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Abstract

In this paper adsorptive removal of hazardous dye C.I. Reactive Black 5 from aqueous solutions was investigated using the mixed silica–alumina oxides including of 4% SiO₂ and 96% Al₂O₃ as well as 97% SiO₂ and 3% Al₂O₃. The kinetic studies revealed that with the increasing initial dye concentration from 10 to 30 mg/l and contact time from 1 to 240 min, the sorption capacities (q_t) increased and the equilibrium was observed after 240 min. Sorption of C.I. Reactive Black 5 on 4% SiO₂ and 96% Al₂O₃ and 97% SiO₂ and 3% Al₂O₃ takes place through the pseudo-second-order mechanism rather than the pseudo-first-order one. The experimental data were fitted using the Langmuir and Freundlich isotherm models. The effect of the auxiliaries such as anionic surfactant (sodium dodecyl sulphate) and sodium chloride on C.I. Reactive Black 5 removal was investigated in the 10 mg/l C.I. Reactive Black 5–0.1–1 g/l sodium dodecyl sulphate or 5–20 g/l NaCl systems. The potentiometric titrations indicated that the presence of C.I. Reactive Black 5 changed the surface charge density of silica–alumina mixed oxides, especially in the case of 4% SiO₂ and 96% Al₂O₃ system.

Keywords

Dye removal Reactive Black 5 anionic dye sorption mixed silica–alumina oxide solid surface charge density

Introduction

Continuous development of industry and still growing demand for effective chemical compounds used in various technological processes makes that the generated wastewaters are characterized by a complex composition. They commonly contain many harmful substances which are present in both molecular dispersion and colloidal state. The toxic effects of these compounds result not only from their chemical structure but also from their colour. Among these substances a large group is represented by inorganic pigments (e.g. metal oxides such as chromium(III) oxide, titanium(IV) oxide, zinc(II) oxide) and organic dyes (Sangeetha et al., 2012). For example, the colloidal chromium(III) oxide is non-toxic. However, due to its intense green colour Cr₂O₃ is hazardous for aquatic ecosystems. Even small amounts of chromium(III) oxide change water colour and significantly reduce the penetration of sunlight into the deeper layers of aquatic reservoirs. As a result, the inhibition of the photosynthesis processes and death of the living organisms take place (Sarker et al., 2013). The same negative effect on the aquatic systems is observed for wastewater streams containing different types of dyes.

The basic source of dyes in industrial wastewaters is technological processes performed in textile, paper, furrier and tanning plants. The largest amounts of organic dyes are produced during the chemical treatment of textile products. This results from the fact that over 100,000 dyes are commercially available and their worldwide production is estimated to be in the range 700,000–1,000,000 ton. Consequently, every year 280,000 ton of textile dyes are released into the environment with industrial wastewaters (Pang and Abdullah, 2013).

Removal of these undesirable substances from wastewaters takes place in many ways using a great variety of physicochemical methods. The most important of them are coagulation, flocculation, co-precipitation, electrolysis, flotation, oxidation, filtration through membranes. Most of these techniques are based on the phenomenon of adsorption. For this purpose, many different types of adsorbents (both porous and non-porous) are used to separate dangerous molecules from the aqueous environment. The group of porous adsorbents includes activated carbons, silica gels, zeolites (natural and synthetic), anion exchangers, porous glasses and metal oxides (Bandura et al., 2015; Chałupnik et al., 2013; Nowicki, 2016; Nowicki et al., 2016, Wawrzkiewicz and Hubicki, 2015).

To obtain the specific surface properties as regards a particulate adsorbate mixed oxides can be also applied. Different textural and physicochemical properties of mixed oxides (compared to simple oxides) make them in many cases very effective adsorbents in various technological processes. It has been shown in the literature that silica-containing mixed oxides can be used as adsorbents for simple molecules and ions, as well as macromolecular compounds (Wawrzkiewicz et al., 2015; Wiśniewska et al., 2016, 2015, 2013).

