Adsorptive and oxidative removal of phenolphthalein by sono-assisted synthesized FeAcC in advanced oxidation-integrated fluidized bed reactor Open Access

The rapid growth of chemical industries and research laboratories around the globe has led to the increased production and discharge of organic pollutants, posing a substantial risk to the environment. The most concerning compounds are the phenolic compounds (phenol, bisphenol-A (BPA), 4-nonylphenol, chlorophenols, phenolphthalein (php), and octylphenols), which show an adverse impact on all living beings and the environment (Panigrahy et al. 2022). Due to the organoleptic characteristics of phenolic compounds, they are hazardous to aquatic species at concentrations as low as parts per billion. Direct contact and consumption of those compounds may cause liver, eye, skin, muscle, and kidney damage (Udoetok et al. 2016). The most widely utilized phenolic substance is php, disrupting the endocrine system in both humans and animals. It was found that php, an active component of several laxatives, causes cancer in animals across various species (Longnecker et al. 1997). As a result, it is essential to remove organic contaminants efficiently using an affordable technique.

Organic contaminants from aqueous solutions can be removed by a variety of conventional techniques, including biological degradation (Cirik et al. 2013), chemical adsorption (Heydari et al. 2021), electrochemical oxidation (Garciá-Orozco et al. 2016), oxidation (Malik et al. 2019), ion-exchange (Edgar & Boyer 2021), reverse osmosis (Jamil et al. 2020), coagulation, flocculation, and sedimentation (Matilainen et al. 2010). However, these methods might be challenging due to some limitations, including secondary byproducts, being more time-consuming (biological method), high installation costs, improper disposal facility and selective ion-exchange resin (ion-exchange), fouling of membrane and high pressure (reverse osmosis), and toxic chemical utilization (coagulation) (Italiya & Subramanian 2022). Advanced oxidation processes (AOPs) have substantially impacted the effective elimination of harmful phenolic compounds. This is attributed to the generation of hydroxyl radicals (·OH), which oxidize these phenolic compounds (Malik et al. 2019). The adsorption mechanism of the adsorbent material depends on its surface charge and physical properties, which could be significantly altered when it undergoes oxidation (Álvarez et al. 2005). Several published studies have discussed the use of advanced oxidation and adsorption for the elimination of phenolic organic pollutants, such as gallic acid, 17β-estradiol, BPA, phenol, p-chlorophenol, and p-nitrophenol (Álvarez et al. 2005; Irmak et al. 2005; Fan et al. 2019; Lu et al. 2022). The most efficient approach for eliminating organic contaminants is using a fluidized bed reactor (FBR) with enhanced oxidation and adsorption.

The most significant concern in removing toxic php from polluted waterways is to find affordable, flexible, and efficient adsorbent materials. There are numerous adsorbent materials including cobalt–aluminum-layered double oxide (Tongchoo et al. 2020), multi-walled carbon nanotube (Vadi & Namavar 2013), β-cyclodextrin–chitosan (Tang et al. 2019), strontium ferrite (Adewuyi et al. 2021), and tetraethylammonium–kaolinite clay (Adewuyi & Oderinde 2022) that show the adsorptive removal of php. Adsorption of pollutants on chitosan and carbon is limited due to the small surface area and weak mechanical strength. Hence, cross-linking and ultrasound were studied and reported to enhance the cavitation, surface area, and mechanical robustness of the adsorbent material (Italiya et al. 2024). Furthermore, the modification of Fe on chitosan forms a stable composite through complexation and electrostatic interaction. This modification enhances the mechanical durability of chitosan, surface net charge, and hydrophilic–hydrophobic properties alteration and imparts resistance to both acidic and basic conditions (Hu et al. 2016b; Zhang et al. 2018). Thus, phenolic compounds, other organic contaminants, and persistent organic pollutants can be eliminated using modified chitosan and carbon materials (Yazdi et al. 2019). Adsorption with a FBR intimates contact between adsorbate and adsorbent through high mass transfer and rate of reaction with less residence time (Mohapatra et al. 2021). The uniform distribution of adsorbents due to fluidization leads to the adsorption of small quantities of pollutants from an aqueous media (Kalaruban et al. 2016). Adsorption with ozonation and FBR is the most promising substitute method for treating organic pollutants that produce oxygen as a byproduct without producing sludge (Cirik et al. 2013; Italiya & Subramanian 2022).

