Electrospun zein/polyvinyl alcohol nanofabrics incorporated with nanosized hydroxyapatite for e�cient heavy metal ion adsorption

In order to solve the problem of heavy metal treatment in water pollution, the biodegradable �lm based on zein/ polyvinyl alcohol (PVA) with incorporation of nano-hydroxyapatite (nHAP) were fabricated through electrospinning in this study, and the copper ion was taken as the representative of heavy metal ions. The result showed that the �ber morphology, hydrophobicity and thermal performance were in�uenced by the contents of nHAP. The incorporated nHAP nanoparticles were dispersed within the zein/PVA matrix and strong hydrogen bonding interactions were formed between the �ller and matrix. The adsorption capacities of nano�brous �lms for Cu 2+ were greatly increased after the addition of nHAP. When the nHAP content was 20% (based on the weight of the solid content), the adsorption capacity could reach up to 23.86 mg/g, and the adsorption e�ciency was 13.94% higher than that of neat zein/PVA nano�bers. In the system of copper ion adsorption, based on the electrostatic interaction and complexation of zein/PVA, the ion exchange effect, electrostatic interaction and complexation of nHAP signi�cantly increased the adsorption capacity of the whole system. This work suggests the potentials of the electrospun zein/PVA/nHAP nano�brous �lms as desirable ecofriendly materials in metal removal applications.


Introduction
Industrial, agricultural wastewater and domestic sewage discharge has result in great damage to ecological environment, social environment and human health, and water pollution has become a global concern (Yap et al. 2021).Heavy metals in water pollution continue to accumulate and enter the food supply chain through rice, vegetables and other agricultural products (Li et al. 2022).After biomagni cation in the food chain, it can cause teratogenicity and cancer, and ultimately pose a serious threat to human health.
Over the past few decades, great efforts have been made to remove heavy metals from wastewater, such as extraction, redox, adsorption and separation, and biochar repair technology (Kong et al. 2021), etc.Among them, adsorption and separation technology has the characteristics of low cost, easy operation, exibility and simple design (Carolin et al. 2017), which has been recognized as an effective way in wastewater treatment (Luo et al. 2019).While due to the signi cantly higher surface area to volume with larger porosity, reliable ltration and adsorption capabilities, nano bers have been used in water treatment (Saleem et al. 2020).Among the various preparation methods of nano brous lms, electrospinning is considered to be the most advantageous and effective method for its advantages of simple operation, high production e ciency and low cost(des Ligneris et al. 2020; Lyu et al. 2021).
For the increased concern of white pollution caused by synthetic nonbiodegradable polymers, natural polymers including cellulose, polysaccharose and protein, etc. have reached quick development in recent years (Deeksha et al. 2021;He et al. 2022;Oyeoka et al. 2021).Proteins have been widely explored as natural biomaterials on account of the sustainable, biocompatible, renewable ability and adjustable structure (Rani et al. 2021), and zein is one of the most widely applied proteins.Zein is a kind of gliadin extracted from maize endosperm, which is abundant in nature (Shukla and Cheryan 2001).On account of high proportion of non-polar hydrophobic amino acid residues (50%) (Wang and Padua 2010), zein exhibits high hydrophobicity.Zein is easily soluble in ethanol solution and acetic acid solution, and its solubility in ethanol increases with the increase of ethanol concentration (Bisharat et al. 2018).Coupled with exibility, toughness, glossiness (Amjadi et al. 2022), non-toxicity, renewability, biodegradability, high availability, excellent biocompatibility and active groups which have strong interactions with pollutants and can trap pollutants (Yu et  Hydroxyapatite (HAP) is a natural mineralization of calcium apatite with the molecular formula Ca 10 (PO 4 ) 6 (OH) 2 .Due to the superior adsorption capacity, acid-base tunability, ion exchange capacity and thermal stability, HAP is widely used as a non-polar adsorbent in the treatment of polluted air, water and soil.Noteworthy, HAP possesses anionic sites, endowing the nano bers with electrostatic adsorption properties and further improving the trapping ability of metal ion (Deng et al. 2022) The well-mixed solution was injected into a syringe pump with a voltage of 20 kV applied to the spinneret and pumped by a monoinject syringe pump.The spinning solution was spun into nano bers under the action of electrostatic eld, and the paper towel substrate xed on a grounded copper mesh was to collect the nano bers.The distance from the spinneret to collector was 12 cm, and an average ow rate of 1.0 mL•h − 1 was utilized.Then zein/PVA/nHAP nano brous lms of ca.0.12mm thickness were prepared after 10h.

