Mass transfer process in the removal of Congo Red (CR) onto Natural Clay (NC): Kinetic, isotherm modeling, and thermodynamic study

Global climate change and population growth have put pressure on water supplies. Wastewater management and potable water purification are crucial in supporting the rapid development of human society and mitigating environmental pollution and health risks. One of the major problems with wastewater is colored effluent. Discharges from different industries such as textile industries, cosmetics, paper and printing, rubber, leather, pharmaceuticals, food, leather tanning, paint manufacturing, battery manufacturing industries, and plastic industries contain many toxic pollutants, such as dyes, which harm the environment (Yu et al. 2021). There are more than 100,000 types of dyes commercially available, with more than 7 × 10⁵ tons of dyes tuff produced annually (Nasuha & Hameed 2011). Dyes can be classified into two categories: non-ionic (vat and disperse dyes) and ionic, i.e. cationic (basic) and anionic (reactive, direct and acidic) (Homaeigohar 2020). The presence of dyes in aquatic ecosystems reduces light penetration and thus decreases the photosynthesis necessary for living organisms (Krishna Moorthy et al. 2021). In addition, industrial dyes are mutagenic, toxic, and dangerous to both humans and animals (Ismail et al. 2022). Congo Red (CR) is a benzidine-based dye with two molecules of naphthenic acid and the first synthetic dye capable of directly dyeing cotton. It is very sensitive to acids and its color changes from red to blue in the presence of inorganic acids; this dye is known to metabolize benzidine, a known human carcinogen (Mall et al. 2005). Therefore, the removal of dyes from aquatic environments is a necessary process to prevent water pollution (Omar et al. 2018). In this regard, several processes, including mechanical (filtration and reverse osmosis), physical (adsorption, extraction and flocculation), chemical (precipitation, oxidation, ion exchange, ozonolysis), and thermal (evaporation and distillation) have been used to remediate water contaminated with dyes (Abbas et al. 2019; Abbas & Trari 2020a, 2020b). However, these methods have certain drawbacks such as high cost, secondary pollution, low efficiency and are not effective on a large scale (remediation of polluted soil) (Chen et al. 2015). Therefore, bioremediation using microbial species is a low-cost alternative with environmentally friendly characteristics (Elnahas et al. 2021). The adsorption is an attractive method for the removal of dyes due to its low maintenance, simple operation and removal effectiveness, especially if the adsorbent is inexpensive and readily available. In this regard, activated carbon (AC) is a versatile adsorbent used regularly for the adsorption process, but remains relatively expensive. Therefore, many researchers have used different wastes for the development of activated carbons (AC) at low economic cost, this allowed us to eliminate them from the natural environment and recover them for water treatment. Therefore, this has prompted a growing research interest in the production of ACs from renewable and cheaper precursors which are mainly industrial and agricultural by-products, for the wastewater treatment. However, the available ACs in commerce are relatively expensive, their production and regeneration cost constitute limiting factors. Hence, most researchers worldwide have focused on the search of new low-cost precursors especially issued from agricultural wastes. The adsorbent used in the present case is Natural Clay (NC) and this study was carried out with the aim to optimize conditions such as initial dye concentration (C₀), pH, contact time, adsorbent dosage, agitation speed, and temperature. In addition, the equilibrium adsorption data were fitted to various equations in order to obtain the constants related to the adsorption phenomena. Equilibrium and kinetic analysis were conducted to determine the factors controlling the adsorption rate, the optimization of various parameters in the dye recovery and to find out the possibility of using this material as low-cost adsorbent for dye removal.

• – The spectrophotometry is a technique which owes its development to progress in the quantum mechanics allowing, among other things, to identify a chemical substance and to determine the concentration of a solute in solution, by using the Beer–Lambert's law.

• – 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 values 4, 7, and 10. The pH was adjusted to by using H₂SO₄ and NaOH, respectively, for acidic and basic media.

• – The FTIR spectroscopy was used to identify the characteristic functional groups of commercial clay. Five mg of NC were mixed with spectroscopic grade dry KBr and pressed under 4,500 psi pressure to form a thin disc. Then, the FTIR spectra were plotted with a Perkin Elmer 2000 infrared spectrometer in the range (4,000–400 cm⁻¹) for 16 times to increase the signal-to-noise ratio.

