SAWDUST: COST EFFECTIVE SCAVENGER FOR THE REMOVAL OF CHROMIUM(III) IONS FROM AQUEOUS SOLUTIONS RASHID AHMAD Nuclear Chemistry Division, Pakistan Institute of Nuclear Science and Technology, P.O. Nilore, Islamabad, Pakistan (e-mail: [email protected])

(Received 14 April 2004; accepted 21 December 2004)

Abstract. Cr(III) ions sorption onto sawdust of spruce (Picea smithiana) has been studied thoroughly using radiotracer technique. Maximum sorption (94%) of Cr(III) ions (8.98 × 10−5 M) onto sorbent surface is achieved from deionized water in 20 min agitation time using 200 mg of sawdust. The sorption data followed the Freundlich, Dubinin-Radushkevich (D-R) and Langmuir isotherms. Freundlich constants l/n = 0.86 ± 0.07 and Ce = 85.0 ± 25.8 mmole g−1 have been estimated. Sorption capacity, X m = 0.82 ± 0.3 mmole g−1 , β = −0.00356 ± 0.00017 kJ2 mole−2 and energy, E = 11.9 ± 0.3 kJ mole−1 have been evaluated using D-R isotherm. The Langmuir constants Q = 5.8 ± 0.2 µmole g−1 and b = (7.4 ± 0.5) × 104 dm3 mole−1 have been calculated. The variation of sorption with temperature yields thermodynamic parameters H = −11.6 ± 0.3 kJ mole−1 , S = −16.2 ± 0.9 J mole−1 K−1 and G = −6.8 ± 0.3 kJ mole−1 at 298 K. The negative value of enthalpy and free energy reflect the exothermic and spontaneous nature of sorption respectively. Among the anions studied oxalate, citrate, carbonate and borate have reduced the sorption. The cations Y(III), Ce(II) and Ca(II) suppressed sorption. The sawdust column can be used to separate Cr(III) ion from Cs(I), I(I),Tc (VII) and Se (IV). Keywords: Cr(III) ions, kinetics, radiotracer technique, sawdust, sorption, sorption isotherms, thermodynamics

1. Introduction Chromium the most widely used metal helps in protecting materials from environmental degradation. About 80% of the mined chromium goes into metallurgical application. About 15% is used in chromium chemicals and the remaining is used in refractory application. Chromium compounds have wide application in leather tanning, dyes, wood preservation, welding batteries and catalysts (James et al., 1997). Although Cr(III) is nutritionally essential in trace quantities, however Cr(VI) have deleterious effects on eyes, liver, kidney, respiratory and gastrointestinal tract and skin. Lung cancer has been reported in workers dealing with chromate (Plunkett, 1987, p. 138). Water is being continuously contaminated with chromium and its compound from industrial effluents and mining run off. Chromium contamination of water bodies must be abated to save the plants and animals from the harmful effects of chromium especially in the hexavalent state. Various methods such as reduction Water, Air, and Soil Pollution (2005) 163: 169–183

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and precipitation, ion exchange, electrolysis and electroplating and adsorption have been used for the removal of chromium from water. Sorption is considered superior over other techniques because of its higher efficiency, low cost and simple operation (Hasany and Ahmad, 2003). Chromium, an important element from environmental point of view has been the subject of several studies wherein its accumulation on cheaper materials like flyashwollastonite (Pandy et al., 1984), zeolites (Foldesova et al., 2000), modified coconut husk (Low et al., 1997), rice husk (Khalid et al., 1999), Haro river sand (Hasany and Chaudhry, 1998), seed of leguminous crops (Voropanova et al., 1998) and modified rice husks (Lee et al., 1998) have been investigated. Chromium pollution is a serious problem facing our country due to the tannery industry. Therefore a comprehensive and low cost sorbent is needed for the removal of chromium. The stability of Spruce and Pinewood under water is from 50 to 100 years (Baraniak et al., 2002). Sawdust of spruce (Picea smithiana) is a byproduct of local sawmills abundantly available at a cost of US$ 20–30 per metric ton. Sawdust has proved a good sorbent for the removal of mercury from aqueous solutions (Hasany and Ahmad, 2002). Therefore, it was selected as sorbent for chromium separation. The results communicated in this paper are the continuation of our earlier work for the exploitation of low cost materials for pollution abatement. The aim of the work is to investigate the sorption capabilities of sawdust for chromium. 2. Materials and Methods 2.1. RADIOTRACER The radiotracer 51 Cr used in the present studies was prepared by irradiating specpure chromium metal in 10 MW swimming pool type research reactor (PARR-1) of this institute for 12 h at a neutron flux of 5 × 1013 n cm−2 s−1 . After suitable cooling time the irradiated metal was dissolved in concentrated nitric acid. In order to remove acid, the sample was diluted with deionized water and heated to dryness and this was repeated thrice. After the removal of acid the tracer was diluted to 10 mL with deionized water. The radionuclidic purity was checked on 4k series of 85 Canberra Multichannel analyzer coupled with a 25-cm3 Ge (Li) detector and was stored properly for further use. 2.2. REAGENTS The reagents used in the present study were of the analar grade. Buffer solution of pH 1–10 having ionic strength of 0.1 M were prepared by mixing an appropriate volume of 0.1 M solutions of HCl and KCl, CH3 COOH and CH3 COONa and H3 BO3 and NaOH for buffer solutions of 1–3, 4–6 and 7–10 respectively.

