Effective removal of Cr(VI) in water by bulk-size polyaniline/polyvinyl alcohol/amyloid fibril composite beads

As an important industrial metal element, chromium (Cr) is widely used in electroplating, metallurgy, leather tanning, and other industries (Gao & Xia 2011). Due to the rapid expansion of these industrial activities, Cr ions are also discharged into the environment in large quantities along with industrial wastewater, which has caused serious heavy metal pollution of water and soil and seriously harmed the natural ecological environment and human health (Peng et al. 2009; Kapoor et al. 2022). Chromium in a water environment mainly exists as trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)). Cr(III) is the most stable oxidation state form of chromium in living organisms and the main form of chromium in living organisms. It is easy to form multi-tooth coordination compounds, but it is not easy to cross the cell membrane and has low reactivity. It is one of the essential trace elements in human and animal bodies. However, Cr(VI) easily crosses the cell membrane and reacts with nucleic acid and protein components, causing oxidative stress, DNA damage, cell apoptosis, or gene mutation in the cell, so it has carcinogenic and mutagenic effects (Wang et al. 2021). Therefore, a widely acknowledged way of reducing Cr(VI) toxicity is to convert its Cr(VI) forms to the less toxic Cr(III) forms and then make them stabilized. In water treatment, using reductants such as ferrites (Wu et al. 2022), zero-valent iron (Quan et al. 2014; Zhang et al. 2018; Liu et al. 2022), sulfides (Wang et al. 2019, 2020), or other materials with reducing ability (Chrysochoou & Reeves 2017; Sangeetha et al. 2019; Farooqi et al. 2021; Dong et al. 2023) has been proved efficient in Cr detoxification.

In recent years, polyaniline (PANI) has been applied in conductive adhesives (Derakhshankhah et al. 2020), rechargeable batteries (Cui et al. 2020), sensors (Anantha-Iyengar et al. 2019), corrosion-resistant material (Perumal et al. 2020), and pollutant adsorbent (Mahanta et al. 2009; Wang et al. 2009; Guo et al. 2011) due to its good environmental stability, biocompatibility, and low production cost. Polyaniline molecules contain a large number of reducing amine ( -NH -) and oxidizing imine ( -N=) groups, which not only have an excellent complexing effect on heavy metal ions (Liu & Huang 2011; Peydayesh et al. 2019) but also react with some heavy metal ions with high oxidation potential (such as Cr) (Leung et al. 2013). Polyaniline can be divided into a reducing state, an oxidation state, and eigenstate according to the proportion of amine and imine groups. During the reaction with Cr(VI), the electron energy is transferred from the reduced state and the eigenstate to the oxidation state, thus promoting the reduction of Cr(VI) to Cr(III) (Li et al. 2020). Polyaniline has been widely used in the remediation of Cr pollution in water. Zhang et al. (2010) and Wang et al. (2014) studied the removal effect of polyaniline doped with sulfuric acid and hydrochloric acid on Cr(VI) in water. The maximum adsorption capacity of the two kinds of polyanilines for Cr(VI) was 95.79 and 182 mg g⁻¹, respectively. The redox potential (E₀) of Cr(VI) in the acidic condition is much higher than that in the alkaline condition, thus accelerating the redox reaction of Cr(VI) with the amine group (Ding et al. 2018). The acidic environment can also protonate the amine group on polyaniline and promote the electrostatic attraction between it and Cr(VI) (mainly ⁠). or Cl⁻ on polyaniline will then undergo anion exchange with to remove chromium from the solution (Zhang et al. 2018).