• – The surface area of the sample clay was determined by the BET method using a AsiQuin, Automated Gas Sorption Analyser Quantachrome Instrument Version 2.02. The specific surface area and pore structure of ACs were characterized by N₂ adsorption–desorption isotherms at −196 °C using the ASAP 2010 Micromeritics equipment.

• – The X-ray diffraction (XRD) patterns of NC was obtained with a Philips X-ray diffractometer (PW 1890 model) operating at 40 kV, 40 mA and equipped with Cu_Kα radiation (λ = 1.54 Å). The patterns were obtained with CONIT T-2 T scan mode at 0.17°/step of step width and 8°/min of scan speed.

• – The chemical analysis was performed by the X-ray fluorescence (XRF) using Horiba XRF

• – The Zero Point Charge pH_(zpc) of the NC, i.e., the pH for which the surface charge is zero, is obtained using a procedure similar to that reported elsewhere (Abbas 2021). Briefly, 20 mL of of KNO₃ solutions (0.01 M) were placed in closed conical flasks; the pH of each solution was adjusted between 2 and 14 by addition of HCl or NaOH solution. Then, 0.1 g of CC was added and the final pH was measured after 24 h under magnetic stirring at ambient temperature; pH₍zpc₎ is the final pH versus initial pH crosses the line at final pH = initial pH.

The anionic dye used as adsorbate is CR purchased from the Nizochem Laboratory, with a formula C₃₂H₂₂N₆Na₂O₆S₂ and a molecular weight of 696.66 g/mol, respectively (Table 1). One hundred mg/L of solution was prepared by adding 0.1 g of CR in 1,000 mL of distilled water, and solutions required for the experimental study were prepared by diluting the CR stock solution to various initial adsorbate concentrations. The adsorbent used in this study is NC, provided by the edible oil refining unit (Algeria).

Table 1

General characteristics of Congo Red

Physical and chemical properties .		Chemical structure .
Brute formula	C32H22N6Na2O6S2
Molecular weight	(696.663 ± 0.004) g/mol
pKa	4
Composition (%)	C: 55.0, N: 12.06, O: 13.78
	H: 3.18, Na: 6.60, S: 9.21
Wavenumber (λmax)	494 nm
Name	Congo Red
Melting temperature	360 °C
Boiling pressure	760 mm Hg
Solubility in water	25 g/L at T = 20 °C
Solubility in alcohol	Very soluble

Physical and chemical properties .		Chemical structure .
Brute formula	C32H22N6Na2O6S2
Molecular weight	(696.663 ± 0.004) g/mol
pKa	4
Composition (%)	C: 55.0, N: 12.06, O: 13.78
	H: 3.18, Na: 6.60, S: 9.21
Wavenumber (λmax)	494 nm
Name	Congo Red
Melting temperature	360 °C
Boiling pressure	760 mm Hg
Solubility in water	25 g/L at T = 20 °C
Solubility in alcohol	Very soluble

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The XRF was performed to determine the chemical composition of the adsorbent. The content of elements present in NC and the main mineralogical constituents are silica and alumina, thus confirming the presence of Si, Al, Mg, Fe, K, Ca and Na. These results corroborate the XRF analysis, which indicates the presence of these elements in oxides: SiO₂ (53.4% in mass), Al₂O₃ (4%), Fe₂O₃ (1.5%), MgO (30.5%), Na₂O (0.3%), CaO (0.7%), and K₂O (1%). The loss on ignition at 1,000 °C of this material is 8.5%.

According to the results of determining pHpzc of clay, the surface of our adsorbent acquires a positive charge in an acidic medium by absorbing H⁺ ions, and adsorbs the RC negatively charged due to the sulfonated group by electrostatic attraction, which leads to greater adsorption capacity of clay. On the other hand, in a basic medium (pH > 8), the clay surface is negatively charged by absorbing OH⁻ ions and can reject the negatively charged RC due to the sulfonated group by electrostatic repulsion, resulting in lower adsorption capacity of the clay (Yandri et al. 2023). The plateau observed between pH 7 and 8, can be explained by the fact that the surface of our adsorbent is neutral in this area (pHpzc of clay corresponds to a pH between 7 and 8).

