Heavy metal cation retention by unconventional sorbents (red muds and fly ashes)

Heavy metal cation retention by unconventional sorbents (red muds and fly ashes)

Pergamon

PII: S0043-1354(97)00204-2

IVat Res Vol 32, No 2, pp 430-440, 1998

© 1998 Elsevier Science Ltd All rights reserved

Printed in Great Britain 0043-1354/98 $19.00 + 0.00

HEAVY METAL CATION UNCONVENTIONAL SORBENTS

ASHES)

RETENTION BY (RED MUDS AND FLY

R E , A T APAK*, ESMA TOTEM, M E H M E T HLIGI]L and J I ] L I D E H I Z A L

Department of Chemistry, Faculty of Engineering, Istanbul University, Avcdar, 34850, Istanbul,

Turkey

(First received April 1996; accepted in revised form June 1997)

Al~raet Toxic heavy metals, i.e copper (II), lead (II) and cadmium (II), can be removed from water

by metallurgical solid wastes, i.e bauxite waste red muds and coal fly ashes acting as sorbents These heavy-metal-loaded solid wastes may then be solidified by adding cement to a durable concrete mass assuring their safe disposal Thus, toxic metals in water have been removed by sorption on to inexpen- sive solid waste materials as a preliminary operation of ultimate fixation Metal uptake (sorption) and release (desorption) have been investigated by thermostatic batch experiments The distribution ratios

of metals between the solid sorbent and aqueous solution have been found as a function of sorbent type, equilibrium aqueous concentration of metal and temperature The breakthrough volumes of the heavy metal solutions have been measured by dynamic column experiments so as to determine the sat- uration capacities of the sorbents The sorption data have been analysed and fitted to linearized adsorp- tion isotherms These observations are believed to constitute a database for the treatment of one industrial plant's effluent with the solid waste of another, and also to utilize unconventional sorbents, i.e metallurgical solid wastes, as cost-effective substitutes in place of the classical hydrous-oxide-type sorbents such as alumina, silica and ferric oxides © 1998 Elsevier Science Ltd All rights reserved

Key words -cadmium (II), lead (II), copper (II), sorption, red muds, fly ashes

INTRODUCTION Cadmium (II), lead (II) and copper (II) are well-

known toxic heavy metals which pose a serious

threat to the fauna and flora of receiving water

bodies when discharged into industrial wastewater

In spite of strict regulations restricting their careless

disposal, these metal cations may still emerge in a

variety of wastewaters stemming from catalyst, elec-

trical apparatus, painting and coating, extractive

metallurgy, antibacterials, insecticides and fungi-

cides, photography, pyrotechnics, smelting, metal

eleetroplating, fertilizer, mining, pigments, stabil-

izers, alloy industries, electrical wiring, plumbing,

heating, roofing and building construction, piping,

water purification, gasoline additive, cable covering,

ammunition and battery industries (Buchauer, 1973;

Low and Lee, 1991; Periasamy and Namasivayam,

1994) and sewage sludge (Bhattacharya and

Venkobachar, 1984) The acute toxicity of these

heavy metals have caused various ecological cata-

strophes in human history, such as the "itai-itai"

disease due to cadmium (Riley and Skirrow, 1975)

Prolonged effect may cause other chronical dis-

orders (Huang and Ostovic, 1978)

*Author to whom all correspondence should be addressed

Various treatment technologies have been devel- oped for the removal o f these metals from water The hydrometallurgical technology extracts and concentrates metals from liquid waste using any of

a variety of processes, such as ion exchange, electro- dialysis, reverse osmosis, membrane filtration, sludge leaching, electrowinning, solvent stripping, precipitation and common adsorption (LaGrega et al., 1994a)

