The studies investigated different adsorption materials based on carbon-based materials, e.g., fly or bottom ash, natural or modified biomass, biochar, ion exchangers (modified or unmodified), magnetic substances, and metal-organic frameworks (see Table 2). As the publications show, materials are often modified to improve the properties of adsorption materials or physical performance. Physical treatment methods, such as heating and autoclaving, and chemical processes, like pre-treatment with acids, alkalis, and organic chemicals, have been reported.

An overview of experimental parameters, optimal testing conditions, used isotherm and kinetic models, and thermodynamic parameters for all five azo dyes is listed in Supplementary Tables S1–S5.

The initial dye concentration has a significant impact on the effectiveness of removing azo dyes from an aqueous solution. Therefore, the effect of increasing initial dye concentration on the adsorption capacity and the percentage of dye removal (% DR) in the different studies was investigated in equilibrium. Supplementary Tables S1–S5 give a detailed overview of each investigated study’s observed initial dye concentration.

Overall, 21 studies investigated the effect of increasing initial dye concentration on % DR, and 49 studies tested the effect of it on adsorption capacity. In summary, increasing the initial dye concentration led to an increase in adsorption capacity in 93.9% and a decrease in adsorption capacity in 6.1% of the studies. 64% studies for CR, 63% studies for RB5, 60% studies for MO, 91% for OII, and 80% studies for MR observed that an increase in initial azo dye concentration led to an increase in adsorption capacity. For example, the adsorption of OII on modified brown macroalgae showed an increase in adsorption capacity from 20.3 to 45.5 mg/g when the initial dye concentration was elevated from 30 to 90 mg/L (Kousha et al., 2012). Similarly, the adsorption capacity of CR onto an adsorbent made of graphene oxide/chitosan fibers with etched-off silica nanoparticles (GO/CS/ETCH) increased from 32.6 to 221 mg/g when increasing initial dye concentration from 20 to 200 mg/L (Du et al., 2014). Initial dye concentration provides the required driving force (concentration gradient) to overcome the mass transfer resistance to transfer dye molecules from the bulk solution to the adsorbent surface (Vimonses et al., 2009; Mohammadi et al., 2011). Therefore, an increase in initial dye concentration intensifies the concentration gradient, enhancing the diffusion rate and number of dye molecules on the adsorbent surface, which ultimately increases the adsorption capacity (Vimonses et al., 2009; Ip et al., 2010; Mohammadi et al., 2011; Kousha et al., 2012; Mu and Ma, 2021). Additionally, higher initial dye concentration may lead to enhanced interactions, where the probability of practical collision between the dye molecule and adsorbent is higher (Mohammadi et al., 2011; Kousha et al., 2012; Gul et al., 2022). In contrast, a decrease in adsorption capacity was observed in one RB5 study and two MO studies. Wong et al. (2019) investigated the adsorption of RB5 and MO on spent tea leaves modified with polyethyleneimine. They reported a decrease in adsorption capacity of RB5 from 24.8 to 6.7 mg/g and of MO from 22.2 to 1.6 mg/g when the initial dye concentration was increased from 50 to 100 mg/L. This opposite behavior can be explained by the limited number of active sites on an adsorbent, which is only able to adsorb a target amount of dye molecules, or by a possible electrostatic repulsion between a negatively charged azo dye and adsorbent (Robati et al., 2016; Wong et al., 2019).

The % DR decreased due to increasing initial dye concentration in 85.7% of the studies and showed no increase in investigated studies. Specifically, a % DR decrease was observed in three CR, four RB5, five MO, three OII, and three MR studies. The adsorption of CR on coir pith carbon demonstrated that an increase in initial dye concentration from 20 to 80 mg/L led to a dye removal decrease from 66.5% to 30.5% (Namasivayam and Kavitha, 2002). Similarly, an adsorption study of MR on eggshell powder found a % DR decrease from 82.2% to 40.3% when increasing MR concentration from 20 to 100 mg/L (Rajoriya et al., 2021). The reason is that the number of active sites is limited to a fixed adsorbent dosage. Nearly complete coverage of binding sites on the adsorbent leads to an apparent reduction of % DR by a further increase of the initial dye concentration because dye molecules remain in the solution (Mohammadi et al., 2011; Zaghouane-Boudiaf et al., 2012; Wong et al., 2019; Zheng et al., 2019; Mu and Ma, 2021).

