Removal of Cr3+ from tanning effluents by adsorption onto phosphate mine waste: Key parameters and mechanisms

Journal of Hazardous Materials 378 (2019) 120718

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Removal of Cr3+ from tanning effluents by adsorption onto phosphate mine waste: Key parameters and mechanisms Amal Oumania,b, Laila Mandia,b, Fatima Berrekhisc, Naaila Ouazzania,b,

T

⁎

a

Laboratory of Hydrobiology, Ecotoxicology, Sanitation and Climate change (LHEAC-URAC33), Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco b National Center for Research and Studies on Water and Energy (CNEREE), Cadi Ayyad University, Marrakech, Morocco c Equipe de Physico-chimie des Matériaux, Ecole Normale Supérieure, Université Cadi Ayyad, Marrakech, Morocco

A R T I C LE I N FO

A B S T R A C T

Keywords: Cr3+removal Tanning wastewater Adsorption Phosphate mine waste COD elimination

The present study aims to investigate key parameters and mechanisms affecting Cr3+ removal from tanning wastewater using phosphate mine waste (PW) as adsorbent in batch mode. The initial Cr3+ concentration was 3920 mg.L−1. The maximum removal capacity of Cr3+ was found to be 97.23 mg.g−1 using 40 g.L−1 of PW at 50 °C and at 200 rpm of stirring speed. Thermodynamic studies indicated that Cr3+ sorption is endothermic reaction of a physico-chemical adsorption process. Kinetic data were satisfactorily described by a pseudo-second order model. Cr3+ removal is probably involving several mechanisms: PW surface dissolution, precipitation, coprecipitation, ion exchange and adsorption. The chromium sorption seems modifying the crystalline structure of the adsorbent. Adsorption isotherm was described by Freundlich, Langmuir and Redlich-Peterson models. But statistically, Freundlich fit better the experimental data. Five error functions were used to check this result. Treatment of chromium effluent using PW as adsorbent can also eliminate more than 60% of organic matter and then can be considered as an effective biomaterial for tanning wastewater treatment.

1. Introduction Several industries use heavy metals (HM) in their manufacturing processes: such as mining, metal processing, tanneries, pharmaceuticals, pesticides, textile, electroplating, organic chemicals, rubber, plastics, lumber and wood products industries [1]. Therefore, their liquid effluents contain huge concentrations of HM and their discharge into the environment causes harmful nuisances to living species [2]. Tannery industry uses chromium salts to transform animal skins into valuable leather and releases very contaminated wastewater that causes serious environmental problems. These effluents are very rich of organic matter, very loaded with chromium [3,4], and are especially not biodegradable (BOD5 / COD ratio is too low). Several studies have been investigated by other authors on the treatment of liquid effluents of tanneries to remedy, especially, their high chromium concentration. Among the most used methods for the chromium removal from an aqueous phase are chemical precipitation [5] and electro-precipitation method [6], electrocoagulation [3], membrane filtration [7], ion exchange [8] and electrochemical ionexchange process [9], liquid-liquid extraction [10], electrodialysis [11],

phytoremediation in constructed wetland [12], infiltration percolation [13], photocatalysis [14] and adsorption [15,16]. However, adsorption process could be considered as the easier method to apply, giving very high removal efficiencies. Many researchers used natural materials as adsorbents such as tree leaves and their bark, needle and cone of conifers [17], fungi [18], alga [19], bacterium [20], wool [21], moss [22] and coals [23]. Other works used bioadsorbent wastes including tires waste [24], agricultural waste materials [25,26], newspaper waste [27], tea factory waste [28], banana peels [29], sawdust [30] and eggshell [16] to remove chromium from wastewaters. Other authors used phosphate rock, which represents one of the best adsorbent of HM in solution. Among these researches we can mention [31] that studied the elimination of Pb; [32] of Cu; [33] of Zn; [34] of U and [35] of Cd. However, at our knowledge, none of these works studied either the chromium trivalent metal adsorption on natural phosphate waste, nor the mechanisms involved in this adsorption processes. This study aims essentially to investigate key parameters and possible mechanisms affecting removal of Cr3+ from highly loaded tanning wastewater using

⁎ Corresponding author at: Laboratory of Hydrobiology, Ecotoxicology, Sanitation and Climate change (LHEAC-URAC33), Faculty of Sciences Semlalia, Cadi Ayyad University, Marrakech, Morocco. E-mail address: [email protected] (N. Ouazzani).

https://doi.org/10.1016/j.jhazmat.2019.05.111 Received 22 October 2018; Received in revised form 29 May 2019; Accepted 30 May 2019 Available online 31 May 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

Journal of Hazardous Materials 378 (2019) 120718

A. Oumani, et al.

measured by atomic absorption spectroscopy AAS (flame mode). The chromium calibration curve was prepared using standard solutions from Sigma-Aldrich (France). The amount of chromium adsorbed per gram of adsorbent (mg. g−1) is calculated according to the following equation:

Table 1 Main characteristics of chromium tanning effluent. Physico-chemical characteristics

Value ± SD

pH Cl− (g.L-1) COD (g.L−1) SO42− (g.L-1) HCO3− (g.L-1) Cr3+ (g.L−1) Cd2+ (mg.L−1) Cu2+ (mg.L−1) Pb2+ (mg.L−1)

3.43 9.46 ± 0.63 3.01 ± 0.21 6.04 ± 0.06 1.33 ± 0.17 3.92 ± 0.41 0.0047 ± 0.0006 0.086 ± 0.010 0.0135 ± 0.0004

Qe = V .

C0 − Ce m

(1)

Where m is the mass of adsorbent (g), V is the volume of the effluent (L), C0 and Ce are, respectively, chromium concentration (mg.L−1) in the initial state and at the equilibrium. The calculation of the adsorption rate (the adsorption efficiency) is done by the following equation:

phosphate mine waste (PW) as adsorbent.

R(%) =

2. Materials and methods

C0 − Ce × 100 C0

(2) 3+

The effect of initial pH on Cr uptake by PW and its evolution during the experiments were studied as follows: chromium tanning effluent or boiled distillated water has been mixed to PW at equilibrium dose. The use of boiled distillated water constitutes the blank treatment. The suspensions were stirring at 200 rpm and at 25 °C for 24 h. The pH is measured at suitable time using pH meter (CONSORT C562). At the end, mixtures were centrifuged (1000 rpm, during 10mn); the obtained solid was dried at 105 °C and analyzed by X-ray diffraction spectroscopy (XDR), Fourier Transform Infra Red spectroscopy (FTIR), and Scanning Electron Microscope (SEM) equipped with an energy dispersive X-ray (EDX) analyzer. All experiments are made in triplicate. The standard deviation is calculated using Microsoft Excel®.

