Laboratory waste liquid treatment method(1)

If metal mercury is accidentally spilled in the laboratory (for example, from a broken thermometer), it must be cleaned up immediately - mercury vapor is highly toxic even at room temperature, so prompt action matters. A dropper can be used to collect visible droplets, or the mercury can be immersed in an acidic mercury nitrate solution to form thin copper flakes and copper wire, which are then collected and covered with water in a beaker.

For mercury particles scattered on the ground, sulfur powder can be sprinkled over the area to convert the mercury into less toxic mercury sulfide. Alternatively, a potassium permanganate solution (5:1000 volume ratio) acidified with hydrochloric acid can be sprayed on the area and removed after 1–2 hours. A third option is spraying a 20% ferric chloride aqueous solution and removing the residue after it dries - note that this method should not be used on metal surfaces, as it will cause corrosion.

For mercury-containing waste liquid treatment, the standard process is:

1. Adjust the waste liquid to pH 8–10
2. Add excess sodium sulfide to precipitate mercury sulfide
3. Add ferrous sulfate as a co-precipitating agent to form iron sulfide, which adsorbs and helps settle the mercury sulfide particles
4. Allow static separation - the clear supernatant can then be discharged
5. Recover the residue via roasting, or process it into mercury salt for reuse

Lab tip: Many facilities keep a dedicated mercury spill kit (sulfur powder, sealed containers, gloves) on hand, since delayed cleanup significantly increases vapor exposure risk.

To treat lead- and cadmium-containing waste liquid, adjust the pH to 8–10 using an alkali to precipitate Pb(OH)₂ and Cd(OH)₂. Add ferrous sulfate as a co-precipitating agent to improve settling efficiency. The resulting precipitate can be mixed with other inorganic substances and processed via sintering, while the clear liquid can be safely discharged.

This method is widely used in labs conducting heavy metal analysis, battery testing, and electroplating waste evaluation, where lead and cadmium residues are common byproducts.

Hexavalent chromium (Cr⁶⁺) is significantly more toxic and mobile than trivalent chromium (Cr³⁺), so the treatment goal is to reduce it before precipitation. Add a reducing agent - such as ferrous sulfate, sodium sulfite, or iron filings - to the chromium-containing waste liquid under acidic conditions to convert hexavalent chromium into trivalent chromium.

Next, add an alkali such as sodium hydroxide, calcium hydroxide, or sodium carbonate to form a Cr(OH)₃ precipitate at trivalent state; the clear liquid can then be discharged. Once dried, the precipitate can be roasted together with coal slag and subsequently landfilled after proper treatment.

This process is standard practice in labs performing electroplating analysis, metal surface treatment testing, and environmental compliance monitoring.

Calcium oxide can be added to raise the pH to approximately 8, producing calcium arsenate and calcium arsenite precipitates that co-precipitate more effectively in the presence of Fe³⁺. Alternatively, raise the solution pH above 10 and add sodium sulfide; this reacts with arsenic to form a low-solubility, low-toxicity arsenic sulfide precipitate.

Safety note: Any test that may generate arsenic gas (such as the Marsh test or similar arsenic detection procedures) must always be performed inside a fume hood, as arsine gas is extremely toxic even at low concentrations.

For low-concentration phenol-containing waste liquid, sodium hypochlorite or bleaching powder can be added to oxidize the phenol into carbon dioxide and water. For higher-concentration waste liquid, extraction with ethyl acetate (or a similar solvent) is recommended, followed by repeated extraction using a small amount of sodium hydroxide solution. After adjusting the pH, the liquid can undergo distillation and purification before reuse or discharge.

Phenol waste treatment is particularly relevant for labs in the plastics, resin, and pharmaceutical intermediate industries, where phenolic byproducts are common.