Environ. Sci. Technol. 2004, 38, 5170-5176
Potential Remediation of Waters Contaminated with Cr(III), Cu, and Zn by Sorption on the Organic Polymeric Fraction of Olive Mill Wastewater (Polymerin) and Its Derivatives RENATO CAPASSO,* MASSIMO PIGNA, ANTONIO DE MARTINO, MARIANNA PUCCI, FILOMENA SANNINO, AND ANTONIO VIOLANTE Dipartimento di Scienze del Suolo della Pianta e dell’Ambiente, Universita` degli Studi di Napoli “Federico II”, 80055 Portici (NA), Italy
A study on the individual sorption of Cr(III), Cu, and Zn on polymerin, the humic-acid-like fraction of olive mill wastewater, and its derivatives, K-polymerin and an Fe(OH)xpolymerin complex, showed that these heavy metals were strongly sorbed on polymerin and K-polymerin in the order Cr(III) > Cu > Zn. The sorption on Fe(OH)xpolymerin was to a lower extent compared with that of the other two sorbents, but to a higher extent compared with ferrihydride [Fe(OH)x]. Combined atomic absorption spectrometry and diffuse reflectance infrared Fourier transform spectroscopy analyses showed that the selected heavy metals were individually sorbed on polymerin by means of a cation exchange mechanism, which was consistent with the replacement of Ca, Mg, K, and H bound to the carboxylate groups of the biosorbent and the concomitant chelation of the heavy metals by the OH groups of polymerin polysaccharide component. In binary combination and equimolar ratio, Cu was sorbed by polymerin more selectively than Zn. In ternary combination and equimolar ratio, Cr(III), Cu, and Zn were sorbed by polymerin in the order Cr(III) > Cu > Zn. The sorbing capacity of Zn and Cu was strongly influenced by Cr(III), whereas the sorbing capacity of Cr(III) was not affected by the presence of the other two metals. The overall sorbing capacity of the binary and ternary mixtures of the three metals on polymerin proved to be considerable and much greater than that on Fe(OH)x-polymerin. Simulated wastewaters contaminated with Cu and Zn were purified after three sorption cycles by polymerin renewed at each cycle, whereas those containing a mixture of Cr(III), Cu, and Zn showed residues of Zn after five cycles. We briefly discuss environmental and industrial advantages for a possible exploitation of polymerin.
Introduction Olive oil mill wastewater (OMW) is a vegetable waste produced in high amounts mainly in the Mediterranean * Corresponding author phone: +39-0812539173; fax: +390812539186; e-mail:
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basin. It causes disposal problems because of its highly polluting properties, which are documented by high chemical oxygen demand (COD) and biological oxygen demand (BOD) (1, 2). These latter are mainly due to their polyphenol content and synergies with other naturally occurring compounds (3, 4) and render OMW unsuitable for discharge to soil and waters. However, OMW may also be regarded as an inexpensive source of inorganic and organic compounds (1, 2) to be recovered because of their potential economic interest and/ or ability to be transformed into products for use in agriculture, environmental biotechnology processes, and/ or industry. In fact, the recycling of OMW as an agricultural amendment, either in raw form or after various treatments (5-10), and recovery of the organic components for use in agriculture and in industry (3, 4, 11, 12) are the approaches commonly proposed for OMW disposal. In the framework of studies of ecologically friendly and useful disposal processes, we recently found (2) that metal cations naturally occurring in OMW were mainly bound to the organic polymeric fraction and that K was the most abundant metal, followed in decreasing order by Ca, Mg, Na, Zn, Fe, and Cu. In addition, the COD and BOD of this biomaterial markedly decreased in comparison with that of the raw OMW (2). These findings prompted the recovery of the humic-acidlike fraction, which was named