In the present paper, the possibilities of adsorptive removal of C.I. Reactive Black 5 (RB5) dye from aqueous solutions and wastewaters were investigated using two mixed silica–alumina oxides of different compositions. They vary in the silica content which results in different surface properties as regards the anionic dye binding with the solid and its separation from the liquid phase.

Experimental

Materials

Two silica–alumina mixed oxides including 4% SiO₂ and 96% Al₂O₃ (SA96), as well as 97% SiO₂ and 3% Al₂O₃ (SA3) were used in the study as adsorbents. These solids were prepared using the pyrogenic method at pilot plant in the Chuiko Institute of Surface Chemistry (Kiev, Ukraine). This type of synthesis allows obtaining a high surface concentration of bridging groups Si–O–Al on the mixed oxides surface, and the surface concentration of components differs significantly from the bulk concentration (Gun'ko et al., 2007; Gun'ko and Turov, 2013; Shpak and Gorbyk, 2010; Zarko et al., 1997). The BET surface area and the mean pore diameter were determined by the low-temperature nitrogen adsorption–desorption isotherm method (Micromeritics ASAP 2405 analyser). SA96 and SA3 adsorbents were characterized by the following values of specific surface area and the mean pore diameter: 75 m²/g, 7.4 nm and 302 m²/g, 7.7 nm, respectively.

The textile dye C.I. Reactive Black 5 (RB5) (C₂₆H₂₁N₅Na₄O₁₉S₆, C.I.20505) is tetrasodium salt of 4-amino-5-hydroxy-3,6-bis((4-((2-(sulphooxy)ethyl)sulphonyl)phenyl)azo)-2,7 naphthalenedisulphonic acid. C.I. Reactive Black 5 dye was selected for the studies because it is extensively used in the textile industry for cotton and other cellulose fibre dyeing; it contributes to 50% of the total world demand for reactive dyes (Mook et al., 2016). It was purchased from Sigma-Aldrich (Germany) and used on commercially available purity level of 55%. The stock solution of the dye was prepared in distilled water, and working solutions were obtained by appropriate dilution.

Methods

The batch technique was applied in order to determine the amounts of dye sorbed after different time intervals (q_t) and at equilibrium (q_e) by the mixed oxides using equations (1) and (2)

q_{t} = \frac{(C_{0} - C_{t})}{m} V

(1)

q_{e} = \frac{(C_{0} - C_{e})}{m} V

(2)

where C₀ (mg/l) is the initial concentration of dye, C_t (mg/l) is the concentration of dye after sorption time t, C_e (mg/l) is the equilibrium concentration of dye, V (l) is the volume of solution and m (g) is the mass of the mixed oxide.

In order to determine kinetic parameters of dye sorption on SA96 or SA3, the experimental conditions such as the phase contact time and the initial dye concentration were as follows: t = 1, 3, 5, 10, 15, 30, 60, 120, 180, 240 min; C₀ = 10, 20, 30 mg RB5/l, pH = 6.0, V = 20 ml, m = 0.02 g, amplitude A = 180 r/min, T = 20℃. Initial pH of the suspensions was as follows: 5.14 for 10 mg/l RB-SA96, 5.63 for 20 mg/l RB5-SA96, 6.54 for 30 mg/l RB5-SA96, 5.44 for 10 mg/l RB5-SA3, 5.5 for 20 mg/l RB5-SA3 and 5.51 for 30 mg/l RB5-SA3.

The equilibrium studies were carried out under the following conditions: t = 24 h, C₀ = 10–250 mg RB5/l, V = 20 ml, m = 0.02 g, A = 180 r/min, T = 20℃. The effect of sodium chloride and anionic surfactant sodium dodecyl sulphate (SDS), frequently present in textile wastewaters, on the dyes uptake at equilibrium was studied in the RB5-SA96 system, under the following experimental conditions: C₀ = 10 mg RB5/l, 5–20 g/l NaCl, 0.1-1.0 g/l SDS, t = 24 h, V = 20 ml, m = 0.02 g, A = 180 r/min, T = 20℃. The dye concentration in aqueous phase was determined spectrophotometrically (Cary 60, Agilent) at 598 nm (for RB5 and RB5–NaCl systems) and 591 nm (for RB5–SDS systems).

