Mercury removal technologies

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Abstract: Mercury poses severe hazards to humans as well as to animals and plants. It can enter the human body through various routes and primarily accumulates in organs such as the liver, kidneys, brain, heart, and bone marrow, eventually leading to death over time. Therefore, how to effectively detect and manage these heavy metal elements, namely mercury, is a topic of great concern to environmental workers. Based on the author’s many years of practical experience, this article provides a brief overview of several methods for detecting and treating the heavy metal element mercury, for reference by colleagues in the field. 　　Keywords: mercury detection ； Mercury wastewater ； Governance Technologies 　　 1. Mercury Detection 　　 1.1 Cold Atomic Absorption Method 　　 This method is suitable for the determination of mercury in various types of water, with a minimum detection concentration of 0.1–0.5 parts per million of mercury (this varies depending on the sensitivity of the instrument and the volume of sample taken). 　　Mercury atomic vapor selectively absorbs ultraviolet light at 253.7 nm. Within a certain concentration range, absorbance is proportional to the mercury concentration. 　　After digestion of the water sample, mercury in various forms is converted into divalent mercury; then stannous chloride is used to reduce the divalent mercury back to elemental mercury. The resulting mercury vapor is carried into the absorption cell of the mercury analyzer using a carrier gas, where its absorbance is measured and compared with that of a mercury standard solution for quantitative analysis. 　　A cold atomic absorption mercury analyzer is used; a low-pressure mercury lamp emits 253.7 nm ultraviolet light, which passes through an ultraviolet filter before entering the absorption cell. The mercury vapor released as a result of reduction in some of the samples absorbs this light, while the remaining ultraviolet light is focused on a photomultiplier tube by a quartz lens. The resulting photocurrent is amplified by an electronic amplification system and sent to an indicator for display or to a recorder for documentation. Once the gauge scale is calibrated with a standard sample, the mercury concentration can be read directly. The mercury vapor generation path is as follows: an exhaust pump draws the carrier gas (air or nitrogen) into a reduction flask containing the pre-treated water sample and stannous chloride, where mercury vapor is generated. This vapor then passes through a molecular sieve bottle to have the water vapor removed, before entering an absorption cell to have its absorbance measured. Afterwards, it is discharged through a flow meter and a mercury removal trap (which absorbs mercury from the exhaust gas). 　　 1.2 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) The inductively coupled plasma optical emission spectrometer (abbreviated as ICP-OES) is one of the main instruments used today for the quantitative analysis of inorganic heavy metals. The principle involves using argon plasma at a temperature of up to 10,000 K to excite the element to be analyzed. When the excited element returns to its stable ground state due to energy instability, it emits a characteristic wavelength specific to that element; quantitative analysis is then performed based on the intensity of this characteristic wavelength signal. 　　ICP-OES is an instrument that allows for the detection of multiple elements at various wavelengths with a single sample injection, which means it offers rapid analysis when multiple element analyses are required for the same sample. Furthermore, thanks to its high excitation energy and precise optical design, it offers high sensitivity and stability, making it the ideal choice for high-precision detection; it is designated as the required analysis equipment in many standard testing methods. 　　 1.3 Atomic Absorption Spectroscopy (AAS) Atomic Absorption Spectroscopy, abbreviated as AAS, is a relatively traditional modern detection technique. It involves atomizing the atoms of the element to be analyzed using a flame or electric heating method (graphite furnace AA), and then quantification is carried out based on the fact that the absorbance of the element is proportional to its concentration. Due to the fact that graphite furnace atomic absorption is used to detect extremely low concentrations of substances, and it requires long processing times, is costly, and involves advanced technology, it is not recommended for the routine analysis of inorganic substances regulated by RoHS; instead, a conventional flame atomic absorption spectrometer is advised. Since AAS requires the use of an air-cathode lamp and is typically used for analyzing a single element at a time, it is time-consuming when multiple elements need to be analyzed in the same sample. The instrumentation industry has now developed a rapid sequential AAS method similar to ICP-OES analysis, which allows for sequential element switching during the analysis of the same sample, effectively overcoming the drawback of AAS’s slower speed. At the same time, thanks to this special detection method, AAS equipped with a rapid sequencing function possesses an internal standard correction feature, which provides further assurance for the accuracy of the detections. 　　 2. Current status of treatment technologies for mercury-containing wastewater 　　 2.1 Chemical precipitation 　　 2.1.1 Neutralization-coagulation precipitation method 　　 Alkaline neutralizing agents are added to heavy metal mercury wastewater, causing the mercury ions to react with hydroxyl groups to form insoluble mercury hydroxide precipitates, which can then be separated. 　　