Adsorptive Properties of Fish Scales of Oreochromis Niloticus (Mojarra

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Ind. Eng. Chem. Res. 2001, 40, 3563-3569

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Adsorptive Properties of Fish Scales of Oreochromis Niloticus (Mojarra Tilapia) for Metallic Ion Removal from Waste Water J. F. Villanueva-Espinosa,† M. Herna´ ndez-Esparza,*,‡ and F. A. Ruiz-Trevin ˜ o‡,§ Facultad de Quı´mica, Universidad Auto´ noma del Estado de Me´ xico, Paseo Colo´ n Esq. Paseo Tollocan, Toluca, Me´ xico 50120, and Departamento de Ciencias y Departamento de Ingenierı´as, Universidad Iberoamericana, Prol. Paseo de la Reforma No. 880, Me´ xico D.F. 01210

The present work presents the experimental study of the adsorptive properties for the removal of Cu2+ , Pb2+, Co2+, and Ni2+ metallic ions from water of thermally pretreated fish scales from a very common and abundant source in Mexico, the Oreachromis Niloticus fish, known as “Mojarra Tilapia”. To determine the influence and importance that the two main fractions (the organic or protein fraction and the inorganic fraction mainly composed of hydroxyapatite) of this material play in this adsorptive capacity, the original pretreated fish scales are subjected to an acid demineralization and a basic deproteinization treatment to modify the organic/ inorganic matter ratios present, and their materials are characterized in terms of their adsorption ability, especially in the removal of the Cu2+ ion. The adsorption isotherms for the thermally pretreated fish scales show that they have a maximum Cu2+ adsorption capacity of 58 mg of Cu2+ /g of adsorbent with an ion selectivity removal in the order Cu2+ > Pb2+ > Co2+ > Ni2+. When the pure organic and inorganic parts of the fish scales are used in the adsorption experiments, the inorganic part has a 75% higher removal capacity than the organic fraction. The adsorption experiments using fish scales with different organic or inorganic fractions show a nonadditive, synergistic effect on the equilibrium amount of metallic ion adsorbed, which is independent of the inorganic content between 30 and 90 wt % of this inorganic fraction. Lowvacuum scanning electron microscopy and elemental analysis reveal that the ion-exchange reaction is the main mechanism for the metallic ion adsorption by fish scales. Introduction One of the alternatives for metallic ion removal from polluted water is adsorption on synthetic polymers,1-5 zeolites, ion-exchange resins,6,7 and biomaterials,8-11 among others. Even though different types of adsorbents have been tested, the search must continue to find better and more easily available materials. From all of them, adsorbent biomaterials which are available from nature and that are subproducts or wastes in the processing of valuable products are being evaluated on their ability to remove metallic ions from wastewater. Biomaterials such as cellulose,12 chitin,13,14 chitosan,8,15 alginate, and their chemical modifications5,9,12,14,16-20 have been tried in the removal of metals from water with low to excellent results. In this search for adsorbent materials, it has been reported that fish, hair, and feathers present in water have the ability to reduce toxicity of polluted water by removal of the toxic metallic ions.21,22 Fish scales such as any other biomaterial are composed of organic and inorganic matter. Specific studies with fish scales from cod, porgy, and flounder have established that the proteins, the organic fraction present in fish scales, seem to be the major factor governing the adsorption ability, because of the nitrogen-containing ligands present.8 Of the proteins present in the organic fraction, it seems * To whom correspondence should be addressed. E-mail: [email protected]. † Universidad Auto ´ noma del Estado de Me´xico. ‡ Universidad Iberoamericana. § E-mail: [email protected].

