Iain Thornton

Chemical partitioning of trace and major elements in soils contaminated by mining and smelting activities

Abstract Soils from historical Pb mining and smelting areas in Derbyshire, England have been analysed by a 5-step sequential extraction procedure, with multielement determination on extraction solutions at each step by ICP-AES. Each of the chemical fractions is operationally defined as: (i) exchangeable; (ii) bound to carbonates or specifically adsorbed; (iii) bound to Fe–Mn oxides; (iv) bound to organic matter and sulphides; (v) residual. The precision was estimated to be about 5%, and the overall recovery rates were between 85 and 110%. The carbonate/specifically adsorbed and Fe–Mn oxide phases are the largest fractions for Pb in soils contaminated by both mining and smelting. Most of the Zn is associated with Fe–Mn oxide and the residual fractions. Cadmium is concentrated in the first 3 extraction steps, particularly in the exchangeable phase. The most marked difference found between soils from the mining and smelting sites is the much higher concentrations and proportions of metals in the exchangeable fraction at the latter sites. This indicates greater mobility and potential bioavailability of Pb, Zn and Cd in soils at the smelting sites than in those in the mining area. The most important fraction for Fe and Al is the residual phase, followed by the Fe–Mn oxide forms. In contrast, the Fe–Mn oxide fraction is the dominant phase for Mn in these soils. In the mining area, most of the Ca is in the carbonate fraction (CaCO3), while the exchangeable and residual phases are the main fractions for Ca at the smelting sites. Phosphorus is mainly in the residual and organic fractions in both areas. The exchangeable fractions of Pb, Zn and Cd in soils were found to be significantly related to the concentrations of these metals in pasture herbage.

Applied Environmental Geochemistry

Since then applied environmental geochemistry has grown enormously to encompass the many factors influencing the sources, dispersion and distribution of elements in the environment, their pathways into foodstuffs and water supplies, and possible effects on health and disease in plants, animals and man. The multi-disciplinary and international nature of this work is reflected by the authorship of the contributed chapters in this volume.Progress has been assisted by two major advances. Firstly the development of rapid low-cost instrumental prosedures enabling the simultaneous analysis of geochemical samples for a large number of elements fed directly to computer format.

Heavy metal distribution in sediment profiles of the Pearl River estuary, South China

The Pearl River estuary is created by the inflow of freshwater from the largest river system that drains into the South China Sea. In recent years, massive economic growth and development in the region has led to excessive release of waste into the environment. The accumulation of contaminants in sediments is likely to pose serious environmental problems in surrounding areas. The study of sediment profiles can provide much information on the metal contamination history and long term potential environmental impacts. In this project, 21 core samples (up to 3.65 m deep) were collected in the Pearl River estuary. About 15 subsamples from each core were analysed for moisture content, total organic matter (L.O.I.), particle size and heavy metal and major element concentrations. The results show that Pb and Zn contents are elevated in the sediments at most of the sampling sites. Compared with historical monitoring results, the sediment metal contents have increased over the last 20 a, particularly for Pb. The west side of the Pearl River estuary tends to be more contaminated than the east side due to the contaminants inputs from the major tributaries and different sedimentation conditions. There are close associations between Fe, Co, Ni and Cu concentrations in the sediments. Zinc and Pb contents in the sediment profiles reflect a combination of the natural geochemical background, anthropogenic influences and the mixing effects within the estuary. The distribution of Pb in the sediments shows strong influences of atmospheric inputs, probably from the coal burning activities in the region.

Sequential extraction of soils for multielement analysis by ICP-AES

Realistic environmental interpretation of soil contamination depends on an understanding of how metals are bound to the various phases in the soil. A five-step sequential chemical extraction scheme, originally designed for sediment analysis by flame atomic adsorption spectroscopy (FAAS), has been developed for the multielement analysis of soils by inductively coupled plasma-atomic emission spectrometry (ICP-AES). Each of the chemical fractions is operationally defined as follows: (1) exchangeable; (2) bound to carbonates or specifically adsorbed; (3) bound to FeMn-oxides; (4) bound to organic matter and sulphides; and (5) residual. The number of elements determined by ICP-AES has been extended to fifteen (Be, Ca, Co, Cr, Cu, Fe, K, Li, Mn, Ni, P, Pb, Ti, V, Zn), which include most of the major elements, thereby increasing the potential of the sequential extraction method by enabling broader studies of geochemical associations in soils. The precision was estimated to be ∼ 5% (2σ) for each extraction step. The overall recovery rates of international reference materials were between 85 and 110% for most elements, with an average of 92%. There is good agreement between the results for the international reference material (USGS MAG-1) in each extraction step and published values. A wide range of soil reference materials, including SO-1-SO-4 and BCR141-BCR143, were also analysed for future comparison. The application of the method to soils contaminated by past mining and smelting activities showed distinctive partitioning patterns of heavy metals from the two sources. The multielement measurements gave useful information to assist in the interpretation of the possible geochemical forms and sources of the trace elements in soils.