Both powdered (Sorg et al., 1978) and granular activated carbon (Huang and Smith, 1981) have been used for the adsorptive removal o f Pb, Cd and similar "soft" heavy metals, especially when associ- ated with common organic particulate matter in water Activated carbon from cheaper and readily available sources, such as coal, coke, peat, wood, nutshell (Freeman, 1989) and rice husk (Srinivason, 1986), may be successfully employed for the removal of heavy metals from aqueous solutions Hydrous oxides such as alumina, iron oxides (hematite and goethite) (Cowan et al., 1991; Gerth and Bruemmer, 1983) manganese (IV) oxide (Hasany and Chaudhary, 1986) and titanium (IV) oxide (Koryukova et al., 1984) have also been used for the adsorption o f the indicated heavy metals The cost of the adsorptive metal removal process

is relatively high when pure sorbents (either acti-

430

Sorption of heavy metal cations 431 vated c a r b o n or hydrated oxides) are used

Therefore, there is an increasing trend for substitut-

ing pure adsorbents with natural by-product or

stabilized solid waste materials for the development

of cost-effective composite sorbents capable of

treating a variety of contaminants F o r example,

recent evidence on the combined use of lime, ferric

a n d a l u m i n i u m coagulants has shown that these

substances are more effective in c o m b i n a t i o n than

individually (Harper a n d Kingham, 1992) A n u m -

ber of metallurgical solid wastes such as bauxite

waste red muds a n d coal-fired thermal plant fly

ashes have been screened in this regard to serve as

versatile a n d cost-effective sorbents for heavy metals

(Apak a n d (0nseren, 1987; A p a k et al., 1993) a n d

radionuclides (Apak et al., 1995; 1996) The ability

of fly ash to remove metal cations from water has

also been demonstrated in the literature

(Bhattacharya a n d Venkobachar, 1984; P a n d a y a n d

Singh, 1985; Yadava et al., 1987) for a limited n u m -

ber of metals

The alternative mechanism for heavy metal

removal by red muds a n d fly ashes (either natural

or in activated form) are assumed to comprise four

steps (Gregory, 1978; A p a k a n d Llnseren, 1987) (i)

surface precipitation (sweep flocculation), where

most hydrolysable heavy metals are removed via co-

precipitation o f their insoluble hydroxides forming

successive layers o n the sorbent surface; (ii) floccu-

lation by adsorption of hydrolytic products, where

multi-nuclear hydrolysis products (formed o n the

adsorbent surface as kinetic intermediates) including

[Fe2(OH)4 ]2+, [Fe3(OH)415+, [AI4(OH)s]4+ a n d

[AIs(OH)20] 4 + act as more effective flocculants t h a n

their parent ions due to their higher charge a n d

strong specific adsorptivities; (iii) chemical adsorp-

tion based on surface-complex formation, where metal ions are usually removed as uncharged hy- droxides condensed on to surfaces of - O H group bearing adsorbents (Lieser, 1975), i.e a l u m i n i u m oxide, silica gel, ferric a n d titanium oxides, existing

as components of the utilized composite sorbents; (iv) ion exchange, where the acid-pretreated sor- bents may function as synthetic cation exchangers

Of these mechanisms, surface precipitation a n d chemical adsorption are believed to play the domi-

n a n t role in heavy metal ions removal (Apak a n d Unseren, 1987)

The aim of the present study is to develop cost- effective unconventional sorbents, preferably metal- lurgical waste solids, for heavy metal removal from contaminated water The heavy metal (Pb, Cd a n d Cu) removal capacity as well as sorption modelling

of red m u d s a n d fly ashes will be evaluated in this regard The irreversible nature of sorption needs to

be shown so as to guarantee non-leachability of metals from the metal-loaded sorbents

EXPEilIMENTAL

Materials and methods

All heavy metal solutions (divalent cations Of Pb, CA and Cu) were prepared in stock solutions up to 10000 ppm 0a g/ml) of metal from the corresponding nitrate salts No further pH adjustment of these solutions was made as their natural acidity due to hydrolysis of metals (i.e to form MOIl + and H +) prevented the precipitation

of the corresponding metal hydroxides All chemicals (E Merck, Darmstadt, Germany) were of analytical reagent grade

Of the metallurgical solid wastes used as sorbents, the red muds were supplied from Etibank Seydi~ehir Aluminum plant, Konya, Turkey and coal fly ashes were from TEK Af~in-Elbistan Thermal Power Plant,