polymerin, with the aim of studying its possible recycling in agriculture and use in environmental technology processes (5). A potential exploitation of this biomaterial in agriculture as bioamendment and/ or metal bio-integrator has been proposed on account of the humic-acid-like nature, the richness in macro- and micronutrients such as K, Ca, Mg, Fe, and Zn and the scarce phytotoxicity compared with that of the raw OMW (4). Polymerin could be also regarded as a very promising bio-organic sorbent for remediation of waters contaminated with toxic metals, on account of its peculiar physicochemical properties and humic-acid-like nature (5). In fact, biosorption technologies in which vegetable biomass is used to accumulate heavy metals (13-16) are methods that can replace conventional processes for remediating metal pollution in wastewaters, which require relatively expensive mineral adsorbents or flocculating agents (17-19). Among the more recent interesting papers on vegetable biomass biosorption, one examined the binding of Cr(III) to oat byproducts (20), and another addressed the remediation of waters contaminated with heavy metals using as biosorbents two biomass byproducts that are commercially available in quantity and low cost, namely “spillage”, a dried yeast and a plant mixture from the production of ethanol from corn (21). In particular, polymerin may be regarded as a bio-organic sorbent rich in negative binding sites and having many chelating functional groups (5), so it would seem to be very suitable for the sorption of heavy metal cations. Consequently, it was also transformed into its semisynthetic derivative, K-polymerin, which is richer in negative sites. This derivative was obtained by reaction of polymerin with KOH. Another sorbent, Fe(OH)x-polymerin, was also prepared by coprecipitation at pH 6.0 of Fe(NO3)3 with polymerin because of the significant capacity of Fe(OH)x-organic matter systems to adsorb heavy metals (22). In this paper we report a study on the individual sorption by the cited sorbents of common heavy metal ions contaminating wastewaters, Cr(III), Cu, and Zn. These metals originate from sources of pollution including agricultural fertilization, manufacturing processes, smelting and refining, 10.1021/es0400234 CCC: $27.50
2004 American Chemical Society Published on Web 08/27/2004
TABLE 1. Chemical Properties of the Sorbents phenol non-phenol aromatic aromatic polysaccharide protein melanin units units (%, w:w) (%, w:w) (%, w:w) (%, w:w) (%, w:w) polymerin
43.07
22.4
30.2b
15.0b1
15.2b2
K-polymerin
44.0
23.8
26.2b
13.1b1
13.1b2
Fe(OH)xpolymerin
metalsa (%, w:w)
rmw (Da)
carbon (%, w:w)
4.33 [Na (0.28), K (1.2), Ca (2.0), Mg (0.65), Fe(0.2)] 6.0 [Na (1.0), K (4.0), Ca (0.5), Mg (0.3), Fe(0.2)] 48.90 [Fe]
first peak: 3500 < rmw < 10000; second peak: 45000 only an individual peak: 3500 < rmw < 10000
45.96
4.35
a The values converted to meq/kg for polymerin and K-polymerin are, respectively: 2068 [Na (120), K (300), Ca (1000), Mg (540), Fe (108)] and 2072 [Na (435), K (1023), Ca (256), Mg (250), Fe (108)]. b Determined by complement to 100 of the sum of the other components. b 1Determined by chemical analysis. b 2Determined by difference between a and a1.
refuse and wastewater treatment, and fuel combustion. The sorption mechanism of these heavy metals on polymerin was investigated. We carried out also a study on the sorption of mixtures of Zn, Cu, and Cr(III) in different combinations by polymerin and its derivative Fe(OH)x-polymerin and on the reciprocal interactions among these metals and the cited sorbents. With the aim to remediate simulated wastewaters contaminated with the selected heavy metals, an investigation was also performed on their cyclic sorption on polymerin renewed at each cycle. Finally, the environmental and industrial advantages deriving from a possible employment of polymerin for this technology is briefly discussed.
