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Bone charcoal (BC) is being developed as a treatment for decontamination of polluted water. In this study, bone charcoal was obtained by pyrolysis of cow bones and tested for the elimination of copper from aqueous solutions. The minimum time to reach the removal equilibrium by adsorption was 10 min with a maximum of copper removal equal to 9615 mg/g. Different kinetics models were applied to fit the experimental data: the pseudo second-order model correlated the results with a linear correlation coefficient equal to 1.

Heavy metals are inorganic pollutants frequently found in the wastewaters of several industries such as electroplating, mining, metal processing, petroleum refining, textile, tanneries, paint manufacture, pesticides, battery manufacturing, pigment manufacture, photographic industries and printing [

The purification of heavy metals contaminated wastewaters can be realized by chemical precipitation [

The objective of this study is to develop cheap and effective adsorbents from the biological wastes, such as cow bone, to replace the classical commercial adsorbents. So cow bone from which the bone charcoal was derived, is used as an adsorbent and investigated for its adsorption properties towards copper ions. The influences of different experimental parameters, such as the amount of BC and the contact time, have been investigated. Different kinetics models were also applied to fit the experimental data.

A stock of copper solution (888.7 ppm) was prepared by dissolving CuSO_{4}∙5H_{2}O in distilled water.

Cortical bovines have been collected from the local slaughter houses in Sfax, Tunisia. The bone samples have been washed and cleaned using boiling method to eliminate organic substances and collagen, to avoid soot developing in the material during the pyrolysis process. Cow bones were boiled in water for 3 h at 99.5˚C. Then the water was eliminated and the bones were washed using fresh water [

Carbonization was carried out in a vertical stainless-steel reactor (length = 170 mm, internal diameter = 22 mm), which was placed into a cylindrical electric furnace Nabertherm. Mineralogical analysis of the BC sample was realized by X-ray diffraction (Bruker D8) and showed the characteristic peaks of amorphous carbon [^{−}^{1} range using a SHIMATZU IR 470 spectrometer. The specific surface area (m^{2}/g) of BC was determined from nitrogen adsorption-de- sorption isotherm with the Brunauer-Emmett-Teller (BET) method. The macroporous volume of BC was measured by mercury intrusion porosimetry (Joschek et al., 2000). The concentration of copper was carried out by atomic absorption spectrometry (HITACHI Z-6100).

The adsorption experiments were determined according to the batch experiments at room temperature (25˚C). 0.1 g of BC was mixed with a 50 mL of copper solution at different concentrations (0 - 500 mg/L). The deviation of the pH of the contaminated heavy metal solution during copper adsorption was measured for all experiments, with an experimental deviation = ±1. The initial pH was fixed <5.5 to avoid precipitation of Cu(OH)_{2}.

The quantity of adsorbed copper, q_{t}, on BC is calculated according to Equation (1):

where C_{o} is the initial concentration of copper (mg/L), C_{e} is the residual concentration of copper (mg/L), m is the mass of BC (g) and V is the volume of the copper solution (L).

In order to examine the mechanism of adsorption processes, the pseudo first- order adsorption, the pseudo second-order adsorption and the intraparticle diffusion models were used to adjust kinetic experimental data. The amount of BC sample (0.25 g) is mixed with a 50 mL of copper solution (10 mg/L) to carry out adsorption experiments in bath mode.

The pseudo first-order rate expression of Lagergren is usually described by the following Equations (2) and (3) [

where q_{e} is the amount of copper adsorbed on BC at the equilibrium time (mg/g), q_{t} is the amount of copper adsorbed on BC at time t (mg/g) and K_{1} is the rate constant of pseudo first-order adsorption (1/min). Integrating and applying the boundary condition, for t = 0, q_{t} = 0 and for t = t, q_{e} = q_{t}, Equation (2) takes the following form:

where K_{1} was determined from the slope of linear plot of 1/q_{t} against 1/t.

The pseudo second-order mechanism for adsorption is shown in Equation (4) [

where q_{t} is the adsorption capacity at time t (mg/g), K_{2} is the rate constant of pseudo second-order adsorption (1/min). Integration and applying the boundary conditions, for t = 0, q_{t} = 0 and for t = t, q_{e} = q_{t}, Equation (4) takes the following form:

If the second order kinetic model is applicable, the plot of t/q_{t} against t of Equation (5) should give a linear relationship from which q_{e} and K_{2} can be established.

The intraparticle diffusion model presented by Allen et al. [

where q_{t} is the adsorption capacity at time t (mg/g), K_{3} is the intraparticle diffusion rate constant (mg/(g∙min^{1/2})) and C is the intercept.

Desorption studies were performed in two phases [

Phase 1: Adsorption. 0.25 g of BC was placed in contact with 50 mL of a 50 mg/L of copper solution. BC was then collected by filtration, washed with distilled water and placed in an oven for 12 h at 60˚C. The liquid phase was analyzed by AAS (HITACHI).

Phase 2: Desorption. The dry and saturated BC was placed in contact with 50 mL of 0.1 M HCl, NaOH, NaCl and distilled water for 2 h. The liquid phase was filtered and analyzed by AAS (HITACHI). The desorbed copper percentage was determined according to Equation (7):

where Q_{des} is the amount of desorbed copper from BC (mg/g) and Q_{ads} is the amount of copper adsorbed onto BC (mg/g).

The bones are composed of inorganic (65 wt%) and organic (35 wt%) components. The principal inorganic component of bone is hydroxyapatite, Ca_{10}(PO_{4})_{6}(OH)_{2}(HA). ^{−1} range of BC. Only the bands characteristic of HA (554 - 960 cm^{−1}) are presented [

The specific surface area of BC, S_{BET}, is equal to 75 m^{2}/g and the macroporous (pore size > 50 nm) volume of BC, determined by mercury porosimetry, is equal to 0.9 cm^{3}/g.

The three kinetic models explained in paragraph 3.2 are used in this study and the results are shown in Figures 3-5. The values of coefficients for the three models are calculated. The correlation coefficient for the pseudo second-order kinetic model is higher (R^{2} = 1, _{2}, represents the number of exchanges between Ca ions in BC and Cu ions present in aqueous solution [_{e} values acquired from the pseudo second-order kinetic model are in accordance with the experimental q_{e} values.

This study investigates heavy metal ions adsorption onto economical Bone Charcoal (BC) adsorbent originating from cow bones. This adsorbent seems to

be efficient to remove the maximum of copper pollution from water, with 0.1 g of BC amount after 10 min. The kinetic studies of copper adsorption on BC indicate that the pseudo second-order model is the most adequate. This result indicates that the copper adsorption is controlled by a chemisorption process at the surface of BC. Although it is potentially a new alternative for the elimination of heavy metals from polluted water, the desorption of copper ions is difficult by the existence of strong covalent bonds between BC and copper ions.

One of us (S. D. L) thanks to the Belgian “Fonds National de Recherche Scientifique” (FRS-FNRS) for her Associate Research position. Furthermore, one other of us (S. G.) thanks to the Laboratory of Water, Environment and Energy, Sfax, Tunisia for the financial funds.

Ghrab, S., Benzina, M. and Lambert, S.D. (2017) Copper Adsorption from Wasterwater Using Bone Charcoal. Advances in Materials Physics and Chemistry, 7, 139-147. https://doi.org/10.4236/ampc.2017.75012