Platinum analysis and speciation in urban gullypots

587

Analytica Chimica Acta, 284 (1994) 587-592 Elsevier Science B.V., Amsterdam

Platinum analysis and speciation in urban gullypots Chen Wei and Gregory M. Morrison Department of Sanitary Engineering, tXhers University of Techdogy, S-412 96 Giiteborg (Sweden) (Received 4th September 1992; revised manuscript received 21st September 1992)

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Concentrations of platinum in the <63-pm fraction of some Swedish road sediients have been shown to increase from 3.0 ng gg’ in 1984 to 8.9 ng g-’ in 1991. Road sediments contained 39-88% more Pt than gullypot sediments and sequential extraction showed a distinct shift from predominantly inorganic Pt on the road surface to wholly organic Pt in the gullypot. Dissolved Pt concentrations in disturbed gullypot liquor were within the range 1.7 ng I-’ to 3.8 ng 1-l and are explained by bacterial action in the gullypot sediment mobilizing organically bound dissolved Pt forms. Keywords: Digestion techniques; Voltammetty; Catalysts; Dry ashing; Platinum

The number of automobiles equipped with catalytic converters is steadily increasing. In 1989 autocatalysts constituted 42%, 8% and 81% of total world demand for Pt, Pd and Rh, respectively [l]. Manufacturers of modern three-way (i.e. removing hydrocarbons, carbon monoxide and nitrogen oxides together) catalysts currently favour Pt and Rh as the active metals in a ratio of 5 : 1. Already catalytic converters are unequivocably the prime source of Pt to road surface sediments 121, albeit in low concentrations. Pt emissions from automobiles are due to the abrasion of the catalytic surface, the emitted particles having a median size of 5-10 pm with an estimated emission rate of 2-40 ng km-’ 131.A comparison of Pt concentrations in size-fractionated road sediments, collected in 1984 and 1991, has shown average increases in all the fractions, but particularly in the < 63-pm fraction with an increase from 3.0 to 8.9 ng g-’ [2]. The low total concentrations of Pt found in Correspondenceto: C. Wei, Department of Sanitary Engineering, Chalmers University of Technology, S-412 96 Giiteborg (Sweden).

road sediments require a sensitivity of analysis at the lower ng 1-l level and this can be achieved by using a highly sensitive voltammetric method [4,5]. Pt is quantified after the catalytic reduction of protons by either the Pt-formazone or the Ptethylenediamine complex preadsorbed to a mercury drop electrode [6]. The results obtained by this method (adsorptive voltammetry) are comparable to those obtained by inductively coupled plasma-mass spectrometry, at least for Pt in blood [51. The adsorptive voltammetric method is highly sensitive for Pt but is negatively affected by even the smallest residual trace of organic material in samples, with extinction of the hydrogen wave [7]. Although aqua regia digestion [B] and ultraviolet irradiation [6] have been tested previously, it is our experience that dry ashing is the most reliable digestion procedure for the total destruction of our highly organic road sediment and gullypot (roadside catchbasin) samples. This agrees with the findings of other research groups [71. In this article we report on the analysis of dissolved/suspended solid associated and solid phase fractionated Pt in road sediments, gullypot

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C. Wei and GM. Morrison /AnaL Chim. Acta 284 (1994) 587-592

liquor and gullypot sediment. Gullypots have been shown previously to act as an accumulative chamber for heavy metals from road runoff and provide a significant contribution to the end-of-pipe “first flush” effect found under wet-weather conditions [9].

EXPERIMENTAL

Sampling Road dusts were collected by means of a wet/dry vacuum cleaner for a four-month period (June to September) at two-week intervals in 1984 and again in 1991. The samples were collected from an asphalt parking area at Chalmers University of Technology with a catchment area of 390 m2. Two representative sub-areas on the catchment were sampled. The first was an 11.7-m2 car parking lot and the second a 215 cm long roadside kerb of area 0.65 m2. After collection

the road dusts were air dried, weighed and sieved prior to analysis. The gullypots, chosen to represent different degrees of road usage by automobiles, were sampled on two occasions (A and B in Table 1). During A (29-04-1992) the gullypot was being washed by rainfall with a total of 11.4 mm for the day and 0.8, 25.2, 4.8, 8.4 and 5 mm on the previous five days. On occasion B (26-05-1992) an antecedent dry period of 14 days was recorded. On both occasions samples of undisturbed gullypot liquor were taken in a 5OO-ml polyethylene bottle secured to an extended stainless steel holder. On occasion A the undisturbed sample was supplemented by a disturbed sample where the gullypot contents were first vigorously stirred with a stainless steel rod. This action was meant to simulate the effect of a high-intensity summer rainfall, which typically mobilizes the gullypot contents [9]. On return to the laboratory, samples were

TABLE 1 Gullypot sample characteristics Sampling site

Sample no.