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2.3. SAWDUST The sawdust of Spruce (Picea smithiana) collected from a sawmill of Matta, Swat, Pakistan was thoroughly washed with deionized water, then dried for 8 h at 100 ◦ C. The dried sawdust was then sieved and 40 mesh size (0.42 mm φ) was used as sorbent. 2.4. SORPTION

MEASUREMENTS

5 cm3 of an electrolyte of known pH or acid concentration was taken in a glass culture tube with a polyethylene cap. A known concentration of Cr (III) tracer was added to it. An aliquot of 0.5 cm3 was taken for gross gamma counts (Ao ). The remaining solution was shaken with 200 mg of sawdust for 5 min on a Stuart Scientific Wrist-Action Shaker. The phases were separated by centrifugation for 3 min. After phase separation 0.5 cm3 of aliquot was again withdrawn for radioassay on a Tennelac gross gamma counter equipped with a 30 cm3 well-type Na (Ti) crystal. The sorbed concentration of chromium was calculated by the difference in the activity of aliquot outdrawn before (Ao ) and after shaking (Ae ). The percent sorption and distribution coefficient (Kd ) were calculated as Ao − Ae × 100 (1) Ao amount of metal in sawdust volume of solution (V ) × = (cm3 /g) (2) Kd = amount of metal in solution weight of dry sawdut (W )

% Adsorption =

The % sorption and the distribution coefficient, can be correlated mathematically as % Adsorption =

100K d K d + V /W

(3)

All the experiments were performed at 26 ± 2◦ C or at temperature specified otherwise. The linear regression analysis was used for slope and intercept evaluation and for statistical analyses of the data. The correlation coefficients for all the regression analysis were in the range of 0.98077–0.9996. The results were the average of at least triplicate independent measurements and precision in most cases is ±3%.

3. Results and Discussion For the sorption of chromium onto sawdust in aqueous solution various optimizing parameters for example effect of electrolyte, equilibration time and the amount of

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sorbent and sorbate were studied. The criterion for optimization was the selection of conditions where maximum sorption occurred. The effect of temperature, anions and cations were also studied. 3.1. EFFECT

OF ELECTROLYTES

The chemical nature of electrolyte, sometimes effects the properties of sorbent surface therefore, the sorption of chromium onto sawdust was studied in electrolytes such as deionized water, hydrochloric acid and nitric acid having the concentration range 0.001–0.1 M and buffer solution of pH 1–10. The concentration of Cr(III) ions, amount of sawdust and agitation time were chosen arbitrarily as 1.4×10−5 M, 100 mg and 30 min. The results are shown in Table I and Figure 1. It is clear from the results that maximum sorption occurred in deionized water while the sorption decreased with an increase in acid concentration. These results are in conformity with Hg (II) ions sorption onto sawdust (Hasany and Ahmad, 2002) and coconut husk (Hasany et al., 2003) and chromium onto rice husk (Khalid et al., 1999). The decrease in chromium sorption at higher acid concentration might be due to the competition of the proton and the positively charged chromium ions. The sorption of chromium increased with the increase in the pH of solution and maximum sorption was recorded at buffer solution of pH 7, after that it decreased. The Kd value increased and decreased accordingly. The sorption at near neutral pH should be due to the cellulose where site-binding sorption might occur. It might be also due to the surface complexation phenomenon, facilitated by dissociation of acidic functional groups (–COOH, –SH etc.) present on the sawdust (Mishra et al., 1997). Among all the electrolytes investigated deionized water was selected as the most effective sorption medium for further sorption studies.