The potentiometric titration method (Janusz, 1999) was used for determination of the surface charge density (σ₀) and pH_pzc point (pzc – point of zero charge) of the mixed oxides without and with the dye. The surface charge density was calculated from the difference in the volume (ΔV) of base (NaOH, 0.1 mol/l) which must be added to bring the pH of suspension and electrolyte to the specified value using the below equation

σ_{0} = \frac{Δ V c_{b} F}{mS}

(3)

where c_b (mol/l) is the base concentration, F (C/mol) is the Faraday constant, m (g) is the solid mass in the suspension and S (m²/g) is the specific surface area of the solid.

The experimental set-up for these measurements included Teflon vessel, automatic burette Dosimat 665 (Metrohm), thermostat RE204 (Lauda), pH-meter 71 pH meter (Beckman) and computer with the special program Titr_v3 (author: W Janusz). A total of 0.05 g of SA-3 (or 0.15 g of SA-96) was added into the thermostated vessel to 50 ml of water (or aqueous solution of RB5 with the concentrations 10, 20 and 30 mg/l).

Results and discussion

Kinetic and equilibrium studies

Kinetic experiments as well as equilibrium isotherm studies are one of the most important parts of adsorption investigations because they reveal the character of interactions between the adsorbent and the adsorbate. In the present paper, the simplified equations of adsorption kinetics such as the pseudo-first-order equation (PFO) proposed by Lagergren (1898) and the pseudo-second order kinetic equation (PSO) by Ho and McKay (1998) were applied as follows

\log (q_{e} - q_{t}) = \log q_{e} - \frac{k_{1}}{2.303} t

(4)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{e}^{2}} + \frac{1}{q_{e}} t

(5)

h = k_{2} q_{e}^{2}

(6)

where q_e and q_t (mg/g) are the amounts of dye adsorbed by the mixed oxides at equilibrium and time t, respectively; k₁ (l/min) is the rate constant of the PFO kinetic model, k₂ (g/mg min) is the rate constant of the PSO kinetic model and h (mg/g min) is the initial adsorption rate.

The equilibrium adsorption capacity (q_e) and adsorption rate constants (k₁ and k₂) were calculated using the experimentally based plots log (q_e−q_t) versus t and plot t/q_t versus t. On the basis of the calculated data, listed in Table 1, it can be stated that the PSO equation described better RB5 adsorption on SA3 and SA96 than the PFO model (low values of R², q_e,exp differ significantly from q_e). The values of q_e determined from the PSO model were close to the experimental ones q_e,exp. The determination coefficients R² being in the range of 0.997–0.999 confirmed the applicability of the Ho and McKay model in the 10–30 mg/l RB5–SA96/SA-3 systems. The PSO rate adsorption constants k₂ decreased from 0.072 to 0.061 g/mg min for SA3 and from 0.0.113 to 0.063 g/mg min for SA96 with the increasing concentration of RB5 in the range from 10 to 30 mg/l. This indicates that the chemisorption is the rate-controlling step in the dye-mixed oxide systems. Confirmation of applicability of PSO model for dyes sorption on different types of silica and alumina oxides was reported by Anbia and Salehi (2012), Arshadi et al. (2013) and Banerjee et al. (2014).

Table 1.

Kinetic parameters and equilibrium adsorption data of RB5 on SA3 and SA96.

SA3
Kinetic parameters
C₀ (mg/l)	q_e,exp (mg/g)	PFO			PSO
C₀ (mg/l)	q_e,exp (mg/g)	q_e (mg/g)	k₁ (l/min)	R ²	q_e (mg/g)	k₂ (g/mg min)	h (mg/g min)	R ²
10	6.4	3.7	0.074	0.943	6.2	0.072	2.7	0.999
20	7.2	3.6	0.048	0.697	7.5	0.068	4.3	0.997
30	9.9	4.0	0.041	0.926	9.7	0.061	6.0	0.998