Experimental Section Preparation and Characterization of the Sorbents [Polymerin, K-Polymerin, and Fe(OH)x-Polymerin]. Polymerin was prepared and characterized according to the procedure previously reported (5). An aliquot of OMW (year 2000) from a pressure processing system located in a Monteroduni (Italy) plant was transformed into the complex polymeric mixture, the chemical composition and molecular sizes of which are reported in Table 1. K-Polymerin was prepared by dissolving 5 g of polymerin in 0.03 M KNO3 (500 mL) and adding to the solution 1 mol L-1 KOH to reach pH 12, to hydrolyze the ester linkages occurring in this biosorbent (5). The solution was left under magnetic stirring for 2 h at 20 °C and then dialyzed using membranes (Spectrum Medical Industry, Houston, TX) with molecular weight cutoffs of 1000 Da, against five sequential volumes (1:5) per time, at 4-h intervals, up to neutral pH. The nonpermeated dialyzed fraction was lyophilized, leaving a black residue (2.5 g) called K-polymerin. Protein, carbohydrate, and melanin contents, metal composition, and relative molecular weight of K-polymerin (Table 1) were measured according to the procedure previously described for polymerin (5). The Fe(OH)x-polymerin complex was obtained by adding 1 g of polymerin into a 0.1 mol L-1 FeCl3 solution (500 mL), and by titrating the solution with 1 mol L-1 NaOH to reach pH 6.0 using an automatic titrator (Vit 90, Abu 93 Triburet). The precipitate obtained was washed, dialyzed against distilled water to reach a conductivity of 2 µS, and, then lyophilized, obtaining 9.7 g of the complex. The elemental carbon, metal composition (Table 1), and IR characterization of all the cited sorbents were determined by HCNS, AAS, and DRIFTS analysis, according to the general methods described below. Chemical Analysis. The carbon elemental analysis was performed using a Fisons EA 1108 elemental analyzer for hydrogen, carbon, nitrogen, and sulfur (HCNS).
The metals were determined by a Perkin-Elmer model 3030 B atomic absorption spectrometer equipped with deuterium-arc background correction. Either air-acetylene or nitrous oxide-acetylene flames were used as the atomization source. Stock standard solutions of each metal cation (1 g L-1) were obtained from BDH Reagents (Poole, U.K.). The average and standard deviation of three absorption measurements were recorded for each sample. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) Analysis. Sample preparation for DRIFTS determinations was as follows: 0.2 mg of sample was mixed with 200 mg of KBr (FTIR grade, Aldrich, Chemical Co., Milwaukee, WI). The mixture was finely ground in an agate mortar and transferred to a sample holder. Its surface was smoothed with a microscope glass slide, and DRIFT spectra were recorded. The DRIFT spectra were obtained using a Perkin-Elmer Spectrum One FT-IR. The instrument had a special resolution of 1 cm-1, which was used in all spectra determinations. Sorption Methodology. A 100-mg aliquot of sorbent (polymerin, K-polymerin, Fe(OH)x-polymerin), previously dried at 100 °C for 1 h, was equilibrated at 20 °C with 20 mL of 0.03 mol L-1 KNO3. Solutions of 0.1 mol L-1 individual heavy metal, adjusted previously to pH 4, were added to the sorbent to give an initial concentration ranging from 0.50 to 7.50 mmol L-1. The pH of each suspension was kept constant for 24 h by addition of 0.1 or 0.01 mol L-1 HCl or KOH. The final suspensions were centrifuged at 10000g for 30 min, then ultrafiltered through an ultrafiltration cell (200 mL) equipped with a magnetic stirrer and a membrane with a cutoff of 1000 Da. The final concentration of the metals was determined in the permeated solution by AAS. The amount of metal retained on the sorbents was determined by the difference between the initial quantity of metal added and that present in the equilibrium solution. These same procedures were followed for all the sorption experiments described below. In the experiments carried out on competitive binary sorption of Zn