Adsorption of nitrophenol onto a novel Fe3O4-κ-carrageenan/MIL-125(Ti) composite: process optimization, isotherms, kinetics, and mechanism

Water pollution is a dreadful affair that has incessantly aggravated, exposing our planet to danger. In particular, the persistent nitro aromatic compound like nitrophenols causes anxiety to the researchers due to their hazardous impacts, excessive usage, and removal difficulty. For this purpose, a novel multi-featured composite was constructed based on κ-Carrageenan (κ-Carr), MOF (MIL-125(Ti)), and magnetic Fe3O4 for efficient adsorptive removal of o-nitrophenol (o-NP). Interestingly, BET measurements revealed the high surface area of Fe3O4-κ-Carr/MIL-125(Ti) of about 163.27 m2/g, while VSM showed its excellent magnetic property (20.34 emu/g). The comparison study pointed out the synergistic effect between Fe3O4, κ-Carr, and MIL-125(Ti), forming a composite with an excellent adsorption performance toward o-NP. The adsorption data obeyed pseudo-second-order kinetic model, and Freundlich isotherm model was better fitted than Langmuir and Temkin. Furthermore, Langmuir verified the supreme adsorption capacity of o-NP onto Fe3O4-κ-Carr/MIL-125(Ti) since the computed qmax reached 320.26 mg/g at pH 6 and 25 °C. Furthermore, the XPS results postulated that the adsorption mechanism pf o-NP proceeded via H-bonding, π-π interaction, and electron donor–acceptor interactions. Interestingly, Fe3O4-κ-Carr/MIL-125(Ti) composite retained good adsorption characteristics after reusing for five cycles, suggesting its viable applicability as an efficient, renewable, and easy-separable adsorbent for removing nitro aromatic pollutants. Supplementary Information The online version contains supplementary material available at 10.1007/s11356-023-25678-2.


