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Mucor biosorbent for chromium removal from tanning effluent
PII: S0043-1354(97)00343-6
Wat. Res. Vol. 32, No. 5, pp. 1407±1416, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
MUCOR BIOSORBENT FOR CHROMIUM REMOVAL FROM TANNING EFFLUENT J. M. TOBIN1* and J. C. ROUX2 School of Biological Sciences, Dublin City University, Dublin 9, Ireland and 2Laboratoire TSV, DBMS, CEA/Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
1
(First received November 1996; accepted in revised form August 1997) AbstractÐWaste industrial Mucor meihi biomass was found to be an eective biosorbent for the removal of chromium from industrial tanning euents. Sorption levels of 1.15 and 0.7 mmol/g were observed at pH 4 and 2 respectively while precipitation eects augmented these values at higher pH ranges. Acid elution of biosorbed chromium increased with decreasing eluant pH to a maximium value of ca. 30% at approximately zero pH. Successive elution stages with increasingly strong acids resulted in a cumulative chromium recovery of in excess of 80%. Both acid and base treatments eluted biosorbed chromium and successive acid/base and base/acid treatments resulted in recovery values approaching 100% at low metal loadings. These values decreased to 80 to 60% at higher biomass metal loadings. In comparative studies with ion exchange resins, the Mucor biomass demonstrated chromium biosorption levels that correspond closely to those of commercial strongly acidic exchange resin while the pH behaviour mirrored that of the weakly acidic resins in solution. The chromium elution characteristics from the Mucor biomass were similar to those of both the weakly and strongly acid resins. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐchromium, biosorption, fungal biomass, tanning euent, metal uptake, Mucor biosorbent
INTRODUCTION
The interactions of microorganisms and metals in aqueous media have in recent years been the focus of a growing number of scienti®c studies (Fourest and Roux, 1992; Tobin et al., 1984; Volesky and Holan, 1995; White and Gadd, 1990). The characteristics of both passive microbial metal binding, commonly termed biosorption (Brady and Tobin, 1995; Holan et al., 1993; Tsezos and Deutschmann, 1990) and metabolically-mediated intracellular accumulation, or bioaccumulation (Avery and Tobin, 1992; Brady and Duncan, 1994; Norris and Kelly, 1977) have been investigated for a wide range of simple metal/organism systems. In addition, competition studies in solutions of multiple cations and/or anions have underscored the complexity of the sorption interactions involved as well as demonstrating the ability of certain co-ions to either reverse or augment metal toxicity and uptake (Karamushka and Gadd, 1994; Venkata Ramana and Sivarama Sastry, 1994). Nonetheless, in spite of increased understanding of the phenomena and the recognition of the potential of microbial metal uptake levels equivalent or exceeding those of ion exchange resins, there have been few successful industrial applications of biosorption systems. To date, the principal exponents of metal-microorganism interactions for environmental purposes remain *Author to whom all correspondence should be addressed.
traditional biological waste treatment systems of the activated sludge/biological ®lter type. Chromium-bearing euents from the tanning industry have been cited as potentially amenable to treatment by biosorption processes (Aksu et al., 1990; Nourbash et al., 1994). The tanning process uses an acidic solution of chromium oxide to transform animal hides into leather. Chromium (III) is adsorbed from the solution by the tissue and crosslinks the collagen ®bres by forming coordinate bonds between protein functional groups. In this way nitrogen detachment from the tissue and rotting is prevented. The euent, which may contain of the order of 3 g Cr(III)/l at ca. pH 4 (Landgrave, 1995), is typically treated by raising the pH, with or without addition of coprecipitants such as Fe or organic coagulants, to recover precipitated chromium hydroxide. While pilot plant studies have reported 99.5% recovery by these methods (Landgrave, 1995), recent analyses of actual tanning euents following treatment have shown residual total Cr levels of in excess of 10 mg/l (Stein and Schwedt, 1994). In addition, the waste waters were found to have high conductivities following the precipitation treatments and high organic levels resulting from contact with the hides. Cr(VI) is widely recognised to be considerably more toxic than Cr(III) (Rapoport and Muter, 1995; Stein and Schwedt, 1994; Venkata Ramana and Sivarama Sastry, 1994). Cr(III), in its stable
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hydrate form, is unable to cross biological membranes and its toxicity to fungi has been related to its speci®c antagonism with iron uptake (Venkata Ramana and Sivarama Sastry, 1994). In man it is recognised as an essential element and diabetes symptoms are reported to be associated with Cr dietary de®ciency (Kaim and Schwederski, 1994). However, the reduction of Cr(VI) to Cr.(III) within cells and the subsequent binding to and disruption of proteins and nucleic acids have recently been recognised as underlining the toxic potential of both chromium forms (Rapoport and Muter, 1995). In the US the maximum permissable concentration of chromium in natural waters recommended by the EPA for the protection of human health is 50 mg/ m3 (Nourbash et al., 1994). Fungi in general, and species of the Mucorales order in particular, are well known for their metal biosorption characteristics (Gadd, 1993; Tobin et al., 1990). Rhizopus arrhizus has been reported to exhibit uptake levels equivalent to commercial exchange resins for a range of divalent metals (Tobin et al., 1984; Treen-Sears et al., 1984) and the closely related Mucor species have been shown to biosorb a range of insoluble metal compounds from acid mine drainage (Gadd, 1993; Singleton et al., 1990). The objectives of this work were to characterise the potential of industrial waste Mucor biomass to remove chromium from real tannery euent and to investigate subsequent desorption processes to facilitate metal recovery. Comparative studies using ion exchange resins were also performed to elucidate the underlying sorption mechanisms.
SA), Amberlite weakly basic anion exchange resin, IRA93, (denoted WB), and Amberlite strongly basic anion exchange resin, IRA 900, (denoted SB). The reference Cr and Na standards used for determinations were purchased from Merck-Clevenot S.A., 5±9 rue Anquetil, 94736 Nogent-Sur-Marne, France. Chromium uptake Serial dilutions of the tanning euent were prepared using deionised distilled water to give solutions ranging in concentration from full strength to 1 in 20 dilution. Aliquots of 100 ml were contacted with 0.3 g quantities of Mucor biomass or ion exchange resin overnight. Samples were ®ltered (0.45 mm ®lters) and the ®ltrates were analysed for remaining metals using a Perkin Elmer 2380 atomic absorption spectrophometer. Both metal-free and biomass-free blanks were used as controls and each experiment was performed in duplicate. Where required, pH adjustment was made using 1N H2SO4 or 1N NaOH. Metal uptake values were calculated by mass balance from the change in ®ltrate metal concentrations as described previously (Tobin et al., 1984). Elution studies The eects of varying elution conditions on the amount of chromium recovered from the biosorbent and ion exchange resins was investigated. Metal loading and elution were carried out in a series of custom-designed contactors each comprising of two concentric polystyrene ¯asks. Known quantities of each adsorbent were added to the inner ¯asks which were ®tted with bases of polystyrene ®lter mesh of pore size 36 mm. Successive loading and elution cycles were achieved by immersion of these ¯asks in outer ¯asks containing euent or eluant solutions as required. The adsorbent was retained on the ®lter mesh when the inner ¯asks were removed and the liquid metal concentration was determined as described above. For elution two ratios of biosorbent weight to eluant volume (S/L ratio in units of mg/ml) were used: 10 and 20. Biomass loading prior to elution was carried out at pH 4 in all cases and at the concentrations speci®ed in the following sections.