Urban Geochemistry: A study of the influence of anthropogenic activity on the heavy metal content of soils in traditionally industrial and non-industrial areas of Britain

Abstract Heavy metal concentrations have been determined in topsoils (0–15cm) in the London Borough of Richmond-upon-Thames, a non-industrial, mainly residential area of approximately 56 km 2 , and Wolverhampton an industrial city in the West Midlands of 70 km 2 . Soil samples were taken on a grid basis at a density of four per km 2 and analysis for 25 elements was carried out by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Topsoils in Richmond were found to have significantly higher concentrations of heavy metals in developed locations compared to areas of open space, whilst in Wolverhampton topsoils a greater degree of contamination with Zn was found than with Pb. GIS-based mapping techniques used in conjunction with statistical analysis of the data have highlighted the influence of land-use on the heavy metal content of topsoils in these two urban areas. The highest concentrations of Pb in Richmond-upon-Thames (> 1000 μg/g) tend to occur close to major road junctions on roads with high traffic densities. High levels of Pb (approx. 500 μg/g) also occur in the areas where the oldest housing is located (> 100 a). In Wolverhampton the highest concentrations of heavy metals, Zn in particular, are generally located to the east of the city in areas of both historical and contemporary industrial activity.

Factors influencing metal bioavailability in soils: preliminary investigations for the development of a critical loads approach for metals

The concept of critical loads, previously applied to acidifying substances, is currently being extended, within the Convention on Long Range Transboundary Air Pollution (CLRTAP), to several metals: Cd, Cu, Hg, Pb and Zn. Receptors in the soil, such as plants and microbiota, are directly exposed to these metals via the soil solution and this should ideally be reflected in environmental quality objectives which, presently, are commonly based on ‘total’ rather than bioavailable metal contents of soils. An understanding of the factors influencing the partition of metals between the solid and solution phases of soils is an essential prerequisite of any attempt to apply the critical loads approach in a manner which relates to the effects of metals on soil organisms.

Heavy metal contamination of soils and plants in the vicinity of a lead-zinc mine, Korea

Current research has been focused on the influence of base metal mining on heavy metal contamination in the surface environment and its implications for soils and plants. Soils were sampled in and around a PbZn mine in Korea from sites including the mine dump, nearby household gardens and uncultivated areas. Various plants were also collected in the household gardens including corn grain, red peppers, soybean leaves, spring onions and tobacco leaves. Similar materials were taken from a nearby control area. Analysis by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) included the metals Cd, Cu, Pb and Zn. Concentrations of heavy metals in soils decreased exponentially with distance from the mine source mainly due to dispersion by water and topography. The metal concentrations in the plant species sampled generally decreased in the order: tobacco leaves > spring onions > soybean leaves > red pepper ≈ corn grain, although this order varied for specific elements. These results agreed with other reports that metal levels in leaves (tobacco and soybean) are usually higher than those in fruit and grain (red pepper and corn). Tobacco leaves had elevated concentrations of the heavy metals with maximum concentrations of 8.5 μg g−1 Cd (DW), 24 μg g−1 Pb (DW) and 1620 μg g−1 Zn (DW). The ratios of metal concentrations in plants to those in soils decreased in the order Zn > Cd > Cu > Pb. Factors influencing the bioavailability of metals in soils and uptake into food crops were examined, including total metal concentration in soil, soil pH, cation exchange capacity, organic matter content, soil texture and interaction among the elements determined. It is concluded that the total metal concentrations in soil and pH are the main factors influencing metal content in plants.

Soil ingestion — a major pathway of heavy metals into livestock grazing contaminated land

An estimated 4000 km2 of agricultural land in England and Wales has been contaminated in varying degrees by past mining and smelting activities. Contaminants include one or more of the metals Cu, Pb, Zn, Cd and As. Studies conducted in southwest and central England conclude that only a small proportion of these metals are taken up into the leaf material of pasture plants and that plant uptake would not seem to constitute a major pathway to grazing animals. Using the titanium content of faeces as a stable indicator of soil ingestion, we found that grazing cattle involuntarily ingest from 1% to nearly 18% of their dry matter intake as soil; sheep may ingest up to 30%. Soil ingestion varies seasonally and with farm management. Calculations based on soil, plant and faecal analyses show that from 9% to 80% percent of the Pb and 34% to 90% of the As intake into cattle on contaminated land is due to ingested soil.

