5310
Ind. Eng. Chem. Res. 2004, 43, 5310-5317
Use of Chromium-Collagen Wastes for the Removal of Tannins from Wastewaters Kalarical Janardhanan Sreeram, Subramani Saravanabhavan, Jonnalagadda Raghava Rao, and Balachandran Unni Nair* Chemical Laboratory, Central Leather Research Institute, Adyar, Chennai 600 020, India
The ability of chromium-collagen compoundsschromium shavings from the leather industrys for the removal of vegetable tannins from mixed effluents containing tannins and chromium has been studied. The experimental equilibrium data for the tannin-shavings system has been analyzed using the linearized forms of Langmuir, Freundlich, and Scatchard isotherms. The Freundlich isotherm was found to provide the best theoretical correlation of the experimental data for the adsorption of tannin. The 1/n value for tannin adsorption was found to be 0.799, indicating the suitability of the material for adsorption process. A theoretical model evaluated in the study provided sufficient correlation and enabled development of parameters such as treatment time, sorbent weight, etc., required for maximum adsorption. The tannin adsorbed shavings were used for the preparation of chromium(III) sulfate. The chromium left behind after near complete removal of tannins was recovered by precipitative techniques and subsequently redissolved in sulfuric acid to generate chromium(III) sulfate. The prepared and recovered chromium(III) sulfate on use in tanning process gave results similar to those of conventional chromium(III) salts, thereby providing a new methodology for the reuse of waste products of the leather industry directly into the leather industry itself. Introduction The hazards to human life from the effects of various metal ions have received extensive attention in the past 20 years. The effect of chromium on the land, water, and mammalian life has been well documented.1-3 Use of chromium for transforming animal hide or skin into leather is followed by more than 90% of the leather industry worldwide.4 During the chromium tanning process, the product of which is called “wet-blue”, stable and inert polynuclear chromium-collagen complexes are formed. The next step in the industrial process is to equalize the thickness of the wet-blue, and to cut uneven parts. As a result, large amounts of shavings and cuttings are produced and rejected; up to 40% of product is rejected at this stage, meaning that hundreds of thousands of tons are released every year into the environment.5 The chromium present in these byproducts is in the 3+ state and is considered harmless. Under noncontrollable conditions, however, it can conceivably be oxidized to the mutagenic chromium(VI).6 Guesstimates for solid wastes containing chromium are about 50 000 metric tons/annum in the United States alone, which includes shavings, trimmings, and splitting wastes.7 From this it is estimated that about 0.8 million tons of chrome shavings could be generated per year globally. Decomplexation into chromium(III) and collagen requires strong chemicals.8 This has forced the leather industry to revert back to the age-old practice of using vegetable tannins (plant polyphenols having molecular weight between 500 and 3000 Daltons).9 However, due to constraints in achieving desired color, strength, and softness, the industry worldwide has resorted to the partial substitution of chromium for vegetable tannins.10 During the tanning process, the * To whom correspondence should be addressed. Tel.: +91 44 2441 1630. Fax: +91 44 2491 1589. E-mail:
[email protected].
tannin fraction is taken up faster than the nontannins, but not completely.11 The effluent discharge from these combination tanning systems contains about 3000 mg/L of chromium(III) and 6000 mg/L of tannins.12 This creates environmental problems owing to the high content of organic matter, coupled with chromium and sulfate ions, discharged in the wastewaters. While the recovery of chromium from such wastewaters is of economic significance to the leather industry, the recovery of vegetable tannins is insignificant as the tanning process results in decreased tannin to nontannin ratio.13 Even though the chromium recovery processes have been well documented and practiced, the presence of vegetable tannins hampers the recovery process, as they are known to complex with metal ions including chromium.14 This results in the coprecipitation of tannins along with chromium during the precipitative recovery of the metal ion from such wastewaters, thereby contaminating the recovered chromium.12 This calls for a predominant removal of either the tannins or chromium from such wastewaters. Adsorption processes have been recognized as effective and economically viable for the removal of pollutants from wastewaters. The widely used activated carbon based processes are expensive, and alternative inexpensive adsorbents from industrial wastes and naturally occurring materials are being investigated.15 In this direction, the chromium-bearing leather wastes such as shavings are a possible choice due to the availability of large numbers of groups such as -CO-, -CO-NH-, and -NH2, etc., which can interact with tannins in the wastewater. Further, the binding of chromium ions, which is reported to be to the -COOH groups in the protein, results in a predominantly positive charge on the surface of protein.16,17 Thus, the chrome tanned shaving dusts are expected to have a