and Cu at initial equimolar ratio, 0.1 mol L-1 solutions of each heavy metal [as Zn(NO3)2 and Cu(NO3)2, respectively], adjusted previously to pH 4, were added to the sorbent at an initial concentration ranging from 0.50 to 5 mmol L-1, corresponding to 100-1000 mmol of each metal per kilogram of sample. The pH of each suspension was kept constant for 24 h by addition of 0.1 or 0.01 mol L-1 HCl or KOH. Experiments with a ternary combination of Zn, Cu, and Cr(III) in equimolar amounts were carried out by adding to the sorbent 0.1 mol L-1 solutions of heavy metals as Zn(NO3)2, Cu(NO3)2, and Cr(NO3)3, respectively, adjusted to pH 4.0, at an initial concentration ranging from 0.50 to 5 mmol L-1, corresponding to 100-1000 mmol of each metal per kilogram of sorbent. VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. DRIFT spectra of polymerin, K-polymerin, and Fe(OH)x-polymerin. Determination of Cr(III), Cu, and Zn and Native Metals Released from Polymerin After Sorption. A 100-mg aliquot of polymerin, previously dried at 100 °C for 1 h, was equilibrated at 20 °C with 18.5 mL of ultrapure water. Predetermined quantities of 0.1 mol L-1 solutions containing Cr(III), Cu, and Zn, as Cr(NO3)3, Cu(NO3)2, and Zn(NO3)2, respectively, whose pH values were adjusted previously to pH 4.0, were pipetted into the flasks to give 7.5 mmol L-1 of each metal. The final suspensions (20 mL) were shaken for 24 h, and their pH values were adjusted periodically to the original value with 0.1 or 0.01 mol L-1 HCl or KOH. Cyclic Sorption on Polymerin of Zn and Cu at Equimolar Ratio. Cyclic experiments for sorption of Zn and Cu at equimolar ratios were carried out using 300 mg of polymerin equilibrated with 57 mL of ultrapure water in reaction flasks. Predetermined quantities of 3 mL of 0.1 mol L-1 solutions containing a mixture of Zn and Cu, whose pH values were previously adjusted to pH 4.0, were pipetted into the flasks to give 2.5 mmol L-1 of each metal. The final suspensions (60 mL) were shaken for 24 h, and their pH values were adjusted periodically to the original value with 0.1 or 0.01 mol L-1 HCl or KOH. The permeated solution, derived from the ultrafiltration process, was utilized for the determination of Cu and Zn by AAS and for the successive cycle. In the second cycle, 40 mL of permeated fraction obtained from the first cycle was added to 200 mg of polymerin. The suspensions were shaken for 24 h at pH 4.0, then centrifuged and ultrafiltered, and these same procedures were followed in the successive cycles. The permeated fraction was analyzed both for the determination of Cu and Zn and for the successive cycle. In the third cycle, 20 mL of permeated solution obtained from the second cycle was added to 100 mg of polymerin. The permeated fraction was analyzed by AAS to determine the residual amounts of metals. Cyclic Sorption on Polymerin of Equimolar Amounts of Cu, Zn, and Cr(III). In the first cycle, 500 mg of polymerin was equilibrated at 20 °C with 92.5 mL of ultrapure water. Suitable amounts of 0.1 mol L-1 solutions containing a 5172
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mixture of Zn, Cu, and Cr(III), whose pH values were previously adjusted to pH 4.0, were pipetted into the flasks to give 500 mmol of each metal per kilogram of sorbent. The final suspensions (100 mL) were shaken for 24 h, and their pH values were adjusted periodically to the original value with 0.1 or 0.01 mol L-1 HCl or KOH. The permeated fraction was analyzed both for the determination of Cu, Zn, and Cr(III) and the successive cycle. In the second, third, fourth, and fifth cycles, different volumes of the permeated solution from each preceding cycle (and precisely 80, 60, 40, and 20 mL) were added to 400, 300, 200, and 100 mg of polymerin in reaction flasks. The permeated solutions were collected and analyzed to determine the residual amounts of Cu, Zn, and Cr(III). Analysis of the Data. All the experiments were performed in triplicate and the relative standard deviation was lower than 5%.