Table 1 Saturation capacities of the sorbents for the metals from column and batch experiments and Langmuir parameters of equilibrium

modelling

Langmuir Parameters b

Qo (rag/8) Equilibrium Qexp (rag/g) Qexp (rag/g) Theoretical Metal ion a Adsorbent pH Column capacity Batch capacity capacity b (litre/mg) Corr coeff (r)

"The initial aqueous metal concentrations for different metal/sorbent combinations were as follows: Cu (II) 50 mM (mmol/litre) for red muds and 90 mM for fly ashes; Pb (If) 50 mM for red muds and 65 mM for fly ashes; Cd (II) 35 ram for red muds and 40 mM for fly

ashes,

bCalculated by the aid of iinearized Langmuir equation (4),

432 Re,at Apak et ,7l

C e (mi/mL)

Fig 1 Distribution coefficient of Cd (II) as a function of equilibrium aqueous concentration on fly

ashes and red muds

Kahramanmara~, Turkey The red muds, obtained as alka-

line leaching wastes of bauxite in the Bayer process of

alumina manufacture, had the following chemical compo-

sition by weight: Fe203 37.3%, A1203 17.6%, SiO2 16.9%,

TiO2 5.6%, Na20 8.3%, CaO 4.4%, loss on ignition

7.2% Red muds, being multicomponent systems, are com-

posed of sodium aluminosilicates, kaolinite, chamosite,

iron oxides (hematite) and hydroxides Basically, Fe is in

the form of hematite, Ti is in the form of Fe-Ti oxides

and AI is in the form of ahiminosilicates 94% of red

muds have less than 10/an grain size

The red muds were thoroughly washed with water to a

neutral pH, dried and sieved (R,) prior to adsorption

tests The red muds were also acid-treated (R~) The acid

treatment was carried out according to a modified version

of Shiao's procedure (Shiao and Akashi, 1977) by boiling

I00 g of water-washed and dried red mud in 2 dm 3 of

10% (by weigh0 HCI solution for 2h, filtering off,

thoroughly washing with water, drying and sieving to

obtain the Ra-sorbents The acid-treatment technique,

which has also been applied by Wahlberg et al (1964) to

clay minerals for improving their surface properties, has

been demonstrated with success in synthesizing a better

adsorbent from red muds in phosphorus (Shiao and Akashi, 1977) and heavy metal (Apak and Unseren, 1987; Apak et al., 1995, 1996) removal However, acid treatment

of red mud sorbents had the drawback of the partial loss

of acid-soluble fractions like hematite The Ra fraction was further subjected to heat treatment at 600°C for 4 h

to obtain the Rah sorbents The red muds (partly agglom- erated due to relative humidity) could not be classified with respect to true grain size as most were of 200 mesh size in wet sieving

The specific areas of Rw, Ra and Rah samples were 14.2, 20.7 and 28.0 m2/g, respectively, measured by the BET/Nz method (Brunauer et al., 1938)

Coal fly ash was recovered from the cyclones and elec- trostatic precipitators of the power plant and had the fol- lowing average composition: CaO 42.5%, SiO2 21.9%, SO3 13.6%, A1203 11.8%, Fe.zO3 2.4%, MgO 1.3%, K20 1.1%, Na20 0.9%, loss on ignition 4.4% Almost 99% of the fly ash could pass through a 200-mesh sieve The raw fly ash (F) was washed with 10-fold distilled water for sev- eral (5-6) times, filtered and dried (Fw) A part of the Fw was further treated several times with acid using 2% (by weight) HCI in boiling solution for 2 h Higher acidity (as

