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 ElsevierScienceLtd All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00 HEAVY METAL CATION RETENTION BY UNCONVENTIONAL SORBENTS (RED MUDS AND FLY ASHES) R E , A T APAK*, ESMA TOTEM, M E H M E T HLIGI]L and JI]LIDE 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 inexpensive 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 saturation capacities of the sorbents The sorption data have been analysed and fitted to linearized adsorption 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 wellknown 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, electrical apparatus, painting and coating, extractive metallurgy, antibacterials, insecticides and fungicides, photography, pyrotechnics, smelting, metal eleetroplating, fertilizer, mining, pigments, stabilizers, 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 catastrophes in human history, such as the "itai-itai" disease due to cadmium (Riley and Skirrow, 1975) Prolonged effect may cause other chronical disorders (Huang and Ostovic, 1978) *Author to whom all correspondence should be addressed Various treatment technologies have been developed for the removal of 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, electrodialysis, 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 of Pb, Cd and similar "soft" heavy metals, especially when associated 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 of the indicated heavy metals The cost of the adsorptive metal removal process is relatively high when pure sorbents (either acti- 430 431 Sorption of heavy metal cations vated carbon or hydrated oxides) are used Therefore, there is an increasing trend for substituting 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 For example, recent evidence on the combined use of lime, ferric and aluminium coagulants has shown that these substances are more effective in combination than individually (Harper and Kingham, 1992) A number of metallurgical solid wastes such as bauxite waste red muds and coal-fired thermal plant fly ashes have been screened in this regard to serve as versatile and cost-effective sorbents for heavy metals (Apak and (0nseren, 1987; Apak et al., 1993) and 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 and Venkobachar, 1984; Panday and Singh, 1985; Yadava et al., 1987) for a limited number of metals The alternative mechanism for heavy metal removal by red muds and fly ashes (either natural or in activated form) are assumed to comprise four steps (Gregory, 1978; Apak a n d Llnseren, 1987) (i) surface precipitation (sweep flocculation), where most hydrolysable heavy metals are removed via coprecipitation of their insoluble hydroxides forming successive layers on the sorbent surface; (ii) flocculation by adsorption of hydrolytic products, where multi-nuclear hydrolysis products (formed on the adsorbent surface as kinetic intermediates) including [Fe2(OH)4 ]2+, [Fe3(OH)415+, [AI4(OH)s]4+ and [AIs(OH)20]4 + act as more effective flocculants than their parent ions due to their higher charge and strong specific adsorptivities; (iii) chemical adsorp- tion based on surface-complex formation, where metal ions are usually removed as uncharged hydroxides condensed on to surfaces of - O H group bearing adsorbents (Lieser, 1975), i.e aluminium oxide, silica gel, ferric and titanium oxides, existing as components of the utilized composite sorbents; (iv) ion exchange, where the acid-pretreated sorbents may function as synthetic cation exchangers Of these mechanisms, surface precipitation and chemical adsorption are believed to play the dominant role in heavy metal ions removal (Apak and Unseren, 1987) The aim of the present study is to develop costeffective unconventional sorbents, preferably metallurgical waste solids, for heavy metal removal from contaminated water The heavy metal (Pb, Cd and Cu) removal capacity as well as sorption modelling of red muds and 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 Saturation capacitiesof the sorbents for the metals from column and batch experimentsand Langmuirparameters of equilibrium modelling Metal iona Adsorbent LangmuirParametersb Qo (rag/8) Equilibrium Qexp.(rag/g) Qexp.(rag/g) Theoretical pH Columncapacity Batch capacity capacity b (litre/mg) Corr.coeff (r) C d (II) C d (II) C d (II) F Fw Fa 7,2 6.7 6.6 220 -122 198.2 195,2 180,4 374.3 223.2 217.2 1.14.10-3 I.17.10-3 6.07.10-3 0.957 0.970 0,953 Cd OI) Cd (II) Cd (II) Cu (If) Cu (II) Cu (II) Cu (II) Cu (II) Cu (II) Rw gab Ra F Fw Fa Rw Rah Ra 6.0 5.9 4.2 6.0 5.8 5.7 6.0 5.7 4.5 160 115 105 -264 187 110 100 63 66.8 66.8 46.9 207.3 205.8 198.5 75.2 65.2 35.2 113.7 112.0 107.5 335.2 328.2 283.9 90.0 87.8 65.4 0.57.10-3 0.65.10-3 0.11.10-3 0.94,10-3 0.75,10-3 0.73.10-3 0.96.10-3 0.79.10-~ 1.00.10-3 0.958 0.994 0.989 0.968 0.961 0.960 0.958 0.956 0.964 Pb (II) Pb (II) F Fw 6.2 6.0 530 444.7 483.4 526.0 490.7 I.I1.10-3 1.10.10-3 0.948 0.976 Pb (II) Pb (II) Pb (II) Pb (II) Fa Rw R.a R, 6.0 6.0 5.7 4.4 -161 164 123 437.0 165.8 138.8 117.3 483.0 158.9 137.2 118.5 0.84.10-3 0.66.10-3 1.17.10-3 1.56.10-3 0.958 0.960 0.970 0.956 "The initial aqueous metal concentrationsfor different metal/sorbentcombinationswere 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, bCalculatedby the aid of iinearizedLangmuirequation (4), 432 Re,at Apak et ,7l .0 , , , , , , 5.0 t A [] CdF CdFw 4.0 ~ caF, 3.0 Jo.'