This research aims to synthesize a sono-assisted Fe-modified activated carbon (Ac)–chitosan (FeAcC) composite to remove php from aqueous solutions. The composite material was characterized using field emission-scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) surface analysis. In the study, the FeAcC demonstrated its efficacy in the batch process for removing php under different conditions, such as pH, initial php concentration, time, and adsorbent dosage. The parameters for isotherm, kinetic, and thermodynamic studies were obtained using the data from the adsorption study. The reactor studies were conducted using an advanced ozone-integrated FBR in the batch and continuous modes of operation. The batch FBR study was carried out with various initial php concentration, flow rate, time, and FeAcC reuse. The continuous FBR study was investigated using the different hydraulic retention times (HRTs). Response surface methodology (RSM) was used to design the optimum experimental condition.

Chitosan (C₅₆H₁₀₃N₉O₃₉, 99.90%), glutaraldehyde ((CH₂)₃(CHO)₂, 98%), and sodium tripolyphosphate (NaTPP, Na₅P₃O₁₀, 99.99%) were purchased from Sigma Aldrich, India. Phenolphthalein (C₂₀H₁₄O₄, 99%), ethanol (CH₃CH₂OH, 99.99%), charcoal, and ferric chloride hexahydrate (FeCl₃·6H₂O, 98%) were purchased from SRL Chemical Pvt. Ltd. Sodium hydroxide (NaOH) and hydrochloric acid (HCl) with 0.1M concentration were used to adjust the pH of the solution. The stock solution of php was prepared using an ethanol solution.

FE-SEM was used to study composites' surface structural morphology at various magnifications (Thermo Fisher FEI-Quanta 250 FEG). The crystalline properties of the composite were examined with 2Ɵ range from 10 to 90° using XRD (Bruker D8 advance model). FTIR with a spectral range from 400 to 4,000 cm⁻¹ was used to determine the presence of functional groups on the surface of FeAcC (IRAffinity-1, Shimadzu, Japan). The specific surface area, volume, and size of the pores of the composite were measured under desorption and adsorption of nitrogen gas using a BET surface analyzer (Quantachrome, USA). Before and after treatment, the php concentration was measured using a UV-vis spectrophotometer (Specord 210 plus).

The 20 mg L⁻¹ of php stock solution was prepared to study its adsorption on FeAcC. Since php is known to be poorly soluble in water, ethanol was used to prepare a stock solution and it was diluted using distilled water to adjust the specific concentration. The various parameters including FeAcC dosage (0.1–1 g L⁻¹), pH (3–12), the initial php concentration (5–25 mg L⁻¹), and various reaction times (15–150 min) were followed by a temperature range between 303 and 323 K to determine the optimum condition for php removal. The final concentration of php after adsorption was measured at 552 nm absorbance wavelength by a UV-vis spectrophotometer (Dehabadi et al. 2014). To comprehend the repulsive and attractive mechanism, the FeAcC point zero surface charge (pH_zpc) was ascertained using this pH drift method (Italiya et al. 2024). For pH_zpc determination, 0.5 g of FeAcC was added to 25 mL of NaCl (0.01M) solution that encompassed a range of pH values (3–12) (Cotman et al. 2016). NaCl was employed as a background electrolyte for the measurement of pH_zpc. In aqueous environments, Na⁺ and Cl⁻ ions are rather inert toward the surface of composite materials. The electrolytes' monovalent interaction with FeAcC results in an increase in surface thickness and a limitation on the mobility of H⁺. That H⁺ limitation helps in the determination of pH_zpc (Kollannur & Arnepalli 2019).

FTIR was used to assess the properties of functional groups present in FeAcC. Figure 4(b) provides FTIR spectra of synthesized FeAcC before and after adsorption of php. The strong and broad spectrum near 3,310 cm⁻¹ ascribed the intermolecular hydrogen bonding and vibrational stretching of the amine (N–H) and hydroxyl (–OH) group. The coordination of Fe⁺³ with hydroxyl and amino groups was suggested by the redshift observed for the FeAcC complex, which resulted in a reduction in hydrogen bonding ability (Eltaweil et al. 2021a). The symmetric and asymmetric vibrational stretching of C–H of methylene and methyl groups are proposed to occur at 2,921 and 2,852 cm⁻¹, respectively (Hu et al. 2016b). The protonation of the amino group of the NHCOCH₃ and NH₂ was responsible for the appearance of bands near 2,337 and 2,363 cm⁻¹ (Hu et al. 2015). The acetylated amino group of chitosan with C = O vibrational stretching in NH = C = O was represented by an infrared peak at 1,648 cm⁻¹ (Mohammadi et al. 2019). The symmetric C–H vibrational stretching and primary alcohol's stretching vibration C–O are the respective adsorption bands located near 1,384 and 1,085 cm⁻¹. After adsorption, the band at 2,360 cm⁻¹ disappeared as a result of dipole interaction, indicating a reaction between php and protonated amino group of chitosan (Yuan et al. 2008).

