3.1 Characterization of adsorbent
The SEM analysis was investigated for the determination of shape, size, and surface morphological structure of the RhBC and MBC. The result reveals the surface morphologies of the two samples are distinctly different, the structures of MBC is a smooth surface, and the RhBC is porosity material (Fig.1). The BC has abundant pore size, the micro-pore is most popular and it’s interspersed on with mesopore to form the capillaries channel in this material. After alginate modification, the pore of BC has covered by silky coating to create MBC.
The surface areas of RhBC and MBC determined by BET analysis were 34.0, 91.5 m2/g, respectively, and the pore volume were 0.289, 0.580 cc/g, respectively (Table 1). The results indicated that, compared with the RhBC the surface area MBC improved significantly after modification. Meanwhile, the analysis of surface morphology indicated that the RhBC has richer pore structure than MBC. The phenomenon can be explained by that after the impregnation alginate will occupied in the pores of biochar, when drop the solution of biochar-alginate in CaCl2 solution, the Ca2+ will crosslink with alginate form the gel fill up the pore.
The results of EDX-image show composition element present in RhBC and MBC material. The carbon and oxygen content of RhBC occupies 72% and 24% respectively. The note is a silicon appearance due to the affection of biomass. However, the material modification with alginate, the peak of Si content is reduced and the peak of O content is increased. It’s indicated that Si was dissolved and removal through the modification process, and the -COO, -OH groups of alginate contributed to O content. Besides that, the weight % Ca has been increased and the new presences of Cl element (Fig.1) indicated that the alginate was loaded onto the biochar composites when Ca(II) ion in solution crosslink with alginate impregnated with RhBC.
The pHpzc is a very important characteristic that determines the pH at which the surface has net electrical neutrality. At pH higher than pHpzc, the surface of the material is negatively charged that increase the adsorption for cation species. The pHpzc of RhBC and MBC is 7.28 and 6.10 (Fig.2) respectively. The decreasing in pHpzc involves the function groups - COOH, and -OH of alginate that it was dissociated hydronium ion (H+). For the removal of lead in the aqueous phase, the pH should be higher than pHpzc because the net negatively charged surfaces are favorable to attract the cation. Generally, adsorbent with greater structured porosity has better adsorption performance [25]. Thereby, MBC with the relatively low structured porosity will not better adsorbent as well as RhBC. In the other hand, functional groups are improved due to alginate loaded, which enhanced the final adsorption efficiency off MBC. The Pb removal capacity of RhBC and MBC were 101.44 mg g-1 and 33.61 mg g-1, respectively (Table 3). The results suggested that functional groups on MBC work better than the pore of RhBC for Pb adsorption.
Table 1
Parameters of specific area alginate modified biochar (MBC) and biochar from rice husk (RhBC)
Parameters
|
BC
|
MBC
|
Specific surface area (m2/g)
|
34.06
|
91.45
|
Volume of the hole (cc/g)
|
0.2891
|
0.5805
|
Average size of pore (nm)
|
9.00
|
6.31
|
3.2 Effect of pH on sorption process
pH value is one of the important factors can affect the adsorption process. The initial pH could influence the removal performance of RhBC or MBC by electrostatic affinity-repulsion between adsorbents and Pb2+, also as the process of ion exchange between adsorbents and lead. pH decides the charge of material surface, also the stability of the metal ion configuration is changed. The effect of solution pH on the adsorbent’s lead sorption (removal) was identified by varying the initial solution pH while keeping other sorption parameters in constants.
The enhancement of adsorption capacity follows with the increasing initial pH shown in Fig 3. This trending is related to the hydrolysis of metal ions under the high pH and the competition reduction of protons (main occur in low pH) combine to active sites on surface of material. Furthermore, according to the point of zero charge theory, at pH higher pHpzc the surfaces of RhBC or MBC will be negatively charged due to the deprotonation of the functional groups, which will facilitate the process of chelation with Pb2+, thereby the adsorption efficiency will enhance. The adsorption capacities of Pb by MBC were higher than those of RhBC at vary initial pH points due to the more functional group loaded into RhBC surfaces, especially, oxygen-containing functional groups as – COOH can form the bond with Pb express as chemically adsorbed. Thereby, usage of alginate as a modifier-agent is improving the adsorption efficiency. The favorable initial pH for Pb removal is 7 (Fig.3).
3.3 Adsorption isotherm
The Langmuir and Freundlich models for adsorption of Pb with the initial concentration range of 10 to 50 mg L-1 was shown in Fig.4. Their constants and correlation coefficients (Table 2) is calculated to have a relatively assessment of adsorption process. Results are the adsorption capacity increased with increasing equilibrium concentration. At low equilibrium solution concentration of Pb, the adsorption increased dramatically. The steadiness of sorption capacity of Pb on material was reached with any increased concentration of equilibrium solution. The adsorption capacities of the MBC were about three times higher than the RhBC. Through the results of previous characterization experiments, explanation for sorption of aqueous lead by the modification biochar can be attributed to the facts that MBC showed much higher surface area, more active sorption sites, more oxygen containing surface functional groups than the RhBC. Several previous studies have demonstrated that surface area, surface functional groups play important roles in controlling the sorption of heavy metals onto carbonaceous materials. With a greater regression coefficient R2 (Table 2), the adsorption process obeys both the Langmuir isotherm and Freundlich isotherm indicate that the adsorption Pb on materials influence by active site (multilayer) and functional groups (monolayer). The calculated adsorption capacities of Pb by MBC and RhBC were 96.89 and 31.50 mg g-1, respectively. It’s close to the experiment data are 101.44 mg g-1 and 33.61 mg g-1.
