Enhanced removal of aqueous Cr(VI) by the in situ iron loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two step method: Mechanism study

In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO4/K2FeO4 impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K2Cr2O7 on the surface of FeOOH was − 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K2Cr2O7 on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr2O72− would electrostatically attract to the positively charged AFPAC surface. (2) Cr2O72− was reduced into Cr2O3 by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO4 and K2FeO4. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.


Introduction
Chromium is one of the most common heavy metals that overloads emissions from electroplating, automobile manufacturing, tanning, mining, and other industries, and exists mainly in two common oxidation states, trivalent Cr(III) and hexavalent Cr(VI). It is noteworthy that Cr(VI) is 300 times higher toxicity than that of Cr(III), which should be ascribed to Cr(VI) has the characteristics of high solubility, biological toxicity, poor degradability, and high carcinogenicity and was easy to be enriched through the food chain ( Li et al. 2020a, b;Tadjenant et al. 2020). Therefore, removing Cr(VI) from aquatic environments is essential for the protection of the environment and public health and devising available technologies for Cr-containing wastewater is currently of great urgency and high priority in many countries (Aljerf 2018).
The commonly used methods for the removal of Cr(VI) from aqueous environments included chemical reduction-precipitation, coagulation/precipitation, photocatalysis, biological, electrochemical, membrane filtration, and adsorption (Li et al. 2018;Roy Choudhury et al. 2018;Yu et al. 2018Yu et al. , 2021Ma et al. 2021;Sun et al. 2021). Among them, the adsorption method had been recognized as a promising technique because of its simplicity of design and operation, high efficiency, low cost, and possible recovery (Taleb et al. 2015). The common adsorbents for Cr(VI) removal include clay minerals, organic polymers, and especially the carbon-based adsorbent materials, which was due to their well-developed porosity and highspecific surface area possessing good adsorption ability and controllable morphology (Liang et al. 2019). Thus, the preparation of a high-capacity Cr(VI) carbon adsorbent is still a major challenge.
Number of modification methods of activated carbon for enhancing Cr(VI) removal have been developed, including alkali (Wang et al. 2020), gas (Sun et al. 2020), ultraviolet (Peng et al. 2018), and iron-loaded (Kang et al. 2022), which were purposed to improve the pore structure and optimize surface properties. Particularly, the impregnating iron (hydr) oxides modification activated carbon method had attracted more attention, which due to the iron (hydr)oxides high affinity and selectivity toward Cr(VI) (Maziarz et al. 2019). Zhang et al. had found that Cr(VI) removal by nano-zerovalent iron-modified oak biochar reached 99.9% at pH 2.0, which was due to the good adsorption of Cr(VI) by various iron oxides formed by zero-valent iron corrosion reaction. Ding et al. (2021) had claimed that metal salt-modified biochar material could enhance Cr(VI) removal because of the modification improved the pores of biochar and the intermediate oxidation products provided effective active sites. However, the iron oxides, especially the hydrated iron oxides with good adsorption performance, had very fine particle size and even nanometer particle size with the disadvantages of low mechanical strength, narrow applicable pH range, and slow adsorption kinetics, which limit their wider industrialscale application. Therefore, the green and safe application of activated carbon and ferric hydroxides adsorption performance, and overcoming the shortcomings of ferric hydroxides, had become a hot research topic of iron-modified activated carbon enhanced adsorption for Cr(VI) removal (Deliyanni et al. 2009). At present, many researchers have proposed methods of iron salt impregnation or combined green oxidant oxidation to enhance the adsorption performance of activated carbon, such as ferrous salt impregnation Fe(II), trivalent iron salt impregnation Fe(III), (Xu et al. 2013) and Fe(II)/H 2 O 2 oxidation (Xu et al. 2016), which effectively improved the removal effect of heavy metals in water by modified activated carbon, especially for Cr(VI) removal at pH 2 (Kaur et al. 2021;Tu et al. 2021).
These studies provided a theoretical basis and technical reference for the study of activated carbon-modified supported iron oxides by the combination of iron salt impregnation and oxidation, and also gave us some special inspirations on how to load a large number of in situ generated iron oxides on the basis of ensuring that the pores and surface structures of activated carbon were fully abundant, so as to further enhanced the decontamination performance of the modified activated carbon. Potassium ferrate (K 2 FeO 4 ) had various functions, such as oxidation, coagulation, and adsorption, which made it attract much attention as an emerging environment-friendly water treatment agent, and the iron (hydrogen) oxide particles produced by in situ hydrolysis showed great ability in removing chromium, selenite, heavy metals, and total organic compounds (Yang et al. 2021;Wan et al. 2022). In addition, researchers had found that the low-cost Fe(II) and Fe(III) could significantly improve the degradation rate and removal efficiency of diclofenac by K 2 FeO 4 , which should be ascribed to Fe(II) and Fe(III) effectively activated K 2 FeO 4 to generate in situ strong oxidizing intermediate valence iron and the final products provided a large number of adsorption sites ). However, little information is available in the literature on using impregnating iron (hydr)oxides combined K 2 FeO 4 modified activated carbons for Cr(VI) removal, especially the appropriate modification method of in situ generated iron oxides with good adsorption properties immobilized on the porous material activated carbon.
Therefore, this study proposed for the first time the appropriate combination modification of activated carbon by FeSO 4 / K 2 FeO 4 . On the one hand, the oxidizing property of K 2 FeO 4 was used to oxidize the activated carbon and optimize the pore structure of the activated carbon. On the other hand, the in situ generated iron oxides through redox reaction between FeSO 4 and K 2 FeO 4 possessed good adsorption performance for heavy metal removal. The main objectives of this paper were as follows: (1) to prepare and characterize the in situ iron-loaded activated carbon by FeSO 4 /K 2 FeO 4 and optimize the preparation parameters; (2) to investigate the adsorption performance of the in situ iron-loaded activated carbon for Cr(VI) removal under different water quality parameters, particularly the kinetic equation, isotherm equation, and the factors controlling adsorption by the particle diffusion model; (3) to suggest the mechanisms on Cr(VI) removal by the in situ iron-loaded activated carbon prepared. The present study provided a new insight into the application of in situ iron-loaded activated carbon by FeSO 4 /K 2 FeO 4 in heavy metal wastewater treatment.

