Nickel removal by biosorption onto medlar male flowers coupled with photocatalysis on the spinel ZnMn2O4
© Chergui et al.; licensee BioMed Central Ltd. 2014
Received: 4 September 2012
Accepted: 15 October 2013
Published: 8 January 2014
Ni2+ is a highly toxic above 0.07 mg/L and its removal is of high significance. The biosorption of Ni2+ onto medlar male flowers (MMF) was studied in relation with the physical parameters like pH, contact time, biosorbent dosage, Ni2+ concentration and temperature. The interaction biosorbent-Ni2+ was examined by the FTIR technique. The equilibrium was achieved within 40 min and the data were well fitted by the Langmuir and Redlich-Peterson (R-P) models. The maximum Ni2+ uptake capacity was 17.073 mg/g at 25°C and the Ni2+ removal follows a pseudo-second order kinetic with activation energy of 13.3 kJ/mol. The thermodynamic parameters: ΔS°, ΔH° and ΔG° showed that the biosorption was spontaneous and endothermic. MMF was used as a post treatment technique and the biosorption was coupled with the visible light driven Ni2+ reduction over the spinel ZnMn2O4. The effect of the pH, ZnMn2O4 loading and light intensity on the photoactivity was investigated. 77.5% of Ni2+ was reduced after ~140 min under optimal conditions. The Ni2+ removal reached a rate conversion of 96% of with the coupled system biosorption/photocatalysis is very promising for the water treatment.
KeywordsBiosorption Nickel Medlar male flower Photocatalytic
Extensive industrial activities have led to tremendous increase in the use of toxic metals over the last decades provoking large scale pollution. Soil and water were continuously contaminated by heavy metals coming from various industries. Ni2+ cannot be biodegraded unlike organic pollutants; it persists indefinitely and accumulates through the food chain, thus posing a serious threat to human health. Nickel is known as one of the most common toxic metals and inhaled nickel compounds are carcinogenic and provoke asthma, chronic bronchitis, pulmonary embolism as well as respiratory and may bring nausea, dizziness and diarrhea[2, 3]. The WHO has drastically reduced at 0.07 mg/L the authorized threshold for nickel in drinking water. Therefore, it was necessary to treat metal-contaminated water prior its discharge in the aquatic environment. Several techniques have been used to this end, like precipitation, ion exchange, biosorption and membranes. In recent years, considerable attention has been focused on the biosorption using agricultural materials[6, 7]. Hence, efforts were done to develop inexpensive biosorbents using agricultural wastes. We have tested several biological materials for the metals removal and studies were carried out to investigate the potential of locally available biosorbent namely the medlar male flowers (MMF) for the nickel removal from aqueous solutions. The contact time, biosorbent dose, pH, Ni2+ concentration and temperature were optimized. To understand the nature of the Ni2+ biosorption on MMF, equilibrium isotherms were analyzed by the Freundlich, Langmuir and Redlich-Peterson (R-P) models. Kinetic and thermodynamic parameters were also evaluated.
On the other hand, the biosorption greatly reduces the pollution but often not enough to comply with the standards of the world health organization. At this level, the photocatalysis appears to be quite promising because of its simplicity. There were only few studies on the combined systems (biosorption/photocatalysis) for the water treatment. So, in a second step, the system was directed to the spinel photocatalyst ZnMn2O4 which was presented as having the required photoelectrochemical parameters. Its conduction band (CB) derives from 3d orbital able to reduce metal ions to the elemental states. In addition, it is low cost, non-toxic and chemically stable under the operating conditions.
MMF obtained from local farms (Algiers) is thoroughly washed with water and dried at 80°C. Then, it was crushed, ground in an agate mortar and sieved to select a particle size of 500 μm by using an ASTM standard sieve. It was stored in closed bottles until use.
The FTIR spectra of unloaded and Ni2+ loaded MMF were recorded on an infrared spectrometer (Jasco-3200) using the KBr routine technique. The specific surface area was measured by the BET method with a Quadrasorb SI apparatus (Micromeritics ASAP 2010). The point of zero charge (pzc) was determined by the immersion technique described elsewhere.
The spinel ZnMn2O4 was synthesized by sol–gel. A stochiometric mixture of Zn(NO3)2, 6H2O/Mn(NO3)2, 6H2O (0.025 M/0.05 M), both of purity greater than 99%, was dissolved in 60 mL of ethylene glycol at room temperature. The solution was heated under stirring at 70°C (6 h) under reflux and the viscous solution was dried at 120°C. The amorphous powder was heated at 850°C and furnace cooled. The phase was confirmed by X-ray diffraction (XRD) using monochromatized Cu Kα radiation. The photocatalytic tests were carried out in the same reactor (Figure 1). The light source was a 200 W tungsten lamp (Osram) disposed at 10 cm above the reactor. The light flux was measured with a commercial light meter (roline RO 1332). The light was turned on after a transition period of 2 h for the dark adsorption onto the spinel.
