Rapid oxidation and deep As(III) purification from water using gelatin-supported iron-based metal-organic framework aerogel coupled with H 2 O 2 Preparation, performance and mechanism

. Quenching experiments, EPR and DFT calculations showed that the active species (surface • OOH and surface • OH) generated by the Fenton-like interaction between the exposed (100) facet of MIL-88A-Fe in PMGA and H 2 O 2 played an essential role in promoting the oxidative removal of As(III). Moreover, the reaction system could work efficiently for As(III) in a wide pH range and was hardly affected by other substances such as chlorides, sulfates, carbonates, silicates, and humic acids. Simulta-neously, the fixed bed column filled with PMGA could stably and continuously treat As(III) in groundwater. Therefore, this study not only broadens the horizon for the development and design of 3D functional materials but also provides a new strategy in water purification to eliminate arsenic pollutants using Fe-MOF aerogels.


Introduction
Inorganic arsenic is one of the most toxic and carcinogenic chemical elements in groundwater caused by various human activities, including ore smelting, industrial wastewater discharge, coal burning, and arsenic-containing pesticides [1].Long-term exposure to or consumption of arsenic-contaminated water poses a serious threat to human health, such as skin, kidney, and liver cancer [2].The main forms of inorganic arsenic in water are arsenate (As(V)) and arsenite (As(III)).Among them, As(III), which has high toxicity and easy migration, is more harmful to human health, the ecosystem and the natural environment [3].Therefore, an urgent need is to develop an economical, simple and efficient method for rapidly removing arsenic, especially As (III), from water.
To date, people have researched and developed various technologies for removing arsenic in water.The adsorption method has received extensive interest due to its high treatment efficiency, low secondary pollution, low cost-effectiveness, and simple operation [4,5].Metal oxides have been reported to exhibit impressive arsenic removal capabilities due to their strong affinity for arsenic species [6,7].Compared with traditional metal oxides, metal-organic frameworks (MOFs), which benefit from the advantages of abundant metal active sites, controllable crystal structure, and huge surface area, have become one of the hotspots in the current research field [8,9].For example, Fe-BTC, ZIF-67 and UIO-66 have been used as powder adsorbents for arsenic removal [4,10,11].Among these MOFs, Fe-MOFs are considered candidates for purifying arsenic pollutants in water due to their strong sustainability, low toxicity, good biocompatibility, and strong water stability [12,13].One of the most active Fe-MOFs is MIL-88A-Fe, made from octahedral polymer composed of fumaric acid and Fe(III) ions and can be synthesized without involving toxic organic solvents [14].Furthermore, the widespread presence of Lewis acid sites in the MIL-88A-Fe structure makes it a potential catalyst in heterogeneous Fenton-like catalytic systems [15,16].Notably, the oxidation of As(III), which usually exists in a non-ionic state, to easily removable As(V) is considered to be an essential step to enhance the removal efficiency and reduce the toxicity during the adsorption process [13,17].Thus, the characteristics of MIL-88A-Fe make it possible to couple with H 2 O 2 to construct a heterogeneous catalytic oxidation system to remove As(III).However, like nonmagnetic metal oxides, most MOFs face difficulties in recycling and disposal due to their powder form, while also causing potential secondary pollution issues.Moreover, the inevitable agglomeration of MOF particles may also decrease the effective surface area, thereby reducing active adsorption sites [18].Therefore, to overcome these shortcomings and improve the applicability of MIL-88A-Fe, immobilizing it on a suitable support is an effective strategy.
As a three-dimensional (3D) macroscopic lightweight material with large specific surface area, high porosity and low density, aerogel can well meet the needs as a carrier [19].Its larger specific area not only enables more catalysts to be immobilized but also the porous nature enables the catalysts immobilized on the surface of the 3D network to be fully exposed to pollutants [20].More importantly, various biomass aerogels represented by chitosan, cellulose, alginate, and gelatin have excellent environmental compatibility and biodegradability [21].In recent years, the application of MOF-based aerogels mainly focuses on catalysis, adsorption, and energy storage and conversion.For example, Shalygin et al. used MOF-HKUST -1 to prepare a HKUST -1 @SiO 2 aerogel that can oxidize styrene to phenylacetaldehyde through isomerization [22].Li et al. explored the adsorption capacity of ZIF-67 modified chitosan/bacterial cellulose composite aerogel for Cr(VI), Cu(II) and organic dyes [23].Despite some progress in the past years, there is little literature focusing on the use of Fe-MOF aerogels for heterogeneous Fenton-like catalytic reaction processes in water purification.Especially Fe-based MOF gelatin aerogels have never been studied for this purpose.Thus, it is worth further exploring and constructing a heterogeneous catalytic oxidation system coupled with MIL-88A-Fe/gelatin aerogel and H 2 O 2 to purify As(III) in water.
To this end, a three-dimensional porous MIL-88A-Fe/gelatin aerogel (PMGA) was synthesized by a direct mixing method using lowtemperature carbonization and foaming techniques to purify As(III) in water.The low-temperature carbonization method was used to increase the degree of cross-linking of gelatin in PMGA instead of using chemical cross-linking agents such as glutaraldehyde or epichlorohydrin to bring potential secondary environmental hazards [24].Furthermore, the crystal phase, morphology, and microstructure of PMGA were characterized by various techniques to understand their effect on As(III) adsorption.The removal performance of As(III) in the PMGA/H 2 O 2 system and the influence of solution chemistry on it were systematically analyzed.The practicality and reliability of the PMGA/H 2 O 2 system were also evaluated.Meanwhile, the main oxidation species and generation mechanism of As(III) were determined using quenching experiments and density functional theory (DFT) calculations.The possible removal mechanism of As(III) was proposed in the reaction system.Finally, As(III) removal experiments in different natural waters and fixed bed column experiments were carried out to investigate the potential of the PMGA/H 2 O 2 system in practical applications.The results provide novel insights into the rational design of Fe-based MOF aerogels for arsenic removal in water purification.