For some studies, the results were not as clear. The adsorption of RB5 on sunflower seed shells showed a decrease of % DR with increasing initial dye concentration in the first 30 min of the adsorption process. However, a higher % DR was observed afterward for higher initial dye concentrations (Osma et al., 2007). On the contrary, OII adsorption on a titania aerogel adsorbent started with an increase in % DR but then decreased above 200 mg/L initial dye concentration (Abramian and El-Rassy, 2009). The adsorption of MO onto a chitosan/MgO composite showed no significant change in % DR with increasing initial dye concentration from 50 to 300 mg/L (Haldorai and Shim, 2014). Haldorai and Shim (2014) explained this by the extensive surface area of the adsorbent and the electrostatic interactions between MO and the adsorbent.

The results generally revealed that the adsorption capacity tended to increase, and the % DR decreased when the initial dye concentration was raised. However, the effect of the initial dye concentration on the adsorption process depends not only on the adsorbent dosage but also on other critical parameters, such as pH, temperature, and surface charge. The investigated studies used various experimental parameters, making a comparison of the results difficult. Too high initial dye concentration and/or too little adsorbent dosage would, for example, lead to a quick coverage of all active adsorption sites and, as a result, remaining high dissolved azo dye concentrations. Nevertheless, the outcome shows the importance of using an adequate amount of adsorbent dosage for an expected initial dye concentration. We recommend the determination of a maximum sorption capacity for the investigated adsorbent.

Investigating the effect of adsorbent dosage helps to understand the effectiveness of an adsorbent and the ability of a dye to be adsorbed with minimal adsorbent dosage (Salleh et al., 2011). Applying an adequate amount of adsorbent is essential to ensure an economically viable adsorption process, as insufficient adsorbent dosage or overdosing would result in either low removal efficiency or high operation cost (Du et al., 2014). Supplementary Tables S1–S5 overview the observed adsorbent dosage in each investigated study.

Overall, 30 studies investigated the effect of increasing adsorbent dosage on % DR, and 23 tested its effect on adsorption capacity. 78.3% of the studies found a decrease in adsorption capacity with increasing adsorbent dosage, whereas the rest found an increase in adsorption capacity.

Specifically, five studies for CR, three for RB5, four for MO, five for OII, and one for MR observed that increased adsorbent dosage led to decreased adsorption capacity. For example, an increase in the adsorbent dosage of raw pinecone powder from 0.01 to 0.03 g led to a decrease in adsorption capacity from 13.4 to 6.28 mg/g (Dawood and Sen, 2012). Comparably, the adsorption of MO and CR on a MnO₂/biomass micro-composite found a decrease of adsorption capacity from 230 to 2.37 mg/g and 250 to 2.42 mg/g, respectively, when increasing the adsorbent dosage from 10 to 1000 mg (Omorogie et al., 2022). Three possible explanations exist for this behavior. Firstly, a high ratio of adsorbent to the azo dye can lead to rapid superficial adsorption of the azo dye(s), resulting in a lower dye concentration in solution, decreasing the concentration gradient as the driving force for the adsorption process (Senthil Kumar et al., 2010; Dawood and Sen, 2012). Secondly, unsaturated adsorption sites derive from the increase in adsorbent dosage with fixed initial dye concentration (Hamzeh et al., 2012; Du et al., 2014; Lai et al., 2020). Consistent with the mass balance equation where adsorption capacity is inversely proportional to adsorbent mass, increasing adsorbent dosage reduces adsorption capacity, as fewer dye molecules adsorb per unit weight (Dawood and Sen, 2012). However, this reduction in adsorption capacity does not necessarily lower % DR, which remains efficient despite higher adsorbent amounts, highlighting a distinct behavior between adsorption capacity and % DR. Thirdly, the reduction in adsorption capacity might be due to adsorbent particle clogging and aggregation, which would lead to a decrease in total adsorbent surface area and, as a result, lengthen the diffusion paths of the dye molecules (Hamzeh et al., 2012; Omorogie et al., 2022). An increase in adsorption capacity with increasing adsorbent dosage, on the other hand, was detected in two MO and OII studies, which both examined the adsorption on bottom ash and de-oiled soya (Gupta et al., 2006; Mittal et al., 2007). One MR adsorption study on an iron-based metal-organic framework loaded onto iron oxide nanoparticle adsorbent also reported the same behavior. It concluded that increased adsorbent dosage with a fixed initial dye concentration can lead to a larger adsorbent surface area, resulting in higher adsorption capacity (Dadfarnia et al., 2015).

