86 Abstracts

Lead phosphate formation in soils

Janet Cotter-Howells Environmental Geochemistry Research, Centre for Environmental Technology and Department of Geology, Imperial College, London SW7 2BP, UK Present address: Departments of Geology and Environmental Biology, Manchester University, Manchester M13 9PL, UK

The influence of the solid speciation of lead in soils on human bioavailability has been investigated in a number of areas of the UK, including soils contaminated by mine-waste.

Elevated concentrations of lead have been found in garden soils and housedusts within an old lead mining village in Derbyshire, England. However, blood lead concentrations of children aged 18 yr are within normal UK ranges.

Investigation of the solid speciation of lead in soils from the lead mining village and other areas of the UK used a density separation technique to separate lead-bearing grains from the soil matrix. Grains were identified and classified using an SEM/EDX technique. An amorphous form of the lead phosphate, pyromorphite (Pbs(PO4)3C1), was found to be a c o m m o n c ons t i t ue n t of the lead-bearing solid phase. It is particularly abundant in soils from the lead mining village, where it represents in excess of 50% of the total soil-lead.

Pyromorphite has formed in these soils as the

endproduct of the weathering of lead compounds, in accordance with thermodynamic predictions. It has an extremely low solubility in acidic environments which approximate to conditions in the human s tomach. Thus, the substant ia l amounts of pyromorphite present in soils from the mining village would reduce the bioavailability of soil-lead to humans. This accounts for the unexceptional blood lead concentrations found in children living in the mining village, despite high environmental lead levels.

The amounts of pyromorphite present in soils increases with increasing residence time of c o n t a m i n a n t and also with total so i l - l ead concentration implying that pyromorphite acts as a reservoir for soil-lead. However, pyromorphite did not form in all soils indicating that geochemical factors can control pyromorphite formation. It is conceivable that the formation of pyromorphite could be used as a "clean up" agent of contaminated land sites.

Environmental protection and the development

of geothermal energy resources

K~ith Nicholson Environment Division, School of Applied Sciences, The Robert Gordon University, Aberdeen AB1 1HG, Scotland, UK

Geothermal resources are natural polluters of the env i ronmen t . Water and steam discharges containing potentially harmful constituents are found over most geothermal fields, and the development of geothermal resources enhances this pollution potential through the transmission and disposal of large masses of waste water and steam extracted from the reservoir. The type and concen t r a t ion of po l lu tan t s carr ied by the geothermal fluid is dependant upon several factors: - hos-trock-water reactions in the reservoir

(organic-rich, sedimentary host rocks provide greatest levels of pollutants );

- flow rate of fluid discharged by the system (grea tes t impac t is caused by sys tems discharging large masses of geothermal water and steam);

- reservoir temperature (the concentration of pollutants increases with increasing reservoir temperature).

Near-neutral-pH waters represent the deep g e o t h e r m a l f luid. They are c o m p o s e d predominantly of chloride, sodium and potassium and can carry significant concentrations of trace elements. Geothermal steam is produced by boiling of the reservoir fluid at depth. Gases typically form about 2% of the steam discharge. Carbon dioxide, compris ing 95-98% of the gas content, and hydrogen sulphide (2-3%) are the main gases (Nicholson, 1993). Minor and variable amounts of ammonia, nitrogen, hydrogen and methane may also be present, together with the volatile species of boron, arsenic and mercury.

Species of most environmental concern include

Abstracts 87

CO2, H2S, As, B and Hg. As the latter three species occur in both the water and steam phases and they can accumulate in soils, geothermal deposits and aquatic sediments to levels far in excess of their dissolved concentrations. Remobilisation of species in such sinks would release an elevated, potentially toxic, flush of the species into the environment emphasise this time-accumulation-remobilisation factor, the term "pollution time-sink" is used to describe these sedimentary elemental reservoirs.

To illustrate the environmental impact of

geothermal waters, the pathways and sinks of arsenic, boron and mercury from New Zealand geothermal systems are examined. All three elements form time-sinks in soils, sediments and geothermal deposits.

R e f e r e n c e

Nicholson, K. 1993. Geothermal Fluids: Chemistry and Exploration Techniques. Springer-Verlag. 263pp.

Determination of boron in environmental

samples by ion-selective electrode: an

evaluation

John Wood and Keith Nicholson

Environment Division School of Applied Sciences, The Robert Gordon University, Aberdeen ABI IHG, Scotland, UK

Boron is an essential trace nutrient but aqueous or soil concentrations in excess of about 4 mg kg q can be toxic to certain plants and crops. As boron is usually present in solution as the neutral species H3BO3, it is not generally removed by standard water treatment processes and with multiple reuse of waters can accumulate in rivers to unacceptable levels. This work is directed to developing a routine method for boron determination that is rapid, cheap and simple, and one that can be adapted to an automated, on-hne monitoring system.

The f luorobora te-se lec t ive electrode was chosen as the method which may best fulfil the above requirements. Although developed in the early 1970s, this technique has received little attention in the literature. Prior to application of the i n s t r u m e n t to e n v i r o n m e n t a l s amp le s and field-based systems, it was therefore necessary to first evaluate the analytical performance of the electrode and it is the results of this evaluation that are reported here.

The time of the electrode to attain equilibrium

emf was evaluated at boron concentrations from 0.05 to 5.0 mg L q. The response time increased with lower boron concentration. Minimum times to equilibrium emf ranged from one minute at 5.0 mg

1 1 L- B, to 5 minutes at 0.05 mg L- B. These represent minimum values and have been incorporated into the analytical procedure being developed.

The electrode calibration departs from linear Nemstian response at 0.35 mg L -I B, but despite curvature in the graph, this remains reproducible to 0.01 mg L 1 B using a 10 minute equilibration time. However, as curvature of the calibration graph increases with decreasing boron concentration, sensitivity of the electrode method is significantly

l reduced at concentrations below 0.03 mg L- B. Reproducibility of the results at 0.1 mg L-I B

and 1.0 mg L -1 B were examined by determinations of at least 20 al iquots of a batch solution. Performance was excellent with reproducibility's within 0.5% at both concentrations using a 10 minute equilibration time.