Isotherm parameters
Freundlich model			Langmuir model
k_F (mg/g)	1/n	R ²	Q₀ (mg/g)	b (l/g)	R ²
3.3	0.470	0.954	14.1	0.435	0.949

SA96
Kinetic parameters
C₀ (mg/l)	q_e,exp (mg/g)	PFO			PSO
C₀ (mg/l)	q_e,exp (mg/g)	q_e (mg/g)	k₁ (l/min)	R ²	q_e (mg/g)	k₂ (g/mg min)	h (mg/g min)	R ²
10	6.8	2.7	0.030	0.881	6.0	0.113	4.1	0.998
20	7.2	3.8	0.044	0.690	6.4	0.077	3.2	0.997
30	9.7	5.1	0.031	0.912	9.0	0.063	5.1	0.998

Isotherm parameters
Freundlich model			Langmuir model
k_F (mg/g)	1/n	R ²	Q₀ (mg/g)	b (l/g)	R ²
9.3	0.351	0.630	47.1	0.176	0.992

PFO: pseudo-first-order equation; PSO: pseudo-second-order equation; RB5: Reactive Black 5; SA3: 97% SiO₂ and 3% Al₂O₃; SA96: 4% SiO₂ and 96% Al₂O₃.

The sorption equilibrium data of RB5 onto SA96 and SA3 were modelled using the most popular isotherm proposed by Langmuir and Freundlich (Foo and Hameed, 2010). Comparing the data presented in Table 1, especially the values of the determination coefficients R², it can be stated that both models fit the experimental sorption data in the RB5–SA3 system. The values of the determination coefficients R² were equal to 0.954 for the Freundlich model and 0.949 for the Langmuir model. However, in the case of the RB5-SA96 system, the data were better described by the Langmuir model (R²=0.992), indicating monolayer coverage of SA96 surface. The monolayer sorption capacity (Q₀) of SA96 was more than three times higher than that determined for SA3 oxide. The results presented by Bradua et al. (2010) confirmed applicability of the Langmuir model for description of adsorption equilibrium of RB5 onto Al₂O₃ and NiO/Al₂O₃. The values of the monolayer sorption capacities for Al₂O₃ and NiO/Al₂O₃ were found to be 71.1 and 29.3 mg/g, respectively (Bradua et al., 2010).

According to the authors the dye uptake by the mixed silica–alumina oxides can involve different types of interactions, e.g. between the electronegative atoms of RB5 anions and the hydroxyl group of SA96 or SA3, between the aromatic ring of RB5 and the hydroxyl group of SA96 or SA3, between the hydroxyl group of SA96 or SA3 and the azo group of RB5, as well as formation of ion pair between the dissociated sulphonic group of RB5 and the protonated hydroxyl group of SA96 or SA3.

Inorganic electrolytes and surfactants such as NaCl and SDS are widely used in chemical treatment of fibres. Sodium chloride increases relatively low fixation of reactive dyes on cellulose fibres, whereas anionic surfactant SDS is applied as wetting, penetrating and dispersing agent in dyeing processes (Wawrzkiewicz and Hubicki, 2015). Therefore, the effect of the presence of above-mentioned auxiliaries on RB5 removal by SA96 (of the higher capacity towards the dye than that of SA3) in the 10 mg/l RB5–5–20 g/l NaCl and 10 mg/l RB5–0.1–1.0 g/l SDS systems was investigated. The salt presence resulted in drop of the sorption capacity by approx. 50% compared to the system without NaCl; however, the increasing NaCl concentration in the range 5–20 g/l did not cause reduction of the q_e values. The dye retention was significantly reduced with the increasing amount of SDS in the 10 mg/l RB5–0.1–1.0 g/l SDS–SA96 system. This can be explained by competitive sorption of Cl⁻ anions or SDS than RB5 anions. It was previously observed that sorption of C.I. Direct Blue 71 on SA96 was reduced with the increasing amount of SDS in the system (Wawrzkiewicz et al., 2015).

Surface charge density and pH_pzc point studies

As can be seen in Figure 1, for both examined mixed oxides adsorption of the dye causes increase of the solid surface charge density in the whole range of the studied pH values (i.e. 3–10).