An increase in % DR when increasing adsorbent dosage was reported in 93.3% of the studies. A decrease in % DR with increasing adsorbent dosage was reported in two studies, where the %DR initially increased with higher adsorbent dosages, reaching a peak at a certain point, before subsequently decreasing. However, these observations did not present a consistent trend across the studies. For example, a study of the adsorption of OII and RB5 on canola stalks reported a % DR increase from 41% to 51% and 41%–95%, respectively, when increasing adsorbent dosage from 2 to 10 g/L (Hamzeh et al., 2012). Similarly, MR adsorption on treated bagasse was investigated and showed a % DR increase from 67.8% to 83.2%, as the adsorbent dosage increased from 2 to 10 g/L (Saad et al., 2010). Similar to above, the increase in % DR with higher adsorbent dosage might be attributed to an increased adsorbent surface area with more active functional groups and pore volume and, thus, more available adsorption sites (Dadfarnia et al., 2015; Hamzeh et al., 2012; S; Liu et al., 2014; Vimonses et al., 2009).

Two studies, one of CR and RB5, do not have similar results. For the adsorption of CR onto a polyacrylamide grafted xanthan gum and silica nanocomposite (XG-g-PAM/SiO₂), it was found that an amount of adsorbent of 50 mg resulted in a maximum dye removal of 96.4%. In contrast, a higher adsorbent dosage of up to 150 mg led to a lower % DR (Ghorai et al., 2013). Comparably, the adsorption of RB5 on a 3D graphene oxide/high molecular weight chitosan (GO/HCS) composite found an increase in % DR when increasing the adsorbent dosage to 15 mg, likely due to more available sorption sites (Lai et al., 2020). However, the % DR decreased with a further increase of adsorbent dosage beyond 15 mg, potentially due to the agglomeration of adsorbent particles (Lai et al., 2020).

Overall, the results indicated that adsorption capacity tends to decrease and % DR increases with increasing adsorbent dosage. However, the investigated studies used a variety of experimental parameters, making comparing the results difficult. Nevertheless, these results confirm the importance of using an adequate adsorbent dosage for an expected initial dye concentration.

Another factor that affects the adsorption process is the contact time between the dye and the adsorbent. At the beginning of the adsorption, the removal rate is higher due to the large surface area of the adsorbent (Al-Amrani et al., 2022). Commonly, dye adsorption increased with longer contact time until equilibrium, and the amount of azo dyes adsorbed at equilibrium represents the adsorbent’s maximum adsorption capacity under the specific operating conditions (Al-Amrani et al., 2022). However, the adsorption rate decreases over time as fewer active sites are left on the adsorbent material until equilibrium is reached.