10.1021/ie034273p CCC: $27.50 © 2004 American Chemical Society Published on Web 07/07/2004
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5311
very strong affinity to the oppositely charged vegetable tannins, resulting in a predominant uptake of tannins. The predominant removal of tannins would then enable the recovery of chromium from wastewaters by conventional precipitation methodologies.18,19 As desorption of the tannins from the adsorbent is of no commercial significance to the leather industry, economically viable alternatives for use of such adsorbents are essential. In this direction, the use of chromium-bearing leather wastes, rich in organic matter, as reductants for chromium(VI), in the preparation of chromium(III) salts for tanning, offers promise.20 This study reports the use of chromium shavings as potential adsorbents for vegetable tannins from mixed effluents containing chromium and vegetable tannins, followed by the use of the adsorbent for manufacture of chromium(III) salts used in leather processing. This work would be of significance especially in developing countries where the concept of leather complexes would provide for the coexistence of waste generators and chemical factories, which can use these wastes. Theoretical and experimental investigations to determine the nature and efficiency of the adsorptive process have been carried out. A theoretical model which would enable the quantification of the weight of sorbent and time required for optimal sorption based on the concentration of tannin in the wastewater has also been evaluated and reported. This work aims at a three-part solution consisting of using a waste (chromium shavings) as adsorbent for tannins present in a chromiumtannin wastewater, followed by recovery of tannin free chromium(III) for reuse in tanning, and use of tannin adsorbed shavings as reductant for chromium(VI) in the manufacture of chromium tanning agents. Experimental Methods Materials. The adsorbent, chromium tanned wastes generated as shaving dust, was obtained from a commercial tanning unit in India. These wastes are currently used in the manufacture of leather boards or disposed of as landfill materials. Mixed effluent containing chromium and vegetable tannins was obtained from five different tanning units converting vegetable-tanned leathers into finished products after re-tanning them using chromium(III) salts. The tannin and chromium content in these wastewaters have been quantified to determine the working range of concentrations. Characterization of Adsorbent: Shaving Dusts. The surface area of the adsorbent was determined using standard nitrogen adsorption porosimetric technique employing the BET method in a Quantachrome NOVA automated gas sorption system as described earlier.21 The pore radius of the adsorbent was also measured using this instrument by employing the BJH method.21 The adsorbent was also characterized for its apparent density, moisture content, nitrogen content, and chromium content by standard procedures.22 Characterization of Chromium-Vegetable Tannin Mixed Effluent. The chromium-tannin mixed effluent sourced from tanning units was filtered through a Whatmann No.1 filter paper and stored in glass containers at pH 3.5 till further use. The pH of the solution was noted and analyses were done for the determination of tannins, chemical oxygen demand (COD), and total chromium by standard procedures.22,23 Adsorption Studies. The studies were carried out using chromium shaving dusts as adsorbents which
were equlibriated in water (1:10) overnight. Next day the free water was squeezed out prior to adsorption studies. Batch mode adsorption studies were carried out by shaking predetermined quantities of the adsorbent (25-200 g/L on dry weight basis) and effluent (100 mL) of varying concentration in 150-mL stoppered glass bottles. At the end of a predetermined time interval a small aliquot was taken, the contents were centrifuged, and the supernatant was analyzed for tannin content using Folin-Ciocalteu agent.23 The amount of tannin adsorbed was calculated from the difference in the tannins remaining in solution and the initial concentration. Studies on the initial tannin concentration (6006000 mg/L) and duration of the treatment (15-180 min) were also carried out. The results reported in this work are an average of five different measurements. Isotherm studies were conducted by bringing a fixed mass of chromium shavings (10 g) in contact with 100 mL of chromium-tannin mixed wastewater in a 150mL stoppered bottle. Desired concentration of tannin was achieved by dilution of the mother liquor. The initial pH of the solutions was adjusted to 3.5 ( 0.1 by the addition of 0.5% hydrochloric acid wherever necessary. The flasks were sealed and agitated at 20 rpm in a shaking apparatus at a constant temperature of 25 ( 1 °C until equilibrium was reached. At time t ) 0 and equilibrium, the tannin content left in the solution was measured. These data were used to calculate the adsorption capacity, qe, of the adsorbent. Finally the adsorption capacity was plotted against the equilibrium concentration, Ce. The tannin concentration at equilibrium, qe, was calculated from
