Simple One-Step Synthesis of Nipa Frond-Derived Magnetic Porous Carbon for Decolorization of Acid Yellow 23

,


Introduction
Today, water pollution is regarded as one of the most signifcant global challenges, endangering the environment and threatening the lives of millions of people worldwide [1,2].During ordinary human activities and industrial operations, a vast range of organic pollutants are released into the water environment [3][4][5].Azo dyes are among the most widespread varieties of water pollutants that are discharged through the efuents of diferent industrial processes [6,7].Because the bulk of the dyes have stable and complex molecular structures, it becomes challenging to degrade them using conventional chemical and biological processes [8].Many of these substances are poisonous, carcinogenic to living organisms, and detrimental to aquatic life [9].Acid yellow 23 (AY23) is also known as tartrazine, which is a sulfonated azo dye widely utilized in the cosmetics, pharmaceutical, and food industries [3,10].Because of its high solubility in water, AY23 is more likely to be found as a pollutant in industrial wastewater.AY23 is toxic at high concentrations and appears to be hazardous to human health, potentially causing hyperactivity, migraine, asthma, urticaria, angioedema, thyroid cancer, etc. [1,11].In light of these possible risks, it is essential to fnd an efective and sustainable solution to treat AY23 in wastewater before discharging it into the environment.
Because AY23 is recalcitrant to biodegradation, adsorption and advanced oxidation are common technologies for its treatment [1,8,[11][12][13].Adsorption is generally simple, but it is unable to mineralize organic molecules.As a result, additional treatment of the used adsorbents may be required.In contrast, advanced oxidation processes (AOPs) have proven their ability to decompose and mineralize diverse organic contaminants.Among AOPs, the low-cost Fenton process, which uses Fe 2+ ions as a catalyst and H 2 O 2 as an oxidation agent, has been widely utilized [14].In this catalysis, Fe 2+ ions can accelerate the breakdown of H 2 O 2 into powerful hydroxyl radicals (•OH) for nonselective oxidation of organic species [15].However, Fe 2+ ions can generate waste sludge that is difcult to reuse [16].In order to overcome the aforementioned issues of homogeneous Fenton oxidation, considerable research has been conducted on the heterogeneous Fenton-like processes utilizing Febased solid catalysts such as Fe 2 O 3 , Fe 3 O 4 , FeOOH, and zero-valent Fe 0 [17].Tese Fe-based particles are afordable, stable, easily prepared, and magnetically separable [18,19].Nevertheless, the particles may aggregate during use, reducing their catalytic activity [20,21].Hence, they must be dispersed onto appropriate supports.Due to their favorable interaction with organic molecules in wastewater, carbonbased materials have been extensively explored as efective supports for Fe-based particles [22].Compared with synthetic carbon materials like graphene, carbon nanotubes, and nanoporous carbon, biomass-derived porous carbon (PC) is recognized as an inexpensive, sustainable, and ecofriendly support for heterogeneous Fenton catalysts [15,23,24].
Biomass-derived PC materials (biochar and activated carbon) are prepared from the carbonization of various biomass resources [25,26].Owing to their advantageous physicochemical characteristics, porous structure, and multiple functional groups [27,28], PCs are utilized for a variety of applications, such as wastewater treatment, gas storage and separation, soil remediation, and catalytic supports [22,29].However, separating and recovering PCs is challenging.Conventional separation techniques are either expensive or inefective, severely limiting their use [22].Recent studies suggest dispersing magnetic particles such as Fe 3 O 4 , Fe 2 O 3 , and Fe 0 on PC to generate magnetic porous carbon (MPC) [30,31].In this manner, magnetic particles can facilitate the separation and recovery of MPC through external magnetic felds [32,33].On the other hand, the carbon support can fx magnetic particles and prevent their aggregation into bigger particles [20,34].Overall, the advanced MPC combines desirable physicochemical properties, including the porous structure and functional groups of carbon supports, as well as the magnetic separability and catalytic activity of Fe-based particles [35][36][37].
MPC is traditionally prepared in two steps: pyrolysis of biomass into pristine PC and chemical dispersion of magnetic particles over pristine PC [31].In general, these procedures are time-and energy-consuming [15].Additionally, magnetic particles could obstruct existing pores in carbon supports, thereby reducing their surface area and pore volume.Recently, an advanced route has been developed for the one-pot synthesis of MPC via direct FeCl 3activation of biomass [35,38].First, magnetic precursors are impregnated into biomass feedstocks.Subsequently, the resulting mixtures are pyrolyzed into MPC [20,39,40].Te advanced procedure could not only load magnetic particles but also activate carbon surfaces simultaneously during pyrolysis.According to Bedia et al. [41], FeCl 3 -activation of biomass yields MPC with well-developed porosity and welldispersed iron-based particles.Do et al. [36] proved that MPC prepared via one-step pyrolysis showed high catalytic activity, long reusability, and good stability in the Fentonlike treatment of Orange G.In spite of this, there are still few reports on MPC fabricated through FeCl 3 -activation of biomass resources for use in catalysis.Further research on AY23 treatment to explore MPC catalysts is necessary.
It is found that FeCl 3 -activation of various biomass resources can yield MPCs with a diversity of porous structures, functional groups, and iron-based products which signifcantly afect their magnetic properties, adsorption capacities, and catalytic activities [22,36,42].To expand the selection of MPC for further applications, increased research on the fabrication conditions, properties, and catalytic performance of MPC from newly prospective biomass resources is desired.Nipa (Nypa fruticans) is a monoecious palm that fourishes in soft mud and slow-moving tidal regions, including river estuaries, coasts, and mangrove forests [43,44].Te palm can be found in a variety of tropical locations, like Nigeria, Sri Lanka, India, Bangladesh, Myanmar, Tailand, Cambodia, Malaysia, Indonesia, Papua New Guinea, Australia, the Philippines, and Vietnam [45][46][47].Unlike most palms, nipa is trunkless.It consists of only fronds, leaves, and fower stalks above the ground [48,49].Nowadays, nipa is considered an unexploited species.Most biomass parts of nipa are left to decay in their living environment, with limited use [48,50].Nipa fronds (NFs), the main part of the palm, can therefore be an abundant and available raw material for further use.In this study, NF was valorized for the frst time as a newly renewable carbon source for the one-step synthesis of MPC.Te properties of the as-synthesized MPC were thoroughly investigated and discussed.) and external surface area (S ext ) were determined by t-plot method.Meso-macropore volume (V meso-macro ) was calculated from the diference between V total and V micro .Micropore surface area (S micro ) was obtained by subtracting S ext from S BET .Te average pore size (d pore ) was calculated from 4V total /S BET .Te scanning electron microscope (SEM) images were observed with a FE-SEM S-4800.An energydispersive X-ray (EDX) spectroscopy instrument (JSM-IT200) was used to examine the elemental mapping of MPC.Functional groups on MPC surface were characterized by Fourier-transform infrared (FTIR) spectroscopy using a spectrometer (VERTEX 70, Bruker Optics).Magnetic properties of MPC were determined by a vibrating sample magnetometer (VSM) at room temperature.Iron content in MPC was determined by the original ferrozine method [51].