Sampling occasion

Status a

Suspended solids (g 1-l)

PH

u D U U U D U U U U D U U D U D U U D U U

0.51 28.7 1.31 0.04 0.17 62.2 0.4 0.31 0.19 0.09 22.4 0.63 1.37 35.6 0.19 57.4 0.036 20.5 1.21 2.03

7.5

Residential

1

A

Parking area

2 3 4

B B A

Roundabout

6

B A B A

Dual carriageway

7

B A

8 9 10

B A B A

11

B B

5

Motorway, E20

a D = disturbed; U = undisturbed.

7.0 6.5 6.5 7.0 6.4 7.0 7.0 7.0 7.4 7.2 7.1 7.8 7.5 7.1 7.0

C. Wei and GM. ~Uorrison/Anal. 0th.

Acta 284 (1994) 587-592

analysed for pH and suspended solids concentration. The latter analysis was by filtration through a GF/C glass fibre filter (Whatman) and drying to constant weight. Sub-samples were filtered through a 0.45pm cellulose acetate filter (Sartorius) and the solids dried at 105°C. All water samples were kept at 4°C before analysis. Platinum &tennimtion Either a dried sediment sample (25-30 mg) or a water sample (lo-20 ml) was transferred to a silica crucible. A volume of 600 ~1 of 15 M HNO, was added and the crucible placed in a muffle furnace with a stepwise program to 800°C [2,5]. When cool, the ashed sample was dissolved in aqua regia (1.5 ml of 12 M HCl and 0.5 ml of 15 M HNO,), left to stand for 4 h and taken just to dryness on a hot plate. A volume of 0.6 ml of 12 M HCl, 8.6 ml of ultrapure water, 0.4 ml of 0.4% hydrazine and 0.4 ml of 3.2% formaldehyde were added to the sample crucible. The crucible was placed directly in the voltammetric cell (Metrohm 647 VA stand). The sample was deaerated with nitrogen for 5 min and preelectrolysed at -800 mV vs. Ag/AgCl for 180 s. A glassy carbon (instead of the more common platinum) auxiliary electrode was used. The stirring was stopped for 10 s and finally cathodic stripping was recorded in the differential-pulse mode with the following instrumental settings (Metrohm 646 VA processor): modulation amplitude, 25 mV; pulse repetition rate, 200 ms; effective scan rate, 20 mV s-l. Pt concentration was determined by comparison with a Pt standard curve and confirmed by standard addition analysis. Sequential extraction of sediments Road surface and gullypot sediments were subjected to a modified form of a sequential extraction scheme [lo]. The fraction identification, extractants and extraction conditions were: (9 exchangeable, 1 M NaAc (10 ml), pH 7, 1 h, room temperature, shaking; (ii) carbonate, 1 M NaAc (10 ml), pH 5, 1 min, microwave digestion; (iii) Fe and Mn hydrous oxide, 0.04 M hydroxylammoniurnchloride, pH 2, 1 min, microwave digestion; (iv) organic, 30% H,O,, pH 2, 5 h, 86°C in a

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water bath; (v) residual, 15 M HNO,, 1 min, microwave digestion. All extractions were carried out in a Parr 45-ml PTFE cup. For microwave extractions the cup was sealed in a Parr 4782 digestion bomb. The digestion bomb was placed in a household microwave oven at full effect. The microwave oven was calibrated [ll] and full effect was found to be equivalent to 474 W. The digestion bomb and PTFE cup were opened carefully after cooling in an ice bath for at least 30 min. The H,O, extraction was not carried out by microwave after initial tests showed the risk for a rapid reaction causing pressure buildup and expulsion of the heated bomb contents into the microwave oven (and the risk for explosion). The extraction scheme was applied initially to 2 g sediment with removal of 200-300 mg for analysis after each extraction. Dried, accurately weighed sub-samples (approximately 30 mg) of the extracted sediment were analysed for the remaining total platinum concentration. In most other sequential extraction schemes the extractant is analysed for metal concentration; on the contrary, we discarded the extractants as their matrix effects on Pt analysis by adsorptive voltammetry have not been investigated. Instead the remaining Pt was analysed and subtracted from total Pt. The retained sediment after each extraction was sufficient to allow five Pt analyses, which we felt to be the minimum necessary for statistical purposes. Three samples each of the < 63-Frn fraction of road sediment and gullypot sediment were analysed.