Therefore, in most studies, especially in the initial phase of the adsorption process, high adsorption rates were observed that eventually slowed down with time before reaching equilibrium. This demonstrates that at a high concentration, which is in the early stages of the cleaning (adsorption process), the “driving force” for adsorption is high. The initial rapid uptake of azo dyes by the adsorbents was explained by the high number of available adsorption sites on the surface area of the adsorbents at the beginning of the adsorption process, which become scarcer over time (Chen et al., 2010; Ghorai et al., 2013; Zheng et al., 2019). Decreasing adsorption rates, which were observed after this initial phase, could be attributed to the decrease in available adsorption sites and to the slower rate of diffusion of azo dye molecules from external adsorption sites on the surface area to internal adsorption sites since internal diffusion processes require more time to reach equilibrium (Chen et al., 2010; Ghorai et al., 2013; Du et al., 2014).

The time needed to reach equilibrium varied greatly among different studies and adsorbents. CR studies analyzed time spans between 10 min and 25 h. The equilibrium time for the investigated RB5 studies was significantly longer and varied between 30 min and 48 h. To reach equilibrium, more time is needed for RB5 than for CR. The reason for this could be that RB5 forms a covalent bond (Lewis, 2011), and, in contrast, CR is only fixed via physical forces, which means that equilibrium starts much more quickly. For MO studies, equilibrium was achieved between 5 min and 6 h. MO has a significantly smaller molecule size than CR and RB5, mirrored in the shorter equilibrium time. For OII, the equilibrium time ranged from 60 min to 24 h. The equilibrium for MR adsorption was achieved between 50 min and 22 h. Even though MR dye has the smallest molecule size, the equilibrium time in the investigated studies tended to be slightly longer than in MO studies, which could be explained by more vital forces/interactions in MO adsorption or due to differences in experimental parameters such as initial dye concentration or adsorbent dosage.

The pH value of a solution is essential for the dye adsorption process since it determines the electrostatic charge of a dye molecule (Salleh et al., 2011). This can impact the degree of ionization, speciation, and structure of a dye as well as affect the dissociation of the functional groups and surface charge of an adsorbent and, consequently, the rate of adsorption (Vimonses et al., 2009; Salleh et al., 2011; Dadfarnia et al., 2015).

Overall, 27 studies investigated the effect of increasing solution pH on % DR, and 32 studies tested the effect of it on adsorption capacity (see also Supplementary Tables S1–S5). 71.9% of the studies observed a decrease in adsorption capacity when increasing solution pH. Likewise, 48.2% of the studies found a reduction in % DR removal due to rising solution pH. Five studies observed no change in either % DR or adsorption capacity when increasing pH. The investigated studies did not describe an increase in adsorption capacity or % DR when increasing solution pH.

Five studies for CR, two for RB5, seven for MO, five for OII, and four for MR observed that an increase in the initial pH of the solution led to decreased adsorption capacity. For example, the adsorption of RB5 and CR on banana peel powder showed a maximum adsorption capacity at pH 3 of 26.9 and 147 mg/g, respectively, which decreased significantly when increasing pH to 10 (Munagapati et al., 2018). Likewise, Du et al. (2014) reported that CR adsorption onto GO/CS/ETCH decreased adsorption capacity from 123.7 to 99.4 mg/g when raising pH from 3 to 10. Another study investigated the effect of pH between 1 and 10 on MR adsorption on hopbush bark and found that the maximum adsorption capacity of 34 mg/g occurred at pH 1 and was reduced to 2.5 mg/g at pH 9 (Gul et al., 2022).

A decrease in % DR was observed in two CR, four RB5, four MO, two OII studies, and one MR study. The adsorption of RB5 on a graphene oxide/cross-linked chitosan composite, for instance, demonstrated a % DR of 86% at pH 2, which decreased to 36% at pH 12 (Travlou et al., 2013). Similarly, CR was almost entirely removed at pH 3 by a zeolite adsorbent but continuously decreased to less than 80% removal at pH 11 (Vimonses et al., 2009). On the contrary, pH was not a critical factor for two CR and three RB5 studies, and no change in either % DR or adsorption capacity could be observed. For example, the adsorption of CR onto a MnFe₂O₄ adsorbent showed a % DR of around 99% from pH 4 to 9 (Yang et al., 2014).