Acid Yellow 23 Removal by Magnetic Porous Carbon.
Acid yellow 23 decolorization was performed in a 600-mL glass fask at ambient temperature (30 °C).260 mL of AY23 (100 ppm) was prepared in the fask.Ten, diferent MPC dosages were added, and a magnetic stirrer was used to mix the obtained suspension continuously.HCl (0.1 M) and NaOH (0.1 M) solutions were used to adjust the initial pH values of the mixture.After the adsorption step in the frst 10 min, diferent H 2 O 2 dosages were rapidly poured into the suspension for the further oxidation step.At diferent time intervals, each 2.5 mL of sample was withdrawn from the reaction mixture and added to a solution of 1.0 mL of Na 2 S 2 O 3 (0.001 M) and 0.5 mL of NaOH (0.01 M) to remove excess H 2 O 2 and adjust pH.A magnet was used to support the separation of the MPC catalyst from the suspension.Finally, AY23 concentrations were measured by a UV-Vis Spectronic Genesys 2 PC at 430 nm.Te adsorption capacity and decolorization efciency of AY23 over MPC were calculated by the following equations: where C MPC was MPC dosage (g/L), C 0 , C 10 , and C 120 (ppm) were AY23 concentrations at the beginning, after 10 min of treatment, and after 120 min of treatment, respectively.

