Methods and precautions for treating waste liquids in inorganic chemistry laboratories

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Is the waste liquid generated in chemical laboratories directly discharged into the sewer? It has a tremendous impact on water quality and the surrounding environment. In this regard, the editors at Tushan Environmental Protection referred to relevant materials and summarized some methods for treating inorganic laboratory waste liquids that laboratory personnel should be aware of and pay attention to, as well as precautions regarding laboratory waste liquid treatment. Classification of inorganic waste liquids in laboratories: Waste liquids in inorganic laboratories generally contain heavy metals such as mercury, chromium, cadmium, lead, copper, silver, etc., as well as waste liquids containing arsenic, cyanide, fluorine, and acids or bases. This paper mainly focuses on the study of some other metal waste liquids such as mercury, chromium, and lead. The general principles for treating inorganic laboratory waste liquids are to classify and collect them based on their characteristics. During experiments, it is necessary to control the amount of reagents used or employ substitutes, with the aim of minimizing the volume of waste generated and reducing pollution. Methods for treating inorganic waste liquids in laboratories. 1. Treatment of mercury-containing waste liquids. The main source of mercury in chemical laboratories is mercury salts. At normal temperature and pressure, mercury tends to combine with oxygen to form mercuric oxide. If mercury is discharged into rivers, microorganisms can convert the mercury floating on the water surface into methyl mercury, which is easily absorbed by most aquatic organisms. Methylmercury is known for causing nerve damage, and fish are the main organisms that absorb methylmercury from water. Methylmercury accumulates in fish and then spreads throughout the entire food chain. Animals that consume these fish suffer from poisoning caused by long-term exposure to mercury, including reduced fertility, damage to the digestive system, gastric erosion, DNA mutations, and kidney failure. Treatment method 1: Principle of the sulfide coprecipitation method – Na2S or NaHS is used to convert Hg2+ into water-insoluble HgS, which is then coprecipitated with Fe(OH)3 for removal. If the reaction is carried out at a pH above 10, HgS turns into a colloidal state. At this point, it is difficult to completely remove it even by filtering with filter paper. If an excess of Na2S is added, 2- is formed, and the precipitate tends to dissolve. If an excess of Na2S is added, 2- is formed, and the precipitate tends to dissolve. Procedure: 1. Add Na2S·9H2O in a 1:1 equivalent amount relative to the concentrations of FeSO4 (10 ppm) and Hg2+ to the waste liquid. Stir thoroughly, and keep the pH of the waste liquid within the range of 6–8. 2. After standing, the above solution is filtered to remove the precipitate, and the filter residue is stored properly (using this method, the Hg concentration can be reduced to below 0.05 ppm). 3. Then, further treat the filtrate using methods such as activated carbon adsorption or ion exchange resins. 4. The treated wastewater may be discharged only after it has been confirmed that no Hg is present. The second treatment method (activated carbon adsorption method) involves diluting the waste liquid first, so that the Hg concentration is below 1 ppm. Then, add NaCl, adjust the pH to around 6, add an excess amount of activated carbon, stir for about 2 hours, then filter. Keep the filter residue safe. This method can also directly remove organic mercury. Principle of the third treatment method (ion exchange resin method): NaCl is added to the mercury-containing waste liquid, causing the formation of 2- complexes that are then adsorbed by an anion exchange resin. However, depending on the form of mercury, this method sometimes fails to produce ideal results. Furthermore, this method is also not applicable in the presence of organic solvents. Additionally, it should be noted that 1. Since mercury readily forms complex ions, its existing form must be considered during treatment. 2. If NaHS and ZnCl2 are used in place of Na2S+FeSO4, mercury can be reduced to extremely trace levels. For example, to 1 liter of waste liquid containing 10 ppm of Hg with a pH of 10.3, 32 mg of NaHS and 80 mg of ZnCl2 were added for treatment. After treatment, the Hg concentration was reduced to 0.003 ppm. II. Treatment of chromium-containing wastewater: Chromium exists mainly in three forms: metallic chromium, trivalent chromium, and hexavalent chromium. The harmful components in chromium waste liquid are mainly soluble sodium chromate and hexavalent chromium ions such as acid-soluble calcium. Hexavalent chromium is easily absorbed by the human body; it enters the body through the digestive system, respiratory tract, skin, and mucous membranes. If laboratories discharge these ions directly into underground channels, it will contaminate groundwater and thus affect human health. Operation steps: The chromium-containing waste liquids collected in the laboratory are generally acidic; a corresponding treatment plan is formulated based on the components present in these waste liquids. 1. Under acidic conditions, a suitable reducing agent is used to reduce hexavalent chromium to trivalent chromium. 