Abstract
Chitosan was deposited on fumed silica without the addition of cross-linkers or activating agents. The chitosan surface layer has a high affinity toward organic molecules, e.g., Acid Orange 8 (AO8) dye, robust to a broad range of simulated conditions (variance with respect to temperature, time, and concentration of solute). Experimental equilibrium data were analyzed by the generalized Langmuir equation taking into consideration the energetic heterogeneity of the adsorption system. The effect of temperature on dye uptake and adsorption rate was studied. According to the calculated thermodynamic functions ΔG°, ΔH°, and ΔS° from the equilibrium data at different temperatures, the adsorption of AO8 onto chitosan–fumed silica composite is exothermic and spontaneous. The studies of temperature effect on adsorption equilibrium show that the maximum adsorption capacity (determined from the Langmuir–Freundlich equation) of synthesized composite toward AO8 is about one-third higher in the case of an isotherm measured at 5 °C than this value obtained for the isotherm measured at 45 °C. The quantitative binding of dye molecules to chitosan coating on the surface of silica was proved by 1H MAS NMR. The deep kinetics study through the application of various theoretical models—the first-order equation, pseudo-first-order equation, second-order equation, pseudo-second-order equation, mixed first, second-order equation, and multiexponential equation—was applied for getting inside the mechanism of AO8 binding to the chitosan coating. Structural characteristics of chitosan-coated silica were obtained from the low-temperature adsorption/desorption isotherms of nitrogen and imaging by scanning electron microscopy. The effects of a synthetic route for polymer coating on thermal stability and the ability to degrade were studied by differential scanning calorimetry.
Highlights
IntroductionThe remaining 15% of dyes are discarded from dye baths as effluent
In textile production, only 85% of the coloring matter gets fixed to cloths
Development of organic−inorganic composites is an effective way to combine physicochemical properties of both components whereby the characteristics of the obtained material favor its use in a wide range of applications
Summary
The remaining 15% of dyes are discarded from dye baths as effluent. This results in producing billions of tonnes of waste waters daily, which cause groundwater depletion and present serious risks of irreparable damage to ecosystems.[1] The sorption of dye onto agriculture and marine byproducts is becoming a potential alternative for inorganic/organic removal from aqueous solution. The biopolymer chitosan, which is produced through deacetylation of chitin, one of the most abundant native polysaccharides,[2−4] has shown potential as a sorbent due to its polycationic structure and physicochemical properties.[5,6] The combination of chitosan with inorganic sorbents, such as silica, provides adsorbent materials with extended pH tolerance, fast adsorption kinetics, and high capacity.[7−12]
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