3.1. Characterization of the Biochar Samples
One of the major advantages of transforming the waste biomass in biochar is the significant reduction of waste quantities, and the use of obtained biochar as adsorbent material. If this transformation is accompanied by an efficient capture of the combustion gases, then this process can be considered a clean one, and could be a solution for waste management [23
In this study, three types of biomass waste (algae biomass waste (ABC), mustard biomass waste (MBC), and soy biomass waste (SBC)) resulted after oil extraction, were pyrolyzed under oxygen-limited conditions at specific temperature, and some characteristics of the pyrolysis process together with the porosity parameters of the obtained biochars are summarized in Table 1
The pyrolysis temperature was selected for each type of waste biomass from derivatograms (Figure S1—Supplementary Materials
), to allow burning of biomass waste without being completely transformed to ash. Thus, it can be observed that mustard waste biomass and soy waste biomass required a high pyrolysis temperature (650 °C) compared with algae waste biomass (600 °C). This can be explained if the biomass of algae waste is considered to have a low content of cellulose and hemicellulose [34
], and therefore the temperature required to carbonize the protein molecules in its structure is lower. However, it should be noted that in the mentioned pyrolysis conditions, the mass decrease is higher than 40% and the order is: Mustard waste biomass > soy waste biomass > algae waste biomass.
Besides high surface area (see Table 1
), SEM images (Figure 1
) demonstrate that all types of biochar have porous and permeable structure, different sizes, and opening shapes, and these characteristics are very important in the removal processes of metal ions from aqueous media.
However, the biosorptive performances of the biochars are not only dependent on their surface area and porosity, but also on the number and nature of superficial functional groups. From this point of view, FTIR spectra are the most useful tools that can be used to characterize the nature of functional groups from the surface of the biochars [35
]. In Figure 2
, FTIR spectra for the three types of biochars used in this study are shown.
In all the biochars samples, the broad band at 3300–3400 cm−1 indicates the presence of hydroxyl groups (from alcohols or phenols) and N–H bonds from aliphatic amines. The bands at 1600–1700 cm−1 can be attributed to C=O bonds from aldehydes, ketones, or carboxylic acids, while the strong bands from 1000–1100 cm−1 correspond to axial deformation of C–O bonds from oxygenated compounds (ethers, esthers, etc.). Also, the bands from 1930 cm−1 denoted the presence of methylene groups from aliphatic radical. The low intensity of these bands indicates that most of the aliphatic chains from all types of waste biomass have been decomposed during of pyrolysis process.
All these observations suggest that all types of biochars have different kinds of functional groups on their surface, and these groups can represent the binding sites for Co(II) ions from aqueous media. The presence of functional groups on the biochars surface, together with their porous structure, are arguments which highlight the possible use of these materials as adsorbents in the removal processes of metal ions.
3.2. Effect of Initial Co(II) Concentration and Isotherms Modeling
Initial metal ions concentration is one the most important factors which influences the efficiency of the sorption process. The experimental results indicate that Co(II) ions sorption on the three types of biochars (Figure 3
) increase with the increase of initial metal ions concentration, and the saturation is not obtained even at the highest initial Co(II) ions concentration (240 mg/L).
In the low initial concentration range (up to 55 mg Co(II)/L), the three biochars have a similar efficiency in the sorption process, and the values of the removal percentage are higher than 85%, in all the cases. When the initial Co(II) ions concentration increases, the differences between sorption capacities obtained for each type of biochar are more significant, and follow the order: ABC < SBC < MBC (Figure 3
a). The better sorption performances of MBC for Co(II) ions from aqueous media can also be seen from the variation of the sorption percentages (Figure 3
b), which decreases from 91% to 60%, compared with SBC (from 89% to 52%) and ABC (from 90% to 50%), respectively. Decreasing the sorption percentages by increasing the initial Co(II) ions concentration is a common variation, and indicates that the number of binding sites on the surface of the biochars, are limited [17
]. However, the different behavior of the three types of biochars in Co(II) ions removal is probably determined by their different structural characteristics. Thus, as can be seen from FTIR spectra (Figure 2
), MBC and SBC have more and diverse functional groups in their structure, compared with ABC. On the other hand, MBC has a higher specific surface area than SBC and ABC (Table 1
). Consequently, the concerted action of these two characteristics (high surface area and numerous and diverse functional groups) makes MBC a more efficient sorbent for removal of Co(II) ions from aqueous media.