Results and Discussion Characterization of the Sorbents. The complex melanin nature and polyelectrolyte characteristics of polymerin, which was recovered from OMW and chemically and spectroscopically (DRIFTS) characterized according to the procedure previously reported (5), were confirmed by the data reported in Table 1 and the DRIFT spectrum of Figure 1. Polymerin was transformed into its derivative saturated with potassium, called K-polymerin (Table 1). This conversion was performed with the aim of producing a sorbent with increased metal cation adsorption capacity with respect to polymerin. In fact, the percentage composition of metal in K-polymerin was higher than that in polymerin (compare the data of Table 1), particularly with regard to K, as a consequence of the hydrolysis of the ester linkages present in polymerin (5), which produced further carboxylate anions, with the consequent increase of negative sorption sites. These characteristics have been confirmed by the analysis of DRIFT spectra (Figure 1) of the two sorbents. In fact, the spectrum of this sample of polymerin compared with that of literature (5) confirmed the following main and characteristic absorp-
tions: a very strong and broad band centered at 3313.95 cm-1 (OH groups), a sharp and medium band at 2924.41 cm-1 (CH and methylester CH3 groups), four weak albeit correlated bands at 1723.93 and 1261.88 cm-1, and 1439.14 and 1354.87 cm-1 (CO and OC-CH3 methylester groups, and symmetric and asymmetric bending of the methylester CH3 groups, respectively), an absorption at 1639.65 cm-1 (aromatic CdC stretching and CO peptide group), and a very strong absorption at 1072.99 cm-1 (C-OH bonding of the polysaccharide component). The spectrum of K-polymerin, compared with that of polymerin, showed the disappearance of the bands at 1439.14 and 1354.87 cm-1 (symmetric and asymmetric bending of the methylester CH3 groups) and the appearance of the two characteristic absorptions of COO- groups at 1594.40 and 1390.20 cm-1, due to the symmetric and asymmetric stretching, respectively. In addition, this spectrum showed the complete disappearance of the absorption at 2924.41 cm-1, corresponding to the loss of CH3, released from the methylester group after the alkaline hydrolysis. Polymerin was also transformed into the organo-mineral complex Fe(OH)x-polymerin to study the influence of Fe(OH)x, a poorly crystalline Fe oxide, on the sorbing capacity of the biosorbent immobilized into a solid matrix. The chemical data of the organo-mineral complex indicated a prevailing presence of the mineral component (Table 1) which was confirmed by its DRIFT spectrum (Figure 1). In fact, the absorptions shown by Fe(OH)x-polymerin spectrum did not show significant variations in comparison with the ferrihydrite spectrum (not shown), due to the very low carbon content in the former sorbent. Sorption of Cr(III), Cu, and Zn on Polymerin, K-Polymerin, and Fe(OH)x-Polymerin. The sorption data were analyzed according to the Langmuir equation. This equation can be written as follows:
x ) xmkc/(1 + kc) where x is the amount of metal sorbed on the adsorbent (meq Kg-1), k is a constant related to the binding energy, xm is the maximum amount of metal adsorbed (meq Kg-1), and c is the equilibrium concentration of metal (meq L-1), according to Giles et al. (23). Figure 2a-c show that the individual sorbing capacity of Cr(III), Cu, and Zn on polymerin was considerable and followed the order Cr(III) > Cu > Zn, as shown by the respective isotherms and the corresponding values of Langmuir parameters (Table 2). The individual sorptions of the three metals on K-polymerin proved to be higher than those on polymerin, following the same order Cr(III) > Cu > Zn, as shown by the corresponding values of Langmuir (Table 2) and by the examination of the isotherms for the sorption of Cr(III) and Zn on the two sorbents reported in comparison in Figure 3 as an example. The high sorption of the selected heavy metals on both polymerins is consistent with the presence of negative sites on their surface. In particular, their greater sorption capacity on K-polymerin is consistent with the increase of negative sites on this sorbent with respect to polymerin, as explained above. The individual sorptions of the selected metals on Fe(OH)x-polymerin proved to be significant, but much lower than those on the polymerins (24). On the other hand, the individual sorption capacity of Cr(III) and Zn on the above organo-mineral complex was much more higher than on Fe(OH)x, (2100 meq Kg-1/480 meq Kg-1 and 400 meq Kg-1/ 100 meq Kg-1, respectively, at the same equilibrium concentration of 3.0 meq L-1), whereas the sorption capacity of Cu showed a very close level (24).
FIGURE 2. Sorption on polymerin of Cr(III) alone and as affected by the presence of equimolar Zn + Cu (a); Cu alone and as affected by the presence of equimolar Zn and Zn + Cr(III) (b); Zn alone and as affected by the presence of equimolar concentrations of Cu and Cu + Cr(III) (c).
TABLE 2. Langmuir Parameters for the Sorption of Cr(III), Cu, and Zn on Polymerin and K-Polymerin polymerin
Cr(III) Cu Zn a
K-polymerin
xm
k
r2 a
2419.0 1501.0
1.76 0.87
0.99 0.99
xm
k
r2 a
2797.0 1888.5
4.38 1.32
0.99 0.99
Correlation coefficient.