MATERIALS AND METHODS
Waste Mucor meihi biomass was supplied by a local fermentation industry. It was washed to a constant pH of ca. 7 with tap water, spread on drying plates and dried overnight at 608C. The dried biomass was ground in a standard cereal grinder and the resulting granules were separated into size fractions by sieving. The porosity of the granules, measured with a mercury porosimeter, had a mean value 60% and the granule density as determined with a helium picnometer had a mean value of 1.4 g/cm3. On the basis of preliminary studies, the size range 100 mm < d < 630 mm was chosen for experimental use to minimise problems associated with settling times and clogging of the ®lters for ®ne particles and long sorption equilibrium times for large particles. Industrial tanning euent was supplied by a tanning company and was stored at 48C without pretreatment for a maximum of three months before use. No pH or colour change or precipitation occurred during storage. Chemicals Analytical grades of H2SO4 and NaOH were supplied by SDS, 13124, Peypin-France. Four types of ion exchange resin were supplied by Sigma-Aldrich Chimie, L'Isle D'Abeau Chesnes, B.P.701, 38297 Saint Quentin Fallavier, France. These were Amberlite weakly acidic cation exchange resin, IRC-50 (denoted WA), Amberlite strongly acidic cation exchange resin, IRA 200, (denoted
RESULTS
Euent analysis. The tannery euent was the deep blue colour characteristic of Cr(III) solutions and had a pH of 4.0. Chromium content, as determined by atomic absorption spectroscopy, was found to be 1.77 g/l. This concentration was veri®ed by the ``method of additions'' test in which the theoretical and experimental concentration values of the augmented samples agreed to within 1%. No Cr(VI) was present in the euent as indicated by phenylcarbazide testing. Sodium concentration was 30.1 g/l. Control experiments con®rmed that no signal interference occurred between the two metals. Mucor metal uptake Mucor biomass exhibited considerable chromium biosorption potential at each of the pH ranges investigated and maximum uptake levels of 1.15 mmole Cr/g biosorbent were observed at pH 4. Typical adsorption isotherms at initial pH 4 and
Mucor biosorbent for tanning euent
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Fig. 1. Cr and H+ uptake/release at pH 2 and pH 4.
pH 2 are shown in Fig. 1. Both curves are concave to the x-axis which is characteristic of microbial metal binding although saturation uptake levels were not attained at the equilibrium solution concentrations involved. In contrast (and in spite of a Na/Cr molar ratio of 11.3), there was no Na binding to the biomass and Na solution concentrations remained unchanged. In addition, appreciable changes in the free H+ concentration were evident during chromium biosorption. During the pH 2 studies, increasing H+ uptake/binding occurred with increasing chromium binding and a maximum pH increase of 0.6 units equivalent to sorption of 2.5 mmolH+/g biomass was recorded. Conversely, during sorption at pH 4
the free hydrogen ion concentration increased by the equivalent of a pH decrease of up to 0.5 units which corresponds to a release of approximately 0.04 mmolH+/g biomass and in this case H+ release decreased very slightly with increasing chromium uptake as illustrated in Fig. 1. In sorption studies at initial pH 5.5 and 7.0 marked precipitation eects augmented the biosorption removal of chromium from solution resulting in apparent sequestration levels of in excess of 2.0 and 4.9 mmol/g (ca. 108 and 250 mg/l) respectively as illustrated in Fig. 2. These data are recorded both as mg/g and mmol/g to facilitate comparison with previously published studies. As can be seen in the ®gure, when the precipitation component is sub-
Fig. 2. Eect of pH and in¯uence of precipitation on uptake (** indicates where precipitation eects have been subtracted).
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Fig. 3. Eect of eluant (H2SO4) pH on desorption eciency.
tracted the net biosorption values are in good agreement with each other and those observed at pH 4. Chromium elution Eects of decreasing pH. Chromium desorption increased with decreasing eluant pH to a maximum value of ca. 30% at approximately zero pH. At pH values of 1.0 and 2.0 only ca. 10 and 5% chromium was recovered respectively. Variation of the solid: eluant (S/L) ratio from 20 to 10 caused no dier-
ence in desorption over the experimental range investigated as shown in Fig. 3. Increasing acid concentrations. At each eluant concentration the percentage chromium desorbed was essentially independent of chromium loading on the biomass as shown in Fig. 4. Successive elution stages with increasing acid concentrations resulted in a cumulative recovery of in excess of 80% of the sequestered chromium although the use of 5M acid caused visible alteration of the biomass structure. As previously described, there was no dierence between desorption levels observed at S/L
Fig. 4. Cumulative desorption eciency with successive treatments of increasing eluant (H2SO4) concentration.
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Fig. 5. Chromium recovery in base, acid and combined elution treatments. (a: 1N NaOH treatment followed by 1N H2SO4 treatment; b: 1N H2SO4 treatment followed by 1N NaOH treatment; c: cumulative treatments to 1N H2SO4 as seen in Fig. 4; d: single 1N H2SO4 treatment; e: single 1N NaOH treatment).
values of 10 and 20 which agreed to within 5%. For clarity mean values are shown in this ®gure. Alternate acid and base elution. Both 1N NaOH and 1N (0.5M) H2SO4 desorbed chromium from the biomass. The percentage recovered in a single acid elution stage agrees closely with the cumulative recovery by equivalent concentrations as shown in Fig. 5 (see curves d and c respectively) and is independent of metal loading on the biomass. NaOH
elution was markedly less ecient and desorbed a maximum of ca. 28% at the lowest chromium loadings investigated. At higher loadings recovery values decreased to approximately 15% as seen in this Figure. When a basic elution stage followed acid elution cumulative chromium recovery ranged from ca. 95 to 60% and decreased markedly with biomass loading. When acid elution followed base elution, cumu-
Fig. 6. Comparison of uptake by Mucor biomass and ion exchange resins. Initial pH = 4.0. Final pH as shown.
