High adsorption capacity of MWCNTs for removal of anionic surfactant SDBS from aqueous solutions

Surfactants have specific physico-chemical properties, such as solubility in polar/non-polar liquids, the ability to form micelles and adsorption to phase boundaries. They are economically important due to specific properties that allow them to be used as washing, wetting, emulsifying and dispersing agents. Surfactants are widely used in industry and in the household. Huge volumes of surfactants are entering the environment, where these compounds and/or their degradation products may cause damage depending on their concentrations (Emmanuel et al. 2005; Murphy et al. 2005). Surfactants are non-degradable and toxic in nature, so their removal from aqueous solutions and industrial effluents is a matter of great concern (Eichhorn et al. 2002). Several treatment processes have been developed over the years to remove surfactants in water and wastewaters, such as chemical precipitation (Shiau et al. 1994), electrochemical treatment (Lissens et al. 2003), membrane filtration (Kowalska et al. 2004), and photocatalytic degradation (Zhang et al. 2003). However, these techniques have disadvantages including incomplete metal removal, high consumption of reagents and energy, low selectivity, high capital and operational costs, and generation of secondary wastes that are difficult to dispose of. For these reasons, cost-effective alternative technologies for treatment of metal-contaminated waste streams are needed. Adsorption has proved to be an appropriate method, as it is a simple, selective and economical process for the removal of surfactants from aqueous solutions. Silica gel (Purakayastha et al. 2005), resins (Yang et al. 2006), bentonite (Gunister et al. 2004), zeolites (Savitsky et al. 1981), sand (Khan & Zareen 2006) and activated carbons (Wu & Pendleton 2001; Gonzalez-Garcia et al. 2004) have been used for removal of surfactants.

Among other adsorbents, carbon nanotubes, due to their large specific surface area (SSA), small size, highly porous, hollow structure and light mass density, have been used for the removal of organic and inorganic contaminants from water (Long & Yang 2001; Merkoci 2006). The aim of this study was to evaluate the adsorption capacity of surface-oxidized multi-walled carbon nanotubes (MWCNTs) for the removal of sodium dodecyl benzene sulfonate (SDBS) from industrial wastewater.

All chemicals used were of analytical reagent grade and were used without further purification. Double-distilled water was used throughout this study. An anionic surfactant, SDBS, used in adsorption studies, was purchased from Merck (Darmstadt, Germany). MWCNT (purity >95%; external and internal diameter: 10–20 nm and 5–10 nm) was purchased from Merck. To prepare oxidized MWCNT, 10 g of MWCNT was added to 100 mL concentrated HNO₃ solution, and the mixture was refluxed for 48 h at 120 °C. After cooling at room temperature, the suspension was filtered through a 0.45 μm filter membrane and then rinsed with deionized water until the pH reached neutral. Finally, the sample was dried in an oven for 2 h at 100 °C. Synthetic wastewater was prepared by dissolving SDBS. The synthetic wastewater was treated using H₂O₂ and ultraviolet to investigate the adsorption process.

SSA may be obtained using the Brunauer, Emmett and Teller (BET) method. The SSA of the sample was determined by measuring the amount of adsorbed gas N₂ on the surface of MWCNT corresponding to a monomolecular layer on the surface. The pore size distribution of MWCNTs and oxidized MWCNTs was measured using the Barrett, Johner and Halenda method.

The effects of pH, adsorbent dosage, dispersion time and temperature on the adsorption of SDBS were studied. To study the effect of pH, 0.1 mol L⁻¹ HCl or NaOH solution were used to vary pH in the range 1–9, at 15 min intervals and with 20 mgL⁻¹ SDBS as the initial surfactant concentration. The effect of varying the MWCNT mass (adsorbent dosage) was studied at room temperature with 20 mgL⁻¹ initial SDBS and a pH of 5.6. The adsorption percentage of SDBS onto oxidized MWCNTs was studied to obtain the dispersion time required to remove 20 mgL⁻¹ initial SDBS at pH 5.6 and 0.01 g of adsorbent.

Adsorption isotherms are used for the design of adsorption systems. Correlation of equilibrium data by either theoretical or empirical equations is important for practical operations. Langmuir and Freundlich equations were used for further interpretation of the adsorption data obtained.

To study the SSA of MWCNTs and oxidized MWCNTs, nitrogen adsorption/desorption isotherms on the adsorbent surfaces were obtained. The data were treated according to the BET theory (Brunauer et al. 1938; Walton & Snurr 2007). The results of the BET method showed that the SSAs of MWCNTs and oxidized MWCNTs were 115 m² g⁻¹ and 158 m² g⁻¹, respectively.

Surfactant removal ratios increased when the pH of the solution increased from 1 to 5.6, and remained constant at higher pH values. This may be due to a chemical reaction between SDBS and the adsorbent. At low pH values, the H⁺ ion concentration in the system increased so the surface of the MWCNTs could acquire a positive charge by protonation. Positively charged surface sites on the MWCNTs do not favor the adsorption of SDBS.

When the experimental temperature was varied, it was found that removal efficiency decreased from 80% at 25 °C to 25% at 50 °C. This suggests that the removal efficiency depends on the temperature, and that the process is exothermic and occurs by the weakening of bonds between SDBS and MWCNTs at high temperatures (Ribeiro et al. 2012).

The Langmuir and Freundlich isotherms were obtained by using the linear least square regression methods. The results showed that the Freundlich isotherm fitted well with the SDBS adsorption process (R² > 0.92). The Langmuir and Freundlich correlation coefficients and constants are shown in Table 1.

This study demonstrated that the optimum pH for SDBS adsorption by MWCNT was 5.6. Equilibrium adsorption data were very well fitted by the Freundlich model. Adsorption of SDBS obeys a pseudo-second-order equation with good correlation. The results show that MWCNTs may be used effectively for removal of SDBS from aqueous media.