that keratin,21 a protein material found in feathers, hair from mammals, and fish scales, with its sulfur groups, may also be responsible for the adsorptive properties of fish scales. However, in other studies it has been reported that hydroxyapatite or HAP, Ca10(PO4)6(OH)2, found and extracted from bones, sea shells, and fish scales,23 and similar materials,24 can also remove metallic ions from wastewater.25 Thus, it is still not totally clear which fraction of the fish scalessthe organic fraction, mainly composed of proteins, or the inorganic fraction, mainly composed of HAPsis responsible or has a stronger influence on the adsorption phenomena observed. In the present research an alternative adsorbent biomaterial was tested, the fish scales from Oreochromis Niloticus (“Mojarra Tilapia”). This fish species is commercially exploited and consumed by the population of Mexico in amounts of up to 77 671 tons/yr (data of 1998 from INEGI). The amount of fish scale generated as waste from this consumption allows an abundant source of biomaterial. Some of the important uses of this biomaterial is as a base for animal feedstock, but a percentage could well be distracted as a cheap source of adsorbent material for the removal of contaminants from water. These thermally pretreated fish scales present an average composition of 49.7% inorganic fraction and 50.3% of organic fraction, with an adsorptive capacity for Cu2+ that competes very favorably with that observed with other fish species scales.8 The present work has three objectives: (1) the first objective is addressed to the treatment of fish scales of

10.1021/ie000884v CCC: $20.00 © 2001 American Chemical Society Published on Web 07/10/2001

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“Mojarra Tilapia”, to obtain adsorbent materials containing different fractions of the organic and inorganic material commonly found in such scales; (2) to evaluate the adsorption capacity for at least one metallic ion, for example, Cu2+, of fish scales containing different proportions of organic and inorganic materials; (3) to evaluate the experimental adsorptive properties of thermally pretreated fish scales, for metallic ions such as Cu2+, Pb2+, Co2+, and Ni2+, using both a pure metal ion solution and a solution containing a mixture of all of them. Experimental Part Materials and Analytical Methods. The raw Oreochromis Niloticus fish scales were collected from the waste of fish cleaning that is originated in Mexican markets. This material was first washed and thermally treated in water at a temperature of 80 °C for 30 min. This was done to stop any bacterial contamination and activity and to get rid of the protein still attached to the main body of the fish scales.13 The pretreated material was dried in a convection oven at 80-90 °C for 18 h and then refrigerated at 4 °C. Fish scales with different proportions of organic and inorganic matter were prepared following two methods. To produce organic-rich materials, the thermally pretreated fish scales were contacted with a warm 5 wt % hydrochloric acid solution, whereas to produce the inorganic-rich materials,13 the thermally pretreated fish scales were contacted with a warm 2 wt % KOH solution.23 In both treatments, time was varied to produce materials containing different ratios of organic/ inorganic matter. Each one of these materials was washed abundantly with deionized water until neutrality was reached and dried in a convection oven at 90 °C for 18 h. For each adsorbent material, the organic and inorganic fractions present in the adsorbent were determined by a gravimetric calcination procedure where the organic fraction was considered to be the volatile fraction of the dried material after a 1 h treatment at 600-700 °C and the inorganic fraction the remaining residue. Aqueous metallic ion solutions for the adsorption experiments were prepared by adjusting the corresponding hydrated chloride salt of the corresponding metal, MCl2‚H2O, to a pH of approximately 5-6. The ion concentration in the original solution and the amount present in the solution after removal were determined in an atomic absorption spectrophotometer (Perkin-Elmer, SpectrAA 250+). In all cases, it was assumed that the amount of metal ion that disappeared from the solution had been adsorbed on the adsorbent material, because no considerable change in pH was observed during the process that would favor metal compounds’ precipitation. Adsorption isotherms were determined from equilibrium adsorption tests in a batch reactor containing 50 mL of the pure metallic ion solution and 1 g of a dried adsorbent material. Every data point in the isotherms represents an equilibrium value which was reached after a 24-h stirring period. Different initial concentrations of metallic ions in solution at a pH of 5-6 and room temperature were used to evaluate the isotherms. The equilibrium affinity and selectivity of the thermally pretreated material toward different metallic ions (Pb2+, Ni2+, Co2+, and Cu2+) was evaluated following the adsorption kinetics of every metallic ion with time.