8.0

[] oaf

0.0

C e ling/roLl

Fig 2 Distribution coefficient of Cu (II) as a function of equilibrium aqueous concentration on fly

ashes and red muds

Sorption of heavy metal cations 433

6.0

PbF,,

1.0 ~

0.0

C e (m mLl Fig 3 Distribution coefficient of Pb (II) as a function of equilibrium aqueous concentration on fly

ashes and red muds

in the activation of red mud) was avoided due to severe

losses of fly ash components by solubilization The solid

product was thoroughly washed with water, filtered, and

oven dried at 100 + 5°C to produce the acid-treated (Fa)

sorbent

The X-ray diffractogram (Apak et al., 1996) of the Fw

identified 51% calcite (CaCO3), 32% anhydrite (CaSO4),

9% quartz (SiO2) and 3% hematite (Fe203) in the crystal-

line phase Elemental analysis of selected spots in the het- erogenous amorphous slag particles by the XRF technique (Apak et al., 1996) yielded 41-52% CaO, 27% SiO2, 13% A1203, 2-5% FeO, 1-4% MgO and up to 2% other ox- ides

The BET/N2 surface area of fly ash were 10.2 and 14.3 m2/g for Fw and Fa, respectively

250

g

@

150

100

50

0

[ ] CdF

)

0

0.0

[]

0

0

2.0

Co(msI L)

!

4.0

Fig 4 Isotherm of Cd (II) adsorption onto fly ashes and red muds

6.0

434 Re,at Apak

160

120

80

40

[ ]

[ ]

[ ]

©

[]

©

®

[ ]

ml,

0.0

©

A

©

O

Ce(mg/mL)

1.2

Fig 5 Isotherm of Cu (II) adsorption onto fly ashes and red muds

CuF

C F a

c,e,, c,e,

Point of zero charge (PZC) measurements by potentio-

metric titration of the sorbent suspensions at different

ionic strengths (Apak et aL, 1995, 1996) yielded approxi-

mate P Z C values of 6.4 and 8.3 for fly ash and red mud

sorbents, respectively

When 1 g of sorbent was equilibrated with 50 ml dis-

tilled water, the indicated sorbents showed the following

approximate final pH in their aqueous leachates:

pH 8.1 4.8 5.3 12.0 10.8 9.3

The acid-treated sorbents contained no free HCI but 15-

20 mg Cl-/g

Batch sorption tests were carried out by agitating a sus-

pension of I g sorbent in 50 ml metal nitrate solution for

8 h (equilibration period) at room temperature

(25 +0.1°C) in stoppered flasks placed on a thermostatic

water-bath/shaker After centrifugation, the remaining

metal concentration in the filtrate was determined by

flame AAS (Perkin Elmer 300, Norwalk, CT, U.S.A.) and

the equilibrium pH was measured by a pH-meter

(Metrohm E-512 Herisau, Switzerland) equipped with a

glass electrode

The metal concentration retained in the sorbent phase

(qe, mg/g) was calculated by

qe = (Co - G ) V / m (1) where Co and C~ are the initial and final (equilibrium) con-

centrations of the metal ion in solution (mg/litre), V is the

solution volume (litres) and m is the mass of sorbent (g)

The solid/water distribution ratios (at equilibrium) of

metals for both sorption and desorption were calculated

by

Ko = q o / G (2)

where KD is the empirical distribution ratio of the metal cation M ((mg/g)/(mg/litre)= litres/g) determined on the approximately linear portion of the corresponding adsorp- tion isotherm

Batch desorption tests were carried out by agitating I g

of metal loaded sorbent with 50 ml of the desired solution until equilibrium (8 h)

The saturation capacities of the sorbents for the uptake

of indicated metals were determined by both batch and column tests For the latter, 40 g of adsorbent was filled at

a height of 8-11 cm in a thermostatic (25 + 0.1°C) column

of dimensions (h = 30 cm, ~b = 3 cm), and the adsorbate solution was fed (counter to gravity) by a peristaltic pump through the fixed bed of sorbent at a constant rate of 0.5 ml/min The metal concentration of the eluate was recorded against throughput volume The dynamic metal uptake capacities of the sorbents were calculated by the in- tegration technique (Apak et al., 1996), i.e the area above the curve up to the line on which the eluate concentration was equalized with the initial concentration of metal was calculated The total amount of retained metal was divided

by the mass of sorbent to yield the saturation capacity (t.)col x

RESULTS AND DISCUSSION

In weakly a c i d i c - n e u t r a l suspensions whose p H was a t t a i n e d n a t u r a l l y by e q u i l i b r a t i n g a q u e o u s

m e t a l n i t r a t e - s o r b e n t mixtures, the d i s t r i b u t i o n ratios generally increased with initial a q u e o u s