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Fig. 7. Comparison of successive desorption eciencies from Mucor biomass and ion exchange resins. Loading at pH 4, initial concentration = 440 mg/l.
lative metal recoveries from 100 to 80% were achieved with a less pronounced decrease as biomass loading was increased.
Comparison with ion exchange resins Tannery euent studies. At the natural pH of the euent (ca. 4.0) all the resins, with the exception of the strong base resin (SB), exhibited considerable chromium removal from solution. Maximum uptake was observed using the weak base resin (WB), at approximately 55 mg/g. For both initial chromium concentrations investigated the order of uptake was WB>SA>WA>SB. Mucor biomass demonstrated uptake levels that correspond closely to those of the strong acid resin (SA), as shown in Fig. 6, while the variation in pH agrees exactly with that seen for the weak acid resin (WA). Chromium was desorbed by both 1N NaOH and 1N H2SO4 from each of the ion exchange resins (see Fig. 7). Using NaOH a maximum recovery of approximately 40% was achieved from the WA resin with corresponding values of ca. 20, 19 and 5% for SA resin, Mucor biomass and WB resin respectively. Subsequent acid elution was eective to varying degrees in desorbing the remaining metal. For the WB resin in excess of 95% of the original metal was desorbed by the acid giving full recovery in the combined treatments. Similarly H2SO4 eluted ca. 80% of chromium from the Mucor biomass resulting in combined recovery of 100%. In the WA and SA resins the secondary acid elution desorbed 25 and 45 of sorbed chromium respectively giving total desorption levels of 68 and 62%.
DISCUSSION
Both the maximum uptake values and the general form of the chromium adsorption isotherms illustrated in Fig. 1 and Fig. 2 dier from previously reported results (Brady and Tobin, 1995; Fourest and Roux, 1992). The present maximum uptake of 1.15 mmol/g is considerably greater than typical biosorption values which are in the range 0.1 to 0.6 mmol/g (de Rome and Gadd, 1987; Gadd, 1993; Nourbash et al., 1994; Volesky and Holan, 1995) and approximately twice the value recorded for chromium uptake by the closely related Mucorale R. arrhizus at a similar pH (Tobin et al., 1984) although that work employed synthetic chromium nitrate solution. However, the present study investigated uptake over a considerably greater solution concentration range. When compared at equivalent equilibrium concentrations the uptake values are in general agreement. Similarly, the magnitude of uptake at pH 2 is unusual and greatly exceeds maximum biosorption capacities reported for comparable studies (Junghans and Straube, 1991; Huang et al., 1990) but, when compared at equivalent concentrations, the present results are in line with earlier studies. These dierences in maximum uptake are attributable to the dierences in the isotherm shape which in turn re¯ect dierences in metal/biomass interactions. In typical biosorption studies uptake saturation is attained at equilibrium solution concentrations of 1 to 2 mmol/l or lower (Brady and Tobin, 1995; Tobin et al., 1984). Moreover, uptake data have frequently been successfully ®tted to the Langmuir adsorption model and a simple
Mucor biosorbent for tanning euent
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Fig. 8. Reciprocal Langmuir plot of pH 4 uptake data.
monolayer, non-interactive binding mechanism proposed (de Rome and Gadd, 1987; Langmuir, 1918; Tobin et al., 1990). In contrast, in the present study uptake is seen to increase with solution concentrations of up to 30 mmol/l and saturation is not evident in Fig. 1 and Fig. 2. When the data were transformed to the reciprocal Langmuir format a clearly non-linear plot resulted as seen in Fig. 8. This non-conformity to idealised Langmuir behaviour may be interpreted as indicating complex adsorption processes involving multilayer, interactive or multiple site type binding or some combination of these phenomena (Brady and Tobin, 1995; Gadd and de Rome, 1988).
The data were further transformed to the Scatchard format as shown in Fig. 9 which also resulted in a clearly non-linear plot. As has been discussed elsewhere, linear Scatchard plots re¯ect binding to sites of a single type while non-linear plots have been interpreted as indicating multiple types of binding (Brady and Tobin, 1995, Huang et al., 1990; Scatchard, 1949). In this study the highly curved nature of the Scatchard plot indicates the existence of multiple types binding sites in the biomass and/or multiple forms of chromium in solution. The pattern of apparent H+ uptake and release seen in Fig. 1 is unexpected. In simple biosorption
Fig. 9. Scatchard plot of pH 4 uptake data.
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systems, metal and hydrogen ions compete for binding sites and uptake of one often results in the displacement of the other (Treen-Sears et al., 1984). By contrast, in this work H+ uptake at pH 2 is concomitant with chromium uptake and the molar ratio of apparent H+ uptake to chromium uptake approximates closely to 3.0. In solution the chromic ion forms a range of mono and polynuclear species y (Baes and Mesner, 1976) of the form Crx (OH)3xÿ y and it is likely that the release of hydroxyl ions during chromium uptake in addition to biomass binding of H+ may be responsible, at least in part, for the observed pH changes. The fact that biosorption potential is not aected by precipitation as illustrated in Fig. 2 is of potential industrial signi®cance. Metal-bearing euents, such as that from tanneries (Stein and Schwedt, 1994) frequently contain residual salts and precipitants following primary treatment. The insensitivity of biosorption processes to both these and high levels of alkali metals (Tobin et al., 1990) highlights their potential as polishing or cotreatment stages in advanced treatment systems.
concentrations greater than 1M irreversible damage and reduction of biosoption potential occurred (Gadd, 1993; Tsezos, 1984). Successive acid and base treatments were also eective in desorption and it is interesting that cumulative recovery for acid followed by base elution is approximately equal to the sum of individual stage recoveries (Fig. 5). In contrast, base followed by acid elution resulted in considerably greater chromium desorption. Chromium solubility increases in extremely alkaline solution (Baes and Mesner, 1976) as CrOÿ 4 and it appears that NaOH elution may resolubilise a limited fraction of the sorbed chromium in this form. At the same time a marked discoloration of the eluant and biomass suggested this treatment altered the biomass structure or composition. Biomass damage by alkali has been described previously (Tsezos, 1984). The greater recovery observed for acid treatment following alkali elution would be consistent with stripping or degradation of the biomass structure which in turn facilitated access to, or elution of, the sequestered chromium.
