How to dispose of waste liquid from chemistry laboratories?

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1. Burning? It can be used for heating. But, are harmful gases produced during the combustion process? 2. Recycling? Good economic method. Isn’t solvent recovery quite troublesome? Otherwise, why do some factories just dispose of it directly? 3. Dispose of it? Convenient, but polluting to the environment. If it is poured into the sewer system, groundwater gets polluted in this way – who would then dare to drink water? 4. How should volatile and highly toxic solvents like acetonitrile be disposed of? Waste liquids must be stored in appropriate containers and at suitable locations, based on their chemical properties; they should be kept in sealed containers and not mixed together. The containers must be labeled with information regarding the type of waste and the storage period, and the waste should be disposed of regularly. General waste liquids can be discharged after being treated by acid-base neutralization, coagulation sedimentation, and sodium hypochlorite oxidation; organic solvent waste liquids should be recycled according to their properties. I. Principles for waste liquid treatment: High-concentration waste acids and alkalis must be neutralized to a neutral pH before being discharged. High-concentration organic solvents containing a small amount of the substance to be measured and other reagents should be recovered and reused. High-concentration waste liquids intended for recycling should be stored separately in order to facilitate recovery; low-concentration waste liquids, after treatment, can be discharged. The storage containers and conditions must be determined based on the properties of the waste liquids. Different types of waste liquids should generally not be mixed together, and they should be kept away from light and heat sources to prevent adverse chemical reactions. Waste liquid storage containers must be labeled with information such as the type and storage duration. II. Treatment methods for waste liquids
Waste liquids containing mercury, chromium, lead, cadmium, arsenic, phenol, and cyanide must be treated to meet standards before they can be discharged. The treatment methods used in laboratories are as follows:
1. Treatment of copper-containing waste liquids
The copper sulfate waste liquid generated from experiments is treated by adding an appropriate amount of iron powder to recover metallic copper. The resulting mother liquor is then subjected to precipitation, filtration, and dilution prior to discharge. 2. Discharge standards for the treatment of mercury-containing waste liquid: The maximum permissible discharge concentration of mercury in the waste liquid is 0.05 mg/L (expressed as Hg). Treatment methods: ① Sulfide coprecipitation method: First, adjust the pH of the wastewater containing mercury salts to 8-10, then add an excess of Na2S to induce the formation of HgS precipitate. Then, FeSO4 (a co-precipitant) is added; it reacts with the excess S2- to form FeS precipitate, which adsorbs and co-precipitates the HgS particles that are difficult to precipitate when suspended in water. After that, the mixture is allowed to stand and then separated. Following centrifugation and filtration, the mercury content in the filtrate can be reduced to below 0.05 mg/L. ②Reduction method: Using copper chips, iron chips, zinc granules, sodium borohydride, etc. as reducing agents, metallic mercury can be directly recovered. 3. Treatment of cadmium-containing wastewater
① Hydroxide precipitation method: Lime is added to the cadmium-containing wastewater to adjust the pH value to above 10.5. After thorough stirring, the mixture is allowed to stand, causing cadmium ions to form insoluble Cd(OH)2 precipitates. The precipitates are separated, and the filtrate is tested for cadmium ions using the dithizone spectrophotometric method. Once the cadmium concentration drops below 0.1 mg/L, the filtrate is neutralized to a pH of approximately 7 before being discharged. ②Ion exchange method: This approach takes advantage of the fact that Cd2+ ions have a stronger binding force with cation exchange resins compared to other ions in water, allowing for preferential exchange. 4. Treatment of lead-containing wastewater: Slaked lime is added to the wastewater to raise its pH value above 11, thereby causing the lead present in it to form Pb(OH)2 precipitates. Then, Al2(SO4)3 (a coagulant) is added to lower the pH value to 7–8; as a result, Pb(OH)2 precipitates together with Al(OH)3. The precipitates are separated, and once they meet the required standards, the wastewater can be discharged. 5. Treatment of arsenic-containing waste liquid: Add FeCl3 to the arsenic-containing waste liquid to make the Fe/As ratio reach 50, and then use slaked lime to control the pH of the waste liquid at 8–10. Arsenic in wastewater is removed by utilizing the adsorption effect of co-precipitation of newly formed hydroxides and arsenic compounds. Let it stand overnight; separate the precipitate. After meeting the standards, discharge the waste liquid. 6. Treatment of phenol-containing waste liquid: Phenol is a highly toxic protoplasmic poison. Treatment method: For low-concentration phenol-containing waste liquid, sodium hypochlorite or bleaching powder can be added and the mixture boiled, thereby decomposing phenol into carbon dioxide and water. For high-concentration phenol-containing waste liquids, they can be extracted using butyl acetate, followed by back-extraction with a small amount of sodium hydroxide solution. After adjusting the pH value, distillation is carried out for recovery. The treated waste liquid can then be discharged. 7. For the treatment of composite waste liquid: Adjust the pH of the waste liquid to 3–4 using acids and bases; add iron powder and stir for 30 minutes. Then, adjust the pH to around 9 using a base and continue stirring for another 10 minutes. Add aluminum sulfate or basic aluminum chloride as a coagulant to induce coagulation and precipitation. The supernatant can be discharged directly, while the sediment is treated as waste residue. 