2.1. Wastewater sampling The effluent used in this study was sampled from a small semi-traditional industrial unit located in the old Medina of Marrakech where the operations of leather processing are made in fullers. Wastewater operations (pre-tanning operations, degreasing, pickling and chrome tanning) are mixed and discharged directly into the public sewerage network. We focused our work only on chrome tanning step wastewater before its mixing with other processing steps effluents. A sample was collected in clean plastic cans, directly after draining the tanning fuller and then was analyzed in the laboratory according to standards methods. Table 1 shows physico chemical characteristics of the studied effluent.

3. Results and discussion

2.2. The adsorbent characterization

3.1. Physico-chemical characterization of the tanning effluent

The adsorbent used in the present study is a mine waste sampled from “Gantour" phosphate mine located at 100 Km northwest of Marrakech. This material represents the economically unprofitable part of the mine residues discharged on a surrounding site. The granulometry of the major part of this material is less than 2 mm. The specific surface area, measured by a nitrogen adsorption using the BrunauerEmmett-Teller method (BET), showed an average of 15.12 m2. g−1. This material is used in the experiments without any prior treatment.

The average results of physicochemical characteristics of the chrome tanning effluent are described in Table 1 and showed a complex composition. It is very acid (pH = 3.43), heavily loaded with chromium (approximately 4 g.L−1), organic matter (COD = 3 g.L−1), chloride and sulfate. These values are far from the limit values allowed by the Moroccan standard (June 2014) for the industrial discharges, which requires a pH between 5.5 and 9.5, COD value not exceeding 0.5 g.L−1 and total chromium contents of less than 2 mg.L−1. Contents of other metal ions (Cd2+, Cu2+, Pb2+) analyzed are considered as negligible (Table 1). For that, our study was only focused on chromium and organic matter removal.

2.3. Batch adsorption tests and pH monitoring Batch experiments has been performed in 100 ml vessels containing 50 ml of chromium effluent, added of PW at a considered dose and stirred during 24 h in a shaker agitator to investigate the effect of several parameters such as adsorbent dose (from 0.5 g to 10 g), stirring speed (100 rpm, 150 rpm and 200 rpm), temperature (15 °C, 25 °C, 30 °C and 50 °C) and initial concentration of the effluent (dilution of the raw effluent: 1/5, 2/5, 3/5, 4/5). Investigations have been carried out successively allowing to applying each already optimized parameter from the previous experiment to the next one. Kinetic tests were carried out in order to determine the equilibrium time for chromium adsorption, and also to study the possible release of calcium and phosphorous by PW during the adsorption process. For that, series of vessels were used, each one containing tanning effluent mixed with the equilibrium dose of PW. All were stirred at 200 rpm in a shaker during a suitable contact time (0–24 h). Concentration of chromium, total phosphorus and calcium were determined in the mixtures. All the studied parameters have been measured after mixture filtration using a 0.45 μm of porosity filter. Concentration of total phosphorus was determined by potassium peroxodisulfate digestion method [36] and calcium according to EDTA titrimetric method [37]. Total COD is analyzed based on dichromate open reflux method [38]. Chromium concentration is

3.2. Batch adsorption tests 3.2.1. Chromium removal efficiency 3.2.1.1. Effect of stirring speed and adsorbent dose.. The study of the absorbent dose effect on the chromium adsorption is particularly important because it determines the adsorbent-adsorbate equilibrium in the system. Fig. 1 shows the relationship between chromium adsorption rate, PW dose and stirring speed. It can be deduced that with a high stirring speed, the rate of the removed chromium increases and the equilibrium state is reached with a low dose of the adsorbent (40 g.L−1 of PW at 200 rpm versus 60 g.L−1 at 150 and 100 rpm). At 200 rpm (Fig. 2), the percentage removal of chromium (R%) increases rapidly with the increasing of the PW mass in the range of 10-40 g.L-−1. The removal percentage increase is related to larger number of PW grains providing further available adsorbent sites. From the PW dose superior to 40 g.L−1, there is a slight increase on the adsorption rate. At this equilibrium point, the chromium adsorption efficiency reached 84.59% and the adsorption capacity was 82.9 mg.g−1. 3.2.1.2. Evolution of the pH and suggestion of probable adsorption 2

Journal of Hazardous Materials 378 (2019) 120718

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Fig. 1. Chromium adsorption efficiency (%) at different stirring speeds and different adsorbent doses. (C0= 3.92 g.L−1, T = 25° C, V=50mL and stirring time=24h).

Fig. 4. Effect of contact time on release of calcium (a) and phosphorus (b) by PW in acidic tanning effluent. (pHi =3.43, V=200 rpm, T=25°C, PW dose= 40 g.L−1).

The increase of the concentration of Ca2+ (Fig. 4a) and total phosphorous (Fig. 4b) in the chrome tanning solution during the adsorption process confirms the theory of dissolution of PW in the acidic tannery effluent. On the other hand, the release of Ca2+ and phosphorus ions as a function of contact time follows qualitatively Cr3+ elimination curve. Probably, Chromium was removed by ion exchange. But quantitatively, this release remains below the amount of sorbed Cr3+. This suggests that the sorption process does not only involve ionexchange reaction, but can also occur through likely surface dissolution, surface adsorption and precipitation. The XDR analysis of PW before adsorption (Fig. 5a) shows the presence of fluorapatite Ca10(PO4)6F2, quartz SiO2 and carbonate CaCO3. The PW analysis after adsorption of chromium from tanning effluent (Fig. 5b) showed formation of some new mineral compounds such as Ca9Cr(PO4)7 and Ca3Cr2(SiO4)3. The presence of carbonate ions in the medium assumes a precipitation of Ca9Cr(PO4)7 with insertion of CO32− in a probably amorphous form. SEM image and EDX spectrum of PW before and after adsorption of Cr3+ ions have been shown in Fig. 6. As can be seen in Fig. 6a, the morphology of PW surface is made mainly by particles of irregular forms. However, the surface of PW after treatment (Fig. 6b) is strongly

Fig. 2. The adsorption efficiency of chromium (R%) and the adsorption capacity (Qe) at different doses of the adsorbent (C0= 3.92 g.L−1, pH = 3.43, V=50mL, stirring speed 200 rpm, stirring time= 24h and T = 25°C).

