Waste-activated sludge (WAS) as Cr(III) sorbent biosolid from wastewater effluent

Colloids and Surfaces B: Biointerfaces 66 (2008) 240–245

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Waste-activated sludge (WAS) as Cr(III) sorbent biosolid from wastewater effluent A. Iddou a,b,∗ , M.S. Ouali b a Université des sciences et de la technologie Mohamed Boudiaf (USTO-MB), Bp. 1505 Bir El Djir, 31000 Oran, Algeria b Laboratoire de valorisation des matériaux et traitement des nuisances, Département de chimie, Faculté des sciences et sciences de l’ingénieur, Université de Mostaganem, B.P. 227, Mostaganem 27000, Algeria

a r t i c l e

i n f o

Article history: Received 24 February 2008 Received in revised form 27 March 2008 Accepted 24 June 2008 Available online 5 July 2008 Keywords: Biological sludge WAS Chromium Wastewater Kinetic Equilibrium Isotherm

a b s t r a c t A biological sludge – waste-activated sludge (WAS) – from a dairy filtering station was investigated for the removal of trivalent chromium from aqueous solution. Kinetic results revealed that chromium adsorption was instantaneous. The removal rate increases up to pH 4 for contact times beyond 20 min. The equilibrium state is attained in 30 min in all the considered systems. The reaction orders as well as the diffusion rate constant were determined. Values adsorption isotherms measured at pH 3 generally followed the Langmuir model. The maximum uptake capacity was 25.64 mg/g. Values of thermodynamic parameters show that chromium (III) sorption on WAS is an exothermic process. This study provides an opportunity for the removal of heavy metals such as chromium from aqueous solutions using a low-cost biosolid as adsorbent support. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The presence of heavy metals in liquid ecosystems is becoming a phenomenon of utmost importance given the increase of their concentration in the wastes, their toxicity and their effects on living organisms. The increase in the presence of the chromium rate in the environment is due to the diversity of the sources of contamination through this metal, the most important of which are tanneries and surface treatment industries. Chromium and its derivatives can provoke intoxication, and range from a simple skin irritation to a carcinogenic effect. Several techniques are proposed for chromium removal from polluted waters. The traditional ones include lime precipitation, adsorption and ion exchange. Separation based on ion-exchanging resin and adsorption on active coal are known for their efficiency in the final treatment of heavy metals, but the elevated cost of these methods as well as some operating parameters limits their use for the low heavy metal concentrations [1]. That is the rea-

∗ Corresponding author at: Université des sciences et de la technologie Mohamed Boudiaf (USTO-MB), Bp. 1505 Bir El Djir, 31000 Oran, Algeria. E-mail addresses: [email protected], [email protected] (A. Iddou). 0927-7765/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2008.06.018

son why several investigations are currently underway, with the aim of discovering new techniques which are cost-effective treatment methods and are capable of removing low concentrations of chromium from solutions. In this perspective, several researchers have proposed heavy metal removal methods using diverse materials such as bacteria [2–4], fungi and yeast [5–14], fermentation waste [15–17], activated carbon [18–21], and clay [22]. Works done for the treatment of wastewater by use of industrial solid waste are rare. The few ones done [23,24] so far show the efficiency of these materials as to removal of some metallic elements from polluted water. Biological materials are known for their potential to adsorb heavy metals [25], and the biosorption presents major advantages like low cost, high efficiency of metal removal from dilute solutions, no additional nutrient requirement, and possibility of metal recovery. In this study, we used waste-activated sludge (WAS) which consists of the non-living microorganisms which are no longer required in the wastewater treatment process and are ready for disposal. Our objectives are to identifying the chromium uptake capacity of the WAS, determining the kinetics and evaluating the influence and importance of solution pH on chromium removal.

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241

Table 1 Physicochemistry analysis of the biosolid Tests

Results

pH Humidity rate (%) Organiques maters (%) Minérales maters (%) Cr (mg/l) Zn (mg/l) Cd (mg/l) Pb (mg/l) Cu (mg/l)

6.5–7 10 73.6 16.4 Traces 3.34 Traces 0.16 Traces

2. Materials and methods Fig. 2. Effect of contact time on the adsorption of Cr(III) onto WAS at various pH (adsorbent dose w/v = 5 g/l, T = 298 K).

