Modified garlic straws as new adsorbents for removing ionic dyes from aqueous solutions

Abstract

In this study, garlic straws (GS) from agricultural by-products were chemically modified by sodium hydroxide and citric acid to yield potential adsorbents for the removal of ionic dyes. Firstly, the physical and chemical properties of obtained adsorbents were characterized by component analysis, X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy. Then the cationic dye crystal violet (CV) and anionic dye Congo red (CR) were applied to evaluate the adsorption capacity of adsorbents under different conditions such as initial solution pH (2–12), adsorbent dose (0.5–10 g/L), initial dye concentration (0.1–6 g/L) and contact time (up to 120 min). Results showed that sodium hydroxide and citric acid treatments could enhance the adsorptions of CV and CR probably through increasing the hydroxyl groups and carboxyl groups on the surface of GS. The maximum adsorption capacities of CV and CR on modified GS at 308.15 K were 63.69 and 998.3 mg/g, respectively. Furthermore, a kinetics study revealed that the adsorption processes could be described by the pseudo-second-order kinetic model. Thermodynamic parameters indicated that the adsorption processes were spontaneous and endothermic. Generally, these results confirmed that sodium hydroxide and citric acid treatments were effective methods to improve the adsorption capacities of GS for ionic dyes.