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This post was last edited by Qingfeng Xuchi on 2022-1-15 10:14. I. What is pH? The hydrogen ion concentration index refers to the ratio of the total number of hydrogen ions to their total amount in a solution, and is commonly referred to as “pH”. The acidity, neutrality, or alkalinity of a solution is determined by the relative concentrations of these substances; at any temperature, a solution is acidic when [H⁺] > [OH⁻] ； 2. It is neutral when = ； 3, 9: When the copper ion concentration is 200 mg/l, there is definitely a problem with these chemical data. Because at high pH values, copper ions precipitate as copper hydroxide, the concentration cannot be very high (Note: ammine complexes of heavy metals can exist at high pH values). At low pH values, salt solutions of heavy metal ions generally have higher solubility and exhibit certain colors; for example, copper sulfate is blue while nickel sulfate is green. Furthermore, the pH value affects the existence forms of certain heavy metals; for example, in the case of chromium Cr, it exists in the form of chromate in alkaline environments, namely (CrO42-) ； In acidic environments, it exists in the form of dichromate, namely (Cr2O72-) ； In acidic environments, the turbidity of the solution is relatively high; the substances present are generally silicates (sediments), oils, etc. It is worth noting that mercury sulfide is also insoluble in acid; therefore, when purifying solutions to remove mercury using high-concentration acids, sulfides can be introduced to precipitate mercury. Furthermore, in the field of chelating resins, an acidic environment is conducive to the proper functioning of the resins. The main reason is as follows: first, functional groups have a strong adsorption capacity for heavy metals such as copper and nickel ； Secondly, in an acidic environment, heavy metals exist in ionic form, and no hydroxide precipitates are formed to clog the resin. III. The pH value is of great significance to water treatment processes. Common methods for wastewater treatment include physical methods, chemical methods, physicochemical methods, and biological methods. Many of these treatment methods involve chemical reactions that need to take place at a specific pH level; otherwise, the desired product will not be obtained. The influence of pH on physicochemical methods: Physicochemical methods used in wastewater treatment include coagulation, air flotation, adsorption, electrochemical methods, etc. The coagulation-sedimentation method is widely used in water supply and wastewater treatment. Coagulants for wastewater treatment fall into two categories: inorganic metal salts and organic polymer compounds. The former mainly includes high-valent metal salts such as those based on iron and aluminum, which can be further divided into ordinary iron and aluminum salts as well as alkalized polymeric salts ； The latter is divided into synthetic and natural categories. The degree to which pH affects coagulation varies depending on the type of coagulant used. When aluminum sulfate is used to remove turbidity from water, the optimal pH range is between 6.5 and 7.5 ； When used for removing chroma, the pH value should be between 4.5 and 5 ； When using trivalent iron salts, the optimal pH range is between 6.0 and 8.4. Polymer coagulants, especially organic polymer coagulants, are less affected by pH value in terms of their coagulation efficiency. As can be seen from the hydrolysis reaction equations of aluminum salts and iron salts, the continuous generation of H+ during hydrolysis will inevitably lower the pH value of water. To keep the pH value within the optimal range, an alkaline substance should be used to neutralize it. 1. The impact of pH on chemical treatment methods is primarily seen in the advanced treatment of wastewater, including chemical demulsification, chemical oxidation (air oxidation), ozone oxidation, chlorine oxidation, hydrogen peroxide oxidation, Fenton reagent oxidation, KMnO4 oxidation, K2FeO4 oxidation, photochemical oxidation, etc., among which Fenton reagent oxidation is a relatively typical example. The Fenton reagent oxidation method involves mixing H2O2 and FeSO4 in a certain ratio to generate hydroxyl radicals with high oxidizing power, which possess both oxidizing and coagulating effects. Based on the results of experiments and practical applications, it is concluded that the best treatment effect is achieved at a pH value of 3; under this acidic-alkaline condition, the COD removal rate exceeds 90%, while the average decolorization rate is also above 95%. 2. The impact of pH on biological treatment methods: Biological treatment of wastewater is divided into aerobic biological treatment and anaerobic biological treatment. The effect of pH on aerobic biological treatment: The optimal pH value for aerobic microorganisms is generally between 6.5 and 9 ； When PH is less than 4.5, fungi will become dominant, leading to sludge bulking; in such cases, adding lime, controlling the water inflow rate, and reducing aeration can be used to suppress sludge bulking. The effect of pH on anaerobic microorganisms: In the application of anaerobic methods for wastewater treatment, since acid production and methane production take place in the same facility, in order to maintain balance and prevent excessive acid accumulation, the pH level in the reactor should be kept within the range of 6.5–7.5 (preferably 6.8–7.2). In summary, pH represents the acidity or alkalinity of a substance, and it plays a crucial role in water treatment. It has a significant impact on areas such as domestic water supply, industrial water use, and industrial wastewater. Therefore, only by fully considering the chemical properties of various substances under different pH levels and selecting an appropriate pH range can water treatment achieve the best possible results.