Sorption of heavy metal cations 435

500

400

300

200

&

0

0

[ ]

O 0

0

0

0

[]

A

0

@

I:bF

~aa, ha,,

• 1 i 1~ i , n II , | 1 II I •

CelmllnnU

Fig 6 Isotherm of Pb (II) adsorption onto fly ashes and red muds

10.0

adsorbate concentration at equilibrium up to a lim-

iting value where the batch capacities of the sor-

/ ' n b a t c h ~ bents for the metals were calculated ~exp ~ The

saturation capacities found by both batch and

dynamic column experiments (the latter symbolized

as Q~L.) are listed in Table 1 The variation o f dis-

tribution ratio (KD) with equilibrium concentration

of the adsorbate in solution is shown in Figs 1, 2

and 3, where KD vs C~ on semi-logarithmic scale

gave roughly linear plots A gradual decrease of dis-

tribution ratios with aqueous concentration was

noted, due to increased occupation of active surface

sites of sorbent with metal loading in the aqueous

solutions (Apak et al., 1995, 1996) As long as a

strict differentiation between true adsorption and

precipitation- masked sorption (McKay et al., 1985)

is not made, both red muds and fly ashes may be

visualized as effective sorbents capable of removing

the studied heavy metal ions from solution with

high distribution ratios, /~D as', ranging up to 10 - 2 -

10 -1 litres/g Naturally the slightly alkaline charac-

ter of aqueous leachate obtained from fly ashes in

conjunction with the CaO and CaSO4 constituents

of this material should account for hydrolytic metal

precipitation reactions (Burgess, 1978; Freeman,

1988) as well as counter-ion adsorption at pH

above PZC accompanying chemical adsorption

(Apak and Unseren, 1987; Apak et al., 1993)

Generally a metal hydroxide may precipitate and

may form at the surface of the hydrous oxide sor- bent prior to its formation in bulk solution and thus contribute to the total apparent sorption The contribution of surface precipitation to the overall sorption increases as the sorbate/sorbent ratio is increased (Stumm and Morgan, 1996) It should be added that raw fly ash (F) cannot be considered as

an EPA-acceptable sorbent (LaGrega, 1994b) as it introduces new contaminants to water in untreated (either water- or acid-washed) form

The adsorption isotherms (q~ vs Ce) of metal uptake at 25°C (see Figs 4, 5 and 6) essentially showed BET (Branauer et aL, 1938) (type II, V) character curves pointing out to the heterogeneity

of the sorbents containing hydrous oxides, silicates and sulfates, resulting in various combinations of linear and nonlinear isotherms (Weber et al., 1996)

It is known from the literature that BET type IV-V isotherms are quite common for the porous hydrox- ides (xerogels) such as siligagel or iron hydroxide (Gregory, 1978)

Although Langmuir and Frenndlich approxi- mations of the observed adsorption data in the line- arized forms gave satisfactory correlation coefficients (r > 0.95) for most of the covered con- centration range, the Langmuir model had more practical utility for representing the limiting sorp- tion capacities of the sorbents than the exponen- tially increasing Freundlich isoterm (McKay et aL,

436 Re,at Apak et al

0.08

Q

0

0.06

0.04

0.02

e

I I

c d e w

c u e w

~ew

0.00 V

Fig 7 Selected isotherms linearized with

C e (mg/mL)

respect to the Langmuir model (red mud)

1985) in spite of the invalidity of the classical

Langmuir assumptions, i.e site-specific and uni-

formly energetic adsorption confined to monolayer

1996) Heavy metal adsorption on heterogenous

sorbents has been interpreted by the aid of the

Langmuir isotherm on various occasions in the en-

vironmental literature (Szymura, 1990; Prasad and

Agarwal, 1991)