qe )
(Co - Ce)V Ws
(1)
where qe is tannin concentration in sorbent at equilibrium; Co is initial tannin concentration in liquid phase; Ce is liquid-phase tannin concentration at equilibrium; V is total volume of tannin solution used; and Ws is mass of the sorbent used. The experimental values were also analyzed to determine the intraparticle diffusion model. A theoretical model24 was evaluated for determination of the percentage tannin removal for known values of tannin concentration, weight of adsorbent, and time of treatment. Bulk Trials. Three trials were run for treating 5 L of commercial wastewater containing chromium along with vegetable tannins. Chromium-bearing shaving (750 g) was taken in a perplex drum which rotated at 15 rpm along with the wastewater and the adsorption was allowed to proceed for 180 min. The shavings were separated out after the process by passing through sieves, and the vegetable tannin content in the treated effluent was determined. The treated effluent as well as the chromium shaving containing the adsorbed vegetable tannins were used for reuse studies. Reuse Studies. The tannin-free chromium-bearing solution was treated with sodium carbonate till pH was 8.0-8.5 to precipitate chromium as chromium(III) hydroxide. The precipitate was allowed to settle overnight, after which the supernatant was decanted and the chromium content in it was estimated. The residue of chromium(III) hydroxide was treated with sulfuric acid till pH 2.8, left overnight, and the basicity of the chromium(III) sulfate was estimated by standard procedures.25 The chromium(III) sulfate solution was used
5312 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
in tanning as a 40% replacement for basic chromium sulfate (BCS) salt as per conventional procedures, and the leathers were evaluated against control leathers tanned using conventional chromium salts.26 The chromium-bearing shaving dusts containing the adsorbed tannins were used as reductant in the manufacture of basic chromium sulfate salts as per a procedure previously reported.20 The generated basic chromium sulfate was evaluated for use in tanning of sheepskins and compared against conventional chrometanned leathers (control).
Table 1. Characteristics of the Adsorbent and Adsorbate parameter
value
adsorbent: chromium tanned shaving dusts specific surface area 3.6786 m2/g average pore radius 32.8 Å apparent density 1.4 ( 0.2 g/mL moisture content 13 ( 2% Cr2O3 2.63 ( 0.2% adsorbate: chromium-tannin mixed effluent pH 3.5 ( 0.3 COD 8056 ( 200 mg/L total chromium as Cr 2760 ( 30 mg/L tannins 6000(25 mg/L
Results and Discussion The chromium-tanned shavings were characterized for their chromium content, surface area available for adsorption, the average pore radius, etc. These values are presented in Table 1. Even though the surface area available for adsorption is lower than that of several other reported adsorbents, the adsorbent under study is a tanned collagen molecule. The triple helical structure of collagen is composed of polypeptide chains of molecular weight 100 000 Daltons each, held together through hydrogen bonds.27 Each of the polypeptide chains has 1052 residues, with amino, carboxyl, and other hydrophobic functionalities.28 The various peptide and amino groups in collagen provide adequate sites for adsorption of vegetable tannins. The pretreatment of the adsorbent with chromium(III) salts during the process of tanning blocks the carboxyl groups, which are required for the binding of chromium onto the fibers.16 In addition, the lowering of pH of collagen during the chromium tanning process has a charging effect on the basic groups of collagen, which are involved in bonding to vegetable tannins through hydrogen and van der Waals forces.17 A comparison of the characteristics of the mixed chromium-tannin effluents over a spectrum of five tanneries indicates varying concentrations of tannins from 500 to 6000 mg/L depending on the nature of leather product manufactured. An ideal case of 6000 mg/L was chosen for further studies. Effect of pH on the Removal of Tannins. The chromium-tannin effluents collected from different tanneries had a pH of 3.5-4.0. To study the influence of pH on the removal of tannins and chromium, the pH of the effluent was varied from 1.5 to 4.5. It can be seen from Figure 1 that the vegetable tannin removal increased with pH. The commonly used mimosa tannin is a condensed tannin with polymeric structure containing four flavanoids units, typified by the formula given below.29
Figure 1. Effect of pH on the removal of tannins by chromium shavings.