Chemicals and reagents
All chemicals were reagent grade and used without any purification. The activated carbon was the coal-activated carbon powder and purchased from Sigma if it was not otherwise specified (average particle size was 50-200 mesh, iodine value was 1000 mg/g, methylene blue adsorption capacity was 120 mg/L, particle size more than 200 mesh accounted for 90%, moisture was 10%, ash was 9%). Potassium dichromate (K 2 Cr 2 O 7 ), ferrous sulfate (FeSO 4 ·7H 2 O), nitric acid (HNO 3 ), sodium hydroxide (NaOH), and ferric chloride (FeCl 3 ) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. Potassium ferrate (K 2 FeO 4 ) was prepared in the laboratory according to a wet method (Li et al. 2005). Briefly, calcium hypochlorite and potassium carbonate were used to produce potassium hypochlorite, and then Fe(VI) was produced by reacting potassium hypochlorite and iron nitrate under alkaline conditions with a purity higher than 95% by ABTS detection method (Dan et al. 2021).

Preparation of in situ iron-loaded activated carbon
Pretreatment of the activated carbon Appropriate amount of powdered activated carbon was taken and washed several times, dried it in a vacuum at 60 °C, and grinded to obtain activated carbon powder PAC for later use. Then, the PAC was immersed in 0.01 mol/L dilute nitric acid or 0.01 mol/L sodium hydroxide for 24 h for pretreatment under the condition of uniform magnetic stirring. After solid-liquid separation, the solid obtained was washed with ultrapure water to keep the pH of the supernatant unchanged, dried it in a vacuum at 60 °C, and grinded to obtain the activated carbon powder APAC or BPAC, respectively.
The in situ iron-loaded activated carbon was prepared by constant temperature infiltration method The appropriate amount of activated carbon powder APAC and a certain amount of FeSO 4 ·7H 2 O were placed in a conical flask containing 100-mL ultrapure water, and adjusted the pH to 4, oscillated at 150 r/min for 12 h in 25 °C water bath. Then slowly added 1 mL K 2 FeO 4 solution with a concentration of 0.10 mol/L into the conical flask and continued to shake in a water bath for 12 h under the above conditions. The conical bottle was taken out after the water bath shaken, and the obtained solid through solid-liquid separation was washed repeatedly with ultrapure water, then matured for 10 h within 90 ℃. After the end, the in situ iron-loaded activated carbon powder AFPAC was obtained after grinding. The specific preparation flow chart is shown in Fig. S1.

Experimental methods and procedures
Static adsorption experiment The appropriate amount of AFPAC was accurately weighed and added into a conical flask with a certain concentration of Cr(VI) solution. The flask was placed in a 150 r/min constant temperature gas bath oscillator for constant temperature oscillation for 24 h at a certain temperature. The supernatant was filtered by a 0.45 μm filter membrane, and the concentration of Cr(VI) in the filtrate was measured.
Study on Cr(VI) removal adsorption performance by in situ iron-loaded activated carbon The influence of the dosage of AFPAC (0.25 ~ 2.5 g/L), initial pH of the solution (2 ~ 7), reaction temperature (25 °C, 35 °C, 45 °C), and initial concentration of Cr(VI) (10 ~ 60 mg/L) on Cr(VI) removal by AFPAC was investigated. The adsorption kinetics and isothermal adsorption process were studied to investigate the adsorption rate and capacity of AFPAC for Cr(VI) removal. In brief, in the process of the adsorption kinetic experiment, 500-mL Cr(VI) solution with an initial concentration of 50 mg/L was added into a 1000-mL conical flask. The pH of the solution was adjusted to 4, and the dosage of AFPAC was 1 g/L. The sample was placed in a water bath oscillator (25 °C, 150 rpm) for 24 h. The concentration of Cr(VI) in the solution was determined at 5, 10,20,40,60,120,240,360,480,600,720,840,960,1080,1200,1320, and 1440 min. While for the isothermal adsorption experiment, 100 mL of Cr(VI) solutions with different concentrations of 10, 20, 30, 40, 50, and 60 mg/L were added into a 250-mL conical flask respectively, adjust the pH to 4 and added 1 g/L AFPAC. Then, the samples were placed in a water bath shaker (temperatures were set at 25 °C, 35 °C, and 45 °C, 150 rpm) for 18 h. After the end, the supernatant was filtered for the determination of Cr(VI).

Adsorption-desorption cycle experiment
In the adsorption-desorption cycle experiment, 0.1 g AFPAC was added into a 250-mL conical flask, and then Cr(VI) solution with an initial concentration of about 20 mg/L was added and placed in a water bath oscillator (25 °C, 150 rpm) for oscillation. After the oscillation, deionized water was used to clean the adsorbed AFPAC for three times. And drying at 60 °C; then, 100 mL desorption solution was added to the conical flask and placed in an air bath oscillator (25 °C, 150 rpm) to shake for 12 h. After that, the desorption AFPAC was washed three times with deionized water and dried at 60 °C. This was a cycle. Three cycles of experiments were repeated to determine the concentration of Cr in the solution before and after adsorption and desorption in each cycle.

Characterization and analytical methods
After each test, the supernatant was sampled, filtrated immediately through a 0.45 μm membrane filter, and the total of Cr and Fe concentrations were measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Perkin Elmer Inc.). The in situ iron-loaded activated carbon AFPAC before and after adsorption was collected and dried, and the surface morphology was observed using a scanning electron microscope (SEM-EDX, JSM-6490LV, JEOL, Japan). X-ray diffraction (XRD, D8ADVANCE, BRUCKNER, Germany) was used to understand the crystal structure and composition of AFPAC. To evaluate the coordination of complexes, a Fourier transform infrared spectroscope (FTIR, Nicolet6700, Nico-let, USA) was applied to determine the distribution of functional groups on the AFPAC. X-photoelectron spectroscopy (XPS, Perkin Elmer, USA) was used to analyze the elemental composition and changes of in situ iron-loaded activated carbons before and after adsorption. The pH of the liquid samples was measured with a pH meter (PSH-3D, China). All experiments were performed in triplicate. Figure 1a and b shows the SEM-EDS results of PAC and AFPAC, respectively. It could be seen from the SEM morphology diagram that the PAC surface was smooth and clean, while the AFPAC surface was rough and accompanied by an irregular pore structure, these rough aggregates might have been formed by the deposition of iron oxides. The formation of these pore structures was due to the acidification of HNO 3 and the oxidation of Fe(VI). The rod-like and blocky aggregates on these surfaces could effectively expand the contact area of AFPAC in water, and these pore structures could also effectively improve the adsorption performance of AFPAC. By comparing the EDS diagram in Fig. 1a, b and Table S1, we could see that Fe(II)/Fe(VI)-modified AFPAC surface had more Fe and O elements, which also proved that Fe(II)/Fe(VI)-modified process was loaded with Fe. Figure 1c mainly shows the information on various functional groups on PAC and AFPAC surfaces. The broadband near 3440 cm −1 was related to the O-H stretching vibration of phenol, carboxyl, alcohol, and chemisorbed water (Nadra and Aljerf 2019). The broadband near 1590 cm −1 was assigned to the stretching vibrations of C = C and C = O bonds in aromatic carbon rings, respectively (Nasseh et al. 2021). The broad band at 1080 cm −1 corresponded to the asymmetric stretching vibration mode of Si-O-Si bonds of the quartz crystalline phase (Moghadam 2016), which was probably affected by the presence of stretching vibration mode of alcoholic C-O groups (Almeida 2015;Michael Chika Egwunyenga 2021). It can be seen from the results that the FTIR spectra of APAC and AFPAC before and after reaction with Cr(VI) had changed obviously. AFPAC had a characteristic Fe-O peak at 465 cm −1 (Kaur et al. 2021), which could be speculated that the effective load of the impregnation process iron oxides. Moreover, the O-H peak at 3440 cm −1 on AFPAC was stronger than that of the original activated carbon PAC, which should be ascribed to the redox reaction between FeSO 4 and K 2 FeO 4 promoted the in situ generation of iron hydroxides and loaded on AFPAC, such as hydrated iron oxide (FeOOH).