Experimental section
The texts S1, S2, and S3 in the Supplementary Materials provide the utilized reagents, characterization instruments and methods, and a detailed description of the Density Functional Theory calculation procedure.

Synthesis of three-dimensional porous MIL-88A-Fe/gelatin aerogel (PMGA)
The synthesis of MIL-88A-Fe was prepared using eco-friendly methods based on predecessors.Briefly, 2.7 g of FeCl 3 ⋅6H 2 O was dissolved in 150 mL of ultrapure water, and 1.2 g of fumaric acid was added under stirring.Then, the mixed precipitate was placed in a Teflon reaction kettle and reacted at 65 • C for 4 h.Finally, the precipitate obtained by centrifugation was collected and washed 5 times with water and ethanol absolute and then dried at 60 • C for 12 h.
In a typical procedure for synthesizing PMGA, 0.06 g of sodium dodecyl sulfate (SDS) and 1.5 g of gelatin were dissolved in 20 mL of ultrapure water at 50 • C.Then, different masses of MIL-88A-Fe were added and stirred slowly for 40 min, followed by rapid stirring for 20 min.Subsequently, the homogeneous solution that produced abundant bubbles was poured into a mold and dried in a vacuum freeze dryer for 36 h.Finally, under the protection of N 2 , carbonization-crosslinking were carried out for 3 h at a heating rate of 5 • C/min at a low temperature.

Batch experiments
Unless otherwise stated, all batch experiments were carried out at room temperature (25 • C) in 1 L beakers at a stirring speed of 200 rpm.The experiment was initiated by adding 0.3 g PMGA and a quantity of H 2 O 2 to 0.5 L of an arsenic solution.The initial pH of the solution prior to the reaction was controlled by adding either dilute HCl or NaOH for adjustment. 2 mL of supernatant was taken out at predetermined time intervals and filtered with a 0.22 µm filter membrane.The concentration of residual arsenic was detected by inductively coupled plasma mass spectroscopy (ICP-MS, Agilent 7700X, USA).The adsorption isotherm studies were carried out in a 100 mL single-necked Erlenmeyer flask where PMGA dose, solution volume, initial arsenic concentration, H 2 O 2 / As molar ratio, pH, shaking speed, and shaking time were 30 mg, 50 mL, 0-30 mg/L, 75, 9.5, 200 rpm, and 24 h, respectively.The residual arsenic concentration in the above solution was detected using inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 8300, PerkinElmer, USA).The kinetic and isotherm data obtained above were fitted using a pseudo-first-order/second-order kinetic model, an intraparticle diffusion model and a Langmuir/Freundlich isotherm model to further understand the heterogeneous catalytic oxidation process.The relevant model information can be found in Supplementary Material Text S4.The effects of coexisting ions, quencher or humic acid (HA) on the As(III) removal were observed by adding a specific amount of Na 2 SiO 3 ⋅9H 2 O, Na 2 HPO 4 , NaCl, Na 2 SO 4 , Na 2 CO 3 , tert-butanol (TBA), formic acid (FA), methanol (MeOH), p-benzoquinone (BQ) or HA to the solution.The test of regenerating capability was to elute the reacted sample with 0.2 M NaOH solution for 4 h.Then, it was washed several times with ultrapure water and freeze-dried for the next desorption cycle.Real water samples from Harbin, Heilongjiang Province, China were taken to assess the ability of the PMGA/H 2 O 2 system to remove actual arsenic contaminated water.The removal experiment procedure was the same as above.The main water quality parameters are shown in Table S1.