TABLE I Sorption of Cr(III) (1.4 × 10−5 M) onto sawdust (100 mg) after 30 min agitation time Electrolyte

Kd (cm3 g−1 )

%Sorption

Deionized water 0.1 M HNO3 0.01 M HNO3 0.001 MHNO3 0.0001 MHNO3 0.1 MHCl 0.01 MHCl 0.001 MHCl 0.0001 MHCl

733 5 12 346 361 3 16 373 437

94.2 10 21.2 88.4 88.9 6.3 25.5 89.2 90.4

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Figure 1. Effect of pH on the sorption of Cr(III) ions onto sawdust.

Figure 2. Sorbed concentration of Cr(III) ions onto sawdust as a function of agitation time.

3.2. EFFECT

OF SHAKING TIME

The distribution of sorbate between sorbent and electrolyte is influenced by the agitation time. The effect of shaking time on sorption was studied between 1 and 60 min. The results are presented in Figure 2. The sorption increased with the increase in shaking time upto 20 min beyond which it attained almost a constant value. Therefore 20 min shaking time was considered to be sufficient for the sorption of chromium ions onto sawdust and was used for all experiments. The results of sorption data was subjected to Morris-Webber equation (Morris and Webber,

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1963) √ qt = R d t

(4)

where qt the adsorbed concentration at time, t, and Rd the rate constant of intraparticle transport, were tested by plotting qt against t1/2 , resulting in a straight line with a correlation coefficient of 0.9929 as shown in Figure 2. The value of Rd , rate constant of intraparticle transport is computed to be 9.7 n mole g−1 min−1/2 and 9.2 n mole g−1 min−1/2 was reported for the sorption of Hg(II) ions onto sawdust (Hasany and Ahmad, 2002). Moreover the sorption data were also evaluated by using Lagergren (Lagergren, 1889) equation log(qe − qt ) = log qe −

kt 2.303

(5)

where qe is the concentration of chromium sorbed at equilibrium and k is the overall rate constant. When log (qe − qt ) vs. t, was plotted a straight line with correlation coefficient of 0.992, was obtained. The value of first order rate constant k computed from the slope of plot is 0.15 min−1 . The Reichenberg equation (Reichenberg, 1953) was applied to check that sorption proceeds via film diffusion or intraparticle diffusion mechanism. Reichenberg equation was tested in the following way.
 6 F = 1 − 2 e−Bt (6) π where F = qt /qe and Bt is a mathematical function of F which can be calculated for each value of F as. Bt = −0.4977 ln (1 − F)

(7)

A plot of Bt vs. t is also shown in Figure 3, which is a straight line. It is clear from it that intraparticle diffusion is the rate controlling step with a small friction of the sorption that occurs through film diffusion because the plot does not pass through origin 3.3. INFLUENCE

OF AMOUNT OF SORBENT

The amount of sorbent affects the efficiency of sorption was studied in the range of 10–300 mg of sawdust by using optimal conditions of 20 min, agitation time from deionized water. The results are depicted in Figure 4. The sorption increased with increasing amount of sorbent and attained almost a maximum value at 200 mg and later it remained constant, while the distribution coefficient Kd decreased with an

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Figure 3. Lagergren and Reichenberg plots of Cr(III) ions sorption onto sawdust.