~ 1.ot 0.0 ~ 0.0 • • I 1.0 2.0 3.0 • • I ~]~ Cd~ I ~ ~ 4.0 5.0 "1 w 6.0 C e (mi/mL) Fig 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 alkaline leaching wastes of bauxite in the Bayer process of alumina manufacture, had the following chemical composition 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 composed 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 dm 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 h to obtain the Rah sorbents The red muds (partly agglomerated 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 electrostatic precipitators of the power plant and had the following 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 several (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 h Higher acidity (as 8.0 [] oaf 5.0 A c~Fw 4.0 • 0.% 2.o3°1.o • 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 C e ling/roLl Fig 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 5.0 A 4.0 • ~'~w PbF,, ~'Rw 1.0 ~ 0.0 0.0 1.0 2.0 3.0 4.0 Ce 5.0 8.0 7.0 8.0 9.0 (m mLl Fig 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 heterogenous 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 oxides The BET/N2 surface area of fly ash were 10.2 and 14.3 m2/g for Fw and Fa, respectively 250 ,' o8 o [] CdF A ca% • cd% 150 g @ ) [] 100 • • 50 0 0.0 ! 2.0 4.0 Co(msI L) Fig Isotherm of Cd (II) adsorption onto fly ashes and red muds 6.0 434 Re,at Apak et al 160 [] [] 120 [] CuF â C.F a đ c,e,, c,e, [] © [] © 80 A © 40 ml, O r • 0.4 0.0 0.8 Ce(mg/mL) 1.2 Fig Isotherm of Cu (II) adsorption onto fly ashes and red muds Point of zero charge (PZC) measurements by potentiometric titration of the sorbent suspensions at different ionic strengths (Apak et aL, 1995, 1996) yielded approximate P Z C values of 6.4 and 8.3 for fly ash and red mud sorbents, respectively When g of sorbent was equilibrated with 50 ml distilled water, the indicated sorbents showed the following approximate final pH in their aqueous leachates: Rw Ra R~ pH 8.1 4.8 5.3 F 12.0 Fw Fa 10.8 9.3 The acid-treated sorbents contained no free HCI but 1520 mg Cl-/g Batch sorption tests were carried out by agitating a suspension of I g sorbent in 50 ml metal nitrate solution for 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 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 adsorption 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 = 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 integration 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 (1) where Co and C~ are the initial and final (equilibrium) concentrations 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 RESULTS AND DISCUSSION In weakly acidic-neutral suspensions whose p H was attained naturally by equilibrating aqueous metal n i t r a t e - s o r b e n t mixtures, the distribution ratios generally increased with initial aqueous Sorption of heavy metal cations 435 500 [] [] 400 [] A [] 0 [] [] & 300 I:bF @ O0 ~aa, ha,, 0 0 200 • •1 0.0 i 1~ 2.0 i , n II , |1 4.0 II I • • 6.0 CelmllnnU 0.0 10.0 Fig Isotherm of Pb (II) adsorption onto fly ashes and red muds adsorbate concentration at equilibrium up to a limiting value where the batch capacities of the sor'nbatch~ 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 The variation of distribution ratio (KD) with equilibrium concentration of the adsorbate in solution is shown in Figs 1, and 3, where KD vs C~ on semi-logarithmic scale gave roughly linear plots A gradual decrease of distribution 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, /~Das', ranging up to 10-210-1 litres/g Naturally the slightly alkaline character 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 sorbent 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, 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 hydroxides (xerogels) such as siligagel or iron hydroxide (Gregory, 1978) Although Langmuir and Frenndlich approximations of the observed adsorption data in the linearized forms gave satisfactory correlation coefficients (r > 0.95) for most of the covered concentration range, the Langmuir model had more practical utility for representing the limiting sorption capacities of the sorbents than the exponentially increasing Freundlich isoterm (McKay et aL, 436 Re,at Apak et al 0.08 e II cde w cue w ~ew 0.06 Q 0.04 0.02 0.00 V 0.0 4.0 2.0 6.0 8.0 C e (mg/mL) Fig Selected isotherms linearized with respect to the Langmuir model (red mud) 1985) in spite of the invalidity of the classical Langmuir assumptions, i.e site-specific and uniformly energetic adsorption confined to monolayer coverage (Weber and DiGiano, 1995; Weber et aL, 1996) Heavy metal adsorption on heterogenous sorbents