Elution with decreasing pH
Comparison of mucor and resin uptake
The general trend of increasing desorption with decreasing eluant pH as shown in Fig. 3 is in keeping with both the present observation of diminished uptake at lower pH and many similar reports (Huang et al., 1990; Treen-Sears et al., 1984). Increased H+ concentrations are recognised either to compete with or displace cationic species from anionic biomass binding sites and pH adjustment has previously been used as an eective method of metal recovery (Holan et al., 1993). However, the present elution values are markedly lower than expected and indicate a non-typical binding mechanism. In comparison, recovery eciencies well in excess of 80% have been achieved using H2SO4 at pH 1±2 to recover uranium from the Mucorale Rhizopus arrhizus at similar S/L ratios (Tsezos, 1984). Similarly copper recovery from the fungi R. arrhizus, Cladosporium resinae, Penicillium italicum and Aureobasidium pullulans was found to be readily achieved by decreasing pH and up to ca. 60% ecient at pH 3, albeit at lower initial metal loading (de Rome and Gadd, 1987; Gadd and de Rome, 1988). The close agreement of the desorption eciencies at S/L 10 and 20 con®rms that it is the H+ concentration rather than the volume of the eluant that is determining and is consistent with previous desorption studies from fungal and algal biomass (Kuyucak and Volesky, 1989; Tsezos, 1984). The cumulative desorption eciencies obtained by successive elution stages shown in Fig. 4 illustrate that values approaching full recovery are attainable with suciently rigorous treatment. However the biomass was visibly altered at the higher eluant concentrations and it is likely that at
The negligible uptake by SB resin suggests that chromium anionic species are either not present or in a non-complexing form in the euent (Fig. 6). For SB resin the active group is quaternary ammonium with a reported eective pH range of 0±14. At pH 4 it is expected to be fully quaternised, which is supported by the small change in pH observed, and not to interact with cationic species. By contrast the WB resin contains polyamine groups which are partially dissociated at pH 4 with sites available for cation binding and chromium is known to complex readily with ammonia and organic ligands (Baes and Mesner, 1976). Moreover, the diminished H+ binding, as re¯ected in decreased pH change, with increased chromium uptake is indicative of a competition mechanism and supports the view that chromium uptake is predominantly by cation sorption. The WA and SA resins contain partially dissociated carboxyl active groups and fully dissociated sulphonic active groups respectively. Although the WA resin has a total exchange capacity greater than the SA resin (10.0 versus 4.2 meq/g) pH 4 is outside its eective pH range of 5±14 as reported in the manufacturer's speci®cations. It is not surprising therefore that the observed uptake for SA resin exceeds that of the WA resin in this work or that the WA resin exhibits some H+ release during chromium sorption while SA resin does not. The concurrence of the pH behaviour in the Mucor and WA resin tests further suggests a similarity in their underlying sorption mechanisms. This is supported by previous work which demonstrated the presence of 0.6 mmol/g of carboxyl groups in Mucor cell walls (Fourest et al., 1996).
Mucor biosorbent for tanning euent
The elution eciency of 1M NaOH shown in Fig. 7 is generally consistent with the Mucor studies described above. Both the elevated pH and sodium levels may be expected to cause resolubilisation of sequestered chromium. The similarity of the recovery eciencies of Mucor and the SA and WA resins (19, 20 and 44% respectively) would tend to indicate similarities in uptake mechanism while recovery from WB resin was only 6%. Subsequent elution by 1N H2SO4 would be expected to result in chromium displacement by H+ from the active groups of each of the resins although, as seen in Fig. 7, high recoveries are only observed for WB resin. As discussed above the high recoveries obtained from Mucor may result from damage to the biomass structure. CONCLUSIONS
In summary, chromium is sequestered from tanning euent in unusually large quantities by this Mucor biomass. Binding is reversible, but total metal recovery is achieved only with harsh and damaging acid/base treatments. Comparison with ion exchange resin binding from the same euent indicates strong similarities to weakly acidic cation exchange binding processes. However, the Mucorales are known to comprise high levels of chitin and chitosan which contain amine functional groups. In view of the known anity for chromium for a variety of ligands (Baes and Mesner, 1976) and the present high uptake exhibited by the WB resin it is likely that biomass amine groups contribute signi®cantly to uptake levels. This is supported by the non-conformity to the Langmuir model and non-linear Scatchard transformations described above and also by the unusually high overall uptake levels at both pH 4 and 2. The insensitivity of the biosorption process to high levels of precipitates and alkali metals underlines the potential for application in ®nal/polishing or cotreatment systems. Continuous ¯ow packed column reactors have been recommended for such operations (Gadd and White, 1993; Volesky, 1987) and testing of the biosorbent columns is currently underway. REFERENCES
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de Rome L. and Gadd G.M. (1987) Copper adsorption by Rhizopus arrhizus, Cladosporium resinae and Penicillium italicum. Appl. Microbiol. Biotechnol. 26, 84±90. Fourest E. and Roux J.C. (1992) Heavy metal biosorption by fungal mycelial by-products: mechanisms and in¯uence of pH. Appl. Microbiol. Biotechnol. 37, 399±403. Fourest E., Serre A. and Roux J.C. (1996) Contribution of carboxyl groups to heavy metal binding sites in fungal wall. Toxic. Environ. Chem. 54, 1±10. Junghans K. and Straube G. (1991) Biosorption of copper by yeasts. Biol. Metals 4, 233±237. Gadd G.M. (1993) Interactions of fungi with toxic metals. New Phytol. 124, 25±60. Gadd G.M. and de Rome L. (1988) Biosorption of copper by fungal melanin. Appl. Microbiol. Biotechnol. 29, 610± 617. Gadd G.M. and White C. (1993) Microbial treatment of metal pollution. Trends Biotechnol. 11, 353±359. Holan Z.R., Volesky B. and Prasetyo I. (1993) Biosorption of cadmium by biomass of marine algae. Biotechnol. Bioeng. 41, 819±825. Huang C.P., Huang C.P. and Morehart A.L. (1990) The removal of Cu(ii) from dilute aqueous solutions by Saccharomyces cerevisiae. Water Research 24, 433± 439. Kaim W. and Schwederski B. (1994) Bioinorganic chemistry: inorganic elements in the chemistry of life. John Wiley and Sons, Chichester, England. Karamushka V.I. and Gadd G.M. (1994) In¯uence of copper on proton eux from Saccharomyces cerevisiae and the protective eect of calcium and magnesium. FEMS Microbiol. Lett. 122, 33±38. Kuyucak N. and Volesky B. (1989) Desorption of cobaltladen algal biosorbent. Biotechnol. Bioeng. 33, 815±822. Landgrave J. (1995) A pilot plant for removing chromium from residual waters of tanneries. Environmental Health Perspectives 103, 63±65. Langmuir I. (1918) The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40, 1361±1403. Nourbash M., Sag Y., Ozer D., Aksu Z., Kutsal T. and Caglar A. (1994) A comparative study of various biosorbents for Chromium(vi) ions from industrial waste waters. Process Biochem. 