Leaching Behavior of Mercury from Spent Fluorescent Lamps Solidified with Cement1

BOBIRICĂ, Constantina, DUMITRESCU, Simonab, OLESCU, Aurorac and STĂNESCU, Rodicad

a Analytical Chemistry and Environmental Engineering, Politehnica University of Bucharest, Bucharest, Romania, e-mail: c_bobirica@yahoo.com
b S.C. Gremlin Computer S.R.L., e-mail: simona.dumitrescu@gremlincom.ro
c S.C. Gremlin Computer S.R.L., e-mail: aurora.olescu@gremlincom.ro
d Analytical Chemistry and Environmental Engineering, Politehnica University of Bucharest, Bucharest, Romania, e-mail: rodica_stanescu_ro@yahoo.com

Abstract The treatment of spent fluorescent lamps with high content of mercury by stabilization/solidification processes was investigated. At the end-of-life of these lamps, mercury may still be in elemental form or could be incorporated as oxide into fluorescent layer called „phosphor”. Due to high mercury content, the spent fluorescent lamps are considered hazardous waste, and therefore dangerous for both human health and environment. Based on both leaching tests and mathematical modeling it was established that the solidification of glass derived from spent fluorescent lamps with cement leads to a solidified material characterized by a high Leachability Index (LI). These results highlight a good behavior to leaching, and therefore a low mobility of mercury in the solidified material. It is also shown that the solidified material could candidate for a non-hazardous waste in relation with mercury.

Keywords dangerous, human health, mercury, fluorescent lamps, stabilization/solidification

1  Introduction

It is well known the extremely harmful effects of mercury and its compounds on the human health (Holmes et al., 2009: 172; Zahir et al., 2005: 351). Elemental mercury and some of its organic compounds affects the central and peripheral nervous system, and are also responsible for some cardiovascular diseases (Virtanen et al., 2007: 76). Inorganic compounds of mercury are also very corrosive to the gastrointestinal tract and accumulate in kidneys (Graeme and Pollack, 1998: 50). There are still many human activities that generate mercury in the environment either indirectly, such as burning of fossil fuels or waste incineration, or directly in mining activities, health care and a lot of manufacturing processes such as pesticides, mirrors and medical equipments, industrial leaks, lighting, etc. (Lee and Lee, 2013: 242; Pavlish et al., 2003: 96).

Mercury is widely used in manufacturing of fluorescent lamps. Its role is to emit ultraviolet light after electrical excitation of its atoms in the vapor phase. The flux of ultraviolet light strikes a luminescent material, called as „phosphor”, attached on the inner walls of the fluorescent lamp, which as response produces a bright visible light (Dunmire et al., 2003: 11). The amount of mercury used in fluorescent lamps depends on the type of lamp (Linear Fluorescent Lamps – LFLs or Compact Fluorescent Lamps – CFLs), brand, year of manufacture, and wattage. For example, the amount of mercury is up to 30 mg/lamp for CFLs and up to 115 mg/lamp for LFLs (Nance et al, 2012: 543). During lamp use, the elemental mercury is either converted into solid mercury compounds or adsorbed onto components as the phosphor powder, end caps, and glass. However, at the end of lifetime of these lamps, which is about four to six years under normal operation, most of the mercury is found in the phosphor powder (Rey-Raap and Gallardo, 2013: 175). Thus, the mercury concentration could reach up to 4.7 g/kg of powder (Truesdale et al., 1992: 2, 8).

Currently, the chemistry of mercury inside the fluorescent lamps during their operation is not fully understood and some controversial issues regarding its chemical speciation were reported. In this respect, it is believed that over time an increasing amount of elemental mercury is converted in mercury compounds, especially mercuric oxide and only a small part of elemental mercury initially present will still be there (approximately 0.2% by mass). In addition, it seems that mercury in vapor phase is especially elemental, while divalent mercury is mainly incorporated into the phosphor powder (US EPA, 1997: 2-4). On the other hand, other studies on the speciation of mercury in spent and new fluorescent lamps by thermal release analysis highlighted that the dominant species present in both phosphor powder and glass are elemental and monovalent mercury (Raposo et al, 2003: 885).

