The Treatment of Lead- Contaminated Travertine

In the urban sites the lead addition in petrol, as an antidetonation agent, allows pollution of exposed surfaces of buildings, soils and vegetation. In a preliminary way a research work has been carried out simulating lead pollution of the slab surfaces of the Roman-Travertine stone. The Roman-Travertine is a limestone dimensional stone widely employed in Italy especially for dressing of the old buildings in Rome. Results achieved by laboratory leaching tests have put in evidence the potential for lead removing from slab surfaces by means of ammonium acetate solutions. Higher lead removal has been obtained for higher temperatures and concentrations of leaching solutions. More than 90% of the lead adsorption has been extracted from polluted stones coming from dismantling of buildings; on the contrary, the limestone rock was poorly affected by that leaching agent. LEAD CONTAMINATION OF SOIL Lead pollution of the environment derives mainly from exhausts of cars using petrol containing tetramethyle and tetraethyle lead as knock suppressers, from industrial effluents using lead or lead compounds and from primary and secondary metallurgical waste. In some areas water pollution (especially water rich in oxygen, in carbonic acid and in nitrates that facilitate lead solubility) is due to the use of old mains made of lead or of lead alloys. Environmental pollution has repercussions on man, who can absorb lead from the air, from water and from foodstuffs. In fact, whatever its source, lead pollutes the air, deposits in the soil and in vegetation and thus reaches man directly and indirectly. Lead concentration in foodstuffs can increase due to the containers in which food is preserved (lead-soldered boxes) and occasionally to protracted use of crockery coated with lead enamel and paints (Abbritti, 1988). Table 1 gives the average contents of the most common metals found in soil and plants (Sequi, 1989; Kabata Pendias et al., 1985; Canova et al., 1995). The lead concentration rates found in street dust largely exceed the average soil rates, ranging between 1600 and 2500 mg/kg (Cariola et al., 1992). Analyses carried out on soil 829 taken from firing ranges and from metallurgical sites evince the presence of lead cations, above all in the smaller(< 20 ~m) and in the larger(> 6000 ~m) size classes, while lead concentrations decrease in median particle size classes (Pruijn et al., 1993). In fact, in soil taken from a firing range the surface soil layer was polluted with balls of lead shot, that also undergo a process of leaching by atmospheric agents. In this way the leached lead is re-absorbed by the clay fraction and the organic matter in the soil. The shots undergo further reduction in size due to mechanical actions and corrosion. Conversely, in soil polluted by residue of industrial metallurgical activity the waste is easily visible, is lighter in weight than the mineral constituents of the soil and in part reduced in size due to the action of atmospheric agents. GENERAL FEATURES OF THE LEAD EXTRACTION PROCESSES Most of the soil decontamination processes can be classified in one of the following groups (Bunge et al., 1995): • biological treatments; • thermal treatments; • immobilization; • soil washing. IMWA Proceedings 1999 | © International Mine Water Association 2012 | www.IMWA.info Reproduced from best available copy Average concentration Average concentration in in soil (mg/kg) plants (mg/kg) Cadmium 0.01 :20 0.07:0.28 Chromium (Ill) 5:1500 0.02:0.2 Manganese 20: 10000 30:500 Mercury 0.01 : 0.5 0.0006 : 0.086 Nickel 10 : 100 0.2:3.7 Lead 16:80 0.05:3.0 Copper 2:250 1 : 20 Zinc 1:900 1:73 Table 1. Average contents of heavy metals in soil and plants. In the case of soil pollution by heavy metals, thermal treatments do not give good results (except for mercury) while they give satisfactory results in all cases of pollution by organic substances in all kinds of soil (Bunge et al., 1995). The remediation processes can further be classified in two general categories: treatments in situ and treatments ex situ (EPA, 1996): • the former type is carried out on the site without removing the soil; • the latter type entail removal of the contaminated soil. The separation by size of the contaminated soil particles is useful in ex situ treatments inasmuch as it enables reduction of the volume of contaminated soil, often comprised above all of the smallest particles (Pruijn et al., 1993; Bunge et al., 1995). In both in situ soil treatment processes and ex situ soil washing, chemical reagents are used to improve extraction of the contaminants (Benker, 1995). In the processes, in which chemical compounds act in the removal of the heavy metals in the soil, are involved (Palmer et al., 1996): • competition in the sites of adsorption by chemical agents acting on the surfaces of solids; • complexation with chemical agents. Studies undertaken in a laboratory scale of soil washing using chemical agents on samples of lead polluted soil have demonstrated the efficacy of 1 N solutions of HCI and 0.01 M solutions of EDT A: it gave lead removal of 92.5 % and 93.5 % respectively, while solutions of 1 N of CH3COOH and solutions of 1 M of CaCI2 gave removal of 57.3 % and 47.2% respectively (Cline et al., 1995). Lead decontamination in sites in which car batteries are stocked, has been realised both by soil washing techniques (EPA, 1996) and by in situ leaching techniques utilising chemical agents (Skinner et al., 1992). REMOTION OF LEAD POLLUTION FROM TRAVERTINE SLABS Travertine is an ornamental dimensional stone, essentially calcium carbonate, widely used, especially in Rome, due to its being easily cut in slabs or hewn, easily polished if used indoors and highly durable when used for outdoor fittings of 830 buildings. The impact of urban car traffic on travertine is degradation and in particular lead contamination by the knock suppresser agents contained in petrol. In order to verify the possibility of removing lead from the surface of travertine used as fitting for building exteriors contaminated by car exhausts, different series of leaching experiments were carried out on laboratory scale. Travertine slabs, coming from the Tivoli area in the Lazio region, were reduced in size in order to increase their specific surface and to enhance the laboratory results. The flakes were placed in glass columns and subjected to a flow of the leaching solutions for fixed periods of time during which the amounts of lead extracted were recorded. The leaching tests were carried out using 7 em glass cylinder columns. The columns were filled at the bottom with 4 em diameter glass balls, kept down with a perforated baffle plate to stop the flow carrying away the finer flakes. To reduce the possible formation of fixed paths of the leaching solution in the travertine bed a waterhead adjustable in height by drainage flow regulation was kept. Figure 1 shows the experiment apparatus utilised, while figure 2 is a schematic description of the elements composing the leaching circuit. Figure 1. Experimental apparatus for the leaching tests.