The coagulation-precipitation method can also effectively remove the heavy metal mercury from wastewater. In alkaline solutions, aluminum salts and iron salts can form colloids with strong adsorption capacity. These colloids can not only adsorb mercury ions in wastewater but also capture and entrap suspended mercury ions, causing them to precipitate together. For example, by adding lime milk and salt coagulants of iron or aluminum to wastewater, under weakly alkaline conditions with a pH of 8–10, mercury and the hydroxide flocs of iron or aluminum precipitate together. If the mercury ion concentration in the wastewater is 2, 5, 10, or 15 mg/L, the corresponding mercury ion concentrations in the effluent are 0.02 mg/L, less than 0.1 mg/L, less than 0.3 mg/L, and less than 0.5 mg/L, respectively. 　　 2.2 Sulfide precipitation method 　　 2.2.1 Basic principles and characteristics of the sulfide precipitation method 　 The method in which sulfides such as sodium sulfide or hydrogen sulfide are added to wastewater, causing heavy metal ions to react with sulfur ions to form insoluble metal sulfide precipitates, is known as the sulfide precipitation method. Since heavy metal ions have a strong affinity for thiosulfate anions, forming sulfides with low solubility products, using sulfides to remove dissolved metallic mercury ions from wastewater is an effective treatment method. 　　The treatment of mercury-containing wastewater must be carried out under weakly alkaline conditions; typically, lime milk and an excess of sodium sulfide are added to the wastewater. At a pH of 9–10, sodium sulfide reacts with the mercury ions in the wastewater to form insoluble mercury sulfide precipitates. 　　Mercuric sulfide has a very fine particle size, and most of it remains suspended in boiling water. To accelerate the sedimentation of mercuric sulfide and simultaneously remove excess sulfur ions remaining in the wastewater, ferrous sulfate is added in appropriate amounts, resulting in the formation of iron sulfide and ferrous hydroxide precipitates. 　　The solubility product of mercury sulfide is much lower than that of iron sulfide; therefore, the precipitate formed is primarily mercury sulfide, which precipitates along with ferrous hydroxide. 　　A certain wastewater contained 0.6–2 mg/L of mercury; after adjusting its pH to 9 using lime milk, a 3% sodium sulfide solution was added and the mixture was stirred for 10 minutes ； Add another 6% ferrous sulfate solution, stir for another 15 minutes, let it settle for half an hour, and the supernatant will meet the discharge standards. The sediment contains 40%–50% mercury; after centrifugal drying, it is sent to a roasting furnace for roasting in order to recover metallic mercury. The mercury content in the calcined mercury slag can be reduced to 0.01%. 　　A certain wastewater contains 5 mg/L of mercury, has a pH of 4.5–6.5, and includes ferrous ions; after treatment with lime milk and sodium sulfide, the mercury concentration in the discharged water is reduced to 0.05 mg/L. 0.5 kg of lime and 0.05 kg of industrial sodium sulfide are consumed per cubic meter of wastewater. 　　The sulfide precipitation method is an effective way to remove heavy metal ions from boiling water. To ensure the complete removal of heavy metal pollutants, it is often necessary to add an excess of sulfides; this excess of sulfides can generate hydrogen sulfide gas, leading to secondary pollution, which limits the widespread use of this method. 　　 3. Improvements and developments in the sulfide precipitation method To separate heavy metal pollutants from wastewater without causing secondary pollution due to the generation of harmful hydrogen sulfide gas, sulfide ions and a certain heavy metal ion are selectively added to the wastewater to be treated. These heavy metal ions form a sulfide with the added sulfide ions, and the ionic equilibrium concentration of this sulfide is higher than that of the sulfide of the heavy metal pollutants to be removed. Since the sulfides of the added heavy metals are more soluble than those of the heavy metals already present in the wastewater, the heavy metal ions originally present in the wastewater are separated first compared to those added. In this way, the added heavy metal ions act like scavengers for the excess sulfides. It also prevents the formation of harmful hydrogen sulfide and sulfide complex ions. Furthermore, under certain conditions, the added heavy metals promote the co-precipitation of other metal sulfides, thereby improving the quality of the treated wastewater. 　 　3.1 Iron oxidation method 　　 Iron salts are added to wastewater containing heavy metal ions, and ferrite powder is produced from the wastewater using co-precipitation. 　　If divalent iron ions coexist with divalent non-iron metal ions in the same water, adding a certain equivalent of base to the solution causes the following reaction: a dark green hydroxide mixture is formed. When this mixture is oxidized in water under certain conditions, it decomposes anew, forming complexes; ultimately, a black spinel compound (ferrite) is formed. Its reaction formula is as follows: Ferrite can also be formed by adding an alkali to wastewater in which trivalent iron ions and divalent iron ions are present in a 2:1 ratio. However, this method is not suitable for producing the powder used in ferrites, as it is difficult to control the composition and particle size. However, during the aforementioned reaction process, by appropriately selecting the concentration of iron ions and controlling the temperature of the wastewater, it is possible to easily obtain ferrites with ideal composition and particle size. 　　