Introduction
Water pollution by the fatal aromatic compounds is swiftly exacerbated, threatening humanity's existence (Das et al. 2021;Kassem et al. 2021;Priyadarshi et al. 2022). Nitrophenol is one of the most pernicious aromatic compounds since it is widely applied in diversified industries such as dyes, insecticides, medicines, and petrochemicals (Abdelfatah et al. 2021a;Liu et al. 2020). Consequently, colossal amounts of nitrophenol have been disposed into waterbodies, causing deleterious impacts on human health, such as damage to kidneys and liver, blurred vision, systemic poisoning, and mouth irritation (Benmaati et al. 2022;Ma et al. 2019). In addition to the dangerous effects on the environment, the presence of the nitro group in nitrophenol enhances its stability in soil and water bodies (Ewis et al. 2022). Hence, several remediation techniques have been fostered to get rid of these detrimental contaminants from water bodies, such as membrane separation, coagulation, electrolysis, ion exchange, and adsorption (Abdelfatah et al. 2021a, Abdelfatah et al. 2021b, Karim et al. 2019, Sadoon and M-Ridha 2019, Saha et al. 2022, Tran et al. 2021. The former technique has been applied more widely than the other techniques owing to its simplicity, low cost, high-efficacy, Responsible Editor: Tito Roberto Cadaval Jr * Eman M. Abd El-Monaem emanabdelmonaem5925@yahoo.com 1 and low-energy consumption (Abdelfatah et al. 2022;Deb et al. 2021;Gomaa et al. 2022b; Mokhtar et al. 2020;Raval et al. 2021;Zhao et al. 2021).
Carrageenan is an anionic polysaccharide polymer extracted from red seaweed . Carrageenan involves sulfate groups, and it is classified into theta, beta, iota, lambda, alpha, and kappa based on the position and numbers of the attached sulfate groups onto the carrageenan skeleton (Sharma et al. 2022). Interestingly, κ-Carr is vastly utilized in the food industries since it has remarkable thickening, stabilizing, and gelling properties (Ammar et al. 2021). Furthermore, the eco-friendly, availability, biodegradability, and biocompatibility characteristics of κ-Carr render it a promising adsorbent for removing contaminants (Li et al. 2019). In spite of the remarkable advantages of κ-Carr, it still suffers some limitations, including an inferior gel strength and poor environmental stability (Lapwanit et al. 2018). Thence, several attempts have been implemented to overcome these flaws, including modification of κ-Carr with carbon materials, magnetic nanoparticles, zeolite, and other polymers (Duman et al. 2019(Duman et al. , 2016Huang et al. 2022;Mittal et al. 2020). Notably, it was reported an enhancement in the κ-Carr properties by impregnating metal-organic frameworks (MOFs) into its matrix (Klongklaew and Bunkoed 2021). Nevertheless, there is a scarcity of research papers that involves the fabrication of κ-Carr/MOF composites.
Strikingly, the applications of MOFs have been raised day-by-day in diversified fields such as drug delivery, solar cell, gas storage, batteries, and especially in water remediation. (Lazaro andForgan 2019, Shen et al. 2021;Xu et al. 2022). Due to their excellent chemical and thermal stability, high surface area, porous structure, water stability, and ease of functionalization, MOFs have gained vast concern as propitious adsorbents (Abd El-Monaem et al. 2022). Titanium-based MIL-125 is a shining member of the remarkable MIL family that possesses excellent adsorption performance owing to excellent chemical stability, redox potential, and thermal stability (Fatima et al. 2020). Interestingly, MIL-125(Ti) has revealed an enhanced adsorption performance toward pharmaceutical residues, organic dyes, and heavy metals (Jiang et al. 2021;Liang et al. 2018;Liu et al. 2021;Omer et al. 2021). However, the difficult separation of MIL-125(Ti) after the adsorption process with the traditional techniques is a big obstacle. To overcome this disadvantage, incorporating magnetic nanoparticles in the adsorbent matrix enable easy separation of them after adsorption. One of the most extensively utilized magnetic nanoparticles is magnetite due to its ease of synthesis, biocompatibility, high surface area, and abundant active sites (Attia et al. 2022;Toto et al. 2022). Despite the construction of various composites from MIL-125(Ti) with many polysaccharides, there are no study reported the fabrication of MIL-125(Ti)/κ-Carr composite and the evaluation of its adsorbability toward organic pollutants or any other water contaminants.
Herein, we adopted a developed avenue to foster the adsorption performance of κ-Carr through its combination with . Furthermore, to overcome the separation difficulty, κ-Carr/MIL-125(Ti) composite was decorated by magnetic nanoparticles (Fe 3 O 4 ). Assorted characterization tools were utilized to infer the successful fabrication of Fe 3 O 4 -κ-Carr/MIL-125(Ti) and study its chemical and physical properties. The adsorbability of Fe 3 O 4 -κ-Carr/ MIL-125(Ti) composite was examined in the adsorption of o-nitrophenol (o-NP) from an aqueous solution. Besides, the reusability of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite was confirmed by executing the recyclability test for five cycles. More importantly, the mechanism of the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite was understood thoroughly based on XPS results.

Fabrication of Fe 3 O 4 nanoparticles
Fe 3 O 4 was prepared as reported in the author's previous study . Under the N 2 atmosphere, the specific amounts of FeCl 3 .6H 2 O and FeCl 2 .4H 2 O were dissolved into 500 mL double-distilled water. Then, ammonium solution was dropped slowly into the Fe 2+ /Fe 3+ solution until pH reached 10. The resultant solution was stirred at 80 °C for 9 min. Finally, the obtained Fe 3 O 4 particles were collected via a magnet, washed, and dried at 70 °C.

Fabrication of MIL-125(Ti)
MIL-125(Ti) was prepared as follows: 995 mg BDC was dissolved into 25 mL DMF under magnetic stirring for 15 min. Then, 1.35 mL Ti(O-iPr) 4 was dipped into the BDC solution and kept under vigorous stirring for 1 h. Next, Ti/BDC solution was poured into a 100 mL autoclave and heated at 130 °C for 24 h. Ultimately, the obtained solid was collected by centrifugation, washed, and dried at 80 °C for 24 h ).

