Atmospheric Enuironment Vol. 23, No. 9, pp. 1939-1943, 1989. M&.5981/89 $3.00+0.00

Printed in Crest Britain. Pergamon Press plc

SURFACE REACTIONS OF SO2 ON METALLIC PARTICLES

CORRADO CONTE,* GIUSEPPE DEVITOFRANCESCO$ and VALERIA Dr CASTRO~

* Laboratorio Chimico Centrale Dogane, Roma, Italy, t Consiglio Nazionale delle Ricerche, Centro Studi Opere d’Arte, Istituto Fisica Tecnica, Via Monte d’Oro, 28,00186 Roma, Italy and §Dipartimento Chimica,

Universitl “La Sapienza”, Roma, Italy

(First received 8 June 1988 and received for publication 21 February 1989)

Abstract-In this note the particle surface transformations are examined, using the results obtained by electronic spectroscopy measurements. The tests concerned the preparation of condensation aerosols from metallic (Zn, Cd) vapours in N, flow (or N, and 0,) so as to obtain clean surfaces immediately brought in contact with the gas. On the surface, new sulphurated compounds and physically adsorbed gas layers were generated and revealed by degassing, using X-ray photoelectron spectroscopy (XPS-ESCA).

The determination of the sulphur chemical state on the surface suggests a model of the particles composed of a metallic core and a layer, which depends on the particles’ reactivity to the gas. These transformations can be responsible for behaviour in the atmosphere and irritant properties, and for dangerous effects for human health.

Key word index: Metallic surface aerosol, adsorption of SO,, coated particles.

INTRODUCTION

The study of aerosols increasingly shows the import- ance of the surface of particles, a parameter which has a fundamental role in a certain number of areas, e.g. in the behaviour of particles in the atmosphere and in their hygienic and toxicological effects. The surface of particles plays an active role in the chemistry of the atmosphere, favouring reactions between gases pre- sent in traces. One may mention the action of particles composed of transition elements: Mn, Fe, V, etc. in the oxidation processes of SO, and also of the nuclei in the formation of condensation aerosol (Barbaray et al., 1977; Devitofrancesco and Conte, 1987; Hulett et al., 1972).

The formation of new surface compounds is inter- esting. In many studies concerning the interaction between SO2 and particles, attention has been direc- ted to the sulphates present, considering the principal compounds containing S in the aerosols (Wagman et al., 1967). Probably the oxidation or reduction of the S deriving from the use of the conventional chemical analysis techniques has contributed in confirming this hypothesis. As a matter of fact seven distinct chemical species containing S have been found (Craig et al., 1974).

Inhalation of metallic vapours (Zn, Ni, Mn, Cd, etc.) causes acute, although transitory, phenomena not clearly explained, with muscular cramps, vomiting and headache, attributed to protein degradation pro- ducts, released by the respiratory membranes when they dome in contact with the surface of metallic particles deposited on them: ‘casting fever’, etc. (Drin- ker and Hatch, 1954; Mayers, 1969). Previous inhala-

$ Author to whom correspondence should be addressed.

tion of particles of Cd0 has also been reported as causing sensitization to bacteria, but desensitization to viruses (Bouley, 1982). This is especially true for particles that have just been generated.

The tests described here concern the analyses of the surface layer of particles formed by condensation of vapours of Zn, Cd and their oxides, kept in contact with SO,; the analyses were carried out with the aid of electronic spectroscopy in order to recognize the new compounds formed.

The procedure adopted consists of the following:

(4

04

IX

formation of the aerosol by homogeneous con- densation of the vapours of the metal in a flow of inert gas (N,) or of a mixture of 0, (25%) and N, (75%); fixation of the particles on a filter; treatment of the aerosol with SO,; analysis by means of photoelectron spectro- scopy.

EXPERIMENTAL

The aerosols were prepared by homogeneous condensa- tion of the metallic vapours in a flow of inert gas (NJ in a line comprising a horizontal cylindrical oven (diameter 2.5 cm, length 304 in which the ceramic vessel containing the granules of the metal was put. The temperature of the oven in the zone where the metal was placed (720°C and 850°C for Cd and Zn, respectively) was controlled by a thermocouple (alumekonstantan). The N, flowed (1 emin-‘) from one end of the oven through a steel capillary; the particles, through an asbestos restriction and a Cu tube for cooling, were collected on the opposite side on a filter of fibreglass without organic binder (Gelman Type A/5,5 cm in diameter). Then the SO, was introduced in the same airtight line. The introduction of the SO, was done in two ways, depending on the concentration desired: for concentrations of the order of ppm, using a permeation tube (O’Keefe and Ortman, 1960);

1939

1940 CORRAD~COP~~E er al.

for concentrations of the order of several per cent, using a glass container fitted with a capillary tube in which the SO,was put after successive dihztions, by movement of Hg in a graduated phial. To obtain &he aerosols of the metal a flow of’ N2 was used as a carrier gas, and for the oxides a mixture of 0, (2504) and of N, (75%) was employed. In fact, in preliminary tests, by putting filtered air in the line after the exit uf the oven, the immediate transformation of the par- ticles of metallic Zn into particles of oxide was abtained; this was confirmed by means of the refraction with X-rays. In the case of Cd this t~ns~o~ation did not take place; instead, for the formation of the oxide, the presence of the 0, was indisponsible right from the time of the gaseous phase of the metal. The aerosol deposited on the filters was analyzed by X-ray photoelectron spectroscopy (ESCA) by means of a VG-ESCA 3 spectrometer using the K. radiation of Al. The values of bond energy have been corrected by C,, (285.0 eV) as to avoid charEe effects. The atomic ratio SL& and S/Cd have been cat&&d using the cross-section tabulated values (Scofield. 1976).