Column experiments
Fixed-bed column experiments were performed using a plexiglass column, in which the adsorbent mass was 2 g, the column diameter was 2 cm, the column length was 9 cm and the flow rate was 1.8 mL/min.The feed solution was taken from natural groundwater with an added arsenic content of 216 µg/L.The peristaltic pump was used to maintain G. Cai et al. the bottom-up water flow in the column.The cotton packed in the column was used to disperse the water flow.Take a sample after filtering 0.2 L of the solution each time and detect the effluent water As concentration.

Characterizations of PMGA
The SEM images of pure gelatin aerogel (PGA) are depicted in Fig. 1a 1 -a 3 .The PGA exhibited a smooth pore wall and a threedimensional porous interconnected network structure through foaming after the surfactant SDS reduced the surface tension.MIL-88A-Fe consisted of hexagonal micro-spindle crystals of approximately 800 nm in length (Fig. 1b 1 -b 3 ).After the two were compounded by simple physical stirring, the synthesized PMGA retained the original threedimensional porous interconnected network skeleton and had a more uniform distribution of MIL-88A-Fe particles anchored on the skeleton surface (Fig. 1c 1 -c 3 ).In addition, the synthetic PMGA could easily stand on the dandelion pompom due to its low density (0.041 g/cm 3 ), indicating that its ultra-light feature would also bring excellent floatability (inset in Fig. 1c 1 ).Therefore, PMGA had excellent recyclability in the actual water treatment process (Fig. S1).EDX spectrum (Fig. S2) and EDS mapping (Fig. 1d 1 -d 5 ) showed that PMGA was mainly composed of C, O, N and Fe elements, and the corresponding elements were uniformly distributed on the aerogel skeleton.Fig. 2a XRD pattern revealed the crystal structure of PMGA.The (1 0 0) and (1 0 1) diffraction planes of MIL-88A-Fe coincided with the characteristic peak positions of MIL-88A-Fe reported in previous literature [25,26], indicating its successful synthesis.The PGA was only observed as a broad peak at 2θ ≈ 25 • due to its amorphous structure.Weak MIL-88A-Fe diffraction peaks were observed after loading MIL-88A-Fe on PGA, which may be related to the lower content of loaded MIL-88A-Fe and the amorphous structure of parent gelatin.
FTIR spectroscopy was employed to explore the molecular structure within PMGA and identify functional groups (Fig. 2b).The PGA showed several characteristic bands for C-O-C bending vibrations (1081 cm − 1 ), C-N/N-H stretching vibrations (1238 cm − 1 ), -COOH symmetric and asymmetric vibrations (1338-1630 cm − 1 ), C-H stretching vibrations (2935 cm − 1 ) and N-H stretching vibrations (3296 cm − 1 ) [8,27,28].The characteristic peaks related to MIL-88A-Fe were observed at 673 (C = C), 796 (C = O), 979 (-COOH), 1398 (-COOH) and 1603 (-COOH) cm − 1 when MIL-88A-Fe was introduced to form PMGA [15].Meanwhile, the Fe-O stretching vibration band at 625 cm − 1 implied the effective combination of gelatin and MIL-88A-Fe [29].The BET surface area, pore volume and pore size of MIL-88A-Fe were 50.10 m 2 /g, 0.26 cm 3 /g and 17.91 nm, respectively, determined by N 2 adsorption-desorption isotherms (Fig. 2c and Table S2).Compared with PGA, the pore volume and BET surface area of PMGA were significantly increased.This suggested that the combination of MIL-88A-Fe and PGA promoted the exposure of PMGA to more active sites and mass transfer channels, thereby potentially enhancing arsenic removal.Furthermore, the thermal stability of PMGA was studied by thermogravimetric (TG) and the loading of MIL-88A-Fe in PMGA was obtained to be 28.79 wt% (Fig. 2d and Table S3).The mass loss rate of PMGA reached 81.79 % during the whole weight loss process.It was mainly attributed to the evaporation of residual solvent and surface adsorbed water in the first stage (25-245 • C), the pyrolysis of aerogel surface functional groups in the second stage (245-475 • C), and the destruction and decomposition of organic ligands and gelatin skeleton in MIL-88A-Fe in the third stage (475-700 • C) [30][31][32].Notably, the introduction of MIL-88A-Fe significantly improved the thermal stability of PMGA, which may be related to the chelation between the surface functional groups of gelatins and Fe 3+ in MIL-88A-Fe [33].

