![]() Typical anode materials for electrocoagulation are iron or aluminium. Electrocoagulation is a process of impurity precipitation using ions passing into purified water from a dissolving anode via application of an electric current. Reagent oxidation is a special case of reagent coagulation in which oxidising reagents (e.g., potassium permanganate or bichromate) are added in purified solution to destroy organic impurities or to change the valence of multi-valent ions following precipitation. In actual use, chemical precipitation can be used as a pretreatment technology for industrial wastewater, and combined with other water treatment methods, it can further reduce the concentration of toxic metal ions.Ĭhemical precipitation or reagent coagulation precipitates impurities from purified water via change of pH, electrooxidising potential or coprecipitation using precipitating agents (coagulants) such as ferrous or aluminium sulphates ( IAEA, 1992). In addition, chemical precipitation requires a long reaction time, and the concentration of metal ions after treatment is higher. However, chemical precipitation requires many chemicals, and the generated sediment sludge needs to be treated afterward, which increases the practical operation time and overhead. Ĭhemical precipitation with low initial investment cost and easy operation can manage high-concentration metal ion sewage. However, using sulfide precipitation agents in acidic environments may generate toxic H 2S, which increases the potential dangers of sulfide precipitation technology. In addition, the precipitated sludge of metal sulfide exhibits better dehydration capacity than its hydroxide, which makes subsequent treatment more convenient. Compared with hydroxide precipitation, sulfide precipitation can achieve highly efficient removal of metal ions in a wider pH range. Traditional sulfide precipitation agents are divided into solid precipitation agents (FeS and CaS), soluble precipitation agents (Na 2S, NaHS, and NH 4S), and gas precipitation agents (H 2S). The solubility of metal sulfide precipitation is lower than that of hydroxide precipitation in an alkaline environment. Sulfide precipitation is also considered to be a mature water treatment technique for removing toxic metal ions from wastewater. Moreover, when other types of pollutants, such as complexing agents, are present in sewage, precipitation may be inhibited under alkaline conditions. When treating wastewater in which multiple metal ions coexist, it is difficult to determine the optimal pH value for the precipitation of different metal hydroxides, which may cause one metal to precipitate while another metal dissociates in aqueous solution. In addition, some metal hydroxides are amphoteric, and single hydroxide precipitation may be ineffective. Although hydroxide precipitation has been completely applied in commercial use, low-density sludge produced by metal precipitation is difficult to dehydrate after subsequent separation, which restricts its practical application. When using a hydroxide precipitation agent such as Ca(OH) 2 or NaOH to treat wastewater, adding aluminum salt (Al 2 2 or KAl 2♱2H 2O), iron salt (FeSO 4 or Fe 2 3), and organic coagulants can effectively improve the removal rate of metal ions. Based on the cost and availability, CaO and CaCO 3 are the most commonly used commercial precipitants. Different types of precipitating agents such as lime (CaO), limestone (CaCO 3), calcium hydroxide (Ca 2), and sodium hydroxide (NaOH) are used to precipitate toxic metal ions from sewage. Under normal circumstances, most metal ions will form metal hydroxides with low solubility when the pH is 8.0–11.0, which can easily be separated afterward. Among them, hydroxide precipitation is widely used for the remediation of industrial wastewater owing to its low cost, convenient operation, and easily adjusted pH of the aqueous solution. Ĭonventional chemical precipitation methods include hydroxide precipitation and sulfide precipitation. The formed precipitate can be further separated through sedimentation or filtration. The specific process of chemical precipitation adjusts the pH of wastewater to a certain range and then adds a precipitating agent to start the precipitation reaction with metal ions. Xiangxue Wang, in Emerging Nanomaterials for Recovery of Toxic and Radioactive Metal Ions from Environmental Media, 2022 3.4 Chemical precipitationĬhemical precipitation is commonly used as a simple strategy to remove heavy metal ions and radionuclide pollution in wastewater because of its low price and simple operation.
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