Efect of Washing on XRD Patterns of MPC.
As reported in the literature [35,36,52], diferent Fe-based particles could be generated during the pyrolysis of FeCl 3 -loaded biomass as follows: biomass Journal of Chemistry Equations ( 3)-( 5) indicate that the formation ability of Fe-based products on MPC could depend on the products of carbonization.As a matter of fact, a lack of water vapor could not completely convert the FeCl 3 reactant into Fe-based products.Hence, the existence of Fe-based matters in nonwashed and washed MPC-R0.2-2.0 h was explored by XRD (Figure 1).Both samples showed characteristic peaks at 30. 40, 35.32, 53.48, 56.96, and 62.44 116) planes of residual FeCl 3 (ICDD 00-001-1059) [53][54][55][56].In addition, the brown color of freshly precipitated Fe(OH) 3 was observed by adding the nonwashed MPC-R0.2-2.0 h sample to an aqueous NaOH solution (Figure 2(a)).pH measurements of the mixture also revealed an acidic medium caused by Fe 3+ leaching.To ensure that all residual FeCl 3 was removed from MPC, washing was repeated until the solution was colorless with the NaOH test (Figure 2(b)) and neutral with the pH test.In later parts, only washed MPC samples were used.

Efect of Pyrolysis Time on XRD Patterns of MPC.
As presented in Figure 3, Fe 3 O 4 and Fe 0 crystals were formed in all MPC samples prepared from diferent pyrolysis time.With high intensity of sharp peaks, Fe 3 O 4 could be the major Fe-based product.A minor part of Fe 3 O 4 could be further reduced into Fe 0 over carbon base.As the pyrolysis time rose from 0.5 to 2.0 h, the formation of Fe 3 O 4 crystals was slightly improved.Reversely, a longer pyrolysis time at 4.0 h did not actually yield better Fe 3 O 4 crystals but seemed to enhance Fe 0 crystals.Tese Fe 3 O 4 crystals might also be converted into other materials in crystal or amorphous phases.Te biomass carbonization might be complete, and prolonging pyrolysis time might not ofer more substances for converting FeCl 3 into Fe 3 O 4 , except for carbon.Tis fnding has been discussed in such reports [20,57].
Table 1 showed that the pyrolysis efciency decreased from 62 to 56% as the pyrolysis time was prolonged from 0.5 to 4.0 h.During pyrolysis, the biomass carbonization, the decomposition of FeCl 3 , and the activation of porous carbon by Fe-based compounds could release volatile matters such as H 2 O, CO, CO 2 , and HCl (equations ( 3)-( 5)), triggering a decrease in pyrolysis efciency [20].Nevertheless, there was a very slight decrease in pyrolysis efciency between 2.0 h (57%) and 4.0 h (56%).Te results proved that the pyrolysis was nearly complete after 2.0 h.Overall, the appropriate pyrolysis time for forming Fe 3 O 4 crystals might be around 2.0 h.

Efect of FeCl 3 /NF Mass Ratio on XRD Patterns of MPC.
Figure 4 shows that increasing the FeCl 3 /NF mass ratio from 0.05 to 0.1 resulted in higher Fe 3 O 4 peaks.However, when the FeCl 3 /NF mass ratio was increased from 0.1 to 0.2, the trend was reversed.According to Do et al. [36], the lack of water released from the biomass carbonization at high ratios may suppress the conversion of FeCl 3 to Fe 3 O 4 .Table 1 reveals that the pyrolysis efciency improved as the FeCl 3 / NF mass ratio increased.Higher residual solids indicated that reactions occurring during pyrolysis were weakened [20,36].

Porous Properties of MPC.
Figure 5 presents the nitrogen adsorption-desorption isotherm of MPC-R0.1-2.0 h.It is found to be a narrow hysteresis loop at relative pressure (P/P o ) from 0.40 to 0.995, possibly relating to capillary condensation in slit-shaped pores [58].Interestingly, BJH analysis revealed that MPC has a hierarchically micro-mesomacro porous structure.Meso-macropores contribute 0.13 cm 3 /g, which corresponds to 50% of V total (Table 2).As compared to PC, MPC has a much higher V micro but a similar V meso-macro .As a result, the S BET and V total of MPC are 330 m 2 /g and 0.26 cm 3 /g, which are 2.2-and 1.4-fold higher than those of PC, respectively.Tese results indicate  4 Journal of Chemistry that the presence of Fe-based compounds could mainly activate micropores during biomass carbonization [35,36,59,60].Moreover, the one-step strategy increased S BET and V total of MPC, as reviewed before.
3.1.6.SEM Images of MPC.SEM images at diferent magnifcations were used to observe the surface morphology of MPC-R0.1-2.0 h (Figure 7).Te material has a smooth surface with faky fragments.Notably, it seems hard to fnd aggregated Fe 3 O 4 and Fe 0 particles on the surface of MPC.Instead of adhering to the carbon surface, Fe-based particles formed by direct pyrolysis of FeCl 3 -loaded biomass can be encapsulated within the carbon base, as stated in such reports [36,41].Consequently, the particles are able to sustain a steady performance over an extended period of time.