Characterization
The morphology and microstructures of the nano brous lms were observed by scanning electron microscopy (SEM, Quanta FEG-250, FEI Nanoports, USA), operated at 10 kV.The samples were pasted on the sample stage with conductive adhesive, sprayed with gold spraying treatment and then tested under high vacuum.The average ber diameter and the size distribution were determined by measuring sizes in the SEM images and statistically analyzed by image-Pro Plus software.The average diameter was calculated by the Eq. ( 1): Where is the average ber diameter; d i is the diameter of the i th ber; n is the number of bers in the SEM images.
The functional groups and their interactions of samples were analyzed by ourier transform infrared spectroscopy (FTIR, Nicolet iN10MX, USA), scanned in adsorption mode at 4 cm − 1 resolution with the wavenumber of 4000 − 400 cm − 1 .The FTIR spectra were analyzed by OMNIC (version 9.0).
The water contact angle (WCA) of all lms was evaluated by contact shape analyzer OCA 35 (Dataphysics Instruments GmbH, Germany).The samples were dried at 60 ℃ for 1h in an oven, then xed on the platform.The deionized water (2 µL) was dropped and deposited on the lm surface.The static water contact angle and drop shape with the different positions of the lm were recorded by CCD video camera and image analysis, calculated by using Ellipse Fitting method.Each lm was measured at least ve times.
The thermodynamic properties of nano brous lms were analyzed by the differential scanning calorimetry (DSC) instrument (Q2500, TA Instruments, United States) in the N 2 atmosphere.The sample weight was about 5-10 mg.
The copper adsorption performance of the lms was tested by inductively coupled plasma optical emission spectrometer (ICP-OES, PerkinElmer Optima 8000).First, standard Cu 2+ solutions with a concentration of 0, 20, 40, 60, 80 and 100mg/g with different gradients were prepared, and a standard curve of Cu 2+ concentration could be obtained by testing them successively.Then, appropriate amount of the heavy metal solution to be measured before and after adsorption was put into the centrifugal tube.
Each sample reported were averaged over three consecutive times.
50 mg of nano brous lms with different nHAP contents were weighed, immersed in 50 ml of 50 mg/L copper standard solution, and oscillated at 200 rpm/min at 25°C for a period of time.The concentration of the liquid to be tested was used to calculate the adsorption capacity.After the Cu 2+ concentration was measured, the adsorption capacity of the lm could be calculated according to the following equation: The conductivity was tested by DDS-307 conductivity meter (Shanghai Yueping Scienti c Instrument Company).Took an appropriate amount of adsorbed copper solution in the centrifuge tube, immersed the probe in the liquid until the solution passed the probe, waited for the indicator to be stable, and then took the average value after consecutive measurements for three times.

Results And Discussion
Figure 1 illustrated the schematic preparation and design of the multifunctional nano brous lm for water treatment based on zein/PVA incorporated with different contents of nHAP.Zein possesses favorable ber-forming and adsorption ability, which facilitates the formation of nano bers by electrospun from the zein solution.To overcome the extremely poor mechanical properties of zein nano bers and further improve the electrospinning properties of zein, PVA was introduced.As one of the most common materials employed in electrospinning, PVA is featured to be biodegradable, processable, spinnable, and eco-friendly.
Besides, nano-hydroxyapatite with remarkable biocompatibility and biological performance, has high speci c surface area and excellent ability of adsorption (Elango et al. 2016).In this work, we focused on the preparation of composite nano brous materials by electrostatic spinning based on the mixed system of zein and PVA, adding different contents of nHAP to improve the copper ions adsorption performance of nano brous lms and explored the in uence of different factors on the adsorption effect of copper ions.The successful preparation of modi ed nano brous lms showed that these composite nano brous materials had broad application prospects in the eld of water treatment.