Removal kinetics and isotherms
The amount of MIL-88A-Fe loading in PMGA influenced its kinetics for As(III) removal.Thus, PMGAs with different MIL-88A-Fe contents (0.1, 0.2, 0.3 and 0.4 g) were synthesized (Fig. S3).As shown in Fig. 3a, increasing the addition of MIL-88A-Fe promoted the removal of As(III) by PMGA, implying that the surface reaction might be the rate-limiting step [34].However, when the content of MIL-88A-Fe was increased to 0.4 g, the As(III) removal ability of PMGA did not further improve.The results indicated that the rate-limiting step at this point may shift to the As(III) diffusion rate.On the other hand, to avoid the swelling phenomenon, we adopted the low-temperature calcination method to improve the cross-linking degree of PMGA through hydrophobic interaction and covalent bonds [35].This was because gelatin chains were rich in highly hydrophilic groups (such as carbonyl and hydroxyl) [36].However, PMGA's TG curves suggested that high temperatures might damage its interconnection network.Therefore, PMGA (Fig. S4) at different calcination temperatures (100, 150, 200 and 250 • C) were synthesized to study the effect of crosslinking temperature on As(III) removal.As shown in Fig. 3b, except that PMGA synthesized at 250 • C had almost no As(III) removal ability, PMGA at other calcination temperatures possessed similar catalytic ability.This phenomenon might be related to the disintegration and destruction of gelatin and MIL-88A-Fe caused by higher temperature.Fig. S4 showed that the PMGA calcined at 250 • C not only significantly reduced in volume but also underwent a sudden color change to black, proving this point.Moreover, dynamic Video S1 showed that PMGA calcined at 200 • C had excellent structural stability and flexibility in water.Therefore, based on the above result analysis, it was determined that the subsequent experiments would be conducted around the PMGA with MIL-88A-Fe content of 0. suggesting that the As(III) removal in PMGA was predominantly due to the loaded MIL-88A-Fe (Fig. S5).Fits to the As(III, V) data were implemented using pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models (Fig. 3d and 3e) and the relevant parameters are listed in Table S4.It was found that the PSO model obtained a higher correlation coefficient (R 2 ), indicating that the As(III, V) adsorption of PMGA may be chemisorption [37].Furthermore, the addition of H 2 O 2 increased the As(III) adsorption rate constant from 0.065 to 0.102 g/mg/min and was comparable to that of As(V) (0.094 g/mg/min).This indicated that H 2 O 2 indeed accelerated the removal of As(III) and the conversion of As(III) to As(V) might have occurred.Subsequently, the intraparticle diffusion model was used to fit the As(III, V) kinetic data in two different to elucidate the diffusion mechanism (Fig. 3f and Table S5).With a steep slope, the first stage indicated that the rapid diffusion and adsorption of As(III, V) from the macropores and mesopores in PMGA was the main factor controlling the rate.The second stage with a lower slope was the process in which As(III, V) ions occupy the adsorption sites and lead to the gradual increase of the steric hindrance effect so that the adsorption reached a dynamic equilibrium process.Meanwhile, all the fitted curves deviated from the origin, indicating that the adsorption process was not only determined by intraparticle diffusion but might also be controlled by steps such as surface diffusion and chemisorption [38].
Adsorption isotherms were often used to understand the adsorption capacity of the adsorbent to the adsorbate.Therefore, two isothermal models, Langmuir and Freundlich, were used to investigate the interaction between As(III, V) ions and the PMGA/H 2 O 2 system and assess the adsorption capacity of PMGA.As shown in Fig. 3g-i and Table S6, the Langmuir model showed the best matching degree and the highest correlation coefficient (R 2 ≥ 0.981), which indicated that the adsorption of As(III, V) by PMGA followed the monolayer adsorption mechanism [39].Furthermore, the maximum adsorption capacity for As(III) of the PMGA/H 2 O 2 system obtained based on the Langmuir model was higher than that of PMGA alone (14.172 vs 24.863 mg/g).Additionally, the adsorption capacity and removal speed were superior to most previously reported similar bio-based or iron-based materials (Table S7).Meanwhile, the equilibrium parameter R L (eq.S7) calculated from the constant K L obtained from the Langmuir model was less than 1, implying that the whole sorption process was spontaneous and favorable [40].Therefore, the anchoring of MIL-88A-Fe on gelatine to form PMGA and coupling with H 2 O 2 was a promising system for As(III) removal from water.