There is a series of complex technologies (dry or wet) developed to remove the mercury from the components of the lamps and recycle it for using in other sectors. The dry technologies rely on heating the components of the fluorescent lamp above the boiling point of mercury (375°C) for several hours followed by condensation of mercury vapors into a scrubber (Jang et al., 2005: 6). Usually, the wet technologies involve some preliminary treatment steps of lamp components such as washing or chemical extraction using a wide range of acidic or alkaline solutions, followed by recovery of mercury by a recovery process such as adsorption, ion exchange, precipitation of highly insoluble mercury compounds or by heterogeneous photocatalysis (Bussi et al., 2010: 478). The main disadvantage of these technologies is that it involves a sophisticated treatment in several stages. Although the results indicate that a high effectiveness can be achieved, a total removal or a concentration in compliance with environmental legislation cannot be achieved. Thus, if some mercury still remains in the lamp components, then the treated spent fluorescent lamps could be considered as hazardous.

European Commission Directive 2002/95/EC on the restriction of the use of certain hazardous substances in electrical and electronic equipment, including fluorescent lamps, states in the case of CFLs that the amount of mercury should not exceed 5 mg/lamp, and in the case of LFLs (halophosphate for normal purposes) the amount of mercury should not exceed 10 mg/lamp (Directive 2002/96/EC, 2003: 7). Also, in the United States the amount of mercury in 40 W/T12 fluorescent lamps (the most used fluorescent lamps) decreased over time from 41 mg/lamp (until 1992) to 21 mg/lamp after 1997 (US EPA, 1997: 2-4).

Cementation is a technique applied for reducing the mobility of specific hazardous chemical species. Cement is a very good binding agent for heavy metals due to its alkalinity and its effectiveness is evaluated by leaching tests. Currently, there are many regulatory compliance tests developed all over the world to compare concentrations or fluxes of emitted pollutants to the criteria of waste acceptance in landfill or criteria of valorisation. These tests are performed in different conditions, and therefore the results for the same type of waste or treated waste could be contradictory. Within this context, the objective of this paper is to establish experimentally as well as using a mathematical model for two different conformity leaching tests, if the solidification with cement of the residual glass from spent fluorescent lamps is an effective treatment technology that could be used for mercury immobilization in low leachability cement-based matrix.

2  Material and Mass

2.1  Samples preparation

The solidified samples were prepared by contacting Portland cement (CEM II 42.5 R) with small cullet glass from spent fluorescent linear lamps (FL), and water under continuous mixing at following mass ratios:

  • 2.25 aggregate/cement (FL/c) and 0.5 water/cement (w/c) (hereinafter ASTM C1260 mixture). These ratios are used to prepare mortars or concretes for establishing if the aggregate are suffering changes due to alkali-silica reaction, (ASTM C1260, 1994: 3);
  • 1.4 aggregate/cement (FL/c) and 0.3 water/cement (w/c) (hereinafter HPM mixture). These ratios were used to prepare a mortar with high performances: low w/c, low porosity and high strength (Ferraris, 1995: 14).

The cullet glass was still covered with „phosphor“ powder and had the role of aggregate in the concrete matrix. The glass particle size was less than 4.75 mm (fine aggregate). The samples were prepared in large quantities to minimize the carbonation process in the laboratory atmosphere. Subsequently, the mortars were placed into the molds (2.2 cm × 2.2 cm polypropylene cylinders) and held for hardening (solidification) in a 98% relative humidity atmosphere over 28 days at 20±2C.

2.2  Determination of Total Content of Mercury

The total content of mercury from spent fluorescent lamp glass used in experiments was analyzed as follows: 0.25 g of finely ground glass (including „phosphor“ powder) was contacted with a solution of HNO3 (2:1 concentration) until the phosphor layer solubilization. The extract was then diluted to 500 mL with distilled water. Mercury was analyzed by atomic absorption spectrophotometry with graphite furnace assembly and generator of hydrides.

2.3  Solidified Waste Testing

Solidification is a technique applied for reducing the mobility of hazardous content of specific chemical species. Cement is a binding agent for heavy metals and its effectiveness is evaluated by leaching tests. The solidified materials and glass cullet were subjected to two compliance standard tests (European Standards EN 12457-1, 2, 3, 4, 2002: 1; TCLP Method 1311, 1992: 1) to determine mercury leachability.