The contact time and initial php concentration are important parameters that influence the adsorption study. By varying the contact time from 15 to 150 min and initial concentration from 5 to 25 mg L⁻¹, the sorption of php was investigated. As depicted in Figure 5(b), the removal of php was initially rapid during the beginning of adsorption, but the rate gradually decreased as it approached equilibrium. The adsorption capacity was enhanced by increasing the initial php concentration due to adequate and stable free sites on the FeAcC that interact with low concentrations of php (Yu et al. 2022). The adsorption rate was decreased because of intense competition between php molecules to bind to the surface of FeAcC, which eventually approached saturation by increasing the initial php concentration (Figure 5(c)) (Tang et al. 2019). All of the free active sites were engaged quickly by php at the highest degree of adsorption, which increased the repulsion and reduced the php removal (Adewuyi et al. 2021). The increased mass gradient of php with increasing php concentration works as a driving force to accelerate the php molecules' movement toward the FeAcC (Hu et al. 2016b). The adsorption capacity was found to be 29.46 mg g⁻¹ at the highest initial php concentration (Figure 5(b)). It was found that ≈98% of php was removed for the lowest php concentration at 120 min of reaction time, and after that, there was no drastic change in adsorption (Figure 5(c)). Furthermore, the effect of only ozonation was carried out to remove various concentrations of php (5–25 mg L⁻¹) from an aqueous solution. The maximum removal of php was found to be ≈40% at 45 min for 20 mg L⁻¹ php concentration. There was no significant difference in the removal of php after 20 min of reaction time. The removal of php for all concentrations has been depicted in Supplementary material S1.

The different FeAcC dosages (0.1–1 g L⁻¹) were tested to determine the ideal quantity of adsorbent for effective php removal (Figure 5(d)). It was observed that the adsorption rate of php was increasing with increasing dose of adsorbent due to the increment of available active sites on the surface of the adsorbent (Eltaweil et al. 2021a). Meanwhile, the adsorption capacity was decreased with increasing the adsorbent dose, because of php aggregation and unsaturation of active sites (Ghadiri et al. 2017). There was a slight increment in the percentage of php adsorption after 0.5 g L⁻¹ of adsorbent dose. Therefore, 0.5 g L⁻¹ adsorbent dose was considered as an optimum dose for the php removal studies.

Using different concentrations of php ranging from 5 to 25 mg L⁻¹ at pH 4, the adsorption by FeAcC was analyzed at regular intervals to determine the equilibrium point. The residual concentrations of php were examined by collecting the final volume after adsorption at the equilibrium level. Reliable experiment design and the relationship between adsorbate and adsorbent were determined at various temperatures (303–323 K) by adsorption isotherms, including Langmuir, Freundlich, and Temkin models (Mohammadi et al. 2019).

All the values of isotherm parameters are displayed in Table 2. The regression coefficient (R²) values of the Langmuir model at 303, 313, and 323 K are 0.99, 0.99, and 0.99, which show better results in comparison to the Freundlich (0.98, 0.90, and 0.97) and Temkin models (0.96, 0.97, and 0.99), respectively. The results demonstrated that the Langmuir isotherm model was perfectly fit for the php adsorption by FeAcC. The favorable adsorption conditions were represented by the R_L values, which ranged from 0 to 1 at different temperatures (Table 2). The Langmuir isotherm parameters indicated that the maximum adsorption capacity (⁠⁠) was found to be 25.18 mg g⁻¹ at 323 K. The values of adsorption intensity (1/n) and separation factor (R_L) between 0 and 1 suggested the favorable php adsorption condition on the FeAcC surface (Hu et al. 2015; Aswin Kumar & Viswanathan 2018). In this study, it was noted that the adsorption capacity of the FeAcC composite diminished as the temperature increased, which is attributed to the physical adsorption process (Cheah et al. 2013).