Most adsorbents used in water purification are nano-size materials that exhibit outstanding performance in metal removal. However, their small size also endows them inactivity owing to the nanomaterial's aggregation phenomenon in solution. Nanomaterials also suffer from the great impediment of separation and recycling. Thus, they need to be grafted onto some stable and larger-size material, such as biochar (Chen et al. 2021, 2022) and graphene oxide (Kong et al. 2019), to prevent losing efficiency and facilitate recycling. Similarly, polyaniline presents in a powder form of a nano- or micro-size structure. Loading polyaniline on stable and big-size supporters will conduce to their removal effect. Recently, the design of bulk-sized materials has caught much attention because they can fix the problems mentioned above with the merits of easy collection, non-aggregation, and non-secondary pollution. However, the major limitation of normal bulk-sized materials for Cr removal is their low adsorption capacities due to limited surface area and porosity. For example, the maximum adsorption capacity (Q_max) was reported in some studies using bulk-sized materials: 6.31 mg g⁻¹ for surfactant-modified zeolites (0.12–0.18 mm) (Song et al. 2015), 3.40–28.88 mg g⁻¹ for chitosan cross-linked silicon materials (0.075–0.25 mm) (Shi et al. 2017), 12.2 mg g⁻¹ for PANI@PS gel beads of 0.4–0.6 mm (Ding et al. 2018), and 3.82 mg g⁻¹ for IPS-AR gel beads of 5 mm (Lv et al. 2013). Therefore, it is necessary to design a bulk material with an adsorption capacity comparable to many existing nano- or micro-sized materials.

We noticed that β-lactoglobulin (β-LG) is the main protein in whey, a byproduct of the cheese industry, and is therefore cheap and readily available (Ramírez-Rodríguez et al. 2020). Protein amyloid fibrils (AFLs) can be obtained from β-LG by combining hydrolysis and aggregation of the original protein folding sequence at low pH and high temperature (Godiya et al. 2020). AFL has high fibrosis, strength, and thermochemical stability, and its surface is rich in natural amino acid groups, which have a strong affinity for Cr(VI) (Arputharaj et al. 2021). In addition, the aerogel derived from the AFL matrix has an abundant porous structure (porosity ∼ 97%) (Peydayesh et al. 2020). AFL can be cross-linked with the amine group in PANI. Thus, AFL/PANI adsorbent can be prepared by a simple one-step chemical cross-linking method (Liu & Huang 2011). Studies have shown that the combination of AFL and polyvinyl alcohol (PVA) can not only overcome their respective shortcomings to form a regular shape of the material but also, through the physical and chemical double cross-linking, the composite can show better mechanical properties and chemical stability (Han et al. 2020). To our knowledge, there has been no study on establishing macroscopic bulk materials using the combination of PANI, PVA, and AFL. The mixture of AFL, PANI, and PVA to form solid bulk composite material is expected to greatly improve its structural stability and recovery value and synergistically improve the overall adsorption performance.

Therefore, the objectives of the study aimed to investigate: (a) the synthesis route of PANI, PVA, and AFL composite (hereafter named polyaniline/polyvinyl alcohol/amyloid fibril (PPA) composite beads); (b) the removal efficiency of PPA for Cr(VI) and Cr(VI) adsorption behaviors (kinetics, thermodynamics, and adsorption isotherms); and (c) examine the removal mechanisms associated with the adsorption process. The study will provide insight into using bulk materials for Cr remediation in the environment.

Polyaniline was prepared with hydrochloric acid (HCl) as doping acid. The specific method was as follows: 10 mL concentrated HCl was added to 336.8 mL MilliQ water and constantly stirred at 25 °C. Then, 1.63 g aniline was added to the above solution and stirred in an ice bath at 150 rpm for 30 min. Afterward, 3.99 g ammonium persulfate was slowly added to the aniline solution and stirred in the ice bath for 24 h. Polyaniline was obtained by centrifugation at 8,000 rpm for 15 min. The oligomers and doped acid were removed by washing with MilliQ water and ethanol alternately several times, and the final pure product was dried in a vacuum drying oven at 60 °C for 12 h.

Ten grams of whey protein isolate (WPI, Fonterra, New Zealand) was dissolved in 90 mL ultra-pure water, and the pH of the solution was adjusted to 4.2 with concentrated hydrochloric acid. The conical flask containing the solution was incubated in a water bath oscillator at 60 °C (150 rpm) for 4 h until the solution became cloudy, then centrifuged (15 min at 8,000 rpm) to obtain a nearly transparent solution and white sediment. The supernatant was collected, and the centrifugation procedure was repeated more than twice. The supernatant was passed through a 0.45 μm microporous filter to further remove the remaining insoluble proteins. The obtained clear solution containing mainly β-LG protein monomers, residual salt, and sugar was transferred to a conical flask, freeze-dried, and stored at −20 °C to synthesize AFLs.

