Study On Efficiency And Mechanism Of Newly-Formed Ferric Coagulant Treating Songhua River Water

Because of serious water pollution, economical efficient water treatment is significant for civil life and industry. Coagulation is easy to handle, and effectively removes various pollutants. These advantages make it an indispensible preliminary part of an integrated water treatment process. Effective and low-cost coagulant is essential for coagulation. Various metal salts and preparation technology has been investigated through years. Polyaluminum chloride (PAC) and polyferric sulfate (PFS) are common recognized coagulants. Attention is being paid to the further investigation of PFS ever since the toxicity of aluminum was discovered.Current preparation of PFS involves aging time for couples of hours which demands reacting vessels of high volume and high resistance against erosion, and also involves solidification of coagulant, and heating of acid solution which contains strong oxidant. The above characteristics not only increase the actual capital in PFS preparation, but also increase virtual capital of pressure and load because of risk brought to the natural and social environment. In order to alleviate these drawbacks, a newly-formed ferric coagulant was trialed with ferrous oxidized by H₂O₂ instantaneously at room temperature and atmospheric pressure. The property of the new coagulant and its performance in treating Songhua river water were investigated.The reaction between H₂O₂ and ferrous is fast. The pH of the mixture decreased fastest in the first 10 minutes as the result of H+ accumulation by hydrolyzation of newly-formed ferric salt. The influence of aging time on the performance of the newly-formed coagulant is minimum in the removal of turbidity, oxygen consuming matter, and pH after sedimentation. Proper dosage of acid during preparation decreased the dosage of coagulant to attain ideal performance.The best acid volume is 0.1 mol/L. Increase of H₂O₂ dosage improved the performance of the new coagulant. The most economical and efficient [H₂O₂]/Fe molar ratio is 0.55 .Songhua River water was treated with the prepared newly-formed ferric salt as coagulant and the result was contrasted with those of PFS and ferric sulfate (FS). Newly-formed ferric coagulant is effective for the removal of turbidity, oxygen consuming matter, and UV254 adsorbance. The pH after sediment ranges from 6 to 8, exposing no great impediment to subsequent procedures. The current disadvantages are color and high residual iron concentration after sedimentation. However, increasing the H₂O₂ dosage would alleviate the drawback while at the same time increase the cost. The density of positive charge is higher on the newly-formed ferric salt than on PFS or FS. The newly-formed ferric salt flocculates with charge neutralization as the main mechanism and adsorption as assistance, which is notably different from the way PFS and FS work.Ferrous, as the main reacting material, is cheap and easy to obtain. As a result, the cost of chemicals is about 50% of the commercial price of PFS. The preparation of the newly-formed ferric coagulant is simple and convenient, without need of heating, high pressure, or any exclusive huge reaction vessel. The price-performance ratio of the newly-formed ferric coagulant is high. This preparation method provide a new approach to considerable decreasing of coagulant cost. As H₂O₂ is non-toxic strong oxidant and no toxic or harmful gas is produced during preparation, this preparation method is safe and environmental-friendly and thus worthy of further investigation toward common application.Decrease of coagulant dosage contributes to reduction of sludge. In order to improve the coagulation efficiency of the newly-formed ferric salt with cost control as premise, to decrease residual iron concentration, and to reduce color, trials of newly-formed composite ferric-aluminum, newly-formed ferric-silicate and newly-formed ferric-polyammonium were implemented. The removal of turbidity and color (residual iron concentration) by both newly-formed composite ferric-aluminum and newly-formed ferric-polyammonium were better than newly-formed ferric salt, probably because bridge-adsorption was strengthened. The coagulation efficiency of newly-formed ferric-silicate is worse than newly-formed ferric salt, probably because silicate molecules didn't polymerize in such a short aging time as 1 min, even though in acid solution.