Table 2
Parameters for Pb sorption by alginate modified biochar (MBC) and biochar from rice husk (RhBC)
Materials
|
Langmuir model
|
|
Freundlich model
|
K
|
Qmax
|
R2
|
RL
|
|
Kf
|
n
|
R2
|
MBC
|
0.7676
|
96.8850
|
0.9745
|
0.1461
|
|
49.4808
|
0.2096
|
0.9814
|
RhBC
|
0.5428
|
31.4950
|
0.9542
|
0.2509
|
|
19.2640
|
0.1284
|
0.9612
|
Table 3
Lead adsorption capacities of class carbonaceous adsorbents
Adsorbents
|
Adsorption capacity (mg g-1)
|
References
|
Activated carbon
|
26.6
|
[26]
|
Alginate modified biochar (MBC)
|
101.44
|
This study
|
Rice husk biochar (RhBC)
|
33.61
|
This study
|
Oak biochar
|
13.10
|
[27]
|
Bagasse biochar
|
5.0-20.5
|
[28]
|
Biochar loaded with nano particles
|
146.8
|
[29]
|
Biochar derived from anaerobic digestion sludge
|
54.0
|
[30]
|
The results indicated that the modification using alginate improved the adsorption performance of pristine biochar. A comparison of the maximum adsorption capacities in Pb removal of absorbents reported in the literature (Table 3) show MBC matched or outperformed than many other carbonaceous adsorbents, moreover the synthesis of MBC material was convenient with friendly agent – easy to treat.
3.4 Adsorption kinetic
The kinetics of adsorption of metal ions depends on the physical and/or chemical properties of materials [13]. To understand the characteristics of the adsorption process, pseudo first-order model, pseudo second- order model was applied to verify the adsorption kinetics [31, 32]. The adsorption kinetic is conducted at the same initial concentration also the pH of Pb solution, but the contact time with RhBC and MBC material is increased. The adsorption capacity of Pb2+ by RhBC and MBC rising in the initial 4 hours and then approached equilibrium (Fig. 5). At the first stage, adsorption may mainly occur on the exterior surfaces of those two sorbents in which the active sites occupied follow increasing reaction time. After that Pb2+ ions were distributed into the capillaries channel (formed by arrangement of pore) through the diffusion process. At the last stage, the Pb2+ particles further reacting with the internal active sites and the diffusion resistance increased so the adsorption process slowly arrives reaching equilibrium. Both kinetics models especially the pseudo-second order model, (both R2 > 0.95) displayed good fit with the measured data (Table 4), suggesting the mechanism of Pb adsorption mainly controlled by physical-chemical adsorption when the pore of material have high capillary attraction with Pb species, beside that functional groups can electrons transference.
Table 4
Parameters of kinetics models for Pb sorption by alginate modified biochar (MBC) and rice husk biochar (RhBC)
Materials
|
Pseudo first order
|
|
Pseudo second order
|
K1
|
Q
|
R2
|
|
K2
|
Q
|
R2
|
MBC
|
0.0146
|
105.0124
|
0.9977
|
|
0.0001
|
126.3114
|
0.9980
|
RhBC
|
0.0130
|
25.5622
|
0.9784
|
|
0.0005
|
30.9660
|
0.9929
|
The sorption capacities of Pb by MBC were approximately four times higher than those by BC300, while the rate constants (k2) of MBC sorption of Pb were lower than that of RhBC. This is probably due to improved efficacy of biochar in the adsorption of heavy metals by changes in its surface area, and ion exchange capacity with the alginate modification.
3.5 Mechanism of adsorption Pb onto material
The adsorption mechanisms usually involve complexation, ion exchange, physical adsorption, and micro-precipitation [33]. Correspondence with presence of oxygen-containing functional groups, biochar reveals greater more attract with cation like Pb2+ [34]. To conceived the adsorption mechanism through above assumption, FTIR spectroscopy was applied to identify the existence of functional groups on materials surface. The relatively considerable broadband at around 3400 cm-1 (Fig. 6) was allocated to the –OH bond [35]. The functional groups existing on RhBC and MBC, by the analysis results on the FT- IR infrared spectrum of these materials (Fig. 6) shows the appearance of OH, C = C, C = CH, ... groups on the surface.
On the infrared spectrum of biochar materials, there are associated spectral lines corresponding to the alignment oscillations of the OH group (3400 cm-1, 1321 cm-1), C=O (1640 cm-1), CO of C-OH (1111 cm-1). These links characterize the straight-chain structure with the appearance of carbonyl and alkyl groups. This structure is completely in accordance with the assumption about the composition of biochar from rice husks. After the modifications, the peak of carbonyl group more sharped indicate that the carboxylate groups on surface of RhBC are enhanced because of that the alginate contains mannuronic and guluronic, besides that the new presence of a peak at 1417 cm-1 (- OH of carboxyl acid) is more clarifying this result. Thereby the MBC from RhBC can make the ability with metal ions by the reaction with carboxylic group. The characteristic peaks of MBC after adsorption also examined by FT-IR (Fig. 6). The peak within 1400 - 1700 cm-1 associate with the vibrations of carboxylic bonds [36]. Carboxylate is polar so it’s can donate and accept both H+ and OH-. In the presence of ligand, metal ions are enabled to substitute H+ for coordination with carboxylic groups. The abovementioned peaks were shifted after Pb adsorption by virtue of the formation layer organometallic complex lie on the surface of MBC adsorbent.
3.6 The regeneration of alginate modified biochar
For investigate the ability of regeneration of MBC, this material was desorbed and using for two other adsorptions under the same optimal condition. From fig. 7 the removal efficiency of MBC has a slight decrease from the first to the second adsorption with data of 80.9% pointed that MBC is still good absorbent. However, the efficiency massive decline to 52.2% in the third adsorption, MBC may not available utilize for the next adsorption. Hence, MBC could be regenerated and used two times.