Fig. 3. Evolution of the pH of chrome (III) effluent and the blank treatment after adding the PW (C0= 3.9 g.L−1, V=200 rpm, T=25°C, PW dose= 40 g.L−1).

mechanism.. The pH is the most critical parameter in the adsorption process. Fig. 3 represents the evolution of pH in the chrome tanning effluent and in the blank treatment after adding PW. It can be seen that the pH increased rapidly and stabilized at around a value of 6 and 7.38 respectively for the mixture of PW with the tanning effluent and the blank treatment. In aqueous medium, under these pH conditions, PW could be affected by dissolution according to the following reaction [39]: − Ca10 (PO4 )3 (CO3)3 FOH(S) + 10H+(aq) ⇌ 10 Ca2 +(aq) + 3H2 PO4( aq)

+ 3HCO3−(aq) + F−(aq) + H2 O(aq)

Fig. 5. XRD analyses of PW before (a) and after adsorption (b).

(3) 3

Journal of Hazardous Materials 378 (2019) 120718

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Fig. 6. SEM and EDX analyzes of PW before (a and c) and after adsorption of Cr3+ (b and d).

Fig. 7. FTIR spectra of the phosphate mine waste (a) and PW loaded with chromium (b).

Fig. 8. The amount of Cr3+ remained in the solution and the adsorbed one at the equilibrium state. (Stirring speed =200 rpm, stirring time = 24h, temperature =25°C, PW dose =40g.L−1).

affected by the presence of crystalline particles. By comparing the EDX spectrum of PW before and after treatment (Fig. 6c and d), it can be concluded that Cr3+ is adsorbed onto the PW surface. This observation confirmed the results found by XRD. FTIR spectra (Fig. 7a) of native PW shows the presence of PO43− at 472.18 cm−1, 571.13 cm−1 and at 104,243 cm−1 [34,40,41]. Moreover, the peak at 1430,63 was attributed to the anti-symmetrical elastic vibrating absorption patterns of CO32− in PW; which indicated the presence of carbonated apatite in PW sample [41]. After adsorption (Fig. 7b), the spectrum of PW loaded with Cr3+ present slight or marginal peak shifts respectively from 3436, 2865, 2514, 1801, 1430, 1042, 710, 571, and 472 cm−1 to 3441, 2514, 1617, 1427, 1095, 1043, 709, 660, and 601 cm−1 due to Cr3+ ions sorption. These shifts may be attributed to the substitution of some metal elements of PW such as calcium by chromium atoms. This phenomenon may indicate structural change of phosphate species. So, The ability of PW to take up Cr(III) suggests the following probable mechanisms :(i) the PW dissolves to release aqueous species (phosphate, silicate, etc.) which probably combined with Cr3+ and precipitated in the form of new metal molecules (co-precipitation). (ii) Cr3+ was removed by adsorption and through ion exchange process on the surface of PW (iii) Cr3+ precipitated as Cr(OH)3 form.

200 rpm for 24 h. Results at the equilibrium (Fig. 8) showed that the function representing relationship between Cr3+ concentration remaining in the solution and the adsorbed one on the solid is linear of the "slope-intercept" form, with a formula of y = 0.3827x + 15.074. So the "C" isotherm, a line of zero-origin [42], is not adequate to describe the mechanism of the adsorbate-adsorbent interaction according to the obtained equation. Fundamentally, the linearity shows that the number of sites for adsorption remains constant: as more solute is adsorbed more sites must be created. According to [43], this phenomenon can be explained by the penetration of the solute into the crystalline regions of the substrate to create new pores and thus modify the texture of the adsorbent. Theoretically, the modification of the structure of the PW will stop abruptly when more highly crystalline regions of the adsorbent are reached to give a horizontal plateau [42,43]. So the study of the models representing concave isotherms proves indispensable. Langmuir, Freundlich, and Redlich–Peterson isotherms have been used to evaluate the equilibrium characteristics of the adsorption processes. The parameters of the isotherm equations were calculated by linear regression analysis according to [44,45] for Langmuir, to [46,47] for Freundlich and to [48] for Redlich–Peterson isotherm. Table 2 shows the linear formulas of the used isotherms and summarizes the calculated adsorption parameters for all the isotherms at 288, 298, 303 and 323 K. The calculated parameters of the three models indicate that there is no

3.2.1.3. Effect of initial concentration of chromium and adsorption isotherms determination.. In order to know the adsorption isotherm type, the evolution of the adsorption capacity as a function of the initial concentration of the adsorbate was carried out with stirring of 4

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Table 2 Langmuir, Freundlich and Redlich–Peterson linear formulas, and theirs calculated parameters for the adsorption of Cr3+ onto natural PW. Parameters Qexp (mg. g

15 °C −1

)

53.04 Langmuir

Qm (mg. g−1) KL (L. mg−1) RL R2 r2Adj CD RMSE χ2 SSE

1 Qe

=

1 Qm . KL . Ce

+

−1

qm (mg. g ) KR.P (L. mg−1) α R2 r2Adj CD RMSE χ2 SSE

1 lnCe n

Ce Qe

=

1 qm . KR . P

50 °C

82.91

88.91

97.23

81.967 7.10−3 0.035 0.9724 0.9274 0.9456 0.0670 10.01.10−3 13.47.10−3

102.04 4.74.10−3 0.051 0.9752 0.9347 0.9510 0.105 10.96.10−3 33.1.10−3

135.14 4,15.10−2 0.0061 0.9785 0.9432 0.9574 0.063 9.8.10−3 11.91.10−3

2.147 0.5644 1.7718 0.9977 0.9939 0.9954 21.2.10−3 8.39.10−4 1.35.10−3

0.784 0.7981 1.253 0.9959 0.9891 0.9918 38.21.10−3 3.308.10−3 4.38.10−3

6.76 0.725 1.3793 0.9931 0.9817 0.9862 39.51.10−3 2.905.10−3 4.68.10−3

0.92 7.71 0.22 0.9958 0.9889 0.9917 38.21.10−3 3.701.10−3 4.38.10−3

6.83 24.38 0.28 0.9930 0.9815 0.9861 39.74.10−3 3.286.10−3 4.74.10−3

(5)