Microtox toxicity test Microtox is an instrument which utilizes the light emitting bacterium, Photobacterium phosphoreum. The results are expressed as an EC,, value which is the toxicant concentration that causes a 50% reduction in light emission. Microtox was performed directly on road and gullypot sediments. Freeze-dried P. phosphorewn was reconstituted for use in the test. All tests were carried out at 15°C. Data for the solid phase test were analysed according to the manufacturers operating manual [12].

C. Wei and GM Morrison /Anal. Chim. Acta 284 (1994) 587-592

590

Chemical oxygen demand Chemical oxygen demand (COD) was used as a surrogate parameter for (oxidizable) organic material and was determined by the sealed-tube method. The commercial sealed-tube method is a Dr Lange cuvette LCK 114 (COD measuring range, 150-1000 mg 0, 1-l). Sediment was added to the cuvette and heated in a LASA Aqua LTK 031 thermostat. Cr3+ was measured in a LASA pocket photometer.

RESULTS

AND DISCUSSION

Recovery and precision of Pt in road sediments The percentage recovery for Pt added to road sediment has been tested previously [2]. For the sediments analysed in this study the absolute quantity of Pt analysed was in the range 0.015-0.3 ng which gives a recovery of between 63% and 85%. Volatile Pt compounds may explain the loss, although HNO, is added before dry ashing to avoid the possible loss of volatile Pt chlorides by conversion to Pt nitrate. Because of the sensitivity of Pt analysis to incomplete digestion, some samples invariably fail

TABLE

and therefore special attention has to be paid to careful replication during digestion and analysis. The analysis of Pt in road sediments, following dry ashing and adsorptive voitammetry, has a detection limit of 0.5 ng g- ’ and a standard deviation of f2.2% [2]. For the gullypot liquor samples the standard deviation between sub-samples was poorer, *29%, due to the heterogeneous nature of suspended solids. Repeated voltammetric analysis of the same digested subsample gave a better standard deviation for the dissolved, f5.1%, and total, f5.7%, measurements. No detectable peaks were found for reagent blanks, which were run with each sample batch.

Pt in sediments A comparison of Pt concentrations and biochemical characteristics between road sediments and gullypot sediments is made in Table 2. Gullypot sediments have an oxidizable organic content which is 39-88% higher than road sediments, but a toxicity which is 260~605% lower. This suggests that road surface sediments do not wholly comprise the gullypot sediment, undoubtedly other material such as fallen leaves are significant. This

2

Platinum concentration, COD and Microtox ECso values for road and gullypot sediments

Pt, (ng g-‘) <63pm

COD

Microtox EC, Wo) <63pm

63-500 pm

500-1000

pm

1.5 a 3.6 a

< 0.5 b 2.8 b

119 128

0.1 0.2

< 0.5 1.4 5.1 1.3 1.8 1.1 0.9

208 143 139 214 232 140 167

0.52 1.04 1.21 0.82 0.68 0.89 0.59

Road sediments

(year) 3.0 8.9

1984 1991

Gulypot sediments (sampk No. ) 1 4 5 6 7 8 10 a 63-250

15.1 3.5 4.6 7.8 7.8 4.0 7.0 pm. b 250-1000

Frn.

< 0.5 2.3 2.1 1.4 1.9 1.6 1.6

591

C. Wei and GM. Morrison /AnaL Chins. Acta 284 (1994) 587-592 Residual

Exchangeable

to carbonate

Bound to organic matter 26s

Fig. 1 Distribution of Pt between geochemical fractions extracted from road sediments ( < 63 rrn).

sediment dilution is reflected in the gullypot sediment which contains a 3948% lower Pt concentration than road surface sediments in the < 63 pm fraction (with the exception of sample 1). Pt fractionation of road sediments