The ‘A’ ring of mimosa tannin serves as very reactive nucleophiles and the ‘B’ ring provides excellent complexation with biopolymers such as collagen. Mimosa tannin at the pH investigated is anionic14 and hence complexes instantaneously with cationic chromium shavings. The complexation of chromium with polyphenols could influence the solubility of chromium and phenol through (a) formation of an uncharged complex from the metal cation, (b) the reduction of polarity of tannin molecule, and (c) formation of high molecular weight complexes. It has been reported that the metal ions catalyze o-dihydroxybenzene oxidation by molecular oxygen at pH 4.5-5.5 at which uncatalyzed autoxidation does not occur. The resulting quinones or semiquinones may polymerize and precipitate with the metal ions.14 Thus, to avoid the precipitation of chromium as well as the formation of quinones and semiquinones it is essential to carry out the adsorption at pH below 4.5. As the observed removal of tannins is around 83% at pH 3.5 (pH of the wastewater), the same has been adopted for further studies. At this pH the tannin removal predominated over that of chromium removal (24%). Effect of Initial Concentration of Tannins and Treatment Period. The removal of tannins by chromium-containing collagen fibers is related to the concentration of tannin content. When the concentration of tannins was varied from 600 to 6000 mg/L, the percentage uptake of tannin decreased from 95 to 90% at 180 min of treatment (Figure 2). The initial uptake of tannins was predominantly fast, reaching about 85% within 30 min of the treatment and reaching an equilibrium value of around 95% at 180 min. No observable change in tannin removal was noticed after 180 min in the entire concentration range selected for investigation in this study.
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5313
Figure 4. S-type adsorption isotherm of vegetable tannins onto chrome shavings. Figure 2. Plot of time versus tannin removal for varying concentrations of tannins: -9-, 600; -b-, 1200; -2-, 2400; -1-, 3600; -[-, 6000 mg/L.
ing 100 g/L adsorbent. Adsorption isotherms thus obtained (Figure 4) are classified as S-type according to the Giles classification system.30 Langmuir’s model of adsorption depends on the assumption that intermolecular forces decrease rapidly with distance and consequently predicts the existence of a monolayer coverage of the adsorbate at the outer surface of adsorbent.31 Further, it is assumed that once a tannin molecule occupies a site, no further adsorption can take place at that site. Moreover, the Langmuir equation is based on the assumption of a structurally homogeneous adsorbent, where all sorption sites are identical and energetically equivalent. The Langmuir adsorption can be used to calculate maximum adsorption capacity Qo (mg/g) and energy of adsorption b (L/ mg) for a given Ce (mg/L) and qe (mg/g), through the expression
Ce Ce 1 + ) qe Qob Qo Figure 3. Plot of tannin removal versus time for various adsorbent-to-adsorbate ratios: -9-, 25; -b-, 50; -2-, 75; -1-, 100; -[-, 150; -left-facing solid triangle-, 200 g/L of adsorbent.
Effect of Weight of Sorbent on Tannin Removal. As the ratio of sorbent to the sorbate was increased from 25 to 200 g/L, the percentage removal of tannins increased from 64 to 98% at 180 min of treatment (for Co ) 3000 mg/L) (Figure 3). Such behavior is expected since the total surface area increases as the amount of sorbent increases and as a result more of the tannin is adsorbed. This trend was found to hold good at all concentrations of tannins investigated in the study. However, the weight of the sorbent needs to be optimally chosen based on the concentration of tannins in the wastewater. For wastewaters with lower tannin concentration, with higher treatment time it is possible to achieve high removal efficiencies at lower adsorbent dosage. Adsorption Isotherms. Adsorption isotherms describe how adsorbates interact with adsorbents and so are critical in optimizing the use of adsorbents. In the present study, the relationship between equilibrium tannin concentration in liquid phase (Ce) and that in the sorbent (qe) was obtained at pH 3.5 for varying concentrations of tannins (600-6000 mg/L) and employ-
