Hg is a heavy metal which can be naturally found in its elemental form (Hgº) as well as organic and inorganic compounds with oxidation states of mercury (Hg+) and mercuric (Hg2+) ions. All forms of mercury are toxic for human as they can cause various diseases. The effects of Hg on the environment are serious as some parts of Hg can be converted into HgCH3 which is highly toxic for humans and other living creatures. This metal has been applied in chemical, electrical and electronic industries (Rodríguez et al., 2012, Viana et al., 2022).
One of the main applications of Hg is in the fabrication of various types of lamps. Hg is utilized in fluorescent lamps to achieve ultraviolet light. The mercury content of the lamp is at its excited and ionic state which is highly reactive; therefore, it can form strong bonds upon interacting with different components of the lamp (Ali et al., 2021, Viana et al., 2022). Investigation of the darkening of the residues of used fluorescent lamps indicated that the mercury may penetrate into the depth of the glass as the glass contains 15 wt.% Na2O whose Na ions have high mobility and can conduct the flow of Hg ions. The results of the studies, however, show that the internal coating of the glass bulb significantly prevented the migration of the Hg ions to the glass, these results also reject the high Hg content in the glass matrix, suggesting that Hg is mostly limited on the surface and the internal coating of the glass bulb such as phosphorous. The total Hg content of the lamps varies depending on the manufacturing company, production year and location, as well as the duration of its operation. The highest penetration of Hg into the phosphorous powder occurs when the lamp is approaching the end of its useful lifetime. Therefore, metallic Hg reacts with the phosphorous powder during the lamp operation. The Hg content of a used lamp is mainly in its divalent state, while the internal coating of the lamp (specifically, the phosphorous powder) is contaminated with Hg (Hildenbrand et al., 2000, Kadam et al., 2019, Ali et al., 2021, Ali et al., 2022).
Natalia Rey-Raap showed that a fluorescent lamp encompasses 24.45 ± 0.4, 204.14 ± 8.9, and 18.47 ± 0.5 ppb Hg in the vapor phase, phosphorous powder, and glass matric, respectively. Therefore, 85.76% of the Hg content of a compact flirecent lamp enter the phosphorous powder; while more than 13.66% is diffused in the glass matrix. By leaching and eliminating the phosphorous powder on the glass surface, it is possible to classify the used fluorescent lamp as non-dangerous wastes (Rey-Raap and Gallardo, 2012).
In most of the developed countries, the recycling of the Hg-containing fluorescent lamp is of crucial significance. In this regard, specific regulations have been established for their recovery. For instance, one of the goals of EC95/2002 regulation of the European commission is to decline the risk of electrical and electronic device wastes including the fluorescent lamps. To improve the effective environmental protection, the membered countries have to make sure that at least 70% of the mean weight of the glass lamp is recycled. Therefore, efficient technologies to eliminate Hg from the lamp components is highly essential (Aucott et al., 2003).
Chang et al. showed that the Hg concentration of each lamp must not exceed 5 mg. Min Jang et al. also investigated different parameters such as temperature and pH in Hg removal from the glass of fluorescent lamps and concluded that the influence of the temperature elevation on the Hg removal is higher than pH variations (Jang et al., 2005, Chang et al., 2007).
Fernandez and Hobohm studied the glass residues of fluorescent lamps at various ratios of solid phase to liquid in an acidic environment and in different years showed that HCl is more capable of extracting Hg from the glass of fluorescent lamps as compared with HNO3. A solution with volume ratio of 1:3 of 5% HCl and HNO3 led to maximum Hg extraction of 68.3% (Fernández-Martínez and Rucandio, 2005, Rey-Raap and Gallardo, 2013, Hobohm et al., 2017).
Tunsa added a solution of HCl and HNO3 citric acid to crushed lamps and reported that the use of a diluted solution of two concentrated acids with the ratio of 1:2 can result in 89.6 ± 3.3% Hg extraction. While acetic acid (25 vol/vol%) managed to extract 2% Hg (Tunsu et al., 2014).
Banz et al. investigated Hg extraction from leachate after its acidification by H2SO4 at various temperatures. Variations in the pH value can increase Hg extraction to 99.84% and the Hg content of leachate was converted into HgS. The toxicity of the high-mercury leachate was controlled by simple cement solidification. Based on the national wastewater discharge standard (GB 8978 − 1996), the Hg concentration of the wastewater should be below 0.05 mg/l. The Hg content of the treated leachate was declined from 3.120 to 0.005 mg/L at pH of 2.98 and temperature of 30\(℃\), suggesting the completely cleaned leachate which can be freely discharged (Lu et al., 2018).
Based on the previous studies, the aim of the present research is to examine the leaching process of residual glasses of fluorescent lamps through a low-cost one-step procedure for effective and simple extraction of Hg. Maximum Hg extraction from the glass residues was achieved using a mixture of HCl and phosphoric acid (5%) at the ratio of 4:1. In this way, glass wastes can be classified as non-dangerous wastes and accumulated in the site of non-dangerous wastes. The Hg-free glass residues of the lamps can be utilized in the production of glass, cement, and ceramics industry, especially in the production of mosaic and glass mosaic.