Sorption of Mixtures of Cu and Zn, or Cu, Zn, and Cr(III) on Polymerin and Fe(OH)x-Polymerin. The results obtained by the study on the individual heavy metal sorption by the above-considered polymerins and Fe(OH)x-polymerin complex encouraged the investigation of the sorption of these metals in binary and ternary combination. The study was specifically carried out on polymerin and Fe(OH)x-polymerin, because of higher convenience in the preparation of the two sorbents in view of their potential application in wastewater remediation processes. Figure 2 also shows the sorption curves on polymerin of each heavy metal when added in the presence of one or both of the other elements. It appears evident that the presence of both Zn and Cu did not affect the capacity of Cr(III) to be sorbed on polymerin (Figure 2a). VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Finally, at an equilibrium concentration of 4 meq L-1 Zn, the presence of Cu or Cu and Cr(III) decreased the sorption of Zn by about 70% and 79%, respectively, indicating that both Cu and Cr(III) strongly inhibited Zn fixation on polymerin (Figure 2c). The Fe(OH)x-polymerin complex showed a lower sorption capacity than polymerin. In fact, Figure 4 shows the overall sorption capacity of Cu + Zn and Cr(III) + Cu +Zn mixtures (in the order listed) on polymerin and Fe(OH)x-polymerin complex.
FIGURE 3. Comparison of the sorbing capacity of Cr(III) and Zn on K-polymerin and polimerin.
FIGURE 4. Comparison of overall adsorbing capacity of the Zn + Cu and Zn + Cu + Cr(III) added in equimolar amount to polymerin and Fe(OH)x-polymerin. Vice versa, the presence of Zn and/or Cr(III) reduced Cu sorption. At an equilibrium concentration of 4 meq L-1, the presence of Zn reduced Cu sorption by 15%, and the concomitant presence of Zn and Cr(III) decreased Cu sorption by about 50% (Figure 2b), showing that Cr(III) was much more effective than Zn to prevent Cu sorption.
Sorption Mechanism of Cr(III), Cu, and Zn on Polymerin. The high sorbing capacity of the selected metals on polymerin suggested that the first sorbent was promising for a possible utilization in wastewater remediation, so the mechanism for the metals sorption was investigated. Table 3a-c and Figure 2a-c show the data on the individual maximum sorption of Cr(III), Cu, and Zn on polymerin obtained by adding to the sorbent the heavy metals at the initial concentrations of 3000, 2000, and 2000 meq/kg, respectively. In addition, the tables report the data for Ca, K, and Mg released from polymerin as affected by the sorption process of the above-considered heavy metals. In particular, by adding 3000 meq/kg of Cr(III) to polymerin (Table 3a and Figure 2a), 2010 meq/kg of Cr(III) were sorbed and 1740 meq/kg of Ca, K, and Mg were replaced. These findings indicate that the sorption mechanism of Cr(III) on polymerin was substantially consistent with the ion exchange of the heavy metal with Ca, K, and Mg (1740/2010 ) 87%). The difference of 270 meq should be equivalent to the replacement of protons by the remaining Cr(III) (13%), according to the model proposed previously for the metal ion exchange in a system at pH 4 on a biosorbent (25). A concomitant complexation with the OH groups proper to the polysaccharide component of this biosorbent (5) should occur, as indicated by the DRIFTS analysis (Figure 5). In fact, in the spectrum of the heavy metal-polymerin, the band of C-OH stretching groups, proper to the polysaccharide component, which occurs in the range 1050-1070 cm-1, decreases strongly. An enlargement of the band of the O-H stretching
TABLE 3. Metal Composition of Raw Polymerin and Polymerin Sorbed with Cr(III), Cu, and Zna Ca meq/kg metal composition of raw polymerin metal composition
1000 40
amounts of the metals and H+ released in solution
960
metal composition
660
amounts of the metals and H+ released in solution
340
metal composition
660
amounts of the metals and H+ released in solution
340
K meq/kg
Mg meq/kg
Na meq/kg
Fe meq/kg
H+ meq/kg
300
540
120
108
ND
120
108
ND
2010b
2338
270c
990
3000
ND
1100d
2218
150e
900
2000
ND
1000f
2113
45g
1000
2000
After Sorption of Cr(III) 10 50 Σ ) 338 290 490
Cr(III) meq/kg
total meq/kg 2068
Σ ) 1740 After Sorption of Cu 30 200 Σ ) 1118 270 340
120
108
Σ ) 950 After Sorption of Zn 25 200 Σ ) 1113 275 340
120
108
Σ ) 955
a
Data were determined in correspondence to the maximum sorption capacity of the metal (Figure 2a). b The initial amount of Cr(III) added to polymerin was 3000 meq/kg. c This amount has been calculated as difference between the Cr(III) ions sorbed on polymerin (2010 meq/kg) and the native alkaline ions released by the biosorbent in solution after the sorption process (1740 meq/kg). d The initial amount of Cu added to polymerin was 2000 meq/kg. e This amount has been calculated as difference between the Cu ions sorbed on polymerin (1100 meq/kg) and the native alkaline ions released by the biosorbent in solution after the sorption process (950 meq/kg). f The initial amount of Zn added to polymerin was 2000 meq/kg. g This amount has been calculated as difference between the Zn ions sorbed on polymerin (1000 meq/kg) and the native alkaline ions releaed by the biosorbent in solution after the sorption process (955 meq/kg).