RESUtl’S AND DISCUSSYON

The vapours of eiements such as Cd and Zn, with no great difference between melting point and boiling point, unlike other metals, are generally formed of non-spherical particles with a crystalline structure and a marked granulometric dispersity (Homma et al., 1980). The polydispersity of the particles is affected by the temperature and by the flow of the carrier gas (in our work conditions CMVID=0.8 pm, ~~=2.2). The condensation of the vapours ofcadmium and zinc in air is an example of the diversified formation process of condensation aerosol, which can happen, starting from the metal? in two sequences: (1) va~~rizatjon~oxjdation~ondensation, for almost all metals; (2) va~rization~ondensation~x~dat~on~ as happens, for example, with the ‘nobler’ metals (Grek- ula et al., 1986). Consequently, there are two models of particles: the first caacerning the homogeneous par- ticle of oxide of the metal (with a thin superficial interface); the second that of a particle composed of a metal core around which a layer of oxide has been formed.

In reality there are “technical’ surfaces of the bodies in contact with the atmosphere, humidity and va- pours, and the ‘clean’ Ones obtained from the former by removing the molecules fixed on the interface between the two phrtsas, solid and gaseous, by means of degreasing, degassing, etc. At the moment of tke formation tke ‘pure’ surface of a metal is instantan- eously covered with a layer of gas.

The method adopted of generating particles in an inert gas environment makes it possible to obtain surfaces that can be considered ‘clean”,

The ESCA analyses of the particles collected on giassfibre filters and kept in contact with SO2 for different times and with concentrations between 1 ppm (see Fig. 1) and 15% (in air), confirm the presence of S in various forms on the surface and make it possible to determine the behaviour of the two metals.

After the treatment with SO, the fiiters are divided into two parts: one half is subjected dire&y to the

167.5 IF?.0

W

Fig. 1. Photoelectron spectra of sulphur (SO,: 1 ppm on Cd aerosol-20 h); eV 167.5 =SO:-, 16Y.O==SO:-.

s~~tros~~i~ analysis, the other is degassed for 5-h in a Aow of N,, at ZWC, and then analyzed too. In every case the S peak seems composite, i.e. made up of different components with close binding energy. In fact, the full width at half maximum (FWHM) of the SzP peak in our samples is in the range 3.54.OeV, while the FWHM of the S,, in ZnSU~, measured in the same experimental conditions, is 2.5 eV, moreover the shape of the S peak is different from the one of a single S signal and changes with the treatment. Vari- ous curve decompositions of the S,, peaks have been checked by an IBM AT computer using a curve fitting program based on a Gaussian curve and the best fittings obtained in this way are used to identify the contributions of the various S species. A comparison of the photoelectronic peaks of the SzEr indicates that the binding energies of the various components are consistent with literature values for the various spe- cies: eV: 167.3 +0.2= SO:-; 168.2 +O.Z = SO,,; 169.3 fO.3=SU:-; 170.5~0.2=SO,,, (Appel et at, 1980; Craig et al., 1974; Novakov, 1972; Wagner et al., 1979). The attribution of the peaks at 168.2 eV and 170.6 eV to adsorbed SO2 and SOS, respectively is supported by the fact that these two peaks are not present in the degassed samples.

The graphs in Figs 2 and 3 refer to aerosols in contact with higher con~ntrations of SO2 (of tke order of 15% in air) for 45 min, concentrations whick permit a more detailed examination of the peaks.

Table 1 shows the binding energies and the percent- age of the various species for the case of the aerosols of metals and of their oxides in contact with a 15% concentration of SO, for 45 min.

Surface reactions of SO, 1941

b

e

e.K

Fig. 2. Photoelectron spectra of sulphur on Cd aerosol and Cd0 aerosol: (a) Cd +SO,, (b) Cdl-S!& after degassing, (c)

Cd0 -I- so,.

The r&z&on af the S band after the degassing (15% of SO2 for 45min) is about 15% for the Zn aerosol, and 80% for the ZnO aerosol treated in the same way.