The adsorption kinetics is important for the development of the adsorption system design, which determines the time required for reaching the equilibrium for the adsorption process (Rangan Sahoo & Prelot 2020). Several models describing the diffusion of solutes at the surface and in the pores of adsorbent have been developed to explain the adsorption kinetics. The pseudo-first-order, pseudo-second-order, Elovich, and intraparticle diffusion models are used in this study to examine the adsorption rates of CR onto NC.

K_in is the intraparticle diffusion rate (mg/gmin^1/2), q_t the amount of iodine adsorbed at time t and C (mg/g) the intercept. The constants of the various kinetic models along with the calculation of statistical errors obtained after modeling are grouped in Table 2. The relation between q_t and t^1/2 displayed a multi-linear plot (i.e., different linear stages) of the iodine experimental data, which reflects different diffusion types. The first sharp stage explains the external mass transfer of iodine from the contaminated solution to the outside surface of the developed NC adsorbent. The second and last stages reflect the pore diffusion and equilibrium phases, respectively. Consequently, the adsorption of iodine molecules on the adsorbent was directed by more than one mechanism (i.e., chemical reaction and pore diffusion are involved in the uptake of iodine by the tested adsorbent). If the intraparticle diffusion occurs, the plot q_t against t^0.5 should be linear and the line should pass by the origin, indicating that intraparticle diffusion is the only rate-limiting parameter controlling the process. Otherwise, some other mechanisms are also involved. The intercept gives an indication of the thickness of the boundary layer, i.e., the larger the intercept the greater is the boundary layer effect (Kannan & Sundaram 2001).

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, R_L is smaller than 1, thus confirming that the adsorption is favourable in both cases as well as the applicability of the Langmuir isotherm.

The viability of any adsorbent on a commercial scale depends primarily on its recyclability. The reactivation of active sites of the adsorbent surface from adsorbed molecules is a basic step to enter new adsorption cycle. In this study, the adsorbent NC washed by acidic solvent followed by drying in incubator at 60 °C for 1 h to desorbed CR molecules from adsorbent to the solution. The removal percentages are given during three continuous cycles. The diagram showed the deactivation effect of adsorption efficiency from 55.5% in the first cycle to 25.3% at three cycles. The decreasing in the adsorption efficiency is due to the partial coverage of NC active sites by the CR molecules which are not easy to desorb from the adsorbent surface.

In order to demonstrate the effectiveness of the adsorbent (NC) for the adsorption of CR in an aqueous medium, the results obtained were compared with other adsorbents reported in the open literature (Table 5). The maximum adsorption capacity is used as a comparative parameter. It should be noted that the maximum absorption capacity obtained for NC is satisfactory compared to other adsorbents, this result shows that our adsorbent is a good attractive candidate for its contribution in the treatment of industrial effluents. Combination tests of this adsorbent with TiO₂, SnO₂, and CaTiO₃ as semiconducting photocatalysts are possible for the development of hybrid compounds with applications in heterogeneous photocatalysis.

The experimental study on the utilization of NC was used to remove CR from aqueous solutions. The influence of different physical parameters such as pH, initial iodine concentration, contact time, adsorbent dose, agitation speed, and temperature was examined. The adsorption capacity increased with augmenting the initial CR concentration, time and the maximum adsorption was obtained at the optimal pH of ∼2. The kinetics of CR removal showed an optimum contact time of 40 min via a two-stage adsorption profile with an initial fast step followed by a slow equilibrium. The adsorption kinetic follows a pseudo-second-order model with a determination coefficient of R² close to unity, which relies on the assumption that the chemisorption is the rate-limiting step where the CR ions are chemically bonded to the adsorbent surface and tend to find sites which maximize their coordination number with the surface. The equilibrium adsorption data were analyzed, indicating that the Langmuir model provides the best correlation (212.766 mg/g at 25 °C) with a homogenous adsorption of CR on monolayer NC sorption sites. The adsorption isotherms at different temperatures have been used for the determination of the free energy ΔG⁰, enthalpy and entropy. The negative ΔG⁰ and positive ΔH⁰ indicated a spontaneous and endothermic nature of the reaction. The comparison of the adsorption capacity of the prepared adsorbent with the literature showed its attractive properties from industrial and economic interests. The combination of high adsorption capacity and fast equilibrium suggests that this material is a noteworthy candidate for the wastewater treatment.

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

The authors declare there is no conflict.