Morphology of nano bers
Figure 2a showed the SEM images and diameter distribution of nano brous lms with varying nHAP content.Only representative pictures (0%, 4.7%, 11.1%) were shown in this paper, and the electron micrographs and diameter distribution of all nano brous lms incorporating different levels of nHAP were shown in Figure S1 and S2.It can be obviously observed that without the addition of nHAP, the nano bers of zein/PVA had smooth surface and uniform diameter distribution.With the addition of nHAP, the surface of the bers became rough and the diameter distribution was no longer uniform which from 450 nm turned to 394 nm, 620 nm, 588 nm and 491 nm, respectively.At higher magni cations, it could be observed that nHAP was encapsulated in the nano bers and formed agglomeration.The agglomeration was attributed to the characteristics of small particle size and high activity of nHAP, which made it prefer to cause mutual attraction due to the increase of the interaction force among particles.With the content of nHAP increased, the agglomeration on the nano bers gradually become apparent.Signi cantly, with the agglomeration and nodules of nHAP, the diameter of nano ber continuously increased.As the content of nHAP was 11.1%, the diameter reached up to the maximum.This because the addition of nanoparticles increased the viscosity of the solution, thus enlarging the nano ber diameter.Then the diameter of nano ber decreased with the continuous addition of nHAP.It has been reported the incorporated nHAP could generate more charge on the surface of ejected jet, thus enhancing the charge density and facilitating the increase of the mass ow of the ejected jet from the spinneret tip to the collector, which resulted in the decrease of average diameter (Ni et al. 2019).The addition of nHAP signi cantly changed the morphology and microstructures of the nano bers.

Structure and interaction
The FTIR spectra was employed to analyze the functional groups vibration and intermolecular interactions of the zein/PVA/nHAP nano brous lms.As shown in Fig. 2b, the peak at 3440 cm − 1 of pure PVA lm spectrum could be assigned to O-H stretching vibration.The C-H from alkyls groups could be observed at 2922 cm − 1 .Absorption peaks at 1734 cm − 1 and 1084 cm − 1 were corresponding to carboxyl groups (C = O) and stretching C-O groups, respectively.In the spectrum of pure zein, amide bond at 1651 was assigned to the C = O stretching, and amide II band at 1538 cm − 1 was attributed to the N-H deformation of the.In the spectrum of the zein/PVA nano brous lm, the 1453 cm − 1 band referred to the -CH 2 bending vibration, and the band at 1080 cm − 1 could be assigned to the C-O stretching vibration.The large and broad peak shifted from 3340 to 3340 cm − 1 .Based on this, we could suggest that interactions, such as hydrogen bonds, had established between zein and PVA.With the addition of nHAP, several new bands appeared on the basis of original spectrum.The peaks at 1022 cm − 1 , 602 cm − 1 and 562 cm − 1 were assigned to the characteristic vibrations of PO 4 3− (P-O: 1022 cm − 1 , O-P-O: 602 cm − 1 , 562 cm − 1 ).
Signi cantly, the typical band intensity of PO 4 3− displayed changes with the addition of nHAP.The peaks corresponding to the O-H and N-H vibrations range from 3340 cm − 1 to 3416 cm − 1 , 3596 cm − 1 and 3605 cm − 1 when the nHAP was 4.7%, 11.1% and 15.8% respectively.When 20% nHAP was added, the peak transferred to 3487 cm − 1 .Thus, the peak shifts at 3340 cm − 1 and 1022 cm − 1 could be assigned to the intermolecular hydrogen bonding of nHAP with zein and PVA.
As the Fig. 2d showed, zein and PVA were crosslinked with nHAP by electrostatic attraction, hydrogen bonding and surface complexation.nHAP was enriched with positively charged calcium ions and negatively charged phosphate ions on the crystal surface, providing ample ion binding sites (Chahkandi 2017).Due to the positive charge of calcium ions on nHAP, it could interact with carboxyl and hydroxyl groups on zein and PVA, while the phosphate group on nHAP can interact with amino groups on zein due to its positive charge.Besides, during the adsorption process of copper ions solution, PO 4 3− was considered to be involved through surface complexation between ≡ POH and copper ions on the HAP surface (Wei et al. 2021).The surface complexation also formed from Ca with -OH groups of zein and PVA.Above all effects made nHAP immobilized well inside the nano bers.Thus, during metal absorption, the nHAP particles would not easily be dispersed into the solutions from the nanofabrics.