Many studies did not observe a clear trend for % DR and adsorption capacity (40.7% of the studies for % DR and 21.9% of the studies for adsorption capacity). For instance, % DR of CR on Kaolin decreased only when pH was higher than 9 (Vimonses et al., 2009). Similar behavior was observed for the adsorption of RB5 on spent tea leaves for pH higher than 7 (Wong et al., 2019) and for MO adsorption on a chitosan/MgO composite for pH higher than 8 (Haldorai and Shim, 2014). The same was reported for different MR adsorption studies on activated carbon from durian seed, activated carbon from custard apple fruit shell activated by H₃PO₄, and phosphoric acid treated sugarcane bagasse for pH above 7, 5, and 6, respectively (Saad et al., 2010; Ahmad et al., 2015; Khan et al., 2018). Omorogie et al. (2022) reported for MO and CR adsorption on a MnO₂/biomass composite that maximum adsorption was attained at pH 3, and it decreased to minimum adsorption capacity when rising pH up to 10 for MO and 8 for CR. However, further increase of pH to 12 led to increasing adsorption capacity (Omorogie et al., 2022).

The optimal pH for maximum CR adsorption ranged from 2 to 4 for 79% of the studies; one study showed maximum adsorption for pH below 9, the results of another study were independent of pH between 3 and 11, and one study did not investigate the effect of changing pH on adsorption. For RB5, the optimal pH was 0.5–3 in 38% of the studies. Eight RB5 studies did not investigate the effect of changing pH; one study found that the optimal pH for RB5 adsorption was at pH 7, and the results of another study were independent of pH with values between 2–10. The optimal pH for MO adsorption was in the range of 1–5 in 80% of the studies. Two studies found the optimal pH between 6 and 9. The optimal pH for maximum OII adsorption extended from 1 to 4.5 for eight studies. The other three studies did not test the effect of pH on adsorption. For MR, the optimal pH was 1–5 in 80% of the studies. One study found it at pH 6, and another reported the highest % DR between pH 2–6.

In conclusion, 53 studies tested the effect of changing the pH of the solution on the adsorption process, and 92.5% found that the adsorption of the five azo dyes was favored at an acidic pH between 1 and 5. In contrast, higher pH resulted in lower adsorption, which agrees with the previously reported pK_a values of the azo dyes. While the values differed for the various dyes, most said pK_a values were below or around 5, suggesting that if pH was higher than 5, the functional groups of the azo dyes were deprotonated. Hence, less acidic or basic conditions could weaken forces, reduce the attraction, or even lead to repulsion between dye molecules and adsorbent (Travlou et al., 2013). Other review studies, such as Salleh et al. (2011), have reported similar findings for the adsorption of anionic dyes by different agricultural solid wastes. They found that low solution pH resulted in a more positively charged adsorbent surface, which increased negatively charged anionic dye adsorption (Salleh et al., 2011).

The results suggest that electrostatic attraction was a vital adsorption mechanism. Nevertheless, many studies did not report a clear trend with changing pH, suggesting that other interactions were also involved in the adsorption process of azo dyes.

Various textile dye effluents have relatively high temperatures (Dawood and Sen, 2012), which could affect the dye adsorption rate and the adsorption equilibrium (Salleh et al., 2011; Dadfarnia et al., 2015). Overall, 38 studies investigated the effect of increasing temperature on adsorption capacity, and five studies tested the impact of it on % DR (see also Supplementary Tables S1–S5). 60.5% observed an increase in adsorption capacity with increasing temperature, whereas 31.6% reported a decrease in adsorption capacity.