A Langmuir equation for adsorption may be

written as

Q° b C~

qe = 1 + bCe (3) which transforms to the linearized form;

Ce/qe = (Q°)-lCe + (Q°b)-I (4)

where the Langmuir parameters, Q0 (rag/g) and b

(L/rag), relating to monolayer adsorption capacity

and energy of adsorption, respectively (Periasamy

and Namasivayam, 1994), are found from the slope

and intercept of C, /qe vs Ce linear plot such that

QO = slope-t and b = intercept -t slope Several line-

arized isotherms with respect to the Langmuir

model are shown in Figs 7 and 8

The Langmuir parameters computed for all metal

in Table 1 (three runs made per isotherm) together with the experimental saturation capacities of batch rn~t~h~ and column (Q~exp.) tests After screening of ol those results where metal hydroxide precipitation could have been effective in metal removal (e.g modelling of Cu(H) sorption has been made up to the concentration edge of Cu(OH)2 precipitation at the studied pH), Langmuir modelling has been quite successful in predicting the experimental satur- ation capacities of the sorbents, especially those obtained from dynamic column tests (see Table 1), although its basic assumptions were not fulfilled

due to heterogeneity of the multicomponent sorbent surfaces Moreover, the presence of a hydrated oxide-type sorbent may delay the precipitation of a metal hydroxide in a saturated solution as, for example, in a suspension containing a silica sorbent where the binding of Cu (II) ions to the SiO2 sur- face would be preferred over precipitation (Park et al., 1995)

The capacities determined by column experiments were generally greater than those by batch tests, i.e QCOL > [ ) b a t c h

exp.- ~xp , due to a number of reasons:

(i) the sorbent column consists of several transfer units, and the height equivalent to one theoreti-

0.020

Sorption of heavy metal cations

i

437

0.015

© O d F w

Cu F w lib F w

o"

0.010

Q

0

0.005

A

O.OOO ~

Fig 8 Selected isotherms linearized with respect to the Langrnuir model (fly ash)

cal plate (HETP) may take quite low values in

efficient columns;

(ii) metal cations are partly held by ion-exchange

while passing through the column causing a

natural pH gradient to develop across the col-

urrm height, whereas pH is a rather conserved

property in batch tests;

(iii) a part of the sorbent surface may be covered

with a hydrous oxide gel containing the heavy

metal hydroxide as the elution proceeds, and

this layer may promote further binding of the

metals enhancing sorption

Generally very high limiting capacities have been

achieved for metal sorption on to the selected

unconventional sorbents giving rise to their possible

utility in heavy metal removal from contaminated

water All the observed metal cations sorption

(except Cd (II) uptake by fly ash) took place at pH

values below the PZC of sorbents indicating specific

adsorption by the hydrous oxide gel layer as the

dominant mechanism of adsorptive uptake (Apak

and Unseren, 1987; Apak et aL, 1993, 1995, 1996)

rather than electrostatic binding The extremely

high capacities of fly ash for Cu (II) and Pb (II)

may be attributed to the contribution by surface

precipitation The pretreatment procedures applied

to red muds and fly ashes (e.g acid activation and subsequent heat treatment) did not significantly increase the metal loading capacities unlike those of

Cs + (Apak et al., 1995) and orthophosphate (Shiao and Akashi, 1977) adsorption by red mud The increased surface area of the pretreated sorbent was not reflected in sorption capacities The only advan- tage of acid activation in this study seems to be the production of clean sorbents compatible with EPA regulations (LaGrega et aL, 1994b)

The order of hydrolysable divalent metal cation retention on the selected sorbents (which actually contained a mixture of hydrated oxides) were as fol- lows in terms of saturation capacities (mmoi/g): Cu