2. Then, the pH is adjusted to an appropriate range to convert trivalent chromium into chromium hydroxide precipitate; subsequently, chromium hydroxide reacts with sulfuric acid to yield chromium sulfate. Scrap iron filings are commonly used as reducing agents; sodium sulfite can also be used for reduction, and the process is fast. At this stage, the substance is in liquid form, and once it has all been converted into trivalent chromium, it is advisable to treat it with lime to form a precipitate, which makes handling easier. III. Treatment of lead-containing wastewater: Lead is the only trace element that the human body does not need; it is a stable, non-degradable pollutant that can accumulate in the environment over time. When accumulated to a certain extent, it can cause chronic poisoning symptoms such as mental disorders, nightmares, insomnia, and headaches ； In severe cases, there may also be fatigue, loss of appetite, nausea, bloating, abdominal pain, or diarrhea. Step 1: Add slaked lime to the waste liquid and adjust the pH value to above 11, so that lead in the waste liquid forms Pb(OH)2 precipitate. 2. Then, add Al2(SO4)3 (coagulant) to lower the pH value to 7–8; this causes Pb(OH)2 and Al(OH)3 to co-precipitate. The precipitate is then separated, and after meeting the standards, the wastewater is discharged. IV. Treatment of arsenic-containing waste liquid: Add FeCl3 to the arsenic-containing waste liquid to make the Fe/As ratio reach 50; then use slaked lime to control the pH value of the waste liquid between 8 and 10. Arsenic in wastewater is removed by utilizing the adsorption effect of co-precipitation of newly formed hydroxides and arsenic compounds. Leave it for one night, separate the precipitate, and discharge the waste liquid once it meets the standards. V. Treatment of phenol-containing waste liquids: Phenol is a highly toxic cytoplasmic poison; the treatment method involves adding sodium hypochlorite or bleaching powder to low-concentration phenol-containing waste liquids and heating them, thereby breaking down 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. VI. Treatment of cyanide-containing wastewater: For low-concentration wastewater, sodium hydroxide can be added to raise the pH to above 10, followed by the addition of potassium permanganate powder (3%) to decompose the cyanides. For high-concentration cases, 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 discarded carelessly or mixed with acids, as this can produce highly toxic volatile hydrogen cyanide gas. VII. Treatment and recovery of silver-containing waste liquids When recovering silver from waste liquids containing various metal ions, adding hydrochloric acid does not result in co-precipitation. Under alkaline conditions, the hydroxides of other metals precipitate along with silver chloride, and acid washing of the precipitate can remove other metal ions. The obtained silver chloride is treated with (4 mol/L) H2SO4 or a 10^(-15) sodium chloride solution; zinc is then used to reduce the silver chloride until no white substance remains in the precipitate. A dark-gray fine metallic silver precipitate is formed. After washing with water and drying, this precipitate can be melted in a graphite crucible to obtain metallic silver. VIII. Treatment of sulfur-containing waste liquids: Most waste liquids containing inorganic sulfides, sulfur, oxygen acids of sulfur, and hydrogen sulfide can be oxidized by H2O2 into sulfates for reuse. Sulfur-containing waste liquid can also be treated with ferrous sulfate and lime to control the pH at 8–9, resulting in the precipitation of iron sulfide. IX. Treatment of waste liquids containing other metal elements and fluoride-containing waste liquids: For the treatment of such waste liquids, lime is added at a pH of 8.5 to form calcium fluoride precipitates; adding alum simultaneously results in even better co-precipitation effects. Barium-containing waste liquid can be removed by forming barium sulfate precipitate. Cadmium-containing wastewater is removed by forming hydroxide precipitates at pH 10–11 or by coprecipitation with iron hydroxide at pH 6.5. Nickel-containing waste liquid is treated with lime at pH 11.5 to produce nickel hydroxide precipitate. Zinc-containing waste liquid is precipitated and removed at pH 11 using calcium oxide or sodium hydroxide. Copper-containing waste liquid is used at pH 8.5 with sodium hydroxide or sulfite to precipitate copper for recovery. Waste acid and waste alkali solutions are neutralized to neutrality before discharge. 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 versus non-volatile acids; ④ acids such as concentrated sulfuric acid, sulfonic acids, hydroxyacids, and polyphosphoric acid versus other acids; ⑤ ammonium salts, volatile amines and bases. Precautions regarding the storage of waste liquid: 1. Choose containers that are undamaged and not susceptible to corrosion by waste liquid for collection. The components and concentrations of the collected waste liquid should be clearly labeled and stored in a safe location. In particular, great care must be taken with highly toxic waste liquids. 2. Waste liquids that emit odors, such as those containing thiols and amines, as well as waste liquids that generate toxic gases like cyanides and phosphine, and highly flammable waste liquids such as carbon disulfide and ether, must be properly treated to prevent leaks; such treatment should be carried out as soon as possible. 3. Waste liquids containing explosive substances such as peroxides and nitroglycerin must be handled with care and disposed of as soon as possible. 4. Waste containing radioactive substances shall be collected using separate methods, and must be handled with great care in strict accordance with relevant regulations to prevent any leakage.