29, 1±5. Norris P.R. and Kelly D.P. (1977) Accumulation of cadmium and cobalt by Saccharomyces cerevisiae. J. Gen. Microbiol. 99, 317±324. Rapoport A.I. and Muter O.A. (1995) Biosorption of hexavalent chromium by yeasts. Process Biochem. 30(2), 145±149. Scatchard G. (1949) The attraction of proteins for small molecules and ions. Ann. N.Y. Acad. Sci. 51, 660± 672. Singleton I., Wainwright M. and Edyvean R.G.J. (1990) Some factors in¯uencing the adsorption of particulates by fungal mycelium. Biorecovery 1, 271±289. Stein K. and Schwedt G. (1994) Speciation of chromium in the waste water from a tannery. Frenius J. Anal. Chem. 350, 38±43. Tobin J.M., Cooper D.G. and Neufeld R.J. (1984) Uptake of metal ions by Rhizopus arrhizus biomass. Appl. Environ. Microbiol. 47(4), 821±824. Tobin J.M., Cooper D.G. and Neufeld R.J. (1990) Investigation of the mechanism of metal uptake by denatured Rhizopus arrhizus biomass. Enzyme Microb. Technol. 12, 591±595. Treen-Sears M., Volesky B. and Neufeld R.J. (1984) Ion exchange/complexation of the uranyl ion by Rhizopus biosorbent. Biotechnol. Bioeng. 26, 1323±1329. Tsezos M. (1984) Recovery of uranium from biological adsorbents - desorption equilibria. Biotechnol. Bioeng. 26, 973±981.
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Wat. Res. Vol. 32, No. 5, pp. 1407±1416, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
MUCOR BIOSORBENT FOR CHROMIUM REMOVAL FROM TANNING EFFLUENT J. M. TOBIN1* and J. C. ROUX2 School of Biological Sciences, Dublin City University, Dublin 9, Ireland and 2Laboratoire TSV, DBMS, CEA/Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
1
(First received November 1996; accepted in revised form August 1997) AbstractÐWaste industrial Mucor meihi biomass was found to be an eective biosorbent for the removal of chromium from industrial tanning euents. Sorption levels of 1.15 and 0.7 mmol/g were observed at pH 4 and 2 respectively while precipitation eects augmented these values at higher pH ranges. Acid elution of biosorbed chromium increased with decreasing eluant pH to a maximium value of ca. 30% at approximately zero pH. Successive elution stages with increasingly strong acids resulted in a cumulative chromium recovery of in excess of 80%. Both acid and base treatments eluted biosorbed chromium and successive acid/base and base/acid treatments resulted in recovery values approaching 100% at low metal loadings. These values decreased to 80 to 60% at higher biomass metal loadings. In comparative studies with ion exchange resins, the Mucor biomass demonstrated chromium biosorption levels that correspond closely to those of commercial strongly acidic exchange resin while the pH behaviour mirrored that of the weakly acidic resins in solution. The chromium elution characteristics from the Mucor biomass were similar to those of both the weakly and strongly acid resins. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐchromium, biosorption, fungal biomass, tanning euent, metal uptake, Mucor biosorbent
INTRODUCTION
The interactions of microorganisms and metals in aqueous media have in recent years been the focus of a growing number of scienti®c studies (Fourest and Roux, 1992; Tobin et al., 1984; Volesky and Holan, 1995; White and Gadd, 1990). The characteristics of both passive microbial metal binding, commonly termed biosorption (Brady and Tobin, 1995; Holan et al., 1993; Tsezos and Deutschmann, 1990) and metabolically-mediated intracellular accumulation, or bioaccumulation (Avery and Tobin, 1992; Brady and Duncan, 1994; Norris and Kelly, 1977) have been investigated for a wide range of simple metal/organism systems. In addition, competition studies in solutions of multiple cations and/or anions have underscored the complexity of the sorption interactions involved as well as demonstrating the ability of certain co-ions to either reverse or augment metal toxicity and uptake (Karamushka and Gadd, 1994; Venkata Ramana and Sivarama Sastry, 1994). Nonetheless, in spite of increased understanding of the phenomena and the recognition of the potential of microbial metal uptake levels equivalent or exceeding those of ion exchange resins, there have been few successful industrial applications of biosorption systems. To date, the principal exponents of metal-microorganism interactions for environmental purposes remain *Author to whom all correspondence should be addressed.
traditional biological waste treatment systems of the activated sludge/biological ®lter type. Chromium-bearing euents from the tanning industry have been cited as potentially amenable to treatment by biosorption processes (Aksu et al., 1990; Nourbash et al., 1994). The tanning process uses an acidic solution of chromium oxide to transform animal hides into leather. Chromium (III) is adsorbed from the solution by the tissue and crosslinks the collagen ®bres by forming coordinate bonds between protein functional groups. In this way nitrogen detachment from the tissue and rotting is prevented. The euent, which may contain of the order of 3 g Cr(III)/l at ca. pH 4 (Landgrave, 1995), is typically treated by raising the pH, with or without addition of coprecipitants such as Fe or organic coagulants, to recover precipitated chromium hydroxide. While pilot plant studies have reported 99.5% recovery by these methods (Landgrave, 1995), recent analyses of actual tanning euents following treatment have shown residual total Cr levels of in excess of 10 mg/l (Stein and Schwedt, 1994). In addition, the waste waters were found to have high conductivities following the precipitation treatments and high organic levels resulting from contact with the hides. Cr(VI) is widely recognised to be considerably more toxic than Cr(III) (Rapoport and Muter, 1995; Stein and Schwedt, 1994; Venkata Ramana and Sivarama Sastry, 1994). Cr(III), in its stable
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hydrate form, is unable to cross biological membranes and its toxicity to fungi has been related to its speci®c antagonism with iron uptake (Venkata Ramana and Sivarama Sastry, 1994). In man it is recognised as an essential element and diabetes symptoms are reported to be associated with Cr dietary de®ciency (Kaim and Schwederski, 1994). However, the reduction of Cr(VI) to Cr.(III) within cells and the subsequent binding to and disruption of proteins and nucleic acids have recently been recognised as underlining the toxic potential of both chromium forms (Rapoport and Muter, 1995). In the US the maximum permissable concentration of chromium in natural waters recommended by the EPA for the protection of human health is 50 mg/ m3 (Nourbash et al., 1994). Fungi in general, and species of the Mucorales order in particular, are well known for their metal biosorption characteristics (Gadd, 1993; Tobin et al., 1990). Rhizopus arrhizus has been reported to exhibit uptake levels equivalent to commercial exchange resins for a range of divalent metals (Tobin et al., 1984; Treen-Sears et al., 1984) and the closely related Mucor species have been shown to biosorb a range of insoluble metal compounds from acid mine drainage (Gadd, 1993; Singleton et al., 1990). The objectives of this work were to characterise the potential of industrial waste Mucor biomass to remove chromium from real tannery euent and to investigate subsequent desorption processes to facilitate metal recovery. Comparative studies using ion exchange resins were also performed to elucidate the underlying sorption mechanisms.
