Characterization of cationic starch flocculants synthesized by dry process with ball milling activating method

Abstract

The cationic starch flocculants were synthesized by the reaction of maize starch which was activated by a ball-milling treatment with 2,3-epoxypropyl trimethyl ammonium chlorides (ETMAC) using the dry method. The cationic starches were characterized by several approaches including scanning electron microscope (SEM), degree of substitution (DS), infrared spectrum (IR), X-ray diffraction (XRD), flocculating activity, electron spin resonance (ESR), and solid-state nuclear magnetic resonance (NMR). The effect of mechanical activation on starch etherifying modification was investigated. The mechanical activation cracked starch granules and destructed their crystal structures. This resulted in enhancements to the reaction activity and reaction efficiency, which was approved by ESR and solid state NMR. The starch flocculants, synthesized by the reaction of mechanically activated starches at 90 °C for 2.5 h with ETMAC at molar ratio of 0.40:1.00, showed good flocculation activity. The substitution degree (0.300) and reaction efficiency (75.06%) of starch flocculants synthesized with mechanically activated starches were significantly greater than those of starch flocculants with native starches (P < 0.05).

Introduction

Cationic polysaccharides are generally synthesized by the reaction of polysaccharides and cationic reaction regents such as amino, imino, ammonium, sulfonium, or phosphonium groups. Cationic starch flocculants with quaternary ammonium type processing positively charged groups are effective flocculants over a wide pH range [1], [2]. They are non-toxic and easily biodegradable. Cationic starch flocculants are widely used for the purification of waste water carrying negative charges, paper making for controlled flocculation, retention and paper strength, and spinning petroleum well drilling [3], [4], [5].

Cationic starches with a high degree of substitution (DS) possess preferable properties and potential applications. Thus, the DS is an important parameter. Conventionally, cationic starch flocculants are commonly prepared by the reaction of starch with cationic moiety in wet, semi-dry, or dry states. The wet method has the advantages of mild reaction conditions, convenience, and high reaction efficiency. However, this method is limited because it only allows for the production of cationic starches with low DS and can induce serious environmental pollution [2].Compared to the wet state method, the dry state approach has several advantages. These advantages include fewer starch losses, pollution-free starch modification, and high reaction efficiency [6], [7]. While the semi-dry state method has the same advantages as the dry process (pollution-free and a high efficiency reaction), the cationic moiety in the reaction mixture is easily degraded and the side reaction occurs at a high temperature under alkaline–water conditions [8], [9]. Compared with the other two approaches, the dry process approach is proposed and applied to industrial applications due to its pollution-free and high efficiency reaction. However, most commercial cationic starches, which were applied to the purification of waste water or paper making, have very low DS (less than 0.2) [10]. As a result, high reaction efficiency (RE) with dry process, using a ball milling activating treatment, was pursued in the current study.

Compared with the other methods, the ball-milling treatment is only a physical method of modification. It has several advantages including its low-cost (when compared with UHP-assisted reactions [11]) and it is environmentally friendly (when compared with the acid–ethanol treatment [12]). Thus, it has attracted much attention in the recent years. The intensive milling force, in conjunction with the pressure and friction action of the ball, breaks the integrity of the granules [13] which shows great advantages in powder activation, surface modification, toxic waste disposal, and the synthesis of organic materials [14], [15], [16]. Currently, research on the mechanical activation of starches mainly focuses on the physically micronized treatment [17], [18], [19]. However, little research has been reported on ball-milling activating cationic flocculants.

In this study, native maize starch was activated by the ball-milling treatment. Cationic starch flocculants were synthesized with a cationic moiety (2,3-epoxypropyl trimethyl ammonium chloride, ETMAC) using the dry process. The conditions for preparing cationic starch flocculants with mechanical activation using the dry process were optimized. The properties of mechanical activation and chemical activation on the starches were characterized with the following methods: a scanning electron microscope (SEM), degree of substitution (DS), infrared spectrum (IR), X-ray diffraction (XRD), flocculating activity, electron spin resonance (ESR), and solid-state nuclear magnetic resonance (NMR).