EDX Spectroscopy and Elemental Mapping of MPC.
Figure 8 depicts the EDX spectroscopy and elemental mapping of MPC-R0.1-2.0 h.C, Fe, and O are the most abundant elements on the MPC surface, accounting for 76.65, 6.63, and 11.37 wt%, respectively.Tis surface Fe content was slightly lower than the bulk Fe content (8.7 wt%) determined by the original ferrozine method.Tis result suggests that the Fe element may be embedded within the carbon structure.Furthermore, the atom ratio of O/Fe on the surface of MPC was 6.0, which was signifcantly more than that of Fe 3 O 4 .Hence, oxygen could exist in both Fe 3 O 4 and oxygen functional groups.
According to Tamunaidu and Saka [48], Na, K, Mg, Ca, P, S, Cl, and Si elements were detected in nipa fronds from the Philippines.Similar inorganic elements were found in NF from Tailand [49].In this research, except for Na and K, the remaining elements were detected in NF-derived MPC.Na and K may have been removed by washing or were not present in NF feedstock.Nevertheless, Cl might partially or completely originate from the added FeCl 3 .As previously stated, MPC was cleaned until no further Fe 3+ and Cl − leaching occurred.Tus, these inorganic elements might be frmly immobilized in the inner parts of the carbon structure by strong mechanical or chemical bonds [20].Lastly, element mapping showed that Fe sites as well as other available elements (O, Mg, Ca, Cl, Si, P, and S) were well dispersed on the MPC surface (C).Te uniform Fe distribution may be due to the efciency with which FeCl 3 was preloaded into NF prior to pyrolysis.

Magnetic Properties of MPC.
As shown in Figure 9, MPC could be efectively attracted by a magnet.It contributes signifcantly to the separation and recovery of MPC from certain suspensions.As previously described, MPC was separated from the treated sample using a magnet prior to the analyzing step.Moreover, the magnetic hysteresis curve revealed that MPC was superparamagnetic due to its magnetizability and demagnetizability.Tese magnetic properties could result from both Fe 3 O 4 and Fe 0 particles coexisting in MPC [65,66].With only 8.7 wt% Fe, the specifc saturation magnetization (M S ) reached 7.65 emu/g (Table 2), not much diferent from other reports for MPC [31,35].Assuming that all Fe in MPC is either 100% Fe 3 O 4 or 100% Fe 0 , their corresponding saturation magnetizations would be 88 or 64 emu/g, respectively.Tese values are roughly comparable to those of Fe 3 O 4 [67][68][69] and Fe 0 nanoparticles [70][71][72].which were favorable for Fenton-like reactions.Tis could lead to the misconception that MPC exhibits efective catalytic activity throughout a broad pH range.To assess right catalytic activity as well as achieve stable heterogeneous catalysis, all MPC samples were meticulously washed to remove all remaining FeCl 3 .

Efect of MPC Prepared from Diferent Pyrolysis Time.
Figure 11 presents the infuence of MPC samples obtained at diferent pyrolysis time on the adsorption capacity and catalytic decolorization of AY23.In the frst adsorption step, the adsorption processes occurred rapidly and nearly achieved equilibrium in 10 min.As the pyrolysis time increased from 0.5 to 4.0 h, the adsorption capacity gradually rose from 5.6 to 11.3 mg/g (Table 3).Pyrolysis time could afect the porous structure of MPC [35], potentially resulting in AY23 adsorption.Regarding the adsorption mechanisms, AY23 might interact with the MPC surface via hydrogen bonding, π-π stacking interaction, and electrostatic interaction [35,73].Contrarily, ionic Fe 3 O 4 and metallic Fe 0 crystals might not bind organic AY23 molecules as well.Tis demonstrated that, under proper conditions, MPC could be a practical adsorbent for AY23 remediation.However, the advantage of MPC comes from its catalytic activity for AY23 decolorization.
Fe 3 O 4 and/or Fe 0 particles residing in MPC might accelerate the breakdown of H 2 O 2 into •OH, according to the following equations [36,74]: As described in XRD results, Fe 3 O 4 and Fe 0 crystals, which could be the dominant catalytic sites of MPC, were grown during the pyrolysis.Te growth was practically complete after 2.0 h and did not improve much thereafter.As a result, when the pyrolysis time was prolonged from 0.5 to 2.0 h, the catalytic activity of the MPC samples increased signifcantly, leading to an increase in the decolorization efciency of AY23 from 33.2 to 81.9% (Table 3).However, the decolorization rate catalyzed by MPC-R0.1-4.0 h was not much higher than that catalyzed by MPC-R0.1-2.0 h.To save energy for the synthesis of MPC, the appropriate pyrolysis time was 2.0 h.

