Coagulation and flocculation of dye-containing solutions using a biopolymer (Chitosan)

Abstract

Chitosan, dissolved in acetic acid, was used for the coagulation–flocculation of an anionic dye (Reactive Black 5). In acidic solutions protonated amine groups of chitosan attract dye sulfonic groups. Increasing chitosan dosage increases dye removal up to a concentration resulting in complete neutralization of anionic charges; above, the excess of cationic charges leads to suspension re-stabilization. Process efficiency increases with decreasing the initial pH of dye solution: the molar ratio between dye molecules and amine groups ([n]) respects the stoichiometry between sulfonic functions and protonated amine groups at initial pH 5; at initial pH 3 a possible dye aggregation phenomenon results in higher molar ratio [n]. The coefficient [n] depends on both the pH and the molecular weight of chitosan. The main mechanism for dye coagulation with chitosan sounds to be charge neutralization at acidic pH.

Introduction

Chitosan is an aminopolysaccharide produced by the deacetylation of chitin, one of the most abundant biopolymers (Fig. 1). The physico-chemical properties of chitosan related to the presence of amine functions (acid–base properties, solubility, cationicity) make it very efficient for binding metal cations in near neutral solutions [1], [2], [3], and for interacting with anionic solutes in acidic solutions [3], [4], [5], [6]. This electrostatic attraction mechanism is responsible for the strong interaction existing between chitosan and anionic dyes such as Reactive Black 5 (RB 5) [7], [8]. Protonated amine groups react with sulfonic functions of the dye in acidic solutions. Indeed, though the intrinsic pK of amine groups of chitosan strongly depends on both the deacetylation degree (DA) and the degree of dissociation (α), for chitosan with a degree of deacetylation above 70%, the pK_a can be approximately fixed to 6.3–6.4 [9]. Hence, at pH below 5, most of amine groups are protonated and they can attract metal anions (molybdate, vanadate, palladate, chromate, …) [10], [11], [12], [13], [14], [15], [16], [17]. This property has been widely used for the sorption of a number of dyes in solution [5], [18], [19], [20], [21], as an alternative to conventional sorbents: activated carbons [22], [23], [24], agriculture waste materials [25], [26], [27]. A much less abundant literature exists on the use of chitosan in the dissolved-state for the removal of dyes [28], [29]. Chitosan has been widely used for the coagulation–flocculation of mineral colloids [30], [31], [32], [33], [34], in food industry [35], [36], bacterial suspensions [37], [38], [39] … These processes mainly focused on the recovery of suspended matters and colloid; in the case of dissolved contaminants there are much less studies.

The coagulation and the flocculation of suspended particles and colloids result from different mechanisms including electrostatic attraction, sorption (related to protonated amine groups), bridging (related to polymer high molecular weight). In some cases, the amount of protonated amine groups added to the solution is far below the number of charges necessary for the neutralization of the anionic charges held by the colloids; the removal of particles can be explained in this case by a combination of distinct mechanisms such as electrostatic patch and bridging. For these reasons the efficiency of chitosan for the treatment of anionic dyes is suspected to depend on its deacetylation degree and its molecular weight (among structural parameters) and its concentration, additionally to solution parameters (dye concentration, pH, ionic strength of the solution).

The present work focuses on the coagulation–flocculation of Reactive Black 5 using chitosan. Experiments have been performed at two initial pHs (pH 3 and 5), with two chitosan samples characterized by the same deacetylation degree but with different molecular weights. The concentration of chitosan added to dye solution was varied in order to determine the optimum molar ratio between sulfonic groups of the dye and amine groups of chitosan for maximum dye removal. This molar ratio served to calculate the ratio dye/amine groups on the polymer at saturation, i.e. the stoichiometric ratio for the reaction between chitosan and the dye. This maximum binding capacity can be compared to the maximum sorption capacity obtained in sorption isotherms with similar materials (RB 5 dye and chitosan, used in the solid state) [8]. The comparison of solid-state application (sorption) and dissolved-state application (coagulation–flocculation) will allow identifying the significant contribution of diffusion properties (accessibility to internal sites) and site availability on the control of sorption properties (versus coagulation–flocculation). The influence of a filtration step before analysis was also investigated.