0.23 0.7201 1.3887 0.9896 0.9725 0.9793 60.69.10−3 10.557.10−3 11.05.10−3 Redlich–Peterson

30 °C

(4)

35.714 2.44.10−3 0.094 0.9406 0.8463 0.8847 0.1068 60.68.10−3 34.22.10−3 Freundlich lnQe = lnKf +

Kf (mg. g−1)/(mg.L−1)1/n 1/n n R2 r2Adj CD RMSE χ2 SSE

1 Qm

25 °C

+

Ce α qm

0.23 8.324 0.27 0.9895 0.9721 0.9791 62.49.10−3 11.95.10−3 11.71.10−3

(6)

1.99 6.36 0.42 0.9975 0.9935 0.9951 277.7.10−3 15.57.10−4 1.56.10−3

Where Ce (mg.L−1) is the equilibrium concentration of Cr3+ in the solution; Qe (mg. g−1) is the amount of chromium adsorbed at the equilibrium; Qm (mg. g−1) is the maximum chromium adsorption capacity; Qexp is the experimental value of the amount of Cr3+ adsorbed per gram of PW of raw effluent. KL is the Langmuir constant related to the energy of adsorption; RL (dimensionless) is a separation factor of Langmuir isotherm calculated according to [45]. Kf and n are Freundlich constants related to the adsorption capacity and the adsorption intensity. qm and KR.P are the parameters of Redlich–Peterson isotherm equation and α (dimensionless) is the Redlich–Peterson exponent.

[50,51,54]. The coefficient of determination (CD) and the adjusted Rsquared (r2Adj) are the more appropriate determination of the goodness of fit than the correlation coefficient R2 often used in the literature: CD measures the fraction of the total variance accounted for by the model [53], and r2Adj has been adjusted for the number of predictors in the model. Results (Table 2) showed that the first model data closer to the experimental one is Freundlich model, the second is Redlich-Peterson; while Langmuir isotherm represents the lowest values of R2, CD and r2Adj; and the highest values of the chi-square test (χ2), RMSE and SSE. So, we consider that the Freundlich model is the most appropriate isotherm to describe the adsorption of Cr3+ onto PW. The theory of Freundlich supposes that the adsorbent has several

big difference between the correlation coefficient of Freundlich, Langmuir and Redlich-Peterson isotherms. Similar results were reported by [49]. But correlation coefficient of Freundlich is bit higher compared to R-P and Langmuir models. In order to confirm which isotherm describes the adsorption of Cr3+ onto PW, a statistical analysis is essential. 3.2.1.4. Statistical analysis: comparison of models.. A comparison between experimental data (Qexp) and those calculated by each model (Qcal) is done by using the most statistical analysis functions widely used in the literature whose formulas are summarized in Table 3. The calculation is done by commercially available software OriginPro 8®. The smaller RMSE (Residual root Mean Square Error) and SSE (Sum of the Squares of the Errors) value indicates the better curve fitting Table 3 Formulas of statistical indices used in the study. Statistical indices The residual root mean square error (RMSE) The chi-square test (χ2) coefficients of determination (CD)

Their Formulas 1 n ∑ n − 2 i=1

RMSE = n

χ 2 = ∑i = 1

References - [50]

(Qiobs − Qical )2 (7)

(Qiobs − Qical Qical

)2

- [51]- [50]

(8)

∑in= 1 (Qiobs − Q¯ eobs )2 − ∑in= 1 (Qiobs − Qical )2 ∑in= 1 (Qiobs − Q¯ eobs )2 n = ∑i = 1 (Qiobs − Qical )2 (10)

CD =

The sum of the squares of the errors (SSE)

SEE

Adjusted R-Squared (r2Adj)

r 2Adj = 1 − (1 − r 2).

(n − 1) (p − 1)

(11)

(9)

- [52]- [53]- [51] - [52]- [51] - [51]

Where n is the number of experimental data points, p is the number of fitted parameters, Qiobs is the ith observation from the batch experiment, Q¯ eobs is the average of observed values, and Qical is the calculated value with the model corresponding to the ith replication. 5

Journal of Hazardous Materials 378 (2019) 120718

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Table 4 Thermodynamic parameters of Cr3+ sorption onto PW (C0 = 3.9 g.L−1, stirring speed = 200 rpm, stirring time t = 24 h, PW dose = 40 g.L−1).

types of adsorption sites; and therefore having a heterogeneous surface. For this model, the value of 1/n gives an indication of the adsorption validity of the adsorbent-adsorbate system. A value of 1/n between 0 and 1 (i.e. n > 1) indicates a favorable adsorption [46]. When 1/ n < 1, it is a chemisorption and when n = 1, all the sites are similar, and so the Freundlich isotherm reduces to a Langmuir isotherm. According to the data in Table 2, the value of 1/n shows that Cr adsorption onto PW is a chemical sorption process. The calculated value of n (> 1) shows that the adsorption sites are heterogeneous, and therefore the model is far to tend toward the Langmuir equation. This ascertainment is proved by other literature studies that have shown that according to the Freundlich equation, the isotherm does not reach a plateau as the concentration increases [42]. Therefore, it can be concluded from the bibliographic analysis and from the calculated statistical parameters that the most appropriate model to describe the adsorbent/adsorbate relationship, in this study, is Freundlich model. But, it is often found that when the Freundlich equation is fitted to data at high and intermediate concentrations, it may provide a poor fit to data at low concentrations [55]. 3.2.1.5. Effect of the temperature variation.. Fig. 9 represents the temperature variation effect on the adsorption efficiency of chromium (R%) and on the adsorption capacity (mg. g−1) at the equilibrium dose of PW (40 g.L−1). It can be seen that both parameters increase with increasing temperature. The best result is observed at 50 °C where more than 99% of chromium is eliminated at the equilibrium dose of the adsorbent with a capacity of 97.23 mg.g−1, versus 53.04 mg.g−1, 82.91 mg.g−1 and 88.91 mg.g−1 at 15 °C, 25 °C, 30 °C respectively.

ΔH ° ΔS° + RT R

ΔG° (kJ. mol−1)

26.72

197.78

288 (K) −31.07

298 (K) −31.48

303 (K) −32.50

323 (K) −37.73

3.2.1.7. Effect of contact time and Kinetic tests.. Effect of contact time on Cr3+ adsorption onto PW is shown in Fig. 10. The adsorption capacity increased rapidly in the initial stages of contact time to rich almost 80 mg.g−1, and that after 6 h of stirring at 200 rpm. Then it slowly increased to approximately the rate 83 mg.g−1 and it became stable. In order to investigate the kinetic mechanism of Cr3+ sorption onto PW, two kinetic models were examined: pseudo-first-order (Eq. (15)) and pseudo-second-order (Eq. (16)).

