Photoadsorption and reaction mechanism of periwinkle shell ash in the removal of hazardous dye

Several types of dyes are commercially available and have found applications in the beverage, pharmaceutical, cosmetics, and clothing industries with the huge volume of discharged effluents (Mansour 2018). As a result, dye effluent treatments have remained a primary concern owing to their bioaccumulative, toxic, and persistent properties (Butler et al. 2016). Consequently, a review study estimated that over 84,000 tons of dyes are discharged annually into water bodies and aquifers (Routoula & Patwardhan 2020). In addition, dyes are carcinogenic and cause serious health problems such as hyperactivity disorder and damage to human organs when ingested/absorbed at certain concentrations (Naseem et al. 2017). Moreover, the disruption of metabolic balance in biological organisms due to the absorption and reflection of solar radiation and light scattering is a common phenomenon (Mariselvam et al. 2016). Furthermore, dyes are biologically resistant, with robust aromatic molecular structures that interfere with aquatic life and disrupt marine life balance (Berradi et al. 2019). Hence, research efforts continue to proffer several treatment technologies for the removal of dyes from wastewater effluents.

Several treatment technologies have been applied for the removal of dyes from wastewater including bioremediation, flocculation, precipitation, adsorption, Fenton oxidation, photocatalysis, combined anaerobic/aerobic digestion, and membrane filters (Chiraibi et al. 2016; Inthapanya et al. 2019; Islam et al. 2019; Castro et al. 2021). Amongst these techniques, photocatalysis and adsorption have been widely studied. Although the two individual treatment processes do not offer a panacea for the removal of dye contaminants, the alternative treatment options, on the other hand, have installation costs, membrane fouling, maintenance problems, and high cleaning expenses. Sometimes they are affected by a lack of modification and repetitive research, which have created gaps in the exploration of other effective methods. Nevertheless, maintaining high separation factors and high throughput, which are often achieved by extra concentrate treatment, is now a challenge (Shamloufard et al. 2022). Subsequently, the combined photocatalysis and adsorption process is cheap, simple, and effective for adsorbents and reduces the dye concentrations to the lowest World Health Organization (WHO) standard. Hence, to the best of our knowledge, there are yet no evaluations available regarding the synergistic impacts of the combined adsorption and photocatalysis processes in the effective and efficient removal of dyes as proposed in this study.

Several investigations have been conducted on the application of many adsorbents. Amidst them are the cheap, affordable, readily available adsorbents, often disposed of as biowaste. These adsorbents are shells that have gained attention in the removal of dyes. They are hard and rigid and form an outer coating for varied animals, including mollusc shells, sea urchins, and crustaceans of marine origin. A study showed that the snail shell is a good adsorbent (AL-Daamy et al. 2018). Also, another work achieved 88% removal efficiency using seashells (Maheesh et al. 2017), while a new study attained 78% dye removal efficiency using CaO from eggshells (Thakur et al. 2021). In addition, activated shellfish was confirmed to be a low-cost adsorbent (Fillo et al. 2021), and calcium-rich biochar from crab shells brilliantly removed malachite green dye (Dai et al. 2018). Hence, the excellent removal of dyes by the materials is attributed to their activated carbon-based properties which have achieved 90–97% efficiency.

Interestingly, activated carbon-based materials have been selected as support materials for TiO₂ photocatalysts due to their chemical inertness and non-reactive nature. Most of the carbon-based adsorbents like the periwinkle shell ash (PSA) materials have high surface properties and a strong adsorption affinity for organic and inorganic compounds. In addition, the regenerative ability of the PSA is advantageous among carbon-based adsorbents (Liu et al. 2022). Moreover, as they grow in size, they retain their basic structural form with several layers and are typically made of an organic matrix (conchiolin). This layer is bonded with calcium carbonate precipitates. These carbonate-filled organic matrix shells are impervious to water, and this makes it possible for periwinkle shells and their derivatives to have very wide applications in the removal of organic and inorganic pollutants (Vijaya Ramnath et al. 2018). The strong interaction between adsorbate and carbon-based materials enables the exploitation of a synergistic effect, increasing adsorption capacity, the efficiency of PSA, and cost savings. (Singh et al. 2020). Likewise, a recent study proposed that the application of TiO₂ catalysts will bolster waste effluent treatment using PSA (Nkwoada et al. 2021). The use of TiO₂ after carbon–adsorbate interaction allows easy recovery of TiO₂ photocatalyst and improves the recovery properties without regeneration of secondary pollutants which improves the overall efficiency of PSA adsorbents (Salgado et al. 2019; Abebe et al. 2020). According to Liu et al. (2021), an improved synergistic effect occurs if the adsorption of the target pollutant onto the activated carbon is followed by a transfer through interphase to TiO₂ particles before obtaining a complete photo-decolourization process. However, data are scarce in this area of study, and neither combined adsorption/photocatalysis studies by PSA are reported in the literature on the treatment of dye effluents. The concept is exploited in this study via a combined adsorption and photocatalysis synergy because the porosity properties of PSA-activated carbon as well as the generation of hydroxyl radicals are the main drivers of the complete adsorption and photodecomposition efficiency of dyes in wastewater treatment (Xing et al. 2016; Mondol et al. 2021).