Surface wettability
The time-dependent water contact angles of nano brous lm with varying nHAP contents were shown in the Fig. 3a.With the extension of time, the water contact angle of all lms decreased in varying degrees.More visual photograph of the water contact angle of nano brous lms with different content of nHAP over time were shown in Figure S3.Obviously, it could be observed that when the water droplets contacted with the nano brous lm, the water droplet could not keep a certain shape all the time.Figure 3b showed the variation of water contact angle of nano brous lms at 10s.With the increase of nHAP, the contact angle rst increased and then decreased, reaching a maximum when the nHAP content was 37.5%, which might be caused by the agglomeration of nHAP.The contact angle between the water droplet and the nano brous lm gradually decreased, and the contact surface became larger until it was completely absorbed by the lm.Comparing the graphs of the water contact angle under different nHAP contents, it was obviously that with the addition of nHAP, the wettability of the material decreased and the hydrophobicity increased.The hydrophobicity of zein/PVA nano brous lm was mainly attributed to the abundant hydrophobic groups in zein and PVA.After adding nHAP, the polar groups were occupied and faced to inside on account of the hydrogen bonding, resulting in the more stable matrix.In addition, water-insoluble inorganic nHAP nanoparticles dispersing on the surface of nano ber, contributing to the increased hydrophobicity (Xu et al. 2013).However, with the extension of time, the nano brous lm gradually became hydrophilic, and the contact angle gradually decreased to 0°, which mainly due to the abundant hydrophilic group -OH in the matrix.

Thermal analysis
Figure 3c and 3d showed the thermodynamic properties of the nano brous lms with different contents of nHAP.Table 1 summarized the detailed thermal data.The nano brous lms showed crystallization peak and melting peak at ca. 146 ℃ and 150 ℃, respectively, indicating the successful cross-linking of PVA and Zein.It could be seen that the melting point (T m ) and crystallization temperature (T c ) of the nano bers exhibited a slight decrease with the addition of nHAP.It showed that the nHAP had good thermal stability which could keep stability and the addition of nHAP rarely changed the melting point and crystallization temperature of neat zein/PVA nano bers.In the melting curve, the nano bers all showed an endothermic peak around 200°C, which was mainly caused by the degradation of PVA.