SA), Amberlite weakly basic anion exchange resin, IRA93, (denoted WB), and Amberlite strongly basic anion exchange resin, IRA 900, (denoted SB). The reference Cr and Na standards used for determinations were purchased from Merck-Clevenot S.A., 5±9 rue Anquetil, 94736 Nogent-Sur-Marne, France. Chromium uptake Serial dilutions of the tanning euent were prepared using deionised distilled water to give solutions ranging in concentration from full strength to 1 in 20 dilution. Aliquots of 100 ml were contacted with 0.3 g quantities of Mucor biomass or ion exchange resin overnight. Samples were ®ltered (0.45 mm ®lters) and the ®ltrates were analysed for remaining metals using a Perkin Elmer 2380 atomic absorption spectrophometer. Both metal-free and biomass-free blanks were used as controls and each experiment was performed in duplicate. Where required, pH adjustment was made using 1N H2SO4 or 1N NaOH. Metal uptake values were calculated by mass balance from the change in ®ltrate metal concentrations as described previously (Tobin et al., 1984). Elution studies The eects of varying elution conditions on the amount of chromium recovered from the biosorbent and ion exchange resins was investigated. Metal loading and elution were carried out in a series of custom-designed contactors each comprising of two concentric polystyrene ¯asks. Known quantities of each adsorbent were added to the inner ¯asks which were ®tted with bases of polystyrene ®lter mesh of pore size 36 mm. Successive loading and elution cycles were achieved by immersion of these ¯asks in outer ¯asks containing euent or eluant solutions as required. The adsorbent was retained on the ®lter mesh when the inner ¯asks were removed and the liquid metal concentration was determined as described above. For elution two ratios of biosorbent weight to eluant volume (S/L ratio in units of mg/ml) were used: 10 and 20. Biomass loading prior to elution was carried out at pH 4 in all cases and at the concentrations speci®ed in the following sections.
MATERIALS AND METHODS
Waste Mucor meihi biomass was supplied by a local fermentation industry. It was washed to a constant pH of ca. 7 with tap water, spread on drying plates and dried overnight at 608C. The dried biomass was ground in a standard cereal grinder and the resulting granules were separated into size fractions by sieving. The porosity of the granules, measured with a mercury porosimeter, had a mean value 60% and the granule density as determined with a helium picnometer had a mean value of 1.4 g/cm3. On the basis of preliminary studies, the size range 100 mm < d < 630 mm was chosen for experimental use to minimise problems associated with settling times and clogging of the ®lters for ®ne particles and long sorption equilibrium times for large particles. Industrial tanning euent was supplied by a tanning company and was stored at 48C without pretreatment for a maximum of three months before use. No pH or colour change or precipitation occurred during storage. Chemicals Analytical grades of H2SO4 and NaOH were supplied by SDS, 13124, Peypin-France. Four types of ion exchange resin were supplied by Sigma-Aldrich Chimie, L'Isle D'Abeau Chesnes, B.P.701, 38297 Saint Quentin Fallavier, France. These were Amberlite weakly acidic cation exchange resin, IRC-50 (denoted WA), Amberlite strongly acidic cation exchange resin, IRA 200, (denoted
RESULTS
Euent analysis. The tannery euent was the deep blue colour characteristic of Cr(III) solutions and had a pH of 4.0. Chromium content, as determined by atomic absorption spectroscopy, was found to be 1.77 g/l. This concentration was veri®ed by the ``method of additions'' test in which the theoretical and experimental concentration values of the augmented samples agreed to within 1%. No Cr(VI) was present in the euent as indicated by phenylcarbazide testing. Sodium concentration was 30.1 g/l. Control experiments con®rmed that no signal interference occurred between the two metals. Mucor metal uptake Mucor biomass exhibited considerable chromium biosorption potential at each of the pH ranges investigated and maximum uptake levels of 1.15 mmole Cr/g biosorbent were observed at pH 4. Typical adsorption isotherms at initial pH 4 and
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Fig. 1. Cr and H+ uptake/release at pH 2 and pH 4.
pH 2 are shown in Fig. 1. Both curves are concave to the x-axis which is characteristic of microbial metal binding although saturation uptake levels were not attained at the equilibrium solution concentrations involved. In contrast (and in spite of a Na/Cr molar ratio of 11.3), there was no Na binding to the biomass and Na solution concentrations remained unchanged. In addition, appreciable changes in the free H+ concentration were evident during chromium biosorption. During the pH 2 studies, increasing H+ uptake/binding occurred with increasing chromium binding and a maximum pH increase of 0.6 units equivalent to sorption of 2.5 mmolH+/g biomass was recorded. Conversely, during sorption at pH 4
the free hydrogen ion concentration increased by the equivalent of a pH decrease of up to 0.5 units which corresponds to a release of approximately 0.04 mmolH+/g biomass and in this case H+ release decreased very slightly with increasing chromium uptake as illustrated in Fig. 1. In sorption studies at initial pH 5.5 and 7.0 marked precipitation eects augmented the biosorption removal of chromium from solution resulting in apparent sequestration levels of in excess of 2.0 and 4.9 mmol/g (ca. 108 and 250 mg/l) respectively as illustrated in Fig. 2. These data are recorded both as mg/g and mmol/g to facilitate comparison with previously published studies. As can be seen in the ®gure, when the precipitation component is sub-
Fig. 2. Eect of pH and in¯uence of precipitation on uptake (** indicates where precipitation eects have been subtracted).