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FIGURE 5. DRIFT spectra of polymerin and Cr(III)-, Zn-, and Cu-polymerin. group is also observed in the region of 3320-3343 cm-1, as a consequence of the complexation. The other two native metals of polymerin, Na and Fe(III), were not involved in the exchange mechanism, consistent with their strong fixation on the sorbent as already demonstrated in our previous work (4) where polymerin was saturated with heavy metals. The same sorption mechanism was also revealed for Cu and Zn (occurring by 86 and 96% for exchange with the alkaline metals, and by 14 and 4% with the protons, respectively) according to the data reported in Table 3b and c and the DRIFT spectra of Figure 5. In conclusion, the sorption process of Cr(III), Cu, and Zn on polymerin occurs mainly by means of the ion exchange of the alkaline metals and, to much lesser extent, with the protons, which are bound to its native carboxylate groups (5), and the concomitant complexation with the OH groups of its polysaccharide component (5). Cyclic Removal of the Heavy Metals by Polymerin from Simulated Wastewaters. The data obtained by analyzing the sorption mechanism on polymerin and the reciprocal influence of the selected metals on the sorption by this sorbent prompted the experimentation with cyclic sorption to totally remove the heavy metals from simulated wastewaters. Figure 6a shows that the total removal of Cu and Zn from a solution containing equimolar amounts of these cations was performed after three cycles, as shown by the two respective curves. Furthermore, the removal of Cu and Cr(III) from a solution containing equimolar amounts of Cu, Cr(III), and Zn was complete in three cycles, whereas about 20% of Zn initially added still remained after five adsorption cycles (Figure 6b). In view of an environmental technology application, the latter findings suggest that greater amounts of polymerin should be employed to render the remediation process more efficient. Potential Exploitation of the Methodology. A possible future exploitation of polymerin should provide environmental and industrial advantages. First, it is the main byproduct originating from the recycling of OMW, and it has a low phytotoxicity (4) and very low BOD and COD values (2). Therefore, its use could contribute to the resolution of the problem of disposing of OMW, which is a highly polluting biomass.
FIGURE 6. Removal of Zn + Cu (a) and Zn + Cu + Cr(III) (b) from equimolar solutions by cyclic sorption on polymerin (the biosorbent was renewed in each cycle). The sorption of Zn, Cu, and Cr(III) on this bio-organic material is very high and not lower than other biomass byproducts used as metal biosorbents, which are widely described in the literature (13-16, 20, 21, 26). In addition, the employment of polymerin in wastewater remediation processes should not only decontaminate the waters of heavy metals, but also should enrich them in nutrients such as Ca, Mg, and K, which are released from polymerin during the cleanup process. In addition, the material obtained after the wastewater cleanup process could potentially be reused as a potential source of metals after desorption or incineration processes. Regarding potential use of Fe(OH)x-polymerin complex, the long-term laboratory studies seem to demonstrate that this complex is a sorbent particularly useful for the removal VOL. 38, NO. 19, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of mixtures of heavy metals and metalloids both in cationic and anionic forms (27).
Acknowledgments This work was supported by grants from the Universita` di Napoli Federico II (Italy). This paper represents Journal Series No. 53 from the DiSSPA.
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Received for review February 23, 2004. Revised manuscript received June 3, 2004. Accepted June 16, 2004. ES0400234