Table 2

Parameters of Langmuir, Freundlich, and Temkin isotherms

Isotherms .	Factors .	Temperature (K) .
		303 .	313 .	323 .
Langmuir	(mg g−1)	6.28	12.15	25.18
		5.70	2.73	1.26
		0.08	0.15	0.28
		0.99	0.99	0.99
Freundlich		0.57	0.48	0.34
		6.22	9.29	13.69
		0.98	0.90	0.97
Temkin	bT	8.12	7.60	9.14
		0.49	1.18	2.70
		0.96	0.97	0.99

Isotherms .	Factors .	Temperature (K) .
		303 .	313 .	323 .
Langmuir	(mg g−1)	6.28	12.15	25.18
		5.70	2.73	1.26
		0.08	0.15	0.28
		0.99	0.99	0.99
Freundlich		0.57	0.48	0.34
		6.22	9.29	13.69
		0.98	0.90	0.97
Temkin	bT	8.12	7.60	9.14
		0.49	1.18	2.70
		0.96	0.97	0.99

View Large

Table 3 lists the different kinetic parameters along with their corresponding R² and constant values. Figure 6(d)–6(f) represents the plots of pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, respectively. Compared to the pseudo-first-order kinetics and intraparticle diffusion models, the pseudo-second-order model fits the data most accurately, as shown by their R² values. Furthermore, the calculated values appear to be almost close to the experimental values, suggesting the suitability of a pseudo-second-order model for php adsorption. The chemisorption mechanism of php adsorption on the surface of FeAcC was suggested by the decreasing values with increasing initial php concentration (Eltaweil et al. 2021b). The occurrence of php adsorption during chemisorption is facilitated by the exchange of electrons under the influence of valence forces (Cheah et al. 2013). Based on the kinetic study, the php adsorption rate was dependent upon available reactive binding sites and porosity of the FeAcC (Italiya et al. 2022).

Table 3

Kinetic parameters for adsorption of php by FeAcC

(mg L−1) .		5 .	10 .	15 .	20 .	25 .
(mg g−1)		9.25	16.97	22.69	25.48	29.47
Pseudo-first-order	(mg g−1)	9.16	14.83	16.76	22.24	26.60
		2.43 × 10−4	2.34 × 10−4	4.02 × 10−4	2.50 × 10−4	2.20 × 10−4
	R2	0.70	0.71	0.51	0.81	0.96
Pseudo-second-order	(mg g−1)	11.30	16.45	21.59	25.65	28.12
		2.10 × 10−3	1.07 × 10−3	1.40 × 10−3	9.11 × 10−4	5.29 × 10−4
	R2	0.97	0.98	0.96	0.95	0.98
Intraparticle diffusion		0.74	1.39	1.55	1.99	1.88
	R2	0.90	0.88	0.94	0.95	0.98

(mg L−1) .		5 .	10 .	15 .	20 .	25 .
(mg g−1)		9.25	16.97	22.69	25.48	29.47
Pseudo-first-order	(mg g−1)	9.16	14.83	16.76	22.24	26.60
		2.43 × 10−4	2.34 × 10−4	4.02 × 10−4	2.50 × 10−4	2.20 × 10−4
	R2	0.70	0.71	0.51	0.81	0.96
Pseudo-second-order	(mg g−1)	11.30	16.45	21.59	25.65	28.12
		2.10 × 10−3	1.07 × 10−3	1.40 × 10−3	9.11 × 10−4	5.29 × 10−4
	R2	0.97	0.98	0.96	0.95	0.98
Intraparticle diffusion		0.74	1.39	1.55	1.99	1.88
	R2	0.90	0.88	0.94	0.95	0.98

View Large

The R² value (0.99) was calculated using the Van't Hoff plot (ln k vs 1/T) (Figure 6(g)). Adsorption appeared to be spontaneous and thermodynamically feasible, as indicated by the negative ΔG° (Supplementary material S2). Furthermore, the high temperature was shown to be more favorable for the adsorption of php, as seen by a rise in negative ΔG° with temperature. The increasing randomness at the solution/solid interface following the adsorption of php was confirmed by the positive value of ΔS° (46.96 kJ K⁻¹ mol⁻¹). The positive value of ΔH° (11.78 kJ mol⁻¹) indicated the endothermic nature of the php adsorption process.