Seven studies for CR, six for RB5, three for MO, three for OII, and four for MR observed that an increase in temperature led to a rise in adsorption capacity, indicating that adsorption was favored at higher temperatures. The adsorption of MR on activated carbon from durian seed, for instance, reported an adsorption capacity increase from 314 mg/g at 30°C to 350 mg/g at 60°C (Ahmad et al., 2015). Similarly, the adsorption of CR on calcined Ni/Mg/Al layered double oxide (NMA-LDO) showed an increase in adsorption capacity with increasing temperature from 1111 mg/g at 20°C to 1385 mg/g at 40°C (Lei et al., 2017). Additionally, the adsorption of CR and RB5 on banana peel powder reported an increase in adsorption capacity from 21.2 to 49.2 mg/g for RB5 and 112–165 mg/g for CR when elevating the temperature from 25°C to 45°C (Munagapati et al., 2018).

Contrarily, in four CR, four MO, three OII studies, and one MR study, a decrease of adsorption capacity with increasing temperature was reported, indicating that adsorption was favored at lower temperatures. For example, the adsorption capacity of OII on sludge-rice husk biochar decreased from 42.1 to 36.2 mg/g when increasing the temperature from 25°C to 45°C (Chen et al., 2019). Similar behavior was observed for MR adsorption on a metal-organic framework. The adsorption capacity decreased from 184 mg/g at 25°C to 163 mg/g at 45°C (Yılmaz et al., 2016).

The % DR was higher with increasing temperature in two out of five cases and decreased in two. In particular, the % DR increased in one MO and MR study. The % DR of MR on eggshell powder increased slightly from 82.2% at 25°C to 85.3% at 55°C (Rajoriya et al., 2021). Contrarily, the % DR decreased in one MO study and one OII study. Wong et al. (2019) reported, for example, that the % DR of MO on modified spent tea leaves reduced from 88.1% at 45 °C to 61% at 60 °C. They also studied the adsorption of RB5 on the same adsorbent and found that the % DR of RB5 was nearly constant over the whole tested temperature range (Wong et al., 2019). Other studies, one for RB5, one for MO, and two for OII, observed an increase in adsorption capacity only until 35°C (308 K) or 30°C, but a decrease for temperatures higher than those (Pedro Silva et al., 2004; Greluk and Hubicki, 2010; Gao et al., 2013; Chen et al., 2020).

On the one hand, an increase in adsorption capacity or % DR with increasing temperature might be attributed to increased surface activity and kinetic energy of the adsorbent particles due to the temperature rise (Ahmad et al., 2015; Munagapati et al., 2018), which might increase the collision frequency between adsorbents and dyes, leading to enhanced adsorption (Munagapati et al., 2018; Omorogie et al., 2022). Besides that, an increase in temperature might also increase the solubility and mobility of the dye molecules, leading to stronger interactions between dye molecules and solvent as well as an increase in diffusion rate (Salleh et al., 2011; Dawood and Sen, 2012; Ghorai et al., 2013; Lei et al., 2017; Wong et al., 2019). Moreover, at elevated temperatures, possible swelling effects within the internal structure of the adsorbent were reported, which could enable further penetration of large dye molecules and, therefore, increase adsorption (Dawood and Sen, 2012; Ghorai et al., 2013).

On the other hand, high temperatures leading to a decrease in adsorption capacity or % DR might be caused by a reduction of surface activity or by desorption due to the higher mobility of dye ions (Ghorai et al., 2013; Chen et al., 2020). Additionally, higher temperatures might weaken physical interactions between dye molecules and adsorption sites (Chatterjee et al., 2007; Vimonses et al., 2009).

Thermodynamic parameters, such as enthalpy (ΔH⁰), entropy (ΔS⁰), and standard Gibbs free energy (ΔG⁰), help to further understand the nature of an adsorption process. Examining ΔH⁰, ΔS⁰, and ΔG⁰ indicates if an adsorption process is endo- or exothermic (ΔH⁰), if it happens spontaneously (ΔG⁰), and if it occurs randomly (ΔS⁰). Supplementary Tables S1–S5 give a detailed overview of the thermodynamic parameters that were investigated in these studies. Enthalpy change was investigated in 50, entropy change in 41, and standard Gibbs free energy change in 42 studies.