Optimization of the modification
In order to find the best way for PAC modification, different pretreatment methods and impregnation with various iron salts were applied modification process. Acid pretreatment (APAC) and alkali treatment (BPAC) were first compared to provide a higher quality activated carbon carrier that contained more oxygen functional groups and a better pore environment (Wan et al. 2022). And then, different iron salt impregnation modification on these activated carbon carriers to get rich species of iron oxides that would provide different roles for Cr(VI) removal. The Cr(VI) adsorption capability of the modified PAC at each modification process was shown in Fig. 2. Obviously, Fig. 2 reveals that Cr(VI) removal by the modified PACs was much better than that of fresh PAC and Cr(VI) removal efficiency was strongly influenced by the pretreatment method and iron salts impregnation. As illustrated in Fig. 2a, b, Cr(VI) removal efficiency by the fresh PAC was 11.7% with the adsorption capacity of 2.4 mg/g, while the removal of Cr(VI) by APAC and BPAC was 53.0% and 43.9% with the adsorption capacity 10.7 mg/g and 8.7 mg/g, respectively. Moreover, Cr(VI) removal efficiency further increased by the iron salt impregnation modification process, and the removal of Cr(VI) by APAC with iron salt impregnation was higher than that of BPAC with iron salt impregnation under the same conditions. Thus, APAC was selected as a pretreatment carrier for further iron salt impregnation modification. The type of iron salts was an important factor that determines the Cr(VI) adsorption efficiency (Liu et al. 2012  Cr(VI) removal rate of 81.5% with the adsorption capacity 16.6 mg/g under this condition. The AFPAC showed much higher Cr(VI) removal than that modified by FeSO 4 /FeCl 3 and FeCl 3 /K 2 FeO 4 , while FeSO 4 impregnation presented similar modification results of 68.2% Cr(VI) removal with FeSO 4 /FeCl 3 and FeCl 3 /K 2 FeO 4 of 66.4% and 66.8% Cr(VI) removal, respectively, and Cr(VI) removal efficiency by the APAC modified through FeCl 3 an K 2 FeO 4 were 61.6% and 51.5%. Figure 2c reveals that the iron content of the iron-loaded activated carbon was positively correlated with its Cr(VI) removal performance, modified activated carbons with higher ferric content tend to have higher Cr(VI) adsorption capacity. AFPAC processed the highest iron content 3.6% and showed the best Cr(VI)removal. Compared with FeCl 3 , FeSO 4 was better immobilized on APAC, which increased the iron content on APAC and did not release into water easily caused by the stirring of water flow. It should be noted that the iron-loaded activated carbon prepared by FeCl 3 / K 2 FeO 4 had an iron content of 3.2%, which is lower than that of FeSO 4 /FeCl 3 the 3.6%, but FeCl 3 /K 2 FeO 4 modification carbon showed better Cr(VI) removal efficiency because of the rich types of iron oxides produced during the oxidation-reduction process of K 2 FeO 4 that possessed higher adsorption capacity for Cr(VI). It can be speculated from the above results that some enhancement effect occurred during FeSO 4 /K 2 FeO 4 combination modification process, especially the in situ interaction between K 2 FeO 4 and the low valence iron salts to generate amorphous iron oxides under acidic conditions which greatly improve its adsorption capacity (Wan et al. 2022). Moreover, K 2 FeO 4 could optimize the APAC structural properties, and more acidic functional groups introduced to provide more adsorption sites. Therefore, the combination modification method of FeSO 4 /K 2 FeO 4 was selected in this study for further Cr(VI) adsorption experiments.
The influence of various levels of FeSO 4 :K 2 FeO 4 ratio and maturation temperatures on the performances of AFPAC for Cr(VI) removal was investigated, as shown in Fig. 3. Figure 3a, b shows that Cr(VI) removal efficiency was significantly increased under certain AFPAC dosage as the impregnated iron salts FeSO 4 dosage increased from 0.05 to 0.20 g and K 2 FeO 4 increased to 0.80 ml (C Fe(VI) = 0.10 mol/L) when carbon carrier APAC was 1.0 g. Previous studies have shown that the difference in the decontamination effect of iron-loaded activated carbon largely depends on its iron content. The higher the iron content, the greater the contact site and collision chance between the loaded iron (hydrogen) oxides and the target pollutants, and thus, the stronger the removal ability of the target pollutants (Xu et al. 2013). However, Cr(VI) removal efficiency was basically stable or even showed a decreasing trend as the impregnated iron salts FeSO 4 dosage exceeded 0.20 g and the dosage of K 2 FeO 4 higher than 0.80 ml (C Fe(VI) = 0.10 mol/L) when carbon carrier APAC was 1.0 g. This phenomenon might be attributed to the excessive iron salt dosage would lead to the blockage of the pore structure of activated carbon and the overloading of iron was easy to fall off from the surface of activated carbon, which could not give full play to the adsorption capacity of activated carbon. Figure 3c shows the effect of maturation temperature and time on Cr(VI) adsorption efficiency by AFPAC. Generally speaking, the maturation temperature and time were positively correlated with the Cr(VI) removal performance of AFPAC. When the maturation temperature reached 90 °C and the maturation time was 10 h, the AFPAC showed the best Cr(VI) removal efficiency. However, as the maturation time was longer, the high temperature above 90 °C was unfavorable to the Cr(VI) adsorption performance of AFPAC. This might be due to the iron (hydrogen) oxides on AFPAC would be transformed from amorphous to crystalline form. Although it was beneficial to the fixation of iron oxides, the amorphous iron oxides with Cr(VI) adsorption performance would reduce. Therefore, the optimal preparation conditions were that the dosage ratio of FeSO 4 /K 2 FeO 4 was 0.20 g: 0.80 mL(C Fe(VI) = 0.10 mol/L, that was the mFe(II): mFe(VI) = 0.20 g: 0.008 g) when 1 g of in situ iron-loaded activated carbon was prepared, and the maturation temperature and time was 90 °C and 10 h, respectively. Under these conditions, the prepared AFPAC adsorbents were utilized for further Cr(VI) adsorption experiment.