An amyloid fibril solution with a mass fraction of 2 wt% was prepared using ultra-pure water with a pH of 2 and purified β-LG protein monomer. AFLs were heated in a 90 °C water bath with constant stirring for 5 h (120 rpm for the first 3 h and 170 rpm for the last 2 h). The synthesized β-LG AFLs were collected by centrifugation, immediately cooled using a water–ice mixture, and stored at 4 °C before use.

The morphology and elemental composition of PPA gel beads in water before and after Cr adsorption were characterized by scanning electron microscopy (SEM) coupled with an X-ray energy dispersion spectrometer (EDX) (Sigma 300, ZEISS, Germany). The PPA beads were crushed to powder and subjected to a zeta potential test at varying pHs 2, 4, 6, 8, and 10 on a Zeta sizer Nano-ZS90 (Malvern, UK). After the adsorption experiment, the beads were also powdered to analyze elemental valence states (Cr, C, and N) using X-ray photoelectron spectrometry (ESCALAB Xi + , Thermo Fisher Scientific, USA). X-ray powder diffraction (XRD) patterns of the materials before and after adsorption were obtained on a diffractometer (EMPYREAN, Holland) using Cu Kα radiation in a scanning range of 10–80° at a scanning rate of 2° min⁻¹. Jade 6.5 software (MDI) was used to analyze the typical mineral phases. Before and after adsorption, the materials were also subject to Fourier Transform Infrared Reflection (FTIR) (iN10, Thermo Fisher Scientific, USA) at the wavenumber of 600–4,000 cm⁻¹. The Brunauer–Emmett–Teller (BET) (ASAP 2460, Micromeritics, USA) method was used to obtain the specific surface area, pore size, and pore volume.

The adsorption performance of PPA composite beads for Cr(VI) in water was studied by batch adsorption experiments. The adsorption kinetics experiment was carried out at the Cr(VI) concentration of 50 mg L⁻¹ at pH = 3. The specific method was as follows: 40 mg of PPA beads was added into a conical flask containing 20 mL Cr(VI) solution with a concentration of 50 mg L⁻¹, and the flasks were oscillated in a constant temperature water bath oscillator (25 °C) at a speed of 150 rpm. Samples were intermittently taken out at different intervals (0–50 h) and then filtered through a 0.22 μm polyethersulfone (PES) membrane filter to determine the residual Cr(VI) concentration. There was no retention effect of Cr(VI) on the PES membrane based on our initial test. Three replicates were set for each sample. The residual Cr(VI) in the solution after adsorption equilibrium was determined by 1,5-dibenzoyl dihydrazine spectrophotometric method at 540 nm on a UV–Visible spectrophotometer (UV-2250, Shimadzu, Japan). The total Cr concentration in the solution was digested with nitric acid–peroxide (HNO₃–H₂O₂) and measured at 357.9 nm on an atomic absorption spectrometry (AAS) (AA700, PerkinElmer, USA). The Cr(III) content in the solution was calculated by subtracting the Cr(VI) content from the total Cr content. The limit of detection and limit of quantification of total Cr on AAS were 0.004 and 0.012 mg L⁻¹, respectively. The limit of detection and limit of quantification of Cr(VI) on UV–Vis were 0.068 and 0.21 mg L⁻¹, respectively.