3.2.1.6. Thermodynamic studies.. The thermodynamic parameters of the sorption process: Gibbs energy, ΔG° (kJ. mol−1), standard entropy change, ΔS° (J. mol−1. K−1) and standard enthalpy change, ΔH° (kJ. mol−1) were determined at 40 g.L−1 of the adsorbent; using the Van't Hoff equation.

LnK e = −

ΔS° (J. mol−1. K−1)

reason for choosing this method to calculate the thermodynamic equilibrium constant (Ke) among several methods very spreads in the literature is discussed in [56]. For this calculation, it is considered that at the equilibrium state, the adsorbate solution is much diluted to consider that the activity coefficient (γ) equals to unitary. In our case, the best model fitted is Freundlich isotherm. It is an empirical model with (Kf) expressed in (mg. g−1)/(mg.L−1)1/n. So, the application of this equation for Freundlich model is difficult, especially for the adjustment of units. Then, the Eq. (14) was applied to R-P model because statistically this model is very close to Freundlich model. The calculation of the values of ΔS◦ and ΔH° is done by plotting Ln Ke versus 1/T. Table 4 summarizes the result obtained. Negative values of ΔG° indicate the spontaneous nature of Cr3+ adsorption onto PW. The increase in negative values of ΔG° with the temperature increase expresses that the removal process is more spontaneous at high temperature [47], and then the efficiency of Cr3+ adsorption is better at high temperatures [57]. Positive value of ΔH° suggests endothermic reaction [58]. The positive value of ΔS° indicated that the solid–liquid interface becomes increasingly disordered during the Cr3+ adsorption process. Similar results were reported for the adsorption of Cr3+ from tanning wastewater on eggshell and powdered marble investigated by [16]. Theoretically, standard enthalpy change ΔH° indicates a physical adsorption if its value range from 2.1 to 20.9 kJ.mol−1, and chemisorption if it is from 80 to 200 kJ.mol−1 [59]. Therefore, it seems that Cr3+ sorption by PW would be attributed to a physico-chemical adsorption process rather than a pure physical or chemical adsorption process. Similar result was obtained by [60] for the adsorption of Ni2+ onto aerobic granules.

Fig. 9. variation of both chromium adsorption rate (R) and capacity Qe (mg.g−1) at different temperatures (C0= 3.9 g.L−1, V=50 mL, stirring speed=200 rpm, stirring time=24h, 40g.L-1 of PW).

ΔG° = −RTLnK e

ΔH° (kJ. mol−1)

log(Qe − Qt) = log Q e − k1.t

(15)

(12)

(13)

Where Ke (dimensionless) is the thermodynamic equilibrium constant calculated by using Eq. (14) [56], R is the universal gas constant (8.314 J.mol−1. K−1), and T is the absolute temperature (K).

Ke =

K g × 1000 × M × [A] ° γ

(14)

−1

Kg (L. mg ) is the constant of the best isotherm model fitted. M is the molecular weight of the adsorbate (in our case, M = 52 g.mol−1). [A]° is the standard concentration of the adsorbate (1 mol.L−1) [56]; and γ is the activity coefficient of the adsorbate (dimensionless). The

Fig. 10. Effect of contact time onto adsorption capacity of chromium (pH= 3.43, stirring speed=200 rpm, PW dose = 40 g.L−1). 6

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Table 5 Kinetic parameters for adsorption of Cr3+ ions onto PW. Pseudo first order Qe.exp (mg. g 82.91

−1

)

Qe. Cal (mg. g 14.32

−1

)

Pseudo second order k1 (min 0.0028

−1

)

2

R 0.563

2

r Adj 0.50832

Qe. Cal (mg. g−1) 86.207

k2 (g. mg−1. min−1) 2.49 × 10−4

R2 0.9958

r2Adj 0.9953

on the temperature. The best result was obtained at 50 °C. At 2 g of PW (40 g.L−1), the COD removal rate is 50.54%, 60%, 62.35% and 66.27% at 15 °C, 25 °C, 30 °C and 50 °C successively. PW 40 g.L−1 is the equilibrium dose for COD removal, which represent also the equilibrium state of chromium removal. 4. Conclusion The present study reports the characteristics and mechanism of Cr3+ sorption from tanning effluent, when using the phosphate mine waste as a sorbent. Obtained results show that PW was able to eliminate effectively the chromium, and reduce considerably the rate of COD. In fact, the treatment of chromium tanning wastewater by adsorption using PW makes it possible to remove more than 90% (90.11% at 30 °C and 99.21% at 50 °C) of chromium; and more than 60% of the COD. Thus this treatment could neutralize the acidic pH which characterizes the chromium tanning effluent. Chromium removal by PW is very efficient at high speed (200 rpm) and at high temperature (50 °C). Thermodynamic studies show that chromium sorption by PW is a physico-chemical adsorption process. This reaction follows Freundlich model isotherm which suggests a multilayer adsorption. Cr3+ was probably removed by ion exchange process, precipitation, co-precipitation and adsorption on the surface PW particles. The treatment of chromium tanning wastewater by PW shows that this waste is an effective adsorbent for chromium. Therefore, this treatment is both economical and ecological: it can eliminate the pollution caused by chromium tanning wastewater, reduce a considerable quantity of COD and, at the same time, valorize PW which is also harm to the environment.

Fig. 11. Effect of stirring speeds (a) and temperatures (b) on COD removal.

t 1 t = + Qt K2. Qe2 Qe

(16)

Where Qe (mg. g−1) and Qt (mg. g−1) are the amount of chromium adsorbed per unit mass of the adsorbent, respectively, at the equilibrium and at current time (min). k1 (min−1) and k2 (g. mg−1. min−1) are the equilibrium rate constant of, respectively, the pseudo-first-order model and the pseudo-second-order model. These kinetic parameters and correlation coefficients can be calculated using linear fitting line. R2 and r2Adj are used to evaluate the applicability of kinetic models. The corresponding parameters are summarized in Table 5. It can be seen that R2 and r2Adj of pseudo-second-order model are much greater than those of pseudo-first-order model. At the same time, the calculated value Qe.cal for pseudo-second-order model is closer to the experimentally observed Qe.exp than the value calculated from pseudo-first-order model. These results suggest that our system follows pseudo-second-order kinetic model; and then Cr3+ sorption onto PW is mainly controlled by the chemical interaction involved covalent/valence forces through exchange or sharing of electrons between Cr3+ and binding sites on the surface of PW [61].