Optimized experimental conditions for Cr(VI) removal
The removal efficiency of Cr(VI) by AFPAC as functions of AFPAC dosage, pH, initial Cr(VI) concentration, and coexisting ions were shown in Fig. 4. As illustrated in Fig. 4a, Cr(VI) removal efficiency increased significantly from 16.5 to 98.2% as AFPAC dosage increased from 0.25 to 1.5 g/L, which should be ascribed to the more unsaturated adsorption sites were provided by the increasing AFPAC. However, Cr(VI) removal efficiency remained basically unchanged and the adsorption capacity decreased sharply with the increase of AFPAC dosage from 1.5 to 2.5 g/L, which should be associated with the adsorption sites cannot be effectively utilized, and the adsorption capacity of Cr(VI) per unit mass of AFPAC decreases with the increase of mass transfer resistance.
pH has a great effect on the adsorption treatment of Cr(VI)-contained wastewater, which was mainly due to the fact that hydrogen ions could affect the morphology of chromium ions in water and the surface properties of adsorbents (Aljerf 2018). As shown in Fig. 4b, AFPAC had a high adsorption capacity for Cr(VI) when the pH value was in the range of 2 to 4, while Cr(VI) removal efficiency experienced a considerable decrease from 81.5 to 25.1% as pH increased from 4 to 7. It was noted that the pH range of Cr(VI) removal was widened by AFPAC, which may be ascribed to different reaction mechanisms. The surface of AFPAC protonated that caused by a large amount of hydrogen ions under acid conditions, which contributed to the adsorption of Cr(VI). AFPAC were positively charged under the action of hydrogen ions and could strongly attract the chromium ions, while as pH increased, the degree of electrical properties of the surface of AFPAC and protonation of these functional groups decreases gradually. Meanwhile, these abundant iron oxides played important role in Cr(VI) removal that were formed on the AFPAC surface under acidic conditions. In addition, the morphology of Cr(VI) depended on pH, when pH > 6; Cr(VI) mainly exists in the form of CrO 4 2− . Under acidic conditions, the two main forms of Cr(VI) were HCr 2 O 7 − and Cr 2 O 7 2− . Secondly, oxygen-containing groups on the PAC surface, such as -OH, would be protonated into -OH 2 + . In addition to these oxygen-containing groups, iron oxides would also be protonated on the surface of the AFPAC. For example, the Fe-OH of FeOOH would become Fe-OH 2 + , and generated electrostatic attraction with HCr 2 O 7 − and Cr 2 O 7 2− , thus deposition of HCr 2 O 7 − and Cr 2 O 7 2− on the surface of AFPAC. As pH increased to 7, the protonation degree of the active groups on the surface of AFPAC was weakened, competing with the OHand Cr 2 O 7 2− in the water, resulting in a decrease in the adsorption rate (Taleb et al. 2015;Kahu et al. 2016;Kazak 2021). Moreover, CrO 4 2− needed to combine with more adsorption site than that of Cr 2 O 7 2− and HCrO 4 − , and Cr(VI) was easy to produce precipitates with Adsorbing capacity(mg/g)  OHin water under alkaline conditions that was more difficult to be adsorbed and removed. Figure 4c shows that the adsorption efficiency of AFPAC for Cr(VI) increased with increasing temperature, which might be due to the increase of temperature promoted the interfacial reaction and increased the mass transfer rate. Meanwhile, it was found that the adsorption capacity of AFPAC for Cr(VI) increased linearly with the increase of Cr(VI) initial concentration from 10 to 40 mg/L at a certain temperature. This was because the active sites on the surface of AFPAC were gradually occupied with the increase of Cr(VI) concentration. When the Cr(VI) concentration was increased to 50 mg/L, the adsorption rate of Cr(VI) by AFPAC at 25 °C began to slow down, and the adsorption amount tended to be saturated, reaching 26.9 mg/g. While at 35 °C and 45 °C, due to higher temperature and faster mass transfer rate, the adsorption efficiency of AFPAC for Cr(VI) was still not saturated. In addition, according to the above three test factors (dosage of AFPAC, solution pH, and experimental temperature), we designed an orthogonal experiment with three factors and five levels, through the response surface curve method, we determined the optimal experimental conditions: Under the condition of 35 °C, when the pH of the solution is about 4, the removal rate of Cr(VI) can reach nearly 99% by adding 1.5 g/L AFPAC, experimental data of response surface curves were shown in Fig. S4. Figure 4d and e shows the influence of coexisting ions on Cr(VI) removal by AFPAC. The cations of Ca 2+ , Mg 2+ , and K + showed a certain promoting effect on Cr(VI) removal by AFPAC with the enhancement trend of Ca 2+ > Mg 2+ > K + , and its promoting effect decreased with the increase of ions concentration. Ca 2+ and Mg 2+ could promote the adsorption, and the promoting effect was enhanced with the increase of concentration, which was due to the co-precipitation of Ca 2+ , Mg 2+ , and Cr(VI) ions. The coexisting anions of Cl -, SO 4 2− and NO 3 − showed little influence on Cr(VI) removal by AFPAC, while PO 4 3− , SiO 3 2− , and CO 3 2− had obvious inhibition effect on Cr(VI) removal by AFPAC with the inhibition trend of PO 4 3− , > SiO 3 2− > CO 3 2− . Studies have shown that ionic strength was an important parameter to determine the specific and non-specific adsorption removal rates of heavy metals by adsorbents (Michael Chika Egwunyenga 2021). The specific adsorption process of heavy metals was basically not affected by the ionic strength of the solution, while the non-specific adsorption of metal ions was generally affected by the change in the ionic strength of the solution (Yanhao Zhang 2020). Therefore, the adsorption of Cr(VI) by AFPAC in this study was a non-specific adsorption behavior. Figure 4f shows the desorption regeneration experiment of AFPAC for Cr(VI) removal. After three desorption regeneration, Cr(VI) removal efficiency by the regeneration AFPAC was 95.63%, 92.47%, and 90.18% with the adsorption capacity was 13.45 mg/g, 14.00 mg/g, and 13.80 mg/g, respectively. That was, the regeneration AFPAC still maintained a high Cr(VI) removal efficiency, indicating that AFPAC had a good regeneration performance and stable Cr(VI) removal performance after multiple cycles.

