How to treat mercury-containing wastewater? What are the properties of mercury compounds?

Overview of treatment methods: I. Treatment of mercury-containing wastewater by reduction methods (1) Sodium borate (NaBH4) reduction method Chemical principle: The non-metallic reducing agent, sodium borate, reacts with mercury at a pH of 9–11 to produce mainly mercury and boric acid, while releasing hydrogen gas. The redox half-reactions are as follows: Hg2+ + 2e– = Hg↓; B5– = B3+ + 8e–; 6H+ + 6e– = 3H2. In mercury-containing wastewater, sodium borate, a non-metallic reducing agent with a concentration of 12%, is added, followed by the incorporation of an alkali (to keep the pH of the wastewater between 9 and 11). This results in the formation of mercury particles with a diameter of about 10 μm, which are then separated and recovered using a hydrocyclone. The mercury remaining in the overflow is separated from water vapor and then trapped by a filter with a pore size of 5 μm. Mercury vapor in the exhaust gas is washed with dilute nitric acid and returned to the original wastewater tank for further recovery and treatment. This method is already used in industrial production in the United States, with residual mercury levels in the wastewater being below 0.01 mg/L. (II) Metal reduction method: Utilizing the redox potentials of certain metals as substitutes for Hg2+, such as Cu, Zn, Fe, Mn, Mg, Al, etc., the corresponding metal shavings are placed in a packed tower to displace the Hg2+ ions from the wastewater. For example, in the reaction of removing mercury using Fe: Fe + Hg2+ = Fe2+ + Hg↓. The E0 value for the Fe2+/Fe couple is –0.44 V, while that for the Hg2+/Hg couple is –0.854 V; therefore, the above reaction can take place. The displacement rate is affected by factors such as pH value, temperature, metal purity, and contact area. The metal reduction method can also be combined with other methods for mercury removal, such as a combined approach using filter cloth filtration and aluminum powder displacement in an alkaline solution to purify mercury-containing water. For example, tests conducted on the clarified water from copper concentrate from a domestic gold-copper mine (using the amalgamation-flotation process) showed that when the mercury content in the clarified water was 7.28 mg/L, the mercury removal rate via filtration using filter cloth was 81.51%, while the overall mercury removal rate was 97.64%. However, organic mercury cannot be treated by direct metal reduction; it is usually first destroyed using an oxidizing agent (such as chlorine) to convert it into inorganic mercury, and then reduced through metal displacement. II. Treatment of mercury-containing wastewater by sulfidation: In this method, sodium sulfide is added to mercury-containing wastewater with a pH of 9–10, causing the sulfur ions to combine with the mercurous ions in the wastewater to form mercurous sulfide precipitates with extremely low solubility. The mercurous sulfide formed in this reaction is unstable and tends to decompose further into mercury sulfide and mercury. The solubility product of the resulting sulfide is very low, allowing HgS to precipitate even under acidic conditions (pH≥1). Since the wastewater contains low levels of mercury, the addition of sodium sulfide results in an excess of S2‑ ions in the wastewater. To avoid adverse consequences, ferrous sulfate can be added to form iron sulfide precipitates. Adding a certain amount of Fe2+ can also combine with the OH– ions in wastewater to form Fe(OH)2 and Fe(OH)3, which together facilitate the co-precipitation and flocculation sedimentation of the small and scarce HgS suspension particles. Adding FeSO4 does not affect the preferential precipitation of HgS. Since the solubility product of FeS (Ksp = 3.7×10⁻¹⁹) is billions of times greater than that of HgS (Ksp = 4×10⁻⁵³), upon adding FeSO₄, sulfur in the solution will first react with mercury to form Hg₂S (Ksp = 1.0×10⁻⁴⁵), and only then will FeS precipitate. For example, when treating acidic wastewater containing 5 mg/L of mercury, the practical approach is to first use lime to adjust the pH to 8–9, making the wastewater alkaline, and then add sodium sulfide (30 mg/L) and FeSO4 (60 mg/L). Sodium sulfide precipitation is used to remove mercury, reducing its concentration in the wastewater to 1–0.1 mg/L. By further employing methods such as iron filings filtration, activated carbon adsorption, and coagulant precipitation, the mercury content in wastewater can be reduced to below 0.05–0.01 mg/L. For organic mercury as well, it is first oxidized to inorganic mercury and then treated as described above. III. Treatment of mercury-containing wastewater by static adsorption: activated carbon is used as an adsorbent, or adsorbents made from kaolin. The specific method is the static adsorption approach, that is, precipitation followed by adsorption. First, sodium sulfide is used to precipitate mercury ions as mercury sulfide, while removing suspended solids such as sediment from the wastewater. The pH value is adjusted using calcium hydroxide, and ferrous sulfate is used as a coagulant. Afterwards, activated carbon is employed to absorb any residual metallic mercury and mercury compounds; the purified liquid resulting from this treatment contains residual mercury levels that are within the allowable emission standards. Abroad, agricultural and sideline products containing tannins are used as adsorbents. Such as walnut fragments, soft peanut skins, peanut shells, straw, sugarcane bagasse, olive pits, etc.; treated clay is also used. Such adsorbents containing tannins. The presence of other metals in mercury-containing wastewater does not affect the adsorption efficiency for mercury, and its adsorption capacity exceeds that of activated carbon by 130%. IV. Treatment of mercury-containing wastewater by solvent extraction: Abroad, triisooctylamine/xylene is currently used to extract mercury from wastewater; after extraction, the residual mercury concentration in the purified liquid is below 0.01 mg/L. The extractant used for extracting mercury is then back-extracted using non-acidic salts in order to recover the mercury. V. Treatment of mercury-containing wastewater by coagulation and precipitation method: Lime is used as the coagulant. Lime is added to mercury-containing wastewater to produce Ca(OH)2, which has a coagulating and adsorbing effect on mercury. The effect is better in the presence of trivalent iron ions. When aluminum sulfate is used as the coagulant, it also yields good results in treating mercury-containing wastewater. After coagulation and sedimentation, the mercury content in the effluent water is reduced to below 0.05 mg/L. VI. Other methods for treating mercury-containing wastewater: It is reported that abroad, methods such as microbial recovery of mercury, electrolytic recovery of mercury, ferrite precipitation for mercury removal, and sulfide precipitation-flotation separation are also used. In China, ion exchange is also used to remove mercury; methods such as conversion for mercury removal and adsorption using humic acid-containing coal are also under investigation. Current gold mines use mercury amalgamation for gold extraction, and no measures are taken to deal with the mercury that ends up in the wastewater discharged from these mines. Thanks to the gravity of mercury, it settles naturally, and as a result, the mercury content in the wastewater discharged at the outlet of the tailings pond meets the standards for surface water quality.