Furthermore, to the best of our knowledge, PSA has not been applied in a combined adsorption and photocatalysis synergy for the removal of crystal violet (CRV) dye. Only one study, for instance, showed successful heterogeneous photocatalytic degradation of aniline within 100 min (Aisien et al. 2014). Likewise, Ikhazuangbe et al. (2019) worked on PSA-activated carbon in the removal of Congo red and showed that calcination at 800 °C created an expanded carbon material having a large surface area and high porosity, and enhanced percentage removal of dyes. In addition, Gunorubon & Chukwunonso (2018) studied the kinetics, equilibrium, and thermodynamics studies of Fe³⁺ ion removal from aqueous solutions using PSA. Their findings showed that the adsorption rate increased with an increase in contact time, adsorbent dosage, and pH of aqueous solution and also decreased with an increase in the particle size. Similarly, Awokoya et al. (2016) successfully activated PSA for the binding of Cr and Zn metal ions from aqueous media, while Olusola & Babayemi (2019) studied Cr³⁺ heavy metal removal using activated bamboo and periwinkle shell. Findings showed the dosage of metal and agitation speed had the most significant effect on removal efficiency. Although the use of PSA over the years in the effective treatment of dye wastewater effluents has received poor attention, its synergistic performance is expected to surpass other activated carbon materials in cost and effectiveness.

Remarkably, PSA possesses a large surface area, large pore sizes, and diameters, which makes it a suitable adsorbent as well as having semi-conductors with photocatalytic substrate attributes (Aisien et al. 2015; Grégorio et al. 2019). The photocatalytic attribute was confirmed in our recent work when used as a nanocomposite photocatalyst in heterogenous photocatalysis (Nkwoada et al. 2022). Furthermore, studies showed that PSA possesses beneficial properties in adsorption. The studies proposed that PSA's extensive exploration will reveal more excellent properties as a nontoxic, cheap, and readily available alternative to organic and inorganic adsorbents (Gbenebor et al. 2017; Adnan et al. 2019). As a consequence, there are continuing challenges inter alia in finding cheap, affordable, and readily available adsorbent material for adsorption/photocatalysis studies. Thus, the primary objective of this work is the study of thermodynamics, kinetics, and adsorption/photocatalysis mechanism in the removal of CRV by PSA and simultaneous photocatalysis (TiO₂) of the remainder concentrations.

A mass of 500 g of periwinkle (Tympanotonus fuscatus) empty shells was purchased from a local seller in the relief market in Owerri (Imo State, Nigeria) and stored in a plastic container. The empty shells were hammered into pieces and soaked in tap water for 5 days to dissolve the dust, sand, and soluble particles’ impurities. The shells were removed and thoroughly washed severally under continuous agitation in flowing tap water to remove any loose particles. The periwinkle shells were dried at 120 °C for 48 h in preparation for milling. The shells were pulverized to powder by a locally fabricated ball mill at workshop 3 of the Federal University of Technology Owerri (FUTO). Afterwards, it was sieved through a 150 μm mesh sieve to get periwinkle shell powder (PSP). The PSP powder was calcined at 800 °C in a muffle furnace for 3 h at 100 °C/10 min to improve the surface properties and uniform size to obtain PSA. PSA was sieved through a 75 μm mesh sieve to remove large particles and then stored in a plastic container.