Mechanism study 3.3.1. Adsorption mechanism
The surface chemical information of PMGA before and after the reaction in the PMGA/H 2 O 2 system with As(III, V) was determined using XPS.As shown in Fig. 4a, compared with fresh PMGA, PMGA, after adsorbing As(III, V), had a new characteristic peak belonging to As 3d at about 45.0 eV.According to the As 3d peak binding energy range of As (III, V) in arsenic oxides [As(III) 44.3-44.5 eV, As(V) 45.2-45.6 eV], we could see that the As(III) adsorbed on the PMGA/H 2 O 2 system were all in the form of As(V) [41].Conversely, no significant As(V) conversion occurred upon As(III) adsorption by PMGA alone.The results showed that As(III) oxidation in the PMGA/H 2 O 2 system occurred.Furthermore, in the FTIR spectrum of As(III, V) adsorbed by the PMGA/H 2 O 2 system, a new peak belonging to the Fe-O-As(V) bond was observed at 806 cm − 1 (Fig. S6) [42].A new peak belonging to the Fe-O-As(III) bond located at 796 cm − 1 was observed for the removal of As(III) by PMGA alone [43].Simultaneously, a slight blue shift of the Fe-O bond peak at 625 cm − 1 indicated the coordination between Fe elements in PMGA and As(III, V) species [9].Thus, the appearance of new peaks and the shift of the Fe-O bond indicated that chemisorption was the main reason for the removal of As(III, V) by PMGA, and the complete oxidation of As(III) occurred in the PMGA/H 2 O 2 system.On the other hand, the Fe 2p spectrum of fresh PMGA was deconvoluted into peaks located at 710.4 and 724.1 eV, corresponding to Fe(II) in Fe 2p3/2 and Fe 2p1/2, as well as peaks located at 713.5 and 729.5 eV, corresponding to Fe(III) in Fe 2p3/2 and Fe 2p1/2 (Fig. 4b) [44].The satellite peak located at 719.0 eV was characteristic of the presence of Fe(III) in the PMGA structure.After the reaction with As(III, V), the characteristic peaks in Fe 2p all shifted to the direction of high binding energy, indicating that the relevant chemical interactions with Fe-O-As bond formation indeed led to As(III, V) adsorption [42].It is noteworthy that the relative intensity of Fe(II) in PMGA increased by 5.3 % after reacting with As(III) and H 2 O 2 , indicating partial reduction of Fe(III) during the Fenton-like reaction process (Table S8).Thus, the Fe component in PMGA served as adsorption sites and might also participate in the oxidation of As(III).
To further elucidate the interaction between As(III, V) and PMGA, the O 1 s spectra of PMGA before and after the reaction were analyzed.As shown in Fig. 4c, the binding energy of the O 1 s spectrum exhibited three peaks, corresponding to the oxygen atoms on the Fe(As)-O bonds (530.6 eV), the carboxyl oxygen component in the gelatin and ligands (531.3 eV), and the surface hydroxyl groups (532.6 eV) [8,45].It was well known that when the metal oxide adsorbent was saturated to adsorb arsenic, the stoichiometric ratio of -OH on its surface to that before the reaction had the following characteristics: 2:1 for the formation of monodentate complexes or 1:2 for bidentate complexes [46].Therefore, when PMGA formed bidentate or monodentate complexes after adsorbing As(III, V), its surface hydroxyl groups should be reduced or increased, respectively.The fitted data showed that the -OH/Fe(As)-O ratio decreased after the reaction of PMGA with As(III, V) regardless of the presence or absence of H 2 O 2 (Table S9).In summary, the adsorption mechanism of As(III, V) on PMGA might be due to the strong coordination between unsaturated iron sites and As(III, V) in MIL-88A-Fe through bidentate complexation.