Efect of MPC Prepared from Diferent FeCl 3 /NF Mass
Ratios.AY23 decolorization was explored with MPC catalysts prepared from diferent FeCl 3 /NF mass ratios of 0.05, 0.1, and 0.2 (Figure 12).To assess the role of Fe-based catalyst and porous carbon support, a porous carbon sample without Fe (PC-2.0h) was used as a reference.In the absence of Fe, PC eliminated a negligible amount of AY23 through adsorption rather than catalytic oxidation.In contrast, all MPC samples exhibited robust AY23 decolorization.Tese results suggest that the catalytic activities of MPC are solely derived from Fe sites in Fe 3 O 4 and Fe 0 particles.However, the porous system and functional groups     of the carbon support might play a certain role in the catalysis process, such as facilitating species in mass transfer and surface interaction [36].When the FeCl 3 /NF mass ratio increased from 0.05 to 0.2, the catalytic activity of the obtained MPC samples did not difer signifcantly.A higher FeCl 3 /NF mass ratio could result in more Fe catalytic sites in MPC, leading to greater AY23 decolorization.Notwithstanding, •OH radicals might be diminished by excessive Fe, as expressed in the following reaction [75,76]:  91.2%, respectively.It is advantageous to raise the MPC dosage in order to improve the decolorization efciency, since this will improve the radical generation.Te catalyst cannot, however, be added without limitation [76].As shown in equation (7), excessive Fe(II) may consume •OH radicals, resulting in a detrimental efect on AY23 decolorization.

Efect of Initial pH.
Te efect of the initial pH values on the adsorption capacity and catalytic decolorization of AY23 by MPC is depicted in Figure 14.Te pH values ranged from 2.0 to 10.0, while all other variables were kept constant, as shown in Table 3.In general, as pH climbed from 2.0 to 10.0, the adsorption capacity reduced from 18.1 to 6.1 mg/g.According to Vargas et al. [73], the electrostatic interaction between AY23 molecules and carbon surface is favorable at   Journal of Chemistry pH ∼2.At pH values greater than ∼2, sulfonic groups and N=N bonds in AY23 molecules become partially negative, causing electrostatic repulsion with the carbon surface charge.Initial pH strongly infuenced the decolorization efciency.In general, the AY23 decolorization rate decreased when pH increased.After 120 min of treatment, the decolorization efciencies at pH 2.0, 3.0, 3.5, 4.0, and 10.0 were 94.5, 89.1, 37.8, 24.4, and 16.5%, respectively.Although pH 2.0 ofered rapid and complete AY23 decolorization within 30 min of H 2 O 2 addition, low pH could promote Fe leaching, leading to a homogeneous mechanism for catalytic decolorization [77].As pH increases, the interaction between AY23 molecules and the MPC surface might decrease [73].Moreover, it is known that as pH increases, the oxidation potential of •OH radicals lowers [76].In basic environments (pH 10.0), CO 2 generated during AY23 oxidation might be transformed into CO 3 2− and HCO 3 − , which could react with •OH [78].Overall, pH 3.0 may be appropriate for achieving a heterogeneous mechanism and maintaining high decolorization efciency.

Conclusion
In this research, magnetic porous carbon was facilely prepared through one-step pyrolysis of FeCl 3 -impregnated nipa frond.Te results showed that Fe 3 O 4 and Fe 0 crystals were formed from FeCl 3 in carbon support.Notably, these reductions activated the porous structure of carbon, providing a high S BET of 330 m 2 /g and a large V total of 0.26 cm 3 /g.MPC was accordingly utilized as an oxidation catalyst for acid yellow 23 decolorization by H 2 O 2 .At pH 3.0, 200 ppm H 2 O 2 , and 0.80 g/L MPC-R0.1-2h, AY23 was removed with an adsorption capacity of 14.5 mg/g.Next, 89.1% of AY23 was removed after H 2 O 2 addition.Furthermore, AY23 decolorization with MPC catalysts approximated pseudo-frst-order kinetics with a typical rate constant of 0.0186 min −1 .With a strong saturation magnetization of 7.65 emu/g, MPC catalyst could be easily separated from the reaction mixture using a magnet.In conclusion, environmentally friendly, sustainable, and cheap magnetic porous carbon has the potential to be an efcient catalyst for acid yellow 23 remediation in wastewater.

Table 3 :
Adsorption capacity and catalytic activity of PC and MPC samples on acid yellow 23.