Adsorption kinetics
To assess the kinetic characteristics of Cr(VI) adsorbed on AFPAC, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model were adopted to analyze the experimental data. The description of the relevant kinetic equations can be expressed in supporting information. The corresponding fitting model plots and related kinetic parameters were shown in Fig. 5a, b and Table S2, respectively. In addition, in order to better understand the kinetic characteristics of the adsorption of Cr(VI) by AFPAC, the effects of external diffusion and intra-particle diffusion on the adsorption rate were compared, which can be expressed as the equations in Eqs. (1) and (2), The parameter F represented the ratio of q t to q e , and the diffusion rate constants for external diffusion and internal diffusion were K 3 and K 4 , respectively. The corresponding fitting model plots and related kinetic parameters were shown in Fig. 5c, d and Table S3, respectively.
The kinetic data were better fitted by the pseudo-secondorder model than the pseudo-first-order model as indicated by higher R 2 values (as shown in Fig. 5a, b and Table S2), which was consistent with many current research reports on the adsorption of heavy metals on activated carbon (Xu et al. 2013;Li et al. 2020a, b;Kaur et al. 2021). It indicated that chemisorption dominated in the surface adsorption process, and the in situ iron-loaded activated carbon AFPAC had a strong binding force with Cr(VI) and was not easy to fall off, consistent with previous studies results had noted that Cr(VI) mainly through the formation of the outer and inner sphere complexations with iron oxyhydroxide onto the ironloaded activated carbon (Rashtbari et al. 2022).
The higher R 2 values of the external diffusion model than that of the intraparticle diffusion model showed that the external diffusion was more suitable to reflect the kinetic characteristics of AFPAC adsorption for Cr(VI). That was, external diffusion had a stronger control effect on the rate of AFPAC adsorption for Cr(VI). To further understand the mechanism of the adsorption process and   (3)) was applied in order to understand how the diffusion of Cr(VI) molecules onto AFPAC took place. The relation between q t and t 0.5 in Eq. (3) qualitatively compares the reaction rates of the three adsorption steps. q t represented the adsorption amount of AFPAC at time t, and K d represented the adsorption rate, the intercept value of the plots, C, characterizes the thickness of the boundary layer. The graph was drawn with q t as ordinate and t 0.5 as abscess, and piecewise linear fitting was performed, as shown in Fig. 5e.
The adsorption process of Cr(VI) by AFPAC can be divided into three stages. The adsorption amount q t and t 0.5 in the first and second stages have a strong linear relationship, and the R 2 values were basically between 0.94 and 0.99. The slope of the first stage (0-20 min) was the largest of the three stages with the highest adsorption rate, and this stage corresponded to surface diffusion, which was mainly controlled by membrane diffusion, influenced by the solution concentration and mass transfer impetus. The second stage (20-840 min) was corresponded to the adsorption process of mass transfer of Cr(VI) to the internal surface of AFPAC, which was controlled by both membrane diffusion and intragranular diffusion. During this adsorption process, the adsorption sites of AFPAC gradually saturate, the concentration of Cr(VI) decreases gradually, and the internal diffusion resistance increases. The third stage (840 min onward) was the final equilibrium stage, where the adsorption sites were saturated, and the adsorption and desorption rates were essentially identical. Our fitting results showed that the regression was linear, but the plot did not pass through the origin (C ≠ 0), which further indicated the diffusion steps were controlled by many factors, such as mass transfer and intraparticle diffusion. The C i values were in order of C 1 < C 2 < C 3 , indicating the lower external diffusion resistance at the first stage of the adsorption process, probably due to the availability of more vacant adsorption sites (Samira Norouzi 2018). Therefore, the adsorption process of Cr(VI) on AFPAC was not a single physical adsorption process but a comprehensive physical-chemical adsorption process dominated by chemical adsorption.

Adsorption isotherms
At present, the Freundlich isotherm and Langmuir isotherm were commonly used to describe the distribution of adsorbate molecules in liquid and solid phases at adsorption equilibrium (Almeida 2015). The Freundlich isotherm describes (3) q t = K d t 0.5 + C multilayer adsorption on a heterogeneous surface, while the Langmuir isotherm assumes that adsorbate molecules form a monolayer on the adsorbent on a homogeneous surface (Moghadam 2016). The description of the formula can be seen in the supporting information. The experimental isotherm data as shown in Fig. S2 and the parameters of the isotherm models were summarized in Table S4. The results showed that the fitted correlation linear coefficients R 2 of the Langmuir model under the three temperature conditions were all 0.99, which were higher than the correlation linear coefficient R 2 (0.98, 0.99, and 0.96) fitted by the Freundlich isotherm adsorption model, indicating that the Langmuir model can better explain the adsorption process of Cr(VI) onto the AFPAC, consistent with the research conclusions of some activated carbon adsorption and removal of heavy metal pollutants (Xu et al. 2013;Li et al. 2020a, b). It could be speculated that the adsorption sites of AFPAC for Cr(VI) were homogeneously distributed with a monolayer coverage of adsorption products. According to the Langmuir model, the maximum adsorption capacity of AFPAC for Cr(VI) in solution was 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C, and 45 °C, respectively. In addition, the Freundlich constant 1/n was between 0.1 and 1, which indicated that there is a strong affinity between AFPAC and Cr(VI) and the adsorption process of Cr(VI) by AFPAC was favorable. When lgC e was low, lgq e increased linearly with the increase of lgC e , but when lgC e was high, the growth rate of lgq e gradually becomes steeper, which should be ascribed to the higher Cr(VI) concentration increased the contact efficiency of AFPAC and Cr(VI), enhancing the interaction force between AFPAC and Cr(VI). When lgC e was further increased, the growth rate of lgq e at 25 °C and 35 °C gradually slowed down, which might be due to the excessive Cr(VI) concentration hindered the interaction between AFPAC and Cr(VI), making the adsorption deviate from the ideal state.

