EVALUATION OF HEAVY METAL EMISSION AND TRANSFER FROM ROAD AND TRAFFIC SOURCES

Michel LEGRET* and Christelle PAGOTTO

Laboratoire Central des Ponts et Chaussées, BP 4129, 44341 Bouguenais Cedex, France

Abstract

A study has been carried out in order to identify and quantify the main sources of pollutants related to road traffic and maintenance. The amounts of materials resulting from wear processes have been evaluated, as well as the emission of metals by exhaust fumes. The heavy metal contents (Cd, Cr, Cu, Pb and Zn) of these materials have been determined by using literature data, car manufacturer information and new laboratory measurements. The mass fluxes of heavy metals have been calculated at an experimental site located in the West of France, with a mean annual daily traffic of about 24,000 vehicles. The most important emissions are those of Cu, Pb and Zn. Cu comes mainly from brake lining wear, Pb from exhaust fumes and Zn from tyre wear, crash barriers corrosion and brake lining wear.

Introduction

Road traffic and maintenance are the origin of many types of environmental pollution. The sources of pollution can be temporary (pollution due to road works), seasonal (road salting in winter), accidental (hazardous chemicals) or chronic (vehicle exhaust, pavement and tyre wear). Estimations of pollutant fluxes from traffic are uncertain because of the complexity of vehicular sources. Literature data are scattered and sometimes old (Brinkman, 1985; Muschack, 1990) and it is necessary to take into account the changes in car manufacturing technologies. A study has been carried out in order to quantify the main sources of pollutants.

Methods

The experimental site is located on the bypass motorway of Nantes (France) (Legret et al., 1997 ; Legret and Pagotto, 1999). This section (A11) opened in August 1993; the mean daily traffic is approximately 12,000 vehicles in both traffic directions and includes about 7% of heavy lorries. The pavement, which originally consisted of a traditional asphalt surface, was replaced by a porous asphalt in June 1996. The motorway consisted of two three-lane carriageways (11.65 m wide), including a hard shoulder, separated by a central reservation.

The main sources of pollution from road traffic and maintenance are the following : tyre wear, wear of brake linings, exhaust fumes, asphalt wear, crash barriers and de-icing agents.

The heavy metal contents (Cd, Cr, Cu, Pb and Zn) of these materials have been determined by using literature data, car manufacturer information and new laboratory measurements.

Determination of metals were carried out either by inductively coupled plasma atomic emission spectrometry (ICP-AES) (Varian Liberty 220), or by flame or electrothermal atomic absorption spectrometry (AAS) (Varian AA 300 and GTA 96 or Perkin Elmer 5000 and HGA 500) depending on the metal and the concentration, after ashing the samples at 550°C and dissolving the residue in a mixture of concentrated hydrochloric and hydrofluoric acids.

Results and discussion

Heavy metal concentrations have been measured in fuel, vehicles and road materials (table 1).

Table 1 : Heavy metal contents in fuel, vehicles and road materials

Mean samples of fuel from different suppliers have been made, proportionally to the volume sold in France by each of them. Samples of brake lining and samples of tyre rubber from two different suppliers have been analysed. The results in table 1 are the mean of the two samples of each materials. A sample of stone used for the construction of the A11 motorway asphalt has been analysed. Likewise, two samples of de-icing salt used on the A11 motorway have been analysed. Nevertheless, the characteristics of stones and de-icing salts depend on their origin and can be different from one site to another. Finally, data related to bitumen come from the literature (Lindgren, 1996).

It appears that brake linings are an important source of metals, mainly Cu, Zn, and Pb, while tyre rubber is essentially a source of Zn. As regards fuel, only leaded gasoline is a source of metal and it is generally assumed that 75 % of the lead contained in gasoline escapes in exhaust fumes (Hewitt and Rashed, 1990).

In order to calculate the fluxes of pollutants it is necessary to estimate the mass of materials produced by wear processes. According to manufacturer information, the amounts of matter resulting of the wear of tyre and brake lining have been estimated :

· considering the total wear in 50,000 km of a common light vehicle tyre 0.54 m in diameter, 0.12 m wide and 6 mm thick rubber abrasion (density = 1) with 30 % void due to tread design, the total quantity of matter produced is 857 g, hence 17 mg/km. It can be assumed that for light vehicles the amount of matter is 68 mg/veh./km, and twice this quantity for heavy vehicles.