This method is suitable for wastewater containing heavy metals with a density of 3.8 g per cubic centimeter or higher, such as mercury. 　　 3.2 Reduction method 　　 According to the theory of electrode potential, a metal with a lower electrode potential can displace a metal with a higher electrode potential in solution. Therefore, iron filings, aluminum filings, zinc, copper, tin dioxide, and the like can all reduce mercury ions to metallic mercury, causing it to precipitate. 　　3.2.1 Copper shavings reduction method The copper shavings reduction method is used to treat wastewater containing mercurous nitrate, mercurous sulfate, as well as mercuric nitrate and mercuric sulfate, with a mercury removal efficiency of around 99%. 　　For example, a certain wastewater contains 100–300 mg/L of mercury, with a pH of 1–4. After clarification, the wastewater passes through two filter columns made of copper shavings at a filtration rate of 5–10 m/h; as a result, the mercury content in the effluent drops to around 0.05 mg/L, achieving a treatment efficiency of 99%. The treatment efficiency decreases significantly when the pH is 10 or higher. 　 3.2.2 Zinc granule reduction method 　　 Abroad, the use of zinc granule filters to treat mercury-containing wastewater has shown excellent results. Reportedly, wastewater with a mercury ion concentration of 100,000 micrograms per liter is passed through a 10-cm-thick filter bed made of zinc granules with a particle size of approximately 2 mm; within 13 seconds, the concentration is reduced to 20 micrograms per liter. A slightly thicker filter bed can reduce the mercury concentration to 2 micrograms per liter within 60 seconds. Treating wastewater within the pH range of 5–10 is effective. 　　 3.2.3 Sodium borohydride method This method involves adding wastewater containing inorganic mercury compounds to an alkaline solution of 12% NaBH in a static mixing tank, with the pH maintained between 9 and 11. Sodium borohydride then reduces the mercury compounds to metallic mercury and hydrogen gas. The hydrogen gas produced is washed with dilute nitric acid to remove any mercury vapor carried along with it; the sludge resulting from dehydrogenation is sent to a cyclone clarifier, where 80%–90% of the mercury is removed along with the sludge ； The clarified water then enters the purification filter, resulting in a mercury content in the wastewater that is usually below 0.01 mg/L. Mercury recovered is purified using vacuum distillation. 1 kg of sodium borohydride can reduce 21 kg of mercury, and the precipitate obtained by this method is metallic mercury, making it relatively easy to recover. 　　 3.3 Electroflotation method The electroflotation method is a purification and separation technique that utilizes electrolytic reactions and secondary precipitation, as well as physical phenomena such as flotation and adsorption. This method is generally used to remove substances with lower density from wastewater, such as oils, emulsions, and organic suspended particles. 　　 3.4 Ion exchange resin method 　　 Mercury in wastewater exists in the form of divalent mercury cations, anionic complexes, and elemental metallic mercury. Ordinary strong-base anion exchange resins can remove mercury complex anions; however, the treatment efficiency is poor, with the mercury concentration in the effluent remaining above 0.1 mg/L. Moreover, the presence of other anions—especially high levels of chlorides—affects the resin’s exchange capacity for mercury. Taking advantage of the fact that mercury and sulfur can combine to form mercury sulfide, which has a very strong binding capacity, a macroporous ion exchange resin containing thiol groups was synthesized. This resin is highly effective at removing mercury; the concentration of mercury in the effluent can be reduced to 0.05–0.005 mg/L. It also has a high exchange capacity and is not affected by other salts present in the wastewater. 　　 3.5 Activated carbon adsorption method 　　 Activated carbon can be used to adsorb and remove mercury from wastewater; the efficiency of this treatment depends on factors such as the concentration and form of mercury in the wastewater, the type and amount of activated carbon used, as well as the contact time. The greater the degree of dissociation in water and the larger the ionic radius of a mercury compound, the easier it is to be adsorbed, resulting in better treatment effects; conversely, the effects are worse. Additionally, summary: Mercury poses a severe threat to humans as well as to animals and plants. It can enter the human body through various routes and mainly accumulates in organs such as the liver, kidneys, brain, heart, and bone marrow; over time this accumulation can lead to death. Therefore, how to effectively detect and manage these heavy metal elements, namely mercury, is a topic of great concern to environmental workers. Based on the author’s many years of practical experience, this article provides a brief overview of several methods for detecting and treating the heavy metal element mercury, for reference by colleagues in the field. Conclusion: Mercury is a toxic heavy metal element that poses serious threats to humans, and it has caused severe harm to humanity in the past. In today’s era of rapid technological advancement, we already have various testing and analysis instruments as well as effective treatment methods. However, the most important way to prevent mercury from polluting the environment is to reduce its use. Therefore, in our daily lives, we should use mercury-containing products wisely to reduce mercury pollution. Furthermore, we must also work on technologies for addressing mercury pollution, hoping that more effective methods for dealing with it will be developed.