Table 1. Inorganic or Organic Fraction That Remains in Thermally Pretreated Fish Scales after Being Treated with a 2 wt % KOH Solution at 60 °C or a 10 wt % HCl Solution at 50 °C

time, min 0 10 10 15 20 30 40 60 120

KOH treated scales/ inorganic-rich adsorbent (mineral fraction)

HCl treated scales/ organic-rich adsorbent (organic fraction)

0.497a

0.503a 0.644b 0.980

0.544 1.0 0.516 1.0 0.842 0.883

1.0

a Fraction present in thermally pretreated fish scales. b As treated with 1 wt % HCl.

These experiments were carried out to determine the kinetic equations and constants for each metal and their affinity toward the material. Additionally, low-vacuum scanning electron microscopy (JEOL JMS-5900LV) and in some cases elemental analysis were performed on the surface and cross sections of the adsorbent materials to determine in which part of the fish scale the metallic ion was being adsorbed. Results and Discussion The first goal of this work was to produce fish scale adsorbent materials made up of different organic/ inorganic ratios. Table 1 reports those types of materials that were obtained by contacting thermally pretreated fish scales with a warm 2 wt % KOH solution and a warm 5 wt % HCl solution. Every treatment produces adsorbent materials containing high ratios of inorganic or organic matter, depending on the treatment and time. However, the dissolution kinetics of the proteins by the KOH treatment is apparently slower than the dissolution kinetics of the inorganic fraction by the HCl treatment, as may be judged by the time required to increase the corresponding inorganic or organic fraction of the fish scales. An important aspect in the production of these materials is related to their treatment time. Inorganicrich materials conserve the form of a scale, change their color from amber to white, and are also more fragile after being treated with 2 wt % KOH for up to 1 h. Longer treatment times produce adsorbent material in the form of a powder that still contains approximately 12 wt % proteins as seen in Table 1. Organic-rich materials produced via a treatment with 5 wt % HCl retain their appearance of fish scales, and they are semitransparent gellike materials. After being washed with water, they still show some adhesion properties, but after being dried at 90 °C for 18 h, this material loses its appearance of a scale and becomes amber and very fragile. In both treatments there must be morphological and structural changes in the fish scales, but these changes were not part of the present study. With respect to the Cu2+ adsorption by the pretreated fish scales, Figure 1 shows the amount of Cu2+ adsorbed per gram of thermally pretreated fish scales, X, as a function of the equilibrium concentration, [Cu2+]eq. The solid line represents the expected behavior when the

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Figure 2. Effect of the inorganic fraction contained in fish scales on the amount of Cu2+ adsorbed at 25 °C and pH ) 5-6. Figure 1. Cu2+ adsorption isotherm of thermally pretreated fish scales. Isotherms measured at 25 °C and pH ) 5-6.

Langmuir model is applied to fit the isothermal experimental data

XmksC Xeq ) 1 + ksC

(1)

where Xm, in mg of Cu2+/g of adsorbent, according to Langmuir’s assumption is the maximum amount of adsorbed material required to give a complete monolayer on the surface, C is the Cu2+ equilibrium concentration, and ks is a constant that relates to the binding strength. It is observed that, for relatively low concentrations, the Cu2+ adsorption isotherm is favorable in this type of material, and it is well defined by the Langmuir model with Xm ) 53.8 mg of Cu2+/g of adsorbent and ks ) 0.23 L/mg of Cu2+ (correlation coefficient R2 ) 0.91). The inset in Figure 1 shows the same experimental data with an additional equilibrium concentration experiment performed to confirm the maximum removal of Cu2+ by thermally pretreated fish scales and to reevaluate the Langmuir parameters using data in the low and high concentration regions. The new values for the Langmuir parameters are Xm ) 58.5 mg of Cu2+/g of adsorbent and ks ) 0.18 L/mg of Cu2+. Because there are practically no significant variations in these parameters, this means that the Cu2+ adsorption process on these fish scales is apparently in a monolayer. The equilibrium amount of Cu2+ adsorbed by fish scales, Xeq, with different proportions of organic and inorganic fractions including the pure organic and inorganic fractions is presented in Figure 2. The solid lines in this figure show the adsorption results of different initial metal ion concentrations with different proportions of the organic/inorganic materials. There are at least two important issues on this plot that may be pointed out using the equilibrium adsorption data of the experiments carried out with an initial concentration of 720 ppm of Cu2+. The first issue is that apparently the pure inorganic fraction, mainly composed of HAP or a calcium phosphate compound, of the fish scales has a 75% higher adsorption capacity for Cu2+ than the organic fraction, as judged by the value of Xeq, when