Table 4(a) and 4(b) display the comparative adsorption capacity of FeAcC with various php initial concentrations and removal of other organic compounds using an ozone-integrated reactor study, respectively. Concerning the small dose and surface area of FeAcC and low ozonation rate, the php removal capacity and rate were high in comparison to other composites listed in Table 4.

Figure 8(b) shows the effect of various recirculation flow rates (0.5–2.5 L min⁻¹) on the php removal. The percentage removal of php increased with the increasing flow rate up to 1.5 L min⁻¹. When the flow rate was low, there was a longer duration for php to engage with FeAcC. However, the interaction was confined to the lower section of the reactor because of the extremely slow movement. At the 1.5 L min⁻¹ flow rate, the removal efficiency was highest due to the distribution of php throughout the reactor with maximum contact with the adsorbent and ozone to get adsorbed and oxidized, respectively. Beyond the flow rate of 1.5 L min⁻¹, the removal rate of php was decreased due to the low residence time of •OH to react and leaching of Fe by the high fluid force. Therefore, it was essential to keep the recirculation flow rate at 1.5 L min⁻¹ to get maximum removal of php (Mohapatra & Ghosh 2023).

The recovery and subsequent utilization of FeAcC offers a cost-effective approach for php removal. The FeAcC reusability experiment was carried out in five cycles (runs) using an optimum dosage of composite (0.5 g L⁻¹) and recirculation flow rate (1.5 L min⁻¹). The FBR introduced the php solution, which was treated with the HRT of 30 min and disposed of at the end of each cycle. Cycle 1 used FeAcC material, which was reused for all the cycles to remove the subsequent php solution. Figure 8(c) shows that the removal rate of php decreased with an increasing number of cycles from 94.74% in run 1 to 56.84% in run 5. This is most likely due to the occupied binding active sites during each cycle (Li et al. 2015). With increasing cycles, repulsion between adsorbed and non-adsorbed php reduces the php uptake rate. The decrease in php removal could also be attributed to the reduced ozone adsorption and decomposition, which may be due to the limited availability of active sites and Fe leaching after multiple uses of FeAcC (Mohapatra & Ghosh 2023).

For the continuous FBR study, HRT is a crucial parameter, representing the average duration that php remains within the reactor. To identify the optimal parameters for php removal, the experiment was conducted with varying initial concentrations of php at different HRTs (Figure 8(d)). The feed rate of php affects HRT, and a low feed rate results in high HRT and vice versa. The removal rate was low at the low HRT due to the high feeding rate of php, less time to interact with FeAcC and •OH, and high dragging force generated by the collision between php solution and FeAcC. At high HRT, php had more time to interact with FeAcC and ·OH without any collision. The removal rate decreased with increased concentration of php (Mohapatra et al. 2021). The optimum HRT was found to be 70 min with the highest php removal for all the php concentrations (Figure 8(d)). According to reports, ideal HRT relies on the type of reactor, fluidized material, pollutant, and feeding rate (Sinharoy et al. 2019).

In this study, a sono-assisted synthesized FeAcC composite material was used for the removal of php from an aqueous solution. The maximum adsorption capacity was found to be 25.18 mg g⁻¹ using the Langmuir adsorption isotherm. The electrostatic mechanism played an important role during the adsorption process. According to the Langmuir isotherm and pseudo-second-order kinetics models, adsorption occurred in a single homogenous layer with a chemisorption mechanism. Ozone-integrated FBR showed around 30% more php removal within 20 min than a single adsorption technique. The Ac and Fe of FeAcC provided greater surface area to react with ozone to produce more •OH radicals for php removal. A combined adsorptive–oxidative removal of php in FBR enhanced the removal efficiency, residence time, removal at the highest and lowest concentration, no byproducts, and high rate of reaction by resolving the drawbacks of each method when employed independently. The RSM study demonstrated the reliability and significance of the developed model from the experimental data to optimize the parameters. Future research must use real-time wastewater as the study topic to understand better the removal of other organic pollutants by fluidized FeAcC using advanced oxidation-integrated FBR.

The authors acknowledge Vellore Institute of Technology, Vellore, Tamil Nadu, for providing a better opportunity to carry out their studies.

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

The authors declare there is no conflict.