8. Treatment of chromium-containing wastewater: A reducing agent such as ferrous sulfate, sodium sulfite, or iron filings is added to the chromium-containing wastewater; under acidic conditions, hexavalent chromium is reduced to trivalent chromium. Then, an alkali such as sodium hydroxide, calcium hydroxide, or sodium carbonate is added to cause the trivalent chromium to form Cr(OH)3 precipitates, and the clear liquid can be discharged. After precipitation and drying, it can be treated by calcination; it is then calcined together with coal slag, and after treatment, it can be landfilled. 9. Treatment of cyanide-containing waste liquid: For low-concentration waste liquid, sodium hydroxide can be added to adjust the pH to above 10; then potassium permanganate powder (3%) is added to decompose the cyanides. For high-concentration solutions, the alkaline chlorination method can be used: first adjust the pH to above 10 with an alkali, then add sodium hypochlorite or bleaching powder. After sufficient agitation, the hydride decomposes into carbon dioxide and nitrogen, which are then released after being left for 24 hours. Cyanide-containing waste must not be dumped indiscriminately or mixed with acids, as this can produce volatile hydrogen cyanide gas, which is highly toxic. 10. Recovery of trichloromethane: The trichloromethane waste liquid is washed sequentially with water, concentrated sulfuric acid (one-tenth the amount of trichloromethane), pure water, and hydroxylamine hydrochloride solution (0.5% AR). Wash twice with doubly distilled water. Dehydrate the washed chloroform with anhydrous calcium chloride, let it stand for a few days, then filter and distill it. The distillation rate is 1–2 drops per second, and the distillate with a boiling range of 60–62 degrees Celsius (indicated by the box) is collected and stored in a brown reagent bottle (a rubber stopper must not be used). 11. Precautions for handling laboratory waste liquids: 1). Whenever possible, recover solvents and reuse them repeatedly, provided that this does not interfere with the experiments. 2). For ease of handling, such waste is typically classified into: a) combustible substances, b) non-combustible substances, c) aqueous waste liquids, d) solid substances, etc. 3). Water-soluble substances are prone to being lost as an aqueous solution. Therefore, care must be taken during recycling. However, solvents such as methanol, ethanol, and acetic acid can be easily decomposed by bacteria. Therefore, dilute solutions of such solvents can be discharged after being diluted with a large amount of water. 4). For waste liquids containing heavy metals, etc., their organic components are decomposed, and then they are treated as inorganic waste liquids. 12. Treatment of waste liquids from biological laboratories: The contamination caused by waste liquids in biological laboratories is mainly chemical and biological in nature; there is also radioactive contamination. Chemical contamination includes organic and inorganic pollutants. Organic pollution mainly consists of organic reagent contamination and organic sample contamination. In most cases, organic reagents in the laboratory do not directly participate in the reactions; they merely serve as solvents. As a result, the organic reagents that are consumed are released into the surrounding environment in various forms, and the total amount released is roughly equivalent to the amount of reagents consumed. Day after day, year after year, the emissions are quite substantial. Contamination by organic samples includes some highly toxic organic substances, such as pesticides, benzo(a)pyrene, aflatoxins, nitrosamines, etc. Inorganic pollution includes contamination by strong acids and strong bases, heavy metal pollution, cyanide pollution, etc. Heavy metals such as mercury, arsenic, lead, cadmium, and chromium are not only highly toxic but also tend to accumulate in the human body. Biological contamination includes biological waste contamination and biological bacterial toxin contamination. Biological waste includes specimens from testing laboratories, such as blood, urine, feces, sputum, and vomit; as well as testing supplies, such as laboratory equipment, bacterial culture media, and positive bacterial specimens. Inadequate design of ventilation systems in biological laboratories, or shortcomings in personal safety measures during experiments, can lead to the spread of bacterial toxins, causing contamination and even serious negative consequences. After the SARS outbreak in 2003, many biological laboratories intensified their research on the SAS virus; subsequent cases of SARS infection were mostly among researchers who became infected while working in the laboratories. Precautions: When the concentration of the waste liquid exceeds the specified level, it must be treated. However, when the treatment facilities are well-equipped, the limits on the concentration of waste liquid that can be treated are often relaxed. It is best to treat the waste liquids separately. If they are stored and then treated together, although the treatment methods will differ, in principle, various compounds that can be treated together should be collected first for processing. When treating waste liquids containing complex ions, chelates, etc., if interfering components are present, such waste liquids should be collected separately. III. The waste liquids listed below must not be mixed with each other: ① Peroxides and organic substances; ② Cyanides, sulfides, hypochlorites and acids; ③ Volatile acids such as hydrochloric acid and hydrofluoric acid, and non-volatile acids; ④ Acids such as concentrated sulfuric acid, sulfonic acids, hydroxy acids, and polyphosphoric acids, and other acids; ⑤ Ammonium salts, volatile amines and alkalis. Select containers that are undamaged and not corroded by waste liquids for collection. The components and concentrations of the collected waste liquid should be clearly labeled and stored in a safe location. Special care must be taken, especially with highly toxic waste liquids. Wastewater that emits odors such as from thiol and amine compounds, as well as wastewater that produces toxic gases like cyanide and phosphine, as well as flammable wastes such as carbon disulfide and ether, must be properly treated to prevent leaks, and should be dealt with as soon as possible. Wastewater containing explosive substances such as peroxides and nitroglycerin must be handled with care and disposed of as soon as possible. Waste containing radioactive materials is collected by separate methods, and must be handled with great care in strict accordance with relevant regulations to prevent leaks.