Acknowledgments This work was supported by the Moroccan -Tunisian cooperation project TM/N°17TM; the National Center for Research and Studies on Water and Energy (Cadi Ayyad University) and the Pole of competences on Water and Environment (PC2E-Morocco). References [1] Neeta Singh, A. Gupta, Adsorption of Heavy Metals: a Review, Int. J. Innov. Res. Sci. Eng. Technol. 5 (2016) 2267–2281 doi:0.15680/IJIRSET.2016.050146. [2] H.K. Alluri, R.R. Srinivasa, S.S. Vijaya, S.B. Jayakumar, V. Suryanarayana, P. Venkateshwar, Biosorption: an eco-friendly alternative for heavy metal removal, Afr. J. Biotechnol. 6 (2007) 2924–2931, https://doi.org/10.5897/AJB2007.0002461. [3] S. Elabbas, N. Ouazzani, L. Mandi, F. Berrekhis, M. Perdicakis, S. Pontvianne, M.N. Pons, F. Lapicque, J.-P. Leclerc, Treatment of highly concentrated tannery wastewater using electrocoagulation: influence of the quality of aluminium used for the electrode, J. Hazard. Mater. 319 (2016) 69–77, https://doi.org/10.1016/j. jhazmat.2015.12.067. [4] M. Fabbricino, B. Naviglio, G. Tortora, L. d’Antonio, An environmental friendly cycle for Cr(III) removal and recovery from tannery wastewater, J. Environ. Manage. 117 (2013) 1–6, https://doi.org/10.1016/j.jenvman.2012.12.012. [5] D. Wang, S. He, C. Shan, Y. Ye, H. Ma, X. Zhang, W. Zhang, B. Pan, Chromium speciation in tannery effluent after alkaline precipitation: isolation and characterization, J. Hazard. Mater. 316 (2016) 169–177, https://doi.org/10.1016/j.jhazmat. 2016.05.021. [6] A. Ramírez-Estrada, V.Y. Mena-Cervantes, J. Fuentes-García, J. Vazquez-Arenas, R. Palma-Goyes, A.I. Flores-Vela, R. Vazquez-Medina, R.H. Altamirano, Cr(III) removal from synthetic and real tanning effluents using an electro-precipitation method, J. Environ. Chem. Eng. 6 (2018) 1219–1225, https://doi.org/10.1016/j. jece.2018.01.038.

3.2.2. COD removal efficiency Organic matter is an important component that influences heavy metals speciation, their bioavailability and therefore their toxicity [62,63]. In general, tannery wastewater is very loaded with organic matter (Table 1). In order to study the influence of PW on the COD removal, different adsorbent doses and stirring speeds were investigated. Fig. 11a and b illustrate the results obtained and show that the removal of COD is independent on the stirring speed, but depends 7

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A. Oumani, et al.