Batch adsorption
The optimum conditions to adsorb o-NP onto Fe 3 O 4 -κ-Carr/ MIL-125(Ti) composite were defined in a batch mode. A series of the o-NP adsorption process proceeded at a pH range from 2 to 10 to identify the optimum pH. Furthermore, the impact of the Fe 3 O 4 -κ-Carr/MIL-125(Ti) dose on the adsorption aptitude of o-NP was determined at a dose range from 0.005 to 0.02 g. Moreover, the influence of the temperature on the removal efficiency of o-NP was examined at a temperature range from 25 to 55 °C. Finally, the impact of the initial concentration of o-NP was studied at a concentration range from 50 to 200 mg/L. The residual concentration of o-NP was measured according to the standard methods to examine water and wastewater (Rice et al. 2012) using UV-Vis spectrophotometer (PG 82 + , UK) at 344 nm, then the adsorption capacity and the removal % of o-NP were calculated by the following equations: where C 0 and C t symbolize the initial concentration of o-NP and the concentration at time t, respectively. m and V symbolize the mass of Fe 3 O 4 -κ-Carr/MIL-125(Ti) and the volume of o-NP solution, respectively.

Recyclability test
To evaluate the regeneration potential of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite, the used composite was separated after the o-NP adsorption and subsequently soaked into 25 mL 1 M NaOH under magnetic stirring to desorb o-NP from its surface. Then, the recycled composite was washed with distilled H 2 O and utilized in the next cycle, repeating this adsorption/desorption cycle five times.

Ionic strength test
The influence of the ionic strength on the o-NP adsorption aptitude was assessed as follows: a specific weight of NaCl (0.2-1.0 mol/L) was soaked in 20 mL of o-NP at pH 6. Then, 10 mg of Fe 3 O 4 -κ-Carr/MIL-125(Ti) was added to o-NP/ NaCl solution under stirring. After 60 min, a sample was withdrawn and measured to determine the concentration of the un-adsorbed o-NP.

Morphology study
SEM was utilized to identify the morphology of the as-synthesized onto the fiber-like particles of κ-Carr, denoting the successful combination between the composite's components.

BET
The N 2 -adsorption/desorption isotherm (Fig. 4C) elucidated that Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite showed a type II with H 4 -type hysteresis loop according to the IUPAC classification, suggesting the mesoporous structure of the composite. Moreover, the specific surface area of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite was 163.27 m 2 /g, and the average pore diameter was 2.861 nm (Fig. 4D).

Comparison test
For assessing the amelioration of the adsorption performance of Carr toward o-NP after blinding with MIL-125(Ti), a comparison test was executed between the pure materials and the three fabricated composites (Fig. 5A). It was found that the removal % of Fe 3 O 4 , κ-Carr, and MIL-125(Ti) were 17.35, 28.35, and 49.55% and the adsorption capacity were 22.86, 33.12, and 52.91 mg/g, respectively. Furthermore, the removal % and the adsorption capacity of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composites with κ-Carr: MIL-125(Ti) ratios 3:1, 1:1, and 1:3 were 60.99, 65.19, and 77.55% and 63.60, 67.51, and 79.05 mg/g, respectively (Table S1). In light of these results, the modification of κ-Carr with efficient material like MIL-125(Ti) is an effective approach as it increased the removal % of o-NP by more than 2.5-fold. In addition to the dual function of Fe 3 O 4 that provides perfect separation and enhances the adsorption aptitude of o-NP, the Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite with a ratio of 1:3 between κ-Carr and MIL-125(Ti) was chosen for the rest of the batch experiments.

