Abstract
The effect of acid treatment on the adsorptive capacity of a Brazilian palygorskite to remove the crystal violet (CV) and congo red (CR) dyes was investigated. The raw palygorskite was acid-treated by different HCl solutions (2, 4, and 6 mol/L). The modifications on the palygorskite structure were investigated using X-ray diffraction, X-ray fluorescence, Fourier-transform infrared spectroscopy, N2 adsorption/desorption, and thermogravimetric and differential thermal analysis. The efficiency of CV and CR adsorption was investigated, and the effect of the initial concentration, contact time, pH, and adsorbent amount was analyzed. The results revealed that CV adsorption in the acid-treated palygorskite was higher than that of the raw material. A Langmuir isotherm model was observed for the adsorption behavior of CV, while a Freundlich isotherm model was verified for the CR adsorption. A pseudo-second-order model was observed for the adsorption kinetics of both dyes. The higher CV adsorption capacity was observed at basic pH, higher than 97%, and the higher CR removal was observed at acidic pH, higher than 50%. The adsorption parameters of enthalpy (ΔH), entropy (ΔS), and Gibbs energy (ΔG) were evaluated. The adsorption process of the CV and CR dyes on the raw and acid-treated Brazilian palygorskite was predominantly endothermic and occurred spontaneously. The studied raw palygorskite has a mild-adsorption capacity to remove anionic dyes, while acid-treated samples effectively remove cationic dyes.
Highlights
The increase in industrial production and the world population has increased water contamination from industrial and domestic sources [1]
Crystal violet (CV) is a cationic dye widely used in the textile, plastics, paint, and leather industries [7,8]
Studies [42] on Algerian palygorskite, with an R2+ /R3+ ratio of ≈0.95 and a dioctahedral-trioctahedral character, obtained an adsorptive capacity of just 15 mg/g for methylene blue (MB). These results indicate that the dioctahedral character, Al-rich palygorskite seems to favor the adsorption of cationic dyes
Summary
The increase in industrial production and the world population has increased water contamination from industrial and domestic sources [1]. Effluents derived from the textile, food, plastics, and cosmetics industries are rich in dyes [2,3]. Dyes have a complex structure, are stable to light and heat, can be highly toxic, potentially carcinogenic, and mutagenic [4], and rapidly accumulate in living cells and damage human health [5,6]. Crystal violet (CV) is a cationic dye widely used in the textile, plastics, paint, and leather industries [7,8]. The CV dye is not biodegradable; it is toxic, genotoxic, and carcinogenic [5,8,9]. The presence of the crystal violet dye in water can promote the growth of tumors and reproductive abnormalities in fishes [10]. CV dye exposure can cause eye irritation and painful sensitization to light, and in extreme cases, respiratory diseases, kidney failure, chemical cystitis, permanent blindness, and cancer [7,11,12,13]
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