Absorption thermodynamics
In order to further determine the reaction type of iron-loaded activated carbon adsorbent to Cr(VI), the thermodynamic parameters of iron-loaded activated carbon adsorbent to Cr(VI) were calculated, including adsorption standard free energy ΔG 0 , adsorption standard enthalpy change ΔH 0 and adsorption standard entropy change ΔS 0 . The results of the calculation were shown in Table S5.
As it could be seen from Table S5, the positive value of ΔH 0 indicated that the adsorption process of Cr(VI) by AFPAC was an endothermic reaction process, and the increase of temperature in the reaction system was conducive to the adsorption, which was consistent with the experimental results observed in this study. The positive value of ΔS 0 indicated that the adsorption was an entropy-increasing process. With the progress of the reaction, the disorder degree of the solid-liquid interface in the system was enhanced. The negative value of ΔG 0 indicated that this process can proceed spontaneously. Our thermodynamic parameters obtained in this study were consistent with Khaled et al. (Taleb et al. 2015), who had reported that the thermodynamic parameters of the adsorption of As(V) by adsorbents loaded with hydrated iron oxide. They believed that ΔG 0 decreased with the increase of temperature, indicating that the high-temperature condition was conducive to the adsorption. At this time, Cr(VI) anion was more likely to overcome the resistance of the liquid film and contacted with the surface of the adsorption material, and the ion transport rate in the solution, material boundary layer, and pores was higher. The positive value of ΔS 0 was due to the fact that different intermolecular interactions contributed to the increase of adsorbent-solution interface disorder under steady-state conditions. In addition, Smith (Bimbo et al. 2021) studied the difference between the activation energy of physical adsorption and chemisorption and found that the activation energy of physical adsorption was less than 4.184 kJ/mol, while the activation energy of chemisorption was generally between 8.4 and 83.7 kJ/mol. The reaction heat of AFPAC adsorbing Cr(VI) was 15.1 kJ/mol, indicating that the adsorption of Cr(VI) by AFPAC was mainly chemical adsorption and supplemented by physical adsorption.

Characterization of adsorbed materials
The iron oxides on the surface of AFPAC and Cr(VI) undergone an obvious adsorption process, resulting in significant changes in the surface morphology and the interface of AFPAC after the reaction with Cr(VI). SEM was used to characterize the surface morphology of AFPAC and Cr(VI) before and after the reaction, and the results were shown in Fig. 6. As illustrated in Fig. 6a, the surface of AFPAC before adsorption was relatively rough and uneven with obvious rods and block particles, which might be due to the loading of iron oxides. Such morphology structures improved the specific surface area of AFPAC, and increased the possibility of interface contact in water, and more importantly, provided abundant adsorption sites. While after adsorption reaction, as shown in Fig. 6b, the surface of AFPAC presented more clumpy particles with a larger diameter and smaller particle spacing and denser arrangement. These particles were Fig. 6 SEM images of AFPAC before and after Cr(VI) adsorption reaction at magnification of 10 K: a before adsorption b after adsorption embedded and stacked on the surface of AFPAC, and the adsorbed Cr(VI) was tightly fixed on the AFPAC, which was not easy to fall off and leached during the shock process.
The distinct surface functional groups of APAC and AFPAC before and after reaction with Cr(VI) were analyzed by FTIR, as illustrated in Fig. 7a. The broadband near 3440 cm −1 was related to the O-H stretching vibration of phenol, carboxyl, alcohol, and chemisorbed water (Nadra and Aljerf 2019). After the adsorption of Cr(VI), the main absorption peak position of AFPAC was basically unchanged, indicating that the adsorption of Cr(VI) did not significantly change its own structure. While a new peak with weaker intensity appears near 895 cm −1 can be observed, corresponding to the characteristic peak of Cr = O, which might be due to the oxygen-containing group (-COOH) on the surface of the adsorbent providing lone pair electrons to coordinate with the empty orbital of Cr(VI) (Chen et al. 2022). In addition, the intensity of the O-H vibrational peak was weakened after the adsorption of Cr(VI), which should be associated with the formation of Fe-O-Cr complexes between FeOOH and Cr(VI) on AFPAC .
PAC and AFPAC samples were examined by XRD to reveal the difference in the crystalline structure, as illustrated in Fig. 7b. The crystal structure of PAC was significantly different from AFPAC. The diffraction peaks at the 2 theta value of 26.2° and 58.1° in PAC matched well with SiO 2 diffraction peaks, while the SiO 2 characteristic peaks in AFPAC disappeared, which indicated that acid treatment and iron-loaded modification could remove impurities on the surface of the original activated carbon and optimize the structure of the original activated carbon ).
The wide diffraction peak appeared at 26.6° in PAC and AFPAC, which was the spectrum of C, indicating that the main constituent structure was an amorphous structure . It should be noted that at least two characteristic peaks of iron oxides appeared in AFPAC, such as FeO characteristic peaks at 33.9°, 41.8°, and 57.7°, and Fe 2 O 3 characteristic peaks at 35.9°, 43.2°, 62.4°, and 64.1°. In addition, the Fe 2 O 3 specie formations might exist as γ-Fe 2 O 3 , α-Fe 2 O 3 , and FeOOH according to the diffraction peaks JCPDS database 89-5894, 00-39-134, and 00-29-0713, respectively (Kaur et al. 2021). Different iron oxides were in situ formed during the redox reaction between FeSO 4 and K 2 FeO 4 , which provided abundant adsorption sites for the removal of Cr(VI). However, the peak intensity was relatively weak, which was speculated to be due to the fact that iron was mainly loaded on activated carbon in an amorphous form.
In order to fully understand the adsorption mechanisms, the XPS test and element analysis of AFPAC before and after reaction with Cr(VI) were carried out. As shown in Fig. S3, the new Cr2p peak appeared on AFPAC after the adsorption reaction with Cr(VI), which indicated that Cr(VI) was effectively adsorbed on AFPAC. The XPS results showed that there were C1s, O1s, and Fe2p characteristic peaks in both materials (Fig. 8). As illustrated in Fig. 8a, the binding energy of C1s did not basically change before and after AFPAC adsorption, which was carbon skeleton peak in the carbon fiber without spectrum peak splitting phenomenon, indicating that C atom does not coordinate with Cr 2 O 7 2− in this process. Figure 8b shows that the binding energy of O1s of AFPAC after adsorption increased, especially the binding energy of O1s with C = O, indicating  Fig. 7 a FTIR spectra of the AFPAC before and after Cr(VI) adsorption reaction; b XRD pattern of activated carbon before and after modification that the O atoms in C = O mainly participated in the adsorption reaction . The coordination complexation made the charge transfer to other ions, and the charge density decreased and the binding energy increased. As illustrated in Fig. 8c, characteristic peaks of Fe appeared at 709.2 eV and 711.9 eV on AFPAC, which fitted well with Fe(II) (FeO) and Fe(III) (Fe 2 O 3 and FeOOH), respectively (Huang et al. 2019;Li et al. 2022a, b;Li et al. 2022a, b), consistent with the results of XRD in this study. Meanwhile, the binding energy of Fe2p enhanced after the adsorption reaction, and the content weight of FeO in AFPAC decreased from 38.16% before adsorption to 20.23% after adsorption, while that of the content weight of Fe 2 O 3 and FeOOH increased from 36.72 to 52.64% after adsorption, indicating that redox reaction was involved during Cr(VI) adsorption. In addition, Fe(III) mainly existed in the form of FeOOH that played high adsorption performance for Cr(VI) removal (Li et al. 2022a, b). Furthermore, Fig. 8d shows the binding energies at around 576.9 eV and 578.0 eV of Cr2p in AFPAC after adsorption corresponded to the Cr(III) and Cr(VI) with the content of 35.95% and 64.05%, respectively, which manifested that the reduction of Cr(VI) played an important role in this removal process (Tu et al. 2021).  On the one hand, as Cr(VI) presented a very high positive redox potential in an acidic solution, carbons could bind to the oxygen functionalities on activated carbon that played a role as electron donors, resulting in Cr(VI) oxyanion reducing to Cr(III) in the presence of activated carbon at acidic solutions, according to the electron transfer reactions in Eqs.
(4)-(6). On the other hand, the change in the binding energy of Fe2p indicated chemical reactions took place, such as the redox reaction between FeSO 4 and K 2 FeO 4 , formed abundant iron oxides. The redox reaction between Fe(II) with Cr(VI), and the adsorption of iron (hydr)oxides formed stable inner-sphere complexes with Cr(VI), as expressed in Eqs. (7)-(10). These in situ generated iron oxides not only introduced more acidic functional groups on the surface of activated carbon that provided more adsorption sites for Cr and enhanced synergistic effect between iron oxides and activated carbon, but also increased the negative charge of the sorbent surface that acquired more positively charged Cr(III), besides, enhanced Cr removal to form stable innersphere complexes.

