Gradient Adsorption of Methylene Blue and Crystal Violet onto Compound Microporous Silica from Aqueous Medium

Microporous silica (MS) materials are a kind of an emerging and promising adsorbent precursor. MS prepared from vermiculite has the advantages of easy preparation, low cost, and low layer charge. In this study, organo-MS (OMS) modified by a typical gemini surfactant 1,2-bis(hexadecyldimethylammonio)ethane dibromide (G₁₆) is first synthesized and proved to have effective retention capacity toward cationic dyes. Fourier transform infrared spectroscopy, TG-DTG, X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Brunauer-Emmett-Teller are used to explore the structural characters of adsorbents. Gradient adsorption of compound MS (MS and OMS) in a binary dye system [methylene blue (MB) and crystal violet (CV)] was investigated. In a single system, the relationship between the adsorption capacity and influencing factors (dye concentration, contact time, temperature, and pH), adsorption kinetics, isotherms, as well as thermodynamics was comprehensively compared to reveal the adsorption mechanism. The adsorption values of MB and CV on MS and OMS are 308 mg g^-1 (R = 77.0%, 15 min) and 250 mg g^-1 (R = 83.3%), respectively, which may be caused by various intermolecular interactions (electrostatic or hydrophobic interactions) between the dye and adsorbent surface. In a binary system, the improved first spectroscopy method is used to calculate the individual concentration of the dye in the binary system. The total removal efficiency of gradient adsorption reaches as high as 89.5% (MB) and 86.4% (CV). In addition, compound MS can be effectively regenerated by HCl solution for several cycles.