| The adsorption behaviors of alizarin red-S, Congo red, crystal violet and malachite green on modified nano-SiO2 (AMNS) were investigated. The adsorption properties, mechanism and dynamics were discussed. The results supported the feasibility that AMNS as an absorbent theoretically. As comparisons, no modified nano-SiO2 and activated carbon was also taken into the experiments. The conclusions were obtained by follow.(1) The factors that affect the adsorption of dyes on AMNS included pH, adsorbent dose, adsorption time, initial concentration of the dyes and temperature. The initial pH of the dyes solution was an important influence factor on adsorption capacity. The adsorption capacities of alizarin red-S and Congo red increased with the medium pH decreased, while those of crystal violet and malachite green decreased. That might be related to adsorbent surface charge state. Adsorption capacities of Congo red, crystal violet and malachite green increased with the temperature increase, while that of alizarin red reduced. Increase the dose of adsorbent could improve removal ratios of dyes, as well as increase adsorption time and the initial concentration of the dyes. The adsorption capacities decreased with adsorbent dose increase and increased with the initial concentration of the dyes increase. The adsorption capacity increased and the adsorption rate decreased with the initial concentration increase of dyes. So, it should be considered in practical process.(2) The adsorption behaviors of the dyes on AMNS were accorded with Langmuir isotherm model. Under the optimum conditions, the maximum adsorption capacities of anionic (alizarin red and Congo red) and cationic dyes (crystal violet and malachite green) were 200.0 mg·g-1, 133.3 mg·g-1, 40.0 mg·g-1 and 102.0 mg·g-1, respectively. Moreover, adsorbent has high adsorption capacity for anionic dyes.(3) All the adsorption processes were rapid. The removal and adsorption capacity of dyes rose rapidest at the beginning of 10 min. The adsorption equilibrium was near to equilibrium at 60 min. The adsorption dynamics were all followed Lagergren pseudo-second-order kinetics. The adsorption mechanisms were shown to be the electrostatic interactions between the anionic dyes (cationic dyes) and adsorbent.(4) The thermodynamic analysis indicated that the standard thermodynamic function changes of the adsorption of AR on modified nano-silica wereΔH0<0,ΔS0<0,ΔG0<0, indicating the adsorption was a spontaneous exothermic process. After the adsorption, the system became order even more;on the other hand, the standard thermodynamic function changes of the adsorption of CR, CV and MG onto the modified nano-silica wereΔH0>0,ΔS0>0,ΔG0<0, which expressed that the adsorption were spontaneous endothermic processes. The increased temperature was advantageous to the adsorption. After the adsorption, the system became more disordered.(5) Comparing the adsorption characteristic of anionic dyes and that of cationic dyes, it indicated that the former had a high adsorption capacity.(6) Under the same experimental conditions, the adsorption efficiencies of AR, CR and MG by AMNS were better than that of unmodified nano-SiO2. It might be attributed to the existed -NH2 on the surface of AMNS, which could react with other functional groups more easily.(7) Comparing with activated carbon, AMNS was more suitable for adsorbing AR, CR and MG from their wastewaters. It had the following advantages: higher adsorption capacity, fast adsorption and regeneration. The adsorpted dye molecules could be released from AMNS with acidic or alkaline desorption agent. Therefore, as a new adsorbent, AMNS has great advantages for the pretreatment or deep processing of dye wastewater. It would reduce costs and improve processing efficiency.
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