[7] T. Riaz, A. Ahmad, S. Saleemi, M. Adrees, F. Jamshed, A.M. Hai, T. Jamil, Synthesis and characterization of polyurethane-cellulose acetate blend membrane for chromium (VI) removal, Carbohydr. Polym. 153 (2016) 582–591, https://doi.org/10. 1016/j.carbpol.2016.08.011. [8] L.-L. Li, X.-Q. Feng, R.-P. Han, S.-Q. Zang, G. Yang, Cr(VI) removal via anion exchange on a silver-triazolate MOF, J. Hazard. Mater. 321 (2017) 622–628, https:// doi.org/10.1016/j.jhazmat.2016.09.029. [9] A.S. Dharnaik, P.K. Ghosh, Hexavalent chromium [Cr(VI)] removal by the electrochemical ion-exchange process, Environ. Technol. 35 (2014) 2272–2279, https://doi.org/10.1080/09593330.2014.902108. [10] S.A. Rim, D.M. Amine, B. Nasr-Eddine, J.P. Canselier, Removal of chromium (III) by two-aqueous phases extraction, J. Hazard. Mater. 167 (2009) 896–903, https://doi. org/10.1016/j.jhazmat.2009.01.087. [11] A. Deghles, U. Kurt, Treatment of tannery wastewater by a hybrid electrocoagulation /electrodialysis process, Chem. Eng. Process. Process Intensif. 104 (2016) 43–50, https://doi.org/10.1016/j.cep.2016.02.009. [12] V.A. Papaevangelou, G.D. Gikas, V.A. Tsihrintzis, Chromium removal from wastewater using HSF and VF pilot-scale constructed wetlands: overall performance, and fate and distribution of this element within the wetland environment, Chemosphere. 168 (2017) 716–730, https://doi.org/10.1016/j.chemosphere.2016. 11.002. [13] S. Tiglyene, A. Jaouad, L. Mandi, Treatment of tannery wastewater by infiltration percolation: chromium removal and speciation in soil, Environ. Technol. 29 (2008) 613–624, https://doi.org/10.1080/09593330801983888. [14] J.-K. Yang, S.-M. Lee, Removal of Cr(VI) and humic acid by using TiO2 photocatalysis, Chemosphere. 63 (2006) 1677–1684, https://doi.org/10.1016/j. chemosphere.2005.10.005. [15] H. Panda, N. Tiadi, M. Mohanty, C.R. Mohanty, Studies on adsorption behavior of an industrial waste for removal of chromium from aqueous solution, South Afr. J. Chem. Eng. 23 (2017) 132–138, https://doi.org/10.1016/j.sajce.2017.05.002. [16] S. Elabbas, L. Mandi, F. Berrekhis, M.N. Pons, J.P. Leclerc, N. Ouazzani, Removal of Cr(III) from chrome tanning wastewater by adsorption using two natural carbonaceous materials: eggshell and powdered marble, J. Environ. Manage. 166 (2016) 589–595, https://doi.org/10.1016/j.jenvman.2015.11.012. [17] D. Park, S.-R. Lim, Y.-S. Yun, J.M. Park, Reliable evidences that the removal mechanism of hexavalent chromium by natural biomaterials is adsorption-coupled reduction, Chemosphere. 70 (2007) 298–305, https://doi.org/10.1016/j. chemosphere.2007.06.007. [18] M. Nouri Sepehr, S. Nasseri, M. Mazaheri Assadi, K. Yaghmaian, Chromium bioremoval from tannery indutries effluent by Aspergillus oryzae, Iran J Env. Health Sci Eng. 2 (2005) 273–279. [19] D. Özer, Z. Aksu, T. Kutsal, A. Çaglar, Adsorption isotherms of lead(II) and chromium(VI) on Cladophora crispata, Environ. Technol. 15 (1994) 439–448, https:// doi.org/10.1080/09593339409385448. [20] S. Srivastava, I.S. Thakur, Biosorption and biotransformation of chromium by Serratia sp. isolated from tannery effluent, Environ. Technol. 33 (2012) 113–122, https://doi.org/10.1080/09593330.2011.551842. [21] N. Balkaya, Variation of Ph, conductivity and potential values in chromium (vi) removal by wool, Environ. Technol. 24 (2003) 11–16, https://doi.org/10.1080/ 09593330309385530. [22] K.S. Low, C.K. Lee, S.G. Tan, Sorption of trivalent chromium from tannery waste by moss, Environ. Technol. 18 (1997) 449–454, https://doi.org/10.1080/ 09593331808616559. [23] G. Arslan, E. Pehlivan, Batch removal of chromium(VI) from aqueous solution by Turkish brown coals, Bioresour. Technol. 98 (2007) 2836–2845, https://doi.org/ 10.1016/j.biortech.2006.09.041. [24] I.A. Bhatti, N. Ahmad, N. Iqbal, M. Zahid, M. Iqbal, Chromium adsorption using waste tire and conditions optimization by response surface methodology, J. Environ. Chem. Eng. 5 (2017) 2740–2751, https://doi.org/10.1016/j.jece.2017.04. 051. [25] S. Dubey, K. Gopal, Adsorption of chromium(VI) on low cost adsorbents derived from agricultural waste material: a comparative study, J. Hazard. Mater. 145 (2007) 465–470, https://doi.org/10.1016/j.jhazmat.2006.11.041. [26] U.K. Garg, M.P. Kaur, V.K. Garg, D. Sud, Removal of hexavalent chromium from aqueous solution by agricultural waste biomass, J. Hazard. Mater. 140 (2007) 60–68, https://doi.org/10.1016/j.jhazmat.2006.06.056. [27] M.H. Dehghani, D. Sanaei, I. Ali, A. Bhatnagar, Removal of chromium(VI) from aqueous solution using treated waste newspaper as a low-cost adsorbent: kinetic modeling and isotherm studies, J. Mol. Liq. 215 (2016) 671–679, https://doi.org/ 10.1016/j.molliq.2015.12.057. [28] E. Malkoc, Y. Nuhoglu, Potential of tea factory waste for chromium(VI) removal from aqueous solutions: thermodynamic and kinetic studies, Sep. Purif. Technol. 54 (2007) 291–298, https://doi.org/10.1016/j.seppur.2006.09.017. [29] G. Vilardi, L. Di Palma, N. Verdone, Heavy metals adsorption by banana peels micro-powder: equilibrium modeling by non-linear models, Chin. J. Chem. Eng. 26 (2018) 455–464, https://doi.org/10.1016/j.cjche.2017.06.026. [30] H.C.P. Srivastava, R.P. Mathur, I. Mehrotra, Removal of chromium from industrial effluents by adsorption on sawdust, Environ. Technol. Lett. 7 (1986) 55–63, https:// doi.org/10.1080/09593338609384391. [31] E. Keleş, A.K. Özer, S. Yörük, Removal of Pb2+ from aqueous solutions by rock phosphate (low-grade), Desalination 253 (2010) 124–128, https://doi.org/10. 1016/j.desal.2009.11.021. [32] M. Sarioglu, ü.A. Atay, Y. Cebeci, Removal of copper from aqueous solutions by phosphate rock, Desalination. 181 (2005) 303–311, https://doi.org/10.1016/j. desal.2005.04.009. [33] S.E. Asri, A. Laghzizil, T. Coradin, A. Saoiabi, A. Alaoui, R. M’hamedi, Conversion of

[34]

[35]

[36] [37] [38]

[39]

[40]

[41]

[42]

[43]

[44]

[45] [46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57]

[58]

