3.1. Effect of cow bone charcoal dosage on adsorption
Figure 1 shows the removal of Mn
2+, Fe
2+, Ni
2+ and Cu
2+ from aqueous solutions of pH 5.1 as a function of added CBC. Cow bone charcoal dosage ranged from 0.01 to 0.08 g for the 100 mL of Mn
2+, Fe
2+, Ni
2+and Cu
2+ test solutions, equilibrated for 60 min. It can be seen that the maximum removal expressed as a percentage was between 75% and 98% from Mn to Cu at dosages between 0.02 g and 0.03 g of CBC. Ion removal increased quickly from 0.01 g to 0.02 g CBC dosages and reached a maximum for 0.03 g CBC. This fact may be associated with the M
2+ ion availability at pH 5.1. From p
Kh values, it can be concluded that, at pH 5.1, Mn
2+ ions have a concentration 1,500 times greater than Cu
2+ ions. On the other hand, hydrated Mn
2+ ions have a volume almost 30% bigger than hydrated Cu
2+ ions. Mn
2+ ions are more likely to be in solution rather than adsorbed. The observed constancy in percentage ion removal beyond 0.03 g/100 mL may be an indicative of a very weak interaction between adsorbent and adsorbate. This interaction appears weaker with Mn
2+ ions than with Cu
2+ ions. Ion solution concentration seems to attain a steady state with adsorbed species, and so, no matter the quantity of adsorbent present, there will be a residual concentration of ions in solution. This fact determines a specific relation between ion concentration and adsorbent quantity.
Figure 1.
CBC adsorbent dosage effect on Mn2+, Fe2+, Ni2+ and Cu2+ removal. Conditions: Co, 20 mg·L–1; time of contact, 60 min; pH 5.1 and temperature, 298 K.
Figure 1.
CBC adsorbent dosage effect on Mn2+, Fe2+, Ni2+ and Cu2+ removal. Conditions: Co, 20 mg·L–1; time of contact, 60 min; pH 5.1 and temperature, 298 K.
Adsorption of metal ions on these types of materials is generally attributed to weak interactions between the adsorbents and adsorbates. Surface charges on substrates, as well as softness or hardness of the solutes are mostly responsible for the intensity of these interactions. Coulombic interactions can be observed for the ionic interexchange of cationic species with anionic sites in the materials and is determined by their surface areas.
3.3. Effect of pH
Mn
2+, Fe
2+, Ni
2+ and Cu
2+ uptake as a function of hydrogen ion concentration was determined for pH values from 2 to 14. Below pH 5.1, hydrogen ions are likely to compete with manganese, iron, nickel and copper ions. At pH values above 8 manganese, iron, nickel and copper might precipitate as hidroxides. pH effects at equilibrium are presented in
Figure 3. Maximum adsorption was observed about pH 5.1. In general, results indicated that the adsorption is highly pH dependant. Similar results have been reported in literature [
20] S.K. Srivastava, R. Tyagi and N. Pant, Water Res. 23 (1989), pp. 1161–1165. Abstract | View Record in Scopus | Cited By in Scopus (106) [
19].
Figure 3.
pH effect on CBC adsorption of Mn2+, Fe2+, Ni2+ and Cu2+. Conditions: C0, 20 mg∙L–1; CBC dose, 0.02 g; contact time, 20 min and temperature, 298 K.
Figure 3.
pH effect on CBC adsorption of Mn2+, Fe2+, Ni2+ and Cu2+. Conditions: C0, 20 mg∙L–1; CBC dose, 0.02 g; contact time, 20 min and temperature, 298 K.
pH values affect the species of heavy metals in aqueous solutions, and heavy metal removal increases as pH value rises, reaching a maximum around 5.1. Solution pH also affects the adsorbent and the surface charge of the CBC changes. Solubility product (Ksp) calculations predict that the formation of Cu(OH)2, occurs at a pH value of 6. Precipitation occurs at pH 6, along with a qe of 26.7 mg∙g–1. On the other hand, the qe has a value of 35 mg∙g–1 when the initial pH was 5.1 (final pH of 2). This means that the removal of copper ions from the solution also contributes to the pH modification. However, at low initial pH values, below 4, the influence of adsorption is the only effect responsible for the reducing of copper ions in the solution. This suggests that the process is a suitable application for heavy metals removal because of its neutral and clean effluent.