As illustrated in Figure 4, 62% of the investigated studies highlighted the adsorption process to be endothermic (+ΔH⁰), whereas 38% reported it as exothermic (-ΔH⁰). The difference was described for nearly all five azo dyes.

92.9% of the studies found the adsorption process to be spontaneous (-ΔG⁰) and feasible, implying that those systems did not require energy from external sources. Only 7.1% reported that the process was not spontaneous (+ΔG⁰), indicating the presence of an energy barrier during the adsorption process.

Furthermore, 80.5% of studies reported an increase in randomness at the solid-liquid interface (+ΔS⁰) during the adsorption process, suggesting an increase in chaos and a higher degree of freedom at the adsorbate-adsorbent interface. Nevertheless, 19.5% reported a decrease in randomness at the solid-liquid interface (-ΔS⁰), which indicates that no change in the internal structure of these adsorbents occurred, which was reported in four CR, two MO, and two OII studies.

Most studies reported that the adsorption process was endothermic and spontaneous. They showed an increase in randomness at the adsorbate-adsorbent interface, which was the case for various adsorbents and azo dyes, indicating similar behavior during the adsorption process. For instance, the adsorption of CR and RB5 on banana peel powder (Munagapati et al., 2018), of RB5 on graphite oxide/cross-linked chitosan (Travlou et al., 2013), of MO and CR on a MnO₂/biomass composite (Omorogie et al., 2022), of OII on modified tea biochar (Mu and Ma, 2021), and of MR on an iron-based metal-organic framework loaded onto iron oxide nanoparticle adsorbent (Dadfarnia et al., 2015) all reported an endothermic, spontaneous adsorption process that showed an increase in randomness at the adsorbate-adsorbent interface.

Studying synergetic or competitive behavior in an adsorption process is vital to assess how well an adsorbent can perform in real wastewater conditions. Few studies investigated the synergetic or competitive adsorption behavior between different types of dyes, but none of them between various azo dyes.

For instance, Yang et al. (2014) investigated the synergetic and competitive interaction between CR and methyl blue for adsorption onto magnetic MnFe₂O₄ adsorbent. They observed competitive and synergetic effects between the two dyes. In mixed solution, methyl blue adsorption capacity decreased because of competitive interaction, whereas CR adsorption increased. The increase of CR adsorption in the presence of methyl blue was explained by the idea that CR could bind to form a 1:1 complex to already adsorbed methyl blue molecules on the adsorbent surface by hydrogen bonds and van der Waals forces, therefore providing additional adsorption sites for CR (Yang et al., 2014).

Chen et al. (2020) investigated the simultaneous adsorption of MO and methylene blue (a cationic dye) on a magnetic chitosan composite. The results indicated that, in the mixed solution, methylene blue adsorption was substantially augmented by the presence of MO molecules. Additionally, the adsorption capacity of methylene blue was proportional to the MO concentration, while MO adsorption was barely affected by the presence of methylene blue in the solution. The suggested mechanism behind this behavior was similar to the ones mentioned above. It is assumed that free MO molecules are adsorbed on the adsorbent surface by electrostatic attraction and then attract methylene blue molecules, e.g., by electrostatic attraction or π–π interaction, increasing the adsorption of methylene blue (Chen et al., 2020).

Some studies also investigated the effect of salts in the solution, which would be present in real textile wastewater. Yılmaz et al. (2016) investigated MR adsorption from simulated industrial sewage. They found that the presence of salts, especially NaCl, increased the amount of MR adsorbed onto the metal-organic framework adsorbent by 6.4% compared to MR adsorption from pure MR solution (Yılmaz et al., 2016). Liu et al. (2014) investigated the effect of ionic strength on CR adsorption onto surfactant-modified zeolite. They found that ionic strength had a weak influence on the adsorption process. Nevertheless, adsorption capacity decreased slightly from 34.9 to 33.0 mg/g as the NaCl concentration increased from 0 to 1.0 mol/L. The reason for the decrease in adsorption capacity could have been competitive adsorption between chloride anions and CR (Liu et al., 2014).