Mass transfer process in the adsorption of Cibacron Blue (CB) on untreated pea pods (UPPs): kinetic and thermodynamic studies

With the rapid development of industry, organic dyes are widely used in fields such as textiles, pharmaceuticals, leather, and cosmetics and it has been stated that at least 15% of the total dye is wasted each year. Substantial amounts of synthetic dyes, extremely resistant to biodegradation, have accumulated in the environment. Dyes enter the human body through food and skin contact, so reducing their application to values below the limit is also an important ecological and medical problem. Several industrial sectors stand out in the environmental scenario as major pollutants, mainly in the textile field, due to the volume of effluents generated by synthetic dyes that do not bind completely to the fiber during the dyeing process (Inyinbor et al. 2016). The widespread use of synthetic dyes has caused serious problems for human health and aquatic life due to their mutagenic, carcinogenic, and toxic properties, and colored wastewater is a direct result of their production. Indeed, many dyes and their degradation products are toxic to living organisms (Abbas 2020, 2021a, 2021b, 2021c, 2022a; Abbas & Trari 2023a). The discharge of industrial effluents generated by many sectors, in particular paints, plastics, paper, leather, and textiles, is becoming a major concern due to their negative effects on the environment. The availability of fresh water is an important requirement for growth and development. It is not only a basic human convenience, but also a prime requirement for development. Various water-intensive activities, such as irrigation, sanitation, and hydropower generation, are dependent on the availability of freshwater resources (Wallace 2000). Developed countries are vulnerable to climate change and critical constraints to food security depend on rainfall. The impacts of climate variability are manifested mainly through changes in temperature and precipitation, which are expected to reduce water availability (Vörösmarty et al. 2010). Among the direct consequences of human-induced climate change, one can cite the following: (i) rising temperatures, (ii) sea level rise, (iii) increasing periods of aridity and drought, and(iv) melting glaciers. More details about the climate changes and their consequences and are given elsewhere (Vörösmarty et al. 2010). Dyes are characterized by high molecular weight and complex chemical structures make organic dyes difficult to degrade under natural conditions. Moreover, most organic dyes are mutagenic, teratogenic, and carcinogenic biological toxins, as well as highly soluble and color saturated, which seriously jeopardizes the safety of biological species and global ecosystems. Cibacron Blue (CB) is one such dye, commonly used in dyeing and textiles, and it is poorly biodegradable. Thus, due to the harmful effects, wastewater containing CB must be treated at the source. Therefore, proper treatment of industrial waters that release lead into aquatic and terrestrial environments is of paramount importance (Singanan et al. 2005). Some processes, including mechanical (e.g., filtration and reverse osmosis), physical (e.g., adsorption, extraction, and flocculation), chemical (e.g., precipitation, oxidation, ion exchange, and sonolysis), and thermal (e.g., evaporation and distillation) methods have been used to remediate the contaminated water by dyes and other pollutants (Abbas & Trari 2020a, 2020b; Abbas et al. 2020; Abbas 2022b). Among the treatment strategies, adsorption is considered the most universal water treatment technology and preferable method for removing pollutants from wastewater; this method is low in cost, with simplicity of design, high removal efficiency, ease of operation, and availability. Agricultural waste that is evaluated for heavy metal and dye removal has advantages as an adsorbent: readily available and existing in abundance, it is cost-effective, renewable, requires less processing time, offers suitable adsorption capability, and can be easily regenerated. Agricultural waste can be modified by treating it with different chemical agents, e.g., alkalis, acids, organic compounds, etc., or thermally – this modification could have beneficial effects on chemical/physical properties including increasing surface area (SA), improving pore structure, adding a functional group, for instance, amino (–NH₂), carboxyl (–COOH), and hydroxyl (–OH) on their surface. Modified adsorbents exhibit adsorption capacity, are more selective and sensitive. Many agricultural wastes such as cotton stalk, olive stones, coconut shell, rice husk, orange peel, pistachio shell, wheat bran, walnut shell, pomegranate peel, natural clay, Lagerstroemia speciosa seed, and biomaterials are used in adsorption applications (Abbas 2022b, c; Abbas & Trari 2023b, 2023c; Merrad et al. 2023; Das et al. 2024). Today, global challenges focus on the following: reducing the environmental impact caused by water pollution, reducing industrial waste that can cause an imbalance in the ecosystem, and raise public awareness of the consequences of global warming. The main objective of this study is to recover natural waste in the water treatment. The study aims to examine the adsorption capacity of untreated pea pods prepared for the adsorption of CB. UPP was characterized by physicochemical methods and the effect of operational parameters, namely pH, contact time, adsorbent dose, particle size, CB concentration, and temperature, were undertaken. Similarly, the experimental data were subjected to equilibrium kinetics and linear regression modeling and validated using a statistical error model. The novelty and ambitions of this study are focused on the following points: (1) reduce the environmental impact caused by water pollution; (2) propose solutions to reduce industrial waste that can cause an imbalance in the ecosystem;(3) raise public awareness of the consequences of global warming; (4) environmental protection means protecting humanity for life on earth; (5) comply with the rules relating to the discharge of industrial waste; (6) show that the prepared adsorbent can constitute a new potential candidate in water treatment comparable to expensive , activated carbon; and (7) finally tests on an industrial scale constitute the logical continuation of this study.

