| China is a large country of coking production all over the world. However, about2.85tons of coking wastewater was discharged per annum, which has brought about shocking environmental pollution. It is a worldwide problem to treat coking wastewater with low-cost and high efficiency. At present, the domestic coking wastewater must be treated by the tertiary treatment in order to meet the standards for emission or reuse. The tertiary treatment methods include absorption, oxidation, membrane separation, coagulation, and etc.. Of them, coagulation is a comparatively more widely-used, economic and efficient water treatment technology. The flagship products-polyacrylamide series of organic flocculants have favourable effects, whereas their prices are unable to afford. Starch is an environmental-friendly, rich and renewable resource, which is also a popular industrial chemical. Starch has been used in many industries, such as paper making, textile manufacturing, food processing and adhesive producing. It is a pity that starch is applied as a cheap chemical material. Using starch to develop high-value-added green chemical materials is one of the main directions of the current and future research. Natural polymer-based flocculants have been acclaimed as "green flocculants of the21st century". They are expected to open a green way for coking wastewater treatment. According to the characteristics of coking wastewater, this study is aimed at the R&D of a tailored flocculant for the treatment of coking wastewater.Due to many advantages of microwave irradiation, such as rapid heating, easy up-scaling, high-energy efficiency and strong selectivity, microwave reactors were hailed as the "Bunsen burner of the21st century" by A. K. Bose, one of the pioneers applying microwave heating to organic synthesis. Compared to the conventional heating method, microwaves accelerate chemical reactions for10-1000times. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. Publications of microwave-assisted polymerization have also mushroomed. Inspired by the booming development of this field, a kind of starch grafted bicopolymer was synthesized by microwave-assisted synthesis.Gelatinization of native starch was undergone by using water-bath heating and microwave heating respectively. Optical microscopy, scanning electron microscopy, Fehling reaction, infrared spectrum, and Zeta potential were used to analyze the similarities and differences of the structure and properties among native starch and gelatinized starches. Under conditions of atmosphere and reflux, water as solvent, microwave gelatinized starch as grafting skeleton, acrylamide (AM) and dimethyldiallylammonium chloride (DADMAC) as grafting monomers, a trace of potassium sulfate as initiator, graft copolymerization (reaction volume80mL) was accomplished under microwave irradiation. Taking grafting efficiency as the main optimization index, the optimal conditions for grafting were obtained through the single-factor experiments: time of microwave irradiation150s, power of microwave irradiation500W, concentration of potassium sulfate2mmol/L, mass of starch on dry basis1g, concentration of AM0.9mol/L and concentration of DADMAC0.1mol/L. Under the optimal conditions, indexes of the graft product are:grafting efficiency68.8%, cationic degree0.227mmol/g, intrinsic viscosity5.64dL/g, weight-average molecular weight2.15×106Da. SEM analysis, EDS analysis, elemental analysis, X-ray diffraction and FTIR spectroscopy being employed to confirm that two vinyl monomers, i.e., AM and DADMAC had been grafted onto the starch backbone.Finally, the starch-graft-bicopolymer with the highest grafting efficiency (68.6%) was used as polymeric flocculant for the coking wastewater treatment. In dealing with raw coking wastewater (CODCr=450.4mg/L, chroma=640°, and turbidity=240NTU) into which dosing1.5g/L PFS,2.5mg/L grafted starch and60mg/L CTAB after being treated by flocculation once, the treated coking wastewater (CODCr=122.5mg/L, chroma=89.6°, and turbidity=19.2NTU) reaches the National Discharge Standard Rank2("Discharge standard of water pollutants for iron and steel industry"(GB13456-92)). Synthetic grafted flocculant is superior to commercial CPAM for CODCr and chroma removal in coking wastewater treatment. |
PDF Full Text Request