2.1. Biosolid The dewatered WAS from biological wastewater treatment in dairy filtering station was used as adsorbent material. This treatment plant was chosen because the origin of wastewater with low background concentrations of chromium. The biosolid was collected from drying beds. The sludge did not undergo any treatment apart from a light grinding. 2.2. Chemicals Chromium solutions were prepared from analytical reagent grade Cr(NO3 )3 . Distilled water was used for all solutions. Stock chromium solution of 1 g/l was initially prepared and acidified with concentrated HNO3 . Diluted solutions of chromium were prepared from the stock solution. The chromium titration method chosen for this study is that using oxidation of Cr(III) into Cr(VI) by a hot potassium permanganate solution, followed by complexation of Cr(VI) formed with a diphenylcarbazide acid solution [26]. Add to a volume V of the solution to be analyzed 1 ml of sulfuric acid and 0.5 ml of a potassium permanganate solution. Heat the mixture in Marie bath for 20 mn. Add to the hot solution sodium azotise to remove the excess of the permanganate. Make sure that the brown colour has disappeared and avoid the excess of the sodium azotise solution. Cool the solution rapidly. Transverse the obtained solution into a 50-ml flask and adjust it to 50 ml. Then, add 2.5 ml of alcoholic diphenylcarbazide solution. After 10 min, measure the absorbance at 540 nm. The Cr(VI) colorimetric titration is carried out on a visible spectrophotometer of the JENWAY 6300 type, at a 540 nm wavelength. pH measures are done on a pH-meter (SCHOTT CG711).

Fig. 1. Evolution of removal rate versus initial pH.

Fig. 3. Removal percentage versus pH for contact times above or equal to 30 min.

2.3. Experimental protocol Optima parameter determination (pH and contact time) is achieved on 100-ml suspensions of 5 ppm of Cr(III). Isotherms are plotted for different temperatures by varying chromium initial concentration (2, 4, 6 and 8 ppm) and keeping a constant solid/solution ratio (5 g/l).

Fig. 4. Plot of first-order equation for Cr(III) removal at different pH values (adsorbent dose w/v = 5 g/l, T = 298 K).

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Table 2 First-order kinetic parameters at different pH values

Slope Intercept R2 K (min−1 )

pH 2

3

4

5

7

−0.040 −0.270 0.970 0.093

−0.044 −0.230 0.820 0.101

−0.040 −0.110 0.950 0.092

−0.032 −0.130 0.890 0.073

−0.060 −0.270 0.800 0.137

3. Results and discussion 3.1. WAS characterisation WAS characterisation has shown an organic aspect since it is composed of about 74% of organic matter. Its chemical contents show the presence of traces of chromium which able as to use it without scaring interferences (Table 1). 3.2. pH effect on Cr(III) removal by WAS The removal of heavy metals from aqueous solution was by adsorption related to the pH of the solution, which affects the surface charge and the degree of ionisation and species of adsorbate. The effect of pH has been studied by varying, from 2 to 7, pH suspensions of the WAS in 5 ppm of Cr(III) solution. The surface aspect of the material is that of bacteria and fungi cell wall characterised by the presence of peptidoglycan and polysaccharide groups, in addition to that of chitin for fungi [27]. The % removal, defined as [(C0 − Ct )/C0 ] × 100, where Ct and C0 are respectively concentrations at time t and initial concentration, in relation to pH is represented on Fig. 1. The plot obtained shows that the removal rate increases up to pH 4 for contact times beyond 20 min, where an adsorption maximum of 92% has been noted. This means that the pH increase reveals negative sites, by the ionisation of especially carboxylic groups producing H+ ions, a fact that increases the removal efficiency of metallic ions. The chromic ions in aqueous solutions exist as [Cr(H2 O)6 ]3+ , depending upon the pH of the solution [28]. We can propose that chromium in our case is removed from solution as the two species existing in the range of pH 3–4: [Cr (H2 O)6 ]

3+

pH=3−4



2+

[Cr (H2 O)5 OH]

On the other hand, up to pH 4, the efficiency of chromium removal decreased by the probable appearance of the surface precipitation phenomenon in the operating conditions such as initial concentra-

Fig. 6. Evolution of diffusion rate constant at different pH values.