3.4. Adsorption isotherms from aqueous solution
When the initial metal concentration rises, adsorption increases, while the binding sites are not saturated. Linear Langmuir isotherm allows the calculation of adsorption capacities and the Langmuir constants and is performed by the following equation:
Linear plots of c
eq/
q vs c
eq (not shown), were used to calculate by means of linear regression equations, the parameters of the Langmuir isotherm. From these regression equations and the linear plots, the values of the Langmuir constants were calculated and are shown on
Table 2.
qmax and
b were obtained from the slope and intercept of the plots. The essential characteristic of the Langmuir isotherms can be expressed in terms of a dimensionless constant separation factor or equilibrium parameter, R
L, which is defined as [
32]:
where
b is the Langmuir constant and c
o is the initial concentration of the metal ions. R
L value indicates the shape of the isotherm. R
L values between 0 and 1 indicate favorable absorption [
33]. R
L equal to 0 indicate irreversible absorption, R
L = 1 is linear and R
L > 1 is unfavorable. From our study, R
L values for Mn
2+, Fe
2+, Ni
2+ and Cu
2+ ions adsorption ranged from 0.0050 to 0.0060. This, for an initial metal ions concentration of 600 mg∙L
–1, therefore, the adsorption process is favorable.
The Freundlich isotherm was chosen to estimate the adsorption intensity of the adsorbent towards the adsorbate. It is represented by the equation [
34]:
where c
eq is the equilibrium concentration (mg∙L
–1),
q is the ion amount adsorbed (mg∙g
–1) and
KF and
n are constants incorporating all parameters affecting the adsorption process, such as adsorption capacity and intensity respectively. Linear form of Freundlich adsorption isotherm was used to evaluate the sorption data and is represented as [
34]:
The linear regression equation for the Freundlich adsorption isotherm is shown on
Table 4. Values of K
F and
n were calculated from the intercepts and slopes of the Freundlich plots respectively and are shown on this table. Adsorption is favorable for values 0.1 < 1/n < 1.0 [
35].
Table 4.
Isotherm parameters of Mn2+, Fe2+,Ni2+ and Cu2 adsorption on cow bone charcoal.
Table 4.
Isotherm parameters of Mn2+, Fe2+,Ni2+ and Cu2 adsorption on cow bone charcoal.
| | Freundlich model | Langmuir model |
---|
Metal | Linear KD (L/g) | KF | 1/n | R2 | qmax (mg/g) | b (L/g) | RL | R2 |
---|
Mn2+ | 6.76 | 14.457 | 0.315 | 0.9587 | 29.56 | 1.12 | 0.006 | 0.9987 |
Fe2+ | 6.99 | 23.545 | 0.425 | 0.9643 | 31.43 | 1.18 | 0.005 | 0.9988 |
Ni2+ | 7.89 | 26.876 | 0.643 | 0.9745 | 32.54 | 1.25 | 0.005 | 0.9988 |
Cu2+ | 8.88 | 34.865 | 0.759 | 0.9876 | 35.44 | 1.34 | 0.005 | 0.9999 |
The Freundlich equation frequently gives an adequate description of adsorption data over a restricted range of concentration, even though it is not based on any theoretical background. Apart from a homogeneous surface, the Freundlich equation is also suitable for a highly heterogeneous surface and an adsorption isotherm lacking a plateau, indicating a multi-layer adsorption [
36]. Values of 1/n, less than unity are an indication that significant adsorption takes place at low concentration but the increase in the amount adsorbed with concentration becomes less significant at higher concentration and vice versa [
37]. The magnitude of K
F and
n, shows that it is possible an easy separation of heavy metal ion from aqueous solution and a high adsorption capacity. Also, as the K
F value increases, the greater the adsorption intensity. Therefore, the higher K
F values for Cu
2+ confirms by these model that the adsorption capacity of is greater than that of the others ions. On the other hand, a relatively high R
2 value indicates that this model is adjusted more confidently; this parameter is shown in the
Table 4. According to the obtained values, the Langmuir model fits better the experimental data of the present study.