Adsorption performance
The physical photo of copper ion solution before and after adsorption were depicted in Fig. 4a.As shown in the photo taken by the digital camera, the copper ion solution before adsorption was blue, and after adsorption, the color of the solution became lighter.The color change was because copper ions were adsorbed by the nano brous lm, and the concentration of free copper ions in the solution decreased.
With the extension of time, the nano bers were partially decomposed, copper ions were adsorbed on the bers and precipitated to the bottom of the bottle, and the whole solution reappeared in a clear and transparent color.The composite nano bers were originally white, and after the adsorption of copper ions, copper ions were attached to the lm and forming a layer of blue precipitate.
Figure 4b showed the change of the adsorption capacity of copper ions with time, and they exhibited the similar adsorption trend.The copper adsorption capacity did not have a linear relationship with time.At rst, the adsorption capacity increased rapidly, and then the curve gradually stabilized and reached a plateau after 12 h adsorption.After that, the adsorption amount exhibited small change with the extension of time.It was obviously that for each adsorption curve, with the extension of the adsorption time, the nano brous lms initially had a great effect on copper ions.
At the same time, the conductivity of the copper ions solution was tested.The level of conductivity re ected the ability of the solution to conduct current in the water, and was often used to infer the concentration of ions in the water.As the concentration of copper ions increased, the conductivity of the solution was enhanced.When the concentration of copper ions in the solution decreased, the conductivity would decrease.With the adsorption of the nano brous lm, the copper ion concentration decreased, and the conductivity curve showed a rapid decrease and then slightly decreased until it was at.It could be seen from Fig. 4c that the curve obtained from the conductivity test was basically corresponded to the adsorption results obtained in Fig. 4b, the adsorption capacity rst rises rapidly, then slowly until it is at.
Figure 4d was a comparison chart of the adsorption performance of nano brous lms with different nHAP contents after 12 h adsorption when the adsorption had reached the equilibrium.The copper adsorption rate was calculated by dividing the capacity of copper ions adsorbed by the nano brous lm by the capacity of copper ions in the original solution.Compared with the zein/PVA nano brous lm, the nHAP nano brous lms showed higher copper adsorption rates.The curves of adsorption capacity and adsorption rate of the nano brous lms were shown in the Fig. 4b and 4d, and the speci c data of adsorption capacity and rate for each time period were shown in Table S1 in the supporting.Since PVA and zein had the ability to adsorb copper ions, the zein/PVA nano brous lm exhibited certain adsorption ability of 20.94 mg/g for copper ions.The addition of nHAP further improved the adsorption capacity of the composite lm.With the increase of nHAP content, the adsorption capacity of the nano brous lms enhanced.When the nHAP content was 20%, the adsorption capacity could reach up to 23.86 mg/g, and the copper adsorption rates was 13.94% higher than that of neat zein/PVA nano bers.
Adsorption kinetics was often used to predict the adsorption e ciency and the control mechanism of the desorption process (Huang et al. 2021;Teng et al. 2022).In this study, the Pseudo-rst-order model and the Pseudo-second-order model were used to analyze the mechanism of copper ions adsorption by nano brous lms.These two models did not follow the ideal kinetic model, but were updated models obtained after correction.The compliance with the Pseudo-rst-order kinetic equation means that the physical adsorption play the main role during the adsorption process(Al Ketife et al. 2021).The hydrogen bonding, van der Waals and electrostatic forces are usually supposed to be the main driving forces of adsorption process.The adsorption process conforms to the pseudo-second-order kinetic equation, indicating that the chemisorption of electron sharing or electron gain and loss play the main role in the adsorption process.The two kins of calculating models are shown in Eq. ( 3) and Eq. ( 4), respectively (Li et al. 2022).
The Pseudo-rst-order model can be calculated as follow: 3 The Pseudo-second-order model can be calculated as follow: t (4) q t (mg/g) is the capacity of adsorption as a function of time (t), q e (mg/g) is the capacity of adsorption at equilibrium, k 1 and k 2 are the rate constant of the Pseudo-rst-order kinetic equation and Pseudo-rstorder kinetic equation, respectively.Figure 4 (e) and 4 (f) were the curves tted by the pseudo-rst-order and pseudo-second-order equations for the adsorption of copper ions on nano brous lms with different nHAP contents.The tting data were shown in Table 2.The R 2 in the table indicated the correlation coe cient of the model.The R 2 in the Pseudo-rst-order model and the Pseudo-second-order model were both larger than 0.9, re ecting that both models could perfectly simulate the adsorption process of copper ions by zein/PVA/nHAP nano brous lms process, and also showed that physical adsorption and chemical adsorption coexisted in the copper adsorption process of nano brous lms.It could be seen from Table 2 that R 1 2 were higher than R 2 2 , result showed that Pseudo-rst-order model was more reliable (Yang et al. 2021).Besides, the similarity of calculated adsorption capacity to the experimental data further testi ed Pseudo-rst-order model was valid to interpret the kinetic adsorption, indicating that physical adsorption played the major role.From the of the Pseudo-rst-order model, it could be found that the k 1 of the nHAP composite nano brous lms were larger than that of the pure zein/PVA nano brous lm, indicating that the physical adsorption rate was accelerated after the addition of nHAP, and the adsorption capacity was increased due to the electrostatic interaction and van der Waals force between nHAP and the adsorbed ions.In the Pseudo-second-order model, k 2 increased with the addition of nHAP, indicating the chemical adsorption enhanced, which was mainly due to the ion exchange and complexation of nHAP in copper ion adsorption.Through the above analysis, it could be seen that in the process of zein/PVA/nHAP nano brous lm adsorption of copper ions, the physical adsorption played the main role due to the electrostatic interaction.At the same time, chemical adsorption such as complexation and ion exchange assisted the adsorption.Overall, due to the capture of copper ion by electrostatic action, complexation and ion exchange, the addition of nHAP could enhance the physical and chemical adsorption of the nano brous lms, thus the ability of removing copper ion of the composite lms were signi cantly enhanced.The nano ber structure changes of nanocomposite lms before and after adsorption of copper ions could be seen more intuitively by scanning electron microscope.After freeze-drying the nano brous lm after adsorption tests, it could be seen from the SEM image in Fig. 5 that the nano ber swelled with water after adsorption, the diameter became larger, and the space between the bers was greatly reduced.The binding between the nano bers and the solvent caused the bers to swell.The SEM images of zein/PVA/nHAP nano ber lms after adsorption at different magni cations could be seen in Figure S4.
With the increase of nHAP content, copper ions attached to the bers, and the ber morphology gradually disappeared and formed stacked morphology.
In Fig. 5, the reasonable copper adsorption mechanism of the zein/PVA/nHAP nano brous lm was depicted.There were three main adsorption mechanisms for copper ions in this system, electrostatic interaction, complexation and ion exchange.In neutral solutions, the zein molecular chain contains a large number of groups with lone pair electrons and some negatively charged groups, which enable zein molecules to absorb copper ions through electrostatic interaction and complexation.PVA molecular chain contains a good deal of hydroxyl groups, which can adsorb copper ions in wastewater by electrostatic interaction and complexation.The exible lattice made nHAP have a strong ability to tolerate defects and vacancies, so that metal ions can replace calcium ions on nHAP and x metal ions, thereby achieving excellent adsorption effect on metal ions.Taking copper ions as a representative, the substitution of calcium ions in the nHAP lattice is transformed into the following equation( Accordingly, in the process of zein/PVA/nHAP nano brous lm for copper ion adsorption, the physical adsorption played the main role due to the electrostatic interaction with copper ions, and chemical adsorption played a supporting role by complexation and ion exchange.Through the joint action of physical adsorption and chemical adsorption, the composite nano brous lms had excellent adsorption effect for copper ion.