tion of adsorbate. The same observation has been reported in the case of copper [29]. 3.3. Contact time effect on Cr(III) removal by WAS Removal has been released at different pH values 2, 3, 4, 5 and 7. The results obtained give a graph representing the removal rate in relation to contact time between WAS and Cr(III) solution at 5 ppm. The plots obtained (Fig. 2) are all characterised by two zones the first of which is a quick step. The equilibrium state is attained in 30 min for the various pH values. The sorption rate at equilibrium increases from 48.3% to 90.6% with increase in the pH solution from 2 to 7. Confirmation of this equilibrium state is given by Fig. 3, representing the removal percentage in relation to pH for contact times above or equal to 30 min. It is effectively observed that the plots obtained are all merged. This confirms our observations as to the necessary time in order to get to an equilibrium state. 3.4. Determination of reaction order In order to express the mechanism of adsorption process onto WAS as adsorbent, we use the Lagergren equation as the pseudofirst-order equation expressed by [30] log (qe − q) = log (qe ) −

kt 2303

(1)

where k is the rate constant of pseudo-first-order adsorption (min−1 ) and qe is the maximum amount of adsorbate that can adsorbed in the WAS (mg/g). This equation enables us to determine the rate constants at different pH values, and consequently determine the order of the reaction. The validity of the model can be checked by the linearized plots of log(qe − q) versus t. The obtained results (Fig. 4) show that the removal of Cr(III) on the WAS obeys to the kinetic model Eq. (1), then the global reaction rate is of first order. Besides, a relative regularity of the rate constants for pH 2–4 can be observed (Table 2). Fluctuation in results can be observed beyond pH 4, a fact Table 3 Diffusion rate constant (K ) at different pH values pH

Fig. 5. Amount of Cr(III) adsorbed versus t1/2 for intraparticle transfer of Cr(III) in WAS at different pH values (adsorbent dose w/v = 5 g/l, T = 298 K).

Slope Intercept R2 K (mg/g min1/2 )

2

3

4

5

7

0.093 −0.029 0.966 0.093

0.147 0.095 0.906 0.147

0.166 0.063 0.968 0.166

0.159 0.060 0.955 0.159

0.161 0.176 0.807 0.161

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Fig. 7. Freundlich isotherms for Cr(III) sorption onto WAS at different temperatures (adsorbent dose w/v = 5 g/l, initial pH 3).

that confirms the appearance of the precipitation phenomenon that hampers adsorption itself. 3.5. pH effect on diffusion rate

243

Fig. 8. Langmuir isotherms for Cr(III) sorption onto WAS at different temperatures (adsorbent dose w/v = 5 g/l, initial pH 3).

modynamic parameters. Our results have also been applied to Freundlich and Langmuir adsorption models [33]. The Freundlich model is defined by the following equation: q = KCen

The rapid process of the present experiments (contact time = 30 min) is an indication of a diffusion-controlled process. To examine the suitability of intraparticle diffusion in fitting our data, the rate constant is given by Weber and Morris [31]. q = K  t 1/2

(2)

where q is the quantity of Cr(III) sorbed per gram of sorbent at time t (mg/g), t the contact time (min), and K the diffusion rate constant (mg/g min1/2 ). The plots of q versus square root of time contact for different pH values are represented in Fig. 5. The slope and intercept of these curves are calculated by linear regression. The results are grouped in Table 3. From this one, we can observe that the results are in adequacy with the diffusion kinetic model used Eq. (2) with a good determination coefficient. Values of the diffusion constant (Table 3) are plotted versus pH in Fig. 6. This figure shows a linear increase of the diffusion constant for pH 2, 3 and 4. From this same figure, it may be observed that the straight lines did not pass through the origin and this further indicates that the intraparticle diffusion is not the only rate-controlling step [32]. It can be observed that above pH 4, a stabilisation of the values of this same constant is noted; this indicates and confirms, once more, the appearance of the precipitation phenomenon. For the remaining investigations, we propose to work at pH 3 in order to avoid precipitation interference that appears beyond pH 4. 3.6. Sorption isotherms Isotherms are determined for four temperatures (293, 303, 313 and 323 K). They are exploited with the aim of calculating ther-

(3)

where q is the amount of Cr(III) sorbed per gram of sorbent (mg/g), Ce the equilibrium concentration of Cr(III) (mg/l), K and n are empirical coefficients. This model can be written by using the decimal logarithm. This will give us log q = n log Ce + log K

(4)

The Langmuir isotherm is based on the following equation: q=

qmax bCe 1 + bCe

(5)

This equation can be written under the following form: 1 1 1 + = q qmax qmax bCe

(6)

where q is the amount of Cr(III) sorbed (mg/g), qmax the maximal adsorption capacity of the sorbent (mg/g), Ce the equilibrium concentration of Cr(III) (mg/l), and b is the constant relative to adsorption energy. The plots of the above Eqs. (4) and (6) are shown in Figs. 7 and 8 and they are linear with a good determination coefficient (R2 > 0.95). The Langmuir adsorption isotherm [33] has been successfully applied to many other sorption processes. A basic assumption of the Langmuir theory is that sorption takes place at specific homogeneous sites within the adsorbent. The monolayer formation was been confirmed by the linear plot of 1/qe versus 1/Ce (Fig. 8) according to the Langmuir isotherm. The details in Table 4 show that the WAS presents a maximal adsorption capacity qmax of 25.64 mg/g at 20 ◦ C. The same values are reported by other authors [9].

