n t s w e r e p e r f o r m e d w i t h c o p p e r s o l u t i o n s : w i t h i nt h e e x p e r i m e n t a l r a n g e o f e r r o r , i. e. a b o u t 5 % , t h ea m o u n t o f m e t a l fi x ed o n p e a t , a s e s t i m a t e d f r o m t h em e t a l r e m o v a l i n s o l u t io n , w a s e q u a l t o t h e a m o u n to f m e t a l r e l e a s e d d u r i n g t h e a c i d i f i c a t i o n s t e p .

CONCLUSIONF o u r m a j o r r e s u l ts w e re f o u n d d u r i n g t h e s e b a t c h

e x p e r i m e n t s w i t h e u t r o p h i c a n d o l i g o t r o p h i c p e a t:( l ) P e a t i s a b l e t o s tr o n g l y b i n d c o p p e r , c a d m i u m ,

z i n c a n d n i ck e l c a t i o n s i n s o l u ti o n , a m a x i m u mc a p ac i ty o f a b o u t 2 0 0 m m o l k g ~ d r y w t b e i n g o b -t a i n e d a t p H l a r g e r t h a n 6 . 7 w h e n t h e i n i ti a l m e t a lc o n c e n t r a t io n s e q u a l 1 0 m M .

( 2 ) T h e m e t a l r e m o v a l e f f ic i en c y i n u n b u f f e r e ds o l u t i o n s i s s i g n i f i c a n t i n a v e r y l a r g e c o n c e n t r a t i o nr a n g e, i .e . f r o m 0 .0 1 t o 1 0 0 m M , t h e m a x i m u me x t r a c t i o n r a t i o s b e i n g o b t a i n e d i n t h e 0 . 1 -1 m Mr a n g e .

( 3 ) B a t c h r e a c t i o n r a t e s a r e s u c h t h a t a r e s i d e n c et i m e s ee m s n e c e s s a ry f o r a c o m p l e t e t r e a t m e n t o fs o l u t i o n s o n c o l u m n s ; t h u s , v e r y t h i n p e a t l a y e rs o rb e d s a s p r o p o s e d b y L a l a n c e t t e i n i t s F r e n c h p a t e n to f 1 9 7 2 s e em v e r y u n l i k e l y t o a c h i e v e a c o m p l e t em e t a l re m o v a l b y c o m p l e x a t i o n , a d s o r p t i o n o r i o n -e x c h a n g e .

( 4 ) E x c e p t f o r n i c k e l c a t i o n w h i c h s e e m s s os t r o n g l y c o m p l e x e d o n p e a t t h a t a t p H 1 .2 --2 h a l f o ft h e m a x i m u m c a p a c i t y is a t t a in e d , c o p p e r , c a d m i u ma n d z i n c m a y b e e a si ly r e m o v e d f r o m p e a t d u r i n g a na c i d t r e a t m e n t .

T a k i n g i n t o a c c o u n t t h e d a t a o b t a i n e d d u r i n g t h e seb a t c h e x p e r i m e n t s , 4 0 1. p e a t c o l u m n s h a v e b e e nc o n s t ru c t e d a n d t h e ir h y d r o d y n a m i c a n d c h e m i c a lp r o p e r t i e s a n d c h a r a c t e r i s t i c s a r e u n d e r i n v e s t i g a t i o n .

REFERENCESA h o M . a n d T u m m a v u o r i J . ( 19 8 4 3 O n t h e i o n -e x c h a n g e

properties of peat --I V. T he effect of experimental cond i-t ions on ion excha nge properties o f sphagn um peat . Suo35, 47 53.Atta l A. , G osse t T . and Th+venot D. R. (19863 Carac-t6risation physico-chimique des tourbes util is6es en