> Pb > Cd for fly ashes and Cu > Cd > Pb for red muds (see Table 1), with Pb (II) replacing Cd (II) in the sequence for the two sorbents The degree of the insolubility of the metal hydroxides (expressed as the pKsp of the corresponding metal hydroxide) approximately followed the same order:

pKsp of M(OH)2 : 1 9 7 14.9 (13.7) 13.6 where (13.7) is the pKsp of Pb(OH)CI, probably showing the role of heavy metal hydrolysis and hy- drolytic precipitation in the observed uptake

438 Re,at Apak et al

Table 2 The distribution coefficients of the metals obtained by batch tests for sorption and desorption

pH 4.75 (H2CO~) KdD ~" pH 7 (H2CO3, NaHCO3) fro ~'

1995) Hydroxo-metal complexes and hydroxides

formed at a p H just below the precipitation limit

tend to sorb on hydrated oxide-type sorbents with

higher affinity due to energetic reasons (Reed and

Cline, 1994) The correlation between the stability

constant o f the surface complex and that o f

hydroxo-complex is linear, especially on a silica sur-

face (Park et al., 1993) The much stronger adsorp-

tion of Cu (II) on TiO2 (s) than of Cd (II) or Zn

(II) has been attributed to the much lower solubility

product of Cu (OH)2 vs Cd(OH)2 or Zn(OH)2

(Zang et al., 1994) Thus, there is a natural corre-

lation as observed in this work between adsorbabil-

ity o f the metal and the pKsp of its hydroxide The

high capacity of fly ash for Pb (II) may have been

additionally affected by PbSO4 formation on the

sorbent surface containing sulphate

If the utilized sorbents are suggested for use in

restricting the expansion o f a metal contaminated

plume in soil, then it will be necessary to show the

leachability of the retained metals from the sorbents

under changing groundwater conditions The poss-

ible p H changes in groundwater have been modelled

by saturated aqueous carbonic acid (pH 4.75) and

H2CO3/NaHCO3 buffer (pH 7.0) solutions, the lat-

ter being prepared by bubbling CO2 through a

4.0 x 10 -3 M NaHCO3 solution until the solution

became neutral (pH 7.0) The distribution coeffi-

cients obtained by batch tests for limiting adsorp-

tion (/~v ds') and for desorption (/~v ~') with both

carbonic acid and p H 7,0 buffer solutions at room

temperature are listed for comparison in Table 2

The fact that the K~D s" values were in general 3-4

orders of magnitude higher than the /~v dS' values

confirmed the essential irreversible character of

metal adsorption (Park et al., 1992; A p a k et al.,

1995) on to the selected sorbents Therefore, the

suggested unconventional sorbents may be used in

confining a subsurface metal contaminant plume in

a restricted zone, and the retained metals would not

be leached out once retained in changing ground- water p H conditions, e.g by CO2 injection

Thus, these sorbents may serve as effective and almost priceless fixation agents for heavy metal removal from water prior to a more sophisticated procedure such as solidification and stabilization as the means of the ultimate disposal F o r example, when metal-loaded solid waste was added up to 20% by mass to Portland cement-based formu- lations, the fixed metals did not leach out from the solidified concrete blocks over extended periods, with the exception o f Cu (II), which reached a con- centration of 0.4 ppm after 8 months in a water lca- chate o f p H 8-9 (Klhnqkale et aL, 1997) A double- fold aim o f heavy metal fixation and metallurgical solid waste disposal would then be achieved with the constraint that fly ashes better serve the purpose

o f heavy metal fixation than red muds

CONCLUSIONS

In investigation o f the possibility o f usage o f met- allurgical solid wastes as cost-effective sorbents in heavy metal removal from contaminated water, red muds and especially fly ashes have been shown to exhibit a high capacity for heavy metals with the sorption sequence Cu > Pb > Cd in accordance with the order of insolubility o f the corresponding metal hydroxides An empirical Langmuir approach could approximate isotherm modelling of metal sorption The metals were essentially held irreversi- bly, and would not leach out into carbonic acid or bicarbonate buffered solutions The metal-loaded solid wastes could be solidified to an environmen- tally safe form, thereby serving the double-fold aim

of water treatment and solid waste disposal

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