Figure 4. Influence of the amount of sorbent on the sorption of Cr(III) ions onto sawdust.

increase in the weight of sorbent. 200 mg of sawdust was chosen as an optimum amount of sorbent for further use. 3.4. EFFECT

OF AMOUNT OF SORBATE CONCENTRATION

The sorption of chromium was studied as a function of its own concentration of 320 folds in the range of 2.81×10−6 to 8.98×10−4 M using 200 mg of sawdust, 20 min shaking time and deionized water as a sorption medium. The percent sorption and distribution coefficient Kd started increasing up to 8.98 × 10−4 M and after that they decreased with the increase of chromium concentration. Similar trend was

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observed when sorbate concentration was varied in the case of Hg (II) ions sorption onto sawdust (Hasany and Ahmad, 2002). The sorption data was subjected to Freundlich, Dubinin–Radushkevich and Langmuir isotherms. All the isotherms were obeyed. The experimental results were fitted to the Freundlich isotherm (Freundlich, 1926) and give an empirical expression encompassing the surface heterogeneity, exponential distribution of active sites and infinite surface coverage. The Freundlich isotherm was followed over the entire range of concentration. Mathematically the linearized form of Freundlich sorption isotherm is log Cads = log Ce + 1/n log Ce

(8)

where Cads and Ce are the concentration of chromium ions at equilibrium sorbed onto sawdust (mole g−1 ) and in aqueous solution (mole L−1 ) respectively and 1/n and Cm are Freundlich constants. When log Cads was plotted against log Ce a linear plot with a correlation coefficient of 0.9807 was obtained as shown in Figure 5. The values of 1/n = 0.86 ± 0.07 the intensity of sorption and Ce = 85.0 ± 25.8 mmole g−1 the maximum sorption, were computed from the slope and intercept of the figure. The fractional value (0 < 1/n < 1) of the constant 1/n shows the heterogeneous nature of the surface. The higher the fractional value the higher is the heterogeneity of the surface and vice versa (Benes and Majar, 1980, p. 175). It is deduced from the higher value of constant 1/n that the surface of sawdust is heterogeneous in nature. To evaluate the nature of sorption the data was fitted to Dubinin-Radushkevich (D-R) isotherm (Dubinin and Radushkevich, 1947). This isotherm postulates

Figure 5. Freundlich sorption isotherm of Cr(III) onto sawdust.

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sorption within a sorption space close to sorbent surface. This model envisages about the heterogeneity of the surface energies. The D-R isotherm was tested in the following linear form ln Cads = ln X m − βε2

(9)

where Cads is the amount of sorbate sorbed by the sawdust (mole g−1 ), Xm is the maximum sorption capacity of sorbent (mole g−1 ) under investigation, β is a constant (kJ2 mole−2 ) related to energy and ε is polany potential which is mathematically equal to ε = RT ln (1 + 1/Ce )

(10)

where R is the gas constant in kJ mole−1 K−1 , T is absolute temperature in Kelvin and Ce is the equilibrium concentration of sorbate in solution (mole L−1 ). The plot of ln Cads versus ε 2 is linear or with a correlation coefficient of 0.9918 as shown in Figure 6. The D-R isotherm was obeyed over the entire range of concentration. The values of β and Xm computed from the slope and intercept of the plot are −0.00356 ± 0.00017 kJ2 mole−2 and 0.82 ± 0.3 mmole g−1 . The value of sorption energy E can be correlated to β by using the relationship (Hobson, 1969).  E = 1/ −2β

Figure 6. D-R sorption isotherm of Cr(III) onto sawdust.

(11)

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Figure 7. Langmuir sorption isotherm of Cr(III) ions onto sawdust.

The value of E sorption energy calculated was 11.9 ± 0.3 kJ mole−1 which is in conformity to 9.79 ± 0.5 kJ mole−1 for mercury sorption onto sawdust (Hasany and Ahmad, 2002) shows the chemisorption nature of sorption. The sorption data was also subjected to Langmuir sorption isotherm (Langmuir, 1918). The Langmuir isotherm assumes that the sorption is monolayer and the strength of the intermolecular attractive forces is believed to fall off rapidly with distance. The linearized form of Langmuir model is Ce /Cads = Ce /Q + 1/Qb

(12)

The Langmuir equation was tested by plotting Ce /Cads vs. Ce (Figure 7). A straight line with correlation coefficient of 0.998 was obtained. The data obeyed the Langmuir isotherm only at lower concentration (2.8 × 10−6 –8.98 × 10−5 M), which confirms the monolayer chemisorption deduced from the results of D-R isotherm. The Langmuir constants Q (5.8 ± 0.2 µmole g−1 ) is a measure of amount of metal sorbed when the monolayer is completed, and b (7.4 ± 0.5 × 104 dm3 mole−1 ) shows the sorption energy were calculated from the slope and intercept of the figure.