(2)
A plot of Ce/qe versus Ce was made. The linearity and correlation coefficients (r2 ) 0.847) do not sufficiently support a Langmuir model of sorption. The values of Qo and b were found to be 111.76 mg/g and 9.5 × 10-4 L/mg, respectively, which is much lower than the values reported for dyes onto similar natural materials such as chitosan.32 Further, the shape of the isotherm does not support a Langmuir model.30 The equilibrium adsorption data were also fitted to the Freundlich and Scatchard isotherm equations given by the expressions
qe ) KFCe1/n
(3)
qe ) KS(Qo - qe) Ce
(4)
and
where KF and 1/n are the Freundlich constants related to the adsorption capacity and intensity of adsorption, and Qo and KS are the Scatchard constants related to the maximum monolayer adsorption capacity and equilibrium sorption constants, respectively. The Scatchard isotherm represents an intermediate situation closer to the Langmuir model. A comparison of the isotherm constants along with regression coefficients is pre-
5314 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 Table 2. Adsorption Isotherm Constants Derived from Various Models along with the Normalized Deviation Factorsa Langmuir Model Q0
(mg/g)
111.8
r2
HYBRID Values
0.847
30.950
b (L/mg) 10-4
9.49 ×
Freundlich Model Kf (mg/g)
1/n
r2
HYBRID Values
0.243
0.799
0.998
23.612
Scatchard Model Q0 (mg/g) 113.2
Ks 9.4 ×
10-4
r2
HYBRID Values
0.795
31.507
a
Weight of sorbent, 100 g/L; time, 180 min. Reported values are an average of five measurements. Figure 6. Intraparticle diffusion kinetics of tannin onto chrome shaving at various initial concentrations of tannin: -9-, 600; -b-, 1200;, -2-, 2400; -1-, 3600; -[-, 6000 mg/L. Table 3. Effect of Initial Tannin Concentration on the Sorption and Intraparticle Diffusion Kineticsa experimental values C0 (mg/L)
Ce (mg/L)
qe (mg/g)
ki (mg/(min)0.5)
ri2
600 1200 2400 3600 6000
55.2 107 240 492 816
5.44 10.93 21.60 31.08 51.84
0.0495 0.1224 0.3155 0.4845 1.0084
0.972 0.955 0.906 0.955 0.935
a
Figure 5. Comparison of experimental and model fits of various isotherms for the adsorption of vegetable tannins onto chrome shavings: -9-, experimental; -b-, Langmuir; -2-, Freundlich; -1-, Scatchard.
sented in Table 2. The Kf and 1/n values obtained from the Freundlich isotherm were found to be 0.2430 and 0.799, respectively, with an r2 value of 0.998. The shape of the isotherm, r2 values, and the heterogeneity factor (1/n) seem to support a Freundlich model for adsorption.33 This has further been substantiated through error analysis. The hybrid fractional error function31 (HYBRID) which included the number of degrees of freedom of the systemsthe number of data points, n, minus the number of parameters, p, of the isotherm equationsas the divisor was chosen for analysis. The equation is given as
100
n
∑
n - pi)1
[
]
(qe,meas - qe,calc)2 qe,meas
(5) i
The values of all the isotherm constants and their HYBRID function values are presented in Table 2. The HYBRID values further substantiate the Freundlich isotherm as the best-fit model for describing the adsorption of tannin on the chromium shavings. By substituting the determined values of the isotherm constants and inputting the values of Ce, the values of qe were calculated and a plot of qe versus Ce was made for both experimental and theoretical values (Figure 5). The plot indicates a sufficient correlation between the experimental and theoretical values.
intraparticle diffusion values
Reported values are an average of five measurements
To further analyze the mass transport mechanisms involved in the sorption process, a simple model reported earlier was used.34 For the rate constant of intraparticle diffusion process,
qt ) kit0.5
(6)