· For private vehicles, the wear was estimated considering disk brake weared in 30,000 km (2 segments per wheel; surface area = 38 cm²; thickness = 1.2 cm; density = 2.2) that equipes front-wheels and for rear wheels, either drum brake weared in 100,000 km (2 segments per wheel; surface area = 240 cm²; thickness = 0.4 cm; density = 2.8) or disk brake weared in 75,000 km (2 segments per wheel; surface area = 25 cm²; thickness = 1.2 cm; density = 2.2). The proportion of each type of rear brake was considered to be 50 % in the calculation. The wear of brake linings is considered to be approximately 20 mg/vehicle/km for tourist vehicles, 29 mg/vehicle/km for light good vehicles and 47 mg/vehicle/km for heavy lorries.

Asphalt wear has been estimated by Muschack (1990) at 3.8 mg/veh./km, and generally asphalt is made of 95 % stone and 5 % bitumen (hydrocarbons).

A study has been carried out on the corrosion of crash barriers in galvanised steel by means of an experimental set up consisting of a 1.50 m long barrier equipped with a gutter installed on the roof of the laboratory. Rain water was collected and heavy metal content was determined for a one-year period. The amount of solids and heavy metals removed by stormwater is presented in table 2 for a one-kilometre long barrier with a total surface area of 0.94 m² (two sides). The contribution from the local atmospheric background has been substracted.

Table 2 : Fluxes of pollutants from a galvanised crash barrier

Fuel consumption depends on the type of traffic : urban or highway. For the calculation we have used data related to traffic on motorway that are lower than mean consumption of vehicles (Joumard et al, 1990 ; Roumegoux, 1994). These data have been corrected due to the decrease of mean fuel consumption observed during the last years :

Private vehicles : Light commercial vehicles : Industrial vehicles :

Gasoline :7.5 l/100 km Gasoline : 7.9 l/100km

Diesel : 6.3 l/100km Diesel : 8.3 l/100km Diesel : 31.5 l/100km

By summing the emissions from the different identified sources it is possible to evaluate the unitary emission of the different types of vehicles for one kilometre of motorway (table 3). Emission of metals in exhaust fumes has been estimated to represent 75 % of the fuel metal content.

Table 3 : Unitary emissions from vehicles (motorway drive)

Considering the traffic data on the experimental sites, it is possible to calculate the mass fluxes of pollutants due to road traffic and maintenance. The distribution of the different types of vehicles has been estimated according to French national stastitics on car fleet composition, mean vehicle drives and fuel consumptions (INSEE, 1998)

The amount of de-icing salt has been estimated at 10t for the two carriageways (Legret et al, 1997). The presence of two crash barriers on each carriageway has been taken into account.

The total loads of heavy metal emissions for the year 1997-1998 are the following, for one carriageway :

Cadmium : 2 g/km/yr Nickel : 99 g/km/yr

Chromium : 24 g/km/yr Lead : 7,700 g/km/yr

Copper : 16,000 g/km/yr Zinc : 7,300 g/km/yr

Emissions of Cd, Cr and Ni appear to be poor compared to Cu, Pb and Zn.

It also appears (figure 1) that except for cadmium and zinc, vehicles are the main souce of pollutants. An important amount of zinc results from the corrosion of the crash barriers, and de-icing salts and crash barriers are also a source of cadmium. Lead mainly comes from exhaust fumes of leaded fuel vehicles , and copper comes from brake lining wear. Tyre rubber is an important source of cadmium and zinc.

Figure 1 : Relative distribution of heavy metal emissions at the A11 experimental site

Nevertheless, some hypothesis should be verified (for example the use of brakes on motorway is not common) and heavy metal emissions could have been ill defined. Further work will be devoted to evaluate the mass balance of metal fluxes considering both runoff and aerial dispersion to the local environment.

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