3.5. INFLUENCE

OF TEMPERATURE ON SORPTION

Temperature has a pronounced effect on sorption. In order to investigate the effect of temperature, the sorption of Cr (III) ions onto sawdust was studied in the temperature range of 278–318 K under optimized conditions. The sorption decreased with increase in temperature. In order to calculate the values of enthalpy H, entropy S and Gibbs free energy G log kc versus 1/K was plotted as depicted in Figure 8. As K c = F/(1−F) where

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Figure 8. Variation of sorption equilibrium of Cr(III) ions onto sawdust with temperature.

F is the fraction sorbed at equilibrium while K is the temperature in Kelvin. The plot of log kc versus 1/K is a straight line with a correlation coefficient of 0.9979. The thermodynamic parameters, H = −11.6 ± 0.3 kJ mole−1 , S = −16.2 ± 0.9 J mole−1 K−1 and G 298 = −6.8 ± 0.3 kJ mole−1 were calculated from the slope

TABLE II Effect of anions on the sorption of Cr(III) (8.98 × 10−5 M) ions onto sawdust (200 mg) after 20 min agitation time from deionized water Anions

Kd (cm3 g−1 )

% Sorption

Nil Fluoride Thiocynate Sulfate Iodide Nitrate Chloride Acetate HEDTA Ascorbate Tartarate Chromate Oxalate Citrate Carbonate Borate

373 317 275 272 237 211 208 183 144 94 25 20 11 3 2 2

94.3 93.4 92.4 92.4 91.3 90.4 90.2 89.1 86.5 80.7 51.7 47.4 32.6 12.7 6.6 8.3

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TABLE III Influence of cations on the sorption of Cr(III) (8.98 × 10−5 M) ions onto sawdust (200 mg) after 20 min agitation time from deionized water Cations

Kd (cm3 g−1 )

% Sorption

Nil

373

94.3

Na(I)

211

90.4

Zr (IV) Ag (I) Kb (I) Sr(II) Ni (II) Zn(II) Co (II) Fe (III) Al(III) Pb (II) Y(III) Ce b (III) Ca(II)

199 149 145 97 84 79 76 45 31 25 18 16 8

89.8 86.8 86.5 81.1 78.8 77.7 77.1 66.3 58.2 52.5 43.1 41.5 25.5

b

Added as chlorides.

and intercept of the plot using the following equations. S −H + 2.303RT 2.303R G = −RT ln K c

log K c =

(13) (14)

It is deduced from the negative value of H and G that the sorption is exothermic and spontaneous in nature, with weak bond formation between the chromium and sawdust. The smaller values of H may be related to the non-heterogeneous nature of sawdust revealed by Freundlich isotherm. 3.6. EFFECT

OF FOREIGN IONS ON SORPTION

Sometimes foreign anions, cations or complexing agents affect the sorption of a specific ion of interest, due to the environment around the central metal ion and subsequently its solution chemistry and sorption behaviors. The sorption of Cr (III) ions was investigated in the presence of such ions under optimized conditions. Concentration of the ions was kept (8.98 × 10−4 M) ten times greater than the concentration of Chromium ion. The anions were added as their sodium salts,

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TABLE IV Sorption of other metal ions onto sawdust (200 mg) after 20 min agitation time from deionized water Metal ions

Kd (cm3 g−1 )

% Sorption

∝ = Kd Cr(III)/Kd M

Cr(III) Zn(II) Eu(III) Hg(II) Co(II) Tm(II) Ag(I) Sb (III) Cs(I) I(I) Tc(VII) Se(IV)

373 463 429 132 109 43 39 15 12 10 6 2

94.3 96.3 94.9 85.4 82.9 65.5 63.2 40.6 33.8 17.1 11.0 7.6

– 1 1 3 3 9 10 25 31 37 62 187

while the cations were added as nitrates or chlorides. The results are presented in Tables II and III. It is clear from the results presented in Table II that oxalate, citrate, carbonate and borate reduced the sorption appreciably. Maximum reduction (8.3%) was caused by borate. So it must be removed from sorptive medium before sorbing Cr (III) onto sawdust. The results of Table III show that none of the cations added have enhanced the sorption while Y(III), Ce(II) and Ca(II) have suppressed the sorption effectively. The foreign ions that reduced the sorption might be due to their competition with Cr(III) ions for the sorption sites on sorbent surface. 3.7. DECONTAMINATION

