Abstract
In this work, the ability of pectin (Pec) to remove direct red 80 (DR80), Congo red (CR), methyl orange (MO), and methyl red (MR) was studied. The removal percentages under adequate pH and ionic strength conditions were as follows: DR80 (99.5%), CR (99.8%), MO (88.6%), and MR (68%), showing that this methodology is efficient to remove azo dyes. The proposed method included the addition of native Pec to the dye aqueous solution and the formation of a gel that occurred when a calcium salt solution was added. This gel retains the molecules adsorbed onto the molecular surface of Pec through hydrogen bonds and electrostatic and hydrophobic interactions. To our knowledge, it is the first time that the Zimm-Bragg model is used to describe the removal of azo dyes with native Pec. This model includes two parameters: [Formula: see text] (nucleation constant), which is related to the tendency exerted by a dye molecule attached to the Pec to bind to other molecules present in the aqueous phase, and [Formula: see text] (cooperativity parameter), which determines the aggregation capacity of the dye molecules already attached to the Pec. This model fits the experimental isotherms very well, suggesting that Pec binds single molecules and dye aggregates. The obtained results in the values of [Formula: see text] ranged from 922 mol/kg (MR) to 1,157,462 mol/kg (CR), and [Formula: see text] varied from 2.51 (MR) to 169.19 (MO). These results suggest that the use of Pec is a viable option to remove azo dyes from aqueous effluents and that the Zimm-Bragg model fits adequately the isotherms of dyes that have a high tendency to form aggregates.
Highlights
The azo dyes correspond to a group of synthetic compounds, which are characterized by including the azo group (-N=N) in their structure and by imparting a wide variety of colors in the different substrates where they are applied
A bathochromic shift is again observed in the two absorption maxima corresponding to the azo and hydrazone forms, located at λ = 531 nm and λ = 559 nm, respectively. These findings suggest that the formation of the calcium pectate (Pec-Ca) gel decreases the repulsion between direct red 80 (DR80) and Pec, and as a consequence, the dye molecules bind to this polysaccharide to a greater extent through the same forces as they intervene in the Pec-DR80 adduct, but in a more intense way
The use of Pec to remove the azo dyes DR80, Congo red (CR), methyl orange (MO), and methyl red (MR) resulted in the following removal percentages under optimal pH and ionic strength (IS) conditions: DR80 (99.5%), CR (99.8%), MO (88.6%), and MR (68%)
Summary
The azo dyes correspond to a group of synthetic compounds, which are characterized by including the azo group (-N=N) in their structure and by imparting a wide variety of colors in the different substrates where they are applied. It is known that these dyes are carcinogenic and it has been reported that when they are degraded anaerobically, toxic amines are produced [2] Due to these adverse effects, alternative methods have been sought in the staining process. Only supercritical carbon dioxide has been applied to reactive and disperse azo dyes, and this has limited application [4,5,6], and nowadays the wet staining process is still used in the dyeing of different textile fibers [7].
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