Oxidation mechanism
Fig. S7 showed that H 2 O 2 alone had a negligible contribution to the oxidation of As(III) under optimal pH reaction conditions.However, the XPS and FTIR analysis results indicated that it was mainly As(V) formed in the PMGA/H 2 O 2 system.Therefore, it could be inferred that other active species might participate in oxidizing As(III) in the PMGA/H 2 O 2 system.To verify the existence of this species, DMPO was used as a spin trapping agent, and the type of active species was determined by EPR.As shown in Fig. 5a, the characteristic signals of DMPO-• OH adducts were clearly detected in the PMGA/H 2 O 2 system.Meanwhile, when As(III) was added to the reaction system, the characteristic signal of DMPO-• OH weakened sharply.The results showed that • OH was produced in the PMGA/H 2 O 2 system and participated in As(III) oxidation.To further analyze the catalytic oxidation mechanism of PMGA, coumarin was added as a probe molecule, and the fluorescence intensity of its reaction with • OH to form 7-hydroxycoumarin was monitored at 445 nm [47].As shown in Fig. 5b, as the reaction time increased, the fluorescence emission intensity of 7-hydroxycoumarin in the system gradually increased.On the contrary, when no PMGA was added, the fluorescence of 7-hydroxycoumarin could hardly be detected in the reaction system (Fig. 5c).These results suggested that PMGA was the main cause of • OH generation in the system and • OH would be continuously generated during the reaction process.
The presence of unsaturated iron sites between Fe 3 -μ 3 -oxo clusters in MIL-88A-Fe had been reported to give it the ability to interact with H 2 O 2 [15,48].Thus, we guessed that ≡Fe(III) on PMGA was reduced to ≡Fe (II) via Eq.(1) (≡ represents surface-bound species).Then, ≡Fe(II) further reacted with H 2 O 2 to generate • OH, leading to the oxidation of As(III) (Eq.(2).To confirm the above conjecture, 1,10-phenanthroline was introduced into the reaction system as a chelating agent for ≡Fe (II) to prevent its reaction with H 2 O 2 [49].The fluorescence intensity of 7-hydroxycoumarin was found to be greatly suppressed (Fig. 5d).This phenomenon provided strong evidence that ≡Fe(II) was indeed produced in the system and led to the generation of • OH.Notably, the addition of formic acid (FA), tert-butanol (TBA), or methanol (MeOH) as quenchers of • OH did not significantly affect the removal of As(III) by the PMGA/H 2 O 2 system (Fig. 5e) [50].This indicated that As(III) oxidation occurred through the PMGA surface-bound • OH, which further proved the establishment of Eqs. ( 1) and ( 2 hand, H 2 O 2 alone excited Fenton-like directly via Eq.( 1), and the generated surface hydroperoxyl radical ( • OOH) might also be an oxidative species of As(III) [47].Thus, quenching experiments were performed using p-benzoquinone (BQ) as a quencher for • OOH [52].As shown in Fig. 5f, adding BQ significantly inhibited As(III) removal in the PMGA/H 2 O 2 system.Note that BQ did not inhibit As(V) removal by PMGA (Fig. S8).In summary, the primary oxide species of As(III) in the PMGA/H 2 O 2 system were surface • OH and surface • OOH.