To quantify the sorption process of Co(II) ions on the three types of biochars (ABC, MBC, and SBC), the experimental sorption isotherms were analyzed using the Langmuir and Freundlich models. Figure 4
shows the experimental data and the fitted isotherms, and the calculated parameters for each isotherm model are given in Table 2
As can be seen from Figure 4
, the Langmuir isotherm model fits better with the experimental results obtained at Co(II) ions sorption on the biochars, than the Freundlich model. This observation is also supported by the R2
values presented in Table 2
, which indicate the applicability of the Langmuir equation in describing the studied sorption processes.
Therefore, it can be said that the sorption of Co(II) ions from aqueous media takes place on the surface of the biochars until a monolayer coverage is formed, after which the driving force of the sorption process decreases drastically, and the equilibrium is reached. The amount of Co(II) ions required to obtain the monolayer coverage depends on the type of biochar, and the values of maximum sorption capacity (qmax
, mg/g), calculated from the Langmuir model (see Table 2
) follow the order: MBC > SBC >ABC.
The different values of maximum sorption capacity indicate that in the sorption process the structural characteristics of biochar play an important role. Thus, MBC, which has a high specific surface area (see Table 1
, Figure 1
) and high number of superficial functional groups (see Figure 2
) can retain larger amounts of Co(II) ions from aqueous solution, and can be considered more effective (2.03 times) compared with SBC (1.64 times), or ABC. This comparison demonstrates that the biochars can be considered as potential sorbents for Co(II) ions, but their effectiveness depends on the structural particularities of the raw biomass and the preparation conditions.
3.3. Effect of Contact Time and Kinetics Modeling
The effect of contact time and sorption kinetics has been studied to highlight the dynamics of the Co(II) sorption process on the three types of biochars. Figure 5
shows the sorption of Co(II) ions by each type of biochar (ABC, MBC, and SBC), at different contact times (5–180 min).
The sorption efficiency increases with contact time increase up to 60 min (Figure 5
) for all three types of biochars, after that the amount of Co(II) ions retained on the biochars remains almost constant. The fast sorption of Co(II) ions at the initial contact time is determined by the abundance of functional groups on the biochars surface, which is the driving forces of the sorption process. When the superficial functional groups are occupied, the retention of Co(II) ions becomes more slower, because the Co(II) ions have to diffuse inside of the biochars pores to find free functional groups to bind [32
]. However, it should be noted that the contact time required to achieve the equilibrium does not depend on the type of biochar, and the optimal value was selected as 60 min for all the sorbent materials.
The kinetics modeling of experimental data has been done to better understand the dynamics of Co(II) sorption processes on these types of biochars and to obtain information on the rate of sorption. As it was mentioned above, three kinetics models (pseudo-first order, pseudo-second order, and intra-particle diffusion models) were used for this, and the overlapping of the experimental data with the kinetic curves obtained by modeling are presented in Figure 6
. Also, the kinetic parameters calculated from mathematical equations of each kinetic model are summarized in Table 3
As can be seen from Figure 6
and Table 3
, the pseudo-second order model better represents the sorption kinetics of Co(II) ions on all types of biochars (ABC, MBC, and SBC, respectively), because the correlation coefficients have the highest values (R2
> 0.999), and the sorption capacities at equilibrium calculated from this model (qe
, mg/g) are very close to those obtained experimentally (qe,exp
This indicates that the Co(II) sorption process involves chemical interactions [32
] between metal ions from aqueous media and superficial functional groups of biochars, and the retention of Co(II) ions required two binding sites, which must have a proper geometric orientation. In addition, the rate constant of pseudo-second order kinetic model (k2
, g/mg min) increases in the order: MBC > SBC > ABC, suggesting that MBC has the most functional groups available for metal ion interactions, and therefore the retention of Co(II) ions is made the easiest. This observation is supported by the FTIR spectra (Figure 2
) which have indicated since the beginning, that MBC has more superficial functional groups than SBC and ABC.
On the other hand, the relative high values of regression coefficients (R2
) obtained in the case of modeling the experimental data using the intra-particle diffusion model suggest that the elementary diffusions processes have also a certain contribution to the sorption of Co(II) ions. However, the deviation from origin indicates that these elementary diffusion processes are not the rate controlling step [38
]. The Co(II) ions diffusion into biochars particles is done quickly, since the rate constants of the diffusion process (kdiff
, mg/g min1/2
) are at least with an order of magnitude larger than the rate constants of the pseudo-second order kinetic model (k2
, mg/g min), and follows the order: MBC > SBC > ABC (see Table 3
Nevertheless, this order is in contradiction with the data presented in Table 1
, which shows that MBC has the largest surface area and therefore, in this case, the diffusion processes should be slower. This contradiction can be explained if the modeling of the experimental kinetic results using the intra-particle diffusion model is analyzed more closely. The linear representations of the intra-particle diffusion model (Figure 7
) indicate that for all types of biochar, retention of Co(II) ions involves several elementary diffusion processes, because they can be divided into two distinct regions.