A scanning electron microscope (SEM; Phenom Pro X) integrated with an energy dispersive X-ray (EDX) detector was employed to determine the surface topography, and the quant technique was used for the SEM/EDX studies. The X-ray diffraction (XRD; RIGAKU Mini Flex 600) was performed to determine the phase angle and crystalline phases, and the Rietveld method was used to examine the collected XRD data. The Fourier transform infrared spectroscopy (FTIR; CARY 630) was used to collect the infrared spectrum and the functional groups by the diffuse reflectance approach for FTIR spectrum capture. The Brunauer–Emmett–Teller (BET; Nova Qunatachrom Instruments) was conducted to determine the specific surface area and pore sizes at isothermal temperature using an inert gas (N₂) at 77k, the released gas molecules were quantified, and surface area/porosity was calculated. Also, the decomposition pattern was measured by Perkin Elmer: STA 8000 thermogravimetric analysis (TGA) by gradually raising the samples’ (mass) temperature in the furnace and weighing against an analytical balance. All the method validations were carried out by taking analytical methods into account following the 2005 revised guidelines established by the international organization for standards (ISO/IEC 17025).

Analytical grade CRV (CI 42535, CAS 548-62-9, purity 85%) and Eriochrome Black T (CI 14645, CAS 260320 G25, purity 85%) have been purchased from Gate Lab Chemicals Nigeria Ltd. Nano-sized titanium dioxide particles of >99% purity in the anatase crystalline form (<25 nm) purchased from Sigma-Aldrich were used in the photocatalytic study. Distilled water was prepared from the MilliQ UF-Plus system (Millipore, Eschborn, Germany) having a resistivity of 18.2 MU cm at 25 °C and used to conduct the experiments. All dye stock solutions were prepared at 1,000 mg/L concentrations. The standard calibration curves were plotted, and wavelengths were measured at λ = 590 nm for CRV dyes at the temperature of 29.5 °C using a Shimadzu-1800 UV–VIS spectrophotometer to determine the concentration of the adsorbates.

The CRV dye concentrations (mg/L) at the start, equilibrium, and time (t), respectively, are shown in the aforementioned equations as C_o, C_e, and C_t. The V stands for the solution volume (L), and the W stands for the adsorbent mass, which is expressed in grams (g). In addition, a batch mode double-jacketed glass reactor with a 0.15 m diameter and 0.25 m height was used to conduct the photocatalytic experiment for the residual concentrations. The reaction was started by loading the reactor with 100 mL of CRV solutions into the reactor and adding a TiO₂ photocatalyst. A stirring paddle was used to stir the reaction at a controlled speed of 250 rpm enabling flow spins to avoid the axis line and straight shape formation. At about 40–45 min, dark equilibrium was achieved in the reactor, and the reaction system was exposed to UV light (275–400 nm) of 60 W UV light irradiation at the intensity of 4 μW/cm at a distance of 2 cm. Also, bland samples left in the dark showed no absorption. After swirling for 30 min, the TiO₂ powder settled towards the back of the reactor, and the concentrations of the remaining clear CRV were measured using a UV–Vis spectrophotometer (Mekatel et al. 2019).

The C = O double bonds in PSP (1,848 and 1,490 cm⁻¹) blue-shifted to triple C ≡ C bonds functional group in PSA as observed in 2,094 and 1,982 cm⁻¹, respectively, owing to bond breaking and new bond formation. Acyl group, C–H bonds, and Si–O bonds in PSA can be found at 1,796, 1,397, and 1,103 cm⁻¹, respectively, but are observed to be amines and amides in PSP owing to improvement in the pore sizes. The prominent peaks at 1,088.4 and 693.3 cm⁻¹ in PSP are blue-shifted to 1,307.8 and 872.2 cm⁻¹ in PSA due to the thermal activation of PSP to PSA, which increased the pore diameter for improved sorption process. This change in absorbance has a consequent increase in ultraviolet light absorbed and increased electrons that will drive the photocatalytic process as proposed (Abdelmalik & Sadiq 2019; Olusola & Babayemi 2019).

The mesoporous pore sizes, width, and diameter of the adsorbent were determined by the BET surface analyzer. The surface area increased from 628 to 831 m²/g, while the pore width almost doubled the width from 2.92 to 3.82 nm. Micropore volume by the Dubinin–Radushkevich method increased from 0.27 to 0.48 cc/g, likewise the BJ method confirmed an increase in diameter from 0.37 to 0.46 nm. From the data (Table 2) obtained, there was a significant improvement in the pore structure of the PSA for enhanced adsorption of dye molecules, owing to the high-temperature treatment of the adsorbent (Gbenebor et al. 2017).