Figure 3. Decrement of metallic ion concentration with time in a solution contacted with thermally pretreated fish scales. Measurements were performed at 25 °C and pH ) 5-6.

such materials are used in the adsorption experiments. The second issue is that there is a nonadditive synergistic effect when both the organic and inorganic fractions are present in the fish scale and, interestingly, the amount of Cu2+ adsorbed is independent of the organic or inorganic fraction between 30 and 90 wt % of the inorganic component. Thermally pretreated fish scales are made up of 49 wt % of organic matter and the rest being inorganic matter, thus falling in the range of concentration where Xeq is independent of the organic or inorganic fraction. This is an important result because it teaches that fish scales will need just a simple thermal treatment to show their maximum capacity for metal adsorption. There is an additional observation on this plot. The adsorption capacity Xeq of fish scales with inorganic fractions between 30 and 90 wt % increases with the initial concentration of Cu 2+ in solution, as would be inferred from the Langmuir behavior shown in Figure 1. Kinetics of Metal Adsorption on Thermally Pretreated Fish Scales. Figure 3 shows the depletion of concentration of the adsorbate in the solution with time as a consequence of the adsorption of the metal ions by thermally pretreated fish scales. Pure metal ion solutions were used to carry out the experiments of this plot. There is a rapid decrease in the concentration of Cu2+ and Pb2+ followed by a gradual equilibrium condition reached in approximately 24 h. In contrast, there is a

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Table 2. Global Kinetics Coefficients and Global Order of the M2+ Concentration Disappearance from a Solution Contacted with Thermally Pretreated Fish Scalesa n)1

n*1

element

kc, h-1

R2

kc, Ln-1 mg1-n h-1

n

R2

Cu2+

0.593 0.179 0.398 1.014

0.995 0.922 0.950 0.972

1.039 0.0001 0.0047 0.0173

0.89 2.54 1.87 1.80

0.998 0.957 0.988 0.990

Ni2+ Co2+ Pb2+

a Parameter values resulted from fitting, by nonlinear regression, eqs 3 and 4 to the experimental data reported in Figure 3.

slow decrease in the concentration of Co2+ and Ni2+, and apparently the equilibrium conditions have not been reached before 24 h. In metal adsorption studies reported on the removal of Cu2+, Pb2+, Cd2+, and Cr3+ ions by porgy, cod, and flounder fish scales,8 similar trends or preferences were observed for Cu2+ and Pb2+. Also, in such studies these two last ions showed the fastest removal ratio as compared with the other two ions (Cd2+ and Cr3+). Several kinetic models were tested with the experimental data. First the Elovich adsorption kinetics equation, which has been used for describing the kinetics of sorption and desorption of various inorganic materials on soils27 and of metal sorption with solvent impregnated resins,28 was tried, but the correlation was not as good as was the case of applying a generalized power-law equation of the form

-

dC ) kc(C - Ceq)n dt

(2)

using the integrated equations to fit the experimental data.

C ) Ceq + (C0 - C)ekct

for n ) 1

(3)

C ) Ceq + [(C0 - Ceq)1-n + tkc(n - 1)]1/n-1 for n * 1 (4) In these equations, Ceq is the equilibrium concentration and kc and n are the corresponding “global” kinetic coefficients and the “global” order of the adsorption process, respectively. Generalized power-law equations have been successfully applied to describe the phosphorus release kinetics and substrate biodegradation kinetics carried out simultaneously by anaerobic microorganisms in a biofilm reactor29 and in the kinetics of copper uptake on chitosan.30 The solid lines in Figure 3 are obtained for the integrated equation when n * 1. As can be seen, these types of models describe satisfactorily the amount of metal adsorbed by these fish scales. When n ) 1, the model represents the liquid-solid film mass-transfer phenomena, and then kc is the overall film mass-transfer coefficient. When n * 1, one can consider that other mechanisms are involved besides the film mass-transfer phenomena. Thus, in this work it is proposed that the use of these types of models may describe the “global” process of adsorption as the adsorbate is gradually removed from the solution and retained in the adsorbent. Table 2 summarizes values for kc and n that are the result of fitting eq 3 or 4, depending on the value of n, to the experimental data and setting Ceq to the known

