Preparation Of Amphoteric Starch Modified Flocculant And Its Application In Water Treatment Research

Water is the most essential substance in the everyday life and production activity of the mankind. The traditional inorganic flocculants, due to the residues of its metal ions which may cause second pollution, leave significant impact on the health and the activity of human beings. Therefore, it has become an important research subject currently to find novel flocculants that are efficient, low-cost, environment-friendly so that the second pollution can be avoided. In recent years, the natural macromolecular substances have received wide attention in the academia with such advantages as abundant availability, renewability, and environment-friendliness. However, it also shows some weaknesses like weak solubility, low surface charge, relatively low molecular weight, and poor flocculation. Thereby, chemical modification is necessary to improve their flocculation effects.This study uses starch (St), an abundant and low-cost natural macromolecular substance, as raw material, and adopts one-step and two-step method, respectively, to prepare two kinds of starch based amphoteric flocculants. The first type one is produced by using low-toxic quaternary ammonium salt (CTA) as cationic etherifying agent and CICH2COOH as anionic etherifying agent. By changing the dosage of the etherifying agent, a series of starch based amphoteric flocculants with different substitution degrees of ionic groups have been prepared. As for the second type flocculants, St-CTA is produced firstly from etherification and CTA as cationic etherifying agent, then utilizing ammonium per sulfate as initiator, and acrylic acid and acryl amide as co-monomer, the grafted starch flocculants have been prepared successfully by graft copolymerization. Then the flocculation properties of aforementioned flocculants have been systematically studied taking kaolin and hematite powder suspensions as simulated wastewater in lab scale. The effects of pH, dosage of flocculants, environmental temperature, and initial turbidity on the flocculation properties have been investigated. In addition, the size and fractal dimension of flocs have measured by low angle laser light scattering method. Furthermore, the flocculation mechanism has been studied from the fractal theory.Some interesting experimental results are showed in below:(1) The flocculation experiment demonstrates that with the increase of the dosage of the flocculants, the flocculation performance improves at the beginning but decrease then. At the optimal dosage, the Zeta potential of the suspension is near zero which indicates that flocculation process is controlled by charge neutralization mechanism. The dosage of the flocculants increases with the increase of original turbidity. Under the same initial turbidity, along with the rise of the temperature, the dosage of the flocculants would decrease firstly and then reach a platform. Under different pH conditions, these two types of flocculants display different flocculation performance for different simulated wastewater. Combined with the Zeta potential of flocculants solutions, it shows that when carrying different surface charges between the flocculants and suspended particles, it has good flocculation properties. However, there is no evident flocculation effect when bearing the same surface charge. It further indicates that charge neutralization mechanism is dominant.(2) From the studies on the flocs properties, under the optimal dosage of flocculants, the flocs size and fractal dimension both show the highest value, indicating most compact and solid structure of flocs. Moreover, the flocs size decreases obviously with the improvement of the shearing force. But, it would recover rapidly when the shear rate decreases. More interestingly, there are different flocs strength and recover speed at various pH conditions. Under the neutral condition, the flocs show the highest strength and the most rapid recovery rate. In some extreme cases, the flocs display the lowest strength and can hardly return to their original size, It agrees with the charge patching theory.Above all, both the flocculation experiments and the flocs properties study demonstrate that aforementioned two types of starch based flocculants show fine flocculation performance and have great potential in real application.