According to the study of Cheung et al., 2007 [38
], the first region represents the film diffusion, while the second region describes the diffusion of metal ions into the pores of the biochars. The higher values of slopes (which represent the rate constants) of the first region compared with those of the second regions shows that the binding sites of biochars are located at the surface and are available to interact with Co(II) ions from aqueous solution [39
Under these conditions, it can be said that even if MBC has the highest surface area (which means a rough surface with many pores), the presence of numerous superficial functional groups (Figure 2
) makes the interactions of these with Co(II) ions to be done quickly and easily. Once the superficial functional groups are occupied, the penetration of Co(II) ions into the biochor granule is prevented, and the importance of elemental diffusion processes decreases considerably. Consequently, the rate constant of the elementary diffusion process is high.
Unlike MBC, in the case of ABC, which has on its surface only few superficial functional groups (Figure 2
), the binding of Co(II) ions on the surface of the particles does not create geometric obstructions. Consequently, Co(II) ions are forced to penetrate inside the ABC particles in search of new free functional groups to interact with. This makes the elemental diffusion processes to be more important, which takes time, and thus the obtained rate constant has a lower value (Table 3
Regardless to the weight of elemental diffusion processes, the retention of Co(II) ions on the three types of biochars (ABC, MBC, and SBC) takes place until the formation of a monolayer coverage, according to the Langmuir model assumptions.
3.4. Recovery of Co(II) Ions and Biochars Regeneration
The recovery of retained Co(II) ions and biochars regeneration should be also discussed to highlight the practical applicability of these adsorbent materials in wastewater treatment at large scale [40
]. In this study, three sorption/desorption cycles have been done for each type of biochar, using the same sample. The desorption agent used every time was HNO3
solution (0.1 mol/L), while the sorption of Co(II) ions was done in considered optimal conditions (pH = 5.0; biochar dose = 8 g/L; c0
= 150 mg Co(II)/L; temperature = 25 °C). The experimental results obtained after each cycle of sorption/desorption are illustrated in Figure 8
It can be seen from Figure 8
that in all the three cycles the recovery of retained Co(II) ions by desorption is a quantitative one for all types of biochars, and after desorption, the biochars can be successfully used in another sorption cycle, without that the efficiency of the sorption process to be significantly affected. These observations highlight the potential applicability of these adsorbent materials in multiple sorption/desorption cycles, which is an important advantage for industrial wastewater treatment systems, both for technological and economical reasons.
On the other hand, from careful analysis of the experimental data presented in Figure 8
, two important observations can be made, namely: (i) The sorption efficiency slightly decreases from cycle 1 to cycle 3, and this decrease follows the order: SBC (up to 20%) > MBC (up to 18%) > ABC (up to 8%), and (ii) the desorption efficiency also shows a tendency to decrease from cycle 1 to cycle 3, but in this case the order is: ABC (up to 12%) > SBC (up to 8%) > MBC (up to 5%). The different variation of the sorption and desorption efficiency as the number of cycles of use increases is mainly determined by the structure of biochars. Thus, the low number of functional groups on the surface of ABC (Figure 2
) determines that the sorption efficiency of Co(II) ions to remains almost constant, while the desorption efficiency decreases from cycle 1 to cycle 3. This is probably because not all Co(II) ions retained on ABC can be desorbed by the treatment with 0.1 mol/L HNO3
solution. In case of MBC and SBC, where the number of superficial functional groups is higher (Figure 2
), the more evident decrease of the sorption efficiency from cycle 1 to cycle 3 is an indication that some functional groups from their surface are destroyed or degraded during desorption. However, almost all retained Co(II) ions are readily recovered by treatment with 0.1 mol/L HNO3
Starting from these observations, it can be said that the efficiency of certain biochar in the sorption processes of metal ions from aqueous media depends on the nature of the biomass used as raw material and preparation conditions. The choice of preparation conditions must be made in such a way as to obtain a material with a porous structure and a larger number of superficial functional groups. In this way the obtained biochar will have high efficiency in removal and recovery of metal ions from aqueous media.