Results and discussion
Characterization of the biosorbent
Physico-chemical characterization of the MMF
Particle size (mm)
BET surface area (m 2 /g)
Effect of contact time
Effect of the biosorbent dose
Effect of temperature
The Ni2+ removal, evaluated for the biosorption isotherms, clearly indicates an endothermic process. However, the biosorption was found to decrease at higher temperatures (55°C) possibly due to the damage of active binding sites in the biomass. In addition, the water vaporization becomes an increasing problem. Nevertheless, the temperature-dependence is somewhat different in our case, and this may be due to the differences in the experimental conditions and the nature of the biosorbent. As a result, the temperature of 40°C is found to be optimal. However, the Ni2+ biosorption at 40°C (84%) is close to that at 25°C (82%). So, to save energy the last temperature is selected for further experiments.
Effect of initial concentration
The isotherm constants for the Ni 2+ biosorption onto MMF under optimized conditions for Langmuir Freundlich and Redlich-Peterson models
The separation factor versus the initial Ni 2+ concentration
- c)The R–P model combines both the Langmuir and Freundlich ones and the hybrid mechanism does not follow a monolayer biosorption :(6)
The exponent g varies between 0 and 1. It is helpful to outline that for g = 1, the equation converts simply to the Langmuir isotherm while for g = 0, it is simplified to the Henry’s law. For, the model is identical to the Freundlich one. Table 2 lists the biosorption parameters together with the R2 values. A comparison between the theoretical and experimental data of Ni2+ biosorption onto MMF was well illustrated in Figure 8. The R–P equation applied over the whole concentrations range gives a coefficient R2 close to 1, thus confirming the best fit for Ni2+ biosorption for the two models. The g values were close to unity and this means that the isotherm approaches the Langmuir model rather than the Freundlich one. Hence, the good fit of the equilibrium data for Langmuir and R–P isotherms confirms the monolayer coverage of Ni2+ onto MMF.
Kinetic parameters for Ni 2+ biosorption onto MMF at different temperatures
χ2 × 103
Thermodynamic parameters of biosorption
Thermodynamic parameters for the biosorption of Ni 2+ onto MMF at different temperatures
As mentioned above, the Ni2+ uptake on MMF greatly reduces the pollution but not sufficiently to comply with the standards of the water quality. However, the biosorption can be used as post treatment for the photocatalyic process. Using colloidal semiconductor particles as light absorbing units is simple and does not need any sophisticated device. The first step was to generate electron/hole (e-/h+) pairs on a semiconductor by energetic photons (hν > Eg). Ni2+ was photo-electrochemically reduced to elemental state. In this respect, the spinel ZnMn2O4 shows light absorption for wavelengths shorter than 730 nm. In addition, the oxide is chemically stable and has been elaborated via sol–gel route in order to increase the active surface.
An agricultural by product namely the Medlar male flower, locally available has been successfully used for Ni2+ biosorption from aqueous solution. The contact time, biosorbent dose, pH and temperature were optimized. The maximum biosorption capacity was obtained at pH 5, and the equilibrium was reached in less than 40 min. The isotherms data were well fitted with both the Langmuir and Redlich-Peterson models. The negative free energy confirmed a favorable Ni2+ biosorption where the positive enthalpy indicated an endothermic nature. The spinel ZnMn2O4 elaborated by co-precipitation, was used to reduce the remaining concentration by photocatalysis into elemental state. A reduction rate of 77.5% was achieved after 2 h irradiation at pH 7.5 with an optimal ZnMn2O4 mass equal to 0.5 g/L. The photoreduction followed a first order kinetic. The results indicated that the coupled system may be a viable alternative and a new way for the Ni2+ removal in aquatic medium.
b Langmuir biosorption constant (L/mg)
Ce Residual Ni2+ concentration at equilibrium (mg/L)
Co Initial Ni2+ concentration (mg/L)
Eg The optical gap
Kf The Freundlich constants denoting the biosorption capacity (mg1-1/n/g L1/n)
k1 Pseudo-first order biosorption rate constant (min-1)
k2 Pseudo-second order biosorption rate constant (g/mg min)
kapp The apparent reaction constant
kc Standard thermodynamic equilibrium constant.
kapp The apparent reaction constant
m Biosorbent dosage (g)
ms semiconducteur dosage (g)
n The Freundlich constants denoting the biosorption intensity
qe Amount of Ni2+ adsorbed on the biosorbent at equilibrium (mg/g)
qmax Langmuir biosorption constant (mg/g)
qt Amount of Ni2+ adsorbed on the sorbent at any time (mg/g)
R Universal gas constant
R2 Correlation coefficient
RL Dimensionless separation factor
T Absolute temperature (K)
t Time of biosorption (min)
ΔG° Free energy (kJ/mol)
ΔH° Enthalpy change (kJ/mol)
ΔS° Entropy change (J/mol K)
The authors are grateful to Dr Nabila Nedjari for technical assistance in the FTIR measurements. This research is financially supported by the Faculty of Chemistry (USTHB, Algiers).
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