Figure 4a and b shows adsorption isotherms related to Mn
2+, Fe
2+, Ni
2+ and Cu
2+ adsorption from aqueous solution on CBC. Continuous lines represent the non-linear regression adjustment of these isotherms fitted the Freundlich adsorption isotherm model and Langmuir isotherm model.
Figure 4.
(a). CBC adsorption isotherms removal of Mn2+, Fe2+,Ni2+ and Cu2+ from aqueous solution, Langmuir model. (b). CBC adsorption isotherms removal of Mn2+, Fe2+,Ni2+ and Cu2+ from aqueous solution, Freundlich model.
Figure 4.
(a). CBC adsorption isotherms removal of Mn2+, Fe2+,Ni2+ and Cu2+ from aqueous solution, Langmuir model. (b). CBC adsorption isotherms removal of Mn2+, Fe2+,Ni2+ and Cu2+ from aqueous solution, Freundlich model.
3.5. Immersion enthalpies
Results show that immersion enthalpies are constant at low initial concentrations. Initial concentrations above 40 mg∙L
–1 exhibited a steady increase up to 90 mg∙L
–1. The highest value of enthalpy was obtained for the immersion of cow bone charcoal in the copper ions solutions, while the lower value of immersion enthalpy was obtained for the immersion of cow bone charcoal in the solutions of manganese. Enthalpy values were between –60 J∙g
–1 (Cu
2+–CBC) and –45 J∙g
–1 (Mn
2+–CBC), as shown in
Figure 5. This behavior agrees with textural characteristics of cow bone charcoal and the sizes of the ions under study. It should be noted that the behavior of immersion enthalpies in the solid prepared in this work, is very similar to that of an isotherm.
Figure 5.
Immersion enthalpies for Mn2+, Fe2+, Ni2+ and Cu2+ aqueous solutions ions concentration at pH 5.1. T = 298 K.
Figure 5.
Immersion enthalpies for Mn2+, Fe2+, Ni2+ and Cu2+ aqueous solutions ions concentration at pH 5.1. T = 298 K.
3.6. Removal of Mn2+, Fe2+, Ni2+ and Cu2+ from wastewater
As an approximation of the results of the present work for application to a real problem, we proposed use an industrial wastewater sample. For that purpose we chose waste from a textile industry for which the content of studied metals was determined. The sample was carefully treated with the aim of performing an analysis of each one of the ions of interest and we evaluated the adsorption capacity as the sole comparison parameter. They are analyzed one by one in order to avoid multicomponent system generation which could produce bias in the obtained results.
Wastewater samples collected in our research laboratory from a textile industry were found to contain more of 500 mg∙L–1 of Mn2+, Fe2+, Ni2+ and Cu2+, among other organic and inorganic components. Six samples were treated with nitric acid, followed by pH adjustment and sorption with CBC under optimized conditions described before. Metal ions were analyzed one at a time by atomic absorption spectrometry, using a complexing agent to avoid interference of ions different from that analyzed. Assay for manganese, iron, nickel and copper in the final effluents indicates 75.0% maximum removal of the ions originally present in the samples. The minimum removal was 53% for Mn(II). Mean standard deviation was 1.0%. These results show that CBC is an suitable material for use in the removal of these ions. However these findings should be analyzed carefully because of, in spite of procedures employed in order to avoid interferences in the assays, the sample complexity does not allow us to assure confidence in the results.