The differential thermal analysis (DTA) confirmed the apparent difference between the adsorbents. The PSA was stable at 500 °C, while the PSP stabilized at 400 °C. The increase in temperature from 200 to 500 °C caused the decomposition and escape of volatile elements and compounds in the adsorbents. In the last phase, both PSP and PSA attained constant weight regardless of a further increase in the temperature. Hence, this confirmed that the thermal treatment of PSP increased the presence of highly stable elements like Fe and Si but reduced other components like Ca and Cl. This difference is consistent and in agreement with the findings in SEM, FTIR, XRD, and BET (Awokoya et al. 2016; Gbenebor et al. 2017).

At a pH of 9, an adsorbate concentration of 50 mg/L in 100 mL, a temperature of 293 K, and a dosage of 0.2 g/L of PSA, CRV dye removal from PSA was tested to see how contact time affected the results (Figure 7(b)). The adsorption kinetics for CRV showed rapid adsorption from 30 to 120 min, and a further increase in contact time showed little adsorption of CRV.

Furthermore, equilibrium was attained at 120 min, and the values remained constant afterwards. The explanation for this behaviour is that there are specific sites available for adsorption, and specific adsorbate molecules occupy the sites. Increasing the contact time increased the kinetic energy for the dye molecules to reach the sites faster and eventual equilibrium at 120 min; hence, the further increase would not drive faster kinetics.

The amount of adsorbent required per unit reaction was conducted to determine the maximum amount of PSA required for the adsorption of CRV dye (Figure 7(c)). The experiments were performed from 1 to 10 g/L using a 100 mL solution of CRV dye, having a 50 mg/L concentration at pH 9. The temperature was kept at 293 K and the contact time of 120 min. The graph showed the rapid adsorption of dyes when the dosage amount increased from 0.05 to 0.20 g to attain maximum equilibrium and afterwards decreased slightly from 90 to 87% of adsorbent. As a result, it was shown that after reaching the maximum adsorbent dose of 0.20 g, an increase in PSA will result in a decrease in reaction kinetics due to the lack of adsorbate molecules or adsorption sites.

The effect of initial concentration was conducted using the optimum parameters derived from this study at dye concentrations in the range of 30–70 mg/L. The pH was kept at 9, the temperature was 293 K, the dosage amount was 0.20 g, and the contact time at 120 min. The data plot (Figure 7(d)) showed that as the concentration increased, the degradation efficiency decreased. The explanation is that increasing the concentration increases the number of dye molecules available against a fixed number of available sites on the PSA adsorbent. Hence, the molecules compete with each other over the available adsorption sites on the PSA and created saturation (repulsion) between the dye molecules, leading to an eventual decrease in adsorption removal.

The pseudo-first-order kinetics failed to adequately characterize the data, according to the regression coefficient (R²) values in Table 3, while the pseudo-second-order kinetics was an excellent fit for the kinetic data, which was close to the unit value. Thus, against the pseudo-first-order that describes a physisorption process, the data were well described by the pseudo-second-order, which is a chemisorption mechanism driving the adsorption of CRV (Gunorubon & Chukwunonso 2018; El maguana et al. 2020). In chemisorption, the dyes stick to the surface of PSA via a covalent chemical bond and get attracted to sites that form dye–metal complexes. This occurs because both dyes contain one or more carboxyl, hydroxyl, and amino groups that form coordination complexes with the metal ions.

The data were plotted (Figure 9) but did not pass through the origin. Therefore, the model confirmed that intra-particle diffusion was not rate determining. But that the process is either controlled by film diffusion or both intra-particle diffusion and film diffusion.

Adsorption isotherms are important in describing the interactive behaviour between CRV over PSA adsorbent and in understanding the reaction mechanism. For this study, the Langmuir and Freundlich isotherm was selected to determine the relationship between the nature and the type of equilibrium concentrations existing between the adsorbent and the adsorbate.

Herein, C_e (mg/L) stands for the equilibrium of dye concentrations, and the rate constant is denoted by k_f. The value of the 1/n parameter indicates the intensity of absorption and the surface heterogeneous measure. It has a value from 0 to 1 and indicates the favourability of the reaction process (Uzosike et al. 2022). The plot of lnC_e (mg/g) against lnC_e (mg/L) represented well-fitted adsorption for Freundlich isotherm from the plotted experimental data (Figure 10) as indicated by the R² value of 0.996 (Table 4). Hence, the graph indicates the monolayer adsorption process in the CRV dyes over the adsorption sites created by the periwinkle adsorbent. The values of k_f were equally determined to be 1, while the values of 1/n were less than 0.5 and strongly suggest heterogeneous surface adsorption.

Hence, the CRV dyes were strongly absorbed by the PSA as similarly observed in adsorption studies of coconut shells (Pires et al. 2020), charred rice husk (Homagai et al. 2022), and enhanced adsorption from shrimp shells (Gopi et al. 2016).