The European set of standards EN 12457 1-4 “Granular waste compliance leaching test” use deionized water as extraction liquid. In accordance with EN 12457-3, the samples was sieved at 4 mm and shaken “end-over-end” with deionized water in two steps for a total leaching time of 24 hours: firstly at liquid/solid ratio 2 for 6 hours and then at liquid/solid ratio 8 for 18 hours. After filtering, the leachates were analyzed separately.

The acetic acid solution in TCLP (Method 1311 in „Test Methods for Evaluating Solid Waste, Physical/Chemical Methods“ EPA Publication SW-846) simulates the leachate produced in a municipal solid waste as a result of infiltrated rainwater and biochemical reactions. The test simulates the behavior of hazardous species placed in a landfill in contact with the acidic leachate. The samples with less than 9.5 mm crushed particles were shaken “end-over-end” in contact with a pH 3 solution of acetic acid at a liquid/solid ratio 20:1 for 18 hours. The leachates were analyzed after filtration. The limit values for toxicity characteristics (40 CFR 261.24) are set for preventing underground water pollution by not sanitary landfills.

To establish the mechanisms for leaching of contaminants, and to quantify their mobility in the solidified matrix, a semi-dynamic tank leaching test was carried out. This type of leaching test is suitable for inorganic contaminants and provides intrinsic material parameters for their release under mass transfer leaching conditions. In this respect, the monolithic cylindrical samples of 2.2 cm × 2.2 cm were placed in closed bottles with distilled water without agitation at 22±2C. The leaching solution was replaced with equal volumes of test solution using a liquid-to-solid ratio of 10 mL per each square centimeter of exposed surface area at cumulative times of 1, 2, 4, 8, 16 and 32 days (a 2N progression). After each leaching period the leachates were checked for mercury concentration (Kosson et al. 2002: 172).

3   Results and Discussion

The test results are presented in Table 1 and Table 2. In the case of untreated glass (Table 1), the concentrations of leached mercury (as mg of mercury/kg of dried material) are always higher then the maximum limit allowed by standard. These results suggest that the glass from spent fluorescent lamps is a hazardous waste and requires an adequate management. Contrary, in the case of solidified samples, the concentrations of mercury are always much smaller then maximum limit allowed by standard. These materials could be considered non-hazardous in relation with mercury. The results from Table 2 related to untreated glass cullet are consistent with those presented in Table 1. Regarding the results obtained for solidified glass cullet, the concentrations of leached mercury (as mg of mercury/L of leaching solution), are different as a function of mixture designed. In this respect, the concentration of leached mercury for HPM mixture is equal with the maximum limit allowed by standard, but for ASTM C1260 mixture the concentration exceeds the limit. This could be due to the more severe leaching conditions imposed by TCLP standard.

Table 1: Results for EN 12457 tests (the highest values recorded are presented)
Glass cullet size, mmMixtureStandardMercury, µg/LMercury, mg/kgRegulatory Level for
mercury limit, mg/kg
< 4.75 ASTM C1260 EN 12457 1.49 0.008 0.2
HPM1.150.007
FL140511.89

The results obtained from semi-dynamic leaching test are presented in Fig. 1. The cumulative amount of mercury leached is much higher for ASTM C1260 mixtures then HPM mixture. In order to investigate the mobility of mercury into solidified materials, the role of diffusion was considered. In this respect, a Fickian diffusion model was used for the interpretation of the semi-dynamic leaching test results (Kosson et al. 2002: 175). The assumptions of the model are as follows: the initial concentration of the species of interest is uniformly distributed throughout the solidified material and zero surface concentration of the hazardous component at solid-liquid interface.

For a one-dimensional geometry, an analytical solution for Fickian diffusion is:

where Mareat is the cumulative mass of the mercury released (surface area bases) at time t, mg/m2; C0 is the initial content of mercury in the solidified material, mg/kg; ρ is the material density, kg/m3; t is the time interval, s; Dobs is the observed diffusivity of mercury, m2/s.