Figure 1.

Values of points of zero charge for SA96 and SA3 mixed oxides without and with RB5 of the concentrations 10, 20, 30 mg/l. RB5: Reactive Black 5; SA3: 97% SiO₂ and 3% Al₂O₃; SA96: 4% SiO₂ and 96% Al₂O₃.

Additionally, the shifts of pH_pzc points towards higher pH values in comparison to those obtained for the solid suspension without RB5 were observed. The values of pH_pzc, at which the concentrations of positively and negatively charged surface hydroxyl groups are the same, change after the dye addition from 4.8 to 6.2 for the SA96 system and from 3.5 to 7.7 for the SA3 system (Figure 2).

Figure 2.

Surface charge density of the SA96 and SA3 mixed oxides without and with RB5 of the concentration 30 mg/l. RB5: Reactive Black 5; SA3: 97% SiO₂ and 3% Al₂O₃; SA96: 4% SiO₂ and 96% Al₂O₃.

The source of surface charge on the metal oxide surface is the reaction of solid hydroxyl groups of amphoteric character with water molecules (leading to proton connecting or disconnecting). The presence of anionic dye with negatively charged groups at the interface promotes the formation of an additional number of positive surface sites (Chibowski and Wiśniewska, 2001; Wawrzkiewicz et al., 2015). As a result, the solid surface charge density of mixed oxide, whose surface is covered with the adsorbed dye molecules, increases. Such mechanism of RB5 adsorption is proved by the obtained rise of the σ₀ values with the increase of dye concentration (Figure 1). The higher the dye concentration is, the greater effect of the surface charge increase is obtained (as evidenced by the greater differences between the location of points of zero charge for the SA systems without RB5 compared to the system with RB5). The greater number of anionic dye molecules in the adsorption layer (at its higher concentration) on the solid surface ensures the higher concentration of positively charged surface groups (manifested by the σ₀ increase).

The most pronounced difference between the silica–alumina surface charge without and with C.I. RB5 is observed for SA96 mixed oxide. This is undoubtedly caused by the considerably greater adsorption of RB5 on the SA96 surface in comparison to that obtained for the SA3 system. The monolayer sorption capacities were found to be 47.1 and 14.1 mg/g for SA96 and SA3 mixed oxides, respectively.

Conclusions

In this study, two mixed silica–alumina oxides of different compositions were synthesized and used as a potential sorbent for the purification of aqueous solutions containing the hazardous RB5 dye. The results showed that the silica–alumina oxide SA96 composed of 96% Al₂O₃ and 4% SiO₂ is characterized by higher adsorption capacity for RB5 than SA3 (of the composition: 3% Al₂O₃ and 97% SiO₂). The experimentally determined equilibrium sorption capacities were found to be 47.1 mg/g SA96 and 14.1 mg/g for SA3. The PSO kinetic model described RB adsorption on both oxides. The addition of auxiliaries such as sodium chloride and anionic surfactant SDS to the system containing 10 mg/l of dye influenced its retention by SA96. The potentiometric titrations indicated that the presence of RB5 changed considerably the surface charge density of silica–alumina mixed oxides. Almost five times greater adsorption of the dye on the SA96 surface (compared to the SA3 suspension) causes a more pronounced effect of the surface charge density increase in this system.

Footnotes

Acknowledgements

First presented at the 15th Ukrainian‐Polish Symposium on Theoretical and Experimental Studies of Interfacial Phenomena and Their Technological Applications, Lviv, Ukraine, 12–15 September 2016.

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union's Seventh Framework Programme FP7/20072013/ under REA grant agreement no. PIRSESGA2013612484.

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Application of silica–alumina oxides of different compositions for removal of C.I. Reactive Black 5 dye from wastewaters

Abstract

Keywords

Introduction

Experimental

Materials

Methods

Results and discussion

Kinetic and equilibrium studies

Surface charge density and pHpzc point studies

Conclusions

Footnotes

Acknowledgements

Declaration of Conflicting Interests

Funding

References

Surface charge density and pH_pzc point studies