Investigation of 3-aminopropyltriethoxysilane modifying attapulgite for Congo red removal: Mechanisms and site energy distribution

Abstract Attapulgite is a promising adsorbent for removal of anionic dyes in water after being modified with 3-aminopropyltriethoxysilane, and four kinetics models and three isotherm models were used to evaluate the adsorption behavior of Congo red onto the adsorbents. It indicated that pseudo-second-order and Sips models were better to describe the adsorption data. Based on Sips model, it was confirmed that the modified attapulgite exhibited a higher adsorption capability for Congo red due to the more high-energy sites. Furthermore, high temperature and acidic environment were conducive to adsorbing Congo red on attapulgite, but the adsorption of the modified attapulgite toward Congo red was not sensitive to the temperature and pH value. A hydrophobic surface of the modified attapulgite reduced the surface tension between adsorbent and dye molecules, and electrostatic interaction played an important role for adsorption of CR on APTES/ATP.