Table 4 Freundlich and Langmuir parameters values at different temperatures Parameters

Freundlich ◦

Temperature ( C) Slope Intercept R2 n k qmax (mg/g) b

20 0.953 −0.330 0.999 0.953 0.469 – –

Langmuir 30 0.925 −0.410 0.989 0.925 0.387 – –

40 0.452 −0.480 0.979 0.452 0.333 – –

50 0.598 −0.750 0.976 0.598 0.177 – –

20 2.093 0.040 0.999 – – 25.64 0.019

30 3.134 0.050 0.996 – – 20.83 0.015

40 1.476 1.34 0.954 – – 0.748 0.906

50 4.655 1.27 0.989 – – 0.788 0.272

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Table 5 Comparative table of the adsorption capacity of Cr(III) form different adsorbents Adsorbents Soils Belen soil Forest soil Blue point Activated carbon GAC-S GAC-E ACF-307 ACF-310 Biomasses Eucalyptus S. Wood Euphorbia ligida Skye peat Sugar beet pulp Waste-activated sludge

Operating conditions

Adsorption capacity (mg/g)

References

pH 3.0–3.3, T = 22 ◦ C, t = 7 days

24.14 9.13 5.65

[34]

pH 5, T = 20 ◦ C

13.31 10.52 7.08 3.52

[35]

3.08 1.96 8.4

[36]

10.81

[37]

25.64

This study

pH 5, T = 20–25 ◦ C, t = 12 days

pH 5.5, T = 20 ◦ C, t = 120 min ◦

pH 3, T = 20 C, t = 30 min

A comparison of results obtained in this study with those of other studies reported in literature for the adsorption of chromium(III) is given in Table 5. We note that our material (WAS) is more effective compared to other materials such as biomass. On the other hand, certain types of soil can present the same capacity.

results show the negative values of H; that confirms the exothermic nature of the chromium (III) sorption on WAS. The negative values of G◦ indicate that the adsorption of Cr(III) on WAS is spontaneous. Negative entropy value indicates an affinity of chromium III from the WAS surface.

3.7. Effect of temperature

4. Conclusion

The temperature range used in this study varied from 283 to 323 K at 10 mg/l Cr(III) solution concentration having a pH 3. The decreasing in the values of the sorption capacities from 25.64 to 0.79 mg/g indicate that the temperature unfavours the sorption of Cr(III). Thermodynamic parameters such as enthalpy, H◦ and entropy, S◦ for Cr(III) sorption on WAS are calculated by using the following equation [22]:

Arising from the biological treatment of a dairy’s wastewater, WAS reveals itself efficient as to Cr(III) removal from water. Cr(III) adsorption is a very quick operation that goes through certain stages: (i) migration of chromic ions towards WAS surface, (ii) diffusion of particles into the solid, and (iii) probable ion precipitation to solid surface. Thermodynamic parameters reflect the possibility of process realisation. This method can be applied for chromium (III) removal from industrial wastewater such as tanneries and metallurgy.

In Kd =

S ◦ H ◦ − R RT

(7)

where T is the temperature (K), R the ideal gas constant = 8.314 (J/mol K), and Kd the distribution coefficient (cm3 /g), calculated as follows: q Kd (cm3 /g) = (8) y/v q the amount of Cr(III) sorbed per gram of sorbent (mg/g), y is the mass of Cr(III) in the sample solution (mg), and v the volume of the sample solution (cm3 ). Free energy values (G) are calculated on the basis of the following equation: G◦ = H ◦ − TS ◦

(9)

The plot of ln Kd versus 1/T (Eq. (7)) enables us to obtain H◦ and S◦ values from the slope and the intercept respectively. Values of thermodynamic parameters are grouped in Table 6. These

Acknowledgments The authors express their thanks to Dr. Abbas Bahous from University of Mostaganem and Dr. Mohamed Hadj Youcef from Mohamed Boudiaf University of Oran (Algeria) for their helpful translation and revision of this paper. References [1] [2] [3] [4] [5] [6] [7] [8]

H◦ (kJ/mol) S◦ (J/mol K)

−41.21 −88.83

[9] [10] [11] [12] [13] [14]

G◦ (kJ/mol) 293 (K) 303 (K) 313 (K) 323 (K)

−15.18 −14.29 −13.41 −12.52

[15] [16] [17] [18] [19]

Table 6 Thermodynamic parameters

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