Figure 4. Amount of metallic ion in solution, Cu2+, Pb2+, Ni2+, and Co2+, in the adsorption process by thermally pretreated fish scales as a function of squared time. Measurements at 25 °C and pH ) 5-6.

equilibrium experimental value. The “global” kinetics coefficients stand in the order

kc,Pb > kc,Cu > kc,Co > kc,Ni

for n ) 1

(5)

kc,Cu > kc,Pb > kc,Co > kc,Ni

for n * 1

(6)

The better adjustment of the equations corresponds to orders different from 1, suggesting that the actions of more than two phenomena are involved in the global process. The nth order different from 1 is clear, because the adsorption process may be due to two main mechanisms additional to the mass-transfer phenomena, binding through a chelating action with the ligands of the proteins,21 or/and ion interchange with the calcium of the HAP.25 In the case of Pb2+ that has a higher value of n, this could be explained by a combination of three possible removal mechanisms including surface precipitation as hydroxypyromorphite or another lead solid phase, as suggested by Lower et al.31 Alternatively, the parabolic diffusion equation of the form8,30,32

X/Xeq ) kDt1/2

(7)

where X is the adsorption of the solute on the adsorbent at a certain time t and kD is the overall diffusion coefficient, was tested in terms of the concentration in the solution

C ) C0 + KDt1/2

(8)

where C0 is the initial ion concentration and KD is the “overall” diffusion coefficient that takes into account the initial and equilibrium concentrations. This equation may be useful to get insight of the limiting step in the adsorption process of metals by bioadsorbents.8,30,32 Figure 4 is a plot that represents the mathematical transformation of the data, where time has been squared, and the solid lines represent the fit of the equation to the experimental data. Because eq 8 describes the data satisfactorily, it suggests that the intraparticle diffusion is the rate-limiting step in the “global” adsorption process, as stated by Weber and Morris,32 when this linear relation is present. Values for the “global” intraparticle diffusion coefficients, KD, for Cu2+ and Pb2+, are summarized in Table 3, and they are compared to those values reported for

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Figure 5. Low-vacuum SEM views of the surface (part a) and cross section (part b) of thermally pretreated fish scales after being exposed to an adsorption process of Cu2+. Table 3. Values of the “Global” Intraparticle Diffusion Coefficients, KD, for the Cu2+ and Pb2+ Metallic Ions Adsorbed by Thermally Pretreated Fish Scalesa

Table 5. Atomic Elemental Analysis of Thermally Pretreated Fish Scalesa regions of pretreated fish scales (atom %)

KD, mmol/(g of adsorbent h0.5) chitin chitosan porgy flounder cod Tilapia

Cu2+

Pb2+

element

surface dark region

0.79 2.31 2.00 0.42 0.31 4.31b

0.26 0.28 0.32 0.49 1.09 0.57b

C O Ca S P Ca/P

70.1 29.6 0.11 0.11 0.05 2.2

a These values are also compared to the corresponding K values D reported for other materials in ref 8. b This work.