has been interpreted by the aid of the Langmuir isotherm on various occasions in the environmental literature (Szymura, 1990; Prasad and Agarwal, 1991) A Langmuir equation for adsorption may be written as Q° b C~ qe = + 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 linearized isotherms with respect to the Langmuir model are shown in Figs and The Langmuir parameters computed for all metal ion-sorbent combinations at 25°C are summarized in Table (three runs made per isotherm) together with the experimental saturation capacities of batch rn~t~h~ and column (Q~exp.) ol tests After screening of 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 saturation capacities of the sorbents, especially those obtained from dynamic column tests (see Table 1), although its basic assumptions were not fulfilled (Weber and DiGiano, 1995; Weber et al., 1996), 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 surface 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 > [)batch exp.~xp , due to a number of reasons: (i) the sorbent column consists of several transfer units, and the height equivalent to one theoreti- Sorption of heavy metal cations 0.020 437 i © O d Fw Cu Fw lib Fw 0.015 o" Q 0.010 0.005 A O.OOO ~ 0.0 2.0 4.0 Ce(mWmL| 8.0 8.0 Fig 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 colurrm 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 advantage 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 follows 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: Metal(II): pKsp of M(OH)2 : Cu > Pb > 14.9 (13.7) Cd 13.6 where (13.7) is the pKsp of Pb(OH)CI, probably showing the role of heavy metal hydrolysis and hydrolytic precipitation in the observed uptake sequence (Apak et al., 1993; TQtem and Apak, 438 Re,at Apak et al Table The distributioncoefficientsof the metalsobtainedby batch tests for sorptionand desorption Metal Absorbent Cd (II) Cd (11) Cd (II) Cd (II) Cd (II) Cd (II) Cu (II) Cu (II) Cu (II) Cu (II) Cu (II) Cu (II) Pb (II) Pb (II) Pb (II) Pb (II) Pb (II) Pb (II) F Fw Fa Rw Rah Ra F Fw Fa Rw Rah Ra F Fw Fa Rw Rah Ra /t~D' (litres/g) 0.372 0.329 0.090 0.026 0.026 0.016 0.132 0.128 0.125 0.045 0.035 0.014 0.126 0.127 0.093 0.024 0.018 0.015 1995) Hydroxo-metal complexes and hydroxides formed at a pH 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 of the surface complex and that of hydroxo-complex is linear, especially on a silica surface (Park et al., 1993) The much stronger adsorption 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 correlation as observed in this work between adsorbability of 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 of 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 possible 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 latter 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 coefficients obtained by batch tests for limiting adsorption (/~vds') and for desorption (/~v~') with both carbonic acid and pH 7,0 buffer solutions at room temperature are listed for comparison in Table The fact that the K~Ds" values were in general 3-4 orders of magnitude higher than the /~vdS' values confirmed the essential irreversible character of metal adsorption (Park et al., 1992; Apak 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 pH 4.75 (H2CO~)KdD ~" (litres/g) pH (H2CO3,NaHCO3) fro~' (litres/g) 249.7 148.8 144.8 I 13.8 112.0 107.4 223,5 218.8 189.3 150.0 146.3 109.0 52.6 49.1 48.3 317.8 342.8 395.0 624.3 111.6 217.2 227.6 86.2 82.6 335.2 328.2 656.8 69.2 125.4 65.4 10.5 377.4 966 be leached out once retained in changing groundwater pH 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 formulations, the fixed metals did not leach out from the solidified concrete blocks over extended periods, with the exception of Cu (II), which reached a concentration of 0.4 ppm after months in a water lcachate of pH 8-9 (Klhnqkale et aL, 1997) A doublefold 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 of the possibility of usage of metallurgical 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 of the corresponding metal hydroxides An empirical Langmuir approach could approximate isotherm modelling of metal sorption The metals were essentially held irreversibly, and would not leach out into carbonic acid or bicarbonate buffered solutions The metal-loaded solid wastes could be solidified to an environmentally safe form, thereby serving the double-fold aim of water treatment and solid waste disposal REFERENCES Apak V and I:lnsvren E (1987) Treatment of waste water 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(Bhattacharya and Venkobachar, 1984; Panday and Singh, 1985; Yadava et al., 1987) for a limited number of metals The alternative mechanism for heavy metal removal by red muds and fly ashes (either... of metallurgical 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... role in heavy metal ions removal (Apak and Unseren, 1987) The aim of the present study is to develop costeffective unconventional sorbents, preferably metallurgical waste solids, for heavy metal