Table 2: Results for TCLP tests
Glass cullet size, mmMixtureStandardMercury in leachate, µg/LRegulatory Level for
mercury limit, mg/L
< 4.75 ASTM C1260 TCLP 0.4 0.2
HPM 0.2
FL 27.7
Figure 1: Leaching behavior of mercury from solidified waste materials

Under the assumptions of the Fickian diffusion model, an observed diffusivity can be calculated for each leaching interval where the slope is 0.5±0.15.

where Dobsi is the observed diffusivity of mercury for leaching interval „i”, m2/s; Mareati is the mass released of mercury (surface area bases) during leaching interval „i”, mg/m2; ti is the contact time after leaching interval „i”, s; ti-1 is the contact time after leaching interval „i-1”, s.

The calculated value of the negative logarithm of the observed diffusivity (pDobs), or so called Leachability Index (LI), reflects the mobility of mercury in the solidified material. A high value of LI represents a lower release of mercury from solidified material. Generally, if LI < 11 the hazardous component has a high mobility, if 11 < LI < 12.5 the hazardous component has a medium mobility, and if LI > 12.5 the hazardous component has a low mobility (EA NEN 7375, 2004: 28). As can be seen from Table 3, the leachability indexes of mercury are different as a function of mixture designed. Thus, the higher value of LI for HPM mixture then that for ASTM C1260 suggests a lower mobility of mercury in this type of mixture. These results could be correlated with those obtained from TCLP conformity test. Both sets of results highlighted that the best mixture in relation with mercury leachability is HPM.

Table 3: Leachability Indexes of mercury
Glass cullet size, mm MixtureStandardLeachability Index (LI)Standard interval
< 4.75 ASTM C1260 NEN 7345 0.4 11 < LI < 12.5
HPM 12.55 > 12.5

4  Conclusions

Mercury pollution prevention is one of the best actions for assuring protection of human, animal and, environment health. Huge quantities of mercury are already spread into environment through spills or emissions, but also it is still contained in various products. Sooner or later all these products will become waste that will pose an important risk during its treatment or disposal. Understanding the behavior of mercury in different environments will make possible not only the reduction of exposure, but also setting methods and technologies for safely storage and disposal of mercury containing waste.

The glass from spent fluorescent lamps used in experimental work was considered as hazardous waste in accordance with the two conformity tests results. The objective of this paper was to investigate if its solidification with cement can lead to some low-leachability cement-based waste materials. The results obtained by conducting leaching experiments and mathematical modeling highlighted that mercury shows a proper leaching behavior in the solidified waste, which seems to be in closed relation with the type of mixture used for sample preparation. As was suggested by TCLP conformity test, the leached mercury concentration is higher than the maximum allowed limit by the standard.

The two mixtures used for samples preparation show different transport properties in the solidified waste, and subsequently, different mobility of mercury. As was suggested by the leaching model, the mercury has a lower mobility in the HPM mixture. If the preparation conditions of the solidified waste are chosen appropriately, especially the choice of mixture, then the solidification with cement could be a proper method of treatment of glass cullet from spent fluorescent lamps with high content of mercury.

In order to establish the characteristics of waste for acceptance in a non-hazardous waste landfill, the two leachability tests can give different results for the same solidified Hg containing waste. So, while performing TCLP the regulatory limits for Hg were exceeded for cemented wastes, while the results obtained with EN 12457 indicate that the same waste could be considered non-hazardous under European legislation. It is difficult to simulate the leaching conditions in a municipal waste landfill, but regulating more stringent conditions could reduce the amount of heavy metals removed from lined landfills with the leachate. Through leachate treatment, mercury could be reintroduced in environment (e.g., by incineration of the concentrate from reverse osmosis), ecosystems and eventually in the food chain.

5  Acknowledgement

The authors acknowledge the financial support from S.C. Gremlin Computer S.R.L., Romania

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Citation

Bobirică, C.; Dumitrescu, S.; Olescu, A.; Stănescu, R. (2014): Leaching Behavior of Mercury from Spent Fluorescent Lamps Solidified with Cement. In: Planet@Risk, 2(4), Special Issue on One Health: 266-270, Davos: Global Risk Forum GRF Davos.


1
This article is based on a presentation given during the 2nd GRF Davos One Health Summit 2013, held 17-20 November 2013 in Davos, Switzerland ( http://onehealth.grforum.org/home/)