Table 4. Metallic Ion Initial and Final Concentrations of an Aqueous Solution of Cu2+, Pb2+, Co2+, and Ni2+ Ions, for Pure Compound Solutions and for a Common Solution, That Have Been Subjected to Adsorption Experiments with Pretreated Fish Scalesa parameters

Cu2+

Pb2+

Co2+

Metallic Ion from Pure Compound Solutions Xeq, mg/g 30.6 29.6 15.8 selectivity: RCu2+/M2+ 1.0 1.03 1.94 Metallic Ion from a Common Solution Xeq, mg/L 20.7 19.9 7.3 selectivity: RCu2+/M2+ 1.0 1.04 2.86

Ni2+ 15.4 2.0 3.7 5.60

a The last row of each section indicates the selectivity calculated according to an equilibrium adsorption ratio. b Selectivity: RCu2+/M2+ 2+ M2+ ) XCu eq /Xeq .

chitin, chitosan, porgy, flounder, and cod fish scales.8 The adsorbent materials from “Mojarra Tilapia” studied in this work compete kinetically very favorably with chitosan which presents the fastest overall diffusion coefficient for Cu2+, and in the case of the Pb2+ ion, it is only surpassed by cod, which presents a much higher response than the rest; nevertheless, the Tilapia fish scales have a higher coefficient than the rest, especially in the case of Cu2+. Table 4 presents values for the adsorption selectivity shown by thermally pretreated fish scales when the equilibrium amount of M2+ adsorbed by the material is

surface white region

cross section

58.9 40.5 0.47 0 0.08 5.88

36.3 46.9 10.6 0 6.2 1.71

a Values are based on atom percentage free of nitrogen and other elements present in relatively very low concentrations.

used to define selectivity. When pure metal solutions are tried in the adsorption experiments, the selectivity stands in the order Cu2+ > Pb2+ > Co2+ > Ni2+. As is also seen in Table 4, the same trend is observed when all of the ions are present in the same solution and compete to take an adsorption site and/or interchange their ions. It is interesting to observe that the ions with the largest size are not only removed faster than the smallest ions but they are also removed in larger quantities. Low-Vacuum Scanning Electron Microscopy (SEM). Low-vacuum SEM and elemental analysis were used to get insight about the morphology and learn about the adsorption and interchange process by thermally pretreated fish scales. Surface and cross-sectional views of thermally pretreated scales that have been subjected to a 24 h adsorption process are presented in Figure 5. The corresponding elemental analysis of the pretreated scale before adsorption is summarized in Table 5. Fish scales as seen in the micrographs are characterized by having two regions, one being white and the other being darker. The white region is rich in inorganic material containing high proportions of calcium and phosphorus, as seen in Table 5, whereas the dark region is rich in proteins, because it has a high proportion of carbon, oxygen, and sulfur. From the elemental analysis before and after

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Table 6. Elemental Analysis Atomic Percentage of Pure Ions Adsorbed from Individual Solutions and from a Common Solution Including Basic Elements in Pretreated Fish Scalesa

previous studies, is that the main removal of metallic ions in the fish scales is mainly due to the ion-exchange reaction with HAP, more than the ligand binding with the proteins.

Pure Compound Solutions atom % surface region

Conclusions cross section

element

dark

white

dark

white

Cu Ni Co Pb CAvg SAvg PAvg Ca/PAvg

0.31 0.1 0.03 0.24 71.6 0.28 0.29 1.81

1.41 0.1 0.43 0.25 56.9 0.23 2.95 1.37

0.53 0.08 0 0 40.3 0.07 2.82 1.48

0.3 0.4 0.72 0.49 37.0 0.04 4.38 1.61

surface region

dark

white

0.15 0 0 0.67 74.0 0.25 0.53 0.25

0.99 0.07 0.4 0 51.9 0 2.5 2.13

1.35 0.23 0.38 0.34 37.1 0 5.97 1.89

Common Solutions Cu Ni Co Pb CAvg SAvg PAvg Ca/PAvg

a Values are based on atom percentage free of nitrogen and other elements present in relatively very low concentrations.