During the degassing the energy supplied in the form of heat leads to an increase of the chemical

so;*+%” II I( il iI

e.v “6%; nsrq.3 I &

4652

Fig. 3. Photoelectron spectra of sulphur on Zn aerosol and ZnO aerosol: (a) Zn + SO,, (b) Zn +SO, after degassing, (c)

ZnO + SO,.

compounds containing S at the expense of part of the SO2 adsorbed physically. chemiadsorbed layer of SO:- and SO:- that pre-

With Zn there is a greater tendency to physical serves the metal, by passivation, from further attack. adsorption that with cadmium; in the case of ZnCY While in the case of Zn and ZnO the SO:- is between there is only physical adsorption (SO, and SO,) 20% and 50% of the SO:-, with Cd and Cd0 the except for small quantities, traces of salts that can SO:- and SOi- are, for the high SOa concentrations hardly be calculated, With cd and Cd0 there is a considered, of the same order of magnitude. For the greater reactivity to the SOs, with the formation of a Cd aerosol in contact with SO2 (1% for 1 min), even

1942 CORRADOCONTE et al.

Table 1. Binding energies (eV), concentrations of the various species identified (%) and sulphur/metal ratio, in the ESCA analyses of aerosols of Zn, ZnO. Cd and Cd0 in

the presence of SO, (15% for 45 min) -

SO: SO,,, so:- SO,,, S/Me 167.3+0.2 168.2+0.2 169.3kO.3 170.6 + 0.2

Zn + SO, 10% 40% 49% 0.9 Zn + SO, (deg) 16% 83% 0.8 ZnO+SO, X6% 14% 0.7 ZnO + SO, (deg) 35% 65% 0.1 Cd+SO, 47% 10% 42% 0.9 Cd + SO, (deg) 48 % 52% 0.8 Cd0 + SO, 41% 9% 49% 0.8 Cd0 + SO, (deg) 45% 55% _ 0.9

with the limitations of this technique, a decrease of the S peak, after 2 and 4 min of sputtering, of 12% and 42%, respectively was noted.

The behaviour with low SO, concentrations is different. For example, for an exposure of Cd aerosol to an SO, concentration of 1 ppm prolonged for 20-h, an S/Cd atomic ratio of about 0.5 is obtained com- pared with one of 0.9 for the action on Cd of SO, in a concentration of 15% for 45 min, and different per- centages of SOi- and of SO:- (23% and 77%, respectively) are also obtained (Fig. 2). A plausible mechanism that can be hypothesized for this latter case is that of an initial fast adsorption followed by a ‘tail’ of slow adsorption, after oxidation of the SO, on the surface. With high concentrations and short ex- posures, with a surface cover O=O, the SO:- and the SOi- are of the same order of magnitude; with a concentration of 1 ppm prolonged for several hours, there is first the most rapid formation of SO:- on the most active sites and then oxidation on the surface of the SO2 at SO:-. This can explain the different percentages found, an oxidation of the SO:-, which are stable products, cannot be hypothesized (Eatough, 1978; Cox, 1986). Zinc has a smaller capacity of reacting with SO, than has Cd: with 1 ppm for 20 h, it is not possible to estimate the peak of the Szp; with 2 ppm for the same time, a peak is obtained whose S/Zn ratio is 0.04 (see Table 2).

CONCLUSIONS

The study of the aerosol surface by means of photoelectron spectroscopy has advantages in many cases, although there are some difficulties and un- certainties, such as those connected with the diversity of the layer formed on the filter by the particles (penetration into the pores, etc.), losses connected with the necessary vacuum (10-7-10-9 torr) and heating of the sample for the bombardment. The most sub- stantial advantage concerns the ascertainment of the speciation of the superficial substances. In the cases examined it is possible to distinguish the presence of molecules of SO2 and SO3 and of the ions SO:- and

Table 2. Ratios S/Me and SOi-/SO:- for several concentrations of SO,

Aerosols

Cd+SO, (15%, 45 min) Zn+SO, (15%, 45 min) Cd + SO, (1 ppm, 20 h) Zn + SO, (2 ppm, 20 h)

S/Me so~-/so:-

0.9 1.1 0.9 0.2 0.55 3.3 0.04

SO:-, although they have the same oxidation state ot 4 and 6, respectively.

An in-depth study can be made of the reactivity from the constitution of the surface of the particles, with the identification of the oxidation state of the elements and the different composition of the surface layer at various depths for some tens of A. In reality one can have a non-homogeneous particle as regards its composition, with an extensive surface concentra- tion of an element; this can lead to error if it is considered as a component uniformly distributed throughout the particle (a layer of 50 8, on a particle of 1 pm represents only 1.5% of the mass).

With the deepening of the study of the surfaces, our knowledge of the behaviour of aerosols with respect to the various phenomena increases, as regards both their presence in the atmosphere (SO:--coated par- ticles are the most effective in the scattering of light (White and Roberts, 1977), and their toxicology. A diversified health risk can be hypothesized when par- ticles are inhaled in the presence of SO,, because of the gas adsorbed and because of the formation, at ambient temperature, of a layer of more soluble sulphurated products with synergic effects. For example, with the SO, adsorbed there occurs a increase in airway resi- stance (Amdur et al., 1988) to the respiratory Bow because of bronchovasoconstriction and the possible arrest of the ciliary action, impairment in the diffusion capacity which is followed by the action proper of the metallic particle.

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Surface reactions of SO1 1943

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