The Impact of Macromolecular Natural Organic Matter on Water Quality

Natural organic matter is an intermediate product formed during the natural metabolic cycle of plants and animals, primarily composed of humic substances. These are large molecular weight condensations containing various functional groups such as phenolic hydroxyl, hydroxyl, and alcoholic hydroxyl groups, with molecular weights generally ranging from 300u to 30000u. Humic substances are typically classified into three parts based on their solubility in acidic and alkaline solutions: 1) humic acid, soluble in alkaline solutions but precipitates upon acidification; 2) fulvic acid, soluble in both acidic and alkaline solutions; and 3) humin, insoluble in both acidic and alkaline solutions. While structurally similar, these three parts differ in molecular weight and functional group content. Compared to humic acid and humin, fulvic acid has a smaller molecular weight and contains more hydrophilic functional groups. Humic substances themselves are harmless to humans, but due to the various functional groups on their surface, they can complex with metal ions in water, affecting water treatment efficiency. Organic matter can adsorb onto the surface of colloidal particles, forming an organic protective film that not only increases the surface charge density of the colloidal particles but also hinders the bonding between them. This is because organic matter generally carries a high surface charge, such as the surface charge density of fulvic acid (10C/mgDOC～15C/mgDOC) which is much higher than that of clay particles (0.1C/mg～1.0C/mg)[1]. It has been reported[2] that when kaolin or silica adsorbs 5mg/L～10mg/L humic acid, its stability in water increases by 1 time, or the collision efficiency during coagulation decreases by 1 time; this effect becomes more significant as the concentration of organic matter increases or the pH of the solution decreases.

The protective effect of organic matter on colloids in water leads to a significant increase in the dosage of coagulants. Edzwald[1] pointed out, based on the difference in charge between fulvic acid and inorganic particles, that if 3mg/L fulvic acid is added to a suspension containing 10mg/L inorganic colloids, the dosage of coagulants needs to be increased by 6 times to destabilize it. Yao Chonghua and Yan Xushi [3] determined through experiments that if the concentration of fulvic acid in clay suspension increases by 3 mg/L (as TOC), the dosage of aluminum sulfate will increase by 5.3 times; if the concentration of fulvic acid increases by 7 mg/L (as TOC), the dosage of aluminum sulfate must be increased to 10.2 times to achieve the same coagulation result. High concentrations of organic matter in water increase the consumption of coagulants and raise water treatment costs. Since most water plants in my country use aluminum coagulants, the concentration of aluminum ions in the treated water is too high, affecting the health of residents (it has been reported that excessive aluminum intake can lead to neurofibrillary tangles, inhibit the secretion of gastric juice and gastric acid, and reduce pepsin activity). Furthermore, because the content of natural organic matter in water is high (on the order of mg/L), it will react with added water treatment agents (such as disinfectants CI2, O3, etc.) and be converted into harmful organic matter or intermediate products.