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Fig. 3. Eect of eluant (H2SO4) pH on desorption eciency.
tracted the net biosorption values are in good agreement with each other and those observed at pH 4. Chromium elution Eects of decreasing pH. Chromium desorption increased with decreasing eluant pH to a maximum value of ca. 30% at approximately zero pH. At pH values of 1.0 and 2.0 only ca. 10 and 5% chromium was recovered respectively. Variation of the solid: eluant (S/L) ratio from 20 to 10 caused no dier-
ence in desorption over the experimental range investigated as shown in Fig. 3. Increasing acid concentrations. At each eluant concentration the percentage chromium desorbed was essentially independent of chromium loading on the biomass as shown in Fig. 4. Successive elution stages with increasing acid concentrations resulted in a cumulative recovery of in excess of 80% of the sequestered chromium although the use of 5M acid caused visible alteration of the biomass structure. As previously described, there was no dierence between desorption levels observed at S/L
Fig. 4. Cumulative desorption eciency with successive treatments of increasing eluant (H2SO4) concentration.
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Fig. 5. Chromium recovery in base, acid and combined elution treatments. (a: 1N NaOH treatment followed by 1N H2SO4 treatment; b: 1N H2SO4 treatment followed by 1N NaOH treatment; c: cumulative treatments to 1N H2SO4 as seen in Fig. 4; d: single 1N H2SO4 treatment; e: single 1N NaOH treatment).
values of 10 and 20 which agreed to within 5%. For clarity mean values are shown in this ®gure. Alternate acid and base elution. Both 1N NaOH and 1N (0.5M) H2SO4 desorbed chromium from the biomass. The percentage recovered in a single acid elution stage agrees closely with the cumulative recovery by equivalent concentrations as shown in Fig. 5 (see curves d and c respectively) and is independent of metal loading on the biomass. NaOH
elution was markedly less ecient and desorbed a maximum of ca. 28% at the lowest chromium loadings investigated. At higher loadings recovery values decreased to approximately 15% as seen in this Figure. When a basic elution stage followed acid elution cumulative chromium recovery ranged from ca. 95 to 60% and decreased markedly with biomass loading. When acid elution followed base elution, cumu-
Fig. 6. Comparison of uptake by Mucor biomass and ion exchange resins. Initial pH = 4.0. Final pH as shown.
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Fig. 7. Comparison of successive desorption eciencies from Mucor biomass and ion exchange resins. Loading at pH 4, initial concentration = 440 mg/l.
lative metal recoveries from 100 to 80% were achieved with a less pronounced decrease as biomass loading was increased.
Comparison with ion exchange resins Tannery euent studies. At the natural pH of the euent (ca. 4.0) all the resins, with the exception of the strong base resin (SB), exhibited considerable chromium removal from solution. Maximum uptake was observed using the weak base resin (WB), at approximately 55 mg/g. For both initial chromium concentrations investigated the order of uptake was WB>SA>WA>SB. Mucor biomass demonstrated uptake levels that correspond closely to those of the strong acid resin (SA), as shown in Fig. 6, while the variation in pH agrees exactly with that seen for the weak acid resin (WA). Chromium was desorbed by both 1N NaOH and 1N H2SO4 from each of the ion exchange resins (see Fig. 7). Using NaOH a maximum recovery of approximately 40% was achieved from the WA resin with corresponding values of ca. 20, 19 and 5% for SA resin, Mucor biomass and WB resin respectively. Subsequent acid elution was eective to varying degrees in desorbing the remaining metal. For the WB resin in excess of 95% of the original metal was desorbed by the acid giving full recovery in the combined treatments. Similarly H2SO4 eluted ca. 80% of chromium from the Mucor biomass resulting in combined recovery of 100%. In the WA and SA resins the secondary acid elution desorbed 25 and 45 of sorbed chromium respectively giving total desorption levels of 68 and 62%.
DISCUSSION
Both the maximum uptake values and the general form of the chromium adsorption isotherms illustrated in Fig. 1 and Fig. 2 dier from previously reported results (Brady and Tobin, 1995; Fourest and Roux, 1992). The present maximum uptake of 1.15 mmol/g is considerably greater than typical biosorption values which are in the range 0.1 to 0.6 mmol/g (de Rome and Gadd, 1987; Gadd, 1993; Nourbash et al., 1994; Volesky and Holan, 1995) and approximately twice the value recorded for chromium uptake by the closely related Mucorale R. arrhizus at a similar pH (Tobin et al., 1984) although that work employed synthetic chromium nitrate solution. However, the present study investigated uptake over a considerably greater solution concentration range. When compared at equivalent equilibrium concentrations the uptake values are in general agreement. Similarly, the magnitude of uptake at pH 2 is unusual and greatly exceeds maximum biosorption capacities reported for comparable studies (Junghans and Straube, 1991; Huang et al., 1990) but, when compared at equivalent concentrations, the present results are in line with earlier studies. These dierences in maximum uptake are attributable to the dierences in the isotherm shape which in turn re¯ect dierences in metal/biomass interactions. In typical biosorption studies uptake saturation is attained at equilibrium solution concentrations of 1 to 2 mmol/l or lower (Brady and Tobin, 1995; Tobin et al., 1984). Moreover, uptake data have frequently been successfully ®tted to the Langmuir adsorption model and a simple
Mucor biosorbent for tanning euent
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Fig. 8. Reciprocal Langmuir plot of pH 4 uptake data.
monolayer, non-interactive binding mechanism proposed (de Rome and Gadd, 1987; Langmuir, 1918; Tobin et al., 1990). In contrast, in the present study uptake is seen to increase with solution concentrations of up to 30 mmol/l and saturation is not evident in Fig. 1 and Fig. 2. When the data were transformed to the reciprocal Langmuir format a clearly non-linear plot resulted as seen in Fig. 8. This non-conformity to idealised Langmuir behaviour may be interpreted as indicating complex adsorption processes involving multilayer, interactive or multiple site type binding or some combination of these phenomena (Brady and Tobin, 1995; Gadd and de Rome, 1988).
The data were further transformed to the Scatchard format as shown in Fig. 9 which also resulted in a clearly non-linear plot. As has been discussed elsewhere, linear Scatchard plots re¯ect binding to sites of a single type while non-linear plots have been interpreted as indicating multiple types of binding (Brady and Tobin, 1995, Huang et al., 1990; Scatchard, 1949). In this study the highly curved nature of the Scatchard plot indicates the existence of multiple types binding sites in the biomass and/or multiple forms of chromium in solution. The pattern of apparent H+ uptake and release seen in Fig. 1 is unexpected. In simple biosorption
Fig. 9. Scatchard plot of pH 4 uptake data.