STUDY

In order to investigate the selectivity of sorbent surface, the sorption of several other nuclides have been measured under the optimized conditions for Cr(III) ions sorption. The results are presented in Table IV, along with their separation factor (∝) with respect to Cr(III) ion. All the metals except Zn(II) and Eu(III) showed lower sorption than Cr(III), especially Cs(I), I(I), Tc(VII) and Se(IV). It is deduced from the results, that these metals can be very easily separated from Cr(III) by using sawdust column. 4. Conclusions The results of this study indicate that: 1. The sorption of Cr(III) ions obeys Reichenberg, Morris-Webber and Lagergren equations.

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2. The sorption of Cr(III) ions onto sawdust follows Freundlich and D-R sorption isotherms over entire range of concentration while the Langmuir isotherm is obeyed at lower concentration. 3. The temperature variation has been used to compute the values of G, H and S. The negative values of H and G indicate exothermic and spontaneous nature of sorption respectively. 4. Y(III), Ce(II), Ca(II), Oxalate, citrate, carbonate and borate have reduced the sorption significantly. 5. The sawdust column can be used to separate Cr (III) ion from Cs (I), I(I),Tc (VII), and Se (IV). 6. The sorbent can be effectively used to remove traces of Cr(III) ions from industrial effluents, to preconcentrate Cr(III) present in small amount to achieve detection limits for analytical measurements and decontaminate large volumes of solutions containing ultra trace levels of Cr(III) ions. References Baraniak, L., Bernhard, G. and Nitsche, H.: 2002, ‘Influence of hydrothermal wood degradation products on the uranium sorption onto metamorphic rocks and sediments’, J. Radioanal. Nucl. Chem. 253(2), 185–190. Benes, P. and Majar, V.: 1980, Trace Chemistry of Aqueous Solutions, Elsevier Science, Amsterdam, 175 pp. Dubinin, M. M. and Radushkevich, L. V.: 1947, ‘The equation of the characteristic curve of activated charcoal’, Proc. Acad. Sci. USSR, Phys. Chem. Sect. 55, 327–329. Foldesova, M., Dillinger, P. and Lukae, P.: 2000, ‘Adsorption and desorption of Cr (III) on natural and chemically modified Slovak Zeolites’, J. Radioanal. Nucl. Chem. 245(2), 435–439. Freundlich, H.: 1926, Colloid and Capillary Chemistry, Methuen, London, p. 397. Hasany, S. M. and Ahmad, R.: 2002, ‘Fixation of micro or submicro amounts of Hg(II) ions onto sawdust from aqueous solutions’, Main Group Met. Chem. 25(12), 719–726. Hasany, S. M. and Ahmad, R.: 2003, ‘Sorption profile of Cd(II) ions onto coconut husk’, Main Group Met. Chem. 26(2), 87–98. Hasany, S. M., Ahmad, R. and Chaudhary, M. H.: 2003, ‘Investigation of sorption of Hg(II) ions onto coconut husk from aqueous solution using radiotracer technique’, Radiochim. Acta 91, 533–538. Hasany, S. M. and Chaudhary, M. H.: 1998, ‘Fixation of Cr(III) traces onto Haro river sand from acidic solution’, J. Radioanal. Nucl. Chem. 230(1–2), 11–15. Hobson, J. P.: 1969, ‘Physical adsorption isotherms extending from ultrahigh vacuum to vapor pressure’, J. Phys. Chem. 73, 2720–2727. James, B. R., Peutra, J. C. and Vitale, R. J.: 1997, ‘Oxidation-reduction chemistry of chromium: Relevance to the regulation and remediation of Chromate-contaminated soils’, J. Soil. Contam. 6(6), 569–580. Khalid, N., Rahman, A., Ahmad, S., Toheed, A. and Ahmed, J.: 1999, ‘Adsorption behavior of rice husk for the decontamination of chromium from industrial effluents’, J. Radioanal. Nucl. Chem. 240(3), 775–781. Lagergren, S.: 1889, ‘Theorie der sogennanten adsorption geloster stoffe’, K. Seveska Vetenskaped Handle. 24, 1–39. Langmuir, I.: 1918, ‘The adsorption of gases on plane surface of glass, mica and platinum’, J. Am. Chem. Soc. 80, 1361–1403.

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