DFT calculation
Based on XRD, MIL-88A-Fe mainly consists of two distinct crystal facets, (1 0 0) and (1 0 1), which were regarded as the primary crystal facets of H 2 O 2 interacting with PMGA.Therefore, the contribution of these two facets to the H 2 O 2 activation ability was compared using DFT calculations, and the H 2 O 2 dissociation process was explored.As shown in Fig. 6a, H 6b  and c).When H 2 O 2 coordinates with Fe sites, the d-p conjugation formed between the p orbitals of H 2 O 2 and the d orbitals of Fe atoms contributes to electron delocalization, increasing the electron cloud density and promoting the coordination reaction.According to the calculation, it could be known that the amounts of electrons transferred from the (1 0 0) and (1 0 1) facets to the H 2 O 2 were 0.19 e and 0.10 e, respectively.This meant that H 2 O 2 molecules would preferentially adsorb on the (1 0 0) facet compared to the (1 0 1) facet.Thus, the (1 0 0) facet of MIL-88A-Fe in PMGA was decisive in realizing the efficient Fenton-like catalytic performance in the reaction system.On the other hand, the adsorption energy of arsenate on (1 0 0) and (1 0 1) facets were evaluated (Fig. 6d and e).The results showed that the adsorption energies on both crystal facets were negative, implying that the As(V) ions formed by oxidation were readily adsorbed on the PMGA surface.Additionally, the adsorption energy on the (1 0 0) facet was more negative (E ads : − 2.13 eV vs − 1.54 eV), indicating that the existence of the (1 0 0) facet was beneficial to the removal of arsenic ions.
In summary, the removal mechanism of As(III) in the PMGA/H 2 O 2 system might involve two processes.That is, the unsaturated iron site ((1 0 0) facet) of MIL-88A-Fe in PMGA triggered a Fenton-like reaction in the presence of H 2 O 2 to produce surface • OH and surface • OOH to oxidize As(III).Subsequently, MIL-88A-Fe adsorbed the As(V) formed by oxidation in the form of a bidentate complex.Among them, the prepared PMGA, with its large porosity, pore size, and hierarchical porous structure, facilitated the rapid adsorption of As(III) on the material surface.

Effect of the solution chemistry
Since the solution pH determines the surface charge properties of PMGA and the form of As(III), its effect on As(III) removal by PMGA/ H 2 O 2 system was investigated.As shown in Fig. 7a, the removal ability of PMGA for As(III) was significantly enhanced with the increase of reaction pH, and reached the maximum at pH = 9.5.According to the pK a value of As(III) (pK a = 9.22), it can be seen that As(III) mainly existed in the form of non-ionic H 3 AsO 3 at pH = 4.0-9.5 (Fig. S9).Therefore, the removal of As(III) by the reaction system was achieved through a surface complexation mechanism (chemisorption) rather than electrostatic mutual attraction (physisorption).In other words, when PMGA formed Fe-O-As bonds with As(III), a higher pH value favoured the release of protons initially bound to PMGA surface oxygen, thereby enhancing the removal of As(III) [5].However, when the pH value increased to 12, As(III) mainly existed in H 2 AsO 3 -, and the isoelectric point of PMGA was about 5.1 (Fig. S10).This led to a gradually enhanced electrostatic repulsion between the negatively charged As(III) and PMGA, which reduced the adsorption capacity of PMGA.Furthermore, the leaching values of Fe ions from PMGA under different pH conditions were almost significantly lower than the maximum allowable contamination level in water, implying its good stability (Fig. S11).Notably, the removal of As(III) by the PMGA/H 2 O 2 system remained largely unaffected by variations in ionic strength (Fig. 7b).The results indicated that the adsorption process of As(III) on PMGA proceeded through a chemisorption mechanism through the formation of complexes on the inner sphere surface.In contrast, the adsorption process by constructing outer-sphere surface complexes was susceptible to changes in ionic strength [12].
Considering the complexity of natural water environments, we also investigated the effects of some common anions and humic acids (HA) on As(III) removal by the PMGA/H 2 O 2 system.As shown in Fig. 7c, apart from the slight inhibitory effect of SiO 3 2-and the significant inhibitory effect of HPO 4

2-
, the impact of other anions on As(III) could be considered negligible.This negative effect was mainly due to their similar chemical properties and atomic structure to arsenite [53].Interestingly, the removal of As(III) was not significantly inhibited by HA (Fig. 7d).This favorable property might be related to the three-dimensional porous structure of PMGA and its negatively charged surface under optimal pH conditions [54].Because they might cause HA with larger size to not diffuse rapidly into the narrow pores of PMGA and occupy the adsorption sites, PMGA was expected to reduce the interference of ubiquitous natural organic matter when remediating As(III) in the actual water environment.