In addition, the behaviour of CRV adsorption mechanisms was determined using other thermodynamic parameters, enthalpy (ΔH⁰), and entropy (ΔS⁰). Using the concentrations provided in Equation (14), the equilibrium constant (K_c) was computed. The intercept and slope of the linear graph of lnK_c against 1/T were used to estimate the values of ΔS⁰ and ΔH⁰ (Table 5) by Equation (15), whereas Equation (13) was used to determine ΔG⁰.

The calculated negative values of Gibbs energy showed that the process was spontaneous. However, an increase in temperature causes a decrease in Gibbs energy for CRV. Although it showed a dynamic and significant force of adsorption; however, it points to the fact that the adsorption is thermodynamically favourable to CRV. The dye's negative enthalpy and entropy values, which indicated a decrease in randomness at the solid–liquid border, proved that the reaction was exothermic. It is interesting to note that CRV dye adsorption has also been claimed to exhibit negative enthalpy (Sakin et al. 2018).

From the studies conducted so far, the adsorption process significantly reduced the concentrations of CRV dyes, but could not lower it comparably to surface water standards. To achieve this, a TiO₂ catalyst was added to the reaction vessel and the photodegradation process proceeded effectively.

To understand the effects of exposure to ultraviolet light (275–400 nm) at a power of 60 W at an intensity of 4 μW/cm at a distance of 2 cm over photodegradation of dye residue concentration by TiO₂, it is crucial to investigate the effects of the two major parameters, catalyst dose and CRV concentration.

As a result, the presence of TiO₂ increased the number of adsorption sites that were available due to the increased synthesis of hydroxyl radicals by TiO₂, which decreased after the suspension of opacity increased and started light scattering. On the other hand, Figure 11(a) achieved maximum degradation efficiency at the catalyst dosage of 0.75 g, and the same amount (0.75 g) was used to determine the function of concentration in Figure 11(b). The graph shows that as concentration increases, photodegradation reduces. The inverse relationship shows that even though catalyst dosage improved photodegradation from 93 to 98%; however, higher concentrations of adsorbate initiated a reverse order and required a higher amount of catalyst, and hence, electron excitation from the valance band to the conduction band is reduced. A similar finding was observed by Zhang (2022) in rhodamine B dye and Osarumwense et al. (2015) in the photodegradation of phenol by periwinkle ash under sunlight.

The calculated R², k, and K constants of Langmuir–Hinshelwood were 0.995, 0.744, and 11, respectively, for the dye. As catalyst dosage increased, the findings showed an excellent correlation coefficient (R²) and improved adsorption capacity (k). From the current study, the data fitted well into the Langmuir isotherm (Figure 10), and the linear correlation between the concentration (Figure 12). It showed the adsorption equilibrium continued during the photodegradation process by TiO₂. Hence, it confirmed that the reaction mechanism was faster than that of the photocatalysis reaction having electrons and electron holes. It is also referred to as limited light intensity which depicts that the combined photoabsorption is the rate-determining step for the removal of CRV dyes in the sorption process.

The effectiveness of PSA as a biosorbent for removing CRV dye from an aqueous solution revealed the material's tremendous potential as a low-cost and sustainable alternative for toxic water treatment. Under all the ideal experimental conditions, CRV removal efficiency from aqueous water samples attained 99% removal efficiency. The usage of thermally modified PSA was a benefit of this method. The results indicated that the rate-determining step of adsorption was combined photoabsorption between the CRV and the surface of the adsorbent, through Langmuir–Hinshelwood kinetics. The pseudo-second-order model best describes the process, and the Langmuir–Freundlich model worked well for the adsorption isotherm. The maximum adsorption capacity, 46.82 mg/g, was commensurate with the amount predicted by theory. The satisfactory maximum adsorption capacity obtained and affordable cost of PSA adsorbent mean that PSA can be considered a reliable material for the removal of CRV from aqueous effluents and is suitable for future research.

The Federal University of Technology Owerri in Owerri, Imo State, Nigeria, provided technicians for this work, and the authors thank them for their efforts.

Amarachi Nkwoada: conceptualization, methodology, data curation, writing – original draft, investigation, formal analysis, and funding acquisition. Gerald Onyedika: methodology, formal analysis, and supervision. Martin Ogwuegbu: project administration, supervision, and review. Emeka E. Oguzie: conceptualization, methodology, writing – review and editing, and supervision.

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

The authors declare there is no conflict.