e pu r a t i on de s e a ux dom e s t i que s . Subm i t t e d f o r pub l i c a -t ion .B l oom P . R . a nd M c B r i de M . B . ( 1979 ) M e t a l i on b i nd i nga nd e xc ha nge w i t h hyd r oge n i ons i n a c i d - w a s he d pe a t .Soil Sci. Soc. Am. J. 43 , 687 - 692 .Boyd S. E . , Sommers L. E. and Ne l son D. W. (1981) Coppe r( I I ) a nd i r on ( I I I) c om pl e xa t i on by t he c a r boxy l a t e g r oupo f hum i c a c i d . Soil Sci. Soc. Am. J . 45, 1241-1242.Bunz l K. (1974a ) Kine t i c s of i on exchange in soi l organicm a t t e r - - l l . I o n e x c h a n g e d u r i n g c o n t i n u o u s a d d i t i o n o fpb2+ - i ons to h um i c a c i d a nd pe a t . J. Soil Sci. 25, 343- 356 .Bunz l K. (1974b) Kine t i c s of i on exchange in soi l organicm a t t e r - - I l l . D i f f e re n t i al ion e xc ha nge r e a c t i ons o fpb2+ - i ons to h um i c a c i d a nd pe a t . J. Soil Sci. 25, 517- 532 .Bunz l K. , Sch mid t W. and S anson i B. (1976) Kin e t i c s of ione xc ha nge i n s o i l o r ga n i c m a t t e r - - I V . A ds o r p t i on a ndde s o r p t i on o f P b -'+ , C u 2+ . C d : ' , a n 2~ a n d C a 2+. J. SoilSci. 27, 32 4l .C ha n e y R . L . a nd H und e m a nn P . T . ( 19793 U s e o f pe a tm os s c o l um ns t o r e m ove c a dm i um f r om w a s t e w a t e r s . J .War. Pollut. Control Fed. 51, 17-21.C ou pa l B . a nd L a l a nc e t t e J . M . ( 1976 ) T he t r e a t m e n t o fw a s t e w a t e r s w i t h pe a t m os s . Water Res. 10, 1071-1076.Dera tani A. and S6bi l l e B. (1981) Me ta l i on ext r ac t ion wi tha th io l hydrophi l i c r e s in . Analvt. Chem. 53, 1742 1746.Dis sanayake C. B. and Weerasoor iya S . V. R. (1981)R e s e a r c h R e po r t . Pe a t a s a m e t a l - t r a pp i ng m a t e r i a l i n t hepur i f i ca t ion of indus t r i a l e f f luent s . J. envir. Stud. 17 ,233 - 238 .G a ngne ux J . L . , G os s e t T . a nd T h6ve no t D . R . ( 19853R e s e a r c h r e po r t . C om pt e xa t i on e n ba t c h de c a t i onsm & a l l ique s pa r l a t ou r be fi pH f i x& U n i ve r s i t6 Pa r i s -V a lde Marne , Cr6te i t , France .Gie sy J . P . (1983 3 M e ta l b in ding capac i ty o f sof t , a c id ,organic - r i ch wa te r s . Toxic. envir. Chem. 6, 203 224.G os s e t T . , T r a nc a r t J . L . a nd T h6ve no t D . R . ( 19843Pr e l i m i na r y r e po r t on k i ne t i c s o f ba t c h m e t a l c om pl e x -a t i on by pe at . C om m u ni c a t i on t o t he Secon d InternationalConference on Hum ic Suhstances. a bs t r a c t pp . 68 - 73 ,B i r m i n g h a m , E n g l a n d .L a l a nc e t t e J . M . ( 1972 ) P r oc 6d6 d ' 6pu r a t i on d ' e a u po l l u t epa r un 616ment m&al l ique e t de r6cup6ra t ion dudi t 616-men t . Breve t fr anqa i s No. 72.43798, 8 D6c . 1972.M c K a y G . ( 19803 Pe a t - - a n a ds o r b e n t f il t r at i on m e d i um f o rw a s t e w a t e r t r e a t m e n t . 142tter Serv. 84, 357- 359 .Me ise l J . , Laka tos B. and Mady G. (1979)B i opo l ym e r - m e t a l c om pl e x s y s t e m s - - V I I . S t udy o f i one xc ha nge a nd r e dox c a pa c i t y o f pe a t hum i c s ubs t a nc e s .Acta agron, hung. 28, 75-83.Poo t s V . J . P . a nd M c K a y G . ( 19803 F l ow c ha r a c t e r i st i c sa nd p a r a m e t e r s r e l a t i ng to u s e o f pe a t a nd w o od a s c he a pa ds o r be n t m a t e r i a l s f o r w a s t e w a t e r pu r i f i c a t i on . Scient.Proc. R. Dubl. Soc. Scr. A 6 , 4 0 7 4 4 1 .Poo t s V. J. P . , Mc Ka y G . and Hea ly J . J. ( 19783 Bas ic dyea ds o r p t i on on pe a t . Scient. Proc. R. Duhl. Soc. Ser. A 6,61- 76 .S!pos S. , Sipos E., Dekany I . , Deer A., Meisel J . andL a ka t o s B . ( 19783 B i opo l ym e r m e t a l c om pl e x s y s t e m s - -I1 . Phys ica l prope r t i e s of humic subs tances and the i rm e t a l c om pl e xe s. Acta agron, hung. 27, 31 42.Sm i t h E . F . , M a c C a r t hy P . , Y u T . C . a nd M a r k H . B . J r(1977) Sul fur i c ac id t r ea tm ent o f pea t for ca t ion ex change .J. Wat. Pollut. Control. Fed. 49, 633.--638.S t um m W . a nd M or ga n J . J . ( 1981 ) Aquatic Chemistry, 2ndedi t ion, p . 625. Wi ley, New York.T a k a m a t s u T . a n d Y o s h i d a T . ( 1 9 7 8 3 D e t e r m i n a t io n o fs t a b i l i t y c ons t a n t s o f m e t a l hum i c a c i d c om pl e xe s bypotent iome t r i c t i t r a t ion and ion- se l ec t ive e l ec t rodes . SoilSci. 125, 377 386.W ol f A. , Bunz l K. , Die t l F . and Schm idt W. F . 119773E f fe ct s o f C a 2 ' i ons on t he a bs o r p t i on o f P b 2 ' , Cu 2~.Cd -'~ and Zn 2~ by hum ic subs tances . Chemosphere 5,207- 213 .

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Batch Metal Removal by Peat