Models and computational methods
In order to further clarify the adsorption mechanism of Cr(VI) by AFPAC, we used FeOOH instead of iron oxide clusters and DFT calculations were carried out to investigate in depth the adsorption performance and interaction strength of K 2 Cr 2 O 7 on different surfaces (PAC, FeOOH). When the initial structure was established, the distance between the C atom on graphene and the O atom on the cluster was about 1.5 Å, which was the normal bonding first guess distance. After VASP optimization, iron oxide clusters were removed from the graphene structure, which was caused by the weak van der Waals forces between them. Therefore, the adsorption configuration of FeOOH clusters was mainly considered in the later DFT (4) Figure 9 a and b shows different perspectives of PAC and FeOOH's structural models, respectively. Figure 9c and d shows the combination of K 2 Cr 2 O 7 on the two models respectively. The adsorption energy of K 2 Cr 2 O 7 on the different surfaces could be ranked as FeOOH > PAC, with FeOOH-K 2 Cr 2 O 7 having the highest adsorption energy at a value of − 2.52 eV, proving that FeOOH had a positive effect on improving the adsorption performance of PAC to adsorb K 2 Cr 2 O 7 .
During the adsorption process, the structure of K 2 Cr 2 O 7 was deformed to a certain extent due to the adsorption, Fig. 9 The side, top views of of K 2 Cr 2 O 7 adsorbed on different model surfaces. a PAC, b AFPAC, c PAC-K 2 Cr 2 O 7 , d AFPAC-K 2 Cr 2 O 7 resulting in a certain change in the bond length. As it could be seen from Table S6, the bond lengths of Cr = O and K-O bonds in initial K 2 Cr 2 O 7 were 1.658 Å and 2.594 Å respectively. The structure and bond length of the K 2 Cr 2 O 7 molecule adsorbed on PAC surface did not change much, only the bond length of the K-O bond was extended from 2.594 to 2.631 Å, At the same time, the C = O of the carboxyl group and the C-O bond length of the phenolic hydroxyl group on PAC surface did not change much, extending from 1.223 to 1.231 and from 1.361 to 1.379, respectively, which was possibly due to the weak interaction between K 2 Cr 2 O 7 and phenolic hydroxyl and carboxyl groups on PAC surface. For the FeOOH-PAC system, Cr = O and K-O bond of K 2 Cr 2 O 7 length changes were more pronounced, which was due to the attraction of the Fe-O in FeOOH. The Cr = O bond of K 2 Cr 2 O 7 increased from 1.658 to 1.711 Å, K-O bond from 2.594 to 3.006 Å, and FeOOH's Fe-O bond from 1.845 to 1.920 Å, respectively, confirming that FeOOH had a stronger attraction force on K 2 Cr 2 O 7 than PAC alone. In addition, by comparing the configurations of the original K 2 Cr 2 O 7 and FeOOH-K 2 Cr 2 O 7 , as shown in Fig. S9, it was found that K 2 Cr 2 O 7 breaks the K2-O6 bond and formed a new Fe3-O8-K2 stable structure with Fe3-O8 bond, confirming that FeOOH had a stronger attraction force on K 2 Cr 2 O 7 than PAC alone.
Charge density difference (CDD) can be used to analyze the electron information in the structure before and after adsorption, helping to understand the electron flow direction and the bond formation. Figure 10 presents the CDD plot of the K 2 Cr 2 O 7 adsorbed on different surfaces. The electron information between the K 2 Cr 2 O 7 and the adsorption surfaces was shown in different colors, where the blue areas represent electron loss and the yellow areas mean electron enrichment. A cross section was also made in the region of the adsorbed atoms for each configuration to create a 2D section view. The isosurfaces chosen for K 2 Cr 2 O 7 adsorption on PAC and FeOOH were 0.0005, 0.002 e/Å 3 , respectively. The isosurface chosen for the FeOOH-K 2 Cr 2 O 7 system was much higher than that of PAC-K 2 Cr 2 O 7 , indicating that more charge was transferred in this system, and the electronic interaction of the K 2 Cr 2 O 7 with the FeOOH-PAC surface was more strongly. Figure 10 shows a yellow region between the carboxyl/phenolic hydroxyl group on the PAC surface and the K 2 Cr 2 O 7 , indicating that electrons are enriched and tend to form a bond. The C, O atom on the carboxyl/phenolic hydroxyl group and the K atom on the K 2 Cr 2 O 7 was surrounded by blue, indicating that they both have a loss of electrons. Therefore, the losing electrons from the C, O atom, and K-O bond were transferred between the K 2 Cr 2 O 7 and the PAC and aggregated to form a bond so that they interacted with each other. Meanwhile, the loss of electrons from K-O would weaken the bond, which coincided with the elongation of the K-O bond obtained in the structural optimization. Similarly, Fig. 10 illustrates the charge transfer and bonding of the K 2 Cr 2 O 7 when adsorbed with FeOOH. Blue areas around the O atom of the K 2 Cr 2 O 7 represented the loss of electrons. And there is also some electron loss on the surface of the Fe atom, and the blue area was similar to the area between the Cr = O bonds, which meant they are losing roughly the same amount of electron. At the same Fig. 10 The 2D sectional views of the charge density difference for the adsorption of K 2 Cr 2 O 7 on different surfaces. a PAC-K 2 Cr 2 O 7 (the vertical section plane of the oxygen atom at the binding site), b PAC-K 2 Cr 2 O 7 (the transverse section of the oxygen atom at the binding site), c FeOOH-K 2 Cr 2 O 7 (the slice of chromium, iron, oxygen atoms), d FeOOH-K 2 Cr 2 O 7 (the slice of oxygen atoms). The green area is the section time, there was a large yellow region between the O atoms and Fe atoms, indicating that the electrons lost from the Cr = O bonds and the surfaces of the Fe atoms were enriched in this region and tend to form bonds. Furthermore, the loss of electrons in the Cr = O bonds and Fe-O bonds corresponded to increased bond length, suggesting that FeOOH had a strong adsorption effect on the K 2 Cr 2 O 7 .
In summary, we proposed the adsorption mechanism of the K 2 Cr 2 O 7 on the FeOOH and PAC surface, respectively. The K 2 Cr 2 O 7 adsorbed on the O atom of the carboxyl group and the phenol hydroxyl group through the K-O bond. During the process of PAC adsorption, the charge of the C = O bond on the carboxyl group and the C-O bond on the phenol hydroxyl group and the K atoms on the K 2 Cr 2 O 7 surface would be transferred during the adsorption process. They would aggregate and form a C-O-K type bond during the process of FeOOH-PAC(AFPAC) adsorption, thus increasing the adsorption affinity and improving the adsorption performance. Moreover, K 2 Cr 2 O 7 was attached to the Fe atom of FeOOH by the K-O and Cr = O bonds, and the charges of the K-O bonds and Cr = O bonds of K 2 Cr 2 O 7 were transferred to the Fe atoms on the surface of FeOOH, then they would aggregate and form Cr = O-Fe and K-O-Fe type bonds for more stable adsorption. Thus, FeOOH could improve the adsorption affinity of PAC. Compared with PAC, FeOOH-PAC(AFPAC) had higher adsorption performance for K 2 Cr 2 O 7 (Cr(VI)).