Effect of the solution pH
In general, pH is the dominant parameter in the uptake processes, so the o-NP uptake onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite was investigated at a wide scale of pH media  (Fig. 5B). The experimental results indicated the superiority of the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) at pH 6. This finding could be explained by the pKa of o-NP = 7.23, meaning that o-NP exists in the molecular form in acidic conditions Marques et al. 2020). Thereby, the electrostatic interaction is not the controlling mechanism on the adsorption of o-NP onto Fe 3 O 4 -κ-Carr/MIL-125(Ti), and there are other chemical and physical interactions such as π-π interaction, H-bonding, and electron donor-acceptor interaction could take place between o-NP and Fe 3 O 4 -κ-Carr/MIL-125(Ti) in the acidic medium (Chen et al. 2017). Conversely, it was observed a dramatic diminution in the o-NP adsorption aptitude when pH > 6 since the removal % and the adsorption capacity of o-NP dwindled from 77.55% and 79.05 mg/g to 26.02% and 34.18 mg/g. This finding may be anticipated by the strong repulsion forces between the anionic o-NP and the negatively charged Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite (Liu et al. 2020). Figure 5C represents the impact of the dosage of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite onto the adsorption efficiency o-NP. It is apparent that the augmentation in the composite dose from 0.005 to 0.02 g causes an increase in the removal % of o-NP from 58.28 to 92.29%, respectively, which is most likely due to the increase in the number of active sites. On the contrary, a decline in the adsorption capacity of o-NP from 122.12 to 46.40 mg/g was observed with the raising in the Fe 3 O 4 -κ-Carr/MIL-125(Ti) dose which may be attributed to the aggregation of the extra amount of the composite, resulting in a diminution in the surface area. Figure 6A reveals the impact of the ionic strength on the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite. It was recorded an enhancement in the adsorption aptitude of o-NP with the increase in the NaCl concentration from 0.2 to 1.0 mol/L since the adsorption capacity and removal % incremented from 79.31% and 81.33 mg/g to 87.93% and 92.45 mg/g, respectively. This behavior is most likely due to the salting out effect as the presence of NaCl declines the o-NP solubility, agreeing with Mengzhi Yang et al. 2018 .

The effect of the temperature
It was deduced the exothermal nature of the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) since the adsorption capacity and removal % of o-NP dwindled from 79.05 mg/g and 77.55% to 65.82 mg/g and 63.38% with raising the process temperature from 25 to 55 °C, respectively (Fig. 6B). This behavior may be assigned to the increase in the system temperature causes an increment in the Brownian motion of the o-NP molecules inside the bulk solution. Hence, the adsorption aptitude o-NP onto the Fe 3 O 4 -κ-Carr/MIL-125(Ti) surface directly diminished. The effect of the initial concentration Figure 6C depicts the influence of the increment of the initial concentration of the bulk solution on the adsorption efficacy of o-NP. It was found that the increase in the o-NP concentration from 50 to 200 mg/g caused an increase in the adsorption capacity from 93.02 to 271.11 mg/g. This finding may be explained by the concentration increase in the bulk solution, generating strong driving forces of o-NP toward the Fe 3 O 4 -κ-Carr/MIL-125(Ti) surface. Thence, such potent forces could overcome the mass transfer resistance to the migration of o-NP from the bulk solution to Fe 3 O 4 -κ-Carr/ MIL-125(Ti) surface (Gomaa et al. 2022a). On the contrary, this increase in the concentration of o-NP solution resulted in a diminution in the removal % from 92.54 to 57.05%, which is most likely due to the inadequate binding sites into the Fe 3 O 4 -κ-Carr/MIL-125(Ti) surface (Fig. 6D) (Eltaweil et al. 2022c). Overall, Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite not only exhibited efficient adsorption performance toward the detrimental o-NP but also fast adsorption since the equilibrium time was 60 min.