where ki is the intraparticle diffusion rate constant (mg/g min0.5). The ki values under various concentration ranges were calculated from the slopes of the straight line plots of qt against t0.5 (Figure 6) and are presented in Table 3. From the figure it can observed that the straight line did not pass through the origin and this further indicates that the intraparticle diffusion is one of the rate controlling steps.35,36 This is possible as the pore volume of the adsorbent is around 64 Å. The reported size of the tannins is around 14-40 Å.37 However, during the process of tanning, the tannin molecules of lower size are taken up faster, leaving behind larger aggregates of 6 × 104 to 13 × 104 Å.37 These larger aggregates found in wastewaters could face problems diffusing into the pores of the adsorbent. The sorption data indicate that the adsorptive removal of the tannin from aqueous solution is a rather complex process, involving both boundary layer diffusion and intraparticle diffusion. The experimental data shown in Figure 3 was fitted to the relationship of the type
t ) a + bt R
(7)
where R is the percentage removal at time t and a and b are coefficients. From a plot of t/R versus t (Figure 7)
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5315 Table 4. Values of Coefficients of a and b with respect to Correlation Coefficient (r2) for Different Adsorbent Dosages (Ws) and Tannin Concentrations (Co) initial tannin concn Co (mg/L) 600 1200 2400 3600 4800 6000
25
50
adsorbent dose, Ws (g/L) 75
100
150
a ) 0.0866 b ) 0.0113 r2 ) 0.9999 a ) 0.1030 b ) 0.0117 r2 ) 0.9999 a ) 0.166 b ) 0.0132 r2 ) 0.9999 a ) 0.2055 b ) 0.0147 r2 ) 0.9999 a ) 0.282 b ) 0.0154 r2 ) 0.9995 a ) 0.310 b ) 0.0159 r2 ) 0.9999
a ) 0.0341 b ) 0.0110 r2 ) 0.9998 a ) 0.0706 b ) 0.0110 r2 ) 0.9997 a ) 0.096 b ) 0.0123 r2 ) 0.9999 a ) 0.1014 b ) 0.01319 r2 ) 0.9996 a ) 0.184 b ) 0.0141 r2 ) 0.9999 a ) 0.148 b ) 0.0148 r2 ) 0.9994
a ) 0.0240 b ) 0.0108 r2 ) 0.9999 a ) 0.0593 b ) 0.0109 r2 ) 0.9999 a ) 0.07 b ) 0.0112 r2 ) 0.9999 a ) 0.0800 b ) 0.0116 r2 ) 0.9999 a ) 0.1 b ) 0.0120 r2 ) 0.9999 a ) 0.1205 b ) 0.0123 r2 ) 0.9999
a ) 0.0208 b ) 0.0107 r2 ) 0.9999 a ) 0.03857 b ) 0.0106 r2 ) 0.9999 a ) 0.013 b ) 0.0103 r2 ) 0.9999 a ) 0.0677 b ) 0.0101 r2 ) 0.9999 a ) 0.084 b ) 0.009 r2 ) 0.9998 a ) 0.0903 b ) 0.0117 r2 ) 0.9999
a ) 0.0202 b ) 0.0106 r2 ) 0.9999 a ) 0.030 b ) 0.0108 r2 ) 0.9999 a ) 0.012 b ) 0.0107 r2 ) 0.9999 a ) 0.0519 b ) 0.0106 r2 ) 0.9998 a ) 0.06 b ) 0.0101 r2 ) 0.9999 a ) 0.066 b ) 0.0099 r2 ) 0.9999
Table 5. Values of Coefficients of e and f along with Correlation Coefficient (r2) for Different Adsorbent Dosages (Ws) adsorbent dose Ws (g/L)
e
f
r2
25 50 75 100 150
16760.51 14978.5 5432.04 5019.6 4878.1
63.39 69.93 84.88 85.74 87.28
0.9708 0.9713 0.9912 0.9972 0.9983
f correlate well with Ws through the relationships
Figure 7. Plot of time (t) versus ratio of time to % removal of tannins for various concentrations of tannins: -9-, 600; -b-, 1200; -2-, 2400; -1-, 3600; -[-, 6000 mg/L.
and using regression analysis, the values of a and b were computed, and these are presented in Table 4. The values of a tend to decrease with Ws for all Co. The average values of a and Ws can be related by the equation
a)
1 c + dWs
(8)
where c and d are coefficients. The regression analysis indicated a sufficiently good fit with a regression coefficient (r2) greater than 0.98. The values of c and d were found to be 3.569 and 0.23, respectively. From Table 4, it can be seen that the values of b increase with Co for all Ws. A linear regression based on the data of this study yields a relationship between b and Co.
b)
Co e + fCo
(9)
The values of e and f were determined for various Ws and are presented along with the coefficients of correlation (r2) in Table 5. This shows that the values of e and
e)
Ws g + hWs
(10)
f)
Ws i + jWs
(11)
where g, h, i, and j are the coefficients. Substitution of the eqs 8-11 in eq 7 yields the relationship of the form
t 1 + ) R c + dWs
C ot Ws Ws + C g + hWs i + jWs o
(
)
(12)
and by inputting the determined values of the constants c, d, g, h, i, and j, the equation takes the form t 1 ) + R 3.569 + 0.23Ws Cot Ws -4
-0.00451 + 2.19x10 hWs
+
(
)
Ws C 0.139815 + 0.009985Ws o
(13) Now by inputting values for t, Co, and Ws, the percentage removal of tannins from the solution can be computed. The model was tested for a range of concentrations, times, and weights of adsorbents. The model provides a reasonable agreement with the experimental values as seen from Figure 8a-c. The model (having been tested under real life effluent conditions) can directly be applied for industrial practice.