The SA of the sample clay was determined by the Brunauer–Emmett–Teller (BET)method using a AsiQuin, Automated Gas Sorption Analyzer Quantachrome Instrument Version 2.02. The specific SA and pore structure of the activated carbons were characterized by N₂ adsorption–desorption isotherms at −196 °C using the ASAP 2010 Micromeritics equipment.

Fourier Transform Infrared (FTIR) spectroscopy was used to identify the characteristic functional groups of commercial clay. Approximately 20mg of (UPP) was mixed with 80mg of dry KBr of spectroscopy quality and pressed under a pressure of 4,500 psi to form a thin disc. Then, the FTIR spectra were plotted with a Perkin Elmer 2000 infrared spectrometer in the range (4,000–400cm⁻¹) 16 times to increase the signal-to-noise ratio.

To observe the surface structure of the adsorbent (UPP) before adsorption of CB, scanning electron micrographs (SEMs) are produced with different resolutions using a Scanning Electron Microscope (JOEL-5910).

The concentration of CB content in the supernatant is determined using UV–visible spectrophotometer, and the pH of the solutions was accurately measured using a microprocessor-based pH meter of the HANNA HI 8521 type. The instrument was calibrated with commercial buffers of pH 4, pH 7, and pH 10.

The zero-point-charge pH_(zpc) of UPP, i.e. the pH for which the surface charge is zero, is determined using a procedure similar to that described elsewhere (Merrad et al. 2023). Approximately 20 mL of KNO₃ solutions (0.01 M) are placed in closed conical flasks; the pH of each solution is adjusted between 2 and 14 by adding HCl or NaOH solution. Then, 0.1 g of UPP is added and the final pH is measured after 24 h under agitation at room temperature. The zero-point-charge pH_(zpc) is the final pH versus initial pH crosses the line at final pH = initial pH.

C₀ is the CB initial concentration and C_t is the CB concentrations (mg/L) at time (t), V is the volume of solution (L), and m is the mass of UPP (g).

The equilibrium times averages 40 min but for practical reasons the adsorption experiments are run up to 50 min. With raising the initial CB concentration (C₀) from 10 to 20 mg/L, the adsorbed quantity of CB onto UPP rose from 3.86 to 8.20 mg/g. The CB ions are adsorbed initially on the external SA of UPP, which makes the adsorption rate easy and fast. When the adsorption of the external surface reaches saturation, the CB ions entered into the pores and absorb on the internal surface of the particles and such a phenomenon takes relatively longer contact time. Similar behavior was reported in the literature (Hammeed et al. 2007). The rapid adsorption kinetics recorded during the first minutes can be explained by the availability of a large number of active sites on the surface of the adsorbent material at the beginning of the adsorption process.

The decrease in the rate of adsorption over time is due to the occupation of the still vacant sites and the appearance of repulsion forces between the lead ions adsorbed by the phosphates and those that are in solution. Finally, saturation is reached after 40 min. This may be attributed to an increase of the driving force due to the concentration gradient with increasing C₀ in order to overcome the mass transfer resistance of CB between the aqueous and solid phases. Therefore, a higher initial CB concentration C₀ increases the adsorption capacity.

R_L indicates the type of isotherm: irreversible (R_L = 0), favorable (0 < R_L < 1), linear (R_L = 1), or unfavorable (R_L > 1). In this contribution, the R_L values are smaller than 1, thus confirming that the adsorption is favorable in both cases as well as the applicability of the Langmuir isotherm.

where is the Temkin adsorption energy change and is the maximum adsorption capacity. T (K) is the absolute temperature and R is the universal gas constant. The adsorption data are analyzed according to Equation (8) and the linear plot q_e versus lnC_e permits to calculate the constants A_T and B_T.

The kinetics of CB adsorption is crucial to determining the operating conditions that are optimized for a full-scale batch process. It gives the uptake rate of the adsorbate, controls the residual time of the global process, and predicts both the adsorption rate and designing of adsorption. Here also, different models were proposed to study the behavior of adsorbents and to propose the mechanisms controlling the adsorption. The experimental data of CB adsorption are examined using the pseudo-first- and pseudo-second-order kinetic models given by (Ho & McKay 1998; Lagergen 1898):

For the pseudo-first-order kinetic, the experimental data deviate from linearity, as evidenced by the low values of q_e and C_o, suggesting the inapplicability of the model for the present system. By contrast, the determination coefficient and q_e,_cal from the pseudo-second-order model agree perfectly with the experimental kinetic data; the corresponding coefficients for UPP are summarized in Table 3.