Mechanism analysis of adsorption of Cr(VI) by AFPAC
It was found that AFPAC had multiple effects on Cr(VI) removal and the schematic of Cr(VI) removal by AFPAC was demonstrated in Fig. 11. As shown in Fig. 11a, the in situ iron-loaded modification process of AFPAC, during the constant temperature impregnation of APAC and FeSO 4 in water, a part of Fe(II) was mainly impregnated on APAC in the form of FeO, and a part was freed in the aqueous solution in the form of Fe 2+ . These free Fe 2+ initially redox with the original oxygen-containing active groups on APAC, and these Fe 2+ participating in the reaction accounted for only a small fraction, most of which were oxidized by Fe(VI) added later, and during this process, the hydrolysis of Fe (VI) was promoted to produce FeOOH and Fe 2 O 3 with good adsorption properties. These iron (hydrogen) oxides in the form of Fe(III) also promoted the adsorption and capture ability of Fe 2+ , so that more and more Fe 2+ was adsorbed onto the activated carbon became FeO, and the remainder was further oxidized to Fe 2 O 3 by unsaturated groups on the activated carbon ( -C = C-, -C = O). These abundant iron oxides made the surface of AFPAC rougher than that of APAC, optimized the pore structure of activated carbon, and increased the surface area of activated carbon, so the modified iron-loaded activated carbon had better adsorption efficiency.
As illustrated in Fig. 11b, various adsorption mechanisms of Cr(VI) by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and coprecipitation. First, in acidic condition, some groups on the surface of AFPAC were gradually protonated and positively charged due to the action of H + , for example, -OH would become -OH 2 + due to protonation. The added Cr(VI) would be hydrolyzed and ionized in an acidic environment, that was, it would coexist in two forms of Cr 2 O 7 2− and HCr 2 O 7 − . Both forms of Cr(VI) would electrostatically attract the positively charged surface of AFPAC. At the same time, the mixing of different valence states and species distribution of iron salts not only provided both electrostatic attraction and electron transfer channels, which promoted electron diffusion, redox, and adsorption and deposition of metal elements (Ling and Zhang 2017). Thus, Cr(VI) was attracted to the surface of activated carbon, which provided conditions for the next reaction. Then, FeO on the activated carbon surface undergone a redox reaction with the free state in the form of Cr(VI), which enhanced the further in situ conversion of iron oxides in the form of Fe(II) to Fe(III), and during the adsorption process, the content of Fe(III) form (hydroxide) oxides on the surface of the activated carbon continued to increase, and the adsorption performance also improved. Meanwhile, carbons could bind to the oxygen functionalities on activated carbon that played a role as electron donors for Cr(VI) reducing to Cr(III) at acidic solutions, and it was also worth noting that Cr(VI) was partially reduced to Cr(III) in the process of FeSO 4 and K 2 FeO 4 redox reaction. Finally, part of Cr(VI) and Cr(III) could stable inner-sphere complexes with the iron (hydr)oxides, and part of Cr(III) became Cr(OH) 3 that was adsorbed and then co-precipitation, which removed by more and more Fe(III) oxides (Fe 2 O 3 , FeOOH) on the surface of AFPAC. Thus, the possible mechanism consisted of the three reaction steps: (1) fast protonation and electrostatic adsorption, (2) reduction of Cr(VI) into Cr(III) through at least two ways, and (3) formation of inner-sphere complexes and then co-precipitation.
Key Laboratory of Urban Water Resource and Environment (Harbin Institute of Technology) (No.ES201917), Engineering Research Center of biomembrane water purification and utilization technology (BWPU2020KF08), and National Natural Science Foundation of China (51878001).

Data availability
The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

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Consent to participate I agree to participate in this called Enhanced removal of aqueous Cr(VI) by the in situ iron-loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two-step method: mechanism study. Signature of participants: Yanli Kong, Zhiyan Huang, Hangyu Chu, Yaqian Ma, Jiangya Ma (corresponding author), Yong Nie, Lei Ding, Zhonglin Chen, Jimin Shen.

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The authors declare no competing interests.