The isothermal adsorption results showed that the removal ability of PPA composite beads for Cr(VI) in water increased with the initial concentration of a solution (Figure 4(b)). The adsorption increased rapidly when the concentration of Cr(VI) increased from 5 to 100 mg L⁻¹. The adsorption tended to reach the plateau when the concentration reached 150 mg L⁻¹. At the low concentrations, the adsorption sites on the PPA beads were sufficient to remove Cr(VI) continuously. With increasing concentrations, the adsorption amount ceased to increase and reached saturation due to the limited number of adsorption sites. The Langmuir, Freundlich, and R-P models were used to analyze the isothermal adsorption process, and the parameters obtained are shown in Table 2. The correlation coefficients (R²) of the Langmuir, Freundlich, and R-P models were 0.9171, 0.8245, and 0.9915, respectively. Therefore, the adsorption process of Cr(VI) on PPA beads can be described better by the R-P model because it was the closest to 1. Noteworthy, the value of β is the R-P constant related to the adsorption strength, and its value is between 0 and 1 and cannot be greater than 1 (Tshemese et al. 2021). When the value of β is 1, the R-P model equation can be simplified to the Langmuir model equation (Yang et al. 2020). In this study, the β value of the R-P model was 1, suggesting that the adsorption was monolayer adsorption with homogeneous surface reaction sites. The maximum adsorption capacity obtained by the Langmuir isothermal model was 51.54 mg g⁻¹. Since the value of R_L and the value of 1/n were between 0 and 1, the adsorption of Cr(VI) by PPA composite beads was favorable (Yang et al. 2020). Table 3 lists the adsorption capacity of different polyaniline composite adsorbents for Cr(VI). Compared with other polyaniline-related composites, the size of PPA was the largest among them, but the adsorption capacity of PPA for Cr(VI) was in the upper middle range. Although the adsorption capacity of PPA was lower than the PPS product made of PANI, PVA, and sodium alginate in our previous study (Li et al. 2022), the lower maximum Cr(VI) concentration (400 mg L⁻¹) in the current study may contribute to the decrease. Noteworthy, the size of PPA was about two times bigger than that of PPS; there is still a likelihood of increasing the PPA's adsorption capacity by size adjustment. Considering the easy aggregation of nano-polyaniline adsorbent when applied in the water environment, the PPA beads have a high potential for water purification because of their excellent separation performance. The large particle size will facilitate recycling and recovery and not cause secondary pollution.

The adsorption capacity of Cr(VI) on PPS material generally followed an increasing trend with the increase in temperature (Figure 4(c)). The thermodynamic parameters, including standard free energy change (ΔG), standard enthalpy change (ΔH), and standard entropy change (ΔS) are summarized in Table 4. The ΔG values were negative, indicating that the adsorption of Cr(VI) by PPA was spontaneous. The ΔH of 9.62 kJ mol⁻¹ > 0 suggested that the adsorption was an endothermic process. Therefore, increasing the temperature is conducive to promoting the adsorption efficiency of PPA for Cr(VI). In addition, the ΔS value was above zero, suggesting that the adsorption of Cr(VI) onto PPA was an entropy increase process, where the chaos of the solid–liquid interface will increase during the adsorption.

In this study, we synthesized a bulk adsorbent PPA with a combination of polyaniline, polyvinyl alcohol, and amyloid fibril. The as-formed material exhibited good adsorption capacity for Cr with Q_m of 51.5 mg g⁻¹. The adsorption capacity was comparable to or even better than some PANI-based nanomaterials. The adsorption process was mainly pseudo-second-order controlling kinetics and followed Langmuir adsorption isotherm, suggesting chemical reactions governed the adsorption chiefly. The XPS analysis further proved the above result, where the redox reactions happened between –NH–, C–OH groups, and Cr(VI) anions caused the formation of Cr(III), which was further stabilized within the PPA beads through complexation. The synthesis and use of PPA beads provide a new way of using bulk-size material for heavy metal remediation, especially in soil remediation, where the bulk material can be easily collected and regenerated. This study provided the initial data of using PPA for Cr remediation. Future research is needed in the aspects of influencing factors, material's reusability, material's stability, etc., to evaluate the potential ability of this material in the broader application.

The study was supported by the Natural Science Foundation of Chongqing (CSTB2022NSCQ-MSX0391), the National Natural Science Foundation of China (No. 52370166), the Natural Science Foundation of Hunan Province (No. 2023JJ30126), and the Natural Science Foundation of Changsha City (No. kq2208020).

J.W.: original draft, methodology, funding acquisition; Y.Z.: data curation, experiment; Y.D.: experiment, data curation.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.