8

natural phosphate rock into mesoporous hydroxyapatite for heavy metals removal from aqueous solution, Colloids Surf. Physicochem. Eng. Asp. 362 (2010) 33–38, https://doi.org/10.1016/j.colsurfa.2010.03.036. B. Chen, J. Wang, L. Kong, X. Mai, N. Zheng, Q. Zhong, J. Liang, D. Chen, Adsorption of uranium from uranium mine contaminated water using phosphate rock apatite (PRA): isotherm, kinetic and characterization studies, Colloids Surf. Physicochem. Eng. Asp. 520 (2017) 612–621, https://doi.org/10.1016/j.colsurfa. 2017.01.055. H. Yaacoubi, O. Zidani, M. Mouflih, M. Gourai, S. Sebti, Removal of cadmium from water using natural phosphate as adsorbent, Procedia Eng. 83 (2014) 386–393, https://doi.org/10.1016/j.proeng.2014.09.039. AFNOR, Recueil De Norme Française: Eau, Méthodes d’essai.2Éme Édition, Paris (1997). J. Rodier, B. Legube, N. Merlet, L’analyse De l’eau, Dunod, Paris (2009). APHA, AWWA, WEF, Standard methods for the examination of Water and wastewater, American Public Health Association/American Water Works Association/ Water Environment Federation, 21st ed., (2005) Washington, DC, USA. H.M. Edenborn, B.H. Howard, J.I. Sams, D.J. Vesper, S.L. Edenborn, Passive detection of Pb in water using rock phosphate agarose beads, J. Hazard. Mater. 336 (2017) 240–248, https://doi.org/10.1016/j.jhazmat.2017.04.036. A. Bouhaouss, A. Bensaoud, M. El Moussaouiti, M. Ferhat, Analyse fine de l’apatite analogue aux biomatériaux par la spectroqcopie infrarouge, Phys. Chem. News. 1 (2001) 125–129. W. Wei, J. Cui, Z. Wei, Effects of low molecular weight organic acids on the immobilization of aqueous Pb(II) using phosphate rock and different crystallized hydroxyapatite, Chemosphere. 105 (2014) 14–23, https://doi.org/10.1016/j. chemosphere.2013.09.121. G. Limousin, J.-P. Gaudet, L. Charlet, S. Szenknect, V. Barthès, M. Krimissa, Sorption isotherms: a review on physical bases, modeling and measurement, Appl. Geochem. 22 (2007) 249–275, https://doi.org/10.1016/j.apgeochem.2006.09.010. C.H. Giles, T.H. MacEwan, S.N. Nakhwa, D. Smith, 786. Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, J. Chem. Soc. Resumed. 3973 (1960), https://doi.org/10.1039/ jr9600003973. S. Alahiane, S. Qourzal, M. El Ouardi, M. Belmouden, A. Assabbane, Y. Ait-Ichou, Adsorption et photodégradation du colorant indigo carmine en milieu aqueux en présence de TiO2/UV/O2 (Adsorption and photocatalytic degradation of indigo carmine dye in aqueous solutions using TiO2/UV/O2), J. Mater. Environ. Sci. 4 (2013) 239–250. K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem. Eng. J. 156 (2010) 2–10, https://doi.org/10.1016/j.cej.2009.09.013. B.H. Hameed, A.A. Ahmad, N. Aziz, Isotherms, kinetics and thermodynamics of acid dye adsorption on activated palm ash, Chem. Eng. J. 133 (2007) 195–203, https:// doi.org/10.1016/j.cej.2007.01.032. N.A. Reddy, R. Lakshmipathy, N.C. Sarada, Application of Citrullus lanatus rind as biosorbent for removal of trivalent chromium from aqueous solution, Alex. Eng. J. 53 (2014) 969–975, https://doi.org/10.1016/j.aej.2014.07.006. F.-C. Wu, B.-L. Liu, K.-T. Wu, R.-L. Tseng, A new linear form analysis of Redlich–Peterson isotherm equation for the adsorptions of dyes, Chem. Eng. J. 162 (2010) 21–27, https://doi.org/10.1016/j.cej.2010.03.006. J.G. Meneguin, M.P. Moisés, T. Karchiyappan, S.H.B. Faria, M.L. Gimenes, M.A.S.D. de Barros, S. Venkatachalam, Preparation and characterization of calcium treated bentonite clay and its application for the removal of lead and cadmium ions: adsorption and thermodynamic modeling, Process Saf. Environ. Prot. 111 (2017) 244–252, https://doi.org/10.1016/j.psep.2017.07.005. K. Vijayaraghavan, T. Padmesh, K. Palanivelu, M. Velan, Biosorption of nickel(II) ions onto Sargassum wightii: application of two-parameter and three-parameter isotherm models, J. Hazard. Mater. 133 (2006) 304–308, https://doi.org/10.1016/ j.jhazmat.2005.10.016. A. Mandal, N. Singh, Kinetic and isotherm error optimization studies for adsorption of atrazine and imidacloprid on bark of Eucalyptus tereticornis L, J. Environ. Sci. Health B 51 (2016) 192–203, https://doi.org/10.1080/03601234.2015.1108817. G. Alberti, V. Amendola, M. Pesavento, R. Biesuz, Beyond the synthesis of novel solid phases: review on modelling of sorption phenomena, Coord. Chem. Rev. 256 (2012) 28–45, https://doi.org/10.1016/j.ccr.2011.08.022. D. Koeppenkastrop, E.H. De Carlo, Uptake of rare earth elements from solution by metal oxides, Environ. Sci. Technol. 27 (1993) 1796–1802, https://doi.org/10. 1021/es00046a006. K. Yamaoka, T. Nakagawa, T. Uno, Application of Akaike’s information criterion (AIC) in the evaluation of linear pharmacokinetic equations, J. Pharmacokinet. Biopharm. 6 (1978) 165–175, https://doi.org/10.1007/BF01117450. C. Ng, J.N. Losso, W.E. Marshall, R.M. Rao, Freundlich adsorption isotherms of agricultural by-product-based powdered activated carbons in a geosmin–water system, Bioresour. Technol. 85 (2002) 131–135, https://doi.org/10.1016/S09608524(02)00093-7. E.C. Lima, A. Hosseini-Bandegharaei, J.C. Moreno-Piraján, I. Anastopoulos, A critical review of the estimation of the thermodynamic parameters on adsorption equilibria. Wrong use of equilibrium constant in the Van’t Hoof equation for calculation of thermodynamic parameters of adsorption, J. Mol. Liq. 273 (2019) 425–434, https://doi.org/10.1016/j.molliq.2018.10.048. C. Fang, T. Zhang, P. Li, R. Jiang, Y. Wang, Application of magnesium modified corn biochar for phosphorus removal and recovery from swine wastewater, Int. J. Environ. Res. Public Health 11 (2014) 9217–9237, https://doi.org/10.3390/ ijerph110909217. Y. Shi, T. Zhang, H. Ren, A. Kruse, R. Cui, Polyethylene imine modified hydrochar

Journal of Hazardous Materials 378 (2019) 120718

A. Oumani, et al.

polyindole nanofiber membrane for effective removal of Cr(VI) from aqueous solution, J. Mater. Sci. 52 (2017) 5417–5434, https://doi.org/10.1007/s10853-0170786-2. [62] J. Buffle, P. Deladoey, F.L. Greter, W. Haerdi, Study of the complex formation of copper(II) by humic and fulvic substances, Anal. Chim. Acta 116 (1980) 255–274, https://doi.org/10.1016/S0003-2670(01)95206-7. [63] A. Tessier, D.R. Turner (Eds.), Metal Speciation and Bioavailability in Aquatic Systems, J. Wiley, Chichester ; New York, 1995.

adsorption for chromium (VI) and nickel (II) removal from aqueous solution, Bioresour. Technol. 247 (2018) 370–379, https://doi.org/10.1016/j.biortech.2017. 09.107. [59] Y. Liu, Y.-J. Liu, Biosorption isotherms, kinetics and thermodynamics, Sep. Purif. Technol. 61 (2008) 229–242, https://doi.org/10.1016/j.seppur.2007.10.002. [60] Y. Liu, H. Xu, Equilibrium, thermodynamics and mechanisms of Ni2+ biosorption by aerobic granules, Biochem. Eng. J. 35 (2007) 174–182, https://doi.org/10. 1016/j.bej.2007.01.020. [61] C. Zhijiang, S. Xianyou, Z. Qing, L. Yuanpei, Amidoxime surface modification of

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