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systems, metal and hydrogen ions compete for binding sites and uptake of one often results in the displacement of the other (Treen-Sears et al., 1984). By contrast, in this work H+ uptake at pH 2 is concomitant with chromium uptake and the molar ratio of apparent H+ uptake to chromium uptake approximates closely to 3.0. In solution the chromic ion forms a range of mono and polynuclear species y (Baes and Mesner, 1976) of the form Crx (OH)3xÿ y and it is likely that the release of hydroxyl ions during chromium uptake in addition to biomass binding of H+ may be responsible, at least in part, for the observed pH changes. The fact that biosorption potential is not aected by precipitation as illustrated in Fig. 2 is of potential industrial signi®cance. Metal-bearing euents, such as that from tanneries (Stein and Schwedt, 1994) frequently contain residual salts and precipitants following primary treatment. The insensitivity of biosorption processes to both these and high levels of alkali metals (Tobin et al., 1990) highlights their potential as polishing or cotreatment stages in advanced treatment systems.
concentrations greater than 1M irreversible damage and reduction of biosoption potential occurred (Gadd, 1993; Tsezos, 1984). Successive acid and base treatments were also eective in desorption and it is interesting that cumulative recovery for acid followed by base elution is approximately equal to the sum of individual stage recoveries (Fig. 5). In contrast, base followed by acid elution resulted in considerably greater chromium desorption. Chromium solubility increases in extremely alkaline solution (Baes and Mesner, 1976) as CrOÿ 4 and it appears that NaOH elution may resolubilise a limited fraction of the sorbed chromium in this form. At the same time a marked discoloration of the eluant and biomass suggested this treatment altered the biomass structure or composition. Biomass damage by alkali has been described previously (Tsezos, 1984). The greater recovery observed for acid treatment following alkali elution would be consistent with stripping or degradation of the biomass structure which in turn facilitated access to, or elution of, the sequestered chromium.
Elution with decreasing pH
Comparison of mucor and resin uptake
The general trend of increasing desorption with decreasing eluant pH as shown in Fig. 3 is in keeping with both the present observation of diminished uptake at lower pH and many similar reports (Huang et al., 1990; Treen-Sears et al., 1984). Increased H+ concentrations are recognised either to compete with or displace cationic species from anionic biomass binding sites and pH adjustment has previously been used as an eective method of metal recovery (Holan et al., 1993). However, the present elution values are markedly lower than expected and indicate a non-typical binding mechanism. In comparison, recovery eciencies well in excess of 80% have been achieved using H2SO4 at pH 1±2 to recover uranium from the Mucorale Rhizopus arrhizus at similar S/L ratios (Tsezos, 1984). Similarly copper recovery from the fungi R. arrhizus, Cladosporium resinae, Penicillium italicum and Aureobasidium pullulans was found to be readily achieved by decreasing pH and up to ca. 60% ecient at pH 3, albeit at lower initial metal loading (de Rome and Gadd, 1987; Gadd and de Rome, 1988). The close agreement of the desorption eciencies at S/L 10 and 20 con®rms that it is the H+ concentration rather than the volume of the eluant that is determining and is consistent with previous desorption studies from fungal and algal biomass (Kuyucak and Volesky, 1989; Tsezos, 1984). The cumulative desorption eciencies obtained by successive elution stages shown in Fig. 4 illustrate that values approaching full recovery are attainable with suciently rigorous treatment. However the biomass was visibly altered at the higher eluant concentrations and it is likely that at
The negligible uptake by SB resin suggests that chromium anionic species are either not present or in a non-complexing form in the euent (Fig. 6). For SB resin the active group is quaternary ammonium with a reported eective pH range of 0±14. At pH 4 it is expected to be fully quaternised, which is supported by the small change in pH observed, and not to interact with cationic species. By contrast the WB resin contains polyamine groups which are partially dissociated at pH 4 with sites available for cation binding and chromium is known to complex readily with ammonia and organic ligands (Baes and Mesner, 1976). Moreover, the diminished H+ binding, as re¯ected in decreased pH change, with increased chromium uptake is indicative of a competition mechanism and supports the view that chromium uptake is predominantly by cation sorption. The WA and SA resins contain partially dissociated carboxyl active groups and fully dissociated sulphonic active groups respectively. Although the WA resin has a total exchange capacity greater than the SA resin (10.0 versus 4.2 meq/g) pH 4 is outside its eective pH range of 5±14 as reported in the manufacturer's speci®cations. It is not surprising therefore that the observed uptake for SA resin exceeds that of the WA resin in this work or that the WA resin exhibits some H+ release during chromium sorption while SA resin does not. The concurrence of the pH behaviour in the Mucor and WA resin tests further suggests a similarity in their underlying sorption mechanisms. This is supported by previous work which demonstrated the presence of 0.6 mmol/g of carboxyl groups in Mucor cell walls (Fourest et al., 1996).
Mucor biosorbent for tanning euent
The elution eciency of 1M NaOH shown in Fig. 7 is generally consistent with the Mucor studies described above. Both the elevated pH and sodium levels may be expected to cause resolubilisation of sequestered chromium. The similarity of the recovery eciencies of Mucor and the SA and WA resins (19, 20 and 44% respectively) would tend to indicate similarities in uptake mechanism while recovery from WB resin was only 6%. Subsequent elution by 1N H2SO4 would be expected to result in chromium displacement by H+ from the active groups of each of the resins although, as seen in Fig. 7, high recoveries are only observed for WB resin. As discussed above the high recoveries obtained from Mucor may result from damage to the biomass structure. CONCLUSIONS
In summary, chromium is sequestered from tanning euent in unusually large quantities by this Mucor biomass. Binding is reversible, but total metal recovery is achieved only with harsh and damaging acid/base treatments. Comparison with ion exchange resin binding from the same euent indicates strong similarities to weakly acidic cation exchange binding processes. However, the Mucorales are known to comprise high levels of chitin and chitosan which contain amine functional groups. In view of the known anity for chromium for a variety of ligands (Baes and Mesner, 1976) and the present high uptake exhibited by the WB resin it is likely that biomass amine groups contribute signi®cantly to uptake levels. This is supported by the non-conformity to the Langmuir model and non-linear Scatchard transformations described above and also by the unusually high overall uptake levels at both pH 4 and 2. The insensitivity of the biosorption process to high levels of precipitates and alkali metals underlines the potential for application in ®nal/polishing or cotreatment systems. Continuous ¯ow packed column reactors have been recommended for such operations (Gadd and White, 1993; Volesky, 1987) and testing of the biosorbent columns is currently underway. REFERENCES
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