adsorption (Tables 5 and 6), it may be interpreted that fish scales have a larger proportion of protein on the surface than in the interior, and there is a higher proportion of inorganic material on the inside. After the adsorption process has been completed, the elemental analysis in Table 6 shows that the metallic ions tend to concentrate more on the white regions, where the original Ca/P ratio is diminished and the M2+/P ratio is increased. In some areas the difference between the Ca/P ratio before and after adsorption is approximately equal to the corresponding increase in the M2+/P ratio, supporting the ion interchange mechanism. When pure ions are in solution they tend to penetrate the structure of the adsorbent as seen in Table 6, being present more on the inside than on the outside of the material with the exception of copper but preferably in higher proportion in the high mineral content zone. When there are competing ions present in solution, as is also seen in Table 6, the smaller ions tend to diffuse into the adsorbent and are found in the internal mineral part of the scale, while the larger ions tend to deposit on the surface. Lead deposits preferably on the surface of the material forming superficial aggregates, again suggesting surface precipitation, although some internal presence is also observed on the mineral zones. Copper, when competing with other ions, deposits in lesser amounts on the surface than internally but always in higher proportion where the mineral material is more abundant. It seems that in this process, where there are two competing mechanisms, ligand binding and ion interchange, the latter may be more favorable because, for the binding of the metal on the proteins, more than one ligand is needed and, for the ion interchange, only one calcium ion is needed for each metallic ion removed. Therefore, one conclusion that may be established in this work, which explains some of the unclear points in

Thermally pretreated fish scales, from the variety known as “Mojarra Tilapia”, show an experimental maximum capacity for the removal of Cu2+ of Xm ) 58 mg of Cu2+/g of adsorbent. This variety contains 49 wt % inorganic matter, with the rest being organic matter. When both the organic and inorganic fractions are separated from the fish scales, the inorganic fraction, mainly composed of HAP, of the fish scales has a 75% higher adsorption capacity for Cu2+ than the organic fraction, mainly composed of keratin, as judged by the value of X when pure mineral and organic material are used in the adsorption experiments. When fish scales with different organic/inorganic ratios are used in the adsorption experiments, the results show that there is a synergistic effect when both the organic and mineral fractions are present in the fish scale and, interestingly, the amount of Cu2+ adsorbed is independent of the organic or mineral fraction between 30 and 90 wt % of the mineral component. There is a linear relationship between the amount of Cu2+ adsorbed/g of fish scales and the initial concentration of Cu2+, which is independent of the amount of mineral or organic fraction. Additionally, the adsorbent materials from “Mojarra Tilapia” studied in this work compete kinetically very favorably with chitosan for Cu2+ adsorption, and for Pb2+, it is only surpassed by cod fish scales. The overall kinetics of the adsorption in this material can be well represented with a generalized power-law equation where n * 1. Low-vacuum SEM carried out with pretreated fish scales before and after removal, supported with elemental analysis, evidences that the metallic ions studied in this work tend to concentrate more in the inorganic fraction, mainly HAP, than in the organic fraction, mainly keratin. Thus, the main removal of metallic ions in the fish scales is due to an ion-exchange and reaction process with HAP rather than the ligand binding with the proteins. The metallic ion adsorption selectivity shown by fish scales stands in the order Cu2+ > Pb2+ > Co2+ > Ni2+ and is the same as those determined for a mixture containing the same ions in solution. Acknowledgment J.F.V.-E. thanks Universidad Iberoamericana, UIA, for the opportunity to carry out his experiments in its laboratories. The authors are thankful for the lowvacuum SEM characterization support from Instituto Nacional de Investigaciones Nucleares. Literature Cited (1) Chanda, M. Chromium(III) Removal by Epoxy-Cross-Linked Poly(ethylenimine) Used as Gel-Coat on Silica. 1. Sorption Characteristics. Ind. Eng. Chem. Res. 1997, 36, 2184. (2) Kawamura, Y.; Mitsuhashi, M.; Tanibe, H.; Yoshida, H. Adsorption of Metal Ions on Polyaminated Highly Porous Chitosan Chelating Resin. Ind. Eng. Chem. Res. 1993, 32, 386. (3) Rivas, B. L.; Maturana, H. A.; Ocampo, X.; Peric, I. M. Adsorption Behavior of Cu2+ and UO22+ Ions on Crosslinked Poly[2,2-(Acrylamide)Acetic Acid]. J. Appl. Polym. Sci. 1995, 58, 2201.

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Received for review October 11, 2000 Revised manuscript received May 30, 2001 Accepted May 30, 2001 IE000884V