Adsorption Kinetics

The kinetic curves and their parameters are shown in Figure 7 and Table 5, respectively.

**Figure 7.** Kinetics of adsorption curves for uptake of RO-16 onto ZnCl2-BBC (**A**) and uptake of RO-16 onto KOH-BBC (**B**), uptake of RB-4 onto ZnCl2-BBC (**C**), uptake of RB-4 onto KOH-BBC (**D**). Initial pH of 5.5 and 4.0 for RO-16 and RB-4, respectively, the adsorbent dosage of 1.5 g L−1. The temperature was 22 ◦C.


**Table 5.** Kinetic parameters of RO-16 and RB-4 adsorption onto the BBC samples.

The general order model had the highest R2adj and lowest SD values for both dyes on both BBCs (Table 5) and was, therefore, considered as the most suitable model type. The general order kinetic equation gives different values for n (order of adsorption rate) when both dyes—RB-4 and RO-16—concentrations change. Hence, it is hard to make an accurate comparison of the model's kinetic parameters. Therefore, t0.5 and t0.95 were utilized to compare the RO-16 and RB-4 adsorption kinetics on the ZnCl2-BBC and KOH-BBC carbons. The t0.5 and t0.95 represent the time (h) when 50% and 95% of saturation (qe) is achieved, respectively [23–25]. For RO-16 on the ZnCl2-BBC and KOH-BBC samples, t0.5 was 1.46 and 0.24 h, respectively, while t0.95 was 6.00 and 2.98 h. For RB-4 on the ZnCl2-BBC and KOH-BBC samples, t0.5 was 1.57 and 0.43 h, respectively, while t0.95 was 6.95 and 3.21 h, respectively (Table 5).

Due to the BBCs' textural properties and chemical surface features, the KOH-BBC had faster kinetics compared to ZnCl2-BBC (Table 5), when the values of t0.5 and t0.95 are considered. KOH-BBC exhibited a much higher SBET and higher amount of micro and mesopores (see Table 2), and this could also be the reason for the better efficiency in the adsorption process. The RB-4 and RO-16 have molecular sizes of 1.59 and 1.68 nm (see Figure 3B), respectively, and are, therefore, readily adsorbed in micro- (<2 nm) and mesopores (2–50 nm). KOH-BBC also has a more hydrophilic surface (Table 2 and Figure 6), which increases the bulk solution's dispersion and the contact between the dyes and available adsorption sites on the KOH-BBC surface.

The adsorption work was further continued by establishing the contact times such as 6.5 and 3.5 h for ZnCl2-BBC and KOH-BBC for RO-16, respectively; and 7.5 and 3.6 h for ZnCl2-BBC and KOH-BBC for RO-16, respectively. The established contact times were slightly higher than the t0.95 to ensure that the adsorption process had enough time to reach the equilibrium between the dyes and the BBCs.

#### *3.3. Equilibrium of Adsorption*

The equilibrium curves and their parameters are shown in Figure 8 and Table 6, respectively.

**Figure 8.** Isotherms of adsorption for RO-16 onto ZnCl2-BBC (**A**) and KOH-BBC (**B**) and for RB-4 onto ZnCl2-BBC (**C**) and KOH-BBC (**D**). Contact time 6.5 and 3.5 h for ZnCl2-BBC and KOH-BBC for RO-16, respectively; and 7.5 and 3.6 h for ZnCl2-BBC and KOH-BBC for RO-16, respectively; Initial pH of 5.5 and 4.0 for RO-16 and RB-4, respectively; the adsorbent dosage of 1.5 g L−1.



For both BBCs and dyes, the Liu isotherm had the best fit. It was, therefore, used to describe the RO-16 and RB-4 removal for both BBCs.

Liu's model assumes that the adsorption has a heterogeneous behavior due to different active sites acting simultaneously and with different free adsorption energies [23,24]. However, a saturation of the adsorbent takes place, attaining the maximum adsorption capacity (Qmax).

For RO-16 on the ZnCl2-BBC and KOH-BBC samples, Qmax was 90.1 and 354.8 mg g<sup>−</sup>1, respectively, while RB-4 was 332.9 and 582.5 mg g<sup>−</sup><sup>1</sup> (Table 6). Thus, the KOH-BBC adsorbed almost three times more RO-16 and 60% more RB-4 than the ZnCl2-BBC. Its higher SBET value and lower hydrophobicity can explain the better performance of KOH-BBC when compared to ZnCl2-BBC, already discussed earlier.

For both BBCs, RB-4 presented higher Qmax when compared to RO-16. Both dyes are water-soluble and carry two anionic sulfonic groups in their molecules and remain anionic in aqueous solutions [42]. On the other hand, both BBCs have their surfaces positively charged (see Table 4, pH are 5.1 and 6.0 for ZnCl2-BBC and KOH-BBC samples, respectively). While the adsorption process is happening, the pH of the solution loaded with the BBCs is around 5.8–6.2; this leads to the presence of H+ in the solution, which leads to the protonation of cationic groups such amino groups present on BBCs surfaces [42–44]. This enhances the adsorption of both dyes RO-16 and RB-4 dyes due to electrostatic interactions [42–44].

Additonally, as mentioned in the kinetic discussion, the RB-4s smaller molecule size may facilitate the diffusion of the RB-4 molecules into the BBC's micro and mesopores.