Sequential extraction of Pt from road sediments shows a significant exchangeable fraction which may be released into the dissolved phase during storm events (Fig. 1). For the gullypot

sediments Pt was only found in the organic fraction (and is therefore not shown in Fig. 1). This suggests that the higher organic content of gullypot sediments results in a transformation of Pt from an inorganic to an organic bound form. Pt in gullypot sampks In gullypot liquor samples (Table 3) the highest total Pt concentrations were found for undisturbed gullypots in a busy car park (site 5) and on a motorway (site 11) with concentrations of 13.6 ng 1-l and 11 ng l-i, respectively. No dissolved Pt was found in the undisturbed gullypot liquor (< 1 ng 1-i). In the disturbed gullypot samples the high suspended solids concentration made reliable total Pt concentrations difficult to achieve and several values of dissolved and total Pt are missing from Table 3 due to difficulties of analysis at the low concentrations found. However, the dissolved concentrations were within the range 1.7 ng 1-l to 3.8 ng 1-i. These Pt concentrations might be expected in the “first flush” of a heavy summer rainfall. Higher concentrations of Pt in a disturbed gullypot can be explained by biochemical activity

TABLE 3 Total platinum concentration and dissolved/suspended solid partitioning in gullypots Sample no.

1 2 3 4

5

6 7

8 9 10 11

Sampling occasion

Status a

A B B A A B A A B A B A A B A B B B

U U U U D U U D U U U U D U D U U U

’ D = disturbed; U = undisturbed.

Total Pt

Dissolved Pt

Suspended solid Pt

(no 1-l)

ins I-’ (%)I

ins s-l (%)I

7.2 7.0

4.9 (34) 6.5 (122) 51 15 (31)

a.2 7.6 13.6

2.02 1.9 (25)

17.2 (91) 13.8 (32)

1.95 6.9 8.1 2.4 5.3

31.9 (88) 33 (38) 40 (1050) 14 (369) 1.7 3.75

2.5 6.5 11.0

2.1 (32) 1.9 (17)

7.41(138)

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in gullypot sediment between storm events. This is reflected in low redox potential and dissolved oxygen ,concentration when gullypot sediments are mobilized [9]. Bacterial action effectively releases organically bound dissolved metal forms into the interstitial waters and eventually into the gullypot liquor. Increasing concentrations of metals have been shown, following an asymptotic curve, between storm events’in the gullypot liquor [9]. A similar situation appears to exist for Pt as dissolved Pt is found after the dry period (B). The gullypots in the busy parking area (site 41, dual carriageway (site 10) and motorway (site 11) had dissolved concentrations amounting to 25%, 32% and 17%, respectively, of total Pt concentration in the gullypot liquor. We have tried, unsuccessfully, to physically separate fractions of Pt in these samples by dialysis. There is certainly a need both to be able to separate dissolved inorganic/organic Pt species and to be able, in the light of possible bioaccumulation, to separate biomethylated Pt.

C. Wei and GM Morrison /AnaL Chim. Acta 284 (1994) 587-592 REFERENCES 1 M.C.F. Steel, in A Crucq (Ed.), Catalysii and Automotive Pollution Control II, Studies in Surface Science and Catalysis, Vol. 71, Elaevier, Amsterdam, 1991, pp. 105-114. 2 C. Wei and GM. Morrison, Sci. Total Bnviron., in press. 3 H.P. Kiinig, R.F. Hertel, W. Koch and G. Rosner, Atmos. Environ., 26A (1992) 741. 4 K Hoppstock, K. Ah, K. Cammann and G. Weber, Fresenius’ Z. Anal. Chem., 335 (1989) 813. 5 0. Nygren, G.T. Vaughan, TM. Florence, GM. Morrison, I.M. Warner and LS. Dale, Anal. Chem., 62 (1990) 1637. 6 C.M.G. van den Berg and G.S. Jacinto, Anal. Chim. Acta, 211(1988) 129. I V. Brabec, 0. Vrana and V. Kleinwiichter, Collect. Czech. Chem. Commun., 48 (198312903. 8 S.B. Adeloju, AM. Bond, S.N. Tan and G. Wei, Analyst, 115 (1990) 1569. 9 G.M. Morrison, D.M. Revitt, J.B. Ellis, G. Svensson and P. Balmer, Water Res., 22 (19881 1417. 10 A. Tessier, P.G.C. Campbell and M. Bisson, Anal. Chem., 51 (1979) 844. 11 A.D. Hewitt and C.M. Reynolds, At. Spectrosc., ll(1990) 187. 12 Microbics manual, How to run Microtox M500, Microbics Company, C&bad, NM.