5316 Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004
at a temperature of 70 °C, till pH 2.8-3.0. The chromium(III) sulfate solution generated was aged overnight. No deposits were found, indicating a complete conversion into chromium(III) sulfate. The basicity of the solution was determined to be 33 ( 0.5%. The generated chromium(III) sulfate solution was used as a 40% replacement for commercial basic chromium sulfate salt. The uptake of chromium from the solution was comparable with that of conventional processing methods, indicating that the adsorptive process for selective tannin removal was efficient and the remaining polyphenolics did not have any influence on the color of the final leather. The leathers made in the study were processed into garment leathers and were found to have organoleptic and physicochemical properties matching that of conventional chromium-tanned leathers. Chromium(III) sulfate used in tanning is commercially produced by reduction of a dichromate salt with molasses or sulfur dioxide. We have previously shown that chromium-bearing wastes such as shavings can be successfully used as reductants in the place of molasses.20 In this study, the adsorbent bearing tannins has been used as a reducing agent for dichromate. Chromium(III) sulfate generated using chromium shavings containing adsorbed vegetable tannins were used as a 100% replacement for conventional commercial chromium(III) sulfate salts. The shrinkage temperatures as well as the uptake of chromium were similar to that of conventional chromium(III) sulfate tanning. The physicochemical properties of the final tanned leathers matched with those prepared using commercial chromium(III) sulfate salts. Conclusions
Figure 8. Comparison of predicted and observed values of tannins removal (a) for various concentrations of tannins, (b) for various adsorbent-to-adsorbate ratios, and (c) at various time intervals (-9- observed, -b- predicted).
Recovery and Reuse Studies. A 5-L portion of a mixed chromium-tannin effluent containing 6000 mg/L of tannins was subjected to adsorption by employing chromium-shaving dusts (150 g/L). The tannin removal efficiency was found to be 94 ( 1%. The tannin-free chromium effluent was treated with sodium carbonate till pH 8.0-8.5 and left overnight. The supernatant was decanted, and the chromium content in the same was found to be 0.8 mg/L. The generated chromium(III) hydroxide was subjected to treatment with sulfuric acid
The use of chromium-bearing leather shavings for adsorptive removal of tannins from chromium-tannin wastewater has been presented. Adsorption efficiency above 80% has been obtained even when the tannin concentrations are as high as 6000 ppm. The adsorption process follows the Freundlich model with the intraparticle diffusion process as the limiting factor. A model tested in this work provides for the optimization of sorbent and time requirements to achieve maximum tannin adsorption. The adsorbent after tannin sorption was effectively used in the manufacture of chromium(III) sulfate for tanning process. The chromium in the wastewater could be regenerated after precipitative recovery. The leathers made in the study by employing the prepared and recycled chromium salts had satisfactory properties, comparable to conventionally processed leathers. This work highlights the use of a solid waste generated in the leather industry to effectively remove a liquid waste and subsequently use the same for preparation of chemicals needed for the industry itself. Nomenclature 1/n ) Exponent in the Freundlich isotherm (dimensionless) a to j ) Constants used in the model study b ) Langmuir constant (L/mg) C0 ) Initial tannin concentration (mg/L) Ce ) Equilibrium solution phase tannin concentration (mg/ L) KF ) Freundlich equilibrium constant (mg/g) ki ) Intraparticle diffusion rate parameter (mg/(min)0.5) Ks ) Scatchard equilibrium constant n ) Number of experimental measures
Ind. Eng. Chem. Res., Vol. 43, No. 17, 2004 5317 p ) Number of parameters qe ) Mass of tannin adsorbed onto chrome shaving at equilibrium (mg/g) Qo ) Langmuir monolayer sorption saturation capacity (mg/g) qt ) mass of tannin sorbed at time t (mg/g) R ) Percentage tannin removal r2 ) Statistical correlation coefficient based on least squares best fit analysis of a straight line correlation t ) Contact time (min) V ) Solution volume (L) Ws ) mass of sorbent (g)
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Resubmitted for review March 4, 2004 Revised manuscript received March 4, 2004 Accepted May 28, 2004 IE034273P