Kinetic study
The experimental results pointed out that the adsorption of o-NP onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite may occur via diverse mechanisms, depending on the heterogeneity of the binding sites onto the composite surface and physicochemical conditions. Therefore, various kinetic models: pseudo-first-order (PFO), pseudo-second-order (PSO), and Elovich, were utilized to analyze the resultant experimental data ( Fig. 7A-C). The linear expressions of the applied models are listed in Table S2.
It was deduced from the computed R 2 values ( Table 1) that PSO is more suitable than PFO to represent the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite since R 2 -PSO > R 2 -PFO (Eltaweil et al. 2022b). Furthermore, the obtained q cal from PSO are closer to q exp than those calculated from PFO. Besides, the favorability of

Isotherm study
The type of interactions between o-NP and Fe 3 O 4 -κ-Carr/ MIL-125(Ti) composite at equilibrium was scrutinized by bountiful isotherm models, including Langmuir, Freundlich, and Temkin ( Fig. 8A-C). The linear expressions of these models were summarized in Table S3. The obtained isotherm parameters ( Table 2) point out that Freundlich model (R 2 = 0.999) is more fitted than Langmuir (R 2 = 0.984) and Temkin (R 2 = 0.983) to model the equilibrium data of the o-NP adsorption onto Fe 3 O 4 -κ-Carr/MIL-125(Ti). Additionally, the b value from Temkin model evinced the same result since b < 80 kJ/mol. Moreover, it was found that n > 2 reflects the favorability of the o-NP adsorption process onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite. Furthermore, the calculated q max of o-NP onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite under Langmuir was 320.26 mg/g at pH 6 and 25 °C.

Reusability study
It is apparent from the recyclability test (Fig. 8D) that Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite has remarkable recyclability where its adsorption performance toward o-NP was still high after the 5th cycle (q = 58.94 mg/g and R% = 56.01%). This finding confirmed the significance of these magnetic adsorbents that provide a fast, easy, and impeccable separation after the adsorption processes using an external magnet.

The proposed adsorption mechanism
Understanding the adsorption mechanism is a quintessential point in any adsorption process, so the controlled mechanism on the adsorption of o-NP onto Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite was studied based on XPS analysis, ZP measurements, and the experimental results.  (ii) π-π interaction between the aromatic ring of BDC in the composite and the benzene ring in o-NP; (iii) electron donor-acceptor interaction between the e-donor groups in Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite (OH, SO 4 2− and benzene ring) and the e-withdrawing group in o-NP (NO 2 ) as well as the e-withdrawing group in the composite (COOH) and the e-donor groups in o-NP (OH and benzene ring). The occurrence of these physicochemical interactions between o-NP and Fe 3 O 4 -κ-Carr/MIL-125(Ti) was confirmed by the peak shift of the XPS spectra of O1s and S2p after the o-NP adsorption (Fig. 9B, C). (iv) The electrostatic repulsion forces between the anionic o-NP and the negatively charged composite as clarified from ZP measurements in an alkaline medium, played a secondary negative role on the adsorption aptitude of o-NP.

Conclusion
This study reported the construction, characterization, and adsorbability evaluation of a new Fe 3 O 4 -κ-Carr/ MIL-125(Ti) composite for removing the organic o-NP pollutant. The characterization stage inferred the successful formulation of the composite adsorbent. Surprisingly, increasing MIL-125(Ti) ratio three times than κ-Carr significantly boosted the removal (%) of o-NP from 60.99 to 77.55%, and the maximal adsorption capacity of o-NP attained 320.26 mg/g at pH 6 and 25 o C. Moreover, data obtained from kinetics and isotherm studies were fitted to Freundlich model and followed the pseudo-second-order model. The reusability test attested the potential capability of Fe 3 O 4 -κ-Carr/MIL-125(Ti) composite to adsorb o-NP after five repeated cycles with removal (%) exceeded 60%. Overall, the higher adsorption performance, facile separation, and recyclability features nominate the potential use of the formulated composite as an efficient adsorbent candidate for advanced water treatment.
Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). A survey before and after the adsorption of o-NP, B O1s, and C S2p after the adsorption process Data availability The datasets used and analyzed during the current study are available from the corresponding author on reasonable request.

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Competing interests
The authors declare no competing interests.
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