#### *3.4. Mechanism of Adsorption*

Taking into account the porosity data such as SBET, pore size distribution, HI, the chemical nature of the adsorbents, initial pH solution, kinetics of adsorption, and equilibrium studies result for the RB-4 and RO-16 dyes onto BBCs samples, it is possible to sugges<sup>t</sup> the primary mechanisms of adsorption for both dyes on BBCs (see Figure 9).

**Figure 9.** Schematic mechanism of adsorption of RO-16 and RB-4 onto BBC structure.

The adsorption process takes place through different physical interactions between BBC surfaces and dyes such as hydrogen bonding, hydrophobic interactions, and π-π and n-π interactions of the aromatic ring of the BBCs with the aromatic rings of the dyes [45]. Donor-acceptor interactions (n-π interaction) occur among aromatic rings in the BBC structures that act as an electron acceptor (see Figure 9). In addition, the aromatic rings of both RB-4 and RO-16 molecules interact with the C=O, OH, COOH, and phenyl groups of the BBCs that act as adsorption sites (see Figure 9) [45].

Another mechanism that takes place on the RB-4 and RO-16 adsorption process onto BBCs is the pore-filling due to the highly developed porosity and high SBET values. The pore-filling can be the most prominent process that contributes to the high adsorption efficiency for both dyes onto highly porous BBCs (see Figure 9).

#### *3.5. Adsorbent Performance: Comparison with Literature*

The spruce bark ZnCl2-BBC and KOH-BBC performances were compared with other adsorbents' literature data (Table 7). Assuming that the literature data displays optimized conditions for each BBC, the KOH-BBC is the second most efficient, having the secondhighest adsorption capacity (Qmax) for RO-16 removal and the highest for RB-4.


**Table 7.** Comparison of KOH-BBC and ZnCl2-BBC concerning the reported literature in terms of capacity.

It is worth highlighting that the spruce bark KOH-BBC's Qmax for RB-4 is comparable to that of the single-walled carbon nanotubes studied by Machado et al. [58] (582.5 vs. 567.7 mg g<sup>−</sup>1), but the production cost of carbon nanotubes is substantially higher when compared to KOH-BBC. Additionally, Table 7 shows and compares the spruce bark BBCs with different adsorbents reported in the literature. It is shown that BBC Brazilian-pine fruit shell [47] exhibited the highest Qmax for RO-16; however, the adsorption conditions were very different when compared to this work, e.g., the temperature was higher (50 ºC vs. 22 ºC) as well as the adsorbent dosage (66.6% more adsorbent than was used by this work), which means increasing the costs involved in the adsorption process. This also needs to

be considered when the effectiveness of adsorbent material is evaluated and compared with others.

Thus, it can be concluded that both BBCs (especially KOH-BBC) are suitable adsorbents for the elimination of dyes with competitive and efficient adsorption capacities.

#### *3.6. Treatment of Synthetic Dye Effluents*

According to the adsorption data (kinetic and equilibrium), both BBCs were very efficient for removing RB-4 and RO-16 from aqueous solutions, indicating that these BBCs could also be employed to treat real effluents. Therefore, both BBCs were tested for the treatment of two synthetic dyeing effluents. The BBCs' removal percentage of dye mixture in the effluents was evaluated from UV–vis spectra of the untreated and treated effluents (see Figure 10).

**Figure 10.** Adsorption of synthetic dyes effluent. (**A**) Effluent A; (**B**) Effluent B. (**C**) Effect of BBC mass dosage on effluent A treated and (**D**) effect of BBC mass dosage on effluent B treated.

ZnCl2-BBC removed 88.2% and 90.4% for the effluent A and B, respectively, while KOH-BBC removed 91.9% and 95.6% at an adsorbent dosage of 1.5 g L−<sup>1</sup> (Figure 10A,B).

With KOH-BBC, only 2.0 g L−<sup>1</sup> was needed to remove almost 100% of all compounds in both effluents (Figure 10C,D). ZnCl2-BBC removed 69.7% and 76.5% at a dosage of 3.5 g L−1. These differences agree with the previously reported adsorption data and discussed in the work where the KOH-BBC had better adsorption properties than ZnCl2- BBC. Still, a good removal percentage was achieved for both BBCs. However, it should point out that the KOH activation could be considered a more interesting method because zinc salts (e.g., ZnCl2) are more expensive and toxic [59] when compared to KOH, which is a corrosive chemical reagen<sup>t</sup> [60]; therefore, it would be preferable to use a cheaper and non-toxic reagent, such as KOH, for BBC preparation.

#### **4. Possible Application of Used BBC after Adsorption of Dyes**

The re-use or final disposal of the BBC materials loaded with the selected adsorbate is an important question when designing an adsorption system or new adsorbent

materials. BBC can be regenerated and reused many times without losing adsorption performance [7,29]. However, after being fully saturated, its final disposal or other utilization must be considered once they no longer can be regenerated for water treatment application [7]. The main employed methods to manage used BBC are landfill disposal and incineration [23,24]. However, in some cases, used adsorbents are used as soil fertilizer [23,24], depending of the type of the adsorbate loaded on the BBc surface. These methods are influenced by some factors such as, cost of the adsorbent, type and toxicity of the pollutant, costs involved with the methods including the cost of the combustion and incineration plant, and fees for disposal. Although landfills have typically been used for the disposal of sorbents, as well as soil fertilizers, these methods might have subsequent pollution risk when toxic compounds leach from adsorbents into the soil.