The applicability of the model suggests that the adsorption CB onto UPP is based on a physical reaction (physisorption), involving an exchange of electrons between the adsorbent and the adsorbate where the CB ions are attached to the adsorbent surface by chemical bond.

The mechanism of adsorption is complex, but the intraparticle diffusion is important in the early stages.

• (i) The first linear portion is due to intraparticle diffusion.

• (ii) The slopes of the linear parts are defined as rate parameters, characteristic of the adsorption rate in the region where the intraparticle diffusion occurs.

Initially and within a short time period, it is postulated that CB is transported to the adsorbent external surface through the film diffusion with a high rate.

After the surface saturation, the CB ions enter inside the adsorbent by intraparticle diffusion through the pores and internal surface diffusion until equilibrium is reached, evidenced by the second straight lines.

The constants of the different models deduced after modeling are grouped in Table 3, which depicts an intraparticle diffusion C value, and based on literature reports, such a high value (C > 1) is synonymous with the occurrence of substantive boundary layer effects within the system. Thus, intraparticle diffusion cannot be regarded as a probable sorption mechanism in the present study.

For pH values greater than 5.5, there is a predominance of negative charges on the surface of the adsorbent and since the CB is negatively charged, we notice repulsive electrostatic forces (two charges with the same signs), therefore the adsorption of iodine is practically impossible except for anions linked by hydrogen bonds or by Kesson forces (very weak adsorption). For pH lower than 5.5, there is a predominance of positive charges on the surface of the adsorbent, and since the CB is negatively charged, we notice attractive electrostatic forces (two charges with opposite signs), therefore the adsorption of iodine in practice is very favorable. Figure 10 clearly illustrates the predominant zones of positives and negatives charges of the adsorbent as a function of pH as well as the nature of the electrostatic forces involved in relation to the negative charge of the pollutant in the pH range studied from 2 to 8.

In order to reveal the reference of the as-prepared UPP for the removal of CB, a comparative study was undertaken with the other adsorbents in the literature (Table 5) where the maximum Langmuir adsorption capacities q_max of the different adsorbents are reported. It is shown that adsorption in the present study are well classified with q_max capacity of 30.30 mg/g at 25 °C. UPP is an interesting adsorbent for basic and acidic dyes owing to its isoelectric point pH_(pzc). Besides the many factors that have been optimized such as pH, temperature, stirring speed, initial dye concentration, and contact time, the regeneration of the prepared UPP is of intrinsic importance for the removal of dyes, not only to increase the reuse of the adsorbent, but also to improve its commercial viability.

Desorption is an unavoidable process and an intermediate stage toward adsorbent regeneration. The latter is an essential step to estimating the reutilization of any adsorbent for industrial applications, owing to the ecological concerns and to needs for sustainable development. Desorption is an inevitable process and an intermediate step toward regeneration of the adsorbent. It is essential for the reuse of any adsorbent for industrial applications, due to ecological concerns and sustainable development needs. Several methods exist for regeneration such as electrochemical, microbiological, thermal, and chemical processes.

This study has shown that the UPP prepared can be employed as an effective adsorbent for the removal of CB from aqueous solution. The Langmuir isotherms model provided a better fit of the equilibrium adsorption data one. They gave a maximum adsorption capacity of 30.30 mg/g at temperature 25 °C. The adsorption of CB ions by UPP follows a pseudo-second-order kinetic model, which relies on the assumption that physisorption may be the rate-limiting step. In physisorption, the CB ions are attached to the adsorbent surface by chemical bond and tend to find sites that maximize their coordination number with the surface. The kinetics and thermodynamic data can be further explored for the design of an adsorber for industrial effluents treatment.

The negative values of ΔG° and negative value of ΔH° indicate that the adsorption of CB onto UPP is spontaneous and exothermic over the studied range of temperatures. The negative value of ΔS° states clearly that the randomness increases at the solid–solution interface during the CB adsorption onto UPP, indicating that some structural exchange may occur among the active sites of the adsorbent and the ions.

This study is a tiny batch that gave rise to encouraging results, and we wish to achieve the adsorption tests in column mode under the conditions applicable to the treatment of industrial effluents. Furthermore, the present investigation showed that UPP is a potentially useful adsorbent for metal, acid, and basic dyes. The elimination of CB in the column mode and the degradation of dye by heterogeneous photocatalysis is the logical next step.

The authors gratefully acknowledge support from the Laboratory of Applied Chemistry and Materials (LabCAM), University of M'hamed Bougara of Boumerdes, Avenue de l'Indépendance Boumerdes, 35000, Algeria.

M.A. contributed to preparation, conceptualization, formal analysis, investigation, writing-original draft and methodology. M.T. and M.A. contributed to validation data curation, supervision, writing-review and editing. All authors have read and agreed to the published version of the paper.

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this paper.

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

The authors declare there is no conflict.