Conclusion
In the study, with the increase of nHAP, hydrogen bonds were formed between the nHAP molecules, and the morphology of the nano bers formed agglomeration, the hydrophobicity also increased, and the melting point and crystallinity did not change signi cantly.The adsorption effect in the nano brous lm was mainly controlled by three mechanisms: electrostatic interaction, complexation and ion exchange.
The zein/PVA nano brous lm with the addition of nHAP showed the improved adsorption performance in the copper ion adsorption, and the adsorption process could be described by the Pseudo-rst-order model and the Pseudo-second-order model.Meanwhile, the adsorption process was dominated by physical adsorption.Therefore, with excellent adsorption performance and environmentally friendly material, the zein/PVA/nHAP-based nano brous lms have a good potential as biodegradable lm for multifunctional and high-e ciency water-adsorption materials and pave a new path for reducing water pollution in industrial applications.

Declarations Figures
Schematic al. 2020), zein not only has a wide range of applications as food packaging(Altan et al. 2018), wound dressings(Fiorentini et al. 2021), drug delivery(Marin et al. 2018; Yang et al. 2021), but also is a desirable natural material for promising adsorbent for mental or dye adsorption(Deng et al. 2019).Compared with zein lms, zein-based nano bers would show better adsorption performance against pollutants for the large surface areas (Deng et al. 2020; Wen et al. 2016).As illustrated in our previous studies, neat zein based nano bers exhibited poor electrospinning ability(Li et al. 2020).Cospinning polymers, including polyvinyl alcohol (PVA), poly(ethylene oxide) (PEO)(Tian et al. 2018), etc. were often incorporated into zein matrix to facilitate the electrospinning process.In this regard, bioplastics make up for the spinning properties while reducing the environmental pollution(Han et al. 2022).PVA is a polyhydroxy environment-friendly polymer, which is easily soluble in water and possessed excellent lm-forming properties(Guo et al. 2015; Niu et al. 2020).In addition, In addition, due to the large number of hydroxyl groups in the PVA molecular chain, heavy metal ions can be adsorbed through cross-linking and hydrogen bonding, thus removed from the wastewater(He et al. 2021).Owing to the distinguished properties, PVA nano ber has wide application for water treatment(Siddiqui and Khan 2020; Tian et al. 2019; Ullah et al. 2020).

2 (
mg/g) indicates the amount of adsorption at any time, (mg/L) is the initial ion concentration of sample, (mg/L) indicates the measured ion concentration of each sample, (L) is the solution volume and (mg) is the quantity of the nano brous lm.
preparation and design of the multi-functional zein/PVA nano bers incorporated with nHAP.

Table 2
Kinetic parameters of copper ions adsorption of Zein/PVA/nHAP nano brous lms Huang et al. 2021; Stoetzel et al. 2009) The nHAP adsorb copper ions through electrostatic interaction due to the PO 4 3− (Zhu et al. 2022).Besides, copper ions can deposit on the surface of nHAP to form an amorphous layer through surface complexation(Casado et al. 2022).