Practical application and regeneration
To understand whether the PMGA/H 2 O 2 system could remediate arsenic contamination in natural waters, we tested its effectiveness in removing arsenic contamination in three water bodies with different As (III) concentrations added.The As(III) removal efficiencies in the three water bodies were found to be almost unchanged from the DI water control, and the residual arsenic concentrations were all lower than the drinking water safety threshold (10 µg/L) (Fig. 8a).Meanwhile, to further investigate the potential of the PMGA/H 2 O 2 system in practical applications, a fixed bed column experiment for the removal of As(III) from groundwater was carried out.As shown in Fig. 8b, the PMGApacked column processed approximately 227 BV (5.4 L) of effluent before the breakthrough point (>10 µg/L) occurred.Correspondingly, the As(III) adsorption density of PMGA at the breakthrough point was about 0.58 mg/g, which was superior to a previously reported commercial product HFO@D201 (0.15 mg/g) that can be used for arsenic removal [54].In addition, the concentration of iron ions in the effluent monitored throughout the continuous flow filtration was lower than 0.05 mg/L, which indicated the good structural stability of PMGA (Fig. S12).Thus, these above results imply that the proposed PMGA/ H 2 O 2 system was a promising method for remediating As(III) in water.On the other hand, the reusability of PMGA was investigated using 0.2 M NaOH as the eluent.As shown in Fig. S13, the adsorption efficiency of As (III) on PMGA decreased to 58.2 % after 5 cycles.This might be related to the fact that MIL-88A-Fe in PMGA undergoes disintegration of the organic linker and incomplete desorption of the active adsorption sites in alkaline solution.Therefore, further exploration for developing ironbased MOF aerogels with good renewability and efficient As(III) removal ability was still needed.

Conclusions
In summary, this study successfully synthesized three-dimensional porous MIL-88A-Fe/gelatin aerogel (PMGA) by low-temperature carbonization and foaming techniques using a simple direct mixing method.For the first time, it was coupled with H 2 O 2 to construct a heterogeneous catalytic oxidation system for As(III) oxidation and adsorption in water.Especially, utilizing biodegradable gelatin as the carrier for MIL-88A-Fe to form aerogel endowed PMGA with recyclable and stable properties and presented a hierarchical porous structure rich in easily accessible adsorption sites after foaming treatment.The surface • OOH and surface • OH generated by the Fenton-like interaction between the (1 0 0) facet exposed by MIL-88A-Fe in PMGA and H 2 O 2 were the main active oxide species of As(III).Furthermore, the PMGA/H 2 O 2 system had a much higher adsorption capacity for As(III) than most similar bio-based or iron-based materials and exhibited a faster As(III) removal rate.Meanwhile, PMGA could effectively remove arsenic pollution in different natural waters and be applied in fixed bed columns to treat arsenic-contaminated groundwater.More importantly, the system could work effectively under different pH, coexisting ions, and natural organic matter.Therefore, the proposed PMGA/H 2 O 2 system might provide a promising strategy for extending aerogels to eliminate arsenic species in water purification.G. Cai et al.

Fig. 6 .
Fig. 6.(a) Energy diagram of H 2 O 2 dissociation process on (1 0 0) and (1 0 1) facets in PMGA.(b, c) The charge density difference of adsorbed H 2 O 2 on the (1 0 0) and (1 0 1) facets of PMGA and the corresponding charge transfer (yellow and blue represent charge accumulation and depletion, respectively).(d, e) Adsorption energy of arsenate on PMGA (1 0 0) and (1 0 1) facets.(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2 O 2 molecules only needed to overcome the energy barrier of 0.80 eV when forming • OH through the cleavage of O-O bond on the (1 0 0) facet.It was lower than the 0.91 eV energy barrier when O-H bond cleavage produced • OOH.Note that both reaction pathways were exothermic.In contrast, H 2 O 2 molecules needed to overcome a higher energy barrier (1.27 eV) to form • OH through O-O bond cleavage on the (1 0 1) facet.Thus, the (1 0 0) facet of MIL-88A-Fe in PMGA was mainly responsible for forming surface • OH and surface • OOH in the Fenton-like reaction.Additionally, the difference in charge density of H 2 O 2 adsorbed on (1 0 0) and (1 0 1) facets in PMGA were compared to investigate the influence of the facets on the corresponding charge transfer (Fig.