Arsenic enrichment in waters and sediments of the Rio Loa (Second Region, Chile)

The Second Region of Chile (126,500 km2) is extremely arid, with a dramatic scarcity of water. The only water resource for the population (about 420,000 habitants) and the mining industry (the most important economic activity in the region) is the 440 km long Rio Loa. Moreover, this is highly enriched in As. In order to assess As concentrations and sources, and to evaluate the impact of mining activity on the water quality in the Rio Loa basin, water and sediment samples were taken at strategic points along the river and its major tributaries. The water in the whole basin is quite saline (total dissolved solids up to 11 g/l) and heavily enriched in As (average: 1400 μg/l) and B (average: 21,000 μg/l). These values are up to 300 and 100 times higher than the respective guidelines suggested by the WHO for drinking water. The quality of water is extremely poor along the tributary Salado, mainly fed by the El Tatio geothermal waters that are very rich in As (up to 27,000 μg/l) and other components. Sediments from the Rio Loa and its tributaries have As contents in the range of 26–2000 mg/kg (mean value of 60 samples: 320 mg/kg), and reach 11,000 mg/kg at El Tatio. Sequential extraction analyses show the As to be mainly associated with Fe–Mn oxy-hydroxides and residual phases, but part of the As (about 20%) is readily available being extracted from the exchangeable and carbonate phases. This result is in agreement with the correlation observed between As content in sediments and As concentration in waters in the area. The extreme arid conditions, high evaporation, and the lack of low-As tributaries contribute to maintain high concentrations of As and other components in the Rio Loa water to the mouth. Due to the oxidising conditions, neutral to alkaline pH, high salinity and high As concentrations, adsorption of As-species is not favoured. The main As source in the Rio Loa basin is considered to be natural, i.e. linked to the lithologies in the area. Smelter emissions and mining wastes, as well as the As-rich effluents from the water treatment plants, possibly represent additional sources.

Environmental contamination and seasonal variation of metals in soils, plants and waters in the paddy fields around a PbZn mine in Korea

The objective of this study is to investigate the extent and degree of heavy metal contamination of paddy fields influenced by metalliferous mining activity. Paddy soils, rice plants and irrigation waters were sampled along six traverse lines in the vicinity of the mine and nearby control site. Soil samples were taken 30, 80 and 150 days after rice transplanting, to study seasonal variation of their chemical properties and heavy metal concentrations. Sampling of rice plants and irrigation waters was also undertaken with seasons. The analysis of the samples were carried out using ICP-AES for 25 elements including Cd, Cu, Pb and Zn. Physical and chemical properties of soils (pH, loss-on-ignition, cation exchange capacity and texture) and waters (pH, Eh and temperature) were also measured. The properties of soils were similar to the average Korean soils, with the exception of some samples taken in the vicinity of the mine. Concentrations of Cd, Cu, Pb and Zn in paddy soils, rice plants and irrigation waters sampled in the immediate vicinity of the mine were relatively high due to the seepage of metals from mining dump sites. Although there was variation between sampling sites, soil pH values under reducing conditions were on average higher than those under oxidising conditions. Relatively low content of organic matter and low cation exchange capacity of soils were found at 80 days after rice transplanting (P < 0.05). No seasonal variations in metal concentrations were found in paddy soils throughout the period of the rice growing, in which soils ranged from flooded reducing conditions through most of the growing season to drained oxidising conditions before and at harvest. Relatively high metal concentrations were found in the rice stalks and leaves under oxidising conditions. The sequential extraction analysis of selected soil samples confirmed that high proportions of exchangeable fractions of the metals were found under oxidising conditions. It was shown that Cd and Zn concentrations in rice leaves and stalks and rice grain increased with increasing metal concentrations in paddy soils to a greater extent than for Cu and Pb. This difference in uptake is in agreement with the greater proportions of Cd and Zn, compared with Cu and Pb, in the exchangeable soil fraction extracted with MgCl2. Average daily intake from locally grown rice by the residents was estimated to be 121 micrograms Cd and 126 micrograms Pb. Thus, long-term metal exposure by regular consumption of the rice poses potential health problems to residents in the vicinity of the mine, although no adverse health effects have as yet been observed.