Adsorptive removal of acid, reactive and direct dyes from aqueous solutions and wastewater using mixed silica–alumina oxide

Highlights

• Mixed silica–alumina oxide as adsorbent for acid, reactive and direct dye removal.

• Kinetic data were well described by the pseudo second-order model.

• C.I. Direct Blue 71 possesses the highest affinity for mixed oxide.

• The dye adsorption cause increase of the solid surface charge density.

Abstract

Untreated or partially purified effluents from the textile, paper, plastic, leather, food and cosmetic industries containing dyes and pigments are a serious environmental problem. Therefore, in this paper, adsorptive removal of acid (C.I. Acid Orange 7, AO7), reactive (C.I. Reactive Black 5, RB5) and direct (C.I. Direct Blue 71, DB71) dyes from aqueous solutions and wastewater was investigated using mixed silica–alumina oxide consisting of 4% SiO₂ and 96% Al₂O₃ (SA96). 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 of adsorption for AO7 and RB5 was observed after 180 min and 240 min for DB71. Sorption of the dyes on SA96 takes place through the pseudo second-order mechanism rather than the pseudo first one or intraparticle diffusion. The experimental data fitted better the Langmuir isotherm model than the Freundlich one. The monolayer sorption capacities (Q₀) were found to be 41.4 mg/g, 47.1 mg/g and 49.2 mg/g for AO7, RB5 and DB71, respectively. The effect of the auxiliaries such as anionic surfactant (SDS) and sodium chloride on removal of DB71 was investigated in the 10 mg/L DB71 containing 0.1–1 g/L SDS or 5–20 g/L NaCl systems. It was observed that DB71 sorption was reduced with the increasing amount of SDS in the system while NaCl does not influence on the dye uptake by SA96. The adsorption of dyes causes increase of the solid surface charge density (σ₀) and shift of pH_pzc point toward higher pH values. This is a result of formation of a greater number of positive surface sites due to the interactions with the anionic adsorbate. The greatest effects were obtained for the system containing DB71. The addition of NaCl to the SA96–DB 71 system results in noticeable lowering of σ₀ in the pH range 4.4–8.3. In the presence of anionic surfactant, the considerable increase of σ₀ was obtained.

Introduction

Synthetic mixed oxides have many important industrial applications. They are widely used as components of ceramics, fine optics, lasers, semiconductors, piezoelectrics, catalysts, nuclear fuels, pigments, etc. [1], [2]. Due to their unique properties such as specific structure, high surface area and pore size, they are currently considered as effective, efficient, economic and eco-friendly adsorbents for removal of both organic and inorganic pollutants such as chlorophenols, complexones, polyelectrolytes and polymers, surfactants, dyes, metal ions and gases [3], [4], [5], [6], [7], [8]. The adsorption of organic compounds like dyes on the mixed oxides provides a great challenge faced by scientists as these substances are dangerous for the environment because of toxicity and resistance to natural degradation. Taking into account the fact that more than 100 000 types of commercially available dyes exist and an annual worldwide production of 700 000–1000 000 tons has been reported, it is difficult to imagine the amount of emitted colored effluents [9]. As estimated 280 000 tons of textile dyes is discharged as industrial wastewaters worldwide every year [9]. Thus, textile manufacturers paid attention to investment in wastewater treatment operation in order to reduce water consumption and residual level of recalcitrant organic pollutants in the fine effluents.

In this regard, the efficiency of mixed oxides toward removal of dye molecules has been studied in recent years. Khosravi and Eftekhar [10] evaluated the effectiveness of Na_0.5Li_0.5CoO₂ as the adsorbent for removal of methylene blue dye. Ninety-two percent of the dye was successfully removed in 10 min using 0.02 g Na_0.5Li_0.5CoO₂ at pH 11. A mixed oxide of cobalt and nickel of the approximate composition Co_0.4Ni_0.4O_0.2 was applied for the methylene blue and procion red uptake with the sorption yield of 20% and 70%, respectively [11]. The azo dye Congo red sorption on the mixed iron and aluminum oxide as well as iron and nickel oxide was investigated by Mahapatra et al. [12] and Zeng et al. [13]. The remarkable sorption capacity of γ-Fe₂O₃-Al₂O₃ amounting 498 mg/g was determined [12]. Ni_0.6Fe_2.4O₄ was characterized by the fast sorption rate of Congo red (92% of the dye was removed within 9 min of contact time), but a lower value of maximum capacity (72.73 mg/g) was obtained [13]. Siliceous-based materials of natural occurrence and artificial origin were widely used for textile wastewaters treatment. Considering their chemical reactivity, porous structure, mechanical stability and high surface area, they can be attractive sorbents for dye-polluted waters [14], [15]. Modified silicon dioxide possessed the highest affinity for C.I. Acid Blue 25 in comparison with the other organic pollutants such as p-nitrophenol, pentachlorophenol or 2,4-dichlorophenoxy acetic acid [16]. Silica-based sorbent (major constituents: 61.1% SiO₂, 22.6% Al₂O₃) was applied by Khan et al. [17] for sorption of methylene blue, malachite green and rhodamine B from aqueous solutions. The removal of dyes between 67.4% and 97.2% indicates that the sorbent is a moderately good one for the color elimination from the textile wastewaters [17]. The fly ash as a by-product generated during the coal combustion in thermal power plant, consisting of silicon dioxide (43.7%), aluminum oxide (15.7%), iron oxide (6.4%), calcium oxide (9.8%) and magnesium oxide (0.9%) was evaluated as efficient sorbent for methylene blue removal [18]. Titania–silica mixed oxide was applied for removal of C.I. Basic Violet 10 with the sorption capacity ranged from 10.5 to 32.1 mg/g depending on the molar ratio of TiO₂ to SiO₂ [19]. TiO₂–SiO₂ combined with manganese or cobalt ions was shown to be more effective in sorption of C.I. Disperse Red 19 than undoped titanium-silica oxide, despite the higher surface area of the latter [20].

As improper treatment and disposal of dye-contaminated effluents provoked serious environmental concerns all over the world, the adsorption behavior of three different textile dyes of anionic type (C.I. Acid Orange 7, C.I. Reactive Black 5, C.I. Direct Blue 71) onto the mixed alumina–silica oxide was investigated. Such parameters as initial dye concentration, phase contact time, solution pH and presence of surfactant influenced the dye adsorption. Mixed oxides are considered not effective adsorbents for dye removal because there is an opinion that they cannot absorb a wide range of dyes and perform poorly in the presence of other additives. Such view is erroneous since at least one successful treatment is known. The relatively good sorption results presented in this paper suggest that mixed silica–alumina oxides could be efficient sorbents for dye removal. Therefore, the authors are convinced that the experimental results contribute to the studies on the